Glycolytic Fibers Research Articles

A maternal high-fat/low-fiber diet impairs glucose tolerance and induces the formation of glycolytic muscle fibers in neonatal offspring.

In our previous study, the maternal high-fat/low-fiber (HF-LF) diet was suggested to induce metabolic disorders and placental dysfunction of the dam, but the effects of this diet on glucose metabolism of neonatal offspring remain largely unknown. Here, a neonatal pig model was used to evaluate the effects of maternal HF-LF diet during pregnancy on glucose tolerance, transition of skeletal muscle fiber types, and mitochondrial function in offspring. A total of 66 pregnant gilts (Guangdong Small-ear Spotted pig) at day 60 of gestation were randomly divided into two groups: control group (CON group; 2.86% crude fat, 9.37% crude fiber), and high-fat/low-fiber diet group (HF-LF group; 5.99% crude fat, 4.13% crude fiber). The maternal HF-LF diet was shown to impair the glucose tolerance of neonatal offspring, downregulate the protein level of slow-twitch fiber myosin heavy chain I (MyHC I), and upregulate the protein levels of fast-twitch fiber myosin heavy chain IIb (MyHC IIb) and IIx (MyHC IIx) in soleus muscle. Additionally, compared with the CON group, the HF-LF offspring showed inhibition of insulin signaling pathway and decrease in mitochondrial function in liver and soleus muscle. Maternal HF-LF diet during pregnancy impairs glucose tolerance, induces the formation of glycolytic muscle fibers, and decreases the hepatic and muscular mitochondrial function in neonatal piglets.

Развитие осевой локомоторной мускулатуры у половозрелого окуня-клювача Sebastes mentella (Sebastidae)

The results of the study of the histological structure of the locomotor axial (skeletal) muscle tissue of the beaked redfish Sebastes mentella are presented and its ontogenetic changes are considered. Its skeletal muscles compose of glycolytic (white) and oxidative (red) fibers. Data show the quantitative distribution of these fibers along the body of sexually mature fish in the cranio-caudal direction, their ratio in the trunk and tail parts. The diameter of these two types of fibers reaches significant sizes, as in a number of deep-sea slow-growing species (> 300 μm - white fibers, > 150 μm - red fibers). The density of glycolytic fibers is higher than that of red ones (80,7±2,0 versus 54,7±1,5 %), which is due to the different content of adipose and connective tissue (endomysium). The variability of white fibers in most fish size groups is significantly lower than that of red ones. A feature that is not noted in other deep-sea fish species with a low growth rate is the relatively frequent occurrence of red fibers with stratification of sarcoplasm into two zones (layers) - external and internal (double structures), the nature of which is not yet clear.

Effect of Excessive or Restrictive Energy on Growth Performance, Meat Quality, and Intramuscular Fat Deposition in Finishing Ningxiang Pigs.

Simple SummaryThe present study indicated that excessive energy enhanced fat deposition by upregulating expression of lipogenic genes in the longissimus thoracis of a Chinese fat-type pig. In addition, impairment in meat quality resulted in reduced meat tenderness and increased cross-sectional area of muscle fiber aroused by promoting glycolytic muscle fibers differentiation in muscles. These results provided a new perspective on the energy needs of Ningxiang pigs.This study investigated the effects of excessive or restrictive energy on growth performance, meat quality, intramuscular fat (IMF) deposition, and related gene expression in finishing Ningxiang pigs. A total of 36 Ningxiang pigs (43.26 ± 3.21 kg) were randomly assigned to three treatments (6 pens of 2 piglets per treatment) and fed by one of three dietary treatments until the pigs of each treatment weighed approximately 75 kg equally. The three treatments were control diet (digestible energy, DE:13.02 MJ/kg, CON), excessive energy diet (DE 15.22 MJ/kg, EE), and restrictive energy diet (DE 10.84 MJ/kg, RE). Results showed that EE improved average daily gain (ADG) and feed conversion ratio (FCR) (p < 0.01), while nothing significantly changed by RE except FCR increasing (p < 0.01). EE increased the content of IMF and triglycerides (TG) (p < 0.05), L*24h and b*45min (p < 0.01), while decreasing cooking loss and meat tenderness in longissimus thoracis (LT) (p < 0.05). b*24h was significantly increased with the increase of energy level (p < 0.01). Meanwhile, EE increased the cross-sectional area (CSA) of muscle fiber and the mRNA expression of myosin heavy chain (MyHC) IIb, acetyl CoA carboxylase (ACC), fatty acid synthase (FAS), and adipocyte fatty-acid binding protein (FABP4) (p < 0.05). In addition, throughout: A diet supplemented with excessive energy promoted IMF deposition by positively changing lipogenic potential while decreasing tenderness by increasing glycolytic muscle fibers, which together affected meat quality. In terms of growth performance and meat quality, the present study suggests that the low-energy diet is suitable for finishing Ningxiang pigs.

