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.

A peptide-based near-infrared fluorescence probe for dynamic monitoring senile plaques in Alzheimer's disease mouse model

In vivo monitoring neuropathological changes in Alzheimer's disease (AD) animal model is critical for drug development. Here, by integrating blood-brain barrier penetrable peptide, we have developed a peptide probe which based on angiopep-2. Angiopep-based probe exhibited high binding affinity to Aβ aggregates and labeled senile plaques in vivo. Remarkably, the in vivo near-infrared imaging data revealed that fluorescence signals of this probe were nearly 3-fold higher in the brains of 16-month-old APP/PS1 transgenic mice compared to C57 mice and exhibited linear correlation with the senile plaques load process in 4-, 8-, 16-month-old APP/PS1 transgenic mice. Moreover, senile plaques load was detected in vivo as early as 4 months of age that even at the very beginning of plaques developed in APP/PS1 transgenic mice. Taken together, this novel peptide-based probe achieved dynamic monitoring senile plaques in APP/PS1 transgenic mice and have been ready to use in drug development in AD mouse model.

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

Selective detection and staining of toxic amyloid plaques, a potential biomarker present in the Alzheimer's disease (AD) brain is crucial for both clinical diagnosis and monitoring AD disease progression. Herein, we report a coumarin-quinoline (CQ) conjugate-based turn-on near-infrared (NIR) fluorescence probe for specific detection of β-amyloid (Aβ) aggregates. CQ probe is highly sensitive and exhibits ~100-fold fluorescence enhancement in vitro upon binding Aβ aggregates with enhanced quantum yield. Furthermore, the probe has ~10-fold higher binding affinity towards Aβ aggregates (86nM) compared to commonly used Thioflavin T. Most importantly, CQ probe displays unambiguous selectivity towards Aβ aggregates compared to other toxic protein aggregates such as tau, α-synuclein (α-Syn) and islet amyloid polypeptide (IAPP). In addition, CQ is nontoxic to neuronal cells and shows significant blood brain barrier permeability. Remarkably, CQ stains Aβ plaques in human brain tissue over co-existing tau aggregates and neurofibrillary tangles (NFTs), which are associated in AD and tauopathies. This is a highly desirable attribute to distinguish AD from tau pathology and mixed dementia.

Substantial fibrin amyloidogenesis in type 2 diabetes assessed using amyloid-selective fluorescent stains

BACKGROUND

We have previously shown that many chronic, inflammatory diseases are accompanied, and possibly partly caused or exacerbated, by various coagulopathies, manifested as anomalous clots in the form of 'dense matted deposits'. More recently, we have shown that these clots can be amyloid in nature, and that the plasma of healthy controls can be induced to form such clots by the addition of tiny amounts of bacterial lipopolysaccharide or lipoteichoic acid. Type 2 diabetes (T2D) is also accompanied by raised levels of LPS.

METHODS

We use superresolution and confocal microscopies to investigate the amyloid nature of clots from healthy and T2D individuals.

RESULTS

We show here, with the established stain thioflavin T and the novel stains Amytracker™ 480 and 680, that the clotting of plasma from type 2 diabetics is also amyloid in nature, and that this may be prevented by the addition of suitable concentrations of LPS-binding protein.

CONCLUSION

This implies strongly that there is indeed a microbial component to the development of type 2 diabetes, and suggests that LBP might be used as treatment for it and its sequelae.

Tuning the stereo-hindrance of a curcumin scaffold for the selective imaging of the soluble forms of amyloid beta species

Amyloid peptides and proteins are associated with the pathologies of numerous diseases. In the progression of a disease, amyloids exist in soluble and insoluble forms, which are the dominant species at different stages of the disease and they have different degrees of toxicity. However, differentiating between the soluble and insoluble forms is very challenging with small molecule probes due to multiple obstacles that need to be overcome. Inspired by the recognition principle of antibodies for sAβ, we hypothesized that the accessibility/tightness of soluble and insoluble amyloids could be utilized to design imaging probes to recognize different amyloid forms and the stereo-hindrance tuning strategy could be used to design imaging probes for selectively detecting the soluble amyloid beta (sAβ) species in Alzheimer's disease (AD). Herein, we demonstrated that tuning the stereo-hindrance of the phenoxy-alkyl chains at the 4-position of a curcumin scaffold could lead to certain selectivity for sAβ over insoluble Aβs (insAβ). Among the designed compounds, CRANAD-102 showed a 68-fold higher affinity for sAβ than for insAβ (7.5 ± 10 nM vs. 505.9 ± 275.9 nM). Moreover, our imaging data indicated that CRANAD-102 was indeed capable of detecting sAβ in vivo using 4 month old APP/PS1 mice, in which sAβ is the predominant species in the brain. In addition, we also demonstrated that CRANAD-102 could be used to monitor the increase in sAβ loading from the ages of 4 months old to 12 months old. We believe that CRANAD-102 can be a useful probe for selectively detecting sAβ species in AD and that our probe designing strategy can be applied to other amyloids and will have tremendous impact on AD drug development and other amyloid research.

