Quantitative comparison of click beetle and firefly luciferases for in vivo bioluminescence imaging

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Abstract
For bioluminescence imaging (BLI) of small animals, the most commonly used luciferase is Fluc from the firefly, but recently, green (CBGr99) and red (CBRed) click beetle luciferases became available. Because signal attenuation by tissues is lower for red light, red luciferases appear to be advantageous for BLI, but this has not been thoroughly tested. We compare different luciferases for BLI. For this purpose, cell transfectants are generated expressing comparable amounts of CBGr99, CBRed, or Fluc. This is achieved by coexpression of the luciferase with eGFP using the bicistronic 2A system, which results in stoichiometric coexpression of the respective proteins. In vitro, the CBGr99 transfectant exhibits the strongest total photon yield. For in vivo BLI, the transfectants are injected into mice at different locations. At a subcutaneous position, CBGr99 is clearly superior to the other luciferases. When the tumor cells are located in the peritoneum or lung, where more absorption by tissue occurs, CBGr99 and CBRed transfected cells emit a comparable number of red photons and are superior to Fluc, but CBGr99 reaches the maximum of the light emission faster than CBRed. Thus, although CBGr99 emits mainly green light, the high yield of total and red photons makes it an excellent candidate for BLI.