Estrogen influences satellite cell activation and proliferation following downhill running in rats

Menstrual cycle phase differences in myofiber damage and macrophage infiltration following electrical stimulation-induced muscle injury

The purpose of this study was to examine the effects of menstrual cycle phase on myofiber injury, regenerative events, and inflammation after electrical stimulation (ES)-induced myofiber damage. Twenty-eight premenopausal women (20.8 ± 2 yr) were randomized into early follicular (EF; n = 14) and late follicular (LF; n = 14) groups. After menstrual cycle tracking and phase confirmation, subjects underwent 200 electrically stimulated eccentric muscle contractions 1 wk after providing a muscle biopsy. Seven days post-ES, subjects provided a final biopsy. Primary outcomes included serum estradiol, indirect markers of muscle damage, direct indicators of myofiber necrosis and regeneration, satellite cell number, and macrophage infiltration. Women in the LF group had higher serum estradiol (122.1 ± 23.4 vs. 81.7 ± 30.8 pg/mL; P < 0.001) than in the EF group on the day of ES. Although the EF group recovered baseline maximal isometric strength by 4 days post-ES, the LF group did not. Only women in the LF group showed significant and consistent evidence of myofiber necrosis and regeneration pre- to post-ES. Despite showing more evidence of myofiber damage, women in the LF group also experienced reduced total and CD206+ macrophage infiltration relative to the EF group. Satellite cell quantity increased significantly post-ES in both groups, with no differences between groups. Collectively, the data suggest that the high-estrogen LF phase may be associated with increased susceptibility to myofiber injury while also limiting the subsequent intramuscular inflammatory response.NEW & NOTEWORTHY The menstrual cycle has widespread physiological effects across many systems, including skeletal muscle. In this study, we show that women in the late follicular phase of the menstrual cycle may be more susceptible to myofiber necrosis following electrical stimulation. We also show reduced evidence of inflammation in the late follicular phase. This is the first study to demonstrate a difference in the response of human skeletal muscle to a necrotic stimulus across a menstrual cycle.

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.

Effects of an extended photoperiod on body composition of young Thoroughbreds in training

The effects of an extended photoperiod (EP) on body composition of Thoroughbreds colts and fillies from December at one year old to April at two years old were investigated. Seventy-three Thoroughbreds reared and trained in Hidaka Training and Research Center, Japan Racing Association, Hokkaido were used. Forty-one horses were under the EP conditions from December 20 to April 15, and the 32 horses were under natural light alone as the control group. Body weight (BW), rump fat thickness (RFT), fat free mass (FFM) and percentage of fat (%F) were used as parameters of body composition. The present study revealed that BW and FFM increased with age in the EP group in colts. In fillies, BW increased with age in both the EP and the control group, however FFM increased with age only in the EP group. From December to April, only colts had a higher rate of increase in both BW and FFM in the EP group than in the control group. However, according to the mean rates of increase in FFM from January to March, the EP group was significantly higher than the control group in both sexes. Furthermore, monthly increase rate of FFM in March was significantly higher in the EP group than in the control group in both sexes. These results suggests that EP treatment to young Thoroughbreds in training at Hokkaido, which is shorter daylength in winter, accelerate the increase of FFM, representing muscle mass.

Menstrual cycle hormones and oral contraceptives: a multimethod systems physiology-based review of their impact on key aspects of female physiology

Hormonal changes around ovulation divide the menstrual cycle (MC) into the follicular and luteal phases. In addition, oral contraceptives (OCs) have active (higher hormone) and placebo phases. Although there are some MC-based effects on various physiological outcomes, we found these differences relatively subtle and difficult to attribute to specific hormones, as estrogen and progesterone fluctuate rather than operating in a complete on/off pattern as observed in cellular or preclinical models often used to substantiate human data. A broad review reveals that the differences between the follicular and luteal phases and between OC active and placebo phases are not associated with marked differences in exercise performance and appear unlikely to influence muscular hypertrophy in response to resistance exercise training. A systematic review and meta-analysis of substrate oxidation between MC phases revealed no difference between phases in the relative carbohydrate and fat oxidation at rest and during acute aerobic exercise. Vascular differences between MC phases are also relatively small or nonexistent. Although OCs can vary in composition and androgenicity, we acknowledge that much more work remains to be done in this area; however, based on what little evidence is currently available, we do not find compelling support for the notion that OC use significantly influences exercise performance, substrate oxidation, or hypertrophy. It is important to note that the study of females requires better methodological control in many areas. Previous studies lacking such rigor have contributed to premature or incorrect conclusions regarding the effects of the MC and systemic hormones on outcomes. While we acknowledge that the evidence in certain research areas is limited, the consensus view is that the impact of the MC and OC use on various aspects of physiology is small or nonexistent.

Influence of sexual dimorphism on satellite cell regulation and inflammatory response during skeletal muscle regeneration

After injury, skeletal muscle regenerates thanks to the key role of satellite cells (SC). The regeneration process is supported and coordinated by other cell types among which immune cells. Among the mechanisms involved in skeletal muscle regeneration, a sexual dimorphism, involving sex hormones and more particularly estrogens, has been suggested. However, the role of sexual dimorphism on skeletal muscle regeneration is not fully understood, likely to the use of various experimental settings in both animals and human. This review aims at addressing how sex and estrogens regulate both the SC and the inflammatory response during skeletal muscle regeneration by considering the different experimental designs used in both animal models (i.e., ovarian hormone deficiency, estrogen replacement or supplementation, treatments with estrogen receptors agonists/antagonists and models knockout for estrogen receptors) and human (hormone therapy replacement, pre vs. postmenopausal, menstrual cycle variation…).

Potential Application of Muscle Precursor Cells from Male Specific-Pathogen-Free (SPF) Chicken Embryos in In Vitro Agriculture

This study examined the potential benefits of male specific-pathogen-free (SPF) White Leghorn embryos in cellular agriculture for sustainable and ethical poultry meat production-addressing traditional farming challenges, including disease outbreaks of Salmonella and Avian influenza. We isolated myogenic precursor cells (MPCs) from the thigh muscles (Musculus femoris) of 12.5-day-old embryos from 10 SPF White Leghorns that tested negative for Salmonella. We randomly selected MPCs from three males and three females, isolated them using a modified pre-plating (pp) method, and compared their in vitro development. After 1 h (pp1) and 2 h (pp2) of incubation, they were transferred to a new dish to remove fast-adhering cells and cultured (pp3). Isolated MPCs had a 69% positive reaction to Pax7. During proliferation, no differences were observed in PAX7, MYF5, or MYOD expression between the male and female MPCs. However, after five days of differentiation, the expression of late myogenic factors-MYOG and MYF6-significantly increased in all MPCs. Notably, MYOG expression was 1.9 times higher in female than in male MPCs. This impacted MYMK's expression pattern. Despite this, the myotube fusion index did not differ between the sexes. Muscle cells from male SPF-laying chicken embryos are promising for developing clean animal-cell-derived protein sources via resource recycling.

Understanding the female athlete: molecular mechanisms underpinning menstrual phase differences in exercise metabolism

Research should equitably reflect responses in men and women. Including women in research, however, necessitates an understanding of the ovarian hormones and menstrual phase variations in both cellular and systems physiology. This review outlines recent advances in the multiplicity of ovarian hormone molecular signaling that elucidates the mechanisms for menstrual phase variability in exercise metabolism. The prominent endogenous estrogen, 17-β-estradiol (E2), molecular structure is bioactive in stabilizing plasma membranes and quenching free radicals and both E2 and progesterone (P4) promote the expression of antioxidant enzymes attenuating exercise-induced muscle damage in the late follicular (LF) and mid-luteal (ML) phases. E2 and P4 bind nuclear hormone receptors and membrane-bound receptors to regulate gene expression directly or indirectly, which importantly includes cross-regulated expression of their own receptors. Activation of membrane-bound receptors also regulates kinases causing rapid cellular responses. Careful analysis of these signaling pathways explains menstrual phase-specific differences. Namely, E2-promoted plasma glucose uptake during exercise, via GLUT4 expression and kinases, is nullified by E2-dominant suppression of gluconeogenic gene expression in LF and ML phases, ameliorated by carbohydrate ingestion. E2 signaling maximizes fat oxidation capacity in LF and ML phases, pending low-moderate exercise intensities, restricted nutrient availability, and high E2:P4 ratios. P4 increases protein catabolism during the luteal phase by indeterminate mechanisms. Satellite cell function supported by E2-targeted gene expression is countered by P4, explaining greater muscle strengthening from follicular phase-based training. In totality, this integrative review provides causative effects, supported by meta-analyses for quantitative actuality, highlighting research opportunities and evidence-based relevance for female athletes.

