Maureen Cabatic

Constant Darkness Induces IL-6-Dependent Depression-Like Behavior through the NF-κB Signaling Pathway

Substantial experimental evidence indicates a major role for the circadian system in mood disorders. Additionally, proinflammatory cytokines have been proposed to be involved in the pathogenesis of depression. However, the molecular elements determining the functional interplay between these two systems in depression have not been described as yet. Here we investigate whether long-term light deprivation in the constant darkness (DD) paradigm affects depression-like behavior in mice and concomitantly modulates the levels of proinflammatory cytokines. We find that after 4 weeks of DD, mice display depression-like behavior, which is paralleled by reduced hippocampal cell proliferation. This chronobiologically induced depressive state is associated with elevated levels of plasma IL-6 (interleukin-6) and IL-6 and Il1-R1 (interleukin 1 receptor, type I) protein levels in the hippocampus and also alters hippocampal protein levels of the clock genes per2 and npas2. Using pharmacological blockers of the NF-κB pathway, we provide evidence that the effects of DD on depression-like behavior, on hippocampal cell proliferation, on altered expressional levels of brain and plasma IL-6, and on the modulation of clock gene expression are mediated through NF-κB signaling. Moreover, NF-κB activity is enhanced in hippocampal tissue of DD mice. Mice with a deletion of IL-6, one of the target genes of NF-κB, are resistant to DD-induced depression-like behavior, which suggests a pivotal role for this cytokine in the constant darkness mouse model of depression. We here first describe some of the molecular elements bridging chronobiological and inflammatory processes in the constant darkness mouse model of depression.

STAT3 controls IL6-dependent regulation of serotonin transporter function and depression-like behavior

Experimental evidence suggests a role for the immune system in the pathophysiology of depression. A specific involvement of the proinflammatory cytokine interleukin 6 (IL6) in both, patients suffering from the disease and pertinent animal models, has been proposed. However, it is not clear how IL6 impinges on neurotransmission and thus contributes to depression. Here we tested the hypothesis that IL6-induced modulation of serotonergic neurotransmission through the STAT3 signaling pathway contributes to the role of IL6 in depression. Addition of IL6 to JAR cells, endogenously expressing SERT, reduced SERT activity and downregulated SERT mRNA and protein levels. Similarly, SERT expression was reduced upon IL6 treatment in the mouse hippocampus. Conversely, hippocampal tissue of IL6-KO mice contained elevated levels of SERT and IL6-KO mice displayed a reduction in depression-like behavior and blunted response to acute antidepressant treatment. STAT3 IL6-dependently associated with the SERT promoter and inhibition of STAT3 blocked the effect of IL6 in-vitro and modulated depression-like behavior in-vivo. These observations demonstrate that IL6 directly controls SERT levels and consequently serotonin reuptake and identify STAT3-dependent regulation of SERT as conceivable neurobiological substrate for the involvement of IL6 in depression.

Focal Adhesion Kinase Regulates Neuronal Growth, Synaptic Plasticity and Hippocampus-Dependent Spatial Learning and Memory

The focal adhesion kinase (FAK) is a non-receptor tyrosine kinase abundantly expressed in the mammalian brain and highly enriched in neuronal growth cones. Inhibitory and facilitatory activities of FAK on neuronal growth have been reported and its role in neuritic outgrowth remains controversial. Unlike other tyrosine kinases, such as the neurotrophin receptors regulating neuronal growth and plasticity, the relevance of FAK for learning and memory in vivo has not been clearly defined yet. A comprehensive study aimed at determining the role of FAK in neuronal growth, neurotransmitter release and synaptic plasticity in hippocampal neurons and in hippocampus-dependent learning and memory was therefore undertaken using the mouse model. Gain- and loss-of-function experiments indicated that FAK is a critical regulator of hippocampal cell morphology. FAK mediated neurotrophin-induced neuritic outgrowth and FAK inhibition affected both miniature excitatory postsynaptic potentials and activity-dependent hippocampal long-term potentiation prompting us to explore the possible role of FAK in spatial learning and memory in vivo. Our data indicate that FAK has a growth-promoting effect, is importantly involved in the regulation of the synaptic function and mediates in vivo hippocampus-dependent spatial learning and memory.

