Platelet activation and interactions with the microvasculature

Fibrinogen binding to activated platelets and its biomimetic thrombus-targeted thrombolytic strategies

Thrombosis is associated with various fatal arteriovenous syndromes including ischemic stroke, myocardial infarction, and pulmonary embolism. However, current clinical thrombolytic treatment strategies still have many problems in targeting and safety to meet the thrombolytic therapy needs. Understanding the molecular mechanism that underlies thrombosis is critical in developing effective thrombolytic strategies. It is well known that platelets play a central role in thrombosis and the binding of fibrinogen to activated platelets is a common pathway in the process of clot formation. Based on this, a concept of biomimetic thrombus-targeted thrombolytic strategy inspired from fibrinogen binding to activated platelets in thrombosis was proposed, which could selectively bind to activated platelets at a thrombus site, thus enabling targeted delivery and local release of thrombolytic agents for effective thrombolysis. In this review, we first summarized the main characteristics of platelets and fibrinogen, and then introduced the classical molecular mechanisms of thrombosis, including platelet adhesion, platelet activation and platelet aggregation through the interactions of activated platelets with fibrinogen. In addition, we highlighted the recent advances in biomimetic thrombus-targeted thrombolytic strategies which inspired from fibrinogen binding to activated platelets in thrombosis. The possible future directions and perspectives in this emerging area are briefly discussed.

Oxidative pathways of arachidonic acid as targets for regulation of platelet activation

Platelet activation plays an important role in acute and chronic cardiovascular disease states. Multiple pathways contribute to platelet activation including those dependent upon arachidonic acid. Arachidonic acid is released from the platelet membrane by phospholipase A2 action and is then metabolized in the cytosol by specific arachidonic acid oxidation enzymes including prostaglandin H synthase, 12-lipoxygenase, and cytochrome P450 to produce pro- and anti-inflammatory eicosanoids. This review aims to analyze the role of arachidonic acid oxidation on platelet activation, the enzymes that use it as a substrate associated as novel therapeutics target for antiplatelet drugs.

Platelets and Cardiovascular Disease

Platelets play an important, but often under-recognized role in cardiovascular disease. For example, the normal response of the platelet can be altered, either by increased pro-aggregatory stimuli or by diminished anti-aggregatory substances to produce conditions of increased platelet activation/aggregation and occur in active cardiovascular disease states both on a chronic (e.g. stable angina pectoris) and acute basis (e.g. acute myocardial infarction). In addition, platelet hyperaggregability is also associated with the risk factors for coronary artery disease (e.g. smoking, hypertension, and hypercholesterolaemia). Finally, the utility of an increasing range of anti-platelet therapies in the management of the above disease states further emphasizes the pivotal role platelets play in the pathogenesis of cardiovascular disease. This paper provides a comprehensive overview of the normal physiologic role of platelets in maintain homeostasis, the pathophysiologic processes that contribute to platelet dysfunction in cardiovascular disease and the associated role and benefits of anti-platelet therapies.

Evaluation of the hemocompatibility and rapid hemostasis of (RADA)4 peptide-based hydrogels

(RADA)4 peptides are promising biomaterials due to their high degree of hydration (<99.5% (w/v)), programmability at the molecular level, and their subsequent potential to respond to external stimuli. Interestingly, these peptides have also demonstrated the ability to cause rapid (∼15s) hemostasis when applied directly to wounds. General hemocompatibility of (RADA)4 nanofibers was investigated systematically using clot formation kinetics, C3a generation, and platelet activation (morphology and CD62P) studies. (RADA)4 nanofibers caused a rapid clot formation, but yielded a low platelet activation and low C3a activation. The study suggests that the rapid hemostasis observed when these materials are employed results principally from humoral coagulation, despite these materials having a net neutral charge and high hydration at physiological conditions. The observed rapid hemostasis may be induced due to the available nanofiber surface area within the hydrogel construct. In conclusion, our experiments strongly support further development of (RADA)4 peptide based biomaterials.

STATEMENT OF SIGNIFICANCE

Biomedicine based applications of (RADA)4 peptides are being extensively studied for the purpose of improving drug carriers, and 3D peptide nanofiber scaffolds. However, this peptide's biocompatibility has not been investigated till now. One particular study has reported a revolutionary and very desirable ability of (RADA)4 peptide to achieve complete and rapid hemostasis, nevertheless, the literature remains inconclusive on the underlying molecular mechanism. In this manuscript we bridge these two main knowledge gaps by providing the much needed systematic biocompatibility analysis (morphology analysis, platelet and C3a activation) of the (RADA)4 based hydrogels, and also investigate the underlying hemostatic mechanism of this peptide-induced hemostasis. Our work not only provides the much-needed biocompatibility of the peptide for applicative research, but also explores the molecular mechanism of hemostasis, which will help us design novel biomaterials to achieve hemostasis.

Toxicological study of the butanol fractionated root extract of Asparagus africanus Lam., on some blood parameter and histopathology of liver and kidney in mice

BACKGROUND

The butanol fractionated root extract of Asparagus africanus Lam., a traditional herb widely used to treat various ailments were analyzed for the presence of potential toxicity after single (acute) and repeated (subchronic) dose oral administration in adult swiss albino mice using gavages.

