ON THE ONTOGENY OF CARDIAC GENE TRANSCRIPTS

As a prerequisite to investigating the specification and differentiati on of cardiac tissue in vitro, the ontogeny of a number of putative ca rdiac-specific, and striated muscle-specific gene transcripts has been studied. The probes used include cDNAs of alpha-actins, myosin heavy chains, myosin light chains, alpha-tropomyosin, troponin-T and atrial natriuretic factor. The expression of these genes was monitored by Nor thern analysis of heart and various other tissues at three development al 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 car diac-specific subset of gene expression in our mammalian model, the gu inea pig. Our results indicate predominantly cardiac expression for th e mRNA transcripts of cardiac alpha-actin (c alpha-actin), cardiac myo sin heavy chain-alpha (MHC alpha), cardiac myosin heavy chain-beta (MH C beta), myosin light chain-1A (MLC(1A)), myosin light chain-1V (MLC(1 V)), alpha-tropomyosin (alpha TM), cardiac troponin-T (cTnT) and atria l natriuretic factor (ANF). Furthermore, cardiac-specific expression a t the midfoetal time point was observed for five gene transcripts, MLC (1V), MHC alpha, MHC beta, striated alpha TM and ANF. No genes were ex pressed exclusively in cardiac tissue; for example, expression of the genes for c alpha-actin, both cardiac MHCs, both MLCs, alpha TM and cT nT was evident in skeletal and vascular smooth muscles at some stages of development. An interesting difference between this species and tho se of previous studies was the minor contribution of skeletal alpha-ac tin to cardiac phenotype. As well, our observation of transcripts of c ardiac MHCs and ANF in vascular smooth muscle has not been previously reported. By providing a comprehensive survey of the ontogenic express ion of this group of genes, our study has been able to identify a numb er of useful cardiac differentiation markers which, as a group, may pr ovide a more definitive indication of authentic cardiac cellular diffe rentiation, appropriate to some in vitro experimental situations.