Biological applications of near infrared fluorescence dye probes in monitoring Alzheimer’s disease

A Bifunctional Fluorescence Probe for the Detection of Hypochlorous Acid and Viscosity in Living Cells and Zebrafish

As two significant indicators in the microenvironment, hypochlorous acid and viscosity play important roles in multitudinous physiological metabolic processes. However, it is challenging to determine the dynamic levels of hypochlorous acid and viscosity in living cells and organisms because of the absence of effective molecular tools that can simultaneously detect hypochlorous acid and viscosity in organisms. Herein, a molecular design strategy was presented to fabricate a single fluorescence probe that can simultaneously detect hypochlorous acid and viscosity by using two different emission channels. In JXR, TICT-based 4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridin-1-ium-iodide chromophore serves as energy acceptor in the FRET process and sensors for hypochlorous acid and viscosity. JXR showed a large Stokes shift, wide emission peak distance, high photostability, and low toxicity. JXR could detect hypochlorous acid and viscosity rapidly, sensitively, and selectively by emitting different fluorescence signals. Importantly, JXR was successfully applied to track the intracellular hypochlorous acid and viscosity in living cells. Meanwhile, the generation of endogenous hypochlorite in living cells can be observed by using JXR.

Fluorescence imaging opens a new window for the diagnosis of early-stage Alzheimer's disease

As the global population ages, the incidence and prevalence of Alzheimer's disease (AD) continues to rise, posing a serious threat to human health. Butyrylcholinesterase (BChE), which is overexpressed in the brains of patients with AD, is a potential drug target and biomarker. However, the molecular mechanism underlying BChE's role in the AD process remains unclear. Therefore, the development of tools for BChE detection can aid in the diagnosis of AD and deepen our understanding of BChE's contribution to disease progression. Motivated by a bioinspired strategy based on the natural substrate of BChE, we designed a BChE fluorescent probe (HCYO) with a novel recognition group for BChE detection to assist in the early diagnosis of AD. This probe can selectively detect endogenous BChE with an excellent detection limit of 28.9 ng/mL. Using HCYO, we successfully imaged four-week-old mice with an ultraearly AD model, the early diagnosis of the disease. Furthermore, using this HCYO probe, we confirmed that BChE influences the inflammation-induced upregulation the levels of phosphorylated tau and Trigger Receptor Expressed on Myeloid Cells 2, impacting AD progression. These findings provide a crucial theoretical basis for the development of BChE inhibitors for AD treatment.

Novel Dual-Functional Half-Curcumin Analogues as a Fluorescent and PET Probe for β-Amyloid Imaging in the Alzheimer's Disease APP/PS1 Model

Noninvasive imaging of β-amyloid in vivo is pivotal for the early diagnosis of Alzheimer's disease (AD). While single imaging methods have been extensively studied for detecting Aβ over the past decade, dual-modal probes have received scant attention. In this study, we synthesized and assessed a series of half-curcumin probes, among which DiFboron-8 demonstrated a high affinity and selectivity for Aβ aggregates. DiFboron-8 effectively served as a dual-functional fluorescent and positron emission tomography (PET) probe for imaging β-amyloid in an AD mouse model. Histological staining results underscored DiFboron-8's potent staining capability for Aβ plaques in APP/PS1 brain slices, while ex vivo biodistribution studies highlighted its rapid clearance rate. In vivo imaging revealed that [18F]-DiFboron-8 could penetrate the blood-brain barrier, displaying higher PET signals in the brains of APP/PS1 mice compared to wild-type mice just 3 min postinjection, a finding corroborated by autoradiography staining. Overall, we propose that [18F]-DiFboron-8 represents an efficient fluorescent/PET dual-modal probe, offering promise for β-amyloid imaging in the early stages of AD.

Synthesis of BODIPYs using organoindium reagents and survey of their cytotoxicity and cell uptake on nervous system cells

In this study, a series of BODIPY dyes were synthesized, containing various substituents at meso position. Further functionalization of the BODIPY framework at C2 and C2-C6 position(s) by palladium-catalysed cross-coupling reactions using organoindium reagents (R3In) was efficiently assessed, starting from C2(6)-halogenated BODIPYs, and their optical properties were measured. The cytotoxicity of BODIPY dyes on SH-SY5Y neuronal cells by MTT assay showed that those compounds bearing thien-2-yl and benzonitrile moieties at meso position, exhibited great efficiency in maintaining cell viability under all tested conditions (up to 50 µM for 24 h and 48 h). Furthermore, nanoliposomal encapsulation of a hydrophobic BODIPY, incorporating bis(trifluoromethyl)phenyl substituents at C2 and C6 positions, through the lipid-extrusion method was addressed. The liposomes exhibited spherical shape as observed in cryo-TEM image, with average particle size of 120 nm (average PdI 0.05) and Zeta potential 54.69 mV by DLS measurements. Simple incubation of gliobastoma U-87 cells with prepared liposomes led to efficient internalization, and visualization of brightness BODIPY in cytoplasm using fluorescence confocal microscopy, demonstrating encapsulation enhance biocompatibility of the hydrophobic BODIPY as preliminary approximation for further biomedical applications.

A novel viscosity sensitive hemicyanine fluorescent dye for real-time imaging of amyloid-β aggregation

Alzheimer's Disease (AD) is a neurodegenerative disease, of which β-amyloid (Aβ) deposition is one of the most important pathological features. It has been reported that during Aβ aggregation, the microenvironment around the Aβ protein is altered in terms of viscosity and polarity. In this work, we developed five novel hemicyanine fluorescent probes (MZs). After screening the photochemical properties, MZ-2 and 3 were found to enable the rapid detection of Aβ42 aggregates, which were also sensitive to ambient viscosity. After comparison the structure of probes, we also observed that extensions of conjugated π-systems effectively cause redshifts of excitation wavelength. In the meanwhile, hydroxyl groups with weaker ionization strengths are more responsive to Aβ42 aggregates than sulfonate groups, probably due to the small size of the hydroxyl group and the acidity. Overall, MZ-2 showed the best response to Aβ42 aggregates (15.35-fold) and viscosity (17.6-fold). MZ-2 can quickly cross the blood-brain barrier (BBB), enabling high-fidelity imaging of Aβ42 aggregates in the mice brain.

