Glia and synapse development in health and disease

Healthy brain development requires coordinated synapse growth and synapse elimination, with disruptions to these processes often resulting in neurodevelopmental disorder. While glia, the non-neuronal cells of the brain, are increasingly recognized as important regulators of both of these processes, the extent of this regulation and, in the case of disorder, dysregulation is still unknown. In this dissertation, I made classic use of the mouse visual system to outline the contours of glial regulation of synapse development in both synapse growth and synapse elimination. First, I examined astrocytes and microglia in the context of normal brain development, characterizing their spatiotemporal expression patterns in and around the mouse optic tract throughout late embryonic and early postnatal development, as RGC axons are growing into their synaptic target, the dLGN (Chapter 2). Next, I examined astrocyte and microglia in the context of disorder. Here, I found that synapses are reduced in size and eye-specific RGC synapse segregation is enhanced in a mouse model of Fragile X Syndrome, the most common single-gene cause of autism and intellectual disability, (Fmr1 KO mouse) during brain development. I identified glial phagocytic genes as disrupted within the developing Fmr1 KO dLGN and demonstrated that both microglial and astrocytic engulfment of synapses were aberrantly increased during this period of enhanced segregation, providing evidence that over-active glial engulfment may drive aberrant synapse refinement during development in a model of Fragile X Syndrome (Chapter 3).