Autophosphorylation of alphaCaMKII downregulates excitability of CA1 pyramidal neurons following synaptic stimulation

A critical role for CaMKII in behavioral timescale synaptic plasticity in hippocampal CA1 pyramidal neurons

Behavioral timescale synaptic plasticity (BTSP) is a type of non-Hebbian synaptic plasticity reported to underlie place field formation. Despite this important function, the molecular mechanisms underlying BTSP are poorly understood. The α-calcium-calmodulin-dependent protein kinase II (αCaMKII) is activated by synaptic transmission-mediated calcium influx, and its subsequent phosphorylation is central to synaptic plasticity. Because the activity of αCaMKII is known to outlast the event triggering phosphorylation, we hypothesized that it could mediate the extended timescale of BTSP. To examine the role of αCaMKII in BTSP, we performed whole-cell in vivo and in vitro recordings in CA1 pyramidal neurons from mice engineered with a point mutation at the autophosphorylation site (T286A) causing accelerated signaling kinetics. Here, we demonstrate a profound deficit in synaptic plasticity, strongly suggesting that αCaMKII signaling is required for BTSP. This study elucidates part of the molecular mechanism of BTSP and provides insight into the function of αCaMKII in place cell formation and ultimately learning and memory.

Heparan Sulfates Regulate Axonal Excitability and Context Generalization through Ca2+/Calmodulin-Dependent Protein Kinase II

Our previous studies demonstrated that enzymatic removal of highly sulfated heparan sulfates with heparinase 1 impaired axonal excitability and reduced expression of ankyrin G at the axon initial segments in the CA1 region of the hippocampus ex vivo, impaired context discrimination in vivo, and increased Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity in vitro. Here, we show that in vivo delivery of heparinase 1 in the CA1 region of the hippocampus elevated autophosphorylation of CaMKII 24 h after injection in mice. Patch clamp recording in CA1 neurons revealed no significant heparinase effects on the amplitude or frequency of miniature excitatory and inhibitory postsynaptic currents, while the threshold for action potential generation was increased and fewer spikes were generated in response to current injection. Delivery of heparinase on the next day after contextual fear conditioning induced context overgeneralization 24 h after injection. Co-administration of heparinase with the CaMKII inhibitor (autocamtide-2-related inhibitory peptide) rescued neuronal excitability and expression of ankyrin G at the axon initial segment. It also restored context discrimination, suggesting the key role of CaMKII in neuronal signaling downstream of heparan sulfate proteoglycans and highlighting a link between impaired CA1 pyramidal cell excitability and context generalization during recall of contextual memories.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Repeated exposure to microcystin-leucine-arginine potentiates excitotoxicity induced by a low dose of kainate

Microcystin-leucine-arginine (MLCR) is a cyanobacterial toxin, and has been demonstrated to cause neurotoxicity. In addition, MCLR has been identified as an inhibitor of protein phosphatase (PP)1 and PP2A, which are known to regulate the phosphorylation of various molecules related to synaptic excitability. Thus, in the present study, we examined whether MCLR exposure affects seizures induced by a low dose of kainic acid (KA; 0.05 μg, i.c.v.) administration. KA-induced seizure occurrence and seizure score significantly increased after repeated exposure to MCLR (2.5 or 5.0 μg/kg, i.p., once a day for 10 days), but not after acute MCLR exposure (2.5 or 5.0 μg/kg, i.p., 2 h and 30 min prior to KA administration), and hippocampal neuronal loss was consistently facilitated by repeated exposure to MCLR. In addition, repeated MCLR significantly elevated the membrane expression of kainate receptor GluK2 subunits, p-pan-protein kinase C (PKC), and p-extracellular signal-related kinase (ERK) at 1 h after KA. However, KA-induced membrane expression of Ca²⁺/calmodulin-dependent kinase II (CaMKII) was significantly reduced by repeated MCLR exposure. Consistent with the enhanced seizures and neurodegeneration, MCLR exposure significantly potentiated KA-induced oxidative stress and microglial activation, which was accompanied by increased expression of p-ERK and p-PKCδ in the hippocampus. The combined results suggest that repeated MCLR exposure potentiates KA-induced excitotoxicity in the hippocampus by increasing membrane GluK2 expression and enhancing oxidative stress and neuroinflammation through the modulation of p-CaMKII, p-PKC, and p-ERK.

