The Role of Autophagy in Skeletal Muscle Diseases

mTORC1 and BMP-Smad1/5 regulation of serum-stimulated myotube hypertrophy: a role for autophagy

Protein synthesis regulation is critical for skeletal muscle hypertrophy, yet other established cellular processes are necessary for growth-related cellular remodeling. Autophagy has a well-acknowledged role in muscle quality control, but evidence for its role in myofiber hypertrophy remains equivocal. Both mammalian target of rapamycin complex I (mTORC1) and bone morphogenetic protein (BMP)-Smad1/5 (Sma and Mad proteins from Caenorhabditis elegans and Drosophila, respectively) signaling are reported regulators of myofiber hypertrophy; however, gaps remain in our understanding of how this regulation is integrated with growth processes and autophagy regulation. Therefore, we investigated the mTORC1 and Smad1/5 regulation of protein synthesis and autophagy flux during serum-stimulated myotube growth. Chronic serum stimulation experiments were performed on day 5 differentiated C2C12 myotubes incubated in differentiation medium [2% horse serum (HS)] or growth medium [5% fetal bovine serum (FBS)] for 48 h. Rapamycin or LDN193189 was dosed for 48 h to inhibit mTORC1 and BMP-Smad1/5 signaling, respectively. Acute serum stimulation was examined in day 7 differentiated myotubes. Protein synthesis was measured by puromycin incorporation. Bafilomycin A1 and immunoblotting for LC3B were used to assess autophagy flux. Chronic serum stimulation increased myotube diameter 22%, total protein 21%, total RNA 100%, and Smad1/5 phosphorylation 404% and suppressed autophagy flux. Rapamycin, but not LDN193189, blocked serum-induced myotube hypertrophy and the increase in total RNA. Acute serum stimulation increased protein synthesis 111%, Smad1/5 phosphorylation 559%, and rpS6 phosphorylation 117% and suppressed autophagy flux. Rapamycin increased autophagy flux during acute serum stimulation. These results provide evidence for mTORC1, but not BMP-Smad1/5, signaling being required for serum-induced myotube hypertrophy and autophagy flux by measuring LC3BII/I expression. Further investigation is warranted to examine the role of autophagy flux in myotube hypertrophy.NEW & NOTEWORTHY The present study demonstrates that myotube hypertrophy caused by chronic serum stimulation requires mammalian target of rapamycin complex 1 (mTORC1) signaling but not bone morphogenetic protein (BMP)-Smad1/5 signaling. The suppression of autophagy flux was associated with serum-induced myotube hypertrophy and mTORC1 regulation of autophagy flux by measuring LC3BII/I expression. Rapamycin is widely investigated for beneficial effects in aging skeletal muscle and sarcopenia; our results provide evidence that rapamycin can regulate autophagy-related signaling during myotube growth, which could benefit skeletal muscle functional and metabolic health.

Gene expression differences associated with intrinsic hindfoot muscle loss in the jerboa, Jaculus jaculus

Vertebrate animals that run or jump across sparsely vegetated habitats, such as horses and jerboas, have reduced the number of distal limb bones, and many have lost most or all distal limb muscle. We previously showed that nascent muscles are present in the jerboa hindfoot at birth and that these myofibers are rapidly and completely lost soon after by a process that shares features with pathological skeletal muscle atrophy. Here, we apply an intra- and interspecies differential RNA-Seq approach, comparing jerboa and mouse muscles, to identify gene expression differences associated with the initiation and progression of jerboa hindfoot muscle loss. We show evidence for reduced hepatocyte growth factor and fibroblast growth factor signaling and an imbalance in nitric oxide signaling; all are pathways that are necessary for skeletal muscle development and regeneration. We also find evidence for phagosome formation, which hints at how myofibers may be removed by autophagy or by nonprofessional phagocytes without evidence for cell death or immune cell activation. Last, we show significant overlap between genes associated with jerboa hindfoot muscle loss and genes that are differentially expressed in a variety of human muscle pathologies and rodent models of muscle loss disorders. All together, these data provide molecular insight into the process of evolutionary and developmental muscle loss in jerboa hindfeet.

Guava leaf extract attenuated muscle proteolysis in dexamethasone induced muscle atrophic mice via ubiquitin proteasome system, mTOR-autophagy, and apoptosis pathway

Muscle atrophy is the waste or loss of muscle mass and is caused by physical inactivity, aging, or diseases such as diabetes, cancer, and heart failure. The number of patients suffering from musculoskeletal disorders is expected to increase in the future. However, intervention for muscle atrophy is limited, so research on treatment for muscle wasting is needed. This study hypothesized that guava leaf (Psidium guajava L. [GL]) would have ameliorative effects on muscle atrophy by regulation of protein degradation pathways in a dexamethasone (DEX)-induced muscle atrophy mice model. Muscle atrophy was induced by DEX injection for 28 days in 7 week-old-male ICR mice. Then, low-dose GL (LGL, 200 mg/kg) or high-dose GL (HGL, 500 mg/kg) extract (GLE) was supplemented by oral gavage for 21 days. Muscle strength, calf thickness, and body composition were analyzed. Histopathological changes in the gastrocnemius muscle were examined using hematoxylin and eosin staining, and molecular pathways related to muscle degradation were analyzed by western blots. GLE treatment regardless of dose increased muscle strength in mice with muscle atrophy accompanied by attenuating autophagy related pathway in the DEX-induced muscle atrophy mice. Moreover, a high dose of GLE treatment ameliorated ubiquitin proteasome system and apoptosis in the DEX-induced muscle atrophy mice. This study suggested that GLE could be helpful to improve muscle health and alleviate proteolysis by regulation of the ubiquitin-proteasome system, autophagy, and apoptosis, which are involved in muscle degradation. In conclusion, GLE could be a potential nutraceutical to prevent muscle atrophy.

Mitochondrial mechanisms in the pathogenesis of chronic inflammatory musculoskeletal disorders

Chronic inflammatory musculoskeletal disorders characterized by prolonged muscle inflammation, resulting in enduring pain and diminished functionality, pose significant challenges for the patients. Emerging scientific evidence points to mitochondrial malfunction as a pivotal factor contributing to these ailments. Mitochondria play a critical role in powering skeletal muscle activity, but in the context of persistent inflammation, disruptions in their quantity, configuration, and performance have been well-documented. Various disturbances, encompassing alterations in mitochondrial dynamics (such as fission and fusion), calcium regulation, oxidative stress, biogenesis, and the process of mitophagy, are believed to play a central role in the progression of these disorders. Additionally, unfolded protein responses and the accumulation of fatty acids within muscle cells may adversely affect the internal milieu, impairing the equilibrium of mitochondrial functioning. The structural discrepancies between different mitochondrial subsets namely, intramyofibrillar and subsarcolemmal mitochondria likely impact their metabolic capabilities and susceptibility to inflammatory influences. The release of signals from damaged mitochondria is known to incite inflammatory responses. Intriguingly, migrasomes and extracellular vesicles serve as vehicles for intercellular transfer of mitochondria, aiding in the removal of impaired mitochondria and regulation of inflammation. Viral infections have been implicated in inducing stress on mitochondria. Prolonged dysfunction of these vital organelles sustains oxidative harm, metabolic irregularities, and heightened cytokine release, impeding the body's ability to repair tissues. This review provides a comprehensive analysis of advancements in understanding changes in the intracellular environment, mitochondrial architecture and distribution, biogenesis, dynamics, autophagy, oxidative stress, cytokines associated with mitochondria, vesicular structures, and associated membranes in the context of chronic inflammatory musculoskeletal disorders. Strategies targeting key elements regulating mitochondrial quality exhibit promise in the restoration of mitochondrial function, alleviation of inflammation, and enhancement of overall outcomes.

Hypoxia-mediated programmed cell death is involved in the formation of wooden breast in broilers

BACKGROUND

Wooden breast (WB) myopathy is a common myopathy found in commercial broiler chickens worldwide. Histological examination has revealed that WB myopathy is accompanied by damage to the pectoralis major (PM) muscle. However, the underlying mechanisms responsible for the formation of WB in broilers have not been fully elucidated. This study aimed to investigate the potential role of hypoxia-mediated programmed cell death (PCD) in the formation of WB myopathy.

RESULTS

Histological examination and biochemical analysis were performed on the PM muscle of the control (CON) and WB groups. A significantly increased thickness of the breast muscle in the top, middle, and bottom portions (P<0.01) was found along with pathological structure damage of myofibers in the WB group. The number of capillaries per fiber in PM muscle, and the levels of pO2 and sO2 in the blood, were significantly decreased (P < 0.01), while the levels of pCO2 and TCO2 in the blood were significantly increased (P < 0.05), suggesting hypoxic conditions in the PM muscle of the WB group. We further evaluated the PCD-related pathways including autophagy, apoptosis, and necroptosis to understand the consequence response to enhanced hypoxic conditions in the PM muscle of birds with WB. The ratio of LC3 II to LC3 I, and the autophagy-related factors HIF-1α, BNIP3, Beclin1, AMPKα, and ULK1 at the mRNA and protein levels, were all significantly upregulated (P < 0.05), showing that autophagy occurred in the PM muscle of the WB group. The apoptotic index, as well as the expressions of Bax, Cytc, caspase 9, and caspase 3, were significantly increased (P < 0.05), whereas Bcl-2 was significantly decreased (P < 0.05) in the WB-affected PM muscle, indicating the occurrence of apoptosis mediated by the mitochondrial pathway. Additionally, the expressions of necroptosis-related factors RIP1, RIP3, and MLKL, as well as NF-κB and the pro-inflammatory cytokines TNF-α, IL-1β, and IL-6, were all significantly enhanced (P < 0.05) in the WB-affected PM muscle.

