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

Ligands for Protein Fibrils of Amyloid-β, α-Synuclein, and Tau

Amyloid fibrils are characteristic features of many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. The use of small molecule ligands that bind to amyloid fibrils underpins both fundamental research aiming to better understand the pathology of neurodegenerative disease, and clinical research aiming to develop diagnostic tools for these diseases. To date, a large number of amyloid-binding ligands have been reported in the literature, predominantly targeting protein fibrils composed of amyloid-β (Aβ), tau, and α-synuclein (αSyn) fibrils. Fibrils formed by a particular protein can adopt a range of possible morphologies, but protein fibrils formed in vivo possess disease-specific morphologies, highlighting the need for morphology-specific amyloid-binding ligands. This review details the morphologies of Aβ, tau, and αSyn fibril polymorphs that have been reported as a result of structural work and describes a database of amyloid-binding ligands containing 4,288 binding measurements for 2,404 unique compounds targeting Aβ, tau, or αSyn fibrils.

Development of Donor-Acceptor Architecture-Based Potential Theranostic Fluorescent Probes for Alzheimer's Disease

The cholinergic deficits and deposition of β-amyloid (Aβ) species are regarded as the key events contributing to the progression of Alzheimer's disease (AD). Herein, a series of novel donor-acceptor architecture-type potential theranostic agents were designed, synthesized, and evaluated for their potential against cholinesterase (ChE) enzymes and detection of Aβ species, which are primary targets in the development of therapeutics for AD. The optimal compound/probe 18 containing a benzothiazolium fluorophore with a bifunctional electron-donating N-aryl piperazine scaffold exhibited potent inhibitory activities against acetylcholinesterase (AChE; IC50 = 0.172 ± 0.011 μM) and butyrylcholinesterase (BuChE; IC50 = 1.376 ± 0.141 μM). Measurement of fluorescence properties showed that probe 18 exhibited emission maxima (λem) of >610 nm in dimethyl sulfoxide (DMSO) and >590 nm in PBS, suitable for the fluorescence imaging. In vitro studies demonstrated a change in fluorescence characteristics and high binding affinities (18; Kd = 0.731 μM) upon binding with Aβ aggregates. The affinity of probe 18 toward Aβ aggregates was further observed in elavGAL4 > UAS Aβ, the Drosophila larval brain sections, using a fluorescence imaging technique. The in vivo acute oral toxicity evaluation indicated a safety profile of the lead probe 18. Moreover, in vivo behavioral studies including Y-maze and novel object recognition tests signified that the administration of compound 18 improved cognitive and spatial memory impairment at a dose of 10 and 20 mg/kg in the scopolamine-induced cognitive deficit model.

Synthesis, Formation Mechanisms, and Molecular Dynamics Simulation of Novel Benzothiazole and Benzo[1,4]oxazin-3(4H)-one as Potential Acetylcholinesterase Inhibitors

A novel series of benzothiazole derivatives was synthesized using straightforward and easily implementable procedures, achieving a high yield. Among these synthesized compounds, amino acids containing the benzothiazole moiety were successfully produced through an 8-step process, with yields reaching as high as 95%. Notably, a serendipitous compound containing both benzothiazole and benzo[1,4]oxazin-3(4H)-one moieties was also synthesized using the same protocol, bypassing purification at step 7 and proceeding directly to hydrolysis. This highlights the unique role of the coupling reagent HATU (hexafluorophosphate azabenzotriazole tetramethyluronium) in the reaction, as it facilitated high yields, reaching up to 90%. The structures of the newly synthesized compounds were confirmed through spectral analysis. Density functional theory calculations suggested that energy barriers can be overcome by utilizing the energy from an exothermic reaction, enabling the thermodynamically favorable formation of this novel structure. Compounds 6d and 6f demonstrated significant inhibitory activity against the enzyme acetylcholinesterase, with IC50 values of 32.00 and 25.33 μg/mL, respectively. Molecular docking and molecular dynamics analyses indicate that compounds 6d and 6f hold potential for combating Alzheimer's disease, due to their interactions with critical amino acid residues and structural stability.

Modulation of AIE and Intramolecular Charge Transfer of a Pyrene-Based Probe for Discriminatory Detection and Imaging of Oligomers and Amyloid Fibrils

