Valerie A. Luyckx Barry M. Brenner
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Fetal Programming of Adult Disease, 654 |
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Nephron Number, 654 |
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Nephron Number and Glomerular Volume, 656 |
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Evidence for Fetal Programming and the Kidney, 657 |
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Programming of Nephron Number in Humans, 658 |
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Programming of Renal Function and Disease, 659 |
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Proposed Mechanisms of Fetal Programming of Nephron Number, 662 |
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Maternal Nutrient Restriction, 664 |
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Increased Apoptosis in the Kidney, 665 |
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Glial-cell Line-derived Neurotrophic Factor and c-ret Receptor Function, 665 |
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Fetal Exposure to Glucocorticoids, 666 |
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Fetal Exposure to Hyperglycemia and the Role of Insulin-like Growth Factors and Their Receptors, 666 |
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Fetal Drug Exposure, 667 |
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Programmed Changes Within the Kidney, 667 |
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Renal Vascular Reactivity, 668 |
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Renin-Angiotensin System, 668 |
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Altered Sodium Handling in the Kidney, 668 |
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Impact of Nephron Endowment on Transplantation Outcomes, 669 |
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Conclusion, 669 |
Genetic factors are important determinants of development and function of major organ systems as well as of susceptibility to disease. Rare genetic and congenital abnormalities leading to abnormal kidney development are associated with the occurrence of subsequent renal dysfunction, often manifest in very early in life. [1] [2] Most renal disease in the general population, however, is not ascribable to genetic mutations, with the most common causes of end-stage renal disease (ESRD) worldwide being the polygenic disorders of diabetes and hypertension. Hypertension and renal disease prevalence vary among populations from different ethnic backgrounds, with very high rates being observed among Aboriginal Australians, Native Americans, and people of African descent. [3] [4] [5] [6] Similarly, renal disease in hypertension and diabetes appear to “run” in families. It is well established that lifestyle factors pose significant risk for the development and persistence of hypertension and diabetes in the general population, with increasing obesity being the most concerning, especially in the developing world.[7] Searches for specific genetic polymorphisms or mutations, however, have not yielded smoking “genes” except in rare kindreds, but instead point to a likely complex interplay between polygenic predisposition and environmental factors in the development of diabetes, hypertension, and renal disease. [7] [8] [9] [10] Furthermore, evidence highlighting the far-reaching effects of the intrauterine environment on organ development, organ function, and subsequent susceptibility to adult disease is becoming more and more compelling. These data suggest that fetal development may be the first in a succession of “hits” that ultimately manifests in overt disease expression. This chapter outlines the effects of fetal programming on renal development (nephrogenesis), nephron endowment, and the risks of hypertension and kidney disease in later life. Low birth weight also predicts later-life diabetes, and therefore, renal function may be affected indirectly as well, through fetal programming effects on other organ systems that are beyond the scope of this discussion. [11] [12]
FETAL PROGRAMMING OF ADULT DISEASE
The process through which an environmental insult experienced early in life, particularly in utero, can predispose to adult disease is known as fetal programming or developmental plasticity. Fetal programming refers to the observation that an environmental stimulus experienced during a critical period of development in utero can induce long-term structural and functional effects in the developing organism.[13] Developmental plasticity is the process whereby different phenotypes may result on a background of a single genotype in response to different environmental stimuli experienced during intrauterine life.[14] These phenomena are intimately linked and have far-reaching implications in that their effects can be transferred and perpetuated across generations.[15]
The association between adverse intrauterine events, for which low birth weight may be a surrogate marker, and subse-quent cardiovascular disease has long been recognized. [13] [14] [16] [17] Adults of low birth weight have higher cardiovascular morbidity and mortality than those of normal birth weight.[18] Subsequently, a large body of evidence from different populations has not only confirmed these initial findings but also expanded them to include other conditions such as hypertension, impaired glucose tolerance, type 2 diabetes, obesity, and chronic kidney disease. [13] [19] [20] [21] [22] [23] Of these, the relation between low birth weight and subsequent hypertension has been the most studied, as demonstrated in Figure 19-1 . [22] [23] [24] [25] [26] It is important to note that re-ported blood pressures tend to be higher in infants, children, and young adults of low birth weight compared with normal birth weight, but do not reach overt hypertensive ranges until well into adulthood in most studies. [26] [27] [28] Blood pressures are also highest in those of low birth weight who “caught up” fastest in postnatal weight.[26] The differences in blood pressure between people of low birth weight and those of normal birth weight also become amplified with age, with the result that adults who had been of low birth weight often develop overt hypertension, which increases with age.[29]
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FIGURE 19-1 Studies reporting multiple regression analysis of change in systolic blood pressure (mm Hg) per kg increase in birth weight in children, adolescents, and adults. (For complete citations for individual studies, please refer to original reference, Figure 1, p 819.) (Adapted from Huxley RR, Shiell AW, Law CM: The role of size at birth and postnatal catch-up growth in determining systolic blood pressure: A systematic review of the literature. J Hypertens 18:815–831, 2000.) |
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NEPHRON NUMBER
The kidney is the organ central to the development of hypertension. The relationship between renal sodium handling, intravascular fluid volume homeostasis, and hypertension is well accepted. [30] [31] In addition, all known monogenetic mutations associated with hypertension involve proteins expressed in the kidney. [32] [33] That factors intrinsic to the kidney itself affect blood pressure has been demonstrated clinically in renal transplantation, in which the blood pressure in the recipient after transplantation has been shown to be related to the blood pressure or hypertension risk factors of the donor: that is, hypertension “follows” the kidney.[34]
In 1988 Garcia and colleagues [35] [36] proposed that a congenital (programmed) variation in nephron number may be a factor explaining why some individuals are susceptible to hypertension and renal injury whereas others may seem relatively resistant under similar circumstances (e.g., sodium excess or diabetes mellitus). A reduction in nephron number and whole kidney glomerular surface area would result in reduced sodium excretory capacity enhancing susceptibility to hypertension and reduced renal reserve limiting compensation for renal injury. This hypothesis is attractive in that an association between a reduced nephron number and low birth weight, a surrogate marker of an adverse intrauterine environment, for example, may explain some of the differences in hypertension and renal disease prevalence observed among populations of different ethnicity, among whom those who tend to have lower birth weights often have higher prevalences and more rapid progression of renal disease. [37] [38] [39] [40] Recent evidence is also pointing not only to an association with low birth weight and subsequent renal disease but also to high birth weight being a risk factor, especially for diabetic nephropathy. [5] [38]
An obstacle to investigation of the nephron number hypothesis has been the difficulty of accurately counting nephron numbers.[41] Review of early studies shows that humans were believed to have an average of approximately one million nephrons per kidney.[42] Such studies, however, were performed using techniques such as acid-maceration or traditional stereologic analysis, which are prone to bias because of required assumptions, extrapolations, and operator sensitivity. [41] [42] [43] More recently, an unbiased fractionator-sampling/dissector-counting method has been developed that is believed to be more objective and reproducible. [41] [42] [43] It is important to recognize that all reported glomerular counting techniques have been performed on autopsy samples and that, to date, no validated technique permits determination of nephron number in vivo. Basgen and associates[44] attempted to develop an in vivo glomerular counting method and compared the fractionator technique with a combined renal biopsy/magnetic resonance imaging (MRI) method on excised canine kidneys. These authors found a good agreement of glomerular number on average between the two methods, but within kidneys, there was a 36% difference, potentially making the renal biopsy/MRI technique less useful in individuals. It is possible that with some refinement this technique may become more accurate and provide a useful technique to determine glomerular numbers in vivo.
Using the fractionator technique, among 37 normal Danish adults, the average glomerular (nephron) number was reported to be 617,000 per kidney (range 331,000–1,424,000).[42] These authors also reported a positive correlation between glomerular number and kidney weight, which has subsequently been used as a surrogate marker for nephron number in vivo. Another study including 78 kidneys from subjects of multiple ethnic origins from the United States and Australia showed somewhat similar results, with a mean of 784,909 glomeruli per kidney, but with a very wide range, from 210,332 to 1,825,380.[45] In both studies, numbers of viable glomeruli were reduced in kidneys from older subjects, owing to age-related glomerulosclerosis and obsolescence. [42] [45] Glomerular number, therefore, appears to vary by up to eightfold within the normal population. The variability of mean nephron number reported in presumed normal subjects in different studies, from 617,000 to 1,429,200, should raise a note of caution about the fractionator technique. [42] [46] Whether these differences reflect true differences in the populations studied or are reflections of small samples sizes will become clearer with time as more studies accumulate or as better techniques evolve.
