Molecular cloning and characterization of human cardiac alpha- and beta-form myosin heavy chain complementary DNA clones. Regulation of expression during development and pressure overload in human atrium

Small Conductance Ca2 +-Activated K+ (SK) Channel mRNA Expression in Human Atrial and Ventricular Tissue: Comparison Between Donor, Atrial Fibrillation and Heart Failure Tissue

In search of more efficacious and safe pharmacological treatments for atrial fibrillation (AF), atria-selective antiarrhythmic agents have been promoted that target ion channels principally expressed in the atria. This concept allows one to engage antiarrhythmic effects in atria, but spares the ventricles from potentially proarrhythmic side effects. It has been suggested that cardiac small conductance Ca²⁺-activated K⁺ (SK) channels may represent an atria-selective target in mammals including humans. However, there are conflicting data concerning the expression of SK channels in different stages of AF, and recent findings suggest that SK channels are upregulated in ventricular myocardium when patients develop heart failure. To address this issue, RNA-sequencing was performed to compare expression levels of three SK channels (KCNN1, KCNN2, and KCNN3) in human atrial and ventricular tissue samples from transplant donor hearts (no cardiac disease), and patients with cardiac disease in sinus rhythm or with AF. In addition, for control purposes expression levels of several genes known to be either chamber-selective or differentially expressed in AF and heart failure were determined. In atria, as compared to ventricle from transplant donor hearts, we confirmed higher expression of KCNN1 and KCNA5, and lower expression of KCNJ2, whereas KCNN2 and KCNN3 were statistically not differentially expressed. Overall expression of KCNN1 was low compared to KCNN2 and KCNN3. Comparing atrial tissue from patients with AF to sinus rhythm samples we saw downregulation of KCNN2 in AF, as previously reported. When comparing ventricular tissue from heart failure patients to non-diseased samples, we found significantly increased ventricular expression of KCNN3 in heart failure, as previously published. The other channels showed no significant difference in expression in either disease. Our results add weight to the view that SK channels are not likely to be an atria-selective target, especially in failing human hearts, and modulators of these channels may prove to have less utility in treating AF than hoped. Whether targeting SK1 holds potential remains to be elucidated.

Expression of cell adhesion molecule, Gicerin/CD146 during the formation of heart and in the cardiac hypertrophy

Gicerin/CD146 is a cell adhesion molecule which belongs to the immunoglobulin (Ig) superfamily. We have reported the existence of gicerin/CD146 in the nervous system, heart, lung and smooth muscles of blood vessels. In this study, we make a cardiac hypertrophy model rat by constricting the rat aorta (AAC, ascending aortic constriction) and examined the effect on the expression of gicerin/CD146 in the heart. We found that the expression level of gicerin/CD146 was increased by the AAC treatment. Next, stretch stimulation was applied to myocardial cell line H9c2 cells to confirm that gicerin/CD146 may participate in the cellular hypertrophy model. We also treated the cells with inhibitors of MAP pathway enzymes. In cultured myocardial cells, the expression level of gicerin/CD146 was increased by the stretch stimulation and decreased by inhibiting the MAP pathway. Based on the above findings, it is suggested that the expression of gicerin/CD146 is involved in cardiac hypertrophy, and that the MAP pathway may be involved in the expression of gicerin/CD146 RNA in the cardiomyocyte. In addition, the expression level of gicerin/CD146 RNA in neonatal rats was upregulated after birth. Therefore, it is suggested that gicerin/CD146 might participate in the increase of myocardial cell volume both in the pathway of cardiac hypertrophy and in the developmental growth of heart.

Precision medicine for heart failure based on molecular mechanisms: The 2019 ISHR Research Achievement Award Lecture

Heart failure is a leading cause of death, and the number of patients with heart failure continues to increase worldwide. To realize precision medicine for heart failure, its underlying molecular mechanisms must be elucidated. In this review summarizing the "The Research Achievement Award Lecture" of the 2019 XXIII ISHR World Congress held in Beijing, China, we would like to introduce our approaches for investigating the molecular mechanisms of cardiac hypertrophy, development, and failure, as well as discuss future perspectives.

A human embryonic stem cell reporter line for monitoring chemical-induced cardiotoxicity

AIMS

Human embryonic stem cells (hESCs) can be used to generate scalable numbers of cardiomyocytes (CMs) for studying cardiac biology, disease modelling, drug screens, and potentially for regenerative therapies. A fluorescence-based reporter line will significantly enhance our capacities to visualize the derivation, survival, and function of hESC-derived CMs. Our goal was to develop a reporter cell line for real-time monitoring of live hESC-derived CMs.

METHODS AND RESULTS

We used CRISPR/Cas9 to knock a mCherry reporter gene into the MYH6 locus of hESC lines, H1 and H9, enabling real-time monitoring of the generation of CMs. MYH6:mCherry+ cells express atrial or ventricular markers and display a range of cardiomyocyte action potential morphologies. At 20 days of differentiation, MYH6:mCherry+ cells show features characteristic of human CMs and can be used successfully to monitor drug-induced cardiotoxicity and oleic acid-induced cardiac arrhythmia.

CONCLUSION

We created two MYH6:mCherry hESC reporter lines and documented the application of these lines for disease modelling relevant to cardiomyocyte biology.

Study of the Expression Transition of Cardiac Myosin Using Polarization-Dependent SHG Microscopy

Detection of the transition between the two myosin isoforms α- and β-myosin in living cardiomyocytes is essential for understanding cardiac physiology and pathology. In this study, the differences in symmetry of polarization spectra obtained from α- and β-myosin in various mammalian ventricles and propylthiouracil-treated rats are explored through polarization-dependent second harmonic generation microscopy. Here, we report for the, to our knowledge, first time that α- and β-myosin, as protein crystals, possess different symmetries: the former has C6 symmetry, and the latter has C3v. A single-sarcomere line scan further demonstrated that the differences in polarization-spectrum symmetry between α- and β-myosin came from their head regions: the head and neck domains of α- and β-myosin account for the differences in symmetry. In addition, the dynamic transition of the polarization spectrum from C6 to C3v line profile was observed in a cell culture in which norepinephrine induced an α- to β-myosin transition.

Developmental increase in β-MHC enhances sarcomere length-dependent activation in the myocardium

The expression of β-myosin heavy chain (β-MHC) in the guinea pig heart increases during postnatal development. Reda et al. show that this increase in β-MHC enhances length-mediated increases in myofilament Ca²⁺ sensitivity and sarcomere length–dependent changes in contractile function.

