Characterization and utilization of the flexor digitorum brevis for assessing skeletal muscle function

Loss of mitochondria long-chain fatty acid oxidation impairs skeletal muscle contractility by disrupting myofibril structure and calcium homeostasis

OBJECTIVE

Abnormal lipid metabolism in mammalian tissues can be highly deleterious, leading to organ failure. Carnitine Palmitoyltransferase 2 (CPT2) deficiency is an inherited metabolic disorder affecting the liver, heart, and skeletal muscle due to impaired mitochondrial oxidation of long-chain fatty acids (mLCFAO) for energy production.

METHODS

However, the basis of tissue damage in mLCFAO disorders is not fully understood. Mice lacking CPT2 in skeletal muscle (Cpt2Sk-/-) were generated to investigate the nexus between mFAO deficiency and myopathy.

RESULTS

Compared to controls, ex-vivo contractile force was reduced by 70% in Cpt2Sk-/- oxidative soleus muscle despite the preserved capacity to couple ATP synthesis to mitochondrial respiration on alternative substrates to long-chain fatty acids. Increased mitochondrial biogenesis, lipid accumulation, and the downregulation of 80% of dystrophin-related and contraction-related proteins severely compromised the structure and function of Cpt2Sk-/- soleus. CPT2 deficiency affected oxidative muscles more than glycolytic ones. Exposing isolated sarcoplasmic reticulum to long-chain acylcarnitines (LCACs) inhibited calcium uptake. In agreement, Cpt2Sk-/- soleus had decreased calcium uptake and significant accumulation of palmitoyl-carnitine, suggesting that LCACs and calcium dyshomeostasis are linked in skeletal muscle.

CONCLUSIONS

Our data demonstrate that loss of CPT2 and mLCFAO compromise muscle structure and function due to excessive mitochondrial biogenesis, downregulation of the contractile proteome, and disruption of calcium homeostasis.

Mitochondria can substitute for parvalbumin to lower cytosolic calcium levels in the murine fast skeletal muscle

AIM

Parvalbumin (PV) is a primary calcium buffer in mouse fast skeletal muscle fibers. Previous work showed that PV ablation has a limited impact on cytosolic Ca2+ ([Ca2+]cyto) transients and contractile response, while it enhances mitochondrial density and mitochondrial matrix-free calcium concentration ([Ca2+]mito). Here, we aimed to quantitatively test the hypothesis that mitochondria act to compensate for PV deficiency.

METHODS

We determined the free Ca2+ redistribution during a 2 s 60 Hz tetanic stimulation in the sarcoplasmic reticulum, cytosol, and mitochondria. Via a reaction-diffusion Ca2+ model, we quantitatively evaluated mitochondrial uptake and storage capacity requirements to compensate for PV lack and analyzed possible extracellular export.

RESULTS

[Ca2+]mito during tetanic stimulation is greater in knock-out (KO) (1362 ± 392 nM) than in wild-type (WT) (855 ± 392 nM), p < 0.05. Under the assumption of a non-linear intramitochondrial buffering, the model predicts an accumulation of 725 μmoles/L fiber (buffering ratio 1:11 000) in KO, much higher than in WT (137 μmoles/L fiber, ratio 1:4500). The required transport rate via mitochondrial calcium uniporter (MCU) reaches 3 mM/s, compatible with available literature. TEM images of calcium entry units and Mn2+ quenching showed a greater capacity of store-operated calcium entry in KO compared to WT. However, levels of [Ca2+]cyto during tetanic stimulation were not modulated to variations of extracellular calcium.

CONCLUSIONS

The model-based analysis of experimentally determined calcium distribution during tetanic stimulation showed that mitochondria can act as a buffer to compensate for the lack of PV. This result contributes to a better understanding of mitochondria's role in modulating [Ca2+]cyto in skeletal muscle fibers.

Concerted regulation of skeletal muscle metabolism and contractile properties by the orphan nuclear receptor Nr2f6

BACKGROUND

The maintenance of skeletal muscle plasticity upon changes in the environment, nutrient supply, and exercise depends on regulatory mechanisms that couple structural and metabolic adaptations. The mechanisms that interconnect both processes at the transcriptional level remain underexplored. Nr2f6, a nuclear receptor, regulates metabolism and cell differentiation in peripheral tissues. However, its role in the skeletal muscle is still elusive. Here, we aimed to investigate the effects of Nr2f6 modulation on muscle biology in vivo and in vitro.

METHODS

Global RNA-seq was performed in Nr2f6 knockdown C2C12 myocytes (N = 4-5). Molecular and metabolic assays and proliferation experiments were performed using stable Nr2f6 knockdown and Nr2f6 overexpression C2C12 cell lines (N = 3-6). Nr2f6 content was evaluated in lipid overload models in vitro and in vivo (N = 3-6). In vivo experiments included Nr2f6 overexpression in mouse tibialis anterior muscle, followed by gene array transcriptomics and molecular assays (N = 4), ex vivo contractility experiments (N = 5), and histological analysis (N = 7). The conservation of Nr2f6 depletion effects was confirmed in primary skeletal muscle cells of humans and mice.

RESULTS

Nr2f6 knockdown upregulated genes associated with muscle differentiation, metabolism, and contraction, while cell cycle-related genes were downregulated. In human skeletal muscle cells, Nr2f6 knockdown significantly increased the expression of myosin heavy chain genes (two-fold to three-fold) and siRNA-mediated depletion of Nr2f6 increased maximal C2C12 myocyte's lipid oxidative capacity by 75% and protected against lipid-induced cell death. Nr2f6 content decreased by 40% in lipid-overloaded myotubes and by 50% in the skeletal muscle of mice fed a high-fat diet. Nr2f6 overexpression in mice resulted in an atrophic and hypoplastic state, characterized by a significant reduction in muscle mass (15%) and myofibre content (18%), followed by an impairment (50%) in force production. These functional phenotypes were accompanied by the establishment of an inflammation-like molecular signature and a decrease in the expression of genes involved in muscle contractility and oxidative metabolism, which was associated with the repression of the uncoupling protein 3 (20%) and PGC-1α (30%) promoters activity following Nr2f6 overexpression in vitro. Additionally, Nr2f6 regulated core components of the cell division machinery, effectively decoupling muscle cell proliferation from differentiation.

