The Role of Calpains in Skeletal Muscle Remodeling with Exercise and Inactivity-induced Atrophy

Physiological and morphological impact of physical activity and nutritional interventions to offset disuse-induced skeletal muscle atrophy

Skeletal muscle tissue acts as a functional unit for physical movements, energy metabolism, thermogenesis, and metabolic homeostasis. In this literature review, the underlying mechanisms of skeletal muscle atrophy and the prevention strategies, including vigorous training and nutritional modifications are focused. Furthermore, the comparative analysis of multiple interventions is briefly explained. Ageing is an inevitable process often associated with cognitive impairment and physical decline due to muscular atrophy. Skeletal muscle atrophy is characterized by low muscle mass due to multiple underlying factors, i.e., genetic predisposition, ageing, inflammation, and trauma. The structural alterations include myofiber shrinkage, changes in myosin isoforms, decrease in myofiber diameter, and total protein loss. Furthermore, there is an imbalance in protein anabolic and catabolic reactions. This may be due to changes in multiple signal transduction pathways of protein degradation (i.e., caspase, calpain, ubiquitin protein degradation system, autophagy) and protein anabolism via the mTOR pathway. Consequently, certain pathophysiological factors associated with health disparities may reduce mobility and functional capacity to perform ADLs. To tackle this issue, novel strategies linked to physical movement, and dietary intake must be incorporated in life. Exercise poses multiple health benefits, including improved muscle mass and mobility. Diet diversification [particularly protein-rich meals] and the "whole food" approach (based on non-protein nutrients) may enhance intramuscular anabolic signaling and tissue remodeling. However, there is a pressing need to fund large-scale evidence-based trials based on modern machine learning techniques (AI-driven nutrition). Additionally, entrepreneurial platforms for commercialization of consumer-friendly food products must be initiated in future.

Postmortem proteolysis and its indicators vary within bovine muscles: Novel insights in muscles that differ in their contractile, metabolic, and connective tissue properties

This study assessed postmortem proteolysis over 14 d in bovine Masseter (MS), Longissimus thoracis (LT), and Cutaneous trunci (CT) muscles. First, the metabolic, contractile, and connective tissue properties were characterized to establish their intrinsic differences. The MS contained the highest levels of oxidative markers and myosin heavy chain-I (MyHC-I), whereas the CT possessed the greatest glycolytic capacity, MyHC-IIx, and connective tissue proteins (P < 0.05). The LT had intermediate metabolic characteristics, a heterogeneous mixture of MyHC isoforms, and the lowest amount of connective tissue proteins (P < 0.05), confirming the muscles' intrinsic divergence. Proteolytic analysis revealed increased desmin and slow troponin-T (TT-slow) degradation, with a higher 110 kDa band intensity in the MS than in the CT (P < 0.05). In comparison, the CT exhibited greater TT-fast degradation and higher 30 kDa fragment intensity (P < 0.05). The LT demonstrated the greatest overall proteolysis, indicated by increased TT-fast and TT-slow degradation and the highest intensity of the 30 kDa band (P < 0.05). This is likely due to protease activity, as the LT and MS exhibited more calpain-1 autolysis and less calpastatin abundance than the CT (P < 0.05). However, caspase-3 activity was highest in the MS and lowest in the LT. A principal component analysis incorporating proteolytic indicators further demonstrated the distinct proteolytic profiles in the three muscles. Overall, findings suggest that the progression of postmortem proteolysis is muscle-specific and that a single proteolytic indicator does not sufficiently describe proteolysis when comparing muscles differing in contractile and metabolic properties.

Sarcopenia and Cardiogeriatrics: The Links Between Skeletal Muscle Decline and Cardiovascular Aging

Sarcopenia, an age-related decline in skeletal muscle mass, strength, and function, is increasingly recognized as a significant condition in the aging population, particularly among those with cardiovascular diseases (CVD). This review provides a comprehensive synthesis of the interplay between sarcopenia and cardiogeriatrics, emphasizing shared mechanisms such as chronic low-grade inflammation (inflammaging), hormonal dysregulation, oxidative stress, and physical inactivity. Despite advancements in diagnostic frameworks, such as the EWGSOP2 and AWGS definitions, variability in criteria and assessment methods continues to challenge standardization. Key diagnostic tools include dual-energy X-ray absorptiometry (DXA) and bioimpedance analysis (BIA) for muscle mass, alongside functional measures such as grip strength and gait speed. The review highlights the bidirectional relationship between sarcopenia and cardiovascular conditions such as heart failure, aortic stenosis, and atherosclerotic cardiovascular disease, which exacerbate each other through complex pathophysiological mechanisms. Emerging therapeutic strategies targeting the mTOR pathway, NAD+ metabolism, and senescence-related processes offer promise in mitigating sarcopenia's progression. Additionally, integrated interventions combining resistance training, nutritional optimization, and novel anti-aging therapies hold significant potential for improving outcomes. This paper underscores critical gaps in the evidence, including the need for longitudinal studies to establish causality and the validation of advanced therapeutic approaches in clinical settings. Future research should leverage multi-omics technologies and machine learning to identify biomarkers and personalize interventions. Addressing these challenges is essential to reducing sarcopenia's burden and enhancing the quality of life for elderly individuals with comorbid cardiovascular conditions. This synthesis aims to guide future research and promote effective, individualized management strategies.

Effect of electrical muscle stimulation on the improvement of deltoid muscle atrophy in a rat shoulder immobilization model

Immobilization following trauma or surgery induces skeletal muscle atrophy, and improvement in the muscle atrophy is critical for successful clinical outcomes. The purpose of this study is to evaluate the effect of electrical muscle stimulation (EMS) on muscle atrophy. The study design is a controlled laboratory study. Eighty rats (56 to establish the deltoid muscle atrophy [DMA] model and 24 to evaluate the effect of EMS on the model) were used. DMA was induced by completely immobilizing the right shoulder of each rat by placing sutures between the scapula and humeral shaft, with the left shoulder as a control. After establishing the DMA model, rats were randomly assigned into three groups: low-frequency EMS (L-EMS, 10 Hz frequency), medium-frequency EMS (M-EMS, 50 Hz frequency), and control (eight rats per group). After 3 weeks, the deltoid muscles of each rat were harvested, alterations in gene expression and muscle cell size were evaluated, and immunohistochemical analysis was performed. DMA was most prominent 3 weeks after shoulder immobilization. Murf1 and Atrogin were significantly induced at the initial phase and gradually decreased at approximately 3 weeks; however, MyoD expressed an inverse relationship with Murf1 and Atrogin. IL6 expression was prominent at 1 week. The time point for the EMS effect evaluation was selected at 3 weeks, when the DMA was the most prominent with a change in relevant gene expression. The M-EMS group cell size was significantly larger than that of L-EMS and control group in both the immobilized and intact shoulders (all p < 0.05), without significant differences between the L-EMS and control groups. The M-EMS group showed significantly lower mRNA expressions of Murf1 and Atrogin and higher expressions of MyoD and Col1A1 than that of the control group (all p < 0.05). In immunohistochemical analysis, similar results were observed with lower Atrogin staining and higher MyoD and Col1A1 staining in the M-EMS group. DMA model was established by complete shoulder immobilization, with the most prominent muscle atrophy observed at 3 weeks. M-EMS improved DMA with changes in the expression of relevant genes. M-EMS might be a solution for strengthening atrophied skeletal muscles and facilitating rehabilitation after trauma or surgery.

Ventilator-induced diaphragm dysfunction: phenomenology and mechanism(s) of pathogenesis

Mechanical ventilation (MV) is used to support ventilation and pulmonary gas exchange in patients during critical illness and surgery. Although MV is a life-saving intervention for patients in respiratory failure, an unintended side-effect of MV is the rapid development of diaphragmatic atrophy and contractile dysfunction. This MV-induced diaphragmatic weakness is labelled as 'ventilator-induced diaphragm dysfunction' (VIDD). VIDD is an important clinical problem because diaphragmatic weakness is a risk factor for the failure to wean patients from MV. Indeed, the inability to remove patients from ventilator support results in prolonged hospitalization and increased morbidity and mortality. The pathogenesis of VIDD has been extensively investigated, revealing that increased mitochondrial production of reactive oxygen species within diaphragm muscle fibres promotes a cascade of redox-regulated signalling events leading to both accelerated proteolysis and depressed protein synthesis. Together, these events promote the rapid development of diaphragmatic atrophy and contractile dysfunction. This review highlights the MV-induced changes in the structure/function of diaphragm muscle and discusses the cell-signalling mechanisms responsible for the pathogenesis of VIDD. This report concludes with a discussion of potential therapeutic opportunities to prevent VIDD and suggestions for future research in this exciting field.

