Recent advances in the development of synthetic chemical probes for glycosidase enzymes

Ratiometric Fluorescent Probe for Real-Time Detection of β-Galactosidase Activity in Lysosomes and Its Application in Drug-Induced Senescence Imaging

Senescence is an important biological process, which leads to the gradual degradation of its physiological function and increases morbidity and mortality. Herein, a novel ratiometric fluorescent probe (P1) was constructed by using benzothiazolyl acetonitrile dye as fluorophore, exhibiting significantly enhanced blue-shifted emission to indicate the activity of β-galactosidase (β-gal), a commonly used biomarker for the detection of senescent cells. After incubation with β-gal, the excimer emission of P1 at 620 nm was weakened, while the emission at 533 nm was significantly enhanced, forming an obvious ratiometric probe with high sensitivity and low detection limit (2.7 mU·mL-1). More importantly, probe P1 can locate lysosomes accurately, allowing us to monitor the emergence of living cell senescence in real time. P1 was successfully used to detect β-gal activity in PC-12 cells, Hep G2 cells, and RAW 264.7 cells. It showed strong green fluorescence signal in senescent cells and red fluorescence signal in normal cells, indicating that it can detect endogenous senescence-related β-gal content in living cells. For in vivo drug-induced senescence imaging, after 5 weeks of injection of D-galactose or hydroxyurea, the mice showed significant fluorescence enhancement in specific channels to indicate the activity of β-gal in vivo. At the same time, the senescence of cell-specific organs and skin tissues at the organ level were also detected, which proved that the drug-induced senescence of brain, skin, and muscle tissues was the most serious. These results supported the important application value of P1 in senescence biomedical research.

Synthesis and Evaluation of Glycosyl Luciferins

Measuring glycosidase activity is important to monitor any aberrations in carbohydrate hydrolase activity, but also for the screening of potential glycosidase inhibitors. To this end, synthetic substrates are needed which provide an enzyme-dependent read-out upon hydrolysis by the glycosidase. Herein, we present two new routes for the synthesis of caged luminescent carbohydrates, which can be used for determining glycosidase activity with a luminescent reporter molecule. The substrates were validated with glycosidase and revealed a clear linear range and enzyme-dependent signal upon the in situ generation of the luciferin moiety from the corresponding nitrile precursors. Besides, we showed that these compounds could directly be synthesized from unprotected glycosyl-α-fluorides in a two-step procedure with yields up to 75 %. The intermediate methyl imidate appeared a key intermediate which also reacted with d-cysteine to give the corresponding d-luciferin substrate rendering this a highly attractive method for synthesizing glycosyl luciferins in good yields.

Glycosidase-targeting small molecules for biological and therapeutic applications

Glycosidases are ubiquitous enzymes that catalyze the hydrolysis of glycosidic linkages in oligosaccharides and glycoconjugates. These enzymes play a vital role in a wide variety of biological events, such as digestion of nutritional carbohydrates, lysosomal catabolism of glycoconjugates, and posttranslational modifications of glycoproteins. Abnormal glycosidase activities are associated with a variety of diseases, particularly cancer and lysosomal storage disorders. Owing to the physiological and pathological significance of glycosidases, the development of small molecules that target these enzymes is an active area in glycoscience and medicinal chemistry. Research efforts carried out thus far have led to the discovery of numerous glycosidase-targeting small molecules that have been utilized to elucidate biological processes as well as to develop effective chemotherapeutic agents. In this review, we describe the results of research studies reported since 2018, giving particular emphasis to the use of fluorescent probes for detection and imaging of glycosidases, activity-based probes for covalent labelling of these enzymes, glycosidase inhibitors, and glycosidase-activatable prodrugs.

Induced-volatolomics for the design of tumour activated therapy

The discovery of tumour-associated markers is of major interest for the development of selective cancer chemotherapy. Within this framework, we introduced the concept of induced-volatolomics enabling to monitor simultaneously the dysregulation of several tumour-associated enzymes in living mice or biopsies. This approach relies on the use of a cocktail of volatile organic compound (VOC)-based probes that are activated enzymatically for releasing the corresponding VOCs. Exogenous VOCs can then be detected in the breath of mice or in the headspace above solid biopsies as specific tracers of enzyme activities. Our induced-volatolomics modality highlighted that the up-regulation of N-acetylglucosaminidase was a hallmark of several solid tumours. Having identified this glycosidase as a potential target for cancer therapy, we designed an enzyme-responsive albumin-binding prodrug of the potent monomethyl auristatin E programmed for the selective release of the drug in the tumour microenvironment. This tumour activated therapy produced a remarkable therapeutic efficacy on orthotopic triple-negative mammary xenografts in mice, leading to the disappearance of tumours in 66% of treated animals. Thus, this study shows the potential of induced-volatolomics for the exploration of biological processes as well as the discovery of novel therapeutic strategies.

