Human thrombomodulin gene is intron depleted: nucleic acid sequences of the cDNA and gene predict protein structure and suggest sites of regulatory control

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.

Exploring traditional and nontraditional roles for thrombomodulin

Thrombomodulin (TM) is an integral component of a multimolecular system, localized primarily to the vascular endothelium, that integrates crucial biological processes and biochemical pathways, including those related to coagulation, innate immunity, inflammation, and cell proliferation. These are designed to protect the host from injury and promote healing. The “traditional” role of TM in hemostasis was determined with its discovery in the 1980s as a ligand for thrombin and a critical cofactor for the major natural anticoagulant protein C system and subsequently for thrombin-mediated activation of the thrombin activatable fibrinolysis inhibitor (also known as procarboxypeptidase B2). Studies in the past 2 decades are redefining TM as a molecule with many properties, exhibited via its multiple domains, through its interacting partners, complex regulated expression, and synthesis by cells other than the endothelium. In this report, we review some of the recently reported diverse properties of TM and how these may impact on our understanding of the pathogenesis of several diseases.

Haemolytic uraemic syndrome

Haemolytic uraemic syndrome (HUS) is defined by the simultaneous occurrence of nonimmune haemolytic anaemia, thrombocytopenia and acute renal failure. This leads to the pathological lesion termed thrombotic microangiopathy, which mainly affects the kidney, as well as other organs. HUS is associated with endothelial cell injury and platelet activation, although the underlying cause may differ. Most cases of HUS are associated with gastrointestinal infection with Shiga toxin-producing enterohaemorrhagic Escherichia coli (EHEC) strains. Atypical HUS (aHUS) is associated with complement dysregulation due to mutations or autoantibodies. In this review, we will describe the causes of HUS. In addition, we will review the clinical, pathological, haematological and biochemical features, epidemiology and pathogenetic mechanisms as well as the biochemical, microbiological, immunological and genetic investigations leading to diagnosis. Understanding the underlying mechanisms of the different subtypes of HUS enables tailoring of appropriate treatment and management. To date, there is no specific treatment for EHEC-associated HUS but patients benefit from supportive care, whereas patients with aHUS are effectively treated with anti-C5 antibody to prevent recurrences, both before and after renal transplantation.

Novel genetic predictors of venous thromboembolism risk in African Americans

Venous thromboembolism (VTE) is the third most common life-threatening cardiovascular condition in the United States, with African Americans (AAs) having a 30% to 60% higher incidence compared with other ethnicities. The mechanisms underlying population differences in the risk of VTE are poorly understood. We conducted the first genome-wide association study in AAs, comprising 578 subjects, followed by replication of highly significant findings in an independent cohort of 159 AA subjects. Logistic regression was used to estimate the association between genetic variants and VTE risk. Through bioinformatics analysis of the top signals, we identified expression quantitative trait loci (eQTLs) in whole blood and investigated the messenger RNA expression differences in VTE cases and controls. We identified and replicated single-nucleotide polymorphisms on chromosome 20 (rs2144940, rs2567617, and rs1998081) that increased risk of VTE by 2.3-fold (P< 6 × 10(-7)). These risk variants were found in higher frequency among populations of African descent (>20%) compared with other ethnic groups (<10%). We demonstrate that SNPs on chromosome 20 are cis-eQTLs for thrombomodulin (THBD), and the expression of THBD is lower among VTE cases compared with controls (P= 9.87 × 10(-6)). We have identified novel polymorphisms associated with increased risk of VTE in AAs. These polymorphisms are predominantly found among populations of African descent and are associated with THBD gene expression. Our findings provide new molecular insight into a mechanism regulating VTE susceptibility and identify common genetic variants that increase the risk of VTE in AAs, a population disproportionately affected by this disease.

Thrombomodulin gene proximal promoter polymorphisms in premature acute coronary syndrome patients in Bahrain

Thrombomodulin is expressed on endothelial cells and monocytes (mTM) where it has an anticoagulant function. Enzymatic cleavage from the cell surface produces soluble thrombomodulin (sTM) in plasma. Abnormal levels of sTM and mutations in the thrombomodulin gene (THBD) are linked to cardiovascular disease. The aim of this study was to investigate THBD proximal promoter mutations and levels of sTM and mTM in men presenting with premature acute coronary syndrome (ACS). This prospective cross-sectional study included 100 adult men with premature ACS (age <55 years) and 60 healthy age-matched controls. Plasma sTM was assayed by ELISA. mTM expression was assessed by flow cytometry with CD141 antibody. The -33 G/A polymorphism was identified by PCR-restriction fragment length polymorphism analysis and the THBD proximal promoter region was sequenced. Significantly lower sTM (P < 0.001) and higher mTM (P < 0.001) were seen in ACS patients. Heterozygous THBD promoter polymorphisms -33 G/A and -9/-10 GG/AT were found in eight patients and five control individuals. In patients and control individuals, allele frequencies of A were 0.02 and 0.025, and that of AT were 0.025 and 0.017, respectively. There were no significant associations of these polymorphisms with ACS, sTM levels or mTM expression. THBD polymorphisms -33 G/A and -9/-10 GG/AT are present in low frequency in our patient population, and are more frequent in the South Asians as compared to the Arabs. The frequency of -33 G/A is lower, whereas that of -9/-10 GG/AT is higher than that reported in the Orientals. The presence of THBD proximal promoter polymorphisms do not explain variations in levels of sTM and mTM in this patient population.

Changes in cardiac thrombomodulin and heat shock transcription factor 1 expression and peripheral thrombomodulin and catecholamines during hypothermia in rats

Effects of hypothermia and rewarming on thrombomodulin, catecholamines and heat shock transcription factor 1 (HSF1) were studied in rats. The aims of this study were to clarify whether cold stress, under anesthesia, is sufficient to change levels of thrombomodulin in healthy endothelium and in the circulation and whether adrenaline, noradrenaline and HSF1 could act as regulators in the process. Rats were divided into control, mild hypothermia (2 and 4.5 hours at + 21 °C; MH1, MH2), severe hypothermia (2 and 4.5 h at + 10 °C; SH1, SH2) and two rewarming groups (2 h at + 10 °C followed by 2 h at + 21 °C or 3 h at + 28 °C; SHW1, SHW2) (n = 15/group, except n = 6 in MH1). Fentanyl-fluanisone-midazolam was used as anesthetic. Low levels of thrombomodulin in plasma and myocardial arterioles/venules measured by ELISA and immunohistochemistry were associated with significant increase of thrombomodulin transcript level in SH1 rats analyzed by quantitative PCR. Plasma adrenaline correlated negatively with the relative amount of myocardial thrombomodulin transcripts and positively with plasma thrombomodulin in SH. Transcript levels of thrombomodulin and HSF1 correlated strongly (r = 0.83; p < 0.001) in SH. Plasma/urine ratio of thrombomodulin and plasma adrenaline (r = 0.87; p = 0.005) or noradrenaline (r = 0.78; p = 0.023) were strongly correlated in SHW1 rats. Hence, cellular and soluble levels of thrombomodulin are modified by cold stress in healthy rats, possibly via catecholamines and HSF1.

