A glitch in the matrix: the pivotal role for extracellular matrix remodeling during muscle hypertrophy

Beta2-Adrenergic Stimulation Induces Resistance Training-Like Adaptations in Human Skeletal Muscle: Potential Role of KLHL41

Skeletal muscle mass plays a pivotal role in metabolic function, but conditions such as bed rest or injury often render resistance training impractical. The beta2-adrenergic receptor has been highlighted as a potential target to promote muscle hypertrophy and treat atrophic conditions. Here, we investigate the proteomic changes associated with beta2-adrenergic-mediated muscle hypertrophy, using resistance training as a hypertrophic comparator. We utilize MS-based proteomics to map skeletal muscle proteome remodeling in response to beta2-adrenergic stimulation or resistance training as well as cell model validation. We report that beta2-adrenergic stimulation mimics multiple features of resistance training in proteome-wide remodeling, comprising systematic upregulation of ribosomal subunits and concomitant downregulation of mitochondrial proteins. Approximately 20% of proteins were regulated in both conditions, comprising proteins involved in steroid metabolism (AKR1C1, AKR1C2, AKRC1C3), protein-folding (SERPINB1), and extracellular matrix organization (COL1A1, COL1A2). Among overall most significantly upregulated proteins were kelch-like family members (KLHL) 40 and 41. In follow-up experiments, we identify KLHL41 as having novel implications for beta2-adrenergic-mediated muscle hypertrophy. Treating C2C12 cells with beta2-agonist for 96 h increased myotube diameter by 48% (p < 0.001). This anabolic effect was abolished by prior knockdown of KLHL41. Using siRNA, KLHL41 abundance was decreased by 60%, and the anabolic response to beta2-agonist was diminished (+ 15%, i.e., greater in the presence of KLHL41, knock-down × treatment: p = 0.004). In conclusion, protein-wide remodeling induced by beta2-adrenergic stimulation mimics multiple features of resistance training, and thus the beta2-adrenergic receptor may be a target with therapeutic potential in the treatment of muscle wasting conditions without imposing mechanical load.

Muscle memory in humans: evidence for myonuclear permanence and long-term transcriptional regulation after strength training

The objective of this work was to investigate myonuclear permanence and transcriptional regulation as mechanisms for cellular muscle memory after strength training in humans. Twelve untrained men and women performed 10 weeks of unilateral elbow-flexor strength training followed by 16 weeks of de-training. Thereafter, 10 weeks' re-training was conducted with both arms: the previously trained arm and the contralateral untrained control arm. Muscle biopsies were taken from the trained arm before and after both training periods and from the control arm before and after re-training. Muscle biopsies were analysed for fibre cross-sectional area (fCSA), myonuclei and global transcriptomics (RNA sequencing). During the first training period, myonuclei increased in type 1 (13 ± 17%) and type 2 (33 ± 23%) fibres together with a 30 ± 43% non-significant increase in mixed fibre fCSA (P = 0.069). Following de-training, fCSA decreased in both fibre types, whereas myonuclei were maintained, resulting in 33% higher myonuclear number in previously trained vs. control muscle in type 2 fibres. Furthermore, in the previously trained muscle, three differentially expressed genes (DEGs; EGR1, MYL5 and COL1A1) were observed. Following re-training, the previously trained muscle showed larger type 2 fCSA compared to the control (P = 0.035). However, delta change in type 2 fCSA was not different between muscles. Gene expression was more dramatically changed in the control arm (1338 DEGs) than in the previously trained arm (822 DEGs). The sustained higher number of myonuclei in the previously trained muscle confirms myonuclear accretion and permanence in humans. Nevertheless, because of the unclear effect on the subsequent hypertrophy with re-training, the physiological benefit remains to be determined. KEY POINTS: Muscle memory is a cellular mechanism that describes the capacity of skeletal muscle fibres to respond differently to training stimuli if the stimuli have been previously encountered. This study overcomes past methodological limitations related to the choice of muscles and analytical procedures. We show that myonuclear number is increased after strength training and maintained during de-training. Increased myonuclear number and differentially expressed genes related to muscle performance and development in the previously trained muscle did not translate into a clearly superior responses during re-training. Because of the unclear effect on the subsequent hypertrophy and muscle strength gain with re-training, the physiological benefit remains to be determined.

