Downstream regulatory element antagonist modulator regulates Ca2+ homeostasis and viability in cerebellar neurons

The plasma membrane Ca²+ ATPase and the plasma membrane sodium calcium exchanger cooperate in the regulation of cell calcium

Calcium is an ambivalent signal: it is essential for the correct functioning of cell life, but may also become dangerous to it. The plasma membrane Ca(2+) ATPase (PMCA) and the plasma membrane Na(+)/Ca(2+) exchanger (NCX) are the two mechanisms responsible for Ca(2+) extrusion. The NCX has low Ca(2+) affinity but high capacity for Ca(2+) transport, whereas the PMCA has a high Ca(2+) affinity but low transport capacity for it. Thus, traditionally, the PMCA pump has been attributed a housekeeping role in maintaining cytosolic Ca(2+), and the NCX the dynamic role of counteracting large cytosolic Ca(2+) variations (especially in excitable cells). This view of the roles of the two Ca(2+) extrusion systems has been recently revised, as the specific functional properties of the numerous PMCA isoforms and splicing variants suggests that they may have evolved to cover both the basal Ca(2+) regulation (in the 100 nM range) and the Ca(2+) transients generated by cell stimulation (in the μM range).

DREAM regulates BDNF-dependent spinal sensitization

Background

The transcriptional repressor DREAM (downstream regulatory element antagonist modulator) controls the expression of prodynorphin and has been involved in the modulation of endogenous responses to pain. To investigate the role of DREAM in central mechanisms of pain sensitization, we used a line of transgenic mice (L1) overexpressing a Ca²⁺- and cAMP-insensitive DREAM mutant in spinal cord and dorsal root ganglia.

Results

L1 DREAM transgenic mice showed reduced expression in the spinal cord of several genes related to pain, including prodynorphin and BDNF (brain-derived neurotrophic factor) and a state of basal hyperalgesia without change in A-type currents. Peripheral inflammation produced enhancement of spinal reflexes and increased expression of BDNF in wild type but not in DREAM transgenic mice. The enhancement of the spinal reflexes was reproduced in vitro by persistent electrical stimulation of C-fibers in wild type but not in transgenic mice. Exposure to exogenous BDNF produced a long-term enhancement of dorsal root-ventral root responses in transgenic mice.

Conclusions

Our results indicate that endogenous BDNF is involved in spinal sensitization following inflammation and that blockade of BDNF induction in DREAM transgenic mice underlies the failure to develop spinal sensitization.

Sumoylation regulates nuclear localization of repressor DREAM

DREAM is a Ca(2+)-binding protein with specific functions in different cell compartments. In the nucleus, DREAM acts as a transcriptional repressor, although the mechanism that controls its nuclear localization is unknown. Yeast two-hybrid assay revealed the interaction between DREAM and the SUMO-conjugating enzyme Ubc9 and bioinformatic analysis identified four sumoylation-susceptible sites in the DREAM sequence. Single K-to-R mutations at positions K26 and K90 prevented in vitro sumoylation of recombinant DREAM. DREAM sumoylation mutants retained the ability to bind to the DRE sequence but showed reduced nuclear localization and failed to regulate DRE-dependent transcription. In PC12 cells, sumoylated DREAM is present exclusively in the nucleus and neuronal differentiation induced nuclear accumulation of sumoylated DREAM. In fully differentiated trigeminal neurons, DREAM and SUMO-1 colocalized in nuclear domains associated with transcription. Our results show that sumoylation regulates the nuclear localization of DREAM in differentiated neurons. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Increased B cell proliferation and reduced Ig production in DREAM transgenic mice

