Satellite Cell Response Research Articles

The Effect of Multi-Ingredient Protein versus Collagen Supplementation on Satellite Cell Properties in Males and Females.

Skeletal muscle satellite cells (SC) contribute to the adaptive process of resistance exercise training (RET) and may be influenced by nutritional supplementation. However, little research exists on the impact of multi-ingredient supplementation on the SC response to RET. We tested the effect of a multi-ingredient supplement (MIS) including whey protein, creatine, leucine, calcium citrate, and vitamin D on SC content and activity as well as myonuclear accretion, SC and myonuclear domain compared with a collagen control (COL) throughout a 10-wk RET program. Twenty-six participants underwent a 10-wk linear RET program while consuming either the MIS or COL supplement twice daily. Muscle biopsies were taken from the vastus lateralis at baseline and 48 h after a bout of damaging exercise, before and after RET. Muscle tissue was analyzed for SC and myonuclear content, domain, acute SC activation, and fiber cross-sectional area (fCSA). MIS resulted in a greater increase in type II fCSA following 10 wk of RET (effect size (ES) = 0.89) but not myonuclear accretion or SC content. Change in myonuclei per fiber was positively correlated with type I and II and total fiber hypertrophy in the COL group only, indicating a robust independent effect of MIS on fCSA. Myonuclear domain increased similarly in both groups, whereas SC domain remained unchanged following RET. SC activation was similar between groups for all fiber types in the untrained state but showed a trend toward greater increases with MIS after RET (ES = 0.70). SC responses to acute damaging exercise and long-term RET are predominantly similar in MIS and COL groups. However, MIS can induce greater increases in type II fCSA with RET and potentially SC activation following damage in the trained state.

Within-subject Comparison Reveals Enhanced Satellite Cell Response To Eccentric Overload During Heavy Resistance Exercise

Within-subject Comparison Reveals Enhanced Satellite Cell Response To Eccentric Overload During Heavy Resistance Exercise

Glutathione Levels are Critical for Satellite Cells Response and Skeletal Muscle Regeneration Following Aseptic Myotrauma in Humans

Glutathione Levels are Critical for Satellite Cells Response and Skeletal Muscle Regeneration Following Aseptic Myotrauma in Humans

Acute hypoxia attenuates resistance exercise-induced ribosome signaling but does not impact satellite cell pool expansion in human skeletal muscle.

Cumulative evidence supports the hypothesis that hypoxia acts as a regulator of muscle mass. However, the underlying molecular mechanisms remain incompletely understood, particularly in human muscle. Here we examined the effect of hypoxia on signaling pathways related to ribosome biogenesis and myogenic activity following an acute bout of resistance exercise. We also investigated whether hypoxia influenced the satellite cell response to resistance exercise. Employing a randomized, crossover design, eight men performed resistance exercise in normoxia (FiO2 21%) or normobaric hypoxia (FiO2 12%). Muscle biopsies were collected in a time-course manner (before, 0, 90, 180 min and 24 h after exercise) and were analyzed with respect to cell signaling, gene expression and satellite cell content using immunoblotting, RT-qPCR and immunofluorescence, respectively. In normoxia, resistance exercise increased the phosphorylation of RPS6, TIF-1A and UBF above resting levels. Hypoxia reduced the phosphorylation of these targets by ~37%, ~43% and ~ 67% throughout the recovery period, respectively (p < .05 vs. normoxia). Resistance exercise also increased 45 S pre-rRNA expression and mRNA expression of c-Myc, Pol I and TAF-1A above resting levels, but no differences were observed between conditions. Similarly, resistance exercise increased mRNA expression of myogenic regulatory factors throughout the recovery period and Pax7+ cells were elevated 24 h following exercise in mixed and type II muscle fibers, with no differences observed between normoxia and hypoxia. In conclusion, acute hypoxia attenuates ribosome signaling, but does not impact satellite cell pool expansion and myogenic gene expression following a bout of resistance exercise in human skeletal muscle.

