Utility of 17-(allylamino)-17-demethoxygeldanamycin treatment for skeletal muscle injury

Adaptability to eccentric exercise training is diminished with age in female mice

The ability of skeletal muscle to adapt to eccentric contractions has been suggested to be blunted in older muscle. If eccentric exercise is to be a safe and efficient training mode for older adults, preclinical studies need to establish if older muscle can effectively adapt and if not, determine the molecular signatures that are causing this impairment. The purpose of this study was to quantify the extent age impacts functional adaptations of muscle and identify genetic signatures associated with adaptation (or lack thereof). The anterior crural muscles of young (4 mo) and older (28 mo) female mice performed repeated bouts of eccentric contractions in vivo (50 contractions/wk for 5 wk) and isometric torque was measured across the initial and final bouts. Transcriptomics was completed by RNA-sequencing 1 wk following the fifth bout to identify common and differentially regulated genes. When torques post eccentric contractions were compared after the first and fifth bouts, young muscle exhibited a robust ability to adapt, increasing isometric torque 20%-36%, whereas isometric torque of older muscle decreased up to 18% (P ≤ 0.047). Using differential gene expression, young and older muscles shared some common transcriptional changes in response to eccentric exercise training, whereas other transcripts appeared to be age dependent. That is, the ability to express particular genes after repeated bouts of eccentric contractions was not the same between ages. These molecular signatures may reveal, in part, why older muscles do not appear to be as adaptive to exercise training as young muscles.NEW & NOTEWORTHY The ability to adapt to exercise training may help prevent and combat sarcopenia. Here, we demonstrate young mouse muscles get stronger whereas older mouse muscles become weaker after repeated bouts of eccentric contractions, and that numerous genes were differentially expressed between age groups following training. These results highlight that molecular and functional plasticity is not fixed in skeletal muscle with advancing age, and the ability to handle or cope with physical stress may be impaired.

Mechanisms of weakness in Mdx muscle following in vivo eccentric contractions

Skeletal muscle of the dystrophin-deficient mdx mouse is hypersensitive to eccentric (ECC) contraction-induced strength loss due to plasmalemmal electrical dysfunction. Despite plasmalemmal inexcitability being a logical mechanism responsible for weakness, it remains unclear if processes up- and/or down-stream remain functionally intact in injured mdx muscle. The purpose of this study was to analyze additional processes necessary for excitation-contraction coupling that are potentially disrupted by ECC contractions. Anterior crural muscles (tibialis anterior, extensor digitorum longus [EDL], and extensor hallucis muscles) of wildtype (WT) and mdx mice were injured in vivo with 50 ECC contractions and torque was measured immediately before and after the contraction bout. Following the in vivo assessment, EDL ex vivo isometric and caffeine forces were analyzed. In vivo isometric torque and ex vivo force in WT muscle were reduced 38 and 30% (p < 0.001), while caffeine force was also reduced (p = 0.021), albeit to a lesser degree (9%). In contrast, in vivo isometric torque, ex vivo isometric force and ex vivo caffeine-induced force were all reduced 56-67% (p < 0.001) in mdx muscle and did not differ from one another (p = 0.114). Disproportional reductions in isometric strength and caffeine-induced force confirm that ECC contractions uncoupled the plasmalemma from the ryanodine receptors (RyRs) in WT muscle. In mdx muscle, the proportional reductions in isometric strength and caffeine-induced force following ECC contractions reveal that dysfunction occurs at and/or distal to the RyRs immediately post-injury. Thus, weakness in injured mdx muscle cannot be isolated to one mechanism, rather several steps of muscle contraction are disrupted.

Membrane Proteins Increase with the Repeated Bout Effect

PURPOSE

The ability of skeletal muscle to adapt to eccentric (ECC) contraction-induced injury is known as the repeated bout effect (RBE). Despite the RBE being a well-established phenomenon observed in skeletal muscle, cellular and molecular events particularly those at the membranes that contribute to the adaptive potential of muscle have yet to be established. Therefore, the purpose of this study was to examine how membrane-associated proteins respond to the RBE.

