The canonical Wnt signaling antagonist DKK2 is an essential effector of PITX2 function during normal eye development

Abstract

Local control of cell signaling activity and integration of inputs from multiple signaling pathways are central for normal development but the underlying mechanisms remain poorly understood. Here we show that Dkk2, encoding an antagonist of canonical Wnt signaling, is an essential downstream target of the PITX2 homeodomain transcription factor in neural crest during eye development. Canonical Wnt signaling is ectopically activated in central ocular surface ectoderm and underlying mesenchyme in Pitx2- and Dkk2-deficient mice. General ocular surface ectoderm identity is maintained during development in Dkk2-deficient mice but peripheral fates, including conjunctival goblet cells and eyelash follicles, are ectopically permitted within more central structures and eyelids are hypomorphic. Loss of DKK2 results in ectopic blood vessels within the periocular mesenchyme and PITX2 expression remains persistently high, providing evidence for a negative feedback loop. Collectively, these data suggest that activation of Dkk2 by PITX2 provides a mechanism to locally suppress canonical Wnt signaling activity during eye development, a paradigm that may be a model for achieving local or transient inhibition of pathway activity elsewhere during embryogenesis. We further propose a model placing PITX2 as an essential integration node between retinoic acid and canonical Wnt signaling during eye development.

Aberrant expression of a β-catenin gain-of-function mutant induces hyperplastic transformation in the mouse cornea

“…Whereas lack of the Wnt antagonist Dkk2 resulted in the ectopic appearance of conjunctiva, epidermis and hair follicles in the corneal epithelium due to direct and/or indirect effect (Mukhopadhyay et al, 2006;Gage et al, 2008), we did not observe these phenotypes in our animal model, regardless of the initial time of Dox-treatment to induce expression of E3-catenin in K12-positive corneal epithelial cells. One of the possibilities is that Wnt/-catenin activation in the Dkk2 null-mutant affects corneal epithelial stem and/or progenitor cells, whereas only the K12-positive cells (including differentiating -or transient amplifying cells -and differentiated corneal epithelial cells) are influenced in our mouse model.…”

“…Repression of the Wnt/-catenin signaling pathway has been implicated as an essential prerequisite for the differentiation of corneal epithelial progenitor cells into a non-keratinizing stratified epithelium during corneal morphogenesis (Mukhopadhyay et al, 2006;Gage et al, 2008). It has been previously shown that stabilized E3-catenin driven by the Le-Cre promoter suppresses surface ectoderm invagination for lens formation at ~E9.5 tõ E10.5.…”

c-Jun, at the crossroad of the signaling network

“…Besides MAP3K1, embryonic eyelid closure depends on signals derived from WNT, Sonic hedgehog, BMP/Activin, FGF and EGF (Luetteke et al, 1993;Mine et al, 2005;Gage et al, 2008;Huang et al, 2009). In addition, eyelid closure requires the participation of a number of intracellular signaling kinases, such as JNK, ROCK and CDH1, and nuclear transcription factors, such as c-Jun, Fra-2, FOXL2, SMAD and GRHL3 (McHenry et al, 1998;Li et al, 2003;Zenz et al, 2003;Zhang et al, 2003;Uda et al, 2004;Thumkeo et al, 2005;Takatori et al, 2008;Yu et al, 2008;Naoe et al, 2010).…”

Wnt/β-catenin signaling modulates corneal epithelium stratification via inhibition of Bmp4 during mouse development

“…It has been shown that loss of Dkk2, an antagonist of canonical Wnt signaling pathways, suppresses corneal differentiation during mouse development (Gage et al, 2008;Mukhopadhyay et al, 2006). This strongly suggests that canonical Wnt signaling might play a crucial role in ocular surface epithelia development.…”