Isoform-specific O-glycosylation of osteopontin and bone sialoprotein by polypeptide N-acetylgalactosaminyltransferase-1

Exogenous bone sialoprotein improves extraction socket healing in Ibsp knockout and wild-type mice

Bone sialoprotein (Ibsp/BSP) is a bone-associated extracellular matrix protein. Ibsp knockout (Ibsp-/-) mice exhibit defective alveolar bone formation, mineralization, and healing. We hypothesized BSP would rescue defective alveolar bone healing in a molar extraction model in Ibsp-/- mice. Collagen gel with or without native rat BSP (nBSP) or recombinant rat BSP (rBSP) was delivered to sockets after first maxillary molar extraction in Ibsp-/- and wild-type (WT) mice. Tissues were harvested 0, 1, 2, 7, and 14 days post-procedure (dpp) and analyzed by micro-computed tomography, histology, and immunohistochemistry (IHC). Histology and IHC demonstrated that collagen and BSP were retained within sockets. At 14 dpp, both bone volume fraction (BV/TV) and bone mineral density (BMD) were increased by both nBSP (over 50 %) and rBSP (over 60 %), compared to collagen alone in Ibsp-/- mice. In WT alveolar bone, BV/TV and BMD were also increased by nBSP (over 30 %) and rBSP (over 60 %) compared to collagen controls. Gene expression analyses revealed few changes from delivery of nBSP, while addition of rBSP resulted in regulation of cell signaling, ECM, mineralization, and osteoblast/osteoclast-associated genes. Exogenous BSP rescued alveolar bone healing defects in Ibsp-/- mice and enhanced bone healing in WT mice. Despite both forms of BSP improving bone healing, the substantial differences in how they regulate gene expression suggests that exogenous BSP acts in a complex, multifunctional manner to promote bone healing. These results support BSP as a novel approach to improve the quantity and quality of new bone in socket healing.

The role of bone sialoprotein in bone healing

Bone sialoprotein (BSP) is a multi-functional extracellular matrix (ECM) protein associated with mineralized tissues, particularly bone and cementum. The amino acid sequence of BSP includes three evolutionarily conserved sequences which contribute to functions of the protein: an N-terminal collagen-binding domain, polyglutamic acid (polyE) sequences involved in hydroxyapatite nucleation and crystal growth, and a C-terminal arginine-glycine-aspartic acid (RGD) integrin-binding domain. BSP promotes attachment and differentiation of osteogenic and osteoclastic cells. Genetic ablation of BSP in mice results in skeletal and dental developmental defects and impaired bone healing in both appendicular bone and alveolar bone of the jaw. Several studies demonstrated positive effects of BSP on bone healing in rodent models, though other experiments show negligible results. Native (harvested from rat bones) BSP cross-linked to collagen induced slight improvements in calvarial bone healing in rats. Recombinant BSP and collagen delivered in a polylactide (PLA) cylinder improved bone defect healing in rat femurs. Both native and recombinant BSP delivered in a collagen gel improved alveolar bone healing in wild-type and BSP-deficient mice. These advances suggest BSP is a new player in bone healing that has potential to be an alternative or complimentary to other bioactive factors. Future studies are necessary to understand mechanisms of how BSP influences bone healing and optimize delivery and dose in different types of bone defects and injuries.

Post-translational modifications: The potential ways for killing cancer stem cells

While strides in cancer treatment continue to advance, the enduring challenges posed by cancer metastasis and recurrence persist as formidable contributors to the elevated mortality rates observed in cancer patients. Among the multifaceted factors implicated in tumor recurrence and metastasis, cancer stem cells (CSCs) emerge as noteworthy entities due to their inherent resistance to conventional therapies and heightened invasive capacities. Characterized by their notable abilities for self-renewal, differentiation, and initiation of tumorigenesis, the eradication of CSCs emerges as a paramount objective. Recent investigations increasingly emphasize the pivotal role of post-translational protein modifications (PTMs) in governing the self-renewal and replication capabilities of CSCs. This review accentuates the critical significance of several prevalent PTMs and the intricate interplay of PTM crosstalk in regulating CSC behavior. Furthermore, it posits that the manipulation of PTMs may offer a novel avenue for targeting and eliminating CSC populations, presenting a compelling perspective on cancer therapeutics with substantial potential for future applications.

Quantitative mapping of the in vivo O-GalNAc glycoproteome in mouse tissues identifies GalNAc-T2 O-glycosites in metabolic disorder

The family of GalNAc-Ts (GalNAcpolypeptide:N-Acetylgalactosaminyl transferases) catalyzes the first committed step in the synthesis of O-glycans, which is an abundant and biologically important protein modification. Abnormalities in the activity of individual GalNAc-Ts can result in congenital disorders of O-glycosylation (CDG) and influence a broad array of biological functions. How site-specific O-glycans regulate biology is unclear. Compiling in vivo O-glycosites would be an invaluable step in determining the function of site-specific O-glycans. We integrated chemical and enzymatic conditions that cleave O-glycosites, a higher-energy dissociation product ions-triggered electron-transfer/higher-energy collision dissociation mass spectrometry (MS) workflow and software to study nine mouse tissues and whole blood. We identified 2,154 O-glycosites from 595 glycoproteins. The O-glycosites and glycoproteins displayed consensus motifs and shared functions as classified by Gene Ontology terms. Limited overlap of O-glycosites was observed with protein O-GlcNAcylation and phosphorylation sites. Quantitative glycoproteomics and proteomics revealed a tissue-specific regulation of O-glycosites that the differential expression of Galnt isoenzymes in tissues partly contributes to. We examined the Galnt2-null mouse model, which phenocopies congenital disorder of glycosylation involving GALNT2 and revealed a network of glycoproteins that lack GalNAc-T2-specific O-glycans. The known direct and indirect functions of these glycoproteins appear consistent with the complex metabolic phenotypes observed in the Galnt2-null animals. Through this study and interrogation of databases and the literature, we have compiled an atlas of experimentally identified mouse O-glycosites consisting of 2,925 O-glycosites from 758 glycoproteins.

