Pattern formation: Wingless on the move

Function of the wingless signaling pathway in Drosophila

Signaling by the wingless pathway has been shown to govern numerous developmental processes. Much of our current understanding of wingless signaling mechanisms comes from studies conducted in Drosophila melanogaster, which offers superior experimental tractability for genetic and developmental studies. Wingless signaling is highly consequential during normal development and patterning of Drosophila. Its earliest identifiable role during development of Drosophila is in the embryonic segmentation cascade, wherein wingless functions as a segment polarity gene and serves to pattern each individual segment along the antero-posterior axis of the developing embryo. Subsequent developmental roles fulfilled by wingless include patterning the developing wings, legs, eyes, CNS, heart, and muscles. Each of these developmental contexts offers excellent systems to query mechanisms regulating different aspects of wingless signal transduction such as synthesis, secretion, reception, and transcription. This chapter presents a brief overview on the functions of wingless signaling during development of Drosophila melanogaster.

Selection of multipotent stem cells during morphogenesis of small intestinal crypts of Lieberkuhn is perturbed by stimulation of Lef-1/beta-catenin signaling

Studies of chimeric mice have disclosed that the stem cell hierarchy in the small intestinal epithelium is established during formation of its proliferative units (crypts of Lieberkühn). This process involves a selection among several multipotential progenitors so that ultimately only one survives to supply descendants to the fully formed crypt. In this report, we examine the hypothesis that the level of beta-catenin (beta-cat)-mediated signaling is an important factor regulating this stem cell selection. In the canonical Wnt signaling pathway, beta-catenin can partner with Lef-1/Tcf high mobility group (HMG) box transcription factors to control gene expression. Both Lef-1 and Tcf-4 mRNAs are produced in the fetal mouse small intestine. Tcf-4 expression is sustained, whereas Lef-1 levels fall as crypt formation is completed during the first two postnatal weeks. A Tcf-4 gene knockout is known to block intestinal epithelial proliferation in late fetal life. Therefore, to test the hypothesis, we enhanced beta-catenin signaling in a chimeric mouse model in which the stem cell selection could be monitored. A fusion protein containing the HMG box domain of Lef-1 linked to the trans-activation domain of beta-catenin (Lef-1/beta-cat) was constructed to promote direct stimulation of signaling without being retained in the cytoplasm through interactions with E-cadherin and Apc/Axin. Lef-1/beta-cat was expressed in 129/Sv embryonic stem cell-derived small intestinal epithelial progenitors present in developing B6-ROSA26<-->129/Sv chimeras. Lef-1/beta-cat stimulated expression of a known beta-catenin target (E-cadherin), suppressed expression of Apc and Axin, and induced apoptosis in 129/Sv but not in neighboring B6-ROSA26 epithelial cells. This apoptotic response was not associated with any detectable changes in cell division within the Lef-1/beta-cat-expressing epithelium. By the time crypt development was completed, all 129/Sv epithelial cells were lost. These results indicate that developmental changes in beta-catenin-mediated signaling can play an important role in establishing a stem cell hierarchy during crypt morphogenesis.

Generating patterns from fields of cells. Examples from Drosophila segmentation

In Drosophila, a cascade of maternal, gap, pair-rule and segment polarity genes subdivides the antero/posterior axis of the embryo into repeating segmental stripes. This review summarizes what happens next, i.e. how an intrasegmental pattern is generated and controls the differentiation of specific cell types in the epidermis. Within each segment, cells secreting the signalling molecules Wingless (the homologue of vertebrate Wnt-1) and Hedgehog are found in narrow stripes on both sides of the parasegmental boundary. The Wingless and Hedgehog organizing activities help to establish two more stripes per segment that localize ligands for the Epidermal Growth Factor and the Notch signalling pathways, respectively. These four signals then act at short range and in concert to control epidermal differentiation at the single cell level across the segment. This example from Drosophila provides a paradigm for how organizers generate precise patterns, and ultimately different cell types, in a naïve field of cells.

RNA localization meets wingless signaling

In many tissues in many organisms, messenger RNA (mRNA) is not randomly distributed throughout a cell, but is targeted and accumulated in specific subcellular locations. Signal transduction pathways can be extremely sensitive to gradients of signaling proteins and represent a cellular phenomenon where RNA localization may be important. Manseau discusses how targeting of the RNA for a secreted ligand, Wingless (Wg), is important in promoting proper Wg signaling in early Drosophila embryos and the mechanism for achieving this subcellular targeting. The possible mechanistic models for how restricted wg mRNA distribution influences Wg signaling are also detailed.