Molecular Brain

MicroRNAs (miRNAs) of the miR-200 family specifically miR-141 and miR-200c regulate neurogenesis, differentiation, and epithelial-mesenchymal transitions in development and several diseases including cancer and stroke. The STOCK Mirc13tm1Mtm /Mmjax mouse line, which targets the miR-141/200c cluster, was originally generated and described by Park et al. 2012 as a conditional "knockout-first" allele requiring a two-step breeding strategy: FLP recombination to excise lacZ/neo cassettes followed by Cre recombination to delete the floxed miRNA cluster (1). However, subsequent studies either bypassed this step and reported knockouts based on direct crosses with Cre mouse lines, leaving residual lacZ/neo sequences that may silence upstream elements or introduce transcriptional artifacts or rare studies used less efficient FLPe Deleter mice. Here, we present a detailed and refined strategy to conditional miR-141/200c knockouts mice using FLPo Deleter mice to efficiently eliminate lacZ/neo cassettes. Our approach not only confirmed complete deletion of miR-141 and miR-200c in various organs such olfactory bulbs and lungs where these miRNAs are robustly expressed using various approach such as genotyping qPCR validation and in situ hybridization but showed that without the use of FLPo deleter mice deletion of miR-141/200c cluster amy also lead to loss of several close proximity physiologically important genes such as ptpn6, phb2, atn1 and eno1. By restoring a clean floxed allele using FLPo deleter mice prior to Cre deletion, we establish a reliable and interpretable mouse model for dissecting the roles of the miR-141/200c cluster miRNA in various disease models.

AbstractExpression of immediate early genes (IEGs) is critical for memory formation and has been widely used to identify the neural substrate of memory traces, termed memory engram cells. Functions of IEGs have been known to be different depending on their types. However, there is limited knowledge about the extent to which different types of IEGs are selectively or concurrently involved in the formation of memory engram. To address this question, we investigated the combinative expression of c-Fos, Arc, and Npas4 proteins using immunohistochemistry following aversive and rewarding experiences across subregions in the prefrontal cortex (PFC), basolateral amygdala (BLA), hippocampal dentate gyrus (DG), and retrosplenial cortex (RSC). Using an automated cell detection algorithm, we found that expression patterns of c-Fos, Npas4, and Arc varied across different brain areas, with a higher increase of IEG expressing cells in the PFC and posterior BLA than in the DG. The combinative expression patterns, along with their learning-induced changes, also differed across brain areas; the co-expression of IEGs increased in the PFC and BLA following learning whereas the increase was less pronounced in the DG and RSC. Furthermore, we demonstrate that different area-to-area functional connectivity networks were extracted by different IEGs. These findings provide insights into how different IEGs and their combinations identify engram cells, which will contribute to a deeper understanding of the functional significance of IEG-tagged memory engram cells.

Synapse-to-nucleus signaling regulates activity-dependent synaptic plasticity underlying memory by linking N-methyl-D-aspartate (NMDA) glutamate receptors (GluN) to gene transcription mediated by the transcription factor cAMP-response element binding protein (CREB), but the underlying gene programs mediating potentiation at excitatory synapses are unknown. Here, we analyzed genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) datasets of mouse and human CREB and the synaptonuclear factor CREB-regulated transcription coactivator1 (CRTC1) to identify relevant target genes and biological pathways coupling neuronal activity to synaptic function/plasticity. Our analyses indicate that CRTC1 specifically couples neuronal activity with synaptic plasticity by binding to conserved promoters of CREB target genes comprising inducible transcription factors (including c fos, Crem, Npas4 and Nr4a1-3), and neuronal excitability and plasticity genes, including Ntrk2, Homer1, Dlg4 (PSD-95) and the NMDA receptor subunit Grin1 (GluN1). CRTC1/CREB target genes were highly enriched in gene ontology (GO) nuclear terms, including several members of the CREB family, and transcriptional modulators and repressors. Interestingly, GO enrichment and protein-protein interaction (PPI) network analyses revealed that genes mediating synapse-to-nucleus signaling (including most known synaptonuclear factors and direct interacting modulators) are collectively regulated by CREB/CRTC1, and that protein kinase C (PKC) is a key interactor of the CRTC1/14-3-3 complex at synapses. In agreement with these in silico analyses, we show that CRTC1 regulates synaptic activity-dependent phosphorylation and synaptic recruitment of GluN1 mediated by PKC in hippocampal neurons, and that PKC activation reverses NMDA receptor-mediated currents and long-term potentiation (LTP) deficits caused by CRTC1 silencing in the hippocampus. Consistent with genomics and functional data, morphological and behavioral analyses show crucial roles of CRTC1 on dendritic spine structure, plasticity, and hippocampal-dependent associative memory. Our results support a model in which neuronal activity and synaptic inputs are integrated in the nucleus through conserved CREB/CRTC1-regulated transcriptional programs sustaining global synapse-to-nucleus signaling pathways impacting on synaptic plasticity and memory.

