Muscle Contractile Function Research Articles

Phosphoproteomics of three exercise modalities identifies canonical signaling and C18ORF25 as an AMPK substrate regulating skeletal muscle function

Exercise induces signaling networks to improve muscle function and confer health benefits. To identify divergent and common signaling networks during and after different exercise modalities, we performed a phosphoproteomic analysis of human skeletal muscle from a cross-over intervention of endurance, sprint, and resistance exercise. This identified 5,486 phosphosites regulated during or after at least one type of exercise modality and only 420 core phosphosites common to all exercise. One of these core phosphosites was S67 on the uncharacterized protein C18ORF25, which we validated as an AMPK substrate. Mice lacking C18ORF25 have reduced skeletal muscle fiber size, exercise capacity, and muscle contractile function, and this was associated with reduced phosphorylation of contractile and Ca2+ handling proteins. Expression of C18ORF25 S66/67D phospho-mimetic reversed the decreased muscle force production. This work defines the divergent and canonical exercise phosphoproteome across different modalities and identifies C18ORF25 as a regulator of exercise signaling and muscle function.

Патогенетическое обоснование и опыт применения серотонина адипината в комплексной терапии функциональной кишечной непроходимости в хирургической практике

Despite considerable achievements in surgical treatment methods and management of patients during the postoperative period, functional bowel obstruction (FBO) is a common complication of abdominal surgery. According to some authors, the prevalence of this complication may be as high as 25% of all postoperative complications. Currently, there is no generally accepted unified scheme for treatment of postoperative FBO. In this regard, the use of pathogenetically substantiated methods with proven efficacy and safety of use in clinical practice for treatment of this condition is relevant in terms of scientific and practical significance. It is believed that the gastrointestinal smooth muscle contractile function impairment due to impaired interaction between serotonin and serotonin receptors plays an important role in FBO. This is a concept of serotonin deficiency. According to the concept, smooth muscle dysfunction in the gastrointestinal tract and blood vessels that occurs in patients during the postoperative period is associated with absolute or relative serotonin deficiency. This review reports pathogenetic substantiation and experience of using serotonin adipate in the complex therapy of FBO in surgical patients during the postoperative period based on the assessment of available information. The analysis of clinical trials and observations pesented in the review shows that the use of serotonin adipate starting from the first hour of postoperative period can significantly improve the outcome and quality of treatment provided to patients with postoperative FBO, since it is a pathogenetically substantiated component of complex therapy of this condition.

Etiology and Recovery of Neuromuscular Function Following Academy Soccer Training.

Aim: To profile the etiology and recovery time-course of neuromuscular function in response to a mixed-content, standard training week in professional academy soccer players. We concurrently examined physical performance, cognitive function, and perceptual measures of mood and wellness states to identify a range of simple tests applied practitioners could use in the field as surrogate measures of neuromuscular function. Methods: Sixteen professional academy soccer players completed a range of neuromuscular, physical, perceptual, mood, and cognitive function tests at baseline and after a strenuous training day (pitch and gym), with retest at 24, 48, and 72 h, and further pitch and gym sessions after 48 h post-baseline. Maximal voluntary contraction force (MVC) and twitch responses to electrical stimulation (femoral nerve) during isometric knee-extensor contractions and at rest were measured to assess central nervous system (voluntary activation, VA) and muscle contractile (potentiated twitch force, Qtw,pot) function. Results: Strenuous training elicited decrements in MVC force post-session (−11%, p = 0.001) that remained unresolved at 72 h (−6%, p = 0.03). Voluntary activation (motor nerve stimulation) was reduced immediately post-training only (−4%, p = 0.03). No change in muscle contractile function (Qtw,pot) was observed post-training, though was reduced at 24 h (−13%, p = 0.01), and had not fully recovered 72 h after (−9%, p = 0.03). Perceptions of wellness were impaired post-training, and recovered by 24 h (sleepiness, energy) and 48 h (fatigue, muscle soreness, readiness to train). Countermovement jump performance declined at 24 h, while RSI (Reactive Strength Index) decrements persisted at 48 h. No changes were evident in adductor squeeze, mood, or cognitive function. Conclusion: Elite youth soccer training elicits substantial decrements in neuromuscular function, which are still present 72 h post-strenuous exercise. Though central processes contribute to post-exercise neuromuscular alterations, the magnitude and prolonged presence of impairments in contractile function indicates it is the restitution of muscular function (peripheral mechanisms) that explains recovery from strenuous training in academy soccer players.

Skeletal Muscle Contractile Function in Heart Failure With Reduced Ejection Fraction-A Focus on Nitric Oxide.

