Fidelity-oriented fluorescence imaging probes for beta-galactosidase: From accurate diagnosis to precise treatment

Beta-galactosidase (β-gal), a typical glycosidase catalyzing the hydrolysis of glycosidic bonds, is regarded as a vital biomarker for cell senescence and cancer occurrence. Given the advantages of high spatiotemporal resolution, high sensitivity, non-invasiveness, and being free of ionizing radiations, fluorescent imaging technology provides an excellent choice for in vivo imaging of β-gal. In this review, we detail the representative biotech advances of fluorescence imaging probes for β-gal bearing diverse fidelity-oriented improvements to elucidate their future potential in preclinical research and clinical application. Next, we propose the comprehensive design strategies of imaging probes for β-gal with respect of high fidelity. Considering the systematic implementation approaches, a range of high-fidelity imaging-guided theragnostic are adopted for the individual β-gal-associated biological scenarios. Finally, current challenges and future trends are proposed to promote the next development of imaging agents for individual and specific application scenarios.

AIE-Active Glycomimetics Triggered Bacterial Agglutination and Membrane-Intercalating toward Efficient Photodynamic Antiseptic

Opportunistic infections caused by Pseudomonas aeruginosa (P. aeruginosa) are particularly difficult to treat due to the altered membrane permeability and inherent resistance to conventional antibiotics. Here, a cationic glycomimetics is designed and synthesized with aggregation-induced emission (AIE) characteristics namely TPyGal, which self-assembles into the spherical aggregates with galactosylated surface. TPyGal aggregates can effectively cluster P. aeruginosa through multivalent carbohydrate-lectin interactions and auxiliary electrostatic interactions and subsequently trigger membrane-intercalating, which results in efficient photodynamic eradication of P. aeruginosa under white light irradiation by in situ singlet oxygen (1 O2 ) burst to disrupt bacterial membrane. Furthermore, the results demonstrate that TPyGal aggregates promote the healing of infected wounds, indicating the potential for clinical treatment of P. aeruginosa infections.

Efficient detection of L-aspartic acid and L-glutamic acid by self-assembled fluorescent microparticles with AIE and FRET activities

Amino acids play an important role in the formation of proteins, enzymes, hormones and peptides in animals. Moreover, aspartic acid and glutamic acid have a critical impact on the central nervous system as excitatory neurotransmitters. Here, we report the highly selective detection of L-glutamic acid (L-Glu) and L-aspartic acid (L-Asp) using fluorescent microparticles constructed by the combination of aggregation-induced emission and self-assembly-induced Förster resonance energy transfer.

A novel red-emitting aggregation-induced emission probe for determination of β-glucosidase activity

β-Glucosidase (β-Glu) is a ubiquitous enzyme which has multiple roles in medical diagnosis, food production, agriculture, etc. Existing β-Glu assays have limitations such as complex operation, long running time, and high background noise. Here we report a red-emissive probe TBPG for measuring the activity of β-Glu. The probe was synthesized through conjugating a β-Glu targeting glucoside to an aggregation-induced emission (AIE) fluorophore. In the presence of β-Glu, TBPG was hydrolyzed and exhibited a fluorescence turn-on process. The detection conditions including time, temperature, pH value, buffer, and probe concentration were optimized systematically. Afterwards, fluorescence titration was conducted showing an excellent linearity (R² = 0.998), a wide linear dynamic range (0-5.0 U/mL), and a limit of detection as low as 0.6 U/L. The detection specificity and ion interference were evaluated by adding various biological species and ions to probe without or with β-Glu. Next, we demonstrate the applicability of probe TBPG in determining the β-Glu activity in living cells using confocal microscopy and flow cytometry. Finally, this newly established assay was applied to real soil samples. Comparable results were obtained as the commercial assay, manifesting its great potential in soil enzyme analysis.

Lipofuscin labeling through biorthogonal strain-promoted azide-alkyne cycloaddition for the detection of senescent cells

A new method for senescent cell detection is described, which is based on lipofuscin labeling with a fluorescent reporter through a biorthogonal strain-promoted azide-alkyne cycloaddition. The sensing protocol involves a first step where the interaction of lipofuscin with a Sudan Black B derivative containing an azide moiety (SBB-N₃ ) is carried out. In the final step, the azide moiety reacts with a fluorophore containing a cyclooctene ring (BODIPY). The efficacy of this two-step protocol is assessed in senescent melanoma SK-MEL-103 cells, senescent triple-negative breast cancer MDA-MB-231 cells and senescent WI-38 fibroblasts. In all cases, a clear fluorescence pattern was observed in senescent cells, compared to proliferative cells, only when the SBB-N₃ -BODIPY probe was formed. Our results provide an alternative tool for the detection of senescent cells, based on an in situ bio-orthogonal reaction for lipofuscin labeling.