The genetic cause of Huntington disease (HD) is attributed to the N-terminal polyglutamine expansion of huntingtin (mHTT). mHTT, which is a ubiquitously expressed protein, induces noticeable damage to the striatum, which affects motor, psychiatric, and cognitive functions in HD individuals. Although inflammatory responses apparently precede striatal damage and an overall progression of HD, the molecular mechanisms at work remain unclear (1-6). In this study, we found that cyclic GMP-AMP synthase (cGAS), a DNA sensor, which regulates inflammation, autophagy, and cellular senescence (7-9), plays a critical role in the inflammatory responses of HD. Ribosome profiling analysis reveals that cGAS mRNA has a high ribosome occupancy at exon 1 and codon-specific pauses at positions 171 (CCG) and 172 (CGT) in HD cells, compared to the control, indicating an altered cGAS expression. Accordingly, cGAS protein levels and activity, as measured by phosphorylation of stimulator of interferon genes (STING) or TANK-binding kinase 1 (TBK1), are increased in HD striatal cells, mouse Q175HD striatum and human postmortem HD striatum, compared to the healthy control. Furthermore, cGAS-dependent inflammatory genes such as Cxcl10 and Ccl5 show enhanced ribosome occupancy at exon 3 and exon 1, respectively and are upregulated in HD cells. Depletion of cGAS via CRISPR/Cas-9 diminishes cGAS activity and decreases expression of inflammatory genes while suppressing the autophagy upregulation in HD cells. We additionally detected the presence of numerous micronuclei, a known inducer of cGAS, in the cytoplasm of HD cells. Overall, the data indicates that cGAS is highly upregulated in HD and mediates inflammatory and autophagy responses. Thus, targeting cGAS may offer therapeutic benefits in HD.

The hippocampus plays a critical role in storing and retrieving spatial information. By targeting the dorsal hippocampus and manipulating specific "candidate" molecules using pharmacological and genetic manipulations, we have previously discovered that long-term active place avoidance memory requires transient activation of particular molecules in dorsal hippocampus. These molecules include amongst others, the persistent kinases Ca-calmodulin kinase II (CaMKII) and the atypical protein kinase C isoform PKC{iota} /{lambda} for acquisition of the conditioned behavior, whereas persistent activation of the other atypical PKC, protein kinase M zeta (PKM{zeta}) is necessary for maintaining the memory for at least a month. It nonetheless remains unclear what other molecules and their interactions maintain active place avoidance long-term memory, and the candidate molecule approach is both impractical and inadequate to identify new candidates since there are so many to survey. Here we use a complementary approach to identify candidates by transcriptional profiling of hippocampus subregions after formation of the long-term active place avoidance memory. Interestingly, 24-h after conditioning and soon after expressing memory retention, immediate early genes were upregulated in the dentate gyrus but not Ammons horn of the memory expressing group. In addition to determining what genes are differentially regulated during memory maintenance, we performed an integrative, unbiased survey of the genes with expression levels that covary with behavioral measures of active place avoidance memory persistence. Gene Ontology analysis of the most differentially expressed genes shows that active place avoidance memory is associated with activation of transcription and synaptic differentiation in dentate gyrus but not CA3 or CA1, whereas hypothesis-driven candidate molecule analyses identified insignificant changes in the expression of many LTP-associated molecules in the various hippocampal subfields, nor did they covary with active place avoidance memory expression, ruling out strong transcriptional regulation but not translational regulation, which was not investigated. These findings and the data set establish an unbiased resource to screen for molecules and evaluate hypotheses for the molecular components of a hippocampus-dependent, long-term active place avoidance memory.

