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

Non-canonical heterogeneous cellular distribution and co-localization of CaMKIIα and CaMKIIβ in the spinal superficial dorsal horn

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a key enzyme in long-term plasticity in many neurons, including in the nociceptive circuitry of the spinal dorsal horn. However, although the role of CaMKII heterooligomers in neuronal plasticity is isoform-dependent, the distribution and co-localization of CaMKII isoforms in the dorsal horn have not been comprehensively investigated. Here, quantitative immunofluorescence analysis was used to examine the distribution of the two major neuronal CaMKII isoforms, α and β, in laminae I–III of the rat dorsal horn, with reference to inhibitory interneurons and neuronal populations defined by expression of parvalbumin, calretinin, and calbindin D28k. Unexpectedly, all or nearly all inhibitory and excitatory neurons showed both CaMKIIα and CaMKIIβ immunoreactivity, although at highly variable levels. Lamina III neurons showed less CaMKIIα immunoreactivity than laminae I–II neurons. Whereas CaMKIIα immunoreactivity was found at nearly similar levels in inhibitory and excitatory neurons, CaMKIIβ generally showed considerably lower immunoreactivity in inhibitory neurons. Distinct populations of inhibitory calretinin neurons and excitatory parvalbumin neurons exhibited high CaMKIIα-to-CaMKIIβ immunoreactivity ratios. CaMKIIα and CaMKIIβ immunoreactivity showed positive correlation at GluA2⁺ puncta in pepsin-treated tissue. These results suggest that, unlike the forebrain, the dorsal horn is characterized by similar expression of CaMKIIα in excitatory and inhibitory neurons, whereas CaMKIIβ is less expressed in inhibitory neurons. Moreover, CaMKII isoform expression varies considerably within and between neuronal populations defined by laminar location, calcium-binding protein expression, and transmitter phenotype, suggesting differences in CaMKII function both between and within neuronal populations in the superficial dorsal horn.

Transient inhibition of LIMKs significantly attenuated central sensitization and delayed the development of chronic pain

Central sensitization represents a key mechanism mediating chronic pain, a major clinical problem lacking effective treatment options. LIM-domain kinases (LIMKs) selectively regulate several substrates, e.g. cofilin and cAMP response element-binding protein (CREB), that profoundly affect neural activities, such as synaptogenesis and gene expression, thus critical in the consolidation of long-term synaptic potentiation and memory in the brain. In this study, we demonstrate that LIMK deficiency significantly impaired the development of multiple forms of chronic pain. Mechanistic studies focusing on spared nerve injury (SNI) model reveal a pivotal role of LIMKs in the up-regulation of spontaneous excitatory synaptic transmission and synaptogenesis after pain induction. Depending on the pain induction methods, LIMKs can be transiently activated with distinct time courses. Accordingly, pharmacological inhibition of LIMKs targeting this critical period remarkably attenuated central sensitization in the spinal cord and alleviated pain behaviors. We propose selectively targeting LIMKs during their activation phase as a potential therapeutic strategy for clinical management of chronic pain, especially for chronic pain with predictable onset and development time courses, such as chronic post-surgical pain (PSP).

CXCL1/CXCR2 signaling in pathological pain: Role in peripheral and central sensitization

Pathological pain conditions can be triggered after peripheral nerve injury and/or inflammation. It is associated with plasticity of nociceptive pathway in which pain is prolonged even after healing of the injured tissue. Generally combinations of analgesic drugs are not sufficient to achieve selective palliation from chronic pain, besides causing a greater number of side effects. In order to identify novel alternatives for more effective treatments, it is necessary to clarify the underlying mechanisms of pathological pain. It is well established that there are two main components in pathological pain development and maintenance: (i) primary sensory neuron sensitization (peripheral sensitization), and (ii) central sensitization. In both components cytokines and chemokines act as key mediators in pain modulation. CXCL1 is a chemokine that promote both nociceptor and central sensitization via its main receptor CXCR2, which is a promising target for novel analgesic drugs. Here, we reviewed and discussed the role of the CXCL1/CXCR2 signaling axis in pathological pain conditions triggered by either peripheral inflammation or nerve injury.

Car8 dorsal root ganglion expression and genetic regulation of analgesic responses are associated with a cis-eQTL in mice

Carbonic anhydrase-8 (Car8 mouse gene symbol) is devoid of enzymatic activity, but instead functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) to regulate this intracellular calcium release channel important in synaptic functions and neuronal excitability. Causative mutations in ITPR1 and carbonic anhydrase-8 in mice and humans are associated with certain subtypes of spinal cerebellar ataxia (SCA). SCA mice are genetically deficient in dorsal root ganglia (DRG) Car8 expression and display mechanical and thermal hypersensitivity and susceptibility to subacute and chronic inflammatory pain behaviors. In this report, we show that DRG Car8 expression is variable across 25 naïve-inbred strains of mice, and this cis-regulated eQTL (association between rs27660559, rs27706398, and rs27688767 and DRG Car8 expression; P < 1 × 10^-11) is correlated with nociceptive responses in mice. Next, we hypothesized that increasing DRG Car8 gene expression would inhibit intracellular calcium release required for morphine antinociception and might correlate with antinociceptive sensitivity of morphine and perhaps other analgesic agents. We show that mean DRG Car8 gene expression is directly related to the dose of morphine or clonidine needed to provide a half-maximal analgesic response (r = 0.93, P < 0.00002; r = 0.83, P < 0.0008, respectively), suggesting that greater DRG Car8 expression increases analgesic requirements. Finally, we show that morphine induces intracellular free calcium release using Fura 2 calcium imaging in a dose-dependent manner; V5-Car8 ^WT overexpression in NBL cells inhibits morphine-induced calcium increase. These findings highlight the 'morphine paradox' whereby morphine provides antinociception by increasing intracellular free calcium, while Car8 and other antinociceptive agents work by decreasing intracellular free calcium. This is the first study demonstrating that biologic variability associated with this cis-eQTL may contribute to differing analgesic responses through altered regulation of ITPR1-dependent calcium release in mice.

