Non-invasive in vivo optical imaging of the lacZ and luc gene expression in mice

The bacterial lacZ gene encoding for β-galactosidase (β-gal) is a common reporter gene used in transgenic mice. Nonetheless, the absence of fluorigenic substrates usable in live animals greatly hampered the non-invasive follow-up of this reporter gene expression. We used far-red fluorescence for imaging β-Gal expression in live cells in vitro or in vivo . The 9 H -(1,3-dichloro-9,9-dimethylacridin- 2-one-7-yl) β- D -galactopyranoside substrate was used to monitor β-Gal expression as a reporter of tumor growth, or of the physiological levels of an endogenous gene or of gene transfer in lung. A quantitative evaluation of this method as well as a comparison of its sensitivity with Firefly Luciferase-based bioluminescence was also performed. In vivo measurements showed that 10 3 β-Gal tumor cells located under the skin were detectable. In deeper organs like lung, as little as 5 ng of β-Gal or Luciferase enzymes per mg of proteins were measured, confirming that both techniques reached similar sensibilities. Nonetheless, quantitative comparison of β-Gal levels measured with far-red imaging or with a standardized enzymatic evaluation after killing revealed that the 2D-fluorescent reflectance imaging method is submitted to a color-dependent disparity of the organs and cannot supply quantitative measurements but that a simple correction can be applied.