Analogous to learning and memory storage, long-term potentiation (LTP) is divided into induction and maintenance phases. Testing the hypothesis that the mechanism of LTP maintenance stores information requires reversing this mechanism in vivo and finding out whether long-term stored information is lost. This was not previously possible. Recently however, persistent phosphorylation by the atypical protein kinase C isoform, protein kinase Mzeta (PKMz), has been found to maintain late LTP in hippocampal slices. Here we show that a cell-permeable PKMz inhibitor, injected in the rat hippocampus, both reverses LTP maintenance in vivo and produces persistent loss of 1-day-old spatial information. Thus, the mechanism maintaining LTP sustains spatial memory.

Most of the molecular mechanisms contributing to long-term memory have been found to consolidate information within a brief time window after learning, but not to maintain information during memory storage. However, with the discovery that synaptic long-term potentiation is maintained by the persistently active protein kinase, protein kinase Mζ (PKMζ), a possible mechanism of memory storage has been identified. Recent research shows how PKMζ might perpetuate information both at synapses and during long-term memory.

The maintenance of long-term memory in hippocampus, neocortex and amygdala requires the persistent action of the atypical protein kinase C isoform, protein kinase Mzeta (PKMzeta). We found that inactivating PKMzeta in the amygdala impaired fear memory in rats and that the extent of the impairment was positively correlated with a decrease in postsynaptic GluR2. Blocking the GluR2-dependent removal of postsynaptic AMPA receptors abolished the behavioral impairment caused by PKMzeta inhibition and the associated decrease in postsynaptic GluR2 expression, which correlated with performance. Similarly, blocking this pathway for removal of GluR2-containing receptors from postsynaptic sites in amygdala slices prevented the reversal of long-term potentiation caused by inactivating PKMzeta. Similar behavioral results were obtained in the hippocampus for unreinforced recognition memory of object location. Together, these findings indicate that PKMzeta maintains long-term memory by regulating the trafficking of GluR2-containing AMPA receptors, the postsynaptic expression of which directly predicts memory retention.

Long-term potentiation in the CAl region of the hippocpus, a model for memory formation in the brain, Is diided into two phases. A transient process (induction) is initiated, which then generates a persistent mechanism (maintenance) for enhancing synaptic strenth. Protein kinase C (PKC), a gene family of multiple isozymes, may play a role in both induction and maintenance. In region CAl from rat hippocampal sices, most of the isozymes of PKC translocated to the partcuate fraction 15 sec after a tetanus. The Increase of PKC in the particulate fraction did not persist into the maintenance phase of long-term potentiation. In contrast, a constitutivey active kinase, PKM, a form specific to a single isozyme (), increased in the cytosol during the maintenance phase. The tramition from trnslocation of PKC to formation of PKM may help to explain the molecular mechanims of induction and maintenance of long-term potentiation.

Protein kinase M (PKM) is a newly described form of PKC that is necessary and sufficient for the maintenance of hippocampal long term potentiation (LTP) and the persistence of memory in Drosophila. PKM is the independent catalytic domain of the atypical PKC isoform and produces long term effects at synapses because it is persistently active, lacking autoinhibition from the regulatory domain of PKC. PKM has been thought of as a proteolytic fragment of PKC. Here we report that brain PKM is a new PKC isoform, synthesized from a PKM mRNA encoding a PKC catalytic domain without a regulatory domain. Multiple -specific antisera show that PKM is expressed in rat forebrain as the major form of in the near absence of full-length PKC. A PKC knockout mouse, in which the regulatory domain was disrupted and catalytic domain spared, still expresses brain PKM, indicating that this form of PKM is not a PKC proteolytic fragment. Furthermore, the distribution of brain PKM does not correlate with PKC mRNA but instead with an alternate RNA transcript thought incapable of producing protein. In vitro translation of this RNA, however, generates PKM of the same molecular weight as that in brain. Metabolic labeling of hippocampal slices shows increased de novo synthesis of PKM in LTP. Because PKM is a kinase synthesized in an autonomously active form and is necessary and sufficient for maintaining LTP, it serves as an example of a link coupling gene expression directly to synaptic plasticity. LTP1 is a persistent enhancement of synaptic transmission widely studied as a physiological model of memory (1). LTP can be divided into two phases: induction, which triggers the potentiation, and maintenance, which sustains it over time. Many molecules have been implicated in LTP induction, which is initiated by the activation of N-methyl-D-aspartate (NMDA) receptors and involves several protein kinases (2). In contrast, very little is known about the molecular mechanism of maintenance. Recently, however, a specific, autonomously active form of the atypical PKC isozyme (3, 4), PKM, has been found both necessary and sufficient for maintaining LTP (5-7). Overexpression of PKM also prolongs memory in Drosophila melanogaster, suggesting it is part of an evolutionarily conserved molecular mechanism for memory storage (8).The unique role of PKM in LTP maintenance is due, in part, to its unusual structural and enzymatic properties as an autonomously active kinase. PKM consists of the independent catalytic domain of a PKC isoform (5). PKC isoforms are divided into three classes: conventional, novel, and atypical (reviewed in Refs. 9 -11). Each isoform is a single polypeptide consisting of an N-terminal regulatory domain and a C-terminal catalytic domain linked by a hinge (Fig. 1A, left). The regulatory domain contains binding sites for second messengers and an autoinhibitory pseudosubstrate sequence, which interacts with and blocks the active site of the catalytic domain. Second messengers stimulate PKC by binding to the regulatory domain, translocating th...

