Dissecting the gut-brain axis It is generally believed that cells in the gut transduce sensory information through the paracrine action of hormones. Kaelberer et al. found that, in addition to the well-described classical paracrine transduction, enteroendocrine cells also form fast, excitatory synapses with vagal afferents (see the Perspective by Hoffman and Lumpkin). This more direct circuit for gut-brain signaling uses glutamate as a neurotransmitter. Thus, sensory cues that stimulate the gut could potentially be manipulated to influence specific brain functions and behavior, including those linked to food choices. Science, this issue p. eaat5236; see also p. 1203 A neuroepithelial circuit that connects the intestinal lumen to the brain stem in one synapse has been identified. INTRODUCTION In 1853, Sydney Whiting wrote in his classic Memoirs of a Stomach, “…and between myself and that individual Mr. Brain, there was established a double set of electrical wires, by which means I could, with the greatest ease and rapidity, tell him all the occurrences of the day as they arrived, and he also could impart to me his own feelings and impressions.” Historically, it is known that the gut must communicate with the brain, but the underlying neural circuits and transmitters mediating gut-brain sensory transduction still remain unknown. In the gut, there is a single layer of epithelial cells separating the lumen from the underlying tissue. Dispersed within this layer reside electrically excitable cells termed enteroendocrine cells, which sense ingested nutrients and microbial metabolites. Like taste or olfactory receptor cells, enteroendocrine cells fire action potentials in the presence of stimuli. However, unlike other sensory epithelial cells, no synaptic link between enteroendocrine cells and a cranial nerve has been described. The cells are thought to act on nerves only indirectly through the slow endocrine action of hormones, like cholecystokinin. Despite its role in satiety, circulating concentrations of cholecystokinin peak only several minutes after food is ingested and often after the meal has ended. Such a discrepancy suggests that the brain perceives gut sensory cues through faster neuronal signaling. Using a mouse model, we sought to identify the underpinnings of this neural circuit that transduces a sense from gut to brain. RATIONALE Our understanding of brain neural circuits is being propelled forward by the emergence of molecular tools that have high topographical and temporal precision. We adapted them for use in the gut. Single-cell quantitative real-time polymerase chain reaction and single-cell Western blot enabled the assessment of synaptic proteins. A monosynaptic rabies virus revealed the neural circuit’s synapse. The neural circuit was recapitulated in vitro by using nodose neurons cocultured with either minigut organoids or purified enteroendocrine cells. This system, coupled to optogenetics and whole-cell patch-clamp recording, served to determine the speed of transduction. Whole-nerve electrophysiology, along with optical excitation and silencing, helped to uncover the neurotransmission properties of the circuit in vivo. The underlying neurotransmitter was revealed by using receptor pharmacology and a fluorescent reporter called iGluSnFR. RESULTS Single-cell analyses showed that a subset of enteroendocrine cells contains presynaptic adhesion proteins, including some necessary for synaptic adhesion. Monosynaptic rabies tracing revealed that enteroendocrine cells synapse with vagal nodose neurons. This neuroepithelial circuit connects the intestinal lumen with the brainstem in one synapse. In coculture, this connection was sufficient to transduce a sugar stimulus from enteroendocrine cells to vagal neurons. Optogenetic activation of enteroendocrine cells elicited excitatory postsynaptic potentials in connected nodose neurons within milliseconds. In vivo recordings showed that enteroendocrine cells are indeed necessary and sufficient to transduce a sugar stimulus to the vagus. By using iGluSnFR, we found that enteroendocrine cells synthesize the neurotransmitter glutamate, and pharmacological inactivation of cholecystokinin and glutamate receptors revealed that these cells use glutamate as a neurotransmitter to transduce fast, sensory signals to vagal neurons. CONCLUSION We identified a type of gut sensory epithelial cell that synapses with vagal neurons. This cell has been referred to as the gut endocrine cell, but its ability to form a neuroepithelial circuit calls for a new name. We term this gut epithelial cell that forms synapses the neuropod cell. By synapsing with the vagus nerve, neuropod cells connect the gut lumen to the brainstem. Neuropod cells transduce sensory stimuli from sugars in milliseconds by using glutamate as a neurotransmitter. The neural circuit they form gives the gut the rapidity to tell the brain of all the occurrences of the day, so that he, too, can make sense of what we eat. The neuropod cells. (Top left) Neuropod cells synapse with sensory neurons in the small intestine, as shown in a confocal microscopy image. Blue indicates all cells in villus; green indicates green fluorescent protein (GFP) in neuropod cell and sensory neurons. (Bottom left) This neural circuit is recapitulated in a coculture system between organoids and vagal neurons. Green indicates GFP in vagal neuron; red indicates tdTomato red fluorescence in neuropod cell. (Right) Neuropod cells transduce fast sensory signals from gut to brain. Scale bars, 10 µm. The brain is thought to sense gut stimuli only via the passive release of hormones. This is because no connection has been described between the vagus and the putative gut epithelial sensor cell—the enteroendocrine cell. However, these electrically excitable cells contain several features of epithelial transducers. Using a mouse model, we found that enteroendocrine cells synapse with vagal neurons to transduce gut luminal signals in milliseconds by using glutamate as a neurotransmitter. These synaptically connected enteroendocrine cells are referred to henceforth as neuropod cells. The neuroepithelial circuit they form connects the intestinal lumen to the brainstem in one synapse, opening a physical conduit for the brain to sense gut stimuli with the temporal precision and topographical resolution of a synapse.