METHODS

For the acute study, butanol fractionated extract of the plant was administered in single doses of 1000, 3000 and 5000 mg/kg body weight. In the sub-chronic dose study, the extract was administered at doses of 300 and 600 mg/kg body weight/day for 42 days. Selected hematological and biochemical parameters of the blood followed by histopathological analysis were investigated after 42 days of daily administrations. The results were expressed as M ± SE, and differences at P < 0.05 was considered significant. One way analysis of variance (ANOVA) and least significant difference tests were employed to check the significant differences between the various parameters of the experimental groups.

RESULTS

In the acute study, the extract did not caused dose-dependent general behavioral adverse effects, body weight change and mortality. The single dose toxicity studies therefore showed that the butanol fraction of the extract has high safety profile when given orally. After 42 days of daily dosing, in the sub-chronic study, no clinically significant changes were observed for hematological and biochemical parameters. Except an occasional small number of focal mononuclear lymphocytic cells infiltrations around the central and portal triad of the liver of a few mice, the histopathological parameters do not show significant change.

CONCLUSION

It is concluded that, the butanol fractionated extract from A. africanus at the given dose does not show significant toxicity. The presence of focal inflammation on the liver of a few mice may be associated to the presence of flavonoid glycoside in the butanol fractionated extract.

Acute oral toxicity of Polyalthia longifolia var. pendula leaf extract in Wistar albino rats

CONTEXT

Polyalthia longifolia (Sonn.) Thw. var. pendula (Annonaceae), a tall evergreen tree, is cultivated all over India. The plant is used in traditional systems of medicine for the treatment of fever, skin diseases, and hypertension.

OBJECTIVE

The present study evaluated the acute oral toxicity of Polyalthia longifolia var. pendula leaf extract in Wistar albino rats.

MATERIAL AND METHODS

The parameters evaluated daily after oral drug administration of the extract (540, 1080, 2160 and 3240 mg/kg body weight) were mortality, signs of toxicity, feed and water consumption and body weight changes up to 14 days. The effect of different doses of the extract on organ weight, biochemical and hematological parameters were evaluated on the 15th day.

RESULTS AND CONCLUSION

Methanol extract of Polyalthia longifolia leaf up to the dose level 3240 mg/kg body weight did not produce any toxic effects or deaths; the extract was well tolerated by the rats. It did not alter body weight, feed and water consumption. The organ weight, biochemical and hematological analysis did not show any dose-dependent changes in any of the parameters examined in animals of both sexes. The acute oral administration of the methanol extract of Polyalthia longifolia leaf was not toxic and safe in a single dose.

Platelet aggregation in humans and nonhuman primates: relevance to xenotransplantation

INTRODUCTION

Platelet activation/aggregation plays a key role in the dysregulation of coagulation and the development of thrombotic microangiopathy in nonhuman primate recipients of pig xenografts. As a preliminary to the study of anti-platelet therapy in vitro and in vivo, the present study aimed to compare platelet aggregation in whole blood from humans, baboons, and cynomolgus monkeys.

METHODS

Using "Chrono-log" technology (two-sample four-channel Chrono-log Whole Blood Aggregometer), we studied aggregation of platelets in healthy humans (n = 8), baboons (n = 5), and monkeys (n = 8). Whole blood (WB) samples were collected, and platelet aggregation was assessed using three different volumes of blood (1, 0.5, and 0.25 ml). Platelet activation was induced using collagen (at 3 and 5 μg/ml), ristocetin (at 0.5 and 1.0 mg/ml), adenosine diphosphate (ADP; at 10, 20, and 40 μm), or thrombin (at 1 and 5 IU/ml). Inhibition of agonist-induced platelet aggregation by heparin and low molecular weight heparin (LMWH) (at 1, 10, and 100 IU/ml) was evaluated.

RESULTS

Mean platelet counts were 222.1, 263.2, and 276.1 (×10(3) /μl) in humans, baboons, and monkeys, respectively. In all three species, platelet aggregation was induced by collagen, ristocetin, ADP, or thrombin in a dose-dependent manner. A blood volume of 0.5 ml provided the most consistent results with all agonists in all three species. Dilution studies indicated that there was a significant positive correlation between platelet count and percent aggregation of platelets (P < 0.05). Collagen (3 and 5 μg/ml), ADP (10, 20, and 40 μm), and thrombin (1 and 5 IU/ml) induced significantly greater platelet aggregation in humans than in baboons. ADP (20 and 40 μm) and thrombin (1 and 5 IU/ml) induced significantly greater platelet aggregation in monkeys than in baboons. There was no species difference with ristocetin (0.5 or 1.0 mg/ml). In all species, thrombin (1 or 5 IU) induced greater platelet aggregation than any of the other reagents. Heparin at 1 IU/ml and LMWH at 10 IU/ml in all species almost completely abrogated thrombin-induced platelet aggregation. Heparin at 100 IU/ml effectively inhibited platelet aggregation induced by collagen, but only partially inhibited aggregation induced by ADP or ristocetin. LMWH only partially inhibited aggregation induced by collagen, ristocetin, and ADP.

CONCLUSIONS

The "Chrono-log" technology proved to be a reliable method of evaluating platelet activation and aggregation in vitro in primates. Species differences may play a role in platelet aggregation, with the monkey being more comparable to the human than the baboon, although overall trends were similar. In all species, thrombin induced greater platelet aggregation than other agonists. Even a concentration of heparin of 1 IU/ml, which is probably the maximal concentration that is clinically-applicable, prevented platelet aggregation induced by thrombin, but was less effective in preventing aggregation induced by collagen, ADP, or, particularly, ristocetin.

