LTP induction by structural rather than enzymatic functions of CaMKII.

Learning and memory are thought to require hippocampal long-term potentiation (LTP), and one of the few central dogmas of molecular neuroscience that has stood undisputed for more than three decades is that LTP induction requires enzymatic activity of the Ca2+/calmodulin-dependent protein kinase II (CaMKII)1-3. However, as we delineate here, the experimental evidence is surprisingly far from conclusive. All previous interventions inhibiting enzymatic CaMKII activity and LTP4-8 also interfere with structural CaMKII roles, in particular binding to the NMDA-type glutamate receptor subunit GluN2B9-14. Thus, we here characterized and utilized complementary sets of new opto-/pharmaco-genetic tools to distinguish between enzymatic and structural CaMKII functions. Several independent lines of evidence demonstrated LTP induction by a structural function of CaMKII rather than by its enzymatic activity. The sole contribution of kinase activity was autoregulation of this structural role via T286 autophosphorylation, which explains why this distinction has been elusive for decades. Directly initiating the structural function in a manner that circumvented this T286 role was sufficient to elicit robust LTP, even when enzymatic CaMKII activity was blocked.

Decreased nitrosylation of CaMKII causes aging-associated impairments in memory and synaptic plasticity in mice.

CaMKII has molecular memory functions because transient calcium ion stimuli can induce long-lasting increases in its synaptic localization and calcium ion-independent (autonomous) activity, thereby leaving memory traces of calcium ion stimuli beyond their duration. The synaptic effects of two mechanisms that induce CaMKII autonomy are well studied: autophosphorylation at threonine-286 and binding to GluN2B. Here, we examined the neuronal functions of additional autonomy mechanisms: nitrosylation and oxidation of the CaMKII regulatory domain. We generated a knock-in mouse line with mutations that render the CaMKII regulatory domain nitrosylation/oxidation-incompetent, CaMKIIΔSNO, and found that it had deficits in memory and synaptic plasticity that were similar to those in aged wild-type mice. In addition, similar to aged wild-type mice, in which CaMKII was hyponitrosylated, but unlike mice with impairments of other CaMKII autonomy mechanisms, CaMKIIΔSNO mice showed reduced long-term potentiation (LTP) when induced by theta-burst stimulation but not high-frequency stimulation (HFS). As in aged wild-type mice, the HFS-LTP in the young adult CaMKIIΔSNO mice required L-type voltage-gated calcium ion channels. The effects in aged mice were likely caused by the loss of nitrosylation because no decline in CaMKII oxidation was detected. In hippocampal neurons, nitrosylation of CaMKII induced its accumulation at synapses under basal conditions in a manner mediated by GluN2B binding, like after LTP stimuli. However, LTP-induced synaptic CaMKII accumulation did not require nitrosylation. Thus, an aging-associated decrease in CaMKII nitrosylation may cause impairments by chronic synaptic effects, such as the decrease in basal synaptic CaMKII.

Aß-induced synaptic impairments require CaMKII activity that is stimulated by indirect signaling events.

Aß bears homology to the CaMKII regulatory domain, and peptides derived from this domain can bind and disrupt the CaMKII holoenzyme, suggesting that Aß could have a similar effect. Notably, Aß impairs the synaptic CaMKII accumulation that is mediated by GluN2B binding, which requires CaMKII assembly into holoenzymes. Furthermore, this Aß-induced impairment is prevented by CaMKII inhibitors that should also inhibit the putative direct Aß binding. However, our study did not find any evidence for direct effects of Aß on CaMKII: Aß did not directly disrupt CaMKII holoenzymes, GluN2B binding, T286 autophosphorylation, or kinase activity in vitro. Most importantly, in neurons, the Aß-induced impairment of CaMKII synaptic accumulation was prevented by an ATP-competitive CaMKII inhibitor that would not interfere with the putative direct Aß binding. Together, our results indicate that synaptic Aß effects are not mediated by direct binding to CaMKII, but instead require CaMKII activation via indirect signaling events.

Conserved and divergent features of neuronal CaMKII holoenzyme structure, function, and high-order assembly.

