Shaping Morphogen Gradients

Shh activity and at different distances from the source. The proper functioning of tissues and organs requires These observations provide unequivocal evidence that that each cell differentiate appropriately for its position. Shh acts directly on distant cells and therefore that Shh In many cases, the positional information that instructs forms a morphogen gradient in the developing neural cells about their prospective fate is conveyed by a mortube. phogen gradient. Morphogens are signaling molecules Evidence for another bona fide vertebrate morphogen that are produced in a restricted region of a tissue and has just been reported by Chen and Schier (2001). They move away from their source to form a long-range conperformed an elegant series of experiments showing centration gradient. Cells differentiate in response to that the Nodal-related TGFprotein Squint acts both morphogen signaling depending on their position within in a concentration-dependent manner and directly at a the gradient and thus on their distance from the morphodistance in the Zebrafish embryo. They injected single gen source. Two criteria have gained acceptance as the cells in embryos with Squint mRNA and showed that evidence needed to qualify a secreted signaling protein target genes were activated in both nearby and distant as a morphogen: concentration dependence and direct cells. High threshold genes were activated only near the action at a distance. The Wingless, Hedgehog (Hh), and Squint source, whereas low threshold genes were also Dpp gradients in the Drosophila wing and leg imaginal activated in more distant cells. Reducing the amount discs fulfill both criteria (for literature, see Teleman and of mRNA injected resulted in only low-threshold genes Cohen, 2000). First, multiple target genes are induced being activated, showing concentration-dependent acat different ligand concentrations, with low threshold tion of Squint. To prove direct action at a distance, they responses elicited in cells at a greater distance from the used embryos mutant for Oep, a cofactor required for source. The second criterion requires a demonstration cells to respond to Squint signaling. By injecting a single that the morphogen acts directly on responsive cells, cell of an Oep mutant embryo with Squint mRNA and and that it does not act at a distance by relay of a implanting at a distance a wild-type cell that could resecondary signal. An example of signal relay is Dpp in spond to Squint signaling, they proved no relay mechathe fly wing. Hh acts directly over 8–10 cells to induce nism is required, indicating that Squint functions as a Dpp, which in turn acts directly on distant cells in the morphogen. form of a long-range symmetric Dpp gradient. There is Morphogen gradients have recently been visualized abundant evidence for concentration-dependent activdirectly by antibody staining or using morphogen-GFP ity of signaling proteins in vertebrate development (refusion proteins that retain signaling activity (Entchev et viewed in Gurdon et al., 1998). However, evidence for al., 2000; Incardona et al., 2000; Strigini and Cohen, direct action at a distance has been lacking. Two new 2000; Teleman and Cohen, 2000; Lewis et al., 2001 [this papers address this issue (Chen and Schier, 2001; issue of Cell]). In spite of considerable interest in morBriscoe et al., 2001). phogen gradients over the last few years, the problem Sonic Hedgehog (Shh) acts in a concentration-depenof how they form in tissues has not been fully explored. dent manner to define domains of gene expression and Gradients could form passively by diffusion of soluble specify neuronal cell fates in the developing central nerprotein or by facilitated diffusion of morphogen bound vous system (Briscoe et al., 2000; Agarwala et al., 2001). to a carrier. As alternatives, mechanisms for active transIn a study soon to be published, Briscoe et al. (2001) port of morphogens have been postulated. One possibilhave made an important step forward in demonstrating ity involves successive rounds of internalization of the that Shh acts directly at a distance to pattern the neural ligand by endocytosis followed by resecretion. This tube. They have made use of a mutant form of the Sonic would serve to pass the molecules from cell to cell and