The myosin alkali light chain proteins and their genes

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.

Genome-wide association studies highlight novel risk loci for septal defects and left-sided congenital heart defects

BACKGROUND

Congenital heart defects (CHD) are structural defects of the heart affecting approximately 1% of newborns. They exhibit low penetrance and non-Mendelian patterns of inheritance as varied and complex traits. While genetic factors are known to play an important role in the development of CHD, the specific genetics remain unknown for the majority of patients. To elucidate the underlying genetic risk, we performed a genome wide association study (GWAS) of CHDs in general and specific CHD subgroups using the FinnGen Release 10 (R10) (N > 393,000), followed by functional fine-mapping through eQTL and co-localization analyses using the GTEx database.

RESULTS

We discovered three genome-wide significant loci associated with general CHD. Two of them were located in chromosome 17: 17q21.32 (rs2316327, intronic: LRRC37A2, Odds ratio (OR) [95% Confidence Interval (CI)] = 1.17[1.12-1.23], p = 1.5 × 10-9) and 17q25.3 (rs1293973611, nearest: BAHCC1, OR[95%CI] = 4.48[2.80-7.17], p = 7.0 × 10-10), respectively, and in addition to general CHD, the rs1293973611 locus was associated with the septal defect subtype. The third locus was in band 1p21.2 (rs35046143, nearest: PALMD, OR[95%CI] = 1.15[1.09-1.21], p = 7.1 × 10-9), and it was associated with general CHD and left-sided lesions. In the subgroup analysis, two additional loci were associated with septal defects (rs75230966 and rs6824295), and one with left-sided lesions (rs1305393195). In the eQTL analysis the variants rs2316327 (general CHD), and rs75230966 (septal defects) both located in 17q21.32 (with a LD r2 of 0.41) were both predicted to significantly associate with the expression of WNT9B in the atrial appendage tissue category. This effect was further confirmed by co-localization analysis, which also implicated WNT3 expression in the atrial appendage. A meta-analysis of general CHD together with the UK Biobank (combined N = 881,678) provided a different genome-wide significant locus in LRRC37A2; rs16941382 (OR[95%CI] = 1.15[1.11-1.20], p = 1.5 × 10-9) which is in significant LD with rs2316327.

CONCLUSIONS

Our results of general CHD and different CHD subcategories identified a complex risk locus on chromosome 17 near BAHCC1 and LRRC37A2, interacting with the genes WNT9B, WNT3 and MYL4, may constitute potential novel CHD risk associated loci, warranting future experimental tests to determine their role.

Transcriptomic analysis the mechanisms of anti-osteoporosis of desert-living Cistanche herb in ovariectomized rats of postmenopausal osteoporosis

Desert-living Cistanche herb (DC), as a traditional Chinese medicine for tonifying kidney yang, is often used to treat postmenopausal osteoporosis (PMOP). Total phenylethanoid glycosides are instruction ingredients for discrimination and assay according to the China pharmacopoeia for DC. This research aimed to reveal the anti-osteoporosis mechanism of total phenylethanoid glycosides of DC (PGC) by transcriptomic analysis of ovariectomized rats. Serum levels of BGP were evaluated by ELISA, the bone weight was measured, and transmission electron microscopy was used to examine the ultrastructure of osteoblasts in rats. In addition, micro-CT was used to detect the bone volume (Tb.BS/BV), bone mineral density (Tb.BMD), and bone mineral content (Tb.BMC) in trabecular bone, and the ratio of cortical bone area to total area (Ct.ar/Tt.ar), and the level of bone mineral content (Ct.BMC) in cortical bone. Differential expressed genes (DEGs) after PGC treatment were analyzed by transcriptomics. Then, a bioinformatics analysis of DEGs was carried out through GO enrichment, KEGG enrichment, and selection of the nucleus gene through the protein-protein interaction network. Through qRT-PCR analysis, the DEGs were verified. The analysis results indicated that PGC increased the secretion of osteogenic markers, and ultrastructural characterization of osteoblasts and bone morphology were improved in ovariectomized rats. A total of 269 genes were differentially expressed, including 201 genes that were downregulated and 68 genes that were upregulated between the model group and the PGC group. Bioinformation analysis results prompt the conclusion that PGC could promote the bone metabolism by muscle cell development, myofibril assembly, etc. In addition, our study also found that PGC has a good effect on osteoporosis complicated with cardiomyopathy, and it also provided evidence for the correlation between sarcopenia and osteoporosis.

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.

Myosin essential light chain 1sa decelerates actin and thin filament gliding on β-myosin molecules

The β-myosin heavy chain expressed in ventricular myocardium and the myosin heavy chain (MyHC) in slow-twitch skeletal Musculus soleus (M. soleus) type-I fibers are both encoded by MYH7. Thus, these myosin molecules are deemed equivalent. However, some reports suggested variations in the light chain composition between M. soleus and ventricular myosin, which could influence functional parameters, such as maximum velocity of shortening. To test for functional differences of the actin gliding velocity on immobilized myosin molecules, we made use of in vitro motility assays. We found that ventricular myosin moved actin filaments with ∼0.9 µm/s significantly faster than M. soleus myosin (0.3 µm/s). Filaments prepared from isolated actin are not the native interaction partner of myosin and are believed to slow down movement. Yet, using native thin filaments purified from M. soleus or ventricular tissue, the gliding velocity of M. soleus and ventricular myosin remained significantly different. When comparing the light chain composition of ventricular and M. soleus β-myosin, a difference became evident. M. soleus myosin contains not only the "ventricular" essential light chain (ELC) MLC1sb/v, but also an additional longer and more positively charged MLC1sa. Moreover, we revealed that on a single muscle fiber level, a higher relative content of MLC1sa was associated with significantly slower actin gliding. We conclude that the ELC MLC1sa decelerates gliding velocity presumably by a decreased dissociation rate from actin associated with a higher actin affinity compared to MLC1sb/v. Such ELC/actin interactions might also be relevant in vivo as differences between M. soleus and ventricular myosin persisted when native thin filaments were used.