Dual-target inhibitors based on acetylcholinesterase: Novel agents for Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia among the elderly, accounting for 60 %-70 % of cases. At present, the pathogenesis of this condition remains unclear, but the hydrolysis of acetylcholine (ACh) is thought to play a role. Acetylcholinesterase (AChE) can break down ACh transmission from the presynaptic membrane and stop neurotransmitters' excitatory effect on the postsynaptic membrane, which plays a key role in nerve conduction. Acetylcholinesterase inhibitors (AChEIs) can delay the hydrolysis of acetylcholine (ACh), which represents a key strategy for treating AD. Due to its complex etiology, AD has proven challenging to treat. Various inhibitors and antagonists targeting key enzymes and proteins implicated in the disease's pathogenesis have been explored as potential therapeutic agents. These include Glycogen Synthase Kinase 3β (GSK-3β) inhibitors, β-site APP Cleaving Enzyme (BACE-1) inhibitors, Monoamine Oxidase (MAO) inhibitors, Phosphodiesterase inhibitors (PDEs), N-methyl--aspartic Acid (NMDA) antagonists, Histamine 3 receptor antagonists (H3R), Serotonin receptor subtype 4 (5-HT4R) antagonists, Sigma1 receptor antagonists (S1R) and soluble Epoxide Hydrolase (sEH) inhibitors. The drug development strategy of multi-target-directed ligands (MTDLs) offers unique advantages in the treatment of complex diseases. On the one hand, it can synergistically enhance the therapeutic efficacy of single-target drugs. On the other hand, it can also reduce the side effects. In this review, we discuss the design strategy of dual inhibitors based on acetylcholinesterase and the structure-activity relationship of these drugs.

Rational design of a high-affinity fluorescent probe for visualizing monitoring the amyloid β clearance effect of anti-Alzheimer's disease drug candidates

Beta-amyloid (Aβ), the most pivotal pathological hallmark for Alzheimer's disease (AD) diagnosis and drug evaluation, was recognized by TZ095, a high-affinity fluorescent probe developed by rational molecular design. With a TICT mechanism, TZ095 exhibited remarkable affinity with Aβ aggregates (Kd = 81.54 nM for oligomers; Kd = 66.70 nM for fibril) and substantial fluorescence enhancement (F/F0 = 44), enabling real-time monitoring of Aβ in live cells and nematodes. Significantly, this work used TZ095 to construct a new protocol that can quickly and conveniently monitor Aβ changes at the cellular and nematode levels to evaluate the anti-AD efficacy of candidate compounds, and four reported Aβ-lowering drug candidates were administrated for validation. Imaging data demonstrated that TZ095 can visually and quantitatively track the effect of Aβ elimination after drug treatment. Furthermore, TZ095 excelled in ex vivo histological staining of 12-month-old APP/PS1 mouse brains, accurately visualizing Aβ plaques. Integrating CUBIC technology, TZ095 facilitated whole-brain, 3D imaging of Aβ distribution in APP/PS1 mice, enabling high-resolution in situ analysis of Aβ plaques. Collectively, these innovative applications of TZ095 offer a promising strategy for rapid, convenient, and real-time monitoring of Aβ levels in preclinical therapeutic assessments.

Proteomic Evidence for Amyloidogenic Cross-Seeding in Fibrinaloid Microclots

In classical amyloidoses, amyloid fibres form through the nucleation and accretion of protein monomers, with protofibrils and fibrils exhibiting a cross-β motif of parallel or antiparallel β-sheets oriented perpendicular to the fibre direction. These protofibrils and fibrils can intertwine to form mature amyloid fibres. Similar phenomena can occur in blood from individuals with circulating inflammatory molecules (and also some originating from viruses and bacteria). Such pathological clotting can result in an anomalous amyloid form termed fibrinaloid microclots. Previous proteomic analyses of these microclots have shown the presence of non-fibrin(ogen) proteins, suggesting a more complex mechanism than simple entrapment. We thus provide evidence against such a simple entrapment model, noting that clot pores are too large and centrifugation would have removed weakly bound proteins. Instead, we explore whether co-aggregation into amyloid fibres may involve axial (multiple proteins within the same fibril), lateral (single-protein fibrils contributing to a fibre), or both types of integration. Our analysis of proteomic data from fibrinaloid microclots in different diseases shows no significant quantitative overlap with the normal plasma proteome and no correlation between plasma protein abundance and their presence in fibrinaloid microclots. Notably, abundant plasma proteins like α-2-macroglobulin, fibronectin, and transthyretin are absent from microclots, while less abundant proteins such as adiponectin, periostin, and von Willebrand factor are well represented. Using bioinformatic tools, including AmyloGram and AnuPP, we found that proteins entrapped in fibrinaloid microclots exhibit high amyloidogenic tendencies, suggesting their integration as cross-β elements into amyloid structures. This integration likely contributes to the microclots' resistance to proteolysis. Our findings underscore the role of cross-seeding in fibrinaloid microclot formation and highlight the need for further investigation into their structural properties and implications in thrombotic and amyloid diseases. These insights provide a foundation for developing novel diagnostic and therapeutic strategies targeting amyloidogenic cross-seeding in blood clotting disorders.

