A recently identified chemokine, fractalkine, is a member of the chemokine gene family, which consists principally of secreted, proinflammatory molecules. Fractalkine is distinguished structurally by the presence of a CX3C motif as well as transmembrane spanning and mucin-like domains and shows atypical constitutive expression in a number of nonhematopoietic tissues, including brain. We undertook an extensive characterization of this chemokine and its receptor CX3CR1 in the brain to gain insights into use of chemokine-dependent systems in the central nervous system. Expression of fractalkine in rat brain was found to be widespread and localized principally to neurons. Recombinant rat CX3CR1, as expressed in Chinese hamster ovary cells, specifically bound fractalkine and signaled in the presence of either membrane-anchored or soluble forms of fractalkine protein. Fractalkine stimulated chemotaxis and elevated intracellular calcium levels of microglia; these responses were blocked by anti-CX3CR1 antibodies. After facial motor nerve axotomy, dramatic changes in the levels of CX3CR1 and fractalkine in the facial nucleus were evident. These included increases in the number and perineuronal location of CX3CR1-expressing microglia, decreased levels of motor neuronexpressed fractalkine mRNA, and an alteration in the forms of fractalkine protein expressed. These data describe mechanisms of cellular communication between neurons and microglia, involving fractalkine and CX3CR1, which occur in both normal and pathological states of the central nervous system.Chemokines mediate the directed migration of a variety of leukocyte subsets and consist of at least four subfamilies based principally on the relative position of highly conserved cysteine residues in their amino acid sequences (1, 2). Most chemokine peptides are characterized as secreted proteins of Ϸ7-10 kDa. The recent discovery of a chemokine termed fractalkine has revealed additional distinctive structural features in this gene family. These features include a CX3C motif and a mucin-like stalk that tethers the chemokine domain to transmembrane (TM) spanning and short intracellular domains (3, 4). Evidence from transfected cell systems indicates that fractalkine can exist as membrane-anchored, pro-adhesive, and secreted, chemotactic forms. Furthermore, unlike most chemokine peptides, fractalkine expression is demonstrable in nonhematopoietic tissues including brain, kidney, lung, and heart. In particular, the relatively high levels of fractalkine in the brain raises questions related to the function of chemokines in the central nervous system (CNS).G-protein coupled receptors for chemokine peptides have been characterized extensively in transfected cells and peripheral leukocytes (2). However, very little is known regarding chemokine receptor expression and function in the CNS. Some chemokine receptors, including CCR5, CCR3 (5-7), CXCR4 (7-10), CXCR1, and DARC (11) have been demonstrated to be expressed in either normal brain tissue or cells derived from the...