Neuronal CaMKII holoenzymes (α and ß isoforms) enable molecular signal computation underlying learning and memory but also mediate excitotoxic neuronal death. Here, we provide a comparative analysis of these signaling devices, using single-particle electron microscopy (EM) in combination with biochemical and live-cell imaging studies. In the basal state, both isoforms assemble mainly as 12-mers (but also 14-mers and even 16-mers for the ß isoform). CaMKIIα and ß isoforms adopt an ensemble of extended activatable states (with average radius of 12.6 versus 16.8 nm, respectively), characterized by multiple transient intra- and inter-holoenzyme interactions associated with distinct functional properties. The extended state of CaMKIIß allows direct resolution of intra-holoenzyme kinase domain dimers. These dimers could enable cooperative activation by calmodulin, which is observed for both isoforms. High-order CaMKII clustering mediated by inter-holoenzyme kinase domain dimerization is reduced for the ß isoform for both basal and excitotoxicity-induced clusters, both in vitro and in neurons.

Characterization of six CaMKIIα variants found in patients with schizophrenia.

The Ca2+/Calmodulin-dependent protein kinase II (CaMKII) is a central regulator of synaptic plasticity and has been implicated in various neurological conditions, including schizophrenia. Here, we characterize six different CaMKIIα variants found in patients with schizophrenia. Only R396stop disrupted the 12-meric holoenzyme structure, GluN2B binding, and synaptic localization. Additionally, R396stop impaired T286 autophosphorylation that generates Ca2+-independent "autonomous" kinase activity. This impairment in T286 autophosphorylation was shared by the R8H mutation, the only mutation that additionally reduced stimulated kinase activity. None of the mutations affected the levels of CaMKII expression in HEK293 cells. Thus, impaired CaMKII function was detected only for R396stop and R8H. However, two of the other mutations have been later identified also in the general population, and not all mutations found in patients with schizophrenia would be expected to cause disease. Nonetheless, for the R396stop mutation, the severity of the biochemical effects found here would predict a neurological phenotype.

GluN2B S1303 phosphorylation by CaMKII or DAPK1: no indication for involvement in ischemia or LTP.

Binding of two different CaM kinases, CaMKII and DAPK1, to the NMDA-type glutamate receptor (NMDAR) subunit GluN2B near S1303 has been implicated in excitotoxic/ischemic neuronal cell death. The GluN2BΔCaMKII mutation (L1298A, R1300Q) is neuroprotective but abolishes only CaMKII but not DAPK1 binding. However, both kinases can additionally phosphorylate GluN2B S1303. Thus, we here tested S1303 phosphorylation for possible contribution to neuronal cell death. The GluN2BΔCaMKII mutation completely abolished phosphorylation by CaMKII and DAPK1, suggesting that the mutation could mediate neuroprotection by disrupting phosphorylation. However, S1303 phosphorylation was not increased by excitotoxic insults in hippocampal slices or by global cerebral ischemia induced by cardiac arrest and cardiopulmonary resuscitation in vivo. In hippocampal cultures, S1303 phosphorylation was induced by chemical LTD but not LTP stimuli. These results indicate that the additional effect of the GluN2BΔCaMKII mutation on phosphorylation needs to be considered only in LTD but not in LTP or ischemia/excitotoxicity.

CaMKII holoenzyme mechanisms that govern the LTP versus LTD decision.

Higher brain functions are thought to require synaptic frequency decoding that can lead to long-term potentiation (LTP) or depression (LTD). We show that the LTP versus LTD decision is determined by complex cross-regulation of T286 and T305/306 autophosphorylation within the 12meric CaMKII holoenzyme, which enabled molecular computation of stimulus frequency, amplitude, and duration. Both LTP and LTD require T286 phosphorylation, but T305/306 phosphorylation selectively promoted LTD. In response to excitatory LTP versus LTD stimuli, the differential T305/306 phosphorylation directed CaMKII movement to either excitatory or inhibitory synapses, thereby coordinating plasticity at both synapse types. Fast T305/306 phosphorylation required prior T286 phosphorylation and then curbed CaMKII activity by two mechanisms: (i) a cis-subunit reaction reduced both Ca2+ stimulation and autonomous activity and (ii) a trans-subunit reaction enabled complete activity shutdown and feed-forward inhibition of further T286 phosphorylation. These are fundamental additions to the long-studied CaMKII regulation and function in neuronal plasticity.

