Madoka Ikemoto

Molecular Signature of Quiescent Satellite Cells in Adult Skeletal Muscle

Skeletal muscle satellite cells play key roles in postnatal muscle growth and regeneration. To study molecular regulation of satellite cells, we directly prepared satellite cells from 8‐ to 12‐week‐old C57BL/6 mice and performed genome‐wide gene expression analysis. Compared with activated/cycling satellite cells, 507 genes were highly upregulated in quiescent satellite cells. These included negative regulators of cell cycle and myogenic inhibitors. Gene set enrichment analysis revealed that quiescent satellite cells preferentially express the genes involved in cell‐cell adhesion, regulation of cell growth, formation of extracellular matrix, copper and iron homeostasis, and lipid transportation. Furthermore, reverse transcription‐polymerase chain reaction on differentially expressed genes confirmed that calcitonin receptor (CTR) was exclusively expressed in dormant satellite cells but not in activated satellite cells. In addition, CTR mRNA is hardly detected in nonmyogenic cells. Therefore, we next examined the expression of CTR in vivo. CTR was specifically expressed on quiescent satellite cells, but the expression was not found on activated/proliferating satellite cells during muscle regeneration. CTR‐positive cells reappeared at the rim of regenerating myofibers in later stages of muscle regeneration. Calcitonin stimulation delayed the activation of quiescent satellite cells. Our data provide roles of CTR in quiescent satellite cells and a solid scaffold to further dissect molecular regulation of satellite cells.

Skeletal muscle gene expression in space-flown rats

Skeletal muscles are vulnerable to marked atrophy under microgravity. This phenomenon is due to the transcriptional alteration of skeletal muscle cells to weightlessness. To further investigate this issue at a subcellular level, we examined the expression of ~26,000 gastrocnemius muscle genes in space‐flown rats by DNA microarray analysis. Comparison of the changes in gene expression among spaceflight, tail‐suspended, and denervated rats revealed that such changes were unique after spaceflight and not just an extension of simulated weightlessness. The microarray data showed two spaceflight‐specific gene expression patterns: 1) imbalanced expression of mitochondrial genes with disturbed expression of cytoskeletal molecules, including putative mitochondria‐anchoring proteins, A‐kinase anchoring protein, and cytoplasmic dynein, and 2) up‐regulated expression of ubiquitin ligase genes, MuRF‐1, Cbl‐b, and Siah‐1A, which are rate‐limiting enzymes of muscle protein degradation. Distorted expression of cytoskeletal genes during spaceflight resulted in dislocation of the mitochondria in the cell. Several oxidative stress‐inducible genes were highly expressed in the muscle of spaceflight rats. We postulate that mitochondrial dislocation during spaceflight has deleterious effects on muscle fibers, leading to atrophy in the form of insufficient energy provision for construction and leakage of reactive oxygen species from the mitochondria.

Space shuttle flight (STS-90) enhances degradation of rat myosin heavy chain in association with activation of ubiquitin–proteasome pathway

To elucidate the mechanisms of microgravity‐induced muscle atrophy, we focused on fast‐type myosin heavy chain (MHC) degradation and expression of proteases in atrophied gastrocnemius muscles of neonatal rats exposed to 16‐d spaceflight (STS‐90). The spaceflight stimulated ubiquitination of proteins, including a MHC molecule, and accumulation of MHC degradation fragments in the muscles. Semiquantitative reverse transcriptase‐polymerase chain reaction revealed that the spaceflight significantly increased mRNA levels of cathepsin L, proteasome components (RC2 and RC9), polyubiquitin, and ubiquitin‐conjugating enzyme in the muscles, compared with those of ground control rats. The levels of μ‐calpain, m‐calpain, cathepsin B, and cathepsin H mRNAs were not changed by the spaceflight. We also found that tail‐suspension of rats for 10 d or longer caused the ubiquitination and degradation of MHC in gastrocnemius muscle, as was observed in the spaceflight rats. In the muscle of suspended rats, these changes were closely associated with activation of proteasome and up‐regulation of expression of mRNA for the proteasome components and polyubiquitin. Administration of a cysteine protease inhibitor, E‐64, to the suspended rats did not prevent the MHC degradation. Our results suggest that spaceflight induces the degradation of muscle contractile proteins, including MHC, possibly through a ubiquitin‐dependent proteolytic pathway.

