Monitoring the Formation of Amyloid Oligomers Using Photoluminescence Anisotropy

An optical ratiometric approach using enantiopure luminescent metal complexes indicates changes in the average quadruplex DNA content as primary cells undergo multiple divisions

The three stereoisomers of a previously reported dinuclear ruthenium(II) complex have been quantitatively separated using cation-exchange chromatography and the individual crystal structures of the racemic pair are reported. Cell-based studies on the three stereoisomers disclosed differences in the rate of uptake of the two chiral forms of the rac diastereoisomer with the ΛΛ-enantiomer being taken up noticeably more rapidly than the ΔΔ-form. Cell viability studies reveal that the three cations show identical cytotoxicity over 24 hours, but over more extended exposure periods, the meso-ΔΛ stereoisomer becomes slightly less active. More significantly, microscopy studies revealed that although both isomers display a near infra-red "light-switch" effect associated with binding to duplex DNA on binding to chromatin in live MCF7 and L5178-R cells, only the ΛΛ enantiomer displays a distinctive, blue-shifted component associated with binding to quadruplex DNA. An analysis of the ratio of "quadruplex emission" compared to "duplex emission" for the ΛΛ-enantiomer indicated that there was a decrease in the average quadruplex DNA content within live primary cells as they undergo multiple cell divisions.

Acid-Resistant and Viscosity-Sensitive Proteome Aggregation Sensor To Visualize Cellular Aggrephagy in Live Cells and Clinical Samples

Aggrephagy in cells is defined as the degradation of intracellular aggregated proteins via the macroautophagy process. This process sequesters protein aggregates into autolysosomes, which bear characteristic viscous and acidic microenvironments. Limited protein aggregation sensors are environmentally compatible with the cellular aggrephagy process. Here, we report an acid-resistant and viscosity-sensitive proteome aggregation sensor to detect cellular aggrephagy in stressed cells and clinical samples. This sensor fluoresces upon selectively and ubiquitously binding to different aggregated proteins. Importantly, unlike other reported protein aggregation sensors, our probe offers unique acid-resistant fluorescence inside aggregated proteins, enabling its application in the acidic autolysosome microenvironment. In live cells under various stressed conditions, the optimal probe (A6) successfully detects aggregated proteome in autolysosomes, as validated by colocalization with a lysosomal tracker. Additionally, we demonstrate that the sensor can detect proteome aggregation in heat-stressed clinical tissue samples biopsied from cancer patients undergoing thermal perfusion treatment. Together, the reported acid-resistant and viscosity-sensitive protein aggregation sensor facilitates the detection of cellular aggrephagy by chemically matching its microenvironmental characteristics.

Blood-brain barrier permeable carbon nano-assemblies for amyloid-β clearance and neurotoxic attenuation

Abnormal amyloid β-protein (Aβ42) fibrillation is a key event in Alzheimer's disease (AD), and photodynamic therapy (PDT) possesses great potential in modulating Aβ42 self-assembly. However, the poor blood-brain barrier (BBB) penetration, low biocompatibility, and limited tissue penetration depth of existing photosensitizers limit the progress of photo-oxidation strategies. In this paper, novel indocyanine green-modified graphene quantum dot nano-assemblies (NBGQDs-ICGs) were synthesized based on a molecular assembly strategy of electrostatic interactions for PDT inhibition of Aβ42 self-assembly process and decomposition of preformed fibrils under near-infrared light. Combining the small-size structure of graphene quantum dots and the near-infrared light-responsive properties of ICGs, the NBGQDs-ICGs could achieve BBB penetration under 808 nm irradiation. More importantly, the neuroprotective mechanism of NBGQDs-ICG was studied for the first time by AFM, which effectively weakened the adhesion of Aβ42 aggregates to the cell surface by blocking the interaction between Aβ42 and the cell membrane, and restored the mechanical stability and adhesion of the neuron membrane. Meanwhile, NBGQDs-ICG promoted phagocytosis of Aβ42 by microglia. In addition, the good biocompatibility and stability ensured the biosafety of NBGQDs-ICG in future clinical applications. We anticipate that such multifunctional nanocomponents may provide promising avenues for the development of novel AD inhibitors.

Transition Metal Complexes as Therapeutics: A New Frontier in Combatting Neurodegenerative Disorders through Protein Aggregation Modulation

Neurodegenerative disorders (NDDs) are a class of debilitating diseases that progressively impair the protein structure and result in neurological dysfunction in the nervous system. Among these disorders, Alzheimer's disease (AD), prion diseases such as Creutzfeldt-Jakob disease (CJD), and Parkinson's disease (PD) are caused by protein misfolding and aggregation at the cellular level. In recent years, transition metal complexes have gained significant attention for their potential applications in diagnosing, imaging, and curing these NDDs. These complexes have intriguing possibilities as therapeutics due to their diverse ligand systems and chemical properties and can interact with biological systems with minimal detrimental effects. This review focuses on the recent progress in transition metal therapeutics as a new era of hope in the battle against AD, CJD, and PD by modulating protein aggregation in vitro and in vivo. It may shed revolutionary insights into unlocking new opportunities for researchers to develop metal-based drugs to combat NDDs.

