Schizophrenia is a devastating neuropsychiatric disorder affecting about 1% of the population. It is characterized by positive symptoms, such as delusions and hallucinations, as well as by negative symptoms, including apathy and social withdrawal. Schizophrenia also includes cognitive symptoms, like deficits in attention and working memory. Several models of schizophrenia are currently discussed. Since the risk of developing this disorder is directly associated with the degree of biological relatedness to an affected individual, one model assumes that genetic risk factors contribute to the susceptibility of developing this disorder (reviewed by Lewis and Levitt, 2002). Another paradigm, the neurodevelopmental model of schizophrenia, states that the illness is the end state of series of defective neurodevelopmental processes, starting years before the onset of the symptoms (reviewed by Rapoport et al., 2012).
There are several links of aberrant NCAM expression and altered polysialylation to schizophrenia. Studies reported on elevated levels of soluble NCAM in the prefrontal cortex, hippocampus, and cerebrospinal fluid of schizophrenic patients that correlate with the severity and duration of the disease (Poltorak et al., 1995; van Kammen et al., 1998; Vawter, 2000; Vawter et al., 2001). In addition, reduced polySia levels have been demonstrated in the hilus region of the hippocampus (Barbeau et al., 1995) and, most recently, in layers 4 and 5 of the dorsolateral prefrontal cortex of schizophrenics (Gilabert-Juan et al., 2012).
Genes for NCAM and both polySTs (NCAM1, ST8SIA2 and ST8SIA4) map to chromosomal regions that harbor susceptibility loci for schizophrenia (11q23.1, 15q26, and 5q21 for NCAM1, ST8SIA2, and ST8SIA4) (Lewis et al., 2003; Lindholm et al., 2004; Maziade et al., 2005). Furthermore, single nucleotide polymorphisms in NCAM1 and ST8SIA2, as well as a risk haplotype within ST8SIA2 have been linked to schizophrenia (Arai et al., 2006; Atz et al., 2007; Sullivan et al., 2007; Tao et al., 2007; McAuley et al., 2012). Similarly, associations with autism spectrum disorders and bipolar disorders have been reported (Anney et al., 2010; McAuley et al., 2012) indicating that variation in the ST8SIA2 gene is linked to increased risk to mental illness.
Some of the clinical parameters in schizophrenia match observed deficits in NCAM- and polySia-deficient mice. Ventricular enlargement has been reported in humans (Shenton et al., 2001) and in mice, deficient for the specific NCAM-180 isoform (Wood et al., 1998). In St8sia2-/- St8sia4-/- mice, ventricular dilatations and as well as a progressive hydrocephalus
were observed (Weinhold et al., 2005). Decreased sizes of the corpus callosum and internal capsule in humans (Innocenti et al., 2003; Hulshoff Pol et al., 2004; Douaud et al., 2007;
Mitelman et al., 2007; Begré and Koenig, 2008) are reminiscent to fiber tract deficits in polySia-compromised mice (Hildebrandt et al., 2009; Schiff et al., 2011). Reduced sizes of olfactory bulbs were reported in schizophrenic patients (Turetsky et al., 2000), as well as in Ncam1-/- (Cremer et al., 1994) and St8sia2-/- St8sia4-/- mice (Weinhold et al., 2005). There are also similarities between cognitive impairments in patients (Heinrichs and Zakzanis, 1998) and deficits in learning, memory formation and long term potentiation in Ncam1-/- and St8sia4-/- mice (Cremer et al., 1994; Cremer et al., 1998; Eckhardt et al., 2000; Bukalo et al., 2004; Senkov et al., 2006).
Dysfunction in schizophrenia involves alterations of interneurons (reviewed by Benes and Berretta, 2001; Reynolds et al., 2001; Eyles et al., 2002; Lewis et al., 2005; Lewis and Sweet, 2009; Marín, 2012). Although results from different studies are conflicting, reduced densities of parvalbumin-positive cells, as well as reduced mRNA levels of parvalbumin and somatostatin have been described in the prefrontal cortex of schizophrenic patients (Beasley et al., 2002; Reynolds et al., 2002; Hashimoto et al., 2003; Morris et al., 2008; Fung et al., 2010).
1.7 Objectives
PolySia and NCAM play important roles during nervous system development and seem to be implicated in the pathophysiology of schizophrenia. Some of the defects observed in mice with aberrant polysialylation are reminiscent to the neuroanatomical defects in patients.
Since polySia and NCAM are involved in the migration of olfactory bulb interneuron precursors and alterations of GABAergic interneurons are frequently observed in schizophrenia, one important question is, if any of the major interneuron populations is affected by altered NCAM polysialylation. The first study of my thesis addressed this question by evaluating selected interneuron populations of the olfactory bulb, prefrontal cortex and hippocampus in mouse models with impaired polysialylation or NCAM deficiency.
As mentioned above, polySia plays an important role for the migration of olfactory bulb interneuron precursors and the results from the first study presented in this thesis indicated that tangential migration of a specific population of MGE-derived cortical interneurons may be compromised in polySia-deficient mouse embryos, which could lead to altered interneuron densities in the adult. Therefore, the second study of this thesis addressed the question, if cortical interneuron migration directly depends on polySia and investigated mechanisms that may cause defective precursor migration. PolyST-deficient mice with genetically labeled interneurons were used to assess a possible loss of interneurons in the prefrontal cortex of 3-month-old mice and different in vitro culture techniques of embryonic brains were applied to evaluate changes of interneuron migration upon polySia deficiency in vitro.
Aberrant polysialylation in mice leads to neuroanatomical defects reminiscent to clinical parameters of schizophrenia. The third study addressed the question whether polyST deficiency might cause a schizophrenia-like phenotype in mice. For this purpose, St8sia2 -/-mice, St8sia4-/- mice and their wildtype littermates were assessed neuroanatomically and subjected to tests of cognition and sensory functions.
2 - Results
Chapter 1 - Altered densities and compromised migration of interneurons in the