CMV-driven expression of green fluorescent protein (GFP) in male germ cells of transgenic mice and its effect on fertility.

To determine if the expression of green fluorescent protein (GFP) during spermatogenesis can compromise the fertility of transgenic animals, we have produced mouse transgenic lines expressing GFP in the testis under the control of the potent immediate early promoter and enhancer region of the human cytomegalovirus (CMV). GFP expression was detected in the germ cells with no apparent effect on the fertility of any of the five transgenic lines studied. We demonstrate that the CMV promoter is transcriptionally active in the testes of mice aged 7 days. However, protein could be visually detected only after day 10. GFP was not found either in mature spermatozoa or in Sertoli cells, but it was evident in round spermatids from seminiferous tubules and in cytoplasmic drops of spermatozoa from the epididymis. We also detected GFP in spermatogonia expressing c-kit, which indicates that GFP expression driven by the CMV promoter takes place during the proliferative phase of spermatogenesis. The expression of GFP during spermatogenesis did not affect the fertility of transgenic mice.