Endothelial protein C receptor (EPCR) also known as activated protein C receptor (APC receptor) is a protein that in humans is encoded by the PROCR gene. PROCR has also recently been designated CD201 (cluster of differentiation 201).
EPCR is a transmembrane glycoprotein receptor that plays a crucial role in regulation of blood coagulation, inflammation, and vascular integrity. Its ability to enhance the anticoagulant activity of protein C, modulate inflammatory responses, and maintain endothelial barrier function highlights its importance in homeostasis maintenance.
The endothelial protein C receptor (EPCR) is a transmembrane glycoprotein which is encoded by the PROCR gene. EPCR is also known as the activated protein C receptor or CD201. EPCR was originally known for its role in the protein C anticoagulant pathway. In this pathway, EPCR helps regulate the formation of blood clots. It is expressed on endothelial cells which line the blood vessels. Research has shown that EPCR can also participate in the regulation of inflammation, survival of endothelial cells, vascular permeability and immune responses.
It functions as a receptor for protein C and activated protein C (APC) EPCR increases the efficiency of protein C activation by positioning protein C near the thrombin-thrombomodulin complex onto the endothelial cells. Once formed, APC decreases the coagulation by inactivating factor Va and factor VIIIa. APC initiates cytoprotective signaling by cleaving protease-activated receptor 1 (PAR1) which results in anti-inflammatory, preventing programmed cell death, and protecting the endothelial barrier.
EPCR is also important in a clinical sense, in infectious and inflammatory diseases. In severe Plasmodium falciparum malaria, the parasite proteins that are expressed on the infected red blood cells bind to EPCR and initiate sequestration in the small blood vessels. This can interfere with the protein C pathway and help with the endothelial cells becoming activated, inflammation, abnormalities in the coagulation cascade and vascular leakage.
Structure EPCR protein is an N-glycosylated type I membrane protein that enhances the activation of protein C. It belongs to the MHC class I/CD1 family of proteins, The structure of CD201 consists of an extracellular domain, a transmembrane domain, and a cytoplasmic tail. The extracellular domain of CD201 contains a high-affinity binding site for activated protein C (APC), a serine protease with anticoagulant properties. The binding site for APC resembles a deep groove with a lipid inside. The bound lipid in EPCR is usually phosphatidylcholine or phosphatidylethanolamine, and it contributes to APC binding.
CD201 is expressed on the surface of endothelial cells, which form the inner lining of blood vessels. CD201 has also been identified as hematopoietic stem cell (HSC) marker.
EPCR is encoded by the PROCR gene. The PROCR gene is located on chromosome 20. Structurally the protein belongs to the major histocompatibility complex class 1 and CD1 family of the family of proteins, but its primary function is not as an antigen presenting molecule. EPCR is a type I transmembrane protein that consists of an extracellular ligand-binding domain, a single membrane-spanning region and a short cytoplasmic tail.
Projecting from the endothelial cell surface, the extracellular domain contains the site responsible for binding protein C and APC. Through their gamma-carboxyglutamic acid (Gla) domain, both protein C and APC interact with EPCR. Occupied by a phospholipid, the extra cellular region of EPCR contains a hydrophobic groove. The hydrophobic groove contributes to the stability of the receptor and its ability to interact with ligands.
EPCR is anchored to the endothelial cell membrane by the transmembrane domain. The cytoplasmic tail is short and does not have a large intracellular signaling region. Because of the inadequate signaling region, EPCR often functions as a docking receptor rather than producing the intracellular signals on its own. EPCR's cytoprotective effects occur through the cooperation with other cell-surface receptors, particularly PAR1.
EPCR can interact with additional molecules, not just protein C and APC which are the best characterized EPCR ligands. Coagulation factor VII and factor VIIA, certain immune-cell proteins and the P. falciparum erythrocyte membrane protein 1 (PfEMP1) are other molecules the EPCR can interact with. EPCR's ability to bind different ligands has led EPCR to be described as a multi glanded and multifunctional receptor.
Soluble EPCR EPCR can go through a process called receptor shedding. Receptor shedding is when EPCR is released from the endothelial-cell surface from proteolytic cleavage. The releases portion is referred to as soluble EPCR (sEPCR). Metalloproteinase enzymes participate in this process, and it may increase during endothelial activation and inflammation.
Soluble EPCR has the ability to bind protein C and APC. It is no longer attached to the membrane, so it does not position protein C next to the thrombin-thrombomodulin complex. It may instead compete with the membrane EPCR for available protein C and APC which potentially could change their anticoagulant and cellular signaling functions.
In Inflammatory, thrombotic and vascular diseases changes in the circulating soluble EPCR have been studied. When the shedding increases, the amount of protective EPCR on the endothelial surface will decrease while the soluble receptor in the circulation increases. Soluble EPCR has been studied as a plausible indicator of endothelial activation or vascular dysfunction, however its exact clinical significance varies between disease settings.
