Search

Previously, we showed that histidine-rich glycoprotein (HRG) binds factor (F) XIIa with high affinity, inhibits FXII autoactivation and FXIIa-mediated activation of FXI, and attenuates ferric chloride-induced arterial thrombosis in mice. Therefore, HRG downregulates the contact pathway in vitro and in vivo.To identify the domains on HRG responsible for contact pathway inhibition.Recombinant HRG domain constructs (N-terminal [N1, N2, and N1N2], proline-rich regions, histidine-rich region [HRR], and C-terminal) were expressed and purified. The affinities of plasma-derived HRG, HRG domain constructs, and synthetic HRR peptides for FXII, FXIIa, β-FXIIa, and polyphosphate (polyP) were determined using surface plasmon resonance, and their effects on polyP-induced FXII autoactivation, FXIIa-mediated activation of FXI and prekallikrein, the activated partial thromboplastin time (APTT), and thrombin generation were examined.HRG and HRG domain constructs bind FXIIa, but not FXII or β-FXII. HRR, N1, and N1N2 bind FXIIa with affinities comparable with that of HRG, whereas the remaining domains bind with lower affinity. Synthetic HRR peptides bind FXIIa and polyP with high affinity. HRG and HRR significantly inhibit FXII autoactivation and prolong the APTT. Like HRG, synthetic HRR peptides inhibit FXII autoactivation, attenuate FXIIa-mediated activation of prekallikrein and FXI, prolong the APTT, and attenuate thrombin generation.The interaction of HRG with FXIIa and polyP is predominantly mediated by the HRR domain. Like intact HRG, HRR downregulates the contact pathway and contributes to HRG-mediated down regulation of coagulation.

Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor (SERPIN) superfamily of proteins, is unique among SERPINs for exhibiting a spontaneous conformational change to a latent or inactive state. The functional half-life for this transition at physiologic temperature and pH is ~1-2 h. To better understand the molecular mechanisms underlying this transition, we now report on the analysis of a comprehensive PAI-1 variant library expressed on filamentous phage and selected for functional stability after 48 h at 37 °C. Of the 7,201 possible single amino acid substitutions in PAI-1, we identify 439 that increase the functional stability of PAI-1 beyond that of the wild-type protein and 1,549 that retain inhibitory activity toward PAI-1’s canonical target protease (urokinase-like plasminogen activator, uPA), while exhibiting functional stability less than or equal to that of wild-type PAI-1. Missense mutations that increase PAI-1 functional stability are concentrated in highly flexible regions within the PAI-1 structure. Finally, we developed a method for simultaneously measuring the functional half-lives of hundreds of PAI-1 variants in a multiplexed, massively parallel manner, quantifying the functional half-lives for 697 single missense variants of PAI-1 by this approach. Overall, these findings provide novel insight into the mechanisms underlying PAI-1’s latency transition and provide a database for interpreting human PAI-1 genetic variants.

Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor superfamily of proteins, is unique among serine protease inhibitors for exhibiting a spontaneous conformational change to a latent or inactive state. The functional half-life for this transition at physiologic temperature and pH is ∼1 to 2 h. To better understand the molecular mechanisms underlying this transition, we now report on the analysis of a comprehensive PAI-1 variant library expressed on filamentous phage and selected for functional stability after 48 h at 37 °C. Of the 7201 possible single amino acid substitutions in PAI-1, we identified 439 that increased the functional stability of PAI-1 beyond that of the WT protein. We also found 1549 single amino acid substitutions that retained inhibitory activity toward the canonical target protease of PAI-1 (urokinase-like plasminogen activator), whereas exhibiting functional stability less than or equal to that of WT PAI-1. Missense mutations that increase PAI-1 functional stability are concentrated in highly flexible regions within the PAI-1 structure. Finally, we developed a method for simultaneously measuring the functional half-lives of hundreds of PAI-1 variants in a multiplexed, massively parallel manner, quantifying the functional half-lives for 697 single missense variants of PAI-1 by this approach. Overall, these findings provide novel insight into the mechanisms underlying the latency transition of PAI-1 and provide a database for interpreting human PAI-1 genetic variants.

The serine protease inhibitor (SERPIN) plasminogen activator inhibitor-1 (PAI-1) is a key regulator of the fibrinolytic system, inhibiting the serine proteases tissue- and urokinase-type plasminogen activator (tPA and uPA, respectively). Missense variants render PAI-1 non-functional through misfolding, leading to its turnover as a protease substrate, or to a more rapid transition to the latent/inactive state. Deep mutational scanning was performed to evaluate the impact of amino acid sequence variation on PAI-1 inhibition of uPA using an M13 filamentous phage display system. Error prone PCR was used to construct a mutagenized PAI-1 library encompassing ~ 70% of potential single amino acid substitutions. The relative effects of 27% of all possible missense variants on PAI-1 inhibition of uPA were determined using high-throughput DNA sequencing. 826 missense variants demonstrated conserved inhibitory activity while 1137 resulted in loss of PAI-1 inhibitory function. The least evolutionarily conserved regions of PAI-1 were also identified as being the most tolerant of missense mutations. The results of this screen confirm previous low-throughput mutational studies, including those of the reactive center loop. These data provide a powerful resource for explaining structure–function relationships for PAI-1 and for the interpretation of human genomic sequence variants.