Detection and disaggregation of amyloid fibrils by luminescent amphiphilic platinum(II) complexes

Cyclometallated Pt(II) complexes possessing hydrophobic 2-phenylpyridine (ppy) ligands and hydrophilic acetonylacetone (acac) ligands have been investigated for their ability to detect amyloid fibrils via luminescence response. Using hen egg-white lysozyme (HEWL) as a model amyloid protein, Pt(II) complexes featuring benzanilide-substituted ppy ligands and ethylene glycol-functionalized acac ligands demonstrated enhanced luminescence in the presence of HEWL fibrils, whereas Pt(II) complexes lacking complementary hydrophobic/hydrophilic ligand sets displayed little to no emission enhancement. An amphiphilic Pt(II) complex incorporating a bis(ethylene glycol)-derivatized acac ligand was additionally found to trigger restructuring of HEWL fibrils into smaller spherical aggregates. Amphiphilic Pt(II) complexes were generally non-toxic to SH-SY5Y neuroblastoma cells, and several complexes also exhibited enhanced luminescence in the presence of Aβ42 fibrils associated with Alzheimer's disease. This study demonstrates that easily prepared and robust (ppy)PtII(acac) complexes show promising reactivity toward amyloid fibrils and represent attractive molecular scaffolds for design of small-molecule probes targeting amyloid assemblies.

Exploring the significance of potassium homeostasis in copper ion binding to human αB-Crystallin

αB-Crystallin (αB-Cry) is a small heat shock protein known for its protective role, with an adaptable structure that responds to environmental changes through oligomeric dynamics. Cu(II) ions are crucial for cellular processes but excessive amounts are linked to diseases like cataracts and neurodegeneration. This study investigated how optimal and detrimental Cu(II) concentrations affect αB-Cry oligomers and their chaperone activity, within the potassium-regulated ionic-strength environment. Techniques including isothermal titration calorimetry, differential scanning calorimetry, fluorescence spectroscopy, inductively coupled plasma atomic emission spectroscopy, cyclic voltammetry, dynamic light scattering, circular dichroism, and MTT assay were employed and complemented by computational methods. Results showed that potassium ions affected αB-Cry's structure, promoting Cu(II) binding at multiple sites and scavenging ability, and inhibiting ion redox reactions. Low concentrations of Cu(II), through modifications of oligomeric interfaces, induce regulation of surface charge and hydrophobicity, resulting in an increase in chaperone activity. Subunit dynamics were regulated, maintaining stable interfaces, thereby inhibiting further aggregation and allowing the functional reversion to oligomers after stress. High Cu(II) disrupted charge/hydrophobicity balance, sewing sizable oligomers together through subunit-subunit interactions, suppressing oligomer dissociation, and reducing chaperone efficiency. This study offers insights into how Cu(II) and potassium ions influence αB-Cry, advancing our understanding of Cu(II)-related diseases.

PdPtB Electrochemiluminescence Nanoenhancer and SiC@Au-PEDOT Nanowires-Based Detection of β-Amyloid Oligomers in Alzheimer's Disease

β-Amyloid oligomers (AβOs) are promising biomarkers for the diagnosis of Alzheimer's disease (AD). The present research introduces a novel electrochemiluminescence (ECL) immunosensor based on PdPtB nanoenhancer and SiC@Au-PEDOT nanowires (NWs) for the specific and ultrasensitive detection of AβOs. The PdPtB nanoenhancer exhibited excellent oxidase-like catalytic activity with in situ generation of reactive oxygen species (ROS) to enhance luminol ECL in neutral media. In addition, SiC@Au-PEDOT NWs were utilized as a biocompatible and conductive substrate for the modification of the glassy carbon electrode (GCE). With this design, the ECL immunosensor showed outstanding AβOs analytical performance without exogenous coreactant. The ECL immunosensor demonstrated a favorable linear range of 20 pM to 20 nM and a detection limit of 10 pM under optimized conditions with potential straightforward clinical application. In general, the developed ECL immunosensor provides a promising strategy for the early diagnosis of AD.

Amphiphilic Molecules Exhibiting Zwitterionic Excited-State Intramolecular Proton Transfer and Near-Infrared Emission for the Detection of Amyloid β Aggregates in Alzheimer's Disease

Chromophores with zwitterionic excited-state intramolecular proton transfer (ESIPT) have been shown to have larger Stock shifts and red-shifted emission wavelengths compared to the conventional π-delocalized ESIPT molecules. However, there is still a dearth of design strategies to expand the current library of zwitterionic ESIPT compounds. Herein, a novel zwitterionic excited-state intramolecular proton transfer system is reported, enabled by addition of 1,4,7-triazacyclononane (TACN) fragments on a dicyanomethylene-4H-pyran (DCM) scaffold. The solvent-dependent steady-state photophysical studies, pKa measurements, and computational analysis strongly support that the ESIPT process is more efficient with two TACN groups attached to the DCM scaffold and not affected by polar protic solvents. Impressively, compound DCM-OH-2-DT exhibits a near-infrared (NIR) emission at 740 nm along with an uncommonly large Stokes shift. Moreover, DCM-OH-2-DT shows high affinity towards soluble amyloid β (Aβ) oligomers in vitro and in 5xFAD mouse brain sections, and we have successfully applied DCM-OH-2-DT for the in vivo imaging of Aβ aggregates and demonstrated its potential use as an early diagnostic agent for AD. Overall, this study can provide a general molecular design strategy for developing new zwitterionic ESIPT compounds with NIR emission in vivo imaging applications.

