A central role for dityrosine crosslinking of Amyloid-β in Alzheimer’s disease
© Al-Hilaly et al.; licensee BioMed Central Ltd. 2013
Received: 3 December 2013
Accepted: 7 December 2013
Published: 18 December 2013
Alzheimer’s disease (AD) is characterized by the deposition of insoluble amyloid plaques in the neuropil composed of highly stable, self-assembled Amyloid-beta (Aβ) fibrils. Copper has been implicated to play a role in Alzheimer’s disease. Dimers of Aβ have been isolated from AD brain and have been shown to be neurotoxic.
We have investigated the formation of dityrosine cross-links in Aβ42 formed by covalent ortho-ortho coupling of two tyrosine residues under conditions of oxidative stress with elevated copper and shown that dityrosine can be formed in vitro in Aβ oligomers and fibrils and that these links further stabilize the fibrils. Dityrosine crosslinking was present in internalized Aβ in cell cultures treated with oligomeric Aβ42 using a specific antibody for dityrosine by immunogold labeling transmission electron microscopy. Results also revealed the prevalence of dityrosine crosslinks in amyloid plaques in brain tissue and in cerebrospinal fluid from AD patients.
Aβ dimers may be stabilized by dityrosine crosslinking. These results indicate that dityrosine cross-links may play an important role in the pathogenesis of Alzheimer’s disease and can be generated by reactive oxygen species catalyzed by Cu2+ ions. The observation of increased Aβ and dityrosine in CSF from AD patients suggests that this could be used as a potential biomarker of oxidative stress in AD.
KeywordsAmyloid Oligomer Aggregation Alzheimer’s disease Dityrosine Oxidative stress Electron microscopy
Amyloid fibrils are associated with a large number of diseases in which proteins or peptides abnormally assemble to form insoluble amyloid deposits in the tissues. The amyloid-β (Aβ) protein is an amyloidogenic peptide that is cleaved from the amyloid precursor protein (APP) . The Aβ peptide is present in all individuals but in Alzheimer’s disease (AD) it abnormally assembles and deposits in amyloid plaques in the neuropil . Amyloid fibrils are characterized by their significant insolubility and resistance to degradation . Aβ is thought to play a central role in AD pathology since several familial AD mutations are related to changes within the Aβ peptide itself, or in the proteins affecting its production . Aβ42 contains a tyrosine at position 10, located close to the fibrillogenic core of the peptide . However, the tyrosine residue can oxidize to many different modification products, such as nitrotyrosine and dityrosine .
Redox-active metal ions, such as Cu2+ and Fe3+, have been suggested to play a role in the pathogenesis of many neurodegenerative disorders, including AD and Parkinson’s disease . There is a large body of evidence pointing to the importance of interactions between redox-active metal ions and proteins in the pathogenesis of many diseases [7–9]. Two mechanisms have been suggested to explain the abnormalities of these interactions in neural tissue: (a) the aggregation of protein mediated by redox-active metal ions; and (b) metal catalyzed oxidation reactions (MCO) . Metal-protein interactions could result in oxidative stress through generation of reactive-oxygen species (ROS), which in turn induces lipid peroxidation, protein oxidation, and DNA damage [11, 12]. The consequences of protein oxidation are protein cross-links, amino acid side chain modifications, and protein fragmentation [11, 13]. The oxidative modification of proteins by ROS has been classified in two groups; (a) Global oxidative modifications, which include oxidation of multiple residues within protein to form several products; (b) Specific oxidative modifications which are very specific in both the residue oxidized and the product generated, e.g. oxidation of tyrosine residue to give dityrosine .
High concentrations of copper (0.4 mM), zinc (1 mM), and iron (1 mM) have been found in amyloid plaques and have been implicated in the pathogenesis of AD [10, 15]. Several studies have shown that Aβ is able to reduce copper and iron ions and to generate ROS [7, 9, 12, 16]. The most common in vivo source of ROS is hydrogen peroxide (H2O2) breakdown according to the Fenton reaction, which is catalyzed by a metal-protein complex .
