Is skeletal mechanotransduction under genetic control?

Studies of twins have established that peak bone mass is about 70% heritable. The skeletal response to exercise contributes to peak bone mass, as mechanical loading increases skeletal mass during growth and development. It is possible that the skeletal responsiveness to mechanical loading is under genetic control, so that some individuals will build stronger bones with exercise. This appears to be the case in mice. Long bones in mice of the C3H/He strain are largely unresponsive to mechanical loading. Ironically, this strain of mice has very high bone density. Perhaps the genes that regulate BMD are not the same as those that regulate mechanical loading response. Studies of recombinant inbred and congenic strains derived from C3H mice will help to identify genes influencing bone size, density and responsiveness to mechanical loading.     

“Slow” skeletal muscles across vertebrate species

Skeletal muscle fibers are generally classified into two groups: slow (type I) and fast (type II). Fibers in each group are uniquely designed for specific locomotory needs based on their intrinsic cellular properties and the types of motor neurons that innervate them. In this review, we will focus on the current concept of slow muscle fibers which, unlike the originally proposed version based purely on amphibian muscles, varies widely depending on the animal model system studied. We will discuss recent findings from zebrafish neuromuscular junction synapses that may provide the framework for establishing a more unified view of slow muscles across mammalian and non-mammalian species.     

PGC-1β regulates angiogenesis in skeletal muscle

Patients who experience acute ischemic stroke may develop hyperglycemia, even in the absence of diabetes, but the exact mechanisms are still unclear. Adipose tissue secretes numerous proinflammatory cytokines and is involved in the regulation of glucose metabolism. This study aimed to determine the effects of acute stroke on adipose inflammatory cytokine expression. In addition, because sympathetic activity is activated after acute stroke and catecholamines can regulate the expression of several adipocytokines, this study also evaluated whether alterations in adipose proinflammatory cytokines following acute stroke, if any, were medicated by sympathetic system. Acute ischemic brain injury was induced by ligating the right middle cerebral artery and bilateral common carotid arteries in male adult Sprague-Dawley rats. Adipose tumor necrosis factor-α (TNF-α) and monocyte chemoattractant protein-1 (MCP-1) mRNA and protein levels were determined by RT-PCR and enzyme-linked immunoassay, respectively. The stroke rats developed glucose intolerance on days 1 and 2 after cerebral ischemic injury. The fasting blood insulin levels and insulin resistance index measured by homeostasis model assessment were higher in the stroke rats compared with the sham group. Epididymal adipose TNF-α and MCP-1 mRNA and protein levels were elevated one- to twofold, in association with increased macrophage infiltration into the adipose tissue. When the rats were treated with a nonselective β-adrenergic receptor blocker, propranolol, before induction of cerebral ischemic injury, the acute stroke-induced increase in TNF-α and MCP-1 was blocked, and fasting blood insulin concentration and homeostasis model assessment-insulin resistance were decreased. These results suggest a potential role of adipose proinflammatory cytokines induced by the sympathetic nervous system in the pathogenesis of glucose metabolic disorder in rats with acute ischemic stroke.     

Are antioxidants useful for treating skeletal muscle atrophy?

Changes in the skeletal muscle protein mass frequently occur in both physiological and pathological states. Muscle hypotrophy, in particular, is commonly observed during aging and is characteristic of several pathological conditions such as neurological diseases, cancer, diabetes, and sepsis. The skeletal muscle protein content depends on the relative rates of synthesis and degradation, which must be coordinately regulated to maintain the equilibrium. Pathological muscle depletion is characterized by a negative nitrogen balance, which results from disruption of this equilibrium due to reduced synthesis, increased breakdown, or both. The current view, mainly based on experimental data, considers hypercatabolism as the major cause of muscle protein depletion. Several signaling pathways that probably contribute to muscle atrophy have been identified, and there is increasing evidence that oxidative stress, due to reactive oxygen species production overwhelming the intracellular antioxidant systems, plays a role in causing muscle depletion both during aging and in chronic pathological states. In particular, oxidative stress has been proposed to enhance protein breakdown, directly or by interacting with other factors. This review focuses on the possibility of using antioxidant treatments to target molecular pathways involved in the pathogenesis of skeletal muscle wasting.     

β-Adrenergic receptors in innervated and denervated skeletal muscle

In order to determine if the development of β-adrenergic receptors may explain the catecholamine evoked contracture of denervated mammalian skeletal muscle, the binding capacities and dissociation constants of β-adrenergic receptors of innervated and denervated rat skeletal muscle membrane preparations were determined by using [³H] dihydroalprenolol. The dissociation constants of [³H] dihydroalprenolol binding to innervated and denervated muscle microsomal suspensions were similar. The maximal number of binding sites increased from 27 pmol/g protein to 85 pmol/g protein following 25 days denervation. These results suggest that motor nerve may be involved in part, in the regulation of β-adrenergic receptors in skeletal muscle membrane preparations.     

