Autophagic degradation of glycogen in skeletal muscles of the newborn rat

Failure of Autophagy in Pompe Disease

Autophagy is an evolutionarily conserved lysosome-dependent degradation of cytoplasmic constituents. The system operates as a critical cellular pro-survival mechanism in response to nutrient deprivation and a variety of stress conditions. On top of that, autophagy is involved in maintaining cellular homeostasis through selective elimination of worn-out or damaged proteins and organelles. The autophagic pathway is largely responsible for the delivery of cytosolic glycogen to the lysosome where it is degraded to glucose via acid α-glucosidase. Although the physiological role of lysosomal glycogenolysis is not fully understood, its significance is highlighted by the manifestations of Pompe disease, which is caused by a deficiency of this lysosomal enzyme. Pompe disease is a severe lysosomal glycogen storage disorder that affects skeletal and cardiac muscles most. In this review, we discuss the basics of autophagy and describe its involvement in the pathogenesis of muscle damage in Pompe disease. Finally, we outline how autophagic pathology in the diseased muscles can be used as a tool to fast track the efficacy of therapeutic interventions.

Interplay of energy metabolism and autophagy

Macroautophagy/autophagy, is widely recognized for its crucial role in enabling cell survival and maintaining cellular energy homeostasis during starvation or energy stress. Its regulation is intricately linked to cellular energy status. In this review, covering yeast, mammals, and plants, we aim to provide a comprehensive overview of the understanding of the roles and mechanisms of carbon- or glucose-deprivation related autophagy, showing how cells effectively respond to such challenges for survival. Further investigation is needed to determine the specific degraded substrates by autophagy during glucose or energy deprivation and the diverse roles and mechanisms during varying durations of energy starvation.Abbreviations: ADP: adenosine diphosphate; AMP: adenosine monophosphate; AMPK: AMP-activated protein kinase; ATG: autophagy related; ATP: adenosine triphosphate; ER: endoplasmic reticulum; ESCRT: endosomal sorting complex required for transport; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GD: glucose deprivation; GFP: green fluorescent protein; GTPases: guanosine triphosphatases; HK2: hexokinase 2; K phaffii: Komagataella phaffii; LD: lipid droplet; MAP1LC3/LC3: microtubule-associated protein1 light chain 3; MAPK: mitogen-activated protein kinase; Mec1: mitosis entry checkpoint 1; MTOR: mechanistic target of rapamycin kinase; NAD (+): nicotinamide adenine dinucleotide; OGD: oxygen and glucose deprivation; PAS: phagophore assembly site; PCD: programmed cell death; PtdIns3K: class III phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; ROS: reactive oxygen species; S. cerevisiae: Saccharomyces cerevisiae; SIRT1: sirtuin 1; Snf1: sucrose non-fermenting 1; STK11/LKB1: serine/threonine kinase 11; TFEB: transcription factor EB; TORC1: target of rapamycin complex 1; ULK1: unc-51 like kinase 1; Vps27: vacuolar protein sorting 27; Vps4: vacuolar protein sorting 4.

The mTOR-Autophagy Axis and the Control of Metabolism

The mechanistic target of rapamycin (mTOR), master regulator of cellular metabolism, exists in two distinct complexes: mTOR complex 1 and mTOR complex 2 (mTORC1 and 2). MTORC1 is a master switch for most energetically onerous processes in the cell, driving cell growth and building cellular biomass in instances of nutrient sufficiency, and conversely, allowing autophagic recycling of cellular components upon nutrient limitation. The means by which the mTOR kinase blocks autophagy include direct inhibition of the early steps of the process, and the control of the lysosomal degradative capacity of the cell by inhibiting the transactivation of genes encoding structural, regulatory, and catalytic factors. Upon inhibition of mTOR, autophagic recycling of cellular components results in the reactivation of mTORC1; thus, autophagy lies both downstream and upstream of mTOR. The functional relationship between the mTOR pathway and autophagy involves complex regulatory loops that are significantly deciphered at the cellular level, but incompletely understood at the physiological level. Nevertheless, genetic evidence stemming from the use of engineered strains of mice has provided significant insight into the overlapping and complementary metabolic effects that physiological autophagy and the control of mTOR activity exert during fasting and nutrient overload.

Advancements in AAV-mediated Gene Therapy for Pompe Disease

Pompe disease (glycogen storage disease type II) is caused by mutations in acid α-glucosidase (GAA) resulting in lysosomal pathology and impairment of the muscular and cardio-pulmonary systems. Enzyme replacement therapy (ERT), the only approved therapy for Pompe disease, improves muscle function by reducing glycogen accumulation but this approach entails several limitations including a short drug half-life and an antibody response that results in reduced efficacy. To address these limitations, new treatments such as gene therapy are under development to increase the intrinsic ability of the affected cells to produce GAA. Key components to gene therapy strategies include the choice of vector, promoter, and the route of administration. The efficacy of gene therapy depends on the ability of the vector to drive gene expression in the target tissue and also on the recipient's immune tolerance to the transgene protein. In this review, we discuss the preclinical and clinical studies that are paving the way for the development of a gene therapy strategy for patients with early and late onset Pompe disease as well as some of the challenges for advancing gene therapy.

Pros and cons of different ways to address dysfunctional autophagy in Pompe disease

Autophagy is a major intracellular self-digestion process that brings cytoplasmic materials to the lysosome for degradation. Defective autophagy has been linked to a broad range of human disorders, including cancer, diabetes, neurodegeneration, autoimmunity, cardiovascular diseases, and myopathies. In Pompe disease, a severe neuromuscular disorder, disturbances in autophagic process manifest themselves as progressive accumulation of undegraded cellular debris in the diseased muscle cells. A growing body of evidence has connected this defect to the decline in muscle function and muscle resistance to the currently available treatment-enzyme replacement therapy (ERT). Both induction and inhibition of autophagy have been tested in pre-clinical studies in a mouse model of the disease. Here, we discuss strengths and weaknesses of different approaches to address autophagic dysfunction in the context of Pompe disease.