Autologous Transplantation of SM/C-2.6+ Satellite Cells Transduced with Micro-dystrophin CS1 cDNA by Lentiviral Vector into mdx Mice

Duchenne muscular dystrophy (DMD) is a lethal muscle disorder caused by mutations in the dystrophin gene. Transplantation of autologous myogenic cells genetically corrected ex vivo is a possible treatment for this disorder. In order to test the regenerative efficiency of freshly isolated satellite cells, we purified quiescent satellite cells from limb muscles of 8-12-week-old green fluorescent protein-transgenic (GFP-Tg) mice using SM/C-2.6 (a recently developed monoclonal antibody) and flow cytometry. Freshly isolated satellite cells were shown to participate in muscle regeneration more efficiently than satellite cell-derived myoblasts passaged in vitro do, when transplanted into tibialis anterior (TA) muscles of 8-12-week-old cardiotoxin-injected C57BL/6 mice and 5-week-old dystrophin-deficient mdx mice, and analyzed at 4 weeks after injection. Importantly, expansion of freshly isolated satellite cells in vitro without passaging had no detrimental effects on their regenerative capacity. Therefore we directly isolated satellite cells from 5-week-old mdx mice using SM/C-2.6 antibody and cultured them with lentiviral vectors expressing micro-dystrophin CS1. The transduced cells were injected into TA muscles of 5-week-old mdx mice. At 4 weeks after transplantation, the grafted cells efficiently contributed to regeneration of mdx dystrophic muscles and expressed micro-dystrophin at the sarcolemma. These results suggest that there is potential for lentiviral vector-mediated ex vivo gene therapy for DMD.

Cysteine supplementation prevents unweighting-induced ubiquitination in association with redox regulation in rat skeletal muscle

Abstract We have previously reported that spaceflight and tail suspension enhanced degradation of rat myosin heavy chain (MHC) in association with activation of a ubiquitindependent proteolytic pathway [Ikemoto et al., FASEB J. 15 (2001), 1279 1281]. To elucidate whether the ubiquitination is accompanied by oxidative stress, we measured markers for oxidative stress, such as thiobarbituric acidreactive substance (TBARS) and glutathione disulfide (GSSG), in gastrocnemius muscle of tailsuspended rats. Glutathione (GSH) concentration in the muscle significantly decreased from day 5 and reached a minimum value on day 10. Tail suspension reciprocally increased concentrations of TBARS and GSSG in parallel with enhancement of protein ubiquitination, suggesting that oxidative stress may play an important role in protein ubiquitination caused by tail suspension. To prevent ubiquitination associated with oxidative stress, we also administered an antioxidative nutrient, cysteine, to tailsuspended rats. Intragastric supplementation of 140 mg/rat of cysteine for 2 weeks or longer normalized the ratio of GSH to GSSG in the muscle and suppressed protein ubiquitination and MHC fragmentation, compared with supplementation of the equimolar amount of alanine. The cysteine supplementation significantly suppressed the loss of hindlimb muscle mass. Our results suggest that supplementation of antioxidative nutrients, such as cysteine, may be beneficial for preventing ubiquitination of muscle proteins caused by unweighting.

