The VASE exon downregulates the neurite growth-promoting activity of NCAM 140

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Abstract
AXONAL growth, guidance and synapse formation are controlled by receptors on neuronal growth cones that can recognize positive and inhibitory cues in the local microenvironment1–3. Four well characterized receptor systems are known that recognize the growth-promoting activities associated with the extracellular matrix and the membranes of cells such as astrocytes, muscle cells and Schwann cells; these are the integrins4 and the homophilically binding cell adhesion molecules neural-cell adhesion molecule (NCAM), N-cadherin and LI (refs 5-12). Alternative splicing generates 20-30 isoforms of NCAM and these can also be differentially glycosylated13. There are two sites where alternative splicing changes the extracellular structure of membrane-bound NCAM and one of these (the MSD1 region) does not obviously affect function6. Here we report that the variable alternatively spliced exon (VASE) in immunoglobulin domain 4 downregulates the neurite outgrowth-promoting activity of NCAM. The high level of VASE expression in the adult central as compared with peripheral nervous system21 could contribute to the poor regenerative capacity of the former.