Comparative Ultrastructural Analyses of Mouse, Rabbit, and Human Platelets and Fibrin Networks

From Bedside to the Bench—A Call for Novel Approaches to Prognostic Evaluation and Treatment of Empyema

Empyema, a severe complication of pneumonia, trauma, and surgery is characterized by fibrinopurulent effusions and loculations that can result in lung restriction and resistance to drainage. For decades, efforts have been focused on finding a universal treatment that could be applied to all patients with practice recommendations varying between intrapleural fibrinolytic therapy (IPFT) and surgical drainage. However, despite medical advances, the incidence of empyema has increased, suggesting a gap in our understanding of the pathophysiology of this disease and insufficient crosstalk between clinical practice and preclinical research, which slows the development of innovative, personalized therapies. The recent trend towards less invasive treatments in advanced stage empyema opens new opportunities for pharmacological interventions. Its remarkable efficacy in pediatric empyema makes IPFT the first line treatment. Unfortunately, treatment approaches used in pediatrics cannot be extrapolated to empyema in adults, where there is a high level of failure in IPFT when treating advanced stage disease. The risk of bleeding complications and lack of effective low dose IPFT for patients with contraindications to surgery (up to 30%) promote a debate regarding the choice of fibrinolysin, its dosage and schedule. These challenges, which together with a lack of point of care diagnostics to personalize treatment of empyema, contribute to high (up to 20%) mortality in empyema in adults and should be addressed preclinically using validated animal models. Modern preclinical studies are delivering innovative solutions for evaluation and treatment of empyema in clinical practice: low dose, targeted treatments, novel biomarkers to predict IPFT success or failure, novel delivery methods such as encapsulating fibrinolysin in echogenic liposomal carriers to increase the half-life of plasminogen activator. Translational research focused on understanding the pathophysiological mechanisms that control 1) the transition from acute to advanced-stage, chronic empyema, and 2) differences in outcomes of IPFT between pediatric and adult patients, will identify new molecular targets in empyema. We believe that seamless bidirectional communication between those working at the bedside and the bench would result in novel personalized approaches to improve pharmacological treatment outcomes, thus widening the window for use of IPFT in adult patients with advanced stage empyema.

Precision targeting of the plasminogen activator inhibitor-1 mechanism increases efficacy of fibrinolytic therapy in empyema

Plasminogen activator inhibitor‐1 (PAI‐1) is an endogenous irreversible inhibitor of tissue‐type (tPA) and urokinase (uPA) plasminogen activators. PAI‐1‐targeted fibrinolytic therapy (PAI‐1‐TFT) is designed to decrease the therapeutic dose of tPA and uPA, attenuating the risk of bleeding and other complications. Docking site peptide (DSP) mimics the part of the PAI‐1 reactive center loop that interacts with plasminogen activators, thereby affecting the PAI‐1 mechanism. We used DSP for PAI‐1‐TFT in two rabbit models: chemically induced pleural injury and Streptococcus pneumoniae induced empyema. These models feature different levels of inflammation and PAI‐1 expression. PAI‐1‐TFT with DSP (2.0 mg/kg) converted ineffective doses of single chain (sc) tPA (72.5 µg/kg) and scuPA (62.5 µg/kg) into effective ones in chemically induced pleural injury. DSP (2.0 mg/kg) was ineffective in S. pneumoniae empyema, where the level of PAI‐1 is an order of magnitude higher. DSP dose escalation to 8.0 mg/kg resulted in effective PAI‐1‐TFT with 0.25 mg/kg sctPA (1/8th of the effective dose of sctPA alone) in empyema. There was no increase in the efficacy of scuPA. PAI‐1‐TFT with DSP increases the efficacy of fibrinolytic therapy up to 8‐fold in chemically induced (sctPA and scuPA) and infectious (sctPA) pleural injury in rabbits. PAI‐1 is a valid molecular target in our model of S. pneumoniae empyema in rabbits, which closely recapitulates key characteristics of empyema in humans. Low‐dose PAI‐1‐TFT is a novel interventional strategy that offers the potential to improve fibrinolytic therapy for empyema in clinical practice.

