Seven days of muscle re-loading and voluntary wheel running following hindlimb suspension in mice restores running performance, muscle morphology and metrics of fatigue but not muscle strength

Proteomic Identification of Seasonally Expressed Proteins Contributing to Heart Function and the Avoidance of Skeletal Muscle Disuse Atrophy in a Hibernating Mammal

Hibernation in the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) takes place over 4-6 months and is characterized by multiday bouts of hypothermic torpor (5-7 °C core body temperature) that are regularly interrupted every 1-2 weeks by brief (12-24 h) normothermic active periods called interbout arousals. Our goal was to gain insight into the molecular mechanisms that underlie the hibernator's ability to preserve heart function and avoid the deleterious effects of skeletal muscle disuse atrophy over prolonged periods of inactivity, starvation, and near-freezing body temperatures. To achieve this goal, we performed organelle enrichment of heart and skeletal muscle at five seasonal time points followed by LC-MS-based label-free quantitative proteomics. In both organs, we saw an increase in the levels of many proteins as ground squirrels transition from an active state to a prehibernation state in the fall. Interestingly, seasonal abundance patterns identified DHRS7C, SRL, TRIM72, RTN2, and MPZ as potential protein candidates for mitigating disuse atrophy in skeletal muscle, and ex vivo contractile mechanics analysis revealed no deleterious effects in the ground squirrel's muscles despite prolonged sedentary activity. Overall, an increased understanding of protein abundance in hibernators may enable novel therapeutic strategies to treat muscle disuse atrophy and heart disease in humans.

Treadmill training does not enhance skeletal muscle recovery following disuse atrophy in older male mice

Introduction: A hallmark of aging is poor muscle recovery following disuse atrophy. Efficacious strategies to enhance muscle recovery following disuse atrophy in aging are non-existent. Prior exercise training could result in favorable muscle morphological and cellular adaptations that may promote muscle recovery in aging. Here, we characterized the impact of exercise training on skeletal muscle inflammatory and metabolic profiles and cellular remodeling and function, together with femoral artery reactivity prior to and following recovery from disuse atrophy in aged male mice. We hypothesized that 12 weeks of treadmill training in aged male mice would improve skeletal muscle cellular remodeling at baseline and during recovery from disuse atrophy, resulting in improved muscle regrowth. Methods: Physical performance, ex vivo muscle and vascular function, tissue and organ mass, hindlimb muscle cellular remodeling (macrophage, satellite cell, capillary, myofiber size, and fibrosis), and proteolytic, inflammatory, and metabolic muscle transcripts were evaluated in aged exercise-trained and sedentary mice. Results: We found that at baseline following exercise training (vs. sedentary mice), exercise capacity and physical function increased, fat mass decreased, and endothelial function improved. However, exercise training did not alter tibialis anterior or gastrocnemius muscle transcriptional profile, macrophage, satellite cell, capillarity or collagen content, or myofiber size and only tended to increase tibialis mass during recovery from disuse atrophy. Conclusion: While exercise training in old male mice improved endothelial function, physical performance, and whole-body tissue composition as anticipated, 12 weeks of treadmill training had limited impact on skeletal muscle remodeling at baseline or in response to recovery following disuse atrophy.

Inhibiting myostatin signaling partially mitigates structural and functional adaptations to hindlimb suspension in mice