Post-natal muscle growth and protein turnover: a narrative review of current understanding

A model explaining the dietary-protein-driven post-natal skeletal muscle growth and protein turnover in the rat is updated, and the mechanisms involved are described, in this narrative review. Dietary protein controls both bone length and muscle growth, which are interrelated through mechanotransduction mechanisms with muscle growth induced both from stretching subsequent to bone length growth and from internal work against gravity. This induces satellite cell activation, myogenesis and remodelling of the extracellular matrix, establishing a growth capacity for myofibre length and cross-sectional area. Protein deposition within this capacity is enabled by adequate dietary protein and other key nutrients. After briefly reviewing the experimental animal origins of the growth model, key concepts and processes important for growth are reviewed. These include the growth in number and size of the myonuclear domain, satellite cell activity during post-natal development and the autocrine/paracrine action of IGF-1. Regulatory and signalling pathways reviewed include developmental mechanotransduction, signalling through the insulin/IGF-1-PI3K-Akt and the Ras-MAPK pathways in the myofibre and during mechanotransduction of satellite cells. Likely pathways activated by maximal-intensity muscle contractions are highlighted and the regulation of the capacity for protein synthesis in terms of ribosome assembly and the translational regulation of 5-TOPmRNA classes by mTORC1 and LARP1 are discussed. Evidence for and potential mechanisms by which volume limitation of muscle growth can occur which would limit protein deposition within the myofibre are reviewed. An understanding of how muscle growth is achieved allows better nutritional management of its growth in health and disease.

Time-course and muscle-specific gene expression of matrix metalloproteinases and inflammatory cytokines in response to acute treadmill exercise in rats

BACKGROUND

The extracellular matrix (ECM) of skeletal muscle plays a pivotal role in tissue repair and growth, and its remodeling tightly regulated by matrix metalloproteinases (MMPs), tissue inhibitors of metalloproteinases (TIMPs), and inflammatory cytokines. This study aimed to investigate changes in the mRNA expression of MMPs (Mmp-2 and Mmp-14), TIMPs (Timp-1 and Timp-2), and inflammatory cytokines (Il-1β, Tnf-α, and Tgfβ1) in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats following acute treadmill exercise. Additionally, muscle morphology was examined using hematoxylin and eosin (H&E) staining.

METHODS AND RESULTS

Male rats were subjected to acute treadmill exercise at 25 m/min for 60 min with a %0 slope. The mRNA expression of ECM components and muscle morphology in the SOL and EDL were assessed in both sedentary and exercise groups at various time points (immediately (0) and 1, 3, 6, 12, and 24 h post-exercise). Our results revealed a muscle-specific response, with early upregulation of the mRNA expression of Mmp-2, Mmp-14, Timp-1, Timp-2, Il-1β, and Tnf-α observed in the SOL compared to the EDL. A decrease in Tgfβ1 mRNA expression was evident in the SOL at all post-exercise time points. Conversely, Tgfβ1 mRNA expression increased at 0 and 3 h post-exercise in the EDL. Histological analysis also revealed earlier cell infiltration in the SOL than in the EDL following acute exercise.

CONCLUSIONS

Our results highlight how acute exercise modulates ECM components and muscle structure differently in the SOL and EDL muscles, leading to distinct muscle-specific responses.

Engineering Stem Cell Fate Controlling Biomaterials to Develop Muscle Connective Tissue Layered Myofibers

Skeletal muscle connective tissue (MCT) surrounds myofiber bundles to provide structural support, produce force transduction from tendons, and regulate satellite cell differentiation during muscle regeneration. Engineered muscle tissue composed of myofibers layered within MCT has not yet been developed. Herein, a bioengineering strategy to create MCT-layered myofibers through the development of stem cell fate-controlling biomaterials that achieve both myogenesis and fibroblast differentiation in a locally controlled manner at the single construct is introduced. The reciprocal role of transforming growth factor-beta 1 (TGF-β1) and its inhibitor as well as 3D matrix stiffness to achieve co-differentiation of MCT fibroblasts and myofibers from a human-induced pluripotent stem cell (hiPSC)-derived paraxial mesoderm is studied. To avoid myogenic inhibition, TGF-β1 is conjugated on the gelatin-based hydrogel to control the fibroblasts' populations locally; the TGF-β1 degrades after 2 weeks, resulting in increased MCT-specific extracellular matrix (ECM) production. The locations of myofibers and fibroblasts are precisely controlled by using photolithography and co-axial wet spinning techniques, which results in the formation of MCT-layered functional myofibers in 3D constructs. This advanced engineering strategy is envisioned as a possible method for obtaining biomimetic human muscle grafts for various biomedical applications.

