Effects of intermittent ladder-climbing exercise training on mitochondrial biogenesis and endoplasmic reticulum stress of the cardiac muscle in obese middle-aged rats

3D Mitochondrial Structure in Aging Human Skeletal Muscle: Insights into MFN-2 Mediated Changes

Age-related atrophy of skeletal muscle, is characterized by loss of mass, strength, endurance, and oxidative capacity during aging. Notably, bioenergetics and protein turnover studies have shown that mitochondria mediate this decline in function. Although exercise has been the only therapy to mitigate sarcopenia, the mechanisms that govern how exercise serves to promote healthy muscle aging are unclear. Mitochondrial aging is associated with decreased mitochondrial capacity, so we sought to investigate how aging affects mitochondrial structure and potential age-related regulators. Specifically, the three-dimensional (3D) mitochondrial structure associated with morphological changes in skeletal muscle during aging requires further elucidation. We hypothesized that aging causes structural remodeling of mitochondrial 3D architecture representative of dysfunction, and this effect is mitigated by exercise. We used serial block-face scanning electron microscopy to image human skeletal tissue samples, followed by manual contour tracing using Amira software for 3D reconstruction and subsequent analysis of mitochondria. We then applied a rigorous in vitro and in vivo exercise regimen during aging. Across 5 human cohorts, we correlate differences in magnetic resonance imaging, mitochondria 3D structure, exercise parameters, and plasma immune markers between young (under 50 years) and old (over 50 years) individuals. We found that mitochondria we less spherical and more complex, indicating age-related declines in contact site capacity. Additionally, aged samples showed a larger volume phenotype in both female and male humans, indicating potential mitochondrial swelling. Concomitantly, muscle area, exercise capacity, and mitochondrial dynamic proteins showed age-related losses. Exercise stimulation restored mitofusin 2 (MFN2), one such of these mitochondrial dynamic proteins, which we show is required for the integrity of mitochondrial structure. Furthermore, we show that this pathway is evolutionarily conserved as Marf, the MFN2 ortholog in Drosophila, knockdown alters mitochondrial morphology and leads to the downregulation of genes regulating mitochondrial processes. Our results define age-related structural changes in mitochondria and further suggest that exercise may mitigate age-related structural decline through modulation of mitofusin 2.

Elucidating the primary mechanisms of high-intensity interval training for improved cardiac fitness in obesity

Obesity is a global and rising multifactorial pandemic associated with the emergence of several comorbidities that are risk factors for malignant cardiac remodeling and disease. High-intensity interval training (HIIT) has gained considerable attention due to its favorable outcomes of cardiometabolic health in individuals with overweight or obese. The primary aim of this review is to discuss the fundamental processes through which HIIT improves cardiac impairment in individuals with obesity to develop viable treatments for obesity management. In this review, a multiple database search and collection were conducted from the earliest record to January 2013 for studies included the qualitative component of HIIT intervention in humans and animals with overweight/obesity related to cardiac remodeling and fitness. We attempt to integrate the main mechanisms of HIIT in cardiac remolding improvement in obesity into an overall sequential hypothesis. This work focus on the ameliorative effects of HIIT on obesity-induced cardiac remodeling with respect to potential and pleiotropic mechanisms, including adipose distribution, energy metabolism, inflammatory response, insulin resistance, and related risk profiles in obesity. In conclusion, HIIT has been shown to reduce obesity-induced risks of cardiac remodeling, but the long-term effects of HIIT on obesity-induced cardiac injury and disease are presently unknown. Collective understanding highlights numerous specific research that are needed before the safety and effectiveness of HIIT can be confirmed and widely adopted in patient with obesity.

Strength training improves insulin resistance and differently affects mitochondria in skeletal muscle and visceral adipose tissue in high-fat fed mice

AIMS

Strength training (ST) improves insulin resistance and glucose tolerance by yet unknown mechanisms. The aims of this study were to investigate the effects of ST on mitochondrial adaptation in skeletal muscle and adipose tissue, on heat shock protein 72 (Hsp72) in skeletal muscle, and on visceral adipocyte size in mice with high-fat diet (HFD)-induced insulin resistance.

MATERIALS AND METHODS

Male Balb/c mice were divided into sedentary control-chow (C-chow), strength trained-chow (ST-chow), sedentary control-HFD (C-HFD) and strength trained-HFD (ST-HFD). Diet was provided for 12 weeks, while ladder climbing ST was performed for the final six weeks of the study at a frequency of three days per week.