Novel treatments for muscle wasting are of significant value to patients with disease states that result in muscle weakness, injury recovery after immobilization and bed rest, and for astronauts participating in long-duration spaceflight. We utilized an anti-myostatin peptibody to evaluate how myostatin signaling contributes to muscle loss in hindlimb suspension. Male C57BL/6 mice were left non-suspended (NS) or were hindlimb suspended (HS) for 14 days and treated with a placebo vehicle (P) or anti-myostatin peptibody (D). Hindlimb suspension (HS-P) resulted in rapid and significantly decreased body mass (-5.6% by day 13) with hindlimb skeletal muscle mass losses between -11.2% and -22.5% and treatment with myostatin inhibitor (HS-D) partially attenuated these losses. Myostatin inhibition increased hindlimb strength with no effect on soleus tetanic strength. Soleus mass and fiber CSA were reduced with suspension and did not increase with myostatin inhibition. In contrast, the gastrocnemius showed histological evidence of wasting with suspension that was partially mitigated with myostatin inhibition. While expression of genes related to protein degradation (Atrogin-1 and Murf-1) in the tibialis anterior increased with suspension, these atrogenes were not significantly reduced by myostatin inhibition despite a modest activation of the Akt/mTOR pathway. Taken together, these findings suggest that myostatin is important in hindlimb suspension but also motivates the study of other factors that contribute to disuse muscle wasting. Myostatin inhibition benefitted skeletal muscle size and function, which suggests therapeutic potential for both spaceflight and terrestrial applications.

Skeletal Muscle Recovery from Disuse Atrophy: Protein Turnover Signaling and Strategies for Accelerating Muscle Regrowth

Skeletal muscle fibers have a unique capacity to adjust their metabolism and phenotype in response to alternations in mechanical loading. Indeed, chronic mechanical loading leads to an increase in skeletal muscle mass, while prolonged mechanical unloading results in a significant decrease in muscle mass (muscle atrophy). The maintenance of skeletal muscle mass is dependent on the balance between rates of muscle protein synthesis and breakdown. While molecular mechanisms regulating protein synthesis during mechanical unloading have been relatively well studied, signaling events implicated in protein turnover during skeletal muscle recovery from unloading are poorly defined. A better understanding of the molecular events that underpin muscle mass recovery following disuse-induced atrophy is of significant importance for both clinical and space medicine. This review focuses on the molecular mechanisms that may be involved in the activation of protein synthesis and subsequent restoration of muscle mass after a period of mechanical unloading. In addition, the efficiency of strategies proposed to improve muscle protein gain during recovery is also discussed.

Neutralizing mitochondrial ROS does not rescue muscle atrophy induced by hindlimb unloading in female mice

Excess reactive oxygen species (ROS) induced by physical inactivity is associated with muscle atrophy and muscle weakness. However, the role of mitochondrial ROS on disuse-induced muscle atrophy is not fully understood. The purpose of this study was to utilize a genetic strategy to examine the effect of neutralizing mitochondrial ROS on disuse-induced skeletal muscle atrophy. This was accomplished by placing wild-type (WT) and mitochondrial-targeted catalase-expressing (MCAT) littermate mice on 7 days of hindlimb unloading. After assessment of body weight and composition, muscles were analyzed for individual muscle mass, force-generating capacity, fiber type, cross-sectional area, and mitochondrial function, including H₂O₂ production. Despite a successful attenuation of mitochondrial ROS, MCAT mice were not protected from muscle atrophy. No differences were observed in body composition, lean mass, individual muscle masses, force-generating capacity, or muscle fiber cross-sectional area. These data suggest that neutralizing mitochondrial ROS is insufficient to suppress disuse-induced loss of skeletal muscle mass and contractile function.NEW & NOTEWORTHY The premise of this study was to examine the efficacy of genetic suppression of mitochondrial reactive oxygen species (ROS) to attenuate disuse-induced muscle atrophy and muscle weakness. Neutralization of mitochondrial ROS by MCAT expression was insufficient to rescue muscle atrophy and muscle weakness.

