Coculture with Colon-26 cancer cells decreases the protein synthesis rate and shifts energy metabolism towards glycolysis dominance in C2C12 myotubes

Cancer cachexia is the result of complex interorgan interactions initiated by cancer cells and changes in patient behavior such as decreased physical activity and energy intake. Therefore, it is crucial to distinguish between the direct and indirect effects of cancer cells on muscle mass regulation and bioenergetics to identify novel therapeutic targets. In this study, we investigated the direct effects of Colon-26 cancer cells on the molecular regulating machinery of muscle mass and its bioenergetics using a coculture system with C2C12 myotubes. Our results demonstrated that coculture with Colon-26 cells induced myotube atrophy and reduced skeletal muscle protein synthesis and its regulating mammalian target of rapamycin complex 1 signal transduction. However, we did not observe any activating effects on protein degradation pathways including ubiquitin-proteasome and autophagy-lysosome systems. From a bioenergetic perspective, coculture with Colon-26 cells decreased the Complex I-driven, but not Complex II-driven, mitochondrial ATP production capacity, while increasing glycolytic enzyme activity and glycolytic metabolites, suggesting a shift in energy metabolism towards glycolysis dominance. Gene expression profiling by RNA-seq showed that the increased activity of glycolytic enzymes was consistent with changes in gene expression. However, the decreased ATP production capacity of mitochondria was not in line with the gene expression. The potential direct interaction between cancer cells and skeletal muscle cells revealed in this study may contribute to a better fundamental understanding of the complex pathophysiology of cancer cachexia.

Monocarboxylate transporter 4 deficiency enhances high-intensity interval training-induced metabolic adaptations in skeletal muscle

High-intensity exercise stimulates glycolysis, subsequently leading to elevated lactate production within skeletal muscle. While lactate produced within the muscle is predominantly released into the circulation via the monocarboxylate transporter 4 (MCT4), recent research underscores lactate's function as an intercellular and intertissue signalling molecule. However, its specific intracellular roles within muscle cells remains less defined. In this study, our objective was to elucidate the effects of increased intramuscular lactate accumulation on skeletal muscle adaptation to training. To achieve this, we developed MCT4 knockout mice and confirmed that a lack of MCT4 indeed results in pronounced lactate accumulation in skeletal muscle during high-intensity exercise. A key finding was the significant enhancement in endurance exercise capacity at high intensities when MCT4 deficiency was paired with high-intensity interval training (HIIT). Furthermore, metabolic adaptations supportive of this enhanced exercise capacity were evident with the combination of MCT4 deficiency and HIIT. Specifically, we observed a substantial uptick in the activity of glycolytic enzymes, notably hexokinase, glycogen phosphorylase and pyruvate kinase. The mitochondria also exhibited heightened pyruvate oxidation capabilities, as evidenced by an increase in oxygen consumption when pyruvate served as the substrate. This mitochondrial adaptation was further substantiated by elevated pyruvate dehydrogenase activity, increased activity of isocitrate dehydrogenase - the rate-limiting enzyme in the TCA cycle - and enhanced function of cytochrome c oxidase, pivotal to the electron transport chain. Our findings provide new insights into the physiological consequences of lactate accumulation in skeletal muscle during high-intensity exercises, deepening our grasp of the molecular intricacies underpinning exercise adaptation. KEY POINTS: We pioneered a unique line of monocarboxylate transporter 4 (MCT4) knockout mice specifically tailored to the ICR strain, an optimal background for high-intensity exercise studies. A deficiency in MCT4 exacerbates the accumulation of lactate in skeletal muscle during high-intensity exercise. Pairing MCT4 deficiency with high-intensity interval training (HIIT) results in a synergistic boost in high-intensity exercise capacity, observable both at the organismal level (via a treadmill running test) and at the muscle tissue level (through an ex vivo muscle contractile function test). Coordinating MCT4 deficiency with HIIT enhances both the glycolytic enzyme activities and mitochondrial capacity to oxidize pyruvate.