Improved efficacy of a next-generation ERT in murine Pompe disease

Pompe disease is a rare inherited disorder of lysosomal glycogen metabolism due to acid α-glucosidase (GAA) deficiency. Enzyme replacement therapy (ERT) using alglucosidase alfa, a recombinant human GAA (rhGAA), is the only approved treatment for Pompe disease. Although alglucosidase alfa has provided clinical benefits, its poor targeting to key disease-relevant skeletal muscles results in suboptimal efficacy. We are developing an rhGAA, ATB200 (Amicus proprietary rhGAA), with high levels of mannose-6-phosphate that are required for efficient cellular uptake and lysosomal trafficking. When administered in combination with the pharmacological chaperone AT2221 (miglustat), which stabilizes the enzyme and improves its pharmacokinetic properties, ATB200/AT2221 was substantially more potent than alglucosidase alfa in a mouse model of Pompe disease. The new investigational therapy is more effective at reversing the primary abnormality - intralysosomal glycogen accumulation - in multiple muscles. Furthermore, unlike the current standard of care, ATB200/AT2221 dramatically reduces autophagic buildup, a major secondary defect in the diseased muscles. The reversal of lysosomal and autophagic pathologies leads to improved muscle function. These data demonstrate the superiority of ATB200/AT2221 over the currently approved ERT in the murine model.

The ER - Glycogen Particle - Phagophore Triangle: A Hub Connecting Glycogenolysis and Glycophagy?

Glycogen particle is an intracellular organelle, which serves as a carbohydrate reserve in various cells. The function of glycogen is not entirely known in several cell types. Glycogen can be mobilized for different purposes, which can be related to cellular metabolic needs, intracellular redox state, metabolic state of the whole organism depending on regulatory aspects and also on cell functions. Essentially there are two different ways of glycogen degradation localized in different cellular organelles: glycogenolysis or lysosomal breakdown by acid alpha-glucosidase. While glycogenolysis occurs in glycogen particles connected to endoplasmic reticulum membrane, glycogen particles can be also combined with phagophores forming autophagosomes. A subdomain of the endoplasmic reticulum membrane - omegasomes - are the sites for phagophore formation. Thus, three organelles, the endoplasmic reticulum, the phagophore and the glycogen particle forms a triangle in which glycogen degradation occurs. The physiological significance, molecular logic and regulation of the two different catabolic paths are summarized and discussed with special aspect on the role of glycogen particles in intracellular organelle homeostasis and on molecular pathology of the cell. Pathological aspects and some diseases connected to the two different degradation pathways of glycogen particles are also detailed.

Sexual dimorphism in the fetal cardiac response to maternal nutrient restriction

Poor maternal nutrition causes intrauterine growth restriction (IUGR); however, its effects on fetal cardiac development are unclear. We have developed a baboon model of moderate maternal undernutrition, leading to IUGR. We hypothesized that the IUGR affects fetal cardiac structure and metabolism. Six control pregnant baboons ate ad-libitum (CTRL)) or 70% CTRL from 0.16 of gestation (G). Fetuses were euthanized at C-section at 0.9G under general anesthesia. Male but not female IUGR fetuses showed left ventricular fibrosis inversely correlated with birth weight. Expression of extracellular matrix protein TSP-1 was increased (p<0.05) in male IUGR. Expression of cardiac fibrotic markers TGFβ, SMAD3 and ALK-1 were downregulated in male IUGRs with no difference in females. Autophagy was present in male IUGR evidenced by upregulation of ATG7 expression and lipidation LC3B. Global miRNA expression profiling revealed 56 annotated and novel cardiac miRNAs exclusively dysregulated in female IUGR, and 38 cardiac miRNAs were exclusively dysregulated in males (p<0.05). Fifteen (CTRL) and 23 (IUGR) miRNAs, were differentially expressed between males and females (p<0.05) suggesting sexual dimorphism, which can be at least partially explained by differential expression of upstream transcription factors (e.g. HNF4α, and NFκB p50). Lipidomics analysis of fetal cardiac tissue exhibited a net increase in diacylglycerol and plasmalogens and a decrease in triglycerides and phosphatidylcholines. In summary, IUGR resulting from decreased maternal nutrition is associated with sex-dependent dysregulations in cardiac structure, miRNA expression, and lipid metabolism. If these changes persist postnatally, they may program offspring for higher later life cardiac risk.

Autophagic regulation in steroid hormone-responsive systems

Two female sex steroid hormones, estrogen and progesterone, are crucial regulators of many physiological functions of reproductive organs. These two hormones are versatile factors linking growth, differentiation, metabolism, and death of cells in the uterus. In recent years, it has become evident that autophagy is involved in the effects of estrogen and progesterone on various cellular events in reproductive organs. Autophagy is the self-eating catabolic process which is linked to cell survival and death in many contexts. In this review, we focus on the new findings concerning the regulation of autophagic response by sex steroid hormones in responsive target organs. We also attempt to further expand our insight into intracellular signaling mediators governing this regulation.