Common genetic risk factors for venous thrombosis in the Chinese population

Venous thrombosis is a major medical disorder caused by both genetic and environmental factors. Little is known about the genetic background of venous thrombosis in the Chinese population. A total of 1,304 individuals diagnosed with a first venous thrombosis and 1,334 age- and sex-matched healthy participants were enrolled in this study. Resequencing of THBD (encoding thrombomodulin) in 60 individuals with venous thrombosis and 60 controls and a functional assay showed that a common variant, c.-151G>T (rs16984852), in the 5' UTR significantly reduced the gene expression and could cause a predisposition to venous thrombosis. Therefore, this variant was genotyped in a case-control study, and results indicated that heterozygotes had a 2.80-fold (95% confidence interval = 1.88-4.29) increased risk of venous thrombosis. The THBD c.-151G>T variant was further investigated in a family analysis involving 176 first-degree relatives from 38 index families. First-degree relatives with this variant had a 3.42-fold increased risk of venous thrombosis, and their probability of remaining thrombosis-free was significantly lower than that of relatives without the variant. In addition, five rare mutations that might be deleterious were also identified in thrombophilic individuals by sequencing. This study is the largest genetic investigation of venous thrombosis in the Chinese population. Further study on genetics of thrombosis should focus on resequencing of THBD and other hemostasis genes in different populations.

Thrombomodulin as a regulator of the anticoagulant pathway: implication in the development of thrombosis

Thrombomodulin is a cell surface-expressed glycoprotein that serves as a cofactor for thrombin-mediated activation of protein C (PC), an event further amplified by the endothelial cell PC receptor. The PC pathway is a major anticoagulant mechanism that downregulates thrombin formation and hedges thrombus formation. The objectives of this review were to review recent findings regarding thrombomodulin structure, its involvement in the regulation of hemostasis and further discuss the implication, if any, of the genetic polymorphisms in the thrombomodulin gene in the risk of development of thrombosis. We performed a literature search by using electronic bibliographic databases. Although the direct evaluation of risk situations associated with thrombomodulin mutations/polymorphisms could be of clinical significance, it appears that mutations that affect the function of thrombomodulin are rarely associated with venous thromboembolism. However, several polymorphisms are reported to be associated with increased risk for arterial thrombosis. Additionally studies on knock out mice as well studies on humans bearing rare mutations suggest that thrombomodulin dysfunction may be implicated in the pathogenesis of myocardial infraction.

Protein C anticoagulant and cytoprotective pathways

Plasma protein C is a serine protease zymogen that is transformed into the active, trypsin-like protease, activated protein C (APC), which can exert multiple activities. For its anticoagulant action, APC causes inactivation of the procoagulant cofactors, factors Va and VIIIa, by limited proteolysis, and APC's anticoagulant activity is promoted by protein S, various lipids, high-density lipoprotein, and factor V. Hereditary heterozygous deficiency of protein C or protein S is linked to moderately increased risk for venous thrombosis, while a severe or total deficiency of either protein is linked to neonatal purpura fulminans. In recent years, the beneficial direct effects of APC on cells which are mediated by several specific receptors have become the focus of much attention. APC-induced signaling can promote multiple cytoprotective actions which can minimize injuries in various preclinical animal injury models. Remarkably, pharmacologic therapy using APC demonstrates substantial neuroprotective effects in various murine injury models, including ischemic stroke. This review summarizes the molecules that are central to the protein C pathways, the relationship of pathway deficiencies to venous thrombosis risk, and mechanisms for the beneficial effects of APC.

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.

Mutagenesis studies toward understanding the mechanism of the cofactor function of thrombomodulin

Thrombomodulin (TM) is as essential cofactor in protein C activation by thrombin. To investigate the cofactor effect of TM on the P3-P3' binding specificity of thrombin, we prepared a Gla-domainless protein C (GDPC) and an antithrombin (AT) mutant in which the P3-P3' residues of both molecules were replaced with the corresponding residues of the factor Xa cleavage site in prethrombin-2. TM is known to interact with GDPC, but not AT in the complex. Thrombin did not react with either mutant in the absence of a cofactor. While the thrombin-TM complex also did not react with the AT mutant, it activated the GDPC mutant with a normal k(cat), but an approximately 4-fold impaired K(m) value. Further studies revealed that the active-site directed inhibitor p-aminobenzamidine acts as a competitive inhibitor of both wild-type and GDPC mutant in reaction with the thrombin-TM complex. These results suggest that the interaction of the P3-P3' residues of GDPC with the active-site pocket of the thrombin-TM complex makes a dominant contribution to the binding specificity of the reaction. Moreover, the observation that the GDPC mutant, but not the AT mutant, functions as an effective substrate for the thrombin-TM complex suggests that GDPC interaction with the thrombin-TM complex may be associated with the alteration of the conformation of the P3-P3' residues of the substrate.

Thrombomodulin: from haemostasis to inflammation and tumourigenesis

Thrombomodulin (TM), a transmembrane endothelial receptor, participates in coagulation, in inflammation, in cancer and plays a role during embryogenesis. The nucleotide sequence of the TM cDNA allows the structure of this protein to be visualized. The protein starts with a signal peptide, followed by the N-terminal globular domain, six repeats of epidermal growth factor-like sequence, a serine/threonine-rich region, a transmembrane domain and a cytoplasmic domain. High-resolution nuclear magnetic resonance (NMR) spectroscopy was employed to define the exact thrombin-binding region. Residues Y(413)ILDD(417) and D(423)IDE(426) are crucial for binding to thrombin; the two critical amino acids for thrombin binding, Ile(414) and Ile(424), are brought into spatial proximity by beta-sheet structure. There also exist some residues for co-factor activity, namely Asp(349), Glu(357), Tyr(358), Phe(376) and Met(388). The complex transcriptional and post-transcriptional control of TM underline its importance in a wide variety of biological systems and pathophysiological processes.

