C-Terminal Tyrosine Residue Modifications Modulate the Protective Phosphorylation of Serine 129 of α-Synuclein in a Yeast Model of Parkinson's Disease

<div><p>Parkinson´s disease (PD) is characterized by the presence of proteinaceous inclusions called Lewy bodies that are mainly composed of α-synuclein (αSyn). Elevated levels of oxidative or nitrative stresses have been implicated in αSyn related toxicity. Phosphorylation of αSyn on serine 129 (S129) modulates autophagic clearance of inclusions and is prominently found in Lewy bodies. The neighboring tyrosine residues Y125, Y133 and Y136 are phosphorylation and nitration sites. Using a yeast model of PD, we found that Y133 is required for protective S129 phosphorylation and for S129-independent proteasome clearance. αSyn can be nitrated and form stable covalent dimers originating from covalent crosslinking of two tyrosine residues. Nitrated tyrosine residues, but not di-tyrosine-crosslinked dimers, contributed to αSyn cytotoxicity and aggregation. Analysis of tyrosine residues involved in nitration and crosslinking revealed that the C-terminus, rather than the N-terminus of αSyn, is modified by nitration and di-tyrosine formation. The nitration level of wild-type αSyn was higher compared to that of A30P mutant that is non-toxic in yeast. A30P formed more dimers than wild-type αSyn, suggesting that dimer formation represents a cellular detoxification pathway in yeast. Deletion of the yeast flavohemoglobin gene <i>YHB1</i> resulted in an increase of cellular nitrative stress and cytotoxicity leading to enhanced aggregation of A30P αSyn. Yhb1 protected yeast from A30P-induced mitochondrial fragmentation and peroxynitrite-induced nitrative stress. Strikingly, overexpression of neuroglobin, the human homolog of <i>YHB1</i>, protected against αSyn inclusion formation in mammalian cells. In total, our data suggest that C-terminal Y133 plays a major role in αSyn aggregate clearance by supporting the protective S129 phosphorylation for autophagy and by promoting proteasome clearance. C-terminal tyrosine nitration increases pathogenicity and can only be partially detoxified by αSyn di-tyrosine dimers. Our findings uncover a complex interplay between S129 phosphorylation and C-terminal tyrosine modifications of αSyn that likely participates in PD pathology.</p></div

The human <i>NGB</i> gene for neuroglobin alters A30P and αSyn aggregation in yeast and mammalian cells.

<p>(A) Spotting analysis of <i>YHB1</i> and Δ<i>yhb1</i> yeast cells co-expressing αSyn and GFP (control) with either empty vector as control or <i>YHB1</i> and <i>NGB</i>, respectively, on non-inducing and galactose-inducing SC-Ura medium after 3 days. (B) Quantification of the percentage of cells displaying αSyn aggregates after 6 h induction in galactose-containing medium (n = 3). (C) Spotting analysis of <i>YHB1</i> and Δ<i>yhb1</i> yeast cells co-expressing A30P and GFP (control) with either empty vector (pME2788) as control or <i>YHB1</i> and <i>NGB</i>, respectively, on non-inducing and galactose-inducing SC-Ura medium after 3 days. (D) Quantification of the percentage of cells displaying A30P aggregates after 6 h induction in galactose-containing medium. Significance of differences was calculated with t-test (**, <i>p</i> < 0.01, n = 3). (E) Fluorescence microscopy of H4 cells co-expressing SynT, synphilin-1 and pcDNA (control) or NGB-mCherry. Nuclei are stained with Hoechst dye (blue). Scale bar = 30 μm. (F) Quantification of the percentage of H4 cells displaying αSyn inclusions after 48 h after transfection. Cells were classified into three groups according to the number of αSyn-immunoreactive inclusions observed: cells with 10 inclusions, cells with less than 10 inclusions and cells without inclusions. Significance of differences was calculated with t-test (*, <i>p</i> < 0.05, n = 3). (G) Lactate dehydrogenase (LDH) activity measurements support that <i>NGB</i> is non-toxic for H4 cells. H4 cells transfected with empty mammalian expression vector pcDNA3.1, with empty pcDNA3.1 or pcDNA3.1 encoding neuroglobin-mCherry (<i>NGB</i>) together with SynT and synphilin-1 (SynT+Synphilin-1) were analyzed. Media from indicated H4 cells were collected and the secretion of lactate LDH was determined as a measure of cytotoxicity. Significance of differences was calculated with t-test (not significant (n.s.); n = 3).</p

Tyrosine mutation of A30P decreases toxicity in <i>Δyhb1</i>.

<p>(A) Spotting analysis of αSyn, A30P, 4(Y/F) αSyn, A30P/4(Y/F) and GFP (control) expressed in <i>YHB1</i> and Δ<i>yhb1</i> yeast on non-inducing and galactose-inducing SC-Ura plates after 3 days of growth. (B) Cell growth analysis of <i>YHB1</i> and Δ<i>yhb1</i> yeast expressing αSyn, A30P, 4(Y/F), A30P/4(Y/F) and GFP (control) at time point 20 h. Significance of differences was calculated with t-test (*, <i>p</i> < 0.05; **, <i>p</i> < 0.01, n = 3). (C) Quantification of the percentage of cells displaying αSyn aggregates after 6 h induction in galactose-containing SC-Ura medium. Significance of differences was calculated with t-test (*, <i>p</i> < 0.05, **, <i>p</i> < 0.01, n = 6).</p

Phospho-peptides identified by MS/MS.