Antimalarial drugs impact chemical messenger secretion by blood platelets

Background

Advances in antimalarial drug development are important for combating malaria. Among the currently identified antimalarial drugs, it is suggested that some interact directly with the malarial parasites while others interact indirectly with the parasites. While this approach leads to parasite elimination, little is known about how these antimalarial drugs impact immune cells that are also critical in malarial response.

Methods

Herein, the effects of two common antimalarial drugs, chloroquine and quinine, on platelets were explored at both the bulk level, using high performance liquid chromatography, and the single cell level, using carbon-fiber microelectrode amperometry, to characterize any changes in chemical messenger secretion.

Results

The data reveal that both drugs cause platelet activation and reduce the number of platelet exocytosis events as well as delay fusion pore opening and closing.

Conclusions

This work demonstrates how chloroquine and quinine quantitatively and qualitatively impact in vitro platelet function.

General significance

Overall, the goal of this work is to promote understanding about how antimalarial drugs impact platelets as this may affect antimalarial drug development as well as therapeutic approaches to treat malarial infection.

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Antimalarial drugs impact platelet function by inducing activation.

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Single cell electrochemistry reveals changes in platelet function.

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It is important to consider platelet behaviors beyond aggregation to understand the side effects of antimalarial drugs.

Targeting plasminogen activator inhibitor-1 in tetracycline-induced pleural injury in rabbits

Elevated active plasminogen activator inhibitor-1 (PAI-1) has an adverse effect on the outcomes of intrapleural fibrinolytic therapy (IPFT) in tetracycline-induced pleural injury in rabbits. To enhance IPFT with prourokinase (scuPA), two mechanistically distinct approaches to targeting PAI-1 were tested: slowing its reaction with urokinase (uPA) and monoclonal antibody (mAb)-mediated PAI-1 inactivation. Removing positively charged residues at the "PAI-1 docking site" (¹⁷⁹RHRGGS¹⁸⁴→¹⁷⁹AAAAAA¹⁸⁴) of uPA results in a 60-fold decrease in the rate of inhibition by PAI-1. Mutant prourokinase (0.0625-0.5 mg/kg; n = 12) showed efficacy comparable to wild-type scuPA and did not change IPFT outcomes ( P > 0.05). Notably, the rate of PAI-1-independent intrapleural inactivation of mutant uPA was 2 times higher ( P < 0.05) than that of the wild-type enzyme. Trapping PAI-1 in a "molecular sandwich"-type complex with catalytically inactive two-chain urokinase with Ser¹⁹⁵Ala substitution (S195A-tcuPA; 0.1 and 0.5 mg/kg) did not improve the efficacy of IPFT with scuPA (0.0625-0.5 mg/kg; n = 11). IPFT failed in the presence of MA-56A7C10 (0.5 mg/kg; n = 2), which forms a stable intrapleural molecular sandwich complex, allowing active PAI-1 to accumulate by blocking its transition to a latent form. In contrast, inactivation of PAI-1 by accelerating the active-to-latent transition mediated by mAb MA-33B8 (0.5 mg/kg; n = 2) improved the efficacy of IPFT with scuPA (0.25 mg/kg). Thus, under conditions of slow (4-8 h) fibrinolysis in tetracycline-induced pleural injury in rabbits, only the inactivation of PAI-1, but not a decrease in the rate of its reaction with uPA, enhances IPFT. Therefore the rate of fibrinolysis, which varies in different pathologic states, could affect the selection of PAI-1 inhibitors to enhance fibrinolytic therapy.

Dose dependency of outcomes of intrapleural fibrinolytic therapy in new rabbit empyema models