The functional significance of the autolysis loop in protein C and activated protein C

The autolysis loop of activated protein C (APC) is five residues longer than the autolysis loop of other vitamin K-dependent coagulation proteases. To investigate the role of this loop in the zymogenic and anticoagulant properties of the molecule, a protein C mutant was constructed in which the autolysis loop of the protein was replaced with the corresponding loop of factor X. The protein C mutant was activated by thrombin with approximately 5-fold higher rate in the presence of Ca2+. Both kinetics and direct binding studies revealed that the Ca2+ affinity of the mutant has been impaired approximately 3-fold. The result of a factor Va degradation assay revealed that the anticoagulant function of the mutant has been improved 4-5-fold in the absence but not in the presence of protein S. The improvement was due to a better recognition of both the P1-Arg506 and P1-Arg306 cleavage sites by the mutant protease. However, the plasma half-life of the mutant was markedly shortened due to faster inactivation by plasma serpins. These results suggest that the autolysis loop of protein C is critical for the Ca(2+)-dependence of activation by thrombin. Moreover, a longer autolysis loop in APC is not optimal for interaction with factor Va in the absence of protein S, but it contributes to the lack of serpin reactivity and longer half-life of the protease in plasma.

The functional significance of the autolysis loop in protein C and activated protein C

The autolysis loop of activated protein C (APC) is five residues longer than the autolysis loop of other vitamin K-dependent coagulation proteases. To investigate the role of this loop in the zymogenic and anticoagulant properties of the molecule, a protein C mutant was constructed in which the autolysis loop of the protein was replaced with the corresponding loop of factor X. The protein C mutant was activated by thrombin with approximately 5-fold higher rate in the presence of Ca2+. Both kinetics and direct binding studies revealed that the Ca2+ affinity of the mutant has been impaired approximately 3-fold. The result of a factor Va degradation assay revealed that the anticoagulant function of the mutant has been improved 4-5-fold in the absence but not in the presence of protein S. The improvement was due to a better recognition of both the P1-Arg506 and P1-Arg306 cleavage sites by the mutant protease. However, the plasma half-life of the mutant was markedly shortened due to faster inactivation by plasma serpins. These results suggest that the autolysis loop of protein C is critical for the Ca(2+)-dependence of activation by thrombin. Moreover, a longer autolysis loop in APC is not optimal for interaction with factor Va in the absence of protein S, but it contributes to the lack of serpin reactivity and longer half-life of the protease in plasma.

Thrombomodulin gene polymorphisms or haplotypes as potential risk factors for venous thromboembolism: a population-based case-control study

Dysfunction of the protein C anticoagulant system is associated with venous thromboembolism (VTE) and thrombomodulin (TM) is a critical cofactor within the protein C system. The aim of this study was to test the hypotheses that polymorphisms or haplotypes within the TM gene are common risk factors for VTE. We screened the TM putative promoter, exon and 3'-untranslated region for sequence variations in a random sample (n = 266) of consecutive idiopathic, objectively confirmed non-Olmsted County VTE patients referred to the Mayo Clinic. We then genotyped a sample of Olmsted County, MN residents with a first lifetime, objectively confirmed VTE in the 25-year period, 1966-90 (n = 223), and a sample of Olmsted County residents without VTE (n = 237) for polymorphisms either discovered in the screening population or previously published, and tested for an association of VTE with TM genotype or haplotypes using unconditional logistic regression and generalized linear models, respectively. We also genotyped these Olmsted County cases and controls at 20 'null' genetic maker loci and tested for population admixture. Nine novel and three previously described mutations were identified in the screening population. Mutations within the TM promoter, EGF(1-5), serine/threonine-rich, transmembrane, and cytoplasm regions were absent or uncommon. TM845G-->A (Ala25Thr; lectin region), TM2136T-->C (Ala455Val; EGF(6) region), TM2470C deletion (3'-untranslated region), and 4363A-->G (3'-flanking region) were more common, but were not associated with VTE by genotype or haplotype. Null genetic marker allele frequencies did not differ significantly among cases and controls. We conclude that polymorphisms or haplotypes within the TM gene are not common risk factors for incident VTE.

Global analysis of RNA expression profile in human vascular cells treated with statins

In addition to a lipid-lowering effect, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) have an effect on the expression levels of many genes. In order to elucidate the range of this effect as comprehensively as possible, we investigated the changes in gene expression profiles brought about by atorvastatin or pitavastatin in cultured human umbilical vein endothelial cells (HUVEC), cultured human coronary artery smooth muscle cells (HCASMC) and cultured human hepatocarcinoma Hep G2 cells by means of DNA microarrays. Among the 6146 genes in the array, statins affected the expression levels of genes involved in coagulation, vascular constriction and cell growth in a cell-type specific manner. In HUVEC, they induced integrin beta4 and thrombomodulin profoundly, and profoundly suppressed pentraxin 3 both at 8 and 24 hours. In HCASMC, the statins induced thrombomodulin and urokinase inhibitor, and potently suppressed the cysteine-rich angiogenic inducer 61 and cyclin B. Many genes related to the cell cycle and/or growth were also regulated in HUVEC and HCASMC by the statins. These results indicate that many aspects of the pleiotropic effect can be mediated by transcriptional control by statins. Genes newly identified by this study may be useful in statin therapy.

Prospective study of the A455V polymorphism in the thrombomodulin gene, plasma thrombomodulin, and incidence of venous thromboembolism: the LITE Study

Plasma thrombomodulin (soluble TM; sTM) is considered to be a marker of endothelial injury, but a recent report indicated that the relationship of sTM with thrombosis is complex. Venous thromboembolic events were identified in adults in two longitudinal cohort studies, the Atherosclerosis Risk in Communities Study and the Cardiovascular Health Study, totaling 21 690 participants. After 8 years of follow-up, sTM was measured in baseline plasma of 305 participants who developed venous thrombosis and 607 who did not. Thrombomodulin A455V genotype was determined in 302 cases and 626 controls. There was no difference in the prevalence of the three TM genotypes between cases and controls and no difference in age-adjusted mean values of sTM by genotype. There were no associations of age-adjusted sTM or TMA455V genotype with overall venous thromboembolism or with thrombosis in any subtype of venous thromboembolism.

