The ability of a neuron to alter its synaptic connections during development is essential to circuit assembly. Synapse remodeling or refinement has been observed in many species and many neuronal circuits, yet the mechanisms defining which neurons undergo remodeling are unclear. Moreover, the molecules that execute the process of remodeling are also obscure. To address this issue, we sought to identify targets of the transcription factor unc-55 COUP-TF, which acts as a cell-specific repressor of synapse remodeling in C. elegans. unc-55 COUP-TF is expressed in VD neurons, where it prevents synapse remodeling. DD neurons can remodel synapses because they do not express unc-55 COUP-TF. Ectopic expression of unc-55 COUP-TF in DD neurons prevents remodeling. We identified the transcription factor Hunchback-like hbl-1 as a target of UNC-55 COUP-TF repression. Differential expression of hbl-1 explains the cell-type specificity of remodeling. hbl-1 is expressed in the DD neurons that are capable of remodeling, and is not expressed in the VD neurons that do not remodel. In unc-55 mutants, hbl-1 expression increases in VD neurons where it promotes ectopic remodeling. Moreover, hbl-1 expression levels bidirectionally regulate the timing of DD remodeling, as increases in hbl-1 cause precocious remodeling while decreases in hbl-1 cause remodeling delays. Finally, hbl-1 coordinates heterochronic microRNA and neuronal activity pathways to regulate the timing of remodeling. Increases or decreases in circuit activity cause increases or decreases in hbl-1 expression, and consequently early or delayed remodeling. Thus, convergent regulation of hbl-1 expression defines a genetic mechanism that patterns activity-dependent synaptic remodeling across cell types and across developmental time. We identified other targets of UNC-55 COUP-TF regulation using gene expression profiling, and implicate some of these factors in the regulation of remodeling using functional genomic screens. Our work suggests roles for conserved networks of transcription factors in the regulation of remodeling. We propose a model in which hbl-1 and other targets of unc-55 COUP-TF transcriptional repression are responsible for regulating synapse remodeling in C. elegans.

<p>The lateral superior olive (LSO) is a binaural nucleus that is critical for azimuthal sound localization. Bipolar principal cells of the LSO compute interaural level differences (ILDs) by comparing converging excitatory and inhibitory inputs driven by either ear. More specifically, this computation relies on integrating excitatory inputs from the ipsilateral cochlear nucleus with inhibitory, GABA/glycinergic inputs from the medial nucleus of the trapezoid body (MNTB), which are driven by sound originating at the contralateral ear. In order to reliably compute ILDs, the converging inputs must represent sounds of the same frequency. This specificity emerges during the first few weeks of postnatal life in rats as a result of functional and anatomical refinement. Interestingly, significant refinement of this auditory circuit occurs in the absence auditory experience. We focused on changes in the subcellular location of MNTB inputs and the expression of vesicular proteins before hearing onset.</p> <p>The subcellular distribution of inputs onto a neuron heavily influences synaptic integration and the mature distribution likely emerges during a period of circuit refinement. Little is known about how the inputs are distributed onto LSO principal cells and how the mature distribution is achieved. We studied the distribution of inhibitory inputs onto LSO neurons and found that significant re- distribution occurs before hearing onset. The mechanisms underlying the refinement of the inhibitory MNTB projection are not well understood but could be related to the transient co-transmission of the excitatory neurotransmitter glutamate. We studied the expression of vesicular proteins that may regulate the release of GABA/glycine and glutamate at the immature MNTB terminal. We found that MNTB terminals transiently express two Ca++ sensors, which may be associated with the different release properties for GABA/glycine and glutamate. Lastly, we asked one specific example of what controls the expression and sorting of vesicular proteins at the immature MNTB terminal.</p> / Doctor of Philosophy (PhD)

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