Exercise and Training Regulation of Autophagy Markers in Human and Rat Skeletal Muscle

Activation of macroautophagy and chaperone-mediated autophagy in human skeletal muscle by high-intensity exercise in normoxia and hypoxia and after recovery with or without post-exercise ischemia

Autophagy is essential for the adaptive response to exercise and physiological skeletal muscle functionality. However, the mechanisms leading to the activation of macroautophagy and chaperone-mediated autophagy in human skeletal muscle in response to high-intensity exercise remain elusive. Our findings demonstrate that macroautophagy and chaperone-mediated autophagy are stimulated by high-intensity exercise in normoxia (PIO2: 143 mmHg) and severe acute hypoxia (PIO2: 73 mmHg) in healthy humans. High-intensity exercise induces macroautophagy initiation through AMPKα phosphorylation, which phosphorylates and activates ULK1. ULK1 phosphorylates BECN1 at Ser15, eliciting the dissociation of BECN1-BCL2 crucial for phagophore formation. Besides, high-intensity exercise elevates the LC3B-II:LC3B-I ratio, reduces total SQSTM1/p62 levels, and induces p-Ser349 SQSTM1/p62 phosphorylation, suggesting heightened autophagosome degradation. PHAF1/MYTHO, a novel macroautophagy biomarker, is highly upregulated in response to high-intensity exercise. The latter is accompanied by elevated LAMP2A expression, indicating chaperone-mediated autophagy activation regardless of post-exercise HSPA8/HSC70 downregulation. Despite increased glycolytic metabolism, severe acute hypoxia does not exacerbate the autophagy signaling response. Signaling changes revert within 1 min of recovery with free circulation, while the application of immediate post-exercise ischemia impedes recovery. Our study concludes that macroautophagy and chaperone-mediated autophagy pathways are strongly activated by high-intensity exercise, regardless of PO2, and that oxygenation is necessary to revert these signals to pre-exercise values. PHAF1/MYTHO emerges as a pivotal exercise-responsive autophagy marker positively associated with the LC3B-II:LC3B-I ratio.

Delineating the contribution of ageing and physical activity to changes in mitochondrial characteristics across the lifespan

Ageing is associated with widespread physiological changes prominent within all tissues, including skeletal muscle and the brain, which lead to a decline in physical function. To tackle the growing health and economic burdens associated with an ageing population, the concept of healthy ageing has become a major research priority. Changes in skeletal muscle mitochondrial characteristics have been suggested to make an important contribution to the reductions in skeletal muscle function with age, and age-related changes in mitochondrial content, respiratory function, morphology, and mitochondrial DNA have previously been reported. However, not all studies report changes in mitochondrial characteristics with ageing, and there is increasing evidence to suggest that physical activity (or inactivity) throughout life is a confounding factor when interpreting age-associated changes. Given that physical activity is a potent stimulus for inducing beneficial adaptations to mitochondrial characteristics, delineating the influence of physical activity on the changes in skeletal muscle that occur with age is complicated. This review aims to summarise our current understanding and knowledge gaps regarding age-related changes to mitochondrial characteristics within skeletal muscle, as well as to provide some novel insights into brain mitochondria, and to propose avenues of future research and targeted interventions. Furthermore, where possible, we incorporate discussions of the modifying effects of physical activity, exercise, and training status, to purported age-related changes in mitochondrial characteristics.

Autophagy and Exercise: Current Insights and Future Research Directions

Autophagy is a cellular process by which proteins and organelles are degraded inside the lysosome. Exercise is known to influence the regulation of autophagy in skeletal muscle. However, as gold standard techniques to assess autophagy flux in vivo are restricted to animal research, important gaps remain in our understanding of how exercise influences autophagy activity in humans. Using available datasets, we show how the gene expression profile of autophagy receptors and ATG8 family members differ between human and mouse skeletal muscle, providing a potential explanation for their differing exercise-induced autophagy responses. Furthermore, we provide a comprehensive view of autophagy regulation following exercise in humans by summarizing human transcriptomic and phosphoproteomic datasets that provide novel targets of potential relevance. These newly identified phosphorylation sites may provide an explanation as to why both endurance and resistance exercise lead to an exercise-induced reduction in LC3B-II, while possibly divergently regulating autophagy receptors, and, potentially, autophagy flux. We also provide recommendations to use ex vivo autophagy flux assays to better understand the influence of exercise, and other stimuli, on autophagy regulation in humans. This review provides a critical overview of the field and directs researchers towards novel research areas that will improve our understanding of autophagy regulation following exercise in humans.

Lactate-induced autophagy activation: unraveling the therapeutic impact of high-intensity interval training on insulin resistance in type 2 diabetic rats

Impaired autophagy is a hallmark of diabetes. The current study proposed to investigate if high intensity interval training (HIIT) induced lactate accumulation could stimulate autophagy in type 2 diabetic male rats. 28 male Wistar rats were randomly assigned into four groups: Healthy Control (CO), Diabetes Control (T2D), Exercise (EX), and Diabetes + Exercise (T2D + EX). Diabetes was induced by feeding high-fat diet and administrating single dose of streptozotocin (35 mg/kg). After becoming diabetic, the animals in the exercise groups (EX and T2D + EX) performed an eight-week HIIT (4-10 interval, 80-100% Vmax, 5 days per week). Serum levels of lactate, glucose and insulin as well as the levels of lactate, pyruvate, lactate transporter monocarboxylate transporter 1 (MCT1), phosphorylated mitogen-activated protein kinases (p-MAP 1 and 2), phosphorylated extracellular signal-regulated protein kinases 1 and 2 (p-ERK 1 and 2), mammalian target of rapamycin (p-mTOR), ribosomal protein S6 kinase beta-1 (p-70S6k), p90 ribosomal S6 kinases (p-90RSK), autophagy related 7 (ATG7), Beclin-1, microtubule-associated protein 1A/1B, and 2A/2B -light chain 3 levels (LC3-I), (LC3- II), (LC3I/LC3II) in soleus muscle were measured. Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) and serum glucose was lower in T2D + EX compared to T2D group (P < 0.0001). While serum and soleus muscle levels of lactate was not different between T2D and T2D + Ex, the levels of Pyruvate (P < 0.01), MCT1, p-ERK1/2, p-mTOR, p70S6k, P-90RSK, ATG7, LC3-II, and LC3-II/LC3I ratios were higher in T2D + EX compared to T2D group (P < 0.0001). We concluded that eight weeks of high-intensity interval training could activated ERK/P90SRK while inhibiting mTOR/P70S6K signaling pathway in lactate dependent manner. It means increased autophagy which resulted in improve insulin resistance (IR) and reduce blood glucose.