ISOLATION OF A NEURONAL CELL-SURFACE RECEPTOR OF HEPARIN-BINDING GROWTH-ASSOCIATED MOLECULE (HB-GAM) - IDENTIFICATION AS N-SYNDECAN (SYNDECAN-3)

HB-GAM (heparin binding growth-associated molecule; pleiotrophin) is a secretory, extracellular matrix-associated protein that is strongly e xpressed in developing nervous tissues and belongs to a novel family o f differentiation/growth factors. It promotes axonal growth from perin atal rat brain neurons and is suggested to be mitogenic for some cell types and to display cell-transforming activity. Since the receptors o f HB-GAM in cells are unknown, we have started isolation of putative c ell surface receptors from brain neurons and from perinatal rat brain. For this purpose, recombinant HB-GAM was produced with the aid of a b aculovirus vector and used as an affinity matrix in receptor isolation . A detergent-solubilized component from cultured brain neurons and fr om brain was identified that binds specifically to HB-GAM and migrates on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a bro ad smear with an apparent molecular mass of about 200 kDa. This cell s urface component was found to contain heparan sulfate chains, which ar e bound to a core protein with an apparent molecular mass of 120 kDa. Gel electrophoretic characteristics, immunochemical analysis, and part ial peptide sequencing revealed that the cell surface component isolat ed as an HB-GAM receptor is N-syndecan (syndecan-3). In a solid phase binding assay, N-syndecan was found to bind to HB-GAM in a similar man ner as to basic fibroblast growth factor (K-D = 0.6 nM). Immunofluores cence microscopy indicated that in brain neurons, N-syndecan occurs at the surface of the cell soma and of the neurites that grow along HB-G AM-coated substrates. Anti-N-syndecan antibodies added to culture medi a had an inhibitory effect on HB-GAM-induced neurite outgrowth. We sug gest that N-syndecan mediates the neurite outgrowth-promoting signal f rom HB-GAM to the cytoskeleton of growing neurites.