Environment-Sensitive Near-Infrared Probe for Fluorescent Discrimination of Aβ and Tau Fibrils in AD Brain

A Small-Molecule NIR-II Probe for the Diagnosis of Hemorrhagic Diseases

Numerous hemorrhagic disorders, particularly those presenting deep hemorrhage, pose diagnostic challenges, often leading to delayed treatment and severe outcomes. Near-infrared (NIR)-II fluorescence imaging offers advantages such as deep tissue penetration, real-time visualization, and a high signal-to-background ratio, making it highly suitable for diagnosing hemorrhagic diseases. In this study, an NIR-II fluorescent probe LJ-2P carrying carboxylic and phosphoric acid groups is successfully applied for imaging hemorrhagic diseases. LJ-2P demonstrates a strong affinity for fibrinogen and fibrin clots both computationally and experimentally, thus exhibiting increased brightness upon coagulation. As compared to Indocyanine Green, LJ-2P provides a longer imaging window, higher imaging specificity, and signal-to-background ratio, as well as superior photobleaching resistance in three disease models: gastric, pulmonary, and cerebral hemorrhages. These results reveal that LJ-2P demonstrates enhanced imaging capabilities, enabling precise identification of hemorrhagic sites.

Manganese-catalyzed Efficient Synthesis of N-heterocycles and Aminoketones Using Glycerol as a C3 Synthon

Glycerol is one of the important biomass-derived feedstocks and the high-value utilizations of glycerol have attracted much attentions in recent years. Herein, we report a manganese catalyzed dehydrogenative coupling of glycerol with amines for the synthesis of substituted 2-methylquinoxalines, 2-ethylbenzimidazoles, and α-aminoketones without any external oxidant. In these reactions, NHC-based pincer manganese complex featuring a pyridine backbone displayed high catalytic activity and selectivity, in which hydrogen and water were produced as the only by-products using glycerol as a C3 synthon.

Spectral Fluorescence Pathology of Protein Misfolding Disorders

Protein misfolding has been extensively studied in the context of neurodegenerative disorders and systemic amyloidoses. Due to misfolding and aggregation of proteins being highly heterogeneous and generating a variety of structures, a growing body of evidence illustrates numerous ways how the aggregates contribute to progression of diseases such as Alzheimer's disease, Parkinson's disease, and prion disorders. Different misfolded species of the same protein, commonly referred to as strains, appear to play a significant role in shaping the disease clinical phenotype and clinical progression. The distinct toxicity profiles of various misfolded proteins underscore their importance. Current diagnostics struggle to differentiate among these strains early in the disease course. This review explores the potential of spectral fluorescence approaches to illuminate the complexities of protein misfolding pathology and discusses the applications of advanced spectral methods in the detection and characterization of protein misfolding disorders. By examining spectrally variable probes, current data analysis approaches, and important considerations for the use of these techniques, this review aims to provide an overview of the progress made in this field and highlights directions for future research.

Dual-environment-sensitive probe to detect protein aggregation in stressed laryngeal carcinoma cells and tissues

The interplay between protein folding and biological activity is crucial, with the integrity of the proteome being paramount to ensuring effective biological function execution. In this study, we report a dual-environment-sensitive probe A1, capable of selectively binding to protein aggregates and dynamically monitoring their formation and degradation. Through in vitro, cellular, and tissue assays, A1 demonstrated specificity in distinguishing aggregated from folded protein states, selectively partitioning into aggregated proteins. Thermal shift assays revealed A1 could monitor the process of protein aggregation upon binding to misfolded proteins and preceding to insoluble aggregate formation. In cellular models, A1 detected stress-induced proteome aggregation in TU212 cells (laryngeal carcinoma cells), revealing a less polar microenvironment within the aggregated proteome. Similarly, tissue samples showed more severe proteome aggregation in cancerous tissues compared to paracancerous tissues. Overall, A1 represents a versatile tool for probing protein aggregation with significant implications for both fundamental research and clinical diagnostics.

