Uncoupling protein 1 expression in adipocytes derived from skeletal muscle fibro/adipogenic progenitors is under genetic and hormonal control

Advances in the interaction between lumbar intervertebral disc degeneration and fat infiltration of paraspinal muscles: critical summarization, classification, and perspectives

More than 619 million people in the world suffer from low back pain (LBP). As two potential inducers of LBP, intervertebral disc degeneration (IVDD) and fat infiltration of paraspinal muscles (PSMs) have attracted extensive attention in recent years. So far, only one review has been presented to summarize their relationship and relevant mechanisms. Nevertheless, it has several noticeable drawbacks, such as incomplete categorization and discussion, lack of practical proposals, etc. Consequently, this paper aims to systematically summarize and classify the interaction between IVDD and fat infiltration of PSMs, thus providing a one-stop search handbook for future studies. As a result, four mechanisms of IVDD leading to fat infiltration of PSMs and three mechanisms of fat infiltration in PSMs causing IVDD are thoroughly analyzed and summarized. The typical reseaches are tabulated and evaluated from four aspects, i.e., methods, conclusions, benefits, and drawbacks. We find that IVDD and fat infiltration of PSMs is a vicious cycle that can promote the occurrence and development of each other, ultimately leading to LBP and disability. Finally, eight perspectives are proposed for future in-depth research.

Fibro-adipogenic progenitors in physiological adipogenesis and intermuscular adipose tissue remodeling

Excessive accumulation of intermuscular adipose tissue (IMAT) is a common pathological feature in various metabolic and health conditions and can cause muscle atrophy, reduced function, inflammation, insulin resistance, cardiovascular issues, and unhealthy aging. Although IMAT results from fat accumulation in muscle, the mechanisms underlying its onset, development, cellular components, and functions remain unclear. IMAT levels are influenced by several factors, such as changes in the tissue environment, muscle type and origin, extent and duration of trauma, and persistent activation of fibro-adipogenic progenitors (FAPs). FAPs are a diverse and transcriptionally heterogeneous population of stromal cells essential for tissue maintenance, neuromuscular stability, and tissue regeneration. However, in cases of chronic inflammation and pathological conditions, FAPs expand and differentiate into adipocytes, resulting in the development of abnormal and ectopic IMAT. This review discusses the role of FAPs in adipogenesis and how they remodel IMAT. It highlights evidence supporting FAPs and FAP-derived adipocytes as constituents of IMAT, emphasizing their significance in adipose tissue maintenance and development, as well as their involvement in metabolic disorders, chronic pathologies and diseases. We also investigated the intricate molecular pathways and cell interactions governing FAP behavior, adipogenesis, and IMAT accumulation in chronic diseases and muscle deconditioning. Finally, we hypothesize that impaired cellular metabolic flexibility in dysfunctional muscles impacts FAPs, leading to IMAT. A deeper understanding of the biology of IMAT accumulation and the mechanisms regulating FAP behavior and fate are essential for the development of new therapeutic strategies for several debilitating conditions.

Studying intramuscular fat deposition and muscle regeneration: insights from a comparative analysis of mouse strains, injury models, and sex differences

Intramuscular fat (IMAT) infiltration, pathological adipose tissue that accumulates between muscle fibers, is a shared hallmark in a diverse set of diseases including muscular dystrophies and diabetes, spinal cord and rotator cuff injuries, as well as sarcopenia. While the mouse has been an invaluable preclinical model to study skeletal muscle diseases, they are also resistant to IMAT formation. To better understand this pathological feature, an adequate pre-clinical model that recapitulates human disease is necessary. To address this gap, we conducted a comprehensive in-depth comparison between three widely used mouse strains: C57BL/6J, 129S1/SvlmJ and CD1. We evaluated the impact of strain, sex and injury type on IMAT formation, myofiber regeneration and fibrosis. We confirm and extend previous findings that a Glycerol (GLY) injury causes significantly more IMAT and fibrosis compared to Cardiotoxin (CTX). Additionally, females form more IMAT than males after a GLY injury, independent of strain. Of all strains, C57BL/6J mice, both females and males, are the most resistant to IMAT formation. In regard to injury-induced fibrosis, we found that the 129S strain formed the least amount of scar tissue. Surprisingly, C57BL/6J of both sexes demonstrated complete myofiber regeneration, while both CD1 and 129S1/SvlmJ strains still displayed smaller myofibers 21 days post injury. In addition, our data indicate that myofiber regeneration is negatively correlated with IMAT and fibrosis. Combined, our results demonstrate that careful consideration and exploration are needed to determine which injury type, mouse model/strain and sex to utilize as preclinical model especially for modeling IMAT formation.

Age-dependent decline of B3AR agonist-mediated activation of FAP UCP-1 expression in murine models of chronic rotator cuff repair

Amibegron, a β3-adrenergic receptor (B3AR) agonist, has recently been shown to provide therapeutic effects for chronic rotator cuff (RC) tears by inducing the expression of uncoupling protein 1 (UCP-1), a marker of brown fat, in fibroadipogenic progenitors (FAPs). However, it remains to be seen if these beneficial effects hold true with age and in older, more clinically relevant populations. This study seeks to understand the impacts of aging on the efficacy of amibegron to treat chronic RC tears. Young (4-month-old) and aged (33-month-old) C57BL/6 mice underwent a RC injury procedure with delayed repair (DR). Mice were equally randomized to receive amibegron or dimethyl sulfoxide (DMSO) treatments after repair. Functional ability was measured at baseline and 6-weeks after DR. Wet muscle weight and histology of injured and contralateral supraspinatus were also analyzed 6-weeks post-DR. For in vitro histology and real-time quantitative PCR experiments, FAPs were isolated from young and aged mice via fluorescence-activated cell sorting. Young and aged FAPs were treated with amibegron or DMSO either immediately after seeding (early exposure) or 8-days after seeding (late exposure). In vitro results showed that amibegron-mediated FAP UCP-1 expression decreases with age. In vivo data demonstrated that aged mice have a decreased responsiveness to amibegron and decreased propensity for intramuscular FAP UCP-1 expression. Further, delayed amibegron treatment with RC repair did not lead to improvements in muscle atrophy and functional outcomes. Our findings demonstrate that age and the timing of interventions play a critical role in FAP-targeted therapeutics for chronic injuries.

