Title: Transcriptional control of somatosensory neuron diversification in Drosophila
Description: Primary sensory neurons deliver information from the periphery to specific circuits in the central nervous system. It is vital that each sensory neuron detects the appropriate type of stimulus and conveys that information to appropriate regions of the sensory neuropil to target second-order neurons. Molecular programs that coordinate sensory morphology in the periphery with axon projection patterns centrally are poorly understood. I have used the multidendritic (md) sensory neurons of the Drosophila melanogaster peripheral nervous system to identify genetic and molecular programs that coordinate dendrite and axonal morphogenesis in individual sensory neurons. The homeodomain transcription factor Cut is expressed in neurons with complex dendrite morphologies that innervate the epidermis and ventral axon projections in the CNS, and is absent from putative proprioceptive neurons that have simpler dendrites and target to more dorsal CNS regions. In this thesis I demonstrate that, in defined subsets of sensory neurons, loss of Cut leads to dendritic transformation to a proprioceptive-type arbor that is accompanied by a dorsal shift in the termination of their axons in the CNS.

Mechanistically, I show that Cut functions at least in part by repressing the expression of the POU domain transcription factors Pdm1 and Pdm2 (Pdm1/2), which are normally expressed only in proprioceptive neurons. Gain and loss of function studies further suggest instructive roles for Pdm1/2 in the development of proprioceptive dendritic arborization and axonal targeting. Together these results identify a transcriptional program that coordinately specifies proprioceptive dendrite morphology and sensory axon targeting to modality-specific domains of the CNS. Using a candidate based approached I have identified three molecular regulators of proprioceptive neuron dendrite morphology. In addition, gene profiling of sensory neurons forced to express Pdm2 has identified over 600 genes that show changes in expression when Pdm2 is misexpressed and that may mediate the effects of Pdm1/2 in directing proprioceptive dendrite and axon development. These profiling experiments pave the way for the identification of novel regulators of dendrite and axon morphogenesis that link transcriptional programs to specific morphologies with consequences for sensory circuit function.