Synapse-to-nucleus signaling

Tracing synaptic loss in Alzheimer's brain with SV2A PET-tracer UCB-J

INTRODUCTION

Synaptic loss is an early prominent feature of Alzheimer's disease (AD). The recently developed novel synaptic vesicle 2A protein (SV2A) PET-tracer UCB-J has shown great promise in tracking synaptic loss in AD. However, there have been discrepancies between the findings and a lack of mechanistic insight.

METHODS

Here we report the first extensive pre-clinical validation studies for UCB-J in control (CN; n = 11) and AD (n = 11) brains using a multidimensional approach of post-mortem brain imaging techniques, radioligand binding, and biochemical studies.

RESULTS AND DISCUSSION

We demonstrate that UCB-J could target SV2A protein with high specificity and depict synaptic loss at synaptosome levels in AD brain regions compared to CNs. UCB-J showed highest synaptic loss in AD hippocampus followed in descending order by frontal cortex, temporal cortex, parietal cortex, and cerebellum. 3H-UCB-J large brain-section autoradiography and cellular/subcellular fractions binding studies indicated potential off-target interaction with phosphorylated tau (p-tau) species in AD brains, which could have subsequent clinical implications for imaging studies.

HIGHLIGHTS

Synaptic positron emission tomography (PET)-tracer UCB-J could target synaptic vesicle 2A protein (SV2A) with high specificity in Alzheimer's disease (AD) and control brains. Synaptic PET-tracer UCB-J could depict synaptic loss at synaptosome levels in AD brain regions compared to control. Potential off-target interaction of UCB-J with phosphorylated tau (p-tau) species at cellular/subcellular levels could have subsequent clinical implications for imaging studies, warranting further investigations.

Protein transport from pre- and postsynapse to the nucleus: Mechanisms and functional implications

The extreme length of neuronal processes poses a challenge for synapse-to-nucleus communication. In response to this challenge several different mechanisms have evolved in neurons to couple synaptic activity to the regulation of gene expression. One of these mechanisms concerns the long-distance transport of proteins from pre- and postsynaptic sites to the nucleus. In this review we summarize current evidence on mechanisms of transport and consequences of nuclear import of these proteins for gene transcription. In addition, we discuss how information from pre- and postsynaptic sites might be relayed to the nucleus by this type of long-distance signaling. When applicable, we highlight how long-distance protein transport from synapse-to-nucleus can provide insight into the pathophysiology of disease or reveal new opportunities for therapeutic intervention.

Dual localization of the carboxy-terminal tail of GLR3.3 in sieve element-companion cell complex

Glutamate receptor-like (GLR) 3.3 and 3.6 proteins are required for mediating wound-induced leaf-to-leaf electrical signaling. In the previous study, we found that the carboxy-terminal tail of GLR3.3 contains key residues that are indispensable for its action in electrical signaling. In the present work, we generated plants that expressed the truncated C-tail fraction of GLR3.3. To our expectation, the truncated C-tail itself was not functional in propagating leaf-to-leaf signals. However, we identified that the C-tail-mVENUS fusion proteins had dual localization patterns in sieve elements and companion cells. In companion cells, the fusion proteins overlapped largely with the nucleus. We speculated that a possible nuclear localization signal is present in the C-tail of GLR3.3, paralleling the C-tails of the ionotropic glutamate receptors in animal cells. Our further findings on the C-tail of GLR3.3 open up new possibilities for the regulatory roles of the C-tails to GLR proteins.

Real-time imaging of Arc/Arg3.1 transcription ex vivo reveals input-specific immediate early gene dynamics

The ability of neurons to process and store salient environmental features underlies information processing in the brain. Long-term information storage requires synaptic plasticity and regulation of gene expression. While distinct patterns of activity have been linked to synaptic plasticity, their impact on immediate early gene (IEG) expression remains poorly understood. The activity regulated cytoskeleton associated (Arc) gene has received wide attention as an IEG critical for long-term synaptic plasticity and memory. Yet, to date, the transcriptional dynamics of Arc in response to compartment and input-specific activity is unclear. By developing a knock-in mouse to fluorescently tag Arc alleles, we studied real-time transcription dynamics after stimulation of dentate granule cells (GCs) in acute hippocampal slices. To our surprise, we found that Arc transcription displayed distinct temporal kinetics depending on the activation of excitatory inputs that convey functionally distinct information, i.e., medial and lateral perforant paths (MPP and LPP, respectively). Moreover, the transcriptional dynamics of Arc after synaptic stimulation was similar to direct activation of GCs, although the contribution of ionotropic glutamate receptors, L-type voltage-gated calcium channel, and the endoplasmic reticulum (ER) differed. Specifically, we observed an ER-mediated synapse-to-nucleus signal that supported elevations in nuclear calcium and, thereby, rapid induction of Arc transcription following MPP stimulation. By delving into the complex excitation-transcription coupling for Arc, our findings highlight how different synaptic inputs may encode information by modulating transcription dynamics of an IEG linked to learning and memory.

Interleukin-4 receptor signaling modulates neuronal network activity

Evidence is emerging that immune responses not only play a part in the central nervous system (CNS) in diseases but may also be relevant for healthy conditions. We discovered a major role for the interleukin-4 (IL-4)/IL-4 receptor alpha (IL-4Rα) signaling pathway in synaptic processes, as indicated by transcriptome analysis in IL-4Rα–deficient mice and human neurons with/without IL-4 treatment. Moreover, IL-4Rα is expressed presynaptically, and locally available IL-4 regulates synaptic transmission. We found reduced synaptic vesicle pools, altered postsynaptic currents, and a higher excitatory drive in cortical networks of IL-4Rα–deficient neurons. Acute effects of IL-4 treatment on postsynaptic currents in wild-type neurons were mediated via PKCγ signaling release and led to increased inhibitory activity supporting the findings in IL-4Rα–deficient neurons. In fact, the deficiency of IL-4Rα resulted in increased network activity in vivo, accompanied by altered exploration and anxiety-related learning behavior; general learning and memory was unchanged. In conclusion, neuronal IL-4Rα and its presynaptic prevalence appear relevant for maintaining homeostasis of CNS synaptic function.

