Megumi Adachi

NMDA receptor blockade at rest triggers rapid behavioural antidepressant responses

Clinical studies consistently demonstrate that a single sub-psychomimetic dose of ketamine, an ionotropic glutamatergic NMDAR (N-methyl-D-aspartate receptor) antagonist, produces fast-acting antidepressant responses in patients suffering from major depressive disorder, although the underlying mechanism is unclear. Depressed patients report the alleviation of major depressive disorder symptoms within two hours of a single, low-dose intravenous infusion of ketamine, with effects lasting up to two weeks, unlike traditional antidepressants (serotonin re-uptake inhibitors), which take weeks to reach efficacy. This delay is a major drawback to current therapies for major depressive disorder and faster-acting antidepressants are needed, particularly for suicide-risk patients. The ability of ketamine to produce rapidly acting, long-lasting antidepressant responses in depressed patients provides a unique opportunity to investigate underlying cellular mechanisms. Here we show that ketamine and other NMDAR antagonists produce fast-acting behavioural antidepressant-like effects in mouse models, and that these effects depend on the rapid synthesis of brain-derived neurotrophic factor. We find that the ketamine-mediated blockade of NMDAR at rest deactivates eukaryotic elongation factor 2 (eEF2) kinase (also called CaMKIII), resulting in reduced eEF2 phosphorylation and de-suppression of translation of brain-derived neurotrophic factor. Furthermore, we find that inhibitors of eEF2 kinase induce fast-acting behavioural antidepressant-like effects. Our findings indicate that the regulation of protein synthesis by spontaneous neurotransmission may serve as a viable therapeutic target for the development of fast-acting antidepressants.

Selective Loss of Brain-Derived Neurotrophic Factor in the Dentate Gyrus Attenuates Antidepressant Efficacy

BACKGROUND Brain-derived neurotrophic factor (BDNF) plays an important role in neural plasticity in the adult nervous system and has been suggested as a target gene for antidepressant treatment. The neurotrophic hypothesis of depression suggests that loss of BDNF from the hippocampus contributes to an increased vulnerability for depression, whereas upregulation of BDNF in the hippocampus is suggested to mediate antidepressant efficacy. METHODS We have used a viral-mediated gene transfer approach to assess the role of BDNF in subregions of the hippocampus in a broad array of behavioral paradigms, including depression-like behavior and antidepressant responses. We have combined the adeno-associated virus (AAV) with the Cre/loxP site-specific recombination system to induce the knockout of BDNF selectively in either the CA1 or dentate gyrus (DG) subregions of the hippocampus. RESULTS We show that the loss of BDNF in either the CA1 or the DG of the hippocampus does not alter locomotor activity, anxiety-like behavior, fear conditioning, or depression-related behaviors. However, the selective loss of BDNF in the DG but not the CA1 region attenuates the actions of desipramine and citalopram in the forced swim test. CONCLUSIONS These data suggest that the loss of hippocampal BDNF per se is not sufficient to mediate depression-like behavior. However, these results support the view that BDNF in the DG might be essential in mediating the therapeutic effect of antidepressants.

Original ArticleSelective Loss of Brain-Derived Neurotrophic Factor in the Dentate Gyrus Attenuates Antidepressant Efficacy

BACKGROUND Brain-derived neurotrophic factor (BDNF) plays an important role in neural plasticity in the adult nervous system and has been suggested as a target gene for antidepressant treatment. The neurotrophic hypothesis of depression suggests that loss of BDNF from the hippocampus contributes to an increased vulnerability for depression, whereas upregulation of BDNF in the hippocampus is suggested to mediate antidepressant efficacy. METHODS We have used a viral-mediated gene transfer approach to assess the role of BDNF in subregions of the hippocampus in a broad array of behavioral paradigms, including depression-like behavior and antidepressant responses. We have combined the adeno-associated virus (AAV) with the Cre/loxP site-specific recombination system to induce the knockout of BDNF selectively in either the CA1 or dentate gyrus (DG) subregions of the hippocampus. RESULTS We show that the loss of BDNF in either the CA1 or the DG of the hippocampus does not alter locomotor activity, anxiety-like behavior, fear conditioning, or depression-related behaviors. However, the selective loss of BDNF in the DG but not the CA1 region attenuates the actions of desipramine and citalopram in the forced swim test. CONCLUSIONS These data suggest that the loss of hippocampal BDNF per se is not sufficient to mediate depression-like behavior. However, these results support the view that BDNF in the DG might be essential in mediating the therapeutic effect of antidepressants.

