Human Skeletal Muscle Biopsies Research Articles

An Association between OXPHOS-Related Gene Expression and Malignant Hyperthermia Susceptibility in Human Skeletal Muscle Biopsies.

Malignant hyperthermia (MH) is a pharmacogenetic condition of skeletal muscle that manifests in hypermetabolic responses upon exposure to volatile anaesthetics. This condition is caused primarily by pathogenic variants in the calcium-release channel RYR1, which disrupts calcium signalling in skeletal muscle. However, our understanding of MH genetics is incomplete, with no variant identified in a significant number of cases and considerable phenotype diversity. In this study, we applied a transcriptomic approach to investigate the genome-wide gene expression in MH-susceptible cases using muscle biopsies taken for diagnostic testing. Baseline comparisons between muscle from MH-susceptible individuals (MHS, n = 8) and non-susceptible controls (MHN, n = 4) identified 822 differentially expressed genes (203 upregulated and 619 downregulated) with significant enrichment in genes associated with oxidative phosphorylation (OXPHOS) and fatty acid metabolism. Investigations of 10 OXPHOS target genes in a larger cohort (MHN: n = 36; MHS: n = 36) validated the reduced expression of ATP5MD and COQ6 in MHS samples, but the remaining 8 selected were not statistically significant. Further analysis also identified evidence of a sex-linked effect in SDHB and UQCC3 expression, and a difference in ATP5MD expression across individuals with MH sub-phenotypes (trigger from in vitro halothane exposure only, MHSh (n = 4); trigger to both in vitro halothane and caffeine exposure, MHShc (n = 4)). Our data support a link between MH-susceptibility and dysregulated gene expression associated with mitochondrial bioenergetics, which we speculate plays a role in the phenotypic variability observed within MH.

Oldies, but goldies-preserved morphology and stability of antigenic determinants in decades-old cryosections of human m. vastus lateralis.

Fibre typing by immunohistochemistry on cryosections from human skeletal muscle biopsies is an essential tool in the diagnosis and research of muscular diseases, ageing, and responses to exercise training and disuse. Preserving a good quality in these frozen specimens can be challenging especially if they are stored for longer periods before histological processing, which is often the case in studies with a large number of test subjects and/or repeated sampling separated by multiple years. We demonstrate in this article that both, the morphology and reactivity of epitopes to myosin heavy chain isoforms and dystrophin are well preserved in up to 18-year-stored unfixed and unstained cryosections of human m. vastus lateralis (n = 241). Any variation in staining intensity between samples was unrelated to the age of the biopsy donor or the storage period of the unstained cryosections, and in all cases, the obtained images were appropriate for image analysis, such as the determination of the fibre type composition and the fibre cross-sectional area, and quantitative analysis of muscle capillarisation.

Cellular activation patterns of CD10+ fibro-adipogenic progenitors across acquired disease states in human skeletal muscle biopsies.

Background: Fibro-adipogenic progenitors (FAP) are muscle resident mesenchymal stem cells pivotal for regulation of myofiber repair. Experimental results show in addition involvement in a range of other pathological conditions and potential for pharmacological intervention. FAP histopathology in human muscle biopsies is largely unknown, but has potential to inform translational research. Methods: CD10+ FAPs in 32 archival muscle biopsies from 8 groups (normal, dermatomyositis, inclusion body myositis (IBM), anti-synthetase syndrome, immune-mediated necrotizing myopathy (IMNM), denervation, type 2 atrophy, rhabdomyolysis) were visualized by CD10 immunohistochemistry and their histology compared. Groups are compared by semi-quantitative scoring. Results: Histological activation of endomysial CD10+ FAPs includes prominent expansion of a network of cell processes surrounding muscle fibers, as well as endomysial cell clusters evidencing proliferation. Prominence of periarteriolar processes is a notable feature in some pathologies. FAP activation is often associated with fiber degeneration/regeneration, foci of inflammation, and denervation in keeping with experimental results. Type 2 atrophy shows no evidence of FAP activation. Dermatomyositis and anti-synthetase syndrome associated myositis demonstrate diffuse activation. Conclusion: Assessment of CD10+ FAP activation is routinely possible using CD10 immunohistochemistry and demonstrates several patterns in keeping with preclinical results. Prominent expansion of FAP processes surrounding myofibers suggests enhanced interaction between myofiber/basement membranes and FAPs during activation. The presence of diffuse FAP activation in dermatomyositis biopsies unrelated to fiber repair raises the possibility of FAP activation as part of the autoimmune process. Future diagnostic applications, clinical significance and therapeutic potential remain to be elucidated.

