In recent years, Parkinson's research has advanced to the point that halting the progression of PD, restoring lost function, and even preventing the disease are all considered realistic goals. While the ultimate goal of preventing PD may take years to achieve, researchers are making great progress in understanding and treating PD.
One of the most exciting areas of PD research is genetics. Studying the genes responsible for inherited cases can help researchers understand both inherited and sporadic cases of the disease. Identifying gene defects can also help researchers understand how PD occurs, develop animal models that accurately mimic the neuronal death in human PD, identify new drug targets, and improve diagnosis.
As discussed in the “What Genes are Linked to Parkinson's Disease?" section, several genes have been definitively linked to PD in some people. Researchers also have identified a number of other genes that may play a role and are working to confirm these findings. In addition, several chromosomal regions have been linked to PD in some families. Researchers hope to identify the genes located in these chromosomal regions and to determine which of them may play roles in PD.
Researchers funded by NINDS are gathering information and
Some scientists have found evidence that specific variations in the
In addition to identifying new genes for PD, researchers are trying to learn how known PD genes function and how the gene mutations cause disease. For example, a 2005 study found that the normal alpha-synuclein protein may help other proteins that are important for nerve transmission to fold correctly. Other studies have suggested that the normal parkin protein protects neurons from a variety of threats, including alpha-synuclein toxicity and excitotoxicity.
Scientists continue to study environmental toxins such as pesticides and herbicides that can cause PD symptoms in animals. They have found that exposing rodents to the pesticide rotenone and several other agricultural chemicals can cause cellular and behavioral changes that mimic those seen in PD. Other studies have suggested that prenatal exposure to certain toxins can increase susceptibility to PD in adulthood. An NIH-sponsored program called the Collaborative Centers for Parkinson's Disease Environmental Research (CCPDER) focuses on how occupational exposure to toxins and use of caffeine and other substances may affect the risk of PD.
Another major area of PD research involves the cell's protein disposal system, called the ubiquitin-proteasome system. If this disposal system fails to work correctly, toxins and other substances may build up to harmful levels, leading to cell death. The ubiquitin-proteasome system requires interactions between several proteins, including parkin and UCH-L1. Therefore, disruption of the ubiquitin-proteasome system may partially explain how mutations in these genes cause PD.
Other studies focus on how Lewy bodies form and what role they play in PD. Some studies suggest that Lewy bodies are a byproduct of degenerative processes within neurons, while others indicate that Lewy bodies are a protective mechanism by which neurons lock away abnormal molecules that might otherwise be harmful. Additional studies have found that alpha-synuclein clumps alter gene expression and bind to vesicles within the cell in ways that could be harmful.
Another common ic of PD research is excitotoxicity – overstimulation of nerve cells that leads to cell damage or death. In excitotoxicity, the brain becomes oversensitized to the neurotransmitter glutamate, which increases activity in the brain. The dopamine deficiency in PD causes overactivity of neurons in the subthalamic nucleus, which may lead to excitotoxic damage there and in other parts of the brain. Researchers also have found that dysfunction of the cells' mitochondria can make dopamine-producing neurons vulnerable to glutamate.
Other researchers are focusing on how inflammation may affect PD. Inflammation is common to a variety of neurodegenerative diseases, including PD, Alzheimer's disease, HIV-1-associated dementia, and amyotrophic lateral sclerosis. Several studies have shown that inflammation-promoting molecules increase cell death after treatment with the toxin MPTP. Inhibiting the inflammation with drugs or by genetic engineering prevented some of the neuronal degeneration in these studies. Other research has shown that dopamine neurons in brains from patients with PD have higher levels of an inflammatory enzyme called
Since the discovery that MPTP causes parkinsonian symptoms in humans, scientists have found that by injecting MPTP and certain other toxins into laboratory animals, they can reproduce the brain lesions that cause these symptoms. This allows them to study the mechanisms of the disease and helps in the development of new treatments. They also have developed animal models with alterations of the alpha-synuclein and parkin genes. Other researchers have used genetic engineering to develop mice with disrupted mitochondrial function in dopamine neurons. These animals have many of the characteristics associated with PD.
Biomarkers for PD – measurable characteristics that can reveal whether the disease is developing or progressing – are another focus of research. Such biomarkers could help doctors detect the disease before symptoms appear and improve diagnosis of the disease. They also would show if medications and other types of therapy have a positive or negative effect on the course of the disease. Some of the most promising biomarkers for PD are brain imaging techniques. For example, some researchers are using positron emission tomography (PET) brain scans to try to identify metabolic changes in the brains of people with PD and to determine how these changes relate to disease symptoms. Other potential biomarkers for PD include alterations in gene expression.
