Regulation of phosphorylation at the postsynaptic density during different activity states of Ca2+/calmodulin-dependent protein kinase II

Zexieyin formula alleviates Alzheimer's disease via post-synaptic CaMKII modulating AMPA receptor: Involved in promoting neurogenesis to strengthen synaptic plasticity in mice hippocampus

BACKGROUND

Alzheimer's disease (AD) is a serious neurodegenerative disease and brings a serious burden to society and families. Due to lack of effective drugs for the treatment of AD, it's urgent to develop new and effective drug for the treatment of AD.

PURPOSE

The study aimed to investigate the potential of Zexieyin formula (ZXYF), a Chinese medicine formula, for the treatment of AD and its potential mechanism of action.

METHODS

We used chronic scopolamine (SCOP) induction mice model and APP/PS1 mice to reveal and confirm ZXYF for the treatment of AD with donepezil (DON) as a positive reference. The learning and memory function were detected by morris water maze test (MWM) and y-maze test. Moreover, western blot and immunofluorescence were used to detect the molecular mechanism of ZXYF for the alleviation of AD in hippocampus. Lastly, pharmacological technology was applied to evaluate AMPA receptor involved in the role of ZXYF in the treatment of AD.

RESULTS

The results showed that ZXYF could improve memory and learning deficits both in two AD models including scopolamine (SCOP)-induced mice model and APP/PS1mice. Moreover, ZXYF or not DON increased expressions of BrdU/DCX and Ki67 positive cells in dentate gyrus (DG), up-regulated the levels of AMPA subunit type (GluA1) and PKA in hippocampus in SCOP-induced mice model, although ZXYF and DON activated CaMKII, CaMKII-phosphorylation, CREB, CREB-phosphorylation and PSD95 in hippocampus in SCOP-induced mice model. ZXYF also activated CaMKII, CaMKII-phosphorylation and GluA1 in HT22 cells. Furthermore, transient inhibiting AMPA receptor was capable of blocking the effects of ZXYF to treat AD in MWM and suppressing the number of BrdU/DCX positive cells increased by ZXYF in DG in SCOP-induced mice model, but had no effect on the alteration of Ki67 positive cells.

CONCLUSION

ZXYF had the therapeutic effects on AD-treatment, which activated CaMKII to promote AMPA receptor (GluA1) and subsequently up-regulated PKA/CREB signaling to facilitate neurogenesis to achieve enhanced postsynaptic protein.

Inhibition of TRPP3 by calmodulin through Ca2+/calmodulin-dependent protein kinase II

Transient receptor potential (TRP) polycystin-3 (TRPP3) is a non-selective cation channel activated by Ca2+ and protons and is involved in regulating ciliary Ca2+ concentration, hedgehog signaling and sour tasting. The TRPP3 channel function and regulation are still not well understood. Here we investigated regulation of TRPP3 by calmodulin (CaM) by means of electrophysiology and Xenopus oocytes as an expression model. We found that TRPP3 channel function is enhanced by calmidazolium, a CaM antagonist, and inhibited by CaM through binding of the CaM N-lobe to a TRPP3 C-terminal domain not overlapped with the EF-hand. We further revealed that the TRPP3/CaM interaction promotes phosphorylation of TRPP3 at threonine 591 by Ca2+/CaM-dependent protein kinase II, which mediates the inhibition of TRPP3 by CaM.

A bidirectional switch in the Shank3 phosphorylation state biases synapses toward up- or downscaling

Homeostatic synaptic plasticity requires widespread remodeling of synaptic signaling and scaffolding networks, but the role of post-translational modifications in this process has not been systematically studied. Using deep-scale quantitative analysis of the phosphoproteome in mouse neocortical neurons, we found widespread and temporally complex changes during synaptic scaling up and down. We observed 424 bidirectionally modulated phosphosites that were strongly enriched for synapse-associated proteins, including S1539 in the autism spectrum disorder-associated synaptic scaffold protein Shank3. Using a parallel proteomic analysis performed on Shank3 isolated from rat neocortical neurons by immunoaffinity, we identified two sites that were persistently hypophosphorylated during scaling up and transiently hyperphosphorylated during scaling down: one (rat S1615) that corresponded to S1539 in mouse, and a second highly conserved site, rat S1586. The phosphorylation status of these sites modified the synaptic localization of Shank3 during scaling protocols, and dephosphorylation of these sites via PP2A activity was essential for the maintenance of synaptic scaling up. Finally, phosphomimetic mutations at these sites prevented scaling up but not down, while phosphodeficient mutations prevented scaling down but not up. These mutations did not impact baseline synaptic strength, indicating that they gate, rather than drive, the induction of synaptic scaling. Thus, an activity-dependent switch between hypo- and hyperphosphorylation at S1586 and S1615 of Shank3 enables scaling up or down, respectively. Collectively, our data show that activity-dependent phosphoproteome dynamics are important for the functional reconfiguration of synaptic scaffolds and can bias synapses toward upward or downward homeostatic plasticity.

