Localization of anticoagulantly active heparan sulfate proteoglycans in vascular endothelium: antithrombin binding on cultured endothelial cells and perfused rat aorta

Role of Endothelium in Cardiovascular Sequelae of Long COVID

The global action against coronavirus disease 2019 (COVID-19), caused by SARS-CoV-2 infection, shed light on endothelial dysfunction. Although SARS-CoV-2 primarily affects the pulmonary system, multiple studies have documented pan-vascular involvement in COVID-19. The virus is able to penetrate the endothelial barrier, damaging it directly or indirectly and causing endotheliitis and multi-organ injury. Several mechanisms cooperate to development of endothelial dysfunction, including endothelial cell injury and pyroptosis, hyperinflammation and cytokine storm syndrome, oxidative stress and reduced nitric oxide bioavailability, glycocalyx disruption, hypercoagulability, and thrombosis. After acute-phase infection, some patients reported signs and symptoms of a systemic disorder known as long COVID, in which a broad range of cardiovascular (CV) disorders emerged. To date, the exact pathophysiology of long COVID remains unclear: in addition to the persistence of acute-phase infection mechanisms, specific pathways of CV damage have been postulated, such as persistent viral reservoirs in the heart or an autoimmune response to cardiac antigens through molecular mimicry. The aim of this review is to provide an overview of the main molecular patterns of enduring endothelial activation following SARS-CoV-2 infection and to offer the latest summary of CV complications in long COVID.

Untargeted metabolomic analysis of ischemic injury in human umbilical vein endothelial cells reveals the involvement of arginine metabolism

OBJECTIVE

In this study, differentially expressed metabolites of vascular endothelial cells were examined to further understand the metabolic regulation of ischemic injury by untargeted metabolomics.

METHOD

Human umbilical vein endothelial cells (HUVECs) were selected to construct an ischemia model using oxygen-glucose deprivation (OGD) and 0, 3, 6, and 9 h of treatment. After that, cell survival levels were determined by CCK8 detection. Flow cytometry, ROS detection, JC-1 detection, and western blotting were used to measure apoptosis and oxidative stress in cells. Then, combined with UPLC Orbitrap/MS, we verified the impacted metabolism pathways by western blotting and RT‒PCR.

RESULTS

CCK8 assays showed that the survival of HUVECs was decreased with OGD treatment. Flow cytometry and the expression of cleaved caspase 3 showed that the apoptosis levels of HUVECs increased following OGD treatment. The ROS and JC-1 results further suggested that oxidative stress injury was aggravated. Then, combined with the heatmap, KEGG and IPA, we found that arginine metabolism was differentially altered during different periods of OGD treatment. Furthermore, the expression of four arginine metabolism-related proteins, ASS1, ARG2, ODC1 and SAT1, was found to change during treatment.

CONCLUSION

Arginine metabolism pathway-related proteins were significantly altered by OGD treatment, which suggests that they may have a potential role in ischemic injury.

Sb-Phenyl-N-methyl-5,6,7,12-tetrahydrodibenz[c,f][1,5]azastibocine Induces Perlecan Core Protein Synthesis in Cultured Vascular Endothelial Cells

Vascular endothelial cells synthesize and secrete perlecan, a large heparan sulfate proteoglycan that increases the anticoagulant activity of vascular endothelium by inducing antithrombin III and intensifying fibroblast growth factor (FGF)-2 activity to promote migration and proliferation in the repair process of damaged endothelium during the progression of atherosclerosis. However, the exact regulatory mechanisms of endothelial perlecan expression remain unclear. Since organic-inorganic hybrid molecules are being developed rapidly as tools to analyze biological systems, we searched for a molecular probe to analyze these mechanisms using a library of organoantimony compounds and found that the Sb-phenyl-N-methyl-5,6,7,12-tetrahydrodibenz[c,f][1,5]azastibocine (PMTAS) molecule promotes the expression of perlecan core protein gene without exhibiting cytotoxicity in vascular endothelial cells. In the present study, we characterized proteoglycans synthesized by cultured bovine aortic endothelial cells using biochemical techniques. The results indicated that PMTAS selectively induced perlecan core protein synthesis, without affecting the formation of its heparan sulfate chain, in vascular endothelial cells. The results also implied that this process is independent of the endothelial cell density, whereas in vascular smooth muscle cells, it occurred only at high cell density. Thus, PMTAS would be a useful tool for further studies on the mechanisms underlying perlecan core protein synthesis in vascular cells, which is critical in the progression of vascular lesions, such as those during atherosclerosis.

Alterations in heparan sulfate proteoglycan synthesis and sulfation and the impact on vascular endothelial function

The glycocalyx attached to the apical surface of vascular endothelial cells is a rich network of proteoglycans, glycosaminoglycans, and glycoproteins with instrumental roles in vascular homeostasis. Given their molecular complexity and ability to interact with the intra- and extracellular environment, heparan sulfate proteoglycans uniquely contribute to the glycocalyx's role in regulating endothelial permeability, mechanosignaling, and ligand recognition by cognate cell surface receptors. Much attention has recently been devoted to the enzymatic shedding of heparan sulfate proteoglycans from the endothelial glycocalyx and its impact on vascular function. However, other molecular modifications to heparan sulfate proteoglycans are possible and may have equal or complementary clinical significance. In this narrative review, we focus on putative mechanisms driving non-proteolytic changes in heparan sulfate proteoglycan expression and alterations in the sulfation of heparan sulfate side chains within the endothelial glycocalyx. We then discuss how these specific changes to the endothelial glycocalyx impact endothelial cell function and highlight therapeutic strategies to target or potentially reverse these pathologic changes.

Physico-mechanical and biological evaluation of heparin/VEGF-loaded electrospun polycaprolactone/decellularized rat aorta extracellular matrix for small-diameter vascular grafts

Although the continuous development of small-diameter vascular grafts (SDVGs) (D < 5 mm) continues, most vascular grafts are made from synthetic polymers, which lead to serious complications from arteriosclerosis, thrombosis, and vascular ischemia. Here, to address these shortcomings, we combine synthetic polymers with natural decellularized small-diameter vessels and loaded with growth factor. We fabricated vascular grafts by electrospinning polycaprolactone (PCL) to decellularized rat aorta matrix (ECM) followed by heparin and vascular endothelial growth factor (VEGF) loading. In- vitro studies showed that PCL/ECM/VEGF vascular grafts, showed excellent hemocompatibility and biocompatibility properties. The vascular grafts implanted into the rat aorta revealed that the PCL/ECM/VEGF grafts promotes endothelial cells and smooth-muscle cells infiltration with a rate of FLK-1, ICAM1, and a-SMA distribution higher than that of the PCL and PCL/ECM vascular grafts at 2 weeks and 4 weeks after implantation. The PCL/ECM/VEGF vascular graft should be considered for potential small-diameter vascular grafts in clinical fields.

Vascular Endothelial Glycocalyx Damage and Potential Targeted Therapy in COVID-19

COVID-19 is a highly infectious respiratory disease caused by a new coronavirus known as SARS-CoV-2. COVID-19 is characterized by progressive respiratory failure resulting from diffuse alveolar damage, inflammatory infiltrates, endotheliitis, and pulmonary and systemic coagulopathy forming obstructive microthrombi with multi-organ dysfunction, indicating that endothelial cells (ECs) play a central role in the pathogenesis of COVID-19. The glycocalyx is defined as a complex gel-like layer of glycosylated lipid–protein mixtures, which surrounds all living cells and acts as a buffer between the cell and the extracellular matrix. The endothelial glycocalyx layer (EGL) plays an important role in vascular homeostasis via regulating vascular permeability, cell adhesion, mechanosensing for hemodynamic shear stresses, and antithrombotic and anti-inflammatory functions. Here, we review the new findings that described EGL damage in ARDS, coagulopathy, and the multisystem inflammatory disease associated with COVID-19. Mechanistically, the inflammatory mediators, reactive oxygen species (ROS), matrix metalloproteases (MMPs), the glycocalyx fragments, and the viral proteins may contribute to endothelial glycocalyx damage in COVID-19. In addition, the potential therapeutic strategies targeting the EGL for the treatment of severe COVID-19 are summarized and discussed.