Antisense oligodeoxynucleotides against thrombomodulin suppress the cell growth of lung adenocarcinoma cell line A549

Thrombomodulin (TM), an anticoagulant factor on endothelial cells, is known to be expressed in non-endothelial cells as well. In neoplastic cells of lung adenocarcinomas, TM is expressed but its correlation with growth potential has not been studied. As TM expression has a negative correlation with cell proliferation in lung squamous cell carcinomas, we examined its growth effect on lung adenocarcinoma cells of the A549 cell line by inhibiting TM expression with antisense oligodeoxynucleotides (ODN). In the antisense ODN transfected cells, the expression of TM mRNA was decreased to 49% at 12 h and 47% at 24 h, which was in accordance with TM expression at the protein level. By IdU (5-iodo-2'-deoxyuridine) incorporation assay, the growth of A549 cells was found to have decreased to 36% of the control level at 24 h post-transfection. The suppression of cell growth was maintained in a concentration-dependent manner for 48 h after transfection, when the expression of TM started to rebound. In the transfected cells, the G1 phase cell count was reduced to 60.7%, compared with 68.2% in the control transfectants. These results suggest that TM expression may play a suppressive role in the proliferation activity of A549 lung adenocarcinoma cells.

Mutations in the thrombomodulin and endothelial protein C receptor genes in women with late fetal loss

Late fetal loss can be associated with placental insufficiency and coagulation defects. Thrombomodulin (TM) and the endothelial protein C receptor (EPCR) are glycoprotein receptors expressed mainly on the endothelial surface of blood vessels and also in the placenta; they both play a key physiological role in the protein C anticoagulant pathway. Defects in these proteins might play an important role in the pathogenesis of late fetal loss. We performed a case-control study in 95 women with unexplained late fetal loss (> 20 weeks), to elucidate whether TM or EPCR gene mutations were associated with an increased risk for this complication of pregnancy. The control group comprised 236 women who gave birth to at least one healthy baby and had no history of late fetal death or obstetrical complications. The entire TM and EPCR genes, including the promoter region, were screened. In total, five mutations were identified in the TM gene in 95 patients and three in 236 control subjects, and two mutations were identified in the EPCR gene in 95 patients and one in 236 control subjects. The relative risk for late fetal loss when having a mutation in the TM or EPCR gene was estimated by an odds ratio of 4.0 (95% CI 1.1-14.9). In conclusion, identified mutations in the TM and EPCR genes of women with unexplained fetal loss are more prevalent compared with women with no obstetrical complications.

Thrombomodulin Ala455Val polymorphism and risk of coronary heart disease

BACKGROUND

Thrombomodulin (TM) is expressed on the endothelial surface and plays an important role in vasoprotection. A common polymorphism of TM at amino acid position 455 with an alanine (A) to valine (V) transition was previously reported to be associated cross-sectionally with acute myocardial infarction. Whether this single nucleotide polymorphism predicts risk of developing coronary heart disease (CHD) is unclear.

METHODS AND RESULTS

Within a large cohort study, we identified 467 incident CHD cases during an average of 5 years of follow-up. We determined TM-455 genotypes on 376 CHD cases (23% black, 77% white) and a reference sample of 461. The AA genotype was significantly more prevalent in noncases than in cases (P:=0.016). The prevalences of the AA genotype in noncase blacks and whites were 93% and 67%, respectively. The AA genotype frequency was significantly reduced in black cases versus noncases (P:=0.018). It was also lower in white cases than in noncases, but the difference was not statistically significant (P:=0.066). Weighted proportional hazards regression analysis after adjustment for age, sex, and other CHD risk factors showed that having the V allele increased risk of CHD by 6.1-fold (risk ratio 6.1, 95% CI 1.7 to 22.9) in blacks but did not significantly increase the risk in whites.

CONCLUSIONS

The TM A455V polymorphism predicts risk of developing CHD in blacks.

Thrombomodulin promoter mutations, venous thrombosis, and varicose veins

We analyzed the distal promoter region of the thrombomodulin (TM) gene (nucleotides -300 to -2052) in subjects from the Paris Thrombosis Study (PATHROS), a French case-control study of venous thrombosis, to identify polymorphisms that might modify TM gene expression. Eight novel mutations were found in the 40 DNA samples initially screened. Two of these mutations (-1748G/C and -1208/-1209 del TT) were frequent. One rare transition (-1166G/A) might have functional consequences owing to its position. These 3 mutations were screened for in the entire study population of 327 patients and 398 controls. None of the 3 was significantly associated with thrombosis. Interestingly, the -1208/-1209 TT deletion was associated with varicose veins in the patients. This mutation was in tight linkage disequilibrium with the +1418 C/T change in the coding sequence, a known polymorphism that predicts an Ala 455 Val substitution in the sixth epidermal growth factor-like TM module, a domain previously implicated in the proliferative functions of TM. This linkage suggests that the Ala 455 Val mutation may promote changes in these functions and thus be involved in varicose vein formation.

RINX(VSX1), a novel homeobox gene expressed in the inner nuclear layer of the adult retina

The locus control region (LCR) of the human red and green visual pigment genes is critical for the formation of functional red and green cones in the retina. A 37-bp core of the LCR is perfectly conserved among mammals and binds specific retinal nuclear proteins. Here, we employed a yeast one-hybrid screen of an adult retinal cDNA library to clone and characterize these proteins. We identified clones encoding homeodomain (HD) transcription factors Pax6, Rx, and Chx10 and a novel paired-like HD protein, RINX. In the adult retina, RINX is exclusively expressed in a subset of cells (likely to be bipolar cells) of the retinal inner nuclear layer (INL). RINX is closely related to Chx10, which is also exclusively expressed in the INL of the adult retina and is critical for retinal development. The RINX gene is expressed in two classes of mRNA. One class encodes proteins that lack either part of or all of the HD, but retain the transcriptional activation domain. The RINX gene maps to chromosome 20p11.2 to which no retinal disease has been assigned. In conclusion, the LCR contains two adjacent motifs that are targets for binding of HD proteins that may specify the development and differentiation of cone photoreceptors and a subset of INL bipolar cells. Mutations in the related human CHX10 gene cause microphthalmia in a subset of families, and, therefore, the RINX gene is a candidate for this phenotype in another subset of patients. Since the RINX gene is likely an ortholog of the goldfish Vsx1 gene, it has been named VSX1 by the Human Gene Nomenclature Committee.

