Deficient hippocampal long-term potentiation in alpha-calcium-calmodulin kinase II mutant mice

Focal adhesion kinase in the brain: novel subcellular localization and specific regulation by Fyn tyrosine kinase in mutant mice

Signaling by tyrosine kinases is required for the induction of synaptic plasticity in the central nervous system. Comparison of fyn, src, yes, and abl nonreceptor tyrosine kinase mutant mice shows a specific requirement for Fyn in the induction of long-term potentiation at CA1 synapses in the hippocampus. To identify components of a Fyn-dependent pathway that may be involved with hippocampus function we examined tyrosine-phosphorylated proteins in kinase mutant mice. We found that nine proteins were hypophosphorylated specifically in fyn mutants. One of the hypophosphorylated proteins was focal adhesion tyrosine kinase (FAK). FAK also showed reduced activity in immunocomplex kinase assays only in fyn mutants. FAK is expressed at very high levels in the brain but in contrast to non-neural cells, FAK was not restricted to focal adhesion contacts. FAK was found in axons, dendrites, and the intermediate filament cytoskeleton of astrocytes. Brain extracts from the mutants also show specific patterns of compensatory changes in the activity of the remaining Src family kinases. Tyrosine phosphorylation is a critical regulator of FAK, and impairments in FAK signal transduction in fyn mutants may contribute to the mutant neural phenotype.

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.

The involvement of metabotropic glutamate receptors and their intracellular signalling pathways in sustained nociceptive transmission in rat dorsal horn neurons

The excitatory responses of individual dorsal horn neurons to cutaneous brush, repeated application of the C-fibre-selective chemical algogen, mustard oil, or to ionophoretic (1S,3R)-ACPD [a metabotropic glutamate receptor (mGluR) agonist] were monitored by extracellular recording. We have previously shown that the responses of dorsal horn neurons to mustard oil are inhibited by several selective antagonists of mGluRs. Effects of ionophoresis of the mGluR antagonists (R,S)-CHPG and L-AP3 and a range of selective inhibitors of intracellular signalling pathways were examined on evoked responses here. The results suggest that protein kinase C, phospholipase A2 and perhaps Ca2+/calmodulin kinase II play a role in mediating the sustained elevated activity of dorsal horn neurons that is incrementally elicited by repeated application of mustard oil, but probably make little contribution to sustained brush-evoked activity. Concurrence in the sensitivity of mustard oil- and (1S,3R)-ACPD-evoked activity to (R,S)-CHPG, L-AP3 and to inhibitors of intracellular signalling pathways, suggests that mGluRs are an important origin of these intracellular signals required for sustained nociception.

PEST sequences in calmodulin-binding proteins

Many short-lived proteins which are devoid of proteolytic activity contain PEST sequences which are segments along the polypeptide chain that are rich in proline (P), glutamate (E), serine (S) and threonine (T). These designated PEST sequences are believed to be putative intramolecular signals for rapid proteolytic degradation. Calmodulin is a ubiquitous, 17 kDa, acidic Ca(2+)-binding protein which plays an important role in the regulation of many physiological processes through its interaction with a wide range of calmodulin-binding proteins. Several calmodulin-binding proteins are known to contain PEST sequences and are susceptible to proteolysis by endogenous neutral proteases such as calpain I and calpain II. In this report, we discuss the functions of PEST sequences in calmodulin-binding proteins and assess the correlation between calmodulin-binding proteins and PEST sequences.

Protein engineering and NMR studies of calmodulin

The calcium regulatory protein calmodulin (CaM) plays a role as an on-off switch in the activation of many enzymes and proteins. CaM has a dumbbell shaped structure with two folded domains, which are connected by a flexible linker in solution. The calmodulin-binding domains of the target proteins are contained in 20 residue long amino acid sequences, that share no obvious amino acid sequence homology. In this contribution, we discuss the features of CaM, which allow it to be rather promiscuous, and bind effectively to all these distinct domains. In particular, we describe the role of the methionine-rich hydrophobic surfaces of the protein in providing a malleable and sticky surface for binding many hydrophobic peptides. The enzyme activation properties of various Met --> Leu mutants of CaM are discussed. In addition, the role of the flexible linker region that connects the two domains is also analyzed. Finally, we describe various NMR and spectroscopic experiments that aid in determining the CaM-bound structures of synthetic peptides containing various CaM-binding domains. All structures analyzed to date are alpha-helical when bound to CaM, and they interact with CaM only through amino acid sidechains. This form of protein-protein interaction is rather unique, and may contribute to CaM's capacity to bind effectively to such a wide range of distinct partners.

