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一个显著特征。一种研究得很好的呼吸道疾病 运动可塑性是指膈神经长期促进(PLTF)，即由急性刺激引起的膈神经活动的持续增加。 间歇性缺氧(AIH)。多种不同的细胞机制参与了AIH诱导的pLTF，这取决于 缺氧发作的严重性。而Q通路需要在膈肌上激活5-HT2受体 对于神经元，S通路需要腺苷2A受体的激活。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；实际上，并发通路激活实际上取消了pLTF 表情。虽然我们已经了解了大量关于AIH诱导的细胞内信号机制的知识 PLTF，我们对细胞间信号转导的作用知之甚少。最近的报道表明，神经胶质细胞调节 多个神经系统的神经可塑性，包括中枢神经系统的先天免疫细胞小胶质细胞。自.以来 小胶质细胞在调节AIH诱导的膈运动可塑性中的作用几乎一无所知，我们的 主要目标是在正常大鼠和全身炎症大鼠中探索这一知识差距。 指导我们建议的基本假设是小胶质细胞对竞争进行差异化调节。 中度和重度AIH诱发的pLTF机制(目标1)。我们提出了一个统一的模型来解释 这种差异小胶质细胞对AIH诱导的pLTF的调节。在严重缺氧的情况下，我们认为膈肌 运动神经元释放Fractalkine(一种神经元特有的趋化因子)，激活小胶质细胞Fractalkine受体 (小胶质细胞独有)，并触发严重的AIH诱导的pLTF所必需的小胶质细胞腺苷释放 (目标2)。对于中度AIH，细胞间Fractalkine和腺苷信号减弱允许表达 5-羟色胺依赖的pLTF(即Q途径)，但带有持久的腺苷限制(目标3)。我们进一步 认为在中度AIH期间，即使是轻微的全身炎症也会促进小胶质细胞腺苷的释放， 增加串扰抑制和抑制pLTF表达(目标4)。最后，由于AIH诱导的pLTF 表现出一种深刻的年龄相关的性二形性，我们将检验这样的假设，即膈运动神经元- 在雌性和雄性大鼠中，小胶质细胞的相互作用受年龄的影响不同(目标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，极大地增加了我们对细胞间信号重要性的机械理解 呼吸运动的可塑性。由于重复性AIH正在成为一种新的治疗手段来恢复 患有颈椎损伤或肌萎缩侧索硬化症等衰弱疾病的人的呼吸(和其他运动)， 更好地了解调节AIH诱导可塑性的因素将有助于优化AIH方案并改进 成功翻译这一前景看好的治疗方式的机会。加深了对年龄和 “性别效应”将为翻译成困扰男性和女性的临床疾病确立“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.