Suppression of autophagy in skeletal muscle uncovers the accumulation of ubiquitinated proteins and their potential role in muscle damage in Pompe disease

AMPK activation stimulates autophagy and ameliorates muscular dystrophy in the mdx mouse diaphragm

Duchenne muscular dystrophy (DMD) is characterized by myofiber death from apoptosis or necrosis, leading in many patients to fatal respiratory muscle weakness. Among other pathological features, DMD muscles show severely deranged metabolic gene regulation and mitochondrial dysfunction. Defective mitochondria not only cause energetic deficiency, but also play roles in promoting myofiber atrophy and injury via opening of the mitochondrial permeability transition pore. Autophagy is a bulk degradative mechanism that serves to augment energy production and eliminate defective mitochondria (mitophagy). We hypothesized that pharmacological activation of AMP-activated protein kinase (AMPK), a master metabolic sensor in cells and on-switch for the autophagy-mitophagy pathway, would be beneficial in the mdx mouse model of DMD. Treatment of mdx mice for 4 weeks with an established AMPK agonist, AICAR (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside), potently triggered autophagy in the mdx diaphragm without inducing muscle fiber atrophy. In AICAR-treated mdx mice, the exaggerated sensitivity of mdx diaphragm mitochondria to calcium-induced permeability transition pore opening was restored to normal levels. There were associated improvements in mdx diaphragm histopathology and in maximal force-generating capacity, which were not linked to increased mitochondrial biogenesis or up-regulated utrophin expression. These findings suggest that agonists of AMPK and other inducers of the autophagy-mitophagy pathway can help to promote the elimination of defective mitochondria and may thus serve as useful therapeutic agents in DMD.

The role of autophagy in the pathogenesis of glycogen storage disease type II (GSDII)

Regulated removal of proteins and organelles by autophagy-lysosome system is critical for muscle homeostasis. Excessive activation of autophagy-dependent degradation contributes to muscle atrophy and cachexia. Conversely, inhibition of autophagy causes accumulation of protein aggregates and abnormal organelles, leading to myofiber degeneration and myopathy. Defects in lysosomal function result in severe muscle disorders such as Pompe (glycogen storage disease type II (GSDII)) disease, characterized by an accumulation of autophagosomes. However, whether autophagy is detrimental or not in muscle function of Pompe patients is unclear. We studied infantile and late-onset GSDII patients and correlated impairment of autophagy with muscle wasting. We also monitored autophagy in patients who received recombinant α-glucosidase. Our data show that infantile and late-onset patients have different levels of autophagic flux, accumulation of p62-positive protein aggregates and expression of atrophy-related genes. Although the infantile patients show impaired autophagic function, the late-onset patients display an interesting correlation among autophagy impairment, atrophy and disease progression. Moreover, reactivation of autophagy in vitro contributes to acid α-glucosidase maturation in both healthy and diseased myotubes. Together, our data suggest that autophagy protects myofibers from disease progression and atrophy in late-onset patients.

Autophagy: more than a nonselective pathway

Autophagy is a catabolic pathway conserved among eukaryotes that allows cells to rapidly eliminate large unwanted structures such as aberrant protein aggregates, superfluous or damaged organelles, and invading pathogens. The hallmark of this transport pathway is the sequestration of the cargoes that have to be degraded in the lysosomes by double-membrane vesicles called autophagosomes. The key actors mediating the biogenesis of these carriers are the autophagy-related genes (ATGs). For a long time, it was assumed that autophagy is a bulk process. Recent studies, however, have highlighted the capacity of this pathway to exclusively eliminate specific structures and thus better fulfil the catabolic necessities of the cell. We are just starting to unveil the regulation and mechanism of these selective types of autophagy, but what it is already clearly emerging is that structures targeted to destruction are accurately enwrapped by autophagosomes through the action of specific receptors and adaptors. In this paper, we will briefly discuss the impact that the selective types of autophagy have had on our understanding of autophagy.

Mouse skeletal muscle fiber-type-specific macroautophagy and muscle wasting are regulated by a Fyn/STAT3/Vps34 signaling pathway

Skeletal muscle atrophy induced by aging (sarcopenia), inactivity, and prolonged fasting states (starvation) is predominantly restricted to glycolytic type II muscle fibers and typical spares oxidative type I fibers. However, the mechanisms accounting for muscle fiber-type specificity of atrophy have remained enigmatic. In the current study, although the Fyn tyrosine kinase activated the mTORC1 signaling complex, it also induced marked atrophy of glycolytic fibers with relatively less effect on oxidative muscle fibers. This was due to inhibition of macroautophagy via an mTORC1-independent but STAT3-dependent reduction in Vps34 protein levels and decreased Vps34/p150/Beclin1/Atg14 complex 1. Physiologically, in the fed state endogenous Fyn kinase activity was increased in glycolytic but not oxidative skeletal muscle. In parallel, Y705-STAT3 phosphorylation increased with decreased Vps34 protein levels. Moreover, fed/starved regulation of Y705-STAT3 phosphorylation and Vps34 protein levels was prevented in skeletal muscle of Fyn null mice. These data demonstrate a Fyn/STAT3/Vps34 pathway that is responsible for fiber-type-specific regulation of macroautophagy and skeletal muscle atrophy.

Autophagy in lysosomal storage disorders

Lysosomes are ubiquitous intracellular organelles that have an acidic internal pH, and play crucial roles in cellular clearance. Numerous functions depend on normal lysosomes, including the turnover of cellular constituents, cholesterol homeostasis, downregulation of surface receptors, inactivation of pathogenic organisms, repair of the plasma membrane and bone remodeling. Lysosomal storage disorders (LSDs) are characterized by progressive accumulation of undigested macromolecules within the cell due to lysosomal dysfunction. As a consequence, many tissues and organ systems are affected, including brain, viscera, bone and cartilage. The progressive nature of phenotype development is one of the hallmarks of LSDs. In recent years biochemical and cell biology studies of LSDs have revealed an ample spectrum of abnormalities in a variety of cellular functions. These include defects in signaling pathways, calcium homeostasis, lipid biosynthesis and degradation and intracellular trafficking. Lysosomes also play a fundamental role in the autophagic pathway by fusing with autophagosomes and digesting their content. Considering the highly integrated function of lysosomes and autophagosomes it was reasonable to expect that lysosomal storage in LSDs would have an impact upon autophagy. The goal of this review is to provide readers with an overview of recent findings that have been obtained through analysis of the autophagic pathway in several types of LSDs, supporting the idea that LSDs could be seen primarily as "autophagy disorders."