Prosapip1 is a brain-specific protein localized to the postsynaptic density, where it promotes dendritic spine maturation in primary hippocampal neurons. However, nothing is known about the role of Prosapip1 in vivo. To examine this, we utilized the Cre-loxP system to develop a Prosapip1 neuronal knockout mouse. We found that Prosapip1 controls the synaptic localization of its binding partner SPAR, along with PSD-95 and the GluN2B subunit of the NMDA receptor (NMDAR) in the dorsal hippocampus (dHP). We next sought to identify the potential contribution of Prosapip1 to the activity and function of the NMDAR and found that Prosapip1 plays an important role in NMDAR-mediated transmission and long-term potentiation (LTP) in the CA1 region of the dHP. As LTP is the cellular hallmark of learning and memory, we examined the consequences of neuronal knockout of Prosapip1 on dHP-dependent memory. We found that global or dHP-specific neuronal knockout of Prosapip1 caused a deficit in learning and memory whereas developmental, locomotor, and anxiety phenotypes were normal. Taken together, Prosapip1 in the dHP promotes the proper localization of synaptic proteins which, in turn, facilitates LTP driving recognition, social, and spatial learning and memory.

Alterations of Ariadne RBR E3 Ubiquitin Protein Ligase 1 (ARIH1), a human homologue of Drosophila Ari, have been associated with a number of human diseases. Given the importance of ubiquitin-proteasome system in learning and memory, whether ARIH1 involves in the process has not been explored. Here we report that ARIH1-deficent mice exhibited a defect in learning and memory evidenced in Morris water maze and in novel object recognition tests without changes in basal motor activity, anxiety, and depressive behaviors. We found that ARIH1 deficiency resulted in an upregulation of G protein-gated inwardly rectifying potassium channel 2 (GIRK2) in dorsal hippocampus that was attributed to the impaired ubiquitination and degradation. Locally injection of ARIH1-expressing lentivirus to restore the ARIH1 expression of dorsal hippocampus in ARIH1+/- mice restored the impaired learning and memory. Moreover, selective knockdown ARIH1 in dorsal hippocampal calcium-calmodulin-dependent protein kinase II (CaMKII)-expressing neurons, but not for parvalbumin+ (PV) or somatostatin+ (SST) neurons, in naive mice was sufficient to mimic the damage in learning and memory of ARIH1+/- mice. Lastly, we demonstrated that systemically or locally inhibition of GIRK activity was able to improve ARIH1 deficiency-induced decline of learning and memory in ARIH1+/- mice. The present study discovered the clear role of ARIH1 in mediating learning and memory, defect of ARIH1 resulted in upregulation of GIRK2 in hippocampal CaMKII-expressing neurons via modulating the ubiquitination and degradation GIRK2.

