Cortical dysplasia and skull defects in mice with a Foxc1 allele reveal the role of meningeal differentiation in regulating cortical development.

We report the identification of a hypomorphic mouse allele for Foxc1 (Foxc1(hith)) that survives into adulthood revealing previously unknown roles for Foxc1 in development of the skull and cerebral cortex. This line of mice was recovered in a forward genetic screen using ENU mutagenesis to identify mutants with cortical defects. In the hith allele a missense mutation substitutes a Leu for a conserved Phe at amino acid 107, leading to destabilization of the protein without substantially altering transcriptional activity. Embryonic and postnatal histological analyses indicate that diminished Foxc1 protein expression in all three layers of meningeal cells in Foxc1(hith/hith) mice contributes to the cortical and skull defects in mutant mice and that the prominent phenotypes appear as the meninges differentiate into pia, arachnoid, and dura. Careful analysis of the cortical phenotypes shows that Foxc1(hith/hith) mice display detachment of radial glial endfeet, marginal zone heterotopias, and cortical dyslamination. These abnormalities have some features resembling defects in type 2 (cobblestone) lissencephaly or congenital muscular dystrophies but appear later in corticogenesis because of the delay in breakdown of the basement membrane. Our data reveal that the meninges regulate the development of the skull and cerebral cortex by controlling aspects of the formation of these neighboring structures. Furthermore, we provide evidence that defects in meningeal differentiation can lead to severe cortical dysplasia.

Loss of the retrograde motor for IFT disrupts localization of Smo to cilia and prevents the expression of both activator and repressor functions of Gli.

Sonic Hedgehog (Shh) signals are transduced into nuclear ratios of Gli transcriptional activator versus repressor. The initial part of this process is accomplished by Shh acting through Patched (Ptc) to regulate Smoothened (Smo) activity. The mechanisms by which Ptc regulates Smo, and Smo activity is transduced to processing of Gli proteins remain unclear. Recently, a forward genetic approach in mice identified a role for intraflagellar transport (IFT) genes in Shh signal transduction, downstream of Patched (Ptc) and Rab23. Here, we show that the retrograde motor for IFT is required in the mouse for the phenotypic expression of both Gli activator and repressor function and for effective proteolytic processing of Gli3. Furthermore, we show that the localization of Smo to primary cilia is disrupted in mutants. These data indicate that primary cilia act as specialized signal transduction organelles required for coupling Smo activity to the biochemical processing of Gli3 protein.

A focused and efficient genetic screening strategy in the mouse: identification of mutations that disrupt cortical development.

Although the mechanisms that regulate development of the cerebral cortex have begun to emerge, in large part through the analysis of mutant mice (Boncinelli et al. 2000; Molnar and Hannan 2000; Walsh and Goffinet 2000), many questions remain unanswered. To provide resources for further dissecting cortical development, we have carried out a focused screen for recessive mutations that disrupt cortical development. One aim of the screen was to identify mutants that disrupt the tangential migration of interneurons into the cortex. At the same time, we also screened for mutations that altered the growth or morphology of the cerebral cortex. We report here the identification of thirteen mutants with defects in aspects of cortical development ranging from the establishment of epithelial polarity to the invasion of thalamocortical axons. Among the collection are three novel alleles of genes for which mutant alleles had already been used to explore forebrain development, and four mutants with defects in interneuron migration. The mutants that we describe here will aid in deciphering the molecules and mechanisms that regulate cortical development. Our results also highlight the utility of focused screens in the mouse, in addition to the large-scale and broadly targeted screens that are being carried out at mutagenesis centers.

A Titin mutation defines roles for circulation in endothelial morphogenesis.

Morphogenesis of the developing vascular network requires coordinated regulation of an extensive array of endothelial cell behaviors. Precisely regulated signaling molecules such as vascular endothelial growth factor (VEGF) direct some of these endothelial behaviors. Newly forming blood vessels also become subjected to novel biomechanical forces upon initiation of cardiac contractions. We report here the identification of a recessive mouse mutation termed shrunken-head (shru) that disrupts function of the Titin gene. Titin was found to be required for the initiation of proper heart contractions as well as for maintaining the correct overall shape and orientation of individual cardiomyocytes. Cardiac dysfunction in shrunken-head mutant embryos provided an opportunity to study the effects of lack of blood circulation on the morphogenesis of endothelial cells. Without blood flow, differentiating endothelial cells display defects in their shapes and patterns of cell-cell contact. These endothelial cells, without exposure to blood circulation, have an abnormal distribution within vasculogenic vessels. Further effects of absent blood flow include abnormal spatial regulation of angiogenesis and elevated VEGF signaling. The shrunken-head mutation has provided an in vivo model to precisely define the roles of circulation on cellular and network aspects of vascular morphogenesis.