Neurotrophin-induced activation of casein kinase 2 in rat hippocampal slices

Casein Kinase 2 Inhibitor, CX-4945, Induces Apoptosis and Restores Blood-Brain Barrier Homeostasis in In Vitro and In Vivo Models of Glioblastoma

Background: In oncology, casein kinase 2 (CK2), a serine/threonine kinase, has a dual action, regulating cellular processes and acting as an oncogenic promoter. Methods: This study examined the effect of CX-4945, a selective CK2 inhibitor, in a human U-87 glioblastoma (GBM) cell line, treated with CX-4945 (5, 10, and 15 μM) for 24 h. Similarly, the hCMEC/D3 cell line was used to mimic the blood-brain barrier (BBB), examining the ability of CX-4945 to restore BBB homeostasis, after stimulation with lipopolysaccharide (LPS) and then treated with CX-4945 (5, 10, and 15 μM). Results: We reported that CX-4945 reduced the proliferative activity and modulated the main pathways involved in tumor progression including apoptosis. Furthermore, in confirmation of the in vitro study, performing a xenograft model, we demonstrated that CX-4945 exerted promising antiproliferative effects, also restoring the tight junctions' expression. Conclusions: These new insights into the molecular signaling of CK2 in GBM and BBB demonstrate that CX-4945 could be a promising approach for future GBM therapy, not only in the tumor microenvironment but also at the BBB level.

Multitarget-directed cotreatment with cilostazol and aripiprazole for augmented neuroprotection against oxidative stress-induced toxicity in HT22 mouse hippocampal cells

Cerebrovascular dysfunction is crucially associated with cognitive impairment and a high prevalence of psychotic symptoms in the vascular dementia characterized by oxidative stress and multifactorial neurodegeneration. In this study, the significant decrease in BDNF expression in HT22 cells due to H₂O₂ (0.25 mM) was little affected by either aripiprazole (1 μM) or cilostazol (1 μM) alone, but significantly increased by cotreatment with both drugs. Even in the presence of H₂O₂, P-CK2α (Tyr 255), nuclear P-CREB (Ser 133), and nuclear P-β-catenin (Ser 675) levels were significantly increased in a synergistic manner by aripiprazole plus cilostazol cotreatment. Aripiprazole and cilostazol cotreatment synergistically increased P-GSK-3β (Ser 9) level. Nrf2/HO-1 expression was significantly elevated time- and concentration-dependently by either aripiprazole or cilostazol. In line with these, concurrent treatment with aripiprazole (1 μM) plus cilostazol (1 μM) significantly increased Nrf2 and HO-1 expression in a synergistic manner, accompanying with increased ARE luciferase activity, while each drug monotherapy showed little effects. Consequently, this cotreatment synergistically ameliorated the attenuated neurite outgrowth induced by H₂O₂ in the HT22 cells, and these were inhibited by K252A (inhibitor of BDNF receptor), TBCA (CK2 inhibitor), imatinib (β-catenin inhibitor) and ZnPP (inhibitor of HO-1), indicating that BDNF, P-CK2α, β-catenin and HO-1 activation are implicated in the enhanced neurite outgrowth. This study highlights that cotreatment with low concentrations of aripiprazole and cilostazol synergistically elicits neuroprotective effects by overcoming oxidative stress-evoked neurotoxicity associated with increased neurite outgrowth, providing a rationale for the use of this combinatorial treatment in vascular dementia.

Circadian control of p75 neurotrophin receptor leads to alternate activation of Nrf2 and c-Rel to reset energy metabolism in astrocytes via brain-derived neurotrophic factor

Circadian clock genes regulate energy metabolism partly through neurotrophins in the body. The low affinity neurotrophin receptor p75^NTR is a clock component directly regulated by the transcriptional factor Clock:Bmal1 complex. Brain-derived neurotrophic factor (BDNF) is expressed in the brain and plays a key role in coordinating metabolic interactions between neurons and astrocytes. BDNF transduces signals through TrkB and p75^NTR receptors. This review highlights a novel molecular mechanism by which BDNF via circadian control of p75^NTR leads to daily resetting of glucose and glycogen metabolism in brain astrocytes to accommodate their functional interaction with neurons. Astrocytes store glycogen as an energy reservoir to provide active neurons with the glycolytic metabolite lactate. Astrocytes predominantly express the truncated receptor TrkB.T1 which lacks an intracellular receptor tyrosine kinase domain. TrkB.T1 retains the capacity to regulate cell morphology through regulation of Rho GTPases. In contrast, p75^NTR mediates generation of the bioactive lipid ceramide upon stimulation with BDNF and inhibits PKA activation. As ceramide directly activates PKCζ, we discuss the importance of the TrkB.T1-p75^NTR-ceramide-PKCζ signaling axis in the stimulation of glycogen and lipid synthesis and activation of RhoA. Ceramide-PKCζ-casein kinase 2 signaling activates Nrf2 to support oxidative phosphorylation via upregulation of antioxidant enzymes. In the absence of p75^NTR, TrkB.T1 functionally interacts with adenosine A_2AR and dopamine D1R receptors to enhance cAMP-PKA signaling and activate Rac1 and NF-κB c-Rel, favoring glycogen hydrolysis, gluconeogenesis and aerobic glycolysis. Thus, diurnal changes in p75^NTR levels in astrocytes resets energy metabolism via BDNF to accommodate their metabolic interaction with neurons.

