EPICARDIN - A NOVEL BASIC HELIX-LOOP-HELIX TRANSCRIPTION FACTOR GENE EXPRESSED IN EPICARDIUM, BRANCHIAL ARCH MYOBLASTS, AND MESENCHYME OF DEVELOPING LUNG, GUT, KIDNEY, AND GONADS

We report the cloning, chromosomal localization, and analysis of the e xpression pattern of epicardin, a member of the basic helix-loop-helix (bHLH) family of transcription factors. Within its bHLH domain, the h uman and murine epicardin genes were most similar to paraxis, a bHLH g ene important for segmentation of embryonic paraxial mesoderm. In situ hybridization studies revealed strong epicardin. expression in murine embryos at 9.5 days postcoitum (dpc) in a region of the septum transv ersum at the base of the heart known as the proepicardial organ. This mesenchymal structure extends villous projections from which epicardia l precursor cells emerge and migrate out over the surface of the myoca rdium. Strong expression was seen in individual migratory cells and cl usters at 9.5 dpc and in a continuous epicardial cell layer in more ma ture hearts. Also from 9.5 dpc, epicardin transcripts were seen in end ocardial cushions of the atrioventricular canal and outflow tract, in skeletal myoblasts within branchial arches and in condensing mesenchym e of gut, kidney, urinary tract, gonads, spleen, and lung. Northern an alysis showed that expression persisted in mature visceral organs and heart, but was transient in skeletal muscle. The central role played b y bHLH factors in pathways for tissue determination in the embryo sugg ests a function for epicardin in specification of select mesodermal ce ll populations associated with heart, cranial skeletal muscle, gut, an d urogenital system. Dev. Dyn. 1998;213:105-113, (C) 1998 Wiley-Liss, Inc.