Muscle stem cell function is impaired in absence of Talpid3 - a gene required for primary cilia formation

Skeletal muscle stem cells (MuSC) are crucial for tissue homeostasis and repair after injury. Following activation, they proliferate to generate differentiating myoblasts. A proportion of cells selfrenew, re-enter the MuSC niche under the basal lamina outside the myofiber and become quiescent. Quiescent MuSC have a primary cilium, which is disassembled upon cell cycle entry. Ex vivo experiments suggest cilia are important for MuSC self-renewal, however, their role in muscle regeneration in vivo remains poorly understood. Talpid3 (TA 3 ) is essential for primary cilia formation and Hedgehog (Hh) signalling. Here we use tamoxifen-inducible conditional deletion of TA 3 in MuSC (iSC-KO) and show that regeneration is impaired in response to cytotoxic injury. Repeat injury exacerbates the regeneration phenotype in TA 3iSC-KO mice, indicating depletion of MuSCs. Single cell transcriptomics of MuSC progeny isolated from myofibers identifies components of several signalling pathways, which are deregulated in absence of TA 3 , including Hh and Wnt. Pharmacological activation of Wnt restores muscle regeneration, while purmorphamine, an activator of the Smoothened (Smo) co-receptor in the Hh pathway, has no effect. Together, our data suggest that TA 3 and primary cilia are important for MuSC self-renewal, and that pharmacological treatment can efficiently restore muscle regeneration.