Fibroblast growth factor 8b (FGF-8b) enhances myogenesis and inhibits adipogenesis in rotator cuff muscle cell populations in vitro

Fatty expansion is one of the features of muscle degeneration due to muscle injuries, and its presence interferes with muscle regeneration. Specifically, poor clinical outcomes have been linked to fatty expansion in rotator cuff tears and repairs. Our group recently found that fibroblast growth factor 8b (FGF-8b) inhibits adipogenic differentiation and promotes myofiber formation of mesenchymal stem cells in vitro. This led us to hypothesize that FGF-8b could similarly control the fate of muscle-specific cell populations derived from rotator cuff muscle involved in muscle repair following rotator cuff injury. In this study, we isolate fibro-adipogenic progenitor cells (FAPs) and satellite stem cells (SCs) from rat rotator cuff muscle tissue and analyzed the effects of FGF-8b supplementation. Utilizing a cell plating protocol, we successfully isolate FAPs-rich fibroblasts (FIBs) and SCs-rich muscle progenitor cells (MPCs). Subsequently, we demonstrate that FIB adipogenic differentiation can be inhibited by FGF-8b, while MPC myogenic differentiation can be enhanced by FGF-8b. We further demonstrate that phosphorylated ERK due to FGF-8b leads to the inhibition of adipogenesis in FIBs and SCs maintenance and myofiber formation in MPCs. Together, these findings demonstrate the powerful potential of FGF-8b for rotator cuff repair by altering the fate of muscle undergoing degeneration.

Development and expansion of intramuscular adipose tissue is not dependent on UCP-1-lineage cells in mice

Accumulation of adipose tissue within and outside of skeletal muscle is associated with orthopedic injury and metabolic disease, where it is thought to impede muscle function. The close juxtaposition between this adipose and myofibers has led to hypotheses that paracrine interactions between the two regulate local physiology. Recent work suggests that intramuscular adipose tissue (IMAT) may have features of beige or brown fat, indicated by the expression of uncoupling protein-1 (UCP-1). However, this is contested by other studies. Clarification of this point is needed to inform our understanding of the relationship between IMAT and muscle health. To achieve this, we examined the effects of constitutive UCP-1+ cell ablation (UCP1-DTA) on IMAT development and homeostasis. IMAT developed normally in UCP1-DTA mice, with no significant differences in quantity compared with wild-type littermates. Likewise, IMAT accumulation in response to glycerol-induced injury was similar between genotypes, with no significant differences in adipocyte size, quantity, or dispersion. This suggests that neither physiological nor pathological IMAT express UCP-1 and that the development of IMAT does not depend on UCP-1 lineage cells. In response to β3-adrenergic stimulation, we find minor, localized UCP-1 positivity in wildtype IMAT, but the bulk of the adipocytes are unresponsive. In contrast, two depots of muscle-adjacent (epi-muscular) adipose tissue have reduced mass in UCP1-DTA mice and UCP-1 positivity in wildtype littermates, comparable to traditional beige and brown adipose depots. Taken together this evidence strongly supports a white adipose phenotype for mouse IMAT and a brown/beige phenotype for some adipose outside the muscle boundary.

Electroconductivity, a regenerative engineering approach to reverse rotator cuff muscle degeneration

Muscle degeneration is one the main factors that lead to the high rate of retear after a successful repair of rotator cuff (RC) tears. The current surgical practices have failed to treat patients with chronic massive rotator cuff tears (RCTs). Therefore, regenerative engineering approaches are being studied to address the challenges. Recent studies showed the promising outcomes of electroactive materials (EAMs) on the regeneration of electrically excitable tissues such as skeletal muscle. Here, we review the most important biological mechanism of RC muscle degeneration. Further, the review covers the recent studies on EAMs for muscle regeneration including RC muscle. Finally, we will discuss the future direction toward the application of EAMs for the augmentation of RCTs.

Intermuscular adipose tissue in obesity and related disorders: cellular origins, biological characteristics and regulatory mechanisms

Intermuscular adipose tissue (IMAT) is a unique adipose depot interspersed between muscle fibers (myofibers) or muscle groups. Numerous studies have shown that IMAT is strongly associated with insulin resistance and muscular dysfunction in people with metabolic disease, such as obesity and type 2 diabetes. Moreover, IMAT aggravates obesity-related muscle metabolism disorders via secretory factors. Interestingly, researchers have discovered that intermuscular brown adipocytes in rodent models provide new hope for obesity treatment by acting on energy dissipation, which inspired researchers to explore the underlying regulation of IMAT formation. However, the molecular and cellular properties and regulatory processes of IMAT remain debated. Previous studies have suggested that muscle-derived stem/progenitor cells and other adipose tissue progenitors contribute to the development of IMAT. Adipocytes within IMAT exhibit features that are similar to either white adipocytes or uncoupling protein 1 (UCP1)-positive brown adipocytes. Additionally, given the heterogeneity of skeletal muscle, which comprises myofibers, satellite cells, and resident mesenchymal progenitors, it is plausible that interplay between these cellular components actively participate in the regulation of intermuscular adipogenesis. In this context, we review recent studies associated with IMAT to offer insights into the cellular origins, biological properties, and regulatory mechanisms of IMAT. Our aim is to provide novel ideas for the therapeutic strategy of IMAT and the development of new drugs targeting IMAT-related metabolic diseases.

Parathyroid Hormone Inhibits Fatty Infiltration and Muscle Atrophy After Rotator Cuff Tear by Browning of Fibroadipogenic Progenitors in a Rodent Model

BACKGROUND

Progressive fatty infiltration and muscle atrophy after rotator cuff tears lead to tendon repair failure and poor outcomes. Fibro-adipogenic progenitors (FAPs) are involved in fatty infiltration and muscle homeostasis of skeletal muscle. Inducing FAP differentiation into brown adipocyte-like "beige adipocytes" suppresses fatty infiltration and muscle atrophy.

HYPOTHESIS

Parathyroid hormone (PTH) suppresses fatty infiltration and muscle atrophy after rotator cuff tears in a rat model by browning of FAPs.

STUDY DESIGN

Controlled laboratory study.

METHODS

PTH was administered subcutaneously for 4 or 8 weeks to a rotator cuff tear model in rats. After treatment, fatty infiltration of supraspinatus muscles was assessed using Oil Red O staining and muscle atrophy using wet muscle weight and muscle fiber cross-sectional area. Costaining of platelet-derived growth factor receptor α (FAP marker) and uncoupling protein 1 (browning marker) was performed to confirm FAP browning by PTH. Mouse-isolated FAPs were cultured with PTH and evaluated for browning-related gene expression and adipogenic differentiation using BODIPY staining. Myogenic differentiation of C2C12 myoblasts was evaluated using coculture of PTH-treated browning FAPs with C2C12.

RESULTS

PTH inhibited fatty infiltration after rotator cuff tear at 8 weeks. Rotator cuff wet muscle loss of PTH-treated rats was inhibited at 4 and 8 weeks. Furthermore, PTH-treated rats demonstrated larger myofiber cross-sectional area than did untreated rats at 4 and 8 weeks. Costaining indicated colocalization of platelet-derived growth factor receptor α and uncoupling protein 1 and promoted PTH-induced FAP browning. PTH increased the expression of browning-related genes in FAPs and suppressed fat droplet accumulation in vitro. Coculture with PTH-treated FAPs promoted C2C12 cell differentiation into myotubes.

