Human skeletal myosin heavy chain genes are tightly linked in the order embryonic-IIa-IId/x-ILb-perinatal-extraocular

Exploring high-resolution chromatin interaction changes and functional enhancers of myogenic marker genes during myogenic differentiation

Skeletal muscle differentiation (myogenesis) is a complex and highly coordinated biological process regulated by a series of myogenic marker genes. Chromatin interactions between gene's promoters and their enhancers have an important role in transcriptional control. However, the high-resolution chromatin interactions of myogenic genes and their functional enhancers during myogenesis remain largely unclear. Here, we used circularized chromosome conformation capture coupled with next generation sequencing (4C-seq) to investigate eight myogenic marker genes in C2C12 myoblasts (C2C12-MBs) and C2C12 myotubes (C2C12-MTs). We revealed dynamic chromatin interactions of these marker genes during differentiation and identified 163 and 314 significant interaction sites (SISs) in C2C12-MBs and C2C12-MTs, respectively. The interacting genes of SISs in C2C12-MTs were mainly involved in muscle development, and histone modifications of the SISs changed during differentiation. Through functional genomic screening, we also identified 25 and 41 putative active enhancers in C2C12-MBs and C2C12-MTs, respectively. Using luciferase reporter assays for putative enhancers of Myog and Myh3, we identified eight activating enhancers. Furthermore, dCas9-KRAB epigenome editing and RNA-Seq revealed a role for Myog enhancers in the regulation of Myog expression and myogenic differentiation in the native genomic context. Taken together, this study lays the groundwork for understanding 3D chromatin interaction changes of myogenic genes during myogenesis and provides insights that contribute to our understanding of the role of enhancers in regulating myogenesis.

Identification of enhancers responsible for the coordinated expression of myosin heavy chain isoforms in skeletal muscle

Background

Skeletal muscles consist of fibers of differing contractility and metabolic properties, which are primarily determined by the content of myosin heavy chain (MYH) isoforms (MYH7, MYH2, MYH1, and MYH4). The regulation of Myh genes transcription depends on three-dimensional chromatin conformation interaction, but the mechanistic details remain to be determined.

Results

In this study, we characterized the interaction profiles of Myh genes using 4C-seq (circular chromosome conformation capture coupled to high-throughput sequencing). The interaction profile of Myh genes changed between fast quadriceps and slow soleus muscles. Combining chromatin immunoprecipitation-sequencing (ChIP-seq) and transposase accessible chromatin with high-throughput sequencing (ATAC-seq), we found that a 38 kb intergenic region interacting simultaneously with fast Myh genes promoters controlled the coordinated expression of fast Myh genes. We also identified four active enhancers of Myh7, and revealed that binding of MYOG and MYOD increased the activity of Myh7 enhancers.

Conclusions

This study provides new insight into the chromatin interactions that regulate Myh genes expression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08737-9.

Fast and slow myofiber-specific expression profiles are affected by noncoding RNAs in Mongolian horses

The heterogeneity and plasticity of muscle fibers are essential for the athletic performance of horses, mainly at the adaption of exercises and the effect on muscle diseases. Skeletal muscle fibers can be generally distinguished by their characteristics of contraction as slow and fast type myofibers. The diversity of contractile properties and metabolism enable skeletal muscles to respond to the variable functional requirements. We investigated the muscle fiber composition and metabolic enzyme activities of splenius muscle and gluteus medius muscle from Mongolian horses. The deep RNA-seq analysis of detecting differentially expressed mRNAs, lncRNAs, circRNAs and their correlation analysis from two muscles were performed. Splenius muscle and gluteus medius muscle from Mongolian horses showed a high divergence of myofiber compositions and metabolic enzyme activities. Corresponding to their phenotypic characteristics, 57 differentially expressed long noncoding RNAs and 12 differentially expressed circle RNAs were found between two muscles. The analysis results indicate multiple binding sites were detected in lncRNAs and circRNAs with myofiber-specific expressed miRNAs. Among which we found significant correlations between the above noncoding RNAs, miRNAs, their target genes, myofiber-specific developmental transcript factors, and sarcomere genes. We suggest that the ceRNA mechanism of differentially expressed noncoding RNAs by acting as miRNA sponges could be fine tuners in regulating skeletal muscle fiber composition and transition in horses, which will operate new protective measures of muscle disease and locomotor adaption for racehorses.

A fast Myosin super enhancer dictates muscle fiber phenotype through competitive interactions with Myosin genes

The contractile properties of adult myofibers are shaped by their Myosin heavy chain isoform content. Here, we identify by snATAC-seq a 42 kb super-enhancer at the locus regrouping the fast Myosin genes. By 4C-seq we show that active fast Myosin promoters interact with this super-enhancer by DNA looping, leading to the activation of a single promoter per nucleus. A rainbow mouse transgenic model of the locus including the super-enhancer recapitulates the endogenous spatio-temporal expression of adult fast Myosin genes. In situ deletion of the super-enhancer by CRISPR/Cas9 editing demonstrates its major role in the control of associated fast Myosin genes, and deletion of two fast Myosin genes at the locus reveals an active competition of the promoters for the shared super-enhancer. Last, by disrupting the organization of fast Myosin, we uncover positional heterogeneity within limb skeletal muscles that may underlie selective muscle susceptibility to damage in certain myopathies.

The contractile properties of adult myofibers are shaped by their Myosin heavy chain isoform content. Here the authors show that a super enhancer controls the spatiotemporal expression of the genes at the fast myosin heavy chain locus by DNA looping and that this expression profile is recapitulated in a rainbow transgenic mouse model of the locus.

Single-nucleus RNA-seq and FISH identify coordinated transcriptional activity in mammalian myofibers

Skeletal muscle fibers are large syncytia but it is currently unknown whether gene expression is coordinately regulated in their numerous nuclei. Here we show by snRNA-seq and snATAC-seq that slow, fast, myotendinous and neuromuscular junction myonuclei each have different transcriptional programs, associated with distinct chromatin states and combinations of transcription factors. In adult mice, identified myofiber types predominantly express either a slow or one of the three fast isoforms of Myosin heavy chain (MYH) proteins, while a small number of hybrid fibers can express more than one MYH. By snRNA-seq and FISH, we show that the majority of myonuclei within a myofiber are synchronized, coordinately expressing only one fast Myh isoform with a preferential panel of muscle-specific genes. Importantly, this coordination of expression occurs early during post-natal development and depends on innervation. These findings highlight a previously undefined mechanism of coordination of gene expression in a syncytium.

