Background: The forebrain consists of multiple structures necessary to achieve elaborate functions. Proper patterning is, therefore, a prerequisite for the generation of optimal functional areas. Only a few factors have been shown to control the genetic networks that establish early forebrain patterning.

No abstract

A large scale insertional mutagenesis experiment was performed in embryonic stem (ES) cells by introducing two types of gene trap vectors into the genome. These cell lines carrying mutations were introduced into the mouse germline. In order to assess the feasibility of a large scale cloning of the targeted genes from these lines, we have isolated and characterized 55 trapped exons from the corresponding ES cells. Analysis of the data has revealed that vectors containing or lacking an internal ribosome entry site (IRES) can integrate into the ES cell genome stochastically. The targeted genes comprise 30% known genes, 20% expressed sequence tags (ESTs) and 50% novel or unknown genes. The known genes belong to several major classes and represent complete or partial knockouts. Using currently available methods or modifications of them, it should be feasible to do a large scale cloning of trapped genes from the mouse ES cell lines.

The extracellular matrix molecule reelin is a crucial molecule in CNS development, in particular in the cerebellum and cerebral cortex. In the cerebral cortex, reelin is provided by a small number of neurons located in the marginal zone (MZ). These neurons belong to the earliest neurons generated, but little is known about the molecular mechanisms of their specification. Here we describe that reelin-positive cells are strongly increased in the developing cortex of the Pax6 mutant mice Small eye. Shortly after the onset of reelin expression, the number of reelin- and calretinin-positive cells is doubled in the cortex of Pax6 mutants and this increase is further enhanced during development. In contrast, calbindin-positive cells in the MZ do not co-express reelin and are not altered in the Pax6 mutant cortex. The split of the preplate cells was also defective in the Pax6 mutant cortex, suggesting that the amount of reelin is crucial for positioning of the cortical plate between the MZ and subplate. We further show that Pax6 mutant cortical cells isolated in vitro do not develop an increase in reelin-positive cells, while cells isolated from the entire telencephalon do. Consistent with non-cell-autonomous mechanisms contributing to the increase in reelin-positive cells in the Pax6-deficient cortex, tangential migration of diverse cell types from the ventral telencephalon into the cortex is enhanced in the Pax6 mutant mice. Taken together, these experiments further elucidate how patterning of the forebrain by the transcription factor Pax6 regulates the specification of distinct neuronal subtypes in the cortical MZ.