Effects of low-load resistance training with blood flow restriction on muscle fiber myofibrillar and extracellular area

Blood flow restriction applied during low-load resistance training (LL-BFR) induces a similar increase in the cross-sectional area of muscle fibers (fCSA) compared to traditional high-load resistance training (HL-RT). However, it is unclear whether LL-BFR leads to differential changes in myofibrillar spacing in muscle fibers and/or extracellular area compared to HL-RT. Therefore, this study aimed to investigate whether the hypertrophy of type I and II fibers induced by LL-BFR or HL-RT is accompanied by differential changes in myofibrillar and non-myofibrillar areas. In addition, we examined if extracellular spacing was differentially affected between these two training protocols. Twenty recreationally active participants were assigned to LL-BFR or HL-RT groups and underwent a 6-week training program. Muscle biopsies were taken before and after the training period. The fCSA of type I and II fibers, the area occupied by myofibrillar and non-myofibrillar components, and extracellular spacing were analyzed using immunohistochemistry techniques. Despite the significant increase in type II and mean (type I + II) fCSA (p < 0.05), there were no significant changes in the proportionality of the myofibrillar and non-myofibrillar areas [∼86% and ∼14%, respectively (p > 0.05)], indicating that initial adaptations to LL-BFR are primarily characterized by conventional hypertrophy rather than disproportionate non-myofibrillar expansion. Additionally, extracellular spacing was not significantly altered between protocols. In summary, our study reveals that LL-BFR, like HL-RT, induces skeletal muscle hypertrophy with proportional changes in the areas occupied by myofibrillar, non-myofibrillar, and extracellular components.

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.

FiNuTyper: Design and validation of an automated deep learning-based platform for simultaneous fiber and nucleus type analysis in human skeletal muscle

AIM

While manual quantification is still considered the gold standard for skeletal muscle histological analysis, it is time-consuming and prone to investigator bias. To address this challenge, we assembled an automated image analysis pipeline, FiNuTyper (Fiber and Nucleus Typer).

METHODS

We integrated recently developed deep learning-based image segmentation methods, optimized for unbiased evaluation of fresh and postmortem human skeletal muscle, and utilized SERCA1 and SERCA2 as type-specific myonucleus and myofiber markers after validating them against the traditional use of MyHC isoforms.

RESULTS

Parameters including cross-sectional area, myonuclei per fiber, myonuclear domain, central myonuclei per fiber, and grouped myofiber ratio were determined in a fiber-type-specific manner, revealing that a large degree of sex- and muscle-related heterogeneity could be detected using the pipeline. Our platform was also tested on pathological muscle tissue (ALS and IBM) and adapted for the detection of other resident cell types (leucocytes, satellite cells, capillary endothelium).

CONCLUSION

In summary, we present an automated image analysis tool for the simultaneous quantification of myofiber and myonuclear types, to characterize the composition and structure of healthy and diseased human skeletal muscle.

Exercise reprograms the inflammatory landscape of multiple stem cell compartments during mammalian aging

Exercise has the ability to rejuvenate stem cells and improve tissue regeneration in aging animals. However, the cellular and molecular changes elicited by exercise have not been systematically studied across a broad range of cell types in stem cell compartments. We subjected young and old mice to aerobic exercise and generated a single-cell transcriptomic atlas of muscle, neural, and hematopoietic stem cells with their niche cells and progeny, complemented by whole transcriptome analysis of single myofibers. We found that exercise ameliorated the upregulation of a number of inflammatory pathways associated with old age and restored aspects of intercellular communication mediated by immune cells within these stem cell compartments. Exercise has a profound impact on the composition and transcriptomic landscape of circulating and tissue-resident immune cells. Our study provides a comprehensive view of the coordinated responses of multiple aged stem cells and niche cells to exercise at the transcriptomic level.

