Activation of multifunctional Ca2+/calmodulin-dependent kinase in intact hippocampal slices

Autophosphorylated CaMKII Facilitates Spike Propagation in Rat Optic Nerve

Repeated spike firing can transmit information at synapses and modulate spike timing, shape, and conduction velocity. These latter effects have been found to result from voltage-induced changes in ion currents and could alter the signals carried by axons. Here, we test whether Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) regulates spike propagation in adult rat optic nerve. We find that small-, medium-, and large-diameter axons bind anti-Thr286-phosphorylated CaMKII (pT286) antibodies and that, in isolated optic nerves, electrical stimulation reduces pT286 levels, spike propagation is hastened by CaMKII autophosphorylation and slowed by CaMKII dephosphorylation, single and multiple spikes slow propagation of subsequently activated spikes, and more frequent stimulation produces greater slowing. Likewise, exposing freely moving animals to flickering illumination reduces pT286 levels in optic nerves and electrically eliciting spikes in vivo in either the optic nerve or optic chiasm slows subsequent spike propagation in the optic nerve. By increasing the time that elapses between successive spikes as they propagate, pT286 dephosphorylation and activity-induced spike slowing reduce the frequency of propagated spikes below the frequency at which they were elicited and would thus limit the frequency at which axons synaptically drive target neurons. Consistent with this, the ability of retinal ganglion cells to drive at least some lateral geniculate neurons has been found to increase when presented with light flashes at low and moderate temporal frequencies but less so at high frequencies. Activity-induced decreases in spike frequency may also reduce the energy required to maintain normal intracellular Na⁺ and Ca²⁺ levels.SIGNIFICANCE STATEMENT By propagating along axons at constant velocities, spikes could drive synapses as frequently as they are initiated. However, the onset of spiking has been found to alter the conduction velocity of subsequent ("follower") spikes in various preparations. Here, we find that spikes reduce spike frequency in rat optic nerve by slowing follower spike propagation and that electrically stimulated spiking ex vivo and spike-generating flickering illumination in vivo produce net decreases in axonal Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation. Consistent with these effects, propagation speed increases and decreases, respectively, with CaMKII autophosphorylation and dephosphorylation. Lowering spike frequency by CaMKII dephosphorylation is a novel consequence of axonal spiking and light adaptation that could decrease synaptic gain as stimulus frequency increases and may also reduce energy use.

The function of GluR1 and GluR2 in cerebellar and hippocampal LTP and LTD is regulated by interplay of phosphorylation and O-GlcNAc modification

Long-term potentiation (LTP) and long-term depression (LTD) are the current models of synaptic plasticity and widely believed to explain how different kinds of memory are stored in different brain regions. Induction of LTP and LTD in different regions of brain undoubtedly involve trafficking of AMPA receptor to and from synapses. Hippocampal LTP involves phosphorylation of GluR1 subunit of AMPA receptor and its delivery to synapse whereas; LTD is the result of dephosphorylation and endocytosis of GluR1 containing AMPA receptor. Conversely the cerebellar LTD is maintained by the phosphorylation of GluR2 which promotes receptor endocytosis while dephosphorylation of GluR2 triggers receptor expression at the cell surface and results in LTP. The interplay of phosphorylation and O-GlcNAc modification is known as functional switch in many neuronal proteins. In this study it is hypothesized that a same phenomenon underlies as LTD and LTP switching, by predicting the potential of different Ser/Thr residues for phosphorylation, O-GlcNAc modification and their possible interplay. We suggest the involvement of O-GlcNAc modification of dephosphorylated GluR1 in maintaining the hippocampal LTD and that of dephosphorylated GluR2 in cerebral LTP.

Reversal of chronic inflammatory pain by acute inhibition of Ca2+/calmodulin-dependent protein kinase II

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a major protein kinase that is capable of regulating the activities of many ion channels and receptors. In the present study, the role of CaMKII in the complete Freund's adjuvant (CFA)-induced inflammatory pain was investigated. Intraplantarly injected CFA was found to induce spinal activity of CaMKII (phosphorylated CaMKII), which was blocked by KN93 [[2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine)], a CaMKII inhibitor. Pretreatment with KN93 (i.t.) dose-dependently prevented the development of CFA-induced thermal hyperalgesia and mechanical allodynia. Acute treatment with KN93 (i.t.) also dose-dependently reversed CFA-induced thermal hyperalgesia and mechanical allodynia. The action of KN93 started in 30 min and lasted for at least 2 to 4 h. KN92 (45 nmol i.t.) [2-[N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine], an inactive analog of KN93, showed no effect on CFA-induced CaMKII activation, allodynia, or hyperalgesia. Furthermore, our previous studies identified trifluoperazine, a clinically used antipsychotic drug, to be a potent CaMKII inhibitor. Inhibition of CaMKII activity by trifluoperazine was confirmed in the study. In addition, trifluoperazine (i.p.) dose-dependently reversed CFA-induced mechanical allodynia and thermal hyperalgesia. The drug was also effectively when given orally. In conclusion, our findings support a critical role of CaMKII in inflammatory pain. Blocking CaMKII or CaMKII-mediated signaling may offer a novel therapeutic target for the treatment of chronic pain.

The signal-induced phospholipid degradation cascade and protein kinase C activation

Acting in synergy with diacylglycerol, unsaturated free fatty acids such as arachidonic, oleic, linoleic, linolenic and docosahexaenoic acids dramatically activate some members of the protein kinase C family at the basal level of Ca2+ concentration. It is plausible that phospholipase C and phospholipase A2, and possibly phospholipase D as well, are involved in the activation of protein kinase C. Presumably, this enzyme activation is integrated into the signal-induced membrane phospholipid degradation cascade, prolonging the activation of protein kinase C. The sustained activity of this enzyme appears to be of importance for long-term cellular responses such as development of neuronal plasticity and gene activation.

CaMKII inactivation by extracellular Ca(2+) depletion in dorsal root ganglion neurons

A mechanism by which Ca(2+)/CaM-dependent protein kinase (CaMKII) is autophosphorylated by changes in extracellular calcium in the absence of detectable changes in cytoplasmic [Ca(2+)] has been identified. We find that when the external Ca(2+) concentration ([Ca(2+)](O)) is lowered, Ca(2+) is released from intracellular stores to maintain a constant cytoplasmic Ca(2+) level, gradually depleting the endoplasmic Ca(2+) stores. Accompanying the store-depletion is a rapid decrease in CaMKII activity. Approximately 25% of the measured CaMKII autophosphorylation in DRG neurons in culture can be regulated by Ca(2+) flux from intracellular stores caused by manipulating [Ca(2+)](O), as shown by blocking refilling of store-operated Ca(2+)-channels with SK&F 96365, Ruthenium Red, and a partial block with Ni(2+). Blocking voltage-gated Ca(2+)-channels with either isradipine or SR 33805, had no effect on CaMKII autophosphorylation induced by restoring Ca(2+)(O) to normal after depleting the intracellular Ca(2+) stores. These results show that removal of Ca(2+)(O) has profound effects on intracellular Ca(2+) signaling and CaMKII autophosphorylation, in the absence of measurable changes in intracellular Ca(2+). These findings have wide-ranging significance, because [Ca(2+)](O) is manipulated in many experimental studies. Moreover, this explanation for the paradoxical changes in CaMKII phosphorylation in response to manipulating [Ca(2+)](O) provides a possible mechanism linking activity-dependent depletion of Ca(2+) from the synaptic cleft to a protein kinase regulating many neuronal properties.

