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Section 5. Transcription Factors, Genes and Proteins

Section 5. Transcription Factors, Genes and Proteins

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CREB



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formation were conducted in Aplysia and Drosophila. Subsequent studies in vertebrate species

(including, in particular, mice and rats) using a variety of molecular-genetic tools suggest that

CREB has a highly conserved role in LTM formation.



Structure

CREB is a member of a family (CREB/ATF) of structurally similar, activity-regulated transcription factors. In mammals, at least three genes encode the CREB-like proteins, CREB,

CREM (cAMP Response Element Modulator) and ATF-1 (Activating Transcription Factor).38,55,94 The mammalian CREB gene comprises 11 exons,26,54,115 and alternative splicing

generates the three major activator isoforms of CREB: α, ∆, and β.11,47,118 Each of these is

highly expressed in all tissues. In addition to these transcriptional activators, the CREB family

also includes repressors of transcription. For example, the CREM gene codes at least four

isoforms that repress CRE-dependent transcription: the CREM α, β and γ proteins as well as

the inducible cyclic AMP early repressor (ICER).37,82

CREB regulates gene expression in response to a wide array of extracellular signals. In its

inactive state, CREB is prebound as a dimer to CRE sites in the promoter regions of target

genes. Neuronal stimulation may lead to the activation of CREB (via activation of various

CREB kinases). In its activated form CREB binds CREB-binding protein (CBP); the recruitment of CBP links CREB directly and indirectly to other components of the basal transcription machinery, thus promoting transcription.24



Activation

A large number of signaling pathways converge on CREB, indicating that the transcriptional activity of CREB is regulated by a wide variety of extracellular signals.31,77,100 Each of

these pathways activate CREB via CREB kinases that phosphorylate CREB at serine 133

(Ser133). This is the critical residue for the transcriptional activity46 since mutation of this

residue to a nonphosphorylatable alanine (Ala) residue abolishes the transcriptional response

to elevated cAMP levels.46,83 Although CREB was initially identified as a transcription factor

that responds to elevated levels of cAMP, it is now clear that CREB may be activated by three

major signaling pathways (Fig. 1): 1) cAMP, 2) Ca2+, and 3) growth factors.

1) cAMP: The activation of G-protein linked receptors (e.g., D1 receptors) leads to the

increases in the second messenger cAMP via activation of adenylate cyclase.44 Rises in levels of

cAMP lead to the activation of protein kinase A (PKA) by dissociating the regulatory (R) from

the catalytic (C) subunits. The C subunits of PKA passively translocate to the nucleus where

they may phosphorylate CREB at Ser133.5,29,50

2) Ca2+: Calcium is a pleiotropic second messenger that is activated via several different

mechanisms following changes in membrane potential. Extracellular Ca2+ may enter the cytoplasm via ligand-gated ion channels of NMDA and AMPA receptors, or via voltage-gated

calcium channels. In addition, Ca2+ may be released from intracellular stores.100 Calcium signals are then transduced via a large number of different CREB kinases which include: CamKII,

CaMKIV, RSK1-3 (via Ras-ERK), PKC and PKA.10,29,33,75,101,107 The different kinetics of

each of these pathways provides a mechanism for sustained CREB activation and CRE-mediated transcription. For example, activation of CaMKIV produces a wave of CREB phosphorylation with rapid on- and offset (lasting only minutes), whereas activation of the Ras-ERK-RSK2

pathway promotes a slower phase of CREB phosphorylation.116 Furthermore, the distinct kinetic properties of these upstream regulatory pathways may allow CREB to compute information regarding the exact nature of the stimuli, perhaps allowing for specific stimuli (or patterns

of stimulation) to be translated into specific patterns of gene expression. For example, recent

data indicate that Ca2+ influx into neurons causes the phosphorylation of CREB at Ser142 and

Ser143 (in addition to Ser133), and that CREB-induced transcription induced by this triple

phosphorylation may not require the participation of CBP.67 Therefore, Ca2+ influx promotes

CREB-mediated transcription via a set of mechanisms that are distinct from those produced by

other extracellular activation.



