The CaMKII K42M and K42R mutations are equivalent in suppressing kinase activity and targeting

Studying CaMKII: Tools and standards

The Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a ubiquitous mediator of cellular Ca2+ signals with both enzymatic and structural functions. Here, we briefly introduce the complex regulation of CaMKII and then provide a comprehensive overview of the expanding toolbox to study CaMKII. Beyond a variety of distinct mutants, these tools now include optical methods for measurement and manipulation, with the latter including light-induced inhibition, stimulation, and sequestration. Perhaps most importantly, there are now three mechanistically distinct classes of specific CaMKII inhibitors, and their combined use enables the interrogation of CaMKII functions in a manner that is powerful and sophisticated yet also accessible. This review aims to provide guidelines for the interpretation of the results obtained with these tools, with careful consideration of their direct and indirect effects.

Real-time single-molecule imaging of CaMKII-calmodulin interactions

The binding of calcium/calmodulin (CAM) to calcium/calmodulin-dependent protein kinase II (CaMKII) initiates an ATP-driven cascade that triggers CaMKII autophosphorylation. The autophosphorylation in turn increases the CaMKII affinity for CAM. Here, we studied the ATP dependence of CAM association with the actin-binding CaMKIIβ isoform using single-molecule total internal reflection fluorescence microscopy. Rhodamine-CAM associations/dissociations to surface-immobilized Venus-CaMKIIβ were resolved with 0.5 s resolution from video records, batch-processed with a custom algorithm. CAM occupancy was determined simultaneously with spot-photobleaching measurement of CaMKII holoenzyme stoichiometry. We show the ATP-dependent increase of the CAM association requires dimer formation for both the α and β isoforms. The study of mutant β holoenzymes revealed that the ATP-dependent increase in CAM affinity results in two distinct states. The phosphorylation-defective (T287.306-307A) holoenzyme resides only in the low-affinity state. CAM association is further reduced in the T287A holoenzyme relative to T287.306-307A. In the absence of ATP, the affinity of CAM for the T287.306-307A mutant and the wild-type monomer are comparable. The affinity of the ATP-binding impaired (K43R) mutant is even weaker. In ATP, the K43R holoenzyme resides in the low-affinity state. The phosphomimetic mutant (T287D) resides only in a 1000-fold higher-affinity state, with mean CAM occupancy of more than half of the 14-mer holoenzyme stoichiometry in picomolar CAM. ATP promotes T287D holoenzyme disassembly but does not elevate CAM occupancy. Single Poisson distributions characterized the ATP-dependent CAM occupancy of mutant holoenzymes. In contrast, the CAM occupancy of the wild-type population had a two-state distribution with both low- and high-affinity states represented. The low-affinity state was the dominant state, a result different from published in vitro assays. Differences in assay conditions can alter the balance between activating and inhibitory autophosphorylation. Bound ATP could be sufficient for CaMKII structural function, while antagonistic autophosphorylations may tune CaMKII kinase-regulated action-potential frequency decoding in vivo.

Activity-Dependent Stabilization of Nascent Dendritic Spines Requires Nonenzymatic CaMKIIα Function