Oligomers and amyloid fibrils formed at different stages of protein aggregation are important biomarkers for a variety of neurodegenerative diseases including Alzheimer's and Parkinson's diseases. The development of probes for the sensitive detection of oligomeric species is important for early stage diagnosis of amyloidogenic diseases. Many small molecular dyes have been developed to probe the dynamic growth of amyloid fibrils. However, there is a lack of discriminatory detection strategies to monitor the dynamics of both oligomers and amyloid fibrils based on the differential modulation of the photophysical properties of a single dye. Here we report a pyrene-based intramolecular charge transfer (ICT) dye with large Stokes shifted red-emitting aggregation induced emission (AIE) for monitoring the dynamic populations of both oligomers and fibrils during the aggregation of hen egg white lysozyme (HEWL) protein. At the early stage of protein aggregation, the accumulation of HEWL oligomers results in a rapid and substantial increase in the red AIE intensity at 660 nm. Later, as the oligomers transform into mature fibrils, the dye exhibits a distinct photophysical change. Binding of the dye to HEWL fibrils strongly suppresses the red AIE and enhances ICT emission. This is evidenced by a gradual decrease in the AIE intensity (∼660 nm) and an increase in LE (∼490 nm) and ICT (∼540 nm) emission intensities during the later stages of protein aggregation. Thus, the dye provides simultaneous measurements of the population dynamics of both HEWL oligomers and fibrils during protein aggregation based on the discriminatory modulation of AIE and ICT of the dye. The dye also enables imaging of both HEWL oligomers and fibrils simultaneously using different emission channels in super-resolution confocal fluorescence microscopy.

Artificial Intelligence-Guided Gut-Microenvironment-Triggered Imaging Sensor Reveals Potential Indicators of Parkinson's Disease

The gut-brain axis has recently emerged as a crucial link in the development and progression of Parkinson's disease (PD). Dysregulation of the gut microbiota has been implicated in the pathogenesis of this disease, sparking growing interest in the quest for non-invasive biomarkers derived from the gut for early PD diagnosis. Herein, an artificial intelligence-guided gut-microenvironment-triggered imaging sensor (Eu-MOF@Au-Aptmer) to achieve non-invasive, accurate screening for various stages of PD is presented. The sensor works by analyzing α-Syn in the gut using deep learning algorithms. By monitoring changes in α-Syn, the sensor can predict the onset of PD with high accuracy. This work has the potential to revolutionize the diagnosis and treatment of PD by allowing for early intervention and personalized treatment plans. Moreover, it exemplifies the promising prospects of integrating artificial intelligence (AI) and advanced sensors in the monitoring and prediction of a broad spectrum of diseases and health conditions.

Morphing cholinesterase inhibitor amiridine into multipotent drugs for the treatment of Alzheimer's disease

The search for novel drugs to address the medical needs of Alzheimer's disease (AD) is an ongoing process relying on the discovery of disease-modifying agents. Given the complexity of the disease, such an aim can be pursued by developing so-called multi-target directed ligands (MTDLs) that will impact the disease pathophysiology more comprehensively. Herewith, we contemplated the therapeutic efficacy of an amiridine drug acting as a cholinesterase inhibitor by converting it into a novel class of novel MTDLs. Applying the linking approach, we have paired amiridine as a core building block with memantine/adamantylamine, trolox, and substituted benzothiazole moieties to generate novel MTDLs endowed with additional properties like N-methyl-d-aspartate (NMDA) receptor affinity, antioxidant capacity, and anti-amyloid properties, respectively. The top-ranked amiridine-based compound 5d was also inspected by in silico to reveal the butyrylcholinesterase binding differences with its close structural analogue 5b. Our study provides insight into the discovery of novel amiridine-based drugs by broadening their target-engaged profile from cholinesterase inhibitors towards MTDLs with potential implications in AD therapy.

A closer look at amyloid ligands, and what they tell us about protein aggregates

The accumulation of amyloid fibrils is characteristic of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease. Detecting these fibrils with fluorescent or radiolabelled ligands is one strategy for diagnosing and better understanding these diseases. A vast number of amyloid-binding ligands have been reported in the literature as a result. To obtain a better understanding of how amyloid ligands bind, we have compiled a database of 3457 experimental dissociation constants for 2076 unique amyloid-binding ligands. These ligands target Aβ, tau, or αSyn fibrils, as well as relevant biological samples including AD brain homogenates. From this database significant variation in the reported dissociation constants of ligands was found, possibly due to differences in the morphology of the fibrils being studied. Ligands were also found to bind to Aβ(1-40) and Aβ(1-42) fibrils with similar affinities, whereas a greater difference was found for binding to Aβ and tau or αSyn fibrils. Next, the binding of ligands to fibrils was shown to be largely limited by the hydrophobic effect. Some Aβ ligands do not fit into this hydrophobicity-limited model, suggesting that polar interactions can play an important role when binding to this target. Finally several binding site models were outlined for amyloid fibrils that describe what ligands target what binding sites. These models provide a foundation for interpreting and designing site-specific binding assays.