CaMKII versus DAPK1 Binding to GluN2B in Ischemic Neuronal Cell Death after Resuscitation from Cardiac Arrest.

DAPK1 binding to GluN2B was prominently reported to mediate ischemic cell death in vivo. DAPK1 and CaMKII bind to the same GluN2B region, and their binding is mutually exclusive. Here, we show that mutating the binding region on GluN2B (L1298A/R1300Q) protected against neuronal cell death induced by cardiac arrest followed by resuscitation. Importantly, the GluN2B mutation selectively abolished only CaMKII, but not DAPK1, binding. During ischemic or excitotoxic insults, CaMKII further accumulated at excitatory synapses, and this accumulation was mediated by GluN2B binding. Interestingly, extra-synaptic GluN2B decreased after ischemia, but its relative association with DAPK1 increased. Thus, ischemic neuronal death requires CaMKII binding to synaptic GluN2B, whereas any potential role for DAPK1 binding is restricted to a different, likely extra-synaptic population of GluN2B.

CaMKII Metaplasticity Drives Aß Oligomer-Mediated Synaptotoxicity.

Alzheimer's disease (AD) is emerging as a synaptopathology driven by metaplasticity. Indeed, reminiscent of metaplasticity, oligomeric forms of the amyloid-ß peptide (oAß) prevent induction of long-term potentiation (LTP) via the prior activation of GluN2B-containing NMDA receptors (NMDARs). However, the downstream Ca2+-dependent signaling molecules that mediate aberrant metaplasticity are unknown. In this study, we show that oAß promotes the activation of Ca2+/calmodulin-dependent kinase II (CaMKII) via GluN2B-containing NMDARs. Importantly, we find that CaMKII inhibition rescues both the LTP impairment and the dendritic spine loss mediated by oAß. Mechanistically resembling metaplasticity, oAß prevents subsequent rounds of plasticity from inducing CaMKII T286 autophosphorylation, as well as the associated anchoring and accumulation of synaptic AMPA receptors (AMPARs). Finally, prolonged oAß treatment-induced CaMKII misactivation leads to dendritic spine loss via the destabilization of surface AMPARs. Thus, our study demonstrates that oAß engages synaptic metaplasticity via aberrant CaMKII activation.

Publisher Correction: The CaMKII holoenzyme structure in activation-competent conformations.

This corrects the article DOI: 10.1038/ncomms15742.

Analysis of the CaMKIIα and ß splice-variant distribution among brain regions reveals isoform-specific differences in holoenzyme formation.

Four CaMKII isoforms are encoded by distinct genes, and alternative splicing within the variable linker-region generates additional diversity. The α and ß isoforms are largely brain-specific, where they mediate synaptic functions underlying learning, memory and cognition. Here, we determined the α and ß splice-variant distribution among different mouse brain regions. Surprisingly, the nuclear variant αB was detected in all regions, and even dominated in hypothalamus and brain stem. For CaMKIIß, the full-length variant dominated in most regions (with higher amounts of minor variants again seen in hypothalamus and brain stem). The mammalian but not fish CaMKIIß gene lacks exon v3N that encodes the nuclear localization signal in αB, but contains three exons not found in the CaMKIIα gene (exons v1, v4, v5). While skipping of exons v1 and/or v5 generated the minor splice-variants ß', ße and ße', essentially all transcripts contained exon v4. However, we instead detected another minor splice-variant (now termed ßH), which lacks part of the hub domain that mediates formation of CaMKII holoenzymes. Surprisingly, in an optogenetic cellular assay of protein interactions, CaMKIIßH was impaired for binding to the ß hub domain, but still bound CaMKIIα. This provides the first indication for isoform-specific differences in holoenzyme formation.

CaMKII regulates the depalmitoylation and synaptic removal of the scaffold protein AKAP79/150 to mediate structural long-term depression.

