Satellite Cell Abundance Research Articles

The Efficacy of Blood Flow Restriction Training to Improve Quadriceps Muscle Function after ACL Reconstruction.

Blood flow restriction training (BFRT) is a popular rehabilitation intervention after anterior cruciate ligament reconstruction (ACLR). However, there are a lack of clinical trials establishing the efficacy of using BFRT during rehabilitation to improve quadriceps muscle function. To evaluate the efficacy of blood flow restriction training to improve quadriceps muscle strength, morphology, and physiology, and knee biomechanics in individuals after ACLR in a double-blind, randomized, placebo-controlled clinical trial (NCT03364647). 48 athletes (20 females/28 males) were randomly assigned to low-load strength training with active BFRT or standard of care strength training with a sham unit. Treatment occurred for 1-month pre-surgery and 4-5 months post-surgery with both groups following the same standard rehabilitation protocol. Outcome variables were measured at baseline and 4-5 months post-surgery. Quadriceps muscle strength (isometric and isokinetic peak torque and rate of torque development) was measured on an isokinetic dynamometer. Quadriceps muscle morphology (physiological cross-sectional area, fibrosis) was determined using magnetic resonance imaging. Quadriceps muscle physiology (fiber type, fiber cross-sectional area, satellite cell abundance, collagen content, fibro-/adipo-genic progenitor cells) was evaluated with muscle biopsies of the vastus lateralis. Knee extensor moment and knee flexion angle were measured via three-dimensional gait analysis. Change scores were calculated as: post-intervention-baseline. Two sample t-tests were then used to assess between-group differences for each outcome variable. No significant between-group differences were found for any outcome variable. The addition of BFRT to a rehabilitation program for athletes pre- and post- ACLR was no more effective than standard rehabilitation for improving quadriceps muscle function. Clinicians should consider the value of BFRT relative to the cost, time, and discomfort for patients in light of these results.

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

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

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

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

Satellite cell depletion does not affect diaphragm adaptations to hypoxia.

The diaphragm is the main skeletal muscle responsible for inspiration and is susceptible to age-associated decline in function and morphology. Satellite cells in diaphragm fuse into unperturbed muscle fibers throughout life, yet their role in adaptations to hypoxia in diaphragm is unknown. Given their continual fusion, we hypothesize that satellite cell depletion will negatively impact adaptations to hypoxia in the diaphragm, particularly with aging. We used the Pax7CreER/CreER:R26RDTA/DTA genetic mouse model of inducible satellite cell depletion to investigate diaphragm responses to hypoxia in adult (6 mo) and aged (22 mo) male mice. The mice were subjected to normobaric hypoxia at 10% [Formula: see text] or normoxia for 4 wk. We showed that satellite cell depletion had no effect on diaphragm muscle fiber cross-sectional area, fiber-type distribution, myonuclear density, or regulation of extracellular matrix in either adult or aged mice. Furthermore, we showed lower muscle fiber cross-sectional area with hypoxia and age (main effects), while extracellular matrix content was higher and satellite cell abundance was lower with age (main effect) in diaphragm. Lastly, a greater number of Pax3-mRNA+ cells was observed in diaphragm muscle of satellite cell-depleted mice independent of hypoxia (main effect), potentially as a compensatory mechanism for the loss of satellite cells. We conclude that satellite cells are not required for diaphragm muscle adaptations to hypoxia in either adult or aged mice.NEW & NOTEWORTHY Satellite cells show consistent fusion into diaphragm muscle fibers throughout life, suggesting a critical role in maintaining homeostasis. Here, we report identical diaphragm adaptations to hypoxia with and without satellite cells in adult and aged mice. In addition, we propose that the higher number of Pax3-positive cells in satellite cell-depleted diaphragm muscle acts as a compensatory mechanism.

Skeletal Muscle Adaptations to Exercise Training in Young and Aged Horses.