CONCLUSION

PTH induced FAP-derived beige adipocytes by upregulating browning-related gene expression, and the browning effect of PTH on FAPs inhibited fatty infiltration and muscle atrophy in the rat rotator cuff tear model. PTH might have potential as a therapeutic drug for fatty infiltration and muscle atrophy after rotator cuff tears.

CLINICAL RELEVANCE

PTH may expand treatment options for rotator cuff tears by reducing fatty infiltration and muscle atrophy after rotator cuff tears by browning of FAPs.

Anti-obesity effects of aerobic exercise along with Rosa canina seed extract supplementation in rats: The role of irisin and adipolin

Aerobic exercise and some bioactive compounds in medicinal plants have anti-obesity effects and can suppress body weight. The aim of this study was to determine the anti-obesity effects of 6 weeks of aerobic exercise (AE) and supplementation of the hydroalcoholic extract of Rosa canina fruit seed (RC) in obese male rats. In this experimental study, 24 high-fat diet (HFD) obese male Wistar rats were used. The animals were randomly divided into 4 groups (6 rat in group), including 1. HFD (the control group), 2. HFD + AE, 3. HFD + RC and 4. HFD + AE + RC. An obesity protocol was implemented for 12 weeks with the consumption of HFD along with the consumption of water containing 1 % fructose. Afterwards, the animals were given access only to HFD food until the end of the study in all the groups. After the obesity protocol, 6 weeks of exercise (50-70 % VO₂ max) and access to the extract (1 % of the consumed food) were given. Bodyweight, subcutaneous adipose tissue mass, and some serum lipid profiles were measured in the experimental groups. The serum levels of irisin and adipolin were evaluated by the ELISA method. Expression of FNDC5 and CTRP12 in adipose tissue were determined by real-time PCR. The findings of this study showed that body weight (P = 0.001), subcutaneous adipose tissue mass (P = 0.001), and lipid profile were significantly reduced in HFD + AE and HFD + AE + RC groups compared with the HFD group. Irisin was significantly increased in the HFD + AE and HFD + AE + RC groups compared with the HFD group (P = 0.019 and P = 0.001; respectively) and in the HFD + AE + RC group compared with the HFD + RC group (P = 0.004). Moreover, adipolin, expression of FNDC5 and CTRP12 were significantly increased in the HFD + AE + RC group compared with the HFD group (P = 0.004, P = 0.023, and P = 0.001; respectively). Altogether, HFD + AE with HFD + RC diet supplementation could reduce weight and the risks of obesity, at least, through the up-regulation of irisin and adipolin.

Bidirectional roles of skeletal muscle fibro-adipogenic progenitors in homeostasis and disease

Sarcopenia and myopathies cause progressive muscle weakness and degeneration, which are closely associated with fat infiltration and fibrosis in muscle. Recently, experimental research has shed light on fibro-adipogenic progenitors (FAPs), also known as muscle-resident mesenchymal progenitors with multiple differentiation potential for adipogenesis, fibrosis, osteogenesis and chondrogenesis. They are considered key regulators of muscle homeostasis and integrity. They play supportive roles in muscle development and repair by orchestrating the regulatory interplay between muscle stem cells (MuSCs) and immune cells. Interestingly, FAPs also contribute to intramuscular fat infiltration, fibrosis and other pathologies when the functional integrity of the network is compromised. In this review, we summarize recent insights into the roles of FAPs in maintenance of skeletal muscle homeostasis, and discuss the underlying mechanisms regulating FAPs behavior and fate, highlighting their roles in participating in efficient muscle repair and fat infiltrated muscle degeneration as well as during muscle atrophy. We suggest that controlling and predicting FAPs differentiation may become a promising strategy to improve muscle function and prevent irreparable muscle damage.

Oral Spermidine Targets Brown Fat and Skeletal Muscle to Mitigate Diet-Induced Obesity and Metabolic Disorders

INTRODUCTION

Obesity causes many life-threatening diseases. It is important to develop effective approaches for obesity treatment. Oral supplementation with spermidine retards age-related processes, but its influences on obesity and various metabolic tissues remain largely unknow. This study aims to investigate the effects of oral spermidine on brown adipose tissue (BAT) and skeletal muscle as well as its roles in counteracting obesity and metabolic disorders.

METHODS AND RESULTS

Spermidine is orally administrated into high-fat diet (HFD)-fed mice. The weight gain, insulin resistance, and hepatic steatosis are attenuated by oral spermidine in HFD-fed mice, accompanied by an alleviation of white adipose tissue inflammation. Oral spermidine promotes BAT activation and metabolic adaptation of skeletal muscle in HFD-fed mice, evidenced by UCP-1 induction and CREB activation in both tissues. Notably, oral spermidine upregulates tyrosine hydroxylase in hypothalamus of HFD-fed mice; spermidine treatment increases tyrosine hydroxylase expression and norepinephrine production in neurocytes, which leads to CREB activation and UCP-1 induction in brown adipocytes and myotubes. Spermidine also directly promotes UCP-1 and PGC-1α expression in brown adipocytes and myotubes.

CONCLUSION

Spermidine serves as an oral supplement to attenuate obesity and metabolic disorders through hypothalamus-dependent or -independent BAT activation and skeletal muscle adaptation.

FGF-2-dependent signaling activated in aged human skeletal muscle promotes intramuscular adipogenesis

Aged skeletal muscle is markedly affected by fatty muscle infiltration, and strategies to reduce the occurrence of intramuscular adipocytes are urgently needed. Here, we show that fibroblast growth factor-2 (FGF-2) not only stimulates muscle growth but also promotes intramuscular adipogenesis. Using multiple screening assays upstream and downstream of microRNA (miR)-29a signaling, 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. 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. Genetic deletions in muscle cells and adeno-associated virus-mediated overexpression of FGF-2 or SPARC in mouse skeletal muscle revealed that this axis regulates differentiation of fibro/adipogenic progenitors in vitro and intramuscular adipose tissue (IMAT) formation in vivo. Skeletal muscle from human donors aged >75 y versus <55 y showed activation of FGF-2-dependent signaling and increased IMAT. Thus, our data highlights a disparate role of FGF-2 in adult skeletal muscle and reveals a pathway to combat fat accumulation in aged human skeletal muscle.