DREAM/KChIP-3 is a calcium-dependent transcriptional repressor highly expressed in immune cells. Transgenic mice expressing a dominant active DREAM mutant show reduced serum Ig levels. In vitro assays show that reduced Ig secretion is an intrinsic defect of transgenic B cells that occurs without impairment in plasma cell differentiation, class switch recombination, or Ig transcription. Surprisingly, transgenic B cells show an accelerated entry in cell division. Transcriptomic analysis of transgenic B cells revealed that hyperproliferative B cell response could be correlated with a reduced expression of Klf9, a cell-cycle regulator. Pulse-chase experiments demonstrated that the defect in Ig production is associated with reduced translation rather than with increased protein degradation. Importantly, transgenic B cells showed reduced expression of the Eif4g3 gene, which encodes a protein related to protein translation. Our results disclose, to our knowledge, a novel function of DREAM in proliferation and Ig synthesis in B lymphocytes.

The DREAM protein negatively regulates the NMDA receptor through interaction with the NR1 subunit

Glutamate-induced excitotoxicity has been implicated in the etiology of stroke, epilepsy, and neurodegenerative diseases. NMDA receptors (NMDARs) play a pivotal role in excitotoxic injury; however, clinical trials testing NMDAR antagonists as neuroprotectants have been discouraging. The development of novel neuroprotectant molecules is being vigorously pursued. Here, we report that downstream regulatory element antagonist modulator (DREAM) significantly inhibits surface expression of NMDARs and NMDAR-mediated current. Overexpression of DREAM showed neuroprotection against excitotoxic neuronal injury, whereas knockdown of DREAM enhanced NMDA-induced toxicity. DREAM could directly bind to the C0 domain of the NR1 subunit. Although DREAM contains multiple binding sites for the NR1 subunit, residues 21-40 of the N terminus are the main binding site for the NR1 subunit. Thus, 21-40 residues might relieve the autoinhibition conferred by residues 1-50 and derepress the DREAM core domain by a competitive mechanism. Intriguingly, the cell-permeable TAT-21-40 peptide, constructed according to the critical binding site of DREAM to the NR1 subunit, inhibits NMDAR-mediated currents in primary cultured hippocampal neurons and has a neuroprotective effect on in vitro neuronal excitotoxic injury and in vivo ischemic brain damage. Moreover, both pretreatment and posttreatment of TAT-21-40 is effective against excitotoxicity. In summary, this work reveals a novel, negative regulator of NMDARs and provides an attractive candidate for the treatment of excitotoxicity-related disease.

DREAM (downstream regulatory element antagonist modulator) contributes to synaptic depression and contextual fear memory

The downstream regulatory element antagonist modulator (DREAM), a multifunctional Ca²⁺-binding protein, binds specifically to DNA and several nucleoproteins regulating gene expression and with proteins outside the nucleus to regulate membrane excitability or calcium homeostasis. DREAM is highly expressed in the central nervous system including the hippocampus and cortex; however, the roles of DREAM in hippocampal synaptic transmission and plasticity have not been investigated. Taking advantage of transgenic mice overexpressing a Ca²⁺-insensitive DREAM mutant (TgDREAM), we used integrative methods including electrophysiology, biochemistry, immunostaining, and behavior tests to study the function of DREAM in synaptic transmission, long-term plasticity and fear memory in hippocampal CA1 region. We found that NMDA receptor but not AMPA receptor-mediated current was decreased in TgDREAM mice. Moreover, synaptic plasticity, such as long-term depression (LTD) but not long-term potentiation (LTP), was impaired in TgDREAM mice. Biochemical experiments found that DREAM interacts with PSD-95 and may inhibit NMDA receptor function through this interaction. Contextual fear memory was significantly impaired in TgDREAM mice. By contrast, sensory responses to noxious stimuli were not affected. Our results demonstrate that DREAM plays a novel role in postsynaptic modulation of the NMDA receptor, and contributes to synaptic plasticity and behavioral memory.

