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改变感觉的神经机制。 将来自外部世界的输入转化为适当和适应性的行为输出。在这里，我们建议采用 通过回答有关嗅觉组织和功能的关键问题来解决这一普遍问题 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，每个嗅觉子系统传达关于 气味空间的一个独特的子集。在两个最大的嗅觉子系统--主嗅觉和犁鼻嗅觉中 系统-感觉神经元通过G蛋白偶联受体(GPCRs)检测气味 每个神经元一个受体的特征模式；气味信息然后被组织到称为 嗅球中的肾小球，并被传递到负责气味处理和 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，表达 单程跨膜受体鸟苷环化酶D(GC-D)，而不是GPCRs。GC-D检测化学物质 信号，包括尿肽和二硫化碳，在特定形式的运动过程中充当无条件刺激 气味学习，食物偏好的社会传递(STFP)。我们最近发现了一个新的家族 4跨膜气味受体被称为跨膜4A(MS4A)，它们是共同的 在项链感觉神经元中表达，可以检测从食物气味到信息素的各种气味。这 非GPCR型气味受体在项链感觉神经元中的共表达提示项链系统 在嗅觉中扮演着与主要的辨别功能完全不同的角色 犁鼻系统似乎得到了优化。在这里，我们将利用我们拥有的新基因工具 我们已经开发了成熟的、新颖的神经跟踪和行为技术，以梳理出独特的 项链系统在气味感知和行为中的功能。我们将首先评估项链的感官 单个MS4A受体基因突变或整个MS4A基因簇缺失的小鼠的反应， 同时询问MS4A和GC-D配体是否能激活项链感官 协同、相加或某种其他模式的神经元(目标I)。然后我们将表演顺行和 逆行追踪(包括特定细胞类型的跨突触追踪)，以识别大脑区域和细胞类型， 从项链系统接收信息(AIM II)。最后，我们将利用遗传学和光遗传学来 操纵项链系统，从而确定MS4A配体、受体和投射的作用 在基于STFP的气味学习中，从项链小球到大脑。这些实验将导致重要的 发现与行为相关的神经回路的功能结构，并将建立 这是检验未来假说的重要平台，这些假说是将感觉与行动相结合的机制。

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.