A bone morphogenetic protein (BMP) signaling gradient patterns cells along the embryonic dorsoventral (DV) axis (Little and Mullins, 2006). BMP signaling is abrogated dorsally by the secreted BMP antagonists Chordin, Noggin and Follistatin to generate low BMP signaling

INTRODUCTION A bone morphogenetic protein (BMP) signaling gradient patterns cells along the embryonic dorsoventral (DV) axis (Little and Mullins, 2006). BMP signaling is abrogated dorsally by the secreted BMP antagonists Chordin, Noggin and Follistatin to generate low BMP signaling levels that allow neurectodermal tissues to develop. These and additional extracellular modulators generate the moderate and high levels of BMP signaling that specify lateral and ventral cell types, respectively (Langdon and Mullins, 2011). In addition to DV patterning of axial tissues, the embryo is also patterned along the anteroposterior (AP) and left-right axes. Although patterning of each of these axes has been well studied, little is known about how patterning is coordinated between each of these axes. We previously showed that DV tissues are patterned temporally progressively from anterior to posterior (Tucker et al., 2008). It is known that AP tissues are also patterned temporally progressively from anterior to posterior (Gamse and Sive, 2000; Gamse and Sive, 2001; Stern et al., 2006). However, it is not known if the patterning of DV and AP tissues is coordinated. Here we investigate whether the temporal patterning of DV and AP tissues is coordinated by the same patterning clock or occurs independently. AP patterning is mediated by fibroblast growth factor (FGF), Wnt and retinoic acid (RA) posteriorizing signals during gastrulation (Gamse and Sive, 2000; Maden, 2002; Schier and Talbot, 2005; Stern et al., 2006; Wilson and Houart, 2004). To allow anterior neural tissues to develop, FGF and Wnt signaling must be repressed while these signals actively specify more posterior neural tissue development (Erter et al., 2001; Houart et al., 2002; Kudoh et al., 2004; Kudoh et al., 2002; Ramel et al., 2005; Rhinn et al., 2005; Shimizu et al., 2005). RA signaling does not posteriorize the most anterior neurectodermal tissues, but acts to posteriorize more caudal neural tissue (Kudoh et al., 2002; Maves and Kimmel, 2005). In zebrafish, expression of FGF, Wnt and RA signaling components overlap in the marginal zone (Schier and Talbot, 2005). In addition, the expression of these AP patterning components in the ventral marginal zone overlaps with that of BMP signaling components, suggesting the potential interaction of DV and AP patterning pathways during gastrulation. In Xenopus, a model has been proposed to coordinate DV and AP patterning in dorsal tissues through integration of FGF and Wnt signaling into the BMP signaling pathway. Following phosphorylation by the BMP receptor, Smad1 is subsequently phosphorylated by FGF/MAPK and Wnt/GSK3, which leads to its degradation (Fuentealba et al., 2007). Although degradation of phosphorylated (P) Smad1/5 is not evident in ventral regions along the AP axis of the zebrafish gastrula (Tucker et al., 2008), a mechanism independent of P-Smad1/5 degradation might integrate BMP signaling with MAPK and GSK3 on P-Smad1/5 to coordinate DV and AP temporal patterning. Here we demonstrate that patterning of DV tissues along the AP axis is intimately coordinated with AP patterning during zebrafish gastrulation. When FGF, Wnt or RA signaling pathways were inhibited in combination with the temporal modulation of BMP signaling, we found that patterning of DV tissues by BMP signaling along the AP axis is coordinated with AP patterning by FGF, Wnt and RA. Furthermore, our results suggest that coordinated patterning is regulated in part by phosphorylation of the P-Smad1/5 linker region via FGF/MAPK. The coordinated patterning of AP and DV tissues allows a cell to adopt both DV and AP positional information simultaneously during gastrulation. Department of Cell and Developmental Biology, University of Pennsylvania, Perelman School of Medicine, 1211 BRBII/III, 421 Curie Blvd., Philadelphia, PA 19104-6058, USA.