Mechanisms of early axonal pathfinding in the olfactory system of larval zebrafish

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Abstract In the olfactory system precise connections create a functional topographic map that translates odorant experience into ordered patterns of odorant-specific neural activity. This map is established early in development. Olfactory Sensory Neurons (OSNs) stochastically choose to mono-allelically express one Odorant Receptor (OR), which belong to the G-Protein Coupled Receptor (GPCR) family, from a large gene repertoire. Each OSN projects a single axon from the Olfactory Epithelium (OE) to the Olfactory Bulb (OB). Axons of OSNs first target larger identifiable neuropils called protoglomeruli. In zebrafish, our lab has found that the protoglomerulus an OSN axon targets correlates with the family or homology clade of its chosen OR. OSNs expressing ORs from different homology clades target different protoglomeruli. Subsequent to this initial protoglomerular targeting, the axons of OSNs expressing the same OR ultimately coalesce into smaller, distinct, and reproducible neuropil regions known as glomeruli. Thus, the target location of each axon on the OB is coordinated with its chosen OR. Previous research has shown that an OSN’s chosen OR and OR-driven neural activity are important for glomerular segregation. However, it is not known how clade-specific protoglomerular targeting and OR-specific glomerular segregation are achieved. Previous studies performed in the mouse may have conflated initial targeting processes with glomerular segregation due to the relative inaccessibility of early developmental stages. The foundational research in olfactory development utilized static imaging after development was complete, often leading to developmental processes being inferred rather than investigated. We believe protoglomerular targeting is determined by the coordinated expression of the OR locus and axon guidance loci, rather than OR protein function. We have shown through single-cell RNA sequencing that OSNs expressing ORs within the same homology clade also express similar transcription factors and axon-guidance-related factors. Further, OSNs expressing closely related ORs target nearby glomeruli. Thus, I hypothesize that regulators controlling OR choice in OSNs also regulate axonal guidance receptor expression, while OR-specific neural activity regulates the subsequent segregation of OR-specific glomeruli. Utilizing a recombinase-mediated cassette exchange approach, my proposed experiments will disambiguate the relative contributions of OR identity and OR-driven neural activity to protoglomerular and glomerular targeting.