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Research Newsletter-Spring/Summer 2013

Kathy Magnusson, D.V.M., Ph.D.

MEMORIES

An interview with Kathy Magnusson, D.V.M., Ph.D.
Professor of Biomedical Sciences
LPI Principal Investigator


Q. When did you decide to become a veterinarian?

A. My father was a veterinarian, and two of my brothers became veterinarians, so it’s the career that I was most exposed to. I decided early on that would be my career.

Q. Where did you grow up?

A. Blooming Prairie, a small town in Minnesota.

Q. Did you practice veterinary medicine after earning your degree?

A. Yes. I took care of dairy cows for a couple of years, as well as small animals like pets, but probably 90 percent of the practice involved dairy cows.

Q. What motivated you to pursue research?

A. I fell in love with neuroanatomy my freshmen year of veterinary school and was amazed at how much was already known about the brain, but there was also still plenty to be learned. I knew then that I would ultimately go back to graduate school.

Q. When you went to graduate school and pursued research, did you continue to practice veterinary medicine or did you give that up in favor of research?

A. I continued to take care of one client. I would go back and do herd health checks for them as a friend while I was in graduate school. What I missed the most from practice was helping the cows to calf. It’s a great feeling to help a new life come into the world.

Q. Most of your research has focused on the influence of alterations in the N-methyl-D-aspartate (NMDA) receptor on age-related changes in brain functions, such as learning and memory. What are NMDA receptors and how do they work?

A. NMDA receptors are located on neurons in the brain. They use the amino acid glutamate as their transmitter, but glutamate binding itself is not sufficient to open up an NMDA receptor channel to make it work—you have to have the neuron already excited or depolarized. And typically that involves other glutamate receptors. Once that occurs—the glutamate is present and the cell has been excited—then the NMDA receptor channel can open up and function to create memories. The reason it doesn’t open without those conditions is that there is a magnesium block. That’s believed to be important for memory because you need a stronger stimulus than just normal signaling to activate the receptor. That way you’re not making memories all the time; you’re making memories when it’s important—there’s enough of a signal.

Q. When you say the glutamate receptor opens up, what does that mean?

A. There are four protein subunits that make up the receptor. When the glutamate binds to the receptor, there’s a change in the relationship of those subunits, such that a channel is formed. Then, when the cell is depolarized, the magnesium block, which is within the channel, is let loose. Then the channel can conduct calcium ions through the NMDA receptor channel into the neuron. The calcium is the key to making changes within the neurons that lead to memory formation.

Q. And without that activity, no memory is formed?

A. The NMDA receptor is important in a lot of different brain regions, but there are some brain regions that don’t require the NMDA receptor for memory formation. There are other receptors involved in making memories, but this is probably the most important.

Q. Are NMDA receptors concentrated in certain areas of the brain, and if so, why?

A. They are concentrated in the higher centers of the brain—the cerebral cortex and the hippocampus—because those are the regions where memories are made and stored.

Q. Are the different types of memory affected by NMDA receptor activity?

A. Well, we know that long-term memory, which is also called reference memory, is influenced by NMDA receptor activity. But we also see that short-term memory is affected, too. If you block NMDA receptor activity in the frontal cortex during and for ten minutes after a learning experience, the memories are not retained. For shorter periods of time, the NMDA receptor doesn’t appear to be involved.

Q. Are there other types of cognitive activity affected by this?

A. Flexibility is affected. There’s evidence that if you block the NMDA receptors in the prefrontal cortex people can’t adjust to new rules.

Q. How do you study these phenomena?

A. I examine the NMDA receptor by receptor-binding autoradiography on tissue sections. This involves labeling glutamate with radioactive isotopes and having it bind to the cellular receptors so they can be visualized.

Q. Are those tissue sections from rodents?

A. Those are mainly from rodents. Amazingly, you can take a frozen brain section, thaw it, and you can still get glutamate to bind to these receptors to then look at the density changes that occur with aging.

Q. How do you correlate that to specific memory function in rodents?

A. In the rodents, I’ve done memory testing using the Morris water maze for spatial long-term memory, and then I correlate that with the receptor-binding changes we observe in tissue sections.

Q. How does the Morris water maze work?

A. It’s basically a horse water tank, and we color the water to make it opaque. We make it white because we use black mice and that makes a nice contrast for our tracking system. There is a platform hidden just below the surface when we’re doing long-term memory testing. There are visual cues on the walls of the room and high up on the tank—there’s evidence from other studies that rodents use those cues like we would use landmarks. So we put the rodents into the tank at different entry points, and they swim around for a maximum of one minute or until they find the platform. It acts as a resting place for them. If they’re going to get better at finding that hidden platform, they’ve got to use the cues around the room, make a spatial map, and use it again. I’ve heard it said that mice aren’t natural swimmers like rats, but we actually have to train them to stay on the platform, which gives them time to make their spatial map. Without that training, some of them would just keep swimming.

