Reduced nephron number and glomerulomegaly in Australian Aborigines: a group at high risk for renal disease and hypertension

Perinatal nutrient restriction reduces nephron endowment increasing renal morbidity in adulthood: a review

Perinatal malnutrition has been included among the causes of renal disease in adulthood. Here, we consider the relationships between early supply of specific nutrients (such as protein, fat, vitamins and electrolytes) and renal endowment. Prenatal and postnatal nutrition mismatch is also discussed. In addition, this article presents the role of nutrition of both mothers and pre-term infants on nephron endowment, with final practical considerations.

A comparison of nephron number, glomerular volume and kidney weight in Senegalese Africans and African Americans

BACKGROUND

Low nephron number is determined in utero and is a proposed risk for essential hypertension. Glomerular volume is inversely correlated with nephron number, and genetic and environmental factors that determine nephron number are thought to determine glomerular volume. This study compared total glomerular (nephron) number (N(glom)), mean glomerular volume (V(glom)) and kidney weight in two geographically separated black populations with significant common genetic ancestry.

METHODS

Unbiased stereology was used to determine N(glom) and V(glom) in kidneys collected at coronial autopsy in an age- and sex-matched sample of 39 adult Africans from Dakar in Senegal, West Africa and 39 African Americans from Mississippi in the USA.

RESULTS

African Americans were taller and heavier than their Senegalese counterparts. N(glom) was remarkably similar-with a geometric mean of 937 967 in Senegalese and 904 412 in African Americans (P = 0.62). V(glom) was correlated inversely with N(glom) and directly with body surface area in both groups, but V(glom) was 54% greater in African Americans than in Senegalese Africans [8.30 +/- 2.92 (SD) and 5.38 +/- 1.25 microm(3) x 10(6), respectively] and remained significantly larger (38%) after adjustment for body size. V(glom) increased with age in African Americans, but not in the Senegalese. Kidney weight was larger in African Americans (P < 0.0001), but kidney-to-body weight ratio was not different between groups.

CONCLUSIONS

Despite similar nephron numbers, a common genetic constitution, and even in relation to current body size, African Americans have larger V(glom) than Senegalese subjects. This may mark exposure to environmental stressors or hereditary traits concentrated in the population's relocation to North America.

The clinical importance of nephron mass

Abundant evidence supports the association between low birth weight (LBW) and renal dysfunction in humans. Anatomic measurements of infants, children, and adults show significant inverse correlation between LBW and nephron number. Nephron numbers are also lower in individuals with hypertension compared with normotension among white and Australian Aboriginal populations. The relationship between nephron number and hypertension among black individuals is still unclear, although the high incidence of LBW predicts low nephron number in this population as well. LBW, a surrogate for low nephron number, also associates with increasing BP from childhood to adulthood and increasing risk for chronic kidney disease in later life. Because nephron numbers can be counted only postmortem, surrogate markers such as birth weight, prematurity, adult height, reduced renal size, and glomerulomegaly are potentially useful for risk stratification, for example, during living-donor assessment. Because early postnatal growth also affects subsequent risk for higher BP or reduced renal function, postnatal nutrition, a potentially modifiable factor, in addition to intrauterine effects, has significant influence on long-term cardiovascular and renal health.

Review: Sex specific programming: a critical role for the renal renin-angiotensin system

The "Developmental Origins of Health and Disease" hypothesis has caused resurgence of interest in understanding the factors regulating fetal development. A multitude of prenatal perturbations may contribute to the onset of diseases in adulthood including cardiovascular and renal diseases. Using animal models such as maternal glucocorticoid exposure, maternal calorie or protein restriction and uteroplacental insufficiency, studies have identified alterations in kidney development as being a common feature. The formation of a low nephron endowment may result in impaired renal function and in turn may contribute to disease. An interesting feature in many animal models of developmental programming is the disparity between males and females in the timing of onset and severity of disease outcomes. The same prenatal insult does not always affect males and females in the same way or to the same degree. Recently, our studies have focused on changes induced in the kidney of both the fetus and the offspring, following a perturbation during pregnancy. We have shown that changes in the renin-angiotensin system (RAS) occur in the kidney. The changes are often sex specific which may in part explain the observed sex differences in disease outcomes and severity. This review explores the evidence suggesting a critical role for the RAS in sex specific developmental programming of disease with particular reference to the immediate and long term changes in the local RAS within the kidney.

