Target recognition and competitive synaptogenesis in the Drosophila giant fiber system

• Main Author: Hill, Jason Joseph
• Language: ENG
• Published: ScholarWorks@UMass Amherst 2012
• Subjects: Neurosciences
• Online Access: https://scholarworks.umass.edu/dissertations/AAI3518241

The development of complex neural networks relies on a careful balance of environmental cues to guide and shape both ends of the eventual connection. However, the correct wiring of circuits whose components share molecular profiles depends on a more elaborate phenomenon, competition. Despite being highly studied, there is still a lack of understanding as to the mechanism that allows molecularly identical cells to form exclusive connections with their targets. To address this complex question, we turned to a simple circuit within the genetically tractable fly. Responsible for the escape reflex, the Giant Fiber System is comprised of bilaterally symmetrical axons that innervate the ipsilateral "jump" motorneuron, TTMn in a 1:1 ratio. However, if a TTMn is unilaterally ablated prior to circuit formation, this ratio is disrupted and the deprived axon forms its presynaptic terminal on the opposite side. Midline crossing by the deprived axon led to exploration of a known pathway in giant fiber development, midline repulsion via Slit and Roundabout. Axons in which Roundabout levels were reduced through a natural pathway antagonist, Commissurelesss, crossed the midline freely, confirming a native, if normally restricted ability to do so. However, unlike the overlapping giant fiber terminals seen following ablation, these axons retained their wild exclusivity, elaborating their terminals toward a single TTMn. This supported our initial aim of uncovering a competitive force in giant fiber target selection. In addition to repulsion, I also examined the attractive pathway of Netrin and Frazzled for a possible role in target identification. Varying the levels of Frazzled receptors led to increased midline crossing and overlapped terminals, suggesting a connection between this attractive receptor and the repulsion pathway first examined. Frazzled has been shown to induce commissureless expression independent of its ligand, making it an important linchpin in the regulation of giant fiber guidance and competition. In fact, when allowed to traverse the midline, giant fibers responded to a Netrin increase with overlapping synaptic terminals. In this dissertation, I present a model in which giant fibers possess competitive machinery, driven by Netrin and triggered by Frazzled, underneath the naturally restrictive midline repulsion.