Cib2 binds integrin 7B 1D and is reduced in laminin 2 chain deficient muscular dystrophy

Mutations in the gene encoding laminin a2 chain cause congenital muscular dystrophy type 1A. In skeletal muscle, laminin a2 chain binds at least two receptor complexes: the dystrophin-glycoprotein complex and integrin a7ß1. To gain insight into the molecular mechanisms underlying this disorder, we performed gene expression profiling of laminin a2 chain-deficient mouse limb muscle. One of the down-regulated genes encodes a protein called Cib2 (calcium- and integrin-binding protein 2) whose expression and function is unknown. However, the closely related Cib1 has been reported to bind integrin aIIb and may be involved in outside-in-signaling in platelets. Since Cib2 might be a novel integrin a7ß1-binding protein in muscle, we have studied Cib2 expression in the developing and adult mouse. Cib2 mRNA is mainly expressed in the developing central nervous system and in developing and adult skeletal muscle. In skeletal muscle, Cib2 colocalizes with the integrin a7B subunit at the sarcolemma and at the neuromuscular and myotendinous junctions. Finally, we demonstrate that Cib2 is a calcium-binding protein that interacts with integrin a7Bß1D. Thus, our data suggest a role for Cib2 as a cytoplasmic effector of integrin a7Bß1D signaling in skeletal muscle.