GLI3 repressor controls nephron number via regulation of Wnt11 and Ret in ureteric tip cells

Truncating GLI3 mutations in Pallister-Hall Syndrome with renal malformation suggests a requirement for Hedgehog signaling during renal development. HH-dependent signaling increases levels of GLI transcriptional activators and decreases processing of GLI3 to a shorter transcriptional repressor. Previously, we showed that Shh-deficiency interrupts early inductive events during renal development in a manner dependent on GLI3 repressor. Here we identify a novel function for GLI3 repressor in controlling nephron number. During renal morphogenesis, HH signaling activity, assayed by expression of Ptc1-lacZ, is localized to ureteric cells of the medulla, but is undetectable in the cortex. Targeted inactivation of Smo, the HH effector, in the ureteric cell lineage causes no detectable abnormality in renal morphogenesis. The functional significance of absent HH signaling activity in cortical ureteric cells was determined by targeted deletion of Ptc1, the SMO inhibitor, in the ureteric cell lineage. Ptc1^−/−UB mice demonstrate ectopic Ptc1-lacZ expression in ureteric branch tips and renal hypoplasia characterized by reduced kidney size and a paucity of mature and intermediate nephrogenic structures. Ureteric tip cells are remarkable for abnormal morphology and impaired expression of Ret and Wnt11, markers of tip cell differentiation. A finding of renal hypoplasia in Gli3^−/− mice suggests a pathogenic role for reduced GLI3 repressor in the Ptc1^−/−UB mice. Indeed, constitutive expression of GLI3 repressor via the Gli3^Δ699 allele in Ptc1^−/−UB mice restores the normal pattern of HH signaling, and expression of Ret and Wnt11 and rescued the renal phenotype. Thus, GLI3 repressor controls nephron number by regulating ureteric tip cell expression of Wnt11 and Ret.

Potential roles of high salt intake and maternal malnutrition in the development of hypertension in disadvantaged populations

1. It has been argued that all major risk factors for cardiovascular disease have been identified. Yet, epidemiological studies undertaken to identify risk factors have largely focused on populations in developed nations or on the urban or relatively affluent rural populations of developing countries. Poor rural populations are seldom studied. 2. Somewhat different risk factors may operate in poor rural populations. Evidence for this is provided by the finding that, in disadvantaged rural India, the prevalence of hypertension is greater than would be expected based on established risk factors in these populations. One risk factor to be considered is a poor intrauterine environment. 3. In animals, maternal macro- and micronutrient malnutrition can lead to reduced nephron endowment. Nephron deficiency, in turn, can render blood pressure salt sensitive. The combination of nephron deficiency and excessive salt intake will predispose to hypertension. 4. Human malnutrition may have similar effects, particularly in regions of the world where malnutrition is endemic and where women are disadvantaged by existing social practices. 5. Moreover, high salt intake is endemic in many parts of Asia, including India. Therefore, we propose that maternal malnutrition (leading to reduced nephron endowment), when combined with excessive salt intake postnatally, will account, at least in part, for the unexpectedly high prevalence of hypertension in disadvantaged rural communities in India and elsewhere.

Indigenous health part 1: determinants and disease patterns

The world's almost 400 million Indigenous people have low standards of health. This poor health is associated with poverty, malnutrition, overcrowding, poor hygiene, environmental contamination, and prevalent infections. Inadequate clinical care and health promotion, and poor disease prevention services aggravate this situation. Some Indigenous groups, as they move from traditional to transitional and modern lifestyles, are rapidly acquiring lifestyle diseases, such as obesity, cardiovascular disease, and type 2 diabetes, and physical, social, and mental disorders linked to misuse of alcohol and of other drugs. Correction of these inequities needs increased awareness, political commitment, and recognition rather than governmental denial and neglect of these serious and complex problems. Indigenous people should be encouraged, trained, and enabled to become increasingly involved in overcoming these challenges.

Common variants of the glial cell-derived neurotrophic factor gene do not influence kidney size of the healthy newborn

Glial cell-derived neurotrophic factor (GDNF) plays an important role in renal development, serving as a trophic factor for outgrowth of the ureteric bud and its continued arborisation. Our previous studies have shown that common variants of the human paired-box 2 (PAX2) gene (a transcriptional activator of GDNF) and rearranged during transfection (RET) gene (encoding the cognate receptor for GDNF) are associated with a subtle reduction in the kidney size of newborns. Since heterozygosity for a mutant GDNF allele causes mild renal hypoplasia and modest hypertension in mice, we considered the possibility that common variants of the GDNF gene might also contribute to renal hypoplasia in humans. We studied the relationship between newborn renal size or umbilical cord cystatin C and 19 common GDNF gene variants [minor allele frequency (MAF) >5%], three single nucleotide polymorphisms (SNPs) related to a putative PAX binding site and one rare SNP (rs36119840 A/G) which changes an amino acid (R93W), based on data from the haplotype map of the human genome (HapMap). However, none of these 23 SNPs was associated with reduced newborn kidney size or function. Among the 163 Caucasians in our cohort, none had the R93W allele.