Belt electrode tetanus muscle stimulation reduces denervation-induced atrophy of rat multiple skeletal muscle groups

Belt electrode-skeletal muscle electrical stimulation (B-SES) involves the use of belt-shaped electrodes to contract multiple muscle groups simultaneously. Twitch contractions have been demonstrated to protect against denervation-induced muscle atrophy in rats, possibly through mitochondrial biosynthesis. This study examined whether inducing tetanus contractions with B-SES suppresses muscle atrophy and identified the underlying molecular mechanisms. We evaluated the effects of acute (60 Hz, 5 min) and chronic (60 Hz, 5 min, every alternate day for one week) B-SES on the tibialis anterior (TA) and gastrocnemius (GAS) muscles in Sprague-Dawley rats using belt electrodes attached to both ankle joints. After acute stimulation, a significant decrease in the glycogen content was observed in the left and right TA and GAS, suggesting that B-SES causes simultaneous contractions in multiple muscle groups. B-SES enhanced p70S6K phosphorylation, an indicator of the mechanistic target of rapamycin complex 1 activity. During chronic stimulations, rats were divided into control (CONT), denervation-induced atrophy (DEN), and DEN + electrically stimulated with B-SES (DEN + ES) groups. After seven days of treatment, the wet weight (n = 8-11 for each group) and muscle fiber cross-sectional area (CSA, n = 6 for each group) of the TA and GAS muscles were reduced in the DEN and DEN + ES groups compared with that in the CON group. The DEN + ES group showed significantly higher muscle weight and CSA than those in the DEN group. Although RNA-seq and pathway analysis suggested that mitochondrial biogenesis is a critical event in this phenomenon, mitochondrial content showed no difference. In contrast, ribosomal RNA 28S and 18S (n = 6) levels in the DEN + ES group were higher than those in the DEN group, even though RNA-seq showed that the ribosome biogenesis pathway was reduced by electrical stimulation. The mRNA levels of the muscle proteolytic molecules atrogin-1 and MuRF1 were significantly higher in DEN than those in CONT. However, they were more suppressed in DEN + ES than those in DEN. In conclusion, tetanic electrical stimulation of both ankles using belt electrodes effectively reduced denervation-induced atrophy in multiple muscle groups. Furthermore, ribosomal biosynthesis plays a vital role in this phenomenon.

Alterations in neuromuscular junction morphology with ageing and endurance training modulate neuromuscular transmission and myofibre composition

Both ageing and exercise training affect the neuromuscular junction (NMJ) structure. Morphological alterations in the NMJ have been considered to influence neuromuscular transmission and myofibre properties, but the direct link between the morphology and function has yet to be established. We measured the neuromuscular transmission, myofibre composition and NMJ structure of 5-month-old (young) and 24-month-old untrained (aged control) and trained (aged trained) mice. Aged trained mice were subjected to 2 months of endurance training before the measurement. Neuromuscular transmission was evaluated in vivo as the ratio of ankle plantar flexion torque evoked by the sciatic nerve stimulation to that by direct muscle stimulation. The torque ratio was significantly lower in aged mice than in young and aged trained mice at high-frequency stimulations, showing a significant positive correlation with voluntary grip strength. The degree of pre- to post-synaptic overlap of the NMJ was also significantly lower in aged mice and positively correlated with the torque ratio. We also found that the proportion of fast-twitch fibres in the soleus muscle decreased with age, and that age-related denervation occurred preferentially in fast-twitch fibres. Age-related denervation and a shift in myofibre composition were partially prevented by endurance training. These results suggest that age-related deterioration of the NMJ structure impairs neuromuscular transmission and alters myofibre composition, but these alterations can be prevented by structural amelioration of NMJ with endurance training. Our findings highlight the importance of the NMJ as a major determinant of age-related deterioration of skeletal muscles and the clinical significance of endurance training as a countermeasure. KEY POINTS: The neuromuscular junction (NMJ) plays an essential role in neuromuscular transmission and the maintenance of myofibre properties. We show that neuromuscular transmission is impaired with ageing but recovered by endurance training, which contributes to alterations in voluntary strength. Neuromuscular transmission is associated with the degree of pre- to post-synaptic overlap of the NMJ. Age-related denervation of fast-twitch fibres and a shift in myofibre composition toward a slower phenotype are partially prevented by endurance training. Our study provides substantial evidence that age-related and exercise-induced alterations in neuromuscular transmission and myofibre properties are associated with morphological changes in the NMJ.

Dietary apple polyphenols enhance mitochondrial turnover and respiratory chain enzymes