Muscle Fiber Properties in Cattle and Their Relationships with Meat Qualities: An Overview

The control of meat quality traits constitutes an important target for any farm animal production, including cattle. Therefore, better understanding of the biochemical properties that drive muscle development and final outcomes constitutes one of the main challenging topics of animal production and meat science. Accordingly, this review has focused on skeletal muscle fibers in cattle and their relationships with beef qualities. It aimed to describe the chemical and structural properties of muscle fibers as well as a comprehensive review of their contractile and metabolic characteristics during the life of the animal. The existing methods for the classification of muscle fibers were reviewed, compared, and discussed. Then, the different stages of myogenesis in cattle were defined. The main factors regulating fetal and postnatal growth and the plasticity of muscle fibers were evidenced, especially the role of myostatin growth factor and the impact of nutritional factors. This review highlights that the knowledge about muscle fibers is paramount for a better understanding of how to control the muscle properties throughout the life of the animal for better management of the final eating qualities of beef. Accordingly, the associations between bovine muscle fibers and different meat eating qualities such as tenderness, pH decline, and color traits were further presented.

Regulation of myosin heavy chain antisense long noncoding RNA in human vastus lateralis in response to exercise training

Alterations to muscle activity or loading state can induce changes in expression of myosin heavy chain (MHC). For example, sedentary individuals that initiate exercise training can induce a pronounced shift from IIx to IIa MHC. We sought to examine the regulatory response of MHC RNA in human subjects in response to exercise training. In particular, we examined how natural antisense RNA transcripts (NATs) are regulated throughout the MHC gene locus that includes MYH2 (IIa), MYH1 (IIx), MYH4 (IIb), and MYH8 (Neonatal) in vastus lateralis before and after a 5-wk training regime that consisted of a combination of aerobic and resistance types of exercise. The exercise program induced a IIx to IIa MHC shift that was associated with a corresponding increase in transcription on the antisense strand of the IIx MHC gene and a decrease in antisense transcription of the IIa MHC gene, suggesting an inhibitory mechanism mediated by NATs. We also report that the absence of expression of IIb MHC in human limb muscle is associated with the abundant expression of antisense transcript overlapping the IIb MHC coding gene, which is the opposite expression pattern as compared with that previously observed in rats. The NAT provides a possible regulatory mechanism for the suppressed expression of IIb MHC in humans. These data indicate that NATs may play a regulatory role with regard to the coordinated shifts in MHC gene expression that occur in human muscle in response to exercise training.

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.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

Six1 homeoprotein drives myofiber type IIA specialization in soleus muscle

BACKGROUND

Adult skeletal muscles are composed of slow and fast myofiber subtypes which each express selective genes required for their specific contractile and metabolic activity. Six homeoproteins are transcription factors regulating muscle cell fate through activation of myogenic regulatory factors and driving fast-type gene expression during embryogenesis.

RESULTS

We show here that Six1 protein accumulates more robustly in the nuclei of adult fast-type muscles than in adult slow-type muscles, this specific enrichment takes place during perinatal growth. Deletion of Six1 in soleus impaired fast-type myofiber specialization during perinatal development, resulting in a slow phenotype and a complete lack of Myosin heavy chain 2A (MyHCIIA) expression. Global transcriptomic analysis of wild-type and Six1 mutant myofibers identified the gene networks controlled by Six1 in adult soleus muscle. This analysis showed that Six1 is required for the expression of numerous genes encoding fast-type sarcomeric proteins, glycolytic enzymes and controlling intracellular calcium homeostasis. Parvalbumin, a key player of calcium buffering, in particular, is a direct target of Six1 in the adult myofiber.

CONCLUSIONS

This analysis revealed that Six1 controls distinct aspects of adult muscle physiology in vivo, and acts as a main determinant of fast-fiber type acquisition and maintenance.

Differential expression of lipid metabolism-related genes and myosin heavy chain isoform genes in pig muscle tissue leading to different meat quality

The aim of this study was to investigate the variations in meat quality, lipid metabolism-related genes, myosin heavy chain (MyHC) isoform genes and peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) gene mRNA expressions in longissimus dorsi muscle (LM) of two different pig breeds. Six Rongchang and six Landrace barrows were slaughtered at 161 days of age. Subsequently, meat quality traits and gene expression levels in LM were observed. Results showed that Rongchang pigs not only exhibited greater pH, CIE a*24 h and intramuscular fat content but also exhibited lower body weight, carcass weight, dressing percentage, LM area and CIE b*24 h compared with Landrace pigs (P<0.05). Meanwhile, the mRNA expression levels of the lipogenesis (peroxisome proliferator-activated receptor gamma, acetyl-CoA carboxylase and fatty acid synthase) and fatty acid uptake (lipoprotein lipase)-related genes were greater in the Rongchang (P<0.05), whereas the lipolysis (adipose triglyceride lipase and hormone sensitive lipase) and fatty acid oxidation (carnitine palmitoyltransferase-1B)-related genes were better expressed in the Landrace. Moreover, compared with the Landrace, the mRNA expression levels of MyHCI, MyHCIIa and MyHCIIx were greater, whereas the mRNA expression levels of MyHCIIb were lower in the Rongchang pigs (P<0.05). In addition, the mRNA expression levels of PGC-1α were greater in Rongchang pigs than in the Landrace (P<0.05), which can partly explain the differences in MyHC isoform gene expressions between Rongchang and Landrace pigs. Although the small number of samples does not allow to obtain a definitive conclusion, we can suggest that Rongchang pigs possess better meat quality, and the underlying molecular mechanisms responsible for the better meat quality in fatty pigs may be partly due to the higher mRNA expression levels of lipogenesis and fatty acid uptake-related genes, as well as the oxidative and intermediate muscle fibers, and due to the lower mRNA expression levels of lipolysis and fatty acid oxidation-related genes, as well as the glycolytic muscle fibers.