Calcium signalling through L-type calcium channels: role in pathophysiology of spinal nociceptive transmission

L-type voltage-gated calcium channels are ubiquitous channels in the CNS. L-type calcium channels (LTCs) are mostly post-synaptic channels regulating neuronal firing and gene expression. They play a role in important physio-pathological processes such as learning and memory, Parkinson's disease, autism and, as recognized more recently, in the pathophysiology of pain processes. Classically, the fundamental role of these channels in cardiovascular functions has limited the use of classical molecules to treat LTC-dependent disorders. However, when applied locally in the dorsal horn of the spinal cord, the three families of LTC pharmacological blockers - dihydropyridines (nifedipine), phenylalkylamines (verapamil) and benzothiazepines (diltiazem) - proved effective in altering short-term sensitization to pain, inflammation-induced hyperexcitability and neuropathy-induced allodynia. Two subtypes of LTCs, Ca_v 1.2 and Ca_v 1.3, are expressed in the dorsal horn of the spinal cord, where Ca_v 1.2 channels are localized mostly in the soma and proximal dendritic shafts, and Ca_v 1.3 channels are more distally located in the somato-dendritic compartment. Together with their different kinetics and pharmacological properties, this spatial distribution contributes to their separate roles in shaping short- and long-term sensitization to pain. Ca_v 1.3 channels sustain the expression of plateau potentials, an input/output amplification phenomenon that contributes to short-term sensitization to pain such as prolonged after-discharges, dynamic receptive fields and windup. The Ca_v 1.2 channels support calcium influx that is crucial for the excitation-transcription coupling underlying nerve injury-induced dorsal horn hyperexcitability. These subtype-specific cellular mechanisms may have different consequences in the development and/or the maintenance of pathological pain. Recent progress in developing more specific compounds for each subunit will offer new opportunities to modulate LTCs for the treatment of pathological pain with reduced side-effects.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

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.

Therapeutic targeting of the pathological triad of extrasynaptic NMDA receptor signaling in neurodegenerations

Activation of extrasynaptic N-methyl-d-aspartate (NMDA) receptors causes neurodegeneration and cell death. The disease mechanism involves a pathological triad consisting of mitochondrial dysfunction, loss of integrity of neuronal structures and connectivity, and disruption of excitation-transcription coupling caused by CREB (cyclic adenosine monophosphate-responsive element-binding protein) shut-off and nuclear accumulation of class IIa histone deacetylases. Interdependency within the triad fuels an accelerating disease progression that culminates in failure of mitochondrial energy production and cell loss. Both acute and slowly progressive neurodegenerative conditions, including stroke, Alzheimer's disease, amyotrophic lateral sclerosis, and Huntington's disease, share increased death signaling by extrasynaptic NMDA receptors caused by elevated extracellular glutamate concentrations or relocalization of NMDA receptors to extrasynaptic sites. Six areas of therapeutic objectives are defined, based on which a broadly applicable combination therapy is proposed to combat the pathological triad of extrasynaptic NMDA receptor signaling that is common to many neurodegenerative diseases.

Exploring cells with targeted biosensors

Cellular signaling networks are composed of multiple pathways, often interconnected, that form complex networks with great potential for cross-talk. Signal decoding depends on the nature of the message as well as its amplitude, temporal pattern, and spatial distribution. In addition, the existence of membrane-bound organelles, which are both targets and generators of messages, add further complexity to the system. The availability of sensors that can localize to specific compartments in live cells and monitor their targets with high spatial and temporal resolution is thus crucial for a better understanding of cell pathophysiology. For this reason, over the last four decades, a variety of strategies have been developed, not only to generate novel and more sensitive probes for ions, metabolites, and enzymatic activity, but also to selectively deliver these sensors to specific intracellular compartments. In this review, we summarize the principles that have been used to target organic or protein sensors to different cellular compartments and their application to cellular signaling.

Exploring novel paths towards protein signatures of chronic pain

Pain is a major symptom of many medical conditions and the worldwide number one reason for people to seek medical assistance. It affects the quality of life of patients and poses a heavy financial burden on society with high costs of treatment and lost productivity. Furthermore, the treatment of chronic pain presents a big challenge as pain therapeutics often lack efficacy and exhibit minimal safety profiles. The latter can be largely attributed to the fact that current therapies target molecules with key physiological functions throughout the body. In light of these difficulties, the identification of proteins specifically involved in chronic pain states is of paramount importance for designing selective interventions. Several profiling efforts have been employed with the aim to dissect the molecular underpinnings of chronic pain, both on the level of the transcriptome and proteome. However, generated results are often inconsistent and non-overlapping, which is largely due to inherent technical constraints. A potential solution may be offered by emerging strategies capable of performing standardized and reproducible proteome analysis, such as data-independent acquisition-mass spectrometry (DIA-MS). We have recently demonstrated the applicability of DIA-MS to interrogate chronic pain-related proteome alterations in mice. Based on our results, we aim to provide an overview on DIA-MS and its potential to contribute to the comprehensive characterization of molecular signatures underlying pain pathologies.

