Endoplasmic Reticulum Stress and Autophagy Markers in Soleus Muscle Disuse-Induced Atrophy of Rats Treated with Fish Oil

Endoplasmic reticulum stress and unfolded protein response: Roles in skeletal muscle atrophy

Skeletal muscle atrophy is commonly present in various pathological states, posing a huge burden on society and patients. Increased protein hydrolysis, decreased protein synthesis, inflammatory response, oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress (ERS) and unfolded protein response (UPR) are all important molecular mechanisms involved in the occurrence and development of skeletal muscle atrophy. The potential mechanisms of ERS and UPR in skeletal muscle atrophy are extremely complex and have not yet been fully elucidated. This article elucidates the molecular mechanisms of ERS and UPR, and discusses their effects on different types of muscle atrophy (muscle atrophy caused by disuse, cachexia, chronic kidney disease (CKD), diabetes mellitus (DM), amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal and bulbar muscular atrophy (SBMA), aging, sarcopenia, obesity, and starvation), and explores the preventive and therapeutic strategies targeting ERS and UPR in skeletal muscle atrophy, including inhibitor therapy and drug therapy. This review aims to emphasize the importance of endoplasmic reticulum (ER) in maintaining skeletal muscle homeostasis, which helps us further understand the molecular mechanisms of skeletal muscle atrophy and provides new ideas and insights for the development of effective therapeutic drugs and preventive measures for skeletal muscle atrophy.

Polystyrene nanoparticles with different particle sizes cause autophagy by ROS/ERS/FOXO1 axis in the Cyprinus carpio kidney affecting immunological function

Microplastic pollution poses challenges for ecosystems worldwide, and nanoplastics (NPs, 1-1000 nm) have been identified as persistent pollutants. However, although some studies have described the hazards of NPs to aquatic organisms, the toxicological processes of NPs in the common carp kidney and the biotoxicity of differently sized NPs remain unclear. In this study, we used juvenile common carp as an in vivo model that were constantly exposed to freshwater at 1000 μg/L polystyrene nanoparticle (PSNP) concentrations (50, 100, and 400 nm) for 28 days. Simultaneously, we constructed an in vitro model utilizing grass fish kidney cells (CIK) to study the toxicological effects of PSNPs of various sizes. We performed RT-PCR and Western blot assays on the genes involved in FOXO1, HMGB1, HIF-1α, endoplasmic reticulum stress, autophagy, and immunoreaction. According to these results, exposure to PSNPs increased reactive oxygen species (ROS) levels, and the carp kidneys experienced endoplasmic reticulum stress. Additionally, PSNPs promoted renal autophagy by activating the ROS/ERS/FOXO1 (ERS: endoplasmic reticulum stress) pathway, and it affected immunological function by stimulating the ROS/HMGB1/HIF-1α signaling pathway. This study provides new insights into the contamination hazards of NPs in freshwater environments, as well as the harm they pose to the human living environments. The relationship between particle size and the degree of damage caused by PSNPs to organisms is a potential future research direction.

Mitigating skeletal muscle wasting in unloading and augmenting subsequent recovery

Skeletal muscle wasting is the hallmark pathophysiological adaptation to unloading or disuse that demonstrates the dependency on frequent mechanical stimulation (e.g. muscle activation and subsequent loading) for homeostasis of normally load-bearing muscles. In the absence of mitigation strategies, no mammalian organism is resistant to muscle atrophy driven by unloading. Given the profound impact of unloading-induced muscle wasting on physical capacity, metabolic health and immune function; mitigation strategies during unloading and/or augmentation approaches during recovery have broad healthcare implications in settings of bed-bound hospitalization, cast immobilization and spaceflight. This topical review aims to: (1) provide a succinct, state-of-the-field summary of seminal and recent findings regarding the mechanisms of unloading-induced skeletal muscle wasting; (2) discuss unsuccessful vs. promising mitigation and recovery augmentation strategies; and (3) identify knowledge gaps ripe for future research. We focus on the rapid muscle atrophy driven by relatively short-term mechanical unloading/disuse, which is in many ways mechanistically distinct from both hypermetabolic muscle wasting and denervation-induced muscle atrophy. By restricting this discussion to mechanical unloading during which all components of the nervous system remain intact (e.g. without denervation models), mechanical loading requiring motor and sensory neural circuits in muscle remain viable targets for both mitigation and recovery augmentation. We emphasize findings in humans with comparative discussions of studies in rodents which enable elaboration of key mechanisms. We also discuss what is currently known about the effects of age and sex as biological factors, and both are highlighted as knowledge gaps and novel future directions due to limited research.

