Proteasome activities in the rectus abdominis muscle of young and older individuals

Molecular mechanisms underlying sarcopenia in heart failure

The loss of skeletal muscle, also known as sarcopenia, is an aging-associated muscle disorder that is disproportionately present in heart failure (HF) patients. HF patients with sarcopenia have poor outcomes compared to the overall HF patient population. The prevalence of sarcopenia in HF is only expected to grow as the global population ages, and novel treatment strategies are needed to improve outcomes in this cohort. Multiple mechanistic pathways have emerged that may explain the increased prevalence of sarcopenia in the HF population, and a better understanding of these pathways may lead to the development of therapies to prevent muscle loss. This review article aims to explore the molecular mechanisms linking sarcopenia and HF, and to discuss treatment strategies aimed at addressing such molecular signals.

Therapeutic Consequences of Targeting the IGF-1/PI3K/AKT/FOXO3 Axis in Sarcopenia: A Narrative Review

The high prevalence of sarcopenia in an aging population has an underestimated impact on quality of life by increasing the risk of falls and subsequent hospitalization. Unfortunately, the application of the major established key therapeutic-physical activity-is challenging in the immobile and injured sarcopenic patient. Consequently, novel therapeutic directions are needed. The transcription factor Forkhead-Box-Protein O3 (FOXO3) may be an option, as it and its targets have been observed to be more highly expressed in sarcopenic muscle. In such catabolic situations, Foxo3 induces the expression of two muscle specific ubiquitin ligases (Atrogin-1 and Murf-1) via the PI3K/AKT pathway. In this review, we particularly evaluate the potential of Foxo3-targeted gene therapy. Foxo3 knockdown has been shown to lead to increased muscle cross sectional area, through both the AKT-dependent and -independent pathways and the reduced impact on the two major downstream targets Atrogin-1 and Murf-1. Moreover, a Foxo3 reduction suppresses apoptosis, activates satellite cells, and initiates their differentiation into muscle cells. While this indicates a critical role in muscle regeneration, this mechanism might exhaust the stem cell pool, limiting its clinical applicability. As systemic Foxo3 knockdown has also been associated with risks of inflammation and cancer progression, a muscle-specific approach would be necessary. In this review, we summarize the current knowledge on Foxo3 and conceptualize a specific and targeted therapy that may circumvent the drawbacks of systemic Foxo3 knockdown. This approach presumably would limit the side effects and enable an activity-independent positive impact on skeletal muscle.

Cancer cachexia and its pathophysiology: links with sarcopenia, anorexia and asthenia

Cancer cachexia is a multifactorial syndrome characterized by a progressive loss of skeletal muscle mass, along with adipose tissue wasting, systemic inflammation and other metabolic abnormalities leading to functional impairment. Cancer cachexia has long been recognized as a direct cause of complications in cancer patients, reducing quality of life and worsening disease outcomes. Some related conditions, like sarcopenia (age-related muscle wasting), anorexia (appetite loss) and asthenia (reduced muscular strength and fatigue), share some key features with cancer cachexia, such as weakness and systemic inflammation. Understanding the interplay and the differences between these conditions is critical to advance basic and translational research in this field, improving the accuracy of diagnosis and contributing to finally achieve effective therapies for affected patients.

Molecular Chaperones and Proteolytic Machineries Regulate Protein Homeostasis In Aging Cells

Throughout their life cycles, cells are subject to a variety of stresses that lead to a compromise between cell death and survival. Survival is partially provided by the cell proteostasis network, which consists of molecular chaperones, a ubiquitin-proteasome system of degradation and autophagy. The cooperation of these systems impacts the correct function of protein synthesis/modification/transport machinery starting from the adaption of nascent polypeptides to cellular overcrowding until the utilization of damaged or needless proteins. Eventually, aging cells, in parallel to the accumulation of flawed proteins, gradually lose their proteostasis mechanisms, and this loss leads to the degeneration of large cellular masses and to number of age-associated pathologies and ultimately death. In this review, we describe the function of proteostasis mechanisms with an emphasis on the possible associations between them.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

The Role of Inflammation in Age-Related Sarcopenia

Many physiological changes occur with aging. These changes often, directly or indirectly, result in a deterioration of the quality of life and even in a shortening of life expectancy. Besides increased levels of reactive oxygen species, DNA damage and cell apoptosis, another important factor affecting the aging process involves a systemic chronic low-grade inflammation. This condition has already been shown to be interrelated with several (sub)clinical conditions, such as insulin resistance, atherosclerosis and Alzheimer's disease. Recent evidence, however, shows that chronic low-grade inflammation also contributes to the loss of muscle mass, strength and functionality, referred to as sarcopenia, as it affects both muscle protein breakdown and synthesis through several signaling pathways. Classic interventions to counteract age-related muscle wasting mainly focus on resistance training and/or protein supplementation to overcome the anabolic inflexibility from which elderly suffer. Although the elderly benefit from these classic interventions, the therapeutic potential of anti-inflammatory strategies is of great interest, as these might add up to/support the anabolic effect of resistance exercise and/or protein supplementation. In this review, the molecular interaction between inflammation, anabolic sensitivity and muscle protein metabolism in sarcopenic elderly will be addressed.