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.

Brain globins in physiology and pathology

Globins are globular proteins for either transport or storage of oxygen which are critical for cellular metabolism. Four globins have been identified in rodent and human brains. Among them, neuroglobin, cytoglobin and hemoglobin chains are constitutively expressed in normal brain, while myoglobin is only expressed in some neurological disorders. Studies on the molecular structure, expression and functional features of these brain globins indicated that they may play crucial roles in maintenance of neural cell survival and activity, including neurons and astrocytes. Their regulation in neurological disorders may help thoroughly understand initiation and progression of ischemia, Alzheimer's disease and glioma, etc. Elucidation of the brain globin functions might remarkably improve medical strategies that sustain neurological homeostasis and treat neurological diseases. Here the expression pattern and functions of brain globins and their involvement in neurological disorders are reviewed.

The Central Analgesic Mechanism of YM-58483 in Attenuating Neuropathic Pain in Rats

Calcium channel antagonists are commonly used to treat neuropathic pain. Their analgesic effects rely on inhibiting long-term potentiation, and neurotransmitters release in the spinal cord. Store-operated Ca(2+)channels (SOCCs) are highly Ca(2+)-selective cation channels broadly expressed in non-excitable cells and some excitable cells. Recent studies have shown that the potent inhibitor of SOCCs, YM-58483, has analgesic effects on neuropathic pain, but its mechanism is unclear. This experiment performed on spinal nerve ligation (SNL)-induced neuropathic pain model in rats tries to explore the mechanism, whereby YM-58483 attenuates neuropathic pain. The left L5 was ligated to produce the SNL neuropathic pain model in male Sprague-Dawley rats. The withdrawal threshold of rats was measured by the up-down method and Hargreaves' method before and after intrathecal administration of YM-58483 and vehicle. The SOCCs in the spinal dorsal horn were located by immunofluorescence. The expression of phosphorylated ERK and phosphorylated CREB, CD11b, and GFAP proteins in spinal level was tested by Western blot, while the release of proinflammatory cytokines (IL-1β, TNF-α, PGE2) was measured by enzyme-linked immunosorbent assay (ELISA). Intrathecal YM-58483 at the concentration of 300 μM (1.5 nmol) and 1000 μM (10 nmol) produced a significant central analgesic effect on the SNL rats, compared with control + vehicle (n = 7, P < 0.001). However, both could not prevent the development of neuropathic pain, compared with normal + saline (P < 0.001). Immunofluorescent staining revealed that Orai1 and STIM1 (the two key components of SOCCs) were located in the spinal dorsal horn neurons. Western blot showed that YM-58483 could decrease the levels of P-ERK and P-CREB (n = 10, #P < 0.05), without affecting the expression of CD11b and GFAP (n = 10, #P > 0.05). YM-58483 also inhibited the release of spinal cord IL-1β, TNF-α, and PGE2, compared with control + vehicle (n = 5, #P < 0.001). The analgesic mechanism of YM-58483 may be via inhibiting central ERK/CREB signaling in the neurons and decreasing central IL-1β, TNF-α, and PGE2 release to reduce neuronal excitability in the spinal dorsal horn of the SNL rats.

Roles of prefrontal cortex and paraventricular thalamus in affective and mechanical components of visceral nociception

Visceral pain represents a major clinical challenge in the management of many gastrointestinal disorders, eg, pancreatitis. However, cerebral neurobiological mechanisms underlying visceral nociception are poorly understood. As a representative model of visceral nociception, we applied cerulein hyperstimulation in C57BL6 mice to induce acute pancreatitis and performed a behavioral test battery and c-Fos staining of brains. We observed a specific pain phenotype and a significant increase in c-Fos immunoreactivity in the paraventricular nucleus of the thalamus (PVT), the periaqueductal gray, and the medial prefrontal cortex (mPFC). Using neuronal tracing, we observed projections of the PVT to cortical layers of the mPFC with contacts to inhibitory GABAergic neurons. These inhibitory neurons showed more activation after cerulein treatment suggesting thalamocortical "feedforward inhibition" in visceral nociception. The activity of neurons in pancreatitis-related pain centers was pharmacogenetically modulated by designer receptors exclusively activated by designer drugs, selectively and cell type specifically expressed in target neurons using adeno-associated virus-mediated gene transfer. Pharmacogenetic inhibition of PVT but not periaqueductal gray neurons attenuated visceral pain and induced an activation of the descending inhibitory pain pathway. Activation of glutamatergic principle neurons in the mPFC, but not inhibitory neurons, also reversed visceral nociception. These data reveal novel insights into central pain processing that underlies visceral nociception and may trigger the development of novel, potent centrally acting analgesic drugs.

Standardized Profiling of The Membrane-Enriched Proteome of Mouse Dorsal Root Ganglia (DRG) Provides Novel Insights Into Chronic Pain

Chronic pain is a complex disease with limited treatment options. Several profiling efforts have been employed with the aim to dissect its molecular underpinnings. However, generated results are often inconsistent and nonoverlapping, which is largely because of inherent technical constraints. Emerging data-independent acquisition (DIA)-mass spectrometry (MS) has the potential to provide unbiased, reproducible and quantitative proteome maps - a prerequisite for standardization among experiments. Here, we designed a DIA-based proteomics workflow to profile changes in the abundance of dorsal root ganglia (DRG) proteins in two mouse models of chronic pain, inflammatory and neuropathic. We generated a DRG-specific spectral library containing 3067 DRG proteins, which enables their standardized quantification by means of DIA-MS in any laboratory. Using this resource, we profiled 2526 DRG proteins in each biological replicate of both chronic pain models and respective controls with unprecedented reproducibility. We detected numerous differentially regulated proteins, the majority of which exhibited pain model-specificity. Our approach recapitulates known biology and discovers dozens of proteins that have not been characterized in the somatosensory system before. Functional validation experiments and analysis of mouse pain behaviors demonstrate that indeed meaningful protein alterations were discovered. These results illustrate how the application of DIA-MS can open new avenues to achieve the long-awaited standardization in the molecular dissection of pathologies of the somatosensory system. Therefore, our findings provide a valuable framework to qualitatively extend our understanding of chronic pain and somatosensation.

