Composition of thrombi in zebrafish: similarities and distinctions with mammals

BACKGROUND

Blood clots are primarily composed of red blood cells (RBCs), platelets/thrombocytes, and fibrin. Despite the similarities observed between mammals and zebrafish, the composition of fish thrombi is not as well known.

OBJECTIVES

To analyze the formation of zebrafish blood clots ex vivo and arterial and venous thrombi in vivo.

METHODS

Transgenic zebrafish lines and laser-mediated endothelial injury were used to determine the relative ratio of RBCs and thrombocytes in clots. Scanning electron and confocal microscopy provided high-resolution images of the structure of adult and larval clots. Adult and larval thrombocyte spreading on fibrinogen was evaluated ex vivo.

RESULTS

RBCs were present in arterial and venous thrombi, making up the majority of cells in both circulations. However, bloodless mutant fish demonstrated that fibrin clots can form in vivo in the absence of blood cells. Scanning electron and confocal microscopy showed that larval and adult zebrafish thrombi and mammalian thrombi look surprisingly similar externally and internally, even though the former have nucleated RBCs and thrombocytes. Although adult thrombocytes spread on fibrinogen, we found that larval cells do not fully activate without the addition of plasma from adult fish, suggesting a developmental deficiency of a plasma activating factor. Finally, mutants lacking αIIbβ3 demonstrated that this integrin mediates thrombocyte spreading on fibrinogen.

CONCLUSION

Our data showed strong conservation of arterial and venous and clot/thrombus formation across species, including developmental regulation of thrombocyte function. This correlation supports the possibility that mammals also do not absolutely require circulating cells to form fibrin clots in vivo.

A novel technique to quantify the kinetics of blood clot contraction based on the expulsion of fluorescently labeled albumin into serum

BACKGROUND

The platelet-driven contraction or retraction of blood clots has been utilized to obtain blood serum for laboratory studies, but now, in vitro clot contraction assays are used in research laboratories and clinics to assess platelet functionality. The static final extent of clot contraction measured using a clot size or expelled serum volume can be supplemented substantially with a dynamic analysis.

OBJECTIVES

To provide a step-by-step protocol for a relatively simple and affordable new automated methodology to follow the kinetics of blood clot contraction, which allows for simultaneous measurements of various samples at a time and requires only a fluorescence plate reader.

METHODS

The kinetics of clot contraction in whole blood was assessed by continuously detecting the fluorescence intensity of fluorescein isothiocyanate-albumin added to a blood sample before clotting and expelled into the serum during clot shrinkage.

RESULTS

The clots are formed and fluorescence is measured in the wells of a black multiwell plate using a standard plate fluorescent reader. The specificity of this technique for clot contraction has been demonstrated by the strong inhibitory effects of blebbistatin, latrunculin A, and abciximab. To validate the new technique, increased fluorescence intensity in the contracting clots was measured in parallel with a visual decrease in clot size performed with the same blood samples.

CONCLUSION

The resulting clot contraction dynamics based on the expulsion of fluorescein isothiocyanate-albumin can be quantified using a number of kinetic parameters as well as a phase kinetics analysis. The advantages and drawbacks of the new technique are discussed.

Megakaryocyte-induced contraction of plasma clots: cellular mechanisms and structural mechanobiology

ABSTRACT

Nonmuscle cell contractility is an essential feature underlying diverse cellular processes such as motility, morphogenesis, division and genome replication, intracellular transport, and secretion. Blood clot contraction is a well-studied process driven by contracting platelets. Megakaryocytes (MKs), which are the precursors to platelets, can be found in bone marrow and lungs. Although they express many of the same proteins and structures found in platelets, little is known about their ability to engage with extracellular proteins such as fibrin and contract. Here, we have measured the ability of MKs to compress plasma clots. Megakaryocytes derived from human induced pluripotent stem cells (iPSCs) were suspended in human platelet-free blood plasma and stimulated with thrombin. Using real-time macroscale optical tracking, confocal microscopy, and biomechanical measurements, we found that activated iPSC-derived MKs (iMKs) caused macroscopic volumetric clot shrinkage, as well as densification and stiffening of the fibrin network via fibrin-attached plasma membrane protrusions undergoing extension-retraction cycles that cause shortening and bending of fibrin fibers. Contraction induced by iMKs involved 2 kinetic phases with distinct rates and durations. It was suppressed by inhibitors of nonmuscle myosin IIA, actin polymerization, and integrin αIIbβ3-fibrin interactions, indicating that the molecular mechanisms of iMK contractility were similar or identical to those in activated platelets. Our findings provide new insights into MK biomechanics and suggest that iMKs can be used as a model system to study platelet contractility. Physiologically, the ability of MKs to contract plasma clots may play a role in the mechanical remodeling of intravascular blood clots and thrombi.

Biomechanics, Energetics, and Structural Basis of Rupture of Fibrin Networks

Fibrin provides the main structural integrity and mechanical strength to blood clots. Failure of fibrin clots can result in life-threating complications, such as stroke or pulmonary embolism. The dependence of rupture resistance of fibrin networks (uncracked and cracked) on fibrin(ogen) concentrations in the (patho)physiological 1-5 g L-1 range is explored by performing the ultrastructural studies and theoretical analysis of the experimental stress-strain profiles available from mechanical tensile loading assays. Fibrin fibers in the uncracked network stretched evenly, whereas, in the cracked network, fibers around the crack tip showed greater deformation. Unlike fibrin fibers in cracked networks formed at the lower 1-2.7 g L-1 fibrinogen concentrations, fibers formed at the higher 2.7-5 g L-1 concentrations align and stretch simultaneously. Cracked fibrin networks formed in higher fibrinogen solutions are tougher yet less extensible. Statistical modeling revealed that the characteristic strain for fiber alignment, crack size, and fracture toughness of fibrin networks control their rupture resistance. The results obtained provide a structural and biomechanical basis to quantitatively understand the material properties of blood plasma clots and to illuminate the mechanisms of their rupture.

Activation of Piezo1 channels in compressed red blood cells augments platelet-driven contraction of blood clots

BACKGROUND

Piezo1 is a mechanosensitive cationic channel that boosts intracellular [Ca2+]i. Compression of red blood cells (RBCs) during platelet-driven contraction of blood clots may cause the activation of Piezo1.

OBJECTIVES

To establish relationships between Piezo1 activity and blood clot contraction.

METHODS

Effects of a Piezo1 agonist, Yoda1, and antagonist, GsMTx-4, on clot contraction in vitro were studied in human blood containing physiological [Ca2+]. Clot contraction was induced by exogenous thrombin. Activation of Piezo1 was assessed by Ca2+ influx in RBCs and with other functional and morphologic features.

RESULTS

Piezo1 channels in compressed RBCs are activated naturally during blood clot contraction and induce an upsurge in the intracellular [Ca2+]i, followed by phosphatidylserine exposure. Adding the Piezo1 agonist Yoda1 to whole blood increased the extent of clot contraction due to Ca2+-dependent volumetric shrinkage of RBCs and increased platelet contractility due to their hyperactivation by the enhanced generation of endogenous thrombin on activated RBCs. Addition of rivaroxaban, the inhibitor of thrombin formation, or elimination of Ca2+ from the extracellular space abrogated the stimulating effect of Yoda1 on clot contraction. The Piezo1 antagonist, GsMTx-4, caused a decrease in the extent of clot contraction relative to the control both in whole blood and in platelet-rich plasma. Activated Piezo1 in compressed and deformed RBCs amplified the platelet contractility as a positive feedback mechanism during clot contraction.

CONCLUSION

The results obtained demonstrate that the Piezo1 channel expressed on RBCs comprises a mechanochemical modulator of blood clotting that may be considered a potential therapeutic target to correct hemostatic disorders.