Roles of Estrogen, Estrogen Receptors, and Estrogen-Related Receptors in Skeletal Muscle: Regulation of Mitochondrial Function

Estrogen is an essential sex steroid hormone that functions primarily in female reproductive system, as well as in a variety of tissues and organs with pleiotropic effects, such as in cardiovascular, nervous, immune, and musculoskeletal systems. Women with low estrogen, as exemplified by those in postmenopause, are therefore prone to suffer from various disorders, i.e., cardiovascular disease, dementia, metabolic syndrome, osteoporosis, sarcopenia, frailty, and so on. Estrogen regulates the expression of its target genes by binding to its cognate receptors, estrogen receptors (ERs) α and β. Notably, the estrogen-related receptors (ERRs) α, β, and γ are originally identified as orphan receptors that share substantial structural homology and common transcriptional targets with ERs. Accumulating evidence suggests that ERs and ERRs play crucial roles in skeletal muscles, such as muscle mass maintenance, muscle exercise physiology, and muscle regeneration. In this article, we review potential regulatory roles of ERs and ERRs in muscle physiology, particularly with regard to mitochondrial function and metabolism.

Estrogen promotes fetal skeletal muscle myofiber development important for insulin sensitivity in offspring

Using our nonhuman primate baboon model, we showed that offspring born to mothers deprived of estrogen during the second half of gestation exhibited insulin resistance and a deficit in first phase insulin release. Although insulin resistance was not due to an impairment of fetal or offspring growth, nor to an alteration in adipose or hepatic sensitivity to insulin, skeletal muscle microvacularization critical for delivery of nutrients/insulin was significantly reduced in fetuses and offspring deprived of estrogen in utero. Skeletal muscle myofiber maturation occurs in utero and estrogen modulates myofiber growth in adults. Therefore, the current study determined whether fetal skeletal muscle development was altered in baboons in which estradiol levels were suppressed/restored during the second half of gestation by maternal treatment with letrozole ± estradiol benzoate. In estrogen-suppressed animals, fetal skeletal muscle fascicles were structurally less organized, smaller, and comprised of slow type I and fast type II fibers, the size, but not the number of which were smaller than in untreated baboons. Moreover, the proportion of non-muscle fiber tissue was greater and that of muscle fibers lower in estrogen-deprived fetuses. Thus, the maintenance of fetal body weight in estrogen-deprived animals was maintained at the expense of muscle fibers and likely reflected increased deposition of non-muscle proteins. Importantly, fetal skeletal muscle development, including fascicle organization, myofiber size and composition was normal in baboons treated with letrozole and estradiol benzoate. Collectively, these and our previous findings support our proposal that exposure of the fetus to estrogen is important for fetal skeletal muscle development and glucose homeostasis in adulthood.

Influence of oestrogen on satellite cells and myonuclear domain size in skeletal muscles following resistance exercise

BACKGROUND

Oestrogen deficiency reduces skeletal muscle mass and force generation in postmenopausal women. Muscle mass is maintained by satellite cells, which are regulated by oestrogen. Although oestrogen therapy enhances muscle hypertrophy induced by resistance training in postmenopausal women, the molecular mechanism is unclear.

METHODS

Adult female rats (10 weeks old) were divided into six groups: sham sedentary (Sham-Sed), sham climbing training (Sham-CT), ovariectomy sedentary (OVX-Sed), ovariectomy climbing training (OVX-CT), ovariectomy plus oestrogen treatment sedentary (OVX+E-Sed), and ovariectomy plus oestrogen treatment climbing training (OVX+E-CT). At 8 weeks after ovariectomy, rats in the training group were trained (one session every 3 days for 8 weeks) to climb a ladder while bearing a load. Oestrogen treatment involved subcutaneous insertion of a 17β-oestradiol pellet. After 8 weeks, the flexor hallucis longus muscle was collected and analysed.

RESULTS

Following climbing training, the flexor hallucis longus muscle mass and muscle-to-body weight ratios were dramatically increased by training (main effect of training, P < 0.01); the OVX+E-CT group showed the highest values (main effect of group, P < 0.01). The cross-sectional area of all muscle fibre types was increased by training (main effect of training, P < 0.01). Particularly, the cross-sectional area of MHC IIa in the OVX+E-CT group was significantly larger than that in the Sham-CT and OVX-CT groups. Satellite cell numbers were increased in all training groups (main effect of training, P < 0.05), and the myonuclear number was increased by training (main effect of training, P < 0.01), but there was no main group effect. The myonuclear domain size of all muscle fibre types and MHC IIa was increased in all training groups (main effect of training, P < 0.01) and showed a main group effect (P < 0.01). The myonuclear domain sizes of all muscle fibre types and MHC IIa in the OVX+E-CT group were significantly larger than those in the Sham-CT and OVX-CT groups. The total RNA contents revealed main effects of training and the group (P < 0.01); the OVX+E-CT group showed the highest contents (main effect of group, P < 0.01). The mRNA and protein levels of rpS6 were increased in the OVX+E-Sed and CT groups (main effects of group, P < 0.05). Particularly, the 28S ribosomal RNA content in OVX+E-Sed group was significantly higher than that in the OVX-Sed group.

CONCLUSIONS

Oestrogen enhanced the resistance training-induced increase in myonuclear domain size but did not affect satellite cells and ribosome biogenesis.

Estrogen Signaling Dictates Musculoskeletal Stem Cell Behavior: Sex Differences in Tissue Repair

Sexual dimorphisms in humans and other species exist in visually evident features such as body size and less apparent characteristics, including disease prevalence. Current research is adding to a growing understanding of sex differences in stem cell function and response to external stimuli, including sex hormones such as estrogens. These differences are proving significant and directly impact both the understanding of stem cell processes in tissue repair and the clinical implementation of stem cell therapies. Adult stem cells of the musculoskeletal system, including those used for development and repair of muscle, bone, cartilage, fibrocartilage, ligaments, and tendons, are no exception. Both in vitro and in vivo studies have found differences in stem cell number, proliferative and differentiation capabilities, and response to estrogen treatment between males and females of many species. Maintaining the stemness and reducing senescence of adult stem cells is an important topic with implications in regenerative therapy and aging. As such, this review discusses the effect of estrogens on musculoskeletal system stem cell response in multiple species and highlights the research gaps that still need to be addressed. The following evidence from investigations of sex-related phenotypes in adult progenitor and stem cells are pieces to the big puzzle of sex-related effects on aging and disease and critical information for both fundamental tissue repair and regeneration studies and safe and effective clinical use of stem cells. Impact Statement This review summarizes current knowledge of sex differences in and the effects of estrogen treatment on musculoskeletal stem cells in the context of tissue engineering. Specifically, it highlights the impact of sex on musculoskeletal stem cell function and ability to regenerate tissue. Furthermore, it discusses the varying effects of estrogen on stem cell properties, including proliferation and differentiation, important to tissue engineering. This review aims to highlight the potential impact of estrogens and the importance of performing sex comparative studies in the field of tissue engineering.

