Identification of the predominant glycosaminoglycan-attachment site in soluble recombinant human thrombomodulin: potential regulation of functionality by glycosyltransferase competition for serine474

Site-specific glycosylation of proteoglycans: a revisited frontier in proteoglycan research

Proteoglycans (PGs), a class of carbohydrate-modified proteins, are present in essentially all metazoan organisms investigated to date. PGs are composed of glycosaminoglycan (GAG) chains attached to various core proteins and are important for embryogenesis and normal homeostasis. PGs exert many of their functions via their GAG chains and understanding the details of GAG-ligand interactions has been an essential part of PG research. Although PGs are also involved in many diseases, the number of GAG-related drugs used in the clinic is yet very limited, indicating a lack of detailed structure-function understanding. Structural analysis of PGs has traditionally been obtained by first separating the GAG chains from the core proteins, after which the two components are analyzed separately. While this strategy greatly facilitates the analysis, it precludes site-specific information and introduces either a "GAG" or a "core protein" perspective on the data interpretation. Mass-spectrometric (MS) glycoproteomic approaches have recently been introduced, providing site-specific information on PGs. Such methods have revealed a previously unknown structural complexity of the GAG linkage regions and resulted in identification of several novel CSPGs and HSPGs in humans and in model organisms, thereby expanding our view on PG complexity. In light of these findings, we discuss here if the use of such MS-based techniques, in combination with various functional assays, can also be used to expand our functional understanding of PGs. We have also summarized the site-specific information of all human PGs known to date, providing a theoretical framework for future studies on site-specific functional analysis of PGs in human pathophysiology.

Circulating Thrombomodulin: Release Mechanisms, Measurements, and Levels in Diseases and Medical Procedures

Thrombomodulin (TM) is a type-I transmembrane protein that is mainly expressed on endothelial cells and plays important roles in many biological processes. Circulating TM of different forms are also present in biofluids, such as blood and urine. Soluble TM (sTM), comprised of several domains of TM, is the major circulating TM which is generated by either enzymatic or chemical cleavage of the intact protein under different conditions. Under normal conditions, sTM is present in low concentrations (<10 ng/mL) in the blood but is elevated in several pathological conditions associated with endothelial dysfunction such as cardiovascular, inflammatory, infection, and metabolic diseases. Therefore, sTM level has been examined for monitoring disease development, such as disseminated intravascular coagulation (DIC), sepsis and multiple organ dysfunction syndrome in patients with novel coronavirus disease 2019 (COVID-19) recently. In addition, microvesicles (MVs) that contain membrane TM (MV-TM) have been found to be released from activated cells which also contribute to levels of circulating TM in certain diseases. Several release mechanisms of sTM and MV-TM have been reported, including enzymatic, chemical, and TM mutation mechanisms. Measurements of sTM and MV-TM have been developed and explored as biomarkers in many diseases. In this review, we summarize all these advances in three categories as follows: (1) release mechanisms of circulating TM, (2) methods for measuring circulating TM in biological samples, and (3) correlation of circulating TM with diseases. Altogether, it provides a whole picture of recent advances on circulating TM in health and disease.

Amino acid sequence surrounding the chondroitin sulfate attachment site of thrombomodulin regulates chondroitin polymerization

Thrombomodulin (TM) is a cell-surface glycoprotein and a critical mediator of endothelial anticoagulant function. TM exists as both a chondroitin sulfate (CS) proteoglycan (PG) form and a non-PG form lacking a CS chain (α-TM); therefore, TM can be described as a part-time PG. Previously, we reported that α-TM bears an immature, truncated linkage tetrasaccharide structure (GlcAβ1-3Galβ1-3Galβ1-4Xyl). However, the biosynthetic mechanism to generate part-time PGs remains unclear. In this study, we used several mutants to demonstrate that the amino acid sequence surrounding the CS attachment site influences the efficiency of chondroitin polymerization. In particular, the presence of acidic residues surrounding the CS attachment site was indispensable for the elongation of CS. In addition, mutants defective in CS elongation did not exhibit anti-coagulant activity, as in the case with α-TM. Together, these data support a model for CS chain assembly in which specific core protein determinants are recognized by a key biosynthetic enzyme involved in chondroitin polymerization.

Overexpression of Galnt3 in chondrocytes resulted in dwarfism due to the increase of mucin-type O-glycans and reduction of glycosaminoglycans

Galnt3, UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3, transfers N-acetyl-D-galactosamine to serine and threonine residues, initiating mucin type O-glycosylation of proteins. We searched the target genes of Runx2, which is an essential transcription factor for chondrocyte maturation, in chondrocytes and found that Galnt3 expression was up-regulated by Runx2 and severely reduced in Runx2(-/-) cartilaginous skeletons. To investigate the function of Galnt3 in chondrocytes, we generated Galnt3(-/-) mice and chondrocyte-specific Galnt3 transgenic mice under the control of the Col2a1 promoter-enhancer. Galnt3(-/-) mice showed a delay in endochondral ossification and shortened limbs at embryonic day 16.5, suggesting that Galnt3 is involved in chondrocyte maturation. Galnt3 transgenic mice presented dwarfism, the chondrocyte maturation was retarded, the cell cycle in chondrocytes was accelerated, premature chondrocyte apoptosis occurred, and the growth plates were disorganized. The binding of Vicia villosa agglutinin, which recognizes the Tn antigen (GalNAc-O-Ser/Thr), was drastically increased in chondrocytes, and aggrecan (Acan) was highly enriched with Tn antigen. However, safranin O staining, which recognizes glycosaminoglycans (GAGs), and Acan were severely reduced. Chondroitin sulfate was reduced in amount, but the elongation of chondroitin sulfate chains had not been severely disturbed in the isolated GAGs. These findings indicate that overexpression of Galnt3 in chondrocytes caused dwarfism due to the increase of mucin-type O-glycans and the reduction of GAGs, probably through competition with xylosyltransferases, which initiate GAG chains by attaching O-linked xylose to serine residues, suggesting a negative effect of Galnt family proteins on Acan deposition in addition to the positive effect of Galnt3 on chondrocyte maturation.

