STDP in a bistable synapse model based on CaMKII and associated signaling pathways

The calcium/calmodulin-dependent protein kinase II (CaMKII) plays a key role in the induction of long-term postsynaptic modifications following calcium entry. Experiments suggest that these long-term synaptic changes are all-or-none switch-like events between discrete states. The biochemical network involving CaMKII and its regulating protein signaling cascade has been hypothesized to durably maintain the evoked synaptic state in the form of a bistable switch. However, it is still unclear whether experimental LTP/LTD protocols lead to corresponding transitions between the two states in realistic models of such a network. We present a detailed biochemical model of the CaMKII autophosphorylation and the protein signaling cascade governing the CaMKII dephosphorylation. As previously shown, two stable states of the CaMKII phosphorylation level exist at resting intracellular calcium concentration, and high calcium transients can switch the system from the weakly phosphorylated (DOWN) to the highly phosphorylated (UP) state of the CaMKII (similar to a LTP event). We show here that increased CaMKII dephosphorylation activity at intermediate Ca²⁺ concentrations can lead to switching from the UP to the DOWN state (similar to a LTD event). This can be achieved if protein phosphatase activity promoting CaMKII dephosphorylation activates at lower Ca²⁺ levels than kinase activity. Finally, it is shown that the CaMKII system can qualitatively reproduce results of plasticity outcomes in response to spike-timing dependent plasticity (STDP) and presynaptic stimulation protocols. This shows that the CaMKII protein network can account for both induction, through LTP/LTD-like transitions, and storage, due to its bistability, of synaptic changes.     

A model of synaptic memory: a CaMKII/PP1 switch that potentiates transmission by organizing an AMPA receptor anchoring assembly

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is localized in the postsynaptic density (PSD) and is necessary for LTP induction. Much has been learned about the autophosphorylation of CaMKII and its dephosphorylation by PSD protein phosphatase-1 (PP1). Here, we show how the CaMKII/PP1 system could function as an energy-efficient, bistable switch that could be activated during LTP induction and remain active despite protein turnover. We also suggest how recently discovered binding interactions could provide a structural readout mechanism: the autophosphorylated state of CaMKII binds tightly to the NMDAR and forms, through CaMKII-actinin-actin-(4.1/SAP97) linkages, additional sites for anchoring AMPARs at synapses. The proposed model has substantial experimental support and elucidates principles by which a local protein complex could produce stable information storage and readout.     

The molecular basis of CaMKII function in synaptic and behavioural memory

Long-term potentiation (LTP) in the CA1 region of the hippocampus has been the primary model by which to study the cellular and molecular basis of memory. Calcium/calmodulin-dependent protein kinase II (CaMKII) is necessary for LTP induction, is persistently activated by stimuli that elicit LTP, and can, by itself, enhance the efficacy of synaptic transmission. The analysis of CaMKII autophosphorylation and dephosphorylation indicates that this kinase could serve as a molecular switch that is capable of long-term memory storage. Consistent with such a role, mutations that prevent persistent activation of CaMKII block LTP, experience-dependent plasticity and behavioural memory. These results make CaMKII a leading candidate in the search for the molecular basis of memory.     

The Delicate Bistability of CaMKII

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a synaptic, autophosphorylating kinase that is essential for learning and memory. Previous models have suggested that CaMKII functions as a bistable switch that could be the molecular correlate of long-term memory, but experiments have failed to validate these predictions. These models involved significant approximations to overcome the combinatorial complexity inherent in a multisubunit, multistate system. Here, we develop a stochastic particle-based model of CaMKII activation and dynamics that overcomes combinatorial complexity without significant approximations. We report four major findings. First, the CaMKII model system is never bistable at resting calcium concentrations, which suggests that CaMKII activity does not function as the biochemical switch underlying long-term memory. Second, the steady-state activation curves are either laserlike or steplike. Both are characterized by a well-defined threshold for activation, which suggests that thresholding is a robust feature of this system. Third, transiently activated CaMKII can maintain its activity over the time course of many experiments, and such slow deactivation may account for the few reports of bistability in the literature. And fourth, under in vivo conditions, increases in phosphatase activity can increase CaMKII activity. This is a surprising and counterintuitive effect, as dephosphorylation is generally associated with CaMKII deactivation.     

