Ubiquitin ligase Cbl-b is a negative regulator for insulin-like growth factor 1 signaling during muscle atrophy caused by unloading. Mol Cell Biol. 2009;29:4798-4811. [PMID: 19546233 DOI: 10.1128/mcb.01347-08]

Molecular Aspects in the Development of Type 2 Diabetes and Possible Preventive and Complementary Therapies

The incidence of diabetes, including type 2 diabetes (T2DM), is increasing sharply worldwide. To reverse this, more effective approaches in prevention and treatment are needed. In our review, we sought to summarize normal insulin action and the pathways that primarily influence the development of T2DM. Normal insulin action involves mitogenic and metabolic pathways, as both are important in normal metabolic processes, regeneration, etc. However, through excess energy, both can be hyperactive or attenuated/inactive leading to disturbances in the cellular and systemic regulation with the consequence of cellular stress and systemic inflammation. In this review, we detailed the beneficial molecular changes caused by some important components of nutrition and by exercise, which act in the same molecular targets as the developed drugs, and can revert the damaged pathways. Moreover, these induce entire networks of regulatory mechanisms and proteins to restore unbalanced homeostasis, proving their effectiveness as preventive and complementary therapies. These are the main steps for success in prevention and treatment of developed diseases to rid the body of excess energy, both from stored fats and from overnutrition, while facilitating fat burning with adequate, regular exercise in healthy people, and together with necessary drug treatment as required in patients with insulin resistance and T2DM.

Cbl and Cbl-b ubiquitin ligases are essential for intestinal epithelial stem cell maintenance

Receptor tyrosine kinases (RTKs) control stem cell maintenance vs. differentiation decisions. Casitas B-lineage lymphoma (CBL) family ubiquitin ligases are negative regulators of RTKs, but their stem cell regulatory roles remain unclear. Here, we show that Lgr5+ intestinal stem cell (ISC)-specific inducible Cbl-knockout (KO) on a Cblb null mouse background (iDKO) induced rapid loss of the Lgr5 Hi ISCs with transient expansion of the Lgr5 Lo transit-amplifying population. LacZ-based lineage tracing revealed increased ISC commitment toward enterocyte and goblet cell fate at the expense of Paneth cells. Functionally, Cbl/Cblb iDKO impaired the recovery from radiation-induced intestinal epithelial injury. In vitro, Cbl/Cblb iDKO led to inability to maintain intestinal organoids. Single-cell RNA sequencing in organoids identified Akt-mTOR (mammalian target of rapamycin) pathway hyperactivation upon iDKO, and pharmacological Akt-mTOR axis inhibition rescued the iDKO defects. Our results demonstrate a requirement for Cbl/Cblb in the maintenance of ISCs by fine-tuning the Akt-mTOR axis to balance stem cell maintenance vs. commitment to differentiation.

Preventive effect of fermented whey protein mediated by Lactobacillus gasseri IM13 via the PI3K/AKT/FOXO pathway in muscle atrophy

This study investigated the preventive effects of whey protein fermented with Lactobacillus gasseri IM13 (F-WP) against dexamethasone (DEX)-induced muscle atrophy. C2C12 muscle cells were treated with F-WP followed by DEX treatment. Dexamethasone treatment inhibited myotube formation and the expression of myogenic regulatory factors; however, pretreatment with F-WP attenuated DEX-induced damage. The F-WP significantly activated the phosphorylation of the IGF-1/PI3K/AKT pathway and improved muscle homeostasis suppressed by DEX. Moreover, F-WP alleviated the phosphorylation of mTOR, S6K1, and 4E-BP1 and enhanced muscle protein synthesis. Muscle-specific ubiquitin ligases and autophagy lysosomes, which were activated by the dephosphorylation of FOXO3a by DEX treatment, were significantly attenuated by F-WP pretreatment of myotubes. For peptidomic analysis, F-WP was fractionated using preparative HPLC (prep-HPLC), and the AA sequences of 11 peptides were identified using MALDI-TOF/MS/MS. In conclusion, fermentation of whey protein by the specific probiotic strain IM13 produced bioactive peptides with high antioxidant and anti-sarcopenic-sarcopenic effects, which markedly enhanced myogenesis and muscle protein synthesis while diminishing muscle protein degradation compared with intact whey protein.

YTHDF2 governs muscle size through a targeted modulation of proteostasis

The regulation of proteostasis is fundamental for maintenance of muscle mass and function. Activation of the TGF-β pathway drives wasting and premature aging by favoring the proteasomal degradation of structural muscle proteins. Yet, how this critical post-translational mechanism is kept in check to preserve muscle health remains unclear. Here, we reveal the molecular link between the post-transcriptional regulation of m6A-modified mRNA and the modulation of SMAD-dependent TGF-β signaling. We show that the m6A-binding protein YTHDF2 is essential to determining postnatal muscle size. Indeed, muscle-specific genetic deletion of YTHDF2 impairs skeletal muscle growth and abrogates the response to hypertrophic stimuli. We report that YTHDF2 controls the mRNA stability of the ubiquitin ligase ASB2 with consequences on anti-growth gene program activation through SMAD3. Our study identifies a post-transcriptional to post-translational mechanism for the coordination of gene expression in muscle.

The gateway reflex regulates tissue-specific autoimmune diseases

The dynamic interaction and movement of substances and cells between the central nervous system (CNS) and peripheral organs are meticulously controlled by a specialized vascular structure, the blood-brain barrier (BBB). Experimental and clinical research has shown that disruptions in the BBB are characteristic of various neuroinflammatory disorders, including multiple sclerosis. We have been elucidating a mechanism termed the "gateway reflex" that details the entry of immune cells, notably autoreactive T cells, into the CNS at the onset of such diseases. This process is initiated through local neural responses to a range of environmental stimuli, such as gravity, electricity, pain, stress, light, and joint inflammation. These stimuli specifically activate neural pathways to open gateways at targeted blood vessels for blood immune cell entry. The gateway reflex is pivotal in managing tissue-specific inflammatory diseases, and its improper activation is linked to disease progression. In this review, we present a comprehensive examination of the gateway reflex mechanism.

Bezafibrate attenuates immobilization-induced muscle atrophy in mice

Muscle atrophy due to fragility fractures or frailty worsens not only activity of daily living and healthy life expectancy, but decreases life expectancy. Although several therapeutic agents for muscle atrophy have been investigated, none is yet in clinical use. Here we report that bezafibrate, a drug used to treat hyperlipidemia, can reduce immobilization-induced muscle atrophy in mice. Specifically, we used a drug repositioning approach to screen 144 drugs already utilized clinically for their ability to inhibit serum starvation-induced elevation of Atrogin-1, a factor related to muscle atrophy, in myotubes in vitro. Two candidates were selected, and here we demonstrate that one of them, bezafibrate, significantly reduced muscle atrophy in an in vivo model of muscle atrophy induced by leg immobilization. In gastrocnemius muscle, immobilization reduced muscle weight by an average of ~ 17.2%, and bezafibrate treatment prevented ~ 40.5% of that atrophy. In vitro, bezafibrate significantly inhibited expression of the inflammatory cytokine Tnfa in lipopolysaccharide-stimulated RAW264.7 cells, a murine macrophage line. Finally, we show that expression of Tnfa and IL-1b is induced in gastrocnemius muscle in the leg immobilization model, an activity significantly antagonized by bezafibrate administration in vivo. We conclude that bezafibrate could serve as a therapeutic agent for immobilization-induced muscle atrophy.

Diabetic sensory neuropathy and insulin resistance are induced by loss of UCHL1 in Drosophila

Diabetic sensory neuropathy (DSN) is one of the most common complications of type 2 diabetes (T2D), however the molecular mechanistic association between T2D and DSN remains elusive. Here we identify ubiquitin C-terminal hydrolase L1 (UCHL1), a deubiquitinase highly expressed in neurons, as a key molecule underlying T2D and DSN. Genetic ablation of UCHL1 leads to neuronal insulin resistance and T2D-related symptoms in Drosophila. Furthermore, loss of UCHL1 induces DSN-like phenotypes, including numbness to external noxious stimuli and axonal degeneration of sensory neurons in flies' legs. Conversely, UCHL1 overexpression improves DSN-like defects of T2D model flies. UCHL1 governs insulin signaling by deubiquitinating insulin receptor substrate 1 (IRS1) and antagonizes an E3 ligase of IRS1, Cullin 1 (CUL1). Consistent with these results, genetic and pharmacological suppression of CUL1 activity rescues T2D- and DSN-associated phenotypes. Therefore, our findings suggest a complete set of genetic factors explaining T2D and DSN, together with potential remedies for the diseases.