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From Messengers to Molecules: Memories Are Made of These



Figure 1. Activation of CREB by a multiple signaling pathways. In the first pathway, a neurotransmitter may

bind to a receptor (R) that is linked to a G-protein (G), which leads to the increases in the second messenger

cAMP via activation of adenylate cyclase (AC). Rises in levels of cAMP leads to the activation of protein

kinase A (PKA) by dissociating the regulatory (R) from the catalytic (C) subunits. The C subunits of PKA

passively translocate to the nucleus where they may phosphorylate CREB at Ser133. In the second pathway

growth factors (such as NGF or BDNF) bind to and activate a Trk receptor. This, in turn, activates Ras and

the downstream kinases Raf, MEK and ERK. Activated ERKs stimulate the activity of MSKs and RSKs

which may then phosphorylate CREB at Ser133. In the third pathway, intracellular increases in Ca2+ which

binds to calmodulin (CaM) which activates CaM kinases (CaMKII, CaMKIV, CaMKK) which may also

phosphorylate CREB at Ser133.



3) Growth Factors: CREB mediates gene expression in response to a wide variety of growth

factors, including nerve growth factor (NGF), fibroblast growth factor (FGF), epidermal growth

factor (EGF) and brain-derived neurotrophic factor (BDNF) (see Brandner, this book). Signaling is then mediated by a large number of growth-factor-induced kinases. For example, NGF

stimulation activates NGF receptors (tyrosine kinase receptor, Trk receptors) that stimulates

guanine-nucleotide release factors (GRFs) that activate Ras, a small G protein. Activated Ras,

in turn, stimulates the serine/threonine kinase, Raf, that triggers activation of MEK, and its

targets, the ERK 1 /2 members of the MAPK family.12 One downstream substrate of the Ras/

ERK pathway is a 90 kDa ribosomal S-6 kinase-2 (RSK-2). Upon activation, both ERKs and

RSKs translocate to the nucleus where they may phosphorylate CREB at Ser133.23,36,117

Just as phosphorylation of Ser133 seems to be critical for activation of CREB, dephosphorylation of this residue is important for inactivation of CREB. As with all other phosphoproteins, therefore, the level of CREB phosphorylation at Ser133 reflects a balance between the

oppositional actions of kinases and phosphatases, such as protein phosphatase 1 and 2 (PP-1

and PP-2).49 For example, dephosphorylation of CREB at Ser 133 may be initiated by the

activation of calcineurin (PP-2B) by the Ca2+-CaM pathway.10 The transcriptional activity of

phosphorylated CREB may also be actively suppressed by transcriptional repressors, such as

CREM α, β and γ isoforms or ICER.37,70,82



CREB



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The complexity of the pathways upstream from CREB may permit tight, fine-tuned regulation of CRE-mediated transcription, allowing it to produce distinct patterns of gene expression in response to different patterns of stimulation. For example, CREB activation may be

moderated by phosphorylation events at sites other than Ser133 (e.g., Ser142 and/or Ser143),

and also indirectly by phosphorylation or dephosphorylation of other components of the transcription machinery that CREB interacts with (e.g., CBP, POL II etc).



CREB and Electrophysiological Studies of Long-Term Plasticity

in Aplysia

The withdrawal of the gill—an external respiratory organ— in the marine mollusk Aplysia

can be produced by mechanical stimulation of either the siphon or mantle shelf. The reflex

serves a defensive purpose: the retraction of the gill protects it from potential damage. This

reflex exhibits a number of forms of plasticity. In particular, the sensitization of the withdrawal

reflex—that is its enhancement following noncontingent shock applied to the tail of the animal—has been instrumental in the identification of many of the cellular and molecular mechanisms mediating synaptic and behavioral plasticity. The persistence of the reflex sensitization is

related to the number of shocks applied to the tail: one shock produces a transient sensitization, lasting minutes, whereas 5 or more shocks produce a LTM lasting days or

longer.6,18,25,39,45,62,91 Long-term sensitization at the synaptic level can be studied in reduced

preparations containing the sensory-motor synapse: short- and long-term facilitation of this

synapse mediates the behavioral sensitization of the reflex.

