Molecular mechanisms of thrombus formation in ischemic stroke: novel insights and targets for treatment

Early detrimental T-cell effects in experimental cerebral ischemia are neither related to adaptive immunity nor thrombus formation

T cells contribute to the pathophysiology of ischemic stroke by yet unknown mechanisms. Mice with transgenic T-cell receptors (TCRs) and mutations in costimulatory molecules were used to define the minimal immunologic requirements for T cell–mediated ischemic brain damage. Stroke was induced in recombination activating gene 1–deficient (RAG1−/−) mice devoid of T and B cells, RAG1−/− mice reconstituted with B cells or T cells, TCR-transgenic mice bearing 1 single CD8+ (2C/RAG2, OTI/RAG1 mice) or CD4+ (OTII/RAG1, 2D2/RAG1 mice) TCR, mice lacking accessory molecules of TCR stimulation (CD28−/−, PD1−/−, B7-H1−/− mice), or mice deficient in nonclassical T cells (natural killer T [NKT] and γδ T cells) by transient middle cerebral artery occlusion (tMCAO). Stroke outcome was assessed at day 1. RAG1−/− mice and RAG1−/− mice reconstituted with B cells developed significantly smaller brain infarctions compared with controls, but thrombus formation after FeCl3-induced vessel injury was unimpaired. In contrast, TCR-transgenic mice and mice lacking costimulatory TCR signals were fully susceptible to tMCAO similar to mice lacking NKT and γδ T cells. These findings were corroborated by adoptive transfer experiments. Our data demonstrate that T cells critically contribute to cerebral ischemia, but their detrimental effect neither depends on antigen recognition nor TCR costimulation or thrombus formation.

ADAMTS13 gene deletion aggravates ischemic brain damage: a possible neuroprotective role of ADAMTS13 by ameliorating postischemic hypoperfusion

Reperfusion after brain ischemia causes thrombus formation and microcirculatory disturbances, which are dependent on the platelet glycoprotein Ib–von Willebrand factor (VWF) axis. Because ADAMTS13 cleaves VWF and limits platelet-dependent thrombus growth, ADAMTS13 may ameliorate ischemic brain damage in acute stroke. We investigated the effects of ADAMTS13 on ischemia-reperfusion injury using a 30-minute middle cerebral artery occlusion model in Adamts13−/− and wild-type mice. After reperfusion for 0.5 hours, the regional cerebral blood flow in the ischemic cortex was decreased markedly in Adamts13−/− mice compared with wild-type mice (P < .05), which also resulted in a larger infarct volume after 24 hours for Adamts13−/− compared with wild-type mice (P < .01). Thus, Adamts13 gene deletion aggravated ischemic brain damage, suggesting that ADAMTS13 may protect the brain from ischemia by regulating VWF-platelet interactions after reperfusion. These results indicate that ADAMTS13 may be a useful therapeutic agent for stroke.

COU254, a specific 3-carboxamide-coumarin inhibitor of coagulation factor XII, does not protect mice from acute ischemic stroke

Background

Anticoagulation is an important means to prevent from acute ischemic stroke but is associated with a significant risk of severe hemorrhages. Previous studies have shown that blood coagulation factor XII (FXII)-deficient mice are protected from pathological thrombus formation during cerebral ischemia without bearing an increased bleeding tendency. Hence, pharmacological blockade of FXII might be a promising and safe approach to prevent acute ischemic stroke and possibly other thromboembolic disorders but pharmacological inhibitors selective over FXII are still lacking. In the present study we investigated the efficacy of COU254, a novel nonpeptidic 3-carboxamide-coumarin that selectively blocks FXII activity, on stroke development and post stroke functional outcome in mice.

Methods

C57Bl/6 mice were treated with COU254 (40 mg/kg i.p.) or vehicle and subjected to 60 min transient middle cerebral artery occlusion (tMCAO) using the intraluminal filament method. After 24 h infarct volumes were determined from 2,3,5-Triphenyltetrazoliumchloride(TTC)-stained brain sections and functional scores were assessed. Hematoxylin and eosin (H&E) staining was used to estimate the extent of neuronal cell damage. Thrombus formation within the infarcted brain areas was analyzed by immunoblot.

Results

Infarct volumes and functional outcomes on day 1 after tMCAO did not significantly differ between COU254 pre-treated mice or untreated controls (p > 0.05). Histology revealed extensive ischemic neuronal damage regularly including the cortex and the basal ganglia in both groups. COU254 treatment did not prevent intracerebral fibrin(ogen) formation.

