Physiology - Research Publications
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ItemDeletion of suppressor of cytokine signaling 3 (SOCS3) in muscle stem cells does not alter muscle regeneration in mice after injury(PUBLIC LIBRARY SCIENCE, 2019-02-27)Muscles of older animals are more susceptible to injury and regenerate poorly, in part due to a persistent inflammatory response. The janus kinase (Jak)/signal transducer and activator of transcription (Stat) pathway mediates inflammatory signaling and is tightly regulated by the suppressor of cytokine signaling (SOCS) proteins, especially SOCS3. SOCS3 expression is altered in the muscle of aged animals and may contribute to the persistent inflammation and impaired regeneration. To test this hypothesis, we performed myotoxic injuries on mice with a tamoxifen-inducible deletion of SOCS3 specifically within the muscle stem cell compartment. Muscle stem cell-specific SOCS3 deletion reduced muscle mass at 14 days post-injury (-14%, P < 0.01), altered the myogenic transcriptional program, and reduced myogenic fusion based on the number of centrally-located nuclei per muscle fiber. Despite the delay in myogenesis, muscles with a muscle stem cell-specific deletion of SOCS3 were still able to regenerate after a single bout or multiple bouts of myotoxic injury. A reduction in SOCS3 expression in muscle stem cells is unlikely to be responsible for the incomplete muscle repair in aged animals.
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ItemGlycine administration attenuates progression of dystrophic pathology in prednisolone-treated dystrophin/utrophin null mice(NATURE PUBLISHING GROUP, 2019-09-10)Duchenne muscular dystrophy (DMD) is an X-linked genetic disease characterized by progressive muscle wasting and weakness and premature death. Glucocorticoids (e.g. prednisolone) remain the only drugs with a favorable impact on DMD patients, but not without side effects. We have demonstrated that glycine preserves muscle in various wasting models. Since glycine effectively suppresses the activity of pro-inflammatory macrophages, we investigated the potential of glycine treatment to ameliorate the dystrophic pathology. Dystrophic mdx and dystrophin-utrophin null (dko) mice were treated with glycine or L-alanine (amino acid control) for up to 15 weeks and voluntary running distance (a quality of life marker and strong correlate of lifespan in dko mice) and muscle morphology were assessed. Glycine increased voluntary running distance in mdx mice by 90% (P < 0.05) after 2 weeks and by 60% (P < 0.01) in dko mice co-treated with prednisolone over an 8 week treatment period. Glycine treatment attenuated fibrotic deposition in the diaphragm by 28% (P < 0.05) after 10 weeks in mdx mice and by 22% (P < 0.02) after 14 weeks in dko mice. Glycine treatment augmented the prednisolone-induced reduction in fibrosis in diaphragm muscles of dko mice (23%, P < 0.05) after 8 weeks. Our findings provide strong evidence that glycine supplementation may be a safe, simple and effective adjuvant for improving the efficacy of prednisolone treatment and improving the quality of life for DMD patients.
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ItemGlycine Protects Muscle Cells From Wasting in vitro via mTORC1 Signaling(FRONTIERS MEDIA SA, 2019-11-13)Glycine supplementation can protect skeletal muscles of mice from cancer-induced wasting, but the mechanisms underlying this protection are not well-understood. The aim of this study was to determine whether exogenous glycine directly protects skeletal muscle cells from wasting. C2C12 muscle cells were exposed to non-inflammatory catabolic stimuli via two models: serum withdrawal (SF) for 48 h; or incubation in HEPES buffered saline (HBS) for up to 5 h. Cells were supplemented with glycine or equimolar concentrations of L-alanine. SF- and HBS-treated myotubes (with or without L-alanine) were ~20% and ~30% smaller than control myotubes. Glycine-treated myotubes were up to 20% larger (P < 0.01) compared to cells treated with L-alanine in both models of muscle cell atrophy. The mTORC1 inhibitor rapamycin prevented the glycine-stimulated protection of myotube diameter, and glycine-stimulated S6 phosphorylation, suggesting that mTORC1 signaling may be necessary for glycine's protective effects in vitro. Increasing glycine availability may be beneficial for muscle wasting conditions associated with inadequate nutrient intake.
