Fractalkine Signalling (CX3CL1/CX3CR1 Axis) as an Emerging Target in Coronary Artery Disease

Abstract

Acute myocardial infarction (MI) is the most common and dramatic complication of atherosclerosis, which, despite successful reperfusion therapy, can lead to incident heart failure (HF). HF occurs when the healing process is impaired due to adverse left ventricular remodelling, and can be the result of so-called ischaemia/reperfusion injury (IRI), visualised by the development of intramyocardial haemorrhage (IMH) or microvascular obstruction (MVO) in cardiac MRI. Thus far, translation of novel pharmacological strategies from preclinical studies to target either IRI or HF post MI have been largely unsuccessful. Anti-inflammatory therapies also carry the risk of affecting the immune system. Fractalkine (FKN, CX₃CL1) is a unique chemokine, present as a transmembrane protein on the endothelium, or following cleavage as a soluble ligand, attracting leukocyte subsets expressing the corresponding receptor CX₃CR1. We have shown previously that the fractalkine receptor CX₃CR1 is associated with MVO in patients undergoing primary PCI. Moreover, inhibition of CX₃CR1 with an allosteric small molecule antagonist (KAND567) in the rat MI model reduces acute infarct size, inflammation, and IMH. Here we review the cellular biology of fractalkine and its receptor, along with ongoing studies that introduce CX₃CR1 as a future target in coronary artery disease, specifically in patients with myocardial infarction.

Keywords: CX3CR1; FRACTAL trial; T lymphocytes; acute myocardial infarction; atherosclerosis; fractalkine; inflammation; monocytes.

Figure 1

Fractalkine (CX₃CL1) is a transmembrane chemokine. Fractalkine contributes to leukocyte binding and arrest on the vascular endothelium. Cleavage by the activity of the metalloproteinases ADAM10 or ADAM17 close to the cell membrane [16] of transmembrane fractalkine (tFKN) results in the release of the mucin-like stalk as well as the chemokine domain, generating a soluble secreted form (sFKN) that acts as a chemoattractant [23]. The remaining C-terminal cleavage fragment is thought to be removed from the cell membrane by intra-membranous cleavage by γ-secretase.

Figure 2

Role of fractalkine in leukocyte recruitment in vascular inflammation. Transmembrane chemokine CX₃CL1 is expressed by inflamed vascular tissue, which then seizes CX₃CR1-positive leukocytes from the circulation. Induced shedding of adhesion molecules would make it easier to separate bound leukocytes from the endothelium after the creation of cellular contact. This might cause the discharge of these cells back into the bloodstream or let leukocytes continue to move across and through the endothelium. Leukocytes would travel to the area of inflammation within the tissue by following a chemotactic gradient of soluble chemokines.

Figure 3

Compound structure of KAND567. * demonstrates that HCl is the salt component of KAND567 and is not a covalent part of the molecule. The HCl salt of KAND567 has a chemical name of (2R)-2-[(2-amino-5-{[(1S)-1-phenylethyl]sulfanyl}[1,3]thiazolo[4,5-d]pyrimidin-7-yl)amino]-4-methylpentan-1-ol hydro chloride.

Figure 4

Mechanism of KAND567-mediated inhibition of the CX₃CR1 receptor. KAND567 antagonises CX₃CL1 binding and G-protein signalling by binding in an allosteric and non-competitive mode, intracellularly and close to the G-protein and beta-arrestin binding sites. As activating CX₃CR1 triggers a cascade of multiple signalling pathways, one of the crucial pathways that KAND567 inhibits to provide its action is by inhibiting Ca²⁺ mobilisation [96,97] and Src activation to activate focal adhesion kinase (FAK) and to prevent cellular migration [98,99].

Figure 5

Flowchart representing the setup of the time points for blood sampling and cardiac MRI.