Vien, K. M. | bioRxiv Science

bioRxiv2025

Deciphering the combinatorial expression pattern and genetic regulatory mechanisms of Beats and Sides in the olfactory circuits of Drosophila

Over the past decades, many critical molecular players have been uncovered to control distinct steps in olfactory circuit assembly in Drosophila. Among these, multi-member gene families of cell surface proteins are of interest because they can act as neuron-specific identification/recognition tags in combinations and contribute to circuit assembly in complex brains through their heterophilic or homophilic interactions. Recently, a multi-protein interactome has been described between the Beat and Side families of IgSF proteins. Here, we use the publicly available single-cell RNA-seq datasets and newly generated gene trap transgenic driver lines to probe the in vivo spatial expression pattern of the beat/side gene families in odorant receptor neurons (ORNs) and their synaptic target projection neurons (PNs). Our results revealed that each ORN and its synaptic target PN class expresses a class-specific combination of beat/side genes, hierarchically regulated by lineage-specific genetic programs. Though ORNs or PNs from closer lineages tend to possess more similar beat/side profiles, we also found many examples of divergence from this pattern among closely related ORNs and closely related PNs. To explore whether the class-specific combination of beats/sides defines ORN-PN matching specificity, we perturbed presynaptic beat-IIa and postsynaptic side-IV in two ORN-PN partners. However, disruption of Beat-IIa-Side-IV interaction did not produce any significant mistargeting in these two examined glomeruli. Though without affecting general glomerular targeting, knockdown of side in ORNs leads to the reduction of synaptic development. Interestingly, we found conserved expression patterns of beat/side orthologs across ORNs in ants and mosquitoes, indicating the shared regulatory strategies specifying the expression of these duplicated paralogs in insect evolution. Overall, this comprehensive analysis of expression patterns lays a foundation for in-depth functional investigations into how Beat/Side combinatorial expression contributes to the olfactory circuit assembly.

neuroscience↗

bioRxiv2023

Conserved atypical cadherin, Fat2, regulates axon terminal organization in the developing Drosophila olfactory sensory neurons

A unique and defining characteristic of the olfactory sensory circuit is its functionally organized topographic map, which requires widely dispersed olfactory sensory neurons with the same identity to converge their axons into one neuropil, a class-specific glomerulus. Understanding the process of how neuronal identity confers circuit organization is a major endeavor in the field of neurobiology due to its intricate connection to neurodegeneration and neuronal dysfunction. In the olfactory system, many aspects of circuit organization, from axon guidance to synaptic matching, are regulated by a variety of cell surface proteins (for example Robo/Slit and Toll receptors). In this paper, weve identified a novel atypical cadherin protein, Fat2 (also known as Kugelei), as a regulator of class-specific axon organization. Fat2 is expressed in olfactory receptor neurons (ORNs) and local interneurons (LNs) within the olfactory circuits, but little to no expression is found in projection neurons (PNs). Fat2 expression levels vary in a neuronal class-specific manner and peak during pupal development. In fat2 null mutants, ORN axon terminals belonging to different ORN classes present with varying phenotypic severity with the highest fat2 expressing classes being most severely affected. In the most extreme cases, fat2 mutations lead to ORN degeneration. We then show evidence that suggests Fat2 intracellular domain is necessary for Fat2 function in ORN axon organization. Within the developmental context, Fat2 is required starting at early stages of olfactory circuit development specifically for precise axon retraction to further condense class-specific glomeruli. Weve also shown that PNs and LNs expression of Fat2 likely does not contribute to ORN organization 1. Lastly, we narrow down potential Fat2 intracellular domain interactors, APC family proteins (Adenomatous polyposis coli) and dop (Drop out), that likely orchestrate the cytoskeletal remodeling required for axon retraction during protoglomerular development. Altogether, we provide a foundational understanding of how Fat2 functions in olfactory circuit organization and implicate the critical role of axon retraction during glomerular maturation.

developmental biology↗