The evolutionarily conserved transcription factor PRDM12 controls sensory neuron development and pain perception

PR homology domain-containing member 12 (PRDM12) belongs to a family of conserved transcription factors implicated in cell fate decisions. Here we show that PRDM12 is a key regulator of sensory neuronal specification in Xenopus. Modeling of human PRDM12 mutations that cause hereditary sensory and autonomic neuropathy (HSAN) revealed remarkable conservation of the mutated residues in evolution. Expression of wild-type human PRDM12 in Xenopus induced the expression of sensory neuronal markers, which was reduced using various human PRDM12 mutants. In Drosophila, we identified Hamlet as the functional PRDM12 homolog that controls nociceptive behavior in sensory neurons. Furthermore, expression analysis of human patient fibroblasts with PRDM12 mutations uncovered possible downstream target genes. Knockdown of several of these target genes including thyrotropin-releasing hormone degrading enzyme (TRHDE) in Drosophila sensory neurons resulted in altered cellular morphology and impaired nociception. These data show that PRDM12 and its functional fly homolog Hamlet are evolutionary conserved master regulators of sensory neuronal specification and play a critical role in pain perception. Our data also uncover novel pathways in multiple species that regulate evolutionary conserved nociception.

The Cannabinoid Receptor CB1 Interacts with the WAVE1 Complex and Plays a Role in Actin Dynamics and Structural Plasticity in Neurons

The molecular composition of the cannabinoid type 1 (CB1) receptor complex beyond the classical G-protein signaling components is not known. Using proteomics on mouse cortex in vivo, we pulled down proteins interacting with CB1 in neurons and show that the CB1 receptor assembles with multiple members of the WAVE1 complex and the RhoGTPase Rac1 and modulates their activity. Activation levels of CB1 receptor directly impacted on actin polymerization and stability via WAVE1 in growth cones of developing neurons, leading to their collapse, as well as in synaptic spines of mature neurons, leading to their retraction. In adult mice, CB1 receptor agonists attenuated activity-dependent remodeling of dendritic spines in spinal cord neurons in vivo and suppressed inflammatory pain by regulating the WAVE1 complex. This study reports novel signaling mechanisms for cannabinoidergic modulation of the nervous system and demonstrates a previously unreported role for the WAVE1 complex in therapeutic applications of cannabinoids.

A proteomic study reveals the actin nucleation complex WAVE1 as a hitherto unknown binding partner of cannabinoid receptor 1 and explores the functional role of this interaction in regulating pain-related structural plasticity.

One of the most interesting features of the endocannabinoid system (a group of neuromodulatory lipids and their receptors, which promotes homeostasis in a variety of physiological processes) is its ability to counteract nociception or pain. This function is largely mediated by the receptor component of the endocannabinoid system. One of the most-studied types of cannabinoid receptors, the cannabinoid receptor 1 (CB1R), exerts its antinociceptive function at all levels of the central nervous system, from the periphery up to the brain. Despite numerous studies on the role of CB1R and its antinociceptive effect, our knowledge of the molecular mechanisms underlying this particular feature is still lacking. In this study, we identify the WAVE1-complex—known to be involved in actin nucleation—as novel interacting partners of CB1R. We observe a functional relationship between the WAVE1-complex and CB1R in the regulation of actin filaments in developing as well as mature cultured neurons. Furthermore, we show that inflammation-induced structural plasticity in spinal neurons that contributes to hyperalgesia is regulated by CB1R in a WAVE1-dependent fashion. These findings expand our understanding of CB1R signaling and of the physiological as well as pathological context of pain.

A new minimally-invasive method for microinjection into the mouse spinal dorsal horn

Noninvasive gene delivery to the spinal dorsal horn (SDH) remains challenging because existing methods to directly microinject vectors require laminectomy, which leads to tissue damage and inflammation. Such responses might hamper accurate readouts of cellular and behavioural effects of an introduced gene. Here we develop a new minimally-invasive SDH microinjection technique without the need of laminectomy in which a microcapillary is inserted into the SDH parenchyma through an intervertebral space. Using this method, we microinjected adeno-associated virus with an astrocytic promoter into the SDH and achieved efficient gene expression in an astrocyte-specific manner without gliosis, neuronal loss or inflammation. Furthermore, astrocytic loss- and gain-of-function of the transcription factor STAT3 by expressing a dominant-negative form and a constitutive-active form of STAT3, respectively, demonstrated the necessity and sufficiency of astrocytic STAT3 in the maintenance of neuropathic pain following peripheral nerve injury, a debilitating chronic pain state in which currently available treatments are frequently ineffective. Thus, our technique enables manipulation of gene expression in cell type- and spatial-specific manners without adverse effects, and may be useful for research in SDH physiology and pathology.

