[Show abstract][Hide abstract] ABSTRACT: During neurogenesis, conserved tissue-specific proneural factors establish a cell's competence to take on neural fate from within a field of unspecified cells. Proneural genes encode basic helix-loop-helix transcription factors that promote the expression of 'core' and subtype-specific target genes. Target genes include both pan-neuronal genes and genes that aid in the process of refinement, known as lateral inhibition. In this process, proneural gene expression is increased in the neural progenitor while simultaneously down-regulated in the surrounding cells, in a Notch signalling-dependent manner. Here, we identify nemo (nmo) as a target of members of both Drosophila Atonal and Achaete-Scute proneural factor families and find that mammalian proneural homologs induce Nemo-like-kinase (Nlk) expression in cell culture. We find that nmo loss of function leads to reduced expression of Notch targets and to perturbations in Notch-mediated lateral inhibition. Furthermore, Notch hyperactivity can compensate for nmo loss in the Drosophila eye. Thus nmo promotes Notch-mediated lateral inhibition downstream of proneural factors during neurogenesis.
[Show abstract][Hide abstract] ABSTRACT: The retinal determination gene network comprises a collection of transcription factors that respond to multiple signaling inputs to direct Drosophila eye development. Previous genetic studies have shown that nemo (nmo), a gene encoding a proline-directed serine/threonine kinase, can promote retinal specification through interactions with the retinal determination gene network, although the molecular point of cross-talk was not defined. Here, we report that the Nemo kinase positively and directly regulates Eyes absent (Eya). Genetic assays show that Nmo catalytic activity enhances Eya-mediated ectopic eye formation and potentiates induction of the Eya-Sine oculis (So) transcriptional targets dachshund and lozenge. Biochemical analyses demonstrate that Nmo forms a complex with and phosphorylates Eya at two consensus mitogen-activated protein kinase (MAPK) phosphorylation sites. These same sites appear crucial for Nmo-mediated activation of Eya function in vivo. Thus, we propose that Nmo phosphorylation of Eya potentiates its transactivation function to enhance transcription of Eya-So target genes during eye specification and development.
[Show abstract][Hide abstract] ABSTRACT: Drosophila nemo (nmo) and other Nemo-like kinase family members (Nlks) are well-established key regulators of numerous conserved signaling pathways, such as Wg and BMP. nmo mutants display pleiotropic defects at different developmental stages, including the embryo. In this study we describe a detailed characterization of embryonic cuticle patterning defects associated with maternal loss of nmo. nmo mutant embryos consistently show segmentation defects, most frequently fusions of pairs of denticle belts in alternating segments. These phenotypes are reminiscent of those associated with defects in pair-rule patterning. Genetic interaction studies demonstrate that Nmo promotes Even-skipped (Eve) activity and is required to promote the expression of the Eve target, engrailed (en), in even numbered parasegments. We find that Nmo regulates a subset of Eve activities by stimulating Eve-mediated suppression of the odd-skipped (odd) repressor. Furthermore, we isolate Nmo in a protein complex with Eve and show that Nmo phosphorylates Eve in in vitro kinase assays. These studies reveal a novel role for the Nmo kinase in embryonic pattern formation through its regulation of the homeodomain-containing transcription factor Eve.
[Show abstract][Hide abstract] ABSTRACT: Drosophila nemo (nmo) is the founding member of the Nemo-like kinase (Nlk) family of serine-threonine kinases. Previous work has characterized nmo's role in planar cell polarity during ommatidial patterning. Here we examine an earlier role for nmo in eye formation through interactions with the retinal determination gene network (RDGN). nmo is dynamically expressed in second and third instar eye imaginal discs, suggesting additional roles in patterning of the eyes, ocelli, and antennae. We utilized genetic approaches to investigate Nmo's role in determining eye fate. nmo genetically interacts with the retinal determination factors Eyeless (Ey), Eyes Absent (Eya), and Dachshund (Dac). Loss of nmo rescues ey and eya mutant phenotypes, and heterozygosity for eya modifies the nmo eye phenotype. Reducing nmo also rescues small-eye defects induced by misexpression of ey and eya in early eye development. nmo can potentiate RDGN-mediated eye formation in ectopic eye induction assays. Moreover, elevated Nmo alone can respecify presumptive head cells to an eye fate by inducing ectopic expression of dac and eya. Together, our genetic analyses reveal that nmo promotes normal and ectopic eye development directed by the RDGN.