Glycosylated quantum dots as fluorometric nanoprobes for trehalase

Trehalase is an important enzyme in the metabolic cascades of many organisms, catalysing the hydrolysis of the disaccharide trehalose. Herein we describe the first examples of fluorometric nanoprobes for detection of trehalase, based on trehalose-functionalised quantum dots (QDs). QDs cross-linked with trehalose form aggregates, which are released upon enzymatic cleavage of the trehalose glycosidic bond proportionally to the enzyme concentration, offering a unique and efficient approach for specific sensing of this biologically important enzyme.

Fluorescence sensing and glycosidase inhibition effect of multivalent glycosidase inhibitors based on Naphthalimide-deoxynojirimycin conjugates

Synthetic glycoconjugates as chemical probes have been widely developed for the detection of glycosidase enzymes. However, the binding interactions between iminosugar derivatives and glycosidases were limited, especially for the binding interactions between multivalent glycosidase inhibitors and α-glycosidases. In this paper, three naphthalimide-DNJ conjugates were synthesized. Furthermore, the binding interactions and glycosidase inhibition effects of them were investigated. It was found that the strong binding interactions of multivalent glycosidase inhibitors with enzymes were related to the efficient inhibitory activity against glycosidase. Moreover, the lengths of the chain between DNJ moieties and the triazole ring for the naphthalimide-DNJ conjugates influenced the self-assembly properties, binding interactions and glycosidase inhibition activities with multisource glycosidases. Compound 13 with six carbons between the DNJ moiety and triazole ring showed the stronger binding interactions and better glycosidase inhibition activities against α-mannosidase (jack bean) and α-glucosidase (aspergillus niger). In addition, compound 13 showed an effective PBG inhibition effect in mice with 51.18 % decrease in blood glucose at 30 min. This result opens a way for detection of multivalent glycosidase inhibition effect by a fluorescent sensing method.

Sensitive and Selective Detection of Enterococcus faecalis Using a New Turn-on Fluorogenic β-glucosidase Substrate Combined with a Modified Selective Broth

A new, simple-to-synthesize and sensitive turn-on fluorogenic substrate (CFMU-Glu) for β-glucosidase activity was developed. This probe was based on a 7-hydroxycoumarin derivative (CFMU) that could emit green fluorescence and had the low pK_a value of 5.61 ± 0.01. CFMU-Glu could be used for sensitive monitoring of the almond βGLU and Enterococcus faecalis (E. faecalis) at the optimal pHs of 6.50 and 7.00, respectively. Moreover, a new sensitive and selective fluorogenic broth (PBF-B) for E. faecalis, utilizing CFMU-Glu and polymyxin B, was also developed. Polymyxin B was discovered to can significantly improve the detection selectivity and signal intensity. The proposed 4-four method using PBF-B and a microcentrifuge tube could provide fluorogenic detection limits of 5.01 × 10⁴ and 1.0 × 10⁵  CFU mL^-1 by fluorescence microplate reader and naked eye, respectively; it could also provide a turn-on chromogenic detection limit of 1.0 × 10⁶  CFU mL^-1 by naked eye. The proposed method could detect 8 CFU mL^-1 of E. faecalis in drinking water, Liangcha (herbal tea) and milk samples within 10 h, without pre-enrichment.

Green-emission nitrogen-doped carbon quantum dots from alkaline N-methyl-2-pyrrolidinone for determination of β-galactosidase and its inhibitors

A new fluorescence method was established for sensitive detection of β-galactosidase (β-gal) activity in spiked human serum and screening of inhibitor. Nitrogen-doped carbon quantum dots (N-CQDs) were prepared by solvothermal polymerization of N-methyl-2-pyrrolidinone in an alkaline condition. The colloidal N-CQDs exhibit good water solubility, stability, and emit bright green fluorescence with a maximum emission peak at 528 nm upon excitation at 420 nm. β-gal specifically catalyzes the decomposition of its substrate P-nitrophenyl-β-D-galactopyranoside into 4-nitrophenol, whose absorption spectrum overlaps well with the excitation spectrum of the N-CQDs. As a result, the fluorescence of the N-CQDs is remarkably quenched by 4-nitrophenol via an inner filter effect. The sensing platform presents a linear response range for β-gal activity from 0.05 to 3.0 U·L^-1 with a low limit of detection of 0.023 U·L^-1. An acceptable precision is obtained with a relative standard deviation (RSD) of 3.1% for 1.0 U·L^-1 β-gal (n = 11). The method was applied to determine β-gal in spiked human serums with recoveries in the range  96.3-104.7%. The method was employed to evaluate inhibitor screening with D-galactal and chloroquine diphosphate as models.

