Fluorescence Imaging of Intracellular Telomerase Activity Using Enzyme-Free Signal Amplification

Biomedical Utility of Non-Enzymatic DNA Amplification Reaction: From Material Design to Diagnosis and Treatment

Nucleic acid nanotechnology has become a promising strategy for disease diagnosis and treatment, owing to remarkable programmability, precision, and biocompatibility. However, current biosensing and biotherapy approaches by nucleic acids exhibit limitations in sensitivity, specificity, versatility, and real-time monitoring. DNA amplification reactions present an advantageous strategy to enhance the performance of biosensing and biotherapy platforms. Non-enzymatic DNA amplification reaction (NEDAR), such as hybridization chain reaction and catalytic hairpin assembly, operate via strand displacement. NEDAR presents distinct advantages over traditional enzymatic DNA amplification reactions, including simplified procedures, milder reaction conditions, higher specificity, enhanced controllability, and excellent versatility. Consequently, research focusing on NEDAR-based biosensing and biotherapy has garnered significant attention. NEDAR demonstrates high efficacy in detecting multiple types of biomarkers, including nucleic acids, small molecules, and proteins, with high sensitivity and specificity, enabling the parallel detection of multiple targets. Besides, NEDAR can strengthen drug therapy, cellular behavior control, and cell encapsulation. Moreover, NEDAR holds promise for constructing assembled diagnosis-treatment nanoplatforms in the forms of pure DNA nanostructures and hybrid nanomaterials, which offer utility in disease monitoring and precise treatment. Thus, this paper aims to comprehensively elucidate the reaction mechanism of NEDAR and review the substantial advancements in NEDAR-based diagnosis and treatment over the past five years, encompassing NEDAR-based design strategies, applications, and prospects.

Telomerase: a nexus between cancer nanotherapy and circadian rhythm

Cancer represents a complex disease category defined by the unregulated proliferation and dissemination of anomalous cells within the human body. According to the GLOBOCAN 2020 report, the year 2020 witnessed the diagnosis of approximately 19.3 million new cases of cancer and 10.0 million individuals succumbed to the disease. A typical cell eventually becomes cancerous because of a long-term buildup of genetic instability and replicative immortality. Telomerase is a crucial regulator of cancer progression as it induces replicative immortality. In cancer cells, telomerase inhibits apoptosis by elongating the length of the telomeric region, which usually protects the genome from shortening. Many nanoparticles are documented as being available for detecting the presence of telomerase, and many were used as delivery systems to transport drugs. Furthermore, telomere homeostasis is regulated by the circadian time-keeping machinery, leading to 24-hour rhythms in telomerase activity and TERT mRNA expression in mammals. This review provides a comprehensive discussion of various kinds of nanoparticles used in telomerase detection, inhibition, and multiple drug-related pathways, as well as enlightens an imperative association between circadian rhythm and telomerase activity from the perspective of nanoparticle-based anticancer therapeutics.

A triggered DNA nanomachine with enzyme-free for the rapid detection of telomerase activity in a one-step method

BACKGROUND

Telomerase is considered a biomarker for the early diagnosis and clinical treatment of cancer. The rapid and sensitive detection of telomerase activity is crucial to biological research, clinical diagnosis, and drug development. However, the main obstacles facing the current telomerase activity assay are the cumbersome and time-consuming procedure, the easy degradation of the telomerase RNA template and the need for additional proteases. Therefore, it is necessary to construct a new method for the detection of telomerase activity with easy steps, efficient reaction and strong anti-interference ability.

RESULTS

Herein, an efficient, enzyme-free, economical, sensitive, fluorometric detection method for telomerase activity in one-step, named triggered-DNA (T-DNA) nanomachine, was created based on target-triggered DNAzyme-cleavage activity and catalytic molecular beacon (CMB). Telomerase served as a switch and extended few numbers of (TTAGGG)n repeat sequences to initiate the signal amplification in the T-DNA nanomachine, resulting in a strong fluorescent signal. The reaction was a one-step method with a shortened time of 1 h and a constant temperature of 37 °C, without the addition of any protease. It also sensitively distinguished telomerase activity in various cell lines. The T-DNA nanomachine offered a detection limit of 12 HeLa cells μL-1, 9 SK-Hep-1 cells μL-1 and 3 HuH-7 cells μL-1 with a linear correlation detection range of 0.39 × 102-6.25 × 102 HeLa cells μL-1 for telomerase activity.