Effects of a Soy Protein Diet on Exercise-Induced Muscle Protein Catabolism in Rats

OBJECTIVE We examined effects of dietary soy protein isolate on muscle calpain activity and myosin heavy chain (MHC) degradation in rats performing an acute running exercise. METHODS In rats fed a 20% casein diet, the treadmill running exercise, fixed at 80 kg/m, transiently increased calpain activity in gastrocnemius muscles in parallel with the release of creatine kinase into plasma. The fixed running also caused an accumulation of immunoreactive degradation fragments of MHC in the muscle. Feeding a 20% soy protein isolate diet as opposed to the control casein diet to rats significantly suppressed the running-induced activation of mu- and m-calpains, fragmentation of MHC, and release of creatine kinase into plasma (P < 0.05). RESULTS Rats fed the soy protein isolate diet had significantly higher calpastatin activity in gastrocnemius muscle than did rats fed the casein diet (P < 0.05), indicating that this increase inhibits the exercise-induced autoactivation of calpain. Activities of proteasome, cathepsin B + L, and antioxidant enzymes and the levels of glutathione and thiobarbituric acid-reactive substances in the muscle did not differ between the diet groups at the end of the exercise. CONCLUSIONS Our results suggest that diets containing soy protein prevent exercise-induced protein degradation in skeletal muscle, possibly through inhibiting the calpain-mediated proteolysis.

253. An AAV Vector-Mediated Gene Transfer into Canine Skeletal Muscle

Duchenne muscular dystrophy (DMD) is an X-linked, lethal muscle disorder caused by mutations in the dystrophin gene (14 kb cDNA). An adeno-associated virus (AAV) vector-mediated gene transfer is one of attractive approaches to the treatment of DMD, but it has a limitation in insertion size up to 4.9 kb. We recently demonstrated that the AAV vector-mediated micro-dystrophin cDNA transfer could ameliorate dystrophic phenotypes in skeletal muscles of dystrophin-deficient mdx mice (Mol Ther 10: 821-828, 2004). For clinical application, it is important to examine therapeutic effects and the safety issue in larger animal models, such as dystrophic dogs. We established a colony of beagle-based canine X-linked muscular dystrophy in Japan (Exp Anim. 52: 93-97, 2003). To investigate transduction efficiency in canine skeletal muscle using an AAV vector, we injected the AAV vector encoding the LacZ gene driven by a CMV promoter (1.0-2.0 |[times]| 1013 vg/ml, 100-500 |[mu]|l/muscle) into skeletal muscles of normal dogs. |[beta]|-galactosidase (|[beta]|-gal) was expressed only in few fibers at 2 weeks after the injection, and not detected at 4 or 8 weeks after the injection. Instead, large numbers of mononuclear cells appeared around |[beta]|-gal-expressing fibers. To clarify mechanisms of low transduction and cellular infiltration in canine muscle after transfer of the AAV vector, we examined viral infectivity in vitro, cytotoxicity and immune responses of AAV vector transduction in vivo. First, we infected the AAV vector into canine primary myotubes. This in vitro study showed that the AAV vector could allow higher transgene expression in canine myotubes than in murine ones. Second, we tested whether injection of AAV particles elicit cytotoxicity or not. When a promoter-less AAV vector expressing no transgene (5 |[times]| 1012 vg/muscle) was injected into canine muscle, almost no infiltrating cells was observed in injected muscle. Third, we investigated immune responses. A lot of CD4- or CD8-positive cells were detected in clusters of infiltrating cells, together with elevated serum level of anti-|[beta]|-gal IgG. To confirm low transduction depending on immune response, dogs received daily oral administration of cyclosporine (20 mg/kg/day) from |[ndash]|5 day of the introduction of the AAV vector. Immunosuppression largely but not completely improves transduction efficiency of the AAV vector. These results suggest that AAV vector-mediated gene transfer elicits stronger immune responses in canine muscle, but immune responses against transgene products can not thoroughly explain the phenomenon. Cellular toxicity of transgene products might also participate in these infiltrations, while cytotoxicity and immunity of the AAV particles themselves can be negligible based on the result of a promoter-less AAV vectors. It is indispensable to know the molecular background of excess immune responses and cellular toxicity in canine models to establish AAV vector-mediated gene transfer in dystrophic patients.