The interaction of integrin αIIbβ3 with fibrin occurs through multiple binding sites in the αIIb β-propeller domain

3D Porous Binary Composites of Collagen, Elastin, and Fibrin Proteins Orchestrate Adipose Tissue Regeneration

The objective for this study is to advance the development of a specialized biomaterial that can effectively facilitate the regeneration of adipose tissue. In prior studies, the assessment of collagen (Col), elastin (Ela), and fibrin (Fib) unary scaffolds has been conducted. However, it is important to note that native adipose tissue is comprised of a diverse array of extracellular matrix (ECM) constituents. To mimic this behavior, binary compositions of collagen, elastin, and fibrin are fabricated in a 1:1 ratio, resulting in the formation of Col/Ela, Col/Fib, and Ela/Fib composites through a customized fabrication procedure. The physical properties of these scaffolds are comprehensively analyzed using a range of material characterization techniques. Additionally, the biological properties of the scaffolds are investigated by examining the survival, proliferation, and phenotype of adipose-derived stem cells. Subsequently, the aforementioned binary scaffolds are implanted into a rodent model for 28 days. the explants are analysed through X-ray microtomography, histology, and immunohistochemistry. The findings of the study demonstrate that the utilization of binary combinations of Col/Ela, Col/Fib, and Ela/Fib has a discernible impact on the physical and biological characteristics of the scaffolds. Nevertheless, Ela/Fib exhibits characteristics that make it a suitable candidate for adipogenesis due to its notable upregulation of caveolin-1 expression in both acellular and cellular cohorts. The combination of two natural polymers in this cell-material interaction has significantly enhanced the comprehension of adipogenesis.

Molecular basis of clot retraction and its role in wound healing

Clot retraction is important for the prevention of bleeding, in the manifestations of thrombosis and for tissue repair. The molecular mechanisms behind clot formation are complex. Platelet involvement begins with adhesion at sites of vessel injury followed by platelet aggregation, thrombin generation and fibrin production. Other blood cells incorporate into a fibrin mesh that is consolidated by FXIIIa-mediated crosslinking and platelet contractile activity. The latter results in the asymmetric redistribution of erythrocytes into a tighter central mass providing the clot with stability and resistance to fibrinolysis. Integrin αIIbβ3 on platelets is the key player in these events, bridging fibrin and the platelet cytoskeleton. Glycoprotein VI participates in thrombus formation but not in the retraction. Rheological and environmental factors influence clot construction with retraction driven by the platelet cytoskeleton with actomyosin acting as the motor. Activated platelets provide procoagulant activity stimulating thrombin generation together with the release of a plethora of biologically active proteins and substances from storage pools; many form chemotactic gradients within the fibrin or the underlying matrix. Also released are newly synthesized metabolites and lipid-rich vesicles that circulate within the vasculature and mimic platelet functions. Platelets and their released elements play key roles in wound healing. This includes promoting stem cell and mesenchymal stromal cell recruitment, fibroblast and endothelial cell migration, angiogenesis and matrix formation. These properties have led to the use of autologous clots in therapies designed to accelerate tissue repair while offering the potential for genetic manipulation in both inherited and acquired diseases.

Systematic mapping of the conformational landscape and dynamism of soluble fibrinogen

BACKGROUND

Fibrinogen is a soluble, multisubunit, and multidomain dimeric protein, which, upon its proteolytic cleavage by thrombin, is converted to insoluble fibrin, initiating polymerization that substantially contributes to clot growth. Fibrinogen contains numerous, transiently accessible "cryptic" epitopes for hemostatic and immunologic proteins, suggesting that fibrinogen exhibits conformational flexibility, which may play functional roles in its temporal and spatial interactions. Hitherto, there have been limited integrative approaches characterizing the solution structure and internal flexibility of fibrinogen.

METHODS

Here, utilizing a multipronged, biophysical approach involving 2 solution-based techniques, temperature-dependent hydrogen-deuterium exchange mass spectrometry and small angle X-ray scattering, corroborated by negative stain electron microscopy, we present a holistic, conformationally dynamic model of human fibrinogen in solution.

RESULTS

Our data reveal 4 major and distinct conformations of fibrinogen accommodated by a high degree of internal protein flexibility along its central scaffold. We propose that the fibrinogen structure in the solution consists of a complex, conformational landscape with multiple local minima. This is further supported by the location of numerous point mutations that are linked to dysfibrinogenemia and posttranslational modifications, residing near the identified fibrinogen flexions.

CONCLUSION

This work provides a molecular basis for the structural "dynamism" of fibrinogen that is expected to influence the broad swath of its functionally diverse macromolecular interactions and fine-tune the structural and mechanical properties of blood clots.

Repeated platelet activation and the potential of previously activated platelets to contribute to thrombus formation

BACKGROUND

Especially in disease conditions, platelets can encounter activating agents in circulation.

OBJECTIVES

To investigate the extent to which previously activated platelets can be reactivated and whether in-and reactivation applies to different aspects of platelet activation and thrombus formation.

METHODS

Short-and long-term effects of glycoprotein VI (GPVI) and G protein-coupled receptor (GPCR) stimulation on platelet activation and aggregation potential were compared via flow cytometry and plate-based aggregation. Using fluorescence and electron microscopy, we assessed platelet morphology and content, as well as thrombus formation.

RESULTS

After 30 minutes of stimulation with thrombin receptor activator peptide 6 (TRAP6) or adenosine diphosphate (ADP), platelets secondarily decreased in PAC-1 binding and were less able to aggregate. The reversibility of platelets after thrombin stimulation was concentration dependent. Reactivation was possible via another receptor. In contrast, cross-linked collagen-related peptide (CRP-XL) or high thrombin stimulation evoked persistent effects in α_IIbβ₃ activation and platelet aggregation. However, after 60 minutes of CRP-XL or high thrombin stimulation, when α_IIbβ₃ activation slightly decreased, restimulation with ADP or CRP-XL, respectively, increased integrin activation again. Compatible with decreased integrin activation, platelet morphology was reversed. Interestingly, reactivation of reversed platelets again resulted in shape change and if not fully degranulated, additional secretion. Moreover, platelets that were previously activated with TRAP6 or ADP regained their potential to contribute to thrombus formation under flow. On the contrary, prior platelet triggering with CRP-XL was accompanied by prolonged platelet activity, leading to a decreased secondary platelet adhesion under flow.

CONCLUSION

This work emphasizes that prior platelet activation can be reversed, whereafter platelets can be reactivated through a different receptor. Reversed, previously activated platelets can contribute to thrombus formation.