Combined computational modeling and experimental study of the biomechanical mechanisms of platelet-driven contraction of fibrin clots

While blood clot formation has been relatively well studied, little is known about the mechanisms underlying the subsequent structural and mechanical clot remodeling called contraction or retraction. Impairment of the clot contraction process is associated with both life-threatening bleeding and thrombotic conditions, such as ischemic stroke, venous thromboembolism, and others. Recently, blood clot contraction was observed to be hindered in patients with COVID-19. A three-dimensional multiscale computational model is developed and used to quantify biomechanical mechanisms of the kinetics of clot contraction driven by platelet-fibrin pulling interactions. These results provide important biological insights into contraction of platelet filopodia, the mechanically active thin protrusions of the plasma membrane, described previously as performing mostly a sensory function. The biomechanical mechanisms and modeling approach described can potentially apply to studying other systems in which cells are embedded in a filamentous network and exert forces on the extracellular matrix modulated by the substrate stiffness.

Fracture toughness of fibrin gels as a function of protein volume fraction: Mechanical origins

The mechanical stability of blood clots necessary for their functions is provided by fibrin, a fibrous gel. Rupture of clots leads to life-threatening thrombotic embolization, which is little understood. Here, we combine experiments and simulations to determine the toughness of plasma clots as a function of fibrin content and correlate toughness with fibrin network structure characterized by confocal and scanning electron microscopy. We develop fibrin constitutive laws that scale with fibrin concentration and capture the force-stretch response of cracked clot specimens using only a few material parameters. Toughness is calculated from the path-independent J* integral that includes dissipative effects due to fluid flow and uses only the constitutive model and overall stretch at crack propagation as input. We show that internal fluid motion, which is not directly measurable, contributes significantly to clot toughness, with its effect increasing as fibrin content increases, because the reduced gel porosity at higher density results in greater expense of energy in fluid motion. Increasing fibrin content (1→10mg/mL) results in a significant increase in clot toughness (3→15 N/m) in accordance with a power law relation reminiscent of cellular solids and elastomeric gels. These results provide a basis for understanding and predicting the tendency for thrombotic embolization. STATEMENT OF SIGNIFICANCE: Fibrin, a naturally occurring biomaterial, is the major determinant of the structural and mechanical integrity of blood clots. We determined that increasing the fibrin content in clots, as in some thrombi and fibrin-based anti-bleeding sealants, results in an increase in clot toughness. Toughness corresponds to the ability to resist rupturing in the presence of a defect. We couple bulk mechanical testing, microstructural measurements, and finite element modeling to capture the force-stretch response of fibrin clots and compute toughness. We show that increased fibrin content in clots reduces porosity and limits fluid motion and that fluid motion drastically alters the clot toughness. These results provide a fundamental understanding of blood clot rupture and could help in rational design of fibrin-containing biomaterials.

Blood clot contraction: Mechanisms, pathophysiology, and disease

A State of the Art lecture titled "Blood Clot Contraction: Mechanisms, Pathophysiology, and Disease" was presented at the International Society on Thrombosis and Haemostasis (ISTH) Congress in 2022. This was a systematic description of blood clot contraction or retraction, driven by activated platelets and causing compaction of the fibrin network along with compression of the embedded erythrocytes. The consequences of clot contraction include redistribution of the fibrin-platelet meshwork toward the periphery of the clot and condensation of erythrocytes in the core, followed by their deformation from the biconcave shape into polyhedral cells (polyhedrocytes). These structural signatures of contraction have been found in ex vivo thrombi derived from various locations, which indicated that clots undergo intravital contraction within the blood vessels. In hemostatic clots, tightly packed polyhedrocytes make a nearly impermeable seal that stems bleeding and is impaired in hemorrhagic disorders. In thrombosis, contraction facilitates the local blood flow by decreasing thrombus obstructiveness, reducing permeability, and changing susceptibility to fibrinolytic enzymes. However, in (pro)thrombotic conditions, continuous background platelet activation is followed by platelet exhaustion, refractoriness, and impaired intravital clot contraction, which is associated with weaker thrombi predisposed to embolization. Therefore, assays that detect imperfect in vitro clot contraction have potential diagnostic and prognostic values for imminent or ongoing thrombosis and thrombotic embolism. Collectively, the contraction of blood clots and thrombi is an underappreciated and understudied process that has a pathogenic and clinical significance in bleeding and thrombosis of various etiologies. Finally, we have summarized relevant new data on this topic presented during the 2022 ISTH Congress.

Biomechanical origins of inherent tension in fibrin networks

Blood clots form at the site of vascular injury to seal the wound and prevent bleeding. Clots are in tension as they perform their biological functions and withstand hydrodynamic forces of blood flow, vessel wall fluctuations, extravascular muscle contraction and other forces. There are several mechanisms that generate tension in a blood clot, of which the most well-known is the contraction/retraction caused by activated platelets. Here we show through experiments and modeling that clot tension is generated by the polymerization of fibrin. Our mathematical model is built on the hypothesis that the shape of fibrin monomers having two-fold symmetry and off-axis binding sites is ultimately the source of inherent tension in individual fibers and the clot. As the diameter of a fiber grows during polymerization the fibrin monomers must suffer axial twisting deformation so that they remain in register to form the half-staggered arrangement characteristic of fibrin protofibrils. This deformation results in a pre-strain that causes fiber and network tension. Our results for the pre-strain in single fibrin fibers is in agreement with experiments that measured it by cutting fibers and measuring their relaxed length. We connect the mechanics of a fiber to that of the network using the 8-chain model of polymer elasticity. By combining this with a continuum model of swellable elastomers we can compute the evolution of tension in a constrained fibrin gel. The temporal evolution and tensile stresses predicted by this model are in qualitative agreement with experimental measurements of the inherent tension of fibrin clots polymerized between two fixed rheometer plates. These experiments also revealed that increasing thrombin concentration leads to increasing internal tension in the fibrin network. Our model may be extended to account for other mechanisms that generate pre-strains in individual fibers and cause tension in three-dimensional proteinaceous polymeric networks.

Contribution of septins to human platelet structure and function

Although septins have been well-studied in nucleated cells, their role in anucleate blood platelets remains obscure. Here, we elucidate the contribution of septins to human platelet structure and functionality. We show that Septin-2 and Septin-9 are predominantly distributed at the periphery of resting platelets and co-localize strongly with microtubules. Activation of platelets by thrombin causes clustering of septins and impairs their association with microtubules. Inhibition of septin dynamics with forchlorfenuron (FCF) reduces thrombin-induced densification of septins and lessens their colocalization with microtubules in resting and activated platelets. Exposure to FCF alters platelet shape, suggesting that septins stabilize platelet cytoskeleton. FCF suppresses platelet integrin αIIbβ3 activation, promotes phosphatidylserine exposure on activated platelets, and induces P-selectin expression on resting platelets, suggesting septin involvement in these processes. Inhibition of septin dynamics substantially reduces platelet contractility and abrogates their spreading on fibrinogen-coated surfaces. Overall, septins strongly contribute to platelet structure, activation and biomechanics.

Illustrated State-of-the-Art Capsules of the ISTH 2022 Congress

The ISTH London 2022 Congress is the first held (mostly) face-to-face again since the COVID-19 pandemic took the world by surprise in 2020. For 2 years we met virtually, but this year's in-person format will allow the ever-so-important and quintessential creativity and networking to flow again. What a pleasure and joy to be able to see everyone! Importantly, all conference proceedings are also streamed (and available recorded) online for those unable to travel on this occasion. This ensures no one misses out. The 2022 scientific program highlights new developments in hemophilia and its treatment, acquired and other inherited bleeding disorders, thromboinflammation, platelets and coagulation, clot structure and composition, fibrinolysis, vascular biology, venous thromboembolism, women's health, arterial thrombosis, pediatrics, COVID-related thrombosis, vaccine-induced thrombocytopenia with thrombosis, and omics and diagnostics. These areas are elegantly reviewed in this Illustrated Review article. The Illustrated Review is a highlight of the ISTH Congress. The format lends itself very well to explaining the science, and the collection of beautiful graphical summaries of recent developments in the field are stunning and self-explanatory. This clever and effective way to communicate research is revolutionary and different from traditional formats. We hope you enjoy this article and will be inspired by its content to generate new research ideas.