Is rat skeletal muscle hexokinase an allosteric enzyme?

A recent report by M. Gregoriou, I. P. Trayer, and A. Cornish-Bowden (1986, Eur. J. Biochem. 161, 171-176) showed that the mechanism for rat skeletal muscle hexokinase contains two allosteric sites: one for ATP and one for glucose 6-phosphate. In this report, we show that the allosteric mechanism is at variance with a large amount of kinetic data for the skeletal muscle hexokinase reaction in the literature. In addition, the allosteric mechanism conflicts with isotope exchange at chemical equilibrium data reported by M. Gregoriou, I. P. Trayer, and A. Cornish-Bowden (1983, Eur. J. Biochem. 134, 283-288).     

The excitation–contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation–contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca²⁺ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca²⁺ from the SR and transient increase of Ca²⁺ concentration in the myoplasm, (6) activation of the myoplasmic Ca²⁺ buffering system and the contractile apparatus, followed by (7) Ca²⁺ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca²⁺ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na⁺/Ca²⁺ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca²⁺ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca²⁺ sparks.     

Gut microbiota–bile acid–skeletal muscle axis

The gut microbiota represents a ‘metabolic organ’ that can regulate human metabolism. Intact gut microbiota contributes to host homeostasis, whereas compositional perturbations, termed dysbiosis, are associated with a wide range of diseases. Recent evidence demonstrates that dysbiosis, and the accompanying loss of microbiota-derived metabolites, results in a substantial alteration of skeletal muscle metabolism. As an example, bile acids, produced in the liver and further metabolized by intestinal microbiota, are of considerable interest since they regulate several host metabolic pathways by activating nuclear receptors, including the farnesoid X receptor (FXR). Indeed, alteration of gut microbiota may lead to skeletal muscle atrophy via a bile acid–FXR pathway. This Review aims to suggest a new pathway that connects different mechanisms, involving the gut–muscle axis, that are often seen as unrelated, and, starting from preclinical studies, we hypothesize new strategies aimed at optimizing skeletal muscle functionality.     

Nitric oxide synthase II in rat skeletal muscles

Constitutive expression of nitric oxide synthase (NOS) II was found in rat hindlimb muscles by immunohistochemistry and western blotting during development from embryonic day 21 to the adult stage of 75 days. The immunohistochemical NOS II expression pattern was related to the physiological metabolic fibre types SO (slow-oxidative), FOG I, II (fast-oxidative glycolytic; I more glycolytic, II more oxidative) and FG (fast-glycolytic) and to the myosin-based fibre types I and IIA, IIB (IIX not separated) identified in serial sections by enzyme histochemistry and immunohistochemistry. In adult muscles only the small population of FOG II fibres, which is a part of both IIA and IIB fibre population, showed NOS II immunoreactivity. This is the reason that only weak NOS II expression in adult hindlimb muscles has been detected by western blotting. Hindlimb muscles of embryonic, neonatal and young rats of 8 days expressed more NOS II as compared with adult rat hindlimb muscles. This can be explained by the findings that before the age of 21 days fast fibres were metabolically undifferentiated, all of them were NOS II positive and contribute to the NOS II expression of the muscle. In muscles of diabetic rats the NOS II expression was elevated indicating an inhibition of glucose uptake into the muscle fibres of diabetic muscles. Our findings suggest that the NOS II may be designated both as constitutive and inducible.     

Brown fat and skeletal muscle: unlikely cousins?

Although the functions of white fat and brown fat are increasingly well understood, their developmental origins remain unclear. A recent study published in Nature (Seale et al., 2008) identifies a population of progenitor cells that gives rise to brown fat and skeletal muscle but not white fat.     

Quantitation of “autophagic flux” in mature skeletal muscle

Reliable and quantitative assays to measure in vivo autophagy are essential. Currently, there are varied methods for monitoring autophagy; however, it is a challenge to measure “autophagic flux” in an in vivo model system. Conversion and subsequent degradation of the microtubule-associated protein 1 light chain 3 (MAP1-LC3/LC3) to the autophagosome associated LC3-II isoform can be evaluated by immunoblot. However, static levels of endogenous LC3-II protein may render possible misinterpretations since LC3-II levels can increase, decrease or remain unchanged in the setting of autophagic induction. Therefore, it is necessary to measure LC3-II protein levels in the presence and absence of lysomotropic agents that block the degradation of LC3-II, a technique aptly named the “autophagometer.” In order to measure autophagic flux in mouse skeletal muscle, we treated animals with the microtubule depolarizing agent colchicine. Two days of 0.4 mg/kg/day intraperitoneal colchicine blocked autophagosome maturation to autolysosomes and increased LC3-II protein levels in mouse skeletal muscle by >100%. the addition of an autophagic stimulus such as dietary restriction or rapamycin led to an additional increase in LC3-II above that seen with colchicine alone. Moreover, this increase was not apparent in the absence of a “colchicine block.” Using this assay, we evaluated the autophagic response in skeletal muscle upon denervation induced atrophy. Our studies highlight the feasibility of performing an “in vivo autophagometer” study using colchicine in skeletal muscle.Key words: autophagy, rapamycin, skeletal muscle     

Cell polarity: The missing link in skeletal morphogenesis?