Six homeoproteins and a Iinc-RNA at the fast MYH locus lock fast myofiber terminal phenotype

Thousands of long intergenic non-coding RNAs (lincRNAs) are encoded by the mammalian genome. However, the function of most of these lincRNAs has not been identified in vivo. Here, we demonstrate a role for a novel lincRNA, linc-MYH, in adult fast-type myofiber specialization. Fast myosin heavy chain (MYH) genes and linc-MYH share a common enhancer, located in the fast MYH gene locus and regulated by Six1 homeoproteins. linc-MYH in nuclei of fast-type myofibers prevents slow-type and enhances fast-type gene expression. Functional fast-sarcomeric unit formation is achieved by the coordinate expression of fast MYHs and linc-MYH, under the control of a common Six-bound enhancer.

Lampreys have a single gene cluster for the fast skeletal myosin heavy chain gene family

Muscle tissues contain the most classic sarcomeric myosin, called myosin II, which consists of 2 heavy chains (MYHs) and 4 light chains. In the case of humans (tetrapod), a total of 6 fast skeletal-type MYH genes (MYHs) are clustered on a single chromosome. In contrast, torafugu (teleost) contains at least 13 fast skeletal MYHs, which are distributed in 5 genomic regions; the MYHs are clustered in 3 of these regions. In the present study, the evolutionary relationship among fast skeletal MYHs is elucidated by comparing the MYHs of teleosts and tetrapods with those of cyclostome lampreys, one of two groups of extant jawless vertebrates (agnathans). We found that lampreys contain at least 3 fast skeletal MYHs, which are clustered in a head-to-tail manner in a single genomic region. Although there was apparent synteny in the corresponding MYH cluster regions between lampreys and tetrapods, phylogenetic analysis indicated that lamprey and tetrapod MYHs have independently duplicated and diversified. Subsequent transgenic approaches showed that the 5′-flanking sequences of Japanese lamprey fast skeletal MYHs function as a regulatory sequence to drive specific reporter gene expression in the fast skeletal muscle of zebrafish embryos. Although zebrafish MYH promoters showed apparent activity to direct reporter gene expression in myogenic cells derived from mice, promoters from Japanese lamprey MYHs had no activity. These results suggest that the muscle-specific regulatory mechanisms are partially conserved between teleosts and tetrapods but not between cyclostomes and tetrapods, despite the conserved synteny.

Evolution of the myosin heavy chain gene MYH14 and its intronic microRNA miR-499: muscle-specific miR-499 expression persists in the absence of the ancestral host gene

Background

A novel sarcomeric myosin heavy chain gene, MYH14, was identified following the completion of the human genome project. MYH14 contains an intronic microRNA, miR-499, which is expressed in a slow/cardiac muscle specific manner along with its host gene; it plays a key role in muscle fiber-type specification in mammals. Interestingly, teleost fish genomes contain multiple MYH14 and miR-499 paralogs. However, the evolutionary history of MYH14 and miR-499 has not been studied in detail. In the present study, we identified MYH14/miR-499 loci on various teleost fish genomes and examined their evolutionary history by sequence and expression analyses.

Results

Synteny and phylogenetic analyses depict the evolutionary history of MYH14/miR-499 loci where teleost specific duplication and several subsequent rounds of species-specific gene loss events took place. Interestingly, miR-499 was not located in the MYH14 introns of certain teleost fish. An MYH14 paralog, lacking miR-499, exhibited an accelerated rate of evolution compared with those containing miR-499, suggesting a putative functional relationship between MYH14 and miR-499. In medaka, Oryzias latipes, miR-499 is present where MYH14 is completely absent in the genome. Furthermore, by using in situ hybridization and small RNA sequencing, miR-499 was expressed in the notochord at the medaka embryonic stage and slow/cardiac muscle at the larval and adult stages. Comparing the flanking sequences of MYH14/miR-499 loci between torafugu Takifugu rubripes, zebrafish Danio rerio, and medaka revealed some highly conserved regions, suggesting that cis-regulatory elements have been functionally conserved in medaka miR-499 despite the loss of its host gene.

Conclusions

This study reveals the evolutionary history of the MYH14/miRNA-499 locus in teleost fish, indicating divergent distribution and expression of MYH14 and miR-499 genes in different teleost fish lineages. We also found that medaka miR-499 was even expressed in the absence of its host gene. To our knowledge, this is the first report that shows the conversion of intronic into non-intronic miRNA during the evolution of a teleost fish lineage.

Proteome dynamics during contractile and metabolic differentiation of bovine foetal muscle

Contractile and metabolic properties of bovine muscles play an important role in meat sensorial quality, particularly tenderness. Earlier studies based on Myosin heavy chain isoforms analyses and measurements of glycolytic and oxidative enzyme activities have demonstrated that the third trimester of foetal life in bovine is characterized by contractile and metabolic differentiation. In order to complete this data and to obtain a precise view of this phase and its regulation, we performed a proteomic analysis of Semitendinosus muscle from Charolais foetuses analysed at three stages of the third trimester of gestation (180, 210 and 260 days). The results complete the knowledge of important changes in the profiles of proteins from metabolic and contractile pathways. They provide new insights about proteins such as Aldehyde dehydrogenase family, Enolase, Dihydrolipoyl dehydrogenase, Troponin T or Myosin light chains isoforms. These data have agronomical applications not only for the management of beef sensorial quality but also in medical context, as bovine myogenesis appears very similar to human one.

Genome-wide association analysis of meat quality traits in a porcine Large White × Minzhu intercross population

Pork quality is an economically important trait and one of the main selection criteria for breeding in the swine industry. In this genome-wide association study (GWAS), 455 pigs from a porcine Large White × Minzhu intercross population were genotyped using the Illumina PorcineSNP60K Beadchip, and phenotyped for intramuscular fat content (IMF), marbling, moisture, color L*, color a*, color b* and color score in the longissimus muscle (LM). Association tests between each trait and the SNPs were performed via the Genome Wide Rapid Association using the Mixed Model and Regression-Genomic Control (GRAMMAR-GC) approach. From the Ensembl porcine database, SNP annotation was implemented using Sus scrofa Build 9. A total of 45 SNPs showed significant association with one or multiple meat quality traits. Of the 45 SNPs, 36 were located on SSC12. These significantly associated SNPs aligned to or were in close approximation to previously reported quantitative trait loci (QTL) and some were located within introns of previously reported candidate genes. Two haplotype blocks ASGA0100525-ASGA0055225-ALGA0067099-MARC0004712-DIAS0000861, and ASGA0085522-H3GA0056170 were detected in the significant region. The first block contained the genes MYH1, MYH2 and MYH4. A SNP (ASGA0094812) within an intron of the USP43 gene was significantly associated with five meat quality traits. The present results effectively narrowed down the associated regions compared to previous QTL studies and revealed haplotypes and candidate genes on SSC12 for meat quality traits in pigs.