CONCLUSIONS

Our findings reveal a novel role for Nr2f6 as a molecular transducer that plays a crucial role in maintaining the balance between skeletal muscle contractile function and oxidative capacity. These results have significant implications for the development of potential therapeutic strategies for metabolic diseases and myopathies.

The effects of muscle starting length on work loop power output of isolated mouse soleus and extensor digitorum longus muscle

Force-length relationships derived from isometric activations may not directly apply to muscle force production during dynamic contractions. As such, different muscle starting lengths between isometric and dynamic conditions could be required to achieve maximal force and power. Therefore, this study examined the effects of starting length [±5-10% of length corresponding to maximal twitch force (L0)] on work loop (WL) power output (PO), across a range of cycle frequencies, of the soleus (SOL) and extensor digitorum longus muscle (EDL; N=8-10) isolated from ∼8 week old C57 mice. Furthermore, passive work was examined at a fixed cycle frequency to determine the association of passive work and active net work. Starting length affected maximal WL PO of the SOL and EDL across evaluated cycle frequencies (P<0.030, ηp2>0.494). For the SOL, PO produced at -5% L0 was greater than that at most starting lengths (P<0.015, Cohen's d>0.6), except -10% L0 (P=0.135, d<0.4). However, PO produced at -10% L0 versus L0 did not differ (P=0.138, d=0.35-0.49), indicating -5% L0 is optimal for maximal SOL WL PO. For the EDL, WL PO produced at -10% L0 was lower than that at most starting lengths (P<0.032, d>1.08), except versus -5% L0 (P=0.124, d<0.97). PO produced at other starting lengths did not differ (P>0.163, d<1.04). For the SOL, higher passive work was associated with reduced PO (Spearman's r=0.709, P<0.001), but no relationship was observed between passive work and PO of the EDL (Pearson's r=0.191, r2=0.04, P=0.184). This study suggests that starting length should be optimised for both static and dynamic contractions and confirms that the force-length curve during dynamic contractions is muscle specific.

Enhanced capacity for CaMKII signaling mitigates calcium release related contractile fatigue with high intensity exercise

BACKGROUND

We tested whether enhancing the capacity for calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling would delay fatigue of excitation-induced calcium release and improve contractile characteristics of skeletal muscle during fatiguing exercise.

METHODS

Fast and slow type muscle, gastrocnemius medialis (GM) and soleus (SOL), of rats and mouse interosseus (IO) muscle fibers, were transfected with pcDNA3-based plasmids for rat α and β CaMKII or empty controls. Levels of CaMKII, its T287-phosphorylation (pT287-CaMKII), and phosphorylation of components of calcium release and re-uptake, ryanodine receptor 1 (pS2843-RyR1) and phospholamban (pT17-PLN), were quantified biochemically. Sarcoplasmic calcium in transfected muscle fibers was monitored microscopically during trains of electrical excitation based on Fluo-4 FF fluorescence (n = 5-7). Effects of low- (n = 6) and high- (n = 8) intensity exercise on pT287-CaMKII and contractile characteristics were studied in situ.

RESULTS

Co-transfection with αCaMKII-pcDNA3/βCaMKII-pcDNA3 increased α and βCaMKII levels in SOL (+45.8 %, +250.5 %) and GM (+40.4 %, +89.9 %) muscle fibers compared to control transfection. High-intensity exercise increased pT287-βCaMKII and pS2843-RyR1 levels in SOL (+269 %, +151 %) and GM (+354 %, +119 %), but decreased pT287-αCaMKII and p17-PLN levels in GM compared to SOL (-76 % vs. +166 %; 0 % vs. +128 %). α/β CaMKII overexpression attenuated the decline of calcium release in muscle fibers with repeated excitation, and mitigated exercise-induced deterioration of rates in force production, and passive force, in a muscle-dependent manner, in correlation with pS2843-RyR1 and pT17-PLN levels (|r| > 0.7).

CONCLUSION

Enhanced capacity for α/β CaMKII signaling improves fatigue-resistance of active and passive contractile muscle properties in association with RyR1- and PLN-related improvements in sarcoplasmic calcium release.

Ca2+ and force during dynamic contractions in mouse intact skeletal muscle fibers

Muscle fiber force production is determined by the excitation frequency of motor nerves, which induce transient increases in cytoplasmic free Ca2+ concentration ([Ca2+]i) and the force-generating capacity of the actomyosin cross-bridges. Previous studies suggest that, in addition to altered cross-bridge properties, force changes during dynamic (concentric or eccentric) contraction might be affected by Ca2+-dependent components. Here we investigated this by measuring [Ca2+]i and force in mouse muscle fibers undergoing isometric, concentric, and eccentric contractions. Intact single muscle fibers were dissected from the flexor digitorum brevis muscle of mice. Fibers were electrically activated isometrically at 30-100 Hz and after reaching the isometric force plateau, they were actively shortened or stretched. We calculated the ratio (relative changes) in force and [Ca2+]i attained in submaximal (30 Hz) and near-maximal (100 Hz) contractions under isometric or dynamic conditions. Tetanic [Ca2+]i was similar during isometric, concentric and eccentric phases of contraction at given stimulation frequencies while the forces were clearly different depending on the contraction types. The 30/100 Hz force ratio was significantly lower in the concentric (44.1 ± 20.3%) than in the isometric (50.3 ± 20.4%) condition (p = 0.005), whereas this ratio did not differ between eccentric and isometric conditions (p = 0.186). We conclude that the larger force decrease by decreasing the stimulation frequency during concentric than during isometric contraction is caused by decreased myofibrillar Ca2+ sensitivity, not by the decreased [Ca2+]i.

Advanced Methodology for Rapid Isolation of Single Myofibers from Flexor Digitorum Brevis Muscle

Isolated individual myofibers are valuable experimental models that can be used in various conditions to understand skeletal muscle physiology and pathophysiology at the tissue and cellular level. This report details a time- and cost-effective method for isolation of single myofibers from the flexor digitorum brevis (FDB) muscle in both young and aged mice. The FDB muscle was chosen for its documented history in single myofiber experiments. By modifying published methods for FDB myofiber isolation, we have optimized the protocol by first separating FDB muscle into individual bundles before the digestion, followed by optimizing the subsequent digestion medium conditions to ensure reproducibility. Morphological and functional assessments demonstrate a high yield of isolated FDB myofibers with sarcolemma integrity achieved in a shorter time frame than previous published procedures. This method could be also adapted to other types of skeletal muscle. Additionally, this highly reproducible method can greatly reduce the number of animals needed to yield adequate numbers of myofibers for experiments. Thus, this advanced method for myofiber isolation has the potential to accelerate research in skeletal muscle physiology and screening potential therapeutics "ex vivo" for muscle diseases and regeneration.