Despite extensive genetic analysis of the dynamic multi-phase process that transforms a small population of lateral plate mesoderm into the mature limb skeleton, the mechanisms by which signaling pathways regulate cellular behaviors to generate morphogenetic forces are not known. Recently, a series of papers have offered the intriguing possibility that regulated cell polarity fine-tunes the morphogenetic process via orienting cell axes, division planes and cell movements. Wnt5a-mediated non-canonical signaling, which may include planar cell polarity, has emerged as a common thread in the otherwise distinct signaling networks that regulate morphogenesis in each phase of limb development. These findings position the limb as a key model to elucidate how global tissue patterning pathways direct local differences in cell behavior that, in turn, generate growth and form.     

Distinct amino acid–sensing mTOR pathways regulate skeletal myogenesis

Signaling through the mammalian target of rapamycin (mTOR) in response to amino acid availability controls many cellular and developmental processes. mTOR is a master regulator of myogenic differentiation, but the pathways mediating amino acid signals in this process are not known. Here we examine the Rag GTPases and the class III phosphoinositide 3-kinase (PI3K) Vps34, two mediators of amino acid signals upstream of mTOR complex 1 (mTORC1) in cell growth regulation, for their potential involvement in myogenesis. We find that, although both Rag and Vps34 mediate amino acid activation of mTORC1 in C2C12 myoblasts, they have opposing functions in myogenic differentiation. Knockdown of RagA/B enhances, whereas overexpression of active RagB/C mutants impairs, differentiation, and this inhibitory function of Rag is mediated by mTORC1 suppression of the IRS1-PI3K-Akt pathway. On the other hand, Vps34 is required for myogenic differentiation. Amino acids activate a Vps34-phospholipase D1 (PLD1) pathway that controls the production of insulin-like growth factor II, an autocrine inducer of differentiation, through the Igf2 muscle enhancer. The product of PLD, phosphatidic acid, activates the enhancer in a rapamycin-sensitive but mTOR kinase–independent manner. Our results uncover amino acid–sensing mechanisms controlling the homeostasis of myogenesis and underline the versatility and context dependence of mTOR signaling.     

Sero‐taxonomy of skeletal macromolecules in living Terebratulid brachiopods

Antibodies prepared against macromolecules isolated from the shells of three living brachiopod genera have proved to be of considerable taxonomic significance, in that the pattern of cross‐reactivity of all three antisera consistently points to a new interpretation for the evolution of the largest extant brachiopod order, the Terebratulida. This new molecular evidence actually complements rather than contradicts the existing morphology‐based taxonomy, since detailed systematic investigation of the taxa in question has already demonstrated subtle but significant morphological differences in the major taxonomic characters which appear to reflect this new interpretation.

As fragments of skeletal macromolecules, including antigenic determinants, are known to survive for many millions of years within the protected micro‐environments provided by enclosing biominerals, these results suggest that such molecular fossils could well provide important insights on at least the high‐level taxonomic relationships of fossil organisms.     

Caries prevalence in skeletal series--is it possible to compare?

Because of the relationship with subsistence, dental caries is a central issue in paleopathological research. Usually, comparisons between caries prevalence exhibited in different skeletal series are made. Dietary variation is the most common explanation for cavities prevalence. The aim of this paper is to verify if it is possible to compare caries prevalence reported on papers for archaeological skeletal series. Another goal is to determine if other factors besides diet are implicated in dental cavity prevalence explanation. Twenty six papers about dental health with caries prevalences published from 1999 to 2004 were analyzed for completeness. This assessment includes carious lesion diagnosis and characteristics, age, sex and size characteristics of samples, and prevalence calculation method. The majority of the analyzed papers do not provide adequate information in the topics listed above. Only very few implicated factors other than diet as a contributor to caries lesions development.     

Adult stem cells derived from skeletal muscle — biology and potential

Skeletal muscle contains at least two distinct populations of adult stem cells — satellite cells and multipotent muscle-derived stem cells. Monopotential satellite cells are located under the basal lamina of muscle fibers. They are capable of giving rise only to cells of myogenic lineage, which play an important role in the processes of muscle regeneration. Multipotent muscle-derived stem cells are considered to be predecessors of the satellite cells. Under proper conditions, both in vitro and in vivo, they undergo myogenic, cardiogenic, chondrogenic, osteogenic and adipogenic differentiation. The main purpose of the present article is to summarize current information about adult stem cells derived from skeletal muscle, and to discuss their isolation and in vitro expansion techniques, biological properties, as well as their potential for regenerative medicine.