Age-dependent stress response DNA demethylation and gene upregulation accompany nuclear and skeletal muscle remodeling following acute resistance-type exercise in rats

Abstract

Efficacy of high-intensity resistance exercise becomes progressively compromised with aging. Previously, to investigate this, we developed a rodent model of high-intensity training consisting of stretch-shortening contractions (SSCs) and determined that following one month of training, young rats exhibit a robust stress response and 20% performance increase, whereas old rats display a muted stress response and 30% performance decrease. Whether these age-specific responses occur early in training and constitute primary factors in adaptation/maladaptation was not addressed. The aim of the present study was to characterize performance, remodeling, and stress response transcriptional profile 6–120 h following acute SSC exposure. For young rats, the stress response pathway was highly regulated (≥20 differentially expressed genes at each time point) and was accompanied by robust DNA demethylation, tissue remodeling, and isometric torque recovery. For old rats, a muted transcriptional profile (13 and 2 differentially expressed genes at 6 and 120 h, respectively) coincided with deficiencies in demethylation, muscle remodeling, and torque recovery. These findings occurred in the context of heightened chronic levels of stress response gene expression with aging. This demonstrates that age-related constitutive elevations in stress response gene expression was accompanied by diminished SSC-induced responsiveness in epigenomic regulation and tissue remodeling.

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

“…Alterations in nuclei number and shape have the potential to impact transcriptional output (Mascetti et al, 2001 ; Murach et al, 2022 ; Rader & Baker, 2020 ; Versaevel et al, 2012 ). Muscle fiber size generally scales with myonuclear number in both rodents and humans during growth and load‐induced hypertrophy (Abou Sawan et al, 2021 ; Goh et al, 2019 ; Hansson et al, 2020 ).…”

“…The expression of genes relevant to growth and energy sensing was investigated using the RT 2 Profiler™ PCR Array for mTOR signaling (Qiagen, Cat# PARN‐098Z) per the manufacturer's instructions. Gene expression was considered significantly differentially regulated when fold regulation exceeded 1.3‐fold regulation (below 0.769‐fold change or above 1.3‐fold change) and p < 0.05(Rader et al, 2017 ; Rader & Baker, 2020 ).…”

“…After 3 × 5 min washes in PBS, sections were mounted with Prolong™ Gold Antifade Reagent (Thermo Fisher Scientific; P36931) with 4′, 6‐diamidino‐2‐phenylindole (DAPI). With the investigator blinded to sample identification, each muscle section was imaged by a standardized method (Baker et al, 2006 ; Rader et al, 2018 ; Rader & Baker, 2020 ; Rader, Naimo, et al, 2016 ). At 2 mm offset from either side of the midpoint, 5 equally spaced fields (at 20× magnification) were imaged for a total of 10 images.…”

“…Muscle fiber area per nucleus was determined for each muscle fiber by dividing the muscle fiber area by the number of nuclei per fiber. ImageJ functions of watershed and particle analysis were utilized for each DAPI image to yield nuclei size and circularity of all the nuclei (2626 ± 70 nuclei per section) in the muscle tissue for the regions imaged (Eidet et al, 2014 ; Rader & Baker, 2020 ). Small nuclear fragments (<5 μm 2 ) were omitted from the measurements by modifying the particle area range (Eidet et al, 2014 ).…”

“…Our research group has established and repeatedly validated a rat research model to investigate resistance‐type training (Cutlip et al, 2006 ; Naimo et al, 2019 ; Rader & Baker, 2017 , 2020 ; Rader, Layner, et al, 2016 ). The model is based on using a dynamometer to precisely expose dorsiflexor muscles of rats to eight sets of 10 repetitions (per set) of stretch‐shortening contractions (SSCs) – a consecutive sequence of isometric, lengthening, and shortening contractions – at a moderate velocity of 60° ankle rotation per second (i.e., contractions and parameters comparable to those in resistance training for human subjects) (Heckel et al, 2019 ; Vaczi et al, 2011 ; Vaczi et al, 2014 ).…”