Mild Acidosis Enhances AMPA Receptor-Mediated Intracellular Zinc Mobilization in Cortical Neurons

Abstract

Overactivation of glutamate receptors and subsequent deregulation of the intraneuronal calcium ([Ca2+]i) levels are critical components of the injurious pathways initiated by cerebral ischemia. Another hallmark of stroke is parenchymal acidosis, and we have previously shown that mild acidosis can act as a switch to decrease NMDAR-dependent neuronal loss while potentiating the neuronal loss mediated by AMPARs. Potentiation of AMPAR-mediated neuronal death in an acidotic environment was originally associated only with [Ca2+]i dyshomeostasis, as assessed by Ca2+ imaging; however, intracellular dyshomeostasis of another divalent cation, Zn2+, has recently emerged as another important co-factor in ischemic neuronal injury. Rises in [Zn2+]i greatly contribute to the fluorescent changes of Ca2+-sensitive fluorescent probes, which also have great affinity for Zn2+. We therefore revisited our original findings (Mcdonald et al., 1998) and investigated if AMPAR-mediated fura-2 signals we observed could also be partially due to [Zn2+]i increases. Fura-2 loaded neuronal cultures were exposed to the AMPAR agonist, kainate, in a physiological buffer at pH 7.4 and then washed either at pH 7.4 or pH 6.2. A delayed recovery of fura-2 signals was observed at both pHs. Interestingly this impaired recovery phase was found to be sensitive to chelation of intracellular Zn2+. Experiments with the Zn2+ sensitive (and Ca2+-insensitive) fluorescent probe FluoZin-3 confirmed the idea that AMPAR activation increases [Zn2+]i, a phenomenon that is potentiated by mild acidosis. Additionally, our results show that selective Ca2+ imaging mandates the use of intracellular heavy metal chelators to avoid confounding effects of endogenous metals such as Zn2+.

Intracellular Zn2+ increases contribute to the progression of excitotoxic Ca2+ increases in apical dendrites of CA1 pyramidal neurons

“…Zn 2+ release from metallothioneins could be triggered by reactive oxygen species generated in a Ca 2+ -dependent manner by mitochondria, nitric oxide synthase or phospholipase C (Frederickson et al, 2004; Dineley et al, 2008). In addition, Ca 2+ -dependent acidification caused by NMDA receptor activation could play a significant role, since intracellular pH decreases are well-established to mobilize Zn 2+ from intracellular stores (Jiang et al, 2000; Frazzini et al, 2007). …”

The Neurophysiology and Pathology of Brain Zinc

“…Ischemic acidosis can increase Zn 2+ influx through VSCC and Ca/ARs and promote Zn 2+ release from MTs, thereby favoring an overall neurotoxic increase in [Zn 2+ ] i levels (Jiang et al, 2000; Sensi et al, 2003; Frazzini et al, 2007). As protons also block NMDARs, ischemic acidosis can therefore serve as a switch to decrease NMDAR-mediated neuronal death while potentiating injury triggered by the activation of VSCC and AMPARs.…”

“…As protons also block NMDARs, ischemic acidosis can therefore serve as a switch to decrease NMDAR-mediated neuronal death while potentiating injury triggered by the activation of VSCC and AMPARs. Data from cultured neurons indicate that, in fact, AMPAR activation promotes ROS-mediated [Zn 2+ ] i rises that are enhanced by mild acidosis (Frazzini et al, 2007). Interestingly, Zn 2+ can itself disrupt the neuronal acid–base equilibrium by blocking the Na + /H + exchanger, thereby creating a feedforward loop as the cation promotes intracellular acidification and also delays recovery from intracellular acidification (Dineley et al, 2002).…”

Acidosis-Induced Zinc-Dependent Death of Cultured Cerebellar Granule Neurons

“…i increase in CGNs after decreased extracellular pH. Importantly, mild acidosis enhanced AMPA receptor-mediated intracellular zinc mobilization in neurons (Frazzini et al 2007). Additionally, elevation of intracellular zinc concentration severely altered mitochondrial function (Brown et al 2000;Link and von Jagow 1995;Lorusso et al 1991), including inhibition of complex I (Sharpley and Hirst 2006).…”