In aged humans, low-intensity exercise increases mitochondrial density, function and oxidative capacity, decreases the prevalence of hybrid fibers, and increases lean muscle mass, but these adaptations have not been studied in aged horses. Effects of age and exercise training on muscle fiber type and size, satellite cell abundance, and mitochondrial volume density (citrate synthase activity; CS), function (cytochrome c oxidase activity; CCO), and integrative (per mg tissue) and intrinsic (per unit CS) oxidative capacities were evaluated in skeletal muscle from aged (n = 9; 22 ± 5 yr) and yearling (n = 8; 9.7 ± 0.7 mo) horses. Muscle was collected from the gluteus medius (GM) and triceps brachii at wk 0, 8, and 12 of exercise training. Data were analyzed using linear models with age, training, muscle, and all interactions as fixed effects. At wk 0, aged horses exhibited a lower percentage of type IIx (p = 0.0006) and greater percentage of hybrid IIa/x fibers (p = 0.002) in the GM, less satellite cells per type II fiber (p = 0.03), lesser integrative and intrinsic (p 0.04) CCO activities, lesser integrative oxidative phosphorylation capacity with complex I (PCI; p = 0.02) and maximal electron transfer system capacity (ECI+II; p = 0.06), and greater intrinsic PCI, ECI+II, and electron transfer system capacity with complex II (ECII; p 0.05) than young horses. The percentage of type IIx fibers increased (p < 0.0001) and of type IIa/x fibers decreased (p = 0.001) in the GM, and the number of satellite cells per type II fiber increased (p = 0.0006) in aged horses following exercise training. Conversely, the percentage of type IIa/x fibers increased (p ≤ 0.01) and of type IIx fibers decreased (p ≤ 0.002) in young horses. Integrative maximal oxidative capacity (p ≤ 0.02), ECI+II (p ≤ 0.07), and ECII (p = 0.0003) increased for both age groups from wk 0 to 12. Following exercise training, aged horses had a greater percentage of IIx (p ≤ 0.002) and lesser percentage of IIa/x fibers (p ≤ 0.07), and more satellite cells per type II fiber (p = 0.08) than young horses, but sustained lesser integrative and intrinsic CCO activities (p 0.04) and greater intrinsic PCI, ECI+II, and ECII (p 0.05). Exercise improved mitochondrial measures in young and aged horses; however, aged horses showed impaired mitochondrial function and differences in adaptation to exercise training.

Dysbiosis of the gut microbiome impairs mouse skeletal muscle adaptation to exercise.

There is emerging evidence of a gut microbiome-skeletal muscle axis. The purpose of this study was to determine if an intact gut microbiome was necessary for skeletal muscle adaptation to exercise. Forty-two 4-month-old female C57BL/6J mice were randomly assigned to untreated (U) or antibiotic-treated (T) non-running controls (CU or CT, respectively) or progressive weighted wheel running (PoWeR, P) untreated (PU) or antibiotic-treated (PT) groups. Antibiotic treatment resulted in disruption of the gut microbiome as indicated by a significant depletion of gut microbiome bacterial species in both CT and PT groups. The training stimulus was the same between PU and PT groups as assessed by weekly (12.35 ± 2.06 vs. 11.09 ± 1.76km/week, respectively) and total (778.9± 130.5 vs. 703.8 ± 112.9km, respectively) running activity. In response to PoWeR, PT showed less hypertrophy of soleus type 1 and 2a fibres and plantaris type 2b/x fibres compared to PU. The higher satellite cell and myonuclei abundance of PU plantaris muscle after PoWeR was not observed in PT. The fibre-type shift of PU plantaris muscle to a more oxidative type 2a fibre composition following PoWeR was blunted in PT. There was no difference in serum cytokine levels among all groups suggesting disruption of the gut microbiome did not induce systemic inflammation. The results of this study provide the first evidence that an intact gut microbiome is necessary for skeletal muscle adaptation to exercise. KEY POINTS: Dysbiosis of the gut microbiome caused by continuous antibiotic treatment did not affect running activity. Continuous treatment with antibiotics did not result in systemic inflammation as indicated by serum cytokine levels. Gut microbiome dysbiosis was associated with blunted fibre type-specific hypertrophy in the soleus and plantaris muscles in response to progressive weighted wheel running (PoWeR). Gut microbiome dysbiosis was associated with impaired PoWeR-induced fibre-type shift in the plantaris muscle. Gut microbiome dysbiosis was associated with a loss of PoWeR-induced myonuclei accretion in the plantaris muscle.

Metformin and leucine increase satellite cells and collagen remodeling during disuse and recovery in aged muscle.

