Sex-Based Differences in Muscle Stem Cell Regulation Following Exercise.

Sexual dimorphism, driven by the sex hormones testosterone and estrogen, influences body composition, muscle fiber type, and inflammation. Research related to muscle stem cell (MuSC) responses to exercise has mainly focused on males. We propose a novel hypothesis that there are sex-based differences in MuSC regulation following exercise, such that males have more MuSCs, whereas females demonstrate a greater capacity for regeneration.

Dietary fiber intake and fecal short-chain fatty acid concentrations are associated with lower plasma lipopolysaccharide-binding protein and inflammation.

Consuming adequate dietary fiber is a promising strategy for reducing systemic inflammation. The objective was to evaluate relationships between dietary fiber intake, markers of metabolic endotoxemia, and systemic inflammation in adults. This was a cross-sectional study of 129 healthy participants (age 33.6 ± 6.1 yr, BMI 30.5 ± 6.9 kg/m2). Dietary fiber intake was assessed by food frequency questionnaire. Adiposity was measured using dual-energy X-ray absorptiometry (DXA). Fecal short-chain fatty acids (SCFA) were quantified using gas chromatography-mass spectrometry. Fecal microbiota sequence data (V4 region, 16S rRNA gene) were analyzed using DADA2 and QIIME2. Inflammatory cytokines were assessed with enzyme-linked immunosorbent assays; flow cytometry was conducted for monocyte surface marker quantification. Bivariate correlations and generalized step-wise linear modeling were used for statistical analyses. Plasma C-reactive protein (CRP) and interleukin (IL)-6 concentrations were positively related to whole body (CRP r = 0.45, P = <0.0001; IL-6 r = 0.34, P = 0.0002) and visceral adiposity (CRP r = 0.33, P = 0.0003; IL-6 r = 0.38, P = 0.0002). Plasma lipopolysaccharide-binding protein (LBP) concentrations were inversely related to dietary fiber intake (r = -0.22, P = 0.03) and fecal SCFA (acetate r = -0.25, P = 0.01; propionate r = -0.28, P = 0.003; butyrate r = -0.23, P = 0.02). Whole body adiposity, dietary fiber, and fecal SCFA were the most predictive of plasma LBS-BP concentrations. Novel findings included associations between dietary fiber intake, the gastrointestinal microbiota, and systemic inflammation.NEW & NOTEWORTHY Dietary fiber intake may reduce the inflammation associated with obesity and metabolic disease. Our cross-sectional analysis revealed that dietary fiber intake and fecal short-chain fatty acids are inversely associated with lipopolysaccharide-binding protein, a marker of systemic inflammation. In addition, plasma interleukin-6 and C-reactive protein were positively related to markers of adiposity.

Muscle injury induces a transient senescence-like state that is required for myofiber growth during muscle regeneration.

Cellular senescence is the irreversible arrest of normally dividing cells and is driven by the cell cycle inhibitors Cdkn2a, Cdkn1a, and Trp53. Senescent cells are implicated in chronic diseases and tissue repair through their increased secretion of pro-inflammatory factors known as the senescence-associated secretory phenotype (SASP). Here, we use spatial transcriptomics and single-cell RNA sequencing (scRNAseq) to demonstrate that cells displaying senescent characteristics are "transiently" present within regenerating skeletal muscle and within the muscles of D2-mdx mice, a model of Muscular Dystrophy. Following injury, multiple cell types including macrophages and fibrog-adipogenic progenitors (FAPs) upregulate senescent features such as senescence pathway genes, SASP factors, and senescence-associated beta-gal (SA-ß-gal) activity. Importantly, when these cells were removed with ABT-263, a senolytic compound, satellite cells are reduced, and muscle fibers were impaired in growth and myonuclear accretion. These results highlight that an "acute" senescent phenotype facilitates regeneration similar to skin and neonatal myocardium.

The role of L-type amino acid transporter 1 (Slc7a5) during in vitro myogenesis.