Noncancerous disease-targeting AIEgens

Noncancerous diseases include a wide plethora of medical conditions beyond cancer and are a major cause of mortality around the world. Despite progresses in clinical research, many puzzles about these diseases remain unanswered, and new therapies are continuously being sought. The evolution of bio-nanomedicine has enabled huge advancements in biosensing, diagnosis, bioimaging, and therapeutics. The recent development of aggregation-induced emission luminogens (AIEgens) has provided an impetus to the field of molecular bionanomaterials. Following aggregation, AIEgens show strong emission, overcoming the problems associated with the aggregation-caused quenching (ACQ) effect. They also have other unique properties, including low background interferences, high signal-to-noise ratios, photostability, and excellent biocompatibility, along with activatable aggregation-enhanced theranostic effects, which help them achieve excellent therapeutic effects as an one-for-all multimodal theranostic platform. This review provides a comprehensive overview of the overall progresses in AIEgen-based nanoplatforms for the detection, diagnosis, bioimaging, and bioimaging-guided treatment of noncancerous diseases. In addition, it details future perspectives and the potential clinical applications of these AIEgens in noncancerous diseases are also proposed. This review hopes to motivate further interest in this topic and promote ideation for the further exploration of more advanced AIEgens in a broad range of biomedical and clinical applications in patients with noncancerous diseases.

Fluorescent Organic Small Molecule Probes for Bioimaging and Detection Applications

The activity levels of key substances (metal ions, reactive oxygen species, reactive nitrogen, biological small molecules, etc.) in organisms are closely related to intracellular redox reactions, disease occurrence and treatment, as well as drug absorption and distribution. Fluorescence imaging technology provides a visual tool for medicine, showing great potential in the fields of molecular biology, cellular immunology and oncology. In recent years, organic fluorescent probes have attracted much attention in the bioanalytical field. Among various organic fluorescent probes, fluorescent organic small molecule probes (FOSMPs) have become a research hotspot due to their excellent physicochemical properties, such as good photostability, high spatial and temporal resolution, as well as excellent biocompatibility. FOSMPs have proved to be suitable for in vivo bioimaging and detection. On the basis of the introduction of several primary fluorescence mechanisms, the latest progress of FOSMPs in the applications of bioimaging and detection is comprehensively reviewed. Following this, the preparation and application of fluorescent organic nanoparticles (FONPs) that are designed with FOSMPs as fluorophores are overviewed. Additionally, the prospects of FOSMPs in bioimaging and detection are discussed.

An enzyme-activatable dual-readout probe for sensitive β-galactosidase sensing and Escherichia coli analysis

Rapid and accurate sensing of β-galactosidase (β-gal) activity is particularly critical for the early detection of many diseases and has become a topic of interest in recent years. However, most traditional probes for β-gal sensing often suffer from the disadvantages of narrow dynamic range, low reaction efficiency and are only employed with either colorimetric or fluorescence sensing. Furthermore, β-galactosidase sensing based assay for efficient detection and antibiotic resistance analysis of Escherichia coli (E.coli) is not available. Here, an enzyme-induced probe assay was reported for dual sensitive fluorescence and colorimetric measurement of β-gal activity, and was further employed for detection of Escherichia coli and their antibiotic resistance analysis. The DCM-βgal probe was virtually non-emissive in aqueous solution, while it could be activated by β-gal to produce bright emission. Under optimized conditions, DCM-βgal displayed high sensitivity, selectivity and rapid response to β-gal with a low detection limit of 1.5 × 10⁻³ U ml⁻¹. Importantly, this assay was successfully applied to sensitive detection of E. coli cells with a fast detection process within 5 h and a low detection concentration of 1 × 10³ CFU ml⁻¹. Furthermore, the enzyme-activatable assay was also successfully applied for high throughput E. coli antibiotic resistance analysis. The DCM-βgal strategy is applied for the first time on the detection of E. coli cells and their antibiotic resistance analysis. It is provided with the advantages of high selectively, a simple operation, low cost and rapid detection. The detection platform can also be extended to analyze the level of β-gal in other types of cells or biological samples. Overall, the simple, effective and dual-readout assay holds promise for efficient sensing of β-gal activity and provides a potential tool for E. coli detection and their antibiotic resistance analysis.

