A novel protein phosphatase that dephosphorylates and regulates Ca2+/calmodulin-dependent protein kinase II

New aspects of neurotransmitter release and exocytosis: dynamic and differential regulation of synapsin I phosphorylation by acute neuronal excitation in vivo

Synapsin I is a synaptic vesicle-associated protein that is phosphorylated at multiple sites by various protein kinases. It has been proposed to play an important role in the regulation of neurotransmitter release and the organization of cytoskeletal architecture in the presynaptic terminal. In the present minireview, I describe the dynamic changes in synapsin I phosphorylation induced by acute neuronal excitation in vivo, and discuss its regulation by protein kinases and phosphatases and its functional significance in vivo. When acute neuronal excitation was induced by electroconvulsive treatment (ECT) in rats, phosphorylation of synapsin I at multiple sites was decreased during brief seizure activity in hippocampal and parieto-cortical homogenates. After termination of the seizure activity, phosphorylation at mitogen-activated protein kinase-dependent sites was increased dramatically. Phosphorylation at a Ca(2+)/calmodulin-dependent protein kinase II-dependent site was also increased moderately afterwards. The dynamic and differential changes in synapsin I phosphorylation induced by acute neuronal excitation may be involved in plastic changes induced by ECT and may have some role in its effectiveness for the treatment of psychiatric diseases in humans.

Regulation of the Multifunctional Ca2+/Calmodulin-dependent Protein Kinase II by the PP2C Phosphatase PPM1F in Fibroblasts

The regulation of the multifunctional calcium/calmodulin dependent protein kinase II (CaMKII) by serine/threonine protein phosphatases has been extensively studied in neuronal cells; however, this regulation has not been investigated previously in fibroblasts. We cloned a cDNA from SV40-transformed human fibroblasts that shares 80% homology to a rat calcium/calmodulin-dependent protein kinase phosphatase that encodes a PPM1F protein. By using extracts from transfected cells, PPM1F, but not a mutant (R326A) in the conserved catalytic domain, was found to dephosphorylate in vitro a peptide corresponding to the auto-inhibitory region of CaMKII. Further analyses demonstrated that PPM1F specifically dephosphorylates the phospho-Thr-286 in autophosphorylated CaMKII substrate and thus deactivates the CaMKII in vitro. Coimmunoprecipitation of CaMKII with PPM1F indicates that the two proteins can interact intracellularly. Binding of PPM1F to CaMKII involves multiple regions and is not dependent on intact phosphatase activity. Furthermore, overexpression of PPM1F in fibroblasts caused a reduction in the CaMKII-specific phosphorylation of the known substrate vimentin(Ser-82) following induction of the endogenous CaM kinase. These results identify PPM1F as a CaM kinase phosphatase within fibroblasts, although it may have additional functions intracellularly since it has been presented elsewhere as POPX2 and hFEM-2. We conclude that PPM1F, possibly together with the other previously described protein phosphatases PP1 and PP2A, can regulate the activity of CaMKII. Moreover, because PPM1F dephosphorylates the critical autophosphorylation site of CaMKII, we propose that this phosphatase plays a key role in the regulation of the kinase intracellularly.

Protein phosphatases that regulate multifunctional Ca2+/calmodulin-dependent protein kinases: from biochemistry to pharmacology

Multifunctional Ca(2+)/calmodulin-dependent protein kinases (CaMKs) play pivotal roles in Ca(2+) signaling pathways, such as the regulation of the neuronal functions of learning, memory, and neuronal cell death. The activities of the kinases are strictly regulated by protein phosphorylation/dephosphorylation. Although the activation mechanisms for multifunctional CaMKs through phosphorylation, which correspond to "switch on," have been extensively studied, the negative regulatory mechanisms through dephosphorylation, which correspond to "switch off," have not. In this review, we focused on the regulation of multifunctional CaMKs by the protein phosphatases responsible. We first summarized the current understanding of negative regulation of CaMKs by known protein phosphatases and their physiological significance. We then discussed newly developed methods for detection of protein phosphatases involved in the regulation of CaMKs. We also summarized the biochemical properties of a novel protein phosphatase, which we isolated with the new methods and designated as CaMK phosphatase (CaMKP), and its homologue. Pharmacological implications for neuronal functions including memory and neuronal cell death are discussed from the viewpoint that regulation of protein kinase activity can be elucidated by focusing on protein phosphatases involved in its "switch off" mechanism.