The mechanism behind BAF60c in myocardial metabolism in rats with heart failure is through the PGC1α-PPARα-mTOR signaling pathway.

Metabolic remodeling in heart failure (HF) is a type of overload cardiomyopathy caused by insufficient energy supply or an imbalance of glucose and lipid metabolism. Therefore, metabolic pathways may serve as potential targets for HF treatment. BRM-associated factor (BAF) 60c (also known as smarcd3) promotes the transformation of oxidative muscle fibers to glycolytic muscle fibers. Our study aimed to test whether BAF60c and the PGC1α-PPARα-mTOR pathway interact to affect myocardial metabolism in HF rats. Established rat models of HF were injected with BAF60c low or overexpression plasmids to assess cardiac contractile proteins, energy metabolism, oxidative metabolism, glycolysis, high-energy phosphate content, mitochondrial function, and apoptosis. BAF60c overexpression/siRNA plasmid was transfected into H9C2 cells. These results suggest that HF rats present decreased levels of BAF60c, increased glycolysis, and reduced levels of cardiac contractile proteins, PGC1α, PPARα, and oxidative metabolism. Overexpression of BAF60c maintained the balance between oxidative metabolism and glycolysis and activated the PGC1α-PPARα-mTOR pathway. PGC1α interacted with BAF60c, and overexpression of PGC1α decreased BAF60c knockdown, damaging H9C2 cells. Collectively, overexpression of BAF60c activated the PGC1α-PPARα-mTOR pathway, maintained the oxidative metabolism/glycolysis balance, and improved mitochondrial function in HF rats. This study offers novel insights into HF treatment.

Morphological study of the integument and corporal skeletal muscles of two psammophilous members of Scincidae (Scincus scincus and Eumeces schneideri)

Sand deserts are common biotopes on the earth's surface. Numerous morphological and physiological adaptations have appeared to cope with the peculiar conditions imposed by sandy substrates, such as abrasion, mechanical resistance and the potential low oxygen levels. The psammophilous scincids (Lepidosauria) Scincus scincus and Eumeces schneideri are among those. S. scincus is a species frequently used to study displacement inside a sandy substrate. E. schneideri is a species phylogenetically closely related to S. scincus with a similar lifestyle. The aims of this study focus on the morphology of the integument and the muscular system. Briefly, we describe interspecific differences at the superficial architecture of the scales pattern and the thickness of the integument. We highlight a high cellular turnover rate at the level of the basal germinal layer of the epidermis, which, we suggest, corresponds to an adaptation to cutaneous wear caused by abrasion. We demonstrate the presence of numerous cutaneous holocrine glands whose secretion probably plays a role in the flow of sand along the integument. Several strata of osteoderms strengthen the skin. We characterize the corporal (M. longissimus dorsi and M. rectus abdominus) and caudal muscular fibers using immunohistochemistry, and quantify them using morphometry. The musculature exhibits a high proportion of glycolytic fast fibers that allow rapid burying and are well adapted to this mechanically resistant and oxygen‐poor substrate. Oxidative slow fibers are low in abundance, less than 10% in S. scincus, but a little higher in E. schneideri.

