INTRODUCTION Heparan sulfate proteoglycans (HSPGs) are abundant cell- surface molecules that consist of a core protein and highly sulfated glycosaminoglycan (GAG) sugar chains

Heparan sulfate proteoglycans (HSPGs) are abundant cellsurface molecules that consist of a core protein and highly sulfated glycosaminoglycan (GAG) sugar chains (Kjellen and Lindahl, 1991; Yanagishita and Hascall, 1992; David, 1993). HS chains bind to various extracellular molecules including growth factors, adhesion molecules, proteases and receptors and can support the functions of HS-binding molecules (Lindahl et al., 1998). HS chains are initially synthesised in the Golgi apparatus as polysaccharides consisting of tandem repeats of D-glucuronic acid (GlcA) and N-acetyl-Dglucosamine (GlcNAc). The nascent polysaccharides are subsequently modified in a series of steps: N-deacetylation/Nsulfation of GlcNAc, C5 epimerisation of GlcA to iduronic acid (IdoA), O-sulfation at C2 of IdoA and GlcA, O-sulfation at C6 of N-sulphoglucosamine (GlcNS) and occasional Osulfation at C3 of GlcNS (Gallagher et al., 1992; Prydz and Dalen, 2000). N-deacetylation/N-sulfation, 2-O-, 6-Oand 3-O-sulfation of HS are mediated by the specific action of HS N-deacetylase/N-sulfotransferase (HSNDST), HS 2-Osulfotransferase (HS2ST), HS 6-O-sulfotransferase (HS6ST) and HS 3-O-sulfotransferase, respectively (Lindahl et al., 1998; Prydz and Dalen, 2000). At each of the modification steps, only a fraction of the potential substrates are modified, resulting in considerable sequence diversity. This structural complexity of HS has made it difficult to determine its sequence and to understand the relationship between HS structure and function. Recent evidence has indicated that HS is involved in various developmental processes interacting with important signalling molecules. Several biochemical studies have shown that fibroblast growth factors (FGFs) form a complex with GAG chains and that complex formation is essential for FGF-FGF receptor mediated mitogenesis (Rapraeger et al., 1991; Yayon et al., 1991; Turnbull et al., 1992; Ishihara, 1994; Pellegrini et al., 2000). Moreover, recent studies indicate important roles for sulfation of HS in developmental processes. Drosophila mutants in the gene sulfateless, encoding a homologue of vertebrate HSNDST, show abnormalities in Wingless-mediated dorsal/ventral patterning and FGF-dependent MAP kinase activation (Lin et al., 1999; Lin and Perrimon, 1999). Another Drosophila mutant in the homologue of HS2ST (pipe) shows abnormal dorsal/ventral polarity in the embryo (Sen et al., 1998). In vertebrates, transgenic mice that lack HS2ST die in the neonatal period showing abnormalities in the kidney, skeleton and eye development (Bullock et al., 1998). Developing axons are guided to their appropriate targets by a succession of guidance molecules variously distributed along the pathway, which act as local cues to attract or repel the 61 Development 129, 61-70 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV1736