NFAT signaling in neural development and axon growth

Blockade of Astrocytic Calcineurin/NFAT Signaling Helps to Normalize Hippocampal Synaptic Function and Plasticity in a Rat Model of Traumatic Brain Injury

Increasing evidence suggests that the calcineurin (CN)-dependent transcription factor NFAT (Nuclear Factor of Activated T cells) mediates deleterious effects of astrocytes in progressive neurodegenerative conditions. However, the impact of astrocytic CN/NFAT signaling on neural function/recovery after acute injury has not been investigated extensively. Using a controlled cortical impact (CCI) procedure in rats, we show that traumatic brain injury is associated with an increase in the activities of NFATs 1 and 4 in the hippocampus at 7 d after injury. NFAT4, but not NFAT1, exhibited extensive labeling in astrocytes and was found throughout the axon/dendrite layers of CA1 and the dentate gyrus. Blockade of the astrocytic CN/NFAT pathway in rats using adeno-associated virus (AAV) vectors expressing the astrocyte-specific promoter Gfa2 and the NFAT-inhibitory peptide VIVIT prevented the injury-related loss of basal CA1 synaptic strength and key synaptic proteins and reduced the susceptibility to induction of long-term depression. In conjunction with these seemingly beneficial effects, VIVIT treatment elicited a marked increase in the expression of the prosynaptogenic factor SPARCL1 (hevin), especially in hippocampal tissue ipsilateral to the CCI injury. However, in contrast to previous work on Alzheimer's mouse models, AAV-Gfa2-VIVIT had no effects on the levels of GFAP and Iba1, suggesting that synaptic benefits of VIVIT were not attributable to a reduction in glial activation per se. Together, the results implicate the astrocytic CN/NFAT4 pathway as a key mechanism for disrupting synaptic remodeling and homeostasis in the hippocampus after acute injury.

SIGNIFICANCE STATEMENT

Similar to microglia, astrocytes become strongly "activated" with neural damage and exhibit numerous morphologic/biochemical changes, including an increase in the expression/activity of the protein phosphatase calcineurin. Using adeno-associated virus (AAV) to inhibit the calcineurin-dependent activation of the transcription factor NFAT (Nuclear Factor of Activated T cells) selectively, we have shown that activated astrocytes contribute to neural dysfunction in animal models characterized by progressive/chronic neuropathology. Here, we show that the suppression of astrocytic calcineurin/NFATs helps to protect synaptic function and plasticity in an animal model in which pathology arises from a single traumatic brain injury. The findings suggest that at least some astrocyte functions impair recovery after trauma and may provide druggable targets for treating victims of acute nervous system injury.

Neuritin Up-regulates Kv4.2 α-Subunit of Potassium Channel Expression and Affects Neuronal Excitability by Regulating the Calcium-Calcineurin-NFATc4 Signaling Pathway

Neuritin is an important neurotrophin that regulates neural development, synaptic plasticity, and neuronal survival. Elucidating the downstream molecular signaling is important for potential therapeutic applications of neuritin in neuronal dysfunctions. We previously showed that neuritin up-regulates transient potassium outward current (IA) subunit Kv4.2 expression and increases IA densities, in part by activating the insulin receptor signaling pathway. Molecular mechanisms of neuritin-induced Kv4.2 expression remain elusive. Here, we report that the Ca(2+)/calcineurin (CaN)/nuclear factor of activated T-cells (NFAT) c4 axis is required for neuritin-induced Kv4.2 transcriptional expression and potentiation of IA densities in cerebellum granule neurons. We found that neuritin elevates intracellular Ca(2+) and increases Kv4.2 expression and IA densities; this effect was sensitive to CaN inhibition and was eliminated in Nfatc4(-/-) mice but not in Nfatc2(-/-) mice. Stimulation with neuritin significantly increased nuclear accumulation of NFATc4 in cerebellum granule cells and HeLa cells, which expressed IR. Furthermore, NFATc4 was recruited to the Kv4.2 gene promoter loci detected by luciferase reporter and chromatin immunoprecipitation assays. More importantly, data obtained from cortical neurons following adeno-associated virus-mediated overexpression of neuritin indicated that reduced neuronal excitability and increased formation of dendritic spines were abrogated in the Nfatc4(-/-) mice. Together, these data demonstrate an indispensable role for the CaN/NFATc4 signaling pathway in neuritin-regulated neuronal functions.

DYRK1A, a Dosage-Sensitive Gene Involved in Neurodevelopmental Disorders, Is a Target for Drug Development in Down Syndrome

Down syndrome (DS) is one of the leading causes of intellectual disability, and patients with DS face various health issues, including learning and memory deficits, congenital heart disease, Alzheimer's disease (AD), leukemia, and cancer, leading to huge medical and social costs. Remarkable advances on DS research have been made in improving cognitive function in mouse models for future therapeutic approaches in patients. Among the different approaches, DYRK1A inhibitors have emerged as promising therapeutics to reduce DS cognitive deficits. DYRK1A is a dual-specificity kinase that is overexpressed in DS and plays a key role in neurogenesis, outgrowth of axons and dendrites, neuronal trafficking and aging. Its pivotal role in the DS phenotype makes it a prime target for the development of therapeutics. Recently, disruption of DYRK1A has been found in Autosomal Dominant Mental Retardation 7 (MRD7), resulting in severe mental deficiency. Recent advances in the development of kinase inhibitors are expected, in the near future, to remove DS from the list of incurable diseases, providing certain conditions such as drug dosage and correct timing for the optimum long-term treatment. In addition the exact molecular and cellular mechanisms that are targeted by the inhibition of DYRK1A are still to be discovered.

Optogenetic Control of Calcium Oscillation Waveform Defines NFAT as an Integrator of Calcium Load

It is known that the calcium-dependent transcription factor NFAT initiates transcription in response to pulsatile loads of calcium signal. However, the relative contributions of calcium oscillation frequency, amplitude, and duty cycle to transcriptional activity remain unclear. Here, we engineer HeLa cells to permit optogenetic control of intracellular calcium concentration using programmable LED arrays. This approach allows us to generate calcium oscillations of constant peak amplitude, in which frequency is varied while holding duty cycle constant, or vice versa. Using this setup and mathematical modeling, we show that NFAT transcriptional activity depends more on duty cycle, defined as the proportion of the integrated calcium concentration over the oscillation period, than on frequency alone. This demonstrates that NFAT acts primarily as a signal integrator of cumulative load rather than a frequency-selective decoder. This approach resolves a fundamental question in calcium encoding and demonstrates the value of optogenetics for isolating individual dynamical components of larger signaling behaviors.

Regulation of neurogenesis by calcium signaling

Calcium (Ca(2+)) signaling has essential roles in the development of the nervous system from neural induction to the proliferation, migration, and differentiation of neural cells. Ca(2+) signaling pathways are shaped by interactions among metabotropic signaling cascades, intracellular Ca(2+) stores, ion channels, and a multitude of downstream effector proteins that activate specific genetic programs. The temporal and spatial dynamics of Ca(2+) signals are widely presumed to control the highly diverse yet specific genetic programs that establish the complex structures of the adult nervous system. Progress in the last two decades has led to significant advances in our understanding of the functional architecture of Ca(2+) signaling networks involved in neurogenesis. In this review, we assess the literature on the molecular and functional organization of Ca(2+) signaling networks in the developing nervous system and its impact on neural induction, gene expression, proliferation, migration, and differentiation. Particular emphasis is placed on the growing evidence for the involvement of store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels in these processes.