The crystal structure of the varicella-zoster Orf24-Orf27 nuclear egress complex spotlights multiple determinants of herpesvirus subfamily specificity

Varicella-zoster virus (VZV) is a human pathogen from the α-subfamily of herpesviruses. The VZV Orf24-Orf27 complex represents the essential viral core nuclear egress complex (NEC) that orchestrates the egress of the preassembled virus capsids from the nucleus. While previous studies have primarily emphasized that the architecture of core NEC complexes is highly conserved among herpesviruses, the present report focuses on subfamily-specific structural and functional features that help explain the differences in the autologous versus nonautologous interaction patterns observed for NEC formation across herpesviruses. Here, we describe the crystal structure of the Orf24-Orf27 complex at 2.1 Å resolution. Coimmunoprecipitation and confocal imaging data show that Orf24-Orf27 complex formation displays some promiscuity in a herpesvirus subfamily-restricted manner. At the same time, analysis of thermodynamic parameters of NEC formation of three prototypical α-, β-, and γ herpesviruses, i.e., VZV, human cytomegalovirus (HCMV), and Epstein–Barr virus (EBV), revealed highly similar binding affinities for the autologous interaction with specific differences in enthalpy and entropy. Computational alanine scanning, structural comparisons, and mutational data highlight intermolecular interactions shared among α-herpesviruses that are clearly distinct from those seen in β- and γ-herpesviruses, including a salt bridge formed between Orf24-Arg167 and Orf27-Asp126. This interaction is located outside of the hook-into-groove interface and contributes significantly to the free energy of complex formation. Combined, these data explain distinct properties of specificity and permissivity so far observed in herpesviral NEC interactions. These findings will prove valuable in attempting to target multiple herpesvirus core NECs with selective or broad-acting drug candidates.

Jacob, a Synapto-Nuclear Protein Messenger Linking N-methyl-D-aspartate Receptor Activation to Nuclear Gene Expression

Pyramidal neurons exhibit a complex dendritic tree that is decorated by a huge number of spine synapses receiving excitatory input. Synaptic signals not only act locally but are also conveyed to the nucleus of the postsynaptic neuron to regulate gene expression. This raises the question of how the spatio-temporal integration of synaptic inputs is accomplished at the genomic level and which molecular mechanisms are involved. Protein transport from synapse to nucleus has been shown in several studies and has the potential to encode synaptic signals at the site of origin and decode them in the nucleus. In this review, we summarize the knowledge about the properties of the synapto-nuclear messenger protein Jacob with special emphasis on a putative role in hippocampal neuronal plasticity. We will elaborate on the interactome of Jacob, the signals that control synapto-nuclear trafficking, the mechanisms of transport, and the potential nuclear function. In addition, we will address the organization of the Jacob/NSMF gene, its origin and we will summarize the evidence for the existence of splice isoforms and their expression pattern.

Nr4a2 Transcription Factor in Hippocampal Synaptic Plasticity, Memory and Cognitive Dysfunction: A Perspective Review

Long-lasting changes of synaptic efficacy are largely mediated by activity-induced gene transcription and are essential for neuronal plasticity and memory. In this scenario, transcription factors have emerged as pivotal players underlying synaptic plasticity and the modification of neural networks required for memory formation and consolidation. Hippocampal synaptic dysfunction is widely accepted to underlie the cognitive decline observed in some neurodegenerative disorders including Alzheimer’s disease. Therefore, understanding the molecular pathways regulating gene expression profiles may help to identify new synaptic therapeutic targets. The nuclear receptor 4A subfamily (Nr4a) of transcription factors has been involved in a variety of physiological processes within the hippocampus, ranging from inflammation to neuroprotection. Recent studies have also pointed out a role for the activity-dependent nuclear receptor subfamily 4, group A, member 2 (Nr4a2/Nurr1) in hippocampal synaptic plasticity and cognitive functions, although the underlying molecular mechanisms are still poorly understood. In this review, we highlight the specific effects of Nr4a2 in hippocampal synaptic plasticity and memory formation and we discuss whether the dysregulation of this transcription factor could contribute to hippocampal synaptic dysfunction, altogether suggesting the possibility that Nr4a2 may emerge as a novel synaptic therapeutic target in brain pathologies associated to cognitive dysfunctions.

Tuberous Sclerosis Complex (TSC) Inactivation Increases Neuronal Network Activity by Enhancing Ca2+ Influx via L-Type Ca2+ Channels

Tuberous sclerosis complex (TSC) is a multisystem developmental disorder characterized by hamartomas in various organs, such as the brain, lungs, and kidneys. Epilepsy, along with autism and intellectual disability, is one of the neurologic impairments associated with TSC that has an intimate relationship with developmental outcomes and quality of life. Sustained activation of the mammalian target of rapamycin (mTOR) via TSC1 or TSC2 mutations is known to be involved in the onset of epilepsy in TSC. However, the mechanism by which mTOR causes seizures remains unknown. In this study, we showed that, human induced pluripotent stem cell-derived TSC2-deficient (TSC2 ^-/-) neurons exhibited elevated neuronal activity with highly synchronized Ca²⁺ spikes. Notably, TSC2 ^-/- neurons presented enhanced Ca²⁺ influx via L-type Ca²⁺ channels (LTCCs), which contributed to the abnormal neurite extension and sustained activation of cAMP response element binding protein (CREB), a critical mediator of synaptic plasticity. Expression of Cav1.3, a subtype of LTCCs, was increased in TSC2 ^-/- neurons, but long-term rapamycin treatment suppressed this increase and reversed the altered neuronal activity and neurite extensions. Thus, we identified Cav1.3 LTCC as a critical downstream component of TSC-mTOR signaling that would trigger enhanced neuronal network activity of TSC2 ^-/- neurons. We suggest that LTCCs could be potential novel targets for the treatment of epilepsy in TSC.SIGNIFICANCE STATEMENT There is a close relationship between elevated mammalian target of rapamycin (mTOR) activity and epilepsy in tuberous sclerosis complex (TSC). However, the underlying mechanism by which mTOR causes epilepsy remains unknown. In this study, using human TSC2 ^-/- neurons, we identified elevated Ca²⁺ influx via L-type Ca²⁺ channels as a critical downstream component of TSC-mTOR signaling and a potential cause of both elevated neuronal activity and neurite extension in TSC2 ^-/- neurons. Our findings demonstrate a previously unrecognized connection between sustained mTOR activation and elevated Ca²⁺ signaling via L-type Ca²⁺ channels in human TSC neurons, which could cause epilepsy in TSC.

Granule Cell Ensembles in Mouse Dentate Gyrus Rapidly Upregulate the Plasticity-Related Protein Synaptopodin after Exploration Behavior

The plasticity-related protein Synaptopodin (SP) has been implicated in neuronal plasticity. SP is targeted to dendritic spines and the axon initial segment, where it organizes the endoplasmic reticulum (ER) into the spine apparatus and the cisternal organelle, respectively. Here, we report an inducible third localization of SP in the somata of activated granule cell ensembles in mouse dentate gyrus. Using immunofluorescence and fluorescence in situ hybridization, we observed a subpopulation of mature granule cells (~1–2%) exhibiting perinuclear SP protein and a strong somatic SP mRNA signal. Double immunofluorescence labeling for Arc demonstrated that ~ 75% of these somatic SP-positive cells are also Arc-positive. Placement of mice into a novel environment caused a rapid (~2–4 h) induction of Arc, SP mRNA, and SP protein in exploration-induced granule cell ensembles. Lesion experiments showed that this induction requires input from the entorhinal cortex. Somatic SP colocalized with α-Actinin2, a known binding partner of SP. Finally, ultrastructural analysis revealed SP immunoprecipitate on dense plates linking cytoplasmic and perinuclear ER cisterns; these structures were absent in granule cells of SP-deficient mice. Our data implicate SP in the formation of contextual representations in the dentate gyrus and the behaviorally induced reorganization of cytoplasmic and perinuclear ER.