This year's Congress of the International Society of Thrombosis and Haemostasis (ISTH) was hosted virtually from Philadelphia July 17-21, 2021. The conference, now held annually, highlighted cutting-edge advances in basic, population and clinical sciences of relevance to the Society. Despite being held virtually, the 2021 congress was of the same scope and quality as an annual meeting held in person. An added feature of the program is that talks streamed at the designated times will then be available on-line for asynchronous viewing. The program included 77 State of the Art (SOA) talks, thematically grouped in 28 sessions, given by internationally recognized leaders in the field. The SOA speakers were invited to prepare brief illustrated reviews of their talks that were peer reviewed and are included in this article. The topics, across the main scientific themes of the congress, include Arterial Thromboembolism, Coagulation and Natural Anticoagulants, COVID-19 and Coagulation, Diagnostics and Omics, Fibrinogen, Fibrinolysis and Proteolysis, Hemophilia and Rare Bleeding Disorders, Hemostasis in Cancer, Inflammation and Immunity, Pediatrics, Platelet Disorders, von Willebrand Disease and Thrombotic Angiopathies, Platelets and Megakaryocytes, Vascular Biology, Venous Thromboembolism and Women's Health. These illustrated capsules highlight the major scientific advances with potential to impact clinical practice. Readers are invited to take advantage of the excellent educational resource provided by these illustrated capsules. They are also encouraged to use the image in social media to draw attention to the high quality and impact of the science presented at the congress. ispartof: RESEARCH AND PRACTICE IN THROMBOSIS AND HAEMOSTASIS vol:5 issue:5 ispartof: location:United States status: published

Sepsis is a life-threatening disease characterized by excessive host response to infection that can lead to activation of the coagulation system. Von Willebrand Factor (VWF) and ADAMTS13 are important regulators of hemostasis and their dysregulation during sepsis progression is not well understood. Herein we characterize ADAMTS13 and VWF in septic and non-septic patients. ADAMTS13 activity, ADAMTS13 antigen, VWF antigen, myeloperoxidase, and protein C, were measured in plasma collected from 40 septic patients (20 non-survivors and 20 survivors) and 40 non-septic patients on the first and last day of their ICU stay. ADAMTS13 activity and ADAMTS13 antigen were reduced, whereas VWF antigen was elevated among septic patients compared to non-septic patients and healthy controls. Non-septic patients also exhibited elevated VWF antigen and reduced ADAMTS13 activity, but to a lesser extent than septic patients. Non-survivor septic patients exhibited the lowest levels of ADAMTS13 activity. ADAMTS13 activity:antigen ratio was similar across all patient cohorts suggesting that the specific activity of ADAMTS13 remains unchanged. Therefore, reduced ADAMTS13 function in circulation is likely due to a reduction in circulating levels. We suggest that massive release of VWF in response to inflammation consumes limited circulating ADAMTS13, resulting in the imbalance observed between VWF and ADAMTS13 among septic and to a lesser extent in non-septic ICU patients. Changes to ADAMTS13 did not correlate with myeloperoxidase or protein C levels. Reduced ADAMTS13 activity and antigen, and elevated VWF antigen observed among all patient cohorts on admission remained unchanged in survivors at ICU discharge. Prolonged reduction in ADAMTS13 activity and antigen in septic patients coincides with elevated levels of VWF. The persistent abnormalities in ADAMTS13 and VWF in sepsis patients discharged from the ICU may contribute to a sustained prothrombotic state.

We have combined random 6 amino acid substrate phage display with high throughput sequencing to comprehensively define the active site specificity of the serine protease thrombin and the metalloprotease ADAMTS13. The substrate motif for thrombin was determined by >6,700 cleaved peptides, and was highly concordant with previous studies. In contrast, ADAMTS13 cleaved only 96 peptides (out of >107 sequences), with no apparent consensus motif. However, when the hexapeptide library was substituted into the P3-P3′ interval of VWF73, an exosite-engaging substrate of ADAMTS13, 1670 unique peptides were cleaved. ADAMTS13 exhibited a general preference for aliphatic amino acids throughout the P3-P3′ interval, except at P2 where Arg was tolerated. The cleaved peptides assembled into a motif dominated by P3 Leu, and bulky aliphatic residues at P1 and P1′. Overall, the P3-P2′ amino acid sequence of von Willebrand Factor appears optimally evolved for ADAMTS13 recognition. These data confirm the critical role of exosite engagement for substrates to gain access to the active site of ADAMTS13, and define the substrate recognition motif for ADAMTS13. Combining substrate phage display with high throughput sequencing is a powerful approach for comprehensively defining the active site specificity of proteases.

The metalloprotease ADAMTS13 cleaves von Willebrand factor (VWF) in circulating blood, limiting the size of VWF multimers and regulating VWF activity. Abnormal regulation of VWF contributes to bleeding and to thrombotic disorders. ADAMTS13 levels in plasma are highly variable among healthy individuals, although the heritability and the genetic determinants of this variation are unclear. We performed genome-wide association studies of plasma ADAMTS13 concentrations in 3244 individuals from 2 independent cohorts of healthy individuals. The heritability of ADAMTS13 levels was between 59.1% (all individuals) and 83.5% (siblings only), whereas tobacco smoking was associated with a decrease in plasma ADAMTS13 levels. Meta-analysis identified common variants near the ADAMTS13 locus on chromosome 9q34.2 that were significantly associated with ADAMTS13 levels and collectively explained 20.0% of the variance. The top single nucleotide polymorphism (SNP), rs28673647, resides in an intron of ADAMTS13 (β, 6.7%; P = 1.3E-52). Conditional analysis revealed 3 additional independent signals represented by rs3739893 (β, -22.3%; P = 1.2E-30) and rs3124762 (β, 3.5%; P = 8.9E-9) close to ADAMTS13 and rs4075970 (β, 2.4%; P = 6.8E-9) on 21q22.3. Linkage analysis also identified the region around ADAMTS13 (9q34.2) as the top signal (LOD 3.5), consistent with our SNP association analyses. Two nonsynonymous ADAMTS13 variants in the top 2 independent linkage disequilibrium blocks (Q448E and A732V) were identified and characterized in vitro. This study uncovered specific common genetic polymorphisms that are key genetic determinants of the variation in plasma ADAMTS13 levels in healthy individuals.