Brief seizure activity alters Ca2+/calmodulin dependent protein kinase II dephosphorylation and subcellular distribution in rat brain for several hours

The effect of a brief pentylenetetrazol (PTZ) convulsive seizure on rat cerebral cortical Ca2+/calmodulin dependent protein kinase II (CaMKII) was investigated. By immunoblot, it was found that a single PTZ seizure, lasting less than a minute, caused translocation of CaMKII alpha-subunit (alpha-CaMKII) from the particulate to the soluble fraction for several hours, paralleled by a dramatic loss of alpha-CaMKII Thr286 phosphorylation. The reduced alpha-CaMKII Thr286 phosphorylation apparently resulted from enhanced phosphatase activity following PTZ seizure, especially in the particulate fraction. CaMKII translocation and phosphatase activation following a brief seizure episode can both contribute to long-lasting CaMKII regulation far outlasting the immediate effects of the seizure on neuronal function.

Influence of immobilization stress on the expression and phosphatase activity of protein phosphatase 2A in the rat brain

BACKGROUND

Protein phosphatase 2A (PP2A) is a major kinase phosphatase that plays an important role in regulating the activities of protein kinase cascades. It has been revealed that stress changes neuronal gene expression by activating these cascades. We examined the expression of the catalytic subunit C and serine and threonine phosphatase activity of PP2A in the rat frontal cortex and hippocampus following various immobilization stress paradigms.

METHODS

Immunoblot and immunohistochemical analyses were performed to examine the expression of PP2A. The level of phosphatase activity of PP2A was determined as the amount of free phosphate generated from a synthetic phosphopeptide.

RESULTS

Immunoblot analysis revealed no significant change in the level of PP2A immunoreactivity in response to either a single or repeated stress. Immunohistochemical analysis revealed that neither a single nor repeated stress changed PP2A immunoreactivity in the hippocampus; however, the levels of serine and threonine phosphatase activity in the frontal cortex and hippocampus were significantly upregulated in response to a single or repeated stress.

CONCLUSIONS

These results demonstrated that both a single and repeated immobilization stress upregulated the activity of PP2A in the rat brain, suggesting that PP2A may be involved, at least in part, in the downregulation of protein kinase activation induced by stress.

p21-Activated Kinase 2 in Neutrophils Can Be Regulated by Phosphorylation at Multiple Sites and by a Variety of Protein Phosphatases

The p21-activated kinase(Pak) 2 undergoes rapid autophosphorylation/activation in neutrophils stimulated with a variety of chemoattractants (e.g., fMLP). Phosphorylation within the activation loop (Thr(402)) and inhibitory domain (Ser(141)) is known to increase the activity of Pak in vitro, whereas phosphorylation within the Nck (Ser(20)) and Pak-interacting guanine nucleotide exchange factor (Ser(192) and Ser(197)) binding sites blocks the interactions of Pak 2 with these proteins. A panel of phosphospecific Abs was used to investigate the phosphorylation of Pak 2 in neutrophils at these sites. Pak 2 underwent rapid (< or =15 s) phosphorylation at Ser(20), Ser(192/197), and Thr(402) in neutrophils stimulated with fMLP. Phosphorylation at Ser(192/197) and Thr(402) were highly transient events, whereas that at Ser(20) was more persistent. In contrast, Pak 2 was constitutively phosphorylated at Ser(141) in unstimulated neutrophils and phosphorylation at this site was less sensitive to cell stimulation than at other residues. Studies with selective inhibitors suggested that a variety of phosphatases might be involved in the rapid dephosphorylation of Pak 2 at Thr(402) in stimulated neutrophils. This was consistent with biochemical studies which showed that the activation loop of GST-Pak 3, which is homologous to that in Pak 2, was a substrate for protein phosphatase 1, 2A, and a Mg(2+)/Mn(2+)-dependent phosphatase(s) which exhibited properties different from those of the conventional isoforms of protein phosphatase 2C. The data indicate that Pak 2 undergoes a complex pattern of phosphorylation in neutrophils and that dephosphorylation at certain sites may involve multiple protein phosphatases that exhibit distinct modes of regulation.