Inhibition of amyloid β1-42 peptide aggregation by newly designed cyclometallated palladium complexes

Uncontrolled amyloid aggregation is a frequent cause of neurodegenerative disorders such as prions and Alzheimer's disease (AD). As a result, many drug development approaches focus on evaluating novel molecules that can alter self-recognition pathways. Herein, we designed and synthesized the cyclometallated pyrene (Pd-1 and Pd-3) and anthracene (Pd-2) based palladium complexes ([Pd((L1)Cl] Pd-1, [Pd(L2)Cl](Pd-2), and [Pd(L3)Cl] (Pd-3)). This study explores the effect of these complexes on the aggregation, fibrillation, and amyloid formation of bovine serum albumin (BSA) and Aβ1-42 peptide. Several spectroscopic methods were used to characterize all the Pd-complexes, and the molecular structure of Pd-3 was determined by X-ray crystallography. The secondary structures were studied using circular dichroism (CD) and transmission electron microscopy (TEM), while amyloid aggregation and inhibitory activities were investigated using the Thioflavin-T (ThT) fluorescence assay. Molecular docking of the Pd-complex (Pd-3) was done using fibril (PDB: 2BEG) and monomeric (PDB: 1IYT) peptides using Auto-dock Vina. As a result, the hydrogen bonding and hydrophobic interaction between the aromatic rings of the Pd-complexes and the amino acids of amyloid-β peptides significantly reduced the production of ordered β-sheets of amyloid fibrils and protein aggregation in the presence of Pd-2 and Pd-3 complexes.

Tailoring the Amphiphilicity of Fluorescent Protein Chromophores to Detect Intracellular Proteome Aggregation in Diverse Biological Samples

The formation of amorphous misfolded and aggregated proteins is a hallmark of proteome stress in diseased cells. Given its lack of defined targeting sites, the rational design of intracellular proteome aggregation sensors has been challenging. Herein, we modulate the amphiphilicity of fluorescent protein chromophores to enable selective detection of aggregated proteins in different biological samples, including recombinant proteins, stressed live cells, intoxicated mouse liver tissue, and human hepatocellular carcinoma tissue. By tuning the number of hydroxyl groups, we optimize the selectivity of fluorescent protein chromophores toward aggregated proteins in these biological samples. In recombinant protein applications, the most hydrophobic P0 (cLogP = 5.28) offers the highest fold change (FC = 31.6), sensitivity (LLOD = 0.1 μM), and brightness (Φ = 0.20) upon binding to aggregated proteins. In contrast, P4 of balanced amphiphilicity (cLogP = 2.32) is required for selective detection of proteome stresses in live cells. In mouse and human liver histology tissues, hydrophobic P1 exhibits the best performance in staining the aggregated proteome. Overall, the amphiphilicity of fluorescent chromophores governs the sensor's performance by matching the diverse nature of different biological samples. Together with common extracellular amyloid sensors (e.g., Thioflavin T), these sensors developed herein for intracellular amorphous aggregation complement the toolbox to study protein aggregation.

A label-free dually-amplified aptamer sensor for the specific detection of amyloid-beta peptide oligomers in cerebrospinal fluids

Amyloid-beta peptide oligomer (Aβo) is widely acknowledged to be associated with Alzheimer's disease (AD). The immediate and accurate detection of Aβo may provide the index for tracking the progress of the state of the disease, as well as some useful information for investigating the pathology of AD. In this work, based on a triple helix DNA which triggers a series of circular amplified reactions in the presence of Aβo, we designed a simple and label-free colorimetric biosensor with dually-amplified signal for the specific detection of Aβo. The sensor displays some advantages including high specificity, high sensitivity, low detection limit down to 0.23 pM, and wide detection range with three orders of magnitude from 0.3472 to 694.44 pM. Furthermore, the proposed sensor was successfully applied for detecting Aβo in artificial and real cerebrospinal fluids with satisfactory results, suggesting the potential application of the proposed sensor for state-monitoring and pathological studies of AD.

Macrophage lineage cells-derived migrasomes activate complement-dependent blood-brain barrier damage in cerebral amyloid angiopathy mouse model

Accumulation of amyloid beta protein (Aβ) in brain vessels damages blood brain barrier (BBB) integrity in cerebral amyloid angiopathy (CAA). Macrophage lineage cells scavenge Aβ and produce disease-modifying mediators. Herein, we report that Aβ40-induced macrophage-derived migrasomes are sticky to blood vessels in skin biopsy samples from CAA patients and brain tissue from CAA mouse models (Tg-SwDI/B and 5xFAD mice). We show that CD5L is packed in migrasomes and docked to blood vessels, and that enrichment of CD5L impairs the resistance to complement activation. Increased migrasome-producing capacity of macrophages and membrane attack complex (MAC) in blood are associated with disease severity in both patients and Tg-SwDI/B mice. Of note, complement inhibitory treatment protects against migrasomes-mediated blood-brain barrier injury in Tg-SwDI/B mice. We thus propose that macrophage-derived migrasomes and the consequent complement activation are potential biomarkers and therapeutic targets in CAA.