The ability to prevent blood loss in response to injury is a critical, evolutionarily conserved function of all vertebrates. Prothrombin (F2) contributes to both primary and secondary hemostasis through the activation of platelets and the conversion of soluble fibrinogen to insoluble fibrin, respectively. Complete prothrombin deficiency has never been observed in humans and is incompatible with life in mice, limiting the ability to understand the entirety of prothrombin’sin vivofunctions. We have previously demonstrated the ability of zebrafish to tolerate loss of both pro- and anticoagulant factors that are embryonic lethal in mammals, making them an ideal model for the study of prothrombin deficiency. Using genome editing with TALENs, we have generated a null allele in zebrafishf2. Homozygous mutant embryos develop normally into early adulthood, but demonstrate eventual complete mortality with the majority of fish succumbing to internal hemorrhage by 2 months of age. We show that despite the extended survival, the mutants are unable to form occlusive thrombi in both the venous and arterial systems as early as 3-5 days of life, and we were able to phenocopy this early hemostatic defect using direct oral anticoagulants. When the equivalent mutation was engineered into the homologous residues of human prothrombin, there were severe reductions in secretion and activation, suggesting a possible role for kringle 1 in thrombin maturation, and the possibility that the F1.2 fragment has a functional role in exerting the procoagulant effects of thrombin. Together, our data demonstrate the conserved function of thrombin in zebrafish, as well as the requirement for kringle 1 for biosynthesis and activation by prothrombinase. Understanding how zebrafish are able to develop normally and survive into early adulthood without prothrombin will provide important insight into its pleiotropic functions as well as the management of patients with bleeding disorders.Key PointsDisruption of the kringle 1 domain of prothrombin leads to severe impairment of biosynthesis, activation, and activityProthrombin deficiency is compatible with normal development in zebrafish but leads to inability to form clots followed by early mortality

Binding to the von Willebrand factor (VWF) D′D3 domains protects factor VIII (FVIII) from rapid clearance. We performed single-particle electron microscopy (EM) analysis of negatively stained specimens to examine the architecture of D′D3 alone and in complex with FVIII. The D′D3 dimer ([D′D3]2) comprises 2 antiparallel D3 monomers with flexibly attached protrusions of D′. FVIII-VWF association is primarily established between the FVIII C1 domain and the VWF D′ domain, whereas weaker interactions appear to be mediated between both FVIII C domains and the VWF D3 core. Modeling the FVIII structure into the three-dimensional EM reconstructions of [D′D3]2-FVIII ternary and quaternary complexes indicates conformational rearrangements of the FVIII C domains compared with their disposition in the unbound state. These results illustrate the cooperative plasticity between VWF and FVIII that coordinate their high-affinity interaction.

Proteases play important roles in many biologic processes and are key mediators of cancer, inflammation, and thrombosis. However, comprehensive and quantitative techniques to define the substrate specificity profile of proteases are lacking. The metalloprotease ADAMTS13 regulates blood coagulation by cleaving von Willebrand factor (VWF), reducing its procoagulant activity. A mutagenized substrate phage display library based on a 73-amino acid fragment of VWF was constructed, and the ADAMTS13-dependent change in library complexity was evaluated over reaction time points, using high-throughput sequencing. Reaction rate constants (kcat/KM) were calculated for nearly every possible single amino acid substitution within this fragment. This massively parallel enzyme kinetics analysis detailed the specificity of ADAMTS13 and demonstrated the critical importance of the P1-P1' substrate residues while defining exosite binding domains. These data provided empirical evidence for the propensity for epistasis within VWF and showed strong correlation to conservation across orthologs, highlighting evolutionary selective pressures for VWF.

Protease specificity is controlled by exosites, which capture and orient the substrate, and the active site, which binds substrate residues near the P1-P1' scissile bond and catalyzes peptide hydrolysis. Techniques used to identify critical contact points between a protease and its substrate can be time consuming and labor-intensive. Screening tools such as phage display have been revitalized with the emergence of high-throughput sequencing technology, and can be used to interrogate protease substrate specificity. This article will outline a method for creating and screening a comprehensive mutagenesis substrate phage display library. High-throughput sequencing of uncleaved phage at various reaction time points enables k

Protease specificity is controlled by exosites, which capture and orient the substrate, and the active site, which binds substrate residues near the P1-P1' scissile bond and catalyzes peptide hydrolysis. Techniques used to identify critical contact points between a protease and its substrate can be time consuming and labor-intensive. Screening tools such as phage display have been revitalized with the emergence of high-throughput sequencing technology, and can be used to interrogate protease substrate specificity. This article will outline a method for creating and screening a comprehensive mutagenesis substrate phage display library. High-throughput sequencing of uncleaved phage at various reaction time points enables k cat/K M determination for every possible single amino acid substitution at each position of the substrate, providing unprecedented resolution for the interaction between a protease and its substrate.

Pathologic blood clotting is a leading cause of morbidity and mortality in the developed world, underlying deep vein thrombosis, myocardial infarction, and stroke. Genetic predisposition to thrombosis is still poorly understood, and we hypothesize that there are many additional risk alleles and modifying factors remaining to be discovered. Mammalian models have contributed to our understanding of thrombosis, but are low throughput and costly. We have turned to the zebrafish, a tool for high-throughput genetic analysis. Using zinc finger nucleases, we show that disruption of the zebrafish antithrombin III (at3) locus results in spontaneous venous thrombosis in larvae. Although homozygous mutants survive into early adulthood, they eventually succumb to massive intracardiac thrombosis. Characterization of null fish revealed disseminated intravascular coagulation in larvae secondary to unopposed thrombin activity and fibrinogen consumption, which could be rescued by both human and zebrafish at3 complementary DNAs. Mutation of the human AT3-reactive center loop abolished the ability to rescue, but the heparin-binding site was dispensable. These results demonstrate overall conservation of AT3 function in zebrafish, but reveal developmental variances in the ability to tolerate excessive clot formation. The accessibility of early zebrafish development will provide unique methods for dissection of the underlying mechanisms of thrombosis.

