A Water-Soluble Coelenterazine for Sensitive In Vivo Imaging of Coelenterate Luciferases

To the editor:

Luciferases have played an integral role in bioluminescence imaging as useful tools in monitoring various biological processes.^1,2,3 The most commonly used luciferase is from the American firefly Photinus pyralis (Fluc), which uses beetle -luciferin as a substrate.^4,5 Other promising luciferases for in vivo imaging are from coelenterates such as the sea pansy Renilla reniformis (Rluc) and the marine copepod Gaussia princeps (Gluc).^6,7,8,9,10 Both of these luciferases catalyze the oxidative decarboxylation of their substrate coelenterazine (CTZ), resulting in emission of blue light.¹ Because Fluc and Gluc/Rluc utilize different substrates, they can be combined as dual reporters for monitoring two distinct biological processes.^11,12 Firefly, Gaussia, and Renilla luciferases are being used as reporters for the in vivo monitoring of numerous biological processes in a variety of fields, including immunology, oncology, virology, neuroscience, and gene and cell therapy.^1,2,3

The major drawback to using coelenterate luciferases as reporters for in vivo imaging has been the unavailability of a CTZ variant that is soluble in aqueous solution, making the use of alcohol essential in animals. The ensuing toxicity/solubility in vivo limits the injected dose (typically up to 100 μg), leading to lower sensitivity. Here, we show that a water-soluble coelenterazine (s-CTZ) that was recently synthesized and is commercially available yields up to 100-fold greater sensitivity as compared with native CTZ for in vivo imaging of Renilla and Gaussia luciferases. This s-CTZ is very useful for monitoring in vivo biological processes in which high sensitivity is required, such as detecting few circulating cells, early tumor metastasis, and apoptosis.^7,10,13,14

We first compared the bioluminescence reaction kinetics for coelenterate luciferases using either CTZ or s-CTZ. We determined the light output for Gluc and Rluc samples over time using a spectrophotometer (Supplementary Methods online). Both substrates showed similar flash-type bioluminescence reactions for Gluc and Rluc (Supplementary Figure S1a). Recently, we characterized a Gluc variant (GlucM43I) that catalyzes a glow-type bioluminescence reaction in the presence of a detergent such as Triton X-100 (ref. 15). We compared the reaction kinetics of this variant as well as wild-type Gluc in the presence of Triton X-100 using CTZ and s-CTZ. The substrates exhibited similar glow-type reactions with GlucM43I and enhanced stable light output with wild-type Gluc in the presence of Triton X-100 (Supplementary Figure S1a). We also investigated whether s-CTZ has any influence on the characteristics of light emission by Gluc or Rluc. We analyzed their emission spectra using either CTZ or s-CTZ and observed that both luciferases exhibited their characteristic peaks around 480 nm, indicating that the water-soluble substrate had no effect on the light emission of coelenterate luciferases (Supplementary Figure S1b). These data suggest that the s-CTZ yields bioluminescence characteristics similar to those of native CTZ.

We then determined the effect of native and water-soluble CTZ concentrations on Gluc and Rluc light production. Gli36 human glioma cells were transduced with a lentivirus vector to express either Rluc or Gluc (Supplementary Methods). Equal aliquots of conditioned medium (for Gluc) or cell lysates (for Rluc) were analyzed with increasing doses of both CTZs using a luminometer. As expected, increasing the substrate dose increased the signal from Gluc in a roughly linear manner, whereas Rluc reached a plateau at around 2 μg/ml. Interestingly, s-CTZ yielded over 10-fold higher activity for Gluc and fourfold for Rluc at all concentrations tested, as compared with CTZ (Supplementary Figure S2a). We also compared the sensitivity of the water-soluble CTZ to the native substrate in detecting viability of mammalian cells. Different numbers of Gli36 glioma cells stably expressing Gluc or Rluc were plated in 96-well plates. Twenty-four hours later, an aliquot of the conditioned medium (for Gluc) or total cell lysate (for Rluc) was assayed using both CTZs. Again, we observed over 10-fold higher photon output with Gluc and fourfold for Rluc using s-CTZ, as compared with the native CTZ substrate. These data collectively suggest that the water-soluble CTZ could be more sensitive for detecting coelenterate luciferases in vitro and in vivo (Supplementary Figure S2b).