Subcellular distributions of rat CaM kinase phosphatase N and other members of the CaM kinase regulatory system

Ca2+/Calmodulin-dependent protein kinase (CaM kinase) regulatory system is composed of multifunctional CaM kinases such as CaM kinases IV and I, upstream CaM kinases such as CaM kinase kinases alpha and beta, which activate multifunctional CaM kinases, and CaM kinase phosphatases such as CaM kinase phosphatase and CaM kinase phosphatase N, which deactivate the activated multifunctional CaM kinases. To understand the combinations of CaM kinases I and IV, CaM kinase kinases alpha and beta, and CaM kinase phosphatases, the locations of the enzymes in the cell were examined by immunocytochemical studies of cultured cells. The results indicate that CaM kinase I, CaM kinase kinase beta, and CaM kinase phosphatase occur in the cytoplasm and that CaM kinase IV, CaM kinase kinase alpha (and CaM kinase kinase beta in some cell types and tissues), and CaM kinase phosphatase N occur inside the cellular nucleus, suggesting that there are at least two different sets of CaM kinase regulatory systems, one consisting of CaM kinase I, CaM kinase kinase beta, and CaM kinase phosphatase in the cytoplasm and the other consisting of CaM kinase IV, CaM kinase kinase alpha (and CaM kinase kinase beta in some cell types and tissues), and CaM kinase phosphatase N in the nucleus.

Stimulation of Ca(2+)/calmodulin-dependent protein kinase phosphatase by polycations

Ca(2+)/calmodulin-dependent protein kinase phosphatase (CaMKPase) dephosphorylates and regulates multifunctional Ca(2+)/calmodulin-dependent protein kinases (CaMKs). One of the prominent features of CaMKPase is stimulation of phosphatase activity by polycations such as poly-L-lysine (poly(Lys)). Using various polycations, basicity and molecular weight of the polymer proved to be important for the stimulation. Surface plasmon resonance (SPR) analysis showed that CaMKIV(T196D), which mimics CaMKPase substrate, and CaMKPase could form tight complexes with poly(Lys). Pull-down binding experiments suggested that the formation of a tightly associated ternary complex consisting of CaMKPase, poly(Lys), and phosphorylated CaMKIV is essential for stimulation. Dilution experiments also supported this contention. Poly(Lys) failed to stimulate a CaMKPase mutant in which a Glu cluster corresponding to residues 101-109 in the N-terminal domain was deleted, and the mutant could not interact with poly(Lys) in the presence of Mn(2+). Thus, the Glu cluster appeared to be the binding site for polycations and to play a pivotal role in the polycation stimulation of CaMKPase activity.

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.

