Commentaries on Viewpoint: The rigorous study of exercise adaptations: Why mRNA might not be enough

Consistent expression pattern of myogenic regulatory factors in whole muscle and isolated human muscle satellite cells after eccentric contractions in humans

Skeletal muscle satellite cells (SC) play an important role in muscle repair following injury. The regulation of SC activity is governed by myogenic regulatory factors (MRF), including MyoD, Myf5, myogenin, and MRF4. The mRNA expression of these MRF in humans following muscle damage has been predominately measured in whole muscle homogenates. Whether the temporal expression of MRF in a whole muscle homogenate reflects SC-specific expression of MRF remains largely unknown. Sixteen young men (23.1 ± 1.0 yr) performed 300 unilateral eccentric contractions (180°/s) of the knee extensors. Percutaneous muscle biopsies from the vastus lateralis were taken before (Pre) and 48 h postexercise. Fluorescence-activated cell sorting analysis was utilized to purify NCAM⁺ muscle SC from the whole muscle homogenate. Forty-eight hours post-eccentric exercise, MyoD, Myf5, and myogenin mRNA expression were increased in the whole muscle homogenate (~1.4-, ~4.0-, ~1.7-fold, respectively, P < 0.05) and in isolated SC (~19.3-, ~17.5-, ~58.9-fold, respectively, P < 0.05). MRF4 mRNA expression was not increased 48 h postexercise in the whole muscle homogenate (P > 0.05) or in isolated SC (P > 0.05). In conclusion, our results suggest that the directional changes in mRNA expression of the MRF in a whole muscle homogenate in response to acute eccentric exercise reflects that observed in isolated muscle SC.NEW & NOTEWORTHY The myogenic program is controlled via transcription factors referred to as myogenic regulatory factors (MRF). Previous studies have derived MRF expression from whole muscle homogenates, but little work has examined whether the mRNA expression of these transcripts reflects the pattern of expression in the actual population of satellite cells (SC). We report that MRF expression from an enriched SC population reflects the directional pattern of expression from skeletal muscle biopsy samples following eccentric contractions.

High-Intensity Exercise and Mitochondrial Biogenesis: Current Controversies and Future Research Directions

It is well established that different types of exercise can provide a powerful stimulus for mitochondrial biogenesis. However, there are conflicting findings in the literature, and a consensus has not been reached regarding the efficacy of high-intensity exercise to promote mitochondrial biogenesis in humans. The purpose of this review is to examine current controversies in the field and to highlight some important methodological issues that need to be addressed to resolve existing conflicts.

Principles of Exercise Prescription, and How They Influence Exercise-Induced Changes of Transcription Factors and Other Regulators of Mitochondrial Biogenesis

Physical inactivity represents the fourth leading risk factor for mortality, and it has been linked with a series of chronic disorders, the treatment of which absorbs ~ 85% of healthcare costs in developed countries. Conversely, physical activity promotes many health benefits; endurance exercise in particular represents a powerful stimulus to induce mitochondrial biogenesis, and it is routinely used to prevent and treat chronic metabolic disorders linked with sub-optimal mitochondrial characteristics. Given the importance of maintaining a healthy mitochondrial pool, it is vital to better characterize how manipulating the endurance exercise dose affects cellular mechanisms of exercise-induced mitochondrial biogenesis. Herein, we propose a definition of mitochondrial biogenesis and the techniques available to assess it, and we emphasize the importance of standardizing biopsy timing and the determination of relative exercise intensity when comparing different studies. We report an intensity-dependent regulation of exercise-induced increases in nuclear peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) protein content, nuclear phosphorylation of p53 (serine 15), and PGC-1α messenger RNA (mRNA), as well as training-induced increases in PGC-1α and p53 protein content. Despite evidence that PGC-1α protein content plateaus within a few exercise sessions, we demonstrate that greater training volumes induce further increases in PGC-1α (and p53) protein content, and that short-term reductions in training volume decrease the content of both proteins, suggesting training volume is still a factor affecting training-induced mitochondrial biogenesis. Finally, training-induced changes in mitochondrial transcription factor A (TFAM) protein content are regulated in a training volume-dependent manner and have been linked with training-induced changes in mitochondrial content.

Training-Induced Changes in Mitochondrial Content and Respiratory Function in Human Skeletal Muscle

A sedentary lifestyle has been linked to a number of metabolic disorders that have been associated with sub-optimal mitochondrial characteristics and an increased risk of premature death. Endurance training can induce an increase in mitochondrial content and/or mitochondrial functional qualities, which are associated with improved health and well-being and longer life expectancy. It is therefore important to better define how manipulating key parameters of an endurance training intervention can influence the content and functionality of the mitochondrial pool. This review focuses on mitochondrial changes taking place following a series of exercise sessions (training-induced mitochondrial adaptations), providing an in-depth analysis of the effects of exercise intensity and training volume on changes in mitochondrial protein synthesis, mitochondrial content and mitochondrial respiratory function. We provide evidence that manipulation of different exercise training variables promotes specific and diverse mitochondrial adaptations. Specifically, we report that training volume may be a critical factor affecting changes in mitochondrial content, whereas relative exercise intensity is an important determinant of changes in mitochondrial respiratory function. As a consequence, a dissociation between training-induced changes in mitochondrial content and mitochondrial respiratory function is often observed. We also provide evidence that exercise-induced changes are not necessarily predictive of training-induced adaptations, we propose possible explanations for the above discrepancies and suggestions for future research.