Synaptic plasticity: stairway to memory

Since the idea that memory is associated with alterations in synaptic strength was accepted, studies on the cellular and molecular mechanisms responsible for the plastic changes in neurons have attracted wide interest in the scientific community. Recent studies on memory processes have also pointed out some unifying themes emerging from a wide range of nervous systems, suggesting that regardless of the species or brain regions, a common denominator for memory may exist. Thus, the present review attempted to create a hypothetical and universal synaptic model valid for a variety of nervous systems, ranging from molluscs to mammals. The cellular and molecular events leading to short- and long-term modifications of memory have been described in a sequential order, from the triggering signals to the gene expression, synthesis of new proteins and neuronal growth. These events are thought to represent the late phases of memory consolidation leading to persistent modifications in synaptic plasticity, thereby facilitating the permanent storage of acquired information throughout the individual's life.

The biochemistry of learning and memory

An overview of some of the biochemical and molecular events involved in the process of learning and memory are presented in a short review. Two invertebrate models of learning are considered: the gill-withdrawal reflex of Aplysia and avoidance learning in Drosophila melanogaster. Particular attention is paid to the biochemical mechanisms underlying both the development of long-term potentiation (LTP) and passive avoidance learning (PAL) in the young chick. The role of several biological molecules in learning and memory are considered, for example, protein kinase C (PKC), Ca(++)-Calmodulin kinase II (CaMKII), GAP-43, and glutamate receptors.

Limbic epilepsy in transgenic mice carrying a Ca2+/calmodulin-dependent kinase II alpha-subunit mutation

Multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMK) phosphorylates proteins pivotally involved in diverse neuronal processes and thereby coordinates cellular responses to external stimuli that regulate intracellular Ca2+ [Hanson, P. I. & Schulman, H. (1992) Annu. Rev. Biochem. 61, 559-664]. Despite extensive study, the impact of this enzyme on control of the excitability of neuron populations in the mammalian nervous system in situ is unknown. To address this question, we studied transgenic mice carrying a null mutation (-/-) for the alpha subunit of CaMK. In contrast to wild-type littermates, null mutants exhibit profound hyperexcitability, evident in epileptic seizures involving limbic structures including the hippocampus. No evidence of increased excitability was detected in mice carrying null mutations of the gamma isoform of protein kinase C, underscoring the specificity of the effect of CaMK. CaMK plays a powerful and previously underappreciated role in control of neuronal excitability in the mammalian nervous system. These insights have important implications for analyses of mechanisms of epilepsy and, perhaps, learning and memory.

N-methyl-D-aspartate and nitric oxide regulate the expression of calcium/calmodulin-dependent kinase II in the hippocampal dentate gyrus

Injection of small volumes of N-methyl-D-aspartate (NMDA) or Sin-1 molsidomine (a nitric oxide releasing agent) onto the dendrites of granule cells in the hippocampal dentate gyrus leads to changes in the level of expression of a number of genes. There is a fall in prodynorphin mRNA levels with a corresponding increase in proenkephalin mRNA levels. Similar changes in opioid gene expression occur following the induction of long-term potentiation (LTP). We report here that at short time periods (1-6 h) after injections of NMDA or sin-1 molsidomine, there is an increase in the levels of the mRNA encoding the alpha subunit of Ca2+/calmodulin-dependent protein kinase II (CaMKII alpha), consistent with a report of elevated CaMKII alpha mRNA in postsynaptic neurons in the CA1 region of the hippocampus following LTP induction [54]. However, we also report that 24 h after injection of NMDA or sin-1, there is a dramatic decrease in CaMKII alpha mRNA levels in the vicinity of the injection. This effect is specific for CaMKII alpha mRNA, in that many other mRNA species are not affected, and occurs in the dendritic population of CaMKII alpha mRNA as well as in the pool of mRNA in the granule cell bodies. The effect is blocked by an inhibitor of cGMP-dependent protein kinase. The biphasic regulation of CaMKII alpha mRNA may be of considerable functional importance for the long-term response of granule cells to local stimulation of NMDA receptors or NO release.

