The origin and function of platelet glycosyltransferases

When a negative (charge) is not a positive: sialylation and its role in cancer mechanics and progression

Sialic acids and sialoglycans are critical actors in cancer progression and metastasis. These terminal sugar residues on glycoproteins and glycolipids modulate key cellular processes such as immune evasion, cell adhesion, and migration. Aberrant sialylation is driven by overexpression of sialyltransferases, resulting in hypersialylation on cancer cell surfaces as well as enhancing tumor aggressiveness. Sialylated glycans alter the structure of the glycocalyx, a protective barrier that fosters cancer cell detachment, migration, and invasion. This bulky glycocalyx also increases membrane tension, promoting integrin clustering and downstream signaling pathways that drive cell proliferation and metastasis. They play a critical role in immune evasion by binding to Siglecs, inhibitory receptors on immune cells, which transmit signals that protect cancer cells from immune-mediated destruction. Targeting sialylation pathways presents a promising therapeutic opportunity to understand the complex roles of sialic acids and sialoglycans in cancer mechanics and progression, which is crucial for developing novel diagnostic and therapeutic strategies that can disrupt these processes and improve cancer treatment outcomes.

Proteolytic cleavage of Golgi glycosyltransferases by SPPL3 and other proteases and its implications for cellular glycosylation

Glycosylation of proteins and lipids is of fundamental importance in multicellular eukaryotes. The vast diversity of glycan structures observed is generated in the Golgi apparatus by the concerted activity of >100 distinct enzymes, which include glycosyltransferases and other glycan-modifying enzymes. Well-known for decades, the majority of these enzymes is released from the Golgi apparatus and subsequently secreted into the extracellular space following endoproteolytic cleavage, but the underlying molecular mechanisms and the physiological implications have remained unexplored. This review will summarize our current knowledge of Golgi enzyme proteolysis and secretion and will discuss its conceptual implications for the regulation of cellular glycosylation and the organization of the Golgi apparatus. A particular focus will lie on the intramembrane protease SPPL3, which recently emerged as key protease facilitating Golgi enzyme release and has since been shown to affect a multitude of glycosylation-dependent physiological processes.

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

Residence of the Nucleotide Sugar Transporter Family Members SLC35F1 and SLC35F6 in the Endosomal/Lysosomal Pathway

The SLC35 (Solute Carrier 35) family members acting as nucleotide sugar transporters are typically localized in the endoplasmic reticulum or Golgi apparatus. It is, therefore, intriguing that some reports document the presence of orphan transporters SLC35F1 and SLC35F6 within the endosomal and lysosomal system. Here, we compared the subcellular distribution of these proteins and found that they are concentrated in separate compartments; i.e., recycling endosomes for SLC35F1 and lysosomes for SLC35F6. Swapping the C-terminal tail of these proteins resulted in a switch of localization, with SLC35F1 being trafficked to lysosomes while SLC35F6 remained in endosomes. This suggested the presence of specific sorting signals in these C-terminal regions. Using site-directed mutagenesis, fluorescence microscopy, and cell surface biotinylation assays, we found that the EQERLL360 signal located in the cytoplasmic tail of human SLC35F6 is involved in its lysosomal sorting (as previously shown for this conserved sequence in mouse SLC35F6), and that SLC35F1 localization in the recycling pathway depends on two YXXΦ-type signals: a Y367KQF sequence facilitates its internalization from the plasma membrane, while a Y392TSL motif prevents its transport to lysosomes, likely by promoting SLC35F1 recycling to the cell surface. Taken together, these results support that some SLC35 members may function at different levels of the endosomal and lysosomal system.

Evolution of the ribbon-like organization of the Golgi apparatus in animal cells

The "ribbon," a structural arrangement in which Golgi stacks connect to each other, is considered to be restricted to vertebrate cells. Although ribbon disruption is linked to various human pathologies, its functional role in cellular processes remains unclear. In this study, we investigate the evolutionary origin of the Golgi ribbon. We observe a ribbon-like architecture in the cells of several metazoan taxa suggesting its early emergence in animal evolution predating the appearance of vertebrates. Supported by AlphaFold2 modeling, we propose that the evolution of Golgi reassembly and stacking protein (GRASP) binding by golgin tethers may have driven the joining of Golgi stacks resulting in the ribbon-like configuration. Additionally, we find that Golgi ribbon assembly is a shared developmental feature of deuterostomes, implying a role in embryogenesis. Overall, our study points to the functional significance of the Golgi ribbon beyond vertebrates and underscores the need for further investigations to unravel its elusive biological roles.

Platelet lifespan and mechanisms for clearance

PURPOSE OF REVIEW

Activated or aged platelets are removed from circulation under (patho)physiologic conditions, the exact mechanism of platelet clearance under such conditions remains unclear and are currently being investigated. This review focuses on recent findings and controversies regarding platelet clearance and the disruption of platelet life cycle.

RECENT FINDINGS

The platelet life span is determined by glycosylation of platelet surface receptors with sialic acid. Recently, it was shown that platelet activation and granule release leads to desialylation of glycans and accelerated clearance of platelets under pathological conditions. This phenomenon was demonstrated to be a main reason for thrombocytopenia being a complication in several infections and immune disorders.

SUMMARY

Although we have recently gained some insight into how aged platelets are cleared from circulation, we are still not seeing the full picture. Further investigations of the platelet clearance pathways under pathophysiologic conditions are needed as well as studies to unravel the connection between platelet clearance and platelet production.

Platelet-localized ST6Gal1 does not impact IgG sialylation

The IgG antibody class forms an important basis of the humoral immune response, conferring reciprocal protection from both pathogens and autoimmunity. IgG function is determined by the IgG subclass, as defined by the heavy chain, as well as the glycan composition at N297, the conserved site of N-glycosylation within the Fc domain. For example, lack of core fucose promotes increased antibody-dependent cellular cytotoxicity, whereas α2,6-linked sialylation by the enzyme ST6Gal1 helps to drive immune quiescence. Despite the immunological significance of these carbohydrates, little is known about how IgG glycan composition is regulated. We previously reported that mice with ST6Gal1-deficient B cells have unaltered IgG sialylation. Likewise, ST6Gal1 released into the plasma by hepatocytes does not significantly impact overall IgG sialylation. Since IgG and ST6Gal1 have independently been shown to exist in platelet granules, it was possible that platelet granules could serve as a B cell-extrinsic site for IgG sialylation. To address this hypothesis, we used a platelet factor 4 (Pf4)-Cre mouse to delete ST6Gal1 in megakaryocytes and platelets alone or in combination with an albumin-Cre mouse to also remove it from hepatocytes and the plasma. The resulting mouse strains were viable and had no overt pathological phenotype. We also found that despite targeted ablation of ST6Gal1, no change in IgG sialylation was apparent. Together with our prior findings, we can conclude that in mice, neither B cells, the plasma, nor platelets have a substantial role in homeostatic IgG sialylation.