Previous studies have demonstrated the beneficial effects of apple polyphenol (AP) intake on muscle endurance. Since mitochondria are critical for muscle endurance, we investigated mitochondrial enzyme activity, biogenesis, degradation and protein quality control. Twenty-four Wistar rats were randomly fed a 5% AP diet (5% AP group, n = 8), a 0.5% AP diet (0.5% AP group, n = 8), or a control diet (control group, n = 8). After a 4-week feeding period, the expression level of peroxisome proliferator-activated receptor γ coactivator-1α, a mitochondrial biosynthetic factor, did not increase, whereas that of transcription factor EB, another regulator of mitochondrial synthesis, significantly increased. Moreover, the mitochondrial count did not differ significantly between the groups. In contrast, mitophagy-related protein levels were significantly increased. The enzymatic activities of mitochondrial respiratory chain complexes II, III and IV were significantly higher in the AP intake group than in the control group. We conclude that AP feeding increases the activity of respiratory chain complex enzymes in rat skeletal muscles. Moreover, mitochondrial biosynthesis and degradation may have increased in AP-treated rats. NEW FINDINGS: What is the central question of this study? Does the administration of apple polyphenols (AP) affect mitochondrial respiratory chain complex enzyme activity, biogenesis, degradation and protein quality control in rat skeletal muscles? What is the main finding and its importance? AP feeding increases respiratory chain complex enzyme activity in rat skeletal muscle. Moreover, AP administration increases transcription factor EB activation, and mitophagy may be enhanced to promote degradation of dysfunctional mitochondria, but mitochondrial protein quality control was not affected.

Association of MMP3 gene polymorphism and sex on recovery of muscle strength after eccentric exercise

Individual differences in recovery of muscle strength after eccentric exercise may be influenced by sex and genotype. A candidate genetic polymorphism associated with response during muscle recovery is the MMP3 gene rs522616 polymorphism, encoding matrix metalloproteinase (MMP-3). Here, we investigated the effect of the MMP3 gene rs522616 polymorphism and sex on recovery of muscle strength after eccentric exercise. A total of 95 healthy subjects (50 men and 45 women) performed five sets of six maximal eccentric elbow flexion exercises. Maximal voluntary contraction (MVC) torque, range of motion (ROM), and muscle soreness, as well as blood parameters [creatine kinase (CK) and interleukin-6 (IL-6)], were assessed immediately before and after and 1, 2, 3, and 5 days after eccentric exercise. No significant time × group interaction in MVC torque after exercise was observed between groups in both sexes. Furthermore, sex differences were identified in the area under the curves (AUC) of CK and IL-6, both of which were higher in men than those in women. A significant genotype-sex interaction was identified in the recovery of MVC, calculated by subtracting the MVC immediately after exercise from the MVC on day 5 after eccentric exercise. The G allele showed a significantly lower recovery of MVC than the AA genotype in men. However, no significant differences were observed in women. This study demonstrated the interaction between the MMP3 rs522616 polymorphism and sex in recovery of muscle strength after eccentric exercise.NEW & NOTEWORTHY Sex differences were identified in the AUC of creatin kinase (CK) and interleukin 6 (IL-6) after eccentric exercise, both of which were greater in men. A genotype-sex interaction was identified in recovery of maximal voluntary contraction (MVC). The G allele showed a significantly lower recovery of MVC than AA genotype in men. To our knowledge, this is the first study to report the interaction between MMP3 gene rs522616 polymorphism and sex difference on recovery of muscle strength after eccentric exercise.

Low-frequency electrical stimulation of bilateral hind legs by belt electrodes is effective for preventing denervation-induced atrophies in multiple skeletal muscle groups in rats

Belt electrode skeletal muscle electrical stimulation (B-SES) can simultaneously contract multiple muscle groups. Although the beneficial effects of B-SES in clinical situations have been elucidated, its molecular mechanism remains unknown. In this study, we developed a novel rodent B-SES ankle stimulation system to test whether low-frequency stimulation prevents denervation-induced muscle atrophy. Electrical stimulations (7‒8 Hz, 30 min) with ankle belt electrodes were applied to Sprague-Dawley rats daily for one week. All animals were assigned to the control (CONT), denervation-induced atrophy (DEN), and DEN + electrical stimulation (ES) groups. The tibialis anterior (TA) and gastrocnemius (GAS) muscles were used to examine the effect of ES treatment. After seven daily sessions of continuous stimulation, muscle wet weight (n = 8-11), and muscle fiber cross-sectional area (CSA, n = 4-6) of TA and GAS muscles were lower in DEN and DEN + ES than in CON. However, it was significantly higher in DEN than DEN + ES, showing that ES partially prevented muscle atrophy. PGC-1α, COX-IV, and citrate synthase activities (n = 6) were significantly higher in DEN + ES than in DEN. The mRNA levels of muscle proteolytic molecules, Atrogin-1 and Murf1, were significantly higher in DEN than in CONT, while B-SES significantly suppressed their expression (p < 0.05). In conclusion, low-frequency electrical stimulation of the bilateral ankles using belt electrodes (but not the pad electrodes) is effective in preventing denervation-induced atrophy in multiple muscles, which has not been observed with pad electrodes. Maintaining the mitochondrial quantity and enzyme activity by low-frequency electrical stimulation is key to suppressing muscle protein degradation.