Spatial and temporal regulation of the rat calmodulin gene III directed by a 877-base promoter and 103-base leader segment in the mature and embryonal central nervous system of transgenic mice

Three non-allelic rat calmodulin (CaM) genes CaMI, CaMII and CaMIII, which share no homology in their 5'-upstream regions, are coordinately expressed in neurons of the central nervous system (CNS). Deletion analysis of the CaMIII promoter showed that the upstream segments longer than 700 bases functioned as efficient promoters, and that the sequence from -133 to -65 was required for the activity of house-keeping type promoter in transient expression assays on a mouse glioma cell line C6. However, the transient expression seemed not to be cell type specific. To determine the temporal and spatial specificity of the promoter function, we produced transgenic mice carrying a fusion gene of the CaMIII segment from -877 to +103 and the lacZ reporter gene. In CNS of the adult transgenic mice, the localization of transgene expression was similar to that of endogenous CaMIII transcripts analyzed by in situ hybridization. The transgene was expressed prominently in pyramidal cells of the cerebral neocortex and the hippocampal regions CA1 to CA3, in Purkinje cells of the cerebellar cortex, and in neurons of the spinal cord, and moderately in granule cells of the dentate gyrus and the cerebellar cortex. In the developing CNS, the overall profiles of neuron-specific expression were also similar for both transgene and endogenous CaMIII that were expressed in the mantle layer and the dorsal root ganglia of the embryonal spinal cord. These results indicated that the neuron-specific expression of rat CaMIII was directed by this 877-base promoter sequence. The CaMIII segment used for the promoter of transgene contained a 29-bp sequence at -410, namely H3, which was conserved in the upstream regions of vertebrate CaMII and CaMIII. H3 seemed to play a pivotal role in the temporal and spatial expression of transgene in CNS, although the deletion of H3 did not decrease CAT activity in the transient expression. The transgene expression was not observed in the external granular cells of the developing cerebellum and in some neurons of the embryonic sensory ganglia in which the endogenous CaMIII was obviously expressed. Therefore, the other cis-acting element(s) located outside of this 877-bp segment seemed to be required for the temporal regulation of CaMIII in certain rudimentary neurons.

The synaptic vesicle cycle: a cascade of protein-protein interactions

The synaptic vesicle cycle at the nerve terminal consists of vesicle exocytosis with neurotransmitter release, endocytosis of empty vesicles, and regeneration of fresh vesicles. Of all cellular transport pathways, the synaptic vesicle cycle is the fastest and the most tightly regulated. A convergence of results now allows formulation of molecular models for key steps of the cycle. These developments may form the basis for a mechanistic understanding of higher neural function.

Impairment of spatial but not contextual memory in CaMKII mutant mice with a selective loss of hippocampal LTP in the range of the theta frequency

We assessed hippocampal-dependent memory in mice with a Ca(2+)-independent form of CaMKII generated by the introduction of an aspartate at amino acid 286. The CaMKII-Asp-286 mice show normal LTP at high frequency stimulation, but in the 5-10 Hz range, they show a shift in the frequency-response curve favoring LTD. This range of frequencies is similar to the theta rhythm, which is associated with exploration in rodents. Using the Barnes maze to assess spatial memory, we found the transgenic mice could not learn to navigate to a specific location using spatial cues. In contrast, one line of transgenic mice performed normally in contextual fear conditioning, a task that is also hippocampal dependent. This dissociation between spatial and contextual memory suggests that even though both require the hippocampus, they may be mediated by different synaptic mechanisms.

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.

Micromolar Ca2+ transients in dendritic spines of hippocampal pyramidal neurons in brain slice

The magnitude and dynamics of [Ca2+] changes in spines and dendrites of hippocampal CA1 pyramidal neurons have been characterized using a low affinity fluorescent indicator, mag-Fura 5, that is sensitive to Ca2+ in the micromolar range. During tetanic stimulation (1 s), we observed progressive [Ca2+] increases in distal CA1 spines to as much as 20-40 microM, both in organotypic slice culture and acute slice. Similar accumulations were reached during continuous depolarization (+10 mV, 1 s) when K+ channels had been blocked, but not with spike trains driven by postsynaptic current injection. The large [Ca2+] increases due to tetanic stimulation were blocked by APV, indicating that NMDA receptor-dependent influx was critical for the large responses. These findings have significant implications for low affinity Ca(2+)-dependent biochemical processes and show a new upper limit for [Ca2+] changes measured in these neurons during stimulation.

Synaptic plasticity: hippocampal LTP

One of the most intensively studied forms of synaptic plasticity is long-term potentiation (LTP). The past year has seen further evidence advanced on both sides of the presynaptic/postsynaptic locus of expression debate, without an obvious path to reconcile the two views. Real progress has been made, however, in clarifying the possible role of nitric oxide as a retrograde messenger and the cellular location of its synthetic enzyme. Intriguing glimpses of the complex involvement of metabotropic glutamate receptors in the induction of LTP have also appeared.