Both long-term potentiation (LTP) and depression (LTD) of excitatory synapse strength require the Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) and its autonomous activity generated by Thr-286 autophosphorylation. Additionally, LTP and LTD are correlated with dendritic spine enlargement and shrinkage that are accompanied by the synaptic accumulation or removal, respectively, of the AMPA-receptor regulatory scaffold protein A-kinase anchoring protein (AKAP) 79/150. We show here that the spine shrinkage associated with LTD indeed requires synaptic AKAP79/150 removal, which in turn requires CaMKII activity. In contrast to normal CaMKII substrates, the substrate sites within the AKAP79/150 N-terminal polybasic membrane-cytoskeletal targeting domain were phosphorylated more efficiently by autonomous compared with Ca2+/CaM-stimulated CaMKII activity. This unusual regulation was mediated by Ca2+/CaM binding to the substrate sites resulting in protection from phosphorylation in the presence of Ca2+/CaM, a mechanism that favors phosphorylation by prolonged, weak LTD stimuli versus brief, strong LTP stimuli. Phosphorylation by CaMKII inhibited AKAP79/150 association with F-actin; it also facilitated AKAP79/150 removal from spines but was not required for it. By contrast, LTD-induced spine removal of AKAP79/150 required its depalmitoylation on two Cys residues within the N-terminal targeting domain. Notably, such LTD-induced depalmitoylation was also blocked by CaMKII inhibition. These results provide a mechanism how CaMKII can indeed mediate not only LTP but also LTD through regulated substrate selection; however, in the case of AKAP79/150, indirect CaMKII effects on palmitoylation are more important than the effects of direct phosphorylation. Additionally, our results provide the first direct evidence for a function of the well-described AKAP79/150 trafficking in regulating LTD-induced spine shrinkage.

DAPK1 Mediates LTD by Making CaMKII/GluN2B Binding LTP Specific.

The death-associated protein kinase 1 (DAPK1) is a potent mediator of neuronal cell death. Here, we find that DAPK1 also functions in synaptic plasticity by regulating the Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII). CaMKII and T286 autophosphorylation are required for both long-term potentiation (LTP) and depression (LTD), two opposing forms of synaptic plasticity underlying learning, memory, and cognition. T286-autophosphorylation induces CaMKII binding to the NMDA receptor (NMDAR) subunit GluN2B, which mediates CaMKII synaptic accumulation during LTP. We find that the LTP specificity of CaMKII synaptic accumulation is due to its LTD-specific suppression by calcineurin (CaN)-dependent DAPK1 activation, which in turn blocks CaMKII binding to GluN2B. This suppression is enabled by competitive DAPK1 versus CaMKII binding to GluN2B. Negative regulation of DAPK1/GluN2B binding by Ca2+/CaM results in synaptic DAPK1 removal during LTP but retention during LTD. A pharmacogenetic approach showed that suppression of CaMKII/GluN2B binding is a DAPK1 function required for LTD.

The CaMKII holoenzyme structure in activation-competent conformations.

The Ca2+/calmodulin-dependent protein kinase II (CaMKII) assembles into large 12-meric holoenzymes, which is thought to enable regulatory processes required for synaptic plasticity underlying learning, memory and cognition. Here we used single particle electron microscopy (EM) to determine a pseudoatomic model of the CaMKIIα holoenzyme in an extended and activation-competent conformation. The holoenzyme is organized by a rigid central hub complex, while positioning of the kinase domains is highly flexible, revealing dynamic holoenzymes ranging from 15-35 nm in diameter. While most kinase domains are ordered independently, ∼20% appear to form dimers and <3% are consistent with a compact conformation. An additional level of plasticity is revealed by a small fraction of bona-fide 14-mers (<4%) that may enable subunit exchange. Biochemical and cellular FRET studies confirm that the extended state of CaMKIIα resolved by EM is the predominant form of the holoenzyme, even under molecular crowding conditions.

CaMKII isoforms differ in their specific requirements for regulation by nitric oxide.

