Lineage-Specific Functions of the Homeodomain Transcription Factor Pitx2 in Eye Development.

Abstract

Numerous inductive patterning events occur in eye development. The periocular mesenchyme plays a role in these processes by patterning the optic stalk, corneal epithelium, and retinal pigmented epithelium during eye development, in addition to contributing cells to many non-neural tissues in the eye. The periocular mesenchyme includes cells from two embryonic lineages, the neural crest and mesoderm, which each form distinct cell types.

The homeodomain transcription factor Pitx2 is required for normal eye development in both mice and humans, and mutations can lead to early onset glaucoma in humans. Pitx2 is expressed in both the neural crest and mesoderm lineages of the periocular mesenchyme, but the mechanisms of its function in each lineage were not known. To test the hypothesis that Pitx2 has unique functions in each lineage during eye development, lineage-specific knockout mice of Pitx2 in the neural crest and mesoderm were created using the Cre-lox system. Pitx2 in the neural crest is cell-autonomously required for anterior segment development, sclera formation and ocular blood vessel growth. Pitx2 also has non-cell autonomous functions in the neural crest in optic stalk development and RPE patterning. The defects in optic stalk development and ocular blood vessel growth represent two new potential mechanisms underlying the glaucoma seen in human patients with PITX2 mutations. In the mesoderm, Pitx2 is cell autonomously required for extraocular muscle precursor survival and non-cell autonomously required for optic fissure closure. Pitx2 function is also required in the mesoderm lineage for eyelid closure. Pitx2, not Pax7, regulates MRF expression in the extraocular muscles, and PITX2 can activate the Myod1 promoter through a novel sequence. This identifies distinct mechanisms of Pitx2 function in the two lineages of the periocular mesenchyme in the developing eye. These findings significantly expand the understanding of the functions of Pitx2 in eye development and its role in human disease.