Researchers also are conducting many studies of new or improved therapies for PD. While deep brain stimulation (DBS) is now FDA-approved and has been used in thousands of people with PD, researchers continue to try to improve the technology and surgical techniques in this therapy. For example, some studies are comparing DBS to the best medical therapy and trying to determine which part of the brain is the best location for stimulation. Another clinical trial is studying how DBS affects depression and quality of life.
Other clinical studies are testing whether transcranial electrical polarization (TEP) or transcranial magnetic stimulation (
One of the enduring questions in PD research has been how treatment with levodopa and other dopaminergic drugs affects progression of the disease. Researchers are continuing to try to clarify these effects. One study has suggested that PD patients with a low-activity variant of the gene for
A variety of new drug treatments are in clinical trials for PD. These include a drug called GM1 ganglioside that increases dopamine levels in the brain. Researchers are testing whether this drug can reduce symptoms, delay disease progression, or partially restore damaged brain cells in PD patients. Other studies are testing whether a drug called istradefylline can improve motor function in PD, and whether a drug called
Some researchers are testing potential neuroprotective drugs to see if they can slow the progression of PD. One study, called
Nerve growth factors, or neurotrophic factors, which support survival, growth, and development of brain cells, are another type of potential therapy for PD. One such drug, glial cell line-derived neurotrophic factor (GDNF), has been shown to protect dopamine neurons and to promote their survival in animal models of PD. This drug has been tested in several clinical trials for people with PD, and the drug appeared to cause regrowth of dopamine nerve fibers in one person who received the drug. However, a phase II clinical study of GDNF was halted in 2004 because the treatment did not show any clinical benefit after 6 months, and some data suggested that it might even be harmful. Other neurotrophins that may be useful for treating PD include neurotrophin-4 (NT-4), brain-derived neurotrophic factor (BDNF), and fibroblast growth factor 2 (FGF-2).
While there is currently no proof that any dietary supplements can slow PD, several clinical studies are testing whether supplementation with vitamin B12 and other substances may be helpful. A 2005 study found that dietary restriction — reducing the number of calories normally consumed – helped to increase abnormally low levels of the neurotransmitter glutamate in a mouse model for early PD. The study also suggested that dietary restriction affected dopamine activity in the brain. Another study showed that dietary restriction before the onset of PD in a mouse model helped to protect dopamine-producing neurons.
Other studies are looking at treatments that might improve some of the secondary symptoms of PD, such as depression and swallowing disorders. One clinical trial is investigating whether a drug called quetiapine can reduce psychosis or agitation in PD patients with dementia and in dementia patients with parkinsonian symptoms. Some studies also are examining whether transcranial magnetic stimulation or a food supplement called s-adenosyl-methionine (
Another approach to treating PD 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.
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 PD. 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.
Gene therapy is yet another approach to treating PD. A study of gene therapy in non-human primate models of PD is testing different genes and gene-delivery techniques in an effort to refine this kind of treatment. An early-phase clinical study is also testing whether using the adeno-associated virus type 2 (AAV2) to deliver the gene for a nerve growth factor called neurturin is safe for use in people with PD. Another study is testing the safety of gene therapy using AAV to deliver a gene for human aromatic L-amino acid decarboxylase, an enzyme that helps convert levodopa to dopamine in the brain. Other investigators are testing whether gene therapy to increase the amount of glutamic acid decarboxylase, which helps produce an inhibitory neurotransmitter called GABA, might reduce the overactivity of neurons in the brain that results from lack of dopamine.
Another potential approach to treating PD is to use a vaccine to modify the immune system in a way that can protect dopamine-producing neurons. One vaccine study in mice used a drug called copolymer-1 that increases the number of immune T cells that secrete anti-inflammatory cytokines and growth factors. The researchers injected copolymer-1-treated immune cells into a mouse model for PD. The vaccine modified the behavior of supporting (glial) cells in the brain so that their responses were beneficial rather than harmful. It also reduced the amount of neurodegeneration in the mice, reduced inflammation, and increased production of nerve growth factors. Another study delivered a vaccine containing alpha-synuclein in a mouse model of PD and showed that the mice developed antibodies that reduced the accumulation of abnormal alpha-synuclein. While these studies are preliminary, investigators hope that similar approaches might one day be tested in humans.