Resistance training-induced changes in muscle proteolysis and extracellular matrix remodeling biomarkers in the untrained and trained states

PURPOSE

Resistance training (RT) induces muscle growth at varying rates across RT phases, and evidence suggests that the muscle-molecular responses to training bouts become refined or attenuated in the trained state. This study examined how proteolysis-related biomarkers and extracellular matrix (ECM) remodeling factors respond to a bout of RT in the untrained (UT) and trained (T) state.

METHODS

Participants (19 women and 19 men) underwent 10 weeks of RT. Biopsies of vastus lateralis were collected before and after (24 h) the first (UT) and last (T) sessions. Vastus lateralis cross-sectional area (CSA) was assessed before and after the experimental period.

RESULTS

There were increases in muscle and type II fiber CSAs. In both the UT and T states, calpain activity was upregulated and calpain-1/-2 protein expression was downregulated from Pre to 24 h. Calpain-2 was higher in the T state. Proteasome activity and 20S proteasome protein expression were upregulated from Pre to 24 h in both the UT and T. However, proteasome activity levels were lower in the T state. The expression of poly-ubiquitinated proteins was unchanged. MMP activity was downregulated, and MMP-9 protein expression was elevated from Pre to 24 h in UT and T. Although MMP-14 protein expression was acutely unchanged, this marker was lower in T state. TIMP-1 protein levels were reduced Pre to 24 h in UT and T, while TIMP-2 protein levels were unchanged.

CONCLUSION

Our results are the first to show that RT does not attenuate the acute-induced response of proteolysis and ECM remodeling-related biomarkers.

Murf1 alters myosin replacement rates in cultured myotubes in a myosin isoform-dependent manner

Skeletal muscle tissue increases or decreases its volume by synthesizing or degrading myofibrillar proteins. The ubiquitin-proteasome system plays a pivotal role during muscle atrophy, where muscle ring finger proteins (Murf) function as E3 ubiquitin ligases responsible for identifying and targeting substrates for degradation. Our previous study demonstrated that overexpression of Ozz, an E3 specific to embryonic myosin heavy chain (Myh3), precisely reduced the Myh3 replacement rate in the thick filaments of myotubes (E. Ichimura et al., Physiol Rep. 9:e15003, 2021). These findings strongly suggest that E3 plays a critical role in regulating myosin replacement. Here, we hypothesized that the Murf isoforms, which recognize Myhs as substrates, reduced the myosin replacement rates through the enhanced Myh degradation by Murfs. First, fluorescence recovery after a photobleaching experiment was conducted to assess whether Murf isoforms affected the GFP-Myh3 replacement. In contrast to Murf2 or Murf3 overexpression, Murf1 overexpression selectively facilitated the GFP-Myh3 myosin replacement. Next, to examine the effects of Murf1 overexpression on the replacement of myosin isoforms, Cherry-Murf1 was coexpressed with GFP-Myh1, GFP-Myh4, or GFP-Myh7 in myotubes. Intriguingly, Murf1 overexpression enhanced the myosin replacement of GFP-Myh4 but did not affect those of GFP-Myh1 or GFP-Myh7. Surprisingly, overexpression of Murf1 did not enhance the ubiquitination of proteins. These results indicate that Murf1 selectively regulated myosin replacement in a Myh isoform-dependent fashion, independent of enhanced ubiquitination. This suggests that Murf1 may have a role beyond functioning as a ubiquitin ligase E3 in thick filament myosin replacement.

Diabetes-induced muscle wasting: molecular mechanisms and promising therapeutic targets

Diabetes has become a serious public health crisis, presenting significant challenges to individuals worldwide. As the largest organ in the human body, skeletal muscle is a significant target of this chronic disease, yet muscle wasting as a complication of diabetes is still not fully understood and effective treatment methods have yet to be developed. Here, we discuss the targets involved in inducing muscle wasting under diabetic conditions, both validated targets and emerging targets. Diabetes-induced skeletal muscle wasting is known to involve changes in various signaling molecules and pathways, such as protein degradation pathways, protein synthesis pathways, mitochondrial function, and oxidative stress inflammation. Recent studies have shown that some of these present potential as promising therapeutic targets, including the neuregulin 1/epidermal growth factor receptor family, advanced glycation end-products, irisin, ferroptosis, growth differentiation factor 15 and more. This study's investigation and discussion of such pathways and their potential applications provides a theoretical basis for the development of clinical treatments for diabetes-induced muscle wasting and a foundation for continued focus on this disease.

Skeletal muscle immobilisation-induced atrophy: mechanistic insights from human studies

Periods of skeletal muscle disuse lead to rapid declines in muscle mass (atrophy), which is fundamentally underpinned by an imbalance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). The complex interplay of molecular mechanisms contributing to the altered regulation of muscle protein balance during disuse have been investigated but rarely synthesised in the context of humans. This narrative review discusses human models of muscle disuse and the ensuing inversely exponential rate of muscle atrophy. The molecular processes contributing to altered protein balance are explored, with a particular focus on growth and breakdown signalling pathways, mitochondrial adaptations and neuromuscular dysfunction. Finally, key research gaps within the disuse atrophy literature are highlighted providing future avenues to enhance our mechanistic understanding of human disuse atrophy.

Alterations of the skeletal muscle contractile apparatus in necrosis induced by myotoxic snake venom phospholipases A2: a mini-review

Skeletal muscle necrosis is a common clinical manifestation of snakebite envenoming. The predominant myotoxic components in snake venoms are catalytically-active phospholipases A2 (PLA2) and PLA2 homologs devoid of enzymatic activity, which have been used as models to investigate various aspects of muscle degeneration. This review addresses the changes in the contractile apparatus of skeletal muscle induced by these toxins. Myotoxic components initially disrupt the integrity of sarcolemma, generating a calcium influx that causes various degenerative events, including hypercontraction of myofilaments. There is removal of specific sarcomeric proteins, owing to the hydrolytic action of muscle calpains and proteinases from invading inflammatory cells, causing an initial redistribution followed by widespread degradation of myofibrillar material. Experiments using skinned cardiomyocytes and skeletal muscle fibers show that these myotoxins do not directly affect the contractile apparatus, implying that hypercontraction is due to cytosolic calcium increase secondary to sarcolemmal damage. Such drastic hypercontraction may contribute to muscle damage by generating mechanical stress and further sarcolemmal damage.

Nutritional Strategies for Muscle Atrophy: Current Evidence and Underlying Mechanisms

Skeletal muscle can undergo detrimental changes in various diseases, leading to muscle dysfunction and atrophy, thus severely affecting people's lives. Along with exercise, there is a growing interest in the potential of nutritional support against muscle atrophy. This review provides a brief overview of the molecular mechanisms driving skeletal muscle atrophy and summarizes recent advances in nutritional interventions for preventing and treating muscle atrophy. The nutritional supplements include amino acids and their derivatives (such as leucine, β-hydroxy, β-methylbutyrate, and creatine), various antioxidant supplements (like Coenzyme Q10 and mitoquinone, resveratrol, curcumin, quercetin, Omega 3 fatty acids), minerals (such as magnesium and selenium), and vitamins (such as vitamin B, vitamin C, vitamin D, and vitamin E), as well as probiotics and prebiotics (like Lactobacillus, Bifidobacterium, and 1-kestose). Furthermore, the study discusses the impact of a combined approach involving nutritional support and physical therapy to prevent muscle atrophy, suggests appropriate multi-nutritional and multi-modal interventions based on individual conditions to optimize treatment outcomes, and enhances the recovery of muscle function for patients. By understanding the molecular mechanisms behind skeletal muscle atrophy and implementing appropriate interventions, it is possible to enhance the recovery of muscle function and improve patients' quality of life.

