Heparan sulfate proteoglycans. Essential co-factors in receptor-mediated processes with relevance to the biology of the vascular wall

Heparan sulfate (HS), a mixed bag of complex, heterogeneous and highly charged polysaccharides, is an essential co-factor in a large number of receptor-ligand interactions and cellular pathways. These co-factor functions depend on the binding-interactions of the HS chains with the ligand or receptor, or both. These binding interactions and the ensuing functional effects often depend on defined carbohydrate sequences within the HS chains, whereby the required sequences are not always represented within all natural forms of the polysaccharide. The proteins that are substituted with HS resort from a limited number of protein families, with different cellular, subcellular and supramolecular associations, and show differential activities in functional assays. It is likely that the natural co-factor functions of the HS proteoglycans depend on glycan-protein and protein-protein interactions that are subject to modulation, both at the glycan and protein levels.