Satellite cells are required for muscle regeneration, remodeling, and repair through their activation, proliferation, and differentiation; however, how dietary factors regulate this process remains poorly understood. The L-type amino acid transporter 1 (LAT1) transports amino acids, such as leucine, into mature myofibers, which then stimulate protein synthesis and anabolic signaling. However, whether LAT1 is expressed on myoblasts and is involved in regulating myogenesis is unknown. The aim of this study was to characterize the expressional and functional relevance of LAT1 during different stages of myogenesis and in response to growth and atrophic conditions in vitro. We determined that LAT1 is expressed by C2C12 and human primary myoblasts, and its gene expression is lower during differentiation (P < 0.05). Pharmacological inhibition and genetic knockdown of LAT1 impaired myoblast viability, differentiation, and fusion (all P < 0.05). LAT1 protein content in C2C12 myoblasts was not significantly altered in response to different leucine concentrations in cell culture media or in two in vitro atrophy models. However, LAT1 content was decreased in myotubes under atrophic conditions in vitro (P < 0.05). These findings indicate that LAT1 is stable throughout myogenesis and in response to several in vitro conditions that induce muscle remodeling. Further, amino acid transport through LAT1 is required for normal myogenesis in vitro.

How Does Lifestyle Affect Hematopoiesis and the Bone Marrow Microenvironment?

Lifestyle factors are modifiable behavioral factors that have a significant impact on health and longevity. Diet-induced obesity and physical activity/exercise are two prevalent lifestyle factors that have strong relationships to overall health. The mechanisms linking obesity to negative health outcomes and the mechanisms linking increased participation in physical activity/exercise to positive health outcomes are beginning to be elucidated. Chronic inflammation, due in part to overproduction of myeloid cells from hematopoietic stem cells (HSCs) in the bone marrow, is an established mechanism responsible for the negative health effects of obesity. Recent work has shown that exercise training can reverse the aberrant myelopoiesis present in obesity in part by restoring the bone marrow microenvironment. Specifically, exercise training reduces marrow adipose tissue, increases HSC retention factor expression, and reduces pro-inflammatory cytokine levels in the bone marrow. Other, novel mechanistic factors responsible for these exercise-induced effects, including intercellular communication using extracellular vesicles (EVs), is beginning to be explored. This review will summarize the recent literature describing the effects of exercise on hematopoiesis in individuals with obesity and introduce the potential contribution of EVs to this process.

Creatine Monohydrate Supplementation, but not Creatyl-L-Leucine, Increased Muscle Creatine Content in Healthy Young Adults: A Double-Blind Randomized Controlled Trial.

Creatine (Cr) supplementation is a well-established strategy to enhance gains in strength, lean body mass, and power from a period of resistance training. However, the effectiveness of creatyl-L-leucine (CLL), a purported Cr amide, is unknown. Therefore, the purpose of this study was to assess the effects of CLL on muscle Cr content. Twenty-nine healthy men (n = 17) and women (n = 12) consumed 5 g/day of either Cr monohydrate (n = 8; 28.5 ± 7.3 years, 172.1 ± 11.0 cm, 76.6 ± 10.7 kg), CLL (n = 11; 29.2 ± 9.3 years, 170.3 ± 10.5 cm, 71.9 ± 14.5 kg), or placebo (n = 10; 30.3 ± 6.9 years, 167.8 ± 9.9 cm, 69.9 ± 11.1 kg) for 14 days in a randomized, double-blind design. Participants completed three bouts of supervised resistance exercise per week. Muscle biopsies were collected before and after the intervention for quantification of muscle Cr. Cr monohydrate supplementation which significantly increased muscle Cr content with 14 days of supplementation. No changes in muscle Cr were observed for the placebo or CLL groups. Cr monohydrate supplementation is an effective strategy to augment muscle Cr content while CLL is not.

Higher protein intake during resistance training does not potentiate strength, but modulates gut microbiota, in middle-aged adults: a randomized control trial.