CONCLUSIONS

The WB myopathy reduces blood supply and induces hypoxia in the PM muscle, which is closely related to the occurrence of PCD including apoptosis, autophagy, and necroptosis within myofibers, and finally leads to abnormal muscle damage and the development of WB in broilers.

Metformin mitigates adipogenesis of fibro-adipogenic progenitors after rotator cuff tears via activating mTOR/ULK1-mediated autophagy

Muscular fatty infiltration is a common issue after rotator cuff tears (RCTs), which impair shoulder function. Females suffer a higher prevalence and a more severe degree of muscular fatty infiltration after RCT when compared with males, with the underlying mechanisms remaining unclear. Fibro-adipogenic progenitors (FAPs) are the primary source of muscular fatty infiltration following RCT. Our findings disclose that gender-specific disparities in muscular fatty infiltration are linked to mTOR/ULK1-mediated autophagy of FAPs. Decreased autophagic activity contributes to adipogenic differentiation in female FAPs after RCT. Furthermore, metformin could enhance mTOR/ULK1-mediated autophagic processes of FAPs, thereby alleviating fatty infiltration and improving shoulder functionality after RCT. Together, our study reveals that gender differences in muscular fatty infiltration arise from distinct autophagic activities. Metformin could be a promising noninvasive intervention to ameliorate muscular fatty infiltration of RCT.NEW & NOTEWORTHY The current study demonstrated that gender-specific disparities in muscular fatty infiltration are attributed to mTOR/ULK1-mediated autophagy of FAPs. Decreased autophagic activity contributes to adipogenic differentiation in female FAPs after RCT. Moreover, metformin could enhance mTOR/ULK1-mediated autophagic processes of FAPs, thereby alleviating fatty infiltration and improving shoulder functionality after RCT. Therefore, metformin could be a promising noninvasive intervention to ameliorate muscular fatty infiltration of RCT.

Thbs1 regulates skeletal muscle mass in a TGFβ-Smad2/3-ATF4-dependent manner

Loss of muscle mass is a feature of chronic illness and aging. Here, we report that skeletal muscle-specific thrombospondin-1 transgenic mice (Thbs1 Tg) have profound muscle atrophy with age-dependent decreases in exercise capacity and premature lethality. Mechanistically, Thbs1 activates transforming growth factor β (TGFβ)-Smad2/3 signaling, which also induces activating transcription factor 4 (ATF4) expression that together modulates the autophagy-lysosomal pathway (ALP) and ubiquitin-proteasome system (UPS) to facilitate muscle atrophy. Indeed, myofiber-specific inhibition of TGFβ-receptor signaling represses the induction of ATF4, normalizes ALP and UPS, and partially restores muscle mass in Thbs1 Tg mice. Similarly, myofiber-specific deletion of Smad2 and Smad3 or the Atf4 gene antagonizes Thbs1-induced muscle atrophy. More importantly, Thbs1-/- mice show significantly reduced levels of denervation- and caloric restriction-mediated muscle atrophy, along with blunted TGFβ-Smad3-ATF4 signaling. Thus, Thbs1-mediated TGFβ-Smad3-ATF4 signaling in skeletal muscle regulates tissue rarefaction, suggesting a target for atrophy-based muscle diseases and sarcopenia with aging.

Skeletal muscle atrophy after sciatic nerve damage: Mechanistic insights

Sciatic nerve injury leads to molecular events that cause muscular dysfunction advancement in atrophic conditions. Nerve damage renders muscles permanently relaxed which elevates intracellular resting Ca2+ levels. Increased Ca2+ levels are associated with several cellular signaling pathways including AMPK, cGMP, PLC-β, CERB, and calcineurin. Also, multiple enzymes involved in the tricarboxylic acid cycle and oxidative phosphorylation are activated by Ca2+ influx into mitochondria during muscle contraction, to meet increased ATP demand. Nerve damage induces mitophagy and skeletal muscle atrophy through increased sensitivity to Ca2+-induced opening of the permeability transition pore (PTP) in mitochondria attributed to Ca2+, ROS, and AMPK overload in muscle. Activated AMPK interacts negatively with Akt/mTOR is a highly prevalent and well-described central pathway for anabolic processes. Over the decade several reports indicate abnormal behavior of signaling machinery involved in denervation-induced muscle loss but end up with some controversial outcomes. Therefore, understanding how the synthesis and inhibitory stimuli interact with cellular signaling to control muscle mass and morphology may lead to new pharmacological insights toward understanding the underlying mechanism of muscle loss after sciatic nerve damage. Hence, the present review summarizes the existing literature on denervation-induced muscle atrophy to evaluate the regulation and expression of differential regulators during sciatic damage.

Combination of Parenteral Amino Acid Infusion and Intermittent Loading Exercise Ameliorates Progression of Postoperative Sarcopenia in Rat Model

Postoperative sarcopenia is associated with poor outcomes in hospitalized patients. However, few studies have focused on short-term postoperative sarcopenia. Furthermore, the influence of nutritional management using amino acids (AAs) comprising a peripheral parenteral nutrition (PPN) solution and its combination with exercise (Exc) is unclear. Hence, we established a postoperative sarcopenic rat model to evaluate the effects of parenteral AA infusion combined with Exc on skeletal muscles and investigate the underlying mechanisms involved in the amelioration of muscle atrophy. Male F344 rats underwent surgery followed by hindlimb suspension (HS) for 5 days. The rats were divided into AA (-), AA (+), AA (-)-Exc, and AA (+)-Exc groups. They were continuously administered a PPN solution with or without AA at 98 kcal/kg/day. The Exc groups were subjected to intermittent loading for 1 h per day. Postoperative sarcopenic rats exhibited decreased muscle strength and mass and an upregulated ubiquitin-proteasome system, autophagy-lysosome system, and fast-twitch fiber-related genes, especially in the AA (-) group. The AA (+)-Exc group exhibited attenuated decreased muscle strength, increased gastrocnemius mass, and a suppressed upregulation of muscle atrophy- and fast-twitch fiber-related genes. Therefore, parenteral AA infusion combined with Exc may be effective in preventing postoperative sarcopenia in hospitalized patients.

LEDT and Idebenone treatment modulate autophagy and improve regenerative capacity in the dystrophic muscle through an AMPK-pathway

PURPOSE

Considering the difficulties and challenges in Duchenne muscular dystrophy (DMD) treatment, such as the adverse effects of glucocorticoids, which are the main medical prescription used by dystrophic patients, new treatment concepts for dystrophic therapy are very necessary. Thus, in this study, we explore the effects of photobiomodulation (PBM; a non-invasive therapy) and Idebenone (IDE) treatment (a potent antioxidant), applied alone or in association, in dystrophic muscle cells and the quadriceps muscle, with special focus on autophagy and regenerative pathways.

METHODS

For the in vitro studies, the dystrophic primary muscle cells received 0.5J LEDT and 0.06μM IDE; and for the in vivo studies, the dystrophic quadriceps muscle received 3J LEDT and the mdx mice were treated with 200mg/kg IDE.

RESULTS

LEDT and IDE treatment modulate autophagy by increasing autophagy markers (SQSTM1/p62, Beclin and Parkin) and signaling pathways (AMPK and TGF-β). Concomitantly, the treatments prevented muscle degeneration by reducing the number of IgG-positive fibers and the fibers with a central nucleus; decreasing the fibrotic area; up-regulating the myogenin and MCH-slow levels; and down-regulating the MyoD and MHC-fast levels.

CONCLUSION

These results suggest that LEDT and IDE treatments enhance autophagy and prevented muscle degeneration in the dystrophic muscle of the experimental model. These findings illustrate the potential efficacy of LEDT and IDE treatment as an alternative therapy focused on muscle recovery in the dystrophic patient.

