Studies of laminin a2 chain deficient mice -muscle sparing, charaterization of Cib2 and defective spermatorgenesis

Sammanfattning: Mutations in the gene encoding laminin ?2 chain, an extracellular matrix protein mainly expressed in the neuromuscular system, cause a severe form of muscular dystrophy, congenital muscular dystrophy type 1A (MDC1A). Laminin ?2 chain is associated to the muscle fibers by two major receptors, where one of them, integrin ?7?1, is diminished upon laminin ?2 chain deficiency. The majority of the skeletal muscles display severe dystrophic phenotypes upon laminin ?2 chain absence, but we report a spared muscle group, the intrinsic laryngeal muscles (ILM). No evidence of muscle degeneration is detected and expression of various laminin chains is similar to that of limb muscles but sustained integrin ?7B expression is noted. We therefore conclude that ILM are spared in the dy3K/dy3K mouse model of congenital muscular dystrophy type 1A. The singling mediated by laminin ?2 chain is poorly understood. To gain insight into the molecular mechanisms underlying MDC1A, we have performed gene expression profiling of laminin ?2 chain deficient mouse limb muscle. One of the down-regulated genes encodes a protein called calcium and integrin binding protein 2 (Cib2) whose expression and function is unknown. However, the closely related Cib1 has been reported to bind integrin ?IIb and may be involved in outside-in-signaling in platelets. Since Cib2 might be a novel integrin ?7?1 binding protein in muscle, we have studied Cib2 expression in the developing and adult mouse. We demonstrate that Cib2 is a calcium binding protein that interacts with integrin ?7B?1D. Thus, our data suggest a role for Cib2 as a cytoplasmic effector of integrin ?7B?1D signaling in skeletal muscle. Moreover, we have in the seminiferous tubules of laminin ?2 chain deficient mice observed a defect in the timing of lumen formation, resulting in production of fewer spermatides. We also demonstrate that overexpression of laminin ?1 chain in testis of dy3K/dy3K mice compensated for laminin ?2 chain deficiency and significantly reversed the appearance of the histopathological features. We thus provide genetic data that laminin ? chains are essential for normal testicular function in vivo. In summary, the increased knowledge about the roles of laminin ?2 chain in development and disease will hopefully help us to design novel therapeutic approaches for MDC1A.