Sci. Aging Knowl. Environ., 19 November 2003
Consequence, Not Cause
Oxidants arise from, but aren't required for, lethal neuron overstimulation
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/sageke;2003/46/nw158
Key Words: cerebellar granule cells dihydroethidium superoxide dismutase mimetic
NEW ORLEANS, LOUISIANA--Overloading a microphone can turn a sweet sax solo into a high-pitched squall. Similarly, a flood of the neurotransmitter glutamate--from stroke, brain injury, or neurodegenerative disease--provokes excess neuron firing and causes cells to die (see Nicholls Perspective). Previous studies hinted that reactive oxygen species (ROS) spurred this slaughter, but new research shows that ROS are a byproduct of the process, according to work presented here on 10 November 2003 at the Society for Neuroscience Annual Meeting. The study helps place ROS in the order of events that lead to neuron death.
After binding to glutamate, which normally stimulates the electrical currents that relay nerve signals, neurons absorb calcium. Mitochondria, the cell's power plants, store this element to regulate its amounts in the cell. But when calcium quantities soar too high for them to handle, mitochondria burst, release calcium, and incite a chain reaction that destroys the cell. Experiments over the past decade have revealed that rising amounts of ROS accompany death by glutamate. Researchers conjectured that ROS spurred the breakdown of mitochondria.
To investigate that possibility, mitochondrial biologist David Nicholls and colleagues at the Buck Institute for Age Research in Novato, California, doused neurons with glutamate and monitored the amounts of calcium and the ROS superoxide in the cells. Only cells that showed the calcium spike that denotes mitochondrial malfunction suffered a rise in superoxide quantities. Moreover, the superoxide concentration in any cell never rocketed up before calcium rose but instead always did at the same time or afterward.
The results suggest that ROS accumulate as a consequence of the calcium debacle but don't cause it. To further test that idea, the researchers added glutamate and a compound that sops up superoxide. The compound prevented superoxide buildup in neurons but didn't blunt calcium deregulation or neuron death. Next, the scientists ramped up oxidative stress in neurons by crippling an antioxidant enzyme, then added glutamate. Twice as many of these cells flooded with calcium as did cells exposed to glutamate only. Together, the results suggest that oxidative damage isn't necessary for cells to overdose on glutamate, but oxidants worsen the problem.
The troublemaking ROS are unlikely to come from mitochondria, says Nicholls, because if the powerhouses malfunction, their generators shut down and they don't make ROS. To probe another potential source, the researchers soaked neurons with a compound that inhibits a lipid-processing enzyme in the cell's cytoplasm that also makes superoxide. After glutamate treatment, these cells produced a smaller spike in superoxide, suggesting that the oxidant flood originates in the cytoplasm rather than in mitochondria.
"It's clear from the data that oxidant production goes up after the mitochondria get screwed up," says pharmacologist Ian Reynolds of the University of Pittsburgh in Pennsylvania. "They've very nicely timed the events." Oxidation might make neurons more susceptible to the hazards of overexcitement, Nicholls says, but the results imply that antioxidant treatment, which some researchers hope might save old brains, won't directly spare neurons from glutamate overload; scientists should explore other avenues for preserving mental acuity. Blocking oxidants might not turn down the volume completely on glutamate poisoning after all.
--R. John Davenport
November 19, 2003
Science of Aging Knowledge Environment. ISSN 1539-6150