Mechanisms of Rapid Reactive Oxygen Species Generation in Response to Cytosolic Ca2+ or Zn2+ Loads in Cortical Neurons

Excessive excitotoxic accumulation of Ca2+ and Zn2+ within neurons contributes to neurodegeneration in pathological conditions including ischemia. Putative early targets of these ions, both of which are linked to increased reactive oxygen species (ROS) generation, are mitochondria and the cytosolic enzyme, NADPH oxidase (NOX). The present study uses primary cortical neuronal cultures to examine respective contributions of mitochondria and NOX to ROS generation in response to Ca2+ or Zn2+ loading. Induction of rapid cytosolic accumulation of either Ca2+ (via NMDA exposure) or Zn2+ (via Zn2+/ZPyrithione exposure in 0 Ca2+) caused sharp cytosolic rises in these ions, as well as a strong and rapid increase in ROS generation. Inhibition of NOX activation significantly reduced the Ca2+-induced ROS production with little effect on the Zn2+-triggered ROS generation. Conversely, dissipation of the mitochondrial electrochemical gradient increased the cytosolic Ca2+ or Zn2+ rises caused by these exposures, consistent with inhibition of mitochondrial uptake of these ions. However, such disruption of mitochondrial function markedly suppressed the Zn2+-triggered ROS, while partially attenuating the Ca2+-triggered ROS. Furthermore, block of the mitochondrial Ca2+ uniporter (MCU), through which Zn2+ as well as Ca2+ can enter the mitochondrial matrix, substantially diminished Zn2+ triggered ROS production, suggesting that the ROS generation occurs specifically in response to Zn2+ entry into mitochondria. Finally, in the presence of the sulfhydryl-oxidizing agent 2,2'-dithiodipyridine, which impairs Zn2+ binding to cytosolic metalloproteins, far lower Zn2+ exposures were able to induce mitochondrial Zn2+ uptake and consequent ROS generation. Thus, whereas rapid acute accumulation of Zn2+ and Ca2+ each can trigger injurious ROS generation, Zn2+ entry into mitochondria via the MCU may do so with particular potency. This may be of particular relevance to conditions like ischemia in which cytosolic Zn2+ buffering is impaired due to acidosis and oxidative stress.