More is more? rDNA gene dosage is correlated with resistance exercise‐induced ribosome biogenesis

Abstract

Many people have started exercise programmes, with either family members, friends or work colleagues, only to find that some seem to make gains in aspects of their physical fitness (e.g. increases in endurance capacity, muscle strength or muscle size) at a rate far faster than the rest of the group. Frustratingly, despite their best efforts, some do not seem to make very significant gains in their physical function or muscle mass at all. This phenomenon has prompted researchers to describe individuals as ‘responders’, ‘non-responders’ or ‘extreme responders’ (Bamman et al. 2007), or as those that have a ‘high sensitivity’ or ‘low sensitivity’ to exercise (Booth & Laye, 2010). A key question that arises from these observations is: what is the underlying molecular basis for this inter-individual variation in the ability to adapt to the stresses, such as exercise? Muscle size/mass is partly determined by the rate of protein synthesis and resistance exercise is well-known to stimulate post-exercise protein synthesis rates. One factor that contributes to the cell’s capacity for protein synthesis is the number of ribosomes, with an increase in the number of ribosomes, via an increase in ribosomal RNA (rRNA) gene transcription (ribosome biogenesis), resulting in an enhanced capacity to translate mRNAs into new proteins. To date, several studies have reported significant inter-individual variation in the induction of ribosome biogenesis that correlates with the hypertrophic response to resistance exercise, with the study by Figueiredo et al. (2015) comprising one example. Although multiple factors could play a role in this heterogeneous response of protein synthesis and ribosome biogenesis to resistance exercise, perhaps there is a genetic and/or epigenetic contribution to this phenomenon? Ribosomes are made up of four different sized rRNA species (5S, 5.8S, 18S and 28S rRNAs) and a host of ribosomal proteins. The mature 5.8S, 18S and 28S rRNA species are derived from a long single 45S pre-rRNA transcript and the polymerase I-mediated transcription of this rDNA gene is considered to be a major rate limiting step in ribosome biogenesis. Interesting, unlike most other genes that only have two copies, this rDNA gene is found on five autosomal chromosomes (chromosomes 13, 14, 15, 21 and 22, in humans) in loci that contain multiple tandem gene copies, with the total number of copies ranging from 10s to 100s (Gibbons et al. 2014). This suggests that someone with a high rDNA copy number has the potential to have a greater number of ribosomes in their muscles under resting/basal conditions and/or undergo a larger increase in ribosome biogenesis in response to resistance exercise. In the current issue of The Journal of Physiology, Figueiredo et al. (2021) report finding a ∼3-fold inter-individual difference in the relative rDNA gene dosage across a group of 30 individuals (Figueiredo et al. 2021). Despite this difference, the relative rDNA dosage did not correlate with ribosome biogenesis at rest; however, rDNA dosage was positively correlated with 45S pre-rRNA expression 24 h after a bout of resistance exercise. These data suggest that there may indeed be a genetic component to the ribosome biogenesis response to resistance exercise that may play a role in determining protein synthesis rates and, ultimately, the extent of muscle hypertrophy. One factor that could play a role in regulating the transcriptional activity of the rDNA gene is epigenetic modifications, such as the methylation status of cytosine nucleotides in the proximal promoter of the rDNA gene, with reduced methylation typically associated with transcriptional activation. Figueiredo et al. (2021) investigated this question by using a mass spectrometry-based array method to compare the degree of rDNA promoter methylation in resting skeletal muscle. It was found that, although there were substantial inter-individual differences in methylation across several sites (4–41%), the average methylation status did not correlate with 45S pre-rRNA levels or total RNA content, suggesting that basal skeletal muscle ribosome biogenesis is not regulated by rDNA promoter methylation. Interestingly, methylation of the rDNA gene proximal promoter was not altered by resistance exercise, which prompted Figueiredo et al. (2021) to look further upstream for changes in methylation in more distal regions, such as enhancers, which also have the potential to regulate rDNA gene transcription. This analysis found reduced methylation (hypomethylation) at 30 min post-resistance exercise in a region of DNA that has previously been characterized as a potential rDNA gene enhancer. In addition, upstream sites of hypomethylation were also identified in close proximity to binding sites of the transcription factor, MYC (c-Myc), which is a known positive regulator of polymerase I transcription activity and ribosome biogenesis (Figueiredo et al. 2021). Importantly, many similar changes to methylation distal to the rDNA promoter were also observed in a mouse model of resistance exercise, suggesting a degree of conservation of the epigenetic responses associated with mechanically-induced muscle hypertrophy (Figueiredo et al. 2021). Combined, the study by Figueiredo et al. (2021) suggests that resistance exercise-induced rDNA transcription may, in part, be determined by rDNA gene copy number and by changes to the methylation status of DNA in distal enhancer regions and in regions in close proximity to MYC binding domains. Moreover, the study may help to explain inter-individual differences in the hypertrophic response to resistance exercise.