The outgrowth and stabilization of nascent dendritic spines are crucial processes underlying learning and memory. Most new spines retract shortly after growth; only a small subset is stabilized and integrated into the new circuit connections that support learning. New spine stabilization has been shown to rely upon activity-dependent molecular mechanisms that also contribute to long-term potentiation (LTP) of synaptic strength. Indeed, disruption of the activity-dependent targeting of the kinase CaMKIIα to the GluN2B subunit of the NMDA-type glutamate receptor disrupts both LTP and activity-dependent stabilization of new spines. Yet it is not known which of CaMKIIα's many enzymatic and structural functions are important for new spine stabilization. Here, we used two-photon imaging and photolysis of caged glutamate to monitor the activity-dependent stabilization of new dendritic spines on hippocampal CA1 neurons from mice of both sexes in conditions where CaMKIIα functional and structural interactions were altered. Surprisingly, we found that inhibiting CaMKIIα kinase activity either genetically or pharmacologically did not impair activity-dependent new spine stabilization. In contrast, shRNA knockdown of CaMKIIα abolished activity-dependent new spine stabilization, which was rescued by co-expressing shRNA-resistant full-length CaMKIIα, but not by a truncated monomeric CaMKIIα. Notably, overexpression of phospho-mimetic CaMKIIα-T286D, which exhibits activity-independent targeting to GluN2B, enhanced basal new spine survivorship in the absence of additional glutamatergic stimulation, even when kinase activity was disrupted. Together, our results support a model in which nascent dendritic spine stabilization requires structural and scaffolding interactions mediated by dodecameric CaMKIIα that are independent of its enzymatic activities.

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.

Distinct synaptic pools of DAPK1 differentially regulate activity-dependent synaptic CaMKII accumulation

The death-associated protein kinase 1 (DAPK1) regulates the synaptic movement of the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII). Synaptic CaMKII accumulation is mediated via binding to the NMDA-receptor subunit GluN2B and is required for long-term potentiation (LTP). By contrast, long-term depression (LTD) instead requires specific suppression of this movement, which is mediated by competitive DAPK1 binding to GluN2B. We find here that DAPK1 localizes to synapses via two distinct mechanisms: basal localization requires F-actin, but retention of DAPK1 at synapses during LTD requires an additional binding mode, likely to GluN2B. While F-actin binding mediates DAPK1 enrichment at synapses, it is not sufficient to suppress synaptic CaMKII movement. However, it is a prerequisite that enables the additional LTD-specific binding mode of DAPK1, which in turn mediates suppression of the CaMKII movement. Thus, both modes of synaptic DAPK1 localization work together to regulate synaptic CaMKII localization and thereby synaptic plasticity.

Short-term CaMKII inhibition with tatCN19o does not erase pre-formed memory in mice and is neuroprotective in pigs

The Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a central regulator of learning and memory, which poses a problem for targeting it therapeutically. Indeed, our study supports prior conclusions that long-term interference with CaMKII signaling can erase pre-formed memories. By contrast, short-term pharmacological CaMKII inhibition with the neuroprotective peptide tatCN19o interfered with learning in mice only mildly and transiently (for less than 1 h) and did not at all reverse pre-formed memories. These results were obtained with ≥500-fold of the dose that protected hippocampal neurons from cell death after a highly clinically relevant pig model of transient global cerebral ischemia: ventricular fibrillation followed by advanced life support and electrical defibrillation to induce the return of spontaneous circulation. Of additional importance for therapy development, our preliminary cardiovascular safety studies in mice and pig did not indicate any concerns with acute tatCN19o injection. Taken together, although prolonged interference with CaMKII signaling can erase memory, acute short-term CaMKII inhibition with tatCN19o did not cause such retrograde amnesia that would pose a contraindication for therapy.

Short-term CaMKII inhibition with tatCN19o does not erase pre-formed memory and is neuroprotective in non-rodents

The Ca ²⁺ /calmodulin-dependent protein kinase II (CaMKII) is a central regulator of learning and memory, which poses a problem for targeting it therapeutically. Indeed, our study supports prior conclusions that long-term interference with CaMKII signaling can erase pre-formed memories. By contrast, short-term pharmacological CaMKII inhibition with tatCN19o interfered with learning in mice only mildly and transiently (for less than 1 h) and did not at all reverse pre-formed memories. This was at ≥500fold of the dose that protected hippocampal neurons from cell death after a highly clinically relevant pig model of transient global cerebral ischemia: ventricular fibrillation followed by advanced life support and electrical defibrillation to induce return of spontaneous circulation. Of additional importance for therapeutic development, cardiovascular safety studies in mice and pig did not indicate any concerns with acute tatCN19o injection. Taken together, even though prolonged interference with CaMKII signaling can erase memory, acute short-term CaMKII inhibition with tatCN19o did not cause such retrograde amnesia that would pose a contraindication for therapy.

