Ribosome accumulation during early phase resistance training in humans

Sex differences in skeletal muscle metabolism in exercise and type 2 diabetes mellitus

This Review focuses on currently available literature describing sex differences in skeletal muscle metabolism in humans, as well as highlighting current research gaps within the field. These discussions serve as a call for action to address the current lack of sufficient sex-balanced studies in skeletal muscle research, and the resulting limitations in understanding sex-specific physiological and pathophysiological responses. Although the participation of women in studies has increased, parity between the sexes remains elusive, affecting the validity of conclusions drawn from studies with limited numbers of participants. Changes in skeletal muscle metabolism contribute to the development of metabolic disease (such as type 2 diabetes mellitus), and maintenance of skeletal muscle mass is a key component for health and the ability to maintain an independent life during ageing. Exercise is an important factor in maintaining skeletal muscle health and insulin sensitivity, and offers promise for both prevention and treatment of metabolic disease. With the increased realization of the promise of precision medicine comes the need to increase patient stratification and improve the understanding of responses in different populations. In this context, a better understanding of sex-dependent differences in skeletal muscle metabolism is essential.

Skeletal muscle ribosome analysis: A comparison of common assay methods and utilization of a novel RiboAb antibody cocktail

While total RNA concentrations putatively represent ribosome content, there is a need to homologize various quantification approaches. Thus, total RNA concentrations ([RNA]) provided through UV-Vis spectroscopy (UV), fluorometry-only (Fluor), and fluorometry-based microfluidic chip electrophoresis (MFGE) were examined in C2C12 myotubes and mouse skeletal muscle to determine if values aligned with [18S + 28S rRNA] (i.e., criterion ribosome metric). A novel antibody cocktail (termed RiboAb) was also tested and compared to [18S + 28S rRNA] in these models. In myotubes, 24-h IGF-1 treatments increased [18S + 28S rRNA] (~2.0-fold) and [RNA] based on UV (~1.9-fold), Fluor (~2.3 fold), and MFGE (~2.1-fold). In C57BL/6 mice, 10 days of mechanical overload (MOV) elevated plantaris [18S + 28S rRNA] (~1.7-fold) and [RNA] according to UV (~1.5-fold), Fluor (~1.6-fold), and MFGE (~1.8-fold). Myotube and mouse plantaris RiboAb levels were significantly higher with IGF-1 treatments and MOV, respectively, versus controls (1.3-fold and 1.7-fold, respectively), and values correlated with [18S + 28S rRNA] (r = 0.637 and r = 0.853, respectively, p ≤ 0.005). UV, Fluor, and MFGE [RNA] are seemingly valid surrogates of cell/tissue ribosome content, although each method has advantages (e.g., ease of use) and disadvantages (e.g., magnitudes of bias) discussed herein. Finally, the RiboAb cocktail may also represent ribosome content, although this should be further explored in other models.

The rRNA epitranscriptome and myonuclear SNORD landscape in skeletal muscle fibers contributes to ribosome heterogeneity and is altered by a hypertrophic stimulus

In cell biology, ribosomal RNA (rRNA) 2'O-methyl (2'-O-Me) is the most prevalent posttranscriptional chemical modification contributing to ribosome heterogeneity. The modification involves a family of small nucleolar RNAs (snoRNAs) and is specified by box C/D snoRNAs (SNORDs). Given the importance of ribosome biogenesis for skeletal muscle growth, we asked if rRNA 2'-O-Me in nascent ribosomes synthesized in response to a growth stimulus is an unrecognized mode of ribosome heterogeneity in muscle. To determine the pattern and dynamics of 2'-O-Me rRNA, we used a sequencing-based profiling method called RiboMeth-seq (RMS). We applied this method to tissue-derived rRNA of skeletal muscle and rRNA specifically from the muscle fiber using an inducible myofiber-specific RiboTag mouse in sedentary and mechanically overloaded conditions. These analyses were complemented by myonuclear-specific small RNA sequencing to profile SNORDs and link the rRNA epitranscriptome to known regulatory elements generated within the muscle fiber. We demonstrate for the first time that mechanical overload of skeletal muscle 1) induces decreased 2'-O-Me at a subset of skeletal muscle rRNA and 2) alters the SNORD profile in isolated myonuclei. These findings point to a transient diversification of the ribosome pool via 2'-O-Me during growth and adaptation in skeletal muscle. These findings suggest changes in ribosome heterogeneity at the 2'-O-Me level during muscle hypertrophy and lay the foundation for studies investigating the functional implications of these newly identified "growth-induced" ribosomes.NEW & NOTEWORTHY Ribosomal RNAs (rRNAs) are posttranscriptionally modified by 2'O-methyl (2'-O-Me). This study applied RiboMeth-seq (RMS) to detect changes in 2'-O-Me levels during skeletal muscle hypertrophy, uncovering transient diversification of the ribosome pool in skeletal muscle fibers. This work implies a role for ribosome heterogeneity in skeletal muscle growth and adaptation.