Recombinant thrombomodulin of different domains for pharmaceutical, biomedical, and cell transplantation applications

Thrombomodulin (TM) is a membrane glycoprotein mainly expressed by vascular endothelial cells and is involved in many physiological and pathological processes, such as coagulation, inflammation, cancer development, and embryogenesis. Human TM consists of 557 amino acids divided into five distinct domains: N-terminal lectin-like domain (designated as TMD1); six epidermal growth factor (EGF)-like domain (TMD2); Ser/Thr-rich domain (TMD3); transmembrane domain (TMD4); and cytoplasmic tail domain (TMD5). The different domains are responsible for different biological functions of TM. In the past decades, various domains of TM have been cloned and expressed for TM structural and functional study. Further, recombinant TMs of different domains show promising antithrombotic and anti-inflammatory activity in both rodents and primates and a recombinant soluble TM has been approved for therapeutic application. This review highlights recombinant TMs of diverse structures and their biological functions, as well as the complex interactions of TM with factors involved in the related biological processes. Particularly, recent advances in exploring recombinant TM of different domains for pharmaceutical, biomedical, and cell transplantation applications are summarized.

Thrombomodulin and its role in inflammation

The goal is to provide an extensive review of the physiologic role of thrombomodulin (TM) in maintaining vascular homeostasis, with a focus on its anti-inflammatory properties. Data were collected from published research. TM is a transmembrane glycoprotein expressed on the surface of all vascular endothelial cells. Expression of TM is tightly regulated to maintain homeostasis and to ensure a rapid and localized hemostatic and inflammatory response to injury. By virtue of its strategic location, its multidomain structure and complex interactions with thrombin, protein C (PC), thrombin activatable fibrinolysis inhibitor (TAFI), complement components, the Lewis Y antigen, and the cytokine HMGB1, TM exhibits a range of physiologically important anti-inflammatory, anti-coagulant, and anti-fibrinolytic properties. TM is an essential cofactor that impacts on multiple biologic processes. Alterations in expression of TM and its partner proteins may be manifest by inflammatory and thrombotic disorders. Administration of soluble forms of TM holds promise as effective therapies for inflammatory diseases, and infections and malignancies that are complicated by disseminated intravascular coagulation.

Human thrombomodulin knock-in mice reveal differential effects of human thrombomodulin on thrombosis and atherosclerosis

OBJECTIVE

We sought to develop a murine model to examine the antithrombotic and antiinflammatory functions of human thrombomodulin in vivo.

METHODS AND RESULTS

Knock-in mice that express human thrombomodulin from the murine thrombomodulin gene locus were generated. Compared with wild-type mice, human thrombomodulin knock-in mice exhibited decreased protein C activation in the aorta (P<0.01) and lung (P<0.001). Activation of endogenous protein C following infusion of thrombin was decreased by 90% in knock-in mice compared with wild-type mice (P<0.05). Carotid artery thrombosis induced by photochemical injury occurred more rapidly in knock-in mice (12±3 minutes) than in wild-type mice (31±6 minutes; P<0.05). No differences in serum cytokine levels were detected between knock-in and wild-type mice after injection of endotoxin. When crossed with apolipoprotein E-deficient mice and fed a Western diet, knock-in mice had a further decrease in protein C activation but did not exhibit increased atherosclerosis.

CONCLUSION

Expression of human thrombomodulin in place of murine thrombomodulin produces viable mice with a prothrombotic phenotype but unaltered responses to systemic inflammatory or atherogenic stimuli. This humanized animal model will be useful for investigating the function of human thrombomodulin under pathophysiological conditions in vivo.

Antibodies associated with heparin-induced thrombocytopenia (HIT) inhibit activated protein C generation: new insights into the prothrombotic nature of HIT

Heparin-induced thrombocytopenia (HIT) is caused by antibodies that recognize complexes between platelet factor 4 (PF4) and heparin or glycosaminoglycan side chains. These antibodies can lead to a limb- and life-threatening prothrombotic state. We now show that HIT antibodies are able to inhibit generation of activated protein C (aPC) by thrombin/thrombomodulin (IIa/TM) in the presence of PF4. Tetrameric PF4 potentiates aPC generation by formation of complexes with chondroitin sulfate (CS) on TM. Formation of these complexes occurs at a specific molar ratio of PF4 to glycosaminoglycan. This observation and the finding that the effect of heparin on aPC generation depends on the concentration of PF4 suggest similarity between PF4/CS complexes and those that bind HIT antibodies. HIT antibodies reduced the ability of PF4 to augment aPC formation. Cationic protamine sulfate, which forms similar complexes with heparin, also enhanced aPC generation, but its activity was not blocked by HIT antibodies. Our studies provide evidence that complexes formed between PF4 and TM's CS may play a physiologic role in potentiating aPC generation. Recognition of these complexes by HIT antibodies reverses the PF4-dependent enhancement in aPC generation and may contribute to the prothrombotic nature of HIT.

Molecular aspects of Plasmodium falciparum Infection during pregnancy

Cytoadherence of Plasmodium-falciparum-parasitized red blood cells (PRBCs) to host receptors is the key phenomenon in the pathological process of the malaria disease. Some of these interactions can originate poor outcomes responsible for 1 to 3 million annual deaths mostly occurring among children in sub-Saharan Africa. Pregnancy-associated malaria (PAM) represents an important exception of the disease occurring at adulthood in malaria endemic settings. Consequences of this are shared between the mother (maternal anemia) and the baby (low birth weight and infant mortality). Demonstrating that parasites causing PAM express specific variant surface antigens (VSA(PAM)), including the P. falciparum erythrocyte membrane protein 1 (P f EMP1) variant VAR2CSA, that are targets for protective immunity has strengthened the possibility for the development of PAM-specific vaccine. In this paper, we review the molecular basis of malaria pathogenesis attributable to the erythrocyte stages of the parasites, and findings supporting potential anti-PAM vaccine components evidenced in PAM.