METHODS

Anterior crural muscles of C57BL/6 female mice (3-5 months) were subjected to repeated bouts of in vivo ECCs, with isometric torque being measured immediately before and after injury. A total of six bouts were completed with 7 d between each bout. Protein content of dystrophin, β-sarcoglycan, and junctophilin were then assessed via immunoblotting in injured and uninjured muscles.

RESULTS

When expressed relative to preinjury isometric torque of bout 1, deficits in postinjury isometric torque during bout 2 (38%) did not differ from bout 1 (36%; P = 0.646) and were attenuated during bouts 3 through 6 (range, 24%-15%; P ≤ 0.014). Contents of dystrophin, β-sarcoglycan, and junctophilin did not change immediately after a single bout of 50 maximal ECCs (P ≥ 0.155); however, as a result of repeated bouts, contents of dystrophin, β-sarcoglycan, and junctophilin all increased compared with muscles that completed one or no bouts of ECC contractions (P ≤ 0.003).

CONCLUSIONS

The RBE represents a physiological measure of skeletal muscle plasticity. Here, we demonstrate that repeated bouts of ECC contractions increase contents of dystrophin, β-sarcoglycan, and junctophilin and attenuate postinjury torque deficits. Given our results, accumulation of membrane-associated proteins likely contributes to strength adaptations observed after repeated bouts of ECC contractions.

Phytoecdysteroids Accelerate Recovery of Skeletal Muscle Function Following in vivo Eccentric Contraction-Induced Injury in Adult and Old Mice

Background: Eccentric muscle contractions are commonly used in exercise regimens, as well as in rehabilitation as a treatment against muscle atrophy and weakness. If repeated multiple times, eccentric contractions may result in skeletal muscle injury and loss of function. Skeletal muscle possesses the remarkable ability to repair and regenerate after an injury or damage; however, this ability is impaired with aging. Phytoecdysteroids are natural plant steroids that possess medicinal, pharmacological, and biological properties, with no adverse side effects in mammals. Previous research has demonstrated that administration of phytoecdysteroids, such as 20-hydroxyecdysone (20E), leads to an increase in protein synthesis signaling and skeletal muscle strength.

Methods: To investigate whether 20E enhances skeletal muscle recovery from eccentric contraction-induced damage, adult (7–8 mo) and old (26–27 mo) mice were subjected to injurious eccentric contractions (EC), followed by 20E or placebo (PLA) supplementation for 7 days. Contractile function via torque-frequency relationships (TF) was measured three times in each mouse: pre- and post-EC, as well as after the 7-day recovery period. Mice were anesthetized with isoflurane and then electrically-stimulated isometric contractions were performed to obtain in vivo muscle function of the anterior crural muscle group before injury (pre), followed by 150 EC, and then again post-injury (post). Following recovery from anesthesia, mice received either 20E (50 mg•kg⁻¹ BW) or PLA by oral gavage. Mice were gavaged daily for 6 days and on day 7, the TF relationship was reassessed (7-day).

Results: EC resulted in significant reductions of muscle function post-injury, regardless of age or treatment condition (p < 0.001). 20E supplementation completely recovered muscle function after 7 days in both adult and old mice (pre vs. 7-day; p > 0.05), while PLA muscle function remained reduced (pre vs. 7-day; p < 0.01). In addition, histological markers of muscle damage appear lower in damaged muscle from 20E-treated mice after the 7-day recovery period, compared to PLA.

Conclusions: Taken together, these findings demonstrate that 20E fully recovers skeletal muscle function in both adult and old mice just 7 days after eccentric contraction-induced damage. However, the underlying mechanics by which 20E contributes to the accelerated recovery from muscle damage warrant further investigation.

