Proteinase-activated receptor 1 (PAR1) also known asprotease-activated receptor 1,coagulation factor II receptor andthrombin receptor is aprotein that in humans is encoded by theF2Rgene.[5] PAR1 is aG protein-coupled receptor and one of fourprotease-activated receptors involved in the regulation ofthrombotic response. Highly expressed in platelets and endothelial cells, PAR1 plays a key role in mediating the interplay betweencoagulation andinflammation, which is important in the pathogenesis of inflammatory and fibrotic lung diseases.[6] It is also involved both in disruption and maintenance ofendothelial barrier integrity, through interaction with eitherthrombin oractivated protein C, respectively.[7]
PAR1 is a transmembrane G-protein-coupled receptor (GPCR) that shares much of its structure with the other protease-activated receptors.[8][9] These characteristics include having seven transmembranealpha helices, four extracellular loops and three intracellular loops.[9] PAR1 specifically contains 425 amino acid residues arranged for optimal binding of thrombin at its extracellularN-terminus. TheC-terminus of PAR1 is located on the intracellular side of the cell membrane as part of its cytoplasmic tail.[8]
This image gives an overview of the cleavage of PAR1 by thrombin. Thrombin, in red, binds to the cleavage site at the extracellular N-terminus of PAR1. Thrombin cleaves the peptide bond between Arg-41 and Ser-42 to reveal a tethered ligand at the new N-terminus and the cleaved peptide, in orange, is released outside of the cell.
PAR1 is activated when the terminal 41amino acids of its N-terminus are cleaved by thrombin, a serine protease.[10] Thrombin recognizes PAR1 by a Lysine-Aspartate-Proline-Arginine-Serine sequence at the N-terminal, where it cuts the peptide bond between Arginine-41 and Serine-42. The affinity of thrombin to this specific cleavage site in PAR1 is further aided by secondary interactions between thrombin's exosite and an acidic region of amino acid residues located C-terminal to Ser-42.[11] This proteolytic cleavage is irreversible and the loose peptide, often referred to as parstatin, is then released outside of the cell.[10] The newly revealed N-terminus acts as a tetheredligand that binds to a binding region between extracellular loops 3 and 4 of PAR1, therefore activating the protein. The binding instigates conformational changes in the protein that ultimately allow for the binding of G-proteins to sites on the intracellular region of PAR1.[12]
Once cleaved, PAR1 can activate G-proteins that bind to several locations on its intracellular loops. For example, PAR1 in conjunction with PAR4 can couple to and activate G-protein G12/13 which in turn activates Rho andRho kinase.[8] This pathway leads to the quick alteration of platelet shape due to actin contractions that lead to platelet mobility, as well as the release of granules which are both necessary forplatelet aggregation.[8] Coupling can also occur with Gq, leading to phospholipase C-β activation; this pathway results in the stimulation of protein kinase C (PKC) which impacts platelet activation.[8]
Additionally, both PAR1 and PAR4 can couple to G-protein q which stimulates intracellular movement for Calcium ions that serve assecond messengers for platelet activation.[8] This also activates protein kinase C which stimulates platelet aggregation and therefore blood coagulation further down the pathway.[11]
The phosphorylation of PAR1's cytoplasmic tail and subsequent binding to arrestin uncouples the protein from G protein signaling.[10][11] These phosphorylated PAR1s are transported back into the cell via endosomes where they are sent to Golgi bodies. The cleaved PAR1s are then sorted and transported to lysosomes where they are degraded.[11] This internalization and degradation process is necessary for the termination of receptor signaling.[10]
In order to regain thrombin responsiveness, PAR1 must be replenished in the cell surface. Uncleaved PAR1 in the cell membrane gets bound by theAP2 adaptor complex at a tyrosine motif on the intracellular C-terminus, which stimulates the endocytosis of the unactivated PAR1.[13] It is then stored inclathrin-coated vesicles within the cytosol and ultimately protected from proteolysis. This ensures that there is a constant supply of uncleaved PAR1 that can be cycled into the plasma membrane independent of PAR1 reproduction, thus resensitizing the cell to thrombin and resetting the signal transduction pathway.[14]
This is a rendering of PAR1's structure when bound to an antagonist, Vorapaxar. The light blue structures represent the seven transmembrane alpha helices of PAR1. The green structures represent the extracellular loops and the orange structures represent the intracellular loops. The red molecule is Vorapaxar. C-terminal tail not pictured.
Finding selective agonists for PAR1 has also been a topic of interest for researchers. A synthetic SFLLRN peptide has been found to serve as an agonist for PAR1. The SFLLRN peptide mimics the first six residues of the N-terminal tethered ligand of activated PAR1 and binds to the same binding site on the second extracellular loop.[15] So, even in the absence of thrombin, SFLLRN binding can garner a response from cleaved or uncleaved PAR1.[16]
Vorapaxar, sold under the brand name Zontivity, is a first-in-class anti-platelet drug used in the treatment of heart disease in patients with a history ofheart attacks andperipheral artery disease.[17] Vorapaxar has been recently shown to attenuate theneutrophilic inflammatory response toStreptococcus pneumoniae by reducing levels of pro-inflammatorycytokines such asIL-1β andchemokinesCXCL1,CCL2 andCCL7.[18] PAR1 is inhibited by Vorapaxar when the molecule binds to a binding pocket between extracellular loop 2 and 3 of the PAR1 where it stabilizes the inactivated protein structure and prevents the switch to the active conformation.[15]
^Hammes SR, Coughlin SR (February 1999). "Protease-activated receptor-1 can mediate responses to SFLLRN in thrombin-desensitized cells: evidence for a novel mechanism for preventing or terminating signaling by PAR1's tethered ligand".Biochemistry.38 (8):2486–93.doi:10.1021/bi982527i.PMID10029543.
Howell DC, Laurent GJ, Chambers RC (April 2002). "Role of thrombin and its major cellular receptor, protease-activated receptor-1, in pulmonary fibrosis".Biochemical Society Transactions.30 (2):211–6.doi:10.1042/BST0300211.PMID12023853.S2CID32822567.
Remillard CV, Yuan JX (May 2005). "PGE2 and PAR-1 in pulmonary fibrosis: a case of biting the hand that feeds you?".American Journal of Physiology. Lung Cellular and Molecular Physiology.288 (5): L789-92.doi:10.1152/ajplung.00016.2005.PMID15821019.S2CID172096.
Vu TK, Hung DT, Wheaton VI, Coughlin SR (March 1991). "Molecular cloning of a functional thrombin receptor reveals a novel proteolytic mechanism of receptor activation".Cell.64 (6):1057–68.doi:10.1016/0092-8674(91)90261-V.PMID1672265.S2CID27467574.
Mathews II, Padmanabhan KP, Ganesh V, Tulinsky A, Ishii M, Chen J, et al. (March 1994). "Crystallographic structures of thrombin complexed with thrombin receptor peptides: existence of expected and novel binding modes".Biochemistry.33 (11):3266–79.doi:10.1021/bi00177a018.PMID8136362.
Hoffman M, Church FC (August 1993). "Response of blood leukocytes to thrombin receptor peptides".Journal of Leukocyte Biology.54 (2):145–51.doi:10.1002/jlb.54.2.145.PMID8395550.S2CID9124992.
Ogino Y, Tanaka K, Shimizu N (November 1996). "Direct evidence for two distinct G proteins coupling with thrombin receptors in human neuroblastoma SH-EP cells".European Journal of Pharmacology.316 (1):105–9.doi:10.1016/S0014-2999(96)00653-X.PMID8982657.