Clinical utility of LC3 and p62 immunohistochemistry in diagnosis of drug-induced autophagic vacuolar myopathies: a case-control study

Background

Some patients treated with chloroquine, hydroxychloroquine, or colchicine develop autophagic vacuolar myopathy, the diagnosis of which currently requires electron microscopy. The goal of the current study was to develop an immunohistochemical diagnostic marker for this pathologic entity.

Methodology

Microtubule-associated protein light chain 3 (LC3) has emerged as a robust marker of autophagosomes. LC3 binds p62/SQSTM1, an adapter protein that is selectively degraded via autophagy. In this study, we evaluated the utility of immunohistochemical stains for LC3 and p62 as diagnostic markers of drug-induced autophagic vacuolar myopathy. The staining was performed on archival muscle biopsy material, with subject assignment to normal control, drug-treated control, and autophagic myopathy groups based on history of drug use and morphologic criteria.

Principal Findings

In all drug-treated subjects, but not in normal controls, LC3 and p62 showed punctate staining characteristic of autophagosome buildup. In the autophagic myopathy subjects, puncta were coarser and tended to coalesce into linear structures aligned with the longitudinal axis of the fiber, often in the vicinity of vacuoles. The percentage of LC3- and p62-positive fibers was significantly higher in the autophagic myopathy group compared to either the normal control (p<0.001) or the drug-treated control group (p<0.05). With the diagnostic threshold set between 8% and 15% positive fibers (depending on the desired level of sensitivity and specificity), immunohistochemical staining for either LC3 or p62 could be used to identify subjects with autophagic vacuolar myopathy within the drug-treated subject group (p≤0.001).

Significance

Immunohistochemistry for LC3 and p62 can facilitate tissue-based diagnosis of drug-induced autophagic vacuolar myopathies. By limiting the need for electron microscopy (a time consuming and costly technique with high specificity, but low sensitivity), clinical use of these markers will improve the speed and accuracy of diagnosis, resulting in significantly improved clinical care.

TWEAK causes myotube atrophy through coordinated activation of ubiquitin-proteasome system, autophagy, and caspases

Proinflammatory cytokine TWEAK has now emerged as a key mediator of skeletal muscle-wasting in many catabolic conditions. However, the mechanisms by which TWEAK induces muscle proteolysis remain poorly understood. Here, we have investigated the role of ubiquitin-proteasome system, autophagy, and caspases in TWEAK-induced muscle wasting. Addition of TWEAK to C2C12 myotubes stimulated the ubiquitination of myosin heavy chain (MyHC) and augmented the expression of E3 ubiquitin ligase MuRF1. Pretreatment of myotubes with proteasome inhibitors MG132 or lactacystin or knockdown of MuRF1 by RNAi blocked the TWEAK-induced degradation of MyHC and myotube atrophy. TWEAK increased the expression of several autophagy-related molecules. Moreover, the inhibitors of autophagy improved the levels of MyHC in TWEAK-treated myotubes. TWEAK also increased activity of caspases in C2C12 myotubes. Pan-caspase or caspase 3 inhibitory peptide inhibited the TWEAK-induced loss of MyHC and myotube diameter. Our study demonstrates that nuclear factor-kappa B (NF-κB) transcription factor is essential for TWEAK-induced expression of MuRF1 and Beclin1. Furthermore, our results suggest that caspases contribute, at least in part, to the activation of NF-κB in response to TWEAK treatment. Collectively, the present study provides novel insight into the mechanisms of action of TWEAK in skeletal muscle.

Aggrephagy: selective disposal of protein aggregates by macroautophagy

Protein aggregation is a continuous process in our cells. Some proteins aggregate in a regulated manner required for different vital functional processes in the cells whereas other protein aggregates result from misfolding caused by various stressors. The decision to form an aggregate is largely made by chaperones and chaperone-assisted proteins. Proteins that are damaged beyond repair are degraded either by the proteasome or by the lysosome via autophagy. The aggregates can be degraded by the proteasome and by chaperone-mediated autophagy only after dissolution into soluble single peptide species. Hence, protein aggregates as such are degraded by macroautophagy. The selective degradation of protein aggregates by macroautophagy is called aggrephagy. Here we review the processes of aggregate formation, recognition, transport, and sequestration into autophagosomes by autophagy receptors and the role of aggrephagy in different protein aggregation diseases.

Glycogen and its metabolism: some new developments and old themes

Glycogen is a branched polymer of glucose that acts as a store of energy in times of nutritional sufficiency for utilization in times of need. Its metabolism has been the subject of extensive investigation and much is known about its regulation by hormones such as insulin, glucagon and adrenaline (epinephrine). There has been debate over the relative importance of allosteric compared with covalent control of the key biosynthetic enzyme, glycogen synthase, as well as the relative importance of glucose entry into cells compared with glycogen synthase regulation in determining glycogen accumulation. Significant new developments in eukaryotic glycogen metabolism over the last decade or so include: (i) three-dimensional structures of the biosynthetic enzymes glycogenin and glycogen synthase, with associated implications for mechanism and control; (ii) analyses of several genetically engineered mice with altered glycogen metabolism that shed light on the mechanism of control; (iii) greater appreciation of the spatial aspects of glycogen metabolism, including more focus on the lysosomal degradation of glycogen; and (iv) glycogen phosphorylation and advances in the study of Lafora disease, which is emerging as a glycogen storage disease.

A mammalian autophagosome maturation mechanism mediated by TECPR1 and the Atg12-Atg5 conjugate

Autophagy is a major catabolic pathway in eukaryotes associated with a broad spectrum of human diseases. In autophagy, autophagosomes carrying cellular cargoes fuse with lysosomes for degradation. However, the molecular mechanism underlying autophagosome maturation is largely unknown. Here we report that TECPR1 binds to the Atg12-Atg5 conjugate and phosphatidylinositol 3-phosphate (PtdIns[3]P) to promote autophagosome-lysosome fusion. TECPR1 and Atg16 form mutually exclusive complexes with the Atg12-Atg5 conjugate, and TECPR1 binds PtdIns(3)P upon association with the Atg12-Atg5 conjugate. Strikingly, TECPR1 localizes to and recruits Atg5 to autolysosome membrane. Consequently, elimination of TECPR1 leads to accumulation of autophagosomes and blocks autophagic degradation of LC3-II and p62. Finally, autophagosome maturation marked by GFP-mRFP-LC3 is defective in TECPR1-deficient cells. Thus, we propose that the concerted interactions among TECPR1, Atg12-Atg5, and PtdIns(3)P provide the fusion specificity between autophagosomes and lysosomes and that the assembly of this complex initiates the autophagosome maturation process.