Maintenance of the branched-chain amino acid transporter LAT1 counteracts myotube atrophy following chemotherapy

Prevention/management of cachexia remains a critical issue in muscle wasting conditions. The branched-chain amino acids (BCAA) have anabolic properties in skeletal muscle, but their use in treating cachexia has minimal benefits. This may be related to altered BCAA metabolism consequent to the use of chemotherapy, a main cancer treatment. Since this topic is minimally studied, we investigated the effect of chemotherapy on BCAA concentrations, transporter expression, and their metabolism. L6 myotubes were treated with vehicle (1.4 μL/mL DMSO) or a chemotherapy drug cocktail, FOLFIRI [CPT-11 (20 μg/mL), leucovorin (10 μg/mL), and 5-fluorouracil (50 μg/mL)] for 24-48 h. Chemotherapy reduced myotube diameter (-43%), myofibrillar protein content (-50%), and phosphorylation of the mechanistic target of rapamycin complex 1 (mTORC1) substrate S6K1thr389 (-80%). Drug-treated myotubes exhibited decreased BCAA concentrations (-52%) and expression of their transporter, L-type amino acid transporter 1 (LAT1; -67%). BCAA transaminase BCAT2 level was increased, but there was a reduction in PP2CM (-54%), along with increased inhibitory phosphorylation of BCKD-E1αser293 (+98%), corresponding with decreased BCKD enzyme activity (-23%) in chemotherapy-treated myotubes. Decreases in BCAA concentrations were a later response, preceded by decreases in LAT1 and BCKD activity. Although supplementation with the BCAA restored myotube BCAA levels, it had minimal effects on preventing the loss of myofibrillar proteins. However, RNAi-mediated depletion of neural precursor cell-expressed developmentally downregulated gene 4 (NEdd4), the protein ligase responsible for ubiquitin-dependent degradation of LAT1, attenuated the effects of chemotherapy on BCAA concentrations, anabolic signaling, protein synthesis, and myofibrillar protein abundance. Thus, if our findings are validated in preclinical models, interventions regulating muscle amino acid transporters might represent a promising strategy to treat cachexia.NEW & NOTEWORTHY This is the first study to attenuate chemotherapy-induced myotube atrophy by manipulating a BCAA transporter. Our findings suggest that positive regulation of amino acid transporters may be a promising strategy to treat cachexia.

Autophagy Determines Distinct Cell Fates in Human Amnion and Chorion Cells

Human fetal membranes (amniochorion) that line the intrauterine cavity consist of two distinct cell layers; single-layer amnion epithelial cells (AEC) and multilayer chorion trophoblast cells (CTC). These layers are connected through a collagen-rich extracellular matrix. Cellular remodeling helps support membrane growth and integrity during gestation and helps to maintain pregnancy. Preterm prelabor rupture of the human amniochorionic (fetal) membrane (pPROM) is antecedent to 40% of all spontaneous preterm birth. Oxidative stress (OS) induced activation of the p38 MAPK due to various maternal risk exposures and the amniochorion cells' senescence are reported pathological features of pPROM. Our transcriptomics analysis implicated dysregulated autophagy and epithelial-mesenchymal transition (EMT) in fetal membranes from pPROM. The molecular interplay between OS-induced p38 MAPK activation, autophagy, and EMT was investigated in AECs and CTCs to better understand the involvement of autophagy and EMT. We report the differential impact of OS on the autophagic machinery in AECs and CTCs, resulting in distinct cell fates. In AECs, OS-induced p38 MAPK activation causes autophagosome accumulation and reduced autophagic flux mediated by decreased ULK1 activity and kinase activity, leading to senescence. In CTCs, induction of autophagy has a limited effect; however, inhibition of autophagy led to SQSTM1-mediated EMT of trophoblast cells. Autophagy, EMT, and senescence were associated with proinflammatory changes. Thus, AECs and CTCs respond differently to OS via differential autophagy response, partly mediated via p38 MAPK. Besides senescence, OS-induced autophagy dysregulation in amniochorion cells may play a mechanistic role in pPROM pathophysiology.

Effects of resistance training on alleviating hypoxia-induced muscle atrophy: Focus on acetylation of FoxO1

This study aims to explore the role of FoxO1 and its acetylation in the alleviation of hypoxia-induced muscle atrophy by resistance training. Forty male Sprague-Dawley rats were randomly divided into four groups: normoxic control group (C), normoxic resistance training group (R), hypoxic control group (H) and hypoxic resistance training group (HR). Rats in R and HR groups were trained on an incremental weight-bearing ladder every other day, while those in H and HR groups were kept in an environment containing 12.4% O2 . After 4 weeks, muscles were collected for analysis. Differentiated L6 myoblasts were analysed in vitro after hypoxia exposure and plasmids transfection (alteration in FoxO1 acetylation). The lean body mass loss, wet weight and fibre cross-sectional area of extensor digitorum longus of rats were decreased after 4 weeks hypoxia, and the adverse reactions above was reversed by resistance training. At the same time, the increase in hypoxia-induced autophagy was suppressed, which was accompanied by a decrease in the expression of nuclear FoxO1 and cytoplasmic Ac-FoxO1 by resistance training. The L6 myotube diameter increased and the expression of autophagic proteins were inhibited under hypoxia via intervening by FoxO1 deacetylation. Overall, resistance training alleviates hypoxia-induced muscle atrophy by inhibiting nuclear FoxO1 and cytoplasmic Ac-FoxO1-mediated autophagy.

Fatigue as hallmark of Fabry disease: role of bioenergetic alterations

Fabry disease (FD) is a lysosomal storage disorder due to the impaired activity of the α-galactosidase A (GLA) enzyme which induces Gb3 deposition and multiorgan dysfunction. Exercise intolerance and fatigue are frequent and early findings in FD patients, representing a self-standing clinical phenotype with a significant impact on the patient's quality of life. Several determinants can trigger fatigability in Fabry patients, including psychological factors, cardiopulmonary dysfunctions, and primary alterations of skeletal muscle. The "metabolic hypothesis" to explain skeletal muscle symptoms and fatigability in Fabry patients is growing acknowledged. In this report, we will focus on the primary alterations of the motor system emphasizing the role of skeletal muscle metabolic disarrangement in determining the altered exercise tolerance in Fabry patients. We will discuss the most recent findings about the metabolic profile associated with Fabry disease offering new insights for diagnosis, management, and therapy.

The role and function of autophagy through signaling and pathogenetic pathways and lncRNAs in ovarian cancer

Lysosomal-driven autophagy is a tightly controlled cellular catabolic process that breaks down and recycles broken or superfluous cell parts. It is involved in several illnesses, including cancer, and is essential in preserving cellular homeostasis. Autophagy prevents DNA mutation and cancer development by actively eliminating pro-oxidative mitochondria and protein aggregates from healthy cells. Oncosuppressor and oncogene gene mutations cause dysregulation of autophagy. Increased autophagy may offer cancer cells a pro-survival advantage when oxygen and nutrients are scarce and resistance to chemotherapy and radiation. This finding justifies the use of autophagy inhibitors in addition to anti-neoplastic treatments. Excessive autophagy levels can potentially kill cells. The diagnosis and treatment of ovarian cancer present many difficulties due to its complexity and heterogeneity. Understanding the role of autophagy, a cellular process involved in the breakdown and recycling of cellular components, in ovarian cancer has garnered increasing attention in recent years. Of particular note is the increasing amount of data indicating a close relationship between autophagy and ovarian cancer. Autophagy either promotes or restricts tumor growth in ovarian cancer. Dysregulation of autophagy signaling pathways in ovarian cancers can affect the development, metastasis, and response to tumor treatment. The precise mechanism underlying autophagy concerning ovarian cancer remains unclear, as does the role autophagy plays in ovarian carcinoma. In this review, we tried to encapsulate and evaluate current findings in investigating autophagy in ovarian cancer.

Ultrasonographic Features of Muscular Weakness and Muscle Wasting in Critically Ill Patients

Muscle wasting begins as soon as in the first week of one's ICU stay and patients with multi-organ failure lose more muscle mass and suffer worse functional impairment as a consequence. Muscle wasting and weakness are mainly characterized by a generalized, bilateral lower limb weakness. However, the impairment of the respiratory and/or oropharyngeal muscles can also be observed with important consequences for one's ability to swallow and cough. Muscle wasting represents the result of the disequilibrium between breakdown and synthesis, with increased protein degradation relative to protein synthesis. It is worth noting that the resulting functional disability can last up to 5 years after discharge, and it has been estimated that up to 50% of patients are not able to return to work during the first year after ICU discharge. In recent years, ultrasound has played an increasing role in the evaluation of muscle. Indeed, ultrasound allows an objective evaluation of the cross-sectional area, the thickness of the muscle, and the echogenicity of the muscle. Furthermore, ultrasound can also estimate the thickening fraction of muscle. The objective of this review is to analyze the current understanding of the pathophysiology of acute skeletal muscle wasting and to describe the ultrasonographic features of normal muscle and muscle weakness.

Transcriptome analysis of skeletal muscle in dermatomyositis, polymyositis, and dysferlinopathy, using a bioinformatics approach

Background

Polymyositis (PM) and dermatomyositis (DM) are two distinct subgroups of idiopathic inflammatory myopathies. Dysferlinopathy, caused by a dysferlin gene mutation, usually presents in late adolescence with muscle weakness, degenerative muscle changes are often accompanied by inflammatory infiltrates, often resulting in a misdiagnosis as polymyositis.

Objective

To identify differential biological pathways and hub genes related to polymyositis, dermatomyositis and dysferlinopathy using bioinformatics analysis for understanding the pathomechanisms and providing guidance for therapy development.

Methods

We analyzed intramuscular ribonucleic acid (RNA) sequencing data from seven dermatomyositis, eight polymyositis, eight dysferlinopathy and five control subjects. Differentially expressed genes (DEGs) were identified by using DESeq2. Enrichment analyses were performed to understand the functions and enriched pathways of DEGs. A protein-protein interaction (PPI) network was constructed, and clarified the gene cluster using the molecular complex detection tool (MCODE) analysis to identify hub genes.

