CD44 sensitivity of platelet activation, membrane scrambling and adhesion under high arterial shear rates

Summary

CD44 is required for signalling of macrophage migration inhibitory factor (MIF), an anti-apoptotic pro-inflammatory cytokine. MIF is expressed and released from blood platelets, key players in the orchestration of occlusive vascular disease. Nothing is known about a role of CD44 in the regulation of platelet function. The present study thus explored whether CD44 modifies degranulation (P-selectin exposure), integrin activation, caspase activity, phosphatidylserine exposure on the platelet surface, platelet volume, Orai1 protein abundance and cytosolic Ca²⁺-activity ([Ca²⁺]i). Platelets from mice lacking CD44 (cd44^-/- ) were compared to platelets from corresponding wild-type mice (cd44^+/+ ). In resting platelets, P-selectin abundance, α_llbβ₃ inte-grin activation, caspase-3 activity and phosphatidylserine exposure were negligible in both genotypes and Orai1 protein abundance, [Ca²⁺]_i, and volume were similar in cd44^-/- and cd44^+/+ platelets. Platelet degranulation and α_llbβ₃ integrin activation were significantly increased by thrombin (0.02 U/ml), collagen related peptide (CRP, 2 µg/ml and Ca²⁺-store depletion with thapsigargin (1 µM), effects more pronounced in cd44^-/- than in cd44^+/+ platelets. Thrombin (0.02 U/ml) increased platelet [Ca²⁺]i, caspase-3 activity, phosphatidylserine exposure and Orai1 surface abundance, effects again significantly stronger in cd44^-/- than in cd44^+/+ platelets. Thrombin further decreased forward scatter in cd44^-/- and cd44^+/+ platelets, an effect which tended to be again more pronounced in cd44^-/- than in cd44^+/+ platelets. Platelet adhesion and in vitro thrombus formation under high arterial shear rates (1,700 s^-1) were significantly augmented in cd44^-/- mice. In conclusion, genetic deficiency of CD44 augments activation, apoptosis and prothrombotic potential of platelets.

Keywords

CD44 - platelet activation - cytosolic Ca²⁺ concentration - phosphatidylserine translocation - thrombus formation

^* Both authors contributed equally and thus share first authorship.

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