The two thrombospondin type I repeat domains of HB-GAM display a cooperative function in N-syndecan binding and regulation of synaptic plasticity.

The TSR (thrombospondin type I repeat) is defined by a conserved tryptophan/cysteine motif and found in a large family of extracellular matrix and cell surface proteins (for review, see [1]). These proteins include thrombospondins 1 and 2, F-spondin, mindin, and semaphorins F and G. HB-GAM (heparin-binding growth-associated molecule, also designated as pleiotrophin) and MK (midgestation-kidney protein) form a subfamily within this type of proteins and have very similar structures (see Fig. 1 for essential structural features of HB-GAM and MK). Structures of the TSR domains of the thrombospondins have been recently resolved using X-ray crystallography[2], and secondary structures of MK[3] and HB-GAM[4] are currently known based on NMR spectroscopy. HB-GAM and MK fold in a very similar manner, but their TSR domains apparently display significant differences from the TSR structures resolved for the thrombospondins. In general, TSR proteins are implicated in cell surface/matrix binding and regulate migratory responses in many types of cells (for reviews, see [1,5]). A wide variety of TSR proteins are highly expressed in the nervous system, but their functional roles and possible significance of the TSRs in these proteins remain elusive. Structure-function studies of HB-GAM[6] have recently shown that the two TSRs of the protein mediate binding to heparin/HS (heparan sulfate). In hippocampal neurons, N-syndecan (neural syndecan, syndecan-3) is the carrier of the HS chains interacting with HB-GAM. Furthermore, studies using hippocampal neurons and slices implicate the di-TSR domain in the regulation of neurite outgrowth/plasticity in the developing and adult hippocampus. The rationale of the structure-function studies using neurite outgrowth/plasticity as a readout derives from previous studies where HB-GAM was shown to enhance neurite extension in hippocampal neurons when presented as a matrix-bound form[7,8] and to inhibit it when an excess of HB-GAM occurs in the culture medium of cells[8]. Furthermore, HB-GAM was shown to be a neuronal activity-induced gene in the hippocampus[9] which, together with the effects on hippocampal neurons in vitro, suggested a role for HB-GAM in the regulation of hippocampal plasticity. To study the possible role of HB-GAM in synaptic plasticity in the hippocampus, its effects on LTP (long-term potentiation; long-lasting, activity-induced