Modulation of autophagy and ubiquitin-proteasome pathways during ultra-endurance running

In this study, the coordinated activation of ubiquitin-proteasome pathway (UPP), autophagy-lysosomal pathway (ALP), and mitochondrial remodeling including mitophagy was assessed by measuring protein markers during ultra-endurance running exercise in human skeletal muscle. Eleven male, experienced ultra-endurance athletes ran for 24 h on a treadmill. Muscle biopsy samples were taken from the vastus lateralis muscle 2 h before starting and immediately after finishing exercise. Athletes ran 149.8 ± 16.3 km with an effective running time of 18 h 42 min ( ± 41 min). The phosphorylation state of Akt (-74 ± 5%; P < 0.001), FOXO3a (-49 ± 9%; P < 0.001), mTOR Ser2448 (-32 ± 14%; P = 0.028), and 4E-BP1 (-34 ± 7%; P < 0.001) was decreased, whereas AMPK phosphorylation state increased by 247 ± 170% (P = 0.042). Proteasome β2 subunit activity increased by 95 ± 44% (P = 0.028), whereas the activities associated with the β1 and β5 subunits remained unchanged. MuRF1 protein level increased by 55 ± 26% (P = 0.034), whereas MAFbx protein and ubiquitin-conjugated protein levels did not change. LC3bII increased by 554 ± 256% (P = 0.005), and the form of ATG12 conjugated to ATG5 increased by 36 ± 17% (P = 0.042). The mitochondrial fission marker phospho-DRP1 increased by 110 ± 47% (P = 0.003), whereas the fusion marker Mfn1 and the mitophagy markers Parkin and PINK1 remained unchanged. These results fit well with a coordinated regulation of ALP and UPP triggered by FOXO3 and AMPK during ultra-endurance exercise.

Autophagy and mitochondria in Pompe disease: nothing is so new as what has long been forgotten

Macroautophagy (often referred to as autophagy) is an evolutionarily conserved intracellular system by which macromolecules and organelles are delivered to lysosomes for degradation and recycling. Autophagy is robustly induced in response to starvation in order to generate nutrients and energy through the lysosomal degradation of cytoplasmic components. Constitutive, basal autophagy serves as a quality control mechanism for the elimination of aggregated proteins and worn-out or damaged organelles, such as mitochondria. Research during the last decade has made it clear that malfunctioning or failure of this system is associated with a wide range of human pathologies and age-related diseases. Our recent data provide strong evidence for the role of autophagy in the pathogenesis of Pompe disease, a lysosomal glycogen storage disease caused by deficiency of acid alpha-glucosidase (GAA). Large pools of autophagic debris in skeletal muscle cells can be seen in both our GAA knockout model and patients with Pompe disease. In this review, we will focus on these recent data, and comment on the not so recent observations pointing to the involvement of autophagy in skeletal muscle damage in Pompe disease.

Histone deacetylases 1 and 2 regulate autophagy flux and skeletal muscle homeostasis in mice

Maintenance of skeletal muscle structure and function requires efficient and precise metabolic control. Autophagy plays a key role in metabolic homeostasis of diverse tissues by recycling cellular constituents, particularly under conditions of caloric restriction, thereby normalizing cellular metabolism. Here we show that histone deacetylases (HDACs) 1 and 2 control skeletal muscle homeostasis and autophagy flux in mice. Skeletal muscle-specific deletion of both HDAC1 and HDAC2 results in perinatal lethality of a subset of mice, accompanied by mitochondrial abnormalities and sarcomere degeneration. Mutant mice that survive the first day of life develop a progressive myopathy characterized by muscle degeneration and regeneration, and abnormal metabolism resulting from a blockade to autophagy. HDAC1 and HDAC2 regulate skeletal muscle autophagy by mediating the induction of autophagic gene expression and the formation of autophagosomes, such that myofibers of mice lacking these HDACs accumulate toxic autophagic intermediates. Strikingly, feeding HDAC1/2 mutant mice a high-fat diet from the weaning age releases the block in autophagy and prevents myopathy in adult mice. These findings reveal an unprecedented and essential role for HDAC1 and HDAC2 in maintenance of skeletal muscle structure and function and show that, at least in some pathological conditions, myopathy may be mitigated by dietary modifications.

Autophagic response to a single bout of moderate exercise in murine skeletal muscle

The effect of a single bout of exercise on autopahgy in murine gastrocnemius muscle was investigated. Autophagy is a process for the degradation system of cytoplasmic components, which may help maintain intracellular quality control of cell survival and turnover under normal conditions. The present study investigated the changes of autophagy-related proteins including microtubule-associated protein 1b light chain 3 (LC3), Beclin-1, Atg7 (autophagy-related gene 7), conjugation form of Atg12 to Atg5, lysosome-associated membrane protein (LAMP2a), and muscle-specific RING finger protein-1 (MURF-1) protein level in gastrocnemius muscle after a single bout of treadmill exercise. Mice exercised on a treadmill for 50 min at a speed of 12.3 m/min with a slope of 5°. The animals were sacrificed by cervical dislocation 0, 3, 6, or 12 h after exercise, and muscle samples were collected immediately. Western blot analysis demonstrated that the autophagy marker LC3-II was significantly decreased during the recovery period (3, 6, and 12 h) whereas there was no decrease immediately after exercise (0 h). To identify factors related to this decrease, autophagosome component proteins were examined in murine gastrocnemius muscle. A decrease in Beclin-1, Atg7, and LAMP2a during recovery period was concomitant with the decreased level of LC3-II. Additionally, MuRF-1 expression was significantly increased after a single bout of exercise. This study is the first to demonstrate autophasic related protein expression after a single bout of treadmill exercise and our results suggest that a single bout of treadmill exercise attenuates the autophagic response in murine skeletal muscle.