Loss of muscle mass and strength after disuse followed by impaired muscle recovery commonly occurs with aging. Metformin (MET) and leucine (LEU) individually have shown positive effects in skeletal muscle during atrophy conditions but have not been evaluated in combination nor tested as a remedy to enhance muscle recovery following disuse atrophy in aging. The purpose of this study was to determine if a dual treatment of metformin and leucine (MET + LEU) would prevent disuse-induced atrophy and/or promote muscle recovery in aged mice and if these muscle responses correspond to changes in satellite cells and collagen remodeling. Aged mice (22-24 months) underwent 14days of hindlimb unloading (HU) followed by 7 or 14days of reloading (7 or 14days RL). MET, LEU, or MET + LEU was administered via drinking water and were compared to Vehicle (standard drinking water) and ambulatory baseline. We observed that during HU, MET + LEU resolved whole body grip strength and soleus muscle specific force decrements caused by HU. Gastrocnemius satellite cell abundance was increased with MET + LEU treatment but did not alter muscle size during disuse or recovery conditions. Moreover, MET + LEU treatment alleviated gastrocnemius collagen accumulation caused by HU and increased collagen turnover during 7 and 14days RL driven by a decrease in collagen IV content. Transcriptional pathway analysis revealed that MET + LEU altered muscle hallmark pathways related to inflammation and myogenesis during HU. Together, the dual treatment of MET and LEU was able to increase muscle function, satellite cell content, and reduce collagen accumulation, thus improving muscle quality during disuse and recovery in aging.

Age-Related Susceptibility to Muscle Damage Following Mechanotherapy in Rats Recovering From Disuse Atrophy.

The inability to fully recover lost muscle mass following periods of disuse atrophy predisposes older adults to lost independence and poor quality of life. We have previously shown that mechanotherapy at a moderate load (4.5 N) enhances muscle mass recovery following atrophy in adult, but not older adult rats. We propose that elevated transverse stiffness in aged muscle inhibits the growth response to mechanotherapy and hypothesize that a higher load (7.6 N) will overcome this resistance to mechanical stimuli. F344/BN adult and older adult male rats underwent 14 days of hindlimb suspension, followed by 7 days of recovery with (RE + M) or without (RE) mechanotherapy at 7.6 N on gastrocnemius muscle. The 7.6 N load was determined by measuring transverse passive stiffness and linearly scaling up from 4.5 N. No differences in protein turnover or mean fiber cross-sectional area were observed between RE and RE + M for older adult rats or adult rats at 7.6 N. However, there was a higher number of small muscle fibers present in older adult, but not adult rats, which was explained by a 16-fold increase in the frequency of small fibers expressing embryonic myosin heavy chain. Elevated central nucleation, satellite cell abundance, and dystrophin-/laminin+ fibers were present in older adult rats only following 7.6 N, while 4.5 N did not induce damage at either age. We conclude that age is an important variable when considering load used during mechanotherapy and age-related transverse stiffness may predispose older adults to damage during the recovery period following disuse atrophy.

Multiomics analysis of the mdx/mTR mouse model of Duchenne muscular dystrophy

ABSTRACT Purpose/Aim Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease characterized by extensive muscle weakness. Patients with DMD lack a functional dystrophin protein, which transmits force and organizes the cytoskeleton of skeletal muscle. Multiomic studies have been proposed as a way to obtain novel insight about disease processes from preclinical models, and we used this approach to study pathological changes in dystrophic muscles. Materials and Methods We evaluated hindlimb muscles of male mdx/mTR mice, which lack a functional dystrophin protein and have deficits in satellite cell abundance and proliferative capacity. Wild type (WT) C57BL/6 J mice served as controls. Muscle fiber contractility was measured, along with changes in the transcriptome using RNA sequencing, and in the proteome, metabolome, and lipidome using mass spectrometry. Results While mdx/mTR mice displayed gross pathological changes and continued cycles of degeneration and regeneration, we found no differences in permeabilized fiber contractility between strains. However, there were numerous changes in the transcriptome and proteome related to protein balance, contractile elements, extracellular matrix, and metabolism. There was only a 53% agreement in fold-change data between the proteome and transcriptome. Numerous changes in markers of skeletal muscle metabolism were observed, with dystrophic muscles exhibiting elevated glycolytic metabolites such as 6-phosphoglycerate, fructose-6-phosphate and glucose-6-phosphate, fructose bisphosphate, phosphorylated hexoses, and phosphoenolpyruvate. Conclusions These findings highlight the utility of multiomics in studying muscle disease, and provide additional insight into the pathological changes in dystrophic muscles that might help to indirectly guide evidence-based nutritional or exercise prescription in DMD patients.