Shifts in myosin heavy chain (MHC) isoforms in cardiac myocytes have been shown to alter cardiac muscle function not only in healthy developing hearts but also in diseased hearts. In guinea pig hearts, there is a large age-dependent shift in MHC isoforms from 80% α-MHC/20% β-MHC at 3 wk to 14% α-MHC/86% β-MHC at 11 wk. Because kinetic differences in α- and β-MHC cross-bridges (XBs) are known to impart different cooperative effects on thin filaments, we hypothesize here that differences in α- and β-MHC expression in guinea pig cardiac muscle impact sarcomere length (SL)–dependent contractile function. We therefore measure steady state and dynamic contractile parameters in detergent-skinned cardiac muscle preparations isolated from the left ventricles of young (3 wk old) or adult (11 wk old) guinea pigs at two different SLs: short (1.9 µm) and long (2.3 µm). Our data show that SL-dependent effects on contractile parameters are augmented in adult guinea pig cardiac muscle preparations. Notably, the SL-mediated increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) is twofold greater in adult guinea pig muscle preparations (ΔpCa₅₀ being 0.11 units in adult preparations but only 0.05 units in young preparations). Furthermore, adult guinea pig cardiac muscle preparations display greater SL-dependent changes than young muscle preparations in (1) the magnitude of length-mediated increase in the recruitment of new force-bearing XBs, (2) XB detachment rate, (3) XB strain-mediated effects on other force-bearing XBs, and (4) the rate constant of force redevelopment. Our findings suggest that increased β-MHC expression enhances length-dependent activation in the adult guinea pig cardiac myocardium.

MicroRNA deep sequencing reveals chamber-specific miR-208 family expression patterns in the human heart

BACKGROUND

Heart chamber-specific mRNA expression patterns have been extensively studied, and dynamic changes have been reported in many cardiovascular diseases. MicroRNAs (miRNAs) are also important regulators of normal cardiac development and functions that generally suppress gene expression at the posttranscriptional level. Recent focus has been placed on circulating miRNAs as potential biomarkers for cardiac disorders. However, miRNA expression levels in human normal hearts have not been thoroughly studied, and chamber-specific miRNA expression signatures in particular remain unclear.

METHODS AND RESULTS

We performed miRNA deep sequencing on human paired left atria (LA) and ventricles (LV) under normal physiologic conditions. Among 438 miRNAs, miR-1 was the most abundant in both chambers, representing 21% of the miRNAs in LA and 26% in LV. A total of 25 miRNAs were differentially expressed between LA and LV; 14 were upregulated in LA, and 11 were highly expressed in LV. Notably, the miR-208 family in particular showed prominent chamber specificity; miR-208a-3p and miR-208a-5p were abundant in LA, whereas miR-208b-3p and miR-208b-5p were preferentially expressed in LV. Subsequent real-time polymerase chain reaction analysis validated the predominant expression of miR-208a in LA and miR-208b in LV.

CONCLUSIONS

Human atrial and ventricular tissues display characteristic miRNA expression signatures under physiological conditions. Notably, miR-208a and miR-208b show significant chamber-specificity as do their host genes, α-MHC and β-MHC, which are mainly expressed in the atria and ventricles, respectively. These findings might also serve to enhance our understanding of cardiac miRNAs and various heart diseases.

Length-dependent effects on cardiac contractile dynamics are different in cardiac muscle containing α- or β-myosin heavy chain

Actomyosin crossbridges (XBs) are the fundamental source of force generation and pressure development in the myocardium. Faster kinetics are imparted on XBs comprised of the fast, α-myosin heavy chain (MHC) isoform, whereas slower kinetics are imparted on XBs comprised of the slow, β-MHC isoform. Other factors, such as sarcomere length (SL), influence XB formation, presumably acting through allosteric effects on the kinetics that regulate the XB cycle. We sought to determine whether the slower XB kinetics of β-MHC were more sensitive to such length-dependent effects than those of α-MHC. We studied the SL effects on mechanical properties of demembranated muscle fibers from normal and propylthiouracil-treated mouse hearts, which expressed predominantly α-MHC or β-MHC, respectively. Interestingly, XB detachment kinetics were more length-sensitive in β-MHC fibers, as estimated by tension cost and XB detachment rate constant (c), and as inferred by ktr. The nonlinearity in force responses to various-amplitude step-like changes in muscle length was more pronounced in β-MHC fibers. This phenomenon is attributed to a greater cooperative/allosteric mechanism in β-MHC fibers, as estimated by model parameter γ. These data suggest a mechanism whereby greater cooperative/allosteric effects impart an enhanced length-sensitivity of XB cycling kinetics in fibers containing the slower cycling β-MHC.

Comparison of human cardiac gene expression profiles in paired samples of right atrium and left ventricle collected in vivo

Studies of expressed genes in human heart provide insight into both physiological and pathophysiological mechanisms. This is of importance for extended understanding of cardiac function as well as development of new therapeutic drugs. Heart tissue for gene expression studies is generally hard to obtain, particularly from the ventricles. Since different parts of the heart have different functions, expression profiles should likely differ between these parts. The aim of the study was therefore to compare the global gene expression in cardiac tissue from the more accessible auricula of the right atrium to expression in tissue from the left ventricle. Tissue samples were collected from five men undergoing aortic valve replacement or coronary artery bypass grafting. Global gene expression analysis identified 542 genes as differentially expressed between the samples extracted from these two locations, corresponding to ∼2% of the genes covered by the microarray; 416 genes were identified as abundantly expressed in right atrium, and 126 genes were abundantly expressed in left ventricle. Further analysis of the differentially expressed genes according to available annotations, information from curated pathways and known protein interactions, showed that genes with higher expression in the ventricle were mainly associated with contractile work of the heart. Transcription in biopsies from the auricula of the right atrium on the other hand indicated a wider area of functions, including immunity and defense. In conclusion, our results suggest that biopsies from the auricula of the right atrium may be suitable for various genetic studies, but not studies directly related to muscle work.

Caenorhabditis elegans muscle: a genetic and molecular model for protein interactions in the heart

The nematode Caenorhabditis elegans has become established as a major experimental organism with applications to many biomedical research areas. The body wall muscle cells are a useful model for the study of human cardiomyocytes and their homologous structures and proteins. The ability to readily identify mutations affecting these proteins and structures in C elegans and to be able to rigorously characterize their genotypes and phenotypes at the cellular and molecular levels permits mechanistic studies of the responsible interactions relevant to the inherited human cardiomyopathies. Future work in C elegans muscle holds great promise in uncovering new mechanisms in the pathogenesis of these cardiac disorders.

Transactivation in Drosophila of human enhancers by human transcription factors involved in congenital heart diseases

BACKGROUND

The human transcription factors (TFs) GATA4, NKX2.5 and TBX5 form part of the core network necessary to build a human heart and are involved in Congenital Heart Diseases (CHDs). The human natriuretic peptide precursor A (NPPA) and α-myosin heavy chain 6 (MYH6) genes are downstream effectors involved in cardiogenesis that have been demonstrated to be in vitro targets of such TFs.

RESULTS

To study the interactions between these human TFs and their target enhancers in vivo, we overexpressed them in the whole Drosophila cardiac tube using the UAS/GAL4 system. We observed that all three TFs up-regulate their natural target enhancers in Drosophila and cause developmental defects when overexpressed in eyes and wings.

