Role of polysialic acid for cortical interneuron migration and dissection of defective brain development in polysialic acid-deficient mice by conditional knock-out of St8sia2

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The neural cell adhesion molecule NCAM and its modification with the sugar polymer polysialic acid (polySia) are major determinants of brain development. PolySia is synthesized by the polysialyltransferases ST8SIA2 and ST8SIA4, which are independently regulated but often co-expressed in the same cell. In mice, ST8SIA2 seems to be the major polysialyltransferase during development, whereas ST8SIA4 prevails in the adult brain. Ablation of both enzymes generated mice that are completely devoid of polySia. These mice reproduce defects of the polySia- and NCAM-depleted Ncam-/- mice, but also show damage of major brain axon tracts, which can be rescued by additional deletion of the polySia carrier protein NCAM. Guided by the severe phenotype of the St8sia2, St8sia4 double knockout mice, we started to comparatively re-evaluate the two single knockout lines. Deficits of thalamocortical connectivity were found specifically in the St8sia2-/- line and may be linked to the observed cognitive impairments. In contrast, both lines displayed reduced interneuron densities in the prefrontal cortex as well as signs of impaired interneuron migration from the medial ganglionic eminence (MGE) into the cortex during embryonic development. Lines of polySia-deficient, GAD67-GFP transgenic mice with genetically labeled interneurons have been established with the aim to study the cellular and molecular mechanisms of polySia-dependent cortical interneuron development in vivo as well as in slice culture and MGE-cortex co-culture systems. High resolution time lapse imaging of GFP-labelled cells in these cultures will be used to study the impact of polySia expression on the dynamics of interneuron migration and on the response to motogenic factors. Other objectives of the current proposal are the dissection of mechanisms which lead to disturbed thalamus-cortex connectivity and interneuron deficits in the prefrontal cortex of St8sia2-/- mice. 'Floxed' St8Sia2Fx/Fx mice have been generated and have been crossed with lines expressing the Cre-recombinase under the Lhx6 promoter or under the Foxb1 promoter for specific deletion of St8Sia2 in MGE-derived cortical interneurons or in the developing thalamus, but not cortex. The interneuron-specific knockout will reveal cell-autonomous consequences of polySia-deficiency. Similarly, the thalamus-specific deletion will allow us to distinguish between the role of polySia on thalamocortical axons and polySia present in their environment. Deviant development due to loss of polySia from the thalamocortical but not the corticothalamic axons may shed light on longstanding questions over the mutual impact of these fibers in development. Finally, comparing behavioral traits between mice with thalamocortical or interneuron deficits, or both, will enable the assignment of defined neurodevelopmental defects to specific cognitive functions.