Skeletal Muscle Regrowth Research Articles

<a name="home"></a>Cyclosporin-A does not affect skeletal muscle mass during disuse and recovery

Cyclosporin-A (CsA) is an immunosuppressive drug that acts as an inhibitor of calcineurin, a calcium phosphatase that has been suggested to play a role in skeletal muscle hypertrophy. The aim of the present study was to determine the effect of CsA administration (25 mg kg(-1) day(-1)) on skeletal muscle mass and phenotype during disuse and recovery. Male Wistar rats received vehicle (N = 8) or CsA (N = 8) during hind limb immobilization (N = 8) and recovery (N = 8). Muscle weight (dry/wet) and cross-sectional area were evaluated to verify the effect of CsA treatment on muscle mass. Muscle phenotype was assessed by histochemistry of myosin ATPase. CsA administration during immobilization and recovery did not change muscle/body weight ratio in the soleus (SOL) or plantaris (PL). Regarding muscle phenotype, we observed a consistent slow-to-fast shift in all experimental groups (immobilized only, receiving CsA only, and immobilized receiving CsA) as compared to control in both SOL and PL (P < 0.05). During recovery, no difference was observed in SOL or PL fiber type composition between the experimental recovered group and recovered group receiving CsA compared to their respective controls. Considering the muscle/body weight ratio, CsA administration does not maximize muscle mass loss induced by immobilization. Our results also indicate that CsA fails to block skeletal muscle regrowth after disuse. The present data suggest that calcineurin inhibition by CsA modulates muscle phenotype rather than muscle mass.

Age-associated decrease in muscle precursor cell differentiation

Muscle precursor cells (MPCs) are required for the regrowth, regeneration, and/or hypertrophy of skeletal muscle, which are deficient in sarcopenia. In the present investigation, we have addressed the issue of age-associated changes in MPC differentiation. MPCs, including satellite cells, were isolated from both young and old rat skeletal muscle with a high degree of myogenic purity (>90% MyoD and desmin positive). MPCs isolated from skeletal muscle of 32-mo-old rats exhibited decreased differentiation into myotubes and demonstrated decreased myosin heavy chain (MHC) and muscle creatine kinase (CK-M) expression compared with MPCs isolated from 3-mo-old rats. p27(Kip1) is a cyclin-dependent kinase inhibitor that has been shown to enhance muscle differentiation in culture. Herein we describe our finding that p27(Kip1) protein was lower in differentiating MPCs from skeletal muscle of 32-mo-old rats than in 3-mo-old rat skeletal muscle. Although MHC and CK-M expression were approximately 50% lower in differentiating MPCs isolated from 32-mo-old rats, MyoD protein content was not different and myogenin protein concentration was twofold higher. These data suggest that there are inherent differences in cell signaling during the transition from cell cycle arrest to the formation of myotubes in MPCs isolated from sarcopenic muscle. Furthermore, there is an age-associated decrease in muscle-specific protein expression in differentiating MPCs despite normal MyoD and elevated myogenin levels.

Effects of Ovarian Hormones on Rat Soleus Skeletal Muscle Regrowth during Reloading after Disuse

1034 PURPOSE: Ovarian hormones can mediate skeletal muscle damage and may play a role in the mediation of regrowth processes regulated by reloading after disuse. The purpose of this study was to determine what role ovarian hormones play in skeletal muscle regrowth after disuse atrophy. METHODS: Female Sprague Dawley rats (N = 65) were randomly assigned to control (Normal) and ovariectomized (OVX) treatments. Animals were then assigned to cage control (Con), 10-days of hindlimb suspension (Sus), 10-days of hindlimb suspension with 7-days of reloading (Sus+7), or 10-days of hindlimb suspension with 14-days of reloading (Sus+14) treatment groups. RESULTS: Muscle wt/tibia length (mg/mm) returned to Con values with Sus+7 in Normal soleus muscle, but remained decreased 32% with Sus+7 and 11% with Sus+14 in OVX soleus muscle. Normal soleus muscle mean cross sectional area (CSA) decreased 46% with Sus (1280 ± 103 μm2) but returned to Con (2363 ± 118 μm2) values with Sus+14 (2245 ± 226 μm2). OVX soleus CSA decreased 52% with Sus (1158 ± 196 μm2) and remained decreased 37% from Con (2423 ± 116 μm2) values with Sus+14 (1518 ± 233 μm2). Sus (9.4 ± 2.3%), Sus+7 (22.4 ± 3.7%), and Sus+14 (17.8 ± 1.7%) increased the percentage of small (< 500 μm2) fibers 1243%, 762%, and 128% respectively, in Normal soleus muscle. Only Sus+14 (30.7 ± 6.9%), however, increased small fibers 333% in the OVX soleus. CONCLUSIONS: The results of this study suggest that ovariectomy impairs the normal regenerative processes associated with reloading after disuse atrophy. Supported by a Grant from the South Carolina Space Grant Consortium and an American College of Sports Medicine/NASA Student Grant.

