Multiple pathways to the same end: mechanisms of myonuclear apoptosis in sarcopenia of aging

Experimental models as a tool for research on sarcopenia: A narrative review

Sarcopenia is a musculoskeletal disorder related to muscle mass and function; as the worldwide population ages, its growing prevalence means a decline in quality of life and an increased burden for public health systems. As sarcopenia is a reversible condition, its early diagnosis is of utmost importance. Consensus definitions and diagnosis protocols for sarcopenia have been evolving for a long time, and the identification of molecular pathways subjacent to sarcopenia is a growing research area. The use of liquid biopsies to identify circulating molecules does not provide information about specific regulatory pathways or biomarkers in relevant tissue, and the use of skeletal muscle biopsies from older people has many limitations. Complementary tools are therefore necessary to advance the knowledge of relevant molecular aspects. The development of experimental models, such as animal, cellular, or bioengineered tissue, together with knock-in or knock-out strategies, could therefore be of great interest. This narrative review will explore experimental models of healthy muscle and aged muscle cells as a tool for research on sarcopenia. We will summarize the literature and present relevant experimental models in terms of their advantages and disadvantages. All of the presented approaches could potentially contribute to the accurate and early diagnosis, follow-up, and possible treatment of sarcopenia.

Molecular and Structural Alterations of Skeletal Muscle Tissue Nuclei during Aging

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. The mechanisms underlying this phenomenon are certainly multifactorial and still remain to be fully elucidated. Changes in the cell nucleus structure and function have been considered among the possible contributing causes. This review offers an overview of the current knowledge on skeletal muscle nuclei in aging, focusing on the impairment of nuclear pathways potentially involved in age-related muscle decline. In skeletal muscle two types of cells are present: fiber cells, constituting the contractile muscle mass and containing hundreds of myonuclei, and the satellite cells, i.e., the myogenic mononuclear stem cells occurring at the periphery of the fibers and responsible for muscle growth and repair. Research conducted on different experimental models and with different methodological approaches demonstrated that both the myonuclei and satellite cell nuclei of aged skeletal muscles undergo several structural and molecular alterations, affecting chromatin organization, gene expression, and transcriptional and post-transcriptional activities. These alterations play a key role in the impairment of muscle fiber homeostasis and regeneration, thus contributing to the age-related decrease in skeletal muscle mass and function.

From Chronodisruption to Sarcopenia: The Therapeutic Potential of Melatonin

Sarcopenia is an age-related condition that involves a progressive decline in muscle mass and function, leading to increased risk of falls, frailty, and mortality. Although the exact mechanisms are not fully understood, aging-related processes like inflammation, oxidative stress, reduced mitochondrial capacity, and cell apoptosis contribute to this decline. Disruption of the circadian system with age may initiate these pathways in skeletal muscle, preceding the onset of sarcopenia. At present, there is no pharmacological treatment for sarcopenia, only resistance exercise and proper nutrition may delay its onset. Melatonin, derived from tryptophan, emerges as an exceptional candidate for treating sarcopenia due to its chronobiotic, antioxidant, and anti-inflammatory properties. Its impact on mitochondria and organelle, where it is synthesized and crucial in aging skeletal muscle, further highlights its potential. In this review, we discuss the influence of clock genes in muscular aging, with special reference to peripheral clock genes in the skeletal muscle, as well as their relationship with melatonin, which is proposed as a potential therapy against sarcopenia.

Vitamin D3 Exerts Beneficial Effects on C2C12 Myotubes through Activation of the Vitamin D Receptor (VDR)/Sirtuins (SIRT)1/3 Axis

The onset of sarcopenia is associated with a decline in vitamin D receptor (VDR) expression, wherein reduced VDR levels contribute to muscle atrophy, while heightened expression promotes muscle hypertrophy. Like VDR, the age-related decline in protein deacetylase sirtuin (SIRT) expression is linked to the development of sarcopenia and age-related muscle dysfunction. This study aimed to investigate whether the VDR agonist 1,25-dihydroxyvitamin D3 (1,25VD3) exerts beneficial effects on muscles through interactions with sirtuins and, if so, the underlying molecular mechanisms. Treatment of 1,25VD3 in differentiating C2C12 myotubes substantially elevated VDR, SIRT1, and SIRT3 expression, enhancing their differentiation. Furthermore, 1,25VD3 significantly enhanced the expression of key myogenic markers, including myosin heavy chain (MyHC) proteins, MyoD, and MyoG, and increased the phosphorylation of AMPK and AKT. Conversely, VDR knockdown resulted in myotube atrophy and reduced SIRT1 and SIRT3 levels. In a muscle-wasting model triggered by IFN-γ/TNF-α in C2C12 myotubes, diminished VDR, SIRT1, and SIRT3 levels led to skeletal muscle atrophy and apoptosis. 1,25VD3 downregulated the increased expression of muscle atrophy-associated proteins, including FoxO3a, MAFbx, and MuRF1 in an IFN-γ/TNF-α induced atrophy model. Importantly, IFN-γ/TNF-α significantly reduced the mtDNA copy number in the C2C12 myotube, whereas the presence of 1,25VD3 effectively prevented this decrease. These results support that 1,25VD3 could serve as a potential preventive or therapeutic agent against age-related muscle atrophy by enhancing the VDR/SIRT1/SIRT3 axis.

Association between handgrip strength and heart failure in adults aged 45 years and older from NHANES 2011-2014

Growing evidence indicates that handgrip strength (HGS) is a conspicuous marker for assessing some diseases affecting middle-aged and elderly individuals. However, research regarding HGS and heart failure (HF) is sparse and controversial. Hence, we aimed to investigate the association between HGS and HF among adults aged 45 years and older in the United States. In this cross-sectional study, we included 4524 adults older than 45 years who were part of the National Health and Nutrition Examination Survey. A generalized additive model was used to estimate the association between HGS and HF. Age, gender, race, income, education, body mass index, smoking status, drinking status, diabetes, hypertension, stroke, vigorous physical activity, total energy intake, total protein intake, total sugars intake, and total fat intake covariates were adjusted using multiple regression models. And further subgroup analysis was conducted. We documented 189 cases of HF, including 106 men and 83 women. HGS was negatively associated with HF after adjusting for all the covariates (odds ratio = 0.97, 95% confidence interval = 0.96-0.99; P < 0.001). Compared with the lowest quintile, the highest quintile was associated with an 82% lower incidence of HF (odds ratio = 0.18, 95% confidence interval = 0.08-0.43; P < 0.001). Subgroup analysis showed that the results remained stable. In US adults older than 45, HGS was negatively associated with HF after adjusting for covariates. This finding had the potential to draw attention to the physiological and pathological effects of decreased muscle function on HF and may influence further prospective studies with intervention trials.

Cardiac valve calcification in patients on maintenance dialysis. The role of malnutrition-inflammation syndrome, adiposity andcomponents of sarcopenia. A cross-sectional study

BACKGROUND & AIMS

Cardiac valve calcification (CVC) is a predictor of cardiovascular disease and all-cause mortality in end stage kidney disease (ESKD) patients. Several risk factors are related to CVC in patients with ESKD including traditional ones as well as inflammation, bone mineral disease and malnutrition. Adiposity is associated with dyslipidemia and proinflammatory activity which could predispose for CVC. Sarcopenia or dynapenia is a state common in patients with ESKD. This study aimed to investigate the relationship of adiposity, sarcopenia and malnutrition-inflammation markers with CVC in patients on maintenance hemodialysis.

METHODS

CVC in aortic (AVC), mitral valves (MVC) and systolic and diastolic dysfunction (DD) were assessed by using two-dimensional echocardiography. Nutritional, adiposity and anthropometric assessments were made using several indices respectively. Creatinine index and muscle strength measurements were also performed. Biochemical parameters such as total proteins, albumin, calcium, phosphate, plasma lipoproteins, C-Reactive Protein and parathyroid hormone were also measured.

