Novel pentameric thiophene derivatives for in vitro and in vivo optical imaging of a plethora of protein aggregates in cerebral amyloidoses

(11)C and (18)F Radiolabeling of Tetra- and Pentathiophenes as PET-Ligands for Amyloid Protein Aggregates

Three oligothiophenes were evaluated as PET ligands for the study of local and systemic amyloidosis ex vivo using tissue from patients with amyloid deposits and in vivo using healthy animals and PET-CT. The ex vivo binding studies revealed that all three labeled compounds bound specifically to human amyloid deposits. Specific binding was found in the heart, kidney, liver, and spleen. To verify the specificity of the oligothiophenes toward amyloid deposits, tissue sections with amyloid pathology were stained using the fluorescence exhibited by the compounds and evaluated with multiphoton microscopy. Furthermore, a in vivo monkey PET-CT study showed very low uptake in the brain, pancreas, and heart of the healthy animal indicating low nonspecific binding to healthy tissue. The biological evaluations indicated that this is a promising group of compounds for the visualization of systemic and localized amyloidosis.

The Luminescent Oligothiophene p-FTAA Converts Toxic Aβ1-42 Species into Nontoxic Amyloid Fibers with Altered Properties

Aggregation of the amyloid-β peptide (Aβ) in the brain leads to the formation of extracellular amyloid plaques, which is one of the pathological hallmarks of Alzheimer disease (AD). It is a general hypothesis that soluble prefibrillar assemblies of the Aβ peptide, rather than mature amyloid fibrils, cause neuronal dysfunction and memory impairment in AD. Thus, reducing the level of these prefibrillar species by using molecules that can interfere with the Aβ fibrillation pathway may be a valid approach to reduce Aβ cytotoxicity. Luminescent-conjugated oligothiophenes (LCOs) have amyloid binding properties and spectral properties that differ when they bind to protein aggregates with different morphologies and can therefore be used to visualize protein aggregates. In this study, cell toxicity experiments and biophysical studies demonstrated that the LCO p-FTAA was able to reduce the pool of soluble toxic Aβ species in favor of the formation of larger insoluble nontoxic amyloid fibrils, there by counteracting Aβ-mediated cytotoxicity. Moreover, p-FTAA bound to early formed Aβ species and induced a rapid formation of β-sheet structures. These p-FTAA generated amyloid fibrils were less hydrophobic and more resistant to proteolysis by proteinase K. In summary, our data show that p-FTAA promoted the formation of insoluble and stable Aβ species that were nontoxic which indicates that p-FTAA might have therapeutic potential.

Cellular uptake and intracellular degradation of poly(alkyl cyanoacrylate) nanoparticles

Background

Poly(alkyl cyanoacrylate) (PACA) nanoparticles have shown promise as drug carriers both to solid tumors and across the blood–brain barrier. Efficient drug delivery requires both high cellular uptake of the nanoparticles and release of the drug from the nanoparticles. Release of hydrophobic drugs from PACA nanoparticles is primarily governed by nanoparticle degradation, and this process has been poorly studied at the cellular level. Here we use the hydrophobic model drug Nile Red 668 (NR668) to investigate intracellular degradation of PACA nanoparticles by measuring changes in NR668 fluorescence emission and lifetime, as the spectral properties of NR668 depend on the hydrophobicity of the dye environment. We also assess the potential of poly(butyl cyanoacrylate) (PBCA) and poly(octyl cyanoacrylate) (POCA) nanoparticles for intracellular drug delivery in the prostate cancer cell line PC3 and rat brain endothelial cell line RBE4 and the role of endocytosis pathways in PACA nanoparticle uptake in those cell lines.

Results

Fluorescence lifetime imaging, emission spectra analysis and Förster resonance energy transfer indicated that the intracellular degradation was in line with the degradation found by direct methods such as gas chromatography and scanning electron microscopy, showing that PBCA has a faster degradation rate compared to POCA. The combined P(BCA/OCA) nanoparticles had an intermediate degradation rate. The uptake of POCA and PBCA nanoparticles was much higher in RBE4 than in PC3 cells. Endocytosis inhibition studies showed that both clathrin- and caveolin-mediated endocytosis were involved in PACA nanoparticle uptake, and that the former played a predominant role, particularly in PC3 cells.

Conclusions

In the present study, we used three different optical techniques to show that within a 24-hour period PBCA nanoparticles degraded significantly inside cells, releasing their payload into the cytosol, while POCA nanoparticles remained intact. This indicates that it is possible to tune the intracellular drug release rate by choosing appropriate monomers from the PACA family or by using hybrid PACA nanoparticles containing different monomers. In addition, we showed that the uptake of PACA nanoparticles depends not only on the monomer material, but also on the cell type, and that different cell lines can use different internalization pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-015-0156-7) contains supplementary material, which is available to authorized users.