The formation of dityrosine cross-links is one of the oxidative modifications that have been implicated in mediating toxicity of Aβ through Aβ aggregation. Several studies [17–20] demonstrate that Aβ can undergo dityrosine formation via two common biochemical pathways. One of them is peroxidase-catalyzed cross-linked tyrosine and the second mechanism is metal-catalyzed oxidative tyrosyl radical formation. The mechanism of dityrosine cross-links involves tyrosyl radical formation, followed by radical isomerisation and then diradical reaction, and finally enolisation . On the other hand, Smith et al. demonstrated that the generation of the Aβ toxic species is modulated by the concentration of Cu2+ ions and the ability to form intermolecular histidine bridges . Interestingly dityrosine has been selected as a biomarker for oxidative stress of proteins due to its chemical stability, as it remains unchanged by exposure to oxygen and high pH . Furthermore, it is highly resistant to acid hydrolysis and proteases [24, 25].
Here we have explored the in vitro formation of dityrosine in Aβ42 using Cu2+ ions and H2O2 in both early oligomeric Aβ42 and preformed Aβ42 fibrils and examined the effect of the cross-linking on the structure of the fibrils and the assembly competence of the oligomers. We have shown that dityrosine formation is inducible in both soluble and fibrillar Aβ42 and suggest that the formation of crosslinks may stabilize assemblies. Neuroblastoma cell cultures treated with non-oxidized Aβ42 oligomers showed formation of Aβ assemblies containing dityrosine surrounding and sometimes internalized into the cells. Examination of samples from AD patients revealed the presence of dityrosine linkages within amyloid plaques and its colocalisation with Aβ in cerebrospinal fluid pointing to a physiological relevance of dityrosine crosslinking in AD, and highlighting the importance of oxidative stress in the disease.
Synthesis of a dityrosine standard
A dityrosine standard was synthesized according to modifications of established procedures [20, 26]. 20 units of horse radish peroxidase (HRP) were added to a clear solution of (10 mM) N-acetyl-3,5-diiodo-L-tyrosine in 40 ml of (0.1 M) phosphate buffer pH 6.0 containing 10% of acetonitrile then mixed gently. Immediately, 0.48 ml of (1 M) H2O2 was added then the reaction mixture was stirred gently for 60 min at 24°C, quenched with 1.2 ml of (1 M) NaHSO3 and the pH of the mixture was adjusted to 7.5 with (1 M) NaOH. After stirring for 10 min the mixture was acidified to pH 3.0 with (2 M) KHSO4 and then extracted with ethyl acetate. The combined organic residues were concentrated under vacuum to give a tan residue, which was purified by flash chromatography to give 3,3′-diiodo-N,N’- diacetyl di-L-tyrosine. In turn, 3,3′-diiodo-N,N’- diacetyl di-L-tyrosine was hydrogenated in 50% methanol:acetic acid to give N,N′- diacetyl di-L-tyrosine, which was heated at reflux in a 1:1 mixture of tetrahydrofuran and concentrated HCl to give dityrosine. Gel filtration on sephadex G-10 was carried out in order to remove the salt and further purification was achieved using RP-HPLC. Finally, the purified dityrosine was dried using a freeze dryer (Edwards, England), and analyzed using a Bruker Daltonics APEX III 4.7 Tesla Fourier Transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) with electrospray source and found to be m/z 361.1392 compared with calculated m/z 361.1394. NMR was carried out and the 1H NMR spectrum contained six proton signals with chemical shifts of: 7.30 ppm (H, d), 7.21 ppm (H, s), 7.07 ppm (H, d), 4.26 ppm (H, m), 3.35 ppm (H, dd), and 3,26 ppm (H’, dd). This spectrum is very similar to that published elsewhere [27–29].
Preparation of fibrillar and oligomeric Aβ42
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) Aβ42 was purchased from rPeptide (Bogart, GA, USA). To remove preformed aggregates, the peptide was prepared as previously described  using HFIP >99.0%, followed by anhydrous DMSO (Fisher Sci.) and then buffer exchanged using a desalting column to remove the solvents. The concentration was determined using a molar extinction coefficient of 1490 M-1 cm-1 and the absorbance was measured at a wavelength of 280 nm using an Eppendorf Biophotometer (Eppendorf UK Ltd., Cambridge, UK). The resulting stock peptide concentrations of 90–130 μM were used immediately for early oligomeric Aβ42 experiments, or incubated for at least two weeks at room temperature (22°C) in order to generate Aβ42 fibrils (confirmed using TEM).
