Transgenic Research 11: 361–372, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 361 Conditional over-expression of estrogen receptor alpha in a transgenic mouse model Kathleen S. Hruska 1,2 , Maddalena T. Tilli 2,3 , Shuxun Ren 2,5 , Ion Cotarla 2,4 , Theresa Kwong 2 , Minglin Li 2,5 , Joseph D. Fondell 6 , Judy A. Hewitt 7 , Robert D. Koos 6 , Priscilla A. Furth 2,3,4,5,6,8, ∗ & Jodi A. Flaws 1 1 Department of Epidemiology and Preventive Medicine, 2 Institute of Human Virology, 3 Graduate Program in Human Genetics, 4 Graduate Program in Molecular and Cellular Biology, 5 Department of Medicine, 6 Department of Physiology, 7 Department of Microbiology and Immunology, School of Medicine, University of Maryland, Baltimore, Baltimore, MD 21201, USA, 8 Lombardi Cancer Center, Georgetown University, Research Building, Room E518, 3970 Reservoir Road NW, Washington, D.C. 20007 Received 24 October 2001; revised 21 January 2002; accepted 1 February 2002 Key words: conditional gene expression, estrogen receptor α, reproduction, tetracycline responsive gene expression system, transgenic mouse Abstract Attempts to delineate the mechanisms of estrogen action have promoted the creation of several estrogen receptor alpha (ERα) mouse models in the past decade. These traditional models are limited by the fact that the receptors are either absent or present throughout all stages of development. The purpose of this work was to develop a conditional transgenic model that would provide an in vivo method of controlling the spatial and temporal regulation of ERα expression. The tetracycline responsive system was utilized. Three lines of transgenic mice carrying a transgene composed of the coding sequence for murine ERα placed under the regulatory control of a tet operator promoter (tet-op) were generated. These three lines of tet-op-mERα mice were each mated to an established line of transgenic mice expressing a tetracycline-dependent transactivator protein (tTA) from the mouse mammary tumor virus-long terminal repeat (MMTV-LTR). Double transgenic MMTV-tTA/tet-op-mERα mice were produced. All three lines demonstrated dominant gain of ERα shown by RT-PCR, immunoprecipitation, and immunohistochemistry. Transgene-specific ERα was expressed in numerous tissues including the mammary gland, salivary gland, testis, seminal vesicle, and epididymis. Expression was silenced by administration of doxy- cycline in the drinking water. This model can be utilized to evaluate the consequences of ERα dominant gain in targeted tissues at specific times during development. In this study dominant gain of ERα was associated with a reduction in epididymal/vas deferens and seminal vesicle weights consistent with the proposed action of ERα on fluid transport in the male reproductive tract. Combining this model with other dominant gain and gene knockout mouse models will be useful for testing effects of ERα action in combination with specific gene products and to evaluate if developmental and stage-specific expression of ERα can rescue identified phenotypes in gene knockout mice. Introduction Estrogens are known to regulate the differentiation, function, and maintenance of a diverse array of ∗ Author for correspondence: E-mail: paf3@georgetown.edu tissues in mammals. Estrogens promote proliferation of the uterine and vaginal epithelium in the female reproductive system and direct development of the mammary gland, external genitalia, appropriate repro- ductive sexual behavior, and other secondary sexual characteristics (George & Wilson, 1988). Estrogen signaling is also required for spermatogenesis, fluid