Reducing agents induce thrombomodulin shedding in human endothelial cells

Vasculature organotropism in drug delivery

Over the past decades, there has been an exponential increase in the development of preclinical and clinical nanodelivery systems, and recently, an accelerating demand to deliver RNA and protein-based therapeutics. Organ-specific vasculature provides a promising intermediary for site-specific delivery of nanoparticles and extracellular vesicles to interstitial cells. Endothelial cells express organ-specific surface marker repertoires that can be used for targeted delivery. This article highlights organ-specific vasculature properties, nanodelivery strategies that exploit vasculature organotropism, and overlooked challenges and opportunities in targeting and simultaneously overcoming the endothelial barrier. Impediments in the clinical translation of vasculature organotropism in drug delivery are also discussed.

Thrombin cleaves recombinant soluble thrombomodulin into a lectin-like domain fragment and a fragment with protein C-activating cofactor activity

Thrombomodulin (TM) is a transmembrane protein that plays an important role in regulating the coagulation system by acting as a cofactor for thrombin in protein C activation. Additionally, TM is involved in inflammation. Previous studies have shown that soluble fragments of TM of varying sizes, which are derived from membrane-bound TM, are present in plasma and urine. Soluble fragments of TM are speculated to exhibit biological activity. Among these, a lectin-like domain fragment (TMD1) is of particular importance. Recombinant TMD1 has previously been shown to attenuate lipopolysaccharide-induced inflammation. Here, we report that thrombin cleaves recombinant soluble TM, which is used for the treatment of disseminated intravascular coagulation associated with sepsis, into TMD1 and a fragment comprising the C-terminal portion of TM (TMD23), the latter of which retains the cofactor activity for activating protein C. Our findings suggest that thrombin not only activates protein C on membrane-bound TM but may also cleave TM to generate TMD1.

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.

The prognostic utility of protein C as a biomarker for adult sepsis: a systematic review and meta-analysis

Background

Sepsis, the dysregulated host response to infection, triggers abnormal pro-coagulant and pro-inflammatory host responses. Limitations in early disease intervention highlight the need for effective diagnostic and prognostic biomarkers. Protein C’s role as an anticoagulant and anti-inflammatory molecule makes it an appealing target for sepsis biomarker studies. This meta-analysis aims to assess the diagnostic and prognostic value of protein C (PC) as a biomarker for adult sepsis.

Methods

We searched MEDLINE, PubMed, EMBASE, CINAHL and Cochrane Library from database inception to September 12, 2021. We included prospective observational studies of (1) adult patients (> 17) with sepsis or suspicion of sepsis that; (2) measured PC levels with 24 h of study admission with; and (3) the goal of examining PC as a diagnostic or prognostic biomarker. Two authors screened articles and conducted risk of bias (RoB) assessment, using the Quality in Prognosis Studies (QUIPS) and the Quality Assessment in Diagnostic Studies-2 (QUADAS-2) tools. If sufficient data were available, meta-analysis was conducted to estimate the standardized mean difference (SMD) between patient populations.

Results

Twelve studies were included, and 8 were synthesized for meta-analysis. Pooled analysis demonstrated moderate certainty of evidence that PC levels were less reduced in sepsis survivors compared to non-survivors (6 studies, 741 patients, SMD = 0.52, 95% CI 0.24–0.81, p = 0.0003, I² = 55%), and low certainty of evidence that PC levels were less reduced in septic patients without disseminated intravascular coagulation (DIC) compared to those with DIC (3 studies, 644 patients, SMD = 0.97, 95% CI 0.62–1.32, p < 0.00001, I² = 67%). PC could not be evaluated as a diagnostic tool due to heterogeneous control populations between studies.

Conclusion and relevance

Our review demonstrates that PC levels were significantly higher in sepsis survivors compared to non-survivors and patients with sepsis but not disseminated intravascular coagulation (DIC). Our evaluation is limited by high RoB in included studies and poor reporting of the sensitivity and specificity of PC as a sepsis biomarker. Future studies are needed to determine the sensitivity and specificity of PC to identify its clinical significance as a biomarker for early sepsis recognition.