Dimorphic effect of TFE3 in determining mitochondrial and lysosomal content in muscle following denervation

BACKGROUND

Muscle atrophy is a common consequence of the loss of innervation and is accompanied by mitochondrial dysfunction. Mitophagy is the adaptive process through which damaged mitochondria are removed via the lysosomes, which are regulated in part by the transcription factor TFE3. The role of lysosomes and TFE3 are poorly understood in muscle atrophy, and the effect of biological sex is widely underreported.

METHODS

Wild-type (WT) mice, along with mice lacking TFE3 (KO), a transcriptional regulator of lysosomal and autophagy-related genes, were subjected to unilateral sciatic nerve denervation for up to 7 days, while the contralateral limb was sham-operated and served as an internal control. A subset of animals was treated with colchicine to capture mitophagy flux.

RESULTS

WT females exhibited elevated oxygen consumption rates during active respiratory states compared to males, however this was blunted in the absence of TFE3. Females exhibited higher mitophagy flux rates and greater lysosomal content basally compared to males that was independent of TFE3 expression. Following denervation, female mice exhibited less muscle atrophy compared to male counterparts. Intriguingly, this sex-dependent muscle sparing was lost in the absence of TFE3. Denervation resulted in 45% and 27% losses of mitochondrial content in WT and KO males respectively, however females were completely protected against this decline. Decreases in mitochondrial function were more severe in WT females compared to males following denervation, as ROS emission was 2.4-fold higher. In response to denervation, LC3-II mitophagy flux was reduced by 44% in females, likely contributing to the maintenance of mitochondrial content and elevated ROS emission, however this response was dysregulated in the absence of TFE3. While both males and females exhibited increased lysosomal content following denervation, this response was augmented in females in a TFE3-dependent manner.

CONCLUSIONS

Females have higher lysosomal content and mitophagy flux basally compared to males, likely contributing to the improved mitochondrial phenotype. Denervation-induced mitochondrial adaptations were sexually dimorphic, as females preferentially preserve content at the expense of function, while males display a tendency to maintain mitochondrial function. Our data illustrate that TFE3 is vital for the sex-dependent differences in mitochondrial function, and in determining the denervation-induced atrophy phenotype.

Addressing cancer anorexia-cachexia in older patients: Potential therapeutic strategies and molecular pathways

Cancer cachexia (CC) syndrome, a feature of cancer-associated muscle wasting, is particularly pronounced in older patients, and is characterised by decreased energy intake and upregulated skeletal muscle catabolic pathways. To address CC, appetite stimulants, anabolic drugs, cytokine mediators, essential amino acid supplementation, nutritional counselling, cognitive behavioural therapy, and enteral nutrition have been utilised. However, pharmacological treatments that have also shown promising results, such as megestrol acetate, anamorelin, thalidomide, and delta-9-tetrahydrocannabinol, have been associated with gastrointestinal and cardiovascular complications. Emerging evidence on the efficacy of probiotics in modulating gut microbiota also presents a promising adjunct to traditional therapies, potentially enhancing nutritional absorption and systemic inflammation control. Additionally, low-dose olanzapine has demonstrated improved appetite and weight management in older patients undergoing chemotherapy, offering a potential refinement to current therapeutic approaches. This review aims to elucidate the molecular mechanisms underpinning CC, with a particular focus on the role of anorexia in exacerbating muscle wasting, and to propose pharmacological and non-pharmacological strategies to mitigate this syndrome, particularly emphasising the needs of an older demographic. Future research targeting CC should focus on refining appetite-stimulating drugs with fewer side-effects, specifically catering to the needs of older patients, and investigating nutritional factors that can either enhance appetite or minimise suppression of appetite in individuals with CC, especially within this vulnerable group.

Assessment of inter-individual variability in hamstring muscle recovery after a sport-specific sprint training in women and men

Background: Hamstring muscles are most affected by multiple sprint-based sports as a result of muscle strain during sprinting, leading to reduced performance and increased risk of injury. Therefore, the purpose of the study was to assess inter-individual variability in hamstrings recovery after a sport-specific repeated-sprint training (RST), through sprint-specific markers of muscle recovery and associated muscle damage biomarkers in women and men. Methods: Healthy females (n = 14) and males (n = 15) underwent 10 repeated 40-m sprints with a 3-min rest pause between each repetition. Force-generating capacity (FGC) by the 90° hip :20° knee test and range of motion Jurdan test, together with serum biomarkers [sarcomeric mitochondrial creatine kinase (sMtCK), oxidative stress, irisin] were tested at baseline and 24-, 48- and 72-h post-exercise through a repeated measures design. Participants were classified according to FGC loss into high responders (HR) and low responders (LR). Results: 21 individuals (10 females, 11 males) were classified as HR (FGC loss >20% and recovery >48 h), while 8 individuals (4 females, 4 males) were classified as LR. HR individuals showed unrecovered maximal voluntary isometric contraction (MVIC) torque until 72 h post-training (p = 0.003, np 2 = 0.170), whereas only HR males showed decreased range of motion (p = 0.026, np 2 = 0.116). HR individuals also showed increased sMtCK (p = 0.016, np 2 = 0.128), oxidative stress (p = 0.038, np 2 = 0.106) and irisin (p = 0.019, np 2 = 0.123). Conclusion: There is inter-individual variability in the muscular response to a sport-specific RST, identifiable by MVIC torque assessment. The findings support that the 90° hip :20° knee test is a powerful indirect test to screen hamstrings recovery in both women and men, in a cost-effective way. However, the Jurdan test might not be able to monitor hamstrings recovery in sportswomen after RST. Decreases in muscle capacity are linked to damage to muscle sarcolemma and mitochondria until 72 h post-exercise. Overall, 72 h will not be adequate time to restore hamstrings structure and function after a sport-specific RST in both female and male responders.

Time Course of Proteolysis Biomarker Responses to Resistance, High-Intensity Interval, and Concurrent Exercise Bouts

ABSTRACT

Godwin, JS, Telles, GD, Vechin, FC, Conceição, MS, Ugrinowitsch, C, Roberts, MD, and Libardi, CA. Time course of proteolysis biomarker responses to resistance, high-intensity interval, and concurrent exercise bouts. J Strength Cond Res 37(12): 2326-2332, 2023-Concurrent exercise (CE) combines resistance exercise (RE) and high-intensity interval exercise (HIIE) in the same training routine, eliciting hypertrophy, strength, and cardiovascular benefits over time. Some studies suggest that CE training may hamper muscle hypertrophy and strength adaptations compared with RE training alone. However, the underlying mechanisms related to protein breakdown are not well understood. The purpose of this study was to examine how a bout of RE, HIIE, or CE affected ubiquitin-proteasome and calpain activity and the expression of a few associated genes, markers of skeletal muscle proteolysis. Nine untrained male subjects completed 1 bout of RE (4 sets of 8-12 reps), HIIE (12 × 1 minute sprints at V̇ o2 peak minimum velocity), and CE (RE followed by HIIE), in a crossover design, separated by 1-week washout periods. Muscle biopsies were obtained from the vastus lateralis before (Pre), immediately post, 4 hours (4 hours), and 8 hours (8 hours) after exercise. FBXO32 mRNA expression increased immediately after exercise (main time effect; p < 0.05), and RE and CE presented significant overall values compared with HIIE ( p < 0.05). There was a marginal time effect for calpain-2 mRNA expression ( p < 0.05), with no differences between time points ( p > 0.05). No significant changes occurred in TRIM63/MuRF-1 and FOXO3 mRNA expression, or 20S proteasome or calpain activities ( p > 0.05). In conclusion, our findings suggest that 1 bout of CE does not promote greater changes in markers of skeletal muscle proteolysis compared with 1 bout of RE or HIIE.

Calpain-3 not only proteolyzes calpain-1 and -2 but also is a substrate for calpain-1 and -2

Calpain is an intracellular cysteine protease that cleaves its specific substrates in a limited region to modulate cellular function. Calpain-1 (C1) and calpain-2 (C2) are ubiquitously expressed in mammalian cells, but calpain-3 (C3) is a skeletal muscle-specific type. In the course of calpain activation, the N-terminal regions of all three isoforms are clipped off in an intramolecular or intermolecular fashion. C1 proteolyzes C2 to promote further proteolysis, but C2 proteolyzes C1 to suspend C1 proteolysis, indicating the presence of C1-C2 reciprocal proteolysis. However, whether C3 is involved in the calpain proteolysis network is unclear. To address this, we examined whether GFP-tagged C3:C129S (GFP-C3:CS), an inactive protease form of C3, was a substrate for C1 or C2 in HEK cells. Intriguingly, the N-terminal region of C3:CS was cleaved by C1 and C2 at the site identical to that of the C3 autoproteolysis site. Furthermore, the N-terminal clipping of C3:CS by C1 and C2 was observed in mouse skeletal muscle lysates. Meanwhile, C3 preferentially cleaved the N-terminus of C1 over that of C2, and the sizes of these cleaved proteins were identical to their autoproteolysis forms. Our findings suggest an elaborate inter-calpain network to prime and suppress proteolysis of other calpains.

