Abstract
Purpose: :
Sonic Hedgehog (Shh), Indian Hedgehog (Ihh) and Desert Hedgehog (Dhh) are extracellular signaling proteins responsible for patterning and tissue formation during embryogenesis. While Ihh signaling has been found to target the developing retinal pigmented epithelium and sclera mesenchyme, Shh ventral midline signaling is critical for proper bilateral division of the forebrain and eye field. In cavefish, excess Shh signaling is implicated in lens and eye degeneration. In the current study we investigated the effects of constitutive activation of Shh, Ihh and Dhh on eye development in a mouse model using an activating mutation in the smoothened (smo) gene encoding for the smo receptor important in hedgehog (Hh) signaling.
Methods: :
An ectodermal cre-recombinase (cre-ect) was used to create a conditional activation of smo in the head ectoderm, including the lens placode, of mice to study the effects of constitutive Hh signaling. Eyes and lenses were examined at embryonic day (E) 12.5, 15.5 and 18.5 using histological and immunofluorescent techniques.
Results: :
At E12.5, activated smo mutant embryos display an anterior lens epithelium that is both misshapen and thicker than that of wild-type (WT) littermates. An aberrant group of cells also occupies the lens vesicle lumen. By E15.5 the mutant lens protrudes outwards away from the optic cup and is disorganized with no distinct lens epithelial and fiber cell compartments. While Pax6 expression appears normal in the mutant lens at E12.5, at E15.5 and E18.5, Pax6 expression is seen in cells throughout the entire mutant lens region, unlike the WT lens in which Pax6 expression is confined to the lens epithelium and early fiber cells. Foxe3 expression resembles that of Pax6, scattered throughout the mutant lens. At E12.5 abnormal amounts of TUNEL staining are seen in the mutant epithelium and primary fiber cells, as well as the cells in the lumen of the lens vesicle, and this persists at E15.5 and E18.5 in mutant lenses.
Conclusions: :
Our studies of activated smo mice show abnormal lens epithelial cell morphology that is correlated with aberrant expression patterns of epithelial markers. Consistent with cavefish studies involving excessive Shh signaling, extensive cell death (TUNEL) is observed in the mutant lens throughout embryogenesis. The activated smo mice provide a useful model to study the effects of Hh signaling in lens and eye development.
Keywords: development • transgenics/knock-outs • genetics