Disease Progression in ALS Linked To Increased Protein Instability

A new study entitled “Aggregation propensities of Superoxide Dismutase G93 hotspot mutants mirror Amyotrophic Lateral Sclerosis clinical phenotypes” identified a mechanism that leads to the aggregation of SOD protein mutant forms that are typically found in Amyotrophic lateral sclerosis (ALS) motor neurons. The study opens new therapeutic avenues for ALS. The work was published in the journal Proceedings of the National Academy of Sciences.

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease, is a neurodegenerative disorder characterized by muscle wasting. The disorder is caused by progressive degeneration and death of motor neurons, responsible for controlling voluntary muscles. Therefore, ALS patients present difficulty in swallowing, speaking and breathing, culminating with patients’ death. In the United States, over 12,000 individuals are diagnosed with ALS, according to data from the National ALS Registry.

The majority of patients (90-95%) are affected with ALS without any genetic association. However, 5 to 10% of all ALS reports are inherited. Mutation in the Superoxide dismutase 1 (Sod1) gene ate detected in about 25% of hereditary ALS cases and 7% of ordinary (non-inherited) ALS. SOD1 binds to copper and zinc ions and is responsible for destroying superoxide free radicals that damage cells.

A particular feature of SOD1-linked forms of ALS is the deposition of aggregates of SOD mutated proteins in both mouse motor neurons and glial cells, observed before the onset of symptoms. Notably, these aggregates are observed even in ALS cases that do not harbor SOD1 mutations.

In this study, the authors proposed to understand how SOD1 mutations cause ALS disease. Previously, work by the same authors proposed a “framework destabilization” hypothesis where mutations in Sod1 gene results in the formation of misfolded SOD1 proteins leading to their miss-assembly and aggregation. In this new study, the team of researchers at The Scripps Research Institute (TSRI), Lawrence Berkeley National Laboratory (Berkeley Lab) and colleagues used advanced biophysical techniques to further understand how SOD mutations impacted SOD protein architecture leading to the disease.

They performed a comparative analysis between wild-type and mutant SOD proteins and measured how the aggregation dynamics changed between the wild-type and a mutant form of SOD, called SOD G93A (one of the most studied mutant forms of the protein). This was achieved by using a technique, developed in the Berkeley Lab’s SIBYLS beamline and termed SAXS (small-angle X-ray scattering) — here, the authors induce SOD aggregation in a graduate manner and they observed that the mutant SOD G93A protein aggregates faster, when compared to the wild-type SOD. When they compared the aggregation profile of mutant SOD G93A with a mutant associated with rapid ALD progression — mutant SOD A4V – the authors observed G93A aggregation was slower. The authors characterized other G93 mutants and always observed that the mutants aggregated in long and rod-shaped forms, when compared to the wild-type structure of SOD protein.

The authors further inquired what caused SOD mutants’ impaired stability, and they focused on the role of copper ions, since they participate in the structure stabilization of the wild-type SOD protein. They observed, with other two techniques — electron-spin resonance (ESR) spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS) — that while G93-mutant SODs were capable of uptaking copper ions as the wild-type, the mutant proteins were less efficient in retaining the same ions, upon stress. Once again, the mutant forms of severe ALS forms, this feature was even more reduced.

The results show, by combing both biophysical and structural analysis, that mutant SOD proteins destabilization is linked to inefficient copper retention rendering these proteins to aggregate more easily, ultimately leading to ALS.

Professor Elizabeth Getzoff, at TSRI and one of the study senior authors noted, “Because mutant SODs get bent out of shape more easily, they don’t hold and release their protein partners properly. By defining these defective partnerships, we can provide new targets for the development of drugs to treat ALS.”

David S. Shin, a research scientist in John A. Tainer laboratory, professor of structural biology at TSRI and study co-senior added, “If our hypothesis is correct, future therapies to treat SOD-linked ALS need not be tailored to each individual mutation–they should be applicable to all of them.”