Comparative aspects of fetal renal development

Ontogeny of the mammalian kidney: expression of aquaporins 1, 2, 3, and 4

BACKGROUND

Determining the expression and functions of aquaporins (AQPs) in the adult kidney has generated important information about the roles of this protein family in the renal regulation of water homeostasis. However, limited information describes the expression of AQPs in fetal kidneys, and most reports on fetal renal AQPs originate from animal studies. Although there are the maturation and regulation of the renal-concentrating mechanism, the ways in which changes in the expression of AQPs contribute to the formation of urine during the perinatal period remain unclear.

DATA SOURCES

This review summarizes current knowledge about the spatial and temporal expression patterns of AQP1, AQP2, AQP3, and AQP4 in the fetal and postnatal kidneys in different animal species and in human beings.

RESULTS

AQP1 and AQP2 expression can be detected earlier in gestation in human beings and sheep compared with mice and rats. AQP1 expression is detected earlier in the proximal tubules than the expression of AQP2, AQP3, and AQP4 in the collecting ducts.

CONCLUSION

Further studies investigating the regulation of AQPs during kidney development may provide insights into normal water-handling mechanisms and the pathophysiology of fetal kidneys, which may determine new directions for the clinical treatment of kidney diseases.

Maternal protein-energy malnutrition during early pregnancy in sheep impacts the fetal ornithine cycle to reduce fetal kidney microvascular development

This paper identifies a common nutritional pathway relating maternal through to fetal protein-energy malnutrition (PEM) and compromised fetal kidney development. Thirty-one twin-bearing sheep were fed either a control (n=15) or low-protein diet (n=16, 17 vs. 8.7 g crude protein/MJ metabolizable energy) from d 0 to 65 gestation (term, ∼145 d). Effects on the maternal and fetal nutritional environment were characterized by sampling blood and amniotic fluid. Kidney development was characterized by histology, immunohistochemistry, vascular corrosion casts, and molecular biology. PEM had little measureable effect on maternal and fetal macronutrient balance (glucose, total protein, total amino acids, and lactate were unaffected) or on fetal growth. PEM decreased maternal and fetal urea concentration, which blunted fetal ornithine availability and affected fetal hepatic polyamine production. For the first time in a large animal model, we associated these nutritional effects with reduced micro- but not macrovascular development in the fetal kidney. Maternal PEM specifically impacts the fetal ornithine cycle, affecting cellular polyamine metabolism and microvascular development of the fetal kidney, effects that likely underpin programming of kidney development and function by a maternal low protein diet.—Dunford, L. J., Sinclair, K. D., Kwong, W. Y., Sturrock, C., Clifford, B. L., Giles, T. C., Gardner, D. S.. Maternal protein-energy malnutrition during early pregnancy in sheep impacts the fetal ornithine cycle to reduce fetal kidney microvascular development.

Protein-energy malnutrition during early gestation in sheep blunts fetal renal vascular and nephron development and compromises adult renal function

Non-technical summary

A poor diet during pregnancy has been linked to long-term health outcomes for the baby, such as an increased risk of diseases of the heart and kidney. We show in an experimental model that recreates a poor diet during pregnancy, i.e. a diet low in protein with adequate energy, that kidney development in the baby is affected in such a way as to reduce the potential for new blood vessels to form. This results in a greater number of important, functional kidney cells spontaneously dying. Later in life, these effects in the kidney manifest as permanently reduced kidney function, especially if the baby subsequently becomes overweight as an adult. The research reinforces advice to pregnant mothers about the importance of eating a nutritionally balanced diet during pregnancy.

Abstract

A nutritionally poor maternal diet can reduce nephron endowment and pre-empt premature expression of markers for chronic renal disease in the offspring. A mechanistic pathway from variation in maternal diet through altered fetal renal development to compromised adult kidney structure and function with adult-onset obesity has not been described. We show that maternal protein-energy malnutrition in sheep blunts nephrogenic potential in the 0.44 gestation (65 days gestation, term ∼147 days) fetus by increasing apoptosis and decreasing angiogenesis in the nephrogenic zone, effects that were more marked in male fetuses. As adults, the low-protein-exposed sheep had reduced glomerular number and microvascular rarefaction in their kidneys compensated for, respectively, by glomerular hypertrophy and increased angiogenic support. In this study, the long-term mild anatomical deficits in the kidney would have remained asymptomatic in the lean state, but when superimposed on the broad metabolic challenge that obesity represents then microalbuminuria and blunted bilateral renal function revealed a long-term physiological compromise, that is only predicted to worsen with age. In conclusion, maternal protein-energy malnutrition specifically impacts fetal kidney vascular development and prevents full functionality of the adult kidney being achieved; these residual deficits are predicted to significantly increase the expected incidence of chronic kidney disease in prenatally undernourished individuals especially when coupled with a Western obesogenic environment.

