SUMMARY The altered metabolism of tumor cells confers a selective advantage for survival and proliferation, and studies have shown that targeting such metabolic shifts may be a useful therapeutic strategy. We developed an intensely fluorescent, rapidly responsive, pH-resistant, genetically encoded sensor of wide dynamic range, denoted SoNar, for tracking cytosolic NAD+ and NADH redox states in living cells and in vivo. SoNar responds to subtle perturbations of various pathways of energy metabolism in real-time, and allowed high-throughput screening for new agents targeting tumor metabolism. Among > 5,500 unique compounds, we identified KP372-1 as a potent NQO1-mediated redox cycling agent that produced extreme oxidative stress, selectively induced cancer cell apoptosis and effectively decreased tumor growth in vivo. This study demonstrates that genetically encoded sensor-based metabolic screening could serve as a valuable approach for drug discovery.

SUMMARY Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is essential for for biosynthetic reactions and antioxidant functions; however, detection of NADPH metabolism in living cells remains technically challenging. We develop and characterize ratiometric, pH-resistant, genetically encoded fluorescent indicators for NADPH (iNap sensors) with various affinities and wide dynamic range. The iNap sensors permitted quantification of cytosolic and mitochondrial NADPH pools that were controlled by cytosolic NAD+ kinase levels, and revealed cellular NADPH dynamics under oxidative stress depending on glucose availability. We find that mammalian cells have a strong tendency to maintain physiological NADPH homeostasis, which is regulated by glucose-6-phosphate dehydrogenase (G6PD) and AMP kinase (AMPK). Moreover, using the iNap sensors we monitor NADPH fluctuations during the activation of macrophage cells or wound response in vivo. These data demonstrate that the iNap sensors will be valuable tools for monitoring NADPH dynamics in live-cells, and gaining new insights into cell metabolism.

Peroxynitrite (OONO(-)) is profoundly implicated in health and disease. The physiological and pathological outcome of OONO(-) is related to its local concentration, and hence, a reliable OONO(-) assay is highly desired. We have developed a FRET-based small-molecule fluorescent probe (PNCy3Cy5), harnessing the differential reactivity of Cy3 and Cy5 toward OONO(-) by fine-tuning. It exhibits high detection sensitivity and yields a ratiometric fluorescent signal. We have exemplified that it can be applied in semiquantitative determination of OONO(-) in living cells. Notably, it specifically localizes in mitochondria, where endogenous OONO(-) is predominantly generated. Thus, PNCy3Cy5 is a promising molecular tool for peroxynitrite biology.

Indicator displacement assays (IDAs) offer a unique and innovative approach to molecular sensing. This Tutorial review discusses the basic concepts of each IDA strategy and illustrates their use in sensing applications.