RESULTS

Adiposity, nutritional, and sarcopenia parameters did not show any difference between patients with or without CVC. Age ≥ 65 years [PR: 1.47 p = 0.012], DD [PR: 2.31, p = 0.005], high CRP/albumin ratio [PR: 1.46, p = 0.01], mid arm circumference (MAC) < 26 cm [PR: 1.37, p = 0.03] were associated with increased prevalence of AVC, while DD [PR: 1.97 p = 0.02], high CRP/albumin ratio [PR: 1.56, p = 0.02], and MAC < 26 cm [PR: 1.52, p = 0.01], showed positive correlation with MVC. Age ≥ 65 years [PR: 1.33, p = 0.028], DD [PR: 1.72, p = 0.01], high CRP/albumin ratio [PR: 1.53, p = 0.003], and MAC < 26 cm [PR: 1.4, p = 0.006], related to greater prevalence of calcification at any valve.

CONCLUSIONS

Ageing, diastolic dysfunction, MAC and increased CRP/albumin ratio were powerful predictors of CVC in patients on hemodialysis.

Suppression of endoplasmic reticulum stress prevents disuse muscle atrophy in a mouse model of microgravity

BACKGROUND

Hind-limb unloaded (HLU) mouse model exhibits skeletal muscle atrophy and weakness mimicking the conditions such as prolonged spaceflight. However, the molecular mechanisms and interventions of muscle loss during muscle unloading remain elusive. Dysfunction of protein folding by ednoplasmic reticulum (ER), a condition called ER stress, is implicated in diseases of various cell types, but its contribution to skeletal muscle detriment remains elusive. In this study, we investigated the contribution of ER stress to muscle atrophy.

METHODS

Sixteen-week-old c57BL/6j male mice were grouped into ground-based controls and HLU group, which was subsequently injected with injected saline (HLU-sal.) or pan-ER stress inhibitor 4-PBA (100mg/kg/d; HLU- 4PBA) via intraperitoneal injections for three weeks.

RESULTS

Three weeks of HLU resulted in reduction in muscle mass and strength, which were restored with 4PBA injections. We also report myofibers atrophy, myonuclear apoptosis, and aterations in the expressions of genes associated with ER stress, apoptosis, and calcium dysregulation. These findings were reversed by 4-PBA treatment.

CONCLUSION

Altogether, our results indicate that ER stress contributes to muscle atrophy in HLU conditions. We suggest that blocking ER stress may be an effective pharmacological therapy to prevent muscle weakness and atrophy during prolonged muscle unloading.

Bibliometric Analysis of the Knowledge Base and Future Trends on Sarcopenia from 1999–2021

Sarcopenia is characterized by progressive loss of muscle mass and function, and it is becoming a serious public health problem with the aging population. However, a comprehensive overview of the knowledge base and future trends is still lacking. The articles and reviews with “sarcopenia” in their title published from 1999 to 2021 in the SCIE database were retrieved. We used Microsoft Excel, VOSviewer, and CiteSpace to conduct a descriptive and bibliometric analysis. A total of 3582 publications were collected, from 4 published in 2000 increasing dramatically to 850 documents in 2021. The USA was the most productive country, with the most citations. The Catholic University of the Sacred Heart and Landi F were the most influential organization and author in this field, respectively. The core journal in this field was the Journal of Cachexia Sarcopenia and Muscle. According to the analysis of keywords and references, we roughly categorized the main research areas into four domains as follows: 1. Definition and diagnosis; 2. Epidemiology; 3. Etiology and pathogenesis; 4. Treatments. Comparing different diagnostic tools and the epidemiology of sarcopenia in different populations are recent hotspots, while more efforts are needed in the underlying mechanism and developing safe and effective treatments. In conclusion, this study provides comprehensive insights into developments and trends in sarcopenia research that can help researchers and clinicians better manage and implement their work.

Caloric Restriction May Help Delay the Onset of Frailty and Support Frailty Management

Frailty is an age-related clinical syndrome that may increase the risk of falls, disability, hospitalization, and death in older adults. Delaying the progression of frailty helps improve the quality of life in older adults. Caloric restriction (CR) may extend lifespan and reduce the risk of age-related diseases. However, few studies have explored the relationship between CR and frailty. In this review, we focused on the impact of CR on frailty and aimed to identify potential associated mechanisms. Although CR may help prevent frailty, further studies are required to determine the underlying mechanisms and specific CR regimens suitable for use in humans.

Redox Signaling and Sarcopenia: Searching for the Primary Suspect

Sarcopenia, the age-related decline in muscle mass and function, derives from multiple etiological mechanisms. Accumulative research suggests that reactive oxygen species (ROS) generation plays a critical role in the development of this pathophysiological disorder. In this communication, we review the various signaling pathways that control muscle metabolic and functional integrity such as protein turnover, cell death and regeneration, inflammation, organismic damage, and metabolic functions. Although no single pathway can be identified as the most crucial factor that causes sarcopenia, age-associated dysregulation of redox signaling appears to underlie many deteriorations at physiological, subcellular, and molecular levels. Furthermore, discord of mitochondrial homeostasis with aging affects most observed problems and requires our attention. The search for the primary suspect of the fundamental mechanism for sarcopenia will likely take more intense research for the secret of this health hazard to the elderly to be unlocked.

The Relation of Inflammaging With Skeletal Muscle Properties in Elderly Men

Aging is associated with a progressive decline of muscle mass and/or the qualitative impairment of the muscle tissue. There is growing evidence of the prominent role of low-grade chronic inflammation in age-related changes in the neuromuscular system. The purpose of the study was to identify the inflammatory mediators responsible for deficit in functional fitness and to explain whether inflammation is related to changes in body composition and the decline of muscle strength in older men. Thirty-three old-aged males (73.5 ± 6.3 years) and twenty young-aged males (21.2 ± 1.3 years) participated in the study. The body composition (bioelectrical impedance analysis), functional capacity (6-min walking test) and knee extension strength (isokinetic test) were estimated. In serum, circulating inflammatory markers H₂O₂, IL-1β, TNFα, and hsCRP as well as growth factors IGF-I and PDGF^BB concentrations were determined (immunoenzymatic methods). The concentrations of H₂O₂, IL-1β, TNFα, and hsCRP were significantly higher in older than young men. The growth factors IGF-I and PDGF^BB were twofold lower and related to high levels of IL-1β and TNFα in the elderly. The changes in cytokines and growth factors levels were correlated with age and peak torque (TQ at 60°/s and 180°/s) in the knee extension. The result of the 6-min walking test was inversely correlated with fat mass index (FMI, r = −.983; p < .001). The generation of inflammatory mediators in older men was related to changes in body composition, maximum strength muscle, and age-related changes in skeletal muscle properties responsible for deficit in functional fitness.

Ameliorative Effect of Melatonin Against Reproductive Toxicity of Tramadol in Rats via the Regulation of Oxidative Stress, Mitochondrial Dysfunction, and Apoptosis-related Gene Expression Signaling Pathway

BACKGROUND

The aim of the present study was to investigate the protective properties of melatonin (MT) against oxidative stress, mitochondrial dysfunction, and apoptosis induced by tramadol-reproductive toxicity in male rats.

METHODS

The rats were divided into the 7 groups of control, melatonin (1.5 mg/kg), tramadol (50 mg/kg), and melatonin (1, 1.5 and 2.5 mg/kg) administered 30 minutes before tramadol and vitamin C group (100 mg/kg). All injections were performed intraperitoneally. After administration for 3 consecutive weeks, the animals were killed and testis tissues were used for assessment of oxidative stress markers including lipid peroxidation (LPO), glutathione (GSH) content and protein carbonyl (PrC), and sperm analysis. Mitochondria were isolated from rat's testis using differential centrifugation technique and were studied in terms of mitochondrial viability, mitochondrial membrane potential (MMP), and mitochondrial swelling. The other part of the tissue sample was placed in RNA protector solution for assessment of Bax and Bcl-2 gene expression through real-time polymerase chain reaction (real-time PCR) assay.

