- Open Access
Pathogenic alpha-synuclein aggregates preferentially bind to mitochondria and affect cellular respiration
© The Author(s). 2019
- Received: 8 March 2019
- Accepted: 10 March 2019
- Published: 14 March 2019
Misfolded alpha-synuclein (αSyn) is a major constituent of Lewy bodies and Lewy neurites, which are pathological hallmarks of Parkinson’s disease (PD). The contribution of αSyn to PD is well established, but the detailed mechanism remains obscure. Using a model in which αSyn aggregation in primary neurons was seeded by exogenously added, preformed αSyn amyloid fibrils (PFF), we found that a majority of pathogenic αSyn (indicated by serine 129 phosphorylated αSyn, ps-αSyn) was membrane-bound and associated with mitochondria. In contrast, only a minuscule amount of physiological αSyn was mitochondrial bound. In vitro, αSyn PFF displayed a stronger binding to purified mitochondria than did αSyn monomer, revealing a preferential mitochondria binding by aggregated αSyn. This selective mitochondrial ps-αSyn accumulation was confirmed in other neuronal and animal αSyn aggregation models that do not require exogenously added PFF and, more importantly, in postmortem brain tissues of patients suffering from PD and other neurodegenerative diseases with αSyn aggregation (α-synucleinopathies). We also showed that the mitochondrial ps-αSyn accumulation was accompanied by defects in cellular respiration in primary neurons, suggesting a link to mitochondrial dysfunction. Together, our results show that, contrary to physiological αSyn, pathogenic αSyn aggregates preferentially bind to mitochondria, indicating mitochondrial dysfunction as the common downstream mechanism for α-synucleinopathies. Our findings suggest a plausible model explaining the formation and the peculiar morphology of Lewy body and reveal that disrupting the interaction between ps-αSyn and the mitochondria is a therapeutic target for α-synucleinopathies.
- Lewy body
- Parkinson’s disease
Parkinson’s disease (PD) is a common neurodegenerative disorder in the elderly and its characteristic motor symptoms result from the loss of dopaminergic neurons in the substantia nigra [11, 37]. Mitochondrial dysfunction is sufficient to cause nigral dopaminergic neuron loss , which is consistent with the idea that these neurons are selectively vulnerable to various toxic insults . Besides mitochondria dysfunction, genetic studies have established a causative role of dominant mutations or multiplication of SNCA (αSyn gene) in familial PD [8, 23, 38, 45] and the association of SNCA polymorphism with a higher risk of idiopathic PD , demonstrating a key contribution of αSyn to PD pathogenesis. αSyn is an intrinsically disordered protein and is abundantly expressed in neurons [18, 23]. During the disease, αSyn is subjected to a variety of post-translational modifications . Among them, the phosphorylation at serine 129 is most relevant to the pathogenic process and is commonly used to indicate the pathogenic αSyn species [1, 14].
In addition to PD, αSyn aggregation is also a common pathological hallmark for a group of neurodegenerative diseases known as α-synucleinopathies, which include PD, dementia with Lewy bodies (DLB), and multiple system atrophy (MSA) [15, 17]. A large body of evidence supports that the aggregation process of αSyn, including both oligomerization and amyloid fibril growth, is closely related to the pathogenesis of α-synucleinopathies . Notably, both increased αSyn expression by SNCA multiplication [8, 45] and the presence of disease-associate αSyn mutations can increase the aggregation propensity of αSyn . Reducing αSyn clearance is also able to increase the amount of αSyn and thereby enhances αSyn aggregation and neurotoxicity . Moreover, inoculating preformed αSyn amyloid fibrils (PFF) into wild-type mice induces endogenous αSyn aggregation and subsequent nigral dopaminergic neuron degeneration , demonstrating that αSyn aggregation is sufficient to cause neurodegeneration. Despite these advances, very little is known about the detailed mechanism of how αSyn aggregation causes neurotoxicity and contributes to the pathogenic process.
It has been reported that αSyn may play a role in the physiology and/or pathology of mitochondrial function . Even though αSyn does not have a mitochondrial targeting sequence, several groups reported its localization in mitochondria or mitochondria-associated membranes (MAMs) [9, 12, 16, 25–27, 44, 47], and showed that αSyn affects a variety of mitochondrial functions, from Ca2+ signaling, to complex I activity, to mitochondrial morphology and dynamics . However, given the well-established presynaptic localization of αSyn and its role in synaptic vesicle release [5–7, 18, 19, 21], it remains unclear how much physiological αSyn is mitochondria-associated and to what extent it affects mitochondrial function. Moreover, whether the mitochondrial αSyn localization contributes to the disease process and how it is related to the other major pathogenic event, αSyn aggregation, are completely unknown.
