For the first time, University of Chicago researchers, using a newly developed technique, report that a molecular mechanism involved in formation of limbs and other body structures is also used to organize the higher functions of the brain.
Specific brain functions, such as vision, touch and memory, are set out in a pattern of distinct areas in the outer layer of the brain, the cerebral cortex. Because these areas can only be seen after birth, it has been impossible until now to determine the molecular mechanisms responsible for forming them in the embryo. "We knew that signaling proteins associated with patterning other parts of the body found in the embryo cortex, but we did not have an easy way to find out what they were doing there," said Elizabeth Grove, assistant professor of neurobiology, pharmacology and physiology at the University of Chicago and principal investigator in the study. "There has been some speculation that cortical patterning depended on completely unique mechanisms."
In the September 20, 2001 edition of Science Express, the researchers report that manipulating one of these signaling proteins in the developing mouse brain causes radical changes in the cortex. Tomomi Fukuchi-Shimogori, Ph.D., research associate in neurobiology, pharmacology and physiology at the University of Chicago, and lead author of the paper, developed a microsurgical technique that allows her to insert genes into the cortex of mice while they are still in utero, about halfway through gestation. The mice are born normally and can be analyzed at any age. Fibroblast Growth Factor 8 (FGF8), a member of a family of signaling proteins involved in forming other structures in the embryo, is normally found near the front of the developing cortex. Using this technique, the researchers were able to manipulate the amount and position of this signaling protein in the embryo and look for changes in the cortical pattern much later. The researchers increased the amount of the signaling protein in its normal position, decreased it by inserting a gene for a receptor able to soak up the protein, or expressed it in a new position. Each manipulation profoundly affected cortical area pattern.
"We found very strong evidence that this signaling protein directs the development of the cortex," said Fukuchi-Shimogori. With increased expression of this protein the sizes and locations of the areas changed. Areas that are towards the front of the cortex and closer to the source of the molecule were enlarged at the expense of areas further away from the source. Reducing the signaling protein caused shifts in the opposite direction. "Most dramatic, when a new source of the signaling protein was generated close to the back of the embryonic cortex, the whole program changed," said Grove. "Now, a region near the back of the cortex was reprogrammed to form a duplicate of a more central region, a second touch area. We saw a identical array of patches that correspond on a one-to-one basis with the mouse's whiskers." (see illustration) The generation of a new cortical area by a molecular manipulation has not been seen before and may provide a clue about how the cerebral cortex changes in evolution. One way that evolution seems to generate more functionally complex brains is by adding new areas to the cortex.
"We have had no idea how evolution achieved this kind of change," said Grove. "So it is exciting to find that you can add a new area by modifying signaling by a single protein."