RESUMO
Nonsense-mediated RNA decay (NMD) degrades transcripts carrying premature termination codons. NMD is thought to prevent the synthesis of toxic truncated proteins. However, whether loss of NMD results in widespread production of truncated proteins is unclear. A human genetic disease, facioscapulohumeral muscular dystrophy (FSHD), features acute inhibition of NMD upon expression of the disease-causing transcription factor, DUX4. Using a cell-based model of FSHD, we show production of truncated proteins from physiological NMD targets and find that RNA-binding proteins are enriched for aberrant truncations. The NMD isoform of one RNA-binding protein, SRSF3, is translated to produce a stable truncated protein, which is detected in FSHD patient-derived myotubes. Ectopic expression of truncated SRSF3 confers toxicity, and its downregulation is cytoprotective. Our results delineate the genome-scale impact of NMD loss. This widespread production of potentially deleterious truncated proteins has implications for FSHD biology as well as other genetic diseases where NMD is therapeutically modulated.
Assuntos
Distrofia Muscular Facioescapuloumeral , Degradação do RNAm Mediada por Códon sem Sentido , Humanos , Regulação da Expressão Gênica , Distrofia Muscular Facioescapuloumeral/genética , Distrofia Muscular Facioescapuloumeral/metabolismo , RNA/metabolismo , Proteínas de Ligação a RNA/genética , Proteínas de Ligação a RNA/metabolismo , Fatores de Processamento de Serina-Arginina/metabolismoRESUMO
OBJECTIVE: Marfan syndrome (MFS) represents a genetic disorder with a range of clinical features, including proximal aortic aneurysms. Extensive research has revealed an abundance of transforming growth factor beta from a mutation in fibrillin-1 to be the key biochemical mechanism of aneurysm formation. Many important signaling pathways downstream of transforming growth factor beta have been further characterized. Our laboratory has previously demonstrated a unique murine model of MFS resulting in the accelerated formation of ascending aortic aneurysms and dilated cardiomyopathies. This study aims to characterize the relevance of this model to known signaling mechanisms in MFS. METHODS: Fibrillin 1C1039G/+ heterozygous mice (ie, MFS), with a mutation in fibrillin-1, were supplemented with 4.5 mg/kg/d angiotensin II to accelerate aneurysm formation. Four mouse groups were analyzed: wild type with or without angiotensin II and MFS with or without angiotensin II. Aortic tissue from these samples were subjected to western blotting and phosphoimaging to query various signaling pathways. RESULTS: Mice with MFS displayed downstream regulation in both the canonical (Smad2) and noncononical (extracellular signal-regulated kinases and P38) pathways characteristic of MFS. However, these downstream signals were exaggerated in the MFS mice supplemented with angiotensin II (accelerated model), matching the observed phenotypic severity of this model. CONCLUSIONS: The murine MFS model depicted here accelerates ascending aortic aneurysm formation and cardiomyopathies via well-characterized MFS signaling cascades. The mechanistic relevance of the accelerated murine MFS model suggests that it could be an important tool in future studies hoping to characterize MFS signaling in an expedited experimental design.