Some recent studies started to explore the potential relationship between pathogenic αSyn species and mitochondria. Using a proximity ligation assay (PLA), Di Maio et al. showed that αSyn oligomer and S129E phosphomimic mutant each bind TOM20 on the mitochondrial outer membrane and impair mitochondrial protein import . Using exogenously added αSyn oligomer, Ludtman et al. showed mitochondrial localization of αSyn oligomer by PLA and the impairment of mitochondrial function . They also showed that increased endogenous αSyn aggregates in neurons derived from an SNCA triplication patient were also in the vicinity of mitochondrial ATP synthase, indicating a mitochondrial localization . Using human pluripotent stem cells expressing mutant SNCA, Ryan et al. reported fragmented mitochondria and the clustering of αSyn aggregates in the mitochondria of SNCA mutant neurons, but not in the isogenic control neurons . The methodologies used in above studies, including transient transfection, exogenously added αSyn oligomers, SNCA mutant cells, and heavy dependence on imaging-based analyses, make it difficult to conclude how much pathogenic αSyn is associated with mitochondria; whether mitochondrial association is a main pathogenic pathway for αSyn aggregates; whether the mitochondrial ps-αSyn accumulation occurs in neurons that express wild-type αSyn at endogenous level, and more importantly, whether such accumulation occurs in α-synucleinopathy patients.
To understand the cellular mechanism of αSyn aggregation and the resulting neurotoxicity, we adopted a highly reproducible primary neuron model developed by Volpicelli-Daley et al., in which the neuronal accumulation of ps-αSyn was induced by exogenously added PFF . We took an unbiased approach to study the subcellular localization of ps-αSyn and found that the majority of ps-αSyn was associated with mitochondria. This finding was verified in other neuronal and mouse models and was also confirmed with postmortem brain tissues from α-synucleinopathy patients. Consistent with these findings, our in vitro results showed a preferential binding of mitochondria by aggregated αSyn. Moreover, we have also showed that the mitochondrial accumulation of ps-αSyn is associated with mitochondrial respiration defects, suggesting mitochondrial dysfunction as a downstream consequence of aggregated αSyn.
Purification of recombinant αSyn and preparation of αSyn PFF
Mouse αSyn was purified as previously described [3, 51]. Purified αSyn was dialyzed against PBS buffer (VWR#97062–732), aliquoted, and stored at − 80 °C. When needed, aliquots of αSyn were thawed, concentrated to 350 μM, and shaken at 37 °C for 7 d at 1000 rpm to prepare mature mouse αSyn preformed fibrils (PFF) . PFF was aliquoted and stored at − 80 °C. All PFF preparations were verified by the thioflavin T fluorescence assay and by imaging using a Tecnai G2 Spirit TWIN transmission electron microscope. The purification of human αSyn and the preparation of hPFF were performed with the same procedures described above.
Primary neuronal culture
P1 mouse or rat cortical neurons were isolated and cultured essentially according to a previously published protocol . At 7 d in vitro (DIV), neurons were treated with or without 140 nM αSyn monomer or PFF. Treated neurons were cultured as usual and collected after 8–9 d or after a specified time. PFF was sonicated before each usage with a water-bath cup-horn sonicator (Misonix XL2020) at 50% power for 5 min. For proteasomal inhibition, primary neurons were treated with DMSO or 100 nM epoxomicin for 8 h, and after incubation, cell lysates or homogenates were prepared.
Immunofluorescence staining and proximity ligation assay (PLA)
At DIV 16, neurons were homogenized into TBS buffer (50 mM Tris, 150 mM NaCl, pH 7.5), sonicated at 50% power for 5 min (Misonix XL2020), and centrifuged at 100,000×g for 30 min at 4 °C. The pellets were resuspended in 1% Triton-100 in TBS, sonicated, and centrifuged as described above. The supernatant was collected as the Triton-soluble fraction. The pellet was resuspended in 2% SDS in TBS.
