Changes In Skeletal Muscle Metabolism Research Articles

Investigating the metabolic changes that accompany skeletal muscle maturation

Skeletal muscle plasticity is imperative for our overall health but diminishes with inactivity and age. As we reach maturity, a shift in nutrient allocation from supporting skeletal muscle hypertrophy to adipogenesis occurs. This pivotal intersection in tissue growth marks the start of a metabolic decline in skeletal muscle and an accumulation of adipose tissue in inactive individuals. If left uncorrected, a loss of skeletal muscle metabolic flexibility can lead to a myriad of metabolic diseases. To define the metabolic changes in skeletal muscle that accompany maturation, samples were collected from the longissimus dorsi (LD, glycolytic muscle), latissimus dorsi (LAT, mixed muscle), and masseter (MS, oxidative muscle) at 20, 53, 87, 120, and 180 days of age from DNA 600 x 241 (DNA Genetics) castrated male pigs that weighed an average of 5.7, 20.8, 42.2, 83.4, and 130.5 kg, respectively. These ages were selected to correspond to tissue growth phases characterized by vital organ growth, intensive muscle hypertrophy, cessation of muscle hypertrophy, and the beginning of unrestricted adipose deposition. Samples were analyzed to determine the abundance of key metabolic enzymes through Western blotting and assess mitochondrial content through real-time PCR. Glucose-6-phosphate dehydrogenase decreased at 87, 120, and 180 d in MS compared to LAT and LD ( P < 0.01), which suggested glycolytic muscles increased nutrient allocation to the pentose phosphate pathway at this time whereas oxidative muscles did not. Moreover, pyruvate dehydrogenase ( P < 0.01) and citrate synthase ( P < 0.01) increased at 120 d, which indicated an increase in metabolites entering oxidative pathways. This increase was accompanied by an increase in mitochondrial DNA at 87 and 120 d ( P < 0.01). Further, glutamic-oxaloacetic transaminase 2 increased between 20 d to 120 d ( P < 0.01) indicating amino acid synthesis peaks at 120 d. These findings have established a metabolic fingerprint associated with the termination of muscle growth and the beginning of adipose tissue growth, which provides the framework for uncovering the regulatory mechanism that modulates nutrient allocation between muscle and fat. Elucidating this mechanism will aid in the discovery of metabolic interventions to combat metabolic diseases. Kansas State University Global Food Systems Seed Grant Program This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Characteristic Metabolic Changes in Skeletal Muscle Due to Vibrio vulnificus Infection in a Wound Infection Model.

Vibrio vulnificus is a bacterium that inhabits warm seawater or brackish water environments and causes foodborne diseases and wound infections. In severe cases, V. vulnificus invades the skeletal muscle tissue, where bacterial proliferation leads to septicemia and necrotizing fasciitis with high mortality. Despite this characteristic, information on metabolic changes in tissue infected with V. vulnificus is not available. Here, we elucidated the metabolic changes in V. vulnificus-infected mouse skeletal muscle using capillary electrophoresis time-of-flight mass spectrometry (CE-TOFMS). Metabolome analysis revealed changes in muscle catabolites and energy metabolites during V. vulnificus infection. In particular, succinic acid accumulated but fumaric acid decreased in the infected muscle. However, the virulence factor deletion mutant revealed that changes in metabolites and bacterial proliferation were abolished in skeletal muscle infected with a multifunctional-autoprocessing repeats-in-toxin (MARTX) mutant. On the other hand, mice that were immunosuppressed via cyclophosphamide (CPA) treatment exhibited a similar level of bacterial counts and metabolites between the wild type and MARTX mutant. Therefore, our data indicate that V. vulnificus induces metabolic changes in mouse skeletal muscle and proliferates by using the MARTX toxin to evade the host immune system. This study indicates a new correlation between V. vulnificus infections and metabolic changes that lead to severe reactions or damage to host skeletal muscle. IMPORTANCE V. vulnificus causes necrotizing skin and soft tissue infections (NSSTIs) in severe cases, with high mortality and sign of rapid deterioration. Despite the severity of the infection, the dysfunction of the host metabolism in skeletal muscle triggered by V. vulnificus is poorly understood. In this study, by using a mouse wound infection model, we revealed characteristic changes in muscle catabolism and energy metabolism in skeletal muscle associated with bacterial proliferation in the infected tissues. Understanding such metabolic changes in V. vulnificus-infected tissue may provide crucial information to identify the mechanism via which V. vulnificus induces severe infections. Moreover, our metabolite data may be useful for the recognition, identification, or detection of V. vulnificus infections in clinical studies.

