Parkin overexpression protects from ageing-related loss of muscle mass and strength

Manifestations of Age on Autophagy, Mitophagy and Lysosomes in Skeletal Muscle

Sarcopenia is the loss of both muscle mass and function with age. Although the molecular underpinnings of sarcopenia are not fully understood, numerous pathways are implicated, including autophagy, in which defective cargo is selectively identified and degraded at the lysosome. The specific tagging and degradation of mitochondria is termed mitophagy, a process important for the maintenance of an organelle pool that functions efficiently in energy production and with relatively low reactive oxygen species production. Emerging data, yet insufficient, have implicated various steps in this pathway as potential contributors to the aging muscle atrophy phenotype. Included in this is the lysosome, the end-stage organelle possessing a host of proteolytic and degradative enzymes, and a function devoted to the hydrolysis and breakdown of defective molecular complexes and organelles. This review provides a summary of our current understanding of how the autophagy-lysosome system is regulated in aging muscle, highlighting specific areas where knowledge gaps exist. Characterization of the autophagy pathway with a particular focus on the lysosome will undoubtedly pave the way for the development of novel therapeutic strategies to combat age-related muscle loss.

CREG1 improves the capacity of the skeletal muscle response to exercise endurance via modulation of mitophagy

CREG1 (cellular repressor of E1A-stimulated genes 1) is involved in tissue homeostasis and influences macroautophagy/autophagy to protect cardiovascular function. However, the physiological and pathological role of CREG1 in the skeletal muscle is not clear. Here, we established a skeletal muscle-specific creg1 knockout mouse model (creg1;Ckm-Cre) by crossing the Creg1-floxed mice (Creg1^fl/fl) with a transgenic line expressing Cre recombinase under the muscle-specific Ckm (creatine kinase, muscle) promoter. In creg1;Ckm-Cre mice, the exercise time to exhaustion and running distance were significantly reduced compared to Creg1^fl/fl mice at the age of 9 months. In addition, the administration of recombinant (re)CREG1 protein improved the motor function of 9-month-old creg1;Ckm-Cre mice. Moreover, electron microscopy images of 9-month-old creg1;Ckm-Cre mice showed that the mitochondrial quality and quantity were abnormal and associated with increased levels of PINK1 (PTEN induced putative kinase 1) and PRKN/PARKIN (parkin RBR E3 ubiquitin protein ligase) but reduced levels of the mitochondrial proteins PTGS2/COX2, COX4I1/COX4, and TOMM20. These results suggested that CREG1 deficiency accelerated the induction of mitophagy in the skeletal muscle. Mechanistically, gain-and loss-of-function mutations of Creg1 altered mitochondrial morphology and function, impairing mitophagy in C2C12 cells. Furthermore, HSPD1/HSP60 (heat shock protein 1) (401-573 aa) interacted with CREG1 (130-220 aa) to antagonize the degradation of CREG1 and was involved in the regulation of mitophagy. This was the first time to demonstrate that CREG1 localized to the mitochondria and played an important role in mitophagy modulation that determined skeletal muscle wasting during the growth process or disease conditions.Abbreviations: CCCP: carbonyl cyanide m-chlorophenylhydrazone; CKM: creatine kinase, muscle; COX4I1/COX4: cytochrome c oxidase subunit 4I1; CREG1: cellular repressor of E1A-stimulated genes 1; DMEM: dulbecco's modified eagle medium; DNM1L/DRP1: dynamin 1-like; FCCP: carbonyl cyanide p-trifluoro-methoxy phenyl-hydrazone; HSPD1/HSP60: heat shock protein 1 (chaperonin); IP: immunoprecipitation; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MFF: mitochondrial fission factor; MFN2: mitofusin 2; MYH1/MHC-I: myosin, heavy polypeptide 1, skeletal muscle, adult; OCR: oxygen consumption rate; OPA1: OPA1, mitochondrial dynamin like GTPase; PINK1: PTEN induced putative kinase 1; PPARGC1A/PGC-1α: peroxisome proliferative activated receptor, gamma, coactivator 1 alpha; PRKN/PARKIN: parkin RBR E3 ubiquitin protein ligase; PTGS2/COX2: prostaglandin-endoperoxide synthase 2; RFP: red fluorescent protein; RT-qPCR: real-time quantitative PCR; SQSTM1/p62: sequestosome 1; TFAM: transcription factor A, mitochondrial; TOMM20: translocase of outer mitochondrial membrane 20; VDAC: voltage-dependent anion channel.