A Synaptic Framework for the Persistence of Memory Engrams

The ability to store and retrieve learned information over prolonged periods of time is an essential and intriguing property of the brain. Insight into the neurobiological mechanisms that underlie memory consolidation is of utmost importance for our understanding of memory persistence and how this is affected in memory disorders. Recent evidence indicates that a given memory is encoded by sparsely distributed neurons that become highly activated during learning, so-called engram cells. Research by us and others confirms the persistent nature of cortical engram cells by showing that these neurons are required for memory expression up to at least 1 month after they were activated during learning. Strengthened synaptic connectivity between engram cells is thought to ensure reactivation of the engram cell network during retrieval. However, given the continuous integration of new information into existing neuronal circuits and the relatively rapid turnover rate of synaptic proteins, it is unclear whether a lasting learning-induced increase in synaptic connectivity is mediated by stable synapses or by continuous dynamic turnover of synapses of the engram cell network. Here, we first discuss evidence for the persistence of engram cells and memory-relevant adaptations in synaptic plasticity, and then propose models of synaptic adaptations and molecular mechanisms that may support memory persistence through the maintenance of enhanced synaptic connectivity within an engram cell network.

Identifying and monitoring neurons that undergo metamorphosis-regulated cell death (metamorphoptosis) by a neuron-specific caspase sensor (Casor) in Drosophila melanogaster

Activation of caspases is an essential step toward initiating apoptotic cell death. During metamorphosis of Drosophila melanogaster, many larval neurons are programmed for elimination to establish an adult central nervous system (CNS) as well as peripheral nervous system (PNS). However, their neuronal functions have remained mostly unknown due to the lack of proper tools to identify them. To obtain detailed information about the neurochemical phenotypes of the doomed larval neurons and their timing of death, we generated a new GFP-based caspase sensor (Casor) that is designed to change its subcellular position from the cell membrane to the nucleus following proteolytic cleavage by active caspases. Ectopic expression of Casor in vCrz and bursicon, two different peptidergic neuronal groups that had been well-characterized for their metamorphic programmed cell death, showed clear nuclear translocation of Casor in a caspase-dependent manner before their death. We found similar events in some cholinergic neurons from both CNS and PNS. Moreover, Casor also reported significant caspase activities in the ventral and dorsal common excitatory larval motoneurons shortly after puparium formation. These motoneurons were previously unknown for their apoptotic fate. Unlike the events seen in the neurons, expression of Casor in non-neuronal cell types, such as glial cells and S2 cells, resulted in the formation of cytoplasmic aggregates, preventing its use as a caspase sensor in these cell types. Nonetheless, our results support Casor as a valuable molecular tool not only for identifying novel groups of neurons that become caspase-active during metamorphosis but also for monitoring developmental timing and cytological changes within the dying neurons.

How the epigenome integrates information and reshapes the synapse

In the past few decades, the field of neuroepigenetics has investigated how the brain encodes information to form long-lasting memories that lead to stable changes in behaviour. Activity-dependent molecular mechanisms, including, but not limited to, histone modification, DNA methylation and nucleosome remodelling, dynamically regulate the gene expression required for memory formation. Recently, the field has begun to examine how a learning experience is integrated at the level of both chromatin structure and synaptic physiology. Here, we provide an overview of key established epigenetic mechanisms that are important for memory formation. We explore how epigenetic mechanisms give rise to stable alterations in neuronal function by modifying synaptic structure and function, and highlight studies that demonstrate how manipulating epigenetic mechanisms may push the boundaries of memory.

TorsinA dysfunction causes persistent neuronal nuclear pore defects

A critical challenge to deciphering the pathophysiology of neurodevelopmental disease is identifying which of the myriad abnormalities that emerge during CNS maturation persist to contribute to long-term brain dysfunction. Childhood-onset dystonia caused by a loss-of-function mutation in the AAA+ protein torsinA exemplifies this challenge. Neurons lacking torsinA develop transient nuclear envelope (NE) malformations during CNS maturation, but no NE defects are described in mature torsinA null neurons. We find that during postnatal CNS maturation torsinA null neurons develop mislocalized and dysfunctional nuclear pore complexes (NPC) that lack NUP358, normally added late in NPC biogenesis. SUN1, a torsinA-related molecule implicated in interphase NPC biogenesis, also exhibits localization abnormalities. Whereas SUN1 and associated nuclear membrane abnormalities resolve in juvenile mice, NPC defects persist into adulthood. These findings support a role for torsinA function in NPC biogenesis during neuronal maturation and implicate altered NPC function in dystonia pathophysiology.

Is plasticity of synapses the mechanism of long-term memory storage?

It has been 70 years since Donald Hebb published his formalized theory of synaptic adaptation during learning. Hebb's seminal work foreshadowed some of the great neuroscientific discoveries of the following decades, including the discovery of long-term potentiation and other lasting forms of synaptic plasticity, and more recently the residence of memories in synaptically connected neuronal assemblies. Our understanding of the processes underlying learning and memory has been dominated by the view that synapses are the principal site of information storage in the brain. This view has received substantial support from research in several model systems, with the vast majority of studies on the topic corroborating a role for synapses in memory storage. Yet, despite the neuroscience community's best efforts, we are still without conclusive proof that memories reside at synapses. Furthermore, an increasing number of non-synaptic mechanisms have emerged that are also capable of acting as memory substrates. In this review, we address the key findings from the synaptic plasticity literature that make these phenomena such attractive memory mechanisms. We then turn our attention to evidence that questions the reliance of memory exclusively on changes at the synapse and attempt to integrate these opposing views.