Percutaneous electrical stimulation-induced muscle contraction prevents the decrease in ribosome RNA and ribosome protein during pelvic hindlimb suspension

Skeletal muscle unloading leads to muscle atrophy. Ribosome synthesis has been implicated as an important skeletal muscle mass regulator owing to its translational capacity. Muscle unloading induces a reduction in ribosome synthesis and content, with muscle atrophy. Percutaneous electrical muscle stimulation (pEMS)-induced muscle contraction is widely used in clinics to improve muscle mass. However, its efficacy in rescuing the reduction in ribosomal synthesis has not been addressed thus far. We examined the effects of daily pEMS treatment on ribosome synthesis and content during mouse hindlimb unloading. Male C57BL/6J mice were randomly assigned to sedentary (SED) and hindlimb unloading by pelvic suspension (HU) groups. Muscle contraction was triggered by pEMS treatment of the right gastrocnemius muscle of a subset of the HU group (HU+pEMS). Hindlimb unloading for 6 days significantly lowered 28S rRNA, rpL10, and rpS3 expression, which was rescued by daily pEMS treatment. The protein expression of phospho-p70S6K and UBF was significantly higher in the HU+pEMS than in the HU group. The mRNA expression of ribophagy receptor Nufip1 increased in both the HU and HU+pEMS groups. Protein light chain 3 (LC3)-II expression and the LC3-II/LC3-I ratio were increased by HU, but pEMS attenuated this increase. Our findings indicate that during HU, daily pEMS treatment prevents the reduction in the levels of some proteins associated with ribosome synthesis. Additionally, the HU-induced activation of ribosome degradation may be attenuated. These data provide insights into ribosome content regulation and the mechanism of attenuation of muscle atrophy by pEMS treatment during muscle disuse.

Effect of different types of muscle activity on the gene and protein expression of ALDH family members in C57BL/6J mouse skeletal muscle

Aldehyde dehydrogenase (ALDH) is an enzyme that detoxifies aldehydes and is primarily involved in alcohol metabolism. Recently, we have shown that ALDH also plays an important role in skeletal muscle homeostasis. To better understand the role of ALDH in skeletal muscle, it is necessary to clarify the adaptability of ALDH. In this study, we examined the effects of endurance training, compensatory hypertrophy by synergist ablation (SA), and denervation-induced atrophy on gene expression and protein levels of selected ALDH isoforms in skeletal muscle. Ten-week-old C57BL/6J mice were subjected to each intervention, and the plantaris muscle was collected. Gene expression levels of Aldh1a1 were decreased by SA and denervation, but ALDH1A1 protein levels were not affected. Protein levels of ALDH1B1 increased after chronic endurance training, SA, and denervation interventions. However, the increase in Aldh1b1 gene expression was observed only after SA. The gene expression of Aldh2 was decreased after SA, but ALDH2 protein levels remained unchanged. Denervation increased both the Aldh2 gene and ALDH2 protein levels. Taken together, each isoform of ALDH undergoes unique quantitative adaptations in skeletal muscle under different conditions.

Aldehyde dehydrogenase 2 deficiency promotes skeletal muscle atrophy in aged mice

Aldehyde dehydrogenase 2 (ALDH2) detoxifies acetaldehyde produced from ethanol. A missense single nucleotide polymorphism (SNP) rs671 in ALDH2 exhibits a dominant-negative form of the ALDH2 protein. Nearly 40% of people in East Asia carry an inactive ALDH2*2 mutation. Previous studies reported that ALDH2*2 is associated with increased risk of several diseases. In this study, we examined the effect of ALDH2 deficiency on age-related muscle atrophy and its underlying mechanisms. We found that ALDH2 deficiency promotes age-related loss of muscle fiber cross-sectional areas, especially in oxidative fibers. Furthermore, ALDH2 deficiency exacerbated age-related accumulation of 4-hydroxy-2-nonenal (4-HNE), a marker of oxidative stress in the gastrocnemius muscle. Similarly, mitochondrial reactive oxygen species (ROS) production increased in aged ALDH2-knockout mice, indicating that ALDH2 deficiency induced mitochondrial dysfunction. In summary, ALDH2 deficiency promotes age-related muscle loss, especially in oxidative fibers, which may be associated with an increased accumulation of oxidative stress via mitochondrial dysfunction.

