Sensitive and continuous screening of inhibitors of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) at single SPR chips

Electrochemiluminescence and electrochemical dual-mode detection of BACE1 activity based on the assembly of peptide and luminol co-functionalized silver nanoparticles induced by cucurbit[8]uril

A novel electrochemiluminescence (ECL) and electrochemical dual-mode sensor was developed for detecting the activity of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) and screening its inhibitor. Specifically, the adamantane (ADA)-functionalized peptide (P1), a designed substrate peptide for BACE1, was immobilized on the electrode surface via host-guest interaction between β-cyclodextrin (β-CD) and ADA. The aggregation of the peptide (P2) and luminol co-functionalized silver nanoparticles could be induced by cucurbit [8]uril (CB[8] due to the ability of CB[8] to accommodate two aromatic residues simultaneously. The obtained (CB[8]-P2-AgNPs-luminol)n aggregates with both ECL and electrochemical activity, used as the dual-mode signal probe, could be captured to the N-terminal of P1 through CB[8]. Once the substrate P1 was cleaved by BACE1, the probe-binding polypeptide fragment detached from the electrode surface, resulting in a remarkable decrease in the ECL and electrochemical signals. Taking advantage of the signal amplification function of the signal probe, the sensitive dual-mode assay for BACE1 activity can be achieved with the low detection limits of 33.11 pM for ECL and 53.19 pM for electrochemical mode. The superior analytical performance of this novel dual-mode sensor toward BACE1 activity suggested the promising application in early diagnosis of Alzheimer's disease (AD).

Platinum-alginate-chitosan nanobioconjugate decorated carbon backbone layered biosensor for highly sensitive and selective detection of BACE-1

Chitosan (CS) and sodium alginates (SA) have been revealed for the design of layer-by-layer (LbL) assembly to develop pharmaceutical dosage forms owing to their versatile characteristics. Recently, the preference for unique LbL assemblies in biosensor development has offered the modified performance for detection interest analyte. Beta (β)-site amyloid precursor protein-cleaving enzyme 1 (BACE-1) is a pivotal biomarker of Alzheimer's disease (AD) and demands high sensitivity and selective identification for the early-stage diagnosis. In this work, CS-SA‑platinum nanoparticles (Pt-NPs) LbL-based nanobioconjugate decorated carbon backbone-layered affinity surface plasmon resonance (Anti-BACE-1-LbL@Pt-NPs-GO-SPR) biosensor was designed for extremely sensitive and selective sensing of BACE-1. Primarily, LbL nanobioconjugate was synthesized by integrating cationic 'CS' and anionic 'SA' on the face of green-made Pt-NPs. Here, the amines of 'CS' offers a softer surface for anti-BACE-1 immobilization that leads to maintaining the bio-functionality of bioreceptors, provides the specific orientation for bioreceptors, etc. As well, the synthesized graphene oxide (GO, 2D carbon backbone) was preferred as non-plasmonic nanomaterials due to their plenty of merits in biosensors. Here, the designed biosensor provides a low detection limit (LOD) of 5.63 fg/mL and a wide linear range from 5 fg/mL to 150 ng/mL. Moreover, selectivity and real-time analyses in spiked samples exhibited their practical usefulness in complex specimens for BACE-1 detection. Hence, the decorating of antibody-immobilized CS-SA coated Pt-NPs nanobioconjugate on the face of GO has various benefits mainly extremely sensitive and superb specificity. Overall, CS and SA coated Pt-NPs bioconjugate decorated GO layered SPR biosensors can provide highly sensitive, selectivity, rapid, label-free, etc. detection of BACE-1 in clinical samples.

