Background
Several neurodegenerative diseases comprise neuronal or glial deposits consisting mainly of protein tau, such as Alzheimer’s neurofibrillary tangles (NFTs) or Pick bodies, and are therefore termed “tauopathies“. In Alzheimer’s Disease (AD), tau exhibits pathological hyperphosphorylation [
1,
2], allowing both histological diagnosis by use of tau antibodies against disease specific phosphorylation sites [
3,
4] and, to a certain extent, even in vivo diagnosis by determination of the protein’s phosphorylation status in cerebrospinal fluid [
5,
6].
One of the most important tau kinases is Glycogen Synthase Kinase 3β (GSK-3β), which has been shown to create AD specific phospho sites on tau in vitro [
7], in cell culture [
8,
9] and in vivo [
10,
11]. Some [
12,
13] but not all [
14] authors reported increased GSK levels in AD brains. GSK-3β is colocalized with NFTs [
15], and the distribution of its active form in AD brains coincides with the appearance of tau pathology [
16].
Tau phosphorylation by GSK-3β promotes the formation of paired helical filaments (PHF) in vitro [
17‐
19], though data concerning the relevance of this effect vary [
20]. An enhancing impact of GSK-3β on tau aggregation was also demonstrated in cell culture and in vivo [
21‐
23], supporting a possible role of this kinase in AD pathogenesis. Furthermore, phosphorylation influences metal ion induced tau aggregation. Several studies demonstrated that tau phosphorylation enhances Al
3+ induced aggregation [
24,
25] or even is a prerequisite for such aggregation [
26,
27].
The influence of aluminium on tau aggregation has been extensively studied, since the metal ion was shown to induce NFT-like deposits in mammalian brain after intracerebral injection [
28]. Though aluminium levels were found to be raised in AD hippocampus [
29] and the metal ion was colocalized with NFTs and early tau deposits in brain sections [
30,
31], its relevance to AD pathogenesis is still unclear, especially due to the inconsistent outcome of epidemiological studies [
32].
In vitro studies examining effects of ferric iron (Fe
3+) yielded results resembling those obtained for aluminium. Fe
3+ also induces the aggregation of phosphorylated protein tau [
27], is colocalized with NFTs [
30,
33,
34] and elevated in AD hippocampus and amygdala [
35]. Furthermore, Fe
3+ induces α-synuclein (α-syn) aggregation [
36‐
38].
Co-deposits of tau and α-syn have been found in several neurodegenerative diseases, and interactions between these two proteins recently gained increasing interest. α-Syn has been detected in NFTs of AD, progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) [
39], whereas tau was located in Lewy bodies of patients with Dementia with Lewy bodies (DLB) [
40]. In vitro, tau in solution requires inducers like heparin for filament formation, whereas the protein readily polymerizes in presence of α-syn without inducers [
41].
Furthermore, the minimal α-syn concentration necessary for fibril formation is reduced in presence of tau, and some of the fibrils formed in presence of both proteins comprise tau and α-syn segments [
41]. Considering that both proteins are located in the cytoplasmic compartment of neurons, and that minimal concentrations of α-syn oligomers can cross-seed tau aggregation [
42], interactions of tau and α-syn may be relevant for pathological protein aggregation in neurodegenerative diseases.
While established methods of monitoring tau and α-syn aggregation like Thioflavin T assay or atomic force microscopy yield important insights in fibril formation and the formation of large oligomers, they are not suitable to directly monitor single protein interactions or interactions of different proteins. It was demonstrated that small oligomer species are on-pathway to tau filament formation [
43]. Furthermore, it is increasingly recognised that prefibrillar small oligomers rather than the large NFTs might be responsible for neuronal and synaptic loss [
44‐
46]. To investigate the influence of phosphorylation on tau oliomer formation and interactions between tau and α-syn, we employed fluorescence correlation spectroscopy (FCS) and scanning for intensely fluorescent targets (SIFT) to investigate the influence of phosphorylation and trivalent metal ions on tau aggregation and its coaggregation with α-syn. These methods allow monitoring of oligomerization processes at the single molecule level even at nanomolar protein concentrations [
47,
48]. Moreover, the possibility to label proteins with different dyes allows the investigation of tau and α-syn interactions at the level of individual oligomers.
