Mechanisms of skeletal muscle aging: insights from Drosophila and mammalian models

Pharmacogenomic analysis indicates potential of 1,5-isoquinolinediol as a universal anti-aging agent for different tissues

The natural aging of multicellular organisms is marked by a progressive decline in the function of cells and tissues. The accumulation of senescent cells in tissues seems to eventually cause aging of the host. Nevertheless, gene expression that influences aging is unlikely to be conserved between tissues, and age-related loss of function seems to depend on a variety of mechanisms. This is a concern when developing anti-aging drugs in geriatric clinical pharmacology. We have sought a universal agent to redundantly cover gene expression despite the variation in differentially expressed genes between tissues. Using a minimally modified connectivity map, the poly (ADP-ribose) polymerase (PARP) inhibitor 1,5-isoquinolinediol was selected as a potent candidate, simultaneously applicable to various tissues. This choice was validated in vitro. Treatment of murine embryonic fibroblasts with 1,5-isoquinolinediol appeared to efficiently suppress the rate of replicative senescence at a concentration of 0.1 µM without resulting in cell death. The appearance of abnormal nuclei and accumulation of β-galactosidase in the cytoplasm were inhibited by daily treatment with the agent. When the aging process was accelerated by hydroxyurea-induced oxidative stress, the effect was even more noticeable. Thus, 1,5-isoquinolinediol may potentially be developed as an agent to prolong life.

Sod1 gene ablation in adult mice leads to physiological changes at the neuromuscular junction similar to changes that occur in old wild-type mice

Reactive oxygen species (ROS) are believed to be important mediators of muscle atrophy and weakness in aging and many degenerative conditions. However, the mechanisms and physiological processes specifically affected by elevated ROS in neuromuscular units that contribute to muscle weakness during aging are not well defined. Here we investigate the effects of chronic oxidative stress on neurotransmission and excitation-contraction (EC) coupling mechanisms in the levator auris longus (LAL) muscle from young (4-8 months) and old (22-28 months) wild-type mice and young adult Cu-Zn superoxide dismutase 1 knockout (Sod1(-/-)) mice. The frequency of spontaneous neurotransmitter release and the amplitude of evoked neurotransmitter release in young Sod1(-/-) and old wild-type LAL neuromuscular junctions were significantly reduced from the young wild-type values, and those declines were mirrored by decreases in synaptic vesicle pool size. Presynaptic cytosolic calcium concentration and mitochondrial calcium uptake amplitudes showed substantial increases in stimulated young Sod1(-/-) and old axon terminals. Surprisingly, LAL muscle fibers from old mice showed a greater excitability than fibers from either young wild-type or young Sod1(-/-) LAL. Both evoked excitatory junction potential (EJP) and spontaneous mini EJP amplitudes were considerably higher in LAL muscles from old mice than in fibers from young Sod1(-/-) LAL muscle. Despite a greater excitability, sarcoplasmic calcium influx in both old wild-type and young Sod1(-/-) LAL muscle fibers was significantly less. Sarcoplasmic reticulum calcium levels were also reduced in both old wild-type and young Sod1(-/-) mice, but the difference was not statistically significant in muscle fibers from old wild-type mice. The protein ratio of triad calcium channels RyR1/DHPR was not different in all groups. However, fibers from both young Sod1(-/-) and old mice had substantially elevated levels of protein carbonylation and S-nitrosylation modifications. Overall, our results suggest that young Sod1(-/-) recapitulate many neuromuscular and muscle fiber changes seen in old mice. We also conclude that muscle weakness in old mice might in part be driven by ROS-mediated EC uncoupling, while both EC uncoupling and reduced neurotransmitter release contribute to muscle weakness in Sod1(-/-) mice.