Aerobic physical training improves skeletal muscle metabolism in heart failure patients

Abstract Purpose To evaluate the efficiency of aerobic physical rehabilitation (PR), selected on the basis of lactate threshold achievement during cardiopulmonary exercising test (CPET), in heart failure patients with normal body mass index (BMI); to study metabolism changes in skeletal muscle fibers (MF) after PR. Methods 100 patients; heart failure with reduced ejection fraction (HFrEF), NYHA III, mean age 52±5.2 years, ejection fraction (EF) 27.3±4.2%, BMI 23.5±2.8 kg/m2. The original estimated CPET, exercise tolerance (ET), quality of life (QOL) and echocardiography (EchoCG). Effects of physical rehabilitation were estimated on the basis of peak oxygen uptake (VO2peak), EF, QOL and ET dynamics after 6 months of training. Shin muscle biopsy was performed at baseline and after 3–6 months of training programm. In muscle samples activity of lactate dehydrogenase (LDH), succinate dehydrogenase (SDG) and alkaline phosphatase (AP) was evaluated with plag-method. Data were statistically processed using software package Statistika, 9.0. Results After 6 months of physical training ET increased by 9.7±0.5 points and QOL improved by 24.8±3.5 points (p1,2 &amp;lt;0.05), EF increased by 10.5±2.3% and VO2 peak increased by 5.2±0.5 ml/min/kg (p3,4 &amp;lt;0.05). After 3–6 months of training AP activity increased by 24.2% (p&amp;lt;0.05). LDH activity in glycolytic MF decreased by 24.4%, in oxidative MF it decreased by only 6.0% (p1 &amp;lt;0.05, p2&amp;gt;0.05). SDG activity in glycolytic MF increased by 30% and in oxidative – by 20% (p1,2 &amp;lt;0.05). There was a positive relation between SDG activity and VO2 peak dynamics (r=0.6, p=0.04), increase in CPET parameters was associated with AP activity (r=0.5, p=0.05). Conclusion Aerobic physical training in stable HFrEF patients with normal BMI was effective in improving values of EF, VO2 peak, QOL and ET; Participating in the rehabilitation program decreased LDH activity in both oxidative and glycolytic muscle fibers; it also significantly increased AP activity and SDG activity in both oxidative and glycolytic MF. This may indicate a decrease in glycolytic metabolism activity and aerobic metabolism activation; The relation between the increase in exercise tolerance and endothelium condition marker (AP activity in muscle tissue), SDG activity and VO2 peak dynamics has been revealed. Funding Acknowledgement Type of funding source: Public grant(s) – EU funding. Main funding source(s): Project was partially funded by the Russian Science Foundation grant 16-15-1017

Muscle-Specific Insulin Receptor Overexpression Protects Mice From Diet-Induced Glucose Intolerance but Leads to Postreceptor Insulin Resistance.

Skeletal muscle insulin resistance is a prominent early feature in the pathogenesis of type 2 diabetes. In attempt to overcome this defect, we generated mice overexpressing insulin receptors (IR) specifically in skeletal muscle (IRMOE). On normal chow, IRMOE mice have body weight similar to that of controls but an increase in lean mass and glycolytic muscle fibers and reduced fat mass. IRMOE mice also show higher basal phosphorylation of IR, IRS-1, and Akt in muscle and improved glucose tolerance compared with controls. When challenged with high-fat diet (HFD), IRMOE mice are protected from diet-induced obesity. This is associated with reduced inflammation in fat and liver, improved glucose tolerance, and improved systemic insulin sensitivity. Surprisingly, however, in both chow and HFD-fed mice, insulin-stimulated Akt phosphorylation is significantly reduced in muscle of IRMOE mice, indicating postreceptor insulin resistance. RNA sequencing reveals downregulation of several postreceptor signaling proteins that contribute to this resistance. Thus, enhancing early insulin signaling in muscle by overexpression of the IR protects mice from diet-induced obesity and its effects on glucose metabolism. However, chronic overstimulation of this pathway leads to postreceptor desensitization, indicating the critical balance between normal signaling and hyperstimulation of the insulin signaling pathway.

The RabGAPs TBC1D1 and TBC1D4 Control Uptake of Long-Chain Fatty Acids Into Skeletal Muscle via Fatty Acid Transporter SLC27A4/FATP4.

