Impact of post-synaptic block of neuromuscular transmission, muscle unloading and mechanical ventilation on skeletal muscle protein and mRNA expression

Compound Muscle Action Potential and Myosin-Loss Pathology in Patients With Critical Illness Myopathy: Correlation and Prognostication

BACKGROUND AND OBJECTIVES

Prolonged compound muscle action potential (CMAP) duration and preferential loss of myosin are considered the diagnostic hallmarks of critical illness myopathy (CIM); however, their correlation and prognostic values have not been studied. We aimed to investigate the correlation between CMAP duration and myosin loss and their effect on mortality by comparing between patients with CIM with and without myosin loss.

METHODS

We searched the Mayo Clinic Electromyography Laboratory databases (1986-2021) for patients diagnosed with CIM on the basis of prolonged distal CMAP durations (>15 msec in fibular motor nerve studies recording over the tibialis anterior or >8 msec in other motor nerves) and needle EMG findings compatible with myopathy. Electrodiagnostic studies were generally performed within 24 hours after weakness became noticeable. We included only patients who underwent muscle biopsy. Clinical, electrophysiologic, and myopathologic data were reviewed. We conducted myosin/actin ratio analysis when muscle tissue was available. We used the Fisher exact test for categorical data comparisons and the Mann-Whitney 2-tailed test for continuous data. We applied the Kaplan-Meier technique to analyze survival rates.

RESULTS

Twenty patients (13 female patients) were identified [median age at diagnosis of 62.5 years (range: 19-80 years)]. The median onset of weakness was 24 days (range: 1-128) from the first day of intensive care unit admission. Muscle biopsy showed myosin loss in 14 patients, 9 of whom had >50% of myofibers affected (high grade). Type 2 fiber atrophy was observed in 19 patients, 13 of whom also had myosin loss. Patients with myosin loss had higher frequency of steroid exposure (14 vs 3; p = 0.004); higher median number of necrotic fibers per low-power field (2.5 vs 1, p = 0.04); and longer median CMAP duration (msec) of fibular (13.4 vs 8.75, p = 0.02), tibial (10 vs 7.8, p = 0.01), and ulnar (11.1 vs 7.95, p = 0.002) nerves compared with those without. Only patients with high-grade myosin loss had reduced myosin/actin ratios (<1.7). Ten patients died during median follow-up of 3 months. The mortality rate was similar between patients with and without myosin loss. Patients with high-grade myosin loss had a lower overall survival rate than those with low-grade or no myosin loss, but this was not statistically significant (p = 0.05).

DISCUSSION

Myosin loss occurred in 70% of the patients with CIM with prolonged CMAP duration. Longer CMAP duration predicts myosin-loss pathology. The extent of myosin loss marginally correlates with the mortality rate. Our findings highlight the potential prognostic values of CMAP duration and myosin loss severity in predicting disease outcome.

Single fibre cytoarchitecture in ventilator-induced diaphragm dysfunction (VIDD) assessed by quantitative morphometry second harmonic generation imaging: Positive effects of BGP-15 chaperone co-inducer and VBP-15 dissociative corticosteroid treatment

Ventilator-induced diaphragm dysfunction (VIDD) is a common sequela of intensive care unit (ICU) treatment requiring mechanical ventilation (MV) and neuromuscular blockade (NMBA). It is characterised by diaphragm weakness, prolonged respirator weaning and adverse outcomes. Dissociative glucocorticoids (e.g., vamorolone, VBP-15) and chaperone co-inducers (e.g., BGP-15) previously showed positive effects in an ICU-rat model. In limb muscle critical illness myopathy, preferential myosin loss prevails, while myofibrillar protein post-translational modifications are more dominant in VIDD. It is not known whether the marked decline in specific force (force normalised to cross-sectional area) is a pure consequence of altered contractility signaling or whether diaphragm weakness also has a structural correlate through sterical remodeling of myofibrillar cytoarchitecture, how quickly it develops, and to which extent VBP-15 or BGP-15 may specifically recover myofibrillar geometry. To address these questions, we performed label-free multiphoton Second Harmonic Generation (SHG) imaging followed by quantitative morphometry in single diaphragm muscle fibres from healthy rats subjected to five or 10 days of MV + NMBA to simulate ICU treatment without underlying confounding pathology (like sepsis). Rats received daily treatment of either Prednisolone, VBP-15, BGP-15 or none. Myosin-II SHG signal intensities, fibre diameters (FD) as well as the parameters of myofibrillar angular parallelism (cosine angle sum, CAS) and in-register of adjacent myofibrils (Vernier density, VD) were computed from SHG images. ICU treatment caused a decline in FD at day 10 as well as a significant decline in CAS and VD from day 5. Vamorolone effectively recovered FD at day 10, while BGP-15 was more effective at day 5. BGP-15 was more effective than VBP-15 in recovering CAS at day 10 although not to control levels. In-register VD levels were restored at day 10 by both compounds. Our study is the first to provide quantitative insights into VIDD-related myofibrillar remodeling unravelled by SHG imaging, suggesting that both VBP-15 and BGP-15 can effectively ameliorate the structure-related dysfunction in VIDD.

Mitochondrial Dysfunction in Intensive Care Unit-Acquired Weakness and Critical Illness Myopathy: A Narrative Review

Mitochondria are key structures providing most of the energy needed to maintain homeostasis. They are the main source of adenosine triphosphate (ATP), participate in glucose, lipid and amino acid metabolism, store calcium and are integral components in various intracellular signaling cascades. However, due to their crucial role in cellular integrity, mitochondrial damage and dysregulation in the context of critical illness can severely impair organ function, leading to energetic crisis and organ failure. Skeletal muscle tissue is rich in mitochondria and, therefore, particularly vulnerable to mitochondrial dysfunction. Intensive care unit-acquired weakness (ICUAW) and critical illness myopathy (CIM) are phenomena of generalized weakness and atrophying skeletal muscle wasting, including preferential myosin breakdown in critical illness, which has also been linked to mitochondrial failure. Hence, imbalanced mitochondrial dynamics, dysregulation of the respiratory chain complexes, alterations in gene expression, disturbed signal transduction as well as impaired nutrient utilization have been proposed as underlying mechanisms. This narrative review aims to highlight the current known molecular mechanisms immanent in mitochondrial dysfunction of patients suffering from ICUAW and CIM, as well as to discuss possible implications for muscle phenotype, function and therapeutic approaches.

Identification of Feature Genes and Key Biological Pathways in Immune-Mediated Necrotizing Myopathy: High-Throughput Sequencing and Bioinformatics Analysis

Background

Immune-mediated necrotizing myopathy (IMNM), a subgroup of idiopathic inflammatory myopathies (IIMs), is characterized by severe proximal muscle weakness and prominent necrotic fibers but no infiltration of inflammatory cells. IMNM pathogenesis is unclear. This study investigated key biomarkers and potential pathways for IMNM using high-throughput sequencing and bioinformatics technology.

