Purification and characterization of active fragment of Ca2+/calmodulin-dependent protein kinase II from the post-synaptic density in the rat forebrain

CaMKIIα as a Promising Drug Target for Ischemic Grey Matter

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a major mediator of Ca2+-dependent signaling pathways in various cell types throughout the body. Its neuronal isoform CaMKIIα (alpha) centrally integrates physiological but also pathological glutamate signals directly downstream of glutamate receptors and has thus emerged as a target for ischemic stroke. Previous studies provided evidence for the involvement of CaMKII activity in ischemic cell death by showing that CaMKII inhibition affords substantial neuroprotection. However, broad inhibition of this central kinase is challenging because various essential physiological processes like synaptic plasticity rely on intact CaMKII regulation. Thus, specific strategies for targeting CaMKII after ischemia are warranted which would ideally only interfere with pathological activity of CaMKII. This review highlights recent advances in the understanding of how ischemia affects CaMKII and how pathospecific pharmacological targeting of CaMKII signaling could be achieved. Specifically, we discuss direct targeting of CaMKII kinase activity with peptide inhibitors versus indirect targeting of the association (hub) domain of CaMKIIα with analogues of γ-hydroxybutyrate (GHB) as a potential way to achieve more specific pharmacological modulation of CaMKII activity after ischemia.

Calpain-Mediated Alterations in Astrocytes Before and During Amyloid Chaos in Alzheimer's Disease

One of the changes found in the brain in Alzheimer's disease (AD) is increased calpain, derived from calcium dysregulation, oxidative stress, and/or neuroinflammation, which are all assumed to be basic pillars in neurodegenerative diseases. The role of calpain in synaptic plasticity, neuronal death, and AD has been discussed in some reviews. However, astrocytic calpain changes sometimes appear to be secondary and consequent to neuronal damage in AD. Herein, we explore the possibility of calpain-mediated astroglial reactivity in AD, both preceding and during the amyloid phase. We discuss the types of brain calpains but focus the review on calpains 1 and 2 and some important targets in astrocytes. We address the signaling involved in controlling calpain expression, mainly involving p38/mitogen-activated protein kinase and calcineurin, as well as how calpain regulates the expression of proteins involved in astroglial reactivity through calcineurin and cyclin-dependent kinase 5. Throughout the text, we have tried to provide evidence of the connection between the alterations caused by calpain and the metabolic changes associated with AD. In addition, we discuss the possibility that calpain mediates amyloid-β clearance in astrocytes, as opposed to amyloid-β accumulation in neurons.

Stimulation-induced changes in diffusion and structure of calmodulin and calmodulin-dependent protein kinase II proteins in neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) and calmodulin (CaM) play essential roles in synaptic plasticity, which is an elementary process of learning and memory. In this study, fluorescence correlation spectroscopy (FCS) revealed diffusion properties of CaM, CaMKIIα and CaMKIIβ proteins in human embryonic kidney 293 (HEK293) cells and hippocampal neurons. A simultaneous multiple-point FCS recording system was developed on a random-access two-photon microscope, which facilitated efficient analysis of molecular dynamics in neuronal compartments. The diffusion of CaM in neurons was slower than that in HEK293 cells at rest, while the diffusion in stimulated neurons was accelerated and indistinguishable from that in HEK293 cells. This implied that activity-dependent binding partners of CaM exist in neurons, which slow down the diffusion at rest. Diffusion properties of CaMKIIα and β proteins implied that major populations of these proteins exist as holoenzymatic forms. Upon stimulation of neurons, the diffusion of CaMKIIα and β proteins became faster with reduced particle brightness, indicating drastic structural changes of the proteins such as dismissal from holoenzyme structure and further fragmentation.

The synaptic proteome

Synapses are focal hot spots for signal transduction and plasticity in the brain. A synapse comprises an axon terminus, the presynapse, the synaptic cleft containing extracellular matrix proteins as well as adhesion molecules, and the postsynaptic density as target structure for chemical signaling. The proteomes of the presynaptic and postsynaptic active zones control neurotransmitter release and perception. These tasks demand short- and long-term structural and functional dynamics of the synapse mediated by its proteinaceous inventory. This review addresses subcellular fractionation protocols and the related proteomic approaches to the various synaptic subcompartments with an emphasis on the presynaptic active zone (PAZ). Furthermore, it discusses major constituents of the PAZ including the amyloid precursor protein family members. Numerous proteins regulating the rearrangement of the cytoskeleton are indicative of the functional and structural dynamics of the pre- and postsynapse. The identification of protein candidates of the synapse provides the basis for further analyzing the interaction of synaptic proteins with their targets, and the effect of their deletion opens novel insights into the functional role of these proteins in neuronal communication. The knowledge of the molecular interactome is also a prerequisite for understanding numerous neurodegenerative diseases.