Kv4.2 channels, which mediate A-type K+ current, exert significant influence on synaptic input signals and synaptic plasticity in the principal cells of the hippocampus. While their influence on activity-dependent regulation of synaptic response is well-established, the impact of Kv4.2 channels on baseline synaptic strength remains elusive. To investigate this, we selectively inhibited postsynaptic Kv4.2 by introducing Kv4.2 antibodies into the hippocampal granule cells and evaluated its impact on the baseline synaptic transmission. Our results demonstrated that Kv4.2 inhibitions led to notable increase in the amplitude of AMPA receptor (AMPAR)-mediated synaptic currents, and this effect was in parallel with the Kv4.2 expression level at dendritic regions. This Kv4.2-dependent synaptic potentiation was effectively abolished by intracellular 10 mM BAPTA or block of R-type calcium channels (RTCC) and downstream signaling molecules including protein kinase A (PKA) and protein kinase C (PKC). Importantly, Kv4.2 inhibitions did not occlude further synaptic strengthening high frequency stimulation, suggesting that synaptic strength regulation by Kv4.2 s distinct from the mechanism of long-term potentiation. Our study highlights the role of Kv4.2 in regulating the baseline synaptic strength, where Kv4.2-mediated inhibition of RTCC is crucial.

cAMP signaling is critical for memory consolidation and certain of forms long-term potentiation (LTP). Phosphodiesterases (PDEs), enzymes that degrade the second messenger cAMP and cGMP, are highly conserved during evolution and represent a unique set of drug targets, given the involvement of these enzymes in several pathophysiological states including brain disorders. The PDE4 family of cAMP selective PDEs, exert regulatory roles in memory and synaptic plasticity, but the specific roles of distinct PDE4 isoforms in these processes are poorly understood. Building on our previous work demonstrating that spatial and contextual memory deficits were caused by expressing selectively the long isoform of the PDE4A subfamily, PDE4A5, in hippocampal excitatory neurons, we now investigate the effects of PDE4A isoforms on different cAMP-dependent forms of LTP. We find that PDE4A5 impairs long-lasting LTP induced by theta burst stimulation (TBS) while sparing long-lasting LTP induced by spaced 4-train stimulation (4X100Hz). This effect requires the unique N-terminus of PDE4A5 and is specific to this long isoform. Targeted overexpression of PDE4A5 in area CA1 is sufficient to impair TBS-LTP, suggesting that cAMP levels in the postsynaptic neuron are critical for TBS-LTP. Our results shed light on the inherent differences among the PDE4A subfamily isoforms, emphasizing the importance of the long isoforms, which have a unique N-terminal region. Advancing our understanding of the function of specific PDE isoforms will pave the way for developing isoform-selective approaches to treat the cognitive deficits that are debilitating aspects of psychiatric, neurodevelopmental, and neurodegenerative disorders. Key PointsO_LIHippocampal overexpression of a PDE4A subfamily long isoform, PDE4A5, but not a short isoform PDE4A1, impairs spatial and contextual fear memory and the N-terminus of PDE4A5 is important for this effect. C_LIO_LIHippocampal overexpression of PDE4A isoforms, PDE4A1 and PDE4A5 do not impair LTP induced by spaced tetanic stimulation at the CA3-CA1 synapses. C_LIO_LIHippocampal overexpression of PDE4A5, but not PDE4A1 or the N-terminus truncated PDE4A5 (PDE4A5{Delta}4) selectively impairs LTP induced by theta burst stimulation (TBS) at the CA3-CA1 synapses and expression of PDE4A5 in area CA1 is sufficient for the TBS-LTP deficit. C_LIO_LIThese results suggest that PDE4A5, through its N-terminus, regulates cAMP pools that are critical for memory consolidation and expression of TBS-LTP at the CA3-CA1 synapses. C_LI O_FIG O_LINKSMALLFIG WIDTH=148 HEIGHT=200 SRC="FIGDIR/small/592525v1_ufig1.gif" ALT="Figure 1"> View larger version (53K): org.highwire.dtl.DTLVardef@1ee4ccdorg.highwire.dtl.DTLVardef@1f43c3corg.highwire.dtl.DTLVardef@19eb64dorg.highwire.dtl.DTLVardef@d2bab0_HPS_FORMAT_FIGEXP M_FIG GRAPHICAL ABSTRACT Spaced tetanic stimulation and TBS induce cAMP synthesis and activation of PKA to promote signaling cascades that facilitate expression of long-lasting LTP at the CA3-CA1 synapses. PDE4A5 overexpression in the hippocampus selectively impairs cAMP and PKA dependent TBS-LTP at the CA3-CA1 synapses, while sparing LTP induced by spaced tetanization. C_FIG