[Pharmacological aspects of pain research in Germany]

In spite of several approved analgesics, the therapy of pain still constitutes a challenge due to the fact that the drugs do not exert sufficient efficacy or are associated with severe side effects. Therefore, the development of new and improved painkillers is still of great importance. A number of highly qualified scientists in Germany are investigating signal transduction pathways in pain, effectivity of new drugs and the so far incompletely investigated mechanisms of well-known analgesics in preclinical and clinical studies. The highlights of pharmacological pain research in Germany are summarized in this article.

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.

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.

A role for Kalirin-7 in nociceptive sensitization via activity-dependent modulation of spinal synapses

Synaptic plasticity is the cornerstone of processes underlying persistent nociceptive activity-induced changes in normal nociceptive sensitivity. Kalirin-7 is a multifunctional guanine-nucleotide-exchange factor (GEF) for Rho GTPases that is characterized by its localization at excitatory synapses, interactions with glutamate receptors and its ability to dynamically modulate the neuronal cytoskeleton. Here we show that spinally expressed Kalirin-7 is required for persistent nociceptive activity-dependent synaptic long-term potentiation as well as activity-dependent remodelling of synaptic spines in the spinal dorsal horn, thereby orchestrating functional and structural plasticity during the course of inflammatory pain.

Carbonic anhydrase-8 regulates inflammatory pain by inhibiting the ITPR1-cytosolic free calcium pathway

Calcium dysregulation is causally linked with various forms of neuropathology including seizure disorders, multiple sclerosis, Huntington’s disease, Alzheimer’s, spinal cerebellar ataxia (SCA) and chronic pain. Carbonic anhydrase-8 (Car8) is an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1), which regulates intracellular calcium release fundamental to critical cellular functions including neuronal excitability, neurite outgrowth, neurotransmitter release, mitochondrial energy production and cell fate. In this report we test the hypothesis that Car8 regulation of ITPR1 and cytoplasmic free calcium release is critical to nociception and pain behaviors. We show Car8 null mutant mice (MT) exhibit mechanical allodynia and thermal hyperalgesia. Dorsal root ganglia (DRG) from MT also demonstrate increased steady-state ITPR1 phosphorylation (pITPR1) and cytoplasmic free calcium release. Overexpression of Car8 wildtype protein in MT nociceptors complements Car8 deficiency, down regulates pITPR1 and abolishes thermal and mechanical hypersensitivity. We also show that Car8 nociceptor overexpression alleviates chronic inflammatory pain. Finally, inflammation results in downregulation of DRG Car8 that is associated with increased pITPR1 expression relative to ITPR1, suggesting a possible mechanism of acute hypersensitivity. Our findings indicate Car8 regulates the ITPR1-cytosolic free calcium pathway that is critical to nociception, inflammatory pain and possibly other neuropathological states. Car8 and ITPR1 represent new therapeutic targets for chronic pain.

Annexin A2 regulates TRPA1-dependent nociception

The transient receptor potential A1 (TRPA1) channel is essential for vertebrate pain. Even though TRPA1 activation by ligands has been studied extensively, the molecular machinery regulating TRPA1 is only poorly understood. Using an unbiased proteomics-based approach we uncovered the physical association of Annexin A2 (AnxA2) with native TRPA1 in mouse sensory neurons. AnxA2 is enriched in a subpopulation of sensory neurons and coexpressed with TRPA1. Furthermore, we observe an increase of TRPA1 membrane levels in cultured sensory neurons from AnxA2-deficient mice. This is reflected by our calcium imaging experiments revealing higher responsiveness upon TRPA1 activation in AnxA2-deficient neurons. In vivo these findings are associated with enhanced nocifensive behaviors specifically in TRPA1-dependent paradigms of acute and inflammatory pain, while heat and mechanical sensitivity as well as TRPV1-mediated pain are preserved in AnxA2-deficient mice. Our results support a model whereby AnxA2 limits the availability of TRPA1 channels to regulate nociceptive signaling in vertebrates.

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.

PKA is required for the modulation of spinal nociceptive information related to ephrinB-EphB signaling in mice

EphB receptors and their ephrinB ligands are implicated in modulating of spinal nociceptive information processing. Here, we investigated whether protein kinase A (PKA), acts as a downstream effector, participates in the modulation spinal nociceptive information related to ephrinB-EphB signaling. Intrathecal injection of ephrinB2-Fc caused thermal hyperalgesia and mechanical allodynia, which were accompanied by increased expression of spinal PKA catalytic subunit (PKAca) and phosphorylated cAMP-response element-binding protein (p-CREB). Pre-treatment with H89, a PKA inhibitor, prevented the activation of CREB by ephrinB2-Fc. Inhibition of spinal PKA signaling prevented and reversed pain behaviors induced by the intrathecal injection of ephrinB2-Fc. Furthermore, blockade of the EphB receptors by intrathecal injection of EphB2-Fc reduced formalin-induced inflammatory, chronic constrictive injury (CCI)-induced neuropathic, and tibia bone cavity tumor cell implantation (TCI)-induced bone cancer pain behaviors, which were accompanied by decreased expression of spinal PKAca and p-CREB. Overall, these results confirmed the important involvement of PKA in the modulation of spinal nociceptive information related to ephrinBs-EphBs signaling. This finding may have important implications for exploring the roles and mechanisms of ephrinB-EphB signaling in physiologic and pathologic pain.