Contribution of calpains to myocardial ischaemia/reperfusion injury

Loss of calcium (Ca(2+)) homeostasis contributes through different mechanisms to cell death occurring during the first minutes of reperfusion. One of them is an unregulated activation of a variety of Ca(2+)-dependent enzymes, including the non-lysosomal cysteine proteases known as calpains. This review analyses the involvement of the calpain family in reperfusion-induced cardiomyocyte death. Calpains remain inactive before reperfusion due to the acidic pHi and increased ionic strength in the ischaemic myocardium. However, inappropriate calpain activation occurs during myocardial reperfusion, and subsequent proteolysis of a wide variety of proteins contributes to the development of contractile dysfunction and necrotic cell death by different mechanisms, including increased membrane fragility, further impairment of Na(+) and Ca(2+) handling, and mitochondrial dysfunction. Recent studies demonstrating that calpain inhibition contributes to the cardioprotective effects of preconditioning and postconditioning, and the beneficial effects obtained with new and more selective calpain inhibitors added at the onset of reperfusion, point to the potential cardioprotective value of therapeutic strategies designed to prevent calpain activation.

Network, cellular, and molecular mechanisms underlying long-term memory formation

The neural network stores information through activity-dependent synaptic plasticity that occurs in populations of neurons. Persistent forms of synaptic plasticity may account for long-term memory storage, and the most salient forms are the changes in the structure of synapses. The theory proposes that encoding should use a sparse code and evidence suggests that this can be achieved through offline reactivation or by sparse initial recruitment of the network units. This idea implies that in some cases the neurons that underwent structural synaptic plasticity might be a subpopulation of those originally recruited; However, it is not yet clear whether all the neurons recruited during acquisition are the ones that underwent persistent forms of synaptic plasticity and responsible for memory retrieval. To determine which neural units underlie long-term memory storage, we need to characterize which are the persistent forms of synaptic plasticity occurring in these neural ensembles and the best hints so far are the molecular signals underlying structural modifications of the synapses. Structural synaptic plasticity can be achieved by the activity of various signal transduction pathways, including the NMDA-CaMKII and ACh-MAPK. These pathways converge with the Rho family of GTPases and the consequent ERK 1/2 activation, which regulates multiple cellular functions such as protein translation, protein trafficking, and gene transcription. The most detailed explanation may come from models that allow us to determine the contribution of each piece of this fascinating puzzle that is the neuron and the neural network.

Calcium/calmodulin dependent protein kinase II bound to NMDA receptor 2B subunit exhibits increased ATP affinity and attenuated dephosphorylation

Calcium/calmodulin dependent protein kinase II (CaMKII) is implicated to play a key role in learning and memory. NR2B subunit of N-methyl-D-aspartate receptor (NMDAR) is a high affinity binding partner of CaMKII at the postsynaptic membrane. NR2B binds to the T-site of CaMKII and modulates its catalysis. By direct measurement using isothermal titration calorimetry (ITC), we show that NR2B binding causes about 11 fold increase in the affinity of CaMKII for ATPγS, an analogue of ATP. ITC data is also consistent with an ordered binding mechanism for CaMKII with ATP binding the catalytic site first followed by peptide substrate. We also show that dephosphorylation of phospho-Thr(286)-α-CaMKII is attenuated when NR2B is bound to CaMKII. This favors the persistence of Thr(286) autophosphorylated state of CaMKII in a CaMKII/phosphatase conjugate system in vitro. Overall our data indicate that the NR2B- bound state of CaMKII attains unique biochemical properties which could help in the efficient functioning of the proposed molecular switch supporting synaptic memory.

