Role of Microglia in Epilepsy – RoME

We use a mouse model of epilepsy to implement electrophysiological approaches and super-resolution microscopy to understand the signaling pathways and physical interactions between microglia and neurons. We use several lines of transgenic mice for visualization of microglia and specific interactions between microglia and neurons and for invalidation of microglia proteins.

Our results indicate that microglial properties quickly (24h) change after a single seizure. These changes include changes in electrophysiological and pharmacological properties, and alterations in the motility properties of the thin extensions of these cells. These modificaitons are accompanied by microglial neuronal death and we identified a membrane receptor of microglia which partly controls this neuronal death. We are currently investigating changes in synaptic activity that occur during the first days of the seizure, and our preliminary results suggest that the contacts between microglia and synapses induces changes in the synaptic activity.
Taken together, these data confirm that microglia affect neuronal remodeling that occurs after a seizure and we are currently working on the signaling pathways between microglia and neurons which may explain these observations.

The second part of our project will be devoted to the study of cellular and molecular mechanisms responsible for the interactions between microglia and neurons after seizure. If we can identify specific signaling pathways of these interactions, it should enable to test new therapeutic approaches, nonneuronal in the treatment of epilepsy

Our work has been featured in several national and international conferences in the form of oral presentations and posters. The first article was submitted to the journal Glia this summer.

Status epilepticus (SE), one of the most common neurological abnormalities, is triggered by a variety of insults to the brain and can lead to the development of epilepsy. Neuronal functional remodelling induced by SE is thought to be a consequence of injury-induced excitotoxicity triggered by massive and repetitive depolarisations. Recently, however, growing evidence challenging this purely neurocentric vision in epilepsy research has indicated that glial cells may influence the development and maintenance of epileptic conditions. In particular, microglia cells which orchestrate inflammatory reactions in the brain become activated after SE, even in the absence of massive neuronal death.

We have recently characterized the functional state of microglia in a mouse model of SE and our working hypothesis is that the specific microglia activation rapidly developing after SE determines neuronal adaptive processes which influence the outcome of the disease. This role of microglia cells could rely on their ability to release inflammatory molecules but also to establish direct contacts with neurons, notably with synapses.

The objective of the present project is thus to understand the consequences of microglia activation after SE on neuronal and synaptic signalling. Our strategy will be to study how properties of hippocampal neurons and of neuron-microglia interactions change in parallel with the development of inflammation and then to analyse how these modifications develop in a situation where microglia activation is impaired. In the absence of really convincing and specific pharmacological tools to inhibit microglia activation, we have identified two knockout (KO) mice for receptors which are mostly microglial, P2X7- and TLR4, and for which preliminary evidence indicates that microglial signalling is disrupted after SE.

We will first analyze gene expression by qPCR and with DNA microarrays at different time points to obtain a precise time course of the inflammatory reaction and to determine the overall transcriptom changes occurring after SE in the hippocampus of WT and KO mice. The results of this first task will identify the genes modulated after SE, regulated by P2X7 and TLR4 and for which we will look for functional correlates in subsequent tasks.

Surprisingly, there no is clear description of the functional changes occurring along the week following SE. We will thus use field potential and patch clamp recordings combined with calcium imaging in acute hippocampal slices and analyse changes in neuronal excitability, synaptic transmission and astrocyte-to-neuron communication at different time after SE. We will then use the most advanced methods in fluorescent microscopy (confocal, 2-photon, STED) to study, with the highest possible resolution, changes in the fine structure of dendritic spines, thought to be important for their function, of thin microglial processes, which contact neurons, and in the dynamics of microglia-neuron interactions in the hippocampus at different time points after SE. Comparison of these functional and structural changes during the inflammatory reaction in wild type mice and when microglial activation is disrupted in KO mice will instruct us on how microglia influences neuronal adaptation after SE. Comparison of these data with those of gene expression profiling will identify putative signalling pathways which will be then tested.

This project associates a team specialized in the functional analysis of neuronal and synaptic networks and of neuron-glia interactions with a team pioneer in superresolution microscopy applied to the study of synapses. It should identify new signalling pathways between microglia and neurons and shed a completely new light on the mechanisms of epileptogenesis.