IMMUNOCYTOCHEMICAL LOCALIZATION OF THE HEPARIN-BINDING GROWTH-ASSOCIATED MOLECULE (HB-GAM) IN THE DEVELOPING AND ADULT-RAT CEREBELLAR CORTEX

The heparin-binding, growth-associated molecule (HB-GAM) is a developm entally regulated protein that belongs to a new family of heparin-bind ing molecules, not related to the fibroblast growth factors (FGFs), wi th putative functions during cell growth and differentiation. In order to further study the functional role of HB-GAM we have used a polyclo nal antiserum, raised against the purified protein to localize HB-GAM in the developing and adult rat cerebellar cortex. During postnatal de velopment HB-GAM-like immunoreactivity (IR) was found to be present in all layers of the cerebellar cortex. IR was mainly associated with pr ocesses or extracellular structures but not with cell bodies. Througho ut all the stages examined the molecular layer was clearly labeled, wh ereas staining in the internal granular layer was diffuse. IR in the e xternal granular layer on postnatal day 1 and 8 was found to be associ ated with radially oriented fibres connecting the internal granular la yer with the pial surface of the cerebellum. The intensity of this sta ining seemed to increase from day 1 to 8. Staining of corresponding ar eas with an antiserum against the glial fibrillary acidic protein (GFA P) suggested that the HB-GAM antiserum in the developing cerebellar co rtex labels Bergmann glia fibres of Golgi epithelial cells. Because of the diffuse staining of the molecular layer in the adult rat it was n ot possible to distinguish whether radial fibres in the adult containe d any HB-GAM IR. Golgi epithelial cells are considered as crucial for the migration of granular cells during the differentiation of the cere bellar cortex. We therefore speculate that the association of HB-GAM-l ike IR may be of functional relevance. The fact that molecules, such a s tenascin, known to be involved in morphogenetic events show a simila r spatiotemporal distribution pattern further underscores this hypothe sis. HB-GAM, which possesses a classical signal sequence, might be rel eased in the extracellular space and could mediate adhesion phenomena by binding to heparin-like molecules associated with the neuronal memb rane. Therefore, it will be important to investigate whether specific antibodies against HB-GAM are able to interfere with normal cerebellar development in vitro and in vivo.