Naphthalimide derivatives as fluorescent probes for imaging endogenous gasotransmitters

Gasotransmitters have gained significant recognition attributed to their evident biological impacts, and is accepted as a promising and less-explored area with immense research scope. The three-member family comprising of nitric oxide, carbon monoxide and hydrogen sulphide as endogenous gaseous signaling molecules have been found to elicit a plethora of crucial biological functions, spawning a new research area. The sensing of these small molecules is vital to gain deeper insights into their functions, as they can act both as a friend or a foe in mammalian systems. The initial sections of the review present the physiological and pathophysiological roles of these endogenous gas transmitters and their synergistic interactions. Further, various detection approaches, especially the usage of fascinating features of 1,8-naphthalimide as fluorescent probe in the detection and monitoring of these small signaling molecules are highlighted. The current limitations and the future scope of improving the sensing of the three gasotransmitters are also discussed.

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position. There are not many applications involving polyfluorinated carbohydrates, here defined as monosaccharides in which more than one carbon has at least one fluorine substituent directly attached to it, with the notable exception of their use as mechanism-based inhibitors. The increasing attention to carbohydrate physical properties, especially around lipophilicity, has resulted in a surge of interest for this class of compounds. This review covers the considerable body of work toward the synthesis of polyfluorinated hexoses, pentoses, ketosugars, and aminosugars including sialic acids and nucleosides. An overview of the current state of the art of their glycosidation is also provided.

Advances in Optical Sensors of N-Acetyl-β-d-hexosaminidase (N-Acetyl-β-d-glucosaminidase)

N-Acetyl-β-d-hexosaminidases (EC 3.2.1.52) are exo-acting glycosyl hydrolases that remove N-acetyl-β-d-glucosamine (Glc-NAc) or N-acetyl-β-d-galactosamine (Gal-NAc) from the nonreducing ends of various biomolecules including oligosaccharides, glycoproteins, and glycolipids. The same enzymes are sometimes called N-acetyl-β-d-glucosaminidases, and this review article employs the shorthand descriptor HEX(NAG) to indicate that the terms HEX or NAG are used interchangeably in the literature. The wide distribution of HEX(NAG) throughout the biosphere and its intracellular location in lysosomes combine to make it an important enzyme in food science, agriculture, cell biology, medical diagnostics, and chemotherapy. For more than 50 years, researchers have employed chromogenic derivatives of N-acetyl-β-d-glucosaminide in basic assays for biomedical research and clinical chemistry. Recent conceptual and synthetic innovations in molecular fluorescence sensors, along with concurrent technical improvements in instrumentation, have produced a growing number of new fluorescent imaging and diagnostics methods. A systematic summary of the recent advances in optical sensors for HEX(NAG) is provided under the following headings: assessing kidney health, detection and treatment of infectious disease, fluorescence imaging of cancer, treatment of lysosomal disorders, and reactive probes for chemical biology. The article concludes with some comments on likely future directions.

Quantifying lysosomal glycosidase activity within cells using bis-acetal substrates

Understanding the function and regulation of enzymes within their physiologically relevant milieu requires quality tools that report on their cellular activities. Here we describe a strategy for glycoside hydrolases that overcomes several limitations in the field, enabling quantitative monitoring of their activities within live cells. We detail the design and synthesis of bright and modularly assembled bis-acetal-based (BAB) fluorescence-quenched substrates, illustrating this strategy for sensitive quantitation of disease-relevant human α-galactosidase and α-N-acetylgalactosaminidase activities. We show that these substrates can be used within live patient cells to precisely measure the engagement of target enzymes by inhibitors and the efficiency of pharmacological chaperones, and highlight the importance of quantifying activity within cells using chemical perturbogens of cellular trafficking and lysosomal homeostasis. These BAB substrates should prove widely useful for interrogating the regulation of glycosidases within cells as well as in facilitating the development of therapeutics and diagnostics for this important class of enzymes.

Effect of Interfacial Properties on Impedimetric Biosensing of the Sialylation Process with a Biantennary N-Glycan-Based Monolayer

Sensing enzymatic sialylation provides new tools for the evaluation of pathological events and pathogen invasion. Enzymatic sialylation is usually monitored via fluorescence or metabolic labeling, which requires relatively large amounts of the glycan substrate with limited availability. Using a label-free biosensor requires smaller quantities of substrates because the interactions induce measurable changes to an interface, which can be translated into a signal. The downside of label-free biosensors is that they are very sensitive to changes at the interface, and the properties of the surface layer can play a major role. Electrochemical impedance spectroscopy was used here to follow the enzymatic sialylation of a biantennary N-glycan acceptor in mixed monolayers. The surfaces contained either neutral, positively or negatively charged, or zwitterionic functional groups. The systems were characterized by contact potential difference, ellipsometry, and contact angle analyses. We found that the characteristics of the mixed monolayer have a profound effect on the biosensing of the enzymatic sialylation. Positively charged layers were found to adsorb the enzyme under the reaction conditions. Negatively charged and zwitterionic surfaces were nonresponsive to enzymatic sialylation. Only the neutral mixed monolayers provided signals that were related directly to enzymatic sialylation. This work demonstrates the importance of appropriate interface properties for monitoring enzymatic sialylation processes.

