Develoment and Stem Cells

INTRODUCTION Planarians are capable of regenerating any missing body part and are an emerging system for investigation of cellular and molecular mechanisms underlying regeneration. Regeneration requires production of new cells and instructions that specify the identity of cell types to be regenerated. The planarian Schmidtea mediterranea utilizes a population of dividing regenerative cells called neoblasts (Reddien and Sánchez Alvarado, 2004), which includes pluripotent stem cells (cNeoblasts) (Wagner et al., 2011), to regenerate any missing body part. Robust regenerative mechanisms exist for restoration of the body plan, involving genes that regulate anteriorposterior (AP), medial-lateral (ML) and dorsal-ventral (DV) polarization of tissues (Reddien and Sánchez Alvarado, 2004; Reddien, 2011). Several signaling pathways and transcription factors are essential for regulation of planarian regeneration. Wnt signaling controls AP regeneration polarity (Gurley et al., 2008; Iglesias et al., 2008; Petersen and Reddien, 2008; Adell et al., 2009; Petersen and Reddien, 2009b; Gurley et al., 2010; Petersen and Reddien, 2011), which is the decision to regenerate a head or tail at transverse amputation planes (Morgan, 1898; Morgan, 1905). Multiple Wnt genes and genes encoding candidate secreted inhibitors of Wnt signaling are expressed in distinct spatial domains along the AP axis (Reddien, 2011; Almuedo-Castillo et al., 2012). Several members of the Hox family, such as DjAbd-Ba and Plox4-Dj, are expressed in the planarian posterior (Orii et al., 1999; Nogi and Watanabe, 2001). Smed-prep, which encodes a TALE (three amino acid loop extension) family homeodomain protein, is expressed at the tip of planarian heads and is required for anterior pole marker expression in regeneration (Felix and Aboobaker, 2010). A LIM-homeobox gene, Djislet, is expressed in the posterior and is required for posterior pole marker expression in regeneration (Hayashi et al., 2011). Regeneration of the DV and ML axes requires Bmp signaling, with Bmp signaling components differentially expressed along the DV and/or ML axes (Orii et al., 1998; Molina et al., 2007; Orii and Watanabe, 2007; Reddien et al., 2007; Molina et al., 2009; Gaviño and Reddien, 2011; Molina et al., 2011). In addition to their functions in regeneration, these signaling pathways and transcription factors also display constitutive expression in the adult planarian body with several being required for homeostatic maintenance of the body plan during natural tissue turnover (Reddien, 2011). For example, RNAi of the Wnt signaling component β-catenin-1 results in ectopic head appearance around the periphery of intact animals (Gurley et al., 2008; Iglesias et al., 2008; Petersen and Reddien, 2008). These observations indicate that actively maintained expression of genes regulating body position instructs tissue turnover, but how these regional expression patterns are maintained and regenerated is poorly understood. pbx encodes a TALE-class homeodomain protein and can regulate gene expression in a variety of developmental contexts (Moens and Selleri, 2006; Laurent et al., 2008). There are four mammalian Pbx genes (Kamps et al., 1990; Nourse et al., 1990; Monica et al., 1991; Wagner et al., 2001), five zebrafish Pbx genes (Pöpperl et al., 2000; Vlachakis et al., 2000), one Drosophila pbx homolog, extradenticle (exd) (Rauskolb et al., 1993) and three Caenorhabditis elegans pbx homologs, ceh-20 (C. elegans homeobox), ceh-40 and ceh-60 (Bürglin and Ruvkun, 1992; Bürglin, 1997; Mukherjee and Bürglin, 2007). pbx was first characterized for regulating antero-posterior patterning during embryonic development as a co-factor of the Hox genes. In Drosophila, mutations in the exd gene cause homeotic transformations without affecting the expression of corresponding Hox genes and Exd controls anterior-posterior patterning in the fly embryo by acting together with Hox proteins (Peifer and Wieschaus, 1990; Rauskolb et al., 1993; Rauskolb et al., 1995). Studies of the zebrafish lzr (pbx4) gene in AP hindbrain patterning suggest that it promotes the action of multiple Hox genes in vertebrates (Pöpperl et al., 2000). In C. elegans, Hox Howard Hughes Medical Institute, MIT Biology, Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02142, USA.