Specific components associated with the endothelial glycocalyx are lost from brain capillaries in cerebral malaria

BACKGROUND

Cerebral malaria (CM) is a rare, but severe and frequently fatal outcome of infections with Plasmodium falciparum. Pathogenetic mechanisms include endothelial activation and sequestration of parasitized erythrocytes in the cerebral microvessels. Increased concentrations of glycosaminoglycans in urine and plasma of malaria patients have been described, suggesting involvement of endothelial glycocalyx.

METHODS

We used lectin histochemistry on postmortem samples to compare the distribution of multiple sugar epitopes on cerebral capillaries in children who died from CM and from non-malarial comas.

RESULTS

N-acetyl glucosamine residues detected by tomato lectin are generally reduced in children with CM compared to controls. We used the vascular expression of intercellular adhesion molecule-1 and mannose residues on brain capillaries of CM as evidence of local vascular inflammation, and both were expressed more highly in CM patients than controls. Sialic acid residues were found to be significantly reduced in patients with CM. By contrast, the levels of other sugar epitopes regularly detected on the cerebral vasculature were unchanged, and this suggests specific remodeling of cerebral microvessels in CM patients.

CONCLUSIONS

Our findings support and expand upon earlier reports of disruptions of the endothelial glycocalyx in children with severe malaria.

Human endothelial cells and fibroblasts express and produce the coagulation proteins necessary for thrombin generation

In a previous study, we reported that human endothelial cells (ECs) express and produce their own coagulation factors (F) that can activate cell surface FX without the additions of external proteins or phospholipids. We now describe experiments that detail the expression and production in ECs and fibroblasts of the clotting proteins necessary for formation of active prothrombinase (FV-FX) complexes to produce thrombin on EC and fibroblast surfaces. EC and fibroblast thrombin generation was identified by measuring: thrombin activity; thrombin-antithrombin complexes; and the prothrombin fragment 1.2 (PF1.2), which is produced by the prothrombinase cleavage of prothrombin (FII) to thrombin. In ECs, the prothrombinase complex uses surface-attached FV and γ-carboxyl-glutamate residues of FX and FII to attach to EC surfaces. FV is also on fibroblast surfaces; however, lower fibroblast expression of the gene for γ-glutamyl carboxylase (GGCX) results in production of vitamin K-dependent coagulation proteins (FII and FX) with reduced surface binding. This is evident by the minimal surface binding of PF1.2, following FII activation, of fibroblasts compared to ECs. We conclude that human ECs and fibroblasts both generate thrombin without exogenous addition of coagulation proteins or phospholipids. The two cell types assemble distinct forms of prothrombinase to generate thrombin.

Heparanase as a potential player in SARS-CoV-2 infection and induced coagulopathy

During the current formidable COVID-19 pandemic, it is appealing to address ideas that may invoke therapeutic interventions. Clotting disorders are well recognized in patients infected with severe acute respiratory syndrome (SARS) caused by a novel coronavirus (SARS-CoV-2), which lead to severe complications that worsen the prognosis in these subjects. Increasing evidence implicate Heparan sulfate proteoglycans (HSPGs) and Heparanase in various diseases and pathologies, including hypercoagulability states. Moreover, HSPGs and Heparanase are involved in several viral infections, in which they enhance cell entry and release of the viruses. Herein we discuss the molecular involvement of HSPGs and heparanase in SARS-CoV-2 infection, namely cell entry and release, and the accompanied coagulopathy complications, which assumedly could be blocked by heparanase inhibitors such as Heparin and Pixatimod.

Endothelial glycocalyx in traumatic brain injury associated coagulopathy: potential mechanisms and impact

Traumatic brain injury (TBI) remains one of the leading causes of death and disability worldwide; more than 10 million people are hospitalized for TBI every year around the globe. While the primary injury remains unavoidable and not accessible to treatment, the secondary injury which includes oxidative stress, inflammation, excitotoxicity, but also complicating coagulation abnormalities, is potentially avoidable and profoundly affects the therapeutic process and prognosis of TBI patients. The endothelial glycocalyx, the first line of defense against endothelial injury, plays a vital role in maintaining the delicate balance between blood coagulation and anticoagulation. However, this component is highly vulnerable to damage and also difficult to examine. Recent advances in analytical techniques have enabled biochemical, visual, and computational investigation of this vascular component. In this review, we summarize the current knowledge on (i) structure and function of the endothelial glycocalyx, (ii) its potential role in the development of TBI associated coagulopathy, and (iii) the options available at present for detecting and protecting the endothelial glycocalyx.

Antithrombin and Its Role in Host Defense and Inflammation

Antithrombin (AT) is a natural anticoagulant that interacts with activated proteases of the coagulation system and with heparan sulfate proteoglycans (HSPG) on the surface of cells. The protein, which is synthesized in the liver, is also essential to confer the effects of therapeutic heparin. However, AT levels drop in systemic inflammatory diseases. The reason for this decline is consumption by the coagulation system but also by immunological processes. Aside from the primarily known anticoagulant effects, AT elicits distinct anti-inflammatory signaling responses. It binds to structures of the glycocalyx (syndecan-4) and further modulates the inflammatory response of endothelial cells and leukocytes by interacting with surface receptors. Additionally, AT exerts direct antimicrobial effects: depending on AT glycosylation it can bind to and perforate bacterial cell walls. Peptide fragments derived from proteolytic degradation of AT exert antibacterial properties. Despite these promising characteristics, therapeutic supplementation in inflammatory conditions has not proven to be effective in randomized control trials. Nevertheless, new insights provided by subgroup analyses and retrospective trials suggest that a recommendation be made to identify the patient population that would benefit most from AT substitution. Recent experiment findings place the role of various AT isoforms in the spotlight. This review provides an overview of new insights into a supposedly well-known molecule.

Therapeutic Implications of COVID-19 for the Interventional Cardiologist

Although COVID-19 is viewed primarily as a respiratory disease, cardiovascular risk factors and disease are prevalent among infected patients and are associated with worse outcomes. In addition, among multiple extra-pulmonary manifestations, there has been an increasing recognition of specific cardiovascular complications of COVID-19. Despite this, in the initial stages of the pandemic there was evidence of a reduction in patients presenting to acute cardiovascular services. In this masterclass review, with the aid of 2 exemplar cases, we will focus on the important therapeutic implications of COVID-19 for interventional cardiologists. We summarize the existing evidence base regarding the varied cardiovascular presentations seen in COVID-19 positive patients and the prognostic importance and potential mechanisms of acute myocardial injury in this setting. Importantly, through the use of a systematic review of the literature, we focus our discussion on the observed higher rates of coronary thrombus burden in patients with COVID-19 and acute coronary syndromes.

Anticoagulant and signaling functions of antithrombin

Antithrombin (AT) is a major plasma glycoprotein of the serpin superfamily that regulates the proteolytic activity of the procoagulant proteases of both intrinsic and extrinsic pathways. Two important structural features that participate in the regulatory function of AT include a mobile reactive center loop that binds to active site of coagulation proteases, trapping them in the form of inactive covalent complexes, and a basic D-helix that binds to therapeutic heparins and heparan sulfate proteoglycans (HSPGs) on vascular endothelial cells. The binding of D-helix of AT by therapeutic heparins promotes the reactivity of the serpin with coagulation proteases by several orders of magnitude by both a conformational activation of the serpin and a template (bridging) mechanism. In addition to its essential anticoagulant function, AT elicits a potent anti-inflammatory signaling response when it binds to distinct vascular endothelial cell HSPGs, thereby inducing prostacyclin synthesis. Syndecans-4 has been found as a specific membrane-bound HSPG receptor on endothelial cells that relays the signaling effect of AT to the relevant second messenger molecules in the signal transduction pathways inside the cell. However, following cleavage by coagulation proteases and/or by spontaneous conversion to a latent form, AT loses both its anti-inflammatory activity and high-affinity interaction with heparin and HSPGs. Interestingly, these low-affinity heparin conformers of AT elicit potent proapoptotic and antiangiogenic activities by also binding to specific HSPGs by unknown mechanisms. This review article will summarize current knowledge about mechanisms through which different conformers of AT exert their serine protease inhibitory and intracellular signaling functions in these biological pathways.