Effects of Ovarian Hormones on Rat Soleus Skeletal Muscle Regrowth during Reloading after Disuse

Effects of Ovarian Hormones on Rat Soleus Skeletal Muscle Regrowth during Reloading after Disuse

Regrowth of skeletal muscle atrophied from inactivity.

The current state of knowledge regarding regrowth of skeletal muscle after inactivity-induced atrophy is reviewed. Muscle regrowth is incomplete after hindlimb suspension in juvenile rats and after limb immobilization in old animals. The process of regrowth from immobilization-induced atrophy likely involves the reversal of directional changes in molecules producing muscle loss while initiating anabolic processes for regrowth of muscle mass. Unfortunately, the molecular mechanisms responsible for successful, or failed, muscle regrowth are not well understood. The purpose of the review is to provide current knowledge about the biology of muscle regrowth from inactivity-induced atrophy.

Temporal alterations in protein signaling cascades during recovery from muscle atrophy.

Currently, the repertoire of cellular and molecular pathways that control skeletal muscle atrophy and hypertrophy are not well defined. It is possible that intracellular regulatory signaling pathways are active at different times during the muscle hypertrophy process. The hypothesis of the given experiments was that cellular signals related to protein translation would be active at early time points of skeletal muscle regrowth, whereas transcriptional signals would be active at later time points of skeletal muscle regrowth. The phosphorylation status of p38 MAPK and JNK increased at the end of limb immobilization but returned to control values at recovery day 3. Transient increases in phosphorylation and in protein concentration occurred during recovery of soleus muscle mass. Phosphorylation of Akt, p70S6k, and signal transducer and activator of transcription 3 (STAT3) peaked on recovery day 3 compared with day 0. Glycogen synthase kinase (GSK)-3beta phosphorylation was increased on the sixth and fifteenth recovery day. In addition, transient peaks in seven protein concentrations occurred at different times of recovery: STAT3, calcineurin A (CaNA), CaNB, and beta4E-BP1 protein concentrations peaked on the third recovery day; p70S6k, STAT3, Akt, and GSK3-beta peaked on the sixth recovery day; and GSK3-beta peaked on the fifteenth recovery day. The apexes of STAT3 and GSK3-beta protein concentrations remained elevated for two recovery time points. Thus the time course of increase in molecules of signaling pathways differed as the young rat soleus muscle regrew from an atrophied state.

P27Kip1: a key regulator of skeletal muscle satellite cell proliferation.

Given the complex process of sarcopenia, it has been postulated that a diminished satellite cell proliferation potential could become rate-limiting to the regrowth of old muscles. Therefore, it is conceivable that if satellite cell proliferative capacity can be maintained or enhanced with advanced age, sarcopenia potentially could be delayed or prevented. The emerging role(s) of a cell cycle regulator, p27Kip, and its role in the regulation of satellite cell proliferative capacity are reviewed. A novel inverse association between the proliferation capacity and p27Kip protein abundance in skeletal muscle satellite cells was observed. Strikingly, with the introduction of ectopic p27Kip via an adenovirus vector, there was a marked growth arrest induced in proliferating cultured satellite cells, despite the presence of a strong mitogenic stimulus. Therefore, p27Kip is proposed to be a key regulatory factor, particularly in its ability to regulate satellite cell cycle progression. In the larger context, such results implicate p27Kip as one of the key molecules governing the regulation of skeletal muscle regrowth, hypertrophy, or both.

