Inactivation of SERCA2 at Cys674 Induces Skeletal Muscle Atrophy by Activating the TGFβ/Smad-S100a4 Axis to Promote Inflammation

Background & Aims Disuse-induced skeletal muscle atrophy is a major clinical challenge lacking effective targeted therapies. Sarco/endoplasmic reticulum Ca 2+ -ATPase 2 (SERCA2) is essential for calcium homeostasis in skeletal muscle, but whether its C674 active site contributes to muscle atrophy remains unclear. This study investigated the role of SERCA2 C674 inactivation in skeletal muscle atrophy and the underlying mechanisms. Methods Oxidized SERCA2 (C674-SO 3 H) expression was assessed in atrophied muscles from bedridden patients and in a murine hindlimb suspension (HLS) model. A SERCA2 C674S knock-in (SKI) mouse model was generated to mimic irreversible oxidation in vivo. Transcriptomic and proteomic analyses, together with in vitro experiments in C2C12 myoblasts and in vivo interventions using Losartan or adeno-associated virus serotype 9 (AAV9)-mediated shRNA, were performed to define downstream signaling pathways and therapeutic potential. Results Total SERCA2 protein levels were unchanged in atrophied muscles from patients and HLS mice, whereas C674-SO 3 H expression was significantly increased. SKI mice developed spontaneous skeletal muscle atrophy, with reduced myofiber cross-sectional area and impaired muscle strength. Multi-omics analyses showed that SERCA2 dysfunction activated the renin-angiotensin system (RAS)-dependent TGF-β/Smad pathway and markedly upregulated S100a4. Mechanistically, S100a4 interacted with Smad3 and acted as a key downstream effector promoting oxidative stress, inflammation, and muscle protein degradation-related pathways. In vivo treatment with Losartan or AAV9-mediated S100a4 knockdown alleviated local inflammation, fibrosis, and muscle atrophy in both SKI and HLS mice. Conclusions The oxidative inactivation of the SERCA2 C674 site constitutes a novel mechanism driving skeletal muscle atrophy. The SERCA2-RAS-TGF-β/Smad-S100a4 signaling axis emerges as a highly promising therapeutic target for mitigating disuse-induced skeletal muscle wasting.