SIGNIFICANCE

In conclusion, our study demonstrated that the triggered-DNA nanomachine fulfills the requirements for rapid detection of telomerase activity in one-step under isothermal and enzyme-free conditions with excellent specificity, and its simple and stable structure makes it ideal for complex systems. These findings indicated the application prospect of DNA nanomachines in clinical diagnostics and provided new insights into the field of DNA nanomachine-based bioanalysis.

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.

Deoxynivalenol (DON)-Triggered Dual-Color Composite Probe Based on Gold Nanoclusters for Simultaneous Imaging of DON and miR-34a in Living Cells

Deoxynivalenol (DON) is a mycotoxin secreted by Fusarium species, posing great harm to food safety and human health. Therefore, it is of great significance to study its toxic effects and mechanism. miR-34a is a representative biomarker during the process of DON-induced apoptosis. Herein, a DON-triggered dual-color composite probe was constructed for simultaneous imaging of DON and miR-34a in living cells. The aptamer blocks the recognition sequence of miR-34a to realize DON-triggered cell imaging. The specific binding of DON with its aptamer and HCR induced by miR-34a resulted in the recovery of fluorescence of the dual-color Au NCs. Under the optimal conditions, the correlation between the relative fluorescence intensities of dual-color Au NCs showed good linear relationships with the logarithm of DON and miR-34a concentration, respectively. With the increase in DON concentration (0-20 μg/mL) and stimulation time (0-12 h), the fluorescence of dual-color Au NCs gradually recovered. This dual-color Au NCs composite probe can realize simultaneous detection of DON and miR-34a induced by DON, which is significant for verifying the cytotoxic mechanism of DON.

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.

Functional Nucleic Acid Probes Based on Two-Photon for Biosensing

Functional nucleic acid (FNA) probes have been widely used in environmental monitoring, food analysis, clinical diagnosis, and biological imaging because of their easy synthesis, functional modification, flexible design, and stable properties. However, most FNA probes are designed based on one-photon (OP) in the ultraviolet or visible regions, and the effectiveness of these OP-based FNA probes may be hindered by certain factors, such as their potential for photodamage and limited light tissue penetration. Two-photon (TP) is characterized by the nonlinear absorption of two relatively low-energy photons of near-infrared (NIR) light with the resulting emission of high-energy ultraviolet or visible light. TP-based FNA probes have excellent properties, including lower tissue self-absorption and autofluorescence, reduced photodamage and photobleaching, and higher spatial resolution, making them more advantageous than the conventional OP-based FNA probes in biomedical sensing. In this review, we summarize the recent advances of TP-excited and -activated FNA probes and detail their applications in biomolecular detection. In addition, we also share our views on the highlights and limitations of TP-based FNA probes. The ultimate goal is to provide design approaches for the development of high-performance TP-based FNA probes, thereby promoting their biological applications.

Trimer structures formed by target-triggered AuNPs self-assembly inducing electromagnetic hot spots for SERS-fluorescence dual-signal detection of intracellular miRNAs

Accurate quantitative, in situ and temporal tracking imaging of tumor-associated miRNAs in living cells could provide a basis for cancer diagnosis and prognosis. In this strategy, a surface-enhanced Raman scattering (SERS)-fluorescence (FL) dual-spectral sensor (DSS) was constructed based on the nanoscale photophysical properties of AuNPs, mediated by functionalized DNA, to achieve rapid imaging of FL and accurate SERS quantification of intracellular miRNAs. The dual-spectrum sensor in the strategy is highly sensitive, specific and reproducibly stable. The LOD values of the dual spectra were 3.58 pM (SERS) as well as 11.8 pM (FL) with RSD values less than 2.69%. The bispectral sensor self-assembled into a trimer by the lapidation of Y-type DNA under the excitation of the target, generating a stable enhanced electric field coupling; and selected adenine located in the enhanced electric field as the reporter molecule, simplifying the labeling process and variables of the Raman reporter molecule, distinguishing it from other traditional methods. This strategy successfully achieved accurate tracking and quantification of miR-21 in cancer cells and showed good stability in the cells. The reported probes are potential tools for reliable monitoring of biomolecular dynamics in living cells.

Functional DNA sensors integrated with nucleic acid signal amplification strategies for non-nucleic acid targets detection

In addition to carrying and transmitting genetic material, some DNA molecules have specific binding ability or catalytic function. DNA with this special function is collectively referred to as functional DNA (fDNA), such as aptamer, DNAzyme and so on. fDNA has the advantages of simple synthetic process, low cost and low toxicity. It also has high chemical stability, recognition specificity and biocompatibility. In recent years, fDNA biosensors have been widely investigated as signal recognition elements and signal transduction elements for the detection of non-nucleic acid targets. However, the main problem of fDNA sensors is their limited sensitivity to trace targets, especially when the affinity of fDNA to the targets is low. To further improve the sensitivity, various nucleic acid signal amplification strategies (NASAS) are explored to improve the limit of detection of fDNA. In this review, we will introduce four NASAS (hybridization chain reaction, entropy-driven catalysis, rolling circle amplification, CRISPR/Cas system) and the corresponding design principles. The principle and application of these fDNA sensors integrated with signal amplification strategies for detection of non-nucleic acid targets are summarized. Finally, the main challenges and application prospects of NASAS integrated fDNA biosensing system are discussed.