Reactive Species-Activatable AIEgens for Biomedical Applications

Precision medicine requires highly sensitive and specific diagnostic strategies with high spatiotemporal resolution. Accurate detection and monitoring of endogenously generated biomarkers at the very early disease stage is of extensive importance for precise diagnosis and treatment. Aggregation-induced emission luminogens (AIEgens) have emerged as a new type of excellent optical agents, which show great promise for numerous biomedical applications. In this review, we highlight the recent advances of AIE-based probes for detecting reactive species (including reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive sulfur species (RSS), and reactive carbonyl species (RCS)) and related biomedical applications. The molecular design strategies for increasing the sensitivity, tuning the response wavelength, and realizing afterglow imaging are summarized, and theranostic applications in reactive species-related major diseases such as cancer, inflammation, and vascular diseases are reviewed. The challenges and outlooks for the reactive species-activatable AIE systems for disease diagnostics and therapeutics are also discussed. This review aims to offer guidance for designing AIE-based specifically activatable optical agents for biomedical applications, as well as providing a comprehensive understanding about the structure-property application relationships. We hope it will inspire more interesting researches about reactive species-activatable probes and advance clinical translations.

Aggregation-Induced Emission (AIE) Photosensitizer Combined Polydopamine Nanomaterials for Organelle-Targeting Photodynamic and Photothermal Therapy by the Recognition of Sialic Acid

The construction of organelle-targeting nanomaterials is an effective way to improve tumor imaging and treatment. Here, a new type of composite nanomaterial named as PTTPB is developed. PTTPB is composed of organelle-targeting aggregation-induced emission photosensitizer TTPB and polydopamine nanomaterials. With the functional modification of TTPB, PTTPB can recognize sialic acid on the cell membrane and present mitochondrial targeted capabilities. The intake of PTTPB in cancerous cells can be increased by the recognition process of cell membrane. PTTPB can generate singlet oxygen for photodynamic therapy (PDT), and present good photothermal conversion ability with irradiation. The PTTPB with organelle-targeting imaging-guided can realize the tumor ablation with the synergistic effect of PDT and photothermal therapy.

Activatable senoprobes and senolytics: Novel strategies to detect and target senescent cells

Pharmacologically active compounds that manipulate cellular senescence (senotherapies) have recently shown great promise in multiple pre-clinical disease models, and some of them are now being tested in clinical trials. Despite promising proof-of-principle evidence, there are known on- and off-target toxicities associated with these compounds, and therefore more refined and novel strategies to improve their efficacy and specificity for senescent cells are being developed. Preferential release of drugs and macromolecular formulations within senescent cells has been predominantly achieved by exploiting one of the most widely used biomarkers of senescence, the increase in lysosomal senescence-associated β-galactosidase (SA-β-gal) activity, a common feature of most reported senescent cell types. Galacto-conjugation is a versatile therapeutic and detection strategy to facilitate preferential targeting of senescent cells by using a variety of existing formulations, including modular systems, nanocarriers, activatable prodrugs, probes, and small molecules. We discuss the benefits and drawbacks of these specific senescence targeting tools and how the strategy of galacto-conjugation might be utilised to design more specific and sophisticated next-generation senotherapeutics, as well as theranostic agents. Finally, we discuss some innovative strategies and possible future directions for the field.

A turn on fluorescent assay for real time determination of β-galactosidase and its application in living cell imaging

In recent years, fluorescent probes based on chemical reactions have been widely investigated as a powerful and noninvasive method for the diagnosis of diseases. β-Galactosidase (β-gal), a typical lysosomal glycosidase, over expressed in senescent cells and primary ovarian cancer cells, which has been considered as an important biomarker cell senescence and primary ovarian cancers. Fluorescent probes for the determination of β-gal provide an excellent choice for visualization of cell senescence. In this work, a turn on fluorescent probe (HBT-gal) for β-gal activity was developed based on the enzymatic hydrolysis of glycosidic bonds. HBT-gal showed little fluorescence in aqueous buffer excited at 415 nm, while emitted green fluorescence centered at ∼ 492 nm upon incubated with β-gal. The sensing scheme showed high selectivity and sensitivity for β-gal activity with a limit of detection calculated as low as 0.19 mU/mL. Moreover, HBT-gal was successfully applied to image β-gal activity in senescent Hep G2 cells treated with H₂O₂. Therefore, probe HBT-gal demonstrated a potential usage for the determination of cell senescence using β-gal as a biomarker.

