Differential mitochondrial roles for α-synuclein in DRP1-dependent fission and PINK1/Parkin-mediated oxidation

Abstract

Mitochondria are highly dynamic organelles with strict quality control processes that maintain cellular homeostasis. Within axons, coordinated cycles of fission-fusion mediated by dynamin related GTPase protein (DRP1) and mitofusins (MFN), together with regulated motility of healthy mitochondria anterogradely and damaged/oxidized mitochondria retrogradely, control mitochondrial shape, distribution and size. Disruption of this tight regulation has been linked to aberrant oxidative stress and mitochondrial dysfunction causing mitochondrial disease and neurodegeneration. Although pharmacological induction of Parkinson’s disease (PD) in humans/animals with toxins or in mice overexpressing α-synuclein (α-syn) exhibited mitochondrial dysfunction and oxidative stress, mice lacking α-syn showed resistance to mitochondrial toxins; yet, how α-syn influences mitochondrial dynamics and turnover is unclear. Here, we isolate the mechanistic role of α-syn in mitochondrial homeostasis in vivo in a humanized Drosophila model of Parkinson’s disease (PD). We show that excess α-syn causes fragmented mitochondria, which persists with either truncation of the C-terminus (α-syn1–120) or deletion of the NAC region (α-synΔNAC). Using in vivo oxidation reporters Mito-roGFP2-ORP1/GRX1 and MitoTimer, we found that α-syn-mediated fragments were oxidized/damaged, but α-syn1–120-induced fragments were healthy, suggesting that the C-terminus is required for oxidation. α-syn-mediated oxidized fragments showed biased retrograde motility, but α-syn1–120-mediated healthy fragments did not, demonstrating that the C-terminus likely mediates the retrograde motility of oxidized mitochondria. Depletion/inhibition or excess DRP1-rescued α-syn-mediated fragmentation, oxidation, and the biased retrograde motility, indicating that DRP1-mediated fragmentation is likely upstream of oxidation and motility changes. Further, excess PINK/Parkin, two PD-associated proteins that function to coordinate mitochondrial turnover via induction of selective mitophagy, rescued α-syn-mediated membrane depolarization, oxidation and cell death in a C-terminus-dependent manner, suggesting a functional interaction between α-syn and PINK/Parkin. Taken together, our findings identify distinct roles for α-syn in mitochondrial homeostasis, highlighting a previously unknown pathogenic pathway for the initiation of PD.

The relationship of alpha-synuclein to mitochondrial dynamics and quality control

“…Membrane remodeling is a critical process required for the sequestration of damaged mitochondrial cargo in terms of both MDV formation and mitophagy, while also an essential mechanism to facilitate mitochondrial dynamics. Excess alphasynuclein influences mitochondrial fission in both animal models and mammalian cells, and a recent study explored the mechanisms potentiating this using specific protein domain mutants of alpha-synuclein (Kamp et al, 2010;Nakamura et al, 2011;Butler et al, 2012;Furlong et al, 2020;Krzystek et al, 2021). Using a humanized Drosophila model, overexpression of full-length alpha-synuclein led to mitochondrial fragmentation which persisted in the absence of both the C-terminus and NAC domain, demonstrating the response was independent of alpha-synuclein's propensity to aggregate (Krzystek et al, 2021).…”

“…Excess alphasynuclein influences mitochondrial fission in both animal models and mammalian cells, and a recent study explored the mechanisms potentiating this using specific protein domain mutants of alpha-synuclein (Kamp et al, 2010;Nakamura et al, 2011;Butler et al, 2012;Furlong et al, 2020;Krzystek et al, 2021). Using a humanized Drosophila model, overexpression of full-length alpha-synuclein led to mitochondrial fragmentation which persisted in the absence of both the C-terminus and NAC domain, demonstrating the response was independent of alpha-synuclein's propensity to aggregate (Krzystek et al, 2021). Fragmentation instead required an intact N-terminus, implying that the response was likely due to alterations in the biophysical properties of mitochondrial membranes resulting from alphasynuclein interactions (Krzystek et al, 2021).…”

Roles of α-Synuclein and Disease-Associated Factors in Drosophila Models of Parkinson’s Disease

“…PINK1 and Parkin work in conjunction to promote mitochondrial quality control; PINK1-dependent phosphorylation of Parkin mediates the clearance of damaged mitochondria, i.e., mitophagy [58]. Similar to Parkin, PINK1 overexpression suppresses αSyn-induced phenotypes in Drosophila, including the loss of climbing ability, shortened lifespan, neurodegeneration, and mitochondrial fragmentation [59][60][61]. These studies suggest the protective role of Parkin and PINK1 in αSyn toxicity, but whether a direct molecular interaction between Parkin/PINK1 and αSyn contributes to these effects remains unclear.…”

Single Image Capture of Bioactive Ion Crosstalk within Inter‐Organelle Membrane Contacts at Nanometer Resolution

“…[ 1,2 ] These inter‐organelle membrane contacts enable rapid and bidirectional exchange of information between mitochondria and lysosomes, thus regulating numerous cellular processes, most notably those involved in the maintenance of cellular homeostasis. [ 3–11 ] Moreover, the mounting evidence implicates the breakdown in mitochondria–lysosome inter‐organelle cross‐talk as relevant in number of human pathologies, most notably neurodegenerative disease, such as Parkinson's disease. [ 3 ] Therefore, understanding MLC processes and the key molecules involved in mediating inter‐organelle communication in normal cellular physiology as well as disease is of critical importance.…”

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mitochondria and lysosomes, thus regulating numerous cellular processes, most notably those involved in the maintenance of cellular homeostasis. [3][4][5][6][7][8][9][10][11] Moreover, the mounting evidence implicates the breakdown in mitochondria-lysosome interorganelle cross-talk as relevant in number of human pathologies, most notably neurodegenerative disease, such as Parkinson's disease. [3] Therefore, understanding MLC processes and the key molecules involved in mediating inter-organelle communication in normal cellular physiology as well as disease is of critical importance.Due to the limitation of Abel diffraction, the structure of subcellular organelles below 200 nm cannot be observed by a traditional fluorescence microscope.

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