Reduced platelet activation and thrombus formation in male transgenic model mice of Alzheimer's disease suggests early sex-specific differences in platelet pathophysiology.

Alzheimer's disease (AD) is the most common form of dementia and characterized by extracellular amyloid-ß (Aß) plaques, intracellular neurofibrillary tau tangles and neurodegeneration. Over 80 % of AD patients also exhibit cerebral amyloid angiopathy (CAA). CAA is a cerebrovascular disease caused by deposition of Aß in the walls of cerebral blood vessels leading to vessel damage and impairment of normal blood flow. To date, different studies suggest that platelet function, including activation, adhesion and aggregation, is altered in AD due to vascular Aß deposition. For example, the transgenic AD model mice APP23 mice that exhibit CAA and parenchymal Aß plaques, show pre-activated platelets in the blood circulation and increased platelet integrin activation leading to a pro-thrombotic phenotype in these mice late stages of AD. However, it is still an open question whether or not platelets exhibit changes in their activation profile before they are exposed to vascular Aß deposits. Therefore, the present study examined platelets from middle-aged transgenic APP23 mice at the age of 8-10 months. At this age, APP23 mice show amyloid plaques in the brain parenchyma but not in the vasculature. Our analyses show that these APP23 mice have unaltered platelet numbers and size, and unaltered surface expression of glycoproteins. However, the number of dense granules in transgenic platelets was increased while the release was unaltered. Male, but not female APP23 mice, exhibited reduced platelet activation after stimulation of the thrombin receptor PAR4 and decreased thrombus stability on collagen under flow conditions ex vivo compared to control mice. In an arterial thrombosis model in vivo, male APP23 mice showed attenuated occlusion of the injured artery compared to controls. These findings provide clear evidence for early changes in platelet activation and thrombus formation in male mice before development of overt CAA. Furthermore, reduced platelet activation and thrombus formation suggest sex-specific differences in platelet physiology in AD that has to be considered in future studies of platelets and their role in AD.

Cryo-EM of Aß fibrils from mouse models find tg-APPArcSwe fibrils resemble those found in patients with sporadic Alzheimer's disease.

The use of transgenic mice displaying amyloid-ß (Aß) brain pathology has been essential for the preclinical assessment of new treatment strategies for Alzheimer's disease. However, the properties of Aß in such mice have not been systematically compared to Aß in the brains of patients with Alzheimer's disease. Here, we determined the structures of nine ex vivo Aß fibrils from six different mouse models by cryogenic-electron microscopy. We found novel Aß fibril structures in the APP/PS1, ARTE10 and tg-SwDI models, whereas the human type II filament fold was found in the ARTE10, tg-APPSwe and APP23 models. The tg-APPArcSwe mice showed an Aß fibril whose structure resembles the human type I filament found in patients with sporadic Alzheimer's disease. A detailed assessment of the Aß fibril structure is key to the selection of adequate mouse models for the preclinical development of novel plaque-targeting therapeutics and positron emission tomography imaging tracers in Alzheimer's disease.

Impact of Amyloid-ß on Platelet Mitochondrial Function and Platelet-Mediated Amyloid Aggregation in Alzheimer's Disease.

BACKGROUND: Alzheimer's disease (AD) is characterized by an accumulation of amyloid ß (Aß) peptides in the brain and mitochondrial dysfunction. Platelet activation is enhanced in AD and platelets contribute to AD pathology by their ability to facilitate soluble Aß to form Aß aggregates. Thus, anti-platelet therapy reduces the formation of cerebral amyloid angiopathy in AD transgenic mice. Platelet mitochondrial dysfunction plays a regulatory role in thrombotic response, but its significance in AD is unknown and explored herein. METHODS: The effects of Aß-mediated mitochondrial dysfunction in platelets were investigated in vitro. RESULTS: Aß40 stimulation of human platelets led to elevated reactive oxygen species (ROS) and superoxide production, while reduced mitochondrial membrane potential and oxygen consumption rate. Enhanced mitochondrial dysfunction triggered platelet-mediated Aß40 aggregate formation through GPVI-mediated ROS production, leading to enhanced integrin αIIbß3 activation during synergistic stimulation from ADP and Aß40. Aß40 aggregate formation of human and murine (APP23) platelets were comparable to controls and could be reduced by the antioxidant vitamin C. CONCLUSIONS: Mitochondrial dysfunction contributes to platelet-mediated Aß aggregate formation and might be a promising target to limit platelet activation exaggerated pathological manifestations in AD.