Continuous Feeding Does Not Blunt Satellite Cell Abundance, Myonuclear Accretion, or Lean Growth in a Neonatal Piglet Model of Prematurity

Abstract Objectives Lean growth in preterm neonates is poor and may increase the lifelong risk for adverse health outcomes. Strategies are needed to promote skeletal muscle growth in the postnatal period. Skeletal muscle growth, which accounts for the largest fraction of lean mass accretion, requires the coordinated activation of protein synthesis, satellite cell (SC; muscle stem cell) proliferation, differentiation, and fusion into muscle fibers. The objective of this study was to determine the effect of feeding modality on SC abundance, myonuclear accretion, and lean growth in preterm neonatal pigs. Methods Pigs delivered 10 d preterm by C-section (952 ± 205 g) were fitted with an umbilical artery catheter (later replaced with jugular vein catheter) and an orogastric tube for parenteral and enteral nutrition, respectively. Pigs were assigned to continuous (CONT; n = 14; 7.5 mL/[kg·h]) or intermittent bolus (INT; n = 30; 30 mL/kg every 4 h) feeding for 21 d (210 kcal/kg and 16 g protein/kg per d); pigs were advanced from parenteral to enteral feeding over 6 d. Bromodeoxyuridine (BrdU; 25 mg·kg−1) was administered to pigs every 12 h from days 19 to 20, inclusive, to label proliferating cells. Body composition was measured by DXA on day 21. Satellite cell (Pax7+) abundance, myonuclear accretion, and muscle fiber cross-sectional area (CSA) were quantified in the longissimus dorsi muscle by immunofluorescence. Results Sublaminal Pax7+ SC abundance was similar between CONT and INT groups (60.9 vs. 58.3 ± 6.0 per 1000 fibers). The proportion of sublaminal relative to total Pax7+ SCs was similar between CONT and INT groups (78.8 vs. 78.8 ± 2.2%). The abundance of BrdU + myonuclei, an index of myonuclear accretion, did not differ between CONT and INT groups (26.7 vs. 26.7 ± 3.4 per 1000 fibers). Total myonuclei did not differ between CONT and INT groups (420 vs. 403 ± 16 per 1000 fibers). Muscle fiber CSA did not differ between CONT and INT groups (210 vs. 237 ± 15 µm2). Lean and fat masses were similar between groups. Conclusions Unlike term pigs, CONT feeding does not blunt lean growth in preterm pigs compared to INT feeding. The absence of increased lean growth with INT feeding is consistent with the similar SC abundance, myonuclear accretion, and muscle fiber CSA between feeding modalities. Funding Sources Research was supported by NIH and USDA.

Mechano‐therapy using high loads enhances regrowth of skeletal muscle in young, but induces muscle damage in old rats.

