Prodrug Approach toward the Development of a PET Radioligand for Imaging the GluN2A Subunits of the NMDA Receptor.

A straightforward synthesis of a fluorine-18-labeled prodrug of AFA233 is reported. The key step in the preparation of [18F]AFA233-prodrug is the selective deprotection of the tert-butyl protection groups of the quinoxalinedione moiety without cleavage of the tert-butyl-S-acyl-2-thioethyl protection groups on the phosphate esters. In addition, the preparation of the nonradioactive prodrug reference compound of AFA233 is reported.

Genetically Encoded Activators of Small Molecules for Imaging and Drug Delivery.

Chemical biologists have developed many tools based on genetically encoded macromolecules and small, synthetic compounds. The two different approaches are extremely useful, but they have inherent limitations. In this Minireview, we highlight examples of strategies that combine both concepts to tackle challenging problems in chemical biology. We discuss applications in imaging, with a focus on super-resolved techniques, and in probe and drug delivery. We propose future directions in this field, hoping to inspire chemical biologists to develop new combinations of synthetic and genetically encoded probes.

Single-Molecule Imaging of Active Mitochondrial Nitroreductases Using a Photo-Crosslinking Fluorescent Sensor.

Many biomacromolecules are known to cluster in microdomains with specific subcellular localization. In the case of enzymes, this clustering greatly defines their biological functions. Nitroreductases are enzymes capable of reducing nitro groups to amines, and play a role in detoxification and pro-drug activation. Although nitroreductase activity has been detected in mammalian cells, the subcellular localization of this activity remains incompletely characterized. Here, we report a fluorescent probe that enables super-resolved imaging of pools of nitroreductase activity within mitochondria. This probe is activated sequentially by nitroreductases and light to give a photo-crosslinked adduct of active enzymes. In combination with a general photoactivatable marker of mitochondria, we performed two-color, three-dimensional, single-molecule localization microscopy. These experiments allowed us to image the sub-mitochondrial organization of microdomains of nitroreductase activity.

Photochemically Active Dyes for Super-Resolution Microscopy.

The development of super-resolved optical microscopies has revolutionized the way we visualize cell biology. These techniques strongly rely on the use of photochemically active fluorophores that display changes in their photophysical properties upon irradiation with light. Many reversible and irreversible photochemical transformations have been explored for this purpose, and different imaging techniques require specific mechanisms of photoconversion. In this review, we provide an overview of the most common strategies used for the development of fluorophores for super-resolution microscopy and give specific examples of state-of-the-art fluorogenic probes. Furthermore, we discuss their main field of application and possible directions for future developments.

Photolabile coumarins with improved efficiency through azetidinyl substitution.

Azetidinyl substituents have been recently used to improve the fluorescence quantum yield of several classes of fluorophores. Herein, we demonstrate that other useful photochemical processes can be modulated using this strategy. In particular, we prepared and measured the quantum yield of photorelease of a series of 7-azetidinyl-4-methyl coumarin esters and compared it to their 7-diethylamino and julolidine-fused analogues. The efficiency of the photorelease reactions of the azetidinyl-substituted compounds was 2- to 5-fold higher than the corresponding diethylamino coumarins. We investigated the origin of this effect in model fluorophores and in the photoactivatable esters, and found that H-bonding with the solvent seems to be the prominent deactivation channel inhibited upon substitution with an azetidinyl ring. We anticipate that this substitution strategy could be used to modulate other photochemical processes with applications in chemical biology, catalysis and materials science.

A Photoactivatable Probe for Super-Resolution Imaging of Enzymatic Activity in Live Cells.

A dual-activatable, fluorogenic probe was developed to sense esterase activity with single-molecule resolution. Without enzymatic pre-activation, the diazoindanone-based probe has an electron-poor core and, upon irradiation, undergoes Wolff rearrangement to give a ring-expanded xanthene core that is nonemissive. If the probe is pre-activated by carboxylesterases, the tricyclic core becomes electron-rich, and the photoinduced Wolff rearrangement produces a highly emissive rhodol dye. Live-cell and solution studies confirmed the selectivity of the probe and revealed that the photoactivated dye does not diffuse away from the original location of activation because the intermediate ketene forms a covalent bond with surrounding macromolecules. Single-molecule localization microscopy was used to reconstruct a super-resolved image of esterase activity. These single-molecule images of enzymatic activity changed significantly upon treatment of the cells with inhibitors of human carboxylesterase I and II, both in terms of total number of signals and intracellular distribution. This proof-of-principle study introduces a sensing mechanism for single-molecule detection of enzymatic activity that could be applied to many other biologically relevant targets.