Selective autophagy mediated by autophagic adapter proteins

Mounting evidence suggests that autophagy is a more selective process than originally anticipated. The discovery and characterization of autophagic adapters, like p62 and NBR1, has provided mechanistic insight into this process. p62 and NBR1 are both selectively degraded by autophagy and able to act as cargo receptors for degradation of ubiquitinated substrates. A direct interaction between these autophagic adapters and the autophagosomal marker protein LC3, mediated by a so-called LIR (LC3-interacting region) motif, their inherent ability to polymerize or aggregate as well as their ability to specifically recognize substrates are required for efficient selective autophagy. These three required features of autophagic cargo receptors are evolutionarily conserved and also employed in the yeast cytoplasm-to-vacuole targeting (Cvt) pathway and in the degradation of P granules in C. elegans. Here, we review the mechanistic basis of selective autophagy in mammalian cells discussing the degradation of misfolded proteins, p62 bodies, aggresomes, mitochondria and invading bacteria. The emerging picture of selective autophagy affecting the regulation of cell signaling with consequences for oxidative stress responses, tumorigenesis and innate immunity is also addressed.

Who needs microtubules? Myogenic reorganization of MTOC, Golgi complex and ER exit sites persists despite lack of normal microtubule tracks

A wave of structural reorganization involving centrosomes, microtubules, Golgi complex and ER exit sites takes place early during skeletal muscle differentiation and completely remodels the secretory pathway. The mechanism of these changes and their functional implications are still poorly understood, in large part because all changes occur seemingly simultaneously. In an effort to uncouple the reorganizations, we have used taxol, nocodazole, and the specific GSK3-β inhibitor DW12, to disrupt the dynamic microtubule network of differentiating cultures of the mouse skeletal muscle cell line C2. Despite strong effects on microtubules, cell shape and cell fusion, none of the treatments prevented early differentiation. Redistribution of centrosomal proteins, conditional on differentiation, was in fact increased by taxol and nocodazole and normal in DW12. Redistributions of Golgi complex and ER exit sites were incomplete but remained tightly linked under all circumstances, and conditional on centrosomal reorganization. We were therefore able to uncouple microtubule reorganization from the other events and to determine that centrosomal proteins lead the reorganization hierarchy. In addition, we have gained new insight into structural and functional aspects of the reorganization of microtubule nucleation during myogenesis.

Pathophysiological role of autophagy: lesson from autophagy-deficient mouse models

Autophagy is a cellular degradation system in which cytoplasmic components including organelles are sequestered by double membrane structures called autophagosomes and sequestered materials are degraded by lysosomal hydrolases for supply of amino acids and for cellular homeostasis. The autophagy induced in response to nutrient deprivation is executed in a nonselective fashion, and adaptation to nutrient-poor conditions is the main purpose of autophagy. On the other hand, recent studies have shed light on another indispensable role for starvation-independent or constitutive autophagy in cellular homeostasis, which is mediated by selective degradation of a specific substrate(s). Herein, we introduce pathophysiological roles of starvation-induced, constitutive, and selective autophagy (in particular, selective turnover of p62 through autophagy) disclosed by autophagy-deficient mouse models.

New insights into therapeutic options for Pompe disease

Glycogen storage disease type II or Pompe disease (GSD II, MIM 232300) is a rare inherited metabolic myopathy caused by a deficiency of lysosomal acid α-glucosidase or acid maltase (GAA; EC 3.2.1.20), resulting in a massive lysosomal glycogen accumulation in cardiac and skeletal muscles. Affected individuals exhibit either severe hypotonia associated with hypertrophic cardiomyopathy (infantile forms) or progressive muscle weakness (late-onset forms). Even if enzyme replacement therapy has recently become a standard treatment, it suffers from several limitations. This review will present the main results of enzyme replacement therapy and the recent findings concerning alternative treatments for Pompe disease, such as gene therapy, enzyme enhancement therapy, and substrate reduction therapy.

Autophagy, nutrition and immunology

Turnover of cellular components in lysosomes or autophagy is an essential mechanism for cellular quality control. Added to this cleaning role, autophagy has recently been shown to participate in the dynamic interaction of cells with the surrounding environment by acting as a point of integration of extracellular cues. In this review, we focus on the relationship between autophagy and two types of environmental factors: nutrients and pathogens. We describe their direct effect on autophagy and discuss how the autophagic reaction to these stimuli allows cells to accommodate the requirements of the cellular response to stress, including those specific to the immune responses.

Autophagy: a core cellular process with emerging links to pulmonary disease

Autophagy is a highly conserved homeostatic pathway by which cells transport damaged proteins and organelles to lysosomes for degradation. Dysregulation of autophagy contributes to the pathogenesis of clinically important disorders in a variety of organ systems but, until recently, little was known about its relationship to diseases of the lung. However, there is now growing evidence at the basic research level that autophagy is linked to the pathogenesis of important pulmonary disorders such as chronic obstructive pulmonary disease, cystic fibrosis, and tuberculosis. In this review, we provide an introduction to the field of autophagy research geared to clinical and research pulmonologists. We focus on the best-studied autophagic mechanism, macroautophagy, and summarize studies that link the regulation of this pathway to pulmonary disease. Last, we offer our perspective on how a better understanding of macroautophagy might be used for designing novel therapies for pulmonary disorders.

Autophagy and cartilage homeostasis mechanisms in joint health, aging and OA

Osteoarthritis (OA) is the most prevalent joint disease, but neither preventive measures nor disease-modifying drugs are available and a continuing need exists for safe and effective symptom-modifying therapies. Clinical trials of candidate disease-modifying OA drugs in patients with established or advanced disease have not demonstrated their efficacy, but these failed trials have motivated investigation into the mechanisms that maintain joint health. The enhancement of such mechanisms could be a novel approach to reducing the risk of OA. Aging is one of the most important risk factors for OA; however, aging of joint cartilage is a process that is distinct from the subsequent cartilage changes that develop following the onset of OA. This Review focuses on the mechanisms that maintain cell and tissue homeostasis, and how these mechanisms fail during the aging process. Autophagy is a cellular homeostasis mechanism for the removal of dysfunctional organelles and macromolecules. Defective autophagy is involved in the pathogenesis of aging-related diseases and recent observations indicate that this process is compromised in aging cartilage. Augmentation of homeostasis mechanisms is discussed as a novel avenue to delay joint aging and reduce OA risk.

