CaMKII Isoforms in Learning and Memory: Localization and Function

1CaMKII is one of the main effectors enzymes involved in calcium signaling in eukaryotic cells. The enzyme is activated as a result of increased intracellular calcium and phosphorylates target proteins involved in various processes such as mobilization of synaptic vesicles, modulation of ion channels, regulation of gene expression, regulation of muscle contraction, and LTP. Over the last two decades CaMKII has become one of the most studied proteins in the nervous system, and it has proven to be a key protein involved in learning, memory, and synaptic plasticity. CaMKII is a holoenzyme composed of 12 subunits of 56–60 kDa that are assembled into 2 rings of 6 subunits each. Interestingly, these subunits are proteins encoded by four distinct but highly related genes termed camk2a, camk2b, camk2d, and camk2g, which give rise to four different CaMKII isoforms: α, β, δ, and γ. Each isoform has different calcium trapping kinetics, sub-cellular localization and affinity for other protein binding, thus enabling CaMKII to have different properties according to its subunit composition, which can be composed of a single type or a combination of isoforms. Furthermore, the RNAs that code for the different isoforms can undergo alternative splicing, resulting in the synthesis of approximately 30 different variants. In the present review, we will first describe general features of CaMKII structure and mechanism of activation and then summarize the main features of each isoform and their splicing variants as well as our current understanding on their role in learning and memory. Studies focused on the interplay between subunit composition and functional outcome will not only contribute to understand why CaMKII is so fundamental for learning and memory processes but they might also reveal key information on the molecular mechanisms involved in memory storage.

2Each of the subunits that comprise CaMKII multimeric enzyme has a conserved structure among the different isoforms: an amino terminal catalytic domain, followed by a regulatory domain that contains a self-inhibitory region and a binding site for the Ca2+/calmodulin (CaM) complex, a variable sequence and finally an associative (or oligomerization) domain in the carboxy-terminal end which allows assembly between the different subunits (Figure 1A;; ). The homology in the catalytic and regulatory domains between the different isoforms and variants of splicing is 89–93%, the main differences between sequences are found within the variable domain. The enzyme is expressed mainly in the brain, but also in the rest of the tissues. In some regions of the brain, such as the hippocampus, the protein levels reach up to 2% of total proteins.

3The entry of calcium into the cell leads to the formation of the Ca2+/CaM complex, which binds approximately 3–4 calcium ions per CaM in a cooperative form. This complex binds to the regulatory region of CaMKII and produces a conformational change, which not only leads to the phosphorylation of its substrates, but also to an inter-subunit, intra-holoenzyme autophosphorylation at threonine 286 in the α isoform and threonine 287 in β, δ, and γ isoforms. Autophosphorylation on this site prevents the enzyme to revert to its inactive conformation and decreases the dissociation rate of the bound CaM. Thus, autophosphorylated CaMKII can remain active even after the intracellular calcium levels decrease and therefore acquire autonomous and Ca2+ independent activity. Interestingly, this autonomous activity can persist even upon degradation or dephosphorylation of some subunits given they can be replaced and rephosphorylated by neighboring autophosphorylated subunits. The discovery of CaMKII mechanism of activation resulted in an increasing interest from the learning and memory field in this enzyme. First, because of the ability to detect small variations in the concentration of intracellular calcium, as action potentials in neurons involve repeated pulses of calcium and memory formation is also tightly linked to the activation of calcium channels like NMDA and Cav1. Second, because its singular ability to acquire persistent autonomous activity has been proposed as a form of molecular memory. Indeed, subsequent studies have proven that CaMKII has a key role in memory as we will describe below.

4The activation of the NMDA receptor in glutamatergic excitatory pathways following a behavioral experience is key for long term memory (LTM) storage. This activation allows the entry of calcium into the neuron which binds to calmodulin to form the Ca2+/CaM complex that is recognized by multiple enzymes, thus inducing a molecular signaling cascade whose main function is to reshape the synaptic structure and physiology, together with regulation of gene expression necessary for the formation of LTM. CaMKII is one of the main targets of Ca2+/CaM. Accordingly, CaMKII activity is increased upon learning and its inhibition causes LTM impairment (;; for reviews see ). Learning to an inhibitory avoidance task increased hippocampal CaMKII activity up to 30 min after training. Studies in synaptic plasticity have shown that CaMKII activity after LTP induction seems to be increased only 1–2 min. This kinetics in CaMKII activity does not match that observed memory, suggesting that LTP may be regulated differently than memory formation. For instance, the longer kinetics observed in behaving animals experiments could suggest that memory formation may involve several “rounds” of LTP-like processes, and not just a single one.

