The protein kinase Nemo integrates specification, proliferation and morphogenesis to shape the Drosophila eye

Development of multi-cellular organisms is orchestrated by genetic circuits, which control proliferation and differentiation. A complex interplay of inductive and inhibitory signals culminates in the expression of unique protein sets, which elicit a distinct cellular response and confer cellular identity. The relatively simple architecture of the Drosophila melanogaster eye provides a powerful model to explore the molecular and regulatory relationships that pattern a complex tissue. This work investigates novel roles for the serine-threonine kinase, Nemo (Nmo), in cell fate specification and morphogenesis of the Drosophila compound eye. Nmo is the founding member of the Nemo-Like Kinase (NLK) family, which regulates conserved signal cascades like Wnt and BMP across species. nmo mutants have developmental defects in processes including embryonic segmentation, wing and eye patterning. Here, I examine the molecular underpinnings of the nmo eye phenotype. nmo is dynamically expressed throughout eye development, suggesting previously uncharacterized functions. Genetic analyses reveal a dose-dependent requirement for Nmo in eye specification directed by the Retinal Determination Gene Network (RDGN). The RDGN comprises a conserved network of transcriptional regulators that is essential for eye development in Drosophila and vertebrates. Loss of nmo reduces the ability of the RD factors to induce ectopic eyes, while co-expressing Nmo synergistically enhances RD-mediated phenotypes. In biochemical assays, Nmo complexes with the RD components Sine oculis (So) and Eyes Absent (Eya). Eya is a novel substrate for Nmo’s kinase activity, and this regulatory interaction promotes Eya’s eye induction activity. Nmo additionally promotes So’s transcriptional activity in culture-based and in vivo assays. Genetic approaches are employed to elucidate the nature of the nmo small-eye phenotype. In genetic interaction studies, manipulating nmo levels modifies the size of the head and eye, indicating that nmo can generally affect proliferation. A requirement for Nmo in normal progression of the morphogenetic furrow, a wave of differentiation that moves anteriorly across the eye epithelium, is also described. As a consequence of retarded furrow movement, photoreceptor differentiation is delayed in nmo mutants. Together, these studies expand our understanding of the nmo eye phenotype and identify Nmo as a novel regulator of the RD signalling network.