Trichostatin A regulates fibro/adipogenic progenitor adipogenesis epigenetically and reduces rotator cuff muscle fatty infiltration

Rotator cuff (RC) muscle fatty infiltration (FI) is an important factor that determines the clinical outcome of patients with RC repair. There is no effective treatment for RC muscle FI at this time. The goal of this study is to define the role Trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor in regulating muscle fibro/adipogenic progenitors (FAPs) adipogenesis and treating muscle fatty degeneration after massive RC tears in a mouse model. We hypothesize that TSA reduces muscle FI after massive RC tears. HDAC activity was measured in FAPs in RC muscle after tendon/nerve transection or sham surgery. FAPs were treated with TSA for 2 weeks and FAP adipogenesis was evaluated with perilipin and Oil Red O staining, as well as reverse transcript-polymerase chain reaction for adipogenesis-related genes. About 0.5 mg/kg TSA or dimethyl sulfoxide was administered to C57B/L6 mice with massive rotator cuff tears through daily intraperitoneal injection for 6 weeks. Supraspinatus muscles were harvested for biochemical and histology analysis. We found that FAPs showed significantly higher HDAC activity after RC tendon/nerve transection. TSA treatment significantly reduced HDAC activity and inhibited adipogenesis of FAPs. TSA also abolished the role of bone morphogenetic protein-7 in inducing FAP adipogenesis and promoted FAP brown/beige adipose tissue (BAT) differentiation. TSA injection significantly increased histone H3 acetylation and reduced FI of rotator cuff muscles after massive tendon tears. Results from this study showed that TSA can regulate FAP adipogenesis and promote FAP BAT differentiation epigenetically. HDAC inhibition may be a new treatment strategy to reduce muscle FI after RC tears and repair.

Origins, potency, and heterogeneity of skeletal muscle fibro-adipogenic progenitors-time for new definitions

Striated muscle is a highly plastic and regenerative organ that regulates body movement, temperature, and metabolism-all the functions needed for an individual's health and well-being. The muscle connective tissue's main components are the extracellular matrix and its resident stromal cells, which continuously reshape it in embryonic development, homeostasis, and regeneration. Fibro-adipogenic progenitors are enigmatic and transformative muscle-resident interstitial cells with mesenchymal stem/stromal cell properties. They act as cellular sentinels and physiological hubs for adult muscle homeostasis and regeneration by shaping the microenvironment by secreting a complex cocktail of extracellular matrix components, diffusible cytokines, ligands, and immune-modulatory factors. Fibro-adipogenic progenitors are the lineage precursors of specialized cells, including activated fibroblasts, adipocytes, and osteogenic cells after injury. Here, we discuss current research gaps, potential druggable developments, and outstanding questions about fibro-adipogenic progenitor origins, potency, and heterogeneity. Finally, we took advantage of recent advances in single-cell technologies combined with lineage tracing to unify the diversity of stromal fibro-adipogenic progenitors. Thus, this compelling review provides new cellular and molecular insights in comprehending the origins, definitions, markers, fate, and plasticity of murine and human fibro-adipogenic progenitors in muscle development, homeostasis, regeneration, and repair.

Diabetic Conditions Confer Metabolic and Structural Modifications to Tissue-Engineered Skeletal Muscle

Skeletal muscle is a tissue that is directly involved in the progression and persistence of type 2 diabetes (T2D), a disease that is becoming increasingly common. Gaining better insight into the mechanisms that are affecting skeletal muscle dysfunction in the context of T2D has the potential to lead to novel treatments for a large number of patients. Through its ability to emulate skeletal muscle architecture while also incorporating aspects of disease, tissue-engineered skeletal muscle (TE-SkM) has the potential to provide a means for rapid high-throughput discovery of therapies to treat skeletal muscle dysfunction, to include that which occurs with T2D. Muscle precursor cells isolated from lean or obese male Zucker diabetic fatty rats were used to generate TE-SkM constructs. Some constructs were treated with adipogenic induction media to accentuate the presence of adipocytes that is a characteristic feature of T2D skeletal muscle. The maturity (compaction and creatine kinase activity), mechanical integrity (Young's modulus), organization (myotube orientation), and metabolic capacity (insulin-stimulated glucose uptake) were all reduced by diabetes. Treating constructs with adipogenic induction media increased the quantity of lipid within the diabetic TE-SkM constructs, and caused changes in construct compaction, cell orientation, and insulin-stimulated glucose uptake in both lean and diabetic samples. Collectively, the findings herein suggest that the recapitulation of structural and metabolic aspects of T2D can be accomplished by engineering skeletal muscle in vitro.

Integrins and extracellular matrix proteins modulate adipocyte thermogenic capacity

Obesity and the metabolic disease epidemic has led to an increase in morbidity and mortality. A rise in adipose thermogenic capacity via activation of brown or beige fat is a potential treatment for metabolic diseases. However, an understanding of how local factors control adipocyte fate is limited. Mice with a null mutation in the laminin α4 (LAMA4) gene (KO) exhibit resistance to obesity and enhanced expression of thermogenic fat markers in white adipose tissue (WAT). In this study, changes in WAT extracellular matrix composition in the absence of LAMA4 were evaluated using liquid chromatography/tandem mass spectrometry. KO-mice showed lower levels of collagen 1A1 and 3A1, and integrins α7 (ITA7) and β1 (ITB1). ITA7-ITB1 and collagen 1A1-3A1 protein levels were lower in brown adipose tissue compared to WAT in wild-type mice. Immunohistochemical staining confirmed lower levels and different spatial distribution of ITA7 in KO-WAT. In culture studies, ITA7 and LAMA4 levels decreased following a 12-day differentiation of adipose-derived stem cells into beige fat, and knock-down of ITA7 during differentiation increased beiging. These results demonstrate that extracellular matrix interactions regulate adipocyte thermogenic capacity and that ITA7 plays a role in beige adipose formation. A better understanding of the mechanisms underlying these interactions can be used to improve systemic energy metabolism and glucose homeostasis.

Single-cell RNA sequencing and lipidomics reveal cell and lipid dynamics of fat infiltration in skeletal muscle

BACKGROUND

Ageing is accompanied by sarcopenia and intramuscular fat (IMAT) infiltration. In skeletal muscle, fat infiltration is a common feature in several myopathies and is associated with muscular dysfunction and insulin resistance. However, the cellular origin and lipidomic and transcriptomic changes during fat infiltration in skeletal muscle remain unclear.

METHODS

In the current study, we generated a high IMAT-infiltrated skeletal muscle model by glycerol (GLY) injection. Single-cell RNA sequencing and lineage tracing were performed on GLY-injured skeletal muscle at 5 days post-injection (DPI) to identify the cell origins and dynamics. Lipidomics and RNA sequencing were performed on IMAT-infiltrated skeletal muscle at 14 DPI (or 17 DPI for the cold treatment) to analyse alterations of lipid compositions and gene expression levels.