Molecular machinery of macroautophagy and its deregulation in diseases

Macroautophagy maintains cellular homeostasis through targeting cytoplasmic contents and organelles into autophagosomes for degradation. This process begins with the assembly of protein complexes on isolation membrane to initiate the formation of autophagosome, followed by its nucleation, elongation and maturation. Fusion of autophagosomes with lysosomes then leads to degradation of the cargo. In the past decade, significant advances have been made on the identification of molecular players that are implicated in various stages of macroautophagy. Post-translational modifications of macroautophagy regulators have also been demonstrated to be critical for the selective targeting of cytoplasmic contents into autophagosomes. In addition, recent demonstration of distinct macroautophagy regulators has led to the identification of different subtypes of macroautophagy. Since deregulation of macroautophagy is implicated in diseases including neurodegenerative disorders, cancers and inflammatory disorders, understanding the molecular machinery of macroautophagy is crucial for elucidating the mechanisms by which macroautophagy is deregulated in these diseases, thereby revealing new potential therapeutic targets and strategies. Here we summarize current knowledge on the regulation of mammalian macroautophagy machineries and their disease-associated deregulation.

Novel therapeutic approaches for inclusion body myositis

PURPOSE OF REVIEW

This review will highlight recent advances in developing strategies to accelerate muscle regeneration and to slow muscle degeneration in myositis, focusing primarily on inclusion body myositis (IBM).

RECENT FINDINGS

Therapies for accelerating muscle regeneration, primarily through inhibition of myostatin, have shown promise in the laboratory and are now entering clinical trials. Recent studies have implicated autophagy, a key cellular process involved in clearance of ubiquitinated aggregates, in the pathogenesis of familial and sporadic inclusion body myositis (sIBM). IBM has joined a growing list of diseases known as TDP-43 proteinopathies, in which this protein becomes mislocalized to the cytoplasm; however, it is unclear whether these protein aggregates or others are pathogenic in this disease.

SUMMARY

New discoveries of biomarkers in sIBM and new insights into the pathogenesis of familial IBM are opening novel therapeutic pathways for these disorders. In particular, drugs that stimulate autophagy, already in development for cancer and neurodegenerative diseases, are candidates for clinical trials. These disease-specific therapies combined with novel therapies to accelerate muscle regeneration hold promise for future therapy for this devastating disease.

Protective effect of caspase inhibition on compression-induced muscle damage

There are currently no effective therapies for treating pressure-induced deep tissue injury. This study tested the efficacy of pharmacological inhibition of caspase in preventing muscle damage following sustained moderate compression. Adult Sprague-Dawley rats were subjected to prolonged moderate compression. Static pressure of 100 mm Hg compression was applied to an area of 1.5 cm2 in the tibialis region of the right limb of the rats for 6 h each day for two consecutive days. The left uncompressed limb served as intra-animal control. Rats were randomized to receive either vehicle (DMSO) as control treatment (n =8) or 6 mg kg⁻¹ of caspase inhibitor (z-VAD-fmk; n =8) prior to the 6 h compression on the two consecutive days.Muscle tissues directly underneath the compression region of the compressed limb and the same region of control limb were harvested after the compression procedure.Histological examination and biochemical/molecular measurement of apoptosis and autophagy were performed. Caspase inhibition was effective in alleviating the compression-induced pathohistology of muscle. The increases in caspase-3 protease activity, TUNEL index, apoptotic DNA fragmentation and pro-apoptotic factors (Bax, p53 and EndoG) and the decreases in anti-apoptotic factors (XIAP and HSP70) observed in compressed muscle of DMSO-treated animals were not found in animals treated with caspase inhibitor. The mRNA content of autophagic factors (Beclin-1, Atg5 and Atg12) and the protein content of LC3, FoxO3 and phospho-FoxO3 that were down-regulated in compressed muscle of DMSO-treated animals were all maintained at their basal level in the caspase inhibitor treated animals. Our data provide evidence that caspase inhibition attenuates compression-induced muscle apoptosis and maintains the basal autophagy level. These findings demonstrate that pharmacological inhibition of caspase/apoptosis is effective in alleviating muscle damage as induced by prolonged compression.

New findings of lysosomal proteolysis in skeletal muscle

PURPOSE OF REVIEW

To discuss the involvement of lysosomes in the control of muscle mass.

RECENT FINDINGS

Lysosomes control the half-life of long-lived proteins and the turnover of organelles and therefore, are critical for cellular homeostasis. Skeletal muscle contraction is a potential source of metabolic, mechanical, and thermal stressors. Therefore, the quality control of proteins and of organelles is particularly active in this tissue. Recent findings have shown that impairment of the degradation systems leads to accumulation of unfolded/misfolded proteins and altered organelles which turns into toxicity for the muscle cells. Conversely, excessive activation of proteolytic machinery, including lysosomal-dependent degradation, contributes to muscle loss, weakness, and finally to death. This article reviews the rapid progress made in the past few years regarding the role of lysosomal-dependent degradation in the homeostasis of adult muscle fibers.

SUMMARY

These findings will help to define the role of the lysosomal system in muscle homeostasis during physiological or pathological conditions.

Pompe disease: design, methodology, and early findings from the Pompe Registry

Pompe disease is an autosomal recessive, progressive, debilitating, and often fatal neuromuscular disorder caused by deficiency of lysosomal acid α-glucosidase (GAA). It is characterized by the accumulation of glycogen in muscle tissue that leads to progressive muscle weakness and loss of function. It presents as a broad spectrum of clinical phenotypes, with varying rates of progression, symptom onset, degree of organ involvement, and severity. The Pompe Registry represents worldwide data collection on this rare and clinically heterogeneous disease. This report describes the design, methodology, and early findings from the Registry and presents an overview of the Registry population over a five-year period from its inception in September 2004 through September 2009. Among the 742 patients from 28 countries in the Registry, 70% (517/742) reported symptom onset >12 months of age and 23% (170/742) reported symptom onset ≤12 months of age. Seventy-eight percent (582/742) of registry patients have received enzyme replacement therapy. Overall, Registry data appear to be consistent with smaller natural history studies in terms of symptoms and disease course in classical infantile Pompe disease (≤12 months of age with cardiomyopathy) and late-onset Pompe disease (>12 months of age). In addition, a subset of patients with symptom onset ≤12 months of age do not have cardiomyopathy (14.7%); these patients appear to have a later age at first symptoms and diagnosis than their peers with cardiomyopathy. As the largest dataset on Pompe disease to date, the Pompe Registry will serve to improve recognition of the disease, enhance understanding of the variable disease course, and offer insights into treated and untreated disease course.