Advantages of Fibrin Polymerization Method without the Use of Exogenous Thrombin for Vascular Tissue Engineering Applications

Fibrin is widely used in vascular tissue engineering. Typically, fibrin polymerization is initiated by adding exogenous thrombin. In this study, we proposed a protocol for the preparation of completely autologous fibrin without the use of endogenous thrombin and compared the properties of the prepared fibrin matrix with that obtained by the traditional method. Fibrinogen was obtained by ethanol precipitation followed by fibrin polymerization by adding either exogenous thrombin and calcium chloride (ExThr), or only calcium chloride (EnThr). We examined the structure, mechanical properties, thrombogenicity, degradation rate and cytocompatibility of fibrin matrices. Factor XIII (FXIII) quantitative assay was performed by ELISA, and FXIII activity was assessed by SDS-PAGE detection of γ-γ cross-links. The results show that network structure of EnThr fibrin was characterized by thinner fibers. The EnThr fibrin matrices had higher strength, stiffness and resistance to proteolytic degradation compared to ExThr fibrin. EnThr fibrin matrices exhibited less thrombogenicity in vitro than ExThr, and retained high cytocompatibility. Thus, the proposed approach has several advantages over the traditional method, namely the fabrication of a completely autologous coating material that has better mechanical properties, higher resistance to proteolysis and lower thrombogenicity.

The P2Y12 Receptor Antagonist Selatogrel Dissolves Preformed Platelet Thrombi In Vivo

Selatogrel, a potent and reversible antagonist of the P2Y12 receptor, inhibited FeCl₃-induced thrombosis in rats. Here, we report the anti-thrombotic effect of selatogrel after subcutaneous applications in guinea pigs and mice. Selatogrel inhibited platelet function only 10 min after subcutaneous application in mice. In addition, in a modified Folts thrombosis model in guinea pigs, selatogrel prevented a decrease in blood-flow, indicative of the inhibition of ongoing thrombosis, approximately 10 min after subcutaneous injection. Selatogrel fully normalised blood flow; therefore, we speculate that it may not only prevent, but also dissolve, platelet thrombi. Thrombus dissolution was investigated using real-time intravital microscopy in mice. The infusion of selatogrel during ongoing platelet thrombus formation stopped growth and induced the dissolution of the preformed platelet thrombus. In addition, platelet-rich thrombi were given 30 min to consolidate in vivo. The infusion of selatogrel dissolved the preformed and consolidated platelet thrombi. Dissolution was limited to the disintegration of the occluding part of the platelet thrombi, leaving small mural platelet aggregates to seal the blood vessel. Therefore, our experiments uncovered a novel advantage of selatogrel: the dissolution of pre-formed thrombi without the disintegration of haemostatic seals, suggesting a bipartite benefit of the early application of selatogrel in patients with acute thrombosis.

Platelet binding to polymerizing fibrin is avidity driven and requires activated αIIbβ3 but not fibrin cross-linking

Platelet interaction with polymerizing fibrin is avidity driven and requires activated αIIbβ3 but not fibrin cross-linking.

The mechanism by which αIIbβ3 interacts with polymerizing fibrin differs subtly from the interaction of αIIbβ3 with fibrinogen.

The molecular basis of platelet-fibrin interactions remains poorly understood despite the predominance of fibrin in thrombi. We have studied the interaction of platelets with polymerizing fibrin by adding thrombin to washed platelets in the presence of the peptide RGDW, which inhibits the initial platelet aggregation mediated by fibrinogen binding to αIIbβ3 but leaves intact a delayed increase in light transmission (delayed wave; DW) as platelets interact with the polymerizing fibrin. The DW was absent in platelets from a patient with Glanzmann thrombasthenia, indicating a requirement for αIIbβ3. The DW required αIIbb3 activation and it was inhibited by the αIIbβ3 antagonists eptifibatide and the monoclonal antibody (mAb) 7E3, but only at much higher concentrations than needed to inhibit platelet aggregation initiated by a thrombin receptor activating peptide (T6). Surface plasmon resonance and scanning electron microscopy studies both supported fibrin having greater avidity for αIIbβ3 than fibrinogen rather than greater affinity, consistent with fibrin’s multivalency. mAb 10E5, a potent inhibitor of T6-induced platelet aggregation, did not inhibit the DW, suggesting that fibrin differs from fibrinogen in its mechanism of binding. Inhibition of factor XIII–mediated fibrin cross-linking by >95% reduced the DW by only 32%. Clot retraction showed a pattern of inhibition similar to that of the DW. We conclude that activated αIIbβ3 is the primary mediator of platelet-fibrin interactions leading to clot retraction, and that the interaction is avidity driven, does not require fibrin cross-linking, and is mediated by a mechanism that differs subtly from that of the interaction of αIIbβ3 with fibrinogen.

Platelet Behavior Contributes to Neuropathologies: A Focus on Alzheimer's and Parkinson's Disease

The functions of platelets are broad. Platelets function in hemostasis and thrombosis, inflammation and immune responses, vascular regulation, and host defense against invading pathogens, among others. These actions are achieved through the release of a wide set of coagulative, vascular, inflammatory, and other factors as well as diverse cell surface receptors involved in the same activities. As active participants in these physiological processes, platelets become involved in signaling pathways and pathological reactions that contribute to diseases that are defined by inflammation (including by pathogen-derived stimuli), vascular dysfunction, and coagulation. These diseases include Alzheimer's and Parkinson's disease, the two most common neurodegenerative diseases. Despite their unique pathological and clinical features, significant shared pathological processes exist between these two conditions, particularly relating to a central inflammatory mechanism involving both neuroinflammation and inflammation in the systemic environment, but also neurovascular dysfunction and coagulopathy, processes which also share initiation factors and receptors. This triad of dysfunction-(neuro)inflammation, neurovascular dysfunction, and hypercoagulation-illustrates the important roles platelets play in neuropathology. Although some mechanisms are understudied in Alzheimer's and Parkinson's disease, a strong case can be made for the relevance of platelets in neurodegeneration-related processes.