Early Diagnosis of Neurodegenerative Diseases: What Has Been Undertaken to Promote the Transition from PET to Fluorescence Tracers

Alzheimer's Disease (AD) and Parkinson's Disease (PD) represent two among the most frequent neurodegenerative diseases worldwide. A common hallmark of these pathologies is the misfolding and consequent aggregation of amyloid proteins into soluble oligomers and insoluble β-sheet-rich fibrils, which ultimately lead to neurotoxicity and cell death. After a hundred years of research on the subject, this is the only reliable histopathological feature in our hands. Since AD and PD are diagnosed only once neuronal death and the first symptoms have appeared, the early detection of these diseases is currently impossible. At present, there is no effective drug available, and patients are left with symptomatic and inconclusive therapies. Several reasons could be associated with the lack of effective therapeutic treatments. One of the most important factors is the lack of selective probes capable of detecting, as early as possible, the most toxic amyloid species involved in the onset of these pathologies. In this regard, chemical probes able to detect and distinguish among different amyloid aggregates are urgently needed. In this article, we will review and put into perspective results from ex vivo and in vivo studies performed on compounds specifically interacting with such early species. Following a general overview on the three different amyloid proteins leading to insoluble β-sheet-rich amyloid deposits (amyloid β1-42 peptide, Tau, and α-synuclein), a list of the advantages and disadvantages of the approaches employed to date is discussed, with particular attention paid to the translation of fluorescence imaging into clinical applications. Furthermore, we also discuss how the progress achieved in detecting the amyloids of one neurodegenerative disease could be leveraged for research into another amyloidosis. As evidenced by a critical analysis of the state of the art, substantial work still needs to be conducted. Indeed, the early diagnosis of neurodegenerative diseases is a priority, and we believe that this review could be a useful tool for better investigating this field.

Recent Developments of Hybrid Fluorescence Techniques: Advances in Amyloid Detection Methods

Amyloid fibrils are formed from various pathological proteins. Monitoring their aggregation process is necessary for early detection and treatment. Among the available detection techniques, fluorescence is simple, intuitive, and convenient due to its sensitive and selective mode of detection. It has certain disadvantages like poor photothermal stability and detection state limitation. Research has focused on minimising the limitation by developing hybrid fluorescence techniques. This review focuses on the two ways fluorescence (intrinsic and extrinsic) has been used to monitor amyloid fibrils. In intrinsic/label free fluorescence: i) The fluorescence emission through aromatic amino acid residues like phenylalanine (F), tyrosine (Y) and tryptophan (W) is present in amyloidogenic peptides/protein sequence. And ii) The structural changes from alpha helix to cross-β-sheet structures during amyloid formation contribute to the fluorescence emission. The second method focuses on the use of extrinsic fluorophores to monitor amyloid fibrils i) organic dyes/small molecules, ii) fluorescent tagged proteins, iii) nanoparticles, iv) metal complexes and v) conjugated polymers. All these fluorophores have their own limitations. Developing them into hybrid fluorescence techniques and converting it into biosensors can contribute to early detection of disease.

Recent Research Progress in Fluorescent Probes for Detection of Amyloid-β In Vivo

Alzheimer's disease (AD) is a neurodegenerative disease. Due to its complex pathological mechanism, its etiology is not yet clear. As one of the main pathological markers of AD, amyloid-β (Aβ) plays an important role in the development of AD. The deposition of Aβ is not only related to the degeneration of neurons, but also can activate a series of pathological events, including the activation of astrocytes and microglia, the breakdown of the blood-brain barrier, and the change in microcirculation, which is the main cause of brain lesions and death in AD patients. Therefore, the development of efficient and reliable Aβ-specific probes is crucial for the early diagnosis and treatment of AD. This paper focuses on reviewing the application of small-molecule fluorescent probes in Aβ imaging in vivo in recent years. These probes efficiently map the presence of Aβ in vivo, providing a pathway for the early diagnosis of AD and providing enlightenment for the design of Aβ-specific probes in the future.

Structure-Activity Relationships of Cyano-substituted Indole Derivatives as Ligands for α-Synuclein Aggregates

α-Synuclein (α-syn) is an essential biomarker for synucleinopathies, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). The development of α-syn imaging probes is of great importance for understanding the pathogenesis mechanism and developing new therapies. In this study, we designed and synthesized a series of cyano-substituted indole derivatives and evaluated their potency to bind to α-syn fibrils by in vitro fibril binding assays. We carried out systematic structure-activity relationship (SAR) studies and obtained a promising candidate 51. The results showed that 51 bound to α-syn fibrils with the affinity of 17.4 ± 5.6 nM, and the biodistribution experiments in normal mice showed [125I]51 exhibited a moderate brain uptake of 3.57 ± 0.28% ID/g at 2 min after injection. In conclusion, the indole derivative [125I]51 showed initial potential as α-syn imaging probes, which needed further development.

An unnatural amino acid modified human insulin derivative for visual monitoring of insulin aggregation

Insulin often forms toxic fibrils during production and transportation, which are deposited as amyloids at repeated injection sites in diabetic patients. Distinguishing early fibrils from non-fibrillated insulin is difficult. Herein, we introduce a chemically modified human insulin derivative with a distinct visual colour transition upon aggregation, facilitating insulin quality assessment.

