Regulation of Nephron Progenitor Cell Self-Renewal and Differentiation

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SUMMARY/ABSTRACT The long-term goal of the proposed work is to determine how the Sall1 transcription factor and its associated chromatin remodeling complexes control gene regulation to generate a normal endowment of nephrons. This will illuminate mechanisms that underlie human genetic syndromes and common sporadic birth defects, such as renal hypoplasia (small kidneys with reduced numbers of nephrons), which result in kidney failure. Formation of the proper complement of nephrons requires a balance between expansion of multi-potent renal progenitor cells and differentiation. Genetic deficiency of Sall1 alters gene expression in the developing kidney, accelerating nephron differentiation leading to a depletion of nephron progenitor cells and renal hypoplasia. These uncommitted nephron precursors, termed cap mesenchyme, must respond to canonical Wnt/ss-catenin signals by either undergoing self-renewal or mesenchymal-to-epithelial transition to form most tubular segments of the nephron. The transcriptional mechanisms that control these opposing responses of the cap mesenchyme are not known, but current evidence argues that Sall1 is pivotal in this critical developmental decision. Our recent studies provide evidence for a novel paradigm. We propose that Sall1 cooperates with the Nucleosome Remodeling and Deacetylase (NuRD) chromatin remodeling complex, to determine nephron progenitor cell fate by regulating the transcriptional output in response to Wnt/ss-catenin. This application will test key predictions of this model using a combination of genetic, genomic, and biochemical approaches. Six2 is an inhibitor of nephron differentiation. In Aim 1, we will determine if Sall1 directly up-regulates Six2 expression in cap mesenchyme to restrain differentiation of progenitor cells. Expression of Wnt9b, the main differentiation signal, is increased in Sall1 mutants. We will determine if the combination of increased Wnt9b and reduced Six2 expression are sufficient to cause accelerated nephron formation, and exhaustion of self-renewing progenitor cells. Aim 2 will determine how Sall1 and NuRD control the transcriptional output in nephron progenitor cells to determine whether the uncommitted precursor cells undergo self-renewal or initiate nephron differentiation. The mechanistic insights gained from these studies will advance efforts to propagate nephron progenitors in vitro and/or promote regeneration of mature tubular epithelial cells in order to correct nephron deficits.