Knockout of c-Cbl/Cbl-b slows c-Met trafficking resulting in enhanced signaling in corneal epithelial cells

In many cell types, the E3 ubiquitin ligases c-Cbl and Cbl-b induce ligand-dependent ubiquitylation of the hepatocyte growth factor (HGF)-stimulated c-Met receptor and target it for lysosomal degradation. This study determines whether c-Cbl/Cbl-b are negative regulators of c-Met in the corneal epithelium (CE) and if their inhibition can augment c-Met-mediated CE homeostasis. Immortalized human corneal epithelial cells were transfected with Cas9 only (Cas9, control cells) or with Cas9 and c-Cbl/Cbl-b guide RNAs to knockout each gene singularly (-c-Cbl or -Cbl-b cells) or both genes (double KO [DKO] cells) and monitored for their responses to HGF. Cells were assessed for ligand-dependent c-Met ubiquitylation via immunoprecipitation, magnitude, and duration of c-Met receptor signaling via immunoblot and receptor trafficking by immunofluorescence. Single KO cells displayed a decrease in receptor ubiquitylation and an increase in phosphorylation compared to control. DKO cells had no detectable ubiquitylation, had delayed receptor trafficking, and a 2.3-fold increase in c-Met phosphorylation. Based on the observed changes in receptor trafficking and signaling, we examined HGF-dependent in vitro wound healing via live-cell time-lapse microscopy in control and DKO cells. HGF-treated DKO cells healed at approximately twice the rate of untreated cells. From these data, we have generated a model in which c-Cbl/Cbl-b mediate the ubiquitylation of c-Met, which targets the receptor through the endocytic pathway toward lysosomal degradation. In the absence of ubiquitylation, the stimulated receptor stays phosphorylated longer and enhances in vitro wound healing. We propose that c-Cbl and Cbl-b are promising pharmacologic targets for enhancing c-Met-mediated CE re-epithelialization.

Cbl and Cbl-b Ubiquitin Ligases are Essential for Intestinal Epithelial Stem Cell Maintenance

Among the signaling pathways that control the stem cell self-renewal and maintenance vs. acquisition of differentiated cell fates, those mediated by receptor tyrosine kinase (RTK) activation are well established as key players. CBL family ubiquitin ligases are negative regulators of RTKs but their physiological roles in regulating stem cell behaviors are unclear. While hematopoietic Cbl/Cblb knockout (KO) leads to a myeloproliferative disease due to expansion and reduced quiescence of hematopoietic stem cells, mammary epithelial KO led to stunted mammary gland development due to mammary stem cell depletion. Here, we examined the impact of inducible Cbl/Cblb double-KO (iDKO) selectively in the Lgr5-defined intestinal stem cell (ISC) compartment. Cbl/Cblb iDKO led to rapid loss of the Lgr5 Hi ISC pool with a concomitant transient expansion of the Lgr5 Lo transit amplifying population. LacZ reporter-based lineage tracing showed increased ISC commitment to differentiation, with propensity towards enterocyte and goblet cell fate at the expense of Paneth cells. Functionally, Cbl/Cblb iDKO impaired the recovery from radiation-induced intestinal epithelial injury. In vitro , Cbl/Cblb iDKO led to inability to maintain intestinal organoids. Single cell RNAseq analysis of organoids revealed Akt-mTOR pathway hyperactivation in iDKO ISCs and progeny cells, and pharmacological inhibition of the Akt-mTOR axis rescued the organoid maintenance and propagation defects. Our results demonstrate a requirement for Cbl/Cblb in the maintenance of ISCs by fine tuning the Akt-mTOR axis to balance stem cell maintenance vs. commitment to differentiation.

Signals for Muscular Protein Turnover and Insulin Resistance in Critically Ill Patients: A Narrative Review

Sarcopenia in critically ill patients is a highly prevalent comorbidity. It is associated with a higher mortality rate, length of mechanical ventilation, and probability of being sent to a nursing home after the Intensive Care Unit (ICU). Despite the number of calories and proteins delivered, there is a complex network of signals of hormones and cytokines that affect muscle metabolism and its protein synthesis and breakdown in critically ill and chronic patients. To date, it is known that a higher number of proteins decreases mortality, but the exact amount needs to be clarified. This complex network of signals affects protein synthesis and breakdown. Some hormones regulate metabolism, such as insulin, insulin growth factor glucocorticoids, and growth hormone, whose secretion is affected by feeding states and inflammation. In addition, cytokines are involved, such as TNF-alpha and HIF-1. These hormones and cytokines have common pathways that activate muscle breakdown effectors, such as the ubiquitin-proteasome system, calpain, and caspase-3. These effectors are responsible for protein breakdown in muscles. Many trials have been conducted with hormones with different results but not with nutritional outcomes. This review examines the effect of hormones and cytokines on muscles. Knowing all the signals and pathways that affect protein synthesis and breakdown can be considered for future therapeutics.

Daily Dietary Supplementation with Steamed Soybean Improves Muscle Volume and Strength in Healthy People Lacking Exercise

Various dietary protein supplements are used by the elderly and bedridden to maintain their skeletal muscle mass and functions. High-quality proteins act as an anabolic driver and help to improve muscle strength and performance. Previously, we reported that soy protein significantly attenuates denervation-induced loss of muscle mass and myofiber cross sectional area in mice with inhibition of ubiquitination and degradation of IRS-1 in tibialis anterior muscle. It also increased muscle volume and strength in bedridden patients. In the present study, we investigated the effects of dietary soybean supplementation on muscle functions in taxi drivers lacking vigorous physical exercise. We conducted a case-control study on 25 healthy, male taxi drivers between the ages of 36 and 71 y performing minimal physical exercise. They were divided into two dietary groups: the soybean diet group (n=13) who ate daily meals (dinner) supplemented with 50 g of steamed soybean for 30 d and the control diet group (n=12) who received no soybean supplement. Next, we measured the muscle cross-sectional area (CSA) and muscle strength and function in both the groups before and after 30 d of soybean intake. The body weights of both diet groups did not differ significantly over time. However, after 30 d of soybean supplementation, the soybean-fed group developed significantly higher muscle CSA and grip strength compared to the control groups. In conclusion, dietary soybean supplementation improved muscle function in taxi drivers who lacked exercise.

Zebrafish Models for Skeletal Muscle Senescence: Lessons from Cell Cultures and Rodent Models

Human life expectancy has markedly increased over the past hundred years. Consequently, the percentage of elderly people is increasing. Aging and sarcopenic changes in skeletal muscles not only reduce locomotor activities in elderly people but also increase the chance of trauma, such as bone fractures, and the incidence of other diseases, such as metabolic syndrome, due to reduced physical activity. Exercise therapy is currently the only treatment and prevention approach for skeletal muscle aging. In this review, we aimed to summarize the strategies for modeling skeletal muscle senescence in cell cultures and rodents and provide future perspectives based on zebrafish models. In cell cultures, in addition to myoblast proliferation and myotube differentiation, senescence induction into differentiated myotubes is also promising. In rodents, several models have been reported that reflect the skeletal muscle aging phenotype or parts of it, including the accelerated aging models. Although there are fewer models of skeletal muscle aging in zebrafish than in mice, various models have been reported in recent years with the development of CRISPR/Cas9 technology, and further advancements in the field using zebrafish models are expected in the future.

Transcriptome and morphological analysis on the heart in gestational protein-restricted aging male rat offspring

Background: Adverse factors that influence embryo/fetal development are correlated with increased risk of cardiovascular disease (CVD), type-2 diabetes, arterial hypertension, obesity, insulin resistance, impaired kidney development, psychiatric disorders, and enhanced susceptibility to oxidative stress and inflammatory processes in adulthood. Human and experimental studies have demonstrated a reciprocal relationship between birthweight and cardiovascular diseases, implying intrauterine adverse events in the onset of these abnormalities. In this way, it is plausible that confirmed functional and morphological heart changes caused by gestational protein restriction could be related to epigenetic effects anticipating cardiovascular disorders and reducing the survival time of these animals. Methods: Wistar rats were divided into two groups according to the protein diet content offered during the pregnancy: a normal protein diet (NP, 17%) or a Low-protein diet (LP, 6%). The arterial pressure was measured, and the cardiac mass, cardiomyocytes area, gene expression, collagen content, and immunostaining of proteins were performed in the cardiac tissue of male 62-weeks old NP compared to LP offspring. Results: In the current study, we showed a low birthweight followed by catch-up growth phenomena associated with high blood pressure development, increased heart collagen content, and cardiomyocyte area in 62-week-old LP offspring. mRNA sequencing analysis identified changes in the expression level of 137 genes, considering genes with a p-value < 0.05. No gene was. Significantly changed according to the adj-p-value. After gene-to-gene biological evaluation and relevance, the study demonstrated significant differences in genes linked to inflammatory activity, oxidative stress, apoptosis process, autophagy, hypertrophy, and fibrosis pathways resulting in heart function disorders. Conclusion: The present study suggests that gestational protein restriction leads to early cardiac diseases in the LP progeny. It is hypothesized that heart dysfunction is associated with fibrosis, myocyte hypertrophy, and multiple abnormal gene expression. Considering the above findings, it may suppose a close link between maternal protein restriction, specific gene expression, and progressive heart failure.