Noninvasive NIR Imaging of Senescence via In Situ Labeling

Cellular senescence, a process that arrests the cell cycle, is a cellular response mechanism for various stresses and is implicated in aging and various age-related diseases. However, the understanding of senescence in living organisms is insufficient, largely due to the scarcity of sensitive tools for the detection of cellular senescence in vivo. Herein, we describe the development of a self-immobilizing near-infrared (NIR) fluorogenic probe that can be activated by senescence-associated β-galactosidase (SA-β-Gal), the most widely used senescence marker. The NIR signal is turned on only in the presence of SA-β-Gal, and the fluorescence signal is retained to the site of activation via in situ labeling, significantly enhancing the sensitivity of the probe. We demonstrate its efficient noninvasive imaging of senescence in mice xenograft models.

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.

Multifunctional fluorescent probes for high-throughput characterization of hexosaminidase enzyme activity

Microbial polysaccharides composed of N-acetylglucosamine (GlcNAc), such as chitin, peptidoglycan and poly-β-(1 → 6)-GlcNAc (dPNAG), play a critical role in maintaining cell integrity or in facilitating biofilm formation in numerous fungal and bacterial pathogens. Glycosyl hydrolase enzymes that catalyze the degradation of these β-GlcNAc containing polysaccharides play important roles in normal microbial cell physiology and can also be exploited as biocatalysts with applications as anti-fungal, anti-bacterial, or biofilm dispersal agents. Assays to rapidly detect and characterize the activity of such glycosyl hydrolase enzymes can facilitate their development as biocatalyst, however, currently available probes such as 4-methylumbelliferyl-β-GlcNAc (4MU-GlcNAc) are not universally accepted as substrates, and their fluorescent signal is sensitive to changes in pH. Here, we present the development of a new multifunctional fluorescent substrate analog for the detection and characterization of hexosaminidase enzyme activity containing a 7-amino-4-methyl coumarin (AMC) carbamate aglycone. This probe is widely tolerated as a substrate for exo-acting β-hexosaminidase, family 19 endo-chitinase, and the dPNAG hydrolase enzyme Dispersin B (DspB) and enables detection of hexosaminidase enzyme activity via either single wavelength fluorescent measurements or ratiometric fluorescent detection. We demonstrate the utility of this probe to screen for recombinant DspB activity in Escherichia coli cell lysates, and for the development of a high-throughput assay to screen for DspB inhibitors.

Molecular probes for selective detection of cysteine cathepsins

Cysteine cathepsins are proteases critical in physiopathological processes and show potential as targets or biomarkers for diseases and medical conditions. The 11 members of the cathepsin family are redundant in some cases but remarkably independent of others, demanding the development of both pan-cathepsin targeting tools as well as probes that are selective for specific cathepsins with little off-target activity. This review addresses the diverse design strategies that have been employed to accomplish this tailored selectivity among cysteine cathepsin targets and the imaging modalities incorporated. The power of these diverse tools is contextualized by briefly highlighting the nature of a few prominent cysteine cathepsins, their involvement in select diseases, and the application of cathepsin imaging probes in research spanning basic biochemical studies to clinical applications.

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.

Recent advances in the direct O-arylation of carbohydrates

Methods for the O-arylation of hydroxyl and hemiacetal groups in carbohydrates via C(sp2)-O bond formation are discussed. Such methods provide an alternative disconnection to the traditional approach of nucleophilic substitution between a sugar-derived electrophile and a phenol or phenoxide nucleophile. They have led to new opportunities for stereoselectivity, site-selectivity and chemoselectivity in the preparation of O-aryl glycosides and carbohydrate-derived aryl ethers, compounds that are useful for a broad range of applications in medicinal chemistry, glycobiology and organic synthesis.

Heme-binding enables allosteric modulation in an ancient TIM-barrel glycosidase

Glycosidases are phylogenetically widely distributed enzymes that are crucial for the cleavage of glycosidic bonds. Here, we present the exceptional properties of a putative ancestor of bacterial and eukaryotic family-1 glycosidases. The ancestral protein shares the TIM-barrel fold with its modern descendants but displays large regions with greatly enhanced conformational flexibility. Yet, the barrel core remains comparatively rigid and the ancestral glycosidase activity is stable, with an optimum temperature within the experimental range for thermophilic family-1 glycosidases. None of the ∼5500 reported crystallographic structures of ∼1400 modern glycosidases show a bound porphyrin. Remarkably, the ancestral glycosidase binds heme tightly and stoichiometrically at a well-defined buried site. Heme binding rigidifies this TIM-barrel and allosterically enhances catalysis. Our work demonstrates the capability of ancestral protein reconstructions to reveal valuable but unexpected biomolecular features when sampling distant sequence space. The potential of the ancestral glycosidase as a scaffold for custom catalysis and biosensor engineering is discussed.

Family 1 glycosidases (GH1) are present in the three domains of life and share classical TIM-barrel fold. Structural and biochemical analyses of a resurrected ancestral GH1 enzyme reveal heme binding, not known in its modern descendants. Heme rigidifies the TIM-barrel and allosterically enhances catalysis.

