Myomesin, M-protein and the structure of the sarcomeric M-band

Fiber-Type Shifting in Sarcopenia of Old Age: Proteomic Profiling of the Contractile Apparatus of Skeletal Muscles

The progressive loss of skeletal muscle mass and concomitant reduction in contractile strength plays a central role in frailty syndrome. Age-related neuronal impairments are closely associated with sarcopenia in the elderly, which is characterized by severe muscular atrophy that can considerably lessen the overall quality of life at old age. Mass-spectrometry-based proteomic surveys of senescent human skeletal muscles, as well as animal models of sarcopenia, have decisively improved our understanding of the molecular and cellular consequences of muscular atrophy and associated fiber-type shifting during aging. This review outlines the mass spectrometric identification of proteome-wide changes in atrophying skeletal muscles, with a focus on contractile proteins as potential markers of changes in fiber-type distribution patterns. The observed trend of fast-to-slow transitions in individual human skeletal muscles during the aging process is most likely linked to a preferential susceptibility of fast-twitching muscle fibers to muscular atrophy. Studies with senescent animal models, including mostly aged rodent skeletal muscles, have confirmed fiber-type shifting. The proteomic analysis of fast versus slow isoforms of key contractile proteins, such as myosin heavy chains, myosin light chains, actins, troponins and tropomyosins, suggests them as suitable bioanalytical tools of fiber-type transitions during aging.

Ca2+ as a coordinator of skeletal muscle differentiation, fusion and contraction

Muscle regeneration is essential for vertebrate muscle homeostasis and recovery after injury. During regeneration, muscle stem cells differentiate into myocytes, which then fuse with pre-existing muscle fibres. Hence, differentiation, fusion and contraction must be tightly regulated during regeneration to avoid the disastrous consequences of premature fusion of myocytes to actively contracting fibres. Cytosolic calcium (Ca²⁺ ), which is coupled to both induction of myogenic differentiation and contraction, has more recently been implicated in the regulation of myocyte-to-myotube fusion. In this viewpoint, we propose that Ca²⁺ -mediated coordination of differentiation, fusion and contraction is a feature selected in the amniotes to facilitate muscle regeneration.

Case Report: Tetralogy of Fallot in a Chinese Family Caused by a Novel Missense Variant of MYOM2

Background

Rare genetic variants have been identified to be important contributors to the risk of Tetralogy of Fallot (TOF), the most common cyanotic congenital heart disease (CHD). But relatively limited familial studies with small numbers of TOF cases have been reported to date. In this study, we aimed to identify novel pathogenic genes and variants that caused TOF in a Chinese family using whole exome sequencing (WES).

Methods

A Chinese family whose twins were affected by TOF were recruited for this study. A WES was performed for the affected twins, their healthy brother, and parents to identify the potential pathogenic mutated gene(s). Heterozygous variants carried by the twins, but not the unaffected brother, were retained. Public databases were used to assess the frequencies of the selected variants, and online prediction tools were accessed to predict the influences of these variants on protein function. The final candidate variant was further confirmed by Sanger sequencing in other members of the family.

Results

After several filtering processes, a heterozygous missense variant in the MYOM2 gene (NM_003970.4:c.3097C>T:p.R1033C) was identified and confirmed by Sanger sequencing in the affected twins and their unaffected father, suggesting an inheritance pattern with incomplete penetrance. The variant was found to be extremely rare in the public databases. Furthermore, the mutated site was highly conserved among mammals, and as shown using multiple online prediction tools, this variant was predicted to be a detrimental variant.

Conclusion

We assessed a family with TOF caused by a rare heterozygous missense variant of MYOM2. Our findings not only further confirm the significant role of genetics in the incidence of TOF but also expand the spectrum of the gene variants that lead to TOF.

