Distinct C-mannosylation of netrin receptor thrombospondin type 1 repeats by mammalian DPY19L1 and DPY19L3

Conserved cysteines prevent C-mannosylation of mucin Cys domains

Mucins are major components of the mucus. Besides the highly O-glycosylated tandem repeat domains, mucins contain Cys domains (CysDs). CysDs contain conserved disulfide-forming cysteine residues as well as a WxxW motif. Since this is the consensus sequence for tryptophan C-mannosylation, mucin CysDs have been suggested to be targets for C-mannosyltransferases, but this has never been directly shown. Here, we recombinantly expressed human mucin CysDs in Chinese hamster ovary (CHO) cells and analyzed the C-mannosylation status. Mass spectrometric analysis revealed that the putative C-mannose site is not or only barely C-mannosylated. However, mutation of the adjacent cysteine residues enabled C-mannosylation to occur. In contrast to mucin CysDs, the homologous CysD of human cartilage intermediate layer protein 1 (CILP1) lacks these cysteine residues preceding the WxxW motif. We show that CILP1 CysD is C-mannosylated, but introducing a cysteine at the -2 position causes this modification to be lost. We thus conclude that the presence of cysteine residues prevents the modification of the WxxW motif in CysDs.

Molecular characterization of the craniosynostosis-associated interleukin-11 receptor variants p.T306_S308dup and p.E364_V368del

Interleukin-11 (IL-11) is a member of the IL-6 family of cytokines and is an important factor for bone homeostasis. IL-11 binds to and signals via the membrane-bound IL-11 receptor (IL-11R, classic signaling) or soluble forms of the IL-11R (sIL-11R, trans-signaling). Mutations in the IL11RA gene, which encodes the IL-11R, are associated with craniosynostosis, a human condition in which one or several of the sutures close prematurely, resulting in malformation of the skull. The biological mechanisms of how mutations within the IL-11R are linked to craniosynostosis are mostly unexplored. In this study, we analyze two variants of the IL-11R described in craniosynostosis patients: p.T306_S308dup, which results in a duplication of three amino-acid residues within the membrane-proximal fibronectin type III domain, and p.E364_V368del, which results in a deletion of five amino-acid residues in the so-called stalk region adjacent to the plasma membrane. The stalk region connects the three extracellular domains to the transmembrane and intracellular region of the IL-11R and contains cleavage sites for different proteases that generate sIL-11R variants. Using a combination of bioinformatics and different biochemical, molecular, and cell biology methods, we show that the IL-11R-T306_S308dup variant does not mature correctly, is intracellularly retained, and does not reach the cell surface. In contrast, the IL-11R-E364_V368del variant is fully biologically active and processed normally by proteases, thus allowing classic and trans-signaling of IL-11. Our results provide evidence that mutations within the IL11RA gene may not be causative for craniosynostosis and suggest that other regulatory mechanism(s) are involved but remain to be identified.

DPY19L3 promotes vasculogenic mimicry by its C-mannosyltransferase activity

C-mannosylation is a post-translational modification that occurs intracellularly in the endoplasmic reticulum. In humans, biosynthesis of C-mannosylation in proteins containing thrombospondin type 1 repeat is catalyzed by the DPY19 family; nonetheless, biological functions of protein C-mannosylation are not yet fully understood, especially in tumor progression. Vasculogenic mimicry (VM) is the formation of fluid-conducting channels by highly invasive and genetically deregulated tumor cells, enabling the tumors to form matrix-embedded vasculogenic structures, containing plasma and blood cells to meet the metabolic demands of rapidly growing tumors. In this study, we focused on DPY19L3, a C-mannosyltransferase, and aimed to unravel its role in VM. Knockout of DPY19L3 inhibited the formation of VM in HT1080 human fibrosarcoma cells. Re-expression of wild-type DPY19L3 recovered VM formation; however, DPY19L3 isoform2, an enzymatic activity-defect mutant, did not restore it, suggesting that the C-mannosyltransferase activity of DPY19L3 is crucial to its function. Furthermore, the knockdown of DPY19L3 in MDA-MB-231 breast cancer cells hindered its network formation ability. Altogether, our findings suggest that DPY19L3 is required for VM formation and stipulate the relevance of C-mannosylation in oncogenesis.

Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications

Protein posttranslational modifications (PTMs) refer to the breaking or generation of covalent bonds on the backbones or amino acid side chains of proteins and expand the diversity of proteins, which provides the basis for the emergence of organismal complexity. To date, more than 650 types of protein modifications, such as the most well-known phosphorylation, ubiquitination, glycosylation, methylation, SUMOylation, short-chain and long-chain acylation modifications, redox modifications, and irreversible modifications, have been described, and the inventory is still increasing. By changing the protein conformation, localization, activity, stability, charges, and interactions with other biomolecules, PTMs ultimately alter the phenotypes and biological processes of cells. The homeostasis of protein modifications is important to human health. Abnormal PTMs may cause changes in protein properties and loss of protein functions, which are closely related to the occurrence and development of various diseases. In this review, we systematically introduce the characteristics, regulatory mechanisms, and functions of various PTMs in health and diseases. In addition, the therapeutic prospects in various diseases by targeting PTMs and associated regulatory enzymes are also summarized. This work will deepen the understanding of protein modifications in health and diseases and promote the discovery of diagnostic and prognostic markers and drug targets for diseases.

The SHDRA syndrome-associated gene TMEM260 encodes a protein-specific O-mannosyltransferase

Mutations in the TMEM260 gene cause structural heart defects and renal anomalies syndrome, but the function of the encoded protein remains unknown. We previously reported wide occurrence of O-mannose glycans on extracellular immunoglobulin, plexin, transcription factor (IPT) domains found in the hepatocyte growth factor receptor (cMET), macrophage-stimulating protein receptor (RON), and plexin receptors, and further demonstrated that two known protein O-mannosylation systems orchestrated by the POMT1/2 and transmembrane and tetratricopeptide repeat-containing proteins 1-4 gene families were not required for glycosylation of these IPT domains. Here, we report that the TMEM260 gene encodes an ER-located protein O-mannosyltransferase that selectively glycosylates IPT domains. We demonstrate that disease-causing TMEM260 mutations impair O-mannosylation of IPT domains and that TMEM260 knockout in cells results in receptor maturation defects and abnormal growth of 3D cell models. Thus, our study identifies the third protein-specific O-mannosylation pathway in mammals and demonstrates that O-mannosylation of IPT domains serves critical functions during epithelial morphogenesis. Our findings add a new glycosylation pathway and gene to a growing group of congenital disorders of glycosylation.

