The main focus of the Harper lab is blood platelets: small cells that play a huge role in normal haemostasis. and pathological thrombosis. Despite their small size, platelets are major anti-thrombotic drug targets. By studying how platelets are activated and how they are affects by drugs, we aim to identify future therapeutic targets.
Procoagulant platelets in thrombosis and haemostasis
During thrombosis, a subpopulation of strongly-activated platelets expose phosphatidylserine on the outer leaflet of their plasma membrane. These procoagulant platelets are essential to the development of arterial thrombosis because phosphatidylserine is an efficient surface for assembly of coagulation complexes, generating a burst of thrombin that is responsible for producing an occlusive thrombus. Phosphatidylserine exposure in this context is driven by a high, sustained intracellular calcium signals, mitochondrial calcium handling and ROS production. We aim to understand how phosphatidylserine exposure is controlled during platelet activation and how it can be inhibited to reduce thrombosis.
Recent research on procoagulant platelets:
- Maintaining flippase activity in procoagulant platelets is a novel approach to reducing thrombin generation. Millington-Burgess SL, Harper MT. J Thromb Haemost (2022) 20: 989-995 (link).
- Supramaximal calcium signaling triggers procoagulant platelet formation. Abbasian N, Millington-Burgess SL, Chabra S, Malcor JD, Harper MT. Blood Adv (2020) 4, 154-164. (link)
- Cytosolic and mitochondrial Ca2+ signalling in procoagulant platelets. Millington-Burgess SL, Harper MT. Platelets (2021) 32, 855-862. (link)
- R5421 is not a selective inhibitor platelet scramblase activity. Millington-Burgess SL, Rahman T, Harper MT. Br J Pharmacol (2020) 177, 4007-4020. (link)
Pharmacology of procoagulant extracellular vesicle (EV) release by platelets
Activated platelets release many extracellular vesicles (EV) during thrombosis. Phosphatidylserine-exposing microparticles are released by procoagulant platelets and promote coagulation. However, the mechanisms and consequences of EV release are less clearly understood that other platelet functions in thrombosis, such as platelet aggregation and granule secretion. We aim to increase our understanding of platelet EV release and whether it is an attractive pharmacological target in thrombosis.
Recent research on platelet EVs:
- Platelet-Derived Extracellular Vesicles in Arterial Thrombosis. Harper MT. Adv Exp Med Biol (2023) 1418:259-275 (link).
- Epigallocatechin gallate inhibits release of extracellular vesicles from platelets without inhibiting phosphatidylserine exposure. Millington-Burgess SL, Harper MT. Sci Rep (2021) 11, 17678. (link)
- 2-Aminoethoxydiphenylborate (2-APB) inhibits release of phosphatidylserine-exposing extracellular vesicles from platelets. Wei H, Davies JE, Harper MT. Cell Death Discov (2020) 6, 10. (link)
- ABT-737 triggers caspase-dependent inhibition of platelet procoagulant extracellular vesicle release during apoptosis and secondary necrosis in vitro. Wei H, Harper MT (2019). Thromb Haemost (2019) 19, 1665-1674. (link)
Microfluidic models of thrombosis, haemostasis and inflammation
We aim to develop new microfluidic models of arterial, venous and microvascular thrombosis and inflammation. These models incorporate microfluidics with thrombogenic substrates, endothelial cells or ex vivo vessel perfusion. Physiological variables such as flow patterns are modelled on in vivo conditions as far as possible, since these are likely to significantly improve the translational relevance of the models.
Recent research on microfluidic models:
- Multicellular vessel-on-a-chip reveals context-dependent roles for platelets in inflammation and inflammatory hemostasis. Riddle RB, Jennbacken K, Hansson KM, Harper MT. Blood Vessels Thromb Haemost (2024)
- Extracellular matrix composition regulates endothelial inflammation and neutrophil transmigration in an inflammation-on-a-chip model. Riddle RB, Jennbacken K, Hansson KM, Harper MT. Sci Rep (2022) 12: 6855 (link).
- An "occlusive thrombosis-on-a-chip" microfluidic device for investigating the effect of anti-thrombotic drugs. Berry J, Peudecerf FJ, Masters NA, Neeves KB, Goldstein RE, Harper MT. Lab Chip (2021) 21, 4104-4117. (link)
- Using Yoda-1 to mimic laminar flow in vitro: a tool to simplify drug testing. Davies JE, Lopresto D, Apta BHR, Lin Z, Ma W, Harper MT. Biochem Pharmacol (2019) 168, 473-480. (link)
Protease-activated receptors in platelets
Thrombin is a potent platelet activator, acting through protease-activated receptors (PAR) 1 and 4. Thrombin, in combination with collagen, is also a powerful trigger for procoagulant platelet formation. We aim to explore how PARs contribute to platelet activation and how they can be pharmacologically modulated.
Recent research on PARs in platelets:
- Protease-activated receptor antagonism prevents occlusion when dual antiplatelet therapy is insufficient in a microfluidic model of occlusive thrombosis. Berry JE, Harper MT. Res Prac Thromb Haemost (2022) 6: e12703. (link)
- Q94 is not a selective modulator of PAR1 in platelets. Francis LRA, Millington-Burgess SL, Rahman T, Harper MT. Platelets (2022) 33: 1090-1095 (link).
- Epigenetic Regulation of F2RL3 Associates with Myocardial Infarction and Platelet Function. Corin LJ, White SJ, Taylor A, Williams CM, Taylor K, van den Bosch MT, Teasdale JE, Jones ML, Bond M, Harper MT, Falk L, Groom A, Haxell GGJ, Paternoster L, Munafo M, Nordestgaard BG, Tybjærg-Hanson A, Bojesen SE, Relton CL, Min JL, Davey Smith G, Mumford AD, Poole AW, Timpson NJ. Circ Res (2022)130: 384-400 (link).