Resembling a seahorse, as its name suggests from the Greek words “hippos” (horse) and “kampus” (sea monster), the hippocampus is a brain region crucial for memory formation. But until recently, scientists were unable to link memory formation to distinct molecular signals.

Now a team of scientists from the Institute of Science and Technology Austria (ISTA) and the Max Planck Institute for Multidisciplinary Sciences has probably opened this black box. Their results are published In Biology PLOS.

Henry Gustav Molaison, known as patient HM, suffered from epilepsy. Riddled with seizures, he was referred to a surgeon, who located the epilepsy in the temporal lobe of his brain, the seat of the hippocampus.

On September 1, 1953, HM suffered brain surgeryremoving his hippocampus to treat his epilepsy. After surgery, the epilepsy and seizures disappeared, but HM developed serious side effects. He now suffered from anterograde amnesia, remembering all the events leading up to the operation but unable to form new memories. His case helped link the hippocampus to brain functions and memory formation.

Nowadays, the hippocampus is recognized as a crucial region of the human brain, involved in memory formation and spatial navigation. It converts short-term memory into long-term memory, making it easier to review personal experience.

In the new study led by Olena Kim, Yuji Okamoto and Magdalena Walz, professor of life sciences at the Austrian Institute of Science and Technology (ISTA) Peter Jonas, an international team of neuroscientists discovered new details about the molecular mechanisms that drive memory processing.

Scientists have precisely examined the mossy fiber synapse, a key connection point between specific nerve cells (neurons) in the hippocampus, by combining approaches to study its structure, essential molecules and functionality.

The center of memory

Inside the hippocampus, several types of neurons participate in memory processing. Granule cells, for example, are important for handling incoming information. “Granule cells receive various signals from other regions of the brain, which they need to further process and propagate,” explains Olena Kim, an ISTA graduate and now a postdoctoral fellow at the Academy’s Institute of Molecular Biotechnology (IMBA). Austrian Institute of Science (ÖAW).

These signals are transmitted via the granule cells‘axons – their arm-like extension, known as mossy fibers. These fibers form a point of contact with the pyramidal cells: the mossy fiber synapse. During this connection, messenger molecules in the form of neurotransmitters facilitate communication, ultimately triggering the formation and storage of memory.

Mossy fiber synapses are characterized by their high plasticity, which means that they can change their activity, structure and connections depending on stimuli. This adaptability helps the hippocampus process information correctly and distinguish between similar signals.

Kim gives an example: “Suppose you encounter a panther and a black cat simultaneously. Both appear black and feline. Yet you can distinguish one as a cat and one as a panther. Mossy fiber synapses play a key role in encoding and processing this information distinctive features, ultimately retrieving memory and information.

Close-up of mossy fiber synapses

The exact molecular details of how signal processing works in mossy fiber synapses are still unknown. In 2020, Peter Jonas, Carolina Borges-Merjane and Olena Kim set out to study the structure of mossy fiber synapses, using a new technique called “Flash and Freeze”, a powerful tool where neurons are frozen immediately after being stimulated.

“At the time, we were able to correlate structural changes in mossy fiber synapses with their functionality,” says Kim. “However, we wanted to take the technique further and examine not only the structure of synapses, but also the changes that occur at the molecular level when signals are processed.”

The scientists were particularly interested in two proteins located in the neurotransmitter release zone: the Cav2.1 calcium channels, which are crucial because the influx of calcium through these channels triggers the release of the neurotransmitter, and Munc13, a key factor , which suggests that the neurotransmitter is ready to be released.

“Before our study, all work on these two proteins was done with chemically fixed brain samples,” Kim continues. As these samples are not living, they do not provide information about dynamic processes. “For our new study, we were eager to use living brain tissue to preserve the dynamics, natural compositions and localization of these proteins.”

A surface similar to that of the moon

With the help of their ISTA colleagues Professor Ryuichi Shigemoto and scientist Walter Kaufmann, the scientists used the gel fracture marking technique. They chemically stimulated granule cells in mouse brain tissue samples to activate the memory formation process. Then the brain tissue was instantly frozen and split into two halves. The inner side of the cup represents the exposed surface of the interior tissue: a 3D imprint of the tissue at that precise moment, with proteins and molecules embedded.

After labeling Cav2.1 and Munc13 to make them visible, the researchers used an electron microscope to find their exact location. The images, resembling a close-up of the moon, revealed that upon stimulation, these two proteins rearranged themselves and moved closer together.

Further testing revealed that the rearrangement is closely correlated with the functionality of the mossy fiber synapse. Peter Jonas summarizes: “During activation, there are two changes. First, the number of vesicles near the membrane increases. Second, there is a nano-rearrangement of Cav2.1 and Munc13, making the synapses more powerful and precise. Both changes can contribute to memory training.”

The study highlights the relationship between structure and function at a key synapse in the hippocampus. Our memories often evoke vivid images. But so far we have failed to capture the molecular signals that trigger memory training. The present study lays the first stone in this regard.