Force-generating capacity of human myosin isoforms extracted from single muscle fibre segments

Muscle, motor unit and muscle fibre type-specific differences in force-generating capacity have been investigated for many years, but there is still no consensus regarding specific differences between slow- and fast-twitch muscles, motor units or muscle fibres. This is probably related to a number of different confounding factors disguising the function of the molecular motor protein myosin. We have therefore studied the force-generating capacity of specific myosin isoforms or combination of isoforms extracted from short single human muscle fibre segments in a modified single fibre myosin in vitro motility assay, in which an internal load (actin-binding protein) was added in different concentrations to evaluate the force-generating capacity. The force indices were the x-axis intercept and the slope of the relationship between the fraction of moving filaments and the α-actinin concentration. The force-generating capacity of the β/slow myosin isoform (type I) was weaker (P < 0.05) than the fast myosin isoform (type II), but the force-generating capacity of the different human fast myosin isoforms types IIa and IIx or a combination of both (IIax) were indistinguishable. A single fibre in vitro motility assay for both speed and force of specific myosin isoforms is described and used to measure the difference in force-generating capacity between fast and slow human myosin isoforms. The assay is proposed as a useful tool for clinical studies on the effects on muscle function of specific mutations or post-translational modifications of myosin.

Early development of medaka Oryzias latipes muscles as revealed by transgenic approaches using embryonic and larval types of myosin heavy chain genes

We cloned three full-length cDNAs encoding myosin heavy chains (MYHs) previously found to be expressed in embryos or larvae of medaka Oryzias latipes. Based on cDNA sequence information, the three medaka MYH genes, mMYH(emb1), mMYH(L1) and mMYH(L2), were localized on the chromosomes. In vivo promoter assay using the gene encoding green or red fluorescent protein and linked to the 5'-flanking region of mMYH demonstrated that the transcripts of fast-type mMYH(emb1), first expressed in embryos but belonging to the adult type in phylogenetic analysis, were located in the horizontal myoseptum. On the other hand, embryonic fast-type mMYH(L1) and mMYH(L2) were expressed in the whole myotomes. Interestingly, cells expressing mMYH(emb1) were localized together with engrailed, and cyclopamine, which blocks hedgehog signaling, inhibited mMYH(emb1) expression as well as the formation of the horizontal myoseptum, suggesting that muscle pioneer cells express mMYH(emb1) as a key protein in the formation of the horizontal myoseptum.

Three embryonic myosin heavy chain genes encoding different motor domain structures from common carp show distinct expression patterns in cranial muscles

Three embryonic myosin heavy chain (MYH) genes >> (MYHs) including MYH(emb1), MYH(emb2) and MYH(emb3) and encoding a C-terminal part of MYH were previously cloned and demonstrated to be expressed transiently in this order during development of common carp Cyprinus carpio embryos. The present study determined the full-length cDNA nucleotide sequences encoding the motor domain of the three MYHs, suggesting the implication of loop 1 and loop 2 sequences for the differences in the motor functions. Phylogenetic analysis based on the full-length amino acid sequences showed that MYH(emb1) and MYH(emb2) both belong to the fast types, though clearly differ from fast-type MYHs expressed in adult fast muscle previously reported. In contrast, MYH(emb3) was in a clade containing slow/cardiac type. Whole-mount immunostaining and in situ hybridization showed that the transcripts of the three embryonic MYHs are localized in the same or different cranial muscles of common carp larvae, suggesting that the three MYHs function cooperatively or individually in various cranial muscles.

Calcineurin plays a modulatory role in loading-induced regulation of type I myosin heavy chain gene expression in slow skeletal muscle

The role of calcineurin (Cn) in skeletal muscle fiber-type expression has been a subject of great interest because of reports indicating that it controls the slow muscle phenotype. To delineate the role of Cn in phenotype remodeling, particularly its role in driving expression of the type I myosin heavy chain (MHC) gene, we used a novel strategy whereby a profound transition from fast to slow fiber type is induced and examined in the absence and presence of cyclosporin A (CsA), a Cn inhibitor. To induce the fast-to-slow transition, we first subjected rats to 7 days of hindlimb suspension (HS) + thyroid hormone [triiodothyronine (T(3))] to suppress nearly all expression of type I MHC mRNA in the soleus muscle. HS + T(3) was then withdrawn, and rats resumed normal ambulation and thyroid state, during which vehicle or CsA (30 mg x kg(-1) x day(-1)) was administered for 7 or 14 days. The findings demonstrate that, despite significant inhibition of Cn, pre-mRNA, mRNA, and protein abundance of type I MHC increased markedly during reloading relative to HS + T(3) (P < 0.05). Type I MHC expression was, however, attenuated by CsA compared with vehicle treatment. In addition, type IIa and IIx MHC pre-mRNA, mRNA, and relative protein levels were increased in Cn-treated compared with vehicle-treated rats. These findings indicate that Cn has a modulatory role in MHC transcription, rather than a role as a primary regulator of slow MHC gene expression.

Expression of multiple slow myosin heavy chain genes reveals a diversity of zebrafish slow twitch muscle fibres with differing requirements for Hedgehog and Prdm1 activity

The zebrafish embryo develops a series of anatomically distinct slow twitch muscle fibres that characteristically express genes encoding lineage-specific isoforms of sarcomeric proteins such as MyHC and troponin. We show here that different subsets of these slow fibres express distinct members of a tandem array of slow MyHC genes. The first slow twitch muscle fibres to differentiate, which are specified by the activity of the transcription factor Prdm1 (also called Ubo or Blimp1) in response to Hedgehog (Hh) signalling, express the smyhc1 gene. Subsequently, secondary slow twitch fibres differentiate in most cases independently of Hh activity. We find that although some of these later-forming fibres also express smyhc1, others express smyhc2 or smyhc3. We show that the smyhc1-positive fibres express the ubo (prdm1) gene and adopt fast twitch fibre characteristics in the absence of Prdm1 activity, whereas those that do not express smyhc1 can differentiate independently of Prdm1 function. Conversely, some smyhc2-expressing fibres, although independent of Prdm1 function, require Hh activity to form. The adult trunk slow fibres express smyhc2 and smyhc3, but lack smyhc1 expression. The different slow fibres in the craniofacial muscles variously express smyhc1, smyhc2 and smyhc3, and all differentiate independently of Prdm1.

