Characterization of fractalkine in rat brain cells: migratory and activation signals for CX3CR-1-expressing microglia.

Molecular analyses of the chemokine fractalkine and its receptor CX3C-R1 in the rat brain have revealed a striking polarization: fractalkine is expressed constitutively in neurons and is up-regulated by TNF-alpha and IL-1beta in astrocytes. Expression of its specific receptor, CX3C-R1, is restricted to astrocytes and microglia. We have analyzed the functional correlates of this expression and demonstrate that fractalkine induces microglial cell migration and activation. However, the activity of this chemokine on astrocytes may also be highly relevant in inducing astrocyte-microglia cell interactions through cytokine/mediator release leading to microglial activation.