A Difluoroboron β-Diketonate Probe Shows "Turn-on" Near-Infrared Fluorescence Specific for Tau Fibrils

Tau aggregation in neuronal cells has recently received significant attention as a robust predictor of the progression of Alzheimer's disease (AD) because of its proven correlation with the degree of cognitive impairment in AD patients. Accordingly, noninvasive imaging of tau aggregates has been highlighted as a promising diagnostic tool for AD. We have previously identified a tau-specific "turn-on" near-infrared fluorescent (NIRF) probe (1), and, in this study, structural modification was performed to optimize its physicochemical as well as fluorescence properties. Thus, a series of fluorescent dyes (2a-2j) composed of a variously substituted difluoroboron β-diketonate and an N,N-dimethylaniline moiety linked by a length-extendable π-bridge were prepared. Among those, isobutyl-substituted difluoroboron β-ketonate with a π-conjugated 1,4-butadienyl linker (2e) showed the most promising properties as a tau-specific NIRF probe. Compared with 1, the "turn-on" fluorescence of 2e was more specific to tau fibrils, and it showed 8.8- and 6.2-times higher tau-over-Aβ and tau-over-BSA specificity, respectively. Also, the fluorescence intensity of 2e upon binding to tau fibrils was substantially higher (∼2.9 times) than that observed from 1. The mechanism for tau-specificity of 2e was investigated, which suggested that the molecular rotor-like property of 2e enables specific recognition of the microenvironment of tau aggregates to emit strong fluorescence. In transgenic cell lines stably expressing GFP-tagged tau proteins, 2e showed good colocalization with tau-GFP. Moreover, the fluorescence from 2e exhibited almost complete overlap with p-Tau antibody staining in the human AD brain tissue section. Collectively, these observations demonstrate the potential of 2e as a tau-specific fluorescent dye in both in vitro and ex vivo settings.

Novel Benzothiazole Derivatives as Fluorescent Probes for Detection of β-Amyloid and α-Synuclein Aggregates

Deposits of β-amyloid (Aβ) and α-synuclein (α-syn) are the hallmark of Alzheimer's disease (AD) and Parkinson's disease (PD), respectively. The detection of these protein aggregates with fluorescent probes is particularly of interest for preclinical studies using fluorescence microscopy on human brain tissue. In this study, we newly designed and synthesized three push-pull benzothiazole (PP-BTA) derivatives as fluorescent probes for detection of Aβ and α-syn aggregates. Fluorescence intensity of all PP-BTA derivatives significantly increased upon binding to Aβ(1-42) and α-syn aggregates in solution. In in vitro saturation binding assays, PP-BTA derivatives demonstrated affinity for both Aβ(1-42) (K_d = 40-148 nM) and α-syn (K_d = 48-353 nM) aggregates. In particular, PP-BTA-4 clearly stained senile plaques composed of Aβ aggregates in the AD brain section. Moreover, it also labeled Lewy bodies composed of α-syn aggregates in the PD brain section. These results suggest that PP-BTA-4 may serve as a promising fluorescent probe for the detection of Aβ and α-syn aggregates.

Self-assembling peptide and protein amyloids: from structure to tailored function in nanotechnology

Self-assembled peptide and protein amyloid nanostructures have traditionally been considered only as pathological aggregates implicated in human neurodegenerative diseases. In more recent times, these nanostructures have found interesting applications as advanced materials in biomedicine, tissue engineering, renewable energy, environmental science, nanotechnology and material science, to name only a few fields. In all these applications, the final function depends on: (i) the specific mechanisms of protein aggregation, (ii) the hierarchical structure of the protein and peptide amyloids from the atomistic to mesoscopic length scales and (iii) the physical properties of the amyloids in the context of their surrounding environment (biological or artificial). In this review, we will discuss recent progress made in the field of functional and artificial amyloids and highlight connections between protein/peptide folding, unfolding and aggregation mechanisms, with the resulting amyloid structure and functionality. We also highlight current advances in the design and synthesis of amyloid-based biological and functional materials and identify new potential fields in which amyloid-based structures promise new breakthroughs.