Muscle-specific deletion of Arid5b causes metabolic changes in skeletal muscle that affect adipose tissue and liver.

Emerging evidence suggests that AT-Rich Interaction Domain 5b (Arid5b) may play a role in energy metabolism in various tissues. To study the metabolic function of Arid5b in skeletal muscle, we generated skeletal muscle-specific Arid5b knockout (Arid5b MKO) mice. We found that Arid5b MKO skeletal muscles preferentially utilized fatty acids for energy generation with a corresponding increase in FABP4 expression. Interestingly, in Arid5b MKO mice, the adipose tissue weight decreased significantly. One possible mechanism for the decrease in adipose tissue weight could be the increase in phospho-HSL and HSL expression in white adipose tissue. While glucose uptake increased in an insulin-independent manner in Arid5b MKO skeletal muscle, glucose oxidation was reduced in conjunction with downregulation of the mitochondrial pyruvate carrier (MPC). We found that glucose was diverted into the pentose phosphate pathway as well as converted into lactate through glycolysis for export to the bloodstream, fueling the Cori cycle. Our data show that muscle-specific deletion of Arid5b leads to changes in fuel utilization in skeletal muscle that influences metabolism in other tissues. These results suggest that Arid5b regulates systemic metabolism by modulating fuel selection.

Noninvasive monitoring of muscle atrophy and bone metabolic disorders using dual-energy X-ray absorptiometry in diabetic mice.

Tracking metabolic changes in skeletal muscle and bone using animal models of diabetes mellitus (DM) provides important insights for the management of DM complications. In this study, we aimed to establish a method for monitoring changes in body composition characteristics, such as fat mass, skeletal muscle mass (lean mass), bone mineral density, and bone mineral content, during DM progression using a dual-energy X-ray absorptiometry (DXA) system in a mouse model of streptozotocin (STZ)-induced type 1 DM. In the DM model, STZ administration resulted in increased blood glucose levels, increased water and food intake, and decreased body weight. Serum insulin levels were significantly decreased on day 30 of STZ administration. The DXA analysis revealed significant and persistent decreases in fat mass, lower limb skeletal muscle mass, and bone mineral content in DM mice. We measured tibialis anterior (TA) muscle weight and performed a quantitative analysis of tibial microstructure by micro-computed tomography imaging in DM mice. The TA muscle weight of DM mice was significantly lower than that of control mice. In addition, the trabecular bone volume fraction, trabecular thickness, trabecular number, and cortical thickness were significantly decreased in DM mice. Pearson's product-moment correlation coefficient analysis showed a high correlation between the DXA-measured and actual body composition. In conclusion, longitudinal measurement of body composition changes using a DXA system may be useful for monitoring abnormalities in muscle and bone metabolism in animal models of metabolic diseases such as DM mice.

AMPK activation is sufficient to increase skeletal muscle glucose uptake and glycogen synthesis but is not required for contraction-mediated increases in glucose metabolism

The AMP-activated protein kinase (AMPK) is a cellular sensor of energetics and when activated in skeletal muscle during contraction can impart changes in skeletal muscle metabolism. Therapeutics that selectively activate AMPK have been developed to lower glucose levels through increased glucose disposal rates as an approach to abrogate the hyperglycemic state of diabetes; however, the metabolic fate of glucose following AMPK activation remains unclear. We have used a combination of in vivo evaluation of glucose homeostasis and ex vivo skeletal muscle incubation to systematically evaluate metabolism following pharmacological activation of AMPK with PF-739, comparing this with AMPK activation through sustained intermittent electrical stimulation of contraction. These methods to activate AMPK result in increased glucose uptake but divergent metabolism of glucose: pharmacological activation results in increased glycogen accumulation while contraction-induced glucose uptake results in increased lactate formation and glucose oxidation. These results provide additional evidence to support a role for AMPK in control of skeletal muscle metabolism and additional insight into the potential for AMPK stimulation with small molecule direct activators.