Pathology of lung-specific thrombosis and inflammation in COVID-19

BACKGROUND

Infection by SARS-CoV-2 produces significant pulmonary pathology including endothelial damage with resultant thrombotic events. While pathologic features were described, there are limited data on the relationship of these changes to the inflammatory response and the production of thromboses.

OBJECTIVE

To investigate pathology of COVID-19-related immunothrombosis.

PATIENTS/METHODS

Tissue samples from lung, kidney, brain and heart that were collected from 45 patients who died of COVID-19. Histopathological examination was performed after H&E and Picro-Mallory staining in combination with (immuno)fluorescence to visualize neutrophil extracellular traps. Ultrastructural alterations in lungs were studied with scanning and transmission electron microscopy.

RESULTS

Inflammatory changes and thrombosis were substantially more pronounced in the lung than in the kidney, heart, and brain. The most common pathologic finding was diffuse alveolar damage. In addition, most lung samples showed thrombi in vessels. The cause of death in single cases was massive pulmonary embolism. Ultrastructural examination revealed neutrophils attached to endothelium, perhaps as a step towards transendothelial migration. In addition, platelets were identified in the midst of fibrin as individual procoagulant balloon-like cells. Ultrastructural examination demonstrated numerous virion-like particles.

CONCLUSIONS

Studying (ultra)structural features of the autopsy lung samples from patients with COVID-19 has provided evidence for a pathogenic link between inflammation and thrombosis. The major features in the lungs of COVID-19 patients comprised primary inflammatory thrombosis associated with diffuse alveolar damage. The lungs had pronounced circulatory changes with inflammation-dependent intravascular blood clotting, whereas heart, brain, and kidneys had predominantly degenerative changes that were distinct from the lung pathology.

Peculiarities of blood coagulation disorders in patients with COVID-19

Aim. To study the relationship of hemostatic disorders with inflammation and estimate their role in the course and outcomes of COVID-19. Materials and methods. We examined 215 consecutive patients with moderate and severe forms of acute COVID-19. The patients were on anticoagulants and immunosuppressive drugs. Hemostasis was assessed using the thrombodynamics assay, thromboelastography, fibrinogen and D-dimer levels, prothrombin time, and soluble fibrin-monomer complexes (ethanol gelation test). The hemostatic parameters were correlated with hematological and biochemical tests, including markers of inflammation (C-reactive protein, interleukins 6 and 8), as well as with the disease severity and outcomes. Results. Laboratory signs of coagulopathy were revealed in the vast majority of the cases. Despite the use of low-molecular-weight heparins in the prophylactic and therapeutic doses, coagulopathy in COVID-19 manifested predominantly as hypercoagulability that correlated directly with the systemic inflammation and metabolic changes due to liver and kidney dysfunction. A direct relationship was found between the grade of coagulopathy and the severity of COVID-19, including comorbidities and the mortality. The chronometric hypocoagulability observed in about 1/4 cases was associated with a high level of C-reactive protein, which may decelerate coagulation in vitro and thereby mask the true inflammatory thrombophilia. Persistent hyperfibrinogenemia and high D-dimer in the absence of consumption coagulopathy suggest the predominance of local and/or regional microthrombosis over disseminated intravascular coagulation. Conclusion. The results obtained substantiate the need for laboratory monitoring of hemostasis and active prophylaxis and treatment of thrombotic complications in COVID-19.

Automated Fiber Diameter and Porosity Measurements of Plasma Clots in Scanning Electron Microscopy Images

Scanning Electron Microscopy (SEM) is a powerful, high-resolution imaging technique widely used to analyze the structure of fibrin networks. Currently, structural features, such as fiber diameter, length, density, and porosity, are mostly analyzed manually, which is tedious and may introduce user bias. A reliable, automated structural image analysis method would mitigate these drawbacks. We evaluated the performance of DiameterJ (an ImageJ plug-in) for analyzing fibrin fiber diameter by comparing automated DiameterJ outputs with manual diameter measurements in four SEM data sets with different imaging parameters. We also investigated correlations between biophysical fibrin clot properties and diameter, and between clot permeability and DiameterJ-determined clot porosity. Several of the 24 DiameterJ algorithms returned diameter values that highly correlated with and closely matched the values of the manual measurements. However, optimal performance was dependent on the pixel size of the images—best results were obtained for images with a pixel size of 8–10 nm (13–16 pixels/fiber). Larger or smaller pixels resulted in an over- or underestimation of diameter values, respectively. The correlation between clot permeability and DiameterJ-determined clot porosity was modest, likely because it is difficult to establish the correct image depth of field in this analysis. In conclusion, several DiameterJ algorithms (M6, M5, T3) perform well for diameter determination from SEM images, given the appropriate imaging conditions (13–16 pixels/fiber). Determining fibrin clot porosity via DiameterJ is challenging.

Strength and deformability of fibrin clots: Biomechanics, thermodynamics, and mechanisms of rupture

Fibrin is the major determinant of the mechanical stability and integrity of blood clots and thrombi. To explore the rupture of blood clots, emulating thrombus breakage, we stretched fibrin gels with single-edge cracks of varying size. Ultrastructural alterations of the fibrin network correlated with three regimes of stress vs. strain profiles: the weakly non-linear regime due to alignment of fibrin fibers; linear regime owing to further alignment and stretching of fibers; and the rupture regime for large deformations reaching the critical strain and stress, at which irreversible breakage of fibers ahead of the crack tip occurs. To interpret the stress-strain curves, we developed a new Fluctuating Spring model, which maps the fibrin alignment at the characteristic strain, network stretching with the Young modulus, and simultaneous cooperative rupture of coupled fibrin fibers into a theoretical framework to obtain the closed-form expressions for the strain-dependent stress profiles. Cracks render network rupture stochastic, and the free energy change for fiber deformation and rupture decreases with the crack length, making network rupture more spontaneous. By contrast, mechanical cooperativity due to the presence of inter-fiber contacts strengthens fibrin networks. The results obtained provide a fundamental understanding of blood clot breakage that underlies thrombotic embolization. STATEMENT OF SIGNIFICANCE: Fibrin, a naturally occurring biomaterial, is the major determinant of mechanical stability and integrity of blood clots and obstructive thrombi. We tested mechanically fibrin gels with single-edge cracks and followed ultrastructural alterations of the fibrin network. Rupture of fibrin gel involves initial alignment and elastic stretching of fibers followed by their eventual rupture for deformations reaching the critical level. To interpret the stress-strain curves, we developed Fluctuating Spring model, which showed that cracks render rupture of fibrin networks more spontaneous; yet, coupled fibrin fibers reinforce cracked fibrin networks. The results obtained provide fundamental understanding of blood clot breakage that underlies thrombotic embolization. Fluctuating Spring model can be applied to other protein networks with cracks and to interpret the stress-strain profiles.

