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

In vivo evidence for GDP-fucose transport in the absence of transporter SLC35C1 and putative transporter SLC35C2

Slc35c1 encodes an antiporter that transports GDP-fucose into the Golgi and returns GMP to the cytoplasm. The closely related gene Slc35c2 encodes a putative GDP-fucose transporter and promotes Notch fucosylation and Notch signaling in cultured cells. Here, we show that HEK293T cells lacking SLC35C1 transferred reduced amounts of O-fucose to secreted epidermal growth factor-like repeats from NOTCH1 or secreted thrombospondin type I repeats from thrombospondin 1. However, cells lacking SLC35C2 did not exhibit reduced fucosylation of these epidermal growth factor-like repeats or thrombospondin type I repeats. To investigate SLC35C2 functions in vivo, WW6 embryonic stem cells were targeted for Slc35c2. Slc35c2[-/-] mice were viable and fertile and exhibited no evidence of defective Notch signaling during skeletal or T cell development. By contrast, mice with inactivated Slc35c1 exhibited perinatal lethality and marked skeletal defects in late embryogenesis, typical of defective Notch signaling. Compound Slc35c1[-/-]Slc35c2[-/-] mutants were indistinguishable in skeletal phenotype from Slc35c1[-/-] embryos and neonates. Double mutants did not exhibit the exacerbated skeletal defects predicted if SLC35C2 was functionally important for Notch signaling in vivo. In addition, NOTCH1 immunoprecipitated from Slc35c1[-/-]Slc35c2[-/-] neonatal lung carried fucose detected by binding of Aleuria aurantia lectin. Given that the absence of both SLC35C1, a known GDP-fucose transporter, and SLC35C2, a putative GDP-fucose transporter, did not lead to afucosylated NOTCH1 nor to the severe Notch signaling defects and embryonic lethality expected if all GDP-fucose transport were abrogated, at least one more mechanism of GDP-fucose transport into the secretory pathway must exist in mammals.

O-Fucosylation of BMP1 promotes endometrial decidualization by activating BMP/Smad signaling pathway

Endometrial decidualization is critical to successful uterine receptivity and embryo implantation. Dysfunction of decidualization is associated with some pregnancy-related disorders, including miscarriage. Protein glycosylation is involved in many physiological and pathological processes. Protein O-fucosyltransferase 1 (poFUT1) is a key enzyme responsible for O-fucosylation biosynthesis on glycoproteins. Bone morphogenetic protein 1 (BMP1) is an essential glycoprotein in reproduction. However, the role and molecular mechanism of fucosylated BMP1 in endometrial stromal cell decidualization are still unknown. In the current study, we found that BMP1 contains a potential O-fucosylation site. Moreover, poFUT1 and BMP1 levels in the secretory phase are higher than those in the proliferative phase, and the highest level was observed in the human uterine tissues of early pregnancy, while a decrease of poFUT1 and BMP1 in the decidua was observed in miscarriage patients. Using human endometrial stromal cells (hESCs), we demonstrated that O-fucosylation of BMP1 was elevated after induced decidualization. Moreover, the increase of BMP1 O-fucosylation by poFUT1 promoted BMP1 secretion to the extracellular matrix, and more actively binds to CHRD. The binding of BMP1 and CHRD further released BMP4 originally bound to CHRD, and activated BMP/Smad signaling pathway, thereby accelerating the decidualization of human endometrial stromal cells. In summary, these results suggest that BMP1 O-fucosylation by poFUT1 could be a potential diagnostic and therapeutic target to predict miscarriage in early pregnancy examinations.