Platelet protein synthesis, regulation, and post-translational modifications: mechanics and function

Dogma had been firmly entrenched in the minds of the scientific community that the anucleate mammalian platelet was incapable of protein biosynthesis since their identification in the late 1880s. These beliefs were not challenged until the 1960s when several reports demonstrated that platelets possessed the capacity to biosynthesize proteins. Even then, many still dismissed the synthesis as trivial and unimportant for at least another two decades. Research in the field expanded after the 1980s and numerous reports have since been published that now clearly demonstrate the potential significance of platelet protein synthesis under normal, pathological, and activating conditions. It is now clear that the platelet proteome is not a static entity but can be altered slowly or rapidly in response to external signals to support physiological requirements to maintain hemostasis and other biological processes. All the necessary biological components to support protein synthesis have been identified in platelets along with post-transcriptional processing of mRNAs, regulators of translation, and post-translational modifications such as glycosylation. The last comprehensive review of the subject appeared in 2009 and much work has been conducted since that time. The current review of the field will briefly incorporate the information covered in earlier reviews and then bring the reader up to date with more recent findings.

Glycosylation-Engineered Platelet Membrane-Coated Interleukin 10 Nanoparticles for Targeted Inhibition of Vascular Restenosis

Purpose

The purpose of this study was to improve the immune compatibility and targeting abilities of IL10 nanoparticles coated with platelet membrane (IL10-PNPs) by glycosylation engineering in order to effectively reduce restenosis after vascular injury.

Materials and Methods

In this study, we removed sialic acids and added α (1,2)-fucose and α (1,3)-fucose to platelet membrane glycoprotein, thus engineering the glycosylation of IL10-PNPs (IL10-GE-PNPs). In vitro and in vivo experiments were conducted to evaluate the targeting and regulatory effects of IL10-GE-PNPs on macrophage polarization, as well as the influence of IL10-GE-PNPs on the phenotypic transformation, proliferation, and migration of smooth muscle cells, and its potential in promoting the repair function of endothelial cells within an inflammatory environment. In order to assess the distribution of IL10-GE-PNP in different organs, in vivo imaging experiments were conducted.

Results

IL10-GE-PNPs were successfully constructed and demonstrated to effectively target and regulate macrophage polarization in both in vitro and in vivo settings. This regulation resulted in reduced proliferation and migration of smooth muscle cells and promoted the repair of endothelial cells in an inflammatory environment. Consequently, restenosis after vascular injury was reduced. Furthermore, the deposition of IL10-GE-PNPs in the liver and spleen was significantly reduced compared to IL10-PNPs.

Conclusion

IL10-GE-PNPs emerged as a promising candidate for targeting vascular injury and exhibited potential as an innovative drug delivery system for suppressing vascular restenosis. The engineered glycosylation of IL10-PNPs improved their immune compatibility and targeting abilities, making them an excellent therapeutic option.

The intracellular and plasma membrane pools of phosphatidylinositol-4-monophosphate control megakaryocyte maturation and proplatelet formation

Background

Megakaryocytes (MKs) develop from hematopoietic stem cells after stimulation by the cytokine thrombopoietin. During megakaryopoiesis, MKs enlarge, undergo the process of endomitosis, and develop intracellular membranes (demarcation membrane system, DMS). During DMS formation, there is active transport of proteins, lipids, and membranes from the Golgi apparatus to the DMS. The most important phosphoinositide that controls anterograde transport from the Golgi apparatus to the plasma membrane (PM) is phosphatidylinositol-4-monophosphate (PI4P), whose levels are controlled by suppressor of actin mutations 1-like protein (Sac1) phosphatase at the Golgi and endoplasmic reticulum.

Objectives

Here we investigated the role of Sac1 and PI4P in megakaryopoiesis.

Methods

We analyzed Sac1 and PI4P localization in primary MKs derived from fetal liver or bone marrow and in the DAMI cell line by immunofluorescence. The intracellular and PM pools of PI4P in primary MKs were modulated by expression of Sac1 constructs from retroviral vector and inhibition of PI4 kinase IIIα, respectively.

Results

We showed that in primary mouse MKs, PI4P is mostly found in the Golgi apparatus and the PM in immature MKs, while in mature MKs, it is found in the cell periphery and at the PM. The exogenous expression of wild-type but not C389S mutant (catalytically dead) Sac1 results in the perinuclear retention of the Golgi apparatus resembling immature MKs, with decreased ability to form proplatelets. The pharmacologic inhibition of PI4P production specifically at the PM also resulted in a significant decrease in MKs that form proplatelets.

Conclusion

These results indicate that both intracellular and PM pools of PI4P mediate MK maturation and proplatelet formation.

Reduced platelet glycoprotein Ibα shedding accelerates thrombopoiesis and COX-1 recovery: implications for aspirin dosing regimen

Cardiovascular (CV) disease prevention with low-dose aspirin can be less effective in patients with a faster recovery of platelet (PLT) cyclooxygenase (COX)-1 activity during the 24-hour dosing interval. We previously showed that incomplete suppression of TXA2 over 24 hours can be rescued by a twice daily aspirin regimen. Here we show that reduced PLT glycoprotein (GP)Ibα shedding characterizes patients with accelerated COX-1 recovery and may contribute to higher thrombopoietin (TPO) production and higher rates of newly formed PLT, escaping aspirin inhibition over 24 hours. Two hundred aspirin-treated patients with high CV risk (100 with type 2 diabetes mellitus) were stratified according to the kinetics of PLT COX-1 activity recovery during the 10- to 24-hour dosing interval. Whole proteome analysis showed that PLT from patients with accelerated COX-1 recovery were enriched in proteins involved in cell survival, inhibition of apoptosis and cellular protrusion formation. In agreement, we documented increased plasma TPO, megakaryocyte maturation and proplatelet formation, and conversely increased PLT galactose and reduced caspase 3, phosphatidylserine exposure and ADAM17 activation, translating into diminished GPIbα cleavage and glycocalicin (GC) release. Treatment of HepG2 cells with recombinant GC led to a dose-dependent reduction of TPO mRNA in the liver, suggesting that reduced GPIbα ectodomain shedding may unleash thrombopoiesis. A cluster of clinical markers, including younger age, non-alcoholic fatty liver disease, visceral obesity and higher TPO/GC ratio, predicted with significant accuracy the likelihood of faster COX-1 recovery and suboptimal aspirin response. Circulating TPO/GC ratio, reflecting a dysregulation of PLT lifespan and production, may provide a simple tool to identify patients amenable to more frequent aspirin daily dosing.

Inherited Thrombocytopenia Caused by Variants in Crucial Genes for Glycosylation

Protein glycosylation, including sialylation, involves complex and frequent post-translational modifications, which play a critical role in different biological processes. The conjugation of carbohydrate residues to specific molecules and receptors is critical for normal hematopoiesis, as it favors the proliferation and clearance of hematopoietic precursors. Through this mechanism, the circulating platelet count is controlled by the appropriate platelet production by megakaryocytes, and the kinetics of platelet clearance. Platelets have a half-life in blood ranging from 8 to 11 days, after which they lose the final sialic acid and are recognized by receptors in the liver and eliminated from the bloodstream. This favors the transduction of thrombopoietin, which induces megakaryopoiesis to produce new platelets. More than two hundred enzymes are responsible for proper glycosylation and sialylation. In recent years, novel disorders of glycosylation caused by molecular variants in multiple genes have been described. The phenotype of the patients with genetic alterations in GNE, SLC35A1, GALE and B4GALT is consistent with syndromic manifestations, severe inherited thrombocytopenia, and hemorrhagic complications.

