Hypothesis: A New Role for the Renin-Angiotensin System in Ureteric Bud Branching

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Abstract
Branching morphogenesis in the developing mammalian kidney involves growth and branching of the ureteric bud (UB), leading to formation of its daughter collecting ducts, calyces, pelvis and ureters. Even subtle defects in the efficiency and/or accuracy of this process have profound effects on the ultimate development of the kidney and result in congenital abnormalities of the kidney and urinary tract. This review summarizes current knowledge regarding a number of genes known to regulate UB development and emphasizes an emerging role for the renin-angiotensin system (RAS) in renal branching morphogenesis. Mutations in the genes encoding components of the RAS in mice cause renal papillary hypoplasia, hydronephrosis, and urinary concentrating defect. These findings imply that UB-derived epithelia are targets for angiotensin (ANG) II actions during metanephric kidney development. Here, it is proposed that papillary hypoplasia in RAS-deficient mice is secondary to an intrinsic defect in the development of the renal medulla. This hypothesis is based on the following observations: a) UB and surrounding stroma express angiotensinogen (AGT) and ANG II AT1 receptors in vivo; b) ANG II stimulates UB cell process extension, branching and cord formation in collagen gel cultures in vitro; and c) AT1 blockade inhibits ANG II-induced UB cell branching. It is further postulated that ANG II is a novel stroma-derived factor involved in stroma/UB cross-talk which regulates UB branching morphogenesis.