Finally, we determined the usefulness of s-CTZ for in vivo bioluminescence imaging in deep tissues. We injected Gli36-Gluc and Gli36-Rluc cells into the leg muscle of nude mice (n = 4). One week later, mice were injected intravenously with 100 μg (4 mg/kg body weight) of CTZ and imaged immediately using a cooled charge-coupled device (CCD) camera. Four hours later, when signal from the previous imaging session had dropped to background levels, mice were injected with s-CTZ and imaged using the same protocol. Interestingly, the water-soluble CTZ showed 10-fold higher photon counts for Gluc and fourfold for Rluc, as compared with the native CTZ at the same dose, showing that s-CTZ is more sensitive in detecting coelenterate luciferases in vivo (Figure 1a–c). We also determined the effect of increasing the amount of s-CTZ on Gluc and Rluc detection in deep tissues. We injected the same mice mentioned above with 500 μg (20 mg/kg) of water-soluble CTZ and imaged them using a CCD camera. Tumors expressing Gluc resulted in a >120-fold increase in photon counts as compared with tumors imaged with native CTZ (Figure 1a–c). Similarly, Rluc-expressing tumors yielded a 30-fold increase in bioluminescence signal intensity (Figure 1a–c). Importantly, mice did not exhibit any sign of toxicity at this higher dose of s-CTZ. These results collectively demonstrate that the water-soluble CTZ is a sensitive substrate for in vivo imaging of coelenterate luciferase and can be used at a higher dose, overcoming one of the primary limitations of native CTZ, which is its insolubility in water.

Figure 1.

Sensitive in vivo imaging of coelenterate luciferases using water-soluble coelenterazine (s-CTZ). A total of 500,000 Gli36-Gluc or Gli36-Rluc cells were injected into the muscle of the left and right leg of nude mice, respectively (n = 4). (a) The mice were injected with 100 μg (4 mg/kg) of native CTZ and imaged using a cooled charge-coupled device camera. Four hours later, when signal from the previous imaging session dropped to background levels, the mice were injected with 100 μg of s-CTZ and imaged again. After another 4 hours, they were imaged after injection of 500 μg of s-CTZ. (b) A typical mouse in which the Gluc-expressing tumor is covered so as to visualize the signal from the Rluc-expressing tumor. (c) Quantitation of tumor-associated signals in a. (d) Blood was collected from the mice, and 5-μl aliquots (in triplicates) were assayed for Gluc activity using 100 μl (50 μg/ml) of either s-CTZ or CTZ and a luminometer. Data are presented as fold differences (in which the signals obtained from native CTZ are set to 1) ± SD (n = 4).

Recently, we demonstrated that Gluc can be used as a blood reporter for ex vivo monitoring of in vivo biological processes such as gene transfer, viral replication, and circulating cells tracking.¹⁰ To evaluate the sensitivity of s-CTZ to detect Gluc in the blood, we collected blood samples from mice bearing Gli36-Gluc intramuscular tumors and analyzed 5-μl aliquots (in triplicates) with 100 μl (50 μg/ml) of either CTZ or s-CTZ, using a luminometer. As compared with the native form, the water-soluble CTZ showed a bioluminescence signal more than eightfold higher, demonstrating a greater sensitivity for Gluc detection in the blood (Figure 1d).

In summary, we showed that water-soluble CTZ, which is commercially available, is a sensitive substrate for in vivo bioluminescence imaging of coelenterate luciferases. s-CTZ proved to be more sensitive in detecting Gluc and Rluc expression in mammalian cells both in culture and in vivo. Because alcohol is not required, s-CTZ could be injected in mice at higher doses compared with native CTZ, yielding up to 100-fold more light output from tumors expressing Gluc or Rluc. Furthermore, s-CTZ yielded more than an eightfold increase in bioluminescence signal when Gluc was assayed in the blood. Together, these results suggest that s-CTZ is a more potent substrate in detecting subtle coelenterate luciferases in vivo, facilitating noninvasive monitoring of various biological processes.

SUPPLEMENTARY MATERIAL Supplementary Methods. Supplementary Figure S1. Time kinetics of Gluc and Rluc light production using s-CTZ. Supplementary Figure S2. Sensitivity of s-CTZ in detecting coelenterate luciferases in culture.

Supplementary Material

Supplementary Figure S1.

Time kinetics of Gluc and Rluc light production using s-CTZ.

Supplementary Figure S2.

Sensitivity of s-CTZ in detecting coelenterate luciferases in culture.