CONCLUSIONS

A strong potential of the present model might be the development of combinatorial and mutational assays to study the interactions between human TFs and their natural target promoters, which are not easily undertaken in tissue culture cells because of the variability in transfection efficiency, especially when multiple constructs are used. Thus, this novel system could be used to determine in vivo the genetic nature of the human mutant forms of these TFs, setting up a powerful tool to unravel the molecular genetic mechanisms that lead to CHDs.

The emerging genetic landscape underlying cardiac conduction system function

Proper function of an organized Cardiac Conduction System (CCS) is vital to the survival of metazoans ranging from fly to man. The routine use of non-invasive electrocardiogram measures in the diagnosis and monitoring of cardiovascular health has established a trove of reliable CCS functional data in both normal and diseased cardiac states. Recent combination of echocardiogram (ECG) data with genome-wide association studies has identified genomic regions implicated in ECG variability which impact CCS function. In this study, we review the substantial recent progress in this area, highlighting the identification of novel loci, confirming the importance of previously implicated loci in CCS function, and exploring potential links between genes with important roles in developmental processes and variation in function of the CCS.

Impaired thymic tolerance to α-myosin directs autoimmunity to the heart in mice and humans

Autoimmunity has long been linked to myocarditis and its sequela, dilated cardiomyopathy, the leading causes of heart failure in young patients. However, the underlying mechanisms are poorly defined, with most clinical investigations focused on humoral autoimmunity as the target for intervention. Here, we show that the α-isoform of myosin heavy chain (α-MyHC, which is encoded by the gene Myh6) is the pathogenic autoantigen for CD4+ T cells in a spontaneous mouse model of myocarditis. Further, we found that Myh6 transcripts were absent in mouse medullary thymic epithelial cells (mTECs) and peripheral lymphoid stromal cells, which have been implicated in mediating central and peripheral T cell tolerance, respectively. Transgenic expression of α-MyHC in thymic epithelium conferred tolerance to cardiac myosin and prevented myocarditis, demonstrating that α-MyHC is a primary autoantigen in this disease process. Remarkably, we found that humans also lacked α-MyHC in mTECs and had high frequencies of α-MyHC-specific T cells in peripheral blood, with markedly augmented T cell responses to α-MyHC in patients with myocarditis. Since α-MyHC constitutes a small fraction of MyHC in human heart, these findings challenge the longstanding notion that autoimmune targeting of MyHC is due to its cardiac abundance and instead suggest that it is targeted as a result of impaired T cell tolerance mechanisms. These results thus support a role for T cell-specific therapies for myocarditis.

Return to the fetal gene program: a suggested metabolic link to gene expression in the heart

A hallmark of cardiac metabolism before birth is the predominance of carbohydrate use for energy provision. After birth, energy substrate metabolism rapidly switches to the oxidation of fatty acids. This switch accompanies the expression of "adult" isoforms of metabolic enzymes and other proteins. However, in a variety of pathophysiologic conditions, including hypoxia, ischemia, hypertrophy, atrophy, diabetes, and hypothyroidism, the postnatal heart returns to the "fetal" gene program. These adaptive mechanisms are also a feature of the failing heart muscle, where at a certain point this fetal-like reprogramming no longer suffices to support cardiac structure and function. We advance the hypothesis that in the postnatal heart, metabolic remodeling triggers the process through glycosylation of transcription factors, potentially protecting the stressed heart from irreversible functional impairment and programmed cell death. In other words, we propose a metabolic link to gene expression in the heart.

Several common variants modulate heart rate, PR interval and QRS duration

Electrocardiographic measures are indicative of the function of the cardiac conduction system. To search for sequence variants that modulate heart rate, PR interval and QRS duration in individuals of European descent, we performed a genome-wide association study in approximately 10,000 individuals and followed up the top signals in an additional approximately 10,000 individuals. We identified several genome-wide significant associations (with P < 1.6 x 10(-7)). We identified one locus for heart rate (MYH6), four for PR interval (TBX5, SCN10A, CAV1 and ARHGAP24) and four for QRS duration (TBX5, SCN10A, 6p21 and 10q21). We tested for association between these loci and subjects with selected arrhythmias in Icelandic and Norwegian case-control sample sets. We observed correlations between TBX5 and CAV1 and atrial fibrillation (P = 4.0 x 10(-5) and P = 0.00032, respectively), between TBX5 and advanced atrioventricular block (P = 0.0067), and between SCN10A and pacemaker implantation (P = 0.0029). We also replicated previously described associations with the QT interval.

Insights into human beta-cardiac myosin function from single molecule and single cell studies

beta-Cardiac myosin is a mechanoenzyme that converts the energy from ATP hydrolysis into a mechanical force that drives contractility in muscle. Thirty percent of the point mutations that result in hypertrophic cardiomyopathy are localized to MYH7, the gene encoding human beta-cardiac myosin heavy chain (beta-MyHC). Force generation by myosins requires a tight and highly conserved allosteric coupling between its different protein domains. Hence, the effects of single point mutations on the force generation and kinetics of beta-cardiac myosin molecules cannot be predicted directly from their location within the protein structure. Great insight would be gained from understanding the link between the functional defect in the myosin protein and the clinical phenotypes of patients expressing them. Over the last decade, several single molecule techniques have been developed to understand in detail the chemomechanical cycle of different myosins. In this review, we highlight the single molecule techniques that can be used to assess the effect of point mutations on beta-cardiac myosin function. Recent bioengineering advances have enabled the micromanipulation of single cardiomyocyte cells to characterize their force-length dynamics. Here, we briefly review single cell micromanipulation as an approach to determine the effect of beta-MyHC mutations on cardiomyocyte function. Finally, we examine the technical challenges specific to studying beta-cardiac myosin function both using single molecule and single cell approaches.

Changes in gene expression during the formation of bioengineered heart muscle

A three-dimensional bioengineered heart muscle (BEHM) construct model had been previously developed, exhibiting contractile forces up to 800 microN. The interest of this study was to determine gene expression levels of biologic markers involved in calcium-handling between BEHM, cell monolayer, and neonatal heart. Cardiac cells were isolated from one litter of F344 rats and organized into groups (n = 5): 4-, 7-, 10-day BEHM and cell monolayer; BEHM was evaluated for cell viability and contractility. Groups were then analyzed for mRNA expression of calcium-handling proteins: myosin heavy chain (MHC) alpha and beta, Sarcoplasmic reticulum Ca++ ATPase (SERCA) 2, phospholamban (PBL), and ryanodine receptor. BEHM exhibited electrically stimulated active force (208 +/- 12 microN day 4, 361 +/- 22 microN day 7, and 344 +/- 29 microN day 10) and no decrease in cell number. Real-time polymerase chain reaction (PCR) showed an increase in gene expression of all calcium-handling proteins in BEHM at 7 and 10 days compared with monolayers, for example, comparing BEHM to monolayer (7 and 10 days, respectively), MHC-alpha: 2600-fold increase and a 100-fold increase; MHC-beta: 70-fold increase at 10 days; ryanodine receptor: 74-fold increase at 10 days; SERCA: 19-fold increase and sixfold increase; PBL: 158-fold increase and 24-fold increase. It was concluded that a three-dimensional environment is a better culturing condition of cardiac cells than a monolayer. Also, BEHM constructs demonstrated a high similarity to a native myocardium, and is, thus, a good starting foundation for engineered heart muscle.

