NEW YORK (July 28, 2006) -- Almost every neurological activity depends on the passage of information between brain cells via the synapse.

Now, biochemists at Weill Medical College of Cornell University in New York City have identified a key event in endocytosis -- a biochemical process that's crucial to keeping the synapse on track.

"Since all thought, emotion and action relies on the synapse, our new finding could have broad implications for research into brain function and dysfunction, as well as the development of drugs targeting this vital mechanism," says senior researcher Dr. Timothy Ryan, professor of biochemistry at Weill Cornell Medical College.

Reporting July 20 in the journal Neuron, his team used cutting-edge technology to illuminate endocytotic processes. The research was featured in Neuron and was selected for commentary in a separate "Preview" section of the issue.

"Endocytosis involves the shuttling back and forth of vesicles -- sac-like objects -- filled with nutrients, neurotransmitters or other compounds; usually from the cell surface to the cell interior," explains Dr. Tomas Fernandez-Alfonso, who led the research while a graduate student at Weill Cornell. He is now a postdoc at University College London.

Once they've dropped off their important cargo of neurotransmitters, synaptic vesicles aren't just thrown away, however.

Instead, they're recycled, including a full replenishment of all eight types of intracellular signaling proteins essential to endocytosis.

But, according to Dr. Ryan, "That left us with a question: During recycling, just where is this new batch of proteins coming from? Is the vesicle somehow reusing the same material, or is it picking up a new supply from outside?"

To help solve that riddle, he and Dr. Fernandez-Alfonso, along with research technician Ricky Kwan, focused on two of the most abundant proteins found in the vesicle, VAMP-2 and synaptotagmin Ia (sytIa).

Both are active on the vesicle's outer membrane. SytIa helps initiate the correct timing for the vesicle to fuse with the cell's membrane, while VAMP-2 is a key player in that fusion process.

In laboratory experiments involving live brain cells obtained from mice, the team engineered each of these two vesicular proteins to carry a special micro-fluorescent tag.

"That meant we could literally spot the presence of these proteins throughout the endocytotic cycle," says Dr. Ryan.

Using high-intensity light, they also "bleached out" the tag under certain conditions, but not others.

"This allowed us to track the movement of VAMP-2 or sytIa, to see if the proteins traveled from once place to another, or stayed put," Dr. Ryan explains.

Their key finding came as a bit of a surprise.

"We discovered that synaptic vesicles interchange their own VAMP and sytIa with that from a large pool lying on the surface membrane of axons -- the long skinny branches that extend out from neurons," Dr. Fernandez-Alfonso says.

"The vesicle releases about half of its VAMP-2 and sytIa into this pool just after fusion," he explains. "The new, recycled vesicle is then able to draw upon this reservoir to help it with the next round of endocytosis."

Work with fluorescent-tagged VAMP-2 confirmed that this relationship involves an active interchange of vesicular proteins with those pooled on the axonal surface.

"The VAMP-2 hadn't just been sitting there -- it had been dropped off before, just after fusion, and was waiting to be picked up by the newly recycled vesicle," Dr. Ryan explains.

Further work with sytIa -- this time tagged with a fluorescent antibody -- also revealed a pool of the protein sitting on the axon.

"The reservoir was there even when we didn't stimulate the cell," Dr. Fernandez-Alfonso adds. "Then, when we did stimulate the neuron, we could actually watch this reservoir of sytIa get concentrated and then internalized within the synaptic vesicle."

The research team stresses that they can't be sure all eight of the protein types necessary for endocytosis are replenished in this way, although it does seem probable. "Further research will clarify if there are exceptions," Dr. Ryan says.

He believes the work provides another crucial piece to the endocytosis puzzle.

"This is exciting, basic science, but it will also have clinical implications down the line," Dr. Ryan says. "Remember, the synapse is an incredibly important point of information control in the brain, and most neurologically active drugs -- either of abuse or for therapeutic use -- work on the synapse."

"Most likely, any aberration in vesicular recycling and endocytosis can and will have downstream effects in terms of neurological function," he explains. "Clues like this one, that can help us better understand and control these processes, are bound to be helpful going forward."

This work was funded by grants from the National Institutes of Health.