Circulating platelets supply ST6Gal-1 in patients with IgA nephropathy

BACKGROUND

Our previous study showed ST6 β-galactoside α2,6-sialyltransferase 1 (ST6Gal-1) levels in plasma were associated with a slower progression of IgA nephropathy (IgAN). Platelets are the crucial regulator of cell surface glycosylation events in circulation by supplying glycosyltransferases.

METHODS

A total of 180 patients with IgAN were included in this study. ST6Gal-1 levels were analyzed before and after activation of platelets by flow cytometry.

RESULTS

We found that IgAN patients in the higher platelet counts group exhibited higher levels of ST6Gal-1 compared with the lower platelet counts group. There was a positive correlation between platelet counts and ST6Gal-1 levels in plasma. Patients with higher platelet counts had higher levels of IgA, serum C3, serum C4 and proteinuria, higher percentages of platelet crits, S1 and T1/2, lower levels of platelet distribution width and the mean platelet volume, as well as a lower percentage of platelet large cell ratio compared with those patients with lower platelet counts. No differences were found in terms of the eGFR decline and composite kidney endpoints between two groups. Furthermore, we investigated whether platelets were activated and released ST6Gal-1 in patients with IgAN. The expression of CD62P in platelets in patients with IgAN was higher than those of healthy controls. There were no obvious changes in ST6Gal-1 levels between the rest and the activated platelets within 1 to 2-hour, however, the difference in ST6Gal-1 levels became more pronounced after 4-hour of incubation.

CONCLUSIONS

In conclusion, human circulating platelets contain ST6Gal-1, which may be released by the activation of platelets in IgAN.

Comparıson of the effect of the autogenıc and xenogenıc use of platelet-rıch plasma on rabbıt chondrocutaneous composıte graft survıval

The platelet-rich plasma (PRP) has become popular in the medical world due to its content of growth factors and numerous studies are experimental. In experimental studies, the preparation and application of PRP are problematic and allogenic PRP transfers have been preffered, because of the difficulties in preparation of autogenic PRP in animal experiments. Xenogenic transfers and their effects have not been studied in this topic. This study aimed to investigate the effect of autogenic and xenogenic use of PRP on composite graft viability.Methods: Two composite grafts are prepared for each ear of nine rabbits. Each ear was randomly divided into three groups. After the procedure, the wound edges and base were injected with 1 cc serum physiologic, autogenic PRP or 1 cc human-derived xenogenic PRP. At 3 weeks, samples were taken, photographic and histopathological evaluations were made.Results: The graft viability was better in autogenic and xenogenic group compared to the control group. In comprasion of autogenic and xenogenic groups, although the macroscopic evaluation revealed better graft viability and less necrosis in the group which had been treated with autogenic PRP, the difference was not statistically significant. The three groups did not significantly differ in terms of inflammation. Vascularization examined histopathologically. CD31 staining, which was used to evaluate angiogenesis, was significantly higher in the autogenic PRP group than the remaining two groups.Conclusion: Although autogenic PRP has better results histopathologically, the xenogenic use of PRP may be an alternative for studies, when macroscopic evaluation is necessary.

ST6Gal1 in plasma is dispensable for IgG sialylation

The glycosylation of IgG has attracted increased attention due to the impact of N-glycan modifications at N297 on IgG function, acting primarily through modulation of Fc domain conformation and Fcγ receptor binding affinities and signaling. However, the mechanisms regulating IgG glycosylation and especially α2,6-sialylation of its N-glycan remain poorly understood. We observed previously that IgG is normally sialylated in mice with B cells lacking the sialyltransferase ST6Gal1. This supported the hypothesis that IgG may be sialylated outside of B cells, perhaps through the action of hepatocyte-released plasma ST6Gal1. Here we demonstrate that this model is incorrect. Animals lacking hepatocyte expressed ST6Gal1 retain normal IgG α2,6-sialylation, despite the lack of detectable ST6Gal1 in plasma. Moreover, we confirmed that B cells were not a redundant source of IgG sialylation. Thus, while α2,6-sialylation is lacking in IgG from mice with germline ablation of ST6Gal1, IgG α2,6-sialylation is normal in mice lacking ST6Gal1 in either hepatocytes or B cells. These results indicate that IgG α2,6-sialylation arises after release from a B cell, but is not dependent on plasma-localized ST6Gal1 activity.

Overlapping and unique substrate specificities of ST3GAL1 and 2 during hematopoietic and megakaryocytic differentiation

ST3GAL1 and ST3GAL2 have both overlapping and unique substrate specificities in O-glycan sialylation during megakaryopoiesis.

O-glycan sialylation is dispensable for MK production but indispensable for MK proplatelet formation.

Although the sialyltransferases ST3GAL1 and ST3GAL2 are known to transfer sialic acid to the galactose residue of type III disaccharides (Galβ1,3GalNAc) in vitro, sialylation of O-linked glycosylated proteins in living cells has been largely attributed to ST3GAL1. To examine the role of ST3GAL2 in O-sialylation, we examined its expression during differentiation of human-induced pluripotent stem cells (iPSCs) into hematopoietic progenitor cells (HPCs) and megakaryocytes (MKs). ST3GAL1 and ST3GAL2 each became highly expressed during the differentiation of iPSCs to HPCs but decreased markedly in their expression upon differentiation into MKs, suggesting coordination of expression during megakaryopoiesis. To further delineate their role in these processes, we generated ST3GAL1-, ST3GAL2-, and doubly deficient human iPSC lines. Binding of the peanut agglutinin lectin, which reports the presence of unsialylated Galβ1,3GalNAc glycan chains, was strongly increased in HPCs and MKs derived from double-knockout iPSCs and remained moderately increased in cells lacking either one of these sialyltransferases, demonstrating that both can serve as functional cellular O-glycan sialyltransferases. Interestingly, the HPC markers CD34 and CD43, as well as MK membrane glycoprotein (GP) GPIbα, were identified as major GP substrates for ST3GAL1 and ST3GAL2. In contrast, O-sialylation of GPIIb relied predominantly on the expression of ST3GAL2. Finally, although disruption of ST3GAL1 and ST3GAL2 had little impact on MK production, their absence resulted in dramatically impaired MK proplatelet formation. Taken together, these data establish heretofore unknown physiological roles for ST3GAL1 and ST3GAL2 in O-linked glycan sialylation in hemato- and megakaryocytopoiesis.

N-terminome analyses underscore the prevalence of SPPL3-mediated intramembrane proteolysis among Golgi-resident enzymes and its role in Golgi enzyme secretion

Golgi membrane proteins such as glycosyltransferases and other glycan-modifying enzymes are key to glycosylation of proteins and lipids. Secretion of soluble Golgi enzymes that are released from their membrane anchor by endoprotease activity is a wide-spread yet largely unexplored phenomenon. The intramembrane protease SPPL3 can specifically cleave select Golgi enzymes, enabling their secretion and concomitantly altering global cellular glycosylation, yet the entire range of Golgi enzymes cleaved by SPPL3 under physiological conditions remains to be defined. Here, we established isogenic SPPL3-deficient HEK293 and HeLa cell lines and applied N-terminomics to identify substrates cleaved by SPPL3 and released into cell culture supernatants. With high confidence, our study identifies more than 20 substrates of SPPL3, including entirely novel substrates. Notably, our N-terminome analyses provide a comprehensive list of SPPL3 cleavage sites demonstrating that SPPL3-mediated shedding of Golgi enzymes occurs through intramembrane proteolysis. Through the use of chimeric glycosyltransferase constructs we show that transmembrane domains can determine cleavage by SPPL3. Using our cleavage site data, we surveyed public proteome data and found that SPPL3 cleavage products are present in human blood. We also generated HEK293 knock-in cells expressing the active site mutant D271A from the endogenous SPPL3 locus. Immunoblot analyses revealed that secretion of select novel substrates such as the key mucin-type O-glycosylation enzyme GALNT2 is dependent on endogenous SPPL3 protease activity. In sum, our study expands the spectrum of known physiological substrates of SPPL3 corroborating its significant role in Golgi enzyme turnover and secretion as well as in the regulation of global glycosylation pathways.

