Trifunctional Fluorogenic Probes for Fluorescence Imaging and Isolation of Glycosidases in Cells

Real-time fluorescent monitoring of phase I xenobiotic-metabolizing enzymes

This article explores the intricate landscape of advanced fluorescent probes crafted for the detection and real-time monitoring of phase I xenobiotic-metabolizing enzymes. Employing state-of-the-art technologies, such as fluorescence resonance energy transfer, intramolecular charge transfer, and solid-state luminescence enhancement, this article unfolds a multifaceted approach to unraveling the dynamics of enzymatic processes within living systems. This encompassing study involves the development and application of a diverse range of fluorescent probes, each intricately designed with tailored mechanisms to heighten sensitivity, providing dynamic insights into phase I xenobiotic-metabolizing enzymes. Understanding the role of phase I xenobiotic-metabolizing enzymes in these pathophysiological processes, is essential for both medical research and clinical practice. This knowledge can guide the development of approaches to prevent, diagnose, and treat a broad spectrum of diseases and conditions. This adaptability underscores their potential clinical applications in cancer diagnosis and personalized medicine. Noteworthy are the trifunctional fluorogenic probes, uniquely designed not only for fluorescence-based cellular imaging but also for the isolation of cellular glycosidases. This innovative feature opens novel avenues for comprehensive studies in enzyme biology, paving the way for potential therapeutic interventions. The research accentuates the selectivity and specificity of the probes, showcasing their proficiency in distinguishing various enzymes and their isoforms. The sophisticated design and successful deployment of these fluorescent probes mark significant advancements in enzymology, providing powerful tools for both researchers and clinicians. Beyond their immediate applications, these probes offer illuminating insights into disease mechanisms, facilitating early detection, and catalyzing the development of targeted therapeutic interventions. This work represents a substantial leap forward in the field, promising transformative implications for understanding and addressing complex biological processes. In essence, this research heralds a new era in the development of fluorescent probes, presenting a comprehensive and innovative approach that not only expands the understanding of cellular enzyme activities but also holds great promise for practical applications in clinical settings and therapeutic endeavors.

Strategies for quantifying the enzymatic activities of glycoside hydrolases within cells and in vivo

Within their native milieu of the cell, the activities of enzymes are controlled by a range of factors including protein interactions and post-translational modifications. The involvement of these factors in fundamental cell biology and the etiology of diseases is stimulating interest in monitoring enzyme activities within tissues. The creation of synthetic substrates, and their use with different imaging modalities, to detect and quantify enzyme activities has great potential to propel these areas of research. Here we describe the latest developments relating to the creation of substrates for imaging and quantifying the activities of glycoside hydrolases, focusing on mammalian systems. The limitations of current tools and the difficulties within the field are summarised, as are prospects for overcoming these challenges.

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.

A fluorescent probe to simultaneously detect both O-GlcNAcase and phosphatase

O-GlcNAc modification of proteins often has crosstalk with protein phosphorylation. These posttranslational modifications are highly dynamic events that modulate a wide range of cellular processes. Owing to the physiological and pathological significance of protein O-GlcNAcylation and phosphorylation, we designed the fluorescent probe, βGlcNAc-CM-Rhod-P, to differentially detect activities of O-GlcNAcase (OGA) and phosphatase, enzymes that are responsible for these modifications. βGlcNAc-CM-Rhod-P was comprised of a βGlcNAc-conjugated coumarin (βGlcNAc-CM) acting as an OGA substrate, a phosphorylated rhodol (Rhod-P) as a phosphatase substrate and a piperazine bridge. Because the emission wavelength maxima of CM and Rhod liberated from the probe are greatly different (100 nm), spectral interference is avoided. The results of this study revealed that treatment of βGlcNAc-CM-Rhod-P with OGA promotes formation of the GlcNAc-cleaved probe, CM-Rhod-P, and a consequent increase in the intensity of fluorescence associated with free CM. Also, it was found that exposure of the probe to phosphatase produces a dephosphorylated probe, βGlcNAc-CM-Rhod, which displays strong fluorescence arising from free Rhod. On the other hand, when incubated with both OGA and phosphatase, βGlcNAc-CM-Rhod-P was converted to CM-Rhod which lacked both βGlcNAc and phosphoryl groups, in conjunction with increases in the intensities of fluorescence arising from both free CM and Rhod. This probe was employed to detect activities of OGA and phosphatase in cell lysates and to fluorescently image both enzymes in cells. Collectively, the findings indicate that βGlcNAc-CM-Rhod-P can be utilized as a chemical tool to simultaneously determine activities of OGA and phosphatase.

Fluorogenic and Mitochondria-Localizable Probe Enables Selective Labeling and Imaging of Nitroreductase

Nitroreductase (NTR), one of the flavin-dependent enzymes and an upregulated enzyme under tumor hypoxia, has been studied for decades. Many fluorescent probes were developed to detect NTR activity; however, these probes tend to diffuse away from their reaction site (NTR) inevitably, leading to inappropriate sample fixation, lower accuracy of NTR localization, and weaker signal-to-noise ratio. Herein, we present the design, synthesis, in vitro evaluation, and biological applications of an NTR-activatable fluorogenic and labeling probe FY. By integrating with quinone methide (QM) proximity-based protein labeling, the additional fluoromethyl group on FY serves as a potential origin of QM. Compared with conventional fluorescent probes, this new NTR probe not only offers mitochondrial localizable and fluorogenic response but also achieves permanent retention on the site of activation with an enhanced spatial resolution to improve the detection sensitivity even after cell fixation. We believe our work could offer an expandable synthetic approach to develop these permanent labeling and imaging fluorescence probes for deciphering complex biological events.

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.

Activatable fluorescent probes for in situ imaging of enzymes

As the main biomarkers of most diseases, enzymes play fundamental but extremely critical roles in biosystems. High-resolution studies of enzymes using activatable in situ fluorescence imaging may help to better elucidate their dynamics in living systems. Currently, most activatable probes can realize changeable imaging of enzymes but inevitably tend to diffuse away from the original active site of the enzyme and even translocate out of cells, seriously impairing in situ high-resolution observation of the enzymes. In situ fluorescence imaging of enzymes can be realized by labelling probes or antibodies with always-on signals that fail to enable activatable imaging of enzymes. Thus, fluorescent probes with both "activatable" and "in situ" properties will enable high-resolution studies of enzymes in living systems. In this tutorial review, we summarize the existing methods ranging from design strategies to bioimaging applications that could be used to develop activatable fluorescent probes for in situ imaging of enzymes. It is expected that this tutorial review will promote the new methods generated to design such probes for better deciphering enzymes in complex biosystems and further extend the application of these methods to other fields of enzymes.

A self-immobilizing near-infrared fluorogenic probe for sensitive imaging of extracellular enzyme activity in vivo

Reported herein is a self-immobilizing near-infrared fluorogenic probe that can be used to image extracellular enzyme activity in vivo. Using a fluorophore as a quinone methide precursor, this probe covalently anchors at sites of activation and greatly enhances the fluorescence intensity at 710 nm upon enzymatic stimulus, significantly boosting detection sensitivity in a highly dynamic in vivo system.

Late-stage difluoromethylation leading to a self-immobilizing fluorogenic probe for the visualization of enzyme activities in live cells

Reported herein is a novel p-quinone methide-based self-immobilizing fluorogenic probe for the visualization of β-galactosidase activities in live cells. This easily prepared imaging reagent massively increases the fluorescence intensity and covalently links to the activation site with high efficiency upon enzymatic trigger.