Indomethacin, ibuprofen and gentamicin administered during late stages of glomerulogenesis do not reduce glomerular number at 14 days of age in the neonatal rat

Premature neonates are frequently administered indomethacin, ibuprofen and gentamicin during the period of active glomerulogenesis. These drugs are known to have nephrotoxic effects, but the morphological effect of these drugs is unknown. The purpose of this study was to determine whether administration of these drugs during the late stages of glomerulogenesis in the rat has an effect on glomerular endowment. Rat pups were given, intraperitoneally, indomethacin, ibuprofen or indomethacin and gentamicin for the first 5 days of their postnatal life. The pups were killed at 14 days of age at completion of glomerulogenesis. The total number of glomeruli in the left kidney was determined by the physical disector/fractionator stereological technique. There was no difference between treatment groups in total number of glomeruli per kidney (P = 0.45). There were significantly fewer glomeruli per gram of kidney in those rat pups that had received indomethacin or ibuprofen (P < 0.0001). The reduction in the number of glomeruli per gram of kidney may indicate augmented growth of nephron tubules and/or collecting ducts, and/or be a consequence of oedema secondary to drug exposure. Further study is required to determine whether reduced glomerular number is seen in older animals or following exposure to these drugs at different time-points in kidney development.

Pathological influence of obesity on renal structural changes in chronic kidney disease

BACKGROUND

Role of obesity in renal pathological and structural changes remains poorly investigated, and this study was designed to examine the pathological effects of obesity on renal structural components in patients with chronic kidney diseases (CKD).

METHODS

The study subjects were obese (body mass index, BMI > or = 25 kg/m2) patients with nonglomerulonephritis (non-GN, n = 26), IgA nephropathy (IgAN, n = 19), benign nephrosclerosis (BNS, n = 15), and thin basement membrane disease (TMD, n = 6), and 65 nonobese controls (n = 20, 20, 10, and 15, respectively). Patients were evaluated for glomerular lesions (mesangial proliferation and focal segmental/global glomerulosclerosis), glomerular size, and thickness of glomerular basement membrane (GBM).

RESULTS

Urinary protein was higher in obese non-GN, IgAN, and BNS groups than in the respective controls. Focal segmental glomerulosclerosis (FSGS) lesions were noted in all obesity groups. The glomeruli were larger in size in obese than in nonobese patients of the non-GN and IgAN groups. The glomeruli of nonobese TMD and BNS patients were significantly larger in size than those of nonobese non-GN patients. GBM were thicker in obese than in nonobese patients irrespective of types of glomerular diseases, but only significantly so in non-GN and BNS groups.

CONCLUSION

In non-GN, IgAN, and BNS, obesity worsens proteinuria and is associated with structural changes such as glomerulomegaly and GBM thickening, similar to changes observed in obesity-related nephropathy. Obesity seems to worsen the renopathological state in CKD.

Obesity-related nephropathy in children

Concurrent with the global obesity epidemic, there is an increasing number of people of all ages developing chronic kidney disease associated with obesity. In adults, the definition of obesity is a BMI greater than 30 kg/m2. Whereas, in children, a BMI greater than the 85th percentile for age is considered overweight and greater than the 95th percentile is classified as obese. Clinical and pathologic characteristics of a distinct nephropathy have emerged independent of that of diabetic or hypertensive glomerulosclerosis. These include a silent presentation in an obese individual with heavy proteinuria, normal serum albumin and the absence of edema. Renal pathologic findings are notable for mesangial matrix expansion, glomerular hypertrophy and reduced density of podocytes with detachment of foot processes from the glomerular basement membrane. These findings are frequently associated with the development of secondary focal segmental glomerulosclerosis. Obesity alone does not appear to be the sole mediator of this nephropathy. It is most likely the ‘second hit’ for individuals who have congenital or acquired reduced nephron mass as well as an inherited genetic vulnerability to the metabolic consequences imposed by cytokines released by adipose tissue. In children, those born of low birthweight, whether small for gestational age and/or preterm, are likely to have reduced nephron mass as well as an increased tendency for early insulin resistance and the development of obesity and the metabolic syndrome. This in turn is perpetuated by the practice of feeding high-calorie fortified formulas to low-birthweight infants. Rapid catch-up growth, early obesity and insulin resistance are major contributors to the emergence of obesity-related glomerulopathy in children and adolescents. Early detection requires recognizing the demographics of high-risk infants and monitoring them for the development of hypertension, elevated glomerular filtration rate, hyperfiltration and proteinuria. After 6 months of age, angiotensin-blocking agents may be used to control blood pressure, glomerular hyperfiltration and proteinuria. If obesity is present, a comprehensive program of weight loss, including diet and exercise, should be the mainstay of treatment. In older children and adolescents, lipid-lowering medications may be indicated. With morbid obesity, bariatric surgery may be an option.

Nephron number and individual glomerular volumes in male Caucasian and African American subjects

BACKGROUND

Glomerular hypertrophy has been described in several populations at high risk of chronic kidney disease. Total nephron (and thereby glomerular) number (N(glom)) varies widely in normal adult human kidneys and is generally inversely correlated with mean glomerular volume (V(glom)). However, little is known about the range of individual glomerular volumes (IV(glom)) within single human kidneys and the association with N(glom). The aim of the present study was to estimate IV(glom) in Caucasian and African Americans and identify any associations between heterogeneity in IV(glom) and nephron number.