Structure, sequon recognition and mechanism of tryptophan C-mannosyltransferase

C-linked glycosylation is essential for the trafficking, folding and function of secretory and transmembrane proteins involved in cellular communication processes. The tryptophan C-mannosyltransferase (CMT) enzymes that install the modification attach a mannose to the first tryptophan of WxxW/C sequons in nascent polypeptide chains by an unknown mechanism. Here, we report cryogenic-electron microscopy structures of Caenorhabditis elegans CMT in four key states: apo, acceptor peptide-bound, donor-substrate analog-bound and as a trapped ternary complex with both peptide and a donor-substrate mimic bound. The structures indicate how the C-mannosylation sequon is recognized by this CMT and its paralogs, and how sequon binding triggers conformational activation of the donor substrate: a process relevant to all glycosyltransferase C superfamily enzymes. Our structural data further indicate that the CMTs adopt an unprecedented electrophilic aromatic substitution mechanism to enable the C-glycosylation of proteins. These results afford opportunities for understanding human disease and therapeutic targeting of specific CMT paralogs.

A structural model of the iRhom-ADAM17 sheddase complex reveals functional insights into its trafficking and activity

Several membrane-anchored signal mediators such as cytokines (e.g. TNFα) and growth factors are proteolytically shed from the cell surface by the metalloproteinase ADAM17, which, thus, has an essential role in inflammatory and developmental processes. The membrane proteins iRhom1 and iRhom2 are instrumental for the transport of ADAM17 to the cell surface and its regulation. However, the structure-function determinants of the iRhom-ADAM17 complex are poorly understood. We used AI-based modelling to gain insights into the structure-function relationship of this complex. We identified different regions in the iRhom homology domain (IRHD) that are differentially responsible for iRhom functions. We have supported the validity of the predicted structure-function determinants with several in vitro, ex vivo and in vivo approaches and demonstrated the regulatory role of the IRHD for iRhom-ADAM17 complex cohesion and forward trafficking. Overall, we provide mechanistic insights into the iRhom-ADAM17-mediated shedding event, which is at the centre of several important cytokine and growth factor pathways.

C-mannosylation regulates stabilization of RAMP1 protein and RAMP1-mediated cell migration

C-mannosylation is a unique type of protein glycosylation via C-C linkage between an α-mannose and a tryptophan residue. This modification has been identified in about 30 proteins and regulates several functions, such as protein secretion and intracellular localization, as well as protein stability. About half of C-mannosylated proteins are categorized as proteins containing thrombospondin type 1 repeat domain or type I cytokine receptors. To evaluate whether C-mannosylation broadly affects protein functions regardless of protein domain or family, we have sought to identify other types of C-mannosylated protein and analyse their functions. In this study, we focused on receptor activity modifying protein 1, which neither contains thrombospondin type 1 repeat domain nor belongs to the type I cytokine receptors. Our mass spectrometry analysis demonstrated that RAMP1 is C-mannosylated at Trp⁵⁶ . It has been shown that RAMP1 transports to the plasma membrane after dimerization with calcitonin receptor-like receptor and is important for ligand-dependent downstream signalling activation. Our results showed that C-mannosylation has no effect on this transport activity. On the other hand, C-mannosylation did enhance protein stability and cell migration activity. Our data may provide new insight into both C-mannosylation research and novel RAMP1 analysis.

Optimising expression and extraction of recombinant proteins in plants

Recombinant proteins are of paramount importance for research, industrial and medical use. Numerous expression chassis are available for recombinant protein production, and while bacterial and mammalian cell cultures are the most widely used, recent developments have positioned transgenic plant chassis as viable and often preferential options. Plant chassis are easily maintained at low cost, are hugely scalable, and capable of producing large quantities of protein bearing complex post-translational modification. Several protein targets, including antibodies and vaccines against human disease, have been successfully produced in plants, highlighting the significant potential of plant chassis. The aim of this review is to act as a guide to producing recombinant protein in plants, discussing recent progress in the field and summarising the factors that must be considered when utilising plants as recombinant protein expression systems, with a focus on optimising recombinant protein expression at the genetic level, and the subsequent extraction and purification of target proteins, which can lead to substantial improvements in protein stability, yield and purity.

GPC3-Unc5 receptor complex structure and role in cell migration

Neural migration is a critical step during brain development that requires the interactions of cell-surface guidance receptors. Cancer cells often hijack these mechanisms to disseminate. Here, we reveal crystal structures of Uncoordinated-5 receptor D (Unc5D) in complex with morphogen receptor glypican-3 (GPC3), forming an octameric glycoprotein complex. In the complex, four Unc5D molecules pack into an antiparallel bundle, flanked by four GPC3 molecules. Central glycan-glycan interactions are formed by N-linked glycans emanating from GPC3 (N241 in human) and C-mannosylated tryptophans of the Unc5D thrombospondin-like domains. MD simulations, mass spectrometry and structure-based mutants validate the crystallographic data. Anti-GPC3 nanobodies enhance or weaken Unc5-GPC3 binding and, together with mutant proteins, show that Unc5/GPC3 guide migrating pyramidal neurons in the mouse cortex, and cancer cells in an embryonic xenograft neuroblastoma model. The results demonstrate a conserved structural mechanism of cell guidance, where finely balanced Unc5-GPC3 interactions regulate cell migration.

A genome-wide CRISPR screen identifies DPM1 as a modifier of DPAGT1 deficiency and ER stress

Partial loss-of-function mutations in glycosylation pathways underlie a set of rare diseases called Congenital Disorders of Glycosylation (CDGs). In particular, DPAGT1-CDG is caused by mutations in the gene encoding the first step in N-glycosylation, DPAGT1, and this disorder currently lacks effective therapies. To identify potential therapeutic targets for DPAGT1-CDG, we performed CRISPR knockout screens in Drosophila cells for genes associated with better survival and glycoprotein levels under DPAGT1 inhibition. We identified hundreds of candidate genes that may be of therapeutic benefit. Intriguingly, inhibition of the mannosyltransferase Dpm1, or its downstream glycosylation pathways, could rescue two in vivo models of DPAGT1 inhibition and ER stress, even though impairment of these pathways alone usually causes CDGs. While both in vivo models ostensibly cause cellular stress (through DPAGT1 inhibition or a misfolded protein), we found a novel difference in fructose metabolism that may indicate glycolysis as a modulator of DPAGT1-CDG. Our results provide new therapeutic targets for DPAGT1-CDG, include the unique finding of Dpm1-related pathways rescuing DPAGT1 inhibition, and reveal a novel interaction between fructose metabolism and ER stress.

Tryptophan C-mannosylation is critical for Plasmodium falciparum transmission

Tryptophan C-mannosylation stabilizes proteins bearing a thrombospondin repeat (TSR) domain in metazoans. Here we show that Plasmodium falciparum expresses a DPY19 tryptophan C-mannosyltransferase in the endoplasmic reticulum and that DPY19-deficiency abolishes C-glycosylation, destabilizes members of the TRAP adhesin family and inhibits transmission to mosquitoes. Imaging P. falciparum gametogenesis in its entirety in four dimensions using lattice light-sheet microscopy reveals defects in ΔDPY19 gametocyte egress and exflagellation. While egress is diminished, ΔDPY19 microgametes still fertilize macrogametes, forming ookinetes, but these are abrogated for mosquito infection. The gametogenesis defects correspond with destabilization of MTRAP, which we show is C-mannosylated in P. falciparum, and the ookinete defect is concordant with defective CTRP secretion on the ΔDPY19 background. Genetic complementation of DPY19 restores ookinete infectivity, sporozoite production and C-mannosylation activity. Therefore, tryptophan C-mannosylation by DPY19 ensures TSR protein quality control at two lifecycle stages for successful transmission of the human malaria parasite.

