Defining the role of beta-catenin in muscle regeneration
- Department of Clinical Pharmacology, Flinders University, South Australia 5042, Australia.
- Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
- Gene Expression Laboratory, The Salk Institute, La Jolla, CA 92037, USA.
Satellite cells are the resident stem cells of skeletal muscle; their activation after injury generates transit-amplifying myoblasts that differentiate to replace damaged myofibers. Canonical Wnt signalling controls myoblast differentiation in vitro and in vivo. However, the role of the Wnt effector β-catenin in adult myogenesis has been contentious, and its mechanism(s) of action unclear. We examined the cell autonomous functions of β-catenin during differentiation using CRISPR-generated β-catenin null adult mouse myoblasts. β-catenin was essential for morphological differentiation and timely deployment of the myogenic gene program. β-catenin null myoblasts showed a rounded phenotype and increased rate of proliferation; after treatment with Wnt, they showed delayed acquisition of the myogenic gene expression program. Fusion, elongation and alignment were impaired, and myogenic gene expression was not coordinated with cytoskeletal and membrane remodelling events. We performed rescue studies with mutant forms of β-catenin that cannot interact with TCF/LEF factors or with α-catenin at membrane junction complexes. Interaction of β-catenin with TCF/LEF factors was not required for differentiation; whereas loss of interaction of β-catenin with α-catenin impaired differentiation. Genome-wide epigenomics showed that β-catenin is required for Wnt-mediated enhancement of MyoD binding to DNA and for activation of myogenic target genes. Overall, we propose that β-catenin acts in the nucleus through its binding partner MyoD to activate the myogenic differentiation program; the membrane-associated β-catenin pool may also control cell-cell interactions. While β-catenin-TCF/LEF complexes are not required for differentiation, they are likely to be involved in feedback to control levels of β-catenin to prevent precocious/excessive myoblast fusion. We also show how specific inhibition of different β-catenin-cofactor-coactivator complexes could be used to curtail pro-fibrotic signalling, whilst maintaining essential pro-differentiation functions.