Olfactory and lens placode formation is controlled by the hedgehog-interacting protein (Xhip) in Xenopus

Abstract

The integration of multiple signaling pathways is a key issue in several aspects of embryonic development. In this context, extracellular inhibitors of secreted growth factors play an important role, which is to antagonize specifically the activity of the corresponding signaling molecule. We provide evidence that the Hedgehog-interacting protein (Hip) from Xenopus, previously described as a Hedgehog-specific antagonist in the mouse, interferes with Wnt-8 and eFgf/Fgf-8 signaling pathways as well. To address the function of Hip during early embryonic development, we performed gain- and loss-of-function studies in the frog. Overexpression of Xhip or mHip1 resulted in a dramatic increase of retinal structures and larger olfactory placodes primarily at the expense of other brain tissues. Furthermore, loss of Xhip function resulted in a suppression of olfactory and lens placode formation. Therefore, the localized expression of Xhip may counteract certain overlapping signaling activities, which inhibit the induction of distinct sensory placodes.

Ablation studies on the developing inner ear reveal a propensity for mirror duplications

“…The ear on the noninjected side was always normal. The smaller eye phenotype previously described after Hip injection (Cornesse et al, 2005) was seen in 50 to 100% of the embryos injected (Table 3).…”

“…One reagent used to inhibit Hh signaling, cyclopamine treatment, did not produce ectopic otic vesicles but instead enlarged the endogenous otocyst. Later work from the same group showed that the effects on otic induction may have been due to some of the reagents used, specifically Hip, disrupting Fgf and Wnt signaling (Cornesse et al, 2005), two pathways known to be involved in otic placode induction (Groves and BronnerFraser, 2000;Noramly and Grainger, 2002). Therefore, we cannot rule out that the mirror duplications we observed could be due to Hip inhibiting other cell signaling pathways such as Fgf and Wnt.…”

“…Hip is a vertebrate-specific inhibitor of Hh signaling that was discovered in mice (Chuang and McMahon, 1999;Koebernick et al, 2003). We blocked Hh signaling by overexpressing the mouse Hip, which has been shown to act the same as the endogenous Xenopus hip (Cornesse et al, 2005), in half of the embryo, leaving the contralateral side as a control. The inner ears of the injected side had anterior mirror duplications in 30 to 35% of the embryos (Table 3; Fig.…”

“…Previous authors have found that Hip gives the same phenotype as the endogenous Xenopus hip (Cornesse et al, 2005). Hip mRNA was injected with the same apparatus described above for styryl dye injection into the ear.…”

“…Lastly, to investigate the potential involvement of the hedgehog signaling pathway on anteroposterior inner ear development, experiments overexpressing Hedgehog interacting protein (Hip) were undertaken. Hip has been demonstrated to be a potent inhibitor of Hh signaling (Koebernick et al, 2003;Cornesse et al, 2005). Hip overexpression resulted in mirror anterior duplications.…”