Here, Lopaticki et al. show that Plasmodium falciparum expresses a Dpy19 C-mannosyltransferase in the endoplasmic reticulum that glycosylates TSR domains. Functional characterization shows that PfDpy19 plays a critical role in transmission through mosquitoes as PfDpy19-deficiency abolishes C-glycosylation and destabilizes proteins relevant for gametogenesis and oocyst formation.

O-fucosylation stabilizes the TSR3 motif in thrombospondin-1 by interacting with nearby amino acids and protecting a disulfide bond

Thrombospondin type-1 repeats (TSRs) are small protein motifs containing six conserved cysteines forming three disulfide bonds that can be modified with an O-linked fucose. Protein O-fucosyltransferase 2 (POFUT2) catalyzes the addition of O-fucose to TSRs containing the appropriate consensus sequence, and the O-fucose modification can be elongated to a Glucose-Fucose disaccharide with the addition of glucose by β3-glucosyltransferase (B3GLCT). Elimination of Pofut2 in mice results in embryonic lethality in mice, highlighting the biological significance of O-fucose modification on TSRs. Knockout of POFUT2 in HEK293T cells has been shown to cause complete or partial loss of secretion of many proteins containing O-fucosylated TSRs. In addition, POFUT2 is localized to the endoplasmic reticulum (ER) and only modifies folded TSRs, stabilizing their structures. These observations suggest that POFUT2 is involved in an ER quality control mechanism for TSR folding and that B3GLCT also participates in quality control by providing additional stabilization to TSRs. However, the mechanisms by which addition of these sugars result in stabilization are poorly understood. Here, we conducted molecular dynamics (MD) simulations and provide crystallographic and NMR evidence that the Glucose-Fucose disaccharide interacts with specific amino acids in the TSR3 domain in thrombospondin-1 that are within proximity to the O-fucosylation modification site resulting in protection of a nearby disulfide bond. We also show that mutation of these amino acids reduces the stabilizing effect of the sugars in vitro. These data provide mechanistic details regarding the importance of O-fucosylation and how it participates in quality control mechanisms inside the ER.

A Bacterial Mannose Binding Lectin as a Tool for the Enrichment of C- and O-Mannosylated Peptides

Mass spectrometry (MS) easily detects C-mannosylated peptides from purified proteins but not from complex biological samples. Enrichment of specific glycopeptides by lectin affinity prior to MS analysis has been widely applied to support glycopeptide identification but was until now not available for C-mannosylated peptides. Here, we used the α-mannose-specific Burkholderia cenocepacia lectin A (BC2L-A) and show that, in addition to its previously demonstrated high-mannose N-glycan binding capability, this lectin is able to retain C- and O-mannosylated peptides. Besides testing binding abilities to standard peptides, we applied BC2L-A affinity to enrich C-mannosylated peptides from complex samples of tryptic digests of HEK293 and MCF10A whole cell extracts, which led to the identification of novel C-mannosylation sites. In conclusion, BC2L-A enabled specific enrichment of C- and O-mannosylated peptides and might have superior properties over other mannose binding lectins for this purpose.

RTN4/NoGo-receptor binding to BAI adhesion-GPCRs regulates neuronal development

RTN4-binding proteins were widely studied as "NoGo" receptors, but their physiological interactors and roles remain elusive. Similarly, BAI adhesion-GPCRs were associated with numerous activities, but their ligands and functions remain unclear. Using unbiased approaches, we observed an unexpected convergence: RTN4 receptors are high-affinity ligands for BAI adhesion-GPCRs. A single thrombospondin type 1-repeat (TSR) domain of BAIs binds to the leucine-rich repeat domain of all three RTN4-receptor isoforms with nanomolar affinity. In the 1.65 Å crystal structure of the BAI1/RTN4-receptor complex, C-mannosylation of tryptophan and O-fucosylation of threonine in the BAI TSR-domains creates a RTN4-receptor/BAI interface shaped by unusual glycoconjugates that enables high-affinity interactions. In human neurons, RTN4 receptors regulate dendritic arborization, axonal elongation, and synapse formation by differential binding to glial versus neuronal BAIs, thereby controlling neural network activity. Thus, BAI binding to RTN4/NoGo receptors represents a receptor-ligand axis that, enabled by rare post-translational modifications, controls development of synaptic circuits.

Thrombospondin type 1 repeat-derived C-mannosylated peptide attenuates synaptogenesis of cortical neurons induced by primary astrocytes via TGF-β

C-Mannosylation is a rare type of protein glycosylation and is reportedly critical for the proper folding and secretion of parental proteins. Still, the effects of C-mannosylation on the biological functions of these modified proteins remain to be elucidated. The Trp-x-x-Trp (WxxW) sequences, whose first tryptophan (Trp) can be C-mannosylated, constitute the consensus motifs for this glycosylation modification and are commonly found in thrombospondin type 1 repeats that regulate molecular functions of thrombospondin 1 in binding and activation of transforming growth factor β (TGF-β). TGF-β plays critical roles in the control of the central nervous system including synaptogenesis. Here, we investigated whether C-mannosylation of the synthetic Trp-Ser-Pro-Trp (WSPW) peptide may confer certain functions to this peptide in TGF-β-mediated synaptogenesis. By using primary cultured rat astrocytes and cortical neurons, we found that the C-mannosylated WSPW (C-Man-WSPW) peptide, but not non-mannosylated WSPW peptide, suppressed astrocyte-conditioned medium (ACM)-stimulated synaptogenesis. C-Man-WSPW peptide inhibited both ACM- and recombinant mature TGF-β1-induced activations of Smad 2, an important mediator in TGF-β signaling. Interactions of recombinant mature TGF-β with the C-Man-WSPW peptide were similar to those with non-C-mannosylated WSPW peptide. Taken together, our results reveal a novel function of C-mannosylation of the WxxW motif in signaling and synaptogenesis mediated by TGF-β. Molecular details of how C-mannosylation affects the biological functions of WxxW motifs deserve future study for clarification.