Protein intake above the recommended dietary allowance (RDA) and resistance training are known anabolic stimuli to support healthy aging. Specifically, protein supplementation after resistance exercise and nightly are strategies to maximize utilization of protein intake above the RDA in healthy adults. As such, the primary objective was to examine the efficacy of protein supplementation and nutritional counseling resulting in either moderate (MOD: ∼1.0 g·kg-1·day-1) or higher (HIGH: ∼1.6 g·kg-1·day-1) protein intake during resistance training on strength (one-repetition maximum, 1-RM; isokinetic and isometric peak torque) in healthy middle-aged adults. Exploratory analyses include diet-exercise effects on lean body mass (LBM), clinical biomarkers, gut microbiota, and diet composition. In all, 50 middle-aged adults (age: 50 ± 8 yr, BMI: 27.2 ± 4.1 kg/m2) were randomized to either MOD or HIGH protein intake during a 10-wk resistance training program (3 × wk). Participants received dietary counseling and consumed either 15 g (MOD) or 30 g (HIGH) of protein from lean beef in the immediate postexercise period and each evening. Maximal strength (1-RM) for all upper and lower body exercises significantly increased with no effect of protein intake (P < 0.050). There was a main effect of time for LBM (P < 0.005). Cardiovascular, renal, or glycemic biomarkers were not affected by the intervention. Gut microbiota were associated with several health outcomes (P < 0.050). In conclusion, higher protein intake above moderate amounts does not potentiate resistance training adaptations in previously untrained middle-aged adults. This trial was registered at clinicaltrials.gov as NCT03029975.NEW & NOTEWORTHY Our research evaluates the efficacy of higher in comparison with moderate animal-based protein intake on resistance exercise training-induced muscle strength, clinical biomarkers, and gut microbiota in middle-aged adults through a dietary counseling-controlled intervention. Higher protein intake did not potentiate training adaptations, nor did the intervention effect disease biomarkers. Both diet and exercise modified gut microbiota composition. Collectively, moderate amounts of high-quality, animal-based protein is sufficient to promote resistance exercise adaptations at the onset of aging.

Impact of Exercise Training on Hematological Outcomes Following Hematopoietic Cell Transplantation: A Scoping Review.

PURPOSE: Hematopoietic cell transplantation (HCT) is associated with significant risk prior to hematopoietic engraftment. Endurance exercise can modify the bone marrow microenvironment, alter hematopoiesis and accelerate hematopoietic regeneration in mouse models of transplantation. METHODS: A systematic review was conducted to clarify the impact of exercise on clinically relevant hemato-logical outcomes in patients following HCT. RESULTS: A systematic search of the literature identified 13 studies (total of 615 participants; 313 in study arms). Studies included exercise regimens that were primarily low-to-moderate intensity. A total of five studies re-ported on engraftment and length of stay, which were largely unchanged with intervention. Rates of graft-ver-sus host disease were reported in six studies whereas red cell and platelet transfusion needs were reported in four studies, neither of which was different with exercise. Survival was reported in four studies and was significantly improved by exercise in one study. CONCLUSIONS: Exercise in patients receiving HCT appears feasible and safe. Heterogeneity in type and intensity of exercise was observed and few studies examined high intensity exercise. Outcome reporting was inconsis-tent regarding transplant-related outcomes. Standardized hematological outcome measures are needed to clarify the impact of higher intensity exercise on HCT.

A Carbohydrate Beverage Reduces Monocytes Expressing TLR4 in Children with Overweight or Obesity.