Longitudinal Changes in Skeletal Muscle Metabolism, Oxygen Uptake, and Myosteatosis During Cardiotoxic Treatment for Early-Stage Breast Cancer

BackgroundWhile cardiotoxic chemotherapy is known to negatively impact cardiac function and hemoglobin levels, the impact on skeletal muscle has been understudied among patients. The purpose was to longitudinally characterize myosteatosis (muscle fat), skeletal muscle metabolism, and oxygen (O2) consumption during cardiotoxic chemotherapy for breast cancer.Patients and MethodsThirty-four patients with stage I-III breast cancer were enrolled before trastuzumab-containing and/or anthracycline-containing chemotherapy. We used magnetic resonance imaging to non-invasively quantify thigh myosteatosis (fat-water imaging), and lower leg metabolism (31P spectroscopy), O2 consumption (custom techniques), and peak power output during single-leg plantarflexion exercise at pre-, mid-, end-chemotherapy, and 1-year. We also measured pulmonary VO2peak and maximal leg press strength.ResultsDuring chemotherapy, VO2peak and leg press strength decreased while peak plantarflexion power output was maintained. At mid-chemotherapy, hemoglobin decreased (16%) and lower leg blood flow increased (37%) to maintain lower leg O2 delivery; exercise Pi:PCr and myosteatosis increased. Between mid- and end-chemotherapy, lower leg O2 extraction (28%) and O2 consumption (21%) increased, while plantarflexion exercise efficiency (watts/O2 consumed) decreased. At one year, VO2peak and leg press strength returned to pre-chemotherapy levels, but lower leg exercise O2 extraction, consumption and Pi:PCr, and myosteatosis remained elevated.ConclusionLower leg skeletal muscle blood flow and O2 extraction adapt to compensate for chemotherapy-related hemoglobin reduction for small muscle mass exercise but are insufficient to maintain large muscle mass exercise (pulmonary VO2peak, leg press strength). The excess O2 required to perform work, increased Pi:PCr ratio and myosteatosis together suggest suppressed fat oxidation during chemotherapy.

Mass spectrometry imaging reveals local metabolic changes in skeletal muscle due to chronic training.

Muscle atrophy is a major health problem that needs effective prevention and treatment approaches. Chronic exercise, an effective treatment strategy for atrophy, promotes muscle hypertrophy, which leads to dynamic metabolic changes; however, the metabolic changes vary among myofiber types. To investigate local metabolic changes due to chronic exercise, we utilized comprehensive proteome and mass spectrometry (MS) imaging analyses. Our training model exhibited hypertrophic features only in glycolytic myofibers. The proteome analyses demonstrated that exercise promoted anabolic pathways, such as protein synthesis, and significant changes in lipid metabolism, but not in glucose metabolism. Furthermore, the fundamental energy sources, glycogen, neutral lipids, and ATP, were sensitive to exercise, and the changes in these sources differed between glycolytic and oxidative myofibers. MS imaging revealed that the lipid composition differs among myofibers; arachidonic acid might be an effective target for promoting lipid metabolism during muscle hypertrophy in oxidative myofibers.

Sex-specific differences in the metabolic enzyme activity and transporter levels in mouse skeletal muscle during postnatal development.

Although sex-associated differences in energy metabolism in adults are well-characterized, developmental sex-specific changes in skeletal muscle metabolism are largely unknown. This study investigated sex differences in high-energy phosphate, glycolytic, and mitochondrial enzyme activities and metabolite transporter protein levels in mouse skeletal muscles during the early postnatal period (day 10), post-weaning (day 28), sexual maturity (day 56), and adult life (day 140). No significant sex-specific differences were observed on days 10 and 28, except for glucose transporter (GLUT) 4 level. The hexokinase, phosphofructokinase, and lactate dehydrogenase activities of skeletal muscle were higher and the citrate synthase, cytochrome c oxidase, and β-hydroxyacyl-CoA dehydrogenase activities were lower in female mice than those in male mice on days 56 and 140. The GLUT4 and FAT/CD36 protein levels were higher and the monocarboxylate transporter 4 level was lower in the skeletal muscles of female mice than those of male mice, particularly on days 56 and 140. At 140 days of age, the respiratory exchange ratio during treadmill running (15m/min, 60min) was lower in females than that in males, despite no sex differences at rest. In summary, sex differences were not evident in the early postnatal and post-weaning periods but became apparent after the mice reached sexual maturity. These findings indicate that sexually mature animals are a better model for investigating sex differences, particularly in the context of studying energy metabolism in mice.