Protein aggregation as an antibiotic design strategy

Taking advantage of the xenobiotic nature of bacterial infections, we tested whether the cytotoxicity of protein aggregation can be targeted to bacterial pathogens without affecting their mammalian hosts. In particular, we examined if peptides encoding aggregation-prone sequence segments of bacterial proteins can display antimicrobial activity by initiating toxic protein aggregation in bacteria, but not in mammalian cells. Unbiased in vitro screening of aggregating peptide sequences from bacterial genomes lead to the identification of several peptides that are strongly bactericidal against methicillin-resistant Staphylococcus aureus. Upon parenteral administration in vivo, the peptides cured mice from bacterial sepsis without apparent toxic side effects as judged from histological and hematological evaluation. We found that the peptides enter and accumulate in the bacterial cytosol where they cause aggregation of bacterial polypeptides. Although the precise chain of events that leads to cell death remains to be elucidated, the ability to tap into aggregation-prone sequences of bacterial proteomes to elicit antimicrobial activity represents a rich and unexplored chemical space to be mined in search of novel therapeutic strategies to fight infectious diseases.

Detection and Monitoring of Amyloid Fibrillation Using a Fluorescence "Switch-On" Probe

Amyloid protein fibrillation is associated with a variety of neurodegenerative and other diseases, and their efficient detection and monitoring can greatly advance early diagnosis and therapy. Herein, we report a fluorescent "switch-on" probe for the reliable detection and monitoring of amyloid fibrils. The probe consists of a peptide component for binding with amyloid structure and a color component with an aggregation-induced green emission property. This probe is nonfluorescent in the presence of amyloid forming monomer protein/peptide, but fluorescence "switch-on" occurs after binding with amyloid fibrils. Compared to conventionally used thioflavin T, this probe offers a high signal-to-noise ratio, which is unaffected by the quencher ion/nanoparticle. The proposed new probe has been used for the detection and monitoring of amyloid fibrils produced by a wide variety of amyloid protein/peptides and can be extended for in vitro diagnostic applications.

Detection of oligomers and fibrils of α-synuclein by AIEgen with strong fluorescence

We report a fluorophore, TPE-TPP, with AIE characteristics which is utilized as a fluorescence probe to monitor the α-synuclein (α-Syn) fibrillation process. Compared with ThT, TPE-TPP shows a higher sensitivity in the detection of α-Syn oligomers as well as fibrils with a stronger fluorescence. The performance of TPE-TPP was evaluated using fluorescence, AFM, dot blot, and SEC.

An imidazole functionalized pentameric thiophene displays different staining patterns in normal and malignant cells

Molecular tools for fluorescent imaging of cells and their components are vital for understanding the function and activity of cells. Here, we report an imidazole functionalized pentameric oligothiophene, p-HTIm, that can be utilized for fluorescent imaging of cells. p-HTIm fluorescence in normal cells appeared in a peripheral punctate pattern partially co-localized with lysosomes, whereas a one-sided perinuclear Golgi associated localization of the dye was observed in malignant cells. The uptake of p-HTIm was temperature dependent and the intracellular target was reached within 1 h after staining. The ability of p-HTIm to stain cells was reduced when the imidazole side chain was chemically altered, verifying that specific imidazole side-chain functionalities are necessary for achieving the observed cellular staining. Our findings confirm that properly functionalized oligothiophenes can be utilized as fluorescent tools for vital staining of cells and that the selectivity toward distinct intracellular targets are highly dependent on the side-chain functionalities along the conjugated thiophene backbone.

Oligo(p-phenylene ethynylene) Electrolytes: A Novel Molecular Scaffold for Optical Tracking of Amyloids

Finding new optical probes to detect and track amyloid protein aggregates is key to understanding and defeating the myriad of neurodegenerative and other diseases associated with these misfolded proteins. Herein we report that a series of fluorescent, soluble oligo(p-phenylene ethynylene)s (OPEs) are able to detect amyloids in vitro by massive binding-activated superluminescence, with low micromolar affinity and high selectivity for the amyloid conformer. The OPEs track the kinetics of amyloid fibril formation from native hen egg white lysozyme (HEWL) similarly to thioflavin T (ThT), and the dependence of binding affinity on OPE length supports the theory of a linear binding groove. We hypothesize, based on spectral properties, induced circular dichroism, and previous work in analogous systems, that the fluorescence turn-on mechanism is a combination of the reduction of static solvent-mediated quenching at the ethyl ester end groups of the phenylene ethynylene fluorophore and the formation of chiral J-type aggregates templated on the amyloid fibril surface.

Pathology and function of nuclear amyloid. Protein homeostasis matters

In aging societies increasing cases of neurodegenerative protein deposit diseases urge for the identification of the underlying mechanisms. Expectations are that in 2050 the percentage of population over age 60 is 42% in Japan, 34% in China, and 27% in the US. The cell nucleus is a major target of amyloid-like protein fibrillation in a variety of disorders that are characterized by widespread aggregation of proteins with instable homopolymeric amino acid repeats, ubiquitin, and other proteinaceous components. Additionally, accumulation of insoluble, SDS-resistant proteins has been identified as an intrinsic property of organismal aging. This review collects current knowledge about the composition and function of insoluble, nuclear protein inclusions from the protein homeostasis perspective. It discusses the occurrence and role of nuclear amyloid in the diseased as well as the healthy cell. Features of nuclear inclusions such as protein composition and locally active protein degradation may predict neural fitness and survival in a variety of health or disease settings.