Oxidation of fibrillar and early oligomeric Aβ42
Stock solutions of soluble or fibrillar Aβ42 were diluted in (a) water or (b) 50 mM phosphate buffer pH 7.4 to obtain 20 μM Aβ42 as a final concentration and incubated with or without (20 μM) Cu2+ and (0.5 mM) H2O2 at 37°C with agitation in a shaking incubator for three days. The oxidation reaction was quenched using a final concentration of 250 μM EDTA.
The fibrils were resuspended by agitation. Fluorescence spectra were recorded over time. Fluorescence measurements were carried out on a Varian Cary Eclipse fluorimeter (Varian Ltd., Oxford, UK) using a 1 cm path length quartz cuvette (Starna, Essex, UK), and dityrosine fluorescence was monitored using an excitation wavelength of 320 nm. Dityrosine emission was monitored between 340 and 500 nm, with maximum fluorescence intensity at around 400–420 nm at a controlled temperature of 21°C. Tyrosine fluorescence signal was monitored using an excitation wavelength of 280 nm and emission wavelength of 305 nm. Excitation and emission slits were both set to 10 nm, and the scan rate was set to 300 nm/min with 2.5 nm data intervals and an averaging time of 0.5 s. The photomultiplier tube detector voltage was set at 500 V. To detect dityrosine fluorescence at early time points, 130 μl of the reaction mixture was removed and EDTA was added to final concentration of 250 μM.
Sample preparation for LC-ESIMS/MS analysis
Oxidized Aβ42 fibrils prepared in water were lyophilized using a Modulyo 4 K Freeze Dryer (Edwards, Crawley, England), and then hydrolysed using evacuated sealed tubes under acidic conditions of (6 M) HCl, 10% TFA, and 1% phenol at 110°C for 48 h. The resulting hydrolysate was then dried under nitrogen gas, dissolved in 100 μl of 0.1% formic acid in water and then filtered using a Millipore 0.22 μm filter into a 0.2 ml tube.
Detection of dityrosine by LC-ESIMS/MS
20 μl of oxidized Aβ42 fibril hydrolysate was injected on to a Phenomenex Gemini 3u C6-phenyl 110 (150 mm × 4.6 mm, 3 micron) column using a High performance liquid chromatography (HPLC) system (Waters Alliance 2695, Ireland) coupled to the mass spectrometer (MicroMass Quattro Premier, Waters, Ireland) operated in the multiple reaction-monitoring (MRM) mode with positive electrospray ionisation (ESI). The solvents for the mobile phase were A: 0.1% formic acid in water; and solvent B: 0.1% formic acid in acetonitrile. The gradients were as follows: t = 0 min, 0% B; t = 1 min, 0% B; t = 15 min, 100% B; t = 20 min, 100% B; t = 25 min, 0% B; t = 30 min, 0% B, and the flow rate was 200 μl/min. Mass spectrometric detection was performed by positive electrospray ionisation (ESI) tandem mass spectrometry on a triple quadrupole mass spectrometer (MicroMass Quattro Premier, Waters, Ireland). The conditions for the mass spectrometer were as follows; electrospray ionization spray voltage 3.5 kV, the cone voltage 35 V, the source temperature at 100°C, whereas the desolvation temperature was 400°C. Argon was used as the collision gas at 5.95 e-003 mbar at 26 ev collision energy.
Thioflavin T fluorescence assay
The fibril formation of Aβ42 was monitored using ThT fluorescence. A 0.2 μm filtered (3.14 mM) aqueous ThT stock solution was prepared and stored frozen at −20°C in 1–10 μl aliquots until required. ThT was added to a 10 μM Aβ42 sample (50 mM phosphate buffer pH 7.4) to a final concentration of 20 μM, gently vortexed, and allowed to bind for 3 minutes before a reading was taken. Using a microvolume cuvette of 1 cm path length, ThT fluorescence was measured using a Varian Cary Eclipse fluorimeter (Varian, Oxford, UK) with excitation wavelength of 450 nm. The emission spectrum was recorded between 460–600 nm at 21°C. Phosphate buffer baselines were subtracted from the data. Excitation and emission slits were set to 5 nm and 10 nm respectively. The scan rate was 600 nm/min with 1 nm data intervals and an averaging time of 0.1 s. The voltage on the photomultiplier tube was set to high (800 v) and experiments were carried out in triplicate to confirm trends.