Kynurenine as a potential biomarker in detecting reduced muscle endurance: metabolomic profiling of patients with heart failure and exercise intolerance

Abstract Aims Reduced muscle endurance (RME) in patients with HFpEF and HFrEF is associated with structural and metabolic changes in skeletal muscle. We investigated the role of kynurenine (Kyn) as a potential marker in detecting RME. Additionally, we described the likely distorted metabolic pathways in serum in patients with RME and both preserved (HFpEF) and reduced (HFrEF) ejection fraction. Methods Fifty-five participants were prospectively recruited (17 HFpEF, 18 HFrEF outpatients and 20 healthy controls, HC). All participants underwent echocardiography, CPET, isokinetic muscle function tests. Quantification of metabolites in serum was performed using liquid chromatography tandem mass spectrometry. Results In a linear regression, Kyn was an independent predictor for RME after adjusting for alanine, glutamate, ornithine, spermine and short-chain-ACs (B: −8.2 per 1μM increase, 95% CI: −13.01, −3.30, p=0.001). Kyn showed 83% sensitivity and 70% specificity (area under the curve 0.83) in detecting RME. Patients with RME and HFpEF showed reduced levels of long-chain-, medium-chain-, medium-/long-chain-ACs ratios and alanine (p<0.05). In patients with RME and HFrEF we observed reduced concentrations of AAs (p<0.05). Compared to HC, patients with HFpEF and HFrEF had reduced amino acid (AA)-concentrations except for branched-chain and aromatic AAs, and higher concentrations of acylcarnitines (ACs) and Kyn (p<0.05). Conclusions Kyn shows high potential as biomarker for detecting RME. RME was associated with impaired fatty acid oxidation rates in HFpEF patients and with reduced concentrations of AAs in those with HFrEF. Funding Acknowledgement Type of funding sources: None.

Hydroxy Selenomethionine Improves Meat Quality through Optimal Skeletal Metabolism and Functions of Selenoproteins of Pigs under Chronic Heat Stress.

Chronic heat stress (CHS) induces metabolic changes in skeletal muscle from growth to maintenance that jeopardizes growth performance, carcass traits, and meat quality of pigs. We investigated the protective effect of dietary organic selenium (hydroxy-4-methylselenobutanoic acid, OH-SeMet) on CHS-induced skeletal muscle damages of growing pigs, and the corresponding responses of selenoproteins. A total of 40 ((Landrace ×Yorkshire) × Duroc) pigs with an average live weight of 49.64 ± 2.48 kg were used in this 4-week trial. Pigs were randomly allotted to 5 groups: The control group was raised on a basal diet in a thermoneutral environment (22 ± 2 °C); and four CHS groups were raised on a basal diet and supplemented with Se 0.0, 0.2, 0.4, and 0.6 mg/kg as OH-SeMet, respectively, in hyperthermal condition (33 ± 2 °C). CHS resulted in significant decrease of growth performance, carcass traits, and meat quality, which were associated with reduced (p < 0.05) serum alkaline phosphatase (ALP) and total superoxide dismutase (T-SOD) and increased (p < 0.05) serum creatine (CK), sarcous heat shock protein 70 (HSP70), glucokinase (GCK), phosphoenolpyruvate carboxykinase (PEPCK), and malondialdehyde (MDA) contents. Meanwhile, four metabolism-related genes and seven selenoprotein encoding genes were abnormally expressed in skeletal muscle. Dietary OH-SeMet addition partially alleviated the negative impact of CHS on carcass traits and improved meat quality. These improvements were accompanied by the increase in Se deposition, the anti-oxidative capacity of serum and muscle, and protein abundance of GPX1, GPX3, GPX4, and SELENOP. Supplementation with 0.6 mg Se/kg (OH-SeMet) restored the sarcous PEPCK, and 0.4 and 0.6 mg Se/kg (OH-SeMet) restored all abnormally expressed metabolism-related and selenoprotein encoding genes. In summary, dietary supplementation with OH-SeMet beyond Se requirement mitigated CHS-induced depression of carcass traits and meat quality of pigs associated with optimal skeletal metabolism, enhanced antioxidant capacity, and regulation of selenoproteins in skeletal muscle of pigs.