Results

A total of 1,048, 179 and 3,807 DEGs were detected in DM, PM and dysferlinopathy, respectively. Enrichment analyses revealed that upregulated DEGs were involved in type 1 interferon (IFN1) signaling pathway in DM, antigen processing and presentation of peptide antigen in PM, and cellular response to stimuli in dysferlinopathy. The PPI network and MCODE cluster identified 23 genes related to type 1 interferon signaling pathway in DM, 4 genes (PDIA3, HLA-C, B2M, and TAP1) related to MHC class 1 formation and quality control in PM, and 7 genes (HSPA9, RPTOR, MTOR, LAMTOR1, LAMTOR5, ATP6V0D1, and ATP6V0B) related to cellular response to stress in dysferliniopathy.

Conclusion

Overexpression of genes related to the IFN1 signaling pathway and major histocompatibility complex (MHC) class I formation was identified in DM and PM, respectively. In dysferlinopathy, overexpression of HSPA9 and the mTORC1 signaling pathway genes was detected.

PRMT5 mediates FoxO1 methylation and subcellular localization to regulate lipophagy in myogenic progenitors

Development is regulated by various factors, including protein methylation status. While PRMT5 is well known for its roles in oncogenesis by mediating symmetric di-methylation of arginine, its role in normal development remains elusive. Using Myod1Cre to drive Prmt5 knockout in embryonic myoblasts (Prmt5MKO), we dissected the role of PRMT5 in myogenesis. The Prmt5MKO mice are born normally but exhibit progressive muscle atrophy and premature death. Prmt5MKO inhibits proliferation and promotes premature differentiation of embryonic myoblasts, reducing the number and regenerative function of satellite cells in postnatal mice. Mechanistically, PRMT5 methylates and destabilizes FoxO1. Prmt5MKO increases the total FoxO1 level and promotes its cytoplasmic accumulation, leading to activation of autophagy and depletion of lipid droplets (LDs). Systemic inhibition of autophagy in Prmt5MKO mice restores LDs in myoblasts and moderately improves muscle regeneration. Together, PRMT5 is essential for muscle development and regeneration at least partially through mediating FoxO1 methylation and LD turnover.

Muscle Involvement in Amyotrophic Lateral Sclerosis: Understanding the Pathogenesis and Advancing Therapeutics

Amyotrophic lateral sclerosis (ALS) is a fatal condition characterized by the selective loss of motor neurons in the motor cortex, brainstem, and spinal cord. Muscle involvement, muscle atrophy, and subsequent paralysis are among the main features of this disease, which is defined as a neuromuscular disorder. ALS is a persistently progressive disease, and as motor neurons continue to degenerate, individuals with ALS experience a gradual decline in their ability to perform daily activities. Ultimately, muscle function loss may result in paralysis, presenting significant challenges in mobility, communication, and self-care. While the majority of ALS research has traditionally focused on pathogenic pathways in the central nervous system, there has been a great interest in muscle research. These studies were carried out on patients and animal models in order to better understand the molecular mechanisms involved and to develop therapies aimed at improving muscle function. This review summarizes the features of ALS and discusses the role of muscle, as well as examines recent studies in the development of treatments.

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.

mTOR Signaling: New Insights into Cancer, Cardiovascular Diseases, Diabetes and Aging

The mechanistic/mammalian target of rapamycin (mTOR), a member of the phosphoinositide 3-kinase (PI3K) related kinase family, integrates intracellular and environmental cues that coordinate a diverse set of cellular/tissue functions, such as cell growth, proliferation, metabolism, autophagy, apoptosis, longevity, protein/lipid/nucleotide synthesis, and tissue regeneration and repair [...].

Autophagy signaling in hypertrophied muscles of diabetic and control rats

Autophagy plays a vital role in cell homeostasis by eliminating nonfunctional components and promoting cell survival. Here, we examined the levels of autophagy signaling proteins after 7 days of overload hypertrophy in the extensor digitorum longus (EDL) and soleus muscles of control and diabetic rats. We compared control and 3-day streptozotocin-induced diabetic rats, an experimental model for type 1 diabetes mellitus (T1DM). EDL muscles showed increased levels of basal autophagy signaling proteins. The diabetic state did not affect the extent of overload-induced hypertrophy or the levels of autophagy signaling proteins (p-ULK1, Beclin-1, Atg5, Atg12-5, Atg7, Atg3, LC3-I and II, and p62) in either muscle. The p-ULK-1, Beclin-1, and p62 protein expression levels were higher in the EDL muscle than in the soleus before the hypertrophic stimulus. On the contrary, the soleus muscle exhibited increased autophagic signaling after overload-induced hypertrophy, with increases in Beclin-1, Atg5, Atg12-5, Atg7, Atg3, and LC3-I expression in the control and diabetic groups, in addition to p-ULK-1 in the control groups. After hypertrophy, Beclin-1 and Atg5 levels increased in the EDL muscle of both groups, while p-ULK1 and LC3-I increased in the control group. In conclusion, the baseline EDL muscle exhibited higher autophagy than the soleus muscle. Although TDM1 promotes skeletal muscle mass loss and strength reduction, it did not significantly alter the extent of overload-induced hypertrophy and autophagy signaling proteins in EDL and soleus muscles, with the two groups exhibiting different patterns of autophagy activation.

FAM210A mediates an inter-organelle crosstalk essential for protein synthesis and muscle growth in mouse

Mitochondria are not only essential for energy production in eukaryocytes but also a key regulator of intracellular signaling. Here, we report an unappreciated role of mitochondria in regulating cytosolic protein translation in skeletal muscle cells (myofibers). We show that the expression of mitochondrial protein FAM210A (Family With Sequence Similarity 210 Member A) is positively associated with muscle mass in mice and humans. Muscle-specific Myl1Cre-driven Fam210a knockout (Fam210aMKO) in mice reduces mitochondrial density and function, leading to progressive muscle atrophy and premature death. Metabolomic and biochemical analyses reveal that Fam210aMKO reverses the oxidative TCA cycle towards the reductive direction, resulting in acetyl-CoA accumulation and hyperacetylation of cytosolic proteins. Specifically, hyperacetylation of several ribosomal proteins leads to disassembly of ribosomes and translational defects. Transplantation of Fam210aMKO mitochondria into wildtype myoblasts is sufficient to elevate protein acetylation in recipient cells. These findings reveal a novel crosstalk between the mitochondrion and ribosome mediated by FAM210A.

A conserved STRIPAK complex is required for autophagy in muscle tissue

Autophagy is important for cellular homeostasis and to prevent the abnormal accumulation of proteins. While many proteins that comprise the canonical autophagy pathway have been characterized, the identification of new regulators may help understand tissue and/or stress-specific responses. Using an in-silico approach, we identified Striatin interacting protein (Strip), MOB kinase activator 4, and fibroblast growth factor receptor 1 oncogene partner 2 as conserved mediators of muscle tissue maintenance. We performed affinity purification-mass spectrometry (AP-MS) experiments with Drosophila melanogaster Strip as a bait protein and copurified additional Striatin-interacting phosphatase and kinase (STRIPAK) complex members from larval muscle tissue. NUAK family kinase 1 (NUAK) and Starvin (Stv) also emerged as Strip-binding proteins and these physical interactions were verified in vivo using proximity ligation assays. To understand the functional significance of the STRIPAK-NUAK-Stv complex, we employed a sensitized genetic assay combined with RNA interference (RNAi) to demonstrate that both NUAK and stv function in the same biological process with genes that encode for STRIPAK complex proteins. RNAi-directed knockdown of Strip in muscle tissue led to the accumulation of ubiquitinated cargo, p62, and Autophagy-related 8a, consistent with a block in autophagy. Indeed, autophagic flux was decreased in Strip RNAi muscles, while lysosome biogenesis and activity were unaffected. Our results support a model whereby the STRIPAK-NUAK-Stv complex coordinately regulates autophagy in muscle tissue.

Mitochondrial dysfunction: roles in skeletal muscle atrophy

Mitochondria play important roles in maintaining cellular homeostasis and skeletal muscle health, and damage to mitochondria can lead to a series of pathophysiological changes. Mitochondrial dysfunction can lead to skeletal muscle atrophy, and its molecular mechanism leading to skeletal muscle atrophy is complex. Understanding the pathogenesis of mitochondrial dysfunction is useful for the prevention and treatment of skeletal muscle atrophy, and finding drugs and methods to target and modulate mitochondrial function are urgent tasks in the prevention and treatment of skeletal muscle atrophy. In this review, we first discussed the roles of normal mitochondria in skeletal muscle. Importantly, we described the effect of mitochondrial dysfunction on skeletal muscle atrophy and the molecular mechanisms involved. Furthermore, the regulatory roles of different signaling pathways (AMPK-SIRT1-PGC-1α, IGF-1-PI3K-Akt-mTOR, FoxOs, JAK-STAT3, TGF-β-Smad2/3 and NF-κB pathways, etc.) and the roles of mitochondrial factors were investigated in mitochondrial dysfunction. Next, we analyzed the manifestations of mitochondrial dysfunction in muscle atrophy caused by different diseases. Finally, we summarized the preventive and therapeutic effects of targeted regulation of mitochondrial function on skeletal muscle atrophy, including drug therapy, exercise and diet, gene therapy, stem cell therapy and physical therapy. This review is of great significance for the holistic understanding of the important role of mitochondria in skeletal muscle, which is helpful for researchers to further understanding the molecular regulatory mechanism of skeletal muscle atrophy, and has an important inspiring role for the development of therapeutic strategies for muscle atrophy targeting mitochondria in the future.