The two closely related RabGTPase-activating proteins (RabGAPs) TBC1D1 and TBC1D4 play a crucial role in the regulation of GLUT4 translocation in response to insulin and contraction in skeletal muscle. In mice, deficiency in one or both RabGAPs leads to reduced insulin- and contraction-stimulated glucose uptake and to elevated fatty acid (FA) uptake and oxidation in both glycolytic and oxidative muscle fibers without altering mitochondrial copy number and the abundance of proteins for oxidative phosphorylation. Here we present evidence for a novel mechanism of skeletal muscle lipid utilization involving the two RabGAPs and the FA transporter SLC27A4/FATP4. Both RabGAPs control the uptake of saturated and unsaturated long-chain FAs (LCFAs) into skeletal muscle and knockdown (Kd) of a subset of RabGAP substrates, Rab8, Rab10, or Rab14, decreased LCFA uptake into these cells. In skeletal muscle from Tbc1d1 and Tbc1d4 knockout animals, SLC27A4/FATP4 abundance was increased and depletion of SLC27A4/FATP4 but not FAT/CD36 completely abrogated the enhanced FA oxidation in RabGAP-deficient skeletal muscle and cultivated C2C12 myotubes. Collectively, our data demonstrate that RabGAP-mediated control of skeletal muscle lipid metabolism converges with glucose metabolism at the level of downstream RabGTPases and involves regulated transport of LCFAs via SLC27A4/FATP4.

Absence of complement factor H reduces physical performance in C57BL6 mice

Complement (C) system is a double edge sword acting as the first line of defense on the one hand and causing aggravation of disease on the other. C activation when unregulated affects different organs including muscle regeneration. However, the effect of factor H (FH), a critical regulator of the alternative C pathway in muscle remains to be studied. FH deficiency results in excessive C activation and generates proinflammatory fragments C5a and C3a as byproducts. C3a and C5a signal through their respective receptors, C5aR and C3aR. In this study, we investigated the role of FH and downstream C5a/C5aR signaling in muscle architecture and function. Using the FH knockout (fh-/-) and fh-/-/C5aR-/double knockout mice we explored the role of C, specifically the alternative C pathway in muscle dysfunction. Substantial C3 and C9 deposits occur along the walls of the fh-/- muscle fibers indicative of unrestricted C activation. Physical performance assessments of the fh-/- mice show reduced grip endurance (76 %), grip strength (14 %) and rotarod balance (36 %) compared to controls. Histological analysis revealed a shift in muscle fiber populations indicated by an increase in glycolytic MHC IIB fibers and reduction in oxidative MHC IIA fibers. Consistent with this finding, mitochondrial DNA (mtDNA) and citrate synthase (CS) expression were both reduced indicating possible reduction in mitochondrial biomass. In addition, our results showed a significant increase in TGFβ expression and altered TGFβ localization in this setting. The architecture of cytoskeletal proteins actin and vimentin in the fh-/- muscle was changed that could lead to contractile weakness and loss of skeletal muscle elasticity. The muscle pathology in fh-/- mice was reduced in fh-/-/C5aR-/- double knockout (DKO) mice, highlighting partial C5aR dependence. Our results for the first time demonstrate an important role of FH in physical performance and skeletal muscle health.

Caveolin-3: A Causative Process of Chicken Muscular Dystrophy.

The etiology of chicken muscular dystrophy is the synthesis of aberrant WW domain containing E3 ubiquitin-protein ligase 1 (WWP1) protein made by a missense mutation of WWP1 gene. The β-dystroglycan that confers stability to sarcolemma was identified as a substrate of WWP protein, which induces the next molecular collapse. The aberrant WWP1 increases the ubiquitin ligase-mediated ubiquitination following severe degradation of sarcolemmal and cytoplasmic β-dystroglycan, and an erased β-dystroglycan in dystrophic αW fibers will lead to molecular imperfection of the dystrophin-glycoprotein complex (DGC). The DGC is a core protein of costamere that is an essential part of force transduction and protects the muscle fibers from contraction-induced damage. Caveolin-3 (Cav-3) and dystrophin bind competitively to the same site of β-dystroglycan, and excessive Cav-3 on sarcolemma will block the interaction of dystrophin with β-dystroglycan, which is another reason for the disruption of the DGC. It is known that fast-twitch glycolytic fibers are more sensitive and vulnerable to contraction-induced small tears than slow-twitch oxidative fibers under a variety of diseased conditions. Accordingly, the fast glycolytic αW fibers must be easy with rapid damage of sarcolemma corruption seen in chicken muscular dystrophy, but the slow oxidative fibers are able to escape from these damages.

