Background

Critical illness myopathy is a common and devastating consequence of critical care, causing dramatic loss of muscle mass and function in intensive care unit patients. Functional deficits often exceed the loss in muscle mass and myosin content. However, the mechanisms underlying the loss of force and emergence of myosin-expressing non-force-generating fibers remain elusive.

Methods

Myosin dysfunction was investigated in six intensive care unit patients exposed to a 12-day mechanical ventilation and immobilization period using mass spectrometry-based proteomics and molecular dynamics simulations.

Results

Previous single muscle fiber analyses revealed decreased fiber size and specific force from the 1st to the 12th days in all patients. A subset of myosin-expressing fibers exhibiting a complete loss of contractile function was identified in three of the patients despite similar atrophy levels (similar to 30%, p < 0.05) after 12 days. All fibers had decreased specific force after 12 days of mechanical ventilation, but 9% to 21% of the fibers were non-force generating. The decline in specific force was linked to 27 post-translational myosin modifications, including oxidation, ubiquitination, acetylation, and methylation. Molecular dynamics simulations indicated oxidation-induced rigidity of the myosin head, predicted to compromise the flexibility of the actin-binding and converter domains. Non-force-generating fibers exhibited a unique proteomic signature predicted to enhance myosin motor domain exposure and rigidity.

Conclusion

In addition to muscle wasting and myosin loss, abnormal myosin post-translational modifications contribute to muscle weakness in ICU patients with CIM, including the development of muscle fibers incapable of generating contractile force.