Systematic Aβ Analysis in Drosophila Reveals High Toxicity for the 1-42, 3-42 and 11-42 Peptides, and Emphasizes N- and C-Terminal Residues

Brain amyloid plaques are a hallmark of Alzheimer's disease (AD), and primarily consist of aggregated Aβ peptides. While Aβ 1-40 and Aβ 1-42 are the most abundant, a number of other Aβ peptides have also been identified. Studies have indicated differential toxicity for these various Aβ peptides, but in vivo toxicity has not been systematically tested. To address this issue, we generated improved transgenic Drosophila UAS strains expressing 11 pertinent Aβ peptides. UAS transgenic flies were generated by identical chromosomal insertion, hence removing any transgenic position effects, and crossed to a novel and robust Gal4 driver line. Using this improved Gal4/UAS set-up, survival and activity assays revealed that Aβ 1-42 severely shortens lifespan and reduces activity. N-terminal truncated peptides were quite toxic, with 3-42 similar to 1-42, while 11-42 showed a pronounced but less severe phenotype. N-terminal mutations in 3-42 (E3A) or 11-42 (E11A) resulted in reduced toxicity for 11-42, and reduced aggregation for both variants. Strikingly, C-terminal truncation of Aβ (1-41, -40, -39, -38, -37) were non-toxic. In contrast, C-terminal extension to 1-43 resulted in reduced lifespan and activity, but not to the same extent as 1-42. Mutating residue 42 in 1-42 (A42D, A42R and A42W) greatly reduced Aβ accumulation and toxicity. Histological and biochemical analysis revealed strong correlation between in vivo toxicity and brain Aβ aggregate load, as well as amount of insoluble Aβ. This systematic Drosophila in vivo and in vitro analysis reveals crucial N- and C-terminal specificity for Aβ neurotoxicity and aggregation, and underscores the importance of residues 1-10 and E11, as well as a pivotal role of A42.

Distinct Spacing Between Anionic Groups: An Essential Chemical Determinant for Achieving Thiophene-Based Ligands to Distinguish β-Amyloid or Tau Polymorphic Aggregates

The accumulation of protein aggregates is associated with many devastating neurodegenerative diseases and the existence of distinct aggregated morphotypes has been suggested to explain the heterogeneous phenotype reported for these diseases. Thus, the development of molecular probes able to distinguish such morphotypes is essential. We report an anionic tetrameric oligothiophene compound that can be utilized for spectral assignment of different morphotypes of β-amyloid or tau aggregates present in transgenic mice at distinct ages. The ability of the ligand to spectrally distinguish between the aggregated morphotypes was reduced when the spacing between the anionic substituents along the conjugated thiophene backbone was altered, which verified that specific molecular interactions between the ligand and the protein aggregate are necessary to detect aggregate polymorphism. Our findings provide the structural and functional basis for the development of new fluorescent ligands that can distinguish between different morphotypes of protein aggregates.

Biochemical markers in early diagnosis and management of systemic amyloidoses

Systemic amyloid diseases are characterized by widespread protein deposition as amyloid fibrils. Precise diagnostic framing is the prerequisite for a correct management of patients. This complex process is achieved through a series of steps, which include detection of the tissue amyloid deposits, identification of the amyloid type, demonstration of the amyloidogenic precursor, and evaluation of organ dysfunction/damage. Laboratory medicine plays a central role in the diagnosis and management of systemic amyloidoses, through the quantification of the amyloidogenic precursor and evaluation of end-organ damage using biomarkers.

The fluorescent pentameric oligothiophene pFTAA identifies filamentous tau in live neurons cultured from adult P301S tau mice

Identification of fluorescent dyes that label the filamentous protein aggregates characteristic of neurodegenerative disease, such as β-amyloid and tau in Alzheimer's disease, in a live cell culture system has previously been a major hurdle. Here we show that pentameric formyl thiophene acetic acid (pFTAA) fulfills this function in living neurons cultured from adult P301S tau transgenic mice. Injection of pFTAA into 5-month-old P301S tau mice detected cortical and DRG neurons immunoreactive for AT100, an antibody that identifies solely filamentous tau, or MC1, an antibody that identifies a conformational change in tau that is commensurate with neurofibrillary tangle formation in Alzheimer's disease brains. In fixed cultures of dorsal root ganglion (DRG) neurons, pFTAA binding, which also identified AT100 or MC1+ve neurons, followed a single, saturable binding curve with a half saturation constant of 0.14 μM, the first reported measurement of a binding affinity of a beta-sheet reactive dye to primary neurons harboring filamentous tau. Treatment with formic acid, which solubilizes filamentous tau, extracted pFTAA, and prevented the re-binding of pFTAA and MC1 without perturbing expression of soluble tau, detected using an anti-human tau (HT7) antibody. In live cultures, pFTAA only identified DRG neurons that, after fixation, were AT100/MC1+ve, confirming that these forms of tau pre-exist in live neurons. The utility of pFTAA to discriminate between living neurons containing filamentous tau from other neurons is demonstrated by showing that more pFTAA+ve neurons die than pFTAA-ve neurons over 25 days. Since pFTAA identifies fibrillar tau and other misfolded proteins in living neurons in culture and in animal models of several neurodegenerative diseases, as well as in human brains, it will have considerable application in sorting out disease mechanisms and in identifying disease-modifying drugs that will ultimately help establish the mechanisms of neurodegeneration in human neurodegenerative diseases.

