Comparative sequence analysis of the complete human sarcomeric myosin heavy chain family: implications for functional diversity

Developmental, Physiological and Phylogenetic Perspectives on the Expression and Regulation of Myosin Heavy Chains in Craniofacial Muscles

This review deals with the developmental origins of extraocular, jaw and laryngeal muscles, the expression, regulation and functional significance of sarcomeric myosin heavy chains (MyHCs) that they express and changes in MyHC expression during phylogeny. Myogenic progenitors from the mesoderm in the prechordal plate and branchial arches specify craniofacial muscle allotypes with different repertoires for MyHC expression. To cope with very complex eye movements, extraocular muscles (EOMs) express 11 MyHCs, ranging from the superfast extraocular MyHC to the slowest, non-muscle MyHC IIB (nmMyH IIB). They have distinct global and orbital layers, singly- and multiply-innervated fibres, longitudinal MyHC variations, and palisade endings that mediate axon reflexes. Jaw-closing muscles express the high-force masticatory MyHC and cardiac or limb MyHCs depending on the appropriateness for the acquisition and mastication of food. Laryngeal muscles express extraocular and limb muscle MyHCs but shift toward expressing slower MyHCs in large animals. During postnatal development, MyHC expression of craniofacial muscles is subject to neural and hormonal modulation. The primary and secondary myotubes of developing EOMs are postulated to induce, via different retrogradely transported neurotrophins, the rich diversity of neural impulse patterns that regulate the specific MyHCs that they express. Thyroid hormone shifts MyHC 2A toward 2B in jaw muscles, laryngeal muscles and possibly extraocular muscles. This review highlights the fact that the pattern of myosin expression in mammalian craniofacial muscles is principally influenced by the complex interplay of cell lineages, neural impulse patterns, thyroid and other hormones, functional demands and body mass. In these respects, craniofacial muscles are similar to limb muscles, but they differ radically in the types of cell lineage and the nature of their functional demands.

MYH13, a superfast myosin expressed in extraocular, laryngeal and syringeal muscles

MYH13 is a unique type of sarcomeric myosin heavy chain (MYH) first detected in mammalian extraocular (EO) muscles and later also in vocal muscles, including laryngeal muscles of some mammals and syringeal muscles of songbirds. All these muscles are specialized in generating very fast contractions while producing relatively low force, a design appropriate for muscles acting against a much lower load than most skeletal muscles inserting into the skeleton. The definition of the physiological properties of muscle fibres containing MYH13 has been complicated by the mixed fibre type composition of EO muscles and the coexistence of different MYH types within the same fibre. A major advance in this area came from studies on isolated recombinant myosin motors and the demonstration that the affinity of actin-bound human MYH13 for ADP is much weaker than those of fast-type MYH1 (type 2X) and MYH2 (type 2A). This property is consistent with a very fast detachment of myosin from actin, a major determinant of shortening velocity. The MYH13 gene arose early during vertebrate evolution but was characterized only in mammals and birds and appears to have been lost in some teleost fish. The MYH13 gene is located at the 3' end of the mammalian fast/developmental gene cluster and in a similar position to the orthologous cluster in syntenic regions of the songbird genome. MYH13 gene regulation is controlled by a super-enhancer in the mammalian locus and deletion of the neighbouring fast MYH1 and MYH4 genes leads to abnormal MYH13 expression in mouse leg muscles.

Myofilament dysfunction in diastolic heart failure

Diastolic heart failure (DHF), in which impaired ventricular filling leads to typical heart failure symptoms, represents over 50% of all heart failure cases and is linked with risk factors, including metabolic syndrome, hypertension, diabetes, and aging. A substantial proportion of patients with this disorder maintain normal left ventricular systolic function, as assessed by ejection fraction. Despite the high prevalence of DHF, no effective therapeutic agents are available to treat this condition, partially because the molecular mechanisms of diastolic dysfunction remain poorly understood. As such, by focusing on the underlying molecular and cellular processes contributing to DHF can yield new insights that can represent an exciting new avenue and propose a novel therapeutic approach for DHF treatment. This review discusses new developments from basic and clinical/translational research to highlight current knowledge gaps, help define molecular determinants of diastolic dysfunction, and clarify new targets for treatment.

Expression of MyHC-15 and -2x in human muscle spindles: An immunohistochemical study

To build on the existing data on the pattern of myosin heavy chain (MyHC) isoforms expression in the human muscle spindles, we aimed to verify whether the 'novel' MyHC-15, -2x and -2b isoforms are co-expressed with the other known isoforms in the human intrafusal fibres. Using a set of antibodies, we attempted to demonstrate nine isoforms (15, slow-tonic, 1, α, 2a, 2x, 2b, embryonic, neonatal) in different regions of intrafusal fibres in the biceps brachii and flexor digitorum profundus muscles. The reactivity of some antibodies with the extrafusal fibres was also tested in the masseter and laryngeal cricothyreoid muscles. In both upper limb muscles, the expression of slow-tonic isoform was a reliable marker for differentiating positive bag fibres from negative chain fibres. Generally, bag1 and bag2 fibres were distinguished in isoform 1 expression; the latter consistently expressed this isoform over their entire length. Although isoform 15 was not abundantly expressed in intrafusal fibres, its expression was pronounced in the extracapsular region of bag fibres. Using a 2x isoform-specific antibody, this isoform was demonstrated in the intracapsular regions of some intrafusal fibres, particularly chain fibres. To the best of our knowledge, this study is the first to demonstrate 15 and 2x isoforms in human intrafusal fibres. However, whether the labelling with an antibody specific for rat 2b isoform reflects the expression of this isoform in bag fibres and some extrafusal ones in the specialised cranial muscles requires further evaluation. The revealed pattern of isoform co-expression only partially agrees with the results of previous, more extensive studies. Nevertheless, it may be inferred that MyHC isoform expression in intrafusal fibres varies along their length, across different muscle spindles and muscles. Furthermore, the estimation of expression may also depend on the antibodies utilised, which may also react differently with intrafusal and extrafusal fibres.

Differences in Mechanical, Electrical and Calcium Transient Performance of the Isolated Right Atrial and Ventricular Myocardium of Guinea Pigs at Different Preloads (Lengths)

There are only a few studies devoted to the comparative and simultaneous study of the mechanisms of the length-dependent regulation of atrial and ventricular contractility. Therefore, an isometric force-length protocol was applied to isolated guinea pig right atrial (RA) strips and ventricular (RV) trabeculae, with a simultaneous measurement of force (Frank-Starling mechanism) and Ca2+ transients (CaT) or transmembrane action potentials (AP). Over the entire length-range studied, the duration of isometric contraction, CaT and AP, were shorter in the RA myocardium than in the RV myocardium. The RA myocardium was stiffer than the RV myocardium. With the increasing length of the RA and RV myocardium, the amplitude and duration of isometric contraction and CaT increased, as well as the amplitude and area of the "CaT difference curves" (shown for the first time). However, the rates of the tension development and relaxation decreased. No contribution of AP duration to the heterometric regulation of isometric tension was found in either the RA or RV myocardium of the guinea pig. Changes in the degree of overlap of the contractile proteins of the guinea pig RA and RV myocardium mainly affect CaT kinetics but not AP duration.

Using whole genome sequence findings to assess gene-disease causality in cardiomyopathy and arrhythmia patients

Aim: The genetic etiologies of cardiomyopathies and arrhythmias have not been fully elucidated. Materials & methods: Research findings from genome analyses in a cardiomyopathy and arrhythmia cohort were gathered. Gene-disease relationships from two databases were compared with patient phenotypes. A literature review was conducted for genes with limited evidence. Results: Of 43 genes with candidate findings from 18 cases, 23.3% of genes had never been curated, 15.0% were curated for cardiomyopathies, 16.7% for arrhythmias and 31.3% for other conditions. 25.5% of candidate findings were curated for the patient's specific phenotype with 11.8% having definitive evidence. MYH6 and TPCN1 were flagged for recuration. Conclusion: Findings from genome sequencing in disease cohorts may be useful to guide gene-curation efforts.

