Fireflies are beetles that generate yellow-green light when their luciferase enzyme activates and oxidizes its substrate, D-luciferin. This bioluminescent reaction is widely used as a sensitive reporter both in vitro and in vivo. However, the light-emitting chemistry is limited by the properties of the small molecule D-luciferin. Our lab has developed a panel of synthetic luciferin analogs that improve on the inherent characteristics of D-luciferin. My thesis work focuses on harnessing these novel substrates to further expand the utility and molecular understanding of firefly bioluminescence. The first part of my thesis focuses on using synthetic luciferins to improve bioluminescence imaging beyond what is possible with D-luciferin. Our substrates emit red-shifted light compared to D-luciferin, bringing the wavelength to a range that is more able to penetrate through tissue, but at a cost of lower signal intensity. I developed mutant luciferases that increase the maximal photon flux with the synthetic luciferins over what is achievable with the wild-type luciferase, and furthermore discriminate between substrates based on their chemical structures. Additionally, I have expanded the bioluminescence toolkit by harnessing the intrinsic properties of the luciferins to non-invasively and specifically assay the activity of a single enzyme (fatty acid amide hydrolase) in live mice. Therefore, my work presents an effective way to generally improve upon bioluminescent reporters, but also to measure the activity of a specific enzyme of interest in the context of a living organism. The second part of my thesis employs synthetic luciferins to more deeply probe the light-emitting chemistry of bioluminescence. Our synthetic substrates reveal latent luciferase activity from multiple luciferase homologs that are inactive with D-luciferin. These enzymes, the fatty acyl-CoA synthetases, are predicted to be luciferase’s evolutionary predecessors, but it was not clear how the light emitting chemistry originated. My work shows that the luciferase must activate the luciferin and provide oxygen access, but the light emitting chemistry is a fundamental property of that activated intermediate. In summary, the work described herein not only expands our understanding of firefly bioluminescence, but also broadens its practical applications to shine bioluminescent light on the dark corners of biology.

The light emission chemistry of firefly luciferase can be harnessed to reveal otherwise invisible biological processes occurring in the brains of live animals. Though powerful, the need for the luciferase substrate D-luciferin to traverse the blood-brain barrier poses limitations on the sensitivity and interpretation of these experiments. In this Viewpoint, we discuss bioluminescent imaging probes for the enzyme fatty acid amide hydrolase (FAAH) and the broader implications for optical imaging and drug delivery in the brain.

Firefly luciferase is homologous to fatty acyl-CoA synthetases. We hypothesized that the firefly luciferase substrate d-luciferin and its analogs are fatty acid mimics that are ideally suited to probe the chemistry of enzymes that release fatty acid products. Here, we synthesized luciferin amides and found that these molecules are hydrolyzed to substrates for firefly luciferase by the enzyme fatty acid amide hydrolase (FAAH). In the presence of luciferase, these molecules enable highly sensitive and selective bioluminescent detection of FAAH activity in vitro, in live cells, and in vivo. The potency and tissue distribution of FAAH inhibitors can be imaged in live mice, and luciferin amides serve as exemplary reagents for greatly improved bioluminescence imaging in FAAH-expressing tissues such as the brain.

Firefly luciferase adenylates and oxidizes d-luciferin to chemically generate visible light and is widely used for biological assays and imaging. Here we show that both luciferase and luciferin can be reengineered to extend the scope of this light-emitting reaction. D-Luciferin can be replaced by synthetic luciferin analogues that increase near-infrared photon flux > 10-fold over that of D-luciferin in live luciferase-expressing cells. Firefly luciferase can be mutated to accept and utilize rigid aminoluciferins with high activity in both live and lysed cells yet exhibit 10,000-fold selectivity over the natural luciferase substrate. These new luciferin analogues thus pave the way to an extended family of bioluminescent reporters.

Beetle luciferases are thought to have evolved from fatty acyl-CoA synthetases present in all insects. Both classes of enzymes activate fatty acids with ATP to form acyl-adenylate intermediates, but only luciferases can activate and oxidize d-luciferin to emit light. Here we show that the Drosophila fatty acyl-CoA synthetase CG6178, which cannot use d-luciferin as a substrate, is able to catalyze light emission from the synthetic luciferin analog CycLuc2. Bioluminescence can be detected from the purified protein, live Drosophila Schneider 2 cells, and from mammalian cells transfected with CG6178. Thus, the nonluminescent fruit fly possesses an inherent capacity for bioluminescence that is only revealed upon treatment with a xenobiotic molecule. This result expands the scope of bioluminescence and demonstrates that the introduction of a new substrate can unmask latent enzymatic activity that differs significantly from an enzyme's normal function without requiring mutation.