Chemical probes for investigating protein liquid-liquid phase separation and aggregation

Protein liquid-liquid phase separation drives the dynamic assembly of membraneless organelles for fulfilling different physiological functions. Under diseased condition, protein may undergo liquid-to-solid condensation to form pathological amyloid aggregates closely associated with neurodegenerative diseases. Chemical probe serves as an important chemical tool not only for exploring the basic principle of the dynamic assembly of different protein condensates in vitro and in cell but also for clinical diagnosis and therapeutics of the related diseases. In this review, we first introduce chemical probes to image and regulate protein condensates. Then, we summarized three different categories of chemical probes including general amyloid dye, selective positron emission tomography tracer, and disaggregating binder, which feature distinct interaction pattern and activity upon binding to different pathological amyloid fibrillar aggregates. Next, we discuss the development of chemical probes for tracking protein amorphous aggregates in cells. Finally, we point out future direction in expanding the probes' chemical space and applications.

Design and synthesis of ruthenium complexes and their studies on the inhibition of amyloid β (1-42) peptide aggregation

Misfolding and protein aggregation have been linked to numerous human neurodegenerative disorders such as Alzheimer's, prions, and Parkinson's. Due to their interesting photophysical properties, ruthenium (Ru) complexes have received considerable attention in studying protein aggregation. In this study, we synthesized the novel Ru complexes ([Ru(p-cymene)Cl(L-1)][PF₆](Ru-1), and [Ru(p-cymene)Cl(L-2)][PF₆](Ru-2)) and investigated their inhibitory activity against the bovine serum albumin (BSA) aggregation and the Aβ_1-42 peptides amyloid formation. Several spectroscopic methods were used to characterize the complexes, and the molecular structure was determined by X-ray crystallography. Amyloid aggregation and inhibition activity were examined using the Thioflavin-T (ThT) assay, and secondary structures were analyzed by circular dichroism (CD) spectroscopy and transmission electron microscopy (TEM). The cell viability assay was carried out on the neuroblastoma cell line, revealing that the Ru-2 complex showed better protective effects against Aβ_1-42 peptide toxicity on neuro-2a cells than the Ru-1 complex. Molecular docking studies elucidate binding sites and interactions between the Ru-complexes and the Aβ_1-42 fibrils. The experimental studies revealed that these complexes significantly inhibited BSA aggregation and Aβ_1-42 amyloid fibril formation at 1:3 and 1:1 equimolar concentrations, respectively. Antioxidant assays demonstrated that these complexes act as antioxidants, protecting from amyloid-induced oxidative stress. Molecular docking studies with the monomeric Aβ_1-42 (PDB: 1IYT) show hydrophobic interaction, and both complexes bind preferably in the central region of the peptide and coordinate with two binding sites of the peptide. Hence, we suggest that the Ru-based complexes could be applied as a potential agent in metallopharmaceutical research against Alzheimer's disease.

Nanoparticle Scanners for the Identification of Key Sequences Involved in the Assembly and Disassembly of β-Amyloid Peptides

The aggregation of β-amyloid peptides (Aβ), implied in the development and progression of Alzheimer's disease, is driven by a complex set of intramolecular and intermolecular interactions involving both hydrophobic and polar residues. The key residues responsible for the forward assembling process may be different from those that should be targeted to disassemble already formed aggregates. Molecularly imprinted nanoparticle (MINP) receptors are reported in this work to strongly and selectively bind specific segments of Aβ₄₀. Combined fluorescence spectroscopy, atomic force microscopy (AFM) imaging, and circular dichroism (CD) spectroscopy indicate that binding residues 21-30 near the loop region is most effective at inhibiting the aggregation of monomeric Aβ₄₀, but residues 11-20 that include the internal β strand closer to the N-terminal represent the best target for disaggregating already formed aggregates in the polymerization phase. Once the aggregation proceeds to the saturation phase, binding residues 1-10 has the largest effect on the disaggregation, likely because of the accessibility of these amino acids relative to others to the MINP receptors.