Methods

RNA sequencing was conducted in 18 IMNM patients and 10 controls. A combination of weighted gene coexpression network analysis (WGCNA) and differentially expressed gene (DEG) analysis was conducted to identify IMNM-related DEGs. Feature genes were screened out by employing the protein-protein interaction (PPI) network, support vector machine-recursive feature elimination (SVM-RFE), and least absolute shrinkage selection operator (LASSO). Quantitative real-time polymerase chain reaction (qRT-PCR) was performed to verify their differential expression, and the receiver operating characteristic curve (ROC) was used to evaluate their diagnostic efficiency. Functional enrichment analysis was applied to reveal the hidden functions of feature genes. Furthermore, 28 immune cell abundance patterns in IMNM samples were measured.

Results

We identified 193 IMNM-related DEGs that were aberrantly upregulated in the IMNM population and were closely associated with immune-inflammatory responses, regulation of skeletal and cardiac muscle contraction, and lipoprotein metabolism. With the help of the PPI network and the LASSO and SVM-RFE algorithms, three feature genes, LTK, MYBPH, and MYL4, were identified and further confirmed by qRT-PCR. ROC curves among IMNM, dermatomyositis (DM), inclusion body myositis (IBM), and polymyositis (PM) samples validated the LTK and MYL4 genes as IMNM-specific feature markers. In addition, all three genes had a notable association with the autophagy-lysosome pathway and immune-inflammatory responses. Ultimately, IMNM displayed a marked immune-cell infiltrative microenvironment. The most significant correlation was found between CD4 T cells, CD8 T cells, macrophages, natural killer (NK) cells, and dendritic cells (DCs).

Conclusions

LTK, MYBPH, and MYL4 were identified as potential key molecules for IMNM and are believed to play a role in the autophagy-lysosome pathway and muscle inflammation.

A prospective clinical study on the mechanisms underlying critical illness myopathy-A time-course approach

BACKGROUND

Critical illness myopathy (CIM) is a consequence of modern critical care resulting in general muscle wasting and paralyses of all limb and trunk muscles, resulting in prolonged weaning from the ventilator, intensive care unit (ICU) treatment and rehabilitation. CIM is associated with severe morbidity/mortality and significant negative socioeconomic consequences, which has become increasingly evident during the current COVID-19 pandemic, but underlying mechanisms remain elusive.

METHODS

Ten neuro-ICU patients exposed to long-term controlled mechanical ventilation were followed with repeated muscle biopsies, electrophysiology and plasma collection three times per week for up to 12 days. Single muscle fibre contractile recordings were conducted on the first and final biopsy, and a multiomics approach was taken to analyse gene and protein expression in muscle and plasma at all collection time points.

RESULTS

(i) A progressive preferential myosin loss, the hallmark of CIM, was observed in all neuro-ICU patients during the observation period (myosin:actin ratio decreased from 2.0 in the first to 0.9 in the final biopsy, P < 0.001). The myosin loss was coupled to a general transcriptional downregulation of myofibrillar proteins (P < 0.05; absolute fold change >2) and activation of protein degradation pathways (false discovery rate [FDR] <0.1), resulting in significant muscle fibre atrophy and loss in force generation capacity, which declined >65% during the 12 day observation period (muscle fibre cross-sectional area [CSA] and maximum single muscle fibre force normalized to CSA [specific force] declined 30% [P < 0.007] and 50% [P < 0.0001], respectively). (ii) Membrane excitability was not affected as indicated by the maintained compound muscle action potential amplitude upon supramaximal stimulation of upper and lower extremity motor nerves. (iii) Analyses of plasma revealed early activation of inflammatory and proinflammatory pathways (FDR < 0.1), as well as a redistribution of zinc ions from plasma.

CONCLUSIONS

The mechanical ventilation-induced lung injury with release of cytokines/chemokines and the complete mechanical silencing uniquely observed in immobilized ICU patients affecting skeletal muscle gene/protein expression are forwarded as the dominant factors triggering CIM.

Metabolomic Profiling of Respiratory Muscles and Lung in Response to Long-Term Controlled Mechanical Ventilation

Critical illness myopathy (CIM) and ventilator-induced diaphragm dysfunction (VIDD) are characterized by severe muscle wasting, muscle paresis, and extubation failure with subsequent increased medical costs and mortality/morbidity rates in intensive care unit (ICU) patients. These negative effects in response to modern critical care have received increasing attention, especially during the current COVID-19 pandemic. Based on experimental and clinical studies from our group, it has been hypothesized that the ventilator-induced lung injury (VILI) and the release of factors systemically play a significant role in the pathogenesis of CIM and VIDD. Our previous experimental/clinical studies have focused on gene/protein expression and the effects on muscle structure and regulation of muscle contraction at the cell and motor protein levels. In the present study, we have extended our interest to alterations at the metabolomic level. An untargeted metabolomics approach was undertaken to study two respiratory muscles (diaphragm and intercostal muscle) and lung tissue in rats exposed to five days controlled mechanical ventilation (CMV). Metabolomic profiles in diaphragm, intercostal muscles and lung tissue were dramatically altered in response to CMV, most metabolites of which belongs to lipids and amino acids. Some metabolites may possess important biofunctions and play essential roles in the metabolic alterations, such as pyruvate, citrate, S-adenosylhomocysteine, alpha-ketoglutarate, glycerol, and cysteine. Metabolic pathway enrichment analysis identified pathway signatures of each tissue, such as decreased metabolites of dipeptides in diaphragm, increased metabolites of branch-chain amino acid metabolism and purine metabolism in intercostals, and increased metabolites of fatty acid metabolism in lung tissue. These metabolite alterations may be associated with an accelerated myofibrillar protein degradation in the two respiratory muscles, an active inflammatory response in all tissues, an attenuated energy production in two respiratory muscles, and enhanced energy production in lung. These results will lay the basis for future clinical studies in ICU patients and hopefully the discovery of biomarkers in early diagnosis and monitoring, as well as the identification of future therapeutic targets.

JAK/STAT inhibition augments soleus muscle function in a rat model of critical illness myopathy via regulation of complement C3/3R

KEY POINTS

Critical illness myopathy (CIM) is a frequently observed negative consequence of modern critical care. Chronic Janus kinase (JAK)/signal transducer and activator of transcription activation impairs muscle size and function and is prominent following mechanical ventilation. We identify pSTAT-3 activation in tibialis anterior of CIM patients, before examining the potential benefits of JAK1/2 inhibition in an experimental model of CIM, where muscle mass and function are impaired. CIM activates complement cascade and increased monocyte infiltration in the soleus muscle, which was ameliorated by JAK1/2 inhibition, leading to reduced muscle degeneration and improved muscle force. Here, we demonstrate that JAK1/2 inhibition augments CIM muscle function through regulation of the complement cascade.