Phosphorylation of calmodulin by Ca2+/calmodulin-dependent protein kinase IV

Calmodulin-dependent protein kinase IV (CaM-kinase IV) phosphorylated calmodulin (CaM), which is its own activator, in a poly-L-Lys [poly(Lys)]-dependent manner. Although CaM-kinase II weakly phosphorylated CaM under the same conditions, CaM-kinase I, CaM-kinase kinase alpha, and cAMP-dependent protein kinase did not phosphorylate CaM. Polycations such as poly(Lys) were required for the phosphorylation. The optimum concentration of poly(Lys) for the phosphorylation of 1 microM CaM was about 10 microg/ml, but poly(Lys) strongly inhibited CaM-kinase IV activity toward syntide-2 at this concentration, suggesting that the phosphorylation of CaM is not due to simple activation of the catalytic activity. Poly-L-Arg could partially substitute for poly(Lys), but protamine, spermine, and poly-L-Glu/Lys/Tyr (6/3/1) could not. When phosphorylation was carried out in the presence of poly(Lys) having various molecular weights, poly(Lys) with a higher molecular weight resulted in a higher degree of phosphorylation. Binding experiments using fluorescence polarization suggested that poly(Lys) mediates interaction between the CaM-kinase IV/CaM complex and another CaM. The 32P-labeled CaM was digested with BrCN and Achromobacter protease I, and the resulting peptides were purified by reversed-phase HPLC. Automated Edman sequence analysis of the peptides, together with phosphoamino acid analysis, indicated that the major phosphorylation site was Thr44. Activation of CaM-kinase II by the phosphorylated CaM was significantly lower than that by the nonphosphorylated CaM. Thus, CaM-kinase IV activated by binding Ca2+/CaM can bind and phosphorylate another CaM with the aid of poly(Lys), leading to a decrease in the activity of CaM.

The atrial natriuretic peptide receptor (NPR-A/GC-A) is dephosphorylated by distinct microcystin-sensitive and magnesium-dependent protein phosphatases

Natriuretic peptide receptor (NPR)-A is the primary signaling receptor for atrial natriuretic peptide and brain natriuretic peptide. Ligand binding to NPR-A rapidly activates its guanylyl cyclase domain, but its rate of cGMP synthesis declines with time. This waning of activity is called homologous desensitization and is mediated in part by receptor dephosphorylation. Here, we characterize two distinct NPR-A phosphatase activities. The serine/threonine protein phosphatase inhibitor, microcystin, inhibited the desensitization of NPR-A in membrane guanylyl cyclase assays in the absence of magnesium. EDTA also inhibited the desensitization, whereas MgCl(2) stimulated the desensitization. Because the effects of microcystin and EDTA were additive, and microcystin did not block the magnesium-dependent desensitization, the targets for these agents appear to be distinct. Incubation of membranes at 37 degrees C stimulated the dephosphorylation of NPR-A, and microcystin blocked the temperature-dependent dephosphorylation. The addition of MgCl(2) or MnCl(2), but not CaCl(2), further stimulated the dephosphorylation of NPR-A, and microcystin failed to inhibit this process. The desensitization required changes in the phosphorylation state of NPR-A because the guanylyl cyclase activity of a receptor variant containing glutamate substitutions at all six phosphorylation sites was unaffected by MgCl(2), EDTA, or microcystin. Together, these data indicate that NPR-A is regulated by two distinct phosphatases, possibly including a member of the protein phosphatase 2C family. Finally, we observed that the desensitization of NPR-A in membranes from mouse kidneys and NIH3T3 cells was increased by prior exposure to atrial natriuretic peptide, suggesting that hormone binding enhances receptor dephosphorylation.

The p21-activated kinase PAK is negatively regulated by POPX1 and POPX2, a pair of serine/threonine phosphatases of the PP2C family

The Rho GTPases are involved in many signaling pathways and cellular functions, including the organization of the actin cytoskeleton, regulation of transcription, cell motility, and cell division. The p21 (Cdc42/Rac)-activated kinase PAK mediates a number of biological effects downstream of these Rho GTPases (reviewed by [1]). The phosphorylation state of mammalian PAK is highly regulated: upon binding of GTPases, PAK is potently activated by autophosphorylation at multiple sites, although the mechanisms of PAK downregulation are not known. We now report two PP2C-like serine/threonine phosphatases (POPX1 and POPX2) that efficiently inactivate PAK. POPX1 was isolated as a binding partner for the PAK interacting guanine nucleotide exchange factor PIX. The dephosphorylating activity of POPX correlates with an ability to block the in vivo effects of active PAK. Consonant with these effects on PAK, POPX can also inhibit actin stress fiber breakdown and morphological changes driven by active Cdc42(V12). The association of the POPX phosphatases with PAK complexes may allow PAK to cycle rapidly between active and inactive states; it represents a unique regulatory component of the signaling pathways of the PAK kinase family.