A novel fluorescent probe for the detection of peroxynitrite and its application in mice epileptic brain model

Herein, a fluorescent probe, GYP, was developed for the detection of ONOO- in KA-induced epileptic brains. In solution, as a ratiometric probe, GYP indicated practical properties including steadiness under wide pH range (3.0-12.0), rapid response (within 20 s), stability over 48 h, high sensitivity (LOD = 0.27 μM) and high selectivity. In living PC12 cells, in spite of the low toxicity, GYP could achieve the time-dependent and dose-dependent imaging of ONOO-, while the generation and elimination were checked by introduction of SIN-1 and NAC, respectively. Further, GYP could cross Blood-Brain Barrier (BBB) rapidly and steadily during the imaging in KA-induced mice epileptic brain model. Thus, this work raised a practical implement for the detection of ONOO- in brain region, which might be helpful for further understanding of the epilepsy mechanism in future.

In vivo three-dimensional brain imaging with chemiluminescence probes in Alzheimer's disease models

Optical three-dimensional (3D) molecular imaging is highly desirable for providing precise distribution of the target-of-interest in disease models. However, such 3D imaging is still far from wide applications in biomedical research; 3D brain optical molecular imaging, in particular, has rarely been reported. In this report, we designed chemiluminescence probes with high quantum yields, relatively long emission wavelengths, and high signal-to-noise ratios to fulfill the requirements for 3D brain imaging in vivo. With assistance from density-function theory (DFT) computation, we designed ADLumin-Xs by locking up the rotation of the double bond via fusing the furan ring to the phenyl ring. Our results showed that ADLumin-5 had a high quantum yield of chemiluminescence and could bind to amyloid beta (Aβ). Remarkably, ADLumin-5's radiance intensity in brain areas could reach 4 × 107 photon/s/cm2/sr, which is probably 100-fold higher than most chemiluminescence probes for in vivo imaging. Because of its strong emission, we demonstrated that ADLumin-5 could be used for in vivo 3D brain imaging in transgenic mouse models of Alzheimer's 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.

Rational design synthesis and evaluation of a novel near-infrared fluorescent probe for selective imaging of amyloid-β aggregates in Alzheimer's disease

Alzheimer's disease (AD) is a degenerative neurological disorder that remains incurable to date, seriously affecting the quality of life and health of those affected. One of the key neuropathological hallmarks of AD is the formation of amyloid-β (Aβ) plaques. Near-infrared (NIR) probes that possess a large Stokes shift show great potential for imaging of Aβ plaques in vivo and in vitro. Herein, we proposed a rational strategy for design and synthesis of a series of NIR fluorescent probes that incorporate a tricarbonitrile group as a strong electron-withdrawing group (EWG) to enable NIR emission and large Stokes shift for optimal imaging of Aβ plaques. The probe TCM-UM exhibited remarkable in vitro performance, including strong NIR emission (λem = 670 nm), large Stokes shift (120-245 nm), and its affinity for Aβ42 aggregates (Kd = 43.78 ± 4.09 nM) was superior to the commercially available probe Thioflavin T (ThT, Kd = 896.04 ± 33.43 nM). Further, TCM-UM was selected for imaging Aβ plaques in brain tissue slices and APP/PS1 transgenic (AD) mice, the results indicated that TCM-UM had an excellent ability to penetrate the blood-brain barrier (BBB) compared with ThT, and it could effectively distinguish wild-type (Wt) mice and APP/PS1 transgenic (AD) mice.

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.

In Vivo Three-dimensional Brain Imaging with Chemiluminescence Probes in Alzheimer's Disease Models

Optical three-dimensional (3D) molecular imaging is highly desirable for providing precise distribution of the target-of-interest in disease models. However, such 3D imaging is still far from wide applications in biomedical research; 3D brain optical molecular imaging, in particular, has rarely been reported. In this report, we designed chemiluminescence probes with high quantum yields (QY), relatively long emission wavelengths, and high signal-to-noise ratios (SNRs) to fulfill the requirements for 3D brain imaging in vivo. With assistance from density-function theory (DFT) computation, we designed ADLumin-Xs by locking up the rotation of the double-bond via fusing the furan ring to the phenyl ring. Our results showed that ADLumin-5 had a high quantum yield of chemiluminescence and could bind to amyloid beta (Aβ). Remarkably, ADLumin-5's radiance intensity in brain areas could reach 4×107 photon/s/cm2/sr, which is probably 100-fold higher than most chemiluminescence probes for in vivo imaging. Because of its strong emission, we demonstrated that ADLumin-5 could be used for in vivo 3D brain imaging in transgenic mouse models of Alzheimer's disease (AD).

