Complicated function of dopamine in Aβ-related neurotoxicity: Dual interactions with Tyr10 and SNK(26–28) of Aβ

Stabilization and Reduced Cytotoxicity of Amyloid Beta Aggregates in the Presence of Catechol Neurotransmitters

Oligomeric aggregates of the amyloid-beta (Aβ) peptide have been implicated as the toxic species for Alzheimer's disease by contributing to oxidative cytotoxicity and physical disruption in cell membranes in the brain. Recent evidence points to the ability of the catecholamine neurotransmitter dopamine in the presence of copper ions to both stabilize oligomers and decrease the toxic effects of these oligomers. Based on these results, physical characterization of aggregates and subsequent cell studies with a neuroblastoma line were performed that show both dopamine and the related neurotransmitter, norepinephrine, can stabilize oligomers and decrease toxicity of Aβ aggregates without copper present. To investigate this reduction of toxicity, structural characterization of oligomers in the presence of neurotransmitters was compared to aggregates formed with Aβ alone. Gel electrophoresis and transmission electron microscopy show higher levels of oligomers in the presence of dopamine and norepinephrine, yet the oligomer structure is largely amorphous. Aβ aggregated alone forms the predicted highly organized fibrillar species, with increased levels of dityrosine covalent linkages, which are largely absent in the presence of the neurotransmitters. A proposed mechanism for the observed decrease in cell death by Aβ in the presence of dopamine and norepinephrine suggests the neurotransmitters both block the formation of organized oligomer structures and dityrosine stabilizing linkages while also behaving as antioxidants, providing a dual mechanism for increased cell viability.

Understanding How Physical Exercise Improves Alzheimer’s Disease: Cholinergic and Monoaminergic Systems

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder, characterized by the accumulation of proteinaceous aggregates and neurofibrillary lesions composed of β-amyloid (Aβ) peptide and hyperphosphorylated microtubule-associated protein tau, respectively. It has long been known that dysregulation of cholinergic and monoaminergic (i.e., dopaminergic, serotoninergic, and noradrenergic) systems is involved in the pathogenesis of AD. Abnormalities in neuronal activity, neurotransmitter signaling input, and receptor function exaggerate Aβ deposition and tau hyperphosphorylation. Maintenance of normal neurotransmission is essential to halt AD progression. Most neurotransmitters and neurotransmitter-related drugs modulate the pathology of AD and improve cognitive function through G protein-coupled receptors (GPCRs). Exercise therapies provide an important alternative or adjunctive intervention for AD. Cumulative evidence indicates that exercise can prevent multiple pathological features found in AD and improve cognitive function through delaying the degeneration of cholinergic and monoaminergic neurons; increasing levels of acetylcholine, norepinephrine, serotonin, and dopamine; and modulating the activity of certain neurotransmitter-related GPCRs. Emerging insights into the mechanistic links among exercise, the neurotransmitter system, and AD highlight the potential of this intervention as a therapeutic approach for AD.

Cytotoxic Aβ Protofilaments Are Generated in the Process of Aβ Fibril Disaggregation

Significant research on Alzheimer's disease (AD) has demonstrated that amyloid β (Aβ) oligomers are toxic molecules against neural cells. Thus, determining the generation mechanism of toxic Aβ oligomers is crucial for understanding AD pathogenesis. Aβ fibrils were reported to be disaggregated by treatment with small compounds, such as epigallocatechin gallate (EGCG) and dopamine (DA), and a loss of fibril shape and decrease in cytotoxicity were observed. However, the characteristics of intermediate products during the fibril disaggregation process are poorly understood. In this study, we found that cytotoxic Aβ aggregates are generated during a moderate disaggregation process of Aβ fibrils. A cytotoxicity assay revealed that Aβ fibrils incubated with a low concentration of EGCG and DA showed higher cytotoxicity than Aβ fibrils alone. Atomic force microscopy imaging and circular dichroism spectrometry showed that short and narrow protofilaments, which were highly stable in the β-sheet structure, were abundant in these moderately disaggregated samples. These results indicate that toxic Aβ protofilaments are generated during disaggregation from amyloid fibrils, suggesting that disaggregation of Aβ fibrils by small compounds may be one of the possible mechanisms for the generation of toxic Aβ aggregates in the brain.

