Mimicking nature's ability to orchestrate molecular self-assembly in living cells is important yet challenging. Molecular self-assembly has found wide applications in cellular activity control, drug delivery, biomarker imaging, etc. Nonetheless, examples of suborganelle-confined supramolecular self-assembly are quite rare and research in this area remains challenging. Herein, we have presented a new strategy to program supramolecular selfassembly specifically in mitochondria by leveraging on a unique enzyme SIRT5. SIRT5 is a mitochondria-localized enzyme belonging to a family of NAD + -dependent histone deacetylases. Accumulating studies suggest that SIRT5 is involved in regulating diverse biological processes, such as reactive oxygen defense, fatty acid metabolism, and apoptosis. In this study, we designed a novel class of succinylated peptide precursors that can be transformed into self-assembling building blocks through SIRT5 catalysis, leading to the formation of supramolecular nanofibers in vitro and in living cells. The increased hydrophobicity arising from self-assembly remarkably enhanced the fluorescence of nitrobenzoxadiazole (NBD) in the nanofibers. With this approach, we have enabled activity-based imaging of SIRT5 in living cells for the first time. Moreover, SIRT5-mediated peptide self-assembly was found to depolarize mitochondria membrane potential and promote ROS formation. Coincubation of the peptide with three different chemotherapeutic agents significantly boosted the anticancer activities of these drugs. Our work has thus illustrated a new way of mitochondria-confined peptide self-assembly for SIRT5 imaging and potential anticancer treatment.

Alkaline phosphatases are a group of enzymes that play important roles in regulating diverse cellular functions and disease pathogenesis. Hence, developing fluorescent probes for in vivo detection of alkaline phosphatase activity is highly desirable for studying the dynamic phosphorylation in living organisms. Here, we developed the very first reaction-based near-infrared (NIR) probe (DHXP) for sensitive detection of alkaline phosphatase activity both in vitro and in vivo. Our studies demonstrated that the probe displayed an up to 66-fold fluorescence increment upon incubation with alkaline phosphatases, and the detection limit of our probe was determined to be 0.07 U/L, which is lower than that of most of alkaline phosphatase probes reported in literature. Furthermore, we demonstrated that the probe can be applied to detecting alkaline phosphatase activity in cells and mice. In addition, our probe possesses excellent biocompatibility and rapid cell-internalization ability. In light of these prominent properties, we envision that DHXP will add useful tools for investigating alkaline phosphatase activity in biomedical research.

Histone deacetylases (HDACs) play important roles in regulating various physiological and pathological processes. Developing fluorescent probes capable of detecting HDAC activity can help further elucidate the roles of HDACs in biology. In this study, we first developed a set of activity-based fluorescent probes by incorporating the Kac residue and the O-NBD group. Upon enzymatic removal of the acetyl group in the Kac residue, the released free amine reacted intramolecularly with the O-NBD moiety, resulting in turn-on fluorescence. These designed probes are capable of detecting HDAC activity in a continuous fashion, thereby eliminating the extra step of fluorescence development. Remarkably, the amount of turn-on fluorescence can be as high as 50-fold, which is superior to the existing one-step HDAC fluorescent probes. Inhibition experiments further proved that the probes can serve as useful tools for screening HDAC inhibitors. Building on these results, we moved on and designed a dual-purpose fluorescent probe by introducing a diazirine photo-cross-linker into the probe. The resulting probe was not only capable of reporting enzymatic activity but also able to directly identify and capture the protein targets from the complex cellular environment. By combining a fluorometric method and in-gel fluorescence scanning technique, we found that epigenetic readers and erasers can be readily identified and differentiated using a single probe. This is not achievable with traditional photoaffinity probes. In light of the prominent properties and the diverse functions of this newly developed probe, we envision that it can provide a robust tool for functional analysis of HDACs and facilitate future drug discovery in epigenetics.