Thrombomodulin with the Asp468Tyr mutation is expressed on the cell surface with normal cofactor activity for protein C activation

Thrombomodulin (TM) is an endothelial cell glycoprotein that acts as an anticoagulant. Mutation in the TM gene is a potential risk factor for thrombosis. The first TM mutation identified was a heterozygous substitution of T for G at nucleotide position 1456, which predicted Asp468 with Tyr in a Ser/Thr-rich domain. To evaluate the reported TM gene mutation as a possible cause of thrombosis, we transiently tranfected a vector for TM gene carrying the mutation to mammalian COS7 cells. TM antigen levels in lysates of cells transfected with variant TM were comparable to those in preparations of normal TM. The TM cofactor activity for protein C (PC) activation on the variant TM-expressing cells was similar to that of the control. The Michaelis constant Km and Vmax. of variant TM for PC activation were shown to be similar compared to those of normal TM. The affinity of each TM for thrombin in PC activation was also similar. We obtained several stable cell lines expressing normal and variant TM. Lysate of the cell lines with normal and variant TM genes had a similar expression level of TM antigen. Pulse-chase analysis showed that normal and variant TM were glycosylated and resistant to endoglycosidase H, indicating that the variant TM was expressed on the cell surface in a mature form. Variant TM protein is apparently expressed on the cell surface with normal cofactor activity for PC activation. It is unlikely that the TM variant directly causes thrombosis by mechanism of reduced expression or impaired cofactor activity for PC activation, which comprises a major anticoagulant activity of TM.

Structure-Function Analyses of Thrombomodulin by Gene-Targeting in Mice: The Cytoplasmic Domain Is Not Required for Normal Fetal Development

Thrombomodulin (TM) is a widely expressed glycoprotein receptor that plays a physiologically important role in maintaining normal hemostatic balance postnatally. Inactivation of the TM gene in mice results in embryonic lethality without thrombosis, suggesting that structures of TM not recognized to be involved in coagulation might be critical for normal fetal development. Therefore, the in vivo role of the cytoplasmic domain of TM was studied by using homologous recombination in ES cells to create mice that lack this region of TM (TMcyt/cyt). Cross-breeding of F1 TMwt/cyt mice (1 wild-type and 1 mutant allele) resulted in more than 300 healthy offspring with a normal Mendelian inheritance pattern of 25.7% TMwt/wt, 46.6% TMwt/cyt, and 27.7% TMcyt/cyt mice, indicating that the tail of TM is not necessary for normal fetal development. Phenotypic analyses showed that the TMcyt/cyt mice responded identically to their wild-type littermates after procoagulant, proinflammatory, and skin wound challenges. Plasma levels of plasminogen, plasminogen activator inhibitor 1 (PAI-1), and 2-antiplasmin were unaltered, but plasmin:2-antiplasmin (PAP) levels were significantly lower in TMcyt/cyt mice than in TMwt/wt mice (0.46 ± 0.2 and 1.99 ± 0.1 ng/mL, respectively; P &lt; .001). Tissue levels of TM antigen were also unaffected. However, functional levels of plasma TM in the TMcyt/cyt mice, as measured by thrombin-dependent activation of protein C, were significantly increased (P &lt; .001). This supported the hypothesis that suppression in PAP levels may be due to augmented activation of thrombin-activatable fibrinolysis inhibitor (TAFI), with resultant inhibition of plasmin generation. In conclusion, these studies exclude the cytoplasmic domain of TM from playing a role in the early embryonic lethality of TM-null mice and support its function in regulating plasmin generation in plasma.

Mutations in promoter region of thrombomodulin and venous thromboembolic disease

The present study was designed to analyze the thrombomodulin proximal promoter region spanning nucleotides -293 to -12 to search for polymorphisms that could modify thrombomodulin gene expression in patients with venous thromboembolic disease. The study population comprised 205 patients and 394 healthy subjects of similar age and sex distribution. No polymorphisms and only 1 point mutation (G-33A) were found. The G-33A mutation was present at the heterozygous state in 2 patients and in 1 control. Being more frequent in the patients (0.97%) than in the controls (0.25%), the G-33A mutation might be a risk factor for venous thrombosis. To investigate the effect of this mutation on the thrombomodulin promoter activity, the proximal promoter region of the gene (bearing or not bearing the G-33A mutation) was inserted into a promotorless expression vector, upstream of the firefly luciferase gene, and transiently transfected into EA.hy926 endothelial cells. Under the conditions of the assay, the G-33A mutation mildly decreased the promoter activity. This study confirms that abnormalities of the thrombomodulin proximal promoter are not frequent in patients with venous thromboembolism.

Reconstitution of the human endothelial cell protein C receptor with thrombomodulin in phosphatidylcholine vesicles enhances protein C activation

Blocking protein C binding to the endothelial cell protein C receptor (EPCR) on the endothelium is known to reduce protein C activation rates. Now we isolate human EPCR and thrombomodulin (TM) and reconstitute them into phosphatidylcholine vesicles. The EPCR increases protein C activation rates in a concentration-dependent fashion that does not saturate at 14 EPCR molecules/TM. Without EPCR, the protein C concentration dependence fits a single class of sites (Km = 2.17 +/- 0.13 microM). With EPCR, two classes of sites are apparent (Km = 20 +/- 15 nM and Km = 3.2 +/- 1.7 microM). Increasing the EPCR concentration at a constant TM concentration increases the percentage of high affinity sites. Holding the TM:EPCR ratio constant while decreasing the density of these proteins results in a decrease in the EPCR enhancement of protein C activation, suggesting that there is little affinity of the EPCR for TM. Negatively charged phospholipids also enhance protein C activation. EPCR acceleration of protein C activation is blocked by anti-EPCR antibodies, but not by annexin V, whereas the reverse is true with negatively charged phospholipids. Human umbilical cord endothelium expresses approximately 7 times more EPCR than TM. Anti-EPCR antibody reduces protein C activation rates 7-fold over these cells, whereas annexin V is ineffective, indicating that EPCR rather than negatively charged phospholipid provide the surface for protein C activation. EPCR expression varies dramatically among vascular beds. The present results indicate that the EPCR concentration will determine the effectiveness of the protein C activation complex.