Heat shock protein 27 regulates myogenic and self-renewal potential of bovine satellite cells under heat stress.

While satellite cells play a key role in the hypertrophy, repair, and regeneration of skeletal muscles, their response to heat exposure remains poorly understood, particularly in beef cattle. This study aimed to investigate the changes in the transcriptome, proteome, and proliferation capability of bovine satellite cells in response to different levels of heat stress (HS) and exposure times. Satellite cells were isolated from 3-mo-old Holstein bulls (body weight: 77.10 ± 2.02 kg) and subjected to incubation under various temperature conditions: 1) control (38 °C; CON), 2) moderate (39.5 °C; MHS), and extreme (41 °C; EHS) for different durations ranging from 0 to 48 h. Following 3 h of exposure to extreme heat (EHS), satellite cells exhibited significantly increased gene expression and protein abundance of heat shock proteins (HSPs; HSP70, HSP90, HSP20) and paired box gene 7 (Pax7; P < 0.05). HSP27 expression peaked at 3 h of EHS and remained elevated until 24 h of exposure (P < 0.05). In contrast, the expression of myogenic factor 5 (Myf5) and paired box gene 3 (Pax3) was decreased by EHS compared to the control at 3 h of exposure (P < 0.05). Notably, the introduction of HSP27 small interference RNA (siRNA) transfection restored Myf5 expression to control levels, suggesting an association between HSP27 and Myf5 in regulating the self-renewal properties of satellite cells upon heat exposure. Immunoprecipitation experiments further confirmed the direct binding of HSP27 to Myf5, supporting its role as a molecular chaperone for Myf5. Protein-protein docking algorithms predicted a high probability of HSP27-Myf5 interaction as well. These findings indicate that extreme heat exposure intrinsically promotes the accumulation of HSPs and modulates the early myogenic regulatory factors in satellite cells. Moreover, HSP27 acts as a molecular chaperone by binding to Myf5, thereby regulating the division or differentiation of satellite cells in response to HS. The results of this study provide a better understanding of muscle physiology in heat-stressed cells, while unraveling the intricate molecular mechanisms that underlie the HS response in satellite cells.

Aerobic conditioning alters the satellite cell and ribosome response to acute eccentric contractions in young men and women.

Satellite cells (SCs) and ribosomes are key determinants of the skeletal muscle adaptive response. Both are thought to increase acutely after resistance exercise and chronically with resistance training. However, the acute SC and ribosome exercise response with prior aerobic conditioning is unknown. Fourteen young men and women underwent 6 wk of single-legged aerobic conditioning followed by an acute bout of 300 eccentric contractions on each leg. Muscle biopsies were taken from the vastus lateralis of the aerobically conditioned (AC) and the control (CTL) legs before (Pre), 24 (24 h), and 48 (48 h) h post-contractions. Pre-eccentric contractions, 45S pre-rRNA and 5.8S internal transcribed spacer (ITS) expression were lower in the AC leg compared with the CTL leg. SC content (PAX7+ cells/100 fibers) in type I and mixed fibers showed a main effect of condition, where values were greater in the AC leg compared with the CTL. A main effect of condition for Pax7 and MyoD1 mRNA expression was observed where expression was greater in the AC leg compared with the CTL. AC had greater RNA concentration and mRNA expression of Ubf and Tif-1a compared with CTL. Only the AC leg increased (Pre-24h) 45S pre-rRNA, 5.8S ITS, and 28S ITS following eccentric contractions. We discovered that aerobic conditioning increased type-I SC abundance and the acute increase in ribosome content following eccentric contractions.

