What talks without being seen or heard?
It sounds like a riddle, but neuronal communication had perplexed biochemists until a few years ago, when scientists discovered ways in which neurons talk to each other — not with cell phones or instant messaging, but through the exchange of virus-like particles. Now, researchers from the University of Illinois have elucidated which pathway is used to facilitate this communication in one essential protein. Their findings are published in PNAS.
Although most neurons communicate by releasing and receiving neurotransmitters, others communicate by delivering materials between cells with the help of an extracellular vesicle. Kritika Mehta (PhD, ’24, biochemistry), currently a postdoctoral researcher at Weill Cornell Medicine and lead author of the paper, became interested in the role of activity-related cytoskeleton-associated protein, or Arc, in facilitating cellular communication and trafficking.
“I was interested in the trafficking of proteins — how they transport through the different regions of the cell,” Mehta said. “Through this research, we’ve discovered that proteins are able to escape the cell. And I wanted to find out what mechanism within the cell was regulating the protein.”
Mehta began by directly studying Arc, which shares a structure similar to Gag, a viral protein that plays a key part in the assembly of retroviruses like HIV. She was surprised to learn that neuronal cells can use Arc to form small capsids, which are then packaged inside a lipid vesicle and released out of the cytosol as an extracellular vesicle. Further investigation revealed that the extracellular vesicles can be released into the outside space of a cell through the multivesicular body (MVB) pathway.
“This was interesting because many viruses also utilize the same pathway to assemble inside a cell and prepare to infect the body,” said Kai Zhang, Associate Professor of Biochemistry and a corresponding author of the paper. “The MVB essentially becomes a capsid-carrying structure before rising to the cell surface and releasing outside of the membrane.”
Because Arc behaves like a virus but lacks toxicity, it can be used as a safe gene delivery tool. It is also associated with memory formation, and deciphering its pathway may be important for neuronal function and communication.
Scientists have historically assumed that Arc functions similarly to the HIV Gag protein, which regulates assembly using phosphatidyl inositol (4,5) bisphosphate (PI(4,5)P2) phospholipids. But the Illinois researchers unexpectedly learned that Arc assembly is actually mediated by Phosphatidylinositol 3-phosphate (PI3P), the only lipid to which it has an affinity.
“You can think about MVB as a big bag containing smaller vesicles,” Zhang said. “It’s actually four layers of lipids. And what we found is that this Arc protein likely assembles the capsid within this pathway. Eventually, when the membrane of the MVB fuses with the plasma membrane, the intraluminal vesicle containing the Arc capsule is released.”
The researchers also used a type of super-resolution imaging called MINFLUX, acquired by the NSF Science and Technology Center for Quantitative Cell Biology, to capture individual vesicles within MVB — a task that can typically only be done with electron microscopy. This imaging modality allowed Mehta and Zhang to resolve multiple Arc-containing vesicles inside an MVB with a size comparable to that resolved by electron microscopy.
Going forward, Zhang’s lab will zero in on the biochemistry of Arc protein to determine how it functions at a molecular level.
“This isn’t just a biochemical study,” Zhang said. “It’s a combination of biochemistry, cell biology, live cell imaging within mammalian cell line and primary neuronal cells. Obtaining data from animal models is an eventual goal of the project.”
Nien-Pei Tsai, Associate Professor of Molecular and Integrative Physiology, is a co-author of the paper.