Hierarchical Structures, with Submillimeter Patterns, Micrometer Wrinkles, and Nanoscale Decorations, Suppress Biofouling and Enable Rapid Droplet Digitization

Liquid repellant surfaces have been shown to play a vital role for eliminating thrombosis on medical devices, minimizing blood contamination on common surfaces as well as preventing non-specific adhesion. Herein, an all solution-based, easily scalable method for producing liquid repellant flexible films, fabricated through nanoparticle deposition and heat-induced thin film wrinkling that suppress blood adhesion, and clot formation is reported. Furthermore, superhydrophobic and hydrophilic surfaces are combined onto the same substrate using a facile streamlined process. The patterned superhydrophobic/hydrophilic surfaces show selective digitization of droplets from various solutions with a single solution dipping step, which provides a route for rapid compartmentalization of solutions into virtual wells needed for high-throughput assays. This rapid solution digitization approach is demonstrated for detection of Interleukin 6. The developed liquid repellant surfaces are expected to find a wide range of applications in high-throughput assays and blood contacting medical devices.

Chikungunya Virus Strains from Each Genetic Clade Bind Sulfated Glycosaminoglycans as Attachment Factors

Alphavirus infections are a global health threat, contributing to outbreaks of disease in many parts of the world. Recent epidemics caused by CHIKV, an arthritogenic alphavirus, resulted in more than 8.5 million cases as the virus has spread into new geographic regions, including the Western Hemisphere. CHIKV causes disease in the majority of people infected, leading to severe and debilitating arthritis. Despite the severity of CHIKV disease, there are no licensed therapeutics. Since attachment factors and receptors are determinants of viral tropism and pathogenesis, understanding these virus-host interactions can enhance our knowledge of CHIKV infection. We analyzed over 670 glycans and identified GAGs as the main glycan bound by CHIKV. We defined specific GAG components required for CHIKV binding and assessed strain-specific differences in GAG binding capacity. These studies provide insight about cell surface molecules that CHIKV binds, which could facilitate the development of antiviral therapeutics targeting the CHIKV attachment step.

Chikungunya virus (CHIKV) is an arthritogenic alphavirus that causes debilitating musculoskeletal disease. CHIKV displays broad cell, tissue, and species tropism, which may correlate with the attachment factors and entry receptors used by the virus. Cell surface glycosaminoglycans (GAGs) have been identified as CHIKV attachment factors. However, the specific types of GAGs and potentially other glycans to which CHIKV binds and whether there are strain-specific differences in GAG binding are not fully understood. To identify the types of glycans bound by CHIKV, we conducted glycan microarray analyses and discovered that CHIKV preferentially binds GAGs. Microarray results also indicate that sulfate groups on GAGs are essential for CHIKV binding and that CHIKV binds most strongly to longer GAG chains of heparin and heparan sulfate. To determine whether GAG binding capacity varies among CHIKV strains, a representative strain from each genetic clade was tested. While all strains directly bound to heparin and chondroitin sulfate in enzyme-linked immunosorbent assays (ELISAs) and depended on heparan sulfate for efficient cell binding and infection, we observed some variation by strain. Enzymatic removal of cell surface GAGs and genetic ablation that diminishes GAG expression reduced CHIKV binding and infectivity of all strains. Collectively, these data demonstrate that GAGs are the preferred glycan bound by CHIKV, enhance our understanding of the specific GAG moieties required for CHIKV binding, define strain differences in GAG engagement, and provide further evidence for a critical function of GAGs in CHIKV cell attachment and infection.

IMPORTANCE Alphavirus infections are a global health threat, contributing to outbreaks of disease in many parts of the world. Recent epidemics caused by CHIKV, an arthritogenic alphavirus, resulted in more than 8.5 million cases as the virus has spread into new geographic regions, including the Western Hemisphere. CHIKV causes disease in the majority of people infected, leading to severe and debilitating arthritis. Despite the severity of CHIKV disease, there are no licensed therapeutics. Since attachment factors and receptors are determinants of viral tropism and pathogenesis, understanding these virus-host interactions can enhance our knowledge of CHIKV infection. We analyzed over 670 glycans and identified GAGs as the main glycan bound by CHIKV. We defined specific GAG components required for CHIKV binding and assessed strain-specific differences in GAG binding capacity. These studies provide insight about cell surface molecules that CHIKV binds, which could facilitate the development of antiviral therapeutics targeting the CHIKV attachment step.

Transcriptional Induction of Cystathionine γ-Lyase, a Reactive Sulfur-Producing Enzyme, by Copper Diethyldithiocarbamate in Cultured Vascular Endothelial Cells

As toxic substances can enter the circulating blood and cross endothelial monolayers to reach parenchymal cells in organs, vascular endothelial cells are an important target compartment for such substances. Reactive sulfur species protect cells against oxidative stress and toxic substances, including heavy metals. Reactive sulfur species are produced by enzymes, such as cystathionine γ-lyase (CSE), cystathionine β-synthase, 3-mercaptopyruvate sulfurtransferase, and cysteinyl-tRNA synthetase. However, little is known about the regulatory mechanisms underlying the expression of these enzymes in vascular endothelial cells. Bio-organometallics is a research field that analyzes biological systems using organic-inorganic hybrid molecules (organometallic compounds and metal coordinating compounds) as molecular probes. In the present study, we analyzed intracellular signaling pathways that mediate the expression of reactive sulfur species-producing enzymes in cultured bovine aortic endothelial cells, using copper diethyldithiocarbamate (Cu10). Cu10 selectively upregulated CSE gene expression in vascular endothelial cells independent of cell density. This transcriptional induction of endothelial CSE required both the diethyldithiocarbamate scaffold and the coordinated copper ion. Additionally, the present study revealed that ERK1/2, p38 MAPK, and hypoxia-inducible factor (HIF)-1α/HIF-1β pathways mediate transcriptional induction of endothelial CSE by Cu10. The transcription factors NF-κB, Sp1, and ATF4 were suggested to act in constitutive CSE expression, although the possibility that they are involved in the CSE induction by Cu10 cannot be excluded. The present study used a copper complex as a molecular probe to reveal that the transcription of CSE is regulated by multiple pathways in vascular endothelial cells, including ERK1/2, p38 MAPK, and HIF-1α/HIF-1β. Bio-organometallics appears to be an effective strategy for analyzing the functions of intracellular signaling pathways in vascular endothelial cells.

Plasmodium falciparum histidine rich protein HRPII inhibits the anti-inflammatory function of antithrombin

BACKGROUND

It has been reported that histidine-rich protein II (HRPII), secreted by the malaria parasite, Plasmodium falciparum (Pf), inhibits the heparin-dependent anticoagulant activity of antithrombin (AT) in vitro and in plasma-based assay systems.

OBJECTIVE

The objective of this study was to test the hypothesis that HRPII may also interact with the AT-binding vascular glycosaminoglycans (GAGs), thereby inhibiting the anti-inflammatory signaling function of the serpin.

METHODS

We expressed HRPII in bacteria, purified it to homogeneity and studied its effect on endothelial cell signaling in the absence and presence of AT employing established signaling assays.