Background ADAMTS13 is a metalloprotease that regulates the platelet-capturing capacity of von Willebrand factor. ADAMTS13 is secreted in its active form and has a circulating half-life of 4-7 days despite the presence of relatively high concentrations of inhibitors known to target other members of the ADAMTS protease family. This suggests that ADAMTS13 is resistant to inhibition by natural protease inhibitors, such as Alpha 2-Macroglobin (A2M) and tissue inhibitors of metalloproteinases (TIMPs). ADAMTS13 is known to adopt a closed conformation whereby C-terminal CUB domains bind to the proximal SPACER domain. The impact of this closed form of ADAMTS13 on accessibilty of the active site is not currently known. Methods To better understand the mechanism of ADAMTS13 resistance to natural protease inhibitors, C-terminal deletion constructs of ADAMTS13 and chimeras between ADAMTS13/ADAMTS5 were generated. Resistance was tested by examining the activity of ADAMTS13 variants in the presence of A2M using FRETS-VWF73; and then confirmed by Western blot examining the presence of the cleaved form of A2M. Results Activity assay showed that both full length (FL) -ADAMTS13 and MDTCS were resistant towards A2M up to 24 hours. Further, FL-ADAMTS13 and MDTCS were unable to cleave A2M; and therefore, were not inhibited. The MD variant in the presence of A2M showed marked reduction in activity and cleaved A2M. Therefore, MD was inhibited by A2M. On-going Studies Additional studies with ADAMTS5 (MD5TCS13, MD5TCST1-CUB2(13) and MD13TCS5) are on-going and should shed additional light on this unique property of ADAMTS13. These experiments will be repeated using the 4 isoforms of TIMPs; which are direct inhibitors of the metalloprotease active site. Conclusion Based on our domain truncations of ADAMTS13, its resistance towards A2M is likely mediated by proximal domains closest to its active site. On-going and future studies will try to determine the exact mechanism of resistance.

The mechanisms by which von Willebrand factor (VWF) achieves hemostasis lie in its structure. Whereas low-molecular-weight forms have diminished hemostatic potential, ultralarge VWF (ULVWF) in excess is potentially thrombogenic. VWF comprises many subunits, which themselves comprise many repeated domains/assemblies possessing characteristic function(s). Organization of these domains/assemblies into a multimeric structure effectively links and replicates these functions. Each domain/assembly influences the synthesis, assembly, secretion, or hemostatic potential of plasma VWF. The C-terminal CT/CK domain mediates dimerization of VWF subunits in the endoplasmic reticulum, while the N-terminal D1D2 assemblies catalyzes disulfide binding between juxtaposed D3 assemblies in the trans-Golgi, creating multimers. The pH-sensitive domains (A2-CT/CK) allow ULVWF multimers to orderly pack into tubules that unravel upon secretion into the circulation. Hemodynamic forces regulate the conformation of the A2 domain and thus, its accessibility to proteolytic enzyme(s) that regulate VWF's hemostatic potential. Binding to the VWF D'D3 assemblies stabilizes coagulation factor VIII. The VWF A1 and A3 domains facilitate platelet capture onto exposed collagen(s) at sites of vascular injury. Our deeper understanding of VWF provided through the recent growth in VWF structure-function studies may potentially guide novel therapeutics for clotting or bleeding disorders.

Background: Scott Syndrome is a rare bleeding disorder characterized by a defect in platelet phosphatidylserine (PS) exposure. The syndrome has recently been linked to mutations in TMEM16F. Tmem16f -/- mice were recently reported to be viable with a prolonged tail snip bleeding time but no spontaneous bleeding. We now report analysis of an additional gene targeted Tmem16f allele generated in C57BL/6 ES cells. Results: JM8 ES cell were obtained from EUCOMM, and successful Tmem16f gene targeting in intron 1 was confirmed by PCR and sequencing. Genotyping of 120 Tmem16f +/gt (+/gt) intercross progeny identified no surviving Tmem16f gt/gt (gt/gt) mice at weaning (p Tail bleeding times for gt/gt were >10min, whereas littermate +/gt and +/+ mice bleeding ceased at 8 ± 1 min and 6 ± 0.8 min, respectively, each significantly different than gt/gt (p Conclusion: These data suggest the existence of a viability-determining genetic modifier of TMEM16F in the 129x1SvJ mouse strain. Identification of the responsible gene may uncover novel functions for TMEM16F and the regulation of hemostatic function.

SummaryHD1, a DNA aptamer, binds exosite 1 on thrombin and blocks its clotting activity. Because HD1 also binds prothrombin and inhibits its activation by prothrombinase, we hypothesised that HD1 would be a more potent inhibitor of coagulation than other exosite 1-directed ligands, such as Hir54–65(SO3 -). Supporting this concept, the effect of HD1 on the prothrombin time and activated partial thromboplastin time was twofold greater than that of Hir54–65(SO3 -) even though both agents inhibited thrombin-mediated factor (F) V and FVIII activation to a similar extent. In thrombin generation assays, HD1 (a) delayed the lag time, (b) reduced peak thrombin concentration, and (c) decreased endogenous thrombin potential to a greater extent than Hir54–65(SO3 -). To eliminate thrombin feedback, studies were repeated in FV- and/or FVIII-deficient plasma supplemented with FVa and/or FVIIIa. Only HD1 prolonged the lag time in FV- and FVIII-deficient plasma supplemented with FVa and FVIIIa. In contrast, HD1 and Hir54–65(SO3 -) inhibited the lag time in FVIII-deficient plasma supplemented with FVIIIa and in normal plasma. The more potent anticoagulant properties of HD1, therefore, reflect its capacity to attenuate FV activation by thrombin and inhibit prothrombinase assembly. These findings identify prothrombin as a potential target for new anticoagulants.

