Inducible gene expression in the lens using tamoxifen and a GFP reporter.

In recent years, P1 phage Cre-recombinase has become an indispensable tool for conditional activation and/or inactivation of genes in the mouse. By fusing Cre to a tamoxifen-sensitive form of the estrogen receptor (Cre-ER) temporal regulation of gene expression can be achieved. Here, we report the initial characterization of the Cre-ER system for controlling gene expression in the lens of the mouse. Cre-ER mice were crossed with a reporter strain (Z/EG; lacZ/EGFP). Following i.p. injection of tamoxifen into Cre-ER;Z/EG mice, green fluorescent protein (GFP) expression was observed in 1-5% of lens epithelial and fiber cells. GFP expression was maintained for at least 6 months although, in the fibers, GFP appeared to diffuse from the cells as they were internalized into the lens. The rate of elongation of the fiber cells was determined by measuring the length of GFP-expressing cells at intervals after tamoxifen injection. The results of this calculation suggested that tamoxifen treatment induced GFP expression predominantly in lens epithelial cells. The GFP-positive fibers were apparently derived from this cell population. The strong induced expression of GFP in a small proportion of lens cells allowed individual lens cells to be identified in the intact tissue and should serve as a useful lineage marker to track the fate of lens cells and their descendents.