ABSTRACT

Critical illness myopathy (CIM) is frequently observed in response to modern critical care with negative consequences for patient quality of life, morbidity, mortality and healthcare costs. Janus kinase (JAK)/signal transducer and activator of transcription (STAT) activation is observed in limb muscles following controlled mechanical ventilation. Chronic JAK/STAT activation promotes loss of muscle mass and function. Thus, we hypothesized that JAK1/2 inhibition would improve muscle outcomes for CIM. Following 12 days of intensive care unit conditions, pSTAT-3 levels increased in tibialis anterior muscle of CIM patients (P = 0.0489). The potential of JAK1/2 inhibition was assessed in an experimental model of CIM, where soleus muscle size and force are impaired. JAK1/2 inhibition restores soleus force (P < 0.0001). CIM activated muscle complement cascade, which was ameliorated by JAK1/2 inhibition (P < 0.05, respectively). Soleus macrophage number corresponded with complement activity, leading to reduced muscle degeneration and augmented muscle function (P < 0.05). Thus, JAK/STAT inhibition improves soleus function by modulating the complement cascade and muscle monocyte infiltration. Collectively, we demonstrate that JAK/STAT inhibition augments muscle function in CIM.

Neurogenic vs. Myogenic Origin of Acquired Muscle Paralysis in Intensive Care Unit (ICU) Patients: Evaluation of Different Diagnostic Methods

Introduction. The acquired muscle paralysis associated with modern critical care can be of neurogenic or myogenic origin, yet the distinction between these origins is hampered by the precision of current diagnostic methods. This has resulted in the pooling of all acquired muscle paralyses, independent of their origin, into the term Intensive Care Unit Acquired Muscle Weakness (ICUAW). This is unfortunate since the acquired neuropathy (critical illness polyneuropathy, CIP) has a slower recovery than the myopathy (critical illness myopathy, CIM); therapies need to target underlying mechanisms and every patient deserves as accurate a diagnosis as possible. This study aims at evaluating different diagnostic methods in the diagnosis of CIP and CIM in critically ill, immobilized and mechanically ventilated intensive care unit (ICU) patients. Methods. ICU patients with acquired quadriplegia in response to critical care were included in the study. A total of 142 patients were examined with routine electrophysiological methods, together with biochemical analyses of myosin:actin (M:A) ratios of muscle biopsies. In addition, comparisons of evoked electromyographic (EMG) responses in direct vs. indirect muscle stimulation and histopathological analyses of muscle biopsies were performed in a subset of the patients. Results. ICU patients with quadriplegia were stratified into five groups based on the hallmark of CIM, i.e., preferential myosin loss (myosin:actin ratio, M:A) and classified as severe (M:A < 0.5; n = 12), moderate (0.5 ≤ M:A < 1; n = 40), mildly moderate (1 ≤ M:A < 1.5; n = 49), mild (1.5 ≤ M:A < 1.7; n = 24) and normal (1.7 ≤ M:A; n = 19). Identical M:A ratios were obtained in the small (4–15 mg) muscle samples, using a disposable semiautomatic microbiopsy needle instrument, and the larger (>80 mg) samples, obtained with a conchotome instrument. Compound muscle action potential (CMAP) duration was increased and amplitude decreased in patients with preferential myosin loss, but deviations from this relationship were observed in numerous patients, resulting in only weak correlations between CMAP properties and M:A. Advanced electrophysiological methods measuring refractoriness and comparing CMAP amplitude after indirect nerve vs. direct muscle stimulation are time consuming and did not increase precision compared with conventional electrophysiological measurements in the diagnosis of CIM. Low CMAP amplitude upon indirect vs. direct stimulation strongly suggest a neurogenic lesion, i.e., CIP, but this was rarely observed among the patients in this study. Histopathological diagnosis of CIM/CIP based on enzyme histochemical mATPase stainings were hampered by poor quantitative precision of myosin loss and the impact of pathological findings unrelated to acute quadriplegia. Conclusion. Conventional electrophysiological methods are valuable in identifying the peripheral origin of quadriplegia in ICU patients, but do not reliably separate between neurogenic vs. myogenic origins of paralysis. The hallmark of CIM, preferential myosin loss, can be reliably evaluated in the small samples obtained with the microbiopsy instrument. The major advantage of this method is that it is less invasive than conventional muscle biopsies, reducing the risk of bleeding in ICU patients, who are frequently receiving anticoagulant treatment, and it can be repeated multiple times during follow up for monitoring purposes.

Intensive Care Unit-Acquired Weakness: Not just Another Muscle Atrophying Condition

Intensive care unit-acquired weakness (ICUAW) occurs in critically ill patients stemming from the critical illness itself, and results in sustained disability long after the ICU stay. Weakness can be attributed to muscle wasting, impaired contractility, neuropathy, and major pathways associated with muscle protein degradation such as the ubiquitin proteasome system and dysregulated autophagy. Furthermore, it is characterized by the preferential loss of myosin, a distinct feature of the condition. While many risk factors for ICUAW have been identified, effective interventions to offset these changes remain elusive. In addition, our understanding of the mechanisms underlying the long-term, sustained weakness observed in a subset of patients after discharge is minimal. Herein, we discuss the various proposed pathways involved in the pathophysiology of ICUAW, with a focus on the mechanisms underpinning skeletal muscle wasting and impaired contractility, and the animal models used to study them. Furthermore, we will explore the contributions of inflammation, steroid use, and paralysis to the development of ICUAW and how it pertains to those with the corona virus disease of 2019 (COVID-19). We then elaborate on interventions tested as a means to offset these decrements in muscle function that occur as a result of critical illness, and we propose new strategies to explore the molecular mechanisms of ICUAW, including serum-related biomarkers and 3D human skeletal muscle culture models.

Maintenance of sarcomeric integrity in adult muscle cells crucially depends on Z-disc anchored titin

The giant protein titin is thought to be required for sarcomeric integrity in mature myocytes, but direct evidence for this hypothesis is limited. Here, we describe a mouse model in which Z-disc-anchored TTN is depleted in adult skeletal muscles. Inactivation of TTN causes sarcomere disassembly and Z-disc deformations, force impairment, myocyte de-stiffening, upregulation of TTN-binding mechanosensitive proteins and activation of protein quality-control pathways, concomitant with preferential loss of thick-filament proteins. Interestingly, expression of the myosin-bound Cronos-isoform of TTN, generated from an alternative promoter not affected by the targeting strategy, does not prevent deterioration of sarcomere formation and maintenance. Finally, we demonstrate that loss of Z-disc-anchored TTN recapitulates muscle remodeling in critical illness ‘myosinopathy’ patients, characterized by TTN-depletion and loss of thick filaments. We conclude that full-length TTN is required to integrate Z-disc and A-band proteins into the mature sarcomere, a function that is lost when TTN expression is pathologically lowered.

Titin is considered an integrator of muscle cell proteins but direct evidence is limited. Here, titin is inactivated in adult mouse muscles, which causes sarcomere disassembly, protein mis-expression and force impairment, recapitulating key alterations in critical illness myopathy patient muscles.

Chaperone co-inducer BGP-15 mitigates early contractile dysfunction of the soleus muscle in a rat ICU model

Aim

Critical illness myopathy (CIM) represents a common consequence of modern intensive care, negatively impacting patient health and significantly increasing health care costs; however, there is no treatment available apart from symptomatic and supportive interventions. The chaperone co‐inducer BGP‐15 has previously been shown to have a positive effect on the diaphragm in rats exposed to the intensive care unit (ICU) condition. In this study, we aim to explore the effects of BGP‐15 on a limb muscle (soleus muscle) in response to the ICU condition.