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.

Morin improves dexamethasone-induced muscle atrophy by modulating atrophy-related genes and oxidative stress in female mice

This study investigated the effect of morin, a flavonoid, on dexamethasone-induced muscle atrophy in C57BL/6J female mice. Dexamethasone (10 mg/kg body weight) for 10 days significantly reduced body weight, gastrocnemius and tibialis anterior muscle mass, and muscle protein in mice. Dexamethasone significantly upregulated muscle atrophy-associated ubiquitin ligases, including atrogin-1 and MuRF-1, and the upstream transcription factors FoxO3a and Klf15. Additionally, dexamethasone significantly induced the expression of oxidative stress-sensitive ubiquitin ligase Cbl-b and the accumulation of the oxidative stress markers malondialdehyde and advanced protein oxidation products in both the plasma and skeletal muscle samples. Intriguingly, morin treatment (20 mg/kg body weight) for 17 days effectively attenuated the loss of muscle mass and muscle protein and suppressed the expression of ubiquitin ligases while reducing the expression of upstream transcriptional factors. Therefore, morin might act as a potential therapeutic agent to attenuate muscle atrophy by modulating atrophy inducing genes and preventing oxidative stress.

MuRF1 deficiency prevents age-related fat weight gain, possibly through accumulation of PDK4 in skeletal muscle mitochondria in older mice

Recent studies show that muscle mass and metabolic function are interlinked. Muscle RING finger 1 (MuRF1) is a critical muscle-specific ubiquitin ligase associated with muscle atrophy. Yet, the molecular target of MuRF1 in atrophy and aging remains unclear. We examined the role of MuRF1 in aging, using MuRF1-deficient (MuRF1^-/- ) mice in vivo, and MuRF1-overexpressing cell in vitro. MuRF1 deficiency partially prevents age-induced skeletal muscle loss in mice. Interestingly, body weight and fat mass of more than 7-month-old MuRF1^-/- mice were lower than in MuRF1^+/+ mice. Serum and muscle metabolic parameters and results of indirect calorimetry suggest significantly higher energy expenditure and enhanced lipid metabolism in 3-month-old MuRF1^-/- mice than in MuRF1^+/+ mice, resulting in suppressed adipose tissue gain during aging. Pyruvate dehydrogenase kinase 4 (PDK4) is crucial for a switch from glucose to lipid metabolism, and the interaction between MuRF1 and PDK4 was examined. PDK4 protein levels were elevated in mitochondria from the skeletal muscle in MuRF1^-/- mice. In vitro, MuRF1 interacted with PDK4 but did not induce degradation through ubiquitination. Instead, SUMO posttranscriptional modification (SUMOylation) of PDK4 was detected in MuRF1-overexpressing cells, in contrast to cells without the RING domain of MuRF1. MuRF1 deficiency enhances lipid metabolism possibly by upregulating PDK4 localization into mitochondrial through prevention of SUMOylation. Inhibition of MuRF1-mediated PDK4 SUMOylation is a potential therapeutic target for age-related dysfunction of lipid metabolism and muscle atrophy.

Additional effects of simultaneous treatment with C14-Cblin and celastrol on the clinorotation-induced rat L6 myotube atrophy

Two novel reagents, N-myristoylated Cbl-b inhibitory peptide (C14-Cblin) and celastrol, a quinone methide triterpene, are reported to be effective in preventing myotube atrophy. The combined effects of C14-Cblin and celastrol on rat L6 myotubes atrophy induced by 3D-clinorotation, a simulated microgravity model, was investigated in the present study. We first examined their effects on expression in atrogenes. Increase in MAFbx1/atrogin-1 and MuRF-1 by 3D-clinorotation was significantly suppressed by treatment with C14-Cblin or celastrol, but there was no additive effect of simultaneous treatment. However, celastrol significantly suppressed the upregulation of Cbl-b and HSP70 by 3D-clinorotation. Whereas 3D-clinorotation decreased the protein level of IRS-1 in L6 myotubes, C14-Cblin and celastrol inhibited the degradation of IRS-1. C14-Cblin and celastrol promoted the phosphorylation of FOXO3a even in microgravity condition. Simultaneous administration of C14-Cblin and celastrol had shown little additive effect in reversing the impairment of IGF-1 signaling by 3D-clinorotation. While 3D-clinorotation-induced marked oxidative stress in L6 myotubes, celastrol suppressed 3D-clinorotation-induced ROS production. Finally, the C14-Cblin and celastrol-treated groups were inhibited decrease in L6 myotube diameter and increased the protein content of slow-twitch MyHC cultured under 3D-clinorotation. The simultaneous treatment of C14-Cblin and celastrol additively prevented 3D-clinorotation-induced myotube atrophy than single treatment. J. Med. Invest. 69 : 127-134, February, 2022.

Cathepsin K activity controls cachexia-induced muscle atrophy via the modulation of IRS1 ubiquitination

BACKGROUND

Cachexia is a complicated metabolic disorder that is characterize by progressive atrophy of skeletal muscle. Cathepsin K (CTSK) is a widely expressed cysteine protease that has garnered attention because of its enzymatic and non-enzymatic functions in signalling in various pathological conditions. Here, we examined whether CTSK participates in cancer-induced skeletal muscle loss and dysfunction, focusing on protein metabolic imbalance.

METHODS

Male 9-week-old wild-type (CTSK^+/+ , n = 10) and CTSK-knockout (CTSK^-/- , n = 10) mice were injected subcutaneously with Lewis lung carcinoma cells (LLC; 5 × 10⁵ ) or saline, respectively. The mice were then subjected to muscle mass and muscle function measurements. HE staining, immunostaining, quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, and western blotting were used to explore the CTSK expression and IRS1/Akt pathway in the gastrocnemius muscle at various time points. In vitro measurements included CTSK expression, IRS1/Akt pathway-related target molecule expressions, and the diameter of C2C12 myotubes with or without LLC-conditioned medium (LCM). An IRS1 ubiquitin assay, and truncation, co-immunoprecipitation, and co-localization experiments were also performed.

RESULTS

CTSK^+/+ cachectic animals exhibited loss of skeletal muscle mass (muscle weight loss of 15%, n = 10, P < 0.01), muscle dysfunction (grip strength loss > 15%, n = 10, P < 0.01), and fibre area (average area reduction > 30%, n = 5, P < 0.01). Compared with that of non-cachectic CTSK^+/+ mice, the skeletal muscle of cachectic CTSK^+/+ mice exhibited greater degradation of insulin receptor substrate 1 (IRS1, P < 0.01). In this setting, cachectic muscles exhibited decreases in the phosphorylation levels of protein kinase B (Akt³⁰⁸ , P < 0.01; Akt⁴⁷³ , P < 0.05) and anabolic-related proteins (the mammalian target of rapamycin, P < 0.01) and increased levels of catabolism-related proteins (muscle RING-finger protein-1, P < 0.01; MAFbx1, P < 0.01) in CTSK^+/+ mice (n = 3). Although there was no difference in LLC tumour growth (n = 10, P = 0.44), CTSK deletion mitigated the IRS1 degradation, loss of the skeletal muscle mass (n = 10, P < 0.01), and dysfunction (n = 10, P < 0.01). In vitro, CTSK silencing prevented the IRS1 ubiquitination and loss of the myotube myosin heavy chain content (P < 0.01) induced by LCM, and these changes were accelerated by CTSK overexpression even without LCM. Immunoprecipitation showed that CTSK selectively acted on IRS1 in the region of amino acids 268 to 574. The results of co-transfection of IRS1-N-FLAG or IRS1-C-FLAG with CTSK suggested that CTSK selectively cleaves IRS1 and causes ubiquitination-related degradation of IRS1.

