Nuclear calcium/calmodulin regulates memory consolidation

Lead exposure induces neurodysfunction through caspase-1-mediated neuronal pyroptosis

Chronic lead (Pb) exposure causes neurodysfunction and contributes to the development of neurodegenerative disease. However, the mechanism of Pb-induced neurological dysfunction have yet to be fully elucidated. This study determined the role pyroptosis plays in Pb-induced neurodysfunction in neurons. We used both in vitro and in vivo approaches to explore whether Pb exposure induces caspase-1-mediated pyroptosis in neurons and its relationship to Pb-induced neurological disorders. Our findings showed that caspase-1-mediated pyroptosis in Pb-exposed neurons activated glycogen synthase kinase 3 protease activity by disrupting Ca2+/calmodulin-dependent protein kinase II/cAMP-response element binding protein pathway, leading to neurological disorders. Moreover, the caspase-1 inhibition VX-765 or the non-steroidal anti-inflammatory drug sodium para-aminosalicylic acid (PAS-Na) attenuated the Pb-induced neurological disorders by alleviating caspase-1 mediated neuronal pyroptosis. Our novel studies suggest that caspase-1-mediated pyroptosis in neurons represents a potential mechanism for Pb-induced neurodysfunction, identifying a putative target for attenuating the neurodegenerative effects induced by this metal.

Brief and diverse excitotoxic insults cause an increase in neuronal nuclear membrane permeability in the neonatal brain

Neuronal swelling after excitotoxic insults is implicated in neuronal injury and death in the developing brain, yet mitigating brain edema with osmotic and surgical interventions yields poor clinical outcomes. Importantly, neuronal swelling and its downstream consequences during early brain development remain poorly investigated. Using multiphoton Ca2+ imaging in vivo (P12-17) and in acute brain slices (P8-12), we explored Ca2+-dependent downstream effects after neuronal cytotoxic edema. We observed the translocation of cytosolic GCaMP6s into the nucleus of a subpopulation of neurons minutes after various excitotoxic insults. We used automated morphology-detection algorithms for neuronal segmentation and quantified the nuclear translocation of GCaMP6s as the ratio of nuclear and cytosolic intensity (N/C ratio). Elevated neuronal N/C ratios were correlated to higher Ca2+ loads and could occur independently of neuronal swelling. Electron microscopy revealed that the nuclear translocation was associated with increased nuclear pore size. Inhibiting calpains prevented elevated N/C ratios and neuronal swelling. Thus, our results indicate altered nuclear transport in a subpopulation of neurons shortly after injury in the developing brain, which can be used as an early biomarker of acute neuronal injury.

Excitation-transcription coupling, neuronal gene expression and synaptic plasticity

Excitation-transcription coupling (E-TC) links synaptic and cellular activity to nuclear gene transcription. It is generally accepted that E-TC makes a crucial contribution to learning and memory through its role in underpinning long-lasting synaptic enhancement in late-phase long-term potentiation and has more recently been linked to late-phase long-term depression: both processes require de novo gene transcription, mRNA translation and protein synthesis. E-TC begins with the activation of glutamate-gated N-methyl-D-aspartate-type receptors and voltage-gated L-type Ca2+ channels at the membrane and culminates in the activation of transcription factors in the nucleus. These receptors and ion channels mediate E-TC through mechanisms that include long-range signalling from the synapse to the nucleus and local interactions within dendritic spines, among other possibilities. Growing experimental evidence links these E-TC mechanisms to late-phase long-term potentiation and learning and memory. These advances in our understanding of the molecular mechanisms of E-TC mean that future efforts can focus on understanding its mesoscale functions and how it regulates neuronal network activity and behaviour in physiological and pathological conditions.

Integration of nuclear Ca2+ transients and subnuclear protein shuttling provides a novel mechanism for the regulation of CREB-dependent gene expression

Nuclear Ca2+ waves elicited by NMDAR and L-type voltage-gated Ca2+-channels as well as protein transport from synapse-to-nucleus are both instrumental in control of plasticity-related gene expression. At present it is not known whether fast [Ca2+]n transients converge in the nucleus with signaling of synapto-nuclear protein messenger. Jacob is a protein that translocate a signalosome from N-methyl-D-aspartate receptors (NMDAR) to the nucleus and that docks this signalosome to the transcription factor CREB. Here we show that the residing time of Jacob in the nucleoplasm strictly correlates with nuclear [Ca2+]n transients elicited by neuronal activity. A steep increase in [Ca2+]n induces instantaneous uncoupling of Jacob from LaminB1 at the nuclear lamina and promotes the association with the transcription factor cAMP-responsive element-binding protein (CREB) in hippocampal neurons. The size of the Jacob pool at the nuclear lamina is controlled by previous activity-dependent nuclear import, and thereby captures the previous history of NMDAR-induced nucleocytoplasmic shuttling. Moreover, the localization of Jacob at the nuclear lamina strongly correlates with synaptic activity and [Ca2+]n waves reflecting ongoing neuronal activity. In consequence, the resulting extension of the nuclear residing time of Jacob amplifies the capacity of the Jacob signalosome to regulate CREB-dependent gene expression and will, thereby, compensate for the relatively small number of molecules reaching the nucleus from individual synapses.

Excessive Protein Accumulation and Impaired Autophagy in the Hippocampus of Angelman Syndrome Modeled in Mice

BACKGROUND

Angelman syndrome (AS), a neurodevelopmental disorder caused by abnormalities of the 15q11.2-q13.1 chromosome region, is characterized by impairment of cognitive and motor functions, sleep problems, and seizures. How the genetic defects of AS produce these neurological symptoms is unclear. Mice modeling AS (AS mice) accumulate activity-regulated cytoskeleton-associated protein (ARC/ARG3.1), a neuronal immediate early gene (IEG) critical for synaptic plasticity. This accumulation suggests an altered protein metabolism.

METHODS

Focusing on the dorsal hippocampus (dHC), a brain region critical for memory formation and cognitive functions, we assessed levels and tissue distribution of IEGs, de novo protein synthesis, and markers of protein synthesis, endosomes, autophagy, and synaptic functions in AS mice at baseline and following learning. We also tested autophagic flux and memory retention following autophagy-promoting treatment.

RESULTS

AS dHC exhibited accumulation of IEGs ARC, FOS, and EGR1; autophagy proteins MLP3B, SQSTM1, and LAMP1; and reduction of the endosomal protein RAB5A. AS dHC also had increased levels of de novo protein synthesis, impaired autophagic flux with accumulation of autophagosome, and altered synaptic protein levels. Contextual fear conditioning significantly increased levels of IEGs and autophagy proteins, de novo protein synthesis, and autophagic flux in the dHC of normal mice, but not in AS mice. Enhancing autophagy in the dHC alleviated AS-related memory and autophagic flux impairments.

CONCLUSIONS

A major biological deficit of AS brain is a defective protein metabolism, particularly that dynamically regulated by learning, resulting in stalled autophagy and accumulation of neuronal proteins. Activating autophagy ameliorates AS cognitive impairments and dHC protein accumulation.

