Homer 1b/c expression correlates with zebrafish olfactory system development.

The zebrafish, (Danio rerio) is an important model organism for the analysis of molecular mechanisms that govern neuronal circuit development. The neuronal circuitry that mediates olfaction is crucial for the development and survival of all teleost fishes. In concert with other sensory systems, olfaction is functional at early stages in zebrafish development and mediates important behavioral and survival strategies in the developing larva. Odorant cues are transduced by an array of signaling molecules from receptors in olfactory sensory neurons. The scaffolding protein family known as Homer is well placed to orchestrate this signaling cascade by interacting with and coupling membrane bound receptors to cytosolic signaling partners. To date, Homer has not been demonstrated in the zebrafish. Here we report that the Homer 1b/c isoform was prominent in the olfactory system from the earliest stages of differentiation. We describe the spatial and temporal distribution of Homer in the zebrafish olfactory system. At 24 hours post fertilization (hpf), Homer expression delineated the boundary of the presumptive olfactory placode. Subsequent expression steadily increased throughout the developing olfactory placode, with a prominent localization to the dendritic knobs of the olfactory sensory neurons. Homer expression in the developing olfactory bulb was punctate and prominent in the glomeruli, displaying an apparent synaptic localization. This work supports the hypothesis that Homer is an important molecule in neuronal circuit development, necessary for crucial behaviors required for development and survival.