Hemostasis, the process of blood clot formation and resolution in response to vascular injury, and thrombosis, the dysregulation of hemostasis leading to pathological clot formation, are widely studied. However, the genetic variability in hemostatic and thrombotic disorders is incompletely understood, suggesting that novel mediators have yet to be uncovered. The zebrafish is developing into a powerful in vivo model to study hemostasis, and its features as a model organism are well suited to (a) develop high-throughput screens to identify novel mediators of hemostasis and thrombosis, (b) validate candidate genes identified in human populations, and (c) characterize the structure/function relationship of gene products. In this review, we discuss conservation of the zebrafish hemostatic system, highlight areas for future study, and outline the utility of this model to study blood coagulation and its dysregulation.

See also Bompiani KM, Monroe DM, Church FC, Sullenger BA. A high affinity, antidote-controllable prothrombin and thrombin-binding RNA aptamer inhibits thrombin generation and thrombin activity. This issue, pp 870–80.

Plasma factor VIII (FVIII) and von Willebrand factor (VWF) circulate together as a complex. We identify VWF fragments sufficient for FVIII stabilization in vivo and show that hepatic expression of the VWF D'D3 domains (S764-P1247), either as a monomer or a dimer, is sufficient to raise FVIII levels in Vwf(-/-) mice from a baseline of ∼5% to 10%, to ∼50% to 100%. These results demonstrate that a fragment containing only ∼20% of the VWF sequence is sufficient to support FVIII stability in vivo. Expression of the VWF D'D3 fragment fused at its C terminus to the Fc segment of immunoglobulin G1 results in markedly enhanced survival in the circulation (t1/2 > 7 days), concomitant with elevated plasma FVIII levels (>25% at 7 days) in Vwf(-/-) mice. Although the VWF D'D3-Fc chimera also exhibits markedly prolonged survival when transfused into FVIII-deficient mice, the cotransfused FVIII is rapidly cleared. Kinetic binding studies show that VWF propeptide processing of VWF D'D3 fragments is required for optimal FVIII affinity. The reduced affinity of VWF D'D3 and VWF D'D3-Fc for FVIII suggests that the shortened FVIII survival in FVIII-deficient mice transfused with FVIII and VWF D'D3/D'D3-Fc is due to ineffective competition of these fragments with endogenous VWF for FVIII binding.

We previously reported the identification and development of novel inhibitors of streptokinase (SK) expression by Group A Streptococcus (GAS), originating from a high throughput cell-based phenotypic screen. Although phenotypic screening is well-suited to identifying compounds that exert desired biological effects in potentially novel ways, it requires follow-up experiments to determine the macromolecular target(s) of active compounds. We therefore designed and synthesized several classes of chemical probes for target identification studies, guided by previously established structure–activity relationships. The probes were designed to first irreversibly photolabel target proteins in the intact bacteria, followed by cell lysis and click ligation with fluorescent tags to allow for visualization on SDS–PAGE gels. This stepwise, ‘tag-free’ approach allows for a significant reduction in molecular weight and polar surface area compared to full-length fluorescent or biotinylated probes, potentially enhancing membrane permeability and the maintenance of activity. Of the seven probes produced, the three most biologically active were employed in preliminary target identification trials. Despite the potent activity of these probes, specific labeling events were not conclusively observed due to a considerable degree of nonspecific protein binding. Nevertheless, the successful synthesis of potent biologically active probe molecules will serve as a starting point for initiating more sensitive methods of probe-based target identification.

Platelet sequestration at sites of vascular injuries and factor VIII (FVIII) stabilization critically relies on von Willebrand factor (VWF). Specific domains within VWF coordinate these functions and may harbor mutations that result in von Willebrand disease (VWD). To survey all possible mutations that may lead to type 2 VWD, we developed prokaryotic and eukaryotic systems to display VWF fragments that are screened for a specific function and identified by DNA sequencing. We constructed an M13 filamentous phage display library consisting of ~2.8x106 independent clones that express random VWF fragments. Following a single round of selection for platelet binding, bound phage were eluted and analyzed by next generation DNA sequencing, revealing an optimal fragment spanning the A1 domain and a second region of weaker enrichment over the D4 domain. All A1 fragments encompassed C1272-C1458, indicating that intramolecular disulfide bridging of these cysteines optimizes platelet binding. Competitive binding assays between phage displaying a fragment of the D4 domain vs. a control phage resulted in enrichment of the former, suggesting a potential, novel role for the VWF D4 domain. Analogous to our previously reported method for analyzing VWF proteolysis (Kretz CA et al. Proc Natl Acad Sci USA. 2015), we mutagenized and displayed the VWF A1 domain to determine the spectrum of mutations that may alter the interaction between VWF and its platelet receptor, GPIbα. The mutant A1 library comprised ~5x106 independent clones, sufficient to represent all 3,933 single substitutions. Screening this library against platelets in the presence of excess control phage resulted in an enrichment of phage displaying an A1 fragment. DNA sequencing identified mutations located outside the VWF A1/platelet GPIbα interface, indicating allosteric control of this interaction. Screening phage displayed VWF fragments for FVIII binding, however, failed to identify critical residues; thus, we adapted our approach for eukaryotes. When displayed on mammalian cells, the VWF D'D3 domains retained their capacity to bind FVIII. Introduction of the VWD subtype 2N mutation, R816W, into the displayed VWF D'D3 domains abolished FVIII binding, demonstrating feasibility for mutational analyses. Together, these results demonstrate the utility of phage and mammalian display for structure-function analyses and that these platforms provide an efficient method for phenotyping variants of uncertain significance. Disclosures No relevant conflicts of interest to declare.

von Willebrand disease (VWD) is the most common inherited human bleeding disorder and is caused by quantitative or qualitative defects in von Willebrand factor (VWF). VWF is a secreted glycoprotein that circulates as large multimers. While reduced VWF is associated with bleeding, elevations in overall level or multimer size are implicated in thrombosis. The zebrafish is a powerful genetic model in which the hemostatic system is well conserved with mammals. The ability of this organism to generate thousands of offspring and its optical transparency make it unique and complementary to mammalian models of hemostasis. Previously, partial clones of zebrafishvwfhave been identified, and some functional conservation has been demonstrated. In this paper we clone the complete zebrafishvwfcDNA and show that there is conservation of domain structure. Recombinant zebrafish Vwf forms large multimers and pseudo-Weibel-Palade bodies (WPBs) in cell culture. Larval expression is in the pharyngeal arches, yolk sac, and intestinal epithelium. These results provide a foundation for continued study of zebrafish Vwf that may further our understanding of the mechanisms of VWD.

