On the ontogeny of cardiac gene transcripts

As a prerequisite to investigating the specification and differentiation of cardiac tissue in vitro, the ontogeny of a number of putative cardiac-specific, and striated muscle-specific gene transcripts has been studied. The probes used include cDNAs of α-actins, myosin heavy chains, myosin light chains, α-tropomyosin, troponin-T and atrial natriuretic factor. The expression of these genes was monitored by Northern analysis of heart and various other tissues at three developmental ages, viz, adult, neonatal and mid-foetal. The aim of this exercise was to confirm the efficacy of a number of markers to represent a cardiac-specific subset of gene expression in our mammalian model, the guinea pig. Our results indicate predominantly cardiac expression for the mRNA transcripts of cardiac α-actin (cα-actin), cardiac myosin heavy chain-α (MHCα), cardiac myosin heavy chain-β (MHCβ), myosin light chain-1A (MLC 1A), myosin light chain-1V (MLC 1v), α-tropomyosin (αTM), cardiac troponin-T (cTnT) and atrial natriuretic factor (ANF). Furthermore, cardiac-specific expression at the midfoetal time point was observed for five gene transcripts, MLC 1v, MHCα, MHCβ, striated αTM and ANF. No genes were expressed exclusively in cardiac tissue; for example, expression of the genes for cα-actin, both cardiac MHCs, both MLCs, αTM and cTnT was evident in skeletal and vascular smooth muscles at some stages of development. An interesting difference between this species and those of previous studies was the minor contribution of skeletal α-actin to cardiac phenotype. As well, our observation of transcripts of cardiac MHCs and ANF in vascular smooth muscle has not been previously reported. By providing a comprehensive survey of the ontogenic expression of this group of genes, our study has been able to identify a number of useful cardiac differentiation markers which, as a group, may provide a more definitive indication of authentic cardiac cellular differentiation, appropriate to some in vitro experimental situations.