Synapse Engulfment by Oligodendrocyte Precursor Cells: A New Mechanism of Circuit Refinement in the Developing Brain - PROJECT SUMMARY The establishment of synaptic connectivity during brain development involves the initial formation of an overabundance of synapses followed by a refinement process in which some of these synapses are maintained while others are eliminated. The precise elimination of excess synapses is driven by sensory experience during critical periods of early postnatal life. Impairments in sensory-dependent synapse elimination contribute to neurodevelopmental disorders such as autism, underscoring the importance of this process for the proper development and function of neural circuits. However, although neurodevelopmental disorders are growing in prevalence at an alarming rate, therapeutic strategies for treating them are scarce in part due to a lack of insight into the factors that control synapse elimination in the healthy brain. A key goal of this proposal is to uncover novel cellular and molecular mechanisms underlying the elimination of synapses downstream of experience, thereby laying the groundwork for new therapeutic approaches to treat disorders of postnatal brain development. Work in the visual system of the mouse has revealed that non-neuronal brain cells, predominantly microglia and astrocytes, coordinate synapse elimination before the onset of visual experience by phagocytosing excess synapses. However, data suggest that these cells may not be major regulators of synapse elimination during late phases of development that are coordinated by visual experience. On the contrary, we recently discovered a key role for a less well understood class of glia, oligodendrocyte precursor cells (OPCs), in eliminating synapses in response to experience through synaptic phagocytosis. This result is consistent with wide-spread speculation that OPCs, while predominantly appreciated for their differentiation into mature oligodendrocytes, play key roles in the brain beyond myelination. In line with this possibility, our data suggest that OPCs are essential for shaping functional neural circuits during the maturation of the brain. In this application, we propose a multi-disciplinary strategy to test the hypothesis that the engulfment of synapses by OPCs is a core mechanism underlying the sensory-dependent elimination of functional synapses in the developing brain. In Aim 1, we will employ viral and transgenic tools to characterize the spatio-temporal dynamics and activity-dependent basis of synaptic engulfment by OPCs using in vivo two-photon microscopy. In Aim 2, we will apply physiological and behavioral assays to determine the consequences of synaptic engulfment by OPCs on brain function in the intact animal. In Aim 3, we will merge unbiased transcriptomic and CRISPR- based screening techniques with a candidate-based approach focused on the phagocytic receptor Lrp1 to reveal molecular pathways underlying the engulfment of synapses by OPCs. Altogether, we expect these studies to establish synapse engulfment by OPCs as a new mechanism linking experience to neural circuit development, and to lay the foundation for future studies geared toward modifying OPC function as a potential therapeutic strategy for treating neurodevelopmental disorders.
