Heparin/heparan sulphate binding in the TGF-β cytokine superfamily

The TGF-β (transforming growth factor-β) cytokine superfamily in mammals contains some 30 members. These dimeric proteins are characterized by a strongly conserved cystine knot-based structure. They regulate the proliferation, differentiation and migration of many cell types, and therefore have important roles in morphogenesis, organogenesis, tissue maintenance and wound healing. Thus far, around one-quarter of these cytokines have been shown to bind to heparin and heparan sulphate. Well-established examples are the TGF-β isoforms 1 and 2, and the BMPs (bone morphogenetic proteins) -2 and -4. In studies in my laboratory, we have shown that GDNF (glial-cell-line-derived neurotrophic factor) and its close relatives neurturin and artemin bind to heparin and heparan sulphate with high affinity. We have reported previously that binding of GDNF is highly dependent on the presence of 2-O-sulphate groups. More recently, we and others have been investigating the heparin/heparan sulphate-binding properties of BMP-7, which is a representative of a distinct BMP subgroup from that of BMPs -2 and -4. Interestingly, several of the various specific BMP antagonist proteins also bind to heparin and heparan sulphate. Much remains to be learnt about the nature and role of glycosaminoglycan interactions in the TGF-β superfamily, but current work suggests that these cytokines do not share a single highly conserved heparin/heparan sulphate-binding site.