Introduction
Disuse-induced bone loss is caused by a suppression of osteoblastic bone formation and an increase in osteoclastic bone resorption. There are few data available for the effects of ...environmental conditions, i.e., atmospheric pressure and/or oxygen concentration, on osteoporosis. This study examined the effects of mild hyperbaric oxygen at 1317 hPa with 40% oxygen on unloading-induced osteoporosis.
Materials and methods
Eighteen 8-week old male Wistar rats were randomly divided into three groups: the control for 21 days without unloading and mild hyperbaric oxygen (NOR,
n
= 6), the unloading for 21 days and recovery for 10 days without mild hyperbaric oxygen (HU + NOR,
n
= 6), and the unloading for 21 days and recovery for 10 days with mild hyperbaric oxygen (HU + MHO,
n
= 6).
Results
The cortical thickness and trabecular bone surface area were decreased in the HU + NOR group compared to the NOR group. There were no differences between the NOR and HU + MHO groups. Osteoclast surface area and
Sclerostin (Sost)
mRNA expression levels were decreased in the HU + MHO group compared to the HU + NOR group. These results suggested that the loss of the cortical and trabecular bone is inhibited by mild hyperbaric oxygen, because of an inhibition of osteoclasts and enhancement of bone formation with decreased
Sost
expression.
Conclusions
We conclude that exposure to mild hyperbaric oxygen partially protects from the osteoporosis induced by hindlimb unloading.
Glycative stress is a type of biological stress caused by non-enzymatic glycation reactions, which include advanced glycation end product (AGE) formation, AGE accumulation, glycation-driven ...dysfunction of proteins and cellular signaling, inflammation, oxidation, and tissue damage. Increased glycative stress derived from hyperglycemia and lifestyle disorders is a risk factor in metabolic and age-related diseases, such as type 2 diabetes, cardiovascular disease, cancer, Alzheimer's disease, osteoporosis, and dementia. Studies have shown that AGE accumulation is correlated with the age-related loss of muscle mass and power output, also called sarcopenia. Mechanistically, dysfunctions of contractile proteins, myogenic capacity, and protein turnover can cause glycative stress-induced skeletal muscle dysfunction. Because the skeletal muscle is the largest metabolic organ in the body, maintaining skeletal muscle health is essential for whole-body health. Increasing awareness and understanding of glycative stress in the skeletal muscle in this review will contribute to the maintenance of better skeletal muscle function.
A single bout of exercise can potentiate the effect of insulin on skeletal muscle glucose uptake via activation of the AMPK-TBC1 domain family member 4 (TBC1D4) pathway, which suggests a positive ...correlation between AMPK activation and insulin sensitization. In addition, prolonged fasting in rodents is known to upregulate and thereby synergistically enhance the effect of exercise on muscle AMPK activation. Therefore, fasting may potentiate the insulin-sensitizing effect of exercise. In the present study, we mimicked exercise by in situ muscle contraction and evaluated the effect of a 36-h fast on muscle contraction-induced insulin sensitization. Male Wistar rats weighing 150-170 g were allocated to either a 36-h fasting or feeding group. The extensor digitorum longus (EDL) muscles were electrically contracted via the common peroneal nerve for 10 min followed by a 3-h recovery period. EDL muscles were dissected and incubated in the presence or absence of submaximal insulin. Our results demonstrated that acute muscle contraction and 36 h of fasting additively upregulated AMPK pathway activation. Insulin-stimulated muscle glucose uptake and site-specific TBC1D4 phosphorylation were enhanced by prior muscle contraction in 36-h-fasted rats, but not in fed rats. Moreover, enhanced insulin-induced muscle glucose uptake and Akt phosphorylation due to 36 h of fasting were associated with a decrease in tribbles homolog 3 (TRB3), a negative regulator of Akt activation. In conclusion, fasting and prior muscle contraction synergistically enhance insulin-stimulated TBC1D4 phosphorylation and glucose uptake, which is associated with augmented AMPK pathway activation in rodents.
In this study, we revealed that 36 h of fasting additively upregulated acute muscle contraction-induced AMPK pathway activation in rats. Besides, fasting and muscle contraction synergistically enhanced insulin-stimulated site-specific TBC1D4 phosphorylation and glucose uptake, which was associated with augmented AMPK pathway activation. These results contribute to understanding the regulation of muscle insulin sensitivity.
Advanced glycation end products (AGEs) have been implicated in several skeletal muscle dysfunctions. However, whether the adverse effects of AGEs on skeletal muscle are because of their direct action ...on the skeletal muscle tissue is unclear. Therefore, this study aimed to investigate the direct and acute effects of AGEs on skeletal muscle using an isolated mouse skeletal muscle to eliminate several confounders derived from other organs. The results showed that the incubation of isolated mouse skeletal muscle with AGEs (1 mg/mL) for 2–6 h suppressed protein synthesis and the mechanistic target of rapamycin signaling pathway. Furthermore, AGEs showed potential inhibitory effects on protein degradation pathways, including autophagy and the ubiquitin–proteasome system. Additionally, AGEs stimulated endoplasmic reticulum (ER) stress by modulating the activating transcription factor 6, PKR‐like ER kinase, C/EBP homologous protein, and altered inflammatory cytokine expression. AGEs also stimulated receptor for AGEs (RAGE)‐associated signaling molecules, including mitogen‐activated protein kinases. These findings suggest that AGEs have direct and acute effect on skeletal muscle and disturb proteostasis by modulating intracellular pathways such as RAGE signaling, protein synthesis, proteolysis, ER stress, and inflammatory cytokines.
