Dosage-dependent hedgehog signals integrated with Wnt/-catenin signaling regulate external genitalia formation as an appendicular program

INTRODUCTION Embryonic development is controlled by a series of basic regulatory processes, including the regulation of protrusion and outgrowth. It has become clear that such developmental processes require coordinated reciprocal interactions between epithelium and the adjacent mesenchyme, frequently mediated through hedgehog, Wnt and fibroblast growth factor (Fgf) pathways. Perturbation of these pathways causes developmental abnormalities in a variety of tissues due, at least in part, to failed cross-talk. Despite the importance of this cross-talk and reciprocal interactions, our understanding of signaling pathway interactions is limited. An embryonic bud structure (an appendage) is a representative organ that is suitable for analyzing reciprocal interactions between signaling pathways. Protruding embryonic buds are often composed of the distal epithelium accompanied by adjacent proliferating mesenchyme, which eventually gives rise to a bud structure. Significant progress has been achieved in understanding the molecular network that regulates limb development (Capdevila and Izpisua Belmonte, 2001; Chen et al., 2004; Johnson and Tabin, 1997; Kmita et al., 2005; Niswander, 2003; Yamaguchi et al., 1999; Yang et al., 2006; Zhu et al., 2008). Vertebrate limb development depends on the establishment and maintenance of the apical ectodermal ridge (AER), a specialized epithelium at the distal tip of the limb bud. Epithelial-mesenchymal interactions between the AER and its adjacent mesenchyme are essential for limb bud outgrowth. Fgf gene family members are expressed specifically in the AER. The cumulative evidence indicates that AER formation and maintenance and Fgf expression are tightly controlled by intricate interplay among several growth factors (Lewandoski et al., 2000; MacArthur et al., 1995; Mariani et al., 2008; Moon and Capecchi, 2000; Sun et al., 2000; Sun et al., 2002; Yu and Ornitz, 2008). Wnt/-catenin signaling in the limb ectoderm regulates AER maintenance and Fgf8 expression (Barrow et al., 2003; Soshnikova et al., 2003). Bmp signaling functions upstream of Wnt/-catenin signaling in this process, as indicated by a failure of AER formation in mice lacking ectodermal bone morphogenetic protein receptor 1A (Bmpr1a) (Ahn et al., 2001). sonic hedgehog (Shh) is expressed in posterior limb bud mesenchyme [zone of polarizing activity (ZPA)] (Riddle et al., 1993), patterns the anterior-posterior axis of the limb, and supports expression of Fgf genes in the AER (Benazet et al., 2009; Laufer et al., 1994; Niswander et al., 1994; Riddle et al., 1993; Zuniga et al., 1999). Another embryonic appendage, the genital tubercle (GT), is the common primordium of male and female external genitalia. GT outgrowth is the result of mesenchymal proliferation around the cloaca, accompanied by formation of the urethral plate epithelium at the ventral midline of the GT. The amniotic cavity and cloacal lumen are separated by two epithelial components: a superficial Dosage-dependent hedgehog signals integrated with Wnt/-catenin signaling regulate external genitalia formation as an appendicular program