Differential expression and localization of CFTR and ENaC in mouse endometrium during pre-implantation Jian Zhi Yang, Louis Chukwuemeka Ajonuma, Lai Ling Tsang, Sun Yee Lam, Dewi Kenneth Rowlands, Lok Sze Ho, Chen Xi Zhou, Yiu Wa Chung, Hsiao Chang Chan ) Epithelial Cell Biology Research Center, Department of Physiology, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR Received 23 September 2003; revised 9 February 2004; accepted 16 March 2004 Abstract Interaction between the cystic fibrosis transmembrane conductance regulator (CFTR), a CAMP-activated Cl ÿ channel, and epithelial Na C channel (ENaC) has been proposed as the major mechanism regulating uterine fluid absorption and secretion. Differential expression of these ion channels may give rise to dynamic changes in the fluid environment affecting various reproductive events in the female reproductive tract. This study investigated the expression and localization of CFTR and ENaC during the pre-implantation period. Semi-quantitative reverse transcriptase polymerase chain reaction and immunohistochemistry were used to study the expression and localization of CFTR and ENaC in uteri collected from mature superovulated female mice. RT-PCR showed maximal ENaC and CFTR expression on day 3 after mating. Maximal immunoreactivity was also observed for both ENaC and CFTR on day 3 after mating. However, ENaC was immunolocalized to the apical membrane of both luminal and glandular epithelia, while CFTR was predominantly found in the stromal cells rather than the epithelial cells. Differential expression and localization of CFTR and ENaC provide a molecular mechanism by which maximal fluid absorption can be achieved immediately prior to implantation, to ensure the immobilization of the blastocyst necessary for implantation. Ó 2004 International Federation for Cell Biology. Published by Elsevier Ltd. All rights reserved. Keywords: CFTR; ENaC; Endometrium; Pre-implantation; Immunohistochemistry; RT-PCR 1. Introduction Implantation is a complex and dynamic process that is initiated by the adhesion between the embryonic trophectoderm and the epithelial cells of the uterine endometrium. This rate-limiting process in reproduction remains a poorly understood phenomenon, but it is thought to be greatly influenced by the uterine fluid, which undergoes dynamic changes during the endome- trial cycle and reproductive events (Long and Evans, 1998). The disappearance of luminal fluid in the pre-implantation period, a characteristic feature of many species, is considered to be a mechanism leading to the ‘‘closure’’ of the lumen, thereby enabling embryos to be held in contact with uterine epithelium before initiation of implantation. While a couple of mechanisms, such as endocytosis (Parr, 1983) and characteristic cytoplasmic projections known as ‘‘pino- podes’’ that appear on the luminal surface before implantation (Nardo et al., 2002; Nikas et al., 1995), have long been thought to play a role in fluid absorption during implantation, recent studies suggest that the dynamic changes in uterine fluid may be largely determined by the interaction of epithelial ion channels in the endometrium (Chan et al., 2002). Fluid movement across epithelia is secondary to the movement of solutes, particularly ions. Na C and Cl ÿ ) Corresponding author. Tel.: C852-26096839; fax: C852- 26035022. E-mail address: hsiaocchan@cuhk.edu.hk (H.C. Chan). www.elsevier.com/locate/cellbi Cell Biology International 28 (2004) 433e439 1065-6995/$ - see front matter Ó 2004 International Federation for Cell Biology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.cellbi.2004.03.011