Risk Factor Analysis and Nomogram for Predicting In-Hospital Mortality in ICU Patients with Heat Stroke: A National Multicenter Study

Objective

The aim of this nationwide multicenter study was to ascertain the risk factors associated with in-hospital mortality in patients with heat stroke admitted to intensive care units (ICUs) and to develop a nomogram for prognostic prediction.

Methods

A retrospective analysis was conducted on clinical data collected from ICU patients diagnosed with heat stroke across multiple centers nationwide. Univariate and multivariate logistic regression analyses were performed to identify significant risk factors for in-hospital mortality. Based on the results of the multivariate analysis, a nomogram was constructed to estimate the individualized probability of mortality. Internal validation of the nomogram was performed, and its performance was assessed using receiver operating characteristic (ROC) curves, calibration plots, and decision curve analysis (DCA).

Results

A total of 292 ICU patients with heat stroke were included in this study. Three risk factors, namely Cr (creatinine), AST (aspartate aminotransferase), and SBP (systolic blood pressure), were found to be significantly associated with in-hospital mortality. These risk factors were incorporated into the nomogram, which exhibited good discriminative ability (area under the ROC curve of the training and validation cohorts were 0.763 and 0.739, respectively) and calibration. Internal validation and decision curve analysis confirmed the stability and reliability of the nomogram.

Conclusion

This nationwide multicenter study identified key risk factors for in-hospital mortality in ICU patients with heat stroke. The developed nomogram provides an individualized prediction of mortality risk and can serve as a valuable tool for clinicians in the assessment and management of ICU patients with heat stroke.

Diet-induced obesity augments ischemic myopathy and functional decline in a murine model of peripheral artery disease

Peripheral artery disease (PAD) causes an ischemic myopathy contributing to patient disability and mortality. Most preclinical models to date use young, healthy rodents with limited translatability to human disease. Although PAD incidence increases with age, and obesity is a common comorbidity, the pathophysiologic association between these risk factors and PAD myopathy is unknown. Using our murine model of PAD, we sought to elucidate the combined effect of age, diet-induced obesity and chronic hindlimb ischemia (HLI) on (1) mobility, (2) muscle contractility, and markers of muscle (3) mitochondrial content and function, (4) oxidative stress and inflammation, (5) proteolysis, and (6) cytoskeletal damage and fibrosis. Following 16-weeks of high-fat, high-sucrose, or low-fat, low-sucrose feeding, HLI was induced in 18-month-old C57BL/6J mice via the surgical ligation of the left femoral artery at 2 locations. Animals were euthanized 4-weeks post-ligation. Results indicate mice with and without obesity shared certain myopathic changes in response to chronic HLI, including impaired muscle contractility, altered mitochondrial electron transport chain complex content and function, and compromised antioxidant defense mechanisms. However, the extent of mitochondrial dysfunction and oxidative stress was significantly greater in obese ischemic muscle compared to non-obese ischemic muscle. Moreover, functional impediments, such as delayed post-surgical recovery of limb function and reduced 6-minute walking distance, as well as accelerated intramuscular protein breakdown, inflammation, cytoskeletal damage, and fibrosis were only evident in mice with obesity. As these features are consistent with human PAD myopathy, our model could be a valuable tool to test new therapeutics.

The emerging role of Piezo1 channels in skeletal muscle physiology

Piezo1 channels are mechanically activated (MA) cation channels that are involved in sensing of various mechanical perturbations, such as membrane stretch and shear stress, and play a crucial role in cell mechanotransduction. In response to mechanical stimuli, these channels open up and allow cations to travel into the cell and induce biochemical reactions that can change the cell's metabolism and function. Skeletal muscle cells/fibers inherently depend upon mechanical cues in the form of fluid shear stress and contractions (physical exercise). For example, an exposure of skeletal muscles to chronic mechanical loading leads to increased anabolism and fiber hypertrophy, while prolonged mechanical unloading results in muscle atrophy. MA Piezo1 channels have recently emerged as key mechanosensors that are capable of linking mechanical signals and intramuscular signaling in skeletal muscle cells/fibers. This review will summarize the emerging role of Piezo1 channels in the development and regeneration of skeletal muscle tissue as well as in the regulation of skeletal muscle atrophy. In addition, an overview of potential Piezo1-related signaling pathways underlying anabolic and catabolic processes will be provided. A better understanding of Piezo1's role in skeletal muscle mechanotransduction may represent an important basis for the development of therapeutic strategies for maintaining muscle functions under disuse conditions and in some disease states.

Intrinsic contractile dysfunction due to impaired sarcoplasmic reticulum Ca2+ release in compensatory hypertrophied muscle fibers following synergist ablation

Synergist ablation (SA) is an experimental procedure for the induction of hypertrophy. However, SA causes a decrease in specific force (i.e., force per cross-sectional area), likely due to excessive muscle use. Here, we investigated the mechanisms behind the SA-induced intrinsic contractile dysfunction, especially focusing on the excitation-contraction (EC) coupling. Male Wistar rats had unilateral surgical ablation of gastrocnemius and soleus muscles to induce compensatory hypertrophy in the plantaris muscles. Two weeks after SA, plantaris muscle was dissected from each animal and used for later analyses. SA significantly increased the mean fiber cross-sectional area (+18%). On the other hand, the ratio of depolarization-induced force to the maximum Ca2+-activated specific force, an indicator of sarcoplasmic reticulum (SR) Ca2+ release, was markedly reduced in mechanically skinned fibers from the SA group (-51%). These functional defects were accompanied by an extensive fragmentation of the SR Ca2+ release channel, the ryanodine receptor 1 (RyR1), and a decrease in the amount of other triad proteins (i.e., DHPR, STAC3, and junctophilin1). SA treatment also caused activation of calpain-1 and increased the amount of NADPH oxidase 2, endoplasmic reticulum (ER) stress proteins (i.e., Grp78, Grp94, PDI, and Ero1), and lipid peroxidation [i.e., 4-hydroxynonenal (4-HNE)] in SA-treated muscles. Our findings show that SA causes skeletal muscle weakness due to impaired EC coupling. This is likely to be induced by Ca2+-dependent degradation of triad proteins, which may result from Ca2+ leak from fragmented RyR1 triggered by increased oxidative stress.NEW & NOTEWORTHY Synergist ablation (SA) has widely been used to understand the mechanisms behind skeletal muscle hypertrophy. However, compensatory hypertrophied muscles display intrinsic contractile dysfunction, i.e., a hallmark of overuse. Here, we demonstrate that SA-induced compensatory hypertrophy is accompanied by muscle weakness due to impaired sarcoplasmic reticulum Ca2+ release. This dysfunction may be caused by the degradation of triad proteins due to the reciprocal amplification of reactive oxygen species and Ca2+ signaling at the junctional space microdomain.

Genetic models for lineage tracing in musculoskeletal development, injury, and healing

Musculoskeletal development and later post-natal homeostasis are highly dynamic processes, marked by rapid structural and functional changes across very short periods of time. Adult anatomy and physiology are derived from pre-existing cellular and biochemical states. Consequently, these early developmental states guide and predict the future of the system as a whole. Tools have been developed to mark, trace, and follow specific cells and their progeny either from one developmental state to the next or between circumstances of health and disease. There are now many such technologies alongside a library of molecular markers which may be utilized in conjunction to allow for precise development of unique cell 'lineages'. In this review, we first describe the development of the musculoskeletal system beginning as an embryonic germ layer and at each of the key developmental stages that follow. We then discuss these structures in the context of adult tissues during homeostasis, injury, and repair. Special focus is given in each of these sections to the key genes involved which may serve as markers of lineage or later in post-natal tissues. We then finish with a technical assessment of lineage tracing and the techniques and technologies currently used to mark cells, tissues, and structures within the musculoskeletal system.

