Platelet secretory products may contribute to neuronal injury

Prospects of modeling poststroke epileptogenesis

This Review describes the current status of poststroke epilepsy (PSE) with an emphasis on poststroke epileptogenesis modeling for testing new therapeutic agents. Stroke is a leading cause of epilepsy in an aging population. Late-onset "epileptic" seizures have been reported in up to 30% cases after stroke. Nevertheless, the overall prevalence of PSE is 2-4%. Rodent models of stroke have contributed to our understanding of the relationship between seizures and the underlying ischemic damage to neurons. To understand whether acutely generated stroke events lead to a chronic phenotype more closely resembling PSE with recurrent seizures, a limited variety of approaches emerged in early 2000s. These limited methods of causing an occlusion in mice and rats show different infarct size and neurological deficits. The most often employed procedure for inducing focal ischemia is the middle cerebral artery occlusion. This mimics the pathophysiology seen in humans in terms of extent of damage to cortex and striatum. Photothrombosis and endothelin-1 models can similarly evoke episodes of ischemic stroke. These models are well suited to studying mechanisms and biomarkers of epileptogenesis or optimizing novel drug discoveries. However, modeling of PSE is tedious, is highly variable, and lacks validity; therefore, it is not widely implemented in epilepsy research. Moreover, the relevance of ischemic models to specific forms of human stroke remains unclear. Stroke modeling in young male rodents lacks clinical relevance to elderly populations and especially to women, likely as a result of sex differences. Nevertheless, because of the neuronal damage and epileptogenic insult that these models trigger, they are helpful tools in studying acquired epilepsy and prophylactic drug therapy. © 2016 Wiley Periodicals, Inc.

The secretome of apoptotic human peripheral blood mononuclear cells attenuates secondary damage following spinal cord injury in rats

After spinal cord injury (SCI), secondary damage caused by oxidative stress, inflammation, and ischemia leads to neurological deterioration. In recent years, therapeutic approaches to trauma have focused on modulating this secondary cascade. There is increasing evidence that the success of cell-based SCI therapy is due mainly to secreted factors rather than to cell implantation per se. This study investigated peripheral blood mononuclear cells as a source of factors for secretome- (MNC-secretome-) based therapy. Specifically, we investigated whether MNC-secretome had therapeutic effects in a rat SCI contusion model and its possible underlying mechanisms. Rats treated with MNC-secretome showed substantially improved functional recovery, attenuated cavity formation, and reduced acute axonal injury compared to control animals. Histological evaluation revealed higher vascular density in the spinal cords of treated animals. Immunohistochemistry showed that MNC-secretome treatment increased the recruitment of CD68(+) cells with concomitant reduction of oxidative stress as reflected by lower expression of inducible nitric oxide synthase. Notably, MNC-secretome showed angiogenic properties ex vivo in aortic rings and spinal cord tissue, and experiments showed that the angiogenic potential of MNC-secretome may be regulated by CXCL-1 upregulation in vivo. Moreover, systemic application of MNC-secretome activated the ERK1/2 pathway in the spinal cord. Taken together, these results indicate that factors in MNC-secretome can mitigate the pathophysiological processes of secondary damage after SCI and improve functional outcomes in rats.

Activated platelets rescue apoptotic cells via paracrine activation of EGFR and DNA-dependent protein kinase

Platelet activation is a frontline response to injury, not only essential for clot formation but also important for tissue repair. Indeed, the reparative influence of platelets has long been exploited therapeutically where application of platelet concentrates expedites wound recovery. Despite this, the mechanisms of platelet-triggered cytoprotection are poorly understood. Here, we show that activated platelets accumulate in the brain to exceptionally high levels following injury and release factors that potently protect neurons from apoptosis. Kinomic microarray and subsequent kinase inhibitor studies showed that platelet-based neuroprotection relies upon paracrine activation of the epidermal growth factor receptor (EGFR) and downstream DNA-dependent protein kinase (DNA-PK). This same anti-apoptotic cascade stimulated by activated platelets also provided chemo-resistance to several cancer cell types. Surprisingly, deep proteomic profiling of the platelet releasate failed to identify any known EGFR ligand, indicating that activated platelets release an atypical activator of the EGFR. This study is the first to formally associate platelet activation to EGFR/DNA-PK – an endogenous cytoprotective cascade.