MEF2C, a transcription factor that facilitates learning and memory by negative regulation of synapse numbers and function

Learning and memory depend on the activity-dependent structural plasticity of synapses and changes in neuronal gene expression. We show that deletion of the MEF2C transcription factor in the CNS of mice impairs hippocampal-dependent learning and memory. Unexpectedly, these behavioral changes were accompanied by a marked increase in the number of excitatory synapses and potentiation of basal and evoked synaptic transmission. Conversely, neuronal expression of a superactivating form of MEF2C results in a reduction of excitatory postsynaptic sites without affecting learning and memory performance. We conclude that MEF2C limits excessive synapse formation during activity-dependent refinement of synaptic connectivity and thus facilitates hippocampal-dependent learning and memory.

An Essential Role for Histone Deacetylase 4 in Synaptic Plasticity and Memory Formation

Histone deacetylases (HDACs), a family of enzymes involved in epigenetic regulation, have been implicated in the control of synaptic plasticity, as well as learning and memory. Previous work has demonstrated administration of pharmacological HDAC inhibitors, primarily those targeted to class I HDACs, enhance learning and memory as well as long-term potentiation. However, a detailed understanding of the role of class II HDACs in these processes remains elusive. Here, we show that selective loss of Hdac4 in brain results in impairments in hippocampal-dependent learning and memory and long-term synaptic plasticity. In contrast, loss of Hdac5 does not impact learning and memory demonstrating unique roles in brain for individual class II HDACs. These findings suggest that HDAC4 is a crucial positive regulator of learning and memory, both behaviorally and at the cellular level, and that inhibition of Hdac4 activity may have unexpected detrimental effects to these processes.

Gender-Specific Impact of Brain-Derived Neurotrophic Factor Signaling on Stress-Induced Depression-Like Behavior

BACKGROUND Major depressive disorder is a leading debilitating disease known to occur at a two-fold higher rate in women than in men. The neurotrophic hypothesis of depression suggests that loss of brain-derived neurotrophic factor (BDNF) may increase susceptibility for depression-like behavior, although direct evidence is lacking. METHODS Using the chronic unpredictable stress (CUS) paradigm, we investigated whether male and female mice with inducible BDNF deletion in the forebrain were more susceptible to depression-related behavior. RESULTS We demonstrate that in certain behavioral measures the loss of BDNF lowers the threshold for female mice studied at random throughout estrus to display anxiogenic and anhedonic behaviors after chronic stress compared with wild-type female mice. However, the loss of BDNF in forebrain does not increase the susceptibility to depression-like behavior in male mice. CONCLUSIONS These gender differences suggest a role for BDNF in mediating some aspects of depression-related behavior in females.

MeCP2-Mediated Transcription Repression in the Basolateral Amygdala May Underlie Heightened Anxiety in a Mouse Model of Rett Syndrome

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder that results from loss of function mutations in the methyl-CpG binding protein 2 (MECP2) gene. Using viral-mediated basolateral amygdala (BLA)-specific deletion of Mecp2 in mice, we show that intact Mecp2 function is required for normal anxiety behavior as well as some types of learning and memory. To examine whether these behavioral deficits are the result of impaired transcriptional repression, because Mecp2 is believed to act as a transcriptional repressor in complex with histone deacetylases (HDACs), we infused a HDAC inhibitor chronically into the BLA of wild-type mice. We found that HDAC inhibition produces behavioral deficits similar to those observed after the deletion of Mecp2 in the BLA. These results suggest a key role for Mecp2 as a transcriptional repressor in the BLA in mediating behavioral features of RTT.

