Background: Reelin is known as an extracellular matrix protein mediating cell migration upon brain development. Recent studies provide evidence of reelin being expressed in platelets, co-localizing to F-actin, binding to amyloid precursor protein (APP) and apolipoprotein E receptor 2 (ApoER2) and being important for glycoprotein Ib (GPIb) mediated platelet adhesion and thrombus formation under high shear. Consequently, defective thrombus formation protects reelin deficient mice (reeler) against arterial thrombosis.

Aims: Defining the role and signaling mechanisms of reelin in platelet cytoskeletal reorganization and glycoprotein VI (GPVI) mediated platelet activation and aggregation.

Methods: In vitro and in vivo analysis of reeler mice.

Results: Reelin activates the small GTPases RhoA and Rac1 in platelets and was shown to be important for PAK1/2 phosphorylation thereby supporting lamellipodia formation upon spreading on fibrinogen. Experiments revealed strongly reduced clot retraction using platelet-rich plasma (PRP) from reeler mice, emphasizing the role of reelin for integrin outside-in signaling. Furthermore, adhesion and cytoskeletal reorganization of reeler platelets were found to be reduced on collagen-related peptide (CRP) and collagen. Correspondingly, reduced phosphorylation of PLCγ2 and Syk as well as platelet aggregation and ATP release were detected using reeler platelets after selective GPVI stimulation. A significant reduction was observed by antibody inhibition of GPVI examining the adhesion of platelets to immobilized reelin. Additionally, immunoprecipitation of reelin using recombinant GPVI clearly showed a direct interaction of GPVI and reelin. It was also observed, that in vivo thrombus formation was completely abrogated in reeler mice treated with a GPVI deleting antibody.

Conclusions: Taken together these data provide the first evidence for reelin modulating GPVI signaling and integrin outside-in signaling of platelets. Thus, reelin affects platelet activation via different signaling pathways involving GPIb, GPVI and the classical reelin receptors APP and ApoER2 pointing to a promising role of reelin as a therapeutic target for antithrombotic therapy.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/reelin-modulates-glycoprotein-vi-and-integrin-outside-in-signaling-of-platelets/