Intergenic bidirectional promoter and cooperative regulation of the IIx and IIb MHC genes in fast skeletal muscle

This study investigated the dynamic regulation of IIx-IIb MHC genes in the fast white medial gastrocnemius (WMG) muscle in response to intermittent resistance exercise training (RE), a model associated with a rapid shift from IIb to IIx expression (11). We investigated the effect of 4 days of RE on the transcriptional activity across the skeletal MHC gene locus in the WMG in female Sprague-Dawley rats. Our results show that RE resulted in significant shifts from IIb to IIx observed at both the pre-mRNA and mRNA levels. An antisense RNA (xII NAT) was detected in the intergenic (IG) region between IIx and IIb, extending across the entire IIx gene and into its promoter. The expression of the xII NAT was positively correlated with IIb pre-mRNA (R = +0.8), and negatively correlated with IIx pre-mRNA (R = -0.8). Transcription mapping of the IIx-IIb IG region revealed the generation of sense IIb and xII NATs from a single promoter region. This bidirectional promoter is highly conserved among species and contains several regulatory elements that may be implicated in its regulation. These results suggest that the IIx and the IIb genes are physically and functionally linked via the bidirectional promoter. In order for the IIx MHC gene to be regulated, a feedback mechanism from the IG xII NAT is needed. In conclusion, the IG bidirectional promoter generating antisense RNA appears to be essential for the coordinated regulation of the skeletal muscle MHC genes during dynamic phenotype shifts.

Hereditary myosin myopathies

Hereditary myosin myopathies have emerged as a new group of muscle diseases with highly variable clinical features and onset during fetal development, childhood or adulthood. They are caused by mutations in skeletal muscle myosin heavy chain (MyHC) genes. Mutations have been reported in two of the three MyHC isoforms expressed in adult limb skeletal muscle: type I (slow/beta-cardiac MyHC; MYH7) and type IIa (MYH2). The majority of more than 200 dominant missense mutations in MYH7 are associated with hypertrophic/dilated cardiomyopathy without signs or symptoms of skeletal myopathy. Several mutations in two different parts of the slow/beta-cardiac MyHC rod region are associated with two distinct skeletal myopathies without cardiomyopathy: Laing early onset distal myopathy and myosin storage myopathy (MSM). However, early onset distal myopathy and MSM caused by MYH7 mutations may also occur together with cardiomyopathy. MSM affects proximal or scapuloperoneal muscles whereas Laing distal myopathy primarily affects the dorsiflexor muscles of the toes and ankles. MSM is morphologically characterized by subsarcolemmal accumulation of myosin in type 1 fibers, whereas Laing distal myopathy is associated with variable and unspecific muscle pathology, frequently with hypotrophic type 1 muscle fibers. A myopathy associated with a specific mutation in MYH2 is associated with congenital joint contractures and external ophthalmoplegia. The disease is mild in childhood but may be progressive in adulthood, with proximal muscle weakness affecting ambulation. Mutations in embryonic MyHC (MYH3) and perinatal MyHC (MYH8), which are myosin isoforms expressed during muscle development, are associated with distal arthrogryposis syndromes with no or minor muscle weakness. Clinical findings, muscle morphology and molecular genetics in hereditary myosin myopathies are summarized in this review.

Fast skeletal muscle myosin heavy chain gene cluster of medaka Oryzias latipes enrolled in temperature adaptation

To disclose mechanisms involved in temperature acclimation of fish muscle, we subjected eurythermal fish of medaka Oryzias latipes to cloning of myosin heavy chain genes (MYHs). We cloned cDNAs encoding fast skeletal muscle myosin heavy chain (MYH) isoforms from cDNA libraries of medaka acclimated to 10 and 30 degrees C and observed that different MYH cDNA clones are expressed in the two temperature-acclimated fish. Subsequently, we isolated several overlapping MYH contigs by shotgun cloning strategy from a medaka genomic library. Contig assembly of the complete medaka MYH (mMYH) locus of 219 kbp revealed a cluster of tandemly arrayed 11 mMYHs, in which eight genes are actually transcribed, with the remaining three being pseudogenes. Expression analysis of the transcribed genes revealed that two genes were each highly expressed in medaka acclimated to 10 and 30 degrees C, whereas comparatively lower expression levels of the three genes were exclusively observed in medaka acclimated to 30 degrees C. cDNAs of the remaining genes were too underrepresented in the libraries to determine the expression levels, and the transcripts could only be obtained by reverse transcription-polymerase chain reaction. Deduced amino acid sequences in the loop 1 and loop 2 regions of mMYHs were highly variable, suggesting that these isoforms were functionally different. The present findings consolidate our knowledge on teleost MYH multigene family and would provide further insight into the mechanisms by which expressions of individual MYH molecules are fine-tuned with environmental temperature fluctuations with further functional analysis of the genes concerned.

cDNA cloning of myosin heavy chain genes from medaka Oryzias latipes embryos and larvae and their expression patterns during development

Several sarcomeric myosin heavy chains (MYHs) were cloned from embryos and larvae of medaka Oryzias latipes. Three genes encoding medaka MYHs (mMYHs) predominantly expressed in embryos (mMYH(emb1)) and larvae (mMYH(L1) and mMYH(L2)), all belonged to fast skeletal MYHs, showing spatiotemporally different expression patterns during development. Besides these mMYHs, a few novel mMYHs were cloned from embryos and larvae at hatching. Whereas mMYH(emb2), mMYH(emb3), and mMYH(L3) belonged to fast skeletal MYH, mMYH(C1) and mMYH(C2) did to slow/cardiac MYH. mMYH(emb1) was expressed ahead of mMYH(L1) and mMYH(L2). In situ hybridization analysis demonstrated that the transcripts of mMYH(emb1) and mMYH(C1) were located in the horizontal myoseptum, whereas those of mMYH(L1) and mMYH(L2) in the inner part of myotomes and pharyngeal muscles, and those of mMYH(C2) in the heart rudiment. In silico cloning based on the medaka genome database showed another mMYHs of the slow/cardiac types, mMYH(C3) and mMYH(C4).