Assuntos
Aorta/metabolismo , Aneurisma Aórtico/metabolismo , Cardiomiopatias/metabolismo , Síndrome de Marfan/metabolismo , Miocárdio/metabolismo , Angiotensina II , Animais , Aorta/patologia , Aneurisma Aórtico/induzido quimicamente , Aneurisma Aórtico/genética , Aneurisma Aórtico/patologia , Cardiomiopatias/genética , Cardiomiopatias/patologia , Dilatação Patológica , Modelos Animais de Doenças , Progressão da Doença , MAP Quinases Reguladas por Sinal Extracelular/metabolismo , Fibrilina-1/genética , Predisposição Genética para Doença , Heterozigoto , Síndrome de Marfan/complicações , Síndrome de Marfan/genética , Camundongos Mutantes , Mutação , Miocárdio/patologia , Fenótipo , Fosforilação , Transdução de Sinais , Proteína Smad2/metabolismo , Fatores de Tempo , Fator de Crescimento Transformador beta/metabolismo , Proteínas Quinases p38 Ativadas por Mitógeno/metabolismoRESUMO
Nonsense-mediated RNA decay (NMD) is an evolutionarily conserved RNA quality control process that serves both as a mechanism to eliminate aberrant transcripts carrying premature stop codons, and to regulate expression of some normal transcripts. For a quality control process, NMD exhibits surprising variability in its efficiency across transcripts, cells, tissues, and individuals in both physiological and pathological contexts. Whether an aberrant RNA is spared or degraded, and by what mechanism, could determine the phenotypic outcome of a disease-causing mutation. Hence, understanding the variability in NMD is not only important for clinical interpretation of genetic variants but also may provide clues to identify novel therapeutic approaches to counter genetic disorders caused by nonsense mutations. Here, we discuss the current knowledge of NMD variability and the mechanisms that allow certain transcripts to escape NMD despite the presence of NMD-inducing features. This article is categorized under: RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA in Disease and Development > RNA in Disease RNA Turnover and Surveillance > Regulation of RNA Stability.
Assuntos
Códon sem Sentido/genética , Degradação do RNAm Mediada por Códon sem Sentido/genética , Variação Genética/genética , HumanosRESUMO
Metabolic cycles are a fundamental element of cellular and organismal function. Among the most critical in higher organisms is the Cori Cycle, the systemic cycling between lactate and glucose. Here, skeletal muscle-specific Mitochondrial Pyruvate Carrier (MPC) deletion in mice diverted pyruvate into circulating lactate. This switch disinhibited muscle fatty acid oxidation and drove Cori Cycling that contributed to increased energy expenditure. Loss of muscle MPC activity led to strikingly decreased adiposity with complete muscle mass and strength retention. Notably, despite decreasing muscle glucose oxidation, muscle MPC disruption increased muscle glucose uptake and whole-body insulin sensitivity. Furthermore, chronic and acute muscle MPC deletion accelerated fat mass loss on a normal diet after high fat diet-induced obesity. Our results illuminate the role of the skeletal muscle MPC as a whole-body carbon flux control point. They highlight the potential utility of modulating muscle pyruvate utilization to ameliorate obesity and type 2 diabetes.
Assuntos
Glucose/metabolismo , Redes e Vias Metabólicas , Mitocôndrias Musculares/metabolismo , Células Musculares/metabolismo , Músculo Esquelético/metabolismo , Ácido Pirúvico/metabolismo , Magreza , Adiposidade , Animais , Proteínas de Transporte de Ânions/deficiência , Deleção de Genes , Lactatos/metabolismo , Camundongos , Camundongos Knockout , Proteínas de Transporte da Membrana Mitocondrial/deficiência , Transportadores de Ácidos Monocarboxílicos/deficiência , Força MuscularRESUMO
Conditions of extended bed rest and limb immobilization can initiate rapid and significant loss of skeletal muscle mass and function. Physical rehabilitation is standard practice following a period of disuse, yet mobility may be severely compromised, and recovery is commonly delayed or incomplete in special populations. Thus, a novel approach toward recovery of muscle mass is highly desired. Pericytes [neuron-glial antigen 2 (NG2)+CD31-CD45- (Lineage- [Lin-]) and CD146+Lin-] demonstrate capacity to facilitate muscle repair, yet the ability to enhance myofiber growth following disuse is unknown. In the current study, 3-4-mo-old mice were unilaterally immobilized for 14 d (IM) or immobilized for 14 d followed by 14 d of remobilization (RE). Flow cytometry and targeted gene expression analyses were completed to assess pericyte quantity and function following IM and RE. In addition, a transplantation study was conducted to assess the impact of pericytes on recovery. Results from targeted analyses suggest minimal impact of disuse on pericyte gene expression, yet NG2+Lin- pericyte quantity is reduced following IM (P < 0.05). Remarkably, pericyte transplantation recovered losses in myofiber cross-sectional area and the capillary-to-fiber ratio following RE, whereas deficits remained with vehicle alone (P = 0.01). These findings provide the first evidence that pericytes effectively rehabilitate skeletal muscle mass following disuse atrophy.-Munroe, M., Dvoretskiy, S., Lopez, A., Leong, J., Dyle, M. C., Kong, H., Adams, C. M., Boppart, M. D. Pericyte transplantation improves skeletal muscle recovery following hindlimb immobilization.