Cardiosphere-derived cells, with and without a biological scaffold, stimulate myogenesis and recovery of muscle function in mice with volumetric muscle loss

Extremity trauma to military personnel and civilians commonly results in volumetric muscle loss (VML), leaving patients suffering chronic physical disability. Biomaterial-based technologies such as extracellular matrices (ECMs) are currently in clinical testing for soft tissue repair, but, in preclinical models of VML, the efficacy of ECMs is equivocal. In a murine model of VML, we investigated the effects of ECM and/or cardiosphere-derived cell (CDC) therapy; the latter improves skeletal myogenesis and muscle function in mdx mice, so we reasoned that CDCs may exert disease-modifying bioactivity in VML. While ECM alone improves functional recovery, CDCs have no additive or synergistic benefits with ECM transplantation following VML injury. However, CDCs alone are sufficient to promote muscle recovery, leading to sustained increases in muscle function throughout the study period. Notably, CDCs stimulate satellite cell accumulation in the muscle defect area and hasten myogenic progression (as evidenced by qPCR gene expression profiling), leading to global increases in myofiber numbers and anterior muscle compartment volume. Together, these data implicate CDCs as a viable therapeutic candidate to regenerate skeletal muscle injured by VML.

Muscle Strength Does Not Adapt From a Second to Third Bout of Eccentric Contractions: A Systematic Review and Meta-Analysis of the Repeated Bout Effect

ABSTRACT

Lindsay, A, Abbott, G, Ingalls, CP, and Baumann, CW. Muscle strength does not adapt from a second to third bout of eccentric contractions: A systematic review and meta-analysis of the repeated bout effect. J Strength Cond Res 35(2): 576-584, 2021-The greatest muscle strength adaptations to repeated bouts of eccentric contractions (ECC) occur after the initial injury, with little to no change in subsequent bouts. However, because of the disparity in injury models, it is unknown whether three or more bouts provide further adaptation. Therefore, we performed a systematic review of the literature to evaluate whether a third bout of skeletal muscle ECC impacts immediate strength loss and rate of strength recovery compared with a second bout. A search of the literature in Web of Science, SCOPUS, Medline, and the American College of Sports Medicine database was conducted between May and September 2019 using the keywords eccentric contraction or lengthening contraction and muscle and repeated or multiple, and bout. Eleven studies with 12 experimental groups, using 72 human subjects, 48 mice, and 11 rabbits, met the inclusion criteria. A meta-analysis using a random effects model and effect sizes (ESs; Hedges' g) calculated from the standardized mean differences was completed. Calculated ESs for immediate strength loss provided no evidence that a third bout of ECC results in greater loss of strength compared with a second bout (ES = -0.12, 95% confidence interval [CI] = -0.41 to 0.17). Furthermore, the rate of strength recovery was not different between a second and third bout (ES = -0.15, 95% CI = -1.01 to 0.70). These results indicate a third bout of skeletal muscle ECC does not improve indices of strength loss or rate of strength recovery compared with a second bout. Therefore, coaches and athletes should expect some level of persistent weakness after each of their initial training sessions involving ECC, and the faster recovery of strength deficits in the second bout documented by previous research is not different from a third bout.

Plasmalemma Function Is Rapidly Restored in Mdx Muscle after Eccentric Contractions

PURPOSE

Muscle that lacks dystrophin, as in the mdx mouse, has a heightened sensitivity to eccentric (ECC) contraction-induced strength loss but an enhanced rate of recovery. However, the timeline and mechanisms underlying why mdx muscle recovers quicker have yet to be determined. We used an EMG approach to analyze plasmalemma electrophysiological function during and after ECC contraction-induced injury to test the hypothesis that loss of plasmalemmal excitability is a transient event in mdx muscle.

METHODS

Mice were implanted with stimulating electrodes on the common peroneal nerve and EMG electrodes on the tibialis anterior muscle. Anterior crural muscles of anesthetized mice performed one or two bouts of 50 injurious ECC contractions, and recovery of maximal isometric torque and M-wave root mean square (RMS) were assessed after each bout.

