Microglia, the resident inflammatory cells of the CNS, are the only CNS cells that express the fractalkine receptor (CX3CR1). Using three different in vivo models, we show that CX3CR1 deficiency dysregulates microglial responses, resulting in neurotoxicity. Following peripheral lipopolysaccharide injections, Cx3cr1-/- mice showed cell-autonomous microglial neurotoxicity. In a toxic model of Parkinson disease and a transgenic model of amyotrophic lateral sclerosis, Cx3cr1-/- mice showed more extensive neuronal cell loss than Cx3cr1+ littermate controls. Augmenting CX3CR1 signaling may protect against microglial neurotoxicity, whereas CNS penetration by pharmaceutical CX3CR1 antagonists could increase neuronal vulnerability.

There is growing evidence that chronic inflammation plays a role in both the development and progression of diabetic retinopathy. There is also evidence that molecules produced as a result of hyperglycemia can activate microglia. However the exact contribution of microglia, the resident immune cells of the central nervous system, to retinal tissue damage during diabetes remains unclear. Current data suggest that dysregulated microglial responses are linked to their deleterious effects in several neurological diseases associated with chronic inflammation. As inflammatory cytokines and hyperglycemia disseminate through the diabetic retina, microglia can change to an activated state, increase in number, translocate through the retina, and themselves become the producers of inflammatory and apoptotic molecules or alternatively exert anti-inflammatory effects. In addition, microglial genetic variations may account for some of the individual differences commonly seen in patient's susceptibility to diabetic retinopathy.

In vitro studies have implicated chemokine receptors in consumption and clearance of specific ligands. We studied the role that various signaling chemokine receptors play during ligand homeostasis in vivo. We examined the levels of ligands in serum and CNS tissue in mice lacking chemokine receptors. Compared with receptor-sufficient controls, Cx3cr1 Ϫ/Ϫ mice exhibited augmented levels of CX3CL1 both in serum and brain, and circulating levels of CXCL1 and CXCL2 were increased in Cxcr2 Ϫ/Ϫ mice. CCR2-deficient mice showed significantly increased amounts of circulating CCL2 compared with wild-type mice. Cxcr3 Ϫ/Ϫ mice revealed increased levels of circulating and brain CXCL10 after experimental autoimmune encephalomyelitis ( IntroductionActions of chemokines through chemokine receptor signaling leads to an array of diverse functions in different tissue compartments. 1,2 Such functions go beyond the original assigned roles of chemokines in leukocyte chemoattraction to inflamed tissues, and involve physiological trafficking to localize surveillant populations in noninflamed tissues, cellular activation, proliferation, adhesion, phagocytosis, apoptosis, and angiogenesis. [3][4][5][6] Chemokine/ chemokine receptor interactions exhibit defined roles during inflammation, atherosclerosis, autoimmunity, viral pathogenesis, cancer, and neurodegeneration. [7][8][9][10][11] Even though the system exhibits apparent redundancy, modulation of chemokine function via chemokine receptor blockade is a challenging area of considerable interest for therapeutic purposes.Among the chemokine receptors, CCR2 and its ligand CCL2 (MCP-1) have been extensively studied, and their role in regulating monocyte and T-cell infiltrations is well established. In 2002, Tylaska et al showed that CCR2-knockout mice manifested extremely high levels of CCL2 at sites of alloinduced inflammation, and in vitro studies confirmed that clearance of ligand was mediated by CCR2. 12 Our group reported that CCL2 is consumed by CCR2 ϩ migrating cells in a human blood-brain barrier model using peripheral blood mononuclear cells from healthy donors. 13 These results suggest an important biologic role of chemokine receptors as scavenger molecules involved in clearance of specific ligands.Chemokine receptor-deficient mouse strains have been instrumental in understanding chemokine biology in health and disease states. 14 In the present study, we evaluated levels of chemokines in 4 receptor-deficient mice, including those lacking CC, CXC, and CX3C receptors. Both circulating and brain tissue levels were studied. Brain was selected for study as a distinct tissue compartment in which chemokines may be produced under physiological and pathological conditions. We found that the levels of circulating and-in some instances-tissue chemokines are dramatically increased in healthy chemokine receptor-deficient mice. Reconstitution with wild-type bone marrow cells restored chemokine homeostasis. Importantly, for chemokines that signal to more than one receptor, absence of one recep...

Fractalkine (CX3CL1 or FKN) is a membrane-bound chemokine expressed on neuronal membranes and is proteolytically cleaved to shed a soluble chemoattractant domain. FKN signals via its unique receptor CX3CR1 expressed on microglia and other peripheral leukocytes. The aim of this study is to determine the role of CX3CR1 in inflammatory-mediated damage to retinal neurons using a model of diabetic retinopathy. For this, we compared neuronal, microglial, and astroglial densities and inflammatory response in nondiabetic and diabetic (Ins2Akita) CX3CR1-wild-type and CX3CR1-deficient mice at 10 and 20 weeks of age. Our results show that Ins2Akita CX3CR1-knockout mice exhibited (a) decreased neuronal cell counts in the retinal ganglion cell layer, (b) increased microglial cell numbers, and (c) decreased astrocyte responses comparable with Ins2Akita CX3CR1-Wild-type mice at 20 weeks of age. Analyses of the inflammatory response using PCR arrays showed several inflammatory genes differentially regulated in diabetic tissues. From those, the response in Ins2Akita CX3CR1-deficient mice at 10 weeks of age revealed a significant upregulation of IL-1β at the transcript level that was confirmed by enzyme-linked immunosorbent assay in soluble retinal extracts. Overall, IL-1β, VEGF, and nitrite levels as a read out of nitric oxide production were abundant in Ins2Akita CX3CR1-deficient retina. Notably, double immunofluorescence staining shows that astrocytes act as a source of IL-1β in the Ins2Akita retina, and CX3CR1-deficient microglia potentiate the inflammatory response via IL-1β release. Collectively, these data demonstrate that dysregulated microglial responses in absence of CX3CR1 contribute to inflammatory-mediated damage of neurons in the diabetic retina.