The C-terminus of NMDAR GluN1-1a Subunit Translocates to Nucleus and Regulates Synaptic Function

NMDARs, the Ca²⁺ permeable channels, play central roles in synaptic plasticity, brain development, learning, and memory. NMDAR binding partners and associated signaling has been extensively studied in synapse-to-nucleus communications. However, whether NMDARs could directly regulate synapse-to-nucleus communications is largely unknown. Here, we analyze the four alternative splicing of the C-terminus isoforms of GluN1 (1a, 2a, 3a, and 4a), and find that C1 domain of GluN1 is necessary for nuclear localization. Besides, we find that the 10 basic amino acids in C1 domain determine the nuclear localization of GluN1 C-terminus. Further investigating the expression patterns of the full length of GluN1 four isoforms shows that only GluN-1a exhibits the cytoplasmic and nucleus distribution in primary hippocampal neurons. Electrophysiological analyses also show that over-expression of GluN1 C-terminus without C1 domain doesn't affect synaptic transmission, whereas GluN1 C-terminus containing C1 domain potentiates NMDAR-mediated synaptic transmission. Our data suggested that the 10 basic amino acids in C1 domain determine translocation of GluN1 C-terminus into nucleus and regulate synaptic transmission.

Cyto-nuclear shuttling of afadin is required for rapid estradiol-mediated modifications of histone H3

Estrogens have been shown to rapidly regulate local signalling at synapses and within the nucleus. The result of these signalling events is to rapidly modulate synapse structure and function, as well as epigenetic mechanisms including histone modifications. Ultimately these mechanisms are thought to contribute to long-lasting changes in neural circuitry, and thus influence cognitive functions such as learning and memory. However, the mechanisms by which estrogen-mediated local synaptic and nuclear signalling events are coordinated are not well understood. In this study we have found that the scaffold protein afadin, (also known as AF-6), undergoes a bi-directional trafficking to both synaptic and nuclear compartment in response to acute 17β-estradiol (estradiol) treatment, in mixed sex neuronal cultures derived from fetal cortex. Interestingly, nuclear accumulation of afadin was coincidental with an increase in the phosphorylation of histone H3 at serine 10 (H3S10p). This epigenetic modification is associated with the remodeling of chromatin into an open euchromatin state, allowing for transcriptional activation and related learning and memory processes. Critically, the cyto-nuclear trafficking of afadin was required for estradiol-dependent H3S10p. We further determined that nuclear accumulation of afadin is sufficient to induce phosphorylation of the mitogentic kinases ERK1/2 (pERK1/2) within the nucleus. Moreover, nuclear pERK1/2 was required for estradiol-dependent H3S10p. Taken together, we propose a model whereby estradiol induces the bi-directional trafficking of afadin to synaptic and nuclear sub-compartments. Within the nucleus, afadin is required for increased pERK1/2 which in turn is required for H3S10p. Therefore this represents a mechanism through which estrogens may be able to coordinate both synaptic and nucleosomal events within the same neuronal population.

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17β-estradiol targets afadin to membrane and nuclear subcompartments.

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Histone H3 is rapidly phosphorylated by 17β-estradiol.

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Histone H3 phosphorylation by 17β-estradiol requires afadin nuclear accumulation.

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17β-estradiol-mediated ERK1/2 activation is required for histone H3 phosphorylation.

JAK/STAT signaling pathway gene expression is reduced following Nelf knockdown in GnRH neurons

Hypothalamic gonadotropin releasing hormone (GnRH) is crucial for the proper function of the hypothalamic-pituitary-gonadal (HPG) axis, subsequent puberty, and reproduction. When GnRH neuron migration or GnRH regulation is impaired, hypogonadotropic hypogonadism results. Mutations in the gene for nasal embryonic luteinizing hormone-releasing factor (NELF) have been identified in GnRH-deficient humans. NELF is a predominantly nuclear protein that may participate in gene transcription, but the genes NELF regulates are unknown. To address this question, RNA was extracted from NLT GnRH neuronal cells following either stable Nelf knockdown or scrambled control and subjected to cDNA arrays. Transcription factors and cell migration gene expression was altered most commonly. Members of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway, including Stat1, Stat2, Stat5a, Jak2, Irf7 and Irf9, were significantly down regulated as assessed by RT-qPCR. Protein levels of STAT1, phospho-STAT1, and JAK2 were reduced, but the protein level of phospho-JAK2 was not. These findings suggest a role for NELF in the regulation of the JAK/STAT signaling pathway, which have important functions in GnRH neurons.

Potential for therapeutic targeting of AKAP signaling complexes in nervous system disorders

A common feature of neurological and neuropsychiatric disorders is a breakdown in the integrity of intracellular signal transduction pathways. Dysregulation of ion channels and receptors in the cell membrane and the enzymatic mediators that link them to intracellular effectors can lead to synaptic dysfunction and neuronal death. However, therapeutic targeting of these ubiquitous signaling elements can lead to off-target side effects due to their widespread expression in multiple systems of the body. A-kinase anchoring proteins (AKAPs) are multivalent scaffolding proteins that compartmentalize a diverse range of receptor and effector proteins to streamline signaling within nanodomain signalosomes. A number of essential neurological processes are known to critically depend on AKAP-directed signaling and an understanding of the role AKAPs play in nervous system disorders has emerged in recent years. Selective targeting of AKAP protein-protein interactions may be a means to uncouple pathologically active signaling pathways in neurological disorders with a greater degree of specificity. In this review we will discuss the role of AKAPs in both regulating normal nervous system function and dysfunction associated with disease, and the potential for therapeutic targeting of AKAP signaling complexes.

CRTC1 Nuclear Translocation Following Learning Modulates Memory Strength via Exchange of Chromatin Remodeling Complexes on the Fgf1 Gene

Memory is formed by synapse-to-nucleus communication that leads to regulation of gene transcription, but the identity and organizational logic of signaling pathways involved in this communication remain unclear. Here we find that the transcription cofactor CRTC1 is a critical determinant of sustained gene transcription and memory strength in the hippocampus. Following associative learning, synaptically localized CRTC1 is translocated to the nucleus and regulates Fgf1b transcription in an activity-dependent manner. After both weak and strong training, the HDAC3-N-CoR corepressor complex leaves the Fgf1b promoter and a complex involving the translocated CRTC1, phosphorylated CREB, and histone acetyltransferase CBP induces transient transcription. Strong training later substitutes KAT5 for CBP, a process that is dependent on CRTC1, but not on CREB phosphorylation. This in turn leads to long-lasting Fgf1b transcription and memory enhancement. Thus, memory strength relies on activity-dependent changes in chromatin and temporal regulation of gene transcription on specific CREB/CRTC1 gene targets.

