Polysialic acid in the vertebrate nervous system: a promoter of plasticity in cell-cell interactions.

Polysialic acid (PSA), a homopolymer attached to the neural cell adhesion molecule (NCAM), serves as a modulator of cell interactions. Polysialic acid exhibits a highly regulated expression pattern. During embryonic development its abundant expression is closely correlated with axon pathfinding and targeting, and with certain aspects of muscle formation. Its level also can be altered by synaptic activity. During neonatal development and in the adult brain, PSA expression is more restricted, being primarily associated with regions capable of morphological or physiological plasticity. The ability to perturb PSA in vivo by a specific glycosidase and by the creation of NCAM-deficient mice has led to extensive analysis of its biological function. These studies suggest that the primary role of PSA is to promote changes in cell interactions and thereby facilitate plasticity in the structure and function of the nervous system.