Six3 acts independently of Pax6 to provide an essential contribution to lens development

The Six3 transcription factor is essential for forebrain and eye development, and SIX3 mutations cause the congenital disorder holoprosencephaly. We created a six3 mutant in Xenopus tropicalis with a mild holoprosencephaly phenotype, and unlike mouse Six3 mutants that are headless/eyeless, the Xenopus mutant forms some eye structures, allowing direct study of Six3 function in eye formation. We focus here on striking deficits in lens formation. Early lens induction occurs normally in the mutant, e.g., the essential eye gene pax6, is activated in lens ectoderm, persisting in the eye to a late developmental stage, but in many embryos the lens fails to form. We found that bmp4, bmp7.1, smad7, dll1, dlc, mab21l1 and/or mab21l2, previously unknown as six3 eye targets, are downregulated in the mutant. We show that six3 is required for lens formation, acting primarily in developing retina during neurulation through BMP and Notch signaling, and that mab21l1/mab21l2 regulate(s) this BMP activity. This work reveals previously unrecognized essential roles for six3 in eye development, identifying its key role in signaling needed for lens formation, and acting independently of pax6 activity. SUMMARY STATEMENTThis study identifies the six3 transcription factor as the mediator of key inductive signals driving lens formation, acting independently of pax6 in early phases of lens formation.