Synthesis and Characterization of Thiophene-based Donor-Acceptor-Donor Heptameric Ligands for Spectral Assignment of Polymorphic Amyloid-β Deposits

Thiophene-Based Ligands for Specific Assignment of Distinct Aβ Pathologies in Alzheimer's Disease

Aggregated species of amyloid-β (Aβ) are one of the pathological hallmarks in Alzheimer's disease (AD), and ligands that selectively target different Aβ deposits are of great interest. In this study, fluorescent thiophene-based ligands have been used to illustrate the features of different types of Aβ deposits found in AD brain tissue. A dual-staining protocol based on two ligands, HS-276 and LL-1, with different photophysical and binding properties, was developed and applied on brain tissue sections from patients affected by sporadic AD or familial AD associated with the PSEN1 A431E mutation. When binding to Aβ deposits, the ligands could easily be distinguished for their different fluorescence, and distinct staining patterns were revealed for these two types of AD. In sporadic AD, HS-276 consistently labeled all immunopositive Aβ plaques, whereas LL-1 mainly stained cored and neuritic Aβ deposits. In the PSEN1 A431E cases, each ligand was binding to specific types of Aβ plaques. The ligand-labeled Aβ deposits were localized in distinct cortical layers, and a laminar staining pattern could be seen. Biochemical characterization of the Aβ aggregates in the individual layers also showed that the variation of ligand binding properties was associated with certain Aβ peptide signatures. For the PSEN1 A431E cases, it was concluded that LL-1 was binding to cotton wool plaques, whereas HS-276 mainly stained diffuse Aβ deposits. Overall, our findings showed that a combination of ligands was essential to identify distinct aggregated Aβ species associated with different forms of AD.

Spectral Fluorescence Pathology of Protein Misfolding Disorders

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

Multifunctional Photoelectroactive Materials for Optoelectronic Applications Based on Thieno[3,4-b]pyrazines and Thieno[1,2,5]thiadiazoles

In this study, we introduce a novel family of symmetrical thiophene-based small molecules with a Donor-Acceptor-Donor structure. These compounds feature three different acceptor units: benzo[c][1,2,5]thiadiazole (Bz), thieno[3,4-b]pyrazine (Pz), and thieno[1,2,5]thiadiazole (Tz), coupled with electron donor units based on a carbazole-thiophene derivative. Using Density Functional Theory (DFT), we investigate how the molecular geometry and strength of the central acceptor unit impact the redox and spectroscopic properties. Notably, the incorporation of Pz and Tz moieties induces a significant redshift in the absorption and emission spectra, which extend into the near-infrared (NIR) region, simultaneously reducing their energy gaps (~1.4-1.6 eV). This shift is attributed to the increased coplanarity of the oligomeric inner core, both in the ground (S0 ) and excited (S1 ) states, due to the enhanced quinoidal character as supported by bond-length alternation (BLA) analysis. These structural changes promote better π-electron delocalization and facilitate photoinduced charge transfer processes in optoelectronic devices. Notably, we show that Pz- and Tz-containing molecules exhibit NIR electrochromic behavior and present ambivalent character in bulk heterojunction (BHJ) solar cells. Finally, theoretical calculations suggest that these molecules could serve as effective two-photon absorption (2PA) probes, further expanding their potential in optoelectronic applications.

Recent Advances in the Development of Near-Infrared Fluorescent Probes for the in Vivo Brain Imaging of Amyloid-β Species in Alzheimer's Disease

The deposition of β-amyloid (Aβ) plaques in the parenchymal and cortical regions of the brain of Alzheimer's disease (AD) patients is considered the foremost pathological hallmark of the disease. The early diagnosis of AD is paramount in order to effective management and treatment of the disease. Developing near-infrared fluorescence (NIRF) probes targeting Aβ species is a potential and attractive approach suitable for the early and timely diagnosis of AD. The advantages of the NIRF probes over other tools include real-time detection, higher sensitivity, resolution, comparatively inexpensive experimental setup, and noninvasive nature. Currently, enormous progress is being observed in the development of NIRF probes for the in vivo imaging of Aβ species. Several strategies, i.e., the classical push-pull approach, "turn-on" effect, aggregation-induced emission (AIE), and resonance energy transfer (RET), have been exploited for development. We have outlined and discussed the recently emerged NIRF probes with different design strategies targeting Aβ species for ex vivo and in vivo imaging. We believe that understanding the recent development enables the prospect of the rational design of probes and will pave the way for developing future novel probes for early diagnosis of AD.