Mechanisms of Estrogen Influence on Skeletal Muscle: Mass, Regeneration, and Mitochondrial Function

Human menopause is widely associated with impaired skeletal muscle quality and significant metabolic dysfunction. These observations pose significant challenges to the quality of life and mobility of the aging population, and are of relevance when considering the significantly greater losses in muscle mass and force-generating capacity of muscle from post-menopausal females relative to age-matched males. In this regard, the influence of estrogen on skeletal muscle has become evident across human, animal, and cell-based studies. Beneficial effects of estrogen have become apparent in mitigation of muscle injury and enhanced post-damage repair via various mechanisms, including prophylactic effects on muscle satellite cell number and function, as well as membrane stability and potential antioxidant influences following injury, exercise, and/or mitochondrial stress. In addition to estrogen replacement in otherwise deficient states, exercise has been found to serve as a means of augmenting and/or mimicking the effects of estrogen on skeletal muscle function in recent literature. Detailed mechanisms behind the estrogenic effect on muscle mass, strength, as well as the injury response are beginning to be elucidated and point to estrogen-mediated molecular cross talk amongst signalling pathways, such as apoptotic signaling, contractile protein modifications, including myosin regulatory light chain phosphorylation, and the maintenance of muscle satellite cells. This review discusses current understandings and highlights new insights regarding the role of estrogen in skeletal muscle, with particular regard to muscle mass, mitochondrial function, the response to muscle damage, and the potential implications for human physiology and mobility.

Sex-related differences and effects of short and long trail running races on resting muscle-tendon mechanical properties

The purpose of the study was to assess sex-related differences in resting mechanical properties and adaptations of skeletal muscles and tendons in response to trail running races of different distances using multi-site shear wave elastography assessments of the lower limb, force capacity and blood analyses. Sex differences in resting mechanical properties of knee extensor and plantar flexor muscles and tendons were characterized by shear wave velocity measurements in healthy males (N=42) and females (N=25) trained in long distance running. Effects of running distance on muscle and tendon properties were assessed in short (<60km, N=23) vs. long (>100km, N=26) distance races. Changes in isometric maximal voluntary contraction torque, serum C-reactive protein and creatine kinase activity were also quantified after running races. Higher shear wave velocity of relaxed triceps surae muscle was detected in females as compared to males before running races (+4.8%, p=0.006), but the significant increases in triceps surae muscle group (+7.0%, p=0.001) and patellar tendon shear wave velocity (+15.4%, p=0.001) after short-distance races were independent of sex. A significant decrease in triceps surae muscle shear wave velocity was found after long-distance races in the whole experimental population (-3.1%, p=0.049). Post-races increase in C-reactive protein and creatine kinase activity were significantly correlated to the relative decreases in triceps surae and quadriceps femoris skeletal muscle shear wave velocity (ρ=-0.56, p=0.001 and ρ=-0.51, p=0.001, respectively). Resting mechanical properties of muscles and tendons are affected by sex, and that adaptations to trail races are related to running distance. Exercise-induced changes in resting skeletal muscle mechanical properties are associated with enhanced indirect markers of inflammation and muscle damage.

Estradiol deficiency reduces the satellite cell pool by impairing cell cycle progression

The size of the satellite cell pool is reduced in estradiol (E2)-deficient female mice and humans. Here, we use a combination of in vivo and in vitro approaches to identify mechanisms, whereby E2 deficiency impairs satellite cell maintenance. By measuring satellite cell numbers in mice at several early time points postovariectomy (Ovx), we determine that satellite cell numbers decline by 33% between 10 and 14 days post-Ovx in tibialis anterior and gastrocnemius muscles. At 14 days post-Ovx, we demonstrate that satellite cells have a reduced propensity to transition from G0/G1 to S and G2/M phases, compared with cells from ovary-intact mice, associated with changes in two key satellite cell cycle regulators, ccna2 and p16INK4a. Further, freshly isolated satellite cells treated with E2 in vitro have 62% greater cell proliferation and require less time to complete the first division. Using clonal and differentiation assays, we measured 69% larger satellite cell colonies and enhanced satellite cell-derived myoblast differentiation with E2 treatment compared with vehicle-treated cells. Together, these results identify a novel mechanism for preservation of the satellite cell pool by E2 via promotion of satellite cell cycling.

Effects of Follicular and Luteal Phase-Based Menstrual Cycle Resistance Training on Muscle Strength and Mass

There is an increasing interest in female athletic performance-especially concerning the impact of the female menstrual cycle on training response. Indeed, fluctuations in female sex hormones, estrogen and progesterone, during the menstrual cycle regulate protein metabolism and recovery processes in skeletal muscle and may thus impact exercise training-related outcomes. Studies demonstrate that anaerobic capacity and muscle strength are greatest during the follicular phase of the menstrual cycle, when estrogen levels peak. In addition, studies indicate that resistance training conducted in the follicular phase of the menstrual cycle (follicular phase-based resistance training) may be superior to luteal phase-based training in terms of enhancing muscle strength and mass. This raises the possibility that the physiological capabilities of skeletal muscle to adapt to exercise training are dependent on the menstrual cycle and can be important for female athletes in optimizing their training. In this paper, we critically review the current state of the art concerning the impact of menstrual cycle phase-based resistance training and highlight why follicular phase-based resistance training possibly is superior to luteal phase-based training in enhancing resistance training outcomes. Finally, we identify directions for further research.

Sex Hormones and Satellite Cell Regulation in Women

Recent years have seen growing scholarly interest in female physiology in general. Moreover, particular attention has been devoted to how concentrations of female sex hormones vary during the menstrual cycle and menopausal transition and how hormonal contraception and hormonal therapy influence skeletal muscle tissue. While much effort has been paid to macro outcomes, such as muscle function or mass, rather less attention has been paid to mechanistic work that may help explain the underlying mechanism through which sex hormones regulate skeletal muscle tissue. Evidence from animal studies shows a strong relationship between the female sex hormone estrogen and satellite cells (SCs), a population of muscle stem cells involved in skeletal muscle regulation. A few human studies investigating this relationship have been published only recently. Thus, the purpose of this study was to bring an updated review on female sex hormones and their role in SC regulation. First, we describe how SCs regulate skeletal muscle maintenance and repair and introduce sex hormone signaling within the muscle. Second, we present evidence from animal studies elucidating how estrogen deficiency and supplementation influence SCs. Third, we present results from investigations from human trials including women whose concentrations of female hormones differ due to menopause, hormone therapy, hormonal contraceptives, and the menstrual cycle. Finally, we discuss research and methodological recommendations for future studies aiming at elucidating the link between female sex hormones and SCs with respect to aging and training.

Estrogen modulates the skeletal muscle regeneration process and myotube morphogenesis: morphological analysis in mice with a low estrogen status

The purpose of this study was to elucidate the functions of estrogen and two estrogen receptors (ERs; ERα and ERβ) in the myoregeneration process and morphogenesis. Cardiotoxin (CTX) was injected into the tibialis anterior (TA) muscles of ovariectomized (OVX) mice to induce muscle injury, and subsequent myoregeneration was morphologically assessed. The diameter of regenerated myotubes in OVX mice was significantly smaller than that in intact mice at all time points of measurement. OVX mice also showed lower muscle recovery rates and slower speeds than did intact mice. ER protein levels showed a predominance of ERβ over ERα in both intact and OVX states. The ERβ level was increased significantly at 7 days after CTX injection in OVX mice and remained at a high level until 14 days. In addition, continuous administration of E2 to OVX mice in which muscle injury was induced resulted in a significantly larger diameter of regenerated myotubes than that in mice that did not receive estrogen. The results indicate that estrogen is an essential factor in the myoregeneration process since estrogen depletion delayed myoregeneration in injured muscles and administration of estrogen under the condition of a low estrogen status rescued delayed myoregeneration. The results strongly suggested that ERβ may be a factor that promotes myoregeneration more than does ERα.