Expression of atrial‑fetal light chains in cultured human cardiomyocytes after chemical ischemia‑reperfusion injury

Atrial light chains (ALC1) are naturally present in adult heart atria, while ventricular light chains (VLC1) are predominant in ventricles. Degradation of VLC1 and re-expression of ALC1 in heart ventricles are associated with heart disorders in response to pressure overload. The aim of the current study was to investigate changes in myosin light chain expression after simulated ischemia and simulated reperfusion (sI/sR). Human cardiomyocytes (HCM) isolated from adult heart ventricles were subjected to chemical ischemia. The control group was maintained under aerobic conditions. Myocyte injury was determined by testing lactate dehydrogenase (LDH) activity. The gene expression of ALC1, VLC1 and MMP-2 were assessed by reverse transcription-quatitive PCR. Additionally, protein synthesis was measured using ELISA kits and MMP-2 activity was measured by zymography. The results revealed that LDH activity was increased in sI/sR cell-conditioned medium (P=0.02), confirming the ischemic damage of HCM. ALC1 gene expression and content in HCM were also increased in the sI/sR group (P=0.03 and P<0.001, respectively), while VLC1 gene expression after sI/sR was decreased (P=0.008). Furthermore, MMP-2 gene expression and synthesis were lower in the sI/sR group when compared with the aerobic control group (P<0.001 and P=0.03, respectively). MMP-2 activity was also increased in sI/sR cell-conditioned medium (P=0.006). In conclusion, sI/sR treatment led to increased ALC1 and decreased VLC1 expression in ventricular cardiomyocytes, which may constitute an adaptive mechanism to altered conditions and contribute to the improvement of heart function.

mRNA expression patterns in human myocardial tissue, pericardial fluid and blood, and its contribution to the diagnosis of cause of death

Gene expression has become an interesting research area in forensic pathology to investigate the process of death at the molecular level. The aims of this study were to analyze changes in gene expression patterns in relation to the cause of death, and to propose new molecular markers of myocardial ischemia of potential use for the postmortem diagnosis of early ischemic heart damage in cases of sudden cardiac death (SCD). We determined mRNA levels of five proteins related with ischemic myocardial damage and repair - TNNI3, MYL3, TGFB1, MMP9 and VEGFA - in specific sites of the myocardium, blood and pericardial fluid in samples from 30 cadavers with different causes of death (SCD, multiple trauma, mechanical asphyxia, and other natural deaths). TNNI3 expression in blood, and MMP9 expression in pericardial fluid, were significantly higher when the cause of death was mechanical asphyxia, probably because of the more sensitive response of these proteins to acute systemic hypoxia/ischemia. Specifically, among SCD cases, increased MYL3, VEGFA and MMP9 values in the anterior wall of the right ventricle were found when the confirmed cause of death was acute myocardial infarction (AMI). Higher TGFB1 expression was found in the interventricular septum when AMI was not the cause of death, most likely as a reflection of the short duration of ischemia. Molecular biology techniques can provide complementary tools for the forensic diagnosis of early ischemic myocardial damage and AMI, and may make it possible to determine the duration and severity of myocardial ischemia.

Convergent evolution of marine mammals is associated with distinct substitutions in common genes

Phenotypic convergence is thought to be driven by parallel substitutions coupled with natural selection at the sequence level. Multiple independent evolutionary transitions of mammals to an aquatic environment offer an opportunity to test this thesis. Here, whole genome alignment of coding sequences identified widespread parallel amino acid substitutions in marine mammals; however, the majority of these changes were not unique to these animals. Conversely, we report that candidate aquatic adaptation genes, identified by signatures of likelihood convergence and/or elevated ratio of nonsynonymous to synonymous nucleotide substitution rate, are characterized by very few parallel substitutions and exhibit distinct sequence changes in each group. Moreover, no significant positive correlation was found between likelihood convergence and positive selection in all three marine lineages. These results suggest that convergence in protein coding genes associated with aquatic lifestyle is mainly characterized by independent substitutions and relaxed negative selection.

Distinct interactions between actin and essential myosin light chain isoforms

Binding of the utmost N-terminus of essential myosin light chains (ELC) to actin slows down myosin motor function. In this study, we investigated the binding constants of two different human cardiac ELC isoforms with actin. We employed circular dichroism (CD) and surface plasmon resonance (SPR) spectroscopy to determine structural properties and protein-protein interaction of recombinant human atrial and ventricular ELC (hALC-1 and hVLC-1, respectively) with α-actin as well as α-actin with alanin-mutated ELC binding site (α-actin(ala3)) as control. CD spectroscopy showed similar secondary structure of both hALC-1 and hVLC-1 with high degree of α-helicity. SPR spectroscopy revealed that the affinity of hALC-1 to α-actin (KD=575 nM) was significantly (p<0.01) lower compared with the affinity of hVLC-1 to α-actin (KD=186 nM). The reduced affinity of hALC-1 to α-actin was mainly due to a significantly (p<0.01) lower association rate (kon: 1,018 M(-1)s(-1)) compared with kon of the hVLC-1/α-actin complex interaction (2,908 M(-1)s(-1)). Hence, differential expression of ELC isoforms could modulate muscle contractile activity via distinct α-actin interactions.

Studies on RNA integrity and gene expression in human myocardial tissue, pericardial fluid and blood, and its postmortem stability

Analyses of gene expression of ischemic myocardial injury and repair related proteins has been carried out for the first time in samples from five specific sites of the myocardium, pericardial fluid and blood from thirty cadavers in relation to post-mortem interval (PMI). RNA integrity was evaluated by RNA integrity number (RIN), with values ranging from 6.57 to 8.11; sufficiently high levels of integrity to permit further gene amplification. No significant correlations between RIN and PMI in any samples were detected. Prior to target gene expression analysis, a normalization strategy was carried out to assess candidate reference gene stability, involving the analysis and comparison of four common housekeeping genes (Glyceraldehide-3-phosphate dehydrogenase, beta-actin, TATA box binding protein and Cyclophilin A). Gene expression of cardiac troponin I (TNNI3), myosin light chain 3 (MYL3), matrix metalloprotease 9 (MMP9), transforming growth factor beta 1 (TGFB1), and vascular endothelial growth factor A (VEGFA) in myocardial zones and body fluids were subsequently studied by real-time quantitative PCR. Expression levels of all the proteins studied in cardiac zone samples were similar. No statistical differences for expression were detected among proteins taken from any myocardial area. No significant differences were detected for TNNI3 and TGFB1 gene expressions when compared with samples at or under 12h-PMI or over 12h-PMI. However, differences in MYL3, MMP9, and VEGFA gene expression in body fluids were found at PMI periods of over 12h. These interesting results may contribute to the refinement of current knowledge regarding cardiac metabolism and improve understanding of the underlying mechanisms involved in myocardium ischemia and its repair.