Screening of Transient Receptor Potential Canonical Channel Activators Identifies Novel Neurotrophic Piperazine Compounds

Transient receptor potential canonical (TRPC) proteins form Ca(2+)-permeable cation channels activated upon stimulation of metabotropic receptors coupled to phospholipase C. Among the TRPC subfamily, TRPC3 and TRPC6 channels activated directly by diacylglycerol (DAG) play important roles in brain-derived neurotrophic factor (BDNF) signaling, promoting neuronal development and survival. In various disease models, BDNF restores neurologic deficits, but its therapeutic potential is limited by its poor pharmacokinetic profile. Elucidation of a framework for designing small molecules, which elicit BDNF-like activity via TRPC3 and TRPC6, establishes a solid basis to overcome this limitation. We discovered, through library screening, a group of piperazine-derived compounds that activate DAG-activated TRPC3/TRPC6/TRPC7 channels. The compounds [4-(5-chloro-2-methylphenyl)piperazin-1-yl](3-fluorophenyl)methanone (PPZ1) and 2-[4-(2,3-dimethylphenyl)piperazin-1-yl]-N-(2-ethoxyphenyl)acetamide (PPZ2) activated, in a dose-dependent manner, recombinant TRPC3/TRPC6/TRPC7 channels, but not other TRPCs, in human embryonic kidney cells. PPZ2 activated native TRPC6-like channels in smooth muscle cells isolated from rabbit portal vein. Also, PPZ2 evoked cation currents and Ca(2+) influx in rat cultured central neurons. Strikingly, both compounds induced BDNF-like neurite growth and neuroprotection, which were abolished by a knockdown or inhibition of TRPC3/TRPC6/TRPC7 in cultured neurons. Inhibitors of Ca(2+) signaling pathways, except calcineurin, impaired neurite outgrowth promotion induced by PPZ compounds. PPZ2 increased activation of the Ca(2+)-dependent transcription factor, cAMP response element-binding protein. These findings suggest that Ca(2+) signaling mediated by activation of DAG-activated TRPC channels underlies neurotrophic effects of PPZ compounds. Thus, piperazine-derived activators of DAG-activated TRPC channels provide important insights for future development of a new class of synthetic neurotrophic drugs.

Transcriptomic and genetic studies identify NFAT5 as a candidate gene for cocaine dependence

Cocaine reward and reinforcing effects are mediated mainly by dopaminergic neurotransmission. In this study, we aimed at evaluating gene expression changes induced by acute cocaine exposure on SH-SY5Y-differentiated cells, which have been widely used as a dopaminergic neuronal model. Expression changes and a concomitant increase in neuronal activity were observed after a 5 μM cocaine exposure, whereas no changes in gene expression or in neuronal activity took place at 1 μM cocaine. Changes in gene expression were identified in a total of 756 genes, mainly related to regulation of transcription and gene expression, cell cycle, adhesion and cell projection, as well as mitogen-activeated protein kinase (MAPK), CREB, neurotrophin and neuregulin signaling pathways. Some genes displaying altered expression were subsequently targeted with predicted functional single-nucleotide polymorphisms (SNPs) in a case-control association study in a sample of 806 cocaine-dependent patients and 817 controls. This study highlighted associations between cocaine dependence and five SNPs predicted to alter microRNA binding at the 3'-untranslated region of the NFAT5 gene. The association of SNP rs1437134 with cocaine dependence survived the Bonferroni correction for multiple testing. A functional effect was confirmed for this variant by a luciferase reporter assay, with lower expression observed for the rs1437134G allele, which was more pronounced in the presence of hsa-miR-509. However, brain volumes in regions of relevance to addiction, as assessed with magnetic resonance imaging, did not correlate with NFAT5 variation. These results suggest that the NFAT5 gene, which is upregulated a few hours after cocaine exposure, may be involved in the genetic predisposition to cocaine dependence.

Effects of embryonic cyclosporine exposures on brain development and behavior

Cyclosporine, a calcineurin inhibitor, is successfully used as an immunosuppressant in transplant medicine. However, the use of this pharmaceutical during pregnancy is concerning since calcineurin is thought to play a role in neural development. The risk for human brain development is difficult to evaluate because of a lack of basic information on the sensitive developmental times and the potentially pleiotropic effects on brain development and behavior. In the present study, we use zebrafish as a model system to examine the effects of embryonic cyclosporine exposures. Early embryonic exposures reduced the size of the eyes and brain. Late embryonic exposures did not affect the size of the eyes or brain, but did lead to substantial behavioral defects at the larval stages. The cyclosporine-exposed larvae displayed a reduced avoidance response to visual stimuli, low swim speeds, increased resting, an increase in thigmotaxis, and changes in the average distance between larvae. Similar results were obtained with the calcineurin inhibitor FK506, suggesting that most, but not all, effects on brain development and behavior are mediated by calcineurin inhibition. Overall, the results show that cyclosporine can induce either structural or functional brain defects, depending on the exposure window. The observed functional brain defects highlight the importance of quantitative behavioral assays when evaluating the risk of developmental exposures.

AM404 inhibits NFAT and NF-κB signaling pathways and impairs migration and invasiveness of neuroblastoma cells

N-Arachidonoylphenolamine (AM404), a paracetamol lipid metabolite, is a modulator of the endocannabinoid system endowed with pleiotropic activities. AM404 is a dual agonist of the Transient Receptor Potential Vanilloid type 1 (TRPV1) and the Cannabinoid Receptor type 1 (CB₁) and inhibits anandamide (AEA) transport and degradation. In addition, it has been shown that AM404 also exerts biological activities through TRPV1- and CB₁ -independent pathways. In the present study we have investigated the effect of AM404 in the NFAT and NF-κB signaling pathways in SK-N-SH neuroblastoma cells. AM404 inhibited NFAT transcriptional activity through a CB₁- and TRPV1-independent mechanism. Moreover, AM404 inhibited both the expression of COX-2 at transcriptional and post-transcriptional levels and the synthesis of PGE₂. AM404 also inhibited NF-κB activation induced by PMA/Ionomycin in SK-N-SH cells by targeting IKKβ phosphorylation and activation. We found that Cot/Tlp-2 induced NFAT and COX-2 transcriptional activities were inhibited by AM404. NFAT inhibition paralleled with the ability of AM404 to inhibit MMP-1, -3 and -7 expression, cell migration and invasion in a cell-type specific dependent manner. Taken together, these data reveal that paracetamol, the precursor of AM404, can be explored not only as an antipyretic and painkiller drug but also as a co-adjuvant therapy in inflammatory and cancer diseases.

Alteration in basal and depolarization induced transcriptional network in iPSC derived neurons from Timothy syndrome

BACKGROUND

Common genetic variation and rare mutations in genes encoding calcium channel subunits have pleiotropic effects on risk for multiple neuropsychiatric disorders, including autism spectrum disorder (ASD) and schizophrenia. To gain further mechanistic insights by extending previous gene expression data, we constructed co-expression networks in Timothy syndrome (TS), a monogenic condition with high penetrance for ASD, caused by mutations in the L-type calcium channel, Cav1.2.

METHODS

To identify patient-specific alterations in transcriptome organization, we conducted a genome-wide weighted co-expression network analysis (WGCNA) on neural progenitors and neurons from multiple lines of induced pluripotent stem cells (iPSC) derived from normal and TS (G406R in CACNA1C) individuals. We employed transcription factor binding site enrichment analysis to assess whether TS associated co-expression changes reflect calcium-dependent co-regulation.

RESULTS

We identified reproducible developmental and activity-dependent gene co-expression modules conserved in patient and control cell lines. By comparing cell lines from case and control subjects, we also identified co-expression modules reflecting distinct aspects of TS, including intellectual disability and ASD-related phenotypes. Moreover, by integrating co-expression with transcription factor binding analysis, we showed the TS-associated transcriptional changes were predicted to be co-regulated by calcium-dependent transcriptional regulators, including NFAT, MEF2, CREB, and FOXO, thus providing a mechanism by which altered Ca(2+) signaling in TS patients leads to the observed molecular dysregulation.

CONCLUSIONS

We applied WGCNA to construct co-expression networks related to neural development and depolarization in iPSC-derived neural cells from TS and control individuals for the first time. These analyses illustrate how a systems biology approach based on gene networks can yield insights into the molecular mechanisms of neural development and function, and provide clues as to the functional impact of the downstream effects of Ca(2+) signaling dysregulation on transcription.

