Watching Memory Formation Uncovers New Biological Processes

Neuroscientists from Johns Hopkins University are using total internal reflection fluorescence microscopy to essentially watch physical neural processes as memories form. Their observations have uncovered that a protein called 4.1N helps the AMPA receptor protein form connections in the brain that are responsible for our memories. Because the involvement of 4.1N was not known before, the new knowledge may help in developing diagnostic tests and treatments for memory disorders like Alzheimer’s.
From Johns Hopkins:

The research team first attached fluorescent tags to AMPA receptor proteins in rat neurons growing on glass slides. Using a laser, they first photobleached the cells to prevent anything from glowing except for newly inserted proteins. When they looked at the slides under a microscope they saw dots appearing on the surface of the neurons, which they interpreted to be the AMPA receptors being inserted into the cell’s surface.
“In the past it was like looking for a shooting star in a bright, night sky full of other stars,” says neuroscience research associate Da-Ting Lin, Ph.D. “Ideally you need a dark sky to clearly see that shooting star, and we’ve now made our dark sky and are starting to find our shooting stars.”
They then asked what other proteins might be leading AMPA receptors to where they should be once they reach the cell’s surface. To do this, they chopped a short bit off the end of the AMPA receptor, put it into neurons and watched the cells under the microscope to see if those same dots appeared. After cutting off more and more and testing each version, they finally ended up with an AMPA receptor that was not inserted into the cell surface. It turns out they had cut off the region of AMPA receptor known to bind another protein, called 4.1N.
“We knew that 4.1N and related proteins can bind to the cell’s skeleton as well as to proteins at the cell surface, but we never knew what role 4.1N played in AMPA receptor movement,” says Lin.
To see if 4.1N is critical for forming memories, the team stimulated neurons and measured electrical connections—signs of memory formation—from neurons containing and missing 4.1N. Neurons containing 4.1N had sustained strong electrical connections while neurons missing 4.1N initially had strong electrical connections but they weakened after 30 minutes.
“Normally, this type of neuronal stimulation makes the cell connections stronger,” says Lin. “But in cells missing 4.1N we see an initial strong connection but it doesn’t hold. So we think 4.1N is required to keep the strong connection going, and therefore make the memory stick.”

Here are clips visualizing AMPAR Insertion with TIRF imaging:

AMPAR Insertion and Diffusion on dendrites, high resolution. Magenta: dendrite and spines morphology; Green: Insertion of AMPAR:

Press release: JOHNS HOPKINS NEUROSCIENTISTS WATCH MEMORIES FORM IN REAL TIME