RESULTS

We identified nine distinct major clusters including myeloid-derived cells (52.13%), fibroblast/fibro/adipogenic progenitors (FAPs) (23.24%), and skeletal muscle stem cells (2.02%) in GLY-injured skeletal muscle. Clustering and pseudotemporal trajectories revealed six subpopulations in fibroblast/FAPs and 10 subclusters in myeloid-derived cells. A subpopulation of myeloid-derived cells expressing adipocyte-enriched genes and Pdgfra- /Cd68+ cells displayed lipid droplets upon adipogenic induction, indicating their adipogenic potential. Lipidomic analysis revealed the changes of overall lipid classes composition (e.g. triglycerides (TAGs) increased by 19.3 times, P = 0.0098; sulfoquinovosyl diacylglycerol decreased by 83%, P = 0.0056) and in the distribution of lipids [e.g. TAGs (18:2/18:2/22:6) increased by 181.6 times, P = 0.021] between GLY-group and saline control. RNA-seq revealed 1847 up-regulated genes and 321 down-regulated genes and significant changes in lipid metabolism-related pathways (e.g. glycerolipid pathway and glycerophospholipid pathway) in our model of GLY-injured skeletal muscle. Notably, short-term cold exposure altered fatty acid composition (e.g. saturated fatty acid decreased by 6.4%, P = 0.058) in fat-infiltrated muscles through directly affecting lipid metabolism pathways including PI3K-AKT and MAPK signalling pathway.

CONCLUSIONS

Our results showed that a subpopulation of myeloid-derived cells may contribute to IMAT infiltration. GLY-induced IMAT infiltration changed the lipid composition and gene expression profiles. Short-term cold exposure might regulate lipid metabolism and its related signalling pathways in fat-infiltrated muscle. Our study provides a comprehensive resource describing the molecular signature of fat infiltration in skeletal muscle.

β3-Adrenergic receptor agonist treats rotator cuff fatty infiltration by activating beige fat in mice

BACKGROUND

Rotator cuff (RC) muscle atrophy and fatty infiltration (FI) are independent factors correlated with failure of attempted tendon repair in larger RC tears. However, there is no effective treatment for RC muscle atrophy and FI at this time. The recent discovery of beige adipose tissue (BAT) in adults shed light on a new avenue in treating obesity and excessive fat deposition by promoting BAT activity. The goal of this study was to define the role of intramuscular BAT in RC muscle FI and the effect of β₃-adrenergic receptor agonists in treating RC muscle FI by promoting BAT activity.

MATERIALS AND METHODS

Three-month-old wild-type C57BL/6J, platelet derived growth factor receptor-alpha (PDGFRα) green fluorescent protein (GFP) reporter and uncoupling protein 1 (UCP-1) knockout mice underwent a unilateral RC injury procedure, which included supraspinatus (SS) and infraspinatus tendon resection and suprascapular nerve transection. To stimulate BAT activity, amibegron, a selective β₃-adrenergic receptor agonist, was administered to C57BL/6J mice either on the same day as surgery or 6 weeks after surgery through daily intraperitoneal injections. Gait analysis was conducted to measure forelimb function at 6 weeks or 12 weeks (in groups receiving delayed amibegron treatment) after surgery. Animals were killed humanely at 6 weeks (or 12 weeks for delayed amibegron groups) after surgery. SS muscles were harvested and analyzed histologically and biochemically.

RESULTS

Histologic analysis of SS muscles from PDGFRα-GFP reporter mice showed that PDGFRα-positive fibroadipogenic progenitors in RC muscle expressed UCP-1, a hallmark of BAT during the development of FI after RC tears. Impairing BAT activity by knocking out UCP-1 resulted in more severe muscle atrophy and FI with inferior forelimb function in UCP-1 knockout mice compared with wild-type mice. Promoting BAT activity with amibegron significantly reduced muscle atrophy and FI after RC tears and improved forelimb function. Delayed treatment with amibegron reversed muscle atrophy and FI in muscle.

CONCLUSIONS

Fat accumulated in muscle after RC tears possesses BAT characteristics. Impairing BAT activity results in worse RC muscle atrophy and FI. Amibegron reduces and reverses RC atrophy and FI by promoting BAT activity.

Human Rotator Cuff Tears Have an Endogenous, Inducible Stem Cell Source Capable of Improving Muscle Quality and Function After Rotator Cuff Repair

BACKGROUND

The muscle quality of the rotator cuff (RC), measured by atrophy and fatty infiltration (FI), is a key determinant of outcomes in RC injury and repair. The ability to regenerate muscle after repair has been shown to be limited.

PURPOSE

To determine if there is a source of resident endogenous stem cells, fibroadipogenic progenitor cells (FAPs), within RC injury patients, and if these cells are capable of adipogenic, fibrogenic, and pro-myogenic differentiation.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 20 patients between the ages of 40 and 75 years with partial- or full-thickness RC tears of the supraspinatus and evidence of atrophy and FI Goutallier grade 1, 2, or 3 were selected from 2 surgeons at an orthopaedic center. During the surgical repair procedure, supraspinatus muscle biopsy specimens were obtained for analysis as were deltoid muscle biopsy specimens to serve as the control. FAPs and satellite cells were quantified using fluorescence-activated cell sorting. Muscle FI and fibrosis was quantified using Oil Red O and Masson trichrome staining. FAP differentiation and gene expression profiles were compared across tear sizes after culture in adipogenic, fibrogenic, and beta-3 agonist (amibegron) conditions. Analysis of variance was used for statistical comparisons between groups, with P < .05 as statistically significant.

RESULTS

Histologic analysis confirmed the presence of fat in biopsy specimens from patients with full-thickness tears. There were more FAPs in the full-thickness tear group compared with the partial-thickness tear group (9.43% ± 4.25% vs 3.84% ± 2.54%; P < .01). Full-thickness tears were divided by tear size, with patients with larger tears having significantly more FAPs than those with smaller tears. FAPs from muscles with full-thickness tendon tears had more adipogenic and fibrogenic potential than those with partial tears. Induction of a beige adipose tissue (BAT) phenotype in FAPs was possible, as demonstrated by increased expression of BAT markers and pro-myogenic genes including insulin-like growth factor 1 and follistatin.

CONCLUSION

Endogenous FAPs are present within the RC and likely are the source of FI. These FAPs were increased in muscles with in larger tears but are capable of adopting a pro-myogenic BAT phenotype that could be utilized to improve muscle quality and patient function after RC repair.

Hallmarks of frailty and osteosarcopenia in prematurely aged PolgA(D257A/D257A) mice

BACKGROUND

Frailty is a geriatric syndrome characterized by increased susceptibility to adverse health outcomes. One major determinant thereof is the gradual weakening of the musculoskeletal system and the associated osteosarcopenia. To improve our understanding of the underlying pathophysiology and, more importantly, to test potential interventions aimed at counteracting frailty, suitable animal models are needed.

METHODS

To evaluate the relevance of prematurely aged PolgA(D257A/D257A) mice as a model for frailty and osteosarcopenia, we quantified the clinical mouse frailty index in PolgA(D257A/D257A) and wild-type littermates (PolgA(+/+) , WT) with age and concertedly assessed the quantity and quality of bone and muscle tissue. Lastly, the anabolic responsiveness of skeletal muscle, muscle progenitors, and bone was assessed.