Assuntos
Transplante de Células , Elevação dos Membros Posteriores , Músculo Esquelético/fisiopatologia , Pericitos/transplante , Animais , Capilares/crescimento & desenvolvimento , Perfilação da Expressão Gênica , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Músculo Esquelético/irrigação sanguínea , Atrofia Muscular/reabilitação , Pericitos/metabolismoRESUMO
PURPOSE OF REVIEW: Here, we discuss recent work focused on the role of activating transcription factor 4 (ATF4) in skeletal muscle atrophy. RECENT FINDINGS: Muscle atrophy involves and requires widespread changes in skeletal muscle gene expression; however, the transcriptional regulatory proteins responsible for those changes are not yet well defined. Recent work indicates that some forms of muscle atrophy require ATF4, a stress-inducible bZIP transcription factor subunit that helps to mediate a broad range of stress responses in mammalian cells. ATF4 expression in skeletal muscle fibers is sufficient to induce muscle fiber atrophy and required for muscle atrophy during several stress conditions, including aging, fasting, and limb immobilization. By helping to activate specific genes in muscle fibers, ATF4 contributes to the expression of numerous mRNAs, including at least two mRNAs (Gadd45a and p21) that encode mediators of muscle fiber atrophy. Gadd45a promotes muscle fiber atrophy by activating the protein kinase MEKK4. p21 promotes atrophy by reducing expression of spermine oxidase, a metabolic enzyme that helps to maintain muscle fiber size under nonstressed conditions. SUMMARY: In skeletal muscle fibers, ATF4 is critical component of a complex and incompletely understood molecular signaling network that causes muscle atrophy during aging, fasting, and immobilization.
Assuntos
Fator 4 Ativador da Transcrição/metabolismo , Envelhecimento/fisiologia , Jejum/fisiologia , Imobilização/fisiologia , Atrofia Muscular/genética , Animais , Humanos , Fibras Musculares Esqueléticas/metabolismo , Músculo Esquelético/metabolismo , Atrofia Muscular/etiologia , Atrofia Muscular/fisiopatologia , Transdução de Sinais/genéticaRESUMO
Skeletal muscle atrophy is a serious and highly prevalent condition that remains poorly understood at the molecular level. Previous work found that skeletal muscle atrophy involves an increase in skeletal muscle Gadd45a expression, which is necessary and sufficient for skeletal muscle fiber atrophy. However, the direct mechanism by which Gadd45a promotes skeletal muscle atrophy was unknown. To address this question, we biochemically isolated skeletal muscle proteins that associate with Gadd45a as it induces atrophy in mouse skeletal muscle fibers in vivo We found that Gadd45a interacts with multiple proteins in skeletal muscle fibers, including, most prominently, MEKK4, a mitogen-activated protein kinase kinase kinase that was not previously known to play a role in skeletal muscle atrophy. Furthermore, we found that, by forming a complex with MEKK4 in skeletal muscle fibers, Gadd45a increases MEKK4 protein kinase activity, which is both sufficient to induce skeletal muscle fiber atrophy and required for Gadd45a-mediated skeletal muscle fiber atrophy. Together, these results identify a direct biochemical mechanism by which Gadd45a induces skeletal muscle atrophy and provide new insight into the way that skeletal muscle atrophy occurs at the molecular level.