A Comprehensive Review of Pathological Mechanisms and Natural Dietary Ingredients for the Management and Prevention of Sarcopenia

Sarcopenia is characterized by an age-related loss of skeletal muscle mass and function and has been recognized as a clinical disease by the World Health Organization since 2016. Substantial evidence has suggested that dietary modification can be a feasible tool to combat sarcopenia. Among various natural dietary ingredients, the present study focused on botanical and marine extracts, phytochemicals, and probiotics. Aims of this review were (1) to provide basic concepts including the definition, diagnosis, prevalence, and adverse effects of sarcopenia, (2) to describe possible pathological mechanisms including protein homeostasis imbalance, inflammation, mitochondrial dysfunction, and satellite cells dysfunction, and (3) to analyze recent experimental studies reporting potential biological functions against sarcopenia. A recent literature review for dietary ingredients demonstrated that protein homeostasis is maintained via an increase in the PI3K/Akt pathway and/or a decrease in the ubiquitin-proteasome system. Regulation of inflammation has primarily targeted inhibition of NF-κB signaling. Elevated Pgc-1α or Pax7 expression reverses mitochondrial or satellite cell dysfunction. This review provides the current knowledge on dietary components with the potential to assist sarcopenia prevention and/or treatment. Further in-depth studies are required to elucidate the role of and develop various dietary materials for healthier aging, particularly concerning muscle health.

mTOR-dependent TFEB activation and TFEB overexpression enhance autophagy-lysosome pathway and ameliorate Alzheimer's disease-like pathology in diabetic encephalopathy

BACKGROUND

Diabetic encephalopathy (DE) is a complication of type 2 diabetes mellitus (T2DM) that features Alzheimer's disease (AD)-like pathology, which can be degraded by the autophagy-lysosome pathway (ALP). Since transcription factor EB (TFEB) is a master regulator of ALP, TFEB-mediated ALP activation might have a therapeutic effect on DE, but this has yet to be investigated.

METHODS

We established T2DM mouse models and cultured HT22 cells under high-glucose (HG) conditions to confirm the role of ALP in DE. To further investigate this, both mice and HT22 cells were treated with 3-methyladenine (3-MA). We also analyzed the content of TFEB in the nucleus and cytoplasm to evaluate its role in ALP. To confirm the effect of TFEB activation at the post-translational level in DE, we used rapamycin to inhibit the mechanistic target of rapamycin (mTOR). We transduced both mice and cells with TFEB vector to evaluate the therapeutic effect of TFEB overexpression on DE. Conversely, we conducted TFEB knockdown to verify its role in DE in another direction.

RESULTS

We found that T2DM mice experienced compromised cognitive function, while HG-cultured HT22 cells exhibited increased cell apoptosis. Additionally, both T2DM mice and HG-cultured HT22 cells showed impaired ALP and heavier AD-like pathology. This pathology worsened after treatment with 3-MA. We also observed decreased TFEB nuclear translocation in both T2DM mice and HG-cultured HT22 cells. However, inhibiting mTOR with rapamycin or overexpressing TFEB increased TFEB nuclear translocation, enhancing the clearance of ALP-targeted AD-like pathology. This contributed to protection against neuronal apoptosis and alleviation of cognitive impairment. Conversely, TFEB knockdown lessened ALP-targeted AD-like pathology clearance and had a negative impact on DE.

CONCLUSION

Our findings suggest that impaired ALP is responsible for the aggravation of AD-like pathology in T2DM. We propose that mTOR-dependent TFEB activation and TFEB overexpression are promising therapeutic strategies for DE, as they enhance the clearance of ALP-targeted AD-like pathology and alleviate neuronal apoptosis. Our study provides insight into the underlying mechanisms of DE and offers potential avenues for the development of new treatments for this debilitating complication of T2DM. Video abstract.

Green Tea Catechol (-)-Epigallocatechin Gallate (EGCG) Conjugated with Phenylalanine Shows Enhanced Autophagy Stimulating Activity in Human Aortic Endothelial Cells

(-)-Epigallocatechin gallate (EGCG) is one of the autophagy stimulators that have been reported to protect vascular endothelial cells from oxidative stress-induced damage. In this study, we attempted potentiation of the autophagy-stimulating activity of EGCG in human aortic epithelial cells (HAECs) by using the EGCG-phenylalanine conjugate, E10. Autophagy-stimulating activity of E10 was evaluated by LC3-II measurement in the absence and presence of the lysosomal blocker chloroquine, CTYO-ID staining, and reporter assay using tandem fluorescence-tagged LC3. These experiments revealed significantly enhanced autophagic flux stimulation in HAECs by E10 compared with EGCG. Further elaboration of E10 showed that activation of AMPK through phosphorylation as the major mechanism of its autophagy stimulation. Like other autophagy stimulators, E10 protected HAECs from lipotoxicity as well as accompanying endothelial senescence. Finally, stimulation of autophagy by E10 was shown to protect HAECs from oxidative stress-induced apoptosis. These findings collectively suggest potential clinical implications of E10 for various cardiovascular complications through stimulation of autophagy.

Chemotherapy-Induced Molecular Changes in Skeletal Muscle

Paraneoplastic conditions such as cancer cachexia are often exacerbated by chemotherapy, which affects the patient’s quality of life as well as the response to therapy. The aim of this narrative review was to overview the body-composition-related changes and molecular effects of different chemotherapy agents used in cancer treatment on skeletal-muscle remodeling. A literature search was performed using the Web of Science, Scopus, and Science Direct databases and a total of 77 papers was retrieved. In general, the literature survey showed that the molecular changes induced by chemotherapy in skeletal muscle have been studied mainly in animal models and mostly in non-tumor-bearing rodents, whereas clinical studies have essentially assessed changes in body composition by computerized tomography. Data from preclinical studies showed that chemotherapy modulates several molecular pathways in skeletal muscle, including the ubiquitin–proteasome pathway, autophagy, IGF-1/PI3K/Akt/mTOR, IL-6/JAK/STAT, and NF-κB pathway; however, the newest chemotherapy agents are underexplored. In conclusion, chemotherapy exacerbates skeletal-muscle wasting in cancer patients; however, the incomplete characterization of the chemotherapy-related molecular effects on skeletal muscle makes the development of new preventive anti-wasting strategies difficult. Therefore, further investigation on molecular mechanisms and clinical studies are necessary.

Salvia plebeia R.Br. and Rosmarinic Acid Attenuate Dexamethasone-Induced Muscle Atrophy in C2C12 Myotubes

Skeletal muscle atrophy occurs when protein degradation exceeds protein synthesis and is associated with increased circulating glucocorticoid levels. Salvia plebeia R.Br. (SPR) has been used as herbal remedy for a variety of inflammatory diseases and has various biological actions such as antioxidant and anti-inflammatory activities. However, there are no reports on the effects of SPR and its bioactive components on muscle atrophy. Herein, we investigated the anti-atrophic effect of SPR and rosmarinic acid (RosA), a major compound of SPR, on dexamethasone (DEX)-induced skeletal muscle atrophy in C2C12 myotubes. Myotubes were treated with 10 μM DEX in the presence or absence of SPR or RosA at different concentrations for 24 h and subjected to immunocytochemistry, western blot, and measurements of ROS and ATP levels. SPR and RosA increased viability and inhibited protein degradation in DEX-treated C2C12 myotubes. In addition, RosA promoted the Akt/p70S6K/mTOR pathway and reduced ROS production, and apoptosis. Furthermore, the treatment of RosA significantly recovered SOD activity, autophagy activity, mitochondrial contents, and APT levels in DEX-treated myotubes. These findings suggest that SPR and RosA may provide protective effects against DEX-induced muscle atrophy and have promising potential as a nutraceutical remedy for the treatment of muscle weakness and atrophy.

Bee Venom Triggers Autophagy-Induced Apoptosis in Human Lung Cancer Cells via the mTOR Signaling Pathway

In oriental medicine, bee venom has long been used as a therapeutic agent against inflammatory diseases. Several studies have reported that isolated and purified bee venom components are effective in treating dementia, arthritis, inflammation, bacterial infections, and cancer. In previous studies, we reported that bee venom inhibits cell growth and induces apoptotic cell death in lung cancer cells. In the present study, we assessed whether bee venom affects autophagy and thereby induces apoptosis. Bee venom treatment increased the levels of autophagy-related proteins (Atg5, Beclin-1, and LC3-II) and the accumulation of LC3 puncta. We found that bee venom could induce autophagy by inhibiting the mTOR signaling pathway. In addition, we found that hydroxychloroquine (HCQ)- or si-ATG5-induced autophagy inhibition further demoted bee venom-induced apoptosis. Bee venom-induced autophagy promotes apoptosis in lung cancer cells and may become a new approach to cancer treatment.