Platelet formation and activation are influenced by neuronal guidance proteins

Platelets are anucleate blood cells derived from megakaryocytes. They link the fundamental functions of hemostasis, inflammation and host defense. They undergo intracellular calcium flux, negatively charged phospholipid translocation, granule release and shape change to adhere to collagen, fibrin and each other, forming aggregates, which are key to several of their functions. In all these dynamic processes, the cytoskeleton plays a crucial role. Neuronal guidance proteins (NGPs) form attractive and repulsive signals to drive neuronal axon navigation and thus refine neuronal circuits. By binding to their target receptors, NGPs rearrange the cytoskeleton to mediate neuron motility. In recent decades, evidence has indicated that NGPs perform important immunomodulatory functions and influence platelet function. In this review, we highlight the roles of NGPs in platelet formation and activation.

Developmental, physiologic and phylogenetic perspectives on the expression and regulation of myosin heavy chains in mammalian skeletal muscles

The kinetics of myosin controls the speed and power of muscle contraction. Mammalian skeletal muscles express twelve kinetically different myosin heavy chain (MyHC) genes which provides a wide range of muscle speeds to meet different functional demands. Myogenic progenitors from diverse craniofacial and somitic mesoderm specify muscle allotypes with different repertoires for MyHC expression. This review provides a brief synopsis on the historical and current views on how cell lineage, neural impulse patterns, and thyroid hormone influence MyHC gene expression in muscles of the limb allotype during development and in adult life and the molecular mechanisms thereof. During somitic myogenesis, embryonic and foetal myoblast lineages form slow and fast primary and secondary myotube ontotypes which respond differently to postnatal neural and thyroidal influences to generate fully differentiated fibre phenotypes. Fibres of a given phenotype may arise from myotubes of different ontotypes which retain their capacity to respond differently to neural and thyroidal influences during postnatal life. This gives muscles physiological plasticity to adapt to fluctuations in thyroid hormone levels and patterns of use. The kinetics of MyHC isoforms vary inversely with animal body mass. Fast 2b fibres are specifically absent in muscles involved in elastic energy saving in hopping marsupials and generally absent in large eutherian mammals. Changes in MyHC expression are viewed in the context of the physiology of the whole animal. The roles of myoblast lineage and thyroid hormone in regulating MyHC gene expression are phylogenetically the most ancient while that of neural impulse patterns the most recent.

Functional and developmental changes in the inner hair cell ribbon synapses caused by Myosin VI knockout and deafness-inducing point mutation

Hearing loss is one of the most common neurosensory disorders in humans, and above half of hearing loss is caused by gene mutations. Among more than 100 genes that cause non-syndromic hearing loss, myosin VI (MYO6) is typical in terms of the complexity of underlying mechanisms, which are not well understood. In this study, we used both knock-out (Myo6^-/-) and point mutation (Myo6^C442Y) mice as animal models, performed whole-cell patch-clamp recording and capacitance measurement in the inner hair cells (IHCs) in the cochlea, and sought to reveal potential functional and developmental changes in their ribbon synapses. In Myo6^-/- cochleae of both before (P8-10) and after hearing onset (P18-20), exocytosis from IHCs, measured in whole-cell capacitance change (ΔC_m), was significantly reduced, Ca²⁺ current amplitude (I_Ca) was unchanged, but Ca²⁺ voltage dependency was differently altered, causing significant increase in Ca²⁺ influx in mature IHCs but not in immature IHCs. In immature IHCs of Myo6^C442Y/C442Y cochleae, neither ΔC_m nor I_Ca was altered, but both were reduced in mature IHCs of the same animal model. Furthermore, while the reduction of exocytosis was caused by a combination of the slower rate of depleting readily releasable (RRP) pool of synaptic vesicles and slower sustained release rate (SRR) in Myo6^-/- immature IHCs, it was likely due to smaller RRP and slower SRR in mature IHCs of both animal models. These results expand our understanding of the mechanisms of deafness caused by MYO6 mutations, and provide a solid theoretical and scientific basis for the diagnosis and treatment of deafness.

Proliferation and Maturation: Janus and the Art of Cardiac Tissue Engineering

During cardiac development and morphogenesis, cardiac progenitor cells differentiate into cardiomyocytes that expand in number and size to generate the fully formed heart. Much is known about the factors that regulate initial differentiation of cardiomyocytes, and there is ongoing research to identify how these fetal and immature cardiomyocytes develop into fully functioning, mature cells. Accumulating evidence indicates that maturation limits proliferation and conversely proliferation occurs rarely in cardiomyocytes of the adult myocardium. We term this oppositional interplay the proliferation-maturation dichotomy. Here we review the factors that are involved in this interplay and discuss how a better understanding of the proliferation-maturation dichotomy could advance the utility of human induced pluripotent stem cell-derived cardiomyocytes for modeling in 3-dimensional engineered cardiac tissues to obtain truly adult-level function.

Hypertrophic cardiomyopathy: Mutations to mechanisms to therapies

Hypertrophic cardiomyopathy (HCM) affects more than 1 in 500 people in the general population with an extensive burden of morbidity in the form of arrhythmia, heart failure, and sudden death. More than 25 years since the discovery of the genetic underpinnings of HCM, the field has unveiled significant insights into the primary effects of these genetic mutations, especially for the myosin heavy chain gene, which is one of the most commonly mutated genes. Our group has studied the molecular effects of HCM mutations on human β-cardiac myosin heavy chain using state-of-the-art biochemical and biophysical tools for the past 10 years, combining insights from clinical genetics and structural analyses of cardiac myosin. The overarching hypothesis is that HCM-causing mutations in sarcomere proteins cause hypercontractility at the sarcomere level, and we have shown that an increase in the number of myosin molecules available for interaction with actin is a primary driver. Recently, two pharmaceutical companies have developed small molecule inhibitors of human cardiac myosin to counteract the molecular consequences of HCM pathogenesis. One of these inhibitors (mavacamten) has recently been approved by the FDA after completing a successful phase III trial in HCM patients, and the other (aficamten) is currently being evaluated in a phase III trial. Myosin inhibitors will be the first class of medication used to treat HCM that has both robust clinical trial evidence of efficacy and that targets the fundamental mechanism of HCM pathogenesis. The success of myosin inhibitors in HCM opens the door to finding other new drugs that target the sarcomere directly, as we learn more about the genetics and fundamental mechanisms of this disease.

The Local Environment of Loop Switch 1 Modulates the Rate of ATP-Induced Dissociation of Human Cardiac Actomyosin

Two isoforms of human cardiac myosin, alpha and beta, share significant sequence similarities but show different kinetics. The alpha isoform is a faster motor; it spends less time being strongly bound to actin during the actomyosin cycle. With alpha isoform, actomyosin dissociates faster upon ATP binding, and the affinity of ADP to actomyosin is weaker. One can suggest that the isoform-specific actomyosin kinetics is regulated at the nucleotide binding site of human cardiac myosin. Myosin is a P-loop ATPase; the nucleotide-binding site consists of P-loop and loops switch 1 and 2. All three loops position MgATP for successful hydrolysis. Loops sequence is conserved in both myosin isoforms, and we hypothesize that the isoform-specific structural element near the active site regulates the rate of nucleotide binding and release. Previously we ran molecular dynamics simulations and found that loop S291-E317 near loop switch 1 is more compact and exhibits larger fluctuations of the position of amino acid residues in beta isoform than in alpha. In alpha isoform, the loop forms a salt bridge with loop switch 1, the bridge is not present in beta isoform. Two isoleucines I303 and I313 of loop S291-E317 are replaced with valines in alpha isoform. We introduced a double mutation I303V:I313V in beta isoform background and studied how the mutation affects the rate of ATP binding and ADP dissociation from actomyosin. We found that ATP-induced actomyosin dissociation occurs faster in the mutant, but the rate of ADP release remains the same as in the wild-type beta isoform. Due to the proximity of loop S291-E317 and loop switch 1, a faster rate of ATP-induced actomyosin dissociation indicates that loop S291-E317 affects structural dynamics of loop switch 1, and that loop switch 1 controls ATP binding to the active site. A similar rate of ADP dissociation from actomyosin in the mutant and wild-type myosin constructs indicates that loop switch 1 does not control ADP release from actomyosin.

