Canonical nuclear factor-κB signaling in satellite stem cell homeostasis and function.

Satellite cells are adult stem cells that are required for the regeneration of skeletal muscle following injury. However, the signaling mechanisms that regulate satellite stem cell homeostasis and function in adult animals remain less understood. Nuclear factor-kappa B (NF-kB) is a major nuclear transcription factor that regulates the gene expression of a plethora of molecules involved in cellular proliferation, differentiation, survival, and the inflammatory immune response. NF-kB can be activated through a canonical or non-canonical pathway. However, the role of canonical NF-kB signaling in the regulation of satellite stem cell function during skeletal muscle regeneration has not been yet investigated using genetic mouse models. In the present work, we demonstrate that physiological levels of activation of the canonical NF-κB pathway promotes satellite cell proliferation, survival, and differentiation. Satellite cell-specific inducible deletion of Inhibitor of Kappa B Kinase β (IKKβ), a critical kinase of the canonical NF-kB pathway, attenuates muscle regeneration in adult mice. Targeted ablation of IKKβ also reduces the number of satellite cells and their fusion to injured skeletal muscle of adult mice. Inhibition of canonical NF-κB pathway causes precocious differentiation of satellite cells both ex vivo and in vitro. We also found that siRNA-mediated knockdown of components of the canonical NF-kB pathway reduces the survival of cultured satellite cells. Intriguingly, our results also demonstrate that supra-physiological activation of canonical NF-kB inhibits satellite stem cell function during skeletal muscle regeneration. Overexpression of a constitutively active mutant of IKKβ (IKKβca) in satellite cells attenuates initial stages of myofiber regeneration following injury. While not affecting their self-renewal, overexpression of IKKβ causes precocious differentiation of satellite cells. Furthermore, our results suggest that constitutive activation of canonical NF-kB pathway inhibits proliferation and reduces survival of satellite cells. Lastly, we found that inducible expression of IKKβca in satellite cells was insufficient to rescue the regenerative deficits observed in satellite cell-specific TAK1-knockout mice. Altogether, our study suggests that a tight regulation of canonical NF-kB pathway is important for maintaining satellite cell pool and skeletal muscle regeneration