PD-MitoQUANT Publications

α-synuclein oligomers and fibrils: a spectrum of species, a spectrum of toxicities

Alam, P., Bousset, L., Melki, R., Otzen, DE. Journal of Neurochemistry (2019)

http://doi.org/10.1111/jnc.14808

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This review article provides an overview of the different species that a-synuclein aggregates can populate. It also attempts to reconcile conflicting views regarding the cytotoxic roles of oligomers versus fibrils. a-synuclein, while highly dynamic in the monomeric state, can access a large number of different assembly states. Depending on assembly conditions, these states can interconvert over different timescales. The fibrillar state is the most thermodynamically favored due to the many stabilizing interactions formed between each monomeric unit, but different fibrillar types form at different rates. The end distribution is likely to reflect kinetic partitioning as much as thermodynamic equilibra. In addition, metastable oligomeric species, some of which are on-pathway and others offpathway, can be populated for remarkably long periods of time. Chemical modifications (phosphorylation, oxidation, covalent links to ligands, etc.) perturb these physical interconversions and invariably destabilize the fibrillar state, leading to small prefibrillar assemblies which can coalesce into amorphous states. Both oligomeric and fibrillar species have been shown to be cytotoxic although firm conclusions require very careful evaluation of particle concentrations and is complicated by the great variety and heterogeneity of different experimentally observed states. The mechanistic relationship between oligomers and fibrils remains to be clarified, both in terms of assembly of oligomers into fibrils and potential dissolution of fibrils into oligomers. While oligomers are possibly implicated in the collapse of neuronal homeostasis, the fibrillar state(s) appears to be the most efficient at propagating itself both in vitro and in vivo, pointing to critical roles for multiple different aggregate species in the progression of Parkinson’s disease. 

The PINK1 kinase-driven ubiquitin ligase Parkin promotes mitochondrial protein import through the presequence pathway in living cells

M. Jacoupy, E. Hamon-Keromen, A.Ordureau, Z. Erpapazoglou1 et al. Scientific Reports 14 August 2019

https://doi.org/10.1038/s41598-019-47352-9

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Most of over a thousand mitochondrial proteins are encoded by nuclear genes and must be imported from the cytosol. Little is known about the cytosolic events regulating mitochondrial protein import, partly due to the lack of appropriate tools for its assessment in living cells. We engineered an inducible biosensor for monitoring the main presequence-mediated import pathway with a quantitative, luminescence-based readout. This tool was used to explore the regulation of mitochondrial import by the PINK1 kinase-driven Parkin ubiquitin ligase, which is dysfunctional in autosomal recessive Parkinson’s disease. We show that mitochondrial import was stimulated by Parkin, but not by disease-causing Parkin variants. This effect was dependent on Parkin activation by PINK1 and accompanied by an increase in the abundance of K11 ubiquitin chains on mitochondria and by ubiquitylation of subunits of the translocase of outer mitochondrial membrane. Mitochondrial import efficiency was abnormally low in cells from patients with PINK1- and PARK2-linked Parkinson’s disease and was restored by phosphomimetic ubiquitin in cells with residual Parkin activity. Altogether, these findings uncover a role of ubiquitylation in mitochondrial import regulation and suggest that loss of this regulatory loop may underlie the pathophysiology of Parkinson’s disease, providing novel opportunities for therapeutic intervention.