Cell Receptor and Cofactor Interactions of the Contact Activation System and Factor XI

Substrates, Cofactors, and Cellular Targets of Coagulation Factor XIa

Coagulation factor XI (FXI) has increasingly been shown to play an integral role in several physiologic and pathological processes. FXI is among several zymogens within the blood coagulation cascade that are activated by proteolytic cleavage, with FXI converting to the active serine protease form (FXIa). The evolutionary origins of FXI trace back to duplication of the gene that transcribes plasma prekallikrein, a key factor in the plasma kallikrein-kinin system, before further genetic divergence led to FXI playing a unique role in blood coagulation. While FXIa is canonically known for activating the intrinsic pathway of coagulation by catalyzing the conversion of FIX into FIXa, it is promiscuous in nature and has been shown to contribute to thrombin generation independent of FIX. In addition to its role in the intrinsic pathway of coagulation, FXI also interacts with platelets, endothelial cells, and mediates the inflammatory response through activation of FXII and cleavage of high-molecular-weight kininogen to generate bradykinin. In this manuscript, we critically review the current body of knowledge surrounding how FXI navigates the interplay of hemostasis, inflammatory processes, and the immune response and highlight future avenues for research. As FXI continues to be clinically explored as a druggable therapeutic target, understanding how this coagulation factor fits into physiological and disease mechanisms becomes increasingly important.

First randomized evaluation of safety, pharmacodynamics, and pharmacokinetics of BAY 1831865, an antibody targeting coagulation factor XI and factor XIa, in healthy men

BACKGROUND

Bleeding is a clinically significant issue with all current anticoagulants. Safer antithrombotic strategies are required.

OBJECTIVES

To investigate the safety, pharmacodynamics, and pharmacokinetics of BAY 1831865, a humanized, factor XI (FXI)-directed monoclonal antibody, after single intravenous (i.v.) or subcutaneous (s.c.) doses in healthy volunteers.

PATIENTS/METHODS

In a first-in-human, phase I study, 70 volunteers were randomly assigned (4:1) to receive single-dose BAY 1831865 (3.5, 7, 17, 35, 75, or 150 mg i.v. or 150 mg s.c.) or placebo. Adverse events, pharmacodynamics, and pharmacokinetics were evaluated.

RESULTS

In this study, no hemorrhage, or hypersensitivity or infusion-/injection-related reactions were reported. Drug-related adverse events occurred in 3 (5.4%) of 56 volunteers; all were mild and self-limited. Dose-dependent prolongation of activated partial thromboplastin time (aPTT) and inhibition of FXI clotting activity was observed with BAY 1831865 i.v. (geometric mean maximum ratio-to-baseline: aPTT, range, 1.09-3.11 vs. 1.05 with placebo; FXI, range, 0.70-0.04 vs. 0.91 with placebo). Onset of effect was rapid after i.v. administration, with duration of effect (up to 55 days) determined by dose. BAY 1831865 s.c. had similar pharmacodynamic effects but a slower onset of action. Terminal half-life increased continuously with increasing i.v. dose (range, 28-208 h), leading to strong and continuous increases in systemic exposure to BAY 1831865. Absolute bioavailability of BAY 1831865 s.c. was 47.2% (95% confidence interval, 30.2-73.7).

CONCLUSIONS

BAY 1831865 i.v. or s.c. was well tolerated, with no evidence of bleeding in healthy volunteers. BAY 1831865 exhibited pronounced, sustained dose-dependent prolongation of aPTT and duration of FXI inhibition.

The rebirth of the contact pathway: a new therapeutic target

PURPOSE OF REVIEW

Anticoagulation with vitamin-K antagonists or direct oral anticoagulants is associated with a significant risk of bleeding. There is a major effort underway to develop antithrombotic drugs that have a smaller impact on hemostasis. The plasma contact proteins factor XI (FXI) and factor XII (FXII) have drawn considerable interest because they contribute to thrombosis but have limited roles in hemostasis. Here, we discuss results of preclinical and clinical trials supporting the hypothesis that the contact system contributes to thromboembolic disease.

RECENT FINDINGS

Numerous compounds targeting FXI or FXII have shown antithrombotic properties in preclinical studies. In phase 2 studies, drugs-targeting FXI or its protease form FXIa compared favorably with standard care for venous thrombosis prophylaxis in patients undergoing knee replacement. While less work has been done with FXII inhibitors, they may be particularly useful for limiting thrombosis in situations where blood comes into contact with artificial surfaces of medical devices.

SUMMARY

Inhibitors of contact activation, and particularly of FXI, are showing promise for prevention of thromboembolic disease. Larger studies are required to establish their efficacy, and to establish that they are safer than current therapy from a bleeding standpoint.

