Structure–Property Relationships of Polyethylene Glycol Modified Fluorophore as Near-Infrared Aβ Imaging Probes

Design and synthesis of hemicyanine-based NIRF probe for detecting Aβ aggregates in Alzheimer's disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder, has garnered increased attention due to its substantial economic burden and the escalating global aging phenomenon. Amyloid-β deposition is a key pathogenic marker observed in the brains of Alzheimer's sufferers. Based on real-time, safe, low-cost, and commonly used, near-infrared fluorescence (NIRF) imaging technology have become an essential technique for the detection of AD in recent years. In this work, NIRF probes with hemicyanine structure were designed, synthesized and evaluated for imaging Aβ aggregates in the brain. We use the hemicyanine structure as the parent nucleus to enhance the probe's optical properties. The introduction of PEG chain is to improve the probe's brain dynamice properties, and the alkyl chain on the N atom is to enhance the fluorescence intensity of the probe after binding to the Aβ aggregates as much as possible. Among these probes, Z2, Z3, Z6, X3, X6 and T1 showed excellent optical properties and high affinity to Aβ aggregates (Kd = 24.31 ∼ 59.60 nM). In vitro brain section staining and in vivo NIRF imaging demonstrated that X6 exhibited superior discrimination between Tg mice and WT mice, and X6 has the best brain clearance rate. As a result, X6 was identified as the optimal probe. Furthermore, the docking theory calculation results aided in describing X6's binding behavior with Aβ aggregates. As a high-affinity, high-selectivity, safe and effective probe of targeting Aβ aggregates, X6 is a promising NIRF probe for in vivo detection of Aβ aggregates in the AD brain.

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.

A Flexible Bivalent Approach to Comprehensively Improve the Performances of Stilbazolium Dyes as Amyloid-β Fluorescent Probes

Exploring new ways to reconstruct the structure and function of inappropriate organic fluorophores for improving amyloid-β (Aβ) fluorescent imaging performance is desired for precise detection and early diagnosis of Alzheimer's disease (AD). With stilbazolium dyes as examples, here, we present a multipronged approach to comprehensively improved the Aβ fluorescent imaging performance through a flexible bivalent method, where a flexible carbon chain was introduced to link two monomers to form a homodimer. Our results reveal a mechanism wherein the flexible linker creates a well-defined probe with specific orientations and distinct photophysical properties. Applying this approach in combination with theoretical simulation, the homodimers exhibited a comprehensive improvement of the Aβ fluorescent imaging performance of the dye monomers, including better photostability and higher signal-to-noise (S/N) ratio, higher "off-on" near-infrared fluorescence (NIRF) response sensitivity, higher specificity and affinity to Aβ deposits, and more reasonable lipophilicity for blood-brain barrier (BBB) penetrability. The results demonstrate that flexible homodimers offer a multipronged approach to obtaining high-performance NIRF imaging reagents for the detection of Aβ deposits both in vitro and in vivo.

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.

Genetic and Molecular Evaluation of SQSTM1/p62 on the Neuropathologies of Alzheimer’s Disease

Sequestosome 1 (SQSTM1)/p62 is a multifunctional scaffolding protein and plays a major role in the cellular processes of autophagy, upregulation of which has been shown in several neurodegenerative disorders, including Alzheimer’s disease (AD). To investigate its genetic effects and relationship with AD pathologies, we analyzed the genetic associations of SQSTM1 rs4935 with the risk of AD and the levels of AD biomarkers using the AD Neuroimaging Initiative (ADNI) Database. We further analyzed the distribution pattern of p62 immunoreactivity in relation to AD pathologies in the postmortem human brain tissues from AD and non-AD controls. We found that SQSTM1 rs4935 was not associated with the risk of AD, but its T allele was significantly associated with decreased β-amyloid (1–42) (Aβ₄₂) levels in the cerebral spinal fluid (CSF) of patients with AD (β = −9.336, p = 0.022). In addition, p62 immunoreactivity in AD is increased, but it shows an inverse relationship to Aβ deposition. A small proportion of senile plaques show p62 positive neurites. Our results suggest that SQSTM1/p62 may play an important role in the progression of AD via associations with Aβ₄₂ levels in CSF and Aβ deposition in the brain of patients with AD.