A Low-Cost, Sensitive Reporter System Using Membrane-Tethered Horseradish Peroxidase for Efficient Gene Expression Analysis

Reporter gene assays are essential for high-throughput analysis, such as drug screening or determining downstream signaling activation/inhibition. However, use of this technology has been hampered by the high cost of the substrate (e.g., d-Luciferin (d-Luc)) in the most common firefly luciferase (FLuc) reporter gene assay. Although alternate luciferase is available worldwide, its substrate has remained expensive, and a more affordable option is still in demand. Here, we present a membrane-tethered horseradish peroxidase (mHRP), a new reporter system composed of a cell membrane expressing HRP that can preserve its enzymatic function on the cell surface, facilitates contact with HRP substrates (e.g., ABTS and TMB), and avoids the cell lysis process and the use of the high-priced luciferase substrate. An evaluation of the light signal sensitivity of mHRP compared to FLuc showed that both had comparable signal sensitivity. We also identified an extended substrate half-life of more than 5-fold that of d-Luc. Of note, this strategy provided a more stable detection signal, and the cell lysis process is not mandatory. Furthermore, with this strategy, we decreased the total amount of time taken for analysis and increased the time of detection limit of the reporter assay. Pricing analysis showed a one-third to one twenty-eighth price drop per single test of reporter assay. Given the convenience and stability of the mHRP reporter system, we believe that our strategy is suitable for use as an alternative to the luciferase reporter assay.

Bone Sialoprotein Is Critical for Alveolar Bone Healing in Mice

Bone sialoprotein (BSP) is an extracellular matrix (ECM) protein associated with mineralized tissues, particularly bone and cementum. BSP includes functional domains implicated in collagen binding, hydroxyapatite nucleation, and cell signaling, although its function(s) in osteoblast and osteoclast differentiation and function remain incompletely understood. Genetic ablation of BSP in Ibsp knockout (Ibsp-/-) mice results in developmental bone mineralization and remodeling defects, with alveolar bone more severely affected than the femurs and tibias of the postcranial skeleton. The role of BSP in alveolar bone healing has not been studied. We hypothesized that BSP ablation would cause defective alveolar bone healing. We employed a maxillary first molar extraction socket healing model in 42-d postnatalIbsp-/- and wild-type (WT) control mice. Tissues were collected at 0, 7, 14, 21, and 56 d postprocedure (dpp) for analysis by micro-computed tomography (microCT), histology, in situ hybridization (ISH), immunohistochemistry (IHC), and quantitative polymerase chain reaction (qPCR) array. As expected, alveolar bone healing progressed in WT mice with increasing bone volume fraction (BV/TV), bone mineral density (BMD), and tissue mineral density (TMD), transitioning from woven to mature bone from 7 to 56 dpp. Ibsp messenger RNA (mRNA) and BSP protein were strongly expressed during alveolar bone healing in parallel with other osteogenic markers. Compared to WT, Ibsp-/- mice exhibited 50% to 70% reduced BV/TV and BMD at all time points, 7% reduced TMD at 21 dpp, abnormally increased Col1a1 and Alpl mRNA expression, and persistent presence of woven bone and increased bone marrow in healing sockets. qPCR revealed substantially dysregulated gene expression in alveolar bone of Ibsp-/- versus WT mice, with significantly disrupted expression of 45% of tested genes in functional groups, including markers for osteoblasts, osteoclasts, mineralization, ECM, cell signaling, and inflammation. We conclude that BSP is a critical and nonredundant factor for alveolar bone healing, and its absence disrupts multiple major pathways involved in appropriate healing.

OPN N-glycosylation Promoted Bone Destruction

OBJECTIVES

Exploring the role of OPN N-glycosylation in osteoblasts and osteoclasts.

METHODS

Immunohistochemistry was used to detect the expression of OPN in mice with apical periodontitis. The asparagine at position 79 of the OPN protein was mutated to glutamine, and the above plasmids were transfected into osteoblasts and osteoclasts. The effect of OPN N-glycosylation on proliferation of osteoblasts and osteoclasts was detected by CCK8 assays. Western blotting was used to detect the expression of OPN N-glycosylation on osteoclasts and osteoblasts. Detection of N-glycosylation of OPN activated the NF-κB signaling pathway to regulate osteoblasts and osteoclasts.

RESULTS

OPN increased expression in a mice model of apical periodontitis. The expression curve of OPN resembled a reverse V shape. The OPN N-glycosylation site was identified as 79 by MS. N-glycosylation of OPN promoted the proliferation of osteoclasts. But the N79 glycosylation site of mutant OPN couldn't increase the proliferation of osteoblasts. OPN N-glycosylation modulated the expression of osteoclast- and osteoblast-associated factors through the NF-κB signaling pathway. N-glycosylation of OPN promoted nuclear translocation of NF-κB in osteoclasts and osteoblasts.

CONCLUSIONS

The N-glycosylation site of OPN is 79. N-glycosylation of OPN played an important role in the biological function of OPN protein.

Structural, Evolutionary, and Functional Analysis of the Protein O-Mannosyltransferase Family in Pathogenic Fungi

Protein O-mannosyltransferases (Pmts) comprise a group of proteins that add mannoses to substrate proteins at the endoplasmic reticulum. This post-translational modification is important for the faithful transfer of nascent glycoproteins throughout the secretory pathway. Most fungi genomes encode three O-mannosyltransferases, usually named Pmt1, Pmt2, and Pmt4. In pathogenic fungi, Pmts, especially Pmt4, are key factors for virulence. Although the importance of Pmts for fungal pathogenesis is well established in a wide range of pathogens, questions remain regarding certain features of Pmts. For example, why does the single deletion of each pmt gene have an asymmetrical impact on host colonization? Here, we analyse the origin of Pmts in fungi and review the most important phenotypes associated with Pmt mutants in pathogenic fungi. Hence, we highlight the enormous relevance of these glycotransferases for fungal pathogenic development.

