Characterization of a neutrophil cell surface glycosaminoglycan that mediates binding of platelet factor 4

Intra-tumoral administration of CHST15 siRNA remodels tumor microenvironment and augments tumor-infiltrating T cells in pancreatic cancer

The dense stroma is one cause of poor efficacy of T cell-mediated immunotherapy in pancreatic ductal adenocarcinoma (PDAC). Carbohydrate sulfotransferase 15 (CHST15) is a proteoglycan-synthetic enzyme responsible for remodeling tumor stroma. Intra-tumoral injection of CHST15 small interfering RNA (siRNA) has been shown to increase the tumor-infiltrating T cells (TILs) in patients with unresectable PDAC. However, the mechanism underlying the enhanced accumulation of TILs is not fully explored. Here, we demonstrate that intra-tumoral injection of CHST15 siRNA locally and remotely diminishes myeloid-derived suppressor cells (MDSCs) and enhances TILs in mice. CHST15 was expressed by tumor cells and MDSCs in both tumor and tumor-draining lymph nodes (TDLNs), and CHST15 siRNA repressed stromal density, neutrophil extracellular traps, and Ly6C/G+ MDSCs in vivo. Remarkably, tumor growth inhibition was only observed in the immunocompetent KPC model, which is associated with enhanced TILs. In vitro, CHST15 siRNA significantly downregulated the levels of CHST15 and indoleamine 2,3-dioxygenase mRNA in CD33+ MDSCs derived from human peripheral blood mononuclear cells. These results suggest a dual role for intra-tumorally injected CHST15 siRNA on modulating the tumor immune microenvironment for T cell entry and remotely diminishing CHST15+ MDSCs, decreasing T cell suppression and expanding T cells in the TDLN, ultimately leading to an enhanced accumulation of TILs.

Add-on multiple submucosal injections of the RNA oligonucleotide GUT-1 to anti-TNF antibody treatment in patients with moderate-to-severe ulcerative colitis: an open-label, proof-of concept study

BACKGROUND

Carbohydrate sulfotransferase 15 (CHST15) is an enzyme biosynthesizing matrix glycosaminoglycan that modulates tissue remodeling. We evaluated the efficacy of add-on submucosal injections of GUT-1, the RNA oligonucleotide inhibitor of CHST15, to ongoing anti-tumor necrosis factor (TNF) antibody treatment in patients with moderate-to-severe ulcerative colitis (UC).

METHODS

This was an open-label study of 250 nM of GUT-1 by endoscopic submucosal injections at weeks 0, 2, 4 in five UC patients who lost response during maintenance treatment to anti-TNF antibodies. The primary endpoint was the rate of endoscopic improvement at week 6 and secondary endpoints included the rates of clinical remission by modified Mayo Score (mMS). Patients received follow-up observation with continuous maintenance treatment by the same anti-TNF antibody till the time of clinical recurrence or for overall 52 weeks.

RESULTS

At week 6, rates of endoscopic improvement and clinical remission were 80% (n = 4/5) and 60% (n = 3/5), respectively. The mean Endoscopy Subscore was reduced from 2.4 (95%CI: 1.7 to 3.1) at baseline, to 1.0 (95%CI: 0.1 to 1.9) at week 6. The mean mMS was reduced from 7.8 (95%CI: 6.2 to 9.4) to 1.3 (95%CI: 2.9 to 4.3). GUT-1 was well tolerated. Three patients did not show clinical recurrence for 52 weeks. All three corticosteroid-dependent patients showed no corticosteroid exposure for at least 24 weeks after achieving clinical remission. Multiple dosing was also well tolerated.

CONCLUSIONS

Add-on multiple injections of GUT-1 to ongoing anti-TNF antibody was able to induce rapid and durable clinical responses in UC patients who lost response to anti-TNF therapy.

TRIAL REGISTRATION

Clinical trial Registration Number (Japan): UMIN000020900.

Inhibitors of dermatan sulfate epimerase 1 decreased accumulation of glycosaminoglycans in mucopolysaccharidosis type I fibroblasts

Genetic deficiency of alpha-L-iduronidase causes mucopolysaccharidosis type I (MPS-I) disease, due to accumulation of glycosaminoglycans (GAGs) including chondroitin/dermatan sulfate (CS/DS) and heparan sulfate (HS) in cells. Currently, patients are treated by infusion of recombinant iduronidase or by hematopoietic stem cell transplantation. An alternative approach is to reduce the L-iduronidase substrate, through limiting the biosynthesis of iduronic acid. Our earlier study demonstrated that ebselen attenuated GAGs accumulation in MPS-I cells, through inhibiting iduronic acid producing enzymes. However, ebselen has multiple pharmacological effects, which prevents its application for MPS-I. Thus, we continued the study by looking for novel inhibitors of dermatan sulfate epimerase 1 (DS-epi1), the main responsible enzyme for production of iduronic acid in CS/DS chains. Based on virtual screening of chemicals towards chondroitinase AC, we constructed a library with 1,064 compounds that were tested for DS-epi1 inhibition. Seventeen compounds were identified to be able to inhibit 27%-86% of DS-epi1 activity at 10 μM. Two compounds were selected for further investigation based on the structure properties. The results show that both inhibitors had a comparable level in inhibition of DS-epi1while they had negligible effect on HS epimerase. The two inhibitors were able to reduce iduronic acid biosynthesis in CS/DS and GAG accumulation in WT and MPS-I fibroblasts. Docking of the inhibitors into DS-epi1 structure shows high affinity binding of both compounds to the active site. The collected data indicate that these hit compounds may be further elaborated to a potential lead drug used for attenuation of GAGs accumulation in MPS-I patients.

Platelet factor 4(PF4) and its multiple roles in diseases

Platelet factor 4 (PF4) combines with heparin to form an antigen that could produce IgG antibodies and participate in heparin-induced thrombocytopenia (HIT). PF4 has attracted wide attention due to its role in novel coronavirus vaccine-19 (COVID-9)-induced immune thrombotic thrombocytopenia (VITT) and cognitive impairments. The electrostatic interaction between PF4 and negatively charged molecules is vital in the progression of VITT, which is similar to HIT. Emerging evidence suggests its multiple roles in hematopoietic and angiogenic inhibition, platelet coagulation interference, host inflammatory response promotion, vascular inhibition, and antitumor properties. The emerging pharmacological effects of PF4 may help deepen the exploration of its mechanism, thus accelerating the development of targeted therapies. However, due to its pleiotropic properties, the development of drugs targeting PF4 is at an early stage and faces many challenges. Herein, we discussed the characteristics and biological functions of PF4, summarized PF4-mediated signaling pathways, and discussed its multiple roles in diseases to inform novel approaches for successful clinical translational research.

Submucosal Injection of the RNA Oligonucleotide GUT-1 in Active Ulcerative Colitis Patients: A Randomized, Double-Blind, Placebo-Controlled Phase 2a Induction Trial

BACKGROUND AND AIMS

Carbohydrate sulfotransferase 15 [CHST15] biosynthesizes sulphated matrix glycosaminoglycans and is implicated in intestinal inflammation and fibrosis. Here, we evaluate the efficacy and safety of the double-stranded RNA oligonucleotide GUT-1, a specific blocker of CHST15, as induction therapy in patients with ulcerative colitis [UC].

METHODS

In this randomized, double-blind, placebo-controlled, phase 2a study, we enrolled endoscopically active UC patients, refractory to conventional therapy, in five hospital centres across Germany. Patients were randomized 1:1:1 using a block randomized technique to receive a single dosing of 25 nM GUT-1, 250 nM GUT-1, or placebo by endoscopic submucosal injections. The primary outcome measure was improvement of endoscopic lesions at weeks 2 or 4. The secondary outcome measures included clinical and histological responses. Safety was assessed in all patients who received treatment.

RESULTS

Twenty-eight patients were screened, 24 were randomized, and 21 were evaluated. Endoscopic improvement at weeks 2 or 4 was achieved by 71.4% in the GUT-1 250 nM, 0% in the GUT-1 25 nM, and 28.6% in the placebo group. Clinical remission was shown by 57.1% in the GUT-1 250 nM, 0% in the GUT-1 25 nM, and 14.3% in the placebo groups. Histological improvement was shown by 42.9% in the GUT-1 250 nM, 0% in the GUT-1 25 nM, and 0% in the placebo groups. GUT-1 250 nM reduced CHST15 expression significantly and suppressed mucosal inflammation and fibrosis. GUT-1 application was well tolerated.

