mRNA localization and regulated translation allow individual neurons to locally regulate the proteome of each of their many subcellular compartments. To investigate the spatial regulation of gene expression during synaptic plasticity, we used a translational reporter system to demonstrate synapse- and stimulus-specific translation during long-term facilitation of Aplysia sensory-motor synapse. These studies revealed a role for a retrograde signal from the postsynaptic motor neuron in regulating translation in the presynaptic sensory neuron. Additional studies with the translational reporter demonstrated that distinct cis-acting localization elements were involved in targeting mRNA to distal neurites and to synapses. Our studies identified a 66 nucleotide long stem loop structure that directs mRNAs to synapses.

In the final part of my thesis research, I addressed the question of whether and how synaptogenic signals direct mRNA targeting and spatially regulate gene expression during synapse formation. I cultured a bifurcated Aplysia sensory neuron contacting a nontarget motor neuron, with which it did not form chemical synapses, and a target motor neuron, with which it formed glutamatergic synapses, and imaged RNA and protein localization. I find that RNAs and translational machinery are delivered throughout the neuron, but that translation is enriched at sites of synaptic contact. Investigation of the molecular mechanisms that promote local translation revealed a role for netrin1-DCC signaling. Together, my research indicates that the spatial regulation of gene expression during synapse formation and during synaptic plasticity is mediated at the level of translation. This mechanism maximizes neuronal plasticity by rendering each compartment capable of locally changing its proteome in response to local cues.