Cell- and tissue-specific expression of putative protein kinase mRNAs in the embryonic leech, Hirudo medicinalis

Protein kinases play important roles in various cellular interactions underlying metazoan development. To complement existing analyses of protein kinase function in the development of members of the three phyla, Chordata, Arthropoda, and Nematoda, we have begun to examine the cell- and tissue-specific localization of protein kinases in another metazoan phylum, the Annelida. For this purpose, we used the polymerase chain reaction to amplify putative protein kinase catalytic domain cDNAs from the medicinal leech, Hirudo medicinalis. This strategy allowed us to identify 11 cytoplasmic and receptor tyrosine kinase catalytic domains, and 2 cytoplasmic serine/threonine kinase catalytic domains. Using these cDNAs as probes for nonradioactive whole-mount in situ hybridization, we examined the embryonic expression pattern of each of the corresponding putative kinase mRNAs. As has been found in other species, most of the Hirudo protein kinase mRNAs were expressed in a highly specific manner in certain embryonic cells and tissues. We found both neuron- and glia-specific kinases within the nervous system, as well as kinases expressed in non-nervous tissues, such as the haemocoelomic, muscular, and excretory systems. These kinase cDNAs encode proteins likely to be critical for proper development, and can be used as cell- and tissue-specific histological probes for the analysis of Hirudo embryogenesis.

Interaction of autophosphorylated Ca2+/calmodulin-dependent protein kinase II with neuronal cytoskeletal proteins. Characterization of binding to a 190-kDa postsynaptic density protein

Subcellular localization of Ca2+/calmodulin-dependent protein kinase II (CaMKII) by interaction with specific anchoring proteins may be an important mechanism contributing to the regulation of CaMKII. Proteins capable of binding CaMKII were identified by the use of a gel overlay assay with recombinant mouse CaMKII alpha (mCaMKII alpha) or Xenopus CaMKII beta (xCaMKII beta) 32P-autophosphorylated at Thr286/287 as a probe. Numerous [32P]CaMKII-binding proteins were identified in various whole rat tissue extracts, but binding was most prominent to forebrain proteins of 190 kDa (p190) and 140 kDa (p140). Fractionation of forebrain extracts localized p190 and p140 to a crude particulate/cytoskeletal fraction and isolated postsynaptic densities. [32P]m-CaMKII alpha-bound to p190 with an apparent Kd of 609 nM (subunit concentration) and a Bmax of 7.0 pmol of mCaMKII alpha subunit bound per mg of P2 protein, as measured using the overlay assay. Binding of 100 nM [32P]m-CaMKII alpha to p190 was competed by nonradioactive mCaMKII alpha autophosphorylated on Thr286 (EC50% = 200 nM), but to a much lesser extent by nonradioactive mCaMKII alpha autophosphorylated on Thr306 (EC50% > 2000 nM). In addition, nonphosphorylated mCaMKII alpha was a poor competitor for [32P]mCaMKII alpha binding to p190. The competition data indicate that Ca2+/CaM-dependent autophosphorylation at Thr286 promotes binding to p190, whereas, Ca2+/CaM-independent autophosphorylation at Thr306 does not enhance binding. Therefore, CaMKII may become localized to postsynaptic densities by p190 following its activation by an increase of dendritic Ca2+ concentration.

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.

Transgenic approaches to cognition

Reverse genetic techniques, including gene 'knockouts' and transgenesis, allow defined mutations to be introduced into the mouse genome. The application of these techniques to neurobiology is beginning to provide a bridge between genes and cognition. Specifically, genetically altered mice make it possible to explore molecular mechanisms underlying implicit and explicit forms of learning, short-term and long-term memory, and emotional behaviors. The analysis of these mutant mice has begun to link specific behavioral deficits to defined changes in synaptic physiology.

Increased phosphorylation of Ca2+/calmodulin-dependent protein kinase II and its endogenous substrates in the induction of long-term potentiation

Induction of long-term potentiation in the CA1 region of hippocampal slices is associated with increased activity of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) (Fukunaga, K., Stoppini, L., Miyamoto, E., and Muller, D. (1993) J. Biol. Chem. 268, 7863-7867). Here we report that application of high but not low frequency stimulation to two groups of afferents in the CA1 region of 32P-labeled slices resulted in the phosphorylation of two major substrates of this enzyme, synapsin I and microtubule-associated protein 2, as well as in the autophosphorylation of CaM kinase II. Furthermore, immunoblotting analysis revealed that long term potentiation induction was associated with an increase in the amount of CaM kinase II in the same region. All these changes were prevented when high frequency stimulation was applied in the presence of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonopentanoate. These results indicate that activation of CaM kinase II is involved in the induction of synaptic potentiation in both the postsynaptic and presynaptic regions.