The role of CREB in memory and plasticity has been studied in cocultured Aplysia sensory

and motor neurons.28 Injection of oligonucleotides with CRE sequences into cultured sensory

neurons blocks long-term facilitation (LTF).28 Presumably, these CRE-oligonucleotides act as

competitive antagonists, trapping the CREB proteins needed for the transcriptional activation

of genes that ultimately mediate LTF.4,61 Moreover, a similar injection of a reporter gene driven

by a CRE-containing promoter shows that stimuli that produce LTF also trigger CREB activation, while stimuli that do not produce LTF similarly do not trigger CREB activation.61

There are several CREB-like proteins in Aplysia. The CREB1 gene encodes three proteins

(ApCREB1a, ApCREB1b and ApCREB1c) by alternative splicing.7 The ApCREB1a shares

structural and functional homology with CREB transactivators in mammals, while ApCREB1b

resembles mammalian ICER, a repressor of CREB transcription. Injection of antibodies or

antisense against CREB1a blocks LTF (but not short-term facilitation) while injection of phosphorylated ApCREB1a protein alone induces LTF.7 Application of ApCREB1b blocks LTF

while decreasing ApCREB1b function lowers the threshold for producing LTF.7 ApCREB1c is

a cytoplasmic protein that lacks a nuclear localization signal. Injection of unphosphorylated

CREB1c followed by a single pulse of serotonin enhances STF and induces LTF. Therefore,

this cytoplasmic form of CREB may play an important role not only in the modulation of

CREB-mediated transcription necessary for LTF but also in STF.7 Aplysia CREB2 is structurally unrelated to Aplysia CREB1 but shares some homology with mouse ATF4.51 Decreasing

ApCREB2 function decreases the threshold for producing LTF.7 However, the precise mechanism underlying the effects that ApCREB2 exerts on LTF is unclear.

One neuron may participate in the storage of multiple memories. Therefore, activity-dependent changes must be synapse-specific so that the same neuron can encode multiple patterns of stimulation. Experiments using a single sensory neuron composed of two branches

that contact two spatially separated motor neurons show that local application of serotonin

onto a single synapse induces LTF that is specific to that branch.22,73 This branch-specific LTF

requires local protein synthesis (presumably at the synapse to be modified) as well as CREB

activation in the nucleus of the presynaptic neuron. Repeated application of serotonin onto the

cell body of the sensory neuron (rather than the branch) induces a transient, cell-wide LTF that

does not persist beyond 48 hours. This transient LTF is CREB-dependent, but is not accompanied by synaptic growth. A similar pattern of transient LTF and no synaptic growth is pro-



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duced by injection of phospho-CREB1 into the sensory neuron. In order for this transient LTF

to become stable and for synaptic growth to appear, a single pulse of serotonin at either synapse

is required. Thus, CREB-mediated transcription cooperatively induces synaptic changes in

concert with local stimulation by serotonin, representing a mechanism by which individual

synapses may be strengthened.