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.

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.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

The intriguing regulators of muscle mass in sarcopenia and muscular dystrophy

Recent advances in our understanding of the biology of muscle have led to new interest in the pharmacological treatment of muscle wasting. Loss of muscle mass and increased intramuscular fibrosis occur in both sarcopenia and muscular dystrophy. Several regulators (mammalian target of rapamycin, serum response factor, atrogin-1, myostatin, etc.) seem to modulate protein synthesis and degradation or transcription of muscle-specific genes during both sarcopenia and muscular dystrophy. This review provides an overview of the adaptive changes in several regulators of muscle mass in both sarcopenia and muscular dystrophy.

Current understanding of sarcopenia: possible candidates modulating muscle mass

The world's elderly population is expanding rapidly, and we are now faced with the significant challenge of maintaining or improving physical activity, independence, and quality of life in the elderly. Sarcopenia, the age-related loss of skeletal muscle mass, is characterized by a deterioration of muscle quantity and quality leading to a gradual slowing of movement, a decline in strength and power, increased risk of fall-related injury, and often, frailty. Since sarcopenia is largely attributed to various molecular mediators affecting fiber size, mitochondrial homeostasis, and apoptosis, the mechanisms responsible for these deleterious changes present numerous therapeutic targets for drug discovery. Muscle loss has been linked with several proteolytic systems, including the ubuiquitin-proteasome, lysosome-autophagy, and tumor necrosis factor (TNF)-α/nuclear factor-kappaB (NF-κB) systems. Although many factors are considered to regulate age-dependent muscle loss, this gentle atrophy is not affected by factors known to enhance rapid atrophy (denervation, hindlimb suspension, etc.). In addition, defects in Akt-mammalian target of rapamycin (mTOR) and serum response factor (SRF)-dependent signaling have been found in sarcopenic muscle. Intriguingly, more recent studies indicated an apparent functional defect in autophagy- and myostatin-dependent signaling in sarcopenic muscle. In this review, we summarize the current understanding of the adaptation of many regulators in sarcopenia.

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.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Sarcopenia: pharmacology of today and tomorrow

Sarcopenia remains largely undiagnosed and undertreated because of the lack of a universally accepted definition, effective ways to measure it, and identification of the outcomes that should guide treatment efficacy. An ever-growing number of clinicians and researchers along with funding and regulatory agencies have gradually recognized that sarcopenia is a human condition that requires both prevention and treatment. In this article, we review sarcopenia and its common and less known pharmacological treatments, attempt to define sarcopenia in its broader context, and present some new ideas for potential future treatment for this devastating condition.

C3KO mouse expression analysis: downregulation of the muscular dystrophy Ky protein and alterations in muscle aging

Mutations in CAPN3 gene cause limb-girdle muscular dystrophy type 2A (LGMD2A) characterized by muscle wasting and progressive degeneration of scapular and pelvic musculature. Since CAPN3 knockout mice (C3KO) display features of muscle pathology similar to those features observed in the earliest-stage or preclinical LGMD2A patients, gene expression profiling analysis in C3KO mice was performed to gain insight into mechanisms of disease. Two different comparisons were carried out in order to determine, first, the differential gene expression between wild-type (WT) and C3KO soleus and, second, to identify the transcripts differentially expressed in aging muscles of WT and C3KO mice. The up/downregulation of two genes, important for normal muscle function, was identified in C3KO mice: the Ky gene, encoding a protease implicated in muscle development, and Park2 gene encoding an E3 ubiquitin ligase (parkin). The Ky gene was downregulated in C3KO muscles suggesting that Ky protease may play a complementary role in regulating muscle cytoskeleton homeostasis in response to changes in muscle activity. Park2 was upregulated in the aged WT muscles but not in C3KO muscles. Taking into account the known functions of parkin E3 ligase, it is possible that it plays a role in ubiquitination and degradation of atrophy-specific and damaged proteins that are necessary to avoid cellular toxicity and a cellular stress response in aging muscles.