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ItemMuscle-specific deletion of SOCS3 does not reduce the anabolic response to leucine in a mouse model of acute inflammation(ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, 2017-08)Excessive inflammation reduces skeletal muscle protein synthesis leading to wasting and weakness. The janus kinase/signal transducers and activators of transcription-3 (JAK/STAT3) pathway is important for the regulation of inflammatory signaling. As such, suppressor of cytokine signaling-3 (SOCS3), the negative regulator of JAK/STAT signaling, is thought to be important in the control of muscle homeostasis. We hypothesized that muscle-specific deletion of SOCS3 would impair the anabolic response to leucine during an inflammatory insult. Twelve week old (n=8 per group) SOCS3 muscle-specific knockout mice (SOCS3-MKO) and littermate controls (WT) were injected with lipopolysaccharide (LPS, 1mg/kg) or saline and were studied during fasted conditions or after receiving 0.5g/kg leucine 3h after the injection of LPS. Markers of inflammation, anabolic signaling, and protein synthesis were measured 4h after LPS injection. LPS injection robustly increased mRNA expression of inflammatory molecules (Socs3, Socs1, Il-6, Ccl2, Tnfα and Cd68). In muscles from SOCS3-MKO mice, the Socs3 mRNA response to LPS was significantly blunted (∼6-fold) while STAT3 Tyr705 phosphorylation was exacerbated (18-fold). Leucine administration increased protein synthesis in both WT (∼1.6-fold) and SOCS3-MKO mice (∼1.5-fold) compared to basal levels. LPS administration blunted this effect, but there were no differences between WT and SOCS3-MKO mice. Muscle-specific SOCS3 deletion did not alter the response of AKT, mTOR, S6 or 4EBP1 under any treatment conditions. Therefore, SOCS3 does not appear to mediate the early inflammatory or leucine-induced changes in protein synthesis in skeletal muscle.
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ItemDietary meat and protection against sarcopenia(ELSEVIER SCI LTD, 2018-10)Sarcopenia describes the age-related loss of skeletal muscle mass and associated muscle weakness. Sarcopenia is a major global health problem given that the number and proportion of older people in the population is escalating worldwide and represent the fastest growing segment of society. The loss of muscle mass compromises physical capacity, increases susceptibility to falls, and impacts on an individual's functional independence and quality of life. Tackling sarcopenia sensibly and effectively will identify strategies that will enable older adults to age well and age productively. The underlying causes of sarcopenia are complex and multifactorial and will likely require combinatorial therapies to address its symptoms. Nutrition, particularly protein intake, is a more easily modifiable factor, especially when combined with structured (resistance) exercise programs. The relative success of protein feeding strategies for sarcopenia, is limited by a so-called anabolic resistance in older people. Meat contains essential amino acids and nutritive compounds of high quality, and even a moderate intake can increase muscle protein synthesis in older men and women. However, health risks have been identified with the consumption of different meats, with high intake of processed meats increasing the risk for cardiovascular disease and different cancers. Risks for fresh white and red meat are considerably less and modest consumption is encouraged as part of a healthy eating plan for many older adults to ensure adequate protein intake. Other nutritive strategies of relevance for sarcopenia involve fortifying the nutrient value of different meats. Studies on muscle cells and animal models of muscle wasting, have identified the therapeutic potential of the amino acid, glycine, to reduce inflammation, attenuate muscle atrophy, and re-sensitize muscle to anabolic stimuli. Glycine supplementation or feeding animal products with a high glycine content (e.g. gelatin), could represent simple and effective nutritional strategies as part of a suite of therapies to attenuate sarcopenia.