An AIE-Active NIR Fluorescent Probe with Good Water Solubility for the Detection of Aβ1-42 Aggregates in Alzheimer's Disease

Alzheimer's disease (AD), an amyloid-related disease, seriously endangers the health of elderly individuals. According to current research, its main pathogenic factor is the amyloid protein, which is a kind of fibrillar aggregate formed by noncovalent self-assembly of proteins. Based on the characteristics of aggregation-induced emission (AIE), a bislactosyl-decorated tetraphenylethylene (TPE) molecule TMNL (TPE + malononitrile + lactose), bearing two malononitrile substituents, was designed and synthesized in this work. The amphiphilic TMNL could self-assemble into fluorescent organic nanoparticles (FONs) with near-infrared (NIR) fluorescence emission in physiological PBS (phosphate buffered saline), achieving excellent fluorescent enhancement (47-fold) upon its combination with Aβ_1-42 fibrils. TMNL was successfully applied to image Aβ_1-42 plaques in the brain tissue of AD transgenic mice, and due to the AIE properties of TMNL, no additional rinsing process was necessary. It is believed that the probe reported in this work should be useful for the sensitive detection and accurate localization mapping of Aβ_1-42 aggregates related to Alzheimer's disease.

Multicomponent Petasis reaction for the identification of pyrazine based multi-target directed anti-Alzheimer's agents: In-silico design, synthesis, and characterization

Multi-target directed ligands (MTDLs) have recently attracted significant interest due to their exceptional effectiveness against multi-factorial Alzheimer's disease. The present work described the development of pyrazine-based MTDLs using multicomponent Petasis reaction for the dual inhibition of tau-aggregation and human acetylcholinesterase (hAChE). The molecular structure of synthesized ligands was validated by ¹H & ¹³C NMR and mass spectrometry. The screened compounds were shown to have a strong inhibitory effect at 10 μM concentration against tau-oligomerization and hAChE, but only moderate inhibitory activity against Aβ₄₂. Among all the compounds, the half-maximal inhibitory concentration (IC₅₀) for 21 and 24 against hAChE were 0.71 μM and 1.09 μM, respectively, while they displayed half-maximal effective concentrations (EC₅₀) values of 2.21 μM and 2.71 μM for cellular tau-oligomerization, respectively. Additionally, an MTT experiment using tau-expressing SH-SY5Y neuroblastoma cells revealed that 21 was more neuroprotective than the FDA-approved medication donepezil. Furthermore, an MD simulation study was performed to investigate the dynamics and stability of AChE-21 and AChE-24 complexes in an aqueous environment. The MM-PBSA calculations were performed to evaluate the binding of 21 and 24 with AChE, and the relative binding energy was calculated as -870.578 and -875.697 kJ mol^-1, respectively. As a result, the study offered insight into the design of new MTDLs and highlighted 21 as a potential roadblock to the development of anti-AD medications.

Design of a Coumarin-Based Fluorescent Probe for Efficient In Vivo Imaging of Amyloid-β Plaques

Amyloid-β (Aβ) is the core constituent protein of senile plaques, which is one of the key pathological hallmarks of Alzheimer's disease (AD). Here we describe the design, synthesis, and evaluation of coumarin-derived small molecule fluorophores for Aβ imaging. By embedding the aromatic coumarin framework into π bridge of a push-pull chromophore, a novel fluorescence probe XCYC-3 applicable to efficient Aβ recognition was discovered. XCYC-3 displays higher fluorescent enhancement for aggregated Aβ than monomeric Aβ, and possesses good blood-brain barrier permeability. In vitro staining and in vivo imaging studies demonstrated that XCYC-3 could efficiently recognize Aβ plaques in the brain of AD transgenic mice. These results suggest that XCYC-3 is a promising fluorescence imaging agent for Aβ, which might provide important clues for the future development of potent NIR fluorescent probes for Aβ diagnosis.