Sensitive and rapid assessment of amyloid by oligothiophene fluorescence in subcutaneous fat tissue

Systemic amyloidosis (SA) is often diagnosed late. Combining clinical and biochemical biomarkers is necessary for raising suspicion of disease. Fine needle aspiration (FNA) of subcutaneous fat enables SA detection by Congo red staining. The luminescent conjugated probe heptameric formic thiophene acetic acid (h-FTAA) is a sensitive alternative to Congo red-staining of tissue samples. Our objective was to compare h-FTAA fluorescence with the Congo red stain for amyloid detection in FNA-obtained fat tissue. Herein, we studied samples from 57 patients with established SA (19 with AA, 20 with AL, and 18 with ATTR) and 17 age-matched controls (34-75 years). Positivity for h-FTAA was graded according to a Congo red-based grading scale ranging from 0 to 4+. Amyloid grading by both methods correlated strongly (r = 0.87). Here h-FTAA was positive in 53 of 54 Congo red-positive cases (sensitivity 98%) and h-FTAA was negative in 7 of 17 Congo red-negative controls (specificity 41%), but was also positive for 3 Congo red-negative SA cases. We conclude that h-FTAA fluorescence is more sensitive than Congo red staining in this small exploratory study of fat tissue samples, implicating potential sensitivity for prodromal amyloidosis, but is less specific for clinical amyloidosis defined by Congo red positivity. Given its simplicity h-FTAA staining may therefore be the most appropriate method for rapid screening of fat tissue samples but should presently treat grade 1+ as only suggestive, whereas 2+ or higher as positive for amyloidosis. Parallel assessment of h-FTAA and Congo red staining appears highly promising for clinical applications.

Differential vital staining of normal fibroblasts and melanoma cells by an anionic conjugated polyelectrolyte

Molecular probes for imaging of live cells are of great interest for studying biological and pathological processes. The anionic luminescent conjugated polythiophene (LCP) polythiophene acetic acid (PTAA), has previously been used for vital staining of cultured fibroblasts as well as transformed cells with results indicating differential staining due to cell phenotype. Herein, we investigated the behavior of PTAA in two normal and five transformed cells lines. PTAA fluorescence in normal cells appeared in a peripheral punctated pattern whereas the probe was more concentrated in a one-sided perinuclear localization in the five transformed cell lines. In fibroblasts, PTAA fluorescence was initially associated with fibronectin and after 24 h partially localized to lysosomes. The uptake and intracellular target in malignant melanoma cells was more ambiguous and the intracellular target of PTAA in melanoma cells is still elusive. PTAA was well tolerated by both fibroblasts and melanoma cells, and microscopic analysis as well as viability assays showed no signs of negative influence on growth. Stained cells maintained their proliferation rate for at least 12 generations. Although the probe itself was nontoxic, photoinduced cellular toxicity was observed in both cell lines upon irradiation directly after staining. However, no cytotoxicity was detected when the cells were irradiated 24 h after staining, indicating that the photoinduced toxicity is dependent on the cellular location of the probe. Overall, these studies certified PTAA as a useful agent for vital staining of cells, and that PTAA can potentially be used to study cancer-related biological and pathological processes.

Acetylcholinesterase-induced fluorescence turn-off of an oligothiophene-grafted quartz surface sensitive to myristoylcholine

Immobilized oligothiophene chains that are able to assemble/disassemble upon interaction with a cationic surfactant to detect acetylcholinesterase activity by altering their photoluminescence.

Biocompatible and biodegradable fluorescent microfibers physiologically secreted by live cells upon spontaneous uptake of thiophene fluorophore

Physiological secretion of fluorescent nanostructured microfibers upon spontaneous uptake of the appropriate organic fluorophore by live cells and the effects of cell seeding on the isolated microfibers.

Curcumin induces structural change and reduces the growth of amyloid-β fibrils: a QCM-D study

Curcumin inhibited Aβ fibril growth through leading to the structural conversion of the growing fibril to a more loosely constructed aggregate.

Diarylethene based fluorescent switchable probes for the detection of amyloid-β pathology in Alzheimer's disease

Two fluorescent switchable diarylethene derivatives which can detect amyloid-β aggregates bothin vitroandin vivowere reported.