Extracellular phosphorylation of membrane protein modifies theta burst-induced long-term potentiation in CA1 neurons of guinea-pig hippocampal slices

The involvement of ecto-protein kinase activity in activity-dependent long-term potentiation (LTP) was studied in CA1 neurons of guinea-pig hippocampal slices. Application of 5 microM K-252b, an ecto-protein kinase inhibitor, blocked LTP induced by a theta-burst stimulation (3 bursts composed of 5 pulses at 100 Hz with inter-burst intervals of 200 ms). On the other hand, under 10 microM RK682, an ecto-phosphatase inhibitor, a robust LTP was induced by a weak theta-burst stimulation (3 bursts composed of 3 pulses) which was just at the threshold for the induction of LTP in the control perfusate. These findings suggest that ATP released from presynaptic terminals during the burst stimulation plays an important role in the induction of LTP through phosphorylation of extracellular domains of synaptic membrane proteins, as the substrate for ecto-protein kinase.

The alpha-Ca2+/calmodulin kinase II: a bidirectional modulator of presynaptic plasticity

The alpha-Ca2+/calmodulin kinase II (alpha CaMKII) is required for long-term potentiation in the CA1 region of the hippocampus. Here, we report that this kinase also has a crucial role in presynaptic plasticity. Paired-pulse facilitation is blunted in the CA1 region of mice heterozygous for a targeted mutation of alpha CaMKII, confirming that this kinase can promote neurotransmitter release. Unexpectedly, field and whole-cell recordings of posttetanic potentiation show that the synaptic responses of mutants are larger than those of controls, indicating that alpha CaMKII can also inhibit transmitter release immediately after tetanic stimulation. Thus, alpha CaMKII has the capacity either to potentiate or to depress excitatory synaptic transmission depending on the pattern of presynaptic activation.

Dephosphorylated synapsin I anchors synaptic vesicles to actin cytoskeleton: an analysis by videomicroscopy

Synapsin I is a synaptic vesicle-associated protein which inhibits neurotransmitter release, an effect which is abolished upon its phosphorylation by Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Based on indirect evidence, it was suggested that this effect on neurotransmitter release may be achieved by the reversible anchoring of synaptic vesicles to the actin cytoskeleton of the nerve terminal. Using video-enhanced microscopy, we have now obtained experimental evidence in support of this model: the presence of dephosphorylated synapsin I is necessary for synaptic vesicles to bind actin; synapsin I is able to promote actin polymerization and bundling of actin filaments in the presence of synaptic vesicles; the ability to cross-link synaptic vesicles and actin is specific for synapsin I and is not shared by other basic proteins; the cross-linking between synaptic vesicles and actin is specific for the membrane of synaptic vesicles and does not reflect either a non-specific binding of membranes to the highly surface active synapsin I molecule or trapping of vesicles within the thick bundles of actin filaments; the formation of the ternary complex is virtually abolished when synapsin I is phosphorylated by CaM kinase II. The data indicate that synapsin I markedly affects synaptic vesicle traffic and cytoskeleton assembly in the nerve terminal and provide a molecular basis for the ability of synapsin I to regulate the availability of synaptic vesicles for exocytosis and thereby the efficiency of neurotransmitter release.

Identification of a Ca2+/calmodulin-dependent protein kinase II regulatory phosphorylation site in non-N-methyl-D-aspartate glutamate receptors

Glutamate receptor ion channels are colocalized in postsynaptic densities with Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II), which can phosphorylate and strongly enhance non-N-methyl-D-aspartate (NMDA) glutamate receptor current. In this study, CaM-kinase II enhanced kainate currents of expressed glutamate receptor 6 in 293 cells and of wild-type glutamate receptor 1, but not the Ser-627 to Ala mutant, in Xenopus oocytes. A synthetic peptide corresponding to residues 620-638 in GluR1 was phosphorylated in vitro by CaM-kinase II but not by cAMP-dependent protein kinase or protein kinase C. The 32P-labeled peptide map of this synthetic peptide appears to be the same as the two-dimensional peptide map of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) glutamate receptors phosphorylated in cultured hippocampal neurons by CaM-kinase II described elsewhere. This CaM-kinase II regulatory phosphorylation site is conserved in all AMPA/kainate-type glutamate receptors, and its phosphorylation may be important in enhancing postsynaptic responsiveness as occurs during synaptic plasticity.