Calcium Pumps in Health and Disease

Ca2+-ATPases (pumps) are key actors in the regulation of Ca2+ in eukaryotic cells and are thus essential to the correct functioning of the cell machinery. They have high affinity for Ca2+ and can efficiently regulate it down to very low concentration levels. Two of the pumps have been known for decades (the SERCA and PMCA pumps); one (the SPCA pump) has only become known recently. Each pump is the product of a multigene family, the number of isoforms being further increased by alternative splicing of the primary transcripts. The three pumps share the basic features of the catalytic mechanism but differ in a number of properties related to tissue distribution, regulation, and role in the cellular homeostasis of Ca2+. The molecular understanding of the function of the pumps has received great impetus from the solution of the three-dimensional structure of one of them, the SERCA pump. These spectacular advances in the structure and molecular mechanism of the pumps have been accompanied by the emergence and rapid expansion of the topic of pump malfunction, which has paralleled the rapid expansion of knowledge in the topic of Ca2+-signaling dysfunction. Most of the pump defects described so far are genetic: when they are very severe, they produce gross and global disturbances of Ca2+ homeostasis that are incompatible with cell life. However, pump defects may also be of a type that produce subtler, often tissue-specific disturbances that affect individual components of the Ca2+-controlling and/or processing machinery. They do not bring cells to immediate death but seriously compromise their normal functioning.

L-type calcium channel C terminus autoregulates transcription

Calcium homeostasis is critical for cardiac myocyte function and must be tightly regulated. The guiding hypothesis of this study is that a carboxyl-terminal cleavage product of the cardiac L-type calcium channel (Ca(V)1.2) autoregulates expression. First, we confirmed that the Ca(V)1.2 C terminus (CCt) is cleaved in murine cardiac myocytes from mature and developing ventricle. Overexpression of full-length CCt caused a 34+/-8% decrease of Ca(V)1.2 promoter activity, and truncated CCt caused an 80+/-3% decrease of Ca(V)1.2 promoter (n=12). The full-length CCt distributes into cytosol and nucleus. A deletion mutant of CCt has a greater relative affinity for the nucleus than full-length CCt, and this is consistent with increased repression of Ca(V)1.2 promoter activity by truncated CCt. Chromatin immunoprecipitation analysis revealed that CCt interacts with the Ca(V)1.2 promoter in adult ventricular cardiac myocytes at promoter modules containing Nkx2.5/Mef2, C/EBp, and a cis regulatory module. The next hypothesis tested was that CCt contributes to transcriptional signaling associated with cellular hypertrophy. We explored whether fetal cardiac myocyte Ca(V)1.2 was regulated by serum in vitro. We tested atrial natriuretic factor promoter activity as a positive control and measured the serum response of Ca(V)1.2 promoter, protein, and L-type current (I(Ca,L)) from fetal mouse ventricular myocytes. Serum increased atrial natriuretic factor promoter activity and cell size as expected. Serum withdrawal increased Ca(V)1.2 promoter activity, mRNA, and I(Ca,L). Moreover, serum withdrawal decreased the relative nuclear localization of CCt. A combination of promoter deletion mutant analyses, and the response of promoter mutants to serum withdrawal support the conclusion that CCt, a proteolytic fragment of Ca(V)1.2, autoregulates Ca(V)1.2 expression in cardiac myocytes. These data support the novel mechanism that a mobile segment of Ca(V)1.2 links Ca handling to nuclear signaling.

PACAP signaling to DREAM: a cAMP-dependent pathway that regulates cortical astrogliogenesis

Astrocytes constitute a very abundant cell type in the mammalian central nervous system and play critical roles in brain function. During development, astrocytes are generated from neural progenitor cells only after these cells have generated neurons. This so called gliogenic switch is tightly regulated by intrinsic factors that inhibit the generation of astrocytes during the neurogenic period. Once neural progenitors acquire gliogenic competence, they differentiate into astrocytes in response to specific extracellular signals. Some of these signals are delivered by neurotrophic cytokines via activation of the gp130-JAK-signal transducer and activator of transcription system, whereas others depend on the activity of pituitary adenylate cyclase-activating polypeptide (PACAP) on specific PAC1 receptors that stimulate the production of cAMP. This results in the activation of the small GTPases Rap1 and Ras, and in the cAMP-dependent entry of extracellular calcium into the cell. Calcium, in turn, stimulates the transcription factor downstream regulatory element antagonist modulator (DREAM), which is bound to specific sites of the promoter of the glial fibrillary acidic protein gene, stimulating its expression during astrocyte differentiation. Lack of DREAM in vivo results in alterations in the number of neurons and astrocytes generated during development. Thus, the PACAP-cAMP-Ca(2+)-DREAM signaling cascade constitutes an important pathway to activate glial-specific gene expression during astrocyte differentiation.