Scott Syndrome is a rare, moderately severe bleeding disorder caused by a defect in platelet, red cell, and lymphocyte phosphatidylserine exposure. The syndrome has been linked to mutations in TMEM16F, a Ca++-activated ion channel. A moderately severe bleeding disorder in German Shephard dogs, characterized by decreased platelet phosphatidylserine exposure, has also linked to mutations in TMEM16F. TMEM16F, is a member of a recently-identified family of calcium-activated chloride channels that are also called anoctamins. Members of the family apparently serve both as ion channels and phospholipid scrambling channels. A crystal structure of one member of the TMEM16 family shows a homo-dimeric structure of an integral membrane protein with each unit containing 10 transmembrane helices. A transmembrane hydrophilic cavity lies at the interface, and contains a cation binding site, and a slot with dimensions that could accommodate acyl chains. Tmem16f-/- mice were recently reported to be viable with a prolonged tail snip bleeding time but no spontaneous bleeding. We have developed an independent gene-targeted (gt) Tmem16f allele generated in C57BL/6 ES cells and find results that both confirm prior reports and contrast with them. JM8 ES cells were obtained from EUCOMM, and Tmem16f gene targeting in intron 1 was confirmed by PCR and sequencing. Genotyping of 120 Tmem16f+/gt intercross progeny identified no surviving Tmem16fgt/gt mice at weaning (p An F2 intercross of +/gt mice outcrossed one generation to 129x16vJ resulted in gt/gt mice surviving to weaning, though at only 30% of the expected Mendelian frequency. Progeny analysis points to a single autosomal dominant 129x1SvJ-associated genetic modifier. Preliminary genetic analysis of these mice appears to map this locus to the proximal region of chromosome 3. Further efforts to localize the responsible gene(s) are underway. Tail bleeding times for gt/gt were >10min, whereas littermate +/gt and +/+ mice bleeding ceased at 8 ± 1 min and 6 ± 0.8 min, respectively, each significantly different than gt/gt (p Our data suggest the existence of a viability-determining genetic modifier of the TMEM16F deficiency phenotype in the 129x1SvJ mouse strain. Identification of the responsible gene may uncover a novel regulator of hemostatic function. The observation that heterozygous deficiency leads to a PS exposure and hemostatic phenotype also suggests the possibility that heterozygous TMEM16F mutations may influence hemostasis or thrombosis in humans. Disclosures No relevant conflicts of interest to declare.

Mouse models of thrombosis have extended our understanding of the role of tissue factor (TF) in thrombogenesis. Because tissue factor deficiency is embryonic lethal in mice, inventive genetic models are required to probe the role of TF in thrombosis. TF is expressed by different cell types, including vascular smooth muscle cells, cardiomyocytes, fibroblasts, and monocytes. Platelets and endothelial cells also express TF under certain conditions, but the importance of this TF remains controversial. Animal models are commonly used to evaluate the contribution of TF from each cell type to thrombogenesis. Although a variety of well-established injury techniques are used to induce thrombosis, it is likely that the sources of TF that drive thrombosis are model dependent. Therefore, rigorous controls are needed before thrombogenesis can be attributed to TF from a particular cell type. This review summarizes data from mouse models that have attempted to delineate the role of TF in thrombus formation in response to various types of vascular injury. We have consolidated this information to generate unifying concepts that require testing in future studies.

Von Willebrand factor (VWF) is a large, multimeric protein that regulates hemostasis by tethering platelets to the subendothelial matrix at sites of vascular damage. The procoagulant activity of plasma VWF correlates with the length of VWF multimers, which is proteolytically controlled by the metalloprotease ADAMTS13. To probe ADAMTS13 substrate specificity, we created phage display libraries containing randomly mutated residues of a minimal ADAMTS13 substrate fragment of VWF, termed VWF73. The libraries were screened for phage particles displaying VWF73 mutant peptides that were resistant to proteolysis by ADAMTS13. These peptides exhibited the greatest mutation frequency near the ADAMTS13 scissile residues. Kinetic assays using mutant and wild-type substrates demonstrated excellent agreement between rates of cleavage for mutant phage particles and the corresponding mutant peptides. Cleavage resistance of selected mutations was tested in vivo using hydrodynamic injection of corresponding full-length expression plasmids into VWF-deficient mice. These studies confirmed the resistance to cleavage resulting from select amino acid substitutions and uncovered evidence of alternate cleavage sites and recognition by other proteases in the circulation of ADAMTS13 deficient mice. Taken together, these studies demonstrate the key role of specific amino acids residues including P3-P2' and P11', for substrate specificity and emphasize the importance in flowing blood of other ADAMTS13-VWF exosite interactions outside of VWF73.