Loss of the AMD-associated B3GLCT gene affects glycosylation of TSP1 without impairing secretion in retinal pigment epithelial cells

Age-related macular degeneration (AMD) has been associated with protective genetic variants in the β1-3 glucosyltransferase (B3GLCT) locus through genome-wide association studies. B3GLCT mediates modification of proteins with thrombospondin type I repeats (TSR) that contain O-linked glucose β1-3 fucose and C-linked mannose glycosylation motifs. B3GLCT-mediated modification is required for proper secretion of TSR-containing proteins. We aimed to start understanding the role of B3GLCT in AMD by evaluating its effect on glycosylation and secretion of proteins from retinal pigment epithelium (RPE) cells. We generated B3GLCT knockout (KO) RPE cells and analyzed glycosylation and secretion of thrombospondin 1 (TSP1), a protein involved in cellular processes highly relevant to AMD. Glycopeptide analysis confirmed the presence of the glucose-β1,3-fucose product of B3GLCT on TSP1 in wildtype (WT) cells and its absence in KO cells. C-mannosylation was variably present on WT TSP1 and increased on TSR domains 1 and 3 in KO cells. Secretion of TSP1 was not affected by the absence of B3GLCT, even not when TSP1 was upregulated by TNFα treatment or when TSP1 was overexpressed in HEK293T cells. Future research is needed to elucidate the effect of the observed glycosylation defects in the context of AMD, which might involve functional loss of TSP1 or effects on other TSR proteins.

C-Mannosylated tryptophan-containing WSPW peptide binds to actinin-4 and alters E-cadherin subcellular localization in lung epithelial-like A549 cells

The Trp-x-x-Trp (W-x-x-W) peptide motif, a consensus site for C-mannosylation, is the functional motif in cytokine type I receptors or thrombospondin type I repeat (TSR) superfamily proteins. W-x-x-W motifs are important for physiological and pathological functions of their parental proteins, but effects of C-mannosylation on protein functions remain to be elucidated. By using chemically synthesized WSPW peptides and C-mannosylated WSPW peptides (C-Man-WSPW), we herein investigated whether C-mannosylation of WSPW peptides confer additional biological functions to WSPW peptides. C-Man-WSPW peptide, but not non-mannosylated WSPW, reduced E-cadherin levels in A549 cells. Via peptide mass fingerprinting analysis, we identified actinin-4 as a C-Man-WSPW-binding protein in A549 cells. Actinin-4 partly co-localized with E-cadherin or β-catenin, despite no direct interaction between actinin-4 and E-cadherin. C-Man-WSPW reduced co-localization of E-cadherin and actinin-4; non-mannosylated WSPW had no effect on localization. In actinin-4-knockdown cells, E-cadherin was upregulated and demonstrated a punctate staining pattern in the cytoplasm, which suggests that actinin-4 regulated cell-surface E-cadherin localization. Thus, C-mannosylation of WSPW peptides is required for interaction with actinin-4 that subsequently alters expression and subcellular localization of E-cadherin and morphology of epithelial-like cells. Our results therefore suggest a regulatory role of C-mannosylation of the W-x-x-W motif in interactions between the motif and its binding partner and will thereby enhance understanding of protein C-mannosylation.

Involvement of DPY19L3 in Myogenic Differentiation of C2C12 Myoblasts

DPY19L3 has been identified as a C-mannosyltransferase for thrombospondin type-1 repeat domain-containing proteins. In this study, we focused on the role of DPY19L3 in the myogenic differentiation of C2C12 mouse myoblast cells. We carried out DPY19L3 gene depletion using the CRISPR/Cas9 system. The result showed that these DPY19L3-knockout cells could not be induced for differentiation. Moreover, the phosphorylation levels of MEK/ERK and p70S6K were suppressed in the DPY19L3-knockout cells compared with that of parent cells, suggesting that the protein(s) that is(are) DPY19L3-mediated C-mannosylated and regulate(s) MEK/ERK or p70S6K signaling is(are) required for the differentiation.

Protein C-Mannosylation and C-Mannosyl Tryptophan in Chemical Biology and Medicine

C-Mannosylation is a post-translational modification of proteins in the endoplasmic reticulum. Monomeric α-mannose is attached to specific Trp residues at the first Trp in the Trp-x-x-Trp/Cys (W-x-x-W/C) motif of substrate proteins, by the action of C-mannosyltransferases, DPY19-related gene products. The acceptor substrate proteins are included in the thrombospondin type I repeat (TSR) superfamily, cytokine receptor type I family, and others. Previous studies demonstrated that C-mannosylation plays critical roles in the folding, sorting, and/or secretion of substrate proteins. A C-mannosylation-defective gene mutation was identified in humans as the disease-associated variant affecting a C-mannosylation motif of W-x-x-W of ADAMTSL1, which suggests the involvement of defects in protein C-mannosylation in human diseases such as developmental glaucoma, myopia, and/or retinal defects. On the other hand, monomeric C-mannosyl Trp (C-Man-Trp), a deduced degradation product of C-mannosylated proteins, occurs in cells and extracellular fluids. Several studies showed that the level of C-Man-Trp is upregulated in blood of patients with renal dysfunction, suggesting that the metabolism of C-Man-Trp may be involved in human kidney diseases. Together, protein C-mannosylation is considered to play important roles in the biosynthesis and functions of substrate proteins, and the altered regulation of protein C-manosylation may be involved in the pathophysiology of human diseases. In this review, we consider the biochemical and biomedical knowledge of protein C-mannosylation and C-Man-Trp, and introduce recent studies concerning their significance in biology and medicine.

The C-Mannosylome of Human Induced Pluripotent Stem Cells Implies a Role for ADAMTS16 C-Mannosylation in Eye Development

C-mannosylation is a modification of tryptophan residues with a single mannose and can affect protein folding, secretion, and/or function. To date, only a few proteins have been demonstrated to be C-mannosylated, and studies that globally assess protein C-mannosylation are scarce. To interrogate the C-mannosylome of human induced pluripotent stem cells, we compared the secretomes of CRISPR–Cas9 mutants lacking either the C-mannosyltransferase DPY19L1 or DPY19L3 to WT human induced pluripotent stem cells using MS-based quantitative proteomics. The secretion of numerous proteins was reduced in these mutants, including that of A Disintegrin And Metalloproteinase with ThromboSpondin Motifs 16 (ADAMTS16), an extracellular protease that was previously reported to be essential for optic fissure fusion in zebrafish eye development. To test the functional relevance of this observation, we targeted dpy19l1 or dpy19l3 in embryos of the Japanese rice fish medaka (Oryzias latipes) by CRISPR–Cas9. We observed that targeting of dpy19l3 partially caused defects in optic fissure fusion, called coloboma. We further showed in a cellular model that DPY19L1 and DPY19L3 mediate C-mannosylation of a recombinantly expressed thrombospondin type 1 repeat of ADAMTS16 and thereby support its secretion. Taken together, our findings imply that DPY19L3-mediated C-mannosylation is involved in eye development by assisting secretion of the extracellular protease ADAMTS16.

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TSR1 of ADAMTS16 can be C-mannosylated.

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Deletion of DPY19L1 or DPY19L3 in hiPSCs caused reduced secretion of ADAMTS16.

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Targeting of dpy19l3 in medaka occasionally led to coloboma.