Nerve growth factor (NGF) regulates activity of nuclear factor of activated T-cells (NFAT) in neurons via the phosphatidylinositol 3-kinase (PI3K)-Akt-glycogen synthase kinase 3β (GSK3β) pathway

The Ca(2+)/calcineurin-dependent transcription factor nuclear factor of activated T-cells (NFAT) plays an important role in regulating many neuronal functions, including excitability, axonal growth, synaptogenesis, and neuronal survival. NFAT can be activated by action potential firing or depolarization that leads to Ca(2+)/calcineurin-dependent dephosphorylation of NFAT and its translocation to the nucleus. Recent data suggest that NFAT and NFAT-dependent functions in neurons can also be potently regulated by NGF and other neurotrophins. However, the mechanisms of NFAT regulation by neurotrophins are not well understood. Here, we show that in dorsal root ganglion sensory neurons, NGF markedly facilitates NFAT-mediated gene expression induced by mild depolarization. The effects of NGF were not associated with changes in [Ca(2+)]i and were independent of phospholipase C activity. Instead, the facilitatory effect of NGF depended on activation of the PI3K/Akt pathway downstream of the TrkA receptor and on inhibition of glycogen synthase kinase 3β (GSK3β), a protein kinase known to phosphorylate NFAT and promote its nuclear export. Knockdown or knockout of NFATc3 eliminated this facilitatory effect. Simultaneous monitoring of EGFP-NFATc3 nuclear translocation and [Ca(2+)]i changes in dorsal root ganglion neurons indicated that NGF slowed the rate of NFATc3 nuclear export but did not affect its nuclear import rate. Collectively, our data suggest that NGF facilitates depolarization-induced NFAT activation by stimulating PI3K/Akt signaling, inactivating GSK3β, and thereby slowing NFATc3 export from the nucleus. We propose that NFAT serves as an integrator of neurotrophin action and depolarization-driven calcium signaling to regulate neuronal gene expression.

Control of neuronal apoptosis by reciprocal regulation of NFATc3 and Trim17

Neuronal apoptosis induced by survival factor deprivation is strongly regulated at the transcriptional level. Notably, the nuclear factor of activated T cell (NFAT) transcription factors have an important role in the control of the survival/death fate of neurons. However, the mechanisms that regulate NFAT activity in response to apoptotic stimuli and the target genes that mediate their effect on neuronal apoptosis are mostly unknown. In a previous study, we identified Trim17 as a crucial E3 ubiquitin ligase that is necessary and sufficient for neuronal apoptosis. Here, we show that Trim17 binds preferentially SUMOylated forms of NFATc3. Nonetheless, Trim17 does not promote the ubiquitination/degradation of NFATc3. NFAT transcription factors are regulated by calcium/calcineurin-dependent nuclear-cytoplasmic shuttling. Interestingly, Trim17 reduced by twofold the calcium-mediated nuclear localization of NFATc3 and, consistent with this, halved NFATc3 activity, as estimated by luciferase assays and by measurement of target gene expression. Trim17 also inhibited NFATc4 nuclear translocation and activity. NFATc4 is known to induce the expression of survival factors and, as expected, overexpression of NFATc4 protected cerebellar granule neurons from serum/KCl deprivation-induced apoptosis. Inhibition of NFATc4 by Trim17 may thus partially mediate the proapoptotic effect of Trim17. In contrast, overexpression of NFATc3 aggravated neuronal death, whereas knockdown of NFATc3 protected neurons from apoptosis. This proapoptotic effect of NFATc3 might be due to a feedback loop in which NFATc3, but not NFATc4, induces the transcription of the proapoptotic gene Trim17. Indeed, we found that overexpression or silencing of NFATc3, respectively, increased or decreased Trim17 levels, whereas NFATc4 had no significant effect on Trim17 expression. Moreover, we showed that NFATc3 binds to the promoter of the Trim17 gene together with c-Jun. Therefore, our results describe a novel mechanism regulating NFAT transcription factors beyond the calcium/calcineurin-dependent pathway and provide a possible explanation for the opposite effects of NFATc3 and NFATc4 on neuronal apoptosis.

Lead induces COX-2 expression in glial cells in a NFAT-dependent, AP-1/NFκB-independent manner

Epidemiologic studies have provided solid evidence for the neurotoxic effect of lead for decades of years. In view of the fact that children are more vulnerable to the neurotoxicity of lead, lead exposure has been an urgent public health concern. The modes of action of lead neurotoxic effects include disturbance of neurotransmitter storage and release, damage of mitochondria, as well as induction of apoptosis in neurons, cerebrovascular endothelial cells, astroglia and oligodendroglia. Our studies here, from a novel point of view, demonstrates that lead specifically caused induction of COX-2, a well known inflammatory mediator in neurons and glia cells. Furthermore, we revealed that COX-2 was induced by lead in a transcription-dependent manner, which relayed on transcription factor NFAT, rather than AP-1 and NFκB, in glial cells. Considering the important functions of COX-2 in mediation of inflammation reaction and oxidative stress, our studies here provide a mechanistic insight into the understanding of lead-associated inflammatory neurotoxicity effect via activation of pro-inflammatory NFAT3/COX-2 axis.

Store-operated CRAC channels regulate gene expression and proliferation in neural progenitor cells

Calcium signals regulate many critical processes during vertebrate brain development including neurogenesis, neurotransmitter specification, and axonal outgrowth. However, the identity of the ion channels mediating Ca(2+) signaling in the developing nervous system is not well defined. Here, we report that embryonic and adult mouse neural stem/progenitor cells (NSCs/NPCs) exhibit store-operated Ca(2+) entry (SOCE) mediated by Ca(2+) release-activated Ca(2+) (CRAC) channels. SOCE in NPCs was blocked by the CRAC channel inhibitors La(3+), BTP2, and 2-APB and Western blots revealed the presence of the canonical CRAC channel proteins STIM1 and Orai1. Knock down of STIM1 or Orai1 significantly diminished SOCE in NPCs, and SOCE was lost in NPCs from transgenic mice lacking Orai1 or STIM1 and in knock-in mice expressing the loss-of-function Orai1 mutant, R93W. Therefore, STIM1 and Orai1 make essential contributions to SOCE in NPCs. SOCE in NPCs was activated by epidermal growth factor and acetylcholine, the latter occurring through muscarinic receptors. Activation of SOCE stimulated gene transcription through calcineurin/NFAT (nuclear factor of activated T cells) signaling through a mechanism consistent with local Ca(2+) signaling by Ca(2+) microdomains near CRAC channels. Importantly, suppression or deletion of STIM1 and Orai1 expression significantly attenuated proliferation of embryonic and adult NPCs cultured as neurospheres and, in vivo, in the subventricular zone of adult mice. These findings show that CRAC channels serve as a major route of Ca(2+) entry in NPCs and regulate key effector functions including gene expression and proliferation, indicating that CRAC channels are important regulators of mammalian neurogenesis.

Analysis of adult cerebral cortex and hippocampus transcriptomes reveals unique molecular changes in the Ts1Cje mouse model of down syndrome

We investigated gene expression and functional differences between Ts1Cje mice and wild-type (WT) littermates in adult cerebral cortex and hippocampus. These two brain regions are affected in people with Down syndrome, but have not been previously molecularly characterized in Ts1Cje mice. Total RNA was prepared from the brains of 8-10-week-old Ts1Cje mice (n = 6) and WT littermates (n = 5) and hybridized to Affymetrix 1.0 ST gene mouse arrays. Differentially regulated genes were identified and used to perform in silico functional analyses to better characterize dysregulated pathways in both brain regions. Hippocampus had more significantly differentially expressed genes compared with cortex (30 vs. 7 at a Benjamini-Hochberg false discovery rate of 20%). We identified novel genes that were differentially regulated in adult brains, including Cyb5r1, Fsbp, Vmn2r110, Snd1 and Zhx2. Functional analyses in Ts1Cje mice highlighted the importance of NFAT signaling, oxidative stress, neuroinflammation and olfactory perception via G-protein signaling. In a comparison of adult Ts1Cje and WT brains, we identified new genes and pathway differences in the cortex and hippocampus. Our analyses identified physiologically relevant pathways that can serve as targets for the development of future treatments to improve neurocognition in Down syndrome.

