Stem cells are a renewable source of tissue that can be coaxed to become different cell types of the body. The best-known examples are the embryonic stem (ES) cells found within an early-stage embryo. These cells can generate all the major cell types of the body (they are "pluripotent"). Stem cells have also been isolated from various other tissues, including bone marrow, muscle, heart, gut and even the brain. These "adult" stem cells help with maintenance and repair by becoming specialized cells types of the tissue or organ where they originate. For example, special stem cells in the bone marrow give rise to all the various types of blood cells (similar blood cell-forming stem cells have also been isolated from umbilical cord blood).
Because adult stem cells become more committed to a particular tissue type during development, unlike embryonic stem cells, they appear to only develop into a limited number of cell types (they are "multipotent").
In addition to ES cells, induced pluripotent stem (iPS) cells, discovered in 2007, represent an important development in stem cell research to treat diseases like Parkinson's disease. Essentially, iPS cells are "man-made" stem cells that share ES cells' ability to become other cell types. IPS cells are created when scientists convert or "reprogram" a mature cell, such as a skin cell, into an embryonic-like state. These cells may have potential both for cell replacement treatment approaches in patients and as disease models that scientists could use in screening new drugs.
IPS cell technology is somewhat related to a previous method called somatic cell nuclear transfer (SCNT) or "therapeutic cloning" (the technology that gave us Dolly the Sheep). Unlike the iPS cell approach, which converts adult cells directly into stem cells, SCNT involves transferring the genetic material of an adult cell into an unfertilized human egg cell, allowing the egg cell to form an early-stage embryo and then collecting its ES cells (which are now genetic "clones" of the person who donated the adult cell). To date, however, this has not been successfully demonstrated with human cells and iPS cell methods may be replacing SCNT as a more viable option.
A potentially exciting use for iPS cells is the development of cell models of Parkinson's disease. In theory, scientists could use cells from people living with Parkinson's disease to create iPS cell models of the disease that have the same intrinsic cellular machinery of a Parkinson's patient. Researchers could use these cell models to evaluate genetic and environmental factors implicated in Parkinson's disease.
Stem cell research has the potential to significantly impact the development of disease-modifying treatments for Parkinson's disease, and considerable progress has been made in creating dopamine-producing cells from stem cells. The development of new cell models of Parkinson's disease is a particularly promising area of stem cell research, as the current lack of progressive, predictive models of Parkinson's disease remains a major barrier to drug development. Cell models of Parkinson's disease generated from stem cells could help researchers screen drugs more efficiently than in currently available animal models, and study the underlying biological mechanisms associated with Parkinson's disease in cells taken from people living with the disease.
However, there are many challenges that need to be overcome before stem cell-based cell replacement therapies for Parkinson's disease are a reality. Work is still needed to generate robust cells, in both quality and quantity, that can also survive and function appropriately in a host brain. Although ES (and now iPS) cells hold great potential, we do not yet know which stem cell type ultimately holds the greatest promise. Thus, researchers require scientific freedom to pursue research on all types -- including ES, adult and IPS cells -- in order to yield results for patients.
The Michael J. Fox Foundation played an early role in supporting work in stem cell research for Parkinson's disease, including funding the original proof of principle demonstrating that ES cells could provide a robust source of dopamine neurons. Since that time, significant other funding resources -- at both the state and federal levels -- have been unleashed to support the whole field, allowing the Foundation to continue to target strategic funding in other critical areas of developing therapies for Parkinson's disease. The Foundation will continue to monitor Parkinson's disease specific stem cell developments for opportunities where the Foundation can help in advancing this research.