Construction of an Enzymatically Controlled DNA Nanomachine for One-Step Imaging of Telomerase in Living Cells

Telomerase is a basic reverse transcriptase that maintains the telomere length in cells, and accurate and specific sensing of telomerase in living cells is critical for medical diagnostics and disease therapeutics. Herein, we demonstrate for the first time the construction of an enzymatically controlled DNA nanomachine with endogenous apurinic/apyrimidinic endonuclease 1 (APE1) as a driving force for one-step imaging of telomerase in living cells. The DNA nanomachine is designed by rational engineering of substrate probes and reporter probes embedded with an enzyme-activatable site (i.e., AP site) and their subsequent assembly on a gold nanoparticle (AuNP). Upon recognition and cleavage of the AP site in the substrate probe by APE1, the loop of the substrate probe unfolds, exposing telomeric primer (TP) with the 3'-OH end. Subsequently, the TP is elongated by telomerase at the 3'-OH end to generate a long telomeric product. The resultant telomeric product acts as a swing arm that can hybridize with a reporter probe to initiate the APE1-powered walking reaction, ultimately generating a significantly enhanced fluorescence signal. Notably, endogenous APE1 is used as the driving force of the DNA nanomachine, avoiding the introduction of exogenous auxiliary cofactors into the cellular microenvironment. Owing to the high kinetics and high amplification efficiency of the APE1-powered DNA nanomachine, this strategy enables one-step sensitive sensing of telomerase in vitro and in vivo. It can successfully discriminate telomerase activity between cancer cells and normal cells, screen telomerase inhibitors, and monitor the variations of telomerase activity in living cells, offering a prospective platform for molecular diagnostics and drug discovery.

Background-filtered telomerase activity assay with cyclic DNA cleavage amplification

Overexpression of telomerase incites the abnormal proliferation of cancer cells. Thus, it has been regarded as a cancer biomarker and a potential therapeutic target. Existing assays suggest a promising sensing scheme to detect telomerase activity. However, they are complicated in terms of assay preparation and implementation. We herein report a Quenching-Exempt invader Signal Amplification Test, termed 'QUEST'. The assay leverages on a high turnover, specific cleaving enzyme, flap endonuclease I (FEN1), and graphene oxide (GO) for background (BG) filtering. In response to the target, FEN1 significantly boosts the signal with invader signal amplification. To distinguish the target signal, GO filters out the BG. It captures residual reporter invader probes (RP) to quench undesired signals. QUEST is straightforward without any pre-preparatory steps and washing/separation. Its probe design is simple and cost-effective. With QUEST, we investigated telomerase activities in various cell lines. Notably, we discriminated cancer cell lines from normal cell lines. In addition, a candidate inhibitor for telomerase was screened, which showed the promising potential of QUEST in real applications.

Amplification-free detection of telomerase activity at the single-cell level via Cas12a-lighting-up single microbeads (Cas12a-LSMBs)

Telomerase overexpresses in almost all cancer cells and has been deemed a universal biomarker for cancer diagnosis and therapy. However, simple and ultrasensitive detection of telomerase activity in single-cells is still a huge challenge. Herein, we wish to report Cas12a-lighting up single microbeads (Cas12a-LSMBs) for ultrasensitive detection of telomerase activity without nucleic acid amplification. In this platform, single-strand DNA reporter (ssDNA reporter)-functionalized single-microbeads (functionalized-SMBs) are employed as a reactor for the trans-cleavage of telomerase-activated CRISPR/Cas12a as well as a reporting unit for fluorescence signal enrichment and visualization. Due to the space-confined effect and signal enrichment mechanism on the surface of the functionalized SMBs, the Cas12a-LSMBs can accurately detect telomerase activity in crude cell lysates with high specificity. Importantly, we have demonstrated that the Cas12a-LSMBs are a reliable and practical tool to detect telomerase activity in single cells and investigate cellular heterogeneity of telomerase activity from cell-to-cell variations.

Deamination-triggered exponential signal amplification for chemiluminescent detection of cytosine deaminase at the single-cell level

We construct a sensitive chemiluminescent biosensor for sensitive detection of cytosine deaminase APOBEC3A based on deamination-triggered exponential signal amplification. This biosensor displays good specificity and high sensitivity, and it can screen APOBEC3A inhibitors and measure endogenous APOBEC3A at the single-cell level, with prospective applications in disease diagnostics and therapy.

