Another approach to treating Parkinson's disease is to implant cells to replace those lost in the disease. Researchers are conducting clinical trials of a cell therapy in which human retinal epithelial cells attached to microscopic gelatin beads are implanted into the brains of people with advanced PD. The retinal epithelial cells produce levodopa. The investigators hope that this therapy will enhance brain levels of dopamine.
Starting in the 1990s, researchers conducted a controlled clinical trial of fetal tissue implants in people with PD. They attempted to replace lost dopamine-producing neurons with healthy ones from fetal tissue in order to improve movement and the response to medications. While many of the implanted cells survived in the brain and produced dopamine, this therapy was associated with only modest functional improvements, mostly in patients under the age of 60. Unfortunately, some of the people who received the transplants developed disabling dyskinesias that could not be relieved by reducing antiparkinsonian medications.
Stem Cells in Parkinson's Research
Another type of cell therapy involves stem cells. Stem cells derived from embryos can develop into any kind of cell in the body, while others, called progenitor cells, are more restricted. One study transplanted neural progenitor cells derived from human embryonic stem cells into a rat model of PD. The cells appeared to trigger improvement on several behavioral tests, although relatively few of the transplanted cells became dopamine-producing neurons. Other researchers are developing methods to improve the number of dopamine-producing cells that can be grown from embryonic stem cells in culture.
Researchers also are exploring whether stem cells from adult brains might be useful in treating Parkinson's. They have shown that the brain's white matter contains multipotent progenitor cells that can multiply and form all the major cell types of the brain, including neurons.