BODIPY in Alzheimer's disease diagnostics: A review

Timely diagnosis and therapy of Alzheimer's disease remains one of the greatest questions in medicinal chemistry of neurodegenerative disease. The lack of low-cost sensors capable of reliable detection of structural changes in AD-related proteins is the driving factor for the development of novel molecules with affinity for AD hallmarks. The development of cheap, safe diagnostic methods is a highly sought-after area of research. Optical fluorescent probes are of great interest due to their non-radioactivity, low cost, and ability of the real-time visualization of AD hallmarks. Boron dipyrromethene (BODIPY)-based fluorophore is one promising fluorescent unit for in vivo labeling due to its high photostability, easy modification, low toxicity, and cell-permeability. In recent years, many fluorescent BODIPY-based probes capable of Aβ plaque, Aβ soluble oligomers, neurofibrillary tangles (NFT) optical detection, as well as probes with copper ion chelating units and viscosity sensors have been developed. In this review, we summarized BODIPY derivatives as fluorescent sensors capable of detecting pathological features of Alzheimer's disease, published from 2009 to 2023, as well as their design strategies, optical properties, and in vitro and in vivo activities.

Construction and evaluation of near-infrared fluorescent probes for imaging lipid droplet and lysosomal viscosity

Microenvironmental viscosity is a crucial parameter for biological systems, and its abnormal fluctuations are closely associated with various functional disorders and diseases. However, it is still important and urgent to develop improved near-infrared fluorescent probes for micro-viscosity with dual-organelle targeting properties, low background noise, and high sensitivity. Herein, two BODIPY-based small-molecule fluorescent probes were designed and synthesized, which were explored for their viscosity- and polarity-responsive properties, and were further applied to imaging sub-cellular viscosity in living cells. Interestingly, BSZ-Ph and BSZ-R displayed near-infrared fluorescence (more than 650 nm) and were sensitive to environmental viscosity and polarity due to the introduction of a benzothiazole at the 2-position and electron-rich aniline groups at the 5-position of the BODIPY core, respectively. The fluorescence intensity increased exponentially with the viscosity changes. Furthermore, the probe BSZ-Ph could successfully target lipid droplets and image cellular viscosity changes by treating lipopolysaccharides (LPS) and nystatin. Comparatively, the probe BSZ-R could successfully target the dual organelles of lipid droplets and lysosomes and image cellular viscosity changes by treating LPS and monensin. Therefore, in this work, we reported two new BODIPY-based near-infrared fluorescent probes, BSZ-Ph and BSZ-R, for cellular viscosity imaging, which could target lipid droplets and the dual organelles of lysosomes and lipid droplets, respectively. The study could provide a reference for the future development of fluorescent probes for viscosity in lipid droplets and lysosomes.

A Mitochondria-Targeting Fluorescent Probe for the Dual Sensing of Hypochlorite and Viscosity without Signal Crosstalk in Living Cells and Zebrafish

Hypochlorite (ClO-) and viscosity both affect the physiological state of mitochondria, and their abnormal levels are closely related to many common diseases. Therefore, it is vitally important to develop mitochondria-targeting fluorescent probes for the dual sensing of ClO- and viscosity. Herein, we have explored a new fluorescent probe, XTAP-Bn, which responds sensitively to ClO- and viscosity with off-on fluorescence changes at 558 and 765 nm, respectively. Because the emission wavelength gap is more than 200 nm, XTAP-Bn can effectively eliminate the signal crosstalk during the simultaneous detection of ClO- and viscosity. In addition, XTAP-Bn has several advantages, including high selectivity, rapid response, good water solubility, low cytotoxicity, and excellent mitochondrial-targeting ability. More importantly, probe XTAP-Bn is successfully employed to monitor the dynamic change in ClO- and viscosity levels in the mitochondria of living cells and zebrafish. This study not only provides a reliable tool for identifying mitochondrial dysfunction but also offers a potential approach for the early diagnosis of mitochondrial-related diseases.

Recent Advances in Fluorescent Theranostics for Alzheimer's Disease: A Comprehensive Survey on Design, Synthesis, and Properties

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia that is rapidly becoming a major health problem, especially in developed countries because of their increasing life expectancy. Two main problems are often associated with the disease: (i) the absence of a widely accessible "gold-standard" for early diagnosis and (ii) lack of effective therapies with disease-modifying effects. The recent success of the monoclonal antibody lecanemab played an important role not only in clarifying a possible druggable pathway but also in spelling the revival of small molecule drug discovery. Unlike bulky biologics, small molecules are structurally less complex, generally cheaper, and compatible with at-home oral consumption, making it feasible for people to start their drug regimen early and stay on it longer. In this sense, small-molecule near-infrared fluorescent theranostics have been gaining more and more attention from the scientific community, as they have the potential to simultaneously provide diagnostic outputs and deliver therapeutic action, paving the way toward personalized medicine in AD patients. They also have the potential to shift the diagnostic "status-quo" from expensive and limited-access PET radiotracers toward inexpensive and handy imaging tools widely available for primary patient screening and preclinical animal studies. Herein, we review the most recent advances in the field of fluorescent theranostics for Alzheimer's disease, detailing their design strategies, synthetic approaches and imaging and therapeutic properties in vitro and in vivo. With this Review, we intend to provide a milestone in the acquired knowledge in the field of AD theranostics, encouraging the future development of properly designed theranostic compounds with improved chances to reach clinical applications.

A conformationally-locked p-hydroxybenzylidene imidazolinone derivative for detecting Aβ42 aggregation

Alzheimer's disease (AD) is a common type of neurodegenerative disease, which can only be symptomatically relieved but does not yet have a cure. Among the different Aβ species, amyloid-β 42 (Aβ42) aggregates are proposed to be more neurotoxic than that of Aβ40, and oligomeric Aβ42 is thought to play a harmful role in the pathophysiology of AD. Therefore, the detection of Aβ42 aggregation is very meaningful in the AD field. We herein report a conformationally-locked p- hydroxybenzylidene imidazolinone derivative, BDI, which exhibits selectivity and specificity towards Aβ42 aggregation and remarkable fluorescent enhancement with a large Stokes shift (more than 100 nm). In the fluorescent co-localization study, BDI can sensitively detect a large population of Aβ42 aggregation over that of Aβ40 in the brain tissues of AD transgenic mouse models. Therefore, this new probe could provide a useful tool for the rapid detection of important Aβ species in AD.

