Currently available antidepressant drugs typically modulate serotonergic, noradrenergic or dopaminergic neurotransmission and take 6-8 weeks to exert their effects. In addition, for each drug, up to 50-60% of patients show inadequate responses in treatment trials. Therefore, it will be important to explore novel antidepressant treatment strategies in pre-clinical studies. It has been suggested that chromatin remodeling mechanisms, including histone modification changes, play an important role for the neurobiology of depressive disorders. The foundations of this hypothesis are built on two observations in the pre-clinical model, (i) commonly used treatments, including antidepressant drugs and electroconvulsive seizures, induce dynamic changes in histone acetylation, and (ii) drugs that inhibit histone deacetylases (HDACs) exhibit antidepressant-like effects, or enhance the therapeutic effect of a conventional antidepressant such as fluoxetine. Based on these findings, it is assumed that drug induced hyperacetylation of histones and the resulting antidepressant effect is linked to "a loosening of chromatin structures and transcriptional activation". However, little is known about the neurological and behavioral phenotypes resulting from experimental manipulation of other histone modifications and histone modifying enzymes, particularly those defined by a genome-wide enrichment pattern that is highly divergent as compared to acetylation. To address these issues, we generated mice with a sustained increase in neuronal expression of Setdb1, encoding a developmentally regulated histone H3-lysine 9 specific histone methyltransferase associated with Sin3a-deacetylase, KAP-1 and NuRD transcriptional repressor complexes. Contrary to our original hypothesis, our preliminary data show that a sustained increase in Setdb1 levels in the forebrain is associated with antidepressant-like changes in behavioral despair and learned helplessness paradigms. Unexpectedly, Setdb1 occupancies in the genome were highly restricted overall, while enriched at several loci encoding glutamate receptor genes. This resulted in decreased levels of the NMDA receptor subunit Grin2b (NR2B) in prefrontal cortex and hippocampus. This finding is interesting because the non-specific NMDA receptor antagonist, ketamine, and the NR2B selective antagonist, CP101,606 elicit rapid antidepressant action in depressed subjects, including a subset of cases who had failed to respond to conventional treatments . The focus of this preclinical proposal is to explore the link between antidepressant-like phenotypes of Setdb1 transgenic mice, and the transcriptional regulation of NMDA receptor subunit Grin2b, and other gene expression changes, and to uncover novel treatment strategies for affective disorders. Our experiments utilize a comprehensive toolbox of in vivo and ex vivo approaches, including (a) multiple lines of genetically engineered mice to target the regulation of histone (H3-lysine 9) methylation in neuronal chromatin, (b) procedures to isolate neuronal chromatin from brain tissue, (c) chromatin immunoprecipitation followed by genome-wide profiling, (d) behavioral assays measuring despair and learned helplessness, anxiety, and learning and memory and (e) functional characterization of NMDA receptors in hippocampus and other brain regions implicated in the neurobiology of depression. This proposal will explore the antidepressant potential of Setdb1-regulated histone methylation, including the potential link to NMDA receptor-mediated signaling, thus offering the perspective of introducing a radically new treatment principle for a major psychiatric disorder.
According to estimates of the World Health Organization, depression could become the second most frequent cause of illness-induced disability by 2020. Currently available antidepressant drugs typically take up to 6-8 weeks to become effective. In addition, for each drug, up to 50-60% of patients show an unsatisfactory antidepressant response to the first drug, requiring switch of medication and other measures. Therefore, it will be very important to explore novel and hitherto unexplored antidepressant treatment strategies in the laboratory. Our research proposal will test the antidepressant potential of enzymes (proteins) that chemically modify histones (which function as the protein "backbone" of chromosomal material). To achieve this, we will change the activity and expression of one of these enzymes, known as "Setdb1" , in genetically engineered mice. Our preliminary data suggest that this approach will explore in this preclinical model a radically new treatment avenue for a major psychiatric disorder, by which a chromatin modifying enzyme (Setdb1) preferentially targets genes that encode certain type of nerve cell receptors (also known as "NMDA receptor"). Then, indirectly, the chromatin changes brought about by Setdb1 result in certain adaptations in the communication between nerve cells, ultimately resulting in an antidepressant-like effect.
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