Trabecular Bone Volume Is Reduced, With Deteriorated Microstructure, With Aging in a Rat Model of Duchenne Muscular Dystrophy

We aimed to clarify the effect of aging on trabecular bone volume and trabecular bone microstructure in a rat model of Duchenne muscular dystrophy (DMD). Six rats each of wild type (WT) and DMD model at 15 weeks of age, and 4 rats each at 30 weeks of age, were analyzed by dual energy X-ray absorptiometry and by micro-CT for analysis of trabecular and cortical bone of the femur. Bone mineral density was significantly lower in the DMD group than in the WT group at both 15 and 30 weeks of age. Micro-CT showed that trabecular bone volume and number were not significantly different between the two groups at 15 weeks, but at 30 weeks both were significantly lower in the DMD group than in the WT group. Connectivity density and structure model index were not significantly different between the two groups at 15 weeks, but at 30 weeks they differed significantly. No significant differences between the WT and DMD groups in cortical thickness and cortical area were evident at both 15 and 30 weeks. In conclusion, trabecular bone volume is significantly reduced, with deteriorated microstructure, with aging in a rat model of DMD.

Effects of endurance training on the expression of host proteins involved in SARS-CoV-2 cell entry in C57BL/6J mouse

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is threatening people's lives and impacting their health. It is still unclear whether people engaged in physical activity are at an increased risk of SARS-CoV-2 infection and severe forms of COVID-19. In order to provide data to help answer this question, we, therefore, investigated the effects of endurance training on the levels of host proteins involved in SARS-CoV-2 infection in mice. Eight-week-old C57BL/6J mice were subjected to treadmill running (17-25 m/min, 60-90 min, 5 sessions/week, 8 weeks). After the intervention, the levels of angiotensin-converting enzyme 2 (ACE2; host receptor for SARS-CoV-2), transmembrane protease serine 2 (TMPRSS2; host protease priming fusion of SARS-CoV-2 to host cell membranes), FURIN (host protease that promotes binding of SARS-CoV-2 to host receptors), and Neuropilin-1 (host coreceptor for SARS-CoV-2) were measured in 10 organs that SARS-CoV-2 can infect (larynx, trachea, lung, heart, jejunum, ileum, colon, liver, kidney, and testis). Six organs (heart, lung, jejunum, liver, trachea, and ileum) showed changes in the levels of at least one of the proteins. Endurance training increased ACE2 levels in heart (+66.4%), lung (+37.1%), jejunum (+24.7%) and liver (+27.4%), and FURIN in liver (+17.9%) tissue. In contrast, endurance training decreased Neuropilin-1 levels in liver (-39.7%), trachea (-41.2%), and ileum (-39.7%), and TMPRSS2 in lung (-11.3%). Taken together, endurance training altered the levels of host proteins involved in SARS-CoV-2 cell entry in an organ-dependent manner.

Repeated bouts of resistance exercise in rats alter mechanistic target of rapamycin complex 1 activity and ribosomal capacity but not muscle protein synthesis

NEW FINDINGS

What is the central question of this study? Is muscle protein synthesis (MPS) additionally activated following exercise when ribosomal capacity is increased after repeated bouts of resistance exercise (RE)? What is the main finding and its importance? Skeletal muscles with increased ribosome content through repeated RE bouts showed sufficient activation of MPS with lower mechanistic target of rapamycin complex 1 signalling. Thus, repeated bouts of RE possibly change the translational capacity and efficiency to optimize translation activation following RE.

ABSTRACT

Resistance exercise (RE) activates ribosome biogenesis and increases ribosome content in skeletal muscles. However, it is unclear whether the increase in ribosome content subsequently causes an increase in RE-induced activation of muscle protein synthesis (MPS). Thus, this study aimed to investigate the relationship between ribosome content and MPS after exercise using a rat RE model. Male Sprague-Dawley rats were categorized into three groups (n = 6 for each group): sedentary (SED) and RE trained with one bout (1B) or three bouts (3B). The RE stimulus was applied to the right gastrocnemius muscle by transcutaneous electrical stimulation under isoflurane anaesthesia. The 3B group underwent stimulation every other day. Our results revealed that 6 h after the last bout of RE, muscles in the 3B group showed an increase in total RNA and 18S+28S rRNA content per muscle weight compared with the SED and 1B groups. In both the 1B and 3B groups, MPS, estimated by puromycin incorporation in proteins, was higher than that in the SED group 6 h after exercise; however, no significant difference was observed between the 1B and 3B groups. In the 1B and 3B groups, phosphorylated p70S6K at Thr-389 increased, indicating mechanistic target of rapamycin complex 1 (mTORC1) activity. p70S6K phosphorylation level was lower in the 3B group than in the 1B group. Finally, protein synthesis per ribosome (indicator of translation efficiency) was lower in the 3B group than in the 1B group. Thus, three bouts of RE changed the ribosome content and mTORC1 activation, but not the degree of RE-induced global MPS activation.

