STAT3-dependent regulation of SERT in the pathophysiology of depression

Monoamine transporters, and in particular the serotonin transporter (SERT, SLC6A4), are the principle site of action of the most commonly prescribed antidepressant drugs. Altered serotonergic neurotransmission is implicated in the pathophysiology of depression. Immune mediators, including proinflammatory cytokines are known to affect brain activity and to thereby modulate mood and associated behavioral states. Thus, a major role has been proposed for the immune system as contributing factor, shaping the susceptibility to, or the resilience against, the development of depression. Our previous experiments indicate that the cytokine interleukin (IL) 6 regulates SERT levels and function in-vitro via recruitment of STAT3 to the SERT promoter and that STAT3 activity is required for the effects of IL6. Moreover, we obtained indirect evidence that this IL6-induced modulation of SERT activity was also relevant for the manifestation of depression-like behavior: IL6 knockout mice displayed a reduction in depression-like behavior and SERT expression was altered in hippocampal tissue obtained from these mice. The objective of the present grant proposal is to understand the implications of STAT3-dependent regulation of serotonin transporter (SERT) function in the pathophysiology of depression at the molecular, cellular and systemic (behavioural) level using specific mouse models. Specifically, we aim to (i) establish a direct cause-and-effect relationship between IL6-dependent regulation of SERT function and depression-like behavior in-vivo by examining the direct behavioral consequences of the action of IL6 on SERT transcription; (ii) investigate whether the repressive function of IL6 on SERT transcription results from a direct inhibitory effect of STAT3 binding at the SERT promoter; (iii) nvestigate whether activity of the STAT3-signaling pathway is required for the control of SERT function and depression-like behavior in- vivo. Results from this project, which is proposing the first detailed analysis of the role of the canonical IL6/STAT3-signaling cascade as modulator of serotonergic neurotransmission may lead to the identification of additional molecular elements to be considered as key players involved in the neurobiological underpinnings of depression. Hence, additional disease susceptibility factors as well as alternative targets for antidepressant drug therapy may be revealed.

The signal transducer and activator of transcription 3 (STAT3) signalling pathway is activated through phosphorylation by Janus kinases in response to a diverse set of immunogenic and non-immunogenic triggers. Several distinct lines of evidence propose an intricate involvement of STAT3 in neural function relevant to behaviour in health and disease. Our previous experiments indicated that the cytokine interleukin (IL) 6 regulates serotonin transporter (SERT) levels and function in-vitro via recruitment of STAT3 to the SERT promoter. Altered serotonergic neurotransmission is highly implicated in the pathophysiology of depression. However, the interplay between the STAT3 signaling pathway and the serotonin system and its relevance for the regulation of emotional behavioural functions has not been explored so far. The objective of the present grant proposal was to understand the implications of STAT3-dependent regulation of serotonergic function in the pathophysiology of affective disorders at the molecular, cellular and systemic (behavioural) level using specific mouse models. In the course of this project we developed a method for [11C]DASB-PET region-specific measurements of SERT density in-vivo. We further established this non-invasive approach for the long-term, repeated evaluation of the dynamic expression of brain SERT in mouse and an animal model of depression. Focussing on the midbrain serotonergic system, a central hub for the regulation of emotions, we examined the relevance of STAT3 signalling for emotional behaviour in mice by selectively knocking down raphe STAT3 expression using germline genetic (STAT3 KO) and viral-mediated approaches. Mice locally lacking STAT3 presented with reduced negative behavioural reactivity and a blunted response to the sensitising effects of amphetamine, alongside alterations in midbrain neuronal firing activity of serotonergic neurons and transcriptional control of gene networks relevant for neuropsychiatric disorders. Viral knockdown of dorsal raphe (DR) STAT3 phenocopied the behavioural alterations of STAT3 KO mice, excluding a developmentally determined effect and suggesting that disruption of STAT3 signalling in the DR of adult mice is sufficient for the manifestation of behavioural traits relevant to psychopathology. Collectively, these results suggest STAT3 as a molecular gate for the control of behavioural reactivity, constituting a mechanistic link between the immune system, serotonergic neurotransmission and psychopathology.