Phd in CLINICAL EXPERIMENTAL NEUROSCIENCE AND PSYCHIATRY

Thesis title: Interplay between microglia and neuronal plasticity along the hippocampal longitudinal axis in male and female mice

The hippocampus is a plastic brain area that shows functional segregation along its longitudinal axis, reflected by a higher level of long-term potentiation (LTP) in the CA1 region of the dorsal hippocampus (DH) compared to the ventral hippocampus (VH), but the mechanisms underlying this difference remain elusive. Numerous studies have highlighted the importance of microglia-neuronal communication in modulating synaptic transmission and hippocampal plasticity, although its role in physiological contexts is still largely unknown. We characterized in depth the features of male mice microglia in the two hippocampal poles and investigated their contribution to CA1 plasticity under physiological conditions. We unveiled the influence of microglia in differentially modulating the amplitude of LTP in the DH and VH, showing that minocycline or PLX5622 treatment reduced LTP amplitude in the DH, while increasing it in the VH. This was recapitulated in Cx3cr1 knockout mice, indicating that microglia have a key role in setting the conditions for plasticity processes in a region-specific manner, and that the CX3CL1-CX3CR1 pathway is a key element in determining the basal level of CA1 LTP in the two regions. The observed LTP differences at the two poles were associated with transcriptional changes in the expression of genes encoding for Il-1, Tnf-α, Il-6, and Bdnf, essential players of neuronal plasticity. Furthermore, microglia in the CA1 SR region showed an increase in soma and a more extensive arborization, an increased prevalence of immature lysosomes accompanied by an elevation in mRNA expression of phagocytic markers Mertk and Cd68 and a surge in the expression of microglial outward K+ currents in the VH compared to DH, suggesting a distinct basal phenotypic state of microglia across the two hippocampal poles. Interestingly, differences in microglia structural and functional properties emerged between sexes; therefore, we extended the characterization to female mice, starting with the analysis of control and Cx3cr1 knockout animals. The LTP at the two hippocampal poles was similar in the control condition and was influenced to an equal extent by the absence of Cx3cr1. Furthermore, microglia in the DH and VH showed similar mRNA levels of Il-1, Tnf-α, and Il-6 and an increased density in the DH with respect to the VH. Overall, we characterized the molecular, morphological, and functional profile of microglia at the two poles, highlighting differences between male and female mice. As sex is a risk factor for several neurological diseases characterized by altered microglial morphology and activity, understanding differences between males and females in microglial features at homeostasis can contribute to identifying mechanisms behind sex-specific neurological diseases.