Cytokines in the Immune Microenvironment Change the Glycosylation of IgG by Regulating Intracellular Glycosyltransferases

Background

Changes in IgG glycosylation, as a novel pathological feature, are observed in various autoimmune diseases (AIDs). The glycosylation patterns of IgG play a critical role in regulating the biological function and stability of IgG involved in the pathophysiology of many AIDs. However, the intracellular regulatory mechanisms underlying the effects of disturbances in various cytokines on IgG glycosylation are poorly understood. Thus, we investigated the regulatory effects of elevated cytokines in AIDs on intracellular IgG glycosylation within B cells.

Methods

First, we established a controlled primary culture system in vitro to differentiate human CD19+ B cells into antibody-secreting cells (ASCs). Then, the IgG concentrations in the supernatants were measured by enzyme-linked immunoassay (ELISA) under IFN-γ, TNF-α, IL-21, IL-17A, BAFF, or APRIL stimulation. Next, the glycosylation levels of IgG under different stimuli were compared via a lectin microarray. The fine carbohydrate structures of IgG were confirmed by matrix-assisted laser desorption/ionization-quadrupole ion trap-time of flight-mass spectrometry (MALDI-TOF-MS). Finally, the expression of glycosyltransferases and glycosidases in B cells under stimulation with several cytokines was detected by real-time PCR and western blotting.

Results

We found that cytokines significantly promoted IgG production in vitro and led to considerably different IgG glycan patterns. Specifically, the results of lectin microarray showed the galactose level of IgG was increased by IFN-γ stimulation (p<0.05), and the sialylation of IgG was increased by IL-21 and IL-17A (p<0.05). The MALDI-TOF-MS data showed that the frequency of agalactosylation was decreased by IFN-γ with the increased frequency of mono-galactosylation and decreased frequency of digalactosylation, accompanied by upregulation of β-1,4-galactosyltransferase 1. Both frequencies of mono-sialylated and disialylated N-glycans were increased by IL-21 and IL-17A with decreased frequency of asialylation, and the expression of β-galactoside α-2,6-sialyltransferase 1 was upregulated by IL-21 and IL-17A.

Conclusion

Abnormally elevated cytokines in the microenvironment regulates IgG glycan patterns by regulating intracellular glycosyltransferases in human B cells.

Imaging of Intracellular and Plasma Membrane Pools of PI(4,5)P2 and PI4P in Human Platelets

Phosphoinositides (PIs) are phosphorylated membrane lipids that have a plethora of roles in the cell, including vesicle trafficking, signaling, and actin reorganization. The most abundant PIs in the cell are phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] and phosphatidylinositol-4-monophosphate (PI4P). The localization and roles of both PI(4,5)P2 and PI4P are well established, is the broadly accepted methodological approach for their immunocytochemical visualization in different cell compartments in several cell lines. However, not much is known about these PIs in platelets (PLTs), the smallest blood cells that detect vessel wall injury, activate, and stop the bleeding. Therefore, we sought to investigate the localization of PI(4,5)P2 and PI4P in resting and activated PLTs by antibody staining. Here, we show that the intracellular pools of PI(4,5)P2 and PI4P can be detected by the established staining protocol, and these pools can be modulated by inhibitors of OCRL phosphatase and PI4KIIIα kinase. However, although resting PLTs readily stain for the plasma membrane (PM) pools of PI(4,5)P2 and PI4P, just a few activated cells were stained with the established protocol. We show that optimized protocol allows for the visualization of PI(4,5)P2 and PI4P at PM in activated PLTs, which could also be modulated by OCRL and PI4KIIIα inhibitors. We conclude that PI(4,5)P2 and PI4P are more sensitive to lipid extraction by permeabilizing agents in activated than in resting human PLTs, which suggests their different roles during PLT activation.

Estrogen-Driven Changes in Immunoglobulin G Fc Glycosylation

Glycosylation within the immunoglobulin G (IgG) Fc region modulates its ability to engage complement and Fc receptors, affording the opportunity to fine-tune effector functions. Mechanisms regulating IgG Fc glycans remain poorly understood. Changes accompanying menarche, menopause, and pregnancy have long implicated hormonal factors. Intervention studies now confirm that estrogens enhance IgG Fc galactosylation, in females and also in males, defining the first pathway modulating Fc glycans and thereby a new link between sex and immunity. This mechanism may participate in fetal-maternal immunity, antibody-mediated inflammation, and other aspects of age- and sex-specific immune function. Here we review the changes affecting the IgG Fc glycome from childhood through old age, the evidence establishing a role for estrogens, and research directions to uncover associated mechanisms that may inform therapeutic intervention.

The Importance of Platelet Glycoside Residues in the Haemostasis of Patients with Immune Thrombocytopaenia

Loss of sialic acid from the carbohydrate side chains of platelet glycoproteins can affect platelet clearance, a proposed mechanism involved in the etiopathogenesis of immune thrombocytopaenia (ITP). We aimed to assess whether changes in platelet glycosylation in patients with ITP affected platelet counts, function, and apoptosis. This observational, prospective, and transversal study included 82 patients with chronic primary ITP and 115 healthy controls. We measured platelet activation markers and assayed platelet glycosylation and caspase activity, analysing samples using flow cytometry. Platelets from patients with ITP with a platelet count <30 × 10³/µL presented less sialic acid. Levels of α1,6-fucose (a glycan residue that can directly regulate antibody-dependent cellular cytotoxicity) and α-mannose (which can be recognised by mannose-binding-lectin and activate the complement pathway) were increased in the platelets from these patients. Platelet surface exposure of other glycoside residues due to sialic acid loss inversely correlated with platelet count and the ability to be activated. Moreover, loss of sialic acid induced the ingestion of platelets by human hepatome HepG2 cells. Changes in glycoside composition of glycoproteins on the platelets’ surface impaired their functional capacity and increased their apoptosis. These changes in platelet glycoside residues appeared to be related to ITP severity.