Alternate Pax7 transcripts are expressed specifically in skeletal muscle, brain and other organs of adult mice

Pax7 is associated with formation of skeletal muscle and the neural tube in developing embryos. Interestingly, in adult mice, rearrangements of Pax7 are associated with differences in the efficiency of skeletal muscle regrowth between mouse strains. The aim of this study was to investigate the possibility that Pax7 is expressed in skeletal muscle or other tissues from adult mice. Total RNA was isolated from adult mouse tissues and the polymerase chain reaction was performed on reverse transcribed mRNA using primers specific for regions that encode the paired and homeodomain of Pax7. At least four different Pax7 transcripts were found. A full-length transcript similar in sequence to that published previously was identified in skeletal muscle, brain and spleen cells of adult mice. Further putative full-length Pax7 transcripts, including one that contains a hexanucleotide insertion in the paired box and one in which approximately 10 bp have been deleted in the homeobox, were found to be expressed in skeletal muscle and brain of adult mice, respectively. A truncated Pax7 splice product comprising the paired box only was found to be expressed in most adult tissues except liver. Results of these studies demonstrate that there are alternate transcripts of Pax7, some of which are expressed exclusively in adult skeletal muscle and brain. It is possible that one of these transcripts may specify an alternate myogenic pathway involved in regeneration of damaged skeletal muscle in adult mice.

Variation in the methylation profile and structure of Pax3 and Pax7 among different mouse strains and during expression

Structural alterations within the myogenic and neurogenic developmental gene Pax7 which involve TaqI recognition sequences have previously been reported. These alterations are associated with differences in the efficiency of regrowth of damaged skeletal muscle. To identify other structural features of Pax genes which may influence skeletal muscle regrowth, variation in the structure and methylation status of Pax7 and the closely related gene Pax3 has been sought among different mouse strains and during gene expression using the restriction endonucleases MspI and HpaII. Following MspI digestion, RFLPs within Pax7 have been found which most likely reflect intron size variability within the paired box. Differences in the size of MspI and HpaII fragments hybridising with Pax7 and Pax3 region specific sub-probes indicate that the paired boxes are hypomethylated, whereas the region encoding the homeodomain of each gene is highly methylated in the spleen and other tissues from adult mice. In the skeletal muscle precursor cell line C 2C 12, which expresses Pax7 but not Pax3, the homeodomain encoding region of Pax7 is hypomethylated. In spleen cells, the Pax7 paired box is transcribed but the homeodomain encoding region is not. By contrast, both the paired box and the homeobox of Pax3 are hypermethylated in C 2C 12 cells indicating that generation of alternate transcripts from Pax genes may be controlled by DNA methylation. In contrast to Pax3, reference to the size of fragments hybridising with a Pax7 homeobox specific probe provides evidence for C pN pG methylation within and immediately downstream from the region encoding the homeodomain. Interestingly, C pN pG methylation remains when the Pax7 homeobox is expressed. Structural variation recognised by MspI digestion and differences in the methylation profile of Pax7 are not associated with the ability to regrow damaged skeletal muscle.

Regrowth of atrophied skeletal muscle in adult rats after ending immobilization.

The recovery time course of muscle atrophied by immobilization was followed after removal of hindlimb casts from adult female rats. Increases of only 9% in body weight, 4% in gastrocnemius weight, and 10% in soleus weight occurred in controls during the 78-day duration of the experiment. There were no increases in the amounts of total protein or of citrate synthase activities in gastrocnemius or soleus during the first 3 days after removal of hindlimb casts; thereafter, there were increases in these paramters. Citrate synthase activities per mg of gastrocnemius protein were significantly higher at the 16th and 50th day of recovery. No significant differences for citrate synthase activity per mg of soleus occurred during recovery. Until the 50th day of recovery, no significant differences for total protein in soleus and for total protein and wet weight of gastrocnemius were observed between control and recovery values. However, the wet weight of the soleus returned rapidly during recovery and was not significantly different from control during recovery.