The effects of lactate on muscle mass and regeneration were investigated using mouse skeletal muscle tissue and cultured C2C12 cells. Male C57BL/6J mice were randomly divided into (1) control, (2) ...lactate (1 mol/L in distilled water, 8.9 mL/g body weight)-administered, (3) cardio toxin (CTX)-injected (CX), and (4) lactate-administered after CTX-injection (LX) groups. CTX was injected into right tibialis anterior (TA) muscle before the oral administration of sodium lactate (five days/week for two weeks) to the mice. Oral lactate administration increased the muscle weight and fiber cross-sectional area, and the population of Pax7-positive nuclei in mouse TA skeletal muscle. Oral administration of lactate also facilitated the recovery process of CTX-associated injured mouse TA muscle mass accompanied with a transient increase in the population of Pax7-positive nuclei. Mouse myoblast-derived C2C12 cells were differentiated for five days to form myotubes with or without lactate administration. C2C12 myotube formation with an increase in protein content, fiber diameter, length, and myo-nuclei was stimulated by lactate. These observations suggest that lactate may be a potential molecule to stimulate muscle hypertrophy and regeneration of mouse skeletal muscle via the activation of muscle satellite cells.
This study investigated the effects of AdipoRon, which is an agonist for adiponectin receptor 1 (AdipoR1) and AdipoR2, on the protein content, myotube diameter, and number of nuclei per myotube of ...C2C12 cells and skeletal muscle mass in C57BL/6J mice. AdipoRon suppressed the protein content, myotube diameter, and number of nuclei per myotube of C2C12 cells of C2C12 myotubes in a dose-dependent manner. Adiponectin-associated decline of protein content, diameter, and number of nuclei per myotube in C2C12 myotubes was partially rescued by knockdown of AdipoR1 and/or AdipoR2. Phosphorylation level of AMPK showed a trend to be increased by AdipoRon. A significant increase in phosphorylation level of AMPK was observed at 20 μM AdipoRon. Knockdown of AdipoR1 and/or AdipoR2 rescued AdipoRon-associated decrease in protein content of C2C12 myotubes. AdipoRon-associated increase in phosphorylation level of AMPK in C2C12 myotubes was suppressed by knockdown of AdipoR1 and/or AdipoR2. Successive intravenous injections of AdipoRon into mice caused a decrease in the wet weight of plantaris muscle (PLA), but not in soleus muscle (SOL). Mean fiber cross-sectional area of PLA, but not of SOL, was significantly decreased by AdipoRon administration. On the one hand, the expression level of phosphorylated AMPK and ubiquitinated protein in SOL and PLA muscles was upregulated by AdipoRon administration. On the other hand, AdipoRon administration induced no changes in the expression level of puromycin-labeled proteins in both SOL and PLA muscles. Expression level of adiponectin in extensor digitorum longus (EDL) muscle was increased by aging, but not in SOL muscle. Aging had no effect on the expression level of AdipoR1 and AdipoR2 in both muscles. Phosphorylation level of AMPK in EDL was increased by aging, but not SOL muscle. Results from this study suggest that high level of circulating adiponectin may induce skeletal muscle atrophy, especially fast-type muscle.
Background
Glycative stress, characterized by the formation and accumulation of advanced glycation end products (AGEs) associated with protein glycation reactions, has been implicated in inducing a ...decline of muscle function. Although the inverse correlation between glycative stress and muscle mass and strength has been demonstrated, the underlying molecular mechanisms are not fully understood. This study aimed to elucidate how glycative stress affects the skeletal muscle, particularly the adaptive muscle response to hypertrophic stimuli and its molecular mechanism.
Methods
Male C57BL/6NCr mice were randomly divided into the following two groups: the bovine serum albumin (BSA)‐treated and AGE‐treated groups. Mice in the AGE‐treated group were intraperitoneally administered AGEs (0.5 mg/g) once daily, whereas those in the BSA‐treated group received an equal amount of BSA (0.5 mg/g) as the vehicle control. After 7 days of continuous administration, the right leg plantaris muscle of mice in each group underwent functional overload treatment by synergist ablation for 7 days to induce muscle hypertrophy. In in vitro studies, cultured C2C12 myocytes were treated with AGEs (1 mg/mL) to examine cell adhesion and cell membrane permeability.
