Regeneration of sensory hair cells and progenitor self-renewal require localized interactions between the Notch and Wnt signaling pathways

Mechanosensory hair cell damage leads to permanent hearing loss. The zebrafish mechanosensory organs (neuromasts) are an excellent model for the study of hair cell regeneration. Neuromasts have a central group of sensory hair cells surrounded by support cells and the outermost mantle cells. While adult mammalian hair cells cannot regenerate, neuromast hair cells have the ability to do so via support cell proliferation. In this study, we used proliferation assays and lineage analyses that demonstrated the existence of proliferative compartments in the posterior lateral line neuromasts where support cells either self-renew or differentiate. These spatially restricted lineage decisions within the progenitor pool resemble the spatial heterogeneities within the intestinal crypt or the hair follicle niche that guide stem cell transit amplification or differentiation. In addition, we identified the mantle cells as a quiescent support cell pool that do not proliferate in response to selective hair cell ablation, but that re-enter the cell cycle when hair cell and support cells numbers are drastically reduced. By combining our lineage analysis with gene expression analyses, genetic manipulations, and the use of chemical inhibitors, we were able to dissect the roles of Notch and Wnt signaling during homeostasis and regeneration. We demonstrate that Notch signaling restricts hair cell differentiation and maintains the spatial pattern of support cell proliferation through Wnt signaling inhibition. iv Thus, Notch-Wnt signaling interactions are required to maintain pools of amplifying support cells in the poles and maintain tissue homeostasis by balancing self-renewal and differentiation.