Calcium/calmodulin-dependent protein kinase IIalpha in optic axons moves with slow axonal transport and undergoes posttranslational modification

In neurons, the mRNA for calcium/calmodulin-dependent protein kinase II alpha (CKIIalpha) is known to be targeted to dendrites-where the enzyme is synthesized and supports postsynaptic functions. We are interested in knowing how neuronal proteins enter axons from the nerve cell body, and the mechanism for protein transport to terminals. Because CKIIalpha immunofluorescence can be demonstrated in over 80% of retinal ganglion cells, we asked whether this regulatory protein is being transported into optic axons. Using Sprague-Dawley rats, [(35)S] methionine was injected into the vitreous humor of the eye. Four days later, the optic nerves, tracts, lateral geniculate ganglia, and superior colliculi were removed and processed for 2D-PAGE and Western blotting. Radiolabeled CKIIalpha appears to move with slow component b (SCb) of axonal transport, as is the case in rodent sciatic motor neurons. In addition, the radiolabeled CKIIalpha isoform that enters the optic nerve is found to be 4 kDa heavier (in SDS-PAGE molecular mass) than the isoform in the optic tract, superior colliculus, and lateral geniculate nucleus. This reduction is likely the result of dephosphorylation, which is a mechanism used to regulate the enzyme's activity.

The Caenorhabditis elegans sex-determining protein FEM-2 and its human homologue, hFEM-2, are Ca2+/calmodulin-dependent protein kinase phosphatases that promote apoptosis

In Caenorhabditis elegans, fem-1, fem-2, and fem-3 play pivotal roles in sex determination. Recently, a mammalian homologue of the C. elegans sex-determining protein FEM-1, F1Aalpha, has been described. Although there is little evidence to link F1Aalpha to sex determination, F1Aalpha and FEM-1 both promote apoptosis in mammalian cells. Here we report the identification and characterization of a human homologue of the C. elegans sex-determining protein FEM-2, hFEM-2. Similar to FEM-2, hFEM-2 exhibited PP2C phosphatase activity and associated with FEM-3. hFEM-2 shows striking similarity (79% amino acid identity) to rat Ca(2+)/calmodulin (CaM)-dependent protein kinase phosphatase (rCaMKPase). hFEM-2 and FEM-2, but not PP2Calpha, were demonstrated to dephosphorylate CaM kinase II efficiently in vitro, suggesting that hFEM-2 and FEM-2 are specific phosphatases for CaM kinase. Furthermore, hFEM-2 and FEM-2 associated with F1Aalpha and FEM-1 respectively. Overexpression of hFEM-2, FEM-2, or rCaMKPase all mediated apoptosis in mammalian cells. The catalytically active, but not the inactive, forms of hFEM-2 induced caspase-dependent apoptosis, which was blocked by Bcl-XL or a dominant negative mutant of caspase-9. Taken together, our data suggest that hFEM-2 and rCaMKPase are mammalian homologues of FEM-2 and they are evolutionarily conserved CaM kinase phosphatases that may have a role in apoptosis signaling.

Calmodulin-dependent kinase II mediates T cell receptor/CD3- and phorbol ester-induced activation of IkappaB kinase

Numerous fundamental biological processes involve the NFkappaB family of transcription factors. The mechanisms by which this family of proteins is regulated are therefore of widespread importance. In most cells, NFkappaB is bound to inhibitory IkappaB proteins and sequestered in the cytoplasm. NFkappaB-inducing signals result in activation of a large multisubunit kinase complex, IKK, which phosphorylates IkappaB. IkappaB is subsequently degraded, releasing NFkappaB, which translocates to the nucleus. We previously reported that inhibitors of the calcium-binding protein calmodulin (CaM) prevent phorbol ester-induced phosphorylation of IkappaB. Here we show that KN93, an inhibitor of CaM-dependent kinases (CaMKs), also inhibits the phosphorylation of IkappaB. The effect of both CaM and CaMK inhibitors on IkappaB phosphorylation is due to the inhibition of the activity of CaMK II because neither drug has any effect when a derivative of CaMK II that is insensitive to these inhibitors is expressed. When CaMK II is inhibited, phorbol ester is no longer able to activate IKK, placing CaMK II in the signaling pathway that leads to IKK activation. CaM and CaMK inhibitors also block T cell receptor/CD3-induced activation but have no effect on the ability of the cytokine tumor necrosis factor alpha or the phosphatase inhibitor calyculin A to induce degradation of IkappaB. Finally we show that expression of a constitutively active CaMK II results in the activation of NFkappaB. The results identify CaMK II as a mediator of IKK activation specifically in response to T cell receptor/CD3 and phorbol ester stimulation.

