Phosphorylation-dependent regulation of axon fasciculation.

Axons often grow along other axons to produce bundles called fascicles, and a number of cell adhesion molecules (CAMs) found on axon surfaces contribute to this process. The surprising observation that Fab fragments against individual CAMs can completely block fascicle formation suggests that the different axon-associated CAMs are functionally linked. The present studies investigate whether such a linkage might reflect intracellular regulatory mechanisms. Results obtained with chicken retinal explants in culture indicate that fasciculation is highly sensitive to cytoplasmic protein phosphorylation by means of a mechanism that does not alter levels of CAM expression. Moreover, the potent effect of individual Fabs on fasciculation disappears with enhanced phosphorylation. These observations suggest that growing axons possess a general regulatory process for the multiple CAMs that participate in fasciculation.