The ultrastructure of muscle fibers and satellite cells in experimental autoimmune encephalomyelitis after treatment with transcranial magnetic stimulation

ABSTRACT In this study, we investigated the effect of transcranial magnetic stimulation (TMS) on the ultrastructure of muscle fibers and satellite cells in rats with experimental autoimmune encephalomyelitis (EAE). EAE-induced animals were treated with TMS (60 Hz at 0.7 mT) for 2 hours in the morning, once a day, 5 days a week, for 3 weeks, starting on day 15 post-immunization. The rats were sacrificed on day 36 post-immunization, and the soleus muscles were evaluated by light microscopy and transmission electron microscopy. Findings were compared with a non-treated EAE group. Electron microscopy analysis showed the presence of degenerated mitochondria, autophagic vacuoles, and altered myofibrils in non-treated EAE group. This correlates with the presence of acid phosphatase activity in muscle fibers and core-targetoid lesions with desmin immunohistochemistry. Most myonuclei in the EAE group showed apoptotic features. In contrast, EAE induced-TMS treated animals had less ultrastructural changes in the mitochondria and the myofibrils, together with less frequent apoptotic nuclear features. Peripheral desmin+ protrusions, as a marker of active satellite cells, were significantly increased in TMS-treated group. This correlates ultrastructurally with the presence of active features in satellite cells in the TMS group. In conclusion, the attenuation of ultrastructural alterations in muscle fibers and activation response of satellite cells caused by EAE indicated that skeletal muscle had a regenerative response to TMS.

Short-term aerobic conditioning prior to resistance training augments muscle hypertrophy and satellite cell content in healthy young men and women.

Factors influencing inter-individual variability of responses to resistance training (RT) remain to be fully elucidated. We have proposed the importance of capillarization in skeletal muscle for the satellite cell (SC) response to RT-induced muscle hypertrophy, and hypothesized that aerobic conditioning (AC) would augment RT-induced adaptations. Fourteen healthy young (22 ± 2 years) men and women underwent AC via 6 weeks of unilateral cycling followed by 10 weeks of bilateral RT to investigate how AC alters SC content, activity, and muscle hypertrophy following RT. Muscle biopsies were taken at baseline (unilateral), post AC (bilateral), and post RT (bilateral) in the aerobically conditioned (AC + RT) and unconditioned (RT) legs. Immunofluorescence was used to determine muscle capillarization, fiber size, SC content, and activity. Type I and type II fiber cross-sectional area (CSA) increased following RT, and when legs were analyzed independently, AC + RT increased type I, type II, and mixed-fiber CSA, where the RT leg tended to increase type II (p=.05), but not type I or mixed-fiber CSA. SC content, activation, and differentiation increased with RT, where type I total and quiescent SC content was greater in AC + RT compared to the RT leg. Those with the greatest capillary-to-fiber perimeter exchange index before RT had the greatest change in CSA following RT and a significant relationship was observed between type II fiber capillarization and the change in type II-fiber CSA with RT (r=0.35). This study demonstrates that AC prior to RT can augment RT-induced muscle adaptions and that these differences are associated with increases in capillarization.

Skeletal Muscle Satellite Cells in Sickle Cell Disease Patients and Their Responses to a Moderate-intensity Endurance Exercise Training Program.

We previously demonstrated that 8 weeks of moderate-intensity endurance training is safe and improves muscle function and characteristics of sickle cell disease (SCD) patients. Here, we investigated skeletal muscle satellite cells (SCs) in SCD patients and their responses to a training program. Fifteen patients followed the training program while 18 control patients maintained a normal lifestyle. Biopsies of the vastus lateralis muscle were performed before and after training. After training, the cross-sectional area and myonuclear content in type I fibers were slightly increased in the training patients compared to non-training patients. The SC pool was unchanged in type I fibers while it was slightly decreased in type II fibers in the training patients compared to non-training patients. No necrotic fibers were detected in patients before or after training. Therefore, the slight myonuclear accretion in type I fibers in trained SCD patients may highlight the contribution of SCs to training-induced slight type I fiber hypertrophy without expansion of the SC pool. The low training intensity and the short duration of training sessions could explain the low SC response to the training program. However, the lack of necrotic fibers suggests that the training program seemed to be safe for patients' muscle tissue.

