CX3CL1 DRIVES CELL-DEPENDENT MODULATION OF HIPPOCAMPAL SYNAPTIC FUNCTIONS

Neuron-microglia communication is essential for the regulation of synaptic function and circuit integrity, yet the precise cellular mechanisms through which microglia sense and modulate neuronal activity remain incompletely understood. The chemokine CX3CL1, predominantly expressed by neurons, and its receptor CX3CR1, selectively expressed by microglia, constitute a key signaling axis at the neuron-microglia interface. Here, we investigate the role of CX3CL1-CX3CR1 signaling in regulating hippocampal synaptic function and connectivity.
Using in vitro hippocampal cultures, we first characterized the subcellular distribution of CX3CL1 under basal conditions and following LTP induction. We find that CX3CL1 displays a dynamic activity-dependent redistribution, suggesting a role in coupling synaptic plasticity to microglial sensing mechanisms.
Electrophysiological analyses in CX3CL1 knockout mice revealed reduced functional connectivity within hippocampal circuits, accompanied by a decreased AMPA/NMDA ratio and altered LTP expression. Morphological analysis showed that microglia in CX3CL1-deficient mice exhibit a hyper-ramified phenotype, consistent with altered neuron-to-microglia signaling. Additionally, time-lapse imaging experiments further demonstrate that CX3CL1 actively promotes the attraction and extension of microglial processes.
To dissect the cellular specificity of this effect, we generated chimeric mice with sparse deletion of CX3CL1 selectively in excitatory pyramidal neurons. Remarkably, this manipulation was sufficient to recapitulate the hippocampal synaptic connectivity deficit observed in constitutive CX3CL1 knockout mice, indicating a cell-autonomous requirement for neuronal CX3CL1 in maintaining network integrity.
Together, these findings identify CX3CL1 as a critical activity-dependent neuronal signal that modulates synaptic function and hippocampal connectivity through precise neuron-microglia interactions, highlighting its role as a key mediator of circuit-level homeostasis.