The roles of miRNAs in adult skeletal muscle satellite cells

Satellite cells are bona fide muscle stem cells that are indispensable for successful post-natal muscle growth and regeneration after severe injury. These cells also participate in adult muscle adaptation in several capacities. MicroRNAs (miRNAs) are post-transcriptional regulators of mRNA that are implicated in several aspects of stem cell function. There is evidence to suggest that miRNAs affect satellite cell behavior in vivo during development and myogenic progenitor behavior in vitro, but the role of miRNAs in adult skeletal muscle satellite cells is less studied. In this review, we provide evidence for how miRNAs control satellite cell function with emphasis on satellite cells of adult skeletal muscle in vivo. We first outline how miRNAs are indispensable for satellite cell viability and control the phases of myogenesis. Next, we discuss the interplay between miRNAs and myogenic cell redox status, senescence, and communication to other muscle-resident cells during muscle adaptation. Results from recent satellite cell miRNA profiling studies are also summarized. In vitro experiments in primary myogenic cells and cell lines have been invaluable for exploring the influence of miRNAs, but we identify a need for novel genetic tools to further interrogate how miRNAs control satellite cell behavior in adult skeletal muscle in vivo.

Extracellular matrix contribution to disease progression and dysfunction in myopathy

Myopathic processes affect skeletal muscle and heart. In the muscular dystrophies, which are a subset of myopathies, muscle cells are gradually replaced by fibrosis and fat, impairing muscle function as well as regeneration and repair. In addition to skeletal muscle, these genetic disorders often also affect the heart, where fibrofatty infiltration progressively accumulates in the myocardium, impairing heart function. Although considerable effort has focused on gene-corrective and gene-replacement approaches to stabilize myofibers and cardiomyocytes, the continual and ongoing deposition of extracellular matrix itself contributes to tissue and organ dysfunction. Transcriptomic and proteomic profiling, along with high-resolution imaging and biophysical measurements, have been applied to define extracellular matrix components and their role in contributing to cardiac and skeletal muscle weakness. More recently, decellularization methods have been adapted to an on-slide format to preserve the spatial geography of the extracellular matrix, allowing new insight into matrix remodeling and its direct role in suppressing regeneration in muscle. This review highlights recent literature with focus on the extracellular matrix and molecular mechanisms that contribute to muscle and heart fibrotic disorders. We will also compare how the myopathic matrix differs from healthy matrix, emphasizing how the pathological matrix contributes to disease.

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

Mechanisms underlying mechanical overload-induced skeletal muscle hypertrophy have been extensively researched since the landmark report by Morpurgo (1897) of "work-induced hypertrophy" in dogs that were treadmill trained. Much of the preclinical rodent and human resistance training research to date supports that involved mechanisms include enhanced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling, an expansion in translational capacity through ribosome biogenesis, increased satellite cell abundance and myonuclear accretion, and postexercise elevations in muscle protein synthesis rates. However, several lines of past and emerging evidence suggest that additional mechanisms that feed into or are independent of these processes are also involved. This review first provides a historical account of how mechanistic research into skeletal muscle hypertrophy has progressed. A comprehensive list of mechanisms associated with skeletal muscle hypertrophy is then outlined, and areas of disagreement involving these mechanisms are presented. Finally, future research directions involving many of the discussed mechanisms are proposed.

