How do the cells in a developing brain know what they'll be when they grow up? How does a tiny, shapeless mass of embryonic cells turn into a human brain? To watch a brain develop in a mammalian embryo, a scientist would have to be able to see through the mother's womb as well as through the developing embryonic skull. For this reason, scientists tend to study these processes in proxy organisms such as the zebrafish. "The zebrafish is a vertebrate, like a human," says Dr. Gil Levkowitz of the Molecular Cell Biology Department. "But the zebrafish embryo develops inside a clear egg, outside the mother's body, and we can observe its brain tissue taking shape under the microscope."
Using genetically engineered fluorescent tags, Levkowitz is able to follow the genes and their proteins that play central roles in brain development in "five dimensions." "We observe how nerve cells are formed in the brain over time and in the three dimensions of space – as well as in a 'fifth dimension' involving interactions between the different types of cell and the areas in which they develop."
Levkowitz and his team focus on a group of cells that produces dopamine – a chemical messenger in the brain that plays an important role in feelings of reward, motor activity and emotions. Malfunction in brain circuits that use dopamine for communication is associated with such diseases as Parkinson's and schizophrenia. The brains of zebrafish contain only a few dozen of these dopamine-producing (dopaminergic) cells (as opposed to hundreds of thousands in a human brain), making them ideal subjects for the study of individual cells. What determines how many cells will be in a brain? The first clue came during Levkowitz's postdoctoral research, in which he discovered that damage to a gene called fezl reduces the number of dopaminergic cells in fishes' brains.
In a follow-up study that recently appeared in the scientific journal Development, Levkowitz, research students Niva Russek-Blum and Amos Gutnick and Drs. Helit Nabel-Rosen and Janna Blechman, together with scientists from King's College London and the University of Utah, scanned a number of proteins that are important for proper brain development to see whether they affect the number of dopaminergic cells. One such protein, called Wnt, is known to be active in many normal developmental processes, as well as in the progression of cancer.
The researchers found that blocking the activity of Wnt raises the dopamine-producing cell count, and this gene also appears to regulate the activities of the fezl gene Levkowitz had previously found. "The final cell amount is determined by the balance between the fezl protein, which increases cell number, and Wnt, which restricts it. We still don't understand the exact mechanism; however, we found that the size of the dopaminergic cell population originates in regulation at the embryonic stem cell stage," says Levkowitz.
Prospective therapies for neurodegenerative diseases, including Parkinson's, are based on "cell-replacement" therapies using such dopaminergic stem cells. Levkowitz believes that a better understanding of the factors regulating the development of dopamine-producing cells will contribute to a better understanding of and eventual treatment for these diseases.
Dr. Gil Levkowitz's research is supported by the Helen and Martin Kimmel Institute for Stem Cell Research; the Dekker Foundation; and the Minna James Heineman Stiftung. Dr. Levkowitz is the incumbent of the Tauro Career Development Chair in Biomedical Research.