Identification of MYH6 as the potential gene for human ischaemic cardiomyopathy

The present study aimed to explore the potential hub genes and pathways of ischaemic cardiomyopathy (ICM) and to investigate the possible associated mechanisms. Two microarray data sets (GSE5406 and GSE57338) were downloaded from the Gene Expression Omnibus (GEO) database. The limma package was used to analyse the differentially expressed genes (DEGs). Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment, Disease Ontology (DO) and Gene Ontology (GO) annotation analyses were performed. A protein‐protein interaction (PPI) network was set up using Cytoscape software. Significant modules and hub genes were identified by the Molecular Complex Detection (MCODE) app. Then, further functional validation of hub genes in other microarrays and survival analysis were performed to judge the prognosis. A total of 1065 genes were matched, with an adjusted p < 0.05, and 17 were upregulated and 25 were downregulated with|log₂ (fold change)|≥1.2. After removing the lengthy entries, GO identified 12 items, and 8 pathways were enriched at adjusted p < 0.05 (false discovery rate, FDR set at <0.05). Three modules with a score >8 after MCODE analysis and MYH6 were ultimately identified. When validated in GSE23561, MYH6 expression was lower in patients with CAD than in healthy controls (p < 0.05). GSE60993 data suggested that MYH6 expression was also lower in AMI patients (p < 0.05). In the GSE59867 data set, MYH6 expression was lower in CAD patients than in AMI patients and lower in heart failure (HF) patients than in non‐HF patients. However, there was no difference at different periods within half a year, and HF was increased when MYH6 expression was low (p < 0.05–0.01). We performed an integrated analysis and validation and found that MYH6 expression was closely related to ICM and HF. However, whether this marker can be used as a predictor in blood samples needs further experimental verification.

Regulation of the Expression of the Myosin Heavy Chain (MYH) Gene myh14 in Zebrafish Development

The human sarcomeric myosin heavy chain gene MYH14 contains an intronic microRNA, miR-499. Our previous studies demonstrated divergent genomic organization and expression patterns of myh14/miR-499 among teleosts; however, the regulatory mechanism is partly known. In this study, we report the regulation of myh14 expression in zebrafish, Danio rerio. Zebrafish myh14 has three paralogs, myh14-1, myh14-2, and myh14-3. Detailed promoter analysis suggested that a 5710-bp 5'-flanking region of myh14-1 and a 5641-bp region of myh14-3 contain a necessary regulatory region to recapitulate specific expression during embryonic development. The 5'-flanking region of zebrafish myh14-1 and its torafugu ortholog shared two distal and a single proximal conserved region. The two distal conserved regions had no effect on zebrafish myh14-1 expression, in contrast to torafugu expression, suggesting an alternative regulatory mechanism among the myh14 orthologs. Comparison among the 5'-flanking regions of the myh14 paralogs revealed two conserved regions. Deletion of these conserved regions significantly reduced the promoter activity of myh14-3 but had no effect on myh14-1, indicating different cis-regulatory mechanisms of myh14 paralogs. Loss of function of miR-499 resulted in a marked reduction in slow muscle fibers in embryonic development. Our study identified different cis-regulatory mechanisms controlling the expression of myh14/miR-499 and an indispensable role of miR-499 in muscle fiber-type specification in zebrafish.

Skeletal muscle myosin and cardiac myosin attenuate heparin's antithrombin-dependent anticoagulant activity

BACKGROUND

Heparin enhances the ability of the plasma protease inhibitor, antithrombin, to neutralize coagulation factor Xa and thrombin. Skeletal muscle myosin binds unfractionated heparin.

OBJECTIVES

The aim of this study was to investigate the influence of myosin binding to heparin on antithrombin's anticoagulant activity.

METHODS

Inhibition of factor Xa and thrombin by antithrombin in the presence of different heparins and skeletal muscle myosin or cardiac myosin was studied by measuring inhibition of each enzyme's chromogenic substrate hydrolysis.

RESULTS AND CONCLUSIONS

Skeletal muscle myosin and cardiac myosin neutralized unfractionated heparin's enhancement of antithrombin's inhibition of purified factor Xa and thrombin. Skeletal muscle myosin also reduced the inhibition of factor Xa and thrombin by antithrombin in the presence of heparan sulfate. These two myosins did not protect factor Xa from antithrombin inhibition when tested in the presence of smaller heparins (eg, low molecular weight heparin, heparin pentasaccharide). This chain length dependence for skeletal muscle myosin's ability to reduce heparin's anticoagulant activity might have potential implications for therapy for patients who experience increases in plasma myosin levels (eg, acute trauma patients). In addition to the chain length, the type and extent of sulfation of glycosaminoglycans influenced the ability of skeletal muscle myosin to neutralize the polysaccharide's ability to enhance antithrombin's activity. In summary, these studies show that skeletal muscle myosin and cardiac myosin can influence antithrombin's anticoagulant activity against factor Xa and thrombin, implying that they may significantly influence the hemostatic balance involving bleeding vs clotting.

Function and Fiber-Type Specific Distribution of Hsp60 and αB-Crystallin in Skeletal Muscles: Role of Physical Exercise

Simple Summary

Skeletal muscle represents about 40% of the body mass in humans and it is a copious and plastic tissue, rich in proteins that are subject to continuous rearrangements. Physical exercise is considered a physiological stressor for different organs, in particular for skeletal muscle, and it is a factor able to stimulate the cellular remodeling processes related to the phenomenon of adaptation. All cells respond to various stress conditions by up-regulating the expression and/or activation of a group of proteins called heat shock proteins (HSPs). Although their expression is induced by several stimuli, they are commonly recognized as HSPs due to the first experiments showing their increased transcription after application of heat shock. These proteins are molecular chaperones mainly involved in assisting protein transport and folding, assembling multimolecular complexes, and triggering protein degradation by proteasome. Among the HSPs, a special attention needs to be devoted to Hsp60 and αB-crystallin, proteins constitutively expressed in the skeletal muscle, where they are known to be important in muscle physiopathology. Therefore, here we provide a critical update on their role in skeletal muscle fibers after physical exercise, highlighting the control of their expression, their biological function, and their specific distribution within skeletal muscle fiber-types.

Abstract

Skeletal muscle is a plastic and complex tissue, rich in proteins that are subject to continuous rearrangements. Skeletal muscle homeostasis can be affected by different types of stresses, including physical activity, a physiological stressor able to stimulate a robust increase in different heat shock proteins (HSPs). The modulation of these proteins appears to be fundamental in facilitating the cellular remodeling processes related to the phenomenon of training adaptations such as hypertrophy, increased oxidative capacity, and mitochondrial activity. Among the HSPs, a special attention needs to be devoted to Hsp60 and αB-crystallin (CRYAB), proteins constitutively expressed in the skeletal muscle, where their specific features could be highly relevant in understanding the impact of different volumes of training regimes on myofiber types and in explaining the complex picture of exercise-induced mechanical strain and damaging conditions on fiber population. This knowledge could lead to a better personalization of training protocols with an optimal non-harmful workload in populations of individuals with different needs and healthy status. Here, we introduce for the first time to the reader these peculiar HSPs from the perspective of exercise response, highlighting the control of their expression, biological function, and specific distribution within skeletal muscle fiber-types.