Resistance exercise: a mighty tool that adapts, destroys, rebuilds and modulates the molecular and structural environment of skeletal muscle

PURPOSE

Skeletal muscle regulates health and performance by maintaining or increasing strength and muscle mass. Although the molecular mechanisms in response to resistance exercise (RE) significantly target the activation of protein synthesis, a plethora of other mechanisms and structures must be involved in orchestrating the communication, repair, and restoration of homeostasis after RE stimulation. In practice, RE can be modulated by variations in intensity, continuity and volume, which affect molecular responses and skeletal muscle adaptation. Knowledge of these aspects is important with respect to planning of training programs and assessing the impact of RE training on skeletal muscle.

METHODS

In this narrative review, we introduce general aspects of skeletal muscle substructures that adapt in response to RE. We further highlighted the molecular mechanisms that control human skeletal muscle anabolism, degradation, repair and memory in response to acute and repeated RE and linked these aspects to major training variables.

RESULTS

Although RE is a key stimulus for the activation of skeletal muscle anabolism, it also induces myofibrillar damage. Nevertheless, to increase muscle mass accompanied by a corresponding adaptation of the essential substructures of the sarcomeric environment, RE must be continuously repeated. This requires the permanent engagement of molecular mechanisms that re-establish skeletal muscle integrity after each RE-induced muscle damage.

CONCLUSION

Various molecular regulators coordinately control the adaptation of skeletal muscle after acute and repeated RE and expand their actions far beyond muscle growth. Variations of key resistance training variables likely affect these mechanisms without affecting muscle growth.

Metformin Pre-Treatment as a Means of Mitigating Disuse-Induced Rat Soleus Muscle Wasting

Currently, no ideal treatment exists to combat skeletal muscle disuse-induced atrophy and loss of strength. Because the activity of AMP-activated protein kinase (AMPK) in rat soleus muscle is suppressed at the early stages of disuse, we hypothesized that pre-treatment of rats with metformin (an AMPK activator) would exert beneficial effects on skeletal muscle during disuse. Muscle disuse was performed via hindlimb suspension (HS). Wistar rats were divided into four groups: (1) control (C), (2) control + metformin for 10 days (C+Met), (3) HS for 7 days (HS), (4) metformin treatment for 7 days before HS and during the first 3 days of 1-week HS (HS+Met). Anabolic and catabolic markers were assessed using WB and RT-PCR. Treatment with metformin partly prevented an HS-induced decrease in rat soleus weight and size of slow-twitch fibers. Metformin prevented HS-related slow-to-fast fiber transformation. Absolute soleus muscle force in the HS+Met group was increased vs. the HS group. GSK-3β (Ser9) phosphorylation was significantly increased in the HS+Met group vs. the HS group. Metformin pre-treatment partly prevented HS-induced decrease in 18S+28S rRNA content and attenuated upregulation of calpain-1 and ubiquitin. Thus, pre-treatment of rats with metformin can ameliorate disuse-induced reductions in soleus muscle weight, the diameter of slow-type fibers, and absolute muscle strength.

Emerging role of TAK1 in the regulation of skeletal muscle mass

Maintenance of skeletal muscle mass and strength throughout life is crucial for heathy living and longevity. Several signaling pathways have been implicated in the regulation of skeletal muscle mass in adults. TGF-β-activated kinase 1 (TAK1) is a key protein, which coordinates the activation of multiple signaling pathways. Recently, it was discovered that TAK1 is essential for the maintenance of skeletal muscle mass and myofiber hypertrophy following mechanical overload. Forced activation of TAK1 in skeletal muscle causes hypertrophy and attenuates denervation-induced muscle atrophy. TAK1-mediated signaling in skeletal muscle promotes protein synthesis, redox homeostasis, mitochondrial health, and integrity of neuromuscular junctions. In this article, we have reviewed the role and potential mechanisms through which TAK1 regulates skeletal muscle mass and growth. We have also proposed future areas of research that could be instrumental in exploring TAK1 as therapeutic target for improving muscle mass in various catabolic conditions and diseases.

Endogenous and Exogenous Antioxidants in Skeletal Muscle Fatigue Development during Exercise

Muscle fatigue is defined as a decrease in maximal force or power generated in response to contractile activity, and it is a risk factor for the development of musculoskeletal injuries. One of the many stressors imposed on skeletal muscle through exercise is the increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which intensifies as a function of exercise intensity and duration. Exposure to ROS/RNS can affect Na⁺/K⁺-ATPase activity, intramyofibrillar calcium turnover and sensitivity, and actin-myosin kinetics to reduce muscle force production. On the other hand, low ROS/RNS concentrations can likely upregulate an array of cellular adaptative responses related to mitochondrial biogenesis, glucose transport and muscle hypertrophy. Consequently, growing evidence suggests that exogenous antioxidant supplementation might hamper exercise-engendering upregulation in the signaling pathways of mitogen-activated protein kinases (MAPKs), peroxisome-proliferator activated co-activator 1α (PGC-1α), or mammalian target of rapamycin (mTOR). Ultimately, both high (exercise-induced) and low (antioxidant intervention) ROS concentrations can trigger beneficial responses as long as they do not override the threshold range for redox balance. The mechanisms underlying the two faces of ROS/RNS in exercise, as well as the role of antioxidants in muscle fatigue, are presented in detail in this review.

ZBP1 and heatstroke

Heatstroke, which is associated with circulatory failure and multiple organ dysfunction, is a heat stress-induced life-threatening condition characterized by a raised core body temperature and central nervous system dysfunction. As global warming continues to worsen, heatstroke is expected to become the leading cause of death globally. Despite the severity of this condition, the detailed mechanisms that underlie the pathogenesis of heatstroke still remain largely unknown. Z-DNA-binding protein 1 (ZBP1), also referred to as DNA-dependent activator of IFN-regulatory factors (DAI) and DLM-1, was initially identified as a tumor-associated and interferon (IFN)-inducible protein, but has recently been reported to be a Z-nucleic acid sensor that regulates cell death and inflammation; however, its biological function is not yet fully understood. In the present study, a brief review of the main regulators is presented, in which the Z-nucleic acid sensor ZBP1 was identified to be a significant factor in regulating the pathological characteristics of heatstroke through ZBP1-dependent signaling. Thus, the lethal mechanism of heatstroke is revealed, in addition to a second function of ZBP1 other than as a nucleic acid sensor.

Invited review: Muscle protein breakdown and its assessment in periparturient dairy cows

Mobilization of body reserves including fat, protein, and glycogen is necessary to overcome phases of negative nutrient balance typical for high-yielding dairy cows during the periparturient period. Skeletal muscle, the largest internal organ in mammals, plays a crucial role in maintaining metabolic homeostasis. However, unlike in liver and adipose tissue, the metabolic and regulatory role of skeletal muscle in the adaptation of dairy cows to the physiological needs of pregnancy and lactation has not been studied extensively. The functional integrity and quality of skeletal muscle are maintained through a constant turnover of protein, resulting from both protein breakdown and protein synthesis. Thus, muscle protein breakdown (MPB) and synthesis are intimately connected and tightly controlled to ensure proper protein homeostasis. Understanding the regulation of MPB, the catabolic component of muscle turnover, and its assessment are therefore important considerations to provide information about the timing and extent of tissue mobilization in periparturient dairy cows. Based on animal models and human studies, it is now evident that MPB occurs via the integration of 3 main systems: autophagy-lysosomal, calpain Ca²⁺-dependent cysteine proteases, and the ubiquitin-proteasome system. These 3 main systems are interconnected and do not work separately, and the regulation is complex. The ubiquitin-proteasomal system is the most well-known cellular proteolytic system and plays a fundamental role in muscle physiology. Complete degradation of a protein often requires a combination of the systems, depending on the physiological situation. Determination of MPB in dairy cows is technically challenging, resulting in a relative dearth of information. The methods for assessing MPB can be divided into either direct or indirect measurements, both having their strengths and limitations. Available information on the direct measures of MPB primarily comes from stable isotopic tracer methods and those of indirect measurements from assessing expression and activity measures of the components of the 3 MPB systems in muscle biopsy samples. Other indirect approaches (i.e., potential indicators of MPB), including ultrasound imaging and measuring metabolites from muscle degradation (i.e., 3-methylhistidine and creatinine), seem to be applicable methods and can provide useful information about the extent and timing of MPB. This review presents our current understanding, including methodological considerations, of the process of MPB in periparturient dairy cows.