RESULTS

We demonstrate that a low concentration of HRPII potently disrupts the barrier permeability function of endothelial cells. Moreover, HRPII competitively inhibits the protective effect of AT by a concentration-dependent manner. Similarly, AT inhibits the pro-inflammatory activity of HRPII by a concentration-dependent manner. The siRNA knockdown of 3-O-sulfotransferase 1 (3-OST-1), the enzyme responsible for the essential 3-O-sulfation of the AT-binding GAGs, downregulates the pro-inflammatory function of HRPII in endothelial cells, supporting the hypothesis that HRPII competitively inhibits the interaction of AT with 3-OS containing vascular GAGs. Histidine-rich protein II elicits its barrier-disruptive effect by the Src-dependent phosphorylation of vascular endothelial (VE)-cadherin and AT counteracts this effect. We further demonstrate that inorganic polyphosphates bind HRPII with a high affinity to amplify the pro-inflammatory signaling function of HRPII in both cellular and in vivo permeability models.

CONCLUSION

We postulate that Pf-derived HRPII and polyphosphate can contribute to the pathogenesis of malaria infection by downregulating the AT-dependent anti-inflammatory and anticoagulant pathways.

PKC (Protein Kinase C)-δ Modulates AT (Antithrombin) Signaling in Vascular Endothelial Cells

OBJECTIVE

Native and latent conformers of AT (antithrombin) induce anti-inflammatory and proapoptotic signaling activities, respectively, in vascular endothelial cells by unknown mechanisms. Synd-4 (syndecan-4) has been identified as a receptor that is involved in transmitting signaling activities of AT in endothelial cells. Approach and Results: In this study, we used flow cytometry, signaling assays, immunoblotting and confocal immunofluorescence microscopy to investigate the mechanism of the paradoxical signaling activities of high-affinity heparin (native) and low-affinity heparin (latent) conformers of AT in endothelial cells. We discovered that native AT binds to glycosaminoglycans on vascular endothelial cells via its heparin-binding D-helix to induce anti-inflammatory signaling responses by recruiting PKC (protein kinase C)-δ to the plasma membrane and promoting phosphorylation of the Synd-4 cytoplasmic domain at Ser179. By contrast, the binding of latent AT to endothelial cells to a site(s), which is not competed by the native AT, induces a proapoptotic effect by localizing PKC-δ to the perinuclear/nuclear compartment in endothelial cells. Overexpression of a dominant-negative form of PKC-δ resulted in inhibition of anti-inflammatory and proapoptotic signaling activities of both native and latent AT.

CONCLUSIONS

These results indicate that the native and latent conformers of AT may exert their distinct intracellular signaling effects through differentially modulating the subcellular localization of PKC-δ in endothelial cells.

Sesamin Induces Endothelial Nitric Oxide Synthase Activation via Transient Receptor Potential Vanilloid Type 1

Sesamin, the most abundant lignan in sesame seed oil, has many biological activities. However, the underlying molecular mechanisms behind the regulatory effects of sesamin on endothelial nitric oxide synthase (eNOS) activity and nitric oxide (NO) generation in endothelial cells (ECs) remain unclear. Sesamin induced the intracellular level of NO and eNOS phosphorylation in ECs in a concentration- and time-dependent manner. Additionally, sesamin induced levels of intracellular calcium, leading to the phosphorylation of calmodulin-dependent protein kinase II (CaMKII) at Thr286, calcium/calmodulin-dependent protein kinase kinase beta (CaMKKβ) at Ser511, protein kinase A (PKA) at Thr197, Akt at Ser473, and AMP-activated protein kinase (AMPK) at Thr172. In particular, blocking of the transient receptor potential vanilloid type 1 (TRPV1) channel by capsazepine (TRPV1 antagonist), as well as TRPV1 knockdown via TRPV1 silencing RNA, abrogated sesamin-induced PKA, Akt, AMPK, CaMKII, CaMKKβ, and eNOS phosphorylation and NO level in ECs. Furthermore, sesamin inhibited TNF-α-induced NF-κB translocation, intercellular adhesion molecule-1 expression, and monocyte adhesion. Sesamin triggered eNOS activity and NO production via activation of TRPV1-calcium signaling, which involved the phosphorylation of PKA, CaMKII, CaMKKβ, Akt, and AMPK. Sesamin may be useful for treating or preventing the endothelial dysfunction correlated with cardiovascular diseases.

Visualizing antithrombin-binding 3-O-sulfated heparan sulfate motifs on cell surfaces

To map the cellular topography of the rare 3-O-sulfated structural motif of heparan sulfate (HS), we constructed quantum dot-based probes for antithrombin and FGF2, which reveal widely different distribution of the targeted HS motifs.

Rosetting revisited: a critical look at the evidence for host erythrocyte receptors in Plasmodium falciparum rosetting

Malaria remains a major cause of mortality in African children, with no adjunctive treatments currently available to ameliorate the severe clinical forms of the disease. Rosetting, the adhesion of infected erythrocytes (IEs) to uninfected erythrocytes, is a parasite phenotype strongly associated with severe malaria, and hence is a potential therapeutic target. However, the molecular mechanisms of rosetting are complex and involve multiple distinct receptor–ligand interactions, with some similarities to the diverse pathways involved in P. falciparum erythrocyte invasion. This review summarizes the current understanding of the molecular interactions that lead to rosette formation, with a particular focus on host uninfected erythrocyte receptors including the A and B blood group trisaccharides, complement receptor one, heparan sulphate, glycophorin A and glycophorin C. There is strong evidence supporting blood group A trisaccharides as rosetting receptors, but evidence for other molecules is incomplete and requires further study. It is likely that additional host erythrocyte rosetting receptors remain to be discovered. A rosette-disrupting low anti-coagulant heparin derivative is being investigated as an adjunctive therapy for severe malaria, and further research into the receptor–ligand interactions underlying rosetting may reveal additional therapeutic approaches to reduce the unacceptably high mortality rate of severe malaria.

Derangement of the endothelial glycocalyx in sepsis

The vascular endothelial surface is coated by the glycocalyx, a ubiquitous gel-like layer composed of a membrane-binding domain that contains proteoglycans, glycosaminoglycan side-chains, and plasma proteins such as albumin and antithrombin. The endothelial glycocalyx plays a critical role in maintaining vascular homeostasis. However, this component is highly vulnerable to damage and is also difficult to examine. Recent advances in analytical techniques have enabled biochemical, visual and computational investigation of this vascular component. The glycocalyx modulates leukocyte-endothelial interactions, thrombus formation and other processes that lead to microcirculatory dysfunction and critical organ injury in sepsis. It also acts as a regulator of vascular permeability and contains mechanosensors as well as receptors for growth factors and anticoagulants. During the initial onset of sepsis, the glycocalyx is damaged and circulating levels of glycocalyx components, including syndecans, heparan sulfate and hyaluronic acid, can be measured and are reportedly useful as biomarkers for sepsis. Also, a new methodology using side-stream dark-field imaging is now clinically available for assessing the glycocalyx. Multiple factors including hypervolemia and hyperglycemia are toxic to the glycocalyx, and several agents have been proposed as therapeutic modalities, although no single treatment has been proven to be clinically effective. In this article, we review the derangement of the glycocalyx in sepsis. Despite the accumulated knowledge regarding the important roles of the glycocalyx, the relationship between derangement of the endothelial glycocalyx and severity of sepsis or disseminated intravascular coagulation has not been adequately elucidated and further work is needed.