Proximity ligation-transcription circuit-powered exponential amplifications for single-molecule monitoring of telomerase in human cells

We develop a new strategy for single-molecule monitoring of telomerase based on proximity ligation-transcription circuit-powered exponential amplifications. This strategy exhibits high sensitivity with a detection limit of 0.1 aM for the synthetic telomerase product TPC4 in vitro and 1 HeLa cell in vivo. Moreover, it can screen potential inhibitors, discriminate telomerase from interferents, and distinguish cancer cells from normal cells.

Entropy-Driven Self-Assembly of Single Quantum Dot Sensor for Catalytic Imaging of Telomerase in Living Cells

Telomerase is a highly valuable cancer diagnosis biomarker and a promising cancer therapy target. So far, most telomerase assays are limited by the involvement of tedious procedures, multiple enzymes, and complicated reaction schemes. Sensitive monitoring of low-abundant telomerase in living cells remains a challenge. Herein, we demonstrate an entropy-driven catalytic assembly of quantum dot (QD) sensors for accurate detection and imaging of telomerase activity in living cells. In this sensor, target telomerase specifically catalyzes extension of telomerase primer, and the extended primer subsequently acts as a catalyst to continuously initiate entropy-driven catalytic reaction, generating a large number of fluorophore- and biotin-labeled DNAs that can be self-assembled on the QD surface to induce an efficient Föster resonance energy transfer signal. The proposed sensor requires a single step for both recognition and amplification of the telomerase signal, eliminating the use of either protein enzymes or laborious procedures. Taking advantage of the inherent superiority of single-molecule fluorescence detection and high amplification efficiency of the entropy-driven reaction, this sensor demonstrates single-cell sensitivity for the in vitro assay. Moreover, it is capable of screening the telomerase inhibitor, discriminating different tumor cells from normal ones, and even real-time imaging telomerase in living cells, providing a novel platform for telomerase-associated cancer diagnosis and drug screening.

A simple and rapid mix-and-read assay for sensitive detection of O6-methylguanine DNA methyltransferase

We have developed a simple and rapid mix-and-read assay for the sensitive detection of O⁶-methylguanine DNA methyltransferase (MGMT) activity based on exonuclease III-assisted signal amplification under completely isothermal conditions (37 °C). This method is very simple and rapid (60 min) with ultrahigh sensitivity and good specificity, and it can detect MGMT activity at the single-cell level. Moreover, this method can be applied for the screening of MGMT inhibitors and the discrimination of MGMT in different cancer cells.

Target-Initiated Cascade Signal Amplification Lights up a G-Quadruplex for a Label-Free Detection of Circular Ribonucleic Acids

Circular ribonucleic acids (circRNAs) are a type of RNA that originates through back-splicing events from linear primary transcripts. CircRNAs display high structural resistance and tissue specificity. Accurate quantification of the circRNA expression level is of vital importance to disease diagnosis. Herein, we construct a label-free fluorescent biosensor for ultrasensitive analysis of circRNAs based on the integration of target-initiated cascade signal amplification strategy with a light-up G-quadruplex. This assay involves only one assistant probe that targets the circRNA-specific back-splice junction. When circRNA is present, it hybridizes with the assistant probe to initiate the duplex-specific nuclease (DSN)-catalyzed cyclic cleavage reaction, producing abundant triggers with 3'OH termini. Then, terminal deoxynucleotidyl transferase (TdT) catalyzes the addition of dGTP and dATP at the 3'-OH termini of the resultant triggers to obtain abundant long G-rich DNA sequences that can form efficient G-quadruplex products. The addition of Thioflavin T (ThT) can light up G-quadruplex, generating an enhanced fluorescence. This assay may be performed isothermally without the involvement of any nucleic acid templates, exogenous primers, and specific labeled probes. Importantly, this biosensor can discriminate target circRNA from one-base mismatched circRNA and exhibits good performance in human serum. Moreover, it can accurately detect circRNA in cancer cells at a single-cell level and even differentiate the circRNA levels in the tissues of healthy persons and nonsmall cell lung cancer (NSCLC) patients, with promising applications in circRNA-related cancer diagnosis and therapeutics.

