Mouse GLI3 Regulates Fgf8 Expression and Apoptosis in the Developing Neural Tube, Face, and Limb Bud

Abstract

The zinc finger transcription factor GLI3 is considered a repressor of vertebrate Hedgehog (Hh) signaling. In humans, the absence of GLI3 function causes Greig cephalopolysyndactyly syndrome, affecting the development of the brain, eye, face, and limb. Because the etiology of these malformations is not well understood, we examined the phenotype of mouse Gli3-/- mutants as a model to investigate this. We observed an up-regulation of Fgf8 in the anterior neural ridge, isthmus, eye, facial primordia, and limb buds of mutant embryos, sites coinciding with the human disease. Intriguingly, endogenous apoptosis was reduced in Fgf8-positive areas in Gli3-/- mutants. Since SHH is thought to be involved in Fgf8 regulation, we compared Fgf8 expression in Shh-/- and Gli3-/-;Shh-/- mutant embryos. Whereas Fgf8 expression was almost absent in Shh-/- mutants, it was up-regulated in Gli3-/-;Shh-/- double mutants, suggesting that SHH is not required for Fgf8 induction, and that GLI3 normally represses Fgf8 independently of SHH. In the limb bud, we provide evidence that ectopic expression of Gremlin in Gli3-/- mutants might contribute to a decrease in apoptosis. Together, our data reveal that GLI3 limits Fgf8-expression domains in multiple tissues, through a mechanism that may include the induction or maintenance of apoptosis.

Gene expression analysis of the hedgehog signaling cascade in the chick midbrain and spinal cord

“…Of interest, several HH-binding factors (MEGALIN, BOC) and HH effectors (SMO,GLI1,2,3,DZIP) are expressed in the dorsal midbrain and the MHB. These expression patterns thus provide a basis for understanding recent reports that implicate HH signaling in the regulation of dorsal midbrain structures (e.g., inferior colliculus) and the MHB (Bayly et al, unpublished observations;Aoto et al, 2002;Ishibashi and McMahon, 2002;Blaess et al, 2006). Finally, we report for the first time the expression of IHH in the intermediate regions of the spinal cord, but not the ventral midbrain between H&H stage 18 and E4.…”

“…3D,DЈ, 3G, 4B, 6G). A role for HH signaling in the maintenance of fgf8 expression in the MHB has recently been established in the Shh Ϫ/Ϫ mouse (Fogel and Agarwala, unpublished observations; Aoto et al, 2002;Blaess et al, 2006). In the mouse limb, the maintenance of fibroblast growth factor-8 signaling in the apical ectodermal ridge by SHH (derived from the zone of polarizing activity) is indirect, by means of the action of the BMP antagonist Gremlin (Khokha et al, 2003).…”

Development of the upper lip: Morphogenetic and molecular mechanisms

“…Cranial neural crest cells lacking Smo migrated and formed facial primordia normally in mouse embryos but exhibited high levels of apoptosis from E9.5 to E10.5 and reduced cell proliferation at E11.5, indicating that Shh expression in the facial ectoderm specifically supports cell survival during early stages and promotes proliferation at later stages to control the size of the facial primordia (Jeong et al, 2004). Interestingly, whereas overactivation of Shh signaling by loss of the inhibitor Gli3 or constitutive activation of Smo in the neural crest causes slight overgrowth of the facial primordia, some patients with mutations in PTCH1 have bilateral CLP (Hahn et al, 1996;Aoto et al, 2002;Jeong et al, 2004), suggesting that Shh signaling is regulated at multiple levels during facial morphogenesis.…”

Birth and death of cells in limb development: A mapping study

“…For example, across species, different patterns of cell death in the INZ correlate with different final morphologies of the interdigital region, and INZ inhibition correlates with survival of interdigit webbing (Saunders and Fallon, 1967;Ede, 1967, Hurle andCliment, 1987;Hurle et al, 1996). Within a single species, modifications in the amount of cell death by naturally occurring mutations have been shown to correlate with syndactyly or other modifications in digital pattern Ede, 1967, 1973;Dvorak and Fallon, 1991;van der Hoeven et al, 1994;Zakeri et al, 1994;Aoto et al, 2002). In general, the areas of cell death are considered to play an important function in regulating the quantity of mesenchymal cells and, therefore, the number of digits (Hinchliffe, 1982).…”