DREAM mediates cAMP-dependent, Ca2+-induced stimulation of GFAP gene expression and regulates cortical astrogliogenesis

In the developing mouse brain, once the generation of neurons is mostly completed during the prenatal period, precisely coordinated signals act on competent neural precursors to direct their differentiation into astrocytes, which occurs mostly after birth. Among these signals, those provided by neurotrophic cytokines and bone morphogenetic proteins appear to have a key role in triggering the neurogenic to gliogenic switch and in regulating astrocyte numbers. In addition, we have reported previously that the neurotrophic peptide pituitary adenylate cyclase-activating polypeptide (PACAP) is able to promote astrocyte differentiation of cortical precursors via activation of a cAMP-dependent pathway. Signals acting on progenitor cells of the developing cortex to generate astrocytes activate glial fibrillary acidic protein (GFAP) gene expression, but the transcriptional mechanisms that regulate this activation are unclear. Here, we identify the previously known transcriptional repressor downstream regulatory element antagonist modulator (DREAM) as an activator of GFAP gene expression. We found that DREAM occupies specific sites on the GFAP promoter before and after differentiation is initiated by exposure of cortical progenitor cells to PACAP. PACAP raises intracellular calcium concentration via a mechanism that requires cAMP, and DREAM-mediated transactivation of the GFAP gene requires the integrity of calcium-binding domains. Cortical progenitor cells from dream(-/-) mice fail to express GFAP in response to PACAP. Moreover, the neonatal cortex of dream(-/-) mice exhibits a reduced number of astrocytes and increased number of neurons. These results identify the PACAP-cAMP-Ca(2+)-DREAM cascade as a new pathway to activate GFAP gene expression during astrocyte differentiation.

Specific effects of KChIP3/calsenilin/DREAM, but not KChIPs 1, 2 and 4, on calcium signalling and regulated secretion in PC12 cells

The KChIPs (K⁺ channel-interacting proteins) are members of the NCS (neuronal calcium sensor) protein family of Ca²⁺-binding proteins. It is unclear to what extent the KChIPs have distinct functions although they all interact with Kv4 K⁺ channels. KChIP3 has also been shown to repress transcription of specific genes via binding to DRE (downstream regulatory element) motifs and all KChIPs may share this function. In the present study, we have compared the function of isoforms of the four KChIPs. KChIPs 1–4 were found to stimulate the traffic of Kv4.2 channels to the plasma membrane. KChIP3 expression in PC12 cells resulted in an increase in exocytosis evoked by activation of purinergic receptors. In contrast, KChIPs 1, 2 and 4, although expressed to the same extent, had no effect on secretion. In addition, KChIP3 but not KChIPs 1, 2 and 4 modified the ATP-induced Ca²⁺ signal resulting in a delay in recovery after the peak Ca²⁺ elevation and also specifically resulted in down-regulation of the Na⁺/Ca²⁺ exchanger NCX3, which could explain the effects on the Ca²⁺ signal and secretion. Regulation of NCX3 by KChIP3 has been shown to occur via its DREAM (DRE antagonist modulator) function [Gomez-Villafuertes, Torres, Barrio, Savignac, Gabellini, Rizzato, Pintado, Gutierrez-Adan, Mellstrom, Carafoli and Naranjo (2005) J. Neurosci. 25, 10822–10830] suggesting that this activity might depend on the cellular context of expression of the various KChIPs. These results reveal a new role for KChIP3 in the regulation of Ca²⁺-regulated secretion and also suggest that the functions of each of the KChIPs may be more specialized than previously appreciated.