Skeletal muscle energy metabolism in obesity.

Comparing energy metabolism in human skeletal muscle and primary skeletal muscle cells in obesity, while focusing on glucose and fatty acid metabolism, shows many common changes. Insulin-mediated glucose uptake in skeletal muscle and primary myotubes is decreased by obesity, whereas differences in basal glucose metabolism are inconsistent among studies. With respect to fatty acid metabolism, there is an increased uptake and storage of fatty acids and a reduced complete lipolysis, suggesting alterations in lipid turnover. In addition, fatty acid oxidation is decreased, probably at the level of complete oxidation, as -oxidation may be enhanced in obesity, which indicates mitochondrial dysfunction. Metabolic changes in skeletal muscle with obesity promote metabolic inflexibility, ectopic lipid accumulation, and formation of toxic lipid intermediates. Skeletal muscle also acts as an endocrine organ, secreting myokines that participate in interorgan cross talk. This review highlights interventions and some possible targets for treatment through action on skeletal muscle energy metabolism. Effects of exercisein vivoon obesity have been compared with simulation of endurance exercisein vitroon myotubes (electrical pulse stimulation). Possible pharmaceutical targets, including signaling pathways and drug candidates that could modify lipid storage and turnover or increase mitochondrial function or cellular energy expenditure through adaptive thermogenic mechanisms, are discussed.

Physiological and pathophysiological concentrations of fatty acids induce lipid droplet accumulation and impair functional performance of tissue engineered skeletal muscle.

Fatty acids (FA) exert physiological and pathophysiological effects leading to changes in skeletal muscle metabolism and function, however, in vitro models to investigate these changes are limited. These experiments sought to establish the effects of physiological and pathophysiological concentrations of exogenous FA upon the function of tissue engineered skeletal muscle (TESkM). Cultured initially for 14 days, C2C12 TESkM was exposed to FA-free bovine serum albumin alone or conjugated to a FA mixture (oleic, palmitic, linoleic, and α-linoleic acids [OPLA] [ratio 45:30:24:1%]) at different concentrations (200 or 800 µM) for an additional 4 days. Subsequently, TESkM morphology, functional capacity, gene expression and insulin signaling were analyzed. There was a dose response increase in the number and size of lipid droplets within the TESkM (p < .05). Exposure to exogenous FA increased the messenger RNA expression of genes involved in lipid storage (perilipin 2 [p < .05]) and metabolism (pyruvate dehydrogenase lipoamide kinase isozyme 4 [p < .01]) in a dose dependent manner. TESkM force production was reduced (tetanic and single twitch) (p < .05) and increases in transcription of type I slow twitch fiber isoform, myosin heavy chain 7, were observed when cultured with 200 µM OPLA compared to control (p < .01). Four days of OPLA exposure results in lipid accumulation in TESkM which in turn results in changes in muscle function and metabolism; thus, providing insight ito the functional and mechanistic changes of TESkM in response to exogenous FA.

Bone metastases induce metabolic changes and mitophagy in mice.

What is the central question of this study? Cachexia causes severe changes in skeletal muscle metabolism and function and is a key predictor of negative outcomes in cancer patients: what are the changes in whole animal energy metabolism and mitochondria in skeletal muscle? What is the main finding and its importance? There is decreased whole animal energy expenditure in mice with cachexia. They displayed highly dysmorphic mitochondria and mitophagy in skeletal muscle. Cachexia causes changes in skeletal muscle metabolism. Mice with MDA-MB-231 breast cancer bone metastases and cachexia have decreased whole animal energy metabolism and increased skeletal muscle mitophagy. We examined whole animal energy metabolism by indirect calorimetry in mice with MDA-MB-231 breast cancer bone metastases, and showed decreased energy expenditure. We also examined skeletal muscle mitochondria and found that mitochondria in mice with MDA-MB-231 bone metastases are highly dysmorphic and have altered protein markers of mitochondrial biogenesis and dynamics. In addition, LC3B protein was increased in mitochondria of skeletal muscle from cachectic mice, and colocalized with the mitochondrial protein Tom20. Our data demonstrate the importance of mitophagy in cachexia. Understanding these changes will help contribute to defining treatments for cancer cachexia.

