Studies on experimental neonatal CNS infection : implications for neuropsychiatric disorders

Abstract

Despite extensive research the etiologies of neuropsychiatric disorders such as schizophrenia remain obscure. It is currently believed that complex interactions between a number of genetic and environmental risk factors contribute to the development of these disorders. The studies in this thesis were undertaken to investigate the potential of a neurotropic influenza A virus infection during early life, to induce changes in the brain leading to functional deficits later in life.

Differential expression of transcripts from human endogenous retrovirus (HERV) elements, including syncytin, has been observed in patients with schizophrenia, however, the function of such transcripts in the brain is not known. We therefore investigated if the suggested analogues in mouse were induced and regulated in the same way as human syncytin. Infection with influenza A/WSN/33 virus induced the expression of syncytin B but not syncytin A as well as the suggested regulator glial cells missing 1 (GCM1) in mouse primary neuron and glial cell cultures. GCM1 seemed to regulate the expression of transcripts encoding syncytin B but not A. Thus, syncytin B appears to be an analogue to human syncytin and the function of such expression can be studied in vivo.

To investigate the short- and long-term effects of a neonatal CNS infection with influenza A/WSN/33 virus in vivo, wild type and Tap1-/- mice, lacking CD8+ T cells, were infected at postnatal day 3 or 4. The spread of the infection in the brain and clearance of virus did not differ between the two genotypes of mice, which contrasts observations in adult infected mice. Elevated levels of transcripts encoding IFN-γ and TNF were observed in the brain of both genotypes, however, the Tap1-/- mice seemed to have a more pronounced innate immune response based on the elevated levels of iNOS and syncytin B only in these mice at P24. The kynurenine pathway metabolizes tryptophan into several immunomodulatory and neuroactive metabolites and is suggested to be part of the innate immune response. To investigate the effects of an influenza A/WSN/33 virus infection on the kynurenine pathway we infected mouse primary neuron and glial cell cultures.

The infection induced several transcripts encoding enzymes in this pathway in both cell culture types. Next we investigated the effects of a neonatal infection with influenza A/WSN/33 on the kynurenine pathway in wild type and Tap1-/- mice. The pathway was induced in both genotypes of mice and a transient elevation of kynurenic acid was observed in the brains at P13. Infected Tap1-/- mice exhibited a prolonged induction of the kynurenine pathway compared to infected wild type mice. When investigating the long-term effects of the neonatal infection in adult mice we found infected Tap1-/- mice to exhibit increased rearing and anxiety-like behavior compared to uninfected Tap1-/- mice. In addition, the infected Tap1-/- mice were deficient in a working memory task and had disrupted sensorimotor gating as measured by prepulse inhibition. No differences in the behavior of infected wild type mice compared to uninfected wild type mice were observed. The infected Tap1-/- mice, with working memory deficits, had lower expression transcripts encoding type III neuregulin 1 in the prefrontal cortex. This is in line with a recent report illustrating a role for type III neuregulin 1 in both working memory and sensorimotor gating.

Taken together, the results in the present thesis show that a viral CNS infection in early life has the potential to induce changes in the brain during development and that this leads to behavioral disturbances related to both emotional and cognitive domains as well as to sensorimotor gating disruption, in genetically vulnerable individuals. Thus, our findings support the notion that both genetic and environmental factors may contribute to the development of certain neuropsychiatric disorders.