Heterogeneous presynaptic distribution of monoacylglycerol lipase, a multipotent regulator of nociceptive circuits in the mouse spinal cord

Monoacylglycerol lipase (MGL) is a multifunctional serine hydrolase, which terminates anti-nociceptive endocannabinoid signaling and promotes pro-nociceptive prostaglandin signaling. Accordingly, both acute nociception and its sensitization in chronic pain models are prevented by systemic or focal spinal inhibition of MGL activity. Despite its analgesic potential, the neurobiological substrates of beneficial MGL blockade have remained unexplored. Therefore, we examined the regional, cellular and subcellular distribution of MGL in spinal circuits involved in nociceptive processing. All immunohistochemical findings obtained with light, confocal or electron microscopy were validated in MGL-knockout mice. Immunoperoxidase staining revealed a highly concentrated accumulation of MGL in the dorsal horn, especially in superficial layers. Further electron microscopic analysis uncovered that the majority of MGL-immunolabeling is found in axon terminals forming either asymmetric glutamatergic or symmetric γ-aminobutyric acid/glycinergic synapses in laminae I/IIo. In line with this presynaptic localization, analysis of double-immunofluorescence staining by confocal microscopy showed that MGL colocalizes with neurochemical markers of peptidergic and non-peptidergic nociceptive terminals, and also with markers of local excitatory or inhibitory interneurons. Interestingly, the ratio of MGL-immunolabeling was highest in calcitonin gene-related peptide-positive peptidergic primary afferents, and the staining intensity of nociceptive terminals was significantly reduced in MGL-knockout mice. These observations highlight the spinal nociceptor synapse as a potential anatomical site for the analgesic effects of MGL blockade. Moreover, the presence of MGL in additional terminal types raises the possibility that MGL may play distinct regulatory roles in synaptic endocannabinoid or prostaglandin signaling according to its different cellular locations in the dorsal horn pain circuitry.

Neuronal calcium signaling in chronic pain

Acute physiological pain, the unpleasant sensory response to a noxious stimulus, is essential for animals and humans to avoid potential injury. Pathological pain that persists after the original insult or injury has subsided, however, not only results in individual suffering but also imposes a significant cost on society. Improving treatments for long-lasting pathological pain requires a comprehensive understanding of the biological mechanisms underlying pain perception and the development of pain chronicity. In this review, we aim to highlight some of the major findings related to the involvement of neuronal calcium signaling in the processes that mediate chronic pain.

Chemokine CXCL1 enhances inflammatory pain and increases NMDA receptor activity and COX-2 expression in spinal cord neurons via activation of CXCR2

Recent studies have shown that CXCL1 upregulation in spinal astrocytes is involved in the maintenance of neuropathic pain. However, whether and how CXCL1 regulates inflammatory pain remains unknown. Here we show that intraplantar injection of CFA increased mRNA and protein expressions of CXCL1 and its major receptor CXCR2 in the spinal cord at 6h and 3days after the injection. Immunofluorescence double staining showed that CXCL1 and CXCR2 were expressed in spinal astrocytes and neurons, respectively. Intrathecal injection of CXCL1 neutralizing antibody or CXCR2 antagonist SB225002 attenuated CFA-induced mechanical and heat hypersensitivity on post-CFA day 3. Patch-clamp recordings showed that CXCL1 potentiated NMDA-induced currents in lamina II neurons via CXCR2, and this potentiation was further increased in CFA-treated mice. Furthermore, intrathecal injection of CXCL1 increased COX-2 expression in dorsal horn neurons, which was blocked by pretreatment with SB225002 or MEK (ERK kinase) inhibitor PD98059. Finally, pretreatment with SB225002 or PD98059 decreased CFA-induced heat hyperalgesia and COX-2 mRNA/protein expression and ERK activation in the spinal cord. Taken together, our data suggest that CXCL1, upregulated and released by spinal astrocytes after inflammation, acts on CXCR2-expressing spinal neurons to increase ERK activation, synaptic transmission and COX-2 expression in dorsal horn neurons and contributes to the pathogenesis of inflammatory pain.

Activation of extracellular signal-regulated kinase1/2 in the medial prefrontal cortex contributes to stress-induced hyperalgesia

Stressful stimuli can exacerbate persistent pain disorder. However, the underlying mechanism is still unknown. Here, to reveal the underlying mechanism for stressful stimuli-induced hyperalgesia in chronic pain, we investigated the effect of extracellular signal-regulated kinase1/2 (ERK1/2) activation on pain hypersensitivity using single-prolonged stress (SPS) model, complete Freund's adjuvant (CFA) model and SPS + CFA model. The experimental results revealed significantly reduced paw withdrawal threshold in the SPS, CFA, and SPS + CFA group compared with the control group. However, the increased phosphorylation of ERK1/2 in the medial prefrontal cortex (mPFC) was observed in the SPS- or SPS + CFA-exposed group but not the CFA group compared with control group. There was also a significant increase in mPFC ERK1/2 phosphorylation and mechanical allodynia after SPS + CFA treatment compared to SPS or CFA treatment alone. Furthermore, inhibiting ERK1/2 phosphorylation by microinjection of U0126, a MAPK kinase (MEK) inhibitor, into the mPFC attenuated SPS + CFA- and SPS- but not CFA-induced mechanical allodynia, anxiety-like behavior, and cognitive impairments. These results suggest that the activation of ERK1/2 in the mPFC may contribute to the process of stress-induced cognitive and emotional disorders, leading to an increase in pain sensitivity.