IntroductionOlder adults have an impaired ability to recover from skeletal muscle atrophy compared to young, predisposing them to loss of independence and poor quality of life. We have previously shown that mechano‐therapy in the form of cyclic, compressive loading (CCL) enhances regrowth of atrophied skeletal muscle in young adult rats, but not in old. Our preliminary data show that passive, transverse stiffness of muscles from old rats is elevated compared to young, potentially blunting the mechanical stimulus and subsequent growth response. In the current study, we hypothesized that an increased CCL load proportional to the higher tissue stiffness in old rats would enhance skeletal muscle regrowth when compared to atrophied muscles of old rats subjected to a lower load of CCL or ambulation alone.MethodsYoung (10 months; N = 6) and old (30 months; N = 6) adult male F344/Brown Norway rats underwent 14 days of hind limb suspension before being randomly assigned to one of the following: reloading for 7 days without CCL (RE); reloading with 4.5 N CCL (RE + CCL, 4.5 N); or reloading with 7.6 N CCL (RE + CCL, 7.6 N). Starting on day 0, four bouts of CCL were administered to the right gastrocnemius using a custom designed device capable of applying a precise load, with 48 hrs between bouts. Gastrocnemius muscle sections were analyzed for muscle fiber cross‐sectional area (CSA), satellite cell abundance (Pax7+/fiber), embryonic myosin heavy chain expression (eMyHC+/fiber) and centro‐nucleation using immunohistochemistry.ResultsMuscle fiber CSA was load dependent in young adult rats, but had no effect at either load in old rats. When compared to muscles from RE rats, 4.5 N CCL had no effect on satellite cell abundance regardless of age; however, satellite cell abundance was higher in both young and old adult muscle after 7.6 N CCL compared to RE with old rats having 2‐fold higher levels compared to RE. Regardless of load, CCL was not associated with differences in eMyHC expression or centrally located nuclei in young adult rat muscle, while muscles receiving 7.6 N CCL in old rats had a higher number of centrally located nuclei and were ~16‐fold higher in eMyHC+ fibers compared to RE.ConclusionsThese results show that CCL induces a load dependent anabolic response in young adult animals during muscle mass recovery following disuse, but has no regrowth effect on muscles from old animals at either load. Furthermore, using a higher CCL load, scaled for elevated transverse stiffness in aged muscle, induces damage in muscle of old but not young rats.Support or Funding InformationThis research was supported by NIH grant RO1AT009268 and R21AG042699.

Resistance exercise training promotes fiber type-specific myonuclear adaptations in older adults.

Aging induces physiological decline in human skeletal muscle function and morphology, including type II fiber atrophy and an increase in type I fiber frequency. Resistance exercise training (RET) is an effective strategy to overcome muscle mass loss and improve strength, with a stronger effect on type II fibers. In the present study, we sought to determine the effect of a 12-wk progressive RET program on the fiber type-specific skeletal muscle hypertrophic response in older adults. Nineteen subjects [10 men and 9 women (71.1 ± 4.3 yr)] were studied before and after the 12-wk program. Immunohistochemical analysis was used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area (CSA), satellite cell abundance, myonuclear content, and lipid droplet density. RET induced an increase in MyHC type II fiber frequency and a concomitant decrease in MyHC type I fiber frequency. Mean CSA increased significantly only in MyHC type II fibers (+23.3%, P < 0.05), but myonuclear content increased only in MyHC type I fibers (P < 0.05), with no change in MyHC type II fibers. Satellite cell content increased ~40% in both fiber types (P > 0.05). RET induced adaptations to the capillary supply to satellite cells, with the distance between satellite cells and the nearest capillary increasing in type I fibers and decreasing in type II fibers. Both fiber types showed similar decrements in intramuscular lipid density with training (P < 0.05). Our data provide intriguing evidence for a fiber type-specific response to RET in older adults and suggest flexibility in the myonuclear domain of type II fibers during a hypertrophic stimulus.NEW & NOTEWORTHY In older adults, progressive resistance exercise training (RET) increased skeletal muscle fiber volume and cross-sectional area independently of myonuclear accretion, leading to an expansion of the myonuclear domain. Fiber type-specific analyses illuminated differential adaptation; type II fibers underwent hypertrophy and exhibited myonuclear domain plasticity, whereas myonuclear accretion occurred in type I fibers in the absence of a robust hypertrophic response. RET also augmented satellite cell-capillary interaction and reduced intramyocellular lipid density to improve muscle quality.

Leucine Supplementation Does Not Restore Diminished Skeletal Muscle Satellite Cell Abundance and Myonuclear Accretion When Protein Intake Is Limiting in Neonatal Pigs