Factor v leiden and inflammation

Factor V Leiden, is a variant of human factor V (FV), also known as proaccelerin, which leads to a hypercoagulable state. Along these years, factor V Leiden (FVL) has been studied from the pathophysiologic point of view, and research has been focused on finding clinical approaches for the management of the FVL associated to a trombophilic state. Less attention has been paid about the possible role of FVL in inflammatory conditions known to be present in different disorders such as uremia, cirrhosis, liver transplantation, depression as well as sepsis, infection or, inflammatory bowel disease (IBD). Whether platelet FVL will increase the activation of coagulation and/or in which proportion is able to determine the final outcome in the previously mentioned inflammatory conditions is a subject that remains uncertain. This paper will review the association of FVL with inflammation. Specifically, it will analyze the important role of the endothelium and the contribution of other inflammatory components involved at both the immune and vascular levels. This paper will also try to emphasize the importance of being a FVL carrier in associations to diseases where a chronic inflammation occurs, and how this condition may be determinant in the progression and outcome of a specific clinic situation.

Liver damage and systemic inflammatory responses are exacerbated by the genetic deletion of CD39 in total hepatic ischemia

Liver ischemia reperfusion injury is associated with both local damage to the hepatic vasculature and systemic inflammatory responses. CD39 is the dominant vascular endothelial cell ectonucleotidase and rapidly hydrolyses both adenosine triphosphate (ATP) and adenosine diphosphate to adenosine monophosphate. These biochemical properties, in tandem with 5'-nucleotidases, generate adenosine and potentially illicit inflammatory vascular responses and thrombosis. We have evaluated the role of CD39 in total hepatic ischemia reperfusion injury (IRI). Wildtype mice, Cd39-hemizygous mice (+/-) and matched Cd39-null mice (-/-); (n = 25 per group) underwent 45 min of total warm ischemia with full inflow occlusion necessitating partial hepatectomy. Soluble nucleoside triphosphate diphosphohydrolase (NTPDases) or adenosine/amrinone were administered to wildtype (n = 6) and Cd39-null mice (n = 6) in order to study protective effects in vivo. Parameters of liver injury, systemic inflammation, hepatic ATP determinations by P(31)-NMR and parameters of lung injury were obtained. All wildtype mice survived up to 7 days with minimal biochemical disturbances and minor evidence for injury. In contrast, 64% of Cd39+/- and 84% of Cd39-null mice required euthanasia or died within 4 h post-reperfusion with liver damage and systemic inflammation associated with hypercytokinemia. Hepatic ATP depletion was pronounced in Cd39-null mice posthepatic IRI. Soluble NTPDase or adenosine administration protected Cd39-deficient mice from acute reperfusion injury. We conclude that CD39 is protective in hepatic IRI preventing local injury and systemic inflammation in an adenosine dependent manner. Our data indicate that vascular CD39 expression has an essential protective role in hepatic IRI.

Profound thrombocytopenia and survival of hematopoietic stem cell transplant patients without clinically significant bleeding, using prophylactic platelet transfusion triggers of 10 x 10(9) or 20 x 10(9) per L

BACKGROUND

A trigger of 10 x 10(9) per L for prophylactic platelet (PLT) transfusions is generally recommended for stable thrombocytopenic patients who receive chemotherapy, based on studies showing similar incidence, severity, and fatality of bleeding compared with the 20 x 10(9) per L trigger. The outcome of thrombocytopenic nonbleeding patients has not been well described. This retrospective analysis evaluates thrombocytopenia and survival of 381 hematopoietic stem cell transplant (HSCT) patients without clinically significant bleeding, with 10 x 10(9) and 20 x 10(9) per L prophylactic triggers.

STUDY DESIGN AND METHODS

A total of 170 patients who received prophylactic PLT transfusions at 20 x 10(9) per L (1997-1998, SP1) and 211 patients who had prophylaxis at 10 x 10(9) per L (1999-2001, SP2) were identified as nonbleeding patients. PLT counts and clinical complications were assessed within 100 days from HSCT.

RESULTS

PLT counts less than or equal to 10 x 10(9) per L were found in 69.2 percent of patients in SP2 and 38.3 percent in SP1 (p < 0.001). Profound thrombocytopenia (4+ PLT counts <or=10 x 10(9)/L) was found in 19.0 percent of patients in SP2 and 7.0 percent in SP1 (p = 0.001). Patients with profound thrombocytopenia had significantly increased early mortality (odds ratio [OR], 3.18; 95% confidence interval [CI], 1.25-8.07) and significantly reduced overall survival (hazard ratio [HR], 1.95; 95% CI, 1.28-2.97) compared to patients with 0 to 3 PLT counts less than or equal to 10 x 10(9) per L. The association of profound thrombocytopenia with early mortality was more notable in SP2.

CONCLUSION

The 10 x 10(9) per L transfusion trigger is associated with significantly greater exposure to low PLT counts. Nonbleeding patients with profound thrombocytopenia were at significantly greater risk of dying compared with nonthrombocytopenic patients. These results suggest that safety of the 10 x 10(9) per L trigger should be more thoroughly evaluated.