Glycogen metabolism has a key role in the cancer microenvironment and provides new targets for cancer therapy

Metabolic reprogramming is a hallmark of cancer cells and contributes to their adaption within the tumour microenvironment and resistance to anticancer therapies. Recently, glycogen metabolism has become a recognised feature of cancer cells since it is upregulated in many tumour types, suggesting that it is an important aspect of cancer cell pathophysiology. Here, we provide an overview of glycogen metabolism and its regulation, with a focus on its role in metabolic reprogramming of cancer cells under stress conditions such as hypoxia, glucose deprivation and anticancer treatment. The various methods to detect glycogen in tumours in vivo as well as pharmacological modulators of glycogen metabolism are also reviewed. Finally, we discuss the therapeutic value of targeting glycogen metabolism as a strategy for combinational approaches in cancer treatment.

AMPK and autophagy in glucose/glycogen metabolism

Glucose/glycogen metabolism is a primary metabolic pathway acting on a variety of cellular needs, such as proliferation, growth, and survival against stresses. The multiple regulatory mechanisms underlying a specific metabolic fate have been documented and explained the molecular basis of various pathophysiological conditions, including metabolic disorders and cancers. AMP-activated protein kinase (AMPK) has been appreciated for many years as a central metabolic regulator to inhibit energy-consuming pathways as well as to activate the compensating energy-producing programs. In fact, glucose starvation is a potent physiological AMPK activating condition, in which AMPK triggers various subsequent metabolic events depending on cells or tissues. Of note, the recent studies show bidirectional interplay between AMPK and glycogen. A growing number of studies have proposed additional level of metabolic regulation by a lysosome-dependent catabolic program, autophagy. Autophagy is a critical degradative pathway not only for maintenance of cellular homeostasis to remove potentially dangerous constituents, such as protein aggregates and dysfunctional subcellular organelles, but also for adaptive responses to metabolic stress, such as nutrient starvation. Importantly, many lines of evidence indicate that autophagy is closely connected with nutrient signaling modules, including AMPK, to fine-tune the metabolic pathways in response to many different cellular cues. In this review, we introduce the studies demonstrating the role of AMPK and autophagy in glucose/glycogen metabolism. Also, we describe the recent advances on their contributions to the metabolic disorders.

Autophagy modulates amino acid signaling network in myotubes: differential effects on mTORC1 pathway and the integrated stress response

Induction of autophagy and the integrated stress response is important for amino acid homeostasis. It remains unknown whether the autophagy coregulates both mechanistic target of rapamycin complex 1 (mTORC1) signaling and the integrated stress response. In mouse C2C12 myotubes, we found that amino acid limitation induced autophagy and that the subsequent release of amino acid is required to sustain mTORC1 signaling. Inhibition of autophagy by bafilomycin A1 or chloroquine treatment during amino acid scarcity abolished mTORC1 signaling, an effect that could be rescued by inhibiting protein synthesis or amino acid supplementation, respectively. Autophagy is required to sustain the balance of both essential and nonessential amino acids during amino acid starvation, and it has a predominant role over the ubiquitin-proteasome system in the regulation of mTORC1. Inhibition of autophagy was found to activate the integrated stress response, as well as eukaryotic initiation factor 2α (eIF2α) and its target genes independent of amino acid availability. Conversely, autophagy induction via mTOR inhibition is sufficient to reduce eIF2α phosphorylation. Thus, autophagy protects the eIF2α-mediated stress response independent of amino acid supply in cultured myotubes. Our results showed that autophagy uniquely modulates mTORC1 and the integrated stress response in an amino acid-dependent and -independent manner, respectively.

Pompe disease: from pathophysiology to therapy and back again

Pompe disease is a lysosomal storage disorder in which acid alpha-glucosidase (GAA) is deficient or absent. Deficiency of this lysosomal enzyme results in progressive expansion of glycogen-filled lysosomes in multiple tissues, with cardiac and skeletal muscle being the most severely affected. The clinical spectrum ranges from fatal hypertrophic cardiomyopathy and skeletal muscle myopathy in infants to relatively attenuated forms, which manifest as a progressive myopathy without cardiac involvement. The currently available enzyme replacement therapy (ERT) proved to be successful in reversing cardiac but not skeletal muscle abnormalities. Although the overall understanding of the disease has progressed, the pathophysiology of muscle damage remains poorly understood. Lysosomal enlargement/rupture has long been considered a mechanism of relentless muscle damage in Pompe disease. In past years, it became clear that this simple view of the pathology is inadequate; the pathological cascade involves dysfunctional autophagy, a major lysosome-dependent intracellular degradative pathway. The autophagic process in Pompe skeletal muscle is affected at the termination stage—impaired autophagosomal-lysosomal fusion. Yet another abnormality in the diseased muscle is the accelerated production of large, unrelated to ageing, lipofuscin deposits—a marker of cellular oxidative damage and a sign of mitochondrial dysfunction. The massive autophagic buildup and lipofuscin inclusions appear to cause a greater effect on muscle architecture than the enlarged lysosomes outside the autophagic regions. Furthermore, the dysfunctional autophagy affects the trafficking of the replacement enzyme and interferes with its delivery to the lysosomes. Several new therapeutic approaches have been tested in Pompe mouse models: substrate reduction therapy, lysosomal exocytosis following the overexpression of transcription factor EB and a closely related but distinct factor E3, and genetic manipulation of autophagy.