Negative stain TEM
Four microliter aliquots of Aβ42 samples were placed onto Formvar/carbon coated 400-mesh copper grids (Agar Scientific, Essex, UK) for 1 min, and the excess was removed using filter paper. Subsequently the grid was washed using 4 μl of Milli-Q water filtered with 0.22 μm filter and blotted dry, then negatively stained twice with 4 μl of filtered 2% (w/v) uranyl acetate for 1 min and blotted dry. The grid was allowed to air-dry before examination on a Hitachi 7100 transmission electron microscope (Hitachi, Germany) fitted with a Gatan Ultrascan 1000 CCD camera (Gatan, Abingdon, UK) at an operating voltage of 100 kV.
Dityrosine cross-linked Aβ42 fibril stability
Aβ42 was assembled under oxidizing and non-oxidizing conditions and stored at −80°C for more than one year and the dityrosine content was assessed using fluorescence with an excitation wavelength of 320 nm as described above. Both oxidized and non-oxidized Aβ42 fibrils were centrifuged for 30 min at 16,000 RCF at 4°C, and then the soluble Aβ42 concentration in the supernatant was measured using absorbance at 280 nm (as described for peptide preparation). TEM grids were prepared to examine the Aβ42 fibrils in the pellet. The oxidized and non-oxidized fibrils were then dissolved in 80% v/v formic acid with agitation. The resulting solution was centrifuged using the same conditions above, and again the dissolved Aβ42 concentration was determined. TEM grids for the pellet were prepared to examine the morphology of the dissolved fibrils and to compare their density.
SDS gel electrophoresis
Aβ was incubated under oxidizing conditions as before for 10 mins and then quenched using EDTA. Oxidized and non-oxidized Aβ peptides were separated by SDS-PAGE and analyzed by densitometry. Samples were heated to 85°C for 5 min and then 5 μl added to each well of a Novex® 1.0 mm 10-20% Tricine gel, (Life Technologies Ltd, Paisley, UK). Oxidized and nonoxidized samples were run on the XCell SureLock® Mini-Cell with a PowerEase® 500 power supply (Life Technologies Ltd, Paisley, UK). A voltage of 125 V, an expected start current of 80 mA/gel and an expected end current of 40 mA/gel was applied. The gel was stained using SilverQuest™ Silver staining kit (Life Technologies Ltd, Paisley, UK) following the manufacturers protocol provided. Briefly, the gel was fixed for 60 min in 40% ethanol, 10% acetic acid, and 50% milliQ water, then rinsed in 30% ethanol (10 min), incubated in sensitizer solution (10 min), rinsed in 30% ethanol (10 min), washed with milliQ water (10 min), incubated in silver stain (15 min), and washed again with milliQ water (60 sec). The gel was then incubated in developer (5–10 min). Finally, the stopper was directly added to the stained gel (10 min) and subsequently washed with milliQ water (10 min). The gel was scanned at 8 bit with 600 dpi resolution. The densitometry was calculated using ImageJ.
Immunogold labeling, negative stain TEM (for fibrils and cerebrospinal fluid)
A modified phosphate-buffered saline, pH 8.2, containing 1% BSA, 500 μl/l Tween-20, 10 mM Na EDTA, and 0.2 g/l NaN3 (henceforward termed PBS+), was used throughout all the following procedures for all dilutions of antibodies and secondary gold probes. Aβ42 fibrils were oxidized and assembled as described in oxidation of fibrillar and early oligomeric Aβ42 and immunogold-labeled ‘on grid’ for dityrosine according to established methods. A monoclonal anti-dityrosine antibody was purchased from Japan Institute for the Control of Aging (JaICA) Shizuoka, Japan (cat. no. MDT-020P). The antibody has been fully characterised and shown to be highly specific and does not show any cross-reactivity with other tyrosine derivatives such as nitrotyrosine, chlorotyrosine . The antibody will detect any protein containing dityrosine. Briefly, 4 μl aliquots of the oxidized Aβ42 fibrils were pipetted onto Formvar/carbon coated 400 mesh copper TEM support grids (Agar Scientific, Essex, UK), left for 1 min, the excess was removed with filter paper, and then blocked in normal goat serum (1:10 in PBS+) for 15 min. Grids were then incubated with (10 μg/ml IgG) mouse dityrosine monoclonal antibody (JaICA, Shizuoka, Japan) for 2 h at room temperature, rinsed in 3×2 min PBS+, and then immunolabeled in a 10 nm gold particle-conjugated goat anti-mouse IgG secondary probe (GaM10 British BioCell International, Cardiff, UK; 1:10 dilution) for 1 h at room temperature. After 5×2 min PBS + and 5×2 min distilled water rinses, the grids were negatively stained as described in negative stain TEM.