Slc35a1 deficiency causes thrombocytopenia due to impaired megakaryocytopoiesis and excessive platelet clearance in the liver

Sialic acid is a common terminal residue of glycans on proteins and acidic sphingolipids such as gangliosides and has important biological functions. The sialylation process is controlled by more than 20 different sialyltransferases, many of which exhibit overlapping functions. Thus, it is difficult to determine the overall biological function of sialylation by targeted deletion of individual sialyltransferases. To address this issue, we established a mouse line with the Slc35a1 gene flanked by loxP sites. Slc35a1 encodes the cytidine-5’-monophosphate (CMP)-sialic acid transporter that transports CMP-sialic acid from the cytoplasm into the Golgi apparatus for sialylation. Here we report our study regarding the role of sialylation on megakaryocytes and platelets using a mouse line with significantly reduced sialylation in megakaryocytes and platelets (Plt Slc35a1– /–). The major phenotype of Plt Slc35a1–/– mice was thrombocytopenia. The number of bone marrow megakaryocytes in Plt Slc35a1–/– mice was reduced, and megakaryocyte maturation was also impaired. In addition, an increased number of desialylated platelets was cleared by Küpffer cells in the liver of Plt Slc35a1–/– mice. This study provides new insights into the role of sialylation in platelet homeostasis and the mechanisms of thrombocytopenia in diseases associated with platelet desialylation, such as immune thrombocytopenia and a rare congenital disorder of glycosylation (CDG), SLC35A1-CDG, which is caused by SLC35A1 mutations.

Sweet Rules: Linking Glycosylation to Antibody Function

Antibodies produced upon infections with pathogenic microorganisms are essential for clearing primary infections and for providing the host with long-lasting immunity. Moreover, antibodies have become the most widely used platform for developing novel therapies against cancer and autoimmunity, requiring an in-depth understanding of how antibodies mediate their activity in vivo and which factors modulate pro- or anti-inflammatory antibody activities. Since the discovery that select residues present in the sugar domain attached to the immunoglobulin G (IgG) fragment crystallizable (Fc) region can modulate both, pro- and anti-inflammatory effector functions, a wealth of studies has focused on understanding how IgG glycosylation is regulated and how this knowledge can be used to optimize therapeutic antibody activity. With the introduction of glycoengineered afucosylated antibodies in cancer therapy and the initiation of clinical testing of highly sialylated anti-inflammatory antibodies the proof-of-concept that understanding antibody glycosylation can lead to clinical innovation has been provided. The focus of this review is to summarize recent insights into how antibody glycosylation is regulated in vivo and how select sugar residues impact IgG function.

Immunoglobulin G Glycosylation Changes in Aging and Other Inflammatory Conditions

Among the multiple roles played by protein glycosylation, the fine regulation of biological interactions is one of the most important. The asparagine 297 (Asn₂₉₇) of IgG heavy chains is decorated by a diantennary glycan bearing a number of galactose and sialic acid residues on the branches ranging from 0 to 2. In addition, the structure can present core-linked fucose and/or a bisecting GlcNAc. In many inflammatory and autoimmune conditions, as well as in metabolic, cardiovascular, infectious, and neoplastic diseases, the IgG Asn₂₉₇-linked glycan becomes less sialylated and less galactosylated, leading to increased expression of glycans terminating with GlcNAc. These conditions alter also the presence of core-fucose and bisecting GlcNAc. Importantly, similar glycomic alterations are observed in aging. The common condition, shared by the above-mentioned pathological conditions and aging, is a low-grade, chronic, asymptomatic inflammatory state which, in the case of aging, is known as inflammaging. Glycomic alterations associated with inflammatory diseases often precede disease onset and follow remission. The aberrantly glycosylated IgG glycans associated with inflammation and aging can sustain inflammation through different mechanisms, fueling a vicious loop. These include complement activation, Fcγ receptor binding, binding to lectin receptors on antigen-presenting cells, and autoantibody reactivity. The complex molecular bases of the glycomic changes associated with inflammation and aging are still poorly understood.

Glycosylation and Aging

Human lifespan has increased significantly in the last 200 years, emphasizing our need to age healthily. Insights into molecular mechanisms of aging might allow us to slow down its rate or even revert it. Similar to aging, glycosylation is regulated by an intricate interplay of genetic and environmental factors. The dynamics of glycopattern variation during aging has been mostly explored for plasma/serum and immunoglobulin G (IgG) N-glycome, as we describe thoroughly in this chapter. In addition, we discuss the potential functional role of agalactosylated IgG glycans in aging, through modulation of inflammation level, as proposed by the concept of inflammaging. We also comment on the potential to use the plasma/serum and IgG N-glycome as a biomarker of healthy aging and on the interventions that modulate the IgG glycopattern. Finally, we discuss the current knowledge about animal models for human plasma/serum and IgG glycosylation and mention other, less explored, instances of glycopattern changes during organismal aging and cellular senescence.

Mucin-Type O-GalNAc Glycosylation in Health and Disease

Mucin-type GalNAc O-glycosylation is one of the most abundant and unique post-translational modifications. The combination of proteome-wide mapping of GalNAc O-glycosylation sites and genetic studies with knockout animals and genome-wide analyses in humans have been instrumental in our understanding of GalNAc O-glycosylation. Combined, such studies have revealed well-defined functions of O-glycans at single sites in proteins, including the regulation of pro-protein processing and proteolytic cleavage, as well as modulation of receptor functions and ligand binding. In addition to isolated O-glycans, multiple clustered O-glycans have an important function in mammalian biology by providing structural support and stability of mucins essential for protecting our inner epithelial surfaces, especially in the airways and gastrointestinal tract. Here the many O-glycans also provide binding sites for both endogenous and pathogen-derived carbohydrate-binding proteins regulating critical developmental programs and helping maintain epithelial homeostasis with commensal organisms. Finally, O-glycan changes have been identified in several diseases, most notably in cancer and inflammation, where the disease-specific changes can be used for glycan-targeted therapies. This chapter will review the biosynthesis, the biology, and the translational perspectives of GalNAc O-glycans.

Global view of human protein glycosylation pathways and functions

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

Platelets in Healthy and Disease States: From Biomarkers Discovery to Drug Targets Identification by Proteomics

Platelets are a heterogeneous small anucleate blood cell population with a central role both in physiological haemostasis and in pathological states, spanning from thrombosis to inflammation, and cancer. Recent advances in proteomic studies provided additional important information concerning the platelet biology and the response of platelets to several pathophysiological pathways. Platelets circulate systemically and can be easily isolated from human samples, making proteomic application very interesting for characterizing the complexity of platelet functions in health and disease as well as for identifying and quantifying potential platelet proteins as biomarkers and novel antiplatelet therapeutic targets. To date, the highly dynamic protein content of platelets has been studied in resting and activated platelets, and several subproteomes have been characterized including platelet-derived microparticles, platelet granules, platelet releasates, platelet membrane proteins, and specific platelet post-translational modifications. In this review, a critical overview is provided on principal platelet proteomic studies focused on platelet biology from signaling to granules content, platelet proteome changes in several diseases, and the impact of drugs on platelet functions. Moreover, recent advances in quantitative platelet proteomics are discussed, emphasizing the importance of targeted quantification methods for more precise, robust and accurate quantification of selected proteins, which might be used as biomarkers for disease diagnosis, prognosis and therapy, and their strong clinical impact in the near future.

Trp53 regulates platelets in bone marrow via the PI3K pathway

The p53 gene is well known as a key tumor suppressor gene; it is vital for hematopoietic stem cell differentiation and growth. In the present study, the change of platelets (PLTs) in p53 knockout mice (p53^-/- mice) was investigated. The peripheral blood cell subsets and PLT parameters in p53^-/-mice were compared with those in age-matched p53^+/+ mice. Bleeding time as well as the alteration of PLT levels, were analyzed with the PLT marker CD41 antibody using flow cytometry. The results revealed that the number of PLTs in p53^-/- mice was significantly lower than that in p53^+/+ mice. Bleeding time was prolonged in the peripheral blood of p53^-/- mice compared with that of p53^+/+ mice. Furthermore, the related gene expression of the PI3K signaling pathway in the bone marrow of p53^-/- mice was shown to be associated with plateletogenesis. PI3K inhibitor (LY294002) was also used to treat p53^-/- mice, and the results demonstrated that LY294002 revert the change of PLTs in these mice. In summary, PLTs were altered in p53^-/- mice, and the PI3K signaling pathway was involved in that process, suggesting that the p53-dependent PI3K signaling pathway is involved in thrombocytopenia or PLT diseases. PLT number is reduced in p53 deficiency; however, this reduction could be reverted by inhibiting the PI3K pathway.