Voltage sensor current, SR Ca2+ release, and Ca2+ channel current during trains of action potential-like depolarizations of skeletal muscle fibers

In skeletal muscle, CaV 1.1 serves as the voltage sensor for both excitation-contraction coupling (ECC) and L-type Ca2+ channel activation. We have recently adapted the technique of action potential (AP) voltage clamp (APVC) to monitor the current generated by the movement of intramembrane voltage sensors (IQ ) during single imposed transverse tubular AP-like depolarization waveforms (IQAP ). We now extend this procedure to monitoring IQAP , and Ca2+ currents during trains of tubular AP-like waveforms in adult murine skeletal muscle fibers, and compare them with the trajectories of APs and AP-induced Ca2+ release measured in other fibers using field stimulation and optical probes. The AP waveform remains relatively constant during brief trains (<1 sec) for propagating APs in non-V clamped fibers. Trains of 10 AP-like depolarizations at 10 Hz (900 ms), 50 Hz (180 ms), or 100 Hz (90 ms) did not alter IQAP amplitude or kinetics, consistent with previous findings in isolated muscle fibers where negligible charge immobilization occurred during 100 ms step depolarizations. Using field stimulation, Ca2+ release did exhibit a considerable decline from pulse to pulse during the train, also consistent with previous findings, indicating that the decline of Ca2+ release during a short train of APs is not correlated to modification of charge movement. Ca2+ currents during single or 10 Hz trains of AP-like depolarizations were hardly detectable, were minimal during 50 Hz trains, and became more evident during 100 Hz trains in some fibers. Our results verify predictions on the behavior of the ECC machinery in response to AP-like depolarizations and provide a direct demonstration that Ca2+ currents elicited by single AP-like waveforms are negligible, but can become more prominent in some fibers during short high-frequency train stimulation that elicits maximal isometric force.

Fast and slow skeletal myosin binding protein-C and aging

Aging is associated with skeletal muscle strength decline and cardiac diastolic dysfunction. The structural arrangements of the sarcomeric proteins, such as myosin binding protein-C (MyBP-C) are shown to be pivotal in the pathogenesis of diastolic dysfunction. Yet, the role of fast (fMyBP-C) and slow (sMyBP-C) skeletal muscle MyBP-C remains to be elucidated. Herein, we aimed to characterize MyBP-C and its paralogs in the fast tibialis anterior (TA) muscle from adult and old mice. Immunoreactivity preparations showed that the relative abundance of the fMyBP-C paralog was greater in the TA of both adult and old, but no differences were noted between groups. We further found that the expression level of cardiac myosin binding protein-C (cMyBP-C), an important modulator of cardiac output, was lowered by age. Standard SDS-PAGE along with Pro-Q Diamond phosphoprotein staining did not identify age-related changes in phosphorylated MyBP-C proteins from TA and cardiac muscles; however, it revealed that MyBP-C paralogs in fast skeletal and cardiac muscle were highly phosphorylated. Mass spectrometry further identified glycogen phosphorylase, desmin, actin, troponin T, and myosin regulatory light chain 2 as phosphorylated myofilament proteins in both ages. MyBP-C protein-bound carbonyls were determined using anti-DNP immunostaining and found the carbonyl level of fMyBP-C, sMyBP-C, and cMyBP-C to be similar between old and adult animals. In summary, our data showed some differences regarding the MyBP-C paralog expression and identified an age-related reduction of cMyBP-C expression. Future studies are needed to elucidate which are the age-driven post-translational modifications in the MyBP-C paralogs.

Defective BVES-mediated feedback control of cAMP in muscular dystrophy

Biological processes incorporate feedback mechanisms to enable positive and/or negative regulation. cAMP is an important second messenger involved in many aspects of muscle biology. However, the feedback mechanisms for the cAMP signaling control in skeletal muscle are largely unknown. Here we show that blood vessel epicardial substance (BVES) is a negative regulator of adenylyl cyclase 9 (ADCY9)-mediated cAMP signaling involved in maintaining muscle mass and function. BVES deletion in mice reduces muscle mass and impairs muscle performance, whereas virally delivered BVES expressed in Bves-deficient skeletal muscle reverses these defects. BVES interacts with and negatively regulates ADCY9's activity. Disruption of BVES-mediated control of cAMP signaling leads to an increased protein kinase A (PKA) signaling cascade, thereby promoting FoxO-mediated ubiquitin proteasome degradation and autophagy initiation. Our study reveals that BVES functions as a negative feedback regulator of ADCY9-cAMP signaling in skeletal muscle, playing an important role in maintaining muscle homeostasis.

Hypoxia Resistance Is an Inherent Phenotype of the Mouse Flexor Digitorum Brevis Skeletal Muscle

The various functions of skeletal muscle (movement, respiration, thermogenesis, etc.) require the presence of oxygen (O2). Inadequate O2 bioavailability (ie, hypoxia) is detrimental to muscle function and, in chronic cases, can result in muscle wasting. Current therapeutic interventions have proven largely ineffective to rescue skeletal muscle from hypoxic damage. However, our lab has identified a mammalian skeletal muscle that maintains proper physiological function in an environment depleted of O2. Using mouse models of in vivo hindlimb ischemia and ex vivo anoxia exposure, we observed the preservation of force production in the flexor digitorum brevis (FDB), while in contrast the extensor digitorum longus (EDL) and soleus muscles suffered loss of force output. Unlike other muscles, we found that the FDB phenotype is not dependent on mitochondria, which partially explains the hypoxia resistance. Muscle proteomes were interrogated using a discovery-based approach, which identified significantly greater expression of the transmembrane glucose transporter GLUT1 in the FDB as compared to the EDL and soleus. Through loss-and-gain-of-function approaches, we determined that GLUT1 is necessary for the FDB to survive hypoxia, but overexpression of GLUT1 was insufficient to rescue other skeletal muscles from hypoxic damage. Collectively, the data demonstrate that the FDB is uniquely resistant to hypoxic insults. Defining the mechanisms that explain the phenotype may provide insight towards developing approaches for preventing hypoxia-induced tissue damage.

