Role of polysialic acid in the genesis of GABAergic neurons of the cerebral cortex and for thalamocortical connectivity

Posttranslational addition of polysialic acid (polySia) by the polysialyltransferases (polySTs) ST8SIA2 and ST8SIA4 is a unique and prominent modification of the neural cell adhesion molecule NCAM and a major determinant of nervous system development. Dysregulation of this system has been linked to schizophrenia and mice with aberrant polysialylation or NCAM expression show neuroanatomical defects reminiscent to observations in patients. The present work consists of three parts dealing with (i) the impact of polySia on selected interneuron populations in brain regions relevant to the pathophysiology of schizophrenia, (ii) the role of polySia for the migration of cortical interneurons and (iii) the development of a schizophrenia-like phenotype in polySia-deficient mice. In the first part, interneuron densities were analyzed in the medial prefrontal cortex (PFC), the hippocampus and the olfactory bulb of mice with differently combined Ncam1 and polyST deletions. Pronounced reductions of specifically the perineuronal net- and parvalbumin (PV)-positive, calbindin-negative and the somatostatin (Sst)-positive interneurons were detected in the medial PFC of all NCAM- or polySia-deficient lines, whereas other types of interneurons were unaffected. Disturbed patterns of tangentially migrating cortical interneuron precursors were observed in polyST-negative lines indicating that migration deficits may cause these alterations. Corresponding to the well-described migration deficits of olfactory interneuron precursors, reductions of calbindin-positive cells were found in the olfactory bulb of all polySia-deficient lines. In contrast, PV-positive cell densities were increased in the hippocampus. Together, these findings demonstrate that attenuation of NCAM-bound polySia causes pathological changes of specific interneuron subtypes. The second part deals with the mechanisms leading to reduced densities of PV- and Sst-positive interneurons in the PFC. The assumed loss of interneurons was corroborated by analyzing polyST-negative mice with genetically labeled interneurons. In ST8SIA2- or ST8SIA4-deficient mouse embryos, reduced levels of polySia were detected in the area of interneuron migration from the medial ganglionic eminence (MGE) into the cortex. Time lapse recordings revealed significantly reduced velocities in slice cultures that have been treated with endosialidase to remove polySia. Compromised migration of interneurons from ST8SIA2-deficient MGE explants into co-cultured wildtype pallium indicated that levels of polySia on the migrating interneurons themselves are critical. Furthermore shorter leading processes of migratory interneurons were observed after enzymatic removal of polySia in slices. The same effect was observed in primary cultures of isolated interneurons suggesting a cell-autonomous mechanism of polySia-dependent interneuron migration. Thus, reduced polySia levels impede the migration of cortical interneurons. This provides a possible link between genetic variation of polyST genes, neurodevelopmental alterations and interneuron dysfunction as observed in neuropsychiatric disease. In the third part of my thesis, mice deficient for ST8SIA2 (St8sia2-/-) or ST8SIA4 (St8sia4-/-) were assessed neuroanatomically and, in a collaborative effort with the laboratory of A. Zharkovsky (Univ. Tartu, Estonia), subjected to tests of cognition and sensory functions. St8sia2-/- but not St8sia4-/- mice displayed enlarged lateral ventricles, a size reduction of the thalamus accompanied by a smaller internal capsule and a highly disorganized pattern of fibers connecting thalamus and cortex. Reduced levels of the vesicular glutamate transporter VGLUT2 were detected in St8sia2-/- mice, implicating a compromised glutamatergic thalamocortical input into the frontal cortex. Recognition memory was impaired in St8sia2-/- and in St8sia4-/- mice, but only St8sia2-/- mice displayed impaired working memory, deficits in prepulse inhibition, which could be attenuated by clozapine treatment, as well as anhedonic behavior and increased sensitivity to amphetamine-induced hyperlocomotion. These data indicate that reduced polysialylation in St8sia2-/- mice causes deficits in thalamocortical connectivity and a schizophrenia-like behavior. We therefore propose that ST8SIA2 deficiency has the potential to cause a neurodevelopmental predisposition to schizophrenia.