Lung ischemia-reperfusion injury: implications of oxidative stress and platelet-arteriolar wall interactions

Pulmonary ischemia-reperfusion (IR) injury may result from trauma, atherosclerosis, pulmonary embolism, pulmonary thrombosis and surgical procedures such as cardiopulmonary bypass and lung transplantation. IR injury induces oxidative stress characterized by formation of reactive oxygen (ROS) and reactive nitrogen species (RNS). Nitric oxide (NO) overproduction via inducible nitric oxide synthase (iNOS) is an important component in the pathogenesis of IR. Reaction of NO with ROS forms RNS as secondary reactive products, which cause platelet activation and upregulation of adhesion molecules. This mechanism of injury is particularly important during pulmonary IR with increased iNOS activity in the presence of oxidative stress. Platelet-endothelial interactions may play an important role in causing pulmonary arteriolar vasoconstriction and post-ischemic alveolar hypoperfusion. This review discusses the relationship between ROS, RNS, P-selectin, and platelet-arteriolar wall interactions and proposes a hypothesis for their role in microvascular responses during pulmonary IR.

Role of P-Selectin in Platelet Sequestration in Pulmonary Capillaries during Endotoxemia

BACKGROUND

There is growing evidence that platelets accumulate in the lung and contribute to the pathogenesis of acute lung injury during endotoxemia. The aims of the present study were to localize platelet sequestration in the pulmonary microcirculation and to investigate the role of P-selectin as a molecular mechanism of platelet endothelial cell interaction.

METHODS

We used in vivo fluorescence microscopy to quantify the kinetics of fluorescently labeled erythrocytes and platelets in alveolar capillary networks in rabbit lungs.

RESULTS

Six hours after onset of endotoxin infusion we observed a massive rolling along and firm adherence of platelets to lung capillary endothelial cells whereas under control conditions no platelet sequestration was detected. P-selectin was expressed on the surface of separated platelets which were incubated with endotoxin and in lung tissue. Pretreatment of platelets with fucoidin, a P-selectin antagonist, significantly attenuated the endotoxin-induced platelet rolling and adherence. In contrast, intravenous infusion of fucoidin in endotoxin-treated rabbits did not inhibit platelet sequestration in pulmonary capillaries.

CONCLUSION

We conclude that platelets accumulate in alveolar capillaries following endotoxemia. P-selectin expressed on the surface of platelets seems to play an important role in mediating this platelet-endothelial cell interaction.

Effects of pulmonary ischemia-reperfusion on platelet adhesion in subpleural arterioles in rabbits

Reperfusion of the ischemic lung is associated with increased pulmonary vascular resistance and reduced alveolar perfusion in conjunction with an inflammatory response. To determine the contribution of platelet-endothelial interactions, we examined effects of pulmonary ischemia-reperfusion (IR) on platelet adhesion and diameter of arterioles and investigated the hypothesis that this process is P-selectin mediated. In anesthetized rabbits with open-chest and ventilated lungs, we examined subpleural arterioles by fluorescence microscopy. Ischemia was caused by reversibly occluding the right pulmonary artery for 2 h. Fluorescently labeled platelets were injected into the right atrium and the right lung was observed after 0.5, 1.0, and 2.0 h of reperfusion. Platelets rolling and adherence along arterioles occurred with a decrease in diameter that was significant during IR, but not after 3- to 5-min occlusion (control). Systemic pretreatment with Fucoidan (a ligand to P- and L-selectin) inhibited platelet rolling, adherence, and the decrease in diameter. Pretreatment of only exogenously labeled platelets with monoclonal antibody (MoAb) to P-selectin prevented platelet rolling and adherence, but not the decrease in diameter. These results indicate that in the intact lung, pulmonary IR causes platelet rolling and adhesion along arteriolar walls, and suggest that this process, which is mediated by P-selectin, contributes to vasoconstriction and hypoperfusion. Thus, it appears that platelet-endothelial interactions may contribute to the development of pulmonary IR injury.

Isosorbide-5-mononitrate treatment prevents cyclosporin A-induced platelet hyperactivation and the underlying nitric oxide-cyclic guanosine-3',5'-monophosphate disturbances

INTRODUCTION

The clinical use of cyclosporin A (CsA) is commonly associated with the development of hypertension and increased risk of thromboembolic events. Decreased endothelium-dependent relaxation and increased platelet activation seems to be involved on those side effects, but the underlying mechanisms are not yet elucidated. The present study aimed to evaluate the CsA effect on the platelet NO-cyclic guanosine-3',5'-monophosphate (cGMP) pathway and the putative benefits of concomitant isosorbide-5-mononitrate (IS-5-MN) administration on CsA-induced hypertension and on platelet hyperactivation.

MATERIALS AND METHODS

Blood pressures, platelet NO synthase activity and cGMP content, intracellular free calcium concentration ([Ca2+]i) and whole blood platelet aggregation were assessed in three rat groups orally treated, during 7 weeks, with the following diets: orange juice (control group), 5 mg/kg/day of CsA (CsA group) and 150 mg/kg/day, b.i.d., of IS-5-MN for 2 weeks and IS-5-MN plus 5 mg/kg/day of CsA for 7 weeks (IS-5-MN+CsA group).

RESULTS

IS-5-MN treatment has prevented hypertension development obtained in the solely CsA-treated rats. CsA treatment has inhibited NOS activity, which was reverted by the concomitant IS-5-MN and CsA administration. On the contrary, platelets from CsA-treated rats had cGMP content increased when compared with the control rats. The variation obtained when ISMN was present was less predominant. Therefore, the organic nitrate treatment has prevented platelet hyperactivation, namely, by decreasing thrombin-evoked [Ca2+]i and collagen-evoked platelet aggregation, when compared with the solely CsA-treated group. The preventive effect of IS-5-MN was reinforced by electron microscopy studies of platelet activation.