Dynamic bi-directional phosphorylation events associated with the reciprocal regulation of synapses during homeostatic up- and down-scaling

Homeostatic synaptic scaling allows for bi-directional adjustment of the strength of synaptic connections in response to changes in their input. Protein phosphorylation modulates many neuronal processes, but it has not been studied on a global scale during synaptic scaling. Here, we use liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses to measure changes in the phosphoproteome in response to up- or down-scaling in cultured cortical neurons over minutes to 24 h. Of ~45,000 phosphorylation events, ~3,300 (associated with 1,285 phosphoproteins) are regulated by homeostatic scaling. Activity-sensitive phosphoproteins are predominantly located at synapses and involved in cytoskeletal reorganization. We identify many early phosphorylation events that could serve as sensors for the activity offset as well as late and/or persistent phosphoregulation that could represent effector mechanisms driving the homeostatic response. Much of the persistent phosphorylation is reciprocally regulated by up- or down-scaling, suggesting that mechanisms underlying these two poles of synaptic regulation make use of a common signaling axis.

•

Global proteome and phosphoproteome dynamics following homeostatic synaptic scaling

•

Approximately 3,300 activity-sensitive, synapse-associated phospho-events

•

Persistent signaling of ~25% of initial phospho-events (min to 24 h)

•

Persistent and reciprocal phosphoregulation links synaptic up- and down-scaling

Twenty Years of SynGAP Research: From Synapses to Cognition

SynGAP is a potent regulator of biochemical signaling in neurons and plays critical roles in neuronal function. It was first identified in 1998, and has since been extensively characterized as a mediator of synaptic plasticity. Because of its involvement in synaptic plasticity, SynGAP has emerged as a critical protein for normal cognitive function. In recent years, mutations in the SYNGAP1 gene have been shown to cause intellectual disability in humans and have been linked to other neurodevelopmental disorders, such as autism spectrum disorders and schizophrenia. While the structure and biochemical function of SynGAP have been well characterized, a unified understanding of the various roles of SynGAP at the synapse and its contributions to neuronal function remains to be achieved. In this review, we summarize and discuss the current understanding of the multifactorial role of SynGAP in regulating neuronal function gathered over the last two decades.

SynGAP isoforms differentially regulate synaptic plasticity and dendritic development

SynGAP is a synaptic Ras GTPase-activating protein (GAP) with four C-terminal splice variants: α1, α2, β, and γ. Although studies have implicated SYNGAP1 in several cognitive disorders, it is not clear which SynGAP isoforms contribute to disease. Here, we demonstrate that SynGAP isoforms exhibit unique spatiotemporal expression patterns and play distinct roles in neuronal and synaptic development in mouse neurons. SynGAP-α1, which undergoes liquid-liquid phase separation with PSD-95, is highly enriched in synapses and is required for LTP. In contrast, SynGAP-β, which does not bind PSD-95 PDZ domains, is less synaptically targeted and promotes dendritic arborization. A mutation in SynGAP-α1 that disrupts phase separation and synaptic targeting abolishes its ability to regulate plasticity and instead causes it to drive dendritic development like SynGAP-β. These results demonstrate that distinct intrinsic biochemical properties of SynGAP isoforms determine their function, and individual isoforms may differentially contribute to the pathogenesis of SYNGAP1-related cognitive disorders.

CaMKIIα phosphorylation of Shank3 modulates ABI1-Shank3 interaction

Protein-protein interactions can be modulated by phosphorylation of either binding partner, thereby altering subcellular localization and/or physiological function. Shank3, a master postsynaptic scaffolding protein that controls the developmental maturation of excitatory synapses, was recently shown to be phosphorylated by Protein Kinase A (PKA) at Ser685 in vivo. Mutation of Shank3 Ser685 was shown to modulate the binding of Abelson interactor 1 (ABI1), a component of the WAVE regulatory complex for actin remodeling, but a direct effect of Ser685 phosphorylation on ABI1 binding was not investigated. Here, we demonstrate that Ca²⁺/calmodulin-dependent protein kinase II alpha (CaMKIIα) also phosphorylates Shank3 at Ser685. Mutation of Ser685 to phospho-null alanine (S685A) prevented both CaMKIIα and PKA phosphorylation of a GST-Shank3 fusion protein. The co-immunoprecipitation of ABI1 with Shank3 from HEK293 cell extracts is reduced by mutation of Ser685 to either Ala or Asp. However, pre-phosphorylation of GST-Shank3 by purified CaMKIIα significantly increased binding of ABI1, and this effect was abrogated by Ser685 to Ala mutation in GST-Shank3. Taken together, our data suggest that neuronal ABI1-Shank3 interactions may be convergently regulated by Shank3 Ser685 phosphorylation in response to both Ca²⁺ and cAMP signaling, potentially modulating dendritic spine morphology.

Dexmedetomidine attenuates the propofol-induced long-term neurotoxicity in the developing brain of rats by enhancing the PI3K/Akt signaling pathway

BACKGROUND

Propofol induces short- and long-term neurotoxicity. Our previous study showed that dexmedetomidine (Dex) can attenuate the propofol-induced acute neurotoxicity in rodents by enhancing the PI3K/Akt signaling. However, whether treatment of young rats with Dex could protect them from long-term neurotoxicity induced by propofol is unclear.