Ca(2+)/CaM-dependent kinases: from activation to function

Calmodulin (CaM) is an essential protein that serves as a ubiquitous intracellular receptor for Ca(2+). The Ca(2+)/CaM complex initiates a plethora of signaling cascades that culminate in alteration of cellular functions. Among the many Ca(2+)/CaM-binding proteins to be discovered, the multifunctional protein kinases CaMKI, II, and IV play pivotal roles. Our review focuses on this class of CaM kinases to illustrate the structural and biochemical basis for Ca(2+)/CaM interaction with and regulation of its target enzymes. Gene transcription has been chosen as the functional endpoint to illustrate the recent advances in Ca(2+)/CaM-mediated signal transduction mechanisms.

Inhibition of PP-2A upregulates CaMKII in rat forebrain and induces hyperphosphorylation of tau at Ser 262/356

The regulation of the activity of CaMKII by PP-1 and PP-2A, as well as the role of this protein kinase in the phosphorylation of tau protein in forebrain were investigated. The treatment of metabolically active rat brain slices with 1.0 microM okadaic acid (OA) inhibited approximately 65% of PP-2A and had no significant effect on PP-1 in the 16000xg tissue extract. Calyculin A (CL-A), 0.1 microM under the same conditions, inhibited approximately 50% of PP-1 and approximately 20% of PP-2A activities. In contrast, a mixture of OA and CL-A practically completely inhibited both PP-2A and PP-1 activities. The inhibition of the two phosphatase activities or PP-2A alone resulted in an approximately 2-fold increase in CaMKII activity and an approximately 8-fold increase in the phosphorylation of tau at Ser 262/356 in 60 min. Treatment of the brain slices with KN-62, an inhibitor of the autophosphorylation of CaMKII at Thr 286/287, produced approximately 60% inhibition in CaMKII activity and no significant effect on tau phosphorylation at Ser 262/356. The KN-62-treated brain slices when further treated with OA and CL-A did not show any change in CaMKII activity. In vitro, both PP-2A and PP-1 dephosphorylated tau at Ser 262/356 that was phosphorylated with purified CaMKII. These studies suggest (i) that in mammalian forebrain the cytosolic CaMKII activity is regulated mainly by PP-2A, (ii) that CaMKII is the major tau Ser 262/356 kinase in brain, and (iii) that a decrease in PP-2A/PP-1 activities in the brain leads to hyperphosphorylation of tau not only by inhibition of its dephosphorylation but also by promoting the CaMKII activity.

Bistability in the Ca(2+)/calmodulin-dependent protein kinase-phosphatase system

A mathematical model is presented of autophosphorylation of Ca(2+)/calmodulin-dependent protein kinase (CaMKII) and its dephosphorylation by a phosphatase. If the total concentration of CaMKII subunits is significantly higher than the phosphatase Michaelis constant, two stable steady states of the CaMKII autophosphorylation can exist in a Ca(2+) concentration range from below the resting value of the intracellular [Ca(2+)] to the threshold concentration for induction of long-term potentiation (LTP). Bistability is a robust phenomenon, it occurs over a wide range of parameters of the model. Ca(2+) transients that switch CaMKII from the low-phosphorylated state to the high-phosphorylated one are in the same range of amplitudes and frequencies as the Ca(2+) transients that induce LTP. These results show that the CaMKII-phosphatase bistability may play an important role in long-term synaptic modifications. They also suggest a plausible explanation for the very high concentrations of CaMKII found in postsynaptic densities of cerebral neurons.