Theaflavins decrease skeletal muscle wasting in disuse atrophy induced by hindlimb suspension in mice.

We previously found that a single dose of theaflavins induced skeletal muscle metabolic changes. In this study, we examined the effect of theaflavins on disuse muscle atrophy model mice by hindlimb suspension. Mice were assigned to 4 groups; ground-vehicle, ground-theaflavins, suspension-vehicle, and suspension-theaflavins, dosed with theaflavins (250 mg/kg/day) for 2 weeks. The peak of myotube size of cross sectional area was significantly moved to the smaller side in the suspension-vehicle group compared with the ground-vehicle group, and these shifts were significantly reduced by the treatment with theaflavins in both soleus and extensor digitorum longus. The level of phosphorylated eukaryotic translation initiation factor 4E-binding protein (4EBP)-1, located downstream of the Akt/mTOR pathway, was significantly different between suspension-vehicle and suspension-theaflavins in soleus. The ratio of forkhead box O (FoxO) 3a to phosphorylated FoxO3a significantly increased in soleus or tended to rise in extensor digitorum longus of suspension-vehicle group compared with ground-vehicle. In contrast, these changes were not observed in suspension-theaflavins group. These results suggested that theaflavins inhibited the progress of disuse muscle atrophy through modulation of protein metabolism.

Exercise intolerance and rapid skeletal muscle energetic decline in human age-associated frailty.

BACKGROUNDPhysical frailty in older individuals is characterized by subjective symptoms of fatigue and exercise intolerance (EI). Objective abnormalities in skeletal muscle (SM) mitochondrial high-energy phosphate (HEP) metabolism contribute to EI in inherited myopathies; however, their presence or link to EI in the frail older adult is unknown.METHODSHere, we studied 3 groups of ambulatory, community-dwelling adults with no history of significant coronary disease: frail older (FO) individuals (81 ± 2.7 years, mean ± SEM), nonfrail older (NFO) individuals (79 ± 2.0 years), and healthy middle-aged individuals, who served as controls (CONT, 51 ± 2.1 years). Lower extremity SM HEP levels and mitochondrial function were measured with 31P magnetic resonance (MR) techniques during graded multistage plantar flexion exercise (PFE). EI was quantified by a 6-minute walk (6MW) and peak oxygen consumption during cardiopulmonary testing (peak VO2).RESULTSDuring graded exercise, FO, NFO, and CONT individuals all fatigued at similar SM HEP levels, as measured by 31P-MR. However, FO individuals fatigued fastest, with several-fold higher rates of PFE-induced HEP decline that correlated closely with shorter exercise duration in the MR scanner and with 6MW distance and lower peak oxygen consumption on cardiopulmonary testing (P < 0.001 for all). SM mitochondrial oxidative capacity was lower in older individuals and correlated with rapid HEP decline but less closely with EI.CONCLUSIONSeveral-fold faster SM energetic decline during exercise occurs in FO individuals and correlates closely with multiple measures of EI. Rapid energetic decline represents an objective, functional measure of SM metabolic changes and a potential new target for mitigating frailty-associated physical limitations.FUNDINGThis work was supported by NIH R21 AG045634, R01 AG063661, R01 HL61912, the Johns Hopkins University Claude D. Pepper Older Americans Independence Center P30AG021334, and the Clarence Doodeman Endowment in Cardiology at Johns Hopkins.