A Telomerase-Activated Magnetic Resonance Imaging Probe for Consecutively Monitoring Tumor Growth Kinetics and In Situ Screening Inhibitors

Telomerase has long been considered as a biomarker for cancer diagnosis and a therapeutic target for drug discovery. Detecting telomerase activity in vivo could provide more direct information of tumor progression and response to drug treatment, which, however, is hampered by the lack of an effective probe that can generate an output signal without a tissue penetration depth limit. In this study, using the principle of distance-dependent magnetic resonance tuning, we constructed a telomerase-activated magnetic resonance imaging probe (TAMP) by connecting superparamagnetic ferroferric oxide nanoparticles (SPFONs) and paramagnetic Gd-DOTA (Gd(III) 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) complexes via telomerase-responsive DNA motifs. Upon telomerase-catalyzed extension of the primer in TAMP, Gd-DOTA-conjugated oligonucleotides can be liberated from the surface of SPFONs through a DNA strand displacement reaction, restoring the T₁ signal of the Gd-DOTA for a direct readout of the telomerase activity. Here we show that, by tracking telomerase activity, this probe provides consistent monitoring of tumor growth kinetics during progression and in response to drug treatment and enables in situ screening of telomerase inhibitors in whole-animal models. This study provides an alternative toolkit for cancer diagnosis, treatment response assessment, and anticancer drug screening.

Genetically encodable tagging and sensing systems for fluorescent RNA imaging

Live cell imaging of RNAs is crucial to interrogate their fundamental roles in various biological processes. The highly spatiotemporal dynamic nature of RNA abundance and localization has presented great challenges for RNA imaging. Genetically encodable tagging and sensing (GETS) systems that can be continuously produced in living systems have afforded promising tools for imaging and sensing RNA dynamics in live cells. Here we review the recent advances of GETS systems that have been developed for RNA tagging and sensing in live cells. We first describe the various GETS systems using MS2-bacteriophage-MS2 coat protein, pumilio homology domain and clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9/13 for RNA labeling and tracking. The progresses of GETS systems for fluorogenic labeling and/or sensing RNAs by engineering light-up RNA aptamers, CRISPR-Cas9 systems and RNA aptamer stabilized fluorogenic proteins are then elaborated. The challenges and future perspectives in this field are finally discussed. With the continuing development, GETS systems will afford powerful tools to elucidate RNA biology in living systems.

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.

Evaluation of molecular signatures in the urinary bladder and upper tract urothelial carcinomas: a prospective controlled clinical study

Background

Urothelial carcinomas (UC) can be either in the upper or in the lower urinary tract or both. Urothelial bladder cancer (UBC) is more common than upper tract urothelial carcinoma (UTUC). This research was designed to study the difference between UBC and UTUC using the molecular pathways including (MAPK/ERK) pathway, cell cycle regulating genes, and oncogenic genes.

Methods

To study the discrepancy between UBC and UTUC, a prospective trial was carried out for 31 radical cystectomy and 19 nephrouretrectomy fresh-frozen specimens of UBC and UTUC patients, respectively. The expression level of mRNA of eight genes namely EGFR, ELK1, c-fos, survivin, TP53, RB1, FGFR3, and hTERT was assessed in normal adjacent tissues, UTUC, and UBC by RT-PCR.

Results

Comparison between UTUC and UBC regarding the expression level of mRNA of the EGFR, ELK1, c-fos, survivin, TP53, and FGFR3 had significant difference (p-value < 0.001), while the expression level of RB1 and hTERT level had no significance. Sensitivity/specificity of EGFR, Elk1, c-fos, survivin, TP53, and FGFR3 was 0.78/0.90, 0.84/0.90, 0.84/0.80, 0.84/0.96, 0.94/0.93, and 0.89/0.93, respectively, to differentiate between UTUC and UBC.

Conclusions

Despite the fact that UTUC and UBC share the same origin, there is a clear evidence that there is a molecular difference between them. This molecular difference could be the reason that UTUC is more aggressive than UBC.

