Institute of Physiology and Pathophysiology, University of Heidelberg, D-69120 Heidelberg, Germany.
Institute of Physiology and Pathophysiology, University of Heidelberg, D-69120 Heidelberg, Germany.
Brain Behav Immun. 2020 Aug;88:802-814. doi: 10.1016/j.bbi.2020.05.052. Epub 2020 May 21.
Microglia are tissue resident macrophages (innate immunity) and universal sensors of alterations in CNS physiology. In response to pathogen or damage signals, microglia feature rapid activation and can acquire different phenotypes exerting neuroprotection or neurotoxicity. Although transcriptional aspects of microglial phenotypic transitions have been described, the underlying metabolic reprogramming is widely unknown. Employing postnatal organotypic hippocampal slice cultures, we describe that microglia transformed into a mild reactive phenotype by single TLR4 stimulation with lipopolysaccharide (LPS), which was boosted into a severe neurotoxic phenotype by IFN-γ (LPS + INF-γ). The two reactive phenotypes associated with reduction of microglial homeostatic "surveillance" markers, increase of cytokine release (IL-6, TNF-α) as well as enhancement of tissue energy demand and lactate production. These reactive phenotypes differed in the pattern of inhibition of the respiratory chain in mitochondria, however. TLR4 stimulation induced succinate dehydrogenase (complex II) inhibition by the metabolite itaconate. By contrast, TLR4 + IFN-γ receptor stimulation mainly resulted in complex IV inhibition by nitric oxide (NO) that also associated with severe oxidative stress, neuronal dysfunction and death. Notably, pharmacological depletion of microglia or treatment with itaconate resulted in effective neuroprotection reflected by well-preserved cytoarchitecture and electrical network activity, i.e., neuronal gamma oscillations (30-70 Hz) that underlie higher cognitive functions in vivo. Our findings provide in situ evidence that (i) proinflammatory microglia can substantially alter brain energy metabolism and (ii) fine-tuning of itaconate and NO metabolism determines microglial reactivity, impairment of neural network function and neurodegeneration. These data add mechanistic insights into microglial activation, with relevance to disorders featuring neuroinflammation and to drug discovery.
小胶质细胞是组织驻留的巨噬细胞（先天免疫）和中枢神经系统生理学改变的通用传感器。小胶质细胞在受到病原体或损伤信号的刺激后，会迅速激活，并获得不同的表型，发挥神经保护或神经毒性作用。虽然已经描述了小胶质细胞表型转变的转录方面，但潜在的代谢重编程还知之甚少。本研究采用产后器官型海马切片培养，描述了小胶质细胞在脂多糖（LPS）单一 TLR4 刺激下转化为轻度反应性表型，然后在 IFN-γ（LPS+INF-γ）刺激下转化为严重的神经毒性表型。这两种反应性表型与小胶质细胞稳态“监视”标志物减少、细胞因子释放（IL-6、TNF-α）增加以及组织能量需求和乳酸产生增加有关。然而，这两种反应性表型在抑制线粒体呼吸链方面存在差异。TLR4 刺激诱导代谢物衣康酸抑制琥珀酸脱氢酶（复合物 II）。相比之下，TLR4+IFN-γ 受体刺激主要导致一氧化氮（NO）抑制复合物 IV，这也与严重的氧化应激、神经元功能障碍和死亡有关。值得注意的是，小胶质细胞的药理学耗竭或衣康酸盐治疗可有效神经保护，表现为细胞结构和电网络活性的良好保存，即神经元γ振荡（30-70 Hz），在体内支持更高的认知功能。本研究结果提供了原位证据，表明（i）促炎小胶质细胞可显著改变大脑能量代谢，（ii）衣康酸和 NO 代谢的精细调节决定了小胶质细胞的反应性、神经网络功能障碍和神经退行性变。这些数据为小胶质细胞激活提供了机制上的见解，与具有神经炎症特征的疾病和药物发现有关。