Biosensors Based on the Binding Events of Nitrilotriacetic Acid-Metal Complexes

Molecular immobilization and recognition are two key events for the development of biosensors. The general ways for the immobilization and recognition of biomolecules include covalent coupling reactions and non-covalent interactions of antigen-antibody, aptamer-target, glycan-lectin, avidin-biotin and boronic acid-diol. Tetradentate nitrilotriacetic acid (NTA) is one of the most common commercial ligands for chelating metal ions. The NTA-metal complexes show high and specific affinity toward hexahistidine tags. Such metal complexes have been widely utilized in protein separation and immobilization for diagnostic applications since most of commercialized proteins have been integrated with hexahistidine tags by synthetic or recombinant techniques. This review focused on the development of biosensors with NTA-metal complexes as the binding units, mainly including surface plasmon resonance, electrochemistry, fluorescence, colorimetry, surface-enhanced Raman scattering spectroscopy, chemiluminescence and so on.

A simple and sensitive electrochemical sensor for the detection of peptidase activity

In this work, a simple and sensitive electrochemical sensor was proposed for the detection of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) activity. Firstly, the BACE1 specific peptide was modified onto the Au electrode to graft a single-strand DNA with polycytosine DNA sequence (dC₁₂) via amide bonding between peptide and dC₁₂. Because the dC₁₂ is abundant in phosphate groups, thus it can react with molybdate to form redox molybdophosphate, which can generate electrochemical current. Using BACE1 as a model peptidase, the proposed sensor shows a linear response range from 1 to 15 U/mL and limit of detection down to 0.05 U/mL. The sensor displays good performance for the BACE1 activity detection in human serum samples, which may have potential applications in the clinical diagnostics of Alzheimer's disease.

Advancement in Analytical Techniques for Determining the Activity of β-Site Amyloid Precursor Protein Cleaving Enzyme 1

Alzheimer's disease (AD) is a degenerative disease of the central nervous system. The pathogenesis is still not fully clear. One of the main histopathological manifestations is senile plaques formed by β-amyloid (Aβ) accumulation. Aβ is generated from the sequential proteolysis of amyloid precursor protein (APP) by β-secretase [i.e. β-site APP cleaving enzyme 1 (BACE1)] and γ-secretase, with a rate-limiting step controlled by BACE1 activity. Therefore, inhibiting BACE1 activity has become a potential therapeutic strategy for AD. The development of reliable detection methods for BACE1 activity plays an important role in early diagnosis of AD and evaluation of the therapeutic effect of new drugs for AD. This article has reviewed the recent advances in BACE1 activity detection techniques. The challenges of applying these analysis techniques to early clinical diagnosis of AD and development trends of the detection techniques have been prospected.

A study of protein–drug interaction based on solvent-induced protein aggregation by fluorescence correlation spectroscopy

Based on the inhibition of protein aggregation by drugs in organic solvent systems, we developed an effective method to study protein–drug interaction by fluorescence correlation spectroscopy.

Self-Assembly of Superquenched Gold Nanoparticle Nanosensors for Lighting up BACE-1 in Live Cells

The β-site amyloid precursor protein-cleaving enzyme 1 (BACE-1) plays a key role in Alzheimer's disease (AD) pathogenesis and is regarded as a valuable biomarker for AD diagnosis and treatment. The reported BACE-1 assay often suffers from laborious procedures, large sample consumption, and unsatisfactory sensitivity with high background signals. Herein, we report the self-assembly of superquenched gold nanoparticle (AuNP) nanosensors for lighting up the BACE-1 in live cells. Through the self-assembly of both fluorophore-labeled peptide probes and quencher-labeled assistant DNAs on the surface of a single AuNP, a superquenched AuNP nanoprobe is obtained with a high quenching efficiency of 98.37% and a near-zero background fluorescence. The presence of target BACE-1 induces a distinct fluorescence signal as a result of the BACE-1-catalyzed cleavage of peptide probe and the subsequent release of abundant fluorophore moieties from the AuNP nanoprobe. The fluorescence signal can be directly visualized by single-molecule imaging and easily quantified by single-molecule counting. This nanosensor involves only a single nanoprobe for the one-step homogeneous detection of the BACE-1 activity without the requirements of any antibodies and separation steps, and it possesses good selectivity and high sensitivity with a low detection limit of 26.48 pM. Moreover, it can be employed to screen BACE-1 inhibitors and analyze kinetic parameters. Especially, this nanoprobe possesses good stability and can be easily transferred into live cells for the real-time imaging of cellular BACE-1 activity, providing a new platform for BACE-1-associated research and early diagnosis of Alzheimer's disease.