AUF1 gene transfer increases exercise performance and improves skeletal muscle deficit in adult mice

Muscle function and mass begin declining in adults long before evidence of sarcopenia and include reduced mitochondrial function, although much remains to be characterized. We found that mRNA decay factor AU-rich mRNA binding factor 1 (AUF1), which stimulates myogenesis, is strongly reduced in skeletal muscle of adult and older mice in the absence of evidence of sarcopenia. Muscle-specific adeno-associated virus (AAV)8-AUF1 gene therapy increased expression of AUF1, muscle function, and mass. AAV8 AUF1 muscle gene transfer in 12-month-old mice increased the levels of activated muscle stem (satellite) cells, increased muscle mass, reduced markers of muscle atrophy, increased markers of mitochondrial content and muscle fiber oxidative capacity, and enhanced exercise performance to levels of 3-month-old mice. With wild-type and AUF1 knockout mice and cultured myoblasts, AUF1 supplementation of muscle fibers was found to increase expression of Peroxisome Proliferator-activated Receptor Gamma Co-activator 1-alpha (PGC1α), a major effector of skeletal muscle mitochondrial oxidative metabolism. AUF1 stabilized and increased translation of the pgc1α mRNA, which is strongly reduced in adult muscle in the absence of AUF1 supplementation. Skeletal muscle-specific gene transfer of AUF1 therefore restores muscle mass, increases exercise endurance, and may provide a therapeutic strategy for age-related muscle loss.

Exploring the Role of Skeletal Muscle in Insulin Resistance: Lessons from Cultured Cells to Animal Models

Skeletal muscle is essential to maintain vital functions such as movement, breathing, and thermogenesis, and it is now recognized as an endocrine organ. Muscles release factors named myokines, which can regulate several physiological processes. Moreover, skeletal muscle is particularly important in maintaining body homeostasis, since it is responsible for more than 75% of all insulin-mediated glucose disposal. Alterations of skeletal muscle differentiation and function, with subsequent dysfunctional expression and secretion of myokines, play a key role in the pathogenesis of obesity, type 2 diabetes, and other metabolic diseases, finally leading to cardiometabolic complications. Hence, a deeper understanding of the molecular mechanisms regulating skeletal muscle function related to energy metabolism is critical for novel strategies to treat and prevent insulin resistance and its cardiometabolic complications. This review will be focused on both cellular and animal models currently available for exploring skeletal muscle metabolism and endocrine function.

Degenerative and regenerative pathways underlying Duchenne muscular dystrophy revealed by single-nucleus RNA sequencing

Duchenne muscular dystrophy (DMD) is a fatal muscle disorder characterized by cycles of degeneration and regeneration of multinucleated myofibers and pathological activation of a variety of other muscle-associated cell types. The extent to which different nuclei within the shared cytoplasm of a myofiber may display transcriptional diversity and whether individual nuclei within a multinucleated myofiber might respond differentially to DMD pathogenesis is unknown. Similarly, the potential transcriptional diversity among nonmuscle cell types within dystrophic muscle has not been explored. Here, we describe the creation of a mouse model of DMD caused by deletion of exon 51 of the dystrophin gene, which represents a prevalent disease-causing mutation in humans. To understand the transcriptional abnormalities and heterogeneity associated with myofiber nuclei, as well as other mononucleated cell types that contribute to the muscle pathology associated with DMD, we performed single-nucleus transcriptomics of skeletal muscle of mice with dystrophin exon 51 deletion. Our results reveal distinctive and previously unrecognized myonuclear subtypes within dystrophic myofibers and uncover degenerative and regenerative transcriptional pathways underlying DMD pathogenesis. Our findings provide insights into the molecular underpinnings of DMD, controlled by the transcriptional activity of different types of muscle and nonmuscle nuclei.

Effect of Silymarin Supplementation on Physical Performance, Muscle and Myocardium Histological Changes, Bodyweight, and Food Consumption in Rats Subjected to Regular Exercise Training

(1) Background: Regular exercise induces physiological and morphological changes in the organisms, but excessive training loads may induce damage and impair recovery or muscle growth. The purpose of the study was to evaluate the impact of Silymarin (SM) consumption on endurance capacity, muscle/cardiac histological changes, bodyweight, and food intake in rats subjected to 60 min of regular exercise training (RET) five days per week. (2) Methods: Male Wistar rats were subjected to an eight-week RET treadmill program and were previously administered SM and vitamin C. Bodyweight and food consumption were measured and registered. The maximal endurance capacity (MEC) test was performed at weeks one and eight. After the last training session, the animals were sacrificed, and samples of quadriceps/gastrocnemius and cardiac tissue were obtained and process for histological analyzes. (3) Results: SM consumption improved muscle recovery, inflammation, and damaged tissue, and promoted hypertrophy, vascularization, and muscle fiber shape/appearance. MEC increased after eight weeks of RET in all trained groups; moreover, the SM-treated group was enhanced more than the group with vitamin C. There were no significant changes in bodyweight and in food and nutrient consumption along the study. (5) Conclusion: SM supplementation may enhance physical performance, recovery, and muscle hypertrophy during the eight-week RET program.