Altered platelet and coagulation function in moderate-to-severe COVID-19

To reveal if coagulopathies relate to the course of COVID-19, we examined 255 patients with moderate and severe COVID-19, receiving anticoagulants and immunosuppressive drugs. Coagulopathy manifested predominantly as hypercoagulability that correlated directly with systemic inflammation, disease severity, comorbidities, and mortality risk. The prolonged clotting tests in about ¼ of cases were associated with high levels of C-reactive protein and antiphospholipid antibodies, which impeded coagulation in vitro. Contraction of blood clots was hindered in about ½ of patients, especially in severe and fatal cases, and correlated directly with prothrombotic parameters. A decrease in platelet contractility was due to moderate thrombocytopenia in combination with platelet dysfunction. Clots with impaired contraction were porous, had a low content of compressed polyhedral erythrocytes (polyhedrocytes) and an even distribution of fibrin, suggesting that the uncompacted intravital clots are more obstructive but patients could also be prone to bleeding. The absence of consumption coagulopathy suggests the predominance of local and/or regional microthrombosis rather than disseminated intravascular coagulation. The results obtained (i) confirm the importance of hemostatic disorders in COVID-19 and their relation to systemic inflammation; (ii) justify monitoring of hemostasis, including the kinetics of blood clot contraction; (iii) substantiate the active prophylaxis of thrombotic complications in COVID-19.

Pathologically stiff erythrocytes impede contraction of blood clots

BACKGROUND

Blood clot contraction, volume shrinkage of the clot, is driven by platelet contraction and accompanied by compaction of the erythrocytes and their gradual shape change from biconcave to polyhedral, with the resulting cells named polyhedrocytes.

OBJECTIVES

Here, we examined the role of erythrocyte rigidity on clot contraction and erythrocyte shape transformation.

METHODS

We used an optical tracking methodology that allowed us to quantify changes in contracting clot size over time.

RESULTS AND CONCLUSIONS

Erythrocyte rigidity has been shown to be increased in sickle cell disease (SCD), and in our experiments erythrocytes from SCD patients were 4-fold stiffer than those from healthy subjects. On average, the final extent of clot contraction was reduced by 53% in the clots from the blood of patients with SCD compared to healthy individuals, and there was significantly less polyhedrocyte formation. To test if this reduction in clot contraction was due to the increase in erythrocyte rigidity, we used stiffening of erythrocytes via chemical cross-linking (glutaraldehyde), rigidifying Wrightb antibodies (Wrb ), and naturally more rigid llama ovalocytes. Results revealed that stiffening erythrocytes result in impaired clot contraction and fewer polyhedrocytes. These results demonstrate the role of erythrocyte rigidity in the contraction of blood clots and suggest that the impaired clot contraction/shrinkage in SCD is due to the reduced erythrocyte deformability, which may be an underappreciated mechanism that aggravates obstructiveness of erythrocyte-rich (micro)thrombi in SCD.

Visualization of Platelet Integrins via Two-Photon Microscopy Using Anti-transmembrane Domain Peptides Containing a Blue Fluorescent Amino Acid

The fluorescent reporters commonly used to visualize proteins can perturb both protein structure and function. Recently, we found that 4-cyanotryptophan (4CN-Trp), a blue fluorescent amino acid, is suitable for one-photon imaging applications. Here, we demonstrate its utility in two-photon fluorescence microscopy by using it to image integrins on cell surfaces. Specifically, we used solid-phase peptide synthesis to generate CHAMP peptides labeled with 4-cyanoindole (4CNI) at their N-termini to image integrins on cell surfaces. CHAMP (computed helical anti-membrane protein) peptides spontaneously insert into membrane bilayers to target integrin transmembrane domains and cause integrin activation. We found that 4CNI labeling did not perturb the ability of CHAMP peptides to insert into membranes, bind to integrins, or cause integrin activation. We then used two-photon fluorescence microscopy to image 4CNI-containing integrins on the surface of platelets. Compared to a 4CNI-labeled scrambled peptide that uniformly decorated cell surfaces, 4CNI-labeled CHAMP peptides were present in discrete blue foci. To confirm that these foci represented CN peptide-containing integrins, we co-stained platelets with integrin-specific fluorescent monoclonal antibodies and found that CN peptide and antibody fluorescence coincided. Because 4CNI can readily be biosynthetically incorporated into proteins with little if any effect on protein structure and function, it provides a facile way to directly monitor protein behavior and protein-protein interactions in cellular environments. In addition, these results clearly demonstrate that the two-photon excitation cross section of 4CN-Trp is sufficiently large to make it a useful two-photon fluorescence reporter for biological applications.

Visualizing thrombosis to improve thrombus resolution

The severity, course, and outcomes of thrombosis are determined mainly by the size and location of the thrombus, but studying thrombus structure and composition has been an important but challenging task. The substantial progress in determination of thrombus morphology has become possible due to new intravital imaging methodologies in combination with mechanical thrombectomy, which allows extraction of a fresh thrombus from a patient followed by microscopy. Thrombi have been found to contain various structural forms of fibrin along with platelet aggregates, leukocytes, and red blood cells, many of which acquire a polyhedral shape (polyhedrocytes) as a result of intravital platelet-driven contraction. The relative volume fractions of thrombus components and their structural forms vary substantially, depending on the clinical and pathogenic characteristics. This review summarizes recent research that describes quantitative and qualitative morphologic characteristics of arterial and venous thrombi that are relevant for the pathogenesis, prophylaxis, diagnosis, and treatment of thrombosis.

Accelerated Spatial Fibrin Growth and Impaired Contraction of Blood Clots in Patients with Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disease associated with thrombotic complications. To elucidate pathogenic mechanisms, hemostatic disorders in RA were correlated with other laboratory and clinical manifestations. Hemostasis was assessed using relatively new complementary tests, the spatial growth of a plasma clot (Thrombodynamics assay), and contraction of whole blood clots. Platelet functionality was assessed with flow cytometry that quantified the expression of P-selectin and the fibrinogen-binding capacity of platelets before and after activation with a thrombin receptor-activating peptide. Parameters of fibrin clot growth and the kinetics of contraction of blood clots were significantly altered in patients with RA compared to the control group. In Thrombodynamics measurements, an increase in the clot growth rate, size, and optical density of plasma clots altogether indicated chronic hypercoagulability. The rate and extent of blood clot contraction in patients with RA was significantly reduced and associated with platelet dysfunction revealed by an impaired response to activation. Changes in the parameters of clot growth and contraction correlated with the laboratory signs of systemic inflammation, including hyperfibrinogenemia. These results confirm the pathogenic role of hemostatic disorders in RA and support the validity of fibrin clot growth and the blood clot contraction assay as indicators of a (pro)thrombotic state.

Quantitative Morphology of Cerebral Thrombi Related to Intravital Contraction and Clinical Features of Ischemic Stroke

BACKGROUND AND PURPOSE

The purpose was to assess quantitatively and qualitatively the composition and structure of cerebral thrombi and correlate them with the signs of intravital clot contraction (retraction), as well as with etiology, severity, duration, and outcomes of acute ischemic stroke.

METHODS

We quantified high-resolution scanning electron micrographs of 41 cerebral thrombi for their detailed cellular and noncellular composition and analyzed histological images for the overall structure with the emphasis on red blood cell compression, fibrin age, and the signs of inflammation.

RESULTS

Cerebral thrombi were quite compact and had extremely low porosity. The prevailing cell type was polyhedral compressed erythrocytes (polyhedrocytes) in the core, and fibrin-platelet aggregates were concentrated at the periphery; both findings are indicative of intravital contraction of the thrombi. The content of polyhedrocytes directly correlated with the stroke severity. The prevalence of fibrin bundles was typical for more severe cases, while the content of fibrin sponge prevailed in cases with a more favorable course. The overall platelet content in cerebral thrombi was surprisingly small, while the higher content of platelet aggregates was a marker of stroke severity. Fibrillar types of fibrin prevailed in atherothrombogenic thrombi. Older fibrin prevailed in thrombi from the patients who received thrombolytics, and younger fibrin dominated in cardioembolic thrombi. Alternating layers of erythrocytes and fibrin mixed with platelets were common for thrombi from the patients with more favorable outcomes. Thrombi with a higher number of leukocytes were associated with fatal cases.