Necrotic enteritis challenge regulates peroxisome proliferator-1 activated receptors signaling and β-oxidation pathways in broiler chickens

Necrotic enteritis (NE) is an important enteric disease in poultry and has become a major concern in poultry production in the post-antibiotic era. The infection with NE can damage the intestinal mucosa of the birds leading to impaired health and, thus, productivity. To gain a better understanding of how NE impacts the gut function of infected broilers, global mRNA sequencing (RNA-seq) was performed in the jejunum tissue of NE challenged and non-challenged broilers to identify the pathways and genes affected by this disease. Briefly, to induce NE, birds in the challenge group were inoculated with 1 mL of Eimeria species on day 9 followed by 1 mL of approximately 10⁸ CFU/mL of a NetB producing Clostridium perfringens on days 14 and 15. On day 16, 2 birds in each treatment were randomly selected and euthanized and the whole intestinal tract was evaluated for lesion scores. Duodenum tissue samples from one of the euthanized birds of each replicate (n = 4) was used for histology, and the jejunum tissue for RNA extraction. RNA-seq analysis was performed with an Illumina RNA HiSeq 2000 sequencer. The differentially expressed genes (DEG) were identified and functional analysis was performed in DAVID to find protein–protein interactions (PPI). At a false discovery rate threshold <0.05, a total of 377 DEG (207 upregulated and 170 downregulated) DEG were identified. Pathway enrichment analysis revealed that DEG were considerably enriched in peroxisome proliferator-activated receptors (PPAR) signaling (P < 0.01) and β-oxidation pathways (P < 0.05). The DEG were mostly related to fatty acid metabolism and degradation (cluster of differentiation 36 [CD36], acyl-CoA synthetase bubblegum family member-1 [ACSBG1], fatty acid-binding protein-1 and -2 [FABP1] and [FABP2]; and acyl-coenzyme A synthetase-1 [ACSL1]), bile acid production and transportation (acyl-CoA oxidase-2 [ACOX2], apical sodium–bile acid transporter [ASBT]) and essential genes in the immune system (interferon-, [IFN-γ], LCK proto-oncogene, Src family tyrosine kinase [LCK], zeta chain of T cell receptor associated protein kinase 70 kDa [ZAP70], and aconitate decarboxylase 1 [ACOD1]). Our data revealed that pathways related to fatty acid digestion were significantly compromised which thereby could have affected metabolic and immune responses in NE infected birds.

Three-dimensional structure of the M-region (bare zone) of vertebrate striated muscle myosin filaments by single-particle analysis

The rods of anti-parallel myosin molecules overlap at the centre of bipolar myosin filaments to produce an M-region (bare zone) that is free of myosin heads. Beyond the M-region edges, myosin molecules aggregate in a parallel fashion to yield the bridge regions of the myosin filaments. Adjacent myosin filaments in striated muscle A-bands are cross-linked by the M-band. Vertebrate striated muscle myosin filaments have a 3-fold rotational symmetry around their long axes. In addition, at the centre of the M-region, there are three 2-fold axes perpendicular to the filament long axis, giving the whole filament dihedral 32-point group symmetry. Here we describe the three-dimensional structure obtained by a single-particle analysis of the M-region of myosin filaments from goldfish skeletal muscle under relaxing conditions and as viewed in negative stain. This is the first single-particle reconstruction of isolated M-regions. The resulting three-dimensional reconstruction reveals details to about 55 Å resolution of the density distribution in the five main nonmyosin densities in the M-band (M6′, M4′, M1, M4 and M6) and in the myosin head crowns (P1, P2 and P3) at the M-region edges. The outermost crowns in the reconstruction were identified specifically by their close similarity to the corresponding crown levels in our previously published bridge region reconstructions. The packing of myosin molecules into the M-region structure is discussed, and some unidentified densities are highlighted.

The bHLH/Per-Arnt-Sim transcription factor SIM2 regulates muscle transcript myomesin2 via a novel, non-canonical E-box sequence

Despite a growing number of descriptive studies that show Single-minded 2 (Sim2) is not only essential for murine survival, but also upregulated in colon, prostate and pancreatic tumours, there is a lack of direct target genes identified for this basic helix-loop-helix/PAS transcription factor. We have performed a set of microarray experiments aimed at identifying genes that are differentially regulated by SIM2, and successfully verified that the Myomesin2 (Myom2) gene is SIM2-responsive. Although SIM2 has been reported to be a transcription repressor, we find that SIM2 induces transcription of Myom2 and activates the Myom2 promoter sequence when co-expressed with the heterodimeric partner protein, ARNT1, in human embryonic kidney cells. Truncation and mutation of the Myom2 promoter sequence, combined with chromatin immunoprecipitation studies in cells, has lead to the delineation of a non-canonical E-box sequence 5'-AACGTG-3' that is bound by SIM2/ARNT1 heterodimers. Interestingly, in immortalized human myoblasts knock down of Sim2 results in increased levels of Myom2 RNA, suggesting that SIM2 is acting as a repressor in these cells and so its activity is likely to be highly context dependent. This is the first report of a direct SIM2/ARNT1 target gene with accompanying analysis of a functional response element.