Multipronged diagnostic and therapeutic strategies for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and a major contributor to dementia cases worldwide. AD is clinically characterized by learning, memory, and cognitive deficits. The accumulation of extracellular amyloid β (Aβ) plaques and neurofibrillary tangles (NFTs) of tau are the pathological hallmarks of AD and are explored as targets for clinical diagnosis and therapy. AD pathology is poorly understood and there are no fully approved diagnosis and treatments. Notwithstanding the gap, decades of research in understanding disease mechanisms have revealed the multifactorial nature of AD. As a result, multipronged and holistic approaches are pertinent to targeting multiple biomarkers and targets for developing effective diagnosis and therapeutics. In this perspective, recent developments in Aβ and tau targeted diagnostic and therapeutic tools are discussed. Novel indirect, combination, and circulating biomarkers as potential diagnostic targets are highlighted. We underline the importance of multiplexing and multimodal detection of multiple biomarkers to generate biomarker fingerprints as a reliable diagnostic strategy. The classical therapeutics targeting Aβ and tau aggregation pathways are described with bottlenecks in the strategy. Drug discovery efforts targeting multifaceted toxicity involving protein aggregation, metal toxicity, oxidative stress, mitochondrial damage, and neuroinflammation are highlighted. Recent efforts focused on multipronged strategies to rationally design multifunctional modulators targeting multiple pathological factors are presented as future drug development strategies to discover potential therapeutics for AD.

Near-Infrared Fluorescent Probes as Imaging and Theranostic Modalities for Amyloid-Beta and Tau Aggregates in Alzheimer's Disease

A person suspected of having Alzheimer's disease (AD) is clinically diagnosed for the presence of principal biomarkers, especially misfolded amyloid-beta (Aβ) and tau proteins in the brain regions. Existing radiotracer diagnostic tools, such as PET imaging, are expensive and have limited availability for primary patient screening and pre-clinical animal studies. To change the status quo, small-molecular near-infrared (NIR) probes have been rapidly developed, which may serve as an inexpensive, handy imaging tool to comprehend the dynamics of pathogenic progression in AD and assess therapeutic efficacy in vivo. This Perspective summarizes the biochemistry of Aβ and tau proteins and then focuses on structurally diverse NIR probes with coverages of their spectroscopic properties, binding affinity toward Aβ and tau species, and theranostic effectiveness. With the summarized information and perspective discussions, we hope that this paper may serve as a guiding tool for designing novel in vivo imaging fluoroprobes with theranostic capabilities in the future.

Non-invasive imaging of tau-targeted probe uptake by whole brain multi-spectral optoacoustic tomography

PURPOSE

Abnormal tau accumulation within the brain plays an important role in tauopathies such as Alzheimer's disease and frontotemporal dementia. High-resolution imaging of tau deposits at the whole-brain scale in animal disease models is highly desired.

METHODS

We approached this challenge by non-invasively imaging the brains of P301L mice of 4-repeat tau with concurrent volumetric multi-spectral optoacoustic tomography (vMSOT) at ~ 115 μm spatial resolution using the tau-targeted pyridinyl-butadienyl-benzothiazole derivative PBB5 (i.v.). In vitro probe characterization, concurrent vMSOT and epi-fluorescence imaging of in vivo PBB5 targeting (i.v.) was performed in P301L and wild-type mice, followed by ex vivo validation using AT-8 antibody for phosphorylated tau.

RESULTS

PBB5 showed specific binding to recombinant K18 tau fibrils by fluorescence assay, to post-mortem Alzheimer's disease brain tissue homogenate by competitive binding against [11C]PBB3 and to tau deposits (AT-8 positive) in post-mortem corticobasal degeneration and progressive supranuclear palsy brains. Dose-dependent optoacoustic and fluorescence signal intensities were observed in the mouse brains following i.v. administration of different concentrations of PBB5. In vivo vMSOT brain imaging of P301L mice showed higher retention of PBB5 in the tau-laden cortex and hippocampus compared to wild-type mice, as confirmed by ex vivo vMSOT, epi-fluorescence, multiphoton microscopy, and immunofluorescence staining.

CONCLUSIONS

We demonstrated non-invasive whole-brain imaging of tau in P301L mice with vMSOT system using PBB5 at a previously unachieved ~ 115 μm spatial resolution. This platform provides a new tool to study tau spreading and clearance in a tauopathy mouse model, foreseeable in monitoring tau targeting putative therapeutics.