MATERIALS AND METHODS

Seven-day-old male Sprague Dawley rats were randomized and injected intraperitoneally with saline (100 μL, NS), propofol (100 mg/kg), Dex (75 μg/kg), propofol (100 mg/kg) plus Dex (25, 50 or 75 μg/kg), 10% dimethyl sulfoxide (DMSO, 100 μL) or TDZD-8 (a GSK3β inhibitor, 1 mg/kg), or intracerebroventricularly with DMSO (5 μL) or LY294002 (a PI3K inhibitor, 25 μg/5 μL DMSO). Other rats in the experimental group were injected with the same doses of propofol, Dex and LY294002 or TDZD-8. All the rats were monitored until they were 9 weeks old. Their spatial learning and memory were tested by Morris water maze. The neuronal apoptosis, expression of PSD₉₅, expression and phosphorylation of Akt and GSK3β and synaptic ultrastructures were determined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, immunohistochemistry, Western blot and transmission electron microscopy assays, respectively.

RESULTS

Compared with the NS control group, young rats injected with intralipid, Dex, TDZD-8, LY294002 or DMSO alone did not show any significant change as they aged. Propofol significantly increased the escape latency time, hippocampal neuroapoptosis and synaptic ultrastructural changes but decreased the relative levels of PSD₉₅ expression, and Akt and GSK3β phosphorylation in the developing hippocampus of the rats. The neuronal toxic effects of propofol were significantly mitigated by the pretreatment with a higher dose of Dex. The neuroprotective effect of Dex was enhanced by the treatment with TDZD-8, but was completely abrogated by the treatment with LY294002.

CONCLUSION

Our results indicated that the pretreatment of young rats with Dex attenuated the propofol-induced long-term neurotoxicity in their developing hippocampus by enhancing the PI3K/Akt signaling.

The DLGAP family: neuronal expression, function and role in brain disorders

The neurotransmitter glutamate facilitates neuronal signalling at excitatory synapses. Glutamate is released from the presynaptic membrane into the synaptic cleft. Across the synaptic cleft glutamate binds to both ion channels and metabotropic glutamate receptors at the postsynapse, which expedite downstream signalling in the neuron. The postsynaptic density, a highly specialized matrix, which is attached to the postsynaptic membrane, controls this downstream signalling. The postsynaptic density also resets the synapse after each synaptic firing. It is composed of numerous proteins including a family of Discs large associated protein 1, 2, 3 and 4 (DLGAP1-4) that act as scaffold proteins in the postsynaptic density. They link the glutamate receptors in the postsynaptic membrane to other glutamate receptors, to signalling proteins and to components of the cytoskeleton. With the central localisation in the postsynapse, the DLGAP family seems to play a vital role in synaptic scaling by regulating the turnover of both ionotropic and metabotropic glutamate receptors in response to synaptic activity. DLGAP family has been directly linked to a variety of psychological and neurological disorders. In this review we focus on the direct and indirect role of DLGAP family on schizophrenia as well as other brain diseases.

CaMKII-mediated displacement of AIDA-1 out of the postsynaptic density core

Ankyrin repeat and sterile alpha motif domain-containing protein 1B (ANKS1B, also known as AIDA-1) is a major component of the postsynaptic density (PSD) in excitatory neurons where it concentrates at the electron-dense core under basal conditions and moves out during activity. This study investigates the molecular mechanism underlying activity-induced displacement of AIDA-1. Experiments with PSD fractions from brain indicate phosphorylation of AIDA-1 upon activation of endogenous CaMKII. Immuno-electron microscopy studies show that treatment of hippocampal neurons with NMDA results in an ~ 30 nm shift in the median distance of the AIDA-1 label from the postsynaptic membrane, an effect that is blocked by the CaMKII inhibitor tatCN21. CaMKII-mediated redistribution of AIDA-1 is similar to that observed for SynGAP. CaMKII-mediated removal of two abundant PSD-95-binding proteins from the PSD core during activity is expected to initiate a molecular reorganization at the PSD.

The Postsynaptic Density: There Is More than Meets the Eye

The postsynaptic density (PSD), apparent in electron micrographs as a dense lamina just beneath the postsynaptic membrane, includes a deeper layer, the “pallium”, containing a scaffold of Shank and Homer proteins. Though poorly defined in traditionally prepared thin-section electron micrographs, the pallium becomes denser and more conspicuous during intense synaptic activity, due to the reversible addition of CaMKII and other proteins. In this Perspective article, we review the significance of CaMKII-mediated recruitment of proteins to the pallium with respect to both the trafficking of receptors and the remodeling of spine shape that follow synaptic stimulation. We suggest that the level and duration of CaMKII translocation and activation in the pallium will shape activity-induced changes in the spine.

Mass spectrometric phosphoproteome analysis of HIV-infected brain reveals novel phosphorylation sites and differential phosphorylation patterns

PURPOSE

To map the phosphoproteome and identify changes in the phosphorylation patterns in the HIV-infected and uninfected brain.

EXPERIMENTAL DESIGN

Parietal cortex from individuals with and without HIV infection were lysed and trypsinized. The peptides were labeled with iTRAQ reagents, combined, phospho-enriched by titanium dioxide chromatography, and analyzed by LC-MS/MS with high resolution.

