BMP-2/ALK3 and HGF signal in parallel to regulate renal collecting duct morphogenesis

Bone morphogenetic protein (BMP)-2 and hepatocyte growth factor (HGF) exert antagonistic effects on renal collecting duct formation during embryogenesis. A current model proposes HGF inhibits BMP-2 signaling at the level of Smad1 in a common target cell. Here, we show that BMP-2 and HGF control collecting duct formation via parallel pathways. We examined the interactions between BMP-2 and HGF in the mIMCD-3 model of collecting duct morphogenesis. During tubule formation, HGF rescued the inhibitory effects of BMP-2 and of a constitutive active form of the BMP-2 receptor, ALK3, stably expressed in mIMCD-3 cells. To determine whether the effect of HGF occurs through known mediators which act downstream of the BMP-2/ALK3 complex, we examined the effect of HGF on BMP-2-induced Smad1 phosphorylation, Smad1/Smad4 complex formation, and Smad1 nuclear translocation. Neither HGF nor other receptor tyrosine kinase ligands (EGF, FGF-4) induced phosphorylation of endogenous Smad1 in mIMCD-3 cells or in Mv1Lu, MC3T3-E1 or P19 cells. Furthermore, none of these ligands blocked induction of the BMP-responsive promoter, Tlx2. Thus, HGF overcomes the inhibitory effects of BMP-2 on collecting duct morphogenesis without interrupting any of the known signaling events in the BMP-2 dependent Smad1 signaling pathway. We conclude that BMP-2/ALK3 and HGF function to control parallel pathways downstream of their respective cell surface receptors. Integration of these signals likely occurs at the level of transcriptional or post-transcriptional events.