A new near-infrared excitation/emission fluorescent probe for the detection of β-galactosidase in living cells and in vivo

Development of noninvasive bioimaging fluorescent probes for detecting particular enzyme activity is greatly recommendable for preclinical diagnosis of cancer. Given that the elevated β-gal activity is positively correlated with several tumors, developing a fluorescent probe for the sensing of β-gal is therefore highly desirable for cancer diagnosis. Herein, a new enzyme-activatable near-infrared (NIR) turn-on fluorescent probe (DMC-βgal) was developed for accurately detecting β-gal activity characterized by excellent selectivity, high sensitivity (LOD = 0.298 U/L), and low toxicity. More importantly, DMC-βgal qualifies remarkable NIR excitation (725 nm) and emission wavelength (770 nm), an ideal tool for restrained photodamage and suppressed autofluorescence. The above excellent performance of DMC-β-gal allowed for the accurate monitoring of endogenous β-gal in living cells. Moreover, the probe was successfully applied to detect intracellular β-gal activity in different types of cancer cells, verifying that SKOV-3 cells had a higher level of β-gal activity than those of A549, HCT-116, MCF-7, and HepG2 cells. Furthermore, DMC-βgal could real-time visualize endogenously β-gal in tumor-bearing nude mice with low auto-fluorescence interference. All results fully demonstrated that DMC-βgal has potential value as a promising strategy for diagnosis of β-gal-related diseases.

Multifunctional stimuli-responsive chemogenetic platform for conditional multicolor cell-selective labeling

Multicolor conditional labeling is a powerful tool that can simultaneously and selectively visualize multiple targets for bioimaging analysis of complex biological processes and cellular features. We herein report a multifunctional stimuli-responsive Fluorescence-Activating and absorption-Shifting Tag (srFAST) chemogenetic platform for multicolor cell-selective labeling. This platform comprises stimuli-responsive fluorogenic ligands and the organelle-localizable FAST. The physicochemical properties of the srFAST ligands can be tailored by modifying the optical-tunable hydroxyl group with diverse reactive groups, and their chemical decaging process caused by cell-specific stimuli induces a conditionally activatable fluorescent labeling upon binding with the FAST. Thus, the resulting switch-on srFASTs were designed for on-demand labeling of cells of interest by spatiotemporally precise photo-stimulation or unique cellular feature-dependent activation, including specific endogenous metabolites or enzyme profiles. Furthermore, diverse enzyme-activatable srFAST ligands with distinct colors were constructed and simultaneously exploited for multicolor cell-selective labeling, which allow discriminating and orthogonal labeling of three different cell types with the same protein tag. Our method provides a promising strategy for designing a stimuli-responsive chemogenetic labeling platform via facile molecular engineering of the synthetic ligands, which has great potential for conditional multicolor cell-selective labeling and cellular heterogeneity evaluation.

Comparison of the stimuli-responsive FAST platform (srFAST) proposed in this work with the reported original FAST system (O-FAST). The srFAST could achieve not only conditional selective labeling, but also multicolor selective labeling.

Glycosidase activated prodrugs for targeted cancer therapy

In this review glycosidase activated prodrugs that target cancer cells are discussed.

A general strategy to the intracellular sensing of glycosidases using AIE-based glycoclusters

Glycosidases, which are the enzymes responsible for the removal of residual monosaccharides from glycoconjugates, are involved in many different biological and pathological events. The ability to detect sensitively the activity and spatiotemporal distribution of glycosidases in cells will provide useful tools for disease diagnosis. However, the currently developed fluorogenic probes for glycosidases are generally based on the glycosylation of the phenol group of a donor-acceptor type fluorogen. This molecular scaffold has potential drawbacks in terms of substrate scope, sensitivity because of aggregation-caused quenching (ACQ), and the inability for long-term cell tracking. Here, we developed glycoclusters characterized by aggregation-induced emission (AIE) properties as a general platform for the sensing of a variety of glycosidases. To overcome the low chemical reactivity associated with phenol glycosylation, here we developed an AIE-based scaffold, which is composed of tetraphenylethylene conjugated with dicyanomethylene-4H-pyran (TPE-DCM) with a red fluorescence emission. Subsequently, a pair of dendritic linkages was introduced to both sides of the fluorophore, to which six copies of monosaccharides (d-glucose, d-galactose or l-fucose) were introduced through azide-alkyne click chemistry. The resulting AIE-active glycoclusters were shown to be capable of (1) fluorogenic sensing of a diverse range of glycosidases including β-d-galactosidase, β-d-glucosidase and α-l-fucosidase through the AIE mechanism, (2) fluorescence imaging of the endogenous glycosidase activities in healthy and cancer cells, and during cell senescence, and (3) glycosidase-activated, long-term imaging of cells. The present study provides a general strategy to the functional, in situ imaging of glycosidase activities through the multivalent display of sugar epitopes of interest onto properly designed AIE-active fluorogens.