Assuntos
Proteínas de Ciclo Celular/metabolismo , MAP Quinase Quinase Quinase 4/metabolismo , Complexos Multiproteicos/metabolismo , Fibras Musculares Esqueléticas/metabolismo , Atrofia Muscular/metabolismo , Proteínas Nucleares/metabolismo , Animais , Proteínas de Ciclo Celular/genética , MAP Quinase Quinase Quinase 4/genética , Camundongos , Complexos Multiproteicos/genética , Fibras Musculares Esqueléticas/patologia , Atrofia Muscular/genética , Atrofia Muscular/patologia , Proteínas Nucleares/genéticaRESUMO
Aging reduces skeletal muscle mass and strength, but the underlying molecular mechanisms remain elusive. Here, we used mouse models to investigate molecular mechanisms of age-related skeletal muscle weakness and atrophy as well as new potential interventions for these conditions. We identified two small molecules that significantly reduce age-related deficits in skeletal muscle strength, quality, and mass: ursolic acid (a pentacyclic triterpenoid found in apples) and tomatidine (a steroidal alkaloid derived from green tomatoes). Because small molecule inhibitors can sometimes provide mechanistic insight into disease processes, we used ursolic acid and tomatidine to investigate the pathogenesis of age-related muscle weakness and atrophy. We found that ursolic acid and tomatidine generate hundreds of small positive and negative changes in mRNA levels in aged skeletal muscle, and the mRNA expression signatures of the two compounds are remarkably similar. Interestingly, a subset of the mRNAs repressed by ursolic acid and tomatidine in aged muscle are positively regulated by activating transcription factor 4 (ATF4). Based on this finding, we investigated ATF4 as a potential mediator of age-related muscle weakness and atrophy. We found that a targeted reduction in skeletal muscle ATF4 expression reduces age-related deficits in skeletal muscle strength, quality, and mass, similar to ursolic acid and tomatidine. These results elucidate ATF4 as a critical mediator of age-related muscle weakness and atrophy. In addition, these results identify ursolic acid and tomatidine as potential agents and/or lead compounds for reducing ATF4 activity, weakness, and atrophy in aged skeletal muscle.
Assuntos
Fator 4 Ativador da Transcrição/antagonistas & inibidores , Envelhecimento/patologia , Músculo Esquelético/metabolismo , Sarcopenia/metabolismo , Fator 4 Ativador da Transcrição/genética , Fator 4 Ativador da Transcrição/fisiologia , Animais , Expressão Gênica/efeitos dos fármacos , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Músculo Esquelético/patologia , RNA Mensageiro/genética , Sarcopenia/patologia , Sarcopenia/prevenção & controle , Tomatina/análogos & derivados , Tomatina/farmacologia , Triterpenos/farmacologia , Ácido UrsólicoRESUMO
PURPOSE OF REVIEW: Here, we discuss a recently developed experimental strategy for discovering small molecules with potential to prevent and treat skeletal muscle atrophy. RECENT FINDINGS: Muscle atrophy involves and requires widespread changes in skeletal muscle gene expression, which generate complex but measurable patterns of positive and negative changes in skeletal muscle mRNA levels (a.k.a. mRNA expression signatures of muscle atrophy). Many bioactive small molecules generate their own characteristic mRNA expression signatures, and by identifying small molecules whose signatures approximate mirror images of muscle atrophy signatures, one may identify small molecules with potential to prevent and/or reverse muscle atrophy. Unlike a conventional drug discovery approach, this strategy does not rely on a predefined molecular target but rather exploits the complexity of muscle atrophy to identify small molecules that counter the entire spectrum of pathological changes in atrophic muscle. We discuss how this strategy has been used to identify two natural compounds, ursolic acid and tomatidine, that reduce muscle atrophy and improve skeletal muscle function. SUMMARY: Discovery strategies based on mRNA expression signatures can elucidate new approaches for preserving and restoring muscle mass and function.
