Abstract
Semaphorins are a family of secreted and membrane-bound proteins, known to control axonal pathfinding. It was recently demonstrated that Semaphorin4A (Sema4A) is crucially involved in immune cell activation. However, the role of Sema4A in the nervous system has not yet been clarified. Our growth cone collapse assay with recombinant Sema4A disclosed that Sema4A induced a significant growth cone collapse in primary hippocampal neurons in a Rho signal-dependent manner. The study also suggested the presence of a receptor on hippocampal neurons that interacts with Sema4A. To identify a high-affinity receptor specific for Sema4A, we firstly prepared molecular probe consisting of the extracellular domain of Sema4A fused to secreted alkaline phosphatase (Sema4A-AP). Then it was examined whether Sema4A-AP could bind as a ligand to COS cells expressing different candidate receptors among plexin family members, by revealing cell-bound AP activity. As a result, Sema4A was found to bind to cells expressing plexin-B1, plexin-B2 or plexin-B3 with high affinity. Sema4A barely bound to cells expressing either plexin-C1 or plexin-D1. Thus the result indicates that Sema4A preferentially binds to members of the plexin-B family, B1-B3. We are currently examining which member of the plexin-B family is functional for Sema4A-induced growth cone collapse in primary hippocampal neurons. [Jpn J Physiol 55 Suppl:S205 (2005)]