The incidence of empyema (EMP) is increasing worldwide; EMP generally occurs with pleural loculation and impaired drainage is often treated with intrapleural fibrinolytic therapy (IPFT) or surgery. A number of IPFT options are used clinically with empiric dosing and variable outcomes in adults. To evaluate mechanisms governing intrapleural fibrinolysis and disease outcomes, models of Pasteurella multocida and Streptococcus pneumoniae were generated in rabbits and the animals were treated with either human tissue (tPA) plasminogen activator or prourokinase (scuPA). Rabbit EMP was characterized by the development of pleural adhesions detectable by chest ultrasonography and fibrinous coating of the pleura. Similar to human EMP, rabbits with EMP accumulated sizable, 20- to 40-ml fibrinopurulent pleural effusions associated with extensive intrapleural organization, significantly increased pleural thickness, suppression of fibrinolytic and plasminogen-activating activities, and accumulation of high levels of plasminogen activator inhibitor 1, plasminogen, and extracellular DNA. IPFT with tPA (0.145 mg/kg) or scuPA (0.5 mg/kg) was ineffective in rabbit EMP (n = 9 and 3 for P. multocida and S. pneumoniae, respectively); 2 mg/kg tPA or scuPA IPFT (n = 5) effectively cleared S. pneumoniae-induced EMP collections in 24 h with no bleeding observed. Although intrapleural fibrinolytic activity for up to 40 min after IPFT was similar for effective and ineffective doses of fibrinolysin, it was lower for tPA than for scuPA treatments. These results demonstrate similarities between rabbit and human EMP, the importance of pleural fluid PAI-1 activity, and levels of plasminogen in the regulation of intrapleural fibrinolysis and illustrate the dose dependency of IPFT outcomes in EMP.

The time course of resolution of adhesions during fibrinolytic therapy in tetracycline-induced pleural injury in rabbits

The time required for the effective clearance of pleural adhesions/organization after intrapleural fibrinolytic therapy (IPFT) is unknown. Chest ultrasonography and computed tomography (CT) were used to assess the efficacy of IPFT in a rabbit model of tetracycline-induced pleural injury, treated with single-chain (sc) urokinase plasminogen activators (scuPAs) or tissue PAs (sctPA). IPFT with sctPA (0.145 mg/kg; n = 10) and scuPA (0.5 mg/kg; n = 12) was monitored by serial ultrasonography alone (n = 12) or alongside CT scanning (n = 10). IPFT efficacy was assessed with gross lung injury scores (GLIS) and ultrasonography scores (USS). Pleural fluids withdrawn at 0-240 min and 24 h after IPFT were assayed for PA and fibrinolytic activities, α-macroglobulin/fibrinolysin complexes, and active PA inhibitor 1 (PAI-1). scuPA and sctPA generated comparable steady-state fibrinolytic activities by 20 min. PA activity in the scuPA group decreased slower than the sctPA group (kobs = 0.016 and 0.042 min(-1)). Significant amounts of bioactive uPA/α-macroglobulin (but not tPA; P < 0.05) complexes accumulated at 0-40 min after IPFT. Despite the differences in intrapleural processing, IPFT with either fibrinolysin was effective (GLIS ≤ 10) in animals imaged with ultrasonography only. USS correlated well with postmortem GLIS (r(2) = 0.85) and confirmed relatively slow intrapleural fibrinolysis after IPFT, which coincided with effective clearance of adhesions/organization at 4-8 h. CT scanning was associated with less effective (GLIS > 10) IPFT and higher levels of active PAI-1 at 24 h following therapy. We concluded that intrapleural fibrinolysis in tetracycline-induced pleural injury in rabbits is relatively slow (4-8 h). In CT-scanned animals, elevated PAI-1 activity (possibly radiation induced) reduced the efficacy of IPFT, buttressing the major impact of active PAI-1 on IPFT outcomes.

Carbon monoxide: Anticoagulant or procoagulant?

Within the past decade there have been several investigations attempting to define the impact of exogenous and endogenous carbon monoxide exposure on hemostasis. Critically, two bodies of literature have emerged, with carbon monoxide mediated platelet inhibition cited as a cause of in vitro human and in vitro/in vivo rodent anticoagulation. In contrast, interaction with heme groups associated with fibrinogen, α₂-antiplasmin and plasmin by carbon monoxide has resulted in enhanced coagulation and decreased fibrinolysis in vitro in human and other species, and in vivo in rabbits. Of interest, the ultrastructure of platelet rich plasma thrombi demonstrates an abnormal increase in fine fiber formation and matting that are obtained from humans exposed to carbon monoxide. Further, thrombi obtained from humans and rabbits have very similar ultrastructures, whereas mice and rats have more fine fibers and matting present. In sum, there may be species specific differences with regard to hemostatic response to carbon monoxide. Carbon monoxide may be a Janus-faced molecule, with potential to attenuate or exacerbate thrombophilic disease.