Heavy and light roles: myosin in the morphogenesis of the heart

Myosin is an essential component of cardiac muscle, from the onset of cardiogenesis through to the adult heart. Although traditionally known for its role in energy transduction and force development, recent studies suggest that both myosin heavy-chain and myosin light-chain proteins are required for a correctly formed heart. Myosins are structural proteins that are not only expressed from early stages of heart development, but when mutated in humans they may give rise to congenital heart defects. This review will discuss the roles of myosin, specifically with regards to the developing heart. The expression of each myosin protein will be described, and the effects that altering expression has on the heart in embryogenesis in different animal models will be discussed. The human molecular genetics of the myosins will also be reviewed.

New insights into the regulation of myosin light chain phosphorylation in retinal pigment epithelial cells

The retinal pigment epithelium (RPE) plays an essential role in the function of the neural retina and the maintenance of vision. Most of the functions displayed by RPE require a dynamic organization of the acto-myosin cytoskeleton. Myosin II, a main cytoskeletal component in muscle and non-muscle cells, is directly involved in force generation required for organelle movement, selective molecule transport within cell compartments, exocytosis, endocytosis, phagocytosis, and cell division, among others. Contractile processes are triggered by the phosphorylation of myosin II light chains (MLCs), which promotes actin-myosin interaction and the assembly of contractile fibers. Considerable evidence indicates that non-muscle myosin II activation is critically involved in various pathological states, increasing the interest in studying the signaling pathways controlling MLC phosphorylation. Particularly, recent findings suggest a role for non-muscle myosin II-induced contraction in RPE cell transformation involved in the establishment of numerous retinal diseases. This review summarizes the current knowledge regarding myosin function in RPE cells, as well as the signaling networks leading to MLC phosphorylation under pathological conditions. Understanding the molecular mechanisms underlying RPE dysfunction would improve the development of new therapies for the treatment or prevention of different ocular disorders leading to blindness.

Changes of gene expression profiles in the cervical spinal cord by acupuncture in an MPTP-intoxicated mouse model: microarray analysis

It has been shown that acupuncture at acupoints GB34 and LR3 inhibits the degeneration of nigrostriatal neurons in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. The degeneration of spinal cord was reported to be induced in the MPTP-treated pre-symptomatic mouse. In this study, the gene expression profile changes following acupuncture at the acupoints were investigated in the cervical spinal cord of an MPTP-induced parkinsonism model using a whole transcript array (Affymetrix GeneChip mouse gene 1.0 ST array). It was shown that 8 of the probes up-regulated in MPTP, as compared to the control, were down-regulated after acupuncture at the acupoints. Of these 8 probes, 6 probes (4 annotated genes in 6 probes: Ctla2a, EG383229, Ppbp and Ube2l6) were exclusively down-regulated by acupuncture at the specific acupoints except for 2 probes as these 2 probes were commonly down-regulated by acupuncture at both the acupoints and the non-acupoints. In addition, 11 of the probes down-regulated in MPTP, as compared to the control, were up-regulated by acupuncture at the acupoints. Of these 11 probes, 10 probes (5 annotated genes in 10 probes: EG665033, ENSMUSG00000055323, Obox6, Pbp2 and Tmem150) were exclusively up-regulated by acupuncture at the specific acupoints except for the Fut11 because the Fut11 was commonly up-regulated by acupuncture at both the acupoints and the non-acupoints. The expression levels of the representative genes in the microarray were validated by real-time RT-PCR. These data suggest that the expression of these exclusively regulated 16 probes (9 genes) may be, at least in part, affected by acupuncture at the acupoints in the cervical spinal cord which can be damaged by MPTP intoxication.

Molecular basis of the phenotypic characteristics of mammalian muscle fibres

Adult mammalian skeletal muscle fibres can be separated into two distinct groups, fast and slow. Within each group there is a continuum of metabolic enzyme activity levels. In addition there are fast and slow isoforms of various myofibrillar proteins such as myosin, tropomyosin and troponin. These proteins are multimeric and multiple isoforms of their subunits assemble to create a continuum of subtypes within each major group. Fibres which coexpress both fast and slow subunit isoforms have an increased number of possible isoform combinations such that an entire spectrum of fibre 'types' is found between the two extremes, fast and slow. Numerous myosin heavy chain and fast troponin T isoforms further increase the diversity of muscle fibres. Such cellular diversity helps to explain the dynamic nature of skeletal muscle. Each individual fibre is able to respond to various functional demands by appropriate changes in its phenotypic expression of specific proteins.

Myosin essential light chain in health and disease

The essential light chain of myosin (ELC) is known to be important for structural stability of the alpha-helical lever arm domain of the myosin head, but its function in striated muscle contraction is poorly understood. Two ELC isoforms are expressed in fast skeletal muscle, a long isoform and its NH(2)-terminal approximately 40 amino acid shorter counterpart, whereas only the long ELC is observed in the heart. Biochemical and structural studies revealed that the NH(2)-terminus of the long ELC can make direct contacts with actin, but the effects of the ELC on the affinity of myosin for actin, ATPase, force, and the kinetics of force generating myosin cross-bridges are inconclusive. Myosin containing the long ELC has been shown to have slower cross-bridge kinetics than myosin with the short isoform. A difference was also reported among myosins with long isoforms. Increased shortening velocity was observed in atrial compared with ventricular muscle fibers. The common findings suggest that ELC provides the fine tuning of the myosin motor function, which is regulated in an isoform and tissue-dependent manner. The functional importance of the ELC is further implicated by the discovery of ELC mutations associated with Familial Hypertrophic Cardiomyopathy. The pathological phenotypes vary in severity, but more notably, almost all ELC mutations result in sudden cardiac death at a young age. This review summarizes the functional roles of striated muscle ELC in normal healthy muscle and in disease. Transgenic animal models and phenotypic characterization of ELC-mediated remodeling of the heart are also discussed.