CREB and Memory in Drosophila

The molecular mechanisms underlying LTM have been successfully studied in Drosophila

(or fruit flies). Learning in flies has been studied using an associative olfactory conditioning

paradigm. Flies will learn to avoid a previously neutral odor that was paired with shock in favor

of another odor that was not paired with shock in a T-maze.112 Both forward and reverse

genetic approaches have been used to study the involvement of CREB in memory in Drosophila.112 Using a forward genetic approach, the progeny of flies that were treated with a mutagen were screened for learning and memory impairments. Two mutants identified by this

screen were subsequently determined to have disruptions in Ca2+/CaM-stimulated adenylate

cyclase (rutabaga) and in cAMP-specific phosphodiesterase (dunce), both key enzymes in the

regulation of intracellular levels of cAMP.16,71,112

Just as in other species, LTM (produced by multiple training trials) is dependent on protein

synthesis.113 Using a reverse genetics approach, Yin and colleagues120 showed that disrupting

CREB function in Drosophila blocks LTM produced by multiple training trials, suggesting

that protein synthesis required for LTM is mediated, at least in part, by CREB. They found

that transgenic over-expression of a CREB transcriptional repressor (dCREB2b) impairs LTM,

but not STM, in this task. The finding that STM is intact indicates that the over-expression of

this CREB repressor does not disrupt acquisition, and furthermore suggests that basic perceptual, motor, and motivational processes required for the task are intact in these flies.120

In species ranging from Aplysia to human, spaced training (training trials presented with

intervening rest intervals) is more effective than massed training (the same number of training

trials presented shorter intervening rest intervals) in producing LTM. The same is true in flies:

multiple spaced training produces maximal LTM, whereas the same number of trials presented

in a massed fashion produces strong STM but weak or no LTM. However, massed training

alone is sufficient to produce maximal LTM if a CREB activator (dCREB2a) is over-expressed

in transgenic flies prior to training. The over-expression of this CREB activator produces robust LTM following even just one training trial,121 perhaps the fly equivalent of ‘photographic’

or ‘flashbulb’ memory.119 Transgenic flies over-expressing a mutant activator, where Ser231

(similar to Ser133 of the mammalian CREB gene) was replaced by an Ala, do not show LTM

after one training trial, indicating that phosphorylation of CREB at this residue is required for

the enhancement of LTM.121 Together, these results show the importance of CREB in LTM

formation in Drosophila and, furthermore, suggest that CREB may be a rate-limiting component of this process.



CREB and LTM in Mammals

Targeted Disruption of CREB Function in a Mouse

The study of the role of CREB in mammalian memory was first made possible by the

generation of a mouse in which the CREB gene was disrupted. A neomycin resistance (neo)

gene was inserted into exon 2 of the CREB gene, which was believed to contain the translation

initiation site for all CREB isoforms.56 This neo insertion resulted in the loss of two main

isoforms of CREB (α and ∆). However, the translation of a previously unknown CREB isoform

(CREBβ) starts from exon 4. Therefore insertion of the neo gene into exon 2 did not disrupt

the translation of this isoform; rather, in these CREBα∆ mice, expression of the CREBβ isoform

is elevated.11 The expression levels of CREM activator (τ) and repressor (α and β) isoforms

were also increased in these mice. However, importantly, CREB-dependent transcription is



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decreased in these CREBα∆ mutant mice.11 The homozygous deletion of all major CREB

isoforms (α, β and ∆; CREBnull) is lethal.96

Since the CREBα∆ mice were generated, they have been exhaustively characterized at the

behavioral level. Consistent with the effects of protein synthesis inhibition,2,13,99 CREBα∆ mice

exhibit normal STM but impaired LTM in several fear conditioning paradigms. For example,

CREBα∆ mice show normal conditioned freezing to both tone and context when tested shortly

(<1 hour) after training. However, both contextual and tone fear conditioning are impaired if

these mice are tested 24 hours after training.14,66 A similar pattern of results has been observed

using a different assay of conditioned fear—fear-potentiated startle.34

A parallel set of findings has been observed in studies examining two forms of social learning in CREBα∆ mice. Rodents develop a preference for foods recently smelled on the breath of

other rodents.15,41,42 Memory for this socially transmitted food preference is normal in CREBα∆

mice when tested immediately following training. However, just as in fear conditioning, CREBα∆

mice are impaired when tested 24 hours following training.42,66 The ability of rodents to remember conspecifics can be assessed in a social recognition task. Recognition is inferred from

a decrease in the amount of time spent investigating a familiar (vs. unfamiliar) conspecific.