Endothelial Function: A Short Guide for the Interventional Cardiologist

An impaired function of the coronary endothelium is an important determinant of all stages of atherosclerosis, from initiation, to mediation of functional phenomena—such as spasm and plaque erosion, to atherothrombotic complications. Endothelial function is modified by therapies, including stent implantation. Finally, endothelial function changes over time, in response to physical stimuli and pharmocotherapies, and its assessment might provide information on how individual patients respond to specific therapies. In this review, we describe the role of the endothelium in the continuum of coronary atherosclerosis, from the perspective of the interventional cardiologist. In the first part, we review the current knowledge of the role of endothelial (dys)function on atherosclerotic plaque progression/instabilization and on the mechanisms of ischemia, in the absence of coronary artery stenosis. In the second part of this review, we describe the impact of coronary artery stenting on endothelial function, platelet aggregation, and inflammation.

Copper(II) Bis(diethyldithiocarbamate) Induces the Expression of Syndecan-4, a Transmembrane Heparan Sulfate Proteoglycan, via p38 MAPK Activation in Vascular Endothelial Cells

Proteoglycans synthesized by vascular endothelial cells are important for regulating cell function and the blood coagulation-fibrinolytic system. Since we recently reported that copper(II) bis(diethyldithiocarbamate) (Cu(edtc)₂) modulates the expression of some molecules involving the antioxidant and blood coagulation systems, we hypothesized that Cu(edtc)₂ may regulate the expression of proteoglycans and examined this hypothesis using a bovine aortic endothelial cell culture system. The experiments showed that Cu(edtc)₂ induced the expression of syndecan-4, a transmembrane heparan sulfate proteoglycan, in a dose- and time-dependent manner. This induction required the whole structure of Cu(edtc)₂—the specific combination of intramolecular copper and a diethyldithiocarbamate structure—as the ligand. Additionally, the syndecan-4 induction by Cu(edtc)₂ depended on the activation of p38 mitogen-activated protein kinase (MAPK) but not the Smad2/3, NF-E2-related factor2 (Nrf2), or epidermal growth factor receptor (EGFR) pathways. p38 MAPK may be a key molecule for inducing the expression of syndecan-4 in vascular endothelial cells.

Responses of Endothelial Cells Towards Ischemic Conditioning Following Acute Myocardial Infarction

One of the primary therapeutic goals of modern cardiology is to design strategies aimed at minimizing myocardial infarct size and optimizing cardiac function following acute myocardial infarction (AMI). Patients with AMI who underwent reperfusion therapy display dysfunction of the coronary endothelium. Consequently, ischemic endothelial cells become more permeable and weaken their natural anti-thrombotic and anti-inflammatory potential. Ischemia-reperfusion injury (IRI) is associated with activation of the humoral and cellular components of the hemostatic and innate immune system, and also with excessive production of reactive oxygen species (ROS), the inhibition of nitric oxide synthase, and with inflammatory processes. Given its essential role in the regulation of vascular homeostasis, involving platelets and leukocytes among others, dysfunctional endothelium can lead to increased risk of coronary vasospasm and thrombosis. Endothelial dysfunction can be prevented by ischemic conditioning with a protective intervention based on limited intermittent periods of ischemia and reperfusion. The molecular mechanisms and signal transduction pathways underlying conditioning phenomena in the coronary endothelium have been described as involving less ROS production, reduced adhesion of neutrophils to endothelial cells and diminished inflammatory reactions. This review summarizes our current understanding of the cellular and molecular mechanisms regulating IRI-affected and -damaged coronary endothelium, and how ischemic conditioning may preserve its function.

Endothelial cell surface limits coagulation without modulating the antithrombin potency

Antithrombin (AT) binds in vitro and in vivo to endothelial cells through various receptors, including heparan sulphate glycosaminoglycan (HSPG) that could modulate the AT activity. A thrombin generation assay (TGA) was set up at the surface of HUVEC and HMVEC evaluating their participation in the coagulation-anticoagulation processes. TGA induced by 0.5 pM Tissue Factor was performed in normal or AT-deficient plasma spiked with various amounts of recombinant or plasma-derived AT (0, 0.1, 0.5 and 1.0 U/ml). To evaluate the role of HSPG or cellular anticoagulant receptors, cells were treated or not with heparin, a mix of heparanase I, II and III, a neutralizing anti-Endothelial Protein C Receptor (EPCR) or with an anti-Tissue Factor Pathway Inhibitor (TFPI) antibody. The presence of the cells diminished the TG in normal plasma and maintained anticoagulation in AT-deficient plasma. Spiking the AT-deficient plasma with different doses of AT demonstrated that the cells did not amplify the anticoagulant activity of AT. The recombinant AT binds the cells with a higher avidity than the plasma-derived one but this did not affect its anticoagulant potency. Moreover both bindings are independent of the HSPG. The antithrombotic activity kept in absence of AT was not inhibited by blocking antibodies directed against EPCR or TFPI. Our data did not reveal a major co-factor activity for AT from endothelial cells that could have been mediated by HSPG. In contrast, it reveals the presence of alternative anti-coagulant system(s) in two venous cell types that maintain an antithrombotic activity.

Cooperative Interactions of Three Hotspot Heparin Binding Residues Are Critical for Allosteric Activation of Antithrombin by Heparin

Heparin allosterically activates the anticoagulant serpin, antithrombin, by binding through a sequence-specific pentasaccharide and inducing activating conformational changes in the protein. Three basic residues of antithrombin, Lys114, Lys125, and Arg129, have been shown to be hotspots for binding the pentasaccharide, but the molecular basis for such hotspot binding has been unclear. To determine whether this results from cooperative interactions, we analyzed the effects of single, double, and triple mutations of the hotspot residues on pentasaccharide binding and activation of antithrombin. Double-mutant cycles revealed that the contribution of each residue to pentasaccharide binding energy was progressively reduced when one or both of the other residues were mutated, indicating strong coupling between each pair of residues that was dependent on the third residue and reflective of the three residues acting as a cooperative unit. Rapid kinetic studies showed that the hotspot residue mutations progressively abrogated the ability of the pentasaccharide to bind productively to native antithrombin and to conformationally activate the serpin by engaging the hotspot residues in an induced-fit interaction. Examination of the antithrombin-pentasaccharide complex structure revealed that the hotspot residues form two adjoining binding pockets for critical sulfates of the pentasaccharide that structurally link these residues. Together, these findings demonstrate that cooperative interactions of Lys114, Lys125, and Arg129 are critical for the productive induced-fit binding of the heparin pentasaccharide to antithrombin that allosterically activates the anticoagulant function of the serpin.

Expression and functional characterization of two natural heparin-binding site variants of antithrombin

Essentials Heparin-binding site (HBS) variants of antithrombin (AT) are associated with thrombosis risk. HSB variants have, in general, normal progressive inhibitory activity but reduced heparin affinity. Thrombosis in HSB carriers has been primarily attributed to the loss of heparin cofactor activity. Results here demonstrate that HSB variants of AT also lack anti-inflammatory signaling functions.

SUMMARY

Background Several heparin-binding site (HBS) variants of antithrombin (AT) have been identified that predispose carriers to a higher incidence of thrombosis. Thrombosis in carriers of HBS variants has been primarily attributed to a loss in their heparin-dependent anticoagulant function. Objective The objective of this study was to determine whether HSB mutations affect the anti-inflammatory functions of variants. Methods Two HBS variants of AT (AT-I7N and AT-L99F), which are known to be associated with a higher incidence of thrombosis, were expressed in mammalian cells and purified to homogeneity. These variants were characterized by kinetic assays followed by analysis of their activities in established cellular and/or in vivo inflammatory models. The possible effects of mutations on AT structure were also evaluated by molecular modeling. Results The results indicated that, whereas progressive inhibitory activities of variants were minimally affected, their heparin affinity and inhibitory activity in the presence of heparin were markedly decreased. Unlike wild-type AT, neither AT variant was capable of inhibiting activation of nuclear factor-κB or downregulation of expression of cell adhesion molecules in response to lipopolysaccharide (LPS). Similarly, neither variant elicited barrier protective activity in response to LPS. Structural analysis suggested that the L99F substitution locally destabilizes AT structure. Conclusions It is concluded that the L99F mutation of AT is associated with destabilization of the serpin structure, and that the loss of anti-inflammatory signaling function of the HBS variants may also contribute to enhanced thrombosis in carriers of HBS mutations.