The malleable brain: plasticity of neural circuits and behavior - a review from students to students

One of the most intriguing features of the brain is its ability to be malleable, allowing it to adapt continually to changes in the environment. Specific neuronal activity patterns drive long-lasting increases or decreases in the strength of synaptic connections, referred to as long-term potentiation and long-term depression, respectively. Such phenomena have been described in a variety of model organisms, which are used to study molecular, structural, and functional aspects of synaptic plasticity. This review originated from the first International Society for Neurochemistry (ISN) and Journal of Neurochemistry (JNC) Flagship School held in Alpbach, Austria (Sep 2016), and will use its curriculum and discussions as a framework to review some of the current knowledge in the field of synaptic plasticity. First, we describe the role of plasticity during development and the persistent changes of neural circuitry occurring when sensory input is altered during critical developmental stages. We then outline the signaling cascades resulting in the synthesis of new plasticity-related proteins, which ultimately enable sustained changes in synaptic strength. Going beyond the traditional understanding of synaptic plasticity conceptualized by long-term potentiation and long-term depression, we discuss system-wide modifications and recently unveiled homeostatic mechanisms, such as synaptic scaling. Finally, we describe the neural circuits and synaptic plasticity mechanisms driving associative memory and motor learning. Evidence summarized in this review provides a current view of synaptic plasticity in its various forms, offers new insights into the underlying mechanisms and behavioral relevance, and provides directions for future research in the field of synaptic plasticity. Read the Editorial Highlight for this article on page 788. Cover Image for this issue: doi: 10.1111/jnc.13815.

Synaptically Localized Transcriptional Regulators in Memory Formation

At the neuronal cell level, long-term memory formation emerges from interactions between initial activity-dependent molecular changes at the synapse and subsequent regulation of gene transcription in the nucleus. This in turn leads to strengthening of the connections back at the synapse that received the initial signal. However, the mechanisms through which this synapse-to-nucleus molecular exchange occurs remain poorly understood. Here we discuss recent studies that delineate nucleocytoplasmic transport of a special class of synaptically localized transcriptional regulators that upon receiving initial external signal by the synapse move to the nucleus to modulate gene transcription.

Neurons and neuronal activity control gene expression in astrocytes to regulate their development and metabolism

The influence that neurons exert on astrocytic function is poorly understood. To investigate this, we first developed a system combining cortical neurons and astrocytes from closely related species, followed by RNA-seq and in silico species separation. This approach uncovers a wide programme of neuron-induced astrocytic gene expression, involving Notch signalling, which drives and maintains astrocytic maturity and neurotransmitter uptake function, is conserved in human development, and is disrupted by neurodegeneration. Separately, hundreds of astrocytic genes are acutely regulated by synaptic activity via mechanisms involving cAMP/PKA-dependent CREB activation. This includes the coordinated activity-dependent upregulation of major astrocytic components of the astrocyte-neuron lactate shuttle, leading to a CREB-dependent increase in astrocytic glucose metabolism and elevated lactate export. Moreover, the groups of astrocytic genes induced by neurons or neuronal activity both show age-dependent decline in humans. Thus, neurons and neuronal activity regulate the astrocytic transcriptome with the potential to shape astrocyte-neuron metabolic cooperation.

Spatially and temporally regulating translation via mRNA-binding proteins in cellular and neuronal function

Coordinated regulation of mRNA localization and local translation are essential steps in cellular asymmetry and function. It is increasingly evident that mRNA-binding proteins play critical functions in controlling the fate of mRNA, including when and where translation occurs. In this review, we discuss the robust and complex roles that mRNA-binding proteins play in the regulation of local translation that impact cellular function in vertebrates. First, we discuss the role of local translation in cellular polarity and possible links to vertebrate development and patterning. Next, we discuss the expanding role for local protein synthesis in neuronal development and function, with special focus on how a number of neurological diseases have given us insight into the importance of translational regulation. Finally, we discuss the ever-increasing set of tools to study regulated translation and how these tools will be vital in pushing forward and addressing the outstanding questions in the field.

If genetic variation could talk: What genomic data may teach us about the importance of gene expression regulation in the genetics of autism

Autism spectrum disorder (ASD) has been long known to have substantial genetic etiology. Much research has attempted to identify specific genes contributing to ASD risk with the goal of tying gene function to a molecular pathological explanation for ASD. A unifying molecular pathology would potentially increase understanding of what is going wrong during development, and could lead to diagnostic biomarkers or targeted preventative or therapeutic directions. We review past and current genetic mapping approaches and discuss major results, leading to the hypothesis that global dysregulation of gene or protein expression may be implicated in ASD rather than disturbance of brain-specific functions. If substantiated, this hypothesis might indicate the need for novel experimental and analytical approaches in order to understand this neurodevelopmental disorder, develop biomarkers, or consider treatment approaches.

Single-particle tracking uncovers dynamics of glutamate-induced retrograde transport of NF-κB p65 in living neurons

Retrograde transport of NF-κB from the synapse to the nucleus in neurons is mediated by the dynein/dynactin motor complex and can be triggered by synaptic activation. The caliber of axons is highly variable ranging down to 100 nm, aggravating the investigation of transport processes in neurites of living neurons using conventional light microscopy. We quantified for the first time the transport of the NF-κB subunit p65 using high-density single-particle tracking in combination with photoactivatable fluorescent proteins in living mouse hippocampal neurons. We detected an increase of the mean diffusion coefficient ([Formula: see text]) in neurites from [Formula: see text] to [Formula: see text] after stimulation with glutamate. We further observed that the relative amount of retrogradely transported p65 molecules is increased after stimulation. Glutamate treatment resulted in an increase of the mean retrograde velocity from [Formula: see text] to [Formula: see text], whereas a velocity increase from [Formula: see text] to [Formula: see text] was observed for anterogradely transported p65. This study demonstrates for the first time that glutamate stimulation leads to an increased mobility of single NF-κB p65 molecules in neurites of living hippocampal neurons.

Ring finger protein 10 is a novel synaptonuclear messenger encoding activation of NMDA receptors in hippocampus

Synapses and nuclei are connected by bidirectional communication mechanisms that enable information transfer encoded by macromolecules. Here, we identified RNF10 as a novel synaptonuclear protein messenger. RNF10 is activated by calcium signals at the postsynaptic compartment and elicits discrete changes at the transcriptional level. RNF10 is enriched at the excitatory synapse where it associates with the GluN2A subunit of NMDA receptors (NMDARs). Activation of synaptic GluN2A-containing NMDARs and induction of long term potentiation (LTP) lead to the translocation of RNF10 from dendritic segments and dendritic spines to the nucleus. In particular, we provide evidence for importin-dependent long-distance transport from synapto-dendritic compartments to the nucleus. Notably, RNF10 silencing prevents the maintenance of LTP as well as LTP-dependent structural modifications of dendritic spines.

DOI:http://dx.doi.org/10.7554/eLife.12430.001

Brain activity depends on the communication between neurons. This process takes place at the junctions between neurons, which are known as synapses, and typically involves one of the cells releasing a chemical messenger that binds to receptors on the other cell. The binding triggers a cascade of events inside the recipient cell, including the production of new receptors and their insertion into the cell membrane. These changes strengthen the synapse and are thought to be one of the ways in which the brain establishes and maintains memories.