Again, LTM, but not STM, for social recognition is disrupted in CREBα∆ mice.65 Together

with the fear conditioning data, these findings show that the CREBα∆ mutation specifically

affects LTM, and not STM, in a variety of behavioral paradigms with widely varying performance demands. The extent of these impairments is influenced by gene dosage.42 Further

disruption of CREB function can be achieved by combining the CREBα∆ and CREBnull mutations to produce mice carrying only a single allele for the CREBβ isoform (CREBcomb). Memory

impairments are more severe in these CREBcomb mice compared to the CREBα∆ mice carrying

two alleles for the CREBβ isoform.

Drawing an intriguing parallel with the fly experiments, Kogan et al66 showed that the

LTM deficits in the CREBα∆ mice were rescued by increasing the spacing between training

trials. This was true in three different forms of LTM: spatial (Morris water maze), contextual

(fear conditioning) and social (socially transmitted food preference). These parallels suggest

that the levels of activated CREB are rate-limiting for memory formation: The over-expression

of the CREB activator (dCREB2a) in the transgenic flies removes the requirement for spaced

training trials for LTM formation; Conversely, the reduced levels of CREB in the CREBα∆

mice necessitates multiple, spaced training, rather than fewer massed trials, to produced stable

LTM.

One difficulty in the analysis of knockout mice in learning and memory studies is distinguishing between the effects of a given mutation on mnemonic vs. nonmnemonic processes.

This problem is largely circumvented in the CREBα∆ and related mice since these mice show

normal learning and STM. Therefore, compromising CREB function alone does not seem to

have nonspecific effects on sensory, motor and motivational processes required for the acquisition and expression of learning. Rather, compromising CREB function appears to specifically

affect the formation of LTM.



Gaining Temporal and Spatial Control of CREB Function

in Mammals

One of the problems with traditional knockout approaches is that the target protein is

deleted throughout development and in all tissues. For example, compensatory upregulation of

the CREBβ and CREM isoforms complicates the analysis and interpretation of the CREBα∆

mice. Therefore, achieving both spatial and temporal control over gene expression has been

one of the major goals, and three approaches have attempted to meet this challenge.

First, two studies have examined the effects of CREB antisense oligonucleotides on learning

and memory in rats. Guzowski and McGaugh48 examined acquisition in the hidden version of

the water maze following injections of antisense against CREB mRNA directly into the dorsal

hippocampus of rats. These injections disrupted acquisition in the hidden version of the water



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maze, a form of learning known to be dependent upon the hippocampus. Similar injections 2

days post-training had no effect on subsequent performance in the water maze, indicating that

decreasing CREB function does not affect the expression of a previously consolidated memory.

Acutely disrupting CREB function in the amygdala has also been shown to disrupt the development of a conditioned taste aversion.69 Injections of antisense directed against CREB blocked

long-term (3-5 days), but not short-term (2 hour), CTA memory. Sense control infusions, as

well as infusions of antisense into brain regions (basal ganglia) not critical for plasticity underlying CTA, had no effect.

Second, a transgenic line of mice has been developed that inducibly expresses a CREB

repressor (αCREBS133A).63 The induciblity of the system is produced by fusing the CREB

repressor to a ligand-binding domain (LBD) of a human estrogen receptor with a G521R

mutation (LBDG521R). The activity of this mutated LBD is regulated not by estrogen but by

the synthetic ligand, tamoxifen.27,35,72 In the absence of tamoxifen, the LBDG521R-CREBS133A

fusion protein is bound to heat shock proteins and is therefore inactive.35 However, administration of tamoxifen activates this inducible CREB repressor (CREBIR) fusion protein, allowing it to compete with endogenous CREB and disrupt CRE-mediated transcription. This mouse

has been used to dissect the role of CREB in potentially dissociable memory processes. By

administering tamoxifen to activate the repressor in CREBIR transgenic mice at key time points

in a fear conditioning protocol, the effects of acutely disrupting CREB function on 1) encoding or STM, 2) consolidation into LTM, 3) storage or maintenance, 4) retrieval were assessed.