Trial Registration PROSPERO registration number: CRD42021229786. The study protocol was published in BMJ Open.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-022-03889-2.

Thrombomodulin in patients with mild to moderate bleeding tendency

Introduction

A massive increase of soluble thrombomodulin (sTM) due to variants in the thrombomodulin gene (THBD) has recently been identified as a novel bleeding disorder.

Aim

To investigate sTM levels and underlying genetic variants as a cause for haemostatic impairment and bleeding in a large number of patients with a mild to moderate bleeding disorder (MBD), including patients with bleeding of unknown cause (BUC).

Patients and methods

In 507 MBD patients, sTM levels, thrombin generation and plasma clot formation were measured and compared to 90 age‐ and sex‐matched healthy controls. In patients, genetic analysis of the THBD gene was performed.

Results

No difference in sTM levels between patients and controls was found overall (median ([IQR] 5.0 [3.8‐6.3] vs. 5.1 [3.7‐6.4] ng/ml, p = .762), and according to specific diagnoses of MBD or BUC, and high sTM levels (≥95th percentile of healthy controls) were not overrepresented in patients. Soluble TM levels had no impact on bleeding severity or global tests of haemostasis, including thrombin generation or plasma clot formation. In the THBD gene, no known pathogenic or novel disease‐causing variants affecting sTM plasma levels were identified in our patient cohort.

Conclusion

TM‐associated coagulopathy appears to be rare, as it was not identified in our large cohort of patients with MBD. Soluble TM did not arise as a risk factor for bleeding or altered haemostasis in these patients.

Thrombomodulin is essential for maintaining quiescence in vascular endothelial cells

Thrombomodulin (TM) is a thrombin receptor on endothelial cells that is involved in promoting activation of the anticoagulant protein C pathway during blood coagulation. TM also exerts protective anti-inflammatory properties through a poorly understood mechanism. In this study, we investigated the importance of TM signaling to cellular functions by deleting it from endothelial cells by CRISPR-Cas9 technology and analyzed the resultant phenotype of TM-deficient (TM ^-/- ) cells. Deficiency of TM in endothelial cells resulted in increased basal permeability and hyperpermeability when stimulated by thrombin and TNF-α. The loss of the basal barrier permeability function was accompanied by increased tyrosine phosphorylation of VE-cadherin and reduced polymerization of F-actin filaments at cellular junctions. A significant increase in basal NF-κB signaling and expression of inflammatory cell adhesion molecules was observed in TM ^-/- cells that resulted in enhanced adhesion of leukocytes to TM ^-/- cells in flow chamber experiments. There was also a marked increase in expression, storage, and release of the von Willebrand factor (VWF) and decreased storage and release of angiopoietin-2 in TM ^-/- cells. In a flow chamber assay, isolated platelets adhered to TM ^-/- cells, forming characteristic VWF-platelet strings. Increased VWF levels and inflammatory foci were also observed in the lungs of tamoxifen-treated ERcre-TM^f/f mice. Reexpression of the TM construct in TM ^-/- cells, but not treatment with soluble TM, normalized the cellular phenotype. Based on these results, we postulate cell-bound TM endows a quiescent cellular phenotype by tightly regulating expression of procoagulant, proinflammatory, and angiogenic molecules in vascular endothelial cells.

Rhomboid-Like-2 Intramembrane Protease Mediates Metalloprotease-Independent Regulation of Cadherins

Cadherins are a major family of cell-cell adhesive receptors, which are implicated in development, tissue homeostasis, and cancer. Here, we show a novel mechanism of post-translational regulation of E-cadherin in cancer cells by an intramembrane protease of the Rhomboid family, RHBDL2, which leads to the shedding of E-cadherin extracellular domain. In addition, our data indicate that RHBDL2 mediates a similar activity on VE-cadherin, which is selectively expressed by endothelial cells. We show that RHBDL2 promotes cell migration, which is consistent with its ability to interfere with the functional role of cadherins as negative regulators of motility; moreover, the two players appear to lie in the same functional pathway. Importantly, we show that RHBDL2 expression is induced by the inflammatory chemokine TNFα. The E-cadherin extracellular domain is known to be released by metalloproteases (MMPs); however, here, we provide evidence of a novel MMP-independent, TNFα inducible, E-cadherin processing mechanism that is mediated by RHBDL2. Thus, the intramembrane protease RHBDL2 is a novel regulator of cadherins promoting cell motility.