Early Dysfunctions in Amyotrophic Lateral Sclerosis: Pathogenetic Mechanisms and the Role in Disease Initiation

Amyotrophic lateral sclerosis (ALS) is a progressive incurable neurodegenerative disease, which is characterized by selective death of the motoneurons of the spinal cord and brain. Currently, ALS is considered as a disease with a wide range of etiological causes, which trigger the cellular and molecular mechanisms leading to the development of similar clinical symptoms associated with degeneration of motoneurons. At the same time, there is no single hypothesis combining various pathogenetic mechanisms, which makes it difficult to develop an effective neuroprotective therapy. This review is devoted to the analysis of the early pathogenetic mechanisms of ALS and their possible role in the initiation of disease on the basis of the published data and our own research. The data on participation of various elements of the motoneuron–skeletal muscle system and membrane disorders in the initiation and development of ALS in humans and in animal models are presented. The analysis of scientific results of the last decades allows us to conclude that a key role in the initiation of pathogenetic mechanisms of ALS is played by dysfunction/destruction of neuromuscular synapses, distal axonopathy, and changes in skeletal muscle metabolism; the methods for correction of these disorders are promising for the development of therapeutic strategies in ALS.

Adipose Tissue Inflammation Is Directly Linked to Obesity-Induced Insulin Resistance, while Gut Dysbiosis and Mitochondrial Dysfunction Are Not Required.

Obesity is associated with adipose tissue hypertrophy, systemic inflammation, mitochondrial dysfunction, and intestinal dysbiosis. Rodent models of high-fat diet (HFD)-feeding or genetic deletion of multifunctional proteins involved in immunity and metabolism are often used to probe the etiology of obesity; however, these models make it difficult to divorce the effects of obesity, diet composition, or immunity on endocrine regulation of blood glucose. We, therefore, investigated the importance of adipose inflammation, mitochondrial dysfunction, and gut dysbiosis for obesity-induced insulin resistance using a spontaneously obese mouse model. We examined metabolic changes in skeletal muscle, adipose tissue, liver, the intestinal microbiome, and whole-body glucose control in spontaneously hyperphagic C57Bl/6J mice compared to lean littermates. A separate subset of lean and obese mice was subject to 8 weeks of obesogenic HFD feeding, or to pair feeding of a standard rodent diet. Hyperphagia, obesity, adipose inflammation, and insulin resistance were present in obese mice despite consuming a standard rodent diet, and these effects were blunted with caloric restriction. However, hyperphagic obese mice had normal mitochondrial respiratory function in all tissues tested and no discernable intestinal dysbiosis relative to lean littermates. In contrast, feeding mice an obesogenic HFD altered the composition of the gut microbiome, impaired skeletal muscle mitochondrial bioenergetics, and promoted poor glucose control. These data show that adipose inflammation and redox stress occurred in all models of obesity, but gut dysbiosis and mitochondrial respiratory dysfunction are not always required for obesity-induced insulin resistance. Rather, changes in the intestinal microbiome and mitochondrial bioenergetics may reflect physiological consequences of HFD feeding.

Thyroid Hormone Status Regulates Skeletal Muscle Response to Chronic Motor Nerve Stimulation.

Although both exercise and thyroid hormone (TH) status can cause cellular and metabolic changes in skeletal muscle, the impact of TH status on exercise-associated changes is not well understood. Here, we examined the effects of TH status on muscle fiber type, cell signaling, and metabolism in a rabbit model of exercise training – chronic motor nerve stimulation (CMNS). Five rabbits were rendered hypothyroid for 7–8 weeks and three rabbits were made hyperthyroid for 2 weeks prior to CMNS of the left peroneal nerve for 10 days. We then measured markers of muscle fiber type, autophagy, and nutrient- or energy-sensing proteins, and metabolic intermediates. CMNS increased MHC-I expression in hypothyroid rabbits, whereas it was unchanged in hyperthyroid rabbits. CMNS also increased p-AMPK, p-ATGL, CPT-1α, p-Akt, GLUT4, and p-70S6K in hypothyroid rabbits. In contrast, p-AMPK and p-AKT were increased at baseline in hyperthyroid rabbits, but CMNS did not further increase them or any of the other markers. CMNS also increased TCA cycle and acylcarnitine metabolites in hypothyroid rabbits; whereas, acylcarnitines were already elevated in hyperthyroid rabbits, and were only slightly increased further by CMNS. In summary, CMNS effects on cell signaling and metabolism of skeletal muscle were more pronounced in the hypothyroid than the hyperthyroid state. Interestingly, in the hypothyroid state, CMNS caused concomitant activation of two signaling pathways that are usually reciprocally regulated – AMPK and mTOR signaling – which manifested as increased β-oxidation, MHC-I expression, and protein synthesis. Thus, our findings provide insight into the role of TH status on exercise response in muscle. Our observations suggest that TH status of patients may be an important determinant and predictor of their response to exercise training in skeletal muscle.