METHODS

Using unbiased stereological techniques, IV(glom) was determined for 30 glomeruli in each of 24 adult male kidneys from Jackson, MS, USA (12 Caucasian and 12 African American). Half of each group had 'high' N(glom) (>1.2 million nephrons per kidney) and the other half had 'low' N(glom) (<600 000).

RESULTS

Caucasians with high N(glom) had a relatively homogeneous distribution of IV(glom) as well as a relatively low mean value, while those with low N(glom) had much greater heterogeneity of IV(glom), as well as a larger IV(glom) (P < 0.0001) compared with those with high N(glom). This disparity was not apparent in African Americans, however, where subjects with both high and low N(glom) showed substantial heterogeneity in IV(glom) and larger mean values (P = 0.95).

CONCLUSIONS

High N(glom) appeared to protect against glomerular enlargement and volume heterogeneity in Caucasians. However, substantial variation in IV(glom) and net enlargement in glomerular size in African Americans with high nephron numbers suggest that additional forces, independent of low N(glom), are driving glomerular enlargement and heterogeneity.

Renal agenesis and unilateral nephrectomy: what are the risks of living with a single kidney?

The long-term outlook for patients with unilateral renal agenesis or following unilateral nephrectomy in childhood is controversial. Animal studies suggest that the resultant compensatory increase in glomerular filtration might lead to progressive damage to the remaining renal tissue and may generate hypertension. Human studies addressing these concerns are limited in number and are difficult to interpret because they are small, retrospective, or cross sectional with significant variations in duration and completeness of follow-up. The published studies suggest that renal function remains stable for several decades in the majority of subjects. The clinical significance of mild-grade proteinuria and hypertension seen in some patients is unknown. Longitudinal studies are needed to understand the long-term effect and significance of the several pathophysiological changes observed in the solitary kidney.

Glomerular surface area is normalized in mice born with a nephron deficit: no role for AT1receptors

We examined whether deficits in glomerular capillary surface area associated with a congenital nephron deficit could be corrected by glomerular hypertrophy. Using unbiased stereological techniques, we examined the time course and mode of glomerular hypertrophy in mice lacking one allele for glial cell line-derived neurotrophic factor (GDNF). These GDNF heterozygous (Het) mice are born with ∼30% less nephrons than wild-type (WT) littermates. An additional group of GDNF Het mice received the angiotensin type 1 (AT1)-receptor antagonist candesartan (Cand; 10 mg·kg−1·day−1) from 5 wk of age to determine the role of AT1receptors in the compensatory hypertrophy. At 10 wk of age, the total volume of renal corpuscles, glomerular capillary surface area, and length of glomerular capillaries in the kidneys of GDNF Het mice were all markedly (∼45%) less than that of WT mice ( P < 0.001). However, by 30 wk, and persisting at 60 wk of age, GDNF Het and WT mice did not significantly differ in any of these parameters. Furthermore, conscious 24-h mean arterial pressure (MAP) did not differ between GDNF Het and WT mice at any time point. MAP of GDNF Het-Cand mice was 20–30 mmHg less than that of GDNF Het-vehicle mice at all three ages, but Cand treatment did not significantly alter glomerular capillary dimensions. In conclusion, we have demonstrated that the deficit in glomerular capillary surface area associated with a congenital nephron deficit can be corrected for in adulthood by an increase in the total length of glomerular capillaries. This process does not require AT1receptor activation.

Birth weight and stages of CKD: a case-control study in an Australian population

BACKGROUND

In view of recent reports of the relationship of kidney disease to birth weight, we evaluate the relationship between birth weight and chronic kidney disease (CKD), including end-stage kidney disease, in Australian adults.

STUDY DESIGN

A case-control study.

SETTING & PARTICIPANTS

Patients attending the nephrology department at a major metropolitan hospital in Australia were asked to recall their birth weight, excluding those with structural kidney abnormalities. Two controls for each patient, matched for sex and within 5 years of age, were selected from participants from the Australian Diabetes, Obesity and Lifestyle (AusDiab) Study, who had also been asked to report their birth weight.

PREDICTOR

Birth weight in kilograms.

OUTCOMES & MEASUREMENTS

CKD and stages were defined using the National Kidney Foundation-Kidney Disease Outcomes Quality Initiative classification, proteinuria as a marker of kidney damage, and glomerular filtration rate estimates, by using the Modification of Diet in Renal Disease Study equation.

RESULTS

Of 189 patients with CKD who reported their birth weights for whom controls were identified, 106 were men. Mean age was 60.3 +/- 15 (SD) years. Mean birth weight overall was 3.27 +/- 0.6 versus 3.46 +/- 0.6 kg for their controls (P < 0.001), and proportions with birth weights less than 2.5 kg were 12.2% and 4.4% (P < 0.001). In patients with CKD, 22.8%, 21.7%, 18%, and 37.6% were in CKD stages 2 (n = 43), 3 (n = 41), 4 (n = 34), and 5 (n = 71), respectively. Birth weights by CKD stage and their AusDiab controls were as follows: stage 2, 3.38 +/- 0.52 versus 3.49 +/- 0.52 kg; P = 0.2; stage 3, 3.28 +/- 0.54 versus 3.44 +/- 0.54 kg; P = 0.1; stage 4, 3.19 +/- 0.72 versus 3.43 +/- 0.56 kg; P = 0.1; and stage 5, 3.09 +/- 0.65 versus 3.47 +/- 0.67 kg; P < 0.001. Differences in birth weights applied to women and men and people younger than 60 and 60 years and older and were present in the major "causal" categories of renal disease.