Effects of endogenous and exogenous N-acetyl-5-methoxy kynuramine on object recognition memory in male C3H mice

Prevention of dementia is important, because it is a leading cause of disability in elderly people. We previously reported that acute intraperitoneal treatment with N-acetyl-5-methoxy kynuramine (AMK), a melatonin (MEL) metabolite, enhanced long-term object recognition memory in ICR mice, a MEL deficient strain. Despite the presumable availability of AMK for dementia, its effects on cognitive performance have not been elucidated. It is unclear whether endogenous AMK is responsible for modulating long-term memory performance. To address this question, we assessed the effects of endogenous AMK on learning and memory using an object recognition test. C3H mice, a MEL-proficient strain, showed peak MEL levels at zeitgeber times (ZT) 19 and 22. Object recognition memory at ZT20 was superior to that at ZT8. Norharmane (NHM, 100 mg/kg), an indoleamine-2,3-dioxygenase (IDO) inhibitor, prevented the transformation of MEL to AMK, thereby suppressing AMK synthesis at ZT20. NHM (100 mg/kg) and another IDO inhibitor, 1-methyl-L-tryptophan (1-MT, 100 mg/kg), disrupted elevated cognitive performance at ZT20. These data imply that endogenous AMK may play a physiological role in the modulation of cognitive function. We also investigated the effects of pharmacological doses of MEL and AMK on object recognition memory in young C3H mice. MEL administration of 0.1 mg/kg, but not 0.01 mg/kg, enhanced object recognition memory, whereas 0.01 and 1 mg/kg AMK enhanced object recognition memory. Administration of 0.1 and 1 mg/kg AMK also enhanced object recognition memory in old C3H mice. These findings in MEL-proficient mice should be confirmed in other learning and memory tests before encouraging the clinical use of AMK.

Missense mutations in inositol 1,4,5-trisphosphate receptor type 3 result in leaky Ca2+ channels and activation of store-operated Ca2+ entry

Mutations in all subtypes of the inositol 1,4,5-trisphosphate receptor Ca2+ release channel are associated with human diseases. In this report, we investigated the functionality of three neuropathy-associated missense mutations in IP3R3 (V615M, T1424M, and R2524C). The mutants only exhibited function when highly over-expressed compared to endogenous hIP3R3. All variants resulted in elevated basal cytosolic Ca2+ levels, decreased endoplasmic reticulum Ca2+ store content, and constitutive store-operated Ca2+ entry in the absence of any stimuli, consistent with a leaky IP3R channel pore. These variants differed in channel function; when stably over-expressed the R2524C mutant was essentially dead, V615M was poorly functional, and T1424M exhibited activity greater than that of the corresponding wild-type following threshold stimulation. These results demonstrate that a common feature of these mutations is decreased IP3R3 function. In addition, these mutations exhibit a novel phenotype manifested as a constitutively open channel, which inappropriately gates SOCE in the absence of stimulation.

Multi-scale modeling toolbox for single neuron and subcellular activity under Transcranial Magnetic Stimulation

BACKGROUND

Transcranial Magnetic Stimulation (TMS) is a widely used non-invasive brain stimulation method. However, its mechanism of action and the neural response to TMS are still poorly understood. Multi-scale modeling can complement experimental research to study the subcellular neural effects of TMS. At the macroscopic level, sophisticated numerical models exist to estimate the induced electric fields. However, multi-scale computational modeling approaches to predict TMS cellular and subcellular responses, crucial to understanding TMS plasticity inducing protocols, are not available so far.

OBJECTIVE

We develop an open-source multi-scale toolbox Neuron Modeling for TMS (NeMo-TMS) to address this problem.

METHODS

NeMo-TMS generates accurate neuron models from morphological reconstructions, couples them to the external electric fields induced by TMS, and simulates the cellular and subcellular responses of single-pulse and repetitive TMS.

RESULTS

We provide examples showing some of the capabilities of the toolbox.

CONCLUSION

NeMo-TMS toolbox allows researchers a previously not available level of detail and precision in realistically modeling the physical and physiological effects of TMS.

The role of nuclear Ca2+ in maintaining neuronal homeostasis and brain health

Nuclear Ca2+ has emerged as one of the most potent mediators of the dialogue between neuronal synapses and the nucleus that regulates heterochromatin states, transcription factor activity, nuclear morphology and neuronal gene expression induced by synaptic activity. Recent studies underline the importance of nuclear Ca2+ signaling in long-lasting, activity-induced adaptation and maintenance of proper brain function. Diverse forms of neuroadaptation require transient nuclear Ca2+ signaling and cyclic AMP-responsive element-binding protein (CREB1, referred to here as CREB) as its prime target, which works as a tunable switch to drive and modulate specific gene expression profiles associated with memory, pain, addiction and neuroprotection. Furthermore, a reduction of nuclear Ca2+ levels has been shown to be neurotoxic and a causal factor driving the progression of neurodegenerative disorders, as well as affecting neuronal autophagy. Because of its central role in the brain, deficits in nuclear Ca2+ signaling may underlie a continuous loss of neuroprotection in the aging brain, contributing to the pathophysiology of Alzheimer's disease. In this Review, we discuss the principles of the 'nuclear calcium hypothesis' in the context of human brain function and its role in controlling diverse forms of neuroadaptation and neuroprotection. Furthermore, we present the most relevant and promising perspectives for future studies.

The melatonin metabolite N1-acetyl-5-methoxykynuramine facilitates long-term object memory in young and aging mice

Melatonin (MEL) has been reported to enhance cognitive processes, making it a potential treatment for cognitive decline. However, the role of MEL's metabolites, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK), in these effects are unknown. The current study directly investigated the acute effects of systemic MEL, AFMK, and AMK on novel object recognition. We also analyzed MEL, AFMK, and AMK levels in hippocampus and temporal lobe containing the perirhinal cortex following systemic MEL and AMK treatment. AMK administered post-training had a more potent effect on object memory than MEL and AFMK. AMK was also able to rescue age-associated declines in memory impairments when object memory was tested up to 4 days following training. Results from administering AMK at varying times around the training trial and the metabolism time course in brain tissue suggest that AMK's memory-enhancing effects reflect memory consolidation. Furthermore, inhibiting the MEL-to-AMK metabolic pathway disrupted object memory at 24 hours post-training, suggesting that endogenous AMK might play an important role in long-term memory formation. This is the first study to report that AMK facilitates long-term object memory performance in mice, and that MEL crosses the blood-brain barrier and is immediately converted to AMK in brain tissue. Overall, these results support AMK as a potential therapeutic agent to improve or prevent memory decline.