Single-Molecule Biosensing of Alkaline Phosphatase in Cells and Serum Based on Dephosphorylation-Triggered Catalytic Assembly and Disassembly of the Fluorescent DNA Chain

Alkaline phosphatase (ALP) is a valuable biomarker and effective therapeutic target for the diagnosis and treatment of diverse human diseases, including bone disorder, cardiovascular disease, and cancers. The reported ALP assays often suffer from laborious procedures, costly reagents, inadequate sensitivity, and large sample consumption. Herein, we report a new single-molecule fluorescent biosensor for the simple and ultrasensitive detection of ALP. In this assay, the ALP-catalyzed dephosphorylation of detection probe can protect the detection probe against lambda exonuclease-mediated digestion, and the remaining detection probes can trigger ceaseless hybridization between two Cy5-labeled hairpin probes through toehold-mediated DNA strand displacement, generating a long fluorescent DNA chain, which can be subsequently separated from unhybridized hairpin probes and disassembled into dispersed Cy5 moieties upon NaOH treatment. The free Cy5 moieties indicate the presence of ALP and can be directly quantified via single-molecule counting. This biosensor enables efficient amplification and transduction of the target ALP signal through enzyme-free assembly and disassembly processes, significantly simplifying the experimental procedure and improving the assay accuracy. The proposed biosensor allows specific and ultrasensitive detection of ALP activity with a detection limit down to 2.61 × 10^-6 U mL^-1 and is suitable for ALP inhibition assay and kinetic analysis. Moreover, this biosensor can be applied for endogenous ALP detection in human cells and clinical human serum, holding the potential in the ALP biological function study and clinical diagnosis.

Recent advances in biological detection with rolling circle amplification: design strategy, biosensing mechanism, and practical applications

Rolling circle amplification (RCA) is a simple and isothermal DNA amplification technique that is used to generate thousands of repeating DNA sequences using circular templates under the catalysis of DNA polymerase.

Poly (A)-specific ribonuclease (PARN): More than just "mRNA stock clearing"

In eukaryotic cells, the balance between the synthesis and the degradation decides the steady-state levels of messenger RNAs (mRNA). The removal of adenosine residues from the poly(A) tail, called deadenylation, is the first and the most crucial step in the process of mRNA degradation. Poly (A)-specific ribonuclease (PARN) is one such enzyme that catalyses the process of deadenylation. Although PARN has been primarily known as the regulator of the mRNA stability, recent evidence clearly suggests several other functions of PARN, including a role in embryogenesis, oocyte maturation, cell-cycle progression, telomere biology, non-coding RNA maturation and ribosome biogenesis. Also, deregulated PARN activity is shown to be a hallmark of specific disease conditions. Pathogenic variants in the PARN gene have been observed in various cancers and inherited bone marrow failure syndromes. The focus in this review is to highlight the emerging functions of PARN, particularly in the context of human diseases.

Accurate Identification and Early Diagnosis of Osteosarcoma through CRISPR-Cas12a-Based Average Telomerase Activity Detection

Sensitive and reliable analysis of telomerase activity is important for clinical diagnosis, therapy, and prognosis of osteosarcoma. Telomerase activity is a complicated concept including both the amount of active telomerases and the length of the telomerases extension product. Still, few of the strategies formerly proposed distinguish the two aspects of telomerase activity. Herein, we propose a novel CRISPR-Cas12a-based fluorescent sensing platform that can output signals of both the amounts of telomerase and length of telomerase extension products with the assistance of an elegantly designed stem-loop probe and CRISPR-Cas12a system. On this basis, we induced a novel index, average telomerase activity, for accurate cancer reporting. Through systematic laboratory and clinical experiments, we have demonstrated that average telomerase activity can accurately distinguish cancer cells and has the potential for osteosarcoma staging.

Aptamer-mediated rolling circle amplification for label-free and sensitive detection of histone acetyltransferase activity

We develop for the first time an aptamer-mediated rolling circle amplification approach for label-free and sensitive detection of histone-modifying enzyme (HME) activity. This method can achieve femtomolar sensitivity for histone acetyltransferase Tip60 assay, which is the most sensitive HME assay reported so far. It can be further applied for inhibitor screening, enzyme kinetic analysis, and endogenous Tip60 measurement in cancer cells.