The PKB/FOXO switch in aging and cancer

Aging is characterized by the general decline in tissue and body function and the increased susceptibility to age-related pathologies, such as cancer. To maintain optimal tissue and body function, organisms have developed complex mechanisms for tissue homeostasis. Importantly, it is becoming apparent that these same mechanisms when deregulated also result in the development of age-related disease. The build in fail safe mechanisms of homeostasis, which prevent skewing toward disease, themselves contribute to aspects of aging. Thus, longevity is limited by an intrinsic trade-off between optimal tissue function and disease. Consequently, aging and age-related diseases, such as cancer and diabetes are driven by the same genetic determinants. Illustrative in this respect is the insulin/IGF-1 signaling pathway acting through PI3K/PKB and FOXO. Loss of PKB signaling contributes to diabetes, whereas gain of function of PKB drives cancer. Enhanced FOXO activity, at least in model organism contributes to extended lifespan and acts as a tumor suppressive mechanism. Here, we focus on the linkage between PKB and FOXO as a central switch in contributing to tissue homeostasis and age-related diseases in particular cancer. This article is part of a Special Issue entitled: P13K-AKT-FoxO axis in cancer and aging.

Mitochondrial biogenesis and fragmentation as regulators of muscle protein degradation

Mitochondria form a dynamic network that rapidly adapts to cellular energy demand. This adaptation is particularly important in skeletal muscle because of its high metabolic rate. Indeed, muscle energy level is one of the cellular checkpoints that lead either to sustained protein synthesis and growth or protein breakdown and atrophy. Mitochondrial function is affected by changes in shape, number, and localization. The dynamics that control the mitochondrial network, such as biogenesis and fusion, or fragmentation and fission, ultimately affect the signaling pathways that regulate muscle mass. Regular exercise and healthy muscles are important players in the metabolic control of human body. Indeed, a sedentary lifestyle is detrimental for muscle function and is one of the major causes of metabolic disorders such as obesity and diabetes. This article reviews the rapid progress made in the past few years regarding the role of mitochondria in the control of proteolytic systems and in the loss of muscle mass and function.

Biochemical and structural study on a S529V mutant acid α-glucosidase responsive to pharmacological chaperones

Recently, pharmacological chaperone therapy for Pompe disease with small molecules such as imino sugars has attracted interest. But mutant acid α-glucosidase (GAA) species responsive to imino sugars are limited. To elucidate the characteristics of a mutant GAA responsive to imino sugars, we performed biochemical and structural analyses. Among cultured fibroblast cell lines derived from Japanese Pompe patients, only one carrying p.S529V/p.S619R amino acid substitutions responded to 1-deoxynojirimycin (DNJ), and an expression study revealed that DNJ, N-butyl-deoxynojirimycin and nojirimycin-1-sulfonic acid increased the enzyme activity of the S529V mutant GAA expressed in Chinese hamster ovary cells. The results of western blotting analysis suggested that these imino sugars facilitated the intracellular transportation of the mutant GAA and stabilized it. Among these imino sugars, DNJ exhibited the strongest action on the mutant GAA. Structural analysis revealed that DNJ almost completely occupied the active site pocket, and interacted with amino acid residues comprising it through van der Waals contacts and hydrogen bonds. This information will be useful for improvement of pharmacological chaperone therapy for Pompe disease.

Suppression of autophagy permits successful enzyme replacement therapy in a lysosomal storage disorder--murine Pompe disease

Autophagy, an intracellular system for delivering portions of cytoplasm and damaged organelles to lysosomes for degradation/recycling, plays a role in many physiological processes and is disturbed in many diseases. We recently provided evidence for the role of autophagy in Pompe disease, a lysosomal storage disorder in which acid alphaglucosidase, the enzyme involved in the breakdown of glycogen, is deficient or absent. Clinically the disease manifests as a cardiac and skeletal muscle myopathy. The current enzyme replacement therapy (ERT) clears lysosomal glycogen effectively from the heart but less so from skeletal muscle. In our Pompe model, the poor muscle response to therapy is associated with the presence of pools of autophagic debris. To clear the fibers of the autophagic debris, we have generated a Pompe model in which an autophagy gene, Atg7, is inactivated in muscle. Suppression of autophagy alone reduced the glycogen level by 50–60%. Following ERT, muscle glycogen was reduced to normal levels, an outcome not observed in Pompe mice with genetically intact autophagy. The suppression of autophagy, which has proven successful in the Pompe model, is a novel therapeutic approach that may be useful in other diseases with disturbed autophagy.

Sporadic inclusion body myositis: possible pathogenesis inferred from biomarkers

PURPOSE OF REVIEW

The relevance of proteins that accumulate and aggregate in the muscle fibers of patients with sporadic inclusion body myositis (sIBM) is unknown. Many of these proteins also aggregate in other disorders, including Alzheimer's disease, leading to speculation that sIBM pathogenesis has similarities to neurodegenerative disorders. Our review will discuss current studies on these protein biomarkers and their utility in sIBM diagnosis.

RECENT FINDINGS

Two 'classical' components of sIBM aggregates (amyloid beta and phospho-tau) have been re-evaluated. Three additional components of aggregates (TDP-43, p62, and LC3) have been identified. The sensitivity and specificity of these biomarkers has been explored. Two studies suggest that TDP-43 may have clinical utility in distinguishing sIBM from other inflammatory myopathies.

SUMMARY

The fact that sIBM muscle accumulates multiple protein aggregates with no single protein appearing in every sIBM patient biopsy suggests that it is not presently possible to place pathogenic blame on any single protein (i.e. amyloid beta or TDP-43). Instead changes in protein homeostasis may lead to the accumulation of different proteins that have a propensity to aggregate in skeletal muscle. Therapies aimed at improving protein homeostasis, instead of targeting a specific protein that may or may not accumulate in all sIBM patients, could be useful future strategies for this devastating and enigmatic disorder.

