Fractalkine cleavage from neuronal membranes represents an acute event in the inflammatory response to excitotoxic brain damage

Fractalkine is a recently identified chemokine that exhibits cell adhesion and chemoattractive properties. It represents a unique member of the chemok ine superfamily because it is located predominantly in the brain in which i t is expressed constitutively on specific subsets of neurons. To elucidate the possible role of neuronally expressed fractalkine in the inflammatory r esponse to neuronal injury, we have analyzed the regulation of fractalkine mRNA expression and protein cleavage under conditions of neurotoxicity. We observed that mRNA encoding fractalkine is unaffected by experimental ische mic stroke (permanent middle cerebral artery occlusion) in the rat. Similar ly, in vitro, levels of fractalkine mRNA were unaffected by ensuing excitot oxicity. However, when analyzed at the protein level, we found that fractal kine is rapidly cleaved from cultured neurons in response to an excitotoxic stimulus. More specifically, fractalkine cleavage preceded actual neuronal death by 2-3 hr, and, when evaluated functionally, fractalkine represented the principal chemokine released from the neurons into the culture medium upon an excitotoxic stimulus to promote chemotaxis of primary microglial an d monocytic cells. We further demonstrate that cleavage of neuron-derived, chemoattractive fractalkine can be prevented by inhibition of matrix metall oproteases. These data strongly suggest that dynamic proteolytic cleavage o f fractalkine from neuronal membranes in response to a neurotoxic insult, a nd subsequent chemoattraction of reactive immune cells, may represent an ea rly event in the inflammatory response to neuronal injury.