A Game of Cellular Clue: SOD1, with oxidative damage, in the--mitochondria? (Oxidative damage; Amyotrophic lateral sclerosis)
Authors: Davenport R. John 
Published: August 2003

Earlier clues linked a deviant protein to Lou Gehrig's disease, but detectives had a hard time placing the suspect molecule at the scene of the crime. It didn't seem to loiter inside the mitochondria, where vandalism likely starts, and scientists were baffled about how it might stir up trouble from afar. Now a closer look reveals that the perpetrator indeed lurks inside mitochondria, suggesting that the protein creates problems directly rather than acting through an intermediary.

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, cripples neurons responsible for movement and eventually kills its victims. Approximately 2.5% of ALS cases are caused by mutations in the gene that produces SOD1, or superoxide dismutase. Normally, this enzyme defuses superoxide, a toxic compound that scars proteins and DNA. The altered form of SOD1 doesn't simply lose an essential function; it acquires the power to actively kill neurons. Scientists have proposed that it wreaks havoc by producing rather than destroying oxidants or by clumping up.

For many years, researchers have known that cells from patients with ALS and rodents that carry mutant SOD1 contain malformed, dysfunctional mitochondria. But that observation has perplexed scientists, because they haven't thought that SOD1 ventures into mitochondria. Controversy about its location began in the 1970s, when researchers separated the cell's compartments and examined them to determine which ones carried SOD1. Although these studies suggested that a small percentage of the enzyme hung out in the mitochondria, they were inconclusive, because it was difficult to ensure that that compartments were pure. Last fall, two independent groups used improved methods and showed that normal SOD1 appears in the mitochondria of yeast and rodent liver cells. But questions lingered because contamination was still a possibility.

Higgins and colleagues isolated spinal cords from engineered mice that produced either normal human SOD1 or a mutated form that's implicated in ALS. The team stained the spinal cord samples with fluorescent antibodies that mark SOD1 and a protein known to reside in mitochondria. By viewing the tissue at different depths under a microscope, the researchers generated three-dimensional images of cells. Using these pictures, they identified where SOD1 and mitochondria overlapped. As expected, the researchers found SOD1 outside of the mitochondria--but a significant portion dwelled inside as well. In additional studies, the team stained similar samples with tiny gold particles and peered at them with an electron microscope. This finer grained view revealed that SOD1 was present at similar concentrations inside and outside the mitochondria.

Because the researchers looked directly at intact cells, the results provide the clearest evidence yet that SOD1 roosts in the mitochondria, says Arlan Richardson, a physiologist at the University of Texas Health Science Center in San Antonio. Furthermore, the team used cells that ALS strikes, he points out. The finding bolsters the notion that SOD1 is directly responsible for the mitochondrial defects associated with ALS, says biochemist Valeria Culotta of Johns Hopkins University in Baltimore, Maryland. And that idea can be tested by finding out whether barring mutant SOD1 from mitochondria prevents ALS-like symptoms in mice. The new handle on SOD1's haunts should help investigators uncover how the enzyme perpetrates this devastating neurodegenerative disease.

--R. John Davenport; suggested by Joy Goto

C. M. J. Higgins, C. Jung, H. Ding, Z. Xu, Mutant Cu, Zn superoxide dismutase that causes motoneuron degeneration is present in mitochondria in the CNS.
J. Neurosci.
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, RC215 (2002) [e-pub only].
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