Deconvoluting binding sites in amyloid nanofibrils using time-resolved spectroscopy

Steady-state fluorescence spectroscopy has a central role not only for sensing applications, but also in biophysics and imaging. Light switching probes, such as ruthenium dipyridophenazine complexes, have been used to study complex systems such as DNA, RNA, and amyloid fibrils. Nonetheless, steady-state spectroscopy is limited in the kind of information it can provide. In this paper, we use time-resolved spectroscopy for studying binding interactions between amyloid-β fibrillar structures and photoluminescent ligands. Using time-resolved spectroscopy, we demonstrate that ruthenium complexes with a pyrazino phenanthroline derivative can bind to two distinct binding sites on the surface of fibrillar amyloid-β, in contrast with previous studies using steady-state photoluminescence spectroscopy, which only identified one binding site for similar compounds. The second elusive binding site is revealed when deconvoluting the signals from the time-resolved decay traces, allowing the determination of dissociation constants of 3 and 2.2 μM. Molecular dynamic simulations agree with two binding sites on the surface of amyloid-β fibrils. Time-resolved spectroscopy was also used to monitor the aggregation of amyloid-β in real-time. In addition, we show that common polypyridine complexes can bind to amyloid-β also at two different binding sites. Information on how molecules bind to amyloid proteins is important to understand their toxicity and to design potential drugs that bind and quench their deleterious effects. The additional information contained in time-resolved spectroscopy provides a powerful tool not only for studying excited state dynamics but also for sensing and revealing important information about the system including hidden binding sites.

Enabling Photo-Crosslinking and Photo-Sensitizing Properties for Synthetic Fluorescent Protein Chromophores

Synthetic fluorescent protein chromophores have been reported for their singlet state fluorescence properties and applications in bioimaging, but rarely for the triplet state chemistries. Herein, we enabled their photo-sensitizing and photo-crosslinking properties through rational modulations. Extension of molecular conjugation and introduction of heavy atoms promoted the generation of reactive oxygen species. Unlike other photosensitizers, these chromophores selectively photo-crosslinked aggregated proteins and uncovered the interactome profiles. We also exemplified their general applications in chromophore-assisted light inactivation, photodynamic therapy and photo induced polymerization. Theoretical calculation, pathway analysis and transient absorption spectroscopy provided mechanistic insights for this triplet state chemistry. Overall, this work expands the function and application of synthetic fluorescent protein chromophores by enabling their triplet excited state properties.

Rational design of photoactivatable metal complexes to target and modulate amyloid-β peptides

The accumulation of amyloid-β (Aβ) aggregates is found in the brains of Alzheimer's disease patients. Thus, numerous efforts have been made to develop chemical reagents capable of targeting Aβ peptides and controlling their aggregation. In particular, tunable coordination and photophysical properties of transition metal complexes, with variable oxidation and spin states on the metal centers, can be utilized to probe Aβ aggregates and alter their aggregation profiles. In this review, we illustrate some rational strategies for designing photoactivatable metal complexes as chemical sensors for Aβ peptides or modulators against their aggregation pathways, with some examples.

Cucurbit[7]uril Macrocyclic Sensors for Optical Fingerprinting: Predicting Protein Structural Changes to Identifying Disease-Specific Amyloid Assemblies

In a three-dimensional (3D) representation, each protein molecule displays a specific pattern of chemical and topological features, which are altered during its misfolding and aggregation pathway. Generating a recognizable fingerprint from such features could provide an enticing approach not only to identify these biomolecules but also to gain clues regarding their folding state and the occurrence of pathologically lethal misfolded aggregates. We report here a universal strategy to generate a fluorescent fingerprint from biomolecules by employing the pan-selective molecular recognition feature of a cucurbit[7]uril (CB[7]) macrocyclic receptor. We implemented a direct sensing strategy by covalently tethering CB[7] with a library of fluorescent reporters. When CB[7] recognizes the chemical and geometrical features of a biomolecule, it brings the tethered fluorophore into the vicinity, concomitantly reporting the nature of its binding microenvironment through a change in their optical signature. The photophysical properties of the fluorophores allow a multitude of probing modes, while their structural features provide additional binding diversity, generating a distinct fluorescence fingerprint from the biomolecule. We first used this strategy to rapidly discriminate a diverse range of protein analytes. The macrocyclic sensor was then applied to probe conformational changes in the protein structure and identify the formation of oligomeric and fibrillar species from misfolded proteins. Notably, the sensor system allowed us to differentiate between different self-assembled forms of the disease-specific amyloid-β (Aβ) aggregates and segregated them from other generic amyloid structures with a 100% identification accuracy. Ultimately, this sensor system predicted clinically relevant changes by fingerprinting serum samples from a cohort of pregnant women.

Fas Apoptosis Inhibitory Molecule Blocks and Dissolves Pathological Amyloid-β Species

A number of neurodegenerative diseases are associated with the accumulation of misfolded proteins, including Alzheimer’s disease (AD). In AD, misfolded proteins such as tau and amyloid-β (Aβ) form pathological insoluble deposits. It is hypothesized that molecules capable of dissolving such protein aggregates might reverse disease progression and improve the lives of afflicted AD patients. Here we report new functions of the highly conserved mammalian protein, Fas Apoptosis Inhibitory Molecule (FAIM). We found that FAIM-deficient Neuro 2A cells accumulate Aβ oligomers/fibrils. We further found that recombinant human FAIM prevents the generation of pathologic Aβ oligomers and fibrils in a cell-free system, suggesting that FAIM functions without any additional cellular components. More importantly, recombinant human FAIM disaggregates and solubilizes established Aβ fibrils. Our results identify a previously unknown, completely novel candidate for understanding and treating irremediable, irreversible, and unrelenting neurodegenerative diseases.

