CLONING AND CHARACTERIZATION OF A BASIC HELIX-LOOP-HELIX PROTEIN EXPRESSED IN EARLY MESODERM AND THE DEVELOPING SOMITES

Basic helix-loop-helix (bHLH) heterodimer protein complexes regulate t ranscription of genes during the processes of differentiation and deve lopment. To study the molecular basis of early mesodermal differentiat ion, we sought to identify bHLH proteins from cells of mesodermal orig in. By using an interaction cloning strategy with a radiolabeled recom binant bHLH protein, E12, a clone encoding a potential heterodimer par tner was isolated from an endothelial cell library. This gene (bHLH-EC 2) is most homologous to Twist but shares similarity within the bHLH d omain with TAL1 and other members of this family. bHLH-EC2 is expresse d in cultured endothelial cells and in embryonic stem cell, erythroleu kemia, and muscle cell lines in a differentiation-dependent manner. In situ hybridization studies of mouse embryos reveal that bHLH-EC2 is e xpressed throughout the primitive mesoderm as early as 7.5 days postco itum. Expression then becomes restricted to the paraxial mesoderm and to the dermamyotome of the developing somite. Expression of bHLH-EC2 i n cells destined to become myoblasts thus predates expression of myoge nic bHLH factors. bHLH-EC2 is expressed in early endothelial and hemat opoietic cells in vivo, as shown by RNA studies of embryonic yolk sac and cultured cells derived from yolk sac explants. These findings sugg est that bHLH-EC2 plays a role in the development of multiple cell typ es derived from the primitive mesoderm.