Although exosites 1 and 2 regulate thrombin activity by binding substrates and cofactors and by allosterically modulating the active site, it is unclear whether there is direct allosteric linkage between the two exosites. To begin to address this, we first titrated a thrombin variant fluorescently labeled at exosite 1 with exosite 2 ligands, HD22 (a DNA aptamer), gamma'-peptide (an analog of the COOH terminus of the gamma'-chain of fibrinogen) or heparin. Concentration-dependent and saturable changes in fluorescence were elicited, supporting inter-exosite linkage. To explore the functional consequences of this phenomenon, we evaluated the capacity of exosite 2 ligands to inhibit thrombin binding to gamma(A)/gamma(A)-fibrin, an interaction mediated solely by exosite 1. When gamma(A)/gamma(A)-fibrinogen was clotted with thrombin in the presence of HD22, gamma'-peptide, or prothrombin fragment 2 there was a dose-dependent and saturable decrease in thrombin binding to the resultant fibrin clots. Furthermore, HD22 reduced the affinity of thrombin for gamma(A)/gamma(A)-fibrin 6-fold and accelerated the dissociation of thrombin from preformed gamma(A)/gamma(A)-fibrin clots. Similar responses were obtained when surface plasmon resonance was used to monitor the interaction of thrombin with gamma(A)/gamma(A)-fibrinogen or fibrin. There is bidirectional communication between the exosites, because exosite 1 ligands, HD1 (a DNA aptamer) or hirudin-(54-65) (an analog of the COOH terminus of hirudin), inhibited the exosite 2-mediated interaction of thrombin with immobilized gamma'-peptide. These findings provide evidence for long range allosteric linkage between exosites 1 and 2 on thrombin, revealing further complexity to the mechanisms of thrombin regulation.

Incorporation of prothrombin into the prothrombinase complex is essential for rapid thrombin generation at sites of vascular injury. Prothrombin binds directly to anionic phospholipid membrane surfaces where it interacts with the enzyme, factor Xa, and its cofactor, factor Va. We demonstrate that HD1, a thrombin-directed aptamer, binds prothrombin and thrombin with similar affinities (K(d) values of 86 and 34 nm, respectively) and attenuates prothrombin activation by prothrombinase by over 90% without altering the activation pathway. HD1-mediated inhibition of prothrombin activation by prothrombinase is factor Va-dependent because (a) the inhibitory activity of HD1 is lost if factor Va is omitted from the prothrombinase complex and (b) prothrombin binding to immobilized HD1 is reduced by factor Va. These data suggest that HD1 competes with factor Va for prothrombin binding. Kinetic analyses reveal that HD1 produces a 2-fold reduction in the k(cat) for prothrombin activation by prothrombinase and a 6-fold increase in the K(m), highlighting the contribution of the factor Va-prothrombin interaction to prothrombin activation. As a high affinity, prothrombin exosite 1-directed ligand, HD1 inhibits prothrombin activation more efficiently than Hir(54-65)(SO(3)(-)). These findings suggest that exosite 1 on prothrombin exists as a proexosite only for ligands whose primary target is thrombin rather than prothrombin.

Scott Syndrome is a rare bleeding disorder characterized by a defect in platelet phosphatidylserine (PS) exposure. The syndrome has recently been linked to mutations in TMEM16F, a Ca++-activated ion channel. Tmem16f-/- mice were recently reported to be viable with a prolonged tail snip bleeding time but no spontaneous bleeding (Yang et al, Cell 151: 111-122; 2012). We now report analysis of an additional gene targeted Tmem16f allele generated in C57BL/6 ES cells. JM8 ES cell were obtained from EUCOMM, and successful Tmem16f gene targeting in intron 1 was confirmed by PCR and sequencing. Genotyping of 120 Tmem16f+/gt (+/gt) intercross progeny identified no surviving Tmem16fgt/gt (gt/gt) mice at weaning (p Tail bleeding times for gt/gt were >10min, whereas littermate +/gt and +/+ mice bleeding ceased at 8 ± 1 min and 6 ± 0.8 min, respectively, each significantly different than gt/gt (p These data suggest the existence of a viability-determining genetic modifier of TMEM16F in the 129x1SvJ mouse strain. Identification of the responsible gene may uncover novel functions for TMEM16F and the regulation of hemostatic function. The observation of a PS exposure and hemostatic phenotype in +/gt mice also suggests the possibility that heterozygous TMEM16F mutations may subtly influence hemostasis or thrombosis in humans. Disclosures No relevant conflicts of interest to declare.

Thrombosis is a leading cause of morbidity and mortality in the developed world, underlying deep vein thrombosis, myocardial infarction, and stroke. Identification of small molecule inhibitors of thrombosis in an in vivo model would facilitate novel and improved therapeutics for patients. The zebrafish is a powerful genetic model in which the hemostatic system is nearly entirely conserved with humans. Its external development, ability to generate thousands of offspring at low cost, and optical transparency all make it a powerful tool to screen for genetic and chemical modifiers of coagulation disorders. We generated a zebrafish model of antithrombin III (AT3) deficiency by targeted mutagenesis using zinc finger nucleases. Homozygous at3 mutants displayed a lethal phenotype due to intracardiac thrombosis between 2 and 7 months of age, yet embryos and larvae appeared grossly normal with no overt evidence of pathologic clotting. Induction of thrombosis at 3-4 days post fertilization (dpf) in homozygous mutant larvae by laser-mediated endothelial ablation resulted in diminished rates of posterior cardinal vein (PCV) occlusion, a bleeding phenotype. To prove functional conservation with mammals, we expressed recombinant zebrafish At3 and demonstrated that it binds human thrombin in vitro. Furthermore, while injection of wild type zebrafish and human cDNAs rescued the laser injury phenotype, zebrafish at3 with a mutation in the putative reactive site failed to do so. We hypothesized that the discrepant larval bleeding and adult thrombotic phenotypes could be accounted for by disseminated intravascular coagulation (DIC). Consistent with this, we observed reduced fibrinogen levels in at3 homozygous mutant plasma, and were able to rescue mutant larvae by injection of human fibrinogen prior to laser injury. To identify the location of consumed fibrinogen, we tagged human fibrinogen with FITC (fluorescein isothiocyanate), followed by infusion into larvae from heterozygous intercrosses at 3 dpf. at3 homozygous mutants displayed extensive PCV fluorescence, which was absent in wild type and heterozygous siblings. Pre-incubation with warfarin completely prevented this phenotype, co-injection of tissue plasminogen activator (TPA) partially prevented fluorescence accumulation, and post-injection of TPA reduced the signal, consistent with our hypothesis of DIC. Our data have uncovered the mechanism underlying the discrepant at3 mutant phenotypes and demonstrated conservation of At3 function in zebrafish. Loss of At3 protein results in DIC in zebrafish larvae secondary to unopposed thrombin activity. Mutants survive, only to succumb to lethal thrombosis as adults. Further study as to why larvae are able to tolerate excessive clot formation, as well as small molecule screens for novel anticoagulants using this model, could potentially lead to innovative therapeutic modalities for affected patients. Disclosures: No relevant conflicts of interest to declare.