Thrombomodulin mutations in atypical hemolytic-uremic syndrome

BACKGROUND

The hemolytic-uremic syndrome consists of the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The common form of the syndrome is triggered by infection with Shiga toxin-producing bacteria and has a favorable outcome. The less common form of the syndrome, called atypical hemolytic-uremic syndrome, accounts for about 10% of cases, and patients with this form of the syndrome have a poor prognosis. Approximately half of the patients with atypical hemolytic-uremic syndrome have mutations in genes that regulate the complement system. Genetic factors in the remaining cases are unknown. We studied the role of thrombomodulin, an endothelial glycoprotein with anticoagulant, antiinflammatory, and cytoprotective properties, in atypical hemolytic-uremic syndrome.

METHODS

We sequenced the entire thrombomodulin gene (THBD) in 152 patients with atypical hemolytic-uremic syndrome and in 380 controls. Using purified proteins and cell-expression systems, we investigated whether thrombomodulin regulates the complement system, and we characterized the mechanisms. We evaluated the effects of thrombomodulin missense mutations associated with atypical hemolytic-uremic syndrome on complement activation by expressing thrombomodulin variants in cultured cells.

RESULTS

Of 152 patients with atypical hemolytic-uremic syndrome, 7 unrelated patients had six different heterozygous missense THBD mutations. In vitro, thrombomodulin binds to C3b and factor H (CFH) and negatively regulates complement by accelerating factor I-mediated inactivation of C3b in the presence of cofactors, CFH or C4b binding protein. By promoting activation of the plasma procarboxypeptidase B, thrombomodulin also accelerates the inactivation of anaphylatoxins C3a and C5a. Cultured cells expressing thrombomodulin variants associated with atypical hemolytic-uremic syndrome had diminished capacity to inactivate C3b and to activate procarboxypeptidase B and were thus less protected from activated complement.

CONCLUSIONS

Mutations that impair the function of thrombomodulin occur in about 5% of patients with atypical hemolytic-uremic syndrome.

cDNA Cloning of Rabbit Thrombomodulin and Characterization of Gene Expression in Cardiovascular Tissue

Thrombomodulin (TM), a component of the protein C anticoagulant pathway, is critical for the maintenance of vascular thromboresistance. To facilitate the study of in vivo TM regulation, we cloned and sequenced the cDNA encoding full-length rabbit TM. Translation of the open reading frame predicts a 580 amino acid protein that contains a 19 amino acid signal peptide, one lectin-like and six EGF-like extracellular domains, a 23 amino acid transmembrane domain and a 36 amino acid cytoplasmic domain. In addition, there are three potential N-linked and six O-linked glycosylation sites. Comparison of the predicted rabbit TM protein with those of human, mouse and rat reveals 67-72% primary sequence conservation with identical domain homology. TM gene expression was quantified in rabbit cardiovascular tissue by real-time PCR using primers and probe based on the derived cDNA sequence and found to correlate with protein expression as determined by Western blot analysis.

Distinct functions of activated protein C differentially attenuate acute kidney injury

Administration of activated protein C (APC) protects from renal dysfunction, but the underlying mechanism is unknown. APC exerts both antithrombotic and cytoprotective properties, the latter via modulation of protease-activated receptor-1 (PAR-1) signaling. We generated APC variants to study the relative importance of the two functions of APC in a model of LPS-induced renal microvascular dysfunction. Compared with wild-type APC, the K193E variant exhibited impaired anticoagulant activity but retained the ability to mediate PAR-1-dependent signaling. In contrast, the L8W variant retained anticoagulant activity but lost its ability to modulate PAR-1. By administering wild-type APC or these mutants in a rat model of LPS-induced injury, we found that the PAR-1 agonism, but not the anticoagulant function of APC, reversed LPS-induced systemic hypotension. In contrast, both functions of APC played a role in reversing LPS-induced decreases in renal blood flow and volume, although the effects on PAR-1-dependent signaling were more potent. Regarding potential mechanisms for these findings, APC-mediated PAR-1 agonism suppressed LPS-induced increases in the vasoactive peptide adrenomedullin and infiltration of iNOS-positive leukocytes into renal tissue. However, the anticoagulant function of APC was responsible for suppressing LPS-induced stimulation of the proinflammatory mediators ACE-1, IL-6, and IL-18, perhaps accounting for its ability to modulate renal hemodynamics. Both variants reduced active caspase-3 and abrogated LPS-induced renal dysfunction and pathology. We conclude that although PAR-1 agonism is solely responsible for APC-mediated improvement in systemic hemodynamics, both functions of APC play distinct roles in attenuating the response to injury in the kidney.

Pig thrombomodulin binds human thrombin but is a poor cofactor for activation of human protein C and TAFI

Incompatibility between pig thrombomodulin (TM) and primate thrombin is thought to be an important factor in the development of microvascular thrombosis in rejecting pig-to-primate xenografts. To examine this interaction at the molecular level, we cloned pig TM and measured its ability to bind human thrombin and act as a cofactor for the activation of human protein C and TAFI. The 579-residue pig TM protein showed approximately 69% sequence identity to human TM. Within the EGF domains necessary for binding of thrombin (EGF56), protein C (EGF4) and TAFI (EGF3), all of the amino acids previously identified as critical for the function of human TM, with the exception of Glu-408 in EGF5, were conserved in pig TM. Comparison of transfected cells expressing pig or human TM demonstrated that both proteins bound human thrombin and inhibited its procoagulant activity. However, pig TM was a poor cofactor for the activation of human protein C and TAFI, with domain swapping showing that EGF5 was the most important determinant of compatibility. Thus, while pig TM may be capable of binding thrombin generated in the vicinity of xenograft endothelium, its failure to promote the activation of human protein C remains a significant problem.