5To further understand the role of CaMKII in learning and memory processes different transgenic mice lines have been generated. developed a transgenic mouse line in which transgene expression of a constitutively active mutant form of CaMKII was inducible and limited to the superficial layers of medial entorhinal cortex, pre- and parasubiculum. The transgene was constitutively expressed and could be inhibited after the administration of doxycycline. Learning to find a visible platform in a water maze was similar across trials between wild-type and transgenic mice, however, long-term memory formation of the platform location, as assessed in a testing session 24 h after training, was impaired in transgenic animals. Mice were also trained to novel object recognition task, and transgenic mice showed recognition memory impairment when tested 3 h after training, thus short-term memory storage for this non-spatial task was also impaired. Finally, using doxycycline to inhibit transgene expression, they performed two different experiments: (i) transgene was expressed immediately after testing to the water maze task and up to 3 weeks after, (ii) transgene was expressed 3–6 weeks after training. These allowed studying the role of CaMKII activity after long-term memory consolidation. In the first case memory was impaired when tested 6 weeks after, suggesting that manipulating CaMKII activity affected the maintenance of the memory trace. In the latter, memory was not affected when tested 6 weeks after, indicating that at longer retention periods memory may have undergone systems consolidation and could be allocated in other cortical structures. More recently, a genetically encoded light-inducible inhibitor of CaMKII, called paAIP2, was designed, which allowed a tight temporal control on the manipulation of CaMKII activity. Mice received an intra-amygdala injection of a virus containing the vector for the expression of paAIP2 and were implanted an optic fiber guided to the amygdala. Inhibiting CaMKII activity during double-trial training to an inhibitory avoidance task led to memory impairment when tested 1 h after. Its inhibition immediately after training had no effect on memory when tested 1 h after training. From these results the authors concluded that CaMKII activity in the amygdala during training, but not after, is necessary for memory formation. It should be noted that, since the testing session took place 1 h after training, they evaluated short-term and not long-term memory storage.

6CaMKII binds to numerous proteins in the post-synaptic density (PSD), including NMDA receptor, synapsin 1, F-actin, and calcium channels. Presumably, the binding and phosphorylation regulates autonomous activity, location and/or transport of these and other proteins to regions of interest. CaMKII can also regulate the number of AMPA receptors, together with their conductance, promoting the rapid growth of dendritic filopodia and spine formation.

7An important issue regarding neural plasticity and memory is the role of CaMKII in synaptic tagging and capture (STC) and behavioral tagging. STC is one of the proposed mechanisms that explain how the plasticity-related proteins (PRP) act only in the activated synapses by means of an activity dependent tag that allows the capture of PRPs in these specific synapses. CaMKII have been proposed as a tag mechanism for LTP and for long-term depression (LTD). In relation with the first finding, found that the tag in LTP is sensitive to CaMKII inhibition. Such tagging function of CaMKII is transient and is then replaced by a PKMζ-mediated mechanism dependent on ryanodine receptor or synaptic activation of metabotropic glutamate receptors that prolongs the durability of the synaptic tag. In studies with behaving animal the learning tag is setting by activation of the glutamatergic NMDA receptors and this machinery further required CaMKII and PKA but not ERK1/2 protein kinase activity.

8δ and γ are isoforms are expressed in different tissues including the brain while α and β isoforms are brain specific. The studies on each isoform allow distinguishing differences and similarities in their affinity for calcium, their location and function. For instance, affinity for calmodulin and the rate of autophosphorylation between the different isoforms is estimated to be γ > β ∼ δ > α and δ > β > α > γ, respectively. Moreover, all the isoforms contain different splicing variants which have different sub-cellular localization and protein binding affinity, and thus even if the holoenzyme is only composed of subunits from a same isoform (homomeric), its overall features and function will depend on the properties of the variants that contains.

