Nuclear-localized androgen receptor content following resistance exercise training is associated with hypertrophy in males but not females.

Androgen receptor (AR) content has been implicated in the differential response between high and low responders following resistance exercise training (RET). However, the influence of AR expression on acute skeletal muscle damage and whether it may influence the adaptive response to RET in females is poorly understood. Thus, the purpose of this exploratory examination was to 1) investigate changes in AR content during skeletal muscle repair and 2) characterize AR-mediated sex-based differences following RET. A skeletal muscle biopsy from the vastus lateralis was obtained from 26 healthy young men (n = 13) and women (n = 13) at baseline and following 300 eccentric kicks. Subsequently, participants performed 10 weeks of full-body RET and a final muscle biopsy was collected. In the untrained state, AR mRNA expression was associated with paired box protein-7 (PAX7) mRNA in males. For the first time in human skeletal muscle, we quantified AR content in the myofiber and localized to the nucleus where AR has been shown to trigger cellular outcomes related to growth. Upon eccentric damage, nuclear-associated AR (nAR) content increased (p < .05) in males and not females. Males with the greatest increase in cross-sectional area (CSA) post-RET had more (p < .05) nAR content than females with the greatest gain CSA. Collectively, skeletal muscle damage and RET increased AR protein, and both gene and hypertrophy measures revealed sex differences in relation to AR. These findings suggest that AR content but more importantly, nuclear localization, is a factor that differentiates RET-induced hypertrophy between males and females.

Low baseline ribosome-related gene expression and resistance training-induced declines in ribosome-related gene expression are associated with skeletal muscle hypertrophy in young men and women.

Ribosomes are essential cellular machinery for protein synthesis. It is hypothesised that ribosome content supports muscle growth and that individuals with more ribosomes have greater increases in muscle size following resistance training (RT). Aerobic conditioning (AC) also elicits distinct physiological adaptations; however, no measures of ribosome content following AC have been conducted. We used ribosome-related gene expression as a proxy measure for ribosome content and hypothesised that AC and RT would increase ribosome-related gene expression. Fourteen young men and women performed 6 weeks of single-legged AC followed by 10 weeks of double-legged RT. Muscle biopsies were taken following AC and following RT in the aerobically conditioned (AC+RT) and unconditioned (RT) legs. No differences in regulatory genes (Ubf, Cyclin D1, Tif-1a and Polr-1b) involved in ribosomal biogenesis or ribosomal RNA (45S, 5.8S, 18S and 28S rRNAs) expression were observed following AC and RT, except for c-Myc (RT > AC+RT) and 5S rRNA (RT < AC+RT at pre-RT) with 18S external transcribed spacer and 5.8S internal transcribed spacer expression decreasing from pre-RT to post-RT in the RT leg only. When divided for change in leg-lean soft tissue mass (ΔLLSTM) following RT, legs with the greatest ΔLLSTM had lower expression in 11/13 measured ribosome-related genes before RT and decreased expression in 9/13 genes following RT. These results indicate that AC and RT did not increase ribosome-related gene expression. Contrary to previous research, the greatest increase in muscle mass was associated with lower changes in ribosome-related gene expression over the course of the 10-week training programme. This may point to the importance of translational efficiency rather than translational capacity (i.e. ribosome content) in mediating long-term exercise-induced adaptations in skeletal muscle.

Bleomycin-treated myoblasts undergo p21-associated cellular senescence and have severely impaired differentiation.

As we age, the ability to regenerate and repair skeletal muscle damage declines, partially due to increasing dysfunction of muscle resident stem cells-satellite cells (SC). Recent evidence implicates cellular senescence, which is the irreversible arrest of proliferation, as a potentiator of SC impairment during aging. However, little is known about the role of senescence in SC, and there is a large discrepancy in senescence classification within skeletal muscle. The purpose of this study was to develop a model of senescence in skeletal muscle myoblasts and identify how common senescence-associated biomarkers respond. Low-passage C2C12 myoblasts were treated with bleomycin or vehicle and then evaluated for cytological and molecular senescence markers, proliferation status, cell cycle kinetics, and differentiation potential. Bleomycin treatment caused double-stranded DNA breaks, which upregulated p21 mRNA and protein, potentially through NF-κB and senescence-associated super enhancer (SASE) signaling (p < 0.01). Consequently, cell proliferation was abruptly halted due to G2/M-phase arrest (p < 0.01). Bleomycin-treated myoblasts displayed greater senescence-associated ß-galactosidase staining (p < 0.01), which increased over several days. These myoblasts remained senescent following 6 days of differentiation and had significant impairments in myotube formation (p < 0.01). Furthermore, our results show that senescence can be maintained despite the lack of p16 gene expression in C2C12 myoblasts. In conclusion, bleomycin treatment provides a valid model of damage-induced senescence that was associated with elevated p21, reduced myoblast proliferation, and aberrant cell cycle kinetics, while confirming that a multi-marker approach is needed for the accurate classification of senescence within skeletal muscle.

