Angelika Schmitt

Neural stem cell proliferation is decreased in schizophrenia, but not in depression

The phenomenon of adult neurogenesis (AN), that is, the generation of functional neurons from neural stem cells in the dentate gyrus of the hippocampus, has attracted remarkable attention, especially as it was shown that this process is also active in the human brain. Based on animal studies, it has been suggested that reduced AN is implicated in the etiopathology of psychiatric disorders, and that stimulation of AN contributes to the mechanism of action of antidepressant therapies. As data from human post-mortem brain are still lacking, we investigated whether the first step of AN, that is, the level of neural stem cell proliferation (NSP; as quantified by Ki-67 immunohistochemistry), is altered in tissue from the Stanley Foundation Neuropathology Consortium comprising brain specimens from patients with bipolar affective disorder, major depression, schizophrenia as well as control subjects (n=15 in each group). The hypothesis was that stem cell proliferation is reduced in affective disorders, and that antidepressant treatment increases NSP. Neither age, brain weight or pH, brain hemisphere investigated nor duration of storage had an effect on NSP. Only in bipolar disorder, post-mortem interval was a significant intervening variable. In disease, onset of the disorder and its duration likewise did not affect NSP. Also, cumulative lifetime dose of fluphenazine was not correlated with NSP, and presence of antidepressant treatment did not result in an increase of NSP. Concerning the different diagnostic entities, reduced amounts of newly formed cells were found in schizophrenia, but not in major depression. Our findings suggest that reduced NSP may contribute to the pathogenesis of schizophrenia, whereas the rate of NSP does not seem to be critical to the etiopathology of affective disorders, nor is it modified by antidepressant drug treatment.

A neuronal nitric oxide synthase (NOS-I) haplotype associated with schizophrenia modifies prefrontal cortex function

Nitric oxide (NO) is a gaseous neurotransmitter thought to play important roles in several behavioral domains. On a neurobiological level, NO acts as the second messenger of the N-methyl-D-aspartate receptor and interacts with both the dopaminergic as well as the serotonergic system. Thus, NO is a promising candidate molecule in the pathogenesis of endogenous psychoses and a potential target in their treatment. Furthermore, the chromosomal locus of the gene for the NO-producing enzyme NOS-I, 12q24.2, represents a major linkage hot spot for schizophrenic and bipolar disorder. To investigate whether the gene encoding NOS-I (NOS1) conveys to the genetic risk for those diseases, five NOS1 polymorphisms as well as a NOS1 mini-haplotype, consisting of two functional polymorphisms located in the transcriptional control region of NOS1, were examined in 195 chronic schizophrenic, 72 bipolar-I patients and 286 controls. Single-marker association analysis showed that the exon 1c promoter polymorphism was linked to schizophrenia (SCZ), whereas synonymous coding region polymorphisms were not associated with disease. Long promoter alleles of the repeat polymorphism were associated with less severe psychopathology. Analysis of the mini-haplotype also revealed a significant association with SCZ. Mutational screening did not detect novel exonic polymorphisms in patients, suggesting that regulatory rather than coding variants convey the genetic risk on psychosis. Finally, promoter polymorphisms impacted on prefrontal functioning as assessed by neuropsychological testing and electrophysiological parameters elicited by a Go-Nogo paradigm in 48 patients (continuous performance test). Collectively these findings suggest that regulatory polymorphisms of NOS1 contribute to the genetic risk for SCZ, and modulate prefrontal brain functioning.

Serotonin transporter function is modulated by brain-derived neurotrophic factor (BDNF) but not nerve growth factor (NGF).

The serotonin transporter (5-HTT) regulates serotonergic neurotransmission by determining the magnitude and duration of serotonergic responses. We have recently described a polymorphism in the 5-HTT gene promoter (5-HTTLPR) which influences the function of the 5-HTT and is associated with several psychiatric disorders. Immortalized B lymphocytes express the 5-HTT, and a B lymphocyte line has been shown to express the receptor for brain-derived neurotrophic factor, trkB. Since brain-derived neurotrophic factor (BDNF) is a specific growth and differentiation factor for serotonergic neurons, we assessed whether BDNF is able to modulate 5-HTT function in B lymphoblasts. Nerve growth factor (NGF), another neurotrophin which acts via the trkA receptor, was also studied. Eight immortalized B lymphoblast lines were generated and genotyped for the 5-HTTLPR. After treatment with BDNF or NGF, 5-HT uptake and proliferation of the cell lines were assessed. Two of the B cell lines showed a dose-dependent reduction of 5-HT uptake after exposure to BDNF. Both of these cell lines were homozygous for the long allele of the 5-HTTLPR. NGF did not influence 5-HT uptake or cellular proliferation in any of the cell lines. Thus, BDNF but not NGF may influence 5-HT uptake in some B lymphocytes. The fact that regulation of the 5-HTT was observed preferentially in cells of the long/long genotype indicates that presence of a short allele confers reduced regulatory capacity on the 5-HTT. In conclusion, B lymphoblasts represent a practical model for functional regulation of the 5-HTT by neurotrophins in serotonergic neurons.