A mechanism for Ca2+/calmodulin-dependent protein kinase II clustering at synaptic and nonsynaptic sites based on self-association

The activity of Ca2+/calmodulin-dependent protein kinase II (CaMKII) plays an integral role in regulating synaptic development and plasticity. We designed a live-cell-imaging approach to monitor an activity-dependent clustering of green fluorescent protein (GFP)-CaMKII holoenzymes, termed self-association, a process that we hypothesize contributes to the translocation of CaMKII to synaptic and nonsynaptic sites in activated neurons. We show that GFP-CaMKII self-association in human embryonic kidney 293 (HEK293) cells requires a catalytic domain and multimeric structure, requires Ca2+ stimulation and a functional Ca2+/CaM-binding domain, is regulated by cellular pH and Thr286 autophosphorylation, and has variable rates of dissociation depending on Ca2+ levels. Furthermore, we show that the same rules that govern CaMKII self-association in HEK293 cells apply for extrasynaptic and postsynaptic translocation of GFP-CaMKII in hippocampal neurons. Our data support a novel mechanism for targeting CaMKII to postsynaptic sites after neuronal activation. As such, CaMKII may form a scaffold that, in combination with other synaptic proteins, recruits and localizes additional proteins to the postsynaptic density. We discuss the potential function of CaMKII self-association as a tag of synaptic activity.

AMPA receptor phosphorylation during synaptic plasticity

A widely studied example of vertebrate plasticity is LTP (long-term potentiation), the persistent synaptic enhancement that follows a brief period of coinciding pre- and post-synaptic activity. During LTP, different kinases, including CaMKII (calcium/calmodulin-dependent protein kinase II) and protein kinase A, become activated and play critical roles in induction and maintenance of enhanced transmission. Biochemical analyses have revealed several regulated phosphorylation sites in the AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor subunits, GluR1 and GluR4. The regulated insertion of these receptors is a key event in the induction of LTP. Here, we discuss the phosphorylation of GluR1 and GluR4 and its role in receptor delivery and neuronal plasticity.

CaMKIIα enhances the desensitization of NR2B-containing NMDA receptors by an autophosphorylation-dependent mechanism

Long-term potentiation or depression of synaptic function often requires Ca2+ influx via NMDA-type glutamate receptors (NMDARs) and changes in the autophosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) at Thr286. Autophosphorylated CaMKII binds directly to NMDAR subunits, co-localizes with NMDARs in the postsynaptic density, and phosphorylates NR2B subunits at Ser1303. Here, we demonstrate that CaMKIIalpha enhances the extent and/or rate of desensitization of NMDA-induced macroscopic currents in HEK293 cells co-expressing NR2B with either the NR1(011) or NR1(101) splice variants, without significantly changing other current parameters. In contrast, the extent of desensitization of NMDARs containing NR2A in place of NR2B is significantly decreased by co-expression of CaMKIIalpha. Kinases harboring K42R (inactive kinase) or T286A (autophosphorylation-deficient) mutations are defective in enhancing the desensitization of NR1/NR2B channels. In addition, the CaMKII-dependent enhancement of NR1/NR2B channel desensitization is abrogated by intracellular loading with BAPTA. These data suggest a novel mechanism for Ca2+-dependent negative-feedback regulation of NR2B-containing NMDARs in a CaMKII activity- and autophosphorylation-dependent manner that may modulate NMDAR-mediated synaptic plasticity.

Expression and phosphorylation of delta-CaM kinase II in cultured Alzheimer fibroblasts

Dysregulation of calcium homeostasis is among the major cellular alterations in Alzheimer's disease (AD). We studied Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II), one of the major effectors regulating neuronal responses to changes in calcium fluxes, in cultured skin fibroblasts from subjects with sporadic AD. We found, by using PCR and Western analysis, that human fibroblasts express the delta-isoform of this kinase, and that CaM kinase II is the major Ca(2+)/calmodulin-dependent kinase in these cells. Protein expression level of the kinase was not significantly different in AD fibroblasts. However, the total activity of the kinase (stimulated by Ca(2+)/calmodulin) was significantly reduced in AD cell lines, whereas Ca(2+)-independent activity was significantly enhanced. The percent autonomy of the kinase (%Ca(2+)-independent/Ca(2+)-dependent activity) in AD cell lines was 62.8%, three-fold the corresponding percentage in control fibroblasts. The abnormal calcium-independent activity was not due to enhanced basal autophosphorylation of Thr(287). The observed abnormalities, if present in brain tissue, may be implicated either in dysfunction of neuroplasticity and cognitive functions or in dysregulation of cell cycle.

Antidepressants activate CaMKII in neuron cell body by Thr286 phosphorylation

CaM kinase II, a regulator of synaptic plasticity, is implicated in pathophysiology and pharmacology of psychiatric disorders. Chronic treatment with antidepressants desipramine and reboxetine up-regulated CaM kinase II in neuronal cell bodies of hippocampus. mRNA/protein expression for alphaCaM kinase II was unchanged, whereas Thr phosphorylation was increased in pyramidal/granular cell bodies, suggesting that increased phosphorylation is responsible for kinase activation. Short-term treatment of neuronal cultures with reboxetine reduced kinase activation in a concentration-dependent manner. The short-term inhibitory effect of reboxetine suggests that kinase up-regulation during antidepressant drug treatment is an adaptive response compensating for initial functional down-regulation.

Presynaptic CaMKII is necessary for synaptic plasticity in cultured hippocampal neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a multifunctional enzyme that is very critical for synaptic plasticity and memory formation. Although significant progress has been made in understanding the role of postsynaptic CaMKII in synaptic plasticity, very little is known about its presynaptic function during plasticity changes. Here we report that KN-93, a membrane-permeable CaMKII inhibitor, blocked glutamate-induced increases in the frequency of miniature excitatory postsynaptic currents (mEPSCs) and the number of presynaptic functional boutons in cultured hippocampal pyramidal neurons. In addition, presynaptic injection of the membrane-impermeable CaMKII inhibitor peptide 281-309 blocked synaptic plasticity induced by tetanus, glutamate, or NO/cGMP pathway activation as expressed by long-lasting increases in EPSC amplitude and functional presynaptic boutons. Presynaptic injection of CaMKII itself coupled with weak tetanus produced an immediate and long-lasting enhancement of EPSC amplitude. Thus, the present results conclusively prove that presynaptic CaMKII is essential for synaptic plasticity in cultured hippocampal neurons.

Ca2+–calmodulin signalling pathway up-regulates GABA synaptic transmission through cytoskeleton-mediated mechanisms

We investigated the role of calcium (Ca(2+))/calmodulin (CaM) signaling pathways in modulating GABA synaptic transmission at CA1 pyramidal neurons in hippocampal slices. Whole-cell pipettes were used to record type A GABA receptor (GABA(A)R)-gated inhibitory postsynaptic currents (IPSCs) and to perfuse intracellularly modulators in the presence of glutamate receptor antagonists. GABA(A)R-gated IPSCs were enhanced by the postsynaptic infusions of adenophostin (1 microM), a potent agonist of inositol-1,4,5-triphosphate receptor (IP(3)R) that induces Ca(2+) release. The enhancement was blocked by co-infusing either 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (10 mM) or CaM-binding peptide (100 microM). Moreover, the postsynaptic infusion of Ca(2+)-CaM (40/10 microM) enhanced both evoked and spontaneous GABA(A)R-gated IPSCs. The enhancement was attenuated by co-infusing 100 microM CaM-KII(281-301), an autoinhibitory peptide of CaM-dependent protein kinases. These results indicate that postsynaptic Ca(2+)-CaM signaling pathways essentially enhance GABAergic synaptic transmission. In the investigation of synaptic targets for the enhancement, we found that IP(3)R agonist-enhanced GABA(A)R-gated IPSCs were attenuated by co-infusing colchicine (30 microM), vincristine (3 microM) or cytochalasin D (1 microM) that inhibits tubulin or actin polymerization, implying that actin filament and microtubules are involved. We conclude that postsynaptic Ca(2+)-CaM signaling pathways strengthen the function of GABAergic synapses via a cytoskeleton-mediated mechanism, probably the recruitment of receptors in the postsynaptic membrane.