However, in order to induce these changes at the synapse, neurons must be able to activate the genes that encode their component parts. These genes are present inside the cell nucleus, which is located some distance away from the synapse. Studies have shown that signals can be sent from the nucleus to the synapse and vice versa, enabling the two parts of the cell to exchange information. Synapses that communicate using a chemical called glutamate have been particularly well studied; but it still remains unclear how the activation of receptors at these “glutamatergic synapses” is linked to activation of genes inside the nucleus at the molecular level.

Dinamarca, Guzzetti et al. have now discovered that this process at glutamatergic synapses involves the movement of a protein messenger to the nucleus. Specifically, activation at synapses of a particularly common subtype of receptor, called NMDA, causes a protein called Ring Finger protein 10 (or RNF10 for short) to move from the synapse to the nucleus. To leave the synapse, RNF10 first has to bind to proteins called importins, which transport RNF10 into the nucleus. Once inside the nucleus, RNF10 binds to another protein that interacts with the DNA to start the production of new synaptic proteins.

Further work is required to identify the molecular mechanisms that trigger RNF10 to leave the synapse. In addition, future studies should evaluate the levels and activity of RNF10 in brain disorders in which synapses are known to function abnormally.

DOI:http://dx.doi.org/10.7554/eLife.12430.002

Importin-7 mediates memory consolidation through regulation of nuclear translocation of training-activated MAPK in Drosophila

Translocation of signaling molecules, MAPK in particular, from the cytosol to nucleus represents a universal key element in initiating the gene program that determines memory consolidation. Translocation mechanisms and their behavioral impact, however, remain to be determined. Here, we report that a highly conserved nuclear transporter, Drosophila importin-7 (DIM-7), regulates import of training-activated MAPK for consolidation of long-term memory (LTM). We show that silencing DIM-7 functions results in impaired LTM, whereas overexpression of DIM-7 enhances LTM. This DIM-7-dependent regulation of LTM is confined to a consolidation time window and in mushroom body neurons. Image data show that bidirectional alteration in DIM-7 expression results in proportional changes in the intensity of training-activated MAPK accumulated within the nuclei of mushroom body neurons during LTM consolidation. Such DIM-7-regulated nuclear accumulation of activated MAPK is observed only in the training specified for LTM induction and determines the amplitude, but not the time course, of memory consolidation.

Dendritic trafficking faces physiologically critical speed-precision tradeoffs

Nervous system function requires intracellular transport of channels, receptors, mRNAs, and other cargo throughout complex neuronal morphologies. Local signals such as synaptic input can regulate cargo trafficking, motivating the leading conceptual model of neuron-wide transport, sometimes called the 'sushi-belt model' (Doyle and Kiebler, 2011). Current theories and experiments are based on this model, yet its predictions are not rigorously understood. We formalized the sushi belt model mathematically, and show that it can achieve arbitrarily complex spatial distributions of cargo in reconstructed morphologies. However, the model also predicts an unavoidable, morphology dependent tradeoff between speed, precision and metabolic efficiency of cargo transport. With experimental estimates of trafficking kinetics, the model predicts delays of many hours or days for modestly accurate and efficient cargo delivery throughout a dendritic tree. These findings challenge current understanding of the efficacy of nucleus-to-synapse trafficking and may explain the prevalence of local biosynthesis in neurons.

Biochemical Computation for Spine Structural Plasticity

The structural plasticity of dendritic spines is considered to be essential for various forms of synaptic plasticity, learning, and memory. The process is mediated by a complex signaling network consisting of numerous species of molecules. Furthermore, the spatiotemporal dynamics of the biochemical signaling are regulated in a complicated manner because of geometrical restrictions from the unique morphology of the dendritic branches and spines. Recent advances in optical techniques have enabled the exploration of the spatiotemporal aspects of the signal regulations in spines and dendrites and have provided many insights into the principle of the biochemical computation that underlies spine structural plasticity.

Deceivingly dynamic: Learning-dependent changes in stathmin and microtubules

Microtubules, one of the major cytoskeletal structures, were previously considered stable and only indirectly involved in synaptic structure and function in mature neurons. However, recent evidence demonstrates that microtubules are dynamic and have an important role in synaptic structure, synaptic plasticity, and memory. In particular, learning induces changes in microtubule turnover and stability, and pharmacological manipulation of microtubule dynamics alters synaptic plasticity and long-term memory. These learning-induced changes in microtubules are controlled by the phosphoprotein stathmin, whose only known cellular activity is to negatively regulate microtubule formation. During the first eight hours following learning, changes in the phosphorylation of stathmin go through two phases causing biphasic shifts in microtubules stability/instability. These shifts, in turn, regulate memory formation by controlling in the second phase synaptic transport of the GluA2 subunit of AMPA receptors. Improper regulation of stathmin and microtubule dynamics has been observed in aged animals and in patients with Alzheimer's disease and depression. Thus, recent work on stathmin and microtubules has identified new molecular players in the early stages of memory encoding.

The RNA-centred view of the synapse: non-coding RNAs and synaptic plasticity

If mRNAs were the only RNAs made by a neuron, there would be a simple mapping of mRNAs to proteins. However, microRNAs and other non-coding RNAs (ncRNAs; endo-siRNAs, piRNAs, BC1, BC200, antisense and long ncRNAs, repeat-related transcripts, etc.) regulate mRNAs via effects on protein translation as well as transcriptional and epigenetic mechanisms. Not only are genes ON or OFF, but their ability to be translated can be turned ON or OFF at the level of synapses, supporting an enormous increase in information capacity. Here, I review evidence that ncRNAs are expressed pervasively within dendrites in mammalian brain; that some are activity-dependent and highly enriched near synapses; and that synaptic ncRNAs participate in plasticity responses including learning and memory. Ultimately, ncRNAs can be viewed as the post-it notes of the neuron. They have no literal meaning of their own, but derive their functions from where (and to what) they are stuck. This may explain, in part, why ncRNAs differ so dramatically from protein-coding genes, both in terms of the usual indicators of functionality and in terms of evolutionary constraints. ncRNAs do not appear to be direct mediators of synaptic transmission in the manner of neurotransmitters or receptors, yet they orchestrate synaptic plasticity—and may drive species-specific changes in cognition.

Rhythms and blues: modulation of oscillatory synchrony and the mechanism of action of antidepressant treatments

Treatments for major depressive disorder (MDD) act at different hierarchical levels of biological complexity, ranging from the individual synapse to the brain as a whole. Theories of antidepressant medication action traditionally have focused on the level of cell-to-cell interaction and synaptic neurotransmission. However, recent evidence suggests that modulation of synchronized electrical activity in neuronal networks is a common effect of antidepressant treatments, including not only medications, but also neuromodulatory treatments such as repetitive transcranial magnetic stimulation. Synchronization of oscillatory network activity in particular frequency bands has been proposed to underlie neurodevelopmental and learning processes, and also may be important in the mechanism of action of antidepressant treatments. Here, we review current research on the relationship between neuroplasticity and oscillatory synchrony, which suggests that oscillatory synchrony may help mediate neuroplastic changes related to neurodevelopment, learning, and memory, as well as medication and neuromodulatory treatment for MDD. We hypothesize that medication and neuromodulation treatments may have related effects on the rate and pattern of neuronal firing, and that these effects underlie antidepressant efficacy. Elucidating the mechanisms through which oscillatory synchrony may be related to neuroplasticity could lead to enhanced treatment strategies for MDD.