Sex-based differences after a single bout of exercise on PGC1α isoforms in skeletal muscle: A pilot study

To date, there are limited and incomplete data on possible sex-based differences in fiber-types of skeletal muscle and their response to physical exercise. Adult healthy male and female mice completed a single bout of endurance exercise to examine the sex-based differences of the peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α), heat shock protein 60 (Hsp60), interleukin 6 (IL-6) expression, as well as the Myosin Heavy Chain (MHC) fiber-type distribution in soleus and extensor digitorum longus (EDL) muscles. Our results showed for the first time that in male soleus, a muscle rich of type IIa fibers, endurance exercise activates specifically genes involved in mitochondrial biogenesis such as PGC1 α1 isoform, Hsp60 and IL-6, whereas the expression of PGC1 α2 and α3 was significantly upregulated in EDL muscle, a fast-twitch skeletal muscle, independently from the gender. Moreover, we found that the acute response of different PGC1α isoforms was muscle and gender dependent. These findings add a new piece to the huge puzzle of muscle response to physical exercise. Given the importance of these genes in the physiological response of the muscle to exercise, we strongly believe that our data could support future research studies to personalize a specific and sex-based exercise training protocol.

Novel blood coagulation molecules: Skeletal muscle myosin and cardiac myosin

Essentials Striated muscle myosins can promote prothrombin activation by FXa or FVa inactivation by APC. Cardiac myosin and skeletal muscle myosin are pro-hemostatic in murine tail cut bleeding models. Infused cardiac myosin exacerbates myocardial injury caused by myocardial ischemia reperfusion. Skeletal muscle myosin isoforms that circulate in human plasma can be grouped into 3 phenotypes. ABSTRACT: Two striated muscle myosins, namely skeletal muscle myosin (SkM) and cardiac myosin (CM), may potentially contribute to physiologic mechanisms for regulation of thrombosis and hemostasis. Thrombin is generated from activation of prothrombin by the prothrombinase (IIase) complex comprising factor Xa, factor Va, and Ca++ ions located on surfaces where these factors are assembled. We discovered that SkM and CM, which are abundant motor proteins in skeletal and cardiac muscles, can provide a surface for thrombin generation by the prothrombinase complex without any apparent requirement for phosphatidylserine or lipids. These myosins can also provide a surface that supports the inactivation of factor Va by activated protein C/protein S, resulting in negative feedback downregulation of thrombin generation. Although the physiologic significance of these reactions remains to be established for humans, substantive insights may be gleaned from murine studies. In mice, exogenously infused SkM and CM can promote hemostasis as they are capable of reducing tail cut bleeding. In a murine myocardial ischemia-reperfusion injury model, exogenously infused CM exacerbates myocardial infarction damage. Studies of human plasmas show that SkM antigen isoforms of different MWs circulate in human plasma, and they can be used to identify three plasma SkM phenotypes. A pilot clinical study showed that one SkM isoform pattern appeared to be linked to isolated pulmonary embolism. These discoveries enable multiple preclinical and clinical studies of SkM and CM, which should provide novel mechanistic insights with potential translational relevance for the roles of CM and SkM in the pathobiology of hemostasis and thrombosis.

Alpha and beta myosin isoforms and human atrial and ventricular contraction

Human atrial and ventricular contractions have distinct mechanical characteristics including speed of contraction, volume of blood delivered and the range of pressure generated. Notably, the ventricle expresses predominantly β-cardiac myosin while the atrium expresses mostly the α-isoform. In recent years exploration of the properties of pure α- & β-myosin isoforms have been possible in solution, in isolated myocytes and myofibrils. This allows us to consider the extent to which the atrial vs ventricular mechanical characteristics are defined by the myosin isoform expressed, and how the isoform properties are matched to their physiological roles. To do this we Outline the essential feature of atrial and ventricular contraction; Explore the molecular structural and functional characteristics of the two myosin isoforms; Describe the contractile behaviour of myocytes and myofibrils expressing a single myosin isoform; Finally we outline the outstanding problems in defining the differences between the atria and ventricles. This allowed us consider what features of contraction can and cannot be ascribed to the myosin isoforms present in the atria and ventricles.

Cardiac Myosin Promotes Thrombin Generation and Coagulation In Vitro and In Vivo

OBJECTIVE

Cardiac myosin (CM) is structurally similar to skeletal muscle myosin, which has procoagulant activity. Here, we evaluated CM's ex vivo, in vivo, and in vitro activities related to hemostasis and thrombosis. Approach and Results: Perfusion of fresh human blood over CM-coated surfaces caused thrombus formation and fibrin deposition. Addition of CM to blood passing over collagen-coated surfaces enhanced fibrin formation. In a murine ischemia/reperfusion injury model, exogenous CM, when administered intravenously, augmented myocardial infarction and troponin I release. In hemophilia A mice, intravenously administered CM reduced tail-cut-initiated bleeding. These data provide proof of concept for CM's in vivo procoagulant properties. In vitro studies clarified some mechanisms for CM's procoagulant properties. Thrombin generation assays showed that CM, like skeletal muscle myosin, enhanced thrombin generation in human platelet-rich and platelet-poor plasmas and also in mixtures of purified factors Xa, Va, and prothrombin. Binding studies showed that CM, like skeletal muscle myosin, directly binds factor Xa, supporting the concept that the CM surface is a site for prothrombinase assembly. In tPA (tissue-type plasminogen activator)-induced plasma clot lysis assays, CM was antifibrinolytic due to robust CM-dependent thrombin generation that enhanced activation of TAFI (thrombin activatable fibrinolysis inhibitor).

CONCLUSIONS

CM in vitro is procoagulant and prothrombotic. CM in vivo can augment myocardial damage and can be prohemostatic in the presence of bleeding. CM's procoagulant and antifibrinolytic activities likely involve, at least in part, its ability to bind factor Xa and enhance thrombin generation. Future work is needed to clarify CM's pathophysiology and its mechanistic influences on hemostasis or thrombosis.

Mixing it up: the biological significance of hybrid skeletal muscle fibers

Skeletal muscle fibers are classified according to the myosin heavy chain (MHC) isoforms and other myofibrillar proteins expressed within these cells. In addition to 'pure' fibers expressing single MHC isoforms, many fibers are 'hybrids' that co-express two or more different isoforms of MHC or other myofibrillar proteins. Although hybrid fibers have been recognized by muscle biologists for more than three decades, uncertainty persists about their prevalence in normal muscles, their role in fiber-type transitions, and what they might tell us about fiber-type regulation at the cellular and molecular levels. This Review summarizes current knowledge on the relative abundance of hybrid fibers in a variety of muscles from different species. Data from more than 150 muscles from 39 species demonstrate that hybrid fibers are common, frequently representing 25% or more of the fibers in normal muscles. Hybrid fibers appear to have two main roles: (1) they function as intermediates during the fiber-type transitions associated with skeletal muscle development, adaptation to exercise and aging; and (2) they provide a functional continuum of fiber phenotypes, as they possess physiological properties that are intermediate to those of pure fiber types. One aspect of hybrid fibers that is not widely recognized is that fiber-type asymmetries - such as dramatic differences in the MHC composition along the length of single fibers - appear to be a common aspect of many fibers. The final section of this Review examines the possible role of differential activities of nuclei in different myonuclear domains in establishing fiber-type asymmetries.