A calcineurin- and NFAT-dependent pathway is involved in α-synuclein-induced degeneration of midbrain dopaminergic neurons

Parkinson's disease (PD), the most common degenerative movement disorder, is caused by a preferential loss of midbrain dopaminergic (mDA) neurons. Both α-synuclein (α-syn) missense and multiplication mutations have been linked to PD. However, the underlying intracellular signalling transduction pathways of α-syn-mediated mDA neurodegeneration remain elusive. Here, we show that transgenic expression of PD-related human α-syn A53T missense mutation promoted calcineurin (CN) activity and the subsequent nuclear translocation of nuclear factor of activated T cells (NFATs) in mDA neurons. α-syn enhanced the phosphatase activity of CN in both cell-free assays and cell lines transfected with either human wild-type or A53T α-syn. Furthermore, overexpression of α-syn A53T mutation significantly increased the CN-dependent nuclear import of NFATc3 in the mDA neurons of transgenic mice. More importantly, a pharmacological inhibition of CN by cyclosporine A (CsA) ameliorated the α-syn-induced loss of mDA neurons. These findings demonstrate an active involvement of CN- and NFAT-mediated signalling pathway in α-syn-mediated degeneration of mDA neurons in PD.

Pharmacology and signaling of MAS-related G protein-coupled receptors

Signaling by heptahelical G protein-coupled receptors (GPCR) regulates many vital body functions. Consequently, dysfunction of GPCR signaling leads to pathologic states, and approximately 30% of all modern clinical drugs target GPCR. One decade ago, an entire new GPCR family was discovered, which was recently named MAS-related G protein-coupled receptors (MRGPR) by the HUGO Gene Nomenclature Committee. The MRGPR family consists of ∼40 members that are grouped into nine distinct subfamilies (MRGPRA to -H and -X) and are predominantly expressed in primary sensory neurons and mast cells. All members are formally still considered "orphan" by the Committee on Receptor Nomenclature and Drug Classification of the International Union of Basic and Clinical Pharmacology. However, several distinct peptides and amino acids are discussed as potential ligands, including β-alanine, angiotensin-(1-7), alamandine, GABA, cortistatin-14, and cleavage products of proenkephalin, pro-opiomelanocortin, prodynorphin, or proneuropeptide-FF-A. The full spectrum of biologic roles of all MRGPR is still ill-defined, but there is evidence pointing to a role of distinct MRGPR subtypes in nociception, pruritus, sleep, cell proliferation, circulation, and mast cell degranulation. This review article summarizes findings published in the last 10 years on the phylogenetic relationships, pharmacology, signaling, physiology, and agonist-promoted regulation of all MRGPR subfamilies. Furthermore, we highlight interactions between MRGPR and other hormonal systems, paying particular attention to receptor multimerization and morphine tolerance. Finally, we discuss the challenges the field faces presently and emphasize future directions of research.

Differential effect of traumatic brain injury on the nuclear factor of activated T Cells C3 and C4 isoforms in the rat hippocampus

The interaction between the phosphatase calcineurin and transcription factor nuclear factor of activated T cells (NFAT) plays an important role numerous signaling and the regulatory events. Although NFAT is mostly known for its transcription function in the immune system, NFAT also has essential functions even in the central nervous system (CNS). The effects of traumatic brain injury (TBI) on NFAT are currently unknown. To determine if there is an alteration in NFAT after TBI, we examined NFATc3 and c4 levels at 6 h, 1 day, 1 week, 2 weeks and 4 weeks post injury. Rats were anesthetized and surgically prepared for controlled cortical impact (CCI) injury or sham surgery. Semi-quantitative measurements of NFATc3 and c4 in the hippocampal homogenates from injured and sham rats sacrificed at the appropriate time after injury were assessed using Western blot analysis. After TBI insult, in the hippocampus ipsilateral to the injury, NFATc3 expression levels were decreased both in the cytoplasmic and nuclear fractions. However, NFATc4 expression levels were increased in the cytoplasmic fraction but decreased in the nuclear fraction. Double labeling (with NeuN and GFAP) immunohistochemistry revealed that NFATc3 was expressed in subset of astrocytes and NFATc4 was expressed primarily in neurons. These differential responses in NFATc3 and c4 expression after TBI insult may indicate long-term changes in hippocampal excitability and may contribute to behavioral deficits. Further study is warranted to illustrate the role of NFATc3 and c4 in the setting of TBI.

Spatiotemporal changes in NFATc4 expression of retinal ganglion cells after light-induced damage

Nuclear factor of activated T cells, cytoplasmic 4 (NFATc4) is one of the four members of the NFAT family, which were described first as essential components of T cells activation and lately as important regulators for the initiation and coordination of the immune response, including B cells and natural killer cells. Accumulating evidence has demonstrated that NFATc4 exerted a pro-apoptotic effect in the pathogenesis of various experimental central nervous system diseases by upregulating Fas ligand (FasL) levels. However, the function of NFATc4 in the retina is still with limited acquaintance. To investigate whether NFATc4 is involved in retinal neuron apoptosis, we performed a light-induced retinal damage model in adult rats. A significant upregulation of NFATc4 was detected in the retina after light-induced damage by using Western blotting and reverse transcriptase PCR (RT-PCR). Besides this, NFATc4 was observed to be localized mainly in the retinal ganglion cells (RGCs). In addition, the expression patterns of active caspase-3, active caspase-8, and FasL were parallel with that of NFATc4. We also found the co-localization of NFATc4 with active caspase-3 and FasL in RGCs after light exposure. Collectively, we hypothesized that NFATc4 might participate in RGCs apoptosis by upregulating FasL levels.

Increased dosage of DYRK1A and DSCR1 delays neuronal differentiation in neocortical progenitor cells

Down's syndrome (DS), a major genetic cause of mental retardation, arises from triplication of genes on human chromosome 21. Here we show that DYRK1A (dual-specificity tyrosine-phosphorylated and -regulated kinase 1A) and DSCR1 (DS critical region 1), two genes lying within human chromosome 21 and encoding for a serine/threonine kinase and calcineurin regulator, respectively, are expressed in neural progenitors in the mouse developing neocortex. Increasing the dosage of both proteins in neural progenitors leads to a delay in neuronal differentiation, resulting ultimately in alteration of their laminar fate. This defect is mediated by the cooperative actions of DYRK1A and DSCR1 in suppressing the activity of the transcription factor NFATc. In Ts1Cje mice, a DS mouse model, dysregulation of NFATc in conjunction with increased levels of DYRK1A and DSCR1 was observed. Furthermore, counteracting the dysregulated pathway ameliorates the delayed neuronal differentiation observed in Ts1Cje mice. In sum, our findings suggest that dosage of DYRK1A and DSCR1 is critical for proper neurogenesis through NFATc and provide a potential mechanism to explain the neurodevelopmental defects in DS.

Hippocampal and thalamic neuronal metabolism in a putative rat model of schizophrenia

The transcription factor early growth response protein 3 (EGR3) is involved in schizophrenia. We developed a putative rat model of schizophrenia by transfecting lentiviral particles carrying the Egr3 gene into bilateral hippocampal dentate gyrus. We assessed spatial working memory using the Morris water maze test, and neuronal metabolite levels in bilateral hippocampus and thalamus were determined by 3.0 T proton magnetic resonance spectroscopy. Choline content was significantly greater in the hippocampus after transfection, while N-acetylaspartate and the ratio of N-acetylaspartate to creatine/phosphocreatine in the thalamus were lower than in controls. This study is the first to report evaluation of brain metabolites using 3.0 T proton magnetic resonance spectroscopy in rats transfected with Egr3, and reveals metabolic abnormalities in the hippocampus and thalamus in this putative model of schizophrenia.

