O-fucosylation of thrombospondin type 1 repeats restricts epithelial to mesenchymal transition (EMT) and maintains epiblast pluripotency during mouse gastrulation

Distinct spatiotemporal expression of ISM1 during mouse and chick development

Isthmin 1 (ISM1) constitutes the founder of a new family of secreted proteins characterized by the presence of 2 functional domains: thrombospondin type 1 repeat (TSR1) and adhesion-associated domain in MUC4 and other proteins (AMOP). ISM1 was identified in the frog embryo as a member of the FGF8 synexpression group due to its expression in the brain midbrain-hindbrain boundary (MHB) or isthmus. In zebrafish, ISM1 was described as a WNT- and NODAL-regulated gene. The function of ISM1 remains largely elusive. So far, ISM1 has been described as an angiogenesis inhibitor that has a dual function in endothelial cell survival and cell death. For a better understanding of ISM1 function, we examined its spatiotemporal distribution in mouse and chick using RT-PCR, ISH, and IHC analyses. In the mouse, ISM1 transcripts are found in tissues such as the anterior mesendoderm, paraxial and lateral plate mesoderm, MHB and trunk neural tube, as well as in the somites and dermomyotome. In the newborn and adult, ISM1 is prominently expressed in the lung and brain. In addition to its putative role during embryonic and postnatal development, ISM1 may also be important for organ homeostasis in the adult. In the chick embryo, ISM1 transcripts are strongly detected in the ear, eye, and spinal cord primordia. Remarkable differences in ISM1 spatiotemporal expression were found during mouse and chick development, despite the high homology of ISM1 orthologs in these species.

Highly coordinated proteome dynamics during reprogramming of somatic cells to pluripotency

Generation of induced pluripotent stem cells (iPSCs) is a process whose mechanistic underpinnings are only beginning to emerge. Here, we applied in-depth quantitative proteomics to monitor proteome changes during the course of reprogramming of fibroblasts to iPSCs. We uncover a two-step resetting of the proteome during the first and last 3 days of reprogramming, with multiple functionally related proteins changing in expression in a highly coordinated fashion. This comprised several biological processes, including changes in the stoichiometry of electron transport-chain complexes, repressed vesicle-mediated transport during the intermediate stage, and an EMT-like process in the late phase. In addition, we demonstrate that the nucleoporin Nup210 is essential for reprogramming by its permitting of rapid cellular proliferation and subsequent progression through MET. Along with the identification of proteins expressed in a stage-specific manner, this study provides a rich resource toward an enhanced mechanistic understanding of cellular reprogramming.

Epithelial-mesenchymal transitions: insights from development

Epithelial-mesenchymal transition (EMT) is a crucial, evolutionarily conserved process that occurs during development and is essential for shaping embryos. Also implicated in cancer, this morphological transition is executed through multiple mechanisms in different contexts, and studies suggest that the molecular programs governing EMT, albeit still enigmatic, are embedded within developmental programs that regulate specification and differentiation. As we review here, knowledge garnered from studies of EMT during gastrulation, neural crest delamination and heart formation have furthered our understanding of tumor progression and metastasis.

6-alkynyl fucose is a bioorthogonal analog for O-fucosylation of epidermal growth factor-like repeats and thrombospondin type-1 repeats by protein O-fucosyltransferases 1 and 2

Protein O-fucosyltransferase 1 (Pofut1) and protein O-fucosyltransferase 2 (Pofut2) add O-linked fucose at distinct consensus sequences in properly folded epidermal growth factor (EGF)-like repeats and thrombospondin type-1 (TSR) repeats, respectively. Glycan chain elongation past O-fucose can occur to yield a tetrasaccharide on EGF repeats and a disaccharide on TSRs. Elimination of Pofut1 in mice causes embryonic lethality with Notch-like phenotypes demonstrating that O-fucosylation of Notch is essential for its function. Similarly, elimination of Pofut2 results in an early embryonic lethal phenotype in mice, although the molecular mechanism for the lethality is unknown. The recent development of sugar analogs has revolutionized the study of glycans by providing a convenient method for labeling and tracking glycosylation. In order to study O-fucosylation, we took advantage of the recently developed reporter, 6-alkynyl fucose. Using the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC), or "click" reaction, azido-biotin allows tagging and detection of 6AF-modified proteins. Here we examine whether proteins containing EGF repeats or TSRs with O-fucose consensus sequences are specifically modified with 6AF in cell culture. Using mass spectrometry (MS), we demonstrate that 6AF is efficiently incorporated onto the appropriate consensus sequences on EGF repeats and TSRs. Furthermore, the elongation of the O-fucose monosaccharide on EGF repeats and TSRs is not hampered when 6AF is used. These results show that 6AF is efficiently utilized in a truly bioorthogonal manner by Pofut1, Pofut2 and the enzymes that elongate O-fucose, providing evidence that 6AF is a significant new tool in the study of protein O-fucosylation.