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.

Chemical Biology Toolbox to Visualize Protein Aggregation in Live Cells

Protein misfolding and aggregation is a complex biochemical process and has been associated with numerous human degenerative diseases. Developing novel chemical and biological tools and approaches to visualize aggregated proteins in live cells is in high demand for mechanistic studies, diagnostics, and therapeutics. In this review, we summarize the recent developments in the chemical biology toolbox applied to protein aggregation studies in live cells. These methods exploited fluorescent protein tags, fluorescent chemical tags, and small-molecule probes to visualize the protein-aggregation process, detect proteome stresses, and quantify the protein homeostasis network capacity. Inspired by these seminal works, we have generalized design principles for the development of new detection methods and probes in the future that will illuminate this important biological process.

A Solvatochromic Fluorescent Probe Reveals Polarity Heterogeneity upon Protein Aggregation in Cells

We report a crystallization-induced emission fluorophore to quantitatively interrogate the polarity of aggregated proteins. This solvatochromic probe, namely "AggRetina" probe, inherently binds to aggregated proteins and exhibits both a polarity-dependent fluorescence emission wavelength shift and a viscosity-dependent fluorescence intensity increase. Regulation of its polarity sensitivity was achieved by extending the conjugation length. Different proteins bear diverse polarity upon aggregation, leading to different resistance to proteolysis. Polarity primarily decreases during protein misfolding but viscosity mainly increases upon the formation of insoluble aggregates. We quantified the polarity of aggregated protein-of-interest in live cells via HaloTag bioorthogonal labeling, revealing polarity heterogeneity within cellular aggregates. The enriched micro-environment details inside misfolded and aggregated proteins may correlate to their bio-chemical properties and pathogenicity.

Exploring the Propensities of Fluorescent Carbazole Analogs toward the Inhibition of Amyloid Aggregation in Type 2 Diabetes: An Experimental and Theoretical Endeavor

Amyloid aggregation is a pathological trait observed in many incurable and fatal neurodegenerative and metabolic diseases associated with misfolding and self-assembly of various proteins. Noncovalent interactions between these structural motifs and small molecules can, however, prevent this aggregation. Herein, five structurally different synthetic (Cz1-Cz4) and naturally occurring (Cz5, mahanimbine) fluorescent carbazole analogs are explored for their comparative amyloid aggregation inhibitory activities. Cz3 inhibited the amyloid deposition on the pancreatic β-cells of diabetic mice. Moreover, Cz3 and Cz5 also showed efficacy as the fluorescent cell (MIN6) imaging agents. Further structural modifications of these carbazoles may lead to development of low-cost and non-toxic therapeutic agents for Type 2 diabetes and other amyloidosis-related diseases.

Illuminating Protein Phase Separation: Reviewing Aggregation-Induced Emission, Fluorescent Molecular Rotor and Solvatochromic Fluorophore based Probes

Protein phase separation process involving protein unfolding, misfolding, condensation and aggregation etc. has been associated with numerous human degenerative diseases. The complexity in protein conformational transitions results in multi-step and multi-species biochemical pathways upon protein phase separation. Recent progresses in designing novel fluorescent probes have unraveled the enriched details of phase separated proteins and provided mechanistic insights towards disease pathology. In this review, we summarized the design and characterizations of fluorescent probes that selectively illuminate proteins at different phase separated states with a focus on aggregation-induced emission probes, fluorescent molecular rotors, and solvatochromic fluorophores. Inspired by these pioneering works, a design blueprint was proposed to further develop fluorescent probes that can potentially shed light on the unresolved protein phase separated states in the future.

Rational Design of Crystallization-Induced-Emission Probes To Detect Amorphous Protein Aggregation in Live Cells

Unlike amyloid aggregates, amorphous protein aggregates with no defined structures have been challenging to target and detect in a complex cellular milieu. In this study, we rationally designed sensors of amorphous protein aggregation from aggregation-induced-emission probes (AIEgens). Utilizing dicyanoisophorone as a model AIEgen scaffold, we first sensitized the fluorescence of AIEgens to a nonpolar and viscous environment mimicking the interior of amorphous aggregated proteins. We identified a generally applicable moiety (dimethylaminophenylene) for selective binding and fluorescence enhancement. Regulation of the electron-withdrawing groups tuned the emission wavelength while retaining selective detection. Finally, we utilized the optimized probe to systematically image aggregated proteome upon proteostasis network regulation. Overall, we present a rational approach to develop amorphous protein aggregation sensors from AIEgens with controllable sensitivity, spectral coverage, and cellular performance.

