Salk Institute scientists have observed for the first time that new cells in the adult brain grow and mature over time, functioning just like any of their neighboring neurons.
The study offers proof that newly born cells integrate into existing neuronal circuitry, providing the brain with a continual reservoir of youthful active cells. Such cells might then replace older neurons or possibly be used to reshape the brain so it may learn and adapt to new experiences.
"This is the first demonstration that new cells that are born in the adult brain are functional," said Fred Gage, a professor at Salk and senior author of a study appearing in today's issue of Nature that describes these results.
It was Gage who, in November 1998, discovered that adult humans, even among the elderly, can generate new brain cells throughout life in a process called neurogenesis. This landmark study upset long-held dogma that stated we are born with a full supply of brain cells that steadily diminish throughout our lives.
Subsequent studies by Gage and his colleagues revealed that the number of new brain cells could be influenced by activity and other environmental stimuli. For example, the Gage team showed that running mice grow more brain cells in a region important for learning and memory -- the hippocampus - than their sedentary counterparts.
Despite such work, scientists still did not know if these new cells actually worked like any other neuron, or even if they grew and matured like other brain cells. The current study, which required the development of a new technique to measure electrical activity in living brain cells, should put those doubts to rest.
Until now, virtually all neurogenesis studies have relied on a method that used certain markers that are taken up and integrated into the DNA of dividing cells. As these immature cells grow over time, the markers then offer a visual representation of that cell's anatomical fate. Thus, months later, scientists can see if the cell ultimately becomes a neuron or something else.
However, this technique doesn't say anything about cell function. Nor does it provide a graphic picture of how, or even if, cells mature to look like any other fully integrated neuron. Some have speculated that these new brain cells die shortly after birth without maturing into normal cells.
To answer these questions, Gage and his team needed to invent a technique that would allow them to view living brain cells as they mature, while they conduct other long-accepted tests for electrical activity.
The group turned to retroviruses, which are known to integrate into dividing cells; however, these organisms are also known to shut down during cell division. To overcome this obstacle, the Gage team engineered new retroviruses whose "on switch" remains active throughout this process and subsequent cell divisions. This altered retrovirus, combined with a glowing marker called green fluorescent protein, or GFP, was then injected into laboratory animals.
At first, they couldn't infect enough brain cells to get clear results.
"We knew we needed to increase the number of cells that are dividing," said Henriette van Praag, a staff scientist in Gage's lab and lead author of the study, supported by grants from the National Institutes of Health, the Christopher Reeve Paralysis Foundation and The Lookout Fund.
"So we thought of the running wheel. If we put the animals in a running wheel, as we learned from previous experiments, we would increase the probability of getting more infected cells. And that's what happened.
"After we injected these animals with the virus, we started seeing many, many more cells."
Four weeks later, the scientists viewed the results under a fluorescent microscope. The green protein taken up by the newly born brain cells were now revealing the anatomy of young neurons, with long axons that transmit electronic messages and dendrites, the antenna that receive signals from the environment.
Then, to see if these cells actually functioned, the scientists turned to a device called a patch clamp that monitors electrical activity. Elelctrophysiological studies conducted by Alexandro F. Schinder and Brian R. Christie, co-authors of the study, showed that these cells generated and received electrical impulses, almost like any other neuron.
"All the electrophysiological parameters of this one-month-old cell were equivalent to cells that were unlabeled, except capacitance which measures volume," said Gage, "It turns out that the volumes were relatively small in these one-month old cells.
"We didn't think our virus would last four months or longer, but we had a separate set of animals, four months old, and so we looked at them."
To the team's surprise, the virus in these older animals survived, and under the microscope, revealed larger and more mature neurons than their youthful counterparts, with thick forests of complex dendrites indistinguishable from other mature neurons.
"The difference that we see between one month and four months reflects the dynamic nature of these cells and the dynamic nature of the central nervous system," said Gage. "This is truly an exciting development."
In their next studies, the Gage team hopes to determine what these new neurons actually do.
"One possible hypothesis is that new neurons may be required to replace dying neurons," he said.
"Another possibility is that young neurons provide a greater degree of plasticity to the mature brain. This enhanced plasticity would become apparent from the integration of new functional units whose connectivity may be shaped by experience."
Also participating in the study were Nicolas Toni and Theo D. Palmer.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation. - By Suzanne Clancy
[Contact: Suzanne Clancy, Warren Froelich]