Noncompaction of the ventricular myocardium is associated with a de novo mutation in the beta-myosin heavy chain gene

Noncompaction of the ventricular myocardium (NVM) is the morphological hallmark of a rare familial or sporadic unclassified heart disease of heterogeneous origin. NVM results presumably from a congenital developmental error and has been traced back to single point mutations in various genes. The objective of this study was to determine the underlying genetic defect in a large German family suffering from NVM. Twenty four family members were clinically assessed using advanced imaging techniques. For molecular characterization, a genome-wide linkage analysis was undertaken and the disease locus was mapped to chromosome 14ptel-14q12. Subsequently, two genes of the disease interval, MYH6 and MYH7 (encoding the α- and β-myosin heavy chain, respectively) were sequenced, leading to the identification of a previously unknown de novo missense mutation, c.842G>C, in the gene MYH7. The mutation affects a highly conserved amino acid in the myosin subfragment-1 (R281T). In silico simulations suggest that the mutation R281T prevents the formation of a salt bridge between residues R281 and D325, thereby destabilizing the myosin head. The mutation was exclusively present in morphologically affected family members. A few members of the family displayed NVM in combination with other heart defects, such as dislocation of the tricuspid valve (Ebstein's anomaly, EA) and atrial septal defect (ASD). A high degree of clinical variability was observed, ranging from the absence of symptoms in childhood to cardiac death in the third decade of life. The data presented in this report provide first evidence that a mutation in a sarcomeric protein can cause noncompaction of the ventricular myocardium.

Factors controlling cardiac myosin-isoform shift during hypertrophy and heart failure

Myosin is a molecular motor, which interacts with actin to convert the energy from ATP hydrolysis into mechanical work. In cardiac myocytes, two myosin isoforms are expressed and their relative distribution changes in different developmental and pathophysiologic conditions of the heart. It has been realized for a long time that a shift in myosin isoforms plays a major role in regulating myocardial contractile activity. With the recent evidence implicating that alteration in myosin isoform ratio may be eventually beneficial for the treatment of a stressed heart, a new interest has developed to find out ways of controlling the myosin isoform shift. This article reviews the published data describing the role of myosin isoforms in the heart and highlighting the importance of various factors shown to influence myosin isofrom shift during physiology and disease states of the heart.

Return to the fetal gene program protects the stressed heart: a strong hypothesis

A common feature of the hemodynamically or metabolically stressed heart is the return to a pattern of fetal metabolism. A hallmark of fetal metabolism is the predominance of carbohydrates as substrates for energy provision in a relatively hypoxic environment. When the normal heart is exposed to an oxygen rich environment after birth, energy substrate metabolism is rapidly switched to oxidation of fatty acids. This switch goes along with the expression of "adult" isoforms of metabolic enzymes and other proteins. However, the heart retains the ability to return to the "fetal" gene program. Specifically, the fetal gene program is predominant in a variety of pathophysiologic conditions including hypoxia, ischemia, hypertrophy, and atrophy. A common feature of all of these conditions is extensive remodeling, a decrease in the rate of aerobic metabolism in the cardiomyocyte, and an increase in cardiac efficiency. The adaptation is associated with a whole program of cell survival under stress. The adaptive mechanisms are prominently developed in hibernating myocardium, but they are also a feature of the failing heart muscle. We propose that in failing heart muscle at a certain point the fetal gene program is no longer sufficient to support cardiac structure and function. The exact mechanisms underlying the transition from adaptation to cardiomyocyte dysfunction are still not completely understood.

Transcriptome-scale similarities between mouse and human skeletal muscles with normal and myopathic phenotypes

BACKGROUND

Mouse and human skeletal muscle transcriptome profiles vary by muscle type, raising the question of which mouse muscle groups have the greatest molecular similarities to human skeletal muscle.

METHODS

Orthologous (whole, sub-) transcriptome profiles were compared among four mouse-human transcriptome datasets: (M) six muscle groups obtained from three mouse strains (wildtype, mdx, mdx5cv); (H1) biopsied human quadriceps from controls and Duchenne muscular dystrophy patients; (H2) four different control human muscle types obtained at autopsy; and (H3) 12 different control human tissues (ten non-muscle).

RESULTS

Of the six mouse muscles examined, mouse soleus bore the greatest molecular similarities to human skeletal muscles, independent of the latters' anatomic location/muscle type, disease state, age and sampling method (autopsy versus biopsy). Significant similarity to any one mouse muscle group was not observed for non-muscle human tissues (dataset H3), indicating this finding to be muscle specific.

CONCLUSION

This observation may be partly explained by the higher type I fiber content of soleus relative to the other mouse muscles sampled.

A 7.1 kbp beta-myosin heavy chain promoter, efficient for green fluorescent protein expression, probably induces lethality when overexpressing a mutated transforming growth factor-beta type II receptor in transgenic mice

The roles of transforming growth factor-beta (TGFbeta) in heart or skeletal muscle development and physiology are still the subject of controversies. Our aim was to block, in transgenic mice, the TGFbeta signalling pathway by a dominant negative mutant of the TGFbeta type II receptor fused to the enhanced green fluorescent protein (TbetaRII-KR-EGFP) under the control of a 7.1 kbp mouse beta-myosin heavy chain (betaMHC) promoter to investigate the roles of TGFbeta in the heart and slow skeletal muscles. First, we generated two transgenic lines overexpressing EGFP under the control of the 7.1 kbp betaMHC promoter. In embryos, EGFP was detectable as early as 7.5 days post coitum. In embryos, newborns and adults, EGFP was expressed mainly in the cardiac ventricles and in slow skeletal muscles. EGFP expression was intense in the bladder but weak in the intestines. In contrast to the endogenous betaMHC promoter, the activity of the 7.1 kbp betaMHC promoter in the transgene was not repressed after birth and remained high in adult transgenic mice. We obtained two founders with the transgene comprising the TbetaRII-KR-EGFP sequence under the control of the 7.1 kbp betaMHC promoter. These founders were generated at a very low frequency and expressed barely detectable levels of TbetaRII-KR-EGFP mRNA. Our failure to obtain transgenic lines overexpressing the dominant negative receptor suggests that the blocking of the TGFbeta signalling pathway in the heart and slow skeletal muscles could be embryonically lethal. To conclude, the 7.1 kbp betaMHC promoter directs high levels of transgene expression in the cardiac ventricles and in slow skeletal muscles of the mouse. Analysis of the consequences of the blocking of the TGFbeta signalling pathway in the heart will require the use of tissue specific means of conditional gene invalidation.