Exploring the Impact of Glyoxal Glycation on β-Amyloid Peptide (Aβ) Aggregation in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by the presence of extracellular senile plaques formed by β-amyloid (Aβ) peptides in the patient's brain. Previous studies have shown that the plaques in the AD brains are colocalized with the advanced glycation end products, which is mainly formed from a series of nonenzymatic reactions of proteins with reducing sugars or reactive dicarbonyls. Glycation was also demonstrated to increase the neurotoxicity of the Aβ peptides. To clarify the impact of glycation on Aβ aggregation, we synthesized two glycated Aβ42 peptides by replacing Lys16 and Lys28 with N^ε-carboxymethyllysine respectively to mimic the occurrence of protein glycation. Afterward, we monitored the aggregation kinetics and conformational change for two glycated peptides. We also used fluorescence correlation spectroscopy to probe the early stage of peptide oligomerization and tested their abilities in copper binding and reactive oxygen species production. Our data show that glycation significantly slows down the aggregation process and induces more cytotoxicity especially at position 28. We speculated that the higher toxicity might result from a relatively stable oligomeric form of peptide and not from ROS production. The data shown here emphasized that glycated proteins would be an important therapeutic target in AD treatments.

Quantitative interrogation of protein co-aggregation using multi-color fluorogenic protein aggregation sensors

Co-aggregation of multiple pathogenic proteins is common in neurodegenerative diseases but deconvolution of such biochemical process is challenging. Herein, we developed a dual-color fluorogenic thermal shift assay to simultaneously report on the aggregation of two different proteins and quantitatively study their thermodynamic stability during co-aggregation. Expansion of spectral coverage was first achieved by developing multi-color fluorogenic protein aggregation sensors. Orthogonal detection was enabled by conjugating sensors of minimal fluorescence crosstalk to two different proteins via sortase-tag technology. Using this assay, we quantified shifts in melting temperatures in a heterozygous model protein system, revealing that the thermodynamic stability of wild-type proteins was significantly compromised by the mutant ones but not vice versa. We also examined how small molecule ligands selectively and differentially interfere with such interplay. Finally, we demonstrated these sensors are suited to visualize how different proteins exert influence on each other upon their co-aggregation in live cells.

Modification of amyloid-beta peptide aggregation via photoactivation of strained Ru(ii) polypyridyl complexes

Alzheimer's disease (AD) is a chronic neurodegenerative disorder characterized by progressive and irreversible damage to the brain. One of the hallmarks of the disease is the presence of both soluble and insoluble aggregates of the amyloid beta (Aβ) peptide in the brain, and these aggregates are considered central to disease progression. Thus, the development of small molecules capable of modulating Aβ peptide aggregation may provide critical insight into the pathophysiology of AD. In this work we investigate how photoactivation of three distorted Ru(ii) polypyridyl complexes (Ru1-3) alters the aggregation profile of the Aβ peptide. Photoactivation of Ru1-3 results in the loss of a 6,6'-dimethyl-2,2'-bipyridyl (6,6'-dmb) ligand, affording cis-exchangeable coordination sites for binding to the Aβ peptide. Both Ru1 and Ru2 contain an extended planar imidazo[4,5-f][1,10]phenanthroline ligand, as compared to a 2,2'-bipyridine ligand for Ru3, and we show that the presence of the phenanthroline ligand promotes covalent binding to Aβ peptide His residues, and in addition, leads to a pronounced effect on peptide aggregation immediately after photoactivation. Interestingly, all three complexes resulted in a similar aggregate size distribution at 24 h, forming insoluble amorphous aggregates as compared to significant fibril formation for peptide alone. Photoactivation of Ru1-3 in the presence of pre-formed Aβ1-42 fibrils results in a change to amorphous aggregate morphology, with Ru1 and Ru2 forming large amorphous aggregates immediately after activation. Our results show that photoactivation of Ru1-3 in the presence of either monomeric or fibrillar Aβ1-42 results in the formation of large amorphous aggregates as a common endpoint, with Ru complexes incorporating the extended phenanthroline ligand accelerating this process and thereby limiting the formation of oligomeric species in the initial stages of the aggregation process that are reported to show considerable toxicity.

Amyloid-Like Aggregation in Diseases and Biomaterials: Osmosis of Structural Information

The discovery that the polypeptide chain has a remarkable and intrinsic propensity to form amyloid-like aggregates endowed with an extraordinary stability is one of the most relevant breakthroughs of the last decades in both protein/peptide chemistry and structural biology. This observation has fundamental implications, as the formation of these assemblies is systematically associated with the insurgence of severe neurodegenerative diseases. Although the ability of proteins to form aggregates rich in cross-β structure has been highlighted by recent studies of structural biology, the determination of the underlying atomic models has required immense efforts and inventiveness. Interestingly, the progressive molecular and structural characterization of these assemblies has opened new perspectives in apparently unrelated fields. Indeed, the self-assembling through the cross-β structure has been exploited to generate innovative biomaterials endowed with promising mechanical and spectroscopic properties. Therefore, this structural motif has become the fil rouge connecting these diversified research areas. In the present review, we report a chronological recapitulation, also performing a survey of the structural content of the Protein Data Bank, of the milestones achieved over the years in the characterization of cross-β assemblies involved in the insurgence of neurodegenerative diseases. A particular emphasis is given to the very recent successful elucidation of amyloid-like aggregates characterized by remarkable molecular and structural complexities. We also review the state of the art of the structural characterization of cross-β based biomaterials by highlighting the benefits of the osmosis of information between these two research areas. Finally, we underline the new promising perspectives that recent successful characterizations of disease-related amyloid-like assemblies can open in the biomaterial field.