Rational Design of Quinoxalinone-Based Red-Emitting Probes for High-Affinity and Long-Term Visualizing Amyloid-β In Vivo

Alzheimer's disease (AD) is a progressive neurodegenerative disease with insidious onset, and the deposition of amyloid-β (Aβ) is believed to be one of the main cause. Fluorescence imaging is a promising technique for this task, but the Aβ gold standard probe ThT developed based on this still has shortcomings. The development of a new fluorescent probe to detect Aβ plaques is thought to be essential. Herein, a series of red to near-infrared emitting fluorescent probes QNO-ADs with newly quinoxalinone skeleton are designed to detect Aβ plaques. They all demonstrate excellent optical properties and high binding affinity (∼K_d = 20 nM) to Aβ aggregates. As the most outstanding candidate, QNO-AD-3 shows significant signal-to-noise (S/N) ratio at the level of in vitro binding studies, and the brilliant fluorescence staining results in favor of grasping the approximate distribution of Aβ plaques in the brain slice. In vivo Aβ plaques imaging suggests that QNO-AD-3 can cross the BBB and have a long retention time in the brain with low biological toxicity. In addition, the results of docking theoretical calculation also provide some references for the design of Aβ probe. Overall, given the high affinity of QNO-AD-3 and the ability to monitor Aβ plaques for a long time that is not common now, we believe QNO-AD-3 will be an effective tool for an Aβ-related matrix and AD disease research 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.

Regulation of Fluorescence Solvatochromism To Resolve Cellular Polarity upon Protein Aggregation

Common solvatochromic fluorophores exhibit a bathochromic fluorescence emission wavelength shift accompanied by intensity attenuation due to the presence of nonradiative decay pathways at the excited state. Such intrinsic but inevitable fluorescence quenching of solvatochromism impedes its applications to faithfully quantify local polarity, especially in a polar environment. Herein, we report a new donor-π-acceptor (D-π-A) type solvatochromic fluorophore scaffold containing a perfluorophenyl group that exhibits both a solvatochromic emission wavelength shift and a controllable emission intensity upon polarity fluctuation. The regulation of fluorescence solvatochromism and colors was achieved by tuning the aryl donors. We exploited such desired solvatochromism of these probes to monitor protein misfolding and aggregation via wavelength shift. Finally, the polarity of pathogenic aggregated proteins was quantified by HaloTag bioorthogonal labeling technology in live cells. While much effort has been devoted to resolving the morphology of pathogenic aggregated proteins, this work provides quantitative hints regarding the chemical information at this disease-related protein interphase.

Quantitative detection of grey and white matter amyloid pathology using a combination of K114 and CRANAD-3 fluorescence

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disease that exacts a huge toll on the patient, the healthcare system and society in general. Abundance and morphology of protein aggregates such as amyloid β plaques and tau tangles, along with cortical atrophy and gliosis are used as measures to assess the changes in the brain induced by the disease. Not all of these parameters have a direct correlation with cognitive decline. Studies have shown that only particular protein conformers can be the main drivers of disease progression, and conventional approaches are unable to distinguish different conformations of disease-relevant proteins.

METHODS AND RESULTS

Using the fluorescent amyloid probes K114 and CRANAD-3 and spectral confocal microscopy, we examined formalin-fixed paraffin-embedded brain samples from different control and AD cases. Based on the emission spectra of the probes used in this study, we found that certain spectral signatures can be correlated with different aggregates formed by different proteins. The combination of spectral imaging and advanced image analysis tools allowed us to detect variability of protein deposits across the samples.

CONCLUSION

Our proposed method offers a quicker and easier neuropathological assessment of tissue samples, as well as introducing an additional parameter by which protein aggregates can be discriminated.

Evaluation of N, O-Benzamide difluoroboron derivatives as near-infrared fluorescent probes to detect β-amyloid and tau tangles

