Modulation of arousal by direct CO2 gating of connexin hemichannels expressed by VTA GABAergic neurons

通过直接 CO2 门控 VTA GABA 能神经元表达的连接蛋白半通道来调节唤醒

• 批准号: BB/V015117/1
• 负责人: Mark Wall
• 金额: $ 83.86万
• 依托单位: University of Warwick
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=BB%2FV015117%2F1
• 关键词: Modulation; arousal; direct; CO2; gating

The regulated removal of the metabolism waste product carbon dioxide (CO2) via breathing is vital for life. If too much CO2 builds up in the blood it becomes acidic and this can lead to death. CO2 levels in blood will be reduced during hyperventilation and endurance sports and can increase with respiratory diseases such as asthma and with smoking. We have recently generated genetic evidence that raised CO2 can be directly sensed in the brain by opening a membrane channel (made up of proteins called connexins), and that this plays an important role in the regulation of breathing. Connexins (Cx) combine to form large-pored channels in the cell membrane to mediate important aspects of cell to cell communication. They are capable of passing ions and small molecules such as adenosine triphosphate (ATP). Connexins can operate in two modes: i) channels in two adjacent cells can dock together to form a passageway between the cells - called a gap junction; or ii) they can simply open into the space outside the cell - called a hemichannel. There are many different connexin (Cx) proteins, but a subset including Cx26 form hemichannels than can be opened by increases in CO2. CO2 binds directly to Cx26 to cause the hemichannels to open. In the brainstem, specific glial cells express Cx26 hemichannels and their opening by high CO2 causes an adaptive change in breathing to reduce the level of CO2 to normal. We have recently discovered that Cx26 hemichannels are also expressed by a subset of neurons in a part of the brain called the ventral tegmental area (VTA). The VTA is well documented to be involved in emotion, motivation and reward and is also an important part of the neural circuitry controlling sleep and wakefulness. Activation of some VTA neurons promotes sleep, with their inhibition promoting arousal. The expression of Cx26 hemichannels makes VTA neurons directly sensitive to CO2 with more CO2 reducing their excitability (as the hemichannels open). Since the neurons that express Cx26 promote sedation and sleep, increases in CO2 will switch them off and this could lead to arousal and wakefulness.We propose that Cx26 hemichannels expressed by these particular neurons in the VTA detect raised levels of CO2 and mediate a life-preserving reflex: hypercapnic arousal. For example, in sleep apnoea when breathing can stop altogether, raised levels of CO2 in the blood lead to abrupt waking from sleep followed by conscious control of breathing. There are known mechanisms for how raised CO2 can lead to arousal but these rely on a change in blood pH (acidification), which only occurs with the accumulation of significant amounts of CO2, and is thus slow to occur. As Cx26 hemichannels are directly opened by only small increases in CO2, we hypothesize that they will provide a rapid arousal response to relatively small increases in CO2. To explore our hypothesis, we will use the genetic tools that we have developed to selectively remove the CO2 sensitivity of Cx26 from the neurons in the VTA and then test how the arousal response to changing levels of CO2 is altered. We predict that this will slow and blunt the arousal response. We shall then determine how the CO2-sensitive VTA neurons connect to other neuronal populations within the VTA and other nuclei in the brain involved in arousal and control of wakefulness. Our project will study the unexpected sensitivity to CO2 of an important population of neurons in the brain that are a control hub for multiple complex behaviours. In doing so, we will shed light on how arousal may be unconsciously modified by the level of CO2 in the body.

通过呼吸调节代谢废物二氧化碳（CO2）的去除对于生命至关重要。如果血液中积聚过多的二氧化碳，就会变成酸性，这可能会导致死亡。在过度换气和耐力运动期间，血液中的二氧化碳水平会降低，并且会因哮喘等呼吸道疾病和吸烟而增加。我们最近获得的遗传证据表明，通过打开膜通道（由称为连接蛋白的蛋白质组成），大脑可以直接感知升高的二氧化碳，这在呼吸调节中发挥着重要作用。连接蛋白 (Cx) 结合在细胞膜中形成大孔通道，介导细胞间通讯的重要方面。它们能够传递离子和小分子，例如三磷酸腺苷 (ATP)。连接蛋白可以以两种模式运行：i）两个相邻细胞中的通道可以对接在一起，形成细胞之间的通道 - 称为间隙连接；或者 ii) 它们可以简单地通向细胞外的空间——称为半通道。有许多不同的连接蛋白 (Cx) 蛋白，但包括 Cx26 在内的一个子集形成半通道，可以通过增加 CO2 来打开。 CO2 直接与 Cx26 结合，导致半通道打开。在脑干中，特定的神经胶质细胞表达 Cx26 半通道，高 CO2 使它们打开，导致呼吸发生适应性变化，从而将 CO2 水平降低至正常水平。我们最近发现 Cx26 半通道也由大脑腹侧被盖区 (VTA) 部分的神经元子集表达。有充分证据表明，VTA 与情绪、动机和奖励有关，也是控制睡眠和觉醒的神经回路的重要组成部分。一些 VTA 神经元的激活可以促进睡眠，而抑制它们则可以促进觉醒。 Cx26 半通道的表达使 VTA 神经元对 CO2 直接敏感，更多的 CO2 会降低其兴奋性（当半通道打开时）。由于表达 Cx26 的神经元可促进镇静和睡眠，因此 CO2 的增加将使它们关闭，这可能导致唤醒和觉醒。我们提出，VTA 中这些特定神经元表达的 Cx26 半通道检测到 CO2 水平升高，并介导一种保护生命的反射：高碳酸血症唤醒。例如，在睡眠呼吸暂停中，当呼吸完全停止时，血液中二氧化碳浓度升高会导致患者突然从睡眠中醒来，然后有意识地控制呼吸。 CO2 升高如何导致性兴奋的机制是已知的，但这些机制依赖于血液 pH 值的变化（酸化），而这种变化仅在大量 CO2 积累时发生，因此发生缓慢。由于 Cx26 半通道仅通过 CO2 的小幅增加而直接打开，因此我们假设它们将对 CO2 相对较小的增加提供快速的唤醒反应。为了探索我们的假设，我们将使用我们开发的遗传工具选择性地去除 VTA 神经元中 Cx26 的 CO2 敏感性，然后测试对 CO2 水平变化的唤醒反应是如何改变的。我们预测这会减慢并减弱唤醒反应。然后，我们将确定 CO2 敏感的 VTA 神经元如何与 VTA 内的其他神经元群以及大脑中参与唤醒和控制觉醒的其他核团连接。我们的项目将研究大脑中重要神经元群对二氧化碳的意外敏感性，这些神经元是多种复杂行为的控制中心。在此过程中，我们将揭示体内二氧化碳水平如何无意识地改变觉醒。