Regulation of dally, an integral membrane proteoglycan, and its function during adult sensory organ formation of Drosophila

Evidence for alternative splicing and developmental regulation of the Drosophila melanogaster Mgat2 (N-acetylglucosaminyltransferase II) gene

We present a molecular study of the Drosophila melanogaster Mgat2 gene that codes for the N-acetylglucosaminyltransferase II. Isolation and analysis of two cDNA clones indicated that the gene is alternatively spliced, generating two transcripts of 3.3 and 2.7kb. Developmental specificity was observed between the two alternative transcripts during the major developmental stages of D. melanogaster. The deduced amino acid sequences of the two proteins show significant homology to the equivalent mammalian proteins, especially in the carboxyterminal region. In situ hybridizations in embryos and embryonic imaginal discs showed that the gene is expressed mainly but not exclusively in the brain.

Heparan sulfate core proteins in cell-cell signaling

Heparan sulfate (HS) binds numerous extracellular ligands, including cell-cell signaling molecules and their signal-transducing receptors. Ligand binding sites in HS have specific sulfation patterns; and several observations suggest that the HS sulfation pattern is the same for every HS chain that a cell synthesizes, regardless of the core protein to which it is attached. Nonetheless, virtually every Drosophila, zebrafish, Xenopus, and mouse that lacks a specific HS core protein has a mutant phenotype, even though other HS core proteins are expressed in the affected cells. Genetic manipulation of HS core protein genes is beginning to indicate that HS core proteins have functional specificities that are required during distinct stages of development.

Drosophila glypicans control the cell-to-cell movement of Hedgehog by a dynamin-independent process

The signalling molecule Hedgehog (Hh) functions as a morphogen to pattern a field of cells in animal development. Previous studies in Drosophila have demonstrated that Tout-velu (Ttv), a heparan sulphate polymerase, is required for Hh movement across receiving cells. However, the molecular mechanism of Ttv- mediated Hh movement is poorly defined. We show that Dally and Dally-like (Dly), two Drosophila glypican members of the heparan sulphate proteoglycan (HSPG) family, are the substrates of Ttv and are essential for Hh movement. We show that embryos lacking dly activity exhibit defects in Hh distribution and its subsequent signalling. However, both Dally and Dly are involved and are functionally redundant in Hh movement during wing development. We further demonstrate that Hh movement in its receiving cells is regulated by a cell-to-cell mechanism that is independent of dynamin-mediated endocytosis. We propose that glypicans transfer Hh along the cell membrane to pattern a field of cells.

Characterization of growth factor-binding structures in heparin/heparan sulfate using an octasaccharide library

Heparan sulfate (HS) chains interact with various growth and differentiation factors and morphogens, and the most interactions occur on the specific regions of the chains with certain monosaccharide sequences and sulfation patterns. Here we generated a library of octasaccharides by semienzymatic methods by using recombinant HS 2-O-sulfotransferase and HS 6-O-sulfotransferase, and we have made a systematic investigation of the specific binding structures for various heparin-binding growth factors. An octasaccharide (Octa-I, DeltaHexA-GlcNSO(3)-(HexA-GlcNSO(3))(3)) was prepared by partial heparitinase digestion from completely desulfated N-resulfated heparin. 2-O- and 6-O-sulfated Octa-I were prepared by enzymatically transferring one to three 2-O-sulfate groups and one to three 6-O-sulfate groups per molecule, respectively, to Octa-I. Another octasaccharide containing 3 units of HexA(2SO(4))-GlcNSO(3)(6SO(4)) was prepared also from heparin. This octasaccharide library was subjected to affinity chromatography for interactions with fibroblast growth factor (FGF)-2, -4, -7, -8, -10, and -18, hepatocyte growth factor, bone morphogenetic protein 6, and vascular endothelial growth factor, respectively. Based upon differences in the affinity to those octasaccharides, the growth factors could be classified roughly into five groups: group 1 needed 2-O-sulfate but not 6-O-sulfate (FGF-2); group 2 needed 6-O-sulfate but not 2-O-sulfate (FGF-10); group 3 had the affinity to both 2-O-sulfate and 6-O-sulfate but preferred 2-O-sulfate (FGF-18, hepatocyte growth factor); group 4 required both 2-O-sulfate and 6-O-sulfate (FGF-4, FGF-7); and group 5 hardly bound to any octasaccharides (FGF-8, bone morphogenetic protein 6, and vascular endothelial growth factor). The approach using the oligosaccharide library may be useful to define specific structures required for binding to various heparin-binding proteins. Octasaccharides with the high affinity to FGF-2 and FGF-10 had the activity to release them, respectively, from their complexes with HS. Thus, the library may provide new reagents to specifically regulate bindings of the growth factors to HS.