Two-photon absorption of polyfluorene aggregates stabilized by insulin amyloid fibrils

We report on the photophysical and optical properties of a polyfluorene derivative (PFO) and its binding to the amyloid-forming protein insulin.

Stable, metastable, and kinetically trapped amyloid aggregate phases

Self-assembly of proteins into amyloid fibrils plays a key role in a multitude of human disorders that range from Alzheimer's disease to type II diabetes. Compact oligomeric species, observed early during amyloid formation, are reported as the molecular entities responsible for the toxic effects of amyloid self-assembly. However, the relation between early-stage oligomeric aggregates and late-stage rigid fibrils, which are the hallmark structure of amyloid plaques, has remained unclear. We show that these different structures occupy well-defined regions in a peculiar phase diagram. Lysozyme amyloid oligomers and their curvilinear fibrils only form after they cross a salt and protein concentration-dependent threshold. We also determine a boundary for the onset of amyloid oligomer precipitation. The oligomeric aggregates are structurally distinct from rigid fibrils and are metastable against nucleation and growth of rigid fibrils. These experimentally determined boundaries match well with colloidal model predictions that account for salt-modulated charge repulsion. The model also incorporates the metastable and kinetic character of oligomer phases. Similarities and differences of amyloid oligomer assembly to metastable liquid-liquid phase separation of proteins and to surfactant aggregation are discussed.

Development and application of methodology for rapid screening of potential amyloid probes

Herein, we demonstrate that it is possible to rapidly screen hydrophobic fluorescent aromatic molecules with regards to their properties as amyloid probes. By grinding the hydrophobic molecule with the amyloidogenic protein insulin, we obtained a water-soluble composite material. When this material is dissolved and exposed to conditions promoting amyloid formation, the protein aggregates into amyloid fibrils incorporating the hydrophobic molecule. As a result, changes in the fluorescence spectra of the hydrophobic molecule can be correlated to the formation of amyloid fibrils, and the suitability of the hydrophobic molecular skeleton as an amyloid probe can thus be assessed. As a result, we discovered two new amyloid probes, of which one is the well-known laser dye DCM. The grinding method can also be used for rapid preparation of novel composite materials between dyes and proteins, which can be used in materials science applications such as organic electronics and photonics.

Contact-mediated intracellular delivery of hydrophobic drugs from polymeric nanoparticles

Encapsulation of drugs in nanoparticles can enhance the accumulation of drugs in tumours, reduce toxicity toward healthy tissue, and improve pharmacokinetics compared to administration of free drug. To achieve efficient delivery and release of drugs at the target site, mechanisms of interaction between the nanoparticles and cells and the mechanism of delivery of the encapsulated drug are crucial to understand. Our aim was to determine the mechanisms for cellular uptake of a fluorescent hydrophobic model drug from poly(butylcyanoacrylate) nanoparticles. Prostate adenocarcinoma cells were incubated with Nile Red-loaded nanoparticles or free Nile Red. Uptake and intracellular distribution were evaluated by flow cytometry and confocal laser scanning microscopy. The nanoparticles mediated a higher intracellular level and more rapid uptake of encapsulated Nile Red compared to model drug administered alone. The main mechanism for delivery was not by endocytosis of nanoparticles but by nanoparticle-cell contact-mediated transfer directly to the cytosol and, to a smaller extent, release of payload from nanoparticles into the medium followed by diffusion into cells. The payload thus avoids entering the endocytic pathway, evading lysosomal degradation and instead gains direct access to intracellular targets. The nanoparticles are promising tools for efficient intracellular delivery of hydrophobic anticancer drugs; therefore, they are clinically relevant for improved cancer therapy.

Multimodal fluorescence microscopy of prion strain specific PrP deposits stained by thiophene-based amyloid ligands

The disease-associated prion protein (PrP) forms aggregates which vary in structural conformation yet share an identical primary sequence. These variations in PrP conformation are believed to manifest in prion strains exhibiting distinctly different periods of disease incubation as well as regionally specific aggregate deposition within the brain. The anionic luminescent conjugated polythiophene (LCP), polythiophene acetic acid (PTAA) has previously been used to distinguish PrP deposits associated with distinct mouse adapted strains via distinct fluorescence emission profiles from the dye. Here, we employed PTAA and 3 structurally related chemically defined luminescent conjugated oligothiophenes (LCOs) to stain brain tissue sections from mice inoculated with 2 distinct prion strains. Our results showed that in addition to emission spectra, excitation, and fluorescence lifetime imaging microscopy (FLIM) can fruitfully be assessed for optical distinction of PrP deposits associated with distinct prion strains. Our findings support the theory that alterations in LCP/LCO fluorescence are due to distinct conformational restriction of the thiophene backbone upon interaction with PrP aggregates associated with distinct prion strains. We foresee that LCP and LCO staining in combination with multimodal fluorescence microscopy might aid in detecting structural differences among discrete protein aggregates and in linking protein conformational features with disease phenotypes for a variety of neurodegenerative proteinopathies.