Exercise Is Muscle Mitochondrial Medicine

Exercise stimulates the biogenesis of mitochondria in muscle. Some literature supports the use of pharmaceuticals to enhance mitochondria as a substitute for exercise. We provide evidence that exercise rejuvenates mitochondrial function, thereby augmenting muscle health with age, in disease, and in the absence of cellular regulators. This illustrates the power of exercise to act as mitochondrial medicine in muscle.

Targeting the Mitochondria-Proteostasis Axis to Delay Aging

Human life expectancy continues to grow globally, and so does the prevalence of age-related chronic diseases, causing a huge medical and economic burden on society. Effective therapeutic options for these disorders are scarce, and even if available, are typically limited to a single comorbidity in a multifaceted dysfunction that inevitably affects all organ systems. Thus, novel therapies that target fundamental processes of aging itself are desperately needed. In this article, we summarize current strategies that successfully delay aging and related diseases by targeting mitochondria and protein homeostasis. In particular, we focus on autophagy, as a fundamental proteostatic process that is intimately linked to mitochondrial quality control. We present genetic and pharmacological interventions that effectively extend health- and life-span by acting on specific mitochondrial and pro-autophagic molecular targets. In the end, we delve into the crosstalk between autophagy and mitochondria, in what we refer to as the mitochondria-proteostasis axis, and explore the prospect of targeting this crosstalk to harness maximal therapeutic potential of anti-aging interventions.

From mitochondria to sarcopenia: Role of inflammaging and RAGE-ligand axis implication

Sarcopenia is characterized by a loss of muscle mass and function that reduces mobility, diminishes quality of life, and can lead to fall-related injuries. At the intracellular level, mitochondrial population alterations are considered as key contributors to the complex etiology of sarcopenia. Mitochondrial dysfunctions lead to reactive oxygen species production, altered cellular proteostasis, and promotes inflammation. Interestingly, the receptor for advanced glycation end-products (RAGE) is a pro-inflammatory receptor involved in inflammaging. In this review, after a brief description of sarcopenia, we will describe how mitochondria and the pathways controlling mitochondrial population quality could participate to age-induced muscle mass and force loss. Finally, we will discuss the RAGE-ligand axis during aging and its possible connection with mitochondria to control inflammaging and sarcopenia.

Ubiquitin Ligases at the Heart of Skeletal Muscle Atrophy Control

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

Mitochondrial Impairment in Sarcopenia

Sarcopenia is defined by the age-related loss of skeletal muscle quality, which relies on mitochondrial homeostasis. During aging, several mitochondrial features such as bioenergetics, dynamics, biogenesis, and selective autophagy (mitophagy) are altered and impinge on protein homeostasis, resulting in loss of muscle mass and function. Thus, mitochondrial dysfunction contributes significantly to the complex pathogenesis of sarcopenia, and mitochondria are indicated as potential targets to prevent and treat this age-related condition. After a concise presentation of the age-related modifications in skeletal muscle quality and mitochondrial homeostasis, the present review summarizes the most relevant findings related to mitochondrial alterations in sarcopenia.

Drp1 knockdown induces severe muscle atrophy and remodelling, mitochondrial dysfunction, autophagy impairment and denervation

KEY POINTS

The maintenance of optimal mitochondrial content and function is critical for muscle health. Mitochondrial dynamics play key roles in mitochondrial quality control; however, the exact role that mitochondrial fission plays in skeletal muscle health remains unclear. Here we report knocking down Drp1 (a protein regulating mitochondrial fission) for 4 months in adult mouse skeletal muscle resulted in severe muscle atrophy (40-50%). Drp1 knockdown also led to a reduction in ADP-stimulated respiration, an increase in markers of impaired autophagy and increased muscle regeneration, denervation, fibrosis and oxidative stress. Our data indicate that Drp1 is crucial for the maintenance of normal mitochondrial function and that Drp1 depletion severely impairs muscle health.

ABSTRACT

Mitochondria play central roles in skeletal muscle physiology, including energy supply, regulation of energy-sensitive signalling pathways, reactive oxygen species production/signalling, calcium homeostasis and the regulation of apoptosis. The maintenance of optimal mitochondrial content and function is therefore critical for muscle cells. Mitochondria are now well known as highly dynamic organelles, able to change their morphology through fusion and fission processes. Solid experimental evidence indicates that mitochondrial dynamics play key roles in mitochondrial quality control, and alteration in the expression of proteins regulating mitochondrial dynamics have been reported in many conditions associated with muscle atrophy and wasting. However, the exact role that mitochondrial fission plays in skeletal muscle health remains unclear. To address this issue, we investigated the impact of Drp1 (a protein regulating mitochondrial fission) knockdown, introduced via intramuscular injection of adeno-associated virus (AAV) on adult mouse skeletal muscle. Knocking down Drp1 for 4 months resulted in very severe muscle atrophy (40-50%). Drp1 knockdown also led to a reduction in ADP-stimulated respiration and increases in markers of muscle regeneration, denervation, fibrosis, oxidative stress and impaired autophagy. Our findings indicate that Drp1 is essential for the maintenance of normal mitochondrial function and that Drp1 suppression severely impairs muscle health.