BackgroundRapid growth of skeletal muscle in the neonate requires the coordination of protein deposition and myonuclear accretion. During this developmental stage, muscle protein synthesis is highly sensitive to amino acid supply, especially Leu, but we do not know if this is true for satellite cells, the source of muscle fiber myonuclei. ObjectiveWe examined whether dietary protein restriction reduces myonuclear accretion in the neonatal pig, and if any reduction in myonuclear accretion is mitigated by restoring Leu intake. MethodsNeonatal pigs (1.53 ± 0.2 kg) were fitted with jugular vein and gastric catheters and fed 1 of 3 isoenergetic milk replacers every 4 h for 21 d: high protein [HP; 22.5 g protein/(kg/d); n = 8]; restricted protein [RP; 11.2 g protein/(kg/d);n= 10]; or restricted protein with Leu [RPL; 12.0 g protein/(kg/d);n= 10]. Pigs were administered 5-bromo-2´-deoxyuridine (BrdU; 15 mg/kg) intravenously every 12 h from days 6 to 8. Blood was sampled on days 6 and 21 to measure plasma Leu concentrations. On day 21, pigs were killed and the longissimus dorsi (LD) muscle was collected to measure cell morphometry, satellite cell abundance, myonuclear accretion, and insulin-like growth factor (IGF) system expression. ResultsCompared with HP pigs, postprandial plasma Leu concentration in RP pigs was 37% and 47% lower on days 6 and 21, respectively (P < 0.05); Leu supplementation in RPL pigs restored postprandial Leu to HP concentrations. Dietary protein restriction reduced LD myofiber cross-sectional area by 21%, satellite cell abundance by 35%, and BrdU+ myonuclear abundance by 25% (P < 0.05); Leu did not reverse these outcomes. Dietary protein restriction reduced LD muscleIGF2expression by 60%, but notIGF1or IGF1Rexpression (P < 0.05); Leu did not rescueIGF2expression. ConclusionsSatellite cell abundance and myonuclear accretion in neonatal pigs are compromised when dietary protein intake is restricted and are not restored with Leu supplementation.

Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: A randomized, double‐blind, placebo‐controlled, multicenter trial: The MASTERS trial

Progressive resistance exercise training (PRT) is the most effective known intervention for combating aging skeletal muscle atrophy. However, the hypertrophic response to PRT is variable, and this may be due to muscle inflammation susceptibility. Metformin reduces inflammation, so we hypothesized that metformin would augment the muscle response to PRT in healthy women and men aged 65 and older. In a randomized, double‐blind trial, participants received 1,700 mg/day metformin (N = 46) or placebo (N = 48) throughout the study, and all subjects performed 14 weeks of supervised PRT. Although responses to PRT varied, placebo gained more lean body mass (p = .003) and thigh muscle mass (p < .001) than metformin. CT scan showed that increases in thigh muscle area (p = .005) and density (p = .020) were greater in placebo versus metformin. There was a trend for blunted strength gains in metformin that did not reach statistical significance. Analyses of vastus lateralis muscle biopsies showed that metformin did not affect fiber hypertrophy, or increases in satellite cell or macrophage abundance with PRT. However, placebo had decreased type I fiber percentage while metformin did not (p = .007). Metformin led to an increase in AMPK signaling, and a trend for blunted increases in mTORC1 signaling in response to PRT. These results underscore the benefits of PRT in older adults, but metformin negatively impacts the hypertrophic response to resistance training in healthy older individuals. ClinicalTrials.gov Identifier: NCT02308228.

Resident muscle stem cells are not required for testosterone-induced skeletal muscle hypertrophy.

It is postulated that testosterone-induced skeletal muscle hypertrophy is driven by myonuclear accretion as the result of satellite cell fusion. To directly test this hypothesis, we utilized the Pax7-DTA mouse model to deplete satellite cells in skeletal muscle followed by testosterone administration. Pax7-DTA mice (6 mo of age) were treated for 5 days with either vehicle [satellite cell replete (SC+)] or tamoxifen [satellite cell depleted (SC-)]. Following a washout period, a testosterone propionate or sham pellet was implanted for 21 days. Testosterone administration caused a significant increase in muscle fiber cross-sectional area in SC+ and SC- mice in both oxidative (soleus) and glycolytic (plantaris and extensor digitorum longus) muscles. In SC+ mice treated with testosterone, there was a significant increase in both satellite cell abundance and myonuclei that was completely absent in testosterone-treated SC- mice. These findings provide direct evidence that testosterone-induced muscle fiber hypertrophy does not require an increase in satellite cell abundance or myonuclear accretion.Listen to a podcast about this Rapid Report with senior author E. E. Dupont-Versteegden (https://ajpcell.podbean.com/e/podcast-on-paper-that-shows-testosterone-induced-skeletal-muscle-hypertrophy-does-not-need-muscle-stem-cells/).