Cytological analysis of integumentary and muscular adaptations in three sand-dwelling marine teleosts, Ammodytes tobianus (Ammodytidae), Gorgasia preclara (Congridae) and Heteroconger hassi (Congridae) (Teleostei; Actinopterygii).

Sandy bottoms are a ubiquitous environment found from sea bottoms to intertidal and freshwater zones. They are inhabited by many invertebrates and vertebrates which have developed morphological and physiological adaptations to sustain life under these particular conditions. Sandy habitats exhibit three potential constraints: abrasion, hypoxia and mechanical resistance. Here, three teleost species living in sandy environments were investigated: Ammodytes tobianus (Ammodytidae), Gorgasia preclara and Heteroconger hassi (Congridae). These teleost fishes were studied for their integument and muscular systems, which are potentially subject to sand abrasion and hypoxia, respectively. Based on histochemistry and transmission electron microscopy, we found the complex mucus system of G. preclara and H. hassi consists of two types of goblet cells and one type of sacciform cell. The secretions of both species are made of complex polysaccharides. In contrast, the scaly integument of A. tobianus has only a few goblet cells and no sacciform cells. We also highlighted, by immunohistochemistry, that the epidermal cell proliferation was much higher for this latter species, potentially resulting from the high rate of sand abrasion when A. tobianus buries itself quickly in the substrate. For all species, the major muscle fibre type was revealed by histoenzymology and corresponds to fast glycolytic fibres followed by intermediate fibres with slow fibres in the lowest proportion. Ammodytes tobianus possesses the highest fast fibre proportion (about 87% for A. tobianus and 75–78% for both garden eels). Our results provide new insights into the previously poorly studied teleost species, such as G. preclara, and allow us to highlight the complex skin histology of both garden eel species. Furthermore, the previously unknown muscle typing of these three species was determined.

The effect of aerobic exercise on muscle tissue in patients with severe heart failure and normal body weight

Aim. To assess the response of skeletal muscle and myocardium to original aerobic exercise (AE) program in patients with heart failure (HF) with reduced ejection fraction (HFrEF); to assess morphometric changes in skeletal muscle fiber after AE.Material and methods. The study included 100 patients with class III HFrEF (age — 52±5,2 years; body mass index (BMI) — 23,5±2,8 kg/m2). At baseline and after 6 months of AE, an echocardiogram, peak oxygen uptake (VO2peak), exercise tolerance and quality of life (QOL) were evaluated. Lactate dehydrogenase (LDH) and alkaline phosphatase (ALP) activity were evaluated in biopsy material of lower leg muscles.Results. After 6 months of AE, the left ventricular ejection fraction (LVEF) increased by 10,5±2,3%, QOL — by 24,8±3,5 points, exercise tolerance — by 9,7±0,5 points, VO2peak — by 5,2±0,5 ml/min/kg (p1,2,3,4&lt; 0,05). In 6 patients, the diameter of muscle fiber decreased slightly. The activity of ALP (initially — 0,33±0,09 D) increased by 24,2% (p&lt; 0,05); LDH in glycolytic fibers was initially 0,213±0,08 D, in oxidative fibers — 0,083±0,04, and after 6 months of AE, decreased by 24,4% and 6,0%, respectively (p1 &lt;0,05, p2 &gt;0,05). A positive relationship was found between the dynamics of HF class and fiber diameter (r=0,4, p=0,05); an increase in сardiopulmonary exercise test was associated with ALP activity (r=0,5, p=0,05).Conclusion. 1. Dosed aerobic exercise in patients with stable class III HFrEF, normal BMI, based on reaching the lactate threshold, had a positive effect on LVEF, QOL, exercise tolerance and VO2peak. 2. With exercise training, a decrease in fiber diameter and LDH activity in both oxidative and glycolytic fibers, an increase in ALP activity were revealed. 3. The functional relationship between the increase in exercise tolerance and ALP content in muscle tissue was revealed.

