Role of Microglia in Sculpting Multisensory Midbrain Circuits

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Microglia are versatile phagocytic cells that play an integral role in establishing mature neural networks. The mechanisms whereby microglial cells (MGCs) respond to signals in their microenvironment to actively engulf extraneous synapses are crucial for topographic map formation, excitatory-inhibitory balance, and synaptic remodeling. MGC perturbation during early critical periods yields unrefined circuits with behavioral and social impairments indicative of certain neurodevelopmental disorders, including autism. Among many activation cues, neuronally-expressed fractalkine (CX3CL1) and complement (C1q, C3) proteins are key players for MGC recruitment to, and active engulfment of underutilized synapses. Knockouts for these and/or their corresponding microglial receptors (CX3CR1; CR3) compromise crosstalk necessary for MGC-mediated synapse elimination. Such interactions have been implicated in shaping visual, somatosensory, and hippocampal connections, but whether similar mechanisms also guide the maturation of auditory and multisensory circuits remains in question. The midbrain inferior colliculus (IC) is an intriguing model structure as its lateral cortex (LCIC) is organized into discrete zones that receive modality-specific inputs. Its modular-extramodular framework and interfacing projection patterns develop concurrently, emerging shortly after birth and becoming well-defined by hearing onset. Initially diffuse and overlapping, somatosensory and auditory inputs segregate as the LCIC matures, targeting complementary modular (GAD-positive) and extramodular (calretinin-positive) domains, respectively. The present study aims: 1) to determine the spatiotemporal patterning of MGCs and fractalkine/complement expression relative to emerging LCIC compartments, 2) to assess fractalkine/complement involvement in sculpting distinct multisensory midbrain circuits, and 3) to test if compromised MGC function during an early critical period of projection shaping results in atypical response behaviors. The first objective utilizes immunocytochemical approaches in control and transgenic mice (GAD67-GFP, CX3CR1-GFP) to visualize MGCs and fractalkine/complement expression with respect to the emerging LCIC microarchitecture, as well as testing if compromised signaling (CX3CR1GFP/GFP, C1qKO, C3KO, CR3KO) alters modular-extramodular development. Proposed anterograde living slice experiments labeling auditory and somatosensory afferents directly test microglial pruning/degradation of supranumerary LCIC synapses. Finally, acoustic startle responses following auditory and/or somatosensory pre-pulse cues (i.e. pre-pulse inhibition) will identify any multimodal psychophysical differences across strains. Project outcomes will advance our understanding of microglial-neuronal signaling in the establishment of multimodal midbrain compartments, their projection maps, and their behavioral significance. Further defining MGC influences during critical periods of circuit assembly will inform how their dysfunction correlates with impaired multisensory processing, communication difficulties, and other neurodevelopmental disorder etiologies.