Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease affecting motor neurons of the central nervous system. Though its biology is poorly understood, one controversial theory suggests that oxidative damage is the root cause of the neuronal destruction. Support for this idea comes from studies on familial ALS which, in some cases, is due to mutation of superoxide dismutase 1 (SOD1), an enzyme that protects tissue against the oxidative effects of the oxygen free radical, superoxide. Evidence from Philip Wong's lab at Johns Hopkins University School of Medicine, Baltimore, now seems to stand that theory on its head.

Wong and coworkers took advantage of the fact that SOD1 needs the assistance of a chaperone to incorporate an essential copper atom into its active site, and they raised transgenic mice lacking the gene for this chaperone (copper chaperone for SOD1, or CCS.) Their results, which are already available from Nature Neuroscience online, were striking. Though the levels of SOD protein remained normal in CCS-negative mice, they had much lower levels of SOD1 activity than their wildtype cousins; this was true for both normal SOD and for SODs with ALS-causing mutations.

But the key observation was that the disease still progressed in CCS-negative mice, suggesting that SOD activity is not required for neurodegeneration.

So what could the role of SOD be? The authors suggest that the answer may lie in SOD-aggregates. However, convincing proponents of the oxidative theory is going to be tough. For starters, though much reduced, there is still SOD activity present in the CCS-mutant mice, so until the rate-limiting steps in the deleterious oxidative pathways are identified, it can always be argued that even a small fraction of the SOD activity is sufficient to cause the damage.—Tom Fagan

Q & A with Philip Wong-Posted 18 March 2002
Q: Could the residual SOD activity found in the CCS-negative mice be sufficient to cause the disease?
A: This is theoretically possible. However, given that the onset of disease is correlated with the amount of mutant SOD1, the significant reduction of copper incorporation into mutant SOD1 in the absence of CCS should have rescued or at least significantly ameliorated the disease if copper plays a key role. Because neither the onset nor progression of disease was altered in three different lines of mutant SOD1 mice lacking CCS, it is more reasonable to argue that CCS-dependent copper toxicity plays no role in the pathogenesis of mutant SOD1 mice.

Comment by Joan Valentine-Posted 18 March 2002
I think this is a very interesting paper that is going to influence everybody's thinking a lot, but I don't think their conclusions are absolute. It is really amazing that it seems to be more important how much of the mutant polypeptide you have, than how much the mutant polypeptide contains copper. But, they have not done the experiment where the mutant polypeptide contains no copper. One could make an argument that all you need is a little bit of copper for the protein to oxidize itself, and this would act as a nucleation site for aggregation.

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  1. The paper by Subramaniam et al. demonstrates for the first time that chaperone-mediated copper loading is not required for mutant SOD protein to produce ALS-like disease. Mutant SOD protein-mediated motor neuron disease was not affected in the absence of the SOD copper chaperone (CCS), although copper-loading was reduced by 85 percent. However, since a residual 15 percent of copper loading remains, it cannot be ruled out for certain that copper-mediated toxicity does not contribute to mutant SOD protein-mediated disease. It is formally possible that the residual CCS-independent copper loading is insufficient for normal SOD-mediated reactions, but sufficient for the aberrant oxidative chemistry implicated with the mutant SOD protein. Although overall SOD activity in spinal cord tissue was dramatically reduced, it remains to be clarified whether aberrant SOD-mediated reactions were altered.

    Some small notes on the data in the paper:

    (1) The number of animals for disease onset are very small in the survival table.

    (2) G85R is included in the survival table, although no copper loading was detected even in the presence of CCS (figure 2b).

    (3) No histology of early disease stage is shown for comparison, only late-disease stage is presented.

  2. Contrary to assumptions that mutant superoxide dismutase (mSOD) causes ALS via peroxynitrite-mediated oxidative injury, Subramaniam et al. provide compelling evidence that mSOD toxicity is not due to the copper-dependent catalytic activity of peroxynitrite-mediated tyrosine nitration. Although a great deal of basic research and drug discovery in ALS has been predicated on the former assumption, the work of several researchers in the last several years has cast doubt on it. Studies have indicated that neuronal nitric oxide synthase is not directly involved in ALS pathogenesis, and the lack of evidence for the efficacy of NOS inhibitors in vivo supports this view. In addition, Doroudchi et al. have demonstrated that inhibiting nitrotyrosine production and protein nitration has little effect on the lifespan of motor neurons carrying the mSOD gene. Subramaniam's article provides more concrete and elegant evidence that the toxic gain of function of mSOD is not related to its pro-oxidative capacity.

    It can be argued that the small amount of altered SOD activity still present in the CCS/SOD1 mice can cause peroxynitrite formation. If the catalyst SOD is not the limiting factor in the reaction, small changes in catalyst concentration should not affect the rate of disease progression. We know that both the low- and high-copy G93A mice possess sufficient SOD activity. However, the onset of disease development is dramatically altered with levels of SOD protein. This suggests that a simple enzymatic process may not be involved in ALS pathogenesis.

    This and other studies merely suggest that the modus operandi of mSOD is not peroxynitrite-induced oxidative injury: the oxidative process may still be involved in ALS pathogenesis, as oxidative stress has long been acknowledged to be a key step in neuronal death in the neurodegenerative diseases.

    A number of questions pertaining to the role of SOD and mSOD in ALS remain unanswered. For example, studies indicate that prion protein (PrP) affects SOD activity, and dysregulation of PrP expression appears to alter Cu Zn-SOD activity. In addition, PrP expression is altered in mSOD mice. It remains to be seen how the two are related and how these two seemingly disparate diseases are linked.

    Alternate models have been suggested to incorporate these latter questions into the story of the toxic gain of function of SOD in ALS. "Conformational disease" models that involve non-native protein structures, such as protein aggregation and high-molecular weight protein complexes, suggest that altered protein conformation may play a role in cellular toxicity. Such models are supported by the growing evidence of a pathogenic role of protein aggregates in Huntington's disease, Alzheimer's disease, alpha1-antitrypsin deficiency, and CJD. These diseases also show the involvement of the ubiquitin-proteasome system in disease pathogenesis. Future efforts to understand the properties of the SOD protein in various conformations will enable an investigation of the possible link between structural properties, enzymatic activity, and ALS pathogenesis, and will ultimately aid in drug discovery efforts in ALS.

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Primary Papers

  1. . Mutant SOD1 causes motor neuron disease independent of copper chaperone-mediated copper loading. Nat Neurosci. 2002 Apr;5(4):301-7. PubMed.