Characterization of a thromboembolic photochemical model of repeated stroke in mice

Many stroke research groups utilize the model of middle cerebral artery occlusion induced by insertion of an intraluminal thread, owing to its pragmatism and reliability of cerebral infarct generation. However, 75% of stroke cases result from a thromboembolic event and 10% from occlusive atherothrombosis in situ. Here, we characterize a mouse model of repeated thromboembolic stroke, which closely mimics the intravascular pathophysiology of arterial thrombus generation from an atherosclerotic plaque, and subsequent release of a thrombus into the cerebral circulation as an embolus. Common carotid artery thrombosis (CCAT) was induced photochemically leading to non-occlusive platelet aggregation in C57/BL6 male mice (n=35), and was followed by mechanical assistance to facilitate release of the thrombus (MRT) and thus promote embolism. Six experimental groups, differing by changes in the surgical protocol, were used for the purpose of determining which such procedure yielded the most reliable and consistent brain infarct volumes with the lowest mortality at 3 days after surgery. The group which best satisfied these conditions was a double insult group which consisted of animals that underwent CCAT for 2 min by means of argon laser irradiation (514.5 nm) at an intensity of ca. 130 W/cm(2), with concomitant injection of erythrosin B (EB) (35 mg/kg infused over those same 2 min), followed by MRT 1 min later; the entire procedure was repeated 24h later. This group showed a percent of brain lesion volume of 15+/-4% (mean+/-S.D.) with no associated 3-day mortality. Compared to a single insult group which sustained a percent brain lesion volume of 7+/-3%, there was a statistically significant (p<0.05) increase in the volume of infarction in the double-insult group.

Cell-derived microparticles and exosomes in neuroinflammatory disorders

All blood cells and the vascular endothelium shed microparticles (MP) from their plasma membranes when suitably stimulated, and assay of MP in patient blood has found increasing application to the monitoring of disease states. In addition, mounting evidence suggests that MP are not mere epiphenomena but play significant roles in the pathophysiology of thromboses, inflammation, and cancers. This chapter endeavors to summarize the limited number of studies thus far done on MP in neurological disorders such as multiple sclerosis (MS), transient ischemic attacks, and the neurological manifestations of antiphospholipid syndrome (APS). In addition, the chapter offers some plausible hypotheses on possible roles of MP in the pathophsyiology of these disorders, chiefly, the hypothesis that MP are indeed important participants in some neuropathologies, especially those which are ischemic in nature, but probably also inflammatory ones. The chapter also goes over the history and general principles of MP studies (e.g., assay methods and pitfalls), comparison with alternative methods (e.g., soluble markers of disease states), subclasses of MP (such as exosomes), and other topics aimed at helping readers to consider MP studies in their own clinical fields. Tables include a listing of bioactive agents known to be carried on MP, many of which were heretofore considered strictly soluble, and some of which can be transferred from cell to cell via MP vectors, for example certain cytokine receptors.

Transcerebral platelet activation after aortic cross-clamp release is linked to neurocognitive decline

BACKGROUND

Neurocognitive decline after cardiac surgery requiring cardiopulmonary bypass (CPB) may be caused in part by highly prothrombotic atheroemboli to the brain; the source of these emboli is likely the ascending aorta and aortic arch. We examined transcerebral platelet activation gradients using simultaneous measurements in arterial and jugular venous blood and then compared gradients with post-CPB-associated neurocognitive injury.

METHODS

Eighty-one patients undergoing elective coronary artery bypass graft surgery requiring CPB were studied. Neurocognitive function was measured preoperatively and again at 6 weeks postoperatively. Paired arterial and jugular venous blood samples were drawn before surgery, immediately before and after aortic cross-clamp removal (an event previously linked to embolic showers), and at the end of the operation. Transcerebral platelet activation gradients (venous minus arterial values) were compared in patients with and without cognitive deficit.

RESULTS

Immediately after aortic cross-clamp removal, there was a significant increase in the transcerebral platelet activation gradient (increased % P-selectin-positive platelets during transcerebral passage) in the subset of patients who subsequently developed post-CPB cognitive deficit; this platelet activation gradient did not occur in patients without cognitive injury. In contrast, there was no transcerebral gradient of platelet activation in CPB patients as an entirety, nor was there a gradient at all other time points in the patient subset who went on to have cognitive deficit develop. This fleeting gradient of transcerebral platelet activation after cross-clamp removal was also significantly correlated with the overall change in cognitive injury score.

CONCLUSIONS

Transient intracerebral platelet activation after removal of the aortic cross-clamp is associated with post-CPB neurocognitive injury.