BACKGROUND: Childhood obesity is increasing, with about one-third of children overweight or obese. Obesity is characterized by a state of chronic low-grade inflammation that is related to cardiometabolic comorbidities. Inflammatory monocytes, which are classified into 3 different groups-classical, intermediate, and nonclassical monocytes, with Toll-like receptor 4 (TLR4+) expression indicating a proinflammatory state-underlie several obesity-associated morbidities. OBJECTIVES: This study aimed to assess the responses of monocyte populations to beverages of differing macronutrient composition in children with healthy weight (HW) or overweight/obesity (OW/OB). METHODS: Ten HW children (5th to 84.9th percentile; mean age 12.29 ± 2.5 y) and 7 children with OW/OB (85th to 99.99th percentile; mean age 11.96 ± 3.8 y) completed the study. Adiposity was determined via DXA. Using a double-blinded, randomized, crossover design, participants consumed either a high-carbohydrate (CHO; 210 kcal; 0 g fat/56 g carbohydrates/0 g protein) or a whole-egg-based high-protein/fat (EGG; 210 kcal; 15 g fat/0 g carbohydrates/18 g protein) beverage. Venous blood was collected at baseline and 2 h postprandially for evaluation of metabolic and inflammatory responses. Repeated measures ANOVA and Pearson correlations were conducted. RESULTS: Consuming the CHO beverage significantly reduced the primary outcome: TLR4+ expression on classical monocytes in children with OW/OB only (25.60% decrease from baseline in OW/OB compared with 1.61% increase in HW). Children with OW/OB had significantly less percentages of TLR4+ nonclassical monocytes than HW (47.66% lower after CHO). Insulin and glucose (secondary outcomes), were significantly higher after the CHO condition compared with baseline (230.61% and 9.93% increase, respectively). Changes in glucose were significantly and negatively related to changes in monocyte populations in the CHO condition. CONCLUSIONS: These data suggest that high-carbohydrate beverages alter monocyte populations in the blood in children with OW/OB, which is related to glucose metabolism. These findings have implications for nutritional recommendations in children with overweight/obesity. National Clinical Trial registry trial number: NCT03597542.

Effects of obesity and exercise on colon cancer induction and hematopoiesis in mice.

Obesity-induced inflammation is associated with increased risk for colorectal cancer (CRC). The role of diet and exercise in modulating increased CRC risk in obesity and the potential role of altered hematopoiesis as a contributor to these effects remain unknown. The purpose of this study was to examine how weight loss induced during CRC induction with or without exercise alters CRC initiation and its relationship to altered hematopoiesis. Mice consumed either a control (CON) or a high-fat diet to induce obesity. All mice were then placed on the control diet during CRC induction with azoxymethane (AOM). Following AOM injection, mice originally on the high-fat diet were randomized into sedentary (HF-SED) or exercise trained (HF-EX) conditions. At euthanasia, body weight and fat mass were similar among all three groups ( P < 0.05). Compared with CON and HF-EX, HF-SED developed increased content of preneoplastic lesions ( P < 0.05), and HF-SED had significantly increased markers of colon inflammation compared with CON. Compared with both CON and HF-EX, HF-SED had decreased content of short-term hematopoietic stem cells and increased content of common myeloid progenitor cells (both P < 0.05). Similarly, HF-SED had increased bone marrow adiposity compared with CON and HF-EX ( P < 0.05), and proteomics analysis revealed an increased marker of bone marrow inflammation in HF-SED compared with CON and HF-EX. Our results suggest that the early removal of a high-fat diet reduces CRC incidence when combined with an exercise training intervention. This reduction in risk was related to lower colon inflammation with anti-inflammatory changes in hematopoiesis induced by exercise.

Effects of endurance exercise training on inflammatory circulating progenitor cell content in lean and obese adults.