CK2-An Emerging Target for Neurological and Psychiatric Disorders

Protein kinase CK2 has received a surge of attention in recent years due to the evidence of its overexpression in a variety of solid tumors and multiple myelomas as well as its participation in cell survival pathways. CK2 is also upregulated in the most prevalent and aggressive cancer of brain tissue, glioblastoma multiforme, and in preclinical models, pharmacological inhibition of the kinase has proven successful in reducing tumor size and animal mortality. CK2 is highly expressed in the mammalian brain and has many bona fide substrates that are crucial in neuronal or glial homeostasis and signaling processes across synapses. Full and conditional CK2 knockout mice have further elucidated the importance of CK2 in brain development, neuronal activity, and behavior. This review will discuss recent advances in the field that point to CK2 as a regulator of neuronal functions and as a potential novel target to treat neurological and psychiatric disorders.

The intracellular domain of L1CAM binds to casein kinase 2α and is neuroprotective via inhibition of the tumor suppressors PTEN and p53

Cell adhesion molecule L1 promotes neuritogenesis and neuronal survival through triggering MAPK pathways. Based on the findings that L1 is associated with casein kinase 2 (CK2), and that deficiency in PTEN promotes neuritogenesis in vitro and regeneration after trauma, we examined the functional relationship between L1 and PTEN. In parallel, we investigated the tumor suppressor p53, which also regulates neuritogenesis. Here, we report that the intracellular domain of L1 binds to the subunit CK2α, and that knockdown of L1 leads to CK2 dephosphorylation and an increase in PTEN and p53 levels. Overexpression of L1, but not the L1 mutants L1 (S1181N, E1184V), which reduced binding between L1 and CK2, reduced expression levels of PTEN and p53 proteins, and enhanced levels of phosphorylated CK2α and mammalian target of rapamycin, which is a downstream effector of PTEN and p53. Treatment of neurons with a CK2 inhibitor or transfection with CK2α siRNA increased levels of PTEN and p53, and inhibited neuritogenesis. The combined observations indicate that L1 downregulates expression of PTEN and p53 via direct binding to CK2α. We suggest that L1 stimulates neuritogenesis by activating CK2α leading to decreased levels of PTEN and p53 via a novel, L1-triggered and CK2α-mediated signal transduction pathway. L1CAM (L1 cell adhesion molecule) is implicated in neural functions through the cognate src/MAP kinase signaling pathway. We now describe a novel signaling platform operating via the alpha subunit of casein kinase 2 which binds to the intracellular domain of L1. Knockdown of L1CAM leads to increased levels of tumor suppressor PTEN (phosphatase and tensin homolog) and p53, known to inhibit neuritogenesis in vitro and recovery from trauma in vivo. By activating this enzyme, L1CAM adds to its beneficial functions by decreasing the levels of PTEN and p53.

Casein kinase 2 phosphorylation of protein kinase C and casein kinase 2 substrate in neurons (PACSIN) 1 protein regulates neuronal spine formation

The PACSIN (protein kinase C and casein kinase 2 substrate in neurons) adapter proteins couple components of the clathrin-mediated endocytosis machinery with regulators of actin polymerization and thereby regulate the surface expression of specific receptors. The brain-specific PACSIN 1 is enriched at synapses and has been proposed to affect neuromorphogenesis and the formation and maturation of dendritic spines. In studies of how phosphorylation of PACSIN 1 contributes to neuronal function, we identified serine 358 as a specific site used by casein kinase 2 (CK2) in vitro and in vivo. Phosphorylated PACSIN 1 was found in neuronal cytosol and membrane fractions. This localization could be modulated by trophic factors such as brain-derived neurotrophic factor (BDNF). We further show that expression of a phospho-negative PACSIN 1 mutant, S358A, or inhibition of CK2 drastically reduces spine formation in neurons. We identified a novel protein complex containing the spine regulator Rac1, its GTPase-activating protein neuron-associated developmentally regulated protein (NADRIN), and PACSIN 1. CK2 phosphorylation of PACSIN 1 leads to a dissociation of the complex upon BDNF treatment and induces Rac1-dependent spine formation in dendrites of hippocampal neurons. These findings suggest that upon BDNF signaling PACSIN 1 is phosphorylated by CK2 which is essential for spine formation.

Brain-derived neurotrophic factor enhances Bcl-xL expression through protein kinase casein kinase 2-activated and nuclear factor kappa B-mediated pathway in rat hippocampus

Brain-derived neurotrophic factor (BDNF) was shown to produce its neuroprotective effect through extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphatidylinositol-3 kinase (PI3-K) signaling. But whether other pathways also mediate the neuroprotective effect of BDNF is less known. In this study, we found that direct administration of BDNF to rat hippocampal CA1 area dose-dependently increased the mRNA and protein levels of Bcl-xL. BDNF also increased protein kinase casein kinase II (CK2) activity and NF-κB phosphorylation at Ser529 dose-dependently. Further, transfection of the wild-type CK2α DNA to CA1 neurons increased nuclear factor kappa B (NF-κB) phosphorylation and Bcl-xL mRNA expression, whereas transfection of CK2α156A, the catalytically inactive mutant of CK2α, decreased these measures. Moreover, transfection of CK2α small interfering RNA (siRNA) blocked the enhancing effect of BDNF on NF-κB phosphorylation and Bcl-xL expression. These results were further confirmed by treatment of 4,5,6,7-tetrabromobenzotriazole (TBB), a specific CK2 inhibitor. Transfection of NF-κBS529A, the dominant negative mutant of NF-κB, prevented the enhancing effect of BDNF on Bcl-xL expression. More importantly, BDNF activation of CK2 is not affected by co-administration of the ERK1/2 inhibitor, PD98059, and the PI3-K inhibitor, LY294002. These results demonstrate a novel BDNF signaling pathway and provide an alternative therapeutic strategy for the protective effect of BDNF on hippocampal neurons in vivo.