Lfng and Dll3 cooperate to modulate protein interactions in cis and coordinate oscillatory Notch pathway activation in the segmentation clock

In mammalian development, oscillatory activation of Notch signaling is required for segmentation clock function during somitogenesis. Notch activity oscillations are synchronized between neighboring cells in the presomitic mesoderm (PSM) and have a period that matches the rate of somite formation. Normal clock function requires cyclic expression of the Lunatic fringe (LFNG) glycosyltransferase, as well as expression of the inhibitory Notch ligand Delta-like 3 (DLL3). How these factors coordinate Notch activation in the clock is not well understood. Recent evidence suggests that LFNG can act in a signal-sending cell to influence Notch activity in the clock, raising the possibility that in this context, glycosylation of Notch pathway proteins by LFNG may affect ligand activity. Here we dissect the genetic interactions of Lfng and Dll3 specifically in the segmentation clock and observe distinctions in the skeletal and clock phenotypes of mutant embryos showing that paradoxically, loss of Dll3 is associated with strong reductions in Notch activity in the caudal PSM. The patterns of Notch activity in the PSM suggest that the loss of Dll3 is epistatic to the loss of Lfng in the segmentation clock, and we present direct evidence for the modification of several DLL1 and DLL3 EGF-repeats by LFNG. We further demonstrate that DLL3 expression in cells co-expressing DLL1 and NOTCH1 can potentiate a cell's signal-sending activity and that this effect is modulated by LFNG, suggesting a mechanism for coordinated regulation of oscillatory Notch activation in the clock by glycosylation and cis-inhibition.

POFUT1 acts as a tumor promoter in glioblastoma by enhancing the activation of Notch signaling

Dysregulation of protein O-fucosyl transferase 1 (POFUT1) contributes to the occurrence and progression of multiple cancers. However, whether POFUT1 has a relationship with the pathogenesis of glioblastoma (GBM) is unknown. This work was aimed at evaluating the detailed relevance of POFUT1 in GBM. Here, we demonstrated high levels of POFUT1 in GBM tissue and elucidated that GBM patients with high levels of POFUT1 had a shorter survival rate than those with low levels of POFUT1. POFUT1 knockdown in GBM cells markedly downregulated the ability to proliferate and invade, while overexpression of POFUT1 potentiated the proliferative and invasive ability of GBM cells. Further mechanistic studies indicated that silencing POFUT1 prohibited the activation of Notch signaling, leading to a reduction in the expression of HES1 and HEY1. On the contrary, overexpression of POFUT1 enhanced the activation of Notch signaling. Notably, inhibition of Notch signaling markedly reversed POFUT1-overexpression-induced tumor promotion effects in GBM cells. In addition, POFUT1 silencing markedly repressed the potential of GBM cells to form tumors in vivo. In conclusion, the data of this work indicates that POFUT1 serves a tumor promotion role in GBM by enhancing the activation of Notch signaling. This study underlines the potential role of the POFUT1/Notch axis in GBM progression and proposes POFUT1 as a promising anticancer target for GBM.

O-Fucosylation of ADAMTSL2 is required for secretion and is impacted by geleophysic dysplasia-causing mutations

ADAMTSL2 mutations cause an autosomal recessive connective tissue disorder, geleophysic dysplasia 1 (GPHYSD1), which is characterized by short stature, small hands and feet, and cardiac defects. ADAMTSL2 is a matricellular protein previously shown to interact with latent transforming growth factor-β binding protein 1 and influence assembly of fibrillin 1 microfibrils. ADAMTSL2 contains seven thrombospondin type-1 repeats (TSRs), six of which contain the consensus sequence for O-fucosylation by protein O-fucosyltransferase 2 (POFUT2). O-fucose-modified TSRs are subsequently elongated to a glucose β1-3-fucose (GlcFuc) disaccharide by β1,3-glucosyltransferase (B3GLCT). B3GLCT mutations cause Peters Plus Syndrome (PTRPLS), which is characterized by skeletal defects similar to GPHYSD1. Several ADAMTSL2 TSRs also have consensus sequences for C-mannosylation. Six reported GPHYSD1 mutations occur within the TSRs and two lie near O-fucosylation sites. To investigate the effects of TSR glycosylation on ADAMTSL2 function, we used MS to identify glycan modifications at predicted consensus sequences on mouse ADAMTSL2. We found that most TSRs were modified with the GlcFuc disaccharide at high stoichiometry at O-fucosylation sites and variable mannose stoichiometry at C-mannosylation sites. Loss of ADAMTSL2 secretion in POFUT2 ^-/- but not in B3GLCT ^-/- cells suggested that impaired ADAMTSL2 secretion is not responsible for skeletal defects in PTRPLS patients. In contrast, secretion was significantly reduced for ADAMTSL2 carrying GPHYSD1 mutations (S641L in TSR3 and G817R in TSR6), and S641L eliminated O-fucosylation of TSR3. These results provide evidence that abnormalities in GPHYSD1 patients with this mutation are caused by loss of O-fucosylation on TSR3 and impaired ADAMTSL2 secretion.