Tenotomy-induced muscle atrophy is sex-specific and independent of NFκB

The nuclear factor-κB (NFκB) pathway is a major thoroughfare for skeletal muscle atrophy and is driven by diverse stimuli. Targeted inhibition of NFκB through its canonical mediator IKKβ effectively mitigates loss of muscle mass across many conditions, from denervation to unloading to cancer. In this study, we used gain- and loss-of-function mouse models to examine the role of NFκB in muscle atrophy following rotator cuff tenotomy - a model of chronic rotator cuff tear. IKKβ was knocked down or constitutively activated in muscle-specific inducible transgenic mice to elicit a twofold gain or loss of NFκB signaling. Surprisingly, neither knockdown of IKKβ nor overexpression of caIKKβ significantly altered the loss of muscle mass following tenotomy. This finding was consistent across measures of morphological adaptation (fiber cross-sectional area, fiber length, fiber number), tissue pathology (fibrosis and fatty infiltration), and intracellular signaling (ubiquitin-proteasome, autophagy). Intriguingly, late-stage tenotomy-induced atrophy was exacerbated in male mice compared with female mice. This sex specificity was driven by ongoing decreases in fiber cross-sectional area, which paralleled the accumulation of large autophagic vesicles in male, but not female muscle. These findings suggest that tenotomy-induced atrophy is not dependent on NFκB and instead may be regulated by autophagy in a sex-specific manner.

Intensive Care Unit Acquired Weakness Is Associated with Rapid Changes to Skeletal Muscle Proteostasis

Intensive care unit (ICU)-acquired weakness is a frequent consequence of critical illness that impacts both the limb and respiratory muscles. The cause of ICU-acquired weakness is multifactorial, but both prolonged limb muscle inactivity and mechanical ventilation are risk factors for muscle wasting, which predisposes ICU patients to both short-term complications and long-term disabilities resulting from muscle weakness. Unfortunately, the current research does not provide a detailed understanding of the cellular etiology of ICU-acquired weakness, and no standard treatment exists. Therefore, improving knowledge of the mechanisms promoting muscle atrophy in critically ill patients is essential to developing therapeutic strategies to protect against ICU-induced skeletal muscle wasting. To advance our understanding of the mechanism(s) responsible for ICU-acquired weakness, we tested the hypothesis that ICU-induced muscle inactivity promotes a rapid decrease in anabolic signaling/protein synthesis and accelerates proteolysis in both limb and respiratory muscles. To investigate ICU-induced changes in skeletal muscle proteostasis, adult Sprague Dawley rats were anesthetized and mechanically ventilated for 12 h to simulate ICU care. Measurements of anabolic signaling, protein synthesis, and proteolytic activity in the limb muscles (plantaris and soleus) and respiratory muscles (parasternal and intercostal) revealed ICU-induced reductions in both anabolic signaling (i.e., AKT/mTOR pathway) and muscle protein synthesis. Moreover, simulated ICU care resulted in increased biomarkers of accelerated proteolysis in both limb and respiratory muscles. These novel findings reveal that disturbances in limb and respiratory muscle proteostasis occur rapidly during ICU-induced muscle inactivity, irrespective of the muscle function or muscle fiber type.

Inhibiting 5-lipoxygenase prevents skeletal muscle atrophy by targeting organogenesis signalling and insulin-like growth factor-1

BACKGROUND

Skeletal muscle atrophy can occur in response to numerous factors, such as ageing and certain medications, and produces a major socio-economic burden. At present, there are no approved drugs for treating skeletal muscle atrophy. Arachidonate 5-lipoxygenase (Alox5) is a drug target for a number of diseases. However, pharmacological targeting of Alox5, and its role in skeletal muscle atrophy, is unclear.

METHODS

The potential effects of gene knockdown and pharmacological targeting of Alox5 on skeletal muscle atrophy were investigated using cell-based models, animal models and human skeletal muscle primary cells. Malotilate, a clinically safe drug developed for enhancing liver regeneration and Alox5 inhibitor, was investigated as a repurposing candidate. Mechanism(s) of action in skeletal muscle atrophy was assessed by measuring the expression level or activation status of key regulatory pathways and validated using gene knockdown and RNA sequencing.

RESULTS

Myotubes treated with the atrophy-inducing glucocorticoid, dexamethasone, were protected from catabolic responses by treatment with malotilate (+41.29%, P < 0.01). Similar anti-atrophy effects were achieved by gene knockdown of Alox5 (+30.4%, P < 0.05). Malotilate produced anti-atrophy effects without affecting the myogenic differentiation programme. In an in vivo model of skeletal muscle atrophy, malotilate treatment preserved muscle force/strength (grip strength: +35.72%, latency to fall: +553.1%, P < 0.05), increased mass and fibre cross-sectional area (quadriceps: +23.72%, soleus: +33.3%, P < 0.01) and down-regulated atrogene expression (Atrogin-1: -61.58%, Murf-1: -66.06%, P < 0.01). Similar, beneficial effects of malotilate treatment were observed in an ageing muscle model, which also showed the preservation of fast-twitch fibres (Type 2a: +56.48%, Type 2b: +37.32%, P < 0.01). Leukotriene B4, a product of Alox5 activity with inflammatory and catabolic functions, was found to be elevated in skeletal muscle undergoing atrophy (quadriceps: +224.4%, P < 0.001). Cellular transcriptome analysis showed that targeting Alox5 up-regulated biological processes regulating organogenesis and increased the expression of insulin-like growth factor-1, a key anti-atrophy hormone (+226.5%, P < 0.05). Interestingly, these effects were restricted to the atrophy condition and not observed in normal skeletal muscle cultures with Alox5 inhibition. Human myotubes were also protected from atrophy by pharmacological targeting of Alox5 (+23.68%, P < 0.05).

CONCLUSIONS

These results shed new light on novel drug targets and mechanisms underpinning skeletal muscle atrophy. Alox5 is a regulator and drug target for muscle atrophy, and malotilate is an attractive compound for repurposing studies to treat this disease.

Mitochondrial Dysfunction as an Underlying Cause of Skeletal Muscle Disorders

Mitochondria are an important energy source in skeletal muscle. A main function of mitochondria is the generation of ATP for energy through oxidative phosphorylation (OXPHOS). Mitochondrial defects or abnormalities can lead to muscle disease or multisystem disease. Mitochondrial dysfunction can be caused by defective mitochondrial OXPHOS, mtDNA mutations, Ca2+ imbalances, mitochondrial-related proteins, mitochondrial chaperone proteins, and ultrastructural defects. In addition, an imbalance between mitochondrial fusion and fission, lysosomal dysfunction due to insufficient biosynthesis, and/or defects in mitophagy can result in mitochondrial damage. In this review, we explore the association between impaired mitochondrial function and skeletal muscle disorders. Furthermore, we emphasize the need for more research to determine the specific clinical benefits of mitochondrial therapy in the treatment of skeletal muscle disorders.

Autophagy in striated muscle diseases

Impaired biomolecules and cellular organelles are gradually built up during the development and aging of organisms, and this deteriorating process is expedited under stress conditions. As a major lysosome-mediated catabolic process, autophagy has evolved to eradicate these damaged cellular components and recycle nutrients to restore cellular homeostasis and fitness. The autophagic activities are altered under various disease conditions such as ischemia-reperfusion cardiac injury, sarcopenia, and genetic myopathies, which impact multiple cellular processes related to cellular growth and survival in cardiac and skeletal muscles. Thus, autophagy has been the focus for therapeutic development to treat these muscle diseases. To develop the specific and effective interventions targeting autophagy, it is essential to understand the molecular mechanisms by which autophagy is altered in heart and skeletal muscle disorders. Herein, we summarize how autophagy alterations are linked to cardiac and skeletal muscle defects and how these alterations occur. We further discuss potential pharmacological and genetic interventions to regulate autophagy activities and their applications in cardiac and skeletal muscle diseases.

Ambra1 deficiency impairs mitophagy in skeletal muscle

BACKGROUND

Maintaining healthy mitochondria is mandatory for muscle viability and function. An essential surveillance mechanism targeting defective and harmful mitochondria to degradation is the selective form of autophagy called mitophagy. Ambra1 is a multifaceted protein with well-known autophagic and mitophagic functions. However, the study of its role in adult tissues has been extremely limited due to the embryonic lethality caused by full-body Ambra1 deficiency.

METHODS

To establish the role of Ambra1 as a positive regulator of mitophagy, we exploited in vivo overexpression of a mitochondria-targeted form of Ambra1 in skeletal muscle. To dissect the consequence of Ambra1 inactivation in skeletal muscle, we generated muscle-specific Ambra1 knockout (Ambra1^fl/fl :Mlc1f-Cre) mice. Mitochondria-enriched fractions were obtained from muscles of fed and starved animals to investigate the dynamics of the mitophagic flux.