Expression of the glutamate transporter GLT1 in neural cells of the rat central nervous system: Non-radioactive in situ hybridization and comparative immunocytochemistry

Non-radioactive in situ hybridization using complementary RNA and oligonucleotide probes was applied in order to clearly identify the cell types expressing GLT1 and to show their regional distribution in the central nervous system of the rat. The results were compared with immunocytochemical data achieved using an antibody against a synthetic GLT1 peptide. The study showed that GLT1 was expressed in astrocytes and Bergmann glia which were identified by the detection of an astrocytic marker protein. Additionally, subsets of neurons in different brain regions (e.g., CA3/4 pyramidal cells of the hippocampus, endopiriform nucleus) were labelled by in situ hybridization. In other cell types of the central nervous system (oligodendrocytes, ependymal cells, epithelal cells of the choroid plexus, tanycytes), GLT1 expression was not detectable. The generally dense astrocytic immunolabelling of the gray matter of the brain showed an even higher intensity in regions reported to show high glutamatergic activity and astrocytic glutamate metabolism (e.g., the termination field of the glutamatergic perforant path in the hippocampus). On the basis of the cellular regional distribution of the GLT1 messenger RNA and protein demonstrated in the present study, it is reasonable to assume that this high affinity transporter is of importance for the maintenance of adequate extraneuronal glutamate levels.

Deficiency of brain 5-HT synthesis but serotonergic neuron formation in Tph2 knockout mice

The relative contribution of the two tryptophan hydroxylase (TPH) isoforms, TPH1 and TPH2, to brain serotonergic system function is controversial. To investigate the respective role of TPH2 in neuron serotonin (5-HT) synthesis and the role of 5-HT in brain development, mice with a targeted disruption of Tph2 were generated. The preliminary results indicate that in Tph2 knockout mice raphe neurons are completely devoid of 5-HT, whereas no obvious alteration in morphology and fiber distribution are observed. The findings confirm the exclusive specificity of Tph2 in brain 5-HT synthesis and suggest that Tph2-synthesized 5-HT is not required for serotonergic neuron formation.

Allelic variation of serotonin transporter expression is associated with depression in Parkinson's disease.

Idiopathic Parkinsons disease (PD) is a common neurodegenerative disorder with prominent motor symptoms. However, depression is common in PD, affecting about 40% of PD patients. Since there is extensive evidence of degeneration of serotonin (5HT) neurons and loss of the 5HT transporter (5HTT) in PD, we assessed whether a functional polymorphism in the promoter of the 5HTT gene (5HTT gene-linked polymorphic region, 5HTTLPR), which determines high or low 5HT uptake, is associated with depressive symptomatology in PD patients. We found that patients with the short allele of the 5HTTLPR had significantly higher scores on the Hamilton Depression Scale. A functional promoter polymorphism of the monoamine oxidase A (MAOA) gene showed no association. Thus, the 5HTTLPR but not the MAOA gene promoter-associated polymorphism may be a risk factor for depression in PD patients, while neither polymorphism increases the risk for development of Parkinsons disease itself.

Spatio-temporal expression of tryptophan hydroxylase isoforms in murine and human brain: convergent data from Tph2 knockout mice.

Dysregulation of tryptophan hydroxylase (TPH)-dependent serotonin (5-HT) synthesis, has been implicated in various neuropsychiatric disorders, although the differential expression pattern of the two isoforms is controversial. Here, we report a comprehensive spatio-temporal isoform-specific analysis of TPH1 and TPH2 expression during pre- and postnatal development of mouse brain and in adult human brain. TPH2 expression was consistently detected in the raphe nuclei, as well as in fibers in the deep pineal gland and in small intestine. Although TPH1 expression was found in these peripheral tissues, no significant TPH1 expression was detected in the brain, neither during murine development, nor in mouse and human adult brain. In support of TPH2 specificity in brain 5-HT synthesis, raphe neurons of Tph2 knockout mice were completely devoid of 5-HT, with no compensatory activation of Tph1 expression. In conclusion, our findings indicate that brain 5-HT synthesis across the lifespan is exclusively maintained by TPH2.

