PB0376 - Inhibition of the p75 pan neurotrophic receptor partly mitigates BDNF-induced platelet aggregation

Background: The Brain-Derived Neurotrophic Factor (BDNF) promotes neuronal growth and survival, acting through alternatively spliced isoforms of the full length TrkB receptor and the 75 kDa pan-neurotrophic receptor (p75NTR). TrkB and p75NTR receptors cross-regulate each other’s signal and ligand bindings. BDNF is present in brain and serum, and platelets can have 1000-fold greater BDNF than neurons. We have shown that washed platelets aggregate in response to BDNF via the activation of a truncated TrkB receptor. Whether p75NTR is present in platelets and regulates BDNF-induced platelet aggregation remains unknown.

Aims: To characterize p75NTR expression and function in platelets.

Methods: Washed platelets were prepared from whole blood of healthy male and female volunteers. p75NTR expression was assessed by immunoblotting, flow cytometry and confocal microscopy, and quantified by ELISA (n=20). Platelets were activated using classical agonists (ADP, collagen, thrombin receptor activating peptide [TRAP]) and BDNF, in the presence of p75NTR inhibitor THX-B or negative control inactive analog, THX. Aggregation was measured by light transmission aggregometry. Signaling pathways downstream of BDNF activation were assessed by anti-phosphoprotein immunoblot.

Results: p75NTR immunoreactivity was detected on the surface of (28.3±19.4%) and within (56.4±18.4%) platelets. p75NTR levels quantified by ELISA were highly variable, with a 10-fold difference between highest (382 pg/ml) and lowest values (38 pg/ml), and did not change following platelet activation. Collagen-, ADP-, and TRAP-induced aggregation were not affected by the p75NTR inhibitor, THX-B. Inhibition of BDNF-induced platelet aggregation showed high interindividual variation (32.4±36.8% inhibition): THX-B-induced responses ranged from no effect in 50%, partial effect in 30% to a complete abrogation of platelet aggregation in 20% of participants.

Conclusion(s): Pharmacological inhibition of p75NTR abrogated BDNF-induced platelet aggregation in some but not all healthy volunteers. Whether this variability in aggregation is related to the high interindividual variation in platelet p75NTR levels remains to be established.