820C Poster - 13. Neural development and physiology
Saturday April 09, 1:30 PM - 3:30 PM

Authors:
Mehmet Neset Ozel ¹; Claudia Skok Gibbs ^1,2; Isabel Holguera ¹; Richard Bonneau ^1,2; Claude Desplan ¹

Affiliations:
1) Department of Biology, New York University, New York, NY; 2) Flatiron Institute, New York, NY

Keywords:
d. neuronal specification; e. neuronal morphogenesis

Cascades of transcription factors (TF) that are transiently expressed in neural stem cells are responsible for generating the enormous diversity of cell types in nervous systems. However, the gene-regulatory mechanisms that establish and maintain these cell fates in postmitotic neurons and instruct their specific morphology, connectivity and physiology remain largely unclear. Using a large scRNA-seq atlas of the Drosophila optic lobes (Özel et al. 2020, Nature), we tracked ~200 neuronal types across 6 stages and found that each one stably expresses a unique combination of 95 TFs (~10 per cell type) that are maintained throughout development to adulthood. We hypothesized that these function as “selector” TFs that are activated in each neuron immediately after their birth and function as top-level regulators of all type-specific gene expression thereafter. Through genetic gain- and loss-of-function experiments, we show that modification of these selector TF codes is sufficient to induce predictable switches of identity between various optic lobe neurons. For instance, pdm3 is necessary and sufficient to determine the fate choice between Tm2 and Tm4 neurons. In addition to their morphological identity, scRNA-seq of perturbed neurons revealed that such conversions are also transcriptomically complete. Similarly, ectopic expression of Vsx genes in Mi15 neurons not only leads their complete morphological conversion to Dm2, but also a loss of their aminergic identity. Thus, continuously maintained TF codes instruct both the adult terminal features and the type-specific development of optic lobe neurons.
To understand the mechanisms that control brain wiring downstream of selector TFs, we combined scRNA and scATAC-seq (chromatin accessibility) datasets to build computational models of gene regulatory networks using Inferelator 3.0 (Gibbs et al. 2021, BioRxiv). Networks inferred at mid-pupal stages suggest that selector TFs interact with hormone-responsive TFs to activate a large repertoire of cell-surface proteins in each neuron prior to synapse formation. For instance, we show that Netrin receptor Frazzled, downstream of pdm3, specifically mediates the unique ‘forking’ of Tm2 dendrites: overexpression of fra in Tm4 neurons is sufficient to produce this feature.
Overall, our results provide a unified framework of how specific fates are maintained in postmitotic neurons, and open up new avenues to understand synaptic specificity through gene regulatory networks.