Comparative molecular analysis of endurance exercise in vivo with electrically stimulated in vitro myotube contraction

Exercise has positive effects on health and improves a variety of disease conditions. An in vitro model of exercise has been developed to better understand its molecular mechanisms. While various conditions have been used to mimic in vivo exercise, no specific conditions have matched a specific type of in vivo exercise. Here, we screened various electrical pulse stimulation (EPS) conditions and compared the molecular events under each condition in myotube culture with that obtained under voluntary wheel running (VWR), a mild endurance exercise, in mice. Both EPS and VWR upregulated the mRNA levels of genes involved in the slow-type twitch ( Myh7 and Myh2) and myogenesis ( Myod and Myog) and increased the protein expression of peroxisome proliferator-activated receptor-γ coactivator-1α, which is involved in mitochondrial biogenesis. These changes were accompanied by activation of p38 and AMPK. However, neither condition induced the expression of muscle-specific E3 ligases such as MAFbx and MuRF1. Both EPS and VWR consistently induced antioxidant genes such as Sod3 and Gpx4 but did not cause similar changes in the expression levels of the calcium channel/pump-related genes Ryr and Serca. Furthermore, both EPS and VWR reduced glycogen levels but not lactate levels as assessed in post-EPS culture medium and post-VWR serum, respectively. Thus we identified an in vitro EPS condition that effectively mimics VWR in mice, which can facilitate further studies of the detailed molecular mechanisms of endurance exercise in the absence of interference from multiple tissues and organs.

NEW & NOTEWORTHY This study establishes an optimal condition for electrical pulse stimulation (EPS) in myotubes that shows a similar molecular signature as voluntary wheel running. The specific EPS condition 1) upregulates the mRNA of slow-twitch muscle components and myogenic transcription factors, 2) induces antioxidant genes without any muscle damage, and 3) promotes peroxisome proliferator-activated receptor-γ coactivator-1α and its upstream regulators involved in mitochondrial biogenesis.

Effects of hindlimb suspension and reloading on gastrocnemius and soleus muscle mass and function in geriatric mice

Reloading of atrophied muscles after hindlimb suspension (HLS) can induce muscle injury and prolong recovery after disuse in old rats, especially in fast contracting muscles. Less is known about the responses in mice and whether fast and slow muscles from geriatric mice will respond in a similar fashion to HLS unloading and recovery (HLS + R). Furthermore, while slow muscles undergo atrophy with disuse, they typically are more resistant to sarcopenia than fast contracting muscles. Geriatric (28 mo. of age) male C57BL/6 mice were randomly placed into 3 groups. These included HLS for 14 days n = 9, and HLS followed by 14 days of reloading recovery (HLS + R; n = 9), or normal ambulatory cage controls (n = 9). Control mice were not exposed to unloading. Electrically evoked maximal muscle function was assessed in vivo in anesthetized mice at baseline, after 14 days of HLS or HLS + R. As expected, HLS significantly reduced body weight, wet weight of gastrocnemius and soleus muscles and in vivo maximal force. There were no differences in vivo fatigability of the plantar flexor muscles and overall fiber size. There were only minor fiber type distribution and frequency distribution of fiber sizes that differ between HLS + R and control gastrocnemius and soleus muscles. Soleus muscle wet weight had recovered to control levels after reloading, but type I/IIA fibers in the soleus muscles were significantly smaller after HLS + R than control muscles. In contrast, gastrocnemius muscle wet weight did not recover to control levels after reloading. Plantar flexion muscle force (primarily influenced by the gastrocnemius muscles) did not recover in HLS + R conditions as compared to HLS conditions and both were lower than control force production signaling for apoptosis, autophagy and anabolic markers were not different between control and HLS + R gastrocnemius and soleus muscles in geriatric mice. These results suggest that molecular signaling does not explain attenuated ability to regain muscle wet weight, fiber size or muscle force production after HLS in geriatric mice. It is possible that fluid shifts, reduced blood flow, or shortened muscle fibers which failed to regain control lengths contributed to the attenuation of muscle wet weight after HLS and reloading and this affected force production. Further work is needed to determine if altered/loss of neural activity contributed to the inability of geriatric mice to regain gastrocnemius muscle weight and function after HLS and reloading.

