Chapter 2 PKMζ, LTP maintenance, and the dynamic molecular biology of memory storage

https://doi.org/10.1016/S0079-6123(07)00002-7Get rights and content

Abstract

How memories persist is a fundamental neurobiological question. The most commonly studied physiological model of memory is long-term potentiation (LTP). The molecular mechanisms of LTP can be divided into two phases: induction, triggering the potentiation; and maintenance, sustaining the potentiation over time. Although many molecules participate in induction, very few have been implicated in the mechanism of maintenance. Understanding maintenance, however, is critical for testing the hypothesis that LTP sustains memory storage in the brain. Only a single molecule has been found both necessary and sufficient for maintaining LTP — the brain-specific, atypical PKC isoform, protein kinase Mzeta (PKMζ). Although full-length PKC isoforms respond to transient second messengers, and are involved in LTP induction, PKMζ is a second messenger-independent kinase, consisting of the independent catalytic domain of PKCζ, and is persistently active to sustain LTP maintenance. PKMζ is produced by a unique PKMζ mRNA, which is generated by an internal promoter within the PKCζ gene and transported to the dendrites of neurons. LTP induction increases new PKMζ synthesis, and the increased level of PKMζ then enhances synaptic transmission by doubling the number of postsynaptic AMPA receptors (AMPAR) through GluR2 subunit-mediated trafficking of the receptors to the synapse. PKMζ mediates synaptic potentiation specifically during the late-phase of LTP, as PKMζ inhibitors can reverse established LTP when applied several hours after tetanization in hippocampal slices or 1 day after tetanization in vivo. These studies set the stage for testing the hypothesis that the mechanism of LTP maintenance sustains memory storage. PKMζ inhibition in the hippocampus after learning eliminates the retention of spatial memory. Once the PKMζ inhibitor has been eliminated, the memory is still erased, but new spatial memories can be learned and stored. Similar results are found for conditioned taste aversion when the inhibitor is injected in the insular neocortex. Thus PKMζ is the first molecule found to be a component of the long-term memory trace.

Section snippets

The discovery of PKMζ

The notion that enzymes might maintain long-term memory was articulated by Crick (1984). Protein kinases have well-established roles in the transient enhancement of synaptic transmission thought to mediate short-term memory (Kandel and Schwartz, 1982). Therefore, one simple mechanism for long-term memory storage is the formation of a persistently active form of a kinase that could then sustain this enhancement over time. This notion was actively explored in the 1980s by the late Dr. James

PKMζ is synthesized from a PKMζ mRNA

Curiously, however, while an increase in PKMζ was seen during LTP maintenance, a decrease in PKCζ was not, i.e., there was no precursor–product relationship, as might be expected if PKMζ were produced by proteolytic cleavage. At that time, we reasoned that there might be new synthesis of PKCζ that could have replaced that cleaved to PKMζ. Therefore, to reveal the putative precursor–product relationship, we added protein synthesis inhibitors during the LTP experiments. To our surprise, however,

PKMζ synthesis is regulated by other protein kinases in LTP induction

As mentioned in the introduction, many molecules are critical for inducing LTP, in particular the protein kinases CaMKII, mitogen-activated protein kinase (MAPK), and PKA (Roberson et al., 1996; Sweatt, 1999). Other kinases important for translational regulation, the lipid kinase, phosphatidyl 3-kinase (PI3-kinase) (Kelly and Lynch, 2000; Sanna et al., 2002; Opazo et al., 2003), and the mammalian target of rapamycin (mTOR) (Tang et al., 2002; Cammalleri et al., 2003; Cracco et al., 2005) are

PKMζ potentiates postsynaptic AMPA receptor responses

The persistent increase in PKMζ correlates with the extent and duration of synaptic potentiation during LTP, but does the persistent kinase action of PKMζ cause synaptic potentiation? To address this question, we recombinantly expressed and purified PKMζ from the baculovirus/Sf9-overexpression system (in which endogenous PDK1 of the Sf9 insect cells phosphorylates the activation loop of the overexpressed PKMζ) (Ling et al., 2002). We then patched CA1 pyramidal cells in hippocampal slices with

