Dose-dependent responses to canonical Wnt transcriptional complexes in the regulation of mammalian nephron progenitors.

In vivo and in vitro studies argue that concentration dependent Wnt signaling regulates mammalian nephron progenitor cell (NPC) programs. Canonical Wnt signaling is regulated through the stabilization of β-catenin, a transcriptional co-activator when complexed with Lef/Tcf DNA binding partners. Utilizing the GSK3β inhibitor CHIR99021 (CHIR), to block GSK3β-dependent destruction of β-catenin, we examined dose-dependent responses to β-catenin in NPCs, using mRNA transduction to modify gene expression. Low CHIR-dependent proliferation of NPCs was blocked on β-catenin removal with evidence of NPCs arresting at the G2-M transition. While NPC identity was maintained following β-catenin removal, mRNA-seq identified low CHIR and β-catenin dependent genes. High CHIR activated nephrogenesis. Nephrogenic programming was dependent on Lef/Tcf factors and β-catenin transcriptional activity. Molecular and cellular features of early nephrogenesis were driven in the absence of CHIR by a mutated, stabilized form of β-catenin. Chromatin association studies indicate low and high CHIR response genes are likely direct targets of canonical Wnt transcriptional complexes. Together these studies provide evidence for concentration dependent Wnt-signaling in the regulation of NPCs and provide new insight into Wnt targets initiating mammalian nephrogenesis.