Regulation of mitochondrial morphology and cristae architecture by the TLR4 pathway in human skeletal muscle.

In skeletal muscle (SkM), a reduced mitochondrial elongate phenotype is associated with several metabolic disorders like type 2 diabetes mellitus (T2DM). However, the mechanisms contributing to this reduction in mitochondrial elongate phenotype in SkM have not been fully elucidated. It has recently been shown in a SkM cell line that toll-like receptor 4 (TLR4) contributes to the regulation of mitochondrial morphology. However, this has not been investigated in human SkM. Here we found that in human SkM biopsies, TLR4 protein correlated negatively with Opa1 (pro-mitochondrial fusion protein). Moreover, the incubation of human myotubes with LPS reduced mitochondrial size and elongation and induced abnormal mitochondrial cristae, which was prevented with the co-incubation of LPS with TAK242. Finally, T2DM myotubes were found to have reduced mitochondrial elongation and mitochondrial cristae density. Mitochondrial morphology, membrane structure, and insulin-stimulated glucose uptake were restored to healthy levels in T2DM myotubes treated with TAK242. In conclusion, mitochondrial morphology and mitochondrial cristae seem to be regulated by the TLR4 pathway in human SkM. Those mitochondrial alterations might potentially contribute to insulin resistance in the SkM of patients with T2DM.

Twenty-four hour rhythmicity in mitochondrial network connectivity and mitochondrial respiration; a study in human skeletal muscle biopsies of young lean and older individuals with obesity

ObjectiveMitochondrial network dynamics may play role in metabolic homeostasis. Whether mitochondrial network dynamics are involved in adaptations to day–night fluctuations in energy supply and demand is unclear. Here we visualized and quantified the mitochondrial network morphology in human skeletal muscle of young healthy lean and older individuals with obesity over the course of 24 h MethodsMuscle biopsies taken at 5 timepoints over a 24-hour period obtained from young healthy lean and older metabolically impaired obese males were analyzed for mitochondrial network integrity with confocal laser scanning microscopy. Variation of level of fragmentation over the course of the day were aligned with variation of mitochondrial respiration over the day ResultsYoung healthy lean individuals displayed a day–night rhythmicity in mitochondrial network morphology, which aligned with the day–night rhythmicity of mitochondrial respiratory capacity, with a more fused network coinciding with higher mitochondrial respiratory capacity. In the older individuals with obesity, the mitochondrial network was more fragmented overall compared to young healthy lean individuals and completely lacked 24 h rhythmicity, which was also true for the mitochondrial respiratory capacity ConclusionsOur data shows a paralleled rhythmicity between mitochondrial network morphology and mitochondrial oxidative capacity, which oscillates over the course of a mimicked real-life day in human skeletal muscle of young, healthy lean individuals. In older individuals with obesity, the lack of a 24-hour rhythmicity in mitochondrial network connectivity was also aligned with a lack in respiratory capacity. This suggests that 24-hour rhythmicity in mitochondrial network connectivity is a determinant of rhythmicity in mitochondrial respiratory capacity. Thus, restoring mitochondrial network integrity may promote mitochondrial respiratory capacity and hence contribute to blunting the metabolic aberrations in individuals with a disturbed 24-hour rhythmicity in metabolism, like older individuals with obesity.

Scanning electrochemical microscopy for the stimulation and investigation of human skeletal muscle-derived mesenchymal stem/stromal cells

IntroductionThe human skeletal muscle-derived mesenchymal stem/stromal cells (SM-MSCs) possess myogenic differentiation potential and participate in the muscle regeneration process. However, the question of how to improve human skeletal muscle regeneration remains actual. In this study, the total SM-MSCs population was separated into subpopulations according to the neural cell adhesion molecule (NCAM, CD56), stimulated with an alternating electric field (AC) using scanning electrochemical microscopy (AC-SECM) and the intracellular redox changes, and myogenic differentiation markers were evaluated. MethodsThe total SM-MSC population was isolated from the postoperative human skeletal muscle biopsies by an enzymatic digestion method. The myogenic differentiation of the total SM-MSCs population before and after AC stimulation was evaluated immunohistochemically by the levels of desmin and myogenin. The effect of AC stimulation on the redox capacity of CD56(+) and CD56(-) cell subpopulations as well as on the level of myogenin on indium tin oxide (ITO) surface were also investigated. ResultsThe total SM-MSCs population grown on a glass coverslip weakly responded to AC stimulus, i.e. the level of desmin was slightly increased by the 3rd day of differentiation, while in the not stimulated cells, it increased only by the 7th day. The level of myogenin did not change after AC stimulation. However, the CD56(+) and CD56(-) subpopulations had different redox activities and myogenic differentiation potential: the CD56(+) cells had stronger natural diffusion and were more redox active compared to the CD56(-) cells; the redox activity of CD56(+) cells was more actively stimulated by an alternating electric field than in CD56(-) cells; at control level, the CD56(+) cells had more myogenic differentiation-regulating transcription factor myogenin, which was more intensively stimulated by AC than in CD56(-) cells. Data show that the total population of human SM-MSCs is heterogeneous with different regenerating potential cells that do not equally respond to extracellular stimuli. The SECM can be used in both ways: (i) for extracellular stimulation and (ii) for the investigation of intracellular redox changes of the human SM-MSCs or their subpopulations allowing a deeper understanding of the mechanisms mediating skeletal tissue regeneration both in vitro and in vivo.

