Beyond Clotting: A Role of Platelets in CNS Repair?

Immune-mediated platelet depletion augments Alzheimer's disease neuropathological hallmarks in APP-PS1 mice

In Alzheimer's disease (AD), platelets become dysfunctional and might contribute to amyloid beta deposition. Here, we depleted platelets in one-year-old APP Swedish PS1 dE9 (APP-PS1) transgenic mice for five days, using intraperitoneal injections of an anti-CD42b antibody, and assessed changes in cerebral amyloidosis, plaque-associated neuritic dystrophy and gliosis. In APP-PS1 female mice, platelet depletion shifted amyloid plaque size distribution towards bigger plaques and increased neuritic dystrophy in the hippocampus. In platelet-depleted females, plaque-associated Iba1+ microglia had lower amounts of fibrillar amyloid beta cargo and GFAP+ astrocytic processes showed a higher overlap with thioflavin S+ amyloid plaques. In contrast to the popular hypothesis that platelets foster plaque pathology, our data suggest that platelets might limit plaque growth and attenuate plaque-related neuritic dystrophy at advanced stages of amyloid plaque pathology in APP-PS1 female mice. Whether the changes in amyloid plaque pathology are due to a direct effect on amyloid beta deposition or are a consequence of altered glial function needs to be further elucidated.

Role of platelet-derived extracellular vesicles in traumatic brain injury-induced coagulopathy and inflammation

Extracellular vesicles are composed of fragments of exfoliated plasma membrane, organelles or nuclei and are released after cell activation, apoptosis or destruction. Platelet-derived extracellular vesicles are the most abundant type of extracellular vesicle in the blood of patients with traumatic brain injury. Accumulated laboratory and clinical evidence shows that platelet-derived extracellular vesicles play an important role in coagulopathy and inflammation after traumatic brain injury. This review discusses the recent progress of research on platelet-derived extracellular vesicles in coagulopathy and inflammation and the potential of platelet-derived extracellular vesicles as therapeutic targets for traumatic brain injury.

Can the administration of platelet lysates to the brain help treat neurological disorders?

Neurodegenerative disorders of the central nervous system (CNS) and brain traumatic insults are characterized by complex overlapping pathophysiological alterations encompassing neuroinflammation, alterations of synaptic functions, oxidative stress, and progressive neurodegeneration that eventually lead to irreversible motor and cognitive dysfunctions. A single pharmacological approach is unlikely to provide a complementary set of molecular therapeutic actions suitable to resolve these complex pathologies. Recent preclinical data are providing evidence-based scientific rationales to support biotherapies based on administering neurotrophic factors and extracellular vesicles present in the lysates of human platelets collected from healthy donors to the brain. Here, we present the most recent findings on the composition of the platelet proteome that can activate complementary signaling pathways in vivo to trigger neuroprotection, synapse protection, anti-inflammation, antioxidation, and neurorestoration. We also report experimental data where the administration of human platelet lysates (HPL) was safe and resulted in beneficial neuroprotective effects in established rodent models of neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, traumatic brain injury, and stroke. Platelet-based biotherapies, prepared from collected platelet concentrates (PC), are emerging as a novel pragmatic and accessible translational therapeutic strategy for treating neurological diseases. Based on this assumption, we further elaborated on various clinical, manufacturing, and regulatory issues that need to be addressed to ensure the ethical supply, quality, and safety of HPL preparations for treating neurodegenerative and traumatic pathologies of the CNS. HPL made from PC may become a unique approach for scientifically based treatments of neurological disorders readily accessible in low-, middle-, and high-income countries.

The role of P2Y12 receptor inhibition in ischemic stroke on microglia, platelets and vascular smooth muscle cells

P2Y12 receptors on platelets have long been the main target of antiplatelet drugs. However, a growing number of studies have revealed that P2Y12 receptor activation on microglia and vascular smooth muscle cells (VSMCs) also aggravates ischemic stroke injury. The proliferation and migration of VSMCs in the vascular wall have important influence on the early lesion of atherosclerosis, which may lead to the origin of cerebral ischemic attack of atherosclerosis. Blockage of cellular P2Y12 receptors could inhibit microglial activation, block formation of platelet-leukocyte aggregates, reduce proinflammatory cytokine levels and suppress migration and proliferation of VSMCs, implying that apart from anti-thrombotic effect, P2Y12 inhibitors have additional neuroprotective, anti-inflammatory and anti-atherosclerotic therapeutic benefits against ischemic stroke. In this review, we will summarize recent advances in studies on P2Y12 receptors and emphatically introduce their significance in microglia, platelets and VSMCs after ischemic stroke, discussing how to exert the beneficial effects of P2Y12 inhibition.