CONCLUSIONS

Most cerebral thrombi undergo intravital clot contraction (retraction) that may be of underestimated clinical importance. Despite the high variability of the composition and structure of cerebral thrombi, the content of certain types of blood cells and fibrin structures combined with the morphological signs of intravital contraction correlate with the clinical course and outcomes of acute ischemic stroke.

Molecular packing structure of fibrin fibers resolved by X-ray scattering and molecular modeling

Fibrin is the major extracellular component of blood clots and a proteinaceous hydrogel used as a versatile biomaterial. Fibrin forms branched networks built of laterally associated double-stranded protofibrils. This multiscale hierarchical structure is crucial for the extraordinary mechanical resilience of blood clots, yet the structural basis of clot mechanical properties remains largely unclear due, in part, to the unresolved molecular packing of fibrin fibers. Here the packing structure of fibrin fibers is quantitatively assessed by combining Small Angle X-ray Scattering (SAXS) measurements of fibrin reconstituted under a wide range of conditions with computational molecular modeling of fibrin protofibrils. The number, positions, and intensities of the Bragg peaks observed in the SAXS experiments were reproduced computationally based on the all-atom molecular structure of reconstructed fibrin protofibrils. Specifically, the model correctly predicts the intensities of the reflections of the 22.5 nm axial repeat, corresponding to the half-staggered longitudinal arrangement of fibrin molecules. In addition, the SAXS measurements showed that protofibrils within fibrin fibers have a partially ordered lateral arrangement with a characteristic transverse repeat distance of 13 nm, irrespective of the fiber thickness. These findings provide fundamental insights into the molecular structure of fibrin clots that underlies their biological and physical properties.

Rupture of blood clots: Mechanics and pathophysiology

Fibrin is the three-dimensional mechanical scaffold of protective blood clots that stop bleeding and pathological thrombi that obstruct blood vessels. Fibrin must be mechanically tough to withstand rupture, after which life-threatening pieces (thrombotic emboli) are carried downstream by blood flow. Despite multiple studies on fibrin viscoelasticity, mechanisms of fibrin rupture remain unknown. Here, we examined mechanically and structurally the strain-driven rupture of human blood plasma-derived fibrin clots where clotting was triggered with tissue factor. Toughness, i.e., resistance to rupture, quantified by the critical energy release rate (a measure of the propensity for clot embolization) of physiologically relevant fibrin gels was determined to be 7.6 ± 0.45 J/m2. Finite element (FE) simulations using fibrin material models that account for forced protein unfolding independently supported this measured toughness and showed that breaking of fibers ahead the crack at a critical stretch is the mechanism of rupture of blood clots, including thrombotic embolization.

Use of electron microscopy to study platelets and thrombi

Electron microscopy has been a valuable tool for the study of platelet biology and thrombosis for more than 70 years. Early studies using conventional transmission and scanning electron microscopy (EM) provided a foundation for our initial understanding of platelet structure and how it changes upon platelet activation. EM approaches have since been utilized to study platelets and thrombi in the context of basic, translational and clinical research, and they are instrumental in the diagnosis of multiple platelet function disorders. In this brief review, we provide a sampling of the many contributions EM based studies have made to the field, including both historical highlights and contemporary applications. We will also discuss exciting new imaging modalities based on EM and their utility for the study of platelets, hemostasis and thrombosis into the future.

Platelet Activation in Heparin-Induced Thrombocytopenia is Followed by Platelet Death via Complex Apoptotic and Non-Apoptotic Pathways

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction characterized by thrombocytopenia and a high risk for venous or arterial thrombosis. HIT is caused by antibodies that recognize complexes of platelet factor 4 and heparin. The pathogenic mechanisms of this condition are not fully understood. In this study, we used flow cytometry, fluorimetry, and Western blot analysis to study the direct effects of pathogenic immune complexes containing platelet factor 4 on human platelets isolated by gel-filtration. HIT-like pathogenic immune complexes initially caused pronounced activation of platelets detected by an increased expression of phosphatidylserine and P-selectin. This activation was mediated either directly through the FcγRIIA receptors or indirectly via protease-activated receptor 1 (PAR1) receptors due to thrombin generated on or near the surface of activated platelets. The immune activation was later followed by the biochemical signs of cell death, such as mitochondrial membrane depolarization, up-regulation of Bax, down-regulation of Bcl-X_L, and moderate activation of procaspase 3 and increased calpain activity. The results show that platelet activation under the action of HIT-like immune complexes is accompanied by their death through complex apoptotic and calpain-dependent non-apoptotic pathways that may underlie the low platelet count in HIT.

The distinctive structure and composition of arterial and venous thrombi and pulmonary emboli

Although arterial and venous thromboembolic disorders are among the most frequent causes of mortality and morbidity, there has been little description of how the composition of thrombi and emboli depends on their vascular origin and age. We quantified the structure and composition of arterial and venous thrombi and pulmonary emboli using high-resolution scanning electron microscopy. Arterial thrombi contained a surprisingly large amount of fibrin, in addition to platelets. The composition of pulmonary emboli mirrored the most distal part of venous thrombi from which they originated, which differed from the structure of the body and head of the same thrombi. All thrombi and emboli contained few biconcave red blood cells but many polyhedrocytes or related forms of compressed red blood cells, demonstrating that these structures are a signature of clot contraction in vivo. Polyhedrocytes and intermediate forms comprised the major constituents of venous thrombi and pulmonary emboli. The structures within all of the thrombi and emboli were very tightly packed, in contrast to clots formed in vitro. There are distinctive, reproducible differences among arterial and venous thrombi and emboli related to their origin, destination and duration, which may have clinical implications for the understanding and treatment of thrombotic disorders.

In systemic lupus erythematosus anti-dsDNA antibodies can promote thrombosis through direct platelet activation

Systemic lupus erythematosus (SLE) is associated with a high risk of venous and arterial thrombosis, not necessarily associated with prothrombotic antiphospholipid antibodies (Abs). Alternatively, thrombosis may be due to an increased titer of anti-dsDNA Abs that presumably promote thrombosis via direct platelet activation. Here, we investigated effects of purified anti-dsDNA Abs from the blood of SLE patients, alone or in a complex with dsDNA, on isolated normal human platelets. We showed that anti-dsDNA Abs and anti-dsDNA Ab/dsDNA complexes induced strong platelet activation assessed by enhanced P-selectin expression and dramatic morphological and ultrastructural changes. Electron microscopy revealed a significantly higher percentage of platelets that lost their discoid shape, formed multiple filopodia and had a shrunken body when treated with anti-dsDNA Abs or anti-dsDNA Ab/dsDNA complexes compared with control samples. In addition, these platelets activated with anti-dsDNA Ab/dsDNA complexes typically contained a reduced number of secretory α-granules that grouped in the middle and often merged into a solid electron dense area. Many activated platelets released plasma membrane-derived microvesicles and/or fell apart into subcellular cytoplasmic fragments. Confocal microscopy revealed that platelets treated with anti-dsDNA Ab/dsDNA complex had a heterogeneous distribution of septin2 compared with the homogeneous distribution in control platelets. Structural perturbations were concomitant with mitochondrial depolarization and a decreased content of platelet ATP, indicating energetic exhaustion. Most of the biochemical and morphological changes in platelets induced by anti-dsDNA Abs and anti-dsDNA Ab/dsDNA complexes were prevented by pre-treatment with a monoclonal mAb against FcγRIIA. The aggregate of data indicates that anti-dsDNA Abs alone or in a complex with dsDNA strongly affect platelets via the FcγRIIA receptor. The immune activation of platelets with antinuclear Abs may comprise a prothrombotic mechanism underlying a high risk of thrombotic complications in patients with SLE.

