A Novel Role for the Parkinson Disease-Linked and Neuromelanin-Associated Parkin Protein as a Cysteine-Dependent Redox-State Regulator
Creators: name: Tokarew, Jacqueline M.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2021
Subjects: Neuroscience
Parkin protein
Parkinson Disease
Oxidative stress

description: Parkinson disease (PD) is an incurable disease, second only to Alzheimer’s disease as the most common neurodegenerative disease in adults. Unfortunately, the course of disease is significantly longer for individuals diagnosed at an early age (20-40 years of age). Although early-onset, recessively inherited cases represent a small subset of individuals with PD (~5- 10%), their clinical presentation is unique, with symptoms being almost exclusively motor-related. The expressivity of early-onset PD is partially explained by post-mortem neuropathological findings, which demonstrate a specific loss of dopamine synthesizing cells in brainstem nuclei that also produce neuromelanin (i.e. Substantia nigra and Locus coeruleus). With the majority of early-onset PD cases being caused by homozygous and biallelic heterozygous mutations in the PRKN gene, its gene product, parkin, has been extensively studied. It is generally accepted that loss of its E3 ligase function leads to neurodegeneration by either one of the following two mechanisms: i) toxic substrate accumulation from the loss of target protein ubiquitination (and related degradation), or ii) accumulation of dysfunctional mitochondria due to impaired mitophagy initiation. However, whether these mechanisms lead to selective neuronal loss within the human brain remains unknown. This thesis represents a body of work that supports a novel role for parkin as a thiol-based anti-oxidant and redox homeostasis regulator, which helps explain the cell-specificity observed in recessive, PRKN-linked PD. These findings include: i) evidence that human brain parkin uniquely and natively undergoes age-associated aggregation beginning at 40 years of age (Chapter 2); ii) identification of multiple, reversible and irreversible oxidative modifications of parkin cysteines, both in cells and tissues, including dopamine-adduct conjugation on primate sequence-specific cysteine 95 (Chapter 2); iii) the demonstration that irreversible oxidation of parkin cysteines causes aggregate formation ii in cells and mice exposed to exogenous and/or endogenous sources of oxidative and dopamine stress (Chapter 2 and 3); iv) evidence that parkin functions as a thiol-dependent anti-oxidant similar to glutathione (Chapter 2), which lowers oxidation state in cells and tissues under native and stress conditions (Chapter 2 and 3); v) the demonstration that parkin cysteines, notably C95, directly bind glutathione and regulate glutathione redox homeostasis in cells and tissues in a dynamic fashion (Chapter 3); and vi) the development of novel, human-specific, anti-parkin monoclonal antibodies that preferentially detect oxidized and aggregated forms of parkin found associated with neuromelanin and lysosomal storage vesicles within neurons of human Substantia nigra (Chapter 2 and 4). Future studies focusing on further validation of in situ oxidative modifications of parkin cysteines and their effect on protein structure, notably the poorly studied linker region that contains C95, will provide insight into how these oxidative modifications affect the function of parkin in vivo, including in adult human brain. Also, identifying the bona fide intracellular redox partners of parkin will be crucial to understanding how this protein regulates cellular redox state. Of clinical importance, the findings presented here indicate a potential, human-specific link between parkin and neuromelanin formation, which deserves to be further explored, such as with parkin mouse models engineered to produce neuromelanin. Finally, designing clinical trials using anti-oxidants specifically in individuals affected by PRKN-associated PD represents a logical, translational treatment approach to explore.