Muscle cell and motor protein function in patients with a IIa myosin missense mutation (Glu-706 to Lys)

The pathogenic events leading to the progressive muscle weakness in patients with a E706K mutation in the head of the myosin heavy chain (MyHC) IIa were analyzed at the muscle cell and motor protein levels. Contractile properties were measured in single muscle fiber segments using the skinned fiber preparation and a single muscle fiber in vitro motility assay. A dramatic impairment in the function of the IIa MyHC isoform was observed at the motor protein level. At the single muscle fiber level, on the other hand, a general decrease was observed in the number of preparations where the specific criteria for acceptance were fulfilled irrespective of MyHC isoform expression. Our results provide evidence that the pathogenesis of the MyHC IIa E706K myopathy involves defective function of the mutated myosin as well as alterations in the structural integrity of all muscle cells irrespective of MyHC isoform expression.

The sarcomeric myosin heavy chain gene family in the dog: analysis of isoform diversity and comparison with other mammalian species

Sarcomeric myosin heavy chains (MyHC) are the major contractile proteins of cardiac and skeletal muscles and belong to class II MyHC. In this study the sequences of nine sarcomeric MyHC isoforms were obtained by combining assembled contigs of the dog genome draft available in the NCBI database. With this information available the dog becomes the second species, after human, for which the sequences of all members of the sarcomeric MyHC gene family are identified. The newly determined sequences of canine MyHC isoforms were aligned with their orthologs in mammals, forming a set of 38 isoforms, to search for the molecular features that determine the structural and functional specificity of each type of isoform. In this way the structural motifs that allow identification of each isoform and are likely determinants of functional properties were identified in six specific regions (surface loop 1, loop 2, loop 3, converter, MLC binding region, and S2 proximal segment).

Diversity of the pufferfish Takifugu rubripes fast skeletal myosin heavy chain genes

Myosin is a highly conserved, ubiquitous actin-based molecular motor that is distributed as diverse as from prokaryotes to mammalian tissues. Among various types in the myosin family proteins, class II, also called sarcomeric, myosin is a classical, conventional molecule that has been extensively studies on its functional and structural properties. It consists of two heavy chains (MYH) of about 200 kDa and four light chains of about 20 kDa. The exon-intron organization was determined for the major subunit of MYH, which contains ATP-hydrolysis and actin-binding sites, from torafugu (tiger pufferfish) Takifugu rubripes fast skeletal muscles. Comprehensive investigation for fast skeletal MYHs based on the fugu (torafugu) genome database and subsequent construction of their physical map revealed that torafugu contains at least 8 putative skeletal MYHs. Furthermore, genomic structural analysis revealed that skeletal MYHs are not clustered in a single locus, but rather spread to at least four loci, with two of them locating at the mammalian syntenic regions. Such arrangement of torafugu MYHs are in a marked contrast to mammalian fast skeletal MYHs that are clustered in a single locus. These data suggest that an ancient segmental duplication or whole-genome duplication occurred in fish lineage as in many other reported torafugu genes.

Biological organization of the extraocular muscles

Extraocular muscle is fundamentally distinct from other skeletal muscles. Here, we review the biological organization of the extraocular muscles with the intent of understanding this novel muscle group in the context of oculomotor system function. The specific objectives of this review are threefold. The first objective is to understand the anatomic arrangement of the extraocular muscles and their compartmental or layered organization in the context of a new concept of orbital mechanics, the active pulley hypothesis. The second objective is to present an integrated view of the morphologic, cellular, and molecular differences between extraocular and the more traditional skeletal muscles. The third objective is to relate recent data from functional and molecular biology studies to the established extraocular muscle fiber types. Developmental mechanisms that may be responsible for the divergence of the eye muscles from a skeletal muscle prototype also are considered. Taken together, a multidisciplinary understanding of extraocular muscle biology in health and disease provides insights into oculomotor system function and malfunction. Moreover, because the eye muscles are selectively involved or spared in a variety of neuromuscular diseases, knowledge of their biology may improve current pathogenic models of and treatments for devastating systemic diseases.

New insights into skeletal muscle fibre types in the dog with particular focus towards hybrid myosin phenotypes

Electrophoresis, immunoblots, immunohistochemistry and image analysis methods were applied to characterise canine trunk and appendicular muscle fibres according to their myosin heavy chain (MyHC) composition and to determine, on a fibre-to-fibre basis, the correlation between contractile [MyHC (s), myofibrillar ATPase (mATPase) and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoforms], metabolic [succinate dehydrogenase (SDH) and glycerol-3-phosphate dehydrogenase (GPDH) activities and glycogen and phospholamban (PLB) content] and morphological (cross-sectional area and capillary and nuclear densities) features of individual myofibres. An accurate delineation of MyHC-based fibre types was obtained with the developed immunohistochemical method, which showed high sensitivity and objectivity to delineate hybrid fibres with overwhelming dominance of one MyHC isoform. Phenotypic differences in contractile, metabolic and morphological properties seen between fibre types were related to MyHC content. All canine skeletal muscle fibre types had a relatively high histochemical SDH activity but significant differences existed in the order IIA>I>IIX. Mean GPDH was ranked according to fibre type such that I<IIA<IIX. Type IIA fibres were the smallest, type IIX fibres the largest and type I of intermediate size. Capillary and nuclear density decreased in the order IIA>I>IIX. Hybrid fibres, which represented nearly one third of the whole pool of skeletal muscle fibres analysed, had mean values intermediate between their respective pure phenotypes. Slow fibres expressed the slow SERCA isoform and PLB, whereas type II fibres expressed the fast SERCA isoform. Discrimination of myofibres according to their MyHC content was possible on the basis of their contractile, metabolic and morphological features. These intrafibre interrelationships suggest that myofibres of control dogs exhibit a high degree of co-ordination in their physiological, biochemical and morphological characteristics. This study demonstrates that canine skeletal muscle fibres have been misclassified in numerous previous studies and offers useful baseline data and new prospects for future work on muscle-fibre-typing in canine experimental studies.

