Polysialic acid on the surface of axons regulates patterns of normal and activity-dependent innervation

Studies of the cell-cell adhesion molecules NCAM and L1 have indicated that their combined action is an important determinant in establishing normal patterns of muscle innervation. Moreover, they participate in activity-dependent changes in axonal sprouting. Recent findings in vivo, however, suggest that the central variable in both events is not altered NCAM or L1 expression, but rather changes in the amount of polysialic acid (PSA) at the cell surface. This finding is consistent with the proposed role of PSA as a regulator of cell-cell interactions. Because these molecular entities are present in most of the nervous system, it is likely that this mechanism can influence many aspects of axonal behavior during development and regeneration.