Wingless and Hedgehog pattern Drosophila denticle belts by regulating the production of short-range signals

Divide and conquer: pattern formation in Drosophila embryonic epidermis

The pattern of differentiated cell types within tissues and organs is often established by organizers, the localized sources of secreted ligands. Although the mechanisms underlying organizer function have been extensively studied, only in a few cases is it clear how an organizer ultimately controls each individual cell's fate across a field of progenitor cells. One of these cases involves the establishment of a precise pattern of cell differentiation across the embryonic epidermis in Drosophila. Here, we review several recent reports that help to elucidate the regulatory principles used to control this pattern. Because organizers are conserved, the same fundamental principles might operate in other organizers.

Wingless signalling: more about the Wingless morphogen

Recent work on pattern formation in the Drosophila embryo reveals a new mechanism which shapes the gradient of the secreted morphogen, Wingless: Wingless protein is degraded more rapidly on one side of its source than on the other.

Wingless capture by Frizzled and Frizzled2 in Drosophila embryos

A variety of factors could influence how far developmental signals spread. For example, the Patched receptor limits the range of its ligand Hedgehog. Somehow, the Frizzled2 receptor has the opposite effect on its ligand. Increasing the level of Frizzled2 stabilizes Wingless and thus extends the Wingless gradient in Drosophila wing imaginal disks. Here we ask whether Frizzled or Frizzled2 affects the spread of Wingless in Drosophila embryos. We show that in the embryonic epidermis, the combined expression of both receptors is lowest in the engrailed domain. This is because expression of Frizzled is repressed by the Engrailed transcription factor, whereas that of Frizzled2 is repressed by Wingless signaling. Receptor downregulation correlates with an early asymmetry in Wingless distribution, characterized by the loss of Wingless staining in the engrailed domain. Raising the expression of either Frizzled or Frizzled2 in this domain prevents the early disappearance of Wingless-containing vesicles. Apparently, Wingless is captured, stabilized, and quickly internalized by either receptor. As far as we can tell, captured Wingless is not passed on to further cells and does not contribute to the spread of Wingless. Receptor downregulation in the posterior compartment may contribute to dampening the signal at the time when cuticular fates are specified.

Regulated endocytic routing modulates wingless signaling in Drosophila embryos

Embryos have evolved various strategies to confine the action of secreted signals. Using an HRP-Wingless fusion protein to track the fate of endocytosed Wingless, we show that degradation by targeting to lysosomes is one such strategy. Wingless protein is specifically degraded at the posterior of each stripe of wingless transcription, even under conditions of overexpression. If lysosomal degradation is compromised genetically or chemically, excess Wingless accumulates and ectopic signaling ensues. In the wild-type, Wingless degradation is slower at the anterior than at the posterior. This follows in part from the segmental activation of signaling by the epidermal growth factor receptor, which accelerates Wingless degradation at the posterior, thus leading to asymmetrical Wingless signaling along the anterior-posterior axis.

Distinct signals generate repeating striped pattern in the embryonic parasegment

How repeating striped patterns arise across cellular fields is unclear. To address this we examined the repeating pattern of Stripe (Sr) expression across the parasegment (PS) in Drosophila. This pattern is generated in two steps. First, the ligands Hedgehog (Hh) and Wingless (Wg) subdivide the PS into smaller territories. Second, the ligands Hh, Spitz (Spi), and Wg each emanate from a specific territory and induce Sr expression in an adjacent territory. We also show that the width of Sr expression is determined by signaling strength. Finally, an enhancer trap in the sr gene detects the response to Spi and Wg, but not to Hh, implying the existence of separable control elements in the sr gene. Thus, a distinct inductive event is used to initiate each element of the repeating striped pattern.

Cubitus interruptus-independent transduction of the Hedgehog signal in Drosophila

The Hedgehog (Hh) family of secreted proteins are key factors that control pattern formation in invertebrates and vertebrates. The manner in which Hh molecules regulate a target cell remains poorly understood. In the Drosophila embryo, Hh is produced in identical stripes of cells in the posterior compartment of each segment. From these cells a Hh signal acts in both anterior and posterior directions. In the anterior cells, the target genes wingless and patched are activated whereas posterior cells respond to Hh by expressing rhomboid and patched. Here, we have examined the role of the transcription factor Cubitus interruptus (Ci) in this process. So far, Ci has been thought to be the most downstream component of the Hh pathway capable of activating all Hh functions. However, our current study of a null ci allele, indicates that it is actually not required for all Hh functions. Whereas Hh and Ci are both required for patched expression, the target genes wingless and rhomboid have unequal requirements for Hh and Ci activity. Hh is required for the maintenance of wingless expression before embryonic stage 11 whereas Ci is necessary only later during stage 11. For rhomboid expression Hh is required positively whereas Ci exhibits negative input. These results indicate that factors other than Ci are necessary for Hh target gene regulation. We present evidence that the zinc-finger protein Teashirt is one candidate for this activity. We show that it is required positively for rhomboid expression and that Teashirt and Ci act in a partially redundant manner before stage 11 to maintain wingless expression in the trunk.

On the range of hedgehog signaling

Hedgehog (Hh) is a secreted signaling protein that regulates the development of many organ systems. It can travel from its site of synthesis, a process that involves covalent attachment of cholesterol to its carboxyl terminus, proteins with putative sterol sensing domains in both sending and receiving cells, and glycosaminoglycans. Understanding how the movement of Hh is controlled and propelled will be key to understanding how it carries out its essential roles.

Tissue- and stage-specific modulation of Wingless signaling by the segment polarity gene lines

Wnt signaling controls a variety of developmental programs but the mechanisms by which the same signal leads to distinct outputs remain unclear. To address this question, we identified stage-specific modulators of Wingless (Wg) signaling in the Drosophilaembryonic epidermis. We show that lines (lin) is essential for Wg-dependent patterning in dorsal epidermis. linencodes a novel protein that acts cell-autonomously, downstream or in parallel to Armadillo (Arm) and upstream of Wg-dependent target genes. Lin can accumulate in nuclei of cells signaled by Wg, suggesting that signaling promotes entry of Lin into the nucleus, where it cooperates with Arm and Pangolin. Thus, a stage-specific modulator is used to mediate Wg signaling activity in dorsal patterning. Hedgehog (Hh) controls half of the parasegmental pattern dorsally and antagonizes Wg function to do so. Lin can accumulate in the cytoplasm of cells signaled by Hh, suggesting that Hh antagonizes Wg function by prohibiting Lin from entering the nucleus.

Pattern formation: Wingless on the move

Wingless is a key morphogen in Drosophila. Although it is evident that Wingless acts at a distance from its site of synthesis, there is considerable debate about how the protein travels across a field of cells. Recent studies have provided important new insights into this process, though the issue is still far from being resolved.