Probing supramolecular protein assembly using covalently attached fluorescent molecular rotors

Changes in microscopic viscosity and macromolecular crowding accompany the transition of proteins from their monomeric forms into highly organised fibrillar states. Previously, we have demonstrated that viscosity sensitive fluorophores termed 'molecular rotors', when freely mixed with monomers of interest, are able to report on changes in microrheology accompanying amyloid formation, and measured an increase in rigidity of approximately three orders of magnitude during aggregation of lysozyme and insulin. Here we extend this strategy by covalently attaching molecular rotors to several proteins capable of assembly into fibrils, namely lysozyme, fibrinogen and amyloid-β peptide (Aβ(1-42)). We demonstrate that upon covalent attachment the molecular rotors can successfully probe supramolecular assembly in vitro. Importantly, our new strategy has wider applications in cellulo and in vivo, since covalently attached molecular rotors can be successfully delivered in situ and will colocalise with the aggregating protein, for example inside live cells. This important advantage allowed us to follow the microscopic viscosity changes accompanying blood clotting and during Aβ(1-42) aggregation in live SH-SY5Y cells. Our results demonstrate that covalently attached molecular rotors are a widely applicable tool to study supramolecular protein assembly and can reveal microrheological features of aggregating protein systems both in vitro and in cellulo not observable through classical fluorescent probes operating in light switch mode.

Graphene quantum dots for detecting monomeric amyloid peptides

Detection of the concentration of amyloid monomers is of great importance in the diagnosis of amyloidogenesis. Herein, we propose a method to detect the concentration of amyloid β (Aβ) peptide monomers by utilizing the fluorescence characteristics of graphene quantum dots (GQDs). The linear dependence of the photoluminescence (PL) intensity of GQDs on the Aβ monomer concentration can be identified. It can be further illustrated that both monomeric and fibrillar Aβ peptides can be monitored by using GQDs. Conventional fluorescent dyes, such as thioflavin T (ThT), usually undergo co-incubation with amyloid peptides, which could lead to disturbance of the aggregation because of their inhibitory effect. Similar Aβ aggregation dynamics observed by using GQDs and ThT demonstrated the feasibility of the GQD-based detection method without co-incubation with soluble amyloid peptide monomers. The utilization of GQDs as a novel probe monitoring amyloid monomers could be applied in pathological detection and diagnosis of degenerative diseases and other conformational disorders.

Development of Phenothiazine-Based Theranostic Compounds That Act Both as Inhibitors of β-Amyloid Aggregation and as Imaging Probes for Amyloid Plaques in Alzheimer's Disease

Early detection of Alzheimer's disease (AD) is imperative in enabling the understanding and clinical treatment of this disorder, as well as in preventing its progression. Imaging agents specifically targeting Aβ plaques in the brain and the retina may lead to the early diagnosis of AD. Among them, near-infrared fluorescent (NIRF) imaging has emerged as an attractive tool to noninvasively identify and monitor diseases during the preclinical and early stages. In the present study, we report the design, synthesis, and evaluation of a series of new near-infrared fluorescent probes. Most of these probes displayed maximum emission in PBS (>650 nm), which falls in the good range for NIRF probes. Among them, 4a1 showed the highest affinity toward Aβ aggregates (K_d = 7.5 nM) and an excellent targeting ability for Aβ plaques in slices of brain and retina tissue from double transgenic mice. These compounds are also found to effectively prevent Aβ fibril formation and disaggregate preformed Aβ fibrils, showing a promising potential as theranostic agents for the diagnosis and therapy of AD.

Targeting β-amyloid plaques and oligomers: development of near-IR fluorescence imaging probes

Evidence indicated that shifting treatment to a presymptomatic stage may produce significant benefits to prevent/alleviate the progression of Alzheimer's disease (AD); in particular, early incorporation of noninvasive imaging and biomarker testing will be significantly beneficial for AD drug development. Based on amyloid cascade hypothesis and its revised version, both β-amyloid deposition and soluble oligomeric species could be good diagnostic biomarkers for AD. Near-IR fluorescence (NIRF) imaging, which so far is limited to animal studies, is a promising method for its incomparable advantages such as low cost, high-throughput and easy operation. This review focuses on recent reported NIRF probes that showed excellent binding to plaques and oligomers. We hope that this review will shed light on the future of NIRF probes' discovery.

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.

The influence of hydrogen bonds on NIAD-4 for use in the optical imaging of amyloid fibrils

The fast and accurate detection of amyloid fibrils, which are associated with many neurodegenerative diseases, is important for their early diagnosis.

Close Correlation of Monoamine Oxidase Activity with Progress of Alzheimer's Disease in Mice, Observed by in Vivo Two-Photon Imaging

Monoamine oxidases (MAOs) play an important role in Alzheimer's disease (AD) pathology. We report in vivo comonitoring of MAO activity and amyloid-β (Aβ) plaques dependent on the aging of live mice with AD, using a two-photon fluorescence probe. The probe under the catalytic action of MAO produces a dipolar fluorophore that senses Aβ plaques, a general AD biomarker, enabling us to comonitor the enzyme activity and the progress of AD indicated by Aβ plaques. The results show that the progress of AD has a close correlation with MAO activity, which can be categorized into three stages: slow initiation stage up to three months, an aggressive stage, and a saturation stage from nine months. Histological analysis also reveals elevation of MAO activity around Aβ plaques in aged mice. The close correlation between the MAO activity and AD progress observed by in vivo monitoring for the first time prompts us to investigate the enzyme as a potential biomarker of AD.