It is known that persons born with severe nephron deficits, for example, unilateral renal agenesis, bilateral renal hypoplasia, and oligomeganephronia, develop progressive proteinuria, glomerulosclerosis, and renal dysfunction with time. [47] [48] [49] [50] [51] Similarly, people born with nephron numbers at or below the median level may be more susceptible to superimposed postnatal factors that act as subsequent “hits”; thus, a significant proportion of the population may be at risk for the development of hypertension and renal disease.[43] This may be a plausible hypothesis given that some 30% of the world's adult population is hypertensive.[7] Drawing on experimental data in animals, surgical removal of more than one kidney under different circumstances and in different species does not always lead to the development of hypertension and renal disease.[43] In humans, uninephrectomy is accompanied by compensatory hypertrophy and function of the remaining contralateral kidney, often with little adverse clinical consequence, although progressive hypertension and proteinuria have been reported. [52] [53] Of interest, however, uninephrectomy on postnatal day 1 in rats or fetal uninephrectomy in sheep, that is, loss of nephrons at a time when nephrogenesis is not yet completed, does lead to adult hypertension prior to any evidence of renal injury. [54] [55] [56] These data support the hypothesis that intrauterine or congenital reduction in nephron number, that is, before nephrogenesis is completed, may be associated with different compensatory mechanisms or a reduced compensatory capacity than occurs in response to later nephron loss, resulting in subsequent development or risk of hypertension. In support of this hypothesis, kidneys from rats that underwent uninephrectomy at 3 days of age showed a similar total glomerular number, but a significantly reduced number of mature glomeruli compared with those who underwent nephrectomy at 120 days of age.[57] Furthermore, the mean glomerular volume in neonatally nephrectomized rats increased by 59% versus 20% in the adult nephrectomized rats, indicating a likely greater burden of compensatory hypertrophy and hyperfunction in response to neonatal nephrectomy.
Nephron Number and Glomerular Volume
Despite the large variation in nephron number seen in the normal population, it has been noted consistently that glomerular volume varies inversely with glomerular number as shown in Figure 19-2 . [46] [58] [59] This observation suggests that larger glomeruli may reflect compensatory hyperfiltration and hypertrophy in subjects with fewer nephrons. [45] [59] In fact, Hoy and co-workers[58] found that, although mean glomerular volume was increased in subjects with reduced nephron numbers, total glomerular tuft volume (a surrogate for total filtration surface area) was not different among groups with different nephron numbers ( Table 19-1 ). This observation suggests that total filtration surface area may initially be maintained in the setting of a reduced nephron number but at the expense of glomerular hypertension and hypertrophy, which are maladaptive and predictors of poor outcomes. [60] [61] [62]Consistent with this possibility, glomerulomegaly is common in renal biopsies from Australian Aborigines, a population with high rates of low birth weight and renal disease, and has also been associated with faster rate of decline of glomerular filtration rate (GFR) in Pima Indians. [63] [64] [65] Furthermore, in a study of donor kidneys, maximal planar area of glomeruli was found to be higher in kidneys from African Americans compared with whites and a predictor of poorer transplant function.[62] In populations at high risk of kidney failure, therefore, large glomeruli are a common finding at early stages of renal disease and may reflect programmed reductions in nephron number in these populations in which access to prenatal and subsequent health care is often suboptimal. [66] [67] [68]
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FIGURE 19-2 Birth weight, glomerular number, and glomerular volume in adults. (From Hughson M, Farris AB, Douglas-Denton R, et al: Glomerular number and size in autopsy kidneys: The relationship to birth weight. Kidney Int 63:2113–2122, 2003.) |
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TABLE 19-1 -- Glomerular Characteristics by Birth Weight in Humans
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Birth Weight |
N |
No Glomeruli[*] |
Mean Glomerular Tuft Volume (μm2 × 106) |
Total Glomerular Tuft Volume (cm2) |
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2.65 Kg (1.81–3.12) |
29 |
770,860 (658,757–882,963) |
9.2 |
6.7 |
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3.27 Kg (3.18–3.38) |
28 |
965,729 (885,714–1,075,744) |
7.2 |
6.8 |
|
3.93 Kg (3.41–4.94) |
30 |
1,005,356 (900,094–1,110,599) |
6.9 |
6.6 |
From Hoy WE, Hughson MD, Bertram JF, et al: Nephron number, hypertension, renal disease, and renal failure. J Am Soc Nephrol 16:2557–2564, 2005.
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* |
Adjusted for age, gender, race, and body surface area. |
EVIDENCE FOR FETAL PROGRAMMING AND THE KIDNEY
Low birth weight is defined by the World Health Organiza-tion as a birth weight less than 2500 grams. Low birth weight can be the result of intrauterine growth restriction (IUGR; birth weight <10th percentile for gestational age) or premature birth. Low birth weight associated with IUGR generally reflects intrauterine stress during late gestation as opposed to low birth weight of prematurity, which may be an appropriate weight for the duration of gestation. Full-term birth low birth weight (i.e., IUGR) has the strongest association with adult disease.[69] Low birth weight is more common among African Americans, Native Americans, and Aboriginal Australians than among whites, the former being populations with disproportionately high rates of hypertension, chronic kidney disease, type 2 diabetes, and cardiovascular disease. [4] [38] [70] [71]
Multiple animal models have demonstrated the association of low birth weight (induced by gestational exposure to low-protein diet, uterine ischemia, dexamethasone, vitamin A deprivation) with subsequent hypertension. [72] [73] [74] [75] [76] [77] [78] The link between adult hypertension and low birth weight in these animal models appears to be mediated, at least in part, by an associated congenital nephron deficit occurring with IUGR. [72] [75] [77] Vehaskari and colleagues[75] demonstrated an almost 30% reduction in glomerular number in offspring of pregnant rats fed a low-protein diet compared with a normal-protein diet during pregnancy. As shown in Figure 19-3 , the maternal low-protein-fed offspring had systolic blood pressures that were 20 to 25 mm Hg higher by 8 weeks of age.[75] Similarly, Celsi and associates[72] found that prenatal administration of dexamethasone in rats was also associated with low birth weight and fewer glomeruli compared with controls. In these nephron-deficient rats, GFR was reduced, albuminuria was increased, and urinary sodium excretion was lower than those with a greater nephron complement.[72]These findings in animals lend credence to the hypothesis that a congenital deficit in nephron number, resulting in a decreased filtra-tion surface area and thus a limitation in renal sodium excretion, is an independent factor determining susceptibility to essential hypertension. Low nephron number alone, however, does not account for all observed programmed hypertension. Langely-Evans and co-workers[24] reported that supplementation of a low-protein diet during gestation with glycine, urea, or alanine resulted in a normalization of nephron number in the offspring but only a normalization of blood pressure in those supplemented with glycine. This finding suggests that there may be factors leading to intrauterine programming of hypertension in addition to, or independent of, nephron number.
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FIGURE 19-3 Fetal programming of hypertension in low-birth-weight rats. (Adapted from Vehaskari VM, Aviles DH, Manning J: Prenatal programming of adult hypertension in the rat. Kidney Int 59:238–245, 2001.) |
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Programming of Nephron Number in Humans
As mentioned previously, nephron numbers vary widely in the normal human population ( Fig. 19-2 ; see also Table 19-1 ). More and more data are emerging supporting a direct relationship between nephron number and birth weight and an inverse relationship between nephron number and glomerular volume. [59] [79] [80] After analysis of 56 kidneys, Hughson and colleagues[80] reported a linear relationship between glomerular number and birth weight and calculated a regression coefficient predicting an increase of 257,426 glomeruli per kilogram increase in birth weight. The applicability of the regression coefficient in populations in which the distribution of nephron number appears bimodal, however, may not be valid. It has also been calculated that in the normal population without renal disease, approximately 4500 glomeruli are lost per kidney per year after the age of 18.[58] Glomerular numbers tend to be lower in females than in males. A kidney starting with a lower nephron number, therefore, would conceivably reach a critical reduction of nephron mass, either with age or in response to an renal insult, earlier than a kidney with a greater nephron complement, contributing to hypertension and/or renal dysfunction.
Kidney development in the human begins during the 9th week of gestation and continues until the 34th to 36th week.[58] Nephron number at birth is, therefore, largely dependent on the intrauterine environment and gestational age. It is generally believed that no new nephrons are formed in humans after birth. In an attempt to investigate whether glomerulogenesis does indeed continue postnatally in premature infants, Rodriguez and colleagues[81] studied kidneys at autopsy from 56 extremely premature infants compared with 10 full-term infants as controls. The radial glomerular counts were lower in premature versus full-term infants and correlated with gestational age. Furthermore, evidence of active glomerulogenesis, indicated by the presence of basophilic S-shaped bodies immediately under the renal capsule, was found in premature infants who died before 40 days but absent in those who died after 40 days of life. The authors concluded that nephrogenesis may continue for up to 40 days after birth in premature infants. Interestingly, these authors also stratified their cases by presence or absence of renal failure in the infants. Among infants surviving longer than 40 days, those with renal failure (serum creatinine ≥2.0 mg/dL) had significantly fewer glomeruli than those without renal failure. This cross-sectional observation may suggest that renal failure inhibited glomerulogenesis or, conversely, that fewer glomeruli lowered the threshold to develop renal failure in these extremely ill infants. Those premature infants surviving longer than 40 days without renal failure exhibited glomerulomegaly, which may reflect, at least in the short term, a compensatory renoprotective response. In contrast to these findings, Hinchliffe and associates [82] [83] studied nephron number in premature or full-term stillbirths or infants who died at 1 year of age and who were born with either appropriate weight for gestational age or small for gestational age. At both time points, growth-restricted infants had fewer nephrons than controls. In addition, the number of nephrons in growth-restricted infants dying at 1 year of age had not increased compared with the growth-restricted stillbirths, demonstrating a lack of postnatal compensation in nephron number ( Fig. 19-4 A). Manalich and co-workers[59] examined the kidneys of neonates dying within 2 weeks of birth in relation to their birth weights ( Fig. 19-4 B). A significant direct correlation was found between glomerular number and birth weight. In addition, there was also a strong inverse correlation between glomerular volume and glomerular number independent of sex and race. These studies, therefore, support the hypothesis that an adverse intrauterine environment, manifest as low birth weight, in infants, is associated with a congenital reduction in nephron number and an early, compensatory increase in glomerular volume.