Chronic doxorubicin administration impacts satellite cell and capillary abundance in a muscle-specific manner.

Anthracycline chemotherapies are effective at reducing disease recurrence and mortality in cancer patients. However, these drugs also contribute to skeletal muscle wasting and dysfunction. The purpose of this study was to assess the impact of chronic doxorubicin (DOX) administration on satellite cell and capillary densities in different skeletal muscles. We hypothesized that DOX would reduce satellite cell and capillary densities of the soleus (SOL) and extensor digitorum longus (EDL) muscles, along with muscle fiber size. Ovariectomized female Sprague‐Dawley rats were randomized to receive three bi‐weekly intraperitoneal injections of DOX (4 mg∙kg−1; cumulative dose 12 mg∙kg−1) or vehicle (VEH; saline). Animals were euthanized 5d following the last injection and the SOL and EDL were dissected and prepared for immunohistochemical and RT‐qPCR analyses. Relative to VEH, CSA of the SOL and EDL fibers were 26% and 33% smaller, respectively, in DOX (P < 0.05). In the SOL, satellite cell and capillary densities were 39% and 35% lower, respectively, in DOX (P < 0.05), whereas in the EDL satellite cell and capillary densities were unaffected by DOX administration (P > 0.05). Proliferating satellite cells were unaffected by DOX in the SOL (P > 0.05). In the SOL, MYF5 mRNA expression was increased in DOX (P < 0.05), while in the EDL MGF mRNA expression was reduced in DOX (P < 0.05). Chronic DOX administration is associated with reduced fiber size in the SOL and EDL; however, DOX appeared to reduce satellite cell and capillary densities only in the SOL. These findings highlight that therapeutic targets to protect skeletal muscle from DOX may vary across muscles.

Blood Flow Restriction Training in Rehabilitation Following Anterior Cruciate Ligament Reconstructive Surgery: A Review

Anterior cruciate ligament (ACL) rupture is a highly prevalent orthopedic injury, resulting in substantial skeletal muscle atrophy because of changes in muscle protein balance and satellite cell abundance. Neural activation problems also contribute to strength loss, impacting upon a patients’ physical function and rehabilitative capacity. Heavy loads typically required for muscle hypertrophy and strength adaptations are contraindicated because of graft strain and concomitant cartilage, meniscal, and bone pathologies associated with ACL reconstruction. Strength of the quadriceps is a fundamental component for the ability to reduce shearing and torsional strains on the ACL with ground contact, and forms a critical component of ACL rehabilitation. Given the dangers of early postoperative heavy-loading, low-load blood flow restriction (BFR) training may provide an alternative rehabilitation tool for practitioners. Passive BFR can attenuate early muscle atrophy and strength loss, and may be more effective with the addition of novel, complementary therapies such as neuromuscular electrical stimulation. Upon ambulation, aerobic, and resistance exercise with BFR can stimulate muscle hypertrophy and strength adaptations and resolve activation problems. This may occur through increasing muscle protein synthesis and satellite cell proliferation, decreasing muscle protein breakdown and improving muscle activation by altered recruitment patterns. Thus, BFR training may provide an effective rehabilitation tool that does not place heavy loads and force through the tibiofemoral joint. This may reduce the risk of damaging the graft, cartilage, meniscus, or other intra-articular structures, providing thorough screening before use is followed by correct, evidence-informed application.

Alterations in quadriceps muscle cellular and molecular properties in adults with moderate knee osteoarthritis