Site-specific O-glucosylation of the epidermal growth factor-like (EGF) repeats of notch: efficiency of glycosylation is affected by proper folding and amino acid sequence of individual EGF repeats

O-Glucosylation of epidermal growth factor-like (EGF) repeats in the extracellular domain of Notch is essential for Notch function. O-Glucose can be elongated by xylose to the trisaccharide, Xylα1-3Xylα1-3Glcβ1-O-Ser, whose synthesis is catalyzed by the consecutive action of three glycosyltransferases. A UDP-glucose:protein O-glucosyltransferase (Poglut/Rumi) transfers O-glucose to serine within the O-glucose consensus. Subsequently, either of two UDP-xylose:glucoside xylosyltransferases (Gxylt1 or Gxylt2) transfers xylose to O-glucose. Finally, a UDP-xylose:xyloside xylosyltransferase (Xxylt1) transfers xylose to Xylα1-3Glcβ1-O-EGF. Our prior site-mapping studies demonstrated that O-glucose consensus sites are modified at high but variable stoichiometries in mouse Notch1 and identified a novel glycosylation site with alanine in place of proline, suggesting a revised, broader consensus sequence (CXSX(P/A)C). Here we examined the molecular basis for this site specificity. A panel of EGF repeats from human coagulation factor 9 (FA9), mouse Notch1, and Notch2 were bacterially expressed and purified by reverse phase HPLC for use in in vitro enzyme assays. We demonstrate that proper folding of EGF repeats is essential for glycosylation by Poglut/Rumi, that alanine can substitute for proline in the context of coagulation factor 9 EGF repeat for O-glucose transfer, confirming the new consensus sequence, and that positively charged residues within the O-glucose consensus sequence reduce efficiency of glycosylation by Poglut/Rumi. Moreover, proper folding of EGF repeats is also important for the activities of Gxylt1, Gxylt2, and Xxylt1. These results indicate that protein folding and amino acid sequences of individual EGF repeats fundamentally affect both attachment and elongation of O-glucose glycans.

Structure of human POFUT2: insights into thrombospondin type 1 repeat fold and O-fucosylation

Protein O-fucosylation is a post-translational modification found on serine/threonine residues of thrombospondin type 1 repeats (TSR). The fucose transfer is catalysed by the protein O-fucosyltransferase 2 (POFUT2) and >40 human proteins contain the TSR consensus sequence for POFUT2-dependent fucosylation. To better understand O-fucosylation on TSR, we carried out a structural and functional analysis of human POFUT2 and its TSR substrate. Crystal structures of POFUT2 reveal a variation of the classical GT-B fold and identify sugar donor and TSR acceptor binding sites. Structural findings are correlated with steady-state kinetic measurements of wild-type and mutant POFUT2 and TSR and give insight into the catalytic mechanism and substrate specificity. By using an artificial mini-TSR substrate, we show that specificity is not primarily encoded in the TSR protein sequence but rather in the unusual 3D structure of a small part of the TSR. Our findings uncover that recognition of distinct conserved 3D fold motifs can be used as a mechanism to achieve substrate specificity by enzymes modifying completely folded proteins of very wide sequence diversity and biological function.

Vertebrate protein glycosylation: diversity, synthesis and function

Protein glycosylation is a ubiquitous post-translational modification found in all domains of life. Despite their significant complexity in animal systems, glycan structures have crucial biological and physiological roles, from contributions in protein folding and quality control to involvement in a large number of biological recognition events. As a result, they impart an additional level of 'information content' to underlying polypeptide structures. Improvements in analytical methodologies for dissecting glycan structural diversity, along with recent developments in biochemical and genetic approaches for studying glycan biosynthesis and catabolism, have provided a greater understanding of the biological contributions of these complex structures in vertebrates.