Disease-associated mutations in inositol 1,4,5-trisphosphate receptor subunits impair channel function

The inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs), which form tetrameric channels, play pivotal roles in regulating the spatiotemporal patterns of intracellular calcium signals. Mutations in IP3Rs have been increasingly associated with many debilitating human diseases such as ataxia, Gillespie syndrome, and generalized anhidrosis. However, how these mutations affect IP3R function, and how the perturbation of as-sociated calcium signals contribute to the pathogenesis and severity of these diseases remains largely uncharacterized. Moreover, many of these diseases occur as the result of autosomal dominant inheritance, suggesting that WT and mutant subunits associate in heterotetrameric channels. How the in-corporation of different numbers of mutant subunits within the tetrameric channels affects its activities and results in different disease phenotypes is also unclear. In this report, we investigated representative disease-associated missense mutations to determine their effects on IP3R channel activity. Additionally, we designed concatenated IP3R constructs to create tetrameric channels with a predefined subunit composition to explore the functionality of heteromeric channels. Using calcium imaging techniques to assess IP3R channel function, we observed that all the mutations studied resulted in severely attenuated Ca2+ release when expressed as homotetramers. However, some heterotetramers retained varied degrees of function dependent on the composition of the tetramer. Our findings suggest that the effect of mutations depends on the location of the mutation in the IP3R structure, as well as on the stoichiometry of mutant subunits assembled within the tetrameric channel. These studies provide insight into the pathogenesis and penetrance of these devastating human diseases.

Functional Consequences of Calcium-Dependent Synapse-to-Nucleus Communication: Focus on Transcription-Dependent Metabolic Plasticity

In the nervous system, calcium signals play a major role in the conversion of synaptic stimuli into transcriptional responses. Signal-regulated gene transcription is fundamental for a range of long-lasting adaptive brain functions that include learning and memory, structural plasticity of neurites and synapses, acquired neuroprotection, chronic pain, and addiction. In this review, we summarize the diverse mechanisms governing calcium-dependent transcriptional regulation associated with central nervous system plasticity. We focus on recent advances in the field of synapse-to-nucleus communication that include studies of the signal-regulated transcriptome in human neurons, identification of novel regulatory mechanisms such as activity-induced DNA double-strand breaks, and the identification of novel forms of activity- and transcription-dependent adaptations, in particular, metabolic plasticity. We summarize the reciprocal interactions between different kinds of neuroadaptations and highlight the emerging role of activity-regulated epigenetic modifiers in gating the inducibility of signal-regulated genes.

Broad domains of histone H3 lysine 4 trimethylation in transcriptional regulation and disease

Histone modifications affect transcription by changing the chromatin structure. In particular, histone H3 lysine 4 trimethylation (H3K4me3) is one of the most recognized epigenetic marks of active transcription. While many studies have provided evidence of the correlation between H3K4me3 and active transcription, details regarding the mechanism involved remain unclear. The first study on the broad H3K4me3 domain was reported in 2014; subsequently, the function of this domain has been studied in various cell types. In this review, we summarized the recent studies on the role of the broad H3K4me3 domain in transcription, development, memory formation, and several diseases, including cancer and autoimmune diseases. The broadest H3K4me3 domains are associated with increased transcriptional precision of cell-type-specific genes related to cell identity and other essential functions. The broad H3K4me3 domain regulates maternal zygotic activation in early mammalian development. In systemic autoimmune diseases, high expression of immune-responsive genes requires the presence of the broad H3K4me3 domain in the promoter-proximal regions. Transcriptional repression of tumor-suppressor genes is associated with the shortening of the broad H3K4me3 domains in cancer cells. Additionally, the broad H3K4me3 domain interacts with the super-enhancer to regulate cancer-associated genes. During memory formation, H3K4me3 breadth is regulated in the hippocampus CA1 neurons. Taken together, these findings indicate that H3K4me3 breadth is essential for the regulation of the transcriptional output across multiple cell types.

Nuclear localization of NCX: Role in Ca2+ handling and pathophysiological implications

Numerous lines of evidence indicate that nuclear calcium concentration ([Ca²⁺]_n) may be controlled independently from cytosolic events by a local machinery. In particular, the perinuclear space between the inner nuclear membrane (INM) and the outer nuclear membrane (ONM) of the nuclear envelope (NE) likely serves as an intracellular store for Ca²⁺ ions. Since ONM is contiguous with the endoplasmic reticulum (ER), the perinuclear space is adjacent to the lumen of ER thus allowing a direct exchange of ions and factors between the two organelles. Moreover, INM and ONM are fused at the nuclear pore complex (NPC), which provides the only direct passageway between the nucleoplasm and cytoplasm. However, due to the presence of ion channels, exchangers and transporters, it has been generally accepted that nuclear ion fluxes may occur across ONM and INM. Within the INM, the Na⁺/Ca²⁺ exchanger (NCX) isoform 1 seems to play an important role in handling Ca²⁺ through the different nuclear compartments. Particularly, nuclear NCX preferentially allows local Ca²⁺ flowing from nucleoplasm into NE lumen thanks to the Na⁺ gradient created by the juxtaposed Na⁺/K⁺-ATPase. Such transfer reduces abnormal elevation of [Ca²⁺]_n within the nucleoplasm thus modulating specific transductional pathways and providing a protective mechanism against cell death. Despite very few studies on this issue, here we discuss those making major contribution to the field, also addressing the pathophysiological implication of nuclear NCX malfunction.

Broad domains of histone 3 lysine 4 trimethylation are associated with transcriptional activation in CA1 neurons of the hippocampus during memory formation

Transcriptional changes in the hippocampus are required for memory formation, and these changes are regulated by numerous post-translational modifications of chromatin-associated proteins. One of the epigenetic marks that has been implicated in memory formation is histone 3 lysine 4 trimethylation (H3K4me3), and this modification is found at the promoters of actively transcribed genes. The total levels of H3K4me3 are increased in the CA1 region of the hippocampus during memory formation, and genetic perturbation of the K4 methyltransferases and demethylases interferes with forming memories. Previous chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) analyses failed to detect changes in H3K4me3 levels at the promoters of memory-linked genes. Since the breadth of H3K4me3 marks was recently reported to be associated with the transcriptional outcome of a gene, we re-analyzed H3K4me3 ChIP-seq data sets to identify the role of H3K4me3 broad domains in CA1 neurons, as well as identify differences in breadth that occur during contextual fear conditioning. We found that, under baseline conditions, broad H3K4me3 peaks mark important learning and memory genes and are often regulated by super-enhancers. The peaks at many learning-associated genes become broader during novel environment exposure and memory formation. Furthermore, the important learning- and memory-associated lysine methyltransferases, Kmt2a and Kmt2b, are involved in maintaining H3K4me3 peak width. Our findings highlight the importance of analyzing H3K4me3 peak shape, and demonstrate that breadth of H3K4me3 marks in neurons of the hippocampus is regulated during memory formation.

γ-Oryzanol Improves Cognitive Function and Modulates Hippocampal Proteome in Mice

Rice (Oryza sativa L.) is the richest source of γ-oryzanol, a compound endowed with antioxidant and anti-inflammatory properties. γ-Oryzanol has been demonstrated to cross the blood-brain barrier in intact form and exert beneficial effects on brain function. This study aimed to clarify the effects of γ-oryzanol in the hippocampus in terms of cognitive function and protein expression. Adult mice were administered with γ-oryzanol 100 mg/kg or vehicle (control) once a day for 21 consecutive days following which cognitive behavior and hippocampal proteome were investigated. Cognitive tests using novel object recognition and Y-maze showed that long-term consumption of γ-oryzanol improves cognitive function in mice. To investigate the hippocampal proteome modulated by γ-oryzanol, 2D-difference gel electrophoresis (2D-DIGE) was performed. Interestingly, we found that γ-oryzanol modulates quantitative changes of proteins involved in synaptic plasticity and neuronal trafficking, neuroprotection and antioxidant activity, and mitochondria and energy metabolism. These findings suggested γ-oryzanol as a natural compound able to maintain and reinforce brain function. Although more intensive studies are needed, we propose γ-oryzanol as a putative dietary phytochemical for preserving brain reserve, the ability to tolerate age-related changes, thereby preventing clinical symptoms or signs of neurodegenerative diseases.