Development and Validation of a Western Blot Method to Quantify Mini-Dystrophin in Human Skeletal Muscle Biopsies

Duchenne muscular dystrophy (DMD) is a degenerative muscular disease affecting roughly one in 5000 males at birth. The disease is often caused by inherited X-linked recessive pathogenic variants in the dystrophin gene, but may also arise from de novo mutations. Disease-causing variants include nonsense, out of frame deletions or duplications that result in loss of dystrophin protein expression. There is currently no cure for DMD and the few treatment options available aim at slowing muscle degradation. New advances in gene therapy and understanding of dystrophin (DYS) expression in other muscular dystrophies have opened new opportunities for treatment. Therefore, reliable methods are needed to monitor dystrophin expression and assess the efficacy of new therapies for muscular dystrophies such as DMD and Becker muscular dystrophy (BMD). Here, we describe the validation of a novel Western blot (WB) method for the quantitation of mini-dystrophin protein in human skeletal muscle tissues that is easy to adopt in most laboratory settings. This WB method was assessed through precision, accuracy, selectivity, dilution linearity, stability, and repeatability. Based on mini-DYS standard performance, the assay has a dynamic range of 0.5–15 ng protein (per 5 µg total protein per lane), precision of 3.3 to 25.5%, and accuracy of − 7.5 to 3.3%. Our stability assessment showed that the protein is stable after 4 F/T cycles, up to 2 h at RT and after 7 months at − 70°C. Furthermore, our WB method was compared to the results from our recently published LC–MS method.Graphical Workflow for our quantitative WB method to determine mini-dystrophin levels in muscle tissues (created in Biorender.com). Step 1 involves protein extraction from skeletal muscle tissue lysates from control, DMD, or BMD biospecimen. Step 2 measures total protein concentrations. Step 3 involves running gel electrophoresis with wild-type dystrophin (wt-DYS) from muscle tissue extracts alongside mini-dystrophin STD curve and mini-DYS and protein normalization with housekeeping GAPDH.

Exercise increases phosphorylation of the putative mTORC2 activity readout NDRG1 in human skeletal muscle.

In mice, exercise is suggested to activate the mechanistic target of rapamycin complex 2 (mTORC2) in skeletal muscle, and mTORC2 is required for normal muscle glucose uptake during exercise. Whether this translates to human skeletal muscle and what signaling pathways facilitate the exercise-induced mTORC2 activation is unknown. We herein tested the hypothesis that exercise increases mTORC2 activity in human skeletal muscle and investigated if β2-adrenergic receptor (AR) activation mediates exercise-induced mTORC2 activation. We examined several mTORC2 activity readouts (p-NDRG1 Thr346, p-Akt Ser473, p-mTOR S2481, and p-Akt Thr450) in human skeletal muscle biopsies after uphill walking or cycling exercise. In mouse muscles, we assessed mTORC2 activity readouts following acute activation of muscle β2-adrenergic or GS signaling and during in vivo and ex vivo muscle contractions. Exercise increased phosphorylation of NDRG1 Thr346 in human soleus, gastrocnemius, and vastus lateralis muscle, without changing p-Akt Ser473, p-Akt Thr450, and p-mTOR Ser2481. In mouse muscle, stimulation of β2-adrenergic or GS signaling and ex vivo contractions failed to increase p-NDRG1 Thr346, whereas in vivo contractions were sufficient to induce p-NDRG1 Thr346. In conclusion, the mTORC2 activity readout p-NDRG1 Thr346 is a novel exercise-responsive signaling protein in human skeletal muscle. Notably, contraction-induced p-NDRG1 Thr346 appears to require a systemic factor. Unlike exercise, and in contrast to published data obtained in cultured muscles cells, stimulation of β2-adrenergic signaling is not sufficient to trigger NDRG1 phosphorylation in mature mouse skeletal muscle.NEW & NOTEWORTHY The mTORC2 readout p-NDRG Thr346 is a novel exercise-responsive protein in human skeletal muscle. β2-AR and GS signaling are not sufficient to induce mTORC2 signaling in adult muscle. In vivo, but not ex vivo, contraction induced p-NDRG Thr346, which indicates requirement of a systemic factor for exercise-induced mTORC2 activation.