Assuntos
Proteínas Musculares/metabolismo , Músculo Esquelético/patologia , Atrofia Muscular/tratamento farmacológico , RNA Mensageiro/metabolismo , Tomatina/análogos & derivados , Triterpenos/uso terapêutico , Descoberta de Drogas/métodos , Expressão Gênica , Humanos , Proteínas Musculares/genética , Músculo Esquelético/metabolismo , Transdução de Sinais , Tomatina/uso terapêutico , Ácido UrsólicoRESUMO
Skeletal muscle atrophy is a common and debilitating condition that remains poorly understood at the molecular level. To better understand the mechanisms of muscle atrophy, we used mouse models to search for a skeletal muscle protein that helps to maintain muscle mass and is specifically lost during muscle atrophy. We discovered that diverse causes of muscle atrophy (limb immobilization, fasting, muscle denervation, and aging) strongly reduced expression of the enzyme spermine oxidase. Importantly, a reduction in spermine oxidase was sufficient to induce muscle fiber atrophy. Conversely, forced expression of spermine oxidase increased muscle fiber size in multiple models of muscle atrophy (immobilization, fasting, and denervation). Interestingly, the reduction of spermine oxidase during muscle atrophy was mediated by p21, a protein that is highly induced during muscle atrophy and actively promotes muscle atrophy. In addition, we found that spermine oxidase decreased skeletal muscle mRNAs that promote muscle atrophy (e.g., myogenin) and increased mRNAs that help to maintain muscle mass (e.g., mitofusin-2). Thus, in healthy skeletal muscle, a relatively low level of p21 permits expression of spermine oxidase, which helps to maintain basal muscle gene expression and fiber size; conversely, during conditions that cause muscle atrophy, p21 expression rises, leading to reduced spermine oxidase expression, disruption of basal muscle gene expression, and muscle fiber atrophy. Collectively, these results identify spermine oxidase as an important positive regulator of muscle gene expression and fiber size, and elucidate p21-mediated repression of spermine oxidase as a key step in the pathogenesis of skeletal muscle atrophy.
Assuntos
GTP Fosfo-Hidrolases/metabolismo , Regulação Enzimológica da Expressão Gênica/fisiologia , Fibras Musculares Esqueléticas/metabolismo , Atrofia Muscular/metabolismo , Miogenina/metabolismo , Oxirredutases atuantes sobre Doadores de Grupo CH-NH/metabolismo , Animais , Jejum/fisiologia , GTP Fosfo-Hidrolases/genética , Immunoblotting , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Denervação Muscular , Fibras Musculares Esqueléticas/enzimologia , Atrofia Muscular/enzimologia , Atrofia Muscular/genética , Miogenina/genética , Oxirredutases atuantes sobre Doadores de Grupo CH-NH/genética , RNA Mensageiro/química , RNA Mensageiro/genética , Reação em Cadeia da Polimerase em Tempo Real , Restrição Física/fisiologia , Poliamina OxidaseRESUMO
Immobilization causes skeletal muscle atrophy via complex signaling pathways that are not well understood. To better understand these pathways, we investigated the roles of p53 and ATF4, two transcription factors that mediate adaptations to a variety of cellular stresses. Using mouse models, we demonstrate that 3 days of muscle immobilization induces muscle atrophy and increases expression of p53 and ATF4. Furthermore, muscle fibers lacking p53 or ATF4 are partially resistant to immobilization-induced muscle atrophy, and forced expression of p53 or ATF4 induces muscle fiber atrophy in the absence of immobilization. Importantly, however, p53 and ATF4 do not require each other to promote atrophy, and coexpression of p53 and ATF4 induces more atrophy than either transcription factor alone. Moreover, muscle fibers lacking both p53 and ATF4 are more resistant to immobilization-induced atrophy than fibers lacking only p53 or ATF4. Interestingly, the independent and additive nature of the p53 and ATF4 pathways allows for combinatorial control of at least one downstream effector, p21. Using genome-wide mRNA expression arrays, we identified p21 mRNA as a skeletal muscle transcript that is highly induced in immobilized muscle via the combined actions of p53 and ATF4. Additionally, in mouse muscle, p21 induces atrophy in a manner that does not require immobilization, p53 or ATF4, and p21 is required for atrophy induced by immobilization, p53, and ATF4. Collectively, these results identify p53 and ATF4 as essential and complementary mediators of immobilization-induced muscle atrophy and discover p21 as a critical downstream effector of the p53 and ATF4 pathways.