Although the maintenance mechanism of late long-term potentiation (LTP) is critical for the storage of long-term memory, the expression mechanism of synaptic enhancement during late-LTP is unknown. The autonomously active protein kinase C isoform, protein kinase M (PKM), is a core molecule maintaining late-LTP. Here we show that PKM maintains late-LTP through persistent N-ethylmaleimide-sensitive factor (NSF)/glutamate receptor subunit 2 (GluR2)-dependent trafficking of AMPA receptors (AMPARs) to the synapse. Intracellular perfusion of PKM into CA1 pyramidal cells causes potentiation of postsynaptic AMPAR responses; this synaptic enhancement is mediated through NSF/GluR2 interactions but not vesicle-associated membrane protein-dependent exocytosis. PKM may act through NSF to release GluR2-containing receptors from a reserve pool held at extrasynaptic sites by protein interacting with C-kinase 1 (PICK1), because disrupting GluR2/PICK1 interactions mimic and occlude PKM-mediated AMPAR potentiation. During LTP maintenance, PKM directs AMPAR trafficking, as measured by NSF/GluR2-dependent increases of GluR2/3-containing receptors in synaptosomal fractions from tetanized slices. Blocking this trafficking mechanism reverses established late-LTP and persistent potentiation at synapses that have undergone synaptic tagging and capture. Thus, PKM maintains late-LTP by persistently modifying NSF/GluR2-dependent AMPAR trafficking to favor receptor insertion into postsynaptic sites.

How long-term memories are stored is a fundamental question in neuroscience. The first molecular mechanism for long-term memory storage in the brain was recently identified as the persistent action of protein kinase Mzeta (PKMζ), an autonomously active atypical protein kinase C (PKC) isoform critical for the maintenance of long-term potentiation (LTP). PKMζ maintains aversively conditioned associations, but what general form of information the kinase encodes in the brain is unknown. We first confirmed the specificity of the action of zeta inhibitory peptide (ZIP) by disrupting long-term memory for active place avoidance with chelerythrine, a second inhibitor of PKMζ activity. We then examined, using ZIP, the effect of PKMζ inhibition in dorsal hippocampus (DH) and basolateral amygdala (BLA) on retention of 1-d-old information acquired in the radial arm maze, water maze, inhibitory avoidance, and contextual and cued fear conditioning paradigms. In the DH, PKMζ inhibition selectively disrupted retention of information for spatial reference, but not spatial working memory in the radial arm maze, and precise, but not coarse spatial information in the water maze. Thus retention of accurate spatial, but not procedural and contextual information required PKMζ activity. Similarly, PKMζ inhibition in the hippocampus did not affect contextual information after fear conditioning. In contrast, PKMζ inhibition in the BLA impaired retention of classical conditioned stimulus–unconditioned stimulus (CS-US) associations for both contextual and auditory fear, as well as instrumentally conditioned inhibitory avoidance. PKMζ inhibition had no effect on postshock freezing, indicating fear expression mediated by the BLA remained intact. Thus, persistent PKMζ activity is a general mechanism for both appetitively and aversively motivated retention of specific, accurate learned information, but is not required for processing contextual, imprecise, or procedural information.

PKMζ is a persistently active PKC isoform proposed to maintain late-LTP and long-term memory. But late-LTP and memory are maintained without PKMζ in PKMζ-null mice. Two hypotheses can account for these findings. First, PKMζ is unimportant for LTP or memory. Second, PKMζ is essential for late-LTP and long-term memory in wild-type mice, and PKMζ-null mice recruit compensatory mechanisms. We find that whereas PKMζ persistently increases in LTP maintenance in wild-type mice, PKCι/λ, a gene-product closely related to PKMζ, persistently increases in LTP maintenance in PKMζ-null mice. Using a pharmacogenetic approach, we find PKMζ-antisense in hippocampus blocks late-LTP and spatial long-term memory in wild-type mice, but not in PKMζ-null mice without the target mRNA. Conversely, a PKCι/λ-antagonist disrupts late-LTP and spatial memory in PKMζ-null mice but not in wild-type mice. Thus, whereas PKMζ is essential for wild-type LTP and long-term memory, persistent PKCι/λ activation compensates for PKMζ loss in PKMζ-null mice.DOI: http://dx.doi.org/10.7554/eLife.14846.001