9CaMKIIα contains three variants termed α, αB, and αKAP, the first two are abundant in the brain while the last one is expressed in skeletal muscle. The αβ variant contains a nuclear localization signal (NLS) which is also shared with variants of the δ and γ isoforms, and is liable to regulation (Figure 2A). The mRNA for α variant is expressed throughout the brain, while αB mRNA is restricted to diencephalon/midbrain regions. Immunohistochemical studies performed in rat brain revealed that indeed, in thalamic and hypothalamic neurons which express both variants, CaMKIIα protein localizes to the nucleus and the cytoplasm, while in caudate putamen, in which only the α variant is expressed, CaMKIIα was exclusively seen in the cytoplasm. The αKAP variant lacks the catalytic domain but contains a domain that facilitates its binding to the sarcoplasmic reticulum and a NLS, although its sub-cellular localization is mainly ruled by the membrane targeting sequence, serving as an anchoring protein of catalytic competent subunits to membranes.

10In neurons, CaMKIIα localizes in pre-synapses and dendrites and is actually one of the main proteins in the PSD. Its location in PSD is calcium-dependent and it has been shown to be induced after NMDA (but not AMPA or metabotropic glutamatergic) receptor stimulation and is regulated by CaMKIIβ F-actin binding (;, see also CaMKIIβ section). Also it is almost exclusive of synapses receiving glutamatergic terminals. In cultured hippocampal neurons expressing a mutant form of CaMKIIα with impaired kinase activity or T286 autophosphorylation, PSD translocation after glutamate stimulation was not altered compared to CaMKIIα wild type, and a CaMKII mutant that simulates the autophosphorylated state did not localize to PSD before stimulation, suggesting that neither kinase activity nor autophosphorylation are necessary for CaMKIIα translocation to PSD. However, it was shown that after external calcium and glutamate removal the mutant CaMKIIα with impaired autophosphorylation dissociated more rapidly from the PSD than CaMKIIα wild type. Thus neither kinase activity nor T286 autophosphorylation are necessary for PSD translocation, but the later can regulate the process.

11Of particular interest is a 30 nucleotide sequence present in the 3′ UTR of the CaMKII mRNA that targets it to dendrites, where it is locally translated (Figure 2B; ). Its transport is inhibited in a resting state and is activated upon neuronal depolarization and NMDA receptor stimulation. These particular characteristics and the fact that the PSD is a key compartment for the molecular events that subserve memory storage turned CaMKIIα to be the most studied isoform in the field of neurobiology of memory.

12Different CaMKIIα mutant mice have been generated, including homozygous and heterozygous knock out (KO) mice, KI mice with impaired Ca2+/CaM binding, KI mice with a mutation blocking the catalytic activity of CaMKIIα, KI mice with a mutation in T286 impeding autophosphorylation (T286A) and transgenic mice expressing a mutated form of CaMKIIα that can be rapidly and reversibly inhibited after the administration of a synthetic inhibitor. The effects of these and other mutations on memory and synaptic plasticity has been reviewed in,, and, here we will present the main findings.

13Homo and heterozygous KO mice showed learning impairment in several hippocampal-dependent tasks which could be sometimes overcome with extended training. However, these transgenic animals had the side effect of showing abnormal behavior, including altered locomotors activity and anxiety-like behavior, as well as an increased targeting of Cambia to PSD which could be compensating for the loss of CaMKIIα. Subsequent studies were done in transgenic mice bearing knock-in mutations on different key regions of the endogenous CaMKIIα gene. KI transgenic mice that express a form of CaMKIIα lacking its kinase activity showed learning and consequent memory impairment in a one-trial inhibitory avoidance task, while KI animals with impaired autophosphorylation at T286 had learning and memory impairments in one-trial passive avoidance tasks, the water Morris maze and contextual fear conditioning. Learning impairment could be overcome with repeated training and, in the case of animals with impaired autophosphorylation at T286, this also rescued memory deficits (it should also be noted that in these animals CaMKII Ca2+-independent activity was reduced by 60% but not completely, so there could be some compensatory effect from the activation of other isoforms). Thus, it was proposed that autophosphorylation at T286 could be related to one-trial learning and not necessarily to memory storage. Regarding the need of CaMKIIα activity in memory storage, it has been shown to be necessary for inhibitory avoidance LTM formation even after repeated training. On the other hand, heterozygous KO mice showed normal contextual fear conditioning learning and LTM 1–3 days after training but this was severely impaired at longer retention delays, when memory becomes independent of hippocampal processing and becomes dependant on storage in the cortex. Therefore, these results suggest a role of cortical CaMKIIα in systems consolidation.