β-Amyloid (Aβ) plaques and Tau tangles are cognitive impairment markers vital for diagnosing and preventing Alzheimer's disease (AD). To systematically explore the relationship between the number or position of nitrogen atoms and their optical properties and biological properties, five series of new N, O-coordinated organo-difluoroboron probes were introduced as binding scaffolds for Aβ plaques and Tau tangles. These probes exhibited suitable optical properties for near-infrared (NIR) imaging. Probe 4PmNO-2 (4-((1E,3E)-4-(1,1-difluoro-1H-1λ⁴,9λ⁴-pyrimido[1,6-c][1,3,5,2]oxadiazaborinin-3-yl)buta-1,3-dien-1-yl)-N,N-dimethylaniline) displayed the excellent emission maximum (716 nm in PBS), a high quantum yield (61.4% in CH₂Cl₂), and a high affinity for synthetic Aβ_1-42 (K_d = 23.64 ± 1.08 nM) and Tau (K18) aggregates (K_d = 26.38 ± 1.29 nM), as well as for native Aβ plaques and NFTs in the brain tissue from AD patients. 4PmNO-2, with significantly enhanced fluorescence (Aβ_1-42, 136 fold; Tau (K18), 96 fold) and the highest initial brain uptake (11.57% ID/g at 2 min) in normal ICR mice, was evaluated further. In vivo NIR fluorescent imaging studies in living Aβ and Tau transgenic mice revealed that it could differentiate healthy and diseased animals. Further ex vivo fluorescent staining studies showed that 4PmNO-2 specifically bound to Aβ plaques and Tau tangles in transgenic mice. In summary, the probe 4PmNO-2 may be a useful near-infrared fluorescence (NIRF) probe for AD biomarkers.

Multimodal Contrast Agents for Optoacoustic Brain Imaging in Small Animals

Optoacoustic (photoacoustic) imaging has demonstrated versatile applications in biomedical research, visualizing the disease pathophysiology and monitoring the treatment effect in an animal model, as well as toward applications in the clinical setting. Given the complex disease mechanism, multimodal imaging provides important etiological insights with different molecular, structural, and functional readouts in vivo. Various multimodal optoacoustic molecular imaging approaches have been applied in preclinical brain imaging studies, including optoacoustic/fluorescence imaging, optoacoustic imaging/magnetic resonance imaging (MRI), optoacoustic imaging/MRI/Raman, optoacoustic imaging/positron emission tomography, and optoacoustic/computed tomography. There is a rapid development in molecular imaging contrast agents employing a multimodal imaging strategy for pathological targets involved in brain diseases. Many chemical dyes for optoacoustic imaging have fluorescence properties and have been applied in hybrid optoacoustic/fluorescence imaging. Nanoparticles are widely used as hybrid contrast agents for their capability to incorporate different imaging components, tunable spectrum, and photostability. In this review, we summarize contrast agents including chemical dyes and nanoparticles applied in multimodal optoacoustic brain imaging integrated with other modalities in small animals, and provide outlook for further research.

Benzothiazolium Derivative-Capped Silica Nanocomposites for β-Amyloid Imaging In Vivo

Alzheimer’s disease (AD) is a neurodegenerative disease, and β-amyloid (Aβ) is believed to be a causative factor in AD pathology. The abnormal deposition of Aβ is believed to be responsible for progression of AD. In order to facilitate the imaging of Aβ in vivo, suitable probe molecules with a near-infrared emission wavelength that can penetrate the blood–brain barrier (BBB) were utilized. The commercial fluorescent probe thioflavin-T (ThT) is used to image Aβ; however, because of its short emission wavelength and poor BBB penetration, ThT can only be used in vitro. With this research, based on ThT, we design three fluorescent probes (SZIs) having a longer emission wavelength in order to image Aβ aggregates. SZIs with different numbers of double bonds respond to Aβ aggregates. The SZIs have a structure similar to ThT, and as such, the SZIs are also unable to penetrate the BBB. To deal with the problem, we develop nanocomposites (MSN-Lf@SZIs) to deliver SZIs into the brain of AD mouse and image Aβ successfully. These new nanocomposites are able to deliver the dyes into the brain and facilitate Aβ imaging in vivo.