KEY POINTS: Chronic inflammation underlies many of the health decrements associated with obesity. Circulating progenitor cells can sense and respond to inflammatory stimuli, increasing the local inflammatory response within tissues. Here we show that 6 weeks of endurance exercise training significantly decreases inflammatory circulating progenitor cells in obese adults. These findings provide novel cellular mechanisms for the beneficial effects of exercise in obese adults. ABSTRACT: Circulating progenitor cells (CPCs) and subpopulations are normally found in the bone marrow, but can migrate to peripheral tissues to participate in local inflammation and/or remodelling. The purpose of this study was to compare the CPC response, particularly the inflammatory-primed haematopoietic stem and progenitor (HSPC) subpopulation, to a 6 week endurance exercise training (EET) intervention between lean and obese adults. Seventeen healthy weight (age: 23.9 ± 5.4 years, body mass index (BMI): 22.0 ± 2.6 kg m-2 ) and 10 obese (age: 29.0 ± 8.0 years, BMI: 33.1 ± 6.0 kg m-2 ) previously sedentary adults participated in an EET. Blood was collected before and after EET for quantification of CPCs and subpopulations via flow cytometry, colony forming unit assays and plasma concentrations of C-X-C motif chemokine 12 (CXCL12), granulocyte-colony stimulating factor (G-CSF), and chemokine (C-C motif) ligand 2 (CCL2). Exercise training reduced the number of circulating HSPCs and adipose tissue-derived mesenchymal stem cells (AT-MSCs). EET increased the colony forming potential of granulocytes and macrophages irrespective of BMI. EET reduced the number of HSPCs expressing the chemokine receptor CCR2 and the pro-inflammatory marker TLR4. EET-induced changes in adipose tissue-derived MSCs and bone marrow-derived MSCs were negatively related to changes in absolute fitness. Our results indicate that EET, regardless of BMI status, decreases CPCs and subpopulations, particularly those primed for contribution to tissue inflammation.

Skeletal Muscle Remodeling: Interconnections Between Stem Cells and Protein Turnover.

Nutrition and exercise are important components of a healthy lifestyle to improve rates of hypertrophic and nonhypertrophic skeletal muscle remodeling. We provide evidence to support the hypothesis that muscle stem cells and protein turnover are collaborative, not separate, mechanisms supporting muscle remodeling by facilitating protein, nuclear, and cellular turnover in response to the ingestion of protein dense foods and exercise.

Hematopoiesis with Obesity and Exercise: Role of the Bone Marrow Niche.

Hematopoietic stem and progenitor cells (HSPC), the most primitive cells of the hematopoietic system responsible for maintaining all mature blood cells, display the hallmark characteristics of self-renewal and multi-potent differentiation into mature cell lineages. HSPC activity is directed by the bone marrow niche, a complex environment composed of heterogeneous cell populations that regulate HSPC function through the secretion of a wide array of cytokines and growth factors. Diet induced obesity results in a dramatic remodeling of the bone marrow niche, skewing HSPC function resulting in a compromised immune system. Exercise is a viable treatment option for deficits imposed by obesity and to combat immune dysfunction; however, the impact of exercise on the bone marrow niche is not well defined. This review summarizes the available information on how obesity disrupts the normal bone marrow niche and HSPC function. In addition, we review the limited data available detailing how exercise may be used to combat obesity induced bone marrow dysfunction, and discuss future directions for research in this field.

Circulating progenitor cells are positively associated with cognitive function among overweight/obese children.

Recent evidence has indicated that overweight/obese children may experience cognitive and immune dysfunction, but the underlying mechanisms responsible for the association between overweight/obesity, immune dysfunction, and cognition have yet to be established. The present study aimed to identify a novel link between obesity-induced immune system dysregulation and cognition in preadolescent children. A total of 27 male children (age: 8-10years) were recruited and separated by body mass index (BMI) into healthy weight (HW: 5th-84.9th percentile, n=16) and overweight/obese (OW: ⩾85th percentile, n=11) groups. Adiposity was assessed using dual energy X-ray absorptiometry (DXA), and aspects of executive function were assessed using the Woodcock-Johnson III Tests of Cognitive Abilities. Monocyte populations (CD14(+)CD16(-), CD14(+)CD16(+)) with and without expression of chemokine receptor type 2 (CCR2), and circulating progenitor cells (CPCs: CD34(+)CD45(dim)), in peripheral blood were quantified by flow cytometry. CPCs were isolated by flow sorting and cultured for 24h for collection of conditioned media (CM) that was applied to SH-SY5Y neuroblastomas to examine the paracrine effects of CPCs on neurogenesis. OW had significantly higher quantities of both populations of monocytes (CD14(+)CD16(-): 57% increase; CD14(+)CD16(+): 95% increase, both p<0.01), monocytes expressing CCR2 (CD14(+)CD16(-)CCR2(+): 66% increase; CD14(+)CD16(+)CCR2(+): 168% increase, both p<0.01), and CPCs (47% increase, p<0.05) than HW. CPCs were positively correlated with abdominal adiposity in OW, and negatively correlated in HW with a significant difference between correlations (p<0.05). CPC content was positively correlated with executive processes in OW, and negatively correlated in HW with a significant difference in the strength of the correlations between groups (p<0.05 for correlation between OW and HW). Finally, CPC-CM from OW trended to increase neuroblast viability in vitro relative to HW (1.79 fold, p=0.07). These novel findings indicate that increased content of CPCs among OW children may play a role in preventing decrements in cognitive function via paracrine mechanisms.