Uncovering the Interaction between Intracellular Telomerase Activity and Hydrogen Peroxide during Cancer Cell Apoptosis Utilizing a Dual-Color Fluorescent Nanoprobe

Uncovering the intrinsic interaction of different bioactive species, i.e., reactive oxygen species (ROS) and telomerase, is of great importance because they play interrelated and interdependent biological roles in living organisms. Nevertheless, exploration of the intracellular ROS/telomerase cross-talk by effective and noninvasive methods remains a great challenge, as it is difficult to simultaneously detect different types of biomolecules (i.e., active small molecules and proteins) in living cells. To address this issue, herein, we report, for the first time, a novel fluorescent nanoprobe for simultaneous determination and in situ imaging of telomerase activity and hydrogen peroxide (H₂O₂) in living cells. With the advantage of high sensitivity and good specificity, this newly fabricated nanoprobe was successfully applied to precisely visualize and monitor the changes in telomerase activity and H₂O₂ concentration in cancer cells. More significantly, by employing the nanoprobe as a one-step incubation tool, it is found that there is a cross-talk between H₂O₂ and telomerase activity in the drug-induced cancer cells' apoptosis process, which provides valuable information for gaining fundamental insights into the relationship between ROS and telomerase activity in cancer treatments. This work affords a promising method for revealing the relevant regulatory mechanisms and roles of ROS and telomerase activity in the occurrence, evolvement, and treatment of diseases.

Construction of an Entropy-Driven Dumbbell-Type DNAzyme Assembly Circuit for Lighting Up Uracil-DNA Glycosylase in Living Cells

Sensitive monitoring of intracellular uracil-DNA glycosylase (UDG) in living cells is essential to understanding the DNA repair pathways and discovery of anticancer drugs. Herein, we demonstrate the construction of an entropy-driven dumbbell-type DNAzyme assembly circuit for lighting up UDG in living cells via the integration of entropy-driven DNA catalysis (EDC) with the DNAzyme biocatalyst. Target UDG excises the damaged uracil base, causing the breakage of detection probe and the release of trigger. The released trigger can initiate the downstream EDC reaction to form two catalytically active DNAzyme units. The resultant dual Mg²⁺-DNAzyme units serve as the signal transducers to cyclically cleave the fluorophore/quenched-modified reporters, generating an enhanced fluorescence signal. In contrast to the single-layered EDC method with a linear amplification, the proposed doublet EDC-DNAzyme strategy exhibits high signal gain and achieves a detection limit of 8.71 × 10^-6 U/mL. Notably, this assay can be performed in one-step manner at room temperature without the requirement of strict temperature control and complicated reaction procedures, and it can further screen the UDG inhibitors, measure kinetic parameters, and discriminate cancer cells from normal cells. Moreover, this strategy can monitor intracellular UDG activity with improved signal gain, and it may be exploited for sensing and imaging of other types of DNA modifying enzymes with the integration of the corresponding detection substrate, providing a facile and robust approach for biological research studies and clinical diagnosis.

DNA-Based Nanoprobes for Simultaneous Detection of Telomerase and Correlated Biomolecules

Telomerase (TE), a ribonucleoprotein reverse transcriptase, is enzymatically activated in most tumor cells and is responsible for promoting tumor progression and malignancy by enabling replicative immortality of cancer cells. TE has become an important hallmark for cancer diagnosis and a potential therapy target. Therefore, accurate and in site detection of TE activity, especially the simultaneous imaging of TE activity and its correlated biomolecules, is highly essential to medical diagnostics and therapeutics. DNA-based nanoprobes, with their effective cell penetration capability and programmability, are the most advantageous for detection of intracellular TE activity. This concept article introduces the recent strategies for in situ sensing and imaging of TE activity, with a focus on simultaneous detection of TE and related biomolecules, and provides challenges and perspectives for the development of new strategies for such correlated imaging.

Metastable DNA hairpins driven isothermal amplification for in situ and intracellular analysis

Intracellular substance analysis is critical for understanding cellular physiological mechanisms and predicting disease progression. Isothermal amplification technologies have been raised to accurately detect intracellular substances due to their low abundance, which is significant for the mechanism analysis and clinical application. However, traditional isothermal method still needs to cell destruction and extraction, resulting in fluctuant results. Moreover, it only works on dead cells. Therefore, non-destructive analysis based on isothermal amplification deserves to be studied, which directly reveals the content and position of relevant molecules. In recent years, metastable DNA hairpins-driven isothermal amplification (Mh-IA) blazes a trail for analysis in living cells. This review tracks the recent advances of Mh-IA strategy in living cell detection and highlights the potential challenges regarding this field, aiming to improve in vivo isothermal amplification. Also, challenges and prospects of Mh-IA for in situ and intracellular analysis are considered.