The Interplay between Non-Esterified Fatty Acids and Plasma Zinc and Its Influence on Thrombotic Risk in Obesity and Type 2 Diabetes

Thrombosis is a major comorbidity of obesity and type-2 diabetes mellitus (T2DM). Despite the development of numerous effective treatments and preventative strategies to address thrombotic disease in such individuals, the incidence of thrombotic complications remains high. This suggests that not all the pathophysiological mechanisms underlying these events have been identified or targeted. Non-esterified fatty acids (NEFAs) are increasingly regarded as a nexus between obesity, insulin resistance, and vascular disease. Notably, plasma NEFA levels are consistently elevated in obesity and T2DM and may impact hemostasis in several ways. A potentially unrecognized route of NEFA-mediated thrombotic activity is their ability to disturb Zn²⁺ speciation in the plasma. Zn²⁺ is a potent regulator of coagulation and its availability in the plasma is monitored carefully through buffering by human serum albumin (HSA). The binding of long-chain NEFAs such as palmitate and stearate, however, trigger a conformational change in HSA that reduces its ability to bind Zn²⁺, thus increasing the ion’s availability to bind and activate coagulation proteins. NEFA-mediated perturbation of HSA-Zn²⁺ binding is thus predicted to contribute to the prothrombotic milieu in obesity and T2DM, representing a novel targetable disease mechanism in these disorders.

Profiling the Genetic and Molecular Characteristics of Glanzmann Thrombasthenia: Can It Guide Current and Future Therapies?

Glanzmann thrombasthenia (GT) is the most widely studied inherited disease of platelet function. Platelets fail to aggregate due to a defect in platelet-to-platelet attachment. The hemostatic plug fails to form and a moderate to severe bleeding diathesis results. Classically of autosomal recessive inheritance, GT is caused by defects within the ITGA2B and ITGB3 genes that encode the αIIbβ3 integrin expressed at high density on the platelet surface and also in intracellular pools. Activated αIIbβ3 acts as a receptor for fibrinogen and other adhesive proteins that hold platelets together in a thrombus. Over 50 years of careful clinical and biological investigation have provided important advances that have improved not only the quality of life of the patients but which have also contributed to an understanding of how αIIbβ3 functions. Despite major improvements in our knowledge of GT and its genetic causes, extensive biological and clinical variability with respect to the severity and intensity of bleeding remains poorly understood. I now scan the repertoire of ITGA2B and ITGB3 gene defects and highlight the wide genetic and biological heterogeneity within the type II and variant subgroups especially with regard to bleeding, clot retraction, the internal platelet Fg storage pool and the nature of the mutations causing the disease. I underline the continued importance of gene profiling and biological studies and emphasize the multifactorial etiology of the clinical expression of the disease. This is done in a manner to provide guidelines for future studies and future treatments of a disease that has not only aided research on rare diseases but also contributed to advances in antithrombotic therapy.

Overcoming challenges in developing small molecule inhibitors for GPVI and CLEC-2

GPVI and CLEC-2 have emerged as promising targets for long-term prevention of both arterial thrombosis and thrombo-inflammation with a decreased bleeding risk relative to current drugs. However, while there are potent blocking antibodies of both receptors, their protein nature comes with decreased bioavailability, making formulation for oral medication challenging. Small molecules are able to overcome these limitations, but there are many challenges in developing antagonists of nanomolar potency, which is necessary when considering the structural features that underlie the interaction of CLEC-2 and GPVI with their protein ligands. In this review, we describe current small-molecule inhibitors for both receptors and strategies to overcome such limitations, including considerations when it comes to in silico drug design and the importance of complex compound library selection.

Platelets Promote Thromboinflammation in SARS-CoV-2 Pneumonia

OBJECTIVE

Pulmonary thrombosis is observed in severe acute respiratory syndrome coronavirus 2 pneumonia. Aim was to investigate whether subpopulations of platelets were programmed to procoagulant and inflammatory activities in coronavirus disease 2019 (COVID-19) patients with pneumonia, without comorbidities predisposing to thromboembolism. Approach and Results: Overall, 37 patients and 28 healthy subjects were studied. Platelet-leukocyte aggregates, platelet-derived microvesicles, the expression of P-selectin, and active fibrinogen receptor on platelets were quantified by flow cytometry. The profile of 45 cytokines, chemokines, and growth factors released by platelets was defined by immunoassay. The contribution of platelets to coagulation factor activity was selectively measured. Numerous platelet-monocyte (mean±SE, 67.9±4.9%, n=17 versus 19.4±3.0%, n=22; P<0.0001) and platelet-granulocyte conjugates (34.2±4.04% versus 8.6±0.7%; P<0.0001) were detected in patients. Resting patient platelets had similar levels of P-selectin (10.9±2.6%, n=12) to collagen-activated control platelets (8.7±1.5%), which was not further increased by collagen activation on patient platelets (12.4±2.5%, P=nonsignificant). The agonist-stimulated expression of the active fibrinogen receptor was reduced by 60% in patients (P<0.0001 versus controls). Cytokines (IL [interleukin]-1α, IL-1β, IL-1RA, IL-4, IL-10, IL-13, IL, 17, IL-27, IFN [interferon]-α, and IFN-γ), chemokines (MCP-1/CCL2 [monocyte chemoattractant protein 1]), and growth factors (VEGF [vascular endothelial growth factor]-A/D) were released in significantly larger amounts upon stimulation of COVID-19 platelets. Platelets contributed to increased fibrinogen, VWF (von Willebrand factor), and factor XII in COVID-19 patients. Patients (28.5±0.7 s, n=32), unlike controls (31.6±0.5 s, n=28; P<0.001), showed accelerated factor XII-dependent coagulation.

CONCLUSIONS

Platelets in COVID-19 pneumonia are primed to spread proinflammatory and procoagulant activities in systemic circulation.

Standardization and Validation of Fluorescence-Based Quantitative Assay to Study Human Platelet Adhesion to Extracellular-Matrix in a 384-Well Plate

Platelets play a crucial role in the immunological response and are involved in the pathological settings of vascular diseases, and their adhesion to the extracellular matrix is important to bring leukocytes close to the endothelial cells and to form and stabilize the thrombus. Currently there are several methods to study platelet adhesion; however, the optimal parameters to perform the assay vary among studies, which hinders their comparison and reproducibility. Here, a standardization and validation of a fluorescence-based quantitative adhesion assay to study platelet-ECM interaction in a high-throughput screening format is proposed. Our study confirms that fluorescence-based quantitative assays can be effectively used to detect platelet adhesion, in which BCECF-AM presents the highest sensitivity in comparison to other dyes.