Predicting mucin-type O-Glycosylation using enhancement value products from derived protein features

Mucin-type O-glycosylation is one of the most common post-translational modifications of proteins. This glycosylation is initiated in the Golgi by the addition of the sugar N-acetylgalactosamine (GalNAc) onto protein Ser and Thr residues by a family of polypeptide GalNAc transferases. In humans there are 20 isoforms that are differentially expressed across tissues that serve multiple important biological roles. Using random peptide substrates, isoform specific amino acid preferences have been obtained in the form of enhancement values (EV). These EVs alone have previously been used to predict O-glycosylation sites via the web based ISOGlyP (Isoform Specific O-Glycosylation Prediction) tool. Here we explore additional protein features to determine whether these can complement the random peptide derived enhancement values and increase the predictive power of ISOGlyP. The inclusion of additional protein substrate features (such as secondary structure and surface accessibility) was found to increase sensitivity with minimal loss of specificity, when tested with three different published in vivo O-glycoproteomics data sets, thus increasing the overall accuracy of the ISOGlyP predictions.

O-glycan initiation directs distinct biological pathways and controls epithelial differentiation

Post-translational modifications (PTMs) greatly expand the function and potential for regulation of protein activity, and O-glycosylation is among the most abundant and diverse PTMs. Initiation of O-GalNAc glycosylation is regulated by 20 distinct GalNAc-transferases (GalNAc-Ts), and deficiencies in individual GalNAc-Ts are associated with human disease, causing subtle but distinct phenotypes in model organisms. Here, we generate a set of isogenic keratinocyte cell lines lacking either of the three dominant and differentially expressed GalNAc-Ts. Through the ability of keratinocytes to form epithelia, we investigate the phenotypic consequences of the loss of individual GalNAc-Ts. Moreover, we probe the cellular responses through global transcriptomic, differential glycoproteomic, and differential phosphoproteomic analyses. We demonstrate that loss of individual GalNAc-T isoforms causes distinct epithelial phenotypes through their effect on specific biological pathways; GalNAc-T1 targets are associated with components of the endomembrane system, GalNAc-T2 targets with cell-ECM adhesion, and GalNAc-T3 targets with epithelial differentiation. Thus, GalNAc-T isoforms serve specific roles during human epithelial tissue formation.

LAMTOR5 raises abnormal initiation of O-glycosylation in breast cancer metastasis via modulating GALNT1 activity

During malignancy, perturbed O-glycosylation confers global influence on cancer progression. As a hallmark of cancer metastasis, GalNAc-type O-glycosylation initiation is aberrantly raised, but the regulatory mechanism is still mysterious. Here, we show that LAMTOR5 raises abnormal initiation of O-glycosylation in breast cancer metastasis. LAMTOR5 was highly expressed in adenocarcinoma and correlated with Tn antigen, a product of O-glycosylation initiation, in both clinical metastatic breast cancer specimens and secondary metastasis mouse model. LAMTOR5-modulated O-glycosylation initiating enzyme GALNT1 conferred Tn accumulation and predicted poor survival. Mechanistically, LAMTOR5 stimulated transcriptions of GALNT1 through coactivating c-Jun, and triggered dislocation of GALNT1 in the endoplasmic reticulum (ER) via LAMTOR5 dependent-activation of c-Src. This unusual initiation of O-glycosylation resulted in the abundance of Tn modified glycoproteins, such as MUC1 and OPN. Collectively, our findings indicate that LAMTOR5/c-Jun/c-Src axis serves as the upstream regulator of abnormal O-glycosylation initiation and potential therapeutic targets in breast cancer metastasis.

Site-specific glycosylation of Ebola virus glycoprotein by human polypeptide GalNAc-transferase 1 induces cell adhesion defects

The surface glycoprotein (GP) of Ebola virus causes many of the virus's pathogenic effects, including a dramatic loss of endothelial cell adhesion associated with widespread hemorrhaging during infection. Although the GP-mediated deadhesion depends on its extracellular mucin-like domain, it is unknown whether any, or all, of this domain's densely clustered O-glycosylation sites are required. It is also unknown whether any of the 20 distinct polypeptide GalNAc-transferases (ppGalNAc-Ts) that initiate mucin-type O-glycosylation in human cells are functionally required. Here, using HEK293 cell lines lacking specific glycosylation enzymes, we demonstrate that GP requires extended O-glycans to exert its deadhesion effect. We also identified ppGalNAc-T1 as largely required for the GP-mediated adhesion defects. Despite its profound effect on GP function, the absence of ppGalNAc-T1 only modestly reduced the O-glycan mass of GP, indicating that even small changes in the bulky glycodomain can cause loss of GP function. Indeed, protein-mapping studies identified a small segment of the mucin-like domain critical for function and revealed that mutation of five glycan acceptor sites within this segment are sufficient to abrogate GP function. Together, these results argue against a mechanism of Ebola GP-induced cell detachment that depends solely on ectodomain bulkiness and identify a single host-derived glycosylation enzyme, ppGalNAc-T1, as a potential target for therapeutic intervention against Ebola virus disease.

Probing the contribution of individual polypeptide GalNAc-transferase isoforms to the O-glycoproteome by inducible expression in isogenic cell lines

The GalNAc-type O-glycoproteome is orchestrated by a large family of polypeptide GalNAc-transferase isoenzymes (GalNAc-Ts) with partially overlapping contributions to the O-glycoproteome besides distinct nonredundant functions. Increasing evidence indicates that individual GalNAc-Ts co-regulate and fine-tune specific protein functions in health and disease, and deficiencies in individual GALNT genes underlie congenital diseases with distinct phenotypes. Studies of GalNAc-T specificities have mainly been performed with in vitro enzyme assays using short peptide substrates, but recently quantitative differential O-glycoproteomics of isogenic cells with and without GALNT genes has enabled a more unbiased exploration of the nonredundant contributions of individual GalNAc-Ts. Both approaches suggest that fairly small subsets of O-glycosites are nonredundantly regulated by specific GalNAc-Ts, but how these isoenzymes orchestrate regulation among competing redundant substrates is unclear. To explore this, here we developed isogenic cell model systems with Tet-On inducible expression of two GalNAc-T genes, GALNT2 and GALNT11, in a knockout background in HEK293 cells. Using quantitative O-glycoproteomics with tandem-mass-tag (TMT) labeling, we found that isoform-specific glycosites are glycosylated in a dose-dependent manner and that induction of GalNAc-T2 or -T11 produces discrete glycosylation effects without affecting the major part of the O-glycoproteome. These results support previous findings indicating that individual GalNAc-T isoenzymes can serve in fine-tuned regulation of distinct protein functions.