The Delicate Bistability of CaMKII

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a synaptic, autophosphorylating kinase that is essential for learning and memory. Previous models have suggested that CaMKII functions as a bistable switch that could be the molecular correlate of long-term memory, but experiments have failed to validate these predictions. These models involved significant approximations to overcome the combinatorial complexity inherent in a multisubunit, multistate system. Here, we develop a stochastic particle-based model of CaMKII activation and dynamics that overcomes combinatorial complexity without significant approximations. We report four major findings. First, the CaMKII model system is never bistable at resting calcium concentrations, which suggests that CaMKII activity does not function as the biochemical switch underlying long-term memory. Second, the steady-state activation curves are either laserlike or steplike. Both are characterized by a well-defined threshold for activation, which suggests that thresholding is a robust feature of this system. Third, transiently activated CaMKII can maintain its activity over the time course of many experiments, and such slow deactivation may account for the few reports of bistability in the literature. And fourth, under in vivo conditions, increases in phosphatase activity can increase CaMKII activity. This is a surprising and counterintuitive effect, as dephosphorylation is generally associated with CaMKII deactivation.     

Lobe-specific functions of Ca2+·calmodulin in alphaCa2+·calmodulin-dependent protein kinase II activation

N-methyl-D-aspartic acid receptor-dependent long term potentiation (LTP), a model of memory formation, requires Ca2+·calmodulin-dependent protein kinase II (αCaMKII) activity and Thr286 autophosphorylation via both global and local Ca2+ signaling, but the mechanisms of signal transduction are not understood. We tested the hypothesis that the Ca2+-binding activator protein calmodulin (CaM) is the primary decoder of Ca2+ signals, thereby determining the output, e.g. LTP. Thus, we investigated the function of CaM mutants, deficient in Ca2+ binding at sites 1 and 2 of the N-terminal lobe or sites 3 and 4 of the C-terminal CaM lobe, in the activation of αCaMKII. Occupancy of CaM Ca2+ binding sites 1, 3, and 4 is necessary and sufficient for full activation. Moreover, the N- and C-terminal CaM lobes have distinct functions. Ca2+ binding to N lobe Ca2+ binding site 1 increases the turnover rate of the enzyme 5-fold, whereas the C lobe plays a dual role; it is required for full activity, but in addition, via Ca2+ binding site 3, it stabilizes ATP binding to αCaMKII 4-fold. Thr286 autophosphorylation is also dependent on Ca2+ binding sites on both the N and the C lobes of CaM. As the CaM C lobe sites are populated by low amplitude/low frequency (global) Ca2+ signals, but occupancy of N lobe site 1 and thus activation of αCaMKII requires high amplitude/high frequency (local) Ca2+ signals, lobe-specific sensing of Ca2+-signaling patterns by CaM is proposed to explain the requirement for both global and local Ca2+ signaling in the induction of LTP via αCaMKII.     

Differential regulation of CaMKII inhibitor β protein expression after exposure to a novel context and during contextual fear memory formation

Understanding of the molecular basis of long‐term fear memory (fear LTM) formation provides targets in the treatment of emotional disorders. Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) is one of the key synaptic molecules involved in fear LTM formation. There are two endogenous inhibitor proteins of CaMKII, CaMKII Nα and Nβ, which can regulate CaMKII activity in vitro. However, the physiological role of these endogenous inhibitors is not known. Here, we have investigated whether CaMKII Nβ protein expression is regulated after contextual fear conditioning or exposure to a novel context. Using a novel CaMKII Nβ‐specific antibody, CaMKII Nβ expression was analysed in the naïve mouse brain as well as in the amygdala and hippocampus after conditioning and context exposure. We show that in naïve mouse forebrain CaMKII Nβ protein is expressed at its highest levels in olfactory bulb, prefrontal and piriform cortices, amygdala and thalamus. The protein is expressed both in dendrites and cell bodies. CaMKII Nβ expression is rapidly and transiently up‐regulated in the hippocampus after context exposure. In the amygdala, its expression is regulated only by contextual fear conditioning and not by exposure to a novel context. In conclusion, we show that CaMKII Nβ expression is differentially regulated by novelty and contextual fear conditioning, providing further insight into molecular basis of fear LTM.     

