How Do Skeletal Muscles Die? An Overview

Clinical Significance of Carnitine in the Treatment of Cancer: From Traffic to the Regulation

Metabolic reprogramming is a common hallmark of cancer cells. Cancer cells exhibit metabolic flexibility to maintain high proliferation and survival rates. In other words, adaptation of cellular demand is essential for tumorigenesis, since a diverse supply of nutrients is required to accommodate tumor growth and progression. Diversity of carbon substrates fueling cancer cells indicate metabolic heterogeneity, even in tumors sharing the same clinical diagnosis. In addition to the alteration of glucose and amino acid metabolism in cancer cells, there is evidence that cancer cells can alter lipid metabolism. Some tumors rely on fatty acid oxidation (FAO) as the primary energy source; hence, cancer cells overexpress the enzymes involved in FAO. Carnitine is an essential cofactor in the lipid metabolic pathways. It is crucial in facilitating the transport of long-chain fatty acids into the mitochondria for β-oxidation. This role and others played by carnitine, especially its antioxidant function in cellular processes, emphasize the fine regulation of carnitine traffic within tissues and subcellular compartments. The biological activity of carnitine is orchestrated by specific membrane transporters that mediate the transfer of carnitine and its derivatives across the cell membrane. The concerted function of carnitine transporters creates a collaborative network that is relevant to metabolic reprogramming in cancer cells. Here, the molecular mechanisms relevant to the role and expression of carnitine transporters are discussed, providing insights into cancer treatment.

Systemic glucose-insulin-potassium reduces skeletal muscle injury, kidney injury, and pain in a murine ischaemia-reperfusion model

Glucose-insulin-potassium (GIK) is protective following cardiac myocyte ischaemia-reperfusion (IR) injury, however the role of GIK in protecting skeletal muscle from IR injury has not been evaluated. Given the similar mechanisms by which cardiac and skeletal muscle sustain an IR injury, we hypothesized that GIK would similarly protect skeletal muscle viability. A total of 20 C57BL/6 male mice (10 control, 10 GIK) sustained a hindlimb IR injury using a 2.5-hour rubber band tourniquet. Immediately prior to tourniquet placement, a subcutaneous osmotic pump was placed which infused control mice with saline (0.9% sodium chloride) and treated mice with GIK (40% glucose, 50 U/l insulin, 80 mEq/L KCl, pH 4.5) at a rate of 16 µl/hr for 26.5 hours. At 24 hours following tourniquet removal, bilateral (tourniqueted and non-tourniqueted) gastrocnemius muscles were triphenyltetrazolium chloride (TTC)-stained to quantify percentage muscle viability. Bilateral peroneal muscles were used for gene expression analysis, serum creatinine and creatine kinase activity were measured, and a validated murine ethogram was used to quantify pain before euthanasia. GIK treatment resulted in a significant protection of skeletal muscle with increased viability (GIK 22.07% (SD 15.48%)) compared to saline control (control 3.14% (SD 3.29%)) (p = 0.005). Additionally, GIK led to a statistically significant reduction in gene expression markers of cell death (CASP3, p < 0.001) and inflammation (NOS2, p < 0.001; IGF1, p = 0.007; IL-1β, p = 0.002; TNFα, p = 0.012), and a significant reduction in serum creatine kinase (p = 0.004) and creatinine (p < 0.001). GIK led to a significant reduction in IR-related pain (p = 0.030). Systemic GIK infusion during and after limb ischaemia protects murine skeletal muscle from cell death, kidneys from reperfusion metabolites, and reduces pain by reducing post-ischaemic inflammation.

Skeletal muscle alterations in HFrEF vs. HFpEF

PURPOSE OF REVIEW

Severe exercise intolerance and early fatigue are hallmarks of heart failure patients either with a reduced (HFrEF) or a still preserved (HFpEF) ejection fraction. This review, therefore, will provide a contemporary summary of the alterations currently known to occur in the skeletal muscles of both HFrEF and HFpEF, and provide some further directions that will be required if we want to improve our current understanding of this area.

RECENT FINDINGS

Skeletal muscle alterations are well documented for over 20 years in HFrEF, and during the recent years also data are presented that in HFpEF muscular alterations are present. Alterations are ranging from a shift in fiber type and capillarization to an induction of atrophy and modulation of mitochondrial energy supply. In general, the molecular alterations are more severe in the skeletal muscle of HFrEF when compared to HFpEF. The alterations occurring in the skeletal muscle at the molecular level may contribute to exercise intolerance in HFrEF and HFpEF. Nevertheless, the knowledge of changes in the skeletal muscle of HFpEF is still sparsely available and more studies in this HF cohort are clearly warranted.

Efficacy of Age-Specific High-Intensity Stretch-Shortening Contractions in Reversing Dynapenia, Sarcopenia, and Loss of Skeletal Muscle Quality

During the aging process, skeletal muscle performance and physiology undergoes alterations leading to decrements in functional capacity, health-span, and independence. Background: The utility and implementation of age-specific exercise is a paramount research agenda focusing on ameliorating the loss of both skeletal muscle performance and physiology; yet, to date, no consensus exists as to the most appropriate mechanical loading protocol design or overall exercise prescription that best meets this need. Thus, the purpose of this review is to highlight the most optimal type of exercise presently available and provide the most current, evidence-based findings for its efficacy. The hypothesis that high-intensity, stretch-shortening contractions (SSCs)-a form of "resistance-type exercise" training-present as the preferred exercise mode for serving as an intervention-based modality to attenuate dynapenia, sarcopenia, and decreased muscle quality with aging, even restoring the overall youthful phenotype, will be demonstrated. Conclusions: Appreciating the fundamental evidence supporting the use of high-intensity SSCs in positively impacting aging skeletal muscle's responsivity and their use as a specific and sensitive countermeasure is crucial. Moreover, from an applied perspective, SSCs may improve skeletal muscle quality and rejuvenate health-span and, ultimately, lead to augmented functional capacity, independence, and quality of life concomitant with decreased morbidity.