Six genes are preferentially transcribed by the circulating and sequestered forms of Plasmodium falciparum parasites that infect pregnant women

In areas of stable malaria transmission, susceptibility to Plasmodium falciparum malaria increases during first pregnancy. Women become resistant to pregnancy malaria over successive pregnancies as they acquire antibodies against the parasite forms that sequester in the placenta, suggesting that a vaccine is feasible. Placental parasites are antigenically distinct and bind receptors, like chondroitin sulfate A (CSA), that are not commonly bound by other parasites. We used whole-genome-expression analysis to find transcripts that distinguish parasites of pregnant women from other parasites and employed a novel approach to define and adjust for cell cycle timing of parasites. Transcription of six genes was substantially higher in both placental parasites and peripheral parasites from pregnant women, and each gene encodes a protein with a putative export sequence and/or transmembrane domain. This cohort of genes includes var2csa, a member of the variant PfEMP1 gene family previously implicated in pregnancy malaria, as well as five conserved genes of unknown functions. Women in East Africa acquire antibodies over successive pregnancies against a protein encoded by one of these genes, PFD1140w, and this protein shows seroreactivity similar to that of VAR2CSA domains. These findings suggest that a suite of genes may be important for the genesis of the placental binding phenotype of P. falciparum and may provide novel targets for therapeutic intervention.

Systematic Analysis of proteoglycan modification sites in Caenorhabditis elegans by scanning mutagenesis

Proteoglycan modification is essential for development and early cell division in Caenorhabditis elegans. The specification of proteoglycan attachment sites is defined by the Golgi enzyme polypeptide xylosyltransferase. Here we evaluate the substrate specificity of this xylosyltransferase for its downstream targets by using reporter proteins containing proteoglycan modification sites from C. elegans syndecan/SDN-1. The N terminus of the SDN-1 contains a Ser-Gly proteoglycan site at Ser(71), flanked by potential mucin and N-glycosylation sites. However, Ser(71) was exclusively used as a proteoglycan site in vivo, based on mapping studies with a Ser(71) reporter protein, glycosyltransferase RNA interference, and co-expression of worm polypeptide xylosyltransferase. To elucidate the substrate requirements of this enzyme, a library of 42 point mutants of the Ser(71) reporter was expressed in tissue culture. The nematode proteoglycan modification site in SDN-1 required serine (not threonine), two flanking glycine residues (positions -1 and +1), and either one proximal acidic N-terminal amino acid (positions -4, -3, and -2) or a pair of distal N-terminal acidic amino acids (positions -6 and -5). C-terminal acidic amino acids, although present in many proteoglycan modification sites, had minimal impact on xylosylation at Ser(71). Proline inhibited glycosylation when present at -1, +1, or +2. The position of glycine, proline, and acidic amino acids allows the glycosylation machinery to discriminate between mucin and proteoglycan modification sites. The key residues that define proteoglycan modification sites also function with the Drosophila polypeptide xylosyltransferase, indicating that the specificity in the glycosylation process is evolutionarily conserved. Using a neural network method, a preliminary proteoglycan predictor has been developed.

The immunology and pathogenesis of malaria during pregnancy

Women in endemic areas become highly susceptible to malaria during first and second pregnancies, despite immunity acquired after years of exposure. Recent insights have advanced our understanding of pregnancy malaria caused by Plasmodium falciparum, which is responsible for the bulk of severe disease and death. Accumulation of parasitized erythrocytes in the blood spaces of the placenta is a key feature of maternal infection with P. falciparum. Placental parasites express surface ligands and antigens that differ from those of other P. falciparum variants, facilitating evasion of existing immunity, and mediate adhesion to specific molecules, such as chondroitin sulfate A, in the placenta. The polymorphic and clonally variant P. falciparum erythrocyte membrane protein 1, encoded by var genes, binds to placental receptors in vitro and may be the target of protective antibodies. An intense infiltration of immune cells, including macrophages, into the placental intervillous spaces, and the production of pro-inflammatory cytokines often occur in response to infection, and are associated with low birth weight and maternal anemia. Expression of alpha and beta chemokines may initiate or facilitate this cellular infiltration during placental malaria. Specific immunity against placental-binding parasites may prevent infection or facilitate clearance of parasites prior to the influx of inflammatory cells, thereby avoiding a cascade of events leading to disease and death. Much less is known about pathogenic processes in P. vivax infections, and corresponding immune responses. Emerging knowledge of the pathogenesis and immunology of malaria in pregnancy will increasingly lead to new opportunities for the development of therapeutic and preventive interventions and new tools for diagnosis and monitoring.

Chondroitin sulphate A as an adherence receptor for Plasmodium falciparum-infected erythrocytes

Until recently, the sequestration of erythrocytes infected with Plasmodium falciparum has been thought to be due to one of a number of protein-protein interactions. In this article, Stephen Rogerson and Graham Brown summarize the emerging evidence that, in vitro, infected erythrocytes can also adhere to the glycosaminoglycan chondroitin sulphate A (CSA) expressed on the surface of cells and immobilized on plastic. In vivo, binding of infected erythrocytes to CSA could be crucial to the development of malarial infection of the placenta, and possibly to sequestration in the lung and brain. The consequences of this may include maternal morbidity and mortality, low birth weight in the infant, pulmonary oedema and cerebral malaria. They discuss the need to characterize the molecular basis of this interaction, and to investigate the possible therapeutic role of CSA in malaria. Chondroitin sulphates are nontoxic compounds already in use for other diseases in humans. Vaccines based on inhibiting this receptor-ligand interaction could also be appropriate.