RESULTS

Maximal isometric torque and M-wave RMS were equally reduced 62% (P < 0.001) in mdx mice immediately after the initial ECC injury. For these mdx mice, M-wave RMS was still reduced at 2 d postinjury (P = 0.034) but was not different from preinjury values by 6 d (P = 0.106), whereas torque took up to 9 d to recover (P = 0.333). M-wave RMS did not change (P = 0.390) in wild-type mice in response to ECC injury, whereas torque decreased 35% (P < 0.001) and recovered by day 2 (P = 0.311). Results from the second bout of ECC contractions were similar to those observed during and after the initial injury.

CONCLUSION

Functional dystrophin is necessary for excitation to occur at the plasmalemma during ECC contractions but is not essential for the complete recovery of plasmalemma electrophysiological function or maximal isometric strength.

Disease-modifying bioactivity of intravenous cardiosphere-derived cells and exosomes in mdx mice

Dystrophin deficiency leads to progressive muscle degeneration in Duchenne muscular dystrophy (DMD) patients. No known cure exists, and standard care relies on the use of antiinflammatory steroids, which are associated with side effects that complicate long-term use. Here, we report that a single intravenous dose of clinical-stage cardiac stromal cells, called cardiosphere-derived cells (CDCs), improves the dystrophic phenotype in mdx mice. CDCs augment cardiac and skeletal muscle function, partially reverse established heart damage, and boost the regenerative capacity of skeletal muscle. We further demonstrate that CDCs work by secreting exosomes, which normalize gene expression at the transcriptome level, and alter cell signaling and biological processes in mdx hearts and skeletal muscle. The work reported here motivated the ongoing HOPE-2 clinical trial of systemic CDC delivery to DMD patients, and identifies exosomes as next-generation cell-free therapeutic candidates for DMD.

Hsp70 Interacts with Mitogen-Activated Protein Kinase (MAPK)-Activated Protein Kinase 2 To Regulate p38MAPK Stability and Myoblast Differentiation during Skeletal Muscle Regeneration

The regenerative process of injured muscle is dependent on the fusion and differentiation of myoblasts derived from muscle stem cells. Hsp70 is important for maintaining skeletal muscle homeostasis and regeneration, but the precise cellular mechanism remains elusive. In this study, we found that Hsp70 was upregulated during myoblast differentiation. Depletion or inhibition of Hsp70/Hsc70 impaired myoblast differentiation. Importantly, overexpression of p38 mitogen-activated protein kinase α (p38MAPKα) but not AKT1 rescued the impairment of myogenic differentiation in Hsp70- or Hsc70-depleted myoblasts. Moreover, Hsp70 interacted with MK2, a substrate of p38MAPK, to regulate the stability of p38MAPK. Knockdown of Hsp70 also led to downregulation of both MK2 and p38MAPK in intact muscles and during cardiotoxin-induced muscle regeneration. Hsp70 bound MK2 to regulate MK2-p38MAPK interaction in myoblasts. We subsequently identified the essential regions required for Hsp70-MK2 interaction. Functional analyses showed that MK2 is essential for both myoblast differentiation and skeletal muscle regeneration. Taken together, our findings reveal a novel role of Hsp70 in regulating myoblast differentiation by interacting with MK2 to stabilize p38MAPK.