A Comprehensive Insight into the Mechanisms of Dopamine in Disrupting Aβ Protofibrils and Inhibiting Aβ Aggregation

Fibrillary aggregates of amyloid-β (Aβ) are the pathological hallmark of Alzheimer's disease (AD). Clearing Aβ deposition or inhibiting Aβ aggregation is a promising approach to treat AD. Experimental studies reported that dopamine (DA), an important neurotransmitter, can inhibit Aβ aggregation and disrupt Aβ fibrils in a dose-dependent manner. However, the underlying molecular mechanisms still remain mostly elusive. Herein, we investigated the effect of DA on Aβ₄₂ protofibrils at three different DA-to-Aβ molar ratios (1:1, 2:1, and 10:1) using all-atom molecular dynamics simulations. Our simulations demonstrate that protonated DA at a DA-to-Aβ ratio of 2:1 exhibits stronger Aβ protofibril disruptive capacity than that at a molar-ratio of 1:1 by mostly disrupting the F4-L34-V36 hydrophobic core. When the ratio of DA-to-Aβ increases to 10:1, DA has a high probability to bind to the outer surface of protofibril and has negligible effect on the protofibril structure. Interestingly, at the same DA-to-Aβ ratio (10:1), a mixture of protonated (DA⁺) and deprotonated (DA⁰) DA molecules significantly disrupts Aβ protofibrils by the binding of DA⁰ to the F4-L34-V36 hydrophobic core. Replica-exchange molecular dynamics simulations of Aβ₄₂ dimer show that DA⁺ inhibits the formation of β-sheets, K28-A42/K28-D23 salt-bridges, and interpeptide hydrophobic interactions and results in disordered coil-rich Aβ dimers, which would inhibit the subsequent fibrillization of Aβ. Further analyses reveal that DA disrupts Aβ protofibril and prevents Aβ dimerization mostly through π-π stacking interactions with residues F4, H6, and H13, hydrogen bonding interactions with negatively charged residues D7, E11, E22 and D23, and cation-π interactions with residues R5. This study provides a complete picture of the molecular mechanisms of DA in disrupting Aβ protofibril and inhibiting Aβ aggregation, which could be helpful for the design of potent drug candidates for the treatment/intervention of AD.

Metal-dependent interactions of metallothionein-3 β-domain with amyloid-β peptide and related physiological implications

Aberrant interactions of metal ions with amyloid-β peptide (Aβ) can potentiate Alzheimer's disease (AD) by participating in the aggregation process of Aβ and in the generation of reactive oxygen species (ROS). Metallothionein-3 (MT3), which is aberrantly expressed in AD brains, is believed to play an important role in the AD progression due to its ability of maintaining metal homeostasis and scavenging ROS. However, the related molecular mechanism is not clear. In this work, the metal-dependent interactions of MT3 β-domain (βMT3) with amyloid-β peptide (Aβ) were systematically studied. The results showed that Zn₃-βMT3 has a higher affinity to Aβ (K_d: ~0.7 μM) than Cu₄-βMT3 (K_d: ~22 μM). In Zn₃-βMT3, both Pro⁷ and Pro⁹ face outwards with their five-member rings in parallel, favoring their binding with aromatic residues via CH/π interactions. Two aromatic residues (Phe⁴ and Tyr¹⁰) in Aβ were identified as the specific binding sites for βMT3. Based on these, we posit a characteristic in-situ protection role of Zn-MT3 in inhibiting the Cu²⁺-induced Aβ neurotoxicity, in which stable Zn-MT3/Aβ complex forms via the Zn₃-βMT3/Aβ interaction and effectively prevents the formation of Cu-Aβ in high viscosity physiological fluids. Our results provide the mechanistic pathway and the specific roles of βMT3 in its protective bioactivity against AD progression, which means significant for elucidating the function of MT3 in AD neuropathology and for designing a MT3-related therapeutic strategy for AD.

Monoamines and their Derivatives on GPCRs: Potential Therapy for Alzheimer's Disease

Albeit cholinergic depletion remains the key event in Alzheimer's Disease (AD), recent information describes stronger links between monoamines (trace amines, catecholamines, histamine, serotonin, and melatonin) and AD than those known in the past century. Therefore, new drug design strategies focus efforts to translate the scope on these topics and to offer new drugs which can be applied as therapeutic tools in AD. In the present work, we reviewed the state-of-art regarding genetic, neuropathology and neurochemistry of AD involving monoamine systems. Then, we compiled the effects of monoamines found in the brain of mammals as well as the reported effects of their derivatives and some structure-activity relationships. Recent derivatives have triggered exciting effects and pharmacokinetic properties in both murine models and humans. In some cases, the mechanism of action is clear, essentially through the interaction on G-protein-coupled receptors as revised in this manuscript. Additional mechanisms are inhibition of enzymes for their biotransformation, regulation of free-radicals in the central nervous system and others for the effects on Tau phosphorylation or amyloid-beta accumulation. All these data make the monoamines and their derivatives attractive potential elements for AD therapy.