VAMP4 directs synaptic vesicles to a pool that selectively maintains asynchronous neurotransmission

Synaptic vesicles in the brain harbor several soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) proteins. With the exception of synaptobrevin2, or VAMP2 (syb2), which is directly involved in vesicle fusion, the role of these SNAREs in neurotransmission is unclear. Here we show that in mice syb2 drives rapid Ca2+-dependent synchronous neurotransmission, whereas the structurally homologous SNARE protein VAMP4 selectively maintains bulk Ca2+-dependent asynchronous release. At inhibitory nerve terminals, up- or downregulation of VAMP4 causes a correlated change in asynchronous release. Biochemically, VAMP4 forms a stable complex with SNAREs syntaxin-1 and SNAP-25 that does not interact with complexins or synaptotagmin-1, proteins essential for synchronous neurotransmission. Optical imaging of individual synapses indicates that trafficking of VAMP4 and syb2 show minimal overlap. Taken together, these findings suggest that VAMP4 and syb2 diverge functionally, traffic independently and support distinct forms of neurotransmission. These results provide molecular insight into how synapses diversify their release properties by taking advantage of distinct synaptic vesicle–associated SNAREs.

A Mouse Model for MeCP2 Duplication Syndrome: MeCP2 Overexpression Impairs Learning and Memory and Synaptic Transmission

Rett syndrome and MECP2 duplication syndrome are neurodevelopmental disorders that arise from loss-of-function and gain-of-function alterations in methyl-CpG binding protein 2 (MeCP2) expression, respectively. Although there have been studies examining MeCP2 loss of function in animal models, there is limited information on MeCP2 overexpression in animal models. Here, we characterize a mouse line with MeCP2 overexpression restricted to neurons (Tau–Mecp2). This MeCP2 overexpression line shows motor coordination deficits, heightened anxiety, and impairments in learning and memory that are accompanied by deficits in long-term potentiation and short-term synaptic plasticity. Whole-cell voltage-clamp recordings of cultured hippocampal neurons from Tau–Mecp2 mice reveal augmented frequency of miniature EPSCs with no change in miniature IPSCs, indicating that overexpression of MeCP2 selectively impacts excitatory synapse function. Moreover, we show that alterations in transcriptional repression mechanisms underlie the synaptic phenotypes in hippocampal neurons from the Tau–Mecp2 mice. These results demonstrate that the Tau–Mecp2 mouse line recapitulates many key phenotypes of MECP2 duplication syndrome and support the use of these mice to further study this devastating disorder.

In Vivo Analysis of MEF2 Transcription Factors in Synapse Regulation and Neuronal Survival

MEF2 (A–D) transcription factors govern development, differentiation and maintenance of various cell types including neurons. The role of MEF2 isoforms in the brain has been studied using in vitro manipulations with only MEF2C examined in vivo. In order to understand specific as well as redundant roles of the MEF2 isoforms, we generated brain-specific deletion of MEF2A and found that Mef2aKO mice show normal behavior in a range of paradigms including learning and memory. We next generated Mef2a and Mef2d brain-specific double KO (Mef2a/dDKO) mice and observed deficits in motor coordination and enhanced hippocampal short-term synaptic plasticity, however there were no alterations in learning and memory, Schaffer collateral pathway long-term potentiation, or the number of dendritic spines. Since previous work has established a critical role for MEF2C in hippocampal plasticity, we generated a Mef2a, Mef2c and Mef2d brain-specific triple KO (Mef2a/c/dTKO). Mef2a/c/d TKO mice have early postnatal lethality with increased neuronal apoptosis, indicative of a redundant role for the MEF2 factors in neuronal survival. We examined synaptic plasticity in the intact neurons in the Mef2a/c/d TKO mice and found significant impairments in short-term synaptic plasticity suggesting that MEF2C is the major isoform involved in hippocampal synaptic function. Collectively, these data highlight the key in vivo role of MEF2C isoform in the brain and suggest that MEF2A and MEF2D have only subtle roles in regulating hippocampal synaptic function.