Suppression of autophagic activation in the mouse uterus by estrogen and progesterone

Autophagy is a major cellular catabolic pathway tightly associated with cell survival. The involvement of autophagy in the prolonged survival of blastocysts in the uterus is well established, and it was assumed that ovarian steroid hormones - progesterone (P4) and estrogens - have important roles in the regulation of autophagy. However, information is scarce regarding whether these hormones regulate autophagy in certain hormone-responsive cellular systems. In this study, we investigated the effects of estrogen and P4 on autophagic response in the uteri of pregnant mice and in ovariectomized (OVX) mice treated with hormones. During pregnancy, autophagic response is high on days 1 and 2 when the uterus shows an inflammatory response to mating, but it subsides around the time of implantation. Dexamethasone treatment to day 1 pregnant mice reduced autophagy in the uterus. In OVX mouse uteri, estrogen or P4 reduces autophagic response within 6 h. Glycogen content in OVX uteri was increased by 3-methyladenine treatment, suggesting that autophagy is involved in glycogen breakdown in the hormone-deprived uterus. The classical nuclear receptor antagonists, ICI 182 780 or mifepristone, lead to the recovery of the autophagic response in OVX uteri. The suppression of autophagy by 17β-estradiol is inversely correlated with the accumulation of phospho-mouse target of rapamycin, and rapamycin treatment is moderately effective in the upregulation of autophagic response in OVX mouse uteri. Collectively, this study establishes that the uterine autophagy is induced in hormone-derived environment and is suppressed by hormone treatment. Uterine autophagy may have multiple functions as a responsive mechanism to acute inflammation and as an energy provider by breaking down glycogen under hormone deprivation.

Cardiomyocyte glycophagy is regulated by insulin and exposure to high extracellular glucose

Disturbed systemic glycemic and insulinemic status elicits cardiomyocyte metabolic stress and altered glucose handling. In diabetes, pathological myocardial glycogen accumulation occurs. Recently, evidence of a specific myocardial autophagic degradation pathway for glycogen ("glycophagy") has been reported, differentiated from the more well-characterized protein "macrophagy" pathway. The goal of this study was to identify potential mechanisms involved in cardiac glycogen accumulation, glycophagy, and macrophagy regulation using cultured neonatal rat ventricular myocytes (NRVMs). In NRVMs, insulin-induced Akt phosphorylation was evident with 5 mM-glucose conditions (∼2.3-fold increased). Under high-glucose (30 mM) conditions, insulin-augmented phosphorylation was not observed. Accumulation of glycogen was observed in response to insulin only in high-glucose conditions (∼2-fold increase). Increased expression of the glycophagy marker starch-binding domain-containing protein-1 (STBD1, 25% increase) was observed under high-glucose and insulin conditions. Expression levels of the macrophagy markers p62 and light chain protein 3BII:I were not increased by insulin at either glucose level. Preliminary results from hearts of streptozotocin-treated diabetic rats are supportive of the findings obtained in NRVMs, suggesting diabetes induced elevated expression of STBD1 and of an additional glycophagy marker GABA(A) receptor-associated protein-like 1. Confocal microscopy demonstrated that light chain protein 3B and STBD1 immunomarkers were not colocalized in NRVMs. These findings provide the first evidence that cardiomyocyte glycophagy induction occurs under the influence of insulin and is responsive to extracellular high glucose. This study suggests that the regulation of glycogen content and glycophagy induction in the cardiomyocyte may be linked, and it is speculated that glycogen pathology in diabetic cardiomyopathy has glycophagic involvement.

Role of autophagy in glycogen breakdown and its relevance to chloroquine myopathy

Several myopathies are associated with defects in autophagic and lysosomal degradation of glycogen, but it remains unclear how glycogen is targeted to the lysosome and what significance this process has for muscle cells. We have established a Drosophila melanogaster model to study glycogen autophagy in skeletal muscles, using chloroquine (CQ) to simulate a vacuolar myopathy that is completely dependent on the core autophagy genes. We show that autophagy is required for the most efficient degradation of glycogen in response to starvation. Furthermore, we show that CQ-induced myopathy can be improved by reduction of either autophagy or glycogen synthesis, the latter possibly due to a direct role of Glycogen Synthase in regulating autophagy through its interaction with Atg8.

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."

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.

Starch-binding domain-containing protein 1 (Stbd1) and glycogen metabolism: Identification of the Atg8 family interacting motif (AIM) in Stbd1 required for interaction with GABARAPL1

Glycogen, a branched polymer of glucose, acts as an intracellular carbon and energy reserve in many tissues and cell types. An important pathway for its degradation is by transport to lysosomes in an autophagy-like process. It has been proposed that starch-binding domain-containing protein 1 (Stbd1) may participate in this mechanism by anchoring glycogen to intracellular membranes. In addition, Stbd1 has been reported to interact with a known autophagy protein, GABARAPL1, a member of the Atg8 family. Here, we confirm this interaction and identify an Atg8 interacting motif (AIM) in Stbd1 necessary for GABARAPL1 binding as judged by co-immunoprecipitation from cell extracts and co-localization in cells as evidenced by immunofluorescence microscopy. The AIM sequence of Stbd1 (200)HEEWEMV(206) lies within a predicted disordered region of the molecule and fits the consensus of other AIM sequences in cargo-specifying proteins such as p62 and Nix. Mutation of the AIM, including single point mutations of either W203 or V206, eliminated the co-localization of Stbd1 with both over-expressed and endogenous GABARAPL1. Stbd1 may therefore function as a novel cargo binding protein that delivers glycogen to lysosomes in an autophagic pathway that could be termed "glycophagy".

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.

The administration of nonmetabolizable glucose analogues fails to suppress the development of glycogen autophagy in newborn rat hepatocytes

The effects of parenteral administration of glucose, 3-methylglucose (3MG), or 2-deoxyglucose (2DG) on the glycogen autophagy were studied in the newborn rat liver using electron microscopy and biochemical methods. The administration of glucose resulted in hyperglycemia and prevented the mobilization of hepatocytic glycogen. It also prevented the development of autophagic vacuoles in general and inhibited the glycogen-degrading activity of acid α-1,4-glucosidase. The nonphosphorylated and not further metabolized glucose analog 3MG also produced hyperglycemia, but increased acid glucosidase. Pretreating the newborns with the β-adrenergic blocker propranolol inhibited the effects of 3MG. The phosphorylated but not fully metabolized glucose analog 2DG produced similar effects. The administration of xylitol to the newborns already treated with 2DG, suppressed acid glucosidase. The results of this and our previous studies suggest that glucose must be metabolized beyond its phosphorylation step to inhibit acid glucosidase activity.