Q. Does NMDA receptor activity affect the performance assessed by the Morris water maze test?

A. I have not done the specific functional studies that showed that; Morris and others have done electro-physiology and have shown correlations between changes in the electrophysiology and performance in the Morris water maze. They have also blocked the function of the NMDA receptor with drugs and shown deficits in the water maze test. What I’ve done is to look at the expression of the different subunits that make up the receptor. Changes in expression in some of them correlate with the long-term and delayed short-term memory declines with age, as assessed in the Morris water maze test, and some of them don’t.

Q. Do NMDA receptors function about the same in rodents and people?

A. As far we know, they function the same.

Q. Are rodents good models to study the molecular basis of memory function?

A. Yes, I think so. There’s little difference in the NMDA receptor between rats and humans. Drugs like PCP or ketamine that block NMDA receptors in humans have similar effects in rodents. We wanted to look at the age-related changes in the GluN2B subunit in humans to see if it was similar to our findings in the mice. Human brain banks have normal brain tissue from older individuals available, but it’s more difficult to find enough normal young brain tissue, probably because young adults don’t have the same motivation to set up a donation plan. So, rodents remain the best model.

Q. Why are the NMDA receptors more vulnerable to dysfunction in older age?

A. That is something I haven’t figured out yet. I think that one of the subunits, the GluN2B subunit, undergoes a programmed decline, meaning that the expression of the gene has been shut down or reduced. This subunit is very important during development of the brain and has very high expression at birth. The expression of the GluN2B subunit declines across post-birth development, and we think that this continues during aging. The GluN1 subunit seems to be more affected by the animal’s environment or experience. The effects of aging are more variable on the GluN1 subunit.

Q. If that particular gene—GluN2B—becomes dysfunctional as animals age, does that mean that the subunit is not being made correctly and that the overall receptor is not functioning properly?

A. Exactly, it’s not as good at making a memory because when the GluN2B subunit is present, the receptor and channel will stay open longer and have a higher affinity for glutamate. When we increase the gene expression in the brain by introducing more copies via a virus, memory is improved. But we have to be careful not to overdo it—too much NMDA receptor activity can be damaging. Our goal is to restore memory similar to that in young adults, not try to super-size it.

Q. The NMDA receptor binds glutamate, which is a non-essential amino acid in proteins that we consume in our diet, and glycine, which is sweet- tasting and the smallest amino acid. Does the binding efficiency of both amino acids decline with age?

A. No. In our studies, the glycine-binding site does not appear to change significantly with age, but the glutamate site does.

mouse

Q. Does supplementing with glutamate have any affect on NMDA receptor activity?

A. That’s not known yet. There are issues with excitotoxicity if you raise glutamate levels excessively in the brain.

Q. Does the number of NMDA receptors decline with age as well?

A. Yes. Studies have shown that there’s a decline in the absolute number, especially in certain regions of the brain. However, there also appear to be changes in the function of the remaining receptors.

Q. Does oxidation play a role in age-related changes in NMDA function?

A. Tom Foster’s work with electrophysiology has shown there is a redox site on the NMDA receptor that is in a more oxidized state in an old rat..

Q. Does that suggest that antioxidants like vitamin C or vitamin E, which is fat-soluble, might have an effect?

A. Vitamin C does not appear to reverse the oxidized state of the NMDA receptor when directly applied to brain slices from aged rats. I don’t believe it has been assessed via treatment of the whole animal. Vitamin E given in the diet can increase the density of NMDA receptor binding across all ages of animals. Although this doesn’t appear to counteract the effects of aging directly, an increase in receptors could still help to restore learning ability.

Q. Is inflammation implicated in the age-related decline of NMDA function?

A. Yes. A study by Mesches and coworkers showed that giving the anti-inflammatory drug sulindac caused an increase in the GluN1 and the GluN2B subunits and improvement in working memory in aged rats.

Q. Do dietary constituents like omega-3 fatty acids, carnitine, lipoic acid, and blueberry extracts improve NMDA function in old animals?

A. Those have been shown in a number of studies by other researchers to increase expression and/or function of the NMDA receptor.

Q. Does dietary restriction affect NMDA receptor activity?

A. We’ve shown that the binding density of the NMDA receptor is improved by dietary restriction, especially involving the GluN1 subunit.

Q. Do you have any plans to work with LPI Principal Investigator Viviana Perez to see if the dietary restriction mimetic rapamycin might have a similar effect?

A. Our Healthy Aging Program group has regular meetings to find ways to collaborate, and I think that would be a perfect project. Rapamycin has been shown to enhance memory in aged animals. It would be interesting to know whether this was through an action on NMDA receptors.