The collagen receptor glycoprotein VI promotes platelet-mediated aggregation of ß-amyloid.

Cerebral amyloid angiopathy (CAA) and ß-amyloid (Aß) deposition in the brain parenchyma are hallmarks of Alzheimer's disease (AD). We previously reported that platelets contribute to Aß aggregation in cerebral vessels by secreting the factor clusterin upon binding of Aß40 to the fibrinogen receptor integrin αIIbß3 Here, we investigated the contribution of the collagen receptor GPVI (glycoprotein VI) in platelet-induced amyloid aggregation. Using platelets isolated from GPVI-wild type and GPVI-deficient human donors and mice, we found that Aß40 bound to GPVI, which induced the release of ATP and fibrinogen, resulting in platelet aggregation. Binding of Aß40 to integrin αIIbß3, fibrinogen, and GPVI collectively contributed to the formation of amyloid clusters at the platelet surface. Consequently, blockade of αIIbß3 or genetic loss of GPVI reduced amyloid fibril formation in cultured platelets and decreased the adhesion of Aß-activated platelets to injured carotid arteries in mice. Application of losartan to inhibit collagen binding to GPVI resulted in decreased Aß40-stimulated platelet activation, factor secretion, and platelet aggregation. Furthermore, the application of GPVI- or integrin-blocking antibodies reduced the formation of platelet-associated amyloid aggregates. Our findings indicate that Aß40 promotes platelet-mediated amyloid aggregation by binding to both GPVI and integrin αIIbß3 Blocking these pathways may therapeutically reduce amyloid plaque formation in cerebral vessels and the brain parenchyma of patients.

Relevance of N-terminal residues for amyloid-ß binding to platelet integrin αIIbß3, integrin outside-in signaling and amyloid-ß fibril formation.

A pathological hallmark of Alzheimer's disease (AD) is the aggregation of amyloid-ß peptides (Aß) into fibrils, leading to deposits in cerebral parenchyma and vessels known as cerebral amyloid angiopathy (CAA). Platelets are major players of hemostasis but are also implicated in AD. Recently we provided strong evidence for a direct contribution of platelets to AD pathology. We found that monomeric Aß40 binds through its RHDS sequence to integrin αIIbß3, and promotes the formation of fibrillar Aß aggregates by the secretion of adenosine diphosphate (ADP) and the chaperone protein clusterin (CLU) from platelets. Here we investigated the molecular mechanisms of Aß binding to integrin αIIbß3 by using Aß11 and Aß16 peptides. These peptides include the RHDS binding motif important for integrin binding but lack the central hydrophobic core and the C-terminal sequence of Aß. We observed platelet adhesion to truncated N-terminal Aß11 and Aß16 peptides that was not mediated by integrin αIIbß3. Thus, no integrin outside-in signaling and reduced CLU release was detected. Accordingly, platelet mediated Aß fibril formation was not observed. Taken together, the RHDS motif of Aß is not sufficient for Aß binding to platelet integrin αIIbß3 and platelet mediated Aß fibril formation but requires other recognition or binding motifs important for platelet mediated processes in CAA. Thus, increased understanding of the molecular mechanisms of Aß binding to platelet integrin αIIbß3 is important to understand the role of platelets in amyloid pathology.

Loss of Reelin protects mice against arterial thrombosis by impairing integrin activation and thrombus formation under high shear conditions.

