Recent insights into the biological roles of mucin-type O-glycosylation

Abstract

In this special issue of the Glycoconjugate Journal focusing on glycosciences and development, we summarize recent advances in our understanding of the role of mucin-type O-glycans in development and disease. The presence of this widespread protein modification has been known for decades, yet identification of its biological functions has been hampered by the redundancy and complexity of the enzyme family controlling the initiation of O-glycosylation, as well as the diversity of extensions of the core sugar. Recent studies in organisms as diverse as mammals and Drosophila have yielded insights into the function of this highly abundant and evolutionarily-conserved protein modification. Gaining an understanding of mucin-type O-glycans in these diverse systems will elucidate crucial conserved processes underlying many aspects of development and homeostasis.

Fig. 1

Biosynthesis of the most common mucin-type O-glycans in mammals (a) and Drosophila melanogaster (b). The initiation of mucin-type O-glycosylation is catalyzed by the addition of GalNAc onto the hydroxyl group of either serine or threonine in protein substrates destined to be membrane-bound or secreted, forming the Tn antigen (Tn Ag). Enzymes responsible for the synthesis of core 1 (T antigen), core 2, core 3 and core 4 structures are shown. Additional extensions of mammalian O-glycans and modifications with sialic acid or fucose are not shown. There is currently no evidence for sialylated O-glycans in Drosophila. ppGalNAcTs or PGANTs, UDP-N-acetylgalactosamine: polypeptide N-acetylgalactosaminyltransferases; Core 1 β3-Gal-T, core 1 β1-3-galactosyltransferase; β3Gn-T6, β1-3 N-acetylglucosaminyltransferase; β6GlcNAc-Ts, β1-6 N-acetylglucosaminyltransferase

Fig. 2

Tn antigen (GalNAcα-S/T) expression in developing tubular tissues during Drosophila embryogenesis. The dashed white lines outline the outer boundaries of each organ shown. Note the intense apical and luminal presence of the O-glycans in each organ. Stages are shown in the lower left corner of each panel. fg foregut, hg hindgut, mp malpighian tubules, sg salivary gland, ts tracheal system. Adapted from Glycobiology, 17, 820–827 (2007) by copyright permission of the Oxford University Press

Fig. 3

A PGANT O-glycosyltransferase is required for proper tracheal development and diffusion barrier formation during Drosophila embryogenesis. Wild-type (A–G), pgant35A^SF32/3775 (A′–G′), pgant35A^HG8/3775 (A″–G″). (A) Tracheal luminal marker 2A12 staining shows the normal development of the tracheal system in wild type (WT) embryos at stage 17 and abnormal tracheal tube formation at stage 17 for pgant35A^SF32/3775 (A′) and pgant35A^HG8/3775 (A″) homozygous maternal/zygotic (m/z) mutants. Magnified views (×40) are shown for wild type (B), pgant35A^SF32/3775 (B′) and pgant35A^HG8/3775 (B″) m/z mutants. Tracheal staining with the luminal marker 2A12 (green) at stage 15 in pgant35A^SF32/3775 (C′) and pgant35A^HG8/3775 (C″) m/z mutants reveals decreased luminal staining and increased cytoplasmic staining relative to wild type (WT) (C). Similar results are seen with Crbs staining at stage 15 in WT (D), pgant35A^SF32/3775 (D′) and pgant35A^HG8/3775 (D″) m/z mutants. At stage 17, the septate junction protein, Sinuous (Sinu), is mislocalized in the pgant35A m/z mutant trachea (E′ and E″) relative to wild type (E), as the mutants show reduced lateral localization and increased apical distribution (arrows in panels E–E″). Tn Ab staining in the apical and luminal regions of the trachea of stage 17 embryos (F–F″) is lost in pgant35A^SF32/3775 (F′) and pgant35A^HG8/3775 (F″) m/z mutants relative to wild type (F). Stage 16– 17 embryos were injected with 10 kD dextran dye and visualized after 30 min. Wild type embryos showed no dye leakage into the tracheal system (between arrows in G), whereas pgant35A homozygous m/z mutants had significant dye present in the tracheal tubes (arrows in G′ and G″), indicating loss of paracellular diffusion barrier formation. Scale bar: 100 µm for A–A″; 50 µm for B–B″; 10 µm for C–F″; 50 µm for G–G″. Embryos are oriented such that anterior is to the left and dorsal is up. Adapted from The Journal of Biological Chemistry, 282, 606–614 (2007) by copyright permission of The American Society for Biochemistry and Molecular Biology, Inc