Microglial regulation of intermittent hypoxia induced phrenic motor plasticity

摘要 可塑性是控制呼吸的神经系统的标志性特征。一种被充分研究的呼吸道疾病 运动可塑性是膈神经长时程易化（pLTF），即由急性运动诱发的膈神经活动的延长增加。 间歇性缺氧（AIH）。多种不同的细胞机制有助于AIH诱导的pLTF，这取决于 缺氧发作的严重程度。而Q通路需要5-HT 2受体激活膈运动 在神经元中，S通路需要腺苷2A受体激活。这些不同的细胞内信号 通路通过强大的串扰抑制相互作用;事实上，同时激活通路实际上取消了pLTF 表情虽然我们已经了解了很多关于AIH诱导的细胞内信号传导机制， pLTF，我们对细胞间信号传导的作用知之甚少。最近的报告表明，神经胶质调节 神经可塑性在多个神经系统，包括小胶质细胞，先天免疫细胞的中枢神经系统。以来 事实上，我们对小胶质细胞在调节AIH诱导的膈运动可塑性中的作用一无所知， 主要目标是在正常大鼠和全身炎症大鼠中探索这种知识差距。 指导我们建议的基本假设是，小胶质细胞差异调节竞争， 中度与重度AIH引起的pLTF机制（目的1）。我们提出了一个统一的模型来解释 这种AIH诱导的pLTF的差异小胶质细胞调节。在严重缺氧时，我们认为膈神经 运动神经元释放Fractalkine（神经元特有的趋化因子），激活小胶质细胞Fractalkine受体 （小胶质细胞特有）和触发小胶质细胞腺苷释放所必需的严重AIH诱导的pLTF (Aim 2）。对于中度AIH，减少的细胞间Fractalkine和腺苷信号传导允许表达 依赖于降钙素的pLTF（即，Q途径），但具有持续的腺苷限制（目的3）。我们进一步 提出在中度AIH期间，即使是轻微的全身炎症也会增强小胶质细胞腺苷的释放， 增加串扰抑制和抑制pLTF表达（目的4）。最后，由于AIH诱导的pLTF 表现出深刻的年龄依赖性的两性异形，我们将测试的假设，膈运动神经元- 在雌性大鼠和雄性大鼠中，年龄对小胶质细胞相互作用的影响不同（目的5）。 该项目将首次尝试确定小胶质细胞在任何形式的呼吸运动中的特定作用 可塑性，大大增加了我们对细胞间信号传导重要性的机械理解 在呼吸运动可塑性方面。由于反复AIH正在成为一种新的治疗干预，以恢复 呼吸（和其他运动）在人与衰弱性疾病，如颈椎损伤或ALS， 更深入地了解调节AIH诱导的可塑性的因素将有助于优化AIH方案， 成功转化这种有前途的治疗方式的机会。增加对年龄的了解， 性效应将为转化为困扰男性和女性的临床疾病建立“基本规则”。

ABSTRACT Plasticity is a hallmark feature of the neural system controlling breathing. One well-studied form of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a prolonged increase in phrenic activity triggered by acute intermittent hypoxia (AIH). Multiple distinct cellular mechanisms contribute to AIH-induced pLTF, depending on the severity of hypoxic episodes. Whereas the Q pathway requires 5-HT2 receptor activation on phrenic motor neurons, the S pathway requires adenosine 2A receptor activation. These distinct intra-cellular signaling pathways interact via powerful cross-talk inhibition; indeed, concurrent pathway activation actually cancels pLTF expression. Although we have learned a great deal about intra-cellular signaling mechanisms of AIH-induced pLTF, we know little concerning the role of inter-cellular signaling. Recent reports demonstrate that glia regulate neuroplasticity in multiple neural systems, including microglia, the innate immune cells of the CNS. Since virtually nothing is known concerning the role of microglia in regulating AIH-induced phrenic motor plasticity, our primary goal is to explore this knowledge gap in normal rats and in rats with systemic inflammation. The fundamental hypothesis guiding our proposal is that microglia differentially regulate competing pLTF mechanisms elicited by moderate versus severe AIH (Aim 1). We propose a unified model to explain such differential microglial regulation of AIH-induced pLTF. During severe hypoxia, we propose that phrenic motor neurons release Fractalkine (a chemokine unique to neurons), activating microglial Fractalkine receptors (unique to microglia) and triggering the microglial adenosine release necessary for severe AIH-induced pLTF (Aim 2). With moderate AIH, diminished inter-cellular Fractalkine and adenosine signaling permit the expression of serotonin-dependent pLTF (ie. Q pathway), but with a persistent adenosine constraint (Aim 3). We further propose that even mild systemic inflammation enhances microglial adenosine release during moderate AIH, increasing cross-talk inhibition and suppressing pLTF expression (Aim 4). Finally, since AIH-induced pLTF exhibits a profound age-dependent sexual dimorphism, we will test the hypothesis that phrenic motor neuron- microglia interactions are differentially affected by age in female versus male rats (Aim 5). This project will be the first attempt to identify a specific role of microglia in any form of respiratory motor plasticity, greatly increasing our mechanistic understanding concerning the importance of inter-cellular signaling in respiratory motor plasticity. Since repetitive AIH is emerging as a novel therapeutic intervention to restore breathing (and other movements) in people with debilitating disorders such as cervical spinal injury or ALS, greater understanding of factors regulating AIH-induced plasticity will help optimize AIH protocols and improve chances for successful translation of this promising therapeutic modality. Increased understanding of age and sex effects will establish “ground rules” for translation to clinical disorders that afflict both men and women.