Extensive research has focused on extracellular-signal regulated kinase (ERK) 1/2 phosphorylation across various memory and plasticity models. However, the precise mechanisms linking ERK activity to memory stabilization and restabilization are still poorly understood, and the role of ERK1/2 dimerization remains unexplored. ERK dimerization is essential for the binding and activation of cytoplasmic targets, many of which are involved in memory and plasticity. In this study, we investigated the role of ERK2 dimerization in long-term memory and synaptic plasticity. We found that reactivation of a weak inhibitory avoidance (wIA) memory led to a significant reduction in hippocampal ERK2 dimerization. Furthermore, intrahippocampal infusion of DEL-22379 (DEL), an ERK dimerization inhibitor, following memory reactivation had a bidirectional effect: it blocked the reconsolidation of a strong inhibitory avoidance (sIA) memory but enhanced the reconsolidation of a wIA memory. Moreover, DEL administration blocked hippocampal ERK2 dimerization in vivo and impaired high-frequency stimulation-induced long-term potentiation (LTP) in hippocampal slices. These findings demonstrate that ERK2 dimerization occurs in the intact mouse nervous system and plays a pivotal role in plasticity and memory. While further research is needed, this study highlights the relevance of ERK dimerization in these processes.

In this study we tested the hypothesis that precision medicine guided therapy targeting glutamatergic neurotransmission could rescue behavioral deficits exhibited by mice carrying a specific mutation in the Iqsec2 gene. The IQSEC2 protein plays a key role in glutamatergic synapses and mutations in the IQSEC2 gene are a frequent cause of neurodevelopmental disorders. We have recently reported on the molecular pathophysiology of one such mutation A350V and demonstrated that this mutation downregulates AMPA type glutamatergic receptors (AMPAR) in A350V mice. Here we sought to identify behavioral deficits in A350V mice and hypothesized that we could rescue these deficits by PF-4778574, a positive AMPAR modulator. We found that A350V Iqsec2 mice exhibit specific deficits in sex preference and emotional state preference behaviors as well as in vocalizations when encountering a female mouse. The social discrimination deficits, but not the impaired vocalization, were rescued with PF-4778574. We conclude that social behavior deficits associated with the A350V Iqsec2 mutation may be rescued by enhancing AMPAR mediated synaptic transmission.

Dendritic spines are plastic structures exhibiting a high degree of morphological variability. Certain morphometric parameters, such as volume, positively correlate with the strength of the synapse in which they participate. Memories, too, are subject to change over time and with experiences. In particular, the presence of a reminder of a learning event can trigger the labilization of the memory trace, followed by a re-stabilization process termed reconsolidation. The underlying mechanisms behind the labilization/reconsolidation processes are of great interest, as they are thought of as possible targets for the treatment of post-traumatic stress disorders. Dendritic spines have long been considered the physical sites for memory formation and storage. Our work aimed at studying the long-term spine morphological plasticity associated with labilization/reconsolidation in the dorsal hippocampus, a brain region relevant for the formation of contextual memories. Our results suggest that labilization/reconsolidation does not affect spine density, but rather induces changes in spine morphology. Furthermore, we show that some of these changes are prevented by the inhibition of the transcription factor NF-{kappa}B inhibition. Finally, we found that NF-{kappa}B negative modulation also affects spine morphology in animals that were not exposed to recall but have undergone the training session, suggesting that there may be a late surge of NF-{kappa}B activity resulting from the consolidation itself.