Quantification of Muscle Satellite Stem Cell Divisions by High-Content Analysis

High-content screening is commonly performed on 2D cultured cells, which is high throughput but has low biological relevance. In contrast, single myofiber culture assay preserves the satellite cell niche between the basal lamina and sarcolemma and consequently has high biological relevance but is low throughput. We describe here a high-content screening method that utilizes single myofiber culture that addresses the caveats of both techniques. Our method utilizes the transgenic reporter allele Myf5-Cre:R26R-eYFP to differentiate stem and committed cells within a dividing couplet that can be quantified by high-content throughput immunodetection and bioinformatic analysis.

Enzymatically dissociated muscle fibers display rapid dedifferentiation and impaired mitochondrial calcium control

Cells rapidly lose their physiological phenotype upon disruption of their extracellular matrix (ECM)-intracellular cytoskeleton interactions. By comparing adult mouse skeletal muscle fibers, isolated either by mechanical dissection or by collagenase-induced ECM digestion, we investigated acute effects of ECM disruption on cellular and mitochondrial morphology, transcriptomic signatures, and Ca2+ handling. RNA-sequencing showed striking differences in gene expression patterns between the two isolation methods with enzymatically dissociated fibers resembling myopathic phenotypes. Mitochondrial appearance was grossly similar in the two groups, but 3D electron microscopy revealed shorter and less branched mitochondria following enzymatic dissociation. Repeated contractions resulted in a prolonged mitochondrial Ca2+ accumulation in enzymatically dissociated fibers, which was partially prevented by cyclophilin inhibitors. Of importance, muscle fibers of mice with severe mitochondrial myopathy show pathognomonic mitochondrial Ca2+ accumulation during repeated contractions and this accumulation was concealed with enzymatic dissociation, making this an ambiguous method in studies of native intracellular Ca2+ fluxes.

Nerve-dependent distribution of subsynaptic type 1 inositol 1,4,5-trisphosphate receptor at the neuromuscular junction

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are enriched at postsynaptic membrane compartments of the neuromuscular junction (NMJ), surrounding the subsynaptic nuclei and close to nicotinic acetylcholine receptors (nAChRs) of the motor endplate. At the endplate level, it has been proposed that nerve-dependent electrical activity might trigger IP3-associated, local Ca2+ signals not only involved in excitation-transcription (ET) coupling but also crucial to the development and stabilization of the NMJ itself. The present study was undertaken to examine whether denervation affects the subsynaptic IP3R distribution in skeletal muscles and which are the underlying mechanisms. Fluorescence microscopy, carried out on in vivo denervated muscles (following sciatectomy) and in vitro denervated skeletal muscle fibers from flexor digitorum brevis (FDB), indicates that denervation causes a reduction in the subsynaptic IP3R1-stained region, and such a decrease appears to be determined by the lack of muscle electrical activity, as judged by partial reversal upon field electrical stimulation of in vitro denervated skeletal muscle fibers.

Sarcomere dynamics revealed by a myofilament integrated FRET-based biosensor in live skeletal muscle fibers

The sarcomere is the functional unit of skeletal muscle, essential for proper contraction. Numerous acquired and inherited myopathies impact sarcomere function causing clinically significant disease. Mechanistic investigations of sarcomere activation have been challenging to undertake in the context of intact, live skeletal muscle fibers during real time physiological twitch contractions. Here, a skeletal muscle specific, intramolecular FRET-based biosensor was designed and engineered into fast skeletal muscle troponin C (TnC) to investigate the dynamics of sarcomere activation. In transgenic animals, the TnC biosensor incorporated into the skeletal muscle fiber sarcomeres by stoichiometric replacement of endogenous TnC and did not alter normal skeletal muscle contractile form or function. In intact single adult skeletal muscle fibers, real time twitch contractile data showed the TnC biosensor transient preceding the peak amplitude of contraction. Importantly, under physiological temperatures, inactivation of the TnC biosensor transient decayed significantly more slowly than the Ca²⁺ transient and contraction. The uncoupling of the TnC biosensor transient from the Ca²⁺ transient indicates the biosensor is not functioning as a Ca²⁺ transient reporter, but rather reports dynamic sarcomere activation/ inactivation that, in turn, is due to the ensemble effects of multiple activating ligands within the myofilaments. Together, these findings provide the foundation for implementing this new biosensor in future physiological studies investigating the mechanism of activation of the skeletal muscle sarcomere in health and disease.

Contractility of permeabilized rat vastus intermedius muscle fibres following high-fat, high-sucrose diet consumption

Obesity is a worldwide health concern associated with impaired physical function. It is not clear if contractile protein dysfunction contributes to the impairment of muscle function observed with obesity. The purpose of this study was to examine if diet-induced obesity affects contractile function of chemically permeabilized vastus intermedius fibres of male Sprague-Dawley rats expressing fast myosin heavy chain (MHC) IIa or slow MHC I. Rats consumed either a high-fat, high sucrose (HFHS) diet or a standard (CHOW) diet beginning as either weanlings (7-week duration: WEAN7 cohort, or 14-week duration: WEAN14 cohort) or young adults (12-week duration: ADULT12 cohort, 24-week duration: ADULT24 cohort). HFHS-fed rats had higher (P < 0.05) whole-body adiposity (derived from dual-energy X-ray absorptiometry) than CHOW-fed rats in all cohorts. Relative to CHOW diet groups, the HFHS diet was associated with impaired force production in (a) MHC I fibres in the ADULT24 cohort; and (b) MHC IIa fibres in the ADULT12 and ADULT24 cohorts combined. However, the HFHS diet did not significantly affect the Ca2+-sensitivity of force production, unloaded shortening velocity, or ratio of active force to active stiffness in any cohort. We conclude that diet-induced obesity can impair force output of permeabilized muscle fibres of adult rats. Novelty: We assessed contractile function of permeabilized skeletal muscle fibres in a rat model of diet-induced obesity. The high-fat, high-sucrose diet was associated with impaired force output of fibres expressing MHC I or MHC IIa in some cohorts of rats. Other measures of contractile function were not significantly affected by diet.