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FIGURE 19-4 A, Effect of IUGR on nephron number in humans. (a) Nephron number in relation to gestational age; (b) lack of postnatal catch-up in nephron number. B, Birth weight, glomerular volume, and glomerular number in neonates. (A, From Hinchliffe SA, Lynch MR, Sargent PH, et al: The effect of intrauterine growth retardation on the development of renal nephrons. Br J Obstet Gynaecol 99:296–301, 1992; B, From Manalich R, Reyes L, Herrera M, et al: Relationship between weight at birth and the number and size of renal glomeruli in humans: A histomorphometric study. Kidney Int 58:770–773, 2000.) |
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Renal mass is known to be proportional to nephron number, and renal volume is proportional to renal mass; therefore, renal volume has been analyzed as a surrogate for nephron endowment in infants in vivo.[42] Renal volume was found to be reduced by anthropomorphic and Doppler flow measurements performed in utero in fetuses subjected to growth restriction.[84] A subsequent analysis of kidney size and growth postnatally, as assessed by ultrasound, in 178 children born premature or small for gestational age as compared with 717 mature children with appropriate weight for gestational age at 0, 3, and 18 months, found that weight for gestational age was positively associated with kidney volume at all three time points.[85] Slight catch-up kidney growth was observed in growth-retarded infants but not in premature infants. In Australian Aboriginal children, low birth weight was also found to be associated with lower renal volumes on ultrasound.[86] A smaller kidney size, therefore, may be a surrogate marker of a reduced nephron endowment, but it must be borne in mind that the growth in kidney size on ultrasound cannot distinguish between the components of normal growth with age and renal hypertrophy.
In a population of 140 adults aged 18 to 65 years old, who died of various causes, a significant correlation was also observed between birth weight and glomerular number.[79] Glomerular volume was again found to be inversely correlated with glomerular number. Mean glomerular numbers did not differ statistically among African American and white subjects, although the distribution among African Americans appeared bimodal, with a few outliers having very high nephron numbers, and several subjects having nephron numbers below 500,000. No white subject had fewer than 500,000 nephrons. Significantly, however, none of the subjects in this study had been of low birth weight; therefore, no conclusion can be drawn as to whether an association with low birth weight and nephron number exists in either population group.[87] In a European study among 26 subjects with non-insulin-dependent diabetes compared with 19 age-matched nondiabetic controls, no difference in glomerular number was found, but again, all subjects had birth weights above 3000 g, and therefore, the impact of low birth weight on nephron number could not be assessed.[88]
In support of the potential association of nephron number and hypertension, a German study of whites aged 35 to 59 years who died in accidents found that, in 10 subjects with a history of essential hypertension, the number of glomeruli per kidney was significantly lower, and glomerular volume significantly higher, than in 10 normotensive-matched controls ( Fig. 19-5 ).[46] Birth weights were not reported in this study, but the authors concluded that a reduced nephron number is associated with susceptibility to essential hypertension. Similarly, among a subset of 63 subjects in whom mean arterial pressures and birth weights were available, Hughson and co-workers[79] reported a significant correlation between birth weight and glomerular number, mean arterial pressure and glomerular number, and mean arterial pressure and birth weight among the white but not the African American subjects. Among African Americans having nephron numbers below the mean, however, twice as many were hypertensive as normotensive, suggesting a possible contribution of lower nephron number in this group as well.[79] In addition, glomerular volumes were found to be higher among the hypertensive African American subjects compared with hypertensive whites.[79] A similar finding was reported among donor kidney biopsies, in which maximal planar area of glomeruli was found to be higher in African Americans than in whites.[62] In this study, glomerulomegaly emerged as an independent predictor of poor allograft function.
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FIGURE 19-5 Nephron number (A) and glomerular volume (B) in Caucasian subjects with primary hypertension compared with controls. (From Keller G, Zimmer G, Mall G, et al: Nephron number in patients with primary hypertension. N Engl J Med 348:101–108, 2003.) |
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Programming of Renal Function and Disease
Experimental Evidence
As opposed to infants of low birth weight, in whom nephron numbers have been shown to be reduced, in adults, there are no data on nephron number specifically in those who had been of low birth weight. The association between nephron number and birth weight, however, does appear to be a consistent finding in infants, so the extrapolation seems reasonable that nephron numbers would also remain reduced in adults of low birth weight.[80] The determination of nephron number in vivo, as mentioned previously, is difficult and not yet reliable enough; therefore, the most utilized in vivo surrogate marker available at present is birth weight. In some animal models, low nephron numbers have been observed also in the setting of normal birth weight; therefore, among humans, if birth weight is the only surrogate marker used, the impact of nephron number on any outcome is likely to be underestimated.[89] Glomerulomegaly is also consistently observed in the setting of a low nephron number. Although this may be a compensatory mechanism to restore filtration surface area, it is conceivable that renal reserve in these kidneys in reduced.[58] If this is the case, these kidneys may be expected to be less able to compensate further in the setting of additional renal insults and to begin to manifest signs of renal dysfunction (i.e., proteinuria, elevations in serum creatinine, and hypertension).
In a provocative study, diabetes was induced by streptozotocin injection in subgroups of low-birth-weight (induced by maternal protein restriction) and normal-birth-weight rats.[90] Low-birth-weight rats, as expected, were found to have reduced nephron numbers and higher blood pressures compared with those of normal birth weight. Among those rendered diabetic, there was a greater proportional increase in renal size and glomerular hypertrophy in the low-birth-weight rats than in normal-birth-weight controls after 1 week ( Fig. 19-6 ).[90] This study demonstrates that the renal response to injury in the setting of a reduced nephron number may be exaggerated and could lead to accelerated loss of renal function. Subsequently, the same authors[91] published outcomes in low-birth-weight versus normal-birth-weight diabetic rats at 40 weeks. Histologically, the podocyte density was reduced and the average area covered by each podocyte was greater in the low-birth-weight diabetic rats than in the normal-birth-weight controls. These findings correlated with urine albumin excretion rate, which was higher in low-birth-weight diabetic rats, although this did not reach statistical significance. In support of the role of altered podocyte physiology in renal disease progression, similar findings were observed in the Munich Wistar-Fromter rat, a strain that has congenitally reduced nephron numbers and develops spontaneous renal disease (see later).[92] Whether these podocyte changes are secondary to an increase in glomerular pressure in the setting of reduced nephron numbers or a primary programmed structural change leading to glomerular injury is not yet known.
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FIGURE 19-6 A and B, Influence of glomerular number on adaptation to diabetes in rats. (From Jones SE, Bilous RW, Flyvbjerg A, Marshall SM: Intra-uterine environment influences glomerular number and the acute renal adaptation to experimental diabetes. Diabetologia 44:721–728, 2001.) |
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As mentioned previously, hypertension has frequently been reported in low-birth-weight rats and sheep, but is not universally observed. [13] [24] [25] [72] [93] Some authors have reported the presence of salt-sensitive hypertension in rats in which low birth weight was induced by maternal uterine artery ligation, whereas others report no salt-sensitivity in rats in which low birth weight was induced by maternal protein restriction. [94] [95] GFR measured in rats in which low birth weight was induced by maternal protein restriction was found to be reduced, concomitant with a reduction in nephron number. [76] [96] Of interest, GFR was reduced by 10%, although nephron number was reduced by 25%, implying some degree of compensatory hyperfunction per nephron ( Fig. 19-7 ).[25] In contrast, in low-birth-weight rats exposed to prenatal dexamethasone and subsequently fed a high-protein diet, GFR was similar to that in normal-birth-weight controls.[97] Nephron numbers were reduced by 13% in only male low-birth-weight rats. This study may suggest that there is a threshold reduction in nephron number above which compensation is adequate or that the high-protein diet induced supranormal GFRs in both groups, masking subtle differences in baseline GFR. Another study that measured GFR in low-birth-weight rats, this time induced by maternal uterine artery ligation during gestation, also failed to demonstrate a difference in GFR in low-birth-weight rats, but they were significantly hypertensive compared with normal-birth-weight controls.[98] Conceivably, in this study, the higher intraglomerular pressure due to elevated blood pressure and reduced nephron mass in low-birth-weight rats may have led to a compensatory increase in single-nephron GFR (SNGFR) and, thus, normalization of whole-kidney GFR. In another study, low-birth-weight rats, which had been subjected to gestational protein restriction, had significantly higher blood pressures and urinary protein excretion at 20 weeks of age than controls, although again GFR was not different.[99] Definitive understanding of the pathophysiologic impact of a reduction in nephron number is difficult to elucidate from the existing literature comprising very varied experimental conditions. Ooverall, however, it is possible that, although whole-kidney GFR may not change, SNGFR is likely to be increased in the setting of a reduced nephron number. In support of this possibility, the Munich-Wistar-Fromter rat is known to develop spontaneously progressive glomerular injury. Interestingly, compared with the control Wistar rat strain, nephron numbers have been found to be significantly reduced, urine protein excretion and systolic blood pressure to be significantly higher, and by micropuncture study, the SN GFR was found to be significantly elevated. A “naturally” occurring (i.e., not experimentally induced) congenital nephron deficit, therefore, does appear to predispose to progressive renal functional decline.