RESULTS

Our data show that Ambra1 has a critical role in the mitophagic flux of adult murine skeletal muscle and that its genetic inactivation leads to mitochondria alterations and myofibre remodelling. Ambra1 overexpression in wild-type muscles is sufficient to enhance mitochondria clearance through the autophagy-lysosome system. Consistently with this, Ambra1-deficient muscles display an abnormal accumulation of the mitochondrial marker TOMM20 by +76% (n = 6-7; P < 0.05), a higher presence of myofibres with swollen mitochondria by +173% (n = 4; P < 0.05), and an alteration in the maintenance of the mitochondrial membrane potential and a 34% reduction in the mitochondrial respiratory complex I activity (n = 4; P < 0.05). Lack of Ambra1 in skeletal muscle leads to impaired mitophagic flux, without affecting the bulk autophagic process. This is due to a significantly decreased recruitment of DRP1 (n = 6-7 mice; P < 0.01) and Parkin (n = 6-7 mice; P < 0.05) to the mitochondrial compartment, when compared with controls. Ambra1-deficient muscles also show a marked dysregulation of the endolysosome compartment, as the incidence of myofibres with lysosomal accumulation is 20 times higher than wild-type muscles (n = 4; P < 0.05). Histologically, Ambra1-deficient muscles of both 3- and 6-month-old animals display a significant decrease of myofibre cross-sectional area and a 52% reduction in oxidative fibres (n = 6-7; P < 0.05), thus highlighting a role for Ambra1 in the proper structure and activity of skeletal muscle.

CONCLUSIONS

Our study indicates that Ambra1 is critical for skeletal muscle mitophagy and for the proper maintenance of functional mitochondria.

VMP1 Regulated by chi-miR-124a Effects Goat Myoblast Proliferation, Autophagy, and Apoptosis through the PI3K/ULK1/mTOR Signaling Pathway

The production of goat meat is determined by the growth speed of muscle fibers, and the autophagy and apoptosis of myoblast cells is a crucial process in the growth of muscle fibers. The rapid growth of muscle fibers occurs from one month old to nine months old in goats; however, the mechanisms of myoblast cells’ autophagy and apoptosis in this process are still unknown. To identify candidate genes and signaling pathway mechanisms involved in myoblast apoptosis and autophagy, we compared the expression characteristics of longissimus dorsi tissues from Wu’an goats—a native goat breed of China—at 1 month old (mon1 group) and 9 months old (mon9 group). Herein, a total of 182 differentially expressed mRNAs (DEGs) in the mon1 vs. mon9 comparison, along with the KEGG enrichments, showed that the PI3K-Akt pathway associated with autophagy and apoptosis was significantly enriched. Among these DEGs, expression of vacuole membrane protein 1 (VMP1)—a key gene for the PI3K-Akt pathway—was significantly upregulated in the older goats relative to the 1-month-old goats. We demonstrated that VMP1 promotes the proliferation and autophagy of myoblasts, and inhibits their apoptosis. The integration analysis of miRNA–mRNA showed that miR-124a was a regulator of VMP1 in muscle tissue, and overexpression and inhibition of miR-124a suppressed the proliferation and autophagy of myoblasts. The PI3K/Akt/mTOR pathway was an important pathway for cell autophagy. Additionally, the activator of the PI3K/Akt/mTOR pathway, the expression of VMP1, and ULK1 were higher than the negative control, and the expression of mTOR was depressed. The expression of VMP1, ULK1, and mTOR was the opposite when the inhibitor was added to the myoblasts. These results show that the PI3K/Akt/mTOR pathway promoted the expression of VMP1 and ULK1. By using adenovirus-mediated apoptosis and proliferation assays, we found that that miR-124a inhibits myoblast proliferation and autophagy, and promotes their apoptosis by targeting VMP1. In conclusion, our results indicated that VMP1 was highly expressed in the LD muscle tissues of nine-month-old goats, and that it was regulated by miR-124a to inhibit myoblast cells’ apoptosis through the PI3K/Akt/mTOR pathway, and to promote proliferation and autophagy. These findings contribute to the understanding of the molecular mechanisms involved in myoblast proliferation, autophagy, and apoptosis.

Integrated Analyses of DNA Methylation and Gene Expression of Rainbow Trout Muscle under Variable Ploidy and Muscle Atrophy Conditions

Rainbow trout, Oncorhynchus mykiss, is an important cool, freshwater aquaculture species used as a model for biological research. However, its genome reference has not been annotated for epigenetic markers affecting various biological processes, including muscle growth/atrophy. Increased energetic demands during gonadogenesis/reproduction provoke muscle atrophy in rainbow trout. We described DNA methylation and its associated gene expression in atrophying muscle by comparing gravid, diploid females to sterile, triploid females. Methyl Mini-seq and RNA-Seq were simultaneously used to characterize genome-wide DNA methylation and its association with gene expression in rainbow trout muscle. Genome-wide enrichment in the number of CpGs, accompanied by depleted methylation levels, was noticed around the gene transcription start site (TSS). Hypermethylation of CpG sites within ±1 kb on both sides of TSS (promoter and gene body) was weakly/moderately associated with reduced gene expression. Conversely, hypermethylation of the CpG sites in downstream regions of the gene body +2 to +10 kb was weakly associated with increased gene expression. Unlike mammalian genomes, rainbow trout gene promotors are poor in CpG islands, at <1% compared to 60%. No signs of genome-wide, differentially methylated (DM) CpGs were observed due to the polyploidy effect; only 1206 CpGs (0.03%) were differentially methylated, and these were primarily associated with muscle atrophy. Twenty-eight genes exhibited differential gene expression consistent with methylation levels of 31 DM CpGs. These 31 DM CpGs represent potential epigenetic markers of muscle atrophy in rainbow trout. The DM CpG-harboring genes are involved in apoptosis, epigenetic regulation, autophagy, collagen metabolism, cell membrane functions, and Homeobox proteins. Our study also identified genes explaining higher water content and modulated glycolysis previously shown as characteristic biochemical signs of rainbow trout muscle atrophy associated with sexual maturation. This study characterized DNA methylation in the rainbow trout genome and its correlation with gene expression. This work also identified novel epigenetic markers associated with muscle atrophy in fish/lower vertebrates.

Effect of Photobiomodulation on Denervation-Induced Skeletal Muscle Atrophy and Autophagy: A Study in Mice

OBJECTIVE

The purpose of this study was to investigate whether photobiomodulation (PBM) can protect against and attenuate muscle atrophy owing to complete peripheral nerve lesion in mice by acting on autophagy.

METHODS

C57BL/10 mice underwent right sciatic nerve transection to induce tibialis anterior muscle atrophy. After 6 hours of denervation, the mice received PBM (wavelength, 830 nm) daily, transcutaneously over the tibialis anterior muscle region for 5 or 14 days. Some mice with sciatic nerve lesion did not receive PBM. Mice that did not have sciatic nerve lesion and PBM were used as controls. After 5 and 14 days, the right tibialis anterior muscle was examined using histomorphometric (cross-sectional area of muscle fibers), Western blot (levels of the autophagy marker LC3), and immunofluorescence analyses (number of LC3 puncta in the muscle fibers).

RESULTS

The cross-sectional area of the tibialis anterior muscle fibers decreased after 5 and 14 days of denervation. PBM protected against muscle fiber atrophy after 5 days of denervation and attenuated muscle fiber atrophy after 14 days of denervation. After 5 days of muscle denervation, autophagy did not change, as demonstrated by the comparable levels of LC3-I/II ratio and LC3 puncta between the controls and the mice with atrophic muscle; PBM did not change this profile. After 14 days of denervation, an increased LC3-I/II ratio suggested an ongoing autophagy, which was not affected by PBM.

CONCLUSION

PBM attenuated the tibialis anterior muscle atrophy induced by sciatic nerve transection in the mice after at least 5 and 14 days of muscle denervation, without affecting autophagy. The transient protective effect of PBM was observed as early as 5 days after the of complete nerve lesion.

Heat shock protein A4 ablation leads to skeletal muscle myopathy associated with dysregulated autophagy and induced apoptosis

BACKGROUND

Molecular chaperones assist protein folding, facilitate degradation of misfolded polypeptides, and thereby maintain protein homeostasis. Impaired chaperone activity leads to defective protein quality control that is implicated in multiple skeletal muscle diseases. The heat shock protein A4 (HSPA4) acts as a co-chaperone for HSP70. Previously, we showed that Hspa4 deletion causes impaired protein homeostasis in the heart. However, its functional role in skeletal muscle has not been explored.

METHODS

We performed a comparative phenotypic and biochemical analyses of Hspa4 knockout (KO) mice with wild-type (WT) littermates.

RESULTS

HSPA4 is markedly upregulated in regenerating WT muscle in vivo, and in differentiated myoblasts in vitro. Hspa4-KO mice are marked by growth retardation and increased variability in body weight, accompanied by 35% mortality rates during the peri-weaning period. The surviving Hspa4-KO mice experienced progressive skeletal muscle myopathy, characterized by increased number of muscle fibers with centralized nuclei, heterogeneous myofiber size distribution, inflammatory cell infiltrates and upregulation of embryonic and perinatal myosin heavy chain transcripts. Hspa4-KO muscles demonstrated an accumulation of autophagosome-associated proteins including microtubule associated protein1 light chain 3-II (LC3-II) and p62/sequestosome accompanied by increased number of TUNEL-positive nuclei.

