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Myosin heavy chains (MHC) are cytoskeletal motor proteins essential to the process of muscle contraction. We have determined the complete sequences of the Xenopus cardiac MHC genes, alpha-MHC and ventricular MHC (vMHC), and have characterized their developmental expression profiles. Whereas alpha-MHC is expressed from the earliest stages of cardiac differentiation, vMHC transcripts are not detected until the heart has undergone chamber formation. Early expression of vMHC appears to mark the cardiac conduction system, but expression expands to include the ventricle and outflow tract myocardium during subsequent development. Sequence comparisons, transgenic expression analysis, and comparative genomic studies indicate that Xenopus alpha-MHC is the true orthologue of the mammalian alpha-MHC gene. On the other hand, we show that the Xenopus vMHC gene is most closely related to chicken ventricular MHC (vMHC1) not the mammalian beta-MHC. Comparative genomic analysis has allowed the detection of a mammalian MHC gene (MyH15) that appears to be the orthologue of vMHC, but evidence suggests that this gene is no longer active.
Figure 2. In situ hybridization analysis of developmental expression of the Xenopus alpha -MHC and vMHC genes. A: Lateral view of stage 26 embryos showing alpha -MHC transcripts in the bisymmetric heart patches. B: Ventral view of embryo in A. C: Transverse section through the heart patch of stage 26 embryo. D: Lateral view of stage 30 embryo showing increased alpha -MHC expression in precardiac tissues (arrow). E: Lateral view of stage 30 embryo showing an absence of vMHC expression at this stage of development. F: Lateral view of stage 38 embryo showing alpha -MHC expression in the heart, muscles of the jaw (jm), and the lymph heart (lh). G: Transverse section through stage 36 heart shows that alpha -MHC expression is confined to the myocardium. H: Transverse section through the lymph heart (stage 38) stained for alpha -MHC. I,J: Whole-mount and section in situ hybridizations, respectively, of stage 45 heart showing that alpha -MHC is expressed in all chambers and outflow tract. K,L: Whole-mount and section in situ hybridizations, respectively, of stage 45 heart, showing restricted expression of vMHC in a ring around the outflow tract and the atria at the junction with the ventricle. M: Section in situ of adult Xenopus heart shows that alpha -MHC is expressed throughout the myocardium, including the atria (a) and outflow tract. N: Section in situ hybridization of adult Xenopus heart shows that vMHC is expressed in the myocardium of the outflow tract and ventricle but not the atria.
myh15 (myosin, heavy chain 15) gene expression in Xenopus laevis embryos, adult from, as assayed by in situ hybridization, transverse section through heart.
myh6 (myosin, heavy chain 6, cardiac muscle, alpha) gene expression in Xenopus laevis embryos, adult frog, as assayed by in situ hybridization, transverse section through heart.
