Cre- loxP –mediated Inactivation of the (cid:97) 6A Integrin Splice Variant In Vivo: Evidence for a Specific Functional Role of (cid:97) 6A in Lymphocyte Migration but Not in Heart Development

Two splice variants of the (cid:97) 6 integrin subunit, (cid:97) 6A and (cid:97) 6B, with different cytoplasmic domains, have previously been described. While (cid:97) 6B is expressed throughout the development of the mouse, the expression of (cid:97) 6A begins at 8.5 days post coitum and is initially restricted to the myocardium. Later in ontogeny, (cid:97) 6A is found in various epithelia and in certain cells of the immune system. In this study, we have investigated the function of (cid:97) 6A in vivo by generating knockout mice deficient for this splice variant. The Cre-loxP system of the bacteriophage P1 was used to specifically remove the exon encoding the cytoplasmic domain of (cid:97) 6A in embryonic stem cells, and the deletion resulted in the expression of (cid:97) 6B in all tissues that normally express (cid:97) 6A . We show that (cid:97) 6A (cid:50) / (cid:50) mice develop normally and are fertile. The substitution of (cid:97) 6A by (cid:97) 6B does not impair the development and function of the heart, hemidesmosome formation in the epidermis, or keratinocyte migration. Furthermore, T cells differentiated normally in (cid:97) 6A (cid:50) / (cid:50) mice. However, the substitution of (cid:97) 6A by (cid:97) 6B leads to a decrease in the migration of lymphocytes through laminin-coated Transwell filters and to a reduction of the number of T cells isolated from the peripheral and mesenteric lymph nodes. Lymphocyte homing to the lymph nodes, which involves various types of integrin–ligand interactions, was not affected in the (cid:97) 6A knockout mice, indicating that the reduced number of lymph node cells could not be directly attributed to defects in lymphocyte trafficking. Nevertheless, the expression of (cid:97) 6A might be necessary for optimal lymphocyte migration on laminin in certain pathological conditions. birth because of of the Moreover, it not possible to investigate the consequences of the lack of (cid:97) 6A during cardiogenesis on the function of the adult heart. We show here that (cid:97) 6B can sustain normal mouse development until ma-turity and that (cid:97) 6A (cid:50) / (cid:50) animals are fertile. We did not de-tect any abnormalities in either the morphology or the his-tology of various organs and tissues, and we argue that the presence of exon A in (cid:97) 6 mRNA is not essential for the proper control of cell migration and proliferation during development. These findings were surprising in the light of the recently published work by Domanico et al. (1997), who have shown that the ectopic expression of (cid:97) 6A in un-differentiated ES cells conferred a highly migratory phenotype to these cells as compared with their normal counterparts, which express only (cid:97) 6B. Importantly, these stimulatory effects on cell migration were found to be independent from adhesion to laminin-1, suggesting that the cytoplasmic domain of (cid:97) 6A was sufficient for promoting motility. However, our own results unequivocally show that the cytoplasmic tail of (cid:97) 6B can support cell migration during development.