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.

Microtubules provide guidance cues for myofibril and sarcomere assembly and growth

BACKGROUND

Muscle myofibrils and sarcomeres present exceptional examples of highly ordered cytoskeletal filament arrays, whose distinct spatial organization is an essential aspect of muscle cell functionality. We utilized ultra-structural analysis to investigate the assembly of myofibrils and sarcomeres within developing myotubes of the indirect flight musculature of Drosophila.

RESULTS

A temporal sequence composed of three major processes was identified: subdivision of the unorganized cytoplasm of nascent, multi-nucleated myotubes into distinct organelle-rich and filament-rich domains; initial organization of the filament-rich domains into myofibrils harboring nascent sarcomeric units; and finally, maturation of the highly-ordered pattern of sarcomeric thick (myosin-based) and thin (microfilament-based) filament arrays in parallel to myofibril radial growth. Significantly, organized microtubule arrays were present throughout these stages and exhibited dynamic changes in their spatial patterns consistent with instructive roles. Genetic manipulations confirm these notions, and imply specific and critical guidance activities of the microtubule-based cytoskeleton, as well as structural interdependence between the myosin- and actin-based filament arrays.

CONCLUSIONS

Our observations highlight a surprisingly significant, behind-the-scenes role for microtubules in establishment of myofibril and sarcomere spatial patterns and size, and provide a detailed account of the interplay between major cytoskeletal elements in generating these essential contractile myogenic units.

Culture conditions influence satellite cell activation and survival of single myofibers

Single myofiber isolation protocols allow to obtain an in vitro system in which the physical association between the myofiber and its stem cells, the satellite cells, is adequately preserved. This technique is an indispensable tool by which the muscle regeneration process can be recapitulated and studied in each specific phase, from satellite cell activation to proliferation, from differentiation to fusion. This study aims to clarify the effect of different culture conditions on single myofibers, their associated satellite cells, and the physiological behavior of the satellite cells upon long term culture. By direct observations of the cultures, we compared different experimental conditions and their effect on both satellite cell behavior and myofiber viability.

Isolation, cultivation and immunostaining of single myofibers: An improved approach to study the behavior of satellite cells

Satellite cells are myogenic cells responsible for muscle growth shortly after birth and muscle repair/regeneration during adulthood. Therapies based on satellite cells hold promise for treating muscular dysfunctions. Studying satellite cells is technically challenging owing to their low abundance, small size and anatomical dispersed location between the basal lamina and the sarcolemma of myofibers. In this article, we present three improved protocol strategies for studying the properties of satellite cells of the mouse during the different stages of muscle regeneration: (1) immunostaining of freshly isolated single myofibers to facilitate the study of quiescent satellite cells, (2) cultivation of single myofibers on Matrigel®-coated dish to study the myogenesis programs initiated by satellite cell activation, and (3) cultivation of single myofibers in floating conditions to analyze activated satellite cells or the doubling time of satellite cells in myofibers. In brief, when compared to previously published protocols, this article presented an improved protocol that requires shorter experimental time and less laborious approach for higher yield of intact single myofibers for downstream analyses.

Metabolic control of myofibers: promising therapeutic target for obesity and type 2 diabetes

Mammalian skeletal muscles are composed of two major fibre types (I and II) that differ in terms of size, metabolism and contractile properties. In general, slow-twitch type I fibres are rich in mitochondria and have a greater insulin sensitivity than fast-twitch type II skeletal muscles. Although not widely appreciated, a forced induction of the slow skeletal muscle phenotype may inhibit the progress of obesity and diabetes. This potentially forms the basis for targeting slow/oxidative myofibers in the treatment of obesity. In this context, a better understanding of the molecular basis of fibre-type specification and plasticity may help to identify potential therapeutic targets for obesity and diabetes.