Progress and Perspective of Solid-State Organic Fluorophores for Biomedical Applications

Fluorescent organic dyes have been extensively used as raw materials for the development of versatile imaging tools in the field of biomedicine. Particularly, the development of solid-state organic fluorophores (SSOFs) in the past 20 years has exhibited an upward trend. In recent years, studies on SSOFs have focused on the development of advanced tools, such as optical contrast agents and phototherapy agents, for biomedical applications. However, the practical application of these tools has been hindered owing to several limitations. Thus, in this Perspective, we have provided insights that could aid researchers to further develop these tools and overcome the limitations such as limited aqueous dispersibility, low biocompatibility, and uncontrolled emission. First, we described the inherent photophysical properties and fluorescence mechanisms of conventional, aggregation-induced emissive, and precipitating SSOFs with respect to their biomedical applications. Subsequently, we highlighted the recent development of functionalized SSOFs for bioimaging, biosensing, and theranostics. Finally, we elucidated the potential prospects and limitations of current SSOF-based tools associated with biomedical applications.

First aggregation-induced emission-active probe for species-specific detection of β-galactosidase

Sensitive detection of β-galactosidase (β-gal) is of great significance for early diagnosis of ovarian cancer. Fluorescent probes for detecting β-gal have received great interest due to the non-invasiveness, excellent sensitivity, high temporal, and superior spatial resolution. However, most reported fluorescent sensors for β-gal suffer from aggregation caused quenching effect when accumulated, and cannot discriminate β-gal from other species, especially, Escherichia coliβ-gal. Herein, we report the first aggregation-induced emission (AIE)-active fluorescent probe HBTTPAG, which achieves species-selective detection of β-gal. Probe HBTTPAG can discriminate Aspergillus oryzae β-gal from Escherichia coliβ-gal, with high sensitivity (detection limit of 3.7 × 10-3 UmL-1), superior selectivity and low cytotoxicity. Furthermore, HBTTPAG is utilized to visualize endogenous β-gal in lysosomes of SKOV-3 cells, as well as to detect β-gal activity in ovarian cancer tissues. Notably, owing to the AIE-active, HBTTPAG realizes long-term (12 h) tracking β-gal in ovarian cancer cells. This work provides a promising method for species-selective detection of β-gal in preclinical.

Fluorescent probes for visualizing ROS-associated proteins in disease

Abnormal expression of proteins, including catalytic and expression dysfunction, is directly related to the development of various diseases in living organisms. Reactive oxygen species (ROS) could regulate protein expression by redox modification or cellular signal pathway and thus influence the development of disease. Determining the expression level and activity of these ROS-associated proteins is of considerable importance in early-stage disease diagnosis and the identification of new drug targets. Fluorescence imaging technology has emerged as a powerful tool for specific in situ imaging of target proteins by virtue of its non-invasiveness, high sensitivity and good spatiotemporal resolution. In this review, we summarize advances made in the past decade for the design of fluorescent probes that have contributed to tracking ROS-associated proteins in disease. We envision that this review will attract significant attention from a wide range of researchers in their utilization of fluorescent probes for in situ investigation of pathological processes synergistically regulated by both ROS and proteins.

Methods of detection of β-galactosidase enzyme in living cells

The application of β-galactosidase enzyme ranges from industrial use as probiotics to medically important application such as cancer detection. The irregular activities of β-galactosidase enzyme are directly related to the development of cancers. Identifying the location and expression levels of enzymes in cancer cells have considerable importance in early-stage cancer diagnosis and monitoring the efficacy of therapies. Most importantly, the knowledge of the efficient method of detection of β-galactosidase enzyme will help in the early-stage treatment of the disease. In this review paper, we provide an overview of recent advances in the detection methods of β-galactosidase enzyme in the living cells, including the detection strategies, and approaches in human beings, plants, and microorganisms such as bacteria. Further, we emphasized on the challenges and opportunities in this rapidly developing field of development of different biomarkers and fluorescent probes based on β-galactosidase enzyme. We found that previously used chromo-fluorogenic methods have been mostly replaced by the new molecular probes, although they have certain drawbacks. Upon comparing the different methods, it was found that near-infrared fluorescent probes are dominating the other detection methods.

Bactericidal activity of a holin-endolysin system derived from Vibrio alginolyticus phage HH109

Vibrio alginolyticus is a common opportunistic pathogen that can cause vibriosis of marine aquatic animals. The application of phages or particularly associated protein products for the treatment of vibriosis has shown prominent advantages compared with the treatment with traditional antibiotics. In this study, the function of a holin-endolysin system from V. alginolyticus phage HH109 was characterized by examining the effect of their overexpression on Escherichia coli and V. alginolyticus. Our data revealed that the endolysin of the phage HH109 has stronger bactericidal activity than the holin, as evidenced by observing more cell death and severe structural damage of cells in the endolysin-expressing E. coli. Furthermore, the two proteins displayed the synergistic effect when the holA and lysin were co-expressed in E. coli, although no interaction between them was detected using the bacterial two-hybrid assay. Transmission electron microscopy observation revealed disruptions of cell envelopes accompanied by leakage of intracellular contents. Similarly, the bactericidal activity of the holin and endolysin against V. alginolyticus was also examined whatever the host is sensitive or resistant to phage HH109. Together, our study contributes to a better understanding of the mechanism of phage HH109 destroying the bacterial cell wall to lyse their host and may offer alternative applications potentially for vibriosis treatment.