Mutations and sequence variation in the human myosin heavy chain IIa gene (MYH2)

We recently described a new autosomal dominant myopathy associated with a missense mutation in the myosin heavy chain (MyHC) IIa gene (MYH2). In this study, we performed mutation analysis of MYH2 in eight Swedish patients with familial myopathy of unknown cause. In two of the eight index cases, we identified novel heterozygous missense mutations in MYH2, one in each case: V970I and L1061V. The mutations were located in subfragment 2 of the MyHC and they changed highly conserved residues. Most family members carrying the mutations had signs and symptoms consisting mainly of mild muscle weakness and myalgia. In addition, we analyzed the extent and distribution of nucleotide variation in MYH2 in 50 blood donors, who served as controls, by the complete sequencing of all 38 exons comprising the coding region. We identified only six polymorphic sites, five of which were synonymous polymorphisms. One variant, which occurred at an allele frequency of 0.01, was identical to the L1061V that was also found in one of the families with myopathy. The results of the analysis of normal variation indicate that there is strong selective pressure against mutations in MYH2. On the basis of these results, we suggest that MyHC genes should be regarded as candidate genes in cases of hereditary myopathies of unknown etiology.

Cluster characterisation and temporal expression of porcine sarcomeric myosin heavy chain genes

Members of the myosin heavy chain (MyHC) gene family are subjected to temporal regulation of gene switching during development. One strategy to the identification of cis-acting regulatory elements that are involved in temporal or fibre-type specific regulation is to undertake a comparative analysis of the MyHC gene family between the pig, an important target species, and other mammals, like human whose entire genome has been recently sequenced. Towards this end, we report here on the isolation, and characterisation of the porcine cardiac (MyHC slow/beta and alpha) and skeletal MyHC (embryonic, 2a, 2x, 2b and perinatal) gene clusters, and their structural comparisons with mouse and human clusters. The genome organisation of both clusters in the pig, human and mouse is conserved as having the same gene order, similar intergenic distances, and in the same head-to-tail orientation. For a period of pre-natal muscle growth, relative expression of MyHC isoforms, as determined by TaqMan real-time RT-PCR, correlated with the gene order in the skeletal MyHC cluster (embryonic > 2a > 2x > 2b) suggesting the possible presence of DNA elements on the same side as the MyHC embryonic gene that direct temporal regulation.

Myosin heavy chain composition of different skeletal muscles in Large White and Meishan pigs1

Four major sarcomeric myosin heavy chains (MyHC) (i.e., I, IIa, IIx, and IIb) are expressed in pig skeletal muscle during postnatal development. The objective of the current study was to compare MyHC composition at mRNA and protein levels in LM, a fast-twitch glycolytic muscle, and rhomboideus (RM), a mixed slow- and fast-twitch oxido-glycolytic muscle, between two pig breeds exhibiting dramatic differences in postnatal muscle growth and meat quality. Eight Large White (LW) and eight Meishan (MS) females were fed under the same standard conditions, and slaughtered at an average BW of 62 kg (131 and 142 d in LW and MS pigs, respectively). In addition to conventional fiber typing by histoenzymology, MyHC composition was analyzed by combining immunocytochemistry, in situ hybridization, and a newly developed real-time PCR assay. Enzyme activities of lactate dehydrogenase, citrate synthase, and beta-hydroxy-acyl-CoA-dehydrogenase were used as markers of glycolytic, oxidative and beta-oxidation capacities, respectively. Results showed that conventional fiber typing in three classes by histoenzymology was insufficient in LM. For the first time, four monoclonal antibodies specific of each MyHC isoform, working in immunocytochemistry, were used. Our results are consistent with the sequential I<-->IIa<-->IIx<-->IIb MyHC transition rule. Breed effect on MyHC composition differed between muscle types. In LM of MS pigs, a shift from IIb to IIx, and to a lesser extent, to IIa, occurred without affecting type I MyHC. In RM, where IIb is absent, a shift from IIx to type I occurred, with a slight decrease in the IIa isoform. Effects were very similar at the mRNA and protein levels, suggesting a transcriptional regulation. In both muscles, MS pigs exhibited a decrease in the relative fiber type specific expression of the fastest isoform (i.e., IIb in LM and IIx in RM). The shift toward a slower phenotype in MS pigs was consistent with a less glycolytic and more oxidative metabolism, potentially using more lipids as fuel. A dramatic increase in cross-sectional area of type I fibers in RM (+27%) and a decrease in that of the fastest IIb fibers in LM (-25%) were observed in MS pigs. Overall, interpretation of earlier data regarding muscle fiber type has been flawed by inaccurate fiber typing in most pig skeletal muscles.

Postnatal myosin heavy chain isoforms in prenatal porcine skeletal muscles: insights into temporal regulation

Our knowledge of the temporal expression of postnatal (adult) fast myosin heavy chain (MyHC) isoforms (2a, 2x, and 2b) in prenatal muscles is limited. Using the pig as a target species and large-animal model, we report on the qualitative and quantitative expression of the major post- and prenatal MyHC isoforms during gestation, as determined by TaqMan real-time PCR and immunohistochemistry. We found that postnatal fast MyHC mRNAs and proteins were expressed much earlier in the pig (gestation day 35) than was previously reported in small mammals. There was a high degree of coexpression and colocalisation of pre- and postnatal MyHC mRNAs and proteins in prenatal muscles. During a period of prenatal muscle growth (gestation days 35-77), relative expression of MyHC isoforms (embryonic > 2a > 2x > 2b) correlated with the gene order in the skeletal MyHC cluster, which suggests the possible presence of cis-acting elements on the same side as the MyHC embryonic gene associated with temporal regulation.

Modular organization of phylogenetically conserved domains controlling developmental regulation of the human skeletal myosin heavy chain gene family

The mammalian skeletal myosin heavy chain locus is composed of a six-membered family of tandemly linked genes whose complex regulation plays a central role in striated muscle development and diversification. We have used publicly available genomic DNA sequences to provide a theoretical foundation for an experimental analysis of transcriptional regulation among the six promoters at this locus. After reconstruction of annotated drafts of the human and murine loci from fragmented DNA sequences, phylogenetic footprint analysis of each of the six promoters using standard and Bayesian alignment algorithms revealed unexpected patterns of DNA sequence conservation among orthologous and paralogous gene pairs. The conserved domains within 2.0 kilobases of each transcriptional start site are rich in putative muscle-specific transcription factor binding sites. Experiments based on plasmid transfection in vitro and electroporation in vivo validated several predictions of the bioinformatic analysis, yielding a picture of synergistic interaction between proximal and distal promoter elements in controlling developmental stage-specific gene activation. Of particular interest for future studies of heterologous gene expression is a 650-base pair construct containing modules from the proximal and distal human embryonic myosin heavy chain promoter that drives extraordinarily powerful transcription during muscle differentiation in vitro.

