Acetylcholine Inhibits Platelet Activation

Protection of β2GPI Deficient Mice from Thrombosis Reflects a Defect in PAR3-facilitated Platelet Activation

Background

Antibodies to β2-glycoprotein I (β2GPI) cause thrombosis in antiphospholipid syndrome, however the role of β2GPI itself in regulation of coagulation pathways in vivo is not well understood.

Methods

We developed β2GPI-deficient mice (Apoh -/- ) by deleting exon 2 and 3 of Apoh using CRISPR/Cas9 and compared the propensity of wild-type (WT) and Apoh -/- mice to develop thrombosis using rose bengal and FeCl 3 -induced carotid thrombosis, laser-induced cremaster arteriolar injury, and inferior vena cava (IVC) stasis models. We also compared tail bleeding times and assessed platelet activation in WT and Apoh -/- mice in the absence and presence of exogenous β2GPI.

Results

Compared to WT littermates, Apoh -/- mice demonstrated a prolonged time to occlusion of the carotid artery after exposure to rose bengal or FeCl 3 , and reduced platelet and fibrin accumulation in cremasteric arterioles after laser injury. Similarly, significantly smaller thrombi were retrieved from the IVC of Apoh -/- mice 48 hours after IVC occlusion. The activated partial thromboplastin time (aPTT) and prothrombin time, as well as aPTT reagent- and tissue factor-induced thrombin generation times using plasma from Apoh -/- and WT mice revealed no differences. However, we observed significant prolongation of tail bleeding in Apoh -/- mice, and reduced P-selectin expression and binding of fibrinogen to the activated α2bβ3 integrin on platelets from these mice after stimulation with low thrombin concentrations; these changes were reversed by exogenous β2GPI. An antibody to PAR3 blocked thrombin-induced activation of WT, but not Apoh -/- platelets, as well as the ability of β2GPI to restore the activation response of Apoh -/- platelets to thrombin. β2GPI deficiency did not affect platelet activation by a PAR4-activator peptide, or ADP.

Conclusions

In mice, β2GPI may mediate procoagulant activity by enhancing the ability of PAR3 to present thrombin to PAR4, promoting platelet activation at low thrombin concentrations.

Key Points

β2GPI deficient mice are protected from experimental arterial, venous, and microvascular thrombosis.β2GPI deficient mice display prolonged tail bleeding times and reduced PAR3-facilitated platelet activation by low concentrations of thrombin.

Effect of stimulated platelets in COVID-19 thrombosis: Role of alpha7 nicotinic acetylcholine receptor

Since early 2020, SARS-CoV-2-induced infection resulted in global pandemics with high morbidity, especially in the adult population. COVID-19 is a highly prothrombotic condition associated with subsequent multiorgan failure and lethal outcomes. The exact mechanism of the prothrombotic state is not well understood and might be multifactorial. Nevertheless, platelets are attributed to play a crucial role in COVID-19-associated thrombosis. To date, platelets' role was defined primarily in thrombosis and homeostasis. Currently, more focus has been set on their part in inflammation and immunity. Moreover, their ability to release various soluble factors under activation as well as internalize and degrade specific pathogens has been highly addressed in viral research. This review article will discuss platelet role in COVID-19-associated thrombosis and their role in the cholinergic anti-inflammatory pathway. Multiple studies confirmed that platelets display a hyperactivated phenotype in COVID-19 patients. Critically ill patients demonstrate increased platelet activation markers such as P-selectin, PF4, or serotonin. In addition, platelets contain acetylcholine and express α7 nicotinic acetylcholine receptors (α7nAchR). Thus, acetylcholine can be released under activation, and α7nAchR can be stimulated in an autocrine manner and support platelet function. α7 receptor is one of the most important mediators of the anti-inflammatory properties as it is associated with humoral and intrinsic immunity and was demonstrated to contribute to better outcomes in COVID-19 patients when under stimulation. Hematopoietic α7nAchR deficiency increases platelet activation and, in experimental studies, α7nAchR stimulation can diminish the pro-inflammatory state and modulate platelet reactiveness via increased levels of NO. NO has been described to inhibit platelet adhesion, activation, and aggregation. In addition, acetylcholine has been demonstrated to decrease platelet aggregation possibly by blocking the e p-38 pathway. SARS-CoV-2 proteins have been found to be similar to neurotoxins which can bind to nAChR and prevent the action of acetylcholine. Concluding, the platelet role in COVID-19 thrombotic events could be explained by their active function in the cholinergic anti-inflammatory pathway.

Platelet olfactory receptor activation limits platelet reactivity and growth of aortic aneurysms

As blood transitions from steady laminar flow (S-flow) in healthy arteries to disturbed flow (D-flow) in aneurysmal arteries, platelets are subjected to external forces. Biomechanical platelet activation is incompletely understood and is a potential mechanism behind antiplatelet medication resistance. Although it has been demonstrated that antiplatelet drugs suppress the growth of abdominal aortic aneurysms (AAA) in patients, we found that a certain degree of platelet reactivity persisted in spite of aspirin therapy, urging us to consider additional antiplatelet therapeutic targets. Transcriptomic profiling of platelets from patients with AAA revealed upregulation of a signal transduction pathway common to olfactory receptors, and this was explored as a mediator of AAA progression. Healthy platelets subjected to D-flow ex vivo, platelets from patients with AAA, and platelets in murine models of AAA demonstrated increased membrane olfactory receptor 2L13 (OR2L13) expression. A drug screen identified a molecule activating platelet OR2L13, which limited both biochemical and biomechanical platelet activation as well as AAA growth. This observation was further supported by selective deletion of the OR2L13 ortholog in a murine model of AAA that accelerated aortic aneurysm growth and rupture. These studies revealed that olfactory receptors regulate platelet activation in AAA and aneurysmal progression through platelet-derived mediators of aortic remodeling.

Nicotine and the nicotinic cholinergic system in COVID-19

There is an urgent need to address the devastating pandemic, COVID‐19, caused by SARS‐CoV‐2. The efforts to understand the details of this disease in hope of providing effective treatments are commendable. It is clear now that the virus can cause far more damage in patients with comorbid conditions—particularly in those with respiratory, cardiovascular, or immune‐compromised system—than in patients without such comorbidities. Drug use can further exacerbate the condition. In this regard, the ill effects of smoking are amply documented, and no doubt can be a confounding factor in COVID‐19 progression. Although conflicting hypotheses on the potential role of nicotine in COVID‐19 pathology have recently been offered, we believe that nicotine itself, through its interaction with the nicotinic cholinergic system, as well as ACE2, may not only be of use in a variety of neuropsychiatric and neurodegenerative diseases, but may also be of potential use in COVID‐19. Thus, on one hand, while we strongly support smoking cessation as a means of harm reduction associated with COVID‐19, on the other hand, we support a potential therapeutic role for nicotine, nicotinic agonists, or positive allosteric modulators of nicotinic cholinergic receptors in COVID‐19, owing to their varied effects including mood regulation, anti‐inflammatory, and purported interference with SARS‐CoV‐2 entry and/or replication.