Estrogen receptor alpha is a major contributor to estrogen-mediated fetal testis dysgenesis and cryptorchidism

Failure of the testes to descend into the scrotum (cryptorchidism) is one of the most common birth defects in humans. In utero exposure to estrogens, such as 17beta-estradiol (E2) or the synthetic estrogen diethylstilbestrol (DES), down-regulates insulin-like 3 (Insl3) expression in embryonic Leydig cells, which in turn results in cryptorchidism in mice. To identify the molecular mechanism whereby xenoestrogens block Insl3 gene transcription, we performed a microarray analysis of wild-type or estrogen receptor (ER) alpha-mutant testes exposed in utero to pharmacological doses of E2 or DES. Six and 31 genes were respectively down-regulated and up-regulated by estrogen exposure (> or =4-fold). All six genes down-regulated by estrogen exposure, including Insl3 and the steroidogenic genes steroidogenic acute regulatory protein and cytochrome P450 17alpha-hydroxylase/17,20-lyase, were done so by an ERalpha-dependent mechanism. In contrast, up-regulation was mediated either by ERalpha for 12 genes or by an independent mechanism for the 19 remaining genes. Finally, we show that Insl3 gene expression and testicular descent were not affected by in utero exposure to E2 or DES in ERalpha mutant mice, whereas absence of ERbeta did not influence the effect of these estrogens. Collectively, these data demonstrate that xenoestrogens inhibit the endocrine functions of fetal Leydig cells through an ERalpha-dependent mechanism.

Gene expression profiling of androgen receptor antagonists in the rat fetal testis reveals few common gene targets

The androgen receptor (AR) is expressed in the fetal testis; however, the role of AR in fetal testicular development is poorly understood. Disrupted AR activity and subsequent gene expression alterations may disturb developmental programming of the fetal testis and result in testicular abnormalities later in life. The present study was performed to examine global gene expression patterns in rat fetal testis following in utero exposure to various AR antagonists. Pregnant Sprague-Dawley rats were treated with flutamide (50 mg/kg/day), linuron (50 mg/kg/day), vinclozolin (200 mg/kg/day), p,p'-DDE (100 mg/kg/day) or corn oil vehicle by gavage daily from gestation day (GD) 12-19. Testes were isolated on GD 19, and AR immunostaining, histology, and global changes in gene expression were determined. There were no alterations in the pattern or expression level of AR and no apparent histological changes in the fetal testes in any treatment group. Microarray analysis using Dunnett's test with multiple testing correction revealed no significant gene expression alterations following exposure to flutamide, linuron, vinclozolin, and p,p'-DDE. A less stringent analysis yielded some chemical specific effects on gene expression, and these effects were further evaluated by real-time RT-PCR. Vinclozolin treatment reduced the expression of several genes involved in cholesterol biosynthesis, though the testosterone levels were unchanged in the fetal testes in any treatment group. In flutamide, linuron, and p,p'-DDE treatment groups, the expression of hemoglobin Y, beta-like embryonic chain (Hbb-y) was reduced. Myomesin 2 (Myom2) expression was increased following linuron treatment. Given the lack of a common set of genes and the absence of overt histopathology, we conclude that the fetal testis is not a major target for AR activity at this stage of development although some cell-type specific gene expression changes cannot be ruled out.

