Mutation and oligomerization of SOD1 can be the reason for Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurological disease that causes the death of motor neurons with consequent muscular paralysis. Even though it is predominantly a rare disease, 10 percent of the ALS cases are determined as familial (fALS). The relation between fALS with mutations in the SOD1 gene was first reported in 1993.

Over one hundred fALS-linked mutations, distributed during the SOD1 gene, are observed with approximately 20 percents of fALS cases. The pathogenicity of SOD1 mutants was demonstrated to be due to the gain of a toxic function and not to the loss of the normal. Thus, SOD1 knockout mice do not show any ALS symptoms, whereas transgenic mice express the fALS associated mutant G93A human SOD1 and develop the symptoms. The analyses of the properties of those isolated ALS-mutant SOD1 proteins have not revealed the type of these toxic function. Some mutations affect protein stability, metal ion affinity and SOD1 activity in different ways. Therefore, the molecular mechanisms by which the mutations cause fALS are now not known.

How SOD1 mutations are related to neurodegenerative disorders?

Protein aggregates and inclusions are a common pathological feature of many neurological diseases, for example: Huntington’s, Alzheimer’s and Parkinson’s diseases. In these neurodegenerative disorders, misfolding, aggregation, and precipitation of proteins appear to be directly associated with neurotoxicity. The detection of proteinaceous aggregates that contain SOD1 in motor neurons of post-mortem fALS patients and transgenic mice has been a major advantage because it has been indicated that aggregation of SOD1 is associated with the pathology of SOD1-linked fALS. Based on recent studies, it seems improbable that the visible SOD1-containing inclusions themselves are more toxic; rather their presence implies that smaller, soluble high molecular weight oligomeric precursor species containing SOD1 are being generated in vivo.

Does the metal free SOD1 form cause the Amyotrophic lateral sclerosis?

Eukaryotic copper, zinc superoxide dismutase (SOD1) is a 32-kDa homodimeric metalloenzyme, found mainly in the cytosol, but also in the mitochondrial intermembrane space, nucleus, and peroxisomes. Each of the two subunits of all SOD1 creates an eight-stranded Greek primary β-barrel and has an active site that binds a catalytic copper ion (binding residues: His46, His48, His63 along with His120) along with a structural zinc ion (binding residues: His63, His71, His80 and Asp83). The functional role is the catalysis of the dismutation of superoxide radical to dioxygen and hydrogen peroxide.  The mature, correctly folded and enzymatically active form of SOD1 is obtained in vivo through several post-translational modifications: acquisition of zinc and copper ions, disulfide bridge formation, and dimerization. It is curious how mutations can affect the phases of SOD1 maturation. Scientists reported that wild type human SOD1, when do not contain any of its metal ions, forms large, stable, soluble, amyloid-like protein oligomers in solutions exposed to air, under physiological conditions (37°C, pH 7, and 100 µM protein concentration). Oligomerization was shown to occur through a combination of oxidation of Cys 6 and Cys 111 and formation of amyloid-like bonds between beta strands, which was demonstrated through the ability of the oligomers to bind the amyloid-binding dye thioflavin T (ThT). ThT is a benzothiazole dye that exhibits increases in fluorescence intensity upon binding to amyloid fibers. A team of scientists selected a few fALS-related mutants, characterized in the apo and zinc-reconstituted states regarding their ability to form soluble large molecular weight oligomers. The results showed that the demetallation is the key factor for fALS-mutant SOD1 oligomerization. The formation of ThT-binding non-covalent interactions and intersubunit disulfide bonds involving the free Cys residues, Cys6 and Cys111 are specific for the formed soluble oligomeric species. Based on these results, it is suggested that metal-free SOD1 itself is a cause of ALS and that a number of mutants associated with fALS may be more susceptible to oligomerization in vivo. All of the fALS mutant SOD1 proteins tested, just like WT SOD1, form high molecular weight oligomers, and that some form oligomers at remarkably fast rates in conditions very close to the physiological ones.

The finding that WT and all the mutants, independently of the nature and location of the mutation, undergo the same type of oligomerization suggest a general picture of SOD1 aggregation that could operate when either wild type or mutant SOD1 proteins are in the metal-free form. Even though, scientists cannot exclude other mechanisms in SOD1-linked familial ALS, but the resent study shows how a large and diverse set of SOD1 mutant proteins all could lead to disease through the same mechanism.