In a step toward using stem cells to treat Parkinson's disease, scientists in South Korea have developed a consistent and efficient way to convert human embryonic stem cells into dopamine-producing nerve cells.

People with Parkinson's disease have lost the natural dopamine-making cells from one brain region, resulting in the disease's characteristic symptoms.

Transplanting the newly made nerve cells into the brains of rats with a Parkinson's-like condition relieved the rats' symptoms, the researchers report in the March 4 Proceedings of the National Academy of Sciences.

While scientists have made dopamine-producing neurons from stem cells before, yields were low and inconsistent. The cell clusters also contained immature cells that often developed into tumors.

"Our method generates the highest yield of dopamine cells from human embryonic stem cells ever reported," says lead scientist Dong-Wook Kim of the Stem Cell Research Center in Seoul. The rats treated by Kim's group showed no signs of brain tumors after 14 weeks, presumably because the implanted cell clusters contained a higher proportion of mature neurons and fewer immature cells, the team reports.

In repeated attempts, the resulting cell masses consistently contained about 66 percent dopamine-producing neurons. With previous methods, percentages ranged from 10 to 30 percent and varied from experiment to experiment.

The team combined existing techniques in a novel way. After transforming stem cells into an intermediate type of cell called neural progenitors, the researchers expanded the number of cells by growing them in dishes. The team teased the cells apart and added signaling proteins to finish the conversion to neurons.

"With this protocol, you have a good shot to get enough cells to do this in primates and potentially in humans," comments Kai-Christian Sonntag of the Center for Neuroregeneration Research at Harvard Medical School in Belmont, Mass.

Kim says that his group plans to improve the efficiency even further and to ensure that immature cells will not eventually form tumors.

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