Calcineurin signaling mediates activity-dependent relocation of the axon initial segment

The axon initial segment (AIS) is a specialized neuronal subcompartment located at the beginning of the axon that is crucially involved in both the generation of action potentials and the regulation of neuronal polarity. We recently showed that prolonged neuronal depolarization produces a distal shift of the entire AIS structure away from the cell body, a change associated with a decrease in neuronal excitability. Here, we used dissociated rat hippocampal cultures, with a major focus on the dentate granule cell (DGC) population, to explore the signaling pathways underlying activity-dependent relocation of the AIS. First, a pharmacological screen of voltage-gated calcium channels (VGCCs) showed that AIS relocation is triggered by activation of L-type Cav1 VGCCs with negligible contribution from any other VGCC subtypes. Additional pharmacological analysis revealed that downstream signaling events are mediated by the calcium-sensitive phosphatase calcineurin; inhibition of calcineurin with either FK506 or cyclosporin A totally abolished both depolarization- and optogenetically-induced activity-dependent AIS relocation. Furthermore, calcineurin activation is sufficient for AIS plasticity, because expression of a constitutively active form of the phosphatase resulted in relocation of the AIS of DGCs without a depolarizing stimulus. Finally, we assessed the role of calcineurin in other forms of depolarization-induced plasticity. Neither membrane resistance changes nor spine density changes were affected by FK506 treatment, suggesting that calcineurin acts via a separate pathway to modulate AIS plasticity. Together, these results emphasize calcineurin as a vital player in the regulation of intrinsic plasticity as governed by the AIS.

Cholecalciferol (vitamin D₃) improves myelination and recovery after nerve injury

Previously, we demonstrated i) that ergocalciferol (vitamin D2) increases axon diameter and potentiates nerve regeneration in a rat model of transected peripheral nerve and ii) that cholecalciferol (vitamin D3) improves breathing and hyper-reflexia in a rat model of paraplegia. However, before bringing this molecule to the clinic, it was of prime importance i) to assess which form - ergocalciferol versus cholecalciferol - and which dose were the most efficient and ii) to identify the molecular pathways activated by this pleiotropic molecule. The rat left peroneal nerve was cut out on a length of 10 mm and autografted in an inverted position. Animals were treated with either cholecalciferol or ergocalciferol, at the dose of 100 or 500 IU/kg/day, or excipient (Vehicle), and compared to unlesioned rats (Control). Functional recovery of hindlimb was measured weekly, during 12 weeks, using the peroneal functional index. Ventilatory, motor and sensitive responses of the regenerated axons were recorded and histological analysis was performed. In parallel, to identify the genes regulated by vitamin D in dorsal root ganglia and/or Schwann cells, we performed an in vitro transcriptome study. We observed that cholecalciferol is more efficient than ergocalciferol and, when delivered at a high dose (500 IU/kg/day), cholecalciferol induces a significant locomotor and electrophysiological recovery. We also demonstrated that cholecalciferol increases i) the number of preserved or newly formed axons in the proximal end, ii) the mean axon diameter in the distal end, and iii) neurite myelination in both distal and proximal ends. Finally, we found a modified expression of several genes involved in axogenesis and myelination, after 24 hours of vitamin supplementation. Our study is the first to demonstrate that vitamin D acts on myelination via the activation of several myelin-associated genes. It paves the way for future randomised controlled clinical trials for peripheral nerve or spinal cord repair.

A role for the tyrosine kinase ACK1 in neurotrophin signaling and neuronal extension and branching

Neurotrophins are involved in many crucial cellular functions, including neurite outgrowth, synapse formation, and plasticity. Although these events have long been known, the molecular determinants underlying neuritogenesis have not been fully characterized. Ack1 (activated Cdc42-associated tyrosine kinase) is a non-receptor tyrosine kinase that is highly expressed in the brain. Here, we demonstrate that Ack1 is a molecular constituent of neurotrophin signaling cascades in neurons and PC12 cells. We report that Ack1 interacts with Trk receptors and becomes tyrosine phosphorylated and its kinase activity is increased in response to neurotrophins. Moreover, our data indicate that Ack1 acts upstream of the Akt and MAPK pathways. We show that Ack1 overexpression induces neuritic outgrowth and promotes branching in neurotrophin-treated neuronal cells, whereas the expression of Ack1 dominant negatives or short-hairpin RNAs counteract neurotrophin-stimulated differentiation. Our results identify Ack1 as a novel regulator of neurotrophin-mediated events in primary neurons and in PC12 cells.

Calcineurin A versus NS5A-TP2/HD domain containing 2: a case study of site-directed low-frequency random mutagenesis for dissecting target specificity of peptide aptamers

We previously identified a peptide aptamer (named R5G42) via functional selection for its capacity to slow cell proliferation. A yeast two-hybrid screen of human cDNA libraries, using R5G42 as “bait,” allowed the identification of two binding proteins with very different functions: calcineurin A (CnA) (PP2B/PPP3CA), a protein phosphatase well characterized for its role in the immune response, and NS5A-TP2/HD domain containing 2, a much less studied protein induced subsequent to hepatitis C virus non-structural protein 5A expression in HepG2 hepatocellular carcinoma cells, with no known activity. Our objective in the present study was to dissect the dual target specificity of R5G42 in order to have tools with which to better characterize the actions of the peptide aptamers toward their individual targets. This was achieved through the selection of random mutants of the variable loop, derived from R5G42, evaluating their specificity toward CnA and NS5A-TP2 and analyzing their sequence. An interdisciplinary approach involving biomolecular computer simulations with integration of the sequence data and yeast two-hybrid binding phenotypes of these mutants yielded two structurally distinct conformers affording the potential molecular basis of the binding diversity of R5G42. Evaluation of the biological impact of CnA- versus NS5A-TP2-specific peptide aptamers indicated that although both contributed to the anti-proliferative effect of R5G42, CnA-binding was essential to stimulate the nuclear translocation of nuclear factor of activated T cells, indicative of the activation of endogenous CnA. By dissecting the target specificity of R5G42, we have generated novel tools with which to study each target individually. Apta-C8 is capable of directly activating CnA independent of binding to NS5A-TP2 and will be an important tool in studying the role of CnA activation in the regulation of different signaling pathways, whereas Apta-E1 will allow dissection of the function of NS5A-TP2, serving as an example of the usefulness of peptide aptamer technology for investigating signaling pathways.

Temporal regulation of nuclear factor one occupancy by calcineurin/NFAT governs a voltage-sensitive developmental switch in late maturing neurons

Dendrite and synapse development are critical for establishing appropriate neuronal circuits, and disrupted timing of these events can alter neural connectivity. Using microarrays, we have identified a nuclear factor I (NFI)-regulated temporal switch program linked to dendrite formation in developing mouse cerebellar granule neurons (CGNs). NFI function was required for upregulation of many synapse-related genes as well as downregulation of genes expressed in immature CGNs. Chromatin immunoprecipitation analysis revealed that a central feature of this program was temporally regulated NFI occupancy of late-expressed gene promoters. Developing CGNs undergo a hyperpolarizing shift in membrane potential, and depolarization inhibits their dendritic and synaptic maturation via activation of calcineurin (CaN) (Okazawa et al., 2009). Maintaining immature CGNs in a depolarized state blocked NFI temporal occupancy of late-expressed genes and the NFI switch program via activation of the CaN/nuclear factor of activated T-cells, cytoplasmic (NFATc) pathway and promotion of late-gene occupancy by NFATc4, and these mechanisms inhibited dendritogenesis. Conversely, inhibition of the CaN/NFATc pathway in CGNs maturing under physiological nondepolarizing conditions upregulated the NFI switch program, NFI temporal occupancy, and dendrite formation. NFATc4 occupied the promoters of late-expressed NFI program genes in immature mouse cerebellum, and its binding was temporally downregulated with development. Further, NFI temporal binding and switch gene expression were upregulated in the developing cerebellum of Nfatc4 (-/-) mice. These findings define a novel NFI switch and temporal occupancy program that forms a critical link between membrane potential/CaN and dendritic maturation in CGNs. CaN inhibits the program and NFI occupancy in immature CGNs by promoting NFATc4 binding to late-expressed genes. As maturing CGNs become more hyperpolarized, NFATc4 binding declines leading to onset of NFI temporal binding and the NFI switch program.

Waking up the sleepers: shared transcriptional pathways in axonal regeneration and neurogenesis

In the last several years, relevant progress has been made in our understanding of the transcriptional machinery regulating CNS repair after acute injury, such as following trauma or stroke. In order to survive and functionally reconnect to the synaptic network, injured neurons activate an intrinsic rescue program aimed to increase their plasticity. Perhaps, in the attempt to switch back to a plastic and growth-competent state, post-mitotic neurons wake up and re-express a set of transcription factors that are also critical for the regulation of their younger brothers, the neural stem cells. Here, we review and discuss the transcriptional pathways regulating both axonal regeneration and neurogenesis highlighting the connection between the two. Clarification of their common molecular substrate may help simultaneous targeting of both neurogenesis and axonal regeneration with the hope to enhance functional recovery following CNS injury.