Methods

Sprague‐Dawley rats were subjected to the ICU condition for 5, 8 and 10 days and compared with untreated sham‐operated controls.

Results

BGP‐15 significantly improved soleus muscle fibre force after 5 days exposure to the ICU condition. This improvement was associated with the protection of myosin from post‐translational myosin modifications, improved mitochondrial structure/biogenesis and reduced the expression of MuRF1 and Fbxo31 E3 ligases. At longer durations (8 and 10 days), BGP‐15 had no protective effect when the hallmark of CIM had become manifest, that is, preferential loss of myosin. Unrelated to the effects on skeletal muscle, BGP‐15 had a strong positive effect on survival compared with untreated animals.

Conclusions

BGP‐15 treatment improved soleus muscle fibre and motor protein function after 5 days exposure to the ICU condition, but not at longer durations (8 and 10 days) when the preferential loss of myosin was manifest. Thus, long‐term CIM interventions targeting limb muscle fibre/myosin force generation capacity need to consider both the post‐translational modifications and the loss of myosin.

Muscle-specific differences in expression and phosphorylation of the Janus kinase 2/Signal Transducer and Activator of Transcription 3 following long-term mechanical ventilation and immobilization in rats

AIM

Muscle wasting is one of the factors most strongly predicting mortality and morbidity in critically ill intensive care unit (ICU). This muscle wasting affects both limb and respiratory muscles, but the understanding of underlying mechanisms and muscle-specific differences remains incomplete. This study aimed at investigating the temporal expression and phosphorylation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway in muscle wasting associated with the ICU condition to characterize the JAK/STAT proteins and the related changes leading or responding to their activation during exposure to the ICU condition.

METHODS

A novel experimental ICU model allowing long-term exposure to the ICU condition, immobilization and mechanical ventilation, was used in this study. Rats were pharmacologically paralysed by post-synaptic neuromuscular blockade and mechanically ventilated for durations varying between 6 hours and 14 days to study muscle-specific differences in the temporal activation of the JAK/STAT pathway in plantaris, intercostal and diaphragm muscles.

RESULTS

The JAK2/STAT3 pathway was significantly activated irrespective of muscle, but muscle-specific differences were observed in the temporal activation pattern between plantaris, intercostal and diaphragm muscles.

CONCLUSION

The JAK2/STAT3 pathway was differentially activated in plantaris, intercostal and diaphragm muscles in response to the ICU condition. Thus, JAK2/STAT3 inhibitors may provide an attractive pharmacological intervention strategy in immobilized ICU patients, but further experimental studies are required in the study of muscle-specific effects on muscle mass and function in response to both short- and long-term exposure to the ICU condition prior to the translation into clinical research and practice.

The chaperone co-inducer BGP-15 alleviates ventilation-induced diaphragm dysfunction

Ventilation-induced diaphragm dysfunction (VIDD) is a marked decline in diaphragm function in response to mechanical ventilation, which has negative consequences for individual patients' quality of life and for the health care system, but specific treatment strategies are still lacking. We used an experimental intensive care unit (ICU) model, allowing time-resolved studies of diaphragm structure and function in response to long-term mechanical ventilation and the effects of a pharmacological intervention (the chaperone co-inducer BGP-15). The marked loss of diaphragm muscle fiber function in response to mechanical ventilation was caused by posttranslational modifications (PTMs) of myosin. In a rat model, 10 days of BGP-15 treatment greatly improved diaphragm muscle fiber function (by about 100%), although it did not reverse diaphragm atrophy. The treatment also provided protection from myosin PTMs associated with HSP72 induction and PARP-1 inhibition, resulting in improvement of mitochondrial function and content. Thus, BGP-15 may offer an intervention strategy for reducing VIDD in mechanically ventilated ICU patients.

Critical Illness Myopathy (CIM) and Ventilator-Induced Diaphragm Muscle Dysfunction (VIDD): Acquired Myopathies Affecting Contractile Proteins

Critical care and intensive care units (ICUs) have undergone dramatic changes and improvements in recent years, and critical care is today one of the fastest growing hospital disciplines. Significant improvements in treatments, removal of inefficient and harmful interventions, and introduction of advanced technological support systems have improved survival among critically ill ICU patients. However, the improved survival is associated with an increased number of patients with complications related to modern critical care. Severe muscle wasting and impaired muscle function are frequently observed in immobilized and mechanically ventilated ICU patients. Approximately 30% of mechanically ventilated and immobilized ICU patients for durations of five days and longer develop generalized muscle paralysis of all limb and trunk muscles. These patients typically have intact sensory and cognitive functions, a condition known as critical illness myopathy (CIM). Mechanical ventilation is a lifesaving treatment in critically ill ICU patients; however, the being on a ventilator creates dependence, and the weaning process occupies as much as 40% of the total time of mechanical ventilation. Furthermore, 20% to 30% of patients require prolonged intensive care due to ventilator-induced diaphragm dysfunction (VIDD), resulting in poorer outcomes, and greatly increased costs to health care providers. Our understanding of the mechanisms underlying both CIM and VIDD has increased significantly in the past decade and intervention strategies are presently being evaluated in different experimental models. This short review is restricted CIM and VIDD pathophysiology rather than giving a comprehensive review of all acquired muscle wasting conditions associated with modern critical care. © 2017 American Physiological Society. Compr Physiol 7:105-112, 2017.

Impaired Ca(2+) release contributes to muscle weakness in a rat model of critical illness myopathy

BACKGROUND

Critical illness myopathy is an acquired skeletal muscle disorder with severe myosin loss and muscle weakness frequently seen in intensive care unit (ICU) patients. It is unknown if impaired excitation-contraction coupling contributes to the muscle weakness.

METHODS

We used a unique ICU model where rats were deeply sedated, post-synaptically pharmacologically paralyzed, mechanically ventilated and closely monitored for up to ten days. Single intact fibers from the flexor digitorum brevis muscle were isolated and used to measure force and free myoplasmic [Ca(2+)] ([Ca(2+)]i) during tetanic contractions.

RESULTS

Fibers from ICU rats had 80 % lower tetanic [Ca(2+)]i and produced only 15 % of the force seen in fibers from sham-operated (SHAM) rats. In the presence of 5 mM caffeine, tetanic [Ca(2+)]i was similar in fibers from ICU and SHAM rats but force was 50 % lower in fibers from ICU rats than SHAM rats. Confocal imaging showed disrupted tetanic [Ca(2+)]i transients in fibers from ICU rats compared to SHAM rats. Western blots showed similar levels of Na(+) channel and dihydropyridine receptor (DHPR) protein expression, whereas ryanodine receptor (RyR) and sarco-endoplasmic reticulum Ca(2+) ATPase 1 (SERCA1) expression was markedly lower in muscle of ICU rats than in SHAM rats. Immunohistochemical analysis showed that distribution of Na(+) channel and DHPR protein on the sarcolemma was disrupted in fibers from ICU rats compared with SHAM rats.