γCaMKII shuttles Ca²⁺/CaM to the nucleus to trigger CREB phosphorylation and gene expression

Activity-dependent CREB phosphorylation and gene expression are critical for long-term neuronal plasticity. Local signaling at CaV1 channels triggers these events, but how information is relayed onward to the nucleus remains unclear. Here, we report a mechanism that mediates long-distance communication within cells: a shuttle that transports Ca(2+)/calmodulin from the surface membrane to the nucleus. We show that the shuttle protein is γCaMKII, its phosphorylation at Thr287 by βCaMKII protects the Ca(2+)/CaM signal, and CaN triggers its nuclear translocation. Both βCaMKII and CaN act in close proximity to CaV1 channels, supporting their dominance, whereas γCaMKII operates as a carrier, not as a kinase. Upon arrival within the nucleus, Ca(2+)/CaM activates CaMKK and its substrate CaMKIV, the CREB kinase. This mechanism resolves long-standing puzzles about CaM/CaMK-dependent signaling to the nucleus. The significance of the mechanism is emphasized by dysregulation of CaV1, γCaMKII, βCaMKII, and CaN in multiple neuropsychiatric disorders.

Synaptic activity controls localization and function of CtBP1 via binding to Bassoon and Piccolo

Persistent experience-driven adaptation of brain function is associated with alterations in gene expression patterns, resulting in structural and functional neuronal remodeling. How synaptic activity-in particular presynaptic performance-is coupled to gene expression in nucleus remains incompletely understood. Here, we report on a role of CtBP1, a transcriptional co-repressor enriched in presynapses and nuclei, in the activity-driven reconfiguration of gene expression in neurons. We demonstrate that presynaptic and nuclear pools of CtBP1 are interconnected and that both synaptic retention and shuttling of CtBP1 between cytoplasm and nucleus are co-regulated by neuronal activity. Finally, we show that CtBP1 is targeted and/or anchored to presynapses by direct interaction with the active zone scaffolding proteins Bassoon and Piccolo. This association is regulated by neuronal activity via modulation of cellular NAD/NADH levels and restrains the size of the CtBP1 pool available for nuclear import, thus contributing to the control of activity-dependent gene expression. Our combined results reveal a mechanism for coupling activity-induced molecular rearrangements in the presynapse with reconfiguration of neuronal gene expression.

'Memory and molecular turnover,' 30 years after inception

In 1984 Sir Francis Crick hypothesized that memory is recorded in the brain as reversible modifications to DNA and protein, but acknowledged that most biomolecules turn over too rapidly to account for long-term memories. To accommodate this possible paradox he modeled an enzymatic mechanism to maintain modifications on hemi-modified multimeric symmetrical molecules. While studies on the turnover of chromatin modifications that may be involved in memory are in their infancy, an exploration of his model in the light of modern epigenetics produced somewhat surprising results. The molecular turnover rates for two classes of chromatin modifications believed to record and store durable memories were approximated from experiments using diverse approaches and were found to be remarkably short. The half-lives of DNA cytosine methylation and post-translationally modified nucleosomal histones are measured in hours and minutes, respectively, for a subset of sites on chromatin controlling gene expression. It appears likely that the turnover of DNA methylation in the brain and in neurons, in particular, is even more rapid than in other cell types and organs, perhaps accommodating neuronal plasticity, learning, and memory. The machinery responsible for the rapid turnover of DNA methylation and nucleosomal histone modifications is highly complex, partially redundant, and appears to act in a sequence specific manner. Molecular symmetry plays an important part in maintaining site-specific turnover, but its particular role in memory maintenance is unknown. Elucidating Crick’s paradox, the contradiction between rapid molecular turnover of modified biomolecules and long-term memory storage, appears fundamental to understanding cognitive function and neurodegenerative disease.

Protein trafficking from synapse to nucleus in control of activity-dependent gene expression

Long-lasting changes in neuronal excitability require activity-dependent gene expression and therefore the transduction of synaptic signals to the nucleus. Synaptic activity is rapidly relayed to the nucleus by membrane depolarization and the propagation of Ca(2+)-waves. However, it is unlikely that Ca(2+)-transients alone can explain the specific genomic response to the plethora of extracellular stimuli that control gene expression. In recent years a steadily growing number of studies report the transport of proteins from synapse to nucleus. Potential mechanisms for active retrograde transport and nuclear targets for these proteins have been identified and recent reports assigned first functions to this type of long-distance signaling. In this review we will discuss how the dissociation of synapto-nuclear protein messenger from synaptic and extrasynaptic sites, their transport, nuclear import and the subsequent genomic response relate to the prevailing concept behind this signaling mechanism, the encoding of signals at their site of origin and their decoding in the nucleus.

Coordinated nuclear and synaptic shuttling of afadin promotes spine plasticity and histone modifications

The ability of a neuron to transduce extracellular signals into long lasting changes in neuronal morphology is central to its normal function. Increasing evidence shows that coordinated regulation of synaptic and nuclear signaling in response to NMDA receptor activation is crucial for long term memory, synaptic tagging, and epigenetic signaling. Although mechanisms have been proposed for synapse-to-nuclear communication, it is unclear how signaling is coordinated at both subcompartments. Here, we show that activation of NMDA receptors induces the bi-directional and concomitant shuttling of the scaffold protein afadin from the cytosol to the nucleus and synapses. Activity-dependent afadin nuclear translocation peaked 2 h post-stimulation, was independent of protein synthesis, and occurred concurrently with dendritic spine remodeling. Moreover, activity-dependent afadin nuclear translocation coincides with phosphorylation of histone H3 at serine 10 (H3S10p), a marker of epigenetic modification. Critically, blocking afadin nuclear accumulation attenuated activity-dependent dendritic spine remodeling and H3 phosphorylation. Collectively, these data support a novel model of neuronal nuclear signaling whereby dual-residency proteins undergo activity-dependent bi-directional shuttling from the cytosol to synapses and the nucleus, coordinately regulating dendritic spine remodeling and histone modifications.