Fluorescent optotracers for bacterial and biofilm detection and diagnostics

Effective treatment of bacterial infections requires methods that accurately and quickly identify which antibiotic should be prescribed. This review describes recent research on the development of optotracing methodologies for bacterial and biofilm detection and diagnostics. Optotracers are small, chemically well-defined, anionic fluorescent tracer molecules that detect peptide- and carbohydrate-based biopolymers. This class of organic molecules (luminescent conjugated oligothiophenes) show unique electronic, electrochemical and optical properties originating from the conjugated structure of the compounds. The photophysical properties are further improved as donor-acceptor-donor (D-A-D)-type motifs are incorporated in the conjugated backbone. Optotracers bind their biopolymeric target molecules via electrostatic interactions. Binding alters the optical properties of these tracer molecules, shown as altered absorption and emission spectra, as well as ON-like switch of fluorescence. As the optotracer provides a defined spectral signature for each binding partner, a fingerprint is generated that can be used for identification of the target biopolymer. Alongside their use for in situ experimentation, optotracers have demonstrated excellent use in studies of a number of clinically relevant microbial pathogens. These methods will find widespread use across a variety of communities engaged in reducing the effect of antibiotic resistance. This includes basic researchers studying molecular resistance mechanisms, academia and pharma developing new antimicrobials targeting biofilm infections and tests to diagnose biofilm infections, as well as those developing antibiotic susceptibility tests for biofilm infections (biofilm-AST). By iterating between the microbial world and that of plants, development of the optotracing technology has become a prime example of successful cross-feeding across the boundaries of disciplines. As optotracers offers a capacity to redefine the way we work with polysaccharides in the microbial world as well as with plant biomass, the technology is providing novel outputs desperately needed for global impact of the threat of antimicrobial resistance as well as our strive for a circular bioeconomy.

Thiophene-Based Ligands: Design, Synthesis and Their Utilization for Optical Assignment of Polymorphic-Disease-Associated Protein Aggregates

The development of ligands for detecting protein aggregates is of great interest, as these aggregated proteinaceous species are the pathological hallmarks of several devastating diseases, including Alzheimer's disease. In this regard, thiophene-based ligands have emerged as powerful tools for fluorescent assessment of these pathological entities. The intrinsic conformationally sensitive photophysical properties of poly- and oligothiophenes have allowed optical assignment of disease-associated protein aggregates in tissue sections, as well as real-time in vivo imaging of protein deposits. Herein, we recount the chemical evolution of different generations of thiophene-based ligands, and exemplify their use for the optical distinction of polymorphic protein aggregates. Furthermore, the chemical determinants for achieving a superior fluorescent thiophene-based ligand, as well as the next generation of thiophene-based ligands targeting distinct aggregated species are described. Finally, the directions for future research into the chemical design of thiophene-based ligands that can aid in resolving the scientific challenges around protein aggregation diseases are discussed.

Thiophene-Based Ligands for Histological Multiplex Spectral Detection of Distinct Protein Aggregates in Alzheimer's Disease

The aggregation and accumulation of proteins in the brain is the defining feature of many devastating neurodegenerative diseases. The development of fluorescent ligands that bind to these accumulations, or deposits, is essential for the characterization of these neuropathological lesions. We report the synthesis of donor-acceptor-donor (D-A-D) thiophene-based ligands with different emission properties. The D-A-D ligands displayed selectivity towards distinct disease-associated protein deposits in histological sections from postmortem brain tissue of individuals affected by Alzheimer's disease (AD). The ability of the ligands to selectively identify AD-associated pathological alterations, such as deposits composed of aggregates of the amyloid-β (Aβ) peptide or tau, was reduced when the chemical composition of the ligands was altered. When combining the D-A-D ligands with conventional thiophene-based ligands, superior spectral separation of distinct protein aggregates in AD tissue sections was obtained. Our findings provide the structural and functional basis for the development of new fluorescent ligands that can distinguish between aggregated proteinaceous species, as well as offer novel strategies for developing multiplex fluorescence detection of protein aggregates in tissue sections.

Contrast Enhanced Magnetic Resonance Imaging of Amyloid-β Plaques in a Murine Alzheimer’s Disease Model

Background: Early detection of amyloid-β (Aβ) aggregates is a critical step to improve the treatment of Alzheimer’s disease (AD) because neuronal damage by the Aβ aggregates occurs before clinical symptoms are apparent. We have previously shown that luminescent conjugated oligothiophenes (LCOs), which are highly specific towards protein aggregates of Aβ, can be used to fluorescently label amyloid plaque in living rodents. Objective: We hypothesize that the LCO can be used to target gadolinium to the amyloid plaque and hence make the plaque detectable by T1-weighted magnetic resonance imaging (MRI). Methods: A novel LCO-gadolinium construct was synthesized to selectively bind to Aβ plaques and give contrast in conventional T1-weighted MR images after intravenous injection in Tg-APPSwe mice. Results: We found that mice with high plaque-burden could be identified using the LCO-Gd constructs by conventional MRI. Conclusion: Our study shows that MR imaging of amyloid plaques is challenging but feasible, and hence contrast-mediated MR imaging could be a valuable tool for early AD detection.