CONCLUSIONS

By increasing [Ca2+]i and aggregation, CsA induces platelet hyperactivation and simultaneously increases cGMP content, which might represent a compensatory inhibitory mechanism. The concomitant IS-5-MN treatment prevents the above-mentioned platelet hyperreactivity and tends to normalize the NO-cGMP pathway as well as the development of hypertension.

Low venular shear rates promote leukocyte-dependent recruitment of adherent platelets

The influence of reductions in venular shear rate on platelet-endothelial (P/E) cell adhesion has not been previously addressed. The objectives of this study were to define the effects of reductions in venular shear rate on P/E cell adhesion and to determine the interdependence of P/E cell adhesion and leukocyte-endothelial (L/E) cell adhesion at low shear rates. Intravital videomicroscopy was used to quantify P/E and L/E cell adhesion in rat mesenteric venules exposed to shear rates ranging between 118 +/- 9 and 835 +/- 44 s(-1). Shear rate was altered in postcapillary venules by rapid, graded blood withdrawal, without retransfusion of shed blood. Reducing shear rate from >600 s(-1) to <200 s(-1) resulted in an eightfold increase in L/E cell adhesion, whereas P/E cell adhesion increased 18-fold. A blocking antibody directed against P-selectin blunted both the P/E and L/E cell adhesion elicited by low shear rates. Immunoneutralization of CD11/CD18 on leukocytes or rendering animals neutropenic also blocked the shear rate-dependent recruitment of both platelets and leukocytes. These findings indicate that 1) low shear rates promote P/E and L/E cell adhesion in mesenteric venules, and 2) adherent neutrophils (mediated by CD11/CD18) create a platform onto which platelets can bind to the venular wall at low shear rates.

The signal transduction pathway of the nonintegrin receptor of 65 kDa is different from glycoprotein VI

The identity and signal pathways of a platelet nonintegrin receptor for type I collagen, 65 kDa, are not established. In this investigation, we have examined whether there is a difference in the signal transduction pathways between the 65-kDa protein and glycoprotein VI (GP VI). Results from this study show that these two proteins are different based on the following facts. First, the anti-65-kDa antibody does not precipitate GP VI and vice versa. Second, the Fc receptor (FcR) gamma chain which associates with GP VI after exposure to collagen does not associate with the 65-kDa protein. Third, tyrosine phosphorylation of the FcR gamma chain was obtained by Fab fragments of anti-GP VI but not by anti-65 kDa. These results suggest that the signal transduction pathway of the platelet receptors for the 65-kDa protein and GP VI are different.

Platelet interactions with titanium: modulation of platelet activity by surface topography

Endosseous implants initially come into contact with blood. Thus, the nature of the interactions between blood and implanted endosseous implants may influence subsequent bone healing events in the peri-implant healing compartment. We conducted studies to address the following question: Does implant surface microtexture modulate platelet activity? We used commercially pure Ti (cpTi) disks with four different surface finishes: dual acid-etched (DAE), 320 grit (320G) abraded, machined, and p1200 polished cpTi. Surfaces were characterized by scanning electron microscopy (SEM) and optical profilometry. The DAE and 320G surfaces presented more complex microtextures than the machined or polished surfaces. Platelet activities were measured by quantifying platelet adherence, platelet-derived microparticle (MP) formation, and P-selectin expression as function of surface type. Platelet adhesion, measured using a lactate dehydrogenase (LDH) assay. was increased on DAE and 320G surfaces compared to machined and polished surfaces (p < 0.05). M P formation and P-selectin expression, assayed by flow cytometry, also showed increased activation of platelets on DAE and 320G surfaces. Because increased activation of platelets may lead to up-regulation of osteogenic responses during bone healing, these results may explain the enhanced osteoconductivity known to occur with DAE cpTi surfaces in comparison with machined cpTi surfaces.

Inhibition of platelet aggregation in baboons: therapeutic implications for xenotransplantation

Activation of endothelial cells and platelet sequestration play major roles in rejection of xenografts. The histopathology of both hyperacute and acute vascular or delayed rejection of vascularized discordant xenografts is characterized by interstitial hemorrhage and intravascular thrombosis. Agents that prevent platelet activation and consequent microthrombus formation have proven beneficial in xenograft rejection but do not fully preclude vascular thrombosis. Recently, several new anti-platelet therapies have undergone extensive clinical testing for atherosclerotic thrombotic vascular disorders; other putative therapies are undergoing pre-clinical evaluation. We have investigated the effect of several of these novel agents on platelet aggregation in baboons in order to screen for future potential in xenograft rejection models.

METHODS

Drugs tested in these experiments were aurintricarboxylic acid (ATA, von Willebrand Factor-GPIb inhibitor), fucoidin (a selectin-inhibitor), 1-benzylimidazole (1-BI, thromboxane synthase antagonist), prostacyclin (PGI2, endothelial stabilizer), heparin (thrombin antagonist), nitroprusside sodium or nicotinamide (NPN or NA, both NO-donors), and eptifibatide (EFT, GPIIb/IIIa receptor antagonist). These were infused intravenously to nine baboons. Coagulation parameters and platelet counts were monitored and baboons were observed for adverse side-effects. The efficacy of these agents in inhibiting platelet aggregation was assayed in a platelet aggregometer.