Intravascular Coagulation Activation in a Murine Model of Thrombomodulin Deficiency: Effects of Lesion Size, Age, and Hypoxia on Fibrin Deposition

We consecutively inactivated both alleles of the thrombomodulin (TM) gene in murine embryonic stem (ES) cells and generated TM-deficient (TM−/−) chimeric mice. Quantitation of an ES-cell marker and protein C cofactor activity indicates that up to 50% of pulmonary endothelial cells are ES-cell derived and therefore TM deficient. Infusions of 125I-fibrinogen into mice show a significant increase (fourfold, P < .005) in radiolabeled cross-linked fibrin in TM−/− chimeric mouse lung as compared with wild-type mice. However, only chimeric mice that exhibit at least a 30% reduction in protein C cofactor activity and are at least 15 months old display this phenotype. Immunocytochemical localization of TM in chimeras shows a mosaic pattern of expression in both large and small blood vessels. Colocalization of cross-linked fibrin and neo (used to replace TM) reveals that fibrin is deposited in TM−/− regions. However, the fibrin deposits were largely restricted to pulmonary vessels with a lumenal area greater than 100 μm2. The hypercoagulable phenotype can be induced in younger chimeric mice by exposure to hypoxia, which causes a fivefold increase in β-fibrin levels in lung. Our findings show that TM chimerism results in spontaneous, intravascular fibrin deposition that is dependent on age and the magnitude of the TM deficiency.

Intravascular Coagulation Activation in a Murine Model of Thrombomodulin Deficiency: Effects of Lesion Size, Age, and Hypoxia on Fibrin Deposition

We consecutively inactivated both alleles of the thrombomodulin (TM) gene in murine embryonic stem (ES) cells and generated TM-deficient (TM−/−) chimeric mice. Quantitation of an ES-cell marker and protein C cofactor activity indicates that up to 50% of pulmonary endothelial cells are ES-cell derived and therefore TM deficient. Infusions of 125I-fibrinogen into mice show a significant increase (fourfold, P &lt; .005) in radiolabeled cross-linked fibrin in TM−/− chimeric mouse lung as compared with wild-type mice. However, only chimeric mice that exhibit at least a 30% reduction in protein C cofactor activity and are at least 15 months old display this phenotype. Immunocytochemical localization of TM in chimeras shows a mosaic pattern of expression in both large and small blood vessels. Colocalization of cross-linked fibrin and neo (used to replace TM) reveals that fibrin is deposited in TM−/− regions. However, the fibrin deposits were largely restricted to pulmonary vessels with a lumenal area greater than 100 μm2. The hypercoagulable phenotype can be induced in younger chimeric mice by exposure to hypoxia, which causes a fivefold increase in β-fibrin levels in lung. Our findings show that TM chimerism results in spontaneous, intravascular fibrin deposition that is dependent on age and the magnitude of the TM deficiency.

A novel recombinant soluble human thrombomodulin, ART-123, activates the protein C pathway in healthy male volunteers

The effect of a novel recombinant soluble human thrombomodulin, ART-123, on protein C activation was investigated by measuring plasma prothrombinase activity in four healthy male volunteers. ART-123 at a dose of 0.3 mg was administered as a bolus intravenous injection for 1 minute. Plasma ART-123 concentration and prothrombinase activity were determined before and immediately, 24, and 48 hours after injection, and thromboelastography was recorded before and immediately and 24 hours after injection. The mean elimination half-life was 19.82 +/- 2.10 hours. Compared with pretreatment levels, ART-123 reduced prothrombinase activity by 44.2 +/- 11.7%, 52.1 +/- 10.8%, and 61.0 +/- 14.7%, respectively, immediately, 24, and 48 hours after injection, suggesting that ART-123 activated the protein C pathway. ART-123 did not affect thromboelastography values. There were no abnormal findings for objective signs or laboratory tests, including blood pressure, heart rate, electrocardiogram, body temperature, hematology, coagulation and hemostatic parameters, blood chemistry, and urinalysis. Based on these observations, ART-123 at a dose of 0.3 mg can activate the protein C pathway in healthy volunteers.

Enhancement of the functional repertoire of the rat parietal peritoneal mesothelium in vivo: directed expression of the anticoagulant and antiinflammatory molecule thrombomodulin

We have used our previously described ex vivo mesothelial cell (MC)-mediated gene therapy strategy (Gene Ther. 2:393-401, 1995) to modify the functional properties of the rat parietal peritoneal mesothelium in vivo by expression of a membrane-bound recombinant protein on the MC surface. Rat primary MCs were stably transfected (using strontium phosphate DNA coprecipitation) with a plasmid containing the gene for rat thrombomodulin (TM), a transmembrane glycoprotein that functions as an essential cofactor for the physiological activation of the anticoagulant protein C by the enzyme thrombin. As demonstrated by immunohistochemistry and by direct equilibrium binding with radiolabeled thrombin, genetically modified MCs expressed high levels of TM antigen on their surface in vitro. As judged by a thrombin-dependent protein C activation assay, such MC membrane-bound TM was biologically active. Once reseeded on the denuded parietal peritoneal surface of syngeneic recipients, these TM-transfected MCs continued to express TM antigen in vivo for at least 90 days. Moreover, the recombinant TM expressed on the reconstituted parietal mesothelium retained its ability to activate protein C in a thrombin-dependent manner. Our data indicate that MC-mediated expression of TM can be used to augment the anticoagulant properties of the parietal peritoneal surface. In general, our results suggest that ex vivo MC-mediated gene therapy can be used to deliver other therapeutic transmembrane proteins to the MC surface to enhance the functional repertoire of the parietal mesothelium in vivo.

Identification of functional endothelial protein C receptor in human plasma

The endothelial cell protein C receptor (EPCR) binds protein C and facilitates activation by the thrombin-thrombomodulin complex. EPCR also binds activated protein C (APC) and inhibits APC anticoagulant activity. In this study, we detected a soluble form of EPCR in normal human plasma. Plasma EPCR appears to be approximately 43, 000 D, and circulates at approximately 100 ng/ml (98.4+/-27.8 ng/ml, n = 22). Plasma EPCR was purified from human citrated plasma using ion exchange, immunoaffinity, and protein C affinity chromatography. Flow cytometry experiments demonstrated that plasma EPCR bound APC with an affinity similar to that previously determined for recombinant soluble EPCR (Kdapp = 30 nM). Furthermore, plasma EPCR inhibited both protein C activation on an endothelial cell line and APC anticoagulant activity in a one-stage Factor Xa clotting assay. The physiological function of plasma EPCR is uncertain, but if the local concentrations are sufficiently high, particularly in disease states, the present data suggest that the soluble plasma EPCR could attenuate the membrane-bound EPCR augmentation of protein C activation and the anticoagulant function of APC.