Voluntary wheel running increases satellite cell abundance and improves recovery from disuse in gastrocnemius muscles from mice

Reloading of atrophied muscles after hindlimb suspension unloading (HSU) can induce injury and prolong recovery. Low-impact exercise, such as voluntary wheel running, has been identified as a nondamaging rehabilitation therapy in rodents, but its effects on muscle function, morphology, and satellite cell activity after HSU are unclear. This study tested the hypothesis that low-impact wheel running would increase satellite cell proliferation and improve recovery of muscle structure and function after HSU in mice. Young adult male and female C57BL/6 mice ( n = 6/group) were randomly placed into five groups. These included HSU without recovery (HSU), normal ambulatory recovery for 14 days after HSU (HSU+NoWR), and voluntary wheel running recovery for 14 days after HSU (HSU+WR). Two control groups were used: nonsuspended mouse cage controls (Control) and voluntary wheel running controls (ControlWR). Satellite cell activation was evaluated by providing mice 5-bromo-2'-deoxyuridine (BrdU) in their drinking water. As expected, HSU significantly reduced in vivo maximal force, decreased in vivo fatigability, and decreased type I and IIa myosin heavy chain (MHC) abundance in plantarflexor muscles. HSU+WR mice significantly improved plantarflexor fatigue resistance, increased type I and IIa MHC abundance, increased fiber cross-sectional area, and increased the percentage of type I and IIA muscle fibers in the gastrocnemius muscle. HSU+WR mice also had a significantly greater percentage of BrdU-positive and Pax 7-positive nuclei inside muscle fibers and a greater MyoD-to-Pax 7 protein ratio compared with HSU+NoWR mice. The mechanotransduction protein Yes-associated protein (YAP) was elevated with reloading after HSU, but HSU+WR mice had lower levels of the inactive phosphorylated YAPserine127, which may have contributed to increased satellite cell activation with reloading after HSU. These results indicate that voluntary wheel running increased YAP signaling and satellite cell activity after HSU and this was associated with improved recovery. NEW & NOTEWORTHY Although satellite cell involvement in muscle remodeling has been challenged, the data in this study suggest that voluntary wheel running increased satellite cell activity and suppressed Yes-associated protein (YAP) protein relative to no wheel running and this was associated with improved muscle recovery of force, fatigue resistance, expression of type I myosin heavy chain, and greater fiber cross-sectional area after disuse.

Musculoskeletal architecture of the prey capture apparatus in salamandrid newts with multiphasic lifestyle: does anatomy change during the seasonal habitat switches?

Some newt species change seasonally between an aquatic and a terrestrial life as adults, and are therefore repeatedly faced with different physical circumstances that affect a wide range of functions of the organism. For example, it has been observed that seasonally habitat-changing newts display notable changes in skin texture and tail fin anatomy, allowing one to distinguish an aquatic and a terrestrial morphotype. One of the main functional challenges is the switch between efficient aquatic and terrestrial prey capture modes. Recent studies have shown that newts adapt quickly by showing a high degree of behavioral flexibility, using suction feeding in their aquatic stage and tongue prehension in their terrestrial stage. As suction feeding and tongue prehension place different functional demands on the prey capture apparatus, this behavioral flexibility may clearly benefit from an associated morphological plasticity. In this study, we provide a detailed morphological analysis of the musculoskeletal system of the prey capture apparatus in the two multiphasic newt species Ichthyosaura alpestris and Lissotriton vulgaris by using histological sections and micro-computed tomography. We then test for quantitative changes of the hyobranchial musculoskeletal system between aquatic and terrestrial morphotypes, The descriptive morphology of the cranio-cervical musculoskeletal system provides new insights on form and function of the prey capture apparatus in newts, and the quantitative approach shows hypertrophy of the hyolingual musculoskeletal system in the terrestrial morphotype of L. vulgaris but hypertrophy in the aquatic morphotype of I. alpestris. It was therefore concluded that the seasonal habitat shifts are accompanied by a species-dependent muscular plasticity of which the potential effect on multiphasic feeding performance in newts remains unclear.