CAPS activity in priming vesicle exocytosis requires CK2 phosphorylation

CAPS (Ca(2+)-dependent activator protein for secretion) functions in priming Ca(2+)-dependent vesicle exocytosis, but the regulation of CAPS activity has not been characterized. Here we show that phosphorylation by protein kinase CK2 is required for CAPS activity. Dephosphorylation eliminated CAPS activity in reconstituting Ca(2+)-dependent vesicle exocytosis in permeable and intact PC12 cells. Ser-5, -6, and -7 and Ser-1281 were identified by mass spectrometry as the major phosphorylation sites in the 1289 residue protein. Ser-5, -6, and -7 but not Ser-1281 to Ala substitutions abolished CAPS activity. Protein kinase CK2 phosphorylated CAPS in vitro at these sites and restored the activity of dephosphorylated CAPS. CK2 is the likely in vivo CAPS protein kinase based on inhibition of phosphorylation by tetrabromo-2-benzotriazole in PC12 cells and by the identity of in vivo and in vitro phosphorylation sites. CAPS phosphorylation by CK2 was constitutive, but the elevation of Ca(2+) in synaptosomes increased CAPS Ser-5 and -6 dephosphorylation, which terminates CAPS activity. These results identify a functionally important N-terminal phosphorylation site that regulates CAPS activity in priming vesicle exocytosis.

Phosphorylation of Nogo receptors suppresses Nogo signaling, allowing neurite regeneration

The myelin-associated proteins Nogo-A, MAG, and OMgp transmit signals from oligodendrocytes into neurons through binding to Nogo receptors. Nogo signaling has critical roles in development and maintenance of the central nervous system (CNS). It can inhibit differentiation, migration, and neurite outgrowth of neurons, causing poor recovery of the adult CNS from damage. Here, I show that phosphorylation of Nogo receptors by casein kinase II (CK2) inhibits binding of the myelin-associated proteins. Brain-derived neurotrophic factor stimulates the phosphorylation, suppressing Nogo-dependent inhibition of neurite outgrowth from neuroblastoma-derived neural cells. Similarly, in rat adult neurons, extracellular CK2 treatment overcomes inhibition of neurite outgrowth by the myelin-associated proteins. These findings provide new strategies to control Nogo signaling and hence neuronal regeneration.

Casein kinase II contributes to the synergistic effects of BMP7 and BDNF on Smad 1/5/8 phosphorylation in septal neurons under hypoglycemic stress

The combination of bone morphogenetic protein 7 (BMP7) and neurotrophins (e.g. brain-derived neurotrophic factor, BDNF) protects septal neurons during hypoglycemic stress. We investigated the signaling mechanisms underlying this synergistic protection. BMP7 (5 nM) increased phosphorylation and nuclear translocation of BMP-responsive Smads 1/5/8 within 30 min in cultures of rat embryonic septal neurons. BDNF (100 ng/mL) enhanced the BMP7-induced increase in phospho-Smad levels in both nucleus and cytoplasm; this effect was more pronounced after a hypoglycemic stress. BDNF increased both Akt and Erk phosphorylation, but pharmacological blockade of these kinase pathways (with wortmannin and U0126, respectively) did not reduce the Smad phosphorylation produced by the BMP7 + BDNF combination. Inhibitors of casein kinase II (CK2) activity reduced the (BMP7 + BDNF)-induced Smad phosphorylation, and this trophic factor combination increased CK2 activity in hypoglycemic cultures. These findings suggest that BDNF can increase BMP-dependent Smad phosphorylation via a mechanism requiring CK2.

Casein kinase II interacts with prion protein in vitro and forms complex with native prion protein in vivo

The most essential and crucial step during the pathogenesis of transmissible spongiform encephalopathy is the conformational change of cellular prion protein to pathologic isoform. Casein kinase II (CK2) is a ubiquitously expressed and evolutionarily conserved pleiotropic protein kinase that is essential for viability. To explore the possible molecular interaction between CK2 and prion protein (PrP), the full-length sequences of human CK2alpha and CK2beta complementary DNA were amplified with reverse transcription-polymerase chain reaction using the total messenger RNA from cell line SH-SY5Y as the template; then, the fusion proteins histidine-CK2alpha and glutathione S-transferase-histidine-CK2beta were expressed in Escherichia coli. The interaction between CK2 and PrP was evaluated with co-immunoprecipitation and pull-down assays. The results demonstrated that recombinant PrP bound specifically with CK2alpha, but not with CK2beta. The native CK2 and PrP in hamster brains interacted with each other, forming protein complexes. Three different glycosylated forms of PrP (diglycosylated, monoglycosylated and unglycosylated PrP) from normal brains interacted with the CK2alpha subunit, though the unglycosylated PrP seemed to have a stronger binding ability with CK2alpha subunit. The domain responsible for interacting with CK2alpha was located at the C-terminal segment of PrP (residues 91-231). This study proposed reliable experimental data for the molecular interaction between PrP and CK2alpha (both in recombinant and native categories), scientific clues for further assessing the potential biological significance of the PrP-CK2 interaction, and the possible role of CK2 in the pathogenesis of prion diseases.