CONCLUSION

Single dosing by submucosal injection of GUT-1 repressed CHST15 mucosal expression and may represent a novel induction therapy by modulating tissue remodelling in UC.

Silencing of tumoral carbohydrate sulfotransferase 15 reactivates lymph node pancreatic cancer T cells in mice

Limited intratumoral T-cell infiltration in pancreatic ductal adenocarcinoma (PDAC) is an obstacle to immunotherapy, yet the efficient approach to enhance tumor-infiltrating T cells is not fully established. Here, we show that tumor-specific knockdown of carbohydrate sulfotransferase 15 (CHST15), a tumor stromal proteoglycan-synthetic enzyme, suppresses tumor growth in a T-cell-dependent manner in a murine model of PDAC. Silencing of tumoral CHST15 unexpectedly expanded CD4+ and CD8+ T cells in tumor draining LN (TDLN), leading to accelerated accumulation of EdU+ proliferating CD4+ and CD8+ T cells and granzyme B+ CD8+ T cells in the tumor. RNA expression analysis indicated that tumoral CHST15 knockdown (KD) downregulated matrix remodeling-related genes, while upregulated anti-tumor T-cell activity-related genes in both tumor and TDLN. CHST15 KD significantly diminished intratumoral and TDLN Ly6C/G+ myeloid-derived suppressor cells prior to TDLN T-cell expansion, suggesting that tumoral CHST15 remotely regulated myeloid-derived suppressor cell mediated T-cell suppression in the TDLN. Our findings illustrate a novel immunotherapeutic potential of tumoral CHST15 blockage by reactivating T cells in immune suppressive TDLN of PDAC.

Chemokine Heteromers and Their Impact on Cellular Function-A Conceptual Framework

Chemoattractant cytokines or chemokines are proteins involved in numerous biological activities. Their essential role consists of the formation of gradient and (immune) cell recruitment. Chemokine biology and its related signaling system is more complex than simple ligand-receptor interactions. Beside interactions with their cognate and/or atypical chemokine receptors, and glycosaminoglycans (GAGs), chemokines form complexes with themselves as homo-oligomers, heteromers and also with other soluble effector proteins, including the atypical chemokine MIF, carbohydrate-binding proteins (galectins), damage-associated molecular patterns (DAMPs) or with chemokine-binding proteins such as evasins. Likewise, nucleic acids have been described as binding targets for the tetrameric form of CXCL4. The dynamic balance between monomeric and dimeric structures, as well as interactions with GAGs, modulate the concentrations of free chemokines available along with the nature of the gradient. Dimerization of chemokines changes the canonical monomeric fold into two main dimeric structures, namely CC- and CXC-type dimers. Recent studies highlighted that chemokine dimer formation is a frequent event that could occur under pathophysiological conditions. The structural changes dictated by chemokine dimerization confer additional biological activities, e.g., biased signaling. The present review will provide a short overview of the known functionality of chemokines together with the consequences of the interactions engaged by the chemokines with other proteins. Finally, we will present potential therapeutic tools targeting the chemokine multimeric structures that could modulate their biological functions.

Platelet factor 4 (CXCL4/PF4) upregulates matrix metalloproteinase-2 (MMP-2) in gingival fibroblasts

Periodontitis is a chronic inflammatory disease characterized by the release of matrix metalloproteinases (MMPs) from resident connective tissue cells in tooth-supporting tissues (periodontium). Platelet activation, and the attendant release of pro-inflammatory chemokines such as platelet factor 4 (CXCL4/PF4), are associated with periodontitis although the associated biochemical pathways remain undefined. Here we report that recombinant PF4 is internalized by cultured human gingival fibroblasts (hGFs), resulting in significant (p < 0.05) upregulation in both the production and release of MMP-2 (gelatinase A). This finding was corroborated by elevated circulating levels of MMP-2 (p < 0.05) in PF4-overexpressing transgenic mice, relative to controls. We also determined that PF4 induces the phosphorylation of NF-κB; notably, the suppression of NF-κB signaling by the inhibitor BAY 11-7082 abrogated PF4-induced MMP-2 upregulation. Moreover, the inhibition of surface glycosaminoglycans (GAGs) blocked both PF4 binding and NF-κB phosphorylation. Partial blockade of PF4 binding to the cells was achieved by treatment with either chondroitinase ABC or heparinase III, suggesting that both chondroitin sulfate and heparan sulfate mediate PF4 signaling. These results identify a novel pathway in which PF4 upregulates MMP-2 release from fibroblasts in an NF-κB- and GAG-dependent manner, and further our comprehension of the role of platelet signaling in periodontal tissue homeostasis.

Functions of chondroitin/dermatan sulfate containing GalNAc4,6-disulfate

Chondroitin sulfate (CS) and dermatan sulfate (DS) containing GalNAc4,6-disulfate (GalNAc4S6S) were initially discovered in marine animals. Following the discovery, these glycosaminoglycans have been found in various animals including human. In the biosynthesis of CS/DS containing GalNAc4S6S, three groups of sulfotransferases are involved; chondroitin 4-sulfotransferases (C4STs), dermatan 4-sulfotransferase-1 (D4ST-1) and GalNAc 4-sulfate 6-O-sulfotransferase (GalNAc4S-6ST). GalNAc4S-6ST and its products have been shown to play important roles in the abnormal pathological conditions such as central nervous system injury, cancer development, abnormal tissue fibrosis, development of osteoporosis, and infection with viruses or nematodes. CS/DS containing GalNAc4S6S has been shown to increase with the functional differentiation of mast cells, macrophages and neutrophils. Genetic approaches using knockout or knockdown of GalNAc4S-6ST, blocking of the epitopes containing GalNAc4S6S by specific antibodies and chemical technology that enabled the synthesis of oligosaccharides with defined sulfation patterns have been applied successfully to these investigations. These studies contributed significantly to the basic understanding of the functional roles of CS/DS containing GalNAc4S6S in various abnormal conditions, and appear to provide promising clues to the development of possible measures to treat them.

Heparin-Functionalized Adsorbents Eliminate Central Effectors of Immunothrombosis, including Platelet Factor 4, High-Mobility Group Box 1 Protein and Histones

Inflammation and thrombosis are closely intertwined in numerous disorders, including ischemic events and sepsis, as well as coronavirus disease 2019 (COVID-19). Thrombotic complications are markers of disease severity in both sepsis and COVID-19 and are associated with multiorgan failure and increased mortality. Immunothrombosis is driven by the complement/tissue factor/neutrophil axis, as well as by activated platelets, which can trigger the release of neutrophil extracellular traps (NETs) and release further effectors of immunothrombosis, including platelet factor 4 (PF4/CXCL4) and high-mobility box 1 protein (HMGB1). Many of the central effectors of deregulated immunothrombosis, including activated platelets and platelet-derived extracellular vesicles (pEVs) expressing PF4, soluble PF4, HMGB1, histones, as well as histone-decorated NETs, are positively charged and thus bind to heparin. Here, we provide evidence that adsorbents functionalized with endpoint-attached heparin efficiently deplete activated platelets, pEVs, PF4, HMGB1 and histones/nucleosomes. We propose that this elimination of central effectors of immunothrombosis, rather than direct binding of pathogens, could be of clinical relevance for mitigating thrombotic complications in sepsis or COVID-19 using heparin-functionalized adsorbents.