Sex Differences in Context-Driven Reinstatement of Methamphetamine Seeking is Associated with Distinct Neuroadaptations in the Dentate Gyrus

The present study examined differences in operant responses in adult male and female rats during distinct phases of addiction. Males and females demonstrated escalation in methamphetamine (0.05 mg/kg, i.v.) intake with females showing enhanced latency to escalate, and bingeing. Following protracted abstinence, females show reduced responses during extinction, and have greater latency to extinguish compared with males, indicating reduced craving. Females demonstrated lower context-driven reinstatement compared to males, indicating that females have less motivational significance to the context associated with methamphetamine. Whole-cell patch-clamp recordings on dentate gyrus (DG) granule cell neurons (GCNs) were performed in acute brain slices from controls and methamphetamine experienced male and female rats, and neuronal excitability was evaluated from GCNs. Reinstatement of methamphetamine seeking reduced spiking in males, and increased spiking in females compared to controls, demonstrating distinct neuroadaptations in intrinsic excitability of GCNs in males and females. Reduced excitability of GCNs in males was associated with enhanced levels of neural progenitor cells, expression of plasticity-related proteins including CaMKII, and choline acetyltransferase in the DG. Enhanced excitability in females was associated with an increased GluN2A/2B ratio, indicating changes in postsynaptic GluN subunit composition in the DG. Altered intrinsic excitability of GCNs was associated with reduced mossy fiber terminals in the hilus and pyramidal projections, demonstrating compromised neuroplasticity in the DG in both sexes. The alterations in excitability, plasticity-related proteins, and mossy fiber density were correlated with enhanced activation of microglial cells in the hilus, indicating neuroimmune responses in both sexes. Together, the present results indicate sexually dimorphic adaptive biochemical changes in excitatory neurotransmitter systems in the DG and highlight the importance of including sex as a biological variable in exploring neuroplasticity and neuroimmune changes that predict enhanced relapse to methamphetamine-seeking behaviors.

Activity-Dependent Facilitation of CaV1.3 Calcium Channels Promotes KCa3.1 Activation in Hippocampal Neurons

Ca_V1 L-type calcium channels are key to regulating neuronal excitability, with the range of functional roles enhanced by interactions with calmodulin, accessory proteins, or CaMKII that modulate channel activity. In hippocampal pyramidal cells, a prominent elevation of Ca_V1 activity is apparent in late channel openings that can last for seconds following a depolarizing stimulus train. The current study tested the hypothesis that a reported interaction among Ca_V1.3 channels, the scaffolding protein densin, and CaMKII could generate a facilitation of channel activity that outlasts a depolarizing stimulus. We found that Ca_V1.3 but not Ca_V1.2 channels exhibit a long-duration calcium-dependent facilitation (L-CDF) that lasts up to 8 s following a brief 50 Hz stimulus train, but only when coexpressed with densin and CaMKII. To test the physiological role for Ca_V1.3 L-CDF, we coexpressed the intermediate-conductance KCa3.1 potassium channel, revealing a strong functional coupling to Ca_V1.3 channel activity that was accentuated by densin and CaMKII. Moreover, the Ca_V1.3-densin-CaMKII interaction gave rise to an outward tail current of up to 8 s duration following a depolarizing stimulus in both tsA-201 cells and male rat CA1 pyramidal cells. A slow afterhyperpolarization in pyramidal cells was reduced by a selective block of Ca_V1 channels by isradipine, a CaMKII blocker, and siRNA knockdown of densin, and spike frequency increased upon selective block of Ca_V1 channel conductance. The results are important in revealing a Ca_V1.3-densin-CaMKII interaction that extends the contribution of Ca_V1.3 calcium influx to a time frame well beyond a brief input train.SIGNIFICANCE STATEMENT Ca_V1 L-type calcium channels play a key role in regulating the output of central neurons by providing calcium influx during repetitive inputs. This study identifies a long-duration calcium-dependent facilitation (L-CDF) of Ca_V1.3 channels that depends on the scaffolding protein densin and CaMKII and that outlasts a depolarizing stimulus by seconds. We further show a tight functional coupling between Ca_V1.3 calcium influx and the intermediate-conductance KCa3.1 potassium channel that promotes an outward tail current of up to 8 s following a depolarizing stimulus. Tests in CA1 hippocampal pyramidal cells reveal that a slow AHP is reduced by blocking different components of the Ca_V1.3-densin-CaMKII interaction, identifying an important role for Ca_V1.3 L-CDF in regulating neuronal excitability.

Memory Erasure Experiments Indicate a Critical Role of CaMKII in Memory Storage

The abundant synaptic protein CaMKII is necessary for long-term potentiation (LTP) and memory. However, whether CaMKII is required only during initial processes or whether it also mediates memory storage remains unclear. The most direct test of a storage role is the erasure test. In this test, a putative memory molecule is inhibited after learning. The key prediction is that this should produce persistent memory erasure even after the inhibitory agent is removed. We conducted this test using transient viral (HSV) expression of dominant-negative CaMKII-alpha (K42M) in the hippocampus. This produced persistent erasure of conditioned place avoidance. As an additional test, we found that expression of activated CaMKII (T286D/T305A/T306A) impaired place avoidance, a result not expected if a process other than CaMKII stores memory. Our behavioral results, taken together with prior experiments on LTP, strongly support a critical role of CaMKII in LTP maintenance and memory storage.