Demographic details of cases from which cerebrospinal fluid were obtained
Alzheimer’s disease HP-tau stage 6 with mild to moderate amyloid angiopathy
Alzheimer’s disease HP-tau stage 6 with moderate amyloid angiopathy
Alzheimer’s disease HP-tau stage 6 with moderate amyloid angiopathy
Control case but with hypoxic-type changes and amyloid angiopathy
Control-some amyloid angiopathy
Immunogold Labeling TEM of sections
Demographic details of cases from which middle frontal gyrus tissues were obtained
Control case but with Hypoxic-type changes and amyloid angiopathy
Control-minimal ageing changes
Alzheimer’s disease HP-tau stage 6 with mild to moderate amyloid angiopathy
Alzheimer’s disease HP-tau stage 6 with moderate amyloid angiopathy
Alzheimer’s disease HP-tau stage 6 with moderate amyloid angiopathy
Alzheimer’s disease HP-tau stage 6 with mild amyloid angiopathy
Alzheimer’s disease -modified Braak BNE stage 5
The grids were examined on a Hitachi 7100 TEM (Hitachi, Germany) fitted with a Gatan Ultrascan 1000 CCD camera (Gatan, Abingdon, UK), and operating with a voltage of 100 kV.
Analysis of immunogold labeled sections
Counting and size analyses of immunogold particles were performed using in house software written in Matlab. Briefly, raw digital electron micrograph images files were uploaded and circular particles detected using inbuilt Matlab circular Hough transforms functions. The detection range diameter was calculated from user input and conversion to pixel size values read from the image header. 15 or 10 nm gold particles were distinguished from 5 nm particles based on a user-selected threshold of 7 nm.
In vitro oxidation of Aβ42 resulting in the formation of dityrosine cross-links
To gain further insight into the morphological changes to Aβ42 fibrils induced by oxidation, samples were examined using negative stain TEM (Figure 4). Aβ42 immediately following preparation (zero hour) showed small globular structures consistent with oligomeric species (5–30 nm) (Figure 4a and 4d). Under oxidation conditions the freshly dissolved Aβ42 oligomers again appeared to have a spherical appearance (5–25 nm) (Figure 4a) and these developed into larger, non-fibrillar structures after 24 hours (Figure 4b) and then into clustered fibrils after 48 hours (Figure 4c). After incubation for 24 hours in phosphate buffer only, fibrils were observed with both flat, striated ribbons, and twisted morphologies (Figure 4e) and these developed further at 48 hours (Figure 4f). TEM did not reveal any obvious differences in fibril density between Aβ incubated under oxidizing and non-oxidizing conditions, although the oxidized sample appears to show shorter, more self-associated fibrils, which may be consistent with some interfibrillar crosslinking. Fibrils were observed following 24 hour incubation in control samples, but not until 48 hours for the oxidized Aβ42.
In order to examine whether dityrosine crosslinks can be detected within the amyloid fibrils and to show the dityrosine distribution, TEM immunogold labeling using a dityrosine specific monoclonal antibody was performed. Figure 4e shows anti-dityrosine labeling on Aβ42 fibrils revealing gold distributed close to the fibrils grown in an oxidizing environment. Control Aβ42 did not label with the dityrosine antibody (Figure 4f). Very strong evidence of dityrosine identity was provided using these specific monoclonal dityrosine antibodies and these clear differences between oxidized and control also provides evidence for the specificity of the antibody. We are not able to rule out formation of trityrosine, but this would be expected to further strengthen the fibrils. Mass-spectrometry data (Figure 1b) shows strong evidence for the presence of dityrosine. These in vitro findings suggest that conditions similar to oxidative stress can promote the formation of dityrosine cross-links in self-assembling Aβ42 samples resulting in a high density within the fibrils. Interestingly, no fibrils were observed in the oxidized sample at 24 hours by TEM (data not shown), although there is a very strong signal by dityrosine fluorescence (Figure 2a & c). This supports the view that dityrosine cross-links form early in the assembly process.