Stabilization of calmodulin-dependent protein kinase II through the autoinhibitory domain

The active 30-kDa chymotryptic fragment of calmodulin-dependent protein kinase II (CaM kinase II), devoid of the autoinhibitory domain, and the enzyme, autothiophosphorylated at Thr286/Thr287, were much more labile than was the original native enzyme. They were markedly stabilized by synthetic peptides, designed after the sequence around the autophosphorylation site in the autoinhibitory domain, such as autocamtide-2 and CaMK-(281-309), but such marked stabilizations were not observed with the ordinary exogenous substrates, such as syntide-2. These results suggest that the autoinhibitory domain of CaM kinase II plays a crucial role in stabilizing the enzyme. A nonphosphorylatable analog of autocamtide-2, AIP, strongly inhibited the activity of the 30-kDa fragment. Kinetic analysis revealed that the inhibition by AIP was competitive with respect to autocamtide-2 and CaMK-(281-289) and noncompetitive with respect to syntide-2 and ATP/Mg2+, suggesting that CaM kinase II possesses at least two distinct substrate-binding sites; one for ordinary exogenous substrates such as syntide-2 and the other for an endogenous substrate, the autophosphorylation site (Thr286/Thr287) in the autoinhibitory domain. Fluorescence analysis of the binding of 7-nitrobenz-2-oxa-1,3-diazole-4-yl labeled AIP to the 30-kDa fragment also supported this contention. Thus, the autoinhibitory domain appears to play a crucial role in keeping the enzyme stable by binding to the substrate-binding site for the autophosphorylation site.

Reduced hippocampal LTP and spatial learning in mice lacking NMDA receptor epsilon 1 subunit

The NMDA (N-methyl-D-aspartate) receptor channel is important for synaptic plasticity, which is thought to underlie learning, memory and development. The NMDA receptor channel is formed by at least two members of the glutamate receptor (GluR) channel subunit families, the GluR epsilon (NR2) and GluR zeta (NR1) subunit families. The four epsilon subunits are distinct in distribution, properties and regulation. On the basis of the Mg2+ sensitivity and expression patterns, we have proposed that the epsilon 1 (NR2A) and epsilon 2 (NR2B) subunits play a role in synaptic plasticity. Here we show that targeted disruption of the mouse epsilon 1 subunit gene resulted in significant reduction of the NMDA receptor channel current and long-term potentiation at the hippocampal CA1 synapses. The mutant mice also showed a moderate deficiency in spatial learning. These results support the notion that the NMDA receptor channel-dependent synaptic plasticity is the cellular basis of certain forms of learning.

Altered behavior and long-term potentiation in type I adenylyl cyclase mutant mice

The murine Ca(2+)-stimulated adenylyl cyclase (type I) (EC 4.6.1.1), which is expressed predominantly in brain, was inactivated by targeted mutagenesis. Ca(2+)-stimulated adenylyl cyclase activity was reduced 40-60% in the hippocampus, neocortex, and cerebellum. Long-term potentiation in the CA1 region of the hippocampus from mutants was perturbed relative to controls. Both the initial slope and maximum extent of changes in synaptic response were reduced. Although mutant mice learned to find a hidden platform in the Morris water task normally, they did not display a preference for the region where the platform had been when it was removed. These results indicate that disruption of the gene for the type I adenylyl cyclase produces changes in behavior and that the cAMP signal transduction pathway may play an important role in synaptic plasticity.

Switching dynamics and the transient memory storage in a model enzyme network involving Ca2+/calmodulin-dependent protein kinase II in synapses

The signal processing through a chain of phosphorylation-dephosphorylations mediated by a pair of enzymes, Ca2+/calmodulin-dependent protein kinase II and the associated phosphatase, is formulated as a nonautonomous dynamical system in the framework of nonautocatalytic, intraholoenzyme reaction dynamics. A classification of switching characteristics of the system is made in the parameter space comprising the three controllable system parameters: an input-pulse intensity and initial concentrations of the two associated enzymes. It is found that a region of parameter space exists termed the transition zone, that exhibits a quasi-switching behaviour characterized by a signal storage time being prolonged by more than several orders of magnitude (10(4) times in certain cases) for the increase of two orders of magnitude in the input signal intensity. The effect of alterations of certain rate constants on the quasi-switching property is explored. It is numerically demonstrated that the Ca2+/calmodulin-dependent kinase II-related phosphatase is the most important key enzyme for regulating the signal storage time.

