Heparin-binding proteins HB-GAM (pleiotrophin) and amphoterin in the regulation of cell motility

Fractionation of proteins from perinatal rat brain was monitored using a ne urite outgrowth assay. Two neurite-promoting proteins, HB-GAM (heparin-bind ing growth-associated molecule; also known as pleiotrophin) and amphoterin, were isolated, cloned and produced by baculovirus expression for structura l and functional studies. HB-GAM is highly expressed in embryonic and early post-natal fiber pathways of the nervous system, and it enhances axonal gr owth/guidance by binding to N-syndecan (syndecan-3) at the neuron surface. N-syndecan in turn communicates with the cytoskeleton through the cortactin /src-kinase pathway to enhance neurite extension. In addition to N-syndecan , the chondroitin sulfate proteoglycan RPTP beta/zeta (receptor-type tyrosi ne phosphatase beta/zeta) is implicated in the receptor mechanism of HB-GAM . HB-GAM is also prominently expressed in developing and regenerating bone as a matrix-bound cue for migration of osteoblasts/osteoblast precursors to the site of bone deposition. HB-GAM is suggested to regulate motility in o steoblasts through a similar mechanism as in neurons. Structural studies us ing heteronuclear :NMR reveal two similar protein domains ire HB-GAM, both consisting of three anti-parallel beta-strands. Search of sequence database s shows that the beta structures of HB-GAM and of the similar domains of MK (midkine) correspond to the thrombospondin type I (TSR) sequence motif. We suggest that the TSR sequence motif, found in several neurite outgrowth-pr omoting and other cell surface and matrix-binding proteins, defines a beta structure similar to those found in HB-GAM and MK. In general, amphoterin i s highly expressed in immature and transformed cells. We suggest a model, a ccording to which amphoterin is an autocrine/paracrine regulator of invasiv e migration. Amphoterin binds to RAGE (receptor of advanced glycation end p roducts), an immunoglubulin superfamily member related to N-CAM (neural cel l adhesion molecule), that communicates with the GTPases Cdc42 and Rac to r egulate cell motility. In addition, ligation of RAGE by amphoterin activate s NF-kappa B to regulate transcription. (C) 2000 Elsevier Science B.V./Inte rnational Society of Matrix Biology. All rights reserved.