Calcium and signal transduction

Cell signaling is an essential process in which a variety of external signals, defined as first messengers, are translated inside the cells into specific responses, which are mediated by a less numerous group of second messengers. The exchange of signals became a necessity when the transition from monocellular to pluricellular life brought with it the division of labor among the cells of the organisms: unicellular organisms do not depend on the mutual exchange of signals, as they essentially only compete with each other for nutrients. Calcium (Ca²⁺) was selected during evolution as second messenger, because its chemistry made it a much more flexible ligand than the other abundant cations in the primordial environment (Na⁺, K⁺, Mg²⁺ ). Ca²⁺ can accept binding sites of irregular geometries and is thus ideally suited to be a carrier of biological information. The Ca²⁺ signal has properties that set it apart from those of all other biological messengers: they will be reviewed in this contribution. Among them, the ambivalent character of the Ca²⁺ signal is the most important: while essential to the viability of the cells, it can also easily become a conveyor of doom.

Searching for a role of NCX/NCKX exchangers in neurodegeneration

Control of intracellular calcium signaling is essential for neuronal development and function. Maintenance of Ca2+ homeostasis depends on the functioning of specific transport systems that remove calcium from the cytosol. Na+/Ca2+ exchange is the main calcium export mechanism across the plasma membrane that restores resting levels of calcium in neurons after stimulation. Two families of Na+/Ca2+ exchangers exist, one of which requires the co-transport of K+ and Ca2+ in exchange for Na+ ions. The malfunctioning of Na+/Ca2+ exchangers has been related to the development of pathological conditions in the regulation of neuronal death after hypoxia-anoxia, brain trauma, and nerve injury. In addition, the Na+/Ca2+ exchanger function has been associated with impaired Ca2+ homeostasis during aging of the brain, as well as with a role in Alzheimer's disease by regulating beta-amyloid toxicity. In this review, we summarize the current knowledge about the Na+/Ca2+ exchanger families and their implications in neurodegenerative disorders.

Ca2+-Operated Transcriptional Networks: Molecular Mechanisms and In Vivo Models

Calcium is the most universal signal used by living organisms to convey information to many different cellular processes. In this review we present well-known and recently identified proteins that sense and decode the calcium signal and are key elements in the nucleus to regulate the activity of various transcriptional networks. When possible, the review also presents in vivo models in which the genes encoding these calcium sensors-transducers have been modified, to emphasize the critical role of these Ca2+-operated mechanisms in many physiological functions.

Targeted disruption of Na+/Ca2+ exchanger 3 (NCX3) gene leads to a worsening of ischemic brain damage

Na+/Ca+ exchanger 3 (NCX3), one of the three isoforms of the NCX family, is highly expressed in the brain and is involved in the maintenance of intracellular Na+ and Ca2+ homeostasis. Interestingly, whereas the function of NCX3 under physiological conditions has been determined, its role under anoxia is still unknown. To assess NCX3 role in cerebral ischemia, we exposed ncx3-/- mice to transient middle cerebral artery occlusion followed by reperfusion. In addition, to evaluate the effect of ncx3 ablation on neuronal survival, organotypic hippocampal cultures and primary cortical neurons from ncx3-/- mice were subjected to oxygen glucose deprivation (OGD) plus reoxygenation. Here we report that ncx3 gene suppression leads to a worsening of brain damage after focal ischemia and to a massive neuronal death in all the hippocampal fields of organotypic cultures as well as in cortical neurons from ncx3-/- mice exposed to OGD plus reoxygenation. In addition, in ncx3-/- cortical neurons exposed to hypoxia, NCX currents, recorded in the reverse mode of operation, were significantly lower than those detected in ncx3+/+. From these results, NCX3 protein emerges as a new molecular target that may have a potential therapeutic value in modulating cerebral ischemia.