The ATPase cycle of human muscle myosin II isoforms: Adaptation of a single mechanochemical cycle for different physiological roles

Striated muscle myosins are encoded by a large gene family in all mammals, including humans. These isoforms define several of the key characteristics of the different striated muscle fiber types, including maximum shortening velocity. We have previously used recombinant isoforms of the motor domains of seven different human myosin isoforms to define the actin·myosin cross-bridge cycle in solution. Here, we present data on an eighth isoform, the perinatal, which has not previously been characterized. The perinatal is distinct from the embryonic isoform, appearing to have features in common with the adult fast-muscle isoforms, including weak affinity of ADP for actin·myosin and fast ADP release. We go on to use a recently developed modeling approach, MUSICO, to explore how well the experimentally defined cross-bridge cycles for each isoform in solution can predict the characteristics of muscle fiber contraction, including duty ratio, shortening velocity, ATP economy, and load dependence of these parameters. The work shows that the parameters of the cross-bridge cycle predict many of the major characteristics of each muscle fiber type and raises the question of what sequence changes are responsible for these characteristics.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Differential ammonia metabolism and toxicity between avian and mammalian species, and effect of ammonia on skeletal muscle: A comparative review

Comparative aspects of ammonia toxicity, specific to liver and skeletal muscle and skeletal muscle metabolism between avian and mammalian species are discussed in the context of models for liver disease and subsequent skeletal muscle wasting. The purpose of this review is to present species differences in ammonia metabolism and to specifically highlight observed differences in skeletal muscle response to excess ammonia in avian species. Ammonia, which is produced during protein catabolism and is an essential component of nucleic acid and protein biosynthesis, is detoxified mainly in the liver. While the liver is consistent as the main organ responsible for ammonia detoxification, there are evolutionary differences in ammonia metabolism and nitrogen excretory products between avian and mammalian species. In patients with liver disease and all mammalian models, inadequate ammonia detoxification and successive increased circulating ammonia concentration, termed hyperammonemia, leads to severe skeletal muscle atrophy, increased apoptosis and reduced protein synthesis, altogether having deleterious effects on muscle size and strength. Previously, an avian embryonic model, designed to determine the effects of increased circulating ammonia on muscle development, revealed that ammonia elicits a positive myogenic response. Specifically, induced hyperammonemia in avian embryos resulted in a reduction in myostatin, a well-known inhibitor of muscle growth, expression, whereas myostatin expression is significantly increased in mammalian models of hyperammonemia. These interesting findings imply that species differences in ammonia metabolism allow avians to utilize ammonia for growth. Understanding the intrinsic physiological mechanisms that allow for ammonia to be utilized for growth has potential to reveal novel approaches to muscle growth in avian species and will provide new targets for preventing muscle degeneration in mammalian species.

Technical note: Protocol for electrophoretic separation of bovine myosin heavy chain isoforms and comparison to immunohistochemistry analysis

Myosin heavy chain (MyHC) isoform composition is a primary determinant of contractile speed of muscle fibers. Currently, bovine MyHC isoforms are evaluated using time-consuming histochemical analysis by immunflourescence or ATPase activity. Electrophoretic separation of MyHC isoforms is more rapid; however, a reliable procedure without use of gradients has not been validated for cattle. Therefore, our objectives were to develop and validate a procedure for separating bovine MyHC isoforms (I, IIa, and IIx) using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and compare results to immunohistochemistry (IHC) analysis. Muscle samples were collected from masseter, sternomandibularis, diaphragm, longissimus lumborum, and cutaneous trunci within 1.5 h postmortem. To determine appropriate conditions for electrophoretic separation, several parameters of gel composition were varied. Bovine MyHC isoforms were clearly separated by increasing glycerol content of polyacrylamide gels to 37%. Identity of MyHC isoforms was confirmed using western blotting. Percent MyHC composition evaluated by gel electrophoresis was consistent with IHC (P > 0.2). Thus, SDS-PAGE produces clear separation of MyHC isoforms, and is a viable alternative to IHC-based methods.

Functional and molecular outcomes of the human masticatory muscles

The masticatory muscles achieve a broad range of different activities such as chewing, sucking, swallowing, and speech. In order to accomplish these duties, masticatory muscles have a unique and heterogeneous structure and fiber composition, enabling them to produce their strength and contraction speed largely dependent on their motor units and myosin proteins that can change in response to genetic and environmental factors. Human masticatory muscles express unique myosin isoforms, including a combination of thick fibers, expressing myosin light chains (MyLC) and myosin class I and II heavy chains (MyHC) -IIA, -IIX, α-cardiac, embryonic and neonatal and thin fibers, respectively. In this review, we discuss the current knowledge regarding the importance of fiber-type diversity in masticatory muscles versus supra- and infrahyoid muscles, and versus limb and trunk muscles. We also highlight new information regarding the adaptive response and specific genetic variations of muscle fibers on the functional significance of the masticatory muscles, which influences craniofacial characteristics, malocclusions, or asymmetry. These findings may offer future possibilities for the prevention of craniofacial growth disturbances.

An E321G MYH1 mutation is strongly associated with nonexertional rhabdomyolysis in Quarter Horses

BACKGROUND

An E321G mutation in MYH1 was recently identified in Quarter Horses (QH) with immune-mediated myositis (IMM) defined by a phenotype of gross muscle atrophy and myofiber lymphocytic infiltrates.

HYPOTHESIS/OBJECTIVES

We hypothesized that the MYH1 mutation also was associated with a phenotype of nonexertional rhabdomyolysis. The objective of this study was to determine the prevalence of the MYH1 mutation in QH with exertional (ER) and nonexertional (nonER) rhabdomyolysis.

ANIMALS

Quarter Horses: 72 healthy controls, 85 ER-no atrophy, 56 ER-atrophy, 167 nonER horses selected regardless of muscle atrophy.

METHODS

Clinical and histopathologic information and DNA was obtained from a database for (1) ER > 2 years of age, with or without atrophy and (2) nonER creatine kinase (CK) ≥ 5000 U/L, <5 years of age. Horses were genotyped for E321G MYH1 by pyrosequencing.

RESULTS

The MYH1 mutation was present in a similar proportion of ER-no atrophy (1/56; 2%) and in a higher proportion of ER-atrophy (25/85; 29%) versus controls (4/72; 5%). The MYH1 mutation was present in a significantly higher proportion of nonER (113/165; 68%) than controls either in the presence (39/42; 93%) or in absence (72/123; 59%) of gross atrophy. Lymphocytes were present in <18% of muscle samples with the MYH1 mutation.

CONCLUSIONS AND CLINICAL IMPORTANCE

Although not associated with ER, the MYH1 mutation is associated with atrophy after ER. The MYH1 mutation is highly associated with nonER regardless of whether muscle atrophy or lymphocytic infiltrates are present. Genetic testing will enhance the ability to diagnose MYH1 myopathies (MYHM) in QH.

A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses

Background

The cause of immune-mediated myositis (IMM), characterized by recurrent, rapid-onset muscle atrophy in Quarter Horses (QH), is unknown. The histopathologic hallmark of IMM is lymphocytic infiltration of myofibers. The purpose of this study was to identify putative functional variants associated with equine IMM.

Methods

A genome-wide association (GWA) study was performed on 36 IMM QHs and 54 breed matched unaffected QHs from the same environment using the Equine SNP50 and SNP70 genotyping arrays.