Differential Effects of Platelet Factor 4 (CXCL4) and Its Non-Allelic Variant (CXCL4L1) on Cultured Human Vascular Smooth Muscle Cells

Platelet factor 4 (CXCL4) is a chemokine abundantly stored in platelets. Upon injury and during atherosclerosis, CXCL4 is transported through the vessel wall where it modulates the function of vascular smooth muscle cells (VSMCs) by affecting proliferation, migration, gene expression and cytokine release. Variant CXCL4L1 is distinct from CXCL4 in function and expression pattern, despite a minor three-amino acid difference. Here, the effects of CXCL4 and CXCL4L1 on the phenotype and function of human VSMCs were compared in vitro. VSMCs were found to constitutively express CXCL4L1 and only exogenously added CXCL4 was internalized by VSMCs. Pre-treatment with heparin completely blocked CXCL4 uptake. A role of the putative CXCL4 receptors CXCR3 and DARC in endocytosis was excluded, but LDL receptor family members appeared to be involved in the uptake of CXCL4. Incubation of VSMCs with both CXCL4 and CXCL4L1 resulted in decreased expression of contractile marker genes and increased mRNA levels of KLF4 and NLRP3 transcription factors, yet only CXCL4 stimulated proliferation and calcification of VSMCs. In conclusion, CXCL4 and CXCL4L1 both modulate gene expression, yet only CXCL4 increases the division rate and formation of calcium-phosphate crystals in VSMCs. CXCL4 and CXCL4L1 may play distinct roles during vascular remodeling in which CXCL4 induces proliferation and calcification while endogenously expressed CXCL4L1 governs cellular homeostasis. The latter notion remains a subject for future investigation.

Inhibition of iduronic acid biosynthesis by ebselen reduces glycosaminoglycan accumulation in mucopolysaccharidosis type I fibroblasts

Mucopolysaccharidosis type I (MPS-I) is a rare lysosomal storage disorder caused by deficiency of the enzyme alpha-L-iduronidase, which removes iduronic acid in both chondroitin/dermatan sulfate (CS/DS) and heparan sulfate (HS) and thereby contributes to the catabolism of glycosaminoglycans (GAGs). To ameliorate this genetic defect, the patients are currently treated by enzyme replacement and bone marrow transplantation, which have a number of drawbacks. This study was designed to develop an alternative treatment by inhibition of iduronic acid formation. By screening the Prestwick drug library, we identified ebselen as a potent inhibitor of enzymes that produce iduronic acid in CS/DS and HS. Ebselen efficiently inhibited iduronic acid formation during CS/DS synthesis in cultured fibroblasts. Treatment of MPS-I fibroblasts with ebselen not only reduced accumulation of CS/DS but also promoted GAG degradation. In early Xenopus embryos, this drug phenocopied the effect of downregulation of DS-epimerase 1, the main enzyme responsible for iduronic production in CS/DS, suggesting that ebselen inhibits iduronic acid production in vivo. However, ebselen failed to ameliorate the CS/DS and GAG burden in MPS-I mice. Nevertheless, the results propose a potential of iduronic acid substrate reduction therapy for MPS-I patients.

Increase in Chondroitin Sulfate and Decline in Arylsulfatase B May Contribute to Pathophysiology of COVID-19 Respiratory Failure

INTRODUCTION

The potential role of accumulation of chondroitin sulfates (CSs) in the pathobiology of COVID-19 has not been examined. Accumulation may occur by increased synthesis or by decline in activity of the enzyme arylsulfatase B (ARSB; N-acetylgalactosamine-4-sulfatase) which requires oxygen for activity.

METHODS

Immunostaining of lung tissue from 28 patients who died due to COVID-19 infection was performed for CS, ARSB, and carbohydrate sulfotransferase (CHST)15. Measurements of mRNA expression of CHST15 and CHST11, sulfotransferase activity, and total sulfated glycosaminoglycans (GAGs) were determined in human vascular smooth muscle cells following angiotensin (Ang) II treatment.

RESULTS

CS immunostaining showed increase in intensity and distribution, and immunostaining of ARSB was diminished in COVID-19 compared to normal lung tissue. CHST15 immunostaining was prominent in vascular smooth muscle cells associated with diffuse alveolar damage due to COVID-19 or other causes. Expression of CHST15 and CHST11 which are required for synthesis of CSE and chondroitin 4-sulfate, total sulfated GAGs, and sulfotransferase activity was significantly increased following AngII exposure in vascular smooth muscle cells. Expression of Interleukin-6 (IL-6), a mediator of cytokine storm in COVID-19, was inversely associated with ARSB expression.

DISCUSSION/CONCLUSION

Decline in ARSB and resulting increases in CS may contribute to the pathobiology of COVID-19, as IL-6 does. Increased expression of CHSTs following activation of Ang-converting enzyme 2 may lead to buildup of CSs.

Chemokine (C-X-C Motif) Ligand 4 Is a Restrictor of Respiratory Syncytial Virus Infection and an Indicator of Clinical Severity

Rationale: Respiratory syncytial virus (RSV) is the leading cause of childhood respiratory infections worldwide; however, no vaccine is available, and treatment options are limited. Identification of host factors pivotal to viral replication may inform the development of novel therapies, prophylaxes, or diagnoses.Objectives: To identify host factors involved in RSV replication and to evaluate their potential for disease management.Methods: A gain-of-function screening was performed on the basis of a genome-wide human complementary DNA library screen for host factors involved in RSV replication. The antiviral mechanism of CXCL4 (chemokine [C-X-C motif] ligand 4) was analyzed. Its clinical role was evaluated via nasopharyngeal aspirates and plasma samples from patients with RSV infection and different disease severities.Measurements and Main Results: Forty-nine host factors restricting RSV replication were identified by gain-of-function screening, with CXCL4 showing the strongest antiviral effect, which was secretion dependent. CXCL4 blocked viral attachment through binding to the RSV main receptor heparan sulfate, instead of through interacting with RSV surface proteins. Intranasal pretreatment with CXCL4 alleviated inflammation in RSV-infected mice, as shown by decreased concentrations of tumor necrosis factor and viral load in BAL fluid samples as well as by viral nucleocapsid protein histological staining in lungs. Compared with non-RSV infections, RSV infections induced elevated CXCL4 concentrations both in plasma and airway samples from mice and pediatric patients. The airway CXCL4 concentration was correlated with viral load and disease severity in patients (P < 0.001).Conclusions: Our results suggest that CXCL4 is an RSV restriction factor that can block viral entry and serve as an indicator of clinical severity in RSV infections.

Structural and functional diversity of neutrophil glycosylation in innate immunity and related disorders

The granulated neutrophils are abundant innate immune cells that utilize bioactive glycoproteins packed in cytosolic granules to fight pathogenic infections, but the neutrophil glycobiology remains poorly understood. Facilitated by technological advances in glycoimmunology, systems glycobiology and glycoanalytics, a considerable body of literature reporting on novel aspects of neutrophil glycosylation has accumulated. Herein, we summarize the building knowledge of the structural and functional diversity displayed by N- and O-linked glycoproteins spatiotemporally expressed and sequentially brought-into-action across the diverse neutrophil life stages during bone marrow maturation, movements to, from and within the blood circulation and microbicidal processes at the inflammatory sites in peripheral tissues. It transpires that neutrophils abundantly decorate their granule glycoproteins including neutrophil elastase, myeloperoxidase and cathepsin G with peculiar glyco-signatures not commonly reported in other areas of human glycobiology such as hyper-truncated chitobiose core- and paucimannosidic-type N-glycans and monoantennary complex-type N-glycans. Sialyl Lewis^x, Lewis^x, poly-N-acetyllactosamine extensions and core 1-/2-type O-glycans are also common neutrophil glyco-signatures. Granule-specific glycosylation is another fascinating yet not fully understood feature of neutrophils. Recent literature suggests that unconventional biosynthetic pathways and functions underpin these prominent neutrophil-associated glyco-phenotypes. The impact of glycosylation on key neutrophil effector functions including extravasation, degranulation, phagocytosis and formation of neutrophil extracellular traps during normal physiological conditions and in innate immune-related diseases is discussed. We also highlight new technologies that are expected to further advance neutrophil glycobiology and briefly discuss the untapped diagnostic and therapeutic potential of neutrophil glycosylation that could open avenues to combat the increasingly prevalent innate immune disorders.

