Expression patterns of laminin receptor splice variants alpha 6A beta 1 and alpha 6B beta 1 suggest different roles in mouse development

The alpha 6 beta 1 integrin is a receptor for laminins and is present from early stages of mouse embryogenesis. In the present study we determined the temporal and spatial expression of the two cytoplasmic splice variants of the alpha 6 integrin subunit, alpha 6A and alpha 6B, in the early- and mid-gestation mouse postimplantation embryo using RT-PCR, in situ hybridization, and immunofluorescence. Our results show that alpha 6B is present in the embryo at all stages studied and is expressed before alpha 6A. alpha 6A expression begins in 8.5 day p.c. embryos and is initially exclusively localized to the developing heart. In 8.5 (and 9.5) day p.c. embryos alpha 6A mRNA and protein are present in a gradient in the myocardium of the heart tube from strongest expression in the sinus venosus and in the common atrial chamber to a weakening expression along the ventricle and bulbus cordis. In 10.5 day p.c. embryos this gradient is less evident and in 12.5 day p.c. embryos alpha 6A mRNA and protein are present in comparable amounts between atria and ventricles. Neither alpha 6A nor alpha 6B is present in endocardial cushion tissue. By day 12.5 p.c. alpha 6A expression is also present in the developing epidermis, dental primordia, lens, gonads, and in a few epithelia such as those of the digestive tract. alpha 6B expression is always much more widespread than alpha 6A expression. For example, only alpha 6B is present in the myotome of the somites of 9.5 day p.c. embryos, in the developing central and peripheral nervous systems, and in the nephrogenic system at all stages studied, except after the differentiation of the gonads when alpha 6A is also present. Furthermore, alpha 6B is the only splice variant present on endothelial cells. We also examined the distribution of the beta 4 integrin subunit to determine whether the alpha 6 beta 4 integrin was present during these stages of development. Beta 4 protein was absent in early postimplantation stages but was present in the epidermis and digestive tract of 12.5 day p.c. embryos. These results show a differential distribution of alpha 6A and alpha 6B during mouse development and thus strongly suggest a different function of these splice variants during embryogenesis. Our results point to a possible role for the alpha 6A beta 1 integrin in the development of the myocardium of the developing heart, but not in the migration of endocardial cushion cells, while alpha 6B beta 1 could be important in the developing nephrogen