PKMζ maintains late-LTP

If PKMζ is sufficient for synaptic potentiation, is it also necessary for the potentiation that maintains LTP? As mentioned in the introduction, although many inhibitors block LTP induction when applied around the time of tetanization, none had reversed established late-phase LTP prior to our work on PKMζ (Ling et al., 2002). In our initial investigation, we examined LTP maintenance 1 h after tetanization by comparing the effects of applications of two PKMζ inhibitors, chelerythrine, an

PKMζ maintains potentiation after synaptic tagging

PKMζ is synthesized by strong, but not weak afferent stimulation of synapses (Osten et al., 1996; Hernandez et al., 2003), and maintains late, but not early LTP. How does newly synthesized PKMζ affect only specific, activated synapses? One possibility is that the kinase might be involved in the process of synaptic tagging and capture (Sossin, 1996; Frey and Morris, 1997; Sajikumar et al., 2005). Synaptic tagging is the hypothesis, proposed by Sossin and by Frey and Morris, to explain how newly

PKMζ maintains long-term spatial memory storage in the hippocampus

The first evidence that PKMζ plays a role in memory came from work by Jerry Yin, Eric Drier, and colleagues (University of Wisconsin, Madison, WI) on odor avoidance conditioning in Drosophila (Drier et al., 2002). They found that overexpression of PKMζ during a narrow time-window after learning enhanced the persistence of memory. Conversely, blocking atypical PKC activity with a dominant negative or chelerythrine prevented persistent memory formation, without affecting short-term memory

PKMζ maintains long-term associative memory storage in the neocortex

Although the hippocampus stores spatial memories and the amygdala fear memories, the repository for most memories in the mammalian brain is thought to be neocortex. To test the role of PKMζ in storage in neocortex, Reut Shema and Yadin Dudai (Weizmann Institute, Rehovot, Israel) examined conditioned taste aversion (CTA), which is stored in the taste cortex located in the insular cortex (Shema et al., 2007). Rats were first presented with a novel taste, the conditioned stimulus (CS, such as

Conclusions

These studies show that PKMζ is the first known component of the storage mechanism for long-term associative memory. The unique structure and function of the PKMζ gene, mRNA, and protein provide a relatively simple, essential molecular mechanism for information storage through synthesis of a constitutively active protein kinase (Sacktor et al., 1993; Hernandez et al., 2003), which persistently maintains enhanced synaptic transmission at synapses. Although this mechanism is quite different from

References (86)

  • S. Hrabetova et al.

    Transient translocation of conventional protein kinase C isoforms and persistent downregulation of atypical protein kinase Mζ in long-term depression

    Brain Res. Mol. Brain Res.

    (2001)
  • M. Inoue et al.

    Studies on a cyclic nucleotide-independent protein kinase and its proenzyme in mammalian tissues. II. Proenzyme and its activation by calcium-dependent protease from rat brain

    J. Biol. Chem.

    (1977)
  • A. Kelly et al.

    Long-term potentiation in dentate gyrus of the rat is inhibited by the phosphoinositide 3-kinase inhibitor, wortmannin

    Neuropharmacology

    (2000)
  • U. Kikkawa et al.

    The common structure and activities of four subspecies of rat brain protein kinase C family

    FEBS Lett.

    (1987)
  • C. Luscher et al.

    Role of AMPA receptor cycling in synaptic transmission and plasticity

    Neuron

    (1999)
  • E. Merlo et al.

    The IkappaB kinase inhibitor sulfasalazine impairs long-term memory in the crab Chasmagnathus

    Neuroscience

    (2002)
  • D. Muller et al.

    Induction of stable long-term potentiation in the presence of the protein kinase C antagonist staurosporine

    Neurosci. Lett.

    (1992)
  • I.A. Muslimov et al.

    Dendritic transport and localization of protein kinase Mζ mRNA: implications for molecular memory consolidation

    J. Biol. Chem.

    (2004)
  • A. Nishimune et al.