Growth factor signals in neural cells: coherent patterns of interaction control multiple levels of molecular and phenotypic responses

Individual neurons express receptors for several different growth factors that influence the survival, growth, neurotransmitter phenotype, and other properties of the cell. Although there has been considerable progress in elucidating the molecular signal transduction pathways and physiological responses of neurons and other cells to individual growth factors, little is known about if and how signals from different growth factors are integrated within a neuron. In this study, we determined the interactive effects of nerve growth factor, insulin-like growth factor 1, and epidermal growth factor on the activation status of downstream kinase cascades and transcription factors, cell survival, and neurotransmitter production in neural cells that express receptors for all three growth factors. We document considerable differences in the quality and quantity of intracellular signaling and eventual phenotypic responses that are dependent on whether cells are exposed to a single or multiple growth factors. Dual stimulations that generated the greatest antagonistic or synergistic actions, compared with a theoretically neutral summation of their two activities, yielded the largest eventual change of neuronal phenotype indicated by the ability of the cell to produce norepinephrine or resist oxidative stress. Combined activation of insulin-like growth factor 1 and epidermal growth factor receptors was particularly notable for antagonistic interactions at some levels of signal transduction and norepinephrine production, but potentiation at other levels of signaling and cytoprotection. Our findings suggest that in true physiological settings where multiple growth factors are present, activation of one receptor type may result in molecular and phenotypic responses that are different from that observed in typical experimental paradigms in which cells are exposed to only a single growth factor at a time.

Protein kinase CK2 modulates synaptic plasticity by modification of synaptic NMDA receptors in the hippocampus

Synaptic plasticity is the foundation of learning and memory. The protein kinase CK2 phosphorylates many proteins related to synaptic plasticity, but whether it is directly involved in it has not been clarified. Here, we examined the role of CK2 in synaptic plasticity in hippocampal slices using the CK2 selective inhibitors 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and 4,5,6,7-tetrabromobenzotriazole (TBB). These significantly inhibited N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). DRB also inhibited NMDA receptor-mediated synaptic transmission, while leaving NMDA receptor-independent LTP unaffected. NMDA receptors thus appear to be the primary targets of CK2. Although both long-term depression (LTD) and LTP are induced by the influx of Ca(2+) through NMDA receptors, surprisingly, LTD was not affected by CK2 inhibitors. We postulated that the LTP-selective modulation by CK2 is due to selective modulation of NMDA receptors, and tested two hypotheses concerning the modulation of NMDA receptors: (i) CK2 selectively modulates NR2A subunits possibly related to LTP, but not NR2B subunits possibly related to LTD; and (ii) CK2 selectively affects synaptic but not extrasynaptic NMDA receptors whose activation is sufficient to induce LTD. DRB decreased NMDA receptor-mediated synaptic transmission in the presence of selective NR2A subunit antagonist. The former hypothesis thus appears unlikely to be correct. However, DRB decreased synaptic NMDA receptor responses in cultured hippocampal neurons without affecting extrasynaptic NMDA receptor current. These findings support the latter hypothesis, that CK2 selectively affects LTP by selective modification of synaptic NMDA receptors in a receptor-location-specific manner.

Central corticosteroid actions: Search for gene targets

Although many of the physiological effects of corticosteroid stress hormones on neuronal function are well recognised, the underlying genomic mechanisms are only starting to be elucidated. Linking physiology and genomics has proven to be a complicated task, despite the emergence of large-scale gene expression profiling technology in the last decade. This is in part due to the complexity of glucocorticoid-signaling, in part due to the complexity of the brain itself. The presence of a binary receptor system for glucocorticoid hormones in limbic brain structures, the coexistence of membrane and intracellular receptors and the highly contextual action of glucocorticoids contribute to this complexity. In addition, the anatomical complexity, extensive cellular heterogeneity of brain and the modest changes in gene expression (mostly in the range of 10-30%) hamper detection of responsive genes, in particular of low abundant transcripts, such as many neurotransmitter receptors and growth factors. Nonetheless, ongoing research into central targets of glucocorticoids has identified many different functional gene classes that underlie the diverse effects of glucocorticoids on brain function. These functional classes include genes involved in energy metabolism, signal transduction, neuronal structure, vesicle dynamics, neurotransmitter catabolism, cell adhesion, genes encoding neurotrophic factors and their receptors and genes involved in regulating glucocorticoid-signalling. The aim of this review is to give an overview of the current status of the field on identification of central corticosteroid targets, discuss the opportunities and pitfalls and highlight new developments in understanding central corticosteroid action.