RESULTS

Our phosphoproteomic workflow resulted in the identification of 112 phosphorylated proteins and 17 novel phosphorylation sites in all the samples that were analyzed. The phosphopeptide sequences were searched for kinase substrate motifs, which revealed potential kinases involved in important signaling pathways. The site-specific phosphopeptide quantification showed that peptides from neurofilament medium polypeptide, myelin basic protein, and 2'-3'-cyclic nucleotide-3' phosphodiesterase have relatively higher phosphorylation levels during HIV infection.

CONCLUSIONS AND CLINICAL RELEVANCE

This study has enriched the global phosphoproteome knowledge of the human brain by detecting novel phosphorylation sites on neuronal proteins and identifying differentially phosphorylated brain proteins during HIV infection. Kinases that lead to unusual phosphorylations could be therapeutic targets for the treatment of HIV-associated neurocognitive disorders.

Signaling pathways relevant to cognition-enhancing drug targets

Aging is generally associated with a certain cognitive decline. However, individual differences exist. While age-related memory deficits can be observed in humans and rodents in the absence of pathological conditions, some individuals maintain intact cognitive functions up to an advanced age. The mechanisms underlying learning and memory processes involve the recruitment of multiple signaling pathways and gene expression, leading to adaptative neuronal plasticity and long-lasting changes in brain circuitry. This chapter summarizes the current understanding of how these signaling cascades could be modulated by cognition-enhancing agents favoring memory formation and successful aging. It focuses on data obtained in rodents, particularly in the rat as it is the most common animal model studied in this field. First, we will discuss the role of the excitatory neurotransmitter glutamate and its receptors, downstream signaling effectors [e.g., calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC), extracellular signal-regulated kinases (ERK), mammalian target of rapamycin (mTOR), cAMP response element-binding protein (CREB)], associated immediate early gene (e.g., Homer 1a, Arc and Zif268), and growth factors [insulin-like growth factors (IGFs) and brain-derived neurotrophic factor (BDNF)] in synaptic plasticity and memory formation. Second, the impact of the cholinergic system and related modulators on memory will be briefly reviewed. Finally, since dynorphin neuropeptides have recently been associated with memory impairments in aging, it is proposed as an attractive target to develop novel cognition-enhancing agents.

NMDA-induced accumulation of Shank at the postsynaptic density is mediated by CaMKII

Shank is a specialized scaffold protein present in high abundance at the postsynaptic density (PSD). Using pre-embedding immunogold electron microscopy on cultured hippocampal neurons, we had previously demonstrated further accumulation of Shank at the PSD under excitatory conditions. Here, using the same experimental protocol, we demonstrate that a cell permeable CaMKII inhibitor, tatCN21, blocks NMDA-induced accumulation of Shank at the PSD. Furthermore we show that NMDA application changes the distribution pattern of Shank at the PSD, promoting a 7-10 nm shift in the median distance of Shank labels away from the postsynaptic membrane. Inhibition of CaMKII with tatCN21 also blocks this shift in the distribution of Shank. Altogether these results imply that upon activation of NMDA receptors, CaMKII mediates accumulation of Shank, preferentially at the distal regions of the PSD complex extending toward the cytoplasm.

Subcellular organization of camkii in rat hippocampal pyramidal neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) plays a key role in N-methyl-D-aspartate (NMDA) receptor-dependent long-term synaptic plasticity; its location is critical for signal transduction, and may provide clues that further elucidate its function. We therefore examined the subcellular localization of CaMKII in CA1 stratum radiatum of adult rat hippocampus, by using immuno-electron microscopy after chemical fixation. When tissue was fixed quickly, the concentration of CaMKIIα (assessed by pre-embedding immunogold) was significantly higher in dendritic shafts than in spine heads. However, when tissue was fixed 5 minutes after perfusion with normal saline, the density of labeling decreased in dendritic shaft while increasing in spine heads, implying rapid translocation into the spine during brief perimortem stress. Likewise, in quickly fixed tissue, CaMKII within spine heads was found at comparable concentrations in the "proximal" half (adjacent to the spine neck) and the "distal" half (containing the postsynaptic density [PSD]), whereas after delayed fixation, label density increased in the distal side of the spine head, suggesting that CaMKII within the spine head moves toward the PSD during this interval. To estimate its distribution at the synapse in vivo, we performed postembedding immunogold staining for CaMKII in quick-fixed tissue, and found that the enzyme did not concentrate primarily within the central matrix of the PSD. Instead, labeling density peaked ∼40 nm inside the postsynaptic membrane, at the cytoplasmic fringe of the PSD. Labeling within 25 nm of the postsynaptic membrane concentrated at the lateral edge of the synapse. This lateral "PSD core" pool of CaMKII may play a special role in synaptic plasticity.