Dual-Channel Imaging of Amyloid-β Plaques and Peroxynitrite To Illuminate Their Correlations in Alzheimer's Disease Using a Unimolecular Two-Photon Fluorescent Probe

Alzheimer's disease (AD) involves multiple pathological factors that mutually cooperate and closely contact to form interaction networks for jointly promoting the AD progression. Therefore, the comonitoring of different factors is particularly valuable for elucidating their level dynamics and complex interactions. However, such significant investigations remain a major challenge due to the lack of unimolecular fluorescent probes capable of simultaneous and discriminative visualization of multiple targets. To address this concern, as proof of principle, we rationally designed a unimolecular fluorescent probe to discriminate and simultaneously profile amyloid-β (Aβ) plaques and peroxynitrite (ONOO^-), which are both the pronounced AD pathological factors. Herein, a novel ONOO^- reaction trigger was installed onto an Aβ plaque binding fluorophore to generate a dual functional fluorescent probe, displaying completely separate spectral responses to Aβ plaques and ONOO^- with high selectivity and sensitivity. With this probe, for the first time, we comonitored the distribution and variation of Aβ plaques and ONOO^- through two independent fluorescence channels, demonstrating their close apposition and tight correlation during AD course in live cell and mouse models through two-photon imaging mode. Notably, Aβ aggregates induce the neuronal ONOO^- generation, which conversely facilitates Aβ aggregation. The two critical events, ONOO^- stress and Aβ aggregation, mutually amplify each other through positive feedback mechanisms and jointly promote the AD onset and progression. Furthermore, by coimaging of the level dynamics of Aβ plaques and ONOO^-, we found that the cerebral ONOO^- is a potential biomarker, which emerges earlier than Aβ plaques in transgenic mouse models. Overall, the dual-channel responsive performance renders this probe as a powerful imaging tool to decipher Aβ plaque-ONOO^- interactions, which will facilitate AD-associated molecular pathogenesis elucidation and multitarget drug discovery.

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.

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.

Nanomaterial synthesis, an enabler of amyloidosis inhibition against human diseases

Amyloid diseases are global epidemics with no cure currently available. In the past decade, the use of engineered nanomaterials as inhibitors or probes against the pathogenic aggregation of amyloid peptides and proteins has emerged as a new frontier in nanomedicine. In this Minireview, we summarize for the first time the pivotal role of chemical synthesis in enabling the development of this multidisciplinary field.

Multifunctional Freestanding Microprobes for Potential Biological Applications

Deep-level sensors for detecting the local temperatures of inner organs and tissues of an animal are rarely reported. In this paper, we present a method to fabricate multifunctional micro-probes with standard cleanroom procedures, using a piece of stainless-steel foil as the substrate. On each of the as-fabricated micro-probes, arrays of thermocouples made of Pd-Cr thin-film stripes with reliable thermal sensing functions were built, together with Pd electrode openings for detecting electrical signals. The as-fabricated sword-shaped freestanding microprobes with length up to 30 mm showed excellent mechanical strength and elastic properties when they were inserted into the brain and muscle tissues of live rats, as well as suitable electrochemical properties and, therefore, are promising for potential biological applications.

Versatile fluorescent probes for near-infrared imaging of amyloid-β species in Alzheimer's disease mouse model

The self-aggregation of amyloid-β peptides into soluble oligomers and then into insoluble fibril-associated amyloid plaques is a key event in the progression of Alzheimer's disease (AD). The imaging of Aβ aggregates in the brain is a powerful and practical approach for the diagnosis and progression monitoring of AD and the evaluation of the effectiveness of novel therapies for this devastating disease. Near-infrared (NIR) imaging is a sensitive and noninvasive method to detect and visualize Aβ aggregates in vivo because of its good penetration depth and low autofluorescence of biological substances. In this article, we comprehensively reviewed the recent progresses made in the development of molecular NIR fluorescent probes for Aβ detection and imaging in vivo with a particular emphasis on the design strategies, optical characteristics, Aβ-binding abilities and potential applications in AD mouse models.