Analsysis of the ENU-generated mouse mutant cbs: Primary cilia play a crucial role in cortical development

Abstract

In the following study the mouse mutant cobblestone (cbs) concerning the development of the forebrain was analyzed. The cbs mutation was uncovered by an ethyl-nitroso-urea (ENU) genetic screen, using a mouseline called tauGFP. At the beginning of the analysis of the cbs mutation, the phenotype of the cbs/cbs mouse mutant was already known, but because of the mutagenic ability of ENU, which causes random mutations, the affected gene was unclear. By applying the method of positional cloning the gene intraflagellar transport 88 (Ift88) was identified as the candidate gene. A detailed analysis of mRNA levels of Ift88 in the cbs/cbs mutant was undertaken by Northen Blot analysis as well as quantitative real-time RT-PCR. At the same time the Ift88 protein levels were also investigated by Western blot analysis. A complementation analysis by crossing cbs heterozygotes to mice heterozygous for a targeted deletion of the Ift88 gene (Ift88tm1.1Bky) (Haycraft et al., 2007) was done to ascertain, if the genetic defect in the cbs/cbs mutant is located in the Ift88 gene. In the represented study it could be shown that cbs is a hypomorphic allele of the gene Ift88, in which both Ift88 mRNA and protein levels are reduced by 70% to 80%, respectively. A detailed analysis by in situ hybridization (ISH) was followed, using different markers, which are specific for various areas of the developing telencephalon such as Ttr1 for the choroid plexus, Wnt2b for the cortical hem, as well as EphB1 and Lhx2 for the hippocampal anlage. Furthermore both the dorsal-ventral and rostral-caudal compartmental boundaries of the forebrain were investigated by ISH. cbs/cbs mutants display defects in the formation of dorsomedial telencephalic structures, such as the choroid plexus, cortical hem and hippocampus. Furthermore mutants exhibit a relaxation of both dorsal-ventral and rostral-caudal compartmental boundaries of the forebrain, resulting in the intermixture of otherwise separated cell populations. It is further demonstrated that the proteolytic processing of Gli3 is reduced in the cbs/cbs mutant, leading to an accumulation of the full-length activator isoform. In addition the cbs/cbs mutant exhibits an upregulation of canonical Wnt signalling in the neocortex and in the caudal forebrain. Primary cilia, microtubule-based organelles that protrude from the surface of most cells of the vertebrate body, are dependent on Ift88 for their formation and maintenance. The ultrastructure and morphology of cilia of the ventricle of the cbs/cbs mutants was therefore simultaneously investigated by transmission and scanning electron microscopy. Surprisingly, examined cilia are still existing and intact in the cbs/cbs mutant. Taken together, these results indicate a fundamental role for primary cilia in the development of the forebrain.