Crumbs is required to achieve proper organ size control duringDrosophilahead development

Abstract

SUMMARYCrumbs (Crb) is a conserved apical polarity determinant required for zonula adherens specification and remodelling during Drosophila development. Interestingly, crb function in maintaining apicobasal polarity appears largely dispensable in primary epithelia such as the imaginal discs. Here, we show that crb function is not required for maintaining epithelial integrity during the morphogenesis of the Drosophila head and eye. However, although crb mutant heads are properly developed, they are also significantly larger than their wild-type counterparts. We demonstrate that in the eye, this is caused by an increase in cell proliferation that can be attributed to an increase in ligand-dependent Notch (N) signalling. Moreover, we show that in crb mutant cells, ectopic N activity correlates with an increase in N and Delta endocytosis. These data indicate a role for Crb in modulating endocytosis at the apical epithelial plasma membrane, which we demonstrate is independent of Crb function in apicobasal polarity. Overall, our work reveals a novel function for Crb in limiting ligand-dependent transactivation of the N receptor at the epithelial cell membrane.

Drosophila Crumbs prevents ectopic Notch activation in developing wings by inhibiting ligand-independent endocytosis

“…Herranz et al (2006) concluded that in larval wing discs absence of Crb enhances intramembrane proteolytic processing of Notch by γ-secretase, called S3 cleavage, which depends on ligand-induced S2 cleavage (Mumm et al, 2000) and is followed by the translocation of the intracellular portion of Notch into the nucleus and activation of downstream genes. Along the same line, Richardson and Pichaud (2010) proposed, based on inhibition experiments, that absence of Crb in eye imaginal discs enhances Dl-mediated, metalloproteasedependent S2 cleavage of Notch, and showed increased internalised Notch in uptake assays in the absence of Crb. Our results differ from those previously published in that: (1) in pupal wings, endocytosis of Notch in the absence of crb does not depend on the ligands Dl and Ser nor on Psn, suggesting that it is independent of the S2 and S3 cleavage of Notch; and (2) endocytosis of Notch in crb mutant pupal cells activates the dx-dependent ligand-independent Notch pathway.…”

“…It has been proposed that in Drosophila eye discs the absence of Crb facilitates incorporation of Notch into early endocytic vesicles marked by Hrs (Richardson and Pichaud, 2010). Therefore, we asked whether increased Notch endocytosis in crb mutant cells observed in pupal wings depends on the Notch ligands Delta (Dl) and Serrate (Ser).…”

“…However, crb has been associated with various other functions in imaginal discs, such as regulation of Notch receptor processing in the wing margin (Herranz et al, 2006), cell survival (Hafezi et al, 2012) and organ size control by the Notch or Hippo pathways (Chen et al, 2010;Ling et al, 2010;Ribeiro et al, 2014;Richardson and Pichaud, 2010). In the pupal retinal epithelium, morphogenesis of photoreceptor cells is impaired upon loss of crb (Izaddoost et al, 2002;Johnson et al, 2002;Pellikka et al, 2002).…”

“…Less is known about the function of the huge extracellular domain. It was reported to modify ligand-dependent Notch activity (Herranz et al, 2006;Richardson and Pichaud, 2010), and to mediate nonautonomous effects on cell survival (Hafezi et al, 2012). Other results suggested that the extracellular domain is engaged in homophilic interactions to stabilise Crb in the membrane, both in the Drosophila follicular epithelium (Fletcher et al, 2012) and in the zebrafish retina (Zou et al, 2012).…”

Ebi modulates wing growth by ubiquitin-dependent downregulation of Crumbs in Drosophila

“…This activation of Notch signaling is correlated with an increased endocytosis of Notch and its ligand Delta. Hence, independent of its function in cell polarity, Crb acts as an inhibitory factor to Notch activation by limiting endocytosis (Richardson and Pichaud, 2010). Crb is also involved in the inhibition of Notch signaling in vertebrates like zebrafish, in which Notch activity is necessary for the apical mitosis of neuroepithelial cells during embryogenesis (Ohata et al, 2011).…”