Platelets as drivers of ischemia/reperfusion injury after stroke

Ischemic stroke is a leading cause of morbidity and mortality worldwide and, despite reperfusion either via thrombolysis or thrombectomy, stroke patients often suffer from lifelong disabilities. These persistent neurological deficits may be improved by treating the ischemia/reperfusion (I/R) injury that occurs following ischemic stroke. There are currently no approved therapies to treat I/R injury, and thus it is imperative to find new targets to decrease the burden of ischemic stroke and related diseases. Platelets, cell fragments from megakaryocytes, are primarily known for their role in hemostasis. More recently, investigators have studied the nonhemostatic role of platelets in inflammatory pathologies, such as I/R injury after ischemic stroke. In this review, we seek to provide an overview of how I/R can lead to platelet activation and how activated platelets, in turn, can exacerbate I/R injury after stroke. We will also discuss potential mechanisms by which platelets may ameliorate I/R injury.

Correlation between Serum Platelet Count and Cognitive Function in Patients with Atrial Fibrillation: A Cross-Sectional Study

BACKGROUND

The risk of cognitive impairment in patients with atrial fibrillation is significantly increased. Its occurrence may be related to blood hypercoagulable state and immune inflammatory reaction. Platelets can mediate immune inflammatory response, but there is no evidence about the relationship between platelet count and cognitive function in patients with atrial fibrillation.

PURPOSE

To explore whether there is a certain correlation between platelet count and cognitive function in patients with atrial fibrillation.

METHODS

A cross-sectional study was conducted in a single center in China, including 254 patients with atrial fibrillation. Cognitive function assessment and clinical and laboratory examinations were performed on all participants. After adjusting the related confounding factors, the relationship between platelet count and cognitive function was analyzed.

RESULTS

A total of 254 subjects with an average age of 59.71 ± 11.14 years were included. The average platelet count was 208.15 ± 68.30, and the average score of cognitive function was 19.29 ± 6.78. Result of fully adjusted binary logistic regression showed platelet count was negatively associated with the cognitive function score after adjusting confounders (hazard ratio (HR) = 0.000, 95%CI -0.01, 0.01). A nonlinear relationship was detected between platelet count and the cognitive function score, whose point was 230. The effect sizes and the confidence intervals of the left and right sides of the inflection point were 0.03 (0.01-0.05, P for nonlinearity = 0.011) and -0.03 (-0.05-0.00, P for nonlinearity = 0.023), respectively.

CONCLUSION

Platelets have a nonlinear relationship with cognitive function in patients with atrial fibrillation. This finding suggests that, in patients with atrial fibrillation, platelets should be maintained at about 230.

Platelets in Amyloidogenic Mice Are Activated and Invade the Brain

Alzheimer’s disease (AD) is a neurodegenerative disease with a complex and not fully understood pathogenesis. Besides brain-intrinsic hallmarks such as abnormal deposition of harmful proteins, i.e., amyloid beta in plaques and hyperphosphorylated Tau in neurofibrillary tangles, blood-derived elements, in particular, platelets have been discussed to be involved in AD pathogenesis. The underlying mechanisms, however, are rather unexplored. Here, we investigate a potential role of platelets in an AD transgenic animal model with severe amyloid plaque formation, the APP-PS1 transgenic mice, and analyzed the presence, spatial location and activation status of platelets within the brain. In APP-PS1 mice, a higher number of platelets were located within the brain parenchyma, i.e., outside the cerebral blood vessels compared to WT controls. Such platelets were activated according to the expression of the platelet activation marker CD62P and to morphological hallmarks such as membrane protrusions. In the brain, platelets were in close contact exclusively with astrocytes suggesting an interaction between these two cell types. In the bloodstream, although the percentage of activated platelets did not differ between transgenic and age-matched control animals, APP-PS1 blood-derived platelets showed remarkable ultrastructural peculiarities in platelet-specific organelles such as the open canalicular system (OCS). This work urges for further investigations on platelets and their yet unknown functional roles in the brain, which might go beyond AD pathogenesis and be relevant for various age-related neurodegenerative diseases.