Myofibrillar remodeling in cardiac hypertrophy, heart failure and cardiomyopathies

BACKGROUND

A wide variety of pathological conditions have been shown to result in cardiac remodelling and myocardial dysfunction. However, the mechanisms of transition from adaptive to maladaptive alterations, as well as those for changes in cardiac performance leading to heart failure, are poorly understood.

OBSERVATIONS

Extensive studies have revealed a broad spectrum of progressive changes in subcellular structures and function, as well as in signal transduction and metabolism in the heart, among different cardiovascular disorders. The present review is focused on identifying the alterations in molecular and biochemical structure of myofibrils (myofibrillar remodelling) in hypertrophied and failing myocardium in different types of heart diseases. Numerous changes at the level of gene expression for both contractile and regulatory proteins have already been reported in failing hearts and heart diseases; these changes are potential precursors for heart failure such as cardiac hypertrophy and cardiomyopathies. Myofibrillar remodelling, as a consequence of proteolysis, oxidation, and phosphorylation of some functional groups in both contractile and regulatory proteins in hearts failing due to different etiologies, has also been described.

CONCLUSIONS

Although myofibrillar remodelling appears to be associated with cardiac dysfunction, alterations in both contractile and regulatory proteins are dependent on the type and stage of heart disease.

Human atrial myosin light chain 1 expression attenuates heart failure

Most patients with hypertrophic cardiomyopathy and congenital heart diseases express the atrial essential myosin light chains (ALC-1) in their ventricles, replacing the ventricular essential light chains (VLC-1). VLC-1/ALC-1 isoform shift is correlated with increases in cardiac contractile parameters of a transgenic rat model overexpressing hALC-1 in the heart (TGR/hALC-1) compared to normal WKY rats. To investigate, whether the benefical effects of the hALC-1 on cardiac contractility could attenuate contractile failure of the overloaded heart, aortocaval shunt operations of 9-10 weeks old WKY and TGR/hALC-1 were performed. 5 weeks later, both animals groups were sacrificed for analysis of cardiac contraction and transgene expression. Control animals were operated but remained normal body and heart weights. The whole heart contractility parameters were evaluated using the Langendorff heart preparation. Shunt-operated TGR/hALC-1 and WKY rats developed comparable levels of cardiac hypertrophy which was associated with significant reduction of contractile parameters of the Langendorff hearts. However, the decline of cardiac contractility was less pronounced in shunt-operated TGR/hALC-1 compared to shunt-operated WKY. In fact, developed left ventricular pressure as well as maximal velocity of pressure development and relaxation were significantly higher in shunt-operated TGR/hALC-1 as compared to shunt-operated WKY. Expression of hALC-1 was 17 microg/mg whole SDS-protein in control (sham-operated) controls and declined significantly to 14 microg/mg whole SDS-protein in hypertrophied TGR/hALC-1. These results demonstrate that the expression of hALC-1 could have a beneficial effect on the overloaded hypertrophied heart.

Molecular defects in cardiac myofibrillar proteins due to thyroid hormone imbalance and diabetesThis paper is a part of a series in the Journal's "Made in Canada" section. The paper has undergone peer review

The heart very often becomes a victim of endocrine abnormalities such as thyroid hormone imbalance and insulin deficiency, which are manifested in a broad spectrum of cardiac dysfunction from mildly compromised function to severe heart failure. These functional changes in the heart are largely independent of alterations in the coronary arteries and instead reside at the level of cardiomyocytes. The status of cardiac function reflects the net of underlying subcellular modifications induced by an increase or decrease in thyroid hormone and insulin plasma levels. Changes in the contractile and regulatory proteins constitute molecular and structural alterations in myofibrillar assembly, called myofibrillar remodeling. These alterations may be adaptive or maladaptive with respect to the functional and metabolic demands on the heart as a consequence of the altered endocrine status in the body. There is a substantial body of information to indicate alterations in myofibrillar proteins including actin, myosin, tropomyosin, troponin, titin, desmin, and myosin-binding protein C in conditions such as hyperthyroidism, hypothyroidism, and diabetes. The present article is focussed on discussion how myofibrillar proteins are altered in response to thyroid hormone imbalance and lack of insulin or its responsiveness, and how their structural and functional changes explain the contractile defects in the heart.

Human fast skeletal myosin light chain 2 cDNA: isolation, tissue specific expression of the single copy gene, comparative sequence analysis of isoforms and evolutionary relationships

A cDNA clone encoding human fast skeletal myosin regulatory light chain (HSRLC) has been isolated and characterized from a fetal muscle cDNA library. The cDNA contains the coding sequence of 170 amino acids (aa) and 58 and 91 nucleotides in the 5' and 3' untranslated regions (UTRs), respectively. HSRLC is encoded by a single copy gene in the human genome and shows a tissue-specific pattern of expression in skeletal muscle. Comparison of derived amino acid sequence of HSRLC with database sequences reveals highly conserved 12 amino acid residues in a putative calcium-binding region. HSRLC is unique among all RLC sequences in having three consecutive potential phosphorylatable serine residues. The Cys-129 of HSRLC corresponds to the critical Gly-117 of scallop RLC that is essential for its regulatory function. The clusters of hydrophobic residues that are believed to stabilize the binding of NH2-terminal of RLC with myosin heavy chain show high sequence conservation in RLCs. Besides identifying specific targets for functional studies of HSRLC by mutagenesis, the results support the concept of an ancestral gene from which the RLC genes have evolved.

