Mechanisms of development of curved refractive surfaces

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Abnormal curvature of the cornea is associated with multiple visual defects including myopia, hyperopia, keratoconus and astigmatism. However, the developmental mechanisms that generate appropriate corneal curvature are not well understood. This proposal will use the Drosophila corneal lens as a simple model system to investigate the development of curvature, and will test whether it shares common molecular and developmental mechanisms with the mammalian cornea. The project is based on data showing that the conserved Blimp-1 transcription factor is essential for external curvature of the Drosophila corneal lens, and that it acts in the peripheral secondary and tertiary pigment cells rather than in the central cells that secrete the bulk of the extracellular corneal lens material. The possibility of an analogous function for mouse Blimp1/Prdm1 is suggested by its enrichment in the limbal epithelial cells at the periphery of the adult cornea. The first aim will investigate the transcriptional network that controls Drosophila corneal lens development. In the absence of Blimp-1, the C/EBP homologue Slow border cells (Slbo) is highly upregulated, and Slbo overexpression, like loss of Blimp-1, results in externally flat corneal lenses. The hypothesis that slbo repression is a major function of Blimp-1 will be tested by inactivating both genes simultaneously. This aim will also identify other critical transcription factors downstream of Blimp-1, and will test whether transient hormonally regulated expression of Blimp-1 in the eye is important for the temporal control of differentiation. The second aim will determine how the peripheral pigment cells contribute to corneal lens curvature. Two possibilities will be investigated: that pigment cells secrete protein components that localize to the peripheral corneal lens and give it structural properties distinct from the central region, and that force exerted on the corneal lens through its attachments to pigment cells controls the pattern of curvature. The third aim will explore possible conservation between insect and mammalian corneal structures. CG43333, a gene that is repressed by Blimp-1 in the Drosophila retina and contributes to the abnormal corneal lens phenotype of Blimp-1 mutants, is homologous to Transforming growth factor beta- induced protein, a frequent target of mutation in human corneal dystrophies. The effect of disease mutations on CG43333 accumulation and localization will be examined to determine whether they reproduce features of corneal dystrophy. Human corneal epithelial cells and corneal organoids will be used to establish whether human BLIMP1 has target genes in common with Drosophila Blimp-1 in the cornea. Finally, the developmental expression pattern of Blimp1 in the mouse cornea will be examined to determine whether it is consistent with a role in corneal curvature. These experiments will elucidate how curvature is imparted to refractive structures composed primarily of extracellular matrix, and will establish the Drosophila corneal lens as a model to understand corneal development and disease.