Screening Platform Based on Inductively Coupled Plasma Mass Spectrometry for β-Site Amyloid Protein Cleaving Enzyme 1 (BACE1) Inhibitors

β-Site amyloid protein cleaving enzyme 1 (BACE1) is a promising therapeutic target for developing inhibitors to alleviate Alzheimer's disease (AD). Herein, we established an inductively coupled plasma mass spectrometry (ICPMS)-based inhibitor screening platform. A biotin-labeled lanthanide-coded peptide probe (LCPP; biotin-PEG₂-EVNLDAEC-DOTA-Ln) was designed to determine the activity of BACE1 and evaluate the degree of inhibition of inhibitors. The platform was first validated with two commercially available inhibitors (BSI I and BSI IV) in terms of IC₅₀ values and then applied to two newly designed inhibitors (inhibitors II and III) based on the crystal structure of BACE1 interacting with inhibitor I, and each of them contained an acylguanidine core structure. We found that their inhibition effects were improved as evaluated by the sensitive and accurate LCPP-ICPMS platform, demonstrating its ability for new drug screening.

Biosensing Amplification by Hybridization Chain Reaction on Phase-Sensitive Surface Plasmon Resonance

Surface Plasmon Resonance (SPR) is widely used in biological and chemical sensing with fascinating properties. However, the application of SPR to detect trace targets is hampered by non-specific binding and poor signal. A variety of approaches for amplification have been explored to overcome this deficiency including DNA aptamers as versatile target detection tools. Hybridization chain reaction (HCR) is a high-efficiency enzyme-free DNA amplification method operated at room temperature, in which two stable species of DNA hairpins coexist in solution until the introduction of the initiator strand triggers a cascade of hybridization events. At an optimal salt condition, as the concentrations of H1 and H2 increased, the HCR signals were enhanced, leading to signal amplification reaching up to 6.5-fold of the detection measure at 30 min. This feature enables DNA to act as an amplifying transducer for biosensing applications to provide an enzyme-free alternative that can easily detect complex DNA sequences. Improvement of more diverse recognition events can be achieved by integrating HCR with a phase-sensitive SPR (pSPR)-tested aptamer stimulus. This work seeks to establish pSPR aptamer system for highly informative sensing by means of an amplification HCR. Thus, combining pSPR and HCR technologies provide an expandable platform for sensitive biosensing.

Self-assembled biotin-phenylalanine nanoparticles for the signal amplification of surface plasmon resonance biosensors

A strategy for amplifying the signal of surface plasmon resonance (SPR) biosensors is reported. Biotinylated phenylalanine (Biotin-Phe) monomers were rapidly self-assembled into nanoparticles in a mild environment. The self-assembled nanoparticles were then used as the carriers of streptavidin-antibody complexes by the streptavidin-biotin interaction. The signal was amplified because of the high molecular weight of the nanoparticle-streptavidin-antibody conjugate. With prostate-specific antigen as a model analyte, the target concentration as low as 1 pg mL^-1 was readily measured. The results of the nanoparticle-enhanced SPR biosensor for analysis of serum samples are well consistent with those achieved by the enzyme-linked immunosorbent assays. This work is valuable for designing of various optical and electronic biosensors through the streptavidin-biotin interaction. Graphical abstract.