Pathway analysis reveals common pro-survival mechanisms of metyrapone and carbenoxolone after traumatic brain injury

Developing new pharmacotherapies for traumatic brain injury (TBI) requires elucidation of the neuroprotective mechanisms of many structurally and functionally diverse compounds. To test our hypothesis that diverse neuroprotective drugs similarly affect common gene targets after TBI, we compared the effects of two drugs, metyrapone (MT) and carbenoxolone (CB), which, though used clinically for noncognitive conditions, improved learning and memory in rats and humans. Although structurally different, both MT and CB inhibit a common molecular target, 11β hydroxysteroid dehydrogenase type 1, which converts inactive cortisone to cortisol, thereby effectively reducing glucocorticoid levels. We examined injury-induced signaling pathways to determine how the effects of these two compounds correlate with pro-survival effects in surviving neurons of the injured rat hippocampus. We found that treatment of TBI rats with MT or CB acutely induced in hippocampal neurons transcriptional profiles that were remarkably similar (i.e., a coordinated attenuation of gene expression across multiple injury-induced cell signaling networks). We also found, to a lesser extent, a coordinated increase in cell survival signals. Analysis of injury-induced gene expression altered by MT and CB provided additional insight into the protective effects of each. Both drugs attenuated expression of genes in the apoptosis, death receptor and stress signaling pathways, as well as multiple genes in the oxidative phosphorylation pathway such as subunits of NADH dehydrogenase (Complex1), cytochrome c oxidase (Complex IV) and ATP synthase (Complex V). This suggests an overall inhibition of mitochondrial function. Complex 1 is the primary source of reactive oxygen species in the mitochondrial oxidative phosphorylation pathway, thus linking the protective effects of these drugs to a reduction in oxidative stress. The net effect of the drug-induced transcriptional changes observed here indicates that suppressing expression of potentially harmful genes, and also, surprisingly, reduced expression of pro-survival genes may be a hallmark of neuroprotective therapeutic effects.

Calcineurin-dependent cofilin activation and increased retrograde actin flow drive 5-HT-dependent neurite outgrowth in Aplysia bag cell neurons

Neurite outgrowth in response to soluble growth factors often involves changes in intracellular Ca(2+); however, mechanistic roles for Ca(2+) in controlling the underlying dynamic cytoskeletal processes have remained enigmatic. Bag cell neurons exposed to serotonin (5-hydroxytryptamine [5-HT]) respond with a threefold increase in neurite outgrowth rates. Outgrowth depends on phospholipase C (PLC) → inositol trisphosphate → Ca(2+) → calcineurin signaling and is accompanied by increased rates of retrograde actin network flow in the growth cone P domain. Calcineurin inhibitors had no effect on Ca(2+) release or basal levels of retrograde actin flow; however, they completely suppressed 5-HT-dependent outgrowth and F-actin flow acceleration. 5-HT treatments were accompanied by calcineurin-dependent increases in cofilin activity in the growth cone P domain. 5-HT effects were mimicked by direct activation of PLC, suggesting that increased actin network treadmilling may be a widespread mechanism for promoting neurite outgrowth in response to neurotrophic factors.

Distinct activation properties of the nuclear factor of activated T-cells (NFAT) isoforms NFATc3 and NFATc4 in neurons

The Ca(2+)/calcineurin-dependent transcription factor NFAT (nuclear factor of activated T-cells) is implicated in regulating dendritic and axonal development, synaptogenesis, and neuronal survival. Despite the increasing appreciation for the importance of NFAT-dependent transcription in the nervous system, the regulation and function of specific NFAT isoforms in neurons are poorly understood. Here, we compare the activation of NFATc3 and NFATc4 in hippocampal and dorsal root ganglion neurons following electrically evoked elevations of intracellular Ca(2+) concentration ([Ca(2+)](i)). We find that NFATc3 undergoes rapid dephosphorylation and nuclear translocation that are essentially complete within 20 min, although NFATc4 remains phosphorylated and localized to the cytosol, only exhibiting nuclear localization following prolonged (1-3 h) depolarization. Knocking down NFATc3, but not NFATc4, strongly diminished NFAT-mediated transcription induced by mild depolarization in neurons. By analyzing NFATc3/NFATc4 chimeras, we find that the region containing the serine-rich region-1 (SRR1) mildly affects initial NFAT translocation, although the region containing the serine-proline repeats is critical for determining the magnitude of NFAT activation and nuclear localization upon depolarization. Knockdown of glycogen synthase kinase 3β (GSK3β) significantly increased the depolarization-induced nuclear localization of NFATc4. In contrast, inhibition of p38 or mammalian target of rapamycin (mTOR) kinases had no significant effect on nuclear import of NFATc4. Thus, electrically evoked [Ca(2+)](i) elevation in neurons rapidly and strongly activates NFATc3, whereas activation of NFATc4 requires a coincident increase in [Ca(2+)](i) and suppression of GSK3β, with differences in the serine-proline-containing region giving rise to these distinct activation properties of NFATc3 and NFATc4.

Nuclear factor of activated T cells (NFATc4) is required for BDNF-dependent survival of adult-born neurons and spatial memory formation in the hippocampus

New neurons generated in the adult dentate gyrus are constantly integrated into the hippocampal circuitry and activated during encoding and recall of new memories. Despite identification of extracellular signals that regulate survival and integration of adult-born neurons such as neurotrophins and neurotransmitters, the nature of the intracellular modulators required to transduce those signals remains elusive. Here, we provide evidence of the expression and transcriptional activity of nuclear factor of activated T cell c4 (NFATc4) in hippocampal progenitor cells. We show that NFATc4 calcineurin-dependent activity is required selectively for survival of adult-born neurons in response to BDNF signaling. Indeed, cyclosporin A injection and stereotaxic delivery of the BDNF scavenger TrkB-Fc in the mouse dentate gyrus reduce the survival of hippocampal adult-born neurons in wild-type but not in NFATc4(-/-) mice and do not affect the net rate of neural precursor proliferation and their fate commitment. Furthermore, associated with the reduced survival of adult-born neurons, the absence of NFATc4 leads to selective defects in LTP and in the encoding of hippocampal-dependent spatial memories. Thus, our data demonstrate that NFATc4 is essential in the regulation of adult hippocampal neurogenesis and identify NFATc4 as a central player of BDNF-driven prosurvival signaling in hippocampal adult-born neurons.

Inhibition of the NFAT pathway alleviates amyloid β neurotoxicity in a mouse model of Alzheimer's disease

Amyloid β (Aβ) peptides, the main pathological species associated with Alzheimer's disease (AD), disturb intracellular calcium homeostasis, which in turn activates the calcium-dependent phosphatase calcineurin (CaN). CaN activation induced by Aβ leads to pathological morphological changes in neurons, and overexpression of constitutively active calcineurin is sufficient to generate a similar phenotype, even without Aβ. Here, we tested the hypothesis that calcineurin mediates neurodegenerative effects via activation of the nuclear transcription factor of activated T-cells (NFAT). We found that both spine loss and dendritic branching simplification induced by Aβ exposure were mimicked by constitutively active NFAT, and abolished when NFAT activation was blocked using the genetically encoded inhibitor VIVIT. When VIVIT was specifically addressed to the nucleus, identical beneficial effects were observed, thus enforcing the role of NFAT transcriptional activity in Aβ-related neurotoxicity. In vivo, when VIVIT or its nuclear counterpart were overexpressed in a transgenic model of Alzheimer's disease via a gene therapy approach, the spine loss and neuritic abnormalities observed in the vicinity of amyloid plaques were blocked. Overall, these results suggest that NFAT/calcineurin transcriptional cascades contribute to Aβ synaptotoxicity, and may provide a new specific set of pathways for neuroprotective strategies.

