Myosin isoenzyme distribution in overloaded human atrial tissue

Sex differences in myosin heavy chain isoforms of human failing and nonfailing atria

Mammalian hearts express two myosin heavy chain (MHC) isoforms, which drive contractions with different kinetics and power-generating ability. The expression of the isoform that is associated with more rapid contraction kinetics and greater power output, MHC-α, is downregulated, with a concurrent increase in the relative amount of the slower isoform, MHC-β, during the progression to experimentally induced or disease-related heart failure. This change in protein expression has been well studied in right and left ventricles in heart failure models and in humans with failure. Relatively little quantitative data exists regarding MHC isoform expression shifts in human failing atria. We previously reported significant increases in the relative amount of MHC-β in the human failing left atrium. The results of that study suggested that there might be a sex-related difference in the level of MHC-β in the left atrium, but the number of female subjects was insufficient for statistical analysis. The objective of this study was to test whether there is, in fact, a sex-related difference in the level of MHC-β in the right and left atria of humans with cardiomyopathy. The results indicate that significant differences exist in atrial MHC isoform expression between men and women who are in failure. The results also revealed an unexpected twofold greater amount of MHC-β in the nonfailing left atrium of women, compared with men. The observed sex-related differences in MHC isoform expression could impact ventricular diastolic filling during normal daily activities, as well as during physiologically stressful events.

Single nucleotide polymorphisms, haplotypes and combined genotypes in MYH₃ gene and their associations with growth and carcass traits in Qinchuan cattle

MYH₃ is a major contractile protein which converts chemical energy into mechanical energy through the ATP hydrolysis. MYH₃ is mainly expressed in the skeletal muscle in different stages especially embryonic period, and it has a role in the development of skeletal muscle and heart. In this study, polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was applied to analyze the genetic variations of the MYH₃ gene and verify the effect on growth and carcass traits in a total of 365 Qinchuan cattles. The PCR product was digested with some restriction enzyme and demonstrated the polymorphism in the population, the single nucleotide polymorphisms (SNPs) at nucleotides g. +1215T>C, g. +3377C>T, and g. +28625C>T were in linkage disequilibrium with each other. The result of haplotype analysis showed that nineteen different haplotypes were identified among the five SNPs. The statistical analyses indicated that the five SNPs were significant association with growth and carcass traits (P < 0.05, N = 365); whereas the five SNPs were no significant association between 18 combined genotypes of MYH₃ gene and growth and carcass traits. Taken together, our results provide the evidence that polymorphisms in MYH₃ are associated with growth and carcass traits in Qinchuan cattle, and may be used as a possible candidate for marker-assisted selection and management in beef cattle breeding program.

Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle

The goal of this mini-review is to summarize findings concerning the role that different models of muscular activity and inactivity play in altering gene expression of the myosin heavy chain (MHC) family of motor proteins in mammalian cardiac and skeletal muscle. This was done in the context of examining parallel findings concerning the role that thyroid hormone (T(3), 3,5,3'-triiodothyronine) plays in MHC expression. Findings show that both cardiac and skeletal muscles of experimental animals are initially undifferentiated at birth and then undergo a marked level of growth and differentiation in attaining the adult MHC phenotype in a T(3)/activity level-dependent fashion. Cardiac MHC expression in small mammals is highly sensitive to thyroid deficiency, diabetes, energy deprivation, and hypertension; each of these interventions induces upregulation of the beta-MHC isoform, which functions to economize circulatory function in the face of altered energy demand. In skeletal muscle, hyperthyroidism, as well as interventions that unload or reduce the weight-bearing activity of the muscle, causes slow to fast MHC conversions. Fast to slow conversions, however, are seen under hypothyroidism or when the muscles either become chronically overloaded or subjected to intermittent loading as occurs during resistance training and endurance exercise. The regulation of MHC gene expression by T(3) or mechanical stimuli appears to be strongly regulated by transcriptional events, based on recent findings on transgenic models and animals transfected with promoter-reporter constructs. However, the mechanisms by which T(3) and mechanical stimuli exert their control on transcriptional processes appear to be different. Additional findings show that individual skeletal muscle fibers have the genetic machinery to express simultaneously all of the adult MHCs, e.g., slow type I and fast IIa, IIx, and IIb, in unique combinations under certain experimental conditions. This degree of heterogeneity among the individual fibers would ensure a large functional diversity in performing complex movement patterns. Future studies must now focus on 1) the signaling pathways and the underlying mechanisms governing the transcriptional/translational machinery that control this marked degree of plasticity and 2) the morphological organization and functional implications of the muscle fiber's capacity to express such a diversity of motor proteins.