Attenuation of diabetes-mediated muscle atrophy in rats by fish oil enriched omega-3 polyunsaturated fatty acids supplementation

Diabetes is associated with an increased risk of muscle wasting/atrophy, which adversely affects quality of life. We hypothesized that long term supplementation of fish oil may have protective effects against sarcopenia or muscle atrophy in streptozotocin (STZ) and high-fat (HF) diet-induced diabetic rat model. Wistar rats at age of 7 weeks were injected with saline or STZ to induce hyperglycemia. After one week, they were fed on a normal control diet or HF diet with/without supplementation of fish oil for 18 weeks. Feeding diabetic rats with a fish oil-enriched diet alleviated body weight loss and the impaired glucose tolerance using OGTT test. Although fish oil did not improve the decreased muscle mass, the muscle atrophy induced by diabetes was attenuated by fish oil in gastrocnemius, soleus, tibialis anterior, and extensor digitorum longus muscles. Fish oil supplementation reversed the decreased expression of phospho (p)-AKT, pmTOR, and p-p70s6k, which are molecules related to protein synthesis. Besides, protein degradation-related signaling pathways were inhibited by fish oil, such as increasing p-FoxO1 and decreasing Atrogin-1 and MURF1 protein expression. Fish oil down-regulated the expression of autophagy-related molecules including ATG5, p62, and LC3B II/I ratio, which may result in less muscle atrophy. Inflammation-related signaling regulators including TNF-α, NF-κB, AGEs, and RAGE were suppressed by fish oil supplementation as well. Moreover, the down-regulated p-AMPKα, SIRT1, and PGC-1 in diabetic rats were counteracted by fish oil, which may improve mitochondrial function and further block FoxO action. These data suggest that long-term fish oil supplementation exerts protective effects against diabetes-induced muscle atrophy, which may in turn ameliorate insulin resistance and impaired glucose tolerance.

Advances in nutritional supplementation for sarcopenia management

Sarcopenia is a syndrome characterized by a decline in muscular mass, strength, and function with advancing age. The risk of falls, fragility, hospitalization, and death is considerably increased in the senior population due to sarcopenia. Although there is no conclusive evidence for drug treatment, resistance training has been unanimously recognized as a first-line treatment for managing sarcopenia, and numerous studies have also pointed to the combination of nutritional supplementation and resistance training as a more effective intervention to improve quality of life for people with sarcopenia. People with both malnutrition and sarcopenia have a higher mortality rate, so identifying people at risk of malnutrition and intervening early is extremely important to avoid sarcopenia and its associated problems. This article provides important information for dietary interventions in sarcopenia by summarizing the discoveries and developments of nutritional supplements such as protein, leucine, β-hydroxy-β-methylbutyric acid, vitamin D, vitamin C, vitamin E, omega-3 fatty acids, creatine, inorganic nitrate, probiotics, minerals, collagen peptides, and polyphenols in the management of sarcopenia.

Endoplasmic reticulum stress promotes sepsis-induced muscle atrophy via activation of STAT3 and Smad3

Endoplasmic reticulum (ER) stress is involved in skeletal muscle atrophy in various conditions, but the role of ER stress in sepsis-induced muscle atrophy is not well understood. In this study, we conducted experiments in wild-type (WT) mice and C/EBP homologous protein knockout (CHOP KO) mice to explore the role and mechanism of ER stress in sepsis-induced muscle atrophy. Cecal ligation and puncture (CLP) was used to establish a mouse model of sepsis. In WT mice, the body weight, muscle mass, and cross-sectional area of muscle fibers in CLP group both decreased significantly compared with sham group, which revealed that sepsis-induced dramatic muscle atrophy. Additionally, sepsis activated the ubiquitin-proteasome system (UPS), accompanied by the activation of ER stress. In vitro, inhibition of ER stress suppressed the activity of E3 ubiquitin ligases and alleviated the myotube atrophy. In vivo, CHOP KO also reduced the expression of E3 ubiquitin ligases and UPS-mediated protein degradation, and significantly attenuated sepsis-induced muscle atrophy. Deletion of CHOP also decreased the phosphorylation of signal transducer and activator of transcription 3 (STAT3) and Smad3, and inhibition of STAT3 and Smad3 partly reduced proteolysis caused by ER stress in vitro. These findings confirm that ER stress activates UPS-mediated proteolysis and promotes sepsis-induced muscle atrophy, which is partly achieved by activating STAT3 and Smad3.