Assuntos
Fator 4 Ativador da Transcrição/metabolismo , Inibidor de Quinase Dependente de Ciclina p21/metabolismo , Imobilização/efeitos adversos , Músculo Esquelético/metabolismo , Atrofia Muscular/etiologia , Proteína Supressora de Tumor p53/metabolismo , Regulação para Cima , Fator 4 Ativador da Transcrição/genética , Substituição de Aminoácidos , Animais , Cruzamentos Genéticos , Inibidor de Quinase Dependente de Ciclina p21/antagonistas & inibidores , Inibidor de Quinase Dependente de Ciclina p21/genética , Resistência à Doença , Estudo de Associação Genômica Ampla , Elevação dos Membros Posteriores , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Camundongos Transgênicos , MicroRNAs , Músculo Esquelético/patologia , Atrofia Muscular/metabolismo , Atrofia Muscular/patologia , Proteínas Mutantes/metabolismo , Regiões Promotoras Genéticas , Proteínas Recombinantes/metabolismo , Proteína Supressora de Tumor p53/genéticaRESUMO
Skeletal muscle atrophy is a common and debilitating condition that lacks an effective therapy. To address this problem, we used a systems-based discovery strategy to search for a small molecule whose mRNA expression signature negatively correlates to mRNA expression signatures of human skeletal muscle atrophy. This strategy identified a natural small molecule from tomato plants, tomatidine. Using cultured skeletal myotubes from both humans and mice, we found that tomatidine stimulated mTORC1 signaling and anabolism, leading to accumulation of protein and mitochondria, and ultimately, cell growth. Furthermore, in mice, tomatidine increased skeletal muscle mTORC1 signaling, reduced skeletal muscle atrophy, enhanced recovery from skeletal muscle atrophy, stimulated skeletal muscle hypertrophy, and increased strength and exercise capacity. Collectively, these results identify tomatidine as a novel small molecule inhibitor of muscle atrophy. Tomatidine may have utility as a therapeutic agent or lead compound for skeletal muscle atrophy.
Assuntos
Fibras Musculares Esqueléticas/efeitos dos fármacos , Músculo Esquelético/efeitos dos fármacos , Atrofia Muscular/prevenção & controle , Tomatina/análogos & derivados , Animais , Linhagem Celular , Linhagem Celular Tumoral , Células Cultivadas , Descoberta de Drogas/métodos , Expressão Gênica/efeitos dos fármacos , Células HL-60 , Humanos , Immunoblotting , Células MCF-7 , Masculino , Alvo Mecanístico do Complexo 1 de Rapamicina , Camundongos , Camundongos Endogâmicos C57BL , Mitocôndrias Musculares/efeitos dos fármacos , Mitocôndrias Musculares/genética , Mitocôndrias Musculares/metabolismo , Complexos Multiproteicos/genética , Complexos Multiproteicos/metabolismo , Fibras Musculares Esqueléticas/citologia , Fibras Musculares Esqueléticas/metabolismo , Proteínas Musculares/genética , Proteínas Musculares/metabolismo , Músculo Esquelético/metabolismo , Músculo Esquelético/patologia , Atrofia Muscular/genética , Atrofia Muscular/metabolismo , Mioblastos Esqueléticos/citologia , Mioblastos Esqueléticos/efeitos dos fármacos , Mioblastos Esqueléticos/metabolismo , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Transdução de Sinais/efeitos dos fármacos , Serina-Treonina Quinases TOR/genética , Serina-Treonina Quinases TOR/metabolismo , Tomatina/farmacologiaRESUMO