Premorbid Hemostasis in Women with a History of Pregnancy Loss

BACKGROUND

 Congenital and acquired hemostatic disorders are among the pathogenic factors of pregnancy loss. Studying mechanistic relations between impaired hemostasis and fetal losses is important for the prognosis and prophylaxis of obstetric complications.

OBJECTIVE

 This article aims to establish latent hemostatic disorders in nonpregnant women as an important premorbid risk factor of pregnancy loss.

METHODS AND RESULTS

 Hemostasis was characterized using two relatively new in vitro assays, namely thrombodynamics (spatial clot growth) and kinetics of blood clot contraction, which together reflect the hemostatic or thrombotic potential. In addition, platelet functionality was assessed using flow cytometry. Our study included 50 women with a history of pregnancy loss and 30 parous women without previous obstetric complications. In patients with pregnancy loss, hypercoagulability was observed along with significant impairment of blood clot contraction associated with chronic platelet activation and dysfunction. Both hypercoagulability and defective clot contraction were significantly more pronounced in patients with a history of three or more miscarriages compared with patients with a history of one or two miscarriages. In addition, a significant inhibition of clot contraction was found in patients with miscarriage occurring after 10 weeks of gestation compared with those who lost a fetus earlier in pregnancy.

CONCLUSION

 These results indicate that chronic hypercoagulability and impaired clot contraction constitute a premorbid status in patients with pregnancy loss. The data confirm a significant pathogenic role of hemostatic disorders in pregnancy loss and suggest the predictive value of thrombodynamics and blood clot contraction assays in evaluating the risk of pregnancy loss.

Contribution of nascent cohesive fiber-fiber interactions to the non-linear elasticity of fibrin networks under tensile load

Fibrin is a viscoelastic proteinaceous polymer that determines the deformability and integrity of blood clots and fibrin-based biomaterials in response to biomechanical forces. Here, a previously unnoticed structural mechanism of fibrin clots' mechanical response to external tensile loads is tested using high-resolution confocal microscopy and recently developed three-dimensional computational model. This mechanism, underlying local strain-stiffening of individual fibers as well as global stiffening of the entire network, is based on previously neglected nascent cohesive pairwise interactions between individual fibers (crisscrossing) in fibrin networks formed under tensile load. Existence of fiber-fiber crisscrossings of reoriented fibers was confirmed using 3D imaging of experimentally obtained stretched fibrin clots. The computational model enabled us to study structural details and quantify mechanical effects of the fiber-fiber cohesive crisscrossing during stretching of fibrin gels at various spatial scales. The contribution of the fiber-fiber cohesive contacts to the elasticity of stretched fibrin networks was characterized by changes in individual fiber stiffness, the length, width, and alignment of fibers, as well as connectivity and density of the entire network. The results show that the nascent cohesive crisscrossing of fibers in stretched fibrin networks comprise an underappreciated important structural mechanism underlying the mechanical response of fibrin to (patho)physiological stresses that determine the course and outcomes of thrombotic and hemostatic disorders, such as heart attack and ischemic stroke. STATEMENT OF SIGNIFICANCE: Fibrin is a viscoelastic proteinaceous polymer that determines the deformability and integrity of blood clots and fibrin-based biomaterials in response to biomechanical forces. In this paper, a novel structural mechanism of fibrin clots' mechanical response to external tensile loads is tested using high-resolution confocal microscopy and newly developed computational model. This mechanism, underlying local strain-stiffening of individual fibers as well as global stiffening of the entire network, is based on previously neglected nascent cohesive pairwise interactions between individual fibers (crisscrossing) in fibrin networks formed under tensile load. Cohesive crisscrossing is an important structural mechanism that influences the mechanical response of blood clots and which can determine the outcomes of blood coagulation disorders, such as heart attacks and strokes.

Lytic Susceptibility, Structure, and Mechanical Properties of Fibrin in Systemic Lupus Erythematosus

Among complications of systemic lupus erythematosus (SLE), thrombotic events are relatively common and contribute significantly to the morbidity and mortality rates. An increased risk of thrombosis in various diseases has been shown to be associated with the lytic stability and mechanical stiffness of the fibrin clot determined by its structure. Here we studied alterations of the fibrin clot properties in relation to disease severity in SLE patients. Plasma clots from 28 SLE patients were characterized by the kinetics of formation and fibrinolytic dissolution (using dynamic turbidimetry), the network and fiber ultrastructure (scanning electron microscopy), viscoelasticity (shear rheometry), and the rate and degree of crosslinking (Western blotting) correlated with the disease activity, blood composition, and compared to clotting of pooled normal human plasma. Clots made from plasma of SLE patients were lysed faster with exogenous t-PA than control clots from normal plasma without a significant difference between those from active (SLEDAI>4) and inactive (SLEDAI<4) SLE patients. Clots from the blood of patients with active SLE were characterized by significantly slower onset, but faster rate of fibrin polymerization and a higher optical density due to thicker fibers compared to those from inactive SLE and control pooled normal plasma. The rheological parameters of the clots (storage and loss moduli) were significantly increased in the active SLE patients along with enhanced fibrin crosslinking and hyperfibrinogenemia. The structural and rheological alterations displayed a strong positive correlation with high fibrinogen levels and other laboratory markers of immune inflammation. In conclusion, changes in the blood composition associated with active systemic inflammation in SLE cause significant alterations in the lytic resistance of fibrin clots associated with changes in polymerization kinetics, viscoelastic properties, and structure. The formation of more rigid prothrombotic fibrin clots in the plasma of SLE patients is likely due to the inflammatory hyperfibrinogenemia and greater extent of crosslinking. However, the higher susceptibility of the SLE clots to fibrinolysis may be a protective and/or compensatory mechanism that reduces the risk of thrombotic complications and improves patient outcomes.

Platelet factor 4-containing immune complexes induce platelet activation followed by calpain-dependent platelet death

Heparin-induced thrombocytopenia (HIT) is a complication of heparin therapy sometimes associated with thrombosis. The hallmark of HIT is antibodies to the heparin/platelet factor 4 (PF4) complex that cause thrombocytopenia and thrombosis through platelet activation. Despite the clinical importance, the molecular mechanisms and late consequences of immune platelet activation are not fully understood. Here, we studied immediate and delayed effects of the complexes formed by human PF4 and HIT-like monoclonal mouse anti-human-PF4/heparin IgG antibodies (named KKO) on isolated human platelets in vitro. Direct platelet-activating effect of the KKO/PF4 complexes was corroborated by the overexpression of phosphatidylserine (PS) and P-selectin on the platelet surface. The immune platelet activation was accompanied by a decrease of the mitochondrial transmembrane potential (ΔΨm), concurrent with a significant gradual reduction of the ATP content in platelets, indicating disruption of energy metabolism. A combination of PS expression and mitochondrial depolarization induced by the PF4-containing immune complexes observed in a substantial fraction of platelets was considered as a sign of ongoing platelet death, as opposed to a subpopulation of activated live platelets with PS on the plasma membrane but normal ΔΨm. Both activated and dying platelets treated with KKO/PF4 formed procoagulant extracellular microvesicles bearing PS on their surface. Scanning and transmission electron microscopy revealed dramatic morphological changes of KKO/PF4-treated platelets, including their fragmentation, another indicator of cell death. Most of the effects of KKO/PF4 were prevented by an anti-FcγRII monoclonal antibody IV.3. The adverse functional and structural changes in platelets induced by the KKO/PF4 complexes were associated with strong time-dependent activation of calpain, but only trace cleavage of caspase 3. The results indicate that the pathogenic PF4-containing HIT-like immune complexes induce direct prothrombotic platelet activation via FcγRIIA receptors followed by non-apoptotic calpain-dependent death of platelets, which can be an important mechanism of thrombocytopenia during HIT development.

