Myosin Post-translational Modifications Associated with Critical Illness Myopathy

BackgroundCritical Illness Myopathy (CIM) is a devastating consequence of modern critical care, causing dramatic loss of muscle mass and function in intensive care unit (ICU) patients. However, the loss in function by far exceeds the loss in muscle mass and myosin content, but the molecular mechanisms underlying the loss of force and myosin-expressing non-force (NF) generating fibers remain elusive. ObjectivesTo explore the mechanisms underlying the compromised myosin function in ICU patients exposed to 12-day mechanical ventilation and immobilization. MethodsMass spectrometry-based proteomics and molecular dynamics simulations were used to explore the pathophysiology underlying the compromised muscle fiber function previously reported at the single muscle fiber level in six ICU patients on 12th day compared with the 1st day of mechanical ventilation and immobilization. ResultsPrevious measurements revealed single muscle fiber size and specific force to be decreased by [~]25% and 35% (p < 0.05), respectively, from the 1st to the 12th days. On the 12th-day, all fibers showed a decreased specific force and a subset of myosin-expressing fibers was identified exhibiting a complete loss of contractile function despite showing comparable fiber atrophy levels ([~]30%, p < 0.05). Compromised force-generating capacity was linked to 27 post-translational myosin modifications, including oxidation, ubiquitination, acetylation, and methylation. Molecular dynamics simulations revealed an oxidative-induced rigidity of the myosin head, compromising the actin-binding and converter domains flexibility. Notably, the non-force generating fibers exhibited a unique proteomic signature linked with increased structural exposure and rigidity of the myosin motor domain. ConclusionsIn addition to muscle wasting and preferential myosin loss, abnormal myosin post-translational modifications contribute to the dramatic loss in muscle function in ICU patients with CIM, including the development of muscle fibers unable to generate contractile force.