Fractalkine regulation of microglial physiology and consequences on the brain and behavior

Abstract

Neural circuits are constantly monitored and supported by the surrounding microglial cells, using finely tuned mechanisms which include both direct contact and release of soluble factors. These bidirectional interactions are not only triggered by pathological conditions as a S.O.S. response to noxious stimuli, but they rather represent an established repertoire of dynamic communication for ensuring continuous immune surveillance and homeostasis in the healthy brain. In addition, recent studies are revealing key tasks for microglial interactions with neurons during normal physiological conditions, especially in regulating the maturation of neural circuits and shaping their connectivity in an activity- and experience-dependent manner. Chemokines, a family of soluble and membrane-bound cytokines, play an essential role in mediating neuron-microglia crosstalk in the developing and mature brain. As part of this special issue on Cytokines as players of neuronal plasticity and sensitivity to environment in healthy and pathological brain, our review focuses on the fractalkine signaling pathway, involving the ligand CX3CL1 which is mainly expressed by neurons, and its receptor CX3CR1 that is exclusively found on microglia within the healthy brain. An extensive literature largely based on transgenic mouse models has revealed that fractalkine signaling plays a critical role in regulating a broad spectrum of microglial properties during normal physiological conditions, especially their migration and dynamic surveillance of the brain parenchyma, in addition to influencing the survival of developing neurons, the maturation, activity and plasticity of developing and mature synapses, the brain functional connectivity, adult hippocampal neurogenesis, as well as learning and memory, and the behavioral outcome.

Keywords: CX3CR1; behavior; development; fractalkine; microglia; neurogenesis; neurons; synapses.

Figure 1

The roles of CX₃CL1-CX₃CR1 interactions in the healthy brain. During postnatal development, fractalkine signaling is promoting microglial recruitment to neuronal circuits by increasing their process dynamics and cellular migration, as well as modulating neuronal survival via the release of trophic factors (A). During postnatal development and adulthood, soluble and/or membrane-bound fractalkine further contribute to the maturation, activity and plasticity of excitatory synapses by promoting the maturation of NMDA receptors (transition from GluN2B to GluN2A subunits), the multiplicity and efficiency of synaptic transmission, the depression of AMPA receptors and potentiation of NMDA receptors, and the modulation of LTP and LTD at the Schaffer collateral synapse in the hippocampus CA1 (B), with lasting consequences on the brain functional connectivity, learning and memory, and the behavioral outcome. Additionally, fractalkine signaling might be influencing the developmental pruning of synapses and adult hippocampal neurogenesis by regulating microglial phagocytosis of synaptic elements and newborn apoptotic cells during normal physiological conditions (C).