Genetics of renal hypoplasia: insights into the mechanisms controlling nephron endowment

Glomerular number and size variability and risk for kidney disease

PURPOSE OF REVIEW

This review discusses current understandings of variability in glomerular number and size, and the implications for renal health.

RECENT FINDINGS

The quantitative microanatomy of the normal human kidney varies widely. Of greatest significance, total nephron number varies at least 13-fold, and several genes and environmental factors that regulate human nephron endowment have been identified. Full or partial deletion of more than 25 genes in mice has been shown to result in renal hypoplasia and, when measured, reduced nephron endowment. Many more will likely be identified. As would be expected, some gene abnormalities increase nephron endowment above that found in control mice. Glomerular volume also varies widely, both between and within kidneys, and increased heterogeneity of glomerular volume within kidneys is associated with risk factors for kidney disease, including birth weight, age, race, body size and hypertension.

SUMMARY

Data from several human populations indicate that the quantitative microanatomy of the human kidney varies considerably: total glomerular number varies at least 13-fold, mean glomerular volume varies up to seven-fold and the volumes of individual glomeruli within single kidneys can vary as much as eight-fold. Human glomerular number, size and size distribution are being found to correlate with risk factors for kidney disease. The genetic and fetal environmental regulators of nephrogenesis, and thereby nephron endowment, are being rapidly identified and will provide the bases for future clinical interventions. In contrast, the molecular regulation of glomerular size remains unclear.

Alk3 controls nephron number and androgen production via lineage-specific effects in intermediate mesoderm

The mammalian kidney and male reproductive system are both derived from the intermediate mesoderm. The spatial and temporal expression of bone morphogenetic protein (BMP) 2 and BMP4 and their cognate receptor, activin like kinase 3 (ALK3), suggests a functional role for BMP-ALK3 signaling during formation of intermediate mesoderm-derivative organs. Here, we define cell autonomous functions for Alk3 in the kidney and male gonad in mice with CRE-mediated Alk3 inactivation targeted to intermediate mesoderm progenitors (Alk3(IMP null)). Alk3-deficient mice exhibit simple renal hypoplasia characterized by decreases in both kidney size and nephron number but normal tissue architecture. These defects are preceded by a decreased contribution of Alk3-deleted cells to the metanephric blastema and reduced expression of Osr1 and SIX2, which mark nephron progenitor cells. Mutant mice are also characterized by defects in intermediate mesoderm-derived genital tissues with fewer mesonephric tubules and testicular Leydig cells, epithelial vacuolization in the postnatal corpus epididymis, and decreased serum testosterone levels and reduced fertility. Analysis of ALK3-dependent signaling effectors revealed lineage-specific reduction of phospho-p38 MAPK in metanephric mesenchyme and phospho-SMAD1/5/8 in the testis. Together, these results demonstrate a requirement for Alk3 in distinct progenitor cell populations derived from the intermediate mesoderm.

Tight regulation of p53 activity by Mdm2 is required for ureteric bud growth and branching

Mdm2 (Murine Double Minute-2) is required to control cellular p53 activity and protein levels. Mdm2 null embryos die of p53-mediated growth arrest and apoptosis at the peri-implantation stage. Thus, the absolute requirement for Mdm2 in organogenesis is unknown. This study examined the role of Mdm2 in kidney development, an organ which develops via epithelial-mesenchymal interactions and branching morphogenesis. Mdm2 mRNA and protein are expressed in the ureteric bud (UB) epithelium and metanephric mesenchyme (MM) lineages. We report here the results of conditional deletion of Mdm2 from the UB epithelium. UB(mdm2-/-) mice die soon after birth and uniformly display severe renal hypodysplasia due to defective UB branching and underdeveloped nephrogenic zone. Ex vivo cultured UB(mdm2-/-) explants exhibit arrested development of the UB and its branches and consequently develop few nephron progenitors. UB(mdm2-/-) cells have reduced proliferation rate and enhanced apoptosis. Although markedly reduced in number, the UB tips of UB(mdm2-/-)metanephroi continue to express c-ret and Wnt11; however, there was a notable reduction in Wnt9b, Lhx-1 and Pax-2 expression levels. We further show that the UB(mdm2-/-) mutant phenotype is mediated by aberrant p53 activity because it is rescued by UB-specific deletion of the p53 gene. These results demonstrate a critical and cell autonomous role for Mdm2 in the UB lineage. Mdm2-mediated inhibition of p53 activity is a prerequisite for renal organogenesis.