Glycosylation: An intrinsic sign of "danger"

The Exploitation of the Glycosylation Pattern in Asthma: How We Alter Ancestral Pathways to Develop New Treatments

Asthma has reached epidemic levels, yet progress in developing specific therapies is slow. One of the main reasons for this is the fact that asthma is an umbrella term for various distinct subsets. Due to its high heterogeneity, it is difficult to establish biomarkers for each subset of asthma and to propose endotype-specific treatments. This review focuses on protein glycosylation as a process activated in asthma and ways to utilize it to develop novel biomarkers and treatments. We discuss known and relevant glycoproteins whose functions control disease development. The key role of glycoproteins in processes integral to asthma, such as inflammation, tissue remodeling, and repair, justifies our interest and research in the field of glycobiology. Altering the glycosylation states of proteins contributing to asthma can change the pathological processes that we previously failed to inhibit. Special emphasis is placed on chitotriosidase 1 (CHIT1), an enzyme capable of modifying LacNAc- and LacdiNAc-containing glycans. The expression and activity of CHIT1 are induced in human diseased lungs, and its pathological role has been demonstrated by both genetic and pharmacological approaches. We propose that studying the glycosylation pattern and enzymes involved in glycosylation in asthma can help in patient stratification and in developing personalized treatment.

A Holistic 4D Approach to Optimize Intrinsic and Extrinsic Factors Contributing to Variability in Microarray Biosensing in Glycomics

Protein-carbohydrate interactions happen to be a crucial facet of biology, discharging a myriad of functions. Microarrays have become a premier choice to discern the selectivity, sensitivity and breadth of these interactions in a high-throughput manner. The precise recognition of target glycan ligands among the plethora of others is central for any glycan-targeting probe being tested by microarray analyses. Ever since the introduction of the microarray as an elemental tool for high-throughput glycoprofiling, numerous distinct array platforms possessing different customizations and assemblies have been developed. Accompanying these customizations are various factors ushering variances across array platforms. In this primer, we investigate the influence of various extrinsic factors, namely printing parameters, incubation procedures, analyses and array storage conditions on the protein-carbohydrate interactions and evaluate these factors for the optimal performance of microarray glycomics analysis. We hereby propose a 4D approach (Design-Dispense-Detect-Deduce) to minimize the effect of these extrinsic factors on glycomics microarray analyses and thereby streamline cross-platform analyses and comparisons. This work will aid in optimizing microarray analyses for glycomics, minimize cross-platform disparities and bolster the further development of this technology.

Cell survival and DNA damage repair are promoted in the human blood thanatotranscriptome shortly after death

RNA analysis of post-mortem tissues, or thanatotranscriptomics, has become a topic of interest in forensic science due to the essential information it can provide in forensic investigations. Several studies have previously investigated the effect of death on gene transcription, but it has never been conducted with samples of the same individual. For the first time, a longitudinal mRNA expression analysis study was performed with post-mortem human blood samples from individuals with a known time of death. The results reveal that, after death, two clearly differentiated groups of up- and down-regulated genes can be detected. Pathway analysis suggests active processes that promote cell survival and DNA damage repair, rather than passive degradation, are the source of early post-mortem changes of gene expression in blood. In addition, a generalized linear model with an elastic net restriction predicted post-mortem interval with a root mean square error of 4.75 h. In conclusion, we demonstrate that post-mortem gene expression data can be used as biomarkers to estimate the post-mortem interval though further validation using independent sample sets is required before use in forensic casework.

Reduced sialylation triggers homeostatic synapse and neuronal loss in middle-aged mice

Sialic acid-binding Ig-like lectin (Siglec) receptors are linked to neurodegenerative processes, but the role of sialic acids in physiological aging is still not fully understood. We investigated the impact of reduced sialylation in the brain of mice heterozygous for the enzyme glucosamine-2-epimerase/N-acetylmannosamine kinase (GNE+/-) that is essential for sialic acid biosynthesis. We demonstrate that GNE+/- mice have hyposialylation in different brain regions, less synapses in the hippocampus and reduced microglial arborization already at 6 months followed by increased loss of neurons at 12 months. A transcriptomic analysis revealed no pro-inflammatory changes indicating an innate homeostatic immune process leading to the removal of synapses and neurons in GNE+/- mice during aging. Crossbreeding with complement C3-deficient mice rescued the earlier onset of neuronal and synaptic loss as well as the changes in microglial arborization. Thus, sialic acids of the glycocalyx contribute to brain homeostasis and act as a recognition system for the innate immune system in the brain.