    NSF binding to GluR2 regulates synaptic transmission

    Neuron

    (1998)
  • Y. Ono et al.

    The structure, expression, and properties of additional members of the protein kinase C family

    J. Biol. Chem.

    (1988)
  • P. Osten et al.

    The AMPA receptor GluR2 C terminus can mediate a reversible, ATP-dependent interaction with NSF and alpha- and beta-SNAPs

    Neuron

    (1998)
  • S. Sajikumar et al.

    Late-associativity, synaptic tagging, and the role of dopamine during LTP and LTD

    Neurobiol. Learn. Mem.

    (2004)
  • I. Song et al.

    Interaction of the N-ethylmaleimide-sensitive factor with AMPA receptors

    Neuron

    (1998)
  • R. Stapulionis et al.

    Efficient mammalian protein synthesis requires an intact F-actin system

    J. Biol. Chem.

    (1997)
  • E. Sublette et al.

    Evidence for a new isoform of protein kinase C in rat hippocampus

    Neurosci. Lett.

    (1993)
  • M.A. Sutton et al.

    Parallel molecular pathways mediate expression of distinct forms of intermediate-term facilitation at tail sensory–motor synapses in Aplysia

    Neuron

    (2000)
  • Y. Takai et al.

    Studies on a cyclic nucleotide-independent protein kinase and its proenzyme in mammalian tissues. I. Purification and characterization of an active enzyme from bovine cerebellum

    J. Biol. Chem.

    (1977)
  • K.L. Thomas et al.

    Spatial and temporal changes in signal transduction pathways during LTP

    Neuron

    (1994)
  • S.F. Traynelis et al.

    Estimated conductance of glutamate receptor channels activated during EPSCs at the cerebellar mossy fiber-granule cell synapse

    Neuron

    (1993)
  • S.H. Yeh et al.

    A requirement of nuclear factor-κB activation in fear-potentiated startle

    J. Biol. Chem.

    (2002)
  • J.C. Yin et al.

    CREB and the formation of long-term memory

    Curr. Opin. Neurobiol.

    (1996)
  • G. Bandyopadhyay et al.

    Evidence for involvement of protein kinase C (PKC)-zeta and noninvolvement of diacylglycerol-sensitive PKCs in insulin-stimulated glucose transport in L6 myotubes

    Endocrinology

    (1997)
  • T.V.P. Bliss et al.

    A synaptic model of memory: long-term potentiation in the hippocampus

    Nature

    (1993)
  • M. Cammalleri et al.

    Time-restricted role for dendritic activation of the mTOR-p70S6K pathway in the induction of late-phase long-term potentiation in the CA1

    Proc. Natl. Acad. Sci. U.S.A.

    (2003)
  • J.M. Cimadevilla et al.

    Inactivating one hippocampus impairs avoidance of a stable room-defined place during dissociation of arena cues from room cues by rotation of the arena

    Proc. Natl. Acad. Sci. U.S.A.

    (2001)
  • J.B. Cracco et al.

    Protein synthesis-dependent LTP in isolated dendrites of CA1 pyramidal cells

    Hippocampus

    (2005)
  • Crick, F. (1984) Memory and molecular turnover. Nature, 312(5990):...
  • P.K. Dash et al.

    Injection of the cAMP-responsive element into the nucleus of Aplysia sensory neurons blocks long-term facilitation

    Nature

    (1990)
  • E.A. Drier et al.

    Memory enhancement and formation by atypical PKM activity in Drosophila melanogaster

    Nat. Neurosci.

    (2002)
  • Y. Dudai

    Memory from A to Z: Keywords, Concepts, and Beyond

    (2002)
  • U. Frey et al.

    Synaptic tagging and long-term potentiation

    Nature

    (1997)
  • S. Hrabetova et al.

    Bidirectional regulation of protein kinase Mζ in the maintenance of long-term potentiation and long-term depression

    J. Neurosci.

    (1996)
  • E.R. Kandel et al.

    Are adult learning mechanisms also used for development?

    Science

    (1992)
  • Cited by (0)

    View full text