Coagulation Pathways in Neurological Diseases: Multiple Sclerosis

Significant progress has been made in understanding the complex interactions between the coagulation system and inflammation and autoimmunity. Increased blood-brain-barrier (BBB) permeability, a key event in the pathophysiology of multiple sclerosis (MS), leads to the irruption into the central nervous system of blood components that include virtually all coagulation/hemostasis factors. Besides their cytotoxic deposition and role as a possible trigger of the coagulation cascade, hemostasis components cause inflammatory response and immune activation, sustaining neurodegenerative events in MS. Early studies showing the contribution of altered hemostasis in the complex pathophysiology of MS have been strengthened by recent studies using methodologies that permitted deeper investigation. Fibrin(ogen), an abundant protein in plasma, has been identified as a key contributor to neuroinflammation. Perturbed fibrinolysis was found to be a hallmark of progressive MS with abundant cortical fibrin(ogen) deposition. The immune-modulatory function of the intrinsic coagulation pathway still remains to be elucidated in MS. New molecular details in key hemostasis components participating in MS pathophysiology, and particularly involved in inflammatory and immune responses, could favor the development of novel therapeutic targets to ameliorate the evolution of MS. This review article introduces essential information on coagulation factors, inhibitors, and the fibrinolytic pathway, and highlights key aspects of their involvement in the immune system and inflammatory response. It discusses how hemostasis components are (dys)regulated in MS, and summarizes histopathological post-mortem human brain evidence, as well as cerebrospinal fluid, plasma, and serum studies of hemostasis and fibrinolytic pathways in MS. Studies of disease-modifying treatments as potential modifiers of coagulation factor levels, and case reports of autoimmunity affecting hemostasis in MS are also discussed.

Bariatric surgery in morbidly obese individuals affects plasma levels of protein C and thrombomodulin

Obesity is associated with a prothrombotic milieu and an increased risk for thrombotic events. Bariatric surgery is the most effective treatment for obesity resulting in dramatic weight loss and reduced inflammation and extrinsic coagulation pathway activation. Blood samples were drawn from 60 patients undergoing Roux-en-Y gastric bypass surgery before and 1 year after the intervention. Protein C (PC), activated PC (APC), soluble thrombomodulin (TM), soluble E-selectin (E-Sel), prothrombin time (PT) and activated partial thromboplastin time (aPTT) were evaluated. Both PC (187.4 ± 64.5% before surgery to 118.1 ± 48% 1 year after surgery, p < 0.001) and APC (138.7 ± 64.4% before surgery to 69.1 ± 65.7% after surgery, p < 0.001) were reduced following surgical intervention. TM showed a similar behavior with a reduction of soluble TM after the procedure from 5.7 ± 2.6 to 3.2 ± 1.4 ng/ml (p < 0.001). Similarly, soluble E-Sel was reduced after surgery from 26.6 ± 12.7 to 5.5 ± 4.1 ng/ml (p < 0.001). In contrast, aPTT was not shortened but slightly increased from 29.1 ± 4.8 s. before surgery to 31 ± 4.4 s. (p = 0.001) after surgery and levels of PT were reduced after surgery to 89.6 ± 15.5% from an initial 97.5 ± 13.5% (p < 0.001). In conclusion, we demonstrate a reduction of PC and APC 1 year after bariatric surgery accompanied by a reduction in soluble TM and soluble E-Sel. The reduction of PC and APC is not paralleled by a reduction but in contrast by a prolongation of aPTT suggesting a compensatory upregulation of PC during obesity. The reduction of TM and E-Sel might hint towards an improved endothelial function in this cohort of patients.