Dominant role of αIIbβ3 in platelet interactions with cross-linked fibrin fragment D-dimer

Although much is known about the interaction of fibrinogen with αIIbβ3, much less is known about the interaction of platelets with cross-linked fibrin. Fibrinogen residue Lys406 plays a vital role in the interaction of fibrinogen with αIIbβ3, but because it participates in fibrin cross-linking, it is not available for interacting with αIIbβ3. We studied the adhesion of platelets and HEK cells expressing normal and constitutively active αIIbβ3 to both immobilized fibrinogen and D-dimer, a proteolytic fragment of cross-linked fibrin, as well as platelet-mediated clot retraction. Nonactivated platelets and HEK cells expressing normal αIIbβ3 adhered to fibrinogen but not D-dimer, whereas activated platelets as well as HEK cells expressing activated αIIbβ3 both bound to D-dimer. Small-molecule antagonists of the αIIbβ3 RGD (Arg-Gly-Asp) binding pocket inhibited adhesion to D-dimer, and an Asp119Ala mutation that disrupts the β3 metal ion-dependent adhesion site inhibited αIIbβ3-mediated adhesion to D-dimer. D-dimer and a polyclonal antibody against D-dimer inhibited clot retraction. The monoclonal antibody (mAb) 10E5, directed at αIIb and a potent inhibitor of platelet interactions with fibrinogen, did not inhibit the interaction of activated platelets with D-dimer or clot retraction, whereas the mAb 7E3, directed at β3, inhibited both phenomena. We conclude that activated, but not nonactivated, αIIbβ3 mediates interactions between platelets and D-dimer, and by extrapolation, to cross-linked fibrin. Although the interaction of αIIbβ3 with D-dimer differs from that with fibrinogen, it probably involves contributions from regions on β3 that are close to, or that are affected by, changes in the RGD binding pocket.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

Structure-guided design of pure orthosteric inhibitors of αIIbβ3 that prevent thrombosis but preserve hemostasis

A prevailing dogma is that inhibition of vascular thrombosis by antagonizing platelet integrin αIIbβ3 cannot be achieved without compromising hemostasis, thus causing serious bleeding and increased morbidity and mortality. It is speculated that these adverse outcomes result from drug-induced activating conformational changes in αIIbβ3 but direct proof is lacking. Here, we report the structure-guided design of peptide Hr10 and a modified form of the partial agonist drug tirofiban that act as "pure" antagonists of αIIbβ3, i.e., they no longer induce the conformational changes in αIIbβ3. Both agents inhibit human platelet aggregation but preserve clot retraction. Hr10 and modified tirofiban are as effective as partial agonist drugs in inhibiting vascular thrombosis in humanized mice, but neither causes serious bleeding, establishing a causal link between partial agonism and impaired hemostasis. Pure orthosteric inhibitors of αIIbβ3 may thus provide safer alternatives for human therapy, and valuable tools to probe structure-activity relationships in integrins.

Novel Role for the AnxA1-Fpr2/ALX Signaling Axis as a Key Regulator of Platelet Function to Promote Resolution of Inflammation

Supplemental Digital Content is available in the text.

Background:

Ischemia reperfusion injury (I/RI) is a common complication of cardiovascular diseases. Resolution of detrimental I/RI-generated prothrombotic and proinflammatory responses is essential to restore homeostasis. Platelets play a crucial part in the integration of thrombosis and inflammation. Their role as participants in the resolution of thromboinflammation is underappreciated; therefore we used pharmacological and genetic approaches, coupled with murine and clinical samples, to uncover key concepts underlying this role.

Methods:

Middle cerebral artery occlusion with reperfusion was performed in wild-type or annexin A1 (AnxA1) knockout (AnxA1^−/−) mice. Fluorescence intravital microscopy was used to visualize cellular trafficking and to monitor light/dye–induced thrombosis. The mice were treated with vehicle, AnxA1 (3.3 mg/kg), WRW4 (1.8 mg/kg), or all 3, and the effect of AnxA1 was determined in vivo and in vitro.

Results:

Intravital microscopy revealed heightened platelet adherence and aggregate formation post I/RI, which were further exacerbated in AnxA1^−/− mice. AnxA1 administration regulated platelet function directly (eg, via reducing thromboxane B₂ and modulating phosphatidylserine expression) to promote cerebral protection post-I/RI and act as an effective preventative strategy for stroke by reducing platelet activation, aggregate formation, and cerebral thrombosis, a prerequisite for ischemic stroke. To translate these findings into a clinical setting, we show that AnxA1 plasma levels are reduced in human and murine stroke and that AnxA1 is able to act on human platelets, suppressing classic thrombin-induced inside-out signaling events (eg, Akt activation, intracellular calcium release, and Ras-associated protein 1 [Rap1] expression) to decrease α_IIbβ₃ activation without altering its surface expression. AnxA1 also selectively modifies cell surface determinants (eg, phosphatidylserine) to promote platelet phagocytosis by neutrophils, thereby driving active resolution. (n=5–13 mice/group or 7–10 humans/group.)

Conclusions:

AnxA1 affords protection by altering the platelet phenotype in cerebral I/RI from propathogenic to regulatory and reducing the propensity for platelets to aggregate and cause thrombosis by affecting integrin (α_IIbβ₃) activation, a previously unknown phenomenon. Thus, our data reveal a novel multifaceted role for AnxA1 to act both as a therapeutic and a prophylactic drug via its ability to promote endogenous proresolving, antithromboinflammatory circuits in cerebral I/RI. Collectively, these results further advance our knowledge and understanding in the field of platelet and resolution biology.

Polyhedrocytes in blood clots of type 2 diabetic patients with high cardiovascular risk: association with glycemia, oxidative stress and platelet activation

Background

Little is known about factors that affect the composition of contracted blood clots in specific diseases. We investigated the content of polyhedral erythrocytes (polyhedrocytes) formed in blood clots and its determinants in type 2 diabetes (T2D) patients.

Methods

In 97 patients with long-standing T2D [median HbA_1c, 6.4% (interquartile range 5.9–7.8)], we measured in vitro the composition of blood clots, including a clot area covered by polyhedrocytes using scanning electron microscopy and the erythrocyte compression index (ECI), defined as a ratio of the mean polyhedrocyte area to the mean native erythrocyte area. Moreover, plasma fibrin clot permeability (K_s), clot lysis time (CLT), thrombin generation, oxidative stress [total protein carbonyl (total PC), total antioxidant capacity and thiobarbituric acid reactive substances (TBARS)], and platelet activation markers were determined. The impact of glucose concentration on polyhedrocytes formation was assessed in vitro.