CONCLUSIONS

These results suggest that impaired SR Ca(2+) release contributes to the muscle weakness seen in patients in ICU.

Mechanical Signaling in the Pathophysiology of Critical Illness Myopathy

The complete loss of mechanical stimuli of skeletal muscles, i.e., the loss of external strain, related to weight bearing, and internal strain, related to the contraction of muscle cells, is uniquely observed in pharmacologically paralyzed or deeply sedated mechanically ventilated intensive care unit (ICU) patients. The preferential loss of myosin and myosin associated proteins in limb and trunk muscles is a significant characteristic of critical illness myopathy (CIM) which separates CIM from other types of acquired muscle weaknesses in ICU patients. Mechanical silencing is an important factor triggering CIM. Microgravity or ground based microgravity models form the basis of research on the effect of muscle unloading-reloading, but the mechanisms and effects may differ from the ICU conditions. In order to understand how mechanical tension regulates muscle mass, it is critical to know how muscles sense mechanical information and convert stimulus to intracellular biochemical actions and changes in gene expression, a process called cellular mechanotransduction. In adult skeletal muscles and muscle fibers, this process may differ, the same stimulus can cause divergent response and the same fiber type may undergo opposite changes in different muscles. Skeletal muscle contains multiple types of mechano-sensors and numerous structures that can be affected differently and hence respond differently in distinct muscles.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

Comparative Proteomics for the Evaluation of Protein Expression and Modifications in Neurodegenerative Diseases

Together with hypothesis-driven approaches, high-throughput differential proteomic analysis performed primarily not only in human cerebrospinal fluid and serum but also on protein content of other tissues (blood cells, muscles, peripheral nerves, etc.) has been used in the last years to investigate neurodegenerative diseases. Even if the goal for these analyses was mainly the discovery of neurodegenerative disorders biomarkers, the characterization of specific posttranslational modifications (PTMs) and the differential protein expression resulted in being very informative to better define the pathological mechanisms. In this chapter are presented and discussed the positive aspects and challenges of the outcomes of some of our investigations on neurological and neurodegenerative disease, in order to highlight the important role of protein PTMs studies in proteomics-based approaches.

Sexually dimorphic myofilament function in a mouse model of nemaline myopathy

Nemaline myopathy, the most common congenital myopathy, is characterized by mutations in genes encoding myofilament proteins such as skeletal α-actin. These mutations are thought to ultimately lead to skeletal muscle weakness. Interestingly, some of the mutations appear to be more potent in males than in females. The underlying mechanisms remain obscure but may be related to sex-specific differences in the myofilament function of both limb and respiratory muscles. To verify this, in the present study, we used skeletal muscles (tibialis anterior and diaphragm) from a transgenic mouse model harbouring the His40Tyr amino acid substitution in skeletal α-actin. In this animal model, 60% of males die by 13weeks of age (the underlying causes of death are obscure but probably due to respiratory insufficiency) whereas females have a normal lifespan. By recording and analysing the mechanics of membrane-permeabilized myofibres, we only observed sex-related differences in the tibialis anterior muscles. Indeed, the concomitant deficits in maximal steady-state isometric force and stiffness of myofibres were less exacerbated in transgenic females than in males, potentially explaining the lower potency in limb muscles. However, the absence of sex-difference in the diaphragm muscles was rather unexpected and suggests that myofilament dysfunction does not solely underlie the sexually dimorphic phenotypes.

Time course analysis of mechanical ventilation-induced diaphragm contractile muscle dysfunction in the rat

Controlled mechanical ventilation (CMV) plays a key role in triggering the impaired diaphragm muscle function and the concomitant delayed weaning from the respirator in critically ill intensive care unit (ICU) patients. To date, experimental and clinical studies have primarily focused on early effects on the diaphragm by CMV, or at specific time points. To improve our understanding of the mechanisms underlying the impaired diaphragm muscle function in response to mechanical ventilation, we have performed time-resolved analyses between 6 h and 14 days using an experimental rat ICU model allowing detailed studies of the diaphragm in response to long-term CMV. A rapid and early decline in maximum muscle fibre force and preceding muscle fibre atrophy was observed in the diaphragm in response to CMV, resulting in an 85% reduction in residual diaphragm fibre function after 9-14 days of CMV. A modest loss of contractile proteins was observed and linked to an early activation of the ubiquitin proteasome pathway, myosin:actin ratios were not affected and the transcriptional regulation of myosin isoforms did not show any dramatic changes during the observation period. Furthermore, small angle X-ray diffraction analyses demonstrate that myosin can bind to actin in an ATP-dependent manner even after 9-14 days of exposure to CMV. Thus, quantitative changes in muscle fibre size and contractile proteins are not the dominating factors underlying the dramatic decline in diaphragm muscle function in response to CMV, in contrast to earlier observations in limb muscles. The observed early loss of subsarcolemmal neuronal nitric oxide synthase activity, onset of oxidative stress, intracellular lipid accumulation and post-translational protein modifications strongly argue for significant qualitative changes in contractile proteins causing the severely impaired residual function in diaphragm fibres after long-term mechanical ventilation. For the first time, the present study demonstrates novel changes in the diaphragm structure/function and underlying mechanisms at the gene, protein and cellular levels in response to CMV at a high temporal resolution ranging from 6 h to 14 days.

The Effect of Nutritional Status in the Pathogenesis of Critical Illness Myopathy (CIM)

The muscle wasting and loss of specific force associated with Critical Illness Myopathy (CIM) is, at least in part, due to a preferential loss of the molecular motor protein myosin. This acquired myopathy is common in critically ill immobilized and mechanically ventilated intensive care patients (ICU). There is a growing understanding of the mechanisms underlying CIM, but the role of nutritional factors triggering this serious complication of modern intensive care remains unknown. This study aims at establishing the effect of nutritional status in the pathogenesis of CIM. An experimental ICU model was used where animals are mechanically ventilated, pharmacologically paralysed post-synaptically and extensively monitored for up to 14 days. Due to the complexity of the experimental model, the number of animals included is small. After exposure to this ICU condition, animals develop a phenotype similar to patients with CIM. The results from this study show that the preferential myosin loss, decline in specific force and muscle fiber atrophy did not differ between low vs. eucaloric animals. In both experimental groups, passive mechanical loading had a sparing effect of muscle weight independent on nutritional status. Thus, this study confirms the strong impact of the mechanical silencing associated with the ICU condition in triggering CIM, overriding any potential effects of caloric intake in triggering CIM. In addition, the positive effects of passive mechanical loading on muscle fiber size and force generating capacity was not affected by the nutritional status in this study. However, due to the small sample size these pilot results need to be validated in a larger cohort.