Despite advances over the past few decades, heart failure with reduced ejection fraction (HFrEF) remains not only a mortal but a disabling disease. Indeed, the New York Heart Association classification of HFrEF severity is based on how much exercise a patient can perform. Moreover, exercise capacity—both aerobic exercise performance and muscle power—are intimately linked with survival in patients with HFrEF. This review will highlight the pathologic changes in skeletal muscle in HFrEF that are related to impaired exercise performance. Next, it will discuss the key role that impaired nitric oxide (NO) bioavailability plays in HFrEF skeletal muscle pathology. Lastly, it will discuss intriguing new data suggesting that the inorganic nitrate ‘enterosalivary pathway’ may be leveraged to increase NO bioavailability via ingestion of inorganic nitrate. This ingestion of inorganic nitrate has several advantages over organic nitrate (e.g., nitroglycerin) and the endogenous nitric oxide synthase pathway. Moreover, inorganic nitrate has been shown to improve exercise performance: both muscle power and aerobic capacity, in some recent small but well-controlled, cross-over studies in patients with HFrEF. Given the critical importance of better exercise performance for the amelioration of disability as well as its links with improved outcomes in patients with HFrEF, further studies of inorganic nitrate as a potential novel treatment is critical.

Gestational Intermittent Hypoxia Induces Mitochondrial Impairment in the Geniohyoid Muscle of Offspring Rats.

IntroductionGestational intermittent hypoxia (IH), a hallmark of obstructive sleep apnea during gestation, alters respiratory neural control and diaphragm muscle contractile function in the offspring. The geniohyoid (GH) muscle is innervated by the respiratory-related hypoglossal nerve and plays a role in tongue traction and suckling, motor behaviors that then give way to chewing. Here, we aimed to investigate the effects of gestational exposure to IH on the muscle development and metabolism of GH and masseter muscles in male offspring rats.Materials and methodsPregnant Sprague-Dawley rats were exposed to IH (3-min periods of 4-21% O2) for eight hours/day during gestational days 7-20. The GH and masseter muscles from 35-day-old male offspring (n = 6 in each group) were analyzed. ResultsGestational IH induction reduced type IIA fiber size in the GH muscle of the offspring but not in the masseter muscle. Western blot analysis showed that gestational IH-induced significant downregulation of peroxisome proliferator-activated receptor (PPAR)-gamma coactivator 1-alpha (PGC1α) protein in the GH muscle but not in the masseter muscle. Moreover, optic atrophy 1 and mitofusin-2 proteins were decreased and mitochondrial fission 1 protein levels were increased in the GH muscle of the offspring exposed to gestational IH. Mitochondrial adenosine triphosphate (ATP) synthase subunit alpha and transcriptional factor A (TFAM) were decreased in the GH muscle post-gestational IH.ConclusionThese findings suggest that gestational IH-induced impaired mitochondrial metabolism and alteration of oxidative myofibers of the GH muscle in the pre-adolescent offspring, but not the masseter muscle, owing to the susceptibility of GH muscular mitochondria to gestational IH.

Exploring the Combined Effect of High‐Fat High‐Sugar Diets and Volumetric Muscle Injury on Skeletal Muscle Metabolic and Contractile Function in Mice

INTRODUCTIONVolumetric muscle loss (VML) injuries due to trauma or surgery cannot fully recover muscle mass and function, resulting in long‐term disability. Western diets, i.e., high‐fat, high‐sugar (HFHS) diets, promote metabolic stress and are associated with impaired skeletal muscle contractile function. The objective of this study was to investigate the extent to which a HFHS diet affected the pathophysiology following a VML injury as diet and disability may influence the effectiveness of rehabilitation. To determine the combined effects of HFHS diet and VML injury on skeletal muscle, metabolic and muscle contractile function was assessed. We hypothesized that a HFHS diet would worsen skeletal muscle metabolic and contractile function after VML injury.METHODSMale C57BL/6 mice (N=23) were randomly divided into four groups: Normal Chow (NC), High‐Fat High‐Sugar (HFHS), Volumetric Muscle Loss (VML), Volumetric Muscle Loss plus High‐Fat High‐Sugar (VML+HFHS). At age 12 weeks, VML animals underwent unilateral VML surgery to the ankle plantarflexors (gastrocnemius, plantaris, soleus muscles). HFHS groups received 60% high‐fat diet and 11% high‐fructose corn syrup for 8 weeks ad libitum. Oxygen respiration and electron conductance of mitochondria within permeabilized gastrocnemius muscle fibers and peak‐isometric torque of the ankle plantarflexors were measured to assess skeletal muscle metabolic and contractile function, respectively. Data were analyzed using a two‐way ANOVA to detect significant interactions between diet and injury, or in the absence of a significant interaction, the main effects of diet or injury.RESULTSIndependent of injury, mice on HFHS diet had 51% greater body mass compared to mice on NC (p<0.001). Additionally, mice on HFHS diet had less endurance capacity on a treadmill test compared to NC, independent of injury (77% less distance covered, p<0.001). Peak‐isometric torque was 6% less in HFHS mice, independent of injury (p=0.011) and peak‐isometric torque was 20% less in VML‐injured mice, independent of diet (p<0.01). There was a significant interaction between diet and injury for permeabilized muscle fiber oxygen respiration (p=0.016) where NC mice had greater oxygen respiration compared to HFHS, VML, and VML+HFMS mice. Electron conductance was 32% less in mice on HFHS diet, independent of injury (p<0.001) and 20% less in VML‐injured mice, independent of diet (p<0.001).CONCLUSIONThe current study indicates that both HFHS diet and VML injury have detrimental consequences on skeletal muscle contractile and metabolic function. Ongoing and future research is needed to determine the extent to which HFHS diets influence skeletal muscle plasticity and if diet impacts the effectiveness of rehabilitation strategies for VML‐injured skeletal muscle.