Results

Polyhedrocytes content in contracted clots was positively correlated with glucose (r = 0.24, p = 0.028), glycated hemoglobin (r = 0.40, p = 0.024), total cholesterol (r = 0.22, p = 0.044), TBARS (r = 0.60, p = 0.0027), P-selectin (r = 0.54, p = 0.0078) and platelet factor-4, PF4 (r = 0.59, p = 0.0032), but not with thrombin generation, platelet count, K_s or CLT. Patients who formed more polyhedrocytes (≥ 10th percentile) (n = 83, 85.6%) had higher glucose (+ 15.7%, p = 0.018), fibrinogen (+ 16.6%, p = 0.004), lower red blood cell distribution width (RDW, − 8.8%, p = 0.034), reduced plasma clot density (− 21.8% K_s, p = 0.011) and impaired fibrinolysis (+ 6.5% CLT, p = 0.037) when compared to patients with lesser amount of polyhedrocytes (< 10th percentile). ECI and the content of polyhedrocytes were strongly associated with total PC (r = 0.79, p = 0.036 and r = 0.67, p = 0.0004, respectively). In vitro an increase of glucose concentration by 10 mmol/L was associated with 94% higher polyhedrocytes content (p = 0.033) when compared to the baseline (7.1 mM). After adjustment for age, sex and fibrinogen, multiple regression analysis showed that RDW was the only independent predictor of polyhedrocytes content in T2D (OR = 0.61, 95% CI 0.39–0.92).

Conclusions

Poor glycemic control, together with enhanced platelet activation and oxidative stress, increase the content of polyhedrocytes in blood clots generated in T2D patients.

Electronic supplementary material

The online version of this article (10.1186/s12933-018-0789-6) contains supplementary material, which is available to authorized users.

Erythrocyte compression index is impaired in patients with residual vein obstruction

Defective clot contraction has been postulated to contribute to thrombosis. We aimed to evaluate the association of residual vein obstruction (RVO) with erythrocyte compression within the whole-blood clot. We studied 32 patients with venous thromboembolism (VTE) taking vitamin K antagonists (VKAs) for at least 3 months (median time in therapeutic range 60%), including 12 (37.5%) with RVO, and 32 age- and sex-matched controls. In all study participants we evaluated whole blood clot retraction, expressed as the erythrocyte compression index (ECI), defined as a ratio of mean polyhedrocyte area to mean native erythrocyte area, along with clot area covered by polyhedrocytes, plasma clot permeability (Ks), clot lysis time (CLT), and thrombin generation. In both groups higher ECI, indicating impaired clot contraction, increased with older age, higher body mass index, red blood cell distribution width, and lower platelet count (all p < 0.05), but not with red blood cell count. In VTE patients ECI was 15.8% higher than in controls (median 63.6 vs. 54.9%, p = 0.021). Subjects with RVO had 20% higher ECI and 155% lower clot area covered by polyhedrocytes. RVO patients had also prolonged CLT by 41%, but not Ks, and elevated peak thrombin generation by 33%, as compared to those without RVO (all p < 0.05). This study is the first to show impaired compression of erythrocytes in RVO patients despite VKA anticoagulation. Altered ECI coexisted with hypolysability and increased thrombin generation. ECI might be useful in the diagnostic process of RVO or post-thrombotic syndrome and can help optimize the anticoagulant therapy.

Extracellular matrix molecules and their potential contribution to the function of transplanted pancreatic islets

Extracellular matrix (ECM) molecules are responsible for structural and biochemical support, as well as for regulation of molecular signalling and tissue repair in many organ structures, including the pancreas. In pancreatic islets, collagen type IV and VI, and laminins are the most abundant molecules, but other ECM molecules are also present. The ECM interacts with specific combinations of integrin α/β heterodimers on islet cells and guides many cellular processes. More specifically, some ECM molecules are involved in beta cell survival, function and insulin production, while others can fine tune the susceptibility of islet cells to cytokines. Further, some ECM induce release of growth factors to facilitate tissue repair. During enzymatic isolation of islets for transplantation, the ECM is damaged, impacting islet function. However, restoration of the ECM in human islets (for example by adding ECM to the interior of immunoprotective capsules) has been shown to enhance islet function. Here, we provide current insight into the role of ECM molecules in islet function and discuss the clinical potential of ECM manipulation to enhance pancreatic islet function and survival.

Congenital hypodysfibrinogenemia associated with a novel deletion of three residues (γAla289_Asp291del) in fibrinogen

Hypodysfibrinogenemia is the least frequently reported congenital fibrinogen disorder, characterized by both quantity and quality defects of fibrinogen. In this study, we investigated the molecular basis of hypodysfibrinogenemia in a Chinese family. Functional fibrinogen was measured by Clauss method, and the antigenic fibrinogen was measured by immunoturbidimetry assay. All the exons and exon-intron boundaries of fibrinogen genes (FGA, FGB and FGG) were analysed by direct DNA sequencing. To further evaluate its molecular and functional characterizations, fibrinogen was purified from the plasma of propositus, then SDS-PAGE, fibrin polymerization, clot lysis, and electron microscopy scanning were all performed. The propositus showed a slight decrease of immunologic fibrinogen (1.52 g/L) but dramatically reduced functional fibrinogen (0.3 g/L). DNA sequencing revealed a novel heterozygous CCTTTGATG deletion in the exon 8 of FGG, leading to the deletion of Ala289, Phe290, and Asp291 in fibrinogen γ-chain. The polymerization of the fibrinogen from the propositus was markedly impaired, with prolonged lag period and decreased final turbidity. The fibrinogen clottability showed a reduced fraction of participating clot formation. While the clot lysis showed normal. Scanning electron microscopy revealed that the fibers of the propositus were thicker than normal, with larger pores and curlier meshworks. We conclude that γAla289_Asp291del is responsible for the hypodysfibrinogenemia in this case.

Fibrinogen as a Pleiotropic Protein Causing Human Diseases: The Mutational Burden of Aα, Bβ, and γ Chains

Fibrinogen is a highly pleiotropic protein that is involved in the final step of the coagulation cascade, wound healing, inflammation, and angiogenesis. Heterozygous mutations in Aα, Bβ, or γ fibrinogen-chain genes (FGA, FGB, FGG) have been described as being responsible for fibrinogen deficiencies (hypofibrinogenemia, hypo-dysfibrinogenemia, dysfibrinogenemia) and for more rare conditions, such as fibrinogen storage disease and hereditary renal amyloidosis. Instead, biallelic mutations have been associated with afibrinogenemia/severe hypofibrinogenemia, i.e., the severest forms of fibrinogen deficiency, affecting approximately 1–2 cases per million people. However, the “true” prevalence for these conditions on a global scale is currently not available. Here, we defined the mutational burden of the FGA, FGB, and FGG genes, and estimated the prevalence of inherited fibrinogen disorders through a systematic analysis of exome/genome data from ~140,000 individuals belonging to the genome Aggregation Database. Our analysis showed that the world-wide prevalence for recessively-inherited fibrinogen deficiencies could be 10-fold higher than that reported so far (prevalence rates vary from 1 in 10⁶ in East Asians to 24.5 in 10⁶ in non-Finnish Europeans). The global prevalence for autosomal-dominant fibrinogen disorders was estimated to be ~11 in 1000 individuals, with heterozygous carriers present at a frequency varying from 3 every 1000 individuals in Finns, to 1–2 every 100 individuals among non-Finnish Europeans and Africans/African Americans. Our analysis also allowed for the identification of recurrent (i.e., FGG-p.Ala108Gly, FGG-Thr47Ile) or ethnic-specific mutations (e.g., FGB-p.Gly103Arg in Admixed Americans, FGG-p.Ser245Phe in Africans/African Americans).