Anoctamin 1 contributes to inflammatory and nerve-injury induced hypersensitivity

BACKGROUND

Various pathological conditions such as inflammation or injury can evoke pain hypersensitivity. That represents the response to innocuous stimuli or exaggerated response to noxious stimuli. The molecular mechanism based on the pain hypersensitivity is associated with changes in many of ion channels in dorsal-root ganglion (DRG) neurons. Anoctamin 1 (ANO1/TMEM16A), a Ca2+ activated chloride channel is highly visible in small DRG neurons and responds to heat. Mice with an abolished function of ANO1 in DRG neurons demonstrated attenuated pain-like behaviors when exposed to noxious heat, suggesting a role in acute thermal nociception. In this study, we further examined the function of ANO1 in mediating inflammation- or injury-induced hyperalgesia or allodynia.

RESULTS

Using Advillin/Ano1fl/fl (Adv/Ano1fl/fl) mice that have a functional ablation of Ano1 mainly in DRG neurons, we were able to determine its role in mediating thermal hyperalgesia and mechanical allodynia induced by inflammation or nerve injury. The thermal hyperalgesia and mechanical allodynia induced by carrageenan injection and spared-nerve injury were significantly reduced in Adv/Ano1fl/fl mice. In addition, flinching or licking behavior after bradykinin or formalin injection was also significantly reduced in Adv/Ano1fl/fl mice. Since pathological conditions augment nociceptive behaviors, we expected ANO1's contribution to the excitability of DRG neurons. Indeed, the application of inflammatory mediators reduced the threshold for action potential (rheobase) or time for induction of the first action potential in DRG neurons isolated from control (Ano1fl/fl) mice. These parameters for neuronal excitability induced by inflammatory mediators were not changed in Adv/Ano1fl/fl mice, suggesting an active contribution of ANO1 in augmenting the neuronal excitability.

CONCLUSIONS

In addition to ANO1's role in mediating acute thermal pain as a heat sensor, ANO1 is also capable of augmenting the excitability of DRG neurons under inflammatory or neuropathic conditions and thereby aggravates inflammation- or tissue injury-induced pathological pain.

Proteome of brain glia: the molecular basis of diverse glial phenotypes

Several different types of nonneuronal glial cells with diverse phenotypes are present in the CNS, and all have distinct indispensible functions. Although glial cells primarily provide neurons with metabolic and structural support in the healthy brain, they may switch phenotype from a "resting" to a "reactive" state in response to pathological insults. Furthermore, this reactive gliosis is an invariant feature of the pathogeneses of CNS maladies. The glial proteome serves as a signature of glial phenotype, and not only executes physiological functions, but also acts as a molecular mediator of the reactive glial phenotype. The glial proteome is also involved in intra- and intercellular communications as exemplified by glia-glia and neuron-glia interactions. The utilization of authoritative proteomic tools and the bioinformatic analyses have helped to profile the brain glial proteome and explore the molecular mechanisms of diverse glial phenotypes. Furthermore, technologic innovations have equipped the field of "glioproteomics" with refined tools for studies of the expression, interaction, and function of glial proteins in the healthy and in the diseased CNS. Glioproteomics is expected to contribute to the elucidation of the molecular mechanisms of CNS pathophysiology and to the discovery of biomarkers and theragnostic targets in CNS disorders.

Pain hypersensitivity mechanisms at a glance

There are two basic categories of pain: physiological pain, which serves an important protective function, and pathological pain, which can have a major negative impact on quality of life in the context of human disease. Major progress has been made in understanding the molecular mechanisms that drive sensory transduction, amplification and conduction in peripheral pain-sensing neurons, communication of sensory inputs to spinal second-order neurons, and the eventual modulation of sensory signals by spinal and descending circuits. This poster article endeavors to provide an overview of how molecular and cellular mechanisms underlying nociception in a physiological context undergo plasticity in pathophysiological states, leading to pain hypersensitivity and chronic pain.

A dramatic increase of C1q protein in the CNS during normal aging

The decline of cognitive function has emerged as one of the greatest health threats of old age. Age-related cognitive decline is caused by an impacted neuronal circuitry, yet the molecular mechanisms responsible are unknown. C1q, the initiating protein of the classical complement cascade and powerful effector of the peripheral immune response, mediates synapse elimination in the developing CNS. Here we show that C1q protein levels dramatically increase in the normal aging mouse and human brain, by as much as 300-fold. This increase was predominantly localized in close proximity to synapses and occurred earliest and most dramatically in certain regions of the brain, including some but not all regions known to be selectively vulnerable in neurodegenerative diseases, i.e., the hippocampus, substantia nigra, and piriform cortex. C1q-deficient mice exhibited enhanced synaptic plasticity in the adult and reorganization of the circuitry in the aging hippocampal dentate gyrus. Moreover, aged C1q-deficient mice exhibited significantly less cognitive and memory decline in certain hippocampus-dependent behavior tests compared with their wild-type littermates. Unlike in the developing CNS, the complement cascade effector C3 was only present at very low levels in the adult and aging brain. In addition, the aging-dependent effect of C1q on the hippocampal circuitry was independent of C3 and unaccompanied by detectable synapse loss, providing evidence for a novel, complement- and synapse elimination-independent role for C1q in CNS aging.