Age-related effect of cell death on fiber morphology and number in tongue muscle

INTRODUCTION

Multiple pathways may exist for age-related tongue muscle degeneration. Cell death is one mechanism contributing to muscle atrophy and decreased function. We hypothesized with aging, apoptosis, and apoptotic regulators would be increased, and muscle fiber size and number would be reduced in extrinsic tongue muscles.

METHODS

Cell death indices, expression of caspase-3 and Bcl-2, and measures of muscle morphology and number were determined in extrinsic tongue muscles of young and old rats.

RESULTS

Significant increases in cell death, caspase-3, and Bcl-2 were observed in all extrinsic tongue muscles along with reductions in muscle fiber number in old rats.

DISCUSSION

We demonstrated that apoptosis indices increase with age in lingual muscles and that alterations in apoptotic regulators may be associated with age-related degeneration in muscle fiber size and number. These observed apoptotic processes may be detrimental to muscle function, and may contribute to degradation of cranial functions with age. Muscle Nerve 57: E29-E37, 2018.

Molecular effects of exercise training in patients with cardiovascular disease: focus on skeletal muscle, endothelium, and myocardium

For decades, we have known that exercise training exerts beneficial effects on the human body, and clear evidence is available that a higher fitness level is associated with a lower incidence of suffering premature cardiovascular death. Despite this knowledge, it took some time to also incorporate physical exercise training into the treatment plan for patients with cardiovascular disease (CVD). In recent years, in addition to continuous exercise training, further training modalities such as high-intensity interval training and pyramid training have been introduced for coronary artery disease patients. The beneficial effect for patients with CVD is clearly documented, and during the last years, we have also started to understand the molecular mechanisms occurring in the skeletal muscle (limb muscle and diaphragm) and endothelium, two systems contributing to exercise intolerance in these patients. In the present review, we describe the effects of the different training modalities in CVD and summarize the molecular effects mainly in the skeletal muscle and cardiovascular system.

Botulinum toxin injection causes hyper-reflexia and increased muscle stiffness of the triceps surae muscle in the rat

Botulinum toxin is used with the intention of diminishing spasticity and reducing the risk of development of contractures. Here, we investigated changes in muscle stiffness caused by reflex activity or elastic muscle properties following botulinum toxin injection in the triceps surae muscle in rats. Forty-four rats received injection of botulinum toxin in the left triceps surae muscle. Control measurements were performed on the noninjected contralateral side in all rats. Acute experiments were performed, 1, 2, 4, and 8 wk following injection. The triceps surae muscle was dissected free, and the Achilles tendon was cut and attached to a muscle puller. The resistance of the muscle to stretches of different amplitudes and velocities was systematically investigated. Reflex-mediated torque was normalized to the maximal muscle force evoked by supramaximal stimulation of the tibial nerve. Botulinum toxin injection caused severe atrophy of the triceps surae muscle at all time points. The force generated by stretch reflex activity was also strongly diminished but not to the same extent as the maximal muscle force at 2 and 4 wk, signifying a relative reflex hyperexcitability. Passive muscle stiffness was unaltered at 1 wk but increased at 2, 4, and 8 wk (P < 0.01). These data demonstrate that botulinum toxin causes a relative increase in reflex stiffness, which is likely caused by compensatory neuroplastic changes. The stiffness of elastic elements in the muscles also increased. The data are not consistent with the ideas that botulinum toxin is an efficient antispastic medication or that it may prevent development of contractures.

Comparative Skeletal Muscle Proteomics Using Two-Dimensional Gel Electrophoresis

The pioneering work by Patrick H. O’Farrell established two-dimensional gel electrophoresis as one of the most important high-resolution protein separation techniques of modern biochemistry (Journal of Biological Chemistry1975, 250, 4007–4021). The application of two-dimensional gel electrophoresis has played a key role in the systematic identification and detailed characterization of the protein constituents of skeletal muscles. Protein changes during myogenesis, muscle maturation, fibre type specification, physiological muscle adaptations and natural muscle aging were studied in depth by the original O’Farrell method or slightly modified gel electrophoretic techniques. Over the last 40 years, the combined usage of isoelectric focusing in the first dimension and sodium dodecyl sulfate polyacrylamide slab gel electrophoresis in the second dimension has been successfully employed in several hundred published studies on gel-based skeletal muscle biochemistry. This review focuses on normal and physiologically challenged skeletal muscle tissues and outlines key findings from mass spectrometry-based muscle proteomics, which was instrumental in the identification of several thousand individual protein isoforms following gel electrophoretic separation. These muscle-associated protein species belong to the diverse group of regulatory and contractile proteins of the acto-myosin apparatus that forms the sarcomere, cytoskeletal proteins, metabolic enzymes and transporters, signaling proteins, ion-handling proteins, molecular chaperones and extracellular matrix proteins.