Extracellular zinc, which is released from hippocampal neurons in response to brain ischaemia, triggers morphological changes in microglia. Under ischaemic conditions, microglia exhibit two opposite activation states (M1 and M2 activation), which may be further regulated by the microenvironment. We examined the role of extracellular zinc on M1 activation of microglia. Pre-treatment of microglia with 30–60 μM ZnCl2 resulted in dose-dependent increases in interleukin-1 beta (IL-1β), interleukin-6 (IL-6), and tumour necrosis factor-alpha (TNFα) secretion when M1 activation was induced by lipopolysaccharide administration. In contrast, the cell-permeable zinc chelator TPEN, the radical scavenger Trolox, and the P2X7 receptor antagonist A438079 suppressed the effects of zinc pre-treatment on microglia. Furthermore, endogenous zinc release was induced by cerebral ischaemia–reperfusion, resulting in increased expression of IL-1β, IL-6, TNFα, and the microglial M1 surface marker CD16/32, without hippocampal neuronal cell loss, in addition to impairments in object recognition memory. However, these effects were suppressed by the zinc chelator CaEDTA. These findings suggest that extracellular zinc may prime microglia to enhance production of pro-inflammatory cytokines via P2X7 receptor activation followed by reactive oxygen species generation in response to stimuli that trigger M1 activation, and that these inflammatory processes may result in deficits in object recognition memory.
Stroke is a major cause of death worldwide, leading to serious disability. Post-ischemic injury, especially in the cerebral ischemia-prone hippocampus, is a serious problem, as it contributes to vascular dementia. Many studies have shown that in the hippocampus, ischemia/reperfusion induces neuronal death through oxidative stress and neuronal zinc (Zn2+) dyshomeostasis. Glutathione (GSH) plays an important role in protecting neurons against oxidative stress as a major intracellular antioxidant. In addition, the thiol group of GSH can function as a principal Zn2+ chelator for the maintenance of Zn2+ homeostasis in neurons. These lines of evidence suggest that neuronal GSH levels could be a key factor in post-stroke neuronal survival. In neurons, excitatory amino acid carrier 1 (EAAC1) is involved in the influx of cysteine, and intracellular cysteine is the rate-limiting substrate for the synthesis of GSH. Recently, several studies have indicated that cysteine uptake through EAAC1 suppresses ischemia-induced neuronal death via the promotion of hippocampal GSH synthesis in ischemic animal models. In this article, we aimed to review and describe the role of GSH in hippocampal neuroprotection after ischemia/reperfusion, focusing on EAAC1.
Microglia, the resident immune cells of the central nervous system, can display a pro-inflammatory M1 phenotype or an anti-inflammatory M2 phenotype. Arginase (Arg)-1 expressed in interleukin-4 (IL-4)-induced M2 microglia reduces nitric oxide (NO) production by competing with inducible NO synthase for l-arginine, which contributes to the attenuation of brain inflammation. Although previous studies have indicated that brain zinc promotes M1 activation, the effect of zinc on M2 microglial activation remains to be determined. In the present study, murine primary microglia treated with 10 ng mL IL-4 exhibited increased Arg-1 mRNA expression and levels of intracellular free zinc. Chelation of this increased intracellular free zinc by the cell permeable zinc chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)ethylenediamine (TPEN) aggravated the IL-4-induced mRNA expression and enzymatic activity of Arg-1. However, the cell impermeable zinc chelator CaEDTA had no effect on Arg-1 expression or cytosolic levels of free zinc in IL-4-induced M2-polarized microglia. Furthermore, treatment with IL-4 resulted in upregulation of phagocytic activity in microglia, while administration of TPEN abolished IL-4-induced phagocytic activity. Moreover, this effect was reversed vial-arginine supplementation. These findings suggest that IL-4 induces an increase in intracellular free zinc in microglia, which may act as a negative regulator of IL-4-induced Arg-1 expression, and that such negative regulation is essential for microglial phagocytic activity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.