The Influence of Androgen Receptors on Muscle Repair and Hypertrophy

The mechanisms underlying inter‐individual variation in adaptation to exercise have yet to be fully elucidated. Recently, high responders to resistance training (RT) have demonstrated greater levels of androgen receptor (AR) content, and therefore AR may be a factor underlying the variation in the adaptive response to resistance exercise. Moreover, there has been inconclusive findings surrounding the impact of AR content in females as well as following damage. In the current study, twenty‐six healthy young men (n=13) and women (n=13) underwent an acute bout of damaging exercise consisting of 300 eccentric kicks, followed by 10 weeks of full‐body RT to investigate changes in AR content following acute muscle damage and following chronic resistance exercise. This was assessed in the context of the satellite cell response and during muscle repair and hypertrophy. Skeletal muscle biopsies from the vastus lateralis were taken at baseline, 48 hours post damage and 48 hours following the last bout of RT. RT‐qPCR and western blots were performed to quantify AR gene expression and protein content, respectively. An immunofluorescence staining protocol was optimized for the visualization of AR protein abundance. Results were considered in the context of the SC response and muscle hypertrophy data from the same cohort. AR content increased from baseline (30457 +/‐ 4399 a.u.) to post‐damage (66448 +/‐ 8083 a.u.) by 218% (p=0.005) and post RT (60889 +/‐ 6636 a.u.) by 199% (p=0.003). Those with the greatest increase in SC content post‐damage also had the greatest increase in AR content (p=0.01). Individuals with the greatest increase in leg‐lean mass following RT also had the greatest increase in AR content (p=0.05). Preliminary data from western blot analysis revealed that AR protein content post‐damage in males was correlated with activated SC number (MyoD+ cells) (p=0.01). We then aimed to investigate sex differences in the context of AR content since there is a paucity of work in females. Males (61550 +/‐ 10961 a.u.) had a 198% greater change in AR content than females (31104 +/‐ 7499 a.u) from baseline to post‐damage (p=0.03). Males also had a greater change in AR content (3.186 +/‐ 0.5443) than females (2.003 +/‐ 0.2625) post‐RT, relative to baseline. In males, those with greater relative gains in muscle fibre CSA (high responders), post‐RT, had a 373% greater change in AR content (64936 +/‐ 11783 a.u.) from baseline to post‐RT than low responders (17388 +/‐ 15438) (p = 0.03). Interestingly in females, those with greater relative gain in CSA post‐RT (1.681 +/‐ 0.4120) had a trend towards lower AR content post‐RT compared to low responders (2.502 +/‐ 0.1946) (p = 0.06). This study is the first to immunofluorescently stain AR content for visualization and quantification in skeletal muscle. Collectively, these findings suggest that AR content influences SC activity and exhibits sex differences following damage and chronic resistance exercise.

Sex-Based Differences in the Myogenic Response and Inflammatory Gene Expression Following Eccentric Contractions in Humans.

After muscle injury, the interaction between muscle satellite cells (SC) and the immune response is instrumental for the repair and regeneration of skeletal muscle tissue. Studies have reported sex-based differences in the skeletal muscle inflammatory and regenerative response following injury. However, many of these studies investigated such differences by manipulating the concentration of estradiol, in rodents and humans, without directly comparing males to females. We sought to explore differences in the myogenic and inflammatory response following unaccustomed eccentric exercise in males and females. We hypothesized that females would have a blunted myogenic and inflammatory response as compared to males. Methods: 26 (13 male, 13 female) healthy young adults (22 ± 0.4 years [mean ± SEM]) performed 300 maximal eccentric contractions (180°/s) of the knee extensors. Muscle biopsies were taken before (pre) and 48 h (post) following eccentric damage. SC content and activation were determined by immunohistochemical and real time-polymerase chain reaction (rt-PCR) analysis. Inflammatory markers were analyzed using rt-PCR. Results: Following eccentric damage, males had a greater expansion of type I-associated SC (p < 0.05), and there was a trend for a greater expansion in total SC (type I + II fibers) (p = 0.06) compared to females. There was a trend for a greater increase in Pax7 and CCL2 gene expression in males compared to females (p = 0.09). Conclusion: We conclude that there are sex-based differences in the myogenic and inflammatory response, where females have a blunted SC and inflammatory response.