A sensitive fluorescent probe for β-galactosidase activity detection and application in ovarian tumor imaging

The development of non-invasive and sensitive optical probes for in vivo bioimaging of cancer-related enzymes is desirable for early diagnosis and effective cancer therapy. β-galactosidase (β-gal) is regarded as a key ovarian cancer biomarker, owing to its overexpression in primary ovarian cancer. Herein, we designed a sensitive near-infrared (NIR) probe (DCMCA-βgal) for the detection and real-time imaging of β-gal activity in ovarian tumors, thereby achieving the visualization of ovarian tumors by β-gal activity detection. DCMCA-β-gal could be triggered by β-gal, resulting in the release of a NIR chromophore, DCM-NH₂; the linear range of fluorescent response to β-gal concentration was 0-1.2 U with a low detection limit of 1.26 × 10^-3 U mL^-1. We used DCMCA-β-gal to detect and visualize β-gal activity in SKOV3 human ovarian cancer cells, as well as for real-time imaging of β-gal activity in ovarian cancer mouse models. DCMCA-β-gal possessed high sensitivity, "turn-on" NIR emission, a large spectral shift, and high photostability in a dynamic living system and thus could serve as a highly sensitive sensor for real-time tracking of β-gal activity in vivo and ovarian tumor imaging.

Ultrasensitive small molecule fluorogenic probe for human heparanase

Heparanase (HPA) is a critical enzyme involved in the remodeling of the extracellular matrix (ECM), and its elevated expression has been linked with diseases such as various types of cancer and inflammation. The detection of heparanase enzymatic activity holds tremendous value in the study of the cellular microenvironment, and search of molecular therapeutics targeting heparanase, however, no structurally defined probes are available for the detection of heparanase activity. Here we present the development of the first ultrasensitive fluorogenic small-molecule probe for heparanase enzymatic activity via tuning the electronic effect of the substrate. The probe exhibits a 756-fold fluorescence turn-on response in the presence of human heparanase, allowing one-step detection of heparanase activity in real-time with a picomolar detection limit. The high sensitivity and robustness of the probe are exemplified in a high-throughput screening assay for heparanase inhibitors.

Design and Development of an HBT-Based Ratiometric Fluorescent Probe to Monitor Stress-Induced Premature Senescence

Stress-induced premature senescence (SIPS) can be induced in tumor cells by reactive oxygen species (ROS) or oncogenes. The antineoplastic drugs cause apoptosis and senescence by damaging the DNA. Although the detection of cellular senescence is important to monitor drug response during anticancer therapy, only a few probes have been studied for imaging SIPS. In this study, we developed 2-(2'-hydroxyphenyl)benzothiazole (HBT)-based fluorescent probes to determine SIPS by monitoring the oxidative stress and β-galactosidase activity. HBT is a commonly used fluorophore because of its luminescence mechanism via excited-state intramolecular proton transfer, and it has attractive properties, such as a four-level photochemical process and large Stokes shift (151 nm). A novel fluorescent probe, (2-(benzo[d]thiazol-2-yl)phenyl)boronic acid, was prepared for the detection of ROS, including H₂O₂, via the oxidation reaction of arylboronic acids to form the fluorescent phenol, HBT. In addition, to determine the enzymatic activity of β-galactosidase, a 2-(4'-chloro-2'-hydroxyphenyl)benzothiazole (CBT)-based enzymatic turn-on probe (CBT-β-Gal) was designed and synthesized. β-Galactosidase catalyzed the hydrolysis of β-galactopyranoside from CBT-β-Gal to release the fluorescent CBT. These probes were capable of ratiometric imaging the accumulation of H₂O₂ and the degree of β-galatosidase activity in contrast to H₂O₂-untreated and H₂O₂-treated HeLa cells. Furthermore, these probes were successfully employed for imaging the increased levels of ROS and β-galactosidase activity in the doxorubicin-treated HeLa cells.

Enzymatically triggered chromogenic cross-linking agents under physiological conditions

Oxidative dimerization of an indoxyl moiety, released by glycosidase action in aqueous solution, yields an indigoid dye in formats that enable bioconjugation and molecular cross-linking.

Illuminating glycoscience: synthetic strategies for FRET-enabled carbohydrate active enzyme probes

Carbohydrates are synthesised, refined and degraded by carbohydrate active enzymes. FRET is emerging as a powerful tool to monitor and quantify their activity as well as to test inhibitors as new drug candidates and monitor disease.

Fluorescent supramolecular hierarchical self-assemblies from glycosylated 4-amino- and 4-bromo-1,8-naphthalimides

The investigation into the self-assembly formation of the glycan based 4-amino- and 4-bromo-1,8-naphthalimide (Nap) structures1–3is presented.

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.