Intravenous Injection of Sodium Hyaluronate Diminishes Basal Inflammatory Gene Expression in Equine Skeletal Muscle

Following strenuous exercise, skeletal muscle experiences an acute inflammatory state that initiates the repair process. Systemic hyaluronic acid (HA) is injected to horses routinely as a joint anti-inflammatory. To gain insight into the effects of HA on skeletal muscle, adult Thoroughbred geldings (n = 6) were injected with a commercial HA product weekly for 3 weeks prior to performing a submaximal exercise test. Gluteal muscle (GM) biopsies were obtained before and 1 h after exercise for gene expression analysis and HA localization. The results from RNA sequencing demonstrate differences in gene expression between non-injected controls (CON; n = 6) and HA horses. Prior to exercise, HA horses contained fewer (p < 0.05) transcripts associated with leukocyte activity and cytokine production than CON. The performance of exercise resulted in the upregulation (p < 0.05) of several cytokine genes and their signaling intermediates, indicating that HA does not suppress the normal inflammatory response to exercise. The transcript abundance for marker genes of neutrophils (NCF2) and macrophages (CD163) was greater (p < 0.05) post-exercise and was unaffected by HA injection. The anti-inflammatory effects of HA on muscle are indirect as no differences (p > 0.05) in the relative amount of the macromolecule was observed between the CON and HA fiber extracellular matrix (ECM). However, exercise tended (p = 0.10) to cause an increase in ECM size suggestive of muscle damage and remodeling. The finding was supported by the increased (p < 0.05) expression of CTGF, TGFβ1, MMP9, TIMP4 and Col4A1. Collectively, the results validate HA as an anti-inflammatory aid that does not disrupt the normal post-exercise muscle repair process.

Extracellular Matrix Proteomics: The mdx-4cv Mouse Diaphragm as a Surrogate for Studying Myofibrosis in Dystrophinopathy

The progressive degeneration of the skeletal musculature in Duchenne muscular dystrophy is accompanied by reactive myofibrosis, fat substitution, and chronic inflammation. Fibrotic changes and reduced tissue elasticity correlate with the loss in motor function in this X-chromosomal disorder. Thus, although dystrophinopathies are due to primary abnormalities in the DMD gene causing the almost-complete absence of the cytoskeletal Dp427-M isoform of dystrophin in voluntary muscles, the excessive accumulation of extracellular matrix proteins presents a key histopathological hallmark of muscular dystrophy. Animal model research has been instrumental in the characterization of dystrophic muscles and has contributed to a better understanding of the complex pathogenesis of dystrophinopathies, the discovery of new disease biomarkers, and the testing of novel therapeutic strategies. In this article, we review how mass-spectrometry-based proteomics can be used to study changes in key components of the endomysium, perimysium, and epimysium, such as collagens, proteoglycans, matricellular proteins, and adhesion receptors. The mdx-4cv mouse diaphragm displays severe myofibrosis, making it an ideal model system for large-scale surveys of systematic alterations in the matrisome of dystrophic fibers. Novel biomarkers of myofibrosis can now be tested for their appropriateness in the preclinical and clinical setting as diagnostic, pharmacodynamic, prognostic, and/or therapeutic monitoring indicators.

Muscle disuse as hindlimb unloading in early postnatal mice negatively impacts grip strength in adult mice: a pilot study

Physical inactivity has many detrimental effects on health, yet the impact of physical inactivity in early life on muscle health in adulthood remains unknown. Early postnatal malnutrition has prolonged effects into adulthood and we propose that early postnatal (P) physical inactivity would have similar negative effects. To test this hypothesis, we exposed postnatal mice (∼P28, C57BL/6J) to 14 days of physical inactivity (shortly after weaning, from ∼P28 to P42 days of age) in the form of muscle disuse with hindlimb unloading (HU). After this early-life physical inactivity, they were allowed to normally ambulate until 5 mo of age (P140, adulthood) when they underwent 14 days of HU with and without 7-day recovery. They were then tested for physical function (grip strength) and muscles were extracted and weighed. Immunofluorescence was carried out on these muscle cross sections for analysis of myofiber cross-sectional area (fCSA), macrophage density (CD68+ cells), and extracellular matrix (ECM) area. Muscle weights and fCSA and myofiber diameter were used to quantify changes in muscle and fiber size. Compared with age-matched controls, no notable effects of early-life physical inactivity (HU) on skeletal muscle and myofiber size were observed. However, a significant reduction in adult grip strength was observed in those exposed to HU early in life. This was associated with reduced muscle macrophages and increased ECM area. Exposure to a short period of early life disuse has negative enduring effects into adulthood impacting grip strength, muscle macrophages, and muscle composition as low muscle quality.NEW & NOTEWORTHY We demonstrate that early life disuse resulted in less grip strength in adulthood. Analysis of muscle composition demonstrated no loss of whole muscle or myofiber size indicating lower muscle quality akin to premature aging. This poor muscle quality was characterized by altered muscle macrophages and extracellular matrix area. We demonstrate intriguing correlations between this loss of grip strength and muscle macrophages and also area of noncontractile tissue in the muscle.