Recent advances in rolling circle amplification-based biosensing strategies-A review

Rolling circle amplification (RCA) is an efficient enzymatic isothermal reaction that using circular probe as a template to generate long tandem single-stranded DNA or RNA products under the initiation of short DNA or RNA primers. As a simplified derivative of natural rolling circle replication which synthesizes copies of circular nucleic acids molecules such as plasmids, RCA amplifies the circular template rapidly without thermal cycling and finds various applications in molecular biology. Compared with other amplification strategies, RCA has many obvious advantages. Firstly, because of the strict complementarity required in ligation of a padlock probe, it endows the RCA reaction with high specificity and can even be utilized to distinguish single base mismatches. Secondly, through the introduction of multiple primers, exponential amplification can be achieved easily and leads to a good sensitivity. Thirdly, RCA products can be customized by manipulating circular templates to generate functional nucleic acids such as aptamer, DNAzymes and restriction enzyme sites. Moreover, the RCA has good biocompatibility and is especially suitable for in situ detection. Therefore, RCA has attracted considerable attention as an efficient and potential tool for highly sensitive detection of biomarkers. Herein, we comprehensively introduce the fundamental principles of RCA technology, summarize it from three aspects including initiation mode, amplification mode and signal output mode, and discuss the recent application of RCA-based biosensor in this review.

A trifunctional split dumbbell probe coupled with ligation-triggered isothermal rolling circle amplification for label-free and sensitive detection of nicotinamide adenine dinucleotide

The nicotinamide adenine dinucleotide (NAD⁺) is an important small biomolecule that participates in a variety of physiological functions, and it has been regarded as a potential biomarker for disease diagnosis and a promising target for disease treatment. The conventional methods for NAD⁺ assay often suffer from complicated procedures, expensive labeling, poor selectivity, and unsatisfactory sensitivity. Herein, we develop a label-free and sensitive method for NAD⁺ assay based on the integration of a trifunctional split dumbbell probe with ligation-triggered isothermal rolling circle amplification (RCA). We design a trifunctional split dumbbell probe that can act as a probe for NAD⁺ recognition, a template for RCA reaction, and a substrate for SYBR Green I binding. In the presence of target NAD⁺, it can serve as a cofactor to active E. coli DNA ligase which subsequently catalyzes the ligation of split dumbbell probe to form a circular template for RCA reaction, generating numerous dumbbell probe amplicons which can be easily and label-free monitored by using SYBR Green I as the fluorescent indicator. Due to the high fidelity of NAD⁺-dependent ligation and high amplification efficiency of RCA amplification, this method exhibits high sensitivity with a detection limit of 85.6 fM and good selectivity with the capability of discriminating target NAD⁺ from its analogs. Moreover, this method can be applied for accurate and sensitive detection of NAD⁺ in complex biological samples and cancer cells, holding great potential in NAD⁺-related biological researches and clinical diagnosis.

Carbon Nanocage/Fe3O4/DNA-Based Magnetically Targeted Intracellular Imaging of Telomerase via Catalyzed Hairpin Assembly and Photodynamic-Photothermal Combination Therapy of Tumor Cells

Human telomerase has been identified as a potential tumor biomarker for early cancer diagnosis and cancer progression monitoring. We construct a novel magnetic targeting carbon nanocage/Fe₃O₄/DNA (CNC/Fe₃O₄/DNA) nanoprobe for intracellular imaging of telomerase via the signal amplification strategy catalyzed hairpin assembly (CHA) and for photodynamic-photothermal therapy of tumor cells. Telomerase primer DNA, trigger DNA, hairpin DNA1 (H1), and hairpin DNA2 (H2) were adsorbed to the surface of CNC/Fe₃O₄ nanoparticles (CNC/Fe₃O₄ NPs), and the fluorescence of (chlorin e6) Ce6 was quenched by CNC/Fe₃O₄ NPs. After entering the living cell through magnetic targeting, the telomerase primer DNA can be extended in the presence of highly activated telomerase, leading to the issue of trigger DNA, which can initiate the CHA cycling process followed by the amplification of the fluorescence intensity. The in vitro detection results justified that the proposed nanoprobe showed good sensitivity and selectivity for telomerase. Confocal microscopy studies indicated that such a nanoprobe can be used to detect the activity of telomerase in living cells and the fluorescence signal was stronger under the guidance of a magnetic field. We successfully employed this nanoprobe to monitor the dynamic activity of telomerase in various types of tumor cells and normal cells and to damage tumor cells by photodynamic-photothermal combination therapy, which evidenced that this is a promising biological method for early cancer diagnosis and tumor cell therapy.

