MONDAY, April 7 (HealthDay News) -- Transplanted neurons derived from reprogrammed adult skin cells successfully integrated into the brains of fetal mice and reduced symptoms of a Parkinson's disease-like condition in adult rats, say U.S. researchers.

"This is the first demonstration that reprogrammed cells can integrate into the neural system or positively affect neurodegenerative disease," study author Marius Wernig, a postdoctoral researcher at the Whitehead Institute for Biomedical Research in Cambridge, Mass., said in a prepared statement.

The induced pluripotent stem (IPS) cells were created by reprogramming adult skin cells using retroviruses that expressed four genes (Oct4, Sox2, c-Myc and Klf4) into the cells' DNA. The IPS cells were then differentiated into neural precursor cells and dopamine neurons.

In one experiment, the neural precursor cells were transplanted into the brains of mouse embryos. When the researchers analyzed the mice nine weeks after birth, they found the transplanted cells had formed clusters at the area they'd been injected and then migrated into surrounding brain tissue.

The researchers also found the migrating neural precursor cells had differentiated into several subtypes of neural cells and had functionally integrated into the brain.

In another experiment, differentiated dopamine neurons were grafted into the brains of rats in which the dopamine-producing neurons on one side of the brain had been destroyed. This is a model of Parkinson's disease in humans, which is caused by insufficient levels of dopamine.

Four weeks after the surgery, eight of the nine rats that received the dopamine neuron transplants showed improvement. Eight weeks after transplantation, the dopamine neurons had extended into the surrounding brain tissue.

The study was published in this week's online issue of the Proceedings of the National Academy of Sciences.

While this work shows promise, much more research needs to be done before there's any likelihood that it will lead to new treatments for humans.

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