CREB is crucial for the consolidation of long-term conditioned fear memories, but not for

encoding, storage or retrieval of these memories. While acute over-expression of a CREB repressor disrupts LTM, chronic over-expression of the same transgene throughout development

has much milder effects.93 The weaker effects associated with chronic over-expression of a

CREB repressor (compared to conditional over-expression of this transgene in the CREBIR

mice) may be due to upregulation or compensation through development. Alternatively, the

weaker phenotype might be due to a milder disruption of CREB function in these mice: for

example, transgene expression levels may not be sufficiently high to compete effectively with

endogenous CREB.

A third approach has used viral vector-mediated gene transfer technology to manipulate

CREB levels.19-21 Josselyn et al60 used herpes simplex viral vector-mediated gene transfer technology to specifically increase CREB expression in the amygdala of rats. This method exploits

the natural ability of the herpes simplex virus to insert DNA into specific neuronal populations.103 These rats were fear-conditioned using massed training that normally only produces

STM but no or weak LTM for a light-shock pairing (Fig. 2). However, the over-expression of

CREB in the amygdala neurons now results in normal LTM. These data are consistent with

results in Drosophila showing that increasing CREB levels reduces the number of training trials

required to produce LTM, or overcomes the requirement for trial spacing to produce LTM.121



Detecting CREB Activation During Learning

Complementary to approaches demonstrating that disruption of CREB function blocks

the formation of LTM are those showing that CREB is activated following learning. These

studies are invaluable since they provide a powerful synergy between systems and molecular

approaches. They not only show that CREB-mediated transcription is critical for the formation of long-term memories, but they identify where and when these processes occur.

Activation of CREB leads to the transcription of genes with CRE sites in their promoter

regions. Transgenic mice with a β-galactosidase reporter construct under the regulation of a

CRE-containing promoter (CRE-LacZ) have been used to identify where in the brain learning-related CREB-mediated transcription occurs.58,59 Following fear conditioning significant

increases in CRE-dependent gene expression are observed in both the hippocampus and the

amygdala, consistent with the idea that plasticity in these structures is critical for learning

context-US and tone-US associations. In a clever control study Impey and colleagues showed

that CRE-dependent gene expression related to tone-US associations was limited to the amygdala



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Figure 2. Effects of CREB over-expression in the amygdala on fear-potentiated startle. A) Massed training

produces weak LTM, as assessed by mean fear-potentiated startle difference scores (difference between mean

startle amplitude on light-tone (LT) trials from tone-alone (TA) trials). B) The same number of trials

presented in a spaced fashion produces robust LTM. C) Infusion of HSV-LacZ herpes simplex viral vectors

encoding LacZ (HSV-LacZ) into the basolateral amygdala produces high expression of β-galactosidase that

is restricted to the basolateral amygdala. D) A high-power image of the amygdala following infusion of

HSV-CREB showing over-expression of CREB that is restricted to the lateral nucleus of the amygdala. E)

Infusion of HSV-LacZ into the amygdala does not change the weak LTM normally induced by massed

training. F) Infusion of HSV-CREB into the amygdala prior to massed training enhances LTM.



using a latent inhibition protocol. To minimize the likelihood of the context becoming associated with shock, mice were pre-exposed to the training context for 12 hours prior to training.

These procedures produced significant increases in CRE-dependent gene expression in the

amygdala, but not the hippocampus. Consistent with this, when these mice were tested they

only showed conditioned freezing when re-exposed to the tone, but not the context.