FINDINGS

Tramadol caused a significant decline in epidermal sperm count, motility, and morphology, as well as a significant decrease in GSH level and mitochondrial function, and a significant evaluation of LPO, PrC, MMP, and mitochondrial swelling. In addition, tramadol induced a significant decrease in Bcl-2 gene expression, and increase in Bax gene expression. However, pretreatment of rats with MT improved sperm analysis, and testicular antioxidative status, and mitochondrial function. Furthermore, MT pretreatment regulated testicular Bcl-2 and Bax expressions.

CONCLUSION

Considering the protective effects of MT against reproductive toxicity induced by tramadol, this compound can be used as a possible agent for the prevention and treatment of tramadol-induced reproductive toxicity.

Sarcopenia and Heart Failure

Modifications of lean mass are a frequent critical determinant in the pathophysiology and progression of heart failure (HF). Sarcopenia may be considered one of the most important causes of low physical performance and reduced cardiorespiratory fitness in older patients with HF. Sarcopenia is frequently misdiagnosed as cachexia. However, muscle wasting in HF has different pathogenetic features in sarcopenic and cachectic conditions. HF may induce sarcopenia through common pathogenetic pathways such as hormonal changes, malnutrition, and physical inactivity; mechanisms that influence each other. In the opposite way, sarcopenia may favor HF development by different mechanisms, including pathological ergoreflex. Paradoxically, sarcopenia is not associated with a sarcopenic cardiac muscle, but the cardiac muscle shows a hypertrophy which seems to be “not-functional.” First-line agents for the treatment of HF, physical activity and nutritional interventions, may offer a therapeutic advantage in sarcopenic patients irrespective of HF. Thus, sarcopenia is highly prevalent in patients with HF, contributing to its poor prognosis, and both conditions could benefit from common treatment strategies based on pharmacological, physical activity, and nutritional approaches.

N-Acetyl Cysteine Attenuates the Sarcopenia and Muscle Apoptosis Induced by Chronic Liver Disease

Background:

Sarcopenia is characterized by the loss of muscle mass and strength (muscle atrophy) because of aging or chronic diseases, such as chronic liver disease (CLD). Different mechanisms are involved in skeletal muscle atrophy, including decreased muscle fibre diameter and myosin heavy chain levels and increased ubiquitin–proteasome pathway activity, oxidative stress and myonuclear apoptosis. We recently found that all these mechanisms, except myonuclear apoptosis, which was not evaluated in the previous study, were involved in muscle atrophy associated with hepatotoxin 5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced CLD.

Objective:

In the present study, we evaluated the involvement of myonuclear apoptosis in CLD-associated sarcopenia and the effect of N-acetyl cysteine (NAC) treatment on muscle strength and apoptosis, using a DDC-supplemented diet-fed mouse model.

Methods:

Four-month-old male C57BL6 mice were fed with a standard or DDCsupplemented diet for six weeks in the absence or presence of NAC treatment.

Results:

Our results showed that NAC attenuated the decrease in muscle fibre diameter and muscle strength associated with CLD-induced muscle wasting in gastrocnemius (GA) muscle of DDC-supplemented diet-fed mice. In addition, in GA muscle of the mice fed with DDC-supplemented diet-induced CLD showed increased myonuclear apoptosis compared with the GA muscle of the control diet-fed mice, as evidenced by increased apoptotic nuclei number, caspase-8 and caspase-9 expression, enzymatic activity of caspase-3 and BAX/BCL-2 ratio. NAC treatment inhibited all the mechanisms associated with myonuclear apoptosis in the GA muscle.

Conclusion:

To our knowledge, this is the first study which reports the redox regulation of muscle strength and myonuclear apoptosis in CLD-induced sarcopenia.

Mitochondria and Aging-The Role of Exercise as a Countermeasure

Mitochondria orchestrate the life and death of most eukaryotic cells by virtue of their ability to supply adenosine triphosphate from aerobic respiration for growth, development, and maintenance of the ‘physiologic reserve’. Although their double-membrane structure and primary role as ‘powerhouses of the cell’ have essentially remained the same for ~2 billion years, they have evolved to regulate other cell functions that contribute to the aging process, such as reactive oxygen species generation, inflammation, senescence, and apoptosis. Biological aging is characterized by buildup of intracellular debris (e.g., oxidative damage, protein aggregates, and lipofuscin), which fuels a ‘vicious cycle’ of cell/DNA danger response activation (CDR and DDR, respectively), chronic inflammation (‘inflammaging’), and progressive cell deterioration. Therapeutic options that coordinately mitigate age-related declines in mitochondria and organelles involved in quality control, repair, and recycling are therefore highly desirable. Rejuvenation by exercise is a non-pharmacological approach that targets all the major hallmarks of aging and extends both health- and lifespan in modern humans.

Exercise Training Attenuates Obesity-Induced Skeletal Muscle Remodeling and Mitochondria-Mediated Apoptosis in the Skeletal Muscle

Obesity is characterized by the induction of skeletal muscle remodeling and mitochondria-mediated apoptosis. Exercise has been reported as a positive regulator of skeletal muscle remodeling and apoptosis. However, the effects of exercise on skeletal muscle remodeling and mitochondria-mediated apoptosis in obese skeletal muscles have not been clearly elucidated. Four-week-old C57BL/6 mice were randomly assigned into four groups: control (CON), control plus exercise (CON + EX), high-fat diet (HFD), and HFD plus exercise groups (HFD + EX). After obesity was induced by 20 weeks of 60% HFD feeding, treadmill exercise was performed for 12 weeks. Exercise ameliorated the obesity-induced increase in extramyocyte space and a decrease in the cross-sectional area of the skeletal muscle. In addition, it protected against increases in mitochondria-mediated apoptosis in obese skeletal muscles. These results suggest that exercise as a protective intervention plays an important role in regulating skeletal muscle structure and apoptosis in obese skeletal muscles.

Influence of Creatine Supplementation on Apoptosis Markers After Downhill Running in Middle-Aged Men: A Crossover Randomized, Double-Blind, and Placebo-Controlled Study

OBJECTIVE

Strenuous exercise can induce apoptosis in a variety of tissues. We investigated the effects of creatine loading on apoptosis markers after downhill running.

DESIGN

Twenty-two middle-aged men were randomly assigned to either a creatine or a placebo group. Crossover design, double-blind controlled supplementation was performed using 20 g/d(-1) of creatine or maltodextrin for 7 days. Downhill running (12% incline) at 70% of heart rate maximum for 40 mins was performed on the eighth day. Blood samples were taken on the day before supplementation, after supplementation and after running.

RESULTS

There were no significant changes in the caspase-3, caspase-9, p53, Bax, and IGF-1 concentrations from presupplementation to postsupplementation in both groups of creatine and placebo (P > 0.05). There were significant increases (P < 0.05) in serum caspase-3, caspase-9, p53, and Bax after running in the placebo group. These markers were not noticeably changed in the creatine group (P > 0.05). Bcl-2 was unchanged in the placebo group but substantially increased (P < 0.05) in the creatine group. No significant changes were observed in IGF-1 concentration after running comparing to prerunning in both groups (P > 0.05). Lactate levels increased similarly in both groups (P < 0.05).

CONCLUSIONS

The findings indicate that creatine supplementation could prevent exercise-induced apoptotic markers.