Targeting Chemokine-Glycosaminoglycan Interactions to Inhibit Inflammation

Leukocyte migration into tissues depends on the activity of chemokines that form concentration gradients to guide leukocytes to a specific site. Interaction of chemokines with their specific G protein-coupled receptors (GPCRs) on leukocytes induces leukocyte adhesion to the endothelial cells, followed by extravasation of the leukocytes and subsequent directed migration along the chemotactic gradient. Interaction of chemokines with glycosaminoglycans (GAGs) is crucial for extravasation in vivo. Chemokines need to interact with GAGs on endothelial cells and in the extracellular matrix in tissues in order to be presented on the endothelium of blood vessels and to create a concentration gradient. Local chemokine retention establishes a chemokine gradient and prevents diffusion and degradation. During the last two decades, research aiming at reducing chemokine activity mainly focused on the identification of inhibitors of the interaction between chemokines and their cognate GPCRs. This approach only resulted in limited success. However, an alternative strategy, targeting chemokine-GAG interactions, may be a promising approach to inhibit chemokine activity and inflammation. On this line, proteins derived from viruses and parasites that bind chemokines or GAGs may have the potential to interfere with chemokine-GAG interactions. Alternatively, chemokine mimetics, including truncated chemokines and mutant chemokines, can compete with chemokines for binding to GAGs. Such truncated or mutated chemokines are characterized by a strong binding affinity for GAGs and abrogated binding to their chemokine receptors. Finally, Spiegelmers that mask the GAG-binding site on chemokines, thereby preventing chemokine-GAG interactions, were developed. In this review, the importance of GAGs for chemokine activity in vivo and strategies that could be employed to target chemokine-GAG interactions will be discussed in the context of inflammation.

Mouse Cytomegalovirus Differentially Exploits Cell Surface Glycosaminoglycans in a Cell Type-Dependent and MCK-2-Independent Manner

Many viruses initiate interaction with target cells by binding to cell surface glycosaminoglycans (GAGs). Heparan sulfate (HS) appears to be particularly important in fibroblasts, epithelial cells and endothelial cells, where it represents the dominant GAG. How GAGs influence viral infectivity in HS-poor target cells such as macrophages has not been clearly defined. Here, we show that mouse cytomegalovirus (MCMV) targets HS in susceptible fibroblasts and cultured salivary gland acinar cells (SGACs), but not in macrophage cell lines and primary bone marrow-derived macrophages, where chondroitin sulfate was the dominant virus-binding GAG. MCK-2, an MCMV-encoded GAG-binding chemokine that promotes infection of macrophages as part of a gH/gL/MCK-2 entry complex, was dispensable for MCMV attachment to the cell surface and for direct infection of SGACs. Thus, MCMV tropism for target cells is markedly influenced by differential GAG expression, suggesting that the specificity of anti-GAG peptides now under development as HCMV therapeutics may need to be broadened for effective application as anti-viral agents.

Galactosaminoglycans: Medical Applications and Drawbacks

Galactosaminoglycans (GalAGs) are sulfated glycans composed of alternating N-acetylgalactosamine and uronic acid units. Uronic acid epimerization, sulfation patterns and fucosylation are modifications observed on these molecules. GalAGs have been extensively studied and exploited because of their multiple biomedical functions. Chondroitin sulfates (CSs), the main representative family of GalAGs, have been used in alternative therapy of joint pain/inflammation and osteoarthritis. The relatively novel fucosylated chondroitin sulfate (FCS), commonly found in sea cucumbers, has been screened in multiple systems in addition to its widely studied anticoagulant action. Biomedical properties of GalAGs are directly dependent on the sugar composition, presence or lack of fucose branches, as well as sulfation patterns. Although research interest in GalAGs has increased considerably over the three last decades, perhaps motivated by the parallel progress of glycomics, serious questions concerning the effectiveness and potential side effects of GalAGs have recently been raised. Doubts have centered particularly on the beneficial functions of CS-based therapeutic supplements and the potential harmful effects of FCS as similarly observed for oversulfated chondroitin sulfate, as a contaminant of heparin. Unexpected components were also detected in CS-based pharmaceutical preparations. This review therefore aims to offer a discussion on (1) the current and potential therapeutic applications of GalAGs, including those of unique features extracted from marine sources, and (2) the potential drawbacks of this class of molecules when applied to medicine.

Leukocyte integrin Mac-1 (CD11b/CD18, αMβ2, CR3) acts as a functional receptor for platelet factor 4

Platelet factor 4 (PF4) is one of the most abundant cationic proteins secreted from α-granules of activated platelets. Based on its structure, PF4 was assigned to the CXC family of chemokines and has been shown to have numerous effects on myeloid leukocytes. However, the receptor for PF4 remains unknown. Here, we demonstrate that PF4 induces leukocyte responses through the integrin Mac-1 (α_Mβ₂, CD11b/CD18). Human neutrophils, monocytes, U937 monocytic and HEK293 cells expressing Mac-1 strongly adhered to immobilized PF4 in a concentration-dependent manner. The cell adhesion was partially blocked by anti-Mac-1 mAb and inhibition was enhanced when anti-Mac-1 antibodies were combined with glycosaminoglycans, suggesting that cell-surface proteoglycans act cooperatively with Mac-1. PF4 also induced Mac-1-dependent migration of human neutrophils and murine WT, but not Mac-1-deficient macrophages. Coating of Escherichia coli bacteria or latex beads with PF4 enhanced their phagocytosis by macrophages by ∼4-fold, and this process was blocked by different Mac-1 antagonists. Furthermore, PF4 potentiated phagocytosis by WT, but not Mac-1-deficient macrophages. As determined by biolayer interferometry, PF4 directly bound the α_MI-domain, the major ligand-binding region of Mac-1, and this interaction was governed by a K_d of 1.3 ± 0.2 μm Using the PF4-derived peptide library, synthetic peptides duplicating the α_MI-domain recognition sequences and recombinant mutant PF4 fragments, the binding sites for α_MI-domain were identified in the PF4 segments Cys¹²-Ser²⁶ and Ala⁵⁷-Ser⁷⁰ These results identify PF4 as a ligand for the integrin Mac-1 and suggest that many immune-modulating effects previously ascribed to PF4 are mediated through its interaction with Mac-1.

Endothelial antigen assembly leads to thrombotic complications in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder initiated by antibodies against complexes between human platelet factor 4 (hPF4) and heparin. A better understanding of the events that initiate the prothrombotic state may improve approaches to antithrombotic management. Here, we visualized thrombus formation in an in vivo murine model and an endothelialized microfluidic system that simulate the pathogenesis of HIT. hPF4 released from platelets predominantly bound to peri-injury endothelium and formed HIT antigenic complexes that were dissociated by heparin. In mice expressing both hPF4+ and human platelet IgG Fc receptor IIA (FcγRIIA), infusion of the HIT-like monoclonal antibody KKO increased fibrin and platelet deposition at sites of injury, followed immediately by antigen formation on proximate endothelial cells. After a few minutes, HIT antigen was detected within the thrombus itself at the interface between the platelet core and the surrounding shell. We observed similar results in the humanized, endothelialized microfluidic system. hPF4 and KKO selectively bound to photochemically injured endothelium at sites where surface glycocalyx was reduced. These studies support the concept that the perithrombus endothelium is the predominant site of HIT antigen assembly. This suggests that disrupting antigen formation along the endothelium or protecting the endothelium may provide a therapeutic opportunity to prevent thrombotic complications of HIT, while sparing systemic hemostatic pathways.

Platelet Factor 4 Binds to Vascular Proteoglycans and Controls Both Growth Factor Activities and Platelet Activation

Platelet factor 4 (PF4) is produced by platelets with roles in both inflammation and wound healing. PF4 is stored in platelet α-granules bound to the glycosaminoglycan (GAG) chains of serglycin. This study revealed that platelet serglycin is decorated with chondroitin/dermatan sulfate and that PF4 binds to these GAG chains. Additionally, PF4 had a higher affinity for endothelial-derived perlecan heparan sulfate chains than serglycin GAG chains. The binding of PF4 to perlecan was found to inhibit both FGF2 signaling and platelet activation. This study revealed additional insight into the ways in which PF4 interacts with components of the vasculature to modulate cellular events.

Intercellular Interactions as Regulators of NETosis

Neutrophil extracellular traps (NETs) are chromatin-derived webs extruded from neutrophils in response to either infection or sterile stimulation with chemicals, cytokines, or microbial products. The vast majority of studies have characterized NET release (also called NETosis) in pure neutrophil cultures in vitro. The situation is surely more complex in vivo as neutrophils constantly sample not only pathogens and soluble mediators but also signals from cellular partners, including platelets and endothelial cells. This complexity is beginning to be explored by studies utilizing in vitro co-culture, as well as animal models of sepsis, infective endocarditis, lung injury, and thrombosis. Indeed, various selectins, integrins, and surface glycoproteins have been implicated in platelet–neutrophil interactions that promote NETosis, albeit with disparate results across studies. NETosis can also clearly be regulated by soluble mediators derived from platelets, such as eicosanoids, chemokines, and alarmins. Beyond platelets, the role of the endothelium in modulating NETosis is being increasingly revealed, with adhesive interactions likely priming neutrophils toward NETosis. The fact that the same selectins and surface glycoproteins may be expressed by both platelets and endothelial cells complicates the interpretation of in vivo data. In summary, we suggest in this review that the engagement of neutrophils with activated cellular partners provides an important in vivo signal or “hit” toward NETosis. Studies should, therefore, increasingly consider the triumvirate of neutrophils, platelets, and the endothelium when exploring NETosis, especially in disease states.

