Quantification of extracellular UDP-galactose

Distinctive domains and activity regulation of core fucosylation enzyme FUT8

BACKGROUND

Core fucose, a structure added to the reducing end N-acetylglucosamine of N-glycans, has been shown to regulate various physiological and pathological processes, including melanoma metastasis, exacerbation of chronic obstructive pulmonary disease, and severe outcomes in COVID-19.

SCOPE OF REVIEW

Recent research has shed light on regulation of the activity and subcellular localization of a1,6-fucosyltransferase (FUT8), the glycosyltransferase responsible for core fucose biosynthesis, unraveling the mechanisms for controlling core fucosylation in vivo.

MAJOR CONCLUSIONS

This review summarizes the various features of FUT8, including its domains, structures, and substrate specificity. Additionally, we discuss the potential involvement of FUT8-binding proteins, such as oligosaccharyltransferase subunits, in the regulation of FUT8 activity, substrate specificity, and the secretion of FUT8.

GENERAL SIGNIFICANCE

We anticipate that this review will contribute to a deeper understanding of the control of core fucose levels in vivo and involvement of core fucosylation in FUT8-relevant functions and diseases.

Identification of a carbohydrate recognition motif of purinergic receptors

As a major class of biomolecules, carbohydrates play indispensable roles in various biological processes. However, it remains largely unknown how carbohydrates directly modulate important drug targets, such as G-protein coupled receptors (GPCRs). Here, we employed P2Y purinoceptor 14 (P2Y14), a drug target for inflammation and immune responses, to uncover the sugar nucleotide activation of GPCRs. Integrating molecular dynamics simulation with functional study, we identified the uridine diphosphate (UDP)-sugar-binding site on P2Y14, and revealed that a UDP-glucose might activate the receptor by bridging the transmembrane (TM) helices 2 and 7. Between TM2 and TM7 of P2Y14, a conserved salt bridging chain (K2.60-D2.64-K7.35-E7.36 [KDKE]) was identified to distinguish different UDP-sugars, including UDP-glucose, UDP-galactose, UDP-glucuronic acid, and UDP-N-acetylglucosamine. We identified the KDKE chain as a conserved functional motif of sugar binding for both P2Y14 and P2Y purinoceptor 12 (P2Y12), and then designed three sugar nucleotides as agonists of P2Y12. These results not only expand our understanding for activation of purinergic receptors but also provide insights for the carbohydrate drug development for GPCRs.

Signalling by extracellular nucleotides in health and disease

Nucleotides are released from all cells through regulated pathways or as a result of plasma membrane damage or cell death. Outside the cell, nucleotides act as signalling molecules triggering multiple responses via specific plasma membrane receptors of the P2 family. In the nervous system, purinergic signalling has a key function in neurotransmission. Outside the nervous system, purinergic signalling is one of the major modulators of basal tissue homeostasis, while its dysregulation contributes to the pathogenesis of various disease, including inflammation and cancer. Pre-clinical and clinical evidence shows that selective P2 agonists or antagonists are effective treatments for many pathologies, thus highlighting the relevance of extracellular nucleotides and P2 receptors as therapeutic targets.

Role of UDP-Sugar Receptor P2Y14 in Murine Osteoblasts

The purinergic (P2) receptor P2Y₁₄ is the only P2 receptor that is stimulated by uridine diphosphate (UDP)-sugars and its role in bone formation is unknown. We confirmed P2Y₁₄ expression in primary murine osteoblasts (CB-Ob) and the C2C12-BMP2 osteoblastic cell line (C2-Ob). UDP-glucose (UDPG) had undiscernible effects on cAMP levels, however, induced dose-dependent elevations in the cytosolic free calcium concentration ([Ca²⁺]_i) in CB-Ob, but not C2-Ob cells. To antagonize the P2Y₁₄ function, we used the P2Y₁₄ inhibitor PPTN or generated CRISPR-Cas9-mediated P2Y₁₄ knockout C2-Ob clones (Y14_KO). P2Y₁₄ inhibition facilitated calcium signalling and altered basal cAMP levels in both models of osteoblasts. Importantly, P2Y₁₄ inhibition augmented Ca²⁺ signalling in response to ATP, ADP and mechanical stimulation. P2Y₁₄ knockout or inhibition reduced osteoblast proliferation and decreased ERK1/2 phosphorylation and increased AMPKα phosphorylation. During in vitro osteogenic differentiation, P2Y₁₄ inhibition modulated the timing of osteogenic gene expression, collagen deposition, and mineralization, but did not significantly affect differentiation status by day 28. Of interest, while P2ry14^-/- mice from the International Mouse Phenotyping Consortium were similar to wild-type controls in bone mineral density, their tibia length was significantly increased. We conclude that P2Y₁₄ in osteoblasts reduces cell responsiveness to mechanical stimulation and mechanotransductive signalling and modulates osteoblast differentiation.

