The role of extracellular and cytoplasmic splice domains of alpha 7-integrin in cell adhesion and migration on laminins

The major laminin-binding integrin of skeletal, smooth, and heart muscle is alpha 7 beta 1-integrin, which is structurally related to alpha 6 beta 1. It occurs in three cytoplasmic splice variants (alpha 7A, -B, and -C) and t wo extracellular forms (X1 and X2) which are developmentally regulated and differentially expressed in skeletal muscle. Previously, we have shown that ectopic expression of the alpha 7B-integrin splice variant in nonmotile HE K293 cells specifically induced cell locomotion on laminin-1 but not on fib ronectin. To investigate the specificity and the mechanism of the alpha 7-m ediated cell motility, we expressed the three alpha 7-chain cytoplasmic spl ice variants, as well as alpha 6A- and alpha 6B-integrin subunits in HEK293 cells. Here we show that all three alpha 7 splice variants (containing the X2 domain), as well as alpha 6A and alpha 6B, promote cell attachment and stimulate cell motility on laminin-1 and its E8 fragment. Deletion of the c ytoplasmic domain (excluding the GFFKR consensus sequence) from alpha 7B re sulted in a loss of the motility-enhancing effect. On laminin-2/4 (merosin) , the predominant isoform in mature skeletal muscle, only alpha 7-expressin g cells showed enhanced motility, whereas cells transfected with alpha 6A a nd alpha 6B neither attached nor migrated on laminin-2. Adhesion of alpha 7 -expressing cells to both laminin-1 and laminin-2 was specifically inhibite d by a new monoclonal antibody (6A11) specific for alpha 7. Expression of t he two extracellular splice variants alpha 7X1 and alpha 7X2 in HEK293 cell s conferred different motilities on laminin isoforms: Whereas alpha 7X2B pr omoted cell migration on both laminin-1 and laminin-2, alpha 7X1B supported motility only on laminin-2 and not on laminin-1, although both X1 and X2 s plice variants revealed similar adhesion rates to laminin-1 and -2. Fluores cence-activated cell sorter analysis revealed a dramatic reduction of surfa ce expression of alpha 6-integrin subunits after alpha 7A or -B trans-fecti on; also, surface expression of alpha 1-, alpha 3-, and alpha 5- integrins was significantly reduced. These results demonstrate selective responses of alpha 6- and alpha 7-integrins and of the alpha 7 splice variants to lamin in-1 and -2 and indicate differential roles in laminin-controlled cell adhe sion and migration. (C) 2000 Academic Press.