A primer extension activating 3D DNAzyme walker for in situ imaging and sensitive detection of telomerase activity

Acquiring information on telomerase activity at multiple levels contributes to a better understanding of its role in various physiological and pathological processes. Herein, a primer extension activating 3D DNAzyme walker is developed for in situ imaging and sensitive detection of telomerase activity. This walker is constructed via co-modifying specially designed hairpin structured walking strands and track strands on a gold nanoparticle (AuNP). The walking strand contains a pre-blocked DNAzyme sequence and a telomerase primer hybridized to its root. The track strand embeds at an RNA cleavage site and is labeled with the FAM group. After this walker is taken up by cells, the telomerase primer is extended under the action of endogenous telomerase to liberate DNAzyme. The liberated DNAzyme cuts track strands in the presence of the cofactor Mn²⁺ to drive the walker's processive operation, resulting in an enhanced fluorescence recovery of the AuNP-quenched FAM fluorophore. In situ imaging of telomerase activity in three different cell lines (MCF-7 cells, HeLa cells and HL-7702 cells) was well implemented. The discrimination of cancer cells from normal cells and the screening of telomerase inhibitors have been achieved. The sensitive detection of telomerase activity in HeLa cell lysate has also been realized with a detection limit of 10 cells. This walker performed a new approach for monitoring telomerase activity from different levels, providing a potential tool for clinical diagnosis, prognostic evaluation and drug screening.

Electrochemiluminescent determination of prostate-specific antigen using Au@(MoS2/GO/o-MWNTs) nanohybrids as co-reaction accelerator and hyperbranched hybridization chain reaction for signal amplification

Three-dimensional flowerlike Au@(MoS₂/GO/o-MWNTs) nanohybrids (abbreviated as AMGMs) were synthesized and then introduced into an electrochemiluminescence (ECL) system as a new co-reaction accelerator for the ultrasensitive prostate-specific antigen (PSA). The AMGMs not only served as a substrate with good conductivity and a large specific surface area for loading abundant primary antibodies but also acted as an effective co-reaction accelerator; the co-reaction accelerator could interact with a co-reactant rather than the luminophore to boost the generation of free radical intermediates, thereby producing abundant excited states of luminophores to amplify the ECL signal response. Additionally, an anticipated signal amplification strategy based on the hybridization chain reaction (HCR) was developed by gathering a large amount of a DNA initiator on gold nanoparticles. These gathered DNA initiators could generate multiple DNA concatemers and attach more signal molecules, which resulted in outstanding exponential signal amplification. Consequently, the ECL immunosensor demonstrated high sensitivity, with a linear range from 0.1 pg mL^-1 to 50 ng mL^-1 and a detection limit of 0.028 pg mL^-1. In addition, the immunosensor displayed excellent stability and selectivity. It was evaluated by analyzing human serum sample. The recovery obtained was 98.80-112.00% and the RSD was 1.73-3.12%, indicating the immunosensor could be applied to the simultaneous detection of PSA in human serum samples. Graphical abstract.

Functionalized nanoprobes for in situ detection of telomerase

Telomerase, a special ribonucleoprotein reverse transcriptase, can maintain the length and stability of telomeres and plays an important role in cell proliferation and differentiation. Due to the distinguishable expression level in normal cells and cancer cells, telomerase has become an important biomarker for cancer diagnosis and prognosis evaluation. Despite major breakthroughs in the field of telomerase detection, the extracts in the cell lysate are still the first choice as the analyte nevertheless, which will bring serious inaccuracies compared with the real intracellular activity. With the development of nanotechnology and nanomaterials, extraordinary progress has been made in telomerase detection by employing different versatile nanoprobes. In this review, we list the superiority of nanoprobes and systematically summarize the applications of nanoprobes in telomerase detection from the aspects of various nanomaterials and discuss the current challenges and potential trends in the future design of nanoprobes.

Single-cell-resolved measurement of enzyme activity at the tissue level using drop-on-demand microkits

Drop-on-demand microkits with a diameter of ∼20 μm are used to measure the activity of acetylcholinesterase (AChE) in a brain slice with single-cell resolution. The relative standard deviation from 25 cellular regions reached 73.3% exhibiting the difference of enzyme activity in the brain slice. Therefore, this approach utilizing the well-established kits provides an alternative single-cell-resolved strategy for the elucidation of enzymatic heterogeneity at the tissue level.

