BACKGROUND The axons of Olfactory Sensory Neurons (OSNs) project to reproducible target locations within the Olfactory Bulb (OB), converting odorant experience into a spatial map of neural activity. We characterized the initial targeting of OSN axons in the zebrafish, a model system suitable for studying axonal targeting early in development. In this system the initial targets of OSN axons are a small number of distinct, individually identifiable neuropilar regions called protoglomeruli. Previously, Olfactory Marker Protein-expressing and TRPC2-expressing classes of OSNs were shown to project to specific, non-overlapping sets of protoglomeruli, indicating that particular subsets of OSNs project to specific protoglomerular targets. We set out to map the relationship between the classical Odorant Receptor (OR) an OSN chooses to express and the protoglomerulus its axon targets. METHODS A panel of BACs were recombineered so that the axons of OSNs choosing to express modified ORs were fluorescently labeled. Axon projections were followed into the olfactory bulb to determine the protoglomeruli in which they terminated. RESULTS RNA-seq demonstrates that OSNs express a surprisingly wide variety of ORs and Trace Amine Associated Receptors (TAARs) very early when sensory axons are arriving in the bulb. Only a single OR is expressed in any given OSN even at these early developmental times. We used a BAC expression technique to map the trajectories of OSNs expressing specific odorant receptors. ORs can be divided into three clades based upon their sequence similarities. OSNs expressing ORs from two of these clades project to the CZ protoglomerulus, while OSNs expressing ORs from the third clade project to the DZ protoglomerulus. In contrast, OSNs expressing a particular TAAR project to multiple protoglomeruli. Neither OR choice nor axonal targeting are related to the position an OSN occupies within the olfactory pit. CONCLUSIONS Our results demonstrate that it is not the choice of a particular OR, but of one from a category of ORs, that is related to initial OSN target location within the olfactory bulb. These choices are not related to OSN position within the olfactory epithelium.

中文翻译：

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嗅觉轴突靶向斑马鱼嗅球中的特定原球体。

背景技术嗅觉感觉神经元（OSN）的轴突投射到嗅球（OB）内可重现的目标位置，将气味体验转换为神经活动的空间图。我们表征了斑马鱼中OSN轴突的初始靶向，该模型系统适用于研究发育早期的轴突靶向。在该系统中，OSN轴突的最初目标是少数几个独特的，可单独识别的神经柱区域，称为原球体。以前，显示嗅觉标记蛋白表达和TRPC2表达的OSN类可投射到特定的，不重叠的原生球小体集，表明OSN的特定子集投射到特定的原生小球靶标。我们着手绘制OSN选择表达的经典气味受体（OR）和其轴突靶标的原球体之间的关系图。方法重组一组BAC，以便荧光标记选择表达修饰OR的OSN轴突。轴突投射到嗅球中，以确定它们终止的原球体。结果RNA-seq证明，当感觉轴突到达球茎时，OSN会很早地表达出令人惊讶的多种OR和痕量胺相关受体（TAAR）。即使在这些早期的开发时代，在任何给定的OSN中也仅表示一个OR。我们使用了BAC表达技术来绘制表达特定气味受体的OSN的轨迹。OR可以根据其序列相似性分为三个进化枝。OSN从其中两个进化支表达的ORs投射到CZ原生小球，而OSN从第三个进化枝表达的ORs投射到DZ原生小球。相反，表达特定TAAR的OSN投影到多个原球体。OR选择或轴突靶向均与OSN在嗅觉凹坑内的位置无关。结论我们的结果表明，与嗅球内OSN初始定位有关的不是选择特定的OR，而是选择特定的OR。这些选择与嗅觉上皮中OSN的位置无关。OR选择或轴突靶向均与OSN在嗅觉凹坑内的位置无关。结论我们的结果表明，与嗅球内OSN初始定位有关的不是选择特定的OR，而是选择特定的OR。这些选择与嗅觉上皮中OSN的位置无关。OR选择或轴突靶向均与OSN在嗅觉凹坑内的位置无关。结论我们的结果表明，与嗅球内OSN初始定位有关的不是选择特定的OR，而是选择特定的OR。这些选择与嗅觉上皮中OSN的位置无关。