Heterodimer formation of wild-type and amyotrophic lateral sclerosis-causing mutant Cu/Zn-superoxide dismutase induces toxicity independent of protein aggregation

Abstract

Recent studies provide evidence that wild-type Cu/Zn-superoxide dismutase (SOD1(hWT)) might be an important factor in mutant SOD1-mediated amyotrophic lateral sclerosis (ALS). In order to investigate its functional role in the pathogenesis of ALS, we designed fusion proteins of two SOD1 monomers linked by a polypeptide. We demonstrated that wild-type-like mutants, but not SOD1(G85R) homodimers, as well as mutant heterodimers including SOD1(G85R)-SOD1(hWT) display dismutase activity. Mutant homodimers showed an increased aggregation compared with the corresponding heterodimers in cell cultures and transgenic Caenorhabditis elegans, although SOD1(G85R) heterodimers are more toxic in functional assays. Our data show that (i) toxicity of mutant SOD1 is not correlated to its aggregation potential; (ii) dismutase-inactive mutants form dismutase-active heterodimers with SOD1(hWT); (iii) SOD1(hWT) can be converted to contribute to disease by forming active heterodimers. Therefore, we conclude that toxicity of mutant SOD1 is at least partially mediated through heterodimer formation with SOD1(hWT) in vivo and does not correlate with the aggregation potential of individual mutants.

Reactive Nitrogen Species in Motor Neuron Apoptosis

“…In addition, Zn-deficient SOD toxicity is prevented by peptides that inhibit nitration (Ye et al 2007;Sahawneh et al 2010), as is the increased toxicity after the addition of Cu,Zn SOD (Sahawneh et al 2010). Biochemical and biophysical studies indicate that the formation of a dimer between Zn-deficient SOD and Cu,Zn SOD increases the stability of the Zn-deficient SOD monomer Roberts et al 2007), in agreement with the in vivo and in vitro studies showing that wild type SOD increases the stability and solubility of the mutant protein (Fukada et al 2001;Witan et al 2008;Witan et al 2009). Moreover, in spite of Cu,Zn SOD being a very stable enzyme, the half-life for exchange between this enzyme and Zn-deficient SOD at 37°C is surprisingly fast at 13-17 min when determined using differential mobility gel electrophoresis and 14 min by FRET (Roberts et al 2007;Sahawneh et al 2010).…”

“…There is some controversy over the effects of wild type SOD on the survival of mice carrying the SOD G85R were one group reported acceleration of disease (Wang et al 2009) and other claims no effect in a different SOD G85R line (Bruijn et al 1998). The mechanism through which coexpression of Cu,Zn SOD wild type increases the toxicity of mutant SOD is controversial, with some groups arguing that aggregation plays a role Furukawa et al 2006), while others conclude that increased solubility of the enzyme contributes to the enhanced toxicity (Fig 8) (Fukada et al 2001;Witan et al 2008;Witan et al 2009;Sahawneh et al 2010). The mechanisms of mutant SOD-induced motor neuron death in the presence of nitric oxide and Zn-deficient SOD stimulation of motor neuron apoptosis in culture seem identical Raoul et al 2002;Sahawneh et al 2010).…”

Cu,Zn-Superoxide Dismutase Increases Toxicity of Mutant and Zinc-deficient Superoxide Dismutase by Enhancing Protein Stability*

“…The mechanism for how co-expression of Cu,Zn-SOD WT increases the toxicity of mutant SOD is controversial, with some groups arguing that aggregation plays a role (27,30) and with others concluding that increased solubility of the enzyme contributes to the enhanced toxicity (26,31,32).…”

Mutant Cu/Zn-Superoxide Dismutase Induced Mitochondrial Dysfunction in Amyotrophic Lateral Sclerosis

“…Importantly, unaffected A4V-SOD1 mutant mice developed the disease only when mated with human wt SOD1 overexpressing mice (Deng et al, 2006). It was also shown recently that the toxicity of mutant SOD1 dimers is not correlated with their capacity to form protein aggregates but rather with their dismutase activity (Witan et al, 2008).…”

“…This activation, at least partly, depends on protein disulphide isomerase (PDI) activity (Iñarrea et al, 2005). On the other hand, the toxicity of mutant SOD1 is not correlated with its aggregation potential but with the ability to form active dimeric molecules (Witan et al, 2008). These findings are in concert with a concept that mitochondrial dysfunction and cell damage are paradoxically induced by SOD1-mediated hydroperoxide production in the IMS (Goldsteins et al, 2008).…”