Inhibitory serpins. New insights into their folding, polymerization, regulation and clearance

Serpins are a widely distributed family of high molecular mass protein proteinase inhibitors that can inhibit both serine and cysteine proteinases by a remarkable mechanism-based kinetic trapping of an acyl or thioacyl enzyme intermediate that involves massive conformational transformation. The trapping is based on distortion of the proteinase in the complex, with energy derived from the unique metastability of the active serpin. Serpins are the favoured inhibitors for regulation of proteinases in complex proteolytic cascades, such as are involved in blood coagulation, fibrinolysis and complement activation, by virtue of the ability to modulate their specificity and reactivity. Given their prominence as inhibitors, much work has been carried out to understand not only the mechanism of inhibition, but how it is fine-tuned, both spatially and temporally. The metastability of the active state raises the question of how serpins fold, whereas the misfolding of some serpin variants that leads to polymerization and pathologies of liver disease, emphysema and dementia makes it clinically important to understand how such polymerization might occur. Finally, since binding of serpins and their proteinase complexes, particularly plasminogen activator inhibitor-1 (PAI-1), to the clearance and signalling receptor LRP1 (low density lipoprotein receptor-related protein 1), may affect pathways linked to cell migration, angiogenesis, and tumour progression, it is important to understand the nature and specificity of binding. The current state of understanding of these areas is addressed here.

Heparin coatings for improving blood compatibility of medical devices

Blood contact with biomaterials triggers activation of multiple reactive mechanisms that can impair the performance of implantable medical devices and potentially cause serious adverse clinical events. This includes thrombosis and thromboembolic complications due to activation of platelets and the coagulation cascade, activation of the complement system, and inflammation. Numerous surface coatings have been developed to improve blood compatibility of biomaterials. For more than thirty years, the anticoagulant drug heparin has been employed as a covalently immobilized surface coating on a variety of medical devices. This review describes the fundamental principles of non-eluting heparin coatings, mechanisms of action, and clinical applications with focus on those technologies which have been commercialized. Because of its extensive publication history, there is emphasis on the CARMEDA^® BioActive Surface (CBAS^® Heparin Surface), a widely used commercialized technology for the covalent bonding of heparin.

HS3ST1 genotype regulates antithrombin's inflammomodulatory tone and associates with atherosclerosis

The HS3ST1 gene controls endothelial cell production of HS^AT+ - a form of heparan sulfate containing a specific pentasaccharide motif that binds the anticoagulant protein antithrombin (AT). HS^AT+ has long been thought to act as an endogenous anticoagulant; however, coagulation was normal in Hs3st1^-/- mice that have greatly reduced HS^AT+ (HajMohammadi et al., 2003). This finding indicates that HS^AT+ is not essential for AT's anticoagulant activity. To determine if HS^AT+ is involved in AT's poorly understood inflammomodulatory activities, Hs3st1^-/- and Hs3st1^+/+ mice were subjected to a model of acute septic shock. Compared with Hs3st1^+/+ mice, Hs3st1^-/- mice were more susceptible to LPS-induced death due to an increased sensitivity to TNF. For Hs3st1^+/+ mice, AT treatment reduced LPS-lethality, reduced leukocyte firm adhesion to endothelial cells, and dilated isolated coronary arterioles. Conversely, for Hs3st1^-/- mice, AT induced the opposite effects. Thus, in the context of acute inflammation, HS^AT+ selectively mediates AT's anti-inflammatory activity; in the absence of HS^AT+, AT's pro-inflammatory effects predominate. To explore if the anti-inflammatory action of HS^AT+ also protects against a chronic vascular-inflammatory disease, atherosclerosis, we conducted a human candidate-gene association study on >2000 coronary catheterization patients. Bioinformatic analysis of the HS3ST1 gene identified an intronic SNP, rs16881446, in a putative transcriptional regulatory region. The rs16881446^G/G genotype independently associated with the severity of coronary artery disease and atherosclerotic cardiovascular events. In primary endothelial cells, the rs16881446^G allele associated with reduced HS3ST1 expression. Together with the mouse data, this leads us to conclude that the HS3ST1 gene is required for AT's anti-inflammatory activity that appears to protect against acute and chronic inflammatory disorders.

Selective disruption of heparin and antithrombin-mediated regulation of human factor IX

BACKGROUND

Interaction with antithrombin and heparin regulates distribution, activity, and clearance of factor IXa (FIXa). Hemophilia B prophylaxis targets plasma FIX levels > 1% but neglects extravascular FIX, which colocalizes with antithrombin-heparan sulfate.

OBJECTIVE

Combined mutagenesis of FIX was undertaken to selectively disrupt heparin- and antithrombin-mediated regulation of the protease.

METHODS

Human FIX alanine substitutions in the heparin (K126A and K132A) and antithrombin (R150A) exosites were characterized with regard to coagulant activity, plasma thrombin generation, antithrombin inhibition, and plasma half-life.

RESULTS

Single or combined (K126A/R150A or K132A/R150A) exosite mutations variably reduced coagulant activity relative to wild-type (WT) for FIX (27-91%) and FIXa (25-91%). Double mutation in the heparin exosite (K126A/K132A and K126A/K132A/R150A) markedly reduced coagulant activity (7-21%) and plasma TG. In contrast to coagulant activity, FIX K126A (1.8-fold), R150 (1.6-fold), and K132A/R150A (1.3-fold) supported increased tissue factor-initiated plasma TG, while FIX K132A and K126A/R150A were similar to WT. FIXa K126A/R150A and K132A/R150A (1.5-fold) demonstrated significantly increased FIXa-initiated TG, while FIXa K132A, R150A, and K126A (0.8-0.9-fold) were similar to WT. Dual mutations in the heparin exosite or combined mutations in both exosites synergistically reduced the inhibition rate for antithrombin-heparin. The half-life of FIXa WT in FIX-deficient plasma was remarkably lengthy (40.9 ±1.4 min) and further prolonged for FIXa R150A, K126A/R150A, and K132A/R150A (> 2 h).

CONCLUSION

Selective disruption of exosite-mediated regulation by heparin and antithrombin can be achieved with preserved or enhanced thrombin generation capacity. These proteins should demonstrate enhanced therapeutic efficacy for hemophilia B.

Antithrombin concentrates use in children on extracorporeal membrane oxygenation: a retrospective cohort study

OBJECTIVE

To investigate whether receipt of any antithrombin concentrate improves laboratory and clinical outcomes in children undergoing extracorporeal membrane oxygenation for respiratory failure during their hospitalization compared with those who did not receive antithrombin.

DESIGN

Retrospective cohort study.

SETTING

Single, tertiary-care pediatric hospital.

PATIENTS

Sixty-four neonatal and pediatric patients who underwent extracorporeal membrane oxygenation for respiratory failure between January 2007 and September 2011.

INTERVENTION

Exposure to any antithrombin concentrate during their extracorporeal membrane oxygenation course compared with similar children who never received antithrombin concentrate.

MEASUREMENTS AND MAIN RESULTS

Thirty patients received at least one dose of antithrombin during their extracorporeal membrane oxygenation course and 34 patients did not receive any. The median age at admission was less than 1-month old. Age, duration of extracorporeal membrane oxygenation, or first antithrombin level did not differ significantly between the two cohorts. The mean plasma antithrombin level in those who never received antithrombin was 42.2% compared with 66% in those who received it. However, few levels reached the targeted antithrombin level of 120% and those who did fell back to deficient levels within an average of 6.8 hours. For those who received antithrombin concentrate, heparin infusion rates decreased by an average of 10.2 U/kg/hr for at least 12 hours following administration. No statistical differences were noted in the number of extracorporeal membrane oxygenation circuit changes, in vivo clots or hemorrhages, transfusion requirements, hospital or ICU length of stay, or in-hospital mortality.