Identification of a receptor for neuropeptide VGF and its role in neuropathic pain

VGF (nonacronymic) is a neuropeptide precursor that plays multiple roles in regulation of energy balance, reproduction, hippocampal synaptic plasticity, and pain. Data from a number of pain models showed significant up-regulation of VGF in sensory neurons. TLQP-21, one of the VGF-derived neuropeptides, has been shown to induce a hyperalgesic response when injected subcutaneously into the hind paw of mice. However, the precise role of VGF-derived neuropeptides in neuropathic pain and the molecular identity of the receptor for VGF-derived peptides are yet to be investigated. Here we identified gC1qR, the globular heads of the C1q receptor, as the receptor for TLQP-21 using chemical cross-linking combined with mass spectrometry analysis. TLQP-21 caused an increase in intracellular Ca²⁺ levels in rat macrophages and microglia. Inoculation of TLQP-21-stimulated macrophages into rat hind paw caused mechanical hypersensitivity. The increase in intracellular Ca²⁺ levels in macrophages was attenuated by either siRNA or neutralizing antibodies against gC1qR. Furthermore, application of the gC1qR-neutralizing antibody to rats with partial sciatic nerve ligation resulted in a delayed onset of nerve injury-associated mechanical hypersensitivity. These results indicate that gC1qR is the receptor for TLQP-21 and plays an important role in chronic pain through activation of macrophages. Because direct association between TLQP-21 and gC1qR is required for activation of macrophages and causes hypersensitivity, disrupting this interaction may be a useful new approach to develop novel analgesics.

Transcriptional mechanisms underlying sensitization of peripheral sensory neurons by granulocyte-/granulocyte-macrophage colony stimulating factors

BACKGROUND

Cancer-associated pain is a major cause of poor quality of life in cancer patients and is frequently resistant to conventional therapy. Recent studies indicate that some hematopoietic growth factors, namely granulocyte macrophage colony stimulating factor (GMCSF) and granulocyte colony stimulating factor (GCSF), are abundantly released in the tumor microenvironment and play a key role in regulating tumor-nerve interactions and tumor-associated pain by activating receptors on dorsal root ganglion (DRG) neurons. Moreover, these hematopoietic factors have been highly implicated in postsurgical pain, inflammatory pain and osteoarthritic pain. However, the molecular mechanisms via which G-/GMCSF bring about nociceptive sensitization and elicit pain are not known.

RESULTS

In order to elucidate G-/GMCSF mediated transcriptional changes in the sensory neurons, we performed a comprehensive, genome-wide analysis of changes in the transcriptome of DRG neurons brought about by exposure to GMCSF or GCSF. We present complete information on regulated genes and validated profiling analyses and report novel regulatory networks and interaction maps revealed by detailed bioinformatics analyses. Amongst these, we validate calpain 2, matrix metalloproteinase 9 (MMP9) and a RhoGTPase Rac1 as well as Tumor necrosis factor alpha (TNFα) as transcriptional targets of G-/GMCSF and demonstrate the importance of MMP9 and Rac1 in GMCSF-induced nociceptor sensitization.

CONCLUSION

With integrative approach of bioinformatics, in vivo pharmacology and behavioral analyses, our results not only indicate that transcriptional control by G-/GMCSF signaling regulates a variety of established pain modulators, but also uncover a large number of novel targets, paving the way for translational analyses in the context of pain disorders.

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.

HDAC inhibitors attenuate the development of hypersensitivity in models of neuropathic pain

Histone deacetylase inhibitors (HDACIs) interfere with the epigenetic process of histone acetylation and are known to have analgesic properties in models of chronic inflammatory pain. The aim of this study was to determine whether these compounds could also affect neuropathic pain. Different class I HDACIs were delivered intrathecally into rat spinal cord in models of traumatic nerve injury and antiretroviral drug–induced peripheral neuropathy (stavudine, d4T). Mechanical and thermal hypersensitivity was attenuated by 40% to 50% as a result of HDACI treatment, but only if started before any insult. The drugs globally increased histone acetylation in the spinal cord, but appeared to have no measurable effects in relevant dorsal root ganglia in this treatment paradigm, suggesting that any potential mechanism should be sought in the central nervous system. Microarray analysis of dorsal cord RNA revealed the signature of the specific compound used (MS-275) and suggested that its main effect was mediated through HDAC1. Taken together, these data support a role for histone acetylation in the emergence of neuropathic pain.

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 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.

Calcium responses to synaptically activated bursts of action potentials and their synapse-independent replay in cultured networks of hippocampal neurons

Both synaptic N-methyl-d-aspartate (NMDA) receptors and voltage-operated calcium channels (VOCCs) have been shown to be critical for nuclear calcium signals associated with transcriptional responses to bursts of synaptic input. However the direct contribution to nuclear calcium signals from calcium influx through NMDA receptors and VOCCs has been obscured by their concurrent roles in action potential generation and synaptic transmission. Here we compare calcium responses to synaptically induced bursts of action potentials with identical bursts devoid of any synaptic contribution generated using the pre-recorded burst as the voltage clamp command input to replay the burst in the presence of blockers of action potentials or ionotropic glutamate receptors. Synapse independent replays of bursts produced nuclear calcium responses with amplitudes around 70% of their original synaptically generated signals and were abolished by the L-type VOCC blocker, verapamil. These results identify a major direct source of nuclear calcium from local L-type VOCCs whose activation is boosted by NMDA receptor dependent depolarization. The residual component of synaptically induced nuclear calcium signals which was both VOCC independent and NMDA receptor dependent showed delayed kinetics consistent with a more distal source such as synaptic NMDA receptors or internal stores. The dual requirement of NMDA receptors and L-type VOCCs for synaptic activity-induced nuclear calcium dependent transcriptional responses most likely reflects a direct somatic calcium influx from VOCCs whose activation is amplified by synaptic NMDA receptor-mediated depolarization and whose calcium signal is boosted by a delayed input from distal calcium sources mostly likely entry through NMDA receptors and release from internal stores. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.