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FIGURE 19-7 Increased blood pressure, decreased glomerular filtration rate, and glomerular number in low-birth-weight adult male rats. (From Vehaskari VM, Woods LL: Prenatal programming of hypertension: Lessons from experimental models. J Am Soc Nephrol 16:2545–2556, 2005.) |
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Evidence in Humans
One consequence of glomerular hyperfiltration is microalbuminuria. Studies from several countries have demonstrated an increased prevalence of microalbuminuria and proteinuria among adults who had been of low birth weight. [4] [5] [64] [100] [101] [102] In long-term follow-up studies of children who had been extremely low-birth-weight premature infants, weighing less than 1000 g a birth, serum creatinine was found to be higher and GFR reduced compared with those in age-matched normal-birth-weight children at ages 6 to12 years.[103] Another study compared blood pressures and renal function in females in their mid-20s who had been preterm, small-for-gestational-age, or normal full-term infants.[104] Blood pressures were significantly higher among those who had been preterm compared with the normal-birth-weight controls. There was no statistically significant difference in GFR or urinary albumin excretion between the groups, but GFR tended to be lower in the small-for-gestational-age group and albuminuria higher in the preterm and small-for-gestational-age groups. This study included fewer than 20 subjects in each group, which might suggest that, with larger numbers, statistical significance may have been reached. In a similar study, 422 19-year-old subjects who had been very premature were stratified according to whether they had been appropriate weight or small for gestational age at birth.[105] Birth weight was found to be negatively associated with serum creatinine and albuminuria and positively associated with GFR. The authors concluded that IUGR is associated with poorer renal function in young adults. Prematurity itself may therefore be associated with poor long-term renal function in some studies, a risk that is increased when there is associated IUGR.
Analysis of 724 subjects aged 48 to 53, who had been subjected to malnutrition in midgestation during the Dutch famine, revealed an increased prevalence of microalbuminuria (12%) when compared with those subjected to malnutrition during early gestation (9%), late gestation (7%), or not exposed to famine (4–8%).[106] Interestingly, size at birth was not associated with the observed increase in microalbuminuria, suggesting that renal development may have been irreversibly affected in midgestation, although sub-sequent intrauterine whole-body growth was able to catch up with restoration of more normal nutrition. These data further emphasize the need for other surrogate markers of the intrauterine environment in addition to birth weight. Furthermore, among all subjects who had microalbuminuria in this cohort, systolic and diastolic blood pressures were increased, glucose intolerance was more prevalent, and GFRs were increased, indicating a degree of compensatory hyperfiltration.
The association between low birth weight and subsequent metabolic syndrome has been well described in many populations around the world. [14] [18] Whether very early renal dysfunction, manifest as microalbuminuria, is a trigger or a consequence of the metabolic syndrome is a topic of significant interest. Recent evidence points to improved cardiovascular outcomes associated with reduction in microalbuminuria, which may support the former possibility, although experimental evidence also supports simultaneous programming of the endocrine pancreas and cardiovascular system during fetal development.[107]
It is not easy to dissect the relative contributions of genetics and the fetal environment to the ultimate manifestation of disease. To address this question, Gielen and colleagues[108] studied 653 twins, comprising 265 twin pairs and 123 individuals whose twin did not participate in the study. Creatinine clearance was significantly lower in low-birth-weight than in normal-birth-weight twins. Furthermore, intrapair birth weight differences were positively correlated with GFR in both monozygotic and dizygotic twin pairs, tha tis, the twin with a higher birth weight had a higher creatinine clearance. These authors concluded that fetoplacental factors have a greater impact than genetic factors on adult renal function.
Early compelling evidence of the relationship between birth weight and renal function was published by Hoy and associates [4] [64] [101] [102] in the Australian Aboriginal population. These authors found that the odds ratio for overt albuminuria was 2.8 in those who had been of low birth weight with a reference value of 1.0 for those of normal birth weight ( Fig. 19-8 ).[101] Furthermore, in addition to the association of albuminuria with low birth weight, the degree of albuminuria predicted loss of renal function and was strongly correlated with both renal and nonrenal deaths. [64] [65] Similarly, among Pima Indians with type 2 diabetes, the prevalence of albuminuria was 63% in those who had a birth weight less than 2500 g, 41% in those of normal birth weight, and 64% among those of high birth weight (≥4500 g).[5] After controlling for maternal diabetes, the odds of albuminuria among those of high birth weight was not increased, indicating a major role for gestational exposure to diabetes in programming of renal disease risk. This finding has been confirmed in other studies.[109] Gestational hyperglycemia often results in high birth weight, and as is discussed later, is also associated with a reduced nephron number in the offspring.
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FIGURE 19-8 Birth weight and albumin-to-creatinine ratio in Tiwi adults. (From Hoy WE, Mathews JD, McCredie DA, et al: The multidimensional nature of renal disease: Rates and associations of albuminuria in an Australian Aboriginal community. Kidney Int 54:1296–1304, 1998.) |
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A handful of studies have examined the relationship between birth weight and diabetic nephropathy and found an increased susceptibility among subjects who had been of low birth weight. [5] [110] [111] [112] [113] Among women with type 1 diabetes mellitus, nephropathy was present in 75% of those with a birth weight below the 10th percentile (≥2700 g), compared with 35% of those with birth weights above the 90th percentile (≥4000 g).[112] This relationship was not present in men, however, but men with diabetic nephropathy were significantly shorter than those without nephropathy, possibly indicating some degree of growth restriction. [111] [112]
A variety of generally small studies have reported a greater severity of renal disease and more rapid progression of diverse renal diseases, including immunoglobulin A (IgA) nephropathy, membranous nephropathy, minimal change disease, nephrotic syndrome, and chronic pyelonephritis, among children and adults who had been of low birth weight. [110] [113] [114] [115] [116] [117] Lackland and co-workers[40] examined the relationship of birth weight and ESRD in 1230 dialysis patients and 2460 age- and sex-matched normal controls in South Carolina. This population has a high prevalence of ESRD among young patients, in whom 70% is attributable to hypertension or diabetes. In this cohort, the odds ratio for ESRD was 1.4 (1.1–1.8) among those with birth weights under 2500 g compared with those of normal birth weight. This association was consistent for all causes of ESRD and was not affected by family history of ESRD. [40] [118] Interestingly, the odds ratio for diabetic renal disease was 2.4 for those having birth weights greater than 4000 g ( Fig. 19-9 ). Although maternal diabetic status is not given, such high birth weights suggest intrauterine exposure to hyperglycemia and its role in programming of renal disease. Epidemiologic data are, therefore, accumulating in support of the impact of fetal programming on subsequent renal disease.