Aβ42 is internalized into neuroblastoma cells and becomes dityrosine crosslinked
The presence of dityrosine within plaques in AD brain
Dityrosine as a potential biomarker of oxidative stress in AD
Given our results showing the colocalisation of antibodies against dityrosine and Aβ over amyloid fibrils deposited in Alzheimer’s brains and images showing that neuroblastoma cells incubated with Aβ42 show dityrosine-Aβ colocalisation over fibrillar material and in lysosomes, we were interested to see if dityrosine crosslinked Aβ could be identified in cerebrospinal fluid (CSF) and could therefore be a useful biomarker for Alzheimer’s disease.
Here, cell culture experiments show that conditions amenable to Aβ dityrosine crosslinking are also present in a cellular environment and that dityrosine crosslinked Aβ fibrils are found around cells and internalized into cells. Aβ and dityrosine co-localize within lysosomes. In previous work, we have shown that internalized oligomeric Aβ42 leads to accumulation of autophagosomes containing Aβ. Considerable evidence supports the view that intralysosomal Aβ accumulation can induce neuronal death [54–56] and here we have shown that oligomers and fibrils can be stabilized by dityrosine crosslinking. Studies using Aβ42 with the tyrosine 10 substituted for alanine mutant showed significantly reduced H2O2 production and prevented toxicity to primary cortical neurons supporting the view that dityrosine crosslinking may be important in mediating oligomer toxicity . Here, electron micrographs show the internalization of dityrosine crosslinked Aβ fibrillar material into the cytoplasm (Figure 7) and into lysosomes (Figure 6). The oligomeric Aβ may be stabilized by covalent crosslinks and provide a stable nucleus for assembly (Figure 10).
Oxidative stress has been widely implicated in AD pathogenesis. Sources of ROS, such as H2O2, superoxide anion, and hydroxyl radical, can be formed from different in vivo sources (e.g. trace metals, photochemical, and enzymatic reactions) although mitochondria represent the main in vivo source of ROS formation . At the same time mitochondria are the main target of ROS attack , resulting in the formation of peroxidized, undegradable macromolecules. H2O2 can easily diffuse into lysosomes and react with iron ions that are released from the degradation of different metalloproteins during their intralysosomal degradation. The interaction of H2O2 with iron ions results in the formation of the highly reactive hydroxyl radicals. The latter would attack intralysosomal macromolecules, such as Aβ, causing cross-linking of these materials. The oxidative modification especially cross-linking of autophagocytosed material, is the most probable cause of non-degradability of these materials. The excessive accumulation of these non-digested materials could result in endosomal/lysosomal leakage and as a consequent acid hydrolase enzymes will be released and ultimately leading to cell death . Recently, Murakami and Shimizu have clarified the role of cytoplasmic superoxide radical as a possible contributing factor to intracellular Aβ oligomerization in AD . It also has been suggested that intraneuronal Aβ oligomers cause neuronal death by activating endoplasmic reticulum stress, endosomal/lysosomal leakage and mitochondria dysfunction [61, 62]. Kurz et al., (2008)  have highlighted a close relation between lysosomes and mitochondria, explaining that accumulation of iron inside mitochondria will result in lysosomal iron loading as a consequence of degradation of mitochondria by lysosomal enzymes. Moreover, normal production of H2O2 by mitochondria results in oxidative stress, which will labilize lysosomes, and further oxidative stress could result from degradation of mitochondria by lysosomal enzymes.