Impact of Heat Therapy on Skeletal Muscle Mass and Function in A Mouse Model of Duchenne Muscular Dystrophy

BackgroundDuchenne muscular dystrophy (DMD) is a lethal X‐linked degenerative disease that is characterized by progressive skeletal muscle wasting and weakness. Few non‐invasive and widely available therapies currently exist to combat the skeletal muscle abnormalities in patients with DMD. Repeated exposure to heat therapy (HT) has been shown to attenuate skeletal muscle atrophy during immobilization and unloading, rescue denervation‐induced atrophy, and increase relative muscle mass and strength in models of ischemia‐induced muscle damage. The goal of the present study was to test the hypothesis that repeated exposure to HT would enhance skeletal muscle mass and contractile function in D2‐mdx mice, a model that recapitulates several of the human characteristics of DMD.MethodsMale D2‐mdx and wild‐type DBA/2J mice (8 weeks of age) were obtained from the Jackson Laboratory. In study 1, we sought to define the optimal temperature for HT application using a heat incubator. Male DBA/2J mice (n=24) were randomly allocated to a control group or one of three different HT regimens (37, 39, 41℃). The treatment regimen consisted of exposure to HT for 30 min/day, 5 days/week during 3 consecutive weeks. Mice assigned to the control group underwent a similar intervention, but the incubator was maintained at room temperature. Based upon the findings of the first experiment, the goal of study 2 was to examine the effects of daily HT at 39℃ for 3 weeks on skeletal muscle mass and function in D2.mdx mice. Twenty‐four D2.mdx mice were randomized to either a HT group (n=12) or a control group (n=12) and were treated as described in study 1. Approximately 48 hrs after the last treatment session, the soleus and extensor digitorum longus (EDL) muscles were isolated and harvested for the measurement of wet mass and the assessment of contractile function.ResultsIn study 1, DBA/2J mice exposed to HT at 39ºC, but not 37º or 41ºC, displayed an increase in relative muscle mass (the ratio of muscle mass to body weight) of both EDL (p=0.03) and soleus (p=0.02) muscles when compared to the control group. Moreover, the EDL maximal specific force was greater in animals treated with HT at 39ºC relative to the 41℃ group (p=0.01). There were no significant differences between groups in soleus muscle force development. In study 2, D2.mdx mice exposed HT at 39ºC tended to display greater relative EDL muscle mass when compared to the control group (p=0.06). However, the relative mass of the soleus muscle was similar between the group treated with HT and the control regimen (p=0.84). The maximal force of both EDL and soleus muscles was also comparable between treated animals and the control group.ConclusionsOur findings indicate that exposure to HT at 39℃ for 3 weeks has limited effects on skeletal muscle mass and strength in a mouse model of DMD. Additional studies are warranted to determine whether different treatment regimens (e.g. higher temperature or longer HT sessions) are necessary to elicit beneficial changes in skeletal muscle function in DMD.