Neuronal mechanism of nociceptin-induced modulation of learning and memory: involvement of N-methyl-D-aspartate receptors

Nociceptin (also called orphanin FQ) is an endogenous heptadecapeptide that activates the opioid receptor-like 1 (ORL1) receptor. Nociceptin system not only affects the nociception and locomotor activity, but also regulates learning and memory in rodents. We have previously reported that long-term potentiation and memory of ORL1 receptor knockout mice are enhanced compared with those in wild-type mice. Here, we show the neuronal mechanism of nociceptin-induced modulation of learning and memory. Retention of fear-conditioned contextual memory was significantly enhanced in the ORL1 receptor knockout mice without any changes in cued conditioned freezing. Inversely, in the wild-type mice retention of contextual, but not cued, conditioning freezing behavior was suppressed by exogenous nociceptin when it was administered into the cerebroventricle immediately after the training. ORL1 receptor knockout mice exhibited a hyperfunction of N-methyl-D-aspartate (NMDA) receptor, as evidenced by an increase in [3H]MK-801 binding, NMDA-evoked 45Ca2+ uptake and activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity and its phosphorylation as compared with those in wild-type mice. The NMDA-induced CaMKII activation in the hippocampal slices of wild-type mice was significantly inhibited by exogenous nociceptin via a pertussis toxin-sensitive pathway. However, the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor GluR1 subunit at Ser831 and Ser845, and NMDA receptor subunit NR2B at Thr286 were phosphorylated similarly after NMDA receptor stimulation in both type of mice. The expressions of GluR1 and GluR2 also did not change, but the levels of polysialylated form of neuronal cell adhesion molecule (N-CAM) were reduced in the ORL1 receptor knockout as compared with wild-type mice. These results suggest that nociceptin system negatively modulates learning and memory through the regulation of NMDA receptor function and the expression of N-CAM.

Selective regulation of presynaptic calcium/calmodulin-dependent protein kinase II by psychotropic drugs

BACKGROUND

Changes in neuroplasticity have been involved in the pathogenesis of psychiatric disorders as well as in psychotropic drug action. Calcium/calmodulin-dependent protein kinase II (CaM kinase II), an enzyme with a pivotal role in synaptic plasticity and cognitive functions, has been implicated in the action of anticonvulsants, benzodiazepines, and antidepressants, but little is known as to its role in the action of different drugs employed for treatment of psychiatric disorders.

METHODS

We studied the function and expression of CaM kinase II following chronic treatment of rats with two antidepressants, fluvoxamine and desipramine, a typical antipsychotic drug, haloperidol, and the typical medication for manic-depressive disorder, lithium.

RESULTS

Antidepressants significantly increased the kinase activity in presynaptic vesicles of frontal/prefrontal cortex. Haloperidol induced no change, whereas lithium significantly decreased the activity. Kinase activation by antidepressants was further demonstrated by increased phosphorylation of exogenously added recombinant synaptotagmin. Immunoreactivity of vesicular kinase (alpha-isoform) was significantly increased by reuptake blockers but not by the two other drugs. Kinetic analysis showed that limiting value of enzymatic velocity (Vmax) of the kinase for substrate was also increased by reuptake blockers and decreased by lithium; however, neither messenger ribonucleic acid nor protein expression level of the kinase was increased in frontal/prefrontal cortex homogenates of antidepressant-treated rats, suggesting the involvement of local synaptic mechanisms.

CONCLUSIONS

These findings show that functional regulation of presynaptic CaM kinase II is selectively affected by different psychotropic drugs, and suggest local synaptic mechanisms for pharmacological regulation of the kinase.

Phosphorylation of spinophilin modulates its interaction with actin filaments

Spinophilin is a protein phosphatase 1 (PP1)- and actin-binding protein that modulates excitatory synaptic transmission and dendritic spine morphology. We report that spinophilin is phosphorylated in vitro by protein kinase A (PKA). Phosphorylation of spinophilin was stimulated by treatment of neostriatal neurons with a dopamine D1 receptor agonist or with forskolin, consistent with spinophilin being a substrate for PKA in intact cells. Using tryptic phosphopeptide mapping, site-directed mutagenesis, and microsequencing analysis, we identified two major sites of phosphorylation, Ser-94 and Ser-177, that are located within the actin-binding domain of spinophilin. Phosphorylation of spinophilin by PKA modulated the association between spinophilin and the actin cytoskeleton. Following subcellular fractionation, unphosphorylated spinophilin was enriched in the postsynaptic density, whereas a pool of phosphorylated spinophilin was found in the cytosol. F-actin co-sedimentation and overlay analysis revealed that phosphorylation of spinophilin reduced the stoichiometry of the spinophilin-actin interaction. In contrast, the ability of spinophilin to bind to PP1 remained unchanged. Taken together, our studies suggest that phosphorylation of spinophilin by PKA modulates the anchoring of the spinophilin-PP1 complex within dendritic spines, thereby likely contributing to the efficacy and plasticity of synaptic transmission.

Neuronal CA2+/calmodulin-dependent protein kinase II: the role of structure and autoregulation in cellular function

Highly enriched in brain tissue and present throughout the body, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is central to the coordination and execution of Ca(2+) signal transduction. The substrates phosphorylated by CaMKII are implicated in homeostatic regulation of the cell, as well as in activity-dependent changes in neuronal function that appear to underlie complex cognitive and behavioral responses, including learning and memory. The architecture of CaMKII holoenzymes is unique in nature. The kinase functional domains (12 per holoenzyme) are attached by stalklike appendages to a gear-shaped core, grouped into two clusters of six. Each subunit contains a catalytic, an autoregulatory, and an association domain. Ca(2+)/calmodulin (CaM) binding disinhibits the autoregulatory domain, allowing autophosphorylation and complex changes in the enzyme's sensitivity to Ca(2+)/CaM, including the generation of Ca(2+)/CaM-independent activity, CaM trapping, and CaM capping. These processes confer a type of molecular memory to the autoregulation and activity of CaMKII. Its function is intimately shaped by its multimeric structure, autoregulation, isozymic type, and subcellular localization; these features and processes are discussed as they relate to known and potential cellular functions of this multifunctional protein kinase.

Regulation of gene expression by Ca2+ signals in neuronal cells

Calcium ions are ubiquitous second messengers that control diverse cellular functions. The versatility of Ca(2+) arises both from the ability of cells to employ a range of mechanisms to generate stimulus-induced Ca(2+) signals with defined characteristics and the existence of a large repertoire of Ca(2+) receptive proteins that mediate the effects of Ca(2+). In neurons, the regulation of gene expression by electrical activity-induced increases in Ca(2+) is critical for the long-term maintenance of neuronal adaptive responses. Different patterns of synaptic activity are able to generate Ca(2+) signals varying in their amplitude, temporal profile, spatial properties and source or site of entry. The information embedded in Ca(2+) signals is decoded by Ca(2+)-responsive transcriptional regulators, including protein kinases, phosphatases and transcription factors, with differing Ca(2+) sensitivities, kinetics of activation and deactivation, and subcellular localisation. The coordinated control of many transcriptional regulators by Ca(2+) signals determines the qualitative and quantitative nature of the genomic response.