Novel hereditary angioedema linked with a heparan sulfate 3-O-sulfotransferase 6 gene mutation

BACKGROUND

Hereditary angioedema (HAE) is a potentially fatal disorder resulting in recurrent attacks of severe swelling. It may be associated with a genetic deficiency of functional C1 inhibitor or with normal C1 inhibitor (HAEnCI). In families with HAEnCI, HAE-linked mutations in the F12, PLG, KNG1, ANGPT1, or MYOF genes have been identified. In many families with HAEnCI the genetic cause of the disease is currently unknown.

OBJECTIVE

The aim of this study was to identify a novel disease-linked mutation for HAEnCI.

METHODS

The study methods comprised whole exome sequencing, Sanger sequencing analysis, pedigree analysis, bioinformatic analysis of the mutation, and biochemical analysis of parameters of the kallikrein-kinin (contact) system.

RESULTS

By performing whole exome sequencing on a multigenerational family with HAEnCI we were able to identify the heparan sulfate (HS)-glucosamine 3-O-sulfotransferase 6 (HS3ST6) mutation c.430A>T (p.Thr144Ser) in all 3 affected family members who were sequenced. This gene encodes HS-glucosamine 3-O-sulfotransferase 6 (3-OST-6), which is involved in the last step of HS biosynthesis. The p.Thr144Ser mutation is likely to affect the interaction between 2 β-sheets stabilizing the active center of the 3-OST-6 protein.

CONCLUSIONS

We conclude that mutant 3-OST-6 fails to transfer sulfo groups to the 3-OH position of HS, resulting in incomplete HS biosynthesis. This likely affects cell surface interactions of key players in angioedema formation and is a novel mechanism for disease development.

Role of platelets in regulating activated coagulation factor XI activity

Factor XI (FXI) has been shown to bind platelets, but the functional significance of this observation remains unknown. Platelets are essential for hemostasis and play a critical role in thrombosis, whereas FXI is not essential for hemostasis but promotes thrombosis. An apparent functional contradiction, platelets are known to support thrombin generation, yet platelet granules release protease inhibitors, including those of activated FXI (FXIa). We aim to investigate the secretory and binding mechanisms by which platelets could support or inhibit FXIa activity. The presence of platelets enhanced FXIa activity in a purified system and increased coagulation Factor IX (FIX) activation by FXIa and fibrin generation in human plasma. In contrast, platelets reduced the activation of FXI by activated coagulation factor XII (FXIIa) and the activation of FXII by kallikrein (PKa). Incubation of FXIa with the platelet secretome, which contains FXIa inhibitors, such as protease nexin-II, abolished FXIa activity, yet in the presence of activated platelets, the secretome was not able to block the activity of FXIa. FXIa variants lacking the anion-binding sites did not alter the effect of platelets on FXIa activity or interaction. Western blot analysis of bound FXIa [by FXIa-platelet membrane immunoprecipitation] showed that the interaction with platelets is zinc dependent and, unlike FXI binding to platelets, not dependent on glycoprotein Ib. FXIa binding to the platelet membrane increases its capacity to activate FIX in plasma likely by protecting it from inhibition by inhibitors secreted by activated platelets. Our findings suggest that an interaction of FXIa with the platelet surface may induce an allosteric modulation of FXIa.

Penicillin causes non-allergic anaphylaxis by activating the contact system

Immediate hypersensitivity reaction (IHR) can be divided into allergic- and non-allergic-mediated, while “anaphylaxis” is reserved for severe IHR. Clinically, true penicillin allergy is rare and most reported penicillin allergy is “spurious”. Penicillin-initiated anaphylaxis is possible to occur in skin test- and specific IgE-negative patients. The contact system is a plasma protease cascade initiated by activation of factor XII (FXII). Many agents with negative ion surface can activate FXII to drive contact system. Our data showed that penicillin significantly induced hypothermia in propranolol- or pertussis toxin-pretreated mice. It also caused a rapid and reversible drop in rat blood pressure, which did not overlap with IgE-mediated hypotension. These effects could be countered by a bradykinin-B2 receptor antagonist icatibant, and consistently, penicillin indeed increased rat plasma bradykinin. Moreover, penicillin not only directly activated contact system FXII-dependently, but also promoted bradykinin release in plasma incubated-human umbilical vein endothelial cells. In fact, besides penicillin, other beta-lactams also activated the contact system in vitro. Since the autoactivation of FXII can be affected by multiple-factors, plasma from different healthy individuals showed vastly different amidolytic activity in response to penicillin, suggesting the necessity of determining the potency of penicillin to induce individual plasma FXII activation. These results clarify that penicillin-initiated non-allergic anaphylaxis is attributed to contact system activation, which might bring more effective diagnosis options for predicting penicillin-induced fatal risk and avoiding costly and inappropriate treatment clinically.