Recombinant human osteopontin expressed in Nicotiana benthamiana stimulates osteogenesis related genes in human periodontal ligament cells

Tissue engineering aims to utilise biologic mediators to facilitate tissue regeneration. Several recombinant proteins have potential to mediate induction of bone production, however, the high production cost of mammalian cell expression impedes patient access to such treatments. The aim of this study is to produce recombinant human osteopontin (hOPN) in plants for inducing dental bone regeneration. The expression host was Nicotiana benthamiana using a geminiviral vector for transient expression. OPN expression was confirmed by Western blot and ELISA, and OPN was purified using Ni affinity chromatography. Structural analysis indicated that plant-produced hOPN had a structure similar to commercial HEK cell-produced hOPN. Biological function of the plant-produced hOPN was also examined. Human periodontal ligament stem cells were seeded on an OPN-coated surface. The results indicated that cells could grow normally on plant-produced hOPN as compared to commercial HEK cell-produced hOPN determined by MTT assay. Interestingly, increased expression of osteogenic differentiation-related genes, including OSX, DMP1, and Wnt3a, was observed by realtime PCR. These results show the potential of plant-produced OPN to induce osteogenic differentiation of stem cells from periodontal ligament in vitro, and suggest a therapeutic strategy for bone regeneration in the future.

Glycosylation status of bone sialoprotein and its role in mineralization

The highly glycosylated bone sialoprotein (BSP) is an abundant non-collagenous phosphoprotein in bone which enhances osteoblast differentiation and new bone deposition in vitro and in vivo. However, the structural details of its different glycosylation linkages have not been well studied and their functions in bone homeostasis are not clear. Previous studies suggested that the O-glycans, but not the N-glycans on BSP, are highly sialylated. Herein, we employed tandem mass spectrometry (MS/MS) to demonstrate that the N-glycanson the recombinant human integrin binding sialoprotein (rhiBSP) are also enriched in sialic acids (SAs) at their termini. We also identified multiple novel sites of N-glycan modification. Treatment of rhiBSP enhances osteoblast differentiation and mineralization of MC3T3-E1 cells and this effect could be partially reversed by efficient enzymatic removal of its N-glycans. Removal of all terminal SAs has a greater effect in reversing the effect of rhiBSP on osteogenesis, especially on mineralization, suggesting that sialylation at the termini of both N-glycans and O-glycans plays an important role in this regulation. Moreover, BSP-conjugated SAs may affect mineralization via ERK activation of VDR expression. Collectively, our results identified novel N-glycans enriched in SAs on the rhiBSP and demonstrated that SAs at both N- and O-glycans are important for BSP regulation of osteoblast differentiation and mineralization in vitro.

Isoform-specific regulation of osteogenic factors by polypeptide N-Acetylgalactosaminyltransferases 1 and 4

The family of UDP-GalNAc polypeptide: N-Acetylgalactosaminlytransfersases (ppGalNAcTs) catalyzes the initial step of O-linked protein glycosylation. Mucin-type O-glycoproteins are abundant in the bone and may play an important role in osteogenesis. Herein, we examined the effects of ppGalNAc-T isoforms on osteogenesis of MC3T3-E1 pre-osteoblasts. We found that ppGalNAc-T1 and -T4 isoforms were highly expressed during osteogenesis of MC3T3-E1 and their knockdown by short hairpin RNA (shRNA) decreased osteoblast formation and bone mineralization. Knockdown of ppGalNAc-T1 or -T4 decreased mRNA and protein levels of bone sialoprotein (BSP). Knockdown of ppGalNAc-T1decreased mRNA levels of osteocalcin (OC), osteoprotegerin (OPG). Knockdown ofppGalNAc-T4 isoform decreased mRNA levels of OC, OPG and vitamin D receptor (VDR). While knockdown of T1 or T4 isoforms did not change the expression of osteopontin (OPN), COLLI, receptor activator for nuclear factor-κB ligand (RANKL) and transforming growth factor-β (TGF-β). Our results demonstrated that the ppGalNAc-T4 was highly expressed in MC3T3-E1 cells during osteogenesis for the first time. We also found that ppGalNAc-T1 and -T4 affected the expression of different osteogenic factors, suggesting distinct roles ppGalNAc-T isoformsplay in regulating osteogenesis in vitro.

Deconstruction of O-glycosylation--GalNAc-T isoforms direct distinct subsets of the O-glycoproteome

GalNAc-type O-glycosylation is found on most proteins trafficking through the secretory pathway in metazoan cells. The O-glycoproteome is regulated by up to 20 polypeptide GalNAc-Ts and the contributions and biological functions of individual GalNAc-Ts are poorly understood. Here, we used a zinc-finger nuclease (ZFN)-directed knockout strategy to probe the contributions of the major GalNAc-Ts (GalNAc-T1 and GalNAc-T2) in liver cells and explore how the GalNAc-T repertoire quantitatively affects the O-glycoproteome. We demonstrate that the majority of the O-glycoproteome is covered by redundancy, whereas distinct subsets of substrates are modified by non-redundant functions of GalNAc-T1 and GalNAc-T2. The non-redundant O-glycoproteome subsets and specific transcriptional responses for each isoform are related to different cellular processes; for the GalNAc-T2 isoform, these support a role in lipid metabolism. The results demonstrate that GalNAc-Ts have different non-redundant glycosylation functions, which may affect distinct cellular processes. The data serves as a comprehensive resource for unique GalNAc-T substrates. Our study provides a new view of the differential regulation of the O-glycoproteome, suggesting that the plurality of GalNAc-Ts arose to regulate distinct protein functions and cellular processes.

Probing polypeptide GalNAc-transferase isoform substrate specificities by in vitro analysis

N-acetylgalactosaminyltransferase (GalNAc)-type (mucin-type) O-glycosylation is an abundant and highly diverse modification of proteins. This type of O-glycosylation is initiated in the Golgi by a large family of up to 20 homologous polypeptide GalNAc-T isoenzymes that transfer GalNAc to Ser, Thr and possibly Tyr residues. These GalNAc residues are then further elongated by a large set of glycosyltransferases to build a variety of complex O-glycan structures. What determines O-glycan site occupancy is still poorly understood, although it is clear that the substrate specificities of individual isoenzymes and the repertoire of GalNAc-Ts in cells are key parameters. The GalNAc-T isoenzymes are differentially expressed in cells and tissues in principle allowing cells to produce unique O-glycoproteomes dependent on the specific subset of isoforms present. In vitro analysis of acceptor peptide substrate specificities using recombinant expressed GalNAc-Ts has been the method of choice for probing activities of individual isoforms, but these studies have been hampered by biological validation of actual O-glycosylation sites in proteins and number of substrate testable. Here, we present a systematic analysis of the activity of 10 human GalNAc-T isoenzymes with 195 peptide substrates covering known O-glycosylation sites and provide a comprehensive dataset for evaluating isoform-specific contributions to the O-glycoproteome.