Conclusions

COU254 at the given concentration of 40 mg/kg failed to demonstrate efficacy in acute ischemic stroke in this preliminary study. Further preclinical evaluation of 3-carboxamide-coumarins is needed before the antithrombotic potential of this novel class of FXII inhibitors can be finally judged.

Accumulation of gene polymorphisms related to plaque disruption and thrombosis is associated with cerebral infarction in subjects with type 2 diabetes

OBJECTIVE

It is believed that disruption of vulnerable atherosclerotic plaque and subsequent thrombus formation play critical roles in the pathogenesis of cerebral infarction. We simultaneously determined four relatively common genetic variants related to plaque rupture or subsequent local thrombus formation and evaluated the combined effect on cerebral infarction.

RESEARCH DESIGN AND METHODS

We enrolled 3,094 Japanese type 2 diabetic subjects (62.7% male; aged 61.5 +/- 8.4 years) and determined their genotypes regarding matrix metalloproteinase 9 C-1562T, coagulation factor XII (F12) C46T, von Willebrand factor (VWF) G-1051A, and plasminogen activator inhibitor (PAI-1) 675 4G/5G polymorphisms. The diagnosis of cerebral infarction was performed based on history, physical examination, and neuroimaging.

RESULTS

The single association analysis revealed that there were no statistically significant associations between each polymorphism and the prevalence of cerebral infarction. Interestingly, the prevalence of cerebral infarction was higher with the increase of the total number of four concomitant unfavorable proatherothrombotic alleles in each subject (P value for linear trend = 0.004). Furthermore, a multiple logistic regression analysis showed that the number of proatherothrombotic alleles was a risk factor for cerebral infarction independently of conventional risk factors (odds ratio for one-point increase in the number of proatherothrombotic allele 1.15 [95% CI 1.05-1.26], P = 0.004).

CONCLUSIONS

Accumulation of gene polymorphisms related to plaque rupture and thrombus formation is likely associated with the prevalence of cerebral infarction in type 2 diabetic patients, suggesting that the combined information about these variants is useful to assess the risk of cerebral infarction.

Repeated inspiratory occlusions in anesthetized rats acutely increase blood coagulability as assessed by thromboelastography

Many of the components contributing to coagulability are enhanced by repeated episodes of hypoxia, as occurs in obstructive sleep apnea, but no one has yet measured the global hemostatic properties of blood in an animal model of this disease. Using thromboelastography, a hemostatic assay, we measured hemostasis in six pentobarbital-anesthetized rats before and after 3h of repeated inspiratory occlusions lasting 30s applied every 2 min and compared the results to those in six identically prepared rats before and after 3h of resting breathing. Rats subjected to occlusions displayed faster onset of clotting (p<0.031) and more rapid coagulation (p<0.031). Thus, repeated inspiratory occlusions acutely cause hypercoagulability in rats. Thromboelastography, a simple test of hemostasis, may help evaluate the factors responsible for this increase and, in patients with obstructive sleep apnea, the risk of future cardiovascular disease.

Impaired alpha(IIb)beta(3) integrin activation and shear-dependent thrombus formation in mice lacking phospholipase D1

Platelet aggregation is essential for hemostasis but can also cause myocardial infarction and stroke. A key but poorly understood step in platelet activation is the shift of the principal adhesive receptor, alpha(IIb)beta(3) integrin, from a low- to high-affinity state for its ligands, a process that enables adhesion and aggregation. In response to stimulation of heterotrimeric guanosine triphosphate-binding protein or immunoreceptor tyrosine-based activation motif-coupled receptors, phospholipases cleave membrane phospholipids to generate lipid and soluble second messengers. An essential role in platelet activation has been established for phospholipase C (PLC) but not for PLD and its product phosphatidic acid. Here, we report that platelets from Pld1(-/-) mice displayed impaired alpha(IIb)beta(3) integrin activation in response to major agonists and defective glycoprotein Ib-dependent aggregate formation under high shear conditions. These defects resulted in protection from thrombosis and ischemic brain infarction without affecting tail bleeding times. These results indicate that PLD1 may be a critical regulator of platelet activity in the setting of ischemic cardiovascular and cerebrovascular events.