Endurance exercise training-responsive miR-19b-3p improves skeletal muscle glucose metabolism

Skeletal muscle is a highly adaptable tissue and remodels in response to exercise training. Using short RNA sequencing, we determine the miRNA profile of skeletal muscle from healthy male volunteers before and after a 14-day aerobic exercise training regime. Among the exercise training-responsive miRNAs identified, miR-19b-3p was selected for further validation. Overexpression of miR-19b-3p in human skeletal muscle cells increases insulin signaling, glucose uptake, and maximal oxygen consumption, recapitulating the adaptive response to aerobic exercise training. Overexpression of miR-19b-3p in mouse flexor digitorum brevis muscle enhances contraction-induced glucose uptake, indicating that miR-19b-3p exerts control on exercise training-induced adaptations in skeletal muscle. Potential targets of miR-19b-3p that are reduced after aerobic exercise training include KIF13A, MAPK6, RNF11, and VPS37A. Amongst these, RNF11 silencing potentiates glucose uptake in human skeletal muscle cells. Collectively, we identify miR-19b-3p as an aerobic exercise training-induced miRNA that regulates skeletal muscle glucose metabolism.

Activation of the NLRP3 Inflammasome Increases the IL-1β Level and Decreases GLUT4 Translocation in Skeletal Muscle during Insulin Resistance

Low-grade chronic inflammation plays a pivotal role in the pathogenesis of insulin resistance (IR), and skeletal muscle has a central role in this condition. NLRP3 inflammasome activation pathways promote low-grade chronic inflammation in several tissues. However, a direct link between IR and NLRP3 inflammasome activation in skeletal muscle has not been reported. Here, we evaluated the NLRP3 inflammasome components and their role in GLUT4 translocation impairment in skeletal muscle during IR. Male C57BL/6J mice were fed with a normal control diet (NCD) or high-fat diet (HFD) for 8 weeks. The protein levels of NLRP3, ASC, caspase-1, gasdermin-D (GSDMD), and interleukin (IL)-1β were measured in both homogenized and isolated fibers from the flexor digitorum brevis (FDB) or soleus muscle. GLUT4 translocation was determined through GLUT4myc-eGFP electroporation of the FBD muscle. Our results, obtained using immunofluorescence, showed that adult skeletal muscle expresses the inflammasome components. In the FDB and soleus muscles, homogenates from HFD-fed mice, we found increased protein levels of NLRP3 and ASC, higher activation of caspase-1, and elevated IL-1β in its mature form, compared to NCD-fed mice. Moreover, GSDMD, a protein that mediates IL-1β secretion, was found to be increased in HFD-fed-mice muscles. Interestingly, MCC950, a specific pharmacological NLRP3 inflammasome inhibitor, promoted GLUT4 translocation in fibers isolated from the FDB muscle of NCD- and HFD-fed mice. In conclusion, we found increased NLRP3 inflammasome components in adult skeletal muscle of obese insulin-resistant animals, which might contribute to the low-grade chronic metabolic inflammation of skeletal muscle and IR development.

Preliminary Observations on Skeletal Muscle Adaptation and Plasticity in Homer 2-/- Mice

Homer represents a diversified family of scaffold and transduction proteins made up of several isoforms. Here, we present preliminary observations on skeletal muscle adaptation and plasticity in a transgenic model of Homer 2^-/- mouse using a multifaceted approach entailing morphometry, quantitative RT-PCR (Reverse Transcription PCR), confocal immunofluorescence, and electrophysiology. Morphometry shows that Soleus muscle (SOL), at variance with Extensor digitorum longus muscle (EDL) and Flexor digitorum brevis muscle (FDB), displays sizable reduction of fibre cross-sectional area compared to the WT counterparts. In SOL of Homer 2^-/- mice, quantitative RT-PCR indicated the upregulation of Atrogin-1 and Muscle ring finger protein 1 (MuRF1) genes, and confocal immunofluorescence showed the decrease of neuromuscular junction (NMJ) Homer content. Electrophysiological measurements of isolated FDB fibres from Homer 2^-/- mice detected the exclusive presence of the adult ε-nAChR isoform excluding denervation. As for NMJ morphology, data were not conclusive, and further work is needed to ascertain whether the null Homer 2 phenotype induces any endplate remodelling. Within the context of adaptation and plasticity, the present data show that Homer 2 is a co-regulator of the normotrophic status in a muscle specific fashion.

NMJ-Analyser identifies subtle early changes in mouse models of neuromuscular disease

The neuromuscular junction (NMJ) is the peripheral synapse formed between a motor neuron axon terminal and a muscle fibre. NMJs are thought to be the primary site of peripheral pathology in many neuromuscular diseases, but innervation/denervation status is often assessed qualitatively with poor systematic criteria across studies, and separately from 3D morphological structure. Here, we describe the development of ‘NMJ-Analyser’, to comprehensively screen the morphology of NMJs and their corresponding innervation status automatically. NMJ-Analyser generates 29 biologically relevant features to quantitatively define healthy and aberrant neuromuscular synapses and applies machine learning to diagnose NMJ degeneration. We validated this framework in longitudinal analyses of wildtype mice, as well as in four different neuromuscular disease models: three for amyotrophic lateral sclerosis (ALS) and one for peripheral neuropathy. We showed that structural changes at the NMJ initially occur in the nerve terminal of mutant TDP43 and FUS ALS models. Using a machine learning algorithm, healthy and aberrant neuromuscular synapses are identified with 95% accuracy, with 88% sensitivity and 97% specificity. Our results validate NMJ-Analyser as a robust platform for systematic and structural screening of NMJs, and pave the way for transferrable, and cross-comparison and high-throughput studies in neuromuscular diseases.

Single cell RNA-seq analysis of the flexor digitorum brevis mouse myofibers

Background

Skeletal muscle myofibers can be separated into functionally distinct cell types that differ in gene and protein expression. Current single cell expression data is generally based upon single nucleus RNA, rather than whole myofiber material. We examined if a whole-cell flow sorting approach could be applied to perform single cell RNA-seq (scRNA-seq) in a single muscle type.

Methods

We performed deep, whole cell, scRNA-seq on intact and fragmented skeletal myofibers from the mouse fast-twitch flexor digitorum brevis muscle utilizing a flow-gated method of large cell isolation. We performed deep sequencing of 763 intact and fragmented myofibers.

Results

Quality control metrics across the different gates indicated only 171 of these cells were optimal, with a median read count of 239,252 and an average of 12,098 transcripts per cell. scRNA-seq identified three clusters of myofibers (a slow/fast 2A cluster and two fast 2X clusters). Comparison to a public skeletal nuclear RNA-seq dataset demonstrated a diversity in transcript abundance by method. RISH validated multiple genes across fast and slow twitch skeletal muscle types.