PUFA supplementation may confer enhanced metabolic capacity in stimulated skeletal muscle cells

Skeletal muscle is an energetically demanding tissue that catabolizes a variety of different substrates, a phenomenon that is largely activity‐dependent. Lipids are an energetically‐rich macromolecule and, in this light, are advantageous for skeletal muscle performance and maintenance. It has long been established that lipids have dichotomous lasting effects on the body that appear to be, at least in part, species‐specific. While lipid‐related pathologies are largely associated with species such as saturated fatty acids, diacylglycerols, and ceramides, other lipid species, such as various unsaturated fatty acids, confer adaptive changes and may actually protect against diseases. Lipids have previously been linked to increased aerobic capacity in skeletal muscle, though the role of fatty acid quality in this context (e.g., degree of saturation) is less clear. In the present study, we used C2C12 cells, an immortalized line of mouse skeletal muscle cells, to better understand how polyunsaturated fatty acids (PUFAs), in combination with exercise, affect metabolism and protein concentration in skeletal muscle. Following standard proliferation protocols, mature myotubes were differentiated for 7 days in either control media or media supplemented with a 5% addition of linoleic acid (PUFA); additionally, to stimulate contractile activity, a subset of cell cohorts from each media group were exposed to caffeine for 20 minutes per day during differentiation days 4–7. All cells were harvested for protein on differentiation day seven. Protein concentration across cohorts was measured using a Bradford protein assay. Metabolic changes were characterized by running functional enzyme assays for lactate dehydrogenase (LDH) and citrate synthase (CS) to compare anaerobic and aerobic changes, respectively. Preliminary data show that non‐exercised cohorts have higher protein concentration when supplemented PUFA, and that exercise, both with and without PUFA supplementation, decreases protein concentration as compared to respective non‐exercised cohorts. Regarding enzyme activity, PUFA supplementation appears to decrease LDH activity in non‐exercised cells, while PUFA supplementation appears to increase LDH activity in exercised cells; CS activity also appears to change in PUFA supplemented cohorts. Collectively, these preliminary data suggest activity‐induced adaptive changes in skeletal muscle metabolism are contingent upon specific substrate availability, whereby PUFA availability may advantageously prime skeletal muscle for a higher overall metabolic capacity.Support or Funding InformationMcNair Scholars Program and The College of Saint ScholasticaThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Mediators of cachexia in cancer patients.

Alterations in amino acid and protein metabolism-particularly in skeletal muscle-are a key feature of cancer that contributes to the cachexia syndrome. Thus, skeletal muscle protein turnover is characterized by an exacerbated rate of protein degradation, promoted by an activation of different proteolytic systems that include the ubiquitin-proteasome and the autophagic-lysosomal pathways. These changes are promoted by both hormonal alterations and inflammatory mediators released as a result of the systemic inflammatory response induced by the tumor. Other events, such as alterations in the rate of myogenesis/apoptosis and decreased regeneration potential also affect skeletal muscle in patients with cancer. Mitochondrial dysfunction also contributes to changes in skeletal muscle metabolism and further contributes to the exacerbation of the cancer-wasting syndrome. Different inflammatory mediators-either released by the tumor or by the patient's healthy cells-are responsible for the activation of these catabolic processes that take place in skeletal muscle and in other tissues/organs, such as liver or adipose tissues. Indeed, white adipose tissue is also subject to extensive wasting and "browning" of some of the white adipocytes into beige cells; therefore increasing the energetic inefficiency of the patient with cancer. Recently, an interest in the role of micromRNAs-either free or transported into exosomes-has been related to the events that take place in white adipose tissue during cancer cachexia.