Race, rituximab, and relapse in TTP.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is characterized by recurring episodes of thrombotic microangiopathy, causing ischemic organ impairment. Black patients are overrepresented in iTTP cohorts in the United States, but racial disparities in iTTP outcome and response to therapy have not been studied. Using the United States Thrombotic Microangiopathies Consortium iTTP Registry, we evaluated the impact of race on mortality and relapse-free survival (RFS) in confirmed iTTP in the United States from 1995 to 2020. We separately examined the impact of rituximab therapy and presentation with newly diagnosed (de novo) or relapsed iTTP on RFS by race. A total of 645 participants with 1308 iTTP episodes were available for analysis. Acute iTTP mortality did not differ by race. When all episodes of iTTP were included, Black race was associated with shorter RFS (hazard ratio [HR], 1.60; 95% CI, 1.16-2.21); the addition of rituximab to corticosteroids improved RFS in White (HR, 0.37; 95% CI, 0.18-0.73) but not Black patients (HR, 0.96; 95% CI, 0.71-1.31). In de novo iTTP, rituximab delayed relapse, but Black patients had shorter RFS than White patients, regardless of treatment. In relapsed iTTP, rituximab significantly improved RFS in White but not Black patients. Race affects overall relapse risk and response to rituximab in iTTP. Black patients may require closer monitoring, earlier retreatment, and alternative immunosuppression after rituximab treatment. How race, racism, and social determinants of health contribute to the disparity in relapse risk in iTTP deserves further study.

Phylogenetic and functional analysis of ADAMTS13 identifies highly conserved domains essential for allosteric regulation.

The metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats member 13) prevents microvascular thrombosis by cleaving von Willebrand factor (VWF) within platelet-rich thrombi, and cleavage depends on allosteric activation of ADAMTS13 by the substrate VWF. Human ADAMTS13 has a short propeptide, metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains (proximal domains), followed by 7 T and 2 CUB (complement components C1r and C1s, sea urchin protein Uegf, and bone morphogenetic protein-1) domains (distal domains). Distal domains inhibit the catalytic proximal domains; binding of distal T8-CUB domains to the VWF D4 domain relieves autoinhibition and promotes cleavage of the nearby VWF A2 domain. However, the role of specific ADAMTS13 distal domains in this allosteric mechanism is not established. Assays of plasma ADAMTS13 from 20 placental mammals, birds, and amphibians show that allosteric regulation is broadly conserved, and phylogenetic analysis of 264 vertebrates shows the long propeptide, T3, T4, T6, and T6a domains have been deleted several times in placental mammals, birds, and fish. Notably, pigeon ADAMTS13 has only 3 distal T domains but was activated normally by human VWF D4 and cleaved VWF multimers, preferentially under fluid shear stress. Human ADAMTS13 constructed to resemble pigeon ADAMTS13 retained normal allosteric regulation and shear-dependent cleavage of VWF. Thus, the T3-T6 domains of human ADAMTS13 are dispensable. Conversely, deletion of T7 or T8 abolished allosteric activation. For most species, some sequence changes in the VWF substrate can markedly increase the rate of cleavage, suggesting that ADAMTS13 and VWF have not evolved to be optimal enzyme-substrate pairs. These properties may reflect evolutionary pressure to balance the risk for VWF-dependent bleeding and thrombosis.

Exploring the "minimal" structure of a functional ADAMTS13 by mutagenesis and small-angle X-ray scattering.