Decoding Ca2+ signals to the nucleus by multifunctional CaM kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is one of the major protein kinases coordinating cellular responses to neurotransmitters and hormones. CaM kinase transduces changes in intracellular free Ca2+ into changes in the phosphorylation state and activity of target proteins involved in neurotransmitter synthesis and release, neuronal plasticity and gene expression. Structure/function analyses of the kinase reveal the kinase is kept inactive in its basal state by a regulatory domain that is displaced by the binding of Ca2+/calmodulin. Once activated by Ca2+/calmodulin, autophosphorylation occurs if a pair of proximate subunits of the decameric kinase have calmodulin bound. The frequency of Ca2+ oscillations or spikes may be decoded by CaM kinase via this autophosphorylation. Calmodulin is essentially trapped by autophosphorylation which converts CaM kinase into a high affinity calmodulin-binding protein. Repetitive stimulation of the kinase may promote recruitment of calmodulin to the kinase so that it becomes increasingly active with each stimulus in a frequency-dependent manner. The association domain at the C-terminal end of CaM kinase contains a variable region that targets isoforms of the kinase to the nucleus or cytoskeleton and assembles the kinase into a decameric structure. Alternative splicing introduces a short nuclear localization signal that targets transfected kinase to the nucleus where it may regulate nuclear functions. The regulatory properties of CaM kinase provide for molecular potentiation of Ca2+ signals and frequency detection whereas its association domain should enable it to decode such Ca2+ fluctuations in the nucleus.

Overexpression of a calcium-binding protein, S100 beta, in astrocytes alters synaptic plasticity and impairs spatial learning in transgenic mice

Recent evidence suggests that slowly propagating Ca2+ waves from astrocytes can modulate the function of neurons. Altering astrocytic calcium processes in vivo may therefore affect neuronal and behavioral phenotypes. Previously, we generated transgenic mice that overexpress an astrocytic calcium-binding protein, S100 beta. Immunocytochemistry and in situ hybridization showed elevated expression in the astrocytes of the hippocampus and other brain regions. Neurons in the hippocampus were negative for S100 beta. In this paper we analyze the hippocampal electrophysiology and learning properties of mice from two transgenic lines. Significant differences were found between the hippocampal slices of normal and transgenic mice in their response to high frequency (100 Hz) stimulation. The overall distribution of post-tetanic excitatory postsynaptic potentials (EPSP) of the slices from the transgenic mice was shifted significantly toward smaller values to a degree that 25% of slices exhibited depression. The altered hippocampal neurophysiology was accompanied by an impairment in a hippocampal-dependent learning task. Transgenic mice showed significant impairment in a spatial version of the Morris water maze, however, they performed normally in non-spatial tasks. Probe trials showed that transgenic mice, though significantly impaired, also acquired spatial information. The results suggested that the impairment was not due to motor dysfunction, impaired vision or motivation of the transgenic mice, findings compatible with a possible hippocampal mechanism. We conclude that overexpression of S100 beta in astrocytes impairs, but does not abolish, the ability to solve a spatial task, and it leads to a significantly decreased post-tetanic potentiation in the hippocampal slice. We hypothesize that the changes are due to calcium mediated processes. Our results support the notion that astrocytes are involved in higher brain functions.

Potentiated transmission and prevention of further LTP by increased CaMKII activity in postsynaptic hippocampal slice neurons

Calcium-calmodulin-dependent protein kinase II (CaMKII) is a necessary component of the cellular machinery underlying learning and memory. Here, a constitutively active form of this enzyme, CaMKII(1-290), was introduced into neurons of hippocampal slices with a recombinant vaccinia virus to test the hypothesis that increased postsynaptic activity of this enzyme is sufficient to produce long-term synaptic potentiation (LTP), a prominent cellular model of learning and memory. Postsynaptic expression of CaMKII(1-290) increased CaMKII activity, enhanced synaptic transmission, and prevented more potentiation by an LTP-inducing protocol. These results, together with previous studies, suggest that postsynaptic CaMKII activity is necessary and sufficient to generate LTP.

