Human α-Synuclein Inhibits Platelets Aggregation in vitro by Interfering with the α-Thrombin/Protease-Activated Receptor 1 Functional Axis

Abstract

α-Synuclein (αSyn) is a small (140 amino acids) disordered, acidic (pI: 4.7) protein, highly conserved in vertebrates and implicated in the pathogenesis of Parkinson’s disease (PD), a neurodegenerative disease characterized by the deposition of αSyn amyloid fibrils in dopaminergic neurons. Beyond the central nervous system, significant expression of αSyn has also been measured in the blood (~1 μM), where platelets are the main cellular hosts of αSyn. Although the pathological implication of αSyn in PD is widely accepted, the physiological role of blood αSyn is still elusive. Starting from the notion that platelets are either the major cellular reservoir of αSyn in the blood and, concomitantly, act as key players in hemostasis, being activated also by α-thrombin (αT) via cleavage of protease-activated receptors (PARs), we decided to investigate the possibility that αSyn could modulate platelet activation by interfering with the αT-PAR functional axis. Using multiple electrode aggregometry, i.e. a fast and specific platelet-function-testing method, as well as steady-state fluorescence spectroscopy, surface plasmon resonance, and fluorescence microscopy, we show here that monomeric αSyn functions as a negative regulator of αT-mediated platelets activation. αSyn acts either directly, via competitive inhibition of PAR1 activation by αT and TRAP6 agonist, and indirectly, by scavenging αT on the platelet plasma membrane. A simple electrostatic model of αSyn platelet antiaggregating effect is proposed and the possible role of the protein at the interplay of amyloidosis and thrombosis is discussed.