CONCLUSIONS

These results demonstrate that CTSK plays a novel role in IRS1 ubiquitination in LLC-induced muscle wasting, and suggest that CTSK could be an effective therapeutic target for cancer-related cachexia.

miR-27b-3p Attenuates Muscle Atrophy by Targeting Cbl-b in Skeletal Muscles

As it is well known, muscle atrophy is a process in which protein degradation increases and protein synthesis decreases. This process is regulated by a variety of links. Among them, microRNAs play an essential role in this process, which has attracted widespread attention. In this paper, we find that miR-27b-3p and Cbl-b genes are significantly differentially expressed in the induced atrophy model. The dual-luciferase experiment and Western blot analysis confirmed that miR-27b-3p could regulate the expression of Cbl-b. In C2C12-differentiated myotubes, the overexpression of the Cbl-b gene showed that Cbl-b could upregulate the expression of MuRF-1 and Atrogin-1, which are related marker genes of muscle atrophy, at both the mRNA and protein levels, indicating that the Cbl-b gene can specifically affect muscle atrophy. The knockdown of the Cbl-b gene after C2C12-differentiated myotubes induced atrophy treatment can downregulate the expression of muscle-atrophy-related genes, indicating that manual intervention to downregulate the expression of Cbl-b has a certain alleviating effect on muscle atrophy. These data suggest that miR-27b-3p can regulate the expression of the Cbl-b gene and then exert a particular influence on muscle atrophy through the Cbl-b gene.

Hyperuricemia contributes to glucose intolerance of hepatic inflammatory macrophages and impairs the insulin signaling pathway via IRS2-proteasome degradation

AIM

Numerous reports have demonstrated the key importance of macrophage-elicited metabolic inflammation in insulin resistance (IR). Our previous studies confirmed that hyperuricemia or high uric acid (HUA) treatment induced an IR state in several peripheral tissues to promote the development of type 2 diabetes mellitus (T2DM). However, the effect of HUA on glucose uptake and the insulin sensitivity of macrophages and its mechanism is unclear.

METHODS

To assess systemic IR, we generated hyperuricemic mice by urate oxidase knockout (UOX-KO). Then, glucose/insulin tolerance, the tissue uptake of 18F-fluorodeoxyglucose, body composition, and energy balance were assessed. Glucose uptake of circulating infiltrated macrophages in the liver was evaluated by glucose transporter type 4 (GLUT-4) staining. Insulin sensitivity and the insulin signaling pathway of macrophages were demonstrated using the 2-NBDG kit, immunoblotting, and immunofluorescence assays. The immunoprecipitation assay and LC-MS analysis were used to determine insulin receptor substrate 2 (IRS2) levels and its interacting protein enrichment under HUA conditions.

RESULTS

Compared to WT mice (10 weeks old), serum uric acid levels were higher in UOX-KO mice (WT, 182.3 ± 5.091 μM versus KO, 421.9 ± 45.47 μM). Hyperuricemic mice with metabolic disorders and systemic IR showed inflammatory macrophage recruitment and increased levels of circulating proinflammatory cytokines. HUA inhibited the nuclear translocation of GLUT-4 in hepatic macrophages, restrained insulin-induced glucose uptake and glucose tolerance, and blocked insulin IRS2/PI3K/AKT signaling. Meanwhile, HUA mediated the IRS2 protein degradation pathway and activated AMPK/mTOR in macrophages. LC-MS analysis showed that ubiquitination degradation could be involved in IRS2 and its interacting proteins to contribute to IR under HUA conditions.

CONCLUSION

The data suggest that HUA-induced glucose intolerance in hepatic macrophages contributed to insulin resistance and impaired the insulin signaling pathway via IRS2-proteasome degradation.

Fermented Oyster Extract Attenuated Dexamethasone-Induced Muscle Atrophy by Decreasing Oxidative Stress

It is well known that oxidative stress induces muscle atrophy, which decreases with the activation of Nrf2/HO-1. Fermented oyster extracts (FO), rich in γ-aminobutyric acid (GABA) and lactate, have shown antioxidative effects. We evaluated whether FO decreased oxidative stress by upregulating Nrf2/HO-1 and whether it decreased NF-κB, leading to decreased IL-6 and TNF-α. Decreased oxidative stress led to the downregulation of Cbl-b ubiquitin ligase, which increased IGF-1 and decreased FoxO3, atrogin1, and Murf1, and eventually decreased muscle atrophy in dexamethasone (Dexa)-induced muscle atrophy animal model. For four weeks, mice were orally administered with FO, GABA, lactate, or GABA+Lactate, and then Dexa was subcutaneously injected for ten days. During Dexa injection period, FO, GABA, lactate, or GABA+Lactate were also administered, and grip strength test and muscle harvesting were performed on the day of the last Dexa injection. We compared the attenuation effect of FO with GABA, lactate, and GABA+lactate treatment. Nrf2 and HO-1 expressions were increased by Dexa but decreased by FO; SOD activity and glutathione levels were decreased by Dexa but increased by FO; NADPH oxidase activity was increased by Dexa but decreased by FO; NF-κB, IL-6, and TNF-α activities were increased by Dexa were decreased by FO; Cbl-b expression was increased by Dexa but restored by FO; IGF-1 expression was decreased by Dexa but increased by FO; FoxO3, Atrogin-1, and MuRF1 expressions were increased by Dexa but decreased by FO. The gastrocnemius thickness and weight were decreased by Dexa but increased by FO. The cross-sectional area of muscle fiber and grip strength were decreased by Dexa but increased by FO. In conclusion, FO decreased Dexa-induced oxidative stress through the upregulation of Nrf2/HO-1. Decreased oxidative stress led to decreased Cbl-b, FoxO3, atrogin1, and MuRF1, which attenuated muscle atrophy.

Programmed cell death 5 improves skeletal muscle insulin resistance by inhibiting IRS-1 ubiquitination through stabilization of MDM2

AIMS

Insulin resistance is defined as the decreased sensitivity of tissues and organs to insulin and it is the main pathological basis of metabolic syndrome. PDCD5 is widely expressed in tissues including skeletal muscle and liver, but its exact function and the role in insulin resistance has not been studied. The present study is to explore the effect of PDCD5 on insulin resistance in skeletal muscle, the largest target organ of insulin, and its mechanism.

MATERIALS AND METHODS

Mice were fed with high-fat diet to establish obesity model. C2C12 myoblasts differentiated into myotubes and then were treated with palmitate to induce insulin resistance. Gain-of-function and loss-of-function experiments were performed by infecting C2C12 with adenovirus containing PDCD5 cDNA or PDCD5 shRNA.

KEY FINDINGS

PDCD5 protein was first increased and then decreased in the skeletal muscle from high-fat diet induced obese mice and consistently in palmitate induced insulin resistance C2C12 myotubes. Overexpression of PDCD5 in C2C12 cells did not affect the sensitivity to insulin but inhibited the palmitate induced insulin resistance, while knockdown of PDCD5 aggravated the insulin resistance. Mechanistically, PDCD5 interacted with ubiquitin ligase MDM2; overexpression of PDCD5 decreased MDM2 protein level, inhibited the increased interaction of MDM2 with IRS-1 and the degradation of IRS-1 by palmitate stimulation.

SIGNIFICANCE

PDCD5 is upregulated during the early stage of insulin resistance in skeletal muscle. The increased PDCD5 inhibits IRS-1 ubiquitination, increases the stability of IRS-1 by interacting with and degrading MDM2, thus providing a protective effect on insulin resistance in skeletal muscle.

Integration of proteomic and genetic approaches to assess developmental muscle atrophy

Muscle atrophy, or a decline in muscle protein mass, is a significant problem in the aging population and in numerous disease states. Unraveling molecular signals that trigger and promote atrophy may lead to a better understanding of treatment options; however, there is no single cause of atrophy identified to date. To gain insight into this problem, we chose to investigate changes in protein profiles during muscle atrophy in Manduca sexta and Drosophila melanogaster. The use of insect models provides an interesting parallel to probe atrophic mechanisms as these organisms undergo a normal developmental atrophy process during the pupal transition stage. Leveraging the inherent advantages of each model organism, we first defined protein signature changes during M. sexta intersegmental muscle (ISM) atrophy and then used genetic approaches to confirm their functional importance in the D. melanogaster dorsal internal oblique muscles (DIOMs). Our data reveal an upregulation of proteasome and peptidase components and a general downregulation of proteins that regulate actin filament formation. Surprisingly, thick filament proteins that comprise the A-band are increased in abundance, providing support for the ordered destruction of myofibrillar components during developmental atrophy. We also uncovered the actin filament regulator ciboulot (Cib) as a novel regulator of muscle atrophy. These insights provide a framework towards a better understanding of global changes that occur during atrophy and may eventually lead to therapeutic targets.

Oral intake of rice overexpressing ubiquitin ligase inhibitory pentapeptide prevents atrophy in denervated skeletal muscle

We previously reported that intramuscular injections of ubiquitin ligase CBLB inhibitory pentapeptide (Cblin; Asp-Gly-pTyr-Met-Pro) restored lost muscle mass caused by sciatic denervation. Here, we detected Cblin on the basolateral side of Caco-2 cells after being placed on the apical side, and found that cytochalasin D, a tight junction opener, enhanced Cblin transport. Orally administered Cblin was found in rat plasma, indicating that intact Cblin was absorbed in vitro and in vivo. Furthermore, transgenic Cblin peptide-enriched rice (CbR) prevented the denervation-induced loss of muscle mass and the upregulation of muscle atrophy-related ubiquitin ligases in mice. These findings indicated that CbR could serve as an alternative treatment for muscle atrophy.