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.

The Role of Supporting Cell Populations in Satellite Cell Mediated Muscle Repair.

Skeletal muscle has a high capacity to repair and remodel in response to damage, largely through the action of resident muscle stem cells, termed satellite cells. Satellite cells are required for the proper repair of skeletal muscle through a process known as myogenesis. Recent investigations have observed relationships between satellite cells and other cell types and structures within the muscle microenvironment. These findings suggest that the crosstalk between inflammatory cells, fibrogenic cells, bone-marrow-derived cells, satellite cells, and the vasculature is essential for the restoration of muscle homeostasis. This review will discuss the influence of the cells and structures within the muscle microenvironment on satellite cell function and muscle repair.

Twist2-expressing cells reside in human skeletal muscle and are responsive to aging and resistance exercise training.

Skeletal muscle is maintained and repaired by sub-laminar, Pax7-expressing satellite cells. However, recent mouse investigations have described a second myogenic progenitor population that resides within the myofiber interstitium and expresses the transcription factor Twist2. Twist2-expressing cells exclusively repair and maintain type IIx/b muscle fibers. Currently, it is unknown if Twist2-expressing cells are present in human skeletal muscle and if they function as myogenic progenitors. Here, we perform a combination of single-cell RNA sequencing analysis and immunofluorescence staining to demonstrate the identity and localization of Twist2-expressing cells in human skeletal muscle. Twist2-expressing cells were identified to be anatomically and transcriptionally comparable to fibro-adipogenic progenitors (FAPs) and lack expression of typical satellite cell markers such as Pax7. Comparative analysis revealed that human and mouse Twist2-expressing cells were highly transcriptionally analogous and resided within the same anatomical structures in vivo. Examination of young and aged skeletal muscle biopsy samples revealed that Twist2-positive cells are more prevalent in aged muscle and increase following 12-weeks of resistance exercise training (RET) in humans. However, the quantity of Twist2-positive cells was not correlated with indices of muscle mass or muscle fiber cross-sectional area (CSA) in young or older muscle, and their abundance was surprisingly, negatively correlated with CSA and myonuclear domain size following RET. Taken together, we have identified cells expressing Twist2 in human skeletal muscle which are responsive to aging and exercise. Further examination of their myogenic potential is warranted.

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.

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.

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.

The Importance of Muscle Capillarization for Optimizing Satellite Cell Plasticity.

Satellite cells are essential for skeletal muscle regeneration, repair, and adaptation. The activity of satellite cells is influenced by their interactions with muscle-resident endothelial cells. We postulate that the microvascular network between muscle fibers plays a critical role in satellite cell function. Exercise-induced angiogenesis can mitigate the decline in satellite cell function with age.

The Effect of a Multi-ingredient Supplement on Resistance Training-induced Adaptations.