A Fluorescent Probe as a Lead Compound for a Selective α-Synuclein PET Tracer: Development of a Library of 2-Styrylbenzothiazoles and Biological Evaluation of [18F]PFSB and [18F]MFSB

A method to detect and quantify aggregated α-synuclein (αSYN) fibrils in vivo would drastically impact the current understanding of multiple neurodegenerative diseases, revolutionizing their diagnosis and treatment. Several efforts have produced promising scaffolds, but a notable challenge has hampered the establishment of a clinically successful αSYN positron emission tomography (PET) tracer: the requirement of high selectivity over the other misfolded proteins amyloid β (Aβ) and tau. By designing and screening a library of 2-styrylbenzothiazoles based on the selective fluorescent probe RB1, this study aimed at developing a selective αSYN PET tracer. [3H]PiB competition binding assays identified PFSB (Ki = 25.4 ± 2.3 nM) and its less lipophilic analogue MFSB, which exhibited enhanced affinity to αSYN (Ki = 10.3 ± 4.7 nM) and preserved selectivity over Aβ. The two lead compounds were labeled with fluorine-18 and evaluated using in vitro autoradiography on human brain slices, where they demonstrated up to 4-fold increased specific binding in MSA cases compared to the corresponding control, reasonably reflecting selective binding to αSYN pathology. In vivo PET imaging showed [18F]MFSB successfully crosses the blood-brain barrier (BBB) and is taken up in the brain (SUV = 1.79 ± 0.02). Although its pharmacokinetic profile raises the need for additional structural optimization, [18F]MFSB represents a critical step forward in the development of a successful αSYN PET tracer by overcoming the major challenge of αSYN/Aβ selectivity.

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.

Multidimensional biomarkers for multiple system atrophy: an update and future directions

Multiple system atrophy (MSA) is a fatal progressive neurodegenerative disease. Biomarkers are urgently required for MSA to improve the diagnostic and prognostic accuracy in clinic and facilitate the development and monitoring of disease-modifying therapies. In recent years, significant research efforts have been made in exploring multidimensional biomarkers for MSA. However, currently few biomarkers are available in clinic. In this review, we systematically summarize the latest advances in multidimensional biomarkers for MSA, including biomarkers in fluids, tissues and gut microbiota as well as imaging biomarkers. Future directions for exploration of novel biomarkers and promotion of implementation in clinic are also discussed.

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.

Small molecule-based fluorescent probes for the detection of α-Synuclein aggregation states

The formation of aggregates due to protein misfolding is encountered in various neurodegenerative diseases. α-Synuclein (α-Syn) aggregation is linked to Parkinson's disease (PD). It is one of the most prevalent neurodegenerative disorders after Alzheimer's disease. Aggregation of α-Syn is associated with Lewy body formation and degeneration of the dopaminergic neurons in the brain. These are the pathological hallmarks of PD progression. α-Syn aggregates in a multi-step process. The native unstructured α-Syn monomers combine to form oligomers, followed by amyloid fibrils, and finally Lewy bodies. Recent evidence suggests that α-Syn oligomerization and fibrils formation play major roles in PD development. α-Syn oligomeric species is the main contributor to neurotoxicity. Therefore, the detection of α-Syn oligomers and fibrils has drawn significant attention for potential diagnostic and therapeutic development. In this regard, the fluorescence strategy has become the most popular approach for following the protein aggregation process. Thioflavin T (ThT) is the most frequently used probe for monitoring amyloid kinetics. Unfortunately, it suffers from several significant drawbacks including the inability to detect neurotoxic oligomers. Researchers developed several small molecule-based advanced fluorescent probes compared to ThT for the detection/monitoring of α-Syn aggregates states. These are summarized here.

A turn-on fluorescent probe based on ESIPT and AIEE mechanisms for the detection of butyrylcholinesterase activity in living cells and in non-alcoholic fatty liver of zebrafish

Butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) are two important cholinesterase enzymes in human metabolism which are closely related to various diseases of the liver. BChE and AChE are difficult to be distinguished due to their similarity in biochemical properties. Therefore, developing BChE-specific probes with high sensitivity and low background reading is desirable for the relevant biological applications. Herein, we reported the design and synthesis of a fluorescent probe HBT-BChE for biological detection and imaging of BChE. The probe is triggered by BChE-mediated hydrolysis, releasing a fluorophore that holds AIEE and ESIPT properties with large Stokes shift (>100 nm), rendering the probe features of low background interference and high sensitivity. The probe can also distinguish BChE from AChE with a low detection limit of 7.540 × 10^-4 U/mL. Further in vitro studies have shown the ability of HBT-BChE to detect intracellular BChE activity, as well as to evaluate the efficiency of the BChE inhibitor. More importantly, the in vivo studies of imaging the BChE activity level in liver tissues using zebrafish as the model animal demonstrated the potential of HBT-BChE as a powerful tool for non-alcoholic fatty liver disease.