Structure-function of the multifunctional Ca2+/calmodulin-dependent protein kinase II

Ca2+/calmodulin (CaM)-dependent protein kinase (CaMKII) is a ubiquitous mediator of Ca2+-linked signalling that phosphorylates a wide range of substrates to co-ordinate and regulate Ca2+-mediated alterations in cellular function. The transmission of information by the kinase from extracellular stimuli and the intracellular Ca2+ rise is not passive. Rather, its multimeric structure and autoregulation enable this enzyme to participate actively in the sensitivity, timing and location of its action. CaMKII can: (i) be activated in a Ca2+-spike frequency-dependent manner; (ii) become independent of its initial Ca2+/CaM activators; and (iii) undergo a 'molecular switch-like' behaviour, which is crucial for certain forms of learning and memory. CaMKII is derived from a family of four homologous but distinct genes, with over 30 alternatively spliced isoforms described at present. These isoforms possess diverse developmental and anatomical expression patterns, as well as subcellular localization. Six independent catalytic/autoregulatory domains are connected by a narrow stalk-like appendage to each hexameric ring within the dodecameric structure. Ca2+/CaM binding activates the enzyme by disinhibiting the autoregulatory domain; this process initiates an intra-holoenzyme autophosphorylation reaction that induces complex changes in the enzyme's sensitivity to Ca2+/CaM, including the generation of Ca2+/CaM-independent (autonomous) activity and marked increase in affinity for CaM. The role of CaMKII in Ca2+ signal transduction is shaped by its autoregulation, isoenzymic type and subcellular localization. The molecular determinants and mechanisms producing these processes are discussed as they relate to the structure-function of this multifunctional protein kinase.

Spike frequency decoding and autonomous activation of Ca2+-calmodulin-dependent protein kinase II in dorsal root ganglion neurons

Autonomous activation of calcium-calmodulin kinase (CaMKII) has been proposed as a molecular mechanism for decoding Ca(2+) spike frequencies resulting from action potential firing, but this has not been investigated in intact neurons. This was studied in mouse DRG neurons in culture using confocal measurements of [Ca(2+)](i) and biochemical measurements of CaMKII autophosphorylation and autonomous activity. Using electrical stimulation at different frequencies, we find that CaMKII autonomous activity reached near maximal levels after approximately 45 impulses, regardless of firing frequency (1-10 Hz), and autonomous activity declined with prolonged stimulation. Frequency-dependent activation of CaMKII was limited to spike frequencies in the range of 0.1-1 Hz, despite marked increases in [Ca(2+)](i) at higher frequencies (1-30 Hz). The high levels of autonomous activity measured before stimulation and the relatively long duration of Ca(2+) spikes induced by action potentials ( approximately 300 msec) are consistent with the lower frequency range of action potential decoding by CaMKII. The high autonomous activity under basal conditions was associated with extracellular [Ca(2+)], independently from changes in [Ca(2+)](i), and unrelated to synaptic or spontaneous impulse activity. CaMKII autonomous activity in response to brief bursts of action potentials correlated better with the frequency of Ca(2+) transients than with the concentration of [Ca(2+)](i). In conclusion, CaMKII may decode frequency-modulated responses between 0.1 and 1 Hz in these neurons, but other mechanisms may be required to decode higher frequencies. Alternatively, CaMKII may mediate high-frequency responses in subcellular microdomains in which the enzyme is maintained at a low level of autonomous activity or the Ca(2+) transients have faster kinetics.

Long-term treatment with S-adenosylmethionine induces changes in presynaptic CaM kinase II and synapsin I

BACKGROUND

According to current hypotheses, antidepressant drug action is the result of adaptive changes in neuronal signaling mechanisms rather than a primary effect on neurotransmitter transporters, receptors, or metabolic enzymes. Among the signaling mechanisms involved, protein kinases and phosphorylation have been shown to be modified by drug treatment. Presynaptic signaling (calcium/calmodulin-dependent protein kinase II [CaMKII]) and the protein machinery regulating transmitter release have been implicated in the action of these drugs.

METHODS

We investigated the effect of S-adenosylmethionine (SAM), a compound with putative antidepressant activity, on presynaptic CaMKII and its synaptic vesicle substrate synapsin I. The activity of CaMKII was assayed in synaptic subcellular fractions prepared from hippocampus (HI), frontal cortex (FCX), striatum (STR), and parieto-temporal cortex.

RESULTS

The kinase activity was increased after SAM treatment in the synaptic vesicle fraction of HI (31.7%), FCX (35.9%), and STR (18.4%). The protein level of CaMKII was also increased in synaptic vesicles of HI (40.4%). The synapsin I level was unchanged in synaptic vesicles but markedly increased in synaptic cytosol of HI (75.8%) and FCX (163.0%). No changes for both CaMKII and synapsin I level were found in homogenates, suggesting that synaptic protein changes are not explained by an increase in total level of proteins, but rather by translocation to nerve terminals.

CONCLUSIONS

Similar to typical antidepressant drugs, SAM induces changes in CaMKII activity and increases synapsin I level in HI and FCX nerve terminals, suggesting a modulatory action on transmitter release.

Calcium-calmodulin signalling pathway up-regulates glutamatergic synaptic function in non-pyramidal, fast spiking rat hippocampal CA1 neurons

1. The role of Ca(2+)-calmodulin (CaM) signalling cascades in modulating glutamatergic synaptic transmission on CA1 non-pyramidal fast-spiking neurons was investigated using whole-cell recording and perfusion in rat hippocampal slices. 2. Paired stimuli (PS), consisting of postsynaptic depolarization to 0 mV and presynaptic stimulation at 1 Hz for 30 s, enhanced excitatory postsynaptic currents (EPSCs) on non-pyramidal neurons in the stratum pyramidale (SP). The potentiation was reduced by the extracellular application of D-amino-5-phosphonovaleric acid (DAP-5, 40 microM), and blocked by the postsynaptic perfusion of 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM), a CaM-binding peptide (100 microM) or CaMKII (281-301) (an autoinhibitory peptide of CaM-dependent protein kinases, 100 microM). 3. The application of adenophostin, an agonist of inositol trisphosphate receptors (IP(3)Rs) that evokes Ca(2+) release, into SP non-pyramidal neurons via the patch pipette (1 microM) enhanced EPSCs and occluded PS-induced synaptic potentiation. The co-application of BAPTA (10 mM) with adenophostin blocked synaptic potentiation. In addition, Ca(2+)-CaM (40:10 microM) induced synaptic potentiation, which occluded PS-induced potentiation and was attenuated by introducing CaMKII (281-301) (100 microM). EPSCs were sensitive to an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR). 4. Application of Ca(2+)-CaM into SP non-pyramidal neurons induced the emergence of AMPAR-mediated EPSCs that were not evoked by low stimulus intensity before perfusion. Ca(2+)-CaM also increased the amplitude and frequency of spontaneous EPSCs. A scavenger of nitric oxide, carboxy-PTIO (30 microM in slice-perfusion solution), did not affect these increases in sEPSCs. 5. The magnitude of PS-, adenophostin- or Ca(2+)-CaM-induced synaptic potentiation in SP non-pyramidal neurons increased during postnatal development. 6. These results indicate that Ca(2+)-CaM signalling pathways in CA1 SP non-pyramidal neurons up-regulate glutamatergic synaptic transmission probably through the conversion of inactive-to-active synapses.

Autonomous activity and autophosphorylation of CAMPK-II in rat hippocampal slices: effects of tissue preparation

Measurement of the proportion of calcium/calmodulin-stimulated protein kinase II (CaMPK-II) that is autonomously active or phosphorylated on Thr(286) is thought to provide an index of the degree to which CaMPK-II in a tissue has been activated. We have examined how various ways of handling hippocampal tissue can alter these properties. Both autonomous activity and phospho-Thr(286) content was high in freshly dissected hippocampus or freshly cut hippocampal slices. After incubation of hippocampal slices in artificial cerebrospinal fluid for 120 min, both properties of CaMPK-II decreased to a steady state level. Freeze-thaw or cutting the equilibrated slices could rapidly increase both autonomous activity and phospho-Thr(286) immunoreactivity of CaMPK-II. These increases were comparable to changes induced by experimental treatment. Therefore, our results suggest that considerable care needs to be taken over the way in which hippocampal slices are handled.