Differential centrifugation to separate mitochondrial and cytosolic/microsomal fractions
Primary cortical neurons were washed once with ice-cold neuronal membrane buffer (NMB, 250 mM sucrose, 20 mM HEPES, 10 mM KCl, 1.5 mM MgCl2, 1 mM EGTA, 1 mM EDTA, and 1 mM DTT, pH 7.5) and then were homogenized in NMB plus proteinase and phosphatase inhibitors (NMB+). Dissected spinal cords of transgenic mice, or brain tissue from patients were directly homogenized in NMB+. The homogenates were centrifuged at 600 × g (for cells) or 700×g (for tissue) at 4 °C for 5 min to remove nuclei and unbroken cells. The supernatants were used as the postnuclear supernatant (PNS). To separate mitochondrial and cytosolic/microsomal fractions, the PNS was centrifuged at 10,000×g for 5 min at 4 °C. The supernatants were collected as cytosolic/microsomal fractions, and the pellets (the mitochondrial fraction) were resuspended in NMB+.
For the iodixanol gradient analysis, either PNS or isolated mitochondria were mixed with iodixanol stock solution to reach the final concentration of 36%. The mixture (0.8 mL) was loaded to the bottom of the ultracentrifuge tube. On top of it, 0.8 mL of 31% and 0.8 mL of 10% iodixanol were sequentially laid. After centrifugation at 52,000×g for 4 h at 4 °C (ThermoFisher rotor S55-S), 0.2 mL fractions were collected from top to bottom. Discontinuous sucrose gradient analysis was performed as previously described .
Samples were separated on 16% SDS polyacrylamide gel or pre-made 4–20% gradient SDS polyacrylamide gel (ThermoFisher) and transferred to PVDF membranes (GE Healthcare). The membranes were fixed with 4% paraformaldehyde and 0.1% glutaraldehyde for 30 min at room temperature and probed with various antibodies. The primary antibodies were anti-αSyn phospho (Ser129) (Abcam 51,253, 1:2500), anti-αSyn (BD Biosciences clone 42, 1:2500), anti-ATPIF1 (ThermoFisher, 1:1500), anti-ATP5A (Abcam, 1:1000), anti-GAPDH (Stressgen, 1:1000), anti-Calnexin (Sressgen, 1:1000), and anti-Syntaxin 6 (Sigma, 1:1000). The HRP-conjugated goat anti-mouse or anti-rabbit IgG antibodies (Bio-Rad) were used as the secondary antibodies. Signals were developed by ECL 2 substrate (Pierce) and scanned with ChemiDoc (Bio-Rad). The signal intensity was measured with the FIJI software.
Mitochondrial stress test
Primary cortical neurons were seeded in Agilent Seahorse 96-well plate at 1.0 × 104 per well. At DIV 7, neurons were treated with either αSyn monomer or PFF. At DIV 14, the oxygen consumption rate was measured with Seahorse XFe96 (Agilent) after sequentially adding 1 μM oligomycin, 1.25 μM FCCP (carbonyl cyanide-p-trifluoromethoxyphenylhydrazone), and 0.5 μM rotenone/antimycin A (XF Cell Mito Stress Test Kit from Agilent).
C57BL/6J mice were from Van Andel Institute internal colony. CD (Sprague Dawley) IGS rats were obtained from Charles River. Transgenic mice expressing human A53T αSyn from the mouse prion protein promoter (line G2–3, kindly provided by Dr. Michael Lee, University of Minnesota) were described previously . The bilateral intramuscular PFF injection was performed according to a previously published protocol  with slight modification. Briefly, the PFF used for the intramuscular injection was prepared by shaking 350 μM αSyn at 1000 rpm for 2 d at 37 °C. PFF was aliquoted and stored at − 80 °C. Before each injection, PFF was diluted to 0.1 μg/μL and sonicated with the water-bath cup-horn sonicator (Misonix XL2020) at 25% power for 2 h, and then 10 μL of PFF (0.1 μg/μl) or 10 μL PBS was injected into biceps femoris on each hindlimb.
Statistical analyses were performed with GraphPad Prism software version 6.05.
PFF-induced ps-αSyn in primary neurons
We first characterized the PFF-induced synucleinopathy model in primary neurons. Our results showed that adding PFF (Additional file 1: Figure S1) was sufficient to induce the phosphorylation of endogenous αSyn and no ps-αSyn was detected in PFF-treated primary neurons derived from αSyn knockout mice (Fig. 1a). Immunofluorescence staining verified that ps-αSyn was accumulated in the neurons labeled with the neuronal marker MAP2 (Fig. 1b). Using an antibody detecting total αSyn, we found that the majority of αSyn in PFF-treated wild-type or αSyn knockout neurons was cleaved (Fig. 1a). Because the only αSyn in the knockout neurons was from PFF, this result suggested that the majority of the exogenously added PFF was truncated in neurons. The induction of ps-αSyn was specific to the addition of αSyn PFF; adding the same amount of αSyn monomer did not produce ps-αSyn. Moreover, the exogenously added monomer was barely detectable by immunoblot analysis after 9 days (Fig. 1a, monomer in αSyn knockout neurons), suggesting a more rapid clearance than that of exogenously added PFF. The same result was obtained with rat primary neurons (Additional file 1: Figure S2).