Extracellular ATP activates NFAT-dependent gene expression in neuronal PC12 cells via P2X receptors

Background

Treatment of neuronal PC12 cells with ATP induces depolarisation and increases intracellular calcium levels via purinergic receptors. In many cell types, sustained elevation of intracellular calcium levels cause changes in gene expression via activation of the transcription factor NFAT (nuclear factor of activated T cells). We have therefore characterised the signalling pathway by which ATP regulates NFAT-dependent gene expression in PC12 cells.

Results

The activation of NFAT transcriptional activity by extracellular ATP was characterised with the help of reporter gene assays. Treatment of PC12 cells with ATP elicited a dose-dependent increase in luciferase activity (EC₅₀ = 78 μM). UTP, 4-benzoylbenzoyl ATP and α,β-methylene ATP did not mimic the effect of ATP, which was abolished by treatment with the P2X receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS). This pharmacological characterisation provides evidence for a critical role of ionotropic P2X receptors. Blockade of L-type voltage-dependent calcium channels by nifedipine reduced the response of NFAT to ATP, indicating that a depolarisation-mediated calcium influx was required for maximal NFAT activation. Inhibition of store-operated calcium entry by the pyrazole derivative BTP2 also diminished ATP-dependent NFAT activation. Furthermore, ATP-induced NFAT activation was associated with the activation of the mitogen-activated protein kinases ERK1/2. Finally, treatment with ATP increased the levels of the NFAT target transcripts, RCAN1-4 (regulator of calcineurin) and BDNF (brain derived neurotrophic factor).

Conclusion

The present data show that ATP induces NFAT-dependent changes in gene expression in PC12 cells by acting on P2X receptors. Maximal NFAT activation depends on both depolarisation-induced calcium influx and store-operated calcium entry and requires the activity of the protein phosphatase calcineurin and the mitogen-activated protein kinase cascade.

Role of calcineurin in neurodegeneration produced by misfolded proteins and endoplasmic reticulum stress

A hallmark event in neurodegenerative diseases is the accumulation of misfolded aggregated proteins in the brain leading to neuronal dysfunction and disease. Compelling evidence suggests that misfolded proteins damage cells by inducing endoplasmic reticulum (ER) stress and alterations in calcium homeostasis. Changes in cytoplasmic calcium concentration lead to unbalances on several signaling pathways. Recent data suggest that calcium-mediated hyperactivation of calcineurin (CaN), a key phosphatase in the brain, triggers synaptic dysfunction and neuronal death, the two central events responsible for brain degeneration in neurodegenerative diseases. Therefore, blocking CaN hyper-activation might be a promising therapeutic strategy to prevent brain damage in neurodegenerative diseases.

Activation of alpha(1) -adrenergic receptors stimulate the growth of small mouse cholangiocytes via calcium-dependent activation of nuclear factor of activated T cells 2 and specificity protein 1

Small cholangiocytes proliferate via activation of calcium (Ca(2+) )-dependent signaling in response to pathological conditions that trigger the damage of large cyclic adenosine monophosphate-dependent cholangiocytes. Although our previous studies suggest that small cholangiocyte proliferation is regulated by the activation of Ca(2+) -dependent signaling, the intracellular mechanisms regulating small cholangiocyte proliferation are undefined. Therefore, we sought to address the role and mechanisms of action by which phenylephrine, an α(1) -adrenergic agonist stimulating intracellular D-myo-inositol-1,4,5-triphosphate (IP(3) )/Ca(2+) levels, regulates small cholangiocyte proliferation. Small and large bile ducts and cholangiocytes expressed all AR receptor subtypes. Small (but not large) cholangiocytes respond to phenylephrine with increased proliferation via the activation of IP(3) /Ca(2+) -dependent signaling. Phenylephrine stimulated the production of intracellular IP(3) . The Ca(2+) -dependent transcription factors, nuclear factor of activated T cells 2 (NFAT2) and NFAT4, were predominantly expressed by small bile ducts and small cholangiocytes. Phenylephrine stimulated the Ca(2+) -dependent DNA-binding activities of NFAT2, NFAT4, and Sp1 (but not Sp3) and the nuclear translocation of NFAT2 and NFAT4 in small cholangiocytes. To determine the relative roles of NFAT2, NFAT4, or Sp1, we knocked down the expression of these transcription factors with small hairpin RNA. We observed an inhibition of phenylephrine-induced proliferation in small cholangiocytes lacking the expression of NFAT2 or Sp1. Phenylephrine stimulated small cholangiocyte proliferation is regulated by Ca(2+) -dependent activation of NFAT2 and Sp1.

CONCLUSION

Selective stimulation of Ca(2+) -dependent small cholangiocyte proliferation may be key to promote the repopulation of the biliary epithelium when large bile ducts are damaged during cholestasis or by toxins.

Cabin1 expression suggests roles in neuronal development

Nervous system assembly and function depends on precise regulation of developmental gene expression. Cabin1, an essential gene in developing mice, is enriched in regions of the developing zebrafish central nervous system (CNS). Cabin1 is a repressor of MEF2- (myocyte enhancer factor 2) and calcineurin-mediated transcription in the immune system, but its function in the CNS during development is unknown. We identified Cabin1 from a library of genes enriched in developing neurons and determined the temporal and spatial expression of Cabin1 mRNA during CNS development. We found Cabin1 mRNA expression in the developing brain at times correlated with later aspects of neuronal differentiation. In some regions of the CNS Cabin1 expression overlaps with regions that also express proteins known to interact with Cabin1: MEF2 and/or calcineurin. We suggest that Cabin1 could act as a regulator of MEF2 and calcineurin activity in the developing nervous system, given their roles in neuronal differentiation and synaptic refinement.

Effect of leukotriene D4 on mouse embryonic stem cell migration and proliferation: involvement of PI3K/Akt as well as GSK-3β/β-catenin signaling pathways

The actual leukotriene D(4) (LTD(4)) signaling pathways that regulate cell proliferation have not been elucidated thoroughly although fatty acid and its metabolites play a key role in regulations of embryonic functions. Thus, this study investigated the response of mouse embryonic stem (ES) cells exposed to LTD(4) and elucidated the signaling pathways as well. LTD(4) increased DNA synthesis in concentration-dependent (≥10(-7) M) and time-dependent (≥12 h) manners, as determined by [(3)H] thymidine incorporation and increased cell number. LTD(4) induced the phosphorylation of signal transducer and activator of transcription-3 (STAT3) and the increase of intracellular Ca(2+) levels via cysteinyl leukotriene (CysLT) 1 and 2 receptors. LTD(4) increased Akt activation and calcineurin expression, which were blocked by STAT3 inhibitor and calcium chelators. LTD(4)-induced glycogen synthase kinase (GSK)-3β phosphorylation was decreased by LY294002, Akt inhibitor, and cyclosporine A. LTD(4) inhibited the phosphorylation of β-catenin. In addition, LTD(4)-stimulated migration through increased activation of focal adhesion kinase (FAK) and paxillin which were blocked by Akt inhibitor and cyclosporine A. LTD(4)-induced increases in protooncogene and cell cycle regulatory proteins were blocked by cyclosporine A, FAK siRNA, and β-catenin siRNA. In conclusion, LTD(4)-stimulated mouse ES cell proliferation and migration via STAT3, phosphoinositide 3-kinases (PI3K)/Akt, Ca(2+)-calcineurin, and GSK-3β/β-catenin pathway.