BF2-Functionalized Benzothiazole Amyloid Markers: Effect of Donor Substituents on One- and Two-Photon Properties

Investigation of amyloids with the aid of fluorescence microscopy provides crucial insights into the development of numerous diseases associated with the formation of aggregates. Here, we present a series of BF2-functionalized benzothiazoles with electron-donating methoxy group(s), which are tested as amyloid fluorescent markers. We evaluate how the position of donor functional group(s) influences optical properties (fluorescence lifetime (τ) and fluorescence quantum yield (FQY)) in a solution and upon binding to amyloids. We elucidate the importance of surrounding environmental factors (hydrogen-bonding network, polarity, and viscosity) on the observed changes in FQY and evaluate how the localization of a donor influences radiative and nonradiative decay pathways. We conclude that a donor attached to the benzothiazole ring contributes to the increment of radiative decay pathways upon binding to amyloids (kr), while the donor attached to the flexible part of a molecule (with rotational freedom) contributes to a decrease in nonradiative decay pathways (knr). We find that the donor-acceptor-donor architecture allows us to obtain 58 times higher FQY of the dye upon binding to bovine insulin amyloids. Finally, we measure two-photon absorption (2PA) cross sections (σ2) of the dyes and their change upon binding by the two-photon excited fluorescence (2PEF) technique. Measurements reveal that dyes that exhibit the increase/decrease of σ2 values when transferred from highly polar solvents to CHCl3 present a similar behavior upon amyloid binding. Our 2PA experimental values are supported by quantum mechanics/molecular mechanics (QM/MM) simulations. Despite this trend, the values of σ2 are not the same, which points out the importance of two-photon absorption measurements of amyloid-dye complexes in order to understand the performance of 2P probes upon binding.

A Novel Near-Infrared Fluorescence Probe THK-565 Enables In Vivo Detection of Amyloid Deposits in Alzheimer's Disease Mouse Model

PURPOSE

Noninvasive imaging of protein aggregates in the brain is critical for the early diagnosis, disease monitoring, and evaluation of the effectiveness of novel therapies for Alzheimer's disease (AD). Near-infrared fluorescence (NIRF) imaging with specific probes is a promising technique for the in vivo detection of protein deposits without radiation exposure. Comprehensive screening of fluorescent compounds identified a novel compound, THK-565, for the in vivo imaging of amyloid-β (Aβ) deposits in the mouse brain. This study assessed whether THK-565 could detect amyloid-β deposits in vivo in the AD mouse model.

PROCEDURES

The fluorescent properties of THK-565 were evaluated in the presence and absence of Aβ fibrils. APP knock-in (APP-KI) mice were used as an animal model of AD. In vivo NIRF images were acquired after the intravenous administration of THK-565 and THK-265 in mice. The binding selectivity of THK-565 to Aβ was evaluated using brain slices obtained from these mouse models.

RESULTS

The fluorescence intensity of the THK-565 solution substantially increased by mixing with Aβ fibrils. The maximum emission wavelength of the complex of THK-565 and Aβ fibrils was 704 nm, which was within the optical window range. THK-565 selectively bound to amyloid deposits in brain sections of APP-KI mice After the intravenous administration of THK-565, the fluorescence signal in the head of APP-KI mice was significantly higher than that of wild-type mice and higher than that after administration of THK-265. Ex vivo analysis confirmed that the THK-565 signal corresponded to Aβ immunostaining in the brain sections of these mice.

CONCLUSIONS

A novel NIRF probe, THK-565, enabled the in vivo detection of Aβ deposits in the brains of the AD mouse model, suggesting that NIRF imaging with THK-565 could non-invasively assess disease-specific pathology in AD.

Synthesis and biological evaluation of indane-based fluorescent probes for detection of amyloid-β aggregates in Alzheimer's disease

In this article, the development of fluorescent imaging probes for the detection of Alzheimer's disease (AD)-associated protein aggregates is described. Indane derivatives with a donor-π-acceptor (D-π-A) structure were designed and synthesized. The probes were evaluated for their ability to bind to β-amyloid (Aβ) protein aggregates, which are a key pathological hallmark of AD. The results showed that several probes exhibited significant changes in fluorescence intensity at wavelengths greater than 600 nm when they were bound to Aβ aggregates compared to the Aβ monomeric form. Among the tested probes, four D-π-A type indane derivatives showed promising binding selectivity to Aβ aggregates over non-specific proteins such as bovine serum albumin (BSA). The molecular docking study showed that our compounds were appropriately located along the Aβ fibril axis through the hydrophobic tunnel structure. Further analysis revealed that the most active compound having dimethylaminopyridyl group as an election donor and dicyano group as an electron acceptor could effectively stain Aβ plaques in brain tissue samples from AD transgenic mice. These findings suggest that our indane-based compounds have the potential to serve as fluorescent probes for the detection and monitoring of Aβ aggregation in AD.

Synthesis and evaluation of curcumin-based near-infrared fluorescent probes for detection of amyloid β peptide in Alzheimer mouse models

The abnormal accumulation of amyloid β protein (Aβ) is one of the most important causes of Alzheimer's disease (AD) and is usually a detecting biomarker. Curcumin and its derivatives have potential Aβ aggregate targeting ability; we synthesized a series of curcumin-based near-infrared fluorescence probes in this study. By characterizing the excitation wavelength and emission wavelength, the imaging characteristics of the investigation in the near-infrared light region were determined; with an increase in the concentration of the probe compounds, the fluorescence intensity showed an upward trend, demonstrating ideal optical characteristics. In vivo, imaging results showed that the synthesized probe compounds could penetrate the blood-brain barrier (BBB) and specifically bind to Aβ in the brain of APP/PS1 mice. Especially for compound 3b, the maximum emission wavelength was around 667 nm, and the fluorescence signal intensity in the brain of the APP/PS1 mice model was more than twice that of the wild control group at 120 min after administration, which could display Aβ pathological changes. The fluorescent probes designed in this study can become an effective tool for early AD diagnosis and visual detection.

