GRADIENT OF INTEGRIN ALPHA-6A DISTRIBUTION IN THE MYOCARDIUM DURING EARLY HEART DEVELOPMENT

The interactions of cells with extracellular matrices (ECM)(1) are lik ely to be key determinants of embryonic development. Integrin adhesion receptors are ideally positioned to mediate some of these interaction s since, in addition to mechanical adhesion, they transduce signals af fecting cell proliferation and differentiation. We investigated expres sion of the integrin alpha 6 beta 1, a receptor for the ECM component, laminin in the early mouse embryo. An intriguing feature of this inte grin is the existence of alpha 6 subunit isoforms. The A and B isoform s, which differ in the cytoplasmic tails, are expressed in cell-type s pecific fashion, and are likely to implement distinct cellular interac tions with laminin. By RT-PCR, alpha 6B but not alpha 6A mRNA was dete ctable in embryo extracts from fertilized oocytes to 6.5 d.p.c. In sub sequent stages, up to 11.5 d.p.c., alpha 6A mRNA was observed in mRNA extracts from whole embryos, but still in significantly lower amounts than alpha 6B. However, in extracts from isolated heart (9.5 to 11.5 d .p.c.), alpha 6A was the predominant alpha 6 isoform, while in extract s from other embryo parts no alpha 6A mRNA was detectable. At the prot ein level, immunostaining with specific antibodies showed alpha 6A pro tein in myocardial cells, at the early stage of heart tube development (8.5 d.p.c.). Localization to the myocardium was tightly restricted, since other structures of the embryonic heart, e.g., endocardium, or o f the remaining embryo did not stain with anti-alpha 6A antibody. Tn t he ventricular myocardium, expression of alpha 6A appeared more intens e than in the subendocardial layer. Quantitation by confocal microscop y unveiled a gradient of expression of alpha 6A, increasing from the o uter to the inner layers of the myocardium. This is the first demonstr ation of a gradient distribution of integrin molecules in a tissue, wh ich appears to be directly connected with the process of organogenesis . The mechanism underlying our observations is not the turning on of a gene, rather it is the activation of a splicing mechanism that substi tutes the cytoplasmic domain of a laminin receptor. Because integrin c ytoplasmic domains are thought to be an important functional end of th e molecule, this may be a mechanism to modulate cellular responses to laminin.