Differential Labeling of Glycoproteins with Alkynyl Fucose Analogs

Fucosylated glycans critically regulate the physiological functions of proteins and cells. Alterations in levels of fucosylated glycans are associated with various diseases. For detection and functional modulation of fucosylated glycans, chemical biology approaches using fucose (Fuc) analogs are useful. However, little is known about how efficiently each unnatural Fuc analog is utilized by enzymes in the biosynthetic pathway of fucosylated glycans. We show here that three clickable Fuc analogs with similar but distinct structures labeled cellular glycans with different efficiency and protein specificity. For instance, 6-alkynyl (Alk)-Fuc modified O-Fuc glycans much more efficiently than 7-Alk-Fuc. The level of GDP-6-Alk-Fuc produced in cells was also higher than that of GDP-7-Alk-Fuc. Comprehensive in vitro fucosyltransferase assays revealed that 7-Alk-Fuc is commonly tolerated by most fucosyltransferases. Surprisingly, both protein O-fucosyltransferases (POFUTs) could transfer all Fuc analogs in vitro, likely because POFUT structures have a larger space around their Fuc binding sites. These findings demonstrate that labeling and detection of fucosylated glycans with Fuc analogs depend on multiple cellular steps, including conversion to GDP form, transport into the ER or Golgi, and utilization by each fucosyltransferase, providing insights into design of novel sugar analogs for specific detection of target glycans or inhibition of their functions.

The Function of Fucosylation in Progression of Lung Cancer

Lung cancer is a disease that influences human health and has become a leading cause of cancer mortality worldwide. However, it is frequently diagnosed at the advanced stage. It is necessary by means of biology to identify specific lung tumor biomarkers with high sensitivity. Glycosylation is one of the most important post-translational modifications and is related to many different diseases. It is involved in numerous essential biological processes, such as cell proliferation, differentiation, migration, cell-cell integrity and recognition, and immune modulation. However, little was known about deregulation of glycosylation in lung cancer and contribution to tumor–microenvironment interactions. Among the numerous glycosylations, fucosylation is the most common modification of glycoproteins and glycosylated oligosaccharides. Increased levels of fucosylation have been detected in various pathological conditions, as well as in lung cancer. In this article, we reviewed the role of fucosylation in lung cancer. We highlighted some of the fucosylation alterations currently being pursued in sera or tissues of lung cancer patients. Moreover, we elaborated on the regulation mechanism of fucosylation in proliferative invasion and metastasis of lung tumor cells. In summary, alterations in fucosylation provide potential biomarkers and therapeutic targets in lung cancer.

In vitro acellular method to reveal O-fucosylation on EGF-like domains

A hundred of human proteins have one or more EGF-like domains (EGF-LD) bearing the O-fucosylation consensus motif C2X4(S/T)C3 but to date, only a few of them have been shown to be O-fucosylated. The protein O-fucosyltransferase (POFUT1) specifically recognizes correctly folded EGF-LD of the human EGF (hEGF) type and transfers fucose on serine or threonine residue within the O-fucosylation motif. Here, we propose a strategy for a rapid screening for ability of any EGF-LD to be O-fucosylated, using copper-catalyzed azide-alkyne cycloaddition (CuAAC). By an oligonucleotide hybridization approach, double-stranded fragments encoding any EGF-LD can be first rapidly cloned into the prokaryotic vector pET-25b to promote its targeting to periplasm and formation of the three conserved disulfide bonds. After protein production and purification, an in vitro POFUT1-mediated O-fucosylation can be performed with azido GDP-fucose. Successful transfer of O-fucose is finally revealed by blotting technique after CuAAC. In this study, we specially focused on mouse NOTCH1 EGF12 and EGF26, which are both known to be O-fucosylated although having different binding affinities towards POFUT1. Indeed, we clearly showed here that addition of O-fucose by POFUT1 was much more efficient for EGF26 than for EGF12. This experimental approach is rapid and sufficiently sensitive to reveal propensity of any EGF-LD to be O-fucosylated; it is thus useful prior to perform structure-function studies on target proteins containing one or several EGF-LD.