Extracellular matrix content and remodeling markers do not differ in college-aged men classified as higher and lower responders to resistance training

We determined if skeletal muscle extracellular matrix (ECM) content and remodeling markers adapted with resistance training or were associated with hypertrophic outcomes. Thirty-eight untrained males (21 ± 3 yr) participated in whole body resistance training (10 wk, 2 × weekly). Participants completed testing [ultrasound, peripheral quantitative computed tomography (pQCT)] and donated a vastus lateralis (VL) biopsy 1 wk before training and 72 h following the last training bout. Higher responders (HR, n = 10) and lower responders (LR, n = 10) were stratified based on a composite score considering changes in pQCT-derived mid-thigh cross-sectional area (mCSA), ultrasound-derived VL thickness, and mean fiber cross-sectional area (fCSA). In all participants, training reduced matrix metalloprotease (MMP)-14 protein (P < 0.001) and increased satellite cell abundance (P < 0.001); however, VL fascial thickness, ECM protein content per myofiber, MMP-2/-9 protein content, tissue inhibitor of metalloproteinase (TIMP)-1/-2 protein content, collagen-1/-4 protein content, macrophage abundance, or fibroadipogenic progenitor cell abundance were not altered. Regarding responder analysis, MMP-14 exhibited an interaction (P = 0.007), and post hoc analysis revealed higher protein content in HR versus LR before training (P = 0.026) and a significant decrease from pre to posttraining in HR only (P = 0.002). In summary, basal skeletal muscle ECM markers are minimally affected with 10 wk of resistance training, and these findings could be related to not capturing more dynamic alterations in the assayed markers earlier in training. However, the downregulation in MMP-14 in college-aged men classified as HR is a novel finding and warrants continued investigation, and further research is needed to delineate muscle connective tissue strength attributes between HR and LR.NEW & NOTEWORTHY Although past studies have examined aspects of extracellular matrix remodeling in relation to mechanical overload or resistance training, this study serves to expand our knowledge on a multitude of extracellular matrix markers and whether these markers adapt to resistance training or are associated with differential hypertrophic responses.

Limited effect of over-the-counter doses of ibuprofen on mechanisms regulating muscle hypertrophy during resistance training in young adults

We have previously shown that maximal over-the-counter doses of ibuprofen, compared with low doses of acetylsalicylic acid, reduce muscle hypertrophy in young individuals after 8 wk of resistance training. Because the mechanism behind this effect has not been fully elucidated, we here investigated skeletal muscle molecular responses and myofiber adaptations in response to acute and chronic resistance training with concomitant drug intake. Thirty-one young (aged 18-35 yr) healthy men (n = 17) and women (n = 14) were randomized to receive either ibuprofen (IBU; 1,200 mg daily; n = 15) or acetylsalicylic acid (ASA; 75 mg daily; n = 16) while undergoing 8 wk of knee extension training. Muscle biopsies from the vastus lateralis were obtained before, at week 4 after an acute exercise session, and after 8 wk of resistance training and analyzed for mRNA markers and mTOR signaling, as well as quantification of total RNA content (marker of ribosome biogenesis) and immunohistochemical analysis of muscle fiber size, satellite cell content, myonuclear accretion, and capillarization. There were only two treatment × time interaction in selected molecular markers after acute exercise (atrogin-1 and MuRF1 mRNA), but several exercise effects. Muscle fiber size, satellite cell and myonuclear accretion, and capillarization were not affected by chronic training or drug intake. RNA content increased comparably (∼14%) in both groups. Collectively, these data suggest that established acute and chronic hypertrophy regulators (including mTOR signaling, ribosome biogenesis, satellite cell content, myonuclear accretion, and angiogenesis) were not differentially affected between groups and therefore do not explain the deleterious effects of ibuprofen on muscle hypertrophy in young adults.NEW & NOTEWORTHY Here we show that mTOR signaling, fiber size, ribosome biogenesis, satellite cell content, myonuclear accretion, and angiogenesis were not differentially affected between groups undergoing 8 wk of resistance training with concomitant anti-inflammatory medication (ibuprofen versus low-dose aspirin). Atrogin-1 and MuRF-1 mRNA were more downregulated after acute exercise in the low-dose aspirin group than in the ibuprofen group. Taken together it appears that these established hypertrophy regulators do not explain the previously reported deleterious effects of high doses of ibuprofen on muscle hypertrophy in young adults.