Rapid glucocorticoid effects on the expression of hippocampal neurotransmission-related genes

We previously assessed corticosterone mediated gene expression in acute explant hippocampal slices and found over 200 responsive genes 1, 3 and 5 h after glucocorticoid receptor (GR) activation by a brief corticosterone pulse. Interestingly, 1 h after GR activation all genes were downregulated, many of which are involved in hippocampal neurotransmission and plasticity. The aim of the current experiment was 1) to measure the expression of several of these neurotransmission-related genes that were corticosterone-responsive 1 h after GR-activation in an in vivo setting, 2) to elucidate in which hippocampal subregion these expression changes take place and 3) to assess the specificity of regulation by activated GRs. For this purpose, rats were subcutaneously injected with vehicle, corticosterone or corticosterone pretreated with GR-antagonist RU38486. One hour after the corticosterone injections, mRNA expression levels of 5 selected genes were measured using in situ hybridization. The mineralocorticoid receptor (MR), MAO-A, casein kinase 2 and voltage dependent potassium mRNA's, but not dynein mRNA, were rapidly downregulated in vivo after corticosterone administration in hippocampal subregions. Furthermore, RU38486 pretreatment reversed in all cases these effects, illustrating the GR-specificity of transcriptional regulation by corticosterone. The results are important for understanding the role of GR in pleiotropic control of hippocampal neurotransmission and plasticity, which is characterized by recovery of function transiently raised by excitatory input.

Nerve growth factor-induced phosphorylation of amphiphysin-1 by casein kinase 2 regulates clathrin-amphiphysin interactions

Amphiphysins interact directly with clathrin and have a function in clathrin-mediated synaptic vesicle recycling and clathrin-mediated endocytosis. The neuronal isoform amphiphysin-1 is a serine/threonine phosphoprotein that is dephosphorylated upon stimulation of synaptic vesicle endocytosis. Rephosphorylation was stimulated by nerve growth factor. We analysed the regulation of amphiphysin-clathrin interactions by phosphorylation. The N-terminal domain of clathrin bound to unphosphorylated amphiphysin-1, but not to the phosphorylated protein. A search for possible phosphorylation sites revealed two casein kinase 2 consensus motifs in close proximity to the clathrin binding sites in amphiphysin-1 and -2. We mutagenized these residues (T350 and T387) to glutamate, mimicking a constitutive phosphorylation. The double mutant showed a strong reduction in clathrin binding. The assumption that casein kinase 2 phosphorylates amphiphysin-1 at T350 and T387 was corroborated by experiments showing that: (i) casein kinase 2 phosphorylated these residues directly in vitro, (ii) when expressed in HeLa cells, the glutamate mutant showed reduced phosphorylation, and (iii) casein kinase 2 inhibitors blocked nerve growth factor-induced phosphorylation of endogenous amphiphysin-1 in PC12 cells. These observations are consistent with the hypothesis that, upon activation by nerve growth factor, casein kinase 2 phosphorylates amphiphysin-1 and thereby regulates the endocytosis of clathrin-coated vesicles via the interaction between clathrin and amphiphysin.

Casein kinase II and calcineurin modulate TRPP function and ciliary localization

Cilia serve as sensory devices in a diversity of organisms and their defects contribute to many human diseases. In primary cilia of kidney cells, the transient receptor potential polycystin (TRPP) channels polycystin-1 (PC-1) and polycystin-2 (PC-2) act as a mechanosensitive channel, with defects resulting in autosomal dominant polycystic kidney disease. In sensory cilia of Caenorhabditis elegans male-specific neurons, the TRPPs LOV-1 and PKD-2 are required for mating behavior. The mechanisms regulating TRPP ciliary localization and function are largely unknown. We identified the regulatory subunit of the serine-threonine casein kinase II (CK2) as a binding partner of LOV-1 and human PC-1. CK2 and the calcineurin phosphatase TAX-6 modulate male mating behavior and PKD-2 ciliary localization. The phospho-defective mutant PKD-2(S534A) localizes to cilia, whereas a phospho-mimetic PKD-2(S534D) mutant is largely absent from cilia. Calcineurin is required for PKD-2 ciliary localization, but is not essential for ciliary gene expression, ciliogenesis, or localization of cilium structural components. This unanticipated function of calcineurin may be important for regulating ciliary protein localization. A dynamic phosphorylation-dephosphorylation cycle may represent a mechanism for modulating TRPP activity, cellular sensation, and ciliary protein localization.

Molecular mechanism of the neurotrophic effect of GDNF on DA neurons: role of protein kinase CK2

Glial cell line-derived neurotrophic factor (GDNF) is suggested as a specific neurotrophic factor for midbrain dopamine (DA) neurons, but the molecular mechanism underlying the neuroprotective action of GDNF is not well known. In the present study, we have shown that GDNF increased protein kinase CK2 activity in rat substantia nigra (SN) in a dose-dependent and time-dependent manner. This effect is prevented by prior treatment of the receptor Ret blocker K-252b. Immunostaining results also revealed that CK2 is expressed in TH-positive neurons in mesencephalon culture. Transfection of the wildtype CK2alpha DNA increased, whereas transfection of the catalytically inactive CK2alphaA156 mutant DNA decreased CK2 activity in the SN. CK2alphaA156 mutant DNA also antagonized the enhancing effect of GDNF on CK2 activity. It also antagonized the enhancing effects of GDNF on tyrosine hydroxylase (TH) protein level in the SN, DA turnover in the striatum and rotarod performance in rats. Further, CK2alpha wildtype DNA increased, whereas CK2alphaA156 mutant DNA decreased TH activity in the SN without altering the TH protein level. On the other hand, the DA neuron toxin 1-methyl-4-phenylpyridinium iodide (MPP+) markedly decreased the number of TH-positive neurons and TH protein level in the SN, decreased DA level in the striatum and impaired rotarod performance in rats. Over-expression of the CK2alpha wildtype DNA partially, but significantly, prevented the deteriorating effect of MPP+ on these measures. Prior administration of MPP+ also antagonized the enhancing effect of GDNF on CK2 activity. These results together suggest that the CK2 signaling pathway contributes to the neuroprotective action of GDNF on DA neurons.