Parkin Overexpression Attenuates Sepsis-Induced Muscle Wasting

Sepsis elicits skeletal muscle weakness and fiber atrophy. The accumulation of injured mitochondria and depressed mitochondrial functions are considered as important triggers of sepsis-induced muscle atrophy. It is unclear whether mitochondrial dysfunctions in septic muscles are due to the inadequate activation of quality control processes. We hypothesized that overexpressing Parkin, a protein responsible for the recycling of dysfunctional mitochondria by the autophagy pathway (mitophagy), would confer protection against sepsis-induced muscle atrophy by improving mitochondrial quality and content. Parkin was overexpressed for four weeks in the limb muscles of four-week old mice using intramuscular injections of adeno-associated viruses (AAVs). The cecal ligation and perforation (CLP) procedure was used to induce sepsis. Sham operated animals were used as controls. All animals were studied for 48 h post CLP. Sepsis resulted in major body weight loss and myofiber atrophy. Parkin overexpression prevented myofiber atrophy in CLP mice. Quantitative two-dimensional transmission electron microscopy revealed that sepsis is associated with the accumulation of enlarged and complex mitochondria, an effect which was attenuated by Parkin overexpression. Parkin overexpression also prevented a sepsis-induced decrease in the content of mitochondrial subunits of NADH dehydrogenase and cytochrome C oxidase. We conclude that Parkin overexpression prevents sepsis-induced skeletal muscle atrophy, likely by improving mitochondrial quality and contents.

ZNF746/PARIS overexpression induces cellular senescence through FoxO1/p21 axis activation in myoblasts

Various stresses, including oxidative stress, impair the proliferative capacity of muscle stem cells leading to declined muscle regeneration related to aging or muscle diseases. ZNF746 (PARIS) is originally identified as a substrate of E3 ligase Parkin and its accumulation is associated with Parkinson’s disease. In this study, we investigated the role of PARIS in myoblast function. PARIS is expressed in myoblasts and decreased during differentiation. PARIS overexpression decreased both proliferation and differentiation of myoblasts without inducing cell death, whereas PARIS depletion enhanced myoblast differentiation. Interestingly, high levels of PARIS in myoblasts or fibroblasts induced cellular senescence with alterations in gene expression associated with p53 signaling, inflammation, and response to oxidative stress. PARIS overexpression in myoblasts starkly enhanced oxidative stress and the treatment of an antioxidant Trolox attenuated the impaired proliferation caused by PARIS overexpression. FoxO1 and p53 proteins are elevated in PARIS-overexpressing cells leading to p21 induction and the depletion of FoxO1 or p53 reduced p21 levels induced by PARIS overexpression. Furthermore, both PARIS and FoxO1 were recruited to p21 promoter region and Trolox treatment attenuated FoxO1 recruitment. Taken together, PARIS upregulation causes oxidative stress-related FoxO1 and p53 activation leading to p21 induction and cellular senescence of myoblasts.

The redox environment and mitochondrial dysfunction in age-related skeletal muscle atrophy

Medical advancements have extended human life expectancy, which is not always accompanied by an improved quality of life or healthspan. A decline in muscle mass and function is a consequence of ageing and can result in a loss of independence in elderly individuals while increasing their risk of falls. Multiple cellular pathways have been implicated in age-related muscle atrophy, including the contribution of reactive oxygen species (ROS) and disrupted redox signalling. Aberrant levels of ROS disrupts the redox environment in older muscle, potentially disrupting cellular signalling and in some cases blunting the adaptive response to exercise. Age-related muscle atrophy is associated with disrupted mitochondrial content and function, one of the hallmarks of age-related diseases. There is a critical link between abnormal ROS generation and dysfunctional mitochondrial dynamics including mitochondrial biogenesis, fusion and fission. In order to develop effective treatments or preventative strategies, it is important to gain a comprehensive understanding of the mechanistic pathways implicated in age associated loss of muscle.