14In non-transgenic animals it has been shown that CaMKIIα expression is also regulated in different types of learning and brain structures. For instance, in the hippocampus, a gene screening (coda array) showed that CaMKIIα mRNA was up-regulated 3 h but not 24 h after one-trial inhibitory avoidance training and its protein levels were increased 24 h but not 3 h after training. In the dentate gyrus, western blot analysis revealed that object-place recognition learning triggered an increment in CaMKIIα protein expression 1 h after training. In the striatum, permanent inhibition of CaMKIIα expression after the stereotaxical injection of a lentivirus expressing a shRNA before training impaired accelerating rotarod and water cross maze test performances. However, these animals were able to learn properly after consecutive training similarly to what it has been seen for transgenic animals.

15A key feature of CaMKIIα relies on its local translation in dendrites. Regarding the role of this process in memory, it has been shown that when the mRNA was confined to the soma by mutation of the 3′-untranslated region in mice, LTM, but not short term memory nor learning, was impaired in cued and contextual fear tasks, water Morris maze and an olfactory associative task. These results point to an important role of CaMKIIα local translation in LTM formation.

16Altogether, these findings support that CaMKIIα proper catalytic activity and T286 autophosphorylation are important for enabling fast learning, that its gene expression is involved in memory consolidation and that this protein may also have a role in remote memory formation.

17CaMKIIβ isoform possesses six splicing variants (Figure 1). The ones that have been described in the brain are β, β′, βe and β′e. A recent publication has also described the existence of a new variant expressed in the brain which was termed βH. None of the splice variants contain a NLS and this could explain why this isoform has not been shown to be present in the nuclear compartment.

18The β and β′ variants contain an F-actin binding domain and are therefore, able to bind F-actin in basal conditions. Glutamate postsynaptic stimulation induces CaMKIIβ/F-actin release and subsequent PSD localization. The dissociation process requires binding of Ca2+/CaM to CaMKIIβ and/or autophosphorylation at T287 and is not dependant on CaMKIIβ kinase activity. Mutated forms of CaMKIIβ with impaired calmodulin binding could bind to F-actin in basal conditions but were unable to dissociate after stimulation. A mutated CaMKIIβ which mimic the autophosphorylation state could not bind to F-actin. Thus F-actin binding requires the enzyme to be in a non-autophosphorylated state (not active) and its dissociation requires Ca2+/CaM binding. CaMKIIβ is usually associated with CaMKIIα to form the heterozyme α/β-CaMKII which translocates jointly to the PSD. Interestingly, translocation kinetics is dependent on the ratio of α/β subunits in the heteromer. It has also been proposed that the δ and γ isoforms can bind F-actin in vitro. Binding of CaMKIIβ to F-actin has been shown to increase F-actin stabilization and bundling and was proposed that CaMKIIβ transient dissociation from F-actin promotes actin re-organization which impacts in dendritic spine remodeling. Figure 3 summarizes CaMKIIβ and α molecular dynamics in the PSD.

19CaMKIIβ has been implicated in a process of inverse synaptic tagging, targeting of Arc to inactive synapses via a high-affinity interaction with the β isoform that is not bound to Ca2+/CaM.

20A transgenic mice line was developed which expressed a mutated form of CaMKIIβ whose activity could be reversibly inhibited upon the administration of a synthetic inhibitor. This over-expression of CaMKIIβ led to an increase in its enzymatic basal activity. In the hippocampus, CaMKIIβ over-expression was seen exclusively in the dentate gyrus. Learning and 1 day retention for novel object recognition task, cued and contextual fear conditioning were normal, but in this last task, 10 days retention was impaired. These animals also presented reversal learning deficits in the radial arm maze task and the water cross maze. Upon administration of the synthetic inhibitor at different time points before and after training, the authors could further conclude that the 10-day memory impairment was a consequence of increased CaMKIIβ expression during the LTM consolidation phase, and not during acquisition or recall. The fact that the memory deficit was observed 10 days but not 24 h after training indicates that CaMKIIβ activity during consolidation modulates the persistence of the memory that is being consolidated. On the other hand, two KO mice for CaMKIIβ were developed, which differed in the locus in which a neomycin cassette was inserted to disrupt CaMKIIβ gene sequence, and where later used to study the role of CaMKIIβ in memory. In one of these studies, animals showed impairment in novel object recognition when tested 4 h after training, suggesting CaMKIIβ could be involved in short-term memory formation. These animals also showed differences compared to wild type in rotarod tests, balance beam and running wheel tasks, elevated plus maze and open field and they had altered body mass composition, among others traits. In another of this studies KO mice showed impaired fear conditioning memory when tested 24 h after training. Interestingly, in this last work the authors found that CaMKIIα expression levels were not altered in the CaMKIIβ KO mice but its location to synapses was decreased by a 40%. This prompted the authors to study if the memory deficits were a side-effect of decreased CaMKIIα localization in PSD. To test this they used a KI mouse line in which Ca2+-dependent activation is blocked by interfering with Ca2+ binding, thus in these animals CaMKIIβ can bind to F-actin but cannot be activated. Fear conditioning performance was normal in these animals and CaMKIIα abnormal distribution was not seen. This last result led to the important conclusion that CaMKIIβ binding to F-actin but not its catalytic activity is necessary for memory processes and CaMKIIα translocation to spines.