Results

A mixed model analysis identified nine SNPs within a ~ 2.87 Mb region on chr11 that were significantly (P_unadjusted < 1.4 × 10^− 6) associated with the IMM phenotype. Associated haplotypes within this region encompassed 38 annotated genes, including four myosin genes (MYH1, MYH2, MYH3, and MYH13). Whole genome sequencing of four IMM and four unaffected QHs identified a single segregating nonsynonymous E321G mutation in MYH1 encoding myosin heavy chain 2X. Genotyping of additional 35 IMM and 22 unaffected QHs confirmed an association (P = 2.9 × 10^− 5), and the putative mutation was absent in 175 horses from 21 non-QH breeds. Lymphocytic infiltrates occurred in type 2X myofibers and the proportion of 2X fibers was decreased in the presence of inflammation. Protein modeling and contact/stability analysis identified 14 residues affected by the mutation which significantly decreased stability.

Conclusions

We conclude that a mutation in MYH1 is highly associated with susceptibility to the IMM phenotype in QH-related breeds. This is the first report of a mutation in MYH1 and the first link between a skeletal muscle myosin mutation and autoimmune disease.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0155-0) contains supplementary material, which is available to authorized users.

Region and cell-type resolved quantitative proteomic map of the human heart

The heart is a central human organ and its diseases are the leading cause of death worldwide, but an in-depth knowledge of the identity and quantity of its constituent proteins is still lacking. Here, we determine the healthy human heart proteome by measuring 16 anatomical regions and three major cardiac cell types by high-resolution mass spectrometry-based proteomics. From low microgram sample amounts, we quantify over 10,700 proteins in this high dynamic range tissue. We combine copy numbers per cell with protein organellar assignments to build a model of the heart proteome at the subcellular level. Analysis of cardiac fibroblasts identifies cellular receptors as potential cell surface markers. Application of our heart map to atrial fibrillation reveals individually distinct mitochondrial dysfunctions. The heart map is available at maxqb.biochem.mpg.de as a resource for future analyses of normal heart function and disease.

The human heart is composed of distinct regions and cell types, but relatively little is known about their specific protein composition. Here, the authors present a region- and cell type-specific proteomic map of the healthy human heart, revealing functional differences and potential cell type markers.

Comparison of myosin heavy chain content determined by label-free quantification between porcine longissimus thoracis, psoas major and semimembranosus muscles

The present study was conducted to quantify MHCs in porcine longissimus thoracis (LT), psoas major (PM) and semimembranosus (SM) muscles through the label-free quantification (LFQ). To estimate the accuracy of LFQ, quantitative RT-PCR (qRT-PCR), immunohistochemistry (IHC), and Western-blotting (WB) were performed. MHCs 2x, 2a, 2b and slow were identified by LC-MS/MS analysis and 279 ion spectra were selected for LFQ analysis. The content of MHC 2b was higher in LT and in SM than in PM (p<0.05), while the content of MHC slow was highest in PM among the muscles (p<0.01) regardless of LFQ types. Positive correlation coefficients of MHC 2b and MHC slow between LFQ and IHC (relative area composition) and qRT-PCR results partially supported the LFQ results. Though low-abundant peptides should be considered to estimate MHC contents via the spectral count method, LFQ enables the determination of MHC contents at protein level regardless of LFQ types.

Altered myocyte contractility and calcium homeostasis in alpha-myosin heavy chain point mutations linked to familial dilated cardiomyopathy

Mutations in the human cardiac motor protein beta-myosin heavy chain (βMHC) have been long recognized as a cause of familial hypertrophic cardiomyopathy. Recently, mutations (P830L and A1004S) in the less abundant but faster isoform alpha-myosin heavy chain (αMHC) have been linked to dilated cardiomyopathy (DCM). In this study, we sought to determine the cellular contractile phenotype associated with these point mutations. Ventricular myocytes were isolated from 2 month male Sprague Dawley rats. Cells were cultured in M199 media and infected with recombinant adenovirus containing the P830L or the A1004S mutant human αMHC at a MOI of 500 for 18 h. Uninfected cells (UI), human βMHC (MOI 500, 18 h), and human αMHC (MOI 500, 18 h) were used as controls. Cells were loaded with fura-2 (1 μM, 15 min) after 48 h. Sarcomere shortening and calcium transients were recorded in CO₂ buffered M199 media (36°±1 C) with and without 10 nM isoproterenol (Iso). The A1004S mutation resulted in decreased peak sarcomere shortening while P830L demonstrated near normal shortening kinetics at baseline. In the presence of Iso, the A1004S sarcomere shortening was identical to the βMHC shortening while the P830L was identical to the αMHC control. All experimental groups had identical calcium transients. Despite a shared association with DCM, the P830L and A1004S αMHC mutations alter myocyte contractility in completely different ways while at the same preserving peak intracellular calcium.

Fiber-type Composition of the Human Jaw Muscles—(Part 1) Origin and Functional Significance of Fiber-type Diversity

This is the first of two articles on the fiber-type composition of the human jaw muscles. The present article discusses the origin of fiber-type composition and its consequences. This discussion is presented in the context of the requirements for functional performance and adaptation that are imposed upon the jaw muscles. The human masticatory system must perform a much larger variety of motor tasks than the average limb or trunk motor system. An important advantage of fiber-type diversity, as observed in the jaw muscles, is that it optimizes the required function while minimizing energy use. The capacity for adaptation is reflected by the large variability in fiber-type composition among muscle groups, individual muscles, and muscle regions. Adaptive changes are related, for example, to the amount of daily activation and/or stretch of fibers. Generally, the number of slow, fatigue-resistant fibers is relatively large in muscles and muscle regions that are subjected to considerable activity and/or stretch.

Bone muscle crosstalk targets muscle regeneration pathway regulated by core circadian transcriptional repressors DEC1 and DEC2

Deletion of proprotein convertase Mbtps1 in bone osteocytes leads to a significant postnatal increase in skeletal muscle size and contractile function, while causing only a 25% increase in stiffness in long bones. Concerns about leakiness in skeletal muscle were discounted since Cre recombinase expression does not account for our findings, and, Mbtps1 protein and mRNA is not deleted. Interestingly, the response of normal skeletal muscle to exercise and the regenerative response of skeletal muscle to the deletion of Mbtps1 in bone share some key regulatory features including a preference for slow twitch muscle fibers. In addition, transcriptional regulators PPAR, PGC-1α, LXR, and repressors DEC1 and DEC2 all occupy central positions within these two pathways. We hypothesize that the age-dependent muscle phenotype in Dmp1-Cre Mbtps1 cKO mice is due to bone→muscle crosstalk. Many of the myogenic genes altered in this larger and functionally improved muscle are regulated by circadian core transcriptional repressors DEC1 and DEC2, and furthermore, display a temporal coordination with Dec1 and Dec2 expression consistent with a regulatory co-dependency. These considerations lead us to propose that Dmp1-Cre Mbtps1 cKO osteocytes activate myogenesis by increased release of an activator of muscle PPAR-gamma, for example, PGE₂ or sphingosine-1-P, or, by diminished release of an inhibitor of LXR, for example, long-chain polyunsaturated fatty acids. We hope that further investigation of these interacting pathways in the Dmp1-Cre Mbtps1 cKO model will lead to clinically translatable findings applicable to age-related sarcopenia and other muscle wasting syndromes.