Platelets mediate protective neuroinflammation and promote neuronal plasticity at the site of neuronal injury

It is generally accepted that inflammation within the CNS contributes to neurodegeneration after traumatic brain injury (TBI), but it is not clear how inflammation is initiated in the absence of infection and whether this neuroinflammation is predominantly beneficial or detrimental. We have previously found that brain-enriched glycosphingolipids within neuronal lipid rafts (NLR) induced platelet degranulation and secretion of neurotransmitters and pro-inflammatory factors. In the present study, we compared TBI-induced inflammation and neurodegeneration in wild-type vs. St3gal5 deficient (ST3^-/-) mice that lack major CNS-specific glycosphingolipids. After TBI, microglial activation and CNS macrophage infiltration were substantially reduced in ST3^-/- animals. However, ST3^-/- mice had a larger area of CNS damage with marked neuronal/axonal loss. The interaction of platelets with NLR stimulated neurite growth, increased the number of PSD95-positive dendritic spines, and intensified neuronal activity. Adoptive transfer and blocking experiments provide further that platelet-derived serotonin and platelet activating factor plays a key role in the regulation of sterile neuroinflammation, hemorrhage and neuronal plasticity after TBI.

Platelets isolated from an Alzheimer mouse damage healthy cortical vessels and cause inflammation in an organotypic ex vivo brain slice model

Platelets are anuclear blood cells and play a major role in hemostasis and thrombosis. Platelets express amyloid-precursor protein (APP), release beta-amyloid (Aβ) and are stimulated (pre-activated) in Alzheimer's disease (AD). We hypothesize that such stimulated platelets severely damage brain vessels which subsequently leads to cerebrovascular damage in AD. In order to study this issue we isolated platelets from AD mice (expressing APP with the Swedish-Dutch-Iowa mutations), labeled them with the red fluorescent dye PKH26 and transcardially infused these freshly isolated platelets into the brains of anesthetized healthy C57BL6 wildtype mice. Brains were immediately taken, 110 µm thick organotypic brain slices prepared and cultured for 1 or 14 days. We observed that red PKH26+ fluorescent platelets were localized in collagen IV and Lectin-649 counterstained cortical brain vessels and that platelets from AD mice severely damaged cortical brain vessels in wildtype mice and entered the brain parenchyma. Confocal microscopy showed immunoreactivity for matrix metalloproteinases (MMP-2 and MMP-9) and beta-amyloid around these platelets. The effect was completely inhibited with an MMP inhibitor. Furthermore, isolated AD platelets caused inflammation and activated microglia around the site where platelets damaged cortical brain vessels. We conclude that AD-derived platelets more aggressively damage healthy vessels which may consequently play a role in the progression of cerebral amyloid angiopathy in AD.

Reduction of relative centrifugal forces increases growth factor release within solid platelet-rich-fibrin (PRF)-based matrices: a proof of concept of LSCC (low speed centrifugation concept)

Purpose The present study evaluated the platelet distribution pattern and growth factor release (VEGF, TGF-β1 and EGF) within three PRF (platelet-rich-fibrin) matrices (PRF, A-PRF and A-PRF+) that were prepared using different relative centrifugation forces (RCF) and centrifugation times. Materials and methods immunohistochemistry was conducted to assess the platelet distribution pattern within three PRF matrices. The growth factor release was measured over 10 days using ELISA. Results The VEGF protein content showed the highest release on day 7; A-PRF+ showed a significantly higher rate than A-PRF and PRF. The accumulated release on day 10 was significantly higher in A-PRF+ compared with A-PRF and PRF. TGF-β1 release in A-PRF and A-PRF+ showed significantly higher values on days 7 and 10 compared with PRF. EGF release revealed a maximum at 24 h in all groups. Toward the end of the study, A-PRF+ demonstrated significantly higher EGF release than PRF. The accumulated growth factor releases of TGF-β1 and EGF on day 10 were significantly higher in A-PRF+ and A-PRF than in PRF. Moreover, platelets were located homogenously throughout the matrix in the A-PRF and A-PRF+ groups, whereas platelets in PRF were primarily observed within the lower portion. ​Discussion the present results show an increase growthfactor release by decreased RCF. However, further studies must be conducted to examine the extent to which enhancing the amount and the rate of released growth factors influence wound healing and biomaterial-based tissue regeneration. ​Conclusion These outcomes accentuate the fact that with a reduction of RCF according to the previously LSCC (described low speed centrifugation concept), growth factor release can be increased in leukocytes and platelets within the solid PRF matrices.