Development and Clinical Evaluation of Bed with Standing-Up Function

The risk of disuse syndrome caused by prolonged supine posture in hemiplegic stroke in- and outpatients has become a social problem. This study aimed to develop a new bed with a standing-up function, allowing medical caregivers and patients to freely take a standing position on the bed to reduce the amount of time spent in the supine position and to clarify its effectiveness through evaluation of its usability and clinical use. In addition to the Gatch function of the developed bed, it allows transition from a supine position to a chair-sitting or standing position on the bed, and from a standing position to walking action. In addition, as with the tilt table used for standing-position training, the bed’s tilt angle can be adjusted, reducing the load on the lower limbs and allowing appropriate rehabilitation to be carried out anytime, consequently reducing the burden of nursing care. The bed was developed with the cooperation of a specialized bed manufacturer and supported by public funds, and clinical evaluation was conducted after confirming its safety. We evaluated the physical and physiological functions of two hemiplegic patients after 4 weeks of standing training using a prototype bed, to which results from the six-item test showed no significant improvement. However, medical professionals, such as doctors, nurses, and physical therapists, who participated in the clinical evaluation indicated that the bed can safely replace the tilt table for standing-position rehabilitation, and it is effective in eliminating related human and time burdens.

Polyphenols and Their Effects on Muscle Atrophy and Muscle Health

Skeletal muscle atrophy is the decrease in muscle mass and strength caused by reduced protein synthesis/accelerated protein degradation. Various conditions, such as denervation, disuse, aging, chronic diseases, heart disease, obstructive lung disease, diabetes, renal failure, AIDS, sepsis, cancer, and steroidal medications, can cause muscle atrophy. Mechanistically, inflammation, oxidative stress, and mitochondrial dysfunction are among the major contributors to muscle atrophy, by modulating signaling pathways that regulate muscle homeostasis. To prevent muscle catabolism and enhance muscle anabolism, several natural and synthetic compounds have been investigated. Recently, polyphenols (i.e., natural phytochemicals) have received extensive attention regarding their effect on muscle atrophy because of their potent antioxidant and anti-inflammatory properties. Numerous in vitro and in vivo studies have reported polyphenols as strongly effective bioactive molecules that attenuate muscle atrophy and enhance muscle health. This review describes polyphenols as promising bioactive molecules that impede muscle atrophy induced by various proatrophic factors. The effects of each class/subclass of polyphenolic compounds regarding protection against the muscle disorders induced by various pathological/physiological factors are summarized in tabular form and discussed. Although considerable variations in antiatrophic potencies and mechanisms were observed among structurally diverse polyphenolic compounds, they are vital factors to be considered in muscle atrophy prevention strategies.

UBE2L3, a Partner of MuRF1/TRIM63, Is Involved in the Degradation of Myofibrillar Actin and Myosin

The ubiquitin proteasome system (UPS) is the main player of skeletal muscle wasting, a common characteristic of many diseases (cancer, etc.) that negatively impacts treatment and life prognosis. Within the UPS, the E3 ligase MuRF1/TRIM63 targets for degradation several myofibrillar proteins, including the main contractile proteins alpha-actin and myosin heavy chain (MHC). We previously identified five E2 ubiquitin-conjugating enzymes interacting with MuRF1, including UBE2L3/UbcH7, that exhibited a high affinity for MuRF1 (KD = 50 nM). Here, we report a main effect of UBE2L3 on alpha-actin and MHC degradation in catabolic C2C12 myotubes. Consistently UBE2L3 knockdown in Tibialis anterior induced hypertrophy in dexamethasone (Dex)-treated mice, whereas overexpression worsened the muscle atrophy of Dex-treated mice. Using combined interactomic approaches, we also characterized the interactions between MuRF1 and its substrates alpha-actin and MHC and found that MuRF1 preferentially binds to filamentous F-actin (KD = 46.7 nM) over monomeric G-actin (KD = 450 nM). By contrast with actin that did not alter MuRF1-UBE2L3 affinity, binding of MHC to MuRF1 (KD = 8 nM) impeded UBE2L3 binding, suggesting that differential interactions prevail with MuRF1 depending on both the substrate and the E2. Our data suggest that UBE2L3 regulates contractile proteins levels and skeletal muscle atrophy.

Natural Compounds Attenuate Denervation-Induced Skeletal Muscle Atrophy

The weight of skeletal muscle accounts for approximately 40% of the whole weight in a healthy individual, and the normal metabolism and motor function of the muscle are indispensable for healthy life. In addition, the skeletal muscle of the maxillofacial region plays an important role not only in eating and swallowing, but also in communication, such as facial expressions and conversations. In recent years, skeletal muscle atrophy has received worldwide attention as a serious health problem. However, the mechanism of skeletal muscle atrophy that has been clarified at present is insufficient, and a therapeutic method against skeletal muscle atrophy has not been established. This review provides views on the importance of skeletal muscle in the maxillofacial region and explains the differences between skeletal muscles in the maxillofacial region and other regions. We summarize the findings to change in gene expression in muscle remodeling and emphasize the advantages and disadvantages of denervation-induced skeletal muscle atrophy model. Finally, we discuss the newly discovered beneficial effects of natural compounds on skeletal muscle atrophy.

Recent advances in measuring and understanding the regulation of exercise-mediated protein degradation in skeletal muscle

Skeletal muscle protein turnover plays a crucial role in controlling muscle mass and protein quality control, including sarcomeric (structural and contractile) proteins. Protein turnover is a dynamic and continual process of protein synthesis and degradation. The ubiquitin proteasome system (UPS) is a key degradative system for protein degradation and protein quality control in skeletal muscle. UPS-mediated protein quality control is known to be impaired in aging and diseases. Exercise is a well-recognized, nonpharmacological approach to promote muscle protein turnover rates. Over the past decades, we have acquired substantial knowledge of molecular mechanisms of muscle protein synthesis after exercise. However, there have been considerable gaps in the mechanisms of how muscle protein degradation is regulated at the molecular level. The main challenge to understand muscle protein degradation is due in part to the lack of solid stable isotope tracer methodology to measure muscle protein degradation rate. Understanding the mechanisms of UPS with the concomitant measurement of protein degradation rate in skeletal muscle will help identify novel therapeutic strategies to ameliorate impaired protein turnover and protein quality control in aging and diseases. Thus, the goal of this present review was to highlight how recent advances in the field may help improve our understanding of exercise-mediated protein degradation. We discuss 1) the emerging roles of protein phosphorylation and ubiquitylation modifications in regulating proteasome-mediated protein degradation after exercise and 2) methodological advances to measure in vivo myofibrillar protein degradation rate using stable isotope tracer methods.

Findings from recent studies by the Japan Aerospace Exploration Agency examining musculoskeletal atrophy in space and on Earth

The musculoskeletal system provides the body with correct posture, support, stability, and mobility. It is composed of the bones, muscles, cartilage, tendons, ligaments, joints, and other connective tissues. Without effective countermeasures, prolonged spaceflight under microgravity results in marked muscle and bone atrophy. The molecular and physiological mechanisms of this atrophy under unloaded conditions are gradually being revealed through spaceflight experiments conducted by the Japan Aerospace Exploration Agency using a variety of model organisms, including both aquatic and terrestrial animals, and terrestrial experiments conducted under the Living in Space project of the Japan Ministry of Education, Culture, Sports, Science, and Technology. Increasing our knowledge in this field will lead not only to an understanding of how to prevent muscle and bone atrophy in humans undergoing long-term space voyages but also to an understanding of countermeasures against age-related locomotive syndrome in the elderly.

Functional rice with tandemly repeated Cbl-b ubiquitin ligase inhibitory pentapeptide prevents denervation-induced muscle atrophy in vivo

Ubiquitin ligase Casitas B-lineage lymphoma-b (Cbl-b) play a critical role in nonloading-mediated skeletal muscle atrophy: Cbl-b ubiquitinates insulin receptor substrate-1 (IRS-1), leading to its degradation and a resulting loss in muscle mass. We reported that intramuscular injection of a pentapeptide, DGpYMP, which acts as a mimic of the phosphorylation site in IRS-1, significantly inhibited denervation-induced skeletal muscle loss. In order to explore the possibility of the prevention of muscle atrophy by diet therapy, we examined the effects of oral administration of transgenic rice containing Cblin (Cbl-b inhibitor) peptide (DGYMP) on denervation-induced muscle mass loss in frogs. We generated transgenic rice seeds in which 15 repeats of Cblin peptides with a WQ spacer were inserted into the rice storage protein glutelin. A diet of the transgenic rice seeds had significant inhibitory effects on denervation-induced atrophy of the leg skeletal muscles in frogs, compared with those receiving a diet of wild-type rice.