RESULTS

PolgA(D257A/D257A) accumulated health deficits at a higher rate compared with WT, resulting in a higher frailty index at 40 and 46 weeks of age (+166%, +278%, P < 0.0001), respectively, with no differences between genotypes at 34 weeks. Concomitantly, PolgA(D257A/D257A) displayed progressive musculoskeletal deterioration such as reduced bone and muscle mass as well as impaired functionality thereof. In addition to lower muscle weights (-14%, P < 0.05, -23%, P < 0.0001) and fibre area (-20%, P < 0.05, -22%, P < 0.0001) at 40 and 46 weeks, respectively, PolgA(D257A/D257A) showed impairments in grip strength and concentric muscle forces (P < 0.05). PolgA(D257A/D257A) mutation altered the acute response to various anabolic stimuli in skeletal muscle and muscle progenitors. While PolgA(D257A/D257A) muscles were hypersensitive to eccentric contractions as well as leucine administration, shown by larger downstream signalling response of the mechanistic target of rapamycin complex 1, myogenic progenitors cultured in vitro showed severe anabolic resistance to leucine and robust impairments in cell proliferation. Longitudinal micro-computed tomography analysis of the sixth caudal vertebrae showed that PolgA(D257A/D257A) had lower bone morphometric parameters (e.g. bone volume fraction, trabecular, and cortical thickness, P < 0.05) as well as reduced remodelling activities (e.g. bone formation and resorption rate, P < 0.05) compared with WT. When subjected to 4 weeks of cyclic loading, young but not aged PolgA(D257A/D257A) caudal vertebrae showed load-induced bone adaptation, suggesting reduced mechanosensitivity with age.

CONCLUSIONS

PolgA(D257A/D257A) mutation leads to hallmarks of age-related frailty and osteosarcopenia and provides a powerful model to better understand the relationship between frailty and the aging musculoskeletal system.

Rotator cuff tear degeneration and the role of fibro-adipogenic progenitors

The high prevalence of rotator cuff tears poses challenges to individual patients and the healthcare system at large. This orthopedic injury is complicated further by high rates of retear after surgical repair. Outcomes following repair are highly dependent upon the quality of the injured rotator cuff muscles, and it is, therefore, crucial that the pathophysiology of rotator cuff degeneration continues to be explored. Fibro-adipogenic progenitors, a major population of resident muscle stem cells, have emerged as the main source of intramuscular fibrosis and fatty infiltration, both of which are key features of rotator cuff muscle degeneration. Improvements to rotator cuff repair outcomes will likely require addressing the muscle pathology produced by these cells. The aim of this review is to summarize the current rotator cuff degeneration assessment tools, the effects of poor muscle quality on patient outcomes, the role of fibro-adipogenic progenitors in mediating muscle pathology, and how these cells could be leveraged for potential therapeutics to augment current rotator cuff surgical and rehabilitative strategies.

Intervertebral disc herniation effects on multifidus muscle composition and resident stem cell populations

BACKGROUND

Paraspinal muscles are crucial for vertebral stabilization and movement. These muscles are prone to develop fatty infiltration (FI), fibrosis, and atrophy in many spine conditions. Fibro-adipogenic progenitors (FAPs), a resident muscle stem cell population, are the main contributors of muscle fibrosis and FI. FAPs are involved in a complex interplay with satellite cells (SCs), the primary myogenic progenitor cells within muscle. Little is known about the stem cell composition of the multifidus. The aim of this study is to examine FAPs and SCs in the multifidus in disc herniation patients. Multifidus muscle samples were collected from 10 patients undergoing decompressive spine surgery for lumbar disc herniation. Hamstring muscle was collected from four patients undergoing hamstring autograft ACL reconstruction as an appendicular control. Multifidus tissue was analyzed for FI and fibrosis using Oil-Red-O and Masson's trichrome staining. FAPs and SCs were visualized using immunostaining and quantified with fluorescence-activated cell sorting (FACS) sorting. Gene expression of these cells from the multifidus were analyzed with reverse transcription-polymerase chain reaction and compared to those from hamstring muscle. FI and fibrosis accounted for 14.2%± 7.4% and 14.8%±4.2% of multifidus muscle, respectively. The multifidus contained more FAPs (11.7%±1.9% vs 1.4%±0.2%; P<.001) and more SCs (3.4%±1.6% vs 0.08%±0.02%; P=.002) than the hamstring. FAPs had greater α Smooth Muscle Actin (αSMA) and adipogenic gene expression than FAPs from the hamstring. SCs from the multifidus displayed upregulated expression of stem, proliferation, and differentiation genes.

CONCLUSION

The multifidus in patients with disc herniation contains large percentages of FAPs and SCs with different gene expression profiles compared to those in the hamstring. These results may help explain the tendency for the multifidus to atrophy and form FI and fibrosis as well as elucidate potential approaches for mitigating these degenerative changes by leveraging these muscle stem cell populations.

Intramuscular Brown Fat Activation Decreases Muscle Atrophy and Fatty Infiltration and Improves Gait After Delayed Rotator Cuff Repair in Mice

BACKGROUND

Successful repair of large and massive rotator cuff (RC) tears remains a challenge at least partially because of secondary muscle atrophy and fatty infiltration. β3 Adrenergic agonists are a group of drugs that promote fat resorption through "white fat browning" of intramuscular stem cells.

PURPOSE

To test the role of a β3 adrenergic receptor agonist, amibegron, in improving muscle quality and forelimb function in a delayed RC repair model via promoting brown/beige adipose tissue activation.

STUDY DESIGN

Controlled laboratory study.

METHODS

Three-month-old PDGFRα-GFP reporter mice, wild type C57BL/6J mice, and uncoupling protein 1 (UCP-1) knockout mice underwent unilateral supraspinatus tendon transection with a 6-week delayed tendon repair. Animals with sham surgery served as controls. Amibegron was given either immediately after tendon transection or after repair. Gait analysis was conducted to measure forelimb function at 6 weeks after tendon repair. Animals were sacrificed at 6 weeks after repair. Supraspinatus muscles were harvested and analyzed histologically. Reverse transcription polymerase chain reaction was performed to quantify gene expression related to atrophy, fibrosis, and fatty infiltration.

RESULTS

Histology of PDGFRα reporter mice showed significantly increased UCP-1 expression, suggesting white fat browning in muscle after RC repair. As administered either immediately after tendon transection or after tendon repair, amibegron significantly reduced muscle atrophy and fatty infiltration and resumed normal upper extremity gait in wild type mice. However, the effect of amibegron was not present in UCP-1 knockout mice, suggesting that the effect of amibegron in treating RC muscle atrophy and fatty infiltration is through a UCP 1-dependent mechanism.

CONCLUSION

Amibegron reduced muscle atrophy and fatty infiltration and improved forelimb function after delayed RC repair through a UCP 1-dependent mechanism. This may be an effective clinical treatment strategy for patients to improve muscle quality after RC repair.

CLINICAL RELEVANCE

β3 Adrenergic agonists may serve as a new pharmacologic modality to treat RC muscle atrophy and fatty infiltration to improve clinical outcome of RC repair.