KEY FINDINGS

Strength training led to increased strength, muscular endurance, and skeletal muscle hypertrophy. Compared to the C-HFD group, mice in the ST-HFD group decreased their whole-body insulin resistance, improved their glucose tolerance, and had higher activation of the insulin pathway in skeletal muscle. ST increased citrate synthase (CS) activity in skeletal muscle, but this increase was blunted in ST-HFD. Conversely, HFD reduced adipose tissue CS activity regardless of training status. Hsp72 content was reduced in C-HFD, but returned to control levels in ST-HFD. Finally, reduced epididymal adipocyte size was observed in ST-HFD.

SIGNIFICANCE

These results suggest that the improvement in insulin resistance induced by ST is related to mitochondrial adaptation in skeletal muscle, but not in adipose tissue. Moreover, this improvement might be related to increased skeletal muscle Hsp72 and reduced epididymal adipocyte size.

Consumption of a high-fat-high-sucrose diet partly diminishes mechanical and structural adaptations of cardiac muscle following resistance training

[Purpose]

The purpose of this study was to investigate the effects of a high-fat high-sucrose (HFHS) diet on previously reported adaptations of cardiac morphological and contractile properties to resistance training.

[Methods]

Twelve-week-old rats participated in 12-weeks of resistance exercise training and consumed an HFHS diet. Echocardiography and skinned cardiac muscle fiber bundle testing were performed to determine the structural and mechanical adaptations.

[Results]

Compared to chow-fed sedentary animals, both HFHS- and chow-fed resistance-trained animals had thicker left ventricular walls. Isolated trabecular fiber bundles from chow-fed resistance-trained animals had greater force output, shortening velocities, and calcium sensitivities than those of chow-fed sedentary controls. However, trabeculae from the HFHS resistance-trained animals had greater force output but no change in unloaded shortening velocity or calcium sensitivity than those of the chow-fed sedentary group animals.

[Conclusion]

Resistance exercise training led to positive structural and mechanical adaptations of the heart, which were partly offset by the HFHS diet.

Mitochondrial Membrane Intracellular Communication in Healthy and Diseased Myocardium

Research efforts in the twenty-first century have been paramount to the discovery and development of novel pharmacological treatments in a variety of diseases resulting in improved life expectancy. Yet, cardiac disease remains a leading cause of morbidity and mortality worldwide. Over time, there has been an expansion in conditions such as atrial fibrillation (AF) and heart failure (HF). Although past research has elucidated specific pathways that participate in the development of distinct cardiac pathologies, the exact mechanisms of action leading to disease remain to be fully characterized. Protein turnover and cellular bioenergetics are integral components of cardiac diseases, highlighting the importance of mitochondria and endoplasmic reticulum (ER) in driving cellular homeostasis. More specifically, the interactions between mitochondria and ER are crucial to calcium signaling, apoptosis induction, autophagy, and lipid biosynthesis. Here, we summarize mitochondrial and ER functions and physical interactions in healthy physiological states. We then transition to perturbations that occur in response to pathophysiological challenges and how this alters mitochondrial–ER and other intracellular organelle interactions. Finally, we discuss lifestyle interventions and innovative therapeutic targets that may be used to restore beneficial mitochondrial and ER interactions, thereby improving cardiac function.

Imbalance of ER and Mitochondria Interactions: Prelude to Cardiac Ageing and Disease?

Cardiac disease is still the leading cause of morbidity and mortality worldwide, despite some exciting and innovative improvements in clinical management. In particular, atrial fibrillation (AF) and heart failure show a steep increase in incidence and healthcare costs due to the ageing population. Although research revealed novel insights in pathways driving cardiac disease, the exact underlying mechanisms have not been uncovered so far. Emerging evidence indicates that derailed proteostasis (i.e., the homeostasis of protein expression, function and clearance) is a central component driving cardiac disease. Within proteostasis derailment, key roles for endoplasmic reticulum (ER) and mitochondrial stress have been uncovered. Here, we describe the concept of ER and mitochondrial stress and the role of interactions between the ER and mitochondria, discuss how imbalance in the interactions fuels cardiac ageing and cardiac disease (including AF), and finally assess the potential of drugs directed at conserving the interaction as an innovative therapeutic target to improve cardiac function.