Searching for novel Cdk5 substrates in brain by comparative phosphoproteomics of wild type and Cdk5-/- mice

Protein phosphorylation is the most common post-translational modification that regulates several pivotal functions in cells. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase which is mostly active in the nervous system. It regulates several biological processes such as neuronal migration, cytoskeletal dynamics, axonal guidance and synaptic plasticity among others. In search for novel substrates of Cdk5 in the brain we performed quantitative phosphoproteomics analysis, isolating phosphoproteins from whole brain derived from E18.5 Cdk5^+/+ and Cdk5^−/− embryos, using an Immobilized Metal-Ion Affinity Chromatography (IMAC), which specifically binds to phosphorylated proteins. The isolated phosphoproteins were eluted and isotopically labeled for relative and absolute quantitation (iTRAQ) and mass spectrometry identification. We found 40 proteins that showed decreased phosphorylation at Cdk5^−/− brains. In addition, out of these 40 hypophosphorylated proteins we characterized two proteins, :MARCKS (Myristoylated Alanine-Rich protein Kinase C substrate) and Grin1 (G protein regulated inducer of neurite outgrowth 1). MARCKS is known to be phosphorylated by Cdk5 in chick neural cells while Grin1 has not been reported to be phosphorylated by Cdk5. When these proteins were overexpressed in N2A neuroblastoma cell line along with p35, serine phosphorylation in their Cdk5 motifs was found to be increased. In contrast, treatments with roscovitine, the Cdk5 inhibitor, resulted in an opposite effect on serine phosphorylation in N2A cells and primary hippocampal neurons transfected with MARCKS. In summary, the results presented here identify Grin 1 as novel Cdk5 substrate and confirm previously identified MARCKS as a a bona fide Cdk5 substrate.

AMPARs and synaptic plasticity: the last 25 years

The study of synaptic plasticity and specifically LTP and LTD is one of the most active areas of research in neuroscience. In the last 25 years we have come a long way in our understanding of the mechanisms underlying synaptic plasticity. In 1988, AMPA and NMDA receptors were not even molecularly identified and we only had a simple model of the minimal requirements for the induction of plasticity. It is now clear that the modulation of the AMPA receptor function and membrane trafficking is critical for many forms of synaptic plasticity and a large number of proteins have been identified that regulate this complex process. Here we review the progress over the last two and a half decades and discuss the future challenges in the field.

Camkii-mediated phosphorylation regulates distributions of Syngap-α1 and -α2 at the postsynaptic density

SynGAP, a protein abundant at the postsynaptic density (PSD) of glutamatergic neurons, is known to modulate synaptic strength by regulating the incorporation of AMPA receptors at the synapse. Two isoforms of SynGAP, α1 and α2, which differ in their C-termini, have opposing effects on synaptic strength. In the present study, antibodies specific for SynGAP-α1 and SynGAP-α2 are used to compare the distribution patterns of the two isoforms at the postsynaptic density (PSD) under basal and excitatory conditions. Western immunoblotting shows enrichment of both isoforms in PSD fractions isolated from adult rat brain. Immunogold electron microscopy of rat hippocampal neuronal cultures shows similar distribution of both isoforms at the PSD, with a high density of immunolabel within the PSD core under basal conditions. Application of NMDA promotes movement of SynGAP-α1 as well as SynGAP-α2 out of the PSD core. In isolated PSDs both isoforms of SynGAP can be phosphorylated upon activation of the endogenous CaMKII. Application of tatCN21, a cell-penetrating inhibitor of CaMKII, to hippocampal neuronal cultures blocks NMDA-induced redistribution of SynGAP-α1 and SynGAP-α2. Thus CaMKII activation promotes the removal of two distinct C-terminal SynGAP variants from the PSD.

Exonic resequencing of the DLGAP3 gene as a candidate gene for schizophrenia

We resequenced the exonic regions of the DLGAP3 gene, which encodes SAP90/PSD95-associated protein 3, in 215 schizophrenic patients and 215 non-psychotic controls. Seven known single-nucleotide polymorphisms (SNPs) were identified, but not associated with schizophrenia. Nevertheless, we identified several rare missense mutations and some of them might be associated with the pathogenesis of schizophrenia.

Direct visualization of CaMKII at postsynaptic densities by electron microscopy tomography

Ca(2+) /calmodulin-dependent protein kinase II (CaMKII) is a major component of postsynaptic densities (PSDs) involved in synaptic regulation. It has been previously shown that upon activity CaMKII from the spine reversibly aggregates at the cytoplasmic surfaces of PSDs, where it encounters various targets for phosphorylation. Targets for CaMKII are also present within the PSD, but there has been no reliable method to pinpoint whether, or where, CaMKII is located inside the PSD. Here we show that CaMKII can be mapped molecule-by-molecule within isolated PSDs using negative stain electron microscopy tomography. CaMKII molecules found in the core of the PSD may represent a pool distinct from the CaMKII residing at the cytoplasmic surface.

Genetic inhibition of CaMKII in dorsal striatal medium spiny neurons reduces functional excitatory synapses and enhances intrinsic excitability

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is abundant in striatal medium spiny neurons (MSNs). CaMKII is dynamically regulated by changes in dopamine signaling, as occurs in Parkinson's disease as well as addiction. Although CaMKII has been extensively studied in the hippocampus where it regulates excitatory synaptic transmission, relatively little is known about how it modulates neuronal function in the striatum. Therefore, we examined the impact of selectively overexpressing an EGFP-fused CaMKII inhibitory peptide (EAC3I) in striatal medium spiny neurons (MSNs) using a novel transgenic mouse model. EAC3I-expressing cells exhibited markedly decreased excitatory transmission, indicated by a decrease in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs). This decrease was not accompanied by changes in the probability of release, levels of glutamate at the synapse, or changes in dendritic spine density. CaMKII regulation of the AMPA receptor subunit GluA1 is a major means by which the kinase regulates neuronal function in the hippocampus. We found that the decrease in striatal excitatory transmission seen in the EAC3I mice is mimicked by deletion of GluA1. Further, while CaMKII inhibition decreased excitatory transmission onto MSNs, it increased their intrinsic excitability. These data suggest that CaMKII plays a critical role in setting the excitability rheostat of striatal MSNs by coordinating excitatory synaptic drive and the resulting depolarization response.