Myogenic Satellite Cells: Biological Milieu and Possible Clinical Applications

Adult skeletal muscle is a post-mitotic terminally differentiated tissue that possesses an immense potential for regeneration after injury. This regeneration can be achieved by adult stem cells named satellite cells that inhabit the muscular tissue. These cells were first identified in 1961 and were described as being wedged between the plasma membrane of the muscle fiber and the surrounding basement membrane. Since their discovery, many researchers investigated their embryological origin and the exact role they play in muscle regeneration and repair. Under normal conditions, satellite cells are retained in a quiescent state and when required, these cells are activated to proliferate and differentiate to repair pre-existing muscle fibers or to a lesser extent fuse with each other to form new myofibers. During skeletal muscle regeneration, satellite cell actions are regulated through a cascade of complex signaling pathways that are influenced by multiple extrinsic factors within the satellite cell micro-environment. Here, the basic concepts were studied about satellite cells, their development, function, distribution and the different cellular and molecular mechanisms that regulate these cells. The recent findings about some of their clinical applications and potential therapeutic use were also discussed.

Muscle-specific androgen receptor deletion shows limited actions in myoblasts but not in myofibers in different muscles in vivo

The aim of this study was to investigate the direct muscle cell-mediated actions of androgens by comparing two different mouse lines. The cre-loxP system was used to delete the DNA-binding activity of the androgen receptor (AR) in mature myofibers (MCK mAR(ΔZF2)) in one model and the DNA-binding activity of the AR in both proliferating myoblasts and myofibers (α-actin mAR(ΔZF2)) in another model. We found that hind-limb muscle mass was normal in MCK mAR(ΔZF2) mice and that relative mass of only some hind-limb muscles was reduced in α-actin mAR(ΔZF2) mice. This suggests that myoblasts and myofibers are not the major cellular targets mediating the anabolic actions of androgens on male muscle during growth and development. Levator ani muscle mass was decreased in both mouse lines, demonstrating that there is a myofiber-specific effect in this unique androgen-dependent muscle. We found that the pattern of expression of genes including c-myc, Fzd4 and Igf2 is associated with androgen-dependent changes in muscle mass; therefore, these genes are likely to be mediators of anabolic actions of androgens. Further research is required to identify the major targets of androgen actions in muscle, which are likely to include indirect actions via other tissues.

Attenuation of p38α MAPK stress response signaling delays the in vivo aging of skeletal muscle myofibers and progenitor cells

Functional competence and self-renewal of mammalian skeletal muscle myofibers and progenitor cells declines with age. Progression of the muscle aging phenotype involves the decline of juvenile protective factors i.e., proteins whose beneficial functions translate directly to the quality of life, and self-renewal of progenitor cells. These characteristics occur simultaneously with the age-associated increase of p38α stress response signaling. This suggests that the maintenance of low levels of p38α activity of juvenile tissues may delay or attenuate aging. We used the dominant negative haploinsufficient p38α mouse (DN-p38α^AF/+) to demonstrate that in vivo attenuation of p38α activity in the gastrocnemius of the aged mutant delays age-associated processes that include: a) the decline of the juvenile protective factors, BubR1, aldehyde dehydrogenase 1A (ALDH1A1), and aldehyde dehydrogenase 2 (ALDH2); b) attenuated expression of p16^Ink4a and p19^Arf tumor suppressor genes of the Cdkn2a locus; c) decreased levels of hydroxynonenal protein adducts, expression of COX2 and iNOS; d) decline of the senescent progenitor cell pool level and d) the loss of gastrocnemius muscle mass. We propose that elevated P-p38α activity promotes skeletal muscle aging and that the homeostasis of p38α impacts the maintenance of a beneficial healthspan.