Changes in Cardiac Levels of Caspase-8, Bcl-2 and NT-proBNP Following 4 Weeks of Aerobic Exercise in Diabetic Rats

Introduction: Cardiac apoptosis is one of the most important cardiovascular complications of diabetes. We aimed to investigate the changes of caspase-8, Bcl-2, and N-terminal pro B-type natriuretic peptide (NT-proBNP) in cardiac tissue after 4 weeks of aerobic exercise in male rats with diabetes. Methods: Forty adult male rats were randomly allocated to healthy control, diabetes, control + exercise and exercise + diabetes groups. Diabetes was induced by intraperitoneal injection of streptozotocin (STZ) solution (55 mg/kg). Two weeks after injection, fasting blood glucose levels were measured. After the induction of diabetes, the exercise program was performed for 4 weeks (5 sessions per week) at a speed of 15 to 18 m/min for 25 to 44 minutes. Forty-eight hours after the last training session, the subjects were anesthetized and the heart muscle was removed. Caspase-8, Bcl-2 and NT-proBNP levels were measured by ELISA method. Results: The induction of diabetes in the control group resulted in a significant increase in caspase-8, and NT-proBNP levels while an insignificant increase was observed for Bcl-2 levels (P<0.05). In non-diabetic groups, exercise caused no changes in caspase-8, NT-proBNP and Bcl-2 (P<0.05). Exercise in diabetic groups significantly decreased NT-proBNP while no changes were observed in caspase-8 and Bcl-2 (P<0.05). Conclusion: Our findings showed that diabetes increases the pro-apoptotic and anti-apoptotic agent. In addition, 4 weeks of regular aerobic exercises can be used as a non-pharmacological strategy to reduce the complications of apoptosis in diabetic cardiomyocytes.

Cellular Senescence in Age-Related Macular Degeneration: Can Autophagy and DNA Damage Response Play a Role?

Age-related macular degeneration (AMD) is the main reason of blindness in developed countries. Aging is the main AMD risk factor. Oxidative stress, inflammation and some genetic factors play a role in AMD pathogenesis. AMD is associated with the degradation of retinal pigment epithelium (RPE) cells, photoreceptors, and choriocapillaris. Lost RPE cells in the central retina can be replaced by their peripheral counterparts. However, if they are senescent, degenerated regions in the macula cannot be regenerated. Oxidative stress, a main factor of AMD pathogenesis, can induce DNA damage response (DDR), autophagy, and cell senescence. Moreover, cell senescence is involved in the pathogenesis of many age-related diseases. Cell senescence is the state of permanent cellular division arrest and concerns only mitotic cells. RPE cells, although quiescent in the retina, can proliferate in vitro. They can also undergo oxidative stress-induced senescence. Therefore, cellular senescence can be considered as an important molecular pathway of AMD pathology, resulting in an inability of the macula to regenerate after degeneration of RPE cells caused by a factor inducing DDR and autophagy. It is too early to speculate about the role of the mutual interplay between cell senescence, autophagy, and DDR, but this subject is worth further studies.

Age-related effect of cell death on fiber morphology and number in tongue muscle

INTRODUCTION

Multiple pathways may exist for age-related tongue muscle degeneration. Cell death is one mechanism contributing to muscle atrophy and decreased function. We hypothesized with aging, apoptosis, and apoptotic regulators would be increased, and muscle fiber size and number would be reduced in extrinsic tongue muscles.

METHODS

Cell death indices, expression of caspase-3 and Bcl-2, and measures of muscle morphology and number were determined in extrinsic tongue muscles of young and old rats.

RESULTS

Significant increases in cell death, caspase-3, and Bcl-2 were observed in all extrinsic tongue muscles along with reductions in muscle fiber number in old rats.

DISCUSSION

We demonstrated that apoptosis indices increase with age in lingual muscles and that alterations in apoptotic regulators may be associated with age-related degeneration in muscle fiber size and number. These observed apoptotic processes may be detrimental to muscle function, and may contribute to degradation of cranial functions with age. Muscle Nerve 57: E29-E37, 2018.

Jaeumganghwa-Tang, a traditional herbal formula, improves muscle function and attenuates muscle loss in aged mice

PURPOSE

Jaeumganghwa-Tang (JGT), a traditional herbal formula composed of 12 medicinal herbs, is used for the treatment of age-related diseases. In the present study, we investigated the effects of JGT on muscle mass and function in aged mice.

METHODS

Young (5-month-old) and old (19-month-old) male C57BL/6 mice were divided into two groups each; one group received JGT (75 mg/d) and the other group received the vehicle for 6 weeks. At the end of the experimental period, muscle strength was examined using the wire hang test, and the tibialis anterior and gastrocnemius muscles were weighed. Muscle samples were further used for histological analysis to assess muscle damage, and the expression of transforming growth factor-beta was investigated via western blotting and immunohistochemistry.

RESULTS

Our results showed that treatment of old mice with JGT improved muscle strength, increased skeletal muscle mass, alleviated muscle damage, and suppressed intramuscular expression of transforming growth factor-beta.

CONCLUSION

In conclusion, JGT has beneficial effects on age-related loss of muscle mass and function. Thus, it might serve as a potential therapeutic agent for sarcopenia.

Physical activity and exercise as countermeasures to physical frailty and sarcopenia

The identification of cost-effective interventions that improve the health status and prevent disability in old age is one of the most important public health challenges. Regular physical activity is the only intervention that has consistently been shown to improve functional health and energy balance and to reduce the risk of cardiovascular disease, stroke, diabetes, several cancers, depression and falls. In advanced age, physical activity is also effective at mitigating sarcopenia, restoring robustness, and preventing/delaying the development of disability. On the other hand, physical inactivity is recognized as one of the leading causes of several chronic degenerative diseases and is also a major contributing factor to sarcopenia and functional disability. This compelling evidence has prompted the World Health Organization to recommend engaging in regular physical activity throughout one's life course. The present review summarizes the available evidence in support of physical activity as a remedy against physical frailty and sarcopenia. The relevant pathways through which the benefits of physical activity are conveyed are also discussed.

Sarcopenia in heart failure: mechanisms and therapeutic strategies

Chronic heart failure (CHF) is a highly prevalent condition among the elderly and is associated with considerable morbidity, institutionalization and mortality. In its advanced stages, CHF is often accompanied by the loss of muscle mass and strength. Sarcopenia is a geriatric syndrome that has been actively studied in recent years due to its association with a wide range of adverse health outcomes. The goal of this review is to discuss the relationship between CHF and sarcopenia, with a focus on shared pathophysiological pathways and treatments. Malnutrition, systemic inflammation, endocrine imbalances, and oxidative stress appear to connect sarcopenia and CHF. At the muscular level, alterations of the ubiquitin proteasome system, myostatin signaling, and apoptosis have been described in both sarcopenia and CHF and could play a role in the loss of muscle mass and function. Possible therapeutic strategies to impede the progression of muscle wasting in CHF patients include protein and vitamin D supplementation, structured physical exercise, and the administration of angiotensin-converting enzyme inhibitors and β-blockers. Hormonal supplementation with growth hormone, testosterone, and ghrelin is also discussed as a potential treatment.

Grip Strength as a Marker of Hypertension and Diabetes in Healthy Weight Adults

INTRODUCTION

Muscle strength may play a role in cardiometabolic disease. We examined the relationship between hand grip strength and diabetes and hypertension in a sample of healthy weight adults.

METHODS

In 2015, we analyzed the National Health and Nutrition Examination Survey 2011-2012 for adults aged ≥20 years with healthy BMIs (between 18.5 and <25) and no history of cardiovascular disease (unweighted n=1,467; weighted n=61,587,139). Hand grip strength was assessed with a dynamometer. Diabetes was based on hemoglobin A1c level and reported diabetes diagnosis. Hypertension was based on measured blood pressure and reported hypertension diagnosis.

RESULTS

Individuals with undiagnosed diabetes compared with individuals without diabetes had lower grip strength (51.9 vs 69.8, p=0.0001), as did individuals with diagnosed diabetes compared with individuals without diabetes (61.7 vs 69.8, p=0.008). Mean grip strength was lower among individuals with undiagnosed hypertension compared with individuals without hypertension (63.5 vs 71.5, p=0.008) as well as among individuals with diagnosed hypertension compared with those without hypertension (60.8 vs 71.5, p<0.0001). In adjusted analyses controlling for age, sex, race, smoking status, and first-degree relative with disease, mean grip strength was lower for undiagnosed diabetes (β=-10.02, p<0.0001) and diagnosed diabetes (β=-8.21, p=0.03) compared with individuals without diabetes. In adjusted analyses, grip strength was lower among individuals with undiagnosed hypertension (β=-6.6, p=0.004) and diagnosed hypertension (β=-4.27, p=0.04) compared with individuals without hypertension.

