Denervation triggers dramatic atrophy of skeletal muscle, accompanied by synaptic, contractile and metabolic changes. Several factors were shown to contribute to genetic reprogramming and proteostasis changes after denervation. However, the mechanisms underlying the coordinated regulation of denervation-induced muscle fiber remodeling remain misunderstood. We investigated the role of the transcriptional co-repressor CtBP1 in the regulation of denervation-induced responses in muscle fibers. To this end, we analyzed its expression and localization in innervated and denervated muscles and assessed the consequences of its knockdown induced in vivo with AAV9, on synaptic, contractile and metabolic properties of muscle fibers. CtBP1 was present both in sub- and non-synaptic myonuclei in innervated muscle. Although CtBP1 levels remained unchanged in denervated muscle, CtBP1 accumulated transiently in myonuclei after 2 days of denervation in TA/EDL muscles. Ctbp1 knockdown perturbed the expression of a large set of activity-independent and -dependent genes in innervated and denervated skeletal muscles. Reducing CtBP1 levels had limited effect on the expression of most synaptic genes, but increased transcript levels of Chrne, encoding the adult ε sub-unit of acetylcholine receptors (AChR). However, it did not affect AChR turnover or maintenance of the post-synaptic compartment upon denervation. Importantly, we uncovered that Ctbp1 knockdown exacerbates denervation-induced changes in metabolic gene expression, including most genes encoding proteins of the respiratory chain complexes. Consistently, it induced a contractile shift towards slower fibers in innervated fast muscle, mimicking denervation, and enhanced the denervation-induced metabolic transition towards oxidative slow-twitch fibers. Moreover, Ctbp1 knockdown precipitated the profound ultrastructural remodeling of mitochondria network induced after denervation. Our study unveils that CtBP1 sustains the innervated muscle pattern and antagonizes the effect of denervation on synaptic, contractile and metabolic muscle properties, with important implications for CtBP1-related muscle diseases.
Cattaneo et al. (Mon,) studied this question.