BCMab1, a monoclonal antibody against aberrantly glycosylated integrin α3β1, has potent antitumor activity of bladder cancer in vivo

PURPOSE

To identify a novel biomarker for bladder cancer targeting therapy.

EXPERIMENTAL DESIGN

The human bladder cancer cell line T24 cells were used as immunogen to generate mouse monoclonal antibodies. We screened and identified a specific antibody BCMab1 against bladder cancer. We examined BCMab1 antigen expression in the patients with bladder cancer through immunohistochemical staining and investigated the BCMab1 antigen association with clinical severity. We detected the antitumor activity of BCMab1 antibody and investigated its therapeutic efficacy by subcutaneous and orthotopic bladder cancer models.

RESULTS

We developed a new monoclonal antibody BCMab1 against bladder cancer that specifically recognized the aberrantly glycosylated Integrin α3β1 epitope on bladder cancer cells. Expression of the BCMab1 antigen was consistent with clinical severity and prognosis of bladder cancer. The glycosyltransferase GALNT1 could contribute to aberrant glycosylation of Integrin α3. The aberrant glycosylation of integrin α3-activated integrin signaling to initiate FAK activation. BCMab1 could block Integrin engagement to inhibit its signaling leading to cell-cycle arrest. In addition, BCMab1 enhanced FcγR-dependent antitumor activity in vivo.

CONCLUSIONS

BCMab1 antigen is a new biomarker for bladder cancer. BCMab1 antibody exhibited potent antitumor activity against bladder cancer in vivo.

Precision genome editing: a small revolution for glycobiology

Precise and stable gene editing in mammalian cell lines has until recently been hampered by the lack of efficient targeting methods. While different gene silencing strategies have had tremendous impact on many biological fields, they have generally not been applied with wide success in the field of glycobiology, primarily due to their low efficiencies, with resultant failure to impose substantial phenotypic consequences upon the final glycosylation products. Here, we review novel nuclease-based precision genome editing techniques enabling efficient and stable gene editing, including gene disruption, insertion, repair, modification and deletion. The nuclease-based techniques comprised of homing endonucleases, zinc finger nucleases, transcription activator-like effector nucleases, as well as the RNA-guided clustered regularly interspaced short palindromic repeat/Cas nuclease system, all function by introducing single or double-stranded breaks at a defined genomic sequence. We here compare and contrast the different techniques and summarize their current applications, highlighting cases from the field of glycobiology as well as pointing to future opportunities. The emerging potential of precision gene editing for the field is exemplified by applications to xenotransplantation; to probing O-glycoproteomes, including differential O-GalNAc glycoproteomes, to decipher the function of individual polypeptide GalNAc-transferases, as well as for engineering Chinese Hamster Ovary host cells for production of improved therapeutic biologics.

The O‑glycosylation mutant osteopontin alters lung cancer cell growth and migration in vitro and in vivo

Osteopontin (OPN) is an acidic, glycosylated and phosphorylated protein that plays an essential role in determining the aggressiveness and oncogenic potential of several types of cancer, including lung cancer. The OPN function is highly dependent on post-translational modification (PTM) and regulation of the processes that involve OPN can be mediated through glycosylation. However, the connection between OPN function and its O-glycosylation in lung cancer cells has yet to be investigated. In the present study, this issue was addressed by studying the effects of wild-type (WT) OPN and a triple mutant (TM) of OPN, which was mutated at three O-glycosylation sites in lung cancer cells. It was shown that OPN WT rather than OPN TM induced the OPN‑mediated signaling pathway. The OPN WT expression enhanced cap-dependent protein translation, NF-κB activity and glucose uptake, whereas a reduction was observed in cells treated with OPN TM. The results clearly demonstrated that unlike OPN WT, OPN TM did not increase lung cancer cell growth and migration both in vitro and in a xenograft mouse model. Thus, results of the present study suggested that targeting OPN by introducing OPN TM may be a good strategy for treating lung cancer.

Aerosol delivery of lentivirus-mediated O-glycosylation mutant osteopontin suppresses lung tumorigenesis in K-ras (LA1) mice

BACKGROUND

Osteopontin (OPN) is a secreted glycophosphoprotein that has been implicated in the regulation of cancer development. The function of OPN is primarily regulated through post-translational modification such as glycosylation. As yet, however, the relationship between OPN glycosylation and lung cancer development has not been investigated. In this study, we addressed this issue by studying the effect of a triple mutant (TM) of OPN, which is mutated at three O-glycosylation sites, on lung cancer development in K-ras (LA1) mice, a murine model for human non-small cell lung cancer.

METHODS

Aerosolized lentivirus-based OPN TM was delivered into the lungs of K-ras (LA1) mice using a nose-only-inhalation chamber 3 times/wk for 4 wks. Subsequently, the effects of repeated delivery of OPN TM on lung tumorigenesis and its concomitant OPN-mediated signaling pathways were investigated.

RESULTS

Aerosol-delivered OPN TM inhibited lung tumorigenesis. In addition, the OPN-mediated Akt signaling pathway was inhibited. OPN TM also decreased NF-κB activity and the phosphorylation of 4E-BP1, while facilitating apoptosis in the lungs of K-ras (LA1) mice.

CONCLUSIONS

Our results show that aerosol delivery of OPN TM successfully suppresses lung cancer development in the K-ras (LA1) mouse model and, therefore, warrant its further investigation as a possible therapeutic strategy for non-small cell lung cancer.