Why patients with THBD c.1611C>A (p.Cys537X) nonsense mutation have high levels of soluble thrombomodulin?

Background

Recently our group has described a new autosomal dominant bleeding disorder characterized by very high plasma levels of soluble thrombomodulin (TM). The THBD c.1611C>A (p.Cys537X) mutation in heterozygous state was found in the propositus. This mutation leads to the synthesis of a truncated TM which has lost the last three amino-acids of the transmembrane domain and the cytoplasmic tail.

Objective

We investigated the mechanism responsible for TM shedding in endothelial cells with THBD c.1611C>A mutation.

Methods

Complementary DNA of TM wild type (TM-WT) was incorporated into a pcDNA3.1 vector for transient transfection in COS-1 cells. Mutagenesis was performed to create the c.1611C<A (TM_1-536) mutant and 4 other TM mutants (TM_1-515, TM_1-525, TM_1-533 and TM_1-537) with a transmembrane domain having different lengths. The effect of shear stress, metalloprotease inhibitor, certain proteases and reducing agents were tested on TM shedding.

Results

Western blot and immunofluorescent analysis showed that TM_1-536 was produced and a certain amount of TM_1-536 was anchored on the cell membrane. A significantly higher levels of soluble TM was observed in the TM_1-536 cell medium in comparison with TM-WT (56.3 +/- 5.2 vs 8.8 +/- 1.6 ng/mL, respectively, p = 0.001). The shedding of TM_1-536 was 75% decreased in cells cultured in the presence of a metalloprotease inhibitor. No difference was observed between TM_1-536 and TM-WT shedding after cell exposure to cathepsin G, elastase, several reducing agents and high shear stress (5000 s^-1). Significantly higher levels of soluble TM were observed in the cell media of TM_1-533, TM_1-525, TM_1-515 in comparison with TM-WT (p < 0.05).

Conclusion

The mechanism responsible for TM shedding is complex and is not completely understood: higher sensitivity of the TM_1-536 to the proteolysis by metalloproteases and a defect of synthesis due to the decreased size of the transmembrane domain might explain the high levels of soluble TM in plasma of the carriers.

Quantitative proteomics screen identifies a substrate repertoire of rhomboid protease RHBDL2 in human cells and implicates it in epithelial homeostasis

Rhomboids are intramembrane serine proteases conserved in all kingdoms of life. They regulate epidermal growth factor receptor signalling in Drosophila by releasing signalling ligands from their transmembrane tethers. Their functions in mammals are poorly understood, in part because of the lack of endogenous substrates identified thus far. We used a quantitative proteomics approach to investigate the substrate repertoire of rhomboid protease RHBDL2 in human cells. We reveal a range of novel substrates that are specifically cleaved by RHBDL2, including the interleukin-6 receptor (IL6R), cell surface protease inhibitor Spint-1, the collagen receptor tyrosine kinase DDR1, N-Cadherin, CLCP1/DCBLD2, KIRREL, BCAM and others. We further demonstrate that these substrates can be shed by endogenously expressed RHBDL2 and that a subset of them is resistant to shedding by cell surface metalloproteases. The expression profiles and identity of the substrates implicate RHBDL2 in physiological or pathological processes affecting epithelial homeostasis.

Interaction between Multimeric von Willebrand Factor and Complement: A Fresh Look to the Pathophysiology of Microvascular Thrombosis