Fiber type conversion by PGC-1α activates lysosomal and autophagosomal biogenesis in both unaffected and Pompe skeletal muscle

PGC-1α is a transcriptional co-activator that plays a central role in the regulation of energy metabolism. Our interest in this protein was driven by its ability to promote muscle remodeling. Conversion from fast glycolytic to slow oxidative fibers seemed a promising therapeutic approach in Pompe disease, a severe myopathy caused by deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA) which is responsible for the degradation of glycogen. The recently approved enzyme replacement therapy (ERT) has only a partial effect in skeletal muscle. In our Pompe mouse model (KO), the poor muscle response is seen in fast but not in slow muscle and is associated with massive accumulation of autophagic debris and ineffective autophagy. In an attempt to turn the therapy-resistant fibers into fibers amenable to therapy, we made transgenic KO mice expressing PGC-1α in muscle (tgKO). The successful switch from fast to slow fibers prevented the formation of autophagic buildup in the converted fibers, but PGC-1α failed to improve the clearance of glycogen by ERT. This outcome is likely explained by an unexpected dramatic increase in muscle glycogen load to levels much closer to those observed in patients, in particular infants, with the disease. We have also found a remarkable rise in the number of lysosomes and autophagosomes in the tgKO compared to the KO. These data point to the role of PGC-1α in muscle glucose metabolism and its possible role as a master regulator for organelle biogenesis - not only for mitochondria but also for lysosomes and autophagosomes. These findings may have implications for therapy of lysosomal diseases and other disorders with altered autophagy.

Chemical and/or biological therapeutic strategies to ameliorate protein misfolding diseases

Inheriting a mutant misfolding-prone protein that cannot be efficiently folded in a given cell type(s) results in a spectrum of human loss-of-function misfolding diseases. The inability of the biological protein maturation pathways to adapt to a specific misfolding-prone protein also contributes to pathology. Chemical and biological therapeutic strategies are presented that restore protein homeostasis, or proteostasis, either by enhancing the biological capacity of the proteostasis network or through small molecule stabilization of a specific misfolding-prone protein. Herein, we review the recent literature on therapeutic strategies to ameliorate protein misfolding diseases that function through either of these mechanisms, or a combination thereof, and provide our perspective on the promise of alleviating protein misfolding diseases by taking advantage of proteostasis adaptation.

Regulation of mammalian autophagy in physiology and pathophysiology

(Macro)autophagy is a bulk degradation process that mediates the clearance of long-lived proteins and organelles. Autophagy is initiated by double-membraned structures, which engulf portions of cytoplasm. The resulting autophagosomes ultimately fuse with lysosomes, where their contents are degraded. Although the term autophagy was first used in 1963, the field has witnessed dramatic growth in the last 5 years, partly as a consequence of the discovery of key components of its cellular machinery. In this review we focus on mammalian autophagy, and we give an overview of the understanding of its machinery and the signaling cascades that regulate it. As recent studies have also shown that autophagy is critical in a range of normal human physiological processes, and defective autophagy is associated with diverse diseases, including neurodegeneration, lysosomal storage diseases, cancers, and Crohn's disease, we discuss the roles of autophagy in health and disease, while trying to critically evaluate if the coincidence between autophagy and these conditions is causal or an epiphenomenon. Finally, we consider the possibility of autophagy upregulation as a therapeutic approach for various conditions.

Autophagy for tissue homeostasis and neuroprotection

Although autophagy has frequently been viewed as a cell death mechanism in the mammalian system, it is now considered as indispensable for the homeostasis of cells, tissues, and organisms. Basal or stress-induced autophagy plays essential and diverse roles in a variety of tissues, due to its cytoprotective properties. In this review, we briefly discuss the different homeostatic functions of autophagy that have been finely dissected in mammals through the generation and characterization of animal models with tissue-specific autophagic alterations. In addition, and given the importance of constitutive autophagy in neuronal tissues, we describe in more detail the specific roles of autophagy in the central nervous system (CNS). Finally, we discuss the contribution of autophagy malfunctions to the development of several common neurological disorders and the potential benefits of pharmacologically induced autophagy for the avoidance of neurodegeneration.

Autophagy in mammalian development and differentiation

It has been known for many decades that autophagy, a conserved lysosomal degradation pathway, is highly active during differentiation and development. However, until the discovery of the autophagy-related (ATG) genes in the 1990s, the functional significance of this activity was unknown. Initially, genetic knockout studies of ATG genes in lower eukaryotes revealed an essential role for the autophagy pathway in differentiation and development. In recent years, the analyses of systemic and tissue-specific knockout models of ATG genes in mice has led to an explosion of knowledge about the functions of autophagy in mammalian development and differentiation. Here we review the main advances in our understanding of these functions.

Inhibition of glycogen biosynthesis via mTORC1 suppression as an adjunct therapy for Pompe disease

Pompe disease, also known as glycogen storage disease (GSD) type II, is caused by deficiency of lysosomal acid alpha-glucosidase (GAA). The resulting glycogen accumulation causes a spectrum of disease severity ranging from a rapidly progressive course that is typically fatal by 1-2years of age to a more slowly progressive course that causes significant morbidity and early mortality in children and adults. Recombinant human GAA (rhGAA) improves clinical outcomes with variable results. Adjunct therapy that increases the effectiveness of rhGAA may benefit some Pompe patients. Co-administration of the mTORC1 inhibitor rapamycin with rhGAA in a GAA knockout mouse reduced muscle glycogen content more than rhGAA or rapamycin alone. These results suggest mTORC1 inhibition may benefit GSDs that involve glycogen accumulation in muscle.

Lysosomal storage disease: revealing lysosomal function and physiology

The discovery over five decades ago of the lysosome, as a degradative organelle and its dysfunction in lysosomal storage disorder patients, was both insightful and simple in concept. Here, we review some of the history and pathophysiology of lysosomal storage disorders to show how they have impacted on our knowledge of lysosomal biology. Although a significant amount of information has been accrued on the molecular genetics and biochemistry of lysosomal storage disorders, we still do not fully understand the mechanistic link between the storage material and disease pathogenesis. However, the accumulation of undegraded substrate(s) can disrupt other lysosomal degradation processes, vesicular traffic, and lysosomal biogenesis to evoke the diverse pathophysiology that is evident in this complex set of disorders.