Na+/Ca2+ exchanger subtype (NCX1, NCX2, NCX3) protein expression in the rat hippocampus following 3 min and 8 min durations of global cerebral ischemia

There is increasing evidence that the sodium-calcium exchanger (NCX) subtypes, NCX1, NCX2 and NCX3 play an important role in intracellular calcium homeostasis/dysregulation following cerebral ischemia. In the present study we examined NCX1, NCX2 and NCX3 protein levels in the rat hippocampus at 3, 6, 12, 18, 24 and 48 h following a 3 min and 8 min duration of global cerebral ischemia. We observed that NCX1 protein levels were significantly increased by 22.3% and 20.6% at the 6 and 12 h respective time points following a 3 min duration of global ischemia, while NCX2 and NCX3 protein levels remained relatively constant. Following a 8 min duration of global ischemia, NCX1 protein levels remained relatively constant, while NCX2 protein levels were down-regulated by 6.9%, 10.8%, 14.4% and 10.3% at the 6, 18, 24 and 48 h respective time points, and NCX3 protein levels were up-regulated by 22.1% at the 18 h time point. Taken together, our results show that NCX subtype protein expression is sensitive to cerebral ischemia, and indicates that changes in NCX activity may be playing an important role in calcium maintenance and neuronal outcome following ischemia.

Na+/Ca2+ exchangers: three mammalian gene families control Ca2+ transport

Mammalian Na+/Ca2+ exchangers are members of three branches of a much larger family of transport proteins [the CaCA (Ca2+/cation antiporter) superfamily] whose main role is to provide control of Ca2+ flux across the plasma membranes or intracellular compartments. Since cytosolic levels of Ca2+ are much lower than those found extracellularly or in sequestered stores, the major function of Na+/Ca2+ exchangers is to extrude Ca2+ from the cytoplasm. The exchangers are, however, fully reversible and thus, under special conditions of subcellular localization and compartmentalized ion gradients, Na+/Ca2+ exchangers may allow Ca2+ entry and may play more specialized roles in Ca2+ movement between compartments. The NCX (Na+/Ca2+ exchanger) [SLC (solute carrier) 8] branch of Na+/Ca2+ exchangers comprises three members: NCX1 has been most extensively studied, and is broadly expressed with particular abundance in heart, brain and kidney, NCX2 is expressed in brain, and NCX3 is expressed in brain and skeletal muscle. The NCX proteins subserve a variety of roles, depending upon the site of expression. These include cardiac excitation-contraction coupling, neuronal signalling and Ca2+ reabsorption in the kidney. The NCKX (Na2+/Ca2+-K+ exchanger) (SLC24) branch of Na+/Ca2+ exchangers transport K+ and Ca2+ in exchange for Na+, and comprises five members: NCKX1 is expressed in retinal rod photoreceptors, NCKX2 is expressed in cone photoreceptors and in neurons throughout the brain, NCKX3 and NCKX4 are abundant in brain, but have a broader tissue distribution, and NCKX5 is expressed in skin, retinal epithelium and brain. The NCKX proteins probably play a particularly prominent role in regulating Ca2+ flux in environments which experience wide and frequent fluctuations in Na+ concentration. Until recently, the range of functions that NCKX proteins play was generally underappreciated. This situation is now changing rapidly as evidence emerges for roles including photoreceptor adaptation, synaptic plasticity and skin pigmentation. The CCX (Ca2+/cation exchanger) branch has only one mammalian member, NCKX6 or NCLX (Na+/Ca2+-Li+ exchanger), whose physiological function remains unclear, despite a broad pattern of expression.