Proteophenes - Amino Acid Functionalized Thiophene-based Fluorescent Ligands for Visualization of Protein Deposits in Tissue Sections with Alzheimer's Disease Pathology

Protein deposits composed of specific proteins or peptides are associated with several neurodegenerative diseases and fluorescent ligands able to detect these pathological hallmarks are vital. Here, we report the synthesis of a class of thiophene-based ligands, denoted proteophenes, with different amino acid side-chain functionalities along the conjugated backbone, which display selectivity towards specific disease-associated protein aggregates in tissue sections with Alzheimer's disease (AD) pathology. The selectivity of the ligands towards AD associated pathological hallmarks, such as aggregates of the amyloid-β (Aβ) peptide or tau filamentous inclusions, was highly dependent on the chemical nature of the amino acid functionality, as well as on the location of the functionality along the pentameric thiophene backbone. Finally, the concept of synthesizing donor-acceptor-donor proteophenes with distinct photophysical properties was shown. Our findings provide the structural and functional basis for the development of new thiophene-based ligands that can be utilized for optical assignment of different aggregated proteinaceous species in tissue sections.

Alpha-Synuclein Strain Variability in Body-First and Brain-First Synucleinopathies

Pathogenic alpha-synuclein (asyn) aggregates are a defining feature of neurodegenerative synucleinopathies, which include Parkinson's disease, Lewy body dementia, pure autonomic failure and multiple system atrophy. Early accurate differentiation between these synucleinopathies is challenging due to the highly heterogeneous clinical profile at early prodromal disease stages. Therefore, diagnosis is often made in late disease stages when a patient presents with a broad range of motor and non-motor symptoms easing the differentiation. Increasing data suggest the clinical heterogeneity seen in patients is explained by the presence of distinct asyn strains, which exhibit variable morphologies and pathological functions. Recently, asyn seed amplification assays (PMCA and RT-QuIC) and conformation-specific ligand assays have made promising progress in differentiating between synucleinopathies in prodromal and advanced disease stages. Importantly, the cellular environment is known to impact strain morphology. And, asyn aggregate pathology can propagate trans-synaptically along the brain-body axis, affecting multiple organs and propagating through multiple cell types. Here, we present our hypothesis that the changing cellular environments, an asyn seed may encounter during its brain-to-body or body-to-brain propagation, may influence the structure and thereby the function of the aggregate strains developing within the different cells. Additionally, we aim to review strain characteristics of the different synucleinopathies in clinical and preclinical studies. Future preclinical animal models of synucleinopathies should investigate if asyn strain morphology is altered during brain-to-body and body-to-brain spreading using these seeding amplification and conformation-specific assays. Such findings would greatly deepen our understanding of synucleinopathies and the potential link between strain and phenotypic variability, which may enable specific diagnosis of different synucleinopathies in the prodromal phase, creating a large therapeutic window with potential future applications in clinical trials and personalized therapeutics.

Structural Properties Dictating Selective Optotracer Detection of S. aureus

Optotracers are conformation‐sensitive fluorescent tracer molecules that detect peptide‐ and carbohydrate‐based biopolymers. Their binding to bacterial cell walls allows selective detection and visualisation of Staphylococcus aureus (S. aureus). Here, we investigated the structural properties providing optimal detection of S. aureus. We quantified spectral shifts and fluorescence intensity in mixes of bacteria and optotracers, using automatic peak analysis, cross‐correlation, and area‐under‐curve analysis. We found that the length of the conjugated backbone and the number of charged groups, but not their distribution, are important factors for selective detection of S. aureus. The photophysical properties of optotracers were greatly improved by incorporating a donor‐acceptor‐donor (D‐A‐D)‐type motif in the conjugated backbone. With significantly reduced background and binding‐induced on‐switch of fluorescence, these optotracers enabled real‐time recordings of S. aureus growth. Collectively, this demonstrates that chemical structure and photophysics are key tunable characteristics in the development of optotracers for selective detection of bacterial species.

Synthesis and Characterization of Thiophene-based Donor-Acceptor-Donor Heptameric Ligands for Spectral Assignment of Polymorphic Amyloid-β Deposits

Protein deposits are associated with many devastating diseases and fluorescent ligands able to visualize these pathological entities are essential. Here, we report the synthesis of thiophene‐based donor–acceptor–donor heptameric ligands that can be utilized for spectral assignment of distinct amyloid‐β (Aβ) aggregates, one of the pathological hallmarks in Alzheimer's disease. The ability of the ligands to selectively distinguish Aβ deposits was abolished when the chemical composition of the ligands was altered. Our findings provide the structural and functional basis for the development of new fluorescent ligands that can distinguish between aggregated proteinaceous species consisting of the same peptide or protein. In addition, such ligands might aid in interpreting the potential role of polymorphic Aβ deposits in the pathogenesis of Alzheimer's disease.