Enzyme overexpression - an exercise toward understanding regulation of heparan sulfate biosynthesis

Biosynthesis of heparan sulfate (HS) involves conversion of D-glucuronic acid (GlcA) to L-iduronic acid (IdoA) units catalyzed by glucuronyl C5-epimerase (Hsepi). IdoA units are the favored substrate for 2-O-sulfotransferase (2OST). We used HEK293 cells as a model to investigate the effects of overexpression of these enzymes on HS structure. Overexpression of Hsepi alone resulted in an unexpected increase in HS chain length. A Hsepi point-mutant (Y168A), devoid of catalytic activity, failed to affect chain length. Moreover, the effect of Hsepi overexpression on HS chain length was abolished by simultaneous overexpression of 2OST. These findings raise novel aspects on regulation of HS biosynthesis. We propose a hypothetical enzyme-binding protein (EBP) with distinct, specific and partly overlapping binding sites, the interactions of which will determine levels of enzymes available to the biosynthetic process.

Role of monocytes and endothelial cells in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an autoimmune disorder characterised by thrombocytopenia and thrombosis. The mechanisms leading to platelet destruction are complex and the thrombotic complications of HIT appear to be due to multiple different intravascular targets. The dual binding of HIT antibodies to platelet surface PF4/GAG complexes and to FcγRIIA likely leads to both platelet clearance and to their direct activation. Monocytes and endothelial cells bind PF4 with higher avidity than platelets and are more resistant to competitive removal of surface-bound PF4 in the presence of heparin. Binding of HIT antibodies to PF4/glycosaminoglycan complexes on the surface on these cells leads to their activation and increased procoagulant activity. Binding of higher levels of PF4 released from activated platelets to the endothelium may lead to changes of the anticoagulant properties of the glycocalyx and target the endothelial cells for HIT antibodies. Pathogenic antibodies bound to endothelial cells further promote prothrombotic conditions by a mechanism that is independent of FcγR activation, yet not completely understood. A more detailed understanding of the role of monocytes and endothelium may identify new targets for intervention to mitigate the risk of thrombosis with less impact on systemic haemostasis than current approaches to treatment for this serious disorder.

The antigenic complex in HIT binds to B cells via complement and complement receptor 2 (CD21)

Heparin-induced thrombocytopenia is a prothrombotic disorder caused by antibodies to platelet factor 4 (PF4)/heparin complexes. The mechanism that incites such prevalent anti-PF4/heparin antibody production in more than 50% of patients exposed to heparin in some clinical settings is poorly understood. To investigate early events associated with antigen exposure, we first examined the interaction of PF4/heparin complexes with cells circulating in whole blood. In healthy donors, PF4/heparin complexes bind preferentially to B cells (>90% of B cells bind to PF4/heparin in vitro) relative to neutrophils, monocytes, or T cells. Binding of PF4 to B cells is heparin dependent, and PF4/heparin complexes are found on circulating B cells from some, but not all, patients receiving heparin. Given the high proportion of B cells that bind PF4/heparin, we investigated complement as a mechanism for noncognate antigen recognition. Complement is activated by PF4/heparin complexes, co-localizes with antigen on B cells from healthy donors, and is present on antigen-positive B cells in patients receiving heparin. Binding of PF4/heparin complexes to B cells is mediated through the interaction between complement and complement receptor 2 (CR2 [CD21]). To the best of our knowledge, these are the first studies to demonstrate complement activation by PF4/heparin complexes, opsonization of PF4/heparin to B cells via CD21, and the presence of complement activation fragments on circulating B cells in some patients receiving heparin. Given the critical contribution of complement to humoral immunity, our observations provide new mechanistic insights into the immunogenicity of PF4/heparin complexes.

NDST2 (N-Deacetylase/N-Sulfotransferase-2) Enzyme Regulates Heparan Sulfate Chain Length

Analysis of heparan sulfate synthesized by HEK 293 cells overexpressing murine NDST1 and/or NDST2 demonstrated that the amount of heparan sulfate was increased in NDST2- but not in NDST1-overexpressing cells. Altered transcript expression of genes encoding other biosynthetic enzymes or proteoglycan core proteins could not account for the observed changes. However, the role of NDST2 in regulating the amount of heparan sulfate synthesized was confirmed by analyzing heparan sulfate content in tissues isolated from Ndst2(-/-) mice, which contained reduced levels of the polysaccharide. Detailed disaccharide composition analysis showed no major structural difference between heparan sulfate from control and Ndst2(-/-) tissues, with the exception of heparan sulfate from spleen where the relative amount of trisulfated disaccharides was lowered in the absence of NDST2. In vivo transcript expression levels of the heparan sulfate-polymerizing enzymes Ext1 and Ext2 were also largely unaffected by NDST2 levels, pointing to a mode of regulation other than increased gene transcription. Size estimation of heparan sulfate polysaccharide chains indicated that increased chain lengths in NDST2-overexpressing cells alone could explain the increased heparan sulfate content. A model is discussed where NDST2-specific substrate modification stimulates elongation resulting in increased heparan sulfate chain length.

Platelet Factor 4 Inhibits and Enhances HIV-1 Infection in a Concentration-Dependent Manner by Modulating Viral Attachment

Platelet factor 4 (PF4) has been recently shown to inhibit infection by a broad range of human immunodeficiency virus type 1 (HIV-1) isolates in vitro. We found that the inhibitory effects of PF4 are limited to a defined concentration range where PF4 exists largely in a monomeric state. Under these conditions, PF4 bound the HIV-1 envelope protein and inhibited HIV-1 attachment to the cell surface. However, as concentrations increased to the point where PF4 exists largely in tetrameric or higher-order forms, viral infection in vitro was enhanced. Enhancement could be inhibited by mutations in PF4 that shift the oligomeric equilibrium toward the monomeric state, or by using soluble glycosaminoglycans (GAGs) to which tetrameric PF4 avidly binds. We conclude that at physiologically relevant concentrations, oligomeric PF4 enhances infection by HIV-1 by interacting with the viral envelope protein as well as cell surface GAGs, enhancing virus attachment to the cell surface. This effect was not specific to HIV-1, as enhancement was seen with some but not all other viruses tested. The biphasic effects of PF4 on HIV-1 infection suggest that native PF4 will not be a useful antiviral agent and that PF4 could contribute to the hematologic abnormalities commonly seen in HIV-infected individuals by enhancing virus infection in the bone marrow.

Heparan Sulfate Modulates Neutrophil and Endothelial Function in Antibacterial Innate Immunity

Recently, we showed that endothelial heparan sulfate facilitates entry of a bacterial pathogen into the central nervous system. Here, we show that normal bactericidal activity of neutrophils is influenced by the sulfation pattern of heparan sulfate. Inactivation of heparan sulfate uronyl 2-O-sulfotransferase (Hs2st) in neutrophils substantially reduced their bactericidal activity, and Hs2st deficiency rendered mice more susceptible to systemic infection with the pathogenic bacterium group B Streptococcus. Specifically, altered sulfation of heparan sulfate in mutant neutrophils affected formation of neutrophil extracellular traps while not influencing phagocytosis, production of reactive oxygen species, or secretion of granular proteases. Heparan sulfate proteoglycan(s) is present in neutrophil extracellular traps, modulates histone affinity, and modulates their microbial activity. Hs2st-deficient brain endothelial cells show enhanced binding to group B Streptococcus and are more susceptible to apoptosis, likely contributing to the observed increase in dissemination of group B Streptococcus into the brain of Hs2st-deficient mice following intravenous challenge. Taken together, our data provide strong evidence that heparan sulfate from both neutrophils and the endothelium plays important roles in modulating innate immunity.