Glycans and glycosaminoglycans in neurobiology: key regulators of neuronal cell function and fate

The aim of the present study was to examine the roles of l-fucose and the glycosaminoglycans (GAGs) keratan sulfate (KS) and chondroitin sulfate/dermatan sulfate (CS/DS) with selected functional molecules in neural tissues. Cell surface glycans and GAGs have evolved over millions of years to become cellular mediators which regulate fundamental aspects of cellular survival. The glycocalyx, which surrounds all cells, actuates responses to growth factors, cytokines and morphogens at the cellular boundary, silencing or activating downstream signaling pathways and gene expression. In this review, we have focused on interactions mediated by l-fucose, KS and CS/DS in the central and peripheral nervous systems. Fucose makes critical contributions in the area of molecular recognition and information transfer in the blood group substances, cytotoxic immunoglobulins, cell fate-mediated Notch-1 interactions, regulation of selectin-mediated neutrophil extravasation in innate immunity and CD-34-mediated new blood vessel development, and the targeting of neuroprogenitor cells to damaged neural tissue. Fucosylated glycoproteins regulate delivery of synaptic neurotransmitters and neural function. Neural KS proteoglycans (PGs) were examined in terms of cellular regulation and their interactive properties with neuroregulatory molecules. The paradoxical properties of CS/DS isomers decorating matrix and transmembrane PGs and the positive and negative regulatory cues they provide to neurons are also discussed.

Biological functions of fucose in mammals

Fucose is a 6-deoxy hexose in the l-configuration found in a large variety of different organisms. In mammals, fucose is incorporated into N-glycans, O-glycans and glycolipids by 13 fucosyltransferases, all of which utilize the nucleotide-charged form, GDP-fucose, to modify targets. Three of the fucosyltransferases, FUT8, FUT12/POFUT1 and FUT13/POFUT2, are essential for proper development in mice. Fucose modifications have also been implicated in many other biological functions including immunity and cancer. Congenital mutations of a Golgi apparatus localized GDP-fucose transporter causes leukocyte adhesion deficiency type II, which results in severe developmental and immune deficiencies, highlighting the important role fucose plays in these processes. Additionally, changes in levels of fucosylated proteins have proven as useful tools for determining cancer diagnosis and prognosis. Chemically modified fucose analogs can be used to alter many of these fucose dependent processes or as tools to better understand them. In this review, we summarize the known roles of fucose in mammalian physiology and pathophysiology. Additionally, we discuss recent therapeutic advances for cancer and other diseases that are a direct result of our improved understanding of the role that fucose plays in these systems.

Synthesis and use of 6,6,6-trifluoro-L-fucose to block core-fucosylation in hybridoma cell lines

Many monoclonal antibodies (mAbs) used in cancer immunotherapy mediate tumour cell lysis by recruiting natural killer (NK) cells; a phenomenon known as antibody-dependent cellular cytotoxicity (ADCC). Eliminating core-fucose from the N-glycans of a mAb enhances its capacity to induce ADCC. As such, inhibitors of fucosylation are highly desirable for the production of mAbs for research and therapeutic use. Herein, we describe a simple synthesis of 6,6,6-trifluoro-l-fucose (F3Fuc), a metabolic inhibitor of fucosylation, and demonstrate the utility of this molecule in the production of low-fucose mAbs from murine hybridoma cell lines.

Inhibition of Delta-induced Notch signaling using fucose analogs

Notch is a cell-surface receptor that controls cell-fate decisions and is regulated by O-glycans attached to epidermal growth factor-like (EGF) repeats in its extracellular domain. Protein O-fucosyltransferase 1 (Pofut1) modifies EGF repeats with O-fucose and is essential for Notch signaling. Constitutive activation of Notch signaling has been associated with a variety of human malignancies. Therefore, tools that inhibit Notch activity are being developed as cancer therapeutics. To this end, we screened L-fucose analogs for their effects on Notch signaling. Two analogs, 6-alkynyl and 6-alkenyl fucose, were substrates of Pofut1 and were incorporated directly into Notch EGF repeats in cells. Both analogs were potent inhibitors of binding to and activation of Notch1 by Notch ligands Dll1 and Dll4, but not by Jag1. Mutagenesis and modeling studies suggest that incorporation of the analogs into EGF8 of Notch1 markedly reduces the ability of Delta ligands to bind and activate Notch1.