Differential CaMKII regulation by voltage-gated calcium channels in the striatum

Calcium signaling regulates synaptic plasticity and many other functions in striatal medium spiny neurons to modulate basal ganglia function. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a major calcium-dependent signaling protein that couples calcium entry to diverse cellular changes. CaMKII activation results in autophosphorylation at Thr286 and sustained calcium-independent CaMKII activity after calcium signals dissipate. However, little is known about the mechanisms regulating striatal CaMKII. To address this, mouse brain slices were treated with pharmacological modulators of calcium channels and punches of dorsal striatum were immunoblotted for CaMKII Thr286 autophosphorylation as an index of CaMKII activation. KCl depolarization increased levels of CaMKII autophosphorylation ~2-fold; this increase was blocked by an LTCC antagonist and was mimicked by treatment with pharmacological LTCC activators. The chelation of extracellular calcium robustly decreased basal CaMKII autophosphorylation within 5min and increased levels of total CaMKII in cytosolic fractions, in addition to decreasing the phosphorylation of CaMKII sites in the GluN2B subunit of NMDA receptors and the GluA1 subunit of AMPA receptors. We also found that the maintenance of basal levels of CaMKII autophosphorylation requires low-voltage gated T-type calcium channels, but not LTCCs or R-type calcium channels. Our findings indicate that CaMKII activity is dynamically regulated by multiple calcium channels in the striatum thus coupling calcium entry to key downstream substrates.

Neuronal excitability and calcium/calmodulin-dependent protein kinase type II: location, location, location

Calcium/calmodulin-dependent protein kinase type II (CaMKII) is a highly abundant serine/threonine kinase comprising a significant fraction of total protein in mammalian forebrain and forming a major component of the postsynaptic density. CaMKII is essential for certain forms of synaptic plasticity and memory consolidation and this is mediated through substrate binding and intramolecular phosphorylation of holoenzyme subunits. CaMKII is multifunctional; it targets a variety of cellular substrates, and this diversity depends on holoenzyme subunit composition. CaMKII comprises homooligomeric and heterooligomeric complexes generated from four subunits (α, β, δ, and γ) encoded by separate genes that are further expanded by extensive alternative splicing to more than 30 different isoforms. Much attention has been paid to understanding the regulation of CaMKII function through its structural diversity and/or substrate specificity. However, given the importance of subunit composition to holoenzyme activity, it is likely that specificity of cellular expression of CaMKII isoforms also plays a major role in regulation of enzyme function. Herein we review the cellular colocalization of CaMKII isoforms with special regard to the cell-type specificity of isoform expression in brain. In addition, we highlight the remarkable specificity of subcellular localization by the CaMKIIα isoform. In addition, we discuss the role that this cellular specificity of expression might play in propagating the type of recurrent neuronal activity associated with disorders such as temporal lobe epilepsy.

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.

α-Calcium calmodulin kinase II modulates the temporal structure of hippocampal bursting patterns

The alpha calcium calmodulin kinase II (α-CaMKII) is known to play a key role in CA1/CA3 synaptic plasticity, hippocampal place cell stability and spatial learning. Additionally, there is evidence from hippocampal electrophysiological slice studies that this kinase has a role in regulating ion channels that control neuronal excitability. Here, we report in vivo single unit studies, with α-CaMKII mutant mice, in which threonine 305 was replaced with an aspartate (α-CaMKII(T305D) mutants), that indicate that this kinase modulates spike patterns in hippocampal pyramidal neurons. Previous studies showed that α-CaMKII(T305D) mutants have abnormalities in both hippocampal LTP and hippocampal-dependent learning. We found that besides decreased place cell stability, which could be caused by their LTP impairments, the hippocampal CA1 spike patterns of α-CaMKII(T305D) mutants were profoundly abnormal. Although overall firing rate, and overall burst frequency were not significantly altered in these mutants, inter-burst intervals, mean number of intra-burst spikes, ratio of intra-burst spikes to total spikes, and mean intra-burst intervals were significantly altered. In particular, the intra burst intervals of place cells in α-CaMKII(T305D) mutants showed higher variability than controls. These results provide in vivo evidence that besides its well-known function in synaptic plasticity, α-CaMKII, and in particular its inhibitory phosphorylation at threonine 305, also have a role in shaping the temporal structure of hippocampal burst patterns. These results suggest that some of the molecular processes involved in acquiring information may also shape the patterns used to encode this information.

Two selected models of missense mutations in mice for the study of learning behaviour

A large number of genome-wide association studies have linked missense mutations, mutations altering the amino acid sequence of proteins, with cognitive impairment in humans. However, these studies are correlative. As there may be multiple mutations for one particular patient, it is essential to address the functional impact of a missense mutation in a model system. The most suitable model system is the generation of knock-in mice with the homologous missense mutation followed by behavioural phenotyping. Here, we review selected mutants demonstrating an impact of single mutations on learning and memory in mice and discuss the relevance of such studies for understanding the role of these polymorphisms in human behaviour. We conclude that using these animal models has been instrumental in decoding the mechanisms underlying behaviour, and assists the design of therapeutic strategies for humans.