What Is Required for Neuronal Calcium Waves? A Numerical Parameter Study

Neuronal calcium signals propagating by simple diffusion and reaction with mobile and stationary buffers are limited to cellular microdomains. The distance intracellular calcium signals can travel may be significantly increased by means of calcium-induced calcium release from internal calcium stores, notably the endoplasmic reticulum. The organelle, which can be thought of as a cell-within-a-cell, is able to sequester large amounts of cytosolic calcium ions via SERCA pumps and selectively release them into the cytosol through ryanodine receptor channels leading to the formation of calcium waves. In this study, we set out to investigate the basic properties of such dendritic calcium waves and how they depend on the three parameters dendrite radius, ER radius and ryanodine receptor density in the endoplasmic membrane. We demonstrate that there are stable and abortive regimes for calcium waves, depending on the above morphological and physiological parameters. In stable regimes, calcium waves can travel across long dendritic distances, similar to electrical action potentials. We further observe that abortive regimes exist, which could be relevant for spike-timing dependent plasticity, as travel distances and wave velocities vary with changing intracellular architecture. For some of these regimes, analytic functions could be derived that fit the simulation data. In parameter spaces, that are non-trivially influenced by the three-dimensional calcium concentration profile, we were not able to derive such a functional description, demonstrating the mathematical requirement to model and simulate biochemical signaling in three-dimensional space.

Calmodulin shuttling mediates cytonuclear signaling to trigger experience-dependent transcription and memory

Learning and memory depend on neuronal plasticity originating at the synapse and requiring nuclear gene expression to persist. However, how synapse-to-nucleus communication supports long-term plasticity and behavior has remained elusive. Among cytonuclear signaling proteins, γCaMKII stands out in its ability to rapidly shuttle Ca²⁺/CaM to the nucleus and thus activate CREB-dependent transcription. Here we show that elimination of γCaMKII prevents activity-dependent expression of key genes (BDNF, c-Fos, Arc), inhibits persistent synaptic strengthening, and impairs spatial memory in vivo. Deletion of γCaMKII in adult excitatory neurons exerts similar effects. A point mutation in γCaMKII, previously uncovered in a case of intellectual disability, selectively disrupts CaM sequestration and CaM shuttling. Remarkably, this mutation is sufficient to disrupt gene expression and spatial learning in vivo. Thus, this specific form of cytonuclear signaling plays a key role in learning and memory and contributes to neuropsychiatric disease.

Activity-dependent gene expression is thought to involve translocation of Ca²⁺/calmodulin (CaM) to the nucleus. Here, the authors examine a translocation-deficient mutant of γCaMKII, a Ca²⁺/CaM shuttle protein, to show that translocation of Ca²⁺/CaM is required for memory and synaptic plasticity.

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.

'Tagging' along memories in aging: Synaptic tagging and capture mechanisms in the aged hippocampus

Aging is accompanied by a general decline in the physiological functions of the body with the deteriorating organ systems. Brain is no exception to this and deficits in cognitive functions are quite common in advanced aging. Though a variety of age-related alterations are observed in the structure and function throughout the brain, certain regions show selective vulnerability. Medial temporal lobe, especially the hippocampus, is one such preferentially vulnerable region and is a crucial structure involved in the learning and long-term memory functions. Hippocampal synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD), are candidate cellular correlates of learning and memory and alterations in these properties have been well documented in aging. A related phenomenon called synaptic tagging and capture (STC) has been proposed as a mechanism for cellular memory consolidation and to account for temporal association of memories. Mounting evidences from behavioral settings suggest that STC could be a physiological phenomenon. In this article, we review the recent data concerning STC and provide a framework for how alterations in STC-related mechanisms could contribute to the age-associated memory impairments. The enormity of impairment in learning and memory functions demands an understanding of age-associated memory deficits at the fundamental level given its impact in the everyday tasks, thereby in the quality of life. Such an understanding is also crucial for designing interventions and preventive measures for successful brain aging.

Cell-Specific PKM Isoforms Contribute to the Maintenance of Different Forms of Persistent Long-Term Synaptic Plasticity

Multiple kinase activations contribute to long-term synaptic plasticity, a cellular mechanism mediating long-term memory. The sensorimotor synapse of Aplysia expresses different forms of long-term facilitation (LTF)-nonassociative and associative LTF-that require the timely activation of kinases, including protein kinase C (PKC). It is not known which PKC isoforms in the sensory neuron or motor neuron L7 are required to sustain each form of LTF. We show that different PKMs, the constitutively active isoforms of PKCs generated by calpain cleavage, in the sensory neuron and L7 are required to maintain each form of LTF. Different PKMs or calpain isoforms were blocked by overexpressing specific dominant-negative constructs in either presynaptic or postsynaptic neurons. Blocking either PKM Apl I in L7, or PKM Apl II or PKM Apl III in the sensory neuron 2 d after 5-hydroxytryptamine (5-HT) treatment reversed persistent nonassociative LTF. In contrast, blocking either PKM Apl II or PKM Apl III in L7, or PKM Apl II in the sensory neuron 2 d after paired stimuli reversed persistent associative LTF. Blocking either classical calpain or atypical small optic lobe (SOL) calpain 2 d after 5-HT treatment or paired stimuli did not disrupt the maintenance of persistent LTF. Soon after 5-HT treatment or paired stimuli, however, blocking classical calpain inhibited the expression of persistent associative LTF, while blocking SOL calpain inhibited the expression of persistent nonassociative LTF. Our data suggest that different stimuli activate different calpains that generate specific sets of PKMs in each neuron whose constitutive activities sustain long-term synaptic plasticity.SIGNIFICANCE STATEMENT Persistent synaptic plasticity contributes to the maintenance of long-term memory. Although various kinases such as protein kinase C (PKC) contribute to the expression of long-term plasticity, little is known about how constitutive activation of specific kinase isoforms sustains long-term plasticity. This study provides evidence that the cell-specific activities of different PKM isoforms generated from PKCs by calpain-mediated cleavage maintain two forms of persistent synaptic plasticity, which are the cellular analogs of two forms of long-term memory. Moreover, we found that the activation of specific calpains depends on the features of the stimuli evoking the different forms of synaptic plasticity. Given the recent controversy over the role of PKMζ maintaining memory, these findings are significant in identifying roles of multiple PKMs in the retention of memory.