Morin attenuates dexamethasone-mediated oxidative stress and atrophy in mouse C2C12 skeletal myotubes

Glucocorticoids are the drugs most commonly used to manage inflammatory diseases. However, they are prone to inducing muscle atrophy by increasing muscle proteolysis and decreasing protein synthesis. Various studies have demonstrated that antioxidants can mitigate glucocorticoid-induced skeletal muscle atrophy. Here, we investigated the effect of a potent antioxidative natural flavonoid, morin, on the muscle atrophy and oxidative stress induced by dexamethasone (Dex) using mouse C2C12 skeletal myotubes. Dex (10 μM) enhanced the production of reactive oxygen species (ROS) in C2C12 myotubes via glucocorticoid receptor. Moreover, Dex administration reduced the diameter and expression levels of the myosin heavy chain protein in C2C12 myotubes, together with the upregulation of muscle atrophy-associated ubiquitin ligases, such as muscle atrophy F-box protein 1/atrogin-1, muscle ring finger protein-1, and casitas B-lineage lymphoma proto-oncogene-b. Dex also significantly decreased phosphorylated Foxo3a and increased total Foxo3a expression. Interestingly, Dex-induced ROS accumulation and Foxo3a expression were inhibited by morin (10 μM) pretreatment. Morin also prevented the Dex-induced reduction of myotube thickness, together with muscle protein degradation and suppression of the upregulation of atrophy-associated ubiquitin ligases. In conclusion, our results suggest that morin effectively prevents glucocorticoid-induced muscle atrophy by reducing oxidative stress.

Ubiquitin Ligases at the Heart of Skeletal Muscle Atrophy Control

Skeletal muscle loss is a detrimental side-effect of numerous chronic diseases that dramatically increases mortality and morbidity. The alteration of protein homeostasis is generally due to increased protein breakdown while, protein synthesis may also be down-regulated. The ubiquitin proteasome system (UPS) is a master regulator of skeletal muscle that impacts muscle contractile properties and metabolism through multiple levers like signaling pathways, contractile apparatus degradation, etc. Among the different actors of the UPS, the E3 ubiquitin ligases specifically target key proteins for either degradation or activity modulation, thus controlling both pro-anabolic or pro-catabolic factors. The atrogenes MuRF1/TRIM63 and MAFbx/Atrogin-1 encode for key E3 ligases that target contractile proteins and key actors of protein synthesis respectively. However, several other E3 ligases are involved upstream in the atrophy program, from signal transduction control to modulation of energy balance. Controlling E3 ligases activity is thus a tempting approach for preserving muscle mass. While indirect modulation of E3 ligases may prove beneficial in some situations of muscle atrophy, some drugs directly inhibiting their activity have started to appear. This review summarizes the main signaling pathways involved in muscle atrophy and the E3 ligases implicated, but also the molecules potentially usable for future therapies.

Master Regulators of Muscle Atrophy: Role of Costamere Components

The loss of muscle mass and force characterizes muscle atrophy in several different conditions, which share the expression of atrogenes and the activation of their transcriptional regulators. However, attempts to antagonize muscle atrophy development in different experimental contexts by targeting contributors to the atrogene pathway showed partial effects in most cases. Other master regulators might independently contribute to muscle atrophy, as suggested by our recent evidence about the co-requirement of the muscle-specific chaperone protein melusin to inhibit unloading muscle atrophy development. Furthermore, melusin and other muscle mass regulators, such as nNOS, belong to costameres, the macromolecular complexes that connect sarcolemma to myofibrils and to the extracellular matrix, in correspondence with specific sarcomeric sites. Costameres sense a mechanical load and transduce it both as lateral force and biochemical signals. Recent evidence further broadens this classic view, by revealing the crucial participation of costameres in a sarcolemmal “signaling hub” integrating mechanical and humoral stimuli, where mechanical signals are coupled with insulin and/or insulin-like growth factor stimulation to regulate muscle mass. Therefore, this review aims to enucleate available evidence concerning the early involvement of costamere components and additional putative master regulators in the development of major types of muscle atrophy.

The E3 ligase TRAF4 promotes IGF signaling by mediating atypical ubiquitination of IRS-1

Insulin-like growth factor (IGF) is a potent mitogen that activates the IGF receptor (IGFR)/insulin receptor substrate (IRS) axis, thus stimulating growth in normal cells and uncontrolled cell proliferation in cancer. Posttranslational modifications of IRS such as ubiquitination tightly control IGF signaling, and we previously identified IRS-1 as a potential substrate for the E3 ubiquitin ligase TRAF4 using an unbiased screen. Here we provide evidence that TRAF4-mediated ubiquitination of IRS-1 is physiologically relevant and crucial for IGF signal transduction. Through site-directed mutagenesis we found that TRAF4 promotes an atypical K29-linked ubiquitination at the C-terminal end of IRS-1. Its depletion abolishes AKT and ERK phosphorylation downstream of IGF-1 and inhibits breast cancer cell proliferation. Overexpression of TRAF4 enhances IGF1-induced IGFR-IRS-1 interaction, IRS-1 tyrosine phosphorylation, and downstream effector protein activation, whereas mutation of IRS-1 ubiquitination sites completely abolishes these effects. Altogether, our studies demonstrate that nonproteolytic ubiquitination of IRS-1 is a key step in conveying IGF-1 stimulation from IGFR to IRS-1.

ALDH2 mutation promotes skeletal muscle atrophy in mice via accumulation of oxidative stress

Muscle atrophy is promoted by various factors including aging, immobilization, unloading and use of drugs such as steroids. However, genetic risk factors for muscle atrophy are less well known. Here, we show that a missense SNP in the ALDH2 gene, rs671 (ALDH2*2), a dominant negative mutation, promotes significant muscle atrophy in the ALDH2*2 mouse model, accompanied by decreased expression of anabolic and catabolic muscle factors and acquisition of a low turnover state. We also demonstrate that expression of LC3, which is require for auto-phagosome formation during autophagy, increases in ALDH2*2 mouse muscles. We show that 4-hydroxynonenal (4HNE), a peroxidated lipid-protein and oxidant, accumulates in ALDH2*2 mouse muscles. We have shown that the rs671 mutation is associated with increased serum levels of acetaldehyde, an alcohol metabolite. We show that expression of the atrogenes Atrogin1 and MuRF1 significantly increased in myogenic cells following acetaldehyde treatment, an outcome significantly inhibited in vitro by Trolox C, an anti-oxidant. Muscle atrophy in ALDH2*2 mice was also significantly rescued by dietary administration of the anti-oxidant vitamin E, which blocked 4HNE accumulation in muscle. Taken together, our data indicate that rs671 is a genetic risk factor for muscle atrophy, but that such atrophy can be rescued by vitamin E treatment.

The ubiquitin-proteasome system in regulation of the skeletal muscle homeostasis and atrophy: from basic science to disorders

Skeletal muscle is one of the most abundant and highly plastic tissues. The ubiquitin-proteasome system (UPS) is recognised as a major intracellular protein degradation system, and its function is important for muscle homeostasis and health. Although UPS plays an essential role in protein degradation during muscle atrophy, leading to the loss of muscle mass and strength, its deficit negatively impacts muscle homeostasis and leads to the occurrence of several pathological phenotypes. A growing number of studies have linked UPS impairment not only to matured muscle fibre degeneration and weakness, but also to muscle stem cells and deficiency in regeneration. Emerging evidence suggests possible links between abnormal UPS regulation and several types of muscle diseases. Therefore, understanding of the role of UPS in skeletal muscle may provide novel therapeutic insights to counteract muscle wasting, and various muscle diseases. In this review, we focussed on the role of proteasomes in skeletal muscle and its regeneration, including a brief explanation of the structure of proteasomes. In addition, we summarised the recent findings on several diseases and elaborated on how the UPS is related to their pathological states.

MasterPATH: network analysis of functional genomics screening data

Background

Functional genomics employs several experimental approaches to investigate gene functions. High-throughput techniques, such as loss-of-function screening and transcriptome profiling, allow to identify lists of genes potentially involved in biological processes of interest (so called hit list). Several computational methods exist to analyze and interpret such lists, the most widespread of which aim either at investigating of significantly enriched biological processes, or at extracting significantly represented subnetworks.