Autophagy in health and disease. 3. Involvement of autophagy in muscle atrophy

Loss of muscle mass aggravates a variety of diseases, and understanding the molecular mechanisms that control muscle wasting is critical for developing new therapeutic approaches. Weakness is caused by loss of muscle proteins, and recent studies have underlined a major role for the autophagy-lysosome system in regulating muscle mass. Some key components of the autophagy machinery are transcriptionally upregulated during muscle wasting, and their induction precedes muscle loss. However, it is unclear whether autophagy is detrimental, causing atrophy, or beneficial, promoting survival during catabolic conditions. This review discusses recent findings on signaling pathways regulating autophagy.

Chaperone-assisted degradation: multiple paths to destruction

Molecular chaperones are well known as facilitators of protein folding and assembly. However, in recent years multiple chaperone-assisted degradation pathways have also emerged, including CAP (chaperone-assisted proteasomal degradation), CASA (chaperone-assisted selective autophagy), and CMA (chaperone-mediated autophagy). Within these pathways chaperones facilitate the sorting of non-native proteins to the proteasome and the lysosomal compartment for disposal. Impairment of these pathways contributes to the development of cancer, myopathies, and neurodegenerative diseases. Chaperone-assisted degradation thus represents an essential aspect of cellular proteostasis, and its pharmacological modulation holds the promise to ameliorate some of the most devastating diseases of our time. Here, we discuss recent insights into molecular mechanisms underlying chaperone-assisted degradation in mammalian cells and highlight its biomedical relevance.

Inclusion body myopathy, Paget's disease of the bone and fronto-temporal dementia: a disorder of autophagy

Inclusion body myopathy associated with Paget's disease of the bone and fronto-temporal dementia (IBMPFD) is a progressive autosomal dominant disorder caused by mutations in p97/VCP (valosin-containing protein). p97/VCP is a member of the AAA+ (ATPase associated with a variety of activities) protein family and participates in multiple cellular processes. One particularly important role for p97/VCP is facilitating intracellular protein degradation. p97/VCP has traditionally been thought to mediate the ubiquitin-proteasome degradation of proteins; however, recent studies challenge this dogma. p97/VCP clearly participates in the degradation of aggregate-prone proteins, a process principally mediated by autophagy. In addition, IBMPFD mutations in p97/VCP lead to accumulation of autophagic structures in patient and transgenic animal tissue. This is likely due to a defect in p97/VCP-mediated autophagosome maturation. The following review will discuss the evidence for p97/VCP in autophagy and how a disruption in this process contributes to IBMPFD pathogenesis.

Myoferlin is required for insulin-like growth factor response and muscle growth

Insulin-like growth factor (IGF) is a potent stimulus of muscle growth. Myoferlin is a membrane-associated protein important for muscle development and regeneration. Myoferlin-null mice have smaller muscles and defective myoblast fusion. To understand the mechanism by which myoferlin loss retards muscle growth, we found that myoferlin-null muscle does not respond to IGF1. In vivo after IGF1 infusion, control muscle increased myofiber diameter by 25%, but myoferlin-null muscle was unresponsive. Myoblasts cultured from myoferlin-null muscle and treated with IGF1 also failed to show the expected increase in fusion to multinucleate myotubes. The IGF1 receptor colocalized with myoferlin at sites of myoblast fusion. The lack of IGF1 responsiveness in myoferlin-null myoblasts was linked directly to IGF1 receptor mistrafficking as well as decreased IGF1 signaling. In myoferlin-null myoblasts, the IGF1 receptor accumulated into large vesicular structures. These vesicles colocalized with a marker of late endosomes/lysosomes, LAMP2, specifying redirection from a recycling to a degradative pathway. Furthermore, ultrastructural analysis showed a marked increase in vacuoles in myoferlin-null muscle. These data demonstrate that IGF1 receptor recycling is required for normal myogenesis and that myoferlin is a critical mediator of postnatal muscle growth mediated by IGF1.-Demonbreun, A. R., Posey, A. D., Heretis, K., Swaggart, K. A., Earley, J. U., Pytel, P., McNally, E. M. Myoferlin is required for insulin-like growth factor response and muscle growth.

Impaired organization and function of myofilaments in single muscle fibers from a mouse model of Pompe disease

Pompe disease, a deficiency of lysosomal acid alpha-glucosidase, is a disorder of glycogen metabolism that can affect infants, children, or adults. In all forms of the disease, there is progressive muscle pathology leading to premature death. The pathology is characterized by accumulation of glycogen in lysosomes, autophagic buildup, and muscle atrophy. The purpose of the present investigation was to determine if myofibrillar dysfunction in Pompe disease contributes to muscle weakness beyond that attributed to atrophy. The study was performed on isolated myofibers dissected from severely affected fast glycolytic muscle in the alpha-glucosidase knockout mouse model. Psoas muscle fibers were first permeabilized, so that the contractile proteins could be directly relaxed or activated by control of the composition of the bathing solution. When normalized by cross-sectional area, single fibers from knockout mice produced 6.3 N/cm2 of maximum Ca2+-activated tension compared with 12.0 N/cm2 produced by wild-type fibers. The total protein concentration was slightly higher in the knockout mice, but concentrations of the contractile proteins myosin and actin remained unchanged. Structurally, X-ray diffraction showed that the actin and myosin filaments, normally arranged in hexagonal arrays, were disordered in the knockout muscle, and a lower fraction of myosin cross bridges was near the actin filaments in the relaxed muscle. The results are consistent with a disruption of actin and myosin interactions in the knockout muscles, demonstrating that impaired myofibrillar function contributes to weakness in the diseased muscle fibers.

Autophagy in health and disease. 1. Regulation and significance of autophagy: an overview

Macroautophagy is a vacuolar degradation pathway that terminates in the lysosomal compartment after formation of a cytoplasmic vacuole or autophagosome that engulfs macromolecules and organelles. The identification of ATG (autophagy-related) genes that are involved in the formation of autophagosomes has greatly increased our knowledge of the molecular basis of macroautophagy, and its roles in cell function, which extend far beyond degradation and quality control of the cytoplasm. Macroautophagy, which plays a major role in tissue homeostasis, is now recognized as contributing to innate and adaptive immune responses. Recently, several mediators of apoptosis have been shown to control macroautophagy. Deciphering the cross talk between macroautophagy and apoptosis probably should help increase understanding of the role of macroautophagy in human disease and is likely to be of therapeutic importance.