Axon degeneration and PGC1α-mediated protection in a vertebrate model of α-synuclein toxicity

Abstract

α-synuclein (aSyn) expression is implicated in neurodegenerative processes, including Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). In animal models of these diseases, axon pathology often precedes cell death, raising the question of whether aSyn has compartment-specific toxic effects that could require early and/or independent therapeutic intervention. The relevance of axonal pathology to degeneration can only be addressed through longitudinal, in vivo monitoring of different neuronal compartments. With current imaging methods, dopaminergic neurons do not readily lend themselves to such a task in any vertebrate system. We therefore expressed human wild-type aSyn in zebrafish peripheral sensory neurons, which project elaborate superficial axons that can be continuously imaged in vivo. Axonal outgrowth was normal in these neurons but, by 2 days post-fertilization (dpf), many aSyn-expressing axons became dystrophic, with focal varicosities or diffuse beading. Approximately 20% of aSyn-expressing cells died by 3 dpf. Time-lapse imaging revealed that focal axonal swelling, but not overt fragmentation, usually preceded cell death. Co-expressing aSyn with a mitochondrial reporter revealed deficits in mitochondrial transport and morphology even when axons appeared overtly normal. The axon-protective protein Wallerian degeneration slow (WldS) delayed axon degeneration but not cell death caused by aSyn. By contrast, the transcriptional coactivator PGC-1α, which has roles in the regulation of mitochondrial biogenesis and reactive-oxygen-species detoxification, abrogated aSyn toxicity in both the axon and the cell body. The rapid onset of axonal pathology in this system, and the relatively moderate degree of cell death, provide a new model for the study of aSyn toxicity and protection. Moreover, the accessibility of peripheral sensory axons will allow effects of aSyn to be studied in different neuronal compartments and might have utility in screening for novel disease-modifying compounds.

Live imaging of mitochondrial dynamics in CNS dopaminergic neurons in vivo demonstrates early reversal of mitochondrial transport following MPP+ exposure

“…Dynamic imaging of mitochondria in live zebrafish has been reported previously in cells of the somites and pharyngeal arches during early development (Kim et al, 2008), and in primary sensory Rohon-Beard neurons (O'Donnell et al, 2014; Plucinska et al, 2012). Imaging these superficially-located cell populations is less challenging than the deeply-located CNS dopaminergic neurons that are relevant to PD.…”

Protein Amphipathic Helix Insertion: A Mechanism to Induce Membrane Fission

“…The function of this protein is not well understood. As already mentioned, α-synuclein drives fragmentation of mitochondria (Kamp et al, 2010;Nakamura et al, 2011;O'Donnell et al, 2014;Martinez et al, 2018). Uncontrolled mitochondrial fragmentation might contribute to the development of Parkinson's disease (Varkey et al, 2010;Panchal and Tiwari, 2019) and, importantly, duplication and triplication of SNCA (α-synuclein) gene cause a severe form of this disease (Olgiati et al, 2015;Konno et al, 2016).…”

“…Proteins belonging to the synuclein family (Lavedan, 1998), namely α-synuclein (Kamp et al, 2010;Nakamura et al, 2011;O'Donnell et al, 2014;Martinez et al, 2018) and β-synuclein (Nakamura et al, 2011;Taschenberger et al, 2013), are involved in mitochondrial fragmentation. α-Synuclein contains a uniquely long α-11/3 AH that forms upon binding to a lipid bilayer (George et al, 1995;Davidson et al, 1998;Drin and Antonny, 2010;Braun et al, 2017).…”

Diverse cellular and molecular modes of axon degeneration

“…Subsequent changes in mitochondria including the production of r eactive o xygen s pecies (ROS) and formation of the m itochondrial p ermeability t ransition p ore (mPTP) ultimately precede and may drive axonal degeneration [66]. Wld S seems to be upstream of all of these events since it can block immediate changes in Ca 2+ signaling [67], mPTP and ROS production [68]. Bursts of Ca 2+ signaling in dendrite branches is predictive of pruning in Drosophila [69], arguing for a role for dynamic Ca 2+ changes in pruning, but whether ROS production or mPTP is important for driving dendritic pruning events is unknown.…”