Recombinant human thrombomodulin inhibits arterial neointimal hyperplasia after balloon injury

OBJECTIVE

Smooth muscle cell proliferation is a major pathophysiologic factor in injury-induced neointimal hyperplasia and recurrent stenosis. We have demonstrated that recombinant human thrombomodulin (rTM) inhibits thrombin-induced arterial smooth muscle cell proliferation in vitro. The purpose of this study was to investigate the effect of rTM on neointimal hyperplasia in vivo.

METHODS

A rabbit femoral artery balloon injury model was used. Bilateral superficial femoral arteries were deendothelialized with a 2F arterial embolectomy catheter. rTM (145 microg/kg; 2.0 microg/mL in circulation) or Tris-hydrochloride vehicle control was administered intravenously during the procedure, then either discontinued (group A) or administered twice daily for an additional 48 hours (group B). Rabbits were euthanized at 4 days and at 1, 2, and 4 weeks, and femoral artery specimens were prepared with in situ perfusion fixation and paraffin embedding. Luminal, intima, media, and whole artery areas were quantitated with digital imaging computerized planimetry. Intima-media and lumen-whole artery ratios were calculated. The injury-induced inflammatory reaction was also evaluated with light microscopy, scanning and transmission electron microscopy, and immunohistochemical and immunohistofluorescence staining.

RESULTS

In the buffer control group, neointimal hyperplasia after femoral artery balloon injury was evident at 2 weeks, and was pronounced at 4 weeks (P <.0001). Infusion of rTM significantly inhibited intimal hyperplasia at both 2 and 4 weeks (P <.0001). In group A, rTM reduced the intima-media ratio by 27% and 39% at 2 and 4 weeks, respectively. Extended administration of rTM (group B) resulted in inhibition of hyperplasia by 57% and 30% at 2 and 4 weeks, respectively, but failed to reach significance compared with the shorter exposure. rTM infusion significantly inhibited thrombosis (8.1-fold) compared with the buffer control group (P =.012). rTM had no significant effect on lumen area or lumen-whole artery ratio, but treated arteries demonstrated significantly less compensatory dilatation (P =.045), as measured by whole artery area in response to less intimal hyperplasia. rTM administration inhibited platelet adhesion and inhibition of neutrophil infiltration to a degree that approached statistical significance (P =.0675).

CONCLUSIONS

Systemic intravenous administration of rTM significantly decreases neointimal hyperplasia and improves patency in the rabbit femoral artery after balloon injury. In addition to exhibiting antithrombotic and antiproliferative effects, rTM may also invoke an anti-inflammatory mechanism, and may alter vascular remodeling in a multidimensional role to inhibit recurrent stenosis after arterial injury.

Decoy receptor 3 (DcR3) is proteolytically processed to a metabolic fragment having differential activities against Fas ligand and LIGHT

Fas ligand (FasL) and Fas receptor are members of the tumor necrosis factor (TNF) receptor and ligand family that play an important role in regulating apoptosis in normal physiology. Decoy receptor 3 (DcR3) is a novel member of the TNF receptor superfamily, which binds to and blocks the activities of the ligands FasL and LIGHT. We have demonstrated that DcR3 was degraded rapidly to a major circulating metabolic fragment after subcutaneous administration in primates and mice. This fragment was also generated in subcutaneous tissue homogenate in vitro. Mass spectrometry and N-terminal sequencing indicated that DcR3 was proteolytically cleaved between R218 and A219 in the primary sequence to yield the fragment DcR3(1-218). While retaining its ability to bind LIGHT and inhibit LIGHT-mediated activities, DcR3(1-218) no longer bound FasL and did not inhibit FasL-mediated apoptosis in vitro. The primary sequence of DcR3 was molecularly engineered, changing the arginine residue at position 218 to glutamine to generate an analog, DcR3(R218Q), which we termed FLINT (LY498919). We demonstrated that FLINT was more stable to proteolytic degradation in vitro and in vivo and maintained its activity against both soluble FasL and soluble LIGHT in vitro. As a result, the modification in the sequence of DcR3 to produce FLINT (LY498919) should result in a pharmacologically superior molecule in the therapeutic intervention of diseases in which the pathogenesis is linked to FasL-mediated apoptotic or inflammatory events.

Naturally occurring mutations in the thrombomodulin gene leading to impaired expression and function

Sporadic mutations in the thrombomodulin (TM) gene occur in patients with both arterial and venous thrombosis, but the effects of these mutations on expression and function are largely unexplored. Full-length wild-type TM complementary DNA (cDNA) was incorporated into vector pcDNA6 for transfection into COS-7 cells for transient expression. Mutagenesis was performed to create 7 TM mutants with natural mutations either previously identified (Ala25Thr, Gly61Ala, Asp468Tyr, Pro477Ser, Pro483Leu) or reported here (an 11-base pair [bp] deletion, del791-801, leading to STOP306, and a missense mutation, Arg385Ser). Four mutations were found to detrimentally affect the level of expression of the TM protein. Of the missense mutations, 3 had reduced expression compared to wild-type TM (100%), Arg385Ser (50.2% +/- 5%, P <.001), Pro477Ser (76.8% +/- 1%, P <.001), Pro483Leu (82.1% +/- 8%, P <.007). No TM protein expression could be detected on the cell surface for mutation del791-801. The cofactor activity of TM in protein C activation was also evaluated. The Michaelis constant (K(m)) for wild-type thrombin-TM complex was 634 +/- 6 nmol/L. Two mutants, with Arg385Ser and Pro477Ser, had increased (P <.0001) K(m), 2967 +/- 283 nM, and 2342 +/- 219 nM, respectively, demonstrating impaired function of the thrombin-TM complex. This work presents biochemical evidence that certain (but not all) natural mutations in the TM gene reduce expression and impair function of the protein on the cell surface, and helps clarify the suggested contribution that these mutations might make to the risk of thromboembolic disease.