The rodent hippocampus is a spatially organized neuronal network that supports the formation of spatial and episodic memories. We conducted bulk RNA sequencing and spatial transcriptomics experiments to measure gene expression changes in the dorsal hippocampus following the recall of active place avoidance (APA) memory. Through bulk RNA sequencing, we examined the gene expression changes following memory recall across the functionally distinct subregions of the dorsal hippocampus. We found that recall induced differentially expressed genes (DEGs) in the CA1 and CA3 hippocampal subregions were enriched with genes involved in synaptic transmission and synaptic plasticity, while DEGs in the dentate gyrus (DG) were enriched with genes involved in energy balance and ribosomal function. Through spatial transcriptomics, we examined gene expression changes following memory recall across an array of spots encompassing putative memory-associated neuronal ensembles marked by the expression of the IEGs Arc, Egr1, and c-Jun. Within samples from both trained and untrained mice, the subpopulations of spatial transcriptomic spots marked by these IEGs were transcriptomically and spatially distinct from one another. DEGs detected between Arc+ and Arc-spots exclusively in the trained mouse were enriched in several memory-related gene ontology terms, including "regulation of synaptic plasticity" and "memory." Our results suggest that APA memory recall is supported by regionalized transcriptomic profiles separating the CA1 and CA3 from the DG, transcriptionally and spatially distinct IEG expressing spatial transcriptomic spots, and biological processes related to synaptic plasticity as a defining the difference between Arc+ and Arc-spatial transcriptomic spots.

Disturbances of gene expression patterns that occur during brain development can severely affect signal transmission, connectivity, and plasticity--key features that underlie memory formation and storage in neurons. Abnormalities at the molecular level can manifest as changes in the structural and functional plasticity of dendritic spines that harbor excitatory synapses. This can lead to such developmental neuropsychiatric conditions as Autism spectrum disorders, intellectual disabilities, and schizophrenia. The present study investigated the role of the major transcriptional regulator serum response factor (SRF) in synapse maturation and its impact on behavioral phenotypes. Using in vitro and in vivo models of early postnatal SRF deletion, we studied its influence on key morphological and physiological hallmarks of spine development. The elimination of SRF in developing neurons resulted in a phenotype of immature dendritic spines and impairments in excitatory transmission. Moreover, using a combination of molecular and imaging techniques, we showed that SRF-depleted neurons exhibited a lower level of specific glutamate receptor mRNAs and a decrease in their surface expression. Additionally, the early postnatal elimination of SRF in hippocampal CA1 excitatory neurons caused spine immaturity and a specific social deficit that is frequently observed in autism patients. Altogether, our data suggest that the regulation of structural and functional dendritic spine maturation begins at the stage of gene transcription, which underpins the crucial role of such transcription factors as SRF. Moreover, disturbances of the postnatal expression of SRF translate to behavioral changes in adult animals.

At the center of the hippocampal tri-synaptic loop are synapses formed between mossy fiber (MF) terminals from granule cells in the dentate gyrus (DG) and proximal dendrites of CA3 pyramidal neurons. However, the molecular mechanism regulating the development and function of these synapses is poorly understood. In this study, we showed that neurotrophin-3 (NT3) was expressed in nearly all mature granule cells but not CA3 cells. We selectively deleted the NT3-encoding Ntf3 gene in the DG during the 1st two postnatal weeks to generate a Ntf3 conditional knockout (Ntf3-cKO). Ntf3-cKO mice had normal hippocampal cytoarchitecture but displayed elevated anxiety level and impairments in contextual memory, spatial reference memory and nest building. As MF-CA3 synapses are essential for encoding of contextual memory, we examined synaptic transmission at these synapses using ex vivo electrophysiological recordings. We found that Ntf3-cKO mice showed impaired basal synaptic transmission due to deficits in excitatory postsynaptic currents mediated by AMPA receptors but normal presynaptic function and intrinsic excitability of CA3 pyramidal neurons. Consistent with this selective postsynaptic deficit, Ntf3-cKO mice had fewer and smaller thorny excrescences on proximal apical dendrites of CA3 neurons and lower GluR1 levels in the stratum lucidum area where MF-CA3 synapses reside but normal MF terminals, compared with control mice. Thus, our study indicates that NT3 expressed in the dentate gyrus is crucial for the postsynaptic structure and function of MF-CA3 synapses and hippocampal-dependent memory.

