SP Transcription factors in psychotic disorders

Abstract

Psychotic disorders including bipolar disorder and schizophrenia are a leading cause of disability across the world but the underlying pathophysiological mechanisms remain poorly understood. Available treatments are inadequate for some sets of symptoms as is the case for negative symptoms in schizophrenia. Alterations in brain connectivity, synaptic plasticity, N-methyl D aspartate receptor (NMDAR) signalling and calcium homeostasis have been suggested to contribute to these disorders. However, the particular transcriptional programmes altered in these disorders are not fully characterised. Previous data suggested that the transcription factors specificity protein 4 (SP4) and SP1 may be involved in the pathophysiology of psychotic disorders. We hypothesized that the expression and/or function of SP4 and SP1 may be altered in psychotic disorders through the regulation of transcriptional programmes involved in neuronal patterning, synaptic plasticity and glutamate signalling. In this doctoral Thesis we aimed to characterise the contribution of SP4 and SP1 transcription factors to the pathophysiology of psychotic disorders. By using real time quantitative RT-PCR and/or immunoblot techniques, we analysed the expression of SP factors, of SP4 S770 phosphorylation and/or of selected SP-regulated gene targets in at least one of the following substrates: (i) rat cerebellar granule neurons (CGNs), (ii) the postmortem brains of bipolar disorder, schizophrenia and control subjects, (iii) peripheral mononuclear blood cells (PMBC) of first-episode psychosis, and (iv) the rodent hippocampus after NMDAR blockade and antipsychotic treatment.

We found that membrane depolarisation regulates SP4 protein levels in CGNs by preventing SP4 degradation via the ubiquitin-proteasoma pathway and that lithium prevents SP4 degradation and increases SP1 gene expression in non-depolarising conditions. In postmortem human tissue, we found a reduction in protein but not mRNA expression of SP4 and SP1 in the cerebellum in subjects with bipolar disorder and in subjects with more severe negative symptoms in schizophrenia. We have also found reduced expression of protein and mRNA levels of SP4 in the prefrontal cortex in bipolar disorder and of SP1 in the same region in schizophrenia, suggesting a disorder-specific regulation in this area. In contrast, both SP4 and SP1 protein and mRNA levels were increased in the hippocampus in schizophrenia. Consistent with this, we also observed an increase of SP1 and SP4 protein levels in the hippocampus of a mouse model of psychosis, but not in the hippocampus of a rat model of chronic antipsychotic treatment, suggesting that this upregulation may be present from the early stages of psychosis.

We further characterised the phosphorylation of SP4 at serine 770 (S770), which is regulated by membrane depolarisation and NMDAR activity. We found an increase of SP4 S770 phosphorylation in conditions where SP4 protein levels are reduced, namely in the cerebellum of bipolar disorder and of schizophrenia patients with more severe negative symptoms, as well as in PMBC in first-episode psychotic patients. These results suggest that an imbalance in SP4 abundance may be regulated by NMDAR-dependent SP4 phosphorylation in the brain. Moreover, we found that reduced expression of NR2A and DRD2 in the cerebellum of schizophrenia patients correlated with more severe negative symptoms and SP protein levels. Additionally, we show here evidence for an imbalance in the SP4-NWK2-NR1 pathway in the cerebellum of patients with bipolar disorder. This pathway is involved in NR1 subunit availability on the cell surface, suggesting that SP4 could contribute to altered NR1 receptor trafficking in psychotic disorders.

Together, the results presented in this Thesis suggest an imbalance in SP4 and SP1 transcription factors in the brains of patients with bipolar disorder and schizophrenia that may contribute to alterations in NMDAR receptor signalling and thereby to the impaired synaptic plasticity and altered brain connectivity observed in psychotic disorders.