Genetic oxygen sensor: GFP as an indicator of intracellular oxygenation

We report in this article a new method for in vivo oxygen measurement using green fluorescence protein (GFP). COS7 cells were transiently transfected with an expression vector, pCMX-GFP, using a polyethylenimine reagent and cultured for 48 hrs. After exposure of the cell to anoxic gas (O2 < .001%), a 1 min illumination of the cell to strong 470-490 nm light evoked a significant red fluorescence (excitation 520-550 nm, emission > 580 nm) that had been negligible before the photoactivation. This red shift of (green) GFP fluorescence was never observed in normoxia. We then examined the validity of this method in transgenic mice in which GFP is stably expressed (green mice). All the ventricular myocytes isolated from the green mice showed significant green fluorescence, although the intensity was approximately 1/200 of the transiently GFP-expressing COS7 cells. The photoactivation in anoxia increased the red fluorescence in these cells, but the magnitude was much smaller than expected. In summary, GFP can be used as an in situ probe for hypoxia. In GFP-expressing transgenic animals, in vivo imaging of anoxic loci with a submicron spatial resolution may be possible.