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ItemSkeletal muscle-specific overexpression of IGFBP-2 promotes a slower muscle phenotype in healthy but not dystrophic mdx mice and does not affect the dystrophic pathology(CHURCHILL LIVINGSTONE, 2016)OBJECTIVE: The insulin-like growth factor binding proteins (IGFBPs) are thought to modulate cell size and homeostasis via IGF-I-dependent and -independent pathways. There is a considerable dearth of information regarding the function of IGFBPs in skeletal muscle, particularly their role in the pathophysiology of Duchenne muscular dystrophy (DMD). In this study we tested the hypothesis that intramuscular IGFBP-2 overexpression would ameliorate the pathology in mdx dystrophic mice. DESIGN: 4week old male C57Bl/10 and mdx mice received a single intramuscular injection of AAV6-empty or AAV6-IGFBP-2 vector into the tibialis anterior muscle. At 8weeks post-injection the effect of IGFBP-2 overexpression on the structure and function of the injected muscle was assessed. RESULTS: AAV6-mediated IGFBP-2 overexpression in the tibialis anterior (TA) muscles of 4-week-old C57BL/10 and mdx mice reduced the mass of injected muscle after 8weeks, inducing a slower muscle phenotype in C57BL/10 but not mdx mice. Analysis of inflammatory and fibrotic gene expression revealed no changes between control and IGFBP-2 injected muscles in dystrophic (mdx) mice. CONCLUSIONS: Together these results indicate that the IGFBP-2-induced promotion of a slower muscle phenotype is impaired in muscles of dystrophin-deficient mdx mice, which contributes to the inability of IGFBP-2 to ameliorate the dystrophic pathology. The findings implicate the dystrophin-glycoprotein complex (DGC) in the signaling required for this adaptation.
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ItemNo Preview AvailableAntibody-directed myostatin inhibition enhances muscle mass and function in tumor-bearing mice(AMER PHYSIOLOGICAL SOC, 2011-09)Cancer cachexia describes the progressive skeletal muscle wasting and weakness in many cancer patients and accounts for >20% of cancer-related deaths. We tested the hypothesis that antibody-directed myostatin inhibition would attenuate the atrophy and loss of function in muscles of tumor-bearing mice. Twelve-week-old C57BL/6 mice received a subcutaneous injection of saline (control) or Lewis lung carcinoma (LLC) tumor cells. One week later, mice received either once weekly injections of saline (control, n = 12; LLC, n = 9) or a mouse chimera of anti-human myostatin antibody (PF-354, 10 mg·kg⁻¹·wk⁻¹, LLC+PF-354, n = 11) for 5 wk. Injection of LLC cells reduced muscle mass and maximum force of tibialis anterior (TA) muscles by 8-10% (P < 0.05), but the muscle atrophy and weakness were prevented with PF-354 treatment (P > 0.05). Maximum specific (normalized) force of diaphragm muscle strips was reduced with LLC injection (P < 0.05) but was not improved with PF-354 treatment (P > 0.05). PF-354 enhanced activity of oxidative enzymes in TA and diaphragm muscles of tumor-bearing mice by 118% and 89%, respectively (P < 0.05). Compared with controls, apoptosis that was not of myofibrillar or satellite cell origin was 140% higher in TA muscle cross sections from saline-treated LLC tumor-bearing mice (P < 0.05) but was not different in PF-354-treated tumor-bearing mice (P > 0.05). Antibody-directed myostatin inhibition attenuated the skeletal muscle atrophy and loss of muscle force-producing capacity in a murine model of cancer cachexia, in part by reducing apoptosis. The improvements in limb muscle mass and function highlight the therapeutic potential of antibody-directed myostatin inhibition for cancer cachexia.
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ItemTranilast administration reduces fibrosis and improves fatigue resistance in muscles of mdx dystrophic mice(BMC, 2014)BACKGROUND: Duchenne muscular dystrophy (DMD) is a severe and progressive muscle-wasting disorder caused by mutations in the dystrophin gene that result in the absence of the membrane-stabilising protein dystrophin. Dystrophic muscle fibres are susceptible to injury and degeneration, and impaired muscle regeneration is associated with fibrotic deposition that limits the efficacy of potential pharmacological, cell- and gene-based therapies. Novel treatments that can prevent or attenuate fibrosis have important clinical merit for DMD and related neuromuscular diseases. We investigated the therapeutic potential for tranilast, an orally bioavailable anti-allergic agent, to prevent fibrosis in skeletal muscles of mdx dystrophic mice. RESULTS: Three-week-old C57Bl/10 and mdx mice received tranilast (~300 mg/kg) in their food for 9 weeks, after which fibrosis was assessed through histological analyses, and functional properties of tibialis anterior muscles were assessed in situ and diaphragm muscle strips in vitro. Tranilast administration did not significantly alter the mass of any muscles in control or mdx mice, but it decreased fibrosis in the severely affected diaphragm muscle by 31% compared with untreated mdx mice (P < 0.05). A similar trend of decreased fibrosis was observed in the tibialis anterior muscles of mdx mice (P = 0.10). These reductions in fibrotic deposition were not associated with improvements in maximum force-producing capacity, but we did observe small but significant improvements in the resistance to fatigue in both the diaphragm and TA muscles of mdx mice treated with tranilast. CONCLUSION: Together these findings demonstrate that administration of potent antifibrotic compounds such as tranilast could help preserve skeletal muscle structure, which could ultimately increase the efficacy of pharmacological, cell and gene replacement/correction therapies for muscular dystrophy and related disorders.