The iRhom homology domain is indispensable for ADAM17-mediated TNFα and EGF receptor ligand release

Membrane-tethered signalling proteins such as TNFα and many EGF receptor ligands undergo shedding by the metalloproteinase ADAM17 to get released. The pseudoproteases iRhom1 and iRhom2 are important for the transport, maturation and activity of ADAM17. Yet, the structural and functional requirements to promote the transport of the iRhom-ADAM17 complex have not yet been thoroughly investigated. Utilising in silico and in vitro methods, we here map the conserved iRhom homology domain (IRHD) and provide first insights into its structure and function. By focusing on iRhom2, we identified different structural and functional factors within the IRHD. We found that the structural integrity of the IRHD is a key factor for ADAM17 binding. In addition, we identified a highly conserved motif within an unstructured region of the IRHD, that, when mutated, restricts the transport of the iRhom-ADAM17 complex through the secretory pathway in in vitro, ex vivo and in vivo systems and also increases the half-life of iRhom2 and ADAM17. Furthermore, the disruption of this IRHD motif was also reflected by changes in the yet undescribed interaction profile of iRhom2 with proteins involved in intracellular vesicle transport. Overall, we provide the first insights into the forward trafficking of iRhoms which is critical for TNFα and EGF receptor signalling.

Yeast- and antibody-based tools for studying tryptophan C-mannosylation

Tryptophan C-mannosylation is an unusual co-translational protein modification performed by metazoans and apicomplexan protists. The prevalence and biological functions of this modification are poorly understood, with progress in the field hampered by a dearth of convenient tools for installing and detecting the modification. Here, we engineer a yeast system to produce a diverse array of proteins with and without tryptophan C-mannosylation and interrogate the modification's influence on protein stability and function. This system also enabled mutagenesis studies to identify residues of the glycosyltransferase and its protein substrates that are crucial for catalysis. The collection of modified proteins accrued during this work facilitated the generation and thorough characterization of monoclonal antibodies against tryptophan C-mannosylation. These antibodies empowered proteomic analyses of the brain C-glycome by enriching for peptides possessing tryptophan C-mannosylation. This study revealed many new modification sites on proteins throughout the secretory pathway with both conventional and non-canonical consensus sequences.

Genetic glycoengineering in mammalian cells

Advances in nuclease-based gene-editing technologies have enabled precise, stable, and systematic genetic engineering of glycosylation capacities in mammalian cells, opening up a plethora of opportunities for studying the glycome and exploiting glycans in biomedicine. Glycoengineering using chemical, enzymatic, and genetic approaches has a long history, and precise gene editing provides a nearly unlimited playground for stable engineering of glycosylation in mammalian cells to explore and dissect the glycome and its many biological functions. Genetic engineering of glycosylation in cells also brings studies of the glycome to the single cell level and opens up wider use and integration of data in traditional omics workflows in cell biology. The last few years have seen new applications of glycoengineering in mammalian cells with perspectives for wider use in basic and applied glycosciences, and these have already led to discoveries of functions of glycans and improved designs of glycoprotein therapeutics. Here, we review the current state of the art of genetic glycoengineering in mammalian cells and highlight emerging opportunities.

Quantification of serum C-mannosyl tryptophan by novel assay to evaluate renal function and vascular complications in patients with type 2 diabetes

C-Mannosyl tryptophan (CMW) is a unique glycosylated amino acid, and a candidate novel biomarker of renal function. In type 2 diabetes (T2D), a combination of metabolites including CMW has recently been the focus of novel biomarkers for the evaluation of renal function and prediction of its decline. However, previous quantification methods for serum CMW have several limitations. We recently established a novel assay for quantifying serum CMW. Serum CMW from 99 Japanese patients with T2D was quantified by this assay using hydrophilic interaction liquid chromatography. The serum CMW levels were cross-sectionally characterized in relation to clinical features, including renal function and vascular complications. Serum CMW level was more strongly correlated with serum creatinine and cystatin C levels and with eGFR than with albumin urea level. The ROC curve to detect eGFR < 60 ml/min/1.73 m2 revealed that the cutoff serum CMW level was 337.5 nM (AUC 0.883). Serum CMW levels were higher in patients with a history of macroangiopathy than in those without history. They correlated with ankle-brachial pressure index, whereas cystatin C did not. Serum CMW levels quantified by the novel assay could be useful in evaluation of glomerular filtration of renal function and peripheral arterial disease in T2D.

Regulation of N-glycosylation and secretion of Isthmin-1 by its C-mannosylation

BACKGROUND

C-mannosylation is a type of protein glycosylation. Human Isthmin-1 (ISM1) is a 52-kDa secreted protein with a thrombospondin type 1 repeat (TSR) domain, containing two consensus C-mannosylation sequences at Trp²²³ and Trp²²⁶. In this study, we sought to examine the role of C-mannosylation in the secretion of ISM1.

METHODS

We established and cultured an ISM1-overexpressing HT1080 cell line and purified recombinant ISM1 for analysis from the conditioned medium by LC-MS/MS. Subcellular localization of ISM1 was observed by confocal fluorescence microscopy.

RESULTS

We found that ISM1 is C-mannosylated at Trp²²³ and Trp²²⁶ in the TSR domain. To determine the functions of the C-mannosylation of ISM1, we established a C-mannosylation-defective mutant ISM1-overexpressing HT1080 cell line and measured its secretion of ISM1. The secretion of ISM1 decreased significantly in this mutant ISM1-overexpressing line compared with wild-type cells. Furthermore, ISM1 was N-glycosylated only in these C-mannosylation-defective cells.

CONCLUSIONS

ISM1 is C-mannosylated in its TSR domain, and the status of the C-mannosylation of ISM1 affects its N-glycosylation.

GENERAL SIGNIFICANCE

The C-mannosylation of ISM1 regulates its N-glycosylation status.

Requirement for C-mannosylation to be secreted and activated a disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS4)

BACKGROUND

C-mannosylation is a unique type of glycosylation. A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS4) is a multidomain extracellular metalloproteinase that contains several potential C-mannosylation sites. Although some ADAMTS family proteins have been reported to be C-mannosylated proteins, whether C-mannosylation affects the activation and protease activity of these proteins is unclear.

METHODS

We established wild-type and mutant ADAMTS4-overexpressing HT1080 cell lines. Recombinant ADAMTS4 was purified from the conditioned medium of the wild-type ADAMTS4-overexpressing cells, and the C-mannosylation sites of ADAMTS4 were identified by LC-MS/MS. The processing, secretion, and intracellular localization of ADAMTS4 were examined by immunoblot and immunofluorescence analyses. ADAMTS4 enzymatic activity was evaluated by assessing the cleavage of recombinant aggrecan.