Human ADAMTS13 is a multidomain protein with metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains, followed by 7 additional T domains and 2 CUB (complement components C1r and C1s, sea urchin protein Uegf, and bone morphogenetic protein-1) domains. ADAMTS13 inhibits the growth of von Willebrand factor (VWF)-platelet aggregates by cleaving the cryptic Tyr1605-Met1606 bond in the VWF A2 domain. ADAMTS13 is regulated by substrate-induced allosteric activation; without shear stress, the distal T8-CUB domains markedly inhibit VWF cleavage, and binding of VWF domain D4 or selected monoclonal antibodies (MAbs) to distal ADAMTS13 domains relieves this autoinhibition. By small angle X-ray scattering (SAXS), ADAMTS13 adopts a hairpin-like conformation with distal T7-CUB domains close to the proximal MDTCS domains and a hinge point between T4 and T5. The hairpin projects like a handle away from the core MDTCS and T7-CUB complex and contains distal T domains that are dispensable for allosteric regulation. Truncated constructs that lack the T8-CUB domains are not autoinhibited and cannot be activated by VWF D4 but retain the hairpin fold. Allosteric activation by VWF D4 requires T7, T8, and the 58-amino acid residue linker between T8 and CUB1. Deletion of T3 to T6 produced the smallest construct (delT3-6) examined that could be activated by MAbs and VWF D4. Columba livia (pigeon) ADAMTS13 (pADAMTS13) resembles human delT3-6, retains normal activation by VWF D4, and has a SAXS envelope consistent with amputation of the hairpin containing the dispensable T domains of human ADAMTS13. Our findings suggest that human delT3-6 and pADAMTS13 approach a "minimal" structure for allosterically regulated ADAMTS13.

Pathophysiology of thrombotic thrombocytopenic purpura.

The discovery of a disintegrin-like and metalloproteinase with thrombospondin type 1 motif, member 13 (ADAMTS13) revolutionized our approach to thrombotic thrombocytopenic purpura (TTP). Inherited or acquired ADAMTS13 deficiency allows the unrestrained growth of microthrombi that are composed of von Willebrand factor and platelets, which account for the thrombocytopenia, hemolytic anemia, schistocytes, and tissue injury that characterize TTP. Most patients with acquired TTP respond to a combination of plasma exchange and rituximab, but some die or acquire irreversible neurological deficits before they can respond, and relapses can occur unpredictably. However, knowledge of the pathophysiology of TTP has inspired new ways to prevent early deaths by targeting autoantibody production, replenishing ADAMTS13, and blocking microvascular thrombosis despite persistent ADAMTS13 deficiency. In addition, monitoring ADAMTS13 has the potential to identify patients who are at risk of relapse in time for preventive therapy.

Warfarin traps human vitamin K epoxide reductase in an intermediate state during electron transfer.

Although warfarin is the most widely used anticoagulant worldwide, the mechanism by which warfarin inhibits its target, human vitamin K epoxide reductase (hVKOR), remains unclear. Here we show that warfarin blocks a dynamic electron-transfer process in hVKOR. A major fraction of cellular hVKOR is in an intermediate redox state containing a Cys51-Cys132 disulfide, a characteristic accommodated by a four-transmembrane-helix structure of hVKOR. Warfarin selectively inhibits this major cellular form of hVKOR, whereas disruption of the Cys51-Cys132 disulfide impairs warfarin binding and causes warfarin resistance. Relying on binding interactions identified by cysteine alkylation footprinting and mass spectrometry coupled with mutagenesis analysis, we conducted structure simulations, which revealed a closed warfarin-binding pocket stabilized by the Cys51-Cys132 linkage. Understanding the selective warfarin inhibition of a specific redox state of hVKOR should enable the rational design of drugs that exploit the redox chemistry and associated conformational changes in hVKOR.

The functions of the A1A2A3 domains in von Willebrand factor include multimerin 1 binding.

Multimerin 1 (MMRN1) is a massive, homopolymeric protein that is stored in platelets and endothelial cells for activation-induced release. In vitro, MMRN1 binds to the outer surfaces of activated platelets and endothelial cells, the extracellular matrix (including collagen) and von Willebrand factor (VWF) to support platelet adhesive functions. VWF associates with MMRN1 at high shear, not static conditions, suggesting that shear exposes cryptic sites within VWF that support MMRN1 binding. Modified ELISA and surface plasmon resonance were used to study the structural features of VWF that support MMRN1 binding, and determine the affinities for VWF-MMRN1 binding. High shear microfluidic platelet adhesion assays determined the functional consequences for VWF-MMRN1 binding. VWF binding to MMRN1 was enhanced by shear exposure and ristocetin, and required VWF A1A2A3 region, specifically the A1 and A3 domains. VWF A1A2A3 bound to MMRN1 with a physiologically relevant binding affinity (KD: 2.0 ± 0.4 nM), whereas the individual VWF A1 (KD: 39.3 ± 7.7 nM) and A3 domains (KD: 229 ± 114 nM) bound to MMRN1 with lower affinities. VWF A1A2A3 was also sufficient to support the adhesion of resting platelets to MMRN1 at high shear, by a mechanism dependent on VWF-GPIbα binding. Our study provides new information on the molecular basis of MMRN1 binding to VWF, and its role in supporting platelet adhesion at high shear. We propose that at sites of vessel injury, MMRN1 that is released following activation of platelets and endothelial cells, binds to VWF A1A2A3 region to support platelet adhesion at arterial shear rates.