To determine whether the findings can be reproduced with human αSyn, we cultured neurons derived from the BAC transgenic mice (OVX mice) that express human wild-type αSyn at twice the endogenous level of mouse αSyn and on a mouse αSyn knockout background . These neurons were either untreated or treated with human αSyn monomer or PFF prepared with human αSyn (hPFF). Immunoblot analysis (Fig. 1c) revealed that ps-αSyn was induced in neurons treated with hPFF, but not in neurons untreated or treated with human αSyn monomer. Immunofluorescence staining confirmed that ps-αSyn was in neurons (Fig. 1d).
To ensure that the observed effects were not because of toxic effects of the amyloid fibrils, we compared αSyn PFF with fibrils of another amyloidogenic protein, lysozyme (Fig. 1e and f). The ps-αSyn was accumulated only in αSyn PFF-treated neurons, not in those treated with lysozyme fibrils, suggesting the ps-αSyn was likely resulted from seeding by αSyn PFF.
We also determined the aggregation status of ps-αSyn by sequential detergent extraction. ps-αSyn was enriched in Triton- and SDS-soluble fractions, whereas total αSyn in untreated cells appeared mainly in the TBS-soluble fraction (Fig. 1g). Immunofluorescence staining revealed that PFF-induced ps-αSyn resisted proteinase K digestion (Fig. 1h). These results led us to conclude that the majority of PFF-induced ps-αSyn in primary neurons was aggregated.
Preferential binding of PFF-induced ps-αSyn to mitochondria
We tested this possibility by separating the cell homogenates into mitochondrial and cytosolic/microsomal fractions by differential centrifugation. We found that majority of ps-αSyn was indeed in the mitochondrial fraction (Fig. 2b and Additional file 1: Figure S4). To rule out the possibility that the appearance of ps-αSyn in mitochondria fraction was simply due to its aggregation status, we subjected isolated mitochondria to a density gradient that was optimized for mitochondria purification. The ps-αSyn co-migrated with mitochondrial marker ATPIF1 to the top of the gradient (Fig. 2c), confirming its association with the membranous mitochondria.
Aggregated αSyn, but not physiological αSyn, preferentially binds to mitochondria
The large amount of αSyn accumulated in the mitochondrial fraction of PFF-treated neurons suggested to us that aggregated αSyn may have a preference to bind to mitochondria. To test this possibility, we purified mitochondria from untreated primary neurons, incubated isolated mitochondria with αSyn monomer or PFF in vitro, and then subjected the mixture to the density-gradient analysis. The purified mitochondria contained very little endogenous αSyn (Fig. 4b, second panel), which is consistent with the idea that very little physiological αSyn binds to mitochondria under normal conditions. After incubation with αSyn monomer or PFF, the majority of αSyn monomer remained at the bottom, but the majority of αSyn PFF co-migrated with the mitochondrial protein ATPIF1 to the upper fractions (Fig. 4b, third and fourth panels). As a control, αSyn PFF alone (i.e. without incubation with mitochondria) was also subject to the gradient separation, and it remained at the bottom (Fig. 4b, bottom panel). Therefore, our results suggested that relative to αSyn monomer, aggregated αSyn preferentially binds to the mitochondria.
Preferential binding of ps-αSyn to mitochondria in other neuronal and transgenic mouse models
The majority of ps-αSyn was associated with mitochondria in postmortem brain tissues from α-synucleinopathy patients
Mitochondrial ps-αSyn accumulation is associated with defects in cellular respiration
Our results revealed a preferential accumulation of ps-αSyn in the mitochondria, connecting two major pathogenic events of α-synucleinopathies, i.e., αSyn misfolding and mitochondrial dysfunction. The fact that ps-αSyn from three different α-synucleinopathies was associated with mitochondria suggests to us that this is a general property of αSyn aggregates and is independent of the variable conformations of misfolded αSyn in different diseases . Moreover, our results also showed that the majority of physiological αSyn is not associated with mitochondria, which is consistent with previous studies [5–7, 18, 19, 21] and suggests that the mitochondrial ps-αSyn accumulation is most likely not due to the phosphorylation of mitochondria-associated physiological αSyn. Instead, it is likely resulted from the inherent preference of pathogenic αSyn aggregates to bind to mitochondria, which is consistent with findings from our in vitro mitochondria binding study (Fig. 4b).