A Cell-Based High-Throughput Screening Identified Two Compounds that Enhance PINK1-Parkin Signaling

Early-onset Parkinson's disease-associated PINK1-Parkin signaling maintains mitochondrial health. Therapeutic approaches for enhancing PINK1-Parkin signaling present a potential strategy for treating various diseases caused by mitochondrial dysfunction. We report two chemical enhancers of PINK1-Parkin signaling, identified using a robust cell-based high-throughput screening system. These small molecules, T0466 and T0467, activate Parkin mitochondrial translocation in dopaminergic neurons and myoblasts at low doses that do not induce mitochondrial accumulation of PINK1. Moreover, both compounds reduce unfolded mitochondrial protein levels, presumably through enhanced PINK1-Parkin signaling. These molecules also mitigate the locomotion defect, reduced ATP production, and disturbed mitochondrial Ca²⁺ response in the muscles along with the mitochondrial aggregation in dopaminergic neurons through reduced PINK1 activity in Drosophila. Our results suggested that T0466 and T0467 may hold promise as therapeutic reagents in Parkinson's disease and related disorders.

PINK1-Parkin signaling in Parkinson's disease: Lessons from Drosophila

The mitochondrial protein kinase PINK1 activates Parkin ubiquitin ligase by phosphorylating Parkin and ubiquitin, which are required for mitochondrial maintenance in dopaminergic (DA) neurons whose degeneration leads to the development of Parkinson's disease (PD). Loss of PINK1 and Parkin leads to mitochondrial degeneration and abnormal wing posture in Drosophila. Modifier screening using the Drosophila wing phenotype showed that the inactivation of Miro, a mitochondrial adaptor protein, suppresses the phenotype caused by mitochondrial degeneration. When activated by PINK1, Parkin suppresses mitochondrial transport by reducing Miro levels in Drosophila DA neurons. In human DA neurons, PINK1-Parkin signaling also regulates axonal mitochondrial re-distribution in response to reduced mitochondrial membrane potential, which is impaired in the DA neurons of patients with PINK1 and Parkin mutations. Phospho-ubiquitin signals amplified by PINK1 and Parkin are stronger in DA neurons than other neurons, suggesting that PINK1-Parkin signaling is particularly important for DA neuron activity. Moreover, the recently identified PD-associated protein CHCHD2 may ensure proper electron transfer during mitochondrial respiration. The genetic interaction between PINK1/Parkin and CHCHD2 in Drosophila indicates that they are not directly associated and CHCHD2-linked PD exhibits a very different pathology to PINK1/Parkin PD. I suggest a complex pathogenesis for mitochondrial dysregulation in PD.

Regulatory Roles of PINK1-Parkin and AMPK in Ubiquitin-Dependent Skeletal Muscle Mitophagy

The selective removal of damaged mitochondria, also known as mitophagy, is an important mechanism that regulates mitochondrial quality control. Evidence suggests that mitophagy is adversely affected in aged skeletal muscle, and this is thought to contribute toward the age-related decline of muscle health. While our knowledge of the molecular mechanisms that regulate mitophagy are derived mostly from work in non-muscle cells, whether these mechanisms are conferred in muscle under physiological conditions has not been thoroughly investigated. Recent findings from our laboratory and those of others have made several novel contributions to this field. Herein, we consolidate current literature, including our recent work, while evaluating how ubiquitin-dependent mitophagy is regulated both in muscle and non-muscle cells through the steps of mitochondrial fission, ubiquitylation, and autophagosomal engulfment. During ubiquitin-dependent mitophagy in non-muscle cells, mitochondrial depolarization activates PINK1-Parkin signaling to elicit mitochondrial ubiquitylation. TANK-binding kinase 1 (TBK1) then activates autophagy receptors, which in turn, tether ubiquitylated mitochondria to autophagosomes prior to lysosomal degradation. In skeletal muscle, evidence supporting the involvement of PINK1-Parkin signaling in mitophagy is lacking. Instead, 5'-AMP-activated protein kinase (AMPK) is emerging as a critical regulator. Mechanistically, AMPK activation promotes mitochondrial fission before enhancing autophagosomal engulfment of damaged mitochondria possibly via TBK1. While TBK1 may be a point of convergence between PINK1-Parkin and AMPK signaling in muscle, the critical question that remains is: whether mitochondrial ubiquitylation is required for mitophagy. In future, improving understanding of molecular processes that regulate mitophagy in muscle will help to develop novel strategies to promote healthy aging.

