PAX2 Activates WNT4 Expression during Mammalian Kidney Development

Abstract

The transcription factor PAX2 is expressed during normal kidney development and is thought to influence outgrowth and branching of the ureteric bud. Mice with homozygous null Pax2 mutations have developmental defects of the midbrain-hindbrain region, optic nerve, and ear and are anephric. During nephrogenesis, PAX2 is also expressed by mesenchymal cells as they cluster and reorganize to form proximal elements of each nephron, but the function of PAX2 in these cells is unknown. In this study we hypothesized that PAX2 activates expression of WNT4, a secreted glycoprotein known to be critical for successful nephrogenesis. PAX2 protein was identified in distal portions of the "S-shaped" body, and the protein persists in the emerging proximal tubules of murine fetal kidney. PAX2 activated WNT4 promoter activity 5-fold in co-transfection assays with JTC12 cells derived from the proximal tubule. Inspection of the 5-flanking sequence of the human WNT4 gene identified three novel PAX2 recognition motifs; each exhibited specific PAX2 protein binding in electromobility shift assays. Two motifs were contained within a completely duplicated 0.66-kb cassette. Transfection of JTC12 cells with a PAX2 expression vector was associated with a 7-fold increase in endogenous WNT4 mRNA. In contrast, Wnt4 mRNA was decreased by 60% in mesenchymal cell condensates of fetal kidney from mice with a heterozygous Pax2 mutation. We speculated that a key function of PAX2 is to activate WNT4 gene expression in metanephric mesenchymal cells as they differentiate to form elements of the renal tubules.PAX2 belongs to the "paired box" family of transcription factors. Like other family members, it is thought to orchestrate the patterns of gene expression in specific cells during organ development. Homozygous inactivation of the Pax2 gene in mice causes malformation of the midbrain-hindbrain region, the optic nerve, and complete absence of the kidneys (1, 2). Although these observations clearly implicate PAX2 in the development of renal and neural tissue, little is known about its precise gene targets or the specific developmental processes in which it plays a role.Studies thus far suggest that the developmental functions of PAX2 may be multiplex, activating distinct panels of genes in different cell lineages at different stages. During development of mammalian kidney, PAX2 first appears during the caudal descent of the nephric duct where it affects the fate of a cell (3). When the nephric duct arrives at about somite 26, a "ureteric bud" (UB) 2 emerges from its wall and grows toward the adjacent lateral mesenchyme. This event is again orchestrated by PAX2 through activation of glial cell line-derived neurotropic Factor (GDNF) in the uninduced mesenchyme and activation of the GDNF receptor (RET) in UB cells (4). Thus, homozygous Pax2 knockout mice lack normal ureteric bud outgrowth and are unable to induce metanephric kidneys (2).A third function of PAX2 in developing kidney involves suppression of programmed cell death in ureteric bud cells. During ...

Suppression of Ureteric Bud Apoptosis Rescues Nephron Endowment and Adult Renal Function in Pax2 Mutant Mice

“…PAX2 effects the expression of genes in the metanephric mesenchyme such as Gdnf (21) and Wnt4 (22). However, the observation that apoptosis of cultured inner medullary collecting duct cells is strongly suppressed by PAX2 suggests a transcriptional target in cells of the UB lineage as well (23).…”

PAX2 is reactivated in urinary tract obstruction and partially protects collecting duct cells from programmed cell death

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“…Pax2 also increases expression of Wnt-4, a secreted glycoprotein that activates the ␤-catenin signaling pathway regulating cell growth. 2 The transcription factor WT1 is expressed in uninduced mesenchyme but is sharply upregulated as cells condense around ureteric bud branches. Wnt-4 is also upregulated in condensing mesenchyme and, together with WT1, expresses through the vesicle and comma-and S-shaped stages, suggesting both of these proteins are key mediators of epithelial differentiation.…”