LIMITATIONS

Birth weight is by self-recall with a significant nonresponse rate to the questionnaire in both cases and controls.

CONCLUSIONS

Urban Australian patients with CKD had lower birth weights than their matched Australian controls. In addition, the more advanced the CKD stage, the lower the birth weight. Thus, lower birth weights appear to predispose to CKD and to its progression. Among possible explanations is the documented association between birth weight and nephron number.

Associations between age, body size and nephron number with individual glomerular volumes in urban West African males

BACKGROUND

Glomerulomegaly has been associated with an increased risk of renal disease. Few reports have investigated the heterogeneity of glomerular size within kidneys and associated risk factors. This study measured the individual glomerular volume (IGV) of 720 non-sclerotic glomeruli in kidneys of adult West African males, and investigated associations of IGV with age, total glomerular (nephron) number and body surface area (BSA).

METHODS

IGVs were determined in the kidneys of 24 Senegalese males from two age groups (12 subjects aged 20- 30 years and 12 subjects aged 50-70 years). Subjects were randomly chosen at autopsies performed at Le Dantec Hospital in Dakar. Volumes of 30 glomeruli per subject were determined using the disector/Cavalieri stereological method.

RESULTS

IGVs ranged from 1.31 x 10(6) microm3 to 12.40 x 10(6) microm3 (a 9.4-fold variation). IGV varied up to 5.3-fold within single kidneys. The trimmed range of IGV within subjects (10th to 90th percentile of IGV) was directly correlated with median glomerular size. The mean and standard deviation (SD) of IGV did not differ significantly between age groups or between subjects with higher (> or =1.78 m2) and lower BSA (<1.78 m2). In older subjects the SD of IGV was significantly and directly correlated with BSA. Kidneys with less than 1 million nephrons had significantly larger mean IGV than kidneys with more than 1 million nephrons, and the trimmed range of IGVs within subjects was inversely correlated with total glomerular number.

CONCLUSION

There was a considerable variation in IGV within kidneys of Senegalese males at autopsy. The heterogeneity of IGV was increased in association with low nephron number and increased BSA, with more pronounced effects in older subjects.

Anthropometry and renal size of children suffering under sustained conflict in Sri Lanka

AIM

To measure anthropometry, blood pressure and renal lengths of children in a war torn region of Sri Lanka and compare results with local and international standards.

METHODS

Measurements of 147 Tamil children in Kilinochchi, north-east Sri Lanka, were compared with the World Health Organization standards using Anthro 2005 software in a period of relative peace in December 2005. Renal lengths were measured by ultrasound and compared with Australian and Indian data.

RESULTS

1 2-5 years of age. The weight-to-height ratio for combined sexes revealed 13.9% were <-3 standard deviations (SD) and 33.3% <-2 SD. The height-to-age ratio revealed 5.6% <-3 SD and 22.2% <-2 SD. The weight-to-age ratio revealed 13.9% <-3 SD and 38.9% <-2 SD. The body mass index of 13.9% of children was <-3 SD, of 25% it was <-2 SD. Stunting and wasting is prevalent, and worse than in other areas of Sri Lanka, except in tea estates. 2 5-12 years. The height-to-age ratio for combined sexes revealed 15.7% were <-3 SD and 38.5% <-2 SD. The weight-to-age ratio revealed 6.1% were <-3 SD and 47.3% <-2 SD. The body mass index of 64.0% of children was <5th percentile. More children are stunted and wasted in this older group. Girls are more affected than boys. 3 Renal lengths fell progressively below -2 SD for both age and height when compared with Australian and Indian children.

CONCLUSION

Tamil children in Kilinochchi are more stunted and wasted than others in Sri Lanka, except those in the tea estates. Progressive renal stunting because of under-nutrition may be a mechanism for later disease. The loss of potential for human development can only be addressed by long-term improvement in access to nutrition.