Electrophysiological effects of 5-hydroxytryptamine on isolated human atrial myocytes, and the influence of chronic beta-adrenoceptor blockade

5-Hydroxytryptamine (5-HT) has been postulated to play a proarrhythmic role in the human atria via stimulation of 5-HT4 receptors. The aims of this study were to examine the effects of 5-HT on the L-type Ca2+ current (ICaL) action potential duration (APD), the effective refractory period (ERP) and arrhythmic activity in human atrial cells, and to assess the effects of prior treatment with beta-adrenoceptor antagonists. Isolated myocytes, from the right atrial appendage of 27 consenting patients undergoing cardiac surgery who were in sinus rhythm, were studied using the whole-cell perforated patch-clamp technique at 37 degrees C. 5-HT (1 nm-10 microm) caused a concentration-dependent increase in ICaL, which was potentiated in cells from beta-blocked (maximum response to 5-HT, Emax=299+/-12% increase above control) compared to non-beta-blocked patients (Emax=220+/-6%, P<0.05), but with no change in either the potency (log EC50: -7.09+/-0.07 vs -7.26+/-0.06) or Hill coefficient (nH: 1.5+/-0.6 vs 1.5+/-0.3) of the 5-HT concentration-response curve. 5-HT (10 microm) produced a greater increase in the APD at 50% repolarisation (APD50) in cells from beta-blocked patients (of 37+/-10 ms, i.e. 589+/-197%) vs non-beta-blocked patients (of 10+/-4 ms, i.e. 157+/-54%; P<0.05). Both the APD90 and the ERP were unaffected by 5-HT. Arrhythmic activity was observed in response to 5-HT in five of 17 cells (29%) studied from beta-blocked, compared to zero of 16 cells from the non-beta-blocked patients (P<0.05). In summary, the 5-HT-induced increase in calcium current was associated with a prolonged early plateau phase of repolarisation, but not late repolarisation or refractoriness, and the enhancement of these effects by chronic beta-adrenoceptor blockade was associated with arrhythmic potential.

Does increased platelet release of Abeta peptide contribute to brain abnormalities in individuals with depression?

Increased platelet activation with release of procoagulant factors from their alpha granules has been demonstrated in individuals with major depression. Platelet activation has also been shown to be associated with release of beta-amyloid peptides, which have been implicated in Alzheimer's disease. Thus, we are hypothesizing that sustained elevations of Abeta peptides might occur in individuals with recurrent depression. We further hypothesize that such elevations contribute to brain abnormalities in depressed individuals through the formation of neurotoxic oligomeric forms of Abeta peptides and amyloid deposition. We also propose that increased amyloid Abeta peptides from platelet activation may be a mechanism underlying the increased risk for cognitive impairment in nondepressed patients who have other reasons for such activation. If true, our hypothesis would imply that platelet inhibitors may have a role in preventing or delaying the neuronal consequences of disorders characterized by activated platelets.

Atypical presentations of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) usually presents in a fulminant fashion with marked hematologic abnormalities often associated with fever and neurologic and renal abnormalities. Two cases of TTP are reported in which neurologic deficits were initially the dominant findings, followed weeks later by the pronounced hematologic manifestations. Young patients with unexplained neurologic findings, even if there is only minimal anemia or thrombocytopenia, should be monitored for subsequent manifestations of TTP so that treatment may be instituted promptly.

Subarachnoid hemorrhage induces dynamic changes in regional cerebral metabolism in rats

Following a subarachnoid hemorrhage (SAH), adult rats exhibit dynamic regional changes in cerebral glucose metabolism characterized by an increase in metabolic rates and a subsequent upregulation of cytochrome oxidase (CO). We evaluated both local cerebral metabolic rates for glucose (ICMRglc: (mol/100 g/min) and CO in 23 brain regions of interest (ROI). Sham animals underwent anesthesia and superficial surgery; saline-controls received an injection of 0.9% saline into the cisterna magna; and SAH rats received an injection of autologous blood into the cisterna magna. This blood, measured by albumin labeled with radioactive carbon 14, distributed throughout the brain but predominated ventrally. After experimental animals were sacrificed at day 0 (3 h), 1, 3, and 7 days postinjection, ROI were analyzed using [14C]2-deoxy-D-glucose autoradiography and CO histochemistry. ICMRglc in SAH rats increased in many regions (ranging from 0.7% to 32.2% above sham levels). Cytochrome oxidase also increased from 1% to 9% above sham levels, peaking on day 3. Conversely, saline-controls exhibited prolonged depression of ICMRglc (ranging from 11% to 35% below sham levels) and CO (ranging from 4% to 11% below sham levels) from day 0 through day 7. All saline-control ROI for all time points showed this metabolic depression, and between 91% and 95% of saline-control ROI presented lower CO levels as compared to sham. Overall, ICMRglc and CO levels were greater in SAH than in saline-control ROI. However, when considering the influence of subarachnoid blood on metabolic changes in SAH animals, both CO and 2DG levels did not correlate well with the amount of 14C-albumin binding. While previous studies have measured both metabolic rates of glucose and CO soon after SAH, this is the first to simultaneously conduct these measurements in the same SAH rat model.