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ItemDysfunctional Muscle and Liver Glycogen Metabolism in mdx Dystrophic Mice(PUBLIC LIBRARY SCIENCE, 2014-03-13)BACKGROUND: Duchenne muscular dystrophy (DMD) is a severe, genetic muscle wasting disorder characterised by progressive muscle weakness. DMD is caused by mutations in the dystrophin (dmd) gene resulting in very low levels or a complete absence of the dystrophin protein, a key structural element of muscle fibres which is responsible for the proper transmission of force. In the absence of dystrophin, muscle fibres become damaged easily during contraction resulting in their degeneration. DMD patients and mdx mice (an animal model of DMD) exhibit altered metabolic disturbances that cannot be attributed to the loss of dystrophin directly. We tested the hypothesis that glycogen metabolism is defective in mdx dystrophic mice. RESULTS: Dystrophic mdx mice had increased skeletal muscle glycogen (79%, (P<0.01)). Skeletal muscle glycogen synthesis is initiated by glycogenin, the expression of which was increased by 50% in mdx mice (P<0.0001). Glycogen synthase activity was 12% higher (P<0.05) but glycogen branching enzyme activity was 70% lower (P<0.01) in mdx compared with wild-type mice. The rate-limiting enzyme for glycogen breakdown, glycogen phosphorylase, had 62% lower activity (P<0.01) in mdx mice resulting from a 24% reduction in PKA activity (P<0.01). In mdx mice glycogen debranching enzyme expression was 50% higher (P<0.001) together with starch-binding domain protein 1 (219% higher; P<0.01). In addition, mdx mice were glucose intolerant (P<0.01) and had 30% less liver glycogen (P<0.05) compared with control mice. Subsequent analysis of the enzymes dysregulated in skeletal muscle glycogen metabolism in mdx mice identified reduced glycogenin protein expression (46% less; P<0.05) as a possible cause of this phenotype. CONCLUSION: We identified that mdx mice were glucose intolerant, and had increased skeletal muscle glycogen but reduced amounts of liver glycogen.
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ItemL-Citrulline Protects Skeletal Muscle Cells from Cachectic Stimuli through an iNOS-Dependent Mechanism(PUBLIC LIBRARY SCIENCE, 2015-10-29)Dietary L-citrulline is thought to modulate muscle protein turnover by increasing L-arginine availability. To date, the direct effects of increased L-citrulline concentrations in muscle have been completely neglected. Therefore, we determined the role of L-citrulline in regulating cell size during catabolic conditions by depriving mature C2C12 myotubes of growth factors (serum free; SF) or growth factors and nutrients (HEPES buffered saline; HBS). Cells were treated with L-citrulline or equimolar concentrations of L-arginine (positive control) or L-alanine (negative control) and changes in cell size and protein turnover were assessed. In myotubes incubated in HBS or SF media, L-citrulline improved rates of protein synthesis (HBS: +63%, SF: +37%) and myotube diameter (HBS: +18%, SF: +29%). L-citrulline treatment substantially increased iNOS mRNA expression (SF: 350%, HBS: 750%). The general NOS inhibitor L-NAME and the iNOS specific inhibitor aminoguanidine prevented these effects in both models. Depriving myotubes in SF media of L-arginine or L-leucine, exacerbated wasting which was not attenuated by L-citrulline. The increased iNOS mRNA expression was temporally associated with increases in mRNA of the endogenous antioxidants SOD1, SOD3 and catalase. Furthermore, L-citrulline prevented inflammation (LPS) and oxidative stress (H2O2) induced muscle cell wasting. In conclusion, we demonstrate a novel direct protective effect of L-citrulline on skeletal muscle cell size independent of L-arginine that is mediated through induction of the inducible NOS (iNOS) isoform. This discovery of a nutritional modulator of iNOS mRNA expression in skeletal muscle cells could have substantial implications for the treatment of muscle wasting conditions.