RESULTS

Treatment with ATA and fucoidin resulted in complete inhibition of platelet aggregation but also in major perturbation of coagulation parameters. 1-BI and PGI2 had no effect when administered alone, but in combination resulted in moderate inhibition of aggregation without disturbance in PT or PTT. NPN and NA had no substantive effects on platelet aggregation. Heparin resulted in specific inhibition of thrombin-induced platelet aggregation and, as anticipated, was associated with moderate prolongation of PTT. Importantly, EFT caused complete inhibition of platelet aggregation without changes in coagulation. Platelet counts, fibrinogen levels, and fibrinogen degradation products remained within the normal ranges in all experiments.

CONCLUSIONS

Although excellent inhibition of platelet activation was obtained with ATA and fucoidin, clinical use may be precluded by concomitant disturbances of coagulation. Combinations of heparin and EFT may prove beneficial in preventing the thrombotic disorders associated with xenograft rejection while maintaining adequate hemostatic responses. These agents are to be evaluated in our pig-to-primate xenotransplantation models.

Platelet activation is increased in cyclosporin A-induced hypertensive rats

One of the most severe side effects of the immunosuppressive agent, cyclosporin A (CsA), is increased risk of thromboembolic complications and drug-related hypertension. Because platelets might be involved in these processes, we tested the possibility of CsA affecting platelet activation, which might contribute to these adverse drug reactions. The experiments were done using Wistar rats, treated or not (control) with CsA (Sandimmun Neoral), 5 and 30 mg/kg/day, for 7 weeks. Systolic, diastolic, and mean blood pressures, intracellular free calcium concentration ([Ca2+]i), platelet serotonin (5-HT) contents, and aggregation were determined, at weeks 0, 2, and 7 of treatment. Inositol phosphates (InsP) production, platelet thromboxane A2 (TXA2) generation, and morphology of platelets, through electron microscopy studies, also were compared. It was demonstrated that blood pressures increased in the CsA-treated groups, when compared with the control group, after 2 and 7 weeks of administration. CsA at both "attack" and "maintenance" doses increased basal, 5-HT, and thrombin-evoked [Ca2+]i after 2 and 7 weeks versus the control group. However, basal and evoked InsP production was stimulated by 5 mg/kg of CsA, but inhibited by 30 mg/kg, when compared with the control. Platelet 5-HT contents decreased significantly after 2 and 7 weeks in the CsA-treated groups, when compared with the control group. Collagen-induced whole blood platelet aggregation increased drastically in the "attack" CsA-treated group, whereas adenosine diphosphate (ADP)-induced platelet aggregation did not reach statistical significance. Finally, in vitro basal, collagen-, and ADP-evoked platelet TXA2 generation increased in both CsA concentrations, versus the control. In conclusion, our study demonstrates that both CsA doses alter platelet calcium homeostasis (even affecting the calcium fluxes differently), 5-HT and TXA2 contents and aggregation, which might contribute to the development and/or maintenance of high blood pressures and increased risk of thromboembolic complications.

The in vitro blood compatibility of poly(ethylene oxide)-grafted polyurethane/polystyrene interpenetrating polymer networks

Poly(ethylene oxide) (PEO)-grafted polyurethane (PU)/polystyrene (PS) interpenetrating polymer networks (IPNs) were synthesized. The effects of the mobile pendant PEO chains with their microphase separated structure on blood-compatibility were investigated. The morphology of both the fracture surface as well as the top surface indicate that the size of the dispersed domains of the PS-rich phase decreased as the grafting with the PEO was increased. The swelling ratio also decreased as the grafting with the PEO was increased. However, the dynamic contact angle and the interfacial energy between IPN surface and water decreased, due to the structural reorganization of the pendant PEO chains. PU/PS IPNs have an excellent mechanical property as compared with PU homopolymers. The adsorption of bovine plasma fibrinogen (BPF) onto the PU/PS IPNs and PU homopolymers was effectively suppressed by the PEO-grafting. In the platelet adhesion test, the amount of platelets adsorbed, activated, and/or coagulated upon the PEO-grafted PU/PS IPNs were reduced when compared to the ungrafted PU homopolymers.

Platelets induce sinusoidal endothelial cell apoptosis upon reperfusion of the cold ischemic rat liver

BACKGROUND & AIMS

Sinusoidal endothelial cell (SEC) apoptosis is a central feature of reperfusion injury in liver transplantation. Platelet sequestration occurs after transplantation with possible deleterious effects. We tested the hypothesis that platelets mediate SEC apoptosis.

METHODS

Livers were perfused after 24 hours of cold preservation in University of Wisconsin solution in an isolated perfused rat liver model. The perfusate contained isolated syngeneic red blood cells and purified platelets. Effects of inhibiting platelet adhesion on SEC apoptosis was tested using sialyl Lewis-X oligosaccharide (sLe(x)), a natural ligand of selectin adhesion molecules. Reperfusion injury was assessed by established markers of injury. Apoptosis was determined by TUNEL and electron microscopy.

RESULTS

A third of the circulating platelets was rapidly sequestered in the liver after reperfusion. This was associated with increased graft injury. Single platelets were adherent to sinusoidal lining without morphological or dynamic evidence of impairment of microcirculation. TUNEL staining revealed a 6-fold increase in the number of apoptotic SECs at 1 hour of reperfusion. No hepatocyte death or evidence of necrosis was detected up to 3 hours of reperfusion. Addition of sLe(x) inhibited adhesion and significantly reduced SEC apoptosis.