Synaptic Signaling in Learning and Memory

Learning and memory require the formation of new neural networks in the brain. A key mechanism underlying this process is synaptic plasticity at excitatory synapses, which connect neurons into networks. Excitatory synaptic transmission happens when glutamate, the excitatory neurotransmitter, activates receptors on the postsynaptic neuron. Synaptic plasticity is a higher-level process in which the strength of excitatory synapses is altered in response to the pattern of activity at the synapse. It is initiated in the postsynaptic compartment, where the precise pattern of influx of calcium through activated glutamate receptors leads either to the addition of new receptors and enlargement of the synapse (long-term potentiation) or the removal of receptors and shrinkage of the synapse (long-term depression). Calcium/calmodulin-regulated enzymes and small GTPases collaborate to control this highly tuned mechanism.

Nuclear calcium signalling in the regulation of brain function

Synaptic activity initiates biochemical processes that have various outcomes, including the formation of memories, increases in neuronal survival and the development of chronic pain and addiction. Virtually all activity-induced, long-lasting adaptations of brain functions require a dialogue between synapses and the nucleus that results in changes in gene expression. Calcium signals that are induced by synaptic activity and propagate into the nucleus are a major route for synapse-to-nucleus communication. Recent findings indicate that diverse forms of neuroadaptation require calcium transients in the nucleus to switch on the necessary genomic programme. Deficits in nuclear calcium signalling as a result of a reduction in synaptic activity or increased extrasynaptic NMDA receptor signalling may underlie the aetiologies of various diseases, including neurodegeneration and cognitive dysfunction.

Cellular distribution of the NMDA-receptor activated synapto-nuclear messenger Jacob in the rat brain

In previous work, we found that the protein messenger Jacob is involved in N-methyl-D-aspartate receptor (NMDAR) signaling to the nucleus and cAMP response element-binding protein (CREB) mediated gene expression in hippocampal primary neurons. Particularly, extrasynaptic NMDAR activation drives Jacob efficiently into the nucleus where it then induces gene expression that promotes neurodegeneration. However, the protein also translocates to the nucleus in CA1 neurons after Schaffer collateral long-term potentiation (LTP) but not long-term depression (LTD), suggesting that Jacob might be involved in hippocampal and LTP-dependent learning and memory processes. Not much is known about the cellular and subcellular distribution of the protein in brain. In this paper, we provide an overview of the expression of Jacob in rat brain with special emphasis on the hippocampus. We show that Jacob is abundant in hippocampal pyramidal neurons and interneurons but absent from astrocytes and microglia. Interestingly, we found that Jacob is also present in mossy fiber axons. Double immunofluorescence confocal laser scans with presynaptic markers demonstrate that Jacob is indeed found at excitatory but not inhibitory presynaptic sites. Accordingly, we found no substantial co-localization of Jacob with a postsynaptic marker of inhibitory synapses, gephyrin. In contrast, almost all postsynaptic density protein 95 (PSD-95) positive excitatory postsynaptic sites also exhibited strong Jacob-immunofluorescence. Taken together, these data support a synaptic and nuclear role of Jacob that implicates long-distance NMDAR signaling to the nucleus in excitatory neurons.

HDAC4 governs a transcriptional program essential for synaptic plasticity and memory

Neuronal activity influences genes involved in circuit development and information processing. However, the molecular basis of this process remains poorly understood. We found that HDAC4, a histone deacetylase that shuttles between the nucleus and cytoplasm, controls a transcriptional program essential for synaptic plasticity and memory. The nuclear import of HDAC4 and its association with chromatin is negatively regulated by NMDA receptors. In the nucleus, HDAC4 represses genes encoding constituents of central synapses, thereby affecting synaptic architecture and strength. Furthermore, we show that a truncated form of HDAC4 encoded by an allele associated with mental retardation is a gain-of-function nuclear repressor that abolishes transcription and synaptic transmission despite the loss of the deacetylase domain. Accordingly, mice carrying a mutant that mimics this allele exhibit deficits in neurotransmission, spatial learning, and memory. These studies elucidate a mechanism of experience-dependent plasticity and define the biological role of HDAC4 in the brain.

Neuronal activity-regulated gene transcription: how are distant synaptic signals conveyed to the nucleus?

Synaptic activity can trigger gene expression programs that are required for the stable change of neuronal properties, a process that is essential for learning and memory. Currently, it is still unclear how the stimulation of dendritic synapses can be coupled to transcription in the nucleus in a timely way given that large distances can separate these two cellular compartments. Although several mechanisms have been proposed to explain long distance communication between synapses and the nucleus, the possible co-existence of these models and their relevance in physiological conditions remain elusive. One model suggests that synaptic activation triggers the translocation to the nucleus of certain transcription regulators localised at postsynaptic sites that function as synapto-nuclear messengers. Alternatively, it has been hypothesised that synaptic activity initiates propagating regenerative intracellular calcium waves that spread through dendrites into the nucleus where nuclear transcription machinery is thereby regulated. It has also been postulated that membrane depolarisation of voltage-gated calcium channels on the somatic membrane is sufficient to increase intracellular calcium concentration and activate transcription without the need for transported signals from distant synapses. Here I provide a critical overview of the suggested mechanisms for coupling synaptic stimulation to transcription, the underlying assumptions behind them and their plausible physiological significance.

Activity-dependent transport of the transcriptional coactivator CRTC1 from synapse to nucleus

Long-lasting changes in synaptic efficacy, such as those underlying long-term memory, require transcription. Activity-dependent transport of synaptically localized transcriptional regulators provides a direct means of coupling synaptic stimulation with changes in transcription. The CREB-regulated transcriptional coactivator (CRTC1), which is required for long-term hippocampal plasticity, binds CREB to potently promote transcription. We show that CRTC1 localizes to synapses in silenced hippocampal neurons but translocates to the nucleus in response to localized synaptic stimulation. Regulated nuclear translocation occurs only in excitatory neurons and requires calcium influx and calcineurin activation. CRTC1 is controlled in a dual fashion with activity regulating CRTC1 nuclear translocation and cAMP modulating its persistence in the nucleus. Neuronal activity triggers a complex change in CRTC1 phosphorylation, suggesting that CRTC1 may link specific types of stimuli to specific changes in gene expression. Together, our results indicate that synapse-to-nuclear transport of CRTC1 dynamically informs the nucleus about synaptic activity.

A cultured affair: HSV latency and reactivation in neurons

After replicating in surface epithelia, herpes simplex virus type-1 (HSV-1) enters the axonal terminals of peripheral neurons. The viral genome translocates to the nucleus, where it establishes a specialized infection known as latency, re-emerging periodically to seed new infections. Studies using cultured neuron models that faithfully recapitulate the molecular hallmarks of latency and reactivation defined in live animal models have provided fresh insight into the control of latency and connections to neuronal physiology. With this comes a growing appreciation for how the life cycles of HSV-1 and other herpesviruses are governed by key host pathways controlling metabolic homeostasis and cell identity.