α-SYNUCLEIN DISRUPTS INTEGRATED SIGNALING BY THE UNFOLDED PROTEIN RESPONSE THROUGH INHIBITION OF ATF6-α INCORPORATION INTO COPII VESICLES

Abstract

Parkinson's Disease (PD) is a neurodegenerative disease characterized by the advanced loss of motor functions as a result of progressive degenerative conditions that develop in many regions of the brain and manifest as a loss of dopaminergic neuronal cells. Parkinson's disease belongs to the group of diseases known as protein conformational disorders. Diseases belonging to this group have the molecular conditions of abnormal protein misfolding and aggregation, and are confined to specific cells or sub-regions of tissue. In PD, α-synuclein (α-syn) is thought to be the primary disease related protein, with cytotoxicity occurring through increased expression, and aggregation occurring within specific neuronal populations.

The unfolded protein response (UPR) is an endoplasmic reticulum (ER) localized system with three proximal sensors: (a) IRE1, (b) ATF6, and (c) PERK. These sensors help maintain the quality and quantity of protein folding within the ER. The UPR functions as a rheostat; constantly adjusting proportionally to stress, by either inducing adaptation, which returns homeostasis to the folding environment of the ER, or by initiating apoptotic pathways. α-Syn has been shown to inhibit ER-to-Golgi transit of COPII vesicles. ATF6, a

protective branch of the UPR, requires processing within the Golgi via COPII transport during ER-stress induced UPR activation.

The consequences of reduced COPII vesicle transit on UPR signaling remain poorly defined. Using cellular and vertebrate PD models together with biochemical reconstitution assays of COPII budding and protein-protein interaction assays; we showed that α-syn inhibited activation and processing of ATF6 during periods of ER stress. α-Syn inhibited this activation directly through physical interactions and indirectly through reduced COPII vesicle maturation. Reduced ATF6 signaling was accompanied by attenuation of IRE1 mediated splicing of xbp1. Alterations in ATF6 and XBP1 signaling pathways resulted in an impaired UPR adaptive response, with decreased ER-associated degradation (ERAD) function and increased execution of proapoptotic pathways. α-Syn is a promiscuous protein with many proven interactions between itself and individual proteins and organelles. In the background of PD, we present evidence that supports α-syn being capable of altering signaling by the UPR through direct and indirect means of perturbing ATF6 processing. The negative effects of α-syn overexpression on proper UPR signaling likely represents an important avenue in which selected neuronal populations incur unrecoverable stress and force a commitment to the execution of proapoptotic pathways. The overall conclusions of this study illustrate how α-syn can alter certain essential signaling pathways thereby preventing the cell from maintaining homeostasis of basic functions, such as protein folding. The mechanism by which α-syn inhibits ATF6 signaling expands our understanding of the roles that ER stress and the UPR play in the development of degenerative diseases, such as PD.