Resistance training variable manipulations are less relevant than intrinsic biology in affecting muscle fiber hypertrophy.

We aimed to investigate whether muscle fiber cross-sectional area (fCSA) and associated molecular processes could be differently affected at the group and individual level by manipulating resistance training (RT) variables. Twenty resistance-trained subjects had each leg randomly allocated to either a standard RT (RT-CON: without specific variables manipulations) or a variable RT (RT-VAR: manipulation of load, volume, muscle action, and rest interval at each RT session). Muscle fCSA, satellite cell (SC) pool, myonuclei content, and gene expression were assessed before and after training (chronic effect). Gene expression was assessed 24h after the last training session (acute effect). RT-CON and RT-VAR increased fCSA and myonuclei domain in type I and II fibers after training (p<0.05). SC and myonuclei content did not change for both conditions (p>0.05). Pax-7, MyoD, MMP-2 and COL3A1 (chronic) and MGF, Pax-7, and MMP-9 (acute) increased similar for RT-CON and RT-VAR (p<0.05). The increase in acute MyoG expression was significantly higher for the RT-VAR than RT-CON (p<0.05). We found significant correlation between RT-CON and RT-VAR for the fCSA changes (r=0.89). fCSA changes were also correlated to satellite cells (r=0.42) and myonuclei (r=0.50) changes. Heatmap analyses showed coupled changes in fCSA, SC, and myonuclei responses at the individual level, regardless of the RT protocol. The high between and low within-subject variability regardless of RT protocol suggests that the intrinsic biological factors seem to be more important to explain the magnitude of fCSA gains in resistance-trained subjects.

Automated image segmentation method to analyse skeletal muscle cross section in exercise-induced regenerating myofibers

Skeletal muscle is an adaptive tissue with the ability to regenerate in response to exercise training. Cross-sectional area (CSA) quantification, as a main parameter to assess muscle regeneration capability, is highly tedious and time-consuming, necessitating an accurate and automated approach to analysis. Although several excellent programs are available to automate analysis of muscle histology, they fail to efficiently and accurately measure CSA in regenerating myofibers in response to exercise training. Here, we have developed a novel fully-automated image segmentation method based on neutrosophic set algorithms to analyse whole skeletal muscle cross sections in exercise-induced regenerating myofibers, referred as MyoView, designed to obtain accurate fiber size and distribution measurements. MyoView provides relatively efficient, accurate, and reliable measurements for CSA quantification and detecting different myofibers, myonuclei and satellite cells in response to the post-exercise regenerating process. We showed that MyoView is comparable with manual quantification. We also showed that MyoView is more accurate and efficient to measure CSA in post-exercise regenerating myofibers as compared with Open-CSAM, MuscleJ, SMASH and MyoVision. Furthermore, we demonstrated that to obtain an accurate CSA quantification of exercise-induced regenerating myofibers, whole muscle cross-section analysis is an essential part, especially for the measurement of different fiber-types. We present MyoView as a new tool to quantify CSA, myonuclei and satellite cells in skeletal muscle from any experimental condition including exercise-induced regenerating myofibers.

Low-Frequency Electrical Stimulation Promotes Satellite Cell Activities to Facilitate Muscle Regeneration at an Early Phase in a Rat Model of Muscle Strain.

