Rap1 maintains adhesion between cells to affect Egfr signaling and planar cell polarity in Drosophila

Abstract

The small GTPase Rap1 affects cell adhesion and cell motility in numerous developmental contexts. Loss of Rap1 in the Drosophila wing epithelium disrupts adherens junction localization, causing mutant cells to disperse, and dramatically alters epithelial cell shape. While the adhesive consequences of Rap1 inactivation have been well described in this system, the effects on cell signaling, cell fate specification, and tissue differentiation are not known. Here we demonstrate that Egfr-dependent cell types are lost from Rap1 mutant tissue as an indirect consequence of DE-cadherin mis-localization. Cells lacking Rap1 in the developing wing and eye are capable of responding to an Egfr signal, indicating that Rap1 is not required for Egfr/Ras/MAPK signal transduction. Instead, Rap1 regulates adhesive contacts necessary for maintenance of Egfr signaling between cells, and differentiation of wing veins and photoreceptors. Rap1 is also necessary for planar cell polarity in these tissues. Wing hair alignment and ommatidial rotation, functional readouts of planar cell polarity in the wing and eye respectively, are both affected in Rap1 mutant tissue. Finally, we show that Rap1 acts through the effector Canoe to regulate these developmental processes.

Apical Accumulation of the Sevenless Receptor Tyrosine Kinase During Drosophila Eye Development Is Promoted by the Small GTPase Rap1

“…Except for the general lack of R7 cells, only a few ommatidia appear to be missing several photoreceptor cells ( Figure 5). This latter observation is consistent with previous work reporting that elimination of Rap1 activity has partial effects on photoreceptor cell differentiation (O'Keefe et al 2009). Together, while these findings indicate that Rap1 signaling plays a role during photoreceptor cell differentiation, this role is particularly critical for setting up the R7 cell differentiation program.…”

“…In two of these, Rap1 is suggested to serve as a direct RAF activator, which would explain how it influences RTK-dependent MAPK signaling (Lee et al 2002;Mishra et al 2005). In contrast, a third study failed to detect such a role downstream of EGFR during eye and wing development (O'Keefe et al 2009). Instead, the authors reported that Rap1 activity is required for cadherin-based adherens junction formation and that elimination of Rap1 impedes EGFR signaling as a consequence of reduced adhesive contacts between differentiating cells.…”

“…Interestingly, three of those (Roughened, PDZ-GEF, and canoe) encode Rap1-signaling components, which together suggest a functional relationship between CNK and Rap1 signaling. Previous work reported that Rap1 activity is required for adherens junction formation and adhesive contacts between developing photoreceptors (O'Keefe et al 2009). This phenomenon appeared to be critical for maintaining EGFR signaling between cells, although Rap1 activity did not seem autonomously required for the commitment of any cell type.…”

Atonal and EGFR signalling orchestraterok- andDrak-dependent adherens junction remodelling during ommatidia morphogenesis

“…This leaves open several interesting questions; for example, it is not presently clear how EGFR signalling can promote discrete AJ suppression and elongation. It is, however, tempting to speculate that previously described links between EGFR signalling and the expression of E-Cad (Brown et al, 2006;O'Keefe et al, 2009) or Rho1 (Brodu and Casanova, 2006) might play a role during this process. (Corrigall et al, 2007;Escudero et al, 2007;Schlichting and Dahmann, 2008).…”

The role of the small GTPase Rap in Drosophila R7 photoreceptor specification

“…1F). Thus, in the absence of rap gene function, photoreceptors fall out of the developing retina, which likely results from the role of Rap in regulating adherens junctions (23) through the PDZ protein Canoe (24). Indeed, canoe mutant clones show similar phenotypes with basally located photoreceptors (25).…”