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FIGURE 19-9 Birth weight and risk of end-stage renal disease. (From Lackland DT, Bendall HE, Osmond C, et al: Low birth weights contribute to high rates of early-onset chronic renal failure in the Southeastern United States. Arch Intern Med 160:1472–1476, 2000.) |
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PROPOSED MECHANISMS OF FETAL PROGRAMMING OF NEPHRON NUMBER
Kidney development is a complex process involving tightly controlled expression of many genes and constant remodeling.[119] Many experimental models, as outlined in Table 19-2 , have been shown to result in a reduced nephron number. In many of the experimental models of programming, as mentioned previously, a reduced nephron number is often associated with low birth weight and, in some cases, with subsequent hypertension and evidence of renal injury. Interestingly, in normal rat litters, those pups with naturally occurring low birth weight (i.e., birth weights < -2 SD from the mean) were found to have a 13% reduction in nephron number, which was also associated with glomerulomegaly and proteinuria.[120] Low birth weight in rodents, therefore, may be associated with a low nephron number even under nonexperimental conditions. Low birth weight in humans is often associated with poor maternal nutrition, smoking, alcohol ingestion, infections, and low socioeconomic status, all factors that, in turn, may affect nephrogenesis.[58] In humans, kidney development begins around 8 weeks of gestation and continues until 36 weeks. Approximately two thirds of the nephrons develop during the last trimester of gestation, making this the window most susceptible to adverse effects, but earlier insults can also have a major impact on subsequent nephrogenesis. [82] [121] In rodents, nephrogenesis continues for up to 10 days after birth, but from most animal studies, the major impact is noted when environmental stimuli are manipulated when nephrogenesis is most active, that is, mid to late gestation.[121]
TABLE 19-2 -- Nephron Endowment
|
Model |
Subject |
Nephron Number (%) |
BW |
BP |
Renal Function |
|
Maternal calorie restriction [129] [130] [156] |
Rat |
↓ 20–40 |
↓ |
↑ |
↓ GFR proteinuria |
|
Uterine artery ligation [120] [137] [187] [188] |
Rat |
↓ 20–30 |
↓ |
↑ |
Impaired proteinuria |
|
Low-protein diet [24] [25] [99] [128] |
Rat |
↓ 25 |
↓/↔ |
↑ |
↓ GFR proteinuria |
|
↓ 17 |
|||||
|
↓ 16 |
↓ Longevity |
||||
|
Iron deficiency[124] |
Rat |
↓ 22 |
↓ |
↑ |
NA |
|
Vitamin A deficiency[125] |
Rat |
↓ 20 |
↔ |
NA |
NA |
|
Glucocorticoids [72] [77] [140] [189] |
Rat |
↓ 20 |
↔ |
↑ |
Glomerulosclerosis |
|
↑ Collagen deposition |
|||||
|
Sheep |
↓ 38 |
↔ |
↑ |
||
|
Maternal diabetes[145] |
Rat |
↓ 10–35 |
↔ |
NA |
NA |
|
Gentamicin[149] |
Rat |
↓ 10–20 |
↓ |
NA |
NA |
|
β-Lactams[150] |
Rat |
↓ 5–10 |
↔ |
NA |
Tubular dilatation |
|
Interstitial inflammation |
|||||
|
Cyclosporine [152] [153] |
Rabbits |
↓ 25–33 |
↓/↔ |
↑ |
↓ GFR |
|
↑ RVR proteinuria |
|||||
|
Dahl salt-sensitive[35] |
Rat |
↓ 15 |
↔ |
↑ With Na intake |
Accelerated FSGS |
|
Munich-Wistar-Fromter rat [35] [190] |
Rat |
↓ 40 |
|
↑ With age |
↑ |
|
SNGFR |
|||||
|
FSGS |
|||||
|
Milan hypertensive rat[35] |
Rat |
↓ 17 |
↑ |
NA |
|
|
PVG/c[35] |
Rat |
↑ 122 |
Resistant |
Resistant to FSGS |
|
|
Os/+ mouse[191] |
Mouse |
↓ 50 |
NA |
Glomerular hypertrophy |
|
|
PAX2 mutations [134] [135] |
Mouse |
↓ 22 |
NA |
Renal coloboma syndrome in |
|
|
GDNF heterozygote [138] [139] |
Mouse |
↓ 30 |
↔ |
↑ |
Normal GFR |
|
Enlarged glomeruli |
|||||
|
c-ret null mutant[119] |
Mouse |
↓ |
NA |
NA |
Severe renal dysplasia |
|
hIGFBP-1 overexpression[146] |
Mouse |
↓ 18–25 |
↓ |
NA |
Glomerulosclerosis |
|
Bcl-2 deficiency[133] |
Mouse |
↓ |
NA |
NA |
↑ Urea and creatinine |
|
BF-2 null mutant[119] |
Mouse |
↓ 75 |
NA |
NA |
NA |
|
BMP 7 null mutant[119] |
Mouse |
↓ |
NA |
NA |
Small kidneys |
|
p53 transgenic[136] |
Mouse |
↓ 50 |
NA |
NA |
Glomerular hypertrophy |
|
Renal failure |
|||||
|
Intrauterine growth retardation [59] [82] |
Human 0–1 yr |
↓ 13–35 |
↓ |
NA |
NA |
|
Hypertensive vs. normotensive Caucasian [46] [79] |
Human 35–59 yr |
↓ 19–50 |
NA |
↑ |
No ↑ glomerulosclerosis |
|
Hypertensive vs. normotensive African American[79] |
Human 35–59 yr |
NS ↓ |
NA |
↑ |
No ↑ glomerulosclerosis |
|
Aboriginal Australians[43] |
Human 0–85 yr |
↓ 23 |
↓ |
NA |
Glomerular hypertrophy |
Adapted from Brenner BM, Garcia DL, Anderson S: Glomeruli and blood pressure. Less of one, more the other? Am J Hypertens 1:335–347, 1988; and Kett MM, Bertram JF: Nephron endowment and blood pressure: What do we really know? Curr Hypertens Rep 6:133–139, 2004.
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BP, blood pressure; BW, birth weight; FSGS, focal segmental glomerulosclerosis; GFR, glomerular filtration rate; NA, not assessed; NS, nonsignificant; RVR, renal vascular resistance; SNGFR, single-nephron GFR. |
Three processes determine ultimate nephron number: branching of the ureteric bud, condensation of mesenchymal cells, and conversion of mesenchymal condensates into epithelium.[119] It has been estimated that a 2% decrease in ureteric bud branching efficiency would result in a 50% reduction in final nephron complement after 20 generations of branching.[122] The specific molecular mechanisms whereby nephron numbers may be affected and/or function altered, how-ever, are not yet completely understood. Several potential mechanisms have been proposed and investigated thus far, as summarized in Table 19-3 and discussed later.
TABLE 19-3 -- Experimental Models and Proposed Mechanisms of Intrauterine Programming of Renal Development
|
Experimental Model |
Possible Mechanism of Nephron Number Reduction |
|||
|
Maternal low-protein diet |
↑ Apoptosis in metanephrons and postnatal kidney |
|||
|
Altered gene expression in developing kidney |
||||
|
Altered gene methylation |
||||
|
↓ Placental 11-βHSD2 expression |
||||
|
Maternal vitamin A restriction |
↓ Branching of ureteric bud |
|||
|
? Maintenance of spatial orientation of vascular development |
||||
|
↓ c-ret expression |
||||
|
Maternal iron restriction |
? Reduced oxygen delivery |
|||
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? Altered glucocorticoid responsiveness |
||||
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? Altered micronutrient availability |
||||
|
Gestational glucocorticoid exposure |
↑ Fetal glucocorticoid exposure |
|||
|
? Enhanced tissue maturation |
||||
|
↑ Glucocorticoid receptor expression |
||||
|
↑ 1α- and 1 |
||||
|
↓ Renal and adrenal 11-βHSD2 expression |
||||
|
Uterine artery ligation/embolization |
↑ Pro-apoptotic gene expression in developing kidney: casepase-3, Bax, p53 |
|||
|
↓ Antiapoptotic gene expression: PAX2, bcl-2 |
||||
|
Altered gene methylation |
||||
|
Maternal diabetes/hyperglycemia |
↓ IGF-II/mannose-6-phosphate receptor expression |
|||
|
Altered IGF-II activity/bioavailability |
||||
|
Gestational drug exposure |
||||
|
↓ Branching morphogenesis |
|||
|
↑ Mesenchymal apoptosis |
|||
|
Arrest of nephron formation |
|||
Maternal Nutrient Restriction
Experimental alterations in maternal dietary composition at different stages of gestation have been shown to program embryonic kidney gene expression early in the course of gestation, which later affects nephron number.[123] Fetal nutrient supply is also affected by alterations in placental blood flow. Maternal protein restriction during pregnancy has been the most widely studied model, but manipulations at different times of gestation and for different periods during gestation make results not always easy to compare and interpret. Furthermore, not all low-protein diets have the same programming effects. It has been proposed that relative deficiencies of specific amino acids—methionine or glycine, for example—may have a greater impact on organ development than total protein restriction per se.[24] Such effects have been proposed to be mediated largely by changes in DNA methylation, depending on amino acid availability, resulting in epigenetic changes in gene expression.[13]
Maternal iron restriction during pregnancy in rats also leads to a reduction in birth weight and nephron number and the development of subsequent hypertension in the offspring.[124] The authors suggest that fetal anemia may result in reduced tissue oxygen delivery, altered fetal kidney glucocorticoid sensitivity, or altered availability of other micronutrients that may affect nephrogenesis. These hypotheses remain to be proved.
Maternal vitamin A restriction has also been associated with a reduction in nephron number in the offspring. [78] [125] Severe vitamin A deficiency during pregnancy is associated with congenital malformations and renal defects in the offspring. Vitamin A and all-trans retinoic acid have been shown to stimulate nephrogenesis through modulation of ureteric bud branching capacity in ureteric epithelial cell culture and in maintenance of spatial organization of blood vessel development in cultured renal cortical explants.[125] In vivo, a vitamin A-deficient diet sufficient to reduce circulating vitamin A levels by 50% in pregnant rats resulted in a 25% reduction in nephron number in the offspring.[125] Intriguingly, supplementation of vitamin A increased nephron numbers. Analysis of 21-day-old fetal rats (just before birth) revealed a direct correlation between plasma retinol concentration and nephron number, as shown in Figure 19-10 .[125] The reduction in nephron number in the setting of vitamin A deficiency is likely mediated at least in part by modulation of genes regulating branching morphogenesis.[125] These genes are discussed in detail later. It is interesting to note that smoking and alcohol intake are associated with reduced levels of circulating vitamin A, and both, in pregnancy, are associated with infant low birth weight. There has been a single abstract suggesting an impact of maternal alcohol ingestion on kidney development, but it is not known whether the effects are mediated by associated vitamin A deficiency or other mechanisms.[58] Subtle differences in vitamin A level during pregnancy, therefore, may be a significant factor contributing to the wide distribution of nephron number in the general population.[58]
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FIGURE 19-10 Nephron number and plasma retinol in term rat fetuses. (From Merlet-Benichou C: Influence of fetal environment on kidney development. Int J Dev Biol 43:453–456, 1999.) |
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Increased Apoptosis in the Kidney
Total calorie restriction and maternal dietary protein restriction in animals result in low birth weight of offspring and frequently associated hypertension and reduced nephron numbers. [13] [75] [126] [127] [128] [129] [130] Vehaskari and colleagues[75] reported a 29% reduction in nephron number in low-birth-weight rat offspring of mothers subjected to a 6% low-protein diet compared with a 20% normal-protein diet during pregnancy. Associated with this reduction, systolic blood pressures were 20 to 25 mm Hg higher at 8 weeks in pups from the low-protein-diet group than in the normal-protein-diet group (see Fig. 19-3 ). These authors also found that, despite the kidneys' looking histologically normal at 8 weeks' postnatally, there was evidence of increased apoptosis without an increase in proliferation in the low-protein-diet group. Welham and co-workers[126] examined embryonic metanephroi to evaluate at which stage of development a low-protein diet affects nephrogenesis. At embryonic day 13, the metanephros has just formed, the ureteric bud has branched once, branch tips are surrounded by condensed mesenchyme that later transforms into tubule epithelium, and the ureteric stalk is surrounded by loose stromal mesenchyme.[123] By day 15, multiple branching cycles have occurred and primitive nephrons begin to be formed.[123] The authors, therefore, examined metanephroi at these two time points. At embryonic day 13, there was no difference in the number of cells in metanephroi from embryos whose mothers had received a normal-protein diet compared with those eating a reduced-protein diet. At day 15, however, there were significantly fewer cells per metanephros in the low-protein group compared with the normal-protein group. Furthermore, when they examined apoptosis at these two time points, they observed a significant increase in the numbers of apoptotic cells in the low-protein group at day 13 but not at day 15. The authors concluded that the increase in early (day 13) apoptosis was most likely responsible for the reduced cell numbers later (day 15). As mentioned previously, Vehaskari and colleagues[75] noted an increase in apoptosis at 8 weeks postnatally in offspring of low-protein-diet-fed rats; therefore, there may be successive waves of apoptosis at different stages of nephrogenesis that may affect final nephron endowment.