Myosin light chain 1 atrial isoform (MLC1A) is expressed in pre-B cells under control of the BOB.1/OBF.1 coactivator

The BOB.1/OBF.1 protein is a B-cell-specific coactivator of the Oct1 and Oct2 transcription factors. It is involved in mediating the transcriptional activity of the Oct proteins. However, animals deficient for BOB.1/OBF.1 showed virtually normal expression of genes that contain octamer motifs in their regulatory regions. To identify new genes that are regulated by BOB.1/OBF.1, we took advantage of a previously described cell system. RNAs differentially expressed in a BOB.1/OBF.1-deficient pre-B cell line and a derivative of this cell line expressing a hormone dependent BOB.1/OBF.1-estrogen receptor (BobER) fusion protein were isolated. Using the cDNA representational difference analysis method we could identify myosin light chain 1 atrial (MLC1A) isoform as a gene regulated by BOB.1/OBF.1. MLC1A was so far unknown to be expressed in tissues other than muscle. Here we demonstrate that MLC1A is indeed expressed in mouse pre-B cells. Analysis of the expressed mRNA revealed an alternative 5' promoter element and an alternative splice product, which had not yet been described for the murine gene. Cotransfection experiments with reporter constructs driven by the MLC1A promoter suggest that the regulation by BOB.1/OBF.1 is indirect. Consistent with this conclusion is the observation that transcriptional induction of the endogenous MLC1A gene by BOB.1/OBF.1 requires de novo protein synthesis.

Analysis of the energetic state of heart cells after adenovirus-mediated expression of hALC-1

Expression of the human atrial myosin light chain 1 (hALC-1) in the cardiac ventricle in vivo as well as in primary cultivated adult cardiomyocytes caused a pronounced positive inotropic effect. Therefore, it is one of the most promising candidate gene to treat congestive heart failure (CHF). In this work, we investigated, whether hALC-1 expression also modifies the energetic state of cardiomyocytes. Primary cultivated neonatal rat hearts cells (NRHC) were infected with adenoviral vectors (Ad vectors) containing a hALC-1 cDNA (AdCMV.hALC-1) or a control Ad vector. Infection efficiency of NRHC reached 100% at 50 multiplicity of infection (MOI). Interestingly and in contrast to primary cultures of liver cells, there were no cytotoxic side effects or induction of apoptosis up to MOI 50 in Ad vector infected NRHC. NRHC expressed large amounts of hALC-1 upon infection with AdCMV.hALC-1 which could easily been detected by protein staining and Western blot analysis. Analysis of intracellular hALC-1 localization by double-labeling immunofluorescence of AdCMV.hALC-1 infected cardiomyocytes revealed the typical myofibrillar striation pattern, as well as co-localization of hALC-1 with myosin heavy chains. There was no difference in the oxygen consumption between controls and AdCMV.hALC-1 infected NRHC. These data suggest that first: adenoviral vectors could be used as a safe and effective tool for gene transfer to cardiomyocytes, and second: that a positive inotropic effect of hALC-1 is not associated with enhanced oxygen consumption.

Thyroid hormone gene targets in ROS 17/2.8 osteoblast-like cells identified by differential display analysis

Thyroid hormone plays an important role in bone development and metabolism. We used a polymerase chain reaction (PCR)-based mRNA differential display (DD) analysis to obtain a profile of thyroid hormone-responsive genes in osteoblast-like cells (ROS 17/2.8). ROS 17/2.8 cells were treated with 10(-8) M triiodothyronine (T(3)) for 2 and 24 hours. Total RNA was isolated, reverse-transcribed, and amplified using a total of 72 combinations (2 hours) and 240 combinations (24 hours) of 5' and 3' primers. At the 2-hour time point, 1 true-positive novel clone was identified and shown to be the mitochondrial gene, subunit 6 of ATP synthase (ATPase-6). At the 24-hour time point, 3 differentially expressed (DE) mRNAs were confirmed as true-positives including; nonmuscle alkali myosin light chain (NM aMLC), ATPase-6, and one novel clone. T(3)-induction of ATPase-6 mRNA in ROS 17/2.8 cells was seen at 2 and 4 hours, but was maximal at 24 hours (2.1-fold). T(3) induction of ATPase-6 mRNA was increased to fourfold in ROS 17/2.8 cells cultured at a low density. NM aMLC mRNA was modestly upregulated by T(3) in ROS 17/2.8 cells by 1.4-fold, and induction was augmented at low cell density to 1.7-fold. T(3) action on NM aMLC and on the mitochondrial gene ATPase 6, represent novel targets and potential mediators of thyroid hormone action on bone. Cell type, and the extent of cell differentiation, influences T(3) regulation of genes in osteoblast-derived cells.

A molecular mechanism improving the contractile state in human myocardial hypertrophy

BACKGROUND

Various molecular mechanisms are operative in altering the sarcomeric function of the heart under increased hemodynamic workload. Expression of the atrial isoform (ALC-1) of the essential myosin light chain, a shift from alpha-myosin heavy chain (MHC) to beta-MHC, increased phosphorylation of the regulatory myosin light chains and increased troponin I (TnI) phosphorylation have been reported to modulate cardiac contractility in rodents.

METHODS

TO ASSESS A POSSIBLE CONTRIBUTION OF THESE SARCOMERIC PROTEINS TO CARDIAC PERFORMANCE IN HUMAN MYOCARDIAL HYPERTROPHY, TWO DIFFERENT FORMS OF CARDIAC HYPERTROPHY WERE INVESTIGATED: 19 patients with hypertropic obstructive cardiomyopathy (HOCM) and 13 patients with aortic stenosis (AS) with marked left ventricular hypertrophy and normal systolic function.

RESULTS

There was no change in MHC gene expression, regulatory myosin light chain or TnI phosphorylation status in normal heart (NH), HOCM and AS patients. However, patients with hypertrophied myocardium expressed ALC-1 that was not detectable in NH. ALC-1 protein expression correlated positively with the left ventricular ejection fraction. In patients with hypertrophied myocardium, there was a mean ALC-1 protein expression of 12.7+/-3% (range 3.6% to 32%).

CONCLUSION

In humans, ALC-1 expression is in vivo a powerful molecular mechanism of the sarcomere to maintain or improve myocardial contractility under increased hemodynamic demands.