Structural changes of atrial myocardium during chronic atrial fibrillation

Of all known arrhythmia's, atrial fibrillation (AF) is the most often met in the clinical setting and it is associated with an increase in mortality risk. Several risk factors for AF have been described and several mechanisms of induction and maintenance have been proposed. Studies in patients with AF have shown that structural changes occur in the atria, but the relationship between the structural remodelling and the chronicity of the arrhythmia are not well understood. The changes mainly concern adaptive (dedifferentiation of cardiomyocytes) and maladaptive (degeneration of cells with replacement fibrosis) features. In order to characterise the time course of the structural remodelling the need for animal models which adequately mimic chronic atrial fibrillation in humans is felt essential. In this review, the structural changes that are observed during prolonged sustained AF in patients and animal models, are described. Furthermore, the time course and potential mechanisms of structural remodelling are discussed and methods for elucidation of the underlying molecular mechanisms are presented.

Force production in mechanically isolated cardiac myocytes from human ventricular muscle tissue

OBJECTIVE

The expression of contractile isoforms changes during various pathological conditions but little is known about the consequences of these changes for the mechanical properties in human ventricular muscle. We investigated the feasibility of simultaneous determination of protein composition and isometric force development in single cardiac myocytes from human ventricular muscle tissue obtained from small biopsies taken during open heart surgery.

METHODS

Small biopsies of about 3 mg wet weight were taken during open heart surgery from patients with aortic valve stenosis. These biopsies were divided in two parts. One part (approximately 2 mg) was used for mechanical isolation of single myocytes and subsequent force measurement while the remaining part was used, in aliquots of 1 microgram dry weight, for protein analysis by polyacrylamide gel electrophoresis. The myocytes were attached with silicon glue to a sensitive force transducer and a piezoelectric motor, mounted on an inverted microscope and permeabilized by means of Triton X-100. Force development was studied at various free calcium concentrations.

RESULTS

From all biopsies, myocytes could be obtained and the composition of contractile proteins could be determined. The average isometric force (+/- s.e.m.) at saturating calcium concentration obtained on 20 myocytes from 5 patients amounted to 51 +/- 8 kN/m2. Force was half maximal at a calcium concentration of 2.47 +/- 0.10 microM.

CONCLUSION

These measurements indicate that it is possible to study the correlation between mechanical properties and protein composition in small biopsies from human ventricular muscle.

Modulation of contractility in human cardiac hypertrophy by myosin essential light chain isoforms

Cardiac hypertrophy is an adaptive response that normalizes wall stress and compensates for increased workload. It is accompanied by distinct qualitative and quantitative changes in the expression of protein isoforms concerning contractility, intracellular Ca(2+)-homeostasis and metabolism. Changes in the myosin subunit isoform expression improves contractility by an increase in force generation at a given Ca(2+)-concentration (increased Ca(2+)-sensitivity) and by improving the economy of the chemo-mechanical transduction process per amount of utilised ATP (increased duty ratio). In the human atrium this is achieved by partial replacement of the endogenous fast myosin by the ventricular slow-type heavy and light chains. In the hypertrophic human ventricle the slow-type beta-myosin heavy chains remain unchanged, but the ectopic expression of the atrial myosin essential light chain (ALC1) partially replaces the endogenous ventricular isoform (VLC1). The ventricular contractile apparatus with myosin containing ALC1 is characterised by faster cross-bridge kinetics, a higher Ca(2+)-sensitivity of force generation and an increased duty ratio. The mechanism for cross-bridge modulation relies on the extended Ala-Pro-rich N-terminus of the essential light chains of which the first eleven residues interact with the C-terminus of actin. A change in charge in this region between ALC1 and VLC1 explains their functional difference. The intracellular Ca(2+)-handling may be impaired in heart failure, resulting in either higher or lower cytosolic Ca(2+)-levels. Thus the state of the cardiomyocyte determines whether this hypertrophic adaptation remains beneficial or becomes detrimental during failure. Also discussed are the effects on contractility of long-term changes in isoform expression of other sarcomeric proteins. Positive and negative modulation of contractility by short-term phosphorylation reactions at multiple sites in the myosin regulatory light chain, troponin-I, troponin-T, alpha-tropomyosin and myosin binding protein-C are considered in detail.

Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. II. An immunohistochemical analysis of myosin heavy chain isoform expression patterns in the embryonic heart

The spatial distribution of alpha- and beta-myosin heavy chain isoforms (MHCs) was investigated immunohistochemically in the embryonic human heart between the 4th and the 8th week of development. The development of the overall MHC isoform expression pattern can be outlined as follows: (1) In all stages examined, beta-MHC is the predominant isoform in the ventricles and outflow tract (OFT), while alpha-MHC is the main isoform in the atria. In addition, alpha-MHC is also expressed in the ventricles at stage 14 and in the OFT from stage 14 to stage 19. This expression pattern is very reminiscent of that found in chicken and rat. (2) In the early embryonic stages the entire atrioventricular canal (AVC) wall expresses alpha-MHC whereas only the lower part expresses beta-MHC. The separation of atria and ventricles by the fibrous annulus takes place at the ventricular margin of the AVC wall. Hence, the beta-MHC expressing part of the AVC wall, including the right atrioventricular ring bundle, is eventually incorporated in the atria. (3) In the late embryonic stages (approx. 8 weeks of development) areas of alpha-MHC reappear in the ventricular myocardium, in particular in the subendocardial region at the top of the interventricular septum. These coexpressing cells are topographically related to the developing ventricular conduction system. (4) In the sinoatrial junction of all hearts examined alpha- and beta-MHC coexpressing cells are observed. In the older stages these cells are characteristically localized at the periphery of the SA node.

The idiopathic dilated cardiomyopathy in man. A biochemical and molecular study on myosin

We studied subunit composition and Ca(++)-activated ATPase activity of myosin isolated from atria and ventricles of hearts explanted from patients suffering from idiopathic dilated cardiomyopathy. At variance with previously published data, we have been unable to detect in the ventricular subendocardial layers a significant amount of myosin atrial-like light chain 1 (ALC1), which has been reported to be related to some hemodynamic features of the hypertrophied and failing heart. Such a subunit was not visible in the septum and in the subepicardial layers either. On the contrary, in both atria a ventricular-like light chain 2 (VLC2) was found. The nature of this additional light chain was confirmed on the basis of two-dimensional electrophoresis and immunoblotting techniques with polyclonal antibodies reacting with VLC2. In these patients we also observed a depressed Ca(++)-activated ATPase activity, both in atrial and ventricular myosin. The explanation for this finding in ventricles still remains obscure since neither myosin light chains, nor myosin heavy chains showed any difference between patients with dilated cardiomyopathy and controls. On the contrary, in atria we clearly identified changes consistent with the expression of myosin heavy chains of ventricular type and VLC2, which can account for the depressed Ca(++)-activated ATPase activity.

Infarcted heart uptake and biodistribution of radiolabelled anti-myosin monoclonal antibody in rat and dog myocardial infarct models

A new mouse monoclonal antibody that recognizes alpha- and beta-heavy chains of human atrial and ventricular myosin and beta-heavy chain of human slow skeletal muscle myosin was obtained. The 125I- and 111In-labelled antibody, and its F(ab')2 and Fab fragments localize in isoproterenol induced infarcted rat heart, with the F(ab')2 fragment showing the highest uptake. Comparison with 99Tc-pyrophosphate uptake in infarcted dog heart, induced by selective obstruction of a coronary artery, suggest that the 111In-labelled F(ab')2 localizes specifically in infarcted myocardium only.