A CRISPR-driven colorimetric code platform for highly accurate telomerase activity assay

Telomeric repeat amplification protocol (TRAP) has been the most widely used method for assessing the telomerase activity from cells and tissues. However, cell lysates, body fluid samples, or tumor tissue samples often contain high concentrations of protein or other complex matrices, which are usually inhibiting the TRAP response, thus leading to false-negative results. Internal control (IC) involved TRAP enables reliable telomerase activity assay but requires time consuming and laborious electrophoretic separation to visualize telomeric repeat DNA and internal control products from TRAP reaction, severely limiting its application in clinical diagnosis. Herein, a colorimetric code system based on programmable CRISPR-Cas12a technology and gold nano-particles (AuNPs) probe has been developed to analyse telomeric repeat DNA and internal control in TRAP products, enabling the rapid detection of telomerase activity and identification of false-negatives with naked-eye. We transform the detection results into three typical colorimetric codes-positive (P), negative (N) and false-negative (FN), making the judgement of detection results more convenient and user-friendly. The platform has also been applied in accurate detection of clinical liver cancer specimens for telomerase activity with a detection sensitivity of 93.75% and a specificity of 93.75% based on Youden index analysis. As a proof of concept, we further demonstrated the feasibility of Cas9-mediated triple-line lateral flow assay (TL-LFA), which enabled the detection of telomeric repeat DNA and internal control on a single triple-line test strip, achieving convenient and accurate telomerase activity assay.

Gelatin nanoparticles transport DNA probes for detection and imaging of telomerase and microRNA in living cells

Telomerase and microRNA (miRNA) are biomarkers closely related to tumors. Simultaneous detection of both markers can improve accuracy and reliability of early diagnosis. Based on the mechanism of fluorescence resonance energy transfer (FRET), two fluorescent DNA probes were designed for telomerase and miRNA-21. The probes were wrapped by gelatin through electrostatic interaction to form nanoparticles. After that, we synthesized molecularly imprinted coating of transferrin on the surface of gelatin nanoparticles, which can avoid the immune stress response and macrophage phagocytosis to help gelatin nanoparticles enter into the cells smoothly through endocytosis. Following with the degradation of gelatin in the cells, DNA probes were released to react with telomerase and miRNA-21 and lead to the change of the fluorescence signal. Thereby the simultaneous imaging of telomerase and miRNA-21 were successfully achieved in HeLa cells and HepG2 cells. The proposed strategy shows the simultaneous imaging for different biological markers with DNA probes by preventing them from being hydrolyzed with nucleases before the determination and achieves reliable method for early diagnosis of cancer.

Portable glucose meter-utilized label-free and washing-free telomerase assay

We herein describe a portable glucose meter (PGM)-utilized label-free and washing-free method for the facile determination of telomerase activity that relies on the kinase-catalyzed cascade enzymatic reaction (KCER) that transduces the telomerase activity to the glucose level. In the sensor, the telomerase that elongates telomere sequences ((TTAGGG)_n) from the 3'-terminus of telomerase substrate primer (TSP) consumes deoxynucleoside triphosphate (dNTP), which serves as a phosphate source for KCER promoted by hexokinase and pyruvate kinase. Thus, the presence of telomerase protects KCER from working effectively, resulting in the maintenance of an initial, high glucose level that is readily determined using hand-held PGM. With this strategy, the telomerase activities in various types of cell lines were successfully determined with high sensitivity. Furthermore, the ability of this method to screen candidate inhibitors for telomerase activity was also verified.

Catalytic Hairpin Assembly-Assisted Rolling Circle Amplification for High-Sensitive Telomerase Activity Detection

Telomerase is a promising biomarker and a potential therapeutic target of malignant tumors. Reliable, facile, and sensitive telomerase activity analysis is of vital importance for both early diagnosis and therapy of malignant tumors. Herein, we proposed a novel fluorescent assay termed catalytic hairpin assembly-assisted rolling circle amplification (CAR) for both in vitro and in situ high-sensitive telomerase activity detection. In the presence of active telomerase, the extension of a designed telomerase primer was limited to five bases (GGGTT), thus forming short telomerase products. Afterward, the obtained telomerase extension products cyclized Padlock and subsequently initiated the rolling circle amplification (RCA). In order to maintain a higher sensitivity, an ingeniously designed catalytic hairpin assembly (CHA) was attached for both signal amplification and result readout. The highlights of the CAR method were concluded as follows: (i) dual signal amplification from CHA and RCA ensures high sensitivity and (ii) the CAR method has the potential for both in vitro and intracellular imaging of telomerase activity. We believe that the CAR method would be of great potential for the diagnosis and therapy of various human diseases.

