The conclusion is based on animal studies that were published May 1 in Science Advances. “These studies show that, if we can downregulate RasGRP1 signaling before dopamine replacement, we have an opportunity to greatly improve [patients’] quality of life,” said Srinivasa Subramaniam, PhD, of the department of neuroscience at Scripps Research in Jupiter, Fla., in a press release. Dr. Subramaniam is one of the investigators.
Parkinson’s disease results from the loss of substantia nigral projections neurons, which causes decreased levels of dopamine in the dorsal striatum. Treatment with L-DOPA reduces the disease’s motor symptoms effectively, but ultimately leads to the onset of LID. Previous data suggest that LID results from the abnormal activation of dopamine-1 (D1)–dependent cyclic adenosine 3´,5´-monophosphate (cAMP)/protein kinase A (PKA), extracellular signal–regulated kinase (ERK), and mammalian target of rapamycin kinase complex 1 (mTORC1) signaling in the dorsal striatum.
Animal and biochemical data
Based on earlier animal studies, Dr. Subramaniam and colleagues hypothesized that RasGRP1 might regulate LID. To test this theory, the investigators created lesions in wild-type and RasGRP1 knockout mice to create models of Parkinson’s disease. The investigators saw similar Parkinsonian symptoms in both groups of mice on the drag, rotarod, turning, and open-field tests. After all mice received daily treatment with L-DOPA, RasGRP1 knockout mice had significantly fewer abnormal involuntary movements, compared with the wild-type mice. All aspects of dyskinesia appeared to be equally dampened in the knockout mice.
To analyze whether RasGRP1 deletion affected the efficacy of L-DOPA, the investigators subjected the treated mice to motor tests. Parkinsonian symptoms were decreased among wild-type and knockout mice on the drag and turning tests. “RasGRP1 promoted the adverse effects of L-DOPA but did not interfere with its therapeutic motor effects,” the investigators wrote. Compared with the wild-type mice, the knockout mice had no changes in basal motor behavior or coordination or amphetamine-induced motor activity.
In addition, Dr. Subramaniam and colleagues observed that RasGRP1 levels were increased in the striatum after L-DOPA injection, but not after injection of vehicle control. This and other biochemical findings indicated that striatal RasGRP1 is upregulated in an L-DOPA–dependent manner and is causally linked to the development of LID, according to the investigators.
Other observations indicated that RasGRP1 physiologically activates mTORC1 signaling, which contributes to LID. Using liquid chromatography and mass spectrometry, Dr. Subramaniam and colleagues saw that RasGRP1 acts upstream in response to L-DOPA and regulates a specific and diverse group of proteins to promote LID. When they examined a nonhuman primate model of Parkinson’s disease, they noted similar findings.
New therapeutic targets
“There is an immediate need for new therapeutic targets to stop LID ... in Parkinson’s disease,” said Dr. Subramaniam in a press release. “The treatments now available work poorly and have many additional unwanted side effects. We believe this [study] represents an important step toward better options for people with Parkinson’s disease.”
Future research should attempt to identify the best method of selectively reducing expression of RasGRP1 in the striatum without affecting its expression in other areas of the body, according to Dr. Subramaniam. “The good news is that in mice a total lack of RasGRP1 is not lethal, so we think that blocking RasGRP1 with drugs, or even with gene therapy, may have very few or no major side effects.”
The study was funded by grants from the National Institutes of Health. The investigators reported no conflicts of interest.
SOURCE: Eshraghi M et al. Sci Adv. 2020;6:eaaz7001.