Thrombomodulin gene mutations associated with myocardial infarction

BACKGROUND

Thrombomodulin is an important receptor for thrombin on the endothelial cell surface of most blood vessels, including those of the heart. Thrombin-bound thrombomodulin activates protein C, which inhibits thrombin generation by degrading factors Va and VIIIa. The aim of this study was to analyze the 5' region of the thrombomodulin gene to determine whether mutations contribute a risk for myocardial infarction.

METHODS AND RESULTS

We screened the promoter region of the thrombomodulin gene by single-stranded conformation polymorphism analysis in 104 patients with diagnosed myocardial infarction. Five mutations (three distinct) were identified (GG-9/-10AT, G-33A, and C-133A). The dinucleotide mutation GG-9/-10AT was identified in 3 individuals (2 heterozygous, 1 homozygous). Only one of the three different mutations was identified in 104 patient control subjects matched for age, sex, and race (G-33A in a single individual). All mutations identified were in close proximity to consensus sequences for transcription control elements within the thrombomodulin gene. In contrast, no difference was observed between patients and control subjects for the allelic frequency of a previously identified neutral polymorphism GCC/GTC coding for Ala/Val455, with 3 individuals homozygous for GTC (Val) in both groups.

CONCLUSIONS

The findings suggest that mutations in the promoter region of the thrombomodulin gene may constitute a risk for arterial thrombosis.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

Effect of a protein phosphatase inhibitor, okadaic acid, on thrombomodulin expression in cultured human umbilical vein endothelial cells

We examined the effects of okadaic acid, a potent specific inhibitor of protein phosphatases 1 and 2A, on the expression of thrombomodulin (TM), a cell surface anti-thrombotic glycoprotein, on cultured human umbilical endothelial cells. Okadaic acid (2.5-10 nM) significantly increased TM antigen levels in parallel with its cofactor activity for thrombin-dependent protein C activation. Incubation of cells with 10 nM okadaic acid for 18 h induced an approximately 240% up-regulation of TM antigen levels that was accompanied by an increase in TM mRNA levels. Co-incubation of cells with okadaic acid and dibutyryl cyclic AMP further increased TM antigen levels. Furthermore, the effect of cAMP on TM expression was augmented by the pretreatment of cells with 10 nM okadaic acid for 18 h. These results provide evidence for the involvement of protein phosphatase in the cellular regulatory mechanisms for TM expression, which is distinct from that by cAMP.

The Amino Terminal Lectin-Like Domain of Thrombomodulin Is Required for Constitutive Endocytosis

Thrombomodulin (TM) is a multidomain protein that serves as a cofactor in a major natural anticoagulant system. To further characterize the structure-function of TM, we have transfected COS cells with different truncated forms of TM. In the first form, COS cells expressing TM that lacks the putative signal peptide (17 residues); the lectin-like, hydrophobic N-terminal domain (226 residues); and 12 residues of the first epidermal growth factor (EGF )-like repeat (COSdel.238 cells) were found to function normally with respect to TM transport to the cell surface and thrombin-dependent protein C activation. However, in contrast to wild-type TM, as visually studied by immunofluorescence and immunogold electron microscopy, the COSdel.238 cells did not constitutively internalize anti-TM–TM or thrombin-TM complexes. To identify the region responsible for mediating the endocytic process, deletant forms of TM lacking either the lectin-like region (residues 2-155) or the hydrophobic region of the N-terminal domain (residues 161-202) were expressed in COS cells (COSdel.2-155 and COSdel.161-202, respectively). Protein C cofactor activity was maintained in both cells. Although the COSdel.161-202 cells behaved similarly to wild-type TM-transfected cells, visual studies showed a lack of constitutive internalization of thrombin-TM or anti-TM–TM complexes in the COSdel.2-155 cells. We conclude that the lectin-like domain of human TM serves to regulate cell surface expression of TM via the endocytic route and therefore may also play a major physiologic role in controlling intracellular and extracellular accumulation of thrombin in a variety of biologic systems.

Targeting of transgene expression to the vascular endothelium of mice by homologous recombination at the thrombomodulin locus

We describe a straightforward gene-targeting technique to achieve uniform, stable, and genetically invariant expression of a transgene in the vascular endothelium of mice. To demonstrate the feasibility of this approach, the reporter gene bacterial beta-galactosidase was inserted via homologous recombination into the intronless thrombomodulin locus of murine embryonic stem cells. In this fashion, the lacZ gene is placed under the regulatory control of the endogenous thrombomodulin promoter. The expression of the transgene in adult mice recapitulated the widespread, stable, and high-level expression of the thrombomodulin gene in vascular endothelium. These data indicate that targeting of cDNAs into the thrombomodulin locus serves as a viable strategy to express transgenes in endothelial cells. Analysis of reporter gene expression revealed a heterogeneous pattern of thrombomodulin gene activity in the endothelium of the aorta and its tributaries. We also show that embryonic stem cells with a targeted thrombomodulin locus contribute in a mosaic fashion to the vascular endothelium of chimeric mice. This method for generating animals with a functionally heterogeneous cardiovascular system should provide an experimental technique for studying how localized genetic abnormalities in endothelial cell function lead to the development of vascular diseases.

Major basic protein binding to thrombomodulin potentially contributes to the thrombosis in patients with eosinophilia

The contribution of an eosinophil granule protein, major basic protein (MBP), to the pathogenesis of thrombosis seen in patients with eosinophilia was investigated. The sera from eosinophilic patients containing elevated levels of MBP inhibited thrombomodulin (TM) function as a cofactor for the thrombin-catalysed activation of protein C more significantly than those from normal individuals (means 48.5% v 17.4%, respectively). It was suggested that the binding of mature MBP in the sera to TM was electrostatic, because mature MBP (pI 10.9) bound to TM, whereas pro-MBP (pI 6.2) did not. The inhibition of TM cofactor activity by eosinophil granule proteins was mainly attributed to the mature MBP, because MBP-depleted eosinophil granule proteins did not inhibit TM cofactor activity significantly. This inhibition seemed to be due to the specific thrombin-binding to TM being blocked. We concluded that eosinophil granule proteins, particularly MBP, potentially contribute to the hypercoagulation seen in some conditions of eosinophilia, at least because of the inhibition of TM function as a cofactor of the anticoagulation system.