Factor XIII topology: organization of B subunits and changes with activation studied with single-molecule atomic force microscopy

Essentials Factor XIII is a heterotetramer with 2 catalytic A subunits and 2 non-catalytic B subunits. Structure of active and inactive factor XIII was studied with atomic force microscopy. Inactive factor XIII is made of an A2 globule and 2 flexible B subunits extending from it. Activated factor XIII separates into a B2 homodimer and 2 monomeric active A subunits. SUMMARY: Background Factor XIII (FXIII) is a precursor of the blood plasma transglutaminase (FXIIIa) that is generated by thrombin and Ca2+ and covalently crosslinks fibrin to strengthen blood clots. Inactive plasma FXIII is a heterotetramer with two catalytic A subunits and two non-catalytic B subunits. Inactive A subunits have been characterized crystallographically, whereas the atomic structure of the entire FXIII and B subunits is unknown and the oligomerization state of activated A subunits remains controversial. Objectives Our goal was to characterize the (sub)molecular structure of inactive FXIII and changes upon activation. Methods Plasma FXIII, non-activated or activated with thrombin and Ca2+ , was studied by single-molecule atomic force microscopy. Additionally, recombinant separate A and B subunits were visualized and compared with their conformations and dimensions in FXIII and FXIIIa. Results and Conclusions We showed that heterotetrameric FXIII forms a globule composed of two catalytic A subunits with two flexible strands comprising individual non-catalytic B subunits that protrude on one side of the globule. Each strand corresponds to seven to eight out of 10 tandem repeats building each B subunit, called sushi domains. The remainder were not seen, presumably because they were tightly bound to the globular A2 dimer. Some FXIII molecules had one or no visible strands, suggesting dissociation of the B subunits from the globular core. After activation of FXIII with thrombin and Ca2+ , B subunits dissociated and formed B2 homodimers, whereas the activated globular A subunits dissociated into monomers. These results characterize the molecular organization of FXIII and changes with activation.

Differential sensitivity of various markers of platelet activation with adenosine diphosphate

A number of techniques have been available to assess platelet activation, but their relative sensitivity is unknown and their usage is variable and not based on any rational criteria. Here, we compared the ability of several techniques based on morphological and biochemical markers to detect the first signs of ADP-induced platelet activation. Scanning electron microscopy of platelets was performed in parallel with flow cytometry to quantify the surface expression of P-selectin (marked by labeled anti-CD62P antibodies), active αIIbβ3-intergrin (assessed by the binding of labeled fibrinogen) and phosphatidylserine (assessed by the binding of labeled Annexin V). When expressed as a fraction of activated platelets, shape changes were the most sensitive to a low ADP concentration compared to the biochemical markers in the following order of sensitivity: morphological changes>fibrinogen binding capacity>P-selectin expression> phosphatidylserine exposure. These results suggest the greater sensitivity of platelet microscopy and the importance of its combination with flow cytometry used to detect surface expression of the molecular markers of platelet activation.

Fatal dysfunction and disintegration of thrombin-stimulated platelets

Platelets play a key role in the formation of hemostatic clots and obstructive thrombi as well as in other biological processes. In response to physiological stimulants, including thrombin, platelets change shape, express adhesive molecules, aggregate, and secrete bioactive substances, but their subsequent fate is largely unknown. Here we examined late-stage structural, metabolic, and functional consequences of thrombin-induced platelet activation. Using a combination of confocal microscopy, scanning and transmission electron microscopy, flow cytometry, biochemical and biomechanical measurements, we showed that thrombin-induced activation is followed by time-dependent platelet dysfunction and disintegration. After ~30 minutes of incubation with thrombin, unlike with collagen or ADP, human platelets disintegrated into cellular fragments containing organelles, such as mitochondria, glycogen granules, and vacuoles. This platelet fragmentation was preceded by Ca²⁺ influx, integrin α_IIbβ₃ activation and phosphatidylserine exposure (activation phase), followed by mitochondrial depolarization, generation of reactive oxygen species, metabolic ATP depletion and impairment of platelet contractility along with dramatic cytoskeletal rearrangements, concomitant with platelet disintegration (death phase). Coincidentally with the platelet fragmentation, thrombin caused calpain activation but not activation of caspases 3 and 7. Our findings indicate that the late functional and structural damage of thrombin-activated platelets comprise a calpain-dependent platelet death pathway that shares some similarities with the programmed death of nucleated cells, but is unique to platelets, therefore representing a special form of cellular destruction. Fragmentation of activated platelets suggests that there is an underappreciated pathway of enhanced elimination of platelets from the circulation in (pro)thrombotic conditions once these cells have performed their functions.

Blood clot contraction differentially modulates internal and external fibrinolysis

Essentials Clot contraction influences the rate of fibrinolysis in vitro. Internal fibrinolysis is enhanced ∼2-fold in contracted vs. uncontracted blood clots. External fibrinolysis is ∼4-fold slower in contracted vs. uncontracted blood clots. Contraction can modulate lytic resistance and potentially the clinical outcome of thrombosis. SUMMARY: Background Fibrinolysis involves dissolution of polymeric fibrin networks that is required to restore blood flow through vessels obstructed by thrombi. The efficiency of lysis depends in part on the susceptibility of fibrin to enzymatic digestion, which is governed by the structure and spatial organization of fibrin fibers. How platelet-driven clot contraction affects the efficacy of fibrinolysis has received relatively little study. Objective Here, we examined the effects of clot contraction on the rate of internal fibrinolysis emanating from within the clot to simulate (patho)physiological conditions and external fibrinolysis initiated from the clot exterior to simulate therapeutic thrombolysis. Methods Clot contraction was prevented by inhibiting platelet myosin IIa activity, actin polymerization or platelet-fibrin(ogen) binding. Internal fibrinolysis was measured by optical tracking of clot size. External fibrinolysis was determined by the release of radioactive fibrin degradation products. Results and Conclusions Clot contraction enhanced the rate of internal fibrinolysis ∼2-fold. In contrast, external fibrinolysis was ~4-fold slower in contracted clots. This dichotomy in the susceptibility of contracted and uncontracted clots to internal vs. external lysis suggests that the rate of lysis is dependent upon the interplay between accessibility of fibrin fibers to fibrinolytic agents, including clot permeability, and the spatial proximity of the fibrin fibers that modulate the effects of the fibrinolytic enzymes. Understanding how compaction of blood clots influences clot lysis might have important implications for prevention and treatment of thrombotic disorders.

Neutrophil α-defensins promote thrombosis in vivo by altering fibrin formation, structure, and stability

Inflammation and thrombosis are integrated, mutually reinforcing processes, but the interregulatory mechanisms are incompletely defined. Here, we examined the contribution of α-defensins (α-defs), antimicrobial proteins released from activated human neutrophils, on clot formation in vitro and in vivo. Activation of the intrinsic pathway of coagulation stimulates release of α-defs from neutrophils. α-Defs accelerate fibrin polymerization, increase fiber density and branching, incorporate into nascent fibrin clots, and impede fibrinolysis in vitro. Transgenic mice (Def++) expressing human α-Def-1 developed larger, occlusive, neutrophil-rich clots after partial inferior vena cava (IVC) ligation than those that formed in wild-type (WT) mice. IVC thrombi extracted from Def++ mice were composed of a fibrin meshwork that was denser and contained a higher proportion of tightly packed compressed polyhedral erythrocytes than those that developed in WT mice. Def++ mice were resistant to thromboprophylaxis with heparin. Inhibiting activation of the intrinsic pathway of coagulation, bone marrow transplantation from WT mice or provision of colchicine to Def++ mice to inhibit neutrophil degranulation decreased plasma levels of α-defs, caused a phenotypic reversion characterized by smaller thrombi comparable to those formed in WT mice, and restored responsiveness to heparin. These data identify α-defs as a potentially important and tractable link between innate immunity and thrombosis.