Effects of Vitamin D Supplementation on Overload‐Induced Muscle Growth and Function in Mice

Muscle hypertrophy induced by functional overload (FO) provides an in vivo model to study muscle growth. Research suggests vitamin D supplementation may stimulate muscle growth and may be associated with increased growth factor levels, such as insulin‐like growth factor‐1 (IGF‐1), fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF). However, it is unclear if vitamin D impacts both muscle hypertrophy and contractile function. We hypothesized that vitamin D supplementation would positively impact the muscle response to a growth stimulus as evidenced by greater hypertrophy, increased IGF‐1, FGF, and VEGF levels, and improved contractile function compared to placebo. Adult mice underwent functional overload (FO) of the plantaris or sham surgery. In vivo plantaris force and fatigue resistance (% of maximal force after 10 contractions) were measured 14d post‐FO or sham in mice receiving daily vitamin D or placebo (n= 8‐10/group). IGF‐1, FGF, and VEGF were measured in the plantaris by ELISA. FO increased plantaris mass, independent of treatment (placebo: 22.8±1.7 vs. 13.4±1.3 and vitamin D: 20.3±3.3 vs. 13.2±0.7 mg, for FO and sham, respectively, p<0.05). Maximal force relative to body mass was unchanged with FO or vitamin D. Significant main effects of FO and vitamin D were found when maximal force was normalized to plantaris mass. With placebo, FO was associated with a significant decrease in force normalized to plantaris mass while vitamin D attenuated this decrease (placebo: 401±48 vs. 871±157 and vitamin D: 679±125 vs. 1062±120 g/g muscle, for FO and sham, respectively, p<0.05). Fatigue resistance was increased with FO, independent of treatment (placebo: 54±3% vs. 40±4% and vitamin D: 63±5% vs. 33±6%, for FO and sham, respectively, p<0.05). IGF‐1 levels were increased in the overloaded plantaris, independent of treatment (placebo: 842±284 vs. 99±9 and vitamin D: 576±138 vs. 115±10 mg/mg, for FO and sham, respectively, p<0.05), but were unchanged in the unloaded tibialis anterior. FGF levels were increased in the overloaded plantaris, independent of treatment (placebo: 378±56 vs. 105±37 and vitamin D: 414±58 vs. 153±82 mg/mg, for FO and sham, respectively, p<0.05). VEGF levels were unchanged with FO and/or Vitamin D. Vitamin D treatment in combination with FO did not significantly improve the ability of skeletal muscle to hypertrophy or increase growth factor expression in response to a growth stimulus. However, vitamin D supplementation may positively impact muscle strength during periods of growth.

IL-6 Deficiency Attenuates Skeletal Muscle Atrophy by Inhibiting Mitochondrial ROS Production through the Upregulation of PGC-1α in Septic Mice.

Current evidences indicate that both inflammation and oxidative stress contribute to the pathogenesis of sepsis-associated skeletal muscle atrophy. However, the interaction between inflammation and oxidative stress has not been completely understood in sepsis-associated skeletal muscle atrophy. Here in the present study, a murine model of sepsis has been established by cecal ligation and puncture (CLP) with wild-type and interleukin- (IL-) 6 knockout (KO) mice. Our results suggested that IL-6 KO largely attenuated skeletal muscle atrophy as reflected by reduced protein degradation, increased cross-sectional area (CSA) of myofibers, and improved muscle contractile function (all P < 0.05). In addition, we observed that IL-6 KO promoted the expression of peroxisome proliferator-activated receptor γ coactivator–1alpha (PGC–1α) and inhibited CLP-induced mitochondrial reactive oxygen species (ROS) production in skeletal muscles (all P < 0.05). However, the knockdown of PGC–1α abolished the protective effects of IL-6 KO in CLP-induced skeletal muscle atrophy and reversed the changes in mitochondrial ROS production (all P < 0.05). Ex vivo experiments found that exogenous IL-6 inhibited PGC–1α expression, promoted mitochondrial ROS production, and induced proteolysis in C2C12 cells (all P < 0.05). Together, these results suggested that IL-6 deficiency attenuated skeletal muscle atrophy by inhibiting mitochondrial ROS production through the upregulation of PGC–1α expression in septic mice.

Endurance exercise attenuates juvenile irradiation-induced skeletal muscle functional decline and mitochondrial stress

