Platelets are expert at patching up leaks in blood vessels, but could they go overboard and seed amyloid-β plaques as well? As reported in Science Signaling on May 24, Aβ monomers bind integrin receptors on human platelets in vitro, triggering the release of the chaperone clusterin, which then facilitates Aβ aggregation. Blocking platelet activation reduced cerebral amyloid angiopathy (CAA) in a mouse model of Alzheimer’s disease. Led by Margitta Elvers at Heinrich Heine University in Düsseldorf, Germany, the researchers proposed that anti-platelet therapy could one day do the same in people.
The findings add mechanistic oomph to previous reports implicating platelets in AD, commented Costantino Iadecola of Weill Cornell Medical College in New York. However, any future attempts to treat AD patients with anti-platelet therapy will be tempered by the potential for bleeding risks, Iadecola and other researchers warned.
Platelets use their anucleated bodies to patch blood vessel leaks—a line of defense that triggers the formation of blood clots. Multiple studies hint that platelets also play a role in AD. In dementia patients platelets seem to be more activated. People with brain emboli—teeny bits of clotting material floating in the brain vessels, a characteristic of overactive platelets—decline faster on cognitive tests than people without them (see Ciabattoni et al., 2007, and Feb 2012 news). Moreover, platelets are a major source of Aβ in the blood, where the peptide can reportedly activate them, triggering their adhesion to each other (see Roher et al., 2009; Bush et al., 1990; and Shen et al., 2008). Interestingly, platelets also affect Aβ; work from Elvers’ lab revealed that Aβ fibrillizes when added to platelets in vitro, and the researchers spotted platelets comingling with vascular amyloid plaques in APP23 mice (see Gowert et al., 2014; and Jarre et al., 2014).
For this study, first author Lili Donner and colleagues wanted to chip away at the mechanisms behind the two-way interactions between Aβ and platelets. As measured by immunofluorescence, Congo red staining, and electron microscopy, platelets incubated with monomeric Aβ40 for three days stimulated formation of Aβ fibrils. This fibrillization depended upon the presence of the platelets themselves, as simply mixing platelet supernatant with Aβ promoted no fibrils. Aβ also stimulated signaling within the platelets, as evidenced by elevated platelet aggregation and the activation of various signaling molecules as well as secretion of the chaperone clusterin (aka Apolipoprotein J). Genetic variants in this gene associate with AD (see Alzgene). Interestingly, platelets from clusterin knockout mice failed to promote Aβ aggregation, hinting that Aβ may stimulate the release of the very factor—clusterin—that triggers its aggregation on platelets. In keeping with this idea, adding clusterin protein to these cultures promoted Aβ aggregation.
Fibrillization Platform. On platelets, Aβ monomers trigger the integrin receptor, which boosts release of ADP and clusterin, accelerating Aβ fibrillization. [Image courtesy of McFayden and Peter, Focus Sci Signaling 2016.]
How might Aβ trigger clusterin release? The researchers found that the peptide bound to the fibrinogen receptor integrin αIIbβ3, tripping off a signaling cascade resulting in the release of the apolipoprotein. Fibrinogen facilitates the adhesion of platelets to each other, and is also the precursor to fibrin, a major component of blood clots. Aβ also triggered the release of adenosine diphosphate from the platelets, which enhanced clusterin release through binding to the P2Y12 receptor on the platelet surface (see image above). The researchers found that blocking this purinergic receptor with the blood-thinning drug clopidogrel severely dampened both clusterin release from platelets and their penchant for aggregating Aβ.
The researchers next sought to strengthen the proposed connection between Aβ and integrin by measuring Aβ aggregation in the presence of platelets from people who have Glanzmann’s thrombasthenia, a rare blood-clotting disorder caused by poor expression of the αIIbβ3 integrin on platelets. Platelets from patients with the most severe form of GT failed to promote Aβ fibrillization; however, fibrils did form when platelets from GT patients expressed a moderate level of this particular integrin.