Nanomaterial-based biosensors for DNA methyltransferase assay

We review the recent advances in the development of nanomaterial-based biosensors for DNA methyltransferase assay.

Label-free and amplified detection of apoptosis-associated caspase activity using branched rolling circle amplification

We develop a label-free fluorescence method for ultrasensitive detection of apoptosis-associated caspase activity based on branched rolling circle amplification.

Dephosphorylation-directed tricyclic DNA amplification cascades for sensitive detection of protein tyrosine phosphatase

A new fluorescence method is developed for the sensitive detection of protein tyrosine phosphatase based on dephosphorylation-directed tricyclic DNA amplification cascades.

Intracellular MicroRNA imaging using telomerase-catalyzed FRET ratioflares with signal amplification

Intracellular microRNA (miRNA) detection has attracted increasing attention, resulting in significant achievements. However, the development of an available tool that possesses a satisfactory signal-to-background ratio and high sensitivity for miRNA detection remains challenging. Herein, a class of telomerase-catalyzed FRET (fluorescence resonance energy transfer) ratioflares has been developed for the accurate sensing of low-abundance cancer-related miRNA both in a fluorescence assay and living cell imaging with signal amplification capacity. In this work, endogenous telomerase is led in a miRNA test system with signal amplification for the first time, wherein telomerase extends hexamer telomeric repeats (TTAGGG) using the 3' end of the capture probe as the primer. The synergetic work of telomerase and the catalyst strand (CS) makes the target miRNA circulate in the system, resulting in high sensitivity and an enhanced FRET signal an improved detection limit of 2.27 × 10-15 M. Meanwhile, telomerase-catalyzed FRET ratioflares allow the difference between cancer cells and normal cells to be increased reliably. Furthermore, low false positive signals resulting from chemical interference and minimized system fluctuations are achieved through ratiometric measurements.

Telomerase Triggered DNA Walker with a Superhairpin Structure for Human Telomerase Activity Sensing

Different from conventional DNA walkers, we designed a telomerase-triggered three-dimensional DNA walker consisting of a superhairpin structure with a bulged loop in the stem as the walking strand and dye-labeled tracks for ultrasensitive detection of telomerase activity. In the presence of telomerase, the primers in the stem of the superhairpin structure were elongated and triggered internal strand displacement, thus activating the superhairpin structure. Subsequently, the open superhairpin structure as a swing arm was able to bind with the track, and the swing arm could be released by enzymatic cleavage of the binding duplex domain, resulting in the fluorescence recovery of dye-labeled fragments from the surface of gold nanoparticles. Based on signal amplification of the telomerase-triggered DNA walker, the walking device was further applied to various cancer lines with a low detection limit of telomerase activity equivalent to 90 cells μL^-1 for HeLa cells. Moreover, the advantage of this DNA walker strategy was confirmed by calculating telomerase activity in a single cell. This telomerase-triggered DNA walker provides a new concept on signal transduction for telomerase detection and is anticipated to stimulate interest in DNA nanomachine design for bioanalysis.

Electrocatalysis of cerium metal-organic frameworks for ratiometric electrochemical detection of telomerase activity

A ratiometric electrochemical biosensor was constructed to detect telomerase activity based on electrocatalysis of cerium-based metal-organic frameworks (CeMOFs) and conformation switch of hairpin DNA. First, the CeMOFs were synthesized using Ce as nodes and 1,3,5-benzenetricarboxylic acid as linker in a green method, and then functionalized with gold nanoparticles. The resulted Au@CeMOF tags demonstrated an excellent electrocatalysis toward hydroquinone oxidation. Meanwhile, a methylene blue (MB) modified hairpin probe was designed with telomerase primer (TP) hybridized "stem" and immobilized on the electrode surface via Au-S chemistry. In the presence of the dNTPs and telomerase, the extended TP can open the hairpin DNA and keep the MB away from the electrode surface, resulting in a decrease of electrochemical signal. In the meantime, the TP-extended part could capture the Au@CeMOF-cDNA tags on the electrode surface via hybridization, leading to the increase electrochemical signal of hydroquinone oxidation catalyzed by Au@CeMOF-cDNA tags. Thus, a ratiometric signal output mode was developed for the electrochemical detection of telomerase activity. This biosensor showed wide dynamic correlation of telomerase activity from 2 × 10² to 2 × 10⁶ cells mL^-1 with the limit of detection of 27 cells mL^-1, and was applied to evaluate telomerase activity in single cell. The ratiometric electrochemical strategy based on the catalysis of MOFs provides a new avenue on signal transduction in telomerase detection.