von Willebrand factor (VWF), a multimeric protein with a central role in hemostasis, has been shown to interact with complement components. However, results are contrasting and inconclusive. By studying 20 patients with congenital thrombotic thrombocytopenic purpura (cTTP) who cannot cleave VWF multimers because of genetic ADAMTS13 deficiency, we investigated the mechanism through which VWF modulates complement and its pathophysiological implications for human diseases. Using assays of ex vivo serum-induced C3 and C5b-9 deposits on endothelial cells, we documented that in cTTP, complement is activated via the alternative pathway (AP) on the cell surface. This abnormality was corrected by restoring ADAMTS13 activity in cTTP serum, which prevented VWF multimer accumulation on endothelial cells, or by an anti-VWF Ab. In mechanistic studies we found that VWF interacts with C3b through its three type A domains and initiates AP activation, although assembly of active C5 convertase and formation of the terminal complement products C5a and C5b-9 occur only on the VWF-A2 domain. Finally, we documented that in the condition of ADAMTS13 deficiency, VWF-mediated formation of terminal complement products, particularly C5a, alters the endothelial antithrombogenic properties and induces microvascular thrombosis in a perfusion system. Altogether, the results demonstrated that VWF provides a platform for the activation of the AP of complement, which profoundly alters the phenotype of microvascular endothelial cells. These findings link hemostasis-thrombosis with the AP of complement and open new therapeutic perspectives in cTTP and in general in thrombotic and inflammatory disorders associated with endothelium perturbation, VWF release, and complement activation.

Carbon monoxide-releasing molecule-2 suppresses thrombomodulin and endothelial protein C receptor expression of human umbilical vein endothelial cells induced by lipopolysaccharide in vitro

OBJECTIVE

The aim of this study was to observe the counter-effect of carbon monoxide-releasing molecule-2 (CORM-2) on lipopolysaccharide (LPS)-suppressed thrombomodulin (TM) and endothelial protein C receptor (EPCR) expressions from human umbilical vein endothelial cell (HUVEC), and to reveal its mechanisms.

METHODS

HUVECs were divided into 5 treatment groups, wherein reagents were added simultaneously. TM and EPCR proteins of the cells and the culture medium levels of soluble TM, soluble EPCR, and matrix metalloproteinase-2 (MMP-2) were detected after administration, whereas mRNA levels of TM and EPCR, as well as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activity among groups, were also evaluated.

RESULTS

No significant difference was observed in any indicator between CORM-2 and sham groups. Addition of LPS produced drastic increase in MMP-2 expression, NF-κB activity, shedding of TM and EPCR (into the culture medium), as well as remarkable decrease in both mRNA and protein expressions of TM and EPCR, and cell viability. LPS + CORM-2 treatment significantly reduced the increase in MMP-2, NF-κB activity, and TM/EPCR shedding, whereas maintained both mRNA and protein levels of TM and EPCR, and preserved cell viability.

CONCLUSIONS

CORM-2 protects HUVEC from LPS-induced injury, by way of suppressing NF-κB activity, which downregulates TM and EPCR mRNAs. It also decreases MMP-2 expression and prevents the shedding of TM and EPCR from the surface of endothelial cells, so as to preserve their protective effect.

SheddomeDB: the ectodomain shedding database for membrane-bound shed markers

BACKGROUND

A number of membrane-anchored proteins are known to be released from cell surface via ectodomain shedding. The cleavage and release of membrane proteins has been shown to modulate various cellular processes and disease pathologies. Numerous studies revealed that cell membrane molecules of diverse functional groups are subjected to proteolytic cleavage, and the released soluble form of proteins may modulate various signaling processes. Therefore, in addition to the secreted protein markers that undergo secretion through the secretory pathway, the shed membrane proteins may comprise an additional resource of noninvasive and accessible biomarkers. In this context, identifying the membrane-bound proteins that will be shed has become important in the discovery of clinically noninvasive biomarkers. Nevertheless, a data repository for biological and clinical researchers to review the shedding information, which is experimentally validated, for membrane-bound protein shed markers is still lacking.

RESULTS

In this study, the database SheddomeDB was developed to integrate publicly available data of the shed membrane proteins. A comprehensive literature survey was performed to collect the membrane proteins that were verified to be cleaved or released in the supernatant by immunological-based validation experiments. From 436 studies on shedding, 401 validated shed membrane proteins were included, among which 199 shed membrane proteins have not been annotated or validated yet by existing cleavage databases. SheddomeDB attempted to provide a comprehensive shedding report, including the regulation of shedding machinery and the related function or diseases involved in the shedding events. In addition, our published tool ShedP was embedded into SheddomeDB to support researchers for predicting the shedding event on unknown or unrecorded membrane proteins.

