Abstract
Skeletal muscle atrophy is a pathological condition characterized by the progressive loss of muscle mass and function, driven by factors such as disuse, inflammation, and aging. While the ubiquitin-proteasome system is established as the central mediator of myofibrillar protein degradation, the role of selective autophagy and the degradation of organelles remains underexplored in this context. To address this, we employed a quantitative, time-resolved in vitro analysis of protein synthesis and degradation in C2C12 myotubes undergoing TNF-α-induced atrophy, using dynamic Stable Isotope Labeling by Amino Acids in Cell Culture (SILAC) coupled with LC-MS/MS. Our data challenges the classical view of atrophy as a uniform, degradation-centric process. Instead, we reveal temporally distinct patterns of selective protein turnover, including differential degradation of myofibrillar, ribosomal, and endoplasmic reticulum (ER)-resident proteins. Early atrophy is characterized by suppressed short-term protein synthesis, increased ubiquitin-ligase expression, proteasomal activation, and ribosome turnover. In contrast, late atrophy features proteasome-dependent myofibrillar protein degradation, selective synthesis, and degradation of mitochondrial and cytoplasmic ribosomes, indicative of metabolic adaptation. Moreover, we identify a temporal shift in autophagic selectivity: from ER homeostasis to a stress-induced ER-degradation program. Notably, autophagy inhibition during atrophy leads to the accumulation of ER-phagy receptors Tex264 and Calcoco1, implicating ER-phagy as a key contributor to atrophic remodeling and highlighting receptor-mediated selective autophagy as a regulatory axis in muscle proteostasis. By elucidating the role of ER-phagy, this study opens avenues for therapeutic interventions targeting proteostasis in inflammation-induced muscle-wasting, contributing to a refined understanding of muscle atrophy beyond proteasomal degradation, particularly in acute inflammatory conditions such as sepsis.