Sarcopenia and cachexia: the adaptations of negative regulators of skeletal muscle mass

Recent advances in our understanding of the biology of muscle, and how anabolic and catabolic stimuli interact to control muscle mass and function, have led to new interest in the pharmacological treatment of muscle wasting. Loss of muscle occurs as a consequence of several chronic diseases (cachexia) as well as normal aging (sarcopenia). Although many negative regulators [Atrogin-1, muscle ring finger-1, nuclear factor-kappaB (NF-κB), myostatin, etc.] have been proposed to enhance protein degradation during both sarcopenia and cachexia, the adaptation of mediators markedly differs among these conditions. Sarcopenic and cachectic muscles have been demonstrated to be abundant in myostatin- and apoptosis-linked molecules. The ubiquitin-proteasome system (UPS) is activated during many different types of cachexia (cancer cachexia, cardiac heart failure, chronic obstructive pulmonary disease), but not many mediators of the UPS change during sarcopenia. NF-κB signaling is activated in cachectic, but not in sarcopenic, muscle. Some studies have indicated a change of autophagic signaling during both sarcopenia and cachexia, but the adaptation remains to be elucidated. This review provides an overview of the adaptive changes in negative regulators of muscle mass in both sarcopenia and cachexia.

Novel intriguing strategies attenuating to sarcopenia

Sarcopenia, the age-related loss of skeletal muscle mass, is characterized by a deterioration of muscle quantity and quality leading to a gradual slowing of movement, a decline in strength and power, increased risk of fall-related injury, and, often, frailty. Since sarcopenia is largely attributed to various molecular mediators affecting fiber size, mitochondrial homeostasis, and apoptosis, the mechanisms responsible for these deleterious changes present numerous therapeutic targets for drug discovery. Resistance training combined with amino acid-containing supplements is often utilized to prevent age-related muscle wasting and weakness. In this review, we summarize more recent therapeutic strategies (myostatin or proteasome inhibition, supplementation with eicosapentaenoic acid (EPA) or ursolic acid, etc.) for counteracting sarcopenia. Myostatin inhibitor is the most advanced research with a Phase I/II trial in muscular dystrophy but does not try the possibility for attenuating sarcopenia. EPA and ursolic acid seem to be effective as therapeutic agents, because they attenuate the degenerative symptoms of muscular dystrophy and cachexic muscle. The activation of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) in skeletal muscle by exercise and/or unknown supplementation would be an intriguing approach to attenuating sarcopenia. In contrast, muscle loss with age may not be influenced positively by treatment with a proteasome inhibitor or antioxidant.

Skeletal muscle wasting in cachexia and sarcopenia: molecular pathophysiology and impact of exercise training

Skeletal muscle is the most abundant tissue in the human body, and the maintenance of its mass is essential to ensure basic function as locomotion, strength and respiration. The decision to synthesize or to break down skeletal muscle proteins is regulated by a network of signaling pathways that transmit external stimuli to intracellular factors regulating gene transcription. The tightly regulated balance of muscle protein breakdown and synthesis is disturbed in several distinct myopathies, but also in two pathologies: sarcopenia and cachexia. In recent years, it became evident that in these two muscle wasting disorders specific regulating molecules are increased in expression (e.g. members of the ubiquitin-proteasome system, myostatin, apoptosis inducing factors), whereas other factors (e.g. insulin-like growth factor 1) are down-regulated. So far, not many treatment options to fight the muscle loss are available. One of the most promising approaches is exercise training that, due to its multifactorial effects, can act on several signaling pathways. Therefore, this review will concentrate on specific alterations discussed in the current literature that are present in the skeletal muscle of both muscle wasting disorders. In addition, we will focus on exercise training as an intervention strategy.

Expression of NF-kappaB and IkappaB proteins in skeletal muscle of gastric cancer patients

The mechanisms eliciting cancer cachexia are not well understood. Wasting of skeletal muscle is problematic because it is responsible for the clinical deterioration in cancer patients and for the ability to tolerate cancer treatment. Studies done on animals suggest that nuclear factor of kappa B (NF-kappaB) signalling is important in the progression of muscle wasting due to several types of tumours. However, there are no published studies in humans on the role of NF-kappaB in cancer cachexia. In this project, we studied the rectus abdominis muscle in patients with gastric tumours (n=14) and in age-matched control subjects (n=10) for markers of NF-kappaB activation. Nuclear levels of p65, p50 and Bcl-3 were the same in both groups of subjects. However, phospho-p65 was elevated by 25% in the muscles of cancer patients. In addition, expression of the inhibitor of kappa B alpha (IkappaBalpha) was decreased by 25% in cancer patients. Decreased expression of IkappaBalpha reflects its degradation by one of the IkappaBalpha kinases and is a marker of NF-kappaB activation. Interestingly, there was no correlation between the stage of cancer and the extent of IkappaBalpha decrease, nor was there a correlation between the degree of cachexia and decreased IkappaBalpha levels. This suggests that the activation of NF-kappaB is an early and sustained event in gastric cancer. The work implicates the NF-kappaB signalling in the initiation and progression of cancer cachexia in humans and demonstrates the need for additional study of this pathway; it also recommends NF-kappaB signalling as a therapeutic target for the amelioration of cachexia as has been suggested from studies done on rodents.