CONCLUSIONS

Among healthy weight adults, combined grip strength is lower in individuals with diagnosed and undiagnosed diabetes and hypertension.

Beneficial effects of exercise on age-related mitochondrial dysfunction and oxidative stress in skeletal muscle

Mitochondria are negatively affected by ageing leading to their inability to adapt to higher levels of oxidative stress and this ultimately contributes to the systemic loss of muscle mass and function termed sarcopenia. Since mitochondria are central mediators of muscle health, they have become highly sought-after targets of physiological and pharmacological interventions. Exercise is the only known strategy to combat sarcopenia and this is largely mediated through improvements in mitochondrial plasticity. More recently a critical role for mitochondrial turnover in preserving muscle has been postulated. Specifically, cellular pathways responsible for the regulation of mitochondrial turnover including biogenesis, dynamics and autophagy may become dysregulated during ageing resulting in the reduced clearance and accumulation of damaged organelles within the cell. When mitochondrial quality is compromised and homeostasis is not re-established, myonuclear cell death is activated and muscle atrophy ensues. In contrast, acute and chronic exercise attenuates these deficits, restoring mitochondrial turnover and promoting a healthier mitochondrial pool that leads to the preservation of muscle. Additionally, the magnitude of these exercise-induced mitochondrial adaptations is currently debated with several studies reporting a lower adaptability of old muscle relative to young, but the processes responsible for this diminished training response are unclear. Based on these observations, understanding the molecular details of how advancing age and exercise influence mitochondria in older muscle will provide invaluable insight into the development of exercise protocols that will maximize beneficial adaptations in the elderly. This information will also be imperative for future research exploring pharmacological targets of mitochondrial plasticity.

17β-Estradiol and testosterone in sarcopenia: Role of satellite cells

The loss of muscle mass and strength with aging, referred to as sarcopenia, is a prevalent condition among the elderly. Although the molecular mechanisms underlying sarcopenia are unclear, evidence suggests that an age-related acceleration of myocyte loss via apoptosis might be responsible for muscle perfomance decline. Interestingly, sarcopenia has been associated to a deficit of sex hormones which decrease upon aging. The skeletal muscle ability to repair and regenerate itself would not be possible without satellite cells, a subpopulation of cells that remain quiescent throughout life. They are activated in response to stress, enabling them to guide skeletal muscle regeneration. Thus, these cells could be a key factor to overcome sarcopenia. Of importance, satellite cells are 17β-estradiol (E2) and testosterone (T) targets. In this review, we summarize potential mechanisms through which these hormones regulate satellite cells activation during skeletal muscle regeneration in the elderly. The advance in its understanding will help to the development of potential therapeutic agents to alleviate and treat sarcopenia and other related myophaties.

Exercise, Autophagy, and Apoptosis

Exercise is a form of physiological stress which is known to induce an adaptational response. It is proposed that both apoptosis and autophagy are processes which are necessary for adaptation to exercise. Apoptosis and autophagy are induced during exercise to limit tissue damage, restore tissue integrity, terminate inflammatory responses, or induce direct signals for adaptation. Apoptosis is induced by specific mediators like reactive oxygen species, cytokines, and hormones. Autophagic pathways are activated by altered proteins/organelles with the aim to conserve and recycle the cellular resources. In this case, the cell is flooded with damaged molecules, the repairing mechanisms are overtaxed, and apoptosis is induced. In conclusion, autophagy seems to be necessary for adaptation by providing locally the conditions for muscle plasticity and apoptosis systemically by mobilizing progenitor cells.

The angiotensin-(1-7)/Mas axis reduces myonuclear apoptosis during recovery from angiotensin II-induced skeletal muscle atrophy in mice

Angiotensin-(1-7) [Ang (1-7)] is a peptide belonging to the non-classical renin-angiotensin system (RAS). Ang (1-7), through its receptor Mas, has an opposite action to angiotensin II (Ang II), the typical peptide of the classical RAS axis. Ang II produces skeletal muscle atrophy, a pathological condition characterised by the loss of strength and muscle mass. A feature of muscle atrophy is the decrease of the myofibrillar proteins produced by the activation of the ubiquitin-proteasome pathway (UPP), evidenced by the increase in the expression of two muscle-specific ubiquitin ligases: atrogin-1 and MuRF-1. In addition, it has been described that Ang II also induces myonuclear apoptosis during muscle atrophy. We assessed the effects of Ang (1-7) and Mas participation on myonuclear apoptosis during skeletal muscle atrophy induced by Ang II. Our results show that Ang (1-7), through Mas, prevents the effects induced by Ang II in the diaphragm muscles and decreases several events associated with apoptosis in the diaphragm (increased apoptotic nuclei, increased expression of caspase-8 and caspase-9, increased caspase-3 activity and increased Bax/Bcl-2 ratio). Concomitantly, Ang (1-7) also attenuates the decrease in fibre diameter and muscle strength, and prevents the increase in atrogin-1 and MuRF-1 during the muscle wasting induced by Ang II. Interestingly, these effects of Ang (1-7) are dependent on the Mas receptor. Thus, we demonstrated for the first time that Ang (1-7) prevents myonuclear apoptosis during the recovery of skeletal muscle atrophy induced by Ang II.

Apoptosis in capillary endothelial cells in ageing skeletal muscle

The age-related loss of skeletal muscle mass and function (sarcopenia) is a consistent hallmark of ageing. Apoptosis plays an important role in muscle atrophy, and the intent of this study was to specify whether apoptosis is restricted to myofibre nuclei (myonuclei) or occurs in satellite cells or stromal cells of extracellular matrix (ECM). Sarcopenia in mouse gastrocnemius muscle was characterized by myofibre atrophy, oxidative type grouping, delocalization of myonuclei and ECM fibrosis. Terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling (TUNEL) indicated a sharp rise in apoptosis during ageing. TUNEL coupled with immunostaining for dystrophin, paired box protein-7 (Pax7) or laminin-2α, respectively, was used to identify apoptosis in myonuclei, satellite cells and stromal cells. In adult muscle, apoptosis was not detected in myofibres, but was restricted to stromal cells. Moreover, the age-related rise in apoptotic nuclei was essentially due to stromal cells. Myofibre-associated apoptosis nevertheless occurred in old muscle, but represented < 20% of the total muscle apoptosis. Specifically, apoptosis in old muscle affected a small proportion (0.8%) of the myonuclei, but a large part (46%) of the Pax7(+) satellite cells. TUNEL coupled with CD31 immunostaining further attributed stromal apoptosis to capillary endothelial cells. Age-dependent rise in apoptotic capillary endothelial cells was concomitant with altered levels of key angiogenic regulators, perlecan and a perlecan domain V (endorepellin) proteolytic product. Collectively, our results indicate that sarcopenia is associated with apoptosis of satellite cells and impairment of capillary functions, which is likely to contribute to the decline in muscle mass and functionality during ageing.

Sarcopenia and Androgens: A Link between Pathology and Treatment

Sarcopenia, the age-related loss of skeletal muscle mass and function, is becoming more prevalent as the lifespan continues to increase in most populations. As sarcopenia is highly disabling, being associated with increased risk of dependence, falls, fractures, weakness, disability, and death, development of approaches to its prevention and treatment are required. Androgens are the main physiologic anabolic steroid hormones and normal testosterone levels are necessary for a range of developmental and biological processes, including maintenance of muscle mass. Testosterone concentrations decline as age increase, suggesting that low plasma testosterone levels can cause or accelerate muscle- and age-related diseases, as sarcopenia. Currently, there is increasing interest on the anabolic properties of testosterone for therapeutic use in muscle diseases including sarcopenia. However, the pathophysiological mechanisms underlying this muscle syndrome and its relationship with plasma level of androgens are not completely understood. This review discusses the recent findings regarding sarcopenia, the intrinsic, and extrinsic mechanisms involved in the onset and progression of this disease and the treatment approaches that have been developed based on testosterone deficiency and their implications.