Quadriceps muscle weakness is common in knee osteoarthritis (OA). While pain, disuse, and atrophy are commonly cited causes for muscle weakness in OA, emerging evidence suggests changes in muscle quality also occur. Alterations in muscle quality are not well understood, but likely include both cellular and morphologic adaptions. The purpose of this study was to conduct the first cellular-level analysis of the vastus lateralis in adults with moderate knee OA. Vastus lateralis biopsies were obtained from 24 subjects with moderate knee OA and 15 healthy controls. Quadriceps strength, muscle fiber cross sectional area (CSA), fiber type distribution, extracellular matrix (ECM) content, satellite cell abundance, and profibrotic gene expression were assessed. Relative to controls, quadriceps strength was significantly lower in OA subjects (OA 62.23, 50.67-73.8Nm vs 91.46, 75.91-107.0Nm, P=0.003) despite no difference in fiber CSA. OA subjects had significantly fewer Type I fibers (OA 41.51, 35.56-47.47% vs 53.07, 44.86-61.29%, P=0.022) and more hybrid IIa/x fibers (OA 24.61, 20.61-28.61% vs 16.4, 11.60-21.20%, P=0.009). Significantly greater ECM content, lower satellite cell density, and higher profibrotic gene expression was observed with OA, and muscle collagen content was inversely correlated to strength and satellite cell (SC) density. Lower quadriceps function with moderate OA may not result from fiber size impairments, but is associated with ECM expansion. Impaired satellite cell density, high profibrotic gene expression, and a slow-to-fast fiber type transition may contribute to reduced muscle quality in OA. These findings can help guide therapeutic interventions to enhance muscle function with OA.

Voluntary wheel running increases satellite cell abundance and improves recovery from disuse in gastrocnemius muscles from mice.

Reloading of atrophied muscles after hindlimb suspension unloading (HSU) can induce injury and prolong recovery. Low-impact exercise, such as voluntary wheel running, has been identified as a nondamaging rehabilitation therapy in rodents, but its effects on muscle function, morphology, and satellite cell activity after HSU are unclear. This study tested the hypothesis that low-impact wheel running would increase satellite cell proliferation and improve recovery of muscle structure and function after HSU in mice. Young adult male and female C57BL/6 mice ( n = 6/group) were randomly placed into five groups. These included HSU without recovery (HSU), normal ambulatory recovery for 14 days after HSU (HSU+NoWR), and voluntary wheel running recovery for 14 days after HSU (HSU+WR). Two control groups were used: nonsuspended mouse cage controls (Control) and voluntary wheel running controls (ControlWR). Satellite cell activation was evaluated by providing mice 5-bromo-2'-deoxyuridine (BrdU) in their drinking water. As expected, HSU significantly reduced in vivo maximal force, decreased in vivo fatigability, and decreased type I and IIa myosin heavy chain (MHC) abundance in plantarflexor muscles. HSU+WR mice significantly improved plantarflexor fatigue resistance, increased type I and IIa MHC abundance, increased fiber cross-sectional area, and increased the percentage of type I and IIA muscle fibers in the gastrocnemius muscle. HSU+WR mice also had a significantly greater percentage of BrdU-positive and Pax 7-positive nuclei inside muscle fibers and a greater MyoD-to-Pax 7 protein ratio compared with HSU+NoWR mice. The mechanotransduction protein Yes-associated protein (YAP) was elevated with reloading after HSU, but HSU+WR mice had lower levels of the inactive phosphorylated YAPserine127, which may have contributed to increased satellite cell activation with reloading after HSU. These results indicate that voluntary wheel running increased YAP signaling and satellite cell activity after HSU and this was associated with improved recovery. NEW & NOTEWORTHY Although satellite cell involvement in muscle remodeling has been challenged, the data in this study suggest that voluntary wheel running increased satellite cell activity and suppressed Yes-associated protein (YAP) protein relative to no wheel running and this was associated with improved muscle recovery of force, fatigue resistance, expression of type I myosin heavy chain, and greater fiber cross-sectional area after disuse.

Altered satellite cell dynamics accompany skeletal muscle atrophy during chronic illness, disuse, and aging.

This review explores recent research investigating the contribution of satellite cells (skeletal muscle stem cells) during muscle fiber atrophy as seen in periods of disuse, illness, and aging. Studies indicate reduced satellite cell activity and density in a variety of acute and chronic conditions characterized by robust muscle wasting. The direct contribution of satellite cells to unloading/denervation and chronic illness-induced atrophy remains controversial. Inflammation that accompanies acute trauma and illness likely impedes proper satellite cell differentiation and myogenesis, promoting the rapid onset of muscle wasting in these conditions. Transgenic mouse studies provide surprising evidence that age-related declines in satellite cell function and abundance are not causally related to the onset of sarcopenia in sedentary animals. Recent clinical and preclinical studies indicate reduced abundance and dysregulated satellite cell activity that accompany muscle atrophy during periods of disuse, illness, and aging, providing evidence for their therapeutic potential.