Reelin is a secreted glycoprotein and essential for brain development and plasticity. Recent studies provide evidence that Reelin modifies platelet actin cytoskeletal dynamics. In this study we sought to dissect the contribution of Reelin in arterial thrombus formation. Here we analyzed the impact of Reelin in arterial thrombosis ex vivo and in vivo using Reelin deficient (reeler) and wildtype mice. We found that Reelin is secreted upon platelet activation and mediates signaling via glycoprotein (GP)Ib, the amyloid precursor protein (APP) and apolipoprotein E receptor 2 (ApoER2) to induce activation of Akt, extracellular signal-regulated kinase (Erk), SYK and Phospholipase Cγ2. Moreover, our data identifies Reelin as first physiological ligand for platelet APP. Platelets from reeler mice displayed attenuated platelet adhesion and significantly reduced thrombus formation under high shear conditions indicating an important role for Reelin in GPIb-dependent integrin αIIbß3 activation. Accordingly, adhesion to immobilized vWF as well as integrin activation and the phosphorylation of Erk and Akt after GPIb engagement was reduced in Reelin deficient platelets. Defective Reelin signaling translated into protection from arterial thrombosis and cerebral ischemia/reperfusion injury beside normal hemostasis. Furthermore, treatment with an antagonistic antibody specific for Reelin protects wildtype mice from occlusive thrombus formation. Mechanistically, GPIb co-localizes to the major Reelin receptor APP in platelets suggesting that Reelin-induced effects on GPIb signaling are mediated by APP-GPIb interaction. These results indicate that Reelin is an important regulator of GPIb-mediated platelet activation and may represent a new therapeutic target for the prevention and treatment of cardio- and cerebrovascular diseases.

Defective Platelet Activation and Bleeding Complications upon Cholestasis in Mice.

BACKGROUND/AIMS: Platelets are essential mediators of hemostasis to avoid excessive blood loss. Cirrhosis and chronic liver diseases are characterized by alterations in hemostasis. Alterations in the secondary hemostasis have been well studied, while defects in primary hemostasis, especially the consequences of cholestatic liver disease on platelet function are not well defined. METHODS: After bile duct ligation (BDL) platelet activation and thrombus formation were analyzed in mice. RESULTS: BDL in mice had a moderate effect on platelet counts; however, intrinsic platelet activation was strongly reduced upon activation of the collagen receptor GPVI at early time points. 7 days after bile duct ligation, platelets displayed an almost complete loss of activation with reduced agonist-triggered release of alpha and dense granules and expression of integrin αIIbß3 on the platelet surface. This activation defects resulted in strongly reduced thrombus formation under flow, reduced platelet adhesion to fibrinogen and bleeding complications in BDL mice as measured by tail bleeding experiments. Mechanistically, elevated nitric oxide and prostacyclin levels induced phosphorylation of Vasodilator-stimulated phosphoprotein (VASP), an established inhibitor of platelet activation. Furthermore increased tissue plasminogen activator in plasma of BDL mice led to enhanced plasmin levels that might be responsible for reduced glycoprotein expression of BDL platelets. Besides, high amounts of bile acids contribute to defective signal transduction as shown in platelets from mice fed with a cholic acid diet. CONCLUSIONS: Cholestatic liver disease induces multiple platelet activation defects and impairs thrombus formation responsible for bleeding complications at least in mice.

Platelets contribute to amyloid-ß aggregation in cerebral vessels through integrin αIIbß3-induced outside-in signaling and clusterin release.

Cerebral amyloid angiopathy (CAA) is a vascular dysfunction disorder characterized by deposits of amyloid-ß (Aß) in the walls of cerebral vessels. CAA and Aß deposition in the brain parenchyma contribute to dementia and Alzheimer's disease (AD). We investigated the contribution of platelets, which accumulate at vascular Aß deposits, to CAA. We found that synthetic monomeric Aß40 bound through its RHDS (Arg-His-Asp-Ser) sequence to integrin αIIbß3, which is the receptor for the extracellular matrix protein fibrinogen, and stimulated the secretion of adenosine diphosphate (ADP) and the chaperone protein clusterin from platelets. Clusterin promoted the formation of fibrillar Aß aggregates, and ADP acted through its receptors P2Y1 and P2Y12 on platelets to enhance integrin αIIbß3 activation, further increasing the secretion of clusterin and Aß40 binding to platelets. Platelets from patients with Glanzmann's thrombasthenia, a bleeding disorder in which platelets have little or dysfunctional αIIbß3, indicated that the abundance of this integrin dictated Aß-induced clusterin release and platelet-induced Aß aggregation. The antiplatelet agent clopidogrel, which irreversibly inhibits P2Y12, inhibited Aß aggregation in platelet cultures; in transgenic AD model mice, this drug reduced the amount of clusterin in the circulation and the incidence of CAA. Our findings indicate that activated platelets directly contribute to CAA by promoting the formation of Aß aggregates and that Aß, in turn, activates platelets, creating a feed-forward loop. Thus, antiplatelet therapy may alleviate fibril formation in cerebral vessels of AD patients.

