Parkinson’s disease (PD) is a chronic and progressive neurodegeneration of dopamine neurons in the substantia nigra. The reason for the death of these neurons is unclear; however, studies have demonstrated the potential involvement of mitochondria, endoplasmic reticulum, α-synuclein or dopamine levels in contributing to cellular oxidative stress as well as PD symptoms. Even though those papers had separately described the individual roles of each element leading to neurodegeneration, recent publications suggest that neurodegeneration is the product of various cellular interactions. This review discusses the role of oxidative stress in mediating separate pathological events that together, ultimately result in cell death in PD. Understanding the multi-faceted relationships between these events, with oxidative stress as a common denominator underlying these processes, is needed for developing better therapeutic strategies.

In our previous study, we clearly demonstrated the roles of pro-inflammatory cytokines, including tumor necrosis factor-␣, interleukin-1␤ (IL-1␤), and IL-6, and subsequent reactive oxygen species (ROS) generation on the pathogenesis of cisplatin ototoxicity in vitro and in vivo. ROS generation in cisplatin-treated HEI-OC1 auditory cells was also correlated with changing mitochondrial membrane potential. However, the roles of NADPH oxidase in cisplatin-induced ROS generation and ototoxicity have not been fully elucidated. Herein, immunohistochemical studies demonstrated that treatment of cisplatin induced the expression of NADPH oxidase isoforms NOX-1 and NOX-4 in HEI-OC1 auditory cells. Expression of mRNA for NOX-1, NOX-4, NOXO1, NOXA1, p47phox , and p67 phox was also increased. Inhibition of NADPH oxidase with diphenyleniodonium chloride or apocynin abolished ROS production and the subsequent apoptotic cell death in cisplatin-treated cells. Furthermore, suppression of NOX1 and NOX4 expression by small interfering RNA transfection markedly abolished the cytotoxicity and ROS generation by cisplatin. Together, our data suggest that ROS generated, in part, through the activation of NADPH oxidase plays an essential role in cisplatin ototoxicity.

SummaryParkinson's disease (PD) is characterized by the selective loss of dopamine neurons in the substantia nigra; however, the mechanism of neurodegeneration in PD remains unclear. A subset of familial PD is linked to mutations in PARK2 and PINK1, which lead to dysfunctional mitochondria-related proteins Parkin and PINK1, suggesting that pathways implicated in these monogenic forms could play a more general role in PD. We demonstrate that the identification of disease-related phenotypes in PD-patient-specific induced pluripotent stem cell (iPSC)-derived midbrain dopamine (mDA) neurons depends on the type of differentiation protocol utilized. In a floor-plate-based but not a neural-rosette-based directed differentiation strategy, iPSC-derived mDA neurons recapitulate PD phenotypes, including pathogenic protein accumulation, cell-type-specific vulnerability, mitochondrial dysfunction, and abnormal neurotransmitter homeostasis. We propose that these form a pathogenic loop that contributes to disease. Our study illustrates the promise of iPSC technology for examining PD pathogenesis and identifying therapeutic targets.

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