Jcb_201312045 1..10

The directed movement of cells is a fundamental aspect of tissue morphogenesis. Cells migrating in vivo often must invade tissue barriers and ultimately form new cellular contacts in their target tissues. Furthermore, in many epithelial tissues proliferation occurs via a population of basal stem cells that divide below the apical surface and move apically to join the epithelium (Rock and Hogan, 2011). This epithelialization requires that cells move in a specific direction and that they incorporate into the epithelium without disrupting its integrity. The skin of Xenopus laevis embryos represents an excellent model to address the invasive behavior of migrating cells (Fig. 1 A). Multiciliated cells (MCCs) and ionocytes (ICs) are specialized cell types that differentiate in a subapical layer of the epidermis (Drysdale and Elinson, 1992; Deblandre et al., 1999; Dubaissi and Papalopulu, 2011; Quigley et al., 2011; Kim et al., 2012). These cells then move in a directed manner toward the outer epithelial cells (OCs; Fig. 1 A). As they meet the apical junctions of the OCs, intercalating cells selectively localize to vertices consisting of three or more cells (Stubbs et al., 2006). Next, these cells undergo radial intercalation where they push through the vertices and form new apical junctions with the OCs, forming a complete intact epithelium (Fig. 1 A). The Par complex is involved in a wide variety of cellular processes including the establishment of apical-basal polarity, the formation of adherens junctions, and directed cell migration (Chen and Macara, 2005; Chihara and Nance, 2012; Tepass, 2012). To accomplish these functions, the core proteins Par3, Par6, and the atypical PKC (aPKC) interact with an assortment of effector proteins (Goldstein and Macara, 2007). Of particular relevance is the role of the Par complex in establishing forcegenerating interactions between centrioles, microtubules (MTs), and the cell cortex (Labbé et al., 2003; Munro, 2006; Feldman and Priess, 2012). In numerous cell types, the centrosome is positioned between the leading edge and the nucleus, creating an enrichment of MTs along the axis of migration (Gomes et al., 2005; Luxton and Gundersen, 2011). This is mediated by Par3’s interaction with the dynein light intermediate chain 2 (LIC2), which can modulate MT dynamics at the cell cortex (Schmoranzer et al., 2009). Additionally, in neurons Par3 has been reported to directly The directed movement of cells is critical for numer­ ous developmental and disease processes. A devel­ opmentally reiterated form of migration is radial intercalation; the process by which cells move in a direc­ tion orthogonal to the plane of the tissue from an inner layer to an outer layer. We use the radial intercala­ tion of cells into the skin of Xenopus laevis embryos as a model to study directed cell migration within an epi­ thelial tissue. We identify a novel function for both the microtubule­binding protein CLAMP and members of the microtubule­regulating Par complex during intercala­ tion. Specifically, we show that Par3 and aPKC promote the apical positioning of centrioles, whereas CLAMP stabilizes microtubules along the axis of migration. We propose a model in which the Par complex defines the orientation of apical migration during intercalation and in which subcellular localization of CLAMP promotes the establishment of an axis of microtubule stability required for the active migration of cells into the outer epithelium. Radial intercalation is regulated by the Par complex and the microtubule-stabilizing protein CLAMP/Spef1