Preferential skeletal muscle myosin loss in response to mechanical silencing in a novel rat intensive care unit model: underlying mechanisms

Abstract

Non-technical summary Wasting and severely impaired function of skeletal muscle is frequently observed in critically ill intensive care unit (ICU) patients, with negative consequences for recovery and quality of life. An experimental rat ICU model has been used to study the mechanisms underlying this unique wasting condition in neuromuscularly blocked and mechanically ventilated animals at durations varying between 6 h and 2 weeks. The complete 'mechanical silencing' of skeletal muscle (removal of both weight bearing and activation) resulted in a specific myopathy frequently observed in ICU patients and characterized by a preferential loss of the motor protein myosin. A highly complex and coordinated protein synthesis and degradation system was observed in the time-resolved analyses. It is suggested the 'mechanical silencing' of skeletal muscle is a dominating factor triggering the specific myopathy associated with the ICU intervention, and strongly supporting the importance of interventions counteracting the complete unloading in ICU patients. AbstractThe muscle wasting and impaired muscle function in critically ill intensive care unit (ICU) patients delay recovery from the primary disease, and have debilitating consequences that can persist for years after hospital discharge. It is likely that, in addition to pernicious effects of the primary disease, the basic life support procedures of long-term ICU treatment contribute directly to the progressive impairment of muscle function. This study aims at improving our understanding of the mechanisms underlying muscle wasting in ICU patients by using a unique experimental rat ICU model where animals are mechanically ventilated, sedated and pharmacologically paralysed for duration varying between 6 h and 14 days. Results show that the ICU intervention induces a phenotype resembling the severe muscle wasting and paralysis associated with the acute quadriplegic myopathy (AQM) observed in ICU patients, i.e. a preferential loss of myosin, transcriptional down-regulation of myosin synthesis, muscle atrophy and a dramatic decrease in muscle fibre force generation capacity. Detailed analyses of protein degradation pathways show that the ubiquitin proteasome pathway is highly involved in this process. A sequential change in localisation of muscle-specific RING finger proteins 1/2 (MuRF1/2) observed during the experimental period is suggested to play an instrumental role in both transcriptional regulation and protein degradation. We propose that, for those critically ill patients who develop AQM, complete mechanical silencing, due to pharmacological paralysis or sedation, is a critical factor underlying the preferential loss of the molecular motor protein myosin that leads to impaired muscle function or persisting paralysis.

Nuclear actin and Mrtfs control disuse muscle atrophy via Srf activity regulation

“…S1B). We then examined Srf subcellular localization within myofibers, as its extranuclear localization has been reported in atrophied rat muscles after 14 days of an 'intensive care unit model' (Ochala et al, 2011) and in beating-arrested neonatal rat cardiomyocytes (Lange et al, 2005); Srf extranuclear localization has also been associated with nuclear accumulation of MuRF2. However, up to 7 days post denervation, Srf protein was located inside myonuclei (supplementary material Fig.…”

Recent research developments in regeneration of skeletal muscle

“…At the M-band, the mechanically modulated kinase domain of titin interacts with a complex of the protein products of the atrogenes NBR1, p62/SQSTM1 and MuRF 73,75) . This complex dissociates under mechanical arrest, and MuRF-1 and MuRF-2 translocate to the cytoplasm and the nucleus 73,76) . One of the probable nuclear targets of MuRFs is SRF 73) , suggesting that the MuRF-induced nuclear export and transcriptional repression of SRF may contribute to amplifying the transcriptional atrophy program 77) .…”

Ubiquitination and Proteolysis in Acute Lung Injury

“…28 Interestingly, the marked decrease in muscle fi ber force generation capacity was associated with a sequential change in the localization of the muscle-specifi c RING fi nger proteins 1/2 leading to degradation and transcriptional downregulation of the molecular motor protein myosin, suggesting that ubiquitination played a central role in both muscle proteolysis and synthesis. 28 Furthermore, it is well known that diaphragmatic function is critically important for successful weaning of patients from mechanical ventilation. However, mechanical ventilation itself has been shown to result in a rapid loss of diaphragmatic strength.…”

“…26 , 27 It is now increasingly evident that muscle weakness during respiratory disease is not a passive event but is rather mediated by ubiquitination and subsequent degradation of muscle fi bers. 28 For example, in an experimental rat ICU model, in which animals were mechanically ventilated, sedated, and pharmacologically paralyzed for up to 14 days, severe muscle wasting and paralysis similar to that observed among ICU patients was reported. 28 Interestingly, the marked decrease in muscle fi ber force generation capacity was associated with a sequential change in the localization of the muscle-specifi c RING fi nger proteins 1/2 leading to degradation and transcriptional downregulation of the molecular motor protein myosin, suggesting that ubiquitination played a central role in both muscle proteolysis and synthesis.…”

“…28 For example, in an experimental rat ICU model, in which animals were mechanically ventilated, sedated, and pharmacologically paralyzed for up to 14 days, severe muscle wasting and paralysis similar to that observed among ICU patients was reported. 28 Interestingly, the marked decrease in muscle fi ber force generation capacity was associated with a sequential change in the localization of the muscle-specifi c RING fi nger proteins 1/2 leading to degradation and transcriptional downregulation of the molecular motor protein myosin, suggesting that ubiquitination played a central role in both muscle proteolysis and synthesis. 28 Furthermore, it is well known that diaphragmatic function is critically important for successful weaning of patients from mechanical ventilation.…”