Regulation of the NMDA receptor complex and trafficking by activity-dependent phosphorylation of the NR2B subunit PDZ ligand

Interactions between NMDA receptors (NMDARs) and the PDZ [postsynaptic density-95 (PSD-95)/Discs large/zona occludens-1] domains of PSD-95/SAP90 (synapse-associated protein with a molecular weight of 90 kDa) family proteins play important roles in the synaptic targeting and signaling of NMDARs. However, little is known about the mechanisms that regulate these PDZ domain-mediated interactions. Here we show that casein kinase II (CK2) phosphorylates the serine residue (Ser1480) within the C-terminal PDZ ligand (IESDV) of the NR2B subunit of NMDAR in vitro and in vivo. Phosphorylation of Ser1480 disrupts the interaction of NR2B with the PDZ domains of PSD-95 and SAP102 and decreases surface NR2B expression in neurons. Interestingly, activity of the NMDAR and Ca2+/calmodulin-dependent protein kinase II regulates CK2 phosphorylation of Ser1480. Furthermore, CK2 colocalizes with NR1 and PSD-95 at synaptic sites. These results indicate that activity-dependent CK2 phosphorylation of the NR2B PDZ ligand regulates the interaction of NMDAR with PSD-95/SAP90 family proteins as well as surface NMDAR expression and may be a critical mechanism for modulating excitatory synaptic function and plasticity.

Regulation of cytosolic prostaglandin E synthase by phosphorylation

cPGES [cytosolic PG (prostaglandin) E synthase] is constitutively expressed in various cells and can regulate COX (cyclo-oxygenase)-1-dependent immediate PGE2 generation. In the present study, we found that cPGES underwent serine phosphorylation, which was accelerated transiently after cell activation. Several lines of evidence suggest that a cPGES-activating protein kinase is CK-II (casein kinase II). Recombinant cPGES was phosphorylated directly by and associated with CK-II in vitro, resulting in marked reduction of the K m for the substrate PGH2. In activated cells, cPGES phosphorylation occurred in parallel with increased cPGES enzymic activity and PGE2 production from exogenous and endogenous arachidonic acid, and these processes were facilitated by Hsp90 (heat-shock protein 90), a molecular chaperone that formed a tertiary complex with cPGES and CK-II. Treatment of cells with inhibitors of CK-II and Hsp90 and with a dominant-negative CK-II attenuated the formation of the cPGES-CK-II-Hsp90 complex and attendant cPGES phosphorylation and activation. Mutations of either of two predicted CK-II phosphorylation sites on cPGES (Ser113 and Ser118) abrogated its phosphorylation and activation both in vitro and in vivo. Moreover, the CK-II-Hsp90-mediated activation of cPGES was ameliorated by the p38 mitogen-activated protein kinase inhibitor SB20358 or by the anti-inflammatory glucocorticoid dexamethasone. Taken together, the results of the present study have provided the first evidence that the cellular function of this eicosanoid-biosynthetic enzyme is under the control of a molecular chaperone and its client protein kinase.

Protein kinase CK2 in postsynaptic densities: phosphorylation of PSD-95/SAP90 and NMDA receptor regulation

Protein kinase CK2 (CK2) is highly expressed in rat forebrain where its function is not well understood. Subcellular distribution studies showed that the catalytic subunit of CK2 (CK2alpha) was enriched in postsynaptic densities (PSDs) by 68%. We studied the putative role of CK2 activity on N-methyl-D-aspartate receptor (NMDAR) function using isolated, patch-clamped PSDs in the presence of 2 mM extracellular Mg(2+). The usual activation by phosphorylation of the NMDARs in the presence of ATP was inhibited by the selective CK2 inhibitor 5,6-dichloro-1-beta-ribofuranosyl benzimidazole (DRB). This inhibition was voltage-dependent, i.e., 100% at positive membrane potentials, while at negative potentials, inhibition was incomplete. Endogenous CK2 substrates were characterized by their ability to use GTP as a phosphoryl donor and susceptibility to inhibition by DRB. Immunoprecipitation assays and 2D gels indicated that PSD-95/SAP90, the NMDAR scaffolding protein, was a CK2 substrate, while the NR2A/B and NR1 NMDAR subunits were not. These results suggest that postsynaptic NMDAR regulation by CK2 is mediated by indirect mechanisms possibly involving PSD-95/SAP90.

