Marsupial cardiac myosins are similar to those of eutherians in subunit composition and in the correlation of their expression with body size

Cardiac myosins and their subunit compositions were studied in ten species of marsupial mammals. Using native gel electrophoresis, ventricular myosin in macropodoids showed three isoforms, V(1), V(2) and V(3), and western blots using specific anti-alpha- and anti-beta-cardiac myosin heavy chain (MyHC) antibodies showed their MyHC compositions to be alphaalpha, alphabeta and betabeta, respectively. Atrial myosin showed alphaalpha MyHC composition but differed from V(1) in light chain composition. Small marsupials (Sminthopsis crassicaudata, Antechinus stuartii, Antechinus flavipes) showed virtually pure V(1), while the larger (1-3 kg) Pseudocheirus peregrinus and Trichosurus vulpecula showed virtually pure V(3). The five macropodoids (Bettongia penicillata, Macropus eugenii, Wallabia bicolour, M. rufus and M. giganteus), ranging in body mass from 2 to 66 kg, expressed considerably more alpha-MyHC (22.8%) than expected for their body size. These results show that cardiac myosins in marsupial mammals are substantially the same as their eutherian counterparts in subunit composition and in the correlation of their expression with body size, the latter feature underlies the scaling of resting heart rate and cardiac cross-bridge kinetics with specific metabolic rate. The data from macropodoids further suggest that expression of cardiac myosins in mammals may also be influenced by their metabolic scope.

Cardiac myosin isoforms from different species have unique enzymatic and mechanical properties

The mammalian heart contains two cardiac myosin isoforms: beta-myosin heavy chain (MHC) is found predominantly in the ventricles of large mammals, and alpha-MHC is expressed in the atria. The sequence identity between these isoforms is approximately 93%, with nonidentical residues clustered in discrete, functionally important domains associated with actin binding and ATPase activity. It is well-established that rabbit alpha-cardiac myosin has a 2-fold greater unloaded shortening velocity than beta-cardiac myosin but a 2-fold lower average isometric force. Here, we test the generality of these relationships for another large mammal, the pig, as well as for a small rodent, the mouse, which expresses alpha-MHC in its ventricles throughout adulthood. Hydrophobic interaction chromatography (HIC) was used to purify myosin from mouse, rabbit, and pig hearts. The superior resolving power of HIC made it possible to prepare highly homogeneous, enzymatically active myosin from small amounts of tissue. The movement of actin filaments by myosin was measured in an in vitro motility assay. The same assay could be used to determine average isometric force by loading the actin filaments with increasing concentrations of alpha-actinin to stop filament motion. We conclude that myosin from the mouse has significantly higher velocities for both alpha and beta isoforms than myosin from rabbits and pigs, even though the 2-fold difference in velocity between isoforms is maintained. Unlike the larger mammals, however, the small rodent generates the same high isometric force for both alpha and beta isoforms. Thus, nature has adapted the function of cardiac myosin isoforms to optimize power output for hearts of a given species.

Natriuretic peptides in ectopic myocardial tissues originating from mouse embryonic stem cells

In a previous report we described the survival and contractile function of mouse embryonic stem cell-derived cardiomyocytes in the host retroperitoneum. To further understand the nature of embryonic stem cell-derived cardiomyocytes, the study assessed the synthesis of natriuretic peptides in ectopic myocardial tissues of embryonic stem cell origin. Cardiomyocytes formed in embryoid body outgrowths were transplanted into the retroperitoneum of adult nude mice, and the myocardial tissues that developed were characterized by RT-PCR and immunohistochemistry concerning atrial and brain natriuretic peptides (ANP, BNP). In the outgrowths of embryoid bodies in vitro, gene expression of ANP and BNP was detected by RT-PCR and granules positive for the peptides were identified in a few cardiomyocytes by light and electron microscopic immunocytochemistry. Seven days after transplantation the transplants exhibited multidifferentiated teratoma tissues. Developing chamber myocardial tissues positive for cardiac troponin I, cadherin, and connexin 43 were evident in the transplants, which contained ANP-positive cardiomyocytes. Transplants with beating bundles were observed 30 days after transplantation, in which gene expression of both natriuretic peptides was detected. Myocardial tissues with abundant ANP-immunoreactivity, as well as with BNP-immunoreactivity to a lesser extent, were evident in the transplants. Also, myocardial tissues without immunoreactivity for natriuretic peptides were observed. Immunoelectron microscopy showed discernible secretory granules containing ANP and/or BNP in the cardiomyocytes. These results showed that part of the cardiomyocytes in embryonic stem cell-derived ectopic myocardial tissues are capable of producing natriuretic peptides, which suggests that they may be used as an endocrine source for cardiac hormones.