The hitchhiker's guide to PGC-1α isoform structure and biological functions

Proteins of the peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1 (PGC-1) family of transcriptional coactivators coordinate physiological adaptations in many tissues, usually in response to demands for higher nutrient and energy supply. Of the founding members of the family, PGC-1α (also known as PPARGC1A) is the most highly regulated gene, using multiple promoters and alternative splicing to produce a growing number of coactivator variants. PGC-1α promoters are selectively active in distinct tissues in response to specific stimuli. To date, more than ten novel PGC-1α isoforms have been reported to be expressed from a novel promoter (PGC-1α-b, PGC-1α-c), to undergo alternative splicing (NT-PGC-1α) or both (PGC-1α2, PGC-1α3, PGC-1α4). The resulting proteins display differential regulation and tissue distribution and, most importantly, exert specific biological functions. In this review we discuss the structural and functional characteristics of the novel PGC-1α isoforms, aiming to provide an integrative view of this constantly expanding system of transcriptional coactivators.

Cross-sectional area of the supraspinatus muscle after rotator cuff repair: an anatomic measure of outcome

BACKGROUND

The change in the cross-sectional area of a repaired muscle, measured with use of magnetic resonance imaging (MRI), could be an indicator of recovery of muscle function. The aims of this study were to evaluate the change in the area of the supraspinatus muscle between the immediately postoperative and one-year postoperative MRIs and to identify factors associated with the change.

METHODS

Eighty-eight patients with a full-thickness rotator cuff tear were included. MRI was performed three days and one year after surgery. Patients were classified into two groups according to whether the area of the supraspinatus increased or decreased between these two time points. Outcomes including pain, shoulder motion, strength, and commonly used clinical scores were assessed preoperatively and at three, six, and twelve months after surgery. Changes in the rotator cuff muscles and retear of the repaired tendon were also evaluated.

RESULTS

The area of the supraspinatus muscle increased in twenty-nine (33%) of the patients and decreased in fifty-nine (67%). The change in area was 36.75 ± 27.94 mm² in the group in which it increased and -94.25 ± 70.38 mm² in the group in which it decreased (p < 0.001). Multiple regression analysis indicated that a lower preoperative Simple Shoulder Test (SST) score, better gross visual grade of the tendon at surgery, and greater strength of the supraspinatus at six months postoperatively were associated with an increase in the area. No retear or Sugaya grade of 3 was found in any patient in whom the area increased, whereas 34% of the patients in whom the area decreased had a retear (p < 0.001).

CONCLUSIONS

This study showed that the cross-sectional area of the supraspinatus muscle could either increase or decrease during the first year after rotator cuff repair and that robust healing (indicated by a Sugaya grade of 1 or 2) and good tendon quality at surgery were important factors associated with an increase in the area.

LEVEL OF EVIDENCE

Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.

A PGC-1α isoform induced by resistance training regulates skeletal muscle hypertrophy

PGC-1α is a transcriptional coactivator induced by exercise that gives muscle many of the best known adaptations to endurance-type exercise but has no effects on muscle strength or hypertrophy. We have identified a form of PGC-1α (PGC-1α4) that results from alternative promoter usage and splicing of the primary transcript. PGC-1α4 is highly expressed in exercised muscle but does not regulate most known PGC-1α targets such as the mitochondrial OXPHOS genes. Rather, it specifically induces IGF1 and represses myostatin, and expression of PGC-1α4 in vitro and in vivo induces robust skeletal muscle hypertrophy. Importantly, mice with skeletal muscle-specific transgenic expression of PGC-1α4 show increased muscle mass and strength and dramatic resistance to the muscle wasting of cancer cachexia. Expression of PGC-1α4 is preferentially induced in mouse and human muscle during resistance exercise. These studies identify a PGC-1α protein that regulates and coordinates factors involved in skeletal muscle hypertrophy.