High-Sensitivity and Low-Toxicity Fucose Probe for Glycan Imaging and Biomarker Discovery

Fucose, a terminal sugar in glycoconjugates, critically regulates various physiological and pathological phenomena, including cancer development and inflammation. However, there are currently no probes for efficient labeling and detection of this sugar. We chemically synthesized a novel series of alkynyl-fucose analogs as probe candidates and found that 7-alkynyl-fucose gave the highest labeling efficiency and low cytotoxicity. Among the fucose analogs, 7-alkynyl-fucose was the best substrate against all five fucosyltransferases examined. We confirmed its conversion to the corresponding guanosine diphosphate derivative in cells and found that cellular glycoproteins were labeled much more efficiently with 7-alkynyl-fucose than with an existing probe. 7-Alkynyl-fucose was detected in the N-glycan core by mass spectrometry, and 7-alkynyl-fucose-modified proteins mostly disappeared in core-fucose-deficient mouse embryonic fibroblasts, suggesting that this analog mainly labeled core fucose in these cells. These results indicate that 7-alkynyl-fucose is a highly sensitive and powerful tool for basic glycobiology research and clinical application for biomarker discovery.

Marine Antibody-Drug Conjugates: Design Strategies and Research Progress

Antibody-drug conjugates (ADCs), constructed with monoclonal antibodies (mAbs), linkers, and natural cytotoxins, are innovative drugs developed for oncotherapy. Owing to the distinctive advantages of both chemotherapy drugs and antibody drugs, ADCs have obtained enormous success during the past several years. The development of highly specific antibodies, novel marine toxins' applications, and innovative linker technologies all accelerate the rapid R&D of ADCs. Meanwhile, some challenges remain to be solved for future ADCs. For instance, varieties of site-specific conjugation have been proposed for solving the inhomogeneity of DARs (Drug Antibody Ratios). In this review, the usages of various natural toxins, especially marine cytotoxins, and the development strategies for ADCs in the past decade are summarized. Representative ADCs with marine cytotoxins in the pipeline are introduced and characterized with their new features, while perspective comments for future ADCs are proposed.

Chemical Glycoproteomics

Chemical tools have accelerated progress in glycoscience, reducing experimental barriers to studying protein glycosylation, the most widespread and complex form of posttranslational modification. For example, chemical glycoproteomics technologies have enabled the identification of specific glycosylation sites and glycan structures that modulate protein function in a number of biological processes. This field is now entering a stage of logarithmic growth, during which chemical innovations combined with mass spectrometry advances could make it possible to fully characterize the human glycoproteome. In this review, we describe the important role that chemical glycoproteomics methods are playing in such efforts. We summarize developments in four key areas: enrichment of glycoproteins and glycopeptides from complex mixtures, emphasizing methods that exploit unique chemical properties of glycans or introduce unnatural functional groups through metabolic labeling and chemoenzymatic tagging; identification of sites of protein glycosylation; targeted glycoproteomics; and functional glycoproteomics, with a focus on probing interactions between glycoproteins and glycan-binding proteins. Our goal with this survey is to provide a foundation on which continued technological advancements can be made to promote further explorations of protein glycosylation.

O-fucosylated glycoproteins form assemblies in close proximity to the nuclear pore complexes of Toxoplasma gondii

