Composition and function of Optix-Cofactor transcription complexes in the eye.

Abstract Drosophila Optix belongs to the evolutionarily conserved SIX family of homeobox transcription factors. Family members control cell fate, morphology, proliferation and/or survival in multiple tissues and organs of metazoans, including the eye. In Drosophila, Optix functions as a critical regulator of visual system development. Optix homozygous mutant tissue shows defect in retina neurogenesis and patterning of the ocelli. In addition, Optix is an essential regulator of neuroepithelial maintenance and patterning in the Drosophila brain, specifically in the optic lobe. The vertebrate homologs SIX3 and SIX6 are also required for the development of the mammalian eye and brain, and SIX3/6 mutations in humans are associated with severe eye and brain malformation. Optix, as other SIX family members, functions as a transcriptional regulator together with a variety of protein cofactors (CoFs) that modify Optix activity, resulting in changes in the activation or repression of target genes and reporters. Hence, to understand Optix protein function in specific developmental contexts, we need to identify the discrete Optix-CoF complexes at work, define their properties, and understand their specific roles. We propose here to characterize Optix-CoF complexes in multiple ways (Aim 1), and then to generate Optix protein variants that fail to engage in either one or few critical protein-protein interactions to demonstrate the context-dependent requirement for the affected complex(es) in cell culture (Aim 2). Lastly, we will develop tools for the dissection of the complexes’ biological roles in vivo (Aim 3). The research proposed here will lay the groundwork for detailed studies of SIX-CoF transcription complexes in vivo under physiological conditions and define their specific developmental roles by uncovering the precise biological processes they control through their transcriptional activity and TC-specific transcriptomes. This work aims to further our understanding of the intricate ways in which these transcription factors control fly eye development, and in which their dysfunction may lead to human disease and birth defects.