Compact Luminol Chemiluminophores for In Vivo Detection and Imaging of β-Sheet Protein Aggregates

Protein aggregation has been found in a wide range of neurodegenerative protein-misfolding diseases. The demand for in vivo technologies to identify protein aggregation is at the leading edge for the pathogenic study, diagnostic development, and therapeutic intervention of these devastating disorders. Herein, we report a series of luminol analogues to construct a facile chemiluminescence (CL)-based approach for in vivo detection and imaging of β-sheet protein aggregates. The synthesized compounds exhibited a distinct chemiluminescent response with long emission wavelengths toward reactive oxygen species under physiological conditions and displayed signal amplification in the presence of β-sheet protein aggregates, including α-synuclein, β-amyloid, and tau. Among them, CyLumi-3 was further evaluated as a chemiluminescent probe in preclinical models. By intravenous administration into the model mice via the tail vein, in vivo CL imaging noninvasively detected the specific CL of the probe targeting the α-synuclein aggregates in the brains of living mice. Based on its structural characteristics, CyLumi-3 can readily interact with α-synuclein aggregates with significantly enhanced fluorescence and can identify α-synuclein aggregates in vivo via distinctive CL amplification, which could pave the way for a more comprehensive understanding of protein aggregation in preclinical studies and would provide new hints for developing small-molecule chemiluminophores for protein aggregates.

Sensing lysozyme fibrils by salicylaldimine substituted BODIPY dyes - A correlation with molecular structure

Quick and efficient detection of protein fibrils has enormous impact on the diagnosis and treatment of amyloid related neurological diseases. Among several methods, fluorescence based techniques have garnered most importance in the detection of amyloid fibrils due to its high sensitivity and extreme simplicity. Among other classes of molecular probes, BODIPY derivatives have been employed extensively for the detection of amyloid fibrils. However, there are very few studies on the relationship between the molecular structure of BODIPY dyes and their amyloid sensing activity. Here in a BODIPY based salicylaldimine Schiff base and its corresponding boron complex have been evaluated for their ability to sense amyloid fibrils from hen-egg white lysozyme using steady state and time-resolved spectroscopic techniques. Both dyes show fluorescence enhancement as well as increase in their excited state lifetime upon their binding with lysozyme fibrils. However, the BODIPY derivative which shows more emission enhancement in fibrillar solution has much lower affinity towards amyloid fibrils as compared to other derivative. This contrasting behaviour in the emission enhancement and binding affinity has been explained on the basis of differences in their photophysical properties in water and amyloid fibril originating from the difference in their molecular structure. Such correlation between the amyloid sensitivity and the molecular structure of the probe can open up a new strategy for designing new efficient amyloid probes.

One-Pot Reductive Methylation of Nitro- and Amino-Substituted (Hetero)Aromatics with DMSO/HCOOH: Concise Synthesis of Fluorescent Dimethylamino-Functionalized Bibenzothiazole Ligands with Tunable Emission Color upon Complexation

One-pot reductive N,N-dimethylation of suitable nitro- and amino-substituted (hetero)arenes can be achieved using a DMSO/HCOOH/Et₃N system acting as a low-cost but efficient reducing and methylating agent. The transformation of heteroaryl-amines can be accelerated by using dimethyl sulfoxide/oxalyl chloride or chloromethyl methyl sulfide as the source of active CH₃SCH₂⁺ species, while the exclusion of HCOOH in the initial stage of the reaction allows avoiding N-formamides as resting intermediates. The developed procedures are applicable in multigram-scale synthesis, and because of the lower electrophilicity of CH₃SCH₂⁺, they also work in pathological cases, where common methylating agents provide N,N-dimethylated products in no yield or inferior yields due to concomitant side reactions. The method is particularly useful in one-pot reductive transformation of 2-H-nitrobenzazoles to corresponding N,N-dimethylamino-substituted heteroarenes. These, upon Cu(II)-catalyzed oxidative homocoupling, afford 2,2'-bibenzazoles substituted with dimethylamino groups as charge-transfer N^N ligands with intensive absorption/emission in the visible region. The fluorescence of NMe₂-functionalized bibenzothiazoles remains intensive even upon complexation with ZnCl₂, while emission maxima are bathochromically shifted from the green/yellow to orange/red spectral region, making these small-molecule fluorophores, exhibiting large emission quantum yields and Stokes shifts, an attractive platform for the construction of various functional dyes and light-harvesting materials with tunable emission color upon complexation.

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.

Current Progress in the Development of Probes for Targeting α-Synuclein Aggregates

α-Synuclein aggregates abnormally into intracellular inclusions in Parkinson's disease (PD), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and many other neurological disorders, closely connecting with their pathogenesis. The accurate tracking of α-synuclein by targeting probes is of great significance for early diagnosis, disease monitoring, and drug development. However, there have been no promising α-synuclein targeting probes for clinical application reported so far. This overview focuses on various potential α-synuclein targeting probes reported in the past two decades, including small-molecule fluorescent probes and radiolabeled probes. We provide the current status of the development of the small molecular α-synuclein imaging probes, including properties of promising imaging molecules, strategies of processing new probes, limited progress, and growth prospects in this field, expecting to help in the further development of α-synuclein targeting probes.