Quantification of nucleotides and their sugar conjugates in biological samples: Purposes, instruments and applications

Nucleotides and their sugar conjugates are fundamental molecules in life, participating in processes of DNA/RNA composition, cell wall build-up, glycosylation reactions, and signal conduction. Therefore, the quantification of these compounds in biological samples significantly benefits the understanding of their functions. However, nucleotides and nucleotide sugars are extremely hydrophilic, causing bad retention and peak symmetry on regular C18 chromatographic columns. To solve this problem, ion-pair (IP) chromatography, ion-exchange (IE) chromatography and hydrophilic interaction chromatography (HILIC) were applied, of which differentiated mechanisms were utilized to increase the retention of the analytes on the stationary phases. IP-HPLC and HILIC were convenient for coupling with many kinds of detectors (ultraviolet, UV or mass spectrometry, MS). Combining these two kinds of techniques, the advantages of better separation and retention were increased, while disadvantages like irreversible adsorption by stationary phases were greatly decreased. Due to the high concentrations of nonvolatile buffer salts used, IE-HPLC was not suitable for MS detectors. Protein precipitation and solid phase extraction were the common methods for sample treatment in the analysis of nucleotides and nucleotide sugars. By carefully optimizing the LCUV or LCMS conditions, high sensitivities could be achieved, and the methods could be applied to the analysis of many kinds of biological samples (cells, tissues, plants, bacteria, etc.). Developing new analyzing techniques may help the utilization of nucleotides and nucleotide sugars in the diagnosis and therapy of diseases.

P2Y6 Receptor Activation Promotes Inflammation and Tissue Remodeling in Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a disease with a poor prognosis and very few available treatment options. The involvement of the purinergic receptor subtypes P2Y₂ and P2X₇ in fibrotic lung disease has been demonstrated recently. In this study, we investigated the role of P2Y₆ receptors in the pathogenesis of IPF in humans and in the animal model of bleomycin-induced lung injury. P2Y₆R expression was upregulated in lung structural cells but not in bronchoalveolar lavage (BAL) cells derived from IPF patients as well as in animals following bleomycin administration. Furthermore, BAL fluid levels of the P2Y₆R agonist uridine-5′-diphosphate were elevated in animals with bleomycin-induced pulmonary fibrosis. Inflammation and fibrosis following bleomycin administration were reduced in P2Y₆R-deficient compared to wild-type animals confirming the pathophysiological relevance of P2Y₆R subtypes for fibrotic lung diseases. Experiments with bone marrow chimeras revealed the importance of P2Y₆R expression on lung structural cells for pulmonary inflammation and fibrosis. Similar effects were obtained when animals were treated with the P2Y₆R antagonist MRS2578. In vitro studies demonstrated that proliferation and secretion of the pro-inflammatory/pro-fibrotic cytokine IL-6 by lung fibroblasts are P2Y₆R-mediated processes. In summary, our results clearly demonstrate the involvement of P2Y₆R subtypes in the pathogenesis of pulmonary fibrosis. Thus, blocking pulmonary P2Y₆ receptors might be a new target for the treatment of IPF.

Gene-based analysis of regulatory variants identifies 4 putative novel asthma risk genes related to nucleotide synthesis and signaling

BACKGROUND

Hundreds of genetic variants are thought to contribute to variation in asthma risk by modulating gene expression. Methods that increase the power of genome-wide association studies (GWASs) to identify risk-associated variants are needed.

OBJECTIVE

We sought to develop a method that aggregates the evidence for association with disease risk across expression quantitative trait loci (eQTLs) of a gene and use this approach to identify asthma risk genes.

METHODS

We developed a gene-based test and software package called EUGENE that (1) is applicable to GWAS summary statistics; (2) considers both cis- and trans-eQTLs; (3) incorporates eQTLs identified in different tissues; and (4) uses simulations to account for multiple testing. We applied this approach to 2 published asthma GWASs (combined n = 46,044) and used mouse studies to provide initial functional insights into 2 genes with novel genetic associations.

RESULTS

We tested the association between asthma and 17,190 genes that were found to have cis- and/or trans-eQTLs across 16 published eQTL studies. At an empirical FDR of 5%, 48 genes were associated with asthma risk. Of these, for 37, the association was driven by eQTLs located in established risk loci for allergic disease, including 6 genes not previously implicated in disease cause (eg, LIMS1, TINF2, and SAFB). The remaining 11 significant genes represent potential novel genetic associations with asthma. The association with 4 of these replicated in an independent GWAS: B4GALT3, USMG5, P2RY13, and P2RY14, which are genes involved in nucleotide synthesis or nucleotide-dependent cell activation. In mouse studies, P2ry13 and P2ry14-purinergic receptors activated by adenosine 5-diphosphate and UDP-sugars, respectively-were upregulated after allergen challenge, notably in airway epithelial cells, eosinophils, and neutrophils. Intranasal exposure with receptor agonists induced the release of IL-33 and subsequent eosinophil infiltration into the lungs.