Pre-activated blood platelets and a pro-thrombotic phenotype in APP23 mice modeling Alzheimer's disease.

Platelet activation and thrombus formation play a critical role in primary hemostasis but also represent a pathophysiological mechanism leading to acute thrombotic vascular occlusions. Besides, platelets modulate cellular processes including inflammation, angiogenesis and neurodegeneration. On the other hand, platelet activation and thrombus formation are altered in different diseases leading to either bleeding complications or pathological thrombus formation. For many years platelets have been considered to play a role in neuroinflammatory diseases such as Alzheimer's disease (AD). AD is characterized by deposits of amyloid-ß (Aß) and strongly related to vascular diseases with platelets playing a critical role in the progression of AD because exposure of platelets to Aß induces platelet activation, platelet Aß release, and enhanced platelet adhesion to collagen in vitro and at the injured carotid artery in vivo. However, the molecular mechanisms and the relation between vascular pathology and amyloid-ß plaque formation in the pathogenesis of AD are not fully understood. Compelling evidence is suggestive for altered platelet activity in AD patients. Thus we analyzed platelet activation and thrombus formation in aged AD transgenic mice (APP23) known to develop amyloid-ß deposits in the brain parenchyma and cerebral vessels. As a result, platelets are in a pre-activated state in blood of APP23 mice and showed strongly enhanced integrin activation, degranulation and spreading kinetics on fibrinogen surfaces upon stimulation. This enhanced platelet signaling translated into almost unlimited thrombus formation on collagen under flow conditions in vitro and accelerated vessel occlusion in vivo suggesting that these mice are at high risk of arterial thrombosis leading to cerebrovascular and unexpectedly to cardiovascular complications that might be also relevant in AD patients.

Blood platelets in the progression of Alzheimer's disease.

Alzheimer's disease (AD) is characterized by neurotoxic amyloid-ß plaque formation in brain parenchyma and cerebral blood vessels known as cerebral amyloid angiopathy (CAA). Besides CAA, AD is strongly related to vascular diseases such as stroke and atherosclerosis. Cerebrovascular dysfunction occurs in AD patients leading to alterations in blood flow that might play an important role in AD pathology with neuronal loss and memory deficits. Platelets are the major players in hemostasis and thrombosis, but are also involved in neuroinflammatory diseases like AD. For many years, platelets were accepted as peripheral model to study the pathophysiology of AD because platelets display the enzymatic activities to generate amyloid-ß (Aß) peptides. In addition, platelets are considered to be a biomarker for early diagnosis of AD. Effects of Aß peptides on platelets and the impact of platelets in the progression of AD remained, however, ill-defined. The present study explored the cellular mechanisms triggered by Aß in platelets. Treatment of platelets with Aß led to platelet activation and enhanced generation of reactive oxygen species (ROS) and membrane scrambling, suggesting enhanced platelet apoptosis. More important, platelets modulate soluble Aß into fibrillar structures that were absorbed by apoptotic but not vital platelets. This together with enhanced platelet adhesion under flow ex vivo and in vivo and platelet accumulation at amyloid deposits of cerebral vessels of AD transgenic mice suggested that platelets are major contributors of CAA inducing platelet thrombus formation at vascular amyloid plaques leading to vessel occlusion critical for cerebrovascular events like stroke.