Genetic studies in alcohol research

The National Institute on Alcohol Abuse and Alcoholism (NIAAA) supports research to elucidate the specific genetic factors, now largely unknown, which underlie susceptibility to alcoholism and its medical complications (including fetal alcohol syndrome). Because of the genetic complexity and heterogeneity of alcoholism, identification of the multiple underlying factors will require the development of new study designs and methods of analysis of data from human families. While techniques of genetic analysis of animal behavioral traits (e.g., targeted gene disruption, quantitative trait locus (QTL) mapping) are more powerful than those applicable to humans (e.g., linkage and allelic association studies), the validation of animal behaviors as models of aspects of human alcoholism has been problematic. Newly developed methods for mapping QTL influencing animal behavioral traits can not only permit analyses of human family data to be directly informed by the results of animal studies, but can also serve as a novel means of validating animal models of aspects of alcoholism.

Concomitant alterations of physiological and developmental plasticity in Drosophila CaM kinase II-inhibited synapses

Ca2+/calmodulin-dependent protein kinase II (CaM kinase) has been implicated in neural plasticity that underlies learning and memory processes. Transformed strains of Drosophila, ala1 and ala2, expressing a specific inhibitor of CaM kinase are known to be impaired in an associative conditioning behavioral paradigm. We found that these transformants had altered short-term plasticity in synaptic transmission along with abnormal nerve terminal sprouting and directionality of outgrowth. These results represent an interesting parallel with the activity-dependent regulation of synaptic physiology and morphology by the cAMP cascade in Aplysia and Drosophila. In contrast to the learning mutants dunce and rutabaga, which are defective in the cAMP cascade, inhibition of CaM kinase in ala transformants caused increased sprouting at larval neuromuscular junctions near the nerve entry point, rather than altering the higher order branch segments. In addition, synaptic facilitation and potentiation were altered in a manner different from that observed in the cAMP mutants. Furthermore, synaptic currents in ala transformants were characterized by greater variability, suggesting an important role of CaM kinase in the stability of transmission.

Neuronal transfection in brain slices using particle-mediated gene transfer

Difficulties in neuronal transfection continue to restrict the applicability of molecular approaches to neurobiology. Conventional transfection techniques have been of limited effectiveness, particularly in intact neural tissues. Viral vectors effectively transfect neurons both in vitro and in vivo but are labor intensive to construct, difficult to control, and often compromise cell viability. We describe here an alternative strategy using particle-mediated gene transfer for the transfection of neurons and glia in intact brain slices. This approach is efficient, reliable, and does not require advanced molecular biological facilities for its application.

Haloperidol-induced morphological alterations are associated with changes in calcium/calmodulin kinase II activity and glutamate immunoreactivity

Administration of haloperidol for 2 weeks causes an increase within the caudate nucleus of asymmetrical synapses associated with a discontinuous or perforated, postsynaptic density (PSD) [Meshul et al. (1992), Psychopharmacology, 106:45-52; Meshul et al. (1992), Neuropsychopharmacology, 7:285-293]. Coadministration of the N-methyl-D-aspartate noncompetitive antagonist, MK-801, with haloperidol blocked the increase in striatal synapses containing a perforated PSD [Meshul et al. (1994), Brain Res., 648:181-195]. Examination of the caudate using immuno-gold electron microscopy revealed the vast majority (90%) of asymmetrical synapses were labelled with a glutamate antibody [Meshul et al. (1994), Brain Res., 648:181-195]. The purpose of this study was to determine if there were any changes in the density of glutamate immunoreactivity within presynaptic terminals of asymmetric synapses within the striatum following treatment with haloperidol for 1 month that would correlate with the previously observed increase in synapses with perforated PSDs. We also determined the activity of striatal calcium/calmodulin kinase II (CaMK II), an enzyme known to be localized within the synaptic region, after administration of haloperidol. We report here that haloperidol causes an increase in the activity of CaMK II and a decrease in the density of immuno-gold labelling for glutamate within the nerve terminals of asymmetrical synapses containing a perforated or nonperforated PSD. These results are consistent with the hypothesis that the haloperidol-induced increase in activity of CaMK II and the increase in glutamate release, as suggested by the decrease in presynaptic glutamate immunoreactivity, may ultimately lead to an increase in the number of synapses displaying a perforated PSD. These results support the speculation that the haloperidol-induced increase in synapses containing a perforated PSD may be associated with enhanced activity at excitatory synapses.

Changes in reliability of synaptic function as a mechanism for plasticity

Synaptic transmission in the hippocampus is rather unreliable, with many presynaptic action potentials failing to release neurotransmitter. How is this unreliability affected by the alterations in synaptic strength seen in long-term potentiation (LTP) and long-term depression (LTD)? We find that LTP increases synaptic reliability, and LTD decreases it, both without a change in the size of those postsynaptic currents that do occur. Thus LTD is a functional inverse of LTP.