Results
Continuous AGE administration increased the levels of fluorescent AGEs, Nε‐(carboxymethyl) lysine, and methylglyoxal‐derived hydroimidazolone‐1 in both plasma and skeletal muscle. Plantaris muscle weight, muscle fibre cross‐sectional area, protein synthesis rate, and the number of myonuclei increased with functional overload in both groups; however, the increase was significantly reduced by AGE treatment. Some muscles of AGE‐treated mice were destroyed by functional overload. Proteomic analysis was performed to explore the mechanisms of muscle hypertrophy suppression and myofibre destruction by AGEs. When principal component analysis was performed on 4659 data obtained by proteomic analysis, AGE treatment was observed to affect protein expression only in functionally overloaded muscles. Enrichment analysis of the 436 proteins extracted using the K‐means method further identified a group of proteins involved in cell adhesion. Consistent with this finding, dystrophin–glycoprotein complex proteins and cell adhesion‐related proteins were confirmed to increase with functional overload; however, this was attenuated by AGE treatment. Additionally, the treatment of C2C12 muscle cells with AGEs inhibited their ability to adhere and increased cell membrane permeability.
Conclusions
This study indicates that glycative stress may be a novel pathogenic factor in skeletal muscle dysfunctions by causing loss of membrane integrity and preventing muscle mass gain.
We explored the interrelationship between a tissue-specific alternative splicing factor muscleblind-like 1 (MBNL1) and peroxisome proliferator-activated receptor-γ coactivator 1-α (PGC-1α), B-cell ...lymphoma 2 (Bcl-2) or Bcl-2-associated X protein (Bax) in C2C12 myotubes and mouse skeletal muscle to investigate a possible physiological role of MBNL1 in mitochondrial-associated apoptosis of skeletal muscle. Expression level of PGC-1α and mitochondrial membrane potential evaluated by the fluorescence ratio of JC-1 aggregate to monomer in C2C12 myotubes were suppressed by knockdown of MBNL1. Conversely, the ratio of Bax to Bcl-2 as well as the apoptotic index in C2C12 myotubes was increased by MBNL1 knockdown. In plantaris muscle, on the other hand, not only the minimum muscle fiber diameter but also the expression level of MBNL1 and PGC-1α in of 100-week-old mice were significantly lower than that of 10-week-old mice. Furthermore, the ratio of Bax to Bcl-2 in mouse plantaris muscle was increased by aging. These results suggest that MBNL1 may play a key role in aging-associated muscle atrophy accompanied with mitochondrial dysfunction and apoptosis via mediating PGC-1α expression in skeletal muscle.
Exercise has beneficial effects on our health by stimulating metabolic activation of skeletal muscle contraction. Caffeine is a powerful metabolic stimulant in the skeletal muscle that has ergogenic ...effects, including enhanced muscle power output and endurance capacity. In the present study, we aim to characterize the metabolic signatures of contracting muscles with or without caffeine stimulation using liquid chromatography-mass spectrometry and capillary electrophoresis coupled to mass spectrometry. Isolated rat epitrochlearis muscle was incubated in the presence or absence or of 3 mM caffeine for 30 min. Electrical stimulation (ES) was used to induce tetanic contractions during the final 10 min of incubation. Principal component analysis and hierarchical clustering analysis detected 184 distinct metabolites across three experimental groups-basal, ES, and ES with caffeine (ES + C). Significance Analysis of Microarray identified a total of 50 metabolites with significant changes in expression, and 23 metabolites significantly changed between the ES and ES + C groups. Changes were observed in metabolite levels of various metabolic pathways, including the pentose phosphate, nucleotide synthesis, β-oxidation, tricarboxylic acid cycle, and amino acid metabolism. In particular, D-ribose 5-phosphate, IMP, O-acetylcarnitine, butyrylcarnitine, L-leucine, L-valine, and L-aspartate levels were higher in the ES + C group than in the ES group. These metabolic alterations induced by caffeine suggest that caffeine accelerates contraction-induced metabolic activations, thereby contributing to muscle endurance performance and exercise benefits to our health.
5'AMP-activated protein kinase (AMPK) plays an important role in the regulation of skeletal muscle mass and fiber-type distribution. However, it is unclear whether AMPK is involved in muscle mass ...change or transition of myosin heavy chain (MyHC) isoforms in response to unloading or increased loading. Here, we checked whether AMPK controls muscle mass change and transition of MyHC isoforms during unloading and reloading using mice expressing a skeletal-muscle-specific dominant-negative AMPKα1 (AMPK-DN). Fourteen days of hindlimb unloading reduced the soleus muscle weight in wild-type and AMPK-DN mice, but reduction in the muscle mass was partly attenuated in AMPK-DN mice. There was no difference in the regrown muscle weight between the mice after 7 days of reloading, and there was concomitantly reduced AMPKα2 activity, however it was higher in AMPK-DN mice after 14 days reloading. No difference was observed between the mice in relation to the levels of slow-type MyHC I, fast-type MyHC IIa/x, and MyHC IIb isoforms following unloading and reloading. The levels of 72-kDa heat-shock protein, which preserves muscle mass, increased in AMPK-DN-mice. Our results indicate that AMPK mediates the progress of atrophy during unloading and regrowth of atrophied muscles following reloading, but it does not influence the transition of MyHC isoforms.
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