Molecular cloning and sequence of Sparus aurata skeletal myosin light chains expressed in white muscle: developmental expression and thyroid regulation

Two full-length cDNA clones encoding the skeletal myosin light chain 2 (MLC2; 1452bp) and myosin light chain 3 (MLC3; 972bp) were isolated from a cDNA library prepared from gilthead sea bream Sparus aurata larvae. The MLC2 cDNA encoded a predicted protein of 170 residues that was 79% identical to rabbit MLC2 over the entire length and 87% identical within the Ca2+-binding region. The deduced amino acid sequence of MLC3 was 153 residues in length and was 91% and 69% identical to the zebrafish and rabbit MLC3, respectively. Northern blot analysis revealed that in adults both transcripts were expressed in fast white muscle only. MLC2 appeared earlier in development: MLC2 transcripts were detectable from the beginning of segmentation, whereas MLC3 transcripts did not appear until 27h post-fertilisation. At this developmental stage, a second MLC2 transcript of 0.89 kilobase-pairs was present. MLCs exhibited a different age-related pattern of response to varied thyroidal states, which were experimentally induced by the administration of 1μgg−1bodymass of thyroxine (T4) or triiodothyronine (T3), or 5ngg−1bodymass of the hypothyroidal compound thiourea; MLC3 expression was not significantly affected, whereas levels of MLC2 transcripts were significantly elevated in the white muscle only of juvenile sea bream after administration of T4. Although the mechanism of thyroidal regulation of MLC expression remains unknown, the present results suggest that different regulatory mechanisms exist for different MLCs.

Hybrid skeletal muscle fibres: a rare or common phenomenon?

1. The main aim of the present review is to raise awareness of the molecular complexity of single skeletal muscle fibres from "normal" and "transforming" muscles, in recognition of the many types of hybrids that have been observed in vertebrate skeletal muscle. The data used to illustrate various points made in the review were taken from studies on mammalian (mostly rat) and amphibian muscles. 2. The review provides a brief overview of the pattern and extent of molecular heterogeneity in hybrid muscle fibres and of the methodological problems encountered when attempting to identify and characterize such fibres. Particular attention is given to four types of skeletal muscle hybrids: (i) myosin heavy chain (MHC) hybrids; (ii) mismatched MHC-myosin light chains (MLC) hybrids; (iii) mismatched MHC-regulatory protein hybrids; and (iv) hybrids containing mismatched MHC-sarcoplasmic reticulum protein isoforms. 3. Some of the current ideas regarding the functional significance, origin and cognitive value of hybrid fibres are examined critically.

The second EF-hand is responsible for the isoform-specific sorting of myosin essential light chain

It has been known that isoforms of myosin essential light chain (LC) exhibit the isoform-specific sorting within cardiac myocytes and fibroblasts. In order to analyze which domain of LC is responsible for the sorting, various chimeric cDNA constructs between human nonmuscle isoform (LC3nm) and chicken fast skeletal muscle isoform (LC3f) were generated and expressed in cultured chicken cardiac myocytes. If chimeras contained LC3f sequence at the place that was restricted by BssHII and PstI, they were preferentially sorted to sarcomeres and precisely localized at A-bands, and their incorporation levels into the A-bands were identical with that of the wild type LC3f. However, other chimeras were distributed throughout the cytoplasm like the wild type LC3nm. Comparison of amino acid sequences revealed that 12 amino acids are different between chicken LC3f and human LC3nm in the BssHII-PstI fragment, and these amino acids are located within the second EF-hand of LC. These results indicated that the second EF-hand is responsible for the isoform-specific sorting of LC. Although the second EF-hand is not included in the key contacts with myosin heavy chain, it is supposed that this domain is important for the relative disposition of neighboring domains. Thus, the 12 amino acids in the second EF-hand might play a key role for modulation of overall configuration of LC, thereby influencing the precise association of the key contacts.

Developmental program expression of myosin alkali light chain and skeletal actin genes in the rainbow trout Oncorhynchus mykiss

We have isolated MLC1(F) (tMLC1(F)), MLC3(F) (tMLC3(F)) and skeletal actin cDNAs from the teleost Oncorhynchus mykiss. Sequence analysis indicates that tMLC1(F) and tMLC3(F) are not produced from differentially spliced mRNAs as reported in avians and rodents but are encoded by different genes. Results from RNase protection analysis showed that the corresponding transcripts are expressed in fast skeletal muscles. Whole-mount in situ hybridisation revealed distinct expression patterns of the myosin alkali light chains and skeletal actin genes during skeletal muscle development in the embryo.

Modular regulation of the MLC1F/3F gene and striated muscle diversity

Isoform diversity in striated muscle is largely controlled at the level of transcription. In this review we will concentrate on studies concerning transcriptional regulation of the alkali myosin light chain 1F/3F gene. Uncoupled activity of the MLC1F and 3F promoters, together with complex patterns of transcription in developing skeletal and cardiac muscle, combine to make analysis of this gene particularly intriguing. In vitro and transgenic studies of MLC1F/3F regulatory elements have revealed an array of cis-acting modules that each drive a subset of the expression pattern of the two promoters. These cis-acting regulatory modules, including the MLC1F and 3F promoter regions and two skeletal muscle enhancers, control tissue-specificity, cell or fibre-type specificity, and the spatiotemporal regulation of gene expression, including positional information. How each of these regulatory modules acts and how their individual activites are integrated to coordinate transcription at this locus are discussed.

Gene up-regulation in heart during mammalian hibernation

A cDNA library prepared from heart of hibernating golden-mantled ground squirrels, Spermophilus lateralis, was differentially screened to clone genes that were up-regulated during hibernation. Two differentially expressed clones were found after three rounds of screening and were confirmed as up-regulated by Northern blotting. Clone Ang6 encoded a polypeptide with 116 amino acids that was identified as the ventricular isoform of myosin light chain 1 (MLC1(v)). Clone Ang19 coded for 274 amino acid residues of the mitochondrially encoded protein subunit 2 of NADH-ubiquinone oxidoreductase (ND2). Both proteins showed high amino acid sequence identity with their human counterparts, 97.5% for MLC1(v) and 66% for ND2. Northern blot hybridization revealed differential expression of these genes in multiple organs during hibernation. Transcript levels of both were approximately twofold higher in heart and three- to fourfold higher in skeletal muscle of hibernating, versus euthermic, animals. ND2 was also up-regulated in hibernator liver. Hibernation-induced up-regulation of MLC1(v) suggests that a restructuring of myosin subunit composition could contribute to changes in muscle contractility needed for hypothermic function, whereas changes in ND subunit composition may affect the function of the electron transport chain during hibernation.