Mitochondrial dysfunction and sarcopenia of aging: from signaling pathways to clinical trials

Sarcopenia, the age-related loss of muscle mass and function, imposes a dramatic burden on individuals and society. The development of preventive and therapeutic strategies against sarcopenia is therefore perceived as an urgent need by health professionals and has instigated intensive research on the pathophysiology of this syndrome. The pathogenesis of sarcopenia is multifaceted and encompasses lifestyle habits, systemic factors (e.g., chronic inflammation and hormonal alterations), local environment perturbations (e.g., vascular dysfunction), and intramuscular specific processes. In this scenario, derangements in skeletal myocyte mitochondrial function are recognized as major factors contributing to the age-dependent muscle degeneration. In this review, we summarize prominent findings and controversial issues on the contribution of specific mitochondrial processes - including oxidative stress, quality control mechanisms and apoptotic signaling - on the development of sarcopenia. Extramuscular alterations accompanying the aging process with a potential impact on myocyte mitochondrial function are also discussed. We conclude with presenting methodological and safety considerations for the design of clinical trials targeting mitochondrial dysfunction to treat sarcopenia. Special emphasis is placed on the importance of monitoring the effects of an intervention on muscle mitochondrial function and identifying the optimal target population for the trial. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Mitochondrial pathways in sarcopenia of aging and disuse muscle atrophy

Muscle loss during aging and disuse is a highly prevalent and disabling condition, but knowledge about cellular pathways mediating muscle atrophy is still limited. Given the postmitotic nature of skeletal myocytes, the maintenance of cellular homeostasis relies on the efficiency of cellular quality control mechanisms. In this scenario, alterations in mitochondrial function are considered a major factor underlying sarcopenia and muscle atrophy. Damaged mitochondria are not only less bioenergetically efficient, but also generate increased amounts of reactive oxygen species, interfere with cellular quality control mechanisms, and display a greater propensity to trigger apoptosis. Thus, mitochondria stand at the crossroad of signaling pathways that regulate skeletal myocyte function and viability. Studies on these pathways have sometimes provided unexpected and counterintuitive results, which suggests that they are organized into a complex, heterarchical network that is currently insufficiently understood. Untangling the complexity of such a network will likely provide clinicians with novel and highly effective therapeutics to counter the muscle loss associated with aging and disuse. In this review, we summarize the current knowledge on the mechanisms whereby mitochondrial dysfunction intervenes in the pathogenesis of sarcopenia and disuse atrophy, and highlight the prospect of targeting specific processes to treat these conditions.

Late-life enalapril administration induces nitric oxide-dependent and independent metabolic adaptations in the rat skeletal muscle

Recently, we showed that administration of the angiotensin-converting enzyme inhibitor enalapril to aged rats attenuated muscle strength decline and mitigated apoptosis in the gastrocnemius muscle. The aim of the present study was to investigate possible mechanisms underlying the muscle-protective effects of enalapril. We also sought to discern the effects of enalapril mediated by nitric oxide (NO) from those independent of this signaling molecule. Eighty-seven male Fischer 344 × Brown Norway rats were randomly assigned to receive enalapril (n = 23), the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; n = 22), enalapril + L-NAME (n = 19), or placebo (n = 23) from 24 to 27 months of age. Experiments were performed on the tibialis anterior muscle. Total NOS activity and the expression of neuronal, endothelial, and inducible NOS isoforms (nNOS, eNOS, and iNOS) were determined to investigate the effects of enalapril on NO signaling. Transcript levels of tumor necrosis factor-alpha (TNF-α) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) were assessed to explore actions of enalapril on inflammation and mitochondrial biogenesis, respectively. Protein expression of energy-sensing and insulin signaling mediators, including protein kinase B (Akt-1), phosphorylated Akt-1 (pAkt-1), mammalian target of rapamycin (mTOR), AMP-activated protein kinase subunit alpha (AMPKα), phosphorylated AMPKα (pAMPKα), and the glucose transporter GLUT-4, was also determined. Finally, the generation of hydrogen peroxide (H2O2) was quantified in subsarcolemmal (SSM) and intermyofibrillar (IFM) mitochondria. Enalapril increased total NOS activity, which was prevented by L-NAME co-administration. eNOS protein content was enhanced by enalapril, but not by enalapril + L-NAME. Gene expression of iNOS was down-regulated by enalapril either alone or in combination with L-NAME. In contrast, protein levels of nNOS were unaltered by treatments. The mRNA abundance of TNF-α was reduced by enalapril relative to placebo, with no differences among any other group. PCG-1α gene expression was unaffected by enalapril and lowered by enalapril + L-NAME. No differences in protein expression of Akt-1, pAkt-1, AMPKα, pAMPKα, or GLUT-4 were detected among groups. However, mTOR protein levels were increased by enalapril compared with placebo. Finally, all treatment groups displayed reduced SSM, but not IFM H2O2 production relative to placebo. Our data indicate that enalapril induces a number of metabolic adaptations in aged skeletal muscle. These effects result from the concerted modulation of NO and angiotensin II signaling, rather than from a dichotomous action of enalapril on the two pathways. Muscle protection by enalapril administered late in life appears to be primarily mediated by mitigation of oxidative stress and pro-inflammatory signaling.

Troponin T nuclear localization and its role in aging skeletal muscle

Troponin T (TnT) is known to mediate the interaction between Tn complex and tropomyosin (Tm), which is essential for calcium-activated striated muscle contraction. This regulatory function takes place in the myoplasm, where TnT binds Tm. However, recent findings of troponin I and Tm nuclear translocation in Drosophila and mammalian cells imply other roles for the Tn-Tm complex. We hypothesized that TnT plays a nonclassical role through nuclear translocation. Immunoblotting with different antibodies targeting the NH2- or COOH-terminal region uncovered a pool of fast skeletal muscle TnT3 localized in the nuclear fraction of mouse skeletal muscle as either an intact or fragmented protein. Construction of TnT3-DsRed fusion proteins led to the further observation that TnT3 fragments are closely related to nucleolus and RNA polymerase activity, suggesting a role for TnT3 in regulating transcription. Functionally, overexpression of TnT3 fragments produced significant defects in nuclear shape and caused high levels of apoptosis. Interestingly, nuclear TnT3 and its fragments were highly regulated by aging, thus creating a possible link between the deleterious effects of TnT3 and sarcopenia. We propose that changes in nuclear TnT3 and its fragments cause the number of myonuclei to decrease with age, contributing to muscle damage and wasting.

Atrogin-1, MuRF-1, and sarcopenia

Sarcopenia is one of the leading causes of disability in the elderly. Despite the growing prevalence of sarcopenia, the molecular mechanisms that control aging-related changes in muscle mass are not fully understood. The ubiquitin proteasome system is one of the major pathways that regulate muscle protein degradation, and this system plays a central role in controlling muscle size. Atrogin-1 and MuRF-1 are two E3 ubiquitin ligases that are important regulators of ubiquitin-mediated protein degradation in skeletal muscle. In this review, we will discuss: (i) aging-related changes to skeletal muscle structure and function; (ii) the regulation of protein synthesis and protein degradation by IGF-1, TGF-β, and myostatin, with emphasis on the control of atrogin-1 and MuRF-1 expression; and (iii) the potential for modulating atrogin-1 and MuRF-1 expression to treat or prevent sarcopenia.

