The Core Protein of the Chondroitin Sulfate Proteoglycan Phosphacan Is a High-affinity Ligand of Fibroblast Growth Factor-2 and Potentiates Its Mitogenic Activity

Abstract

Using a radioligand binding assay we have demonstrated that phosphacan, a chondroitin sulfate proteoglycan of nervous tissue that also represents the extracellular domain of a receptor-type protein tyrosine phosphatase, shows saturable, reversible, high-affinity binding (K d ϳ6 nM) to fibroblast growth factor-2 (FGF-2). Binding was reduced by only ϳ35% following chondroitinase treatment of the proteoglycan, indicating that the interaction is mediated primarily through the core protein rather than the glycosaminoglycan chains. Immunocytochemical studies also showed an overlapping localization of FGF-2 and phosphacan in the developing central nervous system. At concentrations of 10 g protein/ml, both native phosphacan and the core protein obtained by chondroitinase treatment potentiated the mitogenic effect of FGF-2 (5 ng/ml) on NIH/3T3 cells by 75-90%, which is nearly the same potentiation as that produced by heparin at an equivalent concentration. Although studies on the role of proteoglycans in mediating the binding and mitogenic effects of FGF-2 have previously focused on cell surface heparan sulfate, our results indicate that the core protein of a chondroitin sulfate proteoglycan may also regulate the access of FGF-2 to cell surface signaling receptors in nervous tissue.Phosphacan and neurocan are nervous tissue-specific chondroitin sulfate proteoglycans that are high-affinity ligands of several immunoglobulin superfamily neural cell adhesion molecules and of the extracellular matrix proteins tenascin-C and tenascin-R (1-3). These interactions are variously mediated by the chondroitin sulfate chains, N-linked oligosaccharides present on the core glycoproteins, or by other structural features of the proteoglycans that have not yet been specifically identified. Phosphacan and neurocan both bind to neurons and have potent inhibitory effects on cell adhesion and neurite outgrowth, although in certain experimental situations phosphacan may also stimulate neurite growth. Neurocan is synthesized by neurons and is a member of the family of hyaluronan-binding chondroitin sulfate proteoglycans that also includes aggrecan, versican, and brevican, whereas phosphacan, which is produced by astrocytes, is an alternative splicing product representing the extracellular domain of a receptor-type protein tyrosine phosphatase. Phosphacan is also a ligand of the neural differentiation factor HB-GAM 1 (heparin-binding growth-associated molecule)/pleiotrophin (3, 4), and both neurocan and phosphacan bind with high affinity to HB-GAM and to the related differentiation factor amphoterin, with which they colocalize in nervous tissue (3). The interactions with HB-GAM and amphoterin are largely mediated by the chondroitin sulfate chains of the proteoglycans.

Identification and Functions of Chondroitin Sulfate in the Milieu of Neural Stem Cells

“…The cerebral primordium contains a relatively large number of neural stem cells (1,2), and it is feasible that CSPGs participate in the regulation of their functions as major constituents of their milieu, or the niche. To date, phosphacan has been detected in the neuroepithelium and in the ventricular zone of the fetal rat cerebrum (46,47), but it has been shown that immunoreactivities for aggrecan and versican were very low, if any, in the ventricular zone of the developing rat cerebrum (40). We demonstrated the existence of neurocan, RPTP␤, and NGC in the ventricular zone rich in nestin-positive cells, or neural stem/progenitor cells (Figs.…”

Extracellular matrix of the central nervous system: from neglect to challenge

“…It mostly consists of hyaluronan, neurocan, versican V0, versican V1, tenascin-C and HAPLN1/Crtl1 (Milev et al 1998b;Rauch 2004). In addition, prominent CNS-expression of aggrecan has been reported in chick embryos (Schwartz and Domowicz 2004), while only very small amounts have been detected in prenatal rodent brains Milev et al 1998c). Cellular origin of neurocan and the large versican variants are neurons (Engel et al 1996;Yamagata and Sanes 2005), whereas aggrecan appears to be mainly expressed by cells of astroglial lineage at this early time point (Domowicz et al 2008).…”

Membrane Heparan Sulfate Proteoglycan-supported FGF2-FGFR1 Signaling

“…The core proteins of some proteoglycans (e.g. NG2 and phosphacan) bind to FGF2 (37,38). Direct core protein interactions with signaling components might be lost upon tryptic release of the proteoglycan from the cell surface, explaining the failure of these soluble forms.…”