Nitration of Tyrosine Residue Y10 of Aβ1-42 Significantly Inhibits Its Aggregation and Cytotoxicity

Impact of tyrosine amination on the aggregation and neurotoxicity of amyloid-β

The tyrosine residue in amyloid-β (Aβ) is susceptible to attack by various reactive nitrogen intermediates, leading to the formation of 3-nitrotyrosine (3-NT), a post-translational modification associated with the pathophysiology of Alzheimer's disease (AD). Although considered a "dead-end" product, emerging evidence suggests that 3-NT can be reduced to 3-aminotyrosine (3-AT) in vivo. This study aims to validate the amination of Aβ tyrosine under physiological conditions and systematically investigate its impact on the aggregation and neurotoxicity of Aβ42. Our investigations reveal that tyrosine amination mitigates the highly ordered β-structure content of Aβ42, thereby modulating its aggregation pathway, which is primarily dominated by the multi-step secondary nucleation. Aminotyrosine fibrils exhibit enhanced fragmentation, increasing fibril elongation rate, and insoluble aggregate production. Concurrently, tyrosine amination attenuates the neurotoxicity of Aβ42 by reducing intracellular reactive oxygen species (ROS) production and mitigating cell membrane disruption. Tyrosine amination substantially alters the aggregation and physiological properties of Aβ42. Nitration of Aβ42 and subsequent conversion to tyrosine-aminated Aβ42 may represent an intrinsic defensive response against AD under nitrative stress.

Nitration of Tyr37 alters the aggregation pathway of hIAPP and enhances its cytotoxicity

The amyloid aggregation of hIAPP and the increased level of oxidative stress are closely related to the occurrence and development of type 2 diabetes (T2D). Protein tyrosine nitration is a common post-translational modification under oxidative stress conditions. We previously found that tyrosine nitrated hIAPP (3-NT-hIAPP) has higher cytotoxicity than wild type hIAPP. In order to further elucidate the mechanism by which tyrosine nitration enhances the toxicity of hIAPP, we systematically studied the effect of tyrosine nitration on hIAPP aggregation and its impact on INS-1 cells. Collective experimental data from ThT, RLS, DLS, zeta potentials, Bis-ANS, 1H NMR, TEM, dye leakage and hemolysis confirmed that tyrosine nitration accelerates hIAPP aggregation, consistent with tyrosine nitration reducing hIAPP zeta potential, but 3-NT-hIAPP mainly undergoes an off-pathway aggregation to form amorphous aggregates, even in the presence of POPC/POPG LUVs. Further, our results confirmed that the most toxic species are the small amorphous aggregates formed by 3-NT-hIAPP, which is more stable and toxic than hIAPP oligomers. Collectively, these data suggest that tyrosine nitration can increase cytotoxicity of hIAPP by modulating its amyloidogenicity. This study provides new support for the fact that oxidative stress promotes the development of T2D from the view of nitrative stress.

Serine phosphorylation mimics of Aβ form distinct, non-cross-seeding fibril morphs

The self-assembly of amyloid-β peptide (Aβ) into fibrils and oligomers is linked to Alzheimer's disease (AD). Fibrillar aggregates in AD patient's brains contain several post-translational modifications, including phosphorylation at positions 8 and 26. These play a key role in modifying the aggregation propensity of Aβ, yet how they affect the mechanism of aggregation is only poorly understood. Here we elucidate the aggregation mechanism of Aβ42 peptides with phosphomimic mutations at these positions, with glutamine mimicking the size, and glutamate mimicking both the size and charge effect. We find that all variants are less aggregation-prone than wild-type Aβ42 with the glutamate mutants showing the largest reduction. Secondary nucleation is the dominant nucleation route for all variants, as confirmed using seeding experiments; however, its rate is reduced by about an order of magnitude or more for all variants relative to wild-type. S26Q and S26E fibrils fail to catalyse nucleation of wild-type monomers and vice versa, while the S8 variants co-aggregate more readily with wild-type. Ultrastructural analyses by cryo-electron microscopy and small angle X-ray scattering reveal an altered structure with longer node-to-node distance and smaller cross-section dimensions of S26Q fibrils. These results imply that structural compatibility between fibrils and monomer is a key determinant in secondary nucleation, and that small modifications can alter the preferred fibril structure, and thus its potential to induce aggregation of other variants. Overall, our results indicate that phosphorylation could play a key role in controlling aggregation propensity and may lead to the formation of distinct, non-cross-seeding fibril populations.