Time-of-flight secondary ion mass spectrometry with principal component analysis of titania-blood plasma interfaces

Treatment of osseoimplant surfaces with autologous platelet-rich plasma prepared according to the plasma rich in growth factors (PRGF-Endoret) protocol prior to implantation yields promising results in the clinic. Our objective is to understand the organization of complex interfaces between blood plasma preparations of various compositions and model titania surfaces. Here we present the results of the morphological and chemical characterization of TiO(2) surfaces incubated with four types of blood plasma preparations devoid of leukocytes and red blood cells: either enriched in platelets (PRGF-Endoret) or platelet-depleted, and either activated with CaCl(2) to induce clotting, or not. Chemical characterization was done by time-of-flight secondary ion mass spectrometry with principal component analysis (ToF-SIMS/PCA). The interface morphology was studied with scanning electron and atomic force microscopy. Immunofluorescence microscopy was used to identify platelets and infer their activation state. We observe clear differences among the four types of interfaces by ToF-SIMS/PCA. Some of these could be straightforwardly related to the differences in the sample morphology and known effects of platelet activation, but others are more subtle. Strikingly, it was possible to differentiate between these samples by ToF-SIMS/PCA of the protein species alone. This clearly indicates that the composition, orientation, and/or conformation of the proteins in these specimens depend both on the platelets' presence and on their activation. The ToF-SIMS imaging functionality furthermore provides unique insight into the distribution of phospholipid species in these samples.

Intrapleural adenoviral delivery of human plasminogen activator inhibitor-1 exacerbates tetracycline-induced pleural injury in rabbits

Elevated concentrations of plasminogen activator inhibitor-1 (PAI-1) are associated with pleural injury, but its effects on pleural organization remain unclear. A method of adenovirus-mediated delivery of genes of interest (expressed under a cytomegalovirus promoter) to rabbit pleura was developed and used with lacZ and human (h) PAI-1. Histology, β-galactosidase staining, Western blotting, enzymatic and immunohistochemical analyses of pleural fluids (PFs), lavages, and pleural mesothelial cells were used to evaluate the efficiency and effects of transduction. Transduction was selective and limited to the pleural mesothelial monolayer. The intrapleural expression of both genes was transient, with their peak expression at 4 to 5 days. On Day 5, hPAI-1 (40-80 and 200-400 nM of active and total hPAI-1 in lavages, respectively) caused no overt pleural injury, effusions, or fibrosis. The adenovirus-mediated delivery of hPAI-1 with subsequent tetracycline-induced pleural injury resulted in a significant exacerbation of the pleural fibrosis observed on Day 5 (P = 0.029 and P = 0.021 versus vehicle and adenoviral control samples, respectively). Intrapleural fibrinolytic therapy (IPFT) with plasminogen activators was effective in both animals overexpressing hPAI-1 and control animals with tetracycline injury alone. An increase in intrapleural active PAI-1 (from 10-15 nM in control animals to 20-40 nM in hPAI-1-overexpressing animals) resulted in the increased formation of PAI-1/plasminogen activator complexes in vivo. The decrease in intrapleural plasminogen-activating activity observed at 10 to 40 minutes after IPFT correlates linearly with the initial concentration of active PAI-1. Therefore, active PAI-1 in PFs affects the outcome of IPFT, and may be both a biomarker of pleural injury and a molecular target for its treatment.

Platelet signaling-a primer

OBJECTIVE

To review the receptors and signal transduction pathways involved in platelet plug formation and to highlight links between platelets, leukocytes, endothelium, and the coagulation system.

DATA SOURCES

Original studies, review articles, and book chapters in the human and veterinary medical fields.