INTRODUCTION: Resistance exercise training (RET) induces muscle hypertrophy that, when combined with co-temporal protein ingestion, is enhanced. However, fewer studies have been conducted when RET is combined with multi-ingredient supplements. PURPOSE: We aimed to determine the effect of a high-quality multi-ingredient nutritional supplement (SUPP) versus an isonitrogenous (lower protein quality), isoenergetic placebo (PL) on RET-induced gains in lean body mass (LBM), muscle thickness, and muscle cross-sectional area (CSA). We hypothesized that RET-induced gains in LBM and muscle CSA would be greater in SUPP versus PL. METHODS: In a double-blind randomized controlled trial, 26 (13 male, 13 female) healthy young adults (mean ± SD, 22 ± 2 yr) were randomized to either the SUPP group (n = 13; 20 g whey protein, 2 g leucine, 2.5 g creatine monohydrate, 300 mg calcium citrate, 1000 IU vitamin D) or the PL group (n = 13; 20 g collagen peptides, 1.4 g alanine, 0.6 g glycine) groups, ingesting their respective supplements twice daily. Measurements were obtained before and after a 10-wk linear progressive RET program. RESULTS: Greater increases in LBM were observed for SUPP versus PL (SUPP: +4.1 ± 1.3 kg, PL: +2.8 ± 1.7 kg, P < 0.05). No additive effect of the supplement could be detected on vastus lateralis muscle CSA, but SUPP did result in increased biceps brachii muscle CSA and thickness (P < 0.05). CONCLUSIONS: We conclude that when combined with RET, the consumption of SUPP increased LBM and upper-body CSA and thickness to a greater extent than to that observed in the PL group of healthy young adults.

Do Different Ascertainment Techniques Identify the Same Individuals as Sarcopenic in the Canadian Longitudinal Study on Aging?

BACKGROUND/OBJECTIVES: Sarcopenia is associated with poor health outcomes such as disability, institutionalization, and mortality. Efforts to manage sarcopenia clinically have been hindered by challenges in determining how to ascertain sarcopenia status correctly. The objective of this project was to assess the agreement between the different methods of ascertaining sarcopenia recommended by expert groups. DESIGN: Cross-sectional study of baseline data (2011-2015) from the Canadian Longitudinal Study on Aging. SETTING: Population-based multicenter study of community-dwelling participants. PARTICIPANTS: Eligible participants (n = 12,646) aged 65 to 85 living within 25 to 50 km of 11 data collection sites in Canada. The analyses included 10,820 participants with the data required to diagnose sarcopenia. MEASUREMENTS: Sarcopenia was operationalized as appendicular lean mass (ALM), ALM and grip strength, ALM and gait speed, and grip strength and gait speed. Within each combination, ALM was adjusted for height squared, weight, body mass index, and the residual of regressing lean mass on height and fat mass. The lowest 20th sex-specific percentile values were used as the cutoffs for low ALM. Low grip strength cutoffs of 35.5 kg for men and 20 kg for women and a gait speed cutoff of .8 m/s were used. RESULTS: The mean age was 73.0 ± 5.6 years, and 51.9% of the sample was male. The agreement (Cohen's κ) between the different combinations of variables used to ascertain sarcopenia status was below .50. Agreement for the different lean mass adjustment techniques ranged from .04 to .76. CONCLUSION: The combination of variables used to ascertain sarcopenia and many of the ALM adjustment techniques have insufficient agreement to be considered equivalent. This has important clinical implications for the management of sarcopenia because treatments may differ based on how sarcopenia is identified. To improve the clinical utility of sarcopenia, a unified definition of sarcopenia is required.

Correction: A whey protein-based multi-ingredient nutritional supplement stimulates gains in lean body mass and strength in healthy older men: A randomized controlled trial.

[This corrects the article DOI: 10.1371/journal.pone.0181387.].

Corrigendum: Integrated Myofibrillar Protein Synthesis in Recovery From Unaccustomed and Accustomed Resistance Exercise With and Without Multi-ingredient Supplementation in Overweight Older Men.

[This corrects the article DOI: 10.3389/fnut.2019.00040.].

Corrigendum: A Multi-Ingredient Nutritional Supplement in Combination With Resistance Exercise and High-Intensity Interval Training Improves Cognitive Function and Increases N-3 Index in Healthy Older Men: A Randomized Controlled Trial.

[This corrects the article DOI: 10.3389/fnagi.2019.00107.].

The impact of different diagnostic criteria on the association of sarcopenia with injurious falls in the CLSA.