Toxoplasma gondii is an intracellular parasite that causes disseminated infections in fetuses and immunocompromised individuals. Although gene regulation is important for parasite differentiation and pathogenesis, little is known about protein organization in the nucleus. Here we show that the fucose-binding Aleuria aurantia lectin (AAL) binds to numerous punctate structures in the nuclei of tachyzoites, bradyzoites, and sporozoites but not oocysts. AAL also binds to Hammondia and Neospora nuclei but not to more distantly related apicomplexans. Analyses of the AAL-enriched fraction indicate that AAL binds O-linked fucose added to Ser/Thr residues present in or adjacent to Ser-rich domains (SRDs). Sixty-nine Ser-rich proteins were reproducibly enriched with AAL, including nucleoporins, mRNA-processing enzymes, and cell-signaling proteins. Two endogenous SRDs-containing proteins and an SRD-YFP fusion localize with AAL to the nuclear membrane. Superresolution microscopy showed that the majority of the AAL signal localizes in proximity to nuclear pore complexes. Host cells modify secreted proteins with O-fucose; here we describe the O-fucosylation pathway in the nucleocytosol of a eukaryote. Furthermore, these results suggest O-fucosylation is a mechanism by which proteins involved in gene expression accumulate near the NPC.

A proactive role of water molecules in acceptor recognition by protein O-fucosyltransferase 2

Protein O-fucosyltransferase 2 (POFUT2) is an essential enzyme that fucosylates serine and threonine residues of folded thrombospondin type 1 repeats (TSRs). To date, the mechanism by which this enzyme recognizes very dissimilar TSRs has been unclear. By engineering a fusion protein, we report the crystal structure of Caenorhabditis elegans POFUT2 (CePOFUT2) in complex with GDP and human TSR1 that suggests an inverting mechanism for fucose transfer assisted by a catalytic base and shows that nearly half of the TSR1 is embraced by CePOFUT2. A small number of direct interactions and a large network of water molecules maintain the complex. Site-directed mutagenesis demonstrates that POFUT2 fucosylates threonine preferentially over serine and relies on folded TSRs containing the minimal consensus sequence C-X-X-S/T-C. Crystallographic and mutagenesis data, together with atomic-level simulations, uncover a binding mechanism by which POFUT2 promiscuously recognizes the structural fingerprint of poorly homologous TSRs through a dynamic network of water-mediated interactions.

Chemical Lectinology: Tools for Probing the Ligands and Dynamics of Mammalian Lectins In Vivo

The importance and complexity associated with the totality of glycan structures, i.e. the glycome, has garnered significant attention from chemists and biologists alike. However, what is lacking from this biochemical picture is how cells, tissues, and organisms interpret glycan patterns and translate this information into appropriate responses. Lectins, glycan-binding proteins, are thought to bridge this gap by decoding the glycome and dictating cell fate based on the underlying chemical identities and properties of the glycome. Yet, our understanding of the in vivo ligands and function for most lectins is still incomplete. This review focuses on recent advances in chemical tools to study the specificity and dynamics of mammalian lectins in live cells. A picture emerges of lectin function that is highly sensitive to its organization, which in turn drastically shapes immunity and cancer progression. We hope this review will inspire biologists to make use of these new techniques and stimulate chemists to continue developing innovative approaches to probe lectin biology in vivo.

Peters plus syndrome mutations disrupt a noncanonical ER quality-control mechanism

BACKGROUND

O-fucose is added to cysteine-rich domains called thrombospondin type 1 repeats (TSRs) by protein O-fucosyltransferase 2 (POFUT2) and is elongated with glucose by β3-glucosyltransferase (B3GLCT). Mutations in B3GLCT result in Peters plus syndrome (PPS), an autosomal recessive disorder characterized by eye and other developmental defects. Although 49 putative targets are known, the function of the disaccharide and its role in PPS remain unexplored.

RESULTS

Here we show that while POFUT2 is required for secretion of all targets tested, B3GLCT only affects the secretion of a subset, consistent with the observation that B3GLCT mutant phenotypes in PPS patients are less severe than embryonic lethal phenotypes of Pofut2-null mice. O-glycosylation occurs cotranslationally, as TSRs fold. Mass spectral analysis reveals that TSRs from mature, secreted protein are stoichiometrically modified with the disaccharide, whereas TSRs from protein still folding in the ER are partially modified, suggesting that O-glycosylation marks folded TSRs and promotes ER exit. In vitro unfolding assays demonstrate that fucose and glucose stabilize folded TSRs in an additive manner. In vitro refolding assays under redox conditions showed that POFUT2 recognizes, glycosylates, and stabilizes the folded form of TSRs, resulting in a net acceleration of folding.

