Mdm4 controls ureteric bud branching via regulation of p53 activity

The antagonism between Mdm2 and its close homolog Mdm4 (also known as MdmX) and p53 is vital for embryogenesis and organogenesis. Previously, we demonstrated that targeted disruption of Mdm2 in the Hoxb7+ ureteric bud (Ub) lineage, which gives rise to the renal collecting system, causes renal hypodysplasia culminating in perinatal lethality. In this study, we examine the unique role of Mdm4 in establishing the collecting duct system of the murine kidney. Hoxb7Cre driven loss of Mdm4 in the Ub lineage (Ub^Mdm4−/−) disrupts branching morphogenesis and triggers UB cell apoptosis. Ub^Mdm4−/− kidneys exhibit abnormally dilated Ub tips while the medulla is hypoplastic. These structural alterations result in secondary depletion of nephron progenitors and nascent nephrons. As a result, newborn Ub^Mdm4−/− mice have hypo-dysplastic kidneys. Transcriptional profiling revealed downregulation of the Ret-tyrosine kinase pathway components, Gdnf, Wnt11, Sox8, Etv4 and Cxcr4 in the Ub^Mdm4−/− mice relative to controls. Moreover, the expression levels of the canonical Wnt signaling members Axin2 and Wnt9b are downregulated. Mdm4 deletion upregulated p53 activity and p53-target gene expression including Cdkn1a (p21), Gdf15, Ccng1, PERP, and Fas. Germline loss of p53 in Ub^Mdm4−/− mice largely rescues kidney development and terminal differentiation of the collecting duct. We conclude that Mdm4 plays a unique and vital role in Ub branching morphogenesis and collecting system development.