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.

Multifunctional compounds for the treatment of Alzheimer’s disease

Alzheimer’s disease (AD) is the most common form of dementia, and the prevalence of this currently untreatable disease is expected to rise in step with increased global life expectancy. AD is a multifaceted disorder commonly characterized by extracellular amyloid–beta (Aβ) aggregates, oxidative stress, metal ion dysregulation, and intracellular neurofibrillary tangles. This review will focus on medicinal inorganic chemistry strategies to target AD, with a focus on the Aβ peptide and its relation to metal ion dysregulation and oxidative stress. Multifunctional compounds designed to target multiple disease processes have emerged as promising therapeutic options, and recent reports detailing multifunctional metal-binding compounds, as well as discrete metal complexes, will be discussed.

A Novel Nanosystem Realizing Curcumin Delivery Based on Fe3O4@Carbon Dots Nanocomposite for Alzheimer's Disease Therapy

Alzheimer’s disease (AD) is the most common neurodegenerative disease, which seriously affects human health but lacks effective treatment methods. Amyloid β (Aβ) aggregates are considered a possible target for AD treatment. Evidence is increasingly showing that curcumin (CUR) can partly protect cells from Aβ-mediated neurotoxicity by inhibiting Aβ aggregation. However, the efficiency of targeted cellular uptake and bioavailability of CUR is very low due to its poor stability and water-solubility. In order to better improve the cell uptake efficiency and bioavailability of CUR and reduce the cytotoxicity of high-dose CUR, a novel CUR delivery system for AD therapy has been constructed based on the employment of the Fe₃O₄@carbon dots nanocomposite (Fe₃O₄@CDs) as the carrier. CUR-Fe₃O₄@CDs have a strong affinity toward Aβ and effectively inhibit extracellular Aβ fibrillation. In addition, CUR-Fe₃O₄@CDs can inhibit the production of reactive oxygen species (ROS) mediated by Aβ fibrils and the corresponding neurotoxicity in PC12 cells. More importantly, it can restore nerve damage and maintained neuronal morphology. These results indicate that the application of CUR-Fe₃O₄@CDs provides a promising platform for the treatment of AD.

Evaluation of Peptide/Protein Self-Assembly and Aggregation by Spectroscopic Methods

The self-assembly of proteins is an essential process for a variety of cellular functions including cell respiration, mobility and division. On the other hand, protein or peptide misfolding and aggregation is related to the development of Parkinson's disease and Alzheimer's disease, among other aggregopathies. As a consequence, significant research efforts are directed towards the understanding of this process. In this review, we are focused on the use of UV-Visible Absorption Spectroscopy, Fluorescence Spectroscopy and Circular Dichroism to evaluate the self-organization of proteins and peptides in solution. These spectroscopic techniques are commonly available in most chemistry and biochemistry research laboratories, and together they are a powerful approach for initial as well as routine evaluation of protein and peptide self-assembly and aggregation under different environmental stimulus. Furthermore, these spectroscopic techniques are even suitable for studying complex systems like those in the food industry or pharmaceutical formulations, providing an overall idea of the folding, self-assembly, and aggregation processes, which is challenging to obtain with high-resolution methods. Here, we compiled and discussed selected examples, together with our results and those that helped us better to understand the process of protein and peptide aggregation. We put particular emphasis on the basic description of the methods as well as on the experimental considerations needed to obtain meaningful information, to help those who are just getting into this exciting area of research. Moreover, this review is particularly useful to those out of the field who would like to improve reproducibility in their cellular and biomedical experiments, especially while working with peptide and protein systems as an external stimulus. Our final aim is to show the power of these low-resolution techniques to improve our understanding of the self-assembly of peptides and proteins and translate this fundamental knowledge in biomedical research or food applications.

Seeding and Growth of β-Amyloid Aggregates upon Interaction with Neuronal Cell Membranes

In recent years, the prevalence of amyloid neurodegenerative diseases such as Alzheimer's disease (AD) has significantly increased in developed countries due to increased life expectancy. This amyloid disease is characterized by the presence of accumulations and deposits of β-amyloid peptide (Aβ) in neuronal tissue, leading to the formation of oligomers, fibers, and plaques. First, oligomeric intermediates that arise during the aggregation process are currently thought to be primarily responsible for cytotoxicity in cells. This work aims to provide further insights into the mechanisms of cytotoxicity by studying the interaction of Aβ aggregates with Neuro-2a (N2a) neuronal cells and the effects caused by this interaction. For this purpose, we have exploited the advantages of advanced, multidimensional fluorescence microscopy techniques to determine whether different types of Aβ are involved in higher rates of cellular toxicity, and we measured the cellular stress caused by such aggregates by using a fluorogenic intracellular biothiol sensor. Stress provoked by the peptide is evident by N2a cells generating high levels of biothiols as a defense mechanism. In our study, we demonstrate that Aβ aggregates act as seeds for aggregate growth upon interacting with the cellular membrane, which results in cell permeability and damage and induces lysis. In parallel, these damaged cells undergo a significant increase in intracellular biothiol levels.