CONCLUSIONS

Platelets cause SEC apoptosis upon reperfusion of liver grafts. Prevention of adhesion is protective.

The pathogenesis of vascular thrombosis and its impact in microvascular surgery

As the use of free tissue transfer becomes more wide-spread, it is important for both the ablative surgeon and the reconstructive surgeon to understand the factors that contribute to successful revascularized tissue transfer. The purpose of this two part review is to provide a basic science overview of the problem of failed free tissue transfers. The first part will focus on the pathogenesis of thrombosis at the anastomotic site, and part two will discuss the pathogenesis of the no-reflow phenomenon. The pathophysiology and therapeutic interventions to prevent and treat anastomotic thrombosis and the no-reflow phenomenon will be discussed.

spe-12 encodes a sperm cell surface protein that promotes spermiogenesis in Caenorhabditis elegans

During spermiogenesis, Caenorhabditis elegans spermatids activate and mature into crawling spermatozoa without synthesizing new proteins. Mutations in the spe-12 gene block spermatid activation, rendering normally self-fertile hermaphrodites sterile. Mutant males, however, are fertile. Surprisingly, when mutant hermaphrodites mate with a male, their self-spermatids activate and form functional spermatozoa, presumably due to contact with male seminal fluid. Here we show that, in addition to its essential role in normal activation of hermaphrodite-derived spermatids, SPE-12 also plays a supplementary but nonessential role in mating-induced activation. We have identified the spe-12 gene, which encodes a novel protein containing a single transmembrane domain. spe-12 mRNA is expressed in the sperm-producing germ line and the protein localizes to the spermatid cell surface. We propose that SPE-12 functions downstream of both hermaphrodite- and male-derived activation signals in a spermatid signaling pathway that initiates spermiogenesis.

Increased adhesion and aggregation of platelets lacking cyclic guanosine 3',5'-monophosphate kinase I

Atherosclerotic vascular lesions are considered to be a major cause of ischemic diseases, including myocardial infarction and stroke. Platelet adhesion and aggregation during ischemia-reperfusion are thought to be the initial steps leading to remodeling and reocclusion of the postischemic vasculature. Nitric oxide (NO) inhibits platelet aggregation and smooth muscle proliferation. A major downstream target of NO is cyclic guanosine 3', 5'-monophosphate kinase I (cGKI). To test the intravascular significance of the NO/cGKI signaling pathway in vivo, we have studied platelet-endothelial cell and platelet-platelet interactions during ischemia/reperfusion using cGKI-deficient (cGKI-/-) mice. Platelet cGKI but not endothelial or smooth muscle cGKI is essential to prevent intravascular adhesion and aggregation of platelets after ischemia. The defect in platelet cGKI is not compensated by the cAMP/cAMP kinase pathway supporting the essential role of cGKI in prevention of ischemia-induced platelet adhesion and aggregation.

Platelet-Endothelial Cell Interactions During Ischemia/Reperfusion: The Role of P-Selectin

Growing evidence supports a pathophysiological role for platelets during the manifestation of postischemic reperfusion injury; in the current study, we investigated the nature and the molecular determinants of platelet-endothelial cell interactions induced by ischemia/reperfusion (I/R). Platelet-endothelium and leukocyte-endothelium interactions after 1 hour of ischemia were monitored in vivo within mouse small intestine. By intravital fluorescence microscopy, we observed that platelets, like leukocytes, roll along or firmly adhere to postischemic microvascular endothelial cells. In contrast, few leukocyte-endothelial cell interactions were detected in sham-operated controls. Monoclonal antibodies against P-selectin significantly attenuated platelet rolling and adherence in response to I/R. To identify whether platelet or endothelial P-selectin plays the major role in mediating postischemic platelet-endothelial cell interactions, P-selectin-deficient or wild-type platelets were transfused into wild-type or P-selectin-deficient mice, respectively. Whereas platelets lacking P-selectin rolled along or adhered to postischemic wild-type endothelium, interactions between wild-type platelets with mutant endothelium were nearly absent, indicating that I/R-induced platelet-endothelium interactions are dependent on the expression of P-selectin by endothelial cells. Concomitantly, P-selectin expression in the intestinal microvasculature was enhanced in response to I/R, whereas no upregulation of P-selectin was observed on circulating platelets. In summary, we provide first in vivo evidence that platelets accumulate in the postischemic microvasculature early after reperfusion via P-selectin-ligand interactions. Platelet recruitment and subsequent activation might play an important role in the pathogenesis of I/R injury.

Platelet-Endothelial Cell Interactions During Ischemia/Reperfusion: The Role of P-Selectin

