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

摘要 在嗅觉系统中，精确的连接创建了一个功能地形图，可以翻译气味 感受到气味特有的神经活动的有序模式。这张地图是在开发早期建立的。 嗅觉神经元随机选择单等位基因表达一种气味感受器 (或)，属于G蛋白偶联受体(GPCR)家族，来自一个大的基因库。每个OSN 单个轴突从嗅上皮(OE)投射到嗅球(OB)。OSNs第一个目标的轴突 较大的可识别神经毛，称为原肾小球。在斑马鱼中，我们的实验室发现，原肾小球和 OSN轴突靶标与其选择的OR的家族或同源分支相关。表示ORS的OSN 不同的同源分支针对不同的原肾小球。在最初的原肾小球靶向之后， 表达相同或最终的OSN的轴突融合成更小的、不同的和可复制的神经束 被称为肾小球的区域。因此，OB上每个轴突的目标位置与其选择的OR相协调。 先前的研究表明，OSN选择的OR和OR驱动的神经活动对 肾小球分离。然而，尚不清楚分支特异的原肾小球靶向和OR特异的 实现肾小球分离。之前在小鼠身上进行的研究可能合并了最初的 由于早期发育阶段的相对不可及，以肾小球分离为目标的过程。 嗅觉发育的基础研究在发育完成后利用静态成像， 往往导致发育过程被推断而不是被调查。我们认为原肾小球 定位是由OR轨迹和轴突引导轨迹的协调表达决定的，而不是OR 蛋白质的功能。我们已经通过单细胞RNA测序表明，表达ORs的OSN在 同一同源分支也表达相似的转录因子和轴突引导相关因子。此外， 表达密切相关ORs的OSNs靶向肾小球附近。因此，我假设监管机构控制着 OSNs中的OR选择也调节轴突引导受体的表达，而OR特定的神经活动 调节随后的OR特异性肾小球的分离。利用重组酶介导盒 交换方法，我提出的实验将消除OR身份和 或驱动神经活动，以原肾小球和肾小球为靶点。

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.