In the 2nd postnatal week hippocampus, Hebbian-induced long-term potentiation (LTP) of AMPA receptor-mediated transmission in CA3-CA1 synapses is not a genuine potentiation. Instead, it is a de-depression (unsilencing) and temporary stabilization of postsynaptically AMPA-labile synapses silenced by a prior test pulse (0.03 - 0.2 Hz) stimulation. In addition to such an LTP, Hebbian induction at these synapses also results in a labile potentiation that becomes depotentiated by test pulse stimulation, thus appearing as an Hebbian-induced short- term potentiation (STP). Although the induction of this labile potentiation was blocked in the combined presence of N-methyl-D-aspartate (NMDA) and metabotropic glutamate (mGlu) receptor antagonists, the depotentiation was not affected by these drugs. The labile potentiation was not associated with a change in paired-pulse ratio and was, after a depotentiation, fully re-established by a 20 min interruption of test pulse stimulation. These properties are shared with the silencing of previously non-stimulated (naive) AMPA-labile synapses by such test pulse stimulation. However, the depotentiation following an Hebbian induction is not a re-silencing of naive AMPA labile synapses since there is no correlation between the magnitudes of depotentiation and preceding silencing of naive synapses. The present results suggest that Hebbian induction at these neonatal CA3-CA1 synapses, in addition to unsilencing and temporary stabilization of AMPA-labile transmission, creates a labile potentiation based on the insertion/activation of an additional AMPA-labile signaling unit to a pre-existing synapse.

Expression of the immediate early gene cFos modifies the epigenetic landscape of activated neurons with downstream effects on synaptic plasticity. The production of cFos is inhibited by a long-lived isoform of another Fos family gene, {Delta}FosB. It has been speculated that this negative feedback mechanism may be critical for protecting episodic memories from being overwritten by new information. Here, we investigate the influence of {Delta}FosB inhibition on cFos expression and memory. Hippocampal neurons in slice culture produce more cFos on the first day of stimulation compared to identical stimulation on the following day. This downregulation affects all hippocampal subfields and requires histone deacetylation. Overexpression of {Delta}FosB in individual pyramidal neurons effectively suppresses cFos, indicating that accumulation of {Delta}FosB is the causal mechanism. Water maze training of mice over several days leads to accumulation of {Delta}FosB in granule cells of the dentate gyrus, but not in CA3 and CA1. Because the dentate gyrus is thought to support pattern separation and cognitive flexibility, we hypothesized that inhibiting the expression of {Delta}FosB would affect reversal learning, i.e., the ability to successively learn new platform locations in the water maze. The results indicate that pharmacological HDAC inhibition, which prevents cFos repression, impairs reversal learning, while learning and memory of the initial platform location remain unaffected. Our study supports the hypothesis that epigenetic mechanisms tightly regulate cFos expression in individual granule cells to orchestrate the formation of time-stamped memories.

Synaptic plasticity is triggered by different patterns of neuronal network activity. Network activity leads to an increase in ambient GABA concentration and tonic activation of GABAA receptors. How tonic GABAA conductance affects synaptic plasticity during temporal and rate-based coding is poorly understood. Here, we show that tonic GABAA conductance differently affects long-term potentiation (LTP) induced by different stimulation patterns. The LTP based on a temporal spike - EPSP order (spike-timing-dependent [st] LTP) was not affected by exogenous GABA application. Backpropagating action potential, which enables Ca2+ entry through N-methyl-D-aspartate receptors (NMDARs) during stLTP induction, was only slightly reduced by the tonic conductance. In contrast, GABA application impeded LTP dependent on spiking rate (theta-burst-induced [tb] LTP) by reducing the EPSP bust response and, hence, NMDAR-mediated Ca2+ entry during tbLTP induction. Our results may explain the changes in different forms of memory under physiological and pathological conditions that affect tonic GABAA conductance.

