Loss of PAR4 function was confirmed by platelet aggregation studies where it was shown that PAR4?/? platelets were completely unresponsive to both a PAR4\AP (AYPGKF) and thrombin (Sambrano (Sambrano (2015). is known regarding the pharmacology and physiology of PAR3 and PAR4. Recent studies have begun to develop the pharmacological and genetic tools required to study PAR4 function Chlorocresol in detail, and there is now emerging evidence for the function of PAR4 in disease settings. In this review, we detail the discovery, structure, pharmacology, physiological significance and therapeutic potential of PAR4. Linked Articles This article is a part of a themed section on Molecular Pharmacology of G Protein\Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc AbbreviationsDRGdorsal root ganglionIP3inositol trisphosphateMLCmyosin light chainPARproteinase\activated receptorTBDthrombin binding domain name Tables of Links PARs form a small GPCR family characterized by their unique self\activation mechanism following cleavage by specific serine proteases C typically key effector proteases of coagulation and inflammatory pathways. As a result, extensive research has been conducted into the functions of PARs in thrombosis and other cardiovascular diseases, as well as a range of inflammatory conditions. To date, the research spotlight has been strongly focussed on PAR1 and PAR2, with PAR3 and PAR4 largely waiting in the wings. PAR1 was discovered while attempting to elucidate the mechanism by which thrombin activates platelets and other cells (Vu receptor synthesis (Hoxie murine thrombosis model (Leger PAR4 has also recently been shown to form homodimers (De La Fuente PAR4 homodimers also appear vital for normal signalling: mutation of a series of hydrophobic residues in transmembrane domain name 4 reduced PAR4\mediated calcium mobilization in transfected HEK293 cells (De La Fuente (2001) generated the first PAR4?/? mice, in which \galactosidase was expressed in place of PAR4. Loss of PAR4 function was confirmed by platelet aggregation studies where it was shown that PAR4?/? platelets were completely unresponsive to both a PAR4\AP (AYPGKF) and thrombin (Sambrano (Sambrano (2015). Together, these studies help identify PAR4 as a likely mediator of multiple pathological processes and provide rationale for the development of pharmacological reagents targeting PAR4. Pharmacology Agonists Current agonists of PAR4 consist of amidated peptides Chlorocresol based on the native tethered ligand sequence of the receptor (Table?2). A hexapeptide matching the sequence immediately downstream of the thrombin cleavage site of the receptor, GYPGQV\NH2 (Physique?1) selectively activates PAR4, although prohibitively high concentrations of ~500 M are required for effective activation (Faruqi (2014) to raise anti\PAR4 antibodies against the anionic region sequence of PAR4 (C54ANDSDTLTLPD), just downstream of the thrombin cleavage site. The lead antibody to emerge from these studies, CAN12, not only inhibits thrombin\induced aggregation of human platelets but also inhibits aggregation induced by PAR4\AP, ADP or collagen (Mumaw em et al. /em , 2014), suggesting major issues with specificity. CAN12 was shown to inhibit arterial thrombosis in the Rose Bengal carotid artery thrombosis model at 0.5 mg?kg?1 (Mumaw em et al. /em , Chlorocresol 2014), indicating cross\reactivity with murine PAR4. Beneficial effects were seen when CAN12 was infused both 10 min pre\injury and 15 min post\injury. A new series of monoclonal antibodies from the same group has been very recently reported and appear to bind either to the anionic region or to the thrombin cleavage site of PAR4 and partially inhibit thrombin\induced cleavage of the receptor in expression systems (Mumaw em et al. /em , 2015). This diverse set of PAR4 antagonists, developed over recent years, have been invaluable for investigations into the physiological functions of PAR4 and have also served to Rabbit Polyclonal to AQP3 promote targeting of this receptor as a novel therapeutic approach. However, there still remains significant work to be done to develop PAR4 antagonists suitable for potential clinical power. Clinical significance Cardiovascular disease remains a major clinical problem. Increasingly prevalent conditions such as diabetes, hypertension and coronary artery disease eventuate in diseased and pro\thrombotic vasculature and a common endpoint of these, often co\existing, conditions is the formation of pathological arterial thrombosis leading to myocardial infarction or ischaemic stroke. Together, such conditions account for ~40% of all deaths in Western countries (Mozaffarian em et al. /em , 2015). Given that platelets are the key cellular component of arterial thrombi, anti\platelet brokers are the primary therapy.