Abstract 15 Plasma factor VIII (fVIII) circulates in complex with von Willebrand factor (VWF) and is rapidly cleared in the absence of VWF. Previous in vitro studies have 1) localized the fVIII-binding region of VWF to the N-terminus, comprised of the contiguous D' and D3 domains and 2) observed reduced affinity for fVIII upon alterations to the tertiary structure of VWF. To gain insight into the structure-function of VWF for fVIII stabilization, we tested VWF fragments for in vivo fVIII stabilization and investigated the architecture of a VWF fragment in complex with fVIII. For the in vivo study, fragments of murine Vwf cDNA were cloned into the hepatic-specific expression vector, pLIVE, and modified to fuse tandem E and FLAG tags to the C-terminus. The following VWF fragments were expressed in vivo by hydrodynamic tail vein injection into Vwf−/− mice: 1) a monomer of the VWF D'D3 domains (monoD'D3; M1-C22, S764-P1274); 2) a truncation of monomeric D'D3 (truncD'D3; M1-C22, S764-R1035); 3) dimers of D'D3 (diD'D3; M1-P1274); 4) multimers of D'D3 (multiD'D3; M1-P1274, G2713-K2813); 5) dimers of mature VWF subunits (DPro, M1-C22, S764-K2813); or 6) full length, multimeric VWF (wtVWF, M1-K2813). Expression of all VWF fragments persisted throughout the period of observation (4 weeks) with peak antigenic levels at 1 or 3 days post-injection. Prolonged elevation of plasma fVIII activity (fVIIIa) from ∼10% to ∼50–200% were observed (100% defined as the fVIIIa level of pooled platelet poor plasma from 10 wild type C57BL/6 mice) for all but the truncated monomer of D'D3 (Figure 1). Significantly increased fVIIIa levels (p Figure 1. fVIIIa of hydrodynamically injected mice at indicated time points. Figure 1. fVIIIa of hydrodynamically injected mice at indicated time points. Disclosures: Ginsburg: Shire Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Portola Pharmaceuticals: Consultancy; Catalyst Biosciences: Consultancy; Baxter Pharmaceuticals: benefit from payments to Children's Hosptial, Boston, and the University of Michigan Patents & Royalties; Merck Pharmaceuticals: Consultancy.

Functional maturation of exosites 1 and 2 during prothrombin activation endows thrombin with its physiological activities. Thrombin exosite 1 binds ligands such as fibrinogen, the thrombin receptor, and hirudin, and orients them to the active site. In contrast, thrombin exosite 2 binds ligands such as heparin, platelet glycoprotein Iba, and the chondroitin sulfate moiety of thrombomodulin molecules; ligands that do not directly interact with the active site. Neither exosite is fully functional in prothrombin. Exosite 2 only becomes functional when fragment 2 is released. In contrast, there is evidence for existence of a proexosite 1 in prothrombin which contributes to prothrombin activation by prothrombinase. This domain in prothrombin lacks fibrin-binding capacity, but has been implicated in the interaction with factor Va (fVa), a necessary step for enhanced prothrombin activation by prothrombinase. In the present study, we used fluorescent (F) HD1 and HD22, DNA aptamers that bind to exosites 1 and 2, respectively, to study the exosites in prothrombin, prothrombin intermediates and thrombin. F-HD22 does not bind to prothrombin and only binds prothrombin intermediates lacking fragment 2. In contrast, F-HD1 binds to prothrombin and all of the prothrombin intermediates with Kd values ranging from 80 to 25 nM for prothrombin and thrombin, respectively. These findings are different from what has been reported with the hirudin fragment, Hir54–65, which exhibited a 60-fold lower affinity for prothrombin than for thrombin. To better explore the involvement of proexosite 1 in the activation of prothrombin by prothrombinase, we examined the effect of HD1 on prothrombin activation by intact prothrombinase, or by various components of the prothrombinase complex. HD1 inhibits prothrombin activation by prothrombinase with an IC50 of 140 nM. Omitting fVa from the prothrombinase complex abolishes the inhibitory effect of HD1, supporting the concept that the inhibitory effect of HD1 reflects disruption of the interaction between prothrombin and fVa. In similar experiments, Hir54–65 inhibits prothrombin activation by prothrombinase with an IC50 of 225 nM, in agreement with previous studies. These data show that, like hirugen, HD1 binds to the fVa-binding domain on proexosite 1 of prothrombin. In order to refine the location of HD1 and Hir54–65 binding sites on thrombin and prothrombin, competitive binding experiments were performed. Hir54–65 displaces 80% of F-HD1 from thrombin and 70% from prothrombin. In reverse titrations, HD1 displaces 50% of F-Hir54–65 from thrombin, but does not displace F-Hir54–65 from prothrombin. Taken together, these data suggest that the HD1 binding domain is distinct, at least in part, from the Hir54–65 binding domain on prothrombin, an observation supported by examination of the crystal structures of complexes of HD1 or Hir54–65 with thrombin. In conclusion, our data demonstrate that the prsuggests that the HD1 binding site on prothrombin more closely coincides with the fVa-binding site than does the hirugen-binding site. These findings provide new avenues for investigations into the oexosite 1 domain on prothrombin is fully capacitated for binding HD1. This role of exosite 1, not only in thrombin, but also in prothrombin.