Effects of inhibition of Janus kinase signalling during controlled mechanical ventilation on the rate of skeletal muscle protein synthesis.

We employed a unique murine intensive care unit (ICU) model allowing long-term studies of the ICU condition (immobilization, paralysis, sedation and mechanical ventilation). This model resulted in a substantial loss of myofibrillar protein and muscle size. We hypothesized that an inhibitor of Janus kinase (JAK) activation of transcription 3 (STAT3 (signal transducer and activator of transcription 3)) phosphorylation would help to preserve muscle mass by stimulating the rate of protein synthesis. Sprague-Dawley rats were divided into a control group (CON, n = 5) and two groups exposed to the ICU condition for 8 days. One group was treated with a JAK/STAT3 inhibitor (JST, n = 5) and one without a JST (immobilized group, n = 3) inhibitor. To measure the fractional synthesis rate (FSR) of proteins across the muscle proteome, ²H₂O was administered as an intraparitoneal (IP) bolus followed by continuous infusion of 8% ²H₂O to maintain body water enrichment. Soleus, extensor digitorum longus (EDL), tibialis anterior (TA), gastrocnemius and diaphragm were obtained from all animals. Liquid chromatography-mass spectrometry (LC/MS-MS) analysis was used to measure protein FSR. Compared to CON myofibrillar protein FSR was decreased 39%-73%, with the decrease in gastrocnemius > soleus > TA > diaphragm > EDL. Sarcoplasmic protein FSR was decreased 38%-69%, with the decrease in gastrocnemius > TA > EDL > soleus > diaphragm. Mitochondrial protein FSR was decreased 34%-52%, with the decrease in TA > soleus > gastrocnemius > EDL > diaphragm. The decreases in protein flux rates by ontology corresponded broadly with function and fibre types. Immobilization resulted in profound and tissue-specific decreases in protein FSR, with JAK/STAT3 showing a significant effect to preserve FSR, muscle mass and body weight. KEY POINTS: Mechanical silencing of skeletal muscle resulted in a large lowering of protein fractional synthesis rate (FSR) in all muscles measured, the extent of which was muscle- and fibre specific. All muscle protein ontologies were affected. A Janus kinase (JAK)/STAT inhibitor had a positive effect on body mass, muscle size and protein FSR.