What's new in the diagnosis and pathophysiology of thrombotic thrombocytopenic purpura.

Severe ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) deficiency causes thrombotic thrombocytopenic purpura (TTP), which is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and the absence of oliguric or anuric renal failure. However, some patients with this constellation of findings do not have ADAMTS13 deficiency, and some patients with ADAMTS13 deficiency have renal failure or relatively normal blood counts. Consequently, many investigators and clinicians have incorporated severe ADAMTS13 deficiency into the case definition of TTP. This change has facilitated the timely initiation of treatment for patients with atypical clinical features who otherwise would not be recognized as having TTP. Conversely, excluding severe ADAMTS13 deficiency focuses attention on the diagnosis and treatment of other causes of thrombotic microangiopathy that require different treatment. The rapid return of ADAMTS13 data is important to make the best use of this information.

Atypical HUS may become a diagnosis of inclusion.

In this issue of Blood, Gavriilaki and colleagues describe an assay that could convert atypical hemolytic uremic syndrome (aHUS) from a diagnosis of exclusion into a direct pathophysiologic diagnosis.

Allosteric activation of ADAMTS13 by von Willebrand factor.

The metalloprotease ADAMTS13 cleaves von Willebrand factor (VWF) within endovascular platelet aggregates, and ADAMTS13 deficiency causes fatal microvascular thrombosis. The proximal metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains of ADAMTS13 recognize a cryptic site in VWF that is exposed by tensile force. Another seven T and two complement C1r/C1s, sea urchin epidermal growth factor, and bone morphogenetic protein (CUB) domains of uncertain function are C-terminal to the MDTCS domains. We find that the distal T8-CUB2 domains markedly inhibit substrate cleavage, and binding of VWF or monoclonal antibodies to distal ADAMTS13 domains relieves this autoinhibition. Small angle X-ray scattering data indicate that distal T-CUB domains interact with proximal MDTCS domains. Thus, ADAMTS13 is regulated by substrate-induced allosteric activation, which may optimize VWF cleavage under fluid shear stress in vivo. Distal domains of other ADAMTS proteases may have similar allosteric properties.

Single particle tracking of ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type-1 repeats) molecules on endothelial von Willebrand factor strings.

von Willebrand factor (VWF) strings are removed from the endothelial surface by ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type-1 repeats)-mediated proteolysis. To visualize how single ADAMTS13 molecules bind to these long strings, we built a customized single molecule fluorescence microscope and developed single particle tracking software. Extensive analysis of over 6,000 single inactive ADAMTS13(E225Q) enzymes demonstrated that 20% of these molecules could be detected in at least two consecutive 60-ms frames and followed two types of trajectories. ADAMTS13(E225Q) molecules either decelerated in the vicinity of VWF strings, whereas sometimes making brief contact with the VWF string before disappearing again, or readily bound to the VWF strings and this for 120 ms or longer. These interactions were observed at several sites along the strings. Control experiments using an IgG protein revealed that only the second type of trajectory reflected a specific interaction of ADAMTS13 with the VWF string. In conclusion, we developed a dedicated single molecule fluorescence microscope for detecting single ADAMTS13 molecules (nm scale) on their long, flow-stretched VWF substrates (µm scale) anchored on living cells. Comprehensive analysis of all detected enzymes showed a random interaction mechanism for ADAMTS13 with many available binding sites on the VWF strings.

Thrombin-targeted liposomes establish a sustained localized anticlotting barrier against acute thrombosis.