Skeletal muscle denervation causes muscle atrophy via complex molecular mechanisms that are not well understood. To better understand these mechanisms, we investigated how muscle denervation increases growth arrest and DNA damage-inducible 45α (Gadd45a) mRNA in skeletal muscle. Previous studies established that muscle denervation strongly induces Gadd45a mRNA, which increases Gadd45a, a small myonuclear protein that is required for denervation-induced muscle fiber atrophy. However, the mechanism by which denervation increases Gadd45a mRNA remained unknown. Here, we demonstrate that histone deacetylase 4 (HDAC4) mediates induction of Gadd45a mRNA in denervated muscle. Using mouse models, we show that HDAC4 is required for induction of Gadd45a mRNA during muscle denervation. Conversely, forced expression of HDAC4 is sufficient to increase skeletal muscle Gadd45a mRNA in the absence of muscle denervation. Moreover, Gadd45a mediates several downstream effects of HDAC4, including induction of myogenin mRNA, induction of mRNAs encoding the embryonic nicotinic acetylcholine receptor, and, most importantly, skeletal muscle fiber atrophy. Because Gadd45a induction is also a key event in fasting-induced muscle atrophy, we tested whether HDAC4 might also contribute to Gadd45a induction during fasting. Interestingly, however, HDAC4 is not required for fasting-induced Gadd45a expression or muscle atrophy. Furthermore, activating transcription factor 4 (ATF4), which contributes to fasting-induced Gadd45a expression, is not required for denervation-induced Gadd45a expression or muscle atrophy. Collectively, these results identify HDAC4 as an important regulator of Gadd45a in denervation-induced muscle atrophy and elucidate Gadd45a as a convergence point for distinct upstream regulators during muscle denervation and fasting.
Assuntos
Proteínas de Ciclo Celular/metabolismo , Histona Desacetilases/metabolismo , Músculo Esquelético/patologia , Atrofia Muscular/metabolismo , Proteínas Nucleares/metabolismo , Transdução de Sinais/fisiologia , Fator 4 Ativador da Transcrição/genética , Fator 4 Ativador da Transcrição/metabolismo , Animais , Proteínas de Ciclo Celular/genética , Jejum/metabolismo , Histona Desacetilases/genética , Masculino , Camundongos , Camundongos Knockout , Denervação Muscular , Músculo Esquelético/inervação , Músculo Esquelético/metabolismo , Atrofia Muscular/genética , Atrofia Muscular/patologia , Proteínas Nucleares/genéticaRESUMO
Skeletal muscle Akt activity stimulates muscle growth and imparts resistance to obesity, glucose intolerance and fatty liver disease. We recently found that ursolic acid increases skeletal muscle Akt activity and stimulates muscle growth in non-obese mice. Here, we tested the hypothesis that ursolic acid might increase skeletal muscle Akt activity in a mouse model of diet-induced obesity. We studied mice that consumed a high fat diet lacking or containing ursolic acid. In skeletal muscle, ursolic acid increased Akt activity, as well as downstream mRNAs that promote glucose utilization (hexokinase-II), blood vessel recruitment (Vegfa) and autocrine/paracrine IGF-I signaling (Igf1). As a result, ursolic acid increased skeletal muscle mass, fast and slow muscle fiber size, grip strength and exercise capacity. Interestingly, ursolic acid also increased brown fat, a tissue that shares developmental origins with skeletal muscle. Consistent with increased skeletal muscle and brown fat, ursolic acid increased energy expenditure, leading to reduced obesity, improved glucose tolerance and decreased hepatic steatosis. These data support a model in which ursolic acid reduces obesity, glucose intolerance and fatty liver disease by increasing skeletal muscle and brown fat, and suggest ursolic acid as a potential therapeutic approach for obesity and obesity-related illness.
