Corneal avascularity—the absence of blood vessels in the cornea—is required for optical clarity and optimal vision, and has led to the cornea being widely used for validating pro- and anti-angiogenic therapeutic strategies for many disorders1,2,3,4. But the molecular underpinnings of the avascular phenotype have until now remained obscure5,6,7,8,9,10 and are all the more remarkable given the presence in the cornea of vascular endothelial growth factor (VEGF)-A, a potent stimulator of angiogenesis, and the proximity of the cornea to vascularized tissues. Here we show that the cornea expresses soluble VEGF receptor-1 (sVEGFR-1; also known as sflt-1) and that suppression of this endogenous VEGF-A trap11 by neutralizing antibodies, RNA interference or Cre-lox-mediated gene disruption abolishes corneal avascularity in mice. The spontaneously vascularized corneas of corn1 and Pax6+/- mice12,13 and Pax6+/- patients with aniridia14 are deficient in sflt-1, and recombinant sflt-1 administration restores corneal avascularity in corn1 and Pax6+/- mice. Manatees, the only known creatures uniformly to have vascularized corneas15, do not express sflt-1, whereas the avascular corneas of dugongs, also members of the order Sirenia, elephants, the closest extant terrestrial phylogenetic relatives of manatees, and other marine mammals (dolphins and whales) contain sflt-1, indicating that it has a crucial, evolutionarily conserved role. The recognition that sflt-1 is essential for preserving the avascular ambit of the cornea can rationally guide its use as a platform for angiogenic modulators, supports its use in treating neovascular diseases, and might provide insight into the immunological privilege of the cornea.
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We thank the various aquaria, zoos and wildlife rehabilitation centres that donated tissues for comparative studies; R. Groom, S. Joshi, M. Kellogg, R. King, C. K. Lau, P. Lewis, N. Mezei, K.K. Smith and L. Xu for technical assistance; R. J. Kryscio for statistical guidance; and R. Mohan, S. Bondada, M. W. Fannon, L. Mazzaro, Y. Nozaki, P. A. Pearson, L. Peichl, A. M. Rao, G. S. Rao and K. Ambati for discussions. J.A. was supported by the NEI/NIH, the Lew R. Wasserman Merit Award (Research to Prevent Blindness), the Dennis W. Jahnigen Career Development Award (American Geriatrics Society, John A. Hartford Foundation, Atlantic Philanthropies), the Macula Vision Research Foundation, the International Retinal Research Foundation, the E. Matilda Ziegler Foundation for the Blind, the Dr E. Vernon Smith and Eloise C. Smith Macular Degeneration Endowed Chair, a physician–scientist award from University of Kentucky, and a departmental challenge grant from Research to Prevent Blindness; M.N. by ARVO/Japan National Society for the Prevention of Blindness; E.S. by Fight For Sight; R.J.C.A by Research to Prevent Blindness; J.Z.B. and B.R. by the NIDCD/NIH; A.T. by a Japan Young Scientist Award; B.K.A. by a VA Career Development Award, the Knights-Templar Eye Foundation and Fight for Sight; N.S by ARVO/Alcon; S.I. by the NEI/NIH; J.M.C. by the Wellcome Trust and the Birth Defects Foundation; T.S.K by the NEI/NIH and the NIAMS/NIH; S.B. by the Muscular Dystrophy Association; and S.D.F. by the AIRC (Italian Association for Cancer Research). Author Contributions B.K.A. and J.A. conceived and designed the experiments, wrote the manuscript, and are joint senior authors. M.N., N.S., A.T., P.D.J., J.Z.B. and B.J.R. contributed equally.
This file contains Supplementary Figures and Legends 1–11, Supplementary Methods and Supplementary Notes.