PKR Promotes Oxidative Stress and Apoptosis of Human Articular Chondrocytes by Causing Mitochondrial Dysfunction through p38 MAPK Activation-PKR Activation Causes Apoptosis in Human Chondrocytes

Osteoarthritis (OA) is one of the most common types of arthritis in the elderly people. It has been known that chondrocyte apoptosis occurs in OA cartilage; however, the detailed molecular mechanism remains unclear. In the current study, we aimed to elucidate the role of double-stranded RNA-dependent protein kinase R (PKR) in the TNF-α-caused apoptosis in chondrocytes. Human articular chondrocytes were digested from cartilages of OA subjects who accepted arthroplastic knee surgery. Our results showed that phosphorylation of p38 MAPK was increased after TNF-α stimulation or PKR activation using poly (I:C), and TNF-α-induced p38 MAPK upregulation was inhibited by PKR inhibition, suggesting phosphor-p38 MAPK was regulated by PKR. Moreover, we found that PKR participated in the p53-dependent destruction of AKT following activation of p38 MAPK. The inhibition of AKT led to the reduced expression of PGC-1α, which resulted in mitochondrial dysfunction and increased oxidative stress. We showed that the reduction of oxidative stress using antioxidant Mito TEMPO lowered the TNF-α-induced caspase-3 activation and TUNEL-positive apoptotic cells. The diminished apoptotic response was also observed after repression of PKR/p38 MAPK/p53/AKT/PGC-1α signaling. Taken together, we demonstrated that the aberrant mitochondrial biogenesis and increased oxidative stress in chondrocytes after TNF-α stimulation were mediated by PKR, which may contribute to the chondrocyte apoptosis and cartilage degeneration in OA.

PKR Promotes Oxidative Stress and Apoptosis of Human Articular Chondrocytes by Causing Mitochondrial Dysfunction through p38 MAPK Activation-PKR Activation Causes Apoptosis in Human Chondrocytes

Osteoarthritis (OA) is one of the most common types of arthritis in the elderly people. It has been known that chondrocyte apoptosis occurs in OA cartilage; however, the detailed molecular mechanism remains unclear. In the current study, we aimed to elucidate the role of double-stranded RNA-dependent protein kinase R (PKR) in the TNF-α-caused apoptosis in chondrocytes. Human articular chondrocytes were digested from cartilages of OA subjects who accepted arthroplastic knee surgery. Our results showed that phosphorylation of p38 MAPK was increased after TNF-α stimulation or PKR activation using poly (I:C), and TNF-α-induced p38 MAPK upregulation was inhibited by PKR inhibition, suggesting phosphor-p38 MAPK was regulated by PKR. Moreover, we found that PKR participated in the p53-dependent destruction of AKT following activation of p38 MAPK. The inhibition of AKT led to the reduced expression of PGC-1α, which resulted in mitochondrial dysfunction and increased oxidative stress. We showed that the reduction of oxidative stress using antioxidant Mito TEMPO lowered the TNF-α-induced caspase-3 activation and TUNEL-positive apoptotic cells. The diminished apoptotic response was also observed after repression of PKR/p38 MAPK/p53/AKT/PGC-1α signaling. Taken together, we demonstrated that the aberrant mitochondrial biogenesis and increased oxidative stress in chondrocytes after TNF-α stimulation were mediated by PKR, which may contribute to the chondrocyte apoptosis and cartilage degeneration in OA.

Comparison of endoplasmic reticulum stress and mitochondrial biogenesis responses after 12 weeks of treadmill running and ladder climbing exercises in the cardiac muscle of middle-aged obese rats

The purpose of the present study was to compare the influence of aerobic exercise (AE) lasting 12 weeks to that of resistance exercise (RE) of the same duration on endoplasmic reticulum (ER) stress and mitochondrial biogenesis in the cardiac muscle of middle-aged obese rats. Obesity was induced in thirty 50-week-old male Sprague Dawley rats over 6 weeks by administration of a high-fat diet. The rats were then subjected to treadmill-running (AE) and ladder-climbing (RE) exercises 3 times per week for 12 weeks. Rats in the AE group showed significantly lower increases in body weight and intraperitoneal fat than those in the sedentary control (SC) group (P<0.05). The 12-week exercise regimes resulted in a significant increase in expression of mitochondrial biogenesis markers and levels of peroxisome proliferator-activated receptor gamma coactivator 1α in the cardiac muscle (P<0.05). Phosphorylation of PKR-like ER kinase, an ER stress marker, decreased significantly (P<0.05) after the exercise training. Although a trend for decreased C/EBP homologous protein (CHOP) expression was observed in both exercise groups, only the AE group had a statistically significant decrease (P<0.05). Levels of GRP78, an ER stress marker that protects cardiac muscle, did not significantly differ among the groups. Although only the AE group decreased body weight and fat mass, the two exercise regimes had similar effects on cardiac muscle with the exception of CHOP. Therefore, we suggest that both AE, which results in weight loss, and high-intensity RE, though not accompanied by weight loss, protect obese cardiac muscle effectively.