DATA SYNTHESIS

Platelets express numerous surface receptors. Critical among these are glycoprotein VI, the glycoprotein Ib-IX-V complex, integrin α(IIb) β(3) , and the G-protein-coupled receptors for thrombin, ADP, and thromboxane. Activation of these receptors leads to various important functional events, in particular activation of the principal adhesion receptor α(IIb) β(3) . Integrin activation allows binding of ligands such as fibrinogen, mediating platelet-platelet interaction in the process of aggregation. Signals activated by these receptors also couple to 3 other important functional events, secretion of granule contents, change in cell shape through cytoskeletal rearrangement, and procoagulant membrane expression. These processes generate a stable thrombus to limit blood loss and promote restoration of endothelial integrity.

CONCLUSIONS

Improvements in our understanding of how platelets operate through their signaling networks are critical for diagnosis of unusual primary hemostatic disorders and for rational antithrombotic drug design.

Ultrastructural changes of platelets and fibrin networks in human asthma: a qualitative case study

Platelets and fibrin networks play an important role in asthma and the BALB/c asthmatic mouse model has previously been successfully used to study platelet ultrastructure. In control BALB/c mice, major, thick fibers and minor thin fibers and tight, platelet aggregates with typical pseudopodia formation, are present. Minor fibers of asthmatic mice have a netlike appearance covering the major fibers, whereas the platelets seem to form loosely connected, granular aggregates. The question that now arises is whether platelets and fibrin networks of humans with asthma will have the same ultrastructure as seen in the BALB/c asthmatic model. In order to answer this question, ultrastructure of platelets and fibrin networks from two participants (controlled asthma and uncontrolled, chronic asthma) were studied and compared with that of human controls and BALB/c asthmatic mice. Peak flow measurements of the controls and patients were also assessed. Results showed that similar platelet and fibrin network ultrastructure is found in uncontrolled, human participants and BALB/c asthmatic animals. The challenge when using animal models is always whether the model adequately mimics the human disease; the current research, therefore, shows morphological support for the use of this model in the study of asthma. These morphological results may also provide additional information to plan treatment regimes for sufferers of this very debilitating disease.

Ultrastructure of activated mouse platelets: a qualitative scanning electron microscopy study

Platelets form an integral part of the coagulation process, and their ultrastructure can provide valuable information regarding diseases associated with hemostasis. During coagulation, platelets aggregate; this aggregation can be achieved in vitro, by adding thrombin to platelet-rich plasma. Previous research showed that human thrombin could be used successfully to activate mouse platelets. When conservative changes are included, the amino acid similarity between human and mouse thrombin is approximately 75%. In this qualitative study, we compare the ultrastructure of mouse platelet aggregates activated by human thrombin as well as two concentrations of mouse thrombin, using the scanning electron microscope. Results show that both human and mouse thrombin activate platelets to form aggregates with typical pseudopodia formation. Magnification up to 250,000x showed membrane morphology with the open canalicular system pores visible in both the mouse- and human-activated platelets. It is therefore concluded that mouse platelets can be successfully aggregated using either mouse or human thrombin.

Ultrastructural morphology of platelets and fibrin networks of lactating and non-pregnant rabbits

Platelets and fibrin play an important role in the coagulation process where they are involved in the maintenance of haemostasis. Fibrin dysfunction is associated with the development of vascular complications, while proneness to the formation of tight and rigid fibrin networks is independently associated with thrombotic disease. Rabbits have long been used successfully as animal models, and are often the species of choice for models of antithrombotic efficacy. It was previously shown that rabbit and human platelet and fibrin morphology are very similar in ultrastructure and fibrin fibre thickness. It was also previously reported that the thin minor fibres forms a thick fine network cover over the major fibres during pregnancy. According to research, white blood cell counts also changes during pregnancy and stays changed for up to 6 weeks post-partum; where the number of neutrophils increased, and the number of lymphocytes, basophils and eosinophils decreased. Here, we show that the same ultrastructure and white blood cell count changes occur in lactating rabbits (4 weeks post-partum). We therefore suggest that a rabbit morphology model studying platelet and fibrin morphology can be used successfully, either to study the effect of pharmaceutical products to be used during lactation and pregnancy in humans, or used in veterinary research. Furthermore, the effects of pharmaceutical products on immunology and white blood cell counts can possibly also be used successfully.