Site-selective autophosphorylation of Ca2+/calmodulin-dependent protein kinase II as a synaptic encoding mechanism

Pattern and predictability in memory formation: from molecular mechanisms to clinical relevance

Most long-term memories are formed as a consequence of multiple experiences. The temporal spacing of these experiences is of considerable importance: experiences distributed over time (spaced training) are more easily encoded and remembered than either closely spaced experiences, or a single prolonged experience (massed training). In this article, we first review findings from studies in animal model systems that examine the cellular and molecular properties of the neurons and circuits in the brain that underlie training pattern sensitivity during long-term memory (LTM) formation. We next focus on recent findings which have begun to elucidate the mechanisms that support inter-trial interactions during the induction of LTM. Finally, we consider the implications of these findings for developing therapeutic strategies to address questions of direct clinical relevance.

Postsynaptic signal transduction models for long-term potentiation and depression

More than a hundred biochemical species, activated by neurotransmitters binding to transmembrane receptors, are important in long-term potentiation (LTP) and long-term depression (LTD). To investigate which species and interactions are critical for synaptic plasticity, many computational postsynaptic signal transduction models have been developed. The models range from simple models with a single reversible reaction to detailed models with several hundred kinetic reactions. In this study, more than a hundred models are reviewed, and their features are compared and contrasted so that similarities and differences are more readily apparent. The models are classified according to the type of synaptic plasticity that is modeled (LTP or LTD) and whether they include diffusion or electrophysiological phenomena. Other characteristics that discriminate the models include the phase of synaptic plasticity modeled (induction, expression, or maintenance) and the simulation method used (deterministic or stochastic). We find that models are becoming increasingly sophisticated, by including stochastic properties, integrating with electrophysiological properties of entire neurons, or incorporating diffusion of signaling molecules. Simpler models continue to be developed because they are computationally efficient and allow theoretical analysis. The more complex models permit investigation of mechanisms underlying specific properties and experimental verification of model predictions. Nonetheless, it is difficult to fully comprehend the evolution of these models because (1) several models are not described in detail in the publications, (2) only a few models are provided in existing model databases, and (3) comparison to previous models is lacking. We conclude that the value of these models for understanding molecular mechanisms of synaptic plasticity is increasing and will be enhanced further with more complete descriptions and sharing of the published models.

Temporal sensitivity of protein kinase a activation in late-phase long term potentiation

Protein kinases play critical roles in learning and memory and in long term potentiation (LTP), a form of synaptic plasticity. The induction of late-phase LTP (L-LTP) in the CA1 region of the hippocampus requires several kinases, including CaMKII and PKA, which are activated by calcium-dependent signaling processes and other intracellular signaling pathways. The requirement for PKA is limited to L-LTP induced using spaced stimuli, but not massed stimuli. To investigate this temporal sensitivity of PKA, a computational biochemical model of L-LTP induction in CA1 pyramidal neurons was developed. The model describes the interactions of calcium and cAMP signaling pathways and is based on published biochemical measurements of two key synaptic signaling molecules, PKA and CaMKII. The model is stimulated using four 100 Hz tetani separated by 3 sec (massed) or 300 sec (spaced), identical to experimental L-LTP induction protocols. Simulations show that spaced stimulation activates more PKA than massed stimulation, and makes a key experimental prediction, that L-LTP is PKA-dependent for intervals larger than 60 sec. Experimental measurements of L-LTP demonstrate that intervals of 80 sec, but not 40 sec, produce PKA-dependent L-LTP, thereby confirming the model prediction. Examination of CaMKII reveals that its temporal sensitivity is opposite that of PKA, suggesting that PKA is required after spaced stimulation to compensate for a decrease in CaMKII. In addition to explaining the temporal sensitivity of PKA, these simulations suggest that the use of several kinases for memory storage allows each to respond optimally to different temporal patterns.