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A second approach has been to use immunocytochemical procedures to detect

learning-induced changes in levels of phosphorylated CREB (pCREB). For example, levels of

pCREB are elevated in the olfactory bulbs following olfactory conditioning in neonate rat

pups.79 Consistent with the effects intra-amygdala infusions of CREB antisense oligonucleotides on the development of a conditioned taste aversion,69 increases in pCREB levels are

observed in the lateral nucleus of the amygdala following pairing of saccharin (CS) and

LiCl-induced illness (US). Similar increases are not observed if the rats are exposed to the CS

(saccharin) or US (LiCl) alone, indicating that activation of CREB is related to associative

learning.

Several studies have examined pCREB levels in fear-motivated learning paradigms. Inhibitory avoidance training, for example, induces phosphorylation of CREB in the CA1 and Dentate Gyrus regions of the hippocampus.9,17,88,108-110,114 These immunocytochemical data confirm similar findings using the CRE-reporter mouse.59 Contextual fear conditioning increases

pCREB levels in both the hippocampus and amygdala,105 again consistent with the observations of Impey.59

The contribution of these studies is that they show that CREB activation is restricted to the

brain regions that have been shown to critically mediate learning in each of these tasks. Furthermore, they allow us to characterize the time course of CREB activation following a learning event. Indeed, both contextual fear conditioning and inhibitory avoidance training appear

to produce two waves of CREB activation:8,105 pCREB levels are initially increased 0-30 minutes following training, and later 3-6 hours following training. These observations are consistent with the idea that LTM formation may involve multiple rounds of protein synthesis. For

example, protein synthesis inhibition immediately following, or 4 hours following training,

disrupts long-term contextual fear memories.13 It is speculated that these later waves may be

mediated by sustained PKA activity: In Aplysia CREB activation leads to the induction of a

number of immediate response genes, including carboxy-terminal ubiquitin hydrolase. This

hydrolase removes the regulatory subunit of PKA, allowing the kinase to become persistently

active.13



CREB and Reconsolidation



Two studies showed that either CRE-dependent gene expression59 or CREB activation105

are detected in the amygdala following fear conditioning training. A third study has shown that

pCREB levels are elevated in the amygdala following testing.52 Therefore retrieval, as well as

encoding, of fear memories initiates signaling cascades that culminate in CREB activation and

presumably gene expression. These findings support the idea that memories are dynamic and

modifiable entities.81,85-87,98 That is, memory retrieval may induce a state of plasticity in which

memories become labile before becoming stable again. The process of re-stabilization of the

trace, or reconsolidation, following retrieval has been shown to be protein-synthesis dependent.86 Consistent with the role for CREB in regulating gene expression required for initial

consolidation of memories, recent data supports the role for CREB in regulating gene expression required for reconsolidation, implied by the Hall52 study. Using the inducible CREB

repressor transgenic mice, Kida et al63 showed that acutely repressing CREB function following memory reactivation impairs the stability of memory. Although the exact molecular mechanisms mediating consolidation and reconsolidation may differ,108 CREB appears to be necessary for both.



Conclusions

Much effort, using a wide variety of tools, has been focused on identifying the molecular

mechanisms underlying learning and memory. Establishing that a particular molecule participates critically in these processes relies, it might be argued, on presentation of at least two types

of evidence.74,95 First, disruption of normal molecular function should interfere with memory

formation. Second, activation of the molecule, in a predictable, region-specific manner, should



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be observed following learning. Reliance on evidence from one line of inquiry increases the

potential for false-negative and false-positive results.43,64 For example, targeted deletion of a

particular molecule may cause learning impairments not because that molecule directly participates in processes critical for plasticity; rather, the loss of that molecule may produce more

global disruption of cellular processes that indirectly impair the neuron’s ability to respond

appropriately to extracellular signals.