One nuclear calcium transient induced by a single burst of action potentials represents the minimum signal strength in activity-dependent transcription in hippocampal neurons

Neurons undergo dramatic changes in their gene expression profiles in response to synaptic stimulation. The coupling of neuronal excitation to gene transcription is well studied and is mediated by signaling pathways activated by cytoplasmic and nuclear calcium transients. Despite this, the minimum synaptic activity required to induce gene expression remains unknown. To address this, we used cultured hippocampal neurons and cellular compartment analysis of temporal activity by fluorescence in situ hybridization (catFISH) that allows detection of nascent transcripts in the cell nucleus. We found that a single burst of action potentials, consisting of 24.4±5.1 action potentials during a 6.7±1.9s depolarization of 19.5±2.0mV causing a 9.3±0.9s somatic calcium transient, is sufficient to activate transcription of the immediate early gene arc (also known as Arg3.1). The total arc mRNA yield produced after a single burst-induced nuclear calcium transient was very small and, compared to unstimulated control neurons, did not lead to a significant increase in arc mRNA levels measured using quantitative reverse transcriptase PCR (qRT-PCR) of cell lysates. Significantly increased arc mRNA levels became detectable in hippocampal neurons that had undergone 5-8 consecutive burst-induced nuclear calcium transients at 0.05-0.15Hz. These results indicate that a single burst-induced nuclear calcium transient can activate gene expression and that transcription is rapidly shut off after synaptic stimulation has ceased.

Computational comparison of a calcium-dependent jellyfish protein (apoaequorin) and calmodulin-cholesterol in short-term memory maintenance

Memory reconsolidation and maintenance depend on calcium channels and on calcium/calmodulin-dependent kinases regulating protein turnover in the hippocampus. Ingestion of a jellyfish protein, apoaequorin, reportedly protects and/or improves verbal learning in adults and is currently widely advertised for use by the elderly. Apoaequorin is a member of the EF-hand calcium binding family of proteins that includes calmodulin. Calmodulin-1 (148 residues) differs from Apoaequorin (195 residues) in that it contains four rather than three Ca²⁺-binding sites and three rather than four cholesterol-binding (CRAC, CARC) domains. All three cholesterol-binding CARC domains in calmodulin have a high interaction affinity for cholesterol compared to only two high affinity CARC domains in apoaequorin. Both calmodulin and apoaequorin can form dimers with a potential of eight bound Ca²⁺ ions and six high affinity-bound cholesterol molecules in calmodulin with six bound Ca²⁺ ions and a mixed population of eight cholesterols bound to both CARC and CRAC domains in apoaqueorin. MEMSAT-SVM analysis indicates that both calmodulin and apoaqueorin have a pore-lining region. The Peptide-Cutter algorithm predicts that calmodulin-1 contains 11 trypsin-specific cleavage sites (compared to 21 in apoaqueorin), four of which are potentially blocked by cholesterol and three are within the Ca-binding domains and/or the pore-lining region. Three are clustered between the third and fourth Ca²⁺-binding sites. Only calmodulin pore-lining regions contain Ca²⁺ binding sites and as dimers may insert into the plasma membrane of neural cells and act as Ca²⁺ channels. In a dietary supplement, bound cholesterol may protect both apoaequorin and calmodulin from proteolysis in the gut as well as facilitate uptake across the blood-brain barrier. Our results suggest that a physiological calmodulin-cholesterol complex, not cholesterol-free jellyfish protein, may better serve as a dietary supplement to facilitate memory maintenance.

Rubinstein-Taybi Syndrome and Epigenetic Alterations

Rubinstein-Taybi syndrome (RSTS) is a rare genetic disorder in humans characterized by growth and psychomotor delay, abnormal gross anatomy, and mild to severe mental retardation (Rubinstein and Taybi, Am J Dis Child 105:588-608, 1963, Hennekam et al., Am J Med Genet Suppl 6:56-64, 1990). RSTS is caused by de novo mutations in epigenetics-associated genes, including the cAMP response element-binding protein (CREBBP), the gene-encoding protein referred to as CBP, and the EP300 gene, which encodes the p300 protein, a CBP homologue. Recent studies of the epigenetic mechanisms underlying cognitive functions in mice provide direct evidence for the involvement of nuclear factors (e.g., CBP) in the control of higher cognitive functions. In fact, a role for CBP in higher cognitive function is suggested by the finding that RSTS is caused by heterozygous mutations at the CBP locus (Petrij et al., Nature 376:348-351, 1995). CBP was demonstrated to possess an intrinsic histone acetyltransferase activity (Ogryzko et al., Cell 87:953-959, 1996) that is required for CREB-mediated gene expression (Korzus et al., Science 279:703-707, 1998). The intrinsic protein acetyltransferase activity in CBP might directly destabilize promoter-bound nucleosomes, facilitating the activation of transcription. Due to the complexity of developmental abnormalities and the possible genetic compensation associated with this congenital disorder, however, it is difficult to establish a direct role for CBP in cognitive function in the adult brain. Although aspects of the clinical presentation in RSTS cases have been extensively studied, a spectrum of symptoms found in RSTS patients can be accessed only after birth, and, thus, prenatal genetic tests for this extremely rare genetic disorder are seldom considered. Even though there has been intensive research on the genetic and epigenetic function of the CREBBP gene in rodents, the etiology of this devastating congenital human disorder is largely unknown.

Nuclear calcium is required for human T cell activation

Monaco et al. demonstrate that calcium signals in activated human T cells consist of a cytoplasmic and a nuclear component, which are both required for the immune response. Blockade of nuclear calcium signaling inhibits T cell activation and induces an anergy-like state.

Calcium signals in stimulated T cells are generally considered single entities that merely trigger immune responses, whereas costimulatory events specify the type of reaction. Here we show that the “T cell calcium signal” is a composite signal harboring two distinct components that antagonistically control genomic programs underlying the immune response. Using human T cells from healthy individuals, we establish nuclear calcium as a key signal in human T cell adaptogenomics that drives T cell activation and is required for signaling to cyclic adenosine monophosphate response element–binding protein and the induction of CD25, CD69, interleukin-2, and γ-interferon. In the absence of nuclear calcium signaling, cytosolic calcium activating nuclear factor of activated T cells translocation directed the genomic response toward enhanced expression of genes that negatively modulate T cell activation and are associated with a hyporesponsive state. Thus, nuclear calcium controls the T cell fate decision between a proliferative immune response and tolerance. Modulators of nuclear calcium–driven transcription may be used to develop a new type of pro-tolerance immunosuppressive therapy.

The calmodulin-binding transcription activator CAMTA1 is required for long-term memory formation in mice

The formation of long-term memory requires signaling from the synapse to the nucleus to mediate neuronal activity-dependent gene transcription. Synapse-to-nucleus communication is initiated by influx of calcium ions through synaptic NMDA receptors and/or L-type voltage-gated calcium channels and involves the activation of transcription factors by calcium/calmodulin signaling in the nucleus. Recent studies have drawn attention to a new family of transcriptional regulators, the so-called calmodulin-binding transcription activator (CAMTA) proteins. CAMTAs are expressed at particularly high levels in the mouse and human brain, and we reasoned that, as calmodulin-binding transcription factors, CAMTAs may regulate the formation of long-term memory by coupling synaptic activity and calcium/calmodulin signaling to memory-related transcriptional responses. This hypothesis is supported by genetic studies that reported a correlation between Camta gene polymorphisms or mutations and cognitive capability in humans. Here, we show that acute knockdown of CAMTA1, but not CAMTA2, in the hippocampus of adult mice results in impaired performance in two memory tests, contextual fear conditioning and object-place recognition test. Short-term memory and neuronal morphology were not affected by CAMTA knockdown. Gene expression profiling in the hippocampus of control and CAMTA knockdown mice revealed a number of putative CAMTA1 target genes related to synaptic transmission and neuronal excitability. Patch clamp recordings in organotypic hippocampal slice cultures provided further evidence for CAMTA1-dependent changes in electrophysiological properties. In summary, our study provides experimental evidence that confirms previous human genetic studies and establishes CAMTA1 as a regulator of long-term memory formation.