Structural basis of the leukocyte integrin Mac-1 I-domain interactions with the platelet glycoprotein Ib

Cell-surface receptor interactions between leukocyte integrin macrophage-1 antigen (Mac-1, also known as CR3, αMβ2, CD11b/CD18) and platelet glycoprotein Ibα (GPIbα) are critical to vascular inflammation. To define the key residues at the binding interface, we used nuclear magnetic resonance (NMR) to assign the spectra of the mouse Mac-1 I-domain and mapped the residues contacting the mouse GPIbα N-terminal domain (GPIbαN) to the locality of the integrin metal ion-dependant adhesion site (MIDAS) surface. We next determined the crystal structures of the mouse GPIbαN and Mac-1 I-domain to 2 Å and 2.5 Å resolution, respectively. The mouse Mac-1 I-domain crystal structure reveals an active conformation that is stabilized by a crystal contact from the α7-helix with a glutamate side chain completing the octahedral coordination sphere of the MIDAS Mg²⁺ ion. The amino acid sequence of the α7-helix and disposition of the glutamic acid matches the C-terminal capping region α-helix of GPIbα effectively acting as a ligand mimetic. Using these crystal structures in combination with NMR measurements and docking analysis, we developed a model whereby an acidic residue from the GPIbα leucine-rich repeat (LRR) capping α-helix coordinates directly to the Mac-1 MIDAS Mg²⁺ ion. The Mac-1:GPIbαN complex involves additional interactions consolidated by an elongated pocket flanking the GPIbαN LRR capping α-helix. The GPIbαN α-helix has an HxxxE motif, which is equivalent by homology to RxxxD from the human GPIbαN. Subsequent mutagenesis of residues at this interface, coupled with surface plasmon resonance studies, confirmed the importance of GPIbαN residues H218, E222, and the Mac-1 MIDAS residue T209 to formation of the complex.

The contact pathway and sepsis

The contact pathway factors XI (FXI) and XII (FXII) have been demonstrated to be largely dispensable for hemostasis, as their absence results in a mild to absent bleeding diathesis. A growing body of literature, however, suggests that the contact pathway contributes to the pathologic host response to certain infectious organisms that produces the often-fatal syndrome known as sepsis. The contact pathway factors serve as a central node connecting inflammation to coagulation, and may offer a potentially safe therapeutic target to mitigate the morbidity and mortality associated with sepsis. Herein, we summarize published in vivo and in vitro data that have explored the roles of the contact pathway in sepsis, and discuss potential clinical applications of novel FXI- and FXII-inhibiting drugs currently under investigation.

Plasma kallikrein structure reveals apple domain disc rotated conformation compared to factor XI

Essentials Zymogen PK is activated to PKa and cleaves substrates kininogen and FXII contributing to bradykinin generation. Monomeric PKa and dimeric homologue FXI utilize the N-terminal apple domains to recruit substrates. A high-resolution 1.3 Å structure of full-length PKa reveals an active conformation of the protease and apple domains. The PKa protease and four-apple domain disc organization is 180° rotated compared to FXI. SUMMARY: Background Plasma prekallikrein (PK) and factor XI (FXI) are apple domain-containing serine proteases that when activated to PKa and FXIa cleave substrates kininogen, factor XII, and factor IX, respectively, directing plasma coagulation, bradykinin release, inflammation, and thrombosis pathways. Objective To investigate the three-dimensional structure of full-length PKa and perform a comparison with FXI. Methods A series of recombinant full-length PKa and FXI constructs and variants were developed and the crystal structures determined. Results and conclusions A 1.3 Å structure of full-length PKa reveals the protease domain positioned above a disc-shaped assemblage of four apple domains in an active conformation. A comparison with the homologous FXI structure reveals the intramolecular disulfide and structural differences in the apple 4 domain that prevents dimer formation in PK as opposed to FXI. Two latchlike loops (LL1 and LL2) extend from the PKa protease domain to form interactions with the apple 1 and apple 3 domains, respectively. A major unexpected difference in the PKa structure compared to FXI is the 180° disc rotation of the apple domains relative to the protease domain. This results in a switched configuration of the latch loops such that LL2 interacts and buries portions of the apple 3 domain in the FXI zymogen whereas in PKa LL2 interacts with the apple 1 domain. Hydrogen-deuterium exchange mass spectrometry on plasma purified human PK and PKa determined that regions of the apple 3 domain have increased surface exposure in PKa compared to the zymogen PK, suggesting conformational change upon activation.