Recent Advances in the Development of Near-Infrared Fluorescent Probes for the in Vivo Brain Imaging of Amyloid-β Species in Alzheimer's Disease

The deposition of β-amyloid (Aβ) plaques in the parenchymal and cortical regions of the brain of Alzheimer's disease (AD) patients is considered the foremost pathological hallmark of the disease. The early diagnosis of AD is paramount in order to effective management and treatment of the disease. Developing near-infrared fluorescence (NIRF) probes targeting Aβ species is a potential and attractive approach suitable for the early and timely diagnosis of AD. The advantages of the NIRF probes over other tools include real-time detection, higher sensitivity, resolution, comparatively inexpensive experimental setup, and noninvasive nature. Currently, enormous progress is being observed in the development of NIRF probes for the in vivo imaging of Aβ species. Several strategies, i.e., the classical push-pull approach, "turn-on" effect, aggregation-induced emission (AIE), and resonance energy transfer (RET), have been exploited for development. We have outlined and discussed the recently emerged NIRF probes with different design strategies targeting Aβ species for ex vivo and in vivo imaging. We believe that understanding the recent development enables the prospect of the rational design of probes and will pave the way for developing future novel probes for early diagnosis of AD.

Two-Photon Fluorescent Probes for Amyloid-β Plaques Imaging In Vivo

Amyloid-β (Aβ) peptide deposition, hyperphosphorylated tau proteins, reactive astrocytes, high levels of metal ions, and upregulated monoamine oxidases are considered to be the primary pathological markers of Alzheimer's disease (AD). Among them, Aβ peptide deposition or Aβ plaques, is regarded as the initial factor in the pathogenesis of AD and a critical pathological hallmark in AD. This review highlights recently Aβ-specific fluorescent probes for two-photon imaging of Aβ plaques in vivo. It includes the synthesis and detection mechanism of probes, as well as their application to two-photon imaging of Aβ plaques in vivo.

Protein aggregation and neurodegenerative disease: Structural outlook for the novel therapeutics

Before the controversial approval of humanized monoclonal antibody lecanemab, which binds to the soluble amyloid-β protofibrils, all the treatments available earlier, for Alzheimer's disease (AD) were symptomatic. The researchers are still struggling to find a breakthrough in AD therapeutic medicine, which is partially attributable to lack in understanding of the structural information associated with the intrinsically disordered proteins and amyloids. One of the major challenges in this area of research is to understand the structural diversity of intrinsically disordered proteins under in vitro conditions. Therefore, in this review, we have summarized the in vitro applications of biophysical methods, which are aimed to shed some light on the heterogeneity, pathogenicity, structures and mechanisms of the intrinsically disordered protein aggregates associated with proteinopathies including AD. This review will also rationalize some of the strategies in modulating disease-relevant pathogenic protein entities by small molecules using structural biology approaches and biophysical characterization. We have also highlighted tools and techniques to simulate the in vivo conditions for native and cytotoxic tau/amyloids assemblies, urge new chemical approaches to replicate tau/amyloids assemblies similar to those in vivo conditions, in addition to designing novel potential drugs.

Molecular Design of Magnetic Resonance Imaging Agents Binding to Amyloid Deposits

The ability to detect and monitor amyloid deposition in the brain using non-invasive imaging techniques provides valuable insights into the early diagnosis and progression of Alzheimer's disease and helps to evaluate the efficacy of potential treatments. Magnetic resonance imaging (MRI) is a widely available technique offering high-spatial-resolution imaging. It can be used to visualize amyloid deposits with the help of amyloid-binding diagnostic agents injected into the body. In recent years, a number of amyloid-targeted MRI probes have been developed, but none of them has entered clinical practice. We review the advances in the field and deduce the requirements for the molecular structure and properties of a diagnostic probe candidate. These requirements make up the base for the rational design of MRI-active small molecules targeting amyloid deposits. Particular attention is paid to the novel cryo-EM structures of the fibril aggregates and their complexes, with known binders offering the possibility to use computational structure-based design methods. With continued research and development, MRI probes may revolutionize the diagnosis and treatment of neurodegenerative diseases, ultimately improving the lives of millions of people worldwide.

Validating blood tests as a possible routine diagnostic assay of Alzheimer's disease

INTRODUCTION

In recent years, exciting developments in disease modifying treatments for Alzheimer's disease (AD) have made accurate and timely diagnosis of this disease a priority. Blood biomarkers (BBMs) for amyloid pathology using improved immunoassay and mass spectrometry techniques have been an area of intense research for the last 10 years and are coming to the fore, as a real prospect to be used in the clinical diagnostics of the disease.

AREAS COVERED

The following review will update and discuss blood biomarkers that will be most useful in diagnosing AD and the context necessary for their implementation.

EXPERT OPINION

It is clear we now have BBMs, and technology to measure them, that are capable of detecting amyloid pathology in AD. The challenge is to validate them across platforms and populations to incorporate them into clinical practice. It is important that implementation comes with education, we need to give clinicians the tools for appropriate use and interpretation. It is feasible that BBMs will be used to screen populations, initially for clinical trial entry but also therapeutic intervention in the foreseeable future. We now need to focus BBM research on other pathologies to ensure we accelerate the development of therapeutics for all neurodegenerative diseases.

Small-molecule theranostics in Alzheimer's disease

Alzheimer's Disease (AD) remains one of the most challenging health-related issues for our society. It is becoming increasingly prevalent, especially in developed countries, due to the rising life expectancy and, moreover, represents a considerable economic burden worldwide. All efforts at the discovery of new diagnostic and therapeutic tools in the last decades have invariably met with failure, making AD an incurable illness and underscoring the need for new approaches. In recent years, theranostic agents have emerged as an interesting strategy. They are molecules able to simultaneously provide diagnostic information and deliver therapeutic activity, allowing for the assessment of the molecule activity, the organism response and the pharmacokinetics. This makes these compounds promising for streamlining research on AD drugs and for their application in personalized medicine. We review here the field of small-molecule theranostic agents as promising tools for the development of novel diagnostic and therapeutic resources against AD, highlighting the positive and significant impact that theranostics can be expected to have in the near future in clinical practice.