RESULTS

We identified that ADAMTS4 is C-mannosylated at Trp⁴⁰⁴ in the metalloprotease domain and at Trp⁵²³, Trp⁵²⁶, and Trp⁵²⁹ in the thrombospondin type 1 repeat (TSR). The replacement of Trp⁴⁰⁴ with Phe affected ADAMTS4 processing, without affecting secretion and intracellular localization. In contrast, the substitution of Trp⁵²³, Trp⁵²⁶, and Trp⁵²⁹ with Phe residues suppressed ADAMTS4 secretion, processing, intracellular trafficking, and enzymatic activity.

CONCLUSIONS

Our results demonstrated that the C-mannosylation of ADAMTS4 plays important roles in protein processing, intracellular trafficking, secretion, and enzymatic activity.

GENERAL SIGNIFICANCE

Because C-mannosylation appears to regulate many ADAMTS4 functions, C-mannosylation may also affect other members of the ADAMTS superfamily.

NMR Spectroscopic Characterization of the C-Mannose Conformation in a Thrombospondin Repeat Using a Selective Labeling Approach

Despite the great interest in glycoproteins, structural information reporting on conformation and dynamics of the sugar moieties are limited. We present a new biochemical method to express proteins with glycans that are selectively labeled with NMR-active nuclei. We report on the incorporation of 13 C-labeled mannose in the C-mannosylated UNC-5 thrombospondin repeat. The conformational landscape of the C-mannose sugar puckers attached to tryptophan residues of UNC-5 is characterized by interconversion between the canonical 1 C4 state and the B03 / 1 S3 state. This flexibility may be essential for protein folding and stabilization. We foresee that this versatile tool to produce proteins with selectively labeled C-mannose can be applied and adjusted to other systems and modifications and potentially paves a way to advance glycoprotein research by unravelling the dynamical and conformational properties of glycan structures and their interactions.

Global view of human protein glycosylation pathways and functions

Glycosylation is the most abundant and diverse form of post-translational modification of proteins that is common to all eukaryotic cells. Enzymatic glycosylation of proteins involves a complex metabolic network and different types of glycosylation pathways that orchestrate enormous amplification of the proteome in producing diversity of proteoforms and its biological functions. The tremendous structural diversity of glycans attached to proteins poses analytical challenges that limit exploration of specific functions of glycosylation. Major advances in quantitative transcriptomics, proteomics and nuclease-based gene editing are now opening new global ways to explore protein glycosylation through analysing and targeting enzymes involved in glycosylation processes. In silico models predicting cellular glycosylation capacities and glycosylation outcomes are emerging, and refined maps of the glycosylation pathways facilitate genetic approaches to address functions of the vast glycoproteome. These approaches apply commonly available cell biology tools, and we predict that use of (single-cell) transcriptomics, genetic screens, genetic engineering of cellular glycosylation capacities and custom design of glycoprotein therapeutics are advancements that will ignite wider integration of glycosylation in general cell biology.

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.

Monomeric C-mannosyl tryptophan is a degradation product of autophagy in cultured cells

C-Mannosyl tryptophan (C-Man-Trp) is a unique glycosylated amino acid present in various eukaryotes. The C-Man-Trp structure can be found as a monomeric form or a part of post-translational modifications within polypeptide chains in living organisms. However, the mechanism of how monomeric C-Man-Trp is produced has not been fully investigated. In this study, we assessed levels of cellular C-Man-Trp by ultra performance liquid chromatography with a mass spectrometry assay system, and investigated whether the cellular C-Man-Trp is affected by autophagy induction. The intracellular C-Man-Trp level was significantly increased under serum and/or amino acid starvation in A549, HaCaT, HepG2, NIH3T3, and NRK49F cells. The increase in C-Man-Trp was also observed in NIH3T3 cells treated with rapamycin, an autophagy inducer. The up-regulation of C-Man-Trp caused by starvation was reversed by the inhibition of lysosomal enzymes. We further showed that C-Man-Trp is produced by incubating a synthetic C-mannosylated peptide (C-Man-Trp-Ser-Pro-Trp) or thrombospondin (TSP) in a lysosomal fraction that was prepared from a mouse liver, which provides supporting evidence that C-Man-Trp is a degradation product of the C-mannosylated peptide or protein following lysosome-related proteolysis. Taken together, we propose that the autophagic pathway is a novel pathway that at least partly contributes to intracellular C-Man-Trp production under certain conditions, such as nutrient starvation.

C-Mannosylation Enhances the Structural Stability of Human RNase 2

C-Mannosylation is a relatively rare form of protein glycosylation involving the attachment of an α-mannopyranosyl residue to C-2 of the indole moiety of the amino acid tryptophan. This type of linkage was initially discovered in RNase 2 from human urine but later confirmed to be present in many other important proteins. Based on NMR experiments and extensive molecular dynamics simulations on the hundred microsecond timescale we demonstrate that, for isolated glycopeptides and denatured RNase 2, the C-linked mannopyranosyl residue exists as an ensemble of conformations, among which ¹C₄ is the most abundant. However, for native RNase 2, molecular dynamics and NMR studies revealed that the mannopyranosyl residue favors a specific conformation, which optimally stabilizes the protein fold through a network of hydrogen bonds and which leads to a significant reduction of the protein dynamics on the microsecond timescale. Our findings contribute to the understanding of the biological role of C-mannosylation.

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NMR and MD show that C-linked mannose exists as an ensemble of conformations

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Conformation of mannose is influenced by the protein environment and solvent

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In RNase 2 mannose favors a conformation that optimally stabilizes the protein fold

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Efficient methods for analysis of a large number of MD trajectories are presented

C-Mannosylation of Toxoplasma gondii proteins promotes attachment to host cells and parasite virulence

C-Mannosylation is a common modification of thrombospondin type 1 repeats present in metazoans and recently identified also in apicomplexan parasites. This glycosylation is mediated by enzymes of the DPY19 family that transfer α-mannoses to tryptophan residues in the sequence WX ₂WX ₂C, which is part of the structurally essential tryptophan ladder. Here, deletion of the dpy19 gene in the parasite Toxoplasma gondii abolished C-mannosyltransferase activity and reduced levels of the micronemal protein MIC2. The loss of C-mannosyltransferase activity was associated with weakened parasite adhesion to host cells and with reduced parasite motility, host cell invasion, and parasite egress. Interestingly, the C-mannosyltransferase-deficient Δdpy19 parasites were strongly attenuated in virulence and induced protective immunity in mice. This parasite attenuation could not simply be explained by the decreased MIC2 level and strongly suggests that absence of C-mannosyltransferase activity leads to an insufficient level of additional proteins. In summary, our results indicate that T. gondii C-mannosyltransferase DPY19 is not essential for parasite survival, but is important for adhesion, motility, and virulence.