Label-Free Telomerase Detection in Single Cell Using a Five-Base Telomerase Product-Triggered Exponential Rolling Circle Amplification Strategy

Telomerase is a universal biomarker of malignant tumors. Sensitive and reliable analysis for telomerase activity is of vital importance for both early diagnosis and therapy of malignant tumors. Herein, a novel fluorescent strategy was proposed for sensitive and label-free detection of telomerase activity. One highlight of this strategy is that an exponential signal amplification can be triggered by a very short telomerase extension product (TEP). Without adding dATP, the designed telomerase primer can be easily controlled to extend five bases (GGGTT) to give short TEP with definite length. The resulting short TEP can then be constructed as a circular rolling circle amplification (RCA) template and thus initiate a nicking enzyme-mediated exponential RCA, producing G-rich amplification products that can be sensitively probed via specific binding between the fluorescent dye Thioflavin T (ThT) and the nucleic acid G-quadruplexes. Elevated telomerase translocation efficiency, combined with exponential signal amplification and specific probing of RCA products by ThT, endow the sensing platform with extraordinarily high detection sensitivity. The requirement for short TEP increases the possibility to analyze telomerase with low activity. The proposed sensing platform can achieve sensitive telomerase activity detection in individual cells, even with the interference of accumulated normal cells. It was also demonstrated to show excellent capability in screening for the inhibitors of telomerase. Therefore, the proposed sensing platform has great potential for not only clinical diagnosis but also anticancer drug development.

Quantum dot-based electrochemical biosensor for stripping voltammetric detection of telomerase at the single-cell level

Human telomerase is responsible for the maintenance of chromosome end structures and is a valuable biomarker for malignant growth. However, the accurate measurement of telomerase activity at the single-cell level has remained a great challenge. Here we develop a simple quantum dot (QD)-based electrochemical biosensor for stripping voltammetric detection of telomerase activity at the single-cell level. We designed a thiol-modified capture DNA which may be immobilized on the gold electrode by the gold-sulfur bond. The presence of telomerase enables the addition of the telomere repeats of (TTAGGG)_n to the 3' end of the primer, accompanied by the incorporation of abundant biotins in the extension product with the assistance of the biotin-tagged dATP. The subsequent hybridization of extension product with the capture DNA and the addition of streptavidin-coated QDs induce the assembly of large amounts of QDs onto the electrode via specific biotin-streptavidin binding. After the acidic dissolution of QDs, the released Cd (II) can be simply quantified by anodic stripping voltammetry (ASV). Due to the introduction of large amounts of QDs by telomerase-induced primer extension reaction and the synergistic signal amplification induced by the release of Cd (II) from the QDs, this biosensor can detect telomerase activity at the single-cell level without the involvement of any thermal cycling and extra enzymes for signal amplification. Moreover, this assay exhibits a large dynamic range over four orders of magnitude and it is very simple without the involvement of specific hairpin probe design and complicated labelling, holding great potential in point-of-need testing.

Gold Nanobipyramids as Dual-Functional Substrates for in Situ "Turn On" Analyzing Intracellular Telomerase Activity Based on Target-Triggered Plasmon-Enhanced Fluorescence

Herein, we developed a novel plasmon-enhanced fluorescence (PEF)-based telomerase-responsive nanoprobe for in situ fluorescence "turn on" visualization of telomerase activity in live cells. The as-prepared nanoprobe was composed of a nicked molecular beacon (which contains Cy5.5-labeled hairpin-DNA sequences hybridized with telomerase primers)-functionalized gold nanobipyramids (Au NBPs). Au NBPs were selected as both fluorescence resonance energy-transfer and PEF dual-functional substrates, while DNA was selected to be the precise spacer to manage the interval between the Au NBPs and Cy5.5. On the basis of this target-triggered PEF probe, optimal fluorescence enhancement can be obtained with 49 DNA bases, which was higher than gold nanorods. The proposed method accomplishes sensitive telomerase activity detection down to 23 HeLa cells with a dynamic range of 40-1200 HeLa cells. On the basis of this, in situ fluorescence imaging of telomerase activity in live cells and real-time analysis of the variation in intracellular telomerase activity can be achieved. Moreover, cancer cells and normal cells can also be successfully discriminated even in their co-cultured mixtures, indicating promising potential in clinical diagnoses.