Enzyme-activatable fluorescent probes for β-galactosidase: from design to biological applications

β-Galactosidase (β-gal), a typical hydrolytic enzyme, is a vital biomarker for cell senescence and primary ovarian cancers. Developing precise and rapid methods to monitor β-gal activity is crucial for early cancer diagnoses and biological research. Over the past decade, activatable optical probes have become a powerful tool for real-time tracking and in vivo visualization with high sensitivity and specificity. In this review, we summarize the latest advances in the design of β-gal-activatable probes via spectral characteristics and responsiveness regulation for biological applications, and particularly focus on the molecular design strategy from turn-on mode to ratiometric mode, from aggregation-caused quenching (ACQ) probes to aggregation-induced emission (AIE)-active probes, from near-infrared-I (NIR-I) imaging to NIR-II imaging, and from one-mode to dual-mode of chemo-fluoro-luminescence sensing β-gal activity.

In Situ Generated Novel 1H MRI Reporter for β-Galactosidase Activity Detection and Visualization in Living Tumor Cells

For wide applications of the lacZ gene in cellular/molecular biology, small animal investigations, and clinical assessments, the improvement of noninvasive imaging approaches to precisely assay gene expression has garnered much attention. In this study, we investigate a novel molecular platform in which alizarin 2-O-β-d-galactopyranoside AZ-1 acts as a lacZ gene/β-gal responsive 1H-MRI probe to induce significant 1H-MRI contrast changes in relaxation times T 1 and T 2 in situ as a concerted effect for the discovery of β-gal activity with the exposure of Fe3+. We also demonstrate the capability of this strategy for detecting β-gal activity with lacZ-transfected human MCF7 breast and PC3 prostate cancer cells by reaction-enhanced 1H-MRI T 1 and T 2 relaxation mapping.

Chromo-fluorogenic probes for β-galactosidase detection

β-Galactosidase (β-Gal) is a widely used enzyme as a reporter gene in the field of molecular biology which hydrolyzes the β-galactosides into monosaccharides. β-Gal is an essential enzyme in humans and its deficiency or its overexpression results in several rare diseases. Cellular senescence is probably one of the most relevant physiological disorders that involve β-Gal enzyme. In this review, we assess the progress made to date in the design of molecular-based probes for the detection of β-Gal both in vitro and in vivo. Most of the reported molecular probes for the detection of β-Gal consist of a galactopyranoside residue attached to a signalling unit through glycosidic bonds. The β-Gal-induced hydrolysis of the glycosidic bonds released the signalling unit with remarkable changes in color and/or emission. Additional examples based on other approaches are also described. The wide applicability of these probes for the rapid and in situ detection of de-regulation β-Gal-related diseases has boosted the research in this fertile field.

A near-infrared fluorescent probe for monitoring and imaging of β-galactosidase in living cells

β-Galactosidase (β-gal) which is overexpressed in primary ovarian cancer can be employed as a valuable biomarker for ovarian cancer. Thus, monitoring and imaging endogenous β-gal in living cells is of great importance. Herein, a dicyanoisophorone-based near-infrared (NIR) fluorescent probe 2-(5,5-dimethyl-3-((E)-4-(((2R,3S,4R,5S,6S)-3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-2-yl)oxy)styryl)cyclohex-2-en-1-ylidene)malononitrile named DP-βgal, was rationally designed and synthesized for the monitoring of β-gal activity in living cells. In the presence of β-gal, with the breaking of the glycosidic bond, the NIR fluorescence of the dicyanoisophorone derivative gradually recovered, enabling the fluorescence "off-on" quantitative determination of β-gal activity. DP-βgal has the advantages of good selectivity and high sensitivity for the detection of β-gal, with the limit of detection (LOD) of 3.2 × 10^-3 U. Furthermore, based on its advantages of long-wavelength emission and excellent biocompatibility, the practical applications of DP-βgal in NIR imaging of β-gal in living ovarian cancer cells (SKOV-3) were demonstrated.