Computational Investigations into Two-Photon Fibril Imaging Using the DANIR-2c Probe

The design of novel fibril imaging molecules for medical diagnosis requires the simultaneous optimization of fibril-specific optical properties and binding specificity toward amyloid fibrils. Because of the possibility to monitor internal organs and deep tissues, the two-photon probes that can absorb in the infrared (IR) and near-IR (NIR) region with a significant two-photon absorption cross section are of immense interest. To contribute to this exploration of chemical compounds suitable for two-photon fibril imaging, we have computationally studied the one- and two-photon properties of a donor-acceptor-substituted DANIR-2c probe, which was used for in vivo detection of β-amyloid deposits using fluorescence spectroscopy. In particular, a multiscale computational approach was employed involving molecular docking, molecular dynamics, hybrid QM/MM molecular dynamics, and coupled-cluster/MM to study the binding of the studied probe to amyloid fibril and its one- and two-photon absorption properties in the fibrillar environment. Multiple binding sites are available for this probe in amyloid fibril, and the one corresponding to the largest binding affinity exhibits also the largest and experimentally meaningful two-photon absorption cross section, thus demonstrating the potential of the studied probe in two-photon microscopy.

Recent advance on pleiotropic cholinesterase inhibitors bearing amyloid modulation efficacy

Due to the hugely important roles of neurotransmitter acetylcholine (ACh) and amyloid-β (Aβ) in the pathogenesis of Alzheimer's disease (AD), the development of multi-target directed ligands (MTDLs) focused on cholinesterase (ChE) and Aβ becomes one of the most attractive strategies for combating AD. To date, numerous preclinical studies toward multifunctional conjugates bearing ChE inhibition and anti-Aβ aggregation have been reported. Noteworthily, most of the reported multifunctional cholinesterase inhibitors are carbamate-based compounds due to the initial properties of carbamate moiety. However, because their easy hydrolysis in vivo and the instability of the compound-enzyme conjugate, the mechanism of action of these compounds is rare. Thus, non-carbamate compounds are of great need for developing novel cholinesterase inhibitors. Besides, given that Aβ accumulation begins to occur 10-15 years before AD onset, modulating Aβ is ineffective only in inhibiting its aggregation but not eliminate the already accumulated Aβ if treatment is started when the patient has been diagnosed as AD. Considering the limitation of current Aβ accumulation modulators in ameliorating cognitive deficits and ineffectiveness of ChE inhibitors in blocking disease progression, the development of a practically valuable strategy with multiple pharmaceutical properties including ChE inhibition and Aβ modulation for treating AD is indispensable. In this review, we focus on summarizing the scaffold characteristics of reported non-carbamate cholinesterase inhibitors with Aβ modulation since 2020, and understanding the ingenious multifunctional drug design ideas to accelerate the pace of obtaining more efficient anti-AD drugs in the future.

Using Deep Learning Radiomics to Distinguish Cognitively Normal Adults at Risk of Alzheimer's Disease From Normal Control: An Exploratory Study Based on Structural MRI

Objectives

We proposed a novel deep learning radiomics (DLR) method to distinguish cognitively normal adults at risk of Alzheimer's disease (AD) from normal control based on T1-weighted structural MRI images.

Methods

In this study, we selected MRI data from the Alzheimer's Disease Neuroimaging Initiative Database (ADNI), which included 417 cognitively normal adults. These subjects were divided into 181 individuals at risk of Alzheimer's disease (preAD group) and 236 normal control individuals (NC group) according to standard uptake ratio >1.18 calculated by amyloid Positron Emission Tomography (PET). We further divided the preaAD group into APOE+ and APOE- subgroups according to whether APOE ε4 was positive or not. All data sets were divided into one training/validation group and one independent test group. The proposed DLR method included three steps: (1) the pre-training of basic deep learning (DL) models, (2) the extraction, selection and fusion of DLR features, and (3) classification. The support vector machine (SVM) was used as the classifier. In the comparative experiments, we compared our proposed DLR method with three existing models: hippocampal model, clinical model, and traditional radiomics model. Ten-fold cross-validation was performed with 100 time repetitions.

Results

The DLR method achieved the best classification performance between preAD and NC than other models with an accuracy of 89.85% ± 1.12%. In comparison, the accuracies of the other three models were 72.44% ± 1.37%, 82.00% ± 4.09% and 79.65% ± 2.21%. In addition, the DLR model also showed the best classification performance (85.45% ± 9.04% and 92.80% ± 2.61%) in the subgroup experiment.

Conclusion

The results showed that the DLR method provided a potentially clinical value to distinguish preAD from NC.

Development and Challenge of Fluorescent Probes for Bioimaging Applications: From Visualization to Diagnosis

Fluorescent probes have been used widely in bioimaging, including biological substance detection, cell imaging, in vivo biochemical reaction process tracking, and disease biomarker monitoring, and have gradually occupied an indispensable position. Compared with traditional biological imaging technologies, such as positron emission tomography (PET) and nuclear magnetic resonance imaging (MRI), the attractive advantages of fluorescent probes, such as real-time imaging, in-depth visualization, and less damage to biological samples, have made them increasingly popular. Among them, ultraviolet-visible (UV-vis) fluorescent probes still occupy the mainstream in the field of fluorescent probes due to the advantages of available structure, simple synthesis, strong versatility, and wide application. In recent years, fluorescent probes have become an indispensable tool for bioimaging and have greatly promoted the development of diagnostics. In this review, we focus on the structure, design strategies, advantages, representative probes and latest discoveries in application fields of UV-visible fluorescent probes developed in the past 3-5 years based on several fluorophores. We look forward to future development trends of fluorescent probes from the perspective of bioimaging and diagnostics. This comprehensive review may facilitate the development of more powerful fluorescent sensors for broad and exciting applications in the future.

