Student Research: Heather Klintworth
, Environmental Toxicology (Tox), 2005
Faculty Advisor: Zhengui Xia
Signaling Mechanisms of Paraquat-induced Apoptosis
Parkinson's disease (PD) is the second most common age-related neurodegenerative disorder. It is characterized by a loss of dopaminergic neurons in the substantia nigra and leads to decreased dopamine production in the brain. Such a loss can result in bradykinasia, tremor at rest, loss of postural reflex, and a shuffling gait. Currently, the causes and mechanisms underlying PD are still unknown.
Epidemiologic studies have suggested an elevated risk of PD with pesticide exposure. Paraquat (PQ) is an herbicide of particular interest because of its structural similarities with MPTP, a known neurotoxicant that produces Parkinson-like symptoms. Furthermore, PQ has been shown to cause Parkinson-like symptoms in vivo.
In this study, we use PC12 cells, a rat dopaminergic cell line, and PQ as a model to study mechanisms of cell death induced by environmental toxicants in PD. Two mitogen-activated protien (MAP) kinase-signaling pathways of interest are the p38 and the stress-activated protein kinase/ Jun-N-terminal kinase (SAPK/JNK) pathways. These pathways have been shown to be activated in response to stress and toxicants to cause apoptosis. Ventral mesencephalic cultures from embryonic day 14 rats were used to determine if results from PC12 cell experiments apply to real neurons. Such information could prove important for better understanding of the mechanisms of PD.
We determined there is a dose-response relationship between PQ exposure and apoptosis with approximately 25% apoptosis at 200uM PQ. P38, JNK and cJun were shown to be activated with PQ treatment. Also, cell death was attenuated by inhibiting p38 and JNK pathways indicating that these pathways are required for PQ-mediated cell death. Paraquat caused upregulation of the pro-apoptotic Bel-2 family member, Bim. Bim could be upregulated by p38 or JNK pathways but to determine this more research is needed. The MAP kinase ERK1/2 was found to be activated as a cell survival mechanism when PC12 cells were exposed to PQ. Paraquat exposure to primary neurons caused cell death at concentrations as low as 20uM. Axonal and dendritic degeneration of dopaminergic neurons was observed in PQ-treated primary cultures. Dopaminergic neurons were more sensitive to PQ exposure than the general cecll ppulation which includes non-dopaminergic cells and it is possible that p38 could play a role in observed differential sensitivity of primary cultures with PQ exposure. These findings suggest MAP kinase signaling pathways may play an important role in the mechanisms of selective dopaminergic cell death in Parkinson's disease.