The capability of regeneration for skeletal muscle after injury depends on the differentiation and proliferation ability of the resident stem cells called satellite cells. It has been reported that electrical stimulation was widely used in clinical conditions to facilitate muscle regeneration after injury, but the characterization of satellite cell responses to the context of low-frequency electrical stimulation in early-phase muscle strain conditions has not been fully clarified. In this study, we aim to investigate the effects of low-frequency electrical stimulation (frequency: 20 Hz; duration: 30 minutes, twice daily) on satellite cell activities in a rat model for the early phase of muscle strain. Firstly, we adopted our previously developed rat model to mimic the early phase of muscle strain in human. After then, we examined the effects of low-frequency electrical stimulation on histopathological changes of the muscle fiber by hematoxylin and eosin (H&E) staining. Finally, we investigated the effects of low-frequency electrical stimulation on satellite cell proliferation and differentiation by quantification of the expression level of the specific proteins using western blot analyses. The muscle strain in biceps femoris muscles of rats can be induced by high-speed rotation from knee flexion 50° to full knee extension at 960°·s−1 angular velocity during its tetany by activating the sciatic nerve, as evidenced by a widening of the interstitial space between fibers, and more edema or necrosis fibers were detected in the model rats without treatment than in control rats. After treatment with low-frequency electrical stimulation (frequency: 20 Hz; duration: 30 minutes, twice daily), the acute strained biceps femoris muscles of rats showed obvious improvement of histomorphology as indicated by more mature muscle fibers with well-ordered formation with clear boundaries. Consistently, the expression levels of the MyoD and myogenin were marked higher than those in the rats in the animal model group, indicating increased satellite cell proliferating and differentiating activities by low-frequency electrical stimulation. This study shows that low-frequency electrical stimulation provides an effective stimulus to upregulate the protein expression of MyoD/myogenin and accelerate the restoration of structure during the early phase of muscle strain. This may have significance for clinical practice. Optimization of low-frequency electrical stimulation parameters may enhance the therapeutic outcome in patients.

Effect of growth selection of broilers on breast muscle satellite cell function: Response of satellite cells to NOV, COMP, MYBP-C1, and CSRP3

The wooden breast (WB) myopathy is characterized by the palpation of a hard pectoralis major muscle that results in the necrosis and fibrosis of muscle fibers in fast-growing heavy weight meat-type broiler chickens. Necrosis of existing muscle fibers requires the repair and replacement of these myofibers. Satellite cells are responsible for the repair and regeneration of myofibers. To address how WB affects satellite cell function, top differentially expressed genes in unaffected and WB-affected pectoralis major muscle determined by RNA-Sequencing were studied by knocking down their expression by small interfering RNA in proliferating and differentiating commercial Ross 708 and Randombred (RBch) satellite cells. RBch satellite cells are from commercial 1995 broilers before WB appeared in broilers. Genes studied were: Nephroblastoma Overexpressed (NOV); Myosin Binding Protein-C (MYBP-C1); Cysteine-Rich Protein 3 (CSRP3); and Cartilage Oligomeric Matrix Protein (COMP). Ross 708 satellite cells had greatly reduced proliferation and differentiation compared to RBch satellite cells. MYBP-C1, CSRP3, and COMP reduced late proliferation and NOV did not affect proliferation in both lines. The timing of the knockdown differentially affected differentiation. If the expression was reduced at the beginning of proliferation, the effect on differentiation was greater than if the knockdown was at the beginning of differentiation. These data suggest, appropriate gene expression levels during proliferation greatly impact multinucleated myotube formation during differentiation. The effect of slow myofiber genes MYBP-C1 and CSRP3 on proliferation and differentiation suggests the presence of aerobic Type I satellite cells in the pectoralis major muscle which contains anaerobic Type IIb cells.

Response of turkey pectoralis major muscle satellite cells to hot and cold thermal stress: Effect of growth selection on satellite cell proliferation and differentiation

