SYNDECAN FAMILY OF CELL-SURFACE PROTEOGLYCANS - DEVELOPMENTALLY-REGULATED RECEPTORS FOR EXTRACELLULAR EFFECTOR MOLECULES

Syndecans are a family of integral membrane proteoglycans With conserv ed membrane-spanning and intracellular domains but with structurally d istinct extracellular domains (ectodomains). They are known to functio n as heparan sulphate co-receptors in fibroblast growth factor signall ing as well as to link cells directly to the extracellular matrix. The se and other biological activities of syndecans involve specific inter actions of the heparan sulphate side chains of syndecans with cytokine s and extracellular matrix proteins. Four different vertebrate syndeca ns, designated as syndecans 1-4 (or syndecan, fibroglycan, N-syndecan and amphiglycan, respectively); are known. During embryonic developmen t, syndecans have specific and highly regulated expression patterns th at are distinct from the expression in adult tissue, suggesting an act ive role in morphogenetic processes. The developmental expression of s yndecans is particularly intense in mesenchymal condensates and at epi thelium mesenchyine interfaces, where a number of heparan sulphate-bin ding cytokines and matrix components are also expressed in a regulated manner, often spatially and temporally co-ordinated with the syndecan expression. Recent evidence indicates that the regulation of heparan sulphate fine structure (mainly the number and arrangement of sulphate groups along the polymer) provides a mechanism for the cellular contr ol of syndecan-protein interactions. Furthermore, morphogenetically ac tive cytokines such as fibroblast growth factor-2 and transforming gro wth factor-beta participate in the regulation of syndecan expression a nd glycosaminoglycan structure. This review discusses the developmenta l expression and binding functions of syndecans as well as the molecul ar regulation of specific heparan sulphate-protein interactions.