Ativie Frank, Komorowska Joanna A, Beins Eva, Albayram Önder, Zimmer Till, Zimmer Andreas, Tejera Dario, Heneka Michael, Bilkei-Gorzo Andras
Institute of Molecular Psychiatry, Medical Faculty, University of Bonn, Bonn, Germany.
Department of Neurodegenerative Diseases & Gerontopsychiatry, Medical Faculty, University of Bonn, Bonn, Germany.
Front Mol Neurosci. 2018 Aug 28;11:295. doi: 10.3389/fnmol.2018.00295. eCollection 2018.
Microglia, the resident immune cells of the brain, play important roles in defending the brain against pathogens and supporting neuronal circuit plasticity. Chronic or excessive pro-inflammatory responses of microglia damage neurons, therefore their activity is tightly regulated. Pharmacological and genetic studies revealed that cannabinoid type 1 (CB1) receptor activity influences microglial activity, although microglial CB1 receptor expression is very low and activity-dependent. The CB1 receptor is mainly expressed on neurons in the central nervous system (CNS)-with an especially high level on GABAergic interneurons. Here, we determined whether CB1 signaling on this neuronal cell type plays a role in regulating microglial activity. We compared microglia density, morphology and cytokine expression in wild-type (WT) and GABAergic neuron-specific CB1 knockout mice (GABA/CB1) under control conditions (saline-treatment) and after 3 h, 24 h or repeated lipopolysaccharide (LPS)-treatment. Our results revealed that hippocampal microglia from saline-treated GABA/CB1 mice resembled those of LPS-treated WT mice: enhanced density and larger cell bodies, while the size and complexity of their processes was reduced. No further reduction in the size or complexity of microglia branching was detected after LPS-treatment in GABA/CB1 mice, suggesting that microglia in naïve GABA/CB1 mice were already in an activated state. This result was further supported by correlating the level of microglial tumor necrosis factor α (TNFα) with their size. Acute LPS-treatment elicited in both genotypes similar changes in the expression of pro-inflammatory cytokines (TNFα, interleukin-6 (IL-6) and interleukin 1β (IL-1β)). However, TNFα expression was still significantly elevated after repeated LPS-treatment in WT, but not in GABA/CB1 mice, indicating a faster development of tolerance to LPS. We also tested the possibility that the altered microglia activity in GABA/CB1 mice was due to an altered expression of neuron-glia interaction proteins. Indeed, the level of fractalkine (CX3CL1), a neuronal protein involved in the regulation of microglia, was reduced in hippocampal GABAergic neurons in GABA/CB1 mice, suggesting a disturbed neuronal control of microglial activity. Our result suggests that CB1 receptor agonists can modulate microglial activity indirectly, through CB1 receptors on GABAergic neurons. Altogether, we demonstrated that GABAergic neurons, despite their relatively low density in the hippocampus, have a specific role in the regulation of microglial activity and cannabinoid signaling plays an important role in this arrangement.
小胶质细胞是大脑中的常驻免疫细胞，在保护大脑免受病原体侵害以及支持神经元回路可塑性方面发挥着重要作用。小胶质细胞的慢性或过度促炎反应会损害神经元，因此其活性受到严格调控。药理学和遗传学研究表明，1型大麻素（CB1）受体活性会影响小胶质细胞的活性，尽管小胶质细胞CB1受体的表达非常低且依赖于活性。CB1受体主要表达于中枢神经系统（CNS）的神经元上，在γ-氨基丁酸（GABA）能中间神经元上的表达水平尤其高。在此，我们确定这种神经元细胞类型上的CB1信号传导是否在调节小胶质细胞活性中发挥作用。我们比较了野生型（WT）和GABA能神经元特异性CB1基因敲除小鼠（GABA/CB1）在对照条件下（生理盐水处理）以及在3小时、24小时或重复脂多糖（LPS）处理后的小胶质细胞密度、形态和细胞因子表达。我们的结果显示，生理盐水处理的GABA/CB1小鼠海马中的小胶质细胞与LPS处理的WT小鼠的小胶质细胞相似：密度增加且细胞体更大，而其突起的大小和复杂性降低。在GABA/CB1小鼠中，LPS处理后未检测到小胶质细胞分支的大小或复杂性进一步降低，这表明未接触过抗原的GABA/CB1小鼠中的小胶质细胞已经处于激活状态。通过将小胶质细胞肿瘤坏死因子α（TNFα）水平与其大小相关联，进一步支持了这一结果。急性LPS处理在两种基因型中均引起促炎细胞因子（TNFα、白细胞介素-6（IL-6）和白细胞介素1β（IL-1β））表达的类似变化。然而，在WT小鼠中，重复LPS处理后TNFα表达仍显著升高，而在GABA/CB1小鼠中则没有，这表明对LPS的耐受性发展更快。我们还测试了GABA/CB1小鼠中小胶质细胞活性改变是否是由于神经元-胶质细胞相互作用蛋白表达改变的可能性。实际上，GABA/CB1小鼠海马中的GABA能神经元中，参与调节小胶质细胞的神经元蛋白趋化因子（CX3CL1）水平降低，这表明对小胶质细胞活性的神经元控制受到干扰。我们的结果表明，CB1受体激动剂可以通过GABA能神经元上的CB1受体间接调节小胶质细胞活性。总之，我们证明，尽管GABA能神经元在海马中的密度相对较低，但它们在调节小胶质细胞活性方面具有特定作用，并且大麻素信号传导在这种调节中起着重要作用。