Role of activated platelets in excitation of cardiac afferents during myocardial ischemia in cats

Myocardial ischemia activates cardiac spinal afferents that mediate chest pain and excitatory reflex cardiovascular responses. Platelets are activated during myocardial ischemia and release 5-hydroxytryptamine, which stimulates abdominal spinal afferents. This study investigated the role of activated platelets in excitation of cardiac spinal afferents during ischemia. Platelet-rich plasma (PRP) and platelet-poor plasma (PPP) were obtained from cats and incubated with collagen (2 mg/ml) or thrombin (5 U/ml). We observed reduction of platelets in PRP indicative of platelet activation by collagen and thrombin, respectively. Activity of single-unit, ischemia-sensitive cardiac spinal afferents was recorded from the left sympathetic chain in anesthetized cats. Injection of 1.5 ml PRP + collagen (activated platelets) into the left atrium (LA) stimulated 12 of 13 cardiac afferents. PRP + saline (nonactivated platelets, LA) and PPP + collagen did not alter activity of these afferents. PRP + thrombin (1.5 ml, LA) stimulated eight of nine other cardiac afferents, whereas PPP + thrombin did not stimulate any of the nine afferents. Antiplatelet immune serum (1 ml/kg iv) significantly decreased circulating platelets as well as neutrophils (polymorphonuclear leukocytes, PMNs) in eight other cats, and in each animal, attenuated the ischemia-related increase in activity of cardiac afferents. Conversely, responses of five separate cardiac afferents to ischemia were not diminished after treatment with anti-PMN immune serum when concentration of circulating platelets was maintained by infusion of donated PRP despite the decrease in circulating PMNs. These data indicate activated platelets stimulate ischemia-sensitive cardiac spinal afferents and contribute to activation of these afferents during ischemia.

Synergistic effect of basic fibroblast growth factor and methylprednisolone on neurological function after experimental spinal cord injury

The authors evaluated the effects of exogenous basic fibroblast growth factor (bFGF) in combination with intravenous methylprednisolone on neurological function and cord angiogenesis in a model of spinal cord injury. Cord injury was produced by extradural clip compression through a T-1 laminectomy. Rats were randomized to one of six groups. Group A was given sham laminectomy without cord injury or treatment. The remaining animals were divided into five groups: untreated injury (Group B); injury treated with methylprednisolone (Group C); combined methylprednisolone and 1 microgram bFGF administered locally at the site of injury (Group D); methylprednisolone and 3 micrograms bFGF (Group E); or methylprednisolone and 3 micrograms heated bFGF (Group F). Groups C through F received treatment 1 hour after cord injury. At 1, 2, 3, and 4 weeks after surgery, neurological function of hindlimbs was assessed by blinded observers using an established multiple test method (toe spread, reflexes to extension, pain, and pressure as well as inclined plane and swim test) with tests graded and results expressed as a combined behavioral score. Animals were killed to study spinal cord angiogenesis in cord samples (2-mm sections proximal and distal to the injury site) by capillary density determination. Behavioral scores over time showed a significant difference among Groups B, C, D, E, and F (p = 0.0044), with Groups E and B maintaining highest and lowest scores, respectively. There was a linear dose effect of bFGF over time (p = 0.0187). At 4 weeks, scores showed a difference among the five groups (p = 0.006), with Group E showing higher scores than any other treatment group (for example, vs. group F: p = 0.035). There was a significant difference among the groups in gray matter capillary density counts: proximal (p = 0.0192) and distal (p = 0.024), whereas white matter capillary counts were similar across treatment groups. These results show: 1) possible synergism exists between methylprednisolone and bFGF, such that combinations of these drugs significantly enhance neurological recovery, 2) bFGF exhibits a dose-response effect in function but not in capillary density, and 3) heated, inactivated bFGF is not therapeutically effective.

Further studies on platelet-mediated neurotoxicity

The mechanism of ischemic neuronal injury is not fully resolved. The present view is that vascular occlusion per se does not fully account for the extent of neurological dysfunction. We hypothesized that platelet secretory products might contribute to ischemic neuronal injury in the central nervous system (CNS) (Joseph et al., Stroke, 20 (1989) 38-44 and 1316-1319). Our preliminary studies using organotypic rat spinal cord cultures exposed to human platelet and its secretory products, revealed that platelet product(s) had neurotoxicity. Further studies, using the same methods, were conducted here, with the addition of several refinements such as use of gel-filtered platelets (as opposed to washed platelets), adding additional relevant controls including platelet membranes, red blood cells and washed rat platelets. The results confirmed our initial finding that an agent(s) in platelet secretion is neurotoxic. Subsequently, we identified serotonin (5HT), a major platelet product, as having toxic effects on neurons. This toxicity of 5HT appeared to be blocked by ketanserin, a 5HT2 receptor antagonist. Judging by the concentrations of 5HT that demonstrated neurotoxicity in these in vitro studies, it appears that products secreted from activated platelets could have pathological significance in vivo.