Red-Light-Photosensitized Tyrosine 10 Nitration of β-Amyloid1-42 Diverts the Protein from Forming Toxic Aggregates

Several studies have highlighted the presence of nitration damage following neuroinflammation in Alzheimer's disease (AD). Accordingly, post-transcriptional modifications of β-amyloid (Aβ), including peptide nitration, have been explored as a marker of the disease. However, the implications of Aβ nitration in terms of aggregation propensity and neurotoxicity are still debated. Here, we show new data obtained using a photoactivatable peroxynitrite generator (BPT-NO) to overcome the limitations associated with chemical nitration methods. We found that the photoactivation of BPT-NO with the highly biocompatible red light selectively induces the nitration of tyrosine 10 of freshly solubilized full-length Aβ1-42. Photonitrated Aβ1-42 was, therefore, investigated for aggregation states and functions. It resulted that photonitrated Aβ1-42 did not aggregate into small oligomers but rather self-assembled into large amorphous aggregates. When tested on neuronal-like SH-SY5Y cells and microglial C57BL/6 BV2 cells, photonitrated Aβ1-42 showed to be free of neurotoxicity and able to induce phagocytic microglia cells. We propose that light-controlled nitration of the multiple forms in which Aβ occurs (i.e., monomers, oligomers, fibrils) could be a tool to assess in real-time the impact of tyrosine nitration on the amyloidogenic and toxic properties of Aβ1-42.

Structure and mechanism behind the inhibitory effect of water soluble metalloporphyrins on Aβ1-42 aggregation

Although the exact molecular mechanism of the pathogenesis of Alzheimer’s disease (AD) is still unclear, compounds that can inhibit the aggregation of amyloid-β peptide (Aβ1-42) or scavenge the highly toxic...

Post-translational modifications: Regulators of neurodegenerative proteinopathies

One of the hallmark features in the neurodegenerative disorders (NDDs) is the accumulation of aggregated and/or non-functional protein in the cellular milieu. Post-translational modifications (PTMs) are an essential regulator of non-functional protein aggregation in the pathogenesis of NDDs. Any alteration in the post-translational mechanism and the protein quality control system, for instance, molecular chaperone, ubiquitin-proteasome system, autophagy-lysosomal degradation pathway, enhances the accumulation of misfolded protein, which causes neuronal dysfunction. Post-translational modification plays many roles in protein turnover rate, accumulation of aggregate and can also help in the degradation of disease-causing toxic metabolites. PTMs such as acetylation, glycosylation, phosphorylation, ubiquitination, palmitoylation, SUMOylation, nitration, oxidation, and many others regulate protein homeostasis, which includes protein structure, functions and aggregation propensity. Different studies demonstrated the involvement of PTMs in the regulation of signaling cascades such as PI3K/Akt/GSK3β, MAPK cascade, AMPK pathway, and Wnt signaling pathway in the pathogenesis of NDDs. Further, mounting evidence suggests that targeting different PTMs with small chemical molecules, which acts as an inhibitor or activator, reverse misfolded protein accumulation and thus enhances the neuroprotection. Herein, we briefly discuss the protein aggregation and various domain structures of different proteins involved in the NDDs, indicating critical amino acid residues where PTMs occur. We also describe the implementation and involvement of various PTMs on signaling cascade and cellular processes in NDDs. Lastly, we implement our current understanding of the therapeutic importance of PTMs in neurodegeneration, along with emerging techniques targeting various PTMs.

Insights Into the Mechanism of Tyrosine Nitration in Preventing β-Amyloid Aggregation in Alzheimer's Disease

Nitration of tyrosine at the tenth residue (Tyr10) in amyloid-β (Aβ) has been reported to reduce its aggregation and neurotoxicity in our previous studies. However, the exact mechanism remains unclear. Here, we used Aβ_1-42 peptide with differently modified forms at Tyr10 to investigate the molecular mechanism to fill this gap. By using immunofluorescent assay, we confirmed that nitrated Aβ was found in the cortex of 10-month-old female triple transgenic mice of Alzheimer's disease (AD). And then, we used the surface-enhanced Raman scattering (SERS) method and circular dichroism (CD) to demonstrate that the modification and mutation of Tyr10 in Aβ have little impact on conformational changes. Then, with the aids of fluorescence assays of thioflavin T and 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid, transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS), we found that adding a large group to the phenolic ring of Tyr10 of Aβ could not inhibit Aβ fibrilization and aggregation. Nitration of Aβ reduces its aggregation mainly because it could induce the deprotonation of the phenolic hydroxyl group of Tyr10 of Aβ at physiological pH. We proposed that the negatively charged Tyr10 caused by nitration at physiological pH could interact with the salt bridge between Glu11 and His6 or His13 and block the kink around Tyr10, thereby preventing Aβ fibrilization and aggregation. These findings provide us new insights into the relationship between Tyr10 nitration and Aβ aggregation, which would help to further understand that keeping the balance of nitric oxide in vivo is important for preventing AD.