Calcium, Reactive Oxygen Species, and Synaptic Plasticity

In this review article, we address how activity-dependent Ca2+ signaling is crucial for hippocampal synaptic/structural plasticity and discuss how changes in neuronal oxidative state affect Ca2+ signaling and synaptic plasticity. We also analyze current evidence indicating that oxidative stress and abnormal Ca2+ signaling contribute to age-related synaptic plasticity deterioration.

Functional Role of Mitochondrial and Nuclear BK Channels

BK channels are important for the regulation of many cell functions. The significance of plasma membrane BK channels in the control of action potentials, resting membrane potential, and neurotransmitter release is well established; however, the composition and functions of mitochondrial and nuclear BK (nBK) channels are largely unknown. In this chapter, we summarize the recent findings on the subcellular localization, biophysical, and pharmacological properties of mitochondrial and nBK channels and discuss their molecular identity and physiological functions.

Unique Regulatory Properties of Heterotetrameric Inositol 1,4,5-Trisphosphate Receptors Revealed by Studying Concatenated Receptor Constructs

The ability of inositol 1,4,5-trisphosphate receptors (IP3R) to precisely initiate and generate a diverse variety of intracellular Ca(2+) signals is in part mediated by the differential regulation of the three subtypes (R1, R2, and R3) by key functional modulators (IP3, Ca(2+), and ATP). However, the contribution of IP3R heterotetramerization to Ca(2+) signal diversity has largely been unexplored. In this report, we provide the first definitive biochemical evidence of endogenous heterotetramer formation. Additionally, we examine the contribution of individual subtypes within defined concatenated heterotetramers to the shaping of Ca(2+) signals. Under conditions where key regulators of IP3R function are optimal for Ca(2+) release, we demonstrate that individual monomers within heteromeric IP3Rs contributed equally toward generating a distinct 'blended' sensitivity to IP3 that is likely dictated by the unique IP3 binding affinity of the heteromers. However, under suboptimal conditions where [ATP] were varied, we found that one subtype dictated the ATP regulatory properties of heteromers. We show that R2 monomers within a heterotetramer were both necessary and sufficient to dictate the ATP regulatory properties. Finally, the ATP-binding site B in R2 critical for ATP regulation was mutated and rendered non-functional to address questions relating to the stoichiometry of IP3R regulation. Two intact R2 monomers were sufficient to maintain ATP regulation in R2 homotetramers. In summary, we demonstrate that heterotetrameric IP3R do not necessarily behave as the sum of the constituent subunits, and these properties likely extend the versatility of IP3-induced Ca(2+) signaling in cells expressing multiple IP3R isoforms.

Impaired acquisition of conditioned taste aversion memory induced by isoflurane is accompanied with calcineurin activation and Egr-1 down-regulation in amygdala in rats

Compared to neutral memory, emotional memory is extremely strong and persistent immediately after acquisition, therefore it may recruit specific mechanisms during acquisition. The calcineurin-dependent mechanisms engaging early growth response 1 (Egr-1) have been proved to determine the strength of emotional memory during establishment. Isoflurane, a widely used inhalation anesthetic, can interfere with the acquisition of emotional memory. We hypothesized that isoflurane impairs the acquisition of conditioned taste aversion (CTA) memory in rats and the Egr-1 expression regulation via calcineurin (CaN) and ERK signaling pathway is involved in isoflurane-induced repression of CTA memory. To examine this, we investigated the influence of isoflurane on CTA memory and the expression and activity of CaN, the phosphorylation level of ERK and the expression of Egr-1 in amygdala in response to CTA training in rats. The results showed that isoflurane exposure (1.5%, 2h) before training impaired the acquisition of CTA memory in rats. Isoflurane exposure increased the CaN activity and decreased the p-ERK and Egr-1 in amygdala in rats. These findings suggest that isoflurane can disrupt the establishment of aversion memory, and CaN activation associating with p-ERK and Egr-1 down-regulation may contribute to the isoflurane induced impairment of aversion memory acquisition.

Nuclear Calcium Buffering Capacity Shapes Neuronal Architecture

Calcium-binding proteins (CaBPs) such as parvalbumin are part of the cellular calcium buffering system that determines intracellular calcium diffusion and influences the spatiotemporal dynamics of calcium signals. In neurons, CaBPs are primarily localized to the cytosol and function, for example, in nerve terminals in short-term synaptic plasticity. However, CaBPs are also expressed in the cell nucleus, suggesting that they modulate nuclear calcium signals, which are key regulators of neuronal gene expression. Here we show that the calcium buffering capacity of the cell nucleus in mouse hippocampal neurons regulates neuronal architecture by modulating the expression levels of VEGFD and the complement factor C1q-c, two nuclear calcium-regulated genes that control dendrite geometry and spine density, respectively. Increasing the levels of nuclear calcium buffers by means of expression of a nuclearly targeted form of parvalbumin fused to mCherry (PV.NLS-mC) led to a reduction in VEGFD expression and, as a result, to a decrease in total dendritic length and complexity. In contrast, mRNA levels of the synapse pruning factor C1q-c were increased in neurons expressing PV.NLS-mC, causing a reduction in the density and size of dendritic spines. Our results establish a close link between nuclear calcium buffering capacity and the transcription of genes that determine neuronal structure. They suggest that the development of cognitive deficits observed in neurological conditions associated with CaBP deregulation may reflect the loss of necessary structural features of dendrites and spines.

Evolutionary and functional perspectives on signaling from neuronal surface to nucleus

Reliance on Ca(2+) signaling has been well-preserved through the course of evolution. While the complexity of Ca(2+) signaling pathways has increased, activation of transcription factors including CREB by Ca(2+)/CaM-dependent kinases (CaMKs) has remained critical for long-term plasticity. In C. elegans, the CaMK family is made up of only three members, and CREB phosphorylation is mediated by CMK-1, the homologue of CaMKI. CMK-1 nuclear translocation directly regulates adaptation of thermotaxis behavior in response to changes in the environment. In mammals, the CaMK family has been expanded from three to ten members, enabling specialization of individual elements of a signal transduction pathway and increased reliance on the CaMKII subfamily. This increased complexity enables private line communication between Ca(2+) sources at the cell surface and specific cellular targets. Using both new and previously published data, we review the mechanism of a γCaMKII-CaM nuclear translocation. This intricate pathway depends on a specific role for multiple Ca(2+)/CaM-dependent kinases and phosphatases: α/βCaMKII phosphorylates γCaMKII to trap CaM; CaN dephosphorylates γCaMKII to dispatch it to the nucleus; and PP2A induces CaM release from γCaMKII so that CaMKK and CaMKIV can trigger CREB phosphorylation. Thus, while certain basic elements have been conserved from C. elegans, evolutionary modifications offer opportunities for targeted communication, regulation of key nodes and checkpoints, and greater specificity and flexibility in signaling.

