Zn-induced disruption of neuronal mitochondrial function: Synergism with Ca, critical dependence upon cytosolic Zn buffering, and contributions to neuronal injury.

Excitotoxic Zn and Ca accumulation contributes to neuronal injury after ischemia or prolonged seizures. Synaptically released Zn can enter postsynaptic neurons via routes including voltage sensitive Ca channels (VSCC), and, more rapidly, through Ca permeable AMPA channels. There are also intracellular Zn binding proteins which can either buffer neuronal Zn influx or release bound Zn into the cytosol during pathologic conditions. Studies in culture highlight mitochondria as possible targets of Zn; cytosolic Zn can enter mitochondria and induce effects including loss of mitochondrial membrane potential (ΔΨ), mitochondrial swelling, and reactive oxygen species (ROS) generation. While brief (5 min) neuronal depolarization (to activate VSCC) in the presence of 300 μM Zn causes substantial delayed neurodegeneration, it only mildly impacts acute mitochondrial function, raising questions as to contributions of Zn-induced mitochondrial dysfunction to neuronal injury. Using brief high (90 mM) K/Zn exposures to mimic neuronal depolarization and extracellular Zn accumulation as may accompany ischemia in vivo, we examined effects of disrupted cytosolic Zn buffering and/or the presence of Ca, and made several observations: 1. Mild disruption of cytosolic Zn buffering-while having little effects alone-markedly enhanced mitochondrial Zn accumulation and dysfunction (including loss of ∆Ψ, ROS generation, swelling and respiratory inhibition) caused by relatively low (10-50 μM) Zn with high K. 2. The presence of Ca during the Zn exposure decreased cytosolic and mitochondrial Zn accumulation, but markedly exacerbated the consequent dysfunction. 3. Paralleling effects on mitochondria, disruption of buffering and presence of Ca enhanced Zn-induced neurodegeneration. 4. Zn chelation after the high K/Zn exposure attenuated both ROS production and neurodegeneration, supporting the potential utility of delayed interventions. Taken together, these data lend credence to the idea that in pathologic states that impair cytosolic Zn buffering, slow uptake of Zn along with Ca into neurons via VSCC can disrupt the mitochondria and induce neurodegeneration.