Role of the endothelial surface layer in neutrophil recruitment

Neutrophil recruitment in most tissues is limited to postcapillary venules, where E- and P-selectins are inducibly expressed by venular endothelial cells. These molecules support neutrophil rolling via binding of PSGL-1 and other ligands on neutrophils. Selectins extend ≤ 38 nm above the endothelial plasma membrane, and PSGL-1 extends to 50 nm above the neutrophil plasma membrane. However, endothelial cells are covered with an ESL composed of glycosaminoglycans that is ≥ 500 nm thick and has measurable resistance against compression. The neutrophil surface is also covered with a surface layer. These surface layers would be expected to completely shield adhesion molecules; thus, neutrophils should not be able to roll and adhere. However, in the cremaster muscle and in many other models investigated using intravital microscopy, neutrophils clearly roll, and their rolling is easily and quickly induced. This conundrum was thought to be resolved by the observation that the induction of selectins is accompanied by ESL shedding; however, ESL shedding only partially reduces the ESL thickness (to 200 nm) and thus is insufficient to expose adhesion molecules. In addition to its antiadhesive functions, the ESL also presents neutrophil arrest-inducing chemokines. ESL heparan sulfate can also bind L-selectin expressed by the neutrophils, which contributes to rolling and arrest. We conclude that ESL has both proadhesive and antiadhesive functions. However, most previous studies considered either only the proadhesive or only the antiadhesive effects of the ESL. An integrated model for the role of the ESL in neutrophil rolling, arrest, and transmigration is needed.

An optimized nanoparticle delivery system based on chitosan and chondroitin sulfate molecules reduces the toxicity of amphotericin B and is effective in treating tegumentary leishmaniasis

Amphotericin B (AmpB) is active against leishmaniasis, but its use is hampered due to its high toxicity observed in patients. In this study, a nanoparticles-delivery system for AmpB (NQC-AmpB), containing chitosan and chondroitin sulfate molecules, was evaluated in BALB/c mice against Leishmania amazonensis. An in vivo biodistribution study, including biochemical and toxicological evaluations, was performed to evaluate the toxicity of AmpB. Nanoparticles were radiolabeled with technetium-99m and injected in mice. The products presented a similar biodistribution in the liver, spleen, and kidneys of the animals. Free AmpB induced alterations in the body weight of the mice, which, in the biochemical analysis, indicated hepatic and renal injury, as well as morphological damage to the kidneys of the animals. In general, no significant organic alteration was observed in the animals treated with NQC-AmpB. Mice were infected with L. amazonensis and treated with the nanoparticles or free AmpB; then, parasitological and immunological analyses were performed. The NQC-AmpB group, as compared to the control groups, presented significant reductions in the lesion size and in the parasite burden in all evaluated organs. These animals presented significantly higher levels of IFN-γ and IL-12, and low levels of IL-4 and IL-10, when compared to the control groups. The NQC-AmpB system was effective in reducing the infection in the animals, and proved to be effective in diminishing the toxicity evoked by AmpB, which was observed when it was administered alone. In conclusion, NQC-AmpB could be considered a viable possibility for future studies in the treatment of leishmaniasis.

Nano-sized titanium dioxide-induced splenic toxicity: a biological pathway explored using microarray technology

Titanium dioxide nanoparticles (TiO2 NPs) have been widely used in various areas, and its potential toxicity has gained wide attention. However, the molecular mechanisms of multiple genes working together in the TiO2 NP-induced splenic injury are not well understood. In the present study, 2.5, 5, or 10mg/kg body weight TiO2 NPs were administered to the mice by intragastric administration for 90 consecutive days, their immune capacity in the spleen as well as the gene-expressed characteristics in the mouse damaged spleen were investigated using microarray assay. The findings showed that with increased dose, TiO2 NP exposure resulted in the increases of spleen indices, immune dysfunction, and severe macrophage infiltration as well as apoptosis in the spleen. Importantly, microarray data showed significant alterations in the expressions of 1041 genes involved in immune/inflammatory responses, apoptosis, oxidative stress, stress responses, metabolic processes, ion transport, signal transduction, cell proliferation/division, cytoskeleton and translation in the 10 mg/kg TiO2 NP-exposed spleen. Specifically, Cyp2e1, Sod3, Mt1, Mt2, Atf4, Chac1, H2-k1, Cxcl13, Ccl24, Cd14, Lbp, Cd80, Cd86, Cd28, Il7r, Il12a, Cfd, and Fcnb may be potential biomarkers of spleen toxicity following exposure to TiO2 NPs.

Brittlestars contain highly sulfated chondroitin sulfates/dermatan sulfates that promote fibroblast growth factor 2-induced cell signaling

Glycosaminoglycans (GAGs) isolated from brittlestars, Echinodermata class Ophiuroidea, were characterized, as part of attempts to understand the evolutionary development of these polysaccharides. A population of chondroitin sulfate/dermatan sulfate (CS/DS) chains with a high overall degree of sulfation and hexuronate epimerization was the major GAG found, whereas heparan sulfate (HS) was below detection level. Enzymatic digestion with different chondroitin lyases revealed exceptionally high proportions of di- and trisulfated CS/DS disaccharides. The latter unit appears much more abundant in one of four individual species of brittlestars, Amphiura filiformis, than reported earlier in other marine invertebrates. The brittlestar CS/DS was further shown to bind to growth factors such as fibroblast growth factor 2 and to promote FGF-stimulated cell signaling in GAG-deficient cell lines in a manner similar to that of heparin. These findings point to a potential biological role for the highly sulfated invertebrate GAGs, similar to those ascribed to HS in vertebrates.

Comparative glycomics of leukocyte glycosaminoglycans

Glycosaminoglycans (GAGs) vary widely in disaccharide and oligosaccharide content in a tissue-specific manner. Nonetheless, there are common structural features, such as the presence of highly sulfated non-reducing end domains on heparan sulfate (HS) chains. Less clear are the patterns of expression of GAGs on specific cell types. Leukocytes are known to express GAGs primarily of the chondroitin sulfate (CS) type. However, little is known regarding the properties and structures of the GAG chains, their variability among normal subjects, and changes in structure associated with disease conditions. We isolated peripheral blood leukocyte populations from four human donors and extracted GAGs. We determined the relative and absolute disaccharide abundances for HS and CS GAGs classes using size exclusion chromatography-mass spectrometry (SEC-MS). We found that all leukocytes express HS chains with a level of sulfation that is more similar to heparin than to organ-derived HS. The levels of HS expression follows the trend T cells/B cells > monocytes/natural killer cells > polymorphonuclear leukocytes (PMNs). In addition, CS abundances were considerably higher than total HS but varied considerably in a leukocyte cell type-specific manner. Levels of CS were higher for myeloid lineage cells (PMNs and monocytes) than for lymphoid cells (B, T and natural killer (NK) cells). This information establishes the range of GAG structures expressed on normal leukocytes and is necessary for subsequent inquiry into disease conditions.

A patient with 'spontaneous' heparin-induced thrombocytopenia and thrombosis after undergoing knee replacement

Exogenous heparin exposure is the major risk factor for heparin-induced thrombocytopenia and thrombosis (HITT). To date, only five cases of 'spontaneous' HITT have been reported in the literature. We report the case of a 60-year-old man who developed severe thrombocytopenia (15 000/μl) and bilateral deep venous thromboses 10 days after bilateral knee replacement. There was no evidence of heparin exposure upon review of the patient's preoperative and postoperative medication history. Heparin: platelet factor-4 (PF4) antibodies were positive as was the serotonin release assay. Anticoagulation with argatroban along with prednisone and intravenous immune globulin were administered. Anticoagulation with argatroban was bridged to warfarin once his platelet count reached 100 000/μl, and he was continued on oral prednisone taper. Heparin: PF4 antibodies were negative 3 months after the event. Six months later, Doppler ultrasound revealed deep vein thrombosis. This case represents a third 'spontaneous' episode of HITT in the context of knee replacement surgery without heparin administration. It is unclear whether these cases are a result of exposure to a heparin-like proteoglycan such as chondroitin sulfate during surgery that binds PF4, or whether the perioperative pro-inflammatory milieu is the inciting event. Further consideration of precipitating events in these individuals merits further investigation and may lead to valuable insight into the pathophysiology of heparin-independent PF4-related thrombocytopenia and thrombosis.

