PAR4 Overview
Protease-activated receptor 4 (PAR4) is a thrombin-activated G protein-coupled receptor with important roles in platelet activation, vascular signaling, inflammation, and disease. The Hamm Laboratory has studied protease-activated receptor signaling for more than two decades, with an emphasis on defining the signaling mechanisms that distinguish PAR4 from other thrombin receptors and understanding how these differences can be exploited therapeutically.
Our studies have helped establish the importance of PAR4 in platelet thrombin signaling and have contributed to the development of selective PAR4 antagonists. More recently, this work has expanded beyond thrombosis to investigate the involvement of PAR4 in disorders characterized by vascular dysfunction and inflammation, including Alzheimer’s disease, vascular contributions to cognitive impairment and dementia (VCID), and kidney disease.
PAR4 Signaling & Platelet Biology
Thrombin is one of the most potent activators of human platelets and signals through two protease-activated receptors, PAR1 and PAR4. Although both receptors respond to thrombin, they produce distinct signaling responses that influence the magnitude and duration of platelet activation.
The Hamm Laboratory has investigated the molecular mechanisms that distinguish PAR1 and PAR4 signaling in human platelets. Our studies demonstrated differences in receptor coupling, calcium signaling, lipid signaling, integrin activation, and interactions with other platelet signaling pathways. In particular, PAR4 produces sustained signaling that contributes to platelet activation and amplification of thrombin generation.
This work has also helped define how platelet signaling is altered in conditions associated with increased cardiovascular risk. Studies of platelet function in metabolic syndrome and diabetes demonstrated enhanced platelet prothrombinase activity and hypercoagulability and helped establish the importance of PAR4 in amplifying platelet thrombin generation.
Together, these findings have contributed to our understanding of PAR4 as a distinct regulator of platelet function and provided the biological foundation for investigating PAR4 as an antithrombotic therapeutic target.
Selected Publications
Duvernay MT, Temple KJ, Maeng JG, et al. Contributions of PAR1 and PAR4 to thrombin-induced GPIIbIIIa activation in
PAR4 Drug Discovery
The distinct physiological roles of PAR1 and PAR4 have made selective inhibition of PAR4 an attractive strategy for modulating thrombin-mediated platelet activation. Clinical experience with PAR1 inhibition demonstrated that blocking thrombin signaling can reduce thrombotic events but may also increase bleeding risk, motivating the search for alternative approaches to selectively regulate platelet thrombin signaling.
The Hamm Laboratory, in collaboration with medicinal chemists and computational drug discovery investigators, has contributed to the discovery and characterization of multiple generations of small-molecule PAR4 antagonists. Early efforts led to the development of ML354 and related compounds that demonstrated the feasibility of selectively inhibiting PAR4 over PAR1. Subsequent medicinal chemistry efforts produced improved antagonists with activity against thrombin-mediated PAR4 activation and compounds with central nervous system penetration.
More recently, the laboratory has incorporated computational approaches into PAR4 drug discovery. Ultra-large virtual screening identified new chemical scaffolds capable of inhibiting PAR4 signaling and provided opportunities to investigate the unique pharmacology associated with PAR4 activation by its tethered ligand.
Together, these studies combine receptor pharmacology, platelet biology, medicinal chemistry, and computational drug discovery to develop molecular tools for understanding PAR4 and to evaluate its potential as a therapeutic target.
Selected Publications
PAR4 in Alzheimer's Disease & VCID
Increasing evidence suggests that vascular dysfunction and inflammation contribute substantially to the development and progression of Alzheimer’s disease (AD) and vascular contributions to cognitive impairment and dementia (VCID). Because PAR4 participates in platelet activation, thrombosis, vascular signaling, and inflammation, the receptor represents a potential link between vascular pathology and neurodegenerative disease.
Our work identified increased expression of F2RL3, the gene encoding PAR4, in the brains of individuals with Alzheimer’s disease. Elevated F2RL3 expression was associated with cognitive decline and a pro-inflammatory transcriptional profile, supporting a relationship between PAR4 signaling, neuroinflammation, and disease progression.
These findings extend the biological significance of PAR4 beyond its established role in platelet activation and suggest that altered PAR4 signaling may contribute to the vascular and inflammatory components of neurodegenerative disease.
By integrating our expertise in PAR4 pharmacology with studies of Alzheimer’s disease and VCID, we aim to better understand how thrombin-mediated signaling contributes to neurovascular pathology and establish the scientific foundation for evaluating PAR4 as a potential therapeutic target.
Selected Publication
PAR4 in Kidney Disease
Acute kidney injury can initiate inflammatory and fibrotic processes that contribute to the development of chronic kidney disease. Because PAR4 participates in thrombin signaling, inflammation, and vascular responses, the Hamm Laboratory investigated whether PAR4 also contributes directly to the progression of kidney injury.
Our studies demonstrated that PAR4 expression increases following kidney injury and that genetic loss of PAR4 protects against renal pathology in mouse models of both unilateral ureteral obstruction and the transition from acute kidney injury to chronic kidney disease following ischemia-reperfusion injury.
Loss of PAR4 was associated with reduced renal fibrosis and inflammation and improved measures of kidney function, identifying PAR4 signaling as an important contributor to the response to renal injury.
These findings reveal a previously underappreciated role for PAR4 outside of platelet biology and suggest that modulation of PAR4 signaling may provide a strategy for limiting the inflammatory and fibrotic processes that drive the progression of kidney disease.
Selected Publication