As part of a broad effort to develop preclinical models of Parkinson’s disease (PD), MJFF sponsored the generation and initial characterization of knockout pre-clinical models for the Parkin, DJ-1, PINK1 and LRRK2 genes because mutations in these genes are causally linked to inherited forms of PD. The knockout model brain regions and peripheral tissues that were not used for the initial characterization were collected and stored for potential future studies. The objective of this project is to analyze some of these frozen pre-clinical model brain tissue samples to determine whether mutations causally linked to inherited forms of PD cause biochemical or neurochemical abnormalities in the brains of models bearing mutations in the same genes.
We will analyze frozen pre-clinical brain tissue samples provided by MJFF. We will focus on measuring potential biochemical abnormalities in mitochondrial function and measuring potential neurochemical abnormalities in the levels or turnover of neurotransmitters such as dopamine and serotonin because these abnormalities likely underlie the clinical symptoms of PD.
Relevance to Diagnosis/Treatment of Parkinson’s Disease:
One of the greatest impediments to developing and testing more effective treatments for PD is the lack of pre-clinical models that reproduce all the key aspects of PD and that share both the relevant neuroanatomy and the mechanisms of neurodegeneration with people with PD. The neuroanatomy of these pre-clinical models is very similar to humans, including the location and connections of the dopamine neurons that are the most affected brain cells in PD. Our project is important for determining whether the pre-clinical models generated by MJFF share potential mechanisms of neurodegeneration that have been implicated in human parkinsonism, such as mitochondrial dysfunction.
We expect to contribute a greater level of detail to the characterization of the pre-clinical models generated by MJFF. This research is important for guiding future therapeutic development based on targeting mitochondrial dysfunction in PD.
We analyzed frozen brain tissue from pre-clinical models with deletion mutations in the PD-linked genes Parkin, DJ-1, PINK1 and LRRK2 as well as wild-type (non-mutant) controls. We used high pressure liquid chromatography (HPLC) to measure the levels of dopamine and related metabolites because dopamine is the primary neurotransmitter affected in Parkinson’s disease. We found elevated levels of dopamine in the striatum of DJ-1 knockout pre-clinical models and PINK1 knockout pre-clinical models compared to wild-type pre-clinical models at ages 8 months and 6 months, but not at age 4 months. This unexpected result may be due in part to the way the samples were handled or stored prior to our receiving the samples for analysis. It is possible that there is a compensatory increase in dopamine in the mutant pre-clinical models at the ages when dopamine-producing neurons are degenerating. Because mitochondrial dysfunction has been implicated in Parkinson’s disease and because several of these genes are believed to affect mitochondrial function, we analyzed the function of mitochondria in frozen brain tissue samples from the same wild-type and mutant pre-clinical models at age 8 months. We did not observe significant differences between wild-type and mutant pre-clinical models in the activity of mitochondrial electron transport chain complex I, complex II, complex III or complex IV. However, only frontal cortex tissue was available for analysis and it is possible that significant mitochondrial dysfunction occurs in the mutant pre-clinical models in the brains regions most affected in Parkinson’s disease (the substantia nigra and the caudate/putamen).