Beneficial Role of Exercise in the Modulation of mdx Muscle Plastic Remodeling and Oxidative Stress

Duchenne muscular dystrophy (DMD) is an X-linked recessive progressive lethal disorder caused by the lack of dystrophin, which determines myofibers mechanical instability, oxidative stress, inflammation, and susceptibility to contraction-induced injuries. Unfortunately, at present, there is no efficient therapy for DMD. Beyond several promising gene- and stem cells-based strategies under investigation, physical activity may represent a valid noninvasive therapeutic approach to slow down the progression of the pathology. However, ethical issues, the limited number of studies in humans and the lack of consistency of the investigated training interventions generate loss of consensus regarding their efficacy, leaving exercise prescription still questionable. By an accurate analysis of data about the effects of different protocol of exercise on muscles of mdx mice, the most widely-used pre-clinical model for DMD research, we found that low intensity exercise, especially in the form of low speed treadmill running, likely represents the most suitable exercise modality associated to beneficial effects on mdx muscle. This protocol of training reduces muscle oxidative stress, inflammation, and fibrosis process, and enhances muscle functionality, muscle regeneration, and hypertrophy. These conclusions can guide the design of appropriate studies on human, thereby providing new insights to translational therapeutic application of exercise to DMD patients.

Molecular markers of skeletal muscle hypertrophy following 10 wk of resistance training in oral contraceptive users and nonusers

The objective was to determine whether skeletal muscle molecular markers and SC number were influenced differently in users and nonusers of oral contraceptives (OCs) following 10 wk of resistance training. Thirty-eight young healthy untrained users (n = 20) and nonusers of OC (n = 18) completed a 10-wk supervised progressive resistance training program. Before and after the intervention, a muscle tissue sample was obtained from the vastus lateralis muscle for analysis of muscle fiber cross-sectional area (fCSA) and satellite cell (SC) and myonuclei number using immunohistochemistry, gene expression using PCR, protein expression, and myosin heavy chain composition. Following the training period, quadriceps fCSA (P < 0.05), SCs/type I fiber (P = 0.05), and MURF-1 mRNA (P < 0.01) were significantly increased with no difference between the groups. However, SCs/total fiber and SCs/type II fiber increased in OC users only, and SCs/type II fCSA tended (P = 0.055) to be greater in the OC users. Furthermore, in OC users there were a fiber type shift from myosin heavy chain (MHC) IIx to MHC IIa (P < 0.01), and expression of muscle regulatory factor 4 (MRF4) mRNA (P < 0.001) was significantly greater than in non-OC users. Use of second-generation OCs in young untrained women increased skeletal muscle MRF4 expression and SC number following 10 wk of resistance training compared with nonusers.NEW & NOTEWORTHY The effect of oral contraceptive use on the skeletal muscle regulatory pathways in response to resistance training has not been investigated previously. Here we present novel data, demonstrating that use of second-generation oral contraceptives in young untrained women increased skeletal muscle regulatory factor 4 expression and satellite cell number following 10 wk of resistance training compared with nonusers.

Changes in Salivary Levels of Creatine Kinase, Lactate Dehydrogenase, and Aspartate Aminotransferase after Playing Rugby Sevens: The Influence of Gender

Rugby sevens is characterised by continuous exertion and great physical contact per unit of time, leading to muscle damage. It is important to identify markers that can quantify muscle damage in order to improve recovery strategies. The objective of this study was to evaluate the release dynamics of muscle damage markers creatine kinase (CK), lactate dehydrogenase (LDH), and aspartate aminotransferase (AST) in saliva samples when playing rugby sevens, analysing the influence of gender, during the rugby sevens university championship of Spain. The total sample included 27 athletes, divided into two teams of 14 men and 13 women between 18 and 31 years of age. CK, LDH, and AST were quantified from salivary samples collected from each athlete before and after three rugby sevens matches. The modified Borg scale of perceived exertion was also used after each match. When the results were analysed globally, there were no differences in CK and LDH before and after any match, but AST did show differences after two days of completing all matches. In terms of gender, the three enzymes showed different responses in men and women. Regarding the Borg scale, there were only significant differences between men and women after completing all mataches, with a greater perceived exertion in women. Based on our results, it can be stated that that serial matches of rugby sevens can cause changes of different magnitude in AST, CK and LDH activities in saliva, with AST showing the most significant variations and these changes are more pronounced in men than in women.

Ethanol-Impaired Myogenic Differentiation is Associated With Decreased Myoblast Glycolytic Function

BACKGROUND

Myopathy affects nearly half of individuals with alcohol use disorder (AUD), and impaired skeletal muscle regenerative potential is a probable contributing factor. Previous findings from our laboratory indicate that chronic in vivo and in vitro ethanol (EtOH) treatment decreases myogenic potential of skeletal muscle myoblasts. Myogenesis, a highly coordinated process, requires shifts in cellular metabolic state allowing for myoblasts to proliferate and differentiate into mature myotubes. The objective of this study was to determine whether alcohol interferes with myoblast mitochondrial and glycolytic metabolism and impairs myogenic differentiation.

METHODS

Myoblasts were isolated from vastus lateralis muscle excised from alcohol-naïve adult male (n = 5) and female (n = 5) rhesus macaques. Myoblasts were proliferated for 3 days (day 0 differentiation; D0) and differentiated for 5 days (D5) with or without 50 mM EtOH. Metabolism was assessed using a mitochondrial stress test to measure oxygen consumption (OCR) and extracellular acidification (ECAR) rates at D0. Differentiation was examined at D5. Expression of mitochondrial and glycolytic genes and mitochondrial DNA (mtDNA) was measured at D0 and D5.

RESULTS

Ethanol significantly (p < 0.05) increased myoblast maximal OCR and decreased ECAR at D0, and decreased fusion index, myotubes per field, and total nuclei at D5. The EtOH-induced decrease in ECAR was associated with the EtOH-mediated decreases in fusion index and myotubes per field. EtOH did not alter the decrease in glycolytic gene expression and increase in mtDNA from D0 to D5.

CONCLUSION

During myoblast proliferation, EtOH decreased glycolytic metabolism and increased maximal OCR, suggesting that myoblast metabolic phenotype was dysregulated with EtOH. The EtOH-induced decrease in ECAR was associated with decreased differentiation. These findings suggest that EtOH-mediated shifts in metabolic phenotype may underlie impaired differentiation, which has important clinical implications for myogenesis in those affected by alcoholic myopathy.

Estrogen Regulates the Satellite Cell Compartment in Females

Skeletal muscle mass, strength, and regenerative capacity decline with age, with many measures showing a greater deterioration in females around the time estrogen levels decrease at menopause. Here, we show that estrogen deficiency severely compromises the maintenance of muscle stem cells (i.e., satellite cells) as well as impairs self-renewal and differentiation into muscle fibers. Mechanistically, by hormone replacement, use of a selective estrogen-receptor modulator (bazedoxifene), and conditional estrogen receptor knockout, we implicate 17β-estradiol and satellite cell expression of estrogen receptor α and show that estrogen signaling through this receptor is necessary to prevent apoptosis of satellite cells. Early data from a biopsy study of women who transitioned from peri- to post-menopause are consistent with the loss of satellite cells coincident with the decline in estradiol in humans. Together, these results demonstrate an important role for estrogen in satellite cell maintenance and muscle regeneration in females.

Collins et al. show the loss of estrogen in female mice and post-menopausal women leads to a decrease in skeletal muscle stem cells. Using muscle stem cell-specific mutants, it was demonstrated that ERα is necessary for satellite cell maintenance, self-renewal, and protection from apoptosis, thereby promoting optimal muscle regeneration.