Influence of early postnatal cold exposure on myofiber maturation in pig skeletal muscle

Early after birth, piglets rely almost exclusively on muscular shivering thermogenesis to produce heat in the cold and this can possibly modulate skeletal muscle development. An experiment involving 10 individually housed piglets was conducted to determine the influence of cold (24-15 degrees C, D5C group) vs. thermoneutrality (34-30 degrees C, D5TN group) between birth and 5 days on myosin heavy chain (MyHC) polymorphism and metabolic characteristics of longissimus lumborum (LL) and rhomboideus (RH) muscles. Five additional piglets were sacrificed at birth. Piglets exposed to cold received 43% more artificial milk on a liveweight basis in order to achieve similar growth rates. D5C piglets produced 93% more heat and exhibited intense shivering during the whole experiment. Contractile and metabolic characteristics of muscles were determined by immunocytochemistry, electrophoresis and enzyme activities. At least eight MyHC isoforms were detected, including atypical expressions of the alpha-cardiac and extraocular isoforms. Dramatic changes in MyHC composition, myofiber cross-sectional area (CSA) and energy metabolism occurred between birth and 5 days. Cold exposure did not affect either the total number of fibers or the CSA, but it did influence muscle maturation. In particular, it increased the expression of alpha-cardiac and type I MyHC, and decreased that of fetal MyHC, confirming an acceleration in the rate of postnatal maturation. An increase in oxidative enzyme activities was observed in both muscles in the cold, whereas the activity of a glycolytic enzyme, lactate dehydrogenase, remained unchanged. Cold exposure also induced an increase in T3 plasma levels. The extent to which these changes are the result of sustained shivering or are due to the action of hormonal factors, such as thyroid hormones, are discussed.

New insights into muscle fiber types in the pig

The accurate classification of skeletal muscle fiber types according to myosin heavy chain (MyHC) polymorphism remains a difficult task in the pig. Combined myofibrillar ATPase and metabolic enzyme histochemistry, in situ hybridization, and immunocytochemistry were performed on serial transverse sections of pig longissimus (L) and rhomboideus (R) muscles at 100 kg body weight to give a new insight into muscle fiber typing in the pig. Several monoclonal antibodies (MAbs) either specific for a single MyHC (I, IIa, or IIb) or of multiple MyHCs (IIa + IIx or I + IIx + IIb) were used. No monospecific IIx antibody was available for the pig. All three adult Type II isoforms were expressed in the white L muscle, whereas no IIb was observed in the red R muscle, which was confirmed using RNase protection analysis. In most fibers, the distribution of the transcripts closely matched that of the corresponding proteins. When observed, co-expression of MyHCs mostly occured for IIx and IIb in L muscle, and was more common at the protein (11.5%) than at the mRNA (2.2%) level. A minor proportion of myofibers showed a mismatch between MyHC mRNA and protein. According to the type grouping distribution of myofibers encountered in pig muscle, MyHC isoform expression followed the rank order of I-->IIa-->IIx-->IIb from the center to the periphery of the islets, concomitantly with a decrease in oxidative metabolism and an increase in fiber size. The developmental origin and functional significance of the type grouping distribution are discussed.

Changes in myosin structure and function in response to glycation

Nonenzymatic glycosylation (glycation) is recognized as an important post-translational modification underlying alterations of structure and function of extracellular proteins. The effect of glycation on intracellular proteins is, on the other hand, less well known despite the vital importance of intracellular proteins for cell, tissue, and organ function. The aim of this study was to explore the effects of glycation on the structure and function of skeletal muscle myosin. Myosin was incubated for up to 30 min with glucose and subsequently tested for structural and functional modifications by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and a single-fiber in vitro motility assay, respectively. MALDI spectra revealed glycation-related structural alterations as evidenced by the disappearance of specific Lys-C proteolysis products and the appearance of higher mass peaks that are attributed to cross-linking by glucose. This change was paralleled by a significant reduction in the in vitro motility speed, suggesting a structure-related decline in myosin mechanics in response to glucose exposure. Further evidence that early glycation products form in the regulatory regions of the myosin molecule is derived from the fact that there is complete reversal of motility speed after reaction with the Schiff base-cleaving agent hydroxylamine hydrochloride. Thus, glycation of skeletal muscle myosin has a significant effect on both the structural and functional properties of the protein, a finding that is important in understanding the mechanisms underlying the impairment in muscle function associated with aging and diabetes.

Limb myosin heavy chain isoproteins and muscle fiber types in the adult goat (Capra hircus)

The primary focus of this study was the accurate classification of limb skeletal muscle fiber types in adult goats (Capra hircus) according to the myosin heavy chain (MHC) isoform they express. Combined methodologies of gel electrophoresis, immunoblotting, immunohistochemistry, myofibrillar ATPase (mATPase), and quantitative metabolic enzyme histochemistry of M. semitendinosus samples were developed. Three MHCs were identified and tentatively designated as types I, IIA, and IIX. Five fiber types were defined immunohistochemically according to their MHC content: I, I+IIA, IIA, IIAX, and IIX. The hybrid fast-twitch fibers (IIAX) totaled 21% of the fiber population analyzed. The three major pure fibers (I, IIA, and IIX) could be objectively separated upon the basis of their mATPase activities after acid and alkaline preincubations. The prominent number of hybrid fibers, however, could not be delineated with these mATPase methods. Metabolic and size properties of muscle fibers varied according to their MHC content, but overlapped the full range of muscle fiber phenotypes. These integrated data demonstrate that type II skeletal muscle fibers of small ruminants have been misclassified in previous studies. The immunohistochemical approach developed in the present study offers new prospects for muscle fiber typing in caprine experimental studies and meat production technologies.