DNA Nanomachines for Identifying Cancer Biomarkers in Body Fluids and Cells

Identifying molecular biomarkers promises to significantly improve the accuracy in cancer diagnosis at its early stage. DNA nanomachines, which are designable and switchable nanostructures made of DNA, show broad potential to detect tumor biomarkers with noninvasive, inexpensive, highly sensitive, and highly specific advantages. This Feature summarizes the recent DNA nanomachine-based platforms for the early detection of cancer biomarkers, both from body fluids and in cells.

A light-up "G-quadruplex nanostring" for label-free and selective detection of miRNA via duplex-specific nuclease mediated tandem rolling circle amplification

MicroRNA (miRNA) has emerged as a promising genetic marker for cancer diagnosis and therapy because its expression level is closely related to the progression of malignant diseases. Herein, a label-free and selective fluorescence platform was proposed for miRNA based on light-up "G-quadruplex nanostring" via duplex-specific nuclease (DSN) mediated tandem rolling circle amplification (RCA). First, a long DNA generated from upstream RCA was designed with the antisense sequences for miR-21 and downstream RCA primer. Upon recognizing miR-21, the resulting DNA-RNA permitted DSN digestion and triggered downstream two-way RCA, and generation of abundant "G-quadruplex nanostring" binding with ZnPPIX for label-free fluorescent responses. In our strategy, the strong preference of DSN for perfectly matched DNA/RNA ensures its excellent selectivity. The developed method generated wide linear response with LOD of 1.019 fM. Additionally, the miR-21 levels in cell extracts have been evaluated, revealing the utility of this tool for biomedical research and clinical diagnosis.

Core-Shell Multifunctional Nanomaterial-Based All-in-One Nanoplatform for Simultaneous Multilayer Imaging of Dual Types of Tumor Biomarkers and Photothermal Therapy

Versatile all-in-one nanoplatforms that inherently possess both diagnostic imaging and therapeutic capabilities are highly desirable for efficient tumor diagnosis and treatment. Herein, we have developed a novel core-shell multifunctional nanomaterial-based all-in-one nanoplatform composed of gold nanobipyramids@polydopamine (Au NBPs@PDA) and gold nanoclusters (Au NCs) for simultaneous in situ multilayer imaging of dual types of tumor biomarkers (using a single-wavelength excitation) with different intracellular spatial distributions and fluorescence-guided photothermal therapy. The competitive combination between target transmembrane glycoprotein mucin1 (MUC1) and its aptamer caused Au NCs (620 nm) labeled with MUC1 aptamer to detach from the surface of Au NBPs@PDA, turning on the red fluorescence. Meanwhile, the hybridization between microRNA-21 (miRNA-21) and its complementary single-stranded DNA triggered the green fluorescence of Au NCs (515 nm). Based on this, simultaneous in situ multilayer imaging of dual types of tumor biomarkers with different intracellular spatial distributions was achieved. In addition, the potential of Au NBPs@PDA/Au NCs was also confirmed by simultaneous multilayer in situ imaging within not only three cell lines (MCF-7, HepG2, and L02 cells) with different expression levels of MUC1 and miRNA-21 but also cancer cells treated with different inhibitors. Moreover, the remarkable photothermal properties of Au NBPs@PDA resulted in the more efficient killing of cancer cells, demonstrating the great promise of the all-in-one nanoplatform for accurate diagnosis and tumor therapy.

Magnetic bead-enzyme assemble for triple-parameter telomerase detection at single-cell level

In this work, we developed a triple-parameter strategy for the detection of telomerase activity from cancer cells and urine samples. This strategy was developed based on magnetic bead-enzyme hybrids combined with fluorescence analysis, colorimetric assay, or adenosine triphosphate (ATP) meter as readout. The application of magnetic bead-enzyme hybrids has the advantages of magnetic separation and signal amplification. These detection methods can be used individually or in combination to achieve the optimal sensing performance and make the results more convincing. Among them, the ATP meter with portable size had easy operation and low cost, and this response strategy provided a higher sensitivity at the single-cell level. The designed strategy was suitable as naked-eye sensor and point-of-care testing (POCT) for rapid assaying of telomerase activity. Graphical abstract Magnetic bead-enzyme assemble for triple-parameter telomerase detection.