Welham and co-workers[126] described an increase in apoptosis in both the condensing and the loose mesenchyme of the metanephros but did not measure the relative amount of cell death in each compartment. They therefore suggest two possible mechanisms whereby an increase in apoptosis observed in the offspring of low-protein-diet-fed dams at embryonic day 13 could lead to a reduction in nephron number: (1) directly through loss of actual nephron precursors (i.e., in the condensing mesenchyme) or (2) indirectly through loss of cells in the loose mesenchyme (i.e., the stromal compartment, which supports nephrogenesis but does not contribute actual cells to the final epithelial lineage).[126] These hypotheses are as yet unproved, but evidence for impact of changes in the supporting metanephric stroma on nephron development and number is emerging. Mice deficient in the BF-2 transcription factor, expressed in metanephric stroma, have abnormal kidney development and reduced nephron numbers, associated with slower differentiation of condensed mesenchyme into tubule epithelium and decreased ureteric branching.[131]Nephron development therefore depends upon a close relationship between tubule epithelial precursors and surrounding tissue matrix. Both compartments are therefore likely to be susceptible to programming effects.
Other studies have suggested that altered regulation of apoptosis in the developing kidney may be due to down-regulation of anti-apoptotic factors (e.g., Pax-2 or Bcl-2) and/or up-regulation of pro-apoptotic factors in response to environmental or other stimuli (e.g., Bax, p53). [126] [132] [133] [134] Humans with haploinsufficiency of PAX2 have renal coloboma syndrome, which includes renal hypoplasia and early renal failure as well as optic nerve colobomas.[134] [135] PAX2 is an anti-apoptotic transcriptional regulator that is highly expressed in the branching ureteric bud as well as in foci of induced nephrogenic mesenchyme during kidney development.[134] Heterozygous mice with Pax2 mutations were found to be very small at birth and to have significant reductions in nephron number. In addition, there was a significant increase in apoptotic cell death in the developing kidneys. Subsequently, the same group[135] demonstrated that loss of Pax2 anti-apoptotic activity reduced ureteric bud branching and increased ureteric bud apoptosis. Similarly, loss of the anti-apoptotic factor Bcl-2 or gain of function of the pro-apoptotic factor p53 are also associated with a significant reduction in nephron number, associated with increased apoptosis in metanephric blastemas, in Bcl-2 knockout mice and p53 transgenic mice. [133] [136]
Mutant mouse models, however, although providing evidence that an increase in apoptosis results in reduced nephron numbers, do not address the impact of environmental factors in renal programming. Pham and associates[137]examined gene expression in the kidneys of growth-retarded offspring of rats subjected to uterine artery ligation during gestation. These authors found a 25% reduction in glomerular number, associated with increased evidence of apoptosis and increased pro-apoptotic caspase-3 activity in the kidney at birth. Furthermore, they found evidence of increased mRNA expression of the pro-apoptotic genes Bax and p53 and a decreased expression of the anti-apoptotic gene Bcl-2. These authors also found evidence of hypomethylation of the p53 gene, which in addition to a decrease in Bcl-2 expression, would lead to an increase in p53 activity. Alteration in gene methylation has also been proposed as a mechanism of protein-diet-induced programming effects, as mentioned previously. The increase in apoptotic activity in the developing kidney subjected to varied gestational insults therefore appears to be a consistent finding and mediated via modulation of gene expression in a number of different pathways.
Glial-Cell Line-derived Neurotrophic Factor and c-ret Receptor Function
Glial-cell line-derived neurotrophic factor (GDNF), signal-ing through its receptor-tyrosine kinase Ret, is known to be a key ligand-receptor interaction driving initiation of ureteric bud branching. C-ret is expressed on the tips of the ureteric bud branches, and knockout of this receptor in mice leads to severe renal dysplasia and reduction in nephron number.[119] Homozygous GDNF null mutant mice have complete renal agenesis and die shortly after birth.[138]Heterozygous GDNF mice exhibit reduced branching morpho-genesis and have approximately 30% fewer nephrons than wild-type mice. [138] [139] These mice also develop spontaneous hypertension and glomerulomegaly with time. As described previously, maternal dietary vitamin A has a significant impact on nephrogenesis (see Fig. 19-10 ). In cultured metanephroi, the expression of c-ret was found to be regulated by retinoic acid supplementation in a dose-dependent manner.[78] GDNF expression was not affected by vitamin A fluctuations. Modulation of c-ret expression is therefore likely to be a significant pathway through which vitamin A availability regulates nephrogenesis.
Fetal Exposure to Glucocorticoids
Under normal circumstances, the fetus is protected from exposure to excess maternal corticosteroids by the placental enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which metabolizes corticosterone to the inert 11-dehydrocorticosterone.[13] Prenatal administration of dexamethasone, a steroid not metabolized by 11β-HSD2, was found to lead to fetal growth restriction, a 20% to 60% reduction in nephron number, glomerulomegaly, and subsequent hypertension in rats and sheep. [72] [77] [97] [140] Similar effects have been seen with lower levels of placental 11β-HSD2 in rats and humans with mutations in the 11β-HSD2 gene, in whom birth weights are low and hypertension develops prematurely. [141] [142] Interestingly, maternal low-protein diet during gestation has been shown to result in decreased placental expression of 11β-HSD2, therefore likely increasing the exposure of the fetus to maternal corticosteroids ( Fig. 19-11 ). [24] [143] Treatment of pregnant rats fed a low-protein diet with an inhibitor of steroid synthesis abrogates the programming of hypertension in the offspring, suggesting a prominent role for fetal steroid exposure in the low-protein-diet model. [13] [24] Excessive fetal steroid exposure may then drive inappropriate gene expression and affect growth and nephrogenesis, potentially through more rapid maturation of tissue structures.[24]
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FIGURE 19-11 Decreased expression of placental 11bHSD2 in placentas of mothers fed low-protein diets during gestation. (From Bertram C, Trowern AR, Copin N, et al: The maternal diet during pregnancy programs altered expression of the glucocorticoid receptor and type 2 11beta-hydroxysteroid dehydrogenase: Potential molecular mechanisms underlying the programming of hypertension in utero. Endocrinology 142:2841–2853, 2001.) |
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In an attempt to examine the molecular mechanisms through which glucocorticoid exposure may program hypertension, Bertram and associates[143] examined expression of steroid-responsive receptors in offspring of rats fed a low-protein diet during gestation. These authors found a greater than twofold increase in fetal and neonatal glucocorticoid receptor mRNA expression in offspring of mothers fed a low-protein diet compared with those fed a normal-protein diet. This difference increased to threefold as the offspring aged. In addition, the expression of the corticosteroid responsive renal Na/K-ATPase α1- and β1-subunits were also increased in these offspring ( Fig. 19-12 ). Expression of the mineralocorticoid receptors was not different among the two groups. Interestingly, levels of 11β-HSD2 in offspring kidney and adrenal were significantly reduced during fetal and postnatal life in those exposed to a low-protein diet in utero. These authors conclude that the observed changes would result in marked increases in glucocorticoid action in these tissues and is likely a significant mediator of programmed hypertension.[143] The mechanism of glucocorticoid-mediated reduction in nephron number has not yet been elucidated.