BackgroundRadiotherapy is commonly used to treat childhood cancers and can have adverse effects on muscle function, but the underlying mechanisms have yet to be fully elucidated. We hypothesized that endurance exercise following radiation treatment would improve skeletal muscle function.MethodsWe utilized the Small Animal Radiation Research Platform (SARRP) to irradiate juvenile male mice with a clinically relevant fractionated dose of 3× (every other day over 5 days) 8.2 Gy X-ray irradiation locally from the knee to footpad region of the right hindlimb. Mice were then singly housed for 1 month in cages equipped with either locked or free-spinning voluntary running wheels. Ex vivo muscle contractile function, RT-qPCR analyses, resting cytosolic and sarcoplasmic reticulum (SR) store Ca2+ levels, mitochondrial reactive oxygen species levels (MitoSOX), and immunohistochemical and biochemical analyses of muscle samples were conducted to assess the muscle pathology and the relative therapeutic impact of voluntary wheel running (VWR).ResultsIrradiation reduced fast-twitch extensor digitorum longus (EDL) muscle-specific force by 27% compared to that of non-irradiated mice, while VWR post-irradiation improved muscle-specific force by 37%. Radiation treatment similarly reduced slow-twitch soleus muscle-specific force by 14% compared to that of non-irradiated mice, while VWR post-irradiation improved specific force by 18%. We assessed intracellular Ca2+ regulation, oxidative stress, and mitochondrial homeostasis as potential mechanisms of radiation-induced pathology and exercise-mediated rescue. We found a significant reduction in resting cytosolic Ca2+ concentration following irradiation in sedentary mice. Intriguingly, however, SR Ca2+ store content was increased in myofibers from irradiated mice post-VWR compared to mice that remained sedentary. We observed a 73% elevation in the overall protein oxidization in muscle post-irradiation, while VWR reduced protein nitrosylation by 35% and mitochondrial reactive oxygen species (ROS) production by 50%. Finally, we found that VWR significantly increased the expression of PGC1α at both the transcript and protein levels, consistent with an exercise-dependent increase in mitochondrial biogenesis.ConclusionsJuvenile irradiation stunted muscle development, disrupted proper Ca2+ handling, damaged mitochondria, and increased oxidative and nitrosative stress, paralleling significant deficits in muscle force production. Exercise mitigated aberrant Ca2+ handling, mitochondrial homeostasis, and increased oxidative and nitrosative stress in a manner that correlated with improved skeletal muscle function after radiation.

Extreme Variations in Muscle Fiber Composition Enable Detection of Insulin Resistance and Excessive Insulin Secretion.

Muscle fiber composition is associated with peripheral insulin action. We investigated whether extreme differences in muscle fiber composition are associated with alterations in peripheral insulin action and secretion in young, healthy subjects who exhibit normal fasting glycemia and insulinemia. Relaxation time following a tetanic contraction was used to identify subjects with a high or low expression of type I muscle fibers: group 1 (n = 11), area occupied by type I muscle fibers = 61.0 ± 11.8%, and group 2 (n = 8), type I area = 36.0 ± 4.9% (P < 0.001). Biopsies were obtained from the vastus lateralis muscle and analyzed for mitochondrial respiration on permeabilized fibers, muscle fiber composition, and capillary density. An intravenous glucose tolerance test was performed and indices of glucose tolerance, insulin sensitivity, and secretion were determined. Glucose tolerance was similar between groups, whereas whole-body insulin sensitivity was decreased by ~50% in group 2 vs group 1 (P = 0.019). First-phase insulin release (area under the insulin curve during 10 minutes after glucose infusion) was increased by almost 4-fold in group 2 vs group 1 (P = 0.01). Whole-body insulin sensitivity was correlated with percentage area occupied by type I fibers (r = 0.54; P = 0.018) and capillary density in muscle (r = 0.61; P = 0.005) but not with mitochondrial respiration. Insulin release was strongly related to percentage area occupied by type II fibers (r = 0.93; P < 0.001). Assessment of muscle contractile function in young healthy subjects may prove useful in identifying individuals with insulin resistance and enhanced glucose-stimulated insulin secretion prior to onset of clinical manifestations.

Incretin-induced changes in the transcriptome of skeletal muscles of fa/fa Zucker rat (ZFR) with obesity, without diabetes

Glucagon-like peptide-1 receptor agonists (GLP-1ra) are increasingly used in treating type 2 diabetes and obesity. Exendin-4 (Ex-4), a long acting GLP-1ra, was previously reported to decrease oxidative stress in hepatocytes, adipocytes and skeletal muscle cells in obese nondiabetic fa/fa Zucker rats (ZFR), thereby improving insulin resistance. We aimed first to identify Ex-4-induced changes in the transcriptome of skeletal muscle cells in ZFR. Ontology analysis of differentially expressed genes (DEGs) in ZFR versus lean animals (LR) showed that the extracellular matrix (ECM) is the first most affected cellular compartment, followed by myofibrils and endoplasmic reticulum (ER). Interestingly, among 15 genes regulated in ZFR versus LR, 14 of them were inversely regulated by Ex-4, as further confirmed by RT-qPCR. Picro-Sirius red histological staining showed that decreased ECM fiber area in ZFR is partially restored by Ex-4. Ontology analysis of the myofibril compartment revealed that decreased muscle contractile function in ZFR is partially restored by Ex-4, as confirmed by Phalloidin histological staining that showed a partial restoration by Ex-4 of altered contractile apparatus in ZFR. Ontology analysis of ER DEGs in ZFR versus LR showed that some of them are related to the AMP-activated protein kinase (AMPK) signaling pathway. Phosphorylated AMPK levels were strongly increased in Ex-4-treated ZFR. Altogether, our results suggest that GLP-1ra strongly restructure ECM and reinforce contractile capabilities in ZFR, while optimizing the cellular metabolism through AMPK.