Prefrontal Cortex in Learning to Overcome Generalized Fear

Normal brain functioning relies critically on the ability to control appropriate behavioral responses to fearful stimuli. Overgeneralized fear is the major symptom of anxiety disorders including posttraumatic stress disorder. This review describes recent data demonstrating that the medial prefrontal cortex (mPFC) plays a critical role in the refining of cues that drive the acquisition of fear response. Recent studies on molecular mechanisms that underlie the role of mPFC in fear discrimination learning are discussed. These studies suggest that prefrontal N-methyl-D-aspartate receptors expressed in excitatory neurons govern fear discrimination learning via a mechanism involving cAMP response element-binding protein-dependent engagement of acetyltransferase.

Molecular Genetic Strategies in the Study of Corticohippocampal Circuits

The first reproductively viable genetically modified mice were created in 1982 by Richard Palmiter and Ralph Brinster (Palmiter RD, Brinster RL, Hammer RE, Trumbauer ME, Rosenfeld MG, Birnberg NC, Evans RM. 1982. Dramatic growth of mice that develop from eggs microinjected with metallothionein-growth hormone fusion genes. Nature 300: 611-615). In the subsequent 30 plus years, numerous ground-breaking technical advancements in genetic manipulation have paved the way for improved spatially and temporally targeted research. Molecular genetic studies have been especially useful for probing the molecules and circuits underlying how organisms learn and remember—one of the most interesting and intensively investigated questions in neuroscience research. Here, we discuss selected genetic tools, focusing on corticohippocampal circuits and their implications for understanding learning and memory.

Decoding calcium signaling across the nucleus

Calcium (Ca(2+)) is an important multifaceted second messenger that regulates a wide range of cellular events. A Ca(2+)-signaling toolkit has been shown to exist in the nucleus and to be capable of generating and modulating nucleoplasmic Ca(2+) transients. Within the nucleus, Ca(2+) controls cellular events that are different from those modulated by cytosolic Ca(2+). This review focuses on nuclear Ca(2+) signals and their role in regulating physiological and pathological processes.

An animal model of female adolescent cannabinoid exposure elicits a long-lasting deficit in presynaptic long-term plasticity

Cannabis continues to be the most accessible and popular illicit recreational drug. Whereas current data link adolescence cannabinoid exposure to increased risk for dependence on other drugs, depression, anxiety disorders and psychosis, the mechanism(s) underlying these adverse effects remains controversial. Here we show in a mouse model of female adolescent cannabinoid exposure deficient endocannabinoid (eCB)-mediated signaling and presynaptic forms of long-term depression at adult central glutamatergic synapses in the prefrontal cortex. Increasing endocannabinoid levels by blockade of monoacylglycerol lipase, the primary enzyme responsible for degrading the endocannabinoid 2-arachidonoylglycerol (2-AG), with the specific inhibitor JZL 184 ameliorates eCB-LTD deficits. The observed deficit in cortical presynaptic signaling may represent a neural maladaptation underlying network instability and abnormal cognitive functioning. Our study suggests that adolescent cannabinoid exposure may permanently impair brain functions, including the brain's intrinsic ability to appropriately adapt to external influences.

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.

Nuclear calcium signaling induces expression of the synaptic organizers Lrrtm1 and Lrrtm2

Calcium transients in the cell nucleus evoked by synaptic activity in hippocampal neurons function as a signaling end point in synapse-to-nucleus communication. As an important regulator of neuronal gene expression, nuclear calcium is involved in the conversion of synaptic stimuli into functional and structural changes of neurons. Here we identify two synaptic organizers, Lrrtm1 and Lrrtm2, as targets of nuclear calcium signaling. Expression of both Lrrtm1 and Lrrtm2 increased in a synaptic NMDA receptor- and nuclear calcium-dependent manner in hippocampal neurons within 2-4 h after the induction of action potential bursting. Induction of Lrrtm1 and Lrrtm2 occurred independently of the need for new protein synthesis and required calcium/calmodulin-dependent protein kinases and the nuclear calcium signaling target CREB-binding protein. Analysis of reporter gene constructs revealed a functional cAMP response element in the proximal promoter of Lrrtm2, indicating that at least Lrrtm2 is regulated by the classical nuclear Ca(2+)/calmodulin-dependent protein kinase IV-CREB/CREB-binding protein pathway. These results suggest that one mechanism by which nuclear calcium signaling controls neuronal network function is by regulating the expression of Lrrtm1 and Lrrtm2.

Nuclear BK channels regulate gene expression via the control of nuclear calcium signaling

Ion channels are essential for the regulation of neuronal functions. The significance of plasma membrane, mitochondrial, endoplasmic reticulum and lysosomal ion channels in the regulation of Ca(2+) is well established. In contrast, surprisingly little is known about the function of ion channels on the nuclear envelope (NE). Here we demonstrate the presence of functional large-conductance, calcium-activated potassium channels (BK channels) on the NE of rodent hippocampal neurons. Functionally, blockade of nuclear BK channels (nBK channels) induces NE-derived Ca(2+) release, nucleoplasmic Ca(2+) elevation and cyclic AMP response element binding protein (CREB)-dependent transcription. More importantly, blockade of nBK channels regulates nuclear Ca(2+)-sensitive gene expression and promotes dendritic arborization in a nuclear Ca(2+)-dependent manner. These results suggest that the nBK channel functions as a molecular link between neuronal activity and nuclear Ca(2+) to convey signals from synapse to nucleus and is a new modulator, operating at the NE, of synaptic activity-dependent neuronal functions.

Cyclic nucleotide-gated channels, calmodulin, adenylyl cyclase, and calcium/calmodulin-dependent protein kinase II are required for late, but not early, long-term memory formation in the honeybee

Memory is a dynamic process that allows encoding, storage, and retrieval of information acquired through individual experience. In the honeybee Apis mellifera, olfactory conditioning of the proboscis extension response (PER) has shown that besides short-term memory (STM) and mid-term memory (MTM), two phases of long-term memory (LTM) are formed upon multiple-trial conditioning: an early phase (e-LTM) which depends on translation from already available mRNA, and a late phase (l-LTM) which requires de novo transcription and translation. Here we combined olfactory PER conditioning and neuropharmacological inhibition and studied the involvement of the NO-cGMP pathway, and of specific molecules, such as cyclic nucleotide-gated channels (CNG), calmodulin (CaM), adenylyl cyclase (AC), and Ca(2+)/calmodulin-dependent protein kinase (CaMKII), in the formation of olfactory LTM in bees. We show that in addition to NO-cGMP and cAMP-PKA, CNG channels, CaM, AC, and CaMKII also participate in the formation of a l-LTM (72-h post-conditioning) that is specific for the learned odor. Importantly, the same molecules are dispensable for olfactory learning and for the formation of both MTM (in the minute and hour range) and e-LTM (24-h post-conditioning), thus suggesting that the signaling pathways leading to l-LTM or e-LTM involve different molecular actors.