Satellite cell (SCs), the main progenitors for post-hatch poultry muscle growth, has maximal mitotic activity and sensitivity to temperature during the first week after hatch. The objective of the present study was to determine the effect of hot and cold temperatures on the proliferation and differentiation of SCs from pectoralis major (P. major) muscle of fast-growing 1-week-old Nicholas commercial (NC) turkeys compared to Randombred Control Line 2 (RBC2) turkeys representing commercial turkeys from 1966. Three temperature regimens were used: SCs proliferation at 38 °C (control) with differentiation at 43° or 33 °C; proliferation at 43° or 33 °C with differentiation at 38 °C; or both proliferation and differentiation at 43°, 38°, or 33°C. Satellite cell proliferation and differentiation increased at 43 °C and decreased at 33 °C in both lines. When a thermal challenge was administered during proliferation, greater stimulatory or suppressive effects on differentiation were observed compared to if the thermal challenge was applied only during differentiation in both lines. Expression of myoblast determination protein 1 during proliferation showed a higher increase in the NC line compared to the RBC2 line at 43 °C. Increased myogenin expression was observed in all hot treatment groups in the NC line but was only observed in the RBC2 line if the hot treatment was administered throughout proliferation and differentiation. Cold treatment suppressed myogenin expression independent of line. These results suggest turkey P. major muscle SCs are more sensitive to environmental temperatures during proliferation, and SCs from growth-selected NC turkeys are more sensitive to thermal stress compared to the RBC2 turkeys.

Transient activation of mTORC1 signaling in skeletal muscle is independent of Akt1 regulation.

The regulation of cellular protein synthesis is a critical determinant of skeletal muscle growth and hypertrophy in response to an increased workload such as resistance exercise. The mechanistic target of rapamycin complex 1 (mTORC1) and its upstream protein kinase Akt1 have been implicated as a central signaling pathway that regulates protein synthesis in the skeletal muscle; however, the precise molecular regulation of mTORC1 activity is largely unknown. This study employed germline Akt1 knockout (KO) mice to examine whether upstream Akt1 regulation is necessary for the acute activation of mTORC1 signaling in the plantaris muscle following mechanical overload. The phosphorylation states of S6 kinase 1, ribosomal protein S6, and eukaryotic translation initiation factor 4E‐binding protein 1 which show the functional activity of mTORC1 signaling, were significantly increased in the skeletal muscle of both wildtype and Akt1 KO mice following an acute bout (3 and 12 hr) of mechanical overload. Akt1 deficiency did not affect load‐induced alteration of insulin‐like growth factor‐1 (IGF‐1)/IGF receptor mRNA expression. Also, no effect of Akt1 deficiency was observed on the overload‐induced increase in the gene expressions of pax7 and myogenic regulatory factor of myogenin. These observations show that the upstream IGF‐1/Akt1 regulation is dispensable for the acute activation of mTORC1 signaling and regulation of satellite cells in response to mechanical overload.

Effects of Simulated Microgravity on Muscle Stem Cells Activity.

The study of the effects of simulated microgravity on primary cultures of human satellite cells represents a reliable model for identifying the biomolecular processes involved in mechanic load-related muscle mass loss. Therefore, this study aims to investigate the role of myostatin and Bone Morphogenetic Protein-2 in human satellite cells response to simulated microgravity condition. In order to identify the main molecules involved in the phenomena of degeneration/regeneration of muscle tissue related to the alteration of mechanic load, we performed a morphological and immunohistochemical study on 27 muscle biopsies taken from control, osteoporotic and osteoarthritic patients, underwent hip arthroplasty. For each patient, we set up primary satellite cell cultures subjected to normogravity and simulated microgravity (110h) regimens. Cellular functionality has been studied through a morphological evaluation performed by optical microscopy, and an ultrastructural evaluation carried out by transmission electron microscopy. Furthermore, we evaluated the expression of Bone Morphogenetic Protein-2 and myostatin through immunocytochemical reactions. Our results showed that in the very early phases of simulated microgravity condition the satellite cells are more active than those subjected to the normogravity regime, as demonstrated by both the increase in the number of myotubes and the significant increase in the expression of Bone Morphogenetic Protein-2 in all experimental groups. However, with prolongated exposure to simulated microgravity regime (>72h), satellite cells and new formed myotubes underwent to cell death. It is important to note that, in early phases, simulated microgravity can stimulate the formation of new myotubes from satellite cells derived by osteoporotic patients. Furthermore, we observed that simulated microgravity can induce changes in myostatin expression levels by group-dependent variations. The results obtained allowed us to hypothesize a possible molecular mechanism of response to simulated microgravity, confirming the importance of Bone Morphogenetic Protein-2 and myostatin in the physio-pathogenesis of muscle tissue. In addition, these data can lay the foundation for new therapeutic approached in the prevention/cure of osteoporosis and sarcopenia.