Impact of Cell Seeding Density and Cell Confluence on Human Tissue Engineered Skeletal Muscle.

Tissue engineering methodologies have the potential to treat volumetric muscle loss via the growth of exogenous skeletal muscle grafts from small autogenous muscle biopsies. A significant obstacle preventing the widespread use of engineered skeletal muscle grafts in a clinical setting is the high number of skeletal muscle stem cells, known as satellite cells, required for fabrication of human-sized skeletal muscle tissue. Additionally, there is a lack of work adapting engineered constructs created for animal models into skeletal muscle engineered from a primary human skeletal muscle cell source. For this study, we used scaffold-free tissue-engineered skeletal muscle units (SMUs) to determine the impact of cell seeding density on the ability to fabricate functional human engineered skeletal muscle. Following established protocols, human skeletal muscle isolates were cultured into SMUs at five different cell seeding densities: 1000, 2500, 5000, 10,000, and 25,000 cells/cm2. Following previous human SMU work, SMUs prepared at a cell seeding density of 10,000 cells/cm2 served as controls. Additionally, the impact of cell monolayer confluency on the outcome of human cell-sourced SMU fabrication was investigated at both the 1000 and 10,000 cells/cm2 seeding densities. Light microscopy was used to examine myotube formation and hypertrophy in cell monolayers. After the formation of three-dimensional constructs, SMUs underwent maximum tetanic isometric force production measurements and immunohistochemical staining to examine SMU contractile function and muscle-like structure, respectively. Results indicate that the 25,000 cells/cm2 cell seeding density was detrimental to the contractile function of human cell-sourced SMUs, which had significantly lower maximum tetanic forces compared with SMUs seeded at lower densities. Compared with control, low cell seeding densities (1000-5000 cells/cm2) have no detrimental impact on SMU skeletal muscle growth, maturation, or contractility. Cell cultures seeded at 1000 cells/cm2 and allowed to proliferate to 90-100% confluency before treatment in muscle differentiation media (MDM) resulted in SMUs with greater contractile forces and total muscle structure compared with cell cultures switched to MDM when underconfluent or overconfluent. In conclusion, initial cell seeding density for SMU fabrication can be decreased to as low as 1000 cells/cm2 without negatively impacting SMU muscle-like structure and function. Impact Statement Our research suggests that during the translation of skeletal muscle tissue engineering technologies from animal to human cell sources, initial starting cell seeding density can be significantly lowered without negatively impacting engineered skeletal muscle growth, maturation, or contractile function. Decreasing the initial cell density, and, thus, the muscle biopsy size required to fabricate an engineered human skeletal muscle, increases the potential for the clinical adoption of tissue-engineered based therapies for volumetric muscle loss.

Investigating the active contractile function of the rat paraspinal muscles reveals unique cross-bridge kinetics in the multifidus

Various aspects of paraspinal muscle anatomy, biology, and histology have been studied; however, information on paraspinal muscle contractile function is almost nonexistent, thus hindering functional interpretation of these muscles in healthy individuals and those with low back disorders. The aim of this study was to measure and compare the contractile function and force-sarcomere length properties of muscle fibers from the multifidus (MULT) and erector spinae (ES) as well as a commonly studied lower limb muscle (Extensor digitorum longus (EDL)) in the rat. Single muscle fibers (n = 77 total from 6 animals) were isolated from each of the muscles and tested to determine their active contractile function; all fibers used in the analyses were type IIB. There were no significant differences between muscles for specific force (sFo) (p = 0.11), active modulus (p = 0.63), average optimal sarcomere length (p = 0.27) or unloaded shortening velocity (Vo) (p = 0.69). However, there was a significant difference in the rate of force redevelopment (ktr) between muscles (p = < 0.0001), with MULT being significantly faster than both the EDL (p = < 0.0001) and ES (p = 0.0001) and no difference between the EDL and ES (p = 0.41). This finding suggests that multifidus has faster cross-bridge turnover kinetics when compared to other muscles (ES and EDL) when matched for fiber type. Whether the faster cross-bridge kinetics translate to a functionally significant difference in whole muscle performance needs to be studied further.