Status of the "protein kinase CK2-HMG14" system in age-related amnesia in rats

The experiments described here demonstrate that disruption of the phosphorylation of transcription factors of the HMG cAMP/Ca-independent protein kinase CK2 class may be the cause of decreased gene expression in age-related cognitive deficits. Amnesia for a conditioned passive avoidance reaction (CPAR) in aged rats (24 months old) was accompanied by decreases in the synthesis of synaptosomal proteins and transcription in nuclei isolated from cortical, hippocampal, and striatal neurons. There was a decrease in chromatin protein kinase CK2 activity and a significant decrease in the phosphorylation of HMG14 by protein kinase CK2. Selective activators of protein kinase CK2 (1-ethyl-4-carbamoyl-5-methylcarbamoylimidazole and 1-ethyl-4,5-dicarbamoylimidazole) increased HMG14 phosphorylation by protein kinase CK2, increased transcription, increased the synthesis of synaptosomal proteins, and decreased amnesia for the CPAR in aged rats. Thus, activation of the "protein kinase CK2-HMG14" system is accompanied by optimization of synaptic plasticity in aged animals. The results provide evidence for the high therapeutic potential of protein kinase CK2 activators.

Carbon monoxide neurotransmission activated by CK2 phosphorylation of heme oxygenase-2

Carbon monoxide (CO) is a putative gaseous neurotransmitter that lacks vesicular storage and must be synthesized rapidly following neuronal depolarization. We show that the biosynthetic enzyme for CO, heme oxygenase-2 (HO2), is activated during neuronal stimulation by phosphorylation by CK2 (formerly casein kinase 2). Phorbol ester treatment of hippocampal cultures results in the phosphorylation and activation of HO2 by CK2, implicating protein kinase C (PKC) in CK2 stimulation. Odorant treatment of olfactory receptor neurons augments HO2 phosphorylation and activity as well as cyclic guanosine monophosphate (cGMP) levels, with all of these effects selectively blocked by CK2 inhibitors. Likewise, CO-mediated nonadrenergic, noncholinergic (NANC) relaxation of the internal anal sphincter requires CK2 activity. Our findings provide a molecular mechanism for the rapid neuronal activation of CO biosynthesis, as required for a gaseous neurotransmitter.

A calcium/calmodulin kinase pathway connects brain-derived neurotrophic factor to the cyclic AMP-responsive transcription factor in the rat hippocampus

Brain-derived neurotrophic factor (BDNF) plays fundamental roles in synaptic plasticity in rat hippocampus. Recently, using rat hippocampal slices, we found that BDNF induces activation of calcium/calmodulin-dependent protein kinase 2 (CaMKII), a critical mediator of synaptic plasticity. CaMKII in turn activates the p38 subfamily of mitogen-activated protein kinases (MAPK) and its downstream effector, MAPK-activated protein kinase 2 (MAPKAPK-2). Herein, we determined whether some kinases of this pathway connect BDNF to the cyclic AMP response element -binding protein (CREB), a transcription factor also involved in plasticity and survival. Crude cytosolic and nuclear fractions were prepared from hippocampal slices of adult rat, and then kinase involvement in CREB phosphorylation was studied with a combination of pharmacologic inhibition and antibody depletion. In addition, the regional localization of this signaling pathway was immunohistochemically investigated. We show that: (i). the BDNF-stimulated CaMKII cascade phosphorylates the key positive regulatory site of CREB via its end MAPKAPK-2 component; (ii). this process appears to be highly localized in the outermost cell layer of the dentate gyrus. The present findings suggest that CaMKII is involved in neurotrophic-dependent activation of CREB in the dentate gyrus. Such a signaling process could be important for controlling synaptic plasticity in this major area for the afferent inputs to the hippocampal formation.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Expression, purification and characterization of recombinant human choline acetyltransferase: phosphorylation of the enzyme regulates catalytic activity

Choline acetyltransferase synthesizes acetylcholine in cholinergic neurons and, in humans, may be produced in 82- and 69-kDa forms. In this study, recombinant choline acetyltransferase from baculovirus and bacterial expression systems was used to identify protein isoforms by two-dimensional SDS/PAGE and as substrate for protein kinases. Whereas hexa-histidine-tagged 82- and 69-kDa enzymes did not resolve as individual isoforms on two-dimensional gels, separation of wild-type choline acetyltransferase expressed in insect cells revealed at least nine isoforms for the 69-kDa enzyme and at least six isoforms for the 82-kDa enzyme. Non-phosphorylated wild-type choline acetyltransferase expressed in Escherichia coli yielded six (69 kDa) and four isoforms (82 kDa) respectively. Immunofluorescent labelling of insect cells expressing enzyme showed differential subcellular localization with the 69-kDa enzyme localized adjacent to plasma membrane and the 82-kDa enzyme being cytoplasmic at 24 h. By 64 h, the 69-kDa form was in cytoplasm and the 82-kDa form was only present in nucleus. Studies in vitro showed that recombinant 69-kDa enzyme was a substrate for protein kinase C (PKC), casein kinase II (CK2) and alpha-calcium/calmodulin-dependent protein kinase II (alpha-CaM kinase), but not for cAMP-dependent protein kinase (PKA); phosphorylation by PKC and CK2 enhanced enzyme activity. The 82-kDa enzyme was a substrate for PKC and CK2 but not for PKA or alpha-CaM kinase, with only PKC yielding increased enzyme activity. Dephosphorylation of both forms of enzyme by alkaline phosphatase decreased enzymic activity. These studies are of functional significance as they report for the first time that phosphorylation enhances choline acetyltransferase catalytic activity.