The Benefits of Acute Exercise for Children's Cognition are Associated with Trait Anxiety.

INTRODUCTION: Children's anxiety is associated with decreased cognitive performance. One well-established behavioral intervention to transiently improve cognitive performance in children is acute aerobic exercise (AAE). Thus far, however, it is unclear whether the benefits of AAE on cognition vary based on individual differences in children's anxiety level. Therefore, we investigated whether trait anxiety levels mediate the effects of AAE on cognitive outcomes in preadolescent children. METHODS: Ninety-five preadolescent children (9-11 years, 41 females) underwent two experimental interventions in a random, crossover, and counterbalanced design: an exercise protocol (20 minutes of walking at 65-75% of HRpeak) and a non-exercise control (seated rest). Behavioral indices of cognitive performance (accuracy, reaction times, response time variability) were assessed before and after each intervention using a modified flanker task. The effects of each intervention on cognitive performance were calculated as pre- to post-intervention differences (effect scores). These scores were first correlated with children's personal characteristics: anxiety (STAIC), sex, age, BMI, IQ, and aerobic fitness. Significant correlations guided subsequent hierarchical regression models, which specifically tested for associations between the effects scores and anxiety levels while controlling for remaining relevant covariates. RESULTS: Regression analysis revealed that anxiety was a significant predictor of the effects that AAE and rest had on cognitive performance. Specifically, higher anxiety scores were associated with greater exercise-induced cognitive benefits (increased accuracy, decreased response time variability; p's < 0.05) and greater rest-induced cognitive impairments (decreased accuracy, increased response time variability; p's < 0.01). CONCLUSIONS: These findings suggest that children reporting higher trait anxiety might experience greater cognitive benefits from a single bout of AAE.

Mesenchymal stem cells contribute to vascular growth in skeletal muscle in response to eccentric exercise.

The α(7)ß(1)-integrin is an adhesion molecule highly expressed in skeletal muscle that can enhance regeneration in response to eccentric exercise. We have demonstrated that mesenchymal stem cells (MSCs), predominantly pericytes, accumulate in muscle (mMSCs) overexpressing the α(7B)-integrin (MCK:α(7B); α(7)Tg) and contribute to new fiber formation following exercise. Since vascularization is a common event that supports tissue remodeling, we hypothesized that the α(7)-integrin and/or mMSCs may stimulate vessel growth following eccentric exercise. Wild-type (WT) and α(7)Tg mice were subjected to single or multiple (3 times/wk, 4 wk) bouts of downhill running exercise. Additionally, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) -labeled mMSCs were intramuscularly injected into WT recipients. A subset of recipient mice were run downhill before injection to recapitulate the exercised microenvironment. While total number of CD31(+) vessels declined in both WT and α(7)Tg muscle following a single bout of exercise, the number of larger CD31(+) vessels with a visible lumen was preferentially increased in α(7)Tg mice following eccentric exercise training (P < 0.05). mMSC transplantation similarly increased vessel diameter and the total number of neuron-glial antigen-2 (NG2(+)) arterioles postexercise. Secretion of arteriogenic factors from mMSCs in response to mechanical strain, including epidermal growth factor and granulocyte macrophage-colony stimulating factor, may account for vessel remodeling. In conclusion, this study demonstrates that the α(7)-integrin and mMSCs contribute to increased vessel diameter size and arteriolar density in muscle in response to eccentric exercise. The information in this study has implications for the therapeutic treatment of injured muscle and disorders that result in vessel occlusion, including peripheral artery disease.