<p>Phospho-peptides identified by MS/MS.</p

Determination of crosslinked peptides from αSyn and A30P.

<p>(A) Analysis of di-tyrosine dimers. Exemplary heat map diagram of the number (N) of identified di-tyrosine crosslinked peptides of the non-treated αSyn samples. (B) Distribution of all identified di-tyrosine peptides for αSyn. Identified combinations of crosslinked peptides are presented as percentage of n (n = total number of MS2 spectra verified as crosslinked peptides). (C) Distribution of all identified di-tyrosine peptides for A30P.</p

Blocking of αSyn tyrosine nitration decreases aggregation and cytotoxicity.

<p>(A) Expression of αSyn, A30P, 4(Y/F) and A30P/4(Y/F) αSyn was induced for 12 h in galactose-containing medium and the proteins were enriched by Ni²⁺ pull-down from yeast cell extracts. For <i>in vitro</i> nitration, 1 μl peroxynitrite (PON) was mixed with 15 μg of αSyn extracts in the presence of 1 μl 0.3 M HCl. Western blotting with di-tyrosine antibody reveals a major band at about 36 kDa, corresponding to dimers. Additional bands with lower molecular weights are observed, probably due to intramolecular di-tyrosine crosslinking. The same membrane was stripped and re-probed with αSyn antibody. (B) Western blotting using 3-nitro-tyrosine antibody (3-NT). Phenylalanine codon substitutions eliminate immunoreactivity. The same membrane was stripped and re-probed with αSyn antibody. (C) Spotting analysis of yeast cells expressing <i>GAL1</i>-driven αSyn, A30P, 4(Y/F), A30P/4(Y/F) αSyn and GFP (control). Yeast cells were spotted in 10-fold dilutions on SC-Ura plates containing glucose (αSyn ‘OFF’) or galactose (αSyn ‘ON’). (D) Cell growth analysis of yeast cells expressing αSyn, A30P, 4(Y/F), A30P/4(Y/F) αSyn and GFP (control) in galactose-containing SC-Ura medium for 40 h. Error bars represent standard deviations of three independent experiments. (E) Fluorescence microscopy of yeast cells, expressing indicated αSyn-GFP variants after 6 h of induction in galactose-containing medium. Scale bar: 1 μm. (F) Quantification of the percentage of cells displaying aggregates after 6 h induction in galactose-containing medium. Significance of differences was calculated with t-test (*, <i>p</i> < 0.05, n = 6). (G) Western blotting analysis of protein crude extracts of GFP-tagged αSyn, 4(Y/F), A30P and A30P/4(Y/F) after 6 h induction in galactose-containing medium. GAPDH antibody was used as loading control.</p

Yeast strains used in this study.

<p>Yeast strains used in this study.</p

Tyrosine 133 mutation does not alter the accumulation of reactive oxygen and nitrogen species.

<p>(A, B) Quantification of cells expressing different αSyn variants displaying ROS and RNS assessed with flow cytometry analysis. αSyn expression was induced for 6 h and the cells were stained for 1.5 h with DHR123 to visualize ROS (A) or with DAF-2 DA to visualize RNS (B). Forward scatter (FSC) and DHR123 (A) or DAF-2 DA (B) fluorescence of the cells, showing one representative result from at least four independent experiments. The percentage of the sub-populations of yeast cells with higher fluorescent intensities (P1) than the background are presented in the lower panels. Significance of differences was calculated with one-way ANOVA (****, <i>p</i> < 0.0001).</p

αSyn forms dimers <i>in vivo</i>.

<p>(A) Spotting analysis of yeast cells expressing C-terminally HIS₆-tagged αSyn and A30P αSyn on a high copy vector (2μ) driven by the inducible <i>GAL1-</i>promoter on non-inducing (´OFF`: glucose) and inducing (´ON`: galactose) SC-Ura medium after 3 days. Control cells expressed only the empty vector pME2795 (EV). (B) Western blotting of αSyn and A30P enriched from cell extracts by Ni²⁺ pull-down with anti-αSyn antibody. <i>In vitro</i> nitration was carried out with 15 μg of αSyn extracts using 1 μl peroxynitrite (PON) in the presence of 1 μl 0.3 M HCl. (C) Quantification of dimers. Densitometric analysis of the immunodetection of αSyn and A30P αSyn dimers <i>in vivo</i> and in PON-treated samples. The amount of dimers is presented as percent of the total amount of αSyn detected per lane (monomer + dimer). Significance of differences was calculated with t-test (**, <i>p</i> < 0.01, n = 4).</p

Determination of nitrated peptides from αSyn and A30P.

<p>αSyn and A30P were enriched by Ni²⁺ pull-down from yeast crude extracts and separated by SDS-PAGE. Monomeric and dimeric αSyn stained with Coomassie were excised from the gel and digested with trypsin and AspN. Untreated <i>(in vivo)</i> and subsequent peroxynitrite (PON) treated <i>(in vitro)</i> αSyn and A30P protein samples were analyzed with LC-MS for tyrosine nitration. 3-NT (3-nitrotyrosine) indicates identified nitration sites, supported by at least two peptides and two independent experiments.</p