Stem Cells and Parkinson’s Disease
By Susan Garfinkel, Ph.D.

Parkinson’s disease (PD) is the second most common form of neurological disease. PD is caused by the progressive loss of the dopaminergic neurons, the cells in the brain that release dopamine, a chemical involved in the communication of information from one neuron to another and eventually to the muscles. As these cells die, this communication is lost and various symptoms appear, including tremor, rigidity or limb stiffness, hypokinesia (decreased muscle activity) and difficulties with gait and balance. The primary treatment for PD is replacement of dopamine with medications. Although these medications will relieve symptoms for a period of time, they will not slow or stop the natural progression of the disease, and symptoms can often reappear over time. Grafting of dopaminergic neurons from fetal tissue is being explored as an experimental solution to this problem. Studies using this technique have been conducted in about 400 patients worldwide and have shown some success in reducing the symptoms of the disease. However, the responses seen in different patients are very variable, there is a limited supply of fetal tissue and the ethical concerns over using aborted fetal tissue has driven scientists to study both embryonic and adult stem cells as an alternative source of dopamine producing neurons.

Many of the studies using embryonic stem cells to differentiate into dopaminergic neurons have been performed with mouse embryonic stem cells. Scientists have established various strategies to coax the cells to differentiate into neurons, including treatment of cell cultures with certain growth factors, introduction of specific genes into the cells that cause differentiation and growth of stem cells in culture with other cells types (feeder layers) to prompt differentiation. Methods have also been established to identify specific markers within the cells that confirm that dopaminergic neurons have been produced. The presence of tyrosine hydroxylase, for example, is an important marker since it is involved in the synthesis of dopamine. When the differentiated cells are transplanted into animals with PD-type lesions in the brain, the cells have been shown to survive for weeks and some behavioral improvement is seen. It is not yet clear how these studies will apply to human embryonic stem cells. The differentiation of human embryonic stem cells into neurons can occur, but the survival and function of dopaminergic neurons after grafting needs to be optimized and carefully assessed

Adult stem cells that come from already developed brain tissue have also been studied as a source of replacement cells for PD. Depending on the region of the brain from which the cells are isolated, these adult neural stem cells have slightly different properties. Like embryonic stem cells, scientists are working to establish the correct conditions to efficiently grow these neural stem cells and differentiate them into dopaminergic neurons. Many different techniques have been used; however, upon transplantation into animal models of PD the survival of these cells is low. Scientists are attempting to determine the signals in the brain environment may be required to allow the cells to survive, integrate and function correctly.

Adult stem cells from bone marrow may also be useful for PD. It has been suggested that these bone marrow derived cells are capable of changing into neuronal cells that can integrate in the brain. However, these data are controversial because other studies have suggested that the cells fuse with cells already existing in the brain. More studies are needed to determine the clinical value of these cells.

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Updated: March 31, 2005