The effect of repeated bouts of electrical stimulation-induced muscle contractions on proteolytic signaling in rat skeletal muscle

Mechanistic target of rapamycin complex 1 (mTORC1) plays a central role in muscle protein synthesis and repeated bouts of resistance exercise (RE) blunt mTORC1 activation. However, the changes in the proteolytic signaling when recurrent RE bouts attenuate mTORC1 activation are unclear. Using a RE model of electrically stimulated rat skeletal muscle, this study aimed to clarify the effect of repeated RE bouts on acute proteolytic signaling, particularly the calpain, autophagy‐lysosome, and ubiquitin‐proteasome pathway. p70S6K and rpS6 phosphorylation, indicators of mTORC1 activity, were attenuated by repeated RE bouts. Calpain 3 protein was decreased at 6 h post‐RE in all exercised groups regardless of the bout number. Microtubule‐associated protein 1 light chain 3 beta‐II, an indicator of autophagosome formation, was increased at 3 h and repeated RE bouts increased at 6 h, post‐RE. Ubiquitinated proteins were increased following RE, but these increases were independent of the number of RE bouts. These results suggest that the magnitude of autophagosome formation was increased following RE when mTORC1 activity was attenuated with repeated bouts of RE.

Electrically stimulated contractile activity-induced transcriptomic responses and metabolic remodeling in C2C12 myotubes: twitch vs. tetanic contractions

The contraction of myotubes using electrical pulse stimulation is a research tool used to mimic muscle contractile activity and exercise in rodents and humans. Most protocols employed in previous work used low-frequency twitch contractions. However, high-frequency tetanus contractions that are more physiologically relevant to muscle contractions in vivo are poorly characterized. In this report, the similarities and differences in acute responses and chronic adaptations with different contractile modes using twitches (2 Hz, continuous, 3 h) and tetanus (66 Hz, on: 5 s/off: 5 s, 3 h) were investigated. RNA sequencing-based transcriptome analysis and subsequent bioinformatics analysis suggest that tetanus may promote bioenergetic remodeling rather than twitch. Based on in silico analyses, metabolic remodeling after three contractile sessions of twitch and tetanus were investigated. Although twitch and tetanus had no significant effect on glycolysis, both types of contraction upregulated glucose oxidation capacity. Both twitch and tetanus qualitatively caused mitochondrial adaptations (increased content, respiratory chain enzyme activity, and respiratory function). The magnitude of adaptation was much greater under tetanus conditions. Our findings indicate that the contraction of myotubes by tetanus may be a useful experimental model, especially in the study of metabolic adaptations in C₂C₁₂ myotubes.

The distribution of eukaryotic initiation factor 4E after bouts of resistance exercise is altered by shortening of recovery periods

Insufficient duration of recovery between resistance exercise bouts reduces the effects of exercise training, but the influence on muscle anabolic responses is not fully understood. Here, we investigated the changes in the distribution of eukaryotic initiation factor (eIF) 4E, a key regulator of translation initiation, and related factors in mouse skeletal muscle after three successive bouts of resistance exercise with three durations of recovery periods (72 h: conventional, 24 h: shorter, and 8 h: excessively shorter). Bouts of resistance exercise dissociated eIF4E from eIF4E binding protein 1, with the magnitude increasing with shorter recovery. Whereas bouts of resistance exercise with 72 h recovery increased the association of eIF4E and eIF4G, those with shorter recovery did not. Similar results were observed in muscle protein synthesis. These results suggest that insufficient recovery inhibited the association of eIF4E and eIF4G, which might cause attenuation of protein synthesis activation after bouts of resistance exercise.

Acetaldehyde dehydrogenase 2 deficiency increases mitochondrial reactive oxygen species emission and induces mitochondrial protease Omi/HtrA2 in skeletal muscle

Acetaldehyde dehydrogenase 2 (ALDH2) is an enzyme involved in redox homeostasis as well as the detoxification process in alcohol metabolism. Nearly 8% of the world’s population have an inactivating mutation in the ALDH2 gene. However, the expression patterns and specific functions of ALDH2 in skeletal muscles are still unclear. Herein, we report that ALDH2 is expressed in skeletal muscle and is localized to the mitochondrial fraction. Oxidative muscles had a higher amount of ALDH2 protein than glycolytic muscles. We next comprehensively investigated whether ALDH2 knockout in mice induces mitochondrial adaptations in gastrocnemius muscle (for example, content, enzymatic activity, respiratory function, supercomplex formation, and functional networking). We found that ALDH2 deficiency resulted in partial mitochondrial dysfunction in gastrocnemius muscle because it increased mitochondrial reactive oxygen species (ROS) emission (2′,7′-dichlorofluorescein and MitoSOX oxidation rate during respiration) and the frequency of regional mitochondrial depolarization. Moreover, we determined whether ALDH2 deficiency and the related mitochondrial dysfunction trigger mitochondrial stress and quality control responses in gastrocnemius muscle (for example, mitophagy markers, dynamics, and the unfolded protein response). We found that ALDH2 deficiency upregulated the mitochondrial serine protease Omi/HtrA2 (a marker of the activation of a branch of the mitochondrial unfolded protein response). In summary, ALDH2 deficiency leads to greater mitochondrial ROS production, but homeostasis can be maintained via an appropriate stress response.