Close linkage between calcium/calmodulin kinase II alpha/beta and NMDA-2A receptors in the lateral amygdala and significance for retrieval of auditory fear conditioning

The general mechanism underlying memory and learning is an area under intense investigation and debate, yet this mechanism still remains elusive. Auditory fear conditioning (when a tone is paired with a foot shock) is a simple associative form of learning for which many mechanistic details are known. Lesions of the lateral/basolateral nuclei of the amygdala result in the selective impairment of fear conditioning, indicating that this is a key region for this type of learning. Fear conditioning induces a lasting synaptic potentiation in the lateral nuclei of the amygdala. In addition, recent results from several laboratories suggest that N-methyl-D-aspartate (NMDA) receptor activation in the amygdala is required for the acquisition and expression of cue-conditioned fear responses using several kinds of antagonists. Little is known, however, about the signal transduction pathway and molecular substrate underlying fear conditioning. Here we use NMDA receptor-deficient mice to demonstrate that calmodulin-dependent kinase II, CaMKIIbeta, and CaMKIIalpha activation involves the NR2A subunit in the lateral/basolateral amygdala during memory retrieval following auditory fear conditioning. These results suggest that auditory fear conditioning involves a close linkage between NMDA2A receptors and the CaMKII cascade.

Immunohistochemical study of the distribution of Ca(2+)/calmodulin-dependent protein kinase phosphatase in the rat central nervous system

Distribution of Ca(2+)/calmodulin-dependent protein kinase phosphatase (CaM-K Pase) which dephosphorylate multifunctional Ca(2+)/calmodulin-dependent protein kinases (CaM-kinases) in the rat brain and spinal cord were examined immunohistochemically by using an antibody against this enzyme. CaM-K Pase was localized only in the cytoplasm as has been investigated in PC 12 cells, and was never observed in the nucleus. Immunostainability varied from cell group to cell group. Mitral cells in the olfactory bulb, pyramidal neurons in the fifth layer of the cerebral cortex, hippocampal and striatal interneurons, dorsal and ventral pallidal, entopeduncular, and the reticular part of the substantia nigra neurons were intensely immunolabeled. Motoneurons in all the cranial nerve nuclei and the anterior horn of the spinal cord also revealed intense immunolabeling. On the contrary, pyramidal neurons in the Ammon's horn of the hippocampal formation, granule cells in the olfactory bulb, dentate gyrus and cerebellar cortex, Purkinje cells, neurons in the medial habenular nucleus and the inferior olivary nucleus have not shown immunoreactivity. Axons in the white matter or nerve root of the cranial nerve nuclei were immunolabeled. Glial cells in the white matter also showed immunostaining. Because the substrate of CaM-K Pase is multifunctional CaM-kinase II, I and IV, localization of each CaM-kinase was compared with that of CaM-K Pase. The distribution of CaM-K Pase and these CaM-kinases was found to overlap in various regions in the brain and spinal cord. It was concluded, therefore, that CaM-K Pase could regulate the activity of these CaM-kinases by dephosphorylation, when they existed together in neurons.

Mutant mice lacking ryanodine receptor type 3 exhibit deficits of contextual fear conditioning and activation of calcium/calmodulin-dependent protein kinase II in the hippocampus

As it is known that ryanodine receptor type 3 is expressed in the hippocampus, we examined the contribution of this receptor to contextual fear conditioning behavior and to the activation of Ca(2+)/calmodulin-dependent protein kinase II using mice lacking the receptor. Ryanodine receptor type 3-deficient mice exhibited impairments of performance in the contextual fear conditioning test, passive avoidance test, and Y-maze learning test. Both the activities of Ca(2+)/calmodulin-dependent protein kinase IIbeta and Ca(2+)/calmodulin-dependent protein kinase IIalpha were significantly increased in the experimental group compared to the control group in the hippocampus, but not in the cingulate cortex on the testing day 24 h after contextual fear training. However, the activities of Ca(2+)/calmodulin-dependent protein kinase IIbeta and alpha were almost the same in the experimental and control groups in the hippocampus on the training day. Ryanodine receptor type 3-deficient mice did not show the increment of Ca(2+)/calmodulin-dependent protein kinase IIbeta and alpha activities in the hippocampus on the testing day. In addition, these mutant mice showed the reduction of fear response in the elevated plus-maze test. The present results suggest that calcium-induced calcium release through the activation of ryanodine receptor type 3 in the hippocampus is important to the expression of the performance of contextual learning through the elevation of Ca(2+)/calmodulin-dependent protein kinase IIbeta and alpha activities.