Regulation of calpain-2 in neurons: implications for synaptic plasticity

The family of calcium-dependent neutral proteases, calpains, was discovered more than 30 years ago, but their functional roles in the nervous system under physiological or pathological conditions still remain unclear. Although calpain was proposed to participate in synaptic plasticity and in learning and memory in the early 1980s, the precise mechanism regarding its activation, its target(s) and the functional consequences of its activation have remained controversial. A major issue has been the identification of roles of the two major calpain isoforms present in the brain, calpain-1 and calpain-2, and the calcium requirement for their activation, which exceeds levels that could be reached intracellularly under conditions leading to changes in synaptic efficacy. In this review, we discussed the features of calpains that make them ideally suited to link certain patterns of presynaptic activity to the structural modifications of dendritic spines that could underlie synaptic plasticity and learning and memory. We then summarize recent findings that provide critical answers to the various questions raised by the initial hypothesis, and that further support the idea that, in brain, calpain-2 plays critical roles in developmental and adult synaptic plasticity.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Molecular Mechanism of Learning and Memory Based on the Research for Ca2+/Calmodulin-dependent Protein Kinase II

In the central nervous system (CNS), the synapse is a specialized junctional complex by which axons and dendrites emerging from different neuron intercommunicates. Changes in the efficiency of synaptic transmission are important for a number of aspects of neural function. Much has been learned about the activity-dependent synaptic modifications that are thought to underlie memory storage, but the mechanism by which these modifications are stored remains unclear. Thus, it is important to find and characterize "memory molecules," and "memory apparatus or memory forming apparatus." A good candidate for the storage mechanism is Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II). CaM kinase II is one of the most prominent protein kinases, present in essentially every tissue but most concentrated in the brain. Neuronal CaM kinase II regulates important neuronal functions, including neurotransmitter synthesis, neurotransmitter release, modulation of ion channel activity, cellular transport, cell morphology and neurite extension, synaptic plasticity, learning and memory, and gene expression. Studies concerning this kinase open a door of the molecular basis of nerve function, especially learning and memory, and indicate one direction for the studies in the field of neuroscience. This review presents molecular structure, properties and functions of CaM kinase II, as a major component of neuron, which are mainly developed in our laboratory.

Calpain and synaptic function

Proteolysis by calpain is a unique posttranslational modification that can change integrity, localization, and activity of endogenous proteins. Two ubiquitous calpains, mu-calpain and m-calpain, are highly expressed in the central nervous system, and calpain substrates such as membrane receptors, postsynaptic density proteins, kinases, and phosphatases are localized to the synaptic compartments of neurons. By selective cleavage of synaptically localized molecules, calpains may play pivotal roles in the regulation of synaptic processes not only in physiological states but also during various pathological conditions. Activation of calpains during sustained synaptic activity is crucial for Ca2+-dependent neuronal functions, such as neurotransmitter release, synaptic plasticity, vesicular trafficking, and structural stabilization. Overactivation of calpain following dysregulation of Ca2+ homeostasis can lead to neuronal damage in response to events such as epilepsy, stroke, and brain trauma. Calpain may also provide a neuroprotective effect from axotomy and some forms of glutamate receptor overactivation. This article focuses on recent findings on the role of calpain-mediated proteolytic processes in potentially regulating synaptic substrates in physiological and pathophysiological events in the nervous system.

Study on the Regulation of Synaptic Function by Ca2+/Calmodulin-dependent Protein Kinase II

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is one of the most abundant protein kinases in the mammalian brain, especially in the hippocampus. Neuronal CaMKII is a multifunctional mediator of activity dependent on an increase in the Ca(2+) level in excitable cells. It plays an important role in synaptic plasticity, including learning and memory, and is recognized as a "memory molecule." The expression of the kinase increases most rapidly during the most active phase in the formation of synapses in the postnatal brain and remains at a high level after synaptic maturation, indicating that the kinase is carefully regulated in the space-temporal gene expression. It is accumulated in the postsynaptic density (PSD), which is central in synaptic transmission. This review presents the gene expression and alternative splicing of CaMKII during neural differentiation, molecular constituents of PSD, and regulation of CaMKII by activity-regulated cytoskeleton-associated protein (Arc) mainly developed in our study.