Growing evidence supports a pathophysiological role for platelets during the manifestation of postischemic reperfusion injury; in the current study, we investigated the nature and the molecular determinants of platelet-endothelial cell interactions induced by ischemia/reperfusion (I/R). Platelet-endothelium and leukocyte-endothelium interactions after 1 hour of ischemia were monitored in vivo within mouse small intestine. By intravital fluorescence microscopy, we observed that platelets, like leukocytes, roll along or firmly adhere to postischemic microvascular endothelial cells. In contrast, few leukocyte-endothelial cell interactions were detected in sham-operated controls. Monoclonal antibodies against P-selectin significantly attenuated platelet rolling and adherence in response to I/R. To identify whether platelet or endothelial P-selectin plays the major role in mediating postischemic platelet-endothelial cell interactions, P-selectin-deficient or wild-type platelets were transfused into wild-type or P-selectin-deficient mice, respectively. Whereas platelets lacking P-selectin rolled along or adhered to postischemic wild-type endothelium, interactions between wild-type platelets with mutant endothelium were nearly absent, indicating that I/R-induced platelet-endothelium interactions are dependent on the expression of P-selectin by endothelial cells. Concomitantly, P-selectin expression in the intestinal microvasculature was enhanced in response to I/R, whereas no upregulation of P-selectin was observed on circulating platelets. In summary, we provide first in vivo evidence that platelets accumulate in the postischemic microvasculature early after reperfusion via P-selectin-ligand interactions. Platelet recruitment and subsequent activation might play an important role in the pathogenesis of I/R injury.

Platelet concentrate effects on thromboelastography

OBJECTIVE

This study evaluated platelet effects on thromboelastography to determine how morphologically abnormal platelets affected native whole blood analysis.

DESIGN

Prospective, controlled comparison.

SETTING

Tertiary care university hospital.

PARTICIPANTS

Volunteer cardiac surgery patients.

INTERVENTIONS

Fresh platelets were obtained from volunteers and were either treated normally or cryodisrupted with liquid nitrogen. Fresh platelets, liquid nitrogen-treated platelets, or an equivalent quantity of the patient's blood were added to whole blood samples obtained from cardiac surgery patients before heparinization. Thromboelastographic parameters sensitive to platelet effects were measured in each of the three groups.

MEASUREMENTS AND MAIN RESULTS

Maximum amplitude and alpha-angle significantly increased in the two groups receiving added platelets. There were no differences between the fresh platelet and the liquid nitrogen-treated platelet groups (Student's paired t-test). The R-time decreased significantly in both platelet-treated groups compared with the group that did not receive additional platelets.

CONCLUSIONS

Viscoelastic changes in whole blood coagulation after the addition of platelet concentrates are not dependent on morphologically intact or functionally normal platelets. This in vitro study predicts that transfusion of poorly preserved platelet concentrates as well as fresh platelets would increase clot strength on thromboelastography if the recipient's blood were tested immediately after administration.

Suppressed anti-aggregating and cGMP-elevating effects of sodium nitroprusside in platelets from patients with stable angina pectoris

Platelet hyperactivity plays an important role in the pathogenesis of cardio-vascular diseases. In patients with stable angina pectoris, we have recently demonstrated that nitroglycerin suppressed the increased platelet aggregability. The anti-aggregating effect of NTG and other nitrovasodilators is mediated by platelet guanylate cyclase, which generates cyclic GMP (cGMP) in response to nitric oxide (NO) liberated from the nitrovasodilator molecule. In the current study we utilised a more "direct" NO donor, sodium nitroprusside (SNP), to examine reversal of ADP-induced platelet aggregation in comparison with intraplatelet cGMP elevation in platelets from normal subjects (n = 22) and patients with stable angina pectoris (n = 23). Concentrations of SNP associated with 50% reversal of aggregation were 2.7 +/- 0.4 x 10(-7) mol/L with normal subjects and 4.5 +/- 0.5 x 10(-6) mol/L with patients (P < 0.01). SNP produced a concentration-dependent elevation of intraplatelet cGMP content: with 10(-4) mol/L SNP this was 17-fold for normals and 5-fold for patients (P < 0.01). An increase in cAMP content was seen only with 10(-4) mol/L SNP, and was 157 +/- 11% of baseline in platelets from normal subjects and 138 +/- 14% in patients. There was a strong interrelationship between cGMP-stimulating and anti-aggregating effects of SNP. The decrease in cGMP responsiveness to SNP was not related to a dysfunction of platelet guanylate cyclase; neither basal nor SNP-stimulated activity of the enzyme varied significantly between normal subjects and patients. Lipophilic derivatives of cGMP (db-cGMP) and cAMP (db-cAMP) caused reversal of aggregation; there was a nonsignificant trend towards decreased responsiveness of platelets from patients to both db-cGMP and db-cAMP. The observed decrease in responsiveness of platelets from angina patients to anti-aggregating effects of the exogenous NO donor, SNP, can therefore be attributed to suppressed cGMP accumulation. These results imply reduced platelet sensitivity to endogenous NO (endothelium-derived relaxing factor): this might contribute to platelet hyperaggregability observed in angina pectoris.

Endothelial cell injury in cardiovascular surgery: the pathophysiology of vasomotor dysfunction

Impaired vasomotor function has been suggested as playing a role in the pathophysiology of hypertension, diabetes, hypercholesterolemia, and atherosclerosis, all of which are common in cardiovascular surgery patients. In addition to chronic vasomotor dysfunction, alterations in vasomotor tone can result in acute arterial spasm, microcirculatory ischemia, and wide variations in systemic blood pressure. Changes in the health of the vascular endothelium may also impact the late patency of coronary artery bypass grafts, the progression of atherosclerosis in the native coronary circulation, and the long-term success of cardiac transplants. In the resting state the endothelium produces several substances that promote vascular relaxation and inhibition of platelet function, thus assuring the unhindered flow of blood through the capillaries. In response to injury, the endothelium loses some capacity to relax and also releases powerful vasoconstrictive agents. Attempting to understand the contributions that these substances play in the vasomotor dysfunction seen after cardiothoracic surgery is an area of active investigation.