中文翻译：

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用于营养感觉转导的肠脑神经回路

剖析肠脑轴 人们普遍认为，肠道细胞通过激素的旁分泌作用来传递感觉信息。Kaelberer 等人。发现，除了充分描述的经典旁分泌转导外，肠内分泌细胞还与迷走神经传入形成快速、兴奋的突触（参见 Hoffman 和 Lumpkin 的观点）。这种更直接的肠脑信号通路使用谷氨酸作为神经递质。因此，刺激肠道的感官线索可能会被操纵以影响特定的大脑功能和行为，包括与食物选择相关的那些功能和行为。科学，这个问题 p。eaat5236; 另见第。1203 一个神经上皮回路将肠腔连接到一个突触中的脑干。简介 1853 年，Sydney Whiting 在他的经典《胃回忆录》中写道：“……在我和那个大脑先生之间，建立了两套电线，这样我就可以最轻松和迅速地告诉他所有的事情当天发生的事情，他也可以告诉我他自己的感受和印象。” 从历史上看，肠道必须与大脑进行通信，但介导肠-脑感觉转导的潜在神经回路和递质仍然未知。在肠道中，有一层上皮细胞将腔与下面的组织分开。分散在该层内的是称为肠内分泌细胞的可电兴奋细胞，它们感知摄入的营养物质和微生物代谢物。就像味觉或嗅觉受体细胞一样，肠内分泌细胞在存在刺激时会激发动作电位。然而，与其他感觉上皮细胞不同，肠内分泌细胞和脑神经之间没有突触联系的描述。人们认为这些细胞仅通过激素（如胆囊收缩素）的缓慢内分泌作用间接作用于神经。尽管它在饱腹感中起作用，但胆囊收缩素的循环浓度仅在摄入食物后几分钟达到峰值，并且通常在用餐结束后。这种差异表明大脑通过更快的神经元信号感知肠道感觉线索。使用小鼠模型，我们试图确定这种将感觉从肠道传递到大脑的神经回路的基础。基本原理 具有高地形和时间精度的分子工具的出现推动了我们对大脑神经回路的理解。我们调整了它们用于肠道。单细胞定量实时聚合酶链反应和单细胞蛋白质印迹使突触蛋白的评估成为可能。单突触狂犬病病毒揭示了神经回路的突触。通过使用与小肠类器官或纯化的肠内分泌细胞共培养的结节神经元，在体外重现了神经回路。该系统与光遗传学和全细胞膜片钳记录相结合，用于确定转导速度。全神经电生理学以及光激发和沉默有助于揭示体内电路的神经传递特性。通过使用受体药理学和称为 iGluSnFR 的荧光报告基因，揭示了潜在的神经递质。结果 单细胞分析表明，一部分肠内分泌细胞含有突触前粘附蛋白，包括一些突触粘附所必需的蛋白。单突触狂犬病追踪显示肠内分泌细胞与迷走神经结节神经元突触。这种神经上皮回路在一个突触中将肠腔与脑干连接起来。在共培养中，这种连接足以将糖刺激从肠内分泌细胞转导到迷走神经神经元。肠内分泌细胞的光遗传学激活在几毫秒内引发连接的结节神经元中的兴奋性突触后电位。体内记录表明，肠内分泌细胞确实是必要且足以将糖刺激转导至迷走神经的。通过使用 iGluSnFR，我们发现肠内分泌细胞合成神经递质谷氨酸盐，胆囊收缩素和谷氨酸盐受体的药理学失活表明这些细胞使用谷氨酸盐作为神经递质将快速的感觉信号转导至迷走神经元。结论我们鉴定了一种与迷走神经元突触的肠道感觉上皮细胞。这种细胞被称为肠道内分泌细胞，但它形成神经上皮回路的能力需要一个新名称。我们将这种形成突触的肠道上皮细胞称为神经足细胞。通过与迷走神经突触，神经足细胞将肠腔连接到脑干。神经足细胞通过使用谷氨酸作为神经递质，在几毫秒内从糖类中转导感官刺激。它们形成的神经回路使肠道能够迅速将一天中发生的所有事情告诉大脑，这样他也可以理解我们吃的东西。神经足细胞。（左上）神经足细胞与小肠中的感觉神经元突触，如共聚焦显微镜图像所示。蓝色表示绒毛中的所有细胞；绿色表示神经足细胞和感觉神经元中的绿色荧光蛋白 (GFP)。（左下）这个神经回路在类器官和迷走神经元之间的共培养系统中被概括。绿色表示迷走神经元中的 GFP；红色表示神经足细胞中的 tdTomato 红色荧光。（右）神经足细胞将快速的感觉信号从肠道传导到大脑。比例尺，10 µm。人们认为大脑只能通过激素的被动释放来感知肠道刺激。这是因为没有描述迷走神经和假定的肠道上皮传感器细胞——肠内分泌细胞之间的联系。然而，这些电兴奋细胞包含上皮传感器的几个特征。使用小鼠模型，我们发现肠内分泌细胞与迷走神经元突触，通过使用谷氨酸作为神经递质，在几毫秒内转导肠腔信号。这些突触连接的肠内分泌细胞在下文中被称为神经足细胞。它们形成的神经上皮回路将肠腔连接到一个突触中的脑干，为大脑打开一条物理管道，以突触的时间精度和地形分辨率感知肠道刺激。