High level expression and preparation of autonomous Ca2+/calmodulin-dependent protein kinase II in Escherichia coli

The chymotryptic fragment of Ca2+/calmodulin-dependent protein kinase II (30K-CaMKII) is a constitutively active enzyme that phosphorylates a variety of protein substrates in vitro. Although 30K-CaMKII is an often used and powerful tool for protein phosphorylation, the efficient production of catalytically active 30K-CaMKII in Escherichia coli has not yet been successfully realized, probably due to its toxicity in host cells. In this study, we found that a high-level expression of 30K-CaMKII as an insoluble form was attained when the N-terminal 43 amino acid residues of Xenopus CaMKI were fused to the N-terminal end of 30K-CaMKII (CX-30K-CaMKII). The inactive CX-30K-CaMKII thus expressed in E. coli was reactivated by simple denaturation/renaturation processes and purified on a Ni2+-chelating column. The renatured CX-30K-CaMKII exhibited specific activity similar to that of rat brain CaMKII, and phosphorylated various proteins such as histones, myosin light chain, myelin basic protein, and synapsin I, as in case of 30K-CaMKII or purified CaMKII. Thus, CX-30K-CaMKII, an autonomous CaMKII, can be obtained with a simple procedure using E. coli expression system.

Interaction of Arc with CaM kinase II and stimulation of neurite extension by Arc in neuroblastoma cells expressing CaM kinase II

We investigated the relationship between Arc (activity-regulated cytoskeleton-associated protein) and Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II). Arc and CaM kinase II were concentrated in the postsynaptic density. These proteins were accumulated after electroconvulsive treatment. Arc increased about 2.5-fold within 30 min and was maintained at this level for 8h after the stimulation. CaM kinase II also increased within 30 min and remained at this level for at least 24h. The interaction of Arc with CaM kinase II was demonstrated using GST-Arc fusion protein, and confirmed in neuroblastoma cells by immunoprecipitation. We examined the function of Arc by introducing Arc cDNA into neuroblastoma cells expressing CaM kinase II. The cells expressing both Arc and CaM kinase II had longer neurites than those expressing CaM kinase II alone. Arc itself did not promote neurite outgrowth. The growth of neurites by Arc was completely blocked by treatment with KN62, an inhibitor of CaM kinases. These results indicated that Arc potentiated the action of CaM kinase II for neurite extension.

Molecular constituents and phosphorylation-dependent regulation of the post-synaptic density

The post-synaptic density (PSD) contains receptors with associated signaling- and scaffolding-proteins that organize signal-transduction pathways near the post-synaptic membrane. The PSD plays an important role in synaptic plasticity, and protein phosphorylation is critical to the regulation of PSD function, including learning and memory. Recently, studies have investigated the protein constituents of the PSD and substrate proteins for various protein kinases by proteomic analysis. The present review focuses on the molecular properties of PSD proteins, and substrates of protein kinases and their regulation by phosphorylation in order to understand the role of PSD in synaptic plasticity.

Investigation of protein substrates of Ca(2+)/calmodulin-dependent protein kinase II translocated to the postsynaptic density

To elucidate the physiological significance of the translocation of Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II), we investigated substrates of CaM kinase II in the postsynaptic density (PSD). PSD proteins were phosphorylated by CaM kinase II of its PSD complex, and separated by two-dimensional gel electrophoresis. More than 28 proteins were phosphorylated under experimental conditions. Proteins corresponding to CaM kinase II substrates were excised from the gels, eluted electrophoretically, and then sequenced. Several substrates were identified, including PSD95, SAP90, alpha-internexin, neurofilament L chain, cAMP phosphodiesterase, and alpha- and beta-tubulin. Some substrates were also identified by immunoblotting, including N-methyl-D-aspartic acid (NMDA) receptor 2B subunit, 1-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor 1 (GluR1), neurofilament H chain and dynamin. PSD95, SAP90, dynamin, and alpha-internexin were demonstrated for the first time to be substrates of CaM kinase II. NMDA receptor 2B subunit and GluR1 existed as major substrates in the PSD. Moreover, translocation of CaM kinase II was inhibited by phosphorylation of PSD proteins. These results suggest that CaM kinase II plays important roles in the regulation of synaptic functions through phosphorylation of PSD proteins.

Calpain and caspase: can you tell the difference?