The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) mediates physiological and pathological functions by its Ca(2+)-independent autonomous activity. Two novel mechanisms for generating CaMKII autonomy include oxidation and S-nitrosylation, the latter requiring both Cys280 and Cys289 amino acid residues in the brain-specific isoform CaMKIIα. Even though the other CaMKII isoforms have a different amino acid in the position homologous to Cys280, we show here that nitric oxide (NO)-signaling generated autonomy also for the CaMKIIß isoform. Furthermore, although oxidation of the Met280/281 residues is sufficient to generate autonomy for most CaMKII isoforms, oxidation-induced autonomy was also prevented by a Cys289-mutation in the CaMKIIα isoform. Thus, all CaMKII isoforms can be regulated by physiological NO-signaling, but CaMKIIα regulation by oxidation and S-nitrosylation is more stringent.

Autonomous CaMKII requires further stimulation by Ca2+/calmodulin for enhancing synaptic strength.

A hallmark feature of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) is generation of autonomous (Ca(2+)-independent) activity by T286 autophosphorylation. Biochemical studies have shown that "autonomous" CaMKII is ∼5-fold further stimulated by Ca(2+)/CaM, but demonstration of a physiological function for such regulation within cells has remained elusive. In this study, CaMKII-induced enhancement of synaptic strength in rat hippocampal neurons required both autonomous activity and further stimulation. Synaptic strength was decreased by CaMKIIα knockdown and rescued by reexpression, but not by mutants impaired for autonomy (T286A) or binding to NMDA-type glutamate receptor subunit 2B (GluN2B; formerly NR2B; I205K). Full rescue was seen with constitutively autonomous mutants (T286D), but only if they could be further stimulated (additional T305/306A mutation), and not with two other mutations that additionally impair Ca(2+)/CaM binding. Compared to rescue with wild-type CaMKII, the CaM-binding-impaired mutants even had reduced synaptic strength. One of these mutants (T305/306D) mimicked an inhibitory autophosphorylation of CaMKII, whereas the other one (Δstim) abolished CaM binding without introducing charged residues. Inhibitory T305/306 autophosphorylation also reduced GluN2B binding, but this effect was independent of reduced Ca(2+)/CaM binding and was not mimicked by T305/306D mutation. Thus, even autonomous CaMKII activity must be further stimulated by Ca(2+)/CaM for enhancement of synaptic strength.

Nitric oxide induces Ca2+-independent activity of the Ca2+/calmodulin-dependent protein kinase II (CaMKII).

Both signaling by nitric oxide (NO) and by the Ca(2+)/calmodulin (CaM)-dependent protein kinase II α isoform (CaMKIIα) are implicated in two opposing forms of synaptic plasticity underlying learning and memory, as well as in excitotoxic/ischemic neuronal cell death. For CaMKIIα, these functions specifically involve also Ca(2+)-independent autonomous activity, traditionally generated by Thr-286 autophosphorylation. Here, we demonstrate that NO-induced S-nitrosylation of CaMKIIα also directly generated autonomous activity, and that CaMKII inhibition protected from NO-induced neuronal cell death. NO induced S-nitrosylation at Cys-280/289, and mutation of either site abolished autonomy, indicating that simultaneous nitrosylation at both sites was required. Additionally, autonomy was generated only when Ca(2+)/CaM was present during NO exposure. Thus, generation of this form of CaMKIIα autonomy requires simultaneous signaling by NO and Ca(2+). Nitrosylation also significantly reduced subsequent CaMKIIα autophosphorylation specifically at Thr-286, but not at Thr-305. A previously described reduction of CaMKII activity by S-nitrosylation at Cys-6 was also observed here, but only after prolonged (>5 min) exposure to NO donors. These results demonstrate a novel regulation of CaMKII by another second messenger system and indicate its involvement in excitotoxic neuronal cell death.

CaMKII binding to GluN2B is differentially affected by macromolecular crowding reagents.