β-Hydroxy-β-methylbutyrate reduces myonuclear apoptosis during recovery from hind limb suspension-induced muscle fiber atrophy in aged rats

β-Hydroxy-β-methylbutyrate (HMB) is a leucine metabolite shown to reduce protein catabolism in disease states and promote skeletal muscle hypertrophy in response to loading exercise. In this study, we evaluated the efficacy of HMB to reduce muscle wasting and promote muscle recovery following disuse in aged animals. Fisher 344×Brown Norway rats, 34 mo of age, were randomly assigned to receive either Ca-HMB (340 mg/kg body wt) or the water vehicle by gavage (n = 32/group). The animals received either 14 days of hindlimb suspension (HS, n = 8/diet group) or 14 days of unloading followed by 14 days of reloading (R; n = 8/diet group). Nonsuspended control animals were compared with suspended animals after 14 days of HS (n = 8) or after R (n = 8). HMB treatment prevented the decline in maximal in vivo isometric force output after 2 wk of recovery from hindlimb unloading. The HMB-treated animals had significantly greater plantaris and soleus fiber cross-sectional area compared with the vehicle-treated animals. HMB decreased the amount of TUNEL-positive nuclei in reloaded plantaris muscles (5.1% vs. 1.6%, P < 0.05) and soleus muscles (3.9% vs. 1.8%, P < 0.05). Although HMB did not significantly alter Bcl-2 protein abundance compared with vehicle treatment, HMB decreased Bax protein abundance following R, by 40% and 14% (P < 0.05) in plantaris and soleus muscles, respectively. Cleaved caspase-3 was reduced by 12% and 9% (P < 0.05) in HMB-treated reloaded plantaris and soleus muscles, compared with vehicle-treated animals. HMB reduced cleaved caspase-9 by 14% and 30% (P < 0.05) in reloaded plantaris and soleus muscles, respectively, compared with vehicle-treated animals. Although, HMB was unable to prevent unloading-induced atrophy, it attenuated the decrease in fiber area in fast and slow muscles after HS and R. HMB's ability to protect against muscle loss may be due in part to putative inhibition of myonuclear apoptosis via regulation of mitochondrial-associated caspase signaling.

Matrix metalloproteinase-9 deficiency results in decreased fiber cross-sectional area and alters fiber type distribution in mouse hindlimb skeletal muscle

Matrix metalloproteinases (MMPs) play a major role in the degradation of the extracellular matrix (ECM) of skeletal muscle, and the inducible gelatinase MMP-9 in particular appears to be critical for the remodeling of muscle ECM during growth and repair. Here we determined the effects of MMP-9 gene inactivation on fiber type and size in the tibialis anterior (TA), gastrocnemius (GAST), and soleus (SOL) muscles in female mice. In the TA, the cross-sectional area (CSA) of the myosin heavy chain (MyHC) IIb-expressing fibers was significantly smaller in MMP-9 null mice while in the GAST, CSA of all three fast fiber types was decreased. In the SOL, MyHC type I-expressing fibers were significantly smaller in the MMP-9 null mice. The percentage of MyHC type IIb-expressing fibers was significantly increased in the TA and GAST of MMP-9 null mice, while the percentage of MyHC IId-expressing fibers significantly decreased in the GAST of MMP-9 null mice. Fiber percentages in the SOL were not significantly different between the two lines. Despite these changes in fiber size and type, in vivo hindlimb force production was not changed in MMP-9 null mice. Meanwhile, neither expression of the constitutive gelatinase MMP-2 nor immunohistochemical staining for type IV collagen was significantly altered by MMP-9 inactivation in any muscles examined. The present study demonstrates that MMP-9 inactivation results in changes in fiber size and type in adult mouse hindlimb muscles that may depend on indirect mechanisms involving reduced bone growth or nerve changes in response to MMP-9 inactivation.