Expression and function of atrial myosin light chain 1 in the porcine right ventricle of normal and pulmonary hypertensive animals

We investigated the expression of atrial myosin light chain 1 (ALC-1) and myosin cycling kinetics in the normal and hypertrophied right ventricle of the neonatal porcine heart. Right ventricular hypertrophy was induced by exposing piglets immediately after birth to hypobaric hypoxia for 3 days. Control piglets were kept under normal conditions for the same time. ALC-1 expression in the hypertrophied right ventricle was 16.9%. No ALC-1 expression could be observed in the right ventricle of control pigs. Force-velocity of chemically skinned right ventricular fibers was analyzed in order to examine the functional role of ALC-1 expression on myosin cross-bridge kinetics. Force generation per cross-section at maximal Ca2+ activation (pCa 4.5) was significantly higher in the hypertrophied group. Maximal shortening velocity (Vmax) of skinned fibers increased statistically significant from 0.69 muscle length per second (ML/s) in the control to 1.2 ML/s in the hypertrophied right ventricle. We conclude that the expression of ALC-1 in the hypertrophied ventricle of neonatal pigs increased cross-bridge cycling kinetics and contractility.

Binding of myosin essential light chain to the cytoskeleton-associated protein IQGAP1

The 190 kD human IQGAP1 protein, by virtue of its N-terminal calponin-homology domain, is found associated with the actin cytoskeleton, and is capable of cross-linking actin filaments. IQGAP1 complexes with several proteins, including the Rho family GTPases Cdc42 and Rac, as well as calmodulin. It was previously noted that one of the IQ motifs of IQGAP1 displays significant similarity to a myosin heavy chain IQ motif responsible for binding the calmodulin-related myosin essential light chain (ELC). Employing the yeast two-hybrid methodology as well as in vitro binding experiments, we present evidence that a truncated version of IQGAP1 can interact with the myosin ELC. This interaction may have significant consequences for various cellular processes that involve actomyosin contractility, and suggests that the biological targets of the ELC may not be restricted to the myosin heavy chain.

Role of cardiac neural crest cells in cardiovascular development

The discovery in the chick embryo that a specific region of the neural crest, termed the cardiac neural crest, is essential for septation of the cardiac outflow tract and for aortic arch artery development has led to the classification of a whole series of human cardiac defects as neural crest-associated. Recently, several mouse genetic models have been effectively employed to yield new insights into the relationship between cardiac neural crest and structural heart development. In all the animal models of neural crest-related heart defects, prenatal mortality is too high to be attributed to structural defects of the heart alone, and there are obvious signs of severe cardiac dysfunction. The evidence indicates that poor viability is from impaired cardiac excitation-contraction coupling and contractile function at the myocyte level. The continued study of experimental and genetically defined models with neural crest-associated heart defects will prove useful in identifying the common pathways by which the neural crest contributes to normal heart development.

Role of the nerve in determining fetal skeletal muscle phenotype

To determine the role of the nerve on the establishment of myofiber diversity in skeletal muscles, the lumbosacral spinal cord of 14-day gestation mice (E14) was laser ablated, and the accumulation of the myosin alkali light chains (MLC) mRNAs in crural (hindleg) muscles was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each alkali MLC/ng total RNA in the extensor digitorum longus (EDL) and soleus muscles were determined with competitive polymerase chain reaction. Transcripts for all four alkali MLCs accumulate in aneural muscles. Data suggest that: (1) the absence of the nerve to either future fast or slow muscles results in less accumulation of MLC1V transcript. Moreover, the presence of the nerve is required for the enhanced accumulation of this transcript in future slow muscles; (2) the absence of innervation of future slow, but not fast, muscles decreases the accumulation of MLC1A transcript. Since increased accumulation of MLC1A and MLC1V transcripts are found in future slow muscles at birth, the nerve is necessary for the development of the slow phenotype during myogenesis; (3) MLC1F and MLC3F transcripts do not display any preferential accumulation in future fast muscles during the fetal period. Therefore, the establishment of the differential distribution of these mRNAs, based on fiber type, is a postnatal phenomenon. The nerve is required during the fetal period to allow accumulation of MLC3F messages above a basal level in future fast as well as slow muscles; whereas, the absence of the innervation to future fast, but not slow, muscles reduces the accumulation of MLC1F. Thus, the accumulation of the various alkali MLC mRNAs shows a differential, rather than coordinate, response to the absence of the nerve, and this response may vary depending on the future fiber type of the muscles.

Embryonic and fetal myogenic programs act through separate enhancers at the MLC1F/3F locus

Embryonic and fetal stages of skeletal muscle development are characterized by the differential expression of a number of muscle-specific genes. These include the products of independent promoters at the fast myosin light chain 1F/3F locus. In the mouse embryo MLC1F transcripts accumulate in embryonic skeletal muscle from E9, 4-5 days before high-level accumulation of MLC3F transcripts. A 3' enhancer can activate MLC1F and MLC3F promoters in differentiated muscle cells in vitro and in transgenic mice; both promoters, however, are activated at the time of MLC1F transcript accumulation. We now demonstrate the presence of a second muscle-specific enhancer at this locus, located in the intron separating the MLC1F and MLC3F promoters. Transgenic mice containing the intronic, but lacking the 3' enhancer, express high levels of an nlacZ reporter gene from the MLC3F promoter in adult fast skeletal muscle fibers. In contrast to the 3' enhancer, the intronic element is inactive both in embryonic muscle cells in vivo and in embryonic myocyte cultures. The intronic enhancer is activated at the onset of fetal development in both primary and secondary muscle fibers, at the time of endogenous MLC3F transcript accumulation. Late-activated MLC3F transgenes thus provide a novel in toto marker of fetal myogenesis. These results suggest that temporal regulation of transcription at the MLC1F/3F locus is controlled by separate enhancers which are differentially activated during embryonic and fetal development.