Muscle miR-16 deletion results in impaired insulin sensitivity and contractile function in a sex-dependent manner.

microRNAs (miRs) are linked to various human diseases including type 2 diabetes mellitus (T2DM) and emerging evidence suggests that miRs may serve as potential therapeutic targets. Lower miR-16 content is consistent across different models of T2DM; however, the role of miR-16 in muscle metabolic health is still elusive. Therefore, the purpose of this study was to investigate how deletion of miR-16 in mice affects skeletal muscle metabolic health and contractile function in both sexes. This study was conducted using both 1) in vitro and 2) in vivo experiments. In in vitro experiments, we used C2C12 myoblasts to test if inhibition or overexpression of miR-16 affected insulin-mediated glucose handling. In in vivo experiments, we generated muscle-specific miR-16 knockout (KO) mice fed a high-fat diet (HFD) to assess how miR-16 content impacts metabolic and contractile properties including glucose tolerance, insulin sensitivity, muscle contractile function, protein anabolism, and mitochondrial network health. In in vitro experiments, although inhibition of miR-16 induced impaired insulin signaling (P = 0.002) and glucose uptake (P = 0.014), overexpression of miR-16 did not attenuate lipid overload-induced insulin resistance using the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol. In in vivo experiments, miR-16 deletion induced both impaired muscle contractility (P = 0.031-0.033), and mitochondrial network health (P = 0.008-0.018) in both sexes. However, although males specifically exhibited impaired insulin sensitivity following miR-16 deletion (P = 0.030), female KO mice showed pronounced glucose intolerance (P = 0.046), corresponding with lower muscle weights (P = 0.015), and protein hyperanabolism (P = 0.023). Our findings suggest distinct sex differences in muscle adaptation in response to miR-16 deletion and miR-16 may serve as a key regulator for metabolic dysregulation in T2DM.NEW & NOTEWORTHY We set to investigate the role of miR-16 in skeletal muscle during diet-induced insulin resistance. Our data provide novel evidence that the lack of miR-16 induced multiple aberrations in insulin sensitivity, muscle contractility, mitochondrial network health, and protein turnover in a sex-dependent manner. Interestingly, miR-16 deletion leads to insulin resistance in males and exacerbated glucose intolerance in females, suggesting different mechanisms of metabolic dysregulation with a lack of miR-16 between sexes.

Administration of particulate oxygen generators improves skeletal muscle contractile function after ischemia-reperfusion injury in the rat hindlimb.

Extended tourniquet application, often associated with battlefield extremity trauma, can lead to severe ischemia-reperfusion (I/R) injury in skeletal muscle. Particulate oxygen generators (POGs) can be directly injected into tissue to supply oxygen to attenuate the effects of I/R injury in muscle. The goal of this study was to investigate the efficacy of a sodium percarbonate (SPO)-based POG formulation in reducing ischemic damage in a rat hindlimb during tourniquet application. Male Lewis rats were anesthetized and underwent tourniquet application for 3 h at a pressure of 300 mmHg. Shortly after tourniquet inflation, animals received intramuscular injections of either 0.2 mg/mL SPO with catalase (n = 6) or 2.0 mg/mL SPO with catalase (n = 6) directly into the tibialis anterior (TA) muscle. An additional Tourniquet-Only group (n = 12) received no intervention. Functional recovery was monitored by in vivo contractile testing of the hindlimb at 1, 2, and 4 wk after injury. By the 4 wk time point, the Low-Dose POG group continued to show improved functional recovery (85% of baseline) compared with the Tourniquet-Only (48%) and High-Dose POG (56%) groups. In short, the low-dose POG formulation appeared, at least in part, to mitigate the impact of ischemic tissue injury, thus improving contractile function after tourniquet application. Functional improvement correlated with maintenance of larger muscle fiber cross-sectional area and the presence of fewer fibers containing centrally located nuclei. As such, POGs represent a potentially attractive therapeutic solution for addressing I/R injuries associated with extremity trauma.NEW & NOTEWORTHY Skeletal muscle contraction was evaluated in the same animals at multiple time points up to 4 wk after injury, following administration of particulate oxygen generators (POGs) in a clinically relevant rat hindlimb model of tourniquet-induced ischemia. The observed POG-mediated improvement of muscle function over time confirms and extends previous studies to further document the potential clinical applications of POGs. Of particular significance in austere environments, this technology can be applied in the absence of an intact circulation.

Methods for Transepicardial Cell Transplantation in a Swine Myocardial Infarction Model.