Real-time imaging of senescence in tumors with DNA damage

Detection of cellular senescence is important not only in the study of senescence in various biological systems, but also in various practical applications such as image-guided surgical removal of senescent cells, as well as the monitoring of drug-responsiveness during cancer therapies. Due to the lack of suitable imaging probes for senescence detection, particularly in living subjects, we have developed an activatable near-infrared (NIR) molecular probe with far-red excitation, NIR emission, and high "turn-on" ratio upon senescence-associated β-galactosidase (SABG) activation. We present here the first successful demonstration of NIR imaging of DNA damage-induced senescence both in vitro and in human tumor xenograft models.

An enzyme-activatable probe liberating AIEgens: on-site sensing and long-term tracking of β-galactosidase in ovarian cancer cells

Development of fluorescent probes for on-site sensing and long-term tracking of specific biomarkers is particularly desirable for the early detection of diseases. However, available small-molecule probes tend to facilely diffuse across the cell membrane or remain at the activation site but always suffer from the aggregation-caused quenching (ACQ) effect. Here we report an enzyme-activatable aggregation-induced emission (AIE) probe QM-βgal, which is composed of a hydrophilic β-galactosidase (β-gal)-triggered galactose moiety and a hydrophobic AIE-active fluorophore QM-OH. The probe is virtually non-emissive in aqueous media, but when activated by β-gal, specific enzymatic turnover would liberate hydrophobic AIE luminogen (AIEgen) QM-OH, and then highly fluorescent nanoaggregates are in situ generated as a result of the AIE process, allowing for on-site sensing of endogenous β-gal activity in living cells. Notably, taking advantage of the improved intracellular retention of nanoaggregates, we further exemplify QM-βgal for long-term (∼12 h) visualization of β-gal-overexpressing ovarian cancer cells with high fidelity, which is essential for biomedicine and diagnostics. Thus, this enzyme-activatable AIE probe not only is a potent tool for elucidating the roles of β-gal in biological systems, but also offers an enzyme-regulated liberation strategy to exploit multifunctional probes for preclinical applications.

Glycosylated naphthalimides and naphthalimide Tröger's bases as fluorescent aggregation probes for Con A

We report the synthesis of glycosylated naphthalimide compounds and their application as fluorescent probes for Concanavalin A (Con A) lectin.

Application of lanthanide luminescence in probing enzyme activity

Enzymes play critical roles in the regulation of cellular function and are implicated in numerous disease conditions. Reliable and practicable assays are required to study enzyme activity, to facilitate the discovery of inhibitors and activators of enzymes related to disease. In recent years, a variety of enzyme assays have been devised that utilise luminescent lanthanide(iii) complexes, taking advantage of their high detection sensitivities, long luminescence lifetimes, and line-like emission spectra that permit ratiometric and time-resolved analyses. In this Feature article, we focus on recent progress in the development of enzyme activity assays based on lanthanide(iii) luminescence, covering a variety of strategies including Ln(iii)-labelled antibodies and proteins, Ln(iii) ion encapsulation within defined peptide sequences, reactivity-based Ln(iii) probes, and discrete Ln(iii) complexes. Emerging approaches for monitoring enzyme activity are discussed, including the use of anion responsive lanthanide(iii) complexes, capable of molecular recognition and luminescence signalling of polyphosphate anions.

Glycosylated lanthanide cyclen complexes as luminescent probes for monitoring glycosidase enzyme activity

The development of synthetic chemical probes for the detection of enzymes is extremely important for biological, medicinal, and industrial applications. Here we report the synthesis of an array of novel glycosylated Tb(iii) complexes, their photophysical properties in solution, and their ability to function as luminescent probes for observing glycosidase enzyme activity in real time. Our initial studies into the application of these complexes for the detection of the Concanavalin A (ConA) lectin is also reported, highlighting the broad scope of these novel chemical probes.

Readily functionalizable phosphonium-tagged fluorescent coumarins for enhanced detection of conjugates by mass spectrometry

Fluorescent coumarins are an important class of small-molecule organic fluorophores ubiquitous in different well-established and emerging fields of research including, among others, biochemistry and chemical biology. The present work aims at covering the poor detectability of coumarin-based conjugates by mass spectrometry while keeping important photophysical properties of the coumarin core. In this context, the synthesis of readily functionalizable phosphonium-tagged coumarin derivatives enabling a dual mass-tag and fluorescence labelling of analytes or (bio)molecules of interest through a single-step protocol, is reported. The utility of these coumarins is illustrated through the preparation of fluorogenic substrates that facilitated identification of the peptide fragment released by specific proteolytic cleavages.

Recent progresses in small-molecule enzymatic fluorescent probes for cancer imaging

An overview of recent advances in small-molecule enzymatic fluorescent probes for cancer imaging, including design strategies and cancer imaging applications.

