Familial ALS-superoxide dismutases associate with mitochondria and shift their redox potentials

Abstract

Recent studies suggest that the toxicity of familial amyotrophic lateral sclerosis mutant Cu, Zn superoxide dismutase (SOD1) arises from its selective recruitment to mitochondria. Here we demonstrate that each of 12 different familial ALS-mutant SOD1s with widely differing biophysical properties are associated with mitochondria of motoneuronal cells to a much greater extent than wild-type SOD1, and that this effect may depend on the oxidation of Cys residues. We demonstrate further that mutant SOD1 proteins associated with the mitochondria tend to form cross-linked oligomers and that their presence causes a shift in the redox state of these organelles and results in impairment of respiratory complexes. The observation that such a diverse set of mutant SOD1 proteins behave so similarly in mitochondria of motoneuronal cells and so differently from wild-type SOD1 suggests that this behavior may explain the toxicity of ALS-mutant SOD1 proteins, which causes motor neurons to die. motor neuron ͉ neurodegeneration ͉ amyotrophic lateral sclerosis I n the familial form of ALS (fALS), which is linked to mutations in the Cu, Zn superoxide dismutase (SOD1) gene, it is generally considered that the pathological phenotype is because of the acquisition by the mutant SOD1 protein of new properties that transform it from a highly stable, dimeric antioxidant enzyme into a protein with a propensity to form toxic aggregates and cause oxidative damage to neuronal tissue (1). More than 100 different ALS-causing mutations in the SOD1 gene have been reported to date, and the properties of many of the fALS-linked mutant SOD1 proteins (mutSOD1s) have been studied (2-8) in a concerted effort to identify properties common to the mutant proteins but not shared by wild-type SOD1 (wtSOD1), which might explain their toxicity. Instead of similarities, however, biochemical and biophysical studies of the mutSOD1s have revealed a wide range of differences; in fact, some of the mutSOD1s have been found to be very similar to wtSOD1 in all of the properties that have been measured (1). For example, mutSOD1s with substitutions at or near the metalbinding region have altered metal-binding properties and a tendency to crystallize in filamentous structures, but other mutSOD1s with substitutions remote from the metal-binding region are very similar to wtSOD1 in their crystal structures (9-11). Likewise, the global stabilities of some of the metal ion-free mutSOD1 apoproteins are severely compromised, but others have apoproteins that are nearly identical to or even more stable than apo-wtSOD1 (7). Therefore, other abnormal properties of the mutant proteins must be sought to explain the toxicity of all of the mutSOD1s.We reasoned that the common properties shared by the ALS-mutSOD1 proteins and not by wtSOD1 protein might only become apparent when the mutant proteins were studied within the cellular context and also, possibly, only within motoneuronal cells. We have therefore built a collection of inducible cell lines, derived from a mouse motoneuronal line...

Metal-free superoxide dismutase forms soluble oligomers under physiological conditions: A possible general mechanism for familial ALS

“…We also must take into account the fact that SOD1 is present in both cytoplasm and mitochondria, and that its metal ions are acquired independently in the two cell compartments. These two cell compartments have quite different redox properties that further could modulate aggregation in vivo (28). This finding also is consistent with a recent suggestion of Wang et al (7): they report that non-native intermolecular disulfide bonds help to stabilize the aggregates in vivo and that the redox state of the cell may play a role in aggregation.…”

Import, Maturation, and Function of SOD1 and Its Copper Chaperone CCS in the Mitochondrial Intermembrane Space

“…The two other cysteine residues present in human SOD1, C6 and C111, also play a role in mitochondrial SOD1 localization, because their absence results in reduced mitochondrial SOD1 in mammalian cells (17,31). Although the mechanisms whereby these residues modulate SOD1 mitochondrial import or retention are not fully understood, they are likely to involve disulfide-mediated interactions either with SOD1 itself or with other proteins of the IMS.…”

“…5) (31). Expression of mutant forms of SOD1 in motor neuron-like NSC34 cells and transgenic mice results in disulfide-linked oligomeric forms of SOD1 that are trapped in mitochondria (15,17). It was proposed that SOD1 aggregation involves disulfide cross-linking at C111, and substitution of C111 to serine reduces the formation of large-molecular-weight aggregates (12).…”

Mitochondrial dysfunction in familial amyotrophic lateral sclerosis

“…A number of potential scenarios exist. Of particular interest is the observation that mutant SOD1 preferentially accumulates at the cytoplasmic face of the outer membrane and in the intermembrane space of mitochondria (Cozzolino et al 2009;Ferri et al 2006;Higgins et al 2003;Sotelo-Silveira et al 2009). Therefore, it is possible that an aberrant association between mutant SOD1 and mitochondrial proteins may lead to impaired electron transport chain functioning, shortage of ATP, increased ROS formation and Ca 2+ dysregulation.…”