Energy star. When ATP drops to critical levels in the brain and spinal cord, AMPK (above) fires up to help meet energy demands. Image: MRC National Institute for Medical Research,  PDB. 

ALS may occur, at least in part, due to the breakdown of mitochondria that supply power to the motor nerves.  But according to a new study, the ramping up of energy production to compensate for these power outages may actually do more harm than good - further aggravating the disease.

The study is published today in the Journal of Neuroscience.

The Children's Hospital of Philadelphia (CHOP) team found that the level of activated AMP-associated protein kinase (AMPK), an enzyme that helps maintain energy levels in tissues including the CNS, increased by 50% with respect to the inactive enzyme in a cellular mouse model of ALS. And, by reducing the levels of activated AMPK using the inhibitor compound C, the loss of these cultured motor neurons dropped from 50% to nearly 0%.

These results suggest that a spike in AMPK activity might contribute to neurodegeneration in ALS.  Therefore, by reducing these levels, researchers might be able to slow or stop the progression of the disease.

To put this theory to the test, the CHOP team genetically reduced levels of activated AMPK in a roundworm model of ALS. The researchers found that these so-called nematodes moved more easily.

The CHOP team now hopes to take a look at mouse models of ALS to determine whether levels of activated AMPK are indeed elevated and if so, whether dropping these levels delays the progression of the disease.

Looking ahead, scientists might be able to develop drugs that reduce levels of AMPK in the CNS to treat ALS. But, CHOP neurologist Robert Kalb MD, the leader of the study, cautions that there could be downsides to such a strategy such as the exacerbation of certain conditions such as hypermetabolism or type II diabetes.

"There are all sorts of potential complications,” says Kalb. “Nothing in life is simple."

Reference

Lim, M.A., Selak, M.A., Xiang, Z., Krainc, D., Neve, R.L., Kraemer, B.C., Watts, J.L. and Kalb, R.G. (2011) Reduced activity of AMP-activated protein kinase protects against genetic models of motor neuron disease. Journal of Neuroscience, 32(3), 1123-1141. Abstract | Full Text

Further Reading

Carling, D., Mayer, F.V., Sanders, M.J. and Gamblin, S.J. (2011) AMP-activated protein kinase: nature's energy sensor. Nature Chemical Biology, 7(8), 1123-1141. Abstract | Full Text

Updated 1/25/12:  ALS Today talked to CHOP neurologist Robert Kalb MD about the potential of modulating AMPK as a therapy for ALS.

Energy drain. Mitochondria can swell up and ultimately rupture in people with ALS depleting energy reserves in motor neurons. Image: NIA, NIH.

With Biogen Idec’s dexpramipexole now in phase III clinical trials and the phase III results soon to be in for Trophos’ olesoxime, treatment strategies which keep degenerating motor neurons alive by targeting mitochondria are gathering steam. But researchers remain unsure whether or not these drugs target these intracellular power plants or even whether a power outage causes the disease.

Now, researchers from Weill Cornell Medical College put this theory to the test by generating mice that exclusively produce ALS-associated mutant superoxide dismutase 1 (mSOD1) in mitochondria and look for tell tale signs of the disease.  The researchers find that these mice develop muscle weakness and motor impairment but do not develop paralysis or die of respiratory failure.

“We observe a progressive decline in mitochondrial function,” says Giovanni Manfredi PhD, “but the disease is not as severe and not as lethal.”

These results suggest that a breakdown of these intracellular power plants in motor neurons contributes but is the not the sole cause of the disease.

The study is published this month in the Journal of Neuroscience.

Scientists first suspected in the 1990s that mitochondrial malfunction might be involved in ALS when they noticed, looking under the microscope, defects in the skeletal muscle of people with the disease.  Subsequent studies in mSOD1 mice indicated that these intracellular power plants were not working full steam.  But scientists remained unsure whether the failure of mitochondria caused ALS or whether their breakdown was simply a consequence of the disease.

“One of the difficult things that we have been struggling with is the exact role of mitochondria – that’s always been a problem,” says Johns Hopkins University School of Medicine neuroscientist Lee Martin PhD who was not involved in the study.

