Identification and characterization of CEPU-Se - A secreted isoform of theIgLON family protein, CEPU-1

CEPU-1/Neurotrimin is a neuronal glycoprotein thought to play a role in axo n guidance and cell-cell recognition. It is a member of the IgLON family, h as three C2 domains, and is attached to the plasma membrane by a GPI-anchor . We report here the characterisation of an alternatively-spliced isoform o f CEPU-1 that is secreted. This isoform, termed CEPU-se, is coexpressed wit h CEPU-1 in retina, cerebellum and DRG neurons. In the cerebellum CEPU-1/CE PU-Se is expressed predominantly on granule cells and in the molecular laye r. Divalent but not monovalent CEPU-Se interacts with CEPU-1 and other IgLO Ns, suggesting that the ability of CEPU-Se to modify the activity of the Ig LON family may require an additional cofactor. CEPU-Se does not support the outgrowth of DRG neurons or the extension of established growth cones; how ever, neurite outgrowth on laminin is unaffected by CEPU-Se. Our data sugge st that CEPU-Se may act to modulate the ability of CEPU-1, LAMP, and OBCAM to influence neurite outgrowth.