“This finding puts us in a new and important position to exploit the potential of stem cell-based therapies to improve brain function in neurodegenerative diseases such as Alzheimer’s that are accompanied by a loss of memory,” Evans says.

In an earlier collaboration, Evans and Gage had discovered that TLX, a so-called orphan receptor is crucial for maintaining adult neural stem cell in an undifferentiated, proliferative state. Orphan receptors are structurally related to the well-known hormone receptors that mediate steroid and thyroid signaling. In contrast, a TLX regulatory molecule has not yet been identified.

Now, the Salk team wanted to learn more about TLX’s biology and function. However, the global deletion of TLX leads to a variety of developmental problems, so postdoctoral fellow and first author Chun-Li Zhang, Ph.D., had to devise a strategy that would allow them to control when to shut off the gene coding for TLX in neural stem cells kept in Petri dishes as well as in live animals. When he cultured mouse neural stem cells without the gene encoding TLX, the proliferation rate of these cells plummeted and the activity of hundreds of genes changed.

Explains Zhang, “This experiment confirmed that TLX specifically induces the genetic program necessary for maintaining neural stem cells in their stem-like state,” handing the Salk researchers the perfect tool to track the contribution of newborn neurons to normal brain function — a question Gage is particularly interested in.

“In the past, methods to knock out neurogenesis, such as radiation and mitotic inhibitors that block all cell division have been rather crude,” he says. “So, maybe not surprisingly the literature is riddled with contradictory results.”

Adult neural stem cells continually generate new brain cells or neurons in two small areas of mammalian brains: the olfactory bulb, which processes odors, and the central part of the hippocampus, which is involved in the formation of memories and learning. Some of these newborn cells die shortly after they are born but many of them become functionally integrated into the surrounding brain tissue. Whether they live or die is regulated by the animals’ experience.

Combining mouse genetics and gene transfer techniques, Zhang genetically engineered mice that allowed him to specifically delete TLX in the brains of adult mice and thus shut down neurogenesis. He then put the mice through a battery of standard behavioral tests.

The mice passed with flying colors in all but one test: the Morris water maze, a common behavioral test in which mice have to rely on visual cues on the surrounding walls to find and remember the location of a submerged platform hidden in a pool of milky water. This task draws on many cognitive abilities, including analytical skills, learning and memory, and the ability to form strategies.

The more challenging Zhang made the test, the more difficult the altered mice found it to navigate the maze and remember the location of the platform. “The mice showed both learning and memory deficits,” he says. “It’s not that they didn’t learn, they were just slower at learning the task and didn’t retain as much as their normal counterparts,” observes Zhang.

“Whatever these new neurons are doing it is not controlling whether or not these animals learn,” explains Gage. “But these new cells are regulating the efficiency and the strategy that they using to solve the problem.”

Research assistant Yuhua Zou, M.Sc., and postdoctoral researcher Weimin He, Ph.D., both in the Gene Expression laboratory at the Salk also contributed to the study.

The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.