MiRNA\u2013mRNA network regulation in the skeletal muscle fiber phenotype of chickens revealed by integrated analysis of miRNAome and transcriptome

Skeletal muscle fibers are primarily categorized into oxidative and glycolytic fibers, and the ratios of different myofiber types are important factors in determining livestock meat quality. However, the molecular mechanism for determining muscle fiber types in chickens was hardly understood. In this study, we used RNA sequencing to systematically compare mRNA and microRNA transcriptomes of the oxidative muscle sartorius (SART) and glycolytic muscle pectoralis major (PMM) of Chinese Qingyuan partridge chickens. Among the 44,705 identified mRNAs in the two types of muscles, 3,457 exhibited significantly different expression patterns, including 2,364 up-regulated and 1,093 down-regulated mRNAs in the SART. A total of 698 chicken miRNAs were identified, including 189 novel miRNAs, among which 67 differentially expressed miRNAs containing 42 up-regulated and 25 down-regulated miRNAs in the SART were identified. Furthermore, function enrichment showed that the differentially expressed mRNAs and miRNAs were involved in energy metabolism, muscle contraction, and calcium, peroxisome proliferator-activated receptor (PPAR), insulin and adipocytokine signaling. Using miRNA-mRNA integrated analysis, we identified several candidate miRNA-gene pairs that might affect muscle fiber performance, viz, gga-miR-499-5p/SOX6 and gga-miR-196-5p/CALM1, which were supported by target validation using the dual-luciferase reporter system. This study revealed a mass of candidate genes and miRNAs involved in muscle fiber type determination, which might help understand the molecular mechanism underlying meat quality traits in chickens.

Adult-Onset Myopathy with Constitutive Activation of Akt following the Loss of hnRNP-U.

SummarySkeletal muscle has the remarkable ability to modulate its mass in response to changes in nutritional input, functional utilization, systemic disease, and age. This is achieved by the coordination of transcriptional and post-transcriptional networks and the signaling cascades balancing anabolic and catabolic processes with energy and nutrient availability. The extent to which alternative splicing regulates these signaling networks is uncertain. Here we investigate the role of the RNA-binding protein hnRNP-U on the expression and splicing of genes and the signaling processes regulating skeletal muscle hypertrophic growth. Muscle-specific Hnrnpu knockout (mKO) mice develop an adult-onset myopathy characterized by the selective atrophy of glycolytic muscle, the constitutive activation of Akt, increases in cellular and metabolic stress gene expression, and changes in the expression and splicing of metabolic and signal transduction genes. These findings link Hnrnpu with the balance between anabolic signaling, cellular and metabolic stress, and physiological growth.

Muscle BDNF: a potential therapeutic target for Kennedy's disease.

Muscle BDNF: a potential therapeutic target for Kennedy's disease.

The acute effects of cigarette smoke exposure on muscle fiber type dynamics in rats.

Reduced exercise capacity is common in people with chronic obstructive pulmonary diseases (COPD) and chronic smokers and is suggested to be related to skeletal muscle dysfunction. Previous studies using human muscle biopsies have shown fiber-type shifting in chronic smokers particularly those with COPD. These results, however, are confounded with aging effects because people with COPD tend to be older. In the present study, we implemented an acute 7-day cigarette smoke-exposed model using Sprague-Dawley rats to evaluate early effects of cigarette smoking on soleus muscles. Rats (n = 5 per group) were randomly assigned to either a sham air (SA) or cigarette smoking (CS) groups of three different concentrations of total particulate matters (TPM) (CSTPM2.5, CSTPM5, CSTPM10). Significantly lower percentages of type I and higher type IIa fiber were detected in the soleus muscle in CS groups when compared with SA group. Of these, only CSTMP10 group exhibited significantly lower citrate synthase activity and higher muscle tumor necrosis factor-α level than that of SA group. Tumor necrosis factor-α level was correlated with the percentage of type I and IIa fibers. However, no significant between-group differences were found in fiber cross-sectional area, physical activities, or lung function assessments. In conclusion, acute smoking may directly trigger the onset of glycolytic fiber type shift in skeletal muscle independent of aging.