Different usage of the glycosaminoglycan attachment sites of biglycan

Biglycan is a member of the small leucine-rich proteoglycan family. Its core protein comprises two chondroitin/dermatan sulfate attachment sites on serine 42 and serine 47, respectively, which are the fifth and tenth amino acid residues, respectively, after removal of the prepro peptide. Because the regulation of glycosaminoglycan chain assembly is not fully understood and because of the in vivo existence of monoglycanated biglycan, mutant core proteins were stably expressed in human 293 and Chinese hamster ovary cells in which i) either one or both serine residues were converted into alanine or threonine residues, ii) the number of acidic amino acids N-terminal of the respective serine residues was altered, and iii) a hexapeptide was inserted between the mutated site 1 and the unaltered site 2. Labeling experiments with [(35)S]sulfate and [(35)S]methionine indicated that serine 42 was almost fully used as the glycosaminoglycan attachment site regardless of whether site 2 was available or not for chain assembly. In contrast, substitution of site 2 was greatly influenced by the presence or absence of serine 42, although additional mutations demonstrated a direct influence of the amino acid sequence between the two sites. When site 2 was not substituted with a glycosaminoglycan chain, there was also no assembly of the linkage region. These results indicate that xylosyltransferase is the rate-limiting enzyme in glycosaminoglycan chain assembly and implicate a cooperative effect on the xylosyl transfer to site 2 by xylosylation of site 1, which probably becomes manifest before the removal of the propeptide. It is shown additionally that biglycan expressed in 293 cells may still contain the propeptide sequence and may carry heparan sulfate chains as well as sulfated N-linked oligosaccharides.

Cloning and characterization of Rhesus monkey neuropeptide Y receptor subtypes

Neuropeptide Y (NPY) is a 36 amino acid peptide that is abundant in the brain and peripheral nervous system. NPY has a variety of effects when administered into the brain including a pronounced feeding effect, anxiolysis, regulation of neuroendocrine axes and inhibition of neurotransmitter release. These effects are mediated by up to 6 G protein coupled receptors designated Y1, Y2, Y3, Y4, Y5 and y6. To better understand the phylogeny and pharmacology of NPY in non-human primates, we have cloned and expressed the NPY Y1, Y2 and Y5 receptor subtypes from the Rhesus monkey. No cDNA sequence encoding a Y4 receptor was found suggesting substantial sequence differences when compared to the human sequence. Comparison of these sequences with those from human indicated strong sequence conservation of Y1, Y2 and Y5 between the two species. The displacement of (125)I-PYY binding to the Rhesus monkey and human receptors by various peptides was compared to evaluate the pharmacology of the two species. Similar pharmacologies were noted across the species at the various receptor subtypes. These results indicate the Rhesus monkey and human NPY receptor subtypes have a close amino acid sequence conservation and that the peptide recognition domains are conserved as well.

Recombinant thrombomodulin inhibits arterial smooth muscle cell proliferation induced by thrombin

PURPOSE

Restenosis after angioplasty or bypass grafting to restore circulation to ischemic organs is still an unsolved problem. Thrombin generated in high concentrations at the sites of vascular injury plays a central role in thrombosis and hemostasis. alpha-Thrombin has also been implicated as a mitogen for smooth muscle cell (SMC) proliferation that contributes to arterial restenosis. Thrombomodulin has a high affinity of binding with thrombin and converts thrombin from a procoagulant to an anticoagulant. This study was designed to examine whether thrombomodulin could also moderate the thrombin-mediated SMC proliferative response.

METHODS

Porcine carotid artery SMCs (passages 4-7) were plated onto 96-well plates and incubated for 3 days. After growth arrest in a defined serum-free medium for 2 to 3 days, SMCs were subjected to the reagents as follows: (1) human alpha-thrombin, (2) recombinant human soluble thrombomodulin containing a chondroitin sulfate moiety, (3) thrombin receptor agonist peptide (SFLLRNPNDKYEPF), and (4) alpha-thrombin or thrombin receptor agonist peptide combined with recombinant thrombomodulin (rTM). The viability and proliferation status of SMCs were quantified with MTT (thiazolyl blue) mitochondrial function and bromodeoxyuridine (BrdU)-DNA incorporation assays.

RESULTS

Human alpha-thrombin increased SMC proliferation in a dose dependent manner by more than 25% and 30% with thrombin 1 U/mL to 3 U/mL compared with control groups on day 7 (P <.006). rTM concentrations from 0.5 microg/mL to 3 microg/mL have no significant effect on SMC growth. The stimulation of SMC proliferation induced by alpha-thrombin at 0.5 U/mL, 1 U/mL, and 2 U/mL was significantly inhibited with rTM at 2 microg/mL and 3 microg/mL on days 3, 7, and 10 as evaluated with MTT assay (P <.01 to <.05) and BrdU-DNA incorporation assay on day 3 (P <.008). Thrombin receptor agonist peptide increased SMC BrdU-DNA incorporation at 48 hours (P <.007), and its effect was not altered by rTM.

CONCLUSION

rTM containing all of the extracellular domains of thrombomodulin inhibits the effect of thrombin on SMC proliferation in vitro. Because thrombin is a mitogenic mediator of SMC in vascular injury, inhibition of its function in vivo could help to prevent SMC hyperplasia. The success of further studies in vivo may lead to use of rTM for decreasing or preventing arterial restenosis.