Toward a molecular understanding of the detection of amyloid proteins with flexible conjugated oligothiophenes

Molecular and electronic structures and optical absorption properties of oligothiophenes used for spectral assignment of amyloid deposits have been investigated for a family of probes known as luminescent conjugated oligothiophenes (LCOs). Theoretical absorption spectra have been determined using conformational averaging, combining classical molecular dynamics (MD) simulations with quantum mechanical/molecular mechanics (QM/MM) time-dependent density functional theory (TD-DFT) spectrum calculations. Theoretical absorption spectra are in excellent agreement with experiments, showing average errors below 5 nm for absorption maxima. To couple observed properties to molecular structures, a measure of planarity is defined, revealing a strong correlation between the transition wavelength of the first and dominating electronically excited state and dihedral rotations. It is shown that from this correlation, predictions can be made of the absorption properties of probes based only on information from MD trajectories. We show experimentally that red shifts observed in the excitation maxima of LCOs when bound to amyloid protein aggregates are also evident in absorption spectra. We predict that these red shifts are due to conformational restriction of the LCO in a protein binding pocket, causing a planarization of the conjugated backbone. On the basis of our studies of planarity, it is shown that such shifts are both possible and realistic.

Pentameric thiophene-based ligands that spectrally discriminate amyloid-β and tau aggregates display distinct solvatochromism and viscosity-induced spectral shifts

A wide range of neurodegenerative diseases are characterized by the deposition of multiple protein aggregates. Ligands for molecular characterization and discrimination of these pathological hallmarks are thus important for understanding their potential role in pathogenesis as well as for clinical diagnosis of the disease. In this regard, luminescent conjugated oligothiophenes (LCOs) have proven useful for spectral discrimination of amyloid-beta (Aβ) and tau neurofibrillary tangles (NFTs), two of the pathological hallmarks associated with Alzheimer's disease. Herein, the solvatochromism of a library of anionic pentameric thiophene-based ligands, as well as their ability to spectrally discriminate Aβ and tau aggregates, were investigated. Overall, the results from this study identified distinct solvatochromic and viscosity-dependent behavior of thiophene-based ligands that can be applied as indices to direct the chemical design of improved LCOs for spectral separation of Aβ and tau aggregates in brain tissue sections. The results also suggest that the observed spectral transitions of the ligands are due to their ability to conform by induced fit to specific microenvironments within the binding interface of each particular protein aggregate. We foresee that these findings might aid in the chemical design of thiophene-based ligands that are increasingly selective for distinct disease-associated protein aggregates.

Ca2+-controlled assembly for visualized detection of conformation changes of calmodulin

A new strategy has been designed for visualized detection of the conformation changes of calmodulin bound to target peptide (CaM-M13) based on the conformation sensitive property of a water-soluble conjugated polythiophene derivative (PMNT) and the electrostatic interactions of PMNT/CaM-M13. Interestingly, the direct visualized PMNT color changes under UV irradiation and the turbidity changes of samples in aqueous medium can be applied to detect the conformation changes as well as the controllable assembly of PMNT/CaM-M13 with Ca(2+) in aqueous medium. Because of the specific binding of Ca(2+), the assembly of PMNT/CaM-M13 can be applied to sense calcium as well.

QM/MM-MD simulations of conjugated polyelectrolytes: a study of luminescent conjugated oligothiophenes for use as biophysical probes

A methodological development is reported for the study of luminescence properties of conjugated polyelectrolytes, encompassing systems in which dihedral rotational barriers are easily overcome at room temperature. The components of the model include (i) a molecular mechanics (MM) force field description of the solvent in its electronic ground state as well as the chromophore in its electronic ground and excited states, (ii) a conformational sampling by means of classical molecular dynamics (MD) in the respective electronic states, and (iii) spectral response calculations by means of the quantum mechanics/molecular mechanics QM/MM approach. A detailed analysis of the combined polarization effects of the ionic moiety and the polar water solvent is presented. At an increased computational cost of 30% compared to a calculation excluding the solvent, the error in the transition wavelength of the dominant absorption band is kept as small as 1 nm as compared to the high-quality benchmark result, based largely on a QM description of the solvent. At a reduced computational cost the error of the same quantity is kept as small as 6 nm, with the cost reduction being the result of an effective description of the effects of the solvent by means of replacing the carboxylate ions with neutral hydrogens. In absorption spectroscopy, the obtained best theoretical results are in excellent agreement with the experimental benchmark measurement, regarding excitation energies as well as band intensities and profiles. In fluorescence spectroscopy, the experimental spectrum shows a vibrational progression that is not addressed by theory, but the theoretical band position is in excellent agreement with experiment, with a highly accurate description of the Stokes shift as a result.