Lower maternal body condition during pregnancy affects skeletal muscle structure and glut-4 protein levels but not glucose tolerance in mature adult sheep

Suboptimal maternal nutrition and body composition are implicated in metabolic disease risk in adult offspring. We hypothesized that modest disruption of glucose homeostasis previously observed in young adult sheep offspring from ewes of a lower body condition score (BCS) would deteriorate with age, due to changes in skeletal muscle structure and insulin signaling mechanisms. Ewes were fed to achieve a lower (LBCS, n = 10) or higher (HBCS, n = 14) BCS before and during pregnancy. Baseline plasma glucose, glucose tolerance and basal glucose uptake into isolated muscle strips were similar in male offspring at 210 ± 4 weeks. Vastus total myofiber density (HBCS, 343 ± 15; LBCS, 294 ± 14 fibers/mm(2), P < .05) and fast myofiber density (HBCS, 226 ± 10; LBCS 194 ± 10 fibers/mm(2), P < .05), capillary to myofiber ratio (HBCS, 1.5 ± 0.1; LBCS 1.2 ± 0.1 capillary:myofiber, P < .05) were lower in LBCS offspring. Vastus protein levels of Akt1 were lower (83% ± 7% of HBCS, P < .05), and total glucose transporter 4 was increased (157% ± 6% of HBCS, P < .001) in LBCS offspring, Despite the reduction in total myofiber density in LBCS offspring, glucose tolerance was normal in mature adult life. However, such adaptations may lead to complications in metabolic control in an overabundant postnatal nutrient environment.

Autologous myoblasts attenuate atrophy and improve tongue force in a denervated tongue model: a pilot study

OBJECTIVES/HYPOTHESIS

Autologous muscle-derived stem cell (MdSC) therapy is a promising treatment to restore function. No group has evaluated MdSC therapy in a denervated tongue model. The purpose of this pilot investigation was to determine the extent of autologous MdSC survival, effects on tongue muscle atrophy, maximal contractile force, and lingual pressure in a denervated ovine tongue model.

STUDY DESIGN

Pilot animal experiment.

METHODS

Bilateral implantable cuff electrodes were placed around the hypoglossal nerves in two Dorper cross ewes. Tensometer and high-resolution manometry (HRM) testing were performed during supermaximum hypoglossal nerve stimulation to assess baseline tongue strength. Sternocleidomastoid muscle biopsies were acquired to create autologous MdSC cultures. At 1 month, 5 × 10(8) green fluorescent protein (GFP)-labeled autologous MdSCs were injected into the partially denervated tongue. Two-months postinjection, lingual tensometer testing, HRM, and postmortem histological assessment were performed.

RESULTS

GFP+ myofibers were identified in denervated tongue specimens indicating MdSC survival. Muscle fiber diameter was larger in GFP+ fibers for both tongue specimens, suggesting attenuation of muscle atrophy. Myofiber diameter was larger in GFP+ myofibers than preinjury diameters, providing evidence of new muscle formation. These myogenic changes led to a 27% increase in maximal tongue contractile force and a 54% increase in maximum base of tongue pressure in one animal.

CONCLUSIONS

Autologous MdSC therapy may be a viable treatment for the partially denervated tongue, with current findings demonstrating that injected MdSCs survived and fused with tongue myofibers, with a resultant increase in myofiber diameter and an increase in tongue strength.

LEVEL OF EVIDENCE

N/A.

Moderate aging does not modulate morphological responsiveness of the neuromuscular system to chronic overload in Fischer 344 rats

The purpose of this investigation was to determine the effect of aging on neuromuscular adaptations to chronic overload. Eight young adult (8 months old) and eight aged (22 months old) Fischer 344 rats underwent unilateral synergist ablation to overload the plantaris and soleus muscles of that hindlimb and to provide control muscles from the contralateral hindlimb. Cytofluorescent staining and confocal microscopy were used to quantify pre- and post-synaptic features of neuromuscular junctions (NMJs). Histochemical staining and light microscopy were used to assess adaptations of myofibers to chronic overload. Results demonstrate that NMJs of young adult and aged muscles did not undergo morphological remodeling as a result of 4 weeks of chronic overload. In contrast, myofibers of young and aged rats displayed significant (P<0.05), but similar hypertrophy ( approximately 18%) following that 4 week intervention. In both age groups, however, this hypertrophy was detected in the plantaris, but not the soleus. These data indicate that moderate aging (the equivalent of 65 years in human lifetime) does not modify the sensitivity of the neuromuscular system to chronic overload.