Downhill exercise alters immunoproteasome content in mouse skeletal muscle

Content of the immunoproteasome, the inducible form of the standard proteasome, increases in atrophic muscle suggesting it may be associated with skeletal muscle remodeling. However, it remains unknown if the immunoproteasome responds to stressful situations that do not promote large perturbations in skeletal muscle proteolysis. The purpose of this study was to determine how an acute bout of muscular stress influences immunoproteasome content. To accomplish this, wild-type (WT) and immunoproteasome knockout lmp7 -/- /mecl1 -/- (L7M1) mice were run downhill on a motorized treadmill. Soleus muscles were excised 1 and 3 days post-exercise and compared to unexercised muscle (control). Ex vivo physiology, histology and biochemical analyses were used to assess the effects of immunoproteasome knockout and unaccustomed exercise. Besides L7M1 muscle being LMP7/MECL1 deficient, no other major biochemical, histological or functional differences were observed between the control muscles. In both strains, the downhill run shifted the force-frequency curve to the right and reduced twitch force; however, it did not alter tetanic force or inflammatory markers. In the days post-exercise, several of the proteasome's catalytic subunits were upregulated. Specifically, WT muscle increased LMP7 while L7M1 muscle instead increased β5. These findings indicate that running mice downhill results in subtle contractile characteristics that correspond to skeletal muscle injury, yet it does not appear to induce a significant inflammatory response. Interestingly, this minor stress activated the production of specific immunoproteasome subunits that if knocked out were replaced by components of the standard proteasome. These data suggest that the immunoproteasome may be involved in maintaining cellular homeostasis.

BGP-15 improves contractile function of regenerating soleus muscle

This study investigated the effect of the heat shock protein inducer O-[3-piperidino-2-hydroxy-1-propyl]-nicotinic amidoxime (BGP-15) on the morphology and contractile function of regenerating soleus muscles from mice. Cryolesioned soleus muscles from young mice treated daily with BGP-15 (15 mg/Kg) were evaluated on post-cryolesion day 10. At this time point, there was a significant decrease in the cross-sectional area of regenerating myofibers, maximal force, specific tetanic force, and fatigue resistance of regenerating soleus muscles. BGP-15 did not reverse the decrease in myofiber cross-sectional area but effectively prevented the reduction in tetanic force and fatigue resistance of regenerating muscles. In addition, BGP-15 treatment increased the expression of embryonic myosin heavy chain (e-MyHC), MyHC-II and MyHC-I in regenerating muscles. Although BGP-15 did not alter voltage dependent anion-selective channel 2 (VDAC2) expression in cryolesioned muscles, it was able to increase inducible 70-kDa heat shock protein (HSP70) expression. Our results suggest that BGP-15 improves strength recovery in regenerating soleus muscles by accelerating the re-expression of adult MyHC-II and MyHC-I isoforms and HSP70 induction. The beneficial effects of BGP-15 on the contractile function of regenerating muscles reinforce the potential of this molecule to be used as a therapeutic agent.

Resveratrol-Mediated Expression of KLF15 in the Ischemic Myocardium is Associated with an Improved Cardiac Phenotype

PURPOSE

Myocardial infarction results in physiological derangements that lead to structural and functional alterations to the myocardium. In addition, oxidative stress potentiates cardiac remodeling and drives disease progression. Unfortunately, treatment with antioxidants in clinical trials have failed to show any therapeutic benefits despite the positive results reported in animal studies, which warrants further investigation into their mechanism(s) of action. Accordingly, the aim of this study was to elucidate a previously unknown mechanism of action for the antioxidant, resveratrol, in the treatment of the ischemic heart.

METHODS

Male Sprague-Dawley rats underwent four weeks of chronic myocardial ischemia with or without daily resveratrol treatment (10 mg/kg/day). The expression and signaling of Krüppel-like factor 15 (KLF15) were determined by immunoblot and qPCR analyses, respectively.

RESULTS

Chronic myocardial ischemia reduced the protein expression of KLF15. In parallel, mRNA transcripts of KLF15 gene targets actively involved in cardiac remodeling were robustly increased in untreated hearts. Importantly, daily treatment with resveratrol stimulated KLF15 expression, which was associated with attenuated gene expression and an improved cardiac phenotype. Additionally, we describe a novel role for KLF15 in the regulation of redox homeostasis.

CONCLUSION

Based on our current findings, it appears that resveratrol treatment induces KLF15 expression, which may, in part, explain its therapeutic efficacy to improve the cardiac phenotype following ischemic injury.