Eccentric contraction-induced muscle damage in human flexor pollicis brevis is accompanied by impairment of motor nerve

BACKGROUND

Eccentric contractions (ECCs) cause muscle damage. In addition, we showed that ECCs induce nerve dysfunction and damage with rats and human.

PURPOSE

We aimed to evaluate motor nerve conduction velocity (MCV) for flexor pollicis brevis muscle (FPBM) after ECCs.

METHODS

Twelve men (years, 19.8 ± 1.7 years; height, 172.4 ± 7.0 cm; weight, 64.0 ± 8.6 kg) performed maximal 100 ECCs on their FPBM of non-dominant hands with torque dynamometer. The dominant hands were control (CON). Maximal voluntary contraction (MVC), range of motion (ROM), DOMS, and MCV were assessed before, immediately post, and 1, 2, and 5 days after ECCs. MCV was calculated as the distance by stimulation divided by the latencies of the waveforms generated. Values were statistically analyzed by two-way ANOVA, and the significance level was set at P < .05.

RESULTS

Decreases in MVC immediately (-32.9%) to 5 days after ECCs were significantly greater (P < .05) than for the CON group. ROM showed a significant decrease immediately (-21.6%) after ECCs compared with before ECCs and CON group (P < .05). DOMS after ECCs increased at 1 and 2 days (5.0 cm) after ECCs compared with before ECCs and CON (P < .05). Also, MCV after ECCs delayed significantly from immediately (-36.4%), 1, 2, and 5 days after ECCs compared with CON (P < .05), while no significant change in M-wave amplitude was observed over time for both ECCs and CON.

CONCLUSION

The present study showed that ECCs of the FPBM cause a significant delay in MCV of median nerve.

Cast immobilization of hindlimb upregulates sarcolipin expression in atrophied skeletal muscles and increases thermogenesis in C57BL/6J mice

Mechanical unloading impairs cytosolic calcium (Ca²⁺) homeostasis in skeletal muscles. In this study, we investigated whether sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) itself or one of the regulators of the Ca²⁺ SERCA pump, sarcolipin (SLN), is altered to deregulate Ca²⁺ homeostasis in cast immobilized, atrophied muscles. Hindlimb muscles of 8-wk-old male C57BL/6J mice were subjected to bilateral cast immobilization for 2 wk. Two-week-cast immobilization induced both body weight and skeletal muscle loss. Highly phosphorylated Ca²⁺/calmodulin-dependent protein kinase II in the atrophied muscles suggested that cytosolic Ca²⁺ concentration was elevated. Extremely high expression levels of SLN mRNA and protein were observed in the atrophied muscles. Upregulation of SLN at the transcriptional level was supported by low RCAN1 expression, which is a negative regulator of SLN. We treated C₂C₁₂ cells with dexamethasone to mimic muscle atrophy in vitro and showed a direct relationship between high SLN mRNA expression and low Ca²⁺ uptake by sarcoplasmic reticulum. Since SLN reportedly plays a role in nonshivering thermogenesis, we performed a cold tolerance test of the whole body. As a result, we found that mice with cast immobilization showed high cold tolerance, suggesting that cast immobilization promoted whole body thermogenesis. Although the activity level was decreased during cast immobilization without change in food intake, adipose tissue weights also decreased significantly after cast immobilization. Concomitantly, we conclude that cast immobilization of hindlimb increased thermogenesis in C57Bl/6J mice, probably via high expression of SLN.