Exercise and mitochondrial health

Mitochondrial health is an important mediator of cellular function across a range of tissues, and as a result contributes to whole-body vitality in health and disease. Our understanding of the regulation and function of these organelles is of great interest to scientists and clinicians across many disciplines within our healthcare system. Skeletal muscle is a useful model tissue for the study of mitochondrial adaptations because of its mass and contribution to whole body metabolism. The remarkable plasticity of mitochondria allows them to adjust their volume, structure and capacity under conditions such as exercise, which is useful or improving metabolic health in individuals with various diseases and/or advancing age. Mitochondria exist within muscle as a functional reticulum which is maintained by dynamic processes of biogenesis and fusion, and is balanced by opposing processes of fission and mitophagy. The sophisticated coordination of these events is incompletely understood, but is imperative for organelle function and essential for the maintenance of an interconnected organelle network that is finely tuned to the metabolic needs of the cell. Further elucidation of the mechanisms of mitochondrial turnover in muscle could offer potential therapeutic targets for the advancement of health and longevity among our ageing populations. As well, investigating exercise modalities that are both convenient and capable of inducing robust mitochondrial adaptations are useful in fostering more widespread global adherence. To this point, exercise remains the most potent behavioural therapeutic approach for the improvement of mitochondrial health, not only in muscle, but potentially also in other tissues.

Overexpression of pink1 or parkin in indirect flight muscles promotes mitochondrial proteostasis and extends lifespan in Drosophila melanogaster

Dysfunctional mitochondria have been implicated in aging and age-related disorders such as Parkinson’s diseases (PD). We previously showed that pink1 and parkin, two familial PD genes, function in a linear pathway to maintain mitochondrial integrity and function. Studies of mammalian cell lines also suggest that these genes regulate mitochondrial autophagy(mitophagy). Overexpressing Parkin promotes proteostasis and function of aged muscles both in fruit flies and mice, and recent studies also indicated that mitochondrial ubiquitination are accumulated in aged muscles. However, the underlying mechanisms for pink1 and parkin mediated mitophagy on longevity is not fully understood. Here, we found that mitochondrial ubiquitination increased in indirect flight muscles (IFMs) in an age-dependent manner. Overexpression of pink1 or parkin in IFMs can abolish mitochondrial ubiquitination, restore ATP level and extend lifespan, while blocking autophagy via ATG1 knock-down suppress these effects in aged IFMs. Taken together, these results show that pink1/parkin promotes mitophagy of mitochondrial ubiquitination in aged muscles and extend lifespan in an Atg1-dependent manner. Our study provides physiological evidence that mitophagy of mitochondrial ubiquitination mediated by PINK1/ Parkin is crucial for muscle function and highlights the role of mitophagy in the pathogenesis of chronic diseases like PD.

Depletion of HuR in murine skeletal muscle enhances exercise endurance and prevents cancer-induced muscle atrophy

The master posttranscriptional regulator HuR promotes muscle fiber formation in cultured muscle cells. However, its impact on muscle physiology and function in vivo is still unclear. Here, we show that muscle-specific HuR knockout (muHuR-KO) mice have high exercise endurance that is associated with enhanced oxygen consumption and carbon dioxide production. muHuR-KO mice exhibit a significant increase in the proportion of oxidative type I fibers in several skeletal muscles. HuR mediates these effects by collaborating with the mRNA decay factor KSRP to destabilize the PGC-1α mRNA. The type I fiber-enriched phenotype of muHuR-KO mice protects against cancer cachexia-induced muscle loss. Therefore, our study uncovers that under normal conditions HuR modulates muscle fiber type specification by promoting the formation of glycolytic type II fibers. We also provide a proof-of-principle that HuR expression can be targeted therapeutically in skeletal muscles to combat cancer-induced muscle wasting.

HuR is an RNA-binding protein that regulates myotube differentiation in vitro. Here, the authors show that the muscle-specific ablation of HuR in mice leads to enhanced endurance capacity and an increase in oxidative fibres by destabilising PGC1α-mRNA, and show that the mice are protected against cancer cachexia

Autophagy, apoptosis, and mitochondria: molecular integration and physiological relevance in skeletal muscle

Apoptosis and autophagy are processes resulting from the integration of cellular stress and death signals. Their individual importance is highlighted by the lethality of various mouse models missing apoptosis or autophagy-related genes. In addition to their independent roles, significant overlap exists with respect to the signals that stimulate these processes as well as their effector consequences. While these cellular systems exemplify the programming redundancies that underlie many fundamental biological mechanisms, their intertwined relationship means that dysfunction can promote pathology. Although both autophagic and apoptotic signaling are active in skeletal muscle during various diseases and atrophy, their specific roles here are somewhat unique. Given our growing understanding of how specific changes at the cellular level impact whole-organism physiology, there is an equally growing interest in pharmacological manipulation of apoptosis and/or autophagy for altering human physiology and health.