Binding of the Ca2+/calmodulin(CaM)-dependent protein kinase II (CaMKII) to the NMDA-type glutamate receptor (NMDAR) subunit GluN2B controls long-term potentiation (LTP), a form of synaptic plasticity thought to underlie learning and memory. Regulation of this interaction is well-studied biochemically, but not under conditions that mimic the macromolecular crowding found within cells. Notably, previous molecular crowding experiments with lysozyme indicated an effect on the CaMKII holoenzyme conformation. Here, we found that the effect of molecular crowding on Ca2+/CaM-induced CaMKII binding to immobilized GluN2B in vitro depended on the specific crowding reagent. While binding was reduced by lysozyme, it was enhanced by BSA. The ATP content in the BSA preparation caused CaMKII autophosphorylation at T286 during the binding reaction; however, enhanced binding was also observed when autophosphorylation was blocked. Importantly, the positive regulation by nucleotide and BSA (as well as other macromolecular crowding reagents) did not alleviate the requirement for CaMKII stimulation to induce GluN2B binding. The differential effect of lysozyme (14 kDa) and BSA (66 kDa) was not due to size difference, as both dextran-10 and dextran-70 enhanced binding. By contrast, crowding with immunoglobulin G (IgG) reduced binding. Notably, lysozyme and IgG but not BSA directly bound to Ca2+/CaM in an overlay assay, suggesting a competition of lysozyme and IgG with the Ca2+/CaM-stimulus that induces CaMKII/GluN2B binding. However, lysozyme negatively regulated binding even when it was instead induced by CaMKII T286 phosphorylation. Alternative modes of competition would be with CaMKII or GluN2B, and the negative effects of lysozyme and IgG indeed also correlated with specific or non-specific binding to the immobilized GluN2B. Thus, the effect of any specific crowding reagent can differ, depending on its additional direct effects on CaMKII/GluN2B binding. However, the results of this study also indicate that, in principle, macromolecular crowding enhances CaMKII binding to GluN2B.

Autonomous CaMKII mediates both LTP and LTD using a mechanism for differential substrate site selection.

Traditionally, hippocampal long-term potentiation (LTP) of synaptic strength requires Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) and other kinases, whereas long-term depression (LTD) requires phosphatases. Here, we found that LTD also requires CaMKII and its phospho-T286-induced "autonomous" (Ca(2+)-independent) activity. However, whereas LTP is known to induce phosphorylation of the AMPA-type glutamate receptor (AMPAR) subunit GluA1 at S831, LTD instead induced CaMKII-mediated phosphorylation at S567, a site known to reduce synaptic GluA1 localization. GluA1 S831 phosphorylation by "autonomous" CaMKII was further stimulated by Ca(2+)/CaM, as expected for traditional substrates. By contrast, GluA1 S567 represents a distinct substrate class that is unaffected by such stimulation. This differential regulation caused GluA1 S831 to be favored by LTP-type stimuli (strong but brief), whereas GluA1 S567 was favored by LTD-type stimuli (weak but prolonged). Thus, requirement of autonomous CaMKII in opposing forms of plasticity involves distinct substrate classes that are differentially regulated to enable stimulus-dependent substrate-site preference.

Excitotoxic glutamate insults block autophagic flux in hippocampal neurons.

Excitotoxic insults such as cerebral ischemia are thought to enhance neuronal autophagy, which is then thought to promote neuronal cell death. Excitotoxic insults indeed increase autophagy markers. Notably, however, autophagy markers can be increased either by autophagy induction (as this enhances their production) or by late-stage autophagy inhibition (as this prevents their degradation during autophagic flux). By comparing each condition with and without protease inhibitors that prevent autophagic degradation of the autophagy markers, the results of this study show that excitotoxic glutamate increases autophagy markers by a late-stage block of autophagy. Initially, this study set out to test if the CaMKII inhibitor tatCN21 mediates its post-insult neuroprotection by regulating autophagy. While tatCN21 partially inhibited basal autophagy in hippocampal neurons, it had no effects on the already blocked autophagy after excitotoxic glutamate insults, indicating that autophagy inhibition is not its neuroprotective mechanism. Additionally, while the autophagy inhibitor chloroquine had no effect, significant neuroprotection was seen instead with two drugs that enhance autophagy induction by different mechanisms, rapamycin (mTOR-dependent) and trehalose (mTOR-independent). This suggests that therapeutic approaches should seek to enhance rather than inhibit autophagy, not only in neurodegenerative diseases (where such approach is widely accepted) but also after acute excitotoxic insults. Together, these findings significantly reshape the current view on the mutual cross-regulation of autophagy and excitotoxicity.