Procoagulant platelets: generation, function, and therapeutic targeting in thrombosis

Current understanding of how platelets localize coagulation to wound sites has come mainly from studies of a subpopulation of activated platelets. In this review, we summarize data from the last 4 decades that have described these platelets with a range of descriptive titles and attributes. We identify striking overlaps in the reported characteristics of these platelets, which imply a single subpopulation of versatile platelets and thus suggest that their commonality requires unification of their description. We therefore propose the term procoagulant platelet as the unifying terminology. We discuss the agonist requirements and molecular drivers for the dramatic morphological transformation platelets undergo when becoming procoagulant. Finally, we provide perspectives on the biomarker potential of procoagulant platelets for thrombotic events as well as on the possible clinical benefits of inhibitors of carbonic anhydrase enzymes and the water channel Aquaporin-1 for targeting this subpopulation of platelets as antiprocoagulant antithrombotics.

Fibrin and D-dimer bind to monomeric GPVI

Fibrin has recently been shown to activate platelets through the immunoglobulin receptor glycoprotein VI (GPVI). In the present study, we show that spreading of human platelets on fibrin is abolished in patients deficient in GPVI, confirming that fibrin activates human platelets through the immunoglobulin receptor. Using a series of proteolytic fragments, we show that D-dimer, but not the E fragment of fibrin, binds to GPVI and that immobilized D-dimer induces platelet spreading through activation of Src and Syk tyrosine kinases. In contrast, when platelets are activated in suspension, soluble D-dimer inhibits platelet aggregation induced by fibrin and collagen, but not by a collagen-related peptide composed of a repeat GPO sequence or by thrombin. Using surface plasmon resonance, we demonstrate that fibrin binds selectively to monomeric GPVI with a K_D of 302 nM, in contrast to collagen, which binds primarily to dimeric GPVI. These results establish GPVI as the major signaling receptor for fibrin in human platelets and provide evidence that fibrin binds to a distinct configuration of GPVI. This indicates that it may be possible to develop agents that selectively block the interaction of fibrin but not collagen with the immunoglobulin receptor. Such agents are required to establish whether selective targeting of either interaction has the potential to lead to development of an antithrombotic agent with a reduced effect on bleeding relative to current antiplatelet drugs.

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

αIIbβ3 binding to a fibrinogen fragment lacking the γ-chain dodecapeptide is activation dependent and EDTA inducible

Platelet integrin receptor αIIbβ3 supports platelet aggregation by binding fibrinogen. The interaction between the fibrinogen C-terminal γ-chain peptide composed of residues γ-404-411 (GAKQAGDV) and the Arg-Gly-Asp (RGD) binding pocket on αIIbβ3 is required for fibrinogen-mediated platelet aggregation, but data suggest that other ancillary binding sites on both fibrinogen and αIIbβ3 may lead to higher-affinity fibrinogen binding and clot retraction. To identify additional sites, we analyzed the ability of platelets and cells expressing normal and mutant αIIbβ3 to adhere to an immobilized fibrinogen plasmin fragment that lacks intact γ-404-411 ('D98'). We found the following: (1) Activated, but not unactivated, platelets adhere well to immobilized 'D98.' (2) Cells expressing constitutively active αIIbβ3 mutants, but not cells expressing normal αIIbβ3 or αVβ3, adhere well to 'D98.' (3) Monoclonal antibodies 10E5 and 7E3 inhibit the adhesion to 'D98' of activated platelets and cells expressing constitutively active αIIbβ3, as do small-molecule inhibitors that bind to the RGD pocket. (4) EDTA paradoxically induces normal αIIbβ3 to interact with 'D98.' Because molecular modeling and molecular dynamics simulations suggested that the αIIb L151-D159 helix may contribute to the interaction with 'D98,' we studied an αIIbβ3 mutant in which the αIIb 148-166 loop was swapped with the corresponding αV loop; it failed to bind to fibrinogen or 'D98.' Our data support a model in which conformational changes in αIIbβ3 and/or fibrinogen after platelet activation and the interaction between γ-404-411 and the RGD binding pocket make new ancillary sites available that support higher-affinity fibrinogen binding and clot retraction.

Contraction of Blood Clots Is Impaired in Acute Ischemic Stroke

OBJECTIVE

Obstructive thrombi or thrombotic emboli are the pathogenic basis of ischemic stroke. In vitro blood clots and in vivo thrombi can undergo platelet-driven contraction (retraction), resulting in volume shrinkage. Clot contraction can potentially reduce vessel occlusion and improve blood flow past emboli or thrombi. The aim of this work was to examine a potential pathogenic role of clot contraction in ischemic stroke.

APPROACH AND RESULTS

We used a novel automated method that enabled us to quantify time of initiation and extent and rate of clot contraction in vitro. The main finding is that clot contraction from the blood of stroke patients was reduced compared with healthy subjects. Reduced clot contraction correlated with a lower platelet count and their dysfunction, higher levels of fibrinogen and hematocrit, leukocytosis, and other changes in blood composition that may affect platelet function and properties of blood clots. Platelets from stroke patents were spontaneously activated and displayed reduced responsiveness to additional stimulation. Clinical correlations with respect to severity and stroke pathogenesis suggest that the impaired clot contraction has the potential to be a pathogenic factor in ischemic stroke.

CONCLUSIONS

The changeable ability of clots and thrombi to shrink in volume may be a novel unappreciated mechanism that aggravates or alleviates the course and outcomes of ischemic stroke. The clinical importance of clot or thrombus transformations in vivo and the diagnostic and prognostic value of this blood test for clot contraction need further exploration.