Both necrotic and apoptotic neuronal death are observed in various neurological and neurodegenerative disorders. Calpain is activated in various necrotic and apoptotic conditions, while caspase 3 is only activated in neuronal apoptosis. Despite the difference in cleavage-site specificity, an increasing number of cellular proteins are found to be dually susceptible to these cysteine proteases. These include alpha- and beta-fodrin, calmodulin-dependent protein kinases, ADP-ribosyltransferase (ADPRT/PARP) and tau. Intriguingly, calpastatin is susceptible to caspase-mediated fragmentation. Neurotoxic challenges such as hypoxia-hypoglycemia, excitotoxin treatment or metabolic inhibition of cultured neurons result in activation of both proteases. Calpain inhibitors can protect against necrotic neuronal death and, to a lesser extent, apoptotic death. Caspase inhibitors strongly suppress apoptotic neuronal death. Thus, both protease families might contribute to structural derangement and functional loss in neurons under degenerative conditions.

Blockade of NR2B-Containing NMDA Receptors Prevents BDNF Enhancement of Glutamatergic Transmission in Hippocampal Neurons

Application of brain-derived neurotrophic factor (BDNF) to hippocampal neurons has profound effects on glutamatergic synaptic transmission. Both pre- and postsynaptic actions have been identified that depend on the age and type of preparation. To understand the nature of this diversity, we have begun to examine the mechanisms of BDNF action in cultured dissociated embryonic hippocampal neurons. Whole-cell patch-clamp recording during iontophoretic application of glutamate revealed that BDNF doubled the amplitude of induced inward current. Coexposure to BDNF and the NMDA receptor antagonist AP-5 markedly reduced, but did not entirely prevent, the increase in current. Coexposure to BDNF and ifenprodil, an NR2B subunit antagonist, reproduced the response observed with AP-5, suggesting BDNF primarily enhanced activity of NR2B-containing NMDA receptors with a lesser effect on non-NMDA receptors. Protein kinase involvement was confirmed with the broad spectrum inhibitor staurosporine, which prevented the response to BDNF. PKCI19-31 and H-89, selective antagonists of PKC and PKA, had no effect on the response to BDNF, whereas autocamtide-2-related inhibitory peptide, an antagonist of CaM kinase II, reduced response magnitude by 60%. These results demonstrate the predominant role of a specific NMDA receptor subtype in BDNF modulation of hippocampal synaptic transmission.

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.

Phosphorylation-dependent reversible translocation of Ca2+/calmodulin-dependent protein kinase II to the postsynaptic densities

The translocation of soluble Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) to postsynaptic densities (PSDs) was investigated. When soluble CaM kinase II previously autophosphorylated was incubated with PSDs, the kinase was precipitated by centrifugation, indicating that the soluble kinase associated with PSDs and formed a PSD-CaM kinase II complex. Ca2+-independent activity generated by autophosphorylation of the kinase was retained in the complex. A number of PSD proteins were phosphorylated by the kinase associated with PSDs in both the absence and presence of Ca2+. When PSD-CaM kinase II complex was incubated at 30 degrees C, the enzyme was dephosphorylated and released from the complex. These results indicate that CaM kinase II reversibly translocates to PSDs in a phosphorylation-dependent manner.

Phosphorylation-dependent reversible association of Ca2+/calmodulin-dependent protein kinase II with the postsynaptic densities

The association of soluble Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) with postsynaptic densities (PSDs) was determined by activity assay, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and immunoblotting of the enzyme. Soluble CaM kinase II was autophosphorylated with ATP in the presence of Ca2+ and calmodulin, and then it was incubated with PSDs. Autophosphorylated CaM kinase II was precipitated with PSDs by centrifugation. The kinase that was not autophosphorylated did not precipitate with PSDs. These results indicate that the soluble previously autophosphorylated CaM kinase II associates with PSDs and forms PSD-CaM kinase II complex. A maximum of about 60 microg of soluble CaM kinase II bound to 1 mg of PSD protein under the experimental conditions. Ca2+-independent activity generated by autophosphorylation of the kinase was retained in the PSD-CaM kinase II complex. The CaM kinase II thus associated with PSDs phosphorylated a number of PSD proteins in both the absence and presence of Ca2+. When the CaM kinase II-PSD complex was incubated at 30 degrees C, its Ca2+-independent activity was gradually decreased. This decrease was correlated with dephosphorylation of the kinase and its release from PSD-CaM kinase II complex. These results indicate that CaM kinase II reversibly translocates to PSDs in a phosphorylation-dependent manner.