Specific Near-Infrared Probe for Ultrafast Imaging of Lysosomal β-Galactosidase in Ovarian Cancer Cells

Reactivity based fluorescent probes have been widely investigated as a powerful and noninvasive tool for disease diagnosis in recent years. β-Galactosidase (β-gal), one of the typical lysosomal glycosidases, is reported to be a vital biomarker overexpressed in primary ovarian cancer cells. Fluorescent probes with excellent performance for endogenous β-gal detection offer a unique option for visualization and diagnosis of primary ovarian cancer cells. Herein, a near-infrared fluorescent probe Lyso-Gal with lysosome-targeting ability was developed for lysosomal β-gal detection and imaging in ovarian cancer cells (SKOV-3 cells). Lyso-Gal exhibits weak fluorescence in aqueous solution but emits bright NIR fluorescence at 725 nm after incubation with β-gal. Highly selective imaging of ovarian cancer cells has been achieved upon incubation with Lyso-Gal for only 1 min. The detection time is extremely short. In comparison with a similar hemicyanine probe, Hx-Gal, without lysosome-targeting ability, Lyso-Gal realizes endogenous β-gal visualization in lysosomes and shows brighter fluorescence than Hx-Gal in SKOV-3 cells. This work demonstrates the potential of Lyso-Gal for detection of primary ovarian cancer cells by using β-gal as the biomarker.

Semi-Quantitatively Designing Two-Photon High-Performance Fluorescent Probes for Glutathione S-Transferases

Glutathione S-transferases (GSTs), detoxification enzymes that catalyze the addition of glutathione (GSH) to diverse electrophilic molecules, are often overexpressed in various tumor cells. While fluorescent probes for GSTs have often adopted the 2,4-dinitrobenzenesulfonyl (DNs) group as the receptor unit, they usually suffer from considerable background reaction noise with GSH due to excessive electron deficiency. However, weakening this reactivity is generally accompanied by loss of sensitivity for GSTs, and therefore, finely turning down the reactivity while maintaining certain sensitivity is critical for developing a practical probe. Here, we report a rational semiquantitative strategy for designing such a practical two-photon probe by introducing a parameter adopted from the conceptual density functional theory (CDFT), the local electrophilicity ω _k , to characterize this reactivity. As expected, kinetic studies established ω _k as efficient to predict the reactivity with GSH, and probe NI3 showing the best performance was successfully applied to detecting GST activities in live cells and tissue sections with high sensitivity and signal-to-noise ratio. Photoinduced electron transfer of naphthalimide-based probes, captured by femtosecond transient absorption for the first time and unraveled by theoretical calculations, also contributes to the negligible background noise.

Imaging of ovarian cancers using enzyme activatable probes with second near-infrared window emission

We herein develop two β-galactosidase (β-Gal) activatable NIR fluorescent probes for visualizing ovarian cancers. Particularly, probe BOD-M-βGal produced NIR-II emission light at 900-1300 nm upon β-Gal activation. By using our activatable and target specific NIR-II probe for deep-tissue imaging of β-Gal overexpressed ovarian cancer cells, rapid and accurate imaging of ovarian tumors in nude mice was achieved.

A near-infrared fluorescent probe for the ratiometric detection and living cell imaging of β-galactosidase

β-Galactosidase (β-gal) has captured the attention of biologists, chemists, and medical researchers as an important biomarker for cell senescence and primary ovarian cancer. Therefore, many fluorescent probes with visible light emission have been developed for the detection and imaging of β-gal in living cells. However, near-infrared (NIR) ratiometric probes are more suitable for bioimaging because near-infrared light can effectively avoid the interference of autofluorescence and the ratiometric approach can improve sensitivity and accuracy of the detection. In this work, we designed an NIR ratiometric probe (TMG) for the highly sensitive detection of β-gal. Using a spontaneous degradation mechanism based on the ICT effect, the change in ratio (F₆₅₀/F₅₈₀) exhibited a prominent β-gal-dependent performance and proved a strong linear response to the activity of β-gal at an enzyme concentration between 0 and 200 U L^-1, with a limit of detection as low as 0.86 U L^-1, and the response speed is much faster than the same type of probes previously reported. The probe also revealed an excellent biocompatibility and a large Stokes shift. Moreover, fluorescence microscopy imaging experiments confirmed that this probe could be successfully used for the detection of endogenous β-gal in living cells. Graphical abstract.

In vivo ratiometric tracking of endogenous β-galactosidase activity using an activatable near-infrared fluorescent probe

Herein, we developed a dual-channel and light-up near-infrared fluorescent probe for ratiometric sensing of β-galactosidase (β-gal) activity. The well-designed probe, which shows ratiometric optical response with a significant red-shift (from 575 nm to 730 nm), was successfully applied to detect endogenous β-gal activity in SKOV-3 cells and tumor-bearing mice.