Streptavidin sensor and its sensing mechanism based on water-soluble fluorescence conjugated polymer

Fluorescence quenching effect of water-soluble anionic conjugated polymer (CP) (poly[5-methoxy-2-(3-sulfopoxy)-1,4-phenylenevinylene] (MPS-PPV)) by [Re(N-N)(CO)3(py-CH2-NH-biotin)](PF6) [N-N=2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; py-CH2-NH-biotin=N-[(4-pyridyl) methyl] biotinamide] (Re-Biotin) and fluorescence recovery in the presence of streptavidin (or avidin) were investigated using Re-Biotin as quencher tether ligand (QTL) probe. Meanwhile, the mechanisms of fluorescence quenching and recovery were discussed to provide new thoughts to design biosensor based on water-soluble CPs. The results indicate that the sensing mechanisms of streptavidin sensor or avidin sensor, using Re-Biotin as QTL probe, are the same and stable, whether in non-buffer system (aqueous solution) or different buffer systems [0.01 mol·L(-1) phosphate buffered solution (pH=7.4), 0.1 mol·L(-1) ammonium carbonate buffered solution (pH=8.9)]. There exists specific interactions between streptavidin (or avidin) and biotin of Re-Biotin. Fluorescence quenching and recovery processes of MPS-PPV are reversible. Mechanisms of Re-Biotin quenching MPS-PPV fluorescence can be interpreted as strong electrostatic interactions and charge transferences between Re-Biotin and MPS-PPV. Fluorescence recovery mechanisms of Re-Biotin-MPS-PPV system can be interpreted as specific interactions between streptavidin (or avidin) and biotin of Re-Biotin making Re-Biotin far away from MPS-PPV. Avidin or strptavidin as re-Biotin probe can not only be quantitatively determinated, but also be identified.

Synthesis, characterization, and biological affinity of a near-infrared-emitting conjugated oligoelectrolyte

A near-IR-emitting conjugated oligoelectrolyte (COE), ZCOE, was synthesized, and its photophysical features were characterized. The biological affinity of ZCOE is compared to that of an established lipid-membrane-intercalating COE, DSSN+, which has blue-shifted optical properties making it compatible for tracking preferential sites of accumulation. ZCOE exhibits diffuse staining of E. coli cells, whereas it displays internal staining of select yeast cells which also show propidium iodide staining, indicating ZCOE is a “dead” stain for this organism. Staining of mammalian cells reveals complete internalization of ZCOE through endocytosis, as supported by colocalization with LysoTracker and late endosome markers. In all cases DSSN+ persists in the outer membranes, most likely due to its chemical structure more closely resembling a lipid bilayer.

Reduction of synaptojanin 1 accelerates Aβ clearance and attenuates cognitive deterioration in an Alzheimer mouse model

Recent studies link synaptojanin 1 (synj1), the main phosphoinositol (4,5)-biphosphate phosphatase (PI(4,5)P2-degrading enzyme) in the brain and synapses, to Alzheimer disease. Here we report a novel mechanism by which synj1 reversely regulates cellular clearance of amyloid-β (Aβ). Genetic down-regulation of synj1 reduces both extracellular and intracellular Aβ levels in N2a cells stably expressing the Swedish mutant of amyloid precursor protein (APP). Moreover, synj1 haploinsufficiency in an Alzheimer disease transgenic mouse model expressing the Swedish mutant APP and the presenilin-1 mutant ΔE9 reduces amyloid plaque load, as well as Aβ40 and Aβ42 levels in hippocampus of 9-month-old animals. Reduced expression of synj1 attenuates cognitive deficits in these transgenic mice. However, reduction of synj1 does not affect levels of full-length APP and the C-terminal fragment, suggesting that Aβ generation by β- and γ-secretase cleavage is not affected. Instead, synj1 knockdown increases Aβ uptake and cellular degradation through accelerated delivery to lysosomes. These effects are partially dependent upon elevated PI(4,5)P2 with synj1 down-regulation. In summary, our data suggest a novel mechanism by which reduction of a PI(4,5)P2-degrading enzyme, synj1, improves amyloid-induced neuropathology and behavior deficits through accelerating cellular Aβ clearance.

The structural basis for optimal performance of oligothiophene-based fluorescent amyloid ligands: conformational flexibility is essential for spectral assignment of a diversity of protein aggregates

Protein misfolding diseases are characterized by deposition of protein aggregates, and optical ligands for molecular characterization of these disease-associated structures are important for understanding their potential role in the pathogenesis of the disease. Luminescent conjugated oligothiophenes (LCOs) have proven useful for optical identification of a broader subset of disease-associated protein aggregates than conventional ligands, such as thioflavin T and Congo red. Herein, the molecular requirements for achieving LCOs able to detect nonthioflavinophilic Aβ aggregates or non-congophilic prion aggregates, as well as spectrally discriminate Aβ and tau aggregates, were investigated. An anionic pentameric LCO was subjected to chemical engineering by: 1) replacing thiophene units with selenophene or phenylene moieties, or 2) alternating the anionic substituents along the thiophene backbone. In addition, two asymmetric tetrameric ligands were generated. Overall, the results from this study identified conformational freedom and extended conjugation of the conjugated backbone as crucial determinants for obtaining superior thiophene-based optical ligands for sensitive detection and spectral assignment of disease-associated protein aggregates.