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.

Murine muscle cell models for Pompe disease and their use in studying therapeutic approaches

Lysosomes filled with glycogen are a major pathologic feature of Pompe disease, a fatal myopathy and cardiomyopathy caused by a deficiency of the glycogen-degrading lysosomal enzyme, acid alpha-glucosidase (GAA). To facilitate studies germane to this genetic disorder, we developed two in vitro Pompe models: myotubes derived from cultured primary myoblasts isolated from Pompe (GAA KO) mice, and myotubes derived from primary myoblasts of the same genotype that had been transduced with cyclin-dependent kinase 4 (CDK4). This latter model is endowed with extended proliferative capacity. Both models showed extremely large alkalinized, glycogen-filled lysosomes as well as impaired trafficking to lysosomes. Although both Pompe tissue culture models were derived from fast muscles and were fast myosin positive, they strongly resemble slow fibers in terms of their pathologic phenotype and their response to therapy with recombinant human GAA (rhGAA). Autophagic buildup, a hallmark of Pompe disease in fast muscle fibers, was absent, but basal autophagy was functional. To evaluate substrate deprivation as a strategy to prevent the accumulation of lysosomal glycogen, we knocked down Atg7, a gene essential for autophagosome formation, via siRNA, but we observed no effect on the extent of glycogen accumulation, thus confirming our recent observation in autophagy-deficient Pompe mice [N. Raben, V. Hill, L. Shea, S. Takikita, R. Baum, N. Mizushima, E. Ralston, P. Plotz, Suppression of autophagy in skeletal muscle uncovers the accumulation of ubiquitinated proteins and their potential role in muscle damage in Pompe disease, Hum. Mol. Genet. 17 (2008) 3897-3908] that macroautophagy is not the major route of glycogen transport to lysosomes. The in vitro Pompe models should be useful in addressing fundamental questions regarding the pathway of glycogen to the lysosomes and testing panels of small molecules that could affect glycogen biosynthesis or speed delivery of the replacement enzyme to affected lysosomes.

When more is less: excess and deficiency of autophagy coexist in skeletal muscle in Pompe disease

The role of autophagy, a catabolic lysosome-dependent pathway, has recently been recognized in a variety of disorders, including Pompe disease, which results from a deficiency of the glycogen-degrading lysosomal hydrolase acid-alpha glucosidase (GAA). Skeletal and cardiac muscle are most severely affected by the progressive expansion of glycogen-filled lysosomes. In both humans and an animal model of the disease (GAA KO), skeletal muscle pathology also involves massive accumulation of autophagic vesicles and autophagic buildup in the core of myofibers, suggesting an induction of autophagy. Only when we suppressed autophagy in the skeletal muscle of the GAA KO mice did we realize that the excess of autophagy manifests as a functional deficiency. This failure of productive autophagy is responsible for the accumulation of potentially toxic aggregate-prone ubiquitinated proteins, which likely cause profound muscle damage in Pompe mice. Also, by generating muscle-specific autophagy-deficient wild-type mice, we were able to analyze the role of autophagy in healthy skeletal muscle.

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

The role of autophagy, a catabolic lysosome-dependent pathway, has recently been recognized in a variety of disorders, including Pompe disease, the genetic deficiency of the glycogen-degrading lysosomal enzyme acid-alpha glucosidase. Accumulation of lysosomal glycogen, presumably transported from the cytoplasm by the autophagic pathway, occurs in multiple tissues, but pathology is most severe in skeletal and cardiac muscle. Skeletal muscle pathology also involves massive autophagic buildup in the core of myofibers. To determine if glycogen reaches the lysosome via autophagy and to ascertain whether autophagic buildup in Pompe disease is a consequence of induction of autophagy and/or reduced turnover due to defective fusion with lysosomes, we generated muscle-specific autophagy-deficient Pompe mice. We have demonstrated that autophagy is not required for glycogen transport to lysosomes in skeletal muscle. We have also found that Pompe disease involves induction of autophagy but manifests as a functional deficiency of autophagy because of impaired autophagosomal-lysosomal fusion. As a result, autophagic substrates, including potentially toxic aggregate-prone ubiquitinated proteins, accumulate in Pompe myofibers and may cause profound muscle damage.

FoxO3 controls autophagy in skeletal muscle in vivo

Autophagy allows cell survival during starvation through the bulk degradation of proteins and organelles by lysosomal enzymes. However, the mechanisms responsible for the induction and regulation of the autophagy program are poorly understood. Here we show that the FoxO3 transcription factor, which plays a critical role in muscle atrophy, is necessary and sufficient for the induction of autophagy in skeletal muscle in vivo. Akt/PKB activation blocks FoxO3 activation and autophagy, and this effect is not prevented by rapamycin. FoxO3 controls the transcription of autophagy-related genes, including LC3 and Bnip3, and Bnip3 appears to mediate the effect of FoxO3 on autophagy. This effect is not prevented by proteasome inhibitors. Thus, FoxO3 controls the two major systems of protein breakdown in skeletal muscle, the ubiquitin-proteasomal and autophagic/lysosomal pathways, independently. These findings point to FoxO3 and Bnip3 as potential therapeutic targets in muscle wasting disorders and other degenerative and neoplastic diseases in which autophagy is involved.