Study of the mechanical properties of myomesin proteins using dynamic force spectroscopy

Myomesin is the most prominent structural component of the sarcomeric M-Band that is expressed in mammalian heart and skeletal muscles. Like titin, this protein is an intracellular member of the Ig-fibronectin superfamily, which has a flexible filamentous structure and which is largely composed of two types of domain that are similar to immunoglobulin (Ig)-like and fibronectin type III (FNIII) domains. Several myomesin isoforms have been identified, and their expression patterns are highly regulated both spatially and temporally. Particularly, alternative splicing in the central part of the molecule gives rise to an isoform, EH (embryonic heart)-myomesin, containing a serine and proline-rich insertion with no well-defined secondary structure, the EH segment. EH-myomesin represents the major myomesin isoform at embryonic stages of mammalian heart and is rapidly down-regulated around birth, but it is re-expressed in the heart of patients suffering from dilated cardio-myopathy. Here, in order to facilitate a better understanding of the physiological, and possibly pathological, functions of myomesin proteins, we explore the mechanical stability, elasticity and force-driven structural changes of human myomesin's sub-molecular segments using single-molecule force spectroscopy and protein engineering. We find that human myomesin molecules are composed of modules (Ig and FNIII), that are designed to withstand force and we demonstrate that the human cardiac EH segment functions like an additional elastic stretch in the middle part of the EH-myomesin and behaves like a random coil. Consequently myomesin isoforms (proteins with or without the EH segment) have different elastic properties, the EH-myomesin being the more compliant one. These findings imply that the compliance of the M-band increases with the amount of EH-myomesin it contains. So, we provide the evidence that not only titin but also other sarcomeric proteins have complicated visco-elastic properties depending on the contractile parameters in different muscle types.

Paradoxical absence of M lines and downregulation of creatine kinase in mouse extraocular muscle

The M lines are structural landmarks in striated muscles, necessary for sarcomeric stability and as anchoring sites for the M isoform of creatine kinase (CK-M). These structures, especially prominent in fast skeletal muscles, are missing in rodent extraocular muscle, a particularly fast and active muscle group. In this study, we tested the hypotheses that 1). myomesin and M protein (cytoskeletal components of the M lines) and CK-M are downregulated in mouse extraocular muscle compared with the leg muscles, gastrocnemius and soleus; and 2). the expression of other cytosolic and mitochondrial CK isoforms is correspondingly increased. As expected, mouse extraocular muscles expressed lower levels of myomesin, M protein, and CK-M mRNA than the leg muscles. Immunocytochemically, myomesin and M protein were not detected in the banding pattern typically seen in other skeletal muscles. Surprisingly, message abundance for the other known CK isoforms was also lower in the extraocular muscles. Moreover, total CK activity was significantly decreased compared with that in the leg muscles. Based on these data, we reject our second hypothesis and propose that other energy-buffering systems may be more important in the extraocular muscles. The downregulation of major structural and metabolic elements and relative overexpression of two adenylate kinase isoforms suggest that the extraocular muscle group copes with its functional requirements by using strategies not seen in typical skeletal muscles.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Ion-dependence of Z-line and M-line response to calcium in striated muscle fibres in rigor

The calcium-dependent contraction of vertebrate skeletal muscle is thought to be primarily controlled through the interaction of the thick and thin filaments. Through measurement of the Donnan potential, we have shown that an electrical switching mechanism (sensitive to both anions and cations) is present in both A- and I-bands [1]. Here we show that this mechanism is not confined to the contractile apparatus and report for the first time the presence of M-line potentials. The Z-line responds to Ca2+ ions in a similar manner to the A-band under the same solution conditions (phosphate-chloride and imidazole buffers), even though it has no reported Ca2+ binding sites. Z-line potentials were not observed in tris-acetate buffer. The M-line has a markedly different response to any of the other subsarcomeric regions, however, and can only be detected in the phosphate-chloride buffer. Preliminary observations of the M-line potential in creatine kinase-deficient mouse muscle (phosphate-chloride buffer) reveal significant differences in the calcium-induced transitions between two of the genotypes and demonstrate definitively that it is the M-line potential that is being recorded. From these results, it seems likely that the charge response of the Z-line and M-line is being mediated by titin in an anion-dependent manner. Our evidence comes from several observations. First, the similarity between the response of the Z-line potentials to the A-band potentials, where titin is the only link between these structures and second, the differential observation of M-line and Z-line potentials in a range of buffers containing different anion(s). Both Z-line and M-line potentials were seen in phosphate-chloride buffer, but only the Z-line potentials could be detected in chloride-only (imidazole) buffer and neither was observed in the acetate buffer. The latter observations can be attributed to two sources. The first is the effect of acetate buffer on the conformation of myosin [2]; the second is the absence of binding of the M-line protein, myomesin, to titin in the absence of phosphate ions [3].