Regulatory Activities of Dopamine and Its Derivatives toward Metal-Free and Metal-Induced Amyloid-β Aggregation, Oxidative Stress, and Inflammation in Alzheimer's Disease

A catecholamine neurotransmitter, dopamine (DA), is suggested to be linked to the pathology of dementia; however, the involvement of DA and its structural analogues in the pathogenesis of Alzheimer's disease (AD), the most common form of dementia, composed of multiple pathogenic factors has not been clear. Herein, we report that DA and its rationally designed structural derivatives (1-6) based on DA's oxidative transformation are able to modulate multiple pathological elements found in AD [i.e., metal ions, metal-free amyloid-β (Aβ), metal-bound Aβ (metal-Aβ), and reactive oxygen species (ROS)], with demonstration of detailed molecular-level mechanisms. Our multidisciplinary studies validate that the protective effects of DA and its derivatives on Aβ aggregation and Aβ-mediated toxicity are induced by their oxidative transformation with concomitant ROS generation under aerobic conditions. In particular, DA and the derivatives (i.e., 3 and 4) show their noticeable anti-amyloidogenic ability toward metal-free Aβ and/or metal-Aβ, verified to occur via their oxidative transformation that facilitates Aβ oxidation. Moreover, in primary pan-microglial marker (CD11b)-positive cells, the major producers of inflammatory mediators in the brain, DA and its derivatives significantly diminish inflammation and oxidative stress triggered by lipopolysaccharides and Aβ through the reduced induction of inflammatory mediators as well as upregulated expression of heme oxygenase-1, the enzyme responsible for production of antioxidants. Collectively, we illuminate how DA and its derivatives could prevent multiple pathological features found in AD. The overall studies could advance our understanding regarding distinct roles of neurotransmitters in AD and identify key interactions for alleviation of AD pathology.

Light-Up Nonthiolated Aptasensor for Low-Mass, Soluble Amyloid-β40 Oligomers at High Salt Concentrations

Herein, a light-up nonthiolated aptasensor was developed for low-mass, soluble amyloid-β₄₀ oligomers (LS-Aβ_40-O). Au nanoparticles (AuNP) were employed as colorimetric probes, and the nonthiolated aptamers (Apt) were adsorbed on AuNP surfaces, acting as binding elements for LS-Aβ_40-O. The aggregation of AuNPs was induced when Apt-modified AuNPs (Apt@AuNPs) were under high-salt conditions. However, upon the addition of LS-Aβ_40-O into the Apt@AuNP solution, the salt tolerance of the AuNPs was greatly enhanced. Further studies confirmed that the formed LS-Aβ_40-O-Apt complex attached onto the AuNP surfaces via interactions between LS-Aβ_40-O and Au, which led to electrostatic and steric stabilization of the AuNPs under high-salt conditions. On the basis of this outcome, a sensitive light-up nonthiolated aptasensor for LS-Aβ_40-O was achieved with a detection limit of 10.0 nM and a linear range from 35.0 to 700 nM in a 175 mM NaCl solution. Cerebrospinal-fluid (CSF) samples from healthy persons and Alzheimer's disease (AD) patients were successfully distinguished by using this proposed method. The concentrations of LS-Aβ_40-O in the CSF of AD patients were of nanomolar grade, but there was no detectable LS-Aβ_40-O in those of the healthy persons. This work provides a new insight into the interaction between Apt@AuNPs and Aβ_40-O and also develops a simple, rapid, highly selective and sensitive, and applicable method for LS-Aβ_40-O detection in real CSF samples, which is significant for the diagnosis of AD.

Role of norepinephrine in Aβ-related neurotoxicity: dual interactions with Tyr10 and SNK(26-28) of Aβ

With their capability to inhibit the formation of amyloid-β peptide (Aβ) fibril, norepinephrine (NE), and other catechol derivatives have been considered for the potential treatment of Alzheimer's disease (AD). Such treatment, however, remains debatable because of the diverse functions of Aβ and NE in AD pathology. Moreover, the complicated oxidation accompanying NE has caused the majority of the previous research to focus on the binding of NE oxides onto Aβ. The molecular mechanism by which Aβ interacts with the reduction state of NE, which is correlated with the brain function, should be urgently explored. In this work, by controlling rigorous anaerobic experimental conditions, the molecular mechanism of the Aβ/NE interaction was investigated, and two binding sites were revealed. Tyr10 was identified as the strong binding site of NE, and SNK(26-28) segment was the weak binding segment. Furthermore, thioflavin T fluorescence confirmed NE's positive function of inhibiting Aβ aggregation through its weak binding with SNK(26-28) segment. Meanwhile, 7-OHCCA fluorescence exhibited NE's negative function of enhancing ·OH generation through inhibiting the Aβ/Cu2+ coordination. The viability tests of the neuroblastoma SH-SY5Y cells displayed that the coexistence of NE, Cu2+, and Aβ induced lower cell viability than free Cu2+, indicating the significant negative effect of excessive NE on AD progression. These data revealed the possible pathway of NE-induced damage in AD brain, which is significant for understanding the function of NE in Aβ-involved AD neuropathology and for designing an NE-related therapeutic strategy for AD.