Evolution and Distribution of Teleost myomiRNAs: Functionally Diversified myomiRs in Teleosts

Myosin heavy chain (MYH) genes belong to a multigene family, and the regulated expression of each member determines the physiological and contractile muscle properties. Among these, MYH6, MYH7, and MYH14 occupy unique positions in the mammalian MYH gene family because of their specific expression in slow/cardiac muscles and the existence of intronic micro(mi) RNAs. MYH6, MYH7, and MYH14 encode miR-208a, miR-208b, and miR-499, respectively. These MYH encoded miRNAs are designated as myomiRs because of their muscle-specific expression and functions. In mammals, myomiRs and host MYHs form a transcription network involved in muscle fiber-type specification; thus, genomic positions and expression patterns of them are well conserved. However, our previous studies revealed divergent distribution and expression of MYH14/miR-499 among teleosts, suggesting the unique evolution of myomiRs and host MYHs in teleosts. Here, we examined distribution and expression of myomiRs and host MYHs in various teleost species. The major cardiac MYH isoforms in teleosts are an intronless gene, atrial myosin heavy chain (amhc), and ventricular myosin heavy chain (vmhc) gene that encodes an intronic miRNA, miR-736. Phylogenetic analysis revealed that vmhc/miR-736 is a teleost-specific myomiR that differed from tetrapoda MYH6/MYH7/miR-208s. Teleost genomes also contain species-specific orthologs in addition to vmhc and amhc, indicating complex gene duplication and gene loss events during teleost evolution. In medaka and torafugu, miR-499 was highly expressed in slow/cardiac muscles whereas the expression of miR-736 was quite low and not muscle specific. These results suggest functional diversification of myomiRs in teleost with the diversification of host MYHs.

Expression and identification of 10 sarcomeric MyHC isoforms in human skeletal muscles of different embryological origin. Diversity and similarity in mammalian species

In the mammalian genome, among myosin heavy chain (MyHC) isoforms a family can be identified as sarcomeric based on their molecular structure which allows thick filament formation. In this study we aimed to assess the expression of the 10 sarcomeric isoforms in human skeletal muscles, adopting this species as a reference for comparison with all other mammalian species. To this aim, we set up the condition for quantitative Real Time PCR assay to detect and quantify MyHC mRNA expression in a wide variety of human muscles from somitic, presomitic and preotic origin. Specific patterns of expression of the following genes MYH1, MYH2, MYH3, MYH4, MYH6, MYH7, MYH8, MYH13, MYH14/7b and MYH15 were demonstrated in various muscle samples. On the same muscle samples which were analysed for mRNA expression, the corresponding MyHC proteins were studied with SDS PAGE and Western blot. The mRNA-protein comparison allowed the identification of 10 distinct proteins based on the electrophoretic migration rate. Three groups were formed based on the migration rate: fast migrating comprising beta/slow/1, alpha cardiac and fast 2B, slow migrating comprising fast 2X, fast 2A and two developmental isoforms (NEO and EMB), intermediate migrating comprising EO MyHC, slow B (product of MYH15), slow tonic (product of MYH14/7b). Of special interest was the demonstration of a protein band corresponding to 2B-MyHC in laryngeal muscles and the finding that all 10 isoforms are expressed in extraocular muscles. These latter muscles are the unique localization for extraocular, slow B (product of MYH15) and slow tonic (product of MYH14/7b).

Deletion of Mbtps1 (Pcsk8, S1p, Ski-1) Gene in Osteocytes Stimulates Soleus Muscle Regeneration and Increased Size and Contractile Force with Age

Conditional deletion of Mbtps1 (cKO) protease in bone osteocytes leads to an age-related increase in mass (12%) and in contractile force (30%) in adult slow twitch soleus muscles (SOL) with no effect on fast twitch extensor digitorum longus muscles. Surprisingly, bone from 10-12-month-old cKO animals was indistinguishable from controls in size, density, and morphology except for a 25% increase in stiffness. cKO SOL exhibited increased expression of Pax7, Myog, Myod1, Notch, and Myh3 and 6-fold more centralized nuclei, characteristics of postnatal regenerating muscle, but only in type I myosin heavy chain-expressing cells. Increased expression of gene pathways mediating EGF receptor signaling, circadian exercise, striated muscle contraction, and lipid and carbohydrate oxidative metabolism were also observed in cKO SOL. This muscle phenotype was not observed in 3-month-old mice. Although Mbtps1 mRNA and protein expression was reduced in cKO bone osteocytes, no differences in Mbtps1 or cre recombinase expression were observed in cKO SOL, explaining this age-related phenotype. Understanding bone-muscle cross-talk may provide a fresh and novel approach to prevention and treatment of age-related muscle loss.

Developmental myosins: expression patterns and functional significance

Developing skeletal muscles express unique myosin isoforms, including embryonic and neonatal myosin heavy chains, coded by the myosin heavy chain 3 (MYH3) and MYH8 genes, respectively, and myosin light chain 1 embryonic/atrial, encoded by the myosin light chain 4 (MYL4) gene. These myosin isoforms are transiently expressed during embryonic and fetal development and disappear shortly after birth when adult fast and slow myosins become prevalent. However, developmental myosins persist throughout adult stages in specialized muscles, such as the extraocular and jaw-closing muscles, and in the intrafusal fibers of the muscle spindles. These myosins are re-expressed during muscle regeneration and provide a specific marker of regenerating fibers in the pathologic skeletal muscle. Mutations in MYH3 or MYH8 are responsible for distal arthrogryposis syndromes, characterized by congenital joint contractures and orofacial dysmorphisms, supporting the importance of muscle contractile activity and body movements in joint development and in shaping the form of the face during fetal development. The biochemical and biophysical properties of developmental myosins have only partially been defined, and their functional significance is not yet clear. One possibility is that these myosins are specialized in contracting against low loads, and thus, they may be adapted to the prenatal environment, when fetal muscles contract against a very low load compared to postnatal muscles.

Analysis of Myosin Heavy Chain Isoforms from Longissimus Thoracis Muscle of Hanwoo Steer by Electrophoresis and LC-MS/MS

The purpose of this study was to analyze myosin heavy chain (MHC) isoforms in bovine longissimus thoracis (LT) muscle by liquid chromatography (LC) and mass spectrometry (MS). LT muscles taken from Hanwoo (Korean native cattle) steer (n=3) used to separate myosin bands by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The peptide queries were obtained from the myosin bands by LC-MS/MS analysis following in-gel digestion with trypsin. A total of 33 and 43 queries were identified as common and unique peptides, respectively, of MHC isoforms (individual ions scores >43 indicate identity or extensive homology, p<0.05). MHC-1 (IIx), -2 (IIa), -4 (IIb), and -7 (slow/I) were identified based on the Mowse score (5118, 3951, 2526, and 2541 for MHC-1, -2, -4, and -7, respectively). However, more analysis is needed to confirm the expression of MHC-4 in bovine LT muscle because any query identified as a unique peptide of MHC-4 was not found. The queries that were identified as unique peptides could be used as peptide markers to confirm MHC-1 (14 queries), -2 (8 queries), and -7 (21 queries) in bovine LT muscle; no query identified as a unique peptide of MHC-4 was found. LC-MS/MS analysis is a useful approach to study MHC isoforms at the protein level.

A single mutation in the acetylcholine receptor δ-subunit causes distinct effects in two types of neuromuscular synapses

Mutations in AChR subunits, expressed as pentamers in neuromuscular junctions (NMJs), cause various types of congenital myasthenic syndromes. In AChR pentamers, the adult ε subunit gradually replaces the embryonic γ subunit as the animal develops. Because of this switch in subunit composition, mutations in specific subunits result in synaptic phenotypes that change with developmental age. However, a mutation in any AChR subunit is considered to affect the NMJs of all muscle fibers equally. Here, we report a zebrafish mutant of the AChR δ subunit that exhibits two distinct NMJ phenotypes specific to two muscle fiber types: slow or fast. Homozygous fish harboring a point mutation in the δ subunit form functional AChRs in slow muscles, whereas receptors in fast muscles are nonfunctional. To test the hypothesis that different subunit compositions in slow and fast muscles underlie distinct phenotypes, we examined the presence of ε/γ subunits in NMJs using specific antibodies. Both wild-type and mutant larvae lacked ε/γ subunits in slow muscle synapses. These findings in zebrafish suggest that some mutations in human congenital myasthenic syndromes may affect slow and fast muscle fibers differently.