The goal of the present work was to design and test an acute-use nanoparticle-based antithrombotic agent that exhibits sustained local inhibition of thrombin without requiring a systemic anticoagulant effect to function against acute arterial thrombosis. To demonstrate proof of concept, we functionalized the surface of liposomes with multiple copies of the direct thrombin inhibitor, d-phenylalanyl-l-prolyl-l-arginyl-chloromethyl ketone (PPACK), which exhibits high affinity for thrombin as a free agent but manifests too rapid clearance in vivo to be effective alone. The PPACK-liposomes were formulated as single unilamellar vesicles, with a diameter of 170.78 ± 10.59 nm and a near neutral charge. In vitro models confirmed the inhibitory activity of PPACK-liposomes, demonstrating a KI' of 172.6 nM. In experimental clots in vitro, treatment of formed clots completely abrogated any further clotting upon exposure to human plasma. The liposomes were evaluated in vivo in a model of photochemical-induced carotid artery injury, resulting in significantly prolonged arterial occlusion time over that of controls (69.06 ± 5.65 min for saline treatment, N = 6, 71.33 ± 9.46 min for free PPACK treated; N = 4, 85.75 ± 18.24 min for precursor liposomes; N = 4, 139.75 ± 20.46 min for PPACK-liposomes; P = 0.0049, N = 6). Systemic anticoagulant profiles revealed a rapid return to control levels within 50 min, while still maintaining antithrombin activity at the injury site. The establishment of a potent and long-acting anticoagulant surface over a newly forming clot with the use of thrombin targeted nanoparticles that do not require systemic anticoagulation to be effective offers an alternative site-targeted approach to the management of acute thrombosis.

The B subunits of Shiga-like toxins induce regulated VWF secretion in a phospholipase D1-dependent manner.

Shiga toxin (Stx) causes diarrhea-associated hemolytic uremic syndrome by damaging renal microvascular endothelium. The pentameric B subunits of Stx types 1 and 2 (Stx1B and Stx2B) are sufficient to stimulate acute VWF secretion from endothelial cells, but Stx1B and Stx2B exert distinct effects on Ca(2+) and cAMP pathways. Therefore, we investigated other signaling components in StxB-induced VWF exocytosis. Incubation of HUVECs with StxB transiently increased phospholipase D (PLD) activity. Inhibition of PLD activity or shRNA-mediated PLD1 knockdown abolished StxB-induced VWF secretion. In addition, treatment with StxB triggered actin polymerization, enhanced endothelial monolayer permeability, and activated RhoA. PLD activation and VWF secretion induced by Stx1B were abolished on protein kinase Cα (PKCα) inhibition or gene silencing but were only moderately reduced by Rho or Rho kinase inhibitors. Conversely, PLD activation and VWF exocytosis induced by Stx2B were reduced by Rho/Rho kinase inhibitors and dominant-negative RhoA, whereas attenuation of PKCα did not affect either process. Another PLD1 activator, ADP-ribosylation factor 6, was involved in VWF secretion induced by Stx1B or Stx2B, but not histamine. These data indicate that Stx1B and Stx2B induce acute VWF secretion in a PLD1-dependent manner but do so by differentially modulating PKCα, RhoA, and ADP-ribosylation factor 6.

Rearranging exosites in noncatalytic domains can redirect the substrate specificity of ADAMTS proteases.

ADAMTS proteases typically employ some combination of ancillary C-terminal disintegrin-like, thrombospondin-1, cysteine-rich, and spacer domains to bind substrates and facilitate proteolysis by an N-terminal metalloprotease domain. We constructed chimeric proteases and substrates to examine the role of C-terminal domains of ADAMTS13 and ADAMTS5 in the recognition of their physiological cleavage sites in von Willebrand factor (VWF) and aggrecan, respectively. ADAMTS5 cleaves Glu(373)-Ala(374) and Glu(1480)-Gly(1481) bonds in bovine aggrecan but does not cleave VWF. Conversely, ADAMTS13 cleaves the Tyr(1605)-Met(1606) bond of VWF, which is exposed by fluid shear stress but cannot cleave aggrecan. Replacing the thrombospondin-1/cysteine-rich/spacer domains of ADAMTS5 with those of ADAMTS13 conferred the ability to cleave the Glu(1615)-Ile(1616) bond of VWF domain A2 in peptide substrates or VWF multimers that had been sheared; native (unsheared) VWF multimers were resistant. Thus, by recombining exosites, we engineered ADAMTS5 to cleave a new bond in VWF, preserving physiological regulation by fluid shear stress. The results demonstrate that noncatalytic thrombospondin-1/cysteine-rich/spacer domains are principal modifiers of substrate recognition and cleavage by both ADAMTS5 and ADAMTS13. Noncatalytic domains may perform similar functions in other ADAMTS family members.