The hippocampus is a part of the cerebral cortex intimately involved in learning and memory behavior. A common cellular model of learning is a long lasting form of long term potentiation (L-LTP) in the hippocampus, because it shares several characteristics with learning. For example, both learning and long term potentiation exhibit sensitivity to temporal patterns of synaptic inputs and share common intracellular events such as activation of specific intracellular signaling pathways. Therefore, understanding the pivotal molecules in the intracellular signaling pathways underlying temporal sensitivity of L-LTP in the hippocampus may illuminate mechanisms underlying learning. We developed a computational model to evaluate whether the signaling pathways leading to activation of the two critical enzymes: protein kinase A and calcium-calmodulin-dependent kinase II are sufficient to explain the experimentally observed temporal sensitivity. Indeed, the simulations demonstrate that these enzymes exhibit different temporal sensitivities, and make a key experimental prediction, that L-LTP is dependent on protein kinase A for intervals larger than 60 sec. Measurements of hippocampal L-LTP confirm this prediction, demonstrating the value of a systems biology approach to computational neuroscience.

A simulation study on the activation of cardiac CaMKII delta-isoform and its regulation by phosphatases

Although the highly conserved Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is known to play an essential role in cardiac myocytes, its involvement in the frequency-dependent acceleration of relaxation is still controversial. To investigate the functional significance of CaMKII autophosphorylation and its regulation by protein phosphatases (PPs) in heart, we developed a new mathematical model for the CaMKIIdelta isoform. Due to better availability of experimental data, the model was first adjusted to the kinetics of the neuronal CaMKIIalpha isoform and then converted to a CaMKIIdelta model by fitting to kinetic data of the delta isoform. Both models satisfactorily reproduced experimental data of the CaMKII-calmodulin interaction, the autophosphorylation rate, and the frequency dependence of activation. The level of autophosphorylated CaMKII cumulatively increased upon starting the Ca(2+) stimulation at 3 Hz in the delta model. Variations in PP concentration remarkably affected the frequency-dependent activation of CaMKIIdelta, suggesting that cellular PP activity plays a key role in adjusting CaMKII activation in heart. The inhibitory effect of PP was stronger for CaMKIIalpha compared to CaMKIIdelta. Simulation results revealed a potential involvement of CaMKIIdelta autophosphorylation in the frequency-dependent acceleration of relaxation at physiological heart rates and PP concentrations.

STDP in a bistable synapse model based on CaMKII and associated signaling pathways

The calcium/calmodulin-dependent protein kinase II (CaMKII) plays a key role in the induction of long-term postsynaptic modifications following calcium entry. Experiments suggest that these long-term synaptic changes are all-or-none switch-like events between discrete states. The biochemical network involving CaMKII and its regulating protein signaling cascade has been hypothesized to durably maintain the evoked synaptic state in the form of a bistable switch. However, it is still unclear whether experimental LTP/LTD protocols lead to corresponding transitions between the two states in realistic models of such a network. We present a detailed biochemical model of the CaMKII autophosphorylation and the protein signaling cascade governing the CaMKII dephosphorylation. As previously shown, two stable states of the CaMKII phosphorylation level exist at resting intracellular calcium concentration, and high calcium transients can switch the system from the weakly phosphorylated (DOWN) to the highly phosphorylated (UP) state of the CaMKII (similar to a LTP event). We show here that increased CaMKII dephosphorylation activity at intermediate Ca(2+) concentrations can lead to switching from the UP to the DOWN state (similar to a LTD event). This can be achieved if protein phosphatase activity promoting CaMKII dephosphorylation activates at lower Ca(2+) levels than kinase activity. Finally, it is shown that the CaMKII system can qualitatively reproduce results of plasticity outcomes in response to spike-timing dependent plasticity (STDP) and presynaptic stimulation protocols. This shows that the CaMKII protein network can account for both induction, through LTP/LTD-like transitions, and storage, due to its bistability, of synaptic changes.

Role of the neurogranin concentrated in spines in the induction of long-term potentiation

