Characterizing the Functional Architecture of the Necklace Olfactory System

摘要 现代神经生物学的一个中心目标是了解将感觉神经元转化为神经元的神经机制。 从外部世界的输入转化为适当的和适应性的行为输出。在这里，我们建议接近 通过回答关于嗅觉的组织和功能的关键问题， 系统，大多数动物用来询问环境的主要感觉方式。在哺乳动物中， 气味检测的挑战由多个嗅觉子系统来解决， 一个独特的气味空间子集在两个最大的嗅觉子系统--主嗅觉子系统和犁鼻嗅觉子系统中， 系统-感觉神经元检测气味通过g-蛋白偶联受体（GPCR）表达在一个 一个神经元一个受体的模式;气味信息然后被组织成称为 嗅球中的肾小球，并被中继到负责气味处理的高级大脑中心， 行为然而，并不是所有的嗅觉子系统都遵循这种模式：神秘的嗅觉“项链”是 由感觉神经元组成，这些感觉神经元支配嗅球中的一串肾小球，并表达 单次跨膜受体鸟苷酸环化酶-D（GC-D）而不是GPCR。GC-D检测化学品 线索，包括尿肽和二硫化碳，在特定形式的 气味学习，食物偏好的社会传播（STFP）。我们最近发现了一个新的家庭， 4种跨膜气味受体，称为跨膜4As（MS 4As），它们与 在项链感觉神经元中表达，并检测从食物气味到信息素的气味。这 项链感觉神经元中非GPCR气味受体的共表达表明项链系统 在嗅觉感知中发挥着与主要的辨别功能根本不同的作用 犁鼻系统似乎得到了优化。在这里，我们将利用新的遗传工具， 我们已经开发出的新颖的神经追踪和行为技术，可以将独特的 项链系统在气味感知和行为中的作用。我们首先要评估项链的感官 在单个Ms 4a受体基因突变或整个Ms 4a基因簇缺失的小鼠中的反应， 并询问当MS 4A和GC-D配体同时存在时，是否激活项链感觉 神经元协同地、相加地或以某种其他模式（Aim I）。然后我们将进行顺行， 逆行追踪（包括细胞类型特异性跨突触追踪）以识别 从项链系统（Aim II）接收信息。最后，我们将使用遗传学和光遗传学， 操纵项链系统，从而建立MS 4A配体，受体和投射的作用 从项链肾小球到大脑的STFP气味学习。这些实验将导致重要的 关于行为相关神经回路的功能结构的发现，并将建立一个 这是一个重要的平台，可以用来检验未来关于将感觉与行动结合起来的机制的假设。

Abstract A central goal of modern neurobiology is to understand the neural mechanisms that transform sensory inputs from the outside world into appropriate and adaptive behavioral outputs. Here we propose to approach this general problem by answering key questions regarding the organization and function of the olfactory system, the main sensory modality used by most animals to interrogate the environment. In mammals, the challenge of odor detection is addressed by multiple olfactory subsystems that each convey information about a unique subset of odor space. In the two largest olfactory subsystems — the main and the vomeronasal systems — sensory neurons detect odors through g-protein coupled receptors (GPCRs) expressed in a characteristic one-receptor-per-neuron pattern; odor information is then organized into channels called glomeruli in the olfactory bulb, and is relayed to higher brain centers responsible for odor processing and behavior. However, not all olfactory subsystems follow this pattern: the mysterious olfactory “necklace” is comprised of sensory neurons that innervate a string of glomeruli in the olfactory bulb, and which express the single-pass transmembrane receptor guanylate cyclase-D (GC-D) rather than GPCRs. GC-D detects chemical cues, including urinary peptides and carbon disulfide, that act as unconditioned stimuli during a specific form of odor learning, the social transmission of food preferences (STFP). We have recently identified a new family of 4 transmembrane-containing odor receptors called the Membrane Spanning 4As (MS4As), which are co- expressed in necklace sensory neurons, and which detect odors ranging from food scents to pheromones. This co-expression of non-GPCR odor receptors in necklace sensory neurons suggests that the necklace system plays a fundamentally different role in olfactory perception from the discriminative functions for which the main and vomeronasal systems appear to be optimized. Here we will take advantage of new genetic tools we have established, and novel neural tracing and behavioral techniques we have developed, to tease apart the unique function of the necklace system in odor perception and behavior. We will first assess necklace sensory responses in mice in which single Ms4a receptor genes are mutated or the entire Ms4a gene cluster is deleted, and also ask whether MS4A and GC-D ligands, when presented simultaneously, activate necklace sensory neurons synergistically, additively or in some other pattern (Aim I). We will then perform anterograde and retrograde tracing (including cell type-specific trans-synaptic tracing) to identify brain areas and cell types that receive information from the necklace system (Aim II). Finally, we will use genetics and optogenetics to manipulate the necklace system, thereby establishing the roles of MS4A ligands, receptors and projections from the necklace glomeruli to the brain in STFP-based odor learning. These experiments will lead to important discoveries about the functional architecture of a behaviorally relevant neural circuit, and will establish an important platform for testing future hypotheses about the mechanisms that couple sensation to action.