Resistance and endurance exercise training improves muscle mass and the inflammatory/fibrotic transcriptome in a rhabdomyosarcoma model

BACKGROUND

Rhabdomyosarcoma (RMS) is an aggressive soft tissue sarcoma that most often develops in children. Chemoradiation therapy is a standard treatment modality; however, the detrimental long-term skeletal muscle consequences of this therapy in juvenile cancer survivors include muscle atrophy and fibrosis resulting in decreased physical performance. Using a novel model of murine resistance and endurance exercise training, we investigate its role in preventing the long-term effects of juvenile RMS plus therapy.

METHODS

Four-week-old male (n = 10) and female (n = 10) C57Bl/6J mice were injected with M3-9-M RMS cell into the left gastrocnemius with the right limb serving as an internal control (CON). Mice received a systemic vincristine injection and then five doses of 4.8 Gy of gamma radiation localized to the left hindlimb (RMS + Tx). Mice were then randomly divided into either sedentary (SED) or resistance and endurance exercise training (RET) groups. Changes in exercise performance, body composition, myocellular adaptations and the inflammatory/fibrotic transcriptome were assessed.

RESULTS

RET improved endurance performance (P < 0.0001) and body composition (P = 0.0004) compared to SED. RMS + Tx resulted in significantly lower muscle weight (P = 0.015) and significantly smaller myofibre cross-sectional area (CSA) (P = 0.014). Conversely, RET resulted in significantly higher muscle weight (P = 0.030) and significantly larger Type IIA (P = 0.014) and IIB (P = 0.015) fibre CSA. RMS + Tx resulted in significantly more muscle fibrosis (P = 0.028), which was not prevented by RET. RMS + Tx resulted in significantly fewer mononuclear cells (P < 0.05) and muscle satellite (stem) cells (MuSCs) (P < 0.05) and significantly more immune cells (P < 0.05) than CON. RET resulted in significantly more fibro-adipogenic progenitors (P < 0.05), a trend for more MuSCs (P = 0.076) than SED and significantly more endothelial cells specifically in the RMS + Tx limb. Transcriptomic changes revealed significantly higher expression of inflammatory and fibrotic genes in RMS + Tx, which was prevented by RET. In the RMS + Tx model, RET also significantly altered expression of genes involved in extracellular matrix turnover.

CONCLUSIONS

Our study suggests that RET preserves muscle mass and performance in a model of juvenile RMS survivorship while partially restoring cellular dynamics and the inflammatory and fibrotic transcriptome.

Depressed Protein Synthesis and Anabolic Signaling Potentiate ACL Tear-Resultant Quadriceps Atrophy

BACKGROUND

Anterior cruciate ligament (ACL) tear (ACLT) leads to protracted quadriceps muscle atrophy. Protein turnover largely dictates muscle size and is highly responsive to injury and loading. Regulation of quadriceps molecular protein synthetic machinery after ACLT has largely been unexplored, limiting development of targeted therapies.

PURPOSE

To define the effect of ACLT on (1) the activation of protein synthetic and catabolic signaling within quadriceps biopsy specimens from human participants and (2) the time course of alterations to protein synthesis and its molecular regulation in a mouse ACL injury model.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Muscle biopsy specimens were obtained from the ACL-injured and noninjured vastus lateralis of young adult humans after an overnight fast (N = 21; mean ± SD, 19 ± 5 years). Mice had their limbs assigned to ACLT or control, and whole quadriceps were collected 6 hours or 1, 3, or 7 days after injury with puromycin injected before tissue collection for assessment of relative protein synthesis. Muscle fiber size and expression and phosphorylation of protein anabolic and catabolic signaling proteins were assessed at the protein and transcript levels (RNA sequencing).