Phosphorylation and activation of Ca2+/calmodulin-dependent protein kinase phosphatase by Ca2+/calmodulin-dependent protein kinase II

Ca2+/calmodulin-dependent protein kinase phosphatase (CaMKPase) is a protein phosphatase which dephosphorylates autophosphorylated Ca2+/calmodulin-dependent protein kinase II (CaMKII) and deactivates the enzyme (Ishida, A., Kameshita, I. and Fujisawa, H. (1998) J. Biol. Chem. 273, 1904-1910). In this study, a phosphorylation-dephosphorylation relationship between CaMKII and CaMKPase was examined. CaMKPase was not significantly phosphorylated by CaMKII under the standard phosphorylation conditions but was phosphorylated in the presence of poly-L-lysine, which is a potent activator of CaMKPase. The maximal extent of the phosphorylation was about 1 mol of phosphate per mol of the enzyme and the phosphorylation resulted in an about 2-fold increase in the enzyme activity. Thus, the activity of CaMKPase appears to be regulated through phosphorylation by its target enzyme, CaMKII.

Protein phosphatase 1 is involved in the dissociation of Ca2+/calmodulin-dependent protein kinase II from postsynaptic densities

Autophosphorylation-dependent translocation of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) to postsynaptic densities (PSDs) from cytosol may be a physiologically important process during synaptic activation. We investigated a protein phosphatase responsible for dephosphorylation of the kinase. CaM kinase II was shown to be targeted to two sites using the gel overlay method in two-dimensional gel electrophoresis. Protein phosphatase 1 (PP1) was identified to dephosphorylate CaM kinase II from its complex with PSDs using phosphatase inhibitors and activators, and purified phosphatases. The kinase was released from PSDs after its dephosphorylation by PP1.

Expression and characterization of PP7, a novel plant protein Ser/Thr phosphatase distantly related to RdgC/PPEF and PP5

We have recently identified an Arabidopsis thaliana cDNA encoding a putative protein Ser/Thr phosphatase PP7, not closely related to any protein phosphatases in animals or fungi. Here, we describe the characterization of PP7 expressed in a bacterial system. The recombinant protein was inactive unless the longest insert in its catalytic domain was cleaved, suggesting that this insert is an autoinhibitory region. PP7 was resistant to okadaic acid, calyculin and fumonisin B1, and was stimulated by Mn2+ or Fe2+, while Ni2+ and Zn2+ were inhibitory. Polylysine stimulated PP7 activity towards p-nitrophenylphosphate but inhibited activity towards the most efficient protein substrate, myelin basic protein. A tentative model of the control of PP7 activity is proposed.

The detection of enzyme activity following sodium dodecyl sulfate-polyacrylamide gel electrophoresis

More than a hundred different enzymes impinging on aspects of cell function ranging from carbohydrate and lipid metabolism to signal transduction and gene expression to biomolecule degradation have been detected by the assay of their enzymatic activities following SDS-PAGE. The strategies by which this has been accomplished are as varied as the enzymes themselves and offer testimony to the creativeness and ingenuity of life scientists. Assay of enzyme activity following SDS-PAGE is well adapted to identifying the source of catalytic activity in a heterogeneous protein mixture or a heterooligomeric protein (20), or determining if multiple catalytic activities reside in a single polypeptide (60). The alliance of versatile enzyme assay techniques with the molecular resolution of SDS-PAGE offers a powerful means for meeting the increasing demand for the high-throughput screening arising from protein engineering, combinatorial chemistry, and functional genomics.