Smooth muscle myosin light chain kinase is transiently expressed in skeletal muscle during embryogenesis and muscle regeneration both in vivo and in vitro

By using a polyclonal antibody raised against smooth muscle Myosin Light Chain Kinase of adult chicken we show that the 135 kDa smooth muscle Myosin Light Chain Kinase isoform is present in neonatal and regenerating rat skeletal muscle, as well as in adult atrial myocardium. No reaction was evident in adult skeletal muscle fibres. In neonatal and in early regenerating muscle smooth muscle Myosin Light Chain Kinase is associated with embryonic myosin as revealed by their co-presence in muscle fibres. Experiments in vitro show the same results in myotubes. In atrial myocardium there is a patchy positivity in certain group of myocytes. Immunoblotting experiments show in muscle cell cultures, in neonatal and in regenerating skeletal muscle a protein band with electrophoretic mobility corresponding to that of smooth muscle Myosin Light Chain Kinase. These results suggest that the expression of smooth muscle Myosin Light Chain Kinase is not fully tissue-specific and that regulation of the contractile machinery could be different during myogenesis and in adulthood, in relation to the peculiar dynamic characteristics of developing muscles.

Adult fast myosin pattern and Ca2+-induced slow myosin pattern in primary skeletal muscle culture

A primary muscle cell culture derived from newborn rabbit muscle and growing on microcarriers in suspension was established. When cultured for several weeks, the myotubes in this model develop the completely adult pattern of fast myosin light and heavy chains. When Ca2+ ionophore is added to the culture medium on day 11, raising intracellular [Ca2+] about 10-fold, the myotubes develop to exhibit properties of an adult slow muscle by day 30, expressing slow myosin light as well as heavy chains, elevated citrate synthase, and reduced lactate dehydrogenase. The remarkable plasticity of these myotubes becomes apparent, when 8 days after withdrawal of the ionophore a marked slow-to-fast transition, as judged from the expression of isomyosins and metabolic enzymes, occurs.

The heart and development

The perspective from which the developing heart is viewed can lead to differing conclusions about the effects of development on cardiac function. The hearts of the embryo, fetus and adult, viewed from a global perspective, sustain the circulation through the same basic mechanisms of developing pressure and ejecting blood. The failure of the embryonic heart to perform these tasks results in growth failure, edema, and embryonic death, just as in the infant and adult such failure results in premature death. Furthermore, from the viewpoint of gross anatomy, following embryonic morphogenesis, the developing and adult hearts appear in general to be structurally similar, differing only in size and mass. However, a closer view shows, in the molecular and structural makeup of the myocardium, richly complex changes that can modulate the basic physiological properties of the cardiac myocyte. This article focuses on how these changes and the effects of birth and development alter ventricular function.

An essential myosin light chain peptide induces supramaximal stimulation of cardiac myofibrillar ATPase activity

The N-terminal region of skeletal myosin light chain-1 (MLC-1) binds to the C terminus of actin, yet the functional significance of this interaction is unclear. We studied a fragment (MLC-pep; residues 5-14) of the ventricular MLC-1. When added to rat cardiac myofibrils, 10 nM MLC-pep induced a supramaximal increase in the MgATPase activity at submaximal Ca2+ levels with no effect at low and maximal Ca2+ levels. A nonsense, scrambled sequence peptide had no effect at any pCa value. MLC-pep did not affect myosin KEDTA and CaATPase activities or actin-activated MgATPase activities in the absence or presence of tropomyosin. The MLC-pep did not alter the ability of troponin I to inhibit MgATPase activity. Moreover, when troponin I and troponin C were extracted from the myofibrils, the MLC-pep lost its ability to stimulate the ATPase rate. This effect was fully restored upon reconstitution of the extracted myofibrils with troponin I-troponin C complex. Thus, activation of MgATPase activity by the peptide required a full complement of thin filament regulatory proteins. Interestingly, the stimulatory effect occurred at a ratio of 4 peptides to 1 thin filament, suggesting that the peptide engages in a highly cooperative process that may involve activation of the entire thin filament.

Distinct gene expression patterns in skeletal and cardiac muscle are dependent on common regulatory sequences in the MLC1/3 locus

The myosin light-chain 1/3 locus (MLC1/3) is regulated by two promoters and a downstream enhancer element which produce two protein isoforms in fast skeletal muscle at distinct stages of mouse embryogenesis. We have analyzed the expression of transcripts from the internal MLC3 promoter and determined that it is also expressed in the atria of the heart. Expression from the MLC3 promoter in these striated muscle lineages is differentially regulated during development. In transgenic mice, the MLC3 promoter is responsible for cardiac-specific reporter gene expression while the downstream enhancer augments expression in skeletal muscle. Examination of the methylation status of endogenous and transgenic promoter and enhancer elements indicates that the internal promoter is not regulated in a manner similar to that of the MLC1 promoter or the downstream enhancer. A GATA protein consensus sequence in the proximal MLC3 promoter but not the MLC1 promoter binds with high affinity to GATA-4, a cardiac muscle- and gut-specific transcription factor. Mutation of either the MEF2 or GATA motifs in the MLC3 promoter attenuates its activity in both heart and skeletal muscles, demonstrating that MLC3 expression in these two diverse muscle types is dependent on common regulatory elements.

Regulation of human heart contractility by essential myosin light chain isoforms

Most of the patients with congenital heart diseases express the atrial myosin light chain 1 (ALC-1) in the right ventricle. We investigated the functional consequences of ALC-1 expression on the myosin cycling kinetics in the intact sarcomeric structure using multicellular demembranated fibers ("skinned fibers") from the right ventricular infundibulum of patients with Tetralogy of Fallot (TOF), double outlet right ventricle (DORV), and infundibular pulmonary stenosis (IPS), Force-velocity relation was analyzed by the constant-load technique at maximal Ca2+ activation (pCa 4.5). Half-time of tension development (t1/2) was investigated by monitoring contraction initiation upon photolytic release of ATP from caged-ATP in rigor. The patients investigated here expressed between 0 and 27% ALC-1. There was a statistically significant correlation between ALC-l and maximal shortening velocity (Vmax) which rose 1.87-fold from 1.2 muscle length per second (ML/s) to 2.25 ML/s in a normal (0% ALC-1) and diseased (19.9% ALC-1) ventricle. Half-time of tension development decreased 1.85-fold with increasing ALC-1 expression (t1/2) was 0.252 s and 0.136 s at 2 and 18.4% ALC-1, respectively). We conclude that the expression of ALC-1 in the human heart modulates cross-bridge cycling kinetics accelerating shortening velocity and isometric tension production.