Physiological functions The main function of CD201 is to enhance the activation of protein C. The binding of APC to EPCR on the endothelial cell surface facilitates its anticoagulant activity by inhibiting factors Va and VIIIa. Apart from its anticoagulant role, CD201 also participates in an anti-inflammatory signaling. CD201 has been shown to affect the production of inflammatory cytokines upon binding a coagulation factor VIIa.
Protein C is a plasma protein that is dependent on vitamin K and is produced mainly by the liver. As an inactive precursor or zymogen it circulates in the blood. Protein C is activated on the endothelial surface through interactions involving thrombin, thrombomodulin, and EPCR.
Thrombin is most commonly associated with promoting coagulation. Thrombin converts fibrinogen into fibrin which activates platelets and supports the clotting factor activation. When thrombin binds to thrombomodulin, its preferred substrate changes. The thrombin-thrombomodulin complex is able to convert protein C into activated protein C.
EPCR improves this process by binding circulating protein C and then positioning it near the thrombin-thrombomodulin complex. This increases the efficiency of protein C activation. The normal pathway is protein C binds EPCR, thrombin binds thrombomodulin, the complex activates protein C and then the activated protein C is formed.
Activated protein C acts as a natural anticoagulant. Together with cofactor protein S, APC can inactivate factor Va and VIIIa. This inactivation decreases the additional thrombin being generated and limits further formation of fibrin.
The protein C pathway prevents coagulating from spreading beyond the area where it is needed. This is also connected to inflammation because thrombin and other coagulation proteases influence endothelial and immune cell signaling. By decreasing excessive thrombin production, APC can also help limit coagulation and some inflammatory responses.
Cytoprotective signaling APC produces cytoprotective effects, which protects cells from injury and death. These are particularly important when endothelial cells are exposed to cytokines, oxidative stress, hypoxia or coagulation proteases.
Many of the cytoprotective effects of APC happen when APC is associated with EPCR and activates PAR1. PAR1 is a g-protein receptor that gets activated when a protease cleaves part of its extracellular region. Both thrombin and APC can act through PAR1 but the cellular outcomes can be different depending on the specific protease, receptor location and presence of EPCR.
Thrombin mediated PAR1 signaling is associated with endothelial activation, pro-inflammatory responses and increased vascular permeability. Signaling produced by EPCR bound APC is associated with cell survival, reduced apoptosis, decreased inflammatory activation and endothelial barrier is stabilized.
The ability of different proteases to produce effects through the same receptor is referred to as biased signaling. EPCR helps influence the signaling outcomes by organizing APC and PAR1 within particular regions of the endothelial membrane.
APC dependent signaling has been linked to intracellular pathways involving phosphoinositide 3- kinases and Akt, minitogen-activated protein kinases, Rac1 and sphingosine-1-phosphate receptors. These intracellular pathways support endothelial cell survival, regulate the cytoskeleton and strengthen cell to cell junctions. This APC signaling can also modify activation of the inflammatory transcription factor NF-𝜅B.
Immunological functions Regulation of inflammation
The endothelium participates in immune responses. When there is an infection or a tissue injury, the endothelial cells react to microbial molecules, coagulation factors and cytokines. These activated endothelial cells may increase vascular permeability, release inflammatory mediators and express adhesion molecules that attract leukocytes that are circulating.
EPCR dependent APC signaling can counterbalance endothelial activation when it becomes excessive. APC signaling through EPCR and PAR1 has been linked to a decrease in NF-𝜅B activity and also reduced production of the pro inflammatory mediators. NF-𝜅B is a transcription factor that controls the expression of chemokines, cytokines and adhesion molecules that are involved in inflammation.
Pro inflammatory cytokines that are associated with endothelial activation are interleukin-1 beta, interleukin-6 and tumor necrosis factor alpha. TNF-𝛼 promotes the endothelial activation and also increases the expression of the adhesion molecules. IL-1β is linked to fever, local inflammatory responses and the recruitment of leukocytes. IL-6 participates in the acute response and influences both innate and adaptive immunity.
APC-EPCR signaling does not necessarily suppress every cytokine directly in tissues. Instead, it is considered a pathway that regulates and reduces excessive inflammatory signaling, while limiting the activation of endothelial cells and protects tissues from inflammation-associated vascular injury.
Endothelial barrier protection
Endothelial cells control the movement of fluid, immune cells, and proteins between surrounding tissues and the bloodstream. During inflammation, endothelial junctions can become loosened, allowing the plasma proteins and leukocytes to enter the affected tissues. Although this response supports host defense, excessive permeability can cause reduced blood volume, edema, and organ dysfunction.
EPCR- associated APC signaling supports the endothelial barrier by influencing cell-survival, junctional organization and cytoskeletal activity. Activation of Rac1 and related pathways can help stabilize the endothelial junctions and decrease the contraction of abnormal cells. These changes can limit excessive vascular leakage while also still allowing controlled immune cell migration.