| Summary: | <p>Abstract</p> <p>Background</p> <p>Optical imaging is an attractive non-invasive way to evaluate the expression of a transferred DNA, mainly thanks to its lower cost and ease of realization. In this study optical imaging was evaluated for monitoring and quantification of the mouse knee joint and tibial cranial muscle electrotransfer of a luciferase encoding plasmid. Optical imaging was applied to study the kinetics of luciferase expression in both tissues.</p> <p>Results</p> <p>The substrate of luciferase (luciferin) was injected either intraperitonealy (<it>i.p.</it>) or <it>in situ </it>into the muscle or the knee joint. Luminescence resulting from the luciferase-luciferin reaction was measured <it>in vivo </it>with a cooled CCD camera and/or <it>in vitro </it>on tissue lysate. Maximal luminescence of the knee joint and muscle after <it>i.p. </it>(2.5 mg) or local injection of luciferin (50 μg in the knee joint, 100 μg in the muscle) were highly correlated. With the local injection procedure adopted, <it>in vivo </it>and <it>in vitro </it>luminescences measured on the same muscles significantly correlated. Luminescence measurements were reproducible and the signal level was proportional to the amount of plasmid injected. <it>In vivo </it>luciferase activity in the electrotransfered knee joint was detected for two weeks. Intramuscular electrotransfer of 0.3 or 3 μg of plasmid led to stable luciferase expression for 62 days, whereas injecting 30 μg of plasmid resulted in a drop of luminescence three weeks after electrotransfer. These decreases were partially associated with the development of an immune response.</p> <p>Conclusion</p> <p>A particular advantage of the <it>i.p. </it>injection of substrate is a widespread distribution at luciferase production sites. We have also highlighted advantages of local injection as a more sensitive detection method with reduced substrate consumption. Besides, this route of injection is relatively free of uncontrolled parameters, such as diffusion to the target organ, crossing of biological barriers and evidencing variations in local enzymatic kinetics, probably related to the reaction medium in the targeted organ. Optical imaging was shown to be a sensitive and relevant technique to quantify variations of luciferase activity <it>in vivo</it>. Further evaluation of the effective amount of luciferase in a given tissue by <it>in vivo </it>optical imaging relies on conditions of the enzymatic reaction and light absorption and presently requires <it>in vitro </it>calibration for each targeted organ.</p>
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