Synaptic plasticity in CA1 hippocampal neurons depends on Ca2+ elevation and the resulting activation of calmodulin-dependent enzymes. Induction of long-term depression (LTD) depends on calcineurin, whereas long-term potentiation (LTP) depends on Ca2+/calmodulin-dependent protein kinase II (CaMKII). The concentration of calmodulin in neurons is considerably less than the total concentration of the apocalmodulin-binding proteins neurogranin and GAP-43, resulting in a low level of free calmodulin in the resting state. Neurogranin is highly concentrated in dendritic spines. To elucidate the role of neurogranin in synaptic plasticity, we constructed a computational model with emphasis on the interaction of calmodulin with neurogranin, calcineurin, and CaMKII. The model shows how the Ca2+ transients that occur during LTD or LTP induction affect calmodulin and how the resulting activation of calcineurin and CaMKII affects AMPA receptor-mediated transmission. In the model, knockout of neurogranin strongly diminishes the LTP induced by a single 100 Hz, 1 s tetanus and slightly enhances LTD, in accord with experimental data. Our simulations show that exchange of calmodulin between a spine and its parent dendrite is limited. Therefore, inducing LTP with a short tetanus requires calmodulin stored in spines in the form of rapidly dissociating calmodulin-neurogranin complexes.

A role for calcium/calmodulin-dependent protein kinase II in cardiac disease and arrhythmia

More than 20 years have passed since the discovery that a collection of specific calcium/calmodulin-dependent phosphorylation events is the result of a single multifunctional kinase. Since that time, we have learned a great deal about this multifunctional and ubiquitous kinase, known today as calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII is interesting not only for its widespread distribution and broad specificity but also for its biophysical properties, most notably its activation by the critical second messenger complex calcium/calmodulin and its autophosphorylating capability. A central role for CaMKII has been identified in regulating a diverse array of fundamental cellular activities. Furthermore, altered CaMKII activity profoundly impacts function in the brain and heart. Recent findings that CaMKII expression in the heart changes during hypertrophy, heart failure, myocardial ischemia, and infarction suggest that CaMKII may be a viable therapeutic target for patients suffering from common forms of heart disease.

Sensitivity of CaM kinase II to the frequency of Ca2+ oscillations: a simple model

The rules that govern the activation and autophosphorylation of the multifunctional Ca2+-calmodulin kinase II (CaMKII) by Ca2+ and calmodulin (CaM) are thought to underlie its ability to decode Ca2+ oscillations and to control multiple cellular functions. We propose a simple biophysical model for the activation of CaMKII by Ca2+ and calmodulin. The model describes the transition of the subunits of the kinase between their different possible states (inactive, bound to Ca2+-CaM, phosphorylated at Thr(286), trapped and autonomous). All transitions are described by classical kinetic equations except for the autophosphorylation step, which is modeled in an empirical manner. The model quantitatively reproduces the experimentally demonstrated frequency sensitivity of CaMKII [Science 279 (1998) 227]. We further use the model to investigate the role of several characterized features of the kinase--as well as some that are not easily attainable by experiments--in its frequency-dependent responses. In cellular microdomains, CaMKII is expected to sense very brief Ca2+ spikes; our simulations under such conditions reveal that the enzyme response is tuned to optimal frequencies. This prediction is then confirmed by experimental data. This novel and simple model should help in understanding the rules that govern CaMKII regulation, as well as those involved in decoding intracellular Ca2+ signals.

A model synapse that incorporates the properties of short- and long-term synaptic plasticity

We propose a general computer model of a synapse, which incorporates mechanisms responsible for the realization of both short- and long-term synaptic plasticity-the two forms of experimentally observed plasticity that seem to be very significant for the performance of neuronal networks. The model consists of a presynaptic part based on the earlier 'double barrier synapse' model, and a postsynaptic compartment which is connected to the presynaptic terminal via a feedback, the sign and magnitude of which depend on postsynaptic Ca(2+) concentration. The feedback increases or decreases the amount of neurotransmitter which is in a ready for release state. The model adequately reproduced the phenomena of short- and long-term plasticity observed experimentally in hippocampal slices for CA3-CA1 synapses. The proposed model may be used in the investigation of certain real synapses to estimate their physiological parameters, and in the construction of realistic neuronal networks.

