For the first time, researchers have developed a way to view stem cells in the brains of living animals, including humans—a finding that allows scientists to follow the process neurogenesis (the birth of neurons). The discovery comes just months after scientists confirmed that such cells are generated in adult as well as developing brains.
"I was looking for a method that would enable us to study these cells through[out a] life span," says Mirjana Maletic-Savatic, an assistant professor of neurology at Stony Brook University in New York State, who specializes in neurological disorders such as cerebral palsy that premature and low-weight babies are at greater risk of developing. She says the new technique will enable her to track children at risk by monitoring the quantity and behavior of these so-called progenitor cells in their brains.
The key ingredient in this process is a substance unique to immature cells that is neither found in mature neurons nor in glia, the brain's nonneuronal support cells. Maletic-Savatic and her colleagues collected samples of each of the three cell types from rat brains (stem cells from embryonic animals, the others from adults) and cultured the varieties separately in the lab. They were able to determine the chemical makeup of each variety—and isolate the compound unique to stem cells—with nuclear magnetic resonance (NMR) spectroscopy. (NMR helps to determine a molecule's structure by measuring the magnetic properties of its subatomic particles.) Although the NMR could identify the biomarker, but not its makeup, Maletic-Savatic speculates it is a blend of fatty acids in a lipid (fat) or lipid protein.
After pinpointing their marker, the team ran two tests to determine the method's sensitivity and accuracy: First, they injected a bevy of stem cells into a rat's cerebral cortex, an outer brain layer where neurogenesis does not normally occur. They then passed an electric current through the animals' brains; electric currents induce neurogenesis in the hippocampus, a forebrain structure that is one of two sites (the other being the subventricular zone) where new neurons are believed to arise.
After performing each procedure, the team used NMR spectroscopy to capture images of the living rats' brains. There was, however, too much visual interference on the scans to find their biomarker. The researchers called upon Stony Brook electrical engineering professor Petar Djurić to help them come up with an algorithm to cut through the clutter and glean a clear picture of their target compound.
With the analytical method helping to decode their scans, they could clearly see increased biomarker levels in the cortex after a neural stem cell injection. Similarly, after the animals were given electric shocks, levels of the compound clearly went up in the hippocampus.
The team next turned its attention to humans, enlisting 11 healthy volunteers, ranging in age from eight to 35, who each spent 45 minutes in an NMR scanner. Hippocampal scans turned up more of the marker than the cortical images. In addition, the older subjects showed lower levels of the biomarker than younger ones (a finding consistent with earlier studies). "This is the first technique that allows detection of these cells in the living human brain," says Maletic-Savatic.
Fred Gage, a genetics professor at the Salk Institute for Biological Studies in La Jolla, Calif., and co-author a 1998 report in Nature Medicine that announced the discovery of neurogenesis in the adult human brain, praises the new approach. "It seems that they are measuring proliferation rather then maturation based on their results," he says. "It will be important for them to knock down neurogenesis in a mouse and show that [this] signal disappears to confirm the causal link with neurogenesis."