RESULTS

Human quadriceps showed reduced phosphorylation of ribosomal protein S6 (-41%) in the ACL-injured limb (P = .008), in addition to elevated phosphorylation of eukaryotic initiation factor 2α (+98%; P = .006), indicative of depressed protein anabolic signaling in the injured limb. No differences in E3 ubiquitin ligase expression were noted. Protein synthesis was lower at 1 day (P = .01 vs control limb) and 3 days (P = .002 vs control limb) after ACLT in mice. Pathway analyses revealed shared molecular alterations between human and mouse quadriceps after ACLT.

CONCLUSION

(1) Global protein synthesis and anabolic signaling deficits occur in the quadriceps in response to ACL injury, without notable changes in measured markers of muscle protein catabolism. (2) Importantly, these deficits occur before the onset of significant atrophy, underscoring the need for early intervention.

CLINICAL RELEVANCE

These findings suggest that blunted protein anabolism as opposed to increased catabolism likely mediates quadriceps atrophy after ACL injury. Thus, future interventions should aim to restore muscle protein anabolism rapidly after ACLT.

Role of Matrix Metalloproteinases in Musculoskeletal Diseases

Musculoskeletal disorders include rheumatoid arthritis, osteoarthritis, sarcopenia, injury, stiffness, and bone loss. The prevalence of these conditions is frequent among elderly populations with significant mobility and mortality rates. This may lead to extreme discomfort and detrimental effect on the patient’s health and socioeconomic situation. Muscles, ligaments, tendons, and soft tissue are vital for body function and movement. Matrix metalloproteinases (MMPs) are regulatory proteases involved in synthesizing, degrading, and remodeling extracellular matrix (ECM) components. By modulating ECM reconstruction, cellular migration, and differentiation, MMPs preserve myofiber integrity and homeostasis. In this review, the role of MMPs in skeletal muscle function, muscle injury and repair, skeletal muscle inflammation, and muscular dystrophy and future approaches for MMP-based therapies in musculoskeletal disorders are discussed at the cellular and molecule level.

Multi-transcriptome analysis following an acute skeletal muscle growth stimulus yields tools for discerning global and MYC regulatory networks

Myc is a powerful transcription factor implicated in epigenetic reprogramming, cellular plasticity, and rapid growth as well as tumorigenesis. Cancer in skeletal muscle is extremely rare despite marked and sustained Myc induction during loading-induced hypertrophy. Here, we investigated global, actively transcribed, stable, and myonucleus-specific transcriptomes following an acute hypertrophic stimulus in mouse plantaris. With these datasets, we define global and Myc-specific dynamics at the onset of mechanical overload-induced muscle fiber growth. Data collation across analyses reveals an under-appreciated role for the muscle fiber in extracellular matrix remodeling during adaptation, along with the contribution of mRNA stability to epigenetic-related transcript levels in muscle. We also identify Runx1 and Ankrd1 (Marp1) as abundant myonucleus-enriched loading-induced genes. We observed that a strong induction of cell cycle regulators including Myc occurs with mechanical overload in myonuclei. Additionally, in vivo Myc-controlled gene expression in the plantaris was defined using a genetic muscle fiber-specific doxycycline-inducible Myc-overexpression model. We determined Myc is implicated in numerous aspects of gene expression during early-phase muscle fiber growth. Specifically, brief induction of Myc protein in muscle represses Reverbα, Reverbβ, and Myh2 while increasing Rpl3, recapitulating gene expression in myonuclei during acute overload. Experimental, comparative, and in silico analyses place Myc at the center of a stable and actively transcribed, loading-responsive, muscle fiber-localized regulatory hub. Collectively, our experiments are a roadmap for understanding global and Myc-mediated transcriptional networks that regulate rapid remodeling in postmitotic cells. We provide open webtools for exploring the five RNA-seq datasets as a resource to the field.