An increasing body of evidence suggests that mitochondrial dysfunction plays an important role in the pathogenesis of familial amyotrophic lateral sclerosis associated with "gain of function" mutations in Cu/Zn superoxide dismutase 1 (SOD1). SOD1 is mostly a cytosolic protein, but a portion of SOD1 is localized in mitochondria of patients with familial amyotrophic lateral sclerosis and transgenic mouse models of the disease. Despite the finding that mutant SOD1 localizes in mitochondria, the pathogenic significance of the mitochondrial mutant SOD1 remains to be elucidated. Here, we demonstrate that both wild-type and mutant human SOD1 accumulate in brain mitochondria of transgenic mice and that SOD1 displays a very complex intramitochondrial compartmentalization. For the first time, we show that, in addition to being in the mitochondrial outer membrane and intermembrane space, SOD1 is also localized in the mitochondrial matrix. Importantly, we show that aberrant SOD1 macromolecular aggregates are formed in the matrix of brain mitochondria. This suggests that mutant SOD1 in the brain mitochondrial matrix is misfolded and prone to aggregation, which may contribute to selective neuronal degeneration.

A critical step in the development of mammalian erythroblasts into mature red blood cells is the extrusion of the nucleus, followed by intracellular degradation of the remaining organelles. It has been hypothesized that the breakdown of cellular organelles in rabbit reticulocytes is initiated by 15-lipoxygenase. In vitro, the purified rabbit reticulocyte 15-lipoxygenase binds and permeabilizes organellar membranes, thereby releasing the lumenal contents of the organelle. Here, we demonstrate that ectopic expression of 15-lipoxygenase leads to the collapse of the mitochondrial pH gradient in nonerythroid cells, using a novel reporter of mitochondrial pH, mito-pHluorin. No change in mitochondrial pH was observed with a mutant of 15-lipoxygenase that lacks enzymatic activity. These data demonstrate that 15-lipoxygenase is capable of disrupting the pH gradient maintained by mitochondria in living cells without additional factors specific for red blood cell development.