Aβ plaque-selective NIR fluorescence probe to differentiate Alzheimer's disease from tauopathies

A novel near-infrared fluorescent probe for detection of early-stage Aβ protofibrils in Alzheimer's disease

Detection of Aβ protofibrils at the early stage of Alzheimer's disease was realized by a novel near-infrared probe (DCM-AN) based on dicyanomethylene-4H-pyran. This probe exhibits high affinity towards Aβ protofibrils in vitro and in brain sections of transgenic mouse models for Alzheimer's disease.

Simultaneous imaging of mitochondrial viscosity and hydrogen peroxide in Alzheimer's disease by a single near-infrared fluorescent probe with a large Stokes shift

Simultaneous imaging of mitochondrial viscosity and hydrogen peroxide in Alzheimer's disease by a single near-infrared fluorescent probe with a large Stokes shift.

Unambiguous Detection of Elevated Levels of Hypochlorous Acid in Double Transgenic AD Mouse Brain

Alzheimer's disease (AD) is one of the most prevalent forms of dementia. The current diagnosis methods based on the behavior and cognitive decline or imaging of core biomarkers, namely, amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs), in the brain offer poor to moderate success. Detection and imaging of biomarkers that cause additional traits of pathophysiological aberrations in the brain are invaluable to monitor early disease onset and progression of AD pathology. The pathological hallmark of AD is associated with generation of excessive reactive oxygen species (ROS) in the brain, which aggravate oxidative stress and inflammation. ROS production involves elevated levels of hypochlorous acid (HOCl) and can serve as one of the potential biomarkers for the diagnosis of AD. We report the design, synthesis, and characterization of switchable coumarin-morpholine (CM) conjugates as off-on fluorescence probes for the specific detection of HOCl produced and proximally localized with amyloid plaques. The nonfluorescent thioamide probe CM2 undergoes regioselective transformation to fluorescent amide probe CM1 in the presence of HOCl (∼90-fold fluorescence enhancement and 0.32 quantum yield) with high selectivity and sensitivity (detection limit: 0.17 μM). The excellent cellular uptake and blood-brain barrier (BBB) crossing ability of CM2 allowed unambiguous and differential detection, imaging, and quantification of HOCl in cellular milieu and in the wild type (WT) and AD mouse brains. This study demonstrates the elevated level of HOCl in the AD mouse brain and the potential to expand the repertoire of biomarkers for the diagnosis of AD.

Fluorescence Chemicals To Detect Insoluble and Soluble Amyloid-β Aggregates

Misfolded amyloid-β (Aβ) is the key biomarker of Alzheimer's disease (AD), and discoveries of fluorescence chemicals visualizing such Aβ aggregates in the brain have made major contributions in postmortem and antemortem diagnosis of the disorder. Insoluble senile plaques of Aβ in brain tissues are commonly stained with thioflavin and congo red dyes and observed through microscopy, while those in living patient brains are detected via radioisotope-labeled fluorescence chemicals for positron emission tomography. Clinical evidence strongly supports the view that plaques are well-associated with the onset but not with the progression of AD. Plaques could accumulate while cognitive functions of at-risk individuals are still intact, and thus, another biomarker is needed to monitor neurodegeneration. Soluble Aβ oligomers are considered to have strong correlation with neuronal loss and brain atrophy as they are the most neurotoxic forms of misfolded Aβ. However, oligomer-targeting probes encounter several major difficulties in development. There is a significant structural distinction between two Aβ species-plaques are β-sheet-rich while oligomers are unordered-and it is still difficult to isolate and stabilize the oligomeric forms of Aβ. Due to these challenges, soluble oligomer-detecting imaging probes are relatively rare compared to the plaque-targeting chemical probes. This Review describes biochemical and optical characteristics of up-to-date fluorescence chemicals targeting insoluble plaques and soluble oligomers of Aβ. We also highlight the contributions of Aβ fluorescence chemicals to the clinical diagnosis of AD and technical challenges in searching for enhanced imaging probes.