Estrogen Receptor β Controls Muscle Growth and Regeneration in Young Female Mice

Estrogens are female sex hormones that are important for comprehensively maintaining muscle function, and an insufficiency affects muscle strength and regeneration in females. However, it is still unclear whether estrogen signaling is mediated through receptors. To investigate the specific role of estrogen receptor β (ERβ) in skeletal muscle and satellite cells (muscle stem cells), we generated muscle-specific ERβ-knockout (mKO) and satellite cell-specific ERβ-knockout (scKO) mice, respectively. Young female mKO mice displayed a decrease in fast-type dominant muscle mass. Female, but not male, scKO mice exhibited impaired muscle regeneration following acute muscle injury, probably due to reduced proliferation and increased apoptosis of satellite cells. RNA-sequencing analysis revealed that loss of ERβ in satellite cells altered gene expression of extracellular matrix components, including laminin and collagen. The results indicate that the estrogen-ERβ pathway is a sex-specific regulatory mechanism that controls muscle growth and regeneration in female mice.

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ERβ controls muscle growth in young female mice

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ERβ is essential for muscle regeneration in female mice

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Inactivation of ERβ causes an increase in apoptosis

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ERβ is required for satellite cell population expansion

Satellite cell proliferation and myofiber cross-section area increase after electrical stimulation following sciatic nerve crush injury in rats

BACKGROUND

Electrical stimulation has been recommended as an effective therapy to prevent muscle atrophy after nerve injury. However, the effect of electrical stimulation on the proliferation of satellite cells in denervated muscles has not yet been fully elucidated. This study was aimed to evaluate the changes in satellite cell proliferation after electrical stimulation in nerve injury and to determine whether these changes are related to the restoration of myofiber cross-section area (CSA).

METHODS

Sciatic nerve crush injury was performed in 48 male Sprague-Dawley rats. In half (24/48) of the rats, the gastrocnemius was electrically stimulated transcutaneously on a daily basis after injury, while the other half were not stimulated. Another group of 24 male Sprague-Dawley rats were used as sham operation controls without injury or stimulation. The rats were euthanized 2, 4, and 6 weeks later. After 5-bromo-2'-deoxyuridine (BrdU) labeling, the gastrocnemia were harvested for the detection of paired box protein 7 (Pax7), BrdU, myofiber CSA, and myonuclei number per fiber. All data were analyzed using two-way analysis of variance and Bonferroni post-hoc test.

RESULTS

The percentages of Pax7-positive nuclei (10.81 ± 0.56%) and BrdU-positive nuclei (34.29 ± 3.87%) in stimulated muscles were significantly higher compared to those in non-stimulated muscles (2.58 ± 0.33% and 1.30 ± 0.09%, respectively, Bonferroni t = 15.91 and 18.14, P < 0.05). The numbers of myonuclei per fiber (2.19 ± 0.24) and myofiber CSA (1906.86 ± 116.51 μm) were also increased in the stimulated muscles (Bonferroni t = 3.57 and 2.73, P < 0.05), and both were positively correlated with the Pax7-positive satellite cell content (R = 0.52 and 0.60, P < 0.01). There was no significant difference in the ratio of myofiber CSA/myonuclei number per fiber among the three groups.

CONCLUSIONS

Our results indicate that satellite cell proliferation is promoted by electrical stimulation after nerve injury, which may be correlated with an increase in myonuclei number and myofiber CSA.

Correlation Between Serum Activity of Muscle Enzymes and Stage of the Estrous Cycle in Italian Standardbred Horses Susceptible to Exertional Rhabdomyolysis

Equine exertional rhabdomyolysis (ER) is a well-recognized clinical syndrome affecting racehorses. Prevalence analysis of ER showed that female sex was a significant risk factor. The aim of this research was to evaluate the differences and correlations in the serum activity of muscle enzymes and the stage of the estrous cycle in ER-susceptible and control (C) mares. Serum muscle enzyme activity before and after exercise and sex hormones were analyzed in the two groups of mares. Ten cyclic ER and 10 cyclic C mares were examined weekly for 4 weeks. During diestrus, ER horses had significantly higher resting and postexercise aspartate aminotransferase (AST) activity, but not creatine kinase (CK) activity, compared with controls; only postexercise AST activity was significantly higher during estrus compared with activity levels in controls. During estrus, 17β-estradiol and AST activity were significantly negatively correlated in the control but not ER mares. Based on our results, further studies should be performed to characterize the presumptive different roles played by sexual hormones in horses susceptible to ER compared with healthy mares.

Resistance Training and Skeletal Muscle Protein Metabolism in Eumenorrheic Females: Implications for Researchers and Practitioners

Resistance training is essential for health and performance and confers many benefits such as increasing skeletal muscle mass, increasing strength and power output, and improving metabolic health. Resistance training is a major component of the physical activity guidelines, yet research in female populations is limited. Recent increases in the promotion of, and the participation by, females in sport and exercise, highlight the need for an increase in understanding of evidence-based best practice exercise prescription for females. The aim of this review is to provide an overview of the current research regarding resistance training performance and skeletal muscle adaptation in females, with a focus on the hormonal variables that may influence resistance training outcomes. Findings suggest that the menstrual cycle phase may impact strength, but not skeletal muscle protein metabolism. In comparison, oral contraception use in females may reduce skeletal muscle protein synthesis, but not strength outcomes, when compared to non-users. Future research should investigate the role of resistance training in the maintenance of skeletal muscle protein metabolism during pregnancy, menopause and in athletes experiencing relative energy deficiency in sport. The review concludes with recommendations for researchers to assist them in the inclusion of female participants in resistance training research specifically, with commentary on the most appropriate methods of controlling for, or understanding the implications of, hormonal fluctuations. For practitioners, the current evidence suggests possible resistance training practices that could optimise performance outcomes in females, although further research is warranted.

MRI Quantitative Analysis of Eccentric Exercise-induced Skeletal Muscle Injury in Rats

BACKGROUND

The aim of this study was to investigate the value of magnetic resonance (MR) quantitative analysis (MR-T2 mapping technique) for the evaluation of eccentric exercise-induced skeletal muscle injury.

MATERIALS AND METHODS

We established an animal model of eccentric exercise-induced damage of Sprague Dawley rat skeletal muscle and evaluated the animals by MR imaging, determined the serum levels of fast skeletal troponin (fsTnI), and examined muscle histopathology, at 0, 1, 2, 4, and 7 days after eccentric exercise. The associations between MR imaging findings, and histopathological and laboratory results were evaluated.

RESULTS

T2-weighted images (WIs) of quadriceps femoris muscles showed obvious high signal intensities after exercise, and the T2 values and serum fsTnI levels continued to increase, peaking at day 2 after exercise, p< 0.05. The histopathological findings in muscle specimens, which included swollen and ruptured cells, enlarged extracellular spaces, inflammation, and regeneration of muscle fibers, showed similar trends. After day 2, muscle specimens began to show evidence of self-repair, the T2WI signals decreased in intensity, and the T2 values and serum fsTnI levels decreased; however, at day 7 post injury, the values remained slightly higher than those in the control animals, p< 0.05. The T2 value was significantly correlated with the serum fsTnI level (r = 0.896, p< 0.01).

CONCLUSION

T2 mapping technology accurately reflects the histopathological and fsTnI abnormalities and the degree of skeletal muscle damage associated with eccentric exercise followed by recovery. Because T2 mapping technology is noninvasive and can be quantitatively analyzed, it might become the preferred method for performing the diagnosis of eccentric exercise-induced skeletal muscle injury.