Discussion
Several studies have dealt with the influence of protein phosphorylation and the metal ion aluminium (Al
3+) on tau filament formation so far [
17‐
27]. However, the pathophysiological relevance of these factors is still unclear. Whereas most studies were mainly focused on tau filament formation, we applied single molecule fluorescence techniques to investigate the influence of these factors on early tau aggregation steps and oligomer formation. We demonstrate that tau phosphorylation modulates oligomer formation in presence of Al
3+, and that Al
3+ induces the formation of distinct large, SDS resistant tau oligomers.
We further demonstrate that coaggregation of tau and α-syn can be observed at the single molecule level, is differentially modulated by tau phosphorylation, and induced by trivalent metal ions. Such coaggregation might be of pathophysiological relevance, since co-deposits of tau and α-syn have been detected in various neurodegenerative diseases [
39,
40].
Influence of tau phosphorylation on electrophoretic mobility and antibody interaction
It has been demonstrated that in vitro phosphorylation of human protein tau by GSK-3β can be verified using SDS-PAGE by demonstrating a complete band shift of phosphorylated tau compared to the unphosphorylated protein [
17]. We verified efficient tau phosphorylation indicated by a complete band shift compared to the unphosporylated protein (Figure
1B). In addition, we investigated whether fluorescently labeled antibodies can be employed in the SIFT method to verify tau phosphorylation at nanomolar protein concentrations, allowing for possible diagnostic implementations of this method in the future. Our data corroborate that tau phosphorylation was successful and that antibodies can be employed in the SIFT method to identify proteins and posttranslational modifications such as phosphorylation at nanomolar protein concentrations (Figure
1C). Such an assay may provide a valuable tool for future diagnostic applications e.g. the detection of novel specific CSF biomarkers of neurodegeneration.
Differential influence of phosphorylation on tau aggregation
In vitro studies have yielded conflicting results regarding the influence of tau phosphorylation on its assembly to filaments. To date, both inhibitory and promotional effects of GSK-3β mediated phosphorylation on tau filament assembly were found in absence or presence of polyanionic aggregation inducers [
17‐
20,
50,
51]. Notably, for Al
3+ induced tau aggregation, phosphorylation was consistently found to be enhancing or, in some experiments, even a prerequisite [
24‐
27].
However, the methods applied in these studies, such as electron microscopy, thioflavine fluorescence, laser light scattering, SDS-PAGE and western blot, are of limited use in directly examining single molecule interactions at the oligomer level. Furthermore, these techniques often require high concentrations of both protein and aggregation inducer that might not depict physiological conditions, especially given the fact that tau protein readily polymerizes at high concentrations without any inducer [
52,
53]. In addition, the majority of studies investigating brain Al
3+ concentrations did not detect concentrations exceeding 10 μg/g brain mass (dry weight) in AD brains [
54], which approximately corresponds to 60 μmol/kg (wet weight), while many studies examining the influence of Al
3+ on tau aggregation in vitro applied Al
3+ concentrations in the milimolar range [
25,
27,
55].
In this study, we applied confocal single molecule fluorescent techniques to investigate the influence of protein phosphorylation by GSK-3β on tau oligomer formation at the single molecule level at nanomolar concentrations. Exposure to 10 μM Al
3+ induced the rapid formation of large tau oligomers, while DMSO at a final concentration of 1% led to the formation of smaller oligomers. The large Al
3+ triggered oligomers contained an average of 220 to 240 molecules as shown by FIDA analysis, and were resistant to treatment with 0.2% SDS, which in contrast readily dissolved the smaller oligomers formed in presence of DMSO. The oligomer sizes observed here were comparable with data presented earlier [
49]. Interestingly, phosphorylation by GSK-3β yielded an increase in Al
3+ induced tau oligomer formation, while oligomer size was comparable for pTau and mTau.
Thus, our data identify tau phosphorylation and physiological concentrations of the metal ion Al3+ as synergistic inducers of the formation of SDS resistant tau oligomers even at nanomolar protein concentrations. These findings substantiate the hypothesis that Al3+ may play a role in the formation of neurotoxic oligomers even at early stages of neurodegeneration.