Results

Here we propose a novel network analysis method and corresponding computational software that employs the shortest path approach and centrality measure to discover members of molecular pathways leading to the studied phenotype, based on functional genomics screening data. The method works on integrated interactomes that consist of both directed and undirected networks – HIPPIE, SIGNOR, SignaLink, TFactS, KEGG, TransmiR, miRTarBase. The method finds nodes and short simple paths with significant high centrality in subnetworks induced by the hit genes and by so-called final implementers – the genes that are involved in molecular events responsible for final phenotypic realization of the biological processes of interest. We present the application of the method to the data from miRNA loss-of-function screen and transcriptome profiling of terminal human muscle differentiation process and to the gene loss-of-function screen exploring the genes that regulates human oxidative DNA damage recognition. The analysis highlighted the possible role of several known myogenesis regulatory miRNAs (miR-1, miR-125b, miR-216a) and their targets (AR, NR3C1, ARRB1, ITSN1, VAV3, TDGF1), as well as linked two major regulatory molecules of skeletal myogenesis, MYOD and SMAD3, to their previously known muscle-related targets (TGFB1, CDC42, CTCF) and also to a number of proteins such as C-KIT that have not been previously studied in the context of muscle differentiation. The analysis also showed the role of the interaction between H3 and SETDB1 proteins for oxidative DNA damage recognition.

Conclusion

The current work provides a systematic methodology to discover members of molecular pathways in integrated networks using functional genomics screening data. It also offers a valuable instrument to explain the appearance of a set of genes, previously not associated with the process of interest, in the hit list of each particular functional genomics screening.

The Insulin Receptor Adaptor IRS2 is an APC/C Substrate That Promotes Cell Cycle Protein Expression and a Robust Spindle Assembly Checkpoint

Insulin receptor substrate 2 (IRS2) is an essential adaptor that mediates signaling downstream of the insulin receptor and other receptor tyrosine kinases. Transduction through IRS2-dependent pathways is important for coordinating metabolic homeostasis, and dysregulation of IRS2 causes systemic insulin signaling defects. Despite the importance of maintaining proper IRS2 abundance, little is known about what factors mediate its protein stability. We conducted an unbiased proteomic screen to uncover novel substrates of the Anaphase Promoting Complex/Cyclosome (APC/C), a ubiquitin ligase that controls the abundance of key cell cycle regulators. We found that IRS2 levels are regulated by APC/C activity and that IRS2 is a direct APC/C target in G1 Consistent with the APC/C's role in degrading cell cycle regulators, quantitative proteomic analysis of IRS2-null cells revealed a deficiency in proteins involved in cell cycle progression. We further show that cells lacking IRS2 display a weakened spindle assembly checkpoint in cells treated with microtubule inhibitors. Together, these findings reveal a new pathway for IRS2 turnover and indicate that IRS2 is a component of the cell cycle control system in addition to acting as an essential metabolic regulator.

Vitamin D protects against immobilization-induced muscle atrophy via neural crest-derived cells in mice

Vitamin D deficiency is a recognized risk factor for sarcopenia development, but mechanisms underlying this outcome are unclear. Here, we show that low vitamin D status worsens immobilization-induced muscle atrophy in mice. Mice globally lacking vitamin D receptor (VDR) exhibited more severe muscle atrophy following limb immobilization than controls. Moreover, immobilization-induced muscle atrophy was worse in neural crest-specific than in skeletal muscle-specific VDR-deficient mice. Tnfα expression was significantly higher in immobilized muscle of VDR-deficient relative to control mice, and was significantly elevated in neural crest-specific but not muscle-specific VDR-deficient mice. Furthermore, muscle atrophy induced by limb immobilization in low vitamin D mice was significantly inhibited in Tnfα-deficient mice. We conclude that vitamin D antagonizes immobilization-induced muscle atrophy via VDR expressed in neural crest-derived cells.

How Postural Muscle Senses Disuse? Early Signs and Signals

A mammalian soleus muscle along with other "axial" muscles ensures the stability of the body under the Earth's gravity. In rat experiments with hindlimb suspension, zero-gravity parabolic flights as well as in human dry immersion studies, a dramatic decrease in the electromyographic (EMG) activity of the soleus muscle has been repeatedly shown. Most of the motor units of the soleus muscle convert from a state of activity to a state of rest which is longer than under natural conditions. And the state of rest gradually converts to the state of disuse. This review addresses a number of metabolic events that characterize the earliest stage of the cessation of the soleus muscle contractile activity. One to three days of mechanical unloading are accompanied by energy-dependent dephosphorylation of AMPK, accumulation of the reactive oxygen species, as well as accumulation of resting myoplasmic calcium. In this transition period, a rapid rearrangement of the various signaling pathways occurs, which, primarily, results in a decrease in the rate of protein synthesis (primarily via inhibition of ribosomal biogenesis and activation of endogenous inhibitors of mRNA translation, such as GSK3β) and an increase in proteolysis (via upregulation of muscle-specific E3-ubiquitin ligases).

Ubiquitin ligase Cbl-b and inhibitory Cblin peptides

This review focuses on the Cbl-b muscle atrophy-associated ubiquitin ligase and its inhibitors. Herein, the role of E3 ubiquitin ligase-associated muscle atrophy genes (atrogenes), including MAFbx-1/agrogin-1 and MuRF-1, as well as another ubiquitin ligase, Cbl-b and its inhibitors, is discussed. Cbl-b plays an important role in unloading muscle atrophy caused by spaceflight and in bedridden patients: Cbl-b ubiquitinated and induced the degradation of IRS-1, a key intermediate in the IGF-1 signaling. Furthermore, a pentapetpide (DGpYMP), inhibited Cbl-b-mediated IRS-1 ubiquitination. This peptide-based Cbl-b inhibitor Cblin and its homologous peptides in foods presumably affect muscle atrophy under such conditions.

Local myostatin inhibition improves skeletal muscle glucose uptake in insulin-resistant high-fat diet-fed mice

Myostatin inhibition is thought to improve whole body insulin sensitivity and mitigate the development of insulin resistance in models of obesity. However, although myostatin is known to be a major regulator of skeletal muscle mass, the direct effects of myostatin inhibition in muscle on glucose uptake and the mechanisms that may underlie this are still unclear. We investigated the effect of local myostatin inhibition by adeno-associated virus-mediated overexpression of the myostatin propeptide on insulin-stimulated skeletal muscle glucose disposal in chow-fed or high fat diet-fed mice and evaluated the molecular pathways that might mediate this. We found that myostatin inhibition improved glucose disposal in obese high fat diet-fed mice alongside the induction of muscle hypertrophy but did not have an impact in chow-fed mice. This improvement was not associated with greater glucose transporter or peroxisome proliferator-activated receptor-γ coactivator-1α expression or 5' AMP-activated protein kinase activation as previously suggested. Instead, transcriptomic analysis suggested that the improvement in glucose disposal was associated with significant enrichment in genes involved in fatty acid metabolism and translation of mitochondrial genes. Thus, myostatin inhibition improves muscle insulin-stimulated glucose disposal in obese high fat diet-fed mice independent of muscle hypertrophy, potentially involving previously unidentified pathways.

Transcriptional changes in muscle of hibernating arctic ground squirrels (Urocitellus parryii): implications for attenuation of disuse muscle atrophy

Physical inactivity generates muscle atrophy in most mammalian species. In contrast, hibernating mammals demonstrate limited muscle loss over the prolonged intervals of immobility during winter, which suggests that they have adaptive mechanisms to reduce disuse muscle atrophy. To identify transcriptional programs that underlie molecular mechanisms attenuating muscle loss, we conducted a large-scale gene expression profiling in quadriceps muscle of arctic ground squirrels, comparing hibernating (late in a torpor and during torpor re-entry after arousal) and summer active animals using next generation sequencing of the transcriptome. Gene set enrichment analysis showed a coordinated up-regulation of genes involved in all stages of protein biosynthesis and ribosome biogenesis during both stages of hibernation that suggests induction of translation during interbout arousals. Elevated proportion of down-regulated genes involved in apoptosis, NFKB signaling as well as significant under expression of atrogenes, upstream regulators (FOXO1, FOXO3, NFKB1A), key components of the ubiquitin proteasome pathway (FBXO32, TRIM63, CBLB), and overexpression of PPARGC1B inhibiting proteolysis imply suppression of protein degradation in muscle during arousals. The induction of protein biosynthesis and decrease in protein catabolism likely contribute to the attenuation of disuse muscle atrophy through prolonged periods of immobility of hibernation.

Loss of melusin is a novel, neuronal NO synthase/FoxO3-independent master switch of unloading-induced muscle atrophy

BACKGROUND

Unloading/disuse induces skeletal muscle atrophy in bedridden patients and aged people, who cannot prevent it by means of exercise. Because interventions against known atrophy initiators, such as oxidative stress and neuronal NO synthase (nNOS) redistribution, are only partially effective, we investigated the involvement of melusin, a muscle-specific integrin-associated protein and a recognized regulator of protein kinases and mechanotransduction in cardiomyocytes.