Malignant Drosophila tumors interrupt insulin signaling to induce cachexia-like wasting

Tumors kill patients not only through well-characterized perturbations to their local environment but also through poorly understood pathophysiological interactions with distant tissues. Here, we use a Drosophila tumor model to investigate the elusive mechanisms underlying such long-range interactions. Transplantation of tumors into adults induces robust wasting of adipose, muscle, and gonadal tissues that are distant from the tumor, phenotypes that resemble the cancer cachexia seen in human patients. Notably, malignant, but not benign, tumors induce peripheral wasting. We identify the insulin growth factor binding protein (IGFBP) homolog ImpL2, an antagonist of insulin signaling, as a secreted factor mediating wasting. ImpL2 is sufficient to drive tissue loss, and insulin activity is reduced in peripheral tissues of tumor-bearing hosts. Importantly, knocking down ImpL2, specifically in the tumor, ameliorates wasting phenotypes. We propose that the tumor-secreted IGFBP creates insulin resistance in distant tissues, thus driving a systemic wasting response.

A Drosophila model for mito-nuclear diseases generated by an incompatible interaction between tRNA and tRNA synthetase

Communication between the mitochondrial and nuclear genomes is vital for cellular function. The assembly of mitochondrial enzyme complexes, which produce the majority of cellular energy, requires the coordinated expression and translation of both mitochondrially and nuclear-encoded proteins. The joint genetic architecture of this system complicates the basis of mitochondrial diseases, and mutations both in mitochondrial DNA (mtDNA)- and nuclear-encoded genes have been implicated in mitochondrial dysfunction. Previously, in a set of mitochondrial-nuclear introgression strains, we characterized a dual genome epistasis in which a naturally occurring mutation in the Drosophila simulans simw(501) mtDNA-encoded transfer RNA (tRNA) for tyrosine (tRNA(Tyr)) interacts with a mutation in the nuclear-encoded mitochondrially localized tyrosyl-tRNA synthetase from Drosophila melanogaster. Here, we show that the incompatible mitochondrial-nuclear combination results in locomotor defects, reduced mitochondrial respiratory capacity, decreased oxidative phosphorylation (OXPHOS) enzyme activity and severe alterations in mitochondrial morphology. Transgenic rescue strains containing nuclear variants of the tyrosyl-tRNA synthetase are sufficient to rescue many of the deleterious phenotypes identified when paired with the simw(501) mtDNA. However, the severity of this defective mito-nuclear interaction varies across traits and genetic backgrounds, suggesting that the impact of mitochondrial dysfunction might be tissue specific. Because mutations in mitochondrial tRNA(Tyr) are associated with exercise intolerance in humans, this mitochondrial-nuclear introgression model in Drosophila provides a means to dissect the molecular basis of these, and other, mitochondrial diseases that are a consequence of the joint genetic architecture of mitochondrial function.

Muscle wasting in disease: molecular mechanisms and promising therapies

Atrophy occurs in specific muscles with inactivity (for example, during plaster cast immobilization) or denervation (for example, in patients with spinal cord injuries). Muscle wasting occurs systemically in older people (a condition known as sarcopenia); as a physiological response to fasting or malnutrition; and in many diseases, including chronic obstructive pulmonary disorder, cancer-associated cachexia, diabetes, renal failure, cardiac failure, Cushing syndrome, sepsis, burns and trauma. The rapid loss of muscle mass and strength primarily results from excessive protein breakdown, which is often accompanied by reduced protein synthesis. This loss of muscle function can lead to reduced quality of life, increased morbidity and mortality. Exercise is the only accepted approach to prevent or slow atrophy. However, several promising therapeutic agents are in development, and major advances in our understanding of the cellular mechanisms that regulate the protein balance in muscle include the identification of several cytokines, particularly myostatin, and a common transcriptional programme that promotes muscle wasting. Here, we discuss these new insights and the rationally designed therapies that are emerging to combat muscle wasting.

