Chemokine Fractalkine/CX₃CL1 Negatively Modulates Active Glutamatergic Synapses in Rat Hippocampal Neurons

Abstract

We examined the effects of the chemokine fractalkine (CX3CL1) on EPSCs evoked by electrical stimulation of Schaffer collaterals in patch-clamped CA1 pyramidal neurons from rat hippocampal slices. Acute application of CX3CL1 caused a sustained reduction of EPSC amplitude, with partial recovery after washout. CX3CL1-induced EPSC depression is postsynaptic in nature, because paired-pulse ratio was maintained, amplitude distribution of spontaneous excitatory postsynaptic currents shifted to lower values, and whole-cell current responses to AMPA were reversibly inhibited. EPSC depression by CX3CL1 is mediated by CX3CL1 receptor (CX3CR1), because CX3CL1 was unable to influence EPSC amplitude in CA1 pyramidal neurons from CX3CR1 knock-out mice. CX3CL1-induced depression of both EPSC and AMPA current was not observed in the absence of afferent fiber stimulation or AMPA receptor activation, respectively, indicating the requirement of sustained receptor activity for its development. Findings obtained from hippocampal slices, cultured hippocampal neurons, and transfected human embryonic kidney cells indicate that a Ca2+-, cAMP-, and phosphatase-dependent process is likely to modulate CX3CL1 effects because of the following: (1) CX3CL1-induced depression was antagonized by intracellular BAPTA, 8Br-cAMP, phosphatase inhibitors, and pertussis toxin (PTX); (2) CX3CL1 inhibited forskolin-induced cAMP formation sensitive to PTX; and (3) CX3CL1 inhibited forskolin-induced Ser845 GluR1 phosphorylation, which was sensitive to PTX and dependent on Ca2+and phosphatase activity. Together, these findings indicate that CX3CL1 negatively modulates AMPA receptor function at active glutamatergic synapses through cell-signaling pathways by influencing the balance between kinase and phosphatase activity.

Interactions between Chemokines

“…Both constitutive and stimulated fractalkine cleavage would ultimately limit activation of microglia and support neuronal survival as previously reported (5)(6)(7)(8)(9)(10). Evidence indicates that fractalkine provides a tonic inhibitory signal that controls neighboring microglia resulting in reduced release of pro-inflammatory cytokines and increased production of neurotrophins (41); it controls glutamatergic activity by inhibiting excitatory postsynaptic currents (6,25,42); it negatively regulates neuronal migration while promoting their adhesion to ECM (43). The extent to which each of these mechanisms is involved in the physiological role of fractalkine may vary at different stages of life and/or during disease states.…”

“…The relative expression of these chemokines and their receptors by neuronal and nonneuronal cells suggests a role for both SDF-1 and fractalkine in neuronal-glial communication, as supported by in vivo and in vitro evidence (3,23). Furthermore, data from different groups have shown that SDF-1 and fractalkine are both involved in neuroprotection and regulate excitatory neurotransmission (6,24,25). However, while the neuroprotective effect of fractalkine seems to be related to its ability to counteract microglia-induced neurotoxicity, SDF-1 can directly protect neurons via the activation of its specific receptor CXCR4, which is coupled to major survival pathways, such as the phosphoinositide 3-kinase (PI3K)-Akt pathway (26,27).…”

Activity of Adenosine Receptors Type 1 Is Required for CX3CL1-Mediated Neuroprotection and Neuromodulation in Hippocampal Neurons

“…We and others have previously shown that (i) CX 3 CL1 modulates glutamatergic AMPA-currents revealed as a reduction of the amplitude of both synaptic and agonist-evoked currents; (ii) CX 3 CL1 action on AMPA-currents is stringently and specifically related to the activation of CX 3 CR1 (2,8,32). In this study, we provide evidence that CX 3 CL1-induced depression of AMPA-currents in hippocampal neurons is suppressed by a specific antagonist of AR 1 , DPCPX.…”

“…Since we have found that the neuroprotective action of CX 3 CL1 on hippocampal neurons is dependent on AR 1 activation, we were interested to investigate whether AR 1 could be also involved in the previously described CX 3 CL1-induced neuromodulation (2,8). We report that the CX 3 CL1-mediated inhibition of the amplitude of AMPA-currents in cultured hippocampal neurons was abolished by DPCPX treatment.…”

Central Nervous System: (Immunological) Ivory Tower or Not?

“…It is important to note that although many of these effects may be a result of deficient synaptic pruning, they can also be ascribed to a microglial effect on neuronal circuit activity via other mechanisms in response to soluble CX3CL1 (Ragozzino et al, 2006). A more recently described pathway for microglial pruning is the complement system (Stevens et al, 2007), in which weaker synaptic inputs on developing neurons are decorated with C1q and C3 molecules capable of engaging CR3 receptors on microglia for targeted elimination (Schafer et al, 2012;Stevens et al, 2007).…”