As early as 1994, however, neuroscientists discovered mitochondrial defects in mSOD1 mice before these animals developed symptoms suggesting that these intracellular power plants at least contributed to the disease.  In 2009, Lee Martin’s team found that bolstering mitochondria by blocking the so-called mitochondrial permeability transition pore (mPTP) significantly delayed the onset of the disease in mice.  And last July, University of Montreal researchers found that a mitochondrial pile up in motor neurons in these mice occurred before the onset of ALS which might interrupt trafficking of life’s essentials – possibly explaining why motor neurons ultimately die during the course of the disease.

Traffic jam. Mitochondria pileup in motor neurons in mice before ALS onset suggesting that defects in them may contribute to the disease.  Adapted from Vande Velde et al. (2011) PLoS One: e22031.

But although these power outages occurred in the right place and the right time, researchers still could not prove in these mice that mitochondrial breakdown triggered the disease. 

”The problem I think that we were facing is that SOD1 is present abundantly,” explains Manfredi, “so the relative role of mitochondrial localization of SOD1 relative to everything else everywhere else has been difficult to assess.”

Now, Manfredi’s team find that mice which produced mSOD1 only in the intracellular membrane space of mitochondria in certain tissues including skeletal muscle develop both defects in energy production and a 30% motor neuron loss suggesting that albeit weakly such power outages may drive at least some aspects of ALS.  But the team sees no signs of motor neurons unplugging from muscles suggesting that there may be other triggers that together result in the disease.

These findings come at the heels of two previous studies in 2009 that demonstrated mutant SOD1 produced exclusively in the mitochondria of cultured motor neurons made them more susceptible to destruction.

Now, the Weill Cornell team hopes to use these mice to ferret out why these intracellular power plants may fail in people with ALS.  And looking ahead, says Manfredi, these mice could be used to find better drugs to keep the power on in dying motor nerves to slow or stop the progression of the disease.

“I don’t think we are at the stage that we have good mitochondrial drugs,” says Manfredi.   “They target only certain aspects of mitochondrial function.” 

Furthermore, with the growing understanding that there is much more to the disease that simply a power outage, combinatorial treatment strategies for ALS may need to be developed.

“Realistically, I don’t think there is going to be a magic bullet for this disease,” says Martin.  “Targeting mitochondria is only one aspect and there are other aspects that need to be explored.”

References

Igoudjil, A., Magrané, J., Fischer, L.R., Kim, H.J., Hervias, I., Dumont, M., Cortez, C., Glass, J.D., Starkov, A.A., and Manfredi, G. (2011) In Vivo Pathogenic Role of Mutant SOD1 Localized in the Mitochondrial Intermembrane Space.  Journal of Neuroscience 31(44), 15826-15837. Abstract | Full Text (Subscription Required) 

Fischer L.R., Igoudjil, A., Magrané, J., Li, Y., Hansen, J.M., Manfredi, G. and Glass J.D. (2011) SOD1 targeted to the mitochondrial intermembrane space prevents motor neuropathy in the Sod1 knockout mouse.  Brain, 134(1), 196-209. Abstract | Full Text (Subscription Required) 

Vande Velde, C. et al. (2011).  Misfolded SOD1 associated with motor neuron mitochondria alters mitochondrial shape and distribution prior to clinical onset.  PLoS One 6(7), e22031. Abstract | Full Text

Cozzolino, M., Pesaresi, M.G., Amori, I., Crosio, C., Ferri, A., Nencini, M., and Carri, M.T. (2009). Oligomerization of mutant SOD1 in mitochondria of motorneuronal cellsdrives mitochondrial damage and cell toxicity. Antioxidants and Redox Signaling, 11, 1547-1558.  Abstract | Full Text

Magrané, J., Hervias, I., Henning, M.S., Damiano, M., Kawamata, H., and Manfredi G. (2009). Mutant SOD1 in neuronal mitochondria causes toxicity and mitochondrial dynamics abnormalities. Human Molecular Genetics, 18(23), 4552-4564.  Abstract | Full Text

