Wnt7b and Its Direct Target p57kip2 in Renal Medulla Development

The renal medulla is critical for urine concentration and maintaining the body’s salt and water homeostasis. Its development and physiology are dependent on reciprocal signaling between different cell types. However, the way in which the various cells of the renal medulla orchestrate its development is not well understood. For this reason, my work focuses on Wnt7b, a Wnt ligand that is expressed in the ureteric bud (UB) epithelium that gives rise to the collecting ducts. Previous work demonstrated that Wnt7b promotes elongation of the medullary ureteric trunks through regulation of their oriented cell division, and elongation of the loop of Henle through regulation of their proliferation. Wnt7b was shown to signal through the canonical Wnt pathway to the renal interstitium. I set out to characterize the subset of interstitial cells in the medulla that are responsive to Wnt7b canonical Wnt pathway signaling, to identify the role of Wnt7b in their development, and to identify their roles in renal medulla formation. Here I present evidence that canonical Wnt target cells in the medulla are pericytes of the peri-UB capillaries, and that Wnt7b regulates the development of both these pericytes and of the endothelium these pericytes associate with in the renal medulla. In the nascent renal medulla, Wnt7b regulates the proliferation of peri-UB pericytes and endothelial cells, pericyte expression of PDGFRβ, and endothelial cell flattening and capillary lumen formation. To determine the mechanism by which Wnt7b directs renal medulla elongation, I also studied the role of p57kip2. Medullary peri-UB pericyte expression of p57kip2 is lost in Wnt7b mutants, and p57kip2 mutants have a shorter renal medulla. Here I identify p57kip2 as a direct transcriptional target of canonical Wnt signaling, and demonstrate that pericyte expression of p57kip2 is sufficient and necessary for renal medulla elongation. Furthermore, I demonstrate that p57kip2 expression in the interstitium partially mediates Wnt7b’s role in renal medulla elongation, likely via oriented cell division. My work shows that although p57kip2 is not a regulator of peri-UB capillary lumen formation, it does regulate proliferation of pericytes and endothelial cells. Importantly, my work on the function of p7kip2 that is expressed in the peri-UB capillary mural cells demonstrates the significance of this population of cells in renal medulla elongation, and the proliferation of the peri-UB capillaries, downstream of Wnt7b. Finally, I extended my work into the embryonic lungs to compare the functions of Wnt7b in capillary development in the kidney and lungs. My analysis of Wnt7b mutant lungs confirms the effect of Wnt7b on capillary lumen formation and on proliferation is tissue specific. Together, my work identifies additional cells types whose development is regulates by Wnt7b, and has begun to uncover the signaling pathways and tissue interactions regulating renal medulla formation.