Defects in calcium homeostasis and mitochondria can be reversed in Pompe disease

Mitochondria-induced oxidative stress and flawed autophagy are common features of neurodegenerative and lysosomal storage diseases (LSDs). Although defective autophagy is particularly prominent in Pompe disease, mitochondrial function has escaped examination in this typical LSD. We have found multiple mitochondrial defects in mouse and human models of Pompe disease, a life-threatening cardiac and skeletal muscle myopathy: a profound dysregulation of Ca(2+) homeostasis, mitochondrial Ca(2+) overload, an increase in reactive oxygen species, a decrease in mitochondrial membrane potential, an increase in caspase-independent apoptosis, as well as a decreased oxygen consumption and ATP production of mitochondria. In addition, gene expression studies revealed a striking upregulation of the β 1 subunit of L-type Ca(2+) channel in Pompe muscle cells. This study provides strong evidence that disturbance of Ca(2+) homeostasis and mitochondrial abnormalities in Pompe disease represent early changes in a complex pathogenetic cascade leading from a deficiency of a single lysosomal enzyme to severe and hard-to-treat autophagic myopathy. Remarkably, L-type Ca(2+)channel blockers, commonly used to treat other maladies, reversed these defects, indicating that a similar approach can be beneficial to the plethora of lysosomal and neurodegenerative disorders.

p53 suppresses muscle differentiation at the myogenin step in response to genotoxic stress

Acute muscle injury and physiological stress from chronic muscle diseases and aging lead to impairment of skeletal muscle function. This raises the question of whether p53, a cellular stress sensor, regulates muscle tissue repair under stress conditions. By investigating muscle differentiation in the presence of genotoxic stress, we discovered that p53 binds directly to the myogenin promoter and represses transcription of myogenin, a member of the MyoD family of transcription factors that plays a critical role in driving terminal muscle differentiation. This reduction of myogenin protein is observed in G1-arrested cells and leads to decreased expression of late but not early differentiation markers. In response to acute genotoxic stress, p53-mediated repression of myogenin reduces post-mitotic nuclear abnormalities in terminally differentiated cells. This study reveals a mechanistic link previously unknown between p53 and muscle differentiation, and suggests new avenues for managing p53-mediated stress responses in chronic muscle diseases or during muscle aging.

Neuronal involvement in muscular atrophy

The innervation of skeletal myofibers exerts a crucial influence on the maintenance of muscle tone and normal operation. Consequently, denervated myofibers manifest atrophy, which is preceded by an increase in sarcolemma permeability. Recently, de novo expression of hemichannels (HCs) formed by connexins (Cxs) and other none selective channels, including P2X7 receptors (P2X7Rs), and transient receptor potential, sub-family V, member 2 (TRPV2) channels was demonstrated in denervated fast skeletal muscles. The denervation-induced atrophy was drastically reduced in denervated muscles deficient in Cxs 43 and 45. Nonetheless, the transduction mechanism by which the nerve represses the expression of the above mentioned non-selective channels remains unknown. The paracrine action of extracellular signaling molecules including ATP, neurotrophic factors (i.e., brain-derived neurotrophic factor (BDNF)), agrin/LDL receptor-related protein 4 (Lrp4)/muscle-specific receptor kinase (MuSK) and acetylcholine (Ach) are among the possible signals for repression for connexin expression. This review discusses the possible role of relevant factors in maintaining the normal functioning of fast skeletal muscles and suppression of connexin hemichannel expression.

