Alternative Splice Variants of α7β1Integrin Selectively Recognize Different Laminin Isoforms

Abstract

The integrin ␣ 7 ␤ 1 occurs in several cytoplasmic (␣ 7A , ␣ 7B ) and extracellular splice variants (␣ 7X1 , ␣ 7X2 ), which are differentially expressed during development of skeletal and heart muscle. The extracellular variants result from the alternative splicing of exons X1 and X2, corresponding to a segment within the putative ligand binding domain. To study the specificity and affinity of the X1/X2 variants to different laminin isoforms, soluble ␣ 7 ␤ 1 complexes were prepared by recombinant coexpression of the extracellular domains of the ␣-and ␤-subunits. The binding of these complexes to purified ligands was measured by solid phase binding assays. Surprisingly, the alternative splice variants revealed different and specific affinities to different laminin isoforms. While the ␣ 7X2 variant bound much more strongly to laminin-1 than the ␣ 7X1 variant, the latter showed a high affinity binding to laminins-8 and -10/11. Laminin-2, the major laminin isoform in skeletal muscle, was recognized by both variants, whereas none of the two variants were able to interact with laminin-5. A specific blocking antibody inhibited the binding of both variants to all laminins tested, indicating the involvement of common epitopes in ␣ 7X1 ␤ 1 and ␣ 7X2 ␤ 1 . Because laminin-8 and -10/11 as well as ␣ 7X1 are expressed in developing skeletal and cardiac muscle, these findings suggest that ␣ 7X1 ␤ 1 may represent a physiological receptor with novel specificities for laminin-8 and -10.

Vipera lebetina Venom Contains Two Disintegrins Inhibiting Laminin-binding β1 Integrins

“…A high yield expression of soluble ␣ 7 X2␤ 1 integrin was achieved by coexpression of the murine ␣ 7 X2 integrin ectodomain with the human ␤ 1 integrin ectodomain in Drosophila Schneider's cells. Similar to the results of von der Mark et al (28), we showed that soluble ␣ 7 X2␤ 1 integrin bound to laminin-1 and laminin-2/4, but failed to interact with laminin-5. Despite being the major laminin isoform in muscle tissue (11), laminin-2/4 was bound by ␣ 7 X2␤ 1 integrin with a lower affinity than laminin-1.…”

The β1 Cytoplasmic Domain Regulates the Laminin-binding Specificity of the α7X1 Integrin

“…On laminin-10/11, HuX2/A cells adhered poorly whereas HuX1/A cells showed strong affinity. von der Mark and collaborators recently showed that soluble double truncated forms of ␣7X1 and ␣7X2 bind differently to various laminin isoforms (von der Mark et al, 2002). Our results are similar in that ␣7X1 expressing cells bound to laminin-10/11 substrates whereas ␣7X2 bound preferentially to laminin-1.…”

“…The laminin-binding ␣7␤1 integrin, expressed in mouse skeletal muscle as early as E10.5 d of development, mediates myoblast motility on laminin substrates (Kaufman et al, 1980;Kaufman et al, 1991;Yao et al, 1996a,b;Crawley et al, 1997;von der Mark et al, 2002). In mature skeletal muscle, the ␣7 receptor is associated with costameres and the myotendinous and neuromuscular junctions (Bao et al, 1993;Belkin et al, 1996;Martin et al, 1996;van der Flier et al, 1997).…”

“…These results indicated that alternative splicing regulates ␣7␤1 ligand-binding competence in a cell-specific manner. Recently, von der Mark and collaborators reported that the X1/X2 variants have different binding specificity and affinities for laminin isoforms (von der Mark et al, 2002).…”

Laminin-111 protein therapy prevents muscle disease in the mdx mouse model for Duchenne muscular dystrophy

“…Although laminin-111 is not expressed in normal or dystrophic adult skeletal muscle, studies indicate it is a preferred ligand for ␣ 7 ␤ 1 -integrin (21,22). Treatment with laminin-111 stabilized the sarcolemma of mdx skeletal muscle, reduced myofiber degeneration, decreased serum creatine kinase, and protected muscle from exercise-induced injury, suggesting that treatment with laminin-111 may be a potent therapy for DMD.…”