Understanding molecular functions of PINK1, Parkin and VDAC1 in Parkinson ̓s disease

Abstract

This thesis research was conducted in order to understand the molecular mechanism behind Parkinsons disease by investigating the role of PINK1, Parkin and VDAC1 in the cell and animal models for the disease. The activation of Parkin E3 ligase is regulated by interaction between its UBL and RING1 domains. In healthy cells with normal mitochondrial function, the UBL and the RING1 domain of Parkin bind to each other to inhibit its activity. However, under mitochondrial damage conditions, the upstream regulator PINK1 phosphorylates Parkin at serine 65 in the UBL domain, reducing the binding between the UBL and the RING1 domain. Phosphorylation of the serine 65 in Parkin induces ubiquitination of Parkins substrates, such as MFN1, MFN2, and VDAC1, and increases Parkin-mediated mitophagy, suggesting that the phosphorylation plays a critical role in activating the E3 ligase activity of Parkin. In addition, three patient mutations within the UBL domain of Parkin, K27N, R33Q and A46P, sustain the interaction between the UBL and the RING1 domain even after serine 65 phosphorylation by PINK1, which explains how these mutations in Parkin induce Parkinsons disease. These results show that binding between the UBL and the RING1 domain of Parkin plays an important role in the pathogenesis of Parkin-related Parkinsons disease. The two types of VDAC1 ubiquitination by activated Parkin have different impacts on regulation of mitophagy and apoptosis. Mono-ubiquitination of VDAC1 by Parkin on lysine 274 inhibits apoptosis, while poly-ubiquitination of VDAC1 on lysines 12, 20, 53, 109, and 110 promotes mitophagy. Apoptosis, which is regulated by mono-ubiquitination of VDAC1, occurs as a result of increasing mitochondrial calcium uptake. The calcium uptake is regulated by MCU, a calcium channel located in the mitochondrial inner membrane, and mPTP, a mitochondrial pore responsible for apoptosis. The mono- and poly-ubiquitination of VDAC1 occur independently in a PINK1-Parkin pathway-dependent manner. Furthermore, Parkin patient mutation with K211N suppresses the poly-ubiquitination, while Parkin patient mutation with T415N is defective in the mono-ubiquitination of VDAC1. Drosophila model animals containing Parkin K211N mutation consistently show PD-related phenotypes and defective Parkin-mediated mitophagy, while Drosophila model animals with T415N mutation shows increased mitochondrial calcium uptake and increased apoptosis. Taken together, this thesis research suggests a detailed mechanism of PD pathogenesis caused by mutations in PINK1 and Parkin, and underlines an etiological importance of PD-associated genes in the regulation of mitochondrial homeostasis. Also, VDAC1 has been proposed as a novel regulator of PD, which extends our current understanding of PD pathogenesis and proposes VDAC1 as a potential candidate for PD medication.