TIF-IA-dependent regulation of ribosome synthesis in drosophila muscle is required to maintain systemic insulin signaling and larval growth

The conserved TOR kinase signaling network links nutrient availability to cell, tissue and body growth in animals. One important growth-regulatory target of TOR signaling is ribosome biogenesis. Studies in yeast and mammalian cell culture have described how TOR controls rRNA synthesis—a limiting step in ribosome biogenesis—via the RNA Polymerase I transcription factor TIF-IA. However, the contribution of TOR-dependent ribosome synthesis to tissue and body growth in animals is less clear. Here we show in Drosophila larvae that ribosome synthesis in muscle is required non-autonomously to maintain normal body growth and development. We find that amino acid starvation and TOR inhibition lead to reduced levels of TIF-IA, and decreased rRNA synthesis in larval muscle. When we mimic this decrease in muscle ribosome synthesis using RNAi-mediated knockdown of TIF-IA, we observe delayed larval development and reduced body growth. This reduction in growth is caused by lowered systemic insulin signaling via two endocrine responses: reduced expression of Drosophila insulin-like peptides (dILPs) from the brain and increased expression of Imp-L2—a secreted factor that binds and inhibits dILP activity—from muscle. We also observed that maintaining TIF-IA levels in muscle could partially reverse the starvation-mediated suppression of systemic insulin signaling. Finally, we show that activation of TOR specifically in muscle can increase overall body size and this effect requires TIF-IA function. These data suggest that muscle ribosome synthesis functions as a nutrient-dependent checkpoint for overall body growth: in nutrient rich conditions, TOR is required to maintain levels of TIF-IA and ribosome synthesis to promote high levels of systemic insulin, but under conditions of starvation stress, reduced muscle ribosome synthesis triggers an endocrine response that limits systemic insulin signaling to restrict growth and maintain homeostasis.

All animals need adequate nutrition to grow and develop. Studies in tissue culture and model organisms have identified the TOR kinase signaling pathway as a key nutrient-dependent regulator of growth. Under nutrient rich conditions, TOR kinase is active and stimulates metabolic processes that drive growth. Under nutrient poor conditions, TOR is inhibited and animals alter their metabolism to maintain homeostasis and survival. Here we use Drosophila larvae to identify a role for ribosome synthesis—a key metabolic process—in mediating nutrient and TOR effects on body growth. In particular, we show that ribosome synthesis specifically in larval muscle is necessary to maintain organismal growth. We find that inhibition of muscle ribosome synthesis leads to reduced systemic insulin-like growth factor signaling via two endocrine responses—decreased expression of brain derived Drosophila insulin-like peptides (dILPs) and increased expression of Imp-L2, an inhibitor of insulin signaling. As a result of these effects, body growth is reduced and larval development is delayed. These findings suggest that control of ribosome synthesis, and hence protein synthesis, in specific tissues can exert control on overall body growth.

Development of a Drosophila melanogaster spliceosensor system for in vivo high-throughput screening in myotonic dystrophy type 1

Alternative splicing of pre-mRNAs is an important mechanism that regulates cellular function in higher eukaryotes. A growing number of human genetic diseases involve splicing defects that are directly connected to their pathology. In myotonic dystrophy type 1 (DM1), several clinical manifestations have been proposed to be the consequence of tissue-specific missplicing of numerous genes. These events are triggered by an RNA gain-of-function and resultant deregulation of specific RNA-binding factors, such as the nuclear sequestration of muscleblind-like family factors (MBNL1–MBNL3). Thus, the identification of chemical modulators of splicing events could lead to the development of the first valid therapy for DM1 patients. To this end, we have generated and validated transgenic flies that contain a luciferase-reporter-based system that is coupled to the expression of MBNL1-reliant splicing (spliceosensor flies), to assess events that are deregulated in DM1 patients in a relevant disease tissue. We then developed an innovative 96-well plate screening platform to carry out in vivo high-throughput pharmacological screening (HTS) with the spliceosensor model. After a large-scale evaluation (>16,000 chemical entities), several reliable splicing modulators (hits) were identified. Hit validation steps recognized separate DM1-linked therapeutic traits for some of the hits, which corroborated the feasibility of the approach described herein to reveal promising drug candidates to correct missplicing in DM1. This powerful Drosophila-based screening tool might also be applied in other disease models displaying abnormal alternative splicing, thus offering myriad uses in drug discovery.