On the Mechanism of Synaptic Depression Induced by CaMKIIN,an Endogenous Inhibitor of CaMKII

Activity-dependent synaptic plasticity underlies, at least in part, learning and memory processes. NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) is a major synaptic plasticity model. During LTP induction, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is activated, autophosphorylated and persistently translocated to the postsynaptic density, where it binds to the NMDAR. If any of these steps is inhibited, LTP is disrupted. The endogenous CaMKII inhibitor proteins CaMKIINα,β are rapidly upregulated in specific brain regions after learning. We recently showed that transient application of peptides derived from CaMKIINα (CN peptides) persistently depresses synaptic strength and reverses LTP saturation, as it allows further LTP induction in previously saturated pathways. The treatment disrupts basal CaMKII-NMDAR interaction and decreases bound CaMKII fraction in spines. To unravel CaMKIIN function and to further understand CaMKII role in synaptic strength maintenance, here we more deeply investigated the mechanism of synaptic depression induced by CN peptides (CN-depression) in rat hippocampal slices. We showed that CN-depression does not require glutamatergic synaptic activity or Ca²⁺ signaling, thus discarding unspecific triggering of activity-dependent long-term depression (LTD) in slices. Moreover, occlusion experiments revealed that CN-depression and NMDAR-LTD have different expression mechanisms. We showed that CN-depression does not involve complex metabolic pathways including protein synthesis or proteasome-mediated degradation. Remarkably, CN-depression cannot be resolved in neonate rats, for which CaMKII is mostly cytosolic and virtually absent at the postsynaptic densities. Overall, our results support a direct effect of CN peptides on synaptic CaMKII-NMDAR binding and suggest that CaMKIINα,β could be critical plasticity-related proteins that may operate as cell-wide homeostatic regulators preventing saturation of LTP mechanisms or may selectively erase LTP-induced traces in specific groups of synapses.     

Surface dynamics of GluN2B‐NMDA receptors controls plasticity of maturing glutamate synapses

NMDA‐type glutamate receptors (NMDAR) are central actors in the plasticity of excitatory synapses. During adaptive processes, the number and composition of synaptic NMDAR can be rapidly modified, as in neonatal hippocampal synapses where a switch from predominant GluN2B‐ to GluN2A‐containing receptors is observed after the induction of long‐term potentiation (LTP). However, the cellular pathways by which surface NMDAR subtypes are dynamically regulated during activity‐dependent synaptic adaptations remain poorly understood. Using a combination of high‐resolution single nanoparticle imaging and electrophysiology, we show here that GluN2B‐NMDAR are dynamically redistributed away from glutamate synapses through increased lateral diffusion during LTP in immature neurons. Strikingly, preventing this activity‐dependent GluN2B‐NMDAR surface redistribution through cross‐linking, either with commercial or with autoimmune anti‐NMDA antibodies from patient with neuropsychiatric symptoms, affects the dynamics and spine accumulation of CaMKII and impairs LTP. Interestingly, the same impairments are observed when expressing a mutant of GluN2B‐NMDAR unable to bind CaMKII. We thus uncover a non‐canonical mechanism by which GluN2B‐NMDAR surface dynamics plays a critical role in the plasticity of maturing synapses through a direct interplay with CaMKII.     

Variation in assembly stoichiometry in non‐metazoan homologs of the hub domain of Ca2+/calmodulin‐dependent protein kinase II