A simplified method for identification of human cardiac myosin heavy-chain isoforms

Cardiac myosin is a central participant in the cross-bridge cycling that mediates myocyte contraction and consists of multiple subunits that mediate both hydrolysis of ATP and mechanical production of contractile force Two isoforms of myosin heavy chain (MHC- alpha and MHC- beta ) are known to exist in mammalian cardiac tissue, and it is within this myosin subunit that ATPase activity resides. These isoforms differ by less than 0.2% in total molecular mass and amino acid sequence, but, strikingly, influence the rate and efficiency of energy utilization for generation of contractile force. Changes in the MHC- alpha /MHC- beta ratio has been classically viewed as an adaptation of a failing myocyte in both animal models and humans; however, their measurement has traditionally required specialized preparations and materials for sufficient resolution. Here we describe a greatly simplified method for routine assessments of myosin isoform composition in human cardiac tissues. The primary advantages of our approach include higher throughput and reduced supply costs with no apparent loss of statistical power, reproducibility or achieved results. Use of this more convenient method may provide enhanced access to an otherwise specialized technique and could provide additional opportunity for investigation of cardiac myocyte adaptive changes.

Non-radioactive in situ detection of mRNA in ES cell-derived cardiomyocytes and in the developing heart

Non-radioactive in situ hybridisation is an excellent method to visualise mRNA molecules within their topographical context. Recently we have reported a new non-radioactive in situ hybridisation procedure on tissue sections that is essentially based on the whole mount in situ hybridisation procedure. This method is superior in spatial resolution and sensitivity compared to the radioactive in situ hybridisation procedure. Generally, low levels of gene expression, such as found with the developmental onset of gene expression and in differentiating embryonic stem cells, are difficult to detect by in situ hybridisation. Here an application of the protocol is presented which is based on tyramide signal amplification, which enables the detection of very low abundant mRNAs. The significance of this method is two-fold: (1) the molecular phenotype of embryonic stem cell-derived cardiomyocytes can be examined at the cellular level with high sensitivity, and (2) the number of cells that express the gene of interest can be assessed.

Identification and validation of selective upregulation of ventricular myosin light chain type 2 mRNA in idiopathic dilated cardiomyopathy

BACKGROUND AND AIMS

the etiology of idiopathic dilated cardiomyopathy (IDCM) is unknown, methods such as suppression subtractive hybridization (SSH) and DNA microarray technology can help to identify genes which might be involved in the pathogenesis of this disease.

METHODS AND RESULTS

we used SSH which compared mRNA populations extracted from the left ventricular tissue of IDCM hearts and from the control tissue to identify sequences which correspond to genes up-regulated in IDCM. We identified ventricular myosin light chain type 2 (MLC2V), skeletal alpha-actin, long-chain-acyl-CoA-synthetase and mRNA for the protein KIAA0465 as differentially up-regulated genes. Expression of MLC2V mRNA was determined by RT-PCR in patients with end-stage heart failure caused by IDCM (n=11) or coronary artery disease (CAD, n=9) who underwent heart transplantation as well as the controls (n=6). MLC2V/GAPDH ratios were 2.95+/-0.32, 0.69+/-0.03 and 0.28+/-0.08 (arbitrary unit) for the IDCM group, the CAD group and controls, respectively (P<0.05). DNA microarray analysis confirmed the finding of MLC2V upregulation in IDCM (3.7- and 1.8-fold increase in MLC2V mRNA).

CONCLUSIONS

we have demonstrated that SSH is a useful method to identify differential myocardial upregulation of genes. Upregulation of MLC2V can be judged as a specific IDCM related feature, which might be clinically helpful.

TRs have common and isoform-specific functions in regulation of the cardiac myosin heavy chain genes

TRalpha1 and TRbeta mediate the regulatory effects of T3 and have profound effects on the cardiovascular system. We have analyzed the expression of the cardiac myosin heavy chain (MyHC) genes alpha and beta in mouse strains deficient for one or several TR genes to identify specific regulatory functions of TRalpha1 and TRbeta. The results show that TRalpha1 deficiency, which slows the heart rate, causes chronic overexpression of MyHCbeta. However, MyHCbeta was still suppressible by T3 in both TRalpha1- and TRbeta-deficient mice, indicating that either receptor can mediate repression of MyHCbeta. T3-dependent induction of the positively regulated MyHCalpha gene was similar in both TRalpha1- and TRbeta-deficient mice. The data identify a specific role for TRalpha1 in the negative regulation of MyHCbeta, whereas TRalpha1 and TRbeta appear interchangeable for hormone-dependent induction of MyHCalpha. This suggests that TR isoforms exhibit distinct specificities in the genes that they regulate within a given tissue type. Thus, dysregulation of MyHCbeta is likely to contribute to the critical role of TRalpha1 in cardiac function.