Slalom encodes an adenosine 3'-phosphate 5'-phosphosulfate transporter essential for development in Drosophila

Sulfation of all macromolecules entering the secretory pathway in higher organisms occurs in the Golgi and requires the high-energy sulfate donor adenosine 3'-phosphate 5'-phosphosulfate. Here we report the first molecular identification of a gene that encodes a transmembrane protein required to transport adenosine 3'-phosphate 5'-phosphosulfate from the cytosol into the Golgi lumen. Mutations in this gene, which we call slalom, display defects in Wg and Hh signaling, which are likely due to the lack of sulfation of glycosaminoglycans by the sulfotransferase sulfateless. Analysis of mosaic mutant ovaries shows that sll function is also essential for dorsal-ventral axis determination, suggesting that sll transports the sulfate donor required for sulfotransferase activity of the dorsal-ventral determinant pipe.

Dally regulates Dpp morphogen gradient formation in the Drosophila wing

Decapentaplegic (Dpp), a Drosophila TGF beta/bone morphogenetic protein homolog, functions as a morphogen to specify cell fate along the anteroposterior axis of the wing. Dpp is a heparin-binding protein and Dpp signal transduction is potentiated by Dally, a cell-surface heparan sulfate proteoglycan, during assembly of several adult tissues. However, the molecular mechanism by which the Dpp morphogen gradient is established and maintained is poorly understood. We show evidence that Dally regulates both cellular responses to Dpp and the distribution of Dpp morphogen in tissues. In the developing wing, dally expression in the wing disc is controlled by the same molecular pathways that regulate expression of thick veins, which encodes a Dpp type I receptor. Elevated levels of Dally increase the sensitivity of cells to Dpp in a cell autonomous fashion. In addition, dally affects the shape of the Dpp ligand gradient as well as its activity gradient. We propose that Dally serves as a co-receptor for Dpp and contributes to shaping the Dpp morphogen gradient.

Heparan sulfate fine structure and specificity of proteoglycan functions

Heparan sulfate chains have markedly heterogeneous structures in which distinct patterns of sulfation determine the binding specificity for ligand proteins. These "fine structures" of heparan sulfate are mainly produced by the regulated introduction of sulfate groups at the N-, 2-O-, 6-O-, and 3-O-positions of the sugar chain. Recent biochemical, histochemical, and genetic studies have demonstrated that different fine structures mediate distinct molecular recognition events to regulate a variety of cellular functions. In this review, we focus on the molecular basis of growth factor control by the sulfation status of heparan sulfate.

Order out of chaos: assembly of ligand binding sites in heparan sulfate

Virtually every cell type in metazoan organisms produces heparan sulfate. These complex polysaccharides provide docking sites for numerous protein ligands and receptors involved in diverse biological processes, including growth control, signal transduction, cell adhesion, hemostasis, and lipid metabolism. The binding sites consist of relatively small tracts of variably sulfated glucosamine and uronic acid residues in specific arrangements. Their formation occurs in a tissue-specific fashion, generated by the action of a large family of enzymes involved in nucleotide sugar metabolism, polymer formation (glycosyltransferases), and chain processing (sulfotransferases and an epimerase). New insights into the specificity and organization of the biosynthetic apparatus have emerged from genetic studies of cultured cells, nematodes, fruit flies, zebrafish, rodents, and humans. This review covers recent developments in the field and provides a resource for investigators interested in the incredible diversity and specificity of this process.

The contribution of in vivo manipulation of gene expression to the understanding of the function of glypicans

The name glypican identifies a family of heparan sulfate proteoglycans that are linked to the cell surface by a glycosylphosphatidylinositol anchor. Members of this family have been identified in Drosophila, zebrafish, and mammals. The interest in the study of glypicans has increased in the last few years as a result of the discovery that the glypican-3 gene (GPC-3) is mutated in an overgrowth and dysmorphic syndrome. Despite the increased interest, our knowledge about the function of glypicans is still limited, since the molecular basis for the role of glypican-3 in the regulation of body size remains unknown. The in vivo manipulation of glypican expression in lower organisms, however, has demonstrated that these proteoglycans can modulate cellular responses to Wnts and bone morphogenetic factors. Future studies should investigate whether the phenotype of GPC-3-deficient individuals is also due to altered modulation of cellular responses to these factors.

Wnts, signaling and sulfates

Questions remain about the signaling pathways that control pattern formation during development. Blair describes how sulfated glycosaminoglycans affect several developmentally important signaling pathways, including Wnt-Wingless, Fibroblast growth factor, Hedgehog, and Bone morphogenetic protein-4 signaling. A new secreted sulfatase, Qsulf1, regulates the sensitivity of vertebrate cells to Wnts, possibly by modifying the sulfation of glycosaminoglycans.