Synaptic signal transduction and transcriptional control.

Abstract

Synaptic signal transduction regulates synaptic plasticity, and, on a larger scale, memory itself. The aim of this dissertation is to elucidate some of the mechanisms that control synaptic plasticity in the short term by modulating synaptic morphology and in the long term by controlling gene expression. One modification associated with synaptic plasticity is the change in the size of the spine, the micron-scale structure on the dendrite which supports the synapse. The size and shape of the spine are controlled by the actin cytoskeleton. I studied how stimulation of synaptic receptors drives changes in activation of proteins that regulate actin polymerization. We identified neuron-specific aspects of a canonical actin regulation pathway and characterized activity-regulated phosphatase activity. Changes in spine size and other events associated with synaptic plasticity can begin within seconds of synaptic stimuli, but persistent changes require gene expression. For example, Arc, an immediate early gene required for changes in synaptic strength to persist, is the only transcript known to be both transcribed in response to synaptic stimulation and translocated specifically to the site of the stimulation. However, the role of Arc in promoting the plasticity of the synapse is still under investigation. We studied its binding partners and found that an interaction demonstrated in non-neuronal cells was not evident in neurons. We also studied changes in transcription over longer time periods. In order to identify pathways involving the postsynaptic protein densin, we assessed global changes in transcription with RNA-Seq, which uses ultra-high-throughput, short-read sequencing to measure transcript abundance. Compared to wild-type mice, densin knockout mice exhibit increased abundance of CaMKIIα (a densin binding partner), increased abundance of immediate early gene expression including Arc, and downregulated GABA_AR subunits. In summary, we investigated posttranslational modifications that take place within seconds of stimulation, binding interactions occurring in steady-state conditions in wild-type mice, and homeostatic adaptations to the chronic absence of a gene. These investigations into synaptic signaling illustrate not only the complexity of synapse-related regulatory networks but also the range of time scales they span.