Cloning and sequencing of an intronless mouse S-adenosylmethionine decarboxylase gene coding for a functional enzyme strongly expressed in the liver

A genomic clone for a mouse S-adenosylmethionine decarboxylase (AdoMetDC) gene was isolated from a cosmid library. Surprisingly, the gene proved to be intronless. With the exception of three base substitutions (changing 2 amino acids in the deduced protein), the 1002-nucleotide sequence of the open reading frame was identical to that of mouse AdoMetDC cDNA. Moreover, the gene contained a poly(dA) tract at the 3' end and was flanked by 13-base pair direct repeats. Our findings suggest that this gene has arisen by retroposition, in which a fully processed AdoMetDC mRNA has been reverse transcribed into a DNA copy and inserted into the genome. By polymerase chain reaction, we positively identified the intronless gene in the mouse genome, and, by primer extension analysis, we proved the gene to be functional. Thus, its transcripts were found in many cell lines and tissues of the mouse and were particularly abundant in the liver. When the open reading frame of the intronless gene was expressed in Escherichia coli HT551, a strain with no AdoMetDC activity, it was found to encode a 38-kDa protein, corresponding to AdoMetDC proenzyme. Although the change of methionine 70 to isoleucine was close to the cleavage site at serine 68, this protein underwent proenzyme processing, generating a 31-kDa alpha subunit and an 8-kDa beta subunit. Importantly, the protein encoded by the intronless gene was functional, i.e. it catalyzed the decarboxylation of S-adenosylmethionine, and its specific activity was comparable with that of recombinant human AdoMetDC purified according to the same procedure.

Electron microscopic studies of sequence-specific recognition of duplex DNA by different ligands

The following ligands were used to study sequence specific recognition of duplex DNA by electron microscopic techniques: methyltransferases BspR1 and EcoR124 (recognition sequences GGCC and GAAN7RTCG, respectively), a biotinylated deoxyoligonucleotide 5'-CTCTCTCTCTCTCT-3' capable of forming triplex DNA, and PNA oligomer H-T10-LysNH2. For each ligand the best conditions for electron microscopic (EM) detection of stable specific complex formation were determined. It was demonstrated that EM allowed us to determine the position of the individual target site with an error of 15-20 bp, the relative affinities for individual target sites and kinetic parameters of the binding. These results open new possibilities for EM investigations of sequence-specific interactions with a wide range of other ligands of a similar nature. They also imply that a wide range of different sequences can be unambiguously and precisely mapped by EM and greatly extend the scope of EM applications for physical mapping of genomic DNA.

Thrombomodulin lacking the cytoplasmic domain efficiently internalizes thrombin via nonclathrin-coated, pit-mediated endocytosis

Thrombomodulin (TM) is a transmembrane vascular endothelial cell receptor that is a cofactor in a major physiologically relevant natural anticoagulant system. We recently developed a cell model to examine one mechanism of regulation of TM cell surface expression and visually demonstrated that the receptor undergoes internalization predominantly via noncoated pits (Conway et al., 1992, J. Cell. Phys., 151:604-612). We have extended these studies to examine the role of the cytoplasmic domain of TM by deleting this region and expressing the truncated version of the molecule in COS cells (COS.Cyto.Del cells). Electron microscopy demonstrated internalization of gold-labeled anti-TM antibody or thrombin in a time- and temperature-dependent manner, similar to that seen with the wild-type transfected cells (COS.TM-CR). Endocytosis was characterized by initial surface clustering of gold particles, followed by aggregation into noncoated pits, early endosome formation, and, finally, entry into multivesicular bodies and lysosomes. There was a notable absence of gold particles in clathrin-coated pits and vesicles. The kinetics of binding and internalization of 125I-labeled ligand in COS.Cyto.Del cells was compared with that of COS.TM-CR cells and was not significantly different. These studies provide ultrastructural and quantitative data to indicate that TM efficiently undergoes endocytosis via nonclathrin-coated pits when the receptor is lacking the cytoplasmic domain. This finding suggests that there may be alternative regions of the molecule that mediate those signals necessary for internalization.

Glycosaminoglycans and the regulation of blood coagulation

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High-level expression of recombinant human soluble thrombomodulin in serum-free medium by CHO-K1 cells

Cultivation of gene-engineered Chinese hamster ovary (CHO-K1) cells that produce recombinant human soluble thrombomodulin (rsTM) was investigated to optimize conditions for high-level expression of the protein in a serum-free medium. For economic protein production, oxygenation of cultures with pure O2 permitted sufficient cell growth for high rsTM production with only 1 g/l of microcarriers and a low foetal bovine serum concentration. A longer growth phase (over 5 days) with serum was important to establish sufficient growth of this cell line on the microcarriers for subsequent serum-free culture, and to support a long-term production phase (about 2 months). In the production phase, a high glucose concentration (6.15 g/l) in the serum-free medium was very effective for prolonging the harvest cycle interval. Under these conditions, up to 100 mg/l rsTM was expressed in the conditioned medium. The rates of glucose consumption (G) and lactate production (L) were measured periodically and their ratio (L/G ratio) correlated with rsTM productivity. When the average L/G ratio was lower, reflecting a lower lactate production rate due to appropriate oxygenation of the culture, the specific rsTM production rate increased. Thus it may be possible to estimate protein productivity from L/G ratios calculated from the glucose and lactate measurements.

Oxidation of a specific methionine in thrombomodulin by activated neutrophil products blocks cofactor activity. A potential rapid mechanism for modulation of coagulation

Endothelial thrombomodulin (TM) plays a critical role in hemostasis as a cofactor for thrombin-dependent formation of activated protein C, a potent anticoagulant. Chloramine T, H2O2, or hypochlorous acid generated from H2O2 by myeloperoxidase rapidly destroy 75-90% of TM cofactor activity. Activated PMN, the primary in vivo source of biological oxidants, also rapidly inactivate TM. Oxidation of TM by PMN is inhibited by diphenylene iodonium, an inhibitor of NADPH oxidase. Both Met291 and Met388 in the six epidermal growth factor-like repeat domain are oxidized; however, only substitutions of Met388 lead to TM analogues that resist oxidative inactivation. We suggest that in inflamed tissues activated PMN may inactivate TM and demonstrate further evidence of the interaction between the inflammatory process and induction of thrombotic potential.