3-Nitrotyrosine and related derivatives in proteins: precursors, radical intermediates and impact in function

Oxidative post-translational modification of proteins by molecular oxygen (O2)- and nitric oxide (•NO)-derived reactive species is a usual process that occurs in mammalian tissues under both physiological and pathological conditions and can exert either regulatory or cytotoxic effects. Although the side chain of several amino acids is prone to experience oxidative modifications, tyrosine residues are one of the preferred targets of one-electron oxidants, given the ability of their phenolic side chain to undergo reversible one-electron oxidation to the relatively stable tyrosyl radical. Naturally occurring as reversible catalytic intermediates at the active site of a variety of enzymes, tyrosyl radicals can also lead to the formation of several stable oxidative products through radical-radical reactions, as is the case of 3-nitrotyrosine (NO2Tyr). The formation of NO2Tyr mainly occurs through the fast reaction between the tyrosyl radical and nitrogen dioxide (•NO2). One of the key endogenous nitrating agents is peroxynitrite (ONOO-), the product of the reaction of superoxide radical (O2•-) with •NO, but ONOO--independent mechanisms of nitration have been also disclosed. This chemical modification notably affects the physicochemical properties of tyrosine residues and because of this, it can have a remarkable impact on protein structure and function, both in vitro and in vivo. Although low amounts of NO2Tyr are detected under basal conditions, significantly increased levels are found at pathological states related with an overproduction of reactive species, such as cardiovascular and neurodegenerative diseases, inflammation and aging. While NO2Tyr is a well-established stable oxidative stress biomarker and a good predictor of disease progression, its role as a pathogenic mediator has been laboriously defined for just a small number of nitrated proteins and awaits further studies.

Y12 nitration of human calcitonin (hCT): A promising strategy to produce non-aggregation bioactive hCT

Irreversible aggregation can extremely limit the bioavailability and therapeutic activity of peptide-based drugs. There is therefore an urgent demand of effective strategy to control peptide aggregation. Recently, we found that tyrosine nitration at certain sites of peptide can effectively inhibit its aggregation. This minor modification may be an ideal strategy to the rational design of peptide-based drugs with low aggregation propensity yet without loss of bioactivity. Human calcitonin (hCT) is such a peptide hormone known for its hypocalcaemic effect but has limited pharmaceutical potential due to a high tendency to aggregate. In this study, by using multiple techniques including Fluorescence, TEM, Nu-PAGE and CD, we demonstrated that Y12 nitration of hCT would significantly inhibit its self-assembles, and we also found that this modification would not only reduce the cytotoxicity induced by peptide aggregation, but also had little effect on its potency. This finding may provide a novel strategy for clinically application of hCT instead of sCT.

Halogenation of the N-Terminus Tyrosine 10 Promotes Supramolecular Stabilization of the Amyloid-β Sequence 7-12

Here, we demonstrate that introduction of halogen atoms at the tyrosine 10 phenol ring of the DSGYEV sequence derived from the flexible amyloid-β N-terminus, promotes its self-assembly in the solid state. In particular, we report the crystal structures of two halogen-modified sequences, which we found to be stabilized in the solid state by halogen-mediated interactions. The structural study is corroborated by Non-Covalent Interaction (NCI) analysis. Our results prove that selective halogenation of an amino acid enhances the supramolecular organization of otherwise unstructured biologically-relevant sequences. This method may develop as a general strategy for stabilizing highly polymorphic peptide regions.

Nitration of hIAPP promotes its toxic oligomer formation and exacerbates its toxicity towards INS-1 cells

Amyloid formation of human islet amyloid polypeptide (hIAPP) is one of the most common pathological features of type 2 diabetes (T2D). Increasing evidences have shown that the overproduction of reactive oxygen species (ROS) and reactive nitrogen species (RNS) play an important role in the development of the T2D. Interestingly, our previous studies indicated that heme could bind to hIAPP, and the complex might induce the nitration of tyrosine residue (Y37) of hIAPP in the presence of hydrogen peroxide and nitrite. However, it remains unclear about effect of the nitration on the implicated function of hIAPP in the development of T2D. In this study, fluorescent assays, transmission electron microscopy (TEM), atomic force microscope (AFM) were used to demonstrate that nitration of hIAPP significantly decreased its fibril formation. But the decreased fibril formation was not through the diminished aggregation of hIAPP monomer as suggested by the results of circular dichroism spectroscopy (CD) and gel electrophoresis assay. Surface-enhanced raman spectroscopy (SERS) indicated that nitration of hIAPP impaired the intermolecular hydrogen bonding. On the basis of these results, we hypothesize that nitration of hIAPP may block the intermolecular hydrogen bonding, leading to the inhibition of its fibril formation. In addition, cytotoxicity study of native and modified hIAPP was also performed on INS-1 cells, which revealed exacerbated toxicity of hIAPP by its nitration. The findings in this study that nitration of hIAPP promotes its oligomer formation and thus exacerbates its cytotoxicity suggests a possible link between the nitrite (or the sum of nitrite and nitrate) levels and T2D, and ameliorated nitration of hIAPP by diminishing nitrative stress might be a promising therapeutic strategy for T2D.