Recombinant human thrombomodulin(csa+): a tool for analyzing Plasmodium falciparum adhesion to chondroitin-4-sulfate

The proteoglycan thrombomodulin has been shown to be involved, via its chondroitin-sulfate moiety, in the cytoadhesion of chondroitin-4-sulfate-binding-Plasmodium falciparum-infected erythrocytes to endothelial cells and syncytiotrophoblasts. We cloned and expressed in CHO and COS-7 cells a gene encoding soluble human recombinant thrombomodulin, with a chondroitin-4-sulfate moiety. This system is complementary to the in vitro cell models currently used to study the chondroitin-4-sulfate-binding phenotype. It also provides a means of overcoming the lack of specificity observed in interactions of infected erythrocytes with modified chondroitin-4-sulfate. This thrombomodulin displayed normal activity in coagulation, indicating that it was in a functional conformation. The recombinant protein, whether produced in CHO or COS-7 cells, inhibited cytoadhesion to Saimiri brain microvascular endothelial cells 1D infected with Palo-Alto(FUP)1 parasites selected for chondroitin-4-sulfate receptor preference. Thus, the recombinant protein was produced with a chondroitin-sulfate moiety, identified as a chondroitin-4-sulfate, in both cell types. In both cases, the recombinant protein bound to the chondroitin-4-sulfate phenotype, but not to CD36- and ICAM-1-binding parasites. The chondroitin-4-sulfate was 36 kDa in size for CHO and 17.5 kDa for COS-7 cells. There was, however, no difference in the capacities of the recombinant proteins produced by the two cell types to inhibit the cytoadhesion of infected erythrocytes. Thrombomodulin immobilized on plastic or coupled to Dynabeads was used to purify specifically the infected erythrocytes that bind to chondroitin-4-sulfate. These infected erythrocytes were cultured to establish parasite lines of this phenotype. We then showed that the thrombomodulin, labeled with FITC, could be used to detect this phenotype in blood samples. Finally, the direct binding of infected erythrocytes to immobilized thrombomodulin was used to screen for anti-chondroitin-4-sulfate-binding antibodies.

Novel proteoglycan linkage tetrasaccharides of human urinary soluble thrombomodulin, SO4-3GlcAbeta1-3Galbeta1-3(+/-Siaalpha2-6)Galbeta1-4Xyl

O-linked sugar chains with xylose as a reducing end linked to human urinary soluble thrombomodulin were studied. Sugar chains were liberated by hydrazinolysis followed by N-acetylation and tagged with 2-aminopyridine. Two fractions containing pyridylaminated Xyl as a reducing end were collected. Their structures were determined by partial acid hydrolysis, two-dimensional sugar mapping combined with exoglycosidase digestions, methylation analysis, mass spectrometry, and NMR as SO4-3GlcAbeta1-3Galbeta1-3(+/-Siaalpha2-6)Galbeta1+ ++-4Xyl. These sugar chains could bind to an HNK-1 monoclonal antibody. This is believed to be the first example of a proteoglycan linkage tetrasaccharide with glucuronic acid 3-sulfate and sialic acid.

Demonstration of the immature glycosaminoglycan tetrasaccharide sequence GlcAbeta1-3Galbeta1-3Galbeta1-4Xyl on recombinant soluble human alpha-thrombomodulin. An oligosaccharide structure on a "part-time" proteoglycan

Thrombomodulin (TM), a cell surface glycoprotein, is a critical mediator of endothelial anticoagulant defenses occurring both as a chondroitin sulfate proteoglycan (beta-TM) and a protein (alpha-TM) unsubstituted by chondroitin sulfate (CS), hence its description as a "part-time" proteoglycan (PG) (Fransson, L. A. (1987) Trends Biochem. Sci. 12, 406-411). Sugar analysis was performed on alpha-TM to investigate a possible biosynthetic mechanism for part-time PGs. Recombinant human alpha-TM, which was expressed in CHO-K1 cells, separated by anion-exchange chromatography from beta-TM, and purified by immunoaffinity chromatography (Nawa, K., Sakano, K., Fujiwara, H., Sato, Y., Sugiyama, N., Teruuchi, T., Iwamoto, M., and Marumoto, Y. (1990) Biochem. Biophys. Res. Commun. 171, 729-737), was used for analysis. Preliminary sugar composition analysis after acid hydrolysis showed Xyl in addition to Gal, GalNAc, GlcNAc, Man, Fuc, and Glc. O-Glycosidically-linked oligosaccharides were liberated by mild alkaline treatment and purified. The isolated oligosaccharide fraction was derivatized with a fluorophore 2-aminobenzamide (2AB), resulting in two fluorescent components, a 2AB-oligosaccharide and a putative 2AB-Glc. Based on structural analysis by a combination of sequential exoglycosidase digestion and 500-MHz 1H NMR spectroscopy of the 2AB-oligosaccharide, the structure of the oligosaccharide was elucidated as GlcAbeta1-3Galbeta1-3Galbeta1-4Xyl, which turned out to represent a glycosaminoglycan (GAG)-protein linkage region tetrasaccharide common to various PGs and was considered to be a biosynthetic intermediate of an immature GAG chain. The results may indicate that at least one class of the so-called part-time PGs bear the linkage tetrasaccharide at the GAG attachment sites and that the critical determining step or the rate-limiting step for PG biosynthesis is the transfer of the fifth sugar residue, the first hexosamine, rather than xylose.

Effect of high- and low-molecular-weight heparins on thrombin-thrombomodulin interaction and protein C activation

BACKGROUND

Thrombin-thrombomodulin (TM) interaction, which is critical for accelerating the protein C anticoagulant pathway, involves the heparin-like domain of TM. This study was aimed at investigating the possible effect of heparin on thrombin-TM binding and protein C activation.

METHODS AND RESULTS

The affinity of thrombin-TM interaction was studied by a functional method, based on the ability of thrombin-TM adduct to activate protein C, and by evaluation of the binding of thrombin to immobilized TM. Both experimental approaches showed that the affinity of thrombin-TM interaction was decreased by micromolar heparin concentrations. Heparin had no significant effect when a recombinant TM form, lacking the chondroitin sulfate moiety, was used. Furthermore, it was also shown that the inhibitory effect of heparin was directly proportional to the heparin molecular mass (molecular weight range, 3 to 16 kDa), which suggests that the effect was mediated by formation of electrostatic bonds between heparin and thrombin.

CONCLUSIONS

These results indicate that heparin at therapeutic concentrations reduces the affinity of thrombin for TM and the rate of protein C activation. The magnitude of this effect is proportionally linked to the molecular mass of heparin.

