Neuronal Hyperactivity Disturbs ATP Microgradients, Impairs Microglial Motility, and Reduces Phagocytic Receptor Expression Triggering Apoptosis/Microglial Phagocytosis Uncoupling

Phagocytosis is essential to maintain tissue homeostasis in a large number of inflammatory and autoimmune diseases, but its role in the diseased brain is poorly explored. Recent findings suggest that in the adult hippocampal neurogenic niche, where the excess of newborn cells undergo apoptosis in physiological conditions, phagocytosis is efficiently executed by surveillant, ramified microglia. To test whether microglia are efficient phagocytes in the diseased brain as well, we confronted them with a series of apoptotic challenges and discovered a generalized response. When challenged with excitotoxicity in vitro (via the glutamate agonist NMDA) or inflammation in vivo (via systemic administration of bacterial lipopolysaccharides or by omega 3 fatty acid deficient diets), microglia resorted to different strategies to boost their phagocytic efficiency and compensate for the increased number of apoptotic cells, thus maintaining phagocytosis and apoptosis tightly coupled. Unexpectedly, this coupling was chronically lost in a mouse model of mesial temporal lobe epilepsy (MTLE) as well as in hippocampal tissue resected from individuals with MTLE, a major neurological disorder characterized by seizures, excitotoxicity, and inflammation. Importantly, the loss of phagocytosis/apoptosis coupling correlated with the expression of microglial proinflammatory, epileptogenic cytokines, suggesting its contribution to the pathophysiology of epilepsy. The phagocytic blockade resulted from reduced microglial surveillance and apoptotic cell recognition receptor expression and was not directly mediated by signaling through microglial glutamate receptors. Instead, it was related to the disruption of local ATP microgradients caused by the hyperactivity of the hippocampal network, at least in the acute phase of epilepsy. Finally, the uncoupling led to an accumulation of apoptotic newborn cells in the neurogenic niche that was due not to decreased survival but to delayed cell clearance after seizures. These results demonstrate that the efficiency of microglial phagocytosis critically affects the dynamics of apoptosis and urge to routinely assess the microglial phagocytic efficiency in neurodegenerative disorders.

吞噬作用（Phagocytosis）在多种炎症性与自身免疫性疾病中对维持组织稳态至关重要，但其在患病大脑中的作用却尚未得到充分探索。近期研究表明，在成年海马神经发生微环境（adult hippocampal neurogenic niche）中，生理条件下大量新生细胞会发生细胞凋亡（apoptosis），该过程可由巡逻型分枝状小胶质细胞高效完成。为验证小胶质细胞在患病大脑中是否同样具备高效吞噬能力，我们通过一系列细胞凋亡刺激模型开展实验，并观察到一种广泛性应答现象。当在体外给予兴奋性毒性（excitotoxicity）刺激（通过谷氨酸受体激动剂NMDA），或在体内诱导炎症反应（通过全身给药细菌脂多糖，或采用ω-3脂肪酸缺乏饮食）时，小胶质细胞会启动不同策略以提升吞噬效率，代偿凋亡细胞数量的增加，从而维持吞噬作用与细胞凋亡的紧密耦联。出乎意料的是，在内侧颞叶癫痫（MTLE）小鼠模型以及内侧颞叶癫痫患者切除的海马组织中，这种耦联状态发生了慢性丧失。内侧颞叶癫痫是一类以癫痫发作、兴奋性毒性与炎症为核心特征的主要神经系统疾病。值得注意的是，吞噬作用与细胞凋亡的耦联丧失与小胶质细胞促炎致痫细胞因子的表达水平显著相关，提示其在癫痫病理生理过程中发挥了潜在作用。该吞噬功能障碍源于小胶质细胞巡逻能力及凋亡细胞识别受体表达的下调，并非通过小胶质细胞谷氨酸受体信号通路直接介导。相反，其与海马网络过度活跃导致的局部ATP微梯度（ATP microgradients）紊乱密切相关，至少在癫痫急性期是如此。最终，这种耦联丧失导致神经发生微环境中凋亡新生细胞的积聚，该现象并非源于细胞存活率下降，而是源于癫痫发作后细胞清除过程的延迟。本研究结果表明，小胶质细胞吞噬作用的效率对细胞凋亡的动态平衡具有关键调控作用，呼吁在神经退行性疾病的研究中常规评估小胶质细胞的吞噬效率。