High-intensity exercise training induces morphological and biochemical changes in skeletal muscles

In the present study we investigated the effect of two different exercise protocols on fibre composition and metabolism of two specific muscles of mice: the quadriceps and the gastrocnemius. Mice were run daily on a motorized treadmill, at a velocity corresponding to 60% or 90% of the maximal running velocity. Blood lactate and body weight were measured during exercise training. We found that at the end of training the body weight significantly increased in high-intensity exercise mice compared to the control group (P=0.0268), whereas it decreased in low-intensity exercise mice compared to controls (P=0.30). In contrast, the food intake was greater in both trained mice compared to controls (P < 0.0001 and P < 0.0001 for low-intensity and high-intensity exercise mice, respectively). These effects were accompanied by a progressive reduction in blood lactate levels at the end of training in both the exercised mice compared with controls (P=0.03 and P < 0.0001 for low-intensity and high-intensity exercise mice, respectively); in particular, blood lactate levels after high-intensity exercise were significantly lower than those measured in low-intensity exercise mice (P=0.0044). Immunoblotting analysis demonstrated that high-intensity exercise training produced a significant increase in the expression of mitochondrial enzymes contained within gastrocnemius and quadriceps muscles. These changes were associated with an increase in the amount of slow fibres in both these muscles of high-intensity exercise mice, as revealed by the counts of slow fibres stained with specific antibodies (P < 0.0001 for the gastrocnemius; P=0.0002 for the quadriceps). Our results demonstrate that high-intensity exercise, in addition to metabolic changes consisting of a decrease in blood lactate and body weight, induces an increase in the mitochondrial enzymes and slow fibres in different skeletal muscles of mice, which indicates an exercise-induced increase in the aerobic metabolism.

Automated fiber-type-specific cross-sectional area assessment and myonuclei counting in skeletal muscle

Skeletal muscle is an exceptionally adaptive tissue that compromises 40% of mammalian body mass. Skeletal muscle functions in locomotion, but also plays important roles in thermogenesis and metabolic homeostasis. Thus characterizing the structural and functional properties of skeletal muscle is important in many facets of biomedical research, ranging from myopathies to rehabilitation sciences to exercise interventions aimed at improving quality of life in the face of chronic disease and aging. In this paper, we focus on automated quantification of three important morphological features of muscle: 1) muscle fiber-type composition; 2) muscle fiber-type-specific cross-sectional area, and 3) myonuclear content and location. We experimentally prove that the proposed automated image analysis approaches for fiber-type-specific assessments and automated myonuclei counting are fast, accurate, and reliable.

The superfast human extraocular myosin is kinetically distinct from the fast skeletal IIa, IIb, and IId isoforms

Humans express five distinct myosin isoforms in the sarcomeres of adult striated muscle (fast IIa, IId, the slow/cardiac isoform I/β, the cardiac specific isoform α, and the specialized extraocular muscle isoform). An additional isoform, IIb, is present in the genome but is not normally expressed in healthy human muscles. Muscle fibers expressing each isoform have distinct characteristics including shortening velocity. Defining the properties of the isoforms in detail has been limited by the availability of pure samples of the individual proteins. Here we study purified recombinant human myosin motor domains expressed in mouse C₂C₁₂ muscle cells. The results of kinetic analysis show that among the closely related adult skeletal isoforms, the affinity of ADP for actin·myosin (K_AD) is the characteristic that most readily distinguishes the isoforms. The three fast muscle myosins have K_AD values of 118, 80, and 55 μm for IId, IIa, and IIb, respectively, which follows the speed in motility assays from fastest to slowest. Extraocular muscle is unusually fast with a far weaker K_AD = 352 μm. Sequence comparisons and homology modeling of the structures identify a few key areas of sequence that may define the differences between the isoforms, including a region of the upper 50-kDa domain important in signaling between the nucleotide pocket and the actin-binding site.

Digenic mutational inheritance of the integrin alpha 7 and the myosin heavy chain 7B genes causes congenital myopathy with left ventricular non-compact cardiomyopathy

Background

We report an Italian family in which the proband showed a severe phenotype characterized by the association of congenital fiber type disproportion (CFTD) with a left ventricular non-compaction cardiomyopathy (LVNC). This study was focused on the identification of the responsible gene/s.

Methods and results

Using the whole-exome sequencing approach, we identified the proband homozygous missense mutations in two genes, the myosin heavy chain 7B (MYH7B) and the integrin alpha 7 (ITGA7). Both genes are expressed in heart and muscle tissues, and both mutations were predicted to be deleterious and were not found in the healthy population.

The R890C mutation in the MYH7B gene segregated with the LVNC phenotype in the examined family. It was also found in one unrelated patient affected by LVNC, confirming a causative role in cardiomyopathy.

The E882K mutation in the ITGA7 gene, a key component of the basal lamina of muscle fibers, was found only in the proband, suggesting a role in CFTD.

Conclusions

This study identifies two novel disease genes. Mutation in MYH7B causes a classical LVNC phenotype, whereas mutation in ITGA7 causes CFTD. Both phenotypes represent alterations of skeletal and cardiac muscle maturation and are usually not severe. The severe phenotype of the proband is most likely due to a synergic effect of these two mutations.

This study provides new insights into the genetics underlying Mendelian traits and demonstrates a role for digenic inheritance in complex phenotypes.

Skeletal and cardiac α-actin isoforms differently modulate myosin cross-bridge formation and myofibre force production

Multiple congenital myopathies, including nemaline myopathy, can arise due to mutations in the ACTA1 gene encoding skeletal muscle α-actin. The main characteristics of ACTA1 null mutations (absence of skeletal muscle α-actin) are generalized skeletal muscle weakness and premature death. A mouse model (ACTC(Co)/KO) mimicking these conditions has successfully been rescued by transgenic over-expression of cardiac α-actin in skeletal muscles using the ACTC gene. Nevertheless, myofibres from ACTC(Co)/KO animals generate less force than normal myofibres (-20 to 25%). To understand the underlying mechanisms, here we have undertaken a detailed functional study of myofibres from ACTC(Co)/KO rodents. Mechanical and X-ray diffraction pattern analyses of single membrane-permeabilized myofibres showed, upon maximal Ca(2+) activation and under rigor conditions, lower stiffness and disrupted actin-layer line reflections in ACTC(Co)/KO when compared with age-matched wild-types. These results demonstrate that in ACTC(Co)/KO myofibres, the presence of cardiac α-actin instead of skeletal muscle α-actin alters actin conformational changes upon activation. This later finely modulates the strain of individual actomyosin interactions and overall lowers myofibre force production. Taken together, the present findings provide novel primordial information about actin isoforms, their functional differences and have to be considered when designing gene therapies for ACTA1-based congenital myopathies.

Myosin heavy chain-2b transcripts and isoform are expressed in human laryngeal muscles

Three fast myosin heavy chain (MyHC) isoforms, i.e. MyHC-2a, -2x and -2b, are expressed in skeletal muscles of smaller mammals. In contrast, only MyHC-2a and -2x have been revealed in humans so far. The expression of MyHC isoforms is known to be wider in the functionally more specialized laryngeal muscles. Though mRNA transcripts of the MyHC-2b gene were found to be expressed in certain human skeletal and laryngeal muscles, the corresponding isoform has not been demonstrated in these muscles. To our knowledge, we are the first to demonstrate not only the expression of MyHC-2b transcripts using an in situ hybridization technique but also the corresponding protein, i.e. the MyHC-2b isoform, in some human laryngeal muscles by immunohistochemistry but not by polyacrylamide gel electrophoresis. Using a set of antibodies specific to MyHC isoforms, we demonstrated that MyHC-2b was always co-expressed with the major MyHC isoforms, not only with the fast ones (MyHC-2a and -2x) but with the slow isoform (MyHC-1) as well.