A Comparative Study of High-Contrast Fluorescence Lifetime Probes for Imaging Amyloid in Tissue

Optical imaging of protein aggregates in living and post-mortem tissue can often be impeded by unwanted fluorescence, prompting the need for novel methods to extract meaningful signal in complex biological environments. Historically, benzothiazolium derivatives, prominently Thioflavin T, have been the state-of-the-art fluorescent probes for amyloid aggregates, but their optical, structural, and binding properties typically limit them to in vitro applications. This study compares the use of novel uncharged derivative, PAP_1, with parent Thioflavin T as a fluorescence lifetime imaging probe. This is applied specifically to imaging recombinant α-synuclein aggregates doped into brain tissue. Despite the 100-fold lower brightness of PAP_1 compared to that of Thioflavin T, PAP_1 binds to α-synuclein aggregates with an affinity several orders of magnitude greater than Thioflavin T; thus, we observe a specific decrease in the fluorescence lifetime of PAP_1 bound to α-synuclein aggregates, resulting in a separation of >1.4 standard deviations between PAP_1-stained brain tissue background and α-synuclein aggregates that is not observed with Thioflavin T. This enables contrast between highly fluorescent background tissue and amyloid fibrils that is attributed to the greater affinity of PAP_1 for α-synuclein aggregates, avoiding the substantial off-target staining observed with Thioflavin T.

FRET-based assay for intracellular evaluation of α-synuclein aggregation inhibitors

Aggregation of small neuronal protein α-synuclein (αSyn) in amyloid fibrils is considered to be one of the main causes of Parkinson's disease. Inhibition of this aggregation is a promising approach for disease treatment. Dozens of compounds able to inhibit αSyn fibrillization in solution were developed during the last decade. However, the applicability of most of them in the cellular environment was not established because of the absence of a suitable cell-based assay. In this work, we developed an assay for testing αSyn aggregation inhibitors in cells that is based on fluorescence resonance energy transfer (FRET) between labeled αSyn molecules in fibrils. The assay directly reports the amount of fibrillized αSyn and is more reliable than the assays based on cell viability. Moreover, we showed that cell viability decline does not always correlate with the amount of misfolded αSyn. The developed FRET-based assay does not interfere with the aggregation process and is suitable for high-throughput testing of αSyn aggregation inhibitors. Its application can sort out non-specific inhibitors and thus significantly facilitate the development of drugs for Parkinson`s disease.

Alpha-Synuclein PET Tracer Development-An Overview about Current Efforts

Neurodegenerative diseases such as Parkinson’s disease (PD) are manifested by inclusion bodies of alpha-synuclein (α-syn) also called α-synucleinopathies. Detection of these inclusions is thus far only possible by histological examination of postmortem brain tissue. The possibility of non-invasively detecting α-syn will therefore provide valuable insights into the disease progression of α-synucleinopathies. In particular, α-syn imaging can quantify changes in monomeric, oligomeric, and fibrillic α-syn over time and improve early diagnosis of various α-synucleinopathies or monitor treatment progress. Positron emission tomography (PET) is a non-invasive in vivo imaging technique that can quantify target expression and drug occupancies when a suitable tracer exists. As such, novel α-syn PET tracers are highly sought after. The development of an α-syn PET tracer faces several challenges. For example, the low abundance of α-syn within the brain necessitates the development of a high-affinity ligand. Moreover, α-syn depositions are, in contrast to amyloid proteins, predominantly localized intracellularly, limiting their accessibility. Furthermore, another challenge is the ligand selectivity over structurally similar amyloids such as amyloid-beta or tau, which are often co-localized with α-syn pathology. The lack of a defined crystal structure of α-syn has also hindered rational drug and tracer design efforts. Our objective for this review is to provide a comprehensive overview of current efforts in the development of selective α-syn PET tracers.

Supercharging Prions via Amyloid-Selective Lysine Acetylation

Repulsive electrostatic forces between prion-like proteins are a barrier against aggregation. In neuropharmacology, however, a prion's net charge (Z) is not a targeted parameter. Compounds that selectively boost prion Z remain unreported. Here, we synthesized compounds that amplified the negative charge of misfolded superoxide dismutase-1 (SOD1) by acetylating lysine-NH₃ ⁺ in amyloid-SOD1, without acetylating native-SOD1. Compounds resembled a "ball and chain" mace: a rigid amyloid-binding "handle" (benzothiazole, stilbene, or styrylpyridine); an aryl ester "ball"; and a triethylene glycol chain connecting ball to handle. At stoichiometric excess, compounds acetylated up to 9 of 11 lysine per misfolded subunit (ΔZ_fibril =-8100 per 10³ subunits). Acetylated amyloid-SOD1 seeded aggregation more slowly than unacetylated amyloid-SOD1 in vitro and organotypic spinal cord (these effects were partially due to compound binding). Compounds exhibited reactivity with other amyloid and non-amyloid proteins (e.g., fibrillar α-synuclein was peracetylated; serum albumin was partially acetylated; carbonic anhydrase was largely unacetylated).