The multi‐subunit Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) holoenzyme plays a critical role in animal learning and memory. The kinase domain of CaMKII is connected by a flexible linker to a C‐terminal hub domain that assembles into a 12‐ or 14‐subunit scaffold that displays the kinase domains around it. Studies on CaMKII suggest that the stoichiometry and dynamic assembly/disassembly of hub oligomers may be important for CaMKII regulation. Although CaMKII is a metazoan protein, genes encoding predicted CaMKII‐like hub domains, without associated kinase domains, are found in the genomes of some green plants and bacteria. We show that the hub domains encoded by three related green algae, Chlamydomonas reinhardtii, Volvox carteri f. nagarensis, and Gonium pectoral, assemble into 16‐, 18‐, and 20‐subunit oligomers, as assayed by native protein mass spectrometry. These are the largest known CaMKII hub domain assemblies. A crystal structure of the hub domain from C. reinhardtii reveals an 18‐subunit organization. We identified four intra‐subunit hydrogen bonds in the core of the fold that are present in the Chlamydomonas hub domain, but not in metazoan hubs. When six point mutations designed to recapitulate these hydrogen bonds were introduced into the human CaMKII‐α hub domain, the mutant protein formed assemblies with 14 and 16 subunits, instead of the normal 12‐ and 14‐subunit assemblies. Our results show that the stoichiometric balance of CaMKII hub assemblies can be shifted readily by small changes in sequence.     

Long-lasting LTP requires neither repeated trains for its induction nor protein synthesis for its development

Current thinking about LTP triggered in the area CA1 of hippocampal slices is ruled by two "dogmas": (1) A single train of high-frequency stimulation is sufficient to trigger short-lasting LTP (1-3 h), whereas multiple trains are required to induce long-lasting LTP (L-LTP, more than 4 h). (2) The development of the late phase of L-LTP requires the synthesis of new proteins. In this study, we found that a single high-frequency train could trigger an LTP lasting more than 8 h that was not affected by either anisomycin or cycloheximide (two inhibitors of protein synthesis). We ascertained that the induction of this L-LTP made use of the same mechanisms as those usually reported to be involved in LTP induction: it was dependent on NMDA receptors and on the activation of two "core" kinases, CaMKII and PI3K. These findings call into question the two "dogmas" about LTP.     

Genetic removal of eIF2α kinase PERK in mice enables hippocampal L‐LTP independent of mTORC1 activity

Characterization of the molecular signaling pathways underlying protein synthesis‐dependent forms of synaptic plasticity, such as late long‐term potentiation (L‐LTP ), can provide insights not only into memory expression/maintenance under physiological conditions but also potential mechanisms associated with the pathogenesis of memory disorders. Here, we report in mice that L‐LTP failure induced by the mammalian (mechanistic) target of rapamycin complex 1 (mTORC 1) inhibitor rapamycin is reversed by brain‐specific genetic deletion of PKR ‐like ER kinase, PERK (PERK KO ), a kinase for eukaryotic initiation factor 2α (eIF 2α). In contrast, genetic removal of general control non‐derepressible‐2, GCN 2 (GCN 2 KO ), another eIF 2α kinase, or treatment of hippocampal slices with the PERK inhibitor GSK 2606414, does not rescue rapamycin‐induced L‐LTP failure, suggesting mechanisms independent of eIF 2α phosphorylation. Moreover, we demonstrate that phosphorylation of eukaryotic elongation factor 2 (eEF 2) is significantly decreased in PERK KO mice but unaltered in GCN 2 KO mice or slices treated with the PERK inhibitor. Reduction in eEF 2 phosphorylation results in increased general protein synthesis, and thus could contribute to the mTORC 1‐independent L‐LTP in PERK KO mice. We further performed experiments on mutant mice with genetic removal of eEF 2K (eEF 2K KO ), the only known kinase for eEF 2, and found that L‐LTP in eEF 2K KO mice is insensitive to rapamycin. These data, for the first time, connect reduction in PERK activity with the regulation of translation elongation in enabling L‐LTP independent of mTORC 1. Thus, our findings indicate previously unrecognized levels of complexity in the regulation of protein synthesis‐dependent synaptic plasticity.

Read the Editorial Highlight for this article on page 119 . Cover Image for this issue: doi: 10.1111/jnc.14185 .