BACKGROUND: Sarcopenia definitions recommend different combinations of variables (lean mass, strength, and physical function) and different methods of adjusting lean mass. The purpose of this paper was to address the gaps in the literature regarding how differences in the operationalization of sarcopenia impact the association between sarcopenia and injurious falls. METHODS: Participants included 9936 individuals from the Canadian Longitudinal Study on Aging aged ≥65 years at baseline (2012-2015), with complete data for sarcopenia-related variables, injurious falls, and covariates. Sarcopenia was defined using all combinations of muscle variables (lean mass, grip strength, chair rise test, and gait speed) and methods of adjusting lean mass (height2 , weight, body mass index (BMI), and regressing on height and fat mass) recommended by the expert group sarcopenia definitions. Multiple cut off values for the measures were explored. The association between sarcopenia and injurious falls (0, 1, or 2+ falls) measured 18 months after baseline data collection were assessed using proportional odds regression models. RESULTS: In men (n = 5162, 72.9 ± 5.6 years), the odds of having a higher level of injurious falls was between 1.43 and 2.14 greater when sarcopenia was defined as (i) lean mass adjusted for weight only; (ii) grip strength (<30 or <26 kg) only; (iii) lean mass adjusted for weight and grip strength (<30 or <26 kg); (iv) lean mass adjusted for BMI and grip strength (<26 kg); and (v) lean mass adjusted using the regression technique and grip strength (<30 or <26 kg). In women (n = 4774, 72.8 ± 5.6 years), only the combination of lean mass adjusted using regression with gait speed (<0.8 m/s) was associated with a significantly higher odds (1.46, 95% confidence interval: 1.01-2.10, P = 0.04) of having a higher level of injurious falls. CONCLUSIONS: Sarcopenia definitions based on different combinations of muscle variables and methods of adjusting lean mass are not equally associated with injurious falls. In men, definitions including grip strength but not gait speed or the chair rise test, and adjusting lean mass for weight, BMI, or using the residual technique but not height2 , tended to be associated with injurious falls. In women, sarcopenia was generally not associated with injurious falls regardless of the definition used.

Age-related changes to the satellite cell niche are associated with reduced activation following exercise.

Skeletal muscle satellite cell (SC) function and responsiveness is regulated, in part, through interactions within the niche, in which they reside. Evidence suggests that structural changes occur in the SC niche as a function of aging. In the present study, we investigated the impact of aging on SC niche properties. Muscle biopsies were obtained from the vastus lateralis of healthy young (YM; 21 ± 1 yr; n = 10) and older men (OM; 68 ± 1 yr; n = 16) at rest. A separate group of OM performed a single bout of resistance exercise and additional muscle biopsies were taken 24 and 48 hours post-exercise; this was performed before and following 12 wks of combined exercise training (OM-Ex; 73 ± 1; n = 24). Muscle SC niche measurements were assessed using high resolution immunofluorescent confocal microscopy. Type II SC niche laminin thickness was greater in OM (1.86 ± 0.06 µm) as compared to YM (1.55 ± 0.09 µm, P < .05). The percentage of type II-associated SC that were completely surrounded by laminin was greater in OM (13.6%±4.2%) as compared to YM (3.5%±1.5%; P < .05). In non-surrounded SC, the proportion of active MyoD+ /Pax7+ SC were higher compared to surrounded SC (P < .05) following a single bout of exercise. This "incarceration" of the SC niche by laminin appears with aging and may inhibit SC activation in response to exercise.

Variability in skeletal muscle fibre characteristics during repeated muscle biopsy sampling in human vastus lateralis.

The percutaneous muscle biopsy procedure is an invaluable tool for characterizing skeletal muscle and capillarization. Little is known about methodological or biological variation stemming from the technique in heterogeneous muscle. Five muscle biopsies were taken from the vastus lateralis of a group of young men (n = 29, 22 ± 1 years) over a 96-h period. We investigated the repeatability of fibre distribution, indices of muscle capillarization and perfusion, and myofibre characteristics. No differences between the biopsies were reported in myofibre type distribution, cross-sectional area (CSA), and perimeter. Capillary-to-fibre perimeter exchange index and individual capillary-fibre contacts were unchanged with respect to the location of the muscle biopsy and index of capillarization. The variability in the sampling distribution of fibre type specific muscle CSA increased when fewer than 150 muscle fibres were quantified. Variability in fibre type distribution increased when fewer than 150 muscle fibres were quantified. Myofibre characteristics and indices of capillarization are largely consistent throughout the vastus lateralis when assessed via the skeletal muscle biopsy technique. Novelty Markers of muscle capillarization and perfusion were unchanged across multiple sites of the human vastus lateralis. Myofibre characteristics such as muscle cross-sectional area, perimeter, and fibre type distribution were also unchanged. Variation of muscle CSA was higher when fewer than 150 muscle fibres were quantified.