Conclusion

This study introduces and validates a method to isolate intact skeletal muscle myofibers to generate deep expression patterns and expands the known repertoire of fiber-type-specific genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-021-00269-2.

A mouse model of Huntington's disease shows altered ultrastructure of transverse tubules in skeletal muscle fibers

Huntington’s disease (HD) is a fatal and progressive condition with severe debilitating motor defects and muscle weakness. Although classically recognized as a neurodegenerative disorder, there is increasing evidence of cell autonomous toxicity in skeletal muscle. We recently demonstrated that skeletal muscle fibers from the R6/2 model mouse of HD have a decrease in specific membrane capacitance, suggesting a loss of transverse tubule (t-tubule) membrane in R6/2 muscle. A previous report also indicated that Cav1.1 current was reduced in R6/2 skeletal muscle, suggesting defects in excitation–contraction (EC) coupling. Thus, we hypothesized that a loss and/or disruption of the skeletal muscle t-tubule system contributes to changes in EC coupling in R6/2 skeletal muscle. We used live-cell imaging with multiphoton confocal microscopy and transmission electron microscopy to assess the t-tubule architecture in late-stage R6/2 muscle and found no significant differences in the t-tubule system density, regularity, or integrity. However, electron microscopy images revealed that the cross-sectional area of t-tubules at the triad were 25% smaller in R6/2 compared with age-matched control skeletal muscle. Computer simulation revealed that the resulting decrease in the R6/2 t-tubule luminal conductance contributed to, but did not fully explain, the reduced R6/2 membrane capacitance. Analyses of bridging integrator-1 (Bin1), which plays a primary role in t-tubule formation, revealed decreased Bin1 protein levels and aberrant splicing of Bin1 mRNA in R6/2 muscle. Additionally, the distance between the t-tubule and sarcoplasmic reticulum was wider in R6/2 compared with control muscle, which was associated with a decrease in junctophilin 1 and 2 mRNA levels. Altogether, these findings can help explain dysregulated EC coupling and motor impairment in Huntington’s disease.

Effects of myofiber isolation technique on sarcolemma biomechanics

Isolated myofibers are commonly used to understand the function of skeletal muscle in vivo. This can involve single isolated myofibers obtained from dissection or from enzymatic dissociation. Isolation via dissection allows control of sarcomere length and preserves tendon attachment but is labor-intensive, time-consuming and yields few viable myofibers. In contrast, enzymatic dissociation is fast and facile, produces hundreds of myofibers, and more importantly reduces the number of muscles/animals needed for studies. Biomechanical properties of the sarcolemma have been studied using myofibers from the extensor digitorum longus, but this has been limited to dissected myofibers, making data collection slow and difficult. We have modified this tool to perform biomechanical measurements of the sarcolemma in dissociated myofibers from the flexor digitorum brevis.

Developmental demands contribute to early neuromuscular degeneration in CMT2D mice

Dominantly inherited, missense mutations in the widely expressed housekeeping gene, GARS1, cause Charcot-Marie-Tooth type 2D (CMT2D), a peripheral neuropathy characterised by muscle weakness and wasting in limb extremities. Mice modelling CMT2D display early and selective neuromuscular junction (NMJ) pathology, epitomised by disturbed maturation and neurotransmission, leading to denervation. Indeed, the NMJ disruption has been reported in several different muscles; however, a systematic comparison of neuromuscular synapses from distinct body locations has yet to be performed. We therefore analysed NMJ development and degeneration across five different wholemount muscles to identify key synaptic features contributing to the distinct pattern of neurodegeneration in CMT2D mice. Denervation was found to occur along a distal-to-proximal gradient, providing a cellular explanation for the greater weakness observed in mutant Gars hindlimbs compared with forelimbs. Nonetheless, muscles from similar locations and innervated by axons of equivalent length showed significant differences in neuropathology, suggestive of additional factors impacting on site-specific neuromuscular degeneration. Defective NMJ development preceded and associated with degeneration, but was not linked to a delay of wild-type NMJ maturation processes. Correlation analyses indicate that muscle fibre type nor synaptic architecture explain the differential denervation of CMT2D NMJs, rather it is the extent of post-natal synaptic growth that predisposes to neurodegeneration. Together, this work improves our understanding of the mechanisms driving synaptic vulnerability in CMT2D and hints at pertinent pathogenic pathways.

Cryptochromes Suppress HIF1α in Muscles

Muscles preferentially utilize glycolytic or oxidative metabolism depending on the intensity of physical activity. Transcripts required for carbohydrate and lipid metabolism undergo circadian oscillations of expression in muscles, and both exercise capacity and the metabolic response to exercise are influenced by time of day. The circadian repressors CRY1 and CRY2 repress peroxisome proliferator-activated receptor delta (PPARδ), a major driver of oxidative metabolism and exercise endurance. CRY-deficient mice exhibit enhanced PPARδ activation and greater maximum speed when running on a treadmill but no increase in exercise endurance. Here we demonstrate that CRYs limit hypoxia-responsive transcription via repression of HIF1α-BMAL1 heterodimers. Furthermore, CRY2 appeared to be more effective than CRY1 in the reduction of HIF1α protein steady-state levels in primary myotubes and quadriceps in vivo. Finally, CRY-deficient myotubes exhibit metabolic alterations consistent with cryptochrome-dependent suppression of HIF1α, which likely contributes to circadian modulation of muscle metabolism.

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CRY2 plays a unique role in regulating HIF1α protein accumulation in muscle

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HIF1α and BMAL1 heterodimers are transcriptionally active

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CRY1/2 represses transcription driven by HIF1α/BMAL1 heterodimers

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Cryptochromes influence skeletal muscle substrate preference and utilization

Morphological variability is greater at developing than mature mouse neuromuscular junctions

The neuromuscular junction (NMJ) is the highly specialised peripheral synapse formed between lower motor neuron terminals and muscle fibres. Post‐synaptic acetylcholine receptors (AChRs), which are found in high density in the muscle membrane, bind to acetylcholine released into the synaptic cleft of the NMJ, thereby enabling the conversion of motor action potentials to muscle contractions. NMJs have been studied for many years as a general model for synapse formation, development and function, and are known to be early sites of pathological changes in many neuromuscular diseases. However, information is limited on the diversity of NMJs in different muscles, how synaptic morphology changes during development, and the relevance of these parameters to neuropathology. Here, this crucial gap was addressed using a robust and standardised semi‐automated workflow called NMJ‐morph to quantify features of pre‐ and post‐synaptic NMJ architecture in an unbiased manner. Five wholemount muscles from wild‐type mice were dissected and compared at immature (post‐natal day, P7) and early adult (P31−32) timepoints. The inter‐muscular variability was greater in mature post‐synaptic AChR morphology than that of the pre‐synaptic motor neuron terminal. Moreover, the developing NMJ showed greater differences across muscles than the mature synapse, perhaps due to the observed distinctions in synaptic growth between muscles. Nevertheless, the amount of nerve to muscle contact was consistent, suggesting that pathological denervation can be reliably compared across different muscles in mouse models of neurodegeneration. Additionally, mature post‐synaptic endplate diameters correlated with fibre type, independently of muscle fibre diameter. Altogether, this work provides detailed information on healthy pre‐ and post‐synaptic NMJ morphology from five anatomically and functionally distinct mouse muscles, delivering useful reference data for future comparison with neuromuscular disease models.