[Contraction (retraction) of blood clots in patients with ischemic stroke]

AIM

To study a possible pathogenetic role of the blood clot contraction and its disturbances in the acute stage of ischemic stroke (IS).

MATERIAL AND METHODS

Using a new instrumental technique to study the dynamics of clot contraction in vitro, the authors have determined quantitative parameters of clot contraction (the extent and rate of contraction, duration of the lag-period) in the blood of 85 patients with acute IS.

RESULTS AND CONCLUSION

The contractile activity of blood clots was substantially reduced compared to the blood of healthy subjects. Correlations between hemostatic and contractile parameters suggest that the reduced clot contraction in stroke is due to the lower platelet count and impaired platelet functionality, higher levels of fibrinogen and antithrombin III as well as higher hematocrit and hemoglobin contents, leukocytosis, and changes in the biochemical blood composition. The results show that the reduced ability of clots may be a novel pathogenic mechanism that aggravates the course and outcomes of IS.

Fibrinogen Is at the Interface of Host Defense and Pathogen Virulence in Staphylococcus aureus Infection

Fibrinogen not only plays a pivotal role in hemostasis but also serves key roles in antimicrobial host defense. As a rapidly assembled provisional matrix protein, fibrin(ogen) can function as an early line of host protection by limiting bacterial growth, suppressing dissemination of microbes to distant sites, and mediating host bacterial killing. Fibrinogen-mediated host antimicrobial activity occurs predominantly through two general mechanisms, namely, fibrin matrices functioning as a protective barrier and fibrin(ogen) directly or indirectly driving host protective immune function. The potential of fibrin to limit bacterial infection and disease has been countered by numerous bacterial species evolving and maintaining virulence factors that engage hemostatic system components within vertebrate hosts. Bacterial factors have been isolated that simply bind fibrinogen or fibrin, promote fibrin polymer formation, or promote fibrin dissolution. Staphylococcus aureus is an opportunistic gram-positive bacterium, the causative agent of a wide range of human infectious diseases, and a prime example of a pathogen exquisitely sensitive to host fibrinogen. Indeed, current data suggest fibrinogen serves as a context-dependent determinant of host defense or pathogen virulence in Staphylococcus infection whose ultimate contribution is dictated by the expression of S. aureus virulence factors, the path of infection, and the tissue microenvironment.

Expanding the Mutation Spectrum Affecting αIIbβ3 Integrin in Glanzmann Thrombasthenia: Screening of the ITGA2B and ITGB3 Genes in a Large International Cohort

We report the largest international study on Glanzmann thrombasthenia (GT), an inherited bleeding disorder where defects of the ITGA2B and ITGB3 genes cause quantitative or qualitative defects of the αIIbβ3 integrin, a key mediator of platelet aggregation. Sequencing of the coding regions and splice sites of both genes in members of 76 affected families identified 78 genetic variants (55 novel) suspected to cause GT. Four large deletions or duplications were found by quantitative real-time PCR. Families with mutations in either gene were indistinguishable in terms of bleeding severity that varied even among siblings. Families were grouped into type I and the rarer type II or variant forms with residual αIIbβ3 expression. Variant forms helped identify genes encoding proteins mediating integrin activation. Splicing defects and stop codons were common for both ITGA2B and ITGB3 and essentially led to a reduced or absent αIIbβ3 expression; included was a heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B causing exon skipping in seven unrelated families. Molecular modeling revealed how many missense mutations induced subtle changes in αIIb and β3 domain structure across both subunits, thereby interfering with integrin maturation and/or function. Our study extends knowledge of GT and the pathophysiology of an integrin.

Coated platelets function in platelet-dependent fibrin formation via integrin αIIbβ3 and transglutaminase factor XIII

Coated platelets, formed by collagen and thrombin activation, have been characterized in different ways: i) by the formation of a protein coat of α-granular proteins; ii) by exposure of procoagulant phosphatidylserine; or iii) by high fibrinogen binding. Yet, their functional role has remained unclear. Here we used a novel transglutaminase probe, Rhod-A14, to identify a subpopulation of platelets with a cross-linked protein coat, and compared this with other platelet subpopulations using a panel of functional assays. Platelet stimulation with convulxin/thrombin resulted in initial integrin α(IIb)β3 activation, the appearance of a platelet population with high fibrinogen binding, (independently of active integrins, but dependent on the presence of thrombin) followed by phosphatidylserine exposure and binding of coagulation factors Va and Xa. A subpopulation of phosphatidylserine-exposing platelets bound Rhod-A14 both in suspension and in thrombi generated on a collagen surface. In suspension, high fibrinogen and Rhod-A14 binding were antagonized by combined inhibition of transglutaminase activity and integrin α(IIb)β3 Markedly, in thrombi from mice deficient in transglutaminase factor XIII, platelet-driven fibrin formation and Rhod-A14 binding were abolished by blockage of integrin α(IIb)β3. Vice versa, star-like fibrin formation from platelets of a patient with deficiency in α(IIb)β3(Glanzmann thrombasthenia) was abolished upon blockage of transglutaminase activity. We conclude that coated platelets, with initial α(IIb)β3 activation and high fibrinogen binding, form a subpopulation of phosphatidylserine-exposing platelets, and function in platelet-dependent star-like fibrin fiber formation via transglutaminase factor XIII and integrin α(IIb)β3.

Fibrin activates GPVI in human and mouse platelets

The glycoprotein VI (GPVI)-Fc receptor γ (FcRγ) chain is the major platelet signaling receptor for collagen. Paradoxically, in a FeCl3 injury model, occlusion, but not initiation of thrombus formation, is delayed in GPVI-deficient and GPVI-depleted mice. In this study, we demonstrate that GPVI is a receptor for fibrin and speculate that this contributes to development of an occlusive thrombus. We observed a marked increase in tyrosine phosphorylation, including the FcRγ chain and Syk, in human and mouse platelets induced by thrombin in the presence of fibrinogen and the αIIbβ3 blocker eptifibatide. This was not seen in platelets stimulated by a protease activated receptor (PAR)-4 peptide, which is unable to generate fibrin from fibrinogen. The pattern of tyrosine phosphorylation was similar to that induced by activation of GPVI. Consistent with this, thrombin did not induce tyrosine phosphorylation of Syk and the FcRγ chain in GPVI-deficient mouse platelets. Mouse platelets underwent full spreading on fibrin but not fibrinogen, which was blocked in the presence of a Src kinase inhibitor or in the absence of GPVI. Spreading on fibrin was associated with phosphatidylserine exposure (procoagulant activity), and this too was blocked in GPVI-deficient platelets. The ectodomain of GPVI was shown to bind to immobilized monomeric and polymerized fibrin. A marked increase in embolization was seen following FeCl3 injury in GPVI-deficient mice, likely contributing to the delay in occlusion in this model. These results demonstrate that GPVI is a receptor for fibrin and provide evidence that this interaction contributes to thrombus growth and stability.