Multipedal DNA Walker Biosensors Based on Catalyzed Hairpin Assembly and Isothermal Strand-Displacement Polymerase Reaction for the Chemiluminescent Detection of Proteins

In this study, two kinds of sensitive biosensors based on a multipedal DNA walker along a three-dimensional DNA functional magnet particles track for the chemiluminescent detection of streptavidin (SA) are constructed and compared. In the presence of SA, a multipedal DNA walker was constructed by a biotin-modified catalyst as a result of the terminal protection to avoid being digested by exonuclease I. Then, through a toehold-mediated strand exchange, a "leg" of a multipedal DNA walker interacted with a toehold of a catalyzed hairpin assembly (CHA)-H1 coupled with magnetic microparticles (MMPs) and opened its hairpin structure. The newly open stem in CHA-H1 was hybridized with a toehold of biotin-labeled H2. Via the strand displacement process, H2 displaced one "leg" of a multipedal DNA walker, and the other "leg" continued to interact with the neighboring H1 to initiate the next cycle. In order to solve the high background caused by the hybridization between CHA-H1 and H2 without a CHA-catalyst, the other model was designed. The principle of the other model (isothermal strand-displacement polymerase reaction (ISDPR)-DNA walker) was similar to that of the above one. After the terminal protection of SA, a "leg" of a multipedal DNA walker was triggered to open the hairpin of the ISDPR-H1 conjugated with MMPs. Then, the biotin-modified primer hybridized with the newly exposed DNA segment, triggering the polymerization reaction with the assistance of dNTPs/polymerase. As for the extension of the primer, the "leg" of a multipedal DNA walker was displaced so that the other "leg" could trigger the proximal H1 to go onto the next cycle. Due to its lower background and stronger signal, a multipedal DNA walker based on an ISDPR had a lower limit of detection for SA. The limit of detection for SA was 6.5 pM, and for expanding the application of the method, the detections of the folate receptor and thrombin were explored. In addition, these DNA walker methods were applied in complex samples successfully.

Ratiometric Fluorescent Biosensor for Visual Discrimination of Cancer Cells with Different Telomerase Expression Levels

Telomerase is inactive in normal somatic cells but highly activated in tumor cells to maintain their indefinite proliferation and immortal phenotype. As a specific marker for the generation and progress of almost all tumors, the detection of telomerase activity by classical PCR techniques has served in the biological research of tumors. However, the detection of in situ telomerase activity in cell extracts to evaluate the malignancy, progress, and metastasis of tumors remains a daunting challenge. Here, a precisely designed FRET-based ratiometric fluorescent oligonucleotide probe has achieved high-fidelity detection of telomerase activity for accurate discrimination of different cancer cells toward advanced diagnosis of tumors. Our method is superior to other methods in its capabilities to quantify telomerase activity in cell extracts and visualize various tumor cell extracts with different telomerase expression levels by the naked eye for clinical diagnosis. In particular, the ratiometric fluorescent probe used in the assay could exclude other experimental factors influence, and further avoid false positive signal generation. The method reported here could provide a reliable, accurate, and convenient way in medical diagnostics and therapeutic response assessment.

A triple-amplification strategy for sensitive detection of telomerase at the single-cell level

We develop a triple-amplification strategy for sensitive detection of telomerase from cancer cells at the single-cell level.

Protein-responsive rolling circle amplification as a tandem template to drive amplified transduction of fluorescence signal probes for highly sensitive immunoassay

A protein-responsive fluorescence immunosensor is reported based on proximity ligation-initiated rolling circle amplification as tandem template to drive output switch of signal probes.

A dual signal amplification-assisted DNAzyme biosensor for ultrasensitive detection of Argonaute 2 activity

We develop a new dual-signal amplification-assisted DNAzyme biosensor for sensitive detection of Argonaute 2 (Ago2) activity.