Blood-Borne ST6GAL1 Regulates Immunoglobulin Production in B Cells

Humoral immunity is an effective but metabolically expensive defense mechanism. It is unclear whether systemic cues exist to communicate the dynamic need for antigen presentation and immunoglobulin production. Here, we report a novel role for the liver-produced, acute phase reactant ST6GAL1 in IgG production. B cell expression of ST6GAL1, a sialyltransferase mediating the attachment of α2,6-linked sialic acids on N-glycans, is classically implicated in the dysregulated B cell development and immunoglobulin levels of St6gal1-deficient mice. However, the blood-borne pool of ST6GAL1, upregulated during systemic inflammation, can also extrinsically modify leukocyte cell surfaces. We show that B cell independent, extracellular ST6GAL1 enhances B cell IgG production and increases blood IgG titers. B cells of mice lacking the hepatocyte specific St6gal1 promoter have reduced sialylation of cell surface CD22 and CD45 and produce less IgG upon stimulation. Sialylation of B cells by extracellular ST6GAL1 boosts expression of IgM, IgD, and CD86, proliferation, and IgG production in vitro. In vivo, elevation of blood ST6GAL1 enhances B cell development and systemic IgG in a CD22-dependent manner. Our data point to a function of an extracellular glycosyltransferase in promoting humoral immunity. Manipulation of systemic ST6GAL1 may represent an effective therapeutic approach for humoral insufficiency.

Canalicular system reorganization during mouse platelet activation as revealed by 3D ultrastructural analysis

The canalicular system (CS) has been defined as: 1) an inward, invaginated membrane connector that supports entry into and exit from the platelet; 2) a static structure stable during platelet isolation; and 3) the major source of plasma membrane (PM) for surface area expansion during activation. Recent analysis from STEM tomography and serial block face electron microscopy has challenged the relative importance of CS as the route for granule secretion. Here, We used 3D ultrastructural imaging to reexamine the CS in mouse platelets by generating high-resolution 3D reconstructions to test assumptions 2 and 3. Qualitative and quantitative analysis of whole platelet reconstructions, obtained from immediately fixed or washed platelets fixed post-washing, indicated that CS, even in the presence of activation inhibitors, reorganized during platelet isolation to generate a more interconnected network. Further, CS redistribution into the PM at different times, post-activation, appeared to account for only about half the PM expansion seen in thrombin-activated platelets, in vitro, suggesting that CS reorganization is not sufficient to serve as a dominant membrane reservoir for activated platelets. In sum, our analysis highlights the need to revisit past assumptions about the platelet CS to better understand how this membrane system contributes to platelet function.

The platelet surface glycosylation caused by glycosidase has a strong impact on platelet function

: Platelet surface glycosylation defects has been reported to be significantly associated with many diseases. Our previous study found that platelet surface glycosylation is altered in coronary heart disease. In this study, we further investigated whether altered glycosylation affects platelet function. Platelets were obtained from ten healthy volunteers. The platelet surface terminal sialic acid was removed by neuraminidase A, and N-linked oligosaccharides was removed by PNGase F. The function of the enzyme-treated platelet was measured. The activation and platelet adhesion to von Willebrand factor (vWF) was measured by flow cytometry. Platelet aggregation induced by ADP, arachidonic acid and collagen was detected through light transmission aggregometry, and platelet-leukocyte aggregates (PLAs) was detected by flow cytometry. Neuraminidase A treatment caused sialic acid level decrease and β-galactose level increase significantly on platelet surface. Activation marker CD62P did not change. Platelet adhesion to vWF was increased significantly (P < 0.05). ADP-induced platelet aggregation was significantly reduced (P < 0.05). Platelet-granulocytes aggregates and platelet-monocytes aggregates increased (P < 0.05). Platelet surface sialic acid was increased after PNGase F treatment. Platelet aggregation by all agonists were significantly reduced (P < 0.05). There is no difference in the binding of vWF and PLAs for PNGase F treated platelet. We demonstrated that asialoglycosylation enhances platelet binding to vWF and forming PLAs, suggest that it may be associated with high platelet reactivity and the increased risk of thrombosis.

B cells suppress medullary granulopoiesis by an extracellular glycosylation-dependent mechanism

The immune response relies on the integration of cell-intrinsic processes with cell-extrinsic cues. During infection, B cells vacate the marrow during emergency granulopoiesis but return upon restoration of homeostasis. Here we report a novel glycosylation-mediated crosstalk between marrow B cells and hematopoietic progenitors. Human B cells secrete active ST6GAL1 sialyltransferase that remodels progenitor cell surface glycans to suppress granulopoiesis. In mouse models, ST6GAL1 from B cells alters the sialylation profile of bone marrow populations, and mature IgD+ B cells were enriched in sialylated bone marrow niches. In clinical multiple myeloma, ST6GAL1 abundance in the multiple myeloma cells negatively correlated with neutrophil abundance. These observations highlight not only the ability of medullary B cells to influence blood cell production, but also the disruption to normal granulopoiesis by excessive ST6GAL1 in malignancy.

Inherited thrombocytopenia associated with mutation of UDP-galactose-4-epimerase (GALE)

Severe thrombocytopenia, characterized by dysplastic megakaryocytes and intracranial bleeding, was diagnosed in six individuals from a consanguineous kindred. Three of the individuals were successfully treated by bone marrow transplant. Whole-exome sequencing and homozygosity mapping of multiple family members, coupled with whole-genome sequencing to reveal shared non-coding variants, revealed one potentially functional variant segregating with thrombocytopenia under a recessive model: GALE p.R51W (c.C151T, NM_001127621). The mutation is extremely rare (allele frequency = 2.5 × 10-05), and the likelihood of the observed co-segregation occurring by chance is 1.2 × 10-06. GALE encodes UDP-galactose-4-epimerase, an enzyme of galactose metabolism and glycosylation responsible for two reversible reactions: interconversion of UDP-galactose with UDP-glucose and interconversion of UDP-N-acetylgalactosamine with UDP-N-acetylglucosamine. The mutation alters an amino acid residue that is conserved from yeast to humans. The variant protein has both significantly lower enzymatic activity for both interconversion reactions and highly significant thermal instability. Proper glycosylation is critical to normal hematopoiesis, in particular to megakaryocyte and platelet development, as reflected in the presence of thrombocytopenia in the context of congenital disorders of glycosylation. Mutations in GALE have not previously been associated with thrombocytopenia. Our results suggest that GALE p.R51W is inadequate for normal glycosylation and thereby may impair megakaryocyte and platelet development. If other mutations in GALE are shown to have similar consequences, this gene may be proven to play a critical role in hematopoiesis.

Glycosylation in health and disease

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of 'glycomedicine'.

