Defining the interactions between microglia and synapses in brain development and disease

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PROJECT SUMMARY/ ABSTRACT Establishing precise synaptic connections is critical for normal brain function. Synaptic dysfunction can lead to neuronal hyperexcitability, contributing to disorders including epilepsy. Microglia are the dominant immune cells in the brain and play multiple roles in synaptic development, modulating neuronal excitability, and engulfing excess excitatory synapses. However, the mechanisms by which microglia impact synapses have largely been investigated with fixed tissue histology or in limited regions of the adult brain using rodent models. In fact, microglial engulfment of whole synapses has not been directly observed in the developing brain. In this proposal, I will use a zebrafish model system to study microglial-synapse interactions in the intact developing brain. My recently published work identified a population of synapse-associated microglia (SAMs) enriched in the zebrafish hindbrain and defined its transcriptional profile by single-cell and regional bulk sequencing. In this proposal, I will examine this microglial subset using a combination of live imaging and candidate gene deletion in both physiology and in the context of hyperexcitability. Aim 1 will determine if microglia engulf synapses during development and the impact of immune activation or after deletion of a core lysosomal protease known as cathepsin b (ctsba) - a top candidate from my transcriptomic work. Aim 2 will further assess these phenotypes in the context of chemically induced hyperexcitability and use startle behavior recordings to assess the impact on neural circuit function. Finally, in Aim 3 (R00 phase), I will define the molecular mechanisms regulating lysosome activity during microglia phagocytosis and transcriptionally profile microglia following neuronal hyperexcitability. Together these studies will open a distinct direction using a new model to identify molecular pathways that regulate microglia-synaptic interactions with the potential to investigate non-neuronal therapeutic interventions that impact development and disease states such as epilepsy.