Calcium-dependent spontaneously reversible remodeling of brain mitochondria

An exposure of cultured hippocampal neurons expressing mitochondrially targeted enhanced yellow fluorescent protein to excitotoxic glutamate resulted in reversible mitochondrial remodeling that in many instances could be interpreted as swelling. Remodeling was not evident if glutamate receptors were blocked with MK801, if Ca2+ was omitted or substituted for Sr2+ in the bath solution, if neurons were treated with carbonylcyanide p-trifluoromethoxyphenylhydrazone to depolarize mitochondria, or if neurons were pretreated with cyclosporin A or N-methyl-4-isoleucine-cyclosporin (NIM811) to inhibit the mitochondrial permeability transition. In the experiments with isolated brain synaptic or nonsynaptic mitochondria, Ca2+ triggered transient, spontaneously reversible cyclosporin A-sensitive swelling closely resembling remodeling of organelles in cultured neurons. The swelling was accompanied by the release of cytochrome c, Smac/DIABLO, Omi/HtrA2, and AIF but not endonuclease G. Depolarization with carbonylcyanide p-trifluoromethoxyphenylhydrazone or inhibition of the Ca2+ uniporter with Ru360 prevented rapid onset of the swelling. Sr2+ depolarized mitochondria but failed to induce swelling. Neither inhibitors of the large conductance Ca2+-activated K+ channel ( charybdotoxin, iberiotoxin, quinine, and Ba2+)nor inhibitors of the mitochondrial ATP-sensitive K+ channel (5-hydroxydecanoate and glibenclamide) suppressed swelling. Quinine, dicyclohexylcarbodiimide, and Mg2+, inhibitors of the mitochondrial K+/H+ exchanger, as well as external alkalization inhibited a recovery phase of the reversible swelling. In contrast to brain mitochondria, liver and heart mitochondria challenged with Ca2+ experienced sustained swelling without spontaneous recovery. The proposed model suggests an involvement of the Ca2+-dependent transient K+ influx into the matrix causing mitochondrial swelling followed by activation of the K+/H+ exchanger leading to spontaneous mitochondrial contraction both in situ and in vitro.