Quantitative Detection and Imaging of Multiple Biological Molecules in Living Cells for Cell Screening

Because of insufficient information, a single biomarker is not sufficient for early diagnosis of cancer, whereas sensitive and selective detection of multiple biomolecules can significantly reduce analysis time, sample size, and accurately perform cell screening in early cancer. Therefore, the development of a noninvasive strategy that can simultaneously quantify multiple biomarkers (i.e., nucleic acids, proteins, and small molecules) in a single cell is particularly important. Herein, a universal sensing system (functional DNA@mesoporous silica nanoparticles (MSN)-Black Hole Quencher-rhodamine 6G (RhB), FDSBR), which is based on the combination of functionalized DNA and smart responsive nanomaterial, was successfully constructed. After incubation with the cells, different types of targets trigger the strand displacement reaction to release the fluorophore-labeled nucleic acids as the output signals to reflect the intracellular level of the telomerase and adenosine triphosphate (ATP), respectively. Simultaneously, intracellular miR-21 can be clearly indicated by the restored fluorescence of RhB when the caged double-stranded DNA was substituted into single-stranded DNA to open the pore. The concentrations of intracellular telomerase, miR-21, and ATP were identified successfully in three cell lines at the single-cell level. The results show that the contents of three biomolecules have significant differences in the three model cell lines and provide a promising route for developing innovative early disease diagnosis and cell screening assay.

Biodegradable MnO2 nanosheet mediated hybridization chain reaction for imaging of human apurinic/apyrimidinic endonuclease 1 activity in living cells

A highly sensitive enzyme-free amplification assay for the detection of apurinic/apyrimidinic endonuclease 1 activity was developed. By incorporating biodegradable MnO₂ nanosheets, in situ light up intracellular APE 1 activity was achieved.

In situ Analysis of Cancer Cells Based on DNA Signal Amplification and DNA Nanodevices

Cancer is a global disease which has been disturbing researchers in medicine and seriously threatens patients' health and lifetime around the world in the past several decades. Due to the characteristics of cancer cells, such as uncontrollable cell proliferation, cell invasion and metastasis to surrounding tissues, lower grade of differentiation, higher telomerase activity and others, it has been one of the most usual lethal factors, next to heart disease in incidence. Cancer mortality can be decreased by early diagnosis, and the people who with treatment at an early stage have an obvious improved survival rate. Consequently, early detection is significant for better understanding the pathogenesis of cancer and improving the prognosis of patients. In situ detection technique is a vital tool for imaging and cellular pathology research, which can provide effective information about tumor markers in the early cancer detection. In view of low expression of most tumor markers in the early stage of cancers, detection techniques based on DNA signal amplification and DNA nanodevices can provide a strong support for the diagnosis and detection of cancers. In this review, we summarize the research progress of different analytical techniques for detecting various tumor markers that have been reported in recent years. We compare different DNA amplification and nanodevices, then provide guidance and suggestions for better understanding in situ analysis of cancer cells.

A seesaw ratiometric probe for dual-spectrum imaging and detection of telomerase activity in single living cells

Herein, a seesaw ratiometric (SR) probe is designed which integrates fluorescence and surface enhanced Raman scattering (SERS) technology. Fluorescence imaging enables tracking of the spatiotemporal dynamic behaviour of telomerase. Meanwhile, SERS reverse ratiometric measurement can enable sensitive detection of telomerase activity in single living cells.

Highly sensitive fluorescence biosensor for intracellular telomerase detection based on a single patchy gold/carbon nanosphere via the combination of nanoflare and hybridization chain reaction

How to in situ detect intracellular telomerase activity with high sensitivity still faces many challenges. This paper constructs a new fluorescence biosensing platform for the sensitive detection of intracellular telomerase activity via the combination of nanoflare and hybridization chain reaction (HCR)-based signal amplification on a single patchy gold/carbon nanosphere (PG/CNS), which has two or more distinct parts and allows hybridized-DNA (HS-DNA/Primer-DNA/Flare-DNA) and H1/H2-DNA (a pair of cross complementary DNA hairpins) to bind onto their surfaces via Au-S bond and electrostatic interaction, respectively. In the presence of telomerase, Primer-DNA (telomerase primer) extends at its 3' end to produce a telomeric repeated sequence, resulting in the release of Flare-DNA followed by the recovery of the fluorescence. Subsequently, the released Flare-DNA further initiates cross hybridization of H1 and H2 DNA from mimic-HCR system to amplify the fluorescence signal. The in vivo confocal microscopy studies demonstrate that resulting sensor can enter into the cancer cells such as A549 cells, and lead to the increase in luminescence, which is stronger than the sensor without the HCR-based signal amplification system. A linear relationship between the fluorescence intensity and the amount of A549 cells is observed, and the limit of detection of the sensor reaches about 280 A549 cells.