Q. Aside from glutamate-binding efficiency, are there other differences in NMDA function between young and old animals?

A. Others have shown that long-term potentiation is impaired in old animals, and that affects memory. This is observed in hippocampal brain slices. To study this, you do an electrical stimulation in the slices once every 15 seconds to get a baseline response, which is based on the opening up of the non-NMDA glutamate receptors and ions flowing away from the recording electrode. If you then do a high-frequency stimulation—the original one was about 100 Hertz—and then go back to the lower frequency every 15 seconds, there’s a bigger response. And that response in live animals has been shown to last from weeks to months. That’s referred to as long-term potentiation. You can induce long-term potentiation in slices from old animals, but it decays faster than it does in a young animal—it’s not maintained. The NMDA receptor is important for the initiation of long-term potentiation in many brain regions.

Q. Does stress or exercise affect NMDA receptor activity and memory function?

A. I believe that they do. We have evidence that behavioral experience, such as the Morris water maze test, can increase one of the GluN1 splice variants, and this increase is associated with improved NMDA receptor function. At this point we don’t know whether it’s the learning or the exercise that’s influential.

Q. When you were at Colorado State University you assessed the effect of certain metals like lead, magnesium, and zinc on the NMDA receptor and memory function in rats. What did you learn?

A. I learned that we were using way too high concentrations that were non-dietary and non- physiological. I was working with a toxicologist, and he was interested in very high concentrations that you might find in a toxic superfund site. There’s a zinc- binding site on the receptor. Magnesium is important for the blockade of the receptor, and there’s evidence that lead can influence NMDA receptors. So we thought that it would be good to look at a mixture.

Q. With other LPI investigators you’ve looked into the effect of age on dendritic cells and immune function in mice. What are dendritic cells and what role do they play in immune function?

A. Dendritic cells are cells that are in your skin, blood, liver, and lungs. They monitor the body for invaders. They’re job is surveillance, and if they detect a foreign substance they’ll engulf it and go to the nearest lymph node to recruit lymphocytes. In collaboration with Emily Ho and Carmen Wong, we found age-related declines in some types of dendritic cells in the spleen and increases in others. In addition, we found that dendritic cells in aged animals did not respond as robustly to challenge as those from young.

Q. In collaboration with LPI’s Emily Ho and her team, you also investigated the effect of zinc deficiency on immune response and inflammation. What did you find in those studies?

A. We found that decreased zinc within immune cells was associated with age-related increases in inflammation. The decreased zinc appeared to be due to alterations in zinc transporter expression. Dietary supplementation with zinc was able to reduce inflammation in the aged animals.

Q. You participated in another study that assessed the effect of zinc supplementation on immune function in old mice. Why does good zinc status improve immune function in those animals?

A. Zinc supplementation was able to increase the number of thymocytes. This was associated with increased maturation of thymocytes and inhibition of a thymic suppressive cytokine, which is a cellular signaling molecule. Increased thymocytes should enhance immune responsiveness in aged individuals.

Q. Where is your research headed?

A. As far as the NMDA receptor is concerned, we’re still trying to characterize what’s gone wrong with it with age. We’ve also begun to try interventions to see if we can repair or prevent the damage.

Q. Using dietary micronutrients?

A. We’re interested in looking at alpha-lipoic acid and carnitine with LPI’s Tory Hagen because his group has shown that those substances improve memory in rats, so that seems like an obvious thing to try to see if they have effects on the NMDA receptor. We see different changes with age in different regions of the brain. And alpha-lipoic acid and carnitine have an effect on mitochondria, so one of my ideas is that there may be a differential amount of mitochondrial damage in different brain regions. The health of the mitochondria in those areas might affect the oxidative environment, and we already know that the receptor is in a more oxidized state in older animals.

Q. Do you have more collaborations planned with other LPI scientists?

A. Yes. We’re currently collaborating with Fritz Gombart, looking at vitamin D and vitamin D deficiency, which is prevalent in a lot of older individuals. We are also beginning a collaboration with Fred Stevens to see whether the anti-inflammatory or fatty acid-reducing effects of xanthohumol may help NMDA receptor expression and/or memory.

Q. Finally, what do you like to do when you’re not in the lab?

A. I like to walk my dogs and watch them run.

Q. What kind of dogs do you have?

A. Two Springer Spaniels, Lily and Divya.

Q. Is there an advantage in having a veterinary school and hospital here at Oregon State University, in terms of community access to good veterinary services?

A. Oh yes, I think so. I think we’ve got excellent basic health care with the practitioners in this area, but OSU can offer more specialized services that the average practitioner wouldn’t normally do on a daily basis.

Q. Do you think we might commonly have health insurance for pets some day?

A. I don’t know, but the attitude towards pets has really changed over the last 20 years, and it could be possible in the future. More and more people, myself included, consider their pets as part of their family. At the veterinary school, there have been situations in which accommodations were made for a student whose pet was seriously ill, similar to if it had been a child. I don’t think a veterinary school would have done that 20 years ago. So those attitudes are changing.

Last updated May 2013