Complex Photophysical Properties of K114 Make for a Versatile Fluorescent Probe for Amyloid Detection

Protein aggregation is a hallmark of Alzheimer's disease (AD) and many other neurodegenerative disorders. Small organic fluorophores such as Congo Red preferentially bind to cross-β-sheet-rich deposits and have been used to label amyloid plaques and tau tangles in histological samples. However, distinguishing between different conformations of protein aggregates is not trivial. Using silkworm and spider silks (prototypical amyloids) and transgenic AD mouse (5XFAD) and human AD brain samples, we report how spectral confocal microscopy allowed for improved detection and differentiation of protein aggregates based on the unexpected photophysical behavior of the amyloid-specific dye K114. The pH and excitation power had pronounced effects on the emission spectrum and intensity of amyloid-bound K114 fluorescence. When bound to β-sheet-rich assemblies, the emission spectrum of K114 was governed by the local pH of the binding pockets much more than by the pH of the mounting medium, likely due to ionization of titratable phenols. Unexpectedly, exposure to high excitation power caused a permanent increase in fluorescence intensity and a spectral blue-shift. These light-induced fluorescence changes were dependent in a complex manner on laser power, exposure time, pH, and amyloid type examined. The above-mentioned phenomena were observed in silk fibers and Alzheimer brain sections from mouse and human, indicating that this may be a general characteristic of K114 when bound to tightly aggregated macromolecules. Potential mechanisms are discussed, likely involving photoinduced electron transfer. Our findings illustrate how the complex photophysical behavior of amyloid-bound K114 can be exploited for improved detection and differentiation of protein aggregates.

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.

Red-to-blue paper-based colorimetric sensor integrated with smartphone for point-of-use analysis of cerebral AChE upon Cd2+ exposure

Herein, combined with a pervasive smartphone installed with a color recognition app, dual-responsive CDs@Eu/GMP ICPs were designed as a red-to-blue paper-based colorimetric sensor for the point-of-use analysis of cerebral acetylcholinesterase (AChE) upon Cd2+ exposure. Blue-emitting CDs with multi-functional groups as guests were encapsulated into the network of Eu/GMP ICPs to obtain CDs@Eu/GMP ICPs with the sensitized red fluorescence of Eu3+. With the presence of thiocholine (TCh), derived from acetylthiocholine (ATCh) hydrolyzed by AChE, the coordination environment of the CDs@Eu/GMP ICPs was interrupted, leading to the collapse of the CDs@Eu/GMP ICP network and the corresponding release of guest CDs into the surrounding environment. Consequently, the sensitized red fluorescence of Eu3+ decreased and the blue fluorescence of the CDs increased. This obvious red-to-blue fluorescent color changes of CDs@Eu/GMP ICPs on test paper could then be integrated with the smartphone for point-of-use analysis of cerebral AChE upon Cd2+ exposure, which not only offers a new analytical platform for a better understanding of the environmental risk of Alzheimer's Dementia (AD), but also holds great potential in the early diagnosis of AD even at the asymptomatic stage with the decrease in CSF AChE as an early biomarker.

Polarity-active NIR probes with strong two-photon absorption and ultrahigh binding affinity of insulin amyloid fibrils

Amyloid fibrils are associated with many neurodegenerative diseases. In situ and in vivo visualization of amyloid fibrils is important for medical diagnostics and requires fluorescent probes with both excitation and emission wavelengths in the far-red and NIR region, and simultaneously with high binding-affinity to amyloid fibrils and the ability to cross the blood-brain barrier, which, however, remain a challenge. Here, we rationally design and synthesize an excellent polarity-sensitive two-photon excited NIR fluorophore (TZPI) based on a donor (D)-acceptor (A)-ion compound. The electron-rich carbazole group and the ionic pyridinium bromide group, linked by an electron-poor π-conjugated benzothiadiazole group, ensure strong near infrared (NIR) emission. Furthermore, the lipophilic carbazole together with the benzothiadiazole group facilitates docking of the probe in the hydrophobic domains of amyloid aggregates with the dissociation constant K d = 20 nM and 13.5-fold higher binding affinity to insulin fibrils than the commercial probe ThT. On association with the amyloid fibrils, the tiny decrease in polarity leads to a large increase in its NIR emission intensity with an on-off ratio > 10; meanwhile, the TZPI probe exhibits a quantum yield of up to 30% and two-photon absorption cross-section values of up to 467.6 GM at 890 nm. Moreover, the application of TZPI in two-photon imaging is investigated. The ultrahigh binding affinity, the strong NIR emission, the good two-photon absorption properties, the high photo-stability, the appropriate molecular mass of 569 Da and the lipophilicity with log P = 1.66 ± 0.1 to cross the BBB make TZPI promising as an ideal candidate for detecting amyloid plaques in vivo.

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.

