Polysialic acid (PSA) is a negatively charged carbohydrate polymer, which confers antiadhesive properties to the neural cell adhesion molecule NCAM and facilitates cellular plasticity during brain development. In mice, PSA expression decreases drastically during the first postnatal weeks and it gets confined to immature neurons and regions displaying structural plasticity during adulthood. In the brain, PSA is exclusively synthesized by the two polysialyltransferases ST8SiaII and ST8SiaIV. To study their individual contribution to polysialylation in the adult, we analyzed PSA expression in mice deficient for either polysialyltransferase. Focusing on the cerebral cortex, our results indicate that ST8SiaIV is solely responsible for PSA expression in mature interneurons and in most regions of cortical neuropil. By contrast, ST8SiaII is the major polysialyltransferase in immature neurons of the paleocortex layer II and the hippocampal subgranular zone. The numbers of cells expressing PSA or doublecortin, another marker of immature neurons, were increased in the paleocortex layer II of ST8SiaIV-deficient mice, indicating altered differentiation of these cells. Analysis of doublecortin expression also indicated that the production of new granule neurons in the subgranular zone of ST8SiaII-deficient mice is not affected. However, many of the immature granule neurons showed aberrant locations and morphology, suggesting a role of ST8SiaII in their terminal differentiation.
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