CONCLUSIONS

Our findings underscore the indispensable role of HSPA4 in maintenance of muscle integrity through contribution in skeletal muscle autophagy and apoptosis, which might provide a novel therapeutic strategy for skeletal muscle morbidities.

Inflammation: A New Look at an Old Problem

Pro-inflammatory stress is inherent in any cells that are subject to damage or threat of damage. It is defined by a number of universal components, including oxidative stress, cellular response to DNA damage, unfolded protein response to mitochondrial and endoplasmic reticulum stress, changes in autophagy, inflammasome formation, non-coding RNA response, formation of an inducible network of signaling pathways, and epigenetic changes. The presence of an inducible receptor and secretory phenotype in many cells is the cause of tissue pro-inflammatory stress. The key phenomenon determining the occurrence of a classical inflammatory focus is the microvascular inflammatory response (exudation, leukocyte migration to the alteration zone). This same reaction at the systemic level leads to the development of life-critical systemic inflammation. From this standpoint, we can characterize the common mechanisms of pathologies that differ in their clinical appearance. The division of inflammation into alternative variants has deep evolutionary roots. Evolutionary aspects of inflammation are also described in the review. The aim of the review is to provide theoretical arguments for the need for an up-to-date theory of the relationship between key human pathological processes based on the integrative role of the molecular mechanisms of cellular and tissue pro-inflammatory stress.

Autophagy and Diabetic Encephalopathy: Mechanistic Insights and Potential Therapeutic Implications

Diabetic Encephalopathy (DE) is one of the complications of diabetes mellitus (DM) in the central nervous system. Up to now, the mechanisms of DE are not fully discussed by the field. Autophagy is an intracellular degradation pathway crucial to maintain cellular homeostasis by clearing damaged organelles, pathogens, and unwanted protein aggregates. Increasing evidence has demonstrated that autophagy might play an essential role in DE progress. In this review, we summarize the current evidence on autophagy dysfunction under the condition of DE, and provide novel insights of possibly biological mechanisms linking autophagy impairment to DE, as well as discuss autophagy-targeted therapies as potential treatments for DE.

Exercise and Nutrition Impact on Osteoporosis and Sarcopenia-The Incidence of Osteosarcopenia: A Narrative Review

Osteoporosis and sarcopenia are diseases which affect the myoskeletal system and often occur in older adults. They are characterized by low bone density and loss of muscle mass and strength, factors which reduce the quality of life and mobility. Recently, apart from pharmaceutical interventions, many studies have focused on non-pharmaceutical approaches for the prevention of osteoporosis and sarcopenia with exercise and nutrition to being the most important and well studied of those. The purpose of the current narrative review is to describe the role of exercise and nutrition on prevention of osteoporosis and sarcopenia in older adults and to define the incidence of osteosarcopenia. Most of the publications which were included in this review show that resistance and endurance exercises prevent the development of osteoporosis and sarcopenia. Furthermore, protein and vitamin D intake, as well as a healthy diet, present a protective role against the development of the above bone diseases. However, current scientific data are not sufficient for reaching solid conclusions. Although the roles of exercise and nutrition on osteoporosis and sarcopenia seem to have been largely evaluated in literature over the recent years, most of the studies which have been conducted present high heterogeneity and small sample sizes. Therefore, they cannot reach final conclusions. In addition, osteosarcopenia seems to be caused by the effects of osteoporosis and sarcopenia on elderly. Larger meta-analyses and randomized controlled trials are needed designed based on strict inclusion criteria, in order to describe the exact role of exercise and nutrition on osteoporosis and sarcopenia.

Resistance Training as Treatment for Sarcopenia: Examining Sex-Related Differences in Physiology and Response

Sarcopenia or muscle mass atrophy reportedly occurs in up to 50% of those aged >80 years and is a significant risk factor for functional disability and poor physical performance. Over time, the deterioration in both skeletal muscle quality and composition may compromise functional independence and has been shown to independently increase the risk for falls, fractures, and overall poor health in the elderly population. These are seen most obviously in older women. Given these serious consequences, much effort has been directed toward promoting increased activity and resistance training for muscle maintenance or even muscle regeneration in older adults. The Centers for Disease Control and Prevention states that for all adults ≥65 years of age, weekly aerobic and strength training are vital to healthy aging. Older patients who have not previously participated in strength training may be hesitant about starting a resistance training program and resort to simple and familiar aerobic exercise options such as walking, jogging, or cycling. However, the benefits of strength training are too important to miss: it can improve skeletal muscle metabolic capacity, mitigate effects of aging on functional capacity, maintain bone density, and, most importantly, help individuals maintain a higher quality of life and independence. Due to their increased risk of disability and injury, this opportunity for "exercise as medicine" is particularly important to women and must be encouraged by clinicians. As such, the purpose of this commentary was to highlight known sex-related differences in muscle metabolism and potential benefits of resistance training for elderly patients. A comprehensive understanding of the issues and prevention measures presented here may allow clinicians to better serve their patients, especially older female patients, and, ultimately, alleviate the burden placed on our society by our rapidly aging population.

Oxidative Stress and Autophagy as Key Targets in Melanoma Cell Fate

Melanoma originates from the malignant transformation of melanocytes and is one of the most aggressive forms of cancer. The recent approval of several drugs has increased the chance of survival although a significant subset of patients with metastatic melanoma do not show a long-lasting response to these treatments. The complex cross-talk between oxidative stress and the catabolic process autophagy seems to play a central role in all aspects of melanoma pathophysiology, from initiation to progression and metastasis, including drug resistance. However, determining the fine role of autophagy in cancer death and in response to redox disruption is still a fundamental challenge in order to advance both basic and translational aspects of this field. In order to summarize the interactions among reactive oxygen and nitrogen species, autophagy machinery and proliferation/growth/death/apoptosis/survival, we provide here a narrative review of the preclinical evidence for drugs/treatments that modulate oxidative stress and autophagy in melanoma cells. The significance and the potential for pharmacological targeting (also through multiple and combination approaches) of these two different events, which can contribute independently or simultaneously to the fate of melanoma, may help to define new processes and their interconnections underlying skin cancer biology and unravel new reliable approaches.

A muscle-centric view of time-restricted feeding for older adults

PURPOSE OF REVIEW

The practice of time-restricted feeding (TRF) has received fervent interest in recent years as a strategy to mitigate obesity and metabolic disease. We sought to review the implications of TRF for skeletal muscle health and function in aging.

RECENT FINDINGS

TRF has high adherence and can promote body weight loss in older populations. Body weight reductions favor fat mass in the young, however, there is also the potential for undesirable losses in lean mass. There is currently no evidence to support TRF for skeletal muscle function and metabolism in older persons, and only tentative findings in the young. With a narrow eating window of 6-8 h and a prolonged fasting period to minimize daily insulin exposure, TRF may contradict recommended dietary practices for optimizing skeletal muscle anabolism in older people.

SUMMARY

TRF might represent a promising intervention to address obesity and its associated metabolic diseases, however, at present there is insufficient evidence for optimizing skeletal muscle mass or health in older individuals. Further research is needed to: (1) ascertain the impact of TRF on body composition, skeletal muscle anabolism, and autophagy in aging, and; (2) delineate the potentially myoprotective roles of dietary protein and exercise within the framework of TRF in older persons.

1-Deoxysphingolipids, Early Predictors of Type 2 Diabetes, Compromise the Functionality of Skeletal Myoblasts

Metabolic dysfunction, dysregulated differentiation, and atrophy of skeletal muscle occur as part of a cluster of abnormalities associated with the development of Type 2 diabetes mellitus (T2DM). Recent interest has turned to the attention of the role of 1-deoxysphingolipids (1-DSL), atypical class of sphingolipids which are found significantly elevated in patients diagnosed with T2DM but also in the asymptomatic population who later develop T2DM. In vitro studies demonstrated that 1-DSL have cytotoxic properties and compromise the secretion of insulin from pancreatic beta cells. However, the role of 1-DSL on the functionality of skeletal muscle cells in the pathophysiology of T2DM still remains unclear. This study aimed to investigate whether 1-DSL are cytotoxic and disrupt the cellular processes of skeletal muscle precursors (myoblasts) and differentiated cells (myotubes) by performing a battery of in vitro assays including cell viability adenosine triphosphate assay, migration assay, myoblast fusion assay, glucose uptake assay, and immunocytochemistry. Our results demonstrated that 1-DSL significantly reduced the viability of myoblasts in a concentration and time-dependent manner, and induced apoptosis as well as cellular necrosis. Importantly, myoblasts were more sensitive to the cytotoxic effects induced by 1-DSL rather than by saturated fatty acids, such as palmitate, which are critical mediators of skeletal muscle dysfunction in T2DM. Additionally, 1-DSL significantly reduced the migration ability of myoblasts and the differentiation process of myoblasts into myotubes. 1-DSL also triggered autophagy in myoblasts and significantly reduced insulin-stimulated glucose uptake in myotubes. These findings demonstrate that 1-DSL directly compromise the functionality of skeletal muscle cells and suggest that increased levels of 1-DSL observed during the development of T2DM are likely to contribute to the pathophysiology of muscle dysfunction detected in this disease.