Fabrication of Stable and Luminescent Copper Nanocluster-Based AIE Particles and Their Application in β-Galactosidase Activity Assay

Thiolated copper nanoclusters (CuNCs) with aggregation-induced emission characteristic are becoming a novel luminescent material, but it is still a challenging task to retain its bright luminescence in a neutral solution. In this work, we report a new copper nanocluster with aggregation-induced emission (AIE) enhancement property using a hydrophobic molecule as the protecting ligand, and brightly luminescent AIE particles of copper nanocluster were prepared via hydrophobic interaction. These CuNCs AIE particles possess uniform rod-like shapes, with sizes in hundreds of nanometer, and an intense luminescence; more importantly, its luminescence remains stable in neutral and alkaline solutions. It is found that 4-nitrophenol is able to effectively quench the luminescence of CuNC AIE particles through strong hydrophobic interaction and electron transfer between them. This strong quenching effect was adopted to develop a luminescent assay for β-galactosidase at physiological condition. This work presents a demonstration of preparing CuNC AIE particles with bright luminescence at neutral condition and gives an example of the use of AIE particles in monitoring the enzyme activity.

Synthesis of precipitating chromogenic/fluorogenic β-glucosidase/β-galactosidase substrates by a new method and their application in the visual detection of foodborne pathogenic bacteria

We developed a new efficient method for the synthesis of important indoxyl glycoside substrates for β-glucosidase and β-galactosidase by using 1-acetylindol-3-ones as intermediates. This method was used to synthesise novel precipitating fluorogenic substrates for β-glucosidase based on 2-(benzothiazol-2'-yl)-phenols. We also assessed the application of these substrates in the detection of foodborne pathogenic bacteria.

Carbohydrate Bis-acetal-Based Substrates as Tunable Fluorescence-Quenched Probes for Monitoring exo-Glycosidase Activity

Tunable Förster resonance energy transfer (FRET)-quenched substrates are useful for monitoring the activity of various enzymes within their relevant physiological environments. Development of FRET-quenched substrates for exo-glycosidases, however, has been hindered by their constrained pocket-shaped active sites. Here we report the design of a new class of substrate that overcomes this problem. These Bis-Acetal-Based Substrates (BABS) bear a hemiacetal aglycon leaving group that tethers fluorochromes in close proximity, also positioning them distant from the active site pocket. Following cleavage of the glycosidic bond, the liberated hemiacetal spontaneously breaks down, leading to separation of the fluorophore and quencher. We detail the synthesis and characterization of GlcNAc-BABS, revealing a striking 99.9% quenching efficiency. These substrates are efficiently turned over by the human exo-glycosidase O-GlcNAcase (OGA). We find the hemiacetal leaving group rapidly breaks down, enabling quantitative monitoring of OGA activity. We expect this strategy to be broadly useful for the development of substrate probes for monitoring exo-glycosidases, as well as a range of other enzymes having constrained pocket-shaped active sites.

Fluorescent Neuraminidase Assay Based on Supramolecular Dye Capture After Enzymatic Cleavage

A conceptually new type of enzymatic cleavage assay is reported that utilizes in situ supramolecular capture of the fluorescent product. A squaraine-derived substrate with large blocking groups at each end of its structure cannot be threaded by a tetralactam macrocycle until the blocking groups are removed by enzyme cleavage. A prototype design responds to viral neuraminidase, an indicator of influenza infection, and also measures susceptibility of the sample to neuraminidase inhibitor drugs. The substrate structure incorporates three key features: (a) a bis(4-amino-3-hydroxyphenyl)squaraine core with bright deep-red fluorescence and excellent photostability, (b) an N-methyl group at each end of the squaraine core that ensures fast macrocycle threading kinetics, and (c) sialic acid blocking groups that prevent macrocycle threading until they are removed by viral neuraminidase. The enzyme assay can be conducted in aqueous solution where dramatic colorimetric and fluorescence changes are easily observed by the naked eye. Alternatively, affinity capture beads coated with macrocycle can be used to immobilize the liberated squaraine and enable a range of heterogeneous analysis options. With further optimization, this new type of neuraminidase assay may be useful in a point of care clinic to rapidly diagnose influenza infection and also determine which of the approved antiviral inhibitor drugs is likely to be the most effective treatment for an individual patient. The assay design is generalizable and can be readily modified to monitor virtually any type of enzyme-catalyzed cleavage reaction.

Photophysics of Coumarin and Carbostyril-Sensitized Luminescent Lanthanide Complexes: Implications for Complex Design in Multiplex Detection

Luminescent lanthanide (Ln(III)) complexes with coumarin or carbostyril antennae were synthesized and their photophysical properties evaluated using steady-state and time-resolved UV-vis spectroscopy. Ligands bearing distant hydroxycoumarin-derived antennae attached through triazole linkers were modest sensitizers for Eu(III) and Tb(III), whereas ligands with 7-amidocarbostyrils directly linked to the coordination site could reach good quantum yields for multiple Ln(III), including the visible emitters Sm(III) and Dy(III), and the near-infrared emitters Nd(III) and Yb(III). The highest lanthanide-centered luminescence quantum yields were 35% (Tb), 7.9% (Eu), 0.67% (Dy), and 0.18% (Sm). Antennae providing similar luminescence intensities with 2-4 Ln-emitters were identified. Photoredox quenching of the carbostyril antenna excited states was observed for all Eu(III)-complexes and should be sensitizing in the case of Yb(III); the scope of the process extends to Ln(III) for which it has not been seen previously, specifically Dy(III) and Sm(III). The proposed process is supported by photophysical and electrochemical data. A FRET-type mechanism was identified in architectures with both distant and close antennae for all of the Lns. This mechanism seems to be the only sensitizing one at long distance and probably contributes to the sensitization at shorter distances along with the triplet pathway. The complexes were nontoxic to either bacterial or mammalian cells. Complexes of an ester-functionalized ligand were taken up by bacteria in a concentration-dependent manner. Our results suggest that the effects of FRET and photoredox quenching should be taken into consideration when designing luminescent Ln complexes. These results also establish these Ln(III)-complexes for multiplex detection beyond the available two-color systems.

