The way of the sprig: axo-dendritic morphogenesis in the Drosophila visual system

• Applicant: Dr. Neele Wolterhoff
• Subject Area: Developmental Biology ; Cell Biology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 528372640

Brain development encompasses many developmental steps that ultimately lead to specific synaptic connectivity. Axonal and dendritic growth places specific neurons into spatiotemporal vicinity of a subset of other neurons, thereby restricting which neurons “get to see each other” prior to synapse formation. I am interested in how developmental processes that occur days or hours before actual synapse formation set a morphological and dynamical stage that contributes to, and may ensure, correct brain wiring. The specific goal of this proposal is to understand the morphogenesis and dynamics of an essential interneuron in the fly visual system, the amacrine-like cell Dm8, prior to synapse formation as a basis for subsequent connectivity. The amacrine-like Dm8 cells in the Drosophila adult brain exhibit a unique and remarkable morphology that quantitatively contributes to synaptic specificity: first, highly branched axo-dendritic arborizations extend in a plane, orthogonal to the axonal projection; second, a sprig grows “backwards” out of the arborization plane. Sprigs exhibit a fascinating morphology: they always form in the center of the axo-dendritic arborizations, never in the “original” column of the axonal projection but rather one, sometimes two columns away, there is always one sprig per column, never two and finally every column is occupied by one sprig. Sprig formation is of particular importance for brain wiring because sprig location predetermines key aspects of synaptic specificity. However, the underlying developmental rules that lead to sprig formation and how single Dm8s make local choices, are largely unknown. My key approach is to study Dm8 morphogenesis by using 4D ex vivo live imaging in an intact and normally developing brain. In preparation for this proposal, I have established both 4D live imaging and new genetic tools for sparse labeling of single Dm8s throughout development. Using these tools, I will pursue three specific aims: 1) to extract developmental constraints and rules based on wild type live observations; 2) to test developmental rules through spatiotemporal cellular manipulations; and 3) to interrogate a specific molecular interaction during sprig formation. In summary, this work is devised to provide an exemplary test case to quantitatively understand the contribution of dynamic axo-dendritic morphogenesis to brain wiring in an intact, normally developing brain.

• DFG Programme: WBP Position