Role of vitamin A in determining nephron mass and possible relationship to hypertension

Vitamin A (retinol) and its analogs (retinoids) are important regulators of cell proliferation, differentiation, immune function, and apoptosis. The kidneys are target organs for vitamin A action. Retinoic acid (RA), a vitamin A metabolite, is involved in embryonic kidney patterning through the control of receptor tyrosine kinase expression, which modulates ureteric bud branching morphogenesis. Vitamin A status of the mother profoundly affects kidney organogenesis of the newborn. In rodents, mild vitamin A deficiency results in a 20% reduction of nephron number. In adult humans, nephron number varies between 0.3 and 1.3 million per kidney, which is accepted as normal. However, recent studies indicate that humans at the low end of nephron number are predisposed to primary hypertension. Because RA regulates nephron mass, its optimal availability during nephrogenesis is critical. RA levels in the embryo are affected by several factors, such as maternal vitamin A nutrition and disturbances in retinol metabolism. Maternal vitamin A deficiency during pregnancy is widespread in developing countries and segments of these populations may be exposed to low vitamin A during fetal life when nephron number is determined. Infants are likely to be born with suboptimal nephrons and may develop primary hypertension later in life. Although maternal vitamin A deficiency is not common in developed countries, congenital nephron number nevertheless varies widely, indicating low fetal RA levels due to common variants of the enzymes that convert retinol to RA. These infants might require heightened surveillance for hypertension later in life.

Repeated ethanol exposure during late gestation decreases nephron endowment in fetal sheep

Maternal alcohol consumption during pregnancy can affect fetal development, but little is known about the effects on the developing kidney. Our objectives were to determine the effects of repeated ethanol exposure during the latter half of gestation on glomerular (nephron) number and expression of key genes involved in renal development or function in the ovine fetal kidney. Pregnant ewes received daily intravenous infusion of ethanol (0.75 g/kg, n=5) or saline (control, n=5) over 1 h from 95 to 133 days of gestational age (DGA; term is approximately 147 DGA). Maternal and fetal arterial blood samples were taken before and after the start of the daily ethanol infusions for determination of blood ethanol concentration (BEC). Necropsy was performed at 134 DGA, and fetal kidneys were collected for determination of total glomerular number using the physical disector/fractionator technique; at this gestational age nephrogenesis is completed in sheep. Maximal maternal and fetal BECs of 0.12+/-0.01 g/dl (mean+/-SE) and 0.11+/-0.01 g/dl, respectively, were reached 1 h after starting maternal ethanol infusions. Ethanol exposure had no effect on fetal body weight, kidney weight, or the gene expression of members of the renin-angiotensin system, insulin-like growth factors, and sodium channels. However, fetal glomerular number was lower after ethanol exposure (377,585+/-8,325) than in controls (423,177+/-17,178, P<0.001). The data demonstrate that our regimen of fetal ethanol exposure during the latter half of gestation results in an 11% reduction in nephron endowment without affecting the overall growth of the kidney or fetus or the expression of key genes involved in renal development or function. A reduced nephron endowment of this magnitude could have important implications for the cardiovascular health of offspring during postnatal life.

Nephron number, glomerular volume, renal disease and hypertension

PURPOSE OF REVIEW

To discuss studies evaluating associations of glomerular number (Nglom) and glomerular volume with hypertension and kidney disease.

IMPORTANT FINDINGS

The association of low Nglom with hypertension and renal insufficiency was described in the 1930s. Many investigators have noted loss of glomeruli with age, with most disappearing entirely, and have proposed that hypertension follows. In a recent German study, hypertensive patients had fewer glomeruli and larger mean glomerular volumes than nonhypertensive people. Among the 10-fold range of Nglom in our multiracial autopsy series, the lowest were in Australian Aborigines, who have the highest rates of renal failure. Nglom fell with age. There was a five-fold range in mean glomerular volume and considerable heterogeneity in individual glomerular volumes within a patient. Larger mean glomerular volume and greater individual glomerular volume heterogeneity correlated with lower Nglom, larger body size, hypertension, and black race. Hypertension increased with age and was marked by glomerular enlargement, intimal thickening and higher rates of glomerulosclerosis. In whites and Aborigines, but not in US blacks, lower Nglom was associated with hypertension, while robust numbers were highly protective.

SUMMARY

Higher mean glomerular volume and individual glomerular volume heterogeneity mark glomerular stress. Low Nglom is an important determinant of hypertension and renal disease. Many 'missing' nephrons have probably been lost during life, leaving little trace. Additional factors contribute to high rates of hypertension in blacks.

Disparities in renal endowment: causes and consequences

In humans, nephrogenesis is completed by 36 weeks of gestation. Thus, human kidney development is complete at the time of birth in full-term infants. Those infants born before 36 weeks of gestation are still undergoing nephrogenesis for several weeks after their preterm birth and, accordingly, may be exposed to medications that impact the kidney during its final stages of renal development. The ultimate nephron number (nephron endowment) may influence future response to kidney injury, should it occur. The concept that nephron number may strongly influence blood pressure as well as susceptibility to kidney disease in later life developed in parallel with that of perinatal programming, which holds that the perinatal milieu causes changes that permanently alter organ structure and function, preordaining adult physiology to some extent. Both concepts together may help elucidate, at least in part, the pathogenesis of not only primary but secondary hypertension. This article summarizes human data on nephron number and its evaluation and considers the circumstances, implication, and management of persons born with or acquiring a decreased complement of nephrons early in life. Insufficient data exist to predict outcome or guide management. However, a common-sense approach of avoiding nephrotoxins and minimizing renal stress is indicated.