The role of metaplasticity mechanisms in regulating memory destabilization and reconsolidation

Memory allows organisms to predict future events based on prior experiences. This requires encoded information to persist once important predictors are extracted, while also being modifiable in response to changes within the environment. Memory reconsolidation may allow stored information to be modified in response to related experience. However, there are many boundary conditions beyond which reconsolidation may not occur. One interpretation of these findings is that the event triggering memory retrieval must contain new information about a familiar stimulus in order to induce reconsolidation. Presently, the mechanisms that affect the likelihood of reconsolidation occurring under these conditions are not well understood. Here we speculate on a number of systems that may play a role in protecting memory from being destabilized during retrieval. We conclude that few memories may enter a state in which they cannot be modified. Rather, metaplasticity mechanisms may serve to alter the specific reactivation cues necessary to destabilize a memory. This might imply that destabilization mechanisms can differ depending on learning conditions.

CaMKII antisense oligodeoxynucleotides protect against ischemia-induced neuronal death in the rat hippocampus

The present study was performed to investigate the effects of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) antisense oligodeoxynucleotides (ODNs) on the assembly of the CaMKII·GluR6·PSD-95 signaling module, GluR6 serine phosphorylation and c-Jun N-terminal kinase 3 (JNK3) activation. A further aim was to determine the neuroprotective mechanism of CaMKII antisense ODNs against ischemia-reperfusion (I/R)-induced neuronal death in the rat hippocampus. CaMKII antisense ODNs were intracerebroventricularly infused to inhibit CaMKII expression once daily for 3 days prior to the induction of ischemia. Transient cerebral ischemia (15 min) and reperfusion were induced by four-vessel occlusion in Sprague-Dawley rats as an animal model for transient cerebral I/R. The expression of related proteins was examined by immunoprecipitation and immunoblotting. Neuronal death in the rat hippocampus was detected by histology and histochemistry. The results indicate that CaMKII antisense ODNs inhibit several of the processes that are normally induced by cerebral I/R, including CaMKII expression, increased CaMKII·GluR6·PSD-95 signaling module assembly, GluR6 serine phosphorylation and JNK3 activation. Alternatively, CaMKII antisense ODNs also exhibit a significant neuroprotective role against cerebral I/R-induced cell death. These results provide the first evidence that CaMKII antisense ODNs can exert neuroprotective effects on cerebral I/R-induced cell death. The possible molecular mechanisms underlying this effect include 1) an inhibition of CaMKII expression and subsequent suppression of the assembly of the CaMKII·GluR6·PSD-95 signaling module, 2) GluR6 serine phosphorylation, and 3) reduced JNK3 activation.

Dysregulated phosphorylation of Ca(2+) /calmodulin-dependent protein kinase II-α in the hippocampus of subjects with mild cognitive impairment and Alzheimer's disease

Alzheimer's disease (AD) is a progressive, neurodegenerative disorder and the most prevalent senile dementia. The early symptom of memory dysfunction involves synaptic loss, thought to be mediated by soluble amyloid-beta (Aβ) oligomers. These aggregate species target excitatory synapses and their levels correlate with disease severity. Studies in cell culture and rodents have shown that oligomers increase intracellular calcium (Ca(2+)), impairing synaptic plasticity. Yet, the molecular mechanism mediating Aβ oligomers' toxicity in the aged brain remains unclear. Here, we apply quantitative immunofluorescence in human brain tissue from clinically diagnosed mild cognitive impaired (MCI) and AD patients to investigate the distribution of phosphorylated (active) Ca(2+) /calmodulin-dependent protein kinase-α (p(Thr286)CaMKII), a critical enzyme for activity-dependent synaptic remodeling associated with cognitive function. We show that p(Thr286)CaMKII immunoreactivity is redistributed from dendritic arborizations to neural perikarya of both MCI and AD hippocampi. This finding correlates with cognitive assessment scores, suggesting that it may be a molecular read-out of the functional deficits in early AD. Treatment with oligomeric Aβ replicated the observed phenotype in mice and resulted in a loss of p(Thr286)CaMKII from synaptic spines of primary hippocampal neurons. Both outcomes were prevented by inhibiting the phosphatase calcineurin (CaN). Collectively, our results support a model in which the synaptotoxicity of Aβ oligomers in human brain involves the CaN-dependent subcellular redistribution of p(Thr286)CaMKII. Therapies designed to normalize the homeostatic imbalance of neuronal phosphatases and downstream dephosphorylation of synaptic p(Thr286)CaMKII should be considered to prevent and treat early AD.