Telomerase and poly(ADP-ribose) polymerase-1 activity sensing based on the high fluorescence selectivity and sensitivity of TOTO-1 towards G bases in single-stranded DNA and poly(ADP-ribose)

Telomerase and poly(ADP-ribose) polymerase-1 (PARP-1) are two potential cancer biomarkers and are closely related to tumor initiation and malignant progression. TOTO-1 is well-known for differentiating ss-DNA from ds-DNA because it is virtually non-fluorescent without DNA and exhibits very low fluorescence with ss-DNA, while it emits strong fluorescence with ds-DNA. In this paper, for the first time, it was found that TOTO-1 has high fluorescence selectivity and sensitivity towards the G bases in single-stranded DNA and poly(ADP-ribose) (PAR). Poly(dG) was used as the model target to explore its possible mechanism. Molecular dynamics (MD) simulation proved that intramolecular π-π stacking existed in TOTO-1 (in an aqueous solution), while intermolecular π-π stacking formed between TOTO-1 and poly(dG) in a similar way as that observed for dsDNA. Interestingly, telomerase and PARP-1 catalyzed the formation of G-rich DNA and PAR in vivo, respectively. Therefore, TOTO-1 was explored in detecting both of them, obtaining satisfactory results. To the best of our knowledge, no probe has been reported to recognize PAR. It is also the first time where telomerase is detected based on the specific recognition of G bases. Importantly, integrating multiple functions into one probe that can detect not only telomerase but also PARP-1 will significantly raise the specificity of screening cancer and decrease false positive proportion, which make TOTO-1 a promising candidate probe for clinical diagnosis and pharmaceutical screening.

Biomedical applications of nanoflares: Targeted intracellular fluorescence probes

Nanoflares are intracellular probes consisting of oligonucleotides immobilized on various nanoparticles that can recognize intracellular nucleic acids or other analytes, thus releasing a fluorescent reporter dye. Single-stranded DNA (ssDNA) complementary to mRNA for a target gene is constructed containing a 3'-thiol for binding to gold nanoparticles. The ssDNA "recognition sequence" is prehybridized to a shorter DNA complement containing a fluorescent dye that is quenched. The functionalized gold nanoparticles are easily taken up into cells. When the ssDNA recognizes its complementary target, the fluorescent dye is released inside the cells. Different intracellular targets can be detected by nanoflares, such as mRNAs coding for genes over-expressed in cancer (epithelial-mesenchymal transition, oncogenes, thymidine kinase, telomerase, etc.), intracellular levels of ATP, pH values and inorganic ions can also be measured. Advantages include high transfection efficiency, enzymatic stability, good optical properties, biocompatibility, high selectivity and specificity. Multiplexed assays and FRET-based systems have been designed.

Non-invasive diagnosis of bladder cancer by detecting telomerase activity in human urine using hybridization chain reaction and dynamic light scattering

Cystoscopy and histology are the gold standards for detection of bladder cancer. However, these methods are highly subjective, expensive, and invasive. We have developed a non-invasive method for the diagnosis of bladder cancer by detecting telomerase activity in human urine. Telomerase substrate (TS) primer is elongated with repeating sequences of (TTAGGG)n in the presence of telomerase. The elongated primer can trigger hybridization chain reaction between two hairpins H1 and H2, result in the aggregation of AuNPs due to the hybridization between the tail sequence on H1 (or H2) and DNA-AuNPs probe, and accompany with the increase of hydrodynamic diameter of AuNPs, which can be measured with dynamic light scattering (DLS). The biosensor displayed a detection limit of 4 MCF-7 cells (a signal-to-noise ratio of 3) and a dynamic range of 10-1000 cells. Moreover, only urine specimens from bladder cancer patients induced a significant change in the average hydrodynamic diameter, indicating its specificity for the non-invasive diagnosis of bladder cancer.

Visualizing the down-regulation of hTERT mRNA expression using gold-nanoflare probes and verifying the correlation with cancer cell apoptosis

The human telomerase reverse transcriptase catalytic subunit (hTERT) is the rate-limiting subunit of the telomerase holoenzyme. Down-regulating the expression of hTERT mRNA by antisense oligonucleotides would reduce the expression of hTERT, inhibit telomerase activity, and impair the growth of cancer cells in vitro. In this work, we propose a locked nucleic acid-functionalized gold nanoparticle flare probe (AuNP-probe). After transferring these probes into cells by endocytosis of the gold nanoparticles, the binding process of the antisense locked nucleic acid with hTERT mRNA along with gene regulation can be visualized by fluorescence recovery of flare-sequences. A significant decline in hTERT mRNA levels and the hTERT content occurred in cancer cells after treatment with the AuNP-probes, and only approximately 25% of the original level of hTERT mRNA remained after 72 h. AuNP-probe treated cancer cells were arrested in the G1 phase of the cell cycle and underwent apoptosis; cell viability decreased obviously compared with that of telomerase-negative normal cells.