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.

Nitric oxide acts as a postsynaptic signaling molecule in calcium/calmodulin-induced synaptic potentiation in hippocampal CA1 pyramidal neurons

Postsynaptic injection of Ca(2+)/calmodulin (Ca(2+)/CaM) into hippocampal CA1 pyramidal neurons induces synaptic potentiation, which can occlude tetanus-induced potentiation (Wang and Kelly, 1995). Because Ca(2+)/CaM activates the major forms of nitric oxide synthase (NOS) to produce nitric oxide (NO), NO may play a role during Ca(2+)/CaM-induced potentiation. Here we show that extracellular application of the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) or postsynaptic co-injection of L-NAME with Ca(2+)/CaM blocked Ca(2+)/CaM-induced synaptic potentiation. Thus, NO is necessary for Ca(2+)/CaM-induced synaptic potentiation. In contrast, extracellular perfusion of membrane-impermeable NO scavengers N-methyl-D-glucamine dithiocarbamate/ferrous sulfate mixture (MGD-Fe) or 2-(4-carboxyphenyl)-4,4,5, 5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO) did not attenuate Ca(2+)/CaM-induced synaptic potentiation, even though MGD-Fe or carboxy-PTIO blocked tetanus-induced synaptic potentiation. This result indicates that NO is not a retrograde messenger in Ca(2+)/CaM-induced synaptic potentiation. However, postsynaptic co-injection of carboxy-PTIO with Ca(2+)/CaM blocked Ca(2+)/CaM-induced potentiation. Postsynaptic injection of carboxy-PTIO alone blocked tetanus-induced synaptic potentiation without affecting basal synaptic transmission. Our results suggest that NO works as a postsynaptic (intracellular) messenger during Ca(2+)/CaM-induced synaptic potentiation.

Adenylyl cyclase activation modulates activity-dependent changes in synaptic strength and Ca2+/calmodulin-dependent kinase II autophosphorylation

Activation of the Ca2+- and calmodulin-dependent protein kinase II (CaMKII) and its conversion into a persistently activated form by autophosphorylation are thought to be crucial events underlying the induction of long-term potentiation (LTP) by increases in postsynaptic Ca2+. Because increases in Ca2+ can also activate protein phosphatases that oppose persistent CaMKII activation, LTP induction may also require activation of signaling pathways that suppress protein phosphatase activation. Because the adenylyl cyclase (AC)-protein kinase A signaling pathway may provide a mechanism for suppressing protein phosphatase activation, we investigated the effects of AC activators on activity-dependent changes in synaptic strength and on levels of autophosphorylated alphaCaMKII (Thr286). In the CA1 region of hippocampal slices, briefly elevating extracellular Ca2+ induced an activity-dependent, transient potentiation of synaptic transmission that could be converted into a persistent potentiation by the addition of phosphatase inhibitors or AC activators. To examine activity-dependent changes in alphaCaMKII autophosphorylation, we replaced electrical presynaptic fiber stimulation with an increase in extracellular K+ to achieve a more global synaptic activation during perfusion of high Ca2+ solutions. In the presence of the AC activator forskolin or the protein phosphatase inhibitor calyculin A, this treatment induced a LTP-like synaptic potentiation and a persistent increase in autophosphorylated alphaCaMKII levels. In the absence of forskolin or calyculin A, it had no lasting effect on synaptic strength and induced a persistent decrease in autophosphorylated alphaCaMKII levels. Our results suggest that AC activation facilitates LTP induction by suppressing protein phosphatases and enabling a persistent increase in the levels of autophosphorylated CaMKII.

Calcium/calmodulin-dependent phosphorylation and activation of human Cdc25-C at the G2/M phase transition in HeLa cells

The human tyrosine phosphatase (p54(cdc25-c)) is activated by phosphorylation at mitosis entry. The phosphorylated p54(cdc25-c) in turn activates the p34-cyclin B protein kinase and triggers mitosis. Although the active p34-cyclin B protein kinase can itself phosphorylate and activate p54(cdc25-c), we have investigated the possibility that other kinases may initially trigger the phosphorylation and activation of p54(cdc25-c). We have examined the effects of the calcium/calmodulin-dependent protein kinase (CaM kinase II) on p54(cdc25-c). Our in vitro experiments show that CaM kinase II can phosphorylate p54(cdc25-c) and increase its phosphatase activity by 2.5-3-fold. Treatment of a synchronous population of HeLa cells with KN-93 (a water-soluble inhibitor of CaM kinase II) or the microinjection of AC3-I (a specific peptide inhibitor of CaM kinase II) results in a cell cycle block in G2 phase. In the KN-93-arrested cells, p54(cdc25-c) is not phosphorylated, p34(cdc2) remains tyrosine phosphorylated, and there is no increase in histone H1 kinase activity. Our data suggest that a calcium-calmodulin-dependent step may be involved in the initial activation of p54(cdc25-c).

Modifications in brain cAMP- and calcium/calmodulin-dependent protein kinases induced by treatment with S-adenosylmethionine

Several lines of evidence suggest that the mechanism of action of antidepressant drugs (AD) involves adaptive changes occurring in intraneuronal post-receptor signal transduction cascades. Protein phosphorylation has a key role in signal transduction and was previously found to be a target in the action of AD (5-HT and/or NA reuptake blockers). Several studies showed that cAMP- and type II Ca2+/calmodulin-dependent protein kinases (PKA and CaMKII) are markedly affected by typical AD in two different and complementary cellular districts, respectively microtubules (a somatodendritic compartment) and synaptic vesicles (a presynaptic terminal compartment). In order to investigate whether the effect on protein kinases may be involved in the therapeutic action of drugs it is interesting to compare the effect of atypical AD with that of typical drugs. In this study the effect of the atypical AD S-adenosylmethionine (SAMe) was tested. Repeated (12 days) SAMe treatment induced in cerebrocortical microtubules an increase in the binding of cAMP to the RII PKA regulatory subunit and an increase in the endogenous phosphorylation of microtubule-associated protein 2, an effect resembling that of typical AD. In synaptic terminals the treatment induced an increase in the activity of CaMKII and in the endogenous phosphorylation of vesicular substrates. However, this modification was found in the cerebral cortex rather than in the hippocampus, where typical AD affect CaMKII. In addition the synapsin I level was decreased in the hippocampus and increased in the cerebral cortex, an effect not detected with typical AD.

Phosphorylation at the nuclear localization signal of Ca2+/calmodulin-dependent protein kinase II blocks its nuclear targeting

Translocation of protein kinases with broad substrate specificities between different subcellular compartments by activation of signaling pathways is an established mechanism to direct the activity of these enzymes toward particular substrates. Recently, we identified two isoforms of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), which are targeted to the nucleus by an alternatively spliced nuclear localization signal (NLS). Here we report that cotransfection with constitutively active mutants of CaM kinase I or CaM kinase IV specifically blocks nuclear targeting of CaM kinase II as a result of phosphorylation of a Ser immediately adjacent to the NLS of CaM kinase II. Both CaM kinase I and CaM kinase IV are able to phosphorylate this Ser residue in vitro, and mutagenesis studies suggest that this phosphorylation is both necessary and sufficient to block nuclear targeting. Furthermore, we provide experimental evidence that introduction of a negatively charged residue at this phosphorylation site reduces binding of the kinase to an NLS receptor in vitro, thus providing a mechanism that may explain the blockade of nuclear targeting that we have observed in situ.

