Penn State researchers have created an elegantly simple model of an axon–the extension of a neuron that communicates with other neurons–and have used this model to reproduce a change in the axon’s shape that is characteristic of neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases. This achievement is the first of its kind in a highly simplified biophysical model system. The model provides a novel avenue for investigating the specific mechanisms that contribute to complex brain diseases. It also provides a means of discovering new kinds of drugs for the treatment of these disorders. The research will be described in a paper to be published in the 4 April 2006 issue of the Proceedings of the National Academy of Science.
This model, produced in the laboratory of Paul S. Weiss, Distinguished Professor of Chemistry and Physics at Penn State, has the essential features of an axon, including a lipid membrane that encloses a "cytoskeleton" scaffolding, which produces the axon’s shape. The outer membrane was prepared to contain a very small amount of dye molecules that are sensitive to ultraviolet light. Shining light on the artificial axons initiated a photochemical reaction that produced highly reactive "free radicals" and triggered a catastrophic oxidative-stress reaction. The result was that the previously protruding microtubule cytoskeleton collapsed into a constricted and deformed structure resembling a string of beads–the same morphology observed during the degeneration of actual neurons.
Surprisingly, the model reproduced this highly characteristic "beading" or "pearling" even though it does not include proteins that were previously thought to be essential for causing this kind of axon destruction. "One of the beauties of a simplified model is that it allows you to ask very simple questions, which sometimes are difficult to answer in a complex living system, and sometimes to get surprising answers," Weiss said. "What makes this model so exciting is that it generates many more questions than it answers," Weiss said. "It will allow us to test hypotheses of how damage occurs, and importantly, how we might prevent it. There is a real opportunity to come up with novel therapeutic treatments."
"There is tremendous urgency right now to determine which processes cause the destructive mechanisms that we see in neurodegenerative diseases," said coauthor and Assistant Professor of Veterinary and Biomedical Sciences, Anne Milasincic Andrews. "Our study shows that oxidative stress, whatever its origin, is capable of causing the cytoskeleton of this artificial system to collapse in the same way that it does in diseased or aging brains." One of the future experiments planned by the team is to induce oxidative stress in the presence of key proteins thought to be involved in the underlying causes of the brain pathologies associated with Alzheimer’s and Parkinson’s diseases to see whether these proteins accelerate the damaging effects of oxidative stress.
The study also revealed many specifics about the process of axon collapse. For example, the degradation rate is faster when the lipids comprising the membrane have more multiple bonds (they are more highly unsaturated). The researchers also added free-radical scavengers, such as vitamins C, E, and K, to the model system and found that these vitamins delayed or prevented the degradation of the cytoskeleton. "These antioxidant vitamins neutralized the free radicals before they had a chance to degrade the model axon," Weiss explained.
"Simple models also allow us to build more complicated hypotheses, which later can be tested in complex living systems, such as laboratory animals. We plan to build into our model the different brain chemicals that have been implicated in neurodegenerative processes to see which are the good and bad actors–which are the most effective in promoting the radical attack from the membrane to the interior of the axon and which are the best at disabling free radicals."
One of the types of neurons that degenerate in diseases such as Alzheimer’s disease and that also contribute to depression and anxiety disorders are neurons that produce the neurotransmitter serotonin. Andrews and her colleagues have made a specific model of serotonin-axon degeneration using a chemical neurotoxin. Evidence of serotonin axon damage, including beading and pearling, was published recently by Andrews and her colleagues in the journal Neuropharmacology. This study used antibodies to label serotonin axons so that the degenerative process could be visualized. The researchers injected mice with the chemical neurotoxin, 2′-NH2-MPTP, that Andrews discovered and has been studying for nearly two decades. "This latest study shows conclusively that this toxin destroys serotonin-transmitting neurons," Andrews said, "and it currently is one of the best models to destroy this type of neuron. We clearly observed evidence for axonal collapse into the beaded structures in the brains of these animals a short time after we gave them the neurotoxin."
Neurodegenerative disorders typically involve many different types of neurons that produce different neurotransmitter chemicals. "Our chemical model of neurodegeneration gives us a tool to disable just one type of neuron so we can begin to tease apart how each neurotransmitter system participates in these complex disorders," Andrews said. "We then can study the behavioral effects of the degeneration of each system and can test the effectiveness of potential therapeutics to prevent or reverse the damaging effects."
Other researchers involved in the paper to be published in the Proceedings of the National Academy of Science include Anne E. Counterman, previously an NIH postdoctoral fellow in the Weiss laboratory and now a researcher at Yale University; and Terrence G. D’Onofrio, a former graduate student in chemistry in the Weiss group, who is currently a scientist at the U.S. Army laboratory at Edgewood. The scientists who worked on the studies published in Neuropharmacology include Beth A. Luellen, a former neuroscience graduate student in the Andrews group who is now a postdoctoral fellow at the Penn State Neuroscience Institute; Matthew E. Szapacs, a former chemistry graduate student in the Andrews group who currently is a postdoctoral fellow at the Vanderbilt University School of Medicine; and Christopher K. Materese, a former chemistry undergraduate student researcher in the Andrews group who is now a graduate student in chemistry at the University of North Carolina at Chapel Hill.
The research project led by Professor Weiss was funded by the National Science Foundation. The project led by Professor Andrews was funded by the National Institute of Mental Health.