Masseter muscle myofibrillar protein synthesis and degradation in an experimental critical illness myopathy model

Critical illness myopathy (CIM) is a debilitating common consequence of modern intensive care, characterized by severe muscle wasting, weakness and a decreased myosin/actin (M/A) ratio. Limb/trunk muscles are primarily affected by this myopathy while cranial nerve innervated muscles are spared or less affected, but the mechanisms underlying these muscle-specific differences remain unknown. In this time-resolved study, the cranial nerve innervated masseter muscle was studied in a unique experimental rat intensive care unit (ICU) model, where animals were exposed to sedation, neuromuscular blockade (NMB), mechanical ventilation, and immobilization for durations varying between 6 h and 14d. Gel electrophoresis, immunoblotting, RT-PCR and morphological staining techniques were used to analyze M/A ratios, myofiber size, synthesis and degradation of myofibrillar proteins, and levels of heat shock proteins (HSPs). Results obtained in the masseter muscle were compared with previous observations in experimental and clinical studies of limb muscles. Significant muscle-specific differences were observed, i.e., in the masseter, the decline in M/A ratio and muscle fiber size was small and delayed. Furthermore, transcriptional regulation of myosin and actin synthesis was maintained, and Akt phosphorylation was only briefly reduced. In studied degradation pathways, only mRNA, but not protein levels of MuRF1, atrogin-1 and the autophagy marker LC3b were activated by the ICU condition. The matrix metalloproteinase MMP-2 was inhibited and protective HSPs were up-regulated early. These results confirm that the cranial nerve innervated masticatory muscles is less affected by the ICU-stress response than limb muscles, in accordance with clinical observation in ICU patients with CIM, supporting the model' credibility as a valid CIM model.

Impaired autophagy, chaperone expression, and protein synthesis in response to critical illness interventions in porcine skeletal muscle

Critical illness myopathy (CIM) is characterized by a preferential loss of the motor protein myosin, muscle wasting, and impaired muscle function in critically ill intensive care unit (ICU) patients. CIM is associated with severe morbidity and mortality and has a significant negative socioeconomic effect. Neuromuscular blocking agents, corticosteroids, sepsis, mechanical ventilation, and immobilization have been implicated as important risk factors, but the causal relationship between CIM and the risk factors has not been established. A porcine ICU model has been used to determine the immediate molecular and cellular cascades that may contribute to the pathogenesis prior to myosin loss and extensive muscle wasting. Expression profiles have been compared between pigs exposed to the ICU interventions, i.e., mechanically ventilated, sedated, and immobilized for 5 days, with pigs exposed to critical illness interventions, i.e., neuromuscular blocking agents, corticosteroids, and induced sepsis in addition to the ICU interventions for 5 days. Impaired autophagy as well as impaired chaperone expression and protein synthesis were observed in the skeletal muscle in response to critical illness interventions. A novel finding in this study is impaired core autophagy machinery in response to critical illness interventions, which when in concert with downregulated chaperone expression and protein synthesis may collectively affect the proteostasis in skeletal muscle and may exacerbate the disease progression in CIM.

Sparing of muscle mass and function by passive loading in an experimental intensive care unit model

The response to mechanical stimuli, i.e., tensegrity, plays an important role in regulating cell physiological and pathophysiological function, and the mechanical silencing observed in intensive care unit (ICU) patients leads to a severe and specific muscle wasting condition. This study aims to unravel the underlying mechanisms and the effects of passive mechanical loading on skeletal muscle mass and function at the gene, protein and cellular levels. A unique experimental rat ICU model has been used allowing long-term (weeks) time-resolved analyses of the effects of standardized unilateral passive mechanical loading on skeletal muscle size and function and underlying mechanisms. Results show that passive mechanical loading alleviated the muscle wasting and the loss of force-generation associated with the ICU intervention, resulting in a doubling of the functional capacity of the loaded versus the unloaded muscles after a 2-week ICU intervention. We demonstrate that the improved maintenance of muscle mass and function is probably a consequence of a reduced oxidative stress revealed by lower levels of carbonylated proteins, and a reduced loss of the molecular motor protein myosin. A complex temporal gene expression pattern, delineated by microarray analysis, was observed with loading-induced changes in transcript levels of sarcomeric proteins, muscle developmental processes, stress response, extracellular matrix/cell adhesion proteins and metabolism. Thus, the results from this study show that passive mechanical loading alleviates the severe negative consequences on muscle size and function associated with the mechanical silencing in ICU patients, strongly supporting early and intense physical therapy in immobilized ICU patients.

Mechanisms underlying ICU muscle wasting and effects of passive mechanical loading

Introduction

Critically ill ICU patients commonly develop severe muscle wasting and impaired muscle function, leading to delayed recovery, with subsequent increased morbidity and financial costs, and decreased quality of life for survivors. Critical illness myopathy (CIM) is a frequently observed neuromuscular disorder in ICU patients. Sepsis, systemic corticosteroid hormone treatment and post-synaptic neuromuscular blockade have been forwarded as the dominating triggering factors. Recent experimental results from our group using a unique experimental rat ICU model show that the mechanical silencing associated with CIM is the primary triggering factor. This study aims to unravel the mechanisms underlying CIM, and to evaluate the effects of a specific intervention aiming at reducing mechanical silencing in sedated and mechanically ventilated ICU patients.

Methods

Muscle gene/protein expression, post-translational modifications (PTMs), muscle membrane excitability, muscle mass measurements, and contractile properties at the single muscle fiber level were explored in seven deeply sedated and mechanically ventilated ICU patients (not exposed to systemic corticosteroid hormone treatment, post-synaptic neuromuscular blockade or sepsis) subjected to unilateral passive mechanical loading for 10 hours per day (2.5 hours, four times) for 9 ± 1 days.

Results

These patients developed a phenotype considered pathognomonic of CIM; that is, severe muscle wasting and a preferential myosin loss (P < 0.001). In addition, myosin PTMs specific to the ICU condition were observed in parallel with an increased sarcolemmal expression and cytoplasmic translocation of neuronal nitric oxide synthase. Passive mechanical loading for 9 ± 1 days resulted in a 35% higher specific force (P < 0.001) compared with the unloaded leg, although it was not sufficient to prevent the loss of muscle mass.

Conclusion

Mechanical silencing is suggested to be a primary mechanism underlying CIM; that is, triggering the myosin loss, muscle wasting and myosin PTMs. The higher neuronal nitric oxide synthase expression found in the ICU patients and its cytoplasmic translocation are forwarded as a probable mechanism underlying these modifications. The positive effect of passive loading on muscle fiber function strongly supports the importance of early physical therapy and mobilization in deeply sedated and mechanically ventilated ICU patients.

Neuromuscular blockade and skeletal muscle weakness in critically ill patients: time to rethink the evidence?

Neuromuscular blocking agents are commonly used in critical care. However, concern after observational reports of a causal relationship with skeletal muscle dysfunction and intensive care-acquired weakness (ICU-AW) has resulted in a cautionary and conservative approach to their use. This integrative review, interpreted in the context of our current understanding of the pathophysiology of ICU-AW and integrated into our current conceptual framework of clinical practice, challenges the established clinical view of an adverse relationship between the use of neuromuscular blocking agents and skeletal muscle weakness. In addition to discussing data, this review identifies potential confounders and alternative etiological factors responsible for ICU-AW and provides evidence that neuromuscular blocking agents may not be a major cause of weakness in a 21st century critical care setting.