Coordinated development of the mitochondrial reticulum in skeletal muscle

Skeletal muscle cells form a mitochondrial network, or reticulum, in a fiber‐type specific manner with grid‐like and fiber‐perpendicular mitochondrial network configurations found in oxidative and glycolytic fibers, respectively. However, it is not well characterized how the networks form during postnatal muscle development. Using focused ion beam scanning electron microscopy (FIB‐SEM) and machine learning based 3D image segmentation and analysis, we have sought to understand the 3D mitochondrial development in oxidative (soleus) and glycolytic (gastrocnemius) muscles of mice at different postnatal days 1 (P1), 14 (P14), and 42 (P42). Both oxidative and glycolytic fibers show tortuous, parallel mitochondrial formations at P1. At P14, oxidative fibers have more elongated, fiber‐parallel mitochondria, while glycolytic fibers have a more grid‐like mitochondrial reticulum. To better understand the intra‐ and inter‐organelle interactions of the mitochondrial reticulum during development, we additionally assessed intermitochondrial junctions (IMJ) and mitochondrial interactions with lipid droplets (LD). During early maturation (P1‐P14), many IMJ in oxidative fibers are arranged in both diagonal (~40%) and parallel (~40%) directions, yet matured oxidative fibers have almost half (~50%) of IMJ in the perpendicular direction. In contrast, after having ~50% of IMJ along the parallel axis at P1, glycolytic fibers gradually change IMJ orientations more into both perpendicular and diagonal axis and IMJ are located in all directions at P42. Although mitochondria in glycolytic fibers maintain similar sizes of the IMJ contact sites between P1 and P14 and have smaller areas at P42, oxidative fibers have ~2‐fold larger IMJ areas at P14 as compared to early (P1) and matured (P42) fibers. Meanwhile, both oxidative and glycolytic fibers at P1 have many, big LD that sharply decrease in size during the course of postnatal development. In the matured muscles, oxidative fibers have few, tiny LD and glycolytic fibers have no LD. Both fibers maintain similar LD contract areas per mitochondrion across P1‐P42. Notably, mitochondria directly in contact with LD have volumes ~2–3‐fold larger than non‐LD‐contacted mitochondria at all time points. In addition, the LD‐contacted mitochondria have ~2‐fold bigger IMJ areas as compared to non‐contacted one, with the largest differences observed at P14. In conclusion, development of the muscle mitochondrial reticulum occurs in coordination with altered interactions among mitochondria and with LD. However, future studies are warranted to study possible interactive roles of calcium handling structures such as sarcoplasmic reticulum and t‐tubules.Support or Funding InformationThis study is supported by intramural programs within the National Heart, Lung, and Blood Institute and the National Institute of Arthritis and Musculoskeletal and Skin Diseases (grant 1ZIAHL006221‐02).

Exercise promotes satellite cell contribution to myofibers in a load‐dependent manner

Satellite cells (SCs) are required for muscle repair following injury and are involved in muscle remodeling during exercise. Increased muscular activity stimulates SC accumulation and function, but to what extend alternations in mechanical load control SC fusion remains to be elucidated. To investigate SC contribution to myofibers during exercise training, we labeled SCs by excising tdTomato (mT) in Pax7CreERT2/+; Rosa26mTmG/+ mice, resulting in an mEGFP (mG) labeling of all PAX7+ cells and derived myofibers (Figure 1A). Using this genetic lineage tracing model, we here show that SCs extensively contribute to myofibers during exercise and that this occurs exclusively in hindlimb plantar flexors. Upon 8‐weeks of voluntary free running (VRun), 60% of all m. soleus myofibers had at least one SC fused (mG+). Moreover, adding load to the wheel (VResRun) increased the number of mG+ myofibers to 80%, indicating that SC fusion during exercise is load dependent (Figure 1B). Since only the VResRun showed substantial muscle hypertrophy in both m. soleus and m. plantaris, this data suggests that next to hypertrophy, also endurance‐training adaptions are mirrored by extensive SC fusion. Finally, exercise training induced a shift towards an oxidative fiber type, but SC fusion was similar in oxidative and glycolytic fibers. Collectively, we show that voluntary running strongly induces SC fusion in both free and resistance running conditions, but the latter only induced muscle hypertrophy. Thus, our findings suggest that SC fusion cannot only serve hypertrophy, since we also observed significant contribution under ‘endurance’ settings.Support or Funding InformationInternal funding ETH ZurichExercise induces satellite cell fusion in a load‐dependent manner. (A) mouse model and experimental protocol. (B) mG expression in mouse hindlimb muscle under sedentary (control) and running (VRun and VResRun) conditions. *** p&lt;0.001, ** p&lt;0.01.Figure 1