Martin, L.J., Gertz, B., Pan, Y., Price, A.C., Molkentin, J.D., Chang, Q (2009). The mitochondrial permeability transition pore in motor neurons: involvement in the pathobiology of ALS mice.  Experimental Neurobiology, 218(2), 333-346.  Abstract | Full Text

Dal Canto M.C. and Gurney M.E. (1994) Development of central nervous system pathology in a murine transgenic model of human amyotrophic lateral sclerosis.  American Journal of Pathology, 145(6), 1271-1279.  Abstract | Full Text

Further Reading

Martin, L.J. (2010). The mitochondrial permeability transition pore: a molecular target for amyotrophic lateral sclerosis therapy. Biochimica Biophysica Acta, 1802(1), 186-197.  Abstract |Full Text

 

See that aberrant astrocyte (ABA). Scientists identified a new kind of astrocyte in rats called ABA which produces distinct substances including S100β (red) which may contribute to degeneration of motor neurons in the spinal cord. Adapted from Diaz-Amarilla et al. (2011). Courtesy of the National Academy of Sciences Press. All rights reserved.

People with ALS experience muscle weakness and ultimately paralysis due to the degeneration of their motor nerves. Recent studies suggest that neighboring cells called astrocytes could contribute to this deterioration. But why astrocytes turn traitor and kill motor neurons remains unclear.

Now, a new study in rats from Uruguay’s Institut Pasteur de Montevideo suggests instead that a new kind of astrocyte may arise during the onset of ALS and contribute to disease progression.

“There is a new element, a new actor,” says lead author and institute director Luis Barbeito MD, “that is killing motor neurons, triggering neuroinflammation.”

The study is published this month in Proceedings of the National Academy of Sciences.

Scientists isolated astrocytes from the cervical spinal cord of rats before symptoms or at the early or end-stage of the disease. The team found a distinct type of astrocyte that appeared at ALS onset and that produced substances previously implicated in neuronal injury and diseases including ALS.

“There is a perfect correlation between disease progression and the number of [ABA] cells that appear around motor neurons.” What’s more, says Barbeito, “these cells are proliferating. This is strong evidence that these cells are killing motor neurons.”

The researchers are now looking at post-mortem tissue samples obtained from people with ALS to determine whether or not these same astrocytes are associated with the human form of the disease.

Looking ahead, these astrocytes could be used to screen for new ALS drugs by screening for compounds that neutralize their toxicity. And, these astrocytes could be further characterized to identify unique cell surface markers that could be used to design antibody-based therapies directed toward these cells to eliminate them from the spinal cord.

References

Díaz-Amarilla, P., Olivera-Bravo, S., Trias, E., Cragnolini, A., Martínez-Palma, L., Cassina, P., Beckman, J., Barbeito, L. (2011) Phenotypically aberrant astrocytes that promote motoneuron damage in a model of inherited amyotrophic lateral sclerosis.  Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.1110689108  Abstract | Full Text (Subscription Required)

Migheli, A., Cordera, S., Bendotti, C., Atzori, C., Piva, R., and Schiffer, D. (2011) S-100beta protein is upregulated in astrocytes and motor neurons in the spinal cord of patients with amyotrophic lateral sclerosis. Neuroscience Letters, 261(1-2), 25-28.  Abstract | Full Text (Subscription Required)

Further Reading

Yamanaka, K., Chun, S.J., Boillee, S., Fujimori-Tonou, N., Yamashita, H., Gutmann, D.H., Takahashi, R., Misawa, H., and Cleveland, D.W.. (2008). Astrocytes as determinants of disease progression in inherited amyotrophic lateral sclerosis. Nature Neuroscience, 11(3), 251-253.  Abstract | Full Text

Papadeas, S.T., Kraig, S.E., O'Banion, C., Lepore, A.C. and Maragakis N.J. (2011). Astrocytes carrying the superoxide dismutase 1 (SOD1G93A) mutation induce wild-type motor neuron degeneration in vivo. Proceedings of the National Academy of Sciences, 108(43), 17803-17808.  Abstract | Full Text (Subscription Required)

Researcher Resources

ABA cells can be requested for further study by contacting Luis Barbeito MD.   Email