Six1 regulates Grem1 expression in the metanephric mesenchyme to initiate branching morphogenesis

Urinary tract morphogenesis requires subdivision of the ureteric bud (UB) into the intra-renal collecting system and the extra-renal ureter, by responding to signals in its surrounding mesenchyme. BMP4 is a mesenchymal regulator promoting ureter development, while GREM1 is necessary to negatively regulate BMP4 activity to induce UB branching. However, the mechanisms that regulate the GREM1-BMP4 signaling are unknown. Previous studies have shown that Six1-deficient mice lack kidneys, but form ureters. Here, we show that the tip cells of Six1−/− UB fail to form an ampulla for branching. Instead, the UB elongates within Tbx18- and Bmp4-expressing mesenchyme. We find that the expression of Grem1 in the metanephric mesenchyme (MM) is Six1-dependent. Treatment of Six1−/− kidney rudiments with GREM1 protein restores ampulla formation and branching morphogenesis. Furthermore, we demonstrate that genetic reduction of BMP4 levels in Six1−/− (Six1−/−; Bmp4+/−) embryos restores urinary tract morphogenesis and kidney formation. This study uncovers an essential function for Six1 in the MM as an upstream regulator of Grem1 in initiating branching morphogenesis.

Research highlights
► Grem1 expression in the metanephric mesenchyme is Six1-dependent.
► Exogenous GREM1 protein restores branching morphogenesis of Six1−/− kidney primordia.
► Deletion of one copy of Bmp4 in Six1−/− embryos restores urinary tract morphogenesis.