Regulation of Synapse Refinement by Visual Experience-Dependent Transcription

Abstract

Visual input mediates the structural and functional refinement of synapses in the developing visual system. Proper wiring of this circuitry is dependent on new gene transcription that occurs in the post-synaptic cell following stimulus-evoked neuronal activity and presynaptic neurotransmitter release. Post-synaptic calcium influx through voltage-gated calcium channels and glutamate-gated receptors activates signaling cascades that turn on transcriptional regulators in the nucleus, which subsequently initiate the transcription of a later wave of genes that are important for synaptic maturation and refinement. However, in the visual system, the identity and function of these waves of gene expression are incompletely understood. The findings in this dissertation elucidate the genes that are induced in the mouse dorsal lateral geniculate nucleus (dLGN) of the thalamus upon exposure to visual stimuli, and elucidate the way in which one candidate gene mediates the appropriate connectivity and refinement of the retinogeniculate synapse—the connection between retinal ganglion cells (RGCs) to thalamocortical relay (TC) neurons. We hypothesized that genes that are induced by visual experience in post-synaptic TC neurons might be likely candidate regulators of retinogeniculate synaptic refinement. To test this hypothesis, we first performed whole-tissue and single-nucleus RNA sequencing (snRNA-seq) of the dLGN following one week of dark rearing from postnatal day (P)20 to P27, which spans the vision sensitive period of retinogeniculate synapse refinement, and subsequent exposure to light. The results of our snRNA-seq identified hundreds of significantly upregulated genes in excitatory TC neurons after dark rearing and light exposure. We focused our study on a highly induced gene in TC neurons, Tnfrsf12a, which encodes the cell surface pro-inflammatory cytokine receptor TNF receptor superfamily member 12a, referred to as Fn14. Genetic ablation of Fn14 results in structurally smaller and an increased number of functionally weak RGC inputs relative to wild-type mice at P27, suggesting that Fn14 expression is critical for proper functional and anatomical wiring of the retinogeniculate synapse during the vision sensitive period. These findings indicate that visual experience induces the transcription of genes that are critical for normal refinement of the retinogeniculate synapse.