21Little is known regarding CaMKIIβ expression changes in memory processes. In the striatum, it has been shown that CaMKIIβ mRNA expression was up-regulated specifically after extensive training to an accelerated rotarod task, suggesting a delayed effect of training on CaMKIIβ expression.

22Altogether, these results suggest that β isoform has a non-enzymatic role in LTM consolidation. Presumably a key function of this enzyme is to allow CaMKIIα translocation to the PSD. However, the activity-dependent dissociation to F-actin seems to be important to regulate actin dynamics that may allow dendritic spine plasticity and synaptic reorganization. This fact could explain why manipulating CaMKIIβ activity during the consolidation phase can affect memory persistence several days after, as seen in.

23The δ isoform is expressed ubiquitously. To date, 13 different variants have been identified from alternative splicing of the gene which are expressed differentially in the brain, heart, and skeletal muscle. The structures of 10 of these variants have been studied in detail and show that the main difference between them is within two variable regions located next to the regulatory domain and the C-terminal end of the association domain, respectively (,;; Figure 1B). The variants 1, 2, 3, 5, and 7 have been shown to be expressed in the brain. Interestingly, the variants 3 and 7 contain a NLS which is conserved with the isoforms α and γ. The expression of 2, 3, and 7 variants are highly abundant in heart tissue, where they have been shown to be involved in ischemia/reperfusion injury and the activation of NF-κB signaling cascade. A recent publication has shown that this kind of injury in the brain also triggers a similar signaling cascade. In brain, CaMKIIδ protein has been shown to be present in the nucleus of cerebellar granule cells and substantia nigra dopaminergic neurons in rats. It has been proposed that CaMKIIδ nuclear translocation in neurons is promoted by PP1-dependent dephosphorylation at Ser 332 enhancing BDNF transcription. More recently, we found the δ isoform in mice hippocampal pyramidal cells, with an important presence in nuclei.

24We recently found in the hippocampus that CaMKIIδ is also located in pre-synaptic terminals. Even though strong evidence points to a crucial role for hippocampal pre-synaptic CaMKII in synaptic plasticity and neurotransmitter release, little is known about the presence of each isoform in this compartment. The presence of CaMKIIδ in axons has been documented during neuronal development. Effectively, CaMKIIδ is the main isoform present in the axons of developing rodent neurons, related with neurite outgrowth and stability.