Signal transduction molecules at the glutamatergic postsynaptic membrane

We have applied techniques from modern molecular biology and biochemistry to unravel the complex molecular structure of the postsynaptic membrane at glutamatergic synapses in the central nervous system. We have characterized a set of new proteins that are constituents of the postsynaptic density, including PSD-95, densin-180, citron (a rho/rac effector protein), and synaptic gp130 Ras GAP (a new Ras GTPase-activating protein). The structure of PSD-95 revealed a new protein motif, the PDZ domain, that plays an important role in the assembly of signal transduction complexes at intercellular junctions. More recently, we have used new imaging tools to observe the dynamics of autophosphorylation of CaM kinase II in intact hippocampal tissue. We have been able to detect changes in the amount of autophosphorylated CaM kinase II in dendrites, individual synapses, and somas of hippocampal neurons following induction of long-term potentiation by tetanic stimulation. In addition, we have observed a specific increase in the concentration of CaM kinase II in dendrites of neurons receiving tetanic stimulation. This increase appears to be the result of dendritic synthesis of new protein. Over the next several years we will apply similar methods to study regulatory changes that occur at the molecular level in glutamatergic synapses in the CNS as the brain processes and stores new information.

Involvement of calcium/calmodulin-dependent protein kinases in the setting of a molecular switch involved in hippocampal LTP

Long-term potentiation (LTP) is the form of synaptic plasticity most commonly associated with learning and memory. Studies using protein kinase inhibitors have suggested functional roles for several kinases in the induction of LTP in the CA1 region of the hippocampus, though the precise role of any given kinase has yet to be fully established. Here we report that the selective calcium/calmodulin-dependent protein kinase (CaMK) inhibitor KN-62 has two distinct actions on LTP. As reported previously, KN-62 (3 microM) prevented the induction of LTP. Here we show that KN-62 also prevents the setting of a molecular switch, initiated by the synaptic activation of (S)-alpha-methyl-4-carboxyphenylglycine (MCPG)-sensitive metabotropic glutamate (mGlu) receptors. There are two aspects of this work which might be considered surprising. First, the setting of the molecular switch was prevented by a concentration of KN-62 (1 microM) subthreshold for the inhibition of the induction of LTP per se. Second, the setting of the molecular switch, by the delivery of a tetanus (100 Hz, 1 s) in the presence of a specific NMDA receptor antagonist (R)-2-amino-5-phosphonopentanoate (AP5), reduced the sensitivity of LTP to KN-62, such that at a concentration of 3 microM it no longer blocked induction (though at 10 microM it did). This conditioning effect of a tetanus, delivered in the presence of AP5, was prevented by MCPG (200 microM). These data reveal unexpected complexities in the involvement of KN-62-sensitive processes (presumably CaMKII) in the induction of LTP. They suggest that activation of KN-62-sensitive processes leads to (at least) two phosphorylation steps with fundamentally different roles in synaptic plasticity within a single synapse. They also raise the possibility that CaMKII is an integral part of the MCPG-sensitive molecular switch mechanism.

Sensitivity of CaM kinase II to the frequency of Ca2+ oscillations

The transduction of many cellular stimuli results in oscillations in the intracellular concentration of calcium ions (Ca2+). Although information is thought to be encoded in the frequency of such oscillations, no frequency decoder has been identified. Rapid superfusion of immobilized Ca2+- and calmodulin-dependent protein kinase II (CaM kinase II) in vitro showed that the enzyme can decode the frequency of Ca2+ spikes into distinct amounts of kinase activity. The frequency response of CaM kinase II was modulated by several factors, including the amplitude and duration of individual spikes as well as the subunit composition and previous state of activation of the kinase. These features should provide specificity in the activation of this multifunctional enzyme by distinct cellular stimuli and may underlie its pivotal role in activity-dependent forms of synaptic plasticity.

Postsynaptic inhibitors of calcium/calmodulin-dependent protein kinase type II block induction but not maintenance of pairing-induced long-term potentiation

The role of postsynaptic kinases in the induction and maintenance of long-term potentiation (LTP) was studied in the CA1 region of the rat hippocampal slice. A peptide inhibitor for the catalytic domain of calcium/calmodulin-dependent protein kinase type II (CaM-kinase) was applied through a perfused patch pipette. The inhibitor completely blocked both the short-term potentiation and LTP induced by a pairing protocol. This indicates that the kinase or kinases affected by the peptide are downstream from depolarization in the LTP cascade. The ability to block LTP required that measures be taken to interfere with degradation of the peptide kinase inhibitor by endogenous proteases; either addition of protease inhibitors or modifications of the peptide itself greatly enhanced the effectiveness of the peptide. Protease inhibitors by themselves or control peptide did not block LTP induction. To study the effect of kinase inhibitor on LTP maintenance, we induced LTP in one pathway. Subsequent introduction of the kinase inhibitor blocked the induction of LTP in a second pathway, but it did not affect maintenance of LTP in the first. The implications for the role of kinases in LTP maintenance are discussed.

Visualization of the distribution of autophosphorylated calcium/calmodulin-dependent protein kinase II after tetanic stimulation in the CA1 area of the hippocampus

Autophosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) at threonine-286 produces Ca2+-independent kinase activity and has been proposed to be involved in induction of long-term potentiation by tetanic stimulation in the hippocampus. We have used an immunocytochemical method to visualize and quantify the pattern of autophosphorylation of CaMKII in hippocampal slices after tetanization of the Schaffer collateral pathway. Thirty minutes after tetanic stimulation, autophosphorylated CaM kinase II (P-CaMKII) is significantly increased in area CA1 both in apical dendrites and in pyramidal cell somas. In apical dendrites, this increase is accompanied by an equally significant increase in staining for nonphosphorylated CaM kinase II. Thus, the increase in P-CaMKII appears to be secondary to an increase in the total amount of CaMKII. In neuronal somas, however, the increase in P-CaMKII is not accompanied by an increase in the total amount of CaMKII. We suggest that tetanic stimulation of the Schaffer collateral pathway may induce new synthesis of CaMKII molecules in the apical dendrites, which contain mRNA encoding its alpha-subunit. In neuronal somas, however, tetanic stimulation appears to result in long-lasting increases in P-CaMKII independent of an increase in the total amount of CaMKII. Our findings are consistent with a role for autophosphorylation of CaMKII in the induction and/or maintenance of long-term potentiation, but they indicate that the effects of tetanus on the kinase and its activity are not confined to synapses and may involve induction of new synthesis of kinase in dendrites as well as increases in the level of autophosphorylated kinase.

Postsynaptic calcineurin activity downregulates synaptic transmission by weakening intracellular Ca2+ signaling mechanisms in hippocampal CA1 neurons

Protein phosphorylation and dephosphorylation are believed to functionally couple neuronal activity and synaptic plasticity. Our previous results indicated that postsynaptic Ca2+/calmodulin (CaM) signaling pathways play an important role in setting synaptic strength, and calcineurin (CaN) activity limits synaptic responses during basal synaptic transmission and long-term potentiation expression. The inhibition of postsynaptic CaN activity by FK-506 or an autoinhibitory peptide induced synaptic potentiation in hippocampal slices, which occludes tetanus-induced LTP. FK-506-induced synaptic potentiation was expressed in adult but not young rats. To elucidate mechanisms underlying CaN-inhibited synaptic potentiation, we co-injected certain agents affecting Ca2+ signaling pathways with CaN inhibitors into CA1 neurons. Synaptic potentiation induced by FK-506 was significantly attenuated by co-injecting BAPTA, heparin/dantrolene (inhibitors of intracellular Ca2+ release), a CaM-binding peptide, or CaM-KII/PKC pseudosubstrate peptides. These results indicate that postsynaptic CaN activity can downregulate evoked synaptic transmission by weakening intracellular Ca2+ signals and downstream protein kinase activities.