Identification of differentially expressed genes in pancreatic ductal adenocarcinoma and normal pancreatic tissues based on microarray datasets

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive malignant tumor with rapid progression and poor prognosis. In the present study, 11 high‑quality microarray datasets, comprising 334 tumor samples and 151 non‑tumor samples from the Gene Expression Omnibus, were screened, and integrative meta‑analysis of expression data was used to identify gene signatures that differentiate between PDAC and normal pancreatic tissues. Following the identification of differentially expressed genes (DEGs), two‑way hierarchical clustering analysis was performed for all DEGs using the gplots package in R software. Hub genes were then determined through protein‑protein interaction network analysis using NetworkAnalyst. In addition, functional annotation and pathway enrichment analyses of all DEGs were conducted in the Database for Annotation, Visualization, and Integrated Discovery. The expression levels and Kaplan‑Meier analysis of the top 10 upregulated and downregulated genes were verified in The Cancer Genome Atlas. A total of 1,587 DEGs, including 1,004 upregulated and 583 downregulated genes, were obtained by comparing PDAC with normal tissues. Of these, hematological and neurological expressed 1, integrin subunit α2 (ITGA2) and S100 calcium‑binding protein A6 (S100A6) were the top upregulated genes, and kinesin family member 1A, Dymeclin and β‑secretase 1 were the top downregulated genes. Reverse transcription‑quantitative PCR was performed to examine the expression levels of S100A6, KRT19 and GNG7, and the results suggested that S100A6 was significantly upregulated in PDAC compared with normal pancreatic tissues. ITGA2 overexpression was significantly associated with shorter overall survival times, whereas family with sequence similarity 46 member C overexpression was strongly associated with longer overall survival times. In addition, network‑based meta‑analysis confirmed growth factor receptor‑bound protein 2 and histone deacetylase 5 as pivotal hub genes in PDAC compared with normal tissue. In conclusion, the results of the present meta‑analysis identified PDAC‑related gene signatures, providing new perspectives and potential targets for PDAC diagnosis and treatment.

Direct quantification of cancerous exosomes via surface plasmon resonance with dual gold nanoparticle-assisted signal amplification

Sensitive detection of cancerous exosomes is critical to early diseases diagnosis and prognosis. Herein, a sensitive aptasensor was demonstrated for exosomes detection by surface plasmon resonance (SPR) with dual gold nanoparticle (AuNP)-assisted signal amplification. Dual nanoparticle amplification was achieved by controlled hybridization attachment of AuNPs resulting from electronic coupling between the Au film and AuNPs, as well as coupling effects in plasmonic nanostructures. By blocking the Au film surface with 11-Mercapto-1 -undecanol (MCU), nonspecific adsorption of AuNPs onto the SPR chip surface was suppressed and regeneration of the SPR sensor was realized. This method was highly sensitive and we have achieved the limit of detection (LOD) down to 5 × 10³ exosomes/mL, which showed a 10⁴-fold improvement in LOD compared to commercial ELISA. Moreover, the SPR sensor had the capability to differentiate the exosomes secreted by MCF-7 breast cancer cells and MCF-10A normal breast cells. Furthermore, the SPR sensor could effectively detect the exosomes in 30% fetal bovine serum. The work provides a sensitive and efficient quantification approach to detect cancerous exosomes and offers an avenue toward future diagnosis and comprehensive studies of exosomes.

An αIIbβ3- and phosphatidylserine (PS)-binding recombinant fusion protein promotes PS-dependent anticoagulation and integrin-dependent antithrombosis

Blood platelets are required for normal wound healing, but they are also involved in thrombotic diseases, which are usually managed with anticoagulant drugs. Here, using genetic engineering, we coupled the disintegrin protein echistatin, which specifically binds to the platelet integrin α_IIbβ₃ receptor, to annexin V, which binds platelet membrane-associated phosphatidylserine (PS), to create the bifunctional antithrombotic molecule recombinant echistatin-annexin V fusion protein (r-EchAV). Lipid binding and plasma coagulation studies revealed that r-EchAV dose-dependently binds PS and delays plasma clotting time. Moreover, r-EchAV inhibited ADP-induced platelet aggregation in a dose-dependent manner and exhibited potent antiplatelet aggregation effects. r-EchAV significantly prolonged activated partial thromboplastin time, suggesting that it primarily affects the in vivo coagulation pathway. Flow cytometry results indicated that r-EchAV could effectively bind to the platelet α_IIbβ₃ receptor, indicating that r-EchAV retains echistatin's receptor-recognition region. In vivo experiments in mice disclosed that r-EchAV significantly prolongs bleeding time, indicating a significant anticoagulant effect in vivo resulting from the joint binding of r-EchAV to both PS and the α_IIbβ₃ receptor. We also report optimization of the r-EchAV production steps and its purification for high purity and yield. Our findings indicate that r-EchAV retains the active structural regions of echistatin and annexin V and that the whole molecule exhibits multitarget-binding ability arising from the dual functions of echistatin and annexin V. Therefore, r-EchAV represents a new class of anticoagulant that specifically targets the anionic membrane-associated coagulation enzyme complexes at thrombogenesis sites and may be a potentially useful antithrombotic agent.