Role of the CaMKII/NMDA receptor complex in the maintenance of synaptic strength

During long-term potentiation (LTP), synapses undergo stable changes in synaptic strength. The molecular memory processes that maintain strength have not been identified. One hypothesis is that the complex formed by the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and the NMDA-type glutamate receptor (NMDAR) is a molecular memory at the synapse. To establish a molecule as a molecular memory, it must be shown that interfering with the molecule produces a persistent reversal of LTP. We used the CN class of peptides that inhibit CaMKII binding to the NR2B subunit in vitro to test this prediction in rat hippocampal slices. We found that CN peptides can reverse saturated LTP, allowing additional LTP to be induced. The peptide also produced a persistent reduction in basal transmission. We then tested whether CN compounds actually affect CaMKII binding in living cells. Application of CN peptide to slice cultures reduced the amount of CaMKII concentrated in spines, consistent with delocalization of the kinase from a binding partner in the spine. To more specifically assay the binding of CaMKII to the NMDAR, we used coimmunoprecipitation methods. We found that CN peptide decreased synaptic strength only at concentrations necessary to disrupt the CaMKII/NMDAR complex, but not at lower concentrations sufficient to inhibit CaMKII activity. Importantly, both the reduction of the complex and the reduction of synaptic strength persisted after removal of the inhibitor. These results support the hypothesis that the CaMKII/NMDAR complex has switch-like properties that are important in the maintenance of synaptic strength.

Quantitative estimates of the cytoplasmic, PSD, and NMDAR-bound pools of CaMKII in dendritic spines

CaMKII plays a critical role in long-term potentiation (LTP). The kinase is a major component of the postsynaptic density (PSD); however, it is also contained in the spine cytoplasm. CaMKII can now be monitored optically in living neurons, and it is therefore important to understand the contribution of the PSD and cytoplasmic pools to optical signals. Here, we estimate the size of these pools under basal conditions. From EM immunolabeling data, we calculate that the PSD/cytoplasmic ratio is ~5%. A second independent estimate is derived from measurements indicating that the average mushroom spine PSD contains 90 to 240 holoenzymes. A cytoplasmic concentration of 16 μM (~2590 holoenzymes) in the spine can be estimated from the total measured CaMKII content of hippocampal tissue, the relative volume of different compartments, and the spine-dendrite ratio of CaMKII (2:1). These numbers yield a second estimate (6%) of the PSD/spine ratio in good agreement with the first. The CaMKII bound to the NMDAR is important because preventing the formation of this complex blocks LTP induction. We estimate that the percentage of spine CaMKII held active by binding to the NMDAR is ~0.2%. Implications of the high spine concentration of CaMKII (> 100 μM alpha subunits) and the small fraction within the PSD are discussed. Of particular note, the finding that the CaMKII signal in spines shows only transient activation (open state) after LTP induction is subject to the qualification that it does not reflect the small but important pool bound to the NMDAR.

Direct evidence of phosphorylated neuronal intermediate filament proteins in neurofibrillary tangles (NFTs): phosphoproteomics of Alzheimer's NFTs

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by brain pathology of intracellular neurofibrillary tangles (NFTs) and extracellular amyloid plaques. NFTs contain aberrantly hyperphosphorylated Tau as paired helical filaments (PHFs). Although NFs have been shown immunohistologically to be part of NFTs, there has been debate that the identity of NF proteins in NFTs is due to the cross-reactivity of phosphorylated NF antibodies with phospho-Tau. Here, we provide direct evidence on the identity of NFs in NFTs by immunochemical and mass spectrometric analysis. We have purified sarkosyl-insoluble NFTs and performed liquid chromatography/tandem mass spectrometry of NFT tryptic digests. The phosphoproteomics of NFTs clearly identified NF-M phosphopeptides SPVPKS*PVEEAK, corresponding to Ser685, and KAES*PVKEEAVAEVVTITK, corresponding to Ser736, and an NF-H phosphopeptide, EPDDAKAKEPS*KP, corresponding to Ser942. Western blotting of purified tangles with SMI31 showed a 150-kDa band corresponding to phospho-NF-M, while RT97 antibodies detected phospho-NF-H. The proteomics analysis also identified an NF-L peptide (ALYEQEIR, EAEEEKKVEGAGEEQAAAK) and another intermediate filament protein, vimentin (FADLSEAANR). Mass spectrometry revealed Tau phosphopeptides corresponding to Thr231, Ser235, Thr181, Ser184, Ser185, Thr212, Thr217, Ser396, and Ser403. And finally, phosphopeptides corresponding to MAP1B (corresponding to Ser1270, Ser1274, and Ser1779) and MAP2 (corresponding to Thr350, Ser1702, and Ser1706) were identified. In corresponding matched control preparations of PHF/NFTs, none of these phosphorylated neuronal cytoskeletal proteins were found. These studies independently demonstrate that NF proteins are an integral part of NFTs in AD brains.