Glutamate transporter EAAC1 is expressed in neurons and glial cells in the rat nervous system

Oligonucleotide and cRNA probes were used for non‐radioactive in situ hybridization, carried out to identify the cell types in the nervous system of rat expressing the glutamate transporter EAAC1 mRNA. The results were compared with immunocytochemical data obtained using an antibody against a synthetic EAAC1 peptide. The present data confirm that EAAC1 is expressed in neurons of the CNS. Additionally, our findings indicate the localization of EAAC1 mRNA and protein in peripheral neurons (spinal ganglia) and in glial cells, i.e., oligodendrocytes in various white matter regions of the CNS, ependymal cells, and epithelial cells of the plexus choroideus of the four ventricles, as well as in satellite cells of spinal ganglia. Immunolabeling revealed a preferentially cytoplasmic staining of neurons and glial cells. The cytoplasmic staining was frequently granular, suggesting a localization of EAAC1 protein in vesicle membranes. A membrane localization of EAAC1 was also indicated by Western blotting, which showed immunoreactivity only in the 100,000 × g pellet of brain homogenate. We conclude that the glutamate transporter EAAC1 is not restricted to neurons but may also play an important role in glial cells, particularly in oligodendrocytes. GLIA 27:129–142, 1999.

Neurogenesis and schizophrenia: dividing neurons in a divided mind?

Forty years after the initial discovery of neurogenesis in the postnatal brain of the rat, convincing evidence has been accrued that functional neurons are generated throughout the entire lifespan, particularly in the dentate gyrus (DG) and the subventricular zone (SVZ). This phenomenon has been termed adult neurogenesis (AN) and while it was detected in all examined mammalian species including humans, the physiological role of this process remains unknown. Although a plethora of animal studies indicate an involvement of AN in the pathophysiology of depression, this view has recently kindled considerable controversy. Pertinent studies in humans failed to confirm a role of reduced hippocampal neural stem cell proliferation (NSP) in depression but suggest a contribution to the pathophysiology of schizophrenia. The functional relevance of disturbed AN may encompass erroneous temporal encoding of new memory traces, thereby contributing to cognitive deficits observed in schizophrenia. This AN-hypothesis of schizophrenia is supported by neuroimaging, as well as by several genetically modified rodent models, e.g. reelin and NPAS3 knockout mice. Furthermore, several genes impacting on AN, including NPAS3, were also found to be associated with schizophrenia by case-control studies. In conclusion, several lines of evidence suggest that reduced AN may contribute to the etiopathogenesis of schizophrenic disorders, whereas it does not seem to be a critical risk factor for affective disorders.

Differential effect of endothelial nitric oxide synthase (NOS-III) on the regulation of adult neurogenesis and behaviour.

Although it has been postulated that adult neurogenesis, i.e. the generation of functional neurons from progenitor cells in the mammalian brain, is involved in both the pathogenesis of depressive disorders and the therapeutic effect of antidepressant drugs, its regulation is still poorly understood. Nitric oxide, a gaseous messenger molecule, represents a possible modulating agent as it is involved in learning and memory formation as well as synapto‐ and morphogenesis. Here we investigated whether adult neurogenesis is altered in mice lacking endothelial nitric oxide synthase (NOS‐III). Compared to wild‐type littermates, NOS‐III‐deficient mice showed a significant reduction in neuronal progenitor cell proliferation in the dentate gyrus, suggesting a role for NOS‐III in the stimulation of neuroneogenesis. NeuN, β‐III‐tubulin and GFAP double‐immunolabelling demonstrated that proliferating progenitor cells differentiate preferentially into neurons but not into astrocytes. However, when the survival rate of newly formed cells was examined no difference between wild‐type and NOS‐III knockout mice was found, suggesting that NOS‐III selectively exerts its effects on the proliferation of progenitor cells. This might be mediated by a decrease in vascular endothelial growth factor (VEGF) transcripts in the hippocampus of knockout animals. At the behavioural level, while NOS‐III knockout mice displayed better and faster learning in a learned helplessness paradigm, no depression‐like behaviours were observed. In conclusion, our results indicated that NOS‐III is involved in the proliferation of neuronal progenitor cells, although behavioural analysis does not provide evidence for a pro‐depressive effect of reduced neuroneogenesis.