Identification of two persistently activated neurotrophin-regulated pathways in rat hippocampus

Brain-derived neurotrophic factor contributes profoundly to modulate activity-dependent synaptic plasticity in adult brain areas such as the hippocampus, but the mechanisms underlying this important role still remain unclear. Recently, we have shown that two serine/threonine kinases, calcium/calmodulin-dependent protein kinase-2 and casein kinase-2, are capable of mediating brain-derived neurotrophic factor responses in adult rat hippocampus. In the present study, using hippocampal slices from adult rat, we show that phospholipase C-regulated calcium signals couple the brain-derived neurotrophic factor receptor to two distinct pathways: a pathway in which calcium/calmodulin-dependent protein kinase-2 stimulates a signalling module involving the p38 subfamily of mitogen-activated protein kinases and its downstream target, usually named mitogen-activated protein kinase-activated protein kinase-2; and a pathway in which the extracellular signal-regulated kinase subfamily of mitogen-activated protein kinases activates casein kinase-2. Our results suggest that: (i) extracellular signal-regulated kinase is activated by B-Raf in response to a calcium-sensitive adenylate cyclase; and (ii) extracellular signal-regulated kinase activates casein kinase-2 via a protein phosphatase(s) that may be of the PP1 and/or PP2A type. Interestingly, we also show that neurotrophin-induced activation of the two signalling cascades promotes a sustained activation of mitogen-activated protein kinase-activated protein kinase-2 and casein kinase-2 in slices. Considering the ability of these two kinases to be persistently activated, and that most of the protein kinases which lie in these pathways are believed to be important for multiple events underlying neuronal plasticity, it is suggested that the mechanisms described here might contribute both to rapid synaptic changes through local effects and to long-lasting synaptic responses through new gene transcription in the hippocampus.

Phosphorylation of microtubule-associated protein tau is regulated by protein phosphatase 2A in mammalian brain. Implications for neurofibrillary degeneration in Alzheimer's disease

Hyperphosphorylated tau, which is the major protein of the neurofibrillary tangles in Alzheimer's disease brain, is most probably the result of an imbalance of tau kinase and phosphatase activities in the affected neurons. By using metabolically competent rat brain slices as a model, we found that selective inhibition of protein phosphatase 2A by okadaic acid induced an Alzheimer-like hyperphosphorylation and accumulation of tau. The hyperphosphorylated tau had a reduced ability to bind to microtubules and to promote microtubule assembly in vitro. Immunocytochemical staining revealed hyperphosphorylated tau accumulation in pyramidal neurons in cornu ammonis and in neocortical neurons. The topography of these changes recalls the distribution of neurofibrillary tangles in Alzheimer's disease brain. Selective inhibition of protein phosphatase 2B with cyclosporin A did not have any significant effect on tau phosphorylation, accumulation, or function. These studies suggest that protein phosphatase 2A participates in regulation of tau phosphorylation, processing, and function in vivo. A down-regulation of protein phosphatase 2A activity can lead to Alzheimer-like abnormal hyperphosphorylation of tau.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Brain-derived neurotrophic factor exerts opposing effects on beta2-adrenergic receptor according to depolarization status of cerebellar neurons

To investigate the molecular mechanisms underlying brain-derived neurotrophic factor (BDNF)-controlled synaptic plasticity, we studied beta2-adrenergic receptor (beta2-AR) expression in cultured cerebellar granule cells. We show that, depending on the state of depolarization, BDNF exerts opposite effects on beta2-AR expression. In neurons maintained in low K+ medium (5 mM K+) that will enter apoptosis, BDNF increases beta2-AR and beta2-AR transcripts. In contrast, in depolarized neurons (high K+ medium, 25 mM K+) BDNF represses beta2-AR expression. The use of reporter genes (driven by the beta2-AR promoter or restricted regulatory elements) revealed that BDNF exerts its opposite effects at the transcriptional level by recruiting a cyclic AMP response element (CRE) and the trans-acting factor CRE binding protein. These results provide the first evidence that a neurotrophin, e.g., BDNF, may exert an opposite effect on receptor expression and function (beta2-AR) according to the depolarization status of the neuron. Based on this finding, we propose that BDNF not only mediates neuronal survival, but is also involved in the modulation of the general sensitivity of the neuron to external signals, thus maintaining its optimal functional integration within the neuronal network.

Casein kinase-II regulates NMDA channel function in hippocampal neurons

Several second-messenger-regulated protein kinases have been implicated in the regulation of N-methyl-D-aspartate (NMDA) channel function. Yet the role of calcium and cyclic-nucleotide-independent kinases, such as casein kinase II (CKII), has remained unexplored. Here we identify CKII as an endogenous Ser/Thr protein kinase that potently regulates NMDA channel function and mediates intracellular actions of spermine on the channel. The activity of NMDA channels in cell-attached and inside-out recordings was enhanced by CKII or spermine and was decreased by selective inhibition of CKII. In hippocampal slices, inhibitors of CKII reduced synaptic transmission mediated by NMDA but not AMPA receptors. The dependence of NMDA receptor channel activity on tonically active CKII thus permits changes in intracellular spermine levels or phosphatase activities to effectively control channel function.