Effects of fasting on isolated murine skeletal muscle contractile function during acute hypoxia

Stored muscle carbohydrate supply and energetic efficiency constrain muscle functional capacity during exercise and are influenced by common physiological variables (e.g. age, diet, and physical activity level). Whether these constraints affect overall functional capacity or the timing of muscle energetic failure during acute hypoxia is not known. We interrogated skeletal muscle contractile properties in two anatomically distinct rodent hindlimb muscles that have well characterized differences in energetic efficiency (locomotory- extensor digitorum longus (EDL) and postural- soleus muscles) following a 24 hour fasting period that resulted in substantially reduced muscle carbohydrate supply. 180 mins of acute hypoxia resulted in complete energetic failure in all muscles tested, indicated by: loss of force production, substantial reductions in total adenosine nucleotide pool intermediates, and increased adenosine nucleotide degradation product-inosine monophosphate (IMP). These changes occurred in the absence of apparent myofiber structural damage assessed histologically by both transverse section and whole mount. Fasting and the associated reduction of the available intracellular carbohydrate pool (~50% decrease in skeletal muscle) did not significantly alter the timing to muscle functional impairment or affect the overall force/work capacities of either muscle type. Fasting resulted in greater passive tension development in both muscle types, which may have implications for the design of pre-clinical studies involving optimal timing of reperfusion or administration of precision therapeutics.

Estrogen receptor-α in female skeletal muscle is not required for regulation of muscle insulin sensitivity and mitochondrial regulation

OBJECTIVE

Estrogen receptor-α (ERα) is a nuclear receptor family member thought to substantially contribute to the metabolic regulation of skeletal muscle. However, previous mouse models utilized to assess the necessity of ERα signaling in skeletal muscle were confounded by altered developmental programming and/or influenced by secondary effects, making it difficult to assign a causal role for ERα. The objective of this study was to determine the role of skeletal muscle ERα in regulating metabolism in the absence of confounding factors of development.

METHODS

A novel mouse model was developed allowing for induced deletion of ERα in adult female skeletal muscle (ERαKOism). ERαshRNA was also used to knockdown ERα (ERαKD) in human myotubes cultured from primary human skeletal muscle cells isolated from muscle biopsies from healthy and obese insulin-resistant women.

RESULTS

Twelve weeks of HFD exposure had no differential effects on body composition, VO2, VCO2, RER, energy expenditure, and activity counts across genotypes. Although ERαKOism mice exhibited greater glucose intolerance than wild-type (WT) mice after chronic HFD, ex vivo skeletal muscle glucose uptake was not impaired in the ERαKOism mice. Expression of pro-inflammatory genes was altered in the skeletal muscle of the ERαKOism, but the concentrations of these inflammatory markers in the systemic circulation were either lower or remained similar to the WT mice. Finally, skeletal muscle mitochondrial respiratory capacity, oxidative phosphorylation efficiency, and H2O2 emission potential was not affected in the ERαKOism mice. ERαKD in human skeletal muscle cells neither altered differentiation capacity nor caused severe deficits in mitochondrial respiratory capacity.

CONCLUSIONS

Collectively, these results suggest that ERα function is superfluous in protecting against HFD-induced skeletal muscle metabolic derangements after postnatal development is complete.

The contractile properties of motor units in the rat flexor digitorum brevis muscle have continuous distribution

The majority of motor unit studies were performed predominantly on calf muscles, where three types of units: S, FR and FF were found. These muscles are involved in postural activity, walking, running and jumping. The properties of foot muscles that perform other functions, e.g. scratching (in animals), and are purely co-active with calf muscles, are poorly known. The aim of the present study was to investigate the contractile properties of motor units in the flexor digitorum brevis. Fifty-six motor units were studied in male Wistar rats. Several methods of fast/slow motor unit categorization, presence of sag, contraction time values, and 20 Hz index, did not allow the separation of the studied motor units into discrete clusters. Therefore, motor units were divided into two groups: fatigable and resistant to fatigue, based on the fatigue index with the border value of 0.5 (although the distribution of the index was not bimodal). The fatigable motor units were stronger and faster compared to the resistant ones. In conclusion, the distribution of motor unit contractile properties in the studied foot muscle was continuous and indicated a lack of three separate physiological types of motor units that usually occurs for the majority of hindlimb muscles. This discrepancy appears to be associated with differences in the typical forms of motor unit activity in distinct muscles.

Equivalent L-type channel (CaV1.1) function in adult female and male mouse skeletal muscle fibers

Loss of total muscle force during aging has both atrophic and non-atrophic components. The former deficit is a direct consequence of reduced muscle mass while the latter has been attributed to a depression of excitation-contraction (EC) coupling. It is well established that age-onset reductions in sex hormone production regulate the atrophic component in both males and females. However, it is unknown whether the non-atrophic component is influenced by sex hormones. Since the non-atrophic component has been linked mechanistically to reduced expression of the skeletal muscle L-type Ca²⁺ channel (Ca_V1.1), we recorded L-type Ca²⁺ currents, gating charge movements and depolarization-induced changes in myoplasmic Ca²⁺ from flexor digitorum brevis (FDB) fibers of naïve and gonadectomized mice of both sexes. Our first set of experiments sought to identify any basal differences in EC coupling or L-type Ca²⁺ flux between the sexes; no detectable differences in any of the aforementioned parameters were observed between FDB harvested from either naïve males or females. In the latter segments of the study, ovariectomy (OVX) and orchiectomy (ORX) models were used to assess the possible influence of sex hormones on EC coupling and/or L-type Ca²⁺ flux. In these experiments, FDB fibers harvested from OVX and ORX mice both showed no differences in L-type Ca²⁺ current, gating charge movement or depolarization-induced changes in Ca²⁺ release from the sarcoplasmic reticulum. Taken together, our results indicate L-type Ca²⁺ channel function and EC coupling are: 1) equivalent between the sexes, and 2) not significantly regulated by sex hormones. Since recent NIH review guidelines mandate the consideration of sex differences as a criterion for review, our work indicates the suitability of either sex for the study of the fundamental mechanisms of EC coupling. Thus, our findings may accelerate the research process by conserving animals, labor and financial resources.

