Like the beams in a house infested with termites, certain neurons in people with Parkinson's disease slowly disintegrate, causing muscle tremors and stiffness. Now, researchers have repaired such degenerating neurons in mice using stem cells. The results could eventually lead to better treatments for humans.
People with Parkinson's disease slowly lose a group of neurons that help coordinate body movement. These neurons reside in a brain region called the substantia nigra and use dopamine to communicate with each other. Neuroscientists had previously used neural stem cells to replace dead neurons in brains with other kinds of damage, so neurobiologist Evan Snyder of Harvard Medical School in Boston and colleagues reasoned that neural stem cells could replace the dopamine neurons in mice with a Parkinson's-like condition.
To replicate Parkinson's disease, the team injected old mice with the chemical MPTP, which causes dopamine neurons to slowly disintegrate and eventually die. After a week, more than 60% of the dopamine neurons in the substantia nigra of the MPTP mice had stopped producing the neurotransmitter, and the mice had tremors and muscular rigidity. Then the team injected cultured neural stem cells into one side of the mouse brains. Three days later, a few mice started to walk in circles, indicating that neurons on one side of their brains were making dopamine. After 3 weeks, some of the stem cells had migrated to the other side of the brain, and the symptoms disappeared entirely. All of the MPTP-injected mice that received stem cells had doubled the number of dopamine neurons on both sides of their brain, compared to MPTP mice that didn't receive stem cells, the researchers report online 15 October in Nature Biotechnology. To the team's surprise, only 10% of the functioning dopamine neurons originated from the stem cells, indicating that the stem cells had somehow repaired the other 90% instead of replacing them.
"It's really surprising to see so much restoration," says neuroscientist Theo Palmer of Stanford University in Palo Alto, California. "That's, I think, unprecedented." Palmer says it's "pretty astounding" that the neurons can be rescued. Still, he cautions, many questions need to be resolved before the method can be applied to humans.