Site-specific protein O-glycosylation modulates proprotein processing - deciphering specific functions of the large polypeptide GalNAc-transferase gene family

BACKGROUND

Posttranslational modifications (PTMs) greatly expand the function and regulation of proteins, and glycosylation is the most abundant and diverse PTM. Of the many different types of protein glycosylation, one is quite unique; GalNAc-type (or mucin-type) O-glycosylation, where biosynthesis is initiated in the Golgi by up to twenty distinct UDP-N-acetyl-α-d-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). These GalNAc-Ts are differentially expressed in cells and have different (although partly overlapping) substrate specificities, which provide for both unique functions and considerable redundancy. Recently we have begun to uncover human diseases associated with deficiencies in GalNAc-T genes (GALNTs). Thus deficiencies in individual GALNTs produce cell and protein specific effects and subtle distinct phenotypes such as hyperphosphatemia with hyperostosis (GALNT3) and dysregulated lipid metabolism (GALNT2). These phenotypes appear to be caused by deficient site-specific O-glycosylation that co-regulates proprotein convertase (PC) processing of FGF23 and ANGPTL3, respectively.

SCOPE OF REVIEW

Here we summarize recent progress in uncovering the interplay between human O-glycosylation and protease regulated processing and describes other important functions of site-specific O-glycosylation in health and disease.

MAJOR CONCLUSIONS

Site-specific O-glycosylation modifies pro-protein processing and other proteolytic events such as ADAM processing and thus emerges as an important co-regulator of limited proteolytic processing events.

GENERAL SIGNIFICANCE

Our appreciation of this function may have been hampered by our sparse knowledge of the O-glycoproteome and in particular sites of O-glycosylation. New strategies for identification of O-glycoproteins have emerged and recently the concept of SimpleCells, i.e. human cell lines made deficient in O-glycan extension by zinc finger nuclease gene targeting, was introduced for broad O-glycoproteome analysis.

A structural and functional model for human bone sialoprotein

Human bone sialoprotein (BSP) is an essential component of the extracellular matrix of bone. It is thought to be the primary nucleator of hydroxyapatite crystallization, and is known to bind to hydroxyapatite, collagen, and cells. Mature BSP shows extensive post-translational modifications, including attachment of glycans, sulfation, and phosphorylation, and is highly flexible with no specific 2D or 3D structure in solution or the solid state. These features have severely limited the experimental characterization of the structure of this protein. We have therefore developed a 3D structural model for BSP, based on the available literature data, using molecular modelling techniques. The complete model consists of 301 amino acids, including six phosphorylated serines and two sulfated tyrosines, plus 92 N- and O-linked glycan residues. A notable feature of the model is a large acidic patch that provides a surface for binding Ca(2+) ions. Density functional theory quantum calculations with an implicit solvent model indicate that Ca(2+) ions are bound most strongly by the phosphorylated serines within BSP, along with reasonably strong binding to Asp and Glu, but weak binding to His and sulfated tyrosine. The process of early hydroxyapatite nucleation has been studied by molecular dynamics on an acidic surface loop of the protein; the results suggest that the cationic nature of the loop promotes nucleation by attracting Ca(2+) ions, while its flexibility allows for their rapid self-assembly with PO(4)(3-) ions, rather than providing a regular template for crystallization. The binding of a hydroxyapatite crystal at the protein's acidic patch has also been modelled. The relationships between hydroxyapatite, collagen and BSP are discussed.

Osteopontin induces growth of metastatic tumors in a preclinical model of non-small lung cancer

Osteopontin (OPN), also known as SPP1 (secreted phosphoprotein), is an integrin binding glyco-phosphoprotein produced by a variety of tissues. In cancer patients expression of OPN has been associated with poor prognosis in several tumor types including breast, lung, and colorectal cancers. Despite wide expression in tumor cells and stroma, there is limited evidence supporting role of OPN in tumor progression and metastasis. Using phage display technology we identified a high affinity anti-OPN monoclonal antibody (hereafter AOM1). The binding site for AOM1 was identified as SVVYGLRSKS sequence which is immediately adjacent to the RGD motif and also spans the thrombin cleavage site of the human OPN. AOM1 efficiently inhibited OPNa binding to recombinant integrin αvβ3 with an IC50 of 65 nM. Due to its unique binding site, AOM1 is capable of inhibiting OPN cleavage by thrombin which has been shown to produce an OPN fragment that is biologically more active than the full length OPN. Screening of human cell lines identified tumor cells with increased expression of OPN receptors (αvβ3 and CD44v6) such as mesothelioma, hepatocellular carcinoma, breast, and non-small cell lung adenocarcinoma (NSCLC). CD44v6 and αvβ3 were also found to be highly enriched in the monocyte, but not lymphocyte, subset of human peripheral blood mononuclear cells (hPBMCs). In vitro, OPNa induced migration of both tumor and hPBMCs in a transwell migration assay. AOM1 significantly blocked cell migration further validating its specificity for the ligand. OPN was found to be enriched in mouse plasma in a number of pre-clinical tumor model of non-small cell lung cancers. To assess the role of OPN in tumor growth and metastasis and to evaluate a potential therapeutic indication for AOM1, we employed a Kras^G12D-LSLp53^fl/fl subcutaneously implanted in vivo model of NSCLC which possesses a high capacity to metastasize into the lung. Our data indicated that treatment of tumor bearing mice with AOM1 as a single agent or in combination with Carboplatin significantly inhibited growth of large metastatic tumors in the lung further supporting a role for OPN in tumor metastasis and progression.

Generation of intracellular single-chain antibodies directed against polypeptide GalNAc-transferase using a yeast two-hybrid system

Mucin-type O-glycosylation is initiated by a large number of UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferases (GalNAc-T). Although extensive in vitro studies using synthetic peptides as substrates suggest that most GalNAc-Ts exhibit overlapping substrate specificities, many studies have shown that individual GalNAc-Ts play an important role in both animals and humans. Further investigations of the functions of individual GalNAc-Ts including in vivo substrate proteins and O-glycosylation sites are necessary. In this study, we attempted to generate single-chain variable fragment (scFv) antibodies to bind to GalNAc-T1, T2, T3, and T4 using a yeast two-hybrid system for screening a naive chicken scFv library. Several different scFvs were isolated against a single target GalNAc-T isoform specifically under expressed in yeast and were confirmed to be expressed in mammalian cells and to retain binding activity inside the cells. Generation of these specific antibodies provides an opportunity to modify and exploit antibodies for specific applications in investigations of GalNAc-T functions.

