Molecular and Cellular Mechanisms of Neural Superposition in Drosophila

• Applicant: Professor Dr. Peter Robin Hiesinger
• Subject Area: Developmental Neurobiology
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 443637483

Brain wiring is a developmental growth process. Patterned brain architectures, especially columns and layers, are common in all brains and an essential basis for wiring specificity. As in any other growth process, cellular interactions are restricted in space and time. Correspondingly, surface molecular interactions are restricted to those that 'get to see' each other during development. Many of these interactions contribute to such positional effects during brain wiring, others directly ensure synaptic partnerships. Disruption of either of these two types of molecular functions is likely to lead to brain wiring defects and neurodevelopmental disorders. The goal of this proposal is to resolve a well-known instance of the brain wiring problem in fly visual systems, called Neural Superposition, where close to 5,000 axon terminals are sorted in a highly specific and patterned manner to ensure functional connectivity. In Neural Superposition, as in any brain wiring principle, bringing together the correct pre- and postsynaptic partners is key to functional connectivity. In preliminary data for this proposal, we made the surprising discovery that the entire process of sorting ~5,000 axon terminals according to the principle of neural superposition in Drosophila occurs independent of (and even in the absence of) the postsynaptic partners. Our preliminary evidence further suggests that the establishment of the Neural Superposition wiring principle occurs exclusively through axon-axon interactions of the photoreceptor cell axons. We have established improved methods of multiphoton high-resolution 4D live imaging and analysis methods in intact fly brains that revealed a key role of filopodial dynamics in a pruning-like pattern formation process that brings the correct partners together at the right time and place before synapse formation starts.To solve the Neural Superposition sorting problem, we have developed imaging and image analysis methodology as well as several new transgenic fly strains for this proposal; these include surface receptor knock-ins and genetic inroads based on spatiotemporally defined, acute cell biological and signaling perturbation. Specifically, we propose (1) to establish a 4D filopodial interaction map throughout the sorting process in the presence and absence of target cells in the intact developing brain, (2) to characterize photoreceptor cell type dependencies, the role of differential adhesion, and the role of adhesive cell junction signaling in the pattern formation process, and finally (3) to characterize the role of synapse formation in the stabilization of the connectivity pattern. Together, these experiments are designed to understand the cellular and molecular mechanism that can explain a general principle: how pattern formation can lead to precise and robust connectivity.

• DFG Programme: Research Grants