     

Long-term potentiation: outstanding questions and attempted synthesis

This article attempts an overview of the mechanism of NMDAR-dependent long-term potentiation (LTP) and its role in hippocampal networks. Efforts are made to integrate information, often in speculative ways, and to identify unresolved issues about the induction, expression and molecular storage processes. The pre/post debate about LTP expression has been particularly difficult to resolve. The following hypothesis attempts to reconcile the available physiological evidence as well as anatomical evidence that LTP increases synapse size. It is proposed that synapses are composed of a variable number of trans-synaptic modules, each having presynaptic release sites and a postsynaptic structure that can be AMPAfied by the addition of a hyperslot assembly that anchors 10-20 AMPA channels. According to a newly developed view of transmission, the quantal response is generated by AMPA channels near the site of vesicle release and so will depend on whether the module where release occurs has been AMPAfied. LTP expression may involve two structurally mediated processes: (i) the AMPAfication of existing modules by addition of hyperslot assemblies: this is a purely postsynaptic process and produces an increase in the probability of an AMPA response, with no change in the NMDA component; and (ii) the addition of new modules: this is a structurally coordinated pre/post process that leads to LTP-induced synapse enlargement and potentiation of the NMDA component owing to an increase in the number of release sites (the number of NMDA channels is assumed to be fixed). The protocol used for LTP induction appears to affect the proportion of these two processes; pairing protocols that involve low-frequency presynaptic stimulation induce only AMPAfication, making LTP purely postsynaptic, whereas high-frequency stimulation evokes both processes, giving rise to a presynaptic component. This model is capable of reconciling much of the seemingly contradictory evidence in the pre/post debate. The structural nature of the postulated changes is relevant to a second debate: whether a CaMKII switch or protein-dependent structural change is the molecular memory mechanism. A possible reconciliation is that a reversible CaMKII switch controls the construction of modules and hyperslot assemblies from newly synthesized proteins.     

神经颗粒素:一种脑特异性蛋白质

神经颗粒素(Neurogrann,Ng)是一种新发现的由78个氨基酸组成的脑特异性蛋白,主要分布于人类或动物的大脑皮层、海马和嗅球等脑区的神经突触后。作为Calpacitin蛋白家族中的一员,Ng是蛋白激酶C的天然作用底物及钙调蛋白(CaM)的储库。在生理状态下,Ng与CaM结合形成复合体,而在蛋白激酶C或氧化剂的作用下,Ng可被磷酸化、氧化及谷胱甘肽化等化学修饰,降低其与CaM的亲和力,从而参与对CaM及CaM-激活的蛋白酶,如CaM-依赖性NO合酶、CaM-依赖性蛋白激酶Ⅱ(CaMKⅡ)及CaM-依赖性腺苷酸环化酶的调节。同时,由于CaM-依赖性蛋白酶大多参与长时程增强(LTP)和长时程抑制(LTD)的诱导,并且Ng的基因表达和蛋白质合成与神经元的突触形成、分化同步,因此,Ng可能在学习、记忆、神经系统发育(可塑性)等生理性变化中具有重要作用。此外,一些研究表明,Ng还可能参与甲状腺机能减退、睡眠剥夺、衰老及脑低氧预适应等病理生理学变化所造成的神经系统功能的改变。     

Cytoskeletal signaling: is memory encoded in microtubule lattices by CaMKII phosphorylation?

Memory is attributed to strengthened synaptic connections among particular brain neurons, yet synaptic membrane components are transient, whereas memories can endure. This suggests synaptic information is encoded and 'hard-wired' elsewhere, e.g. at molecular levels within the post-synaptic neuron. In long-term potentiation (LTP), a cellular and molecular model for memory, post-synaptic calcium ion (Ca2?) flux activates the hexagonal Ca2?-calmodulin dependent kinase II (CaMKII), a dodacameric holoenzyme containing 2 hexagonal sets of 6 kinase domains. Each kinase domain can either phosphorylate substrate proteins, or not (i.e. encoding one bit). Thus each set of extended CaMKII kinases can potentially encode synaptic Ca2? information via phosphorylation as ordered arrays of binary 'bits'. Candidate sites for CaMKII phosphorylation-encoded molecular memory include microtubules (MTs), cylindrical organelles whose surfaces represent a regular lattice with a pattern of hexagonal polymers of the protein tubulin. Using molecular mechanics modeling and electrostatic profiling, we find that spatial dimensions and geometry of the extended CaMKII kinase domains precisely match those of MT hexagonal lattices. This suggests sets of six CaMKII kinase domains phosphorylate hexagonal MT lattice neighborhoods collectively, e.g. conveying synaptic information as ordered arrays of six "bits", and thus "bytes", with 64 to 5,281 possible bit states per CaMKII-MT byte. Signaling and encoding in MTs and other cytoskeletal structures offer rapid, robust solid-state information processing which may reflect a general code for MT-based memory and information processing within neurons and other eukaryotic cells.     

