Direct comparison of constitutive Rax-Cre transgenic drivers that activate in the mouse embryonic eye field.

Purpose: Multiple mouse lines using constitutive or inducible Cre recombinase expression have taken advantage of the early optic field expression of the Rax/Rx gene and its subsequent, progressive restriction to retinal progenitors, retinal pigmented epithelium and the optic stalk. Among these, the constitutive transgenic lines, Rx3-Cre and Rax-Cre BAC Tg, are currently used by vision researchers. Here we directly compare their prenatal ocular and extra-ocular Cre activities. Methods: Rx3-Cre or Rax-Cre BAC transgenic male mice were mated to the Cre-dependent lineage tracer Ai9/tdTomato. The resulting live and fixed tissue expression for each line was evaluated at multiple stages of development, from the optic vesicle stage onward. Both whole embryo and immunolabeled cryosectioned material were digitally imaged. Results: Rx3-Cre recapitulates endogenous RAX protein expression at the optic vesicle and optic cup stages, but there is also ectopic Cre activity in periocular mesenchyme (POM), within PITX2- and PECAM-1-expressing subpopulations. We also show that ectopic Cre activity can include germline expression. Besides a small ectopic domain in developing limbs, the Rax-Cre BAC mouse driver fully recapitulates endogenous RAX expression in the embryonic and postnatal eye. Conclusions: We provide a systematic analysis of two Rax-Cre drivers during embryonic and postnatal eye development that are very useful to recapitulate severe congenital eye diseases. Data presented here strongly support the inclusion of lineage reporting and evaluation of littermates lacking a Cre transgene, whenever conditional targeting strategies are used in studies of optic vesicle-derived tissues.