Molecular pathways of platelet factor 4/CXCL4 signaling

The platelet-derived chemokine CXCL4 takes a specific and unique position within the family of chemotactic cytokines. Today, much attention is directed to CXCL4's capacity to inhibit angiogenesis and to promote innate immune responses, which makes this chemokine an interesting tool and target for potential intervention in tumor growth and inflammation. However, such attempts demand a comprehensive knowledge on the molecular mechanisms and pathways underlying the corresponding cellular functions. At least two structurally different receptors, CXCR3-B and a chondroitin sulfate proteoglycan, are capable of binding CXCL4 and to induce a specific intracellular signaling machinery. While signaling mediated by CXCR3-B involves Gs proteins, elevated cAMP levels, and p38 MAP kinase, signaling via proteoglycans appears to be more complicated and varies strongly between the cell types analyzed. In CXCL4-activated neutrophils and monocytes, tyrosine kinases of the Src family and Syk as well as monomeric GTPases and members of the MAP kinase family have been identified as essential intracellular signals. Most intriguingly, signaling does not proceed in a linear sequence of events but in a repeated activation of certain transducing elements like Rac2 or sphingosine kinase 1. Depending on the downstream targets, such biphasic kinetics either leads to a redundant and prolonged activation of a single pathway or to a timely separated initiation of disparate signals and functions. Results of the studies reviewed here help to understand the molecular basis of CXCL4's functional diversity and provide insights into integrated signaling processes in general.

The role of the CXC chemokines platelet factor-4 (CXCL4/PF-4) and its variant (CXCL4L1/PF-4var) in inflammation, angiogenesis and cancer

Chemokines are chemotactic cytokines which recruit leukocytes to inflammatory sites. They also affect tumor development and metastasis by acting as growth factor, by attracting pro- or anti-tumoral leukocytes or by influencing angiogenesis. Platelet factor-4 (CXCL4/PF-4) was the first chemokine shown to inhibit angiogenesis. CXCL4L1/PF-4var, recently isolated from thrombin-stimulated platelets, differing from authentic CXCL4/PF-4 in three carboxy-terminally located amino acids, was found to be more potent than CXCL4/PF-4 in inhibiting angiogenesis and tumor growth. Both glycosaminoglycans (GAG) and CXCR3 are implicated in the activities of the PF-4 variants. This report reviews the current knowledge on the role of CXCL4/PF-4 and CXCL4L1/PF-4var in physiological and pathological processes. In particular, the role of CXCL4/PF-4 in cancer, heparin-induced thrombocytopenia and atherosclerosis is described.

CXC chemokine ligand 4 (CXCL4) down-regulates CC chemokine receptor expression on human monocytes

During acute inflammation, monocytes are essential in abolishing invading micro-organisms and encouraging wound healing. Recruitment by CC chemokines is an important step in targeting monocytes to the inflamed tissue. However, cell surface expression of the corresponding chemokine receptors is subject to regulation by various endogenous stimuli which so far have not been comprehensively identified. We report that the platelet-derived CXC chemokine ligand 4 (CXCL4), a known activator of human monocytes, induces down-regulation of CC chemokine receptors (CCR) 1, -2, and -5, resulting in drastic impairment of monocyte chemotactic migration towards cognate CC chemokine ligands (CCL) for these receptors. Interestingly, CXCL4-mediated down-regulation of CCR1, CCR2 and CCR5 was strongly dependent on the chemokine's ability to stimulate autocrine/paracrine release of TNF-α. In turn, TNF-α induced the secretion CCL3 and CCL4, two chemokines selective for CCR1 and CCR5, while the secretion of CCR2-ligand CCL2 was TNF-α-independent. Culture supernatants of CXCL4-stimulated monocytes as well as chemokine-enriched preparations thereof reproduced CXCL4-induced CCR down-regulation. In conclusion, CXCL4 may act as a selective regulator of monocyte migration by stimulating the release of autocrine, receptor-desensitizing chemokine ligands. Our results stress a co-ordinating role for CXCL4 in the cross-talk between platelets and monocytes during early inflammation.

Angiostatic and chemotactic activities of the CXC chemokine CXCL4L1 (platelet factor-4 variant) are mediated by CXCR3

We investigated possible cellular receptors for the human CXC chemokine platelet factor-4 variant/CXCL4L1, a potent inhibitor of angiogenesis. We found that CXCL4L1 has lower affinity for heparin and chondroitin sulfate-E than platelet factor-4 (CXCL4) and showed that CXCL10 and CXCL4L1 could displace each other on microvascular endothelial cells. Labeled CXCL4L1 also bound to CXCR3A- and CXCR3B-transfectants and was displaced by CXCL4L1, CXCL4, and CXCL10. The CXCL4L1 anti-angiogenic activity was blocked by anti-CXCR3 antibodies (Abs) in the Matrigel and cornea micropocket assays. CXCL4L1 application in CXCR3(-/-) or in wild-type mice treated with neutralizing anti-CXCR3 Abs, resulted in reduced inhibitory activity of CXCL4L1 on tumor growth and vascularization of Lewis lung carcinoma. Furthermore, CXCL4L1 and CXCL4 chemoattracted activated T cells, human natural killer cells, and human immature dendritic cells (DCs). Migration of DCs toward CXCL4 and CXCL4L1 was desensitized by preincubation with CXCL10 and CXCL11, inhibited by pertussis toxin, and neutralized by anti-CXCR3 Abs. Chemotaxis of T cells, natural killer cells, and DCs is likely to contribute to the antitumoral action. However, the in vivo data indicate that the angiostatic property of CXCL4L1 is equally important in retarding tumor growth. Thus, both CXCR3A and CXCR3B are implicated in the chemotactic and vascular effects of CXCL4L1.

Structural basis for the growth factor activity of human adenosine deaminase ADA2

Two distinct adenosine deaminases, ADA1 and ADA2, are found in humans. ADA1 has an important role in lymphocyte function and inherited mutations in ADA1 result in severe combined immunodeficiency. The recently isolated ADA2 belongs to the novel family of adenosine deaminase growth factors (ADGFs), which play an important role in tissue development. The crystal structures of ADA2 and ADA2 bound to a transition state analogue presented here reveal the structural basis of the catalytic/signaling activity of ADGF/ADA2 proteins. In addition to the catalytic domain, the structures discovered two ADGF/ADA2-specific domains of novel folds that mediate the protein dimerization and binding to the cell surface receptors. This complex architecture is in sharp contrast with that of monomeric single domain ADA1. An extensive glycosylation and the presence of a conserved disulfide bond and a signal peptide in ADA2 strongly suggest that ADA2, in contrast to ADA1, is specifically designed to act in the extracellular environment. The comparison of catalytic sites of ADA2 and ADA1 demonstrates large differences in the arrangement of the substrate-binding pockets. These structural differences explain the substrate and inhibitor specificity of adenosine deaminases and provide the basis for a rational design of ADA2-targeting drugs to modulate the immune system responses in pathophysiological conditions.

Role of the platelet chemokine platelet factor 4 (PF4) in hemostasis and thrombosis

Chemokines are a family of small proteins that have significant roles in inflammation, angiogenesis and cellular homing. Since inflammation and hemostasis/thrombosis have multiple overlapping roles and pathways, one could expect that some chemokines would also have biologically significant roles in hemostasis/thrombosis as well. This would especially be true for chemokines that are localized solely or predominantly within platelets and released in large amounts at sites of platelet activation such as platelet factor 4 (PF4, CXCL4) and its closely related chemokine, platelet basic protein (PBP, CXCL7). Our group and others have clearly demonstrated an in vivo role for PF4 in hemostasis/thrombosis, but not for PBP, which in contrast has clear proinflammatory properties. This review will focus on PF4 and its potential roles in hemostasis/thrombosis and the underlying pathways by which PF4 may be especially important in such pathologic thrombotic states as heparin-induced thrombocytopenia (HIT) and septic shock.