Isolation of human fibroadipogenic progenitors and satellite cells from frozen muscle biopsies.

Skeletal muscle contains multiple cell types that work together to maintain tissue homeostasis. Among these, satellite cells (SC) and fibroadipogenic progenitors cells (FAPs) are the two main stem cell pools. Studies of these cells using animal models have shown the importance of interactions between these cells in repair of healthy muscle, and degeneration of dystrophic muscle. Due to the unavailability of fresh patient muscle biopsies, similar analysis of interactions between human FAPs and SCs is limited especially among the muscular dystrophy patients. To address this issue here we describe a method that allows the use of frozen human skeletal muscle biopsies to simultaneously isolate and grow SCs and FAPs from healthy or dystrophic patients. We show that while the purified SCs differentiate into mature myotubes, purified FAPs can differentiate into adipocytes or fibroblasts demonstrating their multipotency. We find that these FAPs can be immortalized and the immortalized FAPs (iFAPs) retain their multipotency. These approaches open the door for carrying out personalized analysis of patient FAPs and interactions with the SCs that lead to muscle loss.

Site-specific acetylation of adenine nucleotide translocase 1 at lysine 23 in human muscle

Evidence suggests acetylation of human adenine nucleotide translocase 1 (ANT1) at lysine 23 (Lys23) reduces binding of ADP. Lys23 contributes to the positive charge that facilitates this interaction. This study was undertaken to characterize ANT1 abundance and acetylation by a novel method using small amounts of human skeletal muscle biopsies. Lysates of whole muscle or mitochondria from the same tissue were prepared from needle biopsies of vastus lateralis muscle of healthy volunteers. Lysed proteins were resolved on gels, the section containing ANT1 (surrounding 30 Kd) was excised, digested with trypsin, spiked with labeled unacetylated and acetylated synthetic standard peptides and analyzed by mass spectrometry. Natural logarithm transformation of data linearized ion intensities over a 10-fold range of peptide mass. Coefficients of variation ranged from 7 to 30% for ANT1 abundance and Lys23 acetylation. In three volunteers, ANT1 content was 8.36 ± 0.33 nmol/g wet weight muscle and 0.64 ± 0.05 nmol/mg mitochondria, so mitochondrial content was 13.3 ± 2.4 mg mitochondria per gram muscle. Acetylation of Lys23 averaged 14.3 ± 4.2% and 4.87 ± 1.84% in whole muscle and mitochondria, respectively. This assay makes it possible to assess effects of acetylation on the function of ANT1 in human muscle.

Global Deletion of 11β-HSD1 Prevents Muscle Wasting Associated with Glucocorticoid Therapy in Polyarthritis.

Glucocorticoids provide indispensable anti-inflammatory therapies. However, metabolic adverse effects including muscle wasting restrict their use. The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1) modulates peripheral glucocorticoid responses through pre-receptor metabolism. This study investigates how 11β-HSD1 influences skeletal muscle responses to glucocorticoid therapy for chronic inflammation. We assessed human skeletal muscle biopsies from patients with rheumatoid arthritis and osteoarthritis for 11β-HSD1 activity ex vivo. Using the TNF-α-transgenic mouse model (TNF-tg) of chronic inflammation, we examined the effects of corticosterone treatment and 11β-HSD1 global knock-out (11βKO) on skeletal muscle, measuring anti-inflammatory gene expression, muscle weights, fiber size distribution, and catabolic pathways. Muscle 11β-HSD1 activity was elevated in patients with rheumatoid arthritis and correlated with inflammation markers. In murine skeletal muscle, glucocorticoid administration suppressed IL6 expression in TNF-tg mice but not in TNF-tg11βKO mice. TNF-tg mice exhibited reductions in muscle weight and fiber size with glucocorticoid therapy. In contrast, TNF-tg11βKO mice were protected against glucocorticoid-induced muscle atrophy. Glucocorticoid-mediated activation of catabolic mediators (FoxO1, Trim63) was also diminished in TNF-tg11βKO compared to TNF-tg mice. In summary, 11β-HSD1 knock-out prevents muscle atrophy associated with glucocorticoid therapy in a model of chronic inflammation. Targeting 11β-HSD1 may offer a strategy to refine the safety of glucocorticoids.