New perspective on the fluorescence and sensing mechanism of TNP chemosensor 2-(4,5-bis(4-chlorophenyl)-1H-imidazol-2-yl)-4-chlorolphenol

For TNP chemosensor 2-(4,5-Bis(4-Chlorophenyl)-1H-Imidazol-2-yl)-4-Chlorolphenol (HPICI), previous thought with no theoretical basis was that excited-state intramolecular proton transfer (ESIPT) process and the ground-state HPICI-TNP complex are mainly responsible for its fluorescence emission and the detection of TNP. However, this interpretation has been proved to be wrong by the present theoretical DFT/TDDFT explorations. Actually, the strong fluorescence of HPICI is mainly induced by the local excitation of the enol form HPICI(E) without ESIPT, and the fluorescence quenching by TNP is due to the photo-induced electron transfer (PET) process together with the cooperative effect of hydrogen-bonding interaction and π-π stacking interaction coexisting in the HPICI-TNP complex. The strengthened excited-state hydrogen bond promotes the PET process, thus facilitates the fluorescence quenching. This mechanism is proposed on the basis of the theoretical analyses on molecule geometry, binding energy, Gibbs free energy, electronic transitions, and frontier molecular orbitals (FMOs).

Half-curcumin analogues as PET imaging probes for amyloid beta species

In this report, we demonstrate that half-curcuminoid could be a better scaffold for PET tracer development. F-CRANAD-101 was designed and found to show significant response to both soluble and insoluble Aβs in the fluorescent spectral tests. PET imaging results indicated that 14 month and 5 month old APP/PS1 AD mice had higher signals in the brain than age-matched wild type mice.

NIR fluorescent probes with good water-solubility for detection of amyloid beta aggregates in Alzheimer's disease

Quinoline-malononitrile-based NIR fluorescent probes with good water-solubility were developed for detecting and imaging of Aβ aggregates in Alzheimer's disease.

A lysosome-targeting dual-functional fluorescent probe for imaging intracellular viscosity and beta-amyloid

A lysosome-targeting dual-functional fluorescent probe was rationally designed and developed for imaging intracellular lysosomal viscosity and beta-amyloid.

Novel fluorescence probes based on the chalcone scaffold for in vitro staining of β-amyloid plaques

The β-amyloid (Aβ) plaque is one of the neuropathological hallmarks in the Alzheimer's disease brain. The detection of Aβ plaques with fluorescence probes is useful for preclinical studies of Alzheimer's disease. In this study, we developed four novel fluorescence probes based on chalcone scaffold. In an in vitro binding study, all FCH derivatives showed moderate binding affinity for Aβ(1-42) aggregates (K_i = 72-114 nM). The fluorescence intensities of FCH-3 and FCH-4 dramatically changed in the presence of Aβ(1-42) aggregates (6.7 and 14.2 fold), but the changes of FCH-1 and FCH-2 were minor (2.0 and 2.4 fold). In a fluorescence staining study using Tg2576 mouse brain sections, FCH-3 and FCH-4 clearly visualized Aβ plaques, but FCH-1 and FCH-2 did not stain. Taken together, all FCH derivatives could bind to Aβ aggregates, but only FCH-3 and FCH-4 may be useful fluorescence probes for in vitro staining of Aβ plaques.

Effective Theranostic Cyanine for Imaging of Amyloid Species in Vivo and Cognitive Improvements in Mouse Model