SynGAP moves out of the core of the postsynaptic density upon depolarization

SynGAP is a Ras GTPase activating protein present at the postsynaptic density (PSD) in quantities matching those of the core scaffold protein PSD-95. SynGAP is reported to inhibit synaptic accumulation of AMPA receptors. Here, we characterize by immunogold electron microscopy the distribution of SynGAP at the PSD under basal and depolarizing conditions in rat hippocampal neuronal cultures. The PSD core, extending up to 40 nm from the postsynaptic membrane, typically shows label for SynGAP, while half of the synapses exhibit additional labeling in a zone 40-120 nm from the postsynaptic membrane. Upon depolarization with high K(+), labeling for SynGAP significantly decreases at the core of the PSD and concomitantly increases at the 40-120 nm zone. Under the same depolarization conditions, label for PSD-95, the presumed binding partner of SynGAP, does not change its localization at the PSD. Depolarization-induced redistribution of SynGAP is reversible and also occurs upon application of N-methyl-d-aspartic acid (NMDA). Activity-induced movement of SynGAP could vacate sites in the PSD core allowing other elements to bind to these sites, such as transmembrane AMPA receptor regulatory proteins (TARPs), and simultaneously facilitate access of SynGAP to CaMKII and Ras, elements of a regulatory cascade.

Requirement of the RNA-editing enzyme ADAR2 for normal physiology in mice

ADAR2, an RNA editing enzyme that converts specific adenosines to inosines in certain pre-mRNAs, often leading to amino acid substitutions in the encoded proteins, is mainly expressed in brain. Of all ADAR2-mediated edits, a single one in the pre-mRNA of the AMPA receptor subunit GluA2 is essential for survival. Hence, early postnatal death of mice lacking ADAR2 is averted when the critical edit is engineered into both GluA2 encoding Gria2 alleles. Adar2(-/-)/Gria2(R/R) mice display normal appearance and life span, but the general phenotypic effects of global lack of ADAR2 have remained unexplored. Here we have employed the Adar2(-/-)/Gria2(R/R) mouse line, and Gria2(R/R) mice as controls, to study the phenotypic consequences of loss of all ADAR2-mediated edits except the critical one in GluA2. Our extended phenotypic analysis covering ∼320 parameters identified significant changes related to absence of ADAR2 in behavior, hearing ability, allergy parameters and transcript profiles of brain.

Quantitative phosphoproteomic analysis of neuronal intermediate filament proteins (NF-M/H) in Alzheimer's disease by iTRAQ

Aberrant hyperphosphorylation of neuronal cytoskeletal proteins is one of the major pathological hallmarks of neurodegenerative disorders such as Alzheimer disease (AD), amyotrophic lateral sclerosis (ALS), and Parkinson's disease (PD). Human NF-M/H display a large number of multiple KSP repeats in the carboxy-terminal tail domain, which are phosphorylation sites of proline-directed serine/threonine (pSer/Thr-Pro, KS/T-P) kinases. The phosphorylation sites of NF-M/H have not been characterized in AD brain. Here, we use quantitative phosphoproteomic methodology, isobaric tag for relative and absolute quantitation (iTRAQ), for the characterization of NF-M/H phosphorylation sites in AD brain. We identified 13 hyperphosphorylated sites of NF-M; 9 Lys-Ser-Pro (KSP) sites; 2 variant motifs, Glu-Ser-Pro (ESP) Ser-736 and Leu-Ser-Pro (LSP) Ser-837; and 2 non-S/T-P motifs, Ser-783 and Ser-788. All the Ser/Thr residues are phosphorylated at significantly greater abundance in AD brain compared with control brain. Ten hyperphosphorylated KSP sites have been identified on the C-terminal tail domain of NF-H, with greater abundance of phosphorylation in AD brain compared with control brain. Our data provide the direct evidence that NF-M/H are hyperphosphorylated in AD compared with control brain and suggest the role of both proline-directed and non-proline-directed protein kinases in AD. This study represents the first comprehensive iTRAQ analyses and quantification of phosphorylation sites of human NF-M and NF-H from AD brain and suggests that aberrant hyperphosphorylation of neuronal intermediate filament proteins is involved in AD.

Strategies for quantitation of phosphoproteomic data

Recent developments in phosphoproteomic sample-preparation techniques and sensitive mass spectrometry instrumentation have led to large-scale identifications of phosphoproteins and phosphorylation sites from highly complex samples. This has facilitated the implementation of different quantitation strategies in order to study the biological role of protein phosphorylation during disease progression, differentiation or during external stimulation of a cellular system. In this article, a brief summary of the most popular strategies for phosphoproteomic studies is given; however, the main focus will be on different quantitation strategies. Methods for metabolic labeling, chemical modification and label-free quantitation and their applicability or inapplicability in phosphoproteomic studies are discussed.

Activity induced changes in the distribution of Shanks at hippocampal synapses

Dendritic spines contain a family of abundant scaffolding proteins known as Shanks, but little is known about how their distributions might change during synaptic activity. Here, pre-embedding immunogold electron microscopy is used to localize Shanks in synapses from cultured hippocampal neurons. We find that Shanks are preferentially located at postsynaptic densities (PSDs) as well as in a filamentous network near the PSD, extending up to 120 nm from the postsynaptic membrane. Application of sub-type specific antibodies shows that Shank2 is typically concentrated at and near PSDs while Shank1 is, in addition, distributed throughout the spine head. Depolarization with high K+ for 2 min causes transient, reversible translocation of Shanks towards the PSD that is dependent on extracellular Ca2+. The amount of activity-induced redistribution and subsequent recovery is pronounced for Shank1 but less so for Shank2. Thus, Shank1 appears to be a dynamic element within the spine, whose translocation could be involved in activity-induced, transient structural changes, while Shank2 appears to be a more stable element positioned at the interface of the PSD with the spine cytoplasm.