CONCLUSIONS

Intermittent, on-demand dosing of antithrombin concentrate in pediatric patients on extracorporeal membrane oxygenation for respiratory failure increased antithrombin levels, but not typically to the targeted level. Patients who received antithrombin concentrate also had decreased heparin requirements for at least 12 hours after dosing. However, no differences were noted in the measured clinical endpoints. A prospective, randomized study of this intervention may require different dosing strategies; such a study is warranted given the unproven efficacy of this costly product.

Heparan sulfate proteoglycans mediate factor XIIa binding to the cell surface

Hageman factor (FXIIa) initiates the intrinsic coagulation pathway and triggers the kallikrein-kinin and the complement systems. In addition, it functions as a growth factor by expressing promitogenic activities toward several cell types. FXIIa binds to the cell surface via a number of structurally unrelated surface receptors; however, the underlying mechanisms are not yet fully understood. Here, we demonstrate that FXIIa utilizes cell membrane-bound glycosaminoglycans to interact with the cell surface of human lung fibroblasts (HLF). The combination of enzymatic, inhibitory, and overexpression approaches identified a heparan sulfate (HS) component of proteoglycans as an important determinant of the FXIIa binding capacity of HLF. Moreover, cell-free assays and competition experiments revealed preferential binding of FXIIa to HS and heparin over dextran sulfate, dermatan sulfate, and chondroitin sulfate A and C. Finally, we demonstrate that fibroblasts isolated from the lungs of the patients suffering from idiopathic pulmonary fibrosis (IPF) exhibit enhanced FXIIa binding capacity. Increased sulfation of HS resulting from elevated HS 6-O-sulfotransferase-1 expression in IPF HLF accounted, in part, for this phenomenon. Application of RNA interference technology and inhibitors of intracellular sulfation revealed the cooperative action of cell surface-associated HS and urokinase-type plasminogen activator receptor in the accumulation of FXIIa on the cell surface of IPF HLF. Moreover, FXIIa stimulated IPF HLF migration, which was abrogated by pretreatment of cells with heparinase I. Collectively, our study uncovers a novel role of HS-type glycosaminoglycans in a local accumulation of FXIIa on the cell membrane. The enhanced association of FXIIa with IPF HLF suggests its contribution to fibrogenesis.

Conformational activation of antithrombin by heparin involves an altered exosite interaction with protease

Heparin allosterically activates antithrombin as an inhibitor of factors Xa and IXa by enhancing the initial Michaelis complex interaction of inhibitor with protease through exosites. Here, we investigate the mechanism of this enhancement by analyzing the effects of alanine mutations of six putative antithrombin exosite residues and three complementary protease exosite residues on antithrombin reactivity with these proteases in unactivated and heparin-activated states. Mutations of antithrombin Tyr(253) and His(319) exosite residues produced massive 10-200-fold losses in reactivity with factors Xa and IXa in both unactivated and heparin-activated states, indicating that these residues made critical attractive interactions with protease independent of heparin activation. By contrast, mutations of Asn(233), Arg(235), Glu(237), and Glu(255) exosite residues showed that these residues made both repulsive and attractive interactions with protease that depended on the activation state and whether the critical Tyr(253)/His(319) residues were mutated. Mutation of factor Xa Arg(143), Lys(148), and Arg(150) residues that interact with the exosite in the x-ray structure of the Michaelis complex confirmed the importance of all residues for heparin-activated antithrombin reactivity and Arg(150) for native serpin reactivity. These results demonstrate that the exosite is a key determinant of antithrombin reactivity with factors Xa and IXa in the native as well as the heparin-activated state and support a new model of allosteric activation we recently proposed in which a balance between attractive and repulsive exosite interactions in the native state is shifted to favor the attractive interactions in the activated state through core conformational changes induced by heparin binding.

Investigation of endoglin wild-type and missense mutant protein heterodimerisation using fluorescence microscopy based IF, BiFC and FRET analyses

The homodimeric transmembrane receptor endoglin (CD105) plays an important role in angiogenesis. This is highlighted by mutations in its gene, causing the vascular disorder HHT1. The main role of endoglin function has been assigned to the modulation of transforming growth factor β and bone morphogenetic protein signalling in endothelial cells. Nevertheless, other functions of endoglin have been revealed to be involved in different cellular functions and in other cell types than endothelial cells. Compared to the exploration of its natural function, little experimental data have been gathered about the mode of action of endoglin HHT mutations at the cellular level, especially missense mutations, and to what degree these might interfere with normal endoglin function. In this paper, we have used fluorescence-based microscopic techniques, such as bimolecular fluorescence complementation (BiFC), immunofluorescence staining with the endoglin specific monoclonal antibody SN6, and protein interaction studies by Förster Resonance Energy Transfer (FRET) to investigate the formation and cellular localisation of possible homo- and heterodimers composed of endoglin wild-type and endoglin missense mutant proteins. The results show that all of the investigated missense mutants dimerise with themselves, as well as with wild-type endoglin, and localise, depending on the position of the affected amino acid, either in the rough endoplasmic reticulum (rER) or in the plasma membrane of the cells. We show that the rER retained mutants reduce the amount of endogenous wild-type endoglin on the plasma membrane through interception in the rER when transiently or stably expressed in HMEC-1 endothelial cells. As a result of this, endoglin modulated TGF-β1 signal transduction is also abrogated, which is not due to TGF-β receptor ER trafficking interference. Protein interaction analyses by FRET show that rER located endoglin missense mutants do not perturb protein processing of other membrane receptors, such as TβRII, ALK5 or ALK1.

Contributing role of extracellular vesicles on vascular endothelium haemostatic balance in cancer

Extracellular vesicles (EVs) generated during tumourigenesis are thought to play a major role in the hypercoagulant state observed in cancer patients. They exhibit negatively charged phospholipids and tissue factor (TF) that promote coagulation cascade activation. In addition, they contain surface proteins and cytoplasmic molecules, both originating from the producing cell that can impact target cells’ expression. By targeting endothelial cells of blood vessels, these EVs could disturb the physiological anticoagulant properties of these cells and be partly responsible for the vascular endothelium activation observed in cancer patients. Indeed, vascular endothelium naturally exhibits heparin-like proteoglycan, TF pathway inhibitor and protein C anticoagulant pathway that prevent thrombosis in physiological condition. An overexpression of TF and a decreased expression of coagulation cascade inhibitors have been reported after EVs’ treatment of endothelial cells. The induction of apoptosis and an increased expression of platelet adhesion molecules have also been highlighted. These events may promote thrombus formation in cancer. The aim of this paper is to provide a targeted review on the current evidence and knowledge of roles and impact of EVs on endothelial surface anticoagulant and procoagulant factors and cellular adhesion molecules expression.