Analysis of a lectin microarray identifies altered sialylation of mouse serum glycoproteins induced by whole-body radiation exposure

Microarrays containing 45 different lectins were analyzed to identify global changes in the glycosylation of serum glycoproteins from mice exposed to whole-body γ-radiation. The results showed that radiation exposure increased and decreased the relative amounts of α-2,3- and α-2,6-sialic acids, respectively. The expression of α-2,3- and α-2,6-sialyltransferase genes in the liver was analyzed to determine whether changes in their expression were responsible for the sialic acid changes. The increase in α-2,3-sialic acid correlated with St3gal5 upregulation after radiation exposure; however, a decrease in St6gal1 expression was not observed. Analysis of a PCR array of genes expressed in irradiated mouse livers revealed that irradiation did not alter the expression of most of the included genes. These results suggest that glycomic screening of serum glycoproteins using lectin microarrays can be a powerful tool for identifying radiation-induced changes in the post-translational addition of sugar moieties to proteins. In addition, the results indicate that altered sialylation of glycoproteins may be an initial response to acute radiation exposure.

The Birth and Death of Platelets in Health and Disease

Blood platelets are involved in a wide range of physiological responses and pathological processes. Recent studies have considerably advanced our understanding of the mechanisms of platelet production and clearance, revealing new connections between the birth and death of these tiny, abundant cells. Key insights have also been gained into how physiological challenges such as inflammation, infection, and chemotherapy can affect megakaryocytes, the cells that produce platelets.

The direct and indirect effects of glycans on immune function

The biological impact of glycans is as diverse and complex as the impact of proteins on biology. Familiar roles include those as a protein folding checkpoint in the endoplasmic reticulum and as a modulator of the serum half-life of secreted glycoproteins, but it has become clear over the last several decades that glycans are key signaling moieties, participate in cell-cell interactions and modulate the function of individual proteins, to name but a few examples. In the immune system, the majority of microbial "patterns" are glycans or glycoconjugates, while virtually all cell surface receptors are glycoproteins, and antibody glycosylation critically influences antibody function. In order to provide a simple contextual framework to understand the myriad roles, glycans play in immunity, we propose that glycan effects are considered direct or indirect, depending on their direct participation or their indirect effects on other components in a given biological process or pathway. Here, we present the published evidence that supports this framework, which ultimately leads to the conclusion that we should learn to embrace the complexity inherent to the glycome and its potential as a largely uncharted but target rich area of new therapeutic investigation.

Emerging glycobiology tools: A renaissance in accessibility

The glycobiology of the immune response is a topic that has garnered increased attention due to a number of key discoveries surrounding IgG function, the specificity of some broadly neutralizing anti-HIV antibodies, cancer immunoregulation by galectin molecules and others. This review is the opening article in a Special Edition of Cellular Immunology focused on glycoimmunology, and has the goal of setting the context for these articles by providing a mini-review of how glycans impact immunity. We also focus on some of the technological and methodological advances in the field of glycobiology that are being deployed to lower the barrier of entry into the glycosciences, and to more fully interrogate the glycome and its function.

Platelet Munc13-4 regulates hemostasis, thrombosis and airway inflammation

Platelet degranulation is crucial for hemostasis and may participate in inflammation. Exocytosis in platelets is mediated by SNARE proteins and should be controlled by Munc13 proteins. We found that platelets express Munc13-2 and -4. We assessed platelet granule exocytosis in Munc13-2 and -4 global and conditional knockout (KO) mice, and observed that deletion of Munc13-4 ablates dense granule release and indirectly impairs alpha granule exocytosis. We found no exocytic role for Munc13-2 in platelets, not even in the absence of Munc13-4. In vitro, Munc13-4-deficient platelets exhibited defective aggregation at low doses of collagen. In a flow chamber assay, we observed that Munc13-4 acted as a rate-limiting factor in the formation of thrombi. In vivo, we observed a dose-dependency between Munc13-4 expression in platelets and both venous bleeding time and time to arterial thrombosis. Finally, in a model of allergic airway inflammation, we found that platelet-specific Munc13-4 KO mice had a reduction in airway hyper-responsiveness and eosinophilic inflammation. Taken together, our results indicate that Munc13-4-dependent platelet dense granule release plays essential roles in hemostasis, thrombosis and allergic inflammation.

IgG and leukocytes: Targets of immunomodulatory α2,6 sialic acids

ST6Gal1 is a critical sialyltransferase enzyme that controls the addition of α2,6-linked sialic acids to the termini of glycans. Attachment of sialic acids to glycoproteins as a posttranslational modification influences cellular responses, and is a well-known modifier of immune cell behavior. ST6Gal1 activity impacts processes such as: effector functions of immunoglobulin G via Fc sialylation, hematopoietic capacity by hematopoietic stem and progenitor cell surface sialylation, and lymphocyte activation thresholds though CD22 engagement and inhibition of galectins. This review summarizes recent studies that suggest α2,6 sialylation by ST6Gal1 has an immunoregulatory effect on immune reactions.

Glycobiology of Aging

Glycosylation is one of the most frequent post-translational modification of proteins. Many membrane and secreted proteins are decorated by sugar chains mainly linked to asparagine (N-linked) or to serine or threonine (O-linked). The biosynthesis of the sugar chains is mainly controlled by the activity of their biosynthetic enzymes: the glycosyltransferases. Glycosylation plays multiple roles, including the fine regulation of the biological activity of glycoproteins. Inflammaging is a chronic low grade inflammatory status associated with aging, probably caused by the continuous exposure of the immune system to inflammatory stimuli of endogenous and exogenous origin. The aging-associated glycosylation changes often resemble those observed in inflammatory conditions. One of the most reproducible markers of calendar and biological aging is the presence of N-glycans lacking terminal galactose residues linked to Asn₂₉₇ of IgG heavy chains (IgG-G0). Although the mechanism(s) generating IgG-G0 remain unclear, their presence in a variety of inflammatory conditions suggests a link with inflammaging. In addition, these aberrantly glycosylated IgG can exert a pro-inflammatory effect through different mechanisms, triggering a self-fueling inflammatory loop. A strong association with aging has been documented also for the plasmatic forms of glycosyltrasferases B4GALT1 and ST6GAL1, although their role in the extracellular glycosylation of antibodies does not appear likely. Siglecs, are a group of sialic acid binding mammalian lectins which mainly act as inhibitory receptors on the surface of immune cells. In general activity of Siglecs appears to be associated with long life, probably because of their ability to restrain aging-associated inflammation.

UDP-sugars activate P2Y14 receptors to mediate vasoconstriction of the porcine coronary artery

Aims

UDP-sugars can act as extracellular signalling molecules, but relatively little is known about their cardiovascular actions. The P2Y₁₄ receptor is a G_i/o-coupled receptor which is activated by UDP-glucose and related sugar nucleotides. In this study we sought to investigate whether P2Y₁₄ receptors are functionally expressed in the porcine coronary artery using a selective P2Y₁₄ receptor agonist, MRS2690, and a novel selective P2Y₁₄ receptor antagonist, PPTN (4,7-disubstituted naphthoic acid derivative).