Fluorescent Probe for Selective Imaging of α-Synuclein Fibrils in Living Cells

Plaques of amyloid fibrils composed of neuronal protein α-synuclein are one of the hallmarks of Parkinson's disease, and their selective imaging is crucial to study the mechanism of its pathogenesis. However, the existing fluorescent probes for amyloids are efficient only in solution and tissue systems, and they are not selective enough for the visualization of amyloid fibrils in living cells. In this study, we present two molecular rotor-based probes RB1 and RB2. These thiazolium probes show affinity to α-synuclein fibrils and turn-on fluorescence response upon interactions. Because of its extended π-conjugation and high rotational degree of freedom, RB1 exhibits a 76 nm red-shift of absorption maxima and 112-fold fluorescence enhancement upon binding to amyloid fibrils. Owing to its strong binding affinity to α-synuclein fibrils, RB1 can selectively stain them in the cytoplasm of living HeLa and SH-SY5Y cells with high optical contrast. RB1 is a cell-permeable and noncytotoxic probe. Taken together, we have demonstrated that RB1 is an amyloid probe with an outstanding absorption red-shift that can be used for intracellular imaging of α-synuclein fibrils.

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.

Fluorescent probes for bioimaging of potential biomarkers in Parkinson's disease

Parkinson's disease (PD), as the second most common neurodegenerative disease, is caused by complex pathological processes and currently remains very difficult to treat. PD brings great distress to patients and imposes a heavy economic burden on society. The number of PD patients is growing as the aging population increases worldwide. Therefore, it is crucial to develop new tools for aiding the early diagnosis and treatment of PD. The significant pathological features involved in PD include the abnormal accumulation of α-synuclein, metal ion dyshomeostasis, oxidative stress, mitochondrial dysfunction and neurotransmitter deficiencies. In recent years, fluorescent probes have emerged as a powerful bioimaging tool with potential to help understand the pathological processes of PD via the detection and monitoring of pathological features. In this review, we comprehensively summarize the design and working mechanisms of fluorescent probes along with their applications in the detection of various PD biomarkers. We also discuss the current limitations of fluorescent probes and provide perspectives on how these limitations can be overcome to develop better fluorescent probes suitable for application in clinical trials in the future. We hope that this review provides valuable information and guidance for the development of new fluorescent probes that can be used clinically in the early diagnosis of PD and contributes to the development of efficient PD drugs in the future.

Fluorescent Diagnostic Probes in Neurodegenerative Diseases

Neurodegenerative diseases are debilitating disorders that feature progressive and selective loss of function or structure of anatomically or physiologically associated neuronal systems. Both chronic and acute neurodegenerative diseases are associated with high morbidity and mortality along with the death of neurons in different areas of the brain; moreover, there are few or no effective curative therapy options for treating these disorders. There is an urgent need to diagnose neurodegenerative disease as early as possible, and to distinguish between different disorders with overlapping symptoms that will help to decide the best clinical treatment. Recently, in neurodegenerative disease research, fluorescent-probe-mediated biomarker visualization techniques have been gaining increasing attention for the early diagnosis of neurodegenerative diseases. A survey of fluorescent probes for sensing and imaging biomarkers of neurodegenerative diseases is provided. These imaging probes are categorized based on the different potential biomarkers of various neurodegenerative diseases, and their advantages and disadvantages are discussed. Guides to develop new sensing strategies, recognition mechanisms, as well as the ideal features to further improve neurodegenerative disease fluorescence imaging are also explored.

SYPRO Orange - a new gold standard amyloid probe

SYPRO Orange, a zwitterionic merocyanine dye, can strongly interact with amyloid fibrils and shows enormous (∼1200 times) increase in its emission intensity. The sensitivity of the new probe is several times higher than that of the gold standard amyloid probe thioflavin T. Unlike thioflavin-T, the new probe has spectral properties suitable for in vivo imaging with detection sensitivity in the picomolar range.

Benzimidazole-based fluorophores for the detection of amyloid fibrils with higher sensitivity than Thioflavin-T

Protein aggregation into amyloid fibrils is a key feature of a multitude of neurodegenerative diseases such as Alzheimer's, Parkinson's, and Prion disease. To detect amyloid fibrils, fluorophores with high sensitivity and better efficiency coupled with the low toxicity are in high demand even to date. In this pursuit, we have unveiled two benzimidazole-based fluorescence sensors ([C₁₅ H₁₅ N₃ ] (C1) and [C₁₆ H₁₆ N₃ O₂ ] (C2), which possess exceptional affinity toward different amyloid fibrils in its submicromolar concentration (8 × 10^-9  M), whereas under a similar concentration, the gold standard Thioflavin-T (ThT) fails to bind with amyloid fibrils. These fluorescent markers bind to α-Syn amyloid fibrils as well as amyloid fibrils forming other proteins/peptides including Aβ42 amyloid fibrils. The ¹ H-¹⁵ N heteronuclear quantum correlation spectroscopy nuclear magnetic resonance data collected on wild-type α-Syn monomer with and without the fluorophores (C1 and C2) reveal that there is weak or no interactions between C1 or C2 with residues in α-Syn monomer, which indirectly reflects the specific binding ability of C1 and C2 to the α-Syn amyloid fibrils. Detailed studies further suggest that C1 and C2 can detect/bind with the α-Syn amyloid fibril as low as 100 × 10^-9  M. Extremely low or no cytotoxicity is observed for C1 and C2 and they do not interfere with α-Syn fibrillation kinetics, unlike ThT. Both C1/C2 not only shows selective binding with amyloid fibrils forming various proteins/peptides but also displays excellent affinity and selectivity toward α-Syn amyloid aggregates in SH-SY5Y cells and Aβ42 amyloid plaques in animal brain tissues. Overall, our data show that the developed dyes could be used for the detection of amyloid fibrils including α-Syn and Aβ42 amyloids with higher sensitivity as compared to currently used ThT.