CONCLUSIONS

To the best of our knowledge, SheddomeDB is the first database for the identification of experimentally validated shed membrane proteins and currently may provide the most number of membrane proteins for reviewing the shedding information. The database included membrane-bound shed markers associated with numerous cellular processes and diseases, and some of these markers are potential novel markers because they are not annotated or validated yet in other databases. SheddomeDB may provide a useful resource for discovering membrane-bound shed markers. The interactive web of SheddomeDB is publicly available at http://bal.ym.edu.tw/SheddomeDB/ .

Characterization of an autosomal dominant bleeding disorder caused by a thrombomodulin mutation

We describe a family with an autosomal dominant disorder characterized by severe trauma- and surgery-related bleeding. The proband, who experienced life-threatening bleeding during a routine operation, had normal clotting times, but markedly reduced prothrombin consumption. Plasma levels of all coagulation factors and of the main coagulation inhibitors were normal. Thrombin generation at low triggers was severely impaired and mixing experiments suggested the presence of a coagulation inhibitor. Using whole exome sequencing, the underlying genetic defect was identified as the THBD c.1611C>A mutation (p.Cys537Stop), which predicts a truncated form of thrombomodulin that is shed from the vascular endothelium. The patient had decreased expression of endothelium-bound thrombomodulin, but extremely elevated levels of soluble thrombomodulin in plasma, impairing the propagation phase of coagulation via rapid activation of protein C and consequent inactivation of factors Va and VIIIa. The same thrombomodulin mutation has been recently described in an unrelated British family with strikingly similar features.

Endothelial markers are associated with thrombolysis resistance in acute stroke patients

BACKGROUND AND PURPOSE

The endothelium is crucial in maintaining the haemostatic balance between pro- and anti-thrombotic factors. In this pilot study, the association of endothelial biomarkers with arterial recanalization and clinical outcome in the setting of acute ischaemic stroke (AIS) was evaluated amongst patients treated with recombinant tissue plasminogen activator (rt-PA).

METHODS

Sixty-four AIS patients treated with rt-PA were prospectively recruited. Blood was collected before thrombolysis and analysed for von Willebrand factor (vWF), soluble thrombomodulin (sTM) and soluble endothelial protein C receptor (sEPCR). Complete recanalization was defined by a Thrombolysis in Myocardial Infarction Score of 3. Favourable clinical outcome was defined by a modified Rankin Score of 0-2 at 90 days.

RESULTS

Amongst the 64 patients, 31 had no documented occlusion, 19 had persistent occlusion and 14 had complete recanalization. After adjustment for confounding factors, these patients presented lower sTM and sEPCR levels than patients with persistent occlusion (median sTM, 21 vs. 48 ng/ml, P = 0.008; median sEPCR, 78 vs. 114 ng/ml, P = 0.018), but similar levels compared with patients without occlusion. vWF levels did not differ between groups. None of these biomarkers was significantly associated with favourable outcome.

CONCLUSIONS

Recanalization after thrombolytic therapy is associated with low sTM and sEPCR levels but not with vWF levels. If corroborated in further larger studies, these findings could be helpful in the identification of patients resistant to rt-PA thrombolysis who could benefit from a modified recanalization therapy.

Thrombomodulin and the vascular endothelium: insights into functional, regulatory, and therapeutic aspects

Thrombomodulin (TM) is a 557-amino acid protein with a broad cell and tissue distribution consistent with its wide-ranging physiological roles. When expressed on the lumenal surface of vascular endothelial cells in both large vessels and capillaries, its primary function is to mediate endothelial thromboresistance. The complete integral membrane-bound protein form displays five distinct functional domains, although shorter soluble (functional) variants comprising the extracellular domains have also been reported in fluids such as serum and urine. TM-mediated binding of thrombin is known to enhance the specificity of the latter serine protease toward both protein C and thrombin activatable fibrinolysis inhibitor (TAFI), increasing their proteolytic activation rate by almost three orders of magnitude with concomitant anticoagulant, antifibrinolytic, and anti-inflammatory benefits to the vascular wall. Recent years have seen an abundance of research into the cellular mechanisms governing endothelial TM production, processing, and regulation (including flow-mediated mechanoregulation)--from transcriptional and posttranscriptional (miRNA) regulation of TM gene expression, to posttranslational processing and release of the expressed protein--facilitating greater exploitation of its therapeutic potential. The goal of the present paper is to comprehensively review the endothelial/TM system from these regulatory perspectives and draw some fresh conclusions. This paper will conclude with a timely examination of the current status of TM's growing therapeutic appeal, from novel strategies to improve the clinical efficacy of recombinant TM analogs for resolution of vascular disorders such as disseminated intravascular coagulation (DIC), to an examination of the complex pleiotropic relationship between statin treatment and TM expression.