Methods and results

Isometric tension recordings were used to evaluate the effects of UDP-sugars in porcine isolated coronary artery segments. The effects of the P2 receptor antagonists suramin and PPADS, the P2Y₁₄ receptor antagonist PPTN, and the P2Y₆ receptor antagonist MRS2578, were investigated. Measurement of vasodilator-stimulated phosphoprotein (VASP) phosphorylation using flow cytometry was used to assess changes in cAMP levels. UDP-glucose, UDP-glucuronic acid UDP-N-acetylglucosamine (P2Y₁₄ receptor agonists), elicited concentration-dependent contractions of the porcine coronary artery. MRS2690 was a more potent vasoconstrictor than the UDP-sugars. Concentration dependent contractile responses to MRS2690 and UDP-sugars were enhanced in the presence of forskolin (activator of cAMP), where the level of basal tone was maintained by addition of U46619, a thromboxane A₂ mimetic. Contractile responses to MRS2690 were blocked by PPTN, but not by MRS2578. Contractile responses to UDP-glucose were also attenuated by PPTN and suramin, but not by MRS2578. Forskolin-induced VASP-phosphorylation was reduced in porcine coronary arteries exposed to UDP-glucose and MRS2690, consistent with P2Y₁₄ receptor coupling to G_i/o proteins and inhibition of adenylyl cyclase activity.

Conclusions

Our data support a role of UDP-sugars as extracellular signalling molecules and show for the first time that they mediate contraction of porcine coronary arteries via P2Y₁₄ receptors.

Precise integration of inducible transcriptional elements (PrIITE) enables absolute control of gene expression

Tetracycline-based inducible systems provide powerful methods for functional studies where gene expression can be controlled. However, the lack of tight control of the inducible system, leading to leakiness and adverse effects caused by undesirable tetracycline dosage requirements, has proven to be a limitation. Here, we report that the combined use of genome editing tools and last generation Tet-On systems can resolve these issues. Our principle is based on precise integration of inducible transcriptional elements (coined PrIITE) targeted to: (i) exons of an endogenous gene of interest (GOI) and (ii) a safe harbor locus. Using PrIITE cells harboring a GFP reporter or CDX2 transcription factor, we demonstrate discrete inducibility of gene expression with complete abrogation of leakiness. CDX2 PrIITE cells generated by this approach uncovered novel CDX2 downstream effector genes. Our results provide a strategy for characterization of dose-dependent effector functions of essential genes that require absence of endogenous gene expression.

Extrinsic sialylation is dynamically regulated by systemic triggers in vivo

Recent reports have documented that extracellular sialyltransferases can remodel both cell-surface and secreted glycans by a process other than the canonical cell-autonomous glycosylation that occurs within the intracellular secretory apparatus. Despite association of the abundance of these extracellular sialyltransferases, particularly ST6Gal-1, with disease states such as cancer and a variety of inflammatory conditions, the prevalence of this extrinsic glycosylation pathway in vivo remains unknown. Here we observed no significant extrinsic sialylation in resting mice, suggesting that extrinsic sialylation is not a constitutive process. However, extrinsic sialylation in the periphery could be triggered by inflammatory challenges, such as exposure to ionizing radiation or to bacterial lipopolysaccharides. Sialic acids from circulating platelets were used in vivo to remodel target cell surfaces. Platelet activation was minimally sufficient to elicit extrinsic sialylation, as demonstrated with the FeCl₃ model of mesenteric artery thrombosis. Although extracellular ST6Gal-1 supports extrinsic sialylation, other sialyltransferases are present in systemic circulation. We also observed in vivo extrinsic sialylation in animals deficient in ST6Gal-1, demonstrating that extrinsic sialylation is not mediated exclusively by ST6Gal-1. Together, these observations form an emerging picture of glycans biosynthesized by the canonical cell-autonomous glycosylation pathway, but subjected to remodeling by extracellular glycan-modifying enzymes.

Cigarette Smoking-Associated Alterations in Serotonin/Adrenalin Signaling Pathways of Platelets

Background

Cigarette smoking plays a major role in cardiovascular diseases. The acute effects of cigarette smoking produce central nervous system–mediated activation of the sympathetic nervous system. The overactive sympathetic nervous system stimulates the secretion of serotonin (5‐HT) and catecholamine into blood at supraphysiological levels. The correlation between these pathological conditions induced by smoking and the increased risk of thrombosis has not been thoroughly investigated. The goal of our study was to explore cigarette smoking–associated changes in platelet biology mediated by elevated 5‐HT and catecholamine levels in blood plasma.

Methods and Results

Using blood samples collected from healthy nonsmokers and smokers (15 minutes after smoking), we determined that cigarette smoking increased the plasma 5‐HT/catecholamine concentration by several fold and the percent aggregation of platelets 2‐fold. Liquid chromatography–tandem mass spectrometry analysis of proteins eluted from platelet plasma membranes of smokers and nonsmokers demonstrated that GTPase‐activating proteins and proteins participating in the actin cytoskeletal network were differentially and significantly elevated in smokers' platelet membranes compared with those of nonsmokers. Interestingly, Matrix‐assisted laser desorption/ionization–mass spectrometry analyses of the glycans eluted from platelet plasma membranes of the smokers demonstrated that the level and structures of glycans are different from the nonsmokers' platelet surface glycans. Pharmacological blockade of 5‐HT or catecholamine receptors counteracted the 5‐HT/catecholamine‐mediated aggregation and altered the level and composition of glycan on platelet surfaces.

Conclusions

Based on our findings, we propose that smoking‐associated 5‐HT/catecholamine signaling accelerates the trafficking dynamics of platelets, and this remodels the surface proteins and glycans and predisposes platelets to hyperactive levels. Smokers' platelets also had correspondingly higher resting concentrations of intracellular calcium and transglutaminase activity. These findings suggest a link among smoking, platelet 5‐HT, catecholamine signaling, and their downstream effectors—including phospholipase C and inositol‐1,4,5‐triphosphate pathways—resulting in an increased tonic level of platelet activation in smokers.

Platelet glycoproteins associated with aspirin-treatment upon platelet activation

Platelet glycoproteins are known to play central roles in hemostasis and vascular integrity and have pathologic roles in vascular occlusive diseases such as myocardial infarction and stroke. Characterizing glycoproteins within and secreted by platelets can provide insight into the mechanisms that underlie vascular pathologies and the therapeutic benefits or failure of anti-platelet agents. To study the impact of aspirin, which is commonly prescribed for primary and secondary cardiovascular prevention, on the platelet glycoproteome, we evaluated washed platelets from ten donors. The platelet glycoproteome, was studied using an iTRAQ in resting and stimulated states and with and without aspirin treatment. Using solid phase extraction of glycosite-containing peptides (SPEG), we were able to identify 799 unique N-linked glycosylation sites (glycosites) in platelets, representing the largest and the most comprehensive analysis to date. We were able to identity a number of glycoproteins impacted by aspirin treatment, which we validated using global proteomics analysis of platelets and their secreted proteins. In our analyses, metallopeptidase inhibitor 1 (TIMP1) was the single most significantly affected glycoprotein by aspirin treatment. ELISA assays confirmed proteomic results and validated our strategy. Functional analysis demonstrated that TIMP1 levels were highly correlated with platelet reactivity in vitro, with a correlation coefficient of -0.5. The release of TIMP1 from platelets, which was previously unknown to be affected by aspirin treatment, may play important roles in hemostasis and/or vascular integrity. If validated, our findings may be useful for developing assays that assess platelet response to aspirin or other anti-platelet therapies.