Mutation in myosin heavy chain 6 causes atrial septal defect

Atrial septal defect is one of the most common forms of congenital heart malformation. We identified a new locus linked with atrial septal defect on chromosome 14q12 in a large family with dominantly inherited atrial septal defect. The underlying mutation is a missense substitution, I820N, in alpha-myosin heavy chain (MYH6), a structural protein expressed at high levels in the developing atria, which affects the binding of the heavy chain to its regulatory light chain. The cardiac transcription factor TBX5 strongly regulates expression of MYH6, but mutant forms of TBX5, which cause Holt-Oram syndrome, do not. Morpholino knock-down of expression of the chick MYH6 homolog eliminates the formation of the atrial septum without overtly affecting atrial chamber formation. These data provide evidence for a link between a transcription factor, a structural protein and congenital heart disease.

Differential regulation of carnitine palmitoyltransferase-I gene isoforms (CPT-I alpha and CPT-I beta) in the rat heart

Carnitine palmitoyltransferase-I (CPT-I) is a major control point for fatty acid oxidation. Two kinetically different isoforms, CPT-I alpha and CPT-I beta, have been identified. Cardiac ventricular myocytes are the only cells known to express both CPT-I isoforms. In this study, we characterized the differential regulation of CPT-I alpha and CPT-I beta expression in the heart. Expression of the CPT-I alpha gene was very high in the fetal heart and declined following birth. CPT-I beta was also highly expressed in fetal myocytes and remained so throughout development. CPT-I alpha mRNA abundance was increased in both the liver and heart of diabetic or fasted rats, but CPT-I beta mRNA levels were not altered in these states. A high fat diet elevated expression of the CPT-I alpha gene in the liver but not in the heart. The fat content of the diet did not affect the expression of CPT-I beta. Cultures of neonatal rat cardiac myocytes were transfected with luciferase reporter genes driven by CPT-I alpha or CPT-I beta promoters. Two regions of the CPT-I alpha promoter, including an upstream region (-1300/-960) and a region in the proximal promoter (-193/-52) contributed equally to basal expression in cardiac myocytes. Basal transcription of CPT-I alpha was dependent on Sp1 sites and a CCAAT box in the proximal promoter. Our data indicate that the CPT-I beta gene is expressed in a tissue specific manner, but that it is not subject to the same developmental or hormonal controls imposed on CPT-I alpha. In addition some aspects of CPT-I alpha expression are confined to the liver. The data presented here thus suggest that two types of differential regulation of CPT-I genes exist: (a) differential control of CPT-I alpha and CPT-I beta gene expression in the heart and (b) differential regulation of CPT-I alpha expression in the heart and liver.

Control of cardiac myosin heavy chain gene expression

The alpha- and beta-myosin genes extend over 51 kb on chromosome 14 in human and 11 in mouse separated by about 4.5 kb of intergenic sequence. They are located in tandem in the order of their expression during development. Transcription of each gene is independently controlled but coordinately regulated. During each embryogenesis, the beta-MHC gene is expressed as part of the cardiac myogenic program under the control of NKX-2.5, MEF-2C, and GATA-4/5/6. After birth, thyroid hormone induces expression of alpha-MHC mRNA and inhibits expression of the beta-MHC gene. While a large number of physiological stimuli are capable of modifying this basic paradigm, thyroid hormone is required for expression of alpha-MHC in ventricular muscle. The positive TRE for T(3)-stimulation of alpha-MHC is an imperfect direct repeat located in the proximal promoter of the gene. The negative TRE for the beta-MHC gene is probably a binding half-site that is located adjacent to the TATA box. Binding of TEF-1 to a strong positive element in the proximal promoter is important in basal expression of beta-MHC gene and in the response to alpha(1)-adrenergic stimulation. The beta-MHC gene also is induced together with several other "fetal" genes during cardiac hypertrophy by a mechanism involving Ca(2+)-mediated activation of calcineurin and NF-AT3. Upon activation, NF-AT3 translocates to the nucleus and interacts with GATA-4 to stimulate beta-MHC expression. Changes in chromatin structure mediated by the association of histone acetylases and deacetylases with transcription factors are essential in regulating cell-specific expression of MHC genes.

Ca(2+) handling in isolated human atrial myocardium

Physiologically, human atrial and ventricular myocardium are coupled by an identical beating rate and rhythm. However, contractile behavior in atrial myocardium may be different from that in ventricular myocardium, and little is known about intracellular Ca(2+) handling in human atrium under physiological conditions. We used rapid cooling contractures (RCCs) to assess sarcoplasmic reticulum (SR) Ca(2+) content and the photoprotein aequorin to assess intracellular Ca(2+) transients in atrial and ventricular muscle strips isolated from nonfailing human hearts. In atrial myocardium (n = 19), isometric twitch force frequency dependently (0. 25-3 Hz) increased by 78 +/- 25% (at 3 Hz; P < 0.05). In parallel, aequorin light signals increased by 111 +/- 57% (P < 0.05) and RCC amplitudes by 49 +/- 13% (P < 0.05). Similar results were obtained in ventricular myocardium (n = 13). SR Ca(2+) uptake (relative to Na(+)/Ca(2+) exchange) frequency dependently increased in atrial and ventricular myocardium (P < 0.05). With increasing rest intervals (1-240 s), atrial myocardium (n = 7) exhibited a parallel decrease in postrest twitch force (at 240 s by 68 +/- 5%, P < 0.05) and RCCs (by 49 +/- 10%, P < 0.05). In contrast, postrest twitch force and RCCs significantly increased in ventricular myocardium (n = 6). We conclude that in human atrial and ventricular myocardium the positive force-frequency relation results from increased SR Ca(2+) turnover. In contrast, rest intervals in atrial myocardium are associated with depressed contractility and intracellular Ca(2+) handling, which may be due to rest-dependent SR Ca(2+) loss (Ca(2+) leak) and subsequent Ca(2+) extrusion via Na(+)/Ca(2+) exchange. Therefore, the influence of rate and rhythm on mechanical performance is not uniform in atrial and ventricular myocardium.

Regional alterations in SR Ca(2+)-ATPase, phospholamban, and HSP-70 expression in chronic hibernating myocardium

We sought to identify mechanisms for chronic dysfunction in hibernating myocardium. Pigs were instrumented with a left anterior descending artery stenosis for 3 mo. Angiography demonstrated high-grade stenoses and hibernating myocardium with 1) severe anterior hypokinesis (P < 0.001 vs. shams), 2) reduced subendocardial perfusion [0.73 +/- 0.05 (SE) vs. 1.01 +/- 0.06 ml. min(-1). g(-1) in normal, P < 0.001], and 3) critically reduced adenosine flow (1.0 +/- 0.17 vs. 3.84 +/- 0.26 ml. min(-1). g(-1) in normal, P < 0.001). Histology did not reveal necrosis. Northern blot analysis of hibernating myocardium demonstrated regional downregulation in mRNAs for sarcoplasmic reticulum (SR) proteins phospholamban (0.76 +/- 0.08 vs. 1.07 +/- 0.06, P < 0.02) and SR Ca(2+)-ATPase (0.83 +/- 0.06 vs. 1.02 +/- 0.06, P < 0.05) with no change in calsequestrin (1.08 +/- 0.06 vs. 0.96 +/- 0.05, P = not significant). Heat shock protein (HSP)-70 mRNA was regionally induced in hibernating myocardium (2.4 +/- 0.3 vs. 1.0 +/- 0.11, P < 0.01). Directionally similar changes were confirmed by Western blot analysis of respective proteins. Our results indicate that hibernating myocardium exhibits a molecular phenotype that on a regional basis is similar to end-stage ischemic cardiomyopathy. This supports the hypothesis that SR dysfunction from reversible ischemia may be an early defect in the progression of left ventricular dysfunction.

