NEUREXIN IS EXPRESSED ON NERVES, BUT NOT AT NERVE-TERMINALS, IN THE ELECTRIC ORGAN

Neurexins are highly variable transmembrane proteins hypothesized to b e nerve terminal-specific cell adhesion molecules, As a test of the hy pothesis that neurexin is restricted to the nerve terminal, we examine d neurexins in the electric organ of the elasmobranch electric fish. S pecific antibodies generated against the intracellular domain of elect ric fish neurexin were used in immunocytochemical and Western blot ana lyses of the electromotor neurons that innervate the electric organ. O ur results indicate that neurexin is not expressed at electric organ n erve terminals, as expected by the neurexin hypothesis. Instead, neure xin is expressed by electromotor neurons and on myelinated axons. This neurexin has a molecular weight of 140 kDa, consistent with an alpha- neurexin. In addition, we find that perineurial cells of the electromo tor nerve also express a neurexin, These cells surround bundles of axo ns to form a diffusion barrier and are thought to be a special form of fibroblast, The results of the study argue against a universal role f or neurexins as nerve terminal-specific proteins but suggest that neur exins are involved in axon-Schwann cell and perineurial cell interacti ons.