Ultraviolet B (UVB) irradiation-induced apoptosis in various cell lineages in vitro

Ultraviolet B (UVB) radiation acts as a strong apoptotic trigger in many cell types, in tumor and normal cells. Several studies have demonstrated that UVB-induced cell death occurs through the generation of reactive oxygen species. The consequent oxidative stress includes the impairment of cellular antioxidants, the induction of DNA damage and the occurrence of apoptosis. In this review, we investigated UVB apoptotic action in various cell models by using ultrastructural, molecular and cytofluorimetric techniques. Myeloid leukemia HL-60, T-lymphoblastoid Molt-4 and myelomonocytic U937 human cells, generally affected by apoptotic stimuli, were studied. Human chondrocytes and C2C12 skeletal muscle cells, known to be more resistant to damage, were also considered. All of them, when exposed to UVB radiation, revealed a number of characteristic apoptotic markers. Membrane blebbing, cytoplasm shrinkage and chromatin condensation were detected by means of electron microscopy. DNA cleavage, investigated by using agarose gel electrophoresis and TUNEL reaction, was observed in suspended cells. Differently, in chondrocytes and in skeletal muscle cells, oligonucleosomic DNA fragmentation did not appear, even if a certain TUNEL positivity was detected. These findings demonstrate that UVB radiation appears to be an ideal tool to study the apoptotic behavior.

Heat-shock proteins attenuate SERCA inactivation by the anti-apoptotic protein Bcl-2: possible implications for the ER Ca2+-mediated apoptosis

We have demonstrated previously that Bcl-2 and Bcl-2Δ21, a C-terminally truncated Bcl-2 sequence, inactivate SERCA (sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase) 1 in isolated SR (sarcoplasmic reticulum), accompanied by a translocation from CRDs (caveolae-related domains) of the SR. In the present study, we obtained evidence for the interaction of Bcl-2 with SERCA2b in C2C12 myoblasts and HEK (human embryonic kidney)-293 cells. Bcl-2 and SERCA2b co-immunoprecipitated from lysate and microsomal fractions of Bcl-2-overexpressing cells. However, Bcl-2 overexpression resulted only in a slight translocation from the CRDs and no significant SERCA inactivation. In isolated HEK-293 cell microsomes, incubation with Bcl-2Δ21 afforded SERCA2b inactivation and some translocation. HSP (heat-shock protein) 70, HSP90, HSP27 and α-crystallin attenuated Bcl-2Δ21-dependent SERCA2b inactivation. An in vitro mechanistic study with the SERCA1 isoform shows that HSP70 (i) protects SERCA1 from the inactivation by Bcl-2Δ21, (ii) inhibits SERCA1 translocation from CRD fractions, and (iii) prevents the Bcl-2Δ21-dependent loss of FITC labelling. Our data demonstrate that the mechanism of SERCA inactivation by Bcl-2 established in vitro for the SERCA1 isoform can be extended to the main housekeeping SERCA2b isoform, and that functional interactions of SERCA2b and Bcl-2 in the cell may be modulated by HSP70 and other chaperones and stress-regulated proteins.

Denervation-induced mitochondrial dysfunction and autophagy in skeletal muscle of apoptosis-deficient animals

Skeletal muscle undergoes remarkable adaptations in response to chronic decreases in contractile activity, such as a loss of muscle mass, decreases in both mitochondrial content and function, as well as the activation of apoptosis. Although these adaptations are well known, questions remain regarding the signaling pathways that mediated these changes. Autophagy is an organelle turnover pathway that could contribute to these adaptations. The purpose of this study was to determine whether denervation-induced muscle disuse would result in the activation of autophagy gene expression in both wild-type (WT) and Bax/Bak double knockout (DKO) animals, which display an attenuated apoptotic response. Denervation caused a reduction in muscle mass for WT and DKO animals; however, there was a 40% attenuation in muscle atrophy in DKO animals. Mitochondrial state 3 respiration was significantly reduced, and reactive oxygen species production was increased by two- to threefold in both WT and DKO animals. Apoptotic markers, including cytosolic AIF and DNA fragmentation, were elevated in WT, but not in DKO animals following denervation. Autophagy proteins including LC3II, ULK1, ATG7, p62, and Beclin1 were increased similarly following denervation for both WT and DKO. Interestingly, denervation markedly increased the localization of LC3II to subsarcolemmal mitochondria, and this was more pronounced in the DKO animals. Thus denervation-induced muscle disuse activates both apoptotic and autophagic signaling pathways in muscle, and autophagic protein expression does not exhibit a compensatory increase in the presence of attenuated apoptosis. However, the absence of Bax and Bak may represent a potential signal to trigger mitophagy in muscle.

Inducible nitric oxide synthase (iNOS) in muscle wasting syndrome, sarcopenia, and cachexia

Muscle atrophy—also known as muscle wasting—is a debilitating syndrome that slowly develops with age (sarcopenia) or rapidly appears at the late stages of deadly diseases such as cancer, AIDS, and sepsis (cachexia). Despite the prevalence and the drastic detrimental effects of these two syndromes, there are currently no widely used, effective treatment options for those suffering from muscle wasting. In an attempt to identify potential therapeutic targets, the molecular mechanisms of sarcopenia and cachexia have begun to be elucidated. Growing evidence suggests that inflammatory cytokines may play an important role in the pathology of both syndromes. As one of the key cytokines involved in both sarcopenic and cachectic muscle wasting, tumor necrosis factor α (TNFα) and its downstream effectors provide an enticing target for pharmacological intervention. However, to date, no drugs targeting the TNFα signaling pathway have been successful as a remedial option for the treatment of muscle wasting. Thus, there is a need to identify new effectors in this important pathway that might prove to be more efficacious targets. Inducible nitric oxide synthase (iNOS) has recently been shown to be an important mediator of TNFα-induced cachectic muscle loss, and studies suggest that it may also play a role in sarcopenia. In addition, investigations into the mechanism of iNOS-mediated muscle loss have begun to reveal potential therapeutic strategies. In this review, we will highlight the potential for targeting the iNOS/NO pathway in the treatment of muscle loss and discuss its functional relevance in sarcopenia and cachexia.

Skeletal muscle apoptotic signaling predicts thigh muscle volume and gait speed in community-dwelling older persons: an exploratory study

Background

Preclinical studies strongly suggest that accelerated apoptosis in skeletal myocytes may be involved in the pathogenesis of sarcopenia. However, evidence in humans is sparse. In the present study, we investigated whether apoptotic signaling in the skeletal muscle was associated with indices of muscle mass and function in older persons.

Methodology/Principal Findings

Community-dwelling older adults were categorized into high-functioning (HF) or low-functioning (LF) groups according to their short physical performance battery (SPPB) summary score. Participants underwent an isokinetic knee extensor strength test and 3-dimensional magnetic resonance imaging of the thigh. Vastus lateralis muscle samples were obtained by percutaneous needle biopsy and assayed for the expression of a set of apoptotic signaling proteins. Age, sex, number of comorbid conditions and medications as well as knee extensor strength were not different between groups. HF participants displayed greater thigh muscle volume compared with LF persons. Multivariate partial least squares (PLS) regressions showed significant correlations between caspase-dependent apoptotic signaling proteins and the muscular percentage of thigh volume (R² = 0.78; Q² = 0.61) as well as gait speed (R² = 0.81; Q² = 0.56). Significant variables in the PLS model of percent muscle volume were active caspase-8, cleaved caspase-3, cytosolic cytochrome c and mitochondrial Bak. The regression model of gait speed was mainly described by cleaved caspase-3 and mitochondrial Bax and Bak. PLS predictive apoptotic variables did not differ between functional groups. No correlation was determined between apoptotic signaling proteins and muscle strength or quality (strength per unit volume).

Conclusions/Significance

Data from this exploratory study show for the first time that apoptotic signaling is correlated with indices of muscle mass and function in a cohort of community-dwelling older persons. Future larger-scale studies are needed to corroborate these preliminary findings and determine if down-regulation of apoptotic signaling in skeletal myocytes will provide improvements in the muscle mass and functional status of older persons.