Expression of proto-oncogenes and gene mutation of sarcomeric proteins in patients with hypertrophic cardiomyopathy

Several mutations of cardiac beta-myosin heavy chain (beta-MHC) gene were reported in patients with hypertrophic cardiomyopathy (HCM). Involvement of proto-oncogenes has been shown in the mechanism of experimental cardiac hypertrophy. This study sought to examine the effects of c-H-ras and c-myc expression in the steady-state myocardium on hypertrophic changes and to evaluate the possible interaction between beta-MHC mutation and proto-oncogene expression in HCM. Endomyocardial biopsy was performed in 17 HCM patients (5 beta-MHC mutations and 1 troponin T mutation) and 7 control subjects (no mutation). Reverse transcription-polymerase chain reaction analysis revealed c-H-ras expression in all members of both groups. Cardiomyocyte size was correlated with the expression level of c-H-ras (P<0.001), and c-H-ras expression was upregulated in HCM patients (P<0.01). HCM patients with a beta-MHC mutation had the higher c-H-ras expression than did control subjects or patients without a mutation (P<0.01). c-myc mRNA was expressed in 7 of 17 HCM patients but not in control subjects. Myocyte size was greater in c-myc-positive HCM patients than in control subjects and c-myc-negative HCM patients (P<0.001 and P<0.05, respectively). The proto-oncogene expression did not affect clinical findings, myocardial fibrosis, or disarray. In conclusion, c-H-ras and c-myc expression in the steady-state myocardium may play a role in the hypertrophic mechanism in HCM. It is possible that ss-MHC gene mutation has some effect on the regulation of proto-oncogene expression in HCM.

Modulation of actomyosin ATPase by goniodomin A differs in types of cardiac myosin

Goniodomin A causes the conformational change of actin to modify actomyosin ATPase activity [Furukawa, K.-I., Sakai, K., Watanabe, S., Maruyama, K., Murakami, M., Yamaguchi, K., Ohizumi, Y., 1993. Goniodomin A induces modulation of actomyosin ATPase activity mediated through conformational change of actin. J. Biol. Chem. 268, 26026-26031]. Goniodomin A inhibited the ATPase activities of atrial myofibrils, myosin B and reconstituted actomyosin in a concentration-dependent manner. Interestingly, these ATPase activities of ventricular muscle were enhanced by goniodomin A (3 x 10(-8)-3 x 10(-7) M), but were decreased when the concentration was further raised. The stimulatory effect of goniodomin A was significantly inhibited by troponin tropomyosin complex. These results suggest that goniodomin A affects actin to modify cardiac actomyosin ATPase activity, and that this modulation differs in types of cardiac myosin.

Mechanisms of thyroid hormone action. Implications for the clinical manifestation of thyrotoxicosis

Serum thyroid hormone concentrations alone do not explain the variability and severity of the range of symptoms observed in thyrotoxic patients. Despite gaps in our understanding of the links between the clinical manifestations of thyrotoxicosis and the underlying mechanisms, much has been learned. A limited number of markers directly reflect T3 action. The future elucidation of T3 targets that mediate these effects should ultimately lead to additional clinical markers of tissue-specific T3 action. The availability of such tests should allow for more specific treatment of individual patients.

Efficient inhibition of the development of cardiac remodeling by a long-acting calcium antagonist amlodipine

The purpose of the present study was to examine the effects of a long-acting calcium antagonist, amlodipine, on the development of cardiac remodeling. Dihydropyridine calcium antagonists have been used widely for many years in the treatment of hypertension and angina pectoris. It has been reported, however, that a prototype of dihydropyridines, nifedipine, does not reduce mortality of patients with ischemic heart disease, possibly because of reflex stimulation of the sympathetic nervous system. A calcium antagonist, amlodipine, has been reported to have potential benefits by virtue of a gradual onset of action and a long duration of effects. Amlodipine (8 mg/kg per day, once a day) or nifedipine (24 mg/kg per day, three times a day) was administered to spontaneously hypertensive 12-week-old rats for 12 weeks. Left ventricular wall thickness was measured by echocardiography, and relative amounts of myosin heavy chain isoforms were assessed by pyrophosphate gels. Expressions of "fetal type" genes and type 1 collagen gene were examined by Northern blot analysis. Amlodipine and nifedipine both markedly reduced systolic blood pressure. However, the decrease in systolic blood pressure caused by nifedipine continued for no more than 8 hours, whereas the blood pressure-lowering effect of amlodipine continued for more than 16 hours post dose. Amlodipine markedly reduced left ventricular wall thickness, whereas nifedipine only weakly attenuated an increase in the wall thickness. Amlodipine, but not nifedipine, prevented an increase in the relative amount of V3 myosin heavy chain isoform and suppressed an increase in mRNA levels of beta-myosin heavy chain, skeletal alpha-actin, and type 1 collagen. Unlike nifedipine, amlodipine effectively prevented cardiac remodeling secondary to high blood pressure at biochemical levels and morphological levels. These results suggest that a long-acting calcium antagonist is more effective than a short-acting one in preventing organ injury in hypertensive subjects.

Changes in gene expression in the intact human heart. Downregulation of alpha-myosin heavy chain in hypertrophied, failing ventricular myocardium

Using quantitative RT-PCR in RNA from right ventricular (RV) endomyocardial biopsies from intact nonfailing hearts, and subjects with moderate RV failure from primary pulmonary hypertension (PPH) or idiopathic dilated cardiomyopathy (IDC), we measured expression of genes involved in regulation of contractility or hypertrophy. Gene expression was also assessed in LV (left ventricular) and RV free wall and RV endomyocardium of hearts from end-stage IDC subjects undergoing heart transplantation or from nonfailing donors. In intact failing hearts, downregulation of beta1-receptor mRNA and protein, upregulation of atrial natriuretic peptide mRNA expression, and increased myocyte diameter indicated similar degrees of failure and hypertrophy in the IDC and PPH phenotypes. The only molecular phenotypic difference between PPH and IDC RVs was upregulation of beta2-receptor gene expression in PPH but not IDC. The major new findings were that (a) both nonfailing intact and explanted human ventricular myocardium expressed substantial amounts of alpha-myosin heavy chain mRNA (alpha-MHC, 23-34% of total), and (b) in heart failure alpha-MHC was downregulated (by 67-84%) and beta-MHC gene expression was upregulated. We conclude that at the mRNA level nonfailing human heart expresses substantial alpha-MHC. In myocardial failure this alteration in gene expression of MHC isoforms, if translated into protein expression, would decrease myosin ATPase enzyme velocity and slow speed of contraction.