Strategic Design of Amyloid-β Species Fluorescent Probes for Alzheimer's Disease

Alzheimer's disease (AD) is a high mortality and high disability rates neurodegenerative disease characterized by irreversible progression and poses a significant social and economic burden throughout the world. However, currently approved AD therapeutic agents only alleviate symptoms and there is still a lack of practical therapeutic regimens to stop or slow the progression of this disease. Thus, there is urgently needed novel diagnosis tools and drugs for early diagnosis and treatment of AD. Among several AD pathological hallmarks, amyloid-β (Aβ) peptide deposition is considered a critical initiating factor in AD. In recent years, with the advantages of excellent sensitivity and high resolution, near-infrared fluorescence (NIRF) imaging has attracted the attention of many researchers to develop Aβ plaque probes. This review mainly focused on different NIRF probe design strategies for imaging Aβ species to pave the way for the future design of novel NIRF probes for early diagnosis AD.

Synthesis and evaluation of curcumin-based near-infrared fluorescent probes for the in vivo optical imaging of amyloid-β plaques

The abnormal self-assembly of amyloid-beta (Aβ) peptides into oligomers, as well as insoluble fibrils, has been identified as a key factor for monitoring the progression of Alzheimer's disease (AD). The noninvasive imaging of Aβ aggregates utilizing chemical probes can be a powerful and practical technique for accurately diagnosing and monitoring the progress of AD, as well as evaluating the effectiveness of therapeutic drug candidates in treating or managing it. Particularly, the near-infrared (NIR) fluorescence imaging of Aβ plaques is a potentially promising approach toward the efficient detection of the biomarker. In this study, we describe a new NIR fluorophore, which was based on curcumin derivatives. The fluorophore is equipped with desirable optical properties for in vivo brain imaging. The emission wavelength of the probe, 8b, is 667 nm, and its fluorescent intensity is significantly increased through binding with the Aβ aggregates. The probe allows the clear visualization of the Aβ plaques 10 min post administration, and the intensity of the fluorescent signal in the brain of a 5XFAD transgenic mouse model is more than three times higher than that of the normal control group. These results demonstrate that the designed probe can be an effective tool for visualizing Aβ plaques, as well as investigating the pathological progress of AD.

Aggregation-induced emission materials for protein fibrils imaging

Protein fibrillation is linked to many devastating diseases including neurodegenerative disorders. Fluorescence probes play a significant role in the detection of amyloid aggregates, monitoring amyloid kinetics, and in the development of amyloid inhibitors. Despite the considerable progress in this area, the mechanism of amyloid fibril formation in vivo is not completely understood. Recent studies in amyloidosis indicate that oligomers and prefibrillar species are more cytotoxic than the fibrils. Hence, early diagnosis of fibrillation has high therapeutical relevance. The gold standard for amyloid staining is thioflavin-T and its major drawbacks are aggregation caused quenching and inability in the detection of oligomers. New amyloid staining probes with novel properties are highly desirable as they can give valuable insights into the complicated process and can replace conventional probes. Aggregation-induced emission probes (AIE-probes) with desirable features are promising candidates in protein fibrils imaging. AIE probes in staining different amyloid fibrils, monitoring amyloid kinetics, and early-stage conformers are reported. Other remarkable features are they can be modified as NIR probes, multifunctional probes, theranostic probes, and super-resolution imaging probes. We aim to provide a broad perspective on the progress attained with AIE probes in protein fibrils imaging and thereby emphasizing the scope of these smart probes in translative research.

"Turn-On" Quinoline-Based Fluorescent Probe for Selective Imaging of Tau Aggregates in Alzheimer's Disease: Rational Design, Synthesis, and Molecular Docking

Tau aggregation is believed to have a strong association with the level of cognitive deficits in Alzheimer's disease (AD). Thus, optical brain imaging of tau aggregates has recently gained substantial attention as a promising tool for the early diagnosis of AD. However, selective imaging of tau aggregates is a major challenge due to sharing similar β-sheet structures with homologous Aβ fibrils. Herein, four quinoline-based fluorescent probes (Q-tau) were judiciously designed using the donor-acceptor architecture for selective imaging of tau aggregates. In particular, probe Q-tau 4 exhibited a strong intramolecular charge transfer and favorable photophysical profile, such as a large Stokes' shift and fluorescence emission wavelength of 630 nm in the presence of tau aggregates. The probe also displayed a "turn-on" fluorescence behavior toward tau fibrils with a 3.5-fold selectivity versus Aβ fibrils. In addition, Q-tau 4 exhibited nanomolar binding affinity to tau aggregates (K_d = 16.6 nM), which was 1.4 times higher than that for Aβ fibrils. The mechanism of "turn-on" fluorescence was proposed to be an environment-sensitive molecular rotor-like response. Moreover, ex vivo labeling of human AD brain sections demonstrated favorable colocalization of Q-tau 4 and the phosphorylated tau antibody, while comparable limited staining was observed with Aβ fibrils. Molecular docking was conducted to obtain insights into the tau-binding mode of the probe. Collectively, Q-tau 4 has successfully been used as a tau-specific fluorescent imaging agent with lower background interference.

Dual-target compounds for Alzheimer's disease: Natural and synthetic AChE and BACE-1 dual-inhibitors and their structure-activity relationship (SAR)

Alzheimer's disease (AD) is a chronic neurodegenerative disease and represents the major cause of dementia worldwide. Currently, there are no available treatments capable to deliver disease-modifying effects, and the available drugs can only alleviate the symptoms. The exact pathology of AD is not yet fully understood and several hallmarks such as the presence of amyloid-β (Aβ) senile plaques, neurofibrillary tangles (NFTs) as well as the loss of cholinergic function have been associated to AD. Distinct pharmacological targets have been validated to address AD, with acetylcholinesterase (AChE) and β-secretase-1 (BACE-1) being two of the most explored ones. A great deal of research has been devoted to the development of new AChE and BACE-1 effective inhibitors, tackled separately or in combination of both. The multi-factorial nature of AD conducted to the development of multi-target directed ligands (MTDLs), defined as single molecules capable to modulate more than one biological target, as an alternative approach to the old paradigm one-target one-drug. In this context, this review describes a collection of natural and synthetic compounds with dual-inhibitory properties towards both AChE and BACE-1 in the MTDLs context. Furthermore, this review also provides a critical comprehensive analysis of structure-activity relationships (SAR) of the synthetic compounds.