Fluoxetine normalizes disrupted light-induced entrainment, fragmented ultradian rhythms and altered hippocampal clock gene expression in an animal model of high trait anxiety- and depression-related behavior

ABSTRACT Introduction Disturbances of circadian rhythms are a key symptom of mood and anxiety disorders. Selective serotonin reuptake inhibitors (SSRIs) - commonly used antidepressant drugs – also modulate aspects of circadian rhythmicity. However, their potential to restore circadian disturbances in depression remains to be investigated. Materials and methods The effects of the SSRI fluoxetine on genetically based, depression-related circadian disruptions at the behavioral and molecular level were examined using mice selectively bred for high anxiety-related and co-segregating depression-like behavior (HAB) and normal anxiety/depression behavior mice (NAB). Results The length of the circadian period was increased in fluoxetine-treated HAB as compared to NAB mice while the number of activity bouts and light-induced entrainment were comparable. No difference in hippocampal Cry2 expression, previously reported to be dysbalanced in untreated HAB mice, was observed, while Per2 and Per3 mRNA levels were higher in HAB mice under fluoxetine treatment. Discussion The present findings provide evidence that fluoxetine treatment normalizes disrupted circadian locomotor activity and clock gene expression in a genetic mouse model of high trait anxiety and depression. An interaction between the molecular mechanisms mediating the antidepressant response to fluoxetine and the endogenous regulation of circadian rhythms in genetically based mood and anxiety disorders is proposed.

Maternal immune activation epigenetically regulates hippocampal serotonin transporter levels

Major depressive disorder (MDD) is one of the most debilitating psychiatric diseases, affecting a large percentage of the population worldwide. Currently, the underlying pathomechanisms remain incompletely understood, hampering the development of critically needed alternative therapeutic strategies, which further largely depends on the availability of suitable model systems. Here we used a mouse model of early life stress – a precipitating factor for the development of MDD – featuring infectious stress through maternal immune activation (MIA) by polyinosinic:polycytidilic acid (Poly(I:C)) to examine epigenetic modulations as potential molecular correlates of the alterations in brain structure, function and behavior. We found that in adult female MIA offspring anhedonic behavior was associated with modulations of the global histone acetylation profile in the hippocampus. Morevoer, specific changes at the promoter and in the expression of the serotonin transporter (SERT), critically involved in the etiology of MDD and pharmacological antidepressant treatment were detected. Furthermore, an accompanying reduction in hippocampal levels of histone deacetylase (HDAC) 1 was observed in MIA as compared to control offspring. Based on these results we propose a model in which the long-lasting impact of MIA on depression-like behavior and associated molecular and cellular aberrations in the offspring is brought about by the modulation of epigenetic processes and consequent enduring changes in gene expression. These data provide additional insights into the principles underlying the impact of early infectious stress on the development of MDD and may contribute to the development of new targets for antidepressant therapy.

A role for glucocorticoid-signaling in depression-like behavior of gastrin-releasing peptide receptor knock-out mice

Abstract Background. The gastrin-releasing peptide receptor (GRPR) is highly expressed in the limbic system, where it importantly regulates emotional functions and in the suprachiasmatic nucleus, where it is central for the photic resetting of the circadian clock. Mice lacking GRPR presented with deficient light-induced phase shift in activity as well altered emotional learning and amygdala function. The effect of GRPR deletion on depression-like behavior and its molecular signature in the amygdala, however, has not yet been evaluated. Methods. GRPR knock-out mice (GRPR-KO) were tested in the forced-swim test and the sucrose preference test for depression-like behavior. Gene expression in the basolateral nucleus of the amygdala was evaluated by micorarray analysis subsequent to laser-capture microdissection-assisted extraction of mRNA. The expression of selected genes was confirmed by RT-PCR. Results. GRPR-KO mice were found to present with increased depression-like behavior. Microarray analysis revealed down-regulation of several glucocorticoid-responsive genes in the basolateral amygdala. Acute administration of dexamethasone reversed the behavioral phenotype and alterations in gene expression. Discussion. We propose that deletion of GRPR leads to the induction of depression-like behavior which is paralleled by dysregulation of amygdala gene expression, potentially resulting from deficient light-induced corticosterone release in GRPR-KO.