Muscle wasting in animal models of severe illness

Muscle wasting is a serious complication of various clinical conditions that significantly worsens the prognosis of the illnesses. Clinically relevant models of muscle wasting are essential for understanding its pathogenesis and for selective preclinical testing of potential therapeutic agents. The data presented here indicate that muscle wasting has been well characterized in rat models of sepsis (endotoxaemia, and caecal ligation and puncture), in rat models of chronic renal failure (partial nephrectomy), in animal models of intensive care unit patients (corticosteroid treatment combined with peripheral denervation or with administration of neuromuscular blocking drugs) and in murine and rat models of cancer (tumour cell transplantation). There is a need to explore genetically engineered mouse models of cancer. The degree of protein degradation in skeletal muscle is not well characterized in animal models of liver cirrhosis, chronic heart failure and chronic obstructive pulmonary disease. The major difficulties with all models are standardization and high variation in disease progression and a lack of reflection of clinical reality in some of the models. The translation of the information obtained by using these models to clinical practice may be problematic.

Muscle wasting and the temporal gene expression pattern in a novel rat intensive care unit model

BACKGROUND

Acute quadriplegic myopathy (AQM) or critical illness myopathy (CIM) is frequently observed in intensive care unit (ICU) patients. To elucidate duration-dependent effects of the ICU intervention on molecular and functional networks that control the muscle wasting and weakness associated with AQM, a gene expression profile was analyzed at time points varying from 6 hours to 14 days in a unique experimental rat model mimicking ICU conditions, i.e., post-synaptically paralyzed, mechanically ventilated and extensively monitored animals.

RESULTS

During the observation period, 1583 genes were significantly up- or down-regulated by factors of two or greater. A significant temporal gene expression pattern was constructed at short (6 h-4 days), intermediate (5-8 days) and long (9-14 days) durations. A striking early and maintained up-regulation (6 h-14d) of muscle atrogenes (muscle ring-finger 1/tripartite motif-containing 63 and F-box protein 32/atrogin-1) was observed, followed by an up-regulation of the proteolytic systems at intermediate and long durations (5-14d). Oxidative stress response genes and genes that take part in amino acid catabolism, cell cycle arrest, apoptosis, muscle development, and protein synthesis together with myogenic factors were significantly up-regulated from 5 to 14 days. At 9-14 d, genes involved in immune response and the caspase cascade were up-regulated. At 5-14d, genes related to contractile (myosin heavy chain and myosin binding protein C), regulatory (troponin, tropomyosin), developmental, caveolin-3, extracellular matrix, glycolysis/gluconeogenesis, cytoskeleton/sarcomere regulation and mitochondrial proteins were down-regulated. An activation of genes related to muscle growth and new muscle fiber formation (increase of myogenic factors and JunB and down-regulation of myostatin) and up-regulation of genes that code protein synthesis and translation factors were found from 5 to 14 days.

CONCLUSIONS

Novel temporal patterns of gene expression have been uncovered, suggesting a unique, coordinated and highly complex mechanism underlying the muscle wasting associated with AQM in ICU patients and providing new target genes and avenues for intervention studies.

Diaphragm muscle weakness in an experimental porcine intensive care unit model

In critically ill patients, mechanisms underlying diaphragm muscle remodeling and resultant dysfunction contributing to weaning failure remain unclear. Ventilator-induced modifications as well as sepsis and administration of pharmacological agents such as corticosteroids and neuromuscular blocking agents may be involved. Thus, the objective of the present study was to examine how sepsis, systemic corticosteroid treatment (CS) and neuromuscular blocking agent administration (NMBA) aggravate ventilator-related diaphragm cell and molecular dysfunction in the intensive care unit. Piglets were exposed to different combinations of mechanical ventilation and sedation, endotoxin-induced sepsis, CS and NMBA for five days and compared with sham-operated control animals. On day 5, diaphragm muscle fibre structure (myosin heavy chain isoform proportion, cross-sectional area and contractile protein content) did not differ from controls in any of the mechanically ventilated animals. However, a decrease in single fibre maximal force normalized to cross-sectional area (specific force) was observed in all experimental piglets. Therefore, exposure to mechanical ventilation and sedation for five days has a key negative impact on diaphragm contractile function despite a preservation of muscle structure. Post-translational modifications of contractile proteins are forwarded as one probable underlying mechanism. Unexpectedly, sepsis, CS or NMBA have no significant additive effects, suggesting that mechanical ventilation and sedation are the triggering factors leading to diaphragm weakness in the intensive care unit.

Upregulation of the CaV 1.1-ryanodine receptor complex in a rat model of critical illness myopathy

The processes that trigger severe muscle atrophy and loss of myosin in critical illness myopathy (CIM) are poorly understood. It has been reported that muscle disuse alters Ca(2+) handling by the sarcoplasmic reticulum. Since inactivity is an important contributor to CIM, this finding raises the possibility that elevated levels of the proteins involved in Ca(2+) handling might contribute to development of CIM. CIM was induced in 3- to 5-mo-old rats by sciatic nerve lesion and infusion of dexamethasone for 1 wk. Western blot analysis revealed increased levels of ryanodine receptor (RYR) isoforms-1 and -2 as well as the dihydropyridine receptor/voltage-gated calcium channel type 1.1 (DHPR/Ca(V) 1.1). Immunostaining revealed a subset of fibers with elevation of RYR1 and Ca(V) 1.1 that had severe atrophy and disorganization of sarcomeres. These findings suggest increased Ca(2+) release from the sarcoplasmic reticulum may be an important contributor to development of CIM. To assess the endogenous functional effects of increased intracellular Ca(2+) in CIM, proteolysis of α-fodrin, a well-known target substrate of Ca(2+)-activated proteases, was measured and found to be 50% greater in CIM. There was also selective degradation of myosin heavy chain relative to actin in CIM muscle. Taken together, our findings suggest that increased Ca(2+) release from the sarcoplasmic reticulum may contribute to pathology in CIM.

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

The 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.

Increasing intravenous glucose load in the presence of normoglycemia: effect on outcome and metabolism in critically ill rabbits

OBJECTIVES

Endocrine disturbances and a feeding-resistant wasting syndrome, characterized by a negative protein balance, promote delayed recovery and poor outcome of critical illness. Parenteral nutrition alone cannot counteract the hypercatabolic state, possibly in part as a result of aggravation of the hyperglycemic response to illness. In critically ill rabbits, we investigated the impact of varying amounts of intravenous glucose while maintaining normoglycemia on mortality, organ damage, and markers of catabolism/anabolism.

DESIGN

Prospective, randomized laboratory investigation.

SETTING

University animal and molecular laboratory.

SUBJECTS

Three-month-old male rabbits.

INTERVENTIONS

Critically ill rabbits were randomized into a fasting group, a standard parenteral nutrition group, and two groups receiving either intermediate or high additional physiological amounts of intravenous glucose while maintained normoglycemic with insulin. These groups were compared with a hyperglycemic group and healthy rabbits. Protein and lipid load was equal for all fed groups.