Advances in fluorescent probes for detection and imaging of amyloid-β peptides in Alzheimer's disease

Amyloid plaques generated from the accumulation of amyloid-β peptides (Aβ) fibrils in the brain is one of the main hallmarks of Alzheimer's disease (AD), a most common neurodegenerative disorder. Aβ aggregation can produce neurotoxic oligomers and fibrils, which has been widely accepted as the causative factor in AD pathogenesis. Accordingly, both soluble oligomers and insoluble fibrils have been considered as diagnostic biomarkers for AD. Among the existing analytical methods, fluorometry using fluorescent probes has exhibited promising potential in quantitative detection and imaging of both soluble and insoluble Aβ species, providing a valuable approach for the diagnosis and drug development of AD. In this review, the most recent advances in the fluorescent probes for soluble or insoluble Aβ aggregates are discussed in terms of design strategy, probing mechanism, and potential applications. In the end, future research directions of fluorescent probes for Aβ species are also proposed.

Amyloid-β and tau aggregation dual-inhibitors: A synthetic and structure-activity relationship focused review

Alzheimer's disease (AD) is one of the most common types of dementia, especially in elderly, with an increasing number of people suffering from this disease worldwide. There are no available disease-modifying therapies and only four drugs are approved for the relief of symptoms. Currently, the therapeutic approach used for AD treatment is based on single target drugs, which are not capable to stop its progression. To address this issue, multi-target compounds, combining two or more pharmacophores in a single molecular entity, have gained increasing interest to deal with the multiple factors related to AD. The exact cause of AD is not yet completely disclosed, and several hallmarks have been associated to this neurodegenerative disease. Even though, the accumulation of both amyloid-β plaques (Aβ) and neurofibrillary tangles (NFTs) are fully accepted as the main AD hallmarks, being object of lots of research for early-stage diagnosis and pharmacological therapy. In this context, this review summarizes the state-of-the-art in the field of dual-target inhibitors of both Aβ and tau aggregation simultaneously, including the design and synthetic strategy of the dual-target compounds, as well as a brief structure-activity relationships (SAR) analysis.

Near-Infrared Fluorescent Probes with Rotatable Polyacetylene Chains for the Detection of Amyloid-β Plaques

The plaques of accumulated β-amyloid (Aβ) in the parenchymal brain are accepted as an important biomarker for the early diagnosis of Alzheimer's disease (AD). Many near-infrared (NIR) probes, which were based on the D-π-A structure and bridged by conjugated double bonds, had been reported and displayed a high affinity to Aβ plaques. Considering the isomerization caused by the polyethylene chain, however, the conjugated polyacetylene chain is a better choice for developing new NIR Aβ probes. Hence, in this report, a new series of NIR probes with naphthyl or phenyl rings and different numbers of conjugated triple bonds were designed, synthesized, and evaluated as NIR probes for Aβ plaques. Upon interaction with Aβ aggregates, these probes displayed a significant increase in fluorescence intensity (45- to 360-fold) and a high to moderate affinity (6.05-56.62 nM). Among them, probe 22b displayed excellent fluorescent properties with a 183-fold increase in fluorescence intensity and an emission maximum at 650 nm after incubated with Aβ aggregates. Furthermore, 22b had a high affinity to Aβ aggregates (K_d = 12.96 nM) and could efficiently detect the Aβ plaques in brain sections from both transgenic mice and AD patients in vitro. In summary, this work may lead to a new direction in the development of novel NIR probes for the detection of Aβ plaques.

A quinoline–benzothiazole hybrid as the first near-infrared fluorescent probe for transthyretin

A quinoline-benzothiazole hybrid was rationally developed as the first NIR fluorescent probe for detecting transthyretin.

BODIPY Dyes as Probes and Sensors to Study Amyloid-β-Related Processes

Amyloid formation plays a major role in a number of neurodegenerative diseases, including Alzheimer's disease. Amyloid-β peptides (Aβ) are one of the primary markers associated with this pathology. Aβ aggregates exhibit a diverse range of morphologies with distinct pathological activities. Recognition of the Aβ aggregates by using small molecule-based probes and sensors should not only enhance understanding of the underlying mechanisms of amyloid formation, but also facilitate the development of therapeutic strategies to interfere with amyloid neurotoxicity. BODIPY (boron dipyrrin) dyes are among the most versatile small molecule fluorophores. BODIPY scaffolds could be functionalized to tune their photophysical properties to the desired ranges as well as to adapt these dyes to various types of conditions and environments. Thus, BODIPY dyes could be viewed as unique platforms for the design of probes and sensors that are capable of detecting and tracking structural changes of various Aβ aggregates. This review summarizes currently available examples of BODIPY dyes that have been used to investigate conformational changes of Aβ peptides, self-assembly processes of Aβ, as well as Aβ interactions with various molecules.

Monitoring the formation of insulin oligomers using a NIR emitting glucose-conjugated BODIPY dye

Protein oligomers, which are formed due to the aggregation of protein molecules under physiological stress, are neurotoxic and responsible for several neurological diseases. Early detection of protein oligomers is essential for the timely intervention in the associated diseases. Although several probes have been developed for the detection of insoluble matured protein fibrils, fluorescent probes with emission in the near infrared (NIR) region for probing protein oligomers are very rare. In the present study we have designed and synthesized a glucose-conjugated BODIPY dye with emission in the NIR spectral range. Our detailed studies show that the new probe is not only capable of detecting matured fibrils but can also probe the formation of oligomers from the native protein. The new probe shows a large increase in its emission intensity upon association with oligomers and matured fibrils. Hence, the present probe has a great potential for the in vivo imaging of protein oligomers and matured fibrils. Detailed spectroscopic properties of the new probes in molecular solvents have been performed to understand its oligomers- and fibril- sensing mechanism.