Our study also revealed that the mitochondrial ps-αSyn accumulation is accompanied by defects in mitochondrial activity. Together with the selective vulnerability of mitochondria in the energy-demanding nigral dopaminergic neurons [36, 48, 49], our findings may explain the fact that certain αSyn mutations or SNCA duplication cause disease similar to idiopathic PD  and that polymorphism of αSyn gene could be a risk factor for idiopathic PD . Because the preferential binding by αSyn aggregates is not limited to mitochondria from nigral dopaminergic neurons, the ps-αSyn mitochondrial connection may also contribute to the degeneration of other neuronal types, which may lead to the cognitive and behavioral disturbances in DLB or PDD (Parkinson’s disease dementia) patients and in patients carrying SNCA triplication [32, 35, 45]. This conclusion is consistent with a recent report of fragmented mitochondria and clustering of αSyn aggregates in the mitochondria of neurons differentiated from human iPSCs carrying αSyn mutations  and the report of mitochondrial dysfunction in neurons differentiated from iPSCs derived from a patient carrying αSyn triplication .
Our findings also support the faithfulness of changes in PFF-induced models [29, 52] to the pathogenesis of α-synucleinopathies. The study of postmortem human tissues reflects the end stage of diseases, but the PFF-induced model allows us to study the dynamic cellular consequences of αSyn aggregation. Notably, some ps-αSyn did appear in the TBS-soluble fraction, and a large portion of it appeared in the Triton-soluble fraction (Fig. 1g), which presumably represents various states of polymerization, from a soluble oligomeric state to an amyloid fibril state. Because the majority of ps-αSyn co-migrated with mitochondria (Fig. 2b and c), it is reasonable to predict that not only αSyn aggregates in the fibril state, but also some of the αSyn oligomers, bind to mitochondria as well.
It is important to note that the mitochondria-associated αSyn aggregates remain competent to seed αSyn monomer to form ordered aggregates. While αSyn aggregates in the middle of Lewy bodies are either already coalesced without reactive surface or crowded by membranous structures without the access to αSyn monomer, the αSyn aggregates on the surface remain competent to seed and have the accessibility to αSyn monomer (Fig. 8). Thus, in neurons where αSyn monomer is available, αSyn amyloid fibrils can be grown on the surface of Lewy bodies, resulting in a ring of densely packed αSyn fibrils that reacts strongly with antibody during immunohistochemical staining . Therefore, our model predicts that at the end stage, Lewy bodies or Lewy neurites may contain membranous structures, mitochondria, αSyn aggregates in complex with mitochondria and vesicles, and αSyn fibrils [31, 39, 46].
Our study provides solid evidence supporting the preferential binding of pathogenic αSyn species to mitochondria; suggests mitochondrial dysfunction as the common downstream pathogenic mechanism for αSyn aggregation; and points out that disrupting the binding of pathogenic αSyn species to mitochondria could be beneficial to patients suffering from α-synucleinopathies.
We would like to thank Dr. Corinne Esquibel at optical imaging core of Van Andel Institute for providing technical support for image acquisition and analysis; David Nadziejka at Van Andel Institute for technical editing the manuscript. We also would like to thank the staff at Michigan brain bank for their support.
This work is supported in part by NIH grants R21NS101676, R01NS060729, and P30AG053760.
Availability of data and materials
All data generated in this study are presented in this article and its online supplementary materials.
JM conceived and designed experiments, analyzed data, and wrote the manuscript. XW designed, planned, and performed experiments, analyzed data, contributed to the writing of the manuscript. KB expressed and purified recombinant αSyn, prepared and validated αSyn PFF used in cell culture experiments. NL contributed to the setup of primary neuronal culture and the rat primary neuron culture. MZ performed part of the immunoblot analyses. APL and DJM provided critical reagents for the study and analyzed the data. All authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.
Ethics approval and consent to participate
This study was conducted in compliance with federal and institutional guidelines. All experiments were approved by appropriate committees at the Van Andel Research Institute.
Consent for publication
The authors declare that they have no competing interests.
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