Control of mucin-type O-glycosylation: a classification of the polypeptide GalNAc-transferase gene family

Glycosylation of proteins is an essential process in all eukaryotes and a great diversity in types of protein glycosylation exists in animals, plants and microorganisms. Mucin-type O-glycosylation, consisting of glycans attached via O-linked N-acetylgalactosamine (GalNAc) to serine and threonine residues, is one of the most abundant forms of protein glycosylation in animals. Although most protein glycosylation is controlled by one or two genes encoding the enzymes responsible for the initiation of glycosylation, i.e. the step where the first glycan is attached to the relevant amino acid residue in the protein, mucin-type O-glycosylation is controlled by a large family of up to 20 homologous genes encoding UDP-GalNAc:polypeptide GalNAc-transferases (GalNAc-Ts) (EC 2.4.1.41). Therefore, mucin-type O-glycosylation has the greatest potential for differential regulation in cells and tissues. The GalNAc-T family is the largest glycosyltransferase enzyme family covering a single known glycosidic linkage and it is highly conserved throughout animal evolution, although absent in bacteria, yeast and plants. Emerging studies have shown that the large number of genes (GALNTs) in the GalNAc-T family do not provide full functional redundancy and single GalNAc-T genes have been shown to be important in both animals and human. Here, we present an overview of the GalNAc-T gene family in animals and propose a classification of the genes into subfamilies, which appear to be conserved in evolution structurally as well as functionally.

Rumi functions as both a protein O-glucosyltransferase and a protein O-xylosyltransferase

Mutations in rumi result in a temperature-sensitive loss of Notch signaling in Drosophila. Drosophila Rumi is a soluble, endoplasmic reticulum-retained protein with a CAP10 domain that functions as a protein O-glucosyltransferase. In human and mouse genomes, three potential Rumi homologues exist: one with a high degree of identity to Drosophila Rumi (52%), and two others with lower degrees of identity but including a CAP10 domain (KDELC1 and KDELC2). Here we show that both mouse and human Rumi, but not KDELC1 or KDELC2, catalyze transfer of glucose from UDP-glucose to an EGF repeat from human factor VII. Similarly, human Rumi, but not KDELC1 or KDELC2, rescues the Notch phenotypes in Drosophila rumi clones. During characterization of the Rumi enzymes, we noted that, in addition to protein O-glucosyltransferase activity, both mammalian and Drosophila Rumi also showed significant protein O-xylosyltransferase activity. Rumi transfers Xyl or glucose to serine 52 in the O-glucose consensus sequence ( ) of factor VII EGF repeat. Surprisingly, the second serine (S53) facilitates transfer of Xyl, but not glucose, to the EGF repeat by Rumi. EGF16 of mouse Notch2, which has a diserine motif in the consensus sequence ( ), is also modified with either O-Xyl or O-glucose glycans in cells. Mutation of the second serine (S590A) causes a loss of O-Xyl but not O-glucose at this site. Altogether, our data establish dual substrate specificity for the glycosyltransferase Rumi and provide evidence that amino acid sequences of the recipient EGF repeat significantly influence which donor substrate (UDP-glucose or UDP-Xyl) is used.

O-fucosylation of thrombospondin type 1 repeats

Thrombospondin type 1 repeats (TSRs) are small cysteine-rich motifs with three conserved disulfide bonds originally described as modules in the thrombospondins. Since then, TSRs have been found as tandem repeats in a wide variety of secreted and cell-surface proteins of diverse function. TSRs in many contexts are known to bind a variety of receptors and have antiangiogenic capabilities. They can be modified with O-linked fucose on serine/threonine found in the consensus, CX(2-3)(S/T)CX(2)G. Here we review what is known about O-fucosylation of TSRs and describe in detail mass spectral methods to map sites of O-fucosylation on proteins containing TSRs. These methods include techniques to identify glycosylated peptides and the relative amounts of elongated products by electrospray ionization mass spectrometry of glycopeptides.

MESD is essential for apical localization of megalin/LRP2 in the visceral endoderm

Deletion of the Mesd gene region blocks gastrulation and mesoderm differentiation in mice. MESD is a chaperone for the Wnt co-receptors: low-density lipoprotein receptor-related protein (LRP) 5 and 6 (LRP5/6). We hypothesized that loss of Wnt signaling is responsible for the polarity defects observed in Mesd-deficient embryos. However, because the Mesd-deficient embryo is considerably smaller than Lrp5/6 or Wnt3 mutants, we predicted that MESD function extends more broadly to the LRP family of receptors. Consistent with this prediction, we demonstrated that MESD function in vitro was essential for maturation of the β-propeller/EGF domain common to LRPs. To begin to understand the role of MESD in LRP maturation in vivo, we generated a targeted Mesd knockout and verified that loss of Mesd blocks WNT signaling in vivo. Mesd mutants continue to express the pluripotency markers Oct4, Nanog, and Sox2, suggesting that Wnt signaling is essential for differentiation of the epiblast. Moreover, we demonstrated that MESD was essential for the apical localization of the related LRP2 (Megalin/MEG) in the visceral endoderm, resulting in impaired endocytic function. Combined, our results provide evidence that MESD functions as a general LRP chaperone and suggest that the Mesd phenotype results from both signaling and endocytic defects resulting from misfolding of multiple LRP receptors.