The function of the visual system is to form images in the brain. Correct photoreceptor neuron specification and their patterning within the retina are prerequisites for precise retinotopic axonal projections and thus image formation in the brain. The Drosophila eye serves as an excellent paradigm for many aspects of eye development, retinal biology, and disease. The Drosophila retina is composed of a stereo-typed array of 700- 800 ommatidia, or unit eyes/facets, each containing a precise arrangement of 8 photoreceptor neurons. The establishment of their correct fates and their highly ordered arrangement requires the interplay of several signaling pathways, all of which are conserved and share similar functions during mammalian eye development. In particular, for precise retinal patterning and arrangement, the correct distinction of two subtypes of photoreceptors, R3 and R4, requires an exquisite regulation of the non-canonical Wnt-pathway, the Frizzled(Fz)/planar cell polarity (PCP) pathway and its interplay with Notch-signaling. This Wnt-Fz mediated signaling is distinct from canonical the Wnt-Fz/-catenin pathway, which leads to photoreceptor cell death and reduction of eye size. Thus correct Wnt/Fz pathway selection is essential and must correlate with the proper activation of downstream effectors of Fz. Strikingly, in PCP signaling Fz acts as both a receptor and a ligand. In its ligand function, Fz binds to the Van Gogh/Strabismus (Vang/Stbm; Vangl in mammals) trans-membrane PCP protein and triggers a novel response on the Vang-side of the interaction. The scope of this application addresses regulatory interactions among the core Fz/PCP factors, with focus on events on the Vang/Stbm side, and their interplay with Notch signaling. The Specific Aims are: (1) To dissect molecular read-out(s) of the Fz-Vang/Stbm interaction (Vang acting as receptor) and their effect on Vang/Stbm function in photoreceptor R4 specification; (2) To define the molecular mechanism of Nmo phosphorylation of Pk, a Vang/Stbm binding partner and its impact on Vang/Stbm interactions with other core PCP regulators and effectors during R3-R4 specification; and (3) To address the role of non-canonical Notch (N)-signaling in R4 specification and subsequent ommatidial rotation. These Aims are well integrated and based on exciting preliminary data, including analyses of several kinases in the context of Vang/Stbm PCP and Notch-signaling (Aims 1 and 3), and the role of the Nemo kinase in Pk-Vang/Stbm complex fomation (Aim 2). A combination of in vivo eye studies, cell culture and biochemical experiments, and genetics/genomics will be utilized to achieve these goals. In the human eye, Wnt-Fz and Notch signaling are associated with many human diseases and, moreover, several components are associated with congenital ciliopathies affecting for example retinal function and neurulation defects. Thus the information acquired here will both advance our understanding of retinal patterning and eye biology and will also be of medical relevance in several disease areas.