We report herein an investigation of carbazole-based cyanine, (E)-4-(2-(9-(2-(2-methoxyethoxy)ethyl)-9H-carbazol-3-yl)-vinyl)-1-methyl-quinolin-1-iumiodide (SLM), as an effective theranostic agent for Alzheimer's disease (AD). This cyanine exhibited desirable multifunctional and biological properties, including amyloid-β (Aβ)-oligomerization inhibition, blood-brain barrier permeability, low neurotoxicity, neuroprotective effect against Aβ-induced toxicities, high selectivity and strong binding interactions with Aβ peptide/species, good biostability, as well as strong fluorescence enhancement upon binding to Aβ species for diagnosis and therapy of AD. This cyanine has been successfully applied to perform near-infrared in vivo imaging of Aβ species in transgenic AD mouse model. The triple transgenic AD mice intraperitoneally treated with SLM showed significant recovery of cognitive deficits. Furthermore, those SLM-treated mice exhibited a substantial decrease in both of oligomeric Aβ contents and tau proteins in their brain, which was attributed to the induction of autophagic flux. These findings demonstrated for the first time that SLM is an effective theranostic agent with in vivo efficacy for diagnosis and treatment of AD in mouse models.

Tailoring Hydrophobic Interactions between Probes and Amyloid-β Peptides for Fluorescent Monitoring of Amyloid-β Aggregation

Despite their unique advantages, the full potential of molecular probes for fluorescent monitoring of amyloid-β (Aβ) aggregates has not been fully exploited. This limited utility stems from the lack of knowledge about the hydrophobic interactions between the molecules of Aβ probes, as well as those between the probe and the Aβ aggregate. Herein, we report the first mechanistic study, which firmly establishes a structure-signaling relationship of fluorescent Aβ probes. We synthesized a series of five fluorescent Aβ probes based on an archetypal donor-acceptor-donor scaffold (denoted as SN1-SN5). The arylamino donor moieties were systematically varied to identify molecular factors that could influence the interactions between molecules of each probe and that could influence their fluorescence outcomes in conditions mimicking the biological milieu. Our probes displayed different responses to aggregates of Aβ, Aβ40 and Aβ42, two major isoforms found in Alzheimer's disease: SN2, having pyrrolidine donors, showed noticeable ratiometric fluorescence responses (Δν = 797 cm-1) to the Aβ40 and Aβ42 samples that contained oligomeric species, whereas SN4, having N-methylpiperazine donors, produced significant fluorescence turn-on signaling in response to Aβ aggregates, including oligomers, protofibrils, and fibrils (with turn-on ratios of 14 and 10 for Aβ42 and Aβ40, respectively). Mechanistic investigations were carried out by performing field-emission scanning electron microscopy, X-ray crystallography, UV-vis absorption spectroscopy, and steady-state and transient photoluminescence spectroscopy experiments. The studies revealed that the SN probes underwent preassembly prior to interacting with the Aβ species and that the preassembled structures depended profoundly on the subtle differences between the amino moieties of the different probes. Importantly, the studies demonstrated that the mode of fluorescence signaling (i.e., ratiometric response versus turn-on response) was primarily governed by stacking geometries within the probe preassemblies. Specifically, ratiometric fluorescence responses were observed for probes capable of forming J-assembly, whereas fluorescence turn-on responses were obtained for probes incapable of forming J-aggregates. This finding provides an important guideline to follow in future efforts at developing fluorescent probes for Aβ aggregation. We also conclude, on the basis of our study, that the rational design of such fluorescent probes should consider interactions between the probe molecules, as well as those between Aβ peptides and the probe molecule.

Current progress, challenges and future prospects of diagnostic and therapeutic interventions in Alzheimer's disease

The diverse pathological mechanisms and their implications for the development of effective diagnostic and therapeutic interventions in Alzheimer's disease are presented with current progress, challenges and future prospects.

Unusual binding-site-specific photophysical properties of a benzothiazole-based optical probe in amyloid beta fibrils

Varying electronic structure of BTA-3 probe in different binding sites in amyloid fibrils is the key mechanism behind its site-specific photophysical properties.

Metal complexes for multimodal imaging of misfolded protein-related diseases

Aggregation of misfolded proteins and progressive polymerization of otherwise soluble proteins is a common hallmark of several highly debilitating and increasingly prevalent diseases, including amyotrophic lateral sclerosis, cerebral amyloid angiopathy, type II diabetes and Parkinson's, Huntington's and Alzheimer's diseases.