Receptor-mediated activation of protein kinase C in hippocampal long-term potentiation: facts, problems and implications

During the last decade hippocampal long-term potentiation has become one of the most frequently used models to study cellular mechanisms of learning and memory. Receptor-mediated activation of protein kinase C is thought to be involved in LTP stabilisation. In the present review, 1. the molecular structure and activation mechanisms of PKC isoenzymes, 2. the biochemical evidences for PKC activation after induction of LTP using different stimulation paradigms as well as 3. the involvement of metabotropic glutamate receptors in PKC activation after induction of LTP are critically discussed.

Effect of glutamate receptor phosphorylation by endogenous protein kinases on electrical activity of isolated postsynaptic densities of rat cortex and hippocampus

Postsynaptic densities (PSDs) were isolated from rat brain cortex and hippocampus, purified and incorporated into giant (5-80 microns in diameter) liposomes. Gigaohm seals were obtained with a patch-clamp pipette, and a giant liposome PSD-containing membrane patch, was excised and recorded. The PSD was always oriented in an inside-out configuration. This allowed receptor agonists or antagonists to be added from the interior of the recording pipette, and also the addition of different substances, such as ATP, calcium, calmodulin and others to the 'intracellular' side of the PSD, i.e. to the bath. alpha-Amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA) receptor agonists such as quisqualate or AMPA induced in the PSD a complex pattern of electrical activity, that was blocked by 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but not by 2-aminophosphonovalerate (APV). The currents generated by 0.5-1 microM quisqualate were increased by about 100% when the PSDs were phosphorylated. Similar findings were obtained when the agonist was 0.2-2 microM kainate. These currents were also blocked by a non-N-methyl-D-aspartate (NMDA) receptor antagonist but not by APV, and were increased by about 70% by phosphorylation of the PSDs. Addition of 5-10 microM NMDA plus 1 microM glycine to the 'extracellular' side of the PSD, led to a characteristic pattern of activity, with the opening of multiple receptor ion channels. This was entirely blocked by 10 microM APV. Addition of extracellular Mg2+ (1-2 mM) induced a voltage-dependent block of the currents. Phosphorylation of the PSD led to an increase of Mg(2+)-blocked current of about 80%. The effect of phosphorylation on ion channel activity showed a markedly different requirement for calcium and for calmodulin among the AMPA, kainate and NMDA types of glutamate receptors, thus suggesting that each receptor type is coupled at the synapse with a unique complement of protein phosphokinases.

A biochemist's view of long-term potentiation

This review surveys the molecular mechanisms of long-term potentiation (LTP) from the point of view of a biochemist. On the basis of available data, LTP in area CA1 of the hippocampus is divided into three phases--initial, early, and late--and the mechanisms contributing to the induction and expression of each phase are examined. We focus on evidence for the involvement of various second messengers and their effectors as well as the biochemical strategies employed in each phase to convert a transient signal into a lasting change in the neuron. We also consider, from a biochemical perspective, the implications of a multiphase model for LTP.

Neuronal activity increases the phosphorylation of the transcription factor cAMP response element-binding protein (CREB) in rat hippocampus and cortex

Activity-mediated gene expression is thought to play an important role in many forms of neuronal plasticities. We have used pentylenetetrazol-induced seizure that produces synchronous and sustained neuronal activity as a model to examine the mechanism(s) of gene activation. The transcription factor CREB (Ca2+/cAMP response element-binding protein) is thought to be necessary for long-term memory formation both in invertebrates and vertebrates. When phosphorylated on Ser133 either by cAMP-dependent protein kinase and/or Ca2+/calmodulin-dependent protein kinases, CREB increases transcription of genes containing the CRE (cAMP response element) sequence. Using an antibody that detects Ser133-phosphorylated CREB protein, we show that CREB phosphorylation is maximal between 3 and 8 min after the onset of seizure activity and declines slowly both in the hippocampus and the cortex. The total amount of CREB protein did not change at the time points examined. The increased phosphorylation of CREB protein is preceded by an increase in the amount of cAMP, suggestive of cAMP-dependent protein kinase activation, in the hippocampus and activation of Ca2+/calmodulin-dependent protein kinases in the cortex. Subsequent to CREB phosphorylation, the expression of the CRE-containing gene, c-fos, and the AP-1 complexes (heterodimers of Fos and Jun family members) is increased. These findings support the role of CREB-mediated gene expression in activity-dependent neuronal plasticities.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Expression of Ca2+/calmodulin-dependent protein kinase types II and IV, and reduced DNA synthesis due to the Ca2+/calmodulin-dependent protein kinase inhibitor KN-62 (1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenyl piperazine) in small cell lung carcinoma

Because changes in intracellular Ca2+ affect progression through the mitotic cell cycle, we investigated the role of Ca2+-binding proteins in regulating cell cycle progression. Evidence was found demonstrating that the activation of Ca2+/calmodulin-dependent protein kinase (CaM kinase) inhibits cell cycle progression in small cell lung carcinoma (SCLC) cells. We also demonstrated that SCLC cells express both CaM kinase type II (CaMKII) and CaM kinase type IV (CaMKIV). Five independent SCLC cell lines expressed proteins reactive with antibody to the CaMKII beta subunit, but none expressed detectable proteins reactive with antibody to the CaMKII alpha subunit. All SCLC cell lines tested expressed both the alpha and beta isoforms of CaMKIV. Immunoprecipitation of CaMKII from SCLC cells yielded multiple proteins that autophosphorylated in the presence of Ca2+ / calmodulin. Autophosphorylation was inhibited by the CaMKII(281-302) peptide, which corresponds to the CaMKII autoinhibitory domain, and by 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4- phenylpiperazine (KN-62), a specific CaM kinase antagonist. Influx of Ca2+ through voltage-gated Ca2+ channels stimulated phosphorylation of CaMKII in SCLC cells, and this was inhibited by KN-62. Incubation of SCLC cells of KN-62 potently inhibited DNA synthesis, and slowed progression through S phase. Similar anti-proliferative effects of KN-62 occurred in SK-N-SH human neuroblastoma cells, which express both CaMKII and CaMKIV, and in K562 human chronic myelogenous leukemia cells, which express CaMKII but not CaMKIV. The expression of both CaMKII and CaMKIV by SCLC cells, and the sensitivity of these cells to the anti-proliferative effects of KN-62, suggest a role for CaM kinase in regulating SCLC proliferation.

Calcium- and CaMKII-dependent chloride secretion induced by the microsomal Ca(2+)-ATPase inhibitor 2,5-di-(tert-butyl)-1,4-hydroquinone in cystic fibrosis pancreatic epithelial cells

Microsomal Ca(2+)-ATPase inhibitors such as thapsigargin (THG), cyclopiazonic acid (CPA) and 2,5-di-(tert-butyl)-1,4-hydroquinone (DBHQ) have been shown to inhibit Ca2+ reuptake by the intracellular stores and increase cytosolic free Ca2+ ([Ca2+]i). DBHQ is a commercially available non-toxic synthetic compound chemically unrelated to THG and CPA. In this study, we tested the feasibility of utilizing DBHQ to improve Cl- secretion via the Ca(2+)-dependent pathway, in the cystic fibrosis (CF)-derived pancreatic epithelial cell line CFPAC-1. DBHQ stimulated 125I efflux and mobilized intracellular free Ca2+ in a dose-dependent manner. The maximal effects were seen at concentrations of 25-50 microM. DBHQ (25 microM) caused a short-term rise in [Ca2+]i in the absence of ambient Ca2+, and a sustained elevation of [Ca2+]i in cell monolayers bathed in the efflux solution (1.2 mM Ca2+), which was largely attenuated by Ni2+ (5 mM). Bath-application of DBHQ induced an outwardly-rectifying whole-cell Cl- current, which was abolished by pipette addition of BAPTA (5 mM) or CaMK [273-302] (20 microM), an inhibitory peptide of multifunctional Ca2+/calmodulin-dependent protein kinase (CaMKII). Pretreatment of monolayers of CFPAC-1 cells with DBHQ for 4-5 min significantly increased the Ca(2+)-independent or autonomous activity of CaMKII assayed in the cell homogenates. Thus, DBHQ appears to enhance Cl- channel activity via a Ca(2+)-dependent mechanism involving CaMKII. Pretreatment of CFPAC-1 cells with up to 50 microM DBHQ for 6 h did not cause any detectable change in cell viability and did not significantly affect the cell proliferation rate. These results suggest that appropriate selective microsomal Ca(2+)-ATPase inhibitors may be therapeutically useful in improving Cl- secretion in CF epithelial cells.