The brain-tumor related protein podoplanin regulates synaptic plasticity and hippocampus-dependent learning and memory.

Abstract Introduction: Podoplanin is a cell-surface glycoprotein constitutively expressed in the brain and implicated in human brain tumorigenesis. The intrinsic function of podoplanin in brain neurons remains however uncharacterized. Materials and methods: Using an established podoplanin-knockout mouse model and electrophysiological, biochemical, and behavioral approaches, we investigated the brain neuronal role of podoplanin. Results: Ex-vivo electrophysiology showed that podoplanin deletion impairs dentate gyrus synaptic strengthening. In vivo, podoplanin deletion selectively impaired hippocampus-dependent spatial learning and memory without affecting amygdala-dependent cued fear conditioning. In vitro, neuronal overexpression of podoplanin promoted synaptic activity and neuritic outgrowth whereas podoplanin-deficient neurons exhibited stunted outgrowth and lower levels of p-Ezrin, TrkA, and CREB in response to nerve growth factor (NGF). Surface Plasmon Resonance data further indicated a physical interaction between podoplanin and NGF. Discussion: This work proposes podoplanin as a novel component of the neuronal machinery underlying neuritogenesis, synaptic plasticity, and hippocampus-dependent memory functions. The existence of a relevant cross-talk between podoplanin and the NGF/TrkA signaling pathway is also for the first time proposed here, thus providing a novel molecular complex as a target for future multidisciplinary studies of the brain function in the physiology and the pathology. Key messages Podoplanin, a protein linked to the promotion of human brain tumors, is required in vivo for proper hippocampus-dependent learning and memory functions. Deletion of podoplanin selectively impairs activity-dependent synaptic strengthening at the neurogenic dentate-gyrus and hampers neuritogenesis and phospho Ezrin, TrkA and CREB protein levels upon NGF stimulation. Surface plasmon resonance data indicates a physical interaction between podoplanin and NGF. On these grounds, a relevant cross-talk between podoplanin and NGF as well as a role for podoplanin in plasticity-related brain neuronal functions is here proposed.

Flotillin-1 interacts with the serotonin transporter and modulates chronic corticosterone response.

Aberrant serotonergic neurotransmission in the brain is considered at the core of the pathophysiological mechanisms involved in neuropsychiatric disorders. Gene by environment interactions contribute to the development of depression and involve modulation of the availability and functional activity of the serotonin transporter (SERT). Using behavioral and in vivo electrophysiological approaches together with biochemical, molecular‐biological and molecular imaging tools we establish Flotillin‐1 (Flot1) as a novel protein interacting with SERT and demonstrate its involvement in the response to chronic corticosterone (CORT) treatment. We show that genetic Flot1 depletion augments chronic CORT‐induced behavioral despair and describe concomitant alterations in the expression of SERT, activity of serotonergic neurons and alterations of the glucocorticoid receptor transport machinery. Hence, we propose a role for Flot1 as modulatory factor for the depressogenic consequences of chronic CORT exposure and suggest Flotillin‐1‐dependent regulation of SERT expression and activity of serotonergic neurotransmission at the core of the molecular mechanisms involved.

Corrigendum: STAT3 controls IL6-dependent regulation of serotonin transporter function and depression-like behaviour.

Corrigendum: STAT3 controls IL6-dependent regulation of serotonin transporter function and depression-like behaviour