Myonuclear apoptosis underlies diaphragm atrophy in mechanically ventilated ICU patients

Rationale Mechanical ventilation plays an important role in critical illness-associated diaphragm weakness. Weakness contributes to difficult weaning and is associated with increased morbidity and mortality. Diaphragm weakness is caused by a combination of atrophy and dysfunction of myofibers, which are large syncytial cells maintained by a population of myonuclei. Each myonucleus provides gene transcripts to a finite fiber volume, termed the myonuclear domain. Changes in myonuclear number in myofibers undergoing atrophy has not been investigated in mechanically ventilated ICU patients. Myonuclear number is a determinant of transcriptional capacity, and therefore critical for muscle regeneration after atrophy. Objectives Our objective was to investigate if and how myonuclear number changes in the diaphragm of mechanically ventilated ICU patients and whether changes are associated with myofiber atrophy. Methods We used a combination of transcriptomics, immunohistochemistry, and confocal microscopy to study myonuclear alterations in diaphragm and quadriceps biopsies from mechanically ventilated ICU patients. Results Myonuclear number and myonuclear domain were reduced in patients with diaphragm myofiber atrophy. Intrinsic apoptotic pathway activation was identified as a mechanism underlying myonuclear removal in the diaphragm of mechanically ventilated ICU patients. Total transcriptional activity in myofibers decreased with myonuclear loss. Furthermore, muscle stem cell number was reduced in the patients with diaphragm atrophy. Conclusion We identified myonuclear loss due to intrinsic apoptotic pathway activation as a potential mechanism underlying diaphragm atrophy in mechanically ventilated patients. This provides novel insights in diaphragm weakness of ICU patients. Targeted therapies may limit development of diaphragm weakness and improve weaning outcome.