Fiber type-specific hypertrophy and increased capillarization in skeletal muscle following testosterone administration in young women

It is well established that testosterone administration induces muscle fiber hypertrophy and myonuclear addition in men; however, it remains to be determined whether similar morphological adaptations can be achieved in women. The aim of the present study was therefore to investigate whether exogenously administered testosterone alters muscle fiber morphology in skeletal muscle of young healthy, physically active women. Thirty-five young (20-35 yr), recreationally trained women were randomly assigned to either 10-wk testosterone administration (10 mg daily) or placebo. Before and after the intervention, hormone concentrations and body composition were assessed, and muscle biopsies were obtained from the vastus lateralis. Fiber type composition, fiber size, satellite cell and myonuclei content, as well as muscle capillarization were assessed in a fiber type-specific manner by immunohistochemistry. After the intervention, testosterone administration elevated serum testosterone concentration (5.1-fold increase, P = 0.001) and induced significant accretion of total lean mass (+1.9%, P = 0.002) and leg lean mass (+2.4%, P = 0.001). On the muscle fiber level, testosterone increased mixed-fiber cross-sectional area (+8.2%, P = 0.001), an effect primarily driven by increases in type II fiber size (9.2%, P = 0.006). Whereas myonuclei content remained unchanged, a numerical increase (+30.8%) was found for satellite cells associated with type II fibers in the Testosterone group. In parallel with fiber hypertrophy, testosterone significantly increased capillary contacts (+7.5%, P = 0.015) and capillary-to-fiber ratio (+9.2%, P = 0.001) in type II muscle fibers. The present study provides novel insight into fiber type-specific adaptations present already after 10 wk of only moderately elevated testosterone levels in women.NEW & NOTEWORTHY We have recently demonstrated performance-enhancing effects of moderately elevated testosterone concentrations in young women. Here we present novel evidence that testosterone alters muscle morphology in these women, resulting in type II fiber hypertrophy and improved capillarization. Our findings suggest that low doses of testosterone potently impact skeletal muscle after only 10 wk. These data provide unique insights into muscle adaptation and support the performance-enhancing role of testosterone in women on the muscle fiber level.

The Impact of Skeletal Muscle ERα on Mitochondrial Function and Metabolic Health

The incidence of chronic disease is elevated in women after menopause. Increased expression of ESR1 (the gene that encodes the estrogen receptor alpha, ERα) in muscle is highly associated with metabolic health and insulin sensitivity. Moreover, reduced muscle expression levels of ESR1 are observed in women, men, and animals presenting clinical features of the metabolic syndrome (MetSyn). Considering that metabolic dysfunction elevates chronic disease risk, including type 2 diabetes, heart disease, and certain cancers, treatment strategies to combat metabolic dysfunction and associated pathologies are desperately needed. This review will provide published work supporting a critical and protective role for skeletal muscle ERα in the regulation of mitochondrial function, metabolic homeostasis, and insulin action. We will provide evidence that muscle-selective targeting of ERα may be effective for the preservation of mitochondrial and metabolic health. Collectively published findings support a compelling role for ERα in the control of muscle metabolism via its regulation of mitochondrial function and quality control. Studies identifying ERα-regulated pathways essential for disease prevention will lay the important foundation for the design of novel therapeutics to improve metabolic health of women while limiting secondary complications that have historically plagued traditional hormone replacement interventions.

Skeletal muscles induce recruitment of Ly6C+ macrophage subtypes and release inflammatory cytokines 3 days after downhill exercise

Macrophages are one of the most versatile cells of the immune system that can express distinct subtypes and functions depending on the physiological challenge. After skeletal muscle damage, inflammatory macrophage subtypes aid muscles to regenerate and are implicated in physical training adaption. Based on this information, this study aimed to evaluate two classic mice macrophage subtypes and determine whether some inflammatory cytokines might be involved in the muscle adaption process after exercise. For this purpose, mice were exposed to an intermittent experimental protocol of downhill exercise (18 bouts of running, each bout 5 min with a 2-min rest interval, slope -16°) and were euthanized before [control (CTRL)] and 1, 2 (D2), and 3 (D3) days after exercise. After euthanasia, the triceps brachii was harvested and submitted to protein extraction, immunostaining, and mononuclear digestion procedures. The muscle size, macrophage accumulation, and cytokines were determined. We observed an increase in the Ly6C⁺ macrophage subtype (P ≤ 0.05) in D2 and D3 compared with CTRL, as well as a significant inverse correlation coefficient (-0.52; P ≤ 0.05) between Ly6C⁺ and Ly6C^- macrophage subtypes. Moreover, we also observed elevation in several cytokines (IL-1β, IFN-γ, TNF-α, IL-6, and IL-13) at D3, although not IL-4, which tended to decrease at this time point (P = 0.06). Downhill exercises preferentially recruited Ly6C⁺ macrophages with important proinflammatory cytokine elevation at D3. Moreover, despite the elevation of several cytokines involved with myogenesis, an increase in IL-6 and IL-13, which potentially involve muscle adaption training after acute exercise, was also observed.

Resistance exercise-induced hormonal response promotes satellite cell proliferation in untrained men but not in women

The purpose of this work was to determine the effect of resistance exercise (RE)-induced hormonal changes on the satellite cell (SC) myogenic state in response to muscle damage. Untrained men (n = 10, 22 ± 3 yr) and women (n = 9, 21 ± 4 yr) completed 2 sessions of 80 unilateral maximal eccentric knee extensions followed by either an upper body RE protocol (EX) or a 20-min rest (CON). Muscle samples were collected and analyzed for protein content of Pax7, MyoD, myogenin, cyclin D1, and p21 before (PRE), 12 h, and 24 h after the session was completed. Serum testosterone, growth hormone, cortisol, and myoglobin concentrations were analyzed at PRE, post-damage, immediately after (IP), and 15, 30, and 60 min after the session was completed. Testosterone was significantly (P < 0.05) higher immediately after the session in EX vs. CON for men. A significant time  × sex × condition interaction was found for MyoD with an increase in EX (men) and CON (women) at 12 h. A significant time × condition interaction was found for Pax7, with a decrease in EX and increase in CON at 24 h. A significant time effect was found for myogenin, p21, and cyclin D1. Myogenin and p21 were increased at 12 and 24 h, and cyclin D1 was increased at 12 h. These results suggest that the acute RE-induced hormonal response can be important for men to promote SC proliferation after muscle damage but had no effect in women. Markers of SC differentiation appeared unaffected by the hormonal response but increased in response to muscle damage.

Functions of estrogen and estrogen receptor signaling on skeletal muscle

Activity of estrogen, a sex steroid hormone, is not only limited to the reproductive organs but also involves other organs and tissues, including skeletal muscle. In postmenopausal women, estrogen decline causes endocrine and metabolic dysfunction, leading to a predisposition to osteoporosis, metabolic syndrome, and decreased muscle mass and strength. The decline in skeletal muscle mass often associates with sarcopenia, a popular condition observed in fragile elder people. In addition, varying estrogen levels associated with the menstrual phases may modulate exercise performance in women. Estrogen is thus considered to play a crucial role in skeletal muscle homeostasis and exercise capacity, although its precise mechanisms remain to be elucidated. In this article, we review the role of estrogen in the skeletal muscle, outlining the proposed molecular mechanisms. We especially focus on the current understanding of estrogen actions on mitochondria metabolism in skeletal muscle.

Aging of the musculoskeletal system: How the loss of estrogen impacts muscle strength

Skeletal muscle weakness occurs with aging and in females this is compounded by the loss of estrogen with ovarian failure. Estrogen deficiency mediates decrements in muscle strength from both inadequate preservation of skeletal muscle mass and decrements in the quality of the remaining skeletal muscle. Processes and components of skeletal muscle that are affected by estrogens are beginning to be identified. This review focuses on mechanisms that contribute to the loss of muscle force generation when estrogen is low in females, and conversely the maintenance of strength by estrogen. Evidence is accumulating that estrogen deficiency induces apoptosis in skeletal muscle contributing to loss of mass and thus strength. Estrogen sensitive processes that affect quality, i.e., force generating capacity of muscle, include myosin phosphorylation and satellite cell function. Further detailing these mechanisms and identifying additional mechanisms that underlie estrogenic effects on skeletal muscle is important foundation for the design of therapeutic strategies to minimize skeletal muscle pathologies, such as sarcopenia and dynapenia.