Myosin heavy chain gene expression in human heart failure

Two isoforms of myosin heavy chain (MyHC), alpha and beta, exist in the mammalian ventricular myocardium, and their relative expression is correlated with the contractile velocity of cardiac muscle. Several pathologic stimuli can cause a shift in the MyHC composition of the rodent ventricle from alpha- to beta-MyHC. Given the potential physiological consequences of cardiac MyHC isoform shifts, we determined MyHC gene expression in human heart failure where cardiac contractility is impaired significantly. In this study, we quantitated the relative amounts of alpha- and beta-MyHC mRNA in the left ventricular free walls (LVs) of 14 heart donor candidates with no history of cardiovascular disease or structural cardiovascular abnormalities. This group consisted of seven patients with nonfailing (NF) hearts and seven patients with hearts that exhibited donor heart dysfunction (DHD). These were compared with 19 patients undergoing cardiac transplantation for chronic end-stage heart failure (F). The relative amounts of alpha-MyHC mRNA to total (i.e., alpha + beta) MyHC mRNA in the NF- and DHD-LVs were surprisingly high compared with previous reports (33.3+/-18.9 and 35.4+/-16.5%, respectively), and were significantly higher than those in the F-LVs, regardless of the cause of heart failure (2.2+/-3.5%, P < 0.0001). There was no significant difference in the ratios in NF- and DHD-LVs. Our results demonstrate that a considerable amount of alpha-MyHC mRNA is expressed in the normal heart, and is decreased significantly in chronic end-stage heart failure. If protein and enzymatic activity correlate with mRNA expression, this molecular alteration may be sufficient to explain systolic dysfunction in F-LVs, and therapeutics oriented towards increasing alpha-MyHC gene expression may be feasible.

Functional analysis of the mutations in the human cardiac beta-myosin that are responsible for familial hypertrophic cardiomyopathy. Implication for the clinical outcome

More than 30 missense mutations in the beta-cardiac myosin heavy chain gene have been shown to be responsible for familial hypertrophic cardiomyopathy. To clarify the effects of these point mutations on myosin motor function, we expressed wild-type and mutant human beta-cardiac myosin heavy chains in insect cells with human cardiac light chains. The wild-type myosin was well purified with similar enzymatic and motor activities to those of the naturally isolated V3 cardiac myosin. Arg249-->Gln and Arg453-->Cys mutations resulted in decreased actin translocating activity (61 and 23% of the wild-type, respectively) with decreased intrinsic ATPase activity. Arg403-->Gln mutation greatly decreased actin translocating activity (27% of wild type) with a 3.3-fold increased dissociation constant for actin, while intrinsic ATPase activity was unchanged. Val606-->Met mutation only mildly affected the actin translocating activity as well as ATPase activity of myosin. The degree of deterioration by each mutation was closely correlated with the prognosis of the affected kindreds, indicating that myosin dysfunction caused by the point mutations is responsible for the pathogenesis of the disease. Structure/function relationship of myosin is discussed.

Identification of alpha-cardiac myosin heavy chain mRNA and protein in extraocular muscle of the adult rabbit

Extraocular muscles contain both fast-twitch and multiply-innervated, tonic-contracting fibres. In rat, these fibres collectively express numerous myosin heavy chain isoforms including fast-type embryonic and neonatal, adult slow twitch type I and fast twitch type II, and a fast isoform unique to extraocular muscle. Immunocytochemical and Western blotting results are presented which suggest that, in rabbit, an additional species, the alpha-cardiac myosin heavy chain, is present. The immunoreactive species is found in all rabbit extraocular muscles and in the extraocular muscles is expressed in almost all fibres which do not contain a fast myosin heavy chain. Positive identification of this isoform as the alpha-cardiac myosin heavy chain was obtained by sequencing a cloned PCR product derived from extraocular muscle mRNA unique to the 3'-end of rabbit alpha-cardiac myosin heavy chain mRNA. This is the first unequivocal demonstration of alpha-cardiac myosin heavy chain expression in extraocular muscle.

Cloning of a crustacean myosin heavy chain isoform: exclusive expression in fast muscle

A clone of a fast isoform of myosin heavy chain (HC) gene was isolated from a cDNA1 expression library made from mRNA purified from the deep abdominal flexor muscle of the lobster, Homarus americanus. The cDNA (1.5 kb) contained the 3' untranslated region (UTR) and the coding sequence for the last 413 amino acid residues of the carboxyl terminus of the polypeptide. The deduced amino acid sequence showed high homology with that of myosin HCs from Drosophila (73% identity), nematode (57% identity), and vertebrates (49% identities). Hydropathy plots showed a 28-amino acid periodicity that is consistent with the alpha-helical coiled coil structure of the rod region of native myosin. Northern blot analysis and in situ hybridization showed that the fast myosin HC isoform was expressed only in fast fibers; the probes did not hybridize to mRNA from slow fibers. The message consisted of a single transcript of 6.6 kb. The intracellular localization of the fast myosin mRNA was not uniform. The mRNA was largely confined to the intermyofibrillar spaces and to the subsarcolemmal cytoplasm of the fiber periphery and large infoldings of the cell membrane. Within these regions the mRNA was concentrated in the cytoplasm immediately surrounding the nuclei. This constitutes the first report of the cloning and expression of a myosin HC gene from a crustacean species.

Structural organization of the human cardiac alpha-myosin heavy chain gene (MYH6)

The human myocardium expresses two cardiac myosin heavy chain (MyHC) isoforms, alpha and beta, that exist in tandem array on chromosome 14q12. We have previously sequenced the entire human cardiac beta-MyHC gene and now report the complete nucleotide sequence of the human cardiac alpha-MyHC, encompassing 26,159 bp as well as the entire 4484-bp 5'-flanking intergenic region. The gene (MYH6) consists of 39 exons, 37 of which contain coding information. The 5'-untranslated region is split into 3 exons, with the third exon containing the AUG translation initiation codon. With the exception of the 13th intron of the human cardiac beta-MyHC, which is not present within the alpha-isogene, all exon/intron boundaries are conserved. Conspicuous sequence motifs contained within the alpha-MyHC gene include four Alu repeats, a single (GT)n element, and a homopurine-homopyrimidine tract containing 23 GAA repeating units followed by 10 GAG repeating units. Comparison of the encoded amino acid sequence with a previously reported human alpha-MyHC cDNA sequence reveals several potential polymorphisms.

