DPP is not a savior for fly epithelial cells, according to new reports from Matthew Gibson and Norbert Perrimon (Harvard Medical School, Boston, MA) and from Jie Shen and Christian Dahmann (Max Planck Institute, Dresden, Germany). Once thought to be a survival factor in the wing imaginal disc, the DPP morphogen is now shown to regulate cell architecture.
Previous experiments showed that wing imaginal disc cells forced into low DPP signaling were eliminated by apoptosis. But the Dresden and Boston groups now report that clones of DPP-deprived cells are misshapen and thus extrude from the wing disc epithelium. The Harvard group found that some mutant cells did evenutally undergo apoptosis, but this seems to be a secondary effect, perhaps brought on by the mechanical stress of extrusion.Cells lacking DPP signaling (green) are extruded from the epithelial layer.
Gradients of DPP, and other morphogens, are thought to provide the positional information that cells use during development to define their later fate. But wing imaginal disc cells will all become wing cells, despite varying DPP levels. The defects seen in the new studies reveal a more immediate effect for DPP on cell shape.
At the center of the developing wing disc, where DPP levels are highest, cells normally assume a tall columnar shape. But in mutant clones that did not perceive DPP, the cells were unable either to establish or maintain this elongated shape. They thus looked more like cells found at lower DPP concentrations. “The cool thing about this,” says Gibson, “is that it suggests a spatial mechanism by which epithelial cell sheets can be sculpted into different morphologies during development.”
DPP may produce tall columnar cells via cytoskeletal organization. Dense apical microtubule arrays were found in normal columnar cells but were absent in the mutant clones. Microtubule loss may not be the primary cause of the mutant phenotype, however. For instance, DPP may control cell adhesions to the matrix or to neighboring cells, which may in turn stabilize apical microtubules.