Progress in the research of GPIIb/IIIa antagonists

This review covers the recent advances in the development of small RGD (Arg-Gly-Asp sequence) containing peptides and their mimetics as potential antithrombotic agents. Glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists include monoclonal antibodies, RGD peptides, peptide hybrids and nonpeptide mimetics. The current trend in the development of nonpeptide mimetics is clearly directed toward orally active and safe antithrombotic drug candidates. But several nonpeptide mimetics, being evaluated for their oral activity in human clinical trials, are currently not approved for clinical use due to poor safety profile. It is expected that newer and more effective nonpeptide mimetics will be developed in the near future.

Clinical and molecular characterisation of 21 patients affected by quantitative fibrinogen deficiency

Fibrinogen is a plasma glycoprotein mainly synthesised by hepatocytes and circulating as a 340-kDa hexamer consisting of two sets of three different polypeptide chains (Aα, Bβ, and γ, encoded by the FGA, FGB, and FGG gene, respectively). Congenital afibrinogenaemia and hypofibrinogenaemia are rare bleeding disorders characterised by abnormally low levels of functional and immunoreactive fibrinogen in plasma, associated with haemorrhagic manifestations of variable severity. While afibrinogenaemia is caused by mutations in the homozygous or compound heterozygous state in one of the three fibrinogen genes, hypofibrinogenaemia is generally due to heterozygous mutations, and is usually characterised by a milder phenotype. The mutational spectrum of these quantitative fibrinogen disorders includes large deletions, point mutations causing premature termination codons, and missense mutations often affecting fibrinogen assembly and/or secretion. Here we report the clinical and molecular characterisation of 13 unrelated afibrinogenaemic and eight hypofibrinogenaemic patients, leading to the identification of 17 different mutations (10 hitherto unknown). All the newly-identified missense and splicing mutations werein vitro expressed to verify their pathogenic role. Our data increase the number of mutations causing quantitative fibrinogen deficiencies by about 7 %. The high number of private mutations identified in the analysed probands indicates that the full mutational screening of the three fibrinogen genes is still required for molecular diagnosis.

A novel role for the fibrinogen Asn-Gly-Arg (NGR) motif in platelet function

The integrin αIIbβ3 on resting platelets can bind to immobilised fibrinogen resulting in platelet spreading and activation but requires activation to bind to soluble fibrinogen. αIIbβ3 is known to interact with the general integrin-recognition motif RGD (arginine-glycine-aspartate) as well as the fibrinogen-specific γ-chain dodecapeptide; however, it is not known how fibrinogen binding triggers platelet activation. NGR (asparagine-glycine-arginine) is another integrin-recognition sequence present in fibrinogen and this study aims to determine if it plays a role in the interaction between fibrinogen and αIIbβ3. NGR-containing peptides inhibited resting platelet adhesion to fibrinogen with an IC50 of 175 µM but failed to inhibit the adhesion of activated platelets to fibrinogen (IC50> 500 µM). Resting platelet adhesion to mutant fibrinogens lacking the NGR sequences was reduced compared to normal fibrinogen under both static and shear conditions (200 s⁻¹). However, pre-activated platelets were able to fully spread on all types of fibrinogen. Thus, the NGR motif in fibrinogen is the site that is primarily responsible for the interaction with resting αIIbβ3 and is responsible for triggering platelet activation.

Synergistic effect of signaling from receptors of soluble platelet agonists and outside-in signaling in formation of a stable fibrinogen-integrin αIIbβ3-actin cytoskeleton complex

INTRODUCTION

Thrombus formation in the injured vessel wall is a highly complex process involving various blood-born components that go through specific temporal and spatial changes as observed by intravital videomicroscopy. Platelets bind transiently to the developing thrombus and may either become stably incorporated into or disengage from the thrombus. The aim of the present study was to reveal the processes involved in the formation of a stable thrombus.

METHODS

Platelet-rich plasma and washed platelets were studied by the aggregometer. The aggregate stability was challenged by eptifibatide. Platelet Triton-insoluble fraction was prepared and the actin and αIIb content in the cytoskeleton was analyzed by western blot.

RESULTS

Maximal actin polymerization is achieved 1min after platelet activation while maximal αIIbβ3-actin cytoskeleton association requires 5 to 10min of activation and fibrinogen-mediated platelet-to-platelet bridging. Thus, actin polymerization is dependent on platelet activation and requires neither αIIbβ3 integrin occupation nor platelet aggregation. Formation of a stable aggregate requires platelet activation for more than 1min, complete increase in actin cytoskeleton fraction and partial association of αIIbβ3 with the actin cytoskeleton. However, direct αIIbβ3 activation is not sufficient for cytoskeleton complex formation. Thus, stable αIIbβ3-fibrinogen interaction, representing stable aggregate, is achieved after more than 1min agonist activation, involving inside-out and outside-in signaling but not after direct integrin activation, involving only outside-in signaling.

CONCLUSIONS

Formation of a stable fibrinogen-αIIbβ3-actin cytoskeleton complex is the result of the combined effect of platelet stimulation by soluble agonists, activation of αIIbβ3, fibrinogen binding and platelet-to-platelet bridging.

Fibrin, a scaffold material for islet transplantation and pancreatic endocrine tissue engineering

Fibrin is derived from fibrinogen during injury to produce a blood clot and thus promote wound repair. Composed of different domains, including Arg-Gly-Asp amino acid motifs, fibrin is used extensively as a hydrogel and sealant in the clinic. By binding to cell surface receptors like integrins and acting as a supportive 3D scaffold, fibrin has been useful in promoting cell differentiation, proliferation, function, and survival. In particular, fibrin has been able to maintain islet cell architecture, promote beta cell insulin secretion, and islet angiogenesis, as well as inducing a protective effect against cell death. During islet transplantation, fibrin improved neovascularization and islet function. These improvements resulted in reduced number of transplanted islets necessary to reverse diabetes. Therefore, fibrin, as a biocompatible and biodegradable scaffold, should be considered during subcutaneous islet transplantation and beta cell expansion in vitro to ensure maintenance of islet cell function, proliferation, and survival to develop effective cell-based therapies for the treatment of diabetes.