Association of uncoupling protein (Ucp) gene polymorphisms with cardiometabolic diseases

The hereditary aspect of obesity is a major focus of modern medical genetics. The genetic background is known to determine a higher-than-average prevalence of obesity in certain regions, like Oceania. There is evidence that dysfunction of brown adipose tissue (BAT) may be a risk factor for obesity and type 2 diabetes (T2D). A significant number of studies in the field focus on the UCP family. The Ucp genes code for electron transport carriers. UCP1 (thermogenin) is the most abundant protein of the UCP superfamily and is expressed in BAT, contributing to its capability of generating heat. Single nucleotide polymorphisms (SNPs) of Ucp1-Ucp3 were recently associated with risk of cardiometabolic diseases. This review covers the main Ucp SNPs A-3826G, A-1766G, A-112C, Met229Leu, Ala64Thr (Ucp1), Ala55Val, G-866A (Ucp2), and C-55 T (Ucp3), which may be associated with the development of obesity, disturbance in lipid metabolism, T2D, and cardiovascular diseases.

Beige FAPs Transplantation Improves Muscle Quality and Shoulder Function After Massive Rotator Cuff Tears

Rotator cuff (RC) tears are a common cause of upper extremity disability. Any tear size can result in subsequent muscle atrophy and fatty infiltration (FI). Preoperative muscle degeneration can predict repair and postoperative functional outcomes. Muscle residential fibro-adipogenic progenitors (FAPs) are found to be capable of differentiating into beige adipocytes that release factors to promote muscle growth. This study evaluated the regenerative potential of local cell transplantation of beige FAPs to mitigate muscle degeneration in a murine massive RC tear model. Beige FAPs were isolated from muscle in UCP-1 reporter mice by flow cytometry as UCP-1⁺ /Sca1⁺ /PDGFR⁺ /CD31^- /CD45^- /integrin α7^- . C57/BL6J mice undergoing supraspinatus tendon tear with suprascapular nerve transection (TT + DN) received either no additional treatment, phosphate-buffered saline injection, or beige FAP injection 2 weeks after the initial injury. Forelimb gait analysis was used to assess shoulder function with DigiGait. Mice were sacrificed 6 weeks after cell transplantation. FI, fibrosis, fiber size, vascularity were analyzed and quantified via ImageJ. Our results showed that beige FAP transplantation significantly decreased fibrosis, FI, and atrophy, enhanced vascularization compared with saline injection and non-treatment groups. Beige FAP transplantation also significantly improved shoulder function as measured by gait analysis. This study suggests that beige-differentiated FAPs may serve as a treatment option for RC muscle atrophy and FI, thus improving shoulder function in patients with massive RC tendon tears. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1159-1166, 2020.

Beige fibro-adipogenic progenitor transplantation reduces muscle degeneration and improves function in a mouse model of delayed repair of rotator cuff tears

BACKGROUND

Muscle atrophy and fatty infiltration (FI) are common occurrences following rotator cuff (RC) tears. Tears of all sizes are subject to muscle degeneration. The degree of muscle degeneration following RC tears is highly correlated with repair success and functional outcomes. We have recently discovered that muscle fibro-adipogenic progenitors (FAPs) can differentiate into uncoupling protein 1 (UCP-1)-expressing beige adipocytes and induce muscle regeneration. This study evaluated the potential of local cell transplantation of beige adipose FAPs (BAT-FAPs) to treat RC muscle degeneration in a murine model of RC repair.

METHODS

BAT-FAPs were isolated from muscle in UCP-1 reporter mice by flow cytometry as UCP-1⁺/Sca1⁺/PDGFR⁺/CD31^-/CD45^-/integrin α7^-. C57/BL6J mice underwent supraspinatus tendon tear with suprascapular nerve transection followed by repair 2 or 6 weeks after the initial injury. At the time of repair, mice received either no additional treatment, phosphate-buffered saline injection, or BAT-FAP injection. Functional outcomes were assessed by gait analysis. Mice were humanely killed at 6 weeks after cell transplantation. Supraspinatus muscle FI, fibrosis, muscle fiber size, and vascularity were analyzed and quantified via ImageJ. Analysis of variance with post hoc Tukey test and P <.05 was used to determine statistical significance.

RESULTS

Cell transplantation diminished fibrosis, FI, and atrophy and enhanced vascularization in both delayed repair models. Cell transplantation resulted in improved shoulder function as assessed with gait analysis in both the delayed repair models.

CONCLUSIONS

BAT-FAPs significantly reduced muscle degeneration and improved shoulder function after RC repair. BAT-FAPs hold significant promise as a therapeutic adjunct to repair for patients with advanced RC pathology.

Role of Metabolic Stress and Exercise in Regulating Fibro/Adipogenic Progenitors

Obesity is a major public health concern and is associated with decreased muscle quality (i.e., strength, metabolism). Muscle from obese adults is characterized by increases in fatty, fibrotic tissue that decreases the force producing capacity of muscle and impairs glucose disposal. Fibro/adipogenic progenitors (FAPs) are muscle resident, multipotent stromal cells that are responsible for muscle fibro/fatty tissue accumulation. Additionally, they are indirectly involved in muscle adaptation through their promotion of myogenic (muscle-forming) satellite cell proliferation and differentiation. In conditions similar to obesity that are characterized by chronic muscle degeneration, FAP dysfunction has been shown to be responsible for increased fibro/fatty tissue accumulation in skeletal muscle, and impaired satellite cell function. The role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle is just beginning to be unraveled. Thus, the present review aims to summarize the recent literature on the role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle, and the mechanisms responsible for these effects. Furthermore, we will review the role of physical activity in reversing or ameliorating the detrimental effects of obesity on FAP function.

The emerging roles of lactate as a redox substrate and signaling molecule in adipose tissues

Thermogenic (brown and beige) adipose tissues improve glucose and lipid homeostasis and therefore represent putative targets to cure obesity and related metabolic diseases including type II diabetes. Beside decades of research and the very well-described role of noradrenergic signaling, mechanisms underlying adipocytes plasticity and activation of thermogenic adipose tissues remain incompletely understood. Recent studies show that metabolites such as lactate control the oxidative capacity of thermogenic adipose tissues. Long time viewed as a metabolic waste product, lactate is now considered as an important metabolic substrate largely feeding the oxidative metabolism of many tissues, acting as a signaling molecule and as an inter-cellular and inter-tissular redox carrier. In this review, we provide an overview of the recent findings highlighting the importance of lactate in adipose tissues, from its production to its role as a browning inducer and its metabolic links with brown adipose tissue. We also discuss additional function(s) than thermogenesis ensured by brown and beige adipose tissues, i.e., their ability to dissipate high redox pressure and oxidative stress thanks to the activity of the uncoupling protein-1, helping to maintain tissue and whole organism redox homeostasis and integrity.