Dynamic interplay of excitatory and inhibitory coupling modes of neuronal L-type calcium channels

L-type voltage-gated calcium channels (LTCCs) have long been considered as crucial regulators of neuronal excitability. This role is thought to rely largely on coupling of LTCC-mediated Ca(2+) influx to Ca(2+)-dependent conductances, namely Ca(2+)-dependent K(+) (K(Ca)) channels and nonspecific cation (CAN) channels, which mediate afterhyperpolarizations (AHPs) and afterdepolarizations (ADPs), respectively. However, in which manner LTCCs, K(Ca) channels, and CAN channels co-operate remained scarcely known. In this study, we examined how activation of LTCCs affects neuronal depolarizations and analyzed the contribution of Ca(2+)-dependent potassium- and cation-conductances. With the use of hippocampal neurons in primary culture, pulsed current-injections were applied in the presence of tetrodotoxin (TTX) for stepwise depolarization and the availability of LTCCs was modulated by BAY K 8644 and isradipine. By varying pulse length and current strength, we found that weak depolarizing stimuli tend to be enhanced by LTCC activation, whereas in the course of stronger depolarizations LTCCs counteract excitation. Both effect modes appear to involve the same channels that mediate ADP and AHP, respectively. Indeed, ADPs were activated at lower stimulation levels than AHPs. In the absence of TTX, activation of LTCCs prolonged or shortened burst firing, depending on the initial burst duration, and invariably augmented brief unprovoked (such as excitatory postsynaptic potentials) and provoked electrical events. Hence, regulation of membrane excitability by LTCCs involves synchronous activity of both excitatory and inhibitory Ca(2+)-activated ion channels. The overall enhancing or dampening effect of LTCC stimulation on excitability does not only depend on the relative abundance of the respective coupling partner but also on the stimulus intensity.

NFAT regulates pre-synaptic development and activity-dependent plasticity in Drosophila

The calcium-regulated transcription factor NFAT is emerging as a key regulator of neuronal development and plasticity but precise cellular consequences of NFAT function remain poorly understood. Here, we report that the single Drosophila NFAT homolog is widely expressed in the nervous system including motor neurons and unexpectedly controls neural excitability. Likely due to this effect on excitability, NFAT regulates overall larval locomotion and both chronic and acute forms of activity-dependent plasticity at the larval glutamatergic neuro-muscular synapse. Specifically, NFAT-dependent synaptic phenotypes include changes in the number of pre-synaptic boutons, stable modifications in synaptic microtubule architecture and pre-synaptic transmitter release, while no evidence is found for synaptic retraction or alterations in the level of the synaptic cell adhesion molecule FasII. We propose that NFAT regulates pre-synaptic development and constrains long-term plasticity by dampening neuronal excitability.

In vitro findings of alterations in intracellular calcium homeostasis in schizophrenia

The pathogenesis of schizophrenia involves several complex cellular mechanisms and is not well understood. Recent research has demonstrated an association between primary disturbances characteristic of the disease, including altered dopaminergic and glutamatergic neurotransmission, and impairments in neuronal calcium (Ca(2+)) homeostasis and signaling. Emerging Ca(2+) hypothesis links and unifies various cellular processes involved in the pathogenesis of schizophrenia and suggests a central role of dysregulation of Ca(2+) homeostasis in the etiology of the disease. This review explores the in vitro data on Ca(2+) homeostasis and signaling in schizophrenia. Major limitation in this research is the lack of schizophrenia markers and validated disease models. As indicated in this review, one way to overcome these limitations may be analyses of Ca(2+) signalosomes in peripheral cells from schizophrenia patients. Validation of animal models of schizophrenia may permit the application of advanced Ca(2+) imaging techniques in living animals.

Plasma membrane calcium ATPase proteins as novel regulators of signal transduction pathways

Emerging evidence suggests that plasma membrane calcium ATPases (PMCAs) play a key role as regulators of calcium-triggered signal transduction pathways via interaction with partner proteins. PMCAs regulate these pathways by targeting specific proteins to cellular sub-domains where the levels of intracellular free calcium are kept low by the calcium ejection properties of PMCAs. According to this model, PMCAs have been shown to interact functionally with the calcium-sensitive proteins neuronal nitric oxide synthase, calmodulin-dependent serine protein kinase, calcineurin and endothelial nitric oxidase synthase. Transgenic animals with altered expression of PMCAs are being used to evaluate the physiological significance of these interactions. To date, PMCA interactions with calcium-dependent partner proteins have been demonstrated to play a crucial role in the pathophysiology of the cardiovascular system via regulation of the nitric oxide and calcineurin/nuclear factor of activated T cells pathways. This new evidence suggests that PMCAs play a more sophisticated role than the mere ejection of calcium from the cells, by acting as modulators of signaling transduction pathways.

A novel microRNA and transcription factor mediated regulatory network in schizophrenia

Background

Schizophrenia is a complex brain disorder with molecular mechanisms that have yet to be elucidated. Previous studies have suggested that changes in gene expression may play an important role in the etiology of schizophrenia, and that microRNAs (miRNAs) and transcription factors (TFs) are primary regulators of this gene expression. So far, several miRNA-TF mediated regulatory modules have been verified. We hypothesized that miRNAs and TFs might play combinatory regulatory roles for schizophrenia genes and, thus, explored miRNA-TF regulatory networks in schizophrenia.

Results

We identified 32 feed-forward loops (FFLs) among our compiled schizophrenia-related miRNAs, TFs and genes. Our evaluation revealed that these observed FFLs were significantly enriched in schizophrenia genes. By converging the FFLs and mutual feedback loops, we constructed a novel miRNA-TF regulatory network for schizophrenia. Our analysis revealed EGR3 and hsa-miR-195 were core regulators in this regulatory network. We next proposed a model highlighting EGR3 and miRNAs involved in signaling pathways and regulatory networks in the nervous system. Finally, we suggested several single nucleotide polymorphisms (SNPs) located on miRNAs, their target sites, and TFBSs, which may have an effect in schizophrenia gene regulation.

Conclusions

This study provides many insights on the regulatory mechanisms of genes involved in schizophrenia. It represents the first investigation of a miRNA-TF regulatory network for a complex disease, as demonstrated in schizophrenia.

Molecules, signaling, and schizophrenia

Schizophrenia is one of the most common psychiatric disorders, but despite some progress in identifying the genetic factors implicated in its development, the molecular mechanisms underlying its etiology and pathogenesis remain poorly understood. However, accumulating evidence suggests that regardless of the underlying genetic complexity, the mechanisms of the disease may impact a small number of common signaling pathways. In this review, we discuss the evidence for a role of schizophrenia susceptibility genes in intracellular signaling cascades by focusing on three prominent candidate genes: AKT, PPP3CC (calcineurin), and DISC1. We describe the regulation of a number of signaling cascades by AKT and calcineurin through protein phosphorylation and dephosphorylation, and the recently uncovered functions of DISC1 in cAMP and GSK3beta signaling. In addition, we present independent evidence for the involvement of their downstream signaling pathways in schizophrenia. Finally, we discuss evidence supporting an impact of these susceptibility genes on common intracellular signaling pathways and the convergence of their effects on neuronal processes implicated in schizophrenia.

Cognitive decline in Alzheimer's disease is associated with selective changes in calcineurin/NFAT signaling

Upon activation by calcineurin, the nuclear factor of activated T-cells (NFAT) translocates to the nucleus and guides the transcription of numerous molecules involved in inflammation and Ca(2+) dysregulation, both of which are prominent features of Alzheimer's disease (AD). However, NFAT signaling in AD remains relatively uninvestigated. Using isolated cytosolic and nuclear fractions prepared from rapid-autopsy postmortem human brain tissue, we show that NFATs 1 and 3 shifted to nuclear compartments in the hippocampus at different stages of neuropathology and cognitive decline, whereas NFAT2 remained unchanged. NFAT1 exhibited greater association with isolated nuclear fractions in subjects with mild cognitive impairment (MCI), whereas NFAT3 showed a strong nuclear bias in subjects with severe dementia and AD. Similar to NFAT1, calcineurin-Aalpha also exhibited a nuclear bias in the early stages of cognitive decline. But, unlike NFAT1 and similar to NFAT3, the nuclear bias for calcineurin became more pronounced as cognition worsened. Changes in calcineurin/NFAT3 were directly correlated to soluble amyloid-beta (Abeta((1-42))) levels in postmortem hippocampus, and oligomeric Abeta, in particular, robustly stimulated NFAT activation in primary rat astrocyte cultures. Oligomeric Abeta also caused a significant reduction in excitatory amino acid transporter 2 (EAAT2) protein levels in astrocyte cultures, which was blocked by NFAT inhibition. Moreover, inhibition of astrocytic NFAT activity in mixed cultures ameliorated Abeta-dependent elevations in glutamate and neuronal death. The results suggest that NFAT signaling is selectively altered in AD and may play an important role in driving Abeta-mediated neurodegeneration.