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FIGURE 19-12 Programming of glucocorticoid sensitivity. (From Bertram C, Trowern AR, Copin N, et al: The maternal diet during pregnancy programs altered expression of the glucocorticoid receptor and type 2 11beta-hydroxysteroid dehydrogenase: Potential molecular mechanisms underlying the programming of hypertension in utero. Endocrinology 142:2841–2853, 2001.) |
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Fetal Exposure to Hyperglycemia and the Role of Insulin-like Growth Factors and Their Receptors
As discussed previously, in the South Carolina dialysis population and the Pima Indian population in Arizona, high birth weight is associated with an increased susceptibility to proteinuria and renal disease (see Fig. 19-9 ). [5] [38] [109] High birth weight is a complication of gestational hyperglycemia and diabetes and may therefore also be a surrogate marker of abnormal intrauterine programming. To address the question of whether gestational diabetes affects nephrogenesis in the offspring, Amri and co-workers[144] studied offspring of rats rendered hyperglycemic during pregnancy either by inducing diabetes mellitus with streptozotocin or by infusing glucose from gestational days 12 to 16. Nephron numbers in offspring exposed to maternal hyperglycemia were reduced by 10% to 35%, and the degree of nephron number reduction correlated with the degree of maternal hyperglycemia ( Fig. 19-13 ). Furthermore, in vitro culture of metanephroi subjected to varying glucose concentrations demonstrated that tight glucose control is necessary for optimal metanephric growth and differentiation.
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FIGURE 19-13 Effects of maternal hyperglycemia on nephron number in rat offspring. (From Amri K, Freund N, Vilar J, et al: Adverse effects of hyperglycemia on kidney development in rats: In vivo and in vitro studies. Diabetes 48:2240–2245, 1999.) |
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Offspring of diabetic pregnancies have a higher incidence of congenital malformations, resulting for defects in early organogenesis.[145] Furthermore, it is known that expression and bioavailability of the insulin-like growth factors (IGFs) are altered in diabetic pregnancies, and that IGFs and their binding proteins are important regulators of fetal development.[145] The impact of maternal diabetes on metanephros expression of IGFs and their receptors was studied in rats in which diabetes was induced by streptozotocin compared with gestational-age-matched normal controls.[145] In metanephroi from offspring subjected to maternal diabetes, there was no significant change in IGF-1 or -II or insulin receptor expression at any stage. Throughout nephrogenesis, however, there was a significantly increased expression of the IGF-II/mannose-6-phosphate receptor. The authors postulate that as this receptor tightly regulates the action of IGF-II, a reduction in its expression may lead to enhanced activity of IGF-II, a critical player in renal development. The same group of investigators have examined the impact of IGF binding protein-1 on nephrogenesis in genetically modified mice.[146] Overexpression of human IGF binding protein-1 in adult mice results in glomerulosclerosis. Offspring of females overexpressing human IGF binding protein-1 were found to be growth restricted and to have an 18% to 25% reduction in nephron number depending on whether human IGF binding protein-1 was overexpressed in the mother only, fetus only, or both. When metanephroi from these mice were cultured in the presence of IGF-I or IGF-II, the authors found that IGF-II increased nephron numbers by 25% to 40% in a concentration dependent manner, whereas IGF-1 had no effect.[146] This study did not involve maternal diabetes or hyperglycemia, but the findings are consistent for a role of increased IGF-II activity in nephrogenesis.
Fetal Drug Exposure
Several medications commonly used during pregnancy have been studied for their effects on nephrogenesis. The aminoglycoside antibiotic gentamicin, administered to pregnant rats, results in a permanent nephron deficit in the offspring.[147] In subsequent experiments, the same authors demonstrated a significant reduction in nephron number in metanephric explants cultured in the presence of gentamicin.[148] In cultured metanephroi, within 8 hours of administration, gentamicin was localized to the growing tips of ureteric buds and the surrounding blastema, and within 24 hours, the presence of gentamicin was associated with a significant reduction in the number of branching points.[149] These data suggest that the reduction in nephron number observed after administration of gentamicin during pregnancy is a result of a decrease in branching morphogenesis.
Another group of antibiotics that has been shown to result in impaired nephrogenesis is the β-lactams.[150] Administration of ampicillin to pregnant rats leads to an 11% average reduction in nephron number in the offspring, as well as evidence of focal cystic tubule dilatation and interstitial inflammation. The administration of ceftriaxone in vivo did not result in a nephron deficit, but histologically, there was evidence of renal interstitial inflammation. The penicillins were also found to inhibit nephrogenesis in cultured metanephroi in vitro in a dose-dependent fashion, an effect that was less evident with ceftriaxone. Importantly, nephrogenesis was affected even at therapeutic doses of penicillins in the rats, which warrants further research on such frequently used antibiotics in human pregnancy. The mechanism whereby these antibiotics reduce nephron number is likely through an increase in apoptosis observed in the induced mesenchyme in exposed developing kidneys.[78]
The immunosuppressive medication cyclosporine is a known nephrotoxin in humans; it crosses the placenta.[151] Women treated with this medication may have successful pregnancies, but its effect on the fetal kidney is not well described, although infants tend to have birth weights in the low range.[151] The effects of administration of this medication in varying doses and at different stages of gestation were evaluated in pregnant rabbits compared with rabbits receiving either vehicle or no drug.[152] Cyclosporine administration in the later period, but not the earlier period, of gestation resulted in smaller litters and growth-restricted pups. All pups exposed to cyclosporine in utero had a 25% to 33% reduction in nephron number compared with controls. The reduction in nephron number was accompanied by glomerulomegaly and was independent of birth weight. At 1 month of age, these kidneys also demonstrated foci of glomerulosclerosis. Subsequent functional evaluation of the kidneys of rabbits exposed to cyclosporine in utero demonstrated a reduction in GFR at 18 and 35 weeks of age and an increase in proteinuria at 11, 18, and 35 weeks of age.[153] Rabbits exposed to cyclosporine in utero developed spontaneous hypertension by 11 weeks of age, which worsened progressively with time.[153] It is important to recognize that, despite reduction in nephron number from birth, renal function did not deteriorate until later, an important factor to bear in mind when evaluating children exposed to cyclosporine in utero. Nephron formation was found to be arrested, potentially due to inhibition of conversion of metanephric mesenchyme to epithelium in the presence of cyclosporine.[78]
PROGRAMMED CHANGES WITHIN THE KIDNEY
The kidney is one of the major organs influencing blood pressure, and programming of hypertension does appear to be at least in part mediated by nephron endowment. That congenitally acquired nephron number is not the sole programmable factor responsible for subsequent hypertension has been shown in offspring of low-protein-diet-fed rats in whom diets were supplemented with glycine, alanine, or urea.[24] As mentioned previously, nephron number was restored in all offspring, but hypertension was prevented only in the glycine supplementation group. In humans, in the two studies that have examined nephron number in relation to presence or absence of essential hypertension, a relationship was found among whites but was weaker for African Americans. [46] [79] These studies suggest that programming of hypertension may occur in the absence of an alteration in nephron number. The pressure-natriuresis curve is shifted to the right in most forms of hypertension, and prenatally programmed hypertension has been demonstrated by some investigators to be salt sensitive. [25] [94] [96] [154] A reduction in filtration surface area associated with a reduction in nephron number is one plausible hypothesis to explain this observation, but other programmed effects have also been described that are likely to influence blood pressure and sodium homeostasis.
Renal Vascular Reactivity
An increase in baseline renal vascular resistance has been described by several authors using different models of fetal programming. [153] [155] [156] In addition, renal arterial responses to β-adrenergic stimulation and sensitivity to adenylyl cyclase were found to be increased in 21-day-old growth-restricted offspring of mothers subjected to uterine artery ligation during gestation.[157] The renal expression of β2-adrenoreceptor mRNA was increased in the pups of rats subjected to reduced uteroplacental blood flow, but there was also evidence of adaptations to the signal transduction pathway contributing to the β-adrenergic hyper-responsiveness observed. Intriguingly, these findings were much more marked in the right compared with the left kidney, an observation that remains unexplained but that is not without precedent: asymmetry of renal blood flow was found in 51% of a cohort of hypertensives without renovascular disease. [157] [158] Functionally, in this study, the growth-restricted rats had reduced glomerular numbers, exhibited hyperfiltration and hyperperfusion, and had significantly increased proteinuria compared with the controls.
Renin-Angiotensin System
All of the components of the renin-angiotensin system are expressed in the developing kidney.[159] The importance of angiotensin II in nephrogenesis was demonstrated by the administration of the angiotensin II subtype 1 receptor (AT1R) blocker losartan to normal rats during the first 12 days of life (while nephrogeneisis is proceeding), which resulted in a reduction in final nephron number and subsequent development of hypertension.[160] Interestingly, administration of an angiotensin-converting enzyme inhibitor (ACEI), captopril, or losartan to low-birth-weight rats from 2 to 4 weeks of age, abrogated the development of adult hypertension in these animals. [13] [161] These data suggest upregulation of the AT1R postnatally, which could be a result of increased glucocorticoid exposure.[13] In support of this hypothesis, administration of angiotensin II or ACEI to adult rats subjected to a low-protein diet in utero resulted in a more exaggerated hypertensive or hypotensive response than in control rats. [13] [162] [163] [164]
In neonates and young offspring of rats subjected to gestational protein restriction, renal renin, AT1R, and angiotensin II mRNA and protein levels have all been found to be reduced compared with control rats, but the AT1R expression increases above control levels as the rats reach the prehypertensive stage. [25] [76] [160] [163] [164] [165] The renal expression of the angiotensin II subtype 2 receptor (AT2R) has been found to be down-regulated in young rats and up-regulated in neonatal sheep, an effect that may reflect different stages of renal maturation at birth among these species. [89] [162] Angiotensin II can stimulate the expression of Pax-2 (an anti-apoptotic factor) through AT2R.[166] AT2R expression, therefore, is likely to affect nephrogenesis and kidney development, but its role in programming is still unclear. Overall, programmed suppression of the intrarenal renin-angiotensin system during nephrogenesis is likely to contribute to the reduction in nephron number under adverse circumstances, and postnatal up-regulation of the AT1R, possibly mediated by an increase in glucocorticoid activity or sensitivity, may contribute to the subsequent development of hypertension.