Shiga toxin (Stx)1B and Stx2B induce von Willebrand factor secretion from human umbilical vein endothelial cells through different signaling pathways.

Diarrhea-associated hemolytic uremic syndrome (D(+)HUS) is caused by the ingestion of Escherichia coli that produce Shiga toxin (Stx), which is composed of a cytotoxic A subunit and pentameric B subunits that bind globotriaosylceramide on susceptible cells. Stx occurs in 2 types, Stx1 and Stx2. B subunits of either type stimulate von Willebrand factor (VWF) secretion from human umbilical vein endothelial cells (HUVECs), and Stx2B can cause thrombotic microangiopathy in Adamts13(-/-) mice. We have now determined that Stx1B and Stx2B activate different signaling pathways in HUVECs. VWF secretion induced by Stx1B is associated with a transient rise in intracellular Ca(2+) level that is blocked by chelation with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester, removal of extracellular Ca(2+), the phospholipase C inhibitor U73122, the protein kinase inhibitor staurosporine, or small interfering RNA knockdown of protein kinase Cα. In contrast, Stx2B-induced VWF secretion is associated with activation of protein kinase A (PKA) and is blocked by the PKA inhibitor H89 or small interfering RNA knockdown of PKA. Stx2B does not increase cAMP levels and may activate PKA by a cAMP-independent mechanism. The activation of distinct signaling pathways may be relevant to understanding why E coli that express Stx2 are more likely to cause D(+)HUS than are E coli expressing only Stx1.

Phylogenetic and functional analysis of histidine residues essential for pH-dependent multimerization of von Willebrand factor.

von Willebrand factor (VWF) is a multimeric plasma protein that mediates platelet adhesion to sites of vascular injury. The hemostatic function of VWF depends upon the formation of disulfide-linked multimers, which requires the VWF propeptide (D1D2 domains) and adjacent D'D3 domains. VWF multimer assembly occurs in the trans-Golgi at pH ~ 6.2 but not at pH 7.4, which suggests that protonation of one or more His residues (pK(a) ~6.0) mediates the pH dependence of multimerization. Alignment of 30 vertebrate VWF sequences identified 13 highly conserved His residues in the D1D2D'D3 domains, and His-to-Ala mutagenesis identified His³95 and His46° in the D2 domain as critical for VWF multimerization. Replacement of His³95 with Lys or Arg prevented multimer assembly, suggesting that reversible protonation of this His residue is essential. In contrast, replacement of His46° with Lys or Arg preserved normal multimer assembly, whereas Leu, Met, and Gln did not, indicating that the function of His46° depends primarily upon the presence of a positive charge. These results suggest that pH sensing by evolutionarily conserved His residues facilitates the assembly and packaging of VWF multimers upon arrival in the trans-Golgi.

Functional architecture of Weibel-Palade bodies.

Weibel-Palade bodies (WPBs) are elongated secretory organelles specific to endothelial cells that contain von Willebrand factor (VWF) and a variety of other proteins that contribute to inflammation, angiogenesis, and tissue repair. The remarkable architecture of WPBs is because of the unique properties of their major constituent VWF. VWF is stored inside WPBs as tubules, but on its release, forms strikingly long strings that arrest bleeding by recruiting blood platelets to sites of vascular injury. In recent years considerable progress has been made regarding the molecular events that underlie the packaging of VWF multimers into tubules and the processes leading to the formation of elongated WPBs. Mechanisms directing the conversion of tightly packaged VWF tubules into VWF strings on the surface of endothelial cells are starting to be unraveled. Several modes of exocytosis have now been described for WPBs, emphasizing the plasticity of these organelles. WPB exocytosis plays a role in the pathophysiology and treatment of von Willebrand disease and may have impact on common hematologic and cardiovascular disorders. This review summarizes the major advances made on the biogenesis and exocytosis of WPBs and places these recent discoveries in the context of von Willebrand disease.