CONCLUSIONS

While known ER quality-control machinery rely on identifying and tagging unfolded proteins, we find that POFUT2 and B3GLCT mediate a noncanonical ER quality-control mechanism that recognizes folded TSRs and stabilizes them by glycosylation. Our findings provide a molecular basis for the defects observed in PPS and potential targets that contribute to the pathology.

Protein O-fucosyltransferase 1 expression impacts myogenic C2C12 cell commitment via the Notch signaling pathway

The Notch signaling pathway plays a crucial role in skeletal muscle regeneration in mammals by controlling the transition of satellite cells from quiescence to an activated state, their proliferation, and their commitment toward myotubes or self-renewal. O-fucosylation on Notch receptor epidermal growth factor (EGF)-like repeats is catalyzed by the protein O-fucosyltransferase 1 (Pofut1) and primarily controls Notch interaction with its ligands. To approach the role of O-fucosylation in myogenesis, we analyzed a murine myoblastic C2C12 cell line downregulated for Pofut1 expression by short hairpin RNA (shRNA) inhibition during the time course of differentiation. Knockdown of Pofut1 affected the signaling pathway activation by a reduction of the amount of cleaved Notch intracellular domain and a decrease in downstream Notch target gene expression. Depletion in Pax7(+)/MyoD(-) cells and earlier myogenic program entrance were observed, leading to an increase in myotube quantity with a small number of nuclei, reflecting fusion defects. The rescue of Pofut1 expression in knockdown cells restored Notch signaling activation and a normal course in C2C12 differentiation. Our results establish the critical role of Pofut1 on Notch pathway activation during myogenic differentiation.

O-fucosylation of the notch ligand mDLL1 by POFUT1 is dispensable for ligand function

Fucosylation of Epidermal Growth Factor-like (EGF) repeats by protein O-fucosyltransferase 1 (POFUT1 in vertebrates, OFUT1 in Drosophila) is pivotal for NOTCH function. In Drosophila OFUT1 also acts as chaperone for Notch independent from its enzymatic activity. NOTCH ligands are also substrates for POFUT1, but in Drosophila OFUT1 is not essential for ligand function. In vertebrates the significance of POFUT1 for ligand function and subcellular localization is unclear. Here, we analyze the importance of O-fucosylation and POFUT1 for the mouse NOTCH ligand Delta-like 1 (DLL1). We show by mass spectral glycoproteomic analyses that DLL1 is O-fucosylated at the consensus motif C²XXXX(S/T)C³ (where C² and C³ are the second and third conserved cysteines within the EGF repeats) found in EGF repeats 3, 4, 7 and 8. A putative site with only three amino acids between the second cysteine and the hydroxy amino acid within EGF repeat 2 is not modified. DLL1 proteins with mutated O-fucosylation sites reach the cell surface and accumulate intracellularly. Likewise, in presomitic mesoderm cells of POFUT1 deficient embryos DLL1 is present on the cell surface, and in mouse embryonic fibroblasts lacking POFUT1 the same relative amount of overexpressed wild type DLL1 reaches the cell surface as in wild type embryonic fibroblasts. DLL1 expressed in POFUT1 mutant cells can activate NOTCH, indicating that POFUT1 is not required for DLL1 function as a Notch ligand.

Metabolic engineering of monoclonal antibody carbohydrates for antibody-drug conjugation

The role that carbohydrates play in antibody function and pharmacokinetics has made them important targets for modification. The terminal fucose of the N-linked glycan structure, which has been shown to be involved in modulation of antibody-directed cellular cytotoxicity, is a particularly interesting location for potential modification through incorporation of alternative sugar structures. A library of fucose analogues was evaluated for their ability to incorporate into antibody carbohydrates in place of the native fucose. A number of efficiently incorporated molecules were identified, demonstrating the ability of fucosyltransferase VIII to utilize a variety of non-natural sugars as substrates. Among these structures was a thiolated analogue, 6-thiofucose, which was incorporated into the antibody carbohydrate with good efficiency. This unnatural thio-sugar could then be used for conjugation using maleimide chemistry to produce antibody-drug conjugates with pronounced cytotoxic activities and improved homogeneity compared to drug attachment through hinge disulfides.