Platelet chemokines in vascular disease

Platelets are a rich source of different chemokines and express chemokine receptors. CXCL4 is highly abundant in platelets and involved in promoting monocyte arrest from rolling and monocyte differentiation to macrophages. CXCL4 can also associate with CCL5 and amplify its effect on monocytes. The megakaryocyte CXCL7 gene product is proteolytically cleaved into the strong neutrophil chemoattractant, NAP-2, which has also been implicated in repair cell homing to vascular lesions. Platelet adhesion can induce release of CCL2 and CXCL8 from endothelial cells. Conversely, the chemokines CCL17, CCL22, and CXCL12 made by other cells amplify platelet activation. Platelet chemokines enhance recruitment of various hematopoietic cells to the vascular wall, fostering processes such as neointima formation, atherosclerosis, and thrombosis, but also vessel repair and regeneration after vascular injury.

The role of CXC chemokines and their receptors in cancer

Chemokines, or chemotactic cytokines, and their receptors have been discovered as essential and selective mediators in leukocyte migration to inflammatory sites and to secondary lymphoid organs. Besides their functions in the immune system, they also play a critical role in tumor initiation, promotion and progression. There are four subgroups of chemokines: CXC, CC, CX(3)C, and C chemokine ligands. The CXC or alpha subgroup is further subdivided in the ELR(+) and ELR(-) chemokines. Members that contain the ELR motif bind to CXC chemokine receptor 2 (CXCR2) and are angiogenic. In contrast, most of the CXC chemokines without ELR motif bind to CXCR3 and are angiostatic. An exception is the angiogenic ELR(-)CXC chemokine stromal cell-derived factor-1 (CXCL12/SDF-1), which binds to CXCR4 and CXCR7 and is implicated in tumor metastasis. This review is focusing on the role of CXC chemokines and their receptors in tumorigenesis, including angiogenesis, attraction of leukocytes to tumor sites and induction of tumor cell migration and homing in metastatic sites. Finally, their therapeutic use in cancer treatment is discussed.

Quantitative determination of the glycosaminoglycan Delta-disaccharide composition of serum, platelets and granulocytes by reversed-phase high-performance liquid chromatography

Seven Delta-disaccharide standards from heparan sulfate/heparin (HS/H) and nine Delta-disaccharide standards from chondroitin/dermatan sulfate (CS/DS) and hyaluronic acid (HA) were derivatized with the fluorophore 2-aminoacridone (AMAC) and separated in two runs each by reversed-phase HPLC with baseline separation and very short run times. This novel method facilitates the separation of the largest number of Delta-disaccharides from both CS/DS/HA and HS/H with one column and buffer system after fluorophore labeling in two runs at present. For the first time nine glycosaminoglycan (GAG) Delta-disaccharides from CS/DS/HA were separated after fluorophore labeling in one run. The limits of quantification (LOQs) were below 0.2 pmol for CS/DS/HA and HS/H Delta-disaccharides. We demonstrated applicability of our method for biological samples. Furthermore, normal ranges of the GAG Delta-disaccharide compositions from platelets and granulocytes were determined for the first time.

Orf virus VEGF-E NZ2 promotes paracellular NRP-1/VEGFR-2 coreceptor assembly via the peptide RPPR

Vascular endothelial growth factors (VEGFs) interact with the receptor tyrosine kinases (RTKs) VEGFR-1, -2, and -3; neuropilins (NRPs); and heparan sulfate (HS) proteoglycans. VEGF RTKs signal to downstream targets upon ligand-induced tyrosine phosphorylation, while NRPs and HS act as coreceptors that lack enzymatic activity yet modulate signal output by VEGF RTKs. VEGFs exist in various isoforms with distinct receptor specificity and biological activity. Here, a series of mammalian VEGF-A splice variants and orf virus VEGF-Es, as well as chimeric and mutant VEGF variants, were characterized to determine the motifs required for binding to NRP-1 in the absence (VEGF-E) or presence (VEGF-A(165)) of an HS-binding sequence. We identified the carboxyterminal peptides RPPR and DKPRR as the NRP-1 binding motifs of VEGF-E and VEGF-A, respectively. RPPR had significantly higher affinity for NRP-1 than DKPRR. VEGFs containing an RPPR motif promoted HS-independent coreceptor complex assembly between VEGFR-2 and NRP-1, independent of whether these receptors were expressed on the same or separate cells grown in cocultures. Functional studies showed that stable coreceptor assembly by VEGF correlated with its ability to promote vessel formation in an embryoid body angiogenesis assay.

Activation of p38(MAPK) mediates the angiostatic effect of the chemokine receptor CXCR3-B

Chemokines binding the CXCR3 receptor have been shown to inhibit angiogenesis via the CXCR3-B isoform, but the underlying molecular mechanisms are unknown. Aim of this study was to elucidate the effects of CXCR3-B on activation of members of the mitogen-activated protein kinase family, and to explore the relevance of defined signaling pathways to the angiostatic effects of CXCR3-B ligands. Human embryonic kidney (HEK) 293 cells were transfected with expression vectors encoding for CXCR3-A or CXCR3-B. In cells expressing CXCR3-A, CXCL10 (IP-10) at nanomolar concentrations induced activation of ERK, Akt, and Src, as previously described in human vascular pericytes. In HEK-293 cells expressing CXCR3-B, exposure to CXCL10 in the micromolar concentration range led to activation of the p38(MAPK) pathway, as indicated by phosphorylation of p38(MAPK) itself, and of MKK3/6 and MAPKAPK-2, that lie upstream and downstream of p38(MAPK), respectively. Similar results were obtained in cells stimulated with CXCL4 (PF4), a specific ligand of CXCR3-B. In contrast, CXCL4 was unable to activate p38(MAPK) in mock-transfected HEK-293 cells. Only a modest induction of ERK or JNK was observed upon CXCR3-B activation. In human microvascular endothelial cells, which selectively express CXCR3-B, in a cell cycle-dependent fashion, CXCL10 and CXCL4 increased the enzymatic activity of p38(MAPK). Pharmacologic inhibition of p38(MAPK) by SB302580 resulted in a significant increase in DNA synthesis and in reversal of the inhibitory action of CXCL10. In conclusion, the p38(MAPK) pathway is a downstream effector of CXCR3-B implicated in the angiostatic action of this chemokine receptor.

Platelet factor 4 (CXC chemokine ligand 4) differentially regulates respiratory burst, survival, and cytokine expression of human monocytes by using distinct signaling pathways

Platelet factor 4 (PF4; CXCL4) is an abundant platelet alpha-granule CXC chemokine with unique functions. Although lacking a chemotactic activity, PF4 initiates a signal transduction cascade in human monocytes leading to the induction of a broad spectrum of acute and delayed functions including phagocytosis, respiratory burst, survival, and the secretion of cytokines. Surprisingly, although these monocyte functions are well defined, only very limited information exists on the specific signaling pathways that are involved in the regulation of these biological responses. By using specific inhibitors and direct phosphorylation/activation studies, we show in the present study that PF4-mediated respiratory burst is dependent on a very rapid activation of PI3K, Syk, and p38 MAPK. Moreover, monocyte survival and differentiation instead is controlled by a delayed activation of Erk, with an activity peak after 6 h of stimulation. The inhibition of Erk completely reverted PF4-mediated protection against apoptosis. Finally, even though JNK is rapidly activated in PF4-treated monocytes, it is dispensable for the regulation of survival and respiratory burst. However, PF4-induced up-regulation of chemokine and cytokine mRNA and protein requires a sustained activation of JNK and Erk. Taken together, PF4-stimulated immediate monocyte functions (oxygen radical formation) are regulated by p38 MAPK, Syk, and PI3K, whereas delayed functions (survival and cytokine expression) are controlled by Erk and JNK.

How does chondroitinase promote functional recovery in the damaged CNS?

A number of recent studies have established that the bacterial enzyme chondroitinase ABC promotes functional recovery in the injured CNS. The issue of how it works is rarely addressed, however. The effects of the enzyme are presumed to be due to the degradation of inhibitory chondroitin sulphate GAG chains. Here we review what is known about the composition, structure and distribution of the extracellular matrix in the CNS, and how it changes in response to injury. We summarize the data pertaining to the ability of chondroitinase to promote functional recovery, both in the context of axon regeneration and the reactivation of plasticity. We also present preliminary data on the persistence of the effects of the enzyme in vivo, and its hyaluronan-degrading activity in CNS homogenates in vitro. We then consider precisely how the enzyme might influence functional recovery in the CNS. The ability of chondroitinase to degrade hyaluronan is likely to result in greater matrix disruption than the degradation of chondroitin sulphate alone.