Potential Therapeutic Strategies for Skeletal Muscle Atrophy

The maintenance of muscle homeostasis is vital for life and health. Skeletal muscle atrophy not only seriously reduces people's quality of life and increases morbidity and mortality, but also causes a huge socioeconomic burden. To date, no effective treatment has been developed for skeletal muscle atrophy owing to an incomplete understanding of its molecular mechanisms. Exercise therapy is the most effective treatment for skeletal muscle atrophy. Unfortunately, it is not suitable for all patients, such as fractured patients and bedridden patients with nerve damage. Therefore, understanding the molecular mechanism of skeletal muscle atrophy is crucial for developing new therapies for skeletal muscle atrophy. In this review, PubMed was systematically screened for articles that appeared in the past 5 years about potential therapeutic strategies for skeletal muscle atrophy. Herein, we summarize the roles of inflammation, oxidative stress, ubiquitin-proteasome system, autophagic-lysosomal pathway, caspases, and calpains in skeletal muscle atrophy and systematically expound the potential drug targets and therapeutic progress against skeletal muscle atrophy. This review focuses on current treatments and strategies for skeletal muscle atrophy, including drug treatment (active substances of traditional Chinese medicine, chemical drugs, antioxidants, enzyme and enzyme inhibitors, hormone drugs, etc.), gene therapy, stem cell and exosome therapy (muscle-derived stem cells, non-myogenic stem cells, and exosomes), cytokine therapy, physical therapy (electroacupuncture, electrical stimulation, optogenetic technology, heat therapy, and low-level laser therapy), nutrition support (protein, essential amino acids, creatine, β-hydroxy-β-methylbutyrate, and vitamin D), and other therapies (biomaterial adjuvant therapy, intestinal microbial regulation, and oxygen supplementation). Considering many treatments have been developed for skeletal muscle atrophy, we propose a combination of proper treatments for individual needs, which may yield better treatment outcomes.

New horizons for selenium in animal nutrition and functional foods

Selenium (Se), one of the indispensable nutrients for both human health and animal growth, participates in various physiological functions, such as antioxidant and immune responses and metabolism. The role of dietary Se, in its organic and inorganic forms, has been well documented in domestic animals. Furthermore, many feeding strategies for different animals have been developed to increase the Se concentration in animal products to address Se deficiency and even as a potential nutritional strategy to treat free radical-associated diseases. Nevertheless, studies on investigating the optimum addition of Se in feed, the long-term consequences of Se usage in food for animal nutrition, the mechanism of metallic Se nanoparticle (SeNP) transformation in vivo, and the nutritional effects of SeNPs on feed workers and the environment are urgently needed. Starting from the absorption and metabolism mechanism of Se, this review discusses the antioxidant role of Se in detail. Based on this characteristic, we further investigated the application of Se in animal health and described some unresolved issues and unanswered questions warranting further investigation. This review is expected to provide a theoretical reference for improving the quality of food animal meat as well as for the development of Se-based biological nutrition enhancement technology.

The inflammatory response, a mixed blessing for muscle homeostasis and plasticity

Skeletal muscle makes up almost half the body weight of heathy individuals and is involved in several vital functions, including breathing, thermogenesis, metabolism, and locomotion. Skeletal muscle exhibits enormous plasticity with its capacity to adapt to stimuli such as changes in mechanical loading, nutritional interventions, or environmental factors (oxidative stress, inflammation, and endocrine changes). Satellite cells and timely recruited inflammatory cells are key actors in muscle homeostasis, injury, and repair processes. Conversely, uncontrolled recruitment of inflammatory cells or chronic inflammatory processes leads to muscle atrophy, fibrosis and, ultimately, impairment of muscle function. Muscle atrophy and loss of function are reported to occur either in physiological situations such as aging, cast immobilization, and prolonged bed rest, as well as in many pathological situations, including cancers, muscular dystrophies, and several other chronic illnesses. In this review, we highlight recent discoveries with respect to the molecular mechanisms leading to muscle atrophy caused by modified mechanical loading, aging, and diseases. We also summarize current perspectives suggesting that the inflammatory process in muscle homeostasis and repair is a double-edged sword. Lastly, we review recent therapeutic approaches for treating muscle wasting disorders, with a focus on the RANK/RANKL/OPG pathway and its involvement in muscle inflammation, protection and regeneration processes.

Inflammation: Roles in Skeletal Muscle Atrophy

Various diseases can cause skeletal muscle atrophy, usually accompanied by inflammation, mitochondrial dysfunction, apoptosis, decreased protein synthesis, and enhanced proteolysis. The underlying mechanism of inflammation in skeletal muscle atrophy is extremely complex and has not been fully elucidated, thus hindering the development of effective therapeutic drugs and preventive measures for skeletal muscle atrophy. In this review, we elaborate on protein degradation pathways, including the ubiquitin-proteasome system (UPS), the autophagy-lysosome pathway (ALP), the calpain and caspase pathways, the insulin growth factor 1/Akt protein synthesis pathway, myostatin, and muscle satellite cells, in the process of muscle atrophy. Under an inflammatory environment, various pro-inflammatory cytokines directly act on nuclear factor-κB, p38MAPK, and JAK/STAT pathways through the corresponding receptors, and then are involved in muscle atrophy. Inflammation can also indirectly trigger skeletal muscle atrophy by changing the metabolic state of other tissues or cells. This paper explores the changes in the hypothalamic-pituitary-adrenal axis and fat metabolism under inflammatory conditions as well as their effects on skeletal muscle. Moreover, this paper also reviews various signaling pathways related to muscle atrophy under inflammatory conditions, such as cachexia, sepsis, type 2 diabetes mellitus, obesity, chronic obstructive pulmonary disease, chronic kidney disease, and nerve injury. Finally, this paper summarizes anti-amyotrophic drugs and their therapeutic targets for inflammation in recent years. Overall, inflammation is a key factor causing skeletal muscle atrophy, and anti-inflammation might be an effective strategy for the treatment of skeletal muscle atrophy. Various inflammatory factors and their downstream pathways are considered promising targets for the treatment and prevention of skeletal muscle atrophy.

Calpain-dependent degradation of cytoskeletal proteins as a key mechanism for a reduction in intrinsic passive stiffness of unloaded rat postural muscle

In mammals, prolonged mechanical unloading results in a significant decrease in passive stiffness of postural muscles. The nature of this phenomenon remains unclear. The aim of the present study was to investigate possible causes for a reduction in rat soleus passive stiffness after 7 and 14 days of unloading (hindlimb suspension, HS). We hypothesized that HS-induced decrease in passive stiffness would be associated with calpain-dependent degradation of cytoskeletal proteins or a decrease in actomyosin interaction. Wistar rats were subjected to HS for 7 and 14 days with or without PD150606 (calpain inhibitor) treatment. Soleus muscles were subjected to biochemical analysis and ex vivo measurements of passive tension with or without blebbistatin treatment (an inhibitor of actomyosin interactions). Passive tension of isolated soleus muscle was significantly reduced after 7- and 14-day HS compared to the control values. PD150606 treatment during 7- and 14-day HS induced an increase in alpha-actinin-2 and -3, desmin contents compared to control, partly prevented a decrease in intact titin (T1) content, and prevented a decrease in soleus passive tension. Incubation of soleus muscle with blebbistatin did not affect HS-induced reductions in specific passive tension in soleus muscle. Our study suggests that calpain-dependent breakdown of cytoskeletal proteins, but not a change in actomyosin interaction, significantly contributes to unloading-induced reductions in intrinsic passive stiffness of rat soleus muscle.