Impact of single-nucleotide polymorphisms at the TP53-binding and responsive promoter region of BCL2 gene in modulating the phenotypic variability of LGMD2C patients

Apoptosis of skeletal muscle fibers is a well-known event occurring in patients suffering from muscular dystrophies. In this study, we hypothesized that functional polymorphisms in genes involved in the mitochondrial apoptotic pathway might modulate the apoptotic capacity underlying the muscle loss and contributing to intrafamilial and interfamilial variable phenotypes in LGMD2C (Limb Girdle Muscular Dystrophy type 2C) patients sharing the same c.521delT mutation in SGCG gene. Detection of apoptosis was confirmed on muscle biopsies taken from LGMD2C patients using the TUNEL method. We genotyped then ten potentially functional SNPs in TP53, BCL-2 and BAX genes involved in the mitochondrial apoptotic pathway. Potential genotype-dependent Bcl-2 and p53 protein expressed in skeletal muscle was investigated using western blot and ELISA assays. The result showed that muscle cells carrying the TP53-R72R and TP53-16 bp del/del genotypes displayed an increased p53 level which could be more effective in inducing apoptosis by activation of the pro-apoptotic gene expression. In addition, the BCL2-938 AA genotype was associated with increased Bcl-2 protein expression in muscle from LGMD2C patients compared to -938CC genotype, while there was no evidence of significant difference in the BAX haplotype. Our findings suggest that increased Bcl-2 protein expression may counteract pro-apoptotic pathways and thus reduce the muscle loss. To the best of our knowledge, this is a pioneer study evaluating the role of apoptotic BCL-2 and TP53 genes in contributing to the phenotypic manifestation of c.521delT mutation in LGMD2C patients. Larger studies are needed to validate these findings.

Living in a box or call of the wild? Revisiting lifetime inactivity and sarcopenia

SIGNIFICANCE

The accepted effects of aging in mammalian skeletal muscle are progressive atrophy and weakening, or sarcopenia. Canonical hallmarks of aging in skeletal muscle include a reduction in muscle fiber cross-sectional area, a loss in muscle fibers through apoptosis and denervation, and infiltration of connective tissue or fibrosis. Emerging thought suggests that pro-inflammatory signaling and oxidative stress may contribute to sarcopenia.

CRITICAL ISSUES

Unfortunately, most of the mammalian models used to examine and understand sarcopenia are confounded by the pervasive influence of prolonged physical inactivity. Further, the potential for underlying metabolic disorder and chronic disease (e.g., type II diabetes and cardiovascular disease) may accelerate skeletal muscle wasting. Because physical inactivity may share elevated pro-inflammatory (tumor necrosis factor-alpha and inducible nitric oxide synthase) and insufficient stress response (insulin-like growth factor-1 [IGF-1], heat-shock protein 25 [HSP25], NAD-dependent deacetylase sirtuin-3 [SIRT-3], and peroxisome proliferator-activated receptor-gamma coactivator 1[PGC-1α]) signaling with aging and chronic disease, it is critical to distinguish true aging from chronic inactivity or underlying disease. Conversely, the efficacy of exercise and caloric restrictive interventions against sarcopenia in aging populations appears highly effective when (a) conducted across the lifespan, or (b) at higher intensities when commenced in middle age or later.

RECENT ADVANCES

While the prospective mechanisms by which exercise or daily activity provide have not been elucidated, upregulation of HSPs, PGC-1α, and IGF-1 may ameliorate inflammatory signaling, apoptosis, and sarcopenia. Limited data indicate that the aging phenotype exhibited by mammals living in their natural habitat (Weddell seal and shrews) express limited apoptosis and fiber atrophy, whereas significant collagen accumulation remains. In addition, aging shrews displayed a remarkable ability to upregulate antioxidant enzymes (copper, zinc isoform of superoxide dismutase, manganese isoform of superoxide dismutase, catalase, and glutathione peroxidase).

FUTURE DIRECTIONS

It is possible that in healthy populations requiring daily activity to thrive, fibrosis and weakness, more than atrophy, may be the predominant phenotype of muscle aging until senescence. Elucidating the molecular mechanisms by which lifetime inactivity contributes to sarcopenia and chronic disease will be critical in managing the quality of life and health costs associated with our aging population.

Apoptosis in skeletal myocytes: a potential target for interventions against sarcopenia and physical frailty - a mini-review

BACKGROUND

Sarcopenia, the age-related loss of muscle mass and function, represents a relevant public health issue due to its high prevalence and detrimental consequences. While the exact mechanisms underlying the pathogenesis of sarcopenia are not clear, growing experimental evidence indicates that progressive myonuclear elimination over the course of aging via an apoptosis-like process may represent a converging mechanism through which muscle atrophy and loss of physical function develop. Notably, the proapoptotic environment taking place in aged muscle appears amenable to interventions.

OBJECTIVE

We aimed at providing (1) an overview of signaling pathways of apoptosis relevant to sarcopenia, and (2) a review of the literature supporting myocyte apoptosis as a target for interventions against muscle aging.

METHODS

We summarized findings from studies focused on skeletal myocyte apoptosis as a mechanism in the development of sarcopenia and reports supporting myonuclear apoptosis as a target for interventions against age-related muscle loss.

RESULTS

Advanced age is associated with increased signaling through extrinsic and intrinsic apoptotic pathways in skeletal myocytes. In contrast, downregulation of myocyte apoptosis through calorie restriction, exercise training, hormonal supplementation, drugs (e.g. angiotensin-converting enzyme inhibitors, acetaminophen, antimyostatin antibodies), nutraceuticals or genetic interventions (e.g. PGC-1α overexpression) is linked with preservation of muscle integrity and improved physical performance in late life. Preliminary data also indicate that skeletal myocyte apoptotic signaling may be downregulated by compounds, such as resveratrol, with calorie restriction-mimicking properties. Whether exercise mimetics exert a similar effect has not yet been investigated.

CONCLUSIONS

Available evidence suggests that targeting myonuclear apoptosis might provide novel and effective therapeutic tools to combat sarcopenia. Further research is required to definitely establish if downregulating myonuclear apoptosis is effective in maintaining muscle mass and function in late life, identify the most relevant apoptotic pathway(s) to target, and determine the optimal timing for intervening.

Usefulness of preclinical models for assessing the efficacy of late-life interventions for sarcopenia

Caloric restriction and physical exercise have proven beneficial against age-associated changes in body composition and declining physical performance; however, little is known regarding what benefit these interventions might have when initiated late in life. The study of mimetics of diet and exercise and the combination thereof may provide additional treatments for a vulnerable elderly population; however, how and when to initiate such interventions requires consideration in developing the most safe and efficacious treatment strategies. In this review, we focus on preclinical late-life intervention studies, which assess the relationship between physical function, sarcopenia, and body composition. We provide a conceptual framework for the ever-changing definition of sarcopenia and a rationale for the use of an appropriate rodent model of this condition. We finish by providing our perspective regarding the implications of this body of work and future areas of research that may also contribute to the ultimate goal of extending healthspan.

Exacerbation of pathology by oxidative stress in respiratory and locomotor muscles with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most devastating type of muscular dystrophy, leading to progressive weakness of respiratory (e.g. diaphragm) and locomotor muscles (e.g. gastrocnemius). DMD is caused by X-linked defects in the gene that encodes for dystrophin, a key scaffolding protein of the dystroglycan complex (DCG) within the sarcolemmal cytoskeleton. As a result of a compromised dystroglycan complex, mechanical integrity is impaired and important signalling proteins (e.g. nNOS, caveolin-3) and pathways are disrupted. Disruption of the dystroglycan complex leads to high susceptibility to injury with repeated, eccentric contractions as well as inflammation, resulting in significant damage and necrosis. Chronic damage and repair cycling leads to fibrosis and weakness. While the link between inflammation with damage and weakness in the DMD diaphragm is unresolved, elevated oxidative stress may contribute to damage, weakness and possibly fibrosis. While utilization of non-specific antioxidant interventions has yielded inconsistent results, recent data suggest that NAD(P)H oxidase could play a pivotal role in elevating oxidative stress via integrated changes in caveolin-3 and stretch-activated channels (SACs). Oxidative stress may act as an amplifier, exacerbating disruption of the dystroglycan complex, upregulation of the inflammatory transcription factor NF-B, and thus functional impairment of force-generating capacity.