MEASUREMENTS AND MAIN RESULTS

Varying intravenous glucose load did not affect mortality or organ damage provided hyperglycemia was prevented. Fasted critically ill rabbits lost weight, which was attenuated by increasing intravenous glucose load. As compared with healthy rabbits, mRNA expression and/or activity of several ubiquitin-proteasome pathway components, cathepsin-L and calpain-1, was elevated in skeletal muscle of fasted critically ill rabbits. Intravenous feeding was able to counteract this response. Excessive glucose load and/or hyperglycemia, however, reduced the protective effect of feeding. Genes investigated in the diaphragm and myocardium revealed roughly a similar response. Except in the normoglycemic group with intermediate glucose load, circulating thyroid hormone and insulin-like growth factor-1 levels decreased, most pronounced in hyperglycemic rabbits.

CONCLUSIONS

Increasing intravenous glucose infusion within the physiological range, while maintaining normoglycemia, was safe for organ function and survival of critically ill rabbits. Concomitantly, it reduced the catabolic responses as compared with fasting. Whether this has a beneficial effect on muscle function and mass remains to be investigated.

Myofibrillar protein and gene expression in acute quadriplegic myopathy

The dramatic muscle wasting, preferential loss of myosin and impaired muscle function in intensive care unit (ICU) patients with acute quadriplegic myopathy (AQM) have traditionally been suggested to be the result of proteolysis via specific proteolytic pathways. In this study we aim to investigate the mechanisms underlying the preferential loss of thick vs. thin filament proteins and the reassembly of the sarcomere during the recovery process in muscle samples from ICU patients with AQM. Quantitative and qualitative analyses of myofibrillar protein and mRNA expression were analyzed using SDS-PAGE, confocal microscopy, histochemistry and real-time PCR. The present results demonstrate that the transcriptional regulation of myofibrillar protein synthesis plays an important role in the loss of contractile proteins, as well as the recovery of protein levels during clinical improvement, myosin in particular, presumably in concert with proteolytic pathways, but the mechanisms are specific to the different thick and thin filament proteins studied.

Acute quadriplegic myopathy: an acquired "myosinopathy"

Acquired neuromuscular disorders have been shown to be very common in critically ill patients receiving prolonged mechanical ventilation in the intensive care unit (ICU). Acute Quadriplegic Myopathy (AQM) is a specific acquired myopathy in ICU patients. Patients with AQM are characterized by severe muscle weakness and atrophy of spinal nerve innervated limb and trunk muscles, while cranial nerve innervated craniofacial muscles, sensory and cognitive functions are spared or less affected. The muscle weakness is associated with altered muscle membrane properties and a preferential loss of the motor protein myosin and myosin-associated thick filament proteins. Prolonged mechanical ventilation, muscle unloading, postsynaptic block of neuromuscular transmission, sepsis and systemic corticosteroid hormone treatment have been suggested as important triggering factors in AQM. However, the exact mechanisms underlying the loss of thick filament proteins are not known, though enhanced myofibrillar protein degradation in combination with a downregulation of protein synthesis at the transcriptional level play important roles.

Transcription factors in muscle atrophy caused by blocked neuromuscular transmission and muscle unloading in rats

The muscle wasting associated with long-term intensive care unit (ICU) treatment has a negative effect on muscle function resulting in prolonged periods of rehabilitation and a decreased quality of life. To identify mechanisms behind this form of muscle wasting, we have used a rat model designed to mimic the conditions in an ICU. Rats were pharmacologically paralyzed with a postsynaptic blocker of neuromuscular transmission, and mechanically ventilated for one to two weeks, thereby unloading the limb muscles. Transcription factors were analyzed for cellular localization and nuclear concentration in the fast-twitch muscle extensor digitorum longus (EDL) and in the slow-twitch soleus. Significant muscle wasting and upregulation of mRNA for the ubiquitin ligases MAFbx and MuRF1 followed the treatment. The IkappaB family-member Bcl-3 displayed a concomitant decrease in concentration, suggesting altered kappaB controlled gene expression, although NFkappaB p65 was not significantly affected. The nuclear levels of the glucocorticoid receptor (GR) and the thyroid receptor alpha1 (TRalpha1) were altered and also suggested as potential mediators of the MAFbx- and MuRF1-induction in the absence of induced Foxo1. We believe that this model, and the strategy of quantifying nuclear proteins, will provide a valuable tool for further, more detailed, analyses of the muscle wasting occurring in patients kept on a mechanical ventilator.

Experimental animal models of muscle wasting in intensive care unit patients

The muscle wasting and loss of muscle function associated with critical illness and intensive care have significant negative consequences for weaning from the respirator, duration of hospital stay, and quality of life for long periods after hospital discharge. There is, accordingly, a significant demand for focused research aiming at improving our understanding of the mechanisms underlying the impaired neuromuscular function in intensive care unit (ICU) patients. However, the study of generalized muscle weakness in critically ill ICU patients is further complicated by the coexistence of multiple independent factors, such as different primary diseases, large variability in pharmacologic treatment, collection of muscle samples several weeks after admission to the ICU, and exposure to causative agents. This has led to the design of specific animal models mimicking ICU conditions. These models have often been used to study the mechanisms underlying the paralysis and muscle wasting associated with acute quadriplegic myopathy in ICU patients. This short review aims at presenting existing and recently introduced experimental animal models mimicking the conditions in the ICU (i.e., models designed to determine the mechanisms underlying the muscle wasting associated with ICU treatment).

Differential regulation of myofilament protein isoforms underlying the contractility changes in skeletal muscle unloading

Weight-bearing skeletal muscles change phenotype in response to unloading. Using the hindlimb suspension rat model, we investigated the regulation of myofilament protein isoforms in correlation to contractility. Four weeks of continuous hindlimb unloading produced progressive atrophy and contractility changes in soleus but not extensor digitorum longus muscle. The unloaded soleus muscle also had decreased fatigue resistance. Along with the decrease of myosin heavy chain isoform I and IIa and increase of IIb and IIx, coordinated regulation of thin filament regulatory protein isoforms were observed: gamma- and beta-tropomyosin decreased and alpha-tropomyosin increased, resulting in an alpha/beta ratio similar to that in normal fast twitch skeletal muscle; troponin I and troponin T (TnT) both showed decrease in the slow isoform and increases in the fast isoform. The TnT isoform switching began after 7 days of unloading and TnI isoform showed detectable changes at 14 days while other protein isoform changes were not significant until 28 days of treatment. Correlating to the early changes in contractility, especially the resistance to fatigue, the early response of TnT isoform regulation may play a unique role in the adaptation of skeletal muscle to unloading. When the fast TnT gene expression was upregulated in the unloaded soleus muscle, alternative RNA splicing switched to produce more high molecular weight acidic isoforms, reflecting a potential compensation for the decrease of slow TnT that is critical to skeletal muscle function. The results demonstrate that differential regulation of TnT isoforms is a sensitive mechanism in muscle adaptation to functional demands.