Superresolution imaging of amyloid fibrils with binding-activated probes

Protein misfolding into amyloid-like aggregates underlies many neurodegenerative diseases. Thus, insights into the structure and function of these amyloids will provide valuable information on the pathological mechanisms involved and aid in the design of improved drugs for treating amyloid-based disorders. However, determining the structure of endogenous amyloids at high resolution has been difficult. Here we employ binding-activated localization microscopy (BALM) to acquire superresolution images of α-synuclein amyloid fibrils with unprecedented optical resolution. We propose that BALM imaging can be extended to study the structure of other amyloids, for differential diagnosis of amyloid-related diseases and for discovery of drugs that perturb amyloid structure for therapy.

Highly fluorescent semiconducting polymer dots for biology and medicine

In recent years, semiconducting polymer nanoparticles have attracted considerable attention because of their outstanding characteristics as fluorescent probes. These nanoparticles, which primarily consist of π-conjugated polymers and are called polymer dots (Pdots) when they exhibit small particle size and high brightness, have demonstrated utility in a wide range of applications such as fluorescence imaging and biosensing. In this review, we summarize recent findings of the photophysical properties of Pdots which speak to the merits of these entities as fluorescent labels. This review also highlights the surface functionalization and biomolecular conjugation of Pdots, and their applications in cellular labeling, in vivo imaging, single-particle tracking, biosensing, and drug delivery. We discuss the relationship between the physical properties and performance, and evaluate the merits and limitations of the Pdot probes for certain imaging tasks and fluorescence assays. We also tackle the current challenges of Pdots and share our perspective on the future directions of the field.

Luminescent conjugated oligothiophenes for sensitive fluorescent assignment of protein inclusion bodies

Small hydrophobic ligands identifying intracellular protein deposits are of great interest, as protein inclusion bodies are the pathological hallmark of several degenerative diseases. Here we report that fluorescent amyloid ligands, termed luminescent conjugated oligothiophenes (LCOs), rapidly and with high sensitivity detect protein inclusion bodies in skeletal muscle tissue from patients with sporadic inclusion body myositis (s-IBM). LCOs having a conjugated backbone of at least five thiophene units emitted strong fluorescence upon binding, and showed co-localization with proteins reported to accumulate in s-IBM protein inclusion bodies. Compared with conventional amyloid ligands, LCOs identified a larger fraction of immunopositive inclusion bodies. When the conjugated thiophene backbone was extended with terminal carboxyl groups, the LCO revealed striking spectral differences between distinct protein inclusion bodies. We conclude that 1) LCOs are sensitive, rapid and powerful tools for identifying protein inclusion bodies and 2) LCOs identify a wider range of protein inclusion bodies than conventional amyloid ligands.

Efficient amyloid A clearance in the absence of immunoglobulins and complement factors

Amyloid A amyloidosis is a protein misfolding disease characterized by deposition of extracellular aggregates derived from the acute-phase reactant serum amyloid A protein. If untreated, amyloid A amyloidosis leads to irreversible damage of various organs, including the kidneys, liver, and heart. Amyloid A deposits regress upon reduction of serum amyloid A concentration, indicating that the amyloid can be efficiently cleared by natural mechanisms. Clearance was proposed to be mediated by humoral immune responses to amyloid. Here, we report that amyloid clearance in mice lacking complement factors 3 and 4 (C3C4(-/-)) was equally efficient as in wild-type mice (C57BL/6), and was only slightly delayed in agammaglobulinemic mice (J(H-/-)). Hence, antibodies or complement factors are not necessary for natural amyloid clearance, implying the existence of alternative physiological pathways for amyloid removal.

Prions and lymphoid organs: solved and remaining mysteries

Prion colonization of secondary lymphoid organs (SLOs) is a critical step preceding neuroinvasion in prion pathogenesis. Follicular dendritic cells (FDCs), which depend on both tumor necrosis factor receptor 1 (TNFR1) and lymphotoxin β receptor (LTβR) signaling for maintenance, are thought to be the primary sites of prion accumulation in SLOs. However, prion titers in RML-infected TNFR1 (-/-) lymph nodes and rates of neuroinvasion in TNFR1 (-/-) mice remain high despite the absence of mature FDCs. Recently, we discovered that TNFR1-independent prion accumulation in lymph nodes relies on LTβR signaling. Loss of LTβR signaling in TNFR1 (-/-) lymph nodes coincided with the de-differentiation of high endothelial venules (HEVs)-the primary sites of lymphocyte entry into lymph nodes. These findings suggest that HEVs are the sites through which prions initially invade lymph nodes from the bloodstream. Identification of HEVs as entry portals for prions clarifies a number of previous observations concerning peripheral prion pathogenesis. However, a number of questions still remain: What is the mechanism by which prions are taken up by HEVs? Which cells are responsible for delivering prions to lymph nodes? Are HEVs the main entry site for prions into lymph nodes or do alternative routes also exist? These questions and others are considered in this article.