Changes in calcium dynamics following the reversal of the sodium-calcium exchanger have a key role in AMPA receptor-mediated neurodegeneration via calpain activation in hippocampal neurons

Proteolytic cleavage of the Na(+)/Ca(2+) exchanger (NCX) by calpains impairs calcium homeostasis, leading to a delayed calcium overload and excitotoxic cell death. However, it is not known whether reversal of the exchanger contributes to activate calpains and trigger neuronal death. We investigated the role of the reversal of the NCX in Ca(2+) dynamics, calpain activation and cell viability, in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-stimulated hippocampal neurons. Selective overactivation of AMPA receptors caused the reversal of the NCX, which accounted for approximately 30% of the rise in intracellular free calcium concentration ([Ca(2+)](i)). The NCX reverse-mode inhibitor, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea (KB-R7943), partially inhibited the initial increase in [Ca(2+)](i), and prevented a delayed increase in [Ca(2+)](i). In parallel, overactivation of AMPA receptors strongly activated calpains and led to the proteolysis of NCX3. KB-R7943 prevented calpain activation, cleavage of NCX3 and was neuroprotective. Silencing of NCX3 reduced Ca(2+) uptake, calpain activation and was neuroprotective. Our data show for the first time that NCX reversal is an early event following AMPA receptor stimulation and is linked to the activation of calpains. Since calpain activation subsequently inactivates NCX, causing a secondary Ca(2+) entry, NCX may be viewed as a new suicide substrate operating in a Ca(2+)-dependent loop that triggers cell death and as a target for neuroprotection.

Distribution of downstream regulatory element antagonist modulator (DREAM) in rat spinal cord and upregulation of its expression during inflammatory pain

A previous knockout study revealed the critical role of downstream regulatory element antagonist modulator (DREAM) in pain processing in the spinal cord by transcriptional regulation of prodynorphin (PPD) gene. Here, we report that, in contrast to the nuclear localization of other transcription factors, DREAM showed a punctate staining pattern in rat spinal dorsal horn in immunofluorescent analysis, with a membrane localization profile in some neurons and its expression accumulated in the inner zone of lamina II. In an inflammatory pain model induced by complete Freund's adjuvant (CFA) injection, we used Western blot analysis and detected transient upregulation of DREAM in the nuclear fraction of ipsilateral spinal dorsal horn at 2 h and 6 h post-injection, and a slow upregulation in the membrane fraction for 7 days. These studies suggest that DREAM might have other roles in pain modulation in the spinal cord in addition to its well-known role as a transcriptional repressor.

BHK cells transfected with NCX3 are more resistant to hypoxia followed by reoxygenation than those transfected with NCX1 and NCX2: Possible relationship with mitochondrial membrane potential

The specific role played by NCX1, NCX2, and NCX3, the three isoforms of the Na+/Ca2+ exchanger (NCX), has been explored during hypoxic conditions in BHK cells stably transfected with each of these isoforms. Six major findings emerged from the present study: (1) all the three isoforms were highly expressed on the plasma membranes of BHK cells; (2) under physiological conditions, the three NCX isoforms showed similar functional activity; (3) hypoxia plus reoxygenation induced a lower increase of [Ca2+]i in BHK-NCX3-transfected cells than in BHK-NCX1- and BHK-NCX2-transfected cells; (4) NCX3-transfected cells were more resistant to chemical hypoxia plus reoxygenation than NCX1- and NCX2-transfected cells. Interestingly, such augmented resistance was eliminated by CBDMD (10 microM), an inhibitor of NCX and by the specific silencing of the NCX3 isoform; (5) chemical hypoxia plus reoxygenation produced a loss of mitochondrial membrane potential in NCX1- and NCX2-transfected cells, but not in NCX3-transfected cells; (6) the forward mode of operation in NCX3-transfected cells was not affected by ATP depletion, as it occurred in NCX1- and NCX2-transfected cells. Altogether, these results indicate that the brain specifically expressed NCX3 isoform more significantly contributes to the maintenance of [Ca2+]i homeostasis during experimental conditions mimicking ischemia, thereby preventing mitochondrial delta psi collapses and cell death.