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.     

Parametric and genetic analysis of Drosophila appetitive long‐term memory and sugar motivation

Distinct forms of memory can be highlighted using different training protocols. In Drosophila olfactory aversive learning, one conditioning session triggers memory formation independently of protein synthesis, while five spaced conditioning sessions lead to the formation of long‐term memory (LTM), a long‐lasting memory dependent on de novo protein synthesis. In contrast, one session of odour–sugar association appeared sufficient for the fly to form LTM. We designed and tuned an apparatus that facilitates repeated discriminative conditioning by alternate presentations of two odours, one being associated with sugar, as well as a new paradigm to test sugar responsiveness (SR). Our results show that both SR and short‐term memory (STM) scores increase with starvation length before conditioning. The protein dependency of appetitive LTM is independent of the repetition and the spacing of training sessions, on the starvation duration and on the strength of the unconditioned stimulus. In contrast to a recent report, our test measures an abnormal SR of radish mutant flies, which might initiate their STM and LTM phenotypes. In addition, our work shows that crammer and tequila mutants, which are deficient for aversive LTM, present both an SR and an appetitive STM defect. Using the MB247‐P[switch] system, we further show that tequila is required in the adult mushroom bodies for normal sugar motivation.     

In?Vitro Reconstitution of a CaMKII Memory Switch by an NMDA Receptor-Derived Peptide

Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII) has been shown to play a major role in establishing memories through complex molecular interactions including phosphorylation of multiple synaptic targets. However, it is still controversial whether CaMKII itself serves as a molecular memory because of a lack of direct evidence. Here, we show that a single holoenzyme of CaMKII per se serves as an erasable molecular memory switch. We reconstituted Ca²⁺/Calmodulin-dependent CaMKII autophosphorylation in the presence of protein phosphatase 1 in vitro, and found that CaMKII phosphorylation shows a switch-like response with history dependence (hysteresis) only in the presence of an N-methyl-D-aspartate receptor-derived peptide. This hysteresis is Ca²⁺ and protein phosphatase 1 concentration-dependent, indicating that the CaMKII memory switch is not simply caused by an N-methyl-D-aspartate receptor-derived peptide lock of CaMKII in an active conformation. Mutation of a phosphorylation site of the peptide shifted the Ca²⁺ range of hysteresis. These functions may be crucial for induction and maintenance of long-term synaptic plasticity at hippocampal synapses.     

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. J. Cell. Biochem. 109: 585–597, 2010. © 2010 Wiley‐Liss, Inc.     

In Vitro Reconstitution of a CaMKII Memory Switch by an NMDA Receptor-Derived Peptide

Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII) has been shown to play a major role in establishing memories through complex molecular interactions including phosphorylation of multiple synaptic targets. However, it is still controversial whether CaMKII itself serves as a molecular memory because of a lack of direct evidence. Here, we show that a single holoenzyme of CaMKII per se serves as an erasable molecular memory switch. We reconstituted Ca²⁺/Calmodulin-dependent CaMKII autophosphorylation in the presence of protein phosphatase 1 in vitro, and found that CaMKII phosphorylation shows a switch-like response with history dependence (hysteresis) only in the presence of an N-methyl-D-aspartate receptor-derived peptide. This hysteresis is Ca²⁺ and protein phosphatase 1 concentration-dependent, indicating that the CaMKII memory switch is not simply caused by an N-methyl-D-aspartate receptor-derived peptide lock of CaMKII in an active conformation. Mutation of a phosphorylation site of the peptide shifted the Ca²⁺ range of hysteresis. These functions may be crucial for induction and maintenance of long-term synaptic plasticity at hippocampal synapses.