Platelets at the crossroads of thrombosis, inflammation and haemolysis

Abstract

Platelets play a critical role at the interphase of thrombosis and inflammation, key features in haemolysis-associated disorders. Exercising this role requires expression of pattern recognition receptors by platelets, including toll-like receptor 4 (TLR4) and nucleotide-binding domain leucine rich repeat containing protein 3 (NLRP3), the latter forming intraplatelet multiprotein inflammasome complexes. Platelets are a potential target of various damage-associated molecular pattern (DAMP) molecules, such as free haem, a degradation by-product of haemoglobin oxidation during haemolysis, and high-mobility group box 1 (HMGB1), a DNA-binding protein released by dying or stressed cells and activated platelets. We have recently identified platelet TLR4, NLRP3, and Bruton tyrosine kinase (BTK) as critical regulators of platelet aggregation and thrombus formation, suggesting that the BTK inhibitor ibrutinib is a potential therapeutic target. Increasing evidence suggests that these and other DAMP-driven signalling mechanisms employed by platelets might be key in mediating inflammation and thrombosis encountered in haemolytic disorders. However, the precise regulatory triggers and their clinical relevance are poorly understood. We provide new insights into these less-well characterised platelet mechanisms, which are potentially targetable in haemolytic disorders.

Keywords: haemolysis; inflammation; platelets; sickle cell disease; thrombosis.

Fig 1.

Schematic overview of how platelets may orchestrate thrombosis and inflammation in haemolysis. Extracellular HMGB1 and haem are haemolysis-associated DAMPs that trigger pattern recognition receptor signalling pathways in platelets. HMGB1 signals via platelet TLR4, inducing complex formation between MyD88 and sGC leading to upregulation of enzymatic activity of sGC and upregulated platelet CD62P expression. HMGB1 sensing and secretion by platelets may also be controlled by intraplatelet NLRP3, which cooperates with ASC and caspase-1 within inflammasome complexes that activate IL-1b and IL-18 signalling. BTK and SIRT are critical regulators of the NLRP3 inflammasome. Free haem may activate both plateletspecific TLR4 and NLRP3. Activation of NLRP3 may or may not depend on purinergic P2X receptors. TLR4 and NLRP3 signalling in platelets upregulates expression of CD62P on the platelet surface. Expression of CD62P and secretion/release of IL-1b, IL-18 and HMGB1 by activated platelets are key events in regulating thrombosis and inflammation. Therapeutics that potentially target these platelet mechanisms include: Crizanlizumab, a P-selectin neutralizing antibody; glycyrrhizin, an HMGB1 inhibitor with potent inhibitory effects on platelet-specific HMGB1 signalling; and ibrutinib, an orally administered selective and covalent BTK inhibitor. (+, blue arrows) stimulatory and (-, red arrows) inhibitory signalling effects. ASC, apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain; BTK, Bruton tyrosine kinase; CD62P, P-selectin; cGMP, cyclic guanosine monophosphate; DAMP, damage-associated molecular pattern molecule; GTP, guanosine-5⁰-triphosphate; HMGB1, high-mobility group box 1; IL-18, interleukin-18; IL-1b, interleukin-1b; MyD88, myeloid differentiation primary response gene 88; NLRP3, nucleotide-binding domain leucine rich repeat containing protein 3; sGC, soluble guanylate cyclase; SIRT, sirtuin; TLR4, toll-like receptor 4.