Dietary apple polyphenols increase skeletal muscle capillaries in Wistar rats

Dietary apple polyphenols (AP) have been shown to exhibit beneficial effects on muscle endurance. Fast-to-slow change in the composition of myosin heavy chains was known as one of the molecular mechanisms. Here, we examined the effects of dietary AP on the capillaries and mitochondria in the rat skeletal muscle to elucidate the mechanisms underlying muscular endurance enhancement. Twenty-four Wistar male rats were divided into three groups, namely, the control group, 0.5% AP group, and 5% AP group (n = 8 in each group). After a feeding period of 4 weeks, rats were dissected, gastrocnemius muscles were removed, and the density of capillaries and levels of mitochondrial proteins were analyzed. Capillary density of the gastrocnemius increased to 17.8% in rats fed with 5% AP as compared to the control rats. No significant change was observed in the mitochondrial content and dynamics (fusion/fission) of regulatory proteins. To investigate the mechanisms underlying the increase in the capillary density, positive (vascular endothelial cell growth factor, VEGF) and negative (thrombosponsin-1, TSP-1) factors of angiogenesis were analyzed. TSP-1 expression significantly decreased in rats fed with 0.5% AP and 5% AP by approximately 25% and 40%, respectively, as compared with the control rats. There were no significant differences in VEGF expression. Thus, dietary AP may increase the muscle capillary density by decreasing TSP-1 expression. We concluded that the increase in the capillary density and the fast-to-slow change in myosin heavy chains by AP feeding are the main causes for muscle endurance enhancement in Wistar rats.

Cervical Intervertebral Disc Degeneration and Low Cervical Extension Independently Associated With a History of Stinger Syndrome

Background:

Stinger syndrome frequently occurs in athletes who compete in collision sports. Sharp pain and impairment of neck motion are major symptoms. Cervical intervertebral disc degeneration (CIDD) is also frequently observed in those who compete in collision sports.

Purpose/Hypothesis:

To investigate whether CIDD and neck functionality are related to a history of stinger syndrome. The hypothesis was that a significant relationship exists between CIDD and neck motion and a history of stinger syndrome in Japanese collegiate football players.

Study Design:

Cross-sectional study; Level of evidence, 3.

Methods:

A total of 49 male Japanese collegiate football players (mean age, 20.0 ± 1.1 years; mean athletic experience, 3.8 ± 2.3 years; mean height, 172.3 ± 4.8 cm; mean weight, 83.1 ± 12.2 kg) were subdivided into athletes with stinger syndrome (stinger group) and those without (control group). Stinger syndrome was confirmed based on a questionnaire and interview. CIDD was assessed by using T2-weighted magnetic resonance imaging. Range of motion (ROM) and isometric muscle strength were measured for neck function testing.

Results:

Thirty-nine percent (19/49) of athletes had at least 1 episode of stinger syndrome. The prevalence of CIDD was significantly higher in the stinger group (68%) than in the control group (30%) (P < .01). A statistically significant difference in cervical extension ROM was found between the stinger group (50.9° ± 11.1°) and the control group (60.2° ± 11.4°) (P < .01). Logistic regression analysis showed that CIDD and low cervical extension were independently associated with a history of stinger syndrome.

Conclusion:

Study results suggest that stinger syndrome is associated with CIDD and low cervical extension in collegiate football players.

Sciatic Nerve Conductivity is Impaired by Hamstring Strain Injuries

The aim of this study was to assess sciatic nerve conductivity in athletes with a history of hamstring strain injuries. Twenty-seven athletes with a history of hamstring strain injuries were included in the injured group. The control group consisted of 16 uninjured participants. We measured the proximal and distal latencies and calculated the sciatic nerve conduction velocity to evaluate neuronal conductivity. The results were expressed as median values and interquartile ranges. Both proximal latency and distal latency of the injured limb in the injured group were significantly longer than those of the uninjured limb (p<0.05). The nerve conduction velocity of the injured limb in the injured group was significantly lower than that of the uninjured limb (p<0.05). There were no significant side-to-side differences in the control group. Sciatic nerve conductivity impairments may exist in athletes with a history of hamstring strain injuries.

Repeated bouts of fast eccentric contraction produce sciatic nerve damage in rats

INTRODUCTION

We evaluated sciatic nerve impairment after eccentric contractions (ECs) in rat triceps surae.

METHODS

Wistar rats were randomly assigned to different joint angular velocity: 180°/s (FAST), 30°/s (SLOW), or nontreated control (CNT). FAST and SLOW groups were subjected to multiple (1-4) bouts of 20 (5 reps, 4 sets) ECs. Nerve conduction velocity (NCV) and isometric tetanic ankle torque were measured 24 h after each ECs bout. We also assessed nerve morphology.

RESULTS

After 4 ECs bouts, NCVs and isometric torque in the FAST group were significantly lower than those in the CNT (NCV: 42%, torque: 66%; P < 0.05). After 4 bouts, average nerve diameter was significantly smaller in the FAST group [2.39 ± 0.20 μm vs. 2.69 ± 0.20 μm (CNT) and 2.93 ± 0.24 μm (SLOW); P < 0.05] than that in other two groups.

CONCLUSIONS

Chronic ECs with high angular velocity induce serious nerve damage. Muscle Nerve 54: 936-942, 2016.