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Mutants Versus Mitos

There is an excellent article over at the ALZ Forum written by the equally excellent Amber Dance. Rather than (poorly) paraphrase it, I have reproduced it here for your enjoyment. Please also follow the link to the original because the commentary following it is often as informative as the original article. I have highlighted certain portions in bold. After you have read the article I will discuss my opinions on why the bolded portions are interesting to me.
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4 November 2011. If you have wondered about the true importance of mitochondria in motor neuron disease, read on. Researchers from the Weill Medical College of Cornell University in New York corralled mutant superoxide dismutase 1 (mSOD1), a ubiquitous protein that causes amyotrophic lateral sclerosis (ALS), in these powerhouse organelles to prove that mSOD1 damages the mitochondria, and in turn, the cell and the body as a whole. In the November 2 Journal of Neuroscience, the researchers report that limiting the dismutase to the mitochondrial intermembrane space is sufficient to recapitulate much, but not all, of the ALS pathology caused by the SOD1 mutation, which causes a rare form of inherited ALS.

There is plenty of evidence that mitochondria play a part in the motor neuron degeneration that happens in ALS (reviewed in Hervias et al., 2006), but so do defects in many other areas such as RNA processing (see ARF related news story on Kwiatkowski et al., 2009 and Vance et al., 2009), the endoplasmic reticulum (see ARF related news story on Saxena et al., 2009) and Golgi (Urushitani et al., 2008), neighboring glia (reviewed in Ilieva et al., 2009), glutamate uptake (Rothstein et al., 1995), and axonal transport (see ARF related news story). The study authors, led by joint first authors Anissa Igoudjil and Jordi Magrané and senior author Giovanni Manfredi, sought to delineate the cause and outcomes of only the mitochondrial dysfunction. Researchers wondered whether malformed, underactive mitochondria cause some or much of the widespread pathology in ALS—or whether they are simply a symptom of a struggling cell. The current work confirms, as Manfredi’s team and others had also seen in cultured cells (Magrané et al., 2009; Cozzolino et al., 2009), that mSOD1 directly influences mitochondria for the worse, even if all else in the cell is normal.

The researchers designed a mouse model that makes the G93A mutant human SOD1 (an alanine for glycine at position 93), under control of the prion promoter. They linked the dismutase to the amino terminus of mitofilin, which targeted it to the inner mitochondrial membrane, facing the intermembrane space. The mitochondria of these mito-mSOD1 mice contained about the same amount of mSOD1 seen in mitochondria in the standard SOD1-G93A model with unrestricted mSOD1 targeting.

Mice with pan-cellular SOD1-G93A succumb to disease within a year. Male mice with mitochondrial mSOD1, in contrast, lived a nearly normal lifespan of 18 months, Manfredi said. Females suffered more severe disease, and their illness required they be sacrificed for ethical reasons by one year. That may be because the transgene landed in an estrogen-sensitive locus, not due to the mitochondrial mSOD1, Manfredi said. Males and females aged prematurely, hunching over and losing weight before their time. Compared to non-transgenic mice of either gender, females struggled with the motor coordination rotarod test when first examined, at three months of age, while males had near-normal coordination until six months. On a hang test for muscle strength, females performed worse than non-transgenics starting at three months, while males’ muscle weakness was not statistically significant.

When the researchers examined the mitochondria from the mito-mSOD1 mice under the electron microscope, they observed large, empty spaces, or vacuoles, in the normally densely packed organelles. In biochemistry experiments, mitochondria isolated from the brains of mito-mSOD1 mice were more sensitive to an uncoupling agent, failed to retain calcium ions, and had reduced activity of the respiratory enzyme cytochrome oxidase, as compared to mitochondria from non-transgenic mice. These data indicate that while the mitochondria with mutant SOD1 were functional, they were weakened and easily ran out of energy when stressed. Manfredi compared them to a four-cylinder engine firing on only three.

The effects of the mitochondrial mSOD1 extended beyond that organelle.
Compared to normal mice, fewer motor neurons populated the spinal cord of mito-mSOD1 mice, and they suffered a thinning of the motor cortex. The motor neuron loss was not as bad as in standard SOD1-G93A mice, the authors noted. In addition, one hallmark of ALS was decidedly missing: “What was surprising to us is, despite the fact that the mice lost a proportion of the spinal cord neurons and there was atrophy of skeletal muscle, we did not see denervation,” Manfredi said. He noted that just because neuromuscular junctions were intact, it does not mean they were healthy—the sickened neurons could still fail to send proper signals through the junction without completely detaching from it, explaining the poor rotarod performance.

