Characterization Of Vesicular Monoamine Transporter 2 And Its Role In Parkinson'S Disease Pathogenesis Using Drosophila

Author

Antonio Joel Tito Jr., The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical SciencesFollow
Sheng Zhang, UTHealthFollow

Date of Graduation

12-2016

Document Type

Dissertation (PhD)

Program Affiliation

Human and Molecular Genetics

Degree Name

Doctor of Philosophy (PhD)

Advisor/Committee Chair

Sheng Zhang, Ph.D.

Committee Member

Richard Behringer, Ph.D.

Committee Member

Hugo Bellen, Ph.D.

Committee Member

Andrew Bean, Ph.D.

Committee Member

Nicholas Justice, Ph.D.

Committee Member

Kartik Venkatachalam, Ph.D.

Committee Member

Momiao Xiong, Ph.D.

Abstract

Parkinson’s disease (PD) is a progressive neurodegenerative disorder caused by the selective loss of the dopaminergic neurons in the Substantia nigra pars compacta region of the brain. PD is also the most common neurodegenerative disorder and the second most common movement disorder. PD patients exhibit the cardinal symptoms, including tremor of the extremities, rigidity, slowness of movement, and postural instability, after 70-80% of DA neurons degenerate. It is, therefore, imperative to elucidate the underlying mechanisms involved in the selective degeneration of DA neurons. Although increasing numbers of PD genes have been identified, why these largely widely expressed genes induce selective loss of DA neurons is still not known. Notably, dopamine (DA) itself is a chemically labile molecule and can become oxidized to toxic by-products while induce the accumulation of harmful molecules such as Reactive Oxygen Species (ROS). Accordingly, DA toxicity has long been suspected to play a role in selective neuronal loss in PD. Vesicular Monoamine Transporter (VMAT) is essential for proper vesicular storage of monoamines such as DA and their regulated release. Increasing evidence have linked VMAT dysfunction with Parkinson’s disease. In this study, we re-examine the gain- and loss-of-function phenotypes of the sole VMAT homologue in Drosophila. Our results suggest that the C-terminal sequences in the two encoded VMAT isoforms not only determine their differential subcellular localizations, but also their activities in content release. In particular, VMAT2 orthologue potentially poses a unique, previously unexplored activity in promoting DA release. On the other hand, by examining DA distribution in wildtype and VMAT mutant animals, we find that there exists intrinsic difference in the dynamics of intracellular DA handling among DA neurons clustered in different brain regions. Furthermore, loss of VMAT causes severe loss of total DA levels and a redistribution of DA in Drosophila brain. Lastly, removal of both VMAT and another PD gene parkin, which is also conserved in Drosophila, results in the selective loss of DA neurons, primarily in the protocerebral anterior medial (PAM) clusters of the brain. Our results suggest a potential involvement of cytoplasmic DA in selective degeneration of DA neurons and also implicating a role for a differential intracellular DA handling mechanism underlying the regional specificity of neuronal loss in PD patients.

Keywords

Neuroscience, Neurodegenerative Disease, Drosophila, Brain, Dopamine Neurons, Dopamine, Parkinson's Disease, Parkin, VMAT