Neuronal CA2+/calmodulin-dependent protein kinase II: the role of structure and autoregulation in cellular function

Highly enriched in brain tissue and present throughout the body, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is central to the coordination and execution of Ca(2+) signal transduction. The substrates phosphorylated by CaMKII are implicated in homeostatic regulation of the cell, as well as in activity-dependent changes in neuronal function that appear to underlie complex cognitive and behavioral responses, including learning and memory. The architecture of CaMKII holoenzymes is unique in nature. The kinase functional domains (12 per holoenzyme) are attached by stalklike appendages to a gear-shaped core, grouped into two clusters of six. Each subunit contains a catalytic, an autoregulatory, and an association domain. Ca(2+)/calmodulin (CaM) binding disinhibits the autoregulatory domain, allowing autophosphorylation and complex changes in the enzyme's sensitivity to Ca(2+)/CaM, including the generation of Ca(2+)/CaM-independent activity, CaM trapping, and CaM capping. These processes confer a type of molecular memory to the autoregulation and activity of CaMKII. Its function is intimately shaped by its multimeric structure, autoregulation, isozymic type, and subcellular localization; these features and processes are discussed as they relate to known and potential cellular functions of this multifunctional protein kinase.

Inhibitory autophosphorylation of CaMKII controls PSD association, plasticity, and learning

To investigate the function of the alpha calcium-calmodulin-dependent kinase II (alphaCaMKII) inhibitory autophosphorylation at threonines 305 and/or 306, we generated knockin mice that express alphaCaMKII that cannot undergo inhibitory phosphorylation. In addition, we generated mice that express the inhibited form of alphaCaMKII, which resembles the persistently phosphorylated kinase at these sites. Our data demonstrate that blocking inhibitory phosphorylation increases CaMKII in the postsynaptic density (PSD), lowers the threshold for hippocampal long-term potentiation (LTP), and results in hippocampal-dependent learning that seems more rigid and less fine-tuned. Mimicking inhibitory phosphorylation dramatically decreased the association of CaMKII with the PSD and blocked both LTP and learning. These data demonstrate that inhibitory phosphorylation has a critical role in plasticity and learning.

Structure-function of the multifunctional Ca2+/calmodulin-dependent protein kinase II

Ca2+/calmodulin (CaM)-dependent protein kinase (CaMKII) is a ubiquitous mediator of Ca2+-linked signalling that phosphorylates a wide range of substrates to co-ordinate and regulate Ca2+-mediated alterations in cellular function. The transmission of information by the kinase from extracellular stimuli and the intracellular Ca2+ rise is not passive. Rather, its multimeric structure and autoregulation enable this enzyme to participate actively in the sensitivity, timing and location of its action. CaMKII can: (i) be activated in a Ca2+-spike frequency-dependent manner; (ii) become independent of its initial Ca2+/CaM activators; and (iii) undergo a 'molecular switch-like' behaviour, which is crucial for certain forms of learning and memory. CaMKII is derived from a family of four homologous but distinct genes, with over 30 alternatively spliced isoforms described at present. These isoforms possess diverse developmental and anatomical expression patterns, as well as subcellular localization. Six independent catalytic/autoregulatory domains are connected by a narrow stalk-like appendage to each hexameric ring within the dodecameric structure. Ca2+/CaM binding activates the enzyme by disinhibiting the autoregulatory domain; this process initiates an intra-holoenzyme autophosphorylation reaction that induces complex changes in the enzyme's sensitivity to Ca2+/CaM, including the generation of Ca2+/CaM-independent (autonomous) activity and marked increase in affinity for CaM. The role of CaMKII in Ca2+ signal transduction is shaped by its autoregulation, isoenzymic type and subcellular localization. The molecular determinants and mechanisms producing these processes are discussed as they relate to the structure-function of this multifunctional protein kinase.

Transient versus asymptotic dynamics of CaM kinase II: possible roles of phosphatase

Calmodulin-dependent protein kinase II (CaMKII) is known to play a key role during induction of long-term potentiation (LTP). Given the dependence of LTP on the frequency of synaptic activation, several previous modeling efforts have proposed that biochemical properties of CaMKII itself might be in part responsible for this dependence. Recently, De Koninck and Schulman (1998) have provided direct experimental evidence that the enzyme itself is sensitive to the frequency of Ca(2+) activation. Here we demonstrate the ability of a detailed biophysical model constructed solely on enzyme kinetics of purified proteins to generate the frequency sensitivity demonstrated by De Koninck and Schulman. Quantitative analysis of the model reveals that this frequency sensitivity is provided by a mechanism different from those previously postulated. This analysis leads to specific predictions concerning the effects of mutations on this process. We further employ the model to examine the asymptotic behavior of CaMKII-phosphatase system during longer simulated periods of stimulation. The analyses of the model suggest that the transient and asymptotic frequency sensitivity of this enzyme are dependent on different biochemical mechanisms. These results may be applicable to Ca(2+)/calmodulin signaling pathways in general.