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.

Requirement for nuclear calcium signaling in Drosophila long-term memory

Calcium is used throughout evolution as an intracellular signal transducer. In the mammalian central nervous system, calcium mediates the dialogue between the synapse and the nucleus that is required for transcription-dependent persistent neuronal adaptations. A role for nuclear calcium signaling in similar processes in the invertebrate brain has yet to be investigated. Here, we show by in vivo calcium imaging of adult brain neurons of the fruit fly Drosophila melanogaster, that electrical foot shocks used in olfactory avoidance conditioning evoked transient increases in cytosolic and nuclear calcium concentrations in neurons. These calcium signals were detected in Kenyon cells of the flies' mushroom bodies, which are sites of learning and memory related to smell. Acute blockade of nuclear calcium signaling during conditioning selectively and reversibly abolished the formation of long-term olfactory avoidance memory, whereas short-term, middle-term, or anesthesia-resistant olfactory memory remained unaffected. Thus, nuclear calcium signaling is required in flies for the progression of memories from labile to transcription-dependent long-lasting forms. These results identify nuclear calcium as an evolutionarily conserved signal needed in both invertebrate and vertebrate brains for transcription-dependent memory consolidation.

Nuclear calcium signaling in spinal neurons drives a genomic program required for persistent inflammatory pain

Persistent pain induced by noxious stimuli is characterized by the transition from normosensitivity to hypersensitivity. Underlying mechanisms are not well understood, although gene expression is considered important. Here, we show that persistent nociceptive-like activity triggers calcium transients in neuronal nuclei within the superficial spinal dorsal horn, and that nuclear calcium is necessary for the development of long-term inflammatory hypersensitivity. Using a nucleus-specific calcium signal perturbation strategy in vivo complemented by gene profiling, bioinformatics, and functional analyses, we discovered a pain-associated, nuclear calcium-regulated gene program in spinal excitatory neurons. This includes C1q, a modulator of synaptic spine morphogenesis, which we found to contribute to activity-dependent spine remodelling on spinal neurons in a manner functionally associated with inflammatory hypersensitivity. Thus, nuclear calcium integrates synapse-to-nucleus communication following noxious stimulation and controls a spinal genomic response that mediates the transition between acute and long-term nociceptive sensitization by modulating functional and structural plasticity.

Nuclear calcium signaling regulates nuclear export of a subset of class IIa histone deacetylases following synaptic activity

In neurons, dynamic changes in the subcellular localization of histone deacetylases (HDACs) are thought to contribute to signal-regulated gene expression. Here we show that in mouse hippocampal neurons, synaptic activity-dependent nucleo-cytoplasmic shuttling is a common feature of all members of class IIa HDACs, which distinguishes them from other classes of HDACs. Nuclear calcium, a key regulator in neuronal gene expression, is required for the nuclear export of a subset of class IIa HDACs. We found that inhibition of nuclear calcium signaling using CaMBP4 or increasing the nuclear calcium buffering capacity by means of expression of a nuclear targeted version of parvalbumin (PV.NLS-mC) led to a build-up of HDAC4 and HDAC5 in the cell nucleus, which in the case of PV.NLS-mC can be reversed by nuclear calcium transients triggered by bursts of action potential firing. A similar nuclear accumulation of HDAC4 and HDAC5 was observed in vivo in the mouse hippocampus following stereotaxic delivery of recombinant adeno-associated viruses expressing either CaMBP4 or PV.NLS-mC. The modulation of HDAC4 activity either by RNA interference-mediated reduction of HDAC4 protein levels or by expression of a constitutively nuclear localized mutant of HDAC4 leads to changes in the mRNA levels of several nuclear calcium-regulated genes with known functions in acquired neuroprotection (atf3, serpinb2), memory consolidation (homer1, arc), and the development of chronic pain (ptgs2, c1qc). These results identify nuclear calcium as a regulator of nuclear export of HDAC4 and HDAC5. The reduction of nuclear localized HDACs represents a novel transcription-promoting pathway stimulated by nuclear calcium.

Environmental enrichment and gut inflammation modify stress-induced c-Fos expression in the mouse corticolimbic system

Environmental enrichment (EE) has a beneficial effect on rodent behaviour, neuronal plasticity and brain function. Although it may also improve stress coping, it is not known whether EE influences the brain response to an external (psychological) stressor such as water avoidance stress (WAS) or an internal (systemic) stressor such as gastrointestinal inflammation. This study hence explored whether EE modifies WAS-induced activation of the mouse corticolimbic system and whether this stress response is altered by gastritis or colitis. Male C67BL/6N mice were housed under standard or enriched environment for 9 weeks, after which they were subjected to a 1-week treatment with oral iodoacetamide to induce gastritis or oral dextran sulfate sodium to induce colitis. Following exposure to WAS the expression of c-Fos, a marker of neuronal activation, was measured by immunocytochemistry. EE aggravated experimentally induced colitis, but not gastritis, as shown by an increase in the disease activity score and the colonic myeloperoxidase content. In the brain, EE enhanced the WAS-induced activation of the dentate gyrus and unmasked an inhibitory effect of gastritis and colitis on WAS-evoked c-Fos expression within this part of the hippocampus. Conversely, EE inhibited the WAS-evoked activation of the central amygdala and prevented the inhibitory effect of gastritis and colitis on WAS-evoked c-Fos expression in this region. EE, in addition, blunted the WAS-induced activation of the infralimbic cortex and attenuated the inhibitory effect of gastritis and colitis on WAS-evoked c-Fos expression in this area. These data reveal that EE has a region-specific effect on stress-induced c-Fos expression in the corticolimbic system, which is likely to improve stress resilience. The response of the prefrontal cortex – amygdala – hippocampus circuitry to psychological stress is also modified by the systemic stress of gut inflammation, and this interaction between external and internal stressors is modulated by the housing environment.

Nuclear calcium signaling controls methyl-CpG-binding protein 2 (MeCP2) phosphorylation on serine 421 following synaptic activity

The function of MeCP2, a methylated DNA-interacting protein that may act as a global chromatin modifier, is controlled by its phosphorylation on serine 421. Here we show that in hippocampal neurons, nuclear calcium signaling controls synaptic activity-induced phosphorylation of MeCP2 on serine 421. Pharmacological inhibition of calcium/calmodulin-dependent protein (CaM)kinases blocked activity-induced MeCP2 serine 421 phosphorylation. CaM kinase II (CaMKII) but not CaMKIV, the major nuclear CaM kinase in hippocampal neurons, appeared to mediate this phosphorylation event. Biochemical subcellular fractionations and immunolocalization studies revealed that several isoforms of CaMKII (i.e. CaMKIIα, -β, -γ, and -δ) are expressed in the cytosol but are also detectable in the cell nucleus of hippocampal neurons, suggesting that nuclear CaMKII catalyzes MeCP2 serine 421 phosphorylation. Thus, in addition to the classical nuclear calcium-CaMKIV-CREB/CBP (cAMP-response element-binding protein/CREB-binding protein) pathway that regulates transcription of specific target genes, nuclear calcium may also modulate genome-wide the chromatin state in response to synaptic activity via nuclear CaMKII-MeCP2 signaling.