PACS2 Alleviates Sepsis‐Induced Myopathy by Activating ERK–MAPK Signalling Pathway to Suppress ER‐Phagy

BACKGROUND: Sepsis-induced myopathy (SIM) is a common and life-threatening complication, but its underlying mechanisms remain poorly understood. PACS2, a key resident protein at mitochondria-associated endoplasmic reticulum membranes (MAMs), regulates ER homeostasis under various pathological conditions. However, whether sepsis disrupts PACS2-dependent MAM integrity, thereby triggering ER dysfunction and muscle wasting, remains unexplored. METHODS: We established a sepsis mouse model via cecal ligation and puncture (CLP) and assessed muscle function using compound muscle action potential (CMAP) recording and grip strength measurements. Muscle atrophy was evaluated by H&E staining and Western blotting. PACS2 expression was determined by Western blotting, immunohistochemistry and qRT-PCR. MAM integrity was assessed by immunofluorescence co-localization of IP3R and VDAC1, and ER-phagy (reticulophagy) activation was evaluated by transmission electron microscopy, Western blotting and fluorescence microscopy. To investigate the functional role of PACS2, adeno-associated virus (AAV)-mediated PACS2 overexpression was performed in mouse tibialis anterior muscle and gastrocnemius muscles, followed by RNA-sequencing analysis. The MAPK pathway proteins p-ERK, p-P38 and p-JNK levels were assessed by Western blotting, and the involvement of ERK-MAPK signalling was tested pharmacologically via intraperitoneal injection of the ERK inhibitor SCH772984. RESULTS: Septic mice developed progressive skeletal muscle atrophy (p < 0.001) and dysfunction (p < 0.01), accompanied by 56% reduction in PACS2 expression at 96 h post-CLP (p < 0.01), 25% decrease in MAM integrity (p < 0.05) and subsequent activation of FAM134B-mediated ER-phagy (p < 0.01). AAV-mediated PACS2 overexpression significantly alleviated muscle atrophy by restoring MAM integrity by 28% (p < 0.01), reducing FAM134B expression by 43% (p < 0.01) and attenuating ER-phagy (p < 0.01). Co-immunoprecipitation revealed no detectable direct protein-protein interaction between PACS2 and FAM134B. Transcriptome sequencing and Western blotting analysis demonstrated that PACS2 overexpression specifically activated the ERK-MAPK signalling pathway (55% increase in p-ERK, p < 0.01) without affecting p-P38 or p-JNK levels (p>0.05), which suppressed FAM134B-mediated ER-phagy (p < 0.05) and ameliorated muscle atrophy (p < 0.05) by inhibiting nuclear translocation of TFEB (p < 0.01). Pharmacological ERK inhibition with SCH772984 abolished the protective effects of PACS2 by promoting TFEB nuclear translocation (p < 0.001) and TFEB-mediated FAM134B expression (p < 0.001). CONCLUSIONS: Our findings demonstrate that SIM is closely associated with disrupted MAM integrity. PACS2 plays a critical role in maintaining MAM structural integrity and regulating FAM134B-mediated ER-phagy through the ERK-MAPK-TFEB signalling axis, thereby providing novel mechanistic insights and potential therapeutic targets for SIM.