Spike frequency decoding and autonomous activation of Ca2+-calmodulin-dependent protein kinase II in dorsal root ganglion neurons

Autonomous activation of calcium-calmodulin kinase (CaMKII) has been proposed as a molecular mechanism for decoding Ca(2+) spike frequencies resulting from action potential firing, but this has not been investigated in intact neurons. This was studied in mouse DRG neurons in culture using confocal measurements of [Ca(2+)](i) and biochemical measurements of CaMKII autophosphorylation and autonomous activity. Using electrical stimulation at different frequencies, we find that CaMKII autonomous activity reached near maximal levels after approximately 45 impulses, regardless of firing frequency (1-10 Hz), and autonomous activity declined with prolonged stimulation. Frequency-dependent activation of CaMKII was limited to spike frequencies in the range of 0.1-1 Hz, despite marked increases in [Ca(2+)](i) at higher frequencies (1-30 Hz). The high levels of autonomous activity measured before stimulation and the relatively long duration of Ca(2+) spikes induced by action potentials ( approximately 300 msec) are consistent with the lower frequency range of action potential decoding by CaMKII. The high autonomous activity under basal conditions was associated with extracellular [Ca(2+)], independently from changes in [Ca(2+)](i), and unrelated to synaptic or spontaneous impulse activity. CaMKII autonomous activity in response to brief bursts of action potentials correlated better with the frequency of Ca(2+) transients than with the concentration of [Ca(2+)](i). In conclusion, CaMKII may decode frequency-modulated responses between 0.1 and 1 Hz in these neurons, but other mechanisms may be required to decode higher frequencies. Alternatively, CaMKII may mediate high-frequency responses in subcellular microdomains in which the enzyme is maintained at a low level of autonomous activity or the Ca(2+) transients have faster kinetics.

Bistability in the Ca(2+)/calmodulin-dependent protein kinase-phosphatase system

A mathematical model is presented of autophosphorylation of Ca(2+)/calmodulin-dependent protein kinase (CaMKII) and its dephosphorylation by a phosphatase. If the total concentration of CaMKII subunits is significantly higher than the phosphatase Michaelis constant, two stable steady states of the CaMKII autophosphorylation can exist in a Ca(2+) concentration range from below the resting value of the intracellular [Ca(2+)] to the threshold concentration for induction of long-term potentiation (LTP). Bistability is a robust phenomenon, it occurs over a wide range of parameters of the model. Ca(2+) transients that switch CaMKII from the low-phosphorylated state to the high-phosphorylated one are in the same range of amplitudes and frequencies as the Ca(2+) transients that induce LTP. These results show that the CaMKII-phosphatase bistability may play an important role in long-term synaptic modifications. They also suggest a plausible explanation for the very high concentrations of CaMKII found in postsynaptic densities of cerebral neurons.

Long-term potentiation and depression induced by a stochastic conditioning of a model synapse

Protracted presynaptic activity can induce long-term potentiation (LTP) or long-term depression (LTD) of the synaptic strength. However, virtually all the experiments testing how LTP and LTD depend on the conditioning input are carried out with trains of stimuli at constant frequencies, whereas neurons in vivo most likely experience a stochastic variation of interstimulus intervals. We used a computational model of synaptic transmission to test if and to what extent the stochastic fluctuations of an input signal could alter the probability to change the state of a synapse. We found that, even if the mean stimulation frequency was maintained constant, the probability to induce LTD and LTP could be a function of the temporal variation of the input activity. This mechanism, which depends only on the statistical properties of the input and not on the onset of additional biochemical mechanisms, is not usually considered in the experiments, but it could have an important role to determine the amount of LTP/LTD induction in vivo. In response to a change in the distribution of the interstimulus intervals, as measured by the coefficient of variation, a synapse could be easily adapted to inputs that might require immediate attention, with a shift of the input thresholds required to elicit LTD or LTP, which are restored to their initial conditions as soon as the input pattern returns to the original temporal distribution.