Co-deposits of tau and α-syn have been found in several neurodegenerative diseases, including AD, PSP, CBD and DLB [
39,
40]. Since interactions of different proteins are difficult to monitor in vitro, only few studies investigating the interaction of tau and α-syn have been published so far. It was demonstrated that in mixed micromolar solutions of α-syn and tau, both proteins’ minimal concentrations for filament assembly are decreased [
41,
56]. Moreover, Giasson et al. detected filaments that comprised both tau and α-syn, proving that the two proteins directly interact in mixed solutions [
41]. A more recent study showed that α-syn fibrils can be taken up by tau overexpressing cells and induce the formation of phosphorylated triton-insoluble tau oligomers [
57]. In vivo studies further demonstrated that tau deposits can be found in mice overexpressing pathologic human α-syn mutations [
58].
In this study, we demonstrate that coaggregation of tau and α-syn takes place even at nanomolar protein concentrations, is strongly induced by trivalent metal ions, and differentially modulated by tau’s phosphorylation status. In contrast to the findings of Giasson et al., we did not detect tau and α-syn coaggregation in the absence of aggregation inducers. This apparent discrepancy is most likely explained by the differences in protein concentrations (up to 1000-fold) and the shorter observation time [
41]. However, our data demonstrate that tau and α-syn coaggregation occurs even at nanomolar protein concentrations in presence of aggregation inducers. As in our tau aggregation assay, Al
3+ had the most pronounced effect on coaggregation of α-syn with tau monomers and oligomers. Al
3+ induces rapid coaggregation of tau and α-syn (see Additional file
3). Fe
3+ and DMSO also induce coaggregation of the two proteins, though to an overall lesser extent. In our experiments, tau phosphorylation by GSK-3β strongly enhanced the formation of mixed oligomers induced by Al
3+ or Fe
3+. Moreover, the mixed oligomers resulting from metal ion induced coaggregation proved to be more resistant to SDS treatment than those formed in presence of DMSO.
We further provide data from FIDA analysis and gel filtration experiments demonstrating the presence and size of mixed tau and a-syn oligomers (see Additional files
6 and
7). Earlier studies applying atomic force micoscropy and SIFT established FIDA-based quantification of oligomer sizes as a reliable method [
38].
These results extend the current model of tau and α-syn interaction at early stages of neurodegeneration. As the cytoplasmic concentration of unbound tau and α-syn is low in pre-clinical neurodegeneration, the presence of pro-aggregatory factors may be crucial in the generation of early mixed oligomers. We identify trivalent metal ions and tau phosphorylation by GSK-3β as potential inducers of such oligomer formation even at nanomolar protein concentrations. A time-dependent increase of oligomer concentrations may then induce self-propagating and cross-seeding mechanisms as demonstrated by Giasson and Waxman [
41,
57].
Conclusions
In summary, we demonstrate that tau phosphorylation and trivalent metal ions such as Al
3+ act together in the formation of distinct SDS resistant tau oligomers. Moreover, applying confocal single molecule fluorescence techniques, we show that, under certain conditions, interactions of tau and α-syn can take place even at nanomolar protein concentrations, and result in the formation of mixed oligomers. Notably, the formation of SDS resistant mixed aggregates was induced by physiologically relevant concentrations of trivalent metal ions, and strongly enhanced by tau phosphorylation. Taking into account that a considerable amount of soluble tau exists in a phosphorylated state in neurodegenerative diseases [
1,
2,
59], these findings support a crucial role of specific metal ions such as Al
3+ and Fe
3+ in early cytoplasmic aggregation and coaggregation events.
Moreover, our results indicate common pathophysiologic mechanisms of both tau aggregation and cross-seeding phenomena, and might explain the coincidence of tau and α-syn in neuronal deposits. The mixed aggregates described here may provide an interlink of different pathological pathways leading to neurodegeneration, and may serve as promising therapeutic targets for future drug development. Furthermore, we introduce a novel technique of monitoring post-translational modifications at very low protein concentrations that may provide a powerful diagnostic tool in the future.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
GN1. A, B, C; 2. A, B, C; 3. A, B, BB1. A, B: 2. A, C; 3. B, JL1. A, B; 2. C; 3. B, JH2. A, B, C; 3. A, B, HK1. B; 3. B, AG1. A, B; 2. A, C; 3. B. Author contributions are abbreviated as follows: 1 = Research project: A. Conception, B. Organization, C. Execution. 2 = Statistical Analysis: A. Design, B. Execution, C. Review and Critique. 3 = Manuscript Preparation: A. Writing of the first draft, B. Review and Critique. All authors read and approved the final manuscript.