C-Mannosyl tryptophan increases in the plasma of patients with ovarian cancer

Ovarian cancer survival is poor, in part, because there are no specific biomarkers for early diagnosis. C-Mannosyl tryptophan (CMW) is a structurally unique glycosylated amino acid recently identified as a novel biomarker of renal dysfunction. The present study investigated whether blood CMW is altered in patients with ovarian cancer and whether differences in blood CMW can distinguish benign from malignant ovarian tumors. Plasma samples were obtained from 49 patients with malignant, borderline or benign ovarian tumors as well as from seven age-matched healthy women. CMW was identified and quantified in these samples using ultra-performance liquid chromatography with fluorometry. Plasma CMW was significantly higher in the malignant tumor group than in the borderline and benign tumor groups, and higher in the combined tumor group (malignant, borderline or benign) compared with healthy controls. Receiver operating characteristic curve analysis of plasma CMW distinguished malignant tumors from borderline/benign tumors [area under the curve (AUC)=0.905]. Discrimination performance was greater than that of cancer antigen (CA) 125 (AUC=0.835), and CMW + CA125 combined achieved even greater discrimination (AUC=0.913, 81.8% sensitivity, 87.5% specificity, 93.1% positive predictive value and 70.0% negative predictive value). Plasma CMW differentiates malignant ovarian cancer from borderline or benign ovarian tumors with high accuracy, and performance is further improved by combined CMW and CA125 measurement.

C-mannosylation supports folding and enhances stability of thrombospondin repeats

Previous studies demonstrated importance of C-mannosylation for efficient protein secretion. To study its impact on protein folding and stability, we analyzed both C-mannosylated and non-C-mannosylated thrombospondin type 1 repeats (TSRs) of netrin receptor UNC-5. In absence of C-mannosylation, UNC-5 TSRs could only be obtained at low temperature and a significant proportion displayed incorrect intermolecular disulfide bridging, which was hardly observed when C-mannosylated. Glycosylated TSRs exhibited higher resistance to thermal and reductive denaturation processes, and the presence of C-mannoses promoted the oxidative folding of a reduced and denatured TSR in vitro. Molecular dynamics simulations supported the experimental studies and showed that C-mannoses can be involved in intramolecular hydrogen bonding and limit the flexibility of the TSR tryptophan-arginine ladder. We propose that in the endoplasmic reticulum folding process, C-mannoses orient the underlying tryptophan residues and facilitate the formation of the tryptophan-arginine ladder, thereby influencing the positioning of cysteines and disulfide bridging.

A Plasmodium falciparum C-mannosyltransferase is dispensable for parasite asexual blood stage development

C-mannosylation was recently identified in the thrombospondin-related anonymous protein (TRAP) from Plasmodium falciparum salivary gland sporozoites. A candidate P. falciparum C-mannosyltransferase (PfDPY-19) was demonstrated to modify thrombospondin type 1 repeat (TSR) domains in vitro, exhibiting a different acceptor specificity than their mammalian counterparts. According to the described minimal acceptor of PfDPY19, several TSR domain-containing proteins of P. falciparum could be C-mannosylated in vivo. However, the relevance of this protein modification for the parasite viability remains unknown. In the present study, we used CRISPR/Cas9 technology to generate a PfDPY19 null mutant, demonstrating that this glycosyltransferase is not essential for the asexual blood development of the parasite. PfDPY19 gene disruption was not associated with a growth phenotype, not even under endoplasmic reticulum-stressing conditions that could impair protein folding. The data presented in this work strongly suggest that PfDPY19 is unlikely to play a critical role in the asexual blood stages of the parasite, at least under in vitro conditions.

Membrane Topological Model of Glycosyltransferases of the GT-C Superfamily

Glycosyltransferases that use polyisoprenol-linked donor substrates are categorized in the GT-C superfamily. In eukaryotes, they act in the endoplasmic reticulum (ER) lumen and are involved in N-glycosylation, glypiation, O-mannosylation, and C-mannosylation of proteins. We generated a membrane topology model of C-mannosyltransferases (DPY19 family) that concurred perfectly with the 13 transmembrane domains (TMDs) observed in oligosaccharyltransferases (STT3 family) structures. A multiple alignment of family members from diverse organisms highlighted the presence of only a few conserved amino acids between DPY19s and STT3s. Most of these residues were shown to be essential for DPY19 function and are positioned in luminal loops that showed high conservation within the DPY19 family. Multiple alignments of other eukaryotic GT-C families underlined the presence of similar conserved motifs in luminal loops, in all enzymes of the superfamily. Most GT-C enzymes are proposed to have an uneven number of TDMs with 11 (POMT, TMTC, ALG9, ALG12, PIGB, PIGV, and PIGZ) or 13 (DPY19, STT3, and ALG10) membrane-spanning helices. In contrast, PIGM, ALG3, ALG6, and ALG8 have 12 or 14 TMDs and display a C-terminal dilysine ER-retrieval motif oriented towards the cytoplasm. We propose that all members of the GT-C superfamily are evolutionary related enzymes with preserved membrane topology.

Analyzing the Effects of O-Fucosylation on Secretion of ADAMTS Proteins Using Cell-Based Assays

Metalloproteinases of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin type 1 repeats) superfamily are extensively modified with glycan moieties. Glycosylation occurs as these enzymes are trafficked through the endoplasmic reticulum (ER) and Golgi apparatus on their way to the extracellular space and includes N-linked glycosylation, O-linked fucosylation and C-linked mannosylation. This chapter focuses on O-linked fucose, which is added to properly folded thrombospondin type 1 repeats (TSRs) in the ER by protein O-fucosyltransferase 2 (POFUT2) and elongated to a Glucoseβ1-3Fucose disaccharide by β3-glucosyltransferase (B3GLCT). Knockout of POFUT2 results in embryonic lethality in mice, and inactivating mutations in B3GLCT cause Peters plus syndrome, a congenital disorder of glycosylation in humans. Addition of the disaccharide by POFUT2 and B3GLCT stabilizes folded TSRs, enhancing folding in the ER and secretion efficiency of several ADAMTS proteins from cells. Thus, POFUT2 and B3GLCT both function as an ER quality control pathway for folding of TSRs in ADAMTS proteins. In this chapter we describe in detail the methods developed to analyze secretion defects of ADAMTS proteins upon loss of either POFUT2 or B3GLCT. The methods described include creation of CRISPR/Cas9-mediated knockout cell lines of POFUT2 and B3GLCT and use of these cell lines to analyze and quantify secretion defects of ADAMTS proteins.

Genomewide Association Study of Leisure-Time Exercise Behavior in Japanese Adults

Supplemental digital content is available in the text.

Purpose

Although several genetic factors may play a role in leisure-time exercise behavior, there is currently no evidence of a significant genomewide association, and candidate gene replication studies have produced inconsistent results.

Methods

We conducted a two-stage genomewide association study and candidate single-nucleotide polymorphisms (SNP) association study on leisure-time exercise behavior using 13,980 discovery samples from the Japan Multi-Institutional Collaborative Cohort (J-MICC) study, and 2036 replication samples from the Hospital-based Epidemiologic Research Program at Aichi Cancer Center-2 study. Leisure-time physical activity was measured using a self-administered questionnaire that inquired about the type, frequency and duration of exercise. Participants with ≥4 MET·h·wk⁻¹ of leisure-time physical activity were defined as exhibiting leisure-time exercise behavior. Association testing using mixed linear regression models was performed on the discovery and replication samples, after which the results were combined in a meta-analysis. In addition, we tested six candidate genetic variants derived from previous genomewide association study.