Exercise and hematopoietic stem and progenitor cells: protection, quantity, and function.

The effects of exercise on mature cells of the hematopoietic lineage have been well characterized; however, the effects of exercise on the more primitive cells of the hematopoietic system are unknown. Here, we propose that the benefits of exercise apply to hematopoietic progenitors and that exercise increases hematopoietic stem cell quantity primarily mediated via adaptations in the stem cell niche.

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.

Sex-based comparisons of muscle cellular adaptations after 10 weeks of progressive resistance training in middle-aged adults.

Resistance training combined with adequate protein intake supports skeletal muscle strength and hypertrophy. These adaptations are supported by the action of muscle stem cells (MuSCs), which are regulated, in part, by fibro-adipogenic progenitors (FAPs) and circulating factors delivered through capillaries. It is unclear if middle-aged males and females have similar adaptations to resistance training at the cellular level. To address this gap, 27 (13 males, 14 females) middle-aged (40-64 yr) adults participated in 10 wk of whole body resistance training with dietary counseling. Muscle biopsies were collected from the vastus lateralis pre- and posttraining. Type II fiber cross-sectional area increased similarly with training in both sexes (P = 0.014). MuSC content was not altered with training; however, training increased PDGFRα+/CD90+ FAP content (P < 0.0001) and reduced PDGFRα+/CD90- FAP content (P = 0.044), independent of sex. The number of CD31+ capillaries per fiber also increased similarly in both sexes (P < 0.05). These results suggest that muscle fiber hypertrophy, stem/progenitor cell, and capillary adaptations are similar between middle-aged males and females in response to whole body resistance training.NEW & NOTEWORTHY We demonstrate that resistance training-induced increases in fiber hypertrophy, FAP content, and capillarization are similar between middle-aged males and females.

Radiation induces long-term muscle fibrosis and promotes a fibrotic phenotype in fibro-adipogenic progenitors.

BACKGROUND: Radiation-induced muscle pathology, characterized by muscle atrophy and fibrotic tissue accumulation, is the most common debilitating late effect of therapeutic radiation exposure particularly in juvenile cancer survivors. In healthy muscle, fibro/adipogenic progenitors (FAPs) are required for muscle maintenance and regeneration, while in muscle pathology FAPs are precursors for exacerbated extracellular matrix deposition. However, the role of FAPs in radiation-induced muscle pathology has not previously been explored. METHODS: Four-week-old Male CBA or C57Bl/6J mice received a single dose (16 Gy) of irradiation (IR) to a single hindlimb with the shielded contralateral limb (CLTR) serving as a non-IR control. Mice were sacrificed 3, 7, 14 (acute IR response), and 56 days post-IR (long-term IR response). Changes in skeletal muscle morphology, myofibre composition, muscle niche cellular dynamics, DNA damage, proliferation, mitochondrial respiration, and metabolism and changes in progenitor cell fate where assessed. RESULTS: Juvenile radiation exposure resulted in smaller myofibre cross-sectional area, particularly in type I and IIA myofibres (P < 0.05) and reduced the proportion of type I myofibres (P < 0.05). Skeletal muscle fibrosis (P < 0.05) was evident at 56 days post-IR. The IR-limb had fewer endothelial cells (P < 0.05) and fibro-adipogenic progenitors (FAPs) (P < 0.05) at 56 days post-IR. Fewer muscle satellite (stem) cells were detected at 3 and 56 days in the IR-limb (P < 0.05). IR induced FAP senescence (P < 0.05), increased their fibrogenic differentiation (P < 0.01), and promoted their glycolytic metabolism. Further, IR altered the FAP secretome in a manner that impaired muscle satellite (stem) cell differentiation (P < 0.05) and fusion (P < 0.05). CONCLUSIONS: Our study suggests that following juvenile radiation exposure, FAPs contribute to long-term skeletal muscle atrophy and fibrosis. These findings provide rationale for investigating FAP-targeted therapies to ameliorate the negative late effects of radiation exposure in skeletal muscle.