Postsynaptic injection of CA2+/CaM induces synaptic potentiation requiring CaMKII and PKC activity

CA2+-regulated protein kinases play critical roles in long-term potentiation (LTP). To understand the role of Ca2+/calmodulin (CaM) signaling pathways in synaptic transmission better, Ca2+/CaM was injected into hippocampal CA1 neurons. Ca2+/CaM induced significant potentiation of excitatory synaptic responses, which was blocked by coinjection of a CaM-binding peptide and was not induced by injections of Ca2+ or CaM alone. Reciprocal experiments demonstrated that Ca2+/CaM-induced synaptic potentiation and tetanus-induced LTP occluded one another. Pseudosubstrate inhibitors or high-affinity substrates of CaMKII or PKC blocked Ca2/CaM-induced potentiation, indicating the requirement of CaMKII and PKC activities in synaptic potentiation. We suggest that postsynaptic levels of free Ca2+/CaM is a rate limiting factor and that functional cross-talk between Ca2+/CaM and PKC pathways occurs during the induction of LTP.

CaMKII regulates the frequency-response function of hippocampal synapses for the production of both LTD and LTP

To investigate the function of the autophosphorylated form of CaMKII in synaptic plasticity, we generated transgenic mice that express a kinase that is Ca2+ independent as a result of a point mutation of Thr-286 to aspartate, which mimics autophosphorylation. Mice expressing the mutant form of the kinase show an increased level of Ca(2+)-independent CaMKII activity similar to that seen following LTP. The mice nevertheless exhibit normal LTP in response to stimulation at 100 Hz. However, at lower frequencies, in the range of 1-10 Hz, there is a systematic shift in the size and direction of the resulting synaptic change in the transgenic animals that favors LTD. The regulation of this frequency-response function by Ca(2+)-independent CaMKII activity seems to account for two previously unexplained synaptic phenomena, the relative loss of LTD in adult animals compared with juveniles and the enhanced capability for depression of facilitated synapses.

Age-dependent changes in the regulation by external calcium ions of the phosphorylation of glial fibrillary acidic protein in slices of rat hippocampus

We studied the effect of external Ca2+ on the incorporation of [32P]phosphate into the astrocytic marker protein, glial fibrillary acidic protein (GFAP), in hippocampal slices from rats in the postnatal age range 12-16 days to +60 days (P12-P16 to +P60). At age P12-P16 the presence of Ca2+ in the incubation medium inhibited the incorporation of 32P into GFAP; this inhibition declined to near zero by P21 and subsequently 32P-incorporation became progressively more dependent on Ca2+ until by P60 no GFAP phosphorylation was observed in the absence of Ca2+. With tissue from immature rats inhibition of 32P-incorporation into GFAP started at a medium concentration of 7.5 microM Ca2+, reached 50% at 100 microM and then remained constant up to 1 mM; with adults maximal phosphorylation required 1 mM Ca2+ in the medium. The inorganic Ca(2+)-channel blockers, Co2+ and Ni2+, and a high concentration of the L-type blocker, nifedipine, reversed the effects of external Ca2+ on GFAP phosphorylation. The results suggest a late developmental change in the compartmental disposition of Ca2+ in astrocytes.

Calcium signaling in neurons: molecular mechanisms and cellular consequences

Neuronal activity can lead to marked increases in the concentration of cytosolic calcium, which then functions as a second messenger that mediates a wide range of cellular responses. Calcium binds to calmodulin and stimulates the activity of a variety of enzymes, including calcium-calmodulin kinases and calcium-sensitive adenylate cyclases. These enzymes transduce the calcium signal and effect short-term biological responses, such as the modification of synaptic proteins and long-lasting neuronal responses that require changes in gene expression. Recent studies of calcium signal-transduction mechanisms have revealed that, depending on the route of entry into a neuron, calcium differentially affects processes that are central to the development and plasticity of the nervous system, including activity-dependent cell survival, modulation of synaptic strength, and calcium-mediated cell death.

The CaM kinase II hypothesis for the storage of synaptic memory

Much has been learned about the activity-dependent synaptic modifications (long-term potentiation and long-term depression) that are thought to underlie memory storage, but the mechanism by which these modifications are stored remains unclear. A good candidate for the storage mechanism is Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) because it is localized at synapses, and its known autophosphorylation properties enable it to undergo long-term modification. In this review, John Lisman describes recent tests of the role of CaM kinase II in long-term potentiation. Experiments show that activity of CaM kinase II is increased for long periods of time after induction of long-term potentiation, that enhanced activity mimics long-term potentiation, and that enzyme activity is necessary for induction of long-term potentiation. The crucial question remaining is whether persistent enzyme activity is necessary to maintain stored information. Related issues concerning the mechanism by which synapses are weakened and the role of gene expression and structural changes are also discussed.

Postsynaptic integration of glutamatergic and dopaminergic signals in the striatum

The aim of this study was to achieve a better understanding of the integration in striatal medium-sized spiny neurons (MSNs) of converging signals from glutamatergic and dopaminergic afferents. The review of the literature in the first section shows that these two types of afferents not only contact the same striatal cell type, but that individual MSNs receive both a corticostriatal and a dopaminergic terminal. The most common sites of convergence are dendritic shafts and spines of MSNs with a distance between the terminals of less than 1-2 microns. The second section focuses on synaptic transmission and second messenger activation. Glutamate, the candidate transmitter of corticostriatal terminals, via different types of glutamate receptors can evoke an increase in intracellular free calcium concentrations. The net effect of dopamine in the striatum is a stimulation of adenylate cyclase activity leading to an increase in cAMP. The subsequent sections present information on calcium- and cAMP-sensitive biochemical pathways and review the regional and subcellular distribution of the components in the striatum. The specific biochemical reaction steps were formalized as simplified equilibrium equations. Parameter values of the model were chosen from published experimental data. Major results of this analysis are: at intracellular free calcium concentrations below 1 microM the stimulation of adenylate cyclase by calcium and dopamine is at least additive in the steady state. Free calcium concentrations exceeding 1 microM inhibit adenylate cyclase, which is not overcome by dopaminergic stimulation. The kinases and phosphatases studied can be divided in those that are almost exclusively calcium-sensitive (PP2B and CaMPK), and others that are modulated by both calcium and dopamine (PKA and PP1). Maximal threonine-phosphorylation of the phosphoprotein DARPP requires optimal concentrations of calcium (about 0.3 microM) and dopamine (above 5 microM). It seems favourable if the glutamate signal precedes phasic dopamine release by approximately 100 msec. The phosphorylation of MAP2 is under essentially calcium-dependent control of at least five kinases and phosphatases, which differentially affect its heterogeneous phosphorylation sites. Therefore, MAP2 could respond specifically to the spatio-temporal characteristics of different intracellular calcium fluxes. The quantitative description of the calcium- and dopamine-dependent regulation of DARPP and MAP2 provides insights into the crosstalk between glutamatergic and dopaminergic signals in striatal MSNs. Such insights constitute an important step towards a better understanding of the links between biochemical pathways, physiological processes, and behavioural consequences connected with striatal function. The relevance to long-term potentiation, reinforcement learning, and Parkinson's disease is discussed.