Glycogen autophagy in glucose homeostasis

Glycogen autophagy, the sequestration and degradation of cell glycogen in the autophagic vacuoles, is a selective, hormonally controlled and highly regulated process, representing a mechanism of glucose homeostasis under conditions of demand for the production of this sugar. In the newborn animals, this process is induced by glucagon secreted during the postnatal hypoglycemia and inhibited by insulin and parenteral glucose, which abolishes glucagon secretion. Hormonal action is mediated by the cAMP/protein kinase A (induction) and phosphoinositides/mTOR (inhibition) pathways that converge on common targets, such as the protein phosphatase 2A to regulate autophgosomal glycogen-hydrolyzing acid glucosidase and glycogen autophagy. Intralysosomal phosphate exchange reactions, which are affected by changes in the calcium levels and acid mannose 6- and acid glucose 6-phosphatase activities, can modify the intralysosomal composition in phosphorylated and nonphosphorylated glucose and promote the exit of free glucose through the lysosomal membrane. Glycogen autophagy-derived nonphosphorylated glucose assists the hyaloplasmic glycogen degradation-derived glucose 6-phosphate to combat postnatal hypoglycemia and participates in other metabolic pathways to secure the fine tuning of glucose homeostasis during the neonatal period.

Autophagy in metazoans: cell survival in the land of plenty

Cells require a constant supply of macromolecular precursors and oxidizable substrates to maintain viability. Unicellular eukaryotes lack the ability to regulate nutrient concentrations in their extracellular environment. So when environmental nutrients are depleted, these organisms catabolize existing cytoplasmic components to support ATP production to maintain survival, a process known as autophagy. By contrast, the environment of metazoans normally contains abundant extracellular nutrients, but a cell's ability to take up these nutrients is controlled by growth factor signal transduction. Despite evolving the ability to maintain a constant supply of extracellular nutrients, metazoans have retained a complete set of autophagy genes. The physiological relevance of autophagy in such species is just beginning to be explored.

Heterogeneity of glycogen synthesis upon refeeding following starvation

1. Starvation of rats for 40 hr decreased the body weight, liver weight and blood glucose concentration. The hepatic and skeletal muscle glycogen concentrations were decreased by 95% (from 410 mumol/g tissue to 16 mumol/g tissue) and 55% (from 40 mumol/g tissue to 18.5 mumol/g tissue), respectively. 2. Fine structural analysis of glycogen purified from the liver and skeletal muscle of starved rats suggested that the glycogenolysis included a lysosomal component, in addition to the conventional phosphorolytic pathway. In support of this the hepatic acid alpha-glucosidase activity increased 1.8-fold following starvation. 3. Refeeding resulted in liver glycogen synthesis at a linear rate of 40 mumol/g tissue per hr over the first 13 hr of refeeding. The hepatic glycogen store were replenished by 8 hr of refeeding, but synthesis continued and the hepatic glycogen content peaked at 24 hr (approximately 670 mumol/g tissue). 4. Refeeding resulted in skeletal muscle glycogen synthesis at an initial rate of 40 mumol/g tissue per hr. The muscle glycogen store was replenished by 30 min of refeeding, but synthesis continued and the glycogen content peaked at 13 hr (approximately 50 mumol/g tissue). 5. Both liver and skeletal muscle glycogen synthesis were inhomogeneous with respect to molecular size; high molecular weight glycogen was initially synthesised at a faster rate than low molecular weight glycogen. These observations support suggestions that there is more than a single site of glycogen synthesis.

Changing ultrastructure of thyrotrophs in the rat anterior pituitary after thyroidectomy as studied by immuno-electron microscopy and enzyme cytochemistry

The characteristic ultrastructure of thyrotrophs of the rat anterior pituitary was observed by immuno-electron microscopy and enzyme cytochemistry with increasing time after thyroidectomy (TX). The rough endoplasmic reticulum (ER) became dilated, the intracisternal granules reacted to serum raised against thyroid stimulating hormone (TSH) around 21 days after TX, and lysosomes and peculiar structures with positive acid phosphatase activity were present. The administration of thyroxine (T4) to the thyroidectomized rats resulted in the reformation of secretory granules, a reduction of dilated cisternae of rough ER and the activation of the lysosomal systems. Morphological features indicating that the TX-cells might be derived from growth hormone (GH) cells or cells other than TSH cells, previously suggested by some researchers, were not recognized in the present study. The amount of serum and pituitary TSH was measured by radioimmunoassay (RIA), and correlated well with the morphological changes. These results indicate that the TX-cells are hypertrophied hyperfunctioning TSH cells that have been affected by the lack of negative feedback of thyroid hormone.

Rat skeletal muscle lysosomes contain glycogen

1. A lysosome- and mitochondria-enriched fraction was obtained from rat skeletal muscle using a differential centrifugation procedure. This fraction contained a proportion (3-4%) of the tissue glycogen content. 2. Lysosomes and mitochondria were separated from one another by centrifugation of a cell-free extract upon discontinuous Ficoll-sucrose gradients. Little glycogen was associated with the resulting mitochondrial fraction. The lysosomal fraction, however, contained a significant amount of glycogen, accounting for 5% of the skeletal muscle glycogen. 3. The lysosomal glycogen was purified and found to be enriched in high molecular weight material. 4. The compartmentation of skeletal muscle glycogen metabolism is suggested.

Acid maltase deficiency in the Japanese quail; early morphological event in skeletal muscle

The skeletal muscle of Japanese quails with acid maltase deficiency (AMD) was studied morphologically at various developmental stages, from the 16th embryonal day up to 3 months after hatching. Membrane-bound glycogen particles began to appear in the affected skeletal muscle at the 16th embryonal day. In normal embryonic muscles, a certain amount of free glycogen particles was observed but they were not membrane-bound. Therefore, this is the earliest morphological event in the muscle of Japanese quails with AMD. In muscle at 3 weeks after hatching, the initial focal degeneration of myofibrils was recognizable but it was not associated with autophagic vacuoles. Quails with AMD developed muscle weakness and difficulty in lifting their wings at about 3 months after hatching: then numerous autophagic vacuoles were present. The formation of large autophagic vacuoles followed by fiber loss and fatty replacement seemed to contribute to the progressive muscle weakness. The study of Japanese quail with AMD will greatly facilitate the elucidation of the pathogenetic mechanism and is also a useful model for therapeutic trials in human AMD.