Isolation and characterization of muscle stem cells, fibro-adipogenic progenitors, and macrophages from human skeletal muscle biopsies.

Animal models clearly illustrate that the maintenance of skeletal muscle mass depends on the function and interaction of a heterogeneous population of resident and infiltrating mononuclear cells. Several lines of evidence suggest that mononuclear cells also play a role in muscle wasting in humans, and targeting these cells may open new treatment options for intervention or prevention in sarcopenia. Methodological and ethical constraints have perturbed exploration of the cellular characteristics and function of mononuclear cells in human skeletal muscle. Thus, investigations of cellular phenotypes often depend on immunohistochemical analysis of small tissue samples obtained by needle biopsies, which do not match the deep phenotyping of mononuclear cells obtained from animal models. Here, we have developed a protocol for fluorescence-activated cell sorting (FACS), based on single-cell RNA-sequencing data, for quantifying and characterizing mononuclear cell populations in human skeletal muscle. Muscle stem cells, fibro-adipogenic progenitors, and two subsets of macrophages (CD11c+/-) are present in needle biopsies in comparable quantities per milligram tissue to open surgical biopsies. We find that direct cell isolation is preferable due to a substantial shift in transcriptome when using preculture before the FACS procedure. Finally, in vitro validation of the cellular phenotype of muscle stem cells, fibro-adipogenic progenitors, and macrophages confirms population-specific traits. This study demonstrates that mononuclear cell populations can be quantified and subsequently analyzed from needle biopsy material and opens the perspective for future clinical studies of cellular mechanisms in muscle wasting.

242-OR: Single Nuclei Skeletal Muscle Chromatin and Gene Expression Profiling across 287 Individuals Reveals Cell Type-Specific Genetic Regulatory Architectures at T2D GWAS Loci

Skeletal muscle is the largest organ, by weight (>40%), in the human body; insulin resistance in this and other peripheral tissues is a hallmark of type 2 diabetes (T2D). Recent genome wide association study (GWAS) results show enrichment for T2D and fasting insulin SNPs in skeletal muscle regulatory regions. Identifying gene regulatory mechanisms underlying these associations requires pinpointing the set of causal variants, the target genes these variants regulate, and the specific skeletal muscle cell populations in which they function. Here, we generated single nuclei chromatin (snATAC) and transcription (snRNA) profiles across 287 frozen human skeletal muscle biopsies. We integrated 414,982 nuclei across the modalities with LIGER and identified 13 cell types, ranging in abundance from 38.1% (type 1 fiber) to <1% (adipocyte) of all nuclei. We further integrated genotypes and performed expression and chromatin accessibility quantitative trait loci (e/caQTL) scans in the five most abundant cell types (type 1, type 2a, type 2x, mesenchymal stem cell, endothelial). We identified >70k associations (5% FDR) ranging from shared to cell type-specific and performed colocalization with T2D GWAS signals. Of 73 T2D GWAS signals that colocalize with caQTL, 39 (53.4%) are cell type-specific, highlighting the importance of sn-caQTL maps for GWAS functional studies. For example, in mesenchymal stem cells, we identified a cell type-specific caQTL for an ARL15 intronic peak (rs702634, p=6.6x10-10), which colocalizes with the T2D GWAS signal (coloc H4 posterior probability = 0.95). To nominate target genes for this caQTL peak, we scanned peaks in promoters of genes in the 1 Mb neighborhood and computed co-accessibility scores using Cicero in an allele-specific manner, which identified the gene FST, ~493 kb away. FST encodes for follistatin which increases muscle growth and reduces fat mass and insulin resistance. Disclosure S. Parker: None. A. Varshney: None. Fusion tissue study: n/a. Amp-t2d skeletal muscle working group: n/a. Funding American Diabetes Association/Pathway to Stop Diabetes (1-14-INI-07 to S.P.); National Institutes of Health (R01DK117960, U01DK06237017, UM1DK126185)

FGF-2-Dependent Signaling Activated in Aged Human Skeletal Muscle Promotes Intramuscular Adipogenesis