CONCLUSION

We identified novel associations between asthma and eQTLs for 4 genes related to nucleotide synthesis/signaling and demonstrated the power of gene-based analyses of GWASs.

UDP-glucose promotes neutrophil recruitment in the lung

In addition to their role in glycosylation reactions, UDP-sugars are released from cells and activate widely distributed cell surface P2Y₁₄ receptors (P2Y₁₄R). However, the physiological/pathophysiological consequences of UDP-sugar release are incompletely defined. Here, we report that UDP-glucose levels are abnormally elevated in lung secretions from patients with cystic fibrosis (CF) as well as in a mouse model of CF-like disease, the βENaC transgenic (Tg) mouse. Instillation of UDP-glucose into wild-type mouse tracheas resulted in enhanced neutrophil lung recruitment, and this effect was nearly abolished when UDP-glucose was co-instilled with the P2Y₁₄R antagonist PPTN [4-(piperidin-4-yl)-phenyl)-7-(4-(trifluoromethyl)-phenyl-2-naphthoic acid]. Importantly, administration of PPTN to βENaC-Tg mice reduced neutrophil lung inflammation. These results suggest that UDP-glucose released into the airways acts as a local mediator of neutrophil inflammation.

UDP-Sugars as Extracellular Signaling Molecules: Cellular and Physiologic Consequences of P2Y14 Receptor Activation

UDP-sugars, which are indispensable for protein glycosylation reactions in cellular secretory pathways, also act as important extracellular signaling molecules. We discuss here the broadly expressed P2Y14 receptor, a G-protein-coupled receptor targeted by UDP sugars, and the increasingly diverse set of physiologic responses discovered recently functioning downstream of this receptor in many epithelia as well as in immune, inflammatory, and other cells.

The UDP-sugar-sensing P2Y(14) receptor promotes Rho-mediated signaling and chemotaxis in human neutrophils

The G(i)-coupled P2Y(14) receptor (P2Y(14)-R) is potently activated by UDP-sugars and UDP. Although P2Y(14)-R mRNA is prominently expressed in circulating neutrophils, the signaling pathways and functional responses associated with this receptor are undefined. In this study, we illustrate that incubation of isolated human neutrophils with UDP-glucose resulted in cytoskeleton rearrangement, change of cell shape, and enhanced cell migration. We also demonstrate that UDP-glucose promotes rapid, robust, and concentration-dependent activation of RhoA in these cells. Ecto-nucleotidases expressed on neutrophils rapidly hydrolyzed extracellular ATP, but incubation with UDP-glucose for up to 1 h resulted in negligible metabolism of the nucleotide-sugar. HL60 human promyelocytic leukemia cells do not express the P2Y(14)-R, but neutrophil differentiation of HL60 cells with DMSO resulted in markedly enhanced P2Y(14)-R expression. Accordingly, UDP-glucose, UDP-galactose, and UDP-N-acetylglucosamine promoted Rho activation in differentiated but not in undifferentiated HL60 cells. Stable expression of recombinant human P2Y(14)-R conferred UDP-sugar-promoted responses to undifferentiated HL60 cells. UDP-glucose-promoted RhoA activation also was accompanied by enhanced cell migration in differentiated HL60 cells, and these responses were blocked by Rho kinase inhibitors. These results support the notion that UDP-glucose is a stable and potent proinflammatory mediator that promotes P2Y(14)-R-mediated neutrophil motility via Rho/Rho kinase activation.

Molecular mechanisms of purine and pyrimidine nucleotide release

Given the widespread importance of purinergic receptor-evoked signaling, understanding how ATP and other nucleotides are released from cells in a regulated manner is an essential physiological question. Nonlytic release of ATP, UTP, UDP-glucose, and other nucleotides occurs in all cell types and tissues via both constitutive mechanisms, that is, in the absence of external stimuli, and to a greater extent in response to biochemical or mechanical/physical stimuli. However, a molecular understanding of the processes regulating nucleotide release has only recently begun to emerge. It is generally accepted that nucleotide release occurs in two different scenarios, exocytotic release from the secretory pathway or via conductive/transport mechanisms, and a critical review of our current understanding of these mechanisms is presented in this chapter.

Signalling and pharmacological properties of the P2Y receptor

The P2Y(14) receptor is a relatively broadly expressed G protein-coupled receptor that is prominently associated with immune and inflammatory cells as well as with many epithelia. This receptor historically was thought to be activated selectively by UDP-glucose and other UDP-sugars. However, UDP is also a very potent agonist of this receptor, and may prove to be one of its most important cognate activators.