Drosophila CK2 regulates lateral-inhibition during eye and bristle development

Abstract

Lateral inhibition is critical for cell fate determination and involves the functions of Notch (N) and its effectors, the Enhancer of Split Complex, E(spl)C repressors. Although E(spl) proteins mediate the repressive effects of N in diverse contexts, the role of phosphorylation was unclear. The studies we describe implicate a common role for the highly conserved Ser/Thr protein kinase CK2 during eye and bristle development. Compromising the functions of the catalytic (alpha) subunit of CK2 elicits a rough eye and defects in the interommatidial bristles (IOBs). These phenotypes are exacerbated by mutations in CK2 and suppressed by an increase in the dosage of this protein kinase. The appearance of the rough eye correlates, in time and space, to the specification and refinement of the 'founding' R8 photoreceptor. Consistent with this observation, compromising CK2 elicits supernumerary R8's at the posterior margin of the morphogenetic furrow (MF), a phenotype characteristic of loss of E(spl)C and impaired lateral inhibition. We also show that compromising CK2 elicits ectopic and split bristles. The former reflects the specification of excess bristle SOPs, while the latter suggests roles during asymmetric divisions that drive morphogenesis of this sensory organ. In addition, these phenotypes are exacerbated by mutations in CK2 or E(spl), indicating genetic interactions between these two loci. Given the centrality of E(spl) to the repressive effects of N, our studies suggest conserved roles for this protein kinase during lateral inhibition. Candidates for this regulation are the E(spl) repressors, the terminal effectors of this pathway.

A Screen for Modifiers of Notch Signaling Uncovers Amun, a Protein With a Critical Role in Sensory Organ Development

“…AmunTRFP (red) localizes to nuclei (B), as indicated by the DNA dye Vybrant Green (B9). antennal-eye imaginal disc; Bose et al 2006), we observe a reduction in eye size (see Figure S3) consistent with a role for Amun in eye development (see above). Reduction of Amun levels under control of the sca-Gal4 or ptc-Gal4 driver results in multiple bristle defects including missing, supernumerary, and misplaced macrochaetae, as well as probable shaft-to-socket transformations ( Figure 6, E and F, respectively).…”

“…The following strains were used for the study of CG2446 (Amun): eyeless (ey)-Gal4 (Bose et al 2006) (Bloomington Drosophila Stock Center); decapentaplegic (dpp)-Gal4/TM6B (StaehlingHampton et al 1994); pannier (pnr)-Gal4/TM3 (Heitzler et al 1996), a gift from Gines Morata, Centro de Biología Molecular Severo Ochoa (Madrid); patched (ptc)-Gal4 (Speicher et al 1994) (Bloomington Drosophila Stock Center); scabrous (sca)-Gal4/CyO (Mlodzik et al 1990), a gift from Andrea Brand, University of Cambridge (Cambridge, UK); UAS-myr-mRFP/ TM6B (Bloomington Drosophila Stock Center); stripe MD710 (sr)-Gal4/TM6B (Calleja et al 2002;Usui et al 2004), a gift from Pat Simpson, University of Cambridge (Cambridge, UK); UASAmunRNAi and UAS-Dicer2 (Dietzl et al 2007), obtained from the Vienna Drosophila RNAi Center; and P[lArB]A101.IF3 (neur A101 -LacZ)/TM3 (Bellen et al 1989), a gift of Hugo Bellen, Baylor College of Medicine (Houston).…”

Evidence that the C‐terminal domain (CtD) autoinhibits neural repression by Drosophila E(spl)M8

“…We have recently found that loss of the MC's elicited by ectopically expressed wild type M8 is strongly rescued by coexpression of a UAS-CK2a-RNAi construct or UAS-Tik, a CK2-DN (Bose et al, 2006;Kahali et al, 2009). These results suggested that phosphorylation also regulates antagonism of ASC by M8 during SOP selection (Fig.…”

“…Moreover, suppression by M8-CtD is unlikely to involve competition for, or sequestration of, endogenous CK2. The interaction of CK2 with M8 or M8-CtD is disrupted upon phosphorylation (Kahali and Bidwai, unpublished;Karandikar et al, 2004), and while halved CK2 dosage is sufficient for proper N signaling, its further knockdown leads to supernumerary (''twinned'') SOP's/R8's, and to rough eyes due to impaired lateral inhibition (Bose et al, 2006). Consistent with the possibility that (endogenous) CK2 levels are not rate limiting for lateral inhibition, the reduced eye of M8SD is not suppressed by decreased CK2 levels or E(spl)-dosage (Kahali, unpublished).…”

“…Since phosphorylation of M8 is required for R8 and SOP selection (Bose et al, 2006), we hypothesized that ectopic CtD might bind (in ''trans'') to the noninhibited state of M8 and impair repression. To distinguish the effects of phosphorylation, we used two CtD-variants; M8-CtD retains the phosphorylation site and should possess autoinhibitory activity, whereas M8SD-CtD should lack such activity by virtue of the phosphomimetic Asp at the CK2 site.…”