Regulation of myosin heavy chain genes in the heart

Advances in our knowledge of the regulation of cardiac myosin isoforms made possible by molecular cloning of the alpha- and beta-MHCs genes are reviewed. Expression of these genes in heart does not seem to require MyoD or related proteins of the skeletal muscle myogenic program. Cardiac MHC genes are under the control of T3, which stimulates transcription of the alpha-MHC gene and inhibits beta-MHC mRNA production both in vivo and in cultured heart cells. The responsiveness of the genes to T3 varies in different mammals, however. The genes are most responsive in rat and rabbit, intermediate in sensitivity in calf and subhuman primate (baboon), and very resistant in the dog. The human alpha-MHC gene is T3-inducible in ventricle, but the degree of response has not been quantified. Introduction of chimeric plasmids containing 5' flanking sequences of cardiac MHC genes fused to the CAT gene into cultured heart cells and transgenic animals has permitted identification of regulatory elements. Although the genes are closely linked in genomic DNA, they are controlled independently. The element within the alpha-MHC promoter responsible for induction by T3 is located approximately 160 base pairs from the transcription initiation site. Additional transcriptional activators located 5' upstream amplify the response to T3, probably by looping out intervening DNA sequences. The proximal region of the beta-MHC gene contains important regulatory elements, including those required for repression by T3, muscle-specific expression, a MyoD-independent positive element, and a hormone-independent repressor.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversible alterations in myocardial gene expression in a young man with dilated cardiomyopathy and hypothyroidism

Thyroid hormone effects on myocardial gene expression have been well defined in animal models, but their relationship to the pathogenesis of cardiac dysfunction in hypothyroid humans has been uncertain. We evaluated a profoundly hypothyroid young man with dilated cardiomyopathy. Before and during 9 months of thyroxine therapy, serial assessment of myocardial performance documented substantial improvements in the left ventricular ejection fraction (16-37%), left ventricular end-diastolic diameter (7.8-5.9 cm), and cardiac index (1.4-2.7 liters.min-1.m-2). Steady-state levels of mRNAs encoding selected cardiac proteins were measured in biopsy samples obtained before and after thyroxine replacement. In comparison with myocardium from nonfailing control hearts, this patient's pretreatment alpha-myosin heavy-chain mRNA level was substantially lower, the atrial natriuretic factor mRNA level was markedly elevated, and the phospholamban mRNA level was decreased. All of these derangements were reversed 9 months after restoration of euthyroidism. These observations in an unusual patient with profound myxedema and cardiac dilatation permit correlation between the reversible changes in myocardial function and steady-state mRNA levels in a cardiomyopathy. They suggest that alterations in gene expression in the dilated myopathic heart may be correctable when a treatable cause is identified.

Impact of dual-chamber permanent pacing in patients with obstructive hypertrophic cardiomyopathy with symptoms refractory to verapamil and beta-adrenergic blocker therapy

BACKGROUND

Patients with obstructive hypertrophic cardiomyopathy (HCM) with symptoms refractory to drugs (beta-blockers or verapamil) are candidates for cardiac surgery (left ventricular septal myectomy or mitral valve replacement). The present study examines prospectively the ability of dual-chamber (DDD) pacing to improve symptoms and relieve left ventricular outflow obstruction in such patients.

METHODS AND RESULTS

Forty-four consecutive patients with obstructive HCM who had failed to benefit from pharmacotherapy underwent treadmill exercise tests, echocardiography, and cardiac catheterization before and 1.5-3 months after implantation of a DDD pacemaker. Symptoms (angina, dyspnea, palpitations, presyncope, and syncope), New York Heart Association functional class status (1.7 +/- 0.7 versus 3.4 +/- 0.5, p less than 0.00001), and exercise durations were improved at follow-up evaluation. This was associated with significant reduction in left ventricular outflow tract gradient (38 +/- 38 versus 87 +/- 43 mm Hg, p less than 0.0001) and significant increases in cardiac output and systemic arterial pressures. Notably, when pacing was discontinued and comparisons were made in sinus rhythm, treadmill exercise durations were greater and left ventricular outflow tract gradients were less at the follow-up evaluation compared with the baseline study.

CONCLUSIONS

DDD pacing is an effective alternative to surgery in most patients with obstructive HCM with drug-refractory symptoms. The beneficial effects of pacing continue to be evident when pacing is acutely discontinued.

Complete sequence of human cardiac alpha-myosin heavy chain gene and amino acid comparison to other myosins based on structural and functional differences

We have obtained the 5820 nucleotide sequence encoding all 1939 amino acids of the human cardiac alpha-myosin heavy chain (alpha-MHC), as established by dideoxy sequencing of cloned cDNA, genomic DNA and polymerase chain reaction (PCR) amplification products. This sequence represents overlapping fragments of the entire coding sequence. Amino acid sequence comparison of the human cardiac alpha-MHC with the published human cardiac beta-MHC have demonstrated that there are, at least, 7 isoform-specific divergent regions, including functionally important binding protein-related sites such as ATP, actin and myosin light chain. It has been reported that in the rat, there are 8 isoform-specific divergent regions. The 7th divergent area (residue area 1633-1657, which is thought to mediate thick filament formation) in the light meromyosin region in the rat is not apparent in the human. The amino acid compositions of cardiac alpha- and beta-MHCs in the human and the rat, and human embryonic skeletal muscle and chicken gizzard smooth muscles were compared. Amino acid sequences in cardiac alpha- and beta-MHCs in the human and the rat are well conserved. In the head portion, the amino acid composition divergence of human cardiac alpha-MHC is ranked between rat cardiac alpha-MHC and human cardiac beta- or rat cardiac beta-MHC; human skeletal muscle MHC is the most divergent of the myosin isoform examined. These data predict that human cardiac alpha-MHC may have undergone evolutionary changes toward obtaining the biochemical and physiological properties of cardiac beta-MHC.

Expression of alpha-subunits of G proteins in failing human heart: a reappraisal utilizing quantitative polymerase chain reaction

A decrease in the density of the beta 1-adrenergic receptor and an increase in the functional activity of the G inhibitory protein Gi accompany human heart failure; however, the molecular and biochemical mechanisms responsible for these changes are unclear. We previously reported that the steady-state levels of the mRNAs encoding both alpha Gi-3 and alpha Gs were significantly increased in failing human heart. However, these results are not consistent with recent studies demonstrating that immunodetectable levels of alpha G proteins are not different in failing human hearts when compared with non-failing controls. In addition, analysis of the 5' flanking regions of alpha Gi and alpha Gs suggests that these two genes are unlikely to be co-regulated as their regulatory domains are quite different. Therefore, we hypothesized that the disparity between the measurements of alpha G protein gene expression and assessment of the actual levels of alpha G proteins might be due to technical limitations of the Northern blot technique utilized in previous studies for assessment of the mRNA levels; (i) cytoskeletal beta-actin mRNA was used as a standard for normalization; and (ii) only relative levels of alpha G mRNAs were measured. The recent application of the polymerase chain reaction to quantification of mRNA levels in small quantities of human heart provided the tool with which to test this hypothesis. When expressed in molecules of mRNA per microgram of total RNA, there were no differences in the levels of alpha Gi and alpha Gs mRNAs in failing human heart when compared with non-failing controls.(ABSTRACT TRUNCATED AT 250 WORDS)