25Until recently, the role of CaMKIIδ in memory processes was virtually unknown. In a first study from our laboratory, we showed that training to a Novel Object Recognition (NOR) task induced NF-κB binding and histone acetylation in the promoter of δ isoform gene as well as an increase in its expression 3 h after training. NF-κB is a key transcription factor for long-term memory consolidation. Histone acetylation is a molecular process that enhances gene expression and is also related to memory strength and persistence. This first evidence led us to the hypothesis that CaMKIIδ gene expression could be sustained beyond memory consolidation in order to warrant memory persistence. More recently, we found that, indeed, training to a NOR task increased CaMKIIδ mRNA levels several days after training. Its expression was increased up to 7 days while memory retention was present, and returned to basal levels 20 days after, when recognition memory retention was vanished. These results suggest that CaMKIIδ expression parallels memory retention. This persistent gene expression was also accompanied by long-term changes in nucleosome dynamics on key sites of its promoter, further supporting the presence of long-term molecular events to regulate its expression. Such a sustained gene expression after learning has been rarely reported, with the exception of PKMζ, a protein that has been shown to be up-regulated up to 7 days after training in the insular cortex and for 1 month in the hippocampus. Therefore, has been proposed to participate in memory maintenance. In accordance with these findings, inhibiting CaMKIIδ gene expression 24 h after NOR training, once memory has been consolidated, led to memory impairment at day 7, suggesting that CaMKIIδ is indeed involved in mechanisms that affect maintenance of the memory trace. The fact that inhibition of CaMKIIδ gene expression during consolidation affect memory at 7 days but left 1 day memory intact support a specific action in persistence and maintenance. Previous evidence has implicated CaMKII activity in memory maintenance. In a recent publication it was shown that transient hippocampal expression of an inactive form of CaMKII 3 days after training, once memory is supposed to have been consolidated, affected behavioral performance when tested 9 days after training, a time point in which virally mediated protein expression had ended. On the other hand, it was also shown that the expression of a constitutively active mutant form of CaMKII in parahippocampal regions, during the first 3 weeks after training to a water maze task, affected the maintenance of a previously established spatial memory. On the contrary, inhibiting CaMKII activity 24 h after training to an inhibitory avoidance task, had no effect on a behavioral test performed 1 h after the inhibition. This last result, taking into account the previous two, suggest that while CaMKII activity may be necessary for memory maintenance, it may not have an immediate impact on the memory trace, thus the impairment may be observable several days after the manipulation.

26Interestingly, whilst it is not common to observe sustained gene expression for most proteins, this seems to be a feature of CaMKIIδ. It has been shown that CaMKIIδ protein expression can be increased up to 5 days after brain injury in rats and up to 7 days in homogenates form ventricles after transverse aortic constriction. CaMKIIδ mRNA was also up-regulated 7 and even 21 days after nerve injury in the peripheral system. Moreover, what it is remarkable from our results is not only the novelty on CaMKIIδ involvement in memory persistence, but also that it poses two novel molecular mechanisms by which a memory trace is allowed to persist on time: sustained gene expression and nucleosome positioning dynamics. Figure 4 summarizes our findings and proposes a general mechanism by which a stronger training favors the formation of a more persistent memory trace.

27At this point two key questions arise. First, in which way is CaMKIIδ gene expression functional to memory maintenance? A maintenance molecular mechanism should be sustained over time and should be necessary for memory retention beyond the consolidation time window. In this report we provide evidence that support this role for CaMKIIδ. In addition, such mechanism should be involved in synaptic efficacy modifications. The action of CaMKIIδ in pre-synapses of excitatory pathways could determine an increment in neurotransmitter release and its action in the nucleus could regulate epigenetic mechanisms and transcription of effectors genes. In cardiomyocytes nuclear translocation of CaMKIIδ has been shown to regulate histone H3 phosphorylation at serine 10. Besides, it induces histone deacetylase HDAC4 export to the cytoplasm, increasing gene transcription. Notably, both H3 phosphorylation and HDAC4 have been related to memory processes, with HDAC4 being the most studied deacetylase subtype in neural plasticity and memory, and its role in this process evolutionarily conserved (, p. 4;; ). Therefore, CaMKIIδ may be regulating these proteins in order to promote sustained changes in the expression of genes necessary to maintain the memory trace. The second question is how might CaMKIIδ sustain its gene expression? We propose that there is a feed-back mechanism in the nucleus, in which CaMKIIδ phosphorylates HDAC4 and histone H3 to sustain its own expression over time. Since CaMKIIδ mRNA expression returns to basal levels after 20 days, this feed-back mechanism may decay over time. This last assumption is in accordance with our finding that nucleosome occupancy at the camk2d transcription start site changed 7 days after training in an opposite manner to what was observed in an early phase.

28The γ isoform is expressed in the brain of mammals, in cardiac tissue, in smooth muscle, liver, and cells of the immune system. As with the other isoforms, CaMKIIγ can be spliced to different variants termed γA to γH. The γA variant is brain specific and contains the NLS, present in other CaMKII isoforms, thus can translocate to the nucleus by a mechanism similar to the one established for the other isoforms (Figure 5). Other variants, like γC, have also been shown to be expressed in neurons.

29In cultured neurons, it was recently shown that depolarization induces the translocation of this isoform to the nucleus and it has been postulated that its main function is to serve as a transport protein of the Ca2+/CaM complex to the nucleus in order to induce the signaling cascade dependent on this signal and activate CaM kinase families for the transcriptional regulation mediated in part by CREB.