Relative rDNA copy number is not associated with resistance training-induced skeletal muscle hypertrophy and does not affect myotube anabolism in vitro

Ribosomal DNA (rDNA) copies exist across multiple chromosomes, and interindividual variation in copy number is speculated to influence the hypertrophic response to resistance training. Thus, we examined if rDNA copy number was associated with resistance training-induced skeletal muscle hypertrophy. Participants (n = 53 male, 21 ± 1 yr old; n = 29 female, 21 ± 2 yr old) performed 10-12 wk of full-body resistance training. Hypertrophy outcomes were determined, as was relative rDNA copy number from preintervention vastus lateralis (VL) biopsies. Pre- and postintervention VL biopsy total RNA was assayed in all participants, and mRNA/rRNA markers of ribosome content and biogenesis were also assayed in the 29 female participants before training, 24 h following training bout 1, and in the basal state after 10 wk of training. Across all participants, no significant associations were evident between relative rDNA copy number and training-induced changes in whole body lean mass (r = -0.034, P = 0.764), vastus lateralis thickness (r = 0.093, P = 0.408), mean myofiber cross-sectional area (r = -0.128, P = 0.259), or changes in muscle RNA concentrations (r = 0.026, P = 0.818), and these trends were similar when examining each gender. However, all Pol-I regulon mRNAs as well as 45S pre-rRNA, 28S rRNA, and 18S rRNA increased 24 h following the first training bout in female participants. Follow-up studies using LHCN-M2 myotubes demonstrated that a reduction in relative rDNA copy number induced by bisphenol A did not significantly affect insulin-like-growth factor-induced myotube hypertrophy. These findings suggest that relative rDNA copy number is not associated with myofiber hypertrophy.NEW & NOTEWORTHY We examined ribosomal DNA (rDNA) copy numbers in men and women who resistance trained for 10-12 wk and found no significant associations with skeletal muscle hypertrophy outcomes. These data, along with in vitro data in immortalized human myotubes whereby rDNA copy number was reduced, provide strong evidence that relative rDNA copy number is not associated with anabolism.

Glucose ingestion before and after resistance training sessions does not augment ribosome biogenesis in healthy moderately trained young adults

PURPOSE

Resistance training-induced skeletal muscle hypertrophy seems to depend on ribosome biogenesis and content. High glucose treatment may augment ribosome biogenesis through potentiating resistance training-induced adaptations. This was investigated with total RNA and ribosomal RNA abundances as main outcomes, with relevant transcriptional/translational regulators (c-Myc/UBF/rpS6) as a secondary outcome.

METHODS

Sixteen healthy, moderately trained individuals [male/female, n = 9/7; age, 24.1 (3.3)] participated in a within-participant crossover trial with unilateral resistance training (leg press and knee extension, 3 sets of 10 repetitions maximum) and pre- and post-exercise ingestion of either glucose (3 × 30 g, 90 g total) or placebo supplements (Stevia rebaudiana, 3 × 0.3 g, 0.9 g total), together with protein (2 × 25 g, 50 g total), on alternating days for 12 days. Six morning resistance exercise sessions were conducted per condition, and the sessions were performed in an otherwise fasted state. Micro-biopsies were sampled from m. vastus lateralis before and after the intervention.

RESULTS

Glucose ingestion did not have beneficial effects on resistance training-induced increases of ribosomal content (mean difference 7.6% [- 7.2, 24.9], p = 0.34; ribosomal RNA, 47S/18S/28S/5.8S/5S, range 7.6-37.9%, p = 0.40-0.98) or levels of relevant transcriptional or translational regulators (c-MYK/UBF/rpS6, p = 0.094-0.292). Of note, both baseline and trained state data of total RNA showed a linear relationship with UBF; a ∼14% increase in total RNA corresponded to 1 SD unit increase in UBF (p = 0.003).

CONCLUSION

Glucose ingestion before and after resistance training sessions did not augment ribosomal RNA accumulation during twelve days of heavy-load resistance training in moderately trained young adults.

Effect of resistance training on quadriceps femoris muscle thickness obtained by ultrasound: A systematic review with meta-analysis

OBJECTIVE

The present study aimed to determine the magnitude and intervention time of resistance training required to generate adaptations in the muscle thickness of the quadriceps muscle obtained by ultrasound in healthy adults.

METHOD

A systematic review with meta-analysis was conducted on studies recovered from Pubmed, Web of Science, and Scopus databases up to March 2022. The study selection process was carried out by two independent researchers, with the presence of a third researcher in case of disagreements. The methodological quality of the studies was determined with the TESTEX scale, and the risk of bias analysis was determined using Cochrane's RoB 2.0 tool. The meta-analysis used the inverse of the variance with a fixed model, and the effect size was reported by the standardized mean difference (SMD) with a confidence interval of 95%.

RESULTS

Ten studies were included in a meta-analysis. The overall analysis of the studies demonstrated an SMD = 0.35 [95% CI: 0.13-0.56] (P = 0.002), with a low heterogeneity of I2 = 0% (P = 0.52). No publication bias was detected using a funnel plot followed by Egger's test (P = 0.06). The degree of certainty of the meta-analysis was high using the GRADE tool.

CONCLUSION

We found that resistance training can generate significant average increases of 16.6% in muscle thickness obtained by ultrasound in the quadriceps femoris muscles of healthy adults. However, the subgroup analysis showed that significant effect sizes were only observed after eight weeks of training.

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.

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

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

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

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

Elevated muscle mass accompanied by transcriptional and nuclear alterations several months following cessation of resistance-type training in rats

Rodent studies investigating long-term effects following termination of hypertrophy-inducing loading have predominantly involved exposures such as synergist ablation and weighted wheel running or ladder climbing. This research yielded a spectrum of results regarding the extent of detraining in terms of muscle mass and myonuclei number. The studies were also limited in their lack of sensitive performance measures and indirect relatedness to resistance training. Our research group developed and validated a relevant rat model of resistance-type training that induces increased muscle mass and performance. The aim of the present study was to determine to what extent these features persist 3 months following the termination of this training. While performance returned to baseline, muscle mass remained elevated by 17% and a shift in distribution to larger muscle fibers persisted. A 16% greater total RNA and heightened mRNA levels of ribosomal protein S6 kinases implicated preserved transcriptional output and ribosomal content. Remodeling of muscle fiber nuclei was consistent with these findings - increased nuclear number and a distribution shift to a more circular nuclear shape. These findings indicate that muscle mass detrains at a slower rate than performance and implicates multiple forms of myonuclear remodeling in muscle memory.