Zn²⁺ Induces Permeability Transition Pore Opening and Release of Pro-apoptotic Peptides from Neuronal Mitochondria

Rapid entry of Ca²⁺ or Zn²⁺ kills neurons. Mitochondria are major sites of Ca²⁺-dependent toxicity. This study examines Zn²⁺-initiated mitochondrial cell death signaling. 10 nM Zn²⁺ induced acute swelling of isolated mitochondria, which was much greater than that induced by higher Ca²⁺ levels. Zn²⁺ entry into mitochondria was dependent upon the Ca²⁺ uniporter, and the consequent swelling resulted from opening of the mitochondrial permeability transition pore. Confocal imaging of intact neurons revealed entry of Zn ²⁺ (with Ca²⁺) to cause pronounced mitochondrial swelling, which was far greater than that induced by Ca²⁺ entry alone. Further experiments compared the abilities of Zn²⁺ and Ca ²⁺ to induce mitochondrial release of cytochrome c (Cyt-c) or apoptosis-inducing factor. In isolated mitochondria, 10 nM Zn²⁺ exposures induced Cyt-c release. Induction of Zn²⁺ entry into cortical neurons resulted in distinct increases in cytosolic Cyt-c immunolabeling and in cytosolic and nuclear apoptosis-inducing factor labeling within 60 min. In comparison, higher absolute [Ca²⁺]_i rises were less effective in inducing release of these factors. Addition of the mitochondrial permeability transition pore inhibitors cyclosporin A and bongkrekic acid decreased Zn²⁺-dependent release of the factors and attenuated neuronal cell death as assessed by trypan blue staining 5-6 h after the exposures.