Soluble thrombomodulin levels in plasma of multiple sclerosis patients and their implication

Thrombomodulin (TM) on the cell-surface of cerebrovascular endothelial cells (CECs) is released into blood upon CEC damage. TM promotes activation of protein C (APC), an anticoagulant, anti-inflammatory, neuroprotective molecule that protects CECs and impedes inflammatory cell migration across the blood-brain barrier (BBB). Multiple sclerosis (MS) is associated with CEC damage and BBB dysfunction. We evaluated soluble TM (sTM) levels as a biomarker of BBB integrity and whether glatiramer acetate (GA) influenced sTM levels in MS patients. sTM levels quantified by 2-site ELISA from sera of healthy controls and systemic lupus erythematosus (SLE) patients (CEC-damage positive control) were compared with levels from patients with relapsing-remitting (RRMS) or secondary-progressive MS (SPMS), stratified as: RRMS/GA/no relapse, RRMS/GA/in relapse, RRMS no GA/no relapse, RRMS/no GA/in relapse; and SPMS/no GA. Additionally, soluble endothelial protein C receptor (sEPCR) levels were assessed in the non-stratified MS group, SLE patients, and controls. sTM levels were highest in RRMS patients taking GA with or without relapse, followed in decreasing order by SLE, RRMS/no GA/in relapse, SPMS, RRMS/no GA/no relapse, healthy controls. sEPCR levels were highest in MS patients, then SLE, then controls. sTM may be a useful biomarker of BBB integrity in RRMS patients. Further evaluation of sEPCR is needed. The finding that the highest sTM levels were in RRMS patients taking GA is interesting and warrants further investigation.

Alternative pathway activation of complement by Shiga toxin promotes exuberant C3a formation that triggers microvascular thrombosis

Shiga toxin (Stx)-producing E.coli O157:H7 has become a global threat to public health; it is a primary cause of diarrhea-associated hemolytic uremic syndrome (HUS), a disorder of thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure with thrombi occluding renal microcirculation. In this study, we explored whether Stx triggers complement-dependent microvascular thrombosis in in vitro and in vivo experimental settings of HUS. Stx induced on human microvascular endothelial cell surface the expression of P-selectin, which bound and activated C3 via the alternative pathway, leading to thrombus formation under flow. In the search for mechanisms linking complement activation and thrombosis, we found that exuberant complement activation in response to Stx generated an increased amount of C3a that caused further endothelial P-selectin expression, thrombomodulin (TM) loss, and thrombus formation. In a murine model of HUS obtained by coinjection of Stx2 and LPS and characterized by thrombocytopenia and renal dysfunction, upregulation of glomerular endothelial P-selectin was associated with C3 and fibrin(ogen) deposits, platelet clumps, and reduced TM expression. Treatment with anti-P-selectin Ab limited glomerular C3 accumulation. Factor B-deficient mice after Stx2/LPS exhibited less thrombocytopenia and were protected against glomerular abnormalities and renal function impairment, indicating the involvement of complement activation via the alternative pathway in the glomerular thrombotic process in HUS mice. The functional role of C3a was documented by data showing that glomerular fibrin(ogen), platelet clumps, and TM loss were markedly decreased in HUS mice receiving C3aR antagonist. These results identify Stx-induced complement activation, via P-selectin, as a key mechanism of C3a-dependent microvascular thrombosis in diarrhea-associated HUS.