METHODS

Muscle atrophy was induced in the rat soleus by tail suspension and in the human vastus lateralis by bed rest. Melusin expression was investigated at the protein and transcript level and after treatment of tail-suspended rats with atrophy initiator inhibitors. Myofiber size, sarcolemmal nNOS activity, FoxO3 myonuclear localization, and myofiber carbonylation of the unloaded rat soleus were studied after in vivo melusin replacement by cDNA electroporation, and muscle force, myofiber size, and atrogene expression after adeno-associated virus infection. In vivo interference of exogenous melusin with dominant-negative kinases and other atrophy attenuators (Grp94 cDNA; 7-nitroindazole) on size of unloaded rat myofibers was also explored.

RESULTS

Unloading/disuse reduced muscle melusin protein levels to about 50%, already after 6 h in the tail-suspended rat (P < 0.001), and to about 35% after 8 day bed rest in humans (P < 0.05). In the unloaded rat, melusin loss occurred despite of the maintenance of β1D integrin levels and was not abolished by treatments inhibiting mitochondrial oxidative stress, or nNOS activity and redistribution. Expression of exogenous melusin by cDNA transfection attenuated atrophy of 7 day unloaded rat myofibers (-31%), compared with controls (-48%, P = 0.001), without hampering the decrease in sarcolemmal nNOS activity and the increase in myonuclear FoxO3 and carbonylated myofibers. Infection with melusin-expressing adeno-associated virus ameliorated contractile properties of 7 day unloaded muscles (P ≤ 0.05) and relieved myofiber atrophy (-33%) by reducing Atrogin-1 and MurF-1 transcripts (P ≤ 0.002), despite of a two-fold increase in FoxO3 protein levels (P = 0.03). Atrophy attenuation by exogenous melusin did not result from rescue of Akt, ERK, or focal adhesion kinase activity, because it persisted after co-transfection with dominant-negative kinase forms (P < 0.01). Conversely, melusin cDNA transfection, combined with 7-nitroindazole treatment or with cDNA transfection of the nNOS-interacting chaperone Grp94, abolished 7 day unloaded myofiber atrophy.

CONCLUSIONS

Disuse/unloading-induced loss of melusin is an early event in muscle atrophy which occurs independently from mitochondrial oxidative stress, nNOS redistribution, and FoxO3 activation. Only preservation of melusin levels and sarcolemmal nNOS localization fully prevented muscle mass loss, demonstrating that both of them act as independent, but complementary, master switches of muscle disuse atrophy.

Stat3 activation induces insulin resistance via a muscle-specific E3 ubiquitin ligase Fbxo40

Cellular mechanisms causing insulin resistance (IR) in chronic kidney disease (CKD) are poorly understood. One potential mechanism is that CKD-induced inflammation activates the signal transducer and activator of transcription 3 (Stat3) in muscle. We uncovered increased p-Stat3 in muscles of mice with CKD or mice fed high-fat diet (HFD). Activated Stat3 stimulates the expression of Fbxo40, a muscle-specific E3 ubiquitin ligase that stimulates ubiquitin conjugation leading to degradation of insulin receptor substrate 1 (IRS1). Evidence that Stat3 activates Fbxo40 includes 1) potential Stat3 binding sites in Fbxo40 promoters; 2) Stat3 binding to the Fbxo40 promoter; and 3) constitutively active Stat3 stimulating both Fbxo40 expression and its promoter activity. We found that IL-6 activates Stat3 in myotubes, increasing Fbxo40 expression with reduced IRS1 and p-Akt. Knockdown Fbxo40 using siRNA from myotubes results in higher levels of IRS1 and p-Akt despite the presence of IL-6. We treated mice with a small-molecule inhibitor of Stat3 (TTI-101) and found improved glucose tolerance and insulin signaling in skeletal muscles of mice with CKD or fed an HFD. Finally, we uncovered improved glucose tolerance in mice with muscle-specific Stat3 KO versus results in Stat3^f/f mice in response to the HFD. Thus Stat3 activation in muscle increases IR in mice. Inhibition of Stat3 by TTI-101 could be developed into clinical strategies to improve muscle insulin signaling in inflammation and other catabolic diseases.

Advances in cancer cachexia: Intersection between affected organs, mediators, and pharmacological interventions

Advanced cancer patients exhibit cachexia, a condition characterized by a significant reduction in the body weight predominantly from loss of skeletal muscle and adipose tissue. Cachexia is one of the major causes of morbidity and mortality in cancer patients. Decreased food intake and multi-organ energy imbalance in cancer patients worsen the cachexia syndrome. Cachectic cancer patients have a low tolerance for chemo- and radiation therapies and also have a reduced quality of life. The presence of tumors and the current treatment options for cancer further exacerbate the cachexia condition, which remains an unmet medical need. The onset of cachexia involves crosstalk between different organs leading to muscle wasting. Recent advancements in understanding the molecular mechanisms of skeletal muscle atrophy/hypertrophy and adipose tissue wasting/browning provide a platform for the development of new targeted therapies. Therefore, a better understanding of this multifactorial disorder will help to improve the quality of life of cachectic patients. In this review, we summarize the metabolic mediators of cachexia, their molecular functions, affected organs especially with respect to muscle atrophy and adipose browning and then discuss advanced therapeutic approaches to cancer cachexia.

Sarcopenia in Chronic Kidney Disease: Factors, Mechanisms, and Therapeutic Interventions

Chronic kidney disease (CKD), a chronic catabolic condition, is characterized by muscle wasting and decreased muscle endurance. Many insights into the molecular mechanisms of muscle wasting in CKD have been obtained. A persistent imbalance between protein degradation and synthesis in muscle causes muscle wasting. During muscle wasting, high levels of reactive oxygen species (ROS) and inflammatory cytokines are detected in muscle. These increased ROS and inflammatory cytokine levels induce the expression of myostatin. The myostatin binding to its receptor activin A receptor type IIB stimulates the expression of atrogenes such as atrogin-1 and muscle ring factor 1, members of the muscle-specific ubiquitin ligase family. Impaired mitochondrial function also contributes to reducing muscle endurance. The increased protein-bound uremic toxin, parathyroid hormone, glucocorticoid, and angiotensin II levels that are observed in CKD all have a negative effect on muscle mass and endurance. Among the protein-bound uremic toxins, indoxyl sulfate, an indole-containing compound has the potential to induce muscle atrophy by stimulating ROS-mediated myostatin and atrogenes expression. Indoxyl sulfate also impairs mitochondrial function. Some potential therapeutic approaches based on the muscle wasting mechanisms in CKD are currently in the testing stages.

Cbl Proto-Oncogene B (CBLB) c.197A>T Mutation Induces Mild Metabolic Dysfunction in Partial Type I Multiple Symmetric Lipomatosis (MSL)

BACKGROUND

Multiple symmetric lipomatosis (MSL) is a rare disease showing chronic progression of multiple, symmetrical, and non-encapsulated subcutaneous lipoma. The cause of the disease remains unknown.

PATIENTS AND METHODS

This study reported and summarized 13 sporadic cases of Type I MSL patients in terms of histopathology and cellular and molecular biology and assessed the CBLB c.197A>T mutation in the IRS1-PI3K-Akt pathway.

RESULTS

The clinical data showed that these 13 Type I patients were all male with a mean age of 57.0 ± 6.6 years old and consumed alcohol heavily. The laboratory tests revealed that most of the patients had hyperuricemia, diabetes, hyperinsulinemia, or insulin resistance; however, their blood lipid levels were close to a normal range. The imaging data exhibited lipomas that only occurred subcutaneously but not viscerally, ie, Types Ia (15.4%), Ib (30.8%), and Ic (53.8%). The molecular analyses of adipocytes of isoprenaline stimulated human adipose tissue-derived mesenchymal stromal cells (hADSCs) isolated from the adipose tissue lipoma-like masses (ATLLM) demonstrated that these adipocytes did not express UCP-1. The Cbl proto-oncogene B (CBLB), an E3 ubiquitin-protein ligase, was associated with insulin resistance and obesity and was mutated (ie, CBLB c.197A>T) in four MSL patients after the whole genome and Sanger sequencing of the blood samples. Furthermore, the CBLB c.197A>T mutation induced hADSC resistance to insulin by inactivation of the IRS-1-PI3K-AKT pathway.

CONCLUSION

This study analyzed clinical, histopathological, and cellular and molecular biological characterizations of 13 Type I MSL patients and identified the CBLB c.197A>T heterozygous mutation that could be responsible for MSL metabolic dysfunction or even MSL development.