Basic Research

1. Glutamate in ALS

There is evidence from many directions suggesting excess excitotoxicity in ALS, probably from glutamate. Glutamate is a stimulatory chemical released from some nerve endings to carry the nerve signal from one neuron to the next in a chain. It is also released if nervous tissue is damaged. Too much glutamate over stimulates neurons, and can cause them to die.

Perhaps the strongest evidence for there being excess glutamate in ALS comes from Dr. Jeffrey Rothstein's work showing a partial deficiency of the glial glutamate uptake transporter GLT1. This transporter is present on supporting glial cells around neurons, and is responsible for removing glutamate by absorbing it into the glial cells. If there is insufficient GLT1, there will be excess glutamate to over stimulate neurons, predisposing them to early death.

It is uncertain how this deficiency of GLT1 arises. Studies have predominantly used autopsy tissue. To learn more about glutamate in ALS, we have undertaken studies of ALS patients' brains in life using MRI spectroscopy. Using a regular clinical MRI machine with spectroscopy capacity, we have shown that in the motor cortex and brainstem of the brains of ALS patients there is a progressive loss of a chemical, N-acetyl-aspartate (NAA), that is a marker of the number and health of nerve cells. We searched for evidence of an increased signal from glutamate and glutamine in motor cortex and brainstem of ALS patients. Accurate estimation of the amount of these chemicals requires a higher field-strength magnet MRI than we have available, but our findings suggest that the total levels of glutamate and glutamine, which includes both intracellular and extracellular, are decreased.

Work from other laboratories indicates that GLT1 is a complex protein that exists in several different forms in ALS and normal nervous system tissue, and that there are no mutations of the gene for GLT1 that have been found to cause the decreased level of GLT1 protein. Clearly we need a much greater understanding of the role of glutamate excitotoxicity in ALS.

2. Surrogate Markers for ALS

Currently we have no laboratory test available to prove that a patient has ALS, except in the 1 to 2% of patients with mutations of the gene for superoxide dismutase -1 (SOD1). Also currently the only marker of disease progression is the extent of the abnormal neurological signs measured by a Neurologist, such as the amount of muscle weakness, muscle wasting, breathing tests, and reflex changes. Programs to discover drugs to treat ALS use these abnormal neurological signs to measure rates of disease progression. However, because ALS is so variable, drug trials need very large numbers of patients. It may take a trial of 2,000 patients treated for 18 months to prove whether a drug works or not.

It would greatly assist progress in diagnosing and monitoring ALS, and in new drug development if we had a marker that was relatively specific and sensitive for ALS, and which changed as the disease progressed. We and others have used MR spectroscopy to study NAA, a chemical present in neurons. The motor cortex levels of NAA fall as ALS progresses. Other research groups have found that treatment with riluzole that slows ALS disease progression raise the levels of NAA slightly, though we could not confirm that. More research is clearly needed to find a reliable surrogate marker for ALS to assist in programs for more rapid screening of drugs that may benefit ALS.

3. Oxidative Damage in ALS

Though oxygen is essential to advanced life forms, it is also potentially toxic. It appears that when primordial life forms appeared, they did so in the "primeval ooze" at the bottom of the oceans where the oxygen concentration was much less than in the atmosphere today. All cells of humans have a complex system of enzymes and other substances designed to control the effect of oxygen. One of these enzymes is SOD1. Discovery of mutations of SOD1 in about 20% of patients with familial ALS increased awareness that damage of motor neurons by excess oxygen free radicals may be a cause of ALS. Other research groups have shown that there is oxidative damage of proteins and other macromolecules in the nervous tissue of ALS patients.

Somewhat surprising to all of us was the finding by several other research groups that in fact oxidative damage is not the mechanism by which SOD1 mutations produce the motor neuron degeneration in ALS. A great deal of research is under way to find exactly how the mutant SOD1 protein damages the cells.

Mitochondria are intimately concerned with using oxygen in cells to produce energy, and they play a role in controlling oxidative damage within cells. Other research groups have shown that the mitochondria show structural changes in the nervous tissues of ALS patients and animal models. We have a basic research program studying the mitochondria in cells from ALS patients and animal models of ALS. This research is producing evidence that neuronal mitochondria in mouse models of ALS are more susceptible to lack of oxygen than normal neurons. We are studying the mitochondria to learn more about their role in ALS.