Expression of the Wnt inhibitor, sFRP5, in the gut endoderm of Xenopus Karen E. Pilcher, Paul A. Krieg * Department of Cell Biology and Anatomy, University of Arizona Health Sciences Center, 1501 North Campbell Avenue, P.O. Box 254044, Tucson, AZ 85724-5044, USA Received 14 August 2002; received in revised form 7 October 2002; accepted 9 October 2002 Abstract Signaling by the Wnt family of secreted growth factors is involved in numerous different aspects of embryonic development and also for maintenance of cellular function in adult tissues. In addition to regulation at the transcriptional level, Wnt activity is modulated by a number of different Wnt-binding proteins. Here we report the cloning and developmental expression pattern of the Xenopus orthologue of secreted Frizzled-related protein 5 (sFRP5), an endogenous inhibitor of Wnt signaling. At early stages of endodermal differentiation, sFRP5 is expressed in the developing liver. At later stages however, sFRP5 expression is down-regulated in the liver and becomes strongly expressed in the region corresponding to the junction between the posterior portion of the stomach and the anterior intestines. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Xenopus; Frizzled-related protein; sFRP5; Endoderm; Liver; Stomach; Intestines 1. Results and discussion Members of the Wnt family of growth factors are expressed at many different stages of embryonic develop- ment and in a wide range of adult tissues. Through binding to transmembrane receptors of the Frizzled family, Wnts elicit various downstream cellular responses depending on the spatial and temporal context (Bhanot et al., 1996; Yamaguchi, 2001). A number of endogenous proteins with the ability to interfere with Wnt signaling have been identified. In general, these are secreted proteins that bind to Wnt growth factors in the extracellular environment and prevent specific interactions with the Frizzled receptors (Hoang et al., 1996; Yamaguchi, 2001). One such class of inhibitors is the secreted Frizzled-related proteins. These molecules resemble Frizzled receptors but lack a transmembrane domain and therefore act as efficient nega- tive regulators of the Wnt signaling pathway (Leyns et al., 1997). In a screen for Wnt pathway molecules expressed in the Xenopus heart, we isolated a clone that encodes a sequence similar to mammalian secreted Frizzled-related protein 5 (sFRP5). Excluding the highly variable signal sequence, the Xenopus amino acid sequence is 71% identical to mouse sFRP5 (Chang et al., 1999) (Fig. 1). This is similar to the 74% sequence identity previously determined for Xenopus orthologues of mammalian sequences (Tonissen and Krieg, 1993). In the highly conserved cysteine-rich domain found in Frizzled-related proteins, the Xenopus Frizzled related sequence is 81% identical to the mouse protein. When the Xenopus Frizzled related sequence is used to search the mouse genome, the closest match is to the sFRP5 gene. We conclude therefore, that this clone encodes the Xenopus orthologue of sFRP5. While mammalian sFRP5 sequences have been detected in retinal pigment epithelium (Chang et al., 1999) and adult pancreas (Melkonyan et al., 1997; Chang et al., 1999) the embryonic expression pattern has not been determined. Therefore, we have used in situ hybridization methods to examine the developmental expression of Xenopus sFRP5. Expression is first detected in the late neurula embryo (stage 25) in the developing endoderm (Figs. 2A–C). This expres- sion domain partially overlaps that of the liver marker Hex (Newman et al., 1997), however, it extends more dorsally than Hex and is more limited along the A/P axis (compare Figs. 2A,D). Through to the tailbud stages (stages 37/38), expression continues in the liver primordium (Figs. 2E,F), and remains slightly more dorsally-extended than Hex expression. At approximately stage 39, however, sFRP5 expression rapidly declines in the liver and becomes strongly upregulated in endodermal tissues lying close to the position of the developing stomach (Fig. 2H). To further characterize this posterior domain of sFRP5 Gene Expression Patterns 2 (2002) 369–372 1567-133X/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-133X(02)00023-6 www.elsevier.com/locate/modgep * Corresponding author. Tel.: 11-520-626-9370; fax: 11-520-626-2097. E-mail address: pkrieg@email.arizona.edu (P.A. Krieg).