Turn-on chemiluminescence probes and dual-amplification of signal for detection of amyloid beta species in vivo

Turn-on fluorescence imaging is routinely studied; however, turn-on chemiluminescence has been rarely explored for in vivo imaging. Herein, we report the design and validation of chemiluminescence probe ADLumin-1 as a turn-on probe for amyloid beta (Aβ) species. Two-photon imaging indicates that ADLumin-1 can efficiently cross the blood-brain barrier and provides excellent contrast for Aβ plaques and cerebral amyloid angiopathy. In vivo brain imaging shows that the chemiluminescence signal of ADLumin-1 from 5-month-old transgenic 5xFAD mice is 1.80-fold higher than that from the age-matched wild-type mice. Moreover, we demonstrate that it is feasible to further dually-amplify signal via chemiluminescence resonance energy transfer (DAS-CRET) using two non-conjugated smart probes (ADLumin-1 and CRANAD-3) in solutions, brain homogenates, and in vivo whole brain imaging. Our results show that DAS-CRET can provide a 2.25-fold margin between 5-month-old 5xFAD mice and wild type mice. We believe that our strategy could be extended to other aggregating-prone proteins.

N,O-Benzamide difluoroboron complexes as near-infrared probes for the detection of β-amyloid and tau fibrils

In this study, a series of organo difluoroboron probes with a BF2 benzamide moiety was designed, prepared and evaluated. Among them, 2c displayed the best optical and biological properties, and may be used as a useful near-infrared fluorescent probe for the detection of Aβ plaques and neurofibrillary tangles in AD.

Visualization of endogenous β-galactosidase activity in living cells and zebrafish with a turn-on near-infrared fluorescent probe

β-Galactosidase (β-gal) is an important biomarker for primary ovarian cancers. Developing noninvasive bioimaging probes for studying the activity of β-gal is highly desirable for cancer diagnosis. Herein, a turn-on near-infrared (NIR) fluorescent probe， 2-((6-(((2S, 3R, 4S, 5R, 6R)-3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran -2-yl)oxy)-2,3-dihydro-1H-xanthen-4-yl)methylene)malononitrile named DXM-βgal, was rationally designed based on enzymatic reaction for the detection of β-gal activity both in vitro and in vivo. Upon incubating with β-gal, DXM-βgal displayed a significant fluorescence enhancement at 640 nm, accompanying by a color change of solution color from red to purple. DXM-βgal exhibited high selectivity and sensitively to β-gal with low limit of detection (2.92 × 10^-4 U mL^-1). Besides, based on its advantages of long-wavelength emission and excellent biocompatibility, DXM-βgal was successfully applied to imaging β-gal in living cells and zebrafish. Given these prominent properties, we believe that DXM-βgal will be a potential tool for investigating β-gal activity in biomedical research.

Phase Separation and Cytotoxicity of Tau are Modulated by Protein Disulfide Isomerase and S-nitrosylation of this Molecular Chaperone

Cells have evolved molecular chaperones that modulate phase separation and misfolding of amyloidogenic proteins to prevent neurodegenerative diseases. Protein disulfide isomerase (PDI), mainly located at the endoplasmic reticulum and also present in the cytosol, acts as both an enzyme and a molecular chaperone. PDI is observed to be S-nitrosylated in the brain of Alzheimer's disease patients, but the mechanism has remained elusive. We herein report that both wild-type PDI and its quadruple cysteine mutant only having chaperone activity, significantly inhibit pathological phosphorylation and abnormal aggregation of Tau in cells, and significantly decrease the mitochondrial damage and Tau cytotoxicity resulting from Tau aberrant aggregation, highlighting the chaperone property of PDI. More importantly, we show that wild-type PDI is selectively recruited by liquid droplets of Tau, which significantly inhibits phase separation and stress granule formation of Tau, whereas S-nitrosylation of PDI abrogates the recruitment and inhibition. These findings demonstrate how phase separation of Tau is physiologically regulated by PDI and how S-nitrosylation of PDI, a perturbation in this regulation, leads to disease.

Design principles and fundamental understanding of biosensors for amyloid-β detection

Aβ as biomarker in Alzheimer’s disease (AD) drives the significant research efforts for developing different biosensors with different sensing strategies, materials, and mechanisms for Aβ detection.