Dynamic Expression of Kru ¨ ppel-Like Factor 4 (Klf4), a Target of Transcription Factor AP-2 During Murine Mid-embryogenesis JULIA EHLERMANN, 1 PETRA PFISTERER, 2 AND HUBERT SCHORLE 1 * 1 Institute for Pathology, Department of Developmental Pathology, University of Bonn Medical School, Bonn, Germany 2 InDex Pharmaceuticals ABMTC, Karolinska Institutet, Stockholm, Sweden ABSTRACT Kru ¨ ppel-like factor 4 (Klf4) belongs to the family of transcription factors that are thought to be involved in the regulation of epithelial and germ cell differentiation, based on their expression in postproliferative cells of the skin, gut, and testes. Gene ablation experiments suggest that Klf4 plays a role in keratinocyte differentiation, since mice lacking Klf4 fail to establish proper barrier function and, as a consequence, die postnatally due to dehydration. Recent studies have shown that Klf4 is also expressed in postnatal male mice, in postmeiotic sperm cells undergoing terminal differentiation into sperm cells. However, prior to the current study, the expression pattern of Klf4 during early and mid-embryogenesis had not been examined. Here we demonstrate that Klf4 transcripts can be detected from embryonic day 4.5 (E4.5) on in the developing conceptus, and that Klf4 expression before E10 is restricted to extraembryonic tissues. The embryo proper displays a highly dynamic and changing Klf4 signal from E10 of murine development on. In addition to being expressed in a stripe of mesenchymal cells extending from the forelimb bud rostrally over the branchial arches to the developing eye, Klf4 is also expressed in the mesenchyme surrounding the nasal pit at day E11.5. In addition, Klf4 has been detected in the apical ectodermal ridge and adjacent mesenchymal cells in the limb buds, and in mesenchymal cells of the developing body wall in trunk areas. These findings suggest that Klf4 plays an important role in regulating cellular proliferation, which underlies the morphogenetic changes that shape the developing embryo. Anat Rec Part A 273A:677– 680, 2003. © 2003 Wiley-Liss, Inc. Key words: Kru ¨ ppel-like factor 4 (Klf4); gut-enriched Kru ¨ ppel-like factor (GKLF); expression pattern; mesenchyme; apical ectodermal ridge; craniofacial mesenchyme; transcription factor AP-2; Tcfap2a Kru ¨ ppel-like factor (Klf4; also known as gut-enriched Kru ¨ ppel-like factor (GKLF)) is a member of the family of Kru ¨ ppel proteins, which are named for their homology to the Drosophila Kru ¨ ppel gene (Anderson et al., 1995). Pre- vious reports have shown that Klf4 is expressed in the gastrointestinal tract (Garrett-Sinha et al., 1996; Shields et al., 1996) and in the developing layers of the skin starting from embryonic day 16.5 (E16.5) of murine devel- opment (Segre et al., 1999). Early studies implicating Klf4 in regulating growth arrest (Garrett-Sinha et al., 1996; Shields et al., 1996) have been substantiated by the find- ing that Klf4 is a transcriptional repressor of Cyclin D1,a gene that is involved in cell cycle progression (Shie et al., 2000). Gene-targeting experiments have revealed a role for Klf4 in barrier formation in the skin (Segre et al., 1999). Recently, Klf4 was described as a target gene that is repressed by transcription factor AP-2 (Tcfap2a, MGI Grant sponsor: Deutsche Forschungsgemeinschaft; Grant num- bers: DFG 503-3; DFG 503-6. *Correspondence to: Hubert Schorle, Institute for Pathology, Department of Developmental Pathology, University of Bonn, Sigmund-Freud Strasse 25, 53127 Bonn, Germany. Fax: +49- 228-287-5030. E-mail: Hubert.Schorle@ukb.uni-bonn.de Received 17 March 2003; Accepted 23 April 2003 DOI 10.1002/ar.a.10089 EXPRESSions THE ANATOMICAL RECORD PART A 273A:677– 680 (2003) © 2003 WILEY-LISS, INC.