Abstract Background: A unique feature of muscle during aging, obesity and type 2 diabetes is the appearance of adipose tissue between skeletal muscle fibers, the intramuscular adipose tissue (IMAT). IMAT is generally associated with systemic insulin resistance, decreased muscle strength and, in older adults, impaired mobility. A large body of work over the last years have addressed the origin of IMAT. Skeletal muscle contains preadipocyte progenitors termed fibro/adipogenic progenitors (FAPs) that normally do not form adipocytes, but proliferate during muscle injury to support the commitment of myogenic progenitor cells during muscle repair. However, under the conditions outlined above, FAPs differentiate to adipocytes and give rise to IMAT. The molecular cues that trigger this pathogenesis are unknown and strategies that can modulate the fate of FAPs and their propensity to differentiate to adipocytes in disease states are urgently needed. Methods: We have used molecular cloning and genome editing techniques together with a variety of biochemical, molecular, genomic, transcriptomic and proteomic analyses in pre-adipocyte and muscle cell lines, primary cells isolated from human and mouse skeletal muscle, human skeletal muscle biopsies, and animal models. Results: Using multiple screening assays upstream and downstream of a master regulator of myogenesis, microRNA (miR)-29a, we located the secreted protein and adipogenic inhibitor SPARC to an FGF-2 signaling pathway that is conserved between skeletal muscle cells from mice and humans and that is activated in skeletal muscle of aged mice and humans. We identified FGF-2 as an upstream regulator that increases miR-29a promoter activity and gene expression via MEK1/2/MAPK signaling and consequently decreases the conserved miR-29a target SPARC. FGF-2 induces the miR-29a/SPARC axis through transcriptional activation of FRA-1, which binds and activates an evolutionary conserved AP-1 site element proximal in the miR-29a promoter. CRISPR/Cas9-mediated genomic deletions of miR-29a recognition elements located in the Sparc 3′UTR of muscle cells and AAV-mediated overexpression of FGF-2 or SPARC in mouse skeletal muscle revealed that this axis regulates differentiation of FAPs in vitro and IMAT formation in vivo. Skeletal muscle from human donors aged > 75 years versus < 55 years showed activation of FGF-2-dependent signaling and increased IMAT. Conclusion: FGF-2 not only stimulates muscle growth, but also promotes intramuscular adipogenesis via targeting the fate of FAPs. Thus, our data highlights a disparate role of FGF-2 in adult skeletal muscle and reveals a novel pathway, which has the potential to provide innovative diagnostic and therapeutic approaches to combat fat accumulation in aged human skeletal muscle.

A single session of high‐intensity interval exercise in hypoxia did not change mitochondrial respiration or content in human skeletal muscle biopsies

Both high-intensity interval exercise (HIIE) and hypoxia have been suggested to affect mitochondria. However, it is not yet fully elucidated how a single session of HIIE in hypoxia affects mitochondrial respiration and content in human skeletal muscle. In this current study, we investigated the effects of a single session of HIIE combined with simulated hypoxia (3200 m, oxygen fraction of 14%) on mitochondrial respiration and content. We hypothesized that HIIE in hypoxia would induce substantial changes in mitochondrial respiration, but not in mitochondrial content. Ten healthy male participants (age, 28 ± 5 years; BMI, 26.0 ± 3.4) were recruited and completed three HIIE sessions (six 4-minute exercise bouts, separated by 2 minutes of resting) in a random order, separated by at least one week. Exercise sessions include an HIIE in hypoxia (88.4% of peak oxygen uptake (VO2peak) determined in hypoxia), an HIIE in normoxia matched for relative intensity (88.7% of VO2peak determined in normoxia), and an HIIE in normoxia matched for absolute intensity (74.1% of VO2peak determined in normoxia). Skeletal muscle samples were collected prior to, 3 hours, and 24 hours post-exercise to determine mitochondrial respiration and content. Hypoxia decreased VO2peak (by 20.2 ± 9.1%, p < 0.01), peak power output (PPO, by 9.4 ± 2.1%, p<0.01), and lactate threshold (LT, by 13.1 ± 3.0%, p<0.01). In the skeletal muscle biopsies, mitochondrial respiration, measured by Oroboros O2k with high-resolution respirometry, was similar among the three conditions prior to the HIIE interventions (p > 0.05) and remained unchanged both 3 and 24 hours after HIIE interventions performed in different conditions (p > 0.05). Citrate synthase activity, as a marker of mitochondrial content, did not differ prior to, or 3 and 24 hours after HIIE interventions among the three conditions (p > 0.05) . In conclusion, a single session of HIIE in hypoxia did not alter mitochondrial respiration and content. Gene expression and protein content of mitochondrial biogenesis markers are currently being analyzed.