Ca2+Signals and Neuronal Death in Brain Ischemia

Although Ca(2+) signals are necessary for cell communication and survival, abnormal cellular Ca(2+) load can trigger different cell death programs. Ca(2+) mediates cell death by activating proteases (ie, calpains), by reinforcing signals leading to caspase activation or by triggering other catabolic processes mediated by lipases and nucleases. Failure in the clearance of excitatory amino acid is a critical determinant of neuronal loss in the ischemic brain. Glutamate activates glutamate-ionotropic receptors at synaptic and extra-synaptic sites, causing prolonged neuronal depolarization and triggering deregulation of cellular ion homeostasis, mainly intracellular calcium and sodium. The mechanisms leading to the sustained calcium deregulation in excitotoxic conditions are only in part elucidated. Recently, we have shown that calpains mediate the inhibition of calcium efflux in primary dissociated neurons challenged with excitotoxic glutamate concentrations. Calpains cleave the sodium-calcium exchanger (NCX) and inhibit its capability to remove calcium accumulated as a consequence of the excitotoxic stimulus. Our findings highlight the link between calcium-dependent proteases, calcium overload and neuronal degeneration after an excitotoxic insult.

Split personality of transcription factors inside and outside the nuclear border

Growing evidence indicates that transcription factors may have functions entirely distinct from the regulation of gene transcription. Here we describe three transcription factors that, when outside the nucleus, regulate calcium homeostasis by three independent but convergent mechanisms.

Calcium microdomains in mitochondria and nucleus

Endomembranes modify the progression of the cytosolic Ca(2+) wave and contribute to generate Ca(2+) microdomains, both in the cytosol and inside the own organella. The concentration of Ca(2+) in the cytosol ([Ca(2+)](C)), the mitochondria ([Ca(2+)](M)) and the nucleus ([Ca(2+)](N)) are similar at rest, but may become very different during cell activation. Mitochondria avidly take up Ca(2+) from the high [Ca(2+)](C) microdomains generated during cell activation near Ca(2+) channels of the plasma membrane and/or the endomembranes and prevent propagation of the high Ca(2+) signal to the bulk cytosol. This shaping of [Ca(2+)](C) signaling is essential for independent regulation of compartmentalized cell functions. On the other hand, a high [Ca(2+)](M) signal is generated selectively in the mitochondria close to the active areas, which tunes up respiration to the increased local needs. The progression of the [Ca(2+)](C) signal to the nucleus may be dampened by mitochondria, the nuclear envelope or higher buffering power inside the nucleoplasm. On the other hand, selective [Ca(2+)](N) signals could be generated by direct release of stored Ca(2+) into the nucleoplasm. Ca(2+) release could even be restricted to subnuclear domains. Putative Ca(2+) stores include the nuclear envelope, their invaginations inside the nucleoplasm (nucleoplasmic reticulum) and nuclear microvesicles. Inositol trisphosphate, cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate have all been reported to produce release of Ca(2+) into the nucleoplasm, but contribution of these mechanisms under physiological conditions is still uncertain.

Gas1 Is Related to the Glial Cell-derived Neurotrophic Factor Family Receptors α and Regulates Ret Signaling

The growth arrest-specific gene 1 (Gas1) protein has been proposed to function during development as an inhibitor of growth and a mediator of cell death and is also re-expressed in adult neurons during excitotoxic insult. Here we have demonstrated that the Gas1 protein shows high structural similarity to the glial cell-derived neurotrophic factor (GDNF) family receptors alpha, which mediate GDNF responses through the receptor tyrosine kinase Ret. We found that Gas1 binds Ret in a ligand-independent manner and sequesters Ret in lipid rafts. Signaling downstream of Ret is thus modified through a mechanism that involves the adaptor protein Shc as well as ERK, eventually blocking Akt activation. Consequently, when Gas1 is induced, Ret-mediated GDNF-dependent survival effects are compromised.