Rhodopsin promoter-EGFP fusion transgene expression in photoreceptor neurons of retina and pineal complex in mice

Light detection in vertebrate eyes is mediated through retinal photoreceptor rod and cone cells that transduce light signals into electrical responses. The differentiation and synaptogenesis of photoreceptor cells are especially important since these cells are the main targets of degeneration in retinitis pigmentosa. We produced transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the mouse rhodopsin gene promoter. In Western blot analyses of transgenic retinal homogenates, expression of the endogenous rhodopsin gene was detected from post-natal day (P)8; however, EGFP expression in transgenic retinas was initially detected at P12, indicating delayed expression of the transgene. At P14, fluorescence microscopy showed a weak expression of EGFP in the transgenic retina. In the adult transgenic mice, the strongest EGFP expression was observed in the outer nuclear layer, and to a lesser extent in the outer plexiform layer as well as in the inner and outer segments. EGFP expression was also observed in the pineal stalk. The rhodopsin promoter-EGFP transgenic mice will be not only useful to assess rhodopsin gene promoter activity in vivo, but also for retinal transplant studies as a source of functional photoreceptor cells that are fluorescent green.