Intensive care unit (ICU)-acquired weakness is a frequent consequence of critical illness that impacts both the limb and respiratory muscles. The cause of ICU-acquired weakness is multifactorial, but both prolonged limb muscle inactivity and mechanical ventilation are risk factors for muscle wasting, which predisposes ICU patients to both short-term complications and long-term disabilities resulting from muscle weakness. Unfortunately, the current research does not provide a detailed understanding of the cellular etiology of ICU-acquired weakness, and no standard treatment exists. Therefore, improving knowledge of the mechanisms promoting muscle atrophy in critically ill patients is essential to developing therapeutic strategies to protect against ICU-induced skeletal muscle wasting. To advance our understanding of the mechanism(s) responsible for ICU-acquired weakness, we tested the hypothesis that ICU-induced muscle inactivity promotes a rapid decrease in anabolic signaling/protein synthesis and accelerates proteolysis in both limb and respiratory muscles. To investigate ICU-induced changes in skeletal muscle proteostasis, adult Sprague Dawley rats were anesthetized and mechanically ventilated for 12 h to simulate ICU care. Measurements of anabolic signaling, protein synthesis, and proteolytic activity in the limb muscles (plantaris and soleus) and respiratory muscles (parasternal and intercostal) revealed ICU-induced reductions in both anabolic signaling (i.e., AKT/mTOR pathway) and muscle protein synthesis. Moreover, simulated ICU care resulted in increased biomarkers of accelerated proteolysis in both limb and respiratory muscles. These novel findings reveal that disturbances in limb and respiratory muscle proteostasis occur rapidly during ICU-induced muscle inactivity, irrespective of the muscle function or muscle fiber type.

Angiotensin II (AngII), the primary effector of the renin-angiotensin system, is essential for maintaining blood pressure and fluid-electrolyte homeostasis. However, elevated AngII levels are a feature of disease conditions such as heart failure and chronic kidney disease, where it is associated with pathological tissue remodeling and fibrosis. AngII-mediated fibrosis has been documented in multiple organs and is characterized by fibroblast expansion, myofibroblast differentiation, and excessive extracellular matrix deposition. Periostin has recently emerged as a marker of fibroblast activation. Notably, periostin expression is highly upregulated during fibrotic remodeling in the kidney and lung, which is strongly linked with impaired organ function. While AngII-induced activation of periostin-lineage (Postn^Lin) cells is well established in the heart, the temporal dynamics of Postn^Lin activation in response to AngII infusion in the lung and kidney remain unexplored. Here, we used a Postn-MerCreMer lineage-tracing approach, combined with continuous AngII infusion over an experimental period of one week and two weeks to assess Postn^Lin responses in lung and kidney. Our findings reveal a progressive activation of Postn^Lin cells in both organs, characterized by myofibroblast phenotype, together with increased collagen deposition and macrophage infiltration. These results highlight the potential of Postn^Lin fibroblasts as a key effector of AngII-mediated tissue remodeling and fibrosis in the lung and kidney.