We have shown that amyloid plaques in AD brain tissue show extensive dityrosine crosslinking and this may suggest that these highly stable insoluble deposits are stabilized by the covalent crosslinking resulting in a resistance to degradation. Therefore, the existence of dityrosine may be relevant in AD pathology. Friedrich et al., (2010)  have shown that Aβ internalized to cultured cells can accumulate and assemble resulting in eventual cell death and thus they suggest that the formation of amyloid plaques might arise from accumulated intracellular Aβ. Therefore, there may be a pathway from the lysosomal/autophagsosome accumulation of dityrosine crosslinked Aβ that we have observed in neuroblastoma cells to the eventual deposition as amyloid plaques composed of dityrosine crosslinked Aβ observed here in AD tissue. Previous work has revealed that dimeric (9 kDa) Aβ can be isolated from human AD brain  and this may be a crosslinked dimer. In future work, it would be important to fully characterize these dimers.
Clioquinol is a potent Cu/Zn chelator and has been shown to significantly reduce Aβ amyloid deposition in an APP transgenic mouse  and has shown some efficacy in human AD subjects . These studies have been taken to suggest that copper plays a very significant role in Aβ deposition in AD and could imply that dityrosine formation can stabilize deposits.
A few studies have previously attempted to quantify dityrosine in cerebrospinal fluid. Techniques including HPLC with electrochemical array detection (HPLC-ECD) or fluorescence detection, and liquid chromatography with triple quadrapole mass spectrometric detection (LC-MS/MS) have been applied to quantify dityrosine concentrations in CSF sample from both healthy and disease affected subjects [5, 68, 69]. However, the results of these studies have been conflicting, perhaps due to differences in sample handling and preparation before measuring. Quantitative screening of protein glycation, oxidation and nitration adducts in CSF of AD and healthy age-matched subjects has been taken to suggest that dityrosine concentration do not change in AD patients with respect to control subjects . By contrast, earlier studies using electrochemical detection showed that dityrosine concentration was elevated markedly in CSF of AD patients . Studies have suggested that CSF is depleted of Aβ in AD patients, however, careful consideration of the results shows highly variable amounts of Aβ in CSF from patients . Here we clearly show, using a specific antibody against the free N-terminus of Aβ42, that Aβ more abundant within AD CSF relative to the age matched controls.
Our results have revealed very strong evidence for the presence of both Aβ and dityrosine in CSF from AD patients and both Aβ and dityrosine are very rarely observed in CSF from age-matched controls. Although this observation needs to be further substantiated with further AD cases, this points to the possible use as a potential biomarker for AD.
At equimolar ratio of Cu2+ and Aβ42, Cu2+ can catalyze tyrosine oxidation by H2O2, leading to dityrosine cross-link generation, and the latter can cause Aβ misfolding, resulting in Aβ assembly into oligomers and subsequently into amyloid fibres. The dityrosine cross-links stabilize Aβ fibres once formed.
Here we present a comprehensive study of in vivo and in vitro dityrosine cross-linking associated with Aβ. We have revealed the presence of dityrosine cross-links in amyloid plaques in human AD brain, and also in CSF, and identified significant relationships between dityrosine cross-links and amyloid deposit formation. The in vitro and in vivo results presented here reveal that oligomeric Aβ can undergo oxidative modification in a cellular environment, resulting in cross-linked Aβ through dityrosine followed by intralysosomal Aβ accumulation that could lead to lysosomal leakage and cell death. Our results show the significant accumulation of dityrosine crosslinked Aβ in amyloid plaques, implying a role in stabilization of these insoluble deposits. We have also shown a potential biomarker for AD in CSF, which contains elevated dityrosine crosslinked proteins as well as elevated dityrosine crosslinked Aβ (Figure 9). Research is ongoing to further characterize the crosslinked Aβ as a biomarker in CSF.
This work was supported by funding from Alzheimer’s research UK and Biotechnology and Biological Sciences Research Council, UK. YA is supported by funding Ministry of Higher Education and Scientific Research in Iraq.
The authors would like to acknowledge help with synthesis and characterisation of dityrosine from Dr Matthew Stanley, Dr Iain Day and Prof. Mark Bagley. Brain tissue was provided by the London Neurodegenerative Diseases Brain Bank, Institute of Psychiatry, King’s College, London, and, respectfully, we thank the anonymous tissue donors and their next of kin. The authors thank Prof. Guy Richardson and Dr Richard Goodyear for MAb10 antibody.
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