Results

We found that one novel SNP (rs10252228) located in the intergenic region between NPSR1 and DPY19L1 was significantly associated with leisure-time exercise behavior in discovery samples. This association was also significant in replication samples (combined P value by meta-analysis = 2.2 × 10⁻⁹). Several SNP linked with rs10252228 were significantly associated with gene expression of DPY19L1 and DP19L2P1 in skeletal muscle, heart, whole blood, and the nervous system. Among the candidate SNP, rs12612420 in DNAPTP6 demonstrated nominal significance in discovery samples but not in replication samples.

Conclusions

We identified a novel genetic variant associated with regular leisure-time exercise behavior. Further functional studies are required to validate the role of these variants in exercise behavior.

Multiple distinct O-Mannosylation pathways in eukaryotes

Protein O-mannosylation (O-Man), originally discovered in yeast five decades ago, is an important post-translational modification (PTM) conserved from bacteria to humans, but not found in plants or nematodes. Until recently, the homologous family of ER-located protein O-mannosyl transferases (PMT1-7 in yeast; POMT1/POMT2 in humans), were the only known enzymes involved in directing O-Man biosynthesis in eukaryotes. However, recent studies demonstrate the existence of multiple distinct O-Man glycosylation pathways indicating that the genetic and biosynthetic regulation of O-Man in eukaryotes is more complex than previously envisioned. Introduction of sensitive glycoproteomics strategies provided an expansion of O-Man glycoproteomes in eukaryotes (yeast and mammalian cell lines) leading to the discovery of O-Man glycosylation on important mammalian cell adhesion (cadherin superfamily) and signaling (plexin family) macromolecules, and to the discovery of unique nucleocytoplasmic O-Man glycosylation in yeast. It is now evident that eukaryotes have multiple distinct O-Man glycosylation pathways including: i) the classical PMT1-7 and POMT1/POMT2 pathway conserved in all eukaryotes apart from plants; ii) a yet uncharacterized nucleocytoplasmic pathway only found in yeast; iii) an ER-located pathway directed by the TMTC1-4 genes found in metazoans and protists and primarily dedicated to the cadherin superfamily; and iv) a yet uncharacterized pathway found in metazoans primarily dedicated to plexins. O-Man glycosylation is thus emerging as a much more widespread and evolutionary diverse PTM with complex genetic and biosynthetic regulation. While deficiencies in the POMT1/POMT2 O-Man pathway underlie muscular dystrophies, the TMTC1-4 pathway appear to be involved in distinct congenital disorders with neurodevelopmental phenotypes. Here, we review and discuss the recent discoveries of the new non-classical O-Man glycosylation pathways, their substrates, functions and roles in disease.

C‑mannosylation of R‑spondin2 activates Wnt/β‑catenin signaling and migration activity in human tumor cells

R‑spondin2 (Rspo2), one of the four members of the R‑spondin family of proteins, has agonistic activity in the Wnt/β‑catenin signaling pathway, and it is associated with normal development, as well as disease, such as cancer. The present study focused on the C‑mannosylation of Rspo2, which is a novel and unique type of glycosylation that occurs via a C‑C linkage between the tryptophan residue and an α‑mannose. Although Rspo2 has two putative C‑mannosylation sites at residues Trp150 and Trp153, it had not been reported to date whether these sites are C‑mannosylated. Firstly, results from mass spectrometry demonstrated that Rspo2 was C‑mannosylated at the Trp150 and Trp153 residues. Notably, while this C‑mannosylation of Rspo2 resulted in increased extracellular secretion in human fibrosarcoma HT1080 cells, in other human tumor cell lines it inhibited secretion. However, C‑mannosylation had consistent effects on the activation of Wnt/β‑catenin signaling in PANC1 and MDA‑MB‑231 cells, as well as HT1080 cells. Furthermore, overexpression of wild‑type Rspo2 significantly increased the migratory ability of A549 and HT1080 cells, whereas overexpression of a C‑mannosylation‑defective mutant enhanced migration to a lesser degree. These results suggested that C‑mannosylation of Rspo2 may promote cancer progression and that the inhibition of C‑mannosylation may serve as a potential novel therapeutic approach for cancer therapy.

A novel assay for detection and quantification of C-mannosyl tryptophan in normal or diabetic mice

C-Mannosyl tryptophan (C-Man-Trp) is a unique molecule in that an α-mannose is connected to the indole C2 carbon atom of a Trp residue via C-glycosidic linkage. Although serum C-Man-Trp may be a novel biomarker of renal function in humans, the biological significance of C-Man-Trp has yet to be fully investigated. In this study, a novel assay system for C-Man-Trp was established using hydrophilic-interaction liquid chromatography, followed by detecting the fluorescence intensity or mass abundance of C-Man-Trp. Using this system, we systematically assessed the amount of free monomeric C-Man-Trp in different tissues of mice. The tissue level of C-Man-Trp was high, especially in the ovaries and uterus. Other organs with high levels of C-Man-Trp included the brain, spleen, lungs, bladder, and testes. The level was low in skeletal muscle. We also investigated whether the tissue level of C-Man-Trp is affected in diabetes. In KK-Ay diabetic mice, the level of urinary C-Man-Trp excretion was increased, and the tissue levels of C-Man-Trp were decreased in the liver but increased in the kidney. These results demonstrate that C-Man-Trp is differentially distributed in numerous tissues and organs in mice, and the levels are altered by disordered carbohydrate metabolism such as diabetes.

Protein O- and C-Glycosylation pathways in Toxoplasma gondii and Plasmodium falciparum

Apicomplexan parasites are amongst the most prevalent and morbidity-causing pathogens worldwide. They are responsible for severe diseases in humans and livestock and are thus of great public health and economic importance. Until the sequencing of apicomplexan genomes at the beginning of this century, the occurrence of N- and O-glycoproteins in these parasites was much debated. The synthesis of rudimentary and divergent N-glycans due to lineage-specific gene loss is now well established and has been recently reviewed. Here, we will focus on recent studies that clarified classical O-glycosylation pathways and described new nucleocytosolic glycosylations in Toxoplasma gondii, the causative agents of toxoplasmosis. We will also review the glycosylation of proteins containing thrombospondin type 1 repeats by O-fucosylation and C-mannosylation, newly discovered in Toxoplasma and the malaria parasite Plasmodium falciparum. The functional significance of these post-translational modifications has only started to emerge, but the evidence points towards roles for these protein glycosylation pathways in tissue cyst wall rigidity and persistence in the host, oxygen sensing, and stability of proteins involved in host invasion.