The neurotrophin brain-derived neurotrophic factor (BDNF) plays a key role in neuronal development and synaptic plasticity. The discovery that BDNF mRNA can be transported in neuronal dendrites in an activity-dependent manner has suggested that its local translation may support synapse maturation and plasticity. However, a clear demonstration that BDNF mRNA is locally transported and translated at activated synapses in response to long-term potentiation (LTP) is still lacking. Here, we study the dynamics of BDNF mRNA dendritic trafficking following induction of chemical-LTP (cLTP). Dendritic transport of BDNF transcripts was analysed using the MS2 system for mRNA visualization, and chimeric BDNF-GFP constructs were used to monitor protein synthesis in living neurons. We found that within 15 following cLTP induction, most BDNF mRNA granules become stationary and transiently accumulate in the dendritic shaft at the basis of the spines similarly to the control CamkII mRNA which increased also inside the spines, at 60 post-cLTP. At 60 but not at 15 from cLTP induction, we observed an increase in BDNF protein levels within the spine. Taken together, these findings suggest that BDNF mRNA trafficking is arrested in the early phase of cLTP, providing a local source of mRNA for translation of BDNF at the basis of the spine followed in the late LTP phase, by translocation of the BDNF protein within the spine head. StatementBrain-derived neurotrophic factor (BDNF) plays a key role in neuronal development and synaptic plasticity. In this study, we investigate two unresolved questions in neuronal plasticity: a) whether the post-synaptically released BDNF can be locally synthesized in this compartment, and b) whether the local translation of BDNF occurs in dendrites, or within the spine. Using chimeric constructs ectopically expressed in living primary hippocampal neurons, we tracked BDNF mRNA trafficking within the dendrites and its local translation following induction of chemical-LTP (cLTP) by forskolin. We show that in the early phase of cLTP induction (15), BDNF mRNA becomes confined at the basis of the spines providing a local source for translation of the protein followed in the late LTP phase (60), by translocation of the BDNF protein within the spine head.

Hilar mossy cells (MCs) are principal excitatory neurons of the dentate gyrus (DG) that play critical roles in hippocampal function and have been implicated in brain disorders such as anxiety and epilepsy. However, the mechanisms by which MCs contribute to DG function and disease are poorly understood. Expression from the dopamine D2 receptor (D2R) gene (Drd2) promoter is a defining feature of MCs, and previous work indicates a key role for dopaminergic signaling in the DG. Additionally, the involvement of D2R signaling in cognition and neuropsychiatric conditions is well-known. Surprisingly, though, the function of MC D2Rs remain largely unexplored. In this study, we show that selective and conditional removal of Drd2 from MCs of adult mice impaired spatial memory, promoted anxiety-like behavior and was proconvulsant. To determine the subcellular expression of D2Rs in MCs, we used a D2R knockin mouse which revealed that D2Rs are enriched in the inner molecular layer of the DG, where MCs establish synaptic contacts with granule cells. D2R activation by exogenous and endogenous dopamine reduced MC to dentate granule cells (GC) synaptic transmission, most likely by a presynaptic mechanism. In contrast, removing Drd2 from MCs had no significant impact on MC excitatory inputs and passive and active properties. Our findings support that MC D2Rs are essential for proper DG function by reducing MC excitatory drive onto GCs. Lastly, impairment of MC D2R signaling could promote anxiety and epilepsy, therefore highlighting a potential therapeutic target. SIGNIFICANCEGrowing evidence indicates that hilar mossy cells (MCs) of the dentate gyrus play critical but incompletely understood roles in memory and brain disorders, including anxiety and epilepsy. Dopamine D2 receptors (D2Rs), implicated in cognition and several psychiatric and neurological disorders, are considered to be characteristically expressed by MCs. Still, the subcellular localization and function of MC D2Rs are largely unknown. We report that removing the Drd2 gene specifically from MCs of adult mice impaired spatial memory and was anxiogenic and proconvulsant. We also found that D2Rs are enriched where MCs synaptically contact dentate granule cells (GC) and reduce MC-GC transmission. This work uncovered the functional significance of MC D2Rs, thus highlighting their therapeutic potential in D2R- and MC-associated pathologies.