Proteasomal activity in skeletal muscle: a matter of assay design, muscle type, and age

The ubiquitin-proteasome system (UPS) is a major degradation system for regulatory and misfolded proteins. UPS function has been implicated to exert a central role in the pathogenesis of various human diseases. Because biochemical analyses are often hampered by the amount of available diseased tissue, we report on the establishment and validation of a luminescence-based proteasomal activity assay applicable to 5-mg quantities of skeletal muscle. We demonstrate that the specific proteasomal activity differs in individual muscle groups and decreases with aging. These findings warrant the use of appropriate controls and a careful interpretation of results in mammalian skeletal muscle pathologies.

Muscle atrophy in immobilization and senescence in humans

PURPOSE OF REVIEW

Recent reports exploring the mechanisms thought to be responsible for the determination of muscle mass during health, ageing and immobilization in humans have presented findings that have wide ranging implications. This brief review highlights some of the more important findings.

RECENT FINDINGS

Contrary to expectations, recent findings suggest an apparent dissociation between muscle signalling pathways and their associated events in humans, particularly in relation to muscle protein synthesis. Although debate concerning the relative importance of muscle protein synthesis and degradation to muscle mass loss during immobilization continues, and the mechanisms responsible for this loss and its restoration during rehabilitation remain unclear, new evidence has emerged showing that anabolic resistance to protein nutrition develops during immobilization. This latter observation is in agreement with earlier evidence pointing to anabolic resistance of muscle to protein nutrition existing in the elderly, which is of clinical importance. Recent observations also suggest that a sex difference exists in the rate of muscle protein synthesis under postabsorptive conditions in the elderly and may explain why women lose muscle mass at a slower rate with age than men.

SUMMARY

These recent findings highlight our current lack of understanding of the mechanisms that regulate muscle mass in humans.

Comparison of rat liver and brain proteasomes for oxidative stress-induced inactivation: Influence of ageing and dietary restriction

The present study examined brain and liver derived proteasome complexes to elucidate if there is a differential susceptibility in proteasome complexes from these tissues to undergo inactivation following exposure to oxidative stressors. It then examined the influence of ageing and dietary restriction (DR) on the observed proteasome inactivation. Studies used a filtration based methodology that allows for enrichment of proteasome complexes with less tissue than is required for traditional chromatography procedures. The results indicate that the brain has much lower levels of overall proteasome activity and exhibits increased sensitivity to hydrogen peroxide mediated inactivation as compared to proteasome complexes derived from the liver. Interestingly, the brain proteasome complexes did not appear to have increased susceptibility to 4-hydroxynonenal (HNE)-induced inactivation. Surprisingly, ageing and DR induced minimal effects on oxidative stress mediated proteasome inhibition. These results indicate that the brain not only has lower levels of proteasome activity compared to the liver, but is also more susceptible to inactivation following exposure to some (but certainly not all) oxidative stressors. This data also suggest that ageing and DR may not significantly modulate the resistance of the proteasome to inactivation in some experimental settings.

Skeletal muscle proteolysis in aging

PURPOSE OF REVIEW

To understand age-related changes in proteolysis and apoptosis in skeletal muscle in relation to oxidative stress and mitochondrial alterations.

RECENT FINDINGS

During aging, a progressive loss of muscle mass (sarcopenia) has been described in both human and rodents. Sarcopenia is attributable to an imbalance between protein synthesis and degradation or between apoptosis and regeneration processes or both. Major age-dependent alterations in muscle proteolysis are a lack of responsiveness of the ubiquitin-proteasome-dependent proteolytic pathway to anabolic and catabolic stimuli and alterations in the regulation of autophagy. In addition, increased oxidative stress leads to the accumulation of damaged proteins, which are not properly eliminated, aggregate, and in turn impair proteolytic activities. Finally, the mitochondria-associated apoptotic pathway may be activated. These age-induced changes may contribute to sarcopenia and decreased ability of old individuals to recover from stress.

SUMMARY

Alterations in proteasome-dependent or lysosomal proteolysis, increased oxidative stress, mitochondrial dysfunction, and apoptosis presumably contribute to the development of sarcopenia.