To test if platelets contribute to CAA in vivo, the researchers treated 13-month-old APP23 mice for three months with clopidogrel to reduce platelet activation and prevent clusterin release and Aβ aggregation. While the treated and untreated mice had similar burdens of Aβ plaques in the brain parenchyma, the burden of vascular plaque was only half in the treated mice. This suggested that blocking platelet function slowed vascular amyloid deposition.
These results seemingly conflict with those from John Fryer’s lab at the Mayo Clinic in Jacksonville, Florida. His group recently reported that compared to APP/PS1 mice, those lacking clusterin had more vascular amyloid, suggesting that clusterin prevented, rather than promoted, vascular amyloid deposition (see Nov 2015 conference news).
Elvers proposes anti-platelet therapy as a potential treatment strategy for CAA. The vascular amyloid-specific action of clopidogrel provides support for this idea, commented Ilaria Canobbio of the University of Pavia in Italy, whose previous work implicated Aβ in platelet activation (see Canobbio et al., 2014). However, such drugs are only used under acute conditions such as myocardial infarction to prevent clotting, and pose substantial bleeding risks, Canobbio added. Clopidogrel’s action is also short-lived, as the active metabolite of the compound has a half-life of less than one hour.
“Clopidogrel is typically not considered for CAA or AD. Thus, from a therapeutic standpoint, it could prove important to prevent amyloid deposition in blood vessel walls, which could in turn reduce the complications of the angiopathy,” commented Marwan Sabbagh of Barrow Neurological Institute in Phoenix. “Ironically, this is counterintuitive because we typically avoid anti-platelet treatments in hemorrhagic events such as CAA.”
Newer therapies, such as antibodies that block the integrin receptor without activating it, could potentially prevent Aβ aggregation with a longer half-life, commented James McFadyen and Karlheinz Peter of Monash University in Melbourne, Australia, in an editorial (see Schwarz et al., 2006; McFadyen and Peter, 2016). The pharmacokinetic profiles of these therapies would provide some wiggle room in the extent to which they block platelet activation, thus potentially reducing the bleeding risk, they pointed out.
McFadyen and Peter wondered whether platelets could exacerbate other diseases, including diabetes and atherosclerosis, which are characterized by deposition of different types of protein along blood vessel walls. “In light of the data presented here, it is tempting to speculate that platelet activation in this context may also establish a self-perpetuating cycle that precipitates the formation of further aggregates of misfolded proteins, thereby directly promoting the progression of disease,” they wrote.
Iadecola thought Elvers’ results dovetailed nicely with previous data from his lab, which revealed that CD36, a scavenger receptor on platelets, promoted vascular amyloid deposition in mice (see Park et al., 2013). He noted that the Aβ aggregation via platelets could at least partly explain why people with AD have elevated levels of atherosclerosis in brain blood vessels, since Aβ fibrils may contribute to sticky platelet clots that serve as a scaffold for atherosclerotic plaques.
The most intriguing unanswered question, Iadecola said, is how and where platelets and Aβ meet up and promote CAA. “What doesn’t make sense is how platelets decorated with Aβ cross the endothelium and get into the smooth muscle cells where CAA accumulates,” he said. William Van Nostrand of Stony Brook University in New York stressed the same quandary. “Perhaps platelets play a role later in the disease when there is compromise of vascular integrity and exposure to platelet components,” he suggested (see full comment below).
Furthermore, whether the source of Aβ for the platelet fibrils comes from the brain or from the platelets themselves is up for debate. Van Nostrand pointed out that while strong evidence exists for the role of neuronal Aβ in vascular amyloid deposition, platelets process APP primarily via the non-amyloidogenic α-secretase pathway, producing only small amounts of Aβ40. Elvers and colleagues had to use concentrations of Aβ40 far exceeding that found in the blood, he added. Others also noted this caveat, and lamented the small number of animals used in some experiments.—Jessica Shugart
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