Spontaneous and experimental myofibrillar hypoplasia and its relation to splayleg in newborn pigs

The relation of myofibrillar hypoplasia to clinical splayleg was studied. A strain of Belgian Landrace sows was selected for this study because they produced pigs which had no myofibrillar hypoplasia. Myofibrillar hypoplasia could nevertheless by induced experimentally in these animals by dexamethasone treatment of the sows during late pregnancy. The lesion was observed without clinical signs and was compared to the myofibrillar hypoplasia in clinical cases of splayleg. The differences between these 2 groups may account for the appearance of clinical signs. These differences included the maturity of the myofibrils and the degree of autophagolysosomal glycogen breakdown.

Cytochemical studies of a glycogen-sarcoplasmic reticulum complex

Enzymatically active cardiac sarcoplasmic reticulum (SR) fractions contain glycogen. Previous biochemical and morphological studies indicate that the glycogen particles are membrane associated. In the present study, further evidence for membrane-associated glycogen particles in these cardiac SR fractions is presented: (1) morphological parameters, (2) enzymatic digestion by glucoamylase and alpha-amylase and (3) cytochemical staining by two different methods. Dense granules comparable in size (20-30 nm diameter), electron density and substructure to glycogen particles observed in intact cardiac muscle and in glycogen preparations isolated from skeletal muscle were seen. Most of these glycogen particles were removed by amylase digestion except for glycogen particles closely adhering to vesicle membranes. Two different cytochemical techniques (bismuth subnitrate and silver proteinate) revealed a positive reaction product over the glycogen particles. These findings provide further support for the biochemical finding of a structured enzyme complex involving the SR, glycogenolytic enzymes and glycogen.

Comparative pathology of the canine model of glycogen storage disease type II (Pompe's disease)

The pathology of canine glycogen storage disease type II (acid alpha-glucosidase deficiency, GSD II) was studied in three genetically related Lapland dogs and compared to the pathology of human GSD II (McKusick 23230). Canine GSD II closely parallels the infantile form of the human disease, except for the presence of oesophageal dilatation. Generalized glycogen storage particularly affected muscular tissues (skeletal, oesophageal, cardiac and smooth muscle). The altered cells showed glycogen accumulation in the cytosol and in autophagic membrane-bound vacuoles (glycogenosomes). They also showed increased acid phosphatase activity consistent with the lysosomal nature of this storage disorder. The cytopathology in canine and human GSD II appears to evolve from segregation of glycogen during regular cellular autophagy, phagolysosomal accumulation of the undigested glycogen, and eventually rupture of distended glycogenosomes. This study indicates that the usefulness of canine GSD II as an animal model of human disease, extends to the area of pathogenesis.

Glycogen-membrane complexes in denervated human skeletal muscle

We describe an unusual intracellular complex of glycogen with smooth, cisternal, cytoplasmic membranes (glycogen-membrane complexes, GMC) in denervated human skeletal muscle. Glycogen particles were always intimately associated with these structures, although the staining intensity varied markedly with different fixation conditions. This may account for previous investigations of similar structures in which an association with glycogen was not recognized. Electron micrographs of tannic acid-enhanced specimens, and of freeze-fracture replicas, showed similarities between the GMCs and the terminal cisternae of the sarcoplasmic reticulum: (i) GMCs resembled the terminal cisternae of the sarcoplasmic reticulum in the size and asymmetric distribution of intramembranous particles seen in freeze-fracture replicas; (ii) tannic acid-enhanced thin sections of GMCs showed intense staining of the cisternal contents and irregular staining of the cytoplasmic leaflet, similar to the appearance of the sarcoplasmic reticulum terminal cisternae; and (iii) triad-like junctions were seen between the glycogen-membrane complexes and elements of the transverse tubule system.

Glycogenosomes in fibres of human normal skeletal muscles. An electron-microscopic study

Analysis of muscle biopsies from five patients, with no clinical evidence of neuromuscular disorders, showed the presence of membrane-bound structures containing glycogen (glycogenosomes). Occurrence of these structures within human muscle fibres has been usually considered a pathological finding, common to various types of metabolic diseases. Our results provide evidence that glycogenosomes are a normal constituent of human muscle fibre, thereby emphasizing that any relationship between glycogenosomes and a specific pathological condition should be established with great care.

The muscle satellite cell: a review

Since the first reports of satellite cells in 1961, considerable knowledge has accumulated concerning their phylogenetic distribution and their location, morphology, and function. There is no doubt that satellite cells are capable of undergoing mitosis and that they have considerable motility. These cells function as the progenitors of the myofiber nuclei that must be added during normal (postnatal) growth of muscle. In muscle undergoing or attempting to undergo regeneration, the satellite cell functions as a myogenic stem cell to produce myoblasts that line up and fuse within the scaffolding of the remnant basal lamina or migrate into the interstitium to produce neofibers . A number of problems remain to be solved concerning the regulation of satellite cell function. At this time it is equivocable whether or not the presumptive myoblast and the satellite cell are functionally identical and at the same stage of myogenic differentiation. Apparently there is species variation in terms of the ability of myotubes from embryonic myogenic cells and satellite cells to synthesize protein. The mechanism(s) by which a wide variety of stimuli activate satellite cells is not known, nor is the mechanism(s) by which satellite cells become inactive during the latter stages of growth and adulthood known. Mitogenic factors are present in damaged muscle; but the specific characteristics of these factors and their mechanism of activation are also unknown. Hormones are certainly involved in the regulation of proliferation and differentiation of myogenic cells, but whether presumptive myoblasts and satellite cells or their myotubes respond similarly to hormones in culture has not been adequately examined. Greater understanding of these mechanisms will increase the possibility of total muscle recovery from severe injury or disease. Such knowledge would also have particular application to the production of meat animals and to a greater understanding of the growth process in general.