Inhibition of binding of malaria-infected erythrocytes by a tetradecasaccharide fraction from chondroitin sulfate A

Adherence of parasite-infected erythrocytes (IEs) to the microvascular endothelium of various organs, a process known as sequestration, is a feature of Plasmodium falciparum malaria. This event is mediated by specific adhesive interactions between parasite proteins, expressed on the surface of IEs, and host molecules. P. falciparum IEs can bind to purified chondroitin sulfate A (CS-A), to the proteoglycan thrombomodulin through CS-A side chains, and to CS-A present on the surface of brain and lung endothelial cells and placental syncytiotrophoblasts. In order to identify structural characteristics of CS-A important for binding, oligosaccharide fragments ranging in size from 2 to 20 monosaccharide units were isolated from CS-A and CS-C, following controlled chondroitin lyase digestion, and used as competitive inhibitors of IE binding to immobilized ligands. Inhibition of binding to CS-A was highly dependent on molecular size: a CS-A tetradecasaccharide fraction was the minimum length able to almost completely inhibit binding. The effect was dose dependent and similar to that of the parent polysaccharide, and the same degree of inhibition was not found with the CS-C oligosaccharides. There was no effect on binding of IEs to other ligands, e.g., CD36 and intercellular adhesion molecule 1. Hexadeca- and octadecasaccharide fractions of CS-A were required for maximum inhibition of binding to thrombomodulin. Analyses of oligosaccharide fractions and polysaccharides by electrospray mass spectrometry and high-performance liquid chromatography suggest that the differences between the activities of CS-A and CS-C oligosaccharides can be attributed to differences in sulfate content and sulfation pattern and that iduronic acid is not involved in IE binding.

Plasmodium falciparum-infected erythrocytes adhere to the proteoglycan thrombomodulin in static and flow-based systems

Plasmodium falciparum-infected erythrocytes can bind to the glycosaminoglycan chondroitin sulfate A. In this paper, we demonstrate that thrombomodulin, a proteoglycan present on endothelial cells and placental syncytiotrophoblasts, supports binding of selected lines of P. falciparum-infected erythrocytes in both static and flow-based assays, and that adhesion is dependent on the presence of the chondroitin sulfate A chain of thrombomodulin. Chondroitinase treatment of thrombomodulin abolished binding, and free chondroitin sulfate A prevented it, whereas other soluble glycosaminoglycans had little or no effect. Soluble thrombomodulin (with, but not without, its chondroitin sulfate chain) inhibited binding at 40 micrograms/ml, but not at physiological concentrations. Parasitized erythrocytes bound to cells expressing thrombomodulin, including human umbilical vein endothelial cells and A549 cells, and binding was inhibited by free chondroitin sulfate A. Established binding to A549 cells or to immobilized thrombomodulin was substantially reversed by chondroitin sulfate A at 10 micrograms/ml. The chondroitin sulfate chain of thrombomodulin is a receptor for malaria-infected erythrocytes in static assays and under physiological flow.

Influence of core protein sequence on glycosaminoglycan assembly

Recent studies have revealed a correlation between amino acid sequences around glycosylation sites in proteoglycans and the ability of cells to initiate and process glycosaminoglycan chains. Initiation depends on Ser-Gly/Ala dipeptides that have one or more acidic amino acids in close proximity. The formation of heparan sulfate chains depends on a nearby cluster of acidic residues, hydrophobic amino acids, and the close spacing of glycosylation sites.

A stereocontrolled synthetic approach to glycopeptides corresponding to the carbohydrate-protein linkage region of cell-surface proteoglycans

The glycopeptides 1 (Gly[O-beta-D-Xylp)-L-Ser-Gly-L-Glu) and 2 (Gly[O-[beta-D-GlcpA-(1-->3)-beta-D-Galp-(1-->3)-beta-D-Galp]-(1-- >4) -beta-D-Xylp)-L-Ser-Gly-L-Glu), carrying the core structure of serine-linked cell-surface proteoglycans were synthesized in a stereocontrolled manner. The carbohydrate key imidate xylosyl donors 3 and glycotetraosyl donors 4 and 5, as well as a tetrapeptide glycosyl acceptor 6, were coupled in the crucial glycosylation step. In these reactions, the application of either trimethylsilyl trifluoromethanesulfonate (TMSOTf) or borontrifluoride etherate (BF3-Et2O) as catalysts proved to be highly efficient. The serine linked glycopeptides 34, 36 and 37 thus obtained yielded target compounds 1 and 2 on complete deprotection.

Chondroitin sulfate A is a cell surface receptor for Plasmodium falciparum-infected erythrocytes

Adherence of Plasmodium falciparum-infected erythrocytes to cerebral postcapillary venular endothelium is believed to be a critical step in the development of cerebral malaria. Some of the possible receptors mediating adherence have been identified, but the process of adherence in vivo is poorly understood. We investigated the role of carbohydrate ligands in adherence, and we identified chondroitin sulfate (CS) as a specific receptor for P. falciparum-infected erythrocytes. Parasitized cells bound to Chinese hamster ovary (CHO) cells and C32 melanoma cells in a chondroitin sulfate-dependent manner, whereas glycosylation mutants lacking chondroitin sulfate A (CSA) supported little or no binding. Chondroitinase treatment of wild-type CHO cells reduced binding by up to 90%. Soluble CSA inhibited binding to CHO cells by 99.2 +/- 0.2% at 10 mg/ml and by 72.5 +/- 3.8% at 1 mg/ml, whereas a range of other glycosaminoglycans such as heparan sulfate had no effect. Parasite lines selected for increased binding to CHO cells and most patient isolates bound specifically to immobilized CSA. We conclude that P. falciparum can express or expose proteins at the surface of the infected erythrocyte that mediate specific binding to CSA. This mechanism of adherence may contribute to the pathogenesis of P. falciparum malaria, but has wider implications as an example of an infectious agent with the capacity to bind specifically to cell-associated or immobilized CS.