The intriguing regulators of muscle mass in sarcopenia and muscular dystrophy

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

Vitamin D signaling in myogenesis: potential for treatment of sarcopenia

Muscle mass and strength progressively decrease with age, which results in a condition known as sarcopenia. Sarcopenia would lead to physical disability, poor quality of life, and death. Therefore, much is expected of an effective intervention for sarcopenia. Epidemiologic, clinical, and laboratory evidence suggest an effect of vitamin D on muscle function. However, the precise molecular and cellular mechanisms remain to be elucidated. Recent studies suggest that vitamin D receptor (VDR) might be expressed in muscle fibers and vitamin D signaling via VDR plays a role in the regulation of myoblast proliferation and differentiation. Understanding how vitamin D signaling contributes to myogenesis will provide a valuable insight into an effective nutritional strategy to moderate sarcopenia. Here we will summarize the current knowledge about the effect of vitamin D on skeletal muscle and myogenic cells and discuss the potential for treatment of sarcopenia.

Intertissue control of the nucleolus via a myokine-dependent longevity pathway

Recent evidence indicates that skeletal muscle influences systemic aging, but little is known about the signaling pathways and muscle-released cytokines (myokines) responsible for this intertissue communication. Here, we show that muscle-specific overexpression of the transcription factor Mnt decreases age-related climbing defects and extends lifespan in Drosophila. Mnt overexpression in muscle autonomously decreases the expression of nucleolar components and systemically decreases rRNA levels and the size of the nucleolus in adipocytes. This nonautonomous control of the nucleolus, a regulator of ribosome biogenesis and lifespan, relies on Myoglianin, a myokine induced by Mnt and orthologous to human GDF11 and Myostatin. Myoglianin overexpression in muscle extends lifespan and decreases nucleolar size in adipocytes by activating p38 mitogen-activated protein kinase (MAPK), whereas Myoglianin RNAi in muscle has converse effects. Altogether, these findings highlight a key role for myokine signaling in the integration of signaling events in muscle and distant tissues during aging.

Studying aging in Drosophila

Drosophila melanogaster represents one of the most important genetically accessible model organisms for aging research. Studies in flies have identified single gene mutations that influence lifespan and have characterized endocrine signaling interactions that control homeostasis systemically. Recent studies have focused on the effects of aging on specific tissues and physiological processes, providing a comprehensive picture of age-related tissue dysfunction and the loss of systemic homeostasis. Here we review methodological aspects of this work and highlight technical considerations when using Drosophila to study aging and age-related diseases.

Mechanisms of muscle growth and atrophy in mammals and Drosophila

BACKGROUND

The loss of skeletal muscle mass (atrophy) that accompanies disuse and systemic diseases is highly debilitating. Although the pathogenesis of this condition has been primarily studied in mammals, Drosophila is emerging as an attractive system to investigate some of the mechanisms involved in muscle growth and atrophy.

RESULTS

In this review, we highlight the outstanding unsolved questions that may benefit from a combination of studies in both flies and mammals. In particular, we discuss how different environmental stimuli and signaling pathways influence muscle mass and strength and how a variety of disease states can cause muscle wasting.

CONCLUSIONS

Studies in Drosophila and mammals should help identify molecular targets for the treatment of muscle wasting in humans.

Whole-mount immunostaining of Drosophila skeletal muscle

Skeletal muscle undergoes marked functional decay during aging in humans, but the cell biological mechanisms responsible for this process are only partly known. Age-related muscle dysfunction is also a feature of aging in the fruit fly Drosophila melanogaster. Here we describe a detailed step-by-step protocol, which takes place over 3 d, for whole-mount immunostaining of Drosophila flight muscle. The skeletal muscle is fixed and permeabilized without any tissue freezing and dehydration so that antigens are accessible for staining with appropriate antibodies and the overall tissue ultrastructure is well preserved. This technique can be used to identify age-related cellular changes driving skeletal muscle aging and for characterizing models of human muscle disease in Drosophila.