Molecular priming of Lyn by GPVI enables an immune receptor to adopt a hemostatic role

The immune receptor signaling pathway is used by nonimmune cells, but the molecular adaptations that underlie its functional diversification are not known. Circulating platelets use the immune receptor homologue glycoprotein VI (GPVI) to respond to collagen exposed at sites of vessel injury. In contrast to immune cell responses, platelet activation must take place within seconds to successfully form thrombi in flowing blood. Here, we show that the GPVI receptor utilizes a unique intracellular proline-rich domain (PRD) to accelerate platelet activation, a requirement for efficient platelet adhesion to collagen under flow. The GPVI PRD specifically binds the Src-family kinase Lyn and directly activates it, presumably through SH3 displacement. In resting platelets, Lyn is constitutively bound to GPVI in an activated state and platelets lacking Lyn exhibit defective collagen adhesion like that of platelets with GPVI receptors lacking the PRD. These findings define a molecular priming mechanism that enables an immune-type receptor to adopt a hemostatic function. These studies also demonstrate that active kinases can constitutively associate with immune-type receptors without initiating signal transduction before receptor ligation, consistent with a recent molecular model of immune receptor signaling in which receptor ligation is required to bring active kinases to their receptor substrates.

Calcium signaling in platelets

Agonist-induced elevation in cytosolic Ca2+ concentrations is essential for platelet activation in hemostasis and thrombosis. It occurs through Ca2+ release from intracellular stores and Ca2+ entry through the plasma membrane (PM). Ca2+ store release is a well-established process involving phospholipase (PL)C-mediated production of inositol-1,4,5-trisphosphate (IP3), which in turn releases Ca2+ from the intracellular stores through IP3 receptor channels. In contrast, the mechanisms controlling Ca2+ entry and the significance of this process for platelet activation have been elucidated only very recently. In platelets, as in other non-excitable cells, the major way of Ca2+ entry involves the agonist-induced release of cytosolic sequestered Ca2+ followed by Ca2+ influx through the PM, a process referred to as store-operated calcium entry (SOCE). It is now clear that stromal interaction molecule 1 (STIM1), a Ca2+ sensor molecule in intracellular stores, and the four transmembrane channel protein Orai1 are the key players in platelet SOCE. The other major Ca2+ entry mechanism is mediated by the direct receptor-operated calcium (ROC) channel, P2X1. Besides these, canonical transient receptor potential channel (TRPC) 6 mediates Ca2+ entry through the PM. This review summarizes the current knowledge of platelet Ca2+ homeostasis with a focus on the newly identified Ca2+ entry mechanisms.

Cl- and F- anions regulate the architecture of protofibrils in fibrin gel

Ischemic heart disease is the leading cause of serious morbidity and mortality in Western society. One of the therapeutic approaches is based on the use of thrombolitic drugs that promote clot lysis. Even if the mechanisms leading to clot lysis are not completely understood, it is widely accepted that they depend on the complex biochemical reactions that occur among fibrin fibers and fibrinolitic agents, and by their ready diffusion into the fibers. Here we investigate the effects of specific anions on the architecture of protofibrils within fibrin fibers in fibrin gels prepared in a para-physiological solution. The results obtained through small-angle X-ray scattering (SAXS) demonstrate that the characteristic axial and longitudinal repeat distances among protofibrils are strongly affected by the action of Cl(-) and F(-) anions.

Deficiency of von Willebrand factor protects mice from ischemic stroke

We recently demonstrated that blockade of the platelet adhesion receptor glycoprotein (GP) Ibalpha protects mice from ischemic stroke. Although von Willebrand factor (VWF) is the major ligand for GPIbalpha, GPIbalpha can engage other counterreceptors on endothelial cells, platelets, and leukocytes (eg, Mac-1 or P-selectin) potentially involved in stroke outcome. To further analyze whether VWF is of particular relevance for stroke development, VWF(-/-) mice underwent 60 minutes of middle cerebral artery occlusion. After 24 hours, VWF(-/-) mice had significantly smaller infarctions (P< .05) and less severe neurologic deficits (P< .01) compared with controls. This effect was sustained after 1 week, and intracranial bleeding was absent in VWF(-/-) mice as revealed by serial magnetic resonance imaging. Hydrodynamic injection of a VWF-encoding plasmid restored the susceptibility for stroke in VWF(-/-) mice. This study indicates that VWF is critically involved in cerebral ischemia. Hence, targeted inhibition of the GPIbalpha-VWF pathway might become a promising therapeutic option.