18F-Labeled Benzyldiamine Derivatives as Novel Flexible Probes for Positron Emission Tomography of Cerebral β-Amyloid Plaques

Early noninvasive visualization of cerebral β-amyloid (Aβ) plaques with positron emission tomography (PET) is the most feasible way to diagnose Alzheimer's disease (AD). In this study, a series of flexible benzyldiamine derivatives (BDA) were proposed for binding to aggregated β-amyloid 1-42 (Aβ_1-42) with high adaptability, high binding affinity (6.8 ± 0.6 nM), and rapid body excretion. The methylthio (12) and ethoxyl (10) derivatives were further labeled with ¹⁸F directly on their benzene ring and examined as PET probes for Aβ plaque imaging. [¹⁸F]12 displayed 4.87 ± 0.52% ID/g initial uptake and prompt washout from normal brain in biodistribution studies. MicroPET-CT imaging indicated sufficient retention of [¹⁸F]12 but lower white matter uptake in the brain of an AD transgenic mouse model compared with that of commercial [¹⁸F]AV-45. Our experimental results provide new insights for developing targeting ligands possessing a flexible framework for use as efficient Aβ probes for PET imaging of AD brain.

A High Affinity Red Fluorescence and Colorimetric Probe for Amyloid β Aggregates

A major challenge in the Alzheimer's disease (AD) is its timely diagnosis. Amyloid β (Aβ) aggregates have been proposed as the most viable biomarker for the diagnosis of AD. Here, we demonstrate hemicyanine-based benzothiazole-coumarin (TC) as a potential probe for the detection of highly toxic Aβ42 aggregates through switch-on, enhanced (~30 fold) red fluorescence (Emax = 654 nm) and characteristic colorimetric (light red to purple) optical outputs. Interestingly, TC exhibits selectivity towards Aβ42 fibrils compared to other abnormal protein aggregates. TC probe show nanomolar binding affinity (Ka = 1.72 × 10(7) M(-1)) towards Aβ42 aggregates and also displace ThT bound to Aβ42 fibrils due to its high binding affinity. The Aβ42 fibril-specific red-shift in the absorption spectra of TC responsible for the observed colorimetric optical output has been attributed to micro-environment change around the probe from hydrophilic-like to hydrophobic-like nature. The binding site, binding energy and changes in optical properties observed for TC upon interaction with Aβ42 fibrils have been further validated by molecular docking and time dependent density functional theory studies.

A copper–amyloid-β targeted fluorescent chelator as a potential theranostic agent for Alzheimer's disease

A fluorescent chelator is able to specifically target and attenuate Cu²⁺–Aβ aggregates in the brain of mice with Alzheimer's disease, which can be visualized by fluorescence imaging of the chelator.

Computational study on donor–acceptor optical markers for Alzheimer's disease: a game of charge transfer and electron delocalization

The length of the conjugated double bond chain in DANIR dyes modulates the charge transfer character, non-radiative deactivation pathways and affinity for amyloid-β fibril.

Specific self-monitoring of metal-associated amyloid-β peptide disaggregation by a fluorescent chelator

A dual-functional fluorescent chelator can specifically target and disassemble metal-associated Aβ aggregates and simultaneously self-monitor the disaggregation by fluorescence in brain homogenates of mice with Alzheimer's disease.

In vivo near-infrared and Cerenkov luminescence imaging of amyloid-β deposits in the brain: a fluorinated small molecule used for dual-modality imaging

Three fluorinated (¹⁹F or ¹⁸F) small molecules were evaluated as fluorescent or radiolabeled probes for Aβ deposits in the brain.

A spiropyran-based fluorescent probe for the specific detection of β-amyloid peptide oligomers in Alzheimer's disease

A fluorescent probe for the specific detection of Aβ oligomers in Alzheimer's disease both in vitro and in vivo was developed.

Disaggregation-induced fluorescence enhancement of NIAD-4 for the optical imaging of amyloid-β fibrils

Fluorescence enhancement of the NIAD-4 marker upon interaction with amyloid-β fibrils arises from the disaggregation of the oligomers that spontaneously form in aqueous solution.

The synthesis and evaluation of near-infrared probes with barbituric acid acceptors for in vivo detection of amyloid plaques

A new array of near-infrared probes containing barbituric acid acceptors has been developed as Aβ imaging agents.

In vivo fluorescence imaging of β-amyloid plaques with push–pull dimethylaminothiophene derivatives

We report a newin vivofluorescent imaging probe, DTM-2, that can detect β-amyloid plaques in the brain.