Glycoproteomic Analysis of the Aortic Extracellular Matrix in Marfan Patients

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Marfan syndrome (MFS) is caused by mutations in FBN1 (fibrillin-1), an extracellular matrix (ECM) component, which is modified post-translationally by glycosylation. This study aimed to characterize the glycoproteome of the aortic ECM from patients with MFS and relate it to aortopathy.

Emerging roles of protein mannosylation in inflammation and infection

Proteins are frequently modified by complex carbohydrates (glycans) that play central roles in maintaining the structural and functional integrity of cells and tissues in humans and lower organisms. Mannose forms an essential building block of protein glycosylation, and its functional involvement as components of larger and diverse α-mannosidic glycoepitopes in important intra- and intercellular glycoimmunological processes is gaining recognition. With a focus on the mannose-rich asparagine (N-linked) glycosylation type, this review summarises the increasing volume of literature covering human and non-human protein mannosylation, including their structures, biosynthesis and spatiotemporal expression. The review also covers their known interactions with specialised host and microbial mannose-recognising C-type lectin receptors (mrCLRs) and antibodies (mrAbs) during inflammation and pathogen infection. Advances in molecular mapping technologies have recently revealed novel immuno-centric mannose-terminating truncated N-glycans, termed paucimannosylation, on human proteins. The cellular presentation of α-mannosidic glycoepitopes on N-glycoproteins appears tightly regulated; α-mannose determinants are relative rare glycoepitopes in physiological extracellular environments, but may be actively secreted or leaked from cells to transmit potent signals when required. Simultaneously, our understanding of the molecular basis on the recognition of mannosidic epitopes by mrCLRs including DC-SIGN, mannose receptor, mannose binding lectin and mrAb is rapidly advancing, together with the functional implications of these interactions in facilitating an effective immune response during physiological and pathophysiological conditions. Ultimately, deciphering these complex mannose-based receptor-ligand interactions at the detailed molecular level will significantly advance our understanding of immunological disorders and infectious diseases, promoting the development of future therapeutics to improve patient clinical outcomes.

Sialylation of neurites inhibits complement-mediated macrophage removal in a human macrophage-neuron Co-Culture System

The complement system has been implicated in the removal of dysfunctional synapses and neurites during development and in disease processes in the mouse, but it is unclear how far the mouse data can be transferred to humans. Here, we co-cultured macrophages derived from human THP1 monocytes and neurons derived from human induced pluripotent stem cells, to study the role of the complement system in a human model. Components of the complement system were expressed by the human macrophages and human neuronal culture, while receptors of the complement cascade were expressed by human macrophages as shown via gene transcript analysis and flow cytometry. We mimicked pathological conditions leading to an altered glycocalyx by treatment of human neurons with sialidases. Desialylated human neurites were opsonized by the complement component C1q. Furthermore, human neurites with an intact sialic acid cap remained untouched, while desialylated human neurites were removed and ingested by human macrophages. While blockage of the complement receptor 1 (CD35) had no effect, blockage of CD11b as part of the complement receptor 3 (CR3) reversed the effect on macrophage phagocytosis of desialylated human neurites. Data demonstrate that in the human system sialylation of the neuronal glycocalyx serves as an inhibitory flag for complement binding and CR3-mediated phagocytosis by macrophages.

Towards a liquid self: how time, geography, and life experiences reshape the biological identity

The conceptualization of immunological self is amongst the most important theories of modern biology, representing a sort of theoretical guideline for experimental immunologists, in order to understand how host constituents are ignored by the immune system (IS). A consistent advancement in this field has been represented by the danger/damage theory and its subsequent refinements, which at present represents the most comprehensive conceptualization of immunological self. Here, we present the new hypothesis of "liquid self," which integrates and extends the danger/damage theory. The main novelty of the liquid self hypothesis lies in the full integration of the immune response mechanisms into the host body's ecosystems, i.e., in adding the temporal, as well as the geographical/evolutionary and environmental, dimensions, which we suggested to call "immunological biography." Our hypothesis takes into account the important biological changes occurring with time (age) in the IS (including immunosenescence and inflammaging), as well as changes in the organismal context related to nutrition, lifestyle, and geography (populations). We argue that such temporal and geographical dimensions impinge upon, and continuously reshape, the antigenicity of physical entities (molecules, cells, bacteria, viruses), making them switching between "self" and "non-self" states in a dynamical, "liquid" fashion. Particular attention is devoted to oral tolerance and gut microbiota, as well as to a new potential source of unexpected self epitopes produced by proteasome splicing. Finally, our framework allows the set up of a variety of testable predictions, the most straightforward suggesting that the immune responses to defined molecules representing potentials antigens will be quantitatively and qualitatively quite different according to the immuno-biographical background of the host.

