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

Wnt/β-catenin signaling is a highly conserved molecular pathway that plays a crucial role in stem/progenitor systems and cancer. β-catenin, the main Wnt pathway effector, has two pools within a cell: one for cell-cell adhesion at the membrane and the other for transcriptional functions in the nucleus. However, the mechanism by which β-catenin mediates both roles remain unclear. The tightly controlled, well characterized system of nephrogenesis is an ideal model to decouple the roles of β-catenin at the membrane and in the nucleus. In kidney development, a delicate balance of nephron progenitor cell self-renewal and differentiation is required for the mesenchymal to epithelial transition (MET) in nephrogenesis and is driven by Wnt/β-catenin signaling. Given an ability to isolate and manipulate large numbers of NPCs in tissue culture, we can dissect the dual nature of β-catenin as a transcriptional activator and component of a cell membrane complex in adhesion. We pioneered a method using CRISPR/Cas9 gene editing to rapidly remove β-catenin, Tcf/Lef factors and simultaneous cadherin genes in primary NPCs. We have characterized the effects of modulating β-catenin and integrated RNA-seq results from β-catenin’s removal with mouse ChIP-seq and mouse single cell RNA -seq data. Functional analysis of β-catenin removal provides strong evidence for β-catenin regulation of NPC proliferation, independent of a direct Lef/Tcf associated transcriptional program. Together these data suggest β-catenin mediates aggregation, the first step in MET, through β-catenin mediated cell adhesion complexes while simultaneous transcriptional activation within these structures initiates the nephrogenic program. The studies provide new insight into the direct transcriptional role of Lef/Tcf/β-catenin complexes associated with the initiation of a nephron forming program. Overall, this study enhances an understanding of the molecular mechanisms underlying kidney development and the dual nature of β-catenin stem/progenitor systems at large.