30Few studies have been published showing a connection between this isoform and memory. In humans it has been shown that genetic variability around this gene affects performance in episodic memory tests and intellectual capacity. CaMKIIγ KO mice have been recently developed. This animals showed learning deficits in Morris water maze and Radial arm maze, as well as impaired long-term memory in an inhibitory avoidance task. These tasks are mainly hippocampal-dependent, while inhibitory avoidance is also dependent on the amygdala. It should be noted, that learning curves were analyzed over consecutive days, and is therefore difficult to disentangle a learning deficit from a long-term memory storage deficit. Normal anxiety and locomotor activity were not affected in mutant animals compared to wild type littermates. They further demonstrated that these animals had decreased levels of nuclear CaM and performed different experiments to show that this was an effect of deficient CaMKIIγ-dependent transport of CaM to the nucleus. Altogether these results show that CaMKIIγ is involved in spatial learning and long-term memory storage.

31Although the key role of CaMKII in neural plasticity and memory have been stressed by decades of study since the initial discovery of its role in long-term potentiation (LTP), the specific role of each isoform in different subcellular compartment is still under investigation and required further analysis (Box 1). Recent findings suggest that each of the four isoforms play different roles and, in some cases, this role is not dependent on the enzymatic function. As was described earlier, the α isoform plays an important role associated with the PSD in dendritic spines of excitatory synapses, regulating receptor and channel functions as well as receptor trafficking. The β isoform could have a non-enzymatic role in LTM consolidation, presumably by allowing CaMKIIα translocation to the PSD. The activity-dependent dissociation to F-actin seems to be important to regulate actin dynamics that may allow dendritic spine plasticity and synaptic reorganization. The main function of γ isoform is attributed to a synapse-to-nucleus communication, transporting Ca2+ signaling to regulate gene expression that is essential for the neural plasticity involved in memory. Recent results support a key role of the δ isoform in memory persistence and maintenance by means of the sustained expression of its gene. The presence of CaMKIIδ in the presynapses and in the nucleus is an important finding which deserves further study.

32Which is the subunit composition of endogenous CaMKII in the brain and how does this affect its location and function?

33Little is known on the endogenous assembly of subunits and variants, how this affects the overall function and location of the enzyme and if this could be regulated by behavioral experience. Current evidence indicates that in forebrain, endogenous CaMKIIα forms homomers as well as heteromers with CaMKIIβ, and that the percentage of each subunit on the holoenzyme affects autophosphorylation rate and PSD translocation. Further studies are necessary to learn about the formation of heteromers with the other isoforms, δ and γ, as well as splicing variants composition.

34What is the gene expression kinetics of the different isoforms during the different memory phases?

35Much of the work on the role of CaMKII expression in learning and memory has been done using transgenic animals. Gene expression is a key molecular event in the formation and storage of long-lasting memories, therefore further studies on the expression of the different isoforms will help to provide insight not only on their role in learning and memory but also on the molecular mechanisms that underlie these processes.

36What is the role of the different isoforms in pre-synaptic terminals and nucleus?

37Most of the studies in CaMKII have focused on synaptic plasticity at the post-synaptic terminal. However, learning and memory also requires modifications at the presynapses and changes in gene transcription. CaMKII has been shown to be present in both compartments. Thus, further studies on the localization and function of each isoform in these compartments are required.

38Beyond the individual role of each isoform, CaMKII is multimeric. The assembly of 6 and then 12 subunits into holoenzymes is an important structural feature that can regulate and modify the function and localization of the kinase. The mRNA of α isoform presents 3′ UTR sequences that target the messenger to dendrites, where it is translated locally. This confers the possibility of homomers formation. However, forebrain CaMKII consists mainly of heteromers and the holoenzyme can include subunits from the four CaMKII genes and the multiple splice variants of those genes. The presence of different subunits could regulate the localization of the holoenzyme. For instance, subunits containing NLS could confer the possibility of nuclear translocation but, at the same time, would co-assemble with cytosolic subunits that could possibly retard its translocation. The regulation of heteromers composition is an important issue that deserves further investigation in the role of this key protein kinase in memory.

39GZ and AR proposed the subject, discussed, and wrote the manuscript. NF discussed and revised the manuscript.

40The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.