Abstract
Heparin-binding growth-associated molecule (HB-GAM) is a cell-surface- and extracellular matrix-associated protein that lines developing axons in vivo and promotes neurite outgrowth in vitro. Because N-syndecan (syndecan-3) was found to function as a receptor in HB-GAM-induced neurite outgrowth, we have now studied whether the heparan sulfate side chains of N-syndecan play a role in HB-GAM-neuron interactions. N-Syndecan from postnatal rat brain was found to inhibit HB-GAM-induced but not laminin-induced neurite outgrowth when added to the assay media. The inhibitory activity was abolished by treating N-syndecan with heparitinase, but it was retained in N-syndecan-derived free glycosaminoglycan chains, suggesting that N-syndecan heparan sulfate at the cell surface is involved in HB-GAM-induced neurite outgrowth. Binding to HB-GAM and inhibition of neurite outgrowth was observed with heparin-related polysaccharides only; galactosaminoglycans were inactive. Significant inhibition of neurite outgrowth was induced by heparin and by N-syndecan heparan sulfate but not by heparan sulfates from other sources. A minimum of 10 monosaccharide residues were required for HB-GAM-induced neurite outgrowth. Experiments with selectively desulfated heparins indicated that 2-O-sulfated iduronic acid units, in particular, are of importance to the interaction with HB-GAM, were implicated to a lesser extent. Structural analysis of N-syndecan from 6-day-old rat brain indicated that the heparan sulfate chains contain sequences of contiguous, N-sulfated disaccharide units with an unusually high proportion (82%) of 2-O-sulfated iduronic acid residues. We suggest that this property of N-syndecan heparan sulfate is essential for HB-GAM binding and induction of neurite outgrowth.
Highlights
Cell surface proteoglycans are suggested to function as receptors or co-receptors for molecules that mediate cell-to-extracellular matrix and cell-to-cell interactions and for growth factors of the extracellular milieu
We suggest that the binding of HB-GAM to carbohydrates is specific for heparin-type structures and that N-syndecan heparan sulfate fulfills the requirements for such interaction
Inhibition of HB-GAM-induced Neurite Outgrowth by N-syndecan—N-Syndecan is expressed in cultured brain neurons based on metabolic labeling experiments and on immunohistochemistry [26], and it is found in the cultured neurons by Western blotting using anti-peptide antibodies [28] against N-syndecan
Summary
Cell surface proteoglycans are suggested to function as receptors or co-receptors for molecules that mediate cell-to-extracellular matrix and cell-to-cell interactions and for growth factors of the extracellular milieu (for reviews see Refs. 1–7). HB-GAM (p18) was isolated from rat brain as a neurite outgrowth-promoting protein, the expression of which in brain corresponds to the stage of rapid axonal growth [14]. During embryonic and perinatal development, HB-GAM is strongly expressed in axon pathways of the brain [24] and as a component of basement membranes outside the brain [25]. These findings have raised the question as to the occurrence of cell surface receptors for HB-GAM. Both proteins are preferentially localized to developing fiber tracts of the rat brain [28]
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