BEN as a presumptive target recognition molecule during the development of the olivocerebellar system

It has been shown previously that in the chick embryo the cell adhesion molecule BEN/SC1/DM-GRASP is expressed by neurons in the inferior olive (IO) and by their terminal axonal arbors in the cerebellar cortex, the climbing fibers (Porquié et al., 1992b). Here, new information on the expression of BEN during the formation of the olivocerebellar projection adds the important notion that BEN is also expressed by the cerebellar targets of inferior olivary axons, Purkinje cells (PCs) and deep nuclear neurons. This expression is transient, starting at E7-E8 and vanishing shortly after hatching. More importantly, BEN expression is restricted to precise subsets of IO neurons and PCs. In the cerebellar cortex, BEN-immunoreactive (BEN-IR) structures are not found randomly but are distributed according to a reproducible pattern of parasagittal stripes. A maximum of four distinct sagittal stripes is found in each lobule, along the whole rostrocaudal extent of the cerebellum. Moreover, BEN-expressing stripes belong to two classes; one contains BEN-IR climbing fibers terminating on BEN-IR PCs and the other, more frequent class is solely composed of BEN-IR climbing fibers. Organotypic cultures of isolated cerebella have shown that the expression of BEN in the IO and in the cerebellum arise independently, probably because of an intrinsic developmental program. Thus, the cell adhesion molecule BEN meets all criteria for a recognition molecule involved in the formation of the olivocerebellar projection.