Penetrance of eye defects in mice heterozygous for mutation of Gli3 is enhanced by heterozygous mutation of Pax6.

Background: Knowledge of the consequences of heterozygous mutations of developmentally important genes is important for understanding human genetic disorders. The Gli3 gene encodes a zinc finger transcription factor and homozygous loss-of-function mutations of Gli3 are lethal. Humans heterozygous for mutations in this gene suffer Greig cephalopolysyndactyly or Pallister-Hall syndromes, in which limb defects are prominent, and mice heterozygous for similar mutations have extra digits.

Pax6 controls cerebral cortical cell number by regulating exit from the cell cycle and specifies cortical cell identity by a cell autonomous mechanism.

Many cerebral cortical neurons and glia are produced by apical progenitors dividing at the ventricular surface of the embryonic dorsal telencephalon. Other neurons are produced by basal progenitor cells, which are derived from apical progenitors, dividing away from the ventricular surface. The transcription factor Pax6 is expressed in apical progenitors and is downregulated in basal progenitors, which upregulate the transcription factor Tbr2.

Abnormal positioning of diencephalic cell types in neocortical tissue in the dorsal telencephalon of mice lacking functional Gli3.

The transcription factor Gli3 (glioma-associated oncogene homolog) is essential for normal development of the mammalian forebrain. One extreme requirement for Gli3 is at the dorsomedial telencephalon, which does not form in Gli3(Xt/Xt) mutant mice lacking functional Gli3. In this study, we analyzed expression of Gli3 in the wild-type telencephalon and observed a (high)dorsal-to-(low)ventral gradient of Gli3 expression and predominance of the cleaved form of the Gli3 protein dorsally.

Loss of Gli3 enhances the viability of embryonic telencephalic cells in vitro.

The transcription factor Gli3 is important for brain and limb development. Mice homozygous for a mutation in Gli3 (Gli3Xt/Xt) have severe abnormalities of telencephalic development and previous studies have suggested that aberrant cell death may contribute to the Gli3Xt/Xt phenotype. We demonstrate that telencephalic cells from embryonic Gli3Xt/Xt embryos survive better and are more resistant to death induced by cytosine arabinoside, a nucleoside analogue that induces death in neuronal progenitors and neurons, than are control counterparts in vitro.

Mouse models of telencephalic development.

As the telencephalon first emerges from anterior neuroectoderm, signalling molecules and transcription factors combine to specify the identity and fate of cells in each of its regions. Studies of both naturally occurring and transgenic mutant mice have identified many genes that contribute to this process. The development of telencephalon and its regions is specified by signalling molecules produced at sites both surrounding and within the telencephalon.