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Parkinsons disease is a neurodegenerative disorder that affects the neural cells through a combination of genetic and environmental factors. Mutations in numerous genes are identified to be responsible for Parkinson's disease and some of them can be inherited to the next generation (Crosiers et. Al 2011). In familial form of the disease,there are 6 genes identified (Crosiers et al 2011), of which 2; PTEN-induced putative kinase 1 (PINK1) and leucine-rich repeat kinase 2 (LRRK2) genes were chosen . For LRRK2; there are 2 missense mutations used in this study, G2019S and R1441C (Moore et al. 2008). For PINK1, there's one nonsense mutation, Q456X (Grunewald et al. 2009). This study provides an insight into how different mechanisms in the disease converge and shows how critical oxidative stress and mitochondrial movement are in Parkinson's disease.
The study by Cooper et al. utilized neural cells derived from induced pluripotent stem cells (iPSCs) created from the fibroblasts of the test subjects. This was done by reprogramming the fibroblasts via retrovirus-based technique. The fibroblast cells were taken from 3 different groups of subjects; 3 subjects with symptomatic Parkinson's Disease (homozygous PINK1 Q456X and LRRK2 G2019S genotypes), 2 subjects with asymptomatic familial Parkinson's Disease(heterozygous LRRK2 R1441C) and 2 healthy subjects at the point of clinical study. The neural cells with mutations in PINK1 and LRRK2 genes were compared against healthy control subjects. The iPSC derived neural cells were also compared against the fibroblast cells used. The aspects that were compared against were responses in mitochondrial reactive oxygen species (mROS), basal oxygen consumption rate (OCR) and mitochondrial movement when subjected to chemical stressors that induces oxidative stress. The responses were also observed when the cells were treated with pharmacological rescue compounds such as Coenzyme Q10, rapamycin and LRRK2 inhibitor, GW5074 and grown with the chemical stressors.
The study showed that the mutations in both genes resulted in an increased vulnerability of the mitochondria against various chemical stressors. The amount of reduced glutathione (GSH) produced was also measured. The results of the assays showed that PINK1 Q456X cells were the most vulnerable against chemical stressors due to a large increase in mROS and low production of GSH.
The basal oxygen consumption rate (OCR) was measured at real time and the addition of the different chemical stressors at different intervals. The results showed that PD-associated mutations share a common vulnerability when exposed to oxidative stress as the cells have higher or lower than normal fluctuations of OCR when compared to healthy cells. However, different mutations show different ways of handling as neural cells with PINK1 mutations increase proton leakage in their inner mitochondria membrane whereas in neural cells with LRRK2 R1441S mutations decrease their proton leakage.
The mobility of the mitochondria in the neural cells was measured through their movements within the cells and their direction. The results showed that PINK1 Q456X neural cells have the same mobility as healthy cells whereas LRRK2 neural cells are more mobile than healthy cells. The axon length of the neural cells was also taken to compare if there are differences amongst the different mutation types but there were no significant difference. As for the mitochondrial length, cells with LRRK2 R1441C mutations have 20% shorter mitochondria compared to healthy cells.
When the pharmacological compounds were added, there was a general increase in the tolerance against chemical stressors. The first compound, coenzyme Q10 helps to decrease the vulnerability of all the experimented cell types when exposed to the lowest concentration of valinomycin (1ug). However, it failed at higher concentrations. When rapamycin was added, it reduces the LDH release from LRRK2 mutations but did not have any effect on PINK1 mutations. The results were the same for GW5074. For healthy cells, addition of pharmacological compounds did not affect mROS production.
When compared to fibroblast cells, neural cells were found to be more sensitive to chemical stressors. Fibroblast cells needed greater concentrations of valinomycin in order to induce vulnerability to oxidative stress. PINK1 neural cells were found to release more LDH than healthy cells when under oxidative stress. However, PINK1 fibroblast cells and healthy cells release the same amount of LDH when under oxidative stress. This result was found to be similar with the between LRRK2 neural and fibroblast cells.
The results obtained showed that there's a general vulnerability in the PD-associated neural cells against oxidative stress. However, the different genotypes of the PD-associated neural cells have different responses from each other. This is due to the effect of the different proteins affected in the biochemical pathways within the cells. The effects of the pharmacological rescue compounds works differently on different mutations. Mitochondrial movement in the PD-associated cells reflects the loss of mitochondrial control as they became more mobile. This disrupts cellular organization of mitochondrial dynamics and thus led to the degeneration of the affected cells (Santos et al. 2012). Although the different mutations may exhibit different biochemical responses against oxidative stress, the final effect on Parkinson's disease still remains the same. This suggests that downstream processes are shared and common amongst the different mutations. The interesting point in this study is that the mutations are also reflected in the fibroblast cells although they are less affected as compared to neural cells. The positive point about this is that future experiments on Parkinson's disease can be done by using iPSCs derived from patient fibroblast and it serves as a comparable and controlled model of Parkinson's disease.
As there are multiple other genes that are involved in Parkinson's disease, perhaps a future study with more detailed and converging field of the mechanisms involved in causing Parkinson's disease could be created. This is because patients with other forms of Parkinson's disease could have a summation of various genetic mutations and extensive environmental effects. The observations of the different pharmacological compounds with have different effects on different types of mutations could be translated into personalized treatment for different Parkinson's disease patients so as to reduce the oxidative stress on their cells. With iPSCs becoming a viable method for detecting Parkinson's Disease, it holds a new future to the detection, identification, understanding and treatment of this neurodegenerative disease.