The transplantation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) has garnered significant attention as a potential means of restoring lost muscle mass and contractile function in injured hearts. Early preclinical work with hPSC-CMs employed rodent models, but the field has recently advanced to transplantation studies in more translationally relevant large animal models including non-human primates and swine. The pig is a particularly attractive model for such studies because the size, structure, and physiology of the porcine heart is very similar to that of humans. The pig model has reasonably high throughput, is readily amenable to clinically available cell delivery methods and imaging modalities and has been used frequently to test the safety and efficacy of new cardiac therapies. Here, we describe methods that were established in our laboratory for the specific purpose of testing hPSC-CM transplantation in a pig model of subacute myocardial infarction, but these same techniques should be broadly applicable to the transepicardial delivery of other biologicals including other candidate cell populations, biomaterials, and/or viral vectors.

Hyperbaric oxygen therapy does not alleviate tourniquet-induced acute ischemia-reperfusion injury in mouse skeletal muscles

During tourniquet application, blood flow is restricted to a limb to stop excessive limb hemorrhage in a trauma setting and to create a bloodless operating field in the surgical setting. During tourniquet-related ischemia, aerobic respiration stops, and ATP is depleted, and during subsequent reperfusion, there is an increase in reactive oxygen species (ROS) production and other endogenous substances, which leads to acute ischemia-reperfusion (IR) injuries, including tissue necrosis and skeletal muscle contractile dysfunction. Hyperbaric oxygen (HBO) therapy can increase the arterial oxygen tension in the tissues of patients with general hypoxia/anoxia, including carbon monoxide poisoning, circulatory arrest, and cerebral and myocardial ischemia. Here, we studied the protective effects of HBO pretreatment with 100% oxygen at 2.5 ATA against tourniquet/IR injury in mice. After one hour of HBO therapy with 100% oxygen at 2.5 ATA was administered to C57/BL6 mice, a rubber band was placed at the hip joint of the unilateral hindlimb to induce 3 h of ischemia and then released for 48 h of reperfusion. We analyzed gastrocnemius muscle morphology and contractile function and measured the levels of ATP and ROS accumulation in the muscles. HBO pretreatment did not improve tourniquet/IR-injured gastrocnemius muscle morphology and muscle contraction. Tourniquet/IR mice with HBO pretreatment showed no increase in ATP levels in IR tissues, but they did have a decreased amount of ROS accumulation in the muscles, compared to IR mice with no HBO pretreatment. These data suggest that one hour of HBO pretreatment with 100% oxygen at 2.5 ATA increases the antioxidant response to lower ROS accumulation but does not increase ATP levels in IR muscles and improve tourniquet/IR-injured muscle morphology and contractile function.

Skeletal muscle mitochondrial network dynamics in metabolic disorders and aging

With global demographics trending towards an aging population, the numbers of individuals with an age-associated loss of independence is increasing. A key contributing factor is loss of skeletal muscle mitochondrial, metabolic, and contractile function. Recent advances in imaging technologies have demonstrated the importance of mitochondrial morphology and dynamics in the pathogenesis of disease. In this review, we examine the evidence for altered mitochondrial dynamics as a mechanism in age and obesity-associated loss of skeletal muscle function, with a particular focus on the available human data. We highlight some of the areas where more data are needed to identify the specific mechanisms connecting mitochondrial morphology and skeletal muscle dysfunction.

Impact of Lockdown during COVID-19 Pandemic on Central Activation, Muscle Activity, Contractile Function, and Spasticity in People with Multiple Sclerosis.

Background People with multiple sclerosis (MS) suffer from symptoms related to neural control, such as reduced central activation, lower muscle activity, and accentuated spasticity. A forced 9-week home confinement related to COVID-19 in Spain may have worsened these symptoms. However, no study has demonstrated the impact of home confinement on neuromuscular mechanisms in the MS population. This study was aimed at analyzing the effects of a 9-week home confinement on central activation, muscle activity, contractile function, and spasticity in MS patients. Methods Eighteen participants were enrolled in the study. Left and right knee extensor maximum voluntary isometric contraction (MVIC), maximal neural drive via peak surface electromyography (EMG) of the vastus lateralis, central activation ratio (CAR), and muscle contractile function via electrical stimulation of the knee extensor muscles, as well as spasticity using the pendulum test, were measured immediately before and after home confinement. Results Seventeen participants completed the study. CAR significantly decreased after lockdown (ES = 1.271, p < 0.001). Regarding spasticity, there was a trend to decrease in the number of oscillations (ES = 0.511, p = 0.059) and a significant decrease in the duration of oscillations (ES = 0.568, p = 0.038). Furthermore, in the left leg, there was a significant decrease in the first swing excursion (ES = 0.612, p = 0.027) and in the relaxation index (ES = 0.992, p = 0.001). Muscle contractile properties, MVIC, and EMG variables were not modified after confinement. Conclusions The results suggest that a home confinement period of 9 weeks may lead to an increase in lower limb spasticity and a greater deficit in voluntary activation of the knee extensors.