O-glycoprotein biosynthesis: site localization by Edman degradation and site prediction based on random peptide substrates

The characterization of mucin-type O-glycosylation is fraught with extreme difficulty at almost every level of analysis: from difficulties in obtaining glycopeptides suitable for study, their structural heterogeneity, lack of broad acting glycosidase tools capable of simplifying the glycans, and finally the vast complexity of performing analysis on multiply glycosylated glycopeptides. This, along with a lack of known peptide sequence motif(s) for the transferases that initiate mucin-type O-glycosylation, significantly hinders our understanding of mucin-type O-glycosylation at almost every level from their biosynthesis to their biological and biophysical properties. In this chapter, the use of partial chemical deglycosylation coupled with Edman amino acid sequencing is described to quantify sites of O-glycosylation. In addition, the use of oriented random peptide substrates is described for providing the specificities of the polypeptide α-N-acetylgalactosaminyltransferases, which can be used to estimate transferase-specific sites of O-glycosylation.

Core-glycosylated mucin-like repeats from MUC1 are an apical targeting signal

MUC1 is efficiently delivered to the apical surface of polarized Madin-Darby canine kidney (MDCK) cells by transit through apical recycling endosomes, a route associated with delivery of apical proteins with glycan-dependent targeting signals. However, a role for glycans in MUC1 sorting has not been established. A key feature of MUC1 is a heavily O-glycosylated mucin-like domain with a variable number of nearly perfect tandem repeats and adjacent imperfect repeats. Metabolic labeling, cell surface biotinylation, immobilized lectins, and confocal immunofluorescence microscopy were used to characterize the polarized delivery of MUC1 mutants and chimeras in MDCK cells to identify the apical targeting signal. Both the interleukin-2 receptor α subunit (Tac) and a chimera where the Tac ectodomain replaced that of MUC1 were delivered primarily to the basolateral surface. Attachment of the MUC1 mucin-like domain to the N terminus of Tac enhanced apical but not basolateral delivery when compared with Tac. Conversely, deletions within the mucin-like domain in MUC1 reduced apical but not basolateral delivery when compared with MUC1. In pull-down assays with lectins, we found a notable difference in the presence of core 1 O-glycans, but not poly-N-acetyllactosamine, in apically targeted MUC1 and chimeras when compared with Tac. Consistent with these data, we found no effect on MUC1 targeting when galectin-3, with preference for poly-N-acetyllactosamine, was depleted from polarized MDCK cells. However, we did block the apical targeting activity of the mucin-like repeats when we overexpressed CMP-Neu5Ac:GalNAc-Rα2,6-sialyltransferase-1 to block core O-glycan synthesis. The cumulative data indicate that the core-glycosylated mucin-like repeats of MUC1 constitute an apical targeting signal.

Lectin domains of polypeptide GalNAc transferases exhibit glycopeptide binding specificity

UDP-GalNAc:polypeptide α-N-acetylgalactosaminyltransferases (GalNAc-Ts) constitute a family of up to 20 transferases that initiate mucin-type O-glycosylation. The transferases are structurally composed of catalytic and lectin domains. Two modes have been identified for the selection of glycosylation sites by GalNAc-Ts: confined sequence recognition by the catalytic domain alone, and concerted recognition of acceptor sites and adjacent GalNAc-glycosylated sites by the catalytic and lectin domains, respectively. Thus far, only the catalytic domain has been shown to have peptide sequence specificity, whereas the primary function of the lectin domain is to increase affinity to previously glycosylated substrates. Whether the lectin domain also has peptide sequence selectivity has remained unclear. Using a glycopeptide array with a library of synthetic and recombinant glycopeptides based on sequences of mucins MUC1, MUC2, MUC4, MUC5AC, MUC6, and MUC7 as well as a random glycopeptide bead library, we examined the binding properties of four different lectin domains. The lectin domains of GalNAc-T1, -T2, -T3, and -T4 bound different subsets of small glycopeptides. These results indicate an additional level of complexity in the initiation step of O-glycosylation by GalNAc-Ts.

Expression profile of the matricellular protein osteopontin in primary open-angle glaucoma and the normal human eye

PURPOSE. To characterize the role of osteopontin (OPN) in primary open-angle glaucoma (POAG) and normal eyes. METHODS. OPN quantification was performed by enzyme-linked immunosorbent assay in aqueous humor (AH) obtained from human donor eyes (POAG and normal) and surgical samples (POAG and elective cataract removal). OPN expression and localization in whole eye tissue sections and primary normal human trabecular meshwork (NTM) cells were studied by Western blot and immunohistochemistry. Latanoprost-free acid (LFA)-treated NTM cells were analyzed for OPN gene and protein expression. Intraocular pressure was measured by tonometry, and central corneal thickness was measured by optical coherence tomography in young OPN(-/-) and wild-type mice. RESULTS. OPN levels were significantly reduced in donor POAG AH compared with normal AH (0.54 ± 0.18 ng/μg [n = 8] vs. 0.77 ± 0.23 ng/μg [n = 9]; P = 0.039). A similar trend was observed in surgical AH (1.05 ± 0.31 ng/μg [n = 20] vs. 1.43 ± 0.88 ng/μg [n = 20]; P = 0.083). OPN was present in the trabecular meshwork, corneal epithelium and endothelium, iris, ciliary body, retina, vitreous humor, and optic nerve. LFA increased OPN gene expression, but minimal change in OPN protein expression was observed. No difference in intraocular pressure (17.5 ± 2.0 mm Hg [n = 56] vs. 17.3 ± 1.9 mm Hg [n = 68]) but thinner central corneal thickness (91.7 ± 3.6 μm [n = 50] vs. 99.2 ± 5.5 μm [n = 70]) was noted between OPN(-/-) and wild-type mice. CONCLUSIONS. OPN is widely distributed in the human eye and was found in lower concentrations in POAG AH. Reduction of OPN in young mice does not affect IOP.