The researchers concluded that mitochondrial mSOD1 is only responsible for part of ALS pathology. “It is now becoming clear that a combination of toxic effects are probably necessary to drive motor neuron disease onset and progression,” wrote Adrian Israelson of the University of San Diego, who was not involved in the study, in an e-mail to ARF.

Precisely how mSOD1 disables mitochondria is unknown. It might interact with proteins or other factors in the respiratory chain, Manfredi suggested, or it might promote the formation of free radicals. The new mito-mSOD1 mouse can help answer that question, commented Piera Pasinelli of Thomas Jefferson University in Philadelphia, Pennsylvania. “This is a powerful tool to really dissect the specific contribution for the mutant SOD1 in these organelles,” said Pasinelli, who also was not involved in the current paper.

One pathological event that is relevant to mSOD1 in mitochondria is production of reactive oxygen species, which generate other toxins, such as peroxynitrite, which can drive apoptosis, or programmed cell death. SOD chemically modifies other proteins in the presence of peroxynitrite. In the October 27 Journal of Biological Chemistry online, researchers from the University of Melbourne, Australia, propose a potential treatment for this mSOD1 effect. They discovered that a copper compound that scavenges peroxynitrite extended lifespan, and reduced inflammation, in a mouse model for ALS that expresses lower levels of SOD1-G93A than Manfredi’s mice. “They do not look at mitochondria directly, but it is possible that there is an effect of the drug at the mitochondrial level,” Manfredi wrote in an e-mail to ARF. In addition, Manfredi noted, this study reports for the first time that their low-expressing G93A mice also exhibit TAR DNA Binding Protein 43 (TDP-43) pathology, which up until now had not been seen in mSOD1 mice. TDP-43 accumulated as fragmented, abnormally phosphorylated protein in the spinal cord of these animals. The study was led by first author Cynthia Soon and senior authors Kevin Barnham and Qiao-Xin Li.—Amber Dance.

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…mitochondrial mSOD1 is only part of ALS pathology…
Previously I had discussed mutant or misfolded SOD1 being found in not just PALS with the genetic mutation but also those with the more common sporadic type. In another new study, it was shown that mSOD1 altered the shape and function of mitochondria prior to symptom onset. This is a very nice clue to a possible pathogenesis of ALS. Damaged mitochondria produce massive amounts of ROS and produce a fraction of the energy the cell requires of each mitochondria. Normally damaged mitochondria are destroyed and recycled by the cell but in ALS the recycling mechanism also appears damaged. Before going too far down this path I should remind readers that astrocytes carrying mSOD1 are sufficient to cause disease in healthy motor neurons. So the answer is never as simple as we would wish.

One pathological event that is relevant to mSOD1 in mitochondria is production of reactive oxygen species, which can drive apoptosis, or programmed cell death.
As discussed above, ROS causes damage to cellular machinery if it overwhelms the ability of the cell to fight it. The lack of energy output is a “bonus” in terms of cellular stress. Once this combination hits the Endoplasmic reticulum the cell is in serious trouble.

In addition, Manfredi noted, this study reports for the first time that their low-expressing G93A mice also exhibit TDP-43 pathology, accumulated as fragmented, abnormally phosphorylated protein in the spinal cord of these animals.
As I discussed in my previous post linked above, mSOD1 has been reported to interact with TDP43. The nature and implications of this interaction is very far from clear. A recent study showed that TDP43 binds to an inflammatory protein called NF-kb p65 in PALS but not controls (people without motor neuron disease) In the microglia, this causes upregulation of inflammatory factors in the CNS. Neuroinflammation is a major factor in progression of ALS.

This is a lot to digest, but are only a few of the clues to the etiology and pathology of ALS. They are like moths circling around a lightbulb that we cannot yet see. However, the more moths we see and the more precisely we can chart their paths, the more we can learn about the bulb. We now have many more moths, with much better understanding of their flight, than even a few years ago. The picture of the bulb is getting much clearer and brighter. One day soon we shall see the light.

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