A cellular mechanism of muscle memory facilitates mitochondrial remodelling following resistance training

Abstract

Key points: Referring to the muscle memory theory, previously trained muscles acquire strength and volume much faster than naive muscles. Using extreme experimental models such as synergist ablation or steroid administration, previous studies have demonstrated that the number of nuclei increases when a muscle becomes enlarged, which serves as a cellular muscle memory mechanism for the muscle. In the present study, we found that, when rats were subjected to physiologically relevant resistance training, the number of myonuclei increased and was retained during a long-term detraining period. The acquired myonuclei were related to a greater degree of muscle hypertrophic and mitochondrial biogenesis processes following subsequent hypertrophic conditions. Our data suggest a cellular mechanism supporting the notion that exposing young muscles to resistance training would help to restore age-related muscle loss coupled with mitochondrial dysfunction in later life.

Abstract: Muscle hypertrophy induced by resistance training is accompanied by an increase in the number of myonuclei. The acquired myonuclei are viewed as a cellular component of muscle memory by which muscle enlargement is promoted during a re-training period. In the present study, we investigated the effect of exercise preconditioning on mitochondrial remodelling induced by resistance training. Sprague-Dawley rats were divided into four groups: untrained control, training, pre-training or re-training. The training groups were subjected to weight loaded-ladder climbing exercise training. Myonuclear numbers were significantly greater (up to 20%) in all trained muscles compared to untrained controls. Muscle mass was significantly higher in the re-training group compared to the training group (∼2-fold increase). Mitochondrial content, mitochondrial biogenesis gene expression levels and mitochondrial DNA copy numbers were significantly higher in re-trained muscles compared to the others. Oxidative myofibres (type I) were significantly increased only in the re-trained muscles. Furthermore, in vitro studies using insulin-like growth factor-1-treated L6 rat myotubes demonstrated that myotubes with a higher myonuclear number confer greater expression levels of both mitochondrial and nuclear genes encoding for constitutive and regulatory mitochondrial proteins, which also showed a greater mitochondrial respiratory function. These data suggest that myonuclei acquired from previous training facilitate mitochondrial biogenesis in response to subsequent retraining by (at least in part) enhancing cross-talk between mitochondria and myonuclei in the pre-conditioned myofibres.

Keywords: mitochondrial biogenesis; muscle memory; myonuclei; resistance training.

Figure 1. Experimental design

Schematic diagram of the experimental design.

Figure 2. Pre‐trained myofibres present greater number of myonuclei compared to untrained fibres

A, single myofibres isolated from FHL muscle were stained with DAPI (green). Scale bar = 50 μm (1:200 magnification). B, myonuclei numbers per single myofibre (counts per mm myofibre length) (n = 8). Data are presented as individual data points. C, positive Pax7 staining on a cross‐section of tibialis anterior muscle. Magnifications are 1:200 (left) and 1:630 (right).

Figure 3. Muscle mass and muscle fibre typing

A, myofibres in FHL muscle were visualized by H&E staining. B, CSA of myofibres and (C and D) muscle wet weights with or without normalization to body weight were quantified (n = 8). E, mRNA expression of myosin heavy chain (Myh) isoforms (n = 4). Scale bar = 50 μm. 1:200 magnification. Data are the mean ± SD. ^* P < 0.05. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 4. Mitochondrial respiratory complex enzymatic activities

A and B, SDH staining on FHL muscle. C and D, COX staining on the FHL muscle (n = 8). Scale bar = 50 μm. 1:200 magnification. Data are the mean ± SD. ^* P < 0.05.

Figure 5. Previously trained muscle showed greater mitochondrial adaptations following re‐training compared to non‐pretrained muscles

A–E, western blots for PGC‐1α, Mfn2, Fis1 and Drp1. Plots are the results of quantification. GAPDH was used as a loading control. F, mitochondrial copy number. G and H, mitochondrial content determined by immuohistochemisty using anti‐porin antibody (n = 8). 1:200 magnification. Staining intensity of porin was quantified. FHL muscles were used for all the experiments. Data are the mean ± SD. ^* P < 0.05.

Figure 6. Enhanced mitochondrial gene expression response in myonuclear‐enriched myotubes after AICAR treatment

A, experimental design for in vitro studies. L6 myoblast differentiation was induced by serum deprivation for 48 h, and then the myotubes were further treated with (or without) IGF‐1 (50 ng mL^–1) for an additional 48 h. Myonuclear enriched myotubes were incubated with 1 mm AICAR for either 6 or 24 h. B, relative myonuclei numbers per myotubes. Note that myonuclei numbers peaks at 50 ng mL^–1. ^* P < 0.05 vs. non‐treated. C, representative images of myotubes (green) with nuclei staining (blue). 1:200 magnification. D, mitochondrial genome‐encoded (left) and nuclear genome‐encoded (right) mitochondrial gene expression levels. ^* P < 0.05 vs. DMSO. E, basal (left) and maximal oxygen consumption rate (OCR) (right). Data are the mean ± SD. ^* P < 0.05.

Figure 7. Enhanced nuclear‐mitochondrial cross‐talk in pre‐trained skeletal muscle

Schematic diagram of the proposed mechanism on enhanced nuclear‐mitochondrial cross‐talk in pre‐trained skeletal muscle.