Learning is complicated business. But typical research studies into the molecular basis of learning and memory measure only one or a few proteins. In a study just reported in the journal Molecular & Cellular Proteomics, researchers cast a wider net and looked at 80 proteins in the brain of mice. By looking at more proteins, the study’s leader, Katheleen Gardiner at the University of Colorado in Denver, says researchers can get a better appreciation of “the greater complexity of molecular events underlying learning and memory, how components of a single pathway change in concert, and how many pathways and processes respond.”
Gardiner’s research focus is on Down syndrome, two characteristics of which are that patients suffer from some level of intellectual disability and that they eventually develop Alzheimer’s disease. Gardiner’s group aims to find drugs that can lessen the learning disability. But in order to do that, researchers need to better understand the molecular events associated with learning, memory and neurodegeneration.
To get a grasp of the proteins involved in a particular learning process, the investigators studied context fear conditioning in mice. In this type of experiment, mice are put in a new cage and given a small electrical shock. Researchers can tell when a mouse has learned to be fearful of the same cage when the mouse freezes when put back in the cage. This approach “has the advantage that it requires only a single trial, lasting less than five minutes, for mice to learn,” explains Gardiner. “This means that we have a clear window in time where we know molecular events associated with successful learning occur.”
Context fear conditioning demands that the hippocampus, a region of the brain important for memory formation, be functional. The hippocampus is also a part of the brain that degenerates in Alzheimer’s disease.
The investigators gave the mice a drug called memantine, which is used to treat moderate to severe cases of Alzheimer’s disease. The drug has been shown to correct for learning impairment in a mouse model of Down syndrome.
Gardiner’s group used proteins arrays to see how protein expression changed in the brains of mice that underwent context fear conditioning and were given memantine compared with control mice. They found levels of 37 proteins changed in the nuclear fraction of the hippocampus. Abnormalities in 13 proteins had been reported in brains of Alzheimer’s patients. “One surprise was that many proteins that increased in level with normal learning also increased, although not as much, with treatment with memantine alone,” says Gardiner. “Memantine induces responses in a substantial number of proteins that we measured, and it does this without impairing or enhancing learning. This indicates that there is considerable flexibility in the timing and extent of protein responses that still result in successful learning.”
In particular, Gardiner’s group identified the MAPK and MTOR pathways to be affected in their experiments as well as subunits of glutamate receptors and the NOTCH pathway modulator called NUMB. NUMB is known to be essential for some aspects of brain development.
Gardiner says her group is now looking at data from a similar experiment done with a mouse model of Down syndrome. Those mice were unsuccessful with context fear conditioning, but they did as well as wild-type mice when they were treated with memantine.