Location, location, location: new insights into O-GalNAc protein glycosylation

O-GalNAc glycosylation of proteins confers essential structural, protective and signaling roles in eumetazoans. Addition of O-glycans onto proteins is an extremely complex process that regulates both sites of attachment and the types of oligosaccharides added. Twenty distinct polypeptide GalNAc-transferases (GalNAc-Ts) initiate O-glycosylation and fine-tuning their expression provides a mechanism for regulating this action. Recently, a new mode of regulation has emerged where activation of Src kinase selectively redistributes Golgi-localized GalNAc-Ts to the ER. This relocalization results in a strong increase in the density of O-glycan decoration. In this review, we discuss how different mechanisms can regulate the number and the types of O-glycans decorating proteins. In addition, we speculate how Src-dependent relocation of GalNAc-Ts could play an important role in cancerous cellular transformation.

Emerging paradigms for the initiation of mucin-type protein O-glycosylation by the polypeptide GalNAc transferase family of glycosyltransferases

Mammalian mucin-type O-glycosylation is initiated by a large family of ∼20 UDP-GalNAc:polypeptide α-N-acetylgalactosaminyltransferases (ppGalNAc Ts) that transfer α-GalNAc from UDP-GalNAc to Ser and Thr residues of polypeptide acceptors. Characterizing the peptide substrate specificity of each isoform is critical to understanding their properties, biological roles, and significance. Presently, only the specificities of ppGalNAc T1, T2, and T10 and the fly orthologues of T1 and T2 have been systematically characterized utilizing random peptide substrates. We now extend these studies to ppGalNAc T3, T5, and T12, transferases variously associated with human disease. Our results reveal several common features; the most striking is the similar pattern of enhancements for the three residues C-terminal to the site of glycosylation for those transferases that contain a common conserved Trp. In contrast, residues N-terminal to the site of glycosylation show a wide range of isoform-specific enhancements, with elevated preferences for Pro, Val, and Tyr being the most common at the -1 position. Further analysis reveals that the ratio of positive (Arg, Lys, and His) to negative (Asp and Glu) charged residue enhancements varied among transferases, thus further modulating substrate preference in an isoform-specific manner. By utilizing the obtained transferase-specific preferences, the glycosylation patterns of the ppGalNAc Ts against a series of peptide substrates could roughly be reproduced, demonstrating the potential for predicting isoform-specific glycosylation. We conclude that each ppGalNAc T isoform may be uniquely sensitive to peptide sequence and overall charge, which together dictates the substrate sites that will be glycosylated.

The Tn antigen-structural simplicity and biological complexity

Glycoproteins in animal cells contain a variety of glycan structures that are added co- and/or posttranslationally to proteins. Of over 20 different types of sugar-amino acid linkages known, the two major types are N-glycans (Asn-linked) and O-glycans (Ser/Thr-linked). An abnormal mucin-type O-glycan whose expression is associated with cancer and several human disorders is the Tn antigen. It has a relatively simple structure composed of N-acetyl-D-galactosamine with a glycosidic α linkage to serine/threonine residues in glycoproteins (GalNAcα1-O-Ser/Thr), and was one of the first glycoconjugates to be chemically synthesized. The Tn antigen is normally modified by a specific galactosyltransferase (T-synthase) in the Golgi apparatus of cells. Expression of active T-synthase is uniquely dependent on the molecular chaperone Cosmc, which is encoded by a gene on the X chromosome. Expression of the Tn antigen can arise as a consequence of mutations in the genes for T-synthase or Cosmc, or genes affecting other steps of O-glycosylation pathways. Because of the association of the Tn antigen with disease, there is much interest in the development of Tn-based vaccines and other therapeutic approaches based on Tn expression.

O-glycosylation modulates proprotein convertase activation of angiopoietin-like protein 3: possible role of polypeptide GalNAc-transferase-2 in regulation of concentrations of plasma lipids

The angiopoietin-like protein 3 (ANGPTL3) is an important inhibitor of the endothelial and lipoprotein lipases and a promising drug target. ANGPTL3 undergoes proprotein convertase processing (RAPR(224)↓TT) for activation, and the processing site contains two potential GalNAc O-glycosylation sites immediately C-terminal (TT(226)). We developed an in vivo model system in CHO ldlD cells that was used to show that O-glycosylation in the processing site blocked processing of ANGPTL3. Genome-wide SNP association studies have identified the polypeptide GalNAc-transferase gene, GALNT2, as a candidate gene for low HDL and high triglyceride blood levels. We hypothesized that the GalNAc-T2 transferase performed critical O-glycosylation of proteins involved in lipid metabolism. Screening of a panel of proteins known to affect lipid metabolism for potential sites glycosylated by GalNAc-T2 led to identification of Thr(226) adjacent to the proprotein convertase processing site in ANGPTL3. We demonstrated that GalNAc-T2 glycosylation of Thr(226) in a peptide with the RAPR(224)↓TT processing site blocks in vitro furin cleavage. The study demonstrates that ANGPTL3 activation is modulated by O-glycosylation and that this step is probably controlled by GalNAc-T2.

A high-throughput O-glycopeptide discovery platform for seromic profiling

Biomarker microarrays are becoming valuable tools for serological screening of disease-associated autoantibodies. Post-translational modifications (PTMs) such as glycosylation extend the range of protein function, and a variety of glycosylated proteins are known to be altered in disease progression. Here, we have developed a synthetic screening microarray platform for facile display of O-glycosylated peptides (O-PTMs). By introduction of a capping step during chemical solid-phase glycopeptide synthesis, selective enrichment of N-terminal glycopeptide end products was achieved on an amine-reactive hydrogel-coated microarray glass surface, allowing high-throughput display of large numbers of glycopeptides. Utilizing a repertoire of recombinant glycosyltransferases enabled further diversification of the array libraries in situ and display of a new level of potential biomarker candidates for serological screening. As proof-of-concept, we have demonstrated that MUC1 glycopeptides could be assembled and used to detect autoantibodies in vaccine-induced disease-free breast cancer patients and in patients with confirmed disease at time of diagnosis.

The role of mucin-type O-glycans in eukaryotic development

Newly emerging genetic studies have revealed that a subset of the family of glycosyltransferases responsible for the formation of mucin-type O glycans is essential for normal development. As additional genetic, biochemical and physical tools are developed to interrogate the complex structure and surface location of this under-studied class of carbohydrate, no doubt additional roles will be elucidated.