Congenital muscular dystrophy-associated inflammatory chemokines provide axes for effective recruitment of therapeutic adult stem cell into muscles

BackgroundCongenital muscular dystrophies (CMD) are a clinically and genetically heterogeneous group of neuromuscular disorders characterized by muscle weakness. The two most prevalent forms of CMD, collagen VI-related myopathies (COL6RM) and laminin α2 deficient CMD type 1A (MDC1A), are both caused by deficiency or dysfunction of extracellular matrix proteins. Previously, we showed that an intramuscular transplantation of human adipose-derived stem cells (ADSC) into the muscle of the Col6a1−/− mice results in efficient stem cell engraftment, migration, long-term survival, and continuous production of the collagen VI protein, suggesting the feasibility of the systemic cellular therapy for COL6RM. In order for this therapeutic approach to work however, stem cells must be efficiently targeted to the entire body musculature. Thus, the main goal of this study is to test whether muscle homing of systemically transplanted ADSC can be enhanced by employing muscle-specific chemotactic signals originating from CMD-affected muscle tissue.MethodsProteomic screens of chemotactic molecules were conducted in the skeletal muscles of COL6RM- and MDC1A-affected patients and CMD mouse models to define the inflammatory and immune activities, thus, providing potential markers of disease activity or treatment effect. Also using a pre-clinical animal model, recapitulating mild Ullrich congenital muscular dystrophy (UCMD), the therapeutic relevance of identified chemotactic pathways was investigated in vivo, providing a basis for future clinical investigations.ResultsComprehensive proteomic screens evaluating relevant human and mouse skeletal muscle biopsies offered chemotactic axes to enhance directional migration of systemically transplanted cells into CMD-affected muscles, including CCL5-CCR1/3/5, CCL2-CCR2, CXCL1/2-CXCR1,2, and CXCL7-CXCR2. Also, the specific populations of ADSC selected with an affinity for the chemokines being released by damaged muscle showed efficient migration to injured site and presented their therapeutic effect.ConclusionsCollectively, identified molecules provided insight into the mechanisms governing directional migration and intramuscular trafficking of systemically infused stem cells, thus, permitting broad and effective application of the therapeutic adult stem cells for CMD treatment.

Collection of Skeletal Muscle Biopsies from the Superior Compartment of Human Musculus Tibialis Anterior for Mechanical Evaluation.

The mechanical properties of contracting skeletal fibers are crucial indicators of overall muscle health, function, and performance. Human skeletal muscle biopsies are often collected for these endeavors. However, relatively few technical descriptions of biopsy procedures, outside of the commonly used musculus vastus lateralis, are available. Although the biopsy techniques are often adjusted to accommodate the characteristics of each muscle under study, few technical reports share these changes to the greater community. Thus, muscle tissue from human participants is often wasted as the operator reinvents the wheel. Expanding the available material on biopsies from a variety of muscles can reduce the incident of failed biopsies. This technical report describes a variation of the modified Bergström technique on the musculus tibialis anterior that limits fiber damage and provides fiber lengths adequate for mechanical evaluation. The surgery is an outpatient procedure that can be completed in an hour. The recovery period for this procedure is immediate for light activity (i.e., walking), up to three days for the resumption of normal physical activity, and about one week for wound care. The extracted tissue can be used for mechanical force experiments and here we present representative activation data. This protocol is appropriate for most collection purposes, potentially adaptable to other skeletal muscles, and may be improved by modifications to the collection needle.

Prospective isolation of human fibroadipogenic progenitors with CD73

Skeletal muscle relies on coordination between myogenic and non-myogenic interstitial cells for homeostasis and for regeneration and response to injury. Fibroadipogenic progenitors (FAPs) have recently been recognized as key modulators of signaling to promote myogenesis following injury. FAPs are also responsible for the fibrosis and fatty replacement of muscle tissue seen in many diseased states. While extensive use of surface markers to purify FAPs has been undertaken in the mouse system, in particular PDGFRA, markers for human FAPs are less well understood. Here, we show that CD73 can be used as a single positive marker to purify FAPs from the lineage-negative (CD45-neg, CD31-neg) fraction of skeletal muscle mononuclear cells. Although CD73 was previously found to be expressed in cultured myogenic cells, we find that this marker is only acquired upon culture and that the CD73+ fraction of human skeletal muscle has no myogenic activity. We show that Lin-neg CD73+ cells from human muscle undergo fat differentiation as well as fibrogenesis when exposed to appropriate activating signals in vitro. This simple single positive marker approach effectively enables isolation of human FAPs from fresh human skeletal muscle biopsies.