Anionic biopolyelectrolytes of the syndecan/perlecan superfamily: physicochemical properties and medical significance

In the review article presented here, we demonstrate that the connective tissue is more than just a matrix for cells and a passive scaffold to provide physical support. The extracellular matrix can be subdivided into proteins (collagen, elastin), glycoconjugates (structural glycoproteins, proteoglycans) and glycosaminoglycans (hyaluronan). Our main focus rests on the anionic biopolyelectrolytes of the perlecan/syndecan superfamily which belongs to extracellular matrix and cell membrane integral proteoglycans. Though the extracellular domain of the syndecans may well be performing a structural role within the extracellular matrix, a key function of this class of membrane intercalated proteoglycans may be to act as signal transducers across the plasma membrane and thus be more appropriately included in the group of cell surface receptors. Nevertheless, there is a continuum in functions of syndecans and perlecans, especially with respect to their structural role and biomedical significance. HS/CS proteoglycans are receptor sites for lipoprotein binding thus intervening directly in lipid metabolism. We could show that among all lipoproteins, HDL has the highest affinity to these proteoglycans and thus instals a feedforward forechecking loop against atherogenic apoB100 lipoprotein deposition on surface membranes and in subendothelial spaces. Therefore, HDL is not only responsible for VLDL/IDL/LDL cholesterol exit but also controls thoroughly the entry. This way, it inhibits arteriosclerotic nanoplaque formation. The ternary complex 'lipoprotein receptor (HS/CS-PG) - lipoprotein (LDL, oxLDL, Lp(a)) - calcium' may be interpreted as arteriosclerotic nanoplaque build-up on the molecular level before any cellular reactivity, possibly representing the arteriosclerotic primary lesion combined with endothelial dysfunction. With laser-based ellipsometry we could demonstrate that nanoplaque formation is a Ca(2+)-driven process. In an in vitro biosensor application of HS-PG coated silica surfaces we tested nanoplaque formation and size in clinical trials with cardiovascular high-risk patients who underwent treatment with ginkgo or fluvastatin. While ginkgo reduced nanoplaque formation (size) by 14.3% (23.4%) in the isolated apoB100 lipid fraction at a normal blood Ca(2+) concentration, the effect of the statin with a reduction of 44.1% (25.4%) was more pronounced. In addition, ginkgo showed beneficial effects on several biomarkers of oxidative stress and inflammation. Besides acting as peripheral lipoprotein binding receptor, HS/CS-PG is crucially implicated in blood flow sensing. A sensor molecule has to fulfil certain mechanochemical and mechanoelectrical requirements. It should possess viscoelastic and cation binding properties capable of undergoing conformational changes caused both mechanically and electrostatically. Moreover, the latter should be ion-specific. Under no-flow conditions, the viscoelastic polyelectrolyte at the endothelium - blood interface assumes a random coil form. Blood flow causes a conformational change from the random coil state to the directed filament structure state. This conformational transition effects a protein unfurling and molecular elongation of the GAG side chains like in a 'stretched' spring. This configuration is therefore combined with an increase in binding sites for Na(+) ions. Counterion migration of Na(+) along the polysaccharide chain is followed by transmembrane Na(+) influx into the endothelial cell and by endothelial cell membrane depolarization. The simultaneous Ca(2+) influx releases NO and PGI2, vasodilatation is the consequence. Decrease in flow reverses the process. Binding of Ca(2+) and/or apoB100 lipoproteins (nanoplaque formation) impairs the flow sensor function. The physicochemical and functional properties of proteoglycans are due to their amphiphilicity and anionic polyelectrolyte character. Thus, they potently interact with cations, albeit in a rather complex manner. Utilizing (23)Na(+) and (39)K(+) NMR techniques, we could show that, both in HS-PG solutions and in native vascular connective tissue, the mode of interaction for monovalent cations is competition. Mg(2+) and Ca(2+) ions, however, induced a conformational change leading to an increased allosteric, cooperative K(+) and Na(+) binding, respectively. Since extracellular matrices and basement membranes form a tight-fitting sheath around the cell membrane of muscle and Schwann cells, in particular around sinus node cells of the heart, and underlie all epithelial and endothelial cell sheets and tubes, a release of cations from or an adsorption to these polyanionic macromolecules can transiently lead to fast and drastic activity changes in these tiny extracellular tissue compartments. The ionic currents underlying pacemaker and action potential of sinus node cells are fundamentally modulated. Therefore, these polyelectrolytic ion binding characteristics directly contribute to and intervene into heart rhythm.

Immobilization of actively thromboresistant assemblies on sterile blood-contacting surfaces

Rapid one-step modification of thrombomodulin with alkylamine derivatives such as azide, biotin, and PEG is achieved using an evolved sortase (eSrtA) mutant. The feasibility of a point-of-care scheme is demonstrated herein to site-specifically immobilize azido-thrombomodulin on sterilized commercial ePTFE vascular grafts, which exhibit superior thromboresistance compared with commercial heparin-coated grafts in a primate model of acute graft thrombosis.

Heparan sulfate 3-O-sulfation: a rare modification in search of a function

Many protein ligands bind to heparan sulfate, which results in their presentation, protection, oligomerization or conformational activation. Binding depends on the pattern of sulfation and arrangement of uronic acid epimers along the chains. Sulfation at the C3 position of glucosamine is a relatively rare, yet biologically significant modification, initially described as a key determinant for binding and activation of antithrombin and later for infection by type I herpes simplex virus. In mammals, a family of seven heparan sulfate 3-O-sulfotransferases installs sulfate groups at this position and constitutes the largest group of sulfotransferases involved in heparan sulfate formation. However, to date very few proteins or biological systems have been described that are influenced by 3-O-sulfation. This review describes our current understanding of the prevalence and structure of 3-O-sulfation sites, expression and substrate specificity of the 3-O-sulfotransferase family and the emerging roles of 3-O-sulfation in biology.

Endothelial dysfunction over the course of coronary artery disease

The vascular endothelium regulates blood flow in response to physiological needs. Endothelial dysfunction is closely related to atherosclerosis and its risk factors, and it constitutes an intermediate step on the progression to adverse events throughout the natural history of coronary artery disease (CAD), often affecting clinical outcomes. Understanding the relation of endothelial function with CAD provides an important pathophysiological insight, which can be useful both in clinical and research management. In this review, we summarize the current knowledge on endothelial dysfunction and its prognostic influence throughout the natural history of CAD, from early atherosclerosis to post-transplant management.

Stable complex formation between serine protease inhibitor and zymogen: coagulation factor X cleaves the Arg393-Ser394 bond in a reactive centre loop of antithrombin in the presence of heparin

Antithrombin (AT) inhibits several blood coagulation proteases, including activated factor X (FXa), by forming stable complexes with these proteases. Herein, we demonstrate that AT forms a stable complex with zymogen factor X (FX). Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and size-exclusion chromatography analyses showed that AT and FX formed an SDS-stable complex, which is distinct in apparent molecular mass from an FXa-AT complex, in the presence of heparin. Amino-terminal sequence analysis of the complex following SDS-PAGE under reducing conditions provided clear evidence that AT forms this complex with the heavy chain of FX, because two sequences, HGSPVDI (residues 1-7 of AT) and SVAQATS (residues 1-7 of the heavy chain of FX), were identified. Furthermore, sequence SLNPNRV, which corresponds to residues 394-400 of AT, was identified in the non-reduced FX-AT complex, indicating that FX cleaved the Arg393-Ser394 bond in a reactive centre loop of AT. Unfractionated heparin induced FX-AT complex formation more effectively than low-molecular weight heparin or AT-binding pentasaccharide, and appeared to promote complex formation mainly via a template effect. These data suggest that AT is capable of forming a stable complex with zymogen FX by acting as an inhibitor in the presence of heparin.

In silico discovery of a compound with nanomolar affinity to antithrombin causing partial activation and increased heparin affinity

The medical and socioeconomic relevance of thromboembolic disorders promotes an ongoing effort to develop new anticoagulants. Heparin is widely used as activator of antithrombin but incurs side effects. We screened a large database in silico to find alternative molecules and predicted d-myo-inositol 3,4,5,6-tetrakisphosphate (TMI) to strongly interact with antithrombin. Isothermal titration calorimetry confirmed a TMI affinity of 45 nM, higher than the heparin affinity (273 nM). Functional studies, fluorescence analysis, and citrullination experiments revealed that TMI induced a partial activation of antithrombin that facilitated the interaction with heparin and low affinity heparins. TMI improved antithrombin inhibitory function of plasma from homozygous patients with antithrombin deficiency with a heparin binding defect and also in a model with endothelial cells. Our in silico screen identified a new, non-polysaccharide scaffold able to interact with the heparin binding domain of antithrombin. The functional consequences of this interaction were experimentally characterized and suggest potential anticoagulant therapeutic applications.