Fractalkine Cleavage from Neuronal Membranes Represents an Acute Event in the Inflammatory Response to Excitotoxic Brain Damage

Abstract

Fractalkine is a recently identified chemokine that exhibits cell adhesion and chemoattractive properties. It represents a unique member of the chemokine superfamily because it is located predominantly in the brain in which it is expressed constitutively on specific subsets of neurons. To elucidate the possible role of neuronally expressed fractalkine in the inflammatory response 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 ischemic stroke (permanent middle cerebral artery occlusion) in the rat. Similarly, in vitro, levels of fractalkine mRNA were unaffected by ensuing excitotoxicity. However, when analyzed at the protein level, we found that fractalkine 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 and monocytic cells. We further demonstrate that cleavage of neuron-derived, chemoattractive fractalkine can be prevented by inhibition of matrix metalloproteases. These data strongly suggest that dynamic proteolytic cleavage of fractalkine from neuronal membranes in response to a neurotoxic insult, and subsequent chemoattraction of reactive immune cells, may represent an early event in the inflammatory response to neuronal injury.

Glia in pathological pain: A role for fractalkine

“…Thus, evidence to date suggest that inducible cleavage of fractalkine is regulated post-translationally by matrix metalloproteases and CatS from glia (Cross and Woodroofe, 1999). In brain, strong neuronal excitation releases fractalkine, which then binds to and activates nearby glia (Chapman et al, 2000). As noted previously, in pain sensory relevant areas of the spinal cord (dorsal horn), fractalkine is expressed by neurons while its receptors are expressed mostly on microglia.…”

“…One report demonstrated CX3CR1 expression on hippocampal neurons (Meucci et al, 2000). Strong neuronal excitation induces the cleavage of the extracellular chemokine domain to form a soluble, diffusible signal (Chapman et al, 2000). During pathological pain conditions, a state induced by strong neuronal excitation in the spinal cord, fractalkine receptor expression is increased in microglia in pain relevant areas .…”

“…In vitro exposure of neuronal mixed cultures and/or brain slices to glutamate, TNF, or interferon-gamma is sufficient to induce fractalkine shedding (Chapman et al, 2000, Erichsen et al, 2003. Matrix metalloproteinases are implicated in the process of shedding.…”

Adenosine A1 Receptors and Microglial Cells Mediate CX3CL1-Induced Protection of Hippocampal Neurons Against Glu-Induced Death

“…No significant differences in neuron death were observed between wt and CX3CL1 À/À mice at all tested Glu concentrations (Supplementary Figure S2). This suggests that endogenous levels of CX3CL1, neither before nor after Glu treatment (Chapman et al, 2000;Erichsen et al, 2003;Limatola et al, 2005), are sufficient to protect neurons by excitotoxicity under our in vitro conditions. To analyze whether the effect of the administration of the soluble form of CX3CL1 could be different in wt vs CX3CL1 À/À mice, evidencing a possible cooperative role of the endogenous CX3CL1, excitotoxicity experiments were performed as shown in Figure 2.…”

Microglia, major player in the brain inflammation: their roles in the pathogenesis of Parkinson's disease

“…In the CNS, several populations of neurons express fractalkine mRNA constitutively that is not affected by stimuli such as cytokines, LPS and toxic stimuli (A , glucose deprivation or glutamate) (Maciejewski-Lenoir et al, 1999). Membrane-bound fractalkine protein levels were decreased after excitotoxic glutamate stimuli (Chapman et al, 2000). Its receptor, CX3CR-1 is expressed at high levels in microglia (Nishiyori et al, 1998).…”