Oxytocin is involved in steroid hormone-stimulated bovine satellite cell proliferation and differentiation in vitro

Sex steroid hormones are used in the meat industry due to their ability to regulate muscle hypertrophy. However, the mechanisms underlying their action are not fully elucidated. Recent reports demonstrate that steroid hormones increase oxytocin (OXT) expression in skeletal muscle, indicating that OXT may play a role in satellite cell activity. This hypothesis was tested using steroid hormones (17β-estradiol [E2]; trenbolone acetate [TBA]), tamoxifen (TAM), OXT, and atosiban (A: OXT receptor inhibitor) applied to bovine satellite cells (BSCs) to investigate BSC regulation by OXT. Oxytocin alone increased fusion index (P < 0.05) but not BSC proliferation. Oxytocin reduced (P < 0.05) apoptotic nuclei and stimulated migration rate (P < 0.05). Similarly, E2 and TBA increased (P < 0.05) BSC proliferation rate, fusion index, and migration and decreased (P < 0.05) apoptotic nuclei. 17β-Estradiol or TBA supplemented with A had lower (P < 0.05) BSC proliferation rate, fusion index, and migration and more (P < 0.05) apoptotic nuclei compared with E2 or TBA alone. Furthermore, OXT expression increased (P < 0.05) in E2 or TBA-treated proliferating BSC. Oxytocin, E2, and TBA increased (P < 0.05) MyoD and MyoG expression in proliferating BSC. During BSC differentiation, OXT expression increased (P < 0.05) with E2 or TBA treatments. MyoG expression increased (P < 0.05) in OXT, E2, and TBA compared with control. However, A, OXT + A, TAM, TAM + OXT, E2 + TAM, E2 + A, and TBA + A decreased (P < 0.05) MyoG expression during BSC differentiation. These results indicate that OXT is involved in steroid hormone-stimulated BSC activity.

Sarcopenic obesity in older adults: aetiology, epidemiology and treatment strategies

The prevalence of obesity in combination with sarcopenia (the age-related loss of muscle mass and strength or physical function) is increasing in adults aged 65 years and older. A major subset of adults over the age of 65 is now classified as having sarcopenic obesity, a high-risk geriatric syndrome predominantly observed in an ageing population that is at risk of synergistic complications from both sarcopenia and obesity. This Review discusses pathways and mechanisms leading to muscle impairment in older adults with obesity. We explore sex-specific hormonal changes, inflammatory pathways and myocellular mechanisms leading to the development of sarcopenic obesity. We discuss the evolution, controversies and challenges in defining sarcopenic obesity and present current body composition modalities used to assess this condition. Epidemiological surveys form the basis of defining its prevalence and consequences beyond comorbidity and mortality. Current treatment strategies, and the evidence supporting them, are outlined, with a focus on calorie restriction, protein supplementation and aerobic and resistance exercises. We also describe weight loss-induced complications in patients with sarcopenic obesity that are relevant to clinical management. Finally, we review novel and potential future therapies including testosterone, selective androgen receptor modulators, myostatin inhibitors, ghrelin analogues, vitamin K and mesenchymal stem cell therapy.

The impact of ERα action on muscle metabolism and insulin sensitivity - Strong enough for a man, made for a woman

Background

The incidence of chronic disease is elevated in women after menopause. Natural variation in muscle expression of the estrogen receptor (ER)α is inversely associated with plasma insulin and adiposity. Moreover, reduced muscle ERα expression levels are observed in women and animals presenting clinical features of the metabolic syndrome (MetSyn). Considering that metabolic dysfunction impacts nearly a quarter of the U.S. adult population and elevates chronic disease risk including type 2 diabetes, heart disease, and certain cancers, treatment strategies to combat metabolic dysfunction and associated pathologies are desperately needed.

Scope of the review

This review will provide evidence supporting a critical and protective role for skeletal muscle ERα in the regulation of metabolic homeostasis and insulin sensitivity, and propose novel ERα targets involved in the maintenance of metabolic health.

Major conclusions

Studies identifying ERα-regulated pathways essential for disease prevention will lay the important foundation for the rational design of novel therapeutics to improve the metabolic health of women while limiting secondary complications that have plagued traditional hormone replacement interventions.

Transcriptional profile of breast muscle in heat stressed layers is similar to that of broiler chickens at control temperature

BACKGROUND

In recent years, the commercial importance of changes in muscle function of broiler chickens and of the corresponding effects on meat quality has increased. Furthermore, broilers are more sensitive to heat stress during transport and at high ambient temperatures than smaller egg-laying chickens. We hypothesised that heat stress would amplify muscle damage and expression of genes that are involved in such changes and, thus, lead to the identification of pathways and networks associated with broiler muscle and meat quality traits. Broiler and layer chickens were exposed to control or high ambient temperatures to characterise differences in gene expression between the two genotypes and the two environments.

RESULTS

Whole-genome expression studies in breast muscles of broiler and layer chickens were conducted before and after heat stress; 2213 differentially-expressed genes were detected based on a significant (P < 0.05) genotype × treatment interaction. This gene set was analysed with the BioLayout Express^3D and Ingenuity Pathway Analysis software and relevant biological pathways and networks were identified. Genes involved in functions related to inflammatory reactions, cell death, oxidative stress and tissue damage were upregulated in control broilers compared with control and heat-stressed layers. Expression of these genes was further increased in heat-stressed broilers.

CONCLUSIONS

Differences in gene expression between broiler and layer chickens under control and heat stress conditions suggest that damage of breast muscles in broilers at normal ambient temperatures is similar to that in heat-stressed layers and is amplified when broilers are exposed to heat stress. The patterns of gene expression of the two genotypes under heat stress were almost the polar opposite of each other, which is consistent with the conclusion that broiler chickens were not able to cope with heat stress by dissipating their body heat. The differentially expressed gene networks and pathways were consistent with the pathological changes that are observed in the breast muscle of heat-stressed broilers.

Female hormones: do they influence muscle and tendon protein metabolism?

Due to increased longevity, women can expect to live more than one-third of their lives in a post-menopausal state, which is characterised by low circulating levels of oestrogen and progesterone. The aim of this review is to provide insights into current knowledge of the effect of female hormones (or lack of female hormones) on skeletal muscle protein turnover at rest and in response to exercise. This review is primarily based on data from human trials. Many elderly post-menopausal women experience physical disabilities and loss of independence related to sarcopenia, which reduces life quality and is associated with substantial financial costs. Resistance training and dietary optimisation can counteract or at least decelerate the degenerative ageing process, but lack of oestrogen in post-menopausal women may reduce their sensitivity to these anabolic stimuli and accelerate muscle loss. Tendons and ligaments are also affected by sex hormones, but the effect seems to differ between endogenous and exogenous female hormones. Furthermore, the effect seems to depend on the age, and as a result influence the biomechanical properties of the ligaments and tendons differentially. Based on the present knowledge oestrogen seems to play a significant role with regard to skeletal muscle protein turnover. Therefore, oestrogen/hormonal replacement therapy may counteract the degenerative changes in skeletal muscle. Nevertheless, there is a need for greater insight into the direct and indirect mechanistic effects of female hormones before any evidence-based recommendations regarding type, dose, duration and timing of hormone replacement therapy can be provided.

Soymilk Improves Muscle Weakness in Young Ovariectomized Female Mice

Estrogens play a key role in an extensive range of physiological functions in various types of tissues throughout the body in females. We previously showed that estrogen insufficiency caused muscle weakness that could be rescued by estrogen administration in a young female ovariectomized (OVX) mouse model. However, long-term estrogen replacement therapy increases risks of breast cancer and cardiovascular diseases. Soymilk contains plant-based protein and isoflavones that exert estrogen-like activity. Here we examined the effects of prolonged soymilk intake on muscle and its resident stem cells, called satellite cells, in the estrogen-insufficient model. Six-week-old C57BL/6 OVX female mice were fed with a dried soymilk-containing diet. We found that prolonged soymilk intake upregulated grip strength in OVX mice. Correspondingly, cross-sectional area of tibialis anterior muscle was significantly increased in OVX mice fed with soymilk. Furthermore, soymilk diet mitigated dysfunction of satellite cells isolated from OVX mice. Thus, these results indicated that prolonged soymilk intake is beneficial for improving muscle weakness in an estrogen-insufficient state in females.