Capillary electrophoresis integrated immobilized enzyme reactor for kinetic and inhibition assays of β-secretase as the Alzheimer's disease drug target

Capillary electrophoresis integrated immobilized enzyme reactors are becoming an increasingly popular alternative for enzyme kinetic and inhibition assays thanks to their unique set of features including cost effectiveness, repeated use of the enzyme, minuscule sample consumption, rapid analysis time and easy automation. In this work we present the development and application of a capillary electrophoresis integrated immobilized enzyme reactor based on magnetic particles for kinetic and inhibition studies of β-secretase, a key enzyme in the development of Alzheimer's disease and a promising drug target. We document the optimization of the immobilization procedure, characterization of immobilized β-secretase, optimization of a mutually compatible incubation protocol and separation method as well as the production of the capillary electrophoresis integrated immobilized enzyme reactor. The applicability of the capillary electrophoresis integrated immobilized enzyme reactor was demonstrated by kinetic assay with an unlabelled substrate and by inhibition assays using three structurally different reference inhibitors. The resulting kinetic and inhibition parameters clearly support the applicability of the herein presented method as well as document the fundamental phenomena which need to be taken in account when comparing the results to other methods.

Multidisciplinary approaches for targeting the secretase protein family as a therapeutic route for Alzheimer's disease

The continual increase of the aging population worldwide renders Alzheimer's disease (AD) a global prime concern. Several attempts have been focused on understanding the intricate complexity of the disease's development along with the on- andgoing search for novel therapeutic strategies. Incapability of existing AD drugs to effectively modulate the pathogenesis or to delay the progression of the disease leads to a shift in the paradigm of AD drug discovery. Efforts aimed at identifying AD drugs have mostly focused on the development of disease-modifying agents in which effects are believed to be long lasting. Of particular note, the secretase enzymes, a group of proteases responsible for the metabolism of the β-amyloid precursor protein (βAPP) and β-amyloid (Aβ) peptides production, have been underlined for their promising therapeutic potential. This review article attempts to comprehensively cover aspects related to the identification and use of drugs targeting the secretase enzymes. Particularly, the roles of secretases in the pathogenesis of AD and their therapeutic modulation are provided herein. Moreover, an overview of the drug development process and the contribution of computational (in silico) approaches for facilitating successful drug discovery are also highlighted along with examples of relevant computational works. Promising chemical scaffolds, inhibitors, and modulators against each class of secretases are also summarized herein. Additionally, multitarget secretase modulators are also taken into consideration in light of the current growing interest in the polypharmacology of complex diseases. Finally, challenging issues and future outlook relevant to the discovery of drugs targeting secretases are also discussed.

Dual-Channel Surface Plasmon Resonance for Quantification of ApoE Gene and Genotype Discrimination in Unamplified Genomic DNA Extracts

Identification of gene variation is of great importance for attaining information related to disease susceptibility. A highly sensitive and specific surface plasmon resonance (SPR) method for quantification of the apoE gene and genotype discrimination was demonstrated. The complementary sequences with the specific recognition sites of GCGC bases upon hybridization to the preimmobilized biotinylated probes could be cleaved by the restriction enzyme HhaI, while the existence of the single-base mismatch (GTGC) prevented the cleavage reaction. In both cases, the incorporation of streptavidin increased the sensitivity of the SPR assay, and the detection levels of 10 fM and 50 fM for the complementary and single-base mismatched sequences were attained, respectively. The sensing protocol is simple, label-free, and quantitative, thus avoiding the complicated polymerase chain reaction (PCR) amplification procedures. The proposed method serves as a viable means for facile and sensitive analyses of apoE genes in four unamplified genomic DNA extracts.

