Myonuclear apoptosis underlies diaphragm atrophy in mechanically ventilated ICU patients.

Abstract (236 words) Rationale. Intensive care unit (ICU) acquired diaphragm weakness is a common consequence of mechanical ventilation (MV). It contributes to difficult weaning, which is associated with increased morbidity and mortality. Diaphragm weakness is caused by a combination of atrophy and dysfunction of myofibers, large syncytial cells that are maintained by a population of myonuclei. Each myonucleus provides gene transcripts to a finite fiber volume, termed the myonuclear domain. Myonuclear loss in myofibers undergoing atrophy is subject to debate. 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 is altered in the diaphragm of mechanically ventilated ICU patients. Methods. We used a combination of confocal microscopy, transcriptomics, and immunohistochemistry techniques to study myonuclear alterations in diaphragm and quadriceps biopsies from MV ICU patients. Measurements and Main Results. Patients with established diaphragm atrophy had a reduced myonuclear number and myonuclear domain. Intrinsic apoptotic pathway activation was identified as a potential mechanism underlying myonuclear removal in the diaphragm of mechanically ventilated ICU patients. Total transcription of 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 mechanism underlying diaphragm atrophy in mechanically ventilated patients. The loss of myonuclei may contribute to difficult weaning due to impaired regrowth of myofibers after atrophy.