Overfeeding and Substrate Availability, But Not Age or BMI, Alter Human Satellite Cell Function.

Satellite cells (SC) aid skeletal muscle growth and regeneration. SC-mediated skeletal muscle repair can both be influenced by and exacerbate several diseases linked to a fatty diet, obesity, and aging. The purpose of this study was to evaluate the effects of different lifestyle factors on SC function, including body mass index (BMI), age, and high-fat overfeeding. For this study, SCs were isolated from the vastus lateralis of sedentary young (18–30 years) and sedentary older (60–80 years) men with varying BMIs (18–32 kg/m2), as well as young sedentary men before and after four weeks of overfeeding (OVF) (55% fat/ + 1000 kcal, n = 4). The isolated SCs were then treated in vitro with a control (5 mM glucose, 10% fetal bovine serum (FBS)) or a high substrate growth media (HSM) (10% FBS, 25 mM glucose, and 400 μM 2:1 oleate–palmitate). Cells were assessed on their ability to proliferate, differentiate, and fuel substrate oxidation after differentiation. The effect of HSM was measured as the percentage difference between SCs exposed to HSM compared to control media. In vitro SC function was not affected by donor age. OVF reduced SC proliferation rates (–19% p < 0.05) but did not influence differentiation. Cellular proliferation in response to HSM was correlated to the donor’s body mass index (BMI) (r2 = 0.6121, p < 0.01). When exposed to HSM, SCs from normal weight (BMI 18–25 kg/m2) participants exhibited reduced proliferation and fusion rates with increased fatty-acid oxidation (p < 0.05), while SCs from participants with higher BMIs (BMI 25–32 kg/m2) demonstrated enhanced proliferation in HSM. HSM reduced proliferation and fusion (p < 0.05) in SCs isolated from subjects before OVF, whereas HSM exposure accelerated proliferation and fusion in SCs collected following OVF. These results indicated that diet has a greater influence on SC function than age and BMI. Though age and BMI do not influence in vitro SC function when grown in controlled conditions, both factors influenced the response of SCs to substrate challenges, indicating age and BMI may mediate responses to diet.

Muscle type from which satellite cells are derived plays a role in their damage response.

The aim of this study was to evaluate the response of satellite cells to muscular atrophies which possess different pathological characteristics and which were induced by distinct damages. Right lower limbs of rats were exposed to denervation or disuse and later its tibialis anterior (TA) or soleus (SOL) muscles were analyzed. After confirming their functional impairments indicated by common but distinct pathological and electrophysiological characteristics, the quantitative polymerase chain reaction analysis of Pax7 and Pax3 expressions and the number of Pax7+ve and Pax3+ve cells were analyzed sequentially at day 0, day 7, and day 14. TA muscles of both denervation- and disuse-induced atrophy models showed persisted low level of Pax7 expression from day 7 (0.91 ± 0.23 and 0.31 ± 0.07, P = 0.06, n = 6) through day 14 (1.09 ± 0.15 and 0.4 ± 0.09 [P < 0.05]). On the other hand, significant elevations were observed in Pax3 expression in both atrophy models (2.73 ± 0.46 and 2.75 ± 0.26 [P < 0.05]) at day 7. Similar to TA muscle, resembled pattern of Pax7 and Pax3 expression changes were observed between the SOL muscles of denervation- and disused-atrophy models. These trends were further confirmed by the changes in Pax7+ve and Pax3+ve cell numbers of TA and SOL muscles in both atrophy models. Despite the distinct pathological findings, similar patterns in the changes of Pax3 and Pax7 expressions and the changes of Pax7+ve and Pax3+ve cell numbers were observed between the denervation- and disuse-induced atrophy models and this commonality was admitted among the muscle type. Therefore, we claim that the muscle regeneration orchestrated by satellite cells was governed by the muscle type in which satellite cells reside.