Bone morphogenetic protein signaling in the developing kidney: present and future

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta superfamily. A critical role for BMP signaling in the development of the metanephric kidney is supported by a growing number of studies using in vitro assays and in vivo animal models. Here we review current knowledge of BMPs, BMP receptors and regulators of the BMP signaling pathway in the developing kidney. We highlight major gaps in our knowledge of the roles of BMP signaling in the development of the normal and abnormal kidney and identify areas and techniques likely to improve our understanding.

Vitamin D deficiency during pregnancy and lactation stimulates nephrogenesis in rat offspring

There is increasing evidence of vitamin D insufficiency in women of child-bearing age and their infants. This study examined the effect of maternal vitamin D deficiency on nephron endowment in rat offspring (n=7 per group). Sprague-Dawley dams were fed either a vitamin D deplete diet or a vitamin replete (control) diet prior to pregnancy, during pregnancy and throughout lactation. At 4 weeks of age the offspring were weaned and maintained on their respective diets until they were killed at 7 weeks. In the fixed right kidney, kidney volume, renal corpuscle volume and nephron number were stereologically determined. There was no difference between groups in body weight, kidney weight or kidney volume. There was a significant 20% increase in nephron number in kidneys of vitamin D deplete offspring (vitamin D deficient, 29,000+/-1,858, control, 23,330+/-1,828; P=0.04). This was accompanied by a significant decrease in renal corpuscle size in the vitamin D deplete group compared with the controls (6.125+/-0.576 x 10(-4) mm(3) and 8.178+/-0.247 x 10(-4) mm(3), respectively; P=0.03). We concluded that maternal vitamin D deficiency in rats appears to stimulate nephrogenesis. Whether this confers a renal functional advantage or not is unknown.

Effects of early postnatal hypernutrition on nephron number and long-term renal function and structure in rats

Various antenatal events impair nephrogenesis in humans as well as in several animal models. The consecutive low nephron endowment may contribute to an increased risk for cardiovascular and renal diseases in adulthood. However, little knowledge is available on the influence of the postnatal environment, especially nutrition, on nephrogenesis. Moreover, the consequences of early postnatal nutrition in late adulthood are not clear. We used a model of early postnatal overfeeding (OF) induced by reduction of litter size (3 pups/litter) in rats. Systolic blood pressure (SBP; plethysmography), glomerular filtration rate (clearance of creatinine), glomerular number and volume, and glomerulosclerosis were evaluated in 22-mo-old aging offspring. Early postnatal OF was associated with increased weight gain during the suckling period (+40%, P < 0.01) and a 20% increase in glomerular number (P < 0.05). However, an increase in SBP at 12 mo by an average of 18 mmHg and an increase in proteinuria (2.6-fold) and glomerulosclerosis at 22 mo of age were observed in OF male offspring compared with controls. In conclusion, early postnatal OF in the rat enhances postnatal nephrogenesis, but elevated blood pressure and glomerulosclerosis are still observed in male adults. Factors other than glomerular number reduction are likely to contribute to the arterial hypertension induced by early postnatal OF.

Fetal kidney volume and its association with growth and blood flow in fetal life: The Generation R Study

An adverse fetal environment may lead to smaller kidneys and subsequent hypertension with renal disease in adult life. The aim of our study was to examine whether maternal characteristics, fetal growth, fetal blood flow redistribution, or inadequate placental perfusion in different periods of fetal life affect kidney volume in late fetal life. We also determined if fetal kidney volume was linked to the amount of amniotic fluid. In a population-based prospective study from early fetal life, fetal growth characteristics and fetal blood flow parameters were assessed by ultrasound and Doppler examinations in 1215 women in mid- and late-pregnancy. Kidney volume was measured in late pregnancy. Maternal height and pre-pregnancy weight were associated with kidney volume. After adjustment for the same characteristics in late pregnancy, fetal growth and blood flow in mid-pregnancy were not associated with kidney volume in late pregnancy. In late pregnancy, however, all fetal growth parameters were positively linked with kidney volume. The largest effect on kidney volume was found for abdominal circumference. Signs of fetal blood flow redistribution and increased placental resistance were associated with decreased kidney volume in late pregnancy. Amniotic fluid volume was positively associated with kidney volume. Our study shows that maternal anthropometrics, fetal growth, fetal blood flow redistribution, and raised placental resistance all correlate with kidney volume.