Assuntos
Tecido Adiposo Marrom/metabolismo , Fígado Gorduroso/tratamento farmacológico , Intolerância à Glucose/tratamento farmacológico , Músculo Esquelético/metabolismo , Obesidade/tratamento farmacológico , Triterpenos/uso terapêutico , Tecido Adiposo Marrom/efeitos dos fármacos , Animais , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Músculo Esquelético/efeitos dos fármacos , Ácido UrsólicoRESUMO
Diverse stresses including starvation and muscle disuse cause skeletal muscle atrophy. However, the molecular mechanisms of muscle atrophy are complex and not well understood. Here, we demonstrate that growth arrest and DNA damage-inducible 45a protein (Gadd45a) is a critical mediator of muscle atrophy. We identified Gadd45a through an unbiased search for potential downstream mediators of the stress-inducible, pro-atrophy transcription factor ATF4. We show that Gadd45a is required for skeletal muscle atrophy induced by three distinct skeletal muscle stresses: fasting, muscle immobilization, and muscle denervation. Conversely, forced expression of Gadd45a in muscle or cultured myotubes induces atrophy in the absence of upstream stress. We show that muscle-specific ATF4 knock-out mice have a reduced capacity to induce Gadd45a mRNA in response to stress, and as a result, they undergo less atrophy in response to fasting or muscle immobilization. Interestingly, Gadd45a is a myonuclear protein that induces myonuclear remodeling and a comprehensive program for muscle atrophy. Gadd45a represses genes involved in anabolic signaling and energy production, and it induces pro-atrophy genes. As a result, Gadd45a reduces multiple barriers to muscle atrophy (including PGC-1α, Akt activity, and protein synthesis) and stimulates pro-atrophy mechanisms (including autophagy and caspase-mediated proteolysis). These results elucidate a critical stress-induced pathway that reprograms muscle gene expression to cause atrophy.
Assuntos
Proteínas de Ciclo Celular/metabolismo , Núcleo Celular/metabolismo , Regulação da Expressão Gênica , Proteínas Musculares/metabolismo , Músculo Esquelético/metabolismo , Atrofia Muscular/metabolismo , Proteínas Nucleares/metabolismo , Estresse Fisiológico , Fator 4 Ativador da Transcrição/genética , Fator 4 Ativador da Transcrição/metabolismo , Animais , Proteínas de Ciclo Celular/genética , Linhagem Celular , Núcleo Celular/genética , Núcleo Celular/patologia , Metabolismo Energético/genética , Camundongos , Camundongos Knockout , Proteínas Musculares/genética , Músculo Esquelético/patologia , Atrofia Muscular/genética , Atrofia Muscular/patologia , Proteínas Nucleares/genética , Coativador 1-alfa do Receptor gama Ativado por Proliferador de Peroxissomo , Biossíntese de Proteínas/genética , Proteínas Proto-Oncogênicas c-akt/genética , Proteínas Proto-Oncogênicas c-akt/metabolismo , RNA Mensageiro/biossíntese , RNA Mensageiro/genética , Transdução de Sinais/genética , Transativadores/genética , Transativadores/metabolismo , Fatores de TranscriçãoRESUMO
Inhibition of photosynthesis by heat stress is accompanied by functional impairment of Rubisco's chaperone, activase (RCA), resulting in deactivation of Rubisco. Since activase is extremely sensitive to thermal denaturation, changes in expression of RCA at the transcript or protein level could provide a mechanism for acclimation of photosynthesis to prolonged heat stress. Using quantitative real-time PCR (qPCR) we show steady-state RCA transcript levels in Arabidopsis thaliana are stabilized during prolonged exposure to moderate heat (35 °C). A survey of RCA transcripts indicates heat stress did not alter the relative abundance of transcripts encoding α and ß-isoforms of activase that are produced by alternative splicing of the pre-mRNA. Instead, mRNA stabilization in heat-stressed plants coincided with a significant reduction in the average length of activase 3'-untranslated regions, and was associated with enrichment of an uncharacterized activase mRNA splice variant, AtRCAß2. Transcript-specific qPCR revealed AtRCAß2 mRNA was more stable than AtRCAα and AtRCAß mRNA in heat-stressed plants. Using an inducible transgenic system, we found that RCA transcripts lacking their native 3'-untranslated region were significantly more stable than their full-length counterparts in vivo. Using this system, stability of the RCA protein was examined over 24 h in vivo, in the absence of RCA transcription. At both optimal and elevated temperatures, RCA protein levels remained stable in plants lacking RCA mRNA, but increased when RCA mRNA was present, particularly in heat-stressed plants. This study reveals a possible mechanism, involving post-transcriptional regulation of an important photosynthesis regulatory gene, for acclimation of photosynthesis to heat stress.