Membrane-glycogen complexes in rabbit extraocular muscle

Analysis of 432 electron micrographs of membrane-glycogen complexes revealed that: (1) Golgi apparatus is closely associated with 4.2% of the complexes, such associations occurring irrespective of the degree of glycogen loading in the complex. (2) Apparent ribosomes are seen in association with about 30% of the complexes, either attached to membranes or enclosed between cisternae. (3) In longitudinal sections of the muscle fibers, complexes may form columns which extend for as much as 40 microns along the fiber. (4) Various cytoplasmic organelles may become enclosed within a complex. (5) Some cisternae of a complex may assume the form of randomly oriented tubules, in contrast to the typical systematic array of flattened cisternae. (6) Some cisternae of a complex may become distended in a wide and uneven manner, in contrast to the typical narrow and even distension.

New fine structural features of cardiac rhabdomyoma: report of a case

Electron microscopy of a cardiac rhabdomyoma revealed two varieties of cells: (1) typical "spider cells' characterized by few myofibrils and large aggregates of glycogen particles free in the cytoplasm; and (2) cells with more myofibrils, less glycogen, and glycogen within membrane-limited vacuoles. These differences in myofibril development and glycogen content represent two succeeding stages in the early maturation of normal cardiocytes. The vacuoles containing glycogen were interpreted as autophagic and, consequently, active in the glycogen decrease typical of normal maturing cardiocytes. Rhabdomyoma cells did not show the structures, e.g., T system, characteristic of the late normal heart development. This study supports the theory that arrested cardiocyte maturation is a factor in the pathogenesis of cardiac rhabdomyoma.

Histochemical patterns in normal and splaylegged piglet muscle fibers

The histochemical pattern of muscle fiber types of the longissimus dorsi and biceps femoris muscles was investigated in normal and splaylegged piglets at birth and seven days later. Only slight differences between the muscle fibers at birth were found using histochemical reactions for alkaline adenosine triphosphatase (ATPase), succinate dehydrogenase (SDH), phosphorylase (PH) activities, and for the periodic acid-Schiff (PAS) reaction. With the method for acid-preincubated ATPase activity, high activity was observed in Type I muscle fibers and low activity in Type II muscle fibers in animals of both groups investigated. However, a higher number of Type I fibers was found in muscles of normal piglets, suggesting a faster and more advanced process of transformation of Type II into Type I muscle fibers in unaffected animals. Thus the histochemical conversion appears to be retarded in muscles of splaylegged animals, which have a histochemical pattern similar to that of normal prenatal animals. Cholinesterase activity in motor endplates was well developed; its staining revealed smaller sized and irregularly arranged endplates in muscles of affected piglets. Fiber type differentiation in muscles of animals which recovered from splayleg becomes fully developed and comparable to normal piglets seven days after birth. The number of fibers which became converted from Type II to Type I was increased; the fiber types were differentiated with regard to the PAS reaction and to their ATPase, SDH and PH activities. Morphological features of motor endplates in muscles of normal and surviving splaylegged piglets are similar. Histochemical investigation of the fiber type differentiation thus suggests that full recovery occurs within the first week of postnatal life in muscles affected by pathological changes accompanying splayleg.

The distribution of membrane-glycogen complexes in the orbital surface layer of rabbit superior rectus

The distribution of membrane--glycogen complexes along the length of individual muscle fibers was compared among three fiber populations in the orbital surface layer of rabbit superior rectus. These three populations were (a) 61 singly innervated fibers (SIFs), (b) 10 multiply innervated fibers of relatively constant 10 micrometer diameter (10 micrometer MIFs), and (c) 22 multiply innervated fibers which are of about 5 micrometer diameter toward the middle of their length and of about 15 micrometer diameter toward their proximal and distal segments (5--15 micrometer MIFs). The orbital surface layer was sampled by electron microscopy at 68 sequential locations. Membrane--glycogen complexes were not seen in any of the 1738 samples of the SIFs. In the MIFs, such complexes were observed in 14% of the 1541 samples. However, both the 10 micrometer MIFs and 5--15 micrometer MIFs displayed a preferential concentration of membrane--glycogen complexes toward their distal fiber portions, and such complexes were seen in about 50% of the MIF samples near the beginning of the muscle's distal third. In the distal portion of 5--15 micrometer MIFs, there was a direct relationship between their increasing fiber diameter and their increasing frequency of occurrence of membrane--glycogen complexes.

Experimental (-) emetine myopathy. Ultrastructural and morphometric observations

The ultrastructural changes induced by (-) emetine hydrochloride were studied in two skeletal muscles of the rat. (-) Emetine was administered at a dose level of 2 mg/kg for periods ranging from one to four weeks. Changes were noted after the one week stage and were progressive with all muscle components appearing to be affected. Myofibrillar changes included Z line streaming, rod formation, myofilament loss and contraction clumping. Mitochondrial degeneration was detected, but the most striking feature of the induced myopathy was the extensive membrane proliferation. The membranes were considered to be involved in the isolation of sarcoplasmic constitutents in the production of autophagic vacuoles. Morphometry was carried out in order to quantify some of the changes in the fibre components.