Diabetic sarcopenia: metabolic and molecular appraisal

Myopathy is the missing slot from the routine clinical checkup for diabetic complications. Similarly, its pathophysiological, metabolic, and molecular bases are insufficiently explored. In this review, the above issues are highlighted with a focus on skeletal muscle atrophy (also described as diabetic sarcopenia), in contrast to the normal histological, physiological, and molecular features of the muscles. Literature search using published data from different online resources was used. Several diabetic myopathy etiological factors are discussed explicitly including; inflammation and immunological responses, with emphasis on TNFα and IL-6 overproduction, oxidative stress, neuropathy and vasculopathy, aging sarcopenia, antidiabetic drugs, and insulin resistance as a denominator. The pathophysiological hallmark of diabetic muscle atrophy is the decreased muscle proteins synthesis and increased degradation. The muscle protein degradation is conveyed by 4 systems; ubiquitin-proteasome, lysosomal autophagy, caspase-3, and calpain systems, and is mostly mediated via the IL6/STAT, TNF&IL6/NFκB, myostatin/Smad2/3, and FOXO1/3 signaling pathways, while the protein synthesis inhibition is mediated via suppression of the IGF1-PI3K-Akt-mTOR, and SC-Gαi2-pathways. Moreover, the satellite cells and multilineage muscle mesenchymal progenitor cells differentiation plays a major role on the fate of the affected muscle cells by taking an adipogenic, fibrogenic, or connective tissue lineage. As a conclusion, in this article, the pathological features of diabetic sarcopenia are reviewed at gross level, while at a molecular level the normal protein turnover, signal transduction, and pathways involved in muscle atrophy are described. Finally, an integrated network describing the molecular partakers in diabetic sarcopenia is presented.

Protein Quality Control at the Sarcomere: Titin Protection and Turnover and Implications for Disease Development

Sarcomeres are mainly composed of filament and signaling proteins and are the smallest molecular units of muscle contraction and relaxation. The sarcomere protein titin serves as a molecular spring whose stiffness mediates myofilament extensibility in skeletal and cardiac muscle. Due to the enormous size of titin and its tight integration into the sarcomere, the incorporation and degradation of the titin filament is a highly complex task. The details of the molecular processes involved in titin turnover are not fully understood, but the involvement of different intracellular degradation mechanisms has recently been described. This review summarizes the current state of research with particular emphasis on the relationship between titin and protein quality control. We highlight the involvement of the proteasome, autophagy, heat shock proteins, and proteases in the protection and degradation of titin in heart and skeletal muscle. Because the fine-tuned balance of degradation and protein expression can be disrupted under pathological conditions, the review also provides an overview of previously known perturbations in protein quality control and discusses how these affect sarcomeric proteins, and titin in particular, in various disease states.

The State of the Art of Piezo1 Channels in Skeletal Muscle Regeneration

Piezo1 channels are highly mechanically-activated cation channels that can sense and transduce the mechanical stimuli into physiological signals in different tissues including skeletal muscle. In this focused review, we summarize the emerging evidence of Piezo1 channel-mediated effects in the physiology of skeletal muscle, with a particular focus on the role of Piezo1 in controlling myogenic precursor activity and skeletal muscle regeneration and vascularization. The disclosed effects reported by pharmacological activation of Piezo1 channels with the selective agonist Yoda1 indicate a potential impact of Piezo1 channel activity in skeletal muscle regeneration, which is disrupted in various muscular pathological states. All findings reported so far agree with the idea that Piezo1 channels represent a novel, powerful molecular target to develop new therapeutic strategies for preventing or ameliorating skeletal muscle disorders characterized by an impairment of tissue regenerative potential.

Activation of Calpain Contributes to Mechanical Ventilation-Induced Depression of Protein Synthesis in Diaphragm Muscle

Mechanical ventilation (MV) is a clinical tool that provides respiratory support to patients unable to maintain adequate alveolar ventilation on their own. Although MV is often a life-saving intervention in critically ill patients, an undesired side-effect of prolonged MV is the rapid occurrence of diaphragmatic atrophy due to accelerated proteolysis and depressed protein synthesis. Investigations into the mechanism(s) responsible for MV-induced diaphragmatic atrophy reveal that activation of the calcium-activated protease, calpain, plays a key role in accelerating proteolysis in diaphragm muscle fibers. Moreover, active calpain has been reported to block signaling events that promote protein synthesis (i.e., inhibition of mammalian target of rapamycin (mTOR) activation). While this finding suggests that active calpain can depress muscle protein synthesis, this postulate has not been experimentally verified. Therefore, we tested the hypothesis that active calpain plays a key role in the MV-induced depression of both anabolic signaling events and protein synthesis in the diaphragm muscle. MV-induced activation of calpain in diaphragm muscle fibers was prevented by transgene overexpression of calpastatin, an endogenous inhibitor of calpain. Our findings indicate that overexpression of calpastatin averts MV-induced activation of calpain in diaphragm fibers and rescues the MV-induced depression of protein synthesis in the diaphragm muscle. Surprisingly, deterrence of calpain activation did not impede the MV-induced inhibition of key anabolic signaling events including mTOR activation. However, blockade of calpain activation prevented the calpain-induced cleavage of glutaminyl-tRNA synthetase in diaphragm fibers; this finding is potentially important because aminoacyl-tRNA synthetases play a central role in protein synthesis. Regardless of the mechanism(s) responsible for calpain’s depression of protein synthesis, these results provide the first evidence that active calpain plays an important role in promoting the MV-induced depression of protein synthesis within diaphragm fibers.

Sunitinib-Induced Myositis Detected on 18F-FDG PET/CT

ABSTRACT

Sunitinib is one of the most commonly used multikinase inhibitors for metastatic renal cell carcinoma. Myositis is one of the rarest known adverse effects of sunitinib. Presenting herewith one such case with its relevant imaging findings on FDG PET/CT and ultrasound.

Classic and exertional heatstroke

In the past two decades, record-breaking heatwaves have caused an increasing number of heat-related deaths, including heatstroke, globally. Heatstroke is a heat illness characterized by the rapid rise of core body temperature above 40 °C and central nervous system dysfunction. It is categorized as classic when it results from passive exposure to extreme environmental heat and as exertional when it develops during strenuous exercise. Classic heatstroke occurs in epidemic form and contributes to 9-37% of heat-related fatalities during heatwaves. Exertional heatstroke sporadically affects predominantly young and healthy individuals. Under intensive care, mortality reaches 26.5% and 63.2% in exertional and classic heatstroke, respectively. Pathological studies disclose endothelial cell injury, inflammation, widespread thrombosis and bleeding in most organs. Survivors of heatstroke may experience long-term neurological and cardiovascular complications with a persistent risk of death. No specific therapy other than rapid cooling is available. Physiological and morphological factors contribute to the susceptibility to heatstroke. Future research should identify genetic factors that further describe individual heat illness risk and form the basis of precision-based public health response. Prioritizing research towards fundamental mechanism and diagnostic biomarker discovery is crucial for the design of specific management approaches.

Calpain Inhibitors as Potential Therapeutic Modulators in Neurodegenerative Diseases

It is considered a significant challenge to understand the neuronal cell death mechanisms with a suitable cure for neurodegenerative disorders in the coming years. Calpains are one of the best-considered "cysteine proteases activated" in brain disorders. Calpain is an important marker and mediator in the pathophysiology of neurodegeneration. Calpain activation being the essential neurodegenerative factor causing apoptotic machinery activation, it is crucial to develop reliable and effective approaches to prevent calpain-mediated apoptosis in degenerating neurons. It has been recently seen that the "inhibition of calpain activation" has appeared as a possible therapeutic target for managing neurodegenerative diseases. A systematic literature review of PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was conducted. The present article reviews the basic pathobiology and role of selective calpain inhibitors used in various neurodegenerative diseases as a therapeutic target.

