Filopodial Dynamics and Synapse Specification in the Drosophila Visual System

How is the synaptic specificity achieved in neural circuits comprised of hundreds of different types of neurons? My dissertation aims to advance our knowledge on this overarching question using the complex visual processing circuitry of D. melanogaster. This system not only provides excellent genetic amenability but also a model where almost all connectivity can be built without environmental input, i.e. it is genetically hardwired. Nearly three decades of research has identified a vast array of genes required for various steps of synapse specification. However, it remained unclear how these genes implement the developmental rules that result in the final connectivity and we understand very little of what actually goes wrong between a particular genetic perturbation and the resulting miswired circuit. To that end, I focused on the actual subcellular substrate of connectivity: axonal growth cones. To gain access to the details of their dynamic behavior during development, I developed an imaging technique which allows the monitoring of intact, developing fly brains over long periods in high temporal and spatial resolution. Using live imaging and the axonal terminals of R7 photoreceptor as a model, I performed a detailed analysis of growth cone dynamics during various steps of synaptic specification, in wild-type and perturbed conditions. Interestingly, I found that none of the perturbations that were previously tied to 'layer specific targeting' of R7 axons were actually required for the recognition of or targeting to a specific layer; instead, all displayed a loss of stabilization with various timings of onset. High speed live analysis revealed the stochastic filopodial dynamics of these axons as crucial mediators of this stabilization. First, as the substrate of attachment to the target layer during early development (Chapter 2); second, as the searching agents for postsynaptic partners during synapse formation (Chapter 3). In brief, my research provided a valuable bridge between the genetic factors that instruct the synapse specific wiring of the brain and how they regulate the dynamic properties of axonal growth cones and synaptic terminals in distinct ways to achieve that final outcome.