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

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

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Aβ and the CaMKII regulatory domain share a region of homology

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Suppression of CaMKII movement in neurons by Aβ requires CaMKII activity

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Aβ does not directly affect CaMKII activity, T286 phosphorylation, or GluN2B binding

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Thus, the Aβ effects on CaMKII in neurons require indirect signaling mechanisms

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.

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Two of six CaMKII variants found in patients with schizophrenia showed impairments

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R396stop disrupted holoenzyme structure, T286 autophosphorylation, and GluN2B binding

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R8H reduced T286 autophosphorylation and stimulated activity

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Two of the four other variants were later found also in the general population

Young DAPK1 knockout mice have altered presynaptic function

The death-associated protein kinase 1 (DAPK1) has recently been shown to have a physiological function in long-term depression (LTD) of glutamatergic synapses: acute inhibition of DAPK1 blocked the LTD that is normally seen at the hippocampal CA1 synapse in young mice, and a pharmacogenetic combination approach showed that this specifically required DAPK1-mediated suppression of postsynaptic Ca²⁺/calmodulin-dependent protein kinase II binding to the NMDA-type glutamate receptor (NMDAR) subunit GluN2B during LTD stimuli. Surprisingly, we found here that genetic deletion of DAPK1 (in DAPK1^-/- mice) did not reduce LTD. Paired pulse facilitation experiments indicated a presynaptic compensation mechanism: in contrast to wild-type mice, LTD stimuli in DAPK1^-/- mice decreased presynaptic release probability. Basal synaptic strength was normal in young DAPK1^-/- mice, but basal glutamate release probability was reduced, an effect that normalized with maturation.NEW & NOTEWORTHY Young death-associated protein kinase 1 (DAPK1) knockout mice have reduced basal glutamate release probability, an effect that normalized with maturation. This provided a compensatory mechanism that may have prevented a reduction of long-term depression in the young DAPK1 knockout mice.

CaMKII binding to GluN2B at S1303 has no role in acute or inflammatory pain

Activation of Ca²⁺/calmodulin kinase II (CaMKII) and the N-Methyl D-aspartate receptor (NMDAR), particularly its GluN2B subunit, contribute to the central sensitization of nociceptive pathways and persistent pain. Using mutant mice wherein the activity-driven binding of CaMKII to S1303 in GluN2B is abrogated (GluN2BKI), this study investigated the importance of this interaction for acute and persistent inflammatory nociception. GluN2BKI, wild type and heterozygote mice did not differ in responses to acute noxious heat stimuli as measured with tail flick, paw flick, or hot plate assays, nor did they differ in their responses to mechanical stimulation with von Frey filaments. Surprisingly, the three genotypes exhibited similar spontaneous pain behaviors and hypersensitivity to heat or mechanical stimuli induced by intraplantar injection of capsaicin; however, GluN2BKI mice did not immediately attend to the paw. WT and GluN2BKI mice also did not differ in the nociceptive behaviors elicited by intraplantar injection of formalin, even though MK801 greatly reduced these behaviors in both genotypes concordant with NMDAR dependence. CaMKII binding to GluN2B at S1303 therefore does not appear to be critical for the development of inflammatory nociception. Finally, intrathecal KN93 reduced formalin-induced nociceptive behaviors in GluN2BKI mice. KN93 does not inhibit CaKMII, but rather binds Ca²⁺/calmodulin. It has multiple other targets including Ca²⁺-, Na⁺- and K⁺-channels, as well as various kinases. Therefore, the use of GluN2BKI mice provided genetic specificity in assessing the role of CaMKII in inflammatory pain signaling cascades. These results challenge current thinking on the involvement of the CaMKII-NMDAR interaction in inflammatory pain.