Increased sensitivity of Apolipoprotein E knockout mice to swainsonine dependent immunomodulation

The mechanisms that mediate accelerated atherosclerosis in autoimmune diseases remain unclear. One common mechanism that has been documented in autoimmune diseases and atherosclerosis is formation of hypoglycosyalted N-glycans on the cell surface. In this study we tested the effects of swainsonine, a class II α-mannosidase inhibitor which results in formation of hypoglycosylated N-glycans, on atherogenesis and immune cell dynamics in the atheroprone and hypercholesterolemic ApoE -/- mouse. Wild type or ApoE-/- mice (8 weeks of age) were fed a normal chow diet and administered swainsonine via the drinking water for 8 weeks at which time, atherosclerosis, and systemic markers of markers of inflammation were evaluated. Interestingly, no change in the rate of atherosclerosis development was observed in ApoE -/- mice treated with swainsonine. However, swainsonine significantly increased the number of peripheral blood leukocytes in ApoE -/- mice, with trends toward similar increases in swainsonine treated wild type mice noted. Assessment of leukocyte subsets using specific markers of all major blood lineages indicated that the increase in circulating leukocytes was due to the elevated number of progenitor cells. Consistent with swainsonine having a greater effect in ApoE -/- vs. wild type mice, increases in circulating inflammatory markers (IgA, IgG and chemokines) were observed in the former. Collectively, these data demonstrate that predisposition of ApoE -/- mice to vascular disease is associated with sensitization to the immunomodulatory effects of swainsonine and indicate that changes in N-glycans may provide a mechanism linking autoimmunity to atherogenesis.

Extracellular ATP and Toll-like receptor 2 agonists trigger in human monocytes an activation program that favors T helper 17

Strategically-paired Toll-like receptor (TLR) ligands induce a unique dendritic cell (DC) phenotype that polarizes Th1 responses. We therefore investigated pairing single TLR ligands with a non TLR-mediated danger signal to cooperatively induce distinct DC properties from cultured human monocytes. Adenosine triphosphate (ATP) and the TLR2 ligand lipoteichoic acid (LTA) selectively and synergistically induced expression of IL-23 and IL-1β from cultured monocytes as determined by ELISA assays. Flow cytometric analysis revealed that a sizable sub-population of treated cells acquired DC-like properties including activated surface phenotype with trans-well assays showing enhanced migration towards CCR7 ligands. Such activated cells also preferentially deviated, in an IL-23 and IL-1-dependent manner, CD4^pos T lymphocyte responses toward the IL-22^hi, IL-17^hi/IFN-γ^lo Th17 phenotype in standard in vitro allogeneic sensitization assays. Although pharmacological activation of either ionotropic or cAMP-dependent pathways acted in synergy with LTA to enhance IL-23, only inhibition of the cAMP-dependent pathway antagonized ATP-enhanced cytokine production. ATP plus atypical lipopolysaccharide from P. gingivalis (signaling through TLR2) was slightly superior to E. coli-derived LPS (TLR4 ligand) for inducing the high IL-23-secreting DC-like phenotype, but greatly inferior for inducing IL-12 p70 production when paired with IFN-γ, a distinction reflected in activated DCs’ ability to deviate lymphocytes toward Th1. Collectively, our data suggest TLR2 ligands encountered by innate immune cells in an environment with physiologically-relevant levels of extracellular ATP can induce a distinct activation state favoring IL-23- and IL-1β-dependent Th17 type response.

Role of endothelial N-glycan mannose residues in monocyte recruitment during atherogenesis

OBJECTIVE

Upregulated expression of endothelial adhesion molecules and subsequent binding to cognate monocytic receptors are established paradigms in atherosclerosis. However, these proteins are the scaffolds, with their posttranslational modification with sugars providing the actual ligands. We recently showed that tumor necrosis factor-α increased hypoglycosylated (mannose-rich) N-glycans on the endothelial surface. In the present study, our aim was to determine whether (1) hypoglycosylated N-glycans are upregulated by proatherogenic stimuli (oscillatory flow) in vitro and in vivo, and (2) mannose residues on hypoglycosylated endothelial N-glycans mediate monocyte rolling and adhesion.

METHODS AND RESULTS

Staining with the mannose-specific lectins concanavalin A and lens culinaris agglutinin was increased in human aortic endothelial cells exposed to oscillatory shear or tumor necrosis factor-α and at sites of plaque development and progression in both mice and human vessels. Increasing surface N-linked mannose by inhibiting N-glycan processing potentiated monocyte adhesion under flow during tumor necrosis factor-α stimulation. Conversely, enzymatic removal of high-mannose N-glycans, or masking mannose residues with lectins, significantly decreased monocyte adhesion under flow. These effects occurred without altering induced expression of adhesion molecule proteins.

