Understanding the molecular basis of these processes, and how they can go wrong, may ultimately lead to treatments for many birth defects, such as spina bifida that afflicts between 800 to 1,000 babies born each year in the United States.
Writing in the August issue of the scientific journal Nature Cell Biology, the researchers report the discovery that a single protein facilitates the movements of cells within the developing embryo of the zebrafish, a small fish that has become an important animal model for studying the development of vertebrates, animals with backbones.
The researchers report that this protein plays an essential role in directing the migration of cells within the spherical egg to the head-tail axis where the body is beginning to take shape. They also found that disruption of the same protein inhibits the normal migration of nerve cells within the developing zebrafish brain, a type of motion found in human brain development.
"A great deal is known about the movement of the projections that neurons send out to connect with other neurons, but very little is known about how neurons move from one place to another," says Lilianna Solnica-Krezel, the associate professor of biological sciences at Vanderbilt who led the study with Anand Chandrasekhar, assistant professor of biological sciences at the University of Missouri, Columbia.
Zebrafish have characteristics that make them ideal for developmental research. They lay eggs that are transparent and develop outside the body, making them particularly easy to study. Development is also rapid, proceeding from fertilization to hatching in only three days. The fish are also easy and inexpensive to raise, so scientists can keep thousands of them in a laboratory. The zebrafish genome is currently being sequenced, which allows researchers to employ the powerful tools of genomics to unravel the complex molecular processes involved in development.
One of these methods is to examine the impact of specific mutations. In this case, Solnica-Krezel and her colleagues were exploring what takes place in a mutant called trilobite. (It was given this name because the developing egg forms a pattern shaped like one of these prehistoric marine creatures.) During an early stage of development called gastrulation, the cells begin converging from all sides of the spherical egg to the embryonic axis where the body begins to form. What begins as a disordered, chaotic motion changes into an orderly movement. As this happens the cells also change from a round to an elongated, spindle shape.
"It's something like a mob transforming into an army," says Solnica-Krezel.
Her research group discovered that the trilobite mutations prevent the army from forming. Cell motions continue to be disordered and do not develop the same sense of direction and purpose in the mutant as they do in normal embryos. As a result, trilobite's development is stunted. The scientists determined that the mutations disrupt the activity of a specific membrane protein, called either Strabismus or Van Gogh.
The same protein has previously been identified in the development of the fruit fly, Drosophila melanogaster, where it affects the orientation of cells that form the fly's wings and compound eyes. A closely related protein found in mice is implicated in malformation of the neural tube, the tubular structure that develops into the brain and spinal cord.
Somewhat later in zebrafish development, a number of motor neurons move from one part of the brain to another. "We don't understand why they move because they can form the connections they need from their original location," says Solnica-Krezel. But Chandrasekhar and his Missouri team discovered that this movement does not take place in trilobite embryos.
In order to determine whether the neurons' failure to migrate was due to factors within the cell or the extracellular environment, the researchers transplanted trilobite neurons in the brains of normal embryos and normal neurons in trilobite brains. They found that none of the normal motor neurons migrated when placed in a trilobite brain, whereas a third of the trilobite neurons migrated when placed in normal brains. This led the scientists to conclude that the Strabismus/Van Gogh protein must have both cellular and extracellular effects.
With further study, the researchers determined that the neurons' method of movement was similar to that of an amoeba: they extend their bodies in the direction they want to move and retract them from the opposite side. By labeling the nerve cells with fluorescent protein, the biologists determined that the trilobite cells moved much slower and their movements were more random in nature than normal neurons.
The results of their various tests suggest that the protein Strabismus/Van Gogh acts independently in mediating neuron movement. If this proves to be the case, then it provides "an entry point to elucidate the molecular basis of this class of neuronal migration," they conclude in the article.
Solnica-Krezel's research team included graduate student Florence Marlow along with research associates Jason R. Jessen, Jacek Topczewski and Diane S. Sepich. Graduate student Stephanie Bingham worked with Chandrasekhar. The research was funded by the National Institutes of Health, the National Science Foundation and the Pew Scholars Program in the Biomedical Sciences.
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