Proteasome- and Calpain-Mediated Proteolysis, but Not Autophagy, Is Required for Leucine-Induced Protein Synthesis in C2C12 Myotubes

Muscle protein synthesis and proteolysis are tightly coupled processes. Given that muscle growth is promoted by increases in net protein balance, it stands to reason that bolstering protein synthesis through amino acids while reducing or inhibiting proteolysis could be a synergistic strategy in enhancing anabolism. However, there is contradictory evidence suggesting that the proper functioning of proteolytic systems in muscle is required for homeostasis. To add clarity to this issue, we sought to determine if inhibiting different proteolytic systems in C2C12 myotubes in conjunction with acute and chronic leucine treatments affected markers of anabolism. In Experiment 1, myotubes underwent 1-h, 6-h, and 24-h treatments with serum and leucine-free DMEM containing the following compounds (n = 6 wells per treatment): (i) DMSO vehicle (CTL), (ii) 2 mM leucine + vehicle (Leu-only), (iii) 2 mM leucine + 40 μM MG132 (20S proteasome inhibitor) (Leu + MG132), (iv) 2 mM leucine + 50 μM calpeptin (calpain inhibitor) (Leu + CALP), and (v) 2 mM leucine + 1 μM 3-methyladenine (autophagy inhibitor) (Leu + 3MA). Protein synthesis levels significantly increased (p < 0.05) in the Leu-only and Leu + 3MA 6-h treatments compared to CTL, and levels were significantly lower in Leu + MG132 and Leu + CALP versus Leu-only and CTL. With 24-h treatments, total protein yield was significantly lower in Leu + MG132 cells versus other treatments. Additionally, the intracellular essential amino acid (EAA) pool was significantly greater in 24-h Leu + MG132 treatments versus other treatments. In a follow-up experiment, myotubes were treated for 48 h with CTL, Leu-only, and Leu + MG132 for morphological assessments. Results indicated Leu + MG132 yielded significantly smaller myotubes compared to CTL and Leu-only. Our data are limited in scope due to the utilization of select proteolysis inhibitors. However, this is the first evidence to suggest proteasome and calpain inhibition with MG132 and CALP, respectively, abrogate leucine-induced protein synthesis in myotubes. Additionally, longer-term Leu + MG132 treatments translated to an atrophy phenotype. Whether or not proteasome inhibition in vivo reduces leucine- or EAA-induced anabolism remains to be determined.

Shared and distinct mechanisms of skeletal muscle atrophy: A narrative review

Maintenance of skeletal muscle mass and function is an incredibly nuanced balance of anabolism and catabolism that can become distorted within different pathological conditions. In this paper we intend to discuss the distinct intracellular signaling events that regulate muscle protein atrophy for a given clinical occurrence. Aside from the common outcome of muscle deterioration, several conditions have at least one or more distinct mechanisms that creates unique intracellular environments that facilitate muscle loss. The subtle individuality to each of these given pathologies can provide both researchers and clinicians with specific targets of interest to further identify and increase the efficacy of medical treatments and interventions.

The Role of GSK-3β in the Regulation of Protein Turnover, Myosin Phenotype, and Oxidative Capacity in Skeletal Muscle under Disuse Conditions

Skeletal muscles, being one of the most abundant tissues in the body, are involved in many vital processes, such as locomotion, posture maintenance, respiration, glucose homeostasis, etc. Hence, the maintenance of skeletal muscle mass is crucial for overall health, prevention of various diseases, and contributes to an individual’s quality of life. Prolonged muscle inactivity/disuse (due to limb immobilization, mechanical ventilation, bedrest, spaceflight) represents one of the typical causes, leading to the loss of muscle mass and function. This disuse-induced muscle loss primarily results from repressed protein synthesis and increased proteolysis. Further, prolonged disuse results in slow-to-fast fiber-type transition, mitochondrial dysfunction and reduced oxidative capacity. Glycogen synthase kinase 3β (GSK-3β) is a key enzyme standing at the crossroads of various signaling pathways regulating a wide range of cellular processes. This review discusses various important roles of GSK-3β in the regulation of protein turnover, myosin phenotype, and oxidative capacity in skeletal muscles under disuse/unloading conditions and subsequent recovery. According to its vital functions, GSK-3β may represent a perspective therapeutic target in the treatment of muscle wasting induced by chronic disuse, aging, and a number of diseases.

The Role of Skeletal Muscles in Exertional Heat Stroke Pathophysiology

The active participation of skeletal muscles is a unique characteristic of exertional heat stroke. Nevertheless, the only well-documented link between skeletal muscle activities and exertional heat stroke pathophysiology is the extensive muscle damage (e. g., rhabdomyolysis) and subsequent leakage of intramuscular content into the circulation of exertional heat stroke victims. Here, we will present and discuss rarely explored roles of skeletal muscles in the context of exertional heat stroke pathophysiology and recovery. This includes an overview of heat production that contributes to severe hyperthermia and the synthesis and secretion of bioactive molecules, such as cytokines, chemokines and acute phase proteins. These molecules can alter the overall inflammatory status from pro- to anti-inflammatory, affecting other organ systems and influencing recovery. The activation of innate immunity can determine whether a victim is ready to return to physical activity or experiences a prolonged convalescence. We also provide a brief discussion on whether heat acclimation can shift skeletal muscle secretory phenotype to prevent or aid recovery from exertional heat stroke. We conclude that skeletal muscles should be considered as a key organ system in exertional heat stroke pathophysiology.

Firearms-related skeletal muscle trauma: pathophysiology and novel approaches for regeneration

One major cause of traumatic injury is firearm-related wounds (i.e., ballistic trauma), common in both civilian and military populations, which is increasing in prevalence and has serious long-term health and socioeconomic consequences worldwide. Common primary injuries of ballistic trauma include soft-tissue damage and loss, haemorrhage, bone fracture, and pain. The majority of injuries are of musculoskeletal origin and located in the extremities, such that skeletal muscle offers a major therapeutic target to aid recovery and return to normal daily activities. However, the underlying pathophysiology of skeletal muscle ballistic trauma remains poorly understood, with limited evidence-based treatment options. As such, this review will address the topic of firearm-related skeletal muscle injury and regeneration. We first introduce trauma ballistics and the immediate injury of skeletal muscle, followed by detailed coverage of the underlying biological mechanisms involved in regulating skeletal muscle dysfunction following injury, with a specific focus on the processes of muscle regeneration, muscle wasting and vascular impairments. Finally, we evaluate novel approaches for minimising muscle damage and enhancing muscle regeneration after ballistic trauma, which may have important relevance for primary care in victims of violence.

Disturbances in Calcium Homeostasis Promotes Skeletal Muscle Atrophy: Lessons From Ventilator-Induced Diaphragm Wasting

Mechanical ventilation (MV) is often a life-saving intervention for patients in respiratory failure. Unfortunately, a common and undesired consequence of prolonged MV is the development of diaphragmatic atrophy and contractile dysfunction. This MV-induced diaphragmatic weakness is commonly labeled “ventilator-induced diaphragm dysfunction” (VIDD). VIDD is an important clinical problem because diaphragmatic weakness is a major risk factor for the failure to wean patients from MV; this inability to remove patients from ventilator support results in prolonged hospitalization and increased morbidity and mortality. Although several processes contribute to the development of VIDD, it is clear that oxidative stress leading to the rapid activation of proteases is a primary contributor. While all major proteolytic systems likely contribute to VIDD, emerging evidence reveals that activation of the calcium-activated protease calpain plays a required role. This review highlights the signaling pathways leading to VIDD with a focus on the cellular events that promote increased cytosolic calcium levels and the subsequent activation of calpain within diaphragm muscle fibers. In particular, we discuss the emerging evidence that increased mitochondrial production of reactive oxygen species promotes oxidation of the ryanodine receptor/calcium release channel, resulting in calcium release from the sarcoplasmic reticulum, accelerated proteolysis, and VIDD. We conclude with a discussion of important and unanswered questions associated with disturbances in calcium homeostasis in diaphragm muscle fibers during prolonged MV.

Skeletal Muscle Recovery from Disuse Atrophy: Protein Turnover Signaling and Strategies for Accelerating Muscle Regrowth

Skeletal muscle fibers have a unique capacity to adjust their metabolism and phenotype in response to alternations in mechanical loading. Indeed, chronic mechanical loading leads to an increase in skeletal muscle mass, while prolonged mechanical unloading results in a significant decrease in muscle mass (muscle atrophy). The maintenance of skeletal muscle mass is dependent on the balance between rates of muscle protein synthesis and breakdown. While molecular mechanisms regulating protein synthesis during mechanical unloading have been relatively well studied, signaling events implicated in protein turnover during skeletal muscle recovery from unloading are poorly defined. A better understanding of the molecular events that underpin muscle mass recovery following disuse-induced atrophy is of significant importance for both clinical and space medicine. This review focuses on the molecular mechanisms that may be involved in the activation of protein synthesis and subsequent restoration of muscle mass after a period of mechanical unloading. In addition, the efficiency of strategies proposed to improve muscle protein gain during recovery is also discussed.