5469 Introduction Two closely related single-pass transmembrane proteins Lrp5 and Lrp6 (Brown et al., 1998; Hey et al., 1998) comprise a subfamily of low-density lipoprotein (LDL) receptor-related proteins with diverse functional roles as cell-surface receptors (Nykjaer and Willnow, 2002; Strickland et al., 2002). Previous studies have shown that Lrp5 and Lrp6 function as coreceptors for Wnt ligands (He et al., 2004; Pinson et al., 2000; Tamai et al., 2000; Wehrli et al., 2000; Zorn, 2001). Wnt signaling plays important roles in a wide variety of biological processes during pre- and postnatal life in invertebrates and vertebrates. Downstream of the surface receptors, Wnt signaling is transduced through the so-called canonical pathway, which is dependent on β-catenin, or through other noncanonical pathways. A line of evidence supports that Lrp5/6 mediates only the canonical Wnt/β-catenin signaling pathway (Bafico et al., 2001; Mao et al., 2001a; McEwen and Peifer, 2001; Semenov et al., 2001; Wehrli et al., 2000). In the current model of the canonical Wnt/β-catenin signaling pathway, Wnt ligands bind to the frizzled receptor and form a ternary complex with Lrp5 or Lrp6 on the cell surface. This heterotrimeric complex formation results in the stabilization of β-catenin by inactivating the β-catenin destruction complex in the cytoplasm, which is a large multiprotein complex consisting of glycogen synthase kinase 3β (GSK3β), the tumor suppressor protein APC, the scaffold protein axin and several other proteins. In the absence of Wnt signaling, GSK3β phosphorylates β-catenin, leading to the ubiquitin-mediated degradation of β-catenin by the proteasome. Direct interaction between Lrp5/6 and axin, which is dependent on the Wnt- frizzled interaction, is thought to be important for the inactivation of GSK3β (Mao et al., 2001b). Upon Wnt signaling, stabilized β-catenin translocates into the nucleus and forms a complex with HMG-box containing transcription factors of the TCF/LEF1 family, leading to the activation of Wnt-target genes. The Wnt pathway has recently been implicated in the control Here, we present evidence that Lrp6, a coreceptor for Wnt ligands, is required for the normal formation of somites and bones. By positional cloning, we demonstrate that a novel spontaneous mutation ringelschwanz (rs) in the mouse is caused by a point mutation in Lrp6, leading to an amino acid substitution of tryptophan for the evolutionarily conserved residue arginine at codon 886 (R886W). We show that rs is a hypomorphic Lrp6 allele by a genetic complementation test with Lrp6-null mice, and that the mutated protein cannot efficiently transduce signals through the Wnt/β-catenin pathway. Homozygous rs mice, many of which are remarkably viable, exhibit a combination of multiple Wnt-deficient phenotypes, including dysmorphologies of the axial skeleton, digits and the neural tube. The establishment of the anteroposterior somite compartments, the epithelialization of nascent somites, and the formation of segment borders are disturbed in rs mutants, leading to a characteristic form of vertebral malformations, similar to dysmorphologies in individuals suffering from spondylocostal dysostosis. Marker expression study suggests that Lrp6 is required for the crosstalk between the Wnt and notch-delta signaling pathways during somitogenesis. Furthermore, the Lrp6 dysfunction in rs leads to delayed ossification at birth and to a low bone mass phenotype in adults. Together, we propose that Lrp6 is one of the key genetic components for the pathogenesis of vertebral segmentation defects and of osteoporosis in humans. Key words: Lrp6, Wnt signaling, Somitogenesis, Osteoporosis, Mouse Summary Skeletal defects in ringelschwanz mutant mice reveal that Lrp6 is required for proper somitogenesis and osteogenesis Chikara Kokubu 1, * ,† , Ulrich Heinzmann 2, *, Tomoko Kokubu 1 , Norio Sakai 3 , Takuo Kubota 3 , Masanobu Kawai 3 , Matthias B. Wahl 1 , Juan Galceran 4 , Rudolf Grosschedl 4 , Keiichi Ozono 3 and Kenji Imai 1,‡ 1 Institute of Developmental Genetics, GSF-National Research Center for Environment and Health, Ingolstädter Landstrasse 1, 85764 Neuherberg, Germany 2 Institute of Pathology, GSF-National Research Center for Environment and Health, Ingolstädter Landstrasse 1, 85764 Neuherberg, Germany 3 Department of Developmental Medicine (Pediatrics), Osaka University Graduate School of Medicine, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan 4 Gene Center and Institute of Biochemistry, Ludwig Maximilians University, Feodor Lynenstrasse 25, 81377 Munich, Germany *These authors contributed equally to this work † Present address: Department of Social and Environmental Medicine, Graduate School of Medicine, Osaka University, Suita, Osaka 565-0871, Japan ‡ Author for correspondence (e-mail: imai@gsf.de) Accepted 17 August 2004 Development 131, 5469-5480 Published by The Company of Biologists 2004 doi:10.1242/dev.01405 Research article Development and disease