Differential effects of maternal nutrient restriction through pregnancy on kidney development and later blood pressure control in the resulting offspring

The mechanisms whereby maternal nutritional manipulation through pregnancy result in altered blood pressure in the offspring may include changes in fetal and newborn and adult renal prostaglandin (PG) synthesis, metabolism, and receptor expression. Since the postnatal effects of nutrient restriction on the renal PG synthesis and receptor system during nephrogenesis in conjunction with nephron numbers and blood pressure have not been evaluated in the rat, the present study examined the effect of reducing maternal food intake by 50% of ad libitum through pregnancy on young male rats. Six control-fed mothers and eight nutrient-restricted pregnant rats with single litter mates were used at each sampling time point, most of which occurred during nephrogenesis. Offspring of nutrient-restricted dams were lighter from birth to 3 days. This was accompanied by reduced PGE2, with smaller kidneys up to 14 days. Nutrient restriction also decreased mRNA expression of the PG synthesis enzyme, had little effect on the PG receptors, and increased mRNA expression of the degradation enzyme during nephrogenesis and the glucocorticoid receptor in the adult kidney. These mRNA changes were normally accompanied by similar changes in protein. Nephron number was also reduced from 7 days up to adulthood when blood pressure (measured by telemetry) did not increase as much as in control offspring during the dark, active period. In conclusion, maternal nutrient restriction suppressed renal PG concentrations in the offspring, and this was associated with suppressed kidney growth and development and decreased blood pressure.

Hypertension and impaired renal function accompany juvenile obesity: the effect of prenatal diet

Obesity has been suggested to have a detrimental impact on kidney structure and function, leading to focal glomerulosclerosis and hypertension. It is also associated with hyperleptinemia and elevated renal sympathetic nerve activity. Prenatal undernutrition promotes postnatal obesity, hypertension, and an altered renal structure and function. In this study, we examined the effects of prenatal nutrient restriction and juvenile obesity in sheep. We found that juvenile obesity led to chronic hyperleptinemia and reduced renal function as assessed by nuclear scintigraphy. Additional factors include hypertension, glomerulosclerosis, and increased kidney apoptosis. Prenatal undernutrition, synchronous with early kidney development, coupled postnatally with juvenile obesity had no effect on systemic pathophysiological sequalae associated with obesity per se. Hypertension, hyperleptinemia, and poor renal function were all observed in this group. All indices of renal pathology such as increased expression of proinflammatory cytokines, angiotensin II, glucocorticoid receptors, and increased apoptosis and glomerulosclerosis were entirely absent in obese prenatally undernourished offspring. Our data indicate that juvenile obesity per se leads to systemic hypertension and renal structural and functional pathology. Prenatal undernutrition effectively abolishes any renal histopathology associated with juvenile obesity.

Aquaporins in development -- a review

Water homeostasis during fetal development is of crucial physiologic importance. It depends upon maternal fetal fluid exchange at the placenta and fetal membranes, and some exchange between fetus and amniotic fluid can occur across the skin before full keratinization. Lungs only grow and develop normally with fluid secretion, and there is evidence that cerebral spinal fluid formation is important in normal brain development. The aquaporins are a growing family of molecular water channels, the ontogeny of which is starting to be explored. One question that is of particular importance is how well does the rodent (mouse, rat) fetus serve as a model for long-gestation mammals such as sheep and human? This is particularly important for organs such as the lung and the kidney, whose development before birth is very much less in rodents than in the long-gestation species.

Foetal fluid balance and hormone status following nephrectomy in the foetal sheep

1. The role of the kidneys in the maintenance of normal foetal plasma (FP) composition and hormone concentrations was examined in the present study. Five ovine foetuses were chronically cannulated and nephrectomized (nephx) at 100 +/- 1 days of gestation and maintained for 14 days. These were compared to five intact control foetuses. 2. Four hours after nephx, FP renin concentrations were significantly lower than in control foetuses. By 48 h, renin concentrations in nephx foetuses were below the level of detectability of the assay. Foetal plasma aldosterone concentrations declined in nephx foetuses, but were not significantly different to those in control foetuses (P = 0.08). 3. During the second week, the nephx foetuses were significantly hypoxic, but FP erythropoietin concentrations were not increased. Adrenocorticotropic hormone (ACTH) and cortisol concentrations, when measured on day 14, were not different between the two groups. Adrenocorticotropic hormone levels were correlated with adrenal weight at post-mortem. 4. Foetal plasma creatinine, magnesium and phosphate concentrations in nephx foetuses increased, eventually reaching values approximately twice that in controls. Foetal plasma chloride levels decreased continuously in nephx foetuses, eventually being 23 mmol/L lower than controls. Maternal plasma composition was unchanged. 5. Total foetal fluid (amniotic + allantoic) volumes were reduced when measured at post-mortem on day 14 after nephx. The composition of both fluids was significantly altered in the nephx foetuses compared with controls. 6. Fetuses can survive in utero for 2 weeks after bilateral nephrectomy. However, there are multiple changes in plasma composition that may compromise foetal survival in the long term.