Hedgehog Signaling Is a Principal Inducer of Myosin-II-Driven Cell Ingression in Drosophila Epithelia

Abstract

Cell constriction promotes epithelial sheet invagination during embryogenesis across phyla. However, how this cell response is linked to global patterning information during organogenesis remains unclear. To address this issue, we have used the Drosophila eye and studied the formation of the morphogenetic furrow (MF), which is characterized by cells undergoing a synchronous apical constriction and apicobasal contraction. We show that this cell response relies on microtubules and F-actin enrichment within the apical domain of the constricting cell as well as on the activation of nonmuscle myosin. In the MF, Hedgehog (Hh) signaling is required to promote cell constriction downstream of cubitus interruptus (ci), and, in this context, Ci155 functions redundantly with mad, the main effector of dpp/BMP signaling. Furthermore, ectopically activating Hh signaling in fly epithelia reveals a direct relationship between the duration of exposure to this signaling pathway, the accumulation of activated Myosin II, and the degree of tissue invagination.

Extramacrochaetae imposes order on the Drosophila eye by refining the activity of the Hedgehog signaling gradient

“…Dpp is then used to prepare more anteriorly positioned cells for entry into the furrow (Greenwald and Struhl, 1999). In addition to activating dpp expression, Hh signaling within the furrow is required for apical surface constriction and the transcriptional repression of h, which encodes a bHLH transcription factor that, at elevated levels, can induce numerous structural defects (Brown et al, 1991;Fu and Baker, 2003;Corrigall et al, 2007; Schlichting and Dahmann, 2008). Together with H, Emc is thought to regulate the rate by which the furrow traverses the retinal epithelium (Brown et al, 1995;Bhattacharya and Baker, 2009).…”

“…The constriction of the apical surfaces of cells within the furrow is dependent upon Hh signaling. A decrease in F-actin accumulation and Armadillo (Arm) levels (two apical profile markers) are observed in clones that are mutant for smoothened (smo), which encodes a transducer of the Hh signal (Corrigall et al, 2007). The cell shape changes that are induced by Hh signaling are crucial for limiting the spread of the Hh signal itself.…”

Hedgehog and extramacrochaetae in theDrosophilaeye

“…We have already seen that Hh signaling induces morphological changes within cells of the furrow restricting the distance over which the Hh ligand can travel. 28,30 It is thought that this constriction increases the concentration of apical membrane, thus increasing the likelihood that cells within the furrow will capture the vast majority of Hh molecules. 30 A second method of restricting Hh signaling appears to involve the helix-loop-helix (HLH) transcription factor, Extramacrochaetae (Emc).…”

“…27 It appears that Hh, and to a lesser extent Dpp, signaling is required for apical cell constriction within the furrow. 28 Loss of signaling leads to a decrease in F-actin accumulation at the apical surface as well as reductions in the amount of β-catenin (encoded by armadillo), myosin regulatory light chain (encoded by spaghetti squash), and at least 1 cadherin (encoded by Cad86C). 28,29 The Hh induced apical constriction (and accompanying increase in F-actin accumulation) forms part of a negative feedback loop by restricting the distance that the Hh ligand can travel.…”

A Screen for X-Linked Mutations Affecting Drosophila Photoreceptor Differentiation Identifies Casein Kinase 1α as an Essential Negative Regulator of Wingless Signaling

“…In the third larval instar, photoreceptor differentiation initiates at the posterior margin of the eye disc and propagates toward the anterior under the control of the morphogen Hedgehog (Hh) (Ready et al 1976;Heberlein et al 1993;Ma et al 1993). Hh secreted by differentiating photoreceptors induces immediately anterior cells to form a transient indentation known as the morphogenetic furrow (MF) (Corrigall et al 2007;Escudero et al 2007), and to express the bone morphogenetic protein (BMP) family member Decapentaplegic (Dpp) (Heberlein et al 1993;Ma et al 1993) and the proneural transcription factor Atonal (Ato) (Jarman et al 1994;Domínguez 1999). Notch (N)-mediated lateral inhibition then contributes to refining Ato expression into single cells that differentiate as R8 photoreceptors (Cagan and Ready 1989;Dokucu et al 1996;Baker and Yu 1997).…”