CONCLUSIONS

Hypoglycosylated (high mannose) N-glycans are present on the endothelial cell surface at sites of early human lesion development and are novel effectors of monocyte adhesion during atherogenesis.

Microglial activatory (immunoreceptor tyrosine-based activation motif)- and inhibitory (immunoreceptor tyrosine-based inhibition motif)-signaling receptors for recognition of the neuronal glycocalyx

Microglia sense intact or lesioned cells of the central nervous system (CNS) and respond accordingly. To fulfill this task, microglia express a whole set of recognition receptors. Fc receptors and DAP12 (TYROBP)-associated receptors such as microglial triggering receptor expressed on myeloid cells-2 (TREM2) and the complement receptor-3 (CR3, CD11b/CD18) trigger the immunoreceptor tyrosine-based activation motif (ITAM)-signaling cascade, resulting in microglial activation, migration, and phagocytosis. Those receptors are counter-regulated by immunoreceptor tyrosine-based inhibition motif (ITIM)-signaling receptors, such as sialic acid-binding immunoglobulin superfamily lectins (Siglecs). Siglecs recognize the sialic acid cap of healthy neurons thus leading to an ITIM signaling that turns down microglial immune responses and phagocytosis. In contrast, desialylated neuronal processes are phagocytosed by microglial CR3 signaling via an adaptor protein containing an ITAM. Thus, the aberrant terminal glycosylation of neuronal surface glycoproteins and glycolipids could serve as a flag for microglia, which display a multitude of diverse carbohydrate-binding receptors that monitor the neuronal physical condition and respond via their ITIM- or ITAM-signaling cascade accordingly.

Cross-reactive antibodies to target proteins are dependent upon oligomannose glycosylated epitopes in HTLV-1 associated neurological disease

Our lab recently identified a cross-reactive antibody response between human T-lymphotropic virus type-1-p24-(gag) (HTLV-1-p24-(gag)) and peroxiredoxin-1 (PrX-1) as potentially contributing to the pathogenesis of HTLV-1 associated neurological disease via molecular mimicry. These targets proteins were glycosylated, yet the glycan side chains immunoreactive with the immunoglobulins were unknown. Using a combination of lectin isolation and serial enzymatic deglycosylation of glycoproteins, we determined that the immunoreactive epitopes contained branched oligomannose side chains. These data suggest that post-translational glycosylation specifically related to oligomannose immunoreactivity to both the infecting and host antigens may contribute to molecular mimicry and be important in the pathogenesis of HTLV-1 associated neurological disease.

Endothelial surface N-glycans mediate monocyte adhesion and are targets for anti-inflammatory effects of peroxisome proliferator-activated receptor γ ligands

Endothelial-monocyte interactions are regulated by adhesion molecules and key in the development of vascular inflammatory disease. Peroxisome proliferator-activated receptor (PPAR) γ activation in endothelial cells is recognized to mediate anti-inflammatory effects that inhibit monocyte rolling and adhesion. Herein, evidence is provided for a novel mechanism for the anti-inflammatory effects of PPARγ ligand action that involves inhibition of proinflammatory cytokine-dependent up-regulation of endothelial N-glycans. TNFα treatment of human umbilical vein endothelial cells increased surface expression of high mannose/hybrid N-glycans. A role for these sugars in mediating THP-1 or primary human monocyte rolling and adhesion was indicated by competition studies in which addition of α-methylmannose, but not α-methylglucose, inhibited monocyte rolling and adhesion during flow, but not under static conditions. This result supports the notion that adhesion molecules provide scaffolds for sugar epitopes to mediate adhesion with cognate receptors. A panel of structurally distinct PPARγ agonists all decreased TNFα-dependent expression of endothelial high mannose/hybrid N-glycans. Using rosiglitazone as a model PPARγ agonist, which decreased TNFα-induced high mannose N-glycan expression, we demonstrate a role for these carbohydrate residues in THP-1 rolling and adhesion that is independent of endothelial surface adhesion molecule expression (ICAM-1 and E-selectin). Data from N-glycan processing gene arrays identified α-mannosidases (MAN1A2 and MAN1C1) as targets for down-regulation by TNFα, which was reversed by rosiglitazone, a result consistent with altered high mannose/hybrid N-glycan epitopes. Taken together we propose a novel anti-inflammatory mechanism of endothelial PPARγ activation that involves targeting protein post-translational modification of adhesion molecules, specifically N-glycosylation.