A new study led by scientists from The Scripps Research Institute (TSRI) suggests a cause of amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease. The work supports a common theme whereby loss of protein stability leads to disease.
The team focused on the effects of mutations to a gene coding for a protein called superoxide dismutase (SOD), report their findings in the Proceedings of the National Academy of Sciences. The study provides evidence that those proteins linked to more severe forms of the disease are less stable structurally and more prone to form clusters or aggregates. The suggestion here is that strategies for stabilizing SOD proteins could be useful in treating or preventing SOD-linked ALS.
ALS is notorious for its ability to strike down people in the prime of life. It first leapt into public consciousness when it afflicted baseball star Lou Gehrig, who succumbed to the disease in 1941 at the age of only 38. Recently, the ALS Association’s Ice Bucket Challenge has enhanced public awareness of the disease.
ALS kills by destroying muscle-controlling neurons, ultimately including those that control breathing. At any one time, about 10,000 Americans are living with the disease, according to new data from the Centers for Disease Control and Prevention, but it is almost always lethal within several years of the onset of symptoms.
SOD1 mutations, the most studied factors in ALS, are found in about a quarter of hereditary ALS cases and seven percent of ordinary sporadic ALS cases. SOD-linked ALS has nearly 200 variants, each associated with a distinct SOD1 mutation. Scientists still don’t agree, though, on just how the dozens of different SOD1 mutations all lead to the same disease.
One feature that SOD1-linked forms of ALS do have in common is the appearance of SOD clusters or aggregates in affected motor neurons and their support cells. Aggregates of SOD with other proteins are also found in affected cells, even in ALS cases that are not linked to SOD1 mutations.
In 2003, based on their and others’ studies of mutant SOD proteins, Tainer et al proposed the framework destabilization hypothesis. In this view, ALS-linked mutant SOD1 genes all code for structurally unstable forms of the SOD protein. Inevitably some of these unstable SOD proteins lose their normal folding enough to expose sticky elements that are normally kept hidden, and they begin to aggregate with one another, faster than neuronal cleanup systems can keep up, and that accumulating SOD aggregation somehow triggers disease.
In the new study the team used advanced biophysical methods to probe how different SOD1 gene mutations in a particular genetic ALS ‘hotspot’ affect SOD protein stability.
To start, they examined how the aggregation dynamics of the best-studied mutant form of SOD, known as SOD G93A, differed from that of non-mutant, ‘wild-type’ SOD. To do this, the group developed a method for gradually inducing SOD aggregation, which was measured with an innovative structural imaging system called SAXS (small-angle X-ray scattering) at the SIBYLS beamline at Berkeley Lab’s Advanced Light Source.
The researchers could detect differences between the two proteins even before we accelerated the aggregation process. The G93A SOD aggregated more quickly than wild-type SOD, but more slowly than an SOD mutant called A4V that is associated with a more rapidly progressing form of ALS. Subsequent experiments with G93A and five other G93 mutants (in which the amino acid glycine at position 93 on the protein is replaced with a different amino acid) revealed that the mutants formed long, rod-shaped aggregates, compared to the compact folded structure of wild-type SOD. The mutant SOD proteins that more quickly formed longer aggregates were again those that corresponded to more rapidly progressing forms of ALS.
What could explain these SOD mutants’ diminished stability? Further tests focused on the role of a copper ion that is normally incorporated within the SOD structure and helps stabilize the protein. Using two other techniques, electron-spin resonance (ESR) spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS), the researchers found that the G93-mutant SODs seemed normal in their ability to take up copper ions, but had a reduced ability to retain copper under mildly stressing conditions, and this ability was lower for the SOD mutants associated with more severe ALS.
The team found that there were indications that the mutant SODs are more flexible than wild-type SOD, and the team summise that explains their relative inability to retain the copper ions.
In short, the G93-mutant SODs appear to have looser, floppier structures that are more likely to drop their copper ions, and thus are more likely to misfold and stick together in aggregates.
Along with other researchers in the field, the team suspect that deviant interactions of mutant SOD trigger inflammation and disrupt ordinary protein trafficking and disposal systems, stressing and ultimately killing affected neurons.
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, the current study provides new targets for the development of drugs to treat ALS.
The researchers also plan to confirm the relationship between structural stability and ALS severity in other SOD mutants.
If the 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.
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