Fibro-Adipogenic Progenitors Cross-Talk in Skeletal Muscle: The Social Network

Skeletal muscle is composed of a large and heterogeneous assortment of cell populations that interact with each other to maintain muscle homeostasis and orchestrate regeneration. Although satellite cells (SCs) – which are muscle stem cells – are the protagonists of functional muscle repair following damage, several other cells such as inflammatory, vascular, and mesenchymal cells coordinate muscle regeneration in a finely tuned process. Fibro–adipogenic progenitors (FAPs) are a muscle interstitial mesenchymal cell population, which supports SCs differentiation during tissue regeneration. During the first days following muscle injury FAPs undergo massive expansion, which is followed by their macrophage-mediated clearance and the re-establishment of their steady-state pool. It is during this critical time window that FAPs, together with the other cellular components of the muscle stem cell niche, establish a dynamic network of interactions that culminate in muscle repair. A number of different molecules have been recently identified as important mediators of this cross-talk, and its alteration has been associated with different muscle pathologies. In this review, we will focus on the soluble factors that regulate FAPs activity, highlighting their roles in orchestrating the inter-cellular interactions between FAPs and the other cell populations that participate in muscle regeneration.

EFFECTS OF VARIOUS AVERSIVE ENVIRONMENTS ON OXYGEN CONSUMPTION OF MUSCLE AND BLOOD IN MICE UNDER CONDITIONS OF THE “FORCED SWIMMING” TEST

The aim of the study is to assess the effect of various aversive environments on the oxygen consumption in muscles and blood in mice Under conditions of the “forced swimming” test.

Materials and methods. The study was performed on outbred male mice. Exhausting physical activity was modeled in the “forced swimming” test in various aversive environments. The oxygen consumption by the muscle tissue, as well as the oxygen capacity of the blood, were estimated using the respirometry method (AKPM1-01L (“Alfa Bassens”, Russia)).

Results. In the course of the study it was found out that in the group of the animals swimming in hot water (at the temperature of 41°C) as an aversive environment, there was no significant change in the oxygen consumption by mitochondria of striated muscle and by red blood cells in comparison with the intact group of the animals. At the same time, in the group of the mice, where cold water (at the temperature of 15°C) as an aversive environment was used, a statistically significant (by the end of the experiment) decrease in the swimming time was observed in relation to the intact group of the animals. It was accompanied by a decrease in the oxygen consumption by muscle mitochondria, with a constant level of the blood oxygenation. Under conditions of exhausting physical exertion, in the group of the animals that received Metaprot®, an increase in working capacity was noted in both hot and cold water. After peak days of working capacity, a slight decrease in physical activity was observed in both experimental groups. At the same time, it should be noted that oxygenation of blood and muscle tissue against the background of exhausting physical exertion in the group that received Metaprot®, did not differ from the group of intact animals in various aversive environments.

Conclusion. Thus, based on the obtained data, it can be assumed that under conditions of “forced swimming” with loading, the most profound changes in the structure and functions of the striated muscles are observed in animals in cold (15°С) water That is reflected in a decrease in the physical strain and in reducing the oxygen consumption by muscle tissue. The use of the drug Metaprot® promoted correcting the changes in the physical performance of the animals, which was reflected in its increase by 144.8% (p <0.05), compared with the initial swimming time of this group, without the oxygen consumption by erythrocytes and mitochondria of striated muscles. 

Fibro-Adipogenic Remodeling of the Diaphragm in Obesity-Associated Respiratory Dysfunction

Respiratory dysfunction is a common complication of obesity, conferring cardiovascular morbidity and increased mortality and often necessitating mechanical ventilatory support. While impaired lung expansion in the setting of increased adipose mass and reduced central response to hypercapnia have been implicated as pathophysiological drivers, the impact of obesity on respiratory muscles-in particular, the diaphragm-has not been investigated in detail. Here, we demonstrate that chronic high-fat diet (HFD) feeding impairs diaphragm muscle function, as assessed in vivo by ultrasonography and ex vivo by measurement of contractile force. During an HFD time course, progressive adipose tissue expansion and collagen deposition within the diaphragm parallel contractile deficits. Moreover, intradiaphragmatic fibro-adipogenic progenitors (FAPs) proliferate with long-term HFD feeding while giving rise to adipocytes and type I collagen-depositing fibroblasts. Thrombospondin 1 (THBS1), a circulating adipokine, increases with obesity and induces FAP proliferation. These findings suggest a novel role for FAP-mediated fibro-adipogenic diaphragm remodeling in obesity-associated respiratory dysfunction.

Uncoupling protein 1 expression in adipocytes derived from skeletal muscle fibro/adipogenic progenitors is under genetic and hormonal control

BACKGROUND

Intramuscular fatty infiltration is generally associated with the accumulation of white adipocytes in skeletal muscle and unfavourable metabolic outcomes. It is, however, still unclear whether intramuscular adipocytes could also acquire a brown-like phenotype. Here, we detected intramuscular expression of brown adipocyte markers during fatty infiltration in an obesity-resistant mouse strain and extensively compared the potential of two different stem cell populations residing in skeletal muscle to differentiate into brown-like adipocytes.

METHODS

Fatty infiltration was induced using intramuscular glycerol or cardiotoxin injection in the tibialis anterior muscles of young or aged 129S6/SvEvTac (Sv/129) mice or interleukin-6 (IL-6) knockout mice, and the expression of general and brown adipocyte markers was assessed after 4 weeks. Fibro/adipogenic progenitors (FAPs) and myogenic progenitors were prospectively isolated using fluorescence-activated cell sorting from skeletal muscle of male and female C57Bl6/6J and Sv/129 mice, and monoclonal and polyclonal cultures were treated with brown adipogenic medium. Additionally, FAPs were differentiated with medium supplemented or not with triiodothyronine.

RESULTS

Although skeletal muscle expression of uncoupling protein 1 (Ucp1) was barely detectable in uninjected tibialis anterior muscle, it was drastically induced following intramuscular adipogenesis in Sv/129 mice and further increased in response to beta 3-adrenergic stimulation. Intramuscular Ucp1 expression did not depend on IL-6 and was preserved in aged skeletal muscle. Myogenic progenitors did not form adipocytes neither in polyclonal nor monoclonal cultures. Fibro/adipogenic progenitors, on the other hand, readily differentiated into brown-like, UCP1+ adipocytes. Uncoupling protein 1 expression in differentiated FAPs was regulated by genetic background, sex, and triiodothyronine treatment independently of adipogenic differentiation levels.

CONCLUSIONS

Intramuscular adipogenesis is associated with increased Ucp1 expression in skeletal muscle from obesity-resistant mice. Fibro/adipogenic progenitors provide a likely source for intramuscular adipocytes expressing UCP1 under control of both genetic and hormonal factors. Therefore, FAPs constitute a possible target for therapies aiming at the browning of intramuscular adipose tissue and the metabolic improvement of skeletal muscle affected by fatty infiltration.