Altered Sodium Handling by the Kidney
Another contributor to a shift of the pressure-natriuresis curve to the right is an alteration of sodium transporter expression or activity in the kidney. Administration of dexamethasone to pregnant rats was associated with growth retardation in the offspring, lower nephron number, reduction in GFR, higher blood pressure, lower urinary sodium excretion rate, reduced fractional excretion of sodium, and higher tissue sodium content in liver and skeletal muscle.[72] Similar findings were seen in growth-retarded piglets in which low nephron number was associated with a reduced GFR but a normal fractional excretion of sodium.[167] A lower fractional excretion of sodium in the presence of reduced GFR is strong evidence of sodium retention by the kidney. Consistent with these whole-kidney observations, Vehaskari and colleagues[168] found significant increases in expression of sodium co-transporters Na-K-2Cl (bumetanide-sensitive co-transporter, BSC1, 302%) and Na-Cl (thiazide-sensitive co-transporter, TSC, 157%) in the offspring of rats fed a protein-restricted diet during gestation compared with normals ( Fig. 19-14 ). Other authors reported an increase in glucocorticoid receptor expression and expression of the glucocorticoid responsive α1- and β1-subunits of Na-K-ATPase in offspring of pregnant rats fed a low-protein diet (see Fig. 19-12 ).[143] Taken together, these data suggest that an increased sodium avidity of the fetally programmed kidney, possibly in the setting of an increase in background glucocorticoid activity, is a likely contributor to the development of adult hypertension. This hypothesis requires further validation.
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FIGURE 19-14 Apical Na transporter expression in 4-week-old offspring from maternal low-protein diet. (From Vehaskari VM, Woods LL: Prenatal programming of hypertension: Lessons from experimental models. J Am Soc Nephrol 16:2545–2556, 2005.) |
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IMPACT OF NEPHRON ENDOWMENT ON TRANSPLANTATION OUTCOMES
Prescription of donor kidneys is largely decided based on immunologic matching. In animal experiments of renal transplantation, however, the impact of transplanted nephron mass, independent of immunologic factors, on the subsequent development of chronic allograft nephropathy has been demonstrated. [169] [170] [171] [172] [173] Despite such evidence, prescription of kidneys on the basis of the physiologic capacity of the donor organ to meet the metabolic needs of the recipient has not generally been considered.[174] More and more data are accumulating, however, suggesting a significant impact of transplanted renal mass on long-term post-transplantation outcomes.
Demographic and anthropomorphic factors associated with late renal allograft loss include donor age, sex, and race, as well as recipient body surface area (BSA). [175] [176] [177] In general, kidneys from older, female, and African American donors fare worse and tend to have lower nephron numbers than younger, white, and male donors. [42] [62] [178] [179] Indirectly, these observations suggest that the intrinsic nephron endowment of the transplanted kidney is likely to play a role in the development of chronic allograft nephropathy. To investigate this question, several investigators have compared recipient and donor BSA as surrogates for metabolic demand and kidney size; others have used kidney weights or renal volumetric measurements by ultrasound as surrogates for nephron mass.
Mismatches between donor kidney size and recipient BSA have an impact on long-term allograft outcomes. A retrospective analysis of 32,083 patients who received a first cadaver kidney found that large recipients of kidneys from small donors had a 43% increased risk of late allograft failure compared with medium-sized recipients receiving kidneys from medium-sized donors.[180] The best outcomes tended to be in small recipients receiving kidneys from large donors. Other smaller, and often single-center studies have not consistently found similar results.[180] One such study analyzed 378 paired recipients of cadaver kidneys from 189 donors, in which one recipient had a high and the other a low BSA.[181] These authors did not find a significant association between allograft loss and the ratio between donor and recipient BSA.[181] Importantly, however, the BSA ranges in the “larger” and “smaller” groups overlapped in this study, limiting the power to detect a true effect. Another interesting study evaluated outcomes in patients receiving cadaveric kidneys from donors either below or above 60 years of age.[182] Kidneys from older donors have fewer viable nephrons and may be less able to recover from transplant-related injury. [42] [183] The recipients were also subdivided into two groups according to mean body mass index (BMI) and mean BSA.[182] The authors found that, in patients receiving kidneys from donors over age 60, there was a positive correlation between BMI and BSA and nadir creatinine. At 5 years, graft survival was significantly better in those with smaller BMI and BSA. This study demonstrates that an older kidney with fewer nephrons transplanted into a smaller recipient functions better than an older kidney given to a larger recipient.
Kidney size, however, may not always be directly proportional to BSA; therefore, ratios of donor to recipient BSA may not be an ideal method of estimating nephron mass to recipient mismatch. Kidney weight, however, is an acceptable surrogate for nephron mass. [42] [184] Using this parameter, Kim and associates[185] analyzed the ratio of donor kidney weight to recipient body weight (DKW/RBW) in 259 live-donor transplants. These authors found that a higher DKW/RBW of greater than 4.5 g/kg was significantly associated with improved allograft function at 3 years compared with a ratio of less than 3.0 g/kg. A similar study including 964 recipients of cadaveric kidneys, in whom proteinuria and Cockroft-Gault creatinine clearances were also calculated, found that 10% of the subjects were “strongly” mismatched, having a DKW/RBW ratio of less than 2 g/kg.[179] The DKW/RBW ratio was lowest when male recipients received kidneys from female donors. The risk of having proteinuria higher than 0.5 g/kg was significantly greater, and developed earlier, in those with DKW/RBW below 2 g/kg as compared with those with higher ratios. In fact, proteinuria was present in 50% of those with DKW/RBW less than 2 g/kg, 33% of those with DKW/RBW of 2 to 4 g/kg, and 23% in those with DKW/RBW of 4 g/kg or lower. Furthermore, calculated creatinine clearance, although fraught with imprecision, increased progressively in the subgroup with DKW/RBW less than 2 g/kg, suggesting glomerular hyperfiltration in response to the increased metabolic demand placed on the small kidney by the larger recipient. GFRs remained stable post-transplantation in those with DKW/RBW of 4 g/kg or lower. At 5 years follow-up, however, there was no difference in graft survival among the three DKW/RBW groups, but the authors concede that it is likely that longer follow-up is needed to determine the true impact of donor to recipient mismatch.[179] Other investigators have used renal ultrasonography to measure cadaveric transplant kidney (Tx) cross-sectional area in relation to recipient body weight (W) to calculate a “nephron dose index,” Tx/W.[186] These authors found that, during the first 5 years after transplantation, serum creatinine was significantly lower in patients with a high Tx/W compared with those with lower values, with a trend toward better graft survival. Therefore, the ratio between renal mass and the recipient's metabolic needs does appear to be a determinant of long-term allograft function. A small kidney transplanted into a large recipient may not have an adequate capacity to meet the metabolic needs of the recipient without imposing glomerular hyperfiltration, which ultimately leads to further nephron loss and eventual allograft failure. [23] [183]
Transplanted nephron mass not only may be a function of congenital endowment and attrition of nephrons with age but also is affected by peritransplant renal injury (i.e., donor hypotension, prolonged cold and warm ischemia, nephrotoxic immunosuppressive drugs). All of these factors need to be closely considered, in addition to immunologic matching, in selection of appropriate recipients in whom the allograft is likely to function for the longest time and therefore provide best possible improvement in quality of life.
CONCLUSION
The association between an adverse fetal environment and subsequent hypertension and kidney disease in later life is now quite compelling and appears to be mediated, at least in part, by impaired nephrogenesis. Concomitant glomerular hypertrophy and altered expression of sodium transporters in the programmed kidney also contribute to the vicious circle of glomerular hypertension, glomerular injury, and sclerosis, leading to worsening hypertension and ongoing renal injury ( Fig. 19-15 ). The number of nephrons in humans varies widely, suggesting that a significant proportion of the general population, especially in areas where high or low birth weights are prevalent, may be at increased risk of developing later-life hypertension and renal dysfunction. Measurement of nephron number in vivo remains an obstacle, with the best surrogate markers thus far being a low birth weight, a high birth weight, and in the absence of other known renal diseases, a reduced kidney volume on ultrasound, especially in children, and glomerular enlargement on kidney biopsy. A kidney with a reduced complement of nephrons would have less renal reserve to adapt to dietary excesses or to compensate for renal injury. The molecular mechanisms through which fetal programming exerts its effects on nephrogenesis are varied and likely complementary and intertwined. The fact that even seemingly minor influences, such as composition of maternal diet during fetal life, can have major consequences on renal development in the offspring underscores the critical importance of optimization of perinatal care.
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FIGURE 19-15 Proposed mechanism of fetal programming of hypertension and renal disease. (Adapted from Zandi-Nejad K, Luyckx VA, Brenner BM: Adult hypertension and kidney disease: The role of fetal programming. Hypertension 47:502–508, 2006.) |
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ATPase expression
