Understanding the Molecular Basis of Memories
Current research suggests that a memory is stored in a sparse group of neurons (nerve cells) that are connected to each other. But little is known about where these neurons are located in the brain, or about the molecular details of how neurons change when they encode a memory. Are new connections made, or are existing connections activated or strengthened? What happens when a memory weakens or is forgotten? Can a memory be strengthened or weakened by manipulating levels of molecular compounds involved in memory encoding or retrieval?
Reijmers and his research group are working to answer some of these questions. Since proteins are the building blocks of new neuron connections, one project is looking at what proteins are synthesized by neurons encoding a memory. Another project will visualize and map the connections among memory-encoding neurons. And a third project will see how memory encoding in a mouse model of Alzheimer’s disease differs from that in control mice, in both protein synthesis and new connections. All of these projects are using an innovative transgenic mouse called the TetTag mouse.
The TetTag mouse was developed by Reijmers and his colleagues at The Scripps Research Institute. "TetTag stands for tetracycline-controlled tagging of activated neurons," says Reijmers. The TetTag mouse has two transgenes (non-native genes) inserted into its DNA that allow activated neurons to be tagged during an investigator-controlled time window, and for the tag to remain active for a prolonged period. The tagging time window is controlled by the presence or absence of tetracycline (or derivatives such as doxycycline) in the animal’s food, and hence in its tissues. The tag is a reporter gene that can be detected and visualized upon analysis of brain tissue. For example, doxycycline can be removed from the food of a mouse to open the testing window; the mouse can be provided with a specific sensory experience and the neurons encoding memory of the experience will be tagged; then the mouse can be put back on doxycycline to close the neuron-tagging window. Researchers can analyze specific areas of the brain to visualize and map those neurons involved in the memory of the experience. A diagram illustrating TetTag technology can be found here.
The Reijmers laboratory currently has three major projects involving the TetTag mouse. The Protein Synthesis Project is headed by Joshua Ainsley, a postdoc in Reijmers’ laboratory. He is working on a new genetic tool—a combination of the TetTag mouse and a ribosome-labeling tool—that will allow the messenger RNA of labeled ribosomes from activated neurons to be sequenced to identify proteins being synthesized during memory storage. "This [ribosome-labeling] tool enables the purification of all translated mRNA, which can be sequenced to determine all the ongoing protein synthesis events," says Reijmers. "This tool is not limited to neurons, but can be applied to any genetically defined group of cells."
The Neuron Connection Project is headed by Stéphanie Trouche, another postdoc working in Reijmers’ laboratory, and Jennifer Sasaki, a Sackler graduate student. "We can use the TetTag mouse to see which neurons in different parts of the brain store a specific memory," says Reijmers. "Stéphanie and Jenny are going to use new tools that allow us to visualize the connections between these neurons. Neurons are connected to each other by long processes called axons. We can follow out a neuron in one side of the brain through the axons connected to a neuron on the other side of the brain." Several different approaches will be used to visualize the axons.
The Mouse Model of Alzheimer’s Disease Project is being worked on by Kara Kittelberger, a senior research technician in Reijmers’ laboratory, in collaboration with the Department of Neuroscience laboratory of Giuseppina Tesco. An excessive accumulation of amyloid beta protein has been associated with Alzheimer’s disease. The researchers will inject amyloid beta protein into the brains of TetTag mice, then test their memories, which they expect will be impaired. They will then look at the TetTag markers of neural activity to see how the neurons that normally store the memory are affected by the presence of amyloid beta protein.
The TetTag technology has broad applications and so may be of interest to other researchers at Tufts. "We can express any transgene in neurons that are activated during a selected time window," says Reijmers. "We can replace the tauLacZ reporter with any transgene or add a transgene in addition to the tauLacZ reporter. Examples of other transgenes are fluorescent reporters, which allow for imaging of activated neurons in the living brain, and receptors that activate or silence neurons, which allow for the manipulation of specific neural circuits and thereby manipulation of memory retrieval and behavior. We use this to study neurons that store a memory, but this method can be used to study any neural circuit that is responsible for a specific function of the brain. We are already collaborating with groups that use this method to study pain perception, visual perception, and stress responses." Reijmers welcomes researchers interested in collaboration to contact him.