A peptide–WS2 nanosheet based biosensing platform for determination of β-secretase and screening of its inhibitors

β-Secretase (BACE1) is an important drug target in the treatment of Alzheimer's disease (AD).

Enzyme catalysis-electrophoresis titration for multiplex enzymatic assay via moving reaction boundary chip

In this work, we developed the concept of enzyme catalysis-electrophoresis titration (EC-ET) under ideal conditions, the theory of EC-ET for multiplex enzymatic assay (MEA), and a related method based on a moving reaction boundary (MRB) chip with a collateral channel and cell phone imaging. As a proof of principle, the model enzymes horseradish peroxidase (HRP), laccase and myeloperoxidase (MPO) were chosen for the tests of the EC-ET model. The experiments revealed that the EC-ET model could be achieved via coupling EC with ET within a MRB chip; particularly the MEA analyses of catalysis rate, maximum rate, activity, Km and Kcat could be conducted via a single run of the EC-ET chip, systemically demonstrating the validity of the EC-ET theory. Moreover, the developed method had these merits: (i) two orders of magnitude higher sensitivity than a fluorescence microplate reader, (ii) simplicity and low cost, and (iii) fairly rapid (30 min incubation, 20 s imaging) analysis, fair stability (<5.0% RSD) and accuracy, thus validating the EC-ET method. Finally, the developed EC-ET method was used for the clinical assay of MPO activity in blood samples; the values of MPO activity detected via the EC-ET chip were in agreement with those obtained by a traditional fluorescence microplate reader, indicating the applicability of the EC-ET method. The work opens a window for the development of enzymatic research, enzyme assay, immunoassay, and point-of-care testing as well as titration, one of the oldest methods of analysis, based on a simple chip.

Surface plasmon resonance biosensors for simultaneous monitoring of amyloid-beta oligomers and fibrils and screening of select modulators

Oligomeric amyloid-beta (Aβ) peptides are considered as the most toxic species in Alzheimer's disease (AD). Monitoring of the Aβ aggregation profiles is critical for elucidating the oligomer toxicity and may serve as a therapeutic target for AD. By immobilizing the capture antibodies of A11 and OC that are specific to the oligomers and fibrils, respectively, in separate fluidic channels, a novel surface plasmon resonance (SPR) biosensor was designed for monitoring the oligomeric and fibrillar species of Aβ(1-42) simultaneously. The influence of curcumin, Cu(2+) and methylene blue on the amount of toxic oligomers and fibrils was evaluated. The half maximal inhibitory concentration (IC50) of curcumin and methylene blue was determined. The formation of Aβ fibrils was also validated by the thioflavin T (ThT) fluorescence assay. The results demonstrate the utility of SPR as an analytical tool for rapid and comprehensive monitoring of Aβ aggregation and screening of Aβ modulators.

A sensitive and microscale method for drug screening combining affinity probes and single molecule fluorescence correlation spectroscopy

The theoretical model of drug screening method based on competitive reaction and fluorescence correlation spectroscopy.

DNA methylation detection with end-to-end nanorod assembly-enhanced surface plasmon resonance

The Au nanorod (AuNR) assembly-enhanced SPR system coupling with polymerization and nicking reactions was developed for amplified detection of DNA methylation and Dam MTase activity assay.

A surface plasmon resonance assay coupled with a hybridization chain reaction for amplified detection of DNA and small molecules

A surface plasmon resonance (SPR) detection system based on a hybridization chain reaction (HCR) was developed for amplified detection of DNA and small molecule with high sensitivity.