The Effect of Gender and Menstrual Phase on Serum Creatine Kinase Activity and Muscle Soreness Following Downhill Running

Serum creatine kinase (CK) activity reflects muscle membrane disruption. Oestrogen has antioxidant and membrane stabilising properties, yet no study has compared the CK and muscle soreness (DOMS) response to unaccustomed exercise between genders when all menstrual phases are represented in women. Fifteen eumenorrhoeic women (early follicular, EF (n = 5); late follicular, LF (n = 5); mid-luteal, ML (n = 5) phase) and six men performed 20 min of downhill running (−10% gradient) at 9 km/h. Serum CK activity and visual analogue scale rating of perceived muscle soreness were measured before, immediately, 24-h, 48-h and 72-h after exercise. The 24-h peak CK response (relative to pre-exercise) was similar between women and men (mean change (95% confidence interval): 58.5 (25.2 to 91.7) IU/L; 68.8 (31.3 to 106.3) IU/L, respectively). However, serum CK activity was restored to pre-exercise levels quicker in women (regardless of menstrual phase) than men; after 48-h post exercise in women (16.3 (−4.4 to 37.0) IU/L; 56.3 (37.0 to 75.6) IU/L, respectively) but only after 72-h in men (14.9 (−14.8 to 44.6) IU/L). Parallel to the CK response, muscle soreness recovered by 72-h in men. Conversely, the women still reported muscle soreness at 72-h despite CK levels being restored by 48-h; delayed recovery of muscle soreness appeared mainly in EF and LF. The CK and DOMS response to downhill running is gender-specific. The CK response recovers quicker in women than men. The CK and DOMS response occur in concert in men but not in women. The DOMS response in women is prolonged and may be influenced by menstrual phase.

The Role of Skeletal Muscle Estrogen Receptors in Metabolic Homeostasis and Insulin Sensitivity

Women in the modern era are challenged with facing menopausal symptoms as well as heightened disease risk associated with increasing adiposity and metabolic dysfunction for up to three decades of life. Treatment strategies to combat metabolic dysfunction and associated pathologies have been hampered by our lack of understanding regarding the biological causes of these clinical conditions and our incomplete understanding regarding the effects of estrogens and the tissue-specific functions and molecular actions of its receptors. In this chapter we provide evidence supporting a critical and protective role for skeletal muscle estrogen receptor α in the maintenance of metabolic homeostasis and insulin sensitivity. Studies identifying the critical ER-regulated pathways essential for disease prevention will lay the important foundation for the rational design of novel therapeutic strategies to improve the health of women while limiting secondary complications that have plagued traditional hormone replacement interventions.

Supplementation Strategies to Reduce Muscle Damage and Improve Recovery Following Exercise in Females: A Systematic Review

Exercise-induced muscle damage (EIMD) caused by unaccustomed or strenuous exercise can result in reduced muscle force, increased muscle soreness, increased intramuscular proteins in the blood, and reduced performance. Pre- and post-exercise optimal nutritional intake is important to assist with muscle-damage repair and reconditioning to allow for an accelerated recovery. The increased demand for training and competing on consecutive days has led to a variety of intervention strategies being used to reduce the negative effects of EIMD. Nutritional intervention strategies are largely tested on male participants, and few report on sex-related differences relating to the effects of the interventions employed. This review focuses on nutritional intervention strategies employed to negate the effects of EIMD, focussing solely on females.

The ubiquitin proteasome system in atrophying skeletal muscle: roles and regulation

Muscle atrophy complicates many diseases as well as aging, and its presence predicts both decreased quality of life and survival. Much work has been conducted to define the molecular mechanisms involved in maintaining protein homeostasis in muscle. To date, the ubiquitin proteasome system (UPS) has been shown to play an important role in mediating muscle wasting. In this review, we have collated the enzymes in the UPS whose roles in muscle wasting have been confirmed through loss-of-function studies. We have integrated information on their mechanisms of action to create a model of how they work together to produce muscle atrophy. These enzymes are involved in promoting myofibrillar disassembly and degradation, activation of autophagy, inhibition of myogenesis as well as in modulating the signaling pathways that control these processes. Many anabolic and catabolic signaling pathways are involved in regulating these UPS genes, but none appear to coordinately regulate a large number of these genes. A number of catabolic signaling pathways appear to instead function by inhibition of the insulin/IGF-I/protein kinase B anabolic pathway. This pathway is a critical determinant of muscle mass, since it can suppress key ubiquitin ligases and autophagy, activate protein synthesis, and promote myogenesis through its downstream mediators such as forkhead box O, mammalian target of rapamycin, and GSK3β, respectively. Although much progress has been made, a more complete inventory of the UPS genes involved in mediating muscle atrophy, their mechanisms of action, and their regulation will be useful for identifying novel therapeutic approaches to this important clinical problem.

The role of sex steroid hormones in the pathophysiology and treatment of sarcopenia

Sex steroids influence the maintenance and growth of muscles. Decline in androgens, estrogens and progesterone by aging leads to the loss of muscular function and mass, sarcopenia. These steroid hormones can interact with different signaling pathways through their receptors. To date, sex steroid hormone receptors and their exact roles are not completely defined in skeletal and smooth muscles. Although numerous studies focused on the effects of sex steroid hormones on different types of cells, still many unexplained molecular mechanisms in both skeletal and smooth muscle cells remain to be investigated. In this paper, many different molecular mechanisms that are activated or inhibited by sex steroids and those that influence the growth, proliferation, and differentiation of skeletal and smooth muscle cells are reviewed. Also, the similarities of cellular and molecular pathways of androgens, estrogens and progesterone in both skeletal and smooth muscle cells are highlighted. The reviewed signaling pathways and participating molecules can be targeted in the future development of novel therapeutics.

Absence of CCR2 results in an inflammaging environment in young mice with age-independent impairments in muscle regeneration

Skeletal muscle regeneration requires coordination between dynamic cellular populations and tissue microenvironments. Macrophages, recruited via CCR2, are essential for regeneration; however, the contribution of macrophages and the role of CCR2 on nonhematopoietic cells has not been defined. In addition, aging and sex interactions in regeneration and sarcopenia are unclear. Muscle regeneration was measured in young (3-6 mo), middle (11-15 mo), old (24-32 mo) male and female CCR2^-/- mice. Whereas age-related muscle atrophy/sarcopenia was present, regenerated myofiber cross-sectional area (CSA) in CCR2^-/- mice was comparably impaired across all ages and sexes, with increased adipocyte area compared with wild-type (WT) mice. CCR2^-/- mice myofibers achieved approximately one third of baseline CSA even 84 d after injury. Regenerated CSA and clearance of necrotic tissue were dependent on bone marrow-derived cellular expression of CCR2. Myogenic progenitor cells isolated from WT and CCR2^-/- mice exhibited comparable proliferation and differentiation capacity. The most striking cellular anomaly in injured muscle of CCR2^-/- mice was markedly decreased macrophages, with a predominance of Ly6C^- anti-inflammatory monocytes/macrophages. Ablation of proinflammatory TLR signaling did not affect muscle regeneration or resolution of necrosis. Of interest, many proinflammatory, proangiogenic, and chemotactic cytokines were markedly elevated in injured muscle of CCR2^-/- relative to WT mice despite impairments in macrophage recruitment. Collectively, these results suggest that CCR2 on bone marrow-derived cells, likely macrophages, were essential to muscle regeneration independent of TLR signaling, aging, and sex. Decreased proinflammatory monocytes/macrophages actually promoted a proinflammatory microenvironment, which suggests that inflammaging was present in young CCR2^-/- mice.