A universal fluorometric assay strategy for glycosidases based on functional carbon quantum dots: β-galactosidase activity detection in vitro and in living cells

The development of highly sensitive assays for glycosidases is of critical significance to understand their functions, facilely detect associated diseases and screen potential new drugs. In this work, we develop a universal assay strategy for glycosidase enzymes and inhibitor screening based on functional carbon quantum dots through a combined host-guest recognition and specific static quenching-induced signal transduction mechanism. This detection strategy is established in terms of the following facts: (1) β-cyclodextrin as a perfect host can selectively associate with p-nitrophenol due to its hydrophobic character and right size match of the cavity, which renders specific binding between β-cyclodextrin and p-nitrophenol via a host-guest recognition. (2) The formation of an inclusion complex between β-cyclodextrin modified carbon quantum dots (β-CD-CQDs) and p-nitrophenol results in fluorescence quenching with a high quenching efficiency due to the static quenching mechanism. Glycoconjugates of p-nitrophenol as the substrates could be rapidly hydrolyzed to corresponding glycose and p-nitrophenol in the presence of specific glycosidase, and the resulting p-nitrophenol induces the following host-guest interaction and static quenching leading to a change in the fluorescence signal. The activity of different glycosidase enzymes could be evaluated in the same way as long as the glycosyl unit of glycosylated substrates was changed. Here we take β-galactosidase as an example to demonstrate the applicability of the proposed detection strategy because it can act as a molecular target for primary ovarian cancers. A highly sensitive assay for β-galactosidase activity in terms of linear correlation of the fluorescence change with the β-galactosidase level was established with a low detection limit of 0.6 U L^-1. Its function of inhibitor screening was also assessed by using d-galactal as the inhibitor for β-galactosidase, and the positive results indicated its feasibility to screen potential inhibitors. It is also illustrated that the nanoprobe possesses excellent biocompatibility, and can sensitively monitor the intracellular β-galactosidase level in ovarian cancer cells. This work provides a general detection method for glycosidase activity, demonstrates its applicability of monitoring the enzyme level in living cells, and broadens fluorogenic probes in fluorescence-guided diagnostics.

A glycosylation strategy to develop a low toxic naphthalimide fluorescent probe for the detection of Fe3+in aqueous medium

A glycosylation strategy based on click chemistry was employed to develop a naphthalimide-based Fe³⁺fluorescent probe with low cytotoxicity and good water-solubility.

Glycosidase activated release of fluorescent 1,8-naphthalimide probes for tumor cell imaging from glycosylated 'pro-probes'

Glycosylated 4-amino-1,8-naphthalimide derivatives possess a native glycosidic linkage that can be selectively hydrolysed in situ by glycosidase enzymes to release the naphthalimide as a fluorescent imaging or therapeutic agent. In vitro studies using a variety of cancer cell lines demonstrated that the naphthalimides only get taken up into cells upon enzymatic cleavage from the glycan unit; a mechanism that offers a novel approach for the targeted delivery of probes/drugs.

Efficient O-Functionalization of Carbohydrates with Electrophilic Reagents

Novel methodology for O-functionalization of carbohydrate derivatives has been established using bench-stable and easily prepared iodonium(III) reagents. Both electron-withdrawing and electron-donating aryl groups were introduced under ambient conditions and without precautions to exclude air or moisture. Furthermore, the approach was extended both to full arylation of cyclodextrin, and to trifluoroethylation of carbohydrate derivatives. This is the first general approach to introduce traditionally non-electrophilic groups into any of the OH groups around the sugar backbone. The methodology will be useful both in synthetic organic chemistry and biochemistry, as important functional groups can be incorporated under simple and robust reaction conditions in a fast and efficient manner.

Cyclen lanthanide-based micellar structures for application as luminescent [Eu(iii)] and magnetic [Gd(iii)] resonance imaging (MRI) contrast agents

The synthesis of coordinatively unsaturated tetra-substituted 1,4,7,10-tetraazacyclododecane (cyclen) lanthanide complexes is described; these structures, possessing hydrophobic (C12-alkyl) tails and hydrophilic head groups, self-assemble into supramolecular micellar structures in aqueous solution, and hence can be utilised as novel contrast agents for MRI.