Collapsin-1 /semaphorin-III/D is regulated developmentally in Purkinje cells and collapses pontocerebellar mossy fiber neuronal growth cones

Most axons in the CNS innervate specific subregions or layers of their targ et regions and form contacts with specific types of target neurons, but the molecular basis of this process is not well understood. To determine wheth er collapsin-1/semaphorin-III/D, a molecule known to repel specific axons, might guide afferent axons within their cerebellar targets, we characterize d its expression by in situ hybridization and observed its effects on mossy and climbing fiber extension and growth cone size in vitro. In newborn mic e sema-D is expressed by cerebellar Purkinje cells in parasagittal bands lo cated medially and in some cells of the cerebellar nuclei. Later, sema-D ex pression in Purkinje cells broadens such that banded expression is no longe r prominent, and expression is detected in progressively more lateral regio ns. By postnatal day 16, expression is observed throughout the cerebellar m ediolateral axis. Collapsin-1 protein, the chick ortholog of sema-D, did not inhibit the exte nsion of neurites from explants of inferior olivary nuclei, the source of c limbing fibers that innervate Purkinje cells. In contrast, when it was appl ied to axons extending from basilar pontine explants, a source of mossy fib er afferents of granule cells, collapsin-1 caused most pontine growth cones to collapse, as evidenced by a reduction in growth cone size of up to 59%. Moreover, 63% of pontine growth cones arrested their extension or retracte d. Its effects on mossy fiber extension and its distribution suggest that s ema-D prevents mossy fibers from innervating inappropriate cerebellar targe t regions and cell types.