Total cardiovascular risk management

Hypertension usually clusters with other cardiovascular risk factors, such as insulin resistance, visceral obesity, and dyslipidemia, greatly increasing an individual's risk for cardiovascular morbidity and death. Despite universal recognition that reduction in blood pressure and other cardiovascular risk factors is essential to improving long-term cardiovascular health, <25% of patients diagnosed with hypertension have adequate blood pressure control. Total cardiovascular risk is increased in the presence of risk factors, target organ damage, comorbid conditions, and the metabolic syndrome and may, to some extent, be prenatally determined. Individuals with "borderline" normal blood pressure and blood glucose are also at increased risk for cardiovascular disease, giving rise to the concept of individuals with "prehypertension" and "prediabetes." International treatment guidelines are now incorporating the concept of global cardiovascular risk assessment and management to improve long-term outcomes. Multifactorial intervention has proved to be highly effective at reducing cardiovascular risk and events in patients with type 2 diabetes mellitus, and studies suggest that reducing an array of risk factors by relatively small amounts can be more beneficial than achieving large reductions in a single risk factor. Clearly, isolated treatment of hypertension is no longer sufficient; risk factors and target organ damage need to be actively searched for and treated if long-term cardiovascular health is to be improved.

Nephrogenesis and the renal renin-angiotensin system in fetal sheep: effects of intrauterine growth restriction during late gestation

Previous studies have shown that intrauterine growth restriction (IUGR) can impair nephrogenesis, but uncertainties remain about the importance of the gestational timing of the insult and the effects on the renal renin-angiotensin system (RAS). We therefore hypothesized that induction of IUGR during late gestation alters the RAS, and this is associated with a decrease in nephron endowment. Our aims were to determine the effects of IUGR induced during the later stages of nephrogenesis on 1) nephron number; 2) mRNA expression of angiotensin AT(1) and AT(2) receptors, angiotensinogen, and renin genes in the kidney; and 3) the size of maculae densae. IUGR was induced in fetal sheep (n = 7) by umbilical-placental embolization from 110 to 130 days of the approximately 147-day gestation; saline-infused fetuses served as controls (n = 7). Samples of cortex from the left kidney were frozen, and the right kidney was perfusion fixed. Total kidney volume, nephron number, renal corpuscle volume, total maculae densae volume, and the volume of macula densa per glomerulus were stereologically estimated. mRNA expression of AT(1) and AT(2) receptors, angiotensinogen, and renin in the renal cortex was determined. In IUGR fetuses at 130 days, body and kidney weights were significantly reduced and nephron number was reduced by 24%. There was no difference in renin, angiotensinogen, or AT(1) and AT(2) receptor mRNA expression levels in the IUGR kidneys compared with controls. We conclude that fetal growth restriction late in nephrogenesis can lead to a marked reduction in nephron endowment but does not affect renal corpuscle or macula densa size, or renal RAS gene expression.

Prenatal programming of nephron number and blood pressure

A low nephron number has been advocated to explain the association between intrauterine growth restriction (IUGR) and hypertension in later life. IUGR not only leads to a low birth weight but is also hypothesized to reprogram nephrogenesis, which results in a low nephron endowment. Several methods are used to estimate the total glomerular number, but only stereological techniques result in accurate (unbiased) and precise (reproducible) data. Several studies, both in humans and animal models that have used these methods indeed revealed that IUGR leads to a low nephron number. According to the hyperfiltration hypothesis, this reduction in renal mass is supposed to lead to glomerular hyperfiltration and hypertension in remnant nephrons with subsequent glomerular injury with proteinuria, systemic hypertension and glomerulosclerosis. Even though IUGR is associated with both a low nephron endowment and an increased risk of hypertension, only circumstantial evidence is available to support the hyperfiltration hypothesis after prenatal programming. A prerequisite for establishment of this association in long-term, prospective follow-up studies is the ability to estimate glomerular numbers in living human beings, for which a further advancement in radiological techniques is necessary. Only then can the association between nephron endowment and blood pressure in humans be studied more conclusively.

Are glomerular volume differences between type 1 and type 2 diabetic patients pathologically significant?

The observation that patients with type 2 diabetes tend to have larger glomeruli than patients with type 1 diabetes was first made more than 10 years ago. It has also been noted that type 2 diabetic patients with nephropathy often have more heterogeneous renal function and structure than type 1 patients. However, whether these observations are linked or have any bearing on the progression of nephropathy in the two types of diabetes remains uncertain. Here we put forward several hypotheses as to why glomerular volume in type 1 differs from that in type 2 diabetes. We suggest that although type 1 and type 2 diabetic patients appear to progress through similar stages of diabetic nephropathy, the route they take may differ. Differences in the way in which the glomeruli respond to the diabetic milieu may enable some type 2 diabetic patients to preserve their filtration surface in the face of an expanding mesangium.

Mechanisms of progression of chronic kidney disease

Chronic kidney disease (CKD) occurs in all age groups, including children. Regardless of the underlying cause, CKD is characterized by progressive scarring that ultimately affects all structures of the kidney. The relentless progression of CKD is postulated to result from a self-perpetuating vicious cycle of fibrosis activated after initial injury. We will review possible mechanisms of progressive renal damage, including systemic and glomerular hypertension, various cytokines and growth factors, with special emphasis on the renin-angiotensin-aldosterone system (RAAS), podocyte loss, dyslipidemia and proteinuria. We will also discuss possible specific mechanisms of tubulointerstitial fibrosis that are not dependent on glomerulosclerosis, and possible underlying predispositions for CKD, such as genetic factors and low nephron number.