Aggregation-Induced Emission Luminogens for Activity-Based Sensing

Fluorescent sensing has emerged as a powerful tool for detecting various analytes and visualizing numerous biological processes by virtue of its superb sensitivity, rapidness, excellent temporal resolution, easy operation, and low cost. Of particular interest is activity-based sensing (ABS), a burgeoning sensing approach that is actualized on the basis of dynamic molecular reactivity rather than conventional lock-and-key molecular recognition. ABS has been recognized to possess some distinct advantages, such as high specificity, extraordinary sensitivity, and accurate signal outputs. A majority of ABS sensors are constructed by modifying conventional fluorogens, which are strongly emissive when molecularly dissolved in solvents but experience emission quenching upon aggregate formation or concentration increase. The aggregation-caused quenching (ACQ) phenomenon leads to a limited amount of labeling of the analyte with the sensor and low photobleaching resistance, which could impede practical applications of the ABS protocol. As an anti-ACQ phenomenon, aggregation-induced emission (AIE) provides a straightforward solution to the ACQ problem. Thanks to their intrinsic advantages, including high photobleaching threshold, high signal-to-noise ratio, fluorescence turn-on nature, and large Stokes shift, AIE-active luminogens (AIEgens) represent a class of extraordinary fluorogen alternatives for the ABS protocol. The use of AIEgen-involved ABS can integrate the advantages of AIEgens and ABS, and additionally, the AIE process offers some unique properties to the ABS approach. For instance, in some cases of water-soluble AIEgen-involved ABS, chemical reaction not only leads to a chang in the emission color of the AIEgens but also causes solubility variations, which could result in specific "light-up" signaling. In this Account, the basic concepts and mechanistic insights of the ABS approach involving the AIE principle are briefly summarized, and then we highlight the new breakthroughs, seminal studies, and trends in the area that have been most recently reported by our group. This emerging sensing protocol has been successfully utilized for detecting an array of targets including ions, small molecules, biomacromolecules, and microenvironments, all of which closely relate to human health, medical, and public concerns. These detections are smoothly achieved on the basis of various reactions (e.g., hydrolysis, boronate cleavage, dephosphorylation, addition, cyclization, and rearrangement reactions) through different sensing principles. In these studies, the AIEgen-involved ABS strategy generally shows good biocompatibility, high selectivity, excellent reliability and high signal contrast, strongly indicating its great potential for high-tech innovations in the sensing field, among which bioprobing is of particular interest. With this Account, we hope to spark new ideas and inspire new endeavors in this emerging research area, further promoting state-of-the-art developments in the field of sensing.

Long-Wavelength Fluorescent Chemosensors for Hg2+ based on Pyrene

A novel long-wavelength turn-on fluorescent chemosensor CS based on pyrene was synthesized to detect Hg²⁺. In the presence of other metal ions, CS could effectively recognize Hg²⁺ and produce the turn-on fluorescent emission at 607 nm. Also, the absorption spectrum exhibited red-shift. Meanwhile, the change of the solution color from yellow to orange was directly observed by the naked eye. The interaction between CS and Hg²⁺ was confirmed by the Job's plot, electrospray ionization mass spectrometry, scanning electron microscopy, and density functional theory calculations. It was found that the fluorescence of CS could be reversible when I^- was added into the solution of CS and Hg²⁺. CS illustrated high selectivity and good sensitivity for Hg²⁺ with the limit of detection of 36 nm. Moreover, CS could be utilized as test strips and silica gel plates to identify Hg²⁺.

Biomacromolecule-Functionalized AIEgens for Advanced Biomedical Studies

The advances in bioinformatics and biomedicine have promoted the development of biomedical imaging and theranostic systems to respectively extend the endogenous biomarker imaging with high contrast and enhance the therapeutic effect with high efficiency. The emergence of biomacromolecule-functionalized aggregation-induced emitters (AIEgens), utilizing AIEgens, and biomacromolecules (nucleic acids, peptides, glycans, and lipids), displays specific targeting ability to cancer cell, improved biocompatibility, reduced toxicity, enhanced therapeutic effect, and so forth. This review summarizes the rational design of biomacromolecule-functionalized AIEgens and their biomedical applications in recent ten years, including high-resolution optical imaging of cell, tissue, and small animal model with low background; the biomarker detection for early diagnosis and prognosis; the delivery and monitoring of prodrugs; image-guide photodynamic therapy and its combination with chemotherapy. Through illustrating their functional mechanisms and application, it is hoped that this review would open up a completely new train of research thought for attracted researchers in various fields.