Metabolism as a Target for Age‐Related Muscle Mass and Functional Loss

The progressive age‐related loss of skeletal muscle mass and function, known as sarcopenia, increases the risk for impaired mobility, falls, fractures, and mortality. Currently, there are no pharmacological interventions to prevent or treat sarcopenia. Here we show that stimulation of adiponectin signaling in skeletal muscle by AdipoRon, a pan‐adiponectin receptor agonist, stimulates pathways linked to exercise and improves muscle function in old mice.Intravenous delivery of AdipoRon (1.2mg/kg bw; 3 times per week, 6 weeks) activated the AMPK‐PGC‐1a axis in skeletal muscle and induced changes in fiber type distribution in young (5 months) and old (24 months) male C57Bl6/J mice. In old mice, AdipoRon treatment improved running endurance and muscle contractile force. To explore mechanisms and enhance translational potential, the response to oral delivery through diet of AdipoRon (10, 25, or 50 mg/kg; daily feeding, 16 weeks) was investigated in young male and female C3B6F1 mice. AdipoRon enhanced insulin sensitivity and decreased body mass in females, however, these changes were not displayed in males. Increased expression of Ppargc1a and ACADM were observed in EDL of male mice in response to AdipoRon treatment and a decrease in expression of PDK4 was observed in both gastrocnemius and soleus indicating a shift toward oxidative metabolism in all muscle types. Conversely, these changes at the gene expression level were not observed in the female mice. Overall, AdipoRon treatment induced changes in the relative proportions of oxidative and glycolytic muscle fibers in EDL, gastrocnemius, and soleus; however, the skeletal muscle remodeling response to AdipoRon was muscle type and sex specific. In both sexes, the greatest response to AdipoRon was detected in EDL, which is the more glycolytic of the three muscle groups investigated. These data uncover differences in the underlying biology of skeletal muscle between males and females that may have implications for sarcopenia treatment. Taken together, our studies show that AdipoRon has potential as a therapeutic for sarcopenia in males and although sex dimorphic in its mechanism of action, is likely to also be effective as a treatment for sarcopenia in females.Support or Funding Information1I01BX003846‐01A2

Developing of a Muscle Atrophy Model

Muscle atrophy is one of the most important and frequent problems observed in patients in Intensive Care Units following a surgery, immobilization, extended periods of bed rest as well as in sports medicine. All of these involves loss of muscle mass and decreased muscle strength which increases the risk for future health problems. Immobilization changes the function as well as structure of the peripheral neuromuscular system, and alters both protein degradation and synthesis. Therefore, effective therapeutic strategies to treat muscle wasting induced by immobilization are urgently required but first is necessary to validate and standardize the immobilization technique. In this work we develop a reproducible well‐characterized injury model using a modified Thomas‐ splint validated with mechanical force measurements, histology (H&amp;E and NADH), morphology and ultrasound imaging. Forty young Wistar male rats were divided into two groups: 1) Control‐ no immobilized extremities (CON; n=10) and 2) Unilateral right hindlimb immobilization at 3, 7 or 14 days (Im 3, Im 7, Im 14; n=10 for each group). Results showed a significant increment in body weight for Im 7 and Im 14 compared to CON. Experiments performed in tibialis anterior (TA) of Im muscle showed morphological changes in muscle weight and muscle diameter compared to CON confirmed by Ultrasound imaging. These changes were accompanied with a decrement in peak isometric tetanic force (Po) of immobilized muscles of 20%, 41% and 28% at 3, 7 and 14 days post injury, respectively. Fatigue time for all Im muscles was higher than CON ones (Im 3= 23%, Im 7=33% and Im 14=42%). Metabolic changes using NADH technique denoted an early increase of oxidative fibers and a decrement of glycolytic fibers at 7 days post‐injury compared to CON. In here, we developed, standardized and characterized an immobilization technique that can be used as a first approach to use effective therapeutic strategies to avoid muscle atrophy.Support or Funding InformationSupported partially by UNAM ‐PAPIIT RA207818 and CONACyT AI‐S‐17636 to KPGP.