New Horizons in the Treatment of Age-Associated Obesity, Sarcopenia and Osteoporosis

The rapid increase in both the lifespan and proportion of older adults in developed countries is accompanied by the dramatic growth of age-associated chronic diseases, including obesity, sarcopenia, and osteoporosis. Hence, prevention and treatment of age-associated chronic diseases has become increasingly urgent. The key to achieving this goal is a better understanding of the mechanisms underlying their pathophysiology, some aspects of which, despite extensive investigation, are still not fully understood. Aging, obesity, sarcopenia, and osteoporosis are characterized by the creation of a systemic, chronic, low-grade inflammation (SCLGI). The common mechanisms that govern the development of these chronic conditions include a failed resolution of inflammation. Physiologically, the process of inflammation resolution is provided mainly by specialized pro-resolving mediators (SPMs) acting via cognate G protein-coupled receptors (GPCRs). Noteworthy, SPM levels and the expression of their receptors are significantly reduced in aging and the associated chronic disorders. In preclinical studies, supplementation of SPMs or their stable, small-molecule SPM mimetics and receptor agonists reveals clear beneficial effects in inflammation-related obesity and sarcopenic and osteoporotic conditions, suggesting a translational potential. Age-associated chronic disorders are also characterized by gut dysbiosis and the accumulation of senescent cells in the adipose tissue, skeletal muscle, and bones. Based on these findings, we propose SCLGI resolution as a novel strategy for the prevention/treatment of age-associated obesity, sarcopenia, and osteoporosis. Our approach entails the enhancement of inflammation resolution by SPM mimetics and receptor agonists in concert with probiotics/prebiotics and compounds that eliminate senescent cells and their pro-inflammatory activity.

Low-frequency electrical stimulation alleviates immobilization-evoked disuse muscle atrophy by repressing autophagy in skeletal muscle of rabbits

BACKGROUND

The study aimed to investigate the effect of low-frequency electrical stimulation (LFES) on disuse muscle atrophy and its mechanism in a rabbit model of knee extension contracture.

METHODS

This study involved two experiments. In the time-point experiment, 24 rabbits were randomly divided into 4 groups: Control 1 (Ctrl1 group), immobilization for 2 weeks (I-2 group), immobilization for 4 weeks (I-4 group), and immobilization for 6 weeks (I-6 group). In the intervention experiment, 24 rabbits were randomly divided into 4 groups: Control 2 (Ctrl2 group), electrical stimulation (ESG group), natural recovery (NRG group), and electrical stimulation treatment (ESTG group). All intervention effects were assessed by evaluating the knee joint range of motion (ROM), cross-sectional area (CSA) of the rectus femoris muscle, and expression of autophagy-related proteins.

RESULTS

The time-point experiment showed that immobilization reduced the knee ROM, reduced the rectus femoris muscle CSA, and activated autophagy in skeletal muscle. The levels of five autophagy-related proteins [mammalian target of rapamycin (mTOR), phosphorylated mTOR (p-mTOR), autophagy-related protein 7 (Atg7), p62, and microtubule-associated protein light chain 3B-II (LC3B-II)] were significantly elevated in the skeletal muscle of the I-4 group. The intervention experiment further showed that LFES significantly improved the immobilization-induced reductions in ROM and CSA. Additionally, LFES resulted in a significant decrease in the protein expression of mTOR, p-mTOR, Atg7, p62, and LC3B-II in the rectus femoris muscle.

CONCLUSIONS

LFES alleviates immobilization-evoked disuse muscle atrophy possibly by inhibiting autophagy in the skeletal muscle of rabbits.