Shape changes of erythrocytes during blood clot contraction and the structure of polyhedrocytes

Polyhedral erythrocytes, named polyhedrocytes, are formed in contracted blood clots and thrombi, as a result of compression by activated contractile platelets pulling on fibrin. This deformation was shown to be mechanical in nature and polyhedrocytes were characterized using light and electron microscopy. Through three-dimensional reconstruction, we quantified the geometry of biconcave, intermediate, and polyhedral erythrocytes within contracting blood clots. During compression, erythrocytes became less oblate and more prolate than the biconcave cells and largely corresponded to convex, irregular polyhedra with a total number of faces ranging from 10 to 16. Faces were polygons with 3 to 6 sides. The majority of the faces were quadrilaterals, though not all sides were straight and not all faces were flat. There were no changes in the surface area or volume. These results describe the gradual natural deformation of erythrocytes as a part of compaction into a tightly packed array that is an important but understudied component of mature blood clots and thrombi.

Model predictions of deformation, embolization and permeability of partially obstructive blood clots under variable shear flow

Thromboembolism, one of the leading causes of morbidity and mortality worldwide, is characterized by formation of obstructive intravascular clots (thrombi) and their mechanical breakage (embolization). A novel two-dimensional multi-phase computational model is introduced that describes active interactions between the main components of the clot, including platelets and fibrin, to study the impact of various physiologically relevant blood shear flow conditions on deformation and embolization of a partially obstructive clot with variable permeability. Simulations provide new insights into mechanisms underlying clot stability and embolization that cannot be studied experimentally at this time. In particular, model simulations, calibrated using experimental intravital imaging of an established arteriolar clot, show that flow-induced changes in size, shape and internal structure of the clot are largely determined by two shear-dependent mechanisms: reversible attachment of platelets to the exterior of the clot and removal of large clot pieces. Model simulations predict that blood clots with higher permeability are more prone to embolization with enhanced disintegration under increasing shear rate. In contrast, less permeable clots are more resistant to rupture due to shear rate-dependent clot stiffening originating from enhanced platelet adhesion and aggregation. These results can be used in future to predict risk of thromboembolism based on the data about composition, permeability and deformability of a clot under specific local haemodynamic conditions.

Atomic Structural Models of Fibrin Oligomers

The space-filling fibrin network is a major part of clots and thrombi formed in blood. Fibrin polymerization starts when fibrinogen, a plasma protein, is proteolytically converted to fibrin, which self-assembles to form double-stranded protofibrils. When reaching a critical length, these intermediate species aggregate laterally to transform into fibers arranged into branched fibrin network. We combined multiscale modeling in silico with atomic force microscopy (AFM) imaging to reconstruct complete atomic models of double-stranded fibrin protofibrils with γ-γ crosslinking, A:a and B:b knob-hole bonds, and αC regions-all important structural determinants not resolved crystallographically. Structures of fibrin oligomers and protofibrils containing up to 19 monomers were successfully validated by quantitative comparison with high-resolution AFM images. We characterized the protofibril twisting, bending, kinking, and reversibility of A:a knob-hole bonds, and calculated hydrodynamic parameters of fibrin oligomers. Atomic structures of protofibrils provide a basis to understand mechanisms of early stages of fibrin polymerization.

Abstract 379: Platelet-driven Contraction of Venous Thrombi Modulates Their Obstructiveness and Embologenicity

Contraction (retraction) of the blood clot is a part of the clotting process driven by activated platelets attached to fibrin. The aim of this work was to reveal the pathogenic importance of contraction of clots and thrombi in venous thromboembolism (VTE). We investigated the kinetics of clot contraction in clots made from the blood of 55 patients with VTE not receiving antiplatelet and anticoagulant medications. In addition, we studied the ultrastructure of ex vivo venous thrombi, as well as the morphology and functionality of isolated platelets. Thrombi from VTE patients contained compressed polyhedral erythrocytes, a marker for clot contraction in vivo . The extent and rate of contraction of in vitro clots were reduced by 2-fold in clots from the blood of VTE patients compared to healthy controls. The contraction of clots from the blood of patients with pulmonary embolism was significantly impaired compared to that of those with isolated venous thrombosis, suggesting that less compacted thrombi may be prone to embolization. The reduced ability of clots to contract correlated with continuous platelet activation followed by their partial refractoriness to stimulation. Morphologically, 75% of platelets from VTE patients were spontaneously partially activated (with filopodia) compared to only 21% of those from healthy controls. At the same time, platelets from VTE patients showed a 1.4-fold reduction in activation markers expressed in response to chemical activation when compared to platelets from healthy individuals. The results obtained suggest that the impaired contraction of thrombi resulting from platelet dysfunction is an underappreciated pathogenic mechanism in VTE that may regulate the obstructiveness and embologenicity of venous thrombi. Furthermore, an assay for clot contraction may have diagnostic and prognostic value for venous thromboembolism.

The work was supported by the Program for Competitive Growth at Kazan Federal University.

Abstract 525: Contraction of Blood Clots Influences Their Susceptibility to Fibrinolysis

The success of endogenous and therapeutic fibrinolysis is governed in part by the structure of the fibrin clot. An understudied feature that potentially affects fibrinolysis is the degree of platelet-driven shrinkage, i.e. contraction, of blood clots and thrombi. The aim of this work was to examine the effect of clot contraction on the rate of naturally occurring fibrinolysis from within and external fibrinolysis to simulate therapeutic thrombolysis. Clot contraction, which was initatied with 2mM CaCl 2 and 1U/ml thrombin, was impaired by ~75% by inhibiting platelet non-muscle myosin IIa activity (blebbistatin), actin polymerization (latrunculin A), and platelet-fibin(ogen) binding (abciximab). Internal fibrinolysis measured using dynamic optical tracking of clot size as a function of time occurred 2X faster in contracted clots compared to uncontracted clots in the presence of 75ng/ml tPA. In direct contrast, the dynamic release of radioactive fibrin degradation products as a measure of external fibrinolysis was 4X faster in uncontracted clots when 75 ng/ml tPA was added to the surface of preformed clots. This difference in the susceptibility of contracted and uncontracted clots to internal versus external lysis suggests that the lysis rate is dominated by the interplay of clot permeability to fibrinolytic enzymes as well as the spatial proximity of the fibrin fibers themselves. These results have implications for understanding clot stability in patients with thrombotic disorders and improving clot lysis by impairing platelet-mediated clot contraction.

Reduced Contraction of Blood Clots in Venous Thromboembolism Is a Potential Thrombogenic and Embologenic Mechanism

Contraction (retraction) of the blood clot is a part of the clotting process driven by activated platelets attached to fibrin that can potentially modulate the obstructiveness and integrity of thrombi. The aim of this work was to reveal the pathogenic importance of contraction of clots and thrombi in venous thromboembolism (VTE). We investigated the kinetics of clot contraction in the blood of 55 patients with VTE. In addition, we studied the ultrastructure of ex vivo venous thrombi as well as the morphology and functionality of isolated platelets. Thrombi from VTE patients contained compressed polyhedral erythrocytes, a marker for clot contraction in vivo. The extent and rate of contraction were reduced by twofold in clots from the blood of VTE patients compared with healthy controls. The contraction of clots from the blood of patients with pulmonary embolism was significantly impaired compared with that of those with isolated venous thrombosis, suggesting that less compacted thrombi are prone to embolization. The reduced ability of clots to contract correlated with continuous platelet activation followed by their partial refractoriness. Morphologically, 75% of platelets from VTE patients were spontaneously activated (with filopodia) compared with only 21% from healthy controls. At the same time, platelets from VTE patients showed a 1.4-fold reduction in activation markers expressed in response to chemical activation when compared with healthy individuals. The results obtained suggest that the impaired contraction of thrombi is an underappreciated pathogenic mechanism in VTE that may regulate the obstructiveness and embologenicity of venous thrombi.