Doxorubicin causes lesions in the electron transport system of skeletal muscle mitochondria that are associated with a loss of contractile function

Doxorubicin is an anthracycline-based chemotherapeutic that causes myotoxicity with symptoms persisting beyond treatment. Patients experience muscle pain, weakness, fatigue, and atrophy, but the underlying mechanisms are poorly understood. Studies investigating doxorubicin-induced myotoxicity have reported disrupted mitochondrial function. Mitochondria are responsible for regulating both cellular energy status and Ca²⁺ handling, both of which impact contractile function. Moreover, loss of mitochondrial integrity may initiate muscle atrophy. Skeletal muscle mitochondrial dysregulation may therefore contribute to an overall loss of skeletal muscle quality and performance that may be mitigated by appropriately targeted mitochondrial therapies. We therefore assessed the impact of doxorubicin on muscle performance and applied a multiplexed assay platform to diagnose alterations in mitochondrial respiratory control. Mice received a clinically relevant dose of doxorubicin delivered systemically and were euthanized 72 h later. We measured extensor digitorum longus and soleus muscle forces, fatigue, and contractile kinetics in vitro, along with Ca²⁺ uptake in isolated sarcoplasmic reticulum. Isolated skeletal muscle mitochondria were used for real-time respirometry or frozen for protein content and activity assays. Doxorubicin impaired muscle performance, which was indicated by reduced force production, fatigue resistance, and sarcoplasmic reticulum-Ca²⁺ uptake, which were associated with a substrate-independent reduction in respiration and membrane potential but no changes in the NAD(P)H/NAD(P)⁺ redox state. Protein content and dehydrogenase activity results corroborated these findings, indicating that doxorubicin-induced mitochondrial impairments are located upstream of ATP synthase within the electron transport system. Collectively, doxorubicin-induced lesions likely span mitochondrial complexes I-IV, providing potential targets for alleviating doxorubicin myotoxicity.

Mitochondrial respiration and H2O2 emission in saponin-permeabilized murine diaphragm fibers: optimization of fiber separation and comparison to limb muscle

Diaphragm abnormalities in aging or chronic diseases include impaired mitochondrial respiration and H₂O₂ emission, which can be measured using saponin-permeabilized muscle fibers. Mouse diaphragm presents a challenge for isolation of fibers due to relatively high abundance of connective tissue in healthy muscle that is exacerbated in disease states. We tested a new approach to process mouse diaphragm for assessment of intact mitochondria respiration and ROS emission in saponin-permeabilized fibers. We used the red gastrocnemius (RG) as "standard" limb muscle. Markers of mitochondrial content were two- to fourfold higher in diaphragm (Dia) than in RG (P < 0.05). Maximal O₂ consumption (JO₂: pmol·s^-1·mg^-1) in Dia was higher with glutamate, malate, and succinate (Dia 399 ± 127, RG 148 ± 60; P < 0.05) and palmitoyl-CoA + carnitine (Dia 15 ± 5, RG 7 ± 1; P < 0.05) than in RG, but not different between muscles when JO₂ was normalized to citrate synthase activity. Absolute JO₂ for Dia was two- to fourfold higher than reported in previous studies. Mitochondrial JH₂O₂ was higher in Dia than in RG (P < 0.05), but lower in Dia than in RG when JH₂O₂ was normalized to citrate synthase activity. Our findings are consistent with an optimized diaphragm preparation for assessment of intact mitochondria in permeabilized fiber bundles. The data also suggest that higher mitochondrial content potentially makes the diaphragm more susceptible to "mitochondrial onset" myopathy. Overall, the new approach will facilitate testing and understanding of diaphragm mitochondrial function in mouse models that are used to advance biomedical research and human health.

Temporal Association Between Ischemic Muscle Perfusion Recovery and the Restoration of Muscle Contractile Function After Hindlimb Ischemia

During incomplete skeletal muscle recovery from ischemia, such as that occurs with critical limb ischemia, the temporal relationship between recovery of muscle capillary perfusion and contractile function is poorly defined. We examined this relationship in BALB/cJ mice (N = 24) following unilateral hindlimb ischemia (HLI), which pre-clinically mimics the myopathy observed in critical limb ischemia patients. Specifically, we examined this relationship in two phenotypically distinct muscles (i.e., "oxidative" soleus - Sol and "glycolytic" extensor digitorum longus - EDL) 14- or 56-days after HLI. Although overall limb blood flow (LDPI) reached its' recovery peak (48% of control) by HLI d14, the capillary networks in both the Sol and EDL (whole mount confocal imaging) were disrupted and competent muscle capillary perfusion (perfused lectin⁺μm²/muscle μm²) remained reduced. Interestingly, both Sol and EDL muscles recovered their distinct capillary structures and perfusion (Con Sol; 0.056 ± 0.02 lectin⁺μm²/muscle μm², and Con EDL; 0.039 ± 0.005 lectin⁺μm²/muscle μm²) by HLI d56 (Sol; 0.062 ± 0.011 lectin⁺μm²/muscle μm² and EDL; 0.0035 ± 0.005 lectin⁺μm²/muscle μm²), despite no further improvement in limb blood flow (LDPI). Both muscles suffered severe myopathy, indicated by loss of dystrophin positive immunostaining and the absence of stimulation induced isometric force production at HLI d14. Dystrophin immunofluorescence returned at HLI d56, although neither myofiber CSA (μm²) nor isometric force production (58 and 28% sustained deficits, Sol and EDL, respectively) recovered completely in either muscle. In summary, we reveal that the temporal relationship between the restoration of muscle capillary perfusion and functional ischemic skeletal muscle regeneration favors competent muscle capillary perfusion recovery in BALB/c mice in a phenotypically non-distinct manner.