Molecular Mechanisms Of Microglia-Mediated Phagocytosis In The Central Nervous System

It has emerged in recent years that microglia act as a key player in the correct maturation and maintenance of neuronal network. The triggering receptor expressed on myeloid cells 2 (TREM2) is exclusively expressed by microglia in the central nervous system (CNS), where it mediates phagocytosis, with a striking role in neurodegeneration. Until now, all the studies about TREM2 in the CNS focused on pathological state, while no data were available about its contribution during the early phase of neurodevelopment. Based on these considerations, we investigated the role of TREM2 receptor in microglia-mediated synapse refinement and elimination. For this reason, our work aimed to characterize the post-natal brain development in a Trem2-/- mouse model, taking advantage of ex vivo and in vitro techniques such as, among others, confocal microscopy analysis and electrophysiology. We demonstrated that the lack of TREM2 affected microglial cells density specifically in the CA1 region of the hippocampus, where also its morphology was altered. Furthermore, we investigated the physiological elimination of synaptic elements by microglia, showing that the absence of TREM2 impairs phagocytosis of synaptic structures, both in vitro and in vivo. In the second part of this work, our research focused on a critical question that remained open, i.e. which signal makes microglia able to recognise the correct synapse to be pruned. Since TREM2 has a high affinity for phospholipids, and phosphatidylserine (PS) has a central relevance in neuronal homeostasis, we hypothesized that PS can be a local cue in synaptic pruning during development. We investigated whether PS could be exposed at synaptic sites and, eventually, be targeted by microglial TREM2 during synapse elimination. Our results suggested that TREM2 functionally recognized PS when it is exposed on synthetic liposome and that PS-TREM2 interaction allowed the process of synaptic pruning in vitro. We also showed that PS labelled a subpopulation of hippocampal synapses in vitro and that it is also exposed at synapses in vivo, where PS was found predominantly at presynaptic site in a time dependent way. Thanks to our data, we demonstrated that microglial TREM2 is a critical actor in the process of synaptic pruning in developing hippocampus. Therefore, we present a novel role for PS as targeting signal for microglia during the central phase of circuit development.