1883-P: Metformin Increases Protein Phosphatase 2A Activity in Primary Skeletal Muscle Cells Derived from Lean Healthy Insulin-Sensitive Human Participants

Skeletal muscle insulin resistance is one of the primary contributors of type 2 diabetes (T2D). Metformin is the first-line drug for the treatment of T2D. Main known effects of metformin include inhibition of glucose production in liver and attenuation of insulin resistance in skeletal muscle. However, molecular mechanism of metformin’s action in skeletal muscle is not well understood. Protein phosphatase 2A (PP2A), a major serine/threonine phosphatase, plays a pivotal role in cellular processes (e.g., signal transduction, apoptosis), and acts through dephosphorylating key signaling molecules such as AKT, AMPK, etc. However, whether or not PP2A plays a role in metformin-induced insulin sensitivity improvement in human skeletal muscle cells remains to be elucidated. In the present study, hyperinsulinemic-euglycemic clamp was performed to assess insulin sensitivity in human subjects and skeletal muscle biopsy samples were obtained. Primary human skeletal muscle cells (shown to retain metabolic characteristics of donors) were cultured from these muscle biopsies from 8 lean insulin sensitive participants (4 Female and 4 male; age: 21.4±0.8 yrs; BMI: 22.1±0.7 kg/m2; 2-hour OGTT: 96.5± 6.3 mg/dl; HbA1c: 5.3±0.1 %; fasting plasma glucose: 87.5±1.4 mg/dl; M-value; 10.9±0.8 mg/kgBW/min). The cells were expanded, differentiated into myotubes, and treated with 50µM metformin (the physiologic concentration) for 24 hours before harvesting. PP2A activity was measured by a phosphatase activity assay (Millipore). The results indicated that metformin increased the activity of PP2A in the myotubes derived from all 8 participants, and the average fold increase is 1.59±0.13 (p&amp;lt;0.05). These results provided the first evidence to suggest that metformin promotes activation of PP2A in human skeletal muscle cells derived from lean insulin sensitive participants, and may help to understand metformin’s action in skeletal muscle in humans. Disclosure A.H. Mestareehi: None. B. Seyoum: None. Z. Msallaty: None. A.P. Mallisho: None. X. Zhang: None. A. Kowluru: None. Z. Yi: None. Funding American Diabetes Association (1-13-TS-27 to Z.Y.); National Institutes of Health (R01DK107666, R01DK081750 to Z.Y.), (T32HL120822 to A.H.M.); DOTS Scholars (to A.H.M., Z.Y.)

MicroRNA-204-5p modulates mitochondrial biogenesis in C2C12 myotubes and associates with oxidative capacity in humans.

Using an unbiased high‐throughput microRNA (miRNA)‐silencing screen combined with functional readouts for mitochondrial oxidative capacity in C2C12 myocytes, we previously identified 19 miRNAs as putative regulators of skeletal muscle mitochondrial metabolism. In the current study, we highlight miRNA‐204‐5p, identified from this screen, and further studied its role in the regulation of skeletal muscle mitochondrial function. Following silencing of miRNA‐204‐5p in C2C12 myotubes, gene and protein expression were assessed using quantitative polymerase chain reaction, microarray analysis, and western blot analysis, while morphological changes were studied by confocal microscopy. In addition, miRNA‐204‐5p expression was quantified in human skeletal muscle biopsies and associated with in vivo mitochondrial oxidative capacity. Transcript levels of PGC‐1α (3.71‐fold; p < .01), predicted as an miR‐204‐5p target, as well as mitochondrial DNA copy number (p < .05) and citrate synthase activity (p = .06) were increased upon miRNA‐204‐5p silencing in C2C12 myotubes. Silencing of miRNA‐204‐5p further resulted in morphological changes, induced gene expression of autophagy marker light chain 3 protein b (LC3B; q = .05), and reduced expression of the mitophagy marker FUNDC1 (q = .01). Confocal imaging revealed colocalization between the autophagosome marker LC3B and the mitochondrial marker OxPhos upon miRNA‐204‐5p silencing. Finally, miRNA‐204‐5p was differentially expressed in human subjects displaying large variation in oxidative capacity and its expression levels associated with in vivo measures of skeletal muscle mitochondrial function. In summary, silencing of miRNA‐204‐5p in C2C12 myotubes stimulated mitochondrial biogenesis, impacted on cellular morphology, and altered expression of markers related to autophagy and mitophagy. The association between miRNA‐204‐5p and in vivo mitochondrial function in human skeletal muscle further identifies miRNA‐204‐5p as an interesting modulator of skeletal muscle mitochondrial metabolism.