Phd in CLINICAL EXPERIMENTAL NEUROSCIENCE AND PSYCHIATRY

Thesis title: Microglia modulate glutamatergic synaptic function through CX3CL1-CX3CR1 signaling

Microglia are the resident immune cells of the Central Nervous System (CNS). Recently, several studies have highlighted the role of microglia in monitoring the brain environment in order to maintain the homeostasis and in regulating network formation. The function of microglia is strictly related to their ability to communicate with the neuronal population, through the release of soluble factors or by physically contacting synaptic elements. Indeed, the alteration of microglia-neurons crosstalk leads to the appearance of deficits in synaptic transmission and behavior. Among the different soluble factors involved in microglia-neuron interaction, the chemokine CX3CL1, produced by neurons, is known to bind the microglial receptor CX3CR1, thus regulating microglial function. It is now accepted that microglia actively regulate synaptic development, however, it is still not clear whether and how they control synaptic function in the mature brain. The aim of this work was to study how CX3CL1 signaling to microglia controls synaptic transmission in the adult mice. Through the use of a pharmacological model of microglial depletion (PLX5622), we assessed microglial role in the control of glutamatergic synaptic transmission in hippocampal acute slices. Following PLX-induced microglia depletion, we observed a decrease in the amplitude of both spontaneous and evoked glutamatergic currents and the appearance of immature synaptic features. We also treated a cohort of Cx3cr1-/- mice with PLX and, remarkably, we obtained that synaptic depression caused by microglia depletion was absent in this model of impaired microglia-neuron crosstalk. By fluorescence monitoring of microglial processes, we observed that CX3CL1 is able to attract microglial branches in Cx3cr1GFP/+ mice. Moreover, the Cx3cr1::CreERT2;Rosa26-CAG::LSL-tomato;Thy1::GFP mouse model allowed us the visualization of neurons and microglia, and the conditional knockout of CX3CR1. We studied the distribution of microglia-neuron contacts in Cx3cr1-/- and Cx3cr1+/- mice and we found a decrease in density of microglia processes contacting neurons in Cx3cr1-/- mice. These results suggest a relevant role of CX3CL1 chemoattraction in the regulation of microglial-neuron communication. Furthermore, we observed a decrease in hippocampal connectivity and the presence of immature synaptic features in Cx3cl1-/- mice. These evidences support the hypothesis that microglia is needed in the adult brain to maintain the correct synaptic function. In this regard, CX3CL1 represents a pivotal signal in microglia interaction with synapses that modulates synaptic function likely by attracting microglial processes.