Abstract
Purpose: :
Pitx2/PITX2 encodes a homeodomain transcription factor that is widely required for normal anterior segment development in mice and humans, and is associated with glaucoma in humans. However, no important developmental genes/pathways regulated by PITX2 have previously been identified. In the present work, we test the hypothesis that the canonical Wnt signaling antagonist DKK2 is an essential effector of PITX2 function during eye development.
Methods: :
Timed pregnant and post-natal litters were generated from Pitx2null and Dkk2null mice. Standard techniques of immunohistochemistry, RNA in situ hybridization, and light microscopy were used to compare gene expression and eye histology in wild type and mutant eyes. Dkk2 promoter analysis was conducted using chromatin immunoprecipitation and luciferase reporter assays.
Results: :
Dkk2 expression is lost in eyes of global or neural crest specific Pitx2 knockout mice. Absence of PITX2 or DKK2 results in elevated canonical Wnt signaling throughout the ocular surface ectoderm (OSE) and underlying mesenchyme. PITX2 physically associates with and transactivates Dkk2 promoter sequences. Dkk2-/- mice exhibit defects in the OSE, including corneal conjunctivalization, ectopic eyelash cilia, and hypomorphic eyelids. In the periocular mesenchyme, normal barriers to blood vessel growth are lost and differentiation of the corneal stroma is disrupted. PITX2 levels remain persistently high in Dkk2-/- eyes, providing evidence for an essential auto-regulatory feedback loop between PITX2 and DKK2 during eye development.
Conclusions: :
DKK2 is an essential effector of PITX2 function during eye development. DKK2 locally inhibits canonical Wnt signaling despite concurrent high-level local expression of Wnt ligands and receptors, allowing normal development within the OSE and establishing barriers to blood vessel growth within the mesenchyme. Changes in Dkk2/DKK2 expression likely underlie certain Pitx2 phenotypes mice and humans, and DKK2 is candidate human eye disease gene. (NIH EY014126)
Keywords: transcription factors • transgenics/knock-outs • gene/expression