Deciphering the mechanisms underlying multicolumnar neuron pathfinding and specification in the Drosophila melanogaster optic lobe

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Project Summary/Abstract: How circuits in the brain are assembled is a crucial question in developmental neurobiology. The Drosophila visual system presents an elegant model to study this question; the optic lobes comprise ~60,000 neurons that allow the animal to perform sophisticated visual tasks. These cells are organized into 800 columns representing the 800 unit eyes (ommatidia) that perceive visual information in the retina. The medulla is the most complex neuropil of the visual system. Though each of the ten medulla layers is highly specialized, a consistent neuronal topology is maintained from layer to layer, in a phenomenon known as retinotopy. The host lab has identified three processes that generate the diversity of the over 80 neural types in the medulla. - First, each of the ~800 neuroblasts (NBs) sequentially expresses a series of six temporal transcription factors (tTFs), whose combination specifies different types of neuronal progeny. The integrated output of this system allows each NB to specify about 20 types of uni-columnar (UC) neurons at a 1:1 ratio to medulla columns. - Second, spatial cues within the OPC act in combination with tTFs to specify the fate of a second set of neurons—multicolumnar neurons (MC neurons)—that have a larger receptive field, exist at less than a 1:1 ratio to columns, and that innervate anywhere from two columns to half of the medulla, depending on the cell type. These MC neurons are only produced in subregions of the neuroepithelium. - Finally, Notch signaling (Non or Noff) further diversifies neuronal identity of the two neurons emerging from the division of the ganglion mother cell, the single transit-amplifying descendant of each NB. While MC neurons derive from restricted regions of the OPC, they find their targets and connect to the entire medulla. Although descriptions of MC neuron organization have been reported, the mechanisms behind how they find their targets are mostly unknown. Furthermore, previous research regarding MC neuron specification has focused on descendants of neuroblasts expressing the tTF Homothorax; the descendants of other tTF- expressing NBs have yet to be explored. My work seeks to understand how MC neurons are specified, and how this fate specification informs the cell's decisions in axon guidance, and thus, the establishment of retinotopy in this system. Specific aim 1 will look at the dynamics of MC neuron targeting within the medulla. We will use live imaging and immunofluorescence techniques to determine the lineage of MC neurons, identify the transcription factors expressed in these cells, and observe the mechanisms used by these cells to find their targets. Specific aim 2 will build upon the knowledge unearthed in Specific aim 1, and will use a candidate approach combined with transcriptome analysis of sorted MC cells in order to identify the genes required for MC neuron identity and pathfinding. Our research will provide novel insight into the physical and genetic mechanisms underlying how complex neurons are generated and find multiple targets on the retinotopic map, allowing us to better comprehend basic principles of nervous system assembly.