Fractalkine (CX3CL1) was originally identified on the basis of sequence homology to lymphotactin (XCL1), a C-type chemokine, and MCP-1 (CCL2), a CC chemokine. The structure of fractalkine is unique among chemokines: it is a transmembrane protein in which the chemokine domain sits atop a mucin-like stalk, and the characteristic cysteines are separated by 3 amino acids (Cys-X-X-X-Cys).1,2 The extracellular domain is shed by protease activity to produce a soluble form, with constitutive shedding mediated by ADAM10, and PMA-induced shedding mediated by TACE (TNF-alpha converting enzyme, or ADAM17).3-5 The soluble form acts as a chemoattractant for monocytes and lymphocytes, while the membrane-bound form promotes adhesion of leukocytes to endothelial cells.6 Another unique aspect of fractalkine biology is its predominant expression in brain, specifically in neurons of the CNS.2,7 The receptor for fractalkine, CX3CR1, is a 7-pass transmembrane G-protein coupled receptor. It is expressed in microglia, cells that mediate inflammatory reactions in the CNS.7,8 When activated by injury, microglial cells produce reactive oxygen species and inflammatory cytokines, compounds that can promote neurotoxicity.9 In neuron/microglial co-cultures, addition of fractalkine has been shown to decrease microglial production of these factors and reduce neuronal cell death.10 A separate study shows that fractalkine treatment also protects microglial cells from apoptosis in vitro.11
|Figure 1. Schematic illustrating the role of fractalkine and its receptor in modulating neurotoxicity. Neurotoxic or other inflammatory stimuli lead to activation of microglial cells. In the absence of fractalkine receptor, microglial activation is significantly increased, leading to increased neuronal cell death.
A recent article in Nature Neuroscience provides convincing evidence that fractalkine-CX3CR1 interactions also modulate neurotoxicity in vivo in the context of disease models including Parkinson’s and amyotrophic lateral sclerosis (ALS).12 Cardona et al. studied transgenic mice in which either one or both copies of the gene encoding the fractalkine receptor are replaced with the coding sequence for green fluorescent protein (GFP).12,13 In these mice, cells expressing CX3CR1 are labeled by GFP, and mice in which both copies are replaced lack CX3CR1 function. The authors found that in three separate models of CNS damage, absence of functional CX3CR1 led to increased neuronal loss due to microglial toxicity, indicating that fractalkine signaling through its receptor acts as a neuroprotective agent. Specifically, when systemic inflammation was induced by intraperitoneal injection of lipopolysaccharide (LPS), CX3CR1-/- mice displayed increased microglial activation and significantly higher levels of neuronal apoptosis. The authors showed this effect to be cell-autonomous to microglia by transplanting activated microglia from LPS injected mice into the brains of mice who had not been LPS stimulated. This effect is likely to be mediated at least in part by IL-1 beta, as LPS-stimulated CX3CR1-/- microglia expressed increased levels of IL-1 beta, and microglial transplant into IL-1R null mice failed to cause neuronal cell death.
The ability of fractalkine-CX3CR1 interactions to function in a neuroprotective capacity in the context of neurological disease was also examined. Cardona et al. administered MPTP to kill dopaminergic neurons and model Parkinson’s disease in CX3CR1-/- mice. They observed significantly higher levels of neuronal loss than in MPTP-treated heterozygotes or wild type mice. Similarly, breeding the CX3CR1 null allele into a genetic model of ALS in which a mutant form of the human superoxide dismutase (SOD) gene is overexpressed also demonstrated worsening of disease in the absence of CX3CR1. This was illustrated by motor neuron loss as well as diminished grip strength, body weight, and survival time.
These interesting results suggest a potential role for fractalkine or other CX3CR1 agonists in treatment of neurodegenerative diseases. However, such an approach would have to be used with caution. Signaling through CX3CR1 also promotes atherosclerosis,14 suggesting that potential therapeutic applications of fractalkine-CX3CR1 activation would have to be carefully titrated to avoid increasing the risk of cardiovascular disease.
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