Facile synthesis of benzazoles through biocatalytic cyclization and dehydrogenation employing catalase in water

The benzazoles are very important entities having immense biological activities, hence; the synthesis of benzazoles is one of the prime areas for synthetic chemists. In pursuit of sustainable protocol, herein an oxidative enzyme i.e. catalase mediated sustainable synthesis is presented. Catalase is a metalloenzyme which is essential for the breakdown of toxic hydrogen peroxide into water and oxygen inside the cell. Despite the higher activity and turnover number of catalase inside the cell, its activity outside the cell is unexplored. Therefore, to explore the hidden potential of catalase for catalyzing the organic transformations, here we reported a green and efficient method for synthesis of benzazoles by the cyclocondensation of o-aminothiophenol or o-phenylenediammine and various aryl aldehydes with ensuing dehydrogenation. This protocol is greener, sustainable and rapid with excellent yields of the products and in addition to this, the catalase demonstrates good functional group tolerance.

Advances in the development of imaging probes and aggregation inhibitors for alpha-synuclein

Abnormal protein aggregation has been linked to many neurodegenerative diseases, including Parkinson’s disease (PD). The main pathological hallmark of PD is the formation of Lewy bodies (LBs) and Lewy neurites, both of which contain the presynaptic protein alpha-synuclein (α-syn). Under normal conditions, native α-syn exists in a soluble unfolded state but undergoes misfolding and aggregation into toxic aggregates under pathological conditions. Toxic α-syn species, especially oligomers, can cause oxidative stress, membrane penetration, synaptic and mitochondrial dysfunction, as well as other damage, leading to neuronal death and eventually neurodegeneration. Early diagnosis and treatments targeting PD pathogenesis are urgently needed. Given its critical role in PD, α-syn is an attractive target for the development of both diagnostic tools and effective therapeutics. This review summarizes the progress toward discovering imaging probes and aggregation inhibitors for α-syn. Relevant strategies and techniques in the discovery of α-syn-targeted drugs are also discussed.

Sensing Performance Investigations on Two-Photon Fluorescent Probes for Detecting β-Amyloid in Alzheimer's Disease

Alzheimer’s disease (AD) is one of the most common forms of senile disease. In recent years, the incidence of AD has been increasing significantly with the acceleration of the aging process of the global population. However, current clinical drugs can only alleviate the symptoms of AD patients without healing the disease fundamentally. Therefore, it is of great significance to develop an effective small molecule diagnostic reagent for the early diagnosis of AD. In this paper, we employ an integrated approach, including molecular docking simulation and quantum mechanics/molecular mechanics calculation, to investigate the sensing performance of a series of donor–acceptor structural probes for the marker protein of AD (β-amyloid). Results show that the probes display evident fluorescence enhancement when bound to the β-amyloid, suggesting the effect of the environment on the molecular properties. Especially, the two-photon absorption cross-section of the probes increase drastically in the β-amyloid compared to that in vacuum, which results from the larger electron delocalization and dipole moment in the fibrillary-like environment. Thus, one can propose that the studied probes are capable of application in two-photon fluorescent imaging, particularly those containing naphthalene rings as the donor or with a longer spacer group. Our calculations elucidate the experimental measurements reasonably, and further establish possible structure–property relationships that can be used to design novel biocompatible two-photon fluorescent probes for the diagnosis of Alzheimer’s.

PET Radiopharmaceuticals for Alzheimer's Disease and Parkinson's Disease Diagnosis, the Current and Future Landscape

Ironically, population aging which is considered a public health success has been accompanied by a myriad of new health challenges, which include neurodegenerative disorders (NDDs), the incidence of which increases proportionally to age. Among them, Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common, with the misfolding and the aggregation of proteins being common and causal in the pathogenesis of both diseases. AD is characterized by the presence of hyperphosphorylated τ protein (tau), which is the main component of neurofibrillary tangles (NFTs), and senile plaques the main component of which is β-amyloid peptide aggregates (Aβ). The neuropathological hallmark of PD is α-synuclein aggregates (α-syn), which are present as insoluble fibrils, the primary structural component of Lewy body (LB) and neurites (LN). An increasing number of non-invasive PET examinations have been used for AD, to monitor the pathological progress (hallmarks) of disease. Notwithstanding, still the need for the development of novel detection tools for other proteinopathies still remains. This review, although not exhaustively, looks at the timeline of the development of existing tracers used in the imaging of Aβ and important moments that led to the development of these tracers.

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.

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.