Epithelial planar cell polarity (PCP) allows epithelial cells to coordinate their development to that of the tissue in which they reside. The mechanisms that impart PCP as well as effectors that execute the polarizing instructions are being sought in many tissues. We report that the epidermal epithelium of Drosophila embryos exhibits PCP. Cells of the prospective denticle field, but not the adjacent smooth field, align precisely. This requires Myosin II (zipper) function, and we find that Myosin II is enriched in a bipolar manner, across the parasegment, on both smooth and denticle field cells during denticle field alignment. This implies that actomyosin contractility, in combination with denticle-field-specific effectors, helps execute the cell rearrangements involved. In addition to this parasegment-wide polarity, prospective denticle field cells express an asymmetry, uniquely recognizing one cell edge over others as these cells uniquely position their actin-based protrusions (ABPs; which comprise each denticle) at their posterior edge. Cells of the prospective smooth field appear to be lacking proper effectors to elicit this unipolar response. Lastly, we identify fringe function as a necessary effector for high fidelity placement of ABPs and show that Myosin II (zipper) activity is necessary for ABP placement and shaping as well.

No abstract

No abstract

Induction and patterning of the mesodermal germ layer is a key early step of vertebrate embryogenesis. We report that FoxD3 function in the Xenopus gastrula is essential for dorsal mesodermal development and for Nodal expression in the Spemann organizer. In embryos and explants, FoxD3 induced mesodermal genes, convergent extension movements and differentiation of axial tissues. Engrailed-FoxD3, but not VP16-FoxD3, was identical to native FoxD3 in mesoderm-inducing activity, indicating that FoxD3 functions as a transcriptional repressor to induce mesoderm. Antagonism of FoxD3 with VP16-FoxD3 or morpholinoknockdown of FoxD3 protein resulted in a complete block to axis formation, a loss of mesodermal gene expression, and an absence of axial mesoderm, indicating that transcriptional repression by FoxD3 is required for mesodermal development. FoxD3 induced mesoderm in a non-cell-autonomous manner, indicating a role for secreted inducing factors in the response to FoxD3. Consistent with this mechanism, FoxD3 was necessary and sufficient for the expression of multiple Nodal-related genes, and inhibitors of Nodal signaling blocked mesoderm induction by FoxD3. Therefore, FoxD3 is required for Nodal expression in the Spemann organizer and this function is essential for dorsal mesoderm formation. KEY WORDS:Xenopus, FoxD3, Forkhead, Nodal, Mesoderm, Transcription Development 133, 4827-4838 (2006) DEVELOPMENT 4828 formation, and antagonism or knockdown of FoxD3 results in severe axial defects and loss of dorsal mesodermal gene expression. FoxD3 induction of mesoderm is non-cell-autonomous and requires the Nodal signaling pathway. Consistent with the co-expression of FoxD3 and Nodal genes in the organizer, FoxD3 is necessary and sufficient for the expression of several Nodal-related genes. Taken together, our results demonstrate a novel mode of Nodal regulation in the Spemann organizer, where transcriptional repression by FoxD3 maintains Nodal expression to promote mesoderm induction and axial development. MATERIALS AND METHODS Embryos and microinjectionEmbryos were collected, fertilized, injected and cultured as previously described (Yao and Kessler, 1999), and embryonic stage was determined according to Nieuwkoop and Faber (Nieuwkoop and Faber, 1967). Dorsal and ventral blastomeres were identified by pigmentation differences (Klein, 1987). Explants were prepared using a Gastromaster microsurgery instrument (Xenotek Engineering). Capped, in vitro transcribed RNA for microinjection was synthesized from linearized template DNA using the Message Machine kit (Ambion) and 10 nl of RNA solution was injected.Templates for in vitro transcription were pCS2-FoxD3, pCS2-mFoxD3, pCS2-Eng-FoxD3, pCS2-VP16-FoxD3, pCS2-FoxD3(N140A/H144A), pCS2-Eng-FoxD3(N140A/H144A), pCS2-VP16-FoxD3(N140A/H144A), pCS2-NLS-FoxD3WH, pCS2-FoxD3-utr (this study), pCS2-Eng, pCS2-VP16 (Kessler, 1997), pCS2-MT-SID (Chen et al., 1997), pCS2-Cer-S (Piccolo et al., 1999), pCS2-Xnr1 (Sampath et al., 1997), and pCS2-VegT⌬UTR (Engleka et al., 2001). FoxD3 expression ...