The case for the critical involvement of CREB in LTM is compelling since both types of

evidence have been brought to bear on the problem. That is, disrupting CREB function, be it

via the generation and testing of genetically-engineered mice or via the infusion of oligonucleotides, specifically disrupts LTM, but not learning. Secondly, studies using reporter mice or

immunocytochemical approaches, have shown that CREB is activated following learning in a

temporally- and region-restricted manner. In rodents, this conclusion is strengthened since

these observations are drawn from a wide variety of tasks, each with widely varying stimulus

properties and performance demands.

Similar manipulations of CREB function produce qualitatively similar effects in a wide

variety of species including Aplysia, Drosophila, Chasmagnathus crab, honey bees, and song

birds.1,3,57,68,84,92,97,102,119 In humans, it is particularly noteworthy that the cognitive disabilities in several disorders appear to be directly related to disruption of CREB-mediated transcription. Mutations in RSK2, a protein kinase that activates CREB by phosphorylation at

Ser133, are associated with Coffin-Lowry syndrome,111 as well as nonspecific mental retardation.80 For example, in tissue from Coffin-Lowry patients, reductions in RSK2-mediated CREB

phosphorylation (following EGF stimulation) are linearly related to severity of cognitive deficits.53 In addition, Rubinstein-Taybi syndrome, which is caused by a mutation of CBP—the

cofactor that is essential for transcriptional activation of CREB—is associated with mental

retardation.90 Consistent with this, mice that are heterozygous for CBP mutation exhibit learning

impairments.89 These studies, along with those from sea slugs and flies, mice and rats, suggest

an evolutionarily conserved role for CREB-transcription in role in long-term memory formation.



Acknowledgements

We thank Rui M. Costa for comments and discussion.



References

1. Abel T, Martin KC, Bartsch D et al. Memory suppressor genes: Inhibitory constraints on the

storage of long- term memory. Science 1998; 279:338-341.

2. Abel T, Nguyen PV, Barad M et al. Genetic demonstration of a role for PKA in the late phase of

LTP and in hippocampus-based long-term memory. Cell 1997; 885:615-26.

3. Alberini CM. Genes to remember. J Exp Biol 1999; 21:2887-2891.

4. Alberini CM, Ghirardi M, Metz R et al. C/EBP is an immediate-early gene required for the consolidation of long-term facilitation in Aplysia. Cell 1994; 76:1099-1114.

5. Bacskai BJ, Hochner B, Mahaut-Smith M et al. Spatially resolved dynamics of cAMP and protein

kinase A subunits in Aplysia sensory neurons. Science 1993; 105:222-226.

6. Bailey CH, Montarolo P, Chen M et al. Inhibitors of protein and RNA synthesis block structural

changes that accompany long-term heterosynaptic plasticity in Aplysia. Neuron 1992; 94:749-58.

7. Bartsch D, Ghirardi M, Skehel PA et al. Aplysia CREB2 represses long-term facilitation: Relief of

repression converts transient facilitation into long-term functional and structural change. Cell 1995;

83:979-992.

8. Bernabeu R, Cammarota M, Izquierdo I et al. Involvement of hippocampal AMPA glutamate receptor changes and the cAMP/protein kinase A/CREB-P signalling pathway in memory consolidation of an avoidance task in rats. Braz J Med Biol Res 1997; 308:961-965.

9. Bevilaqua LR, Cammarota M, Paratcha G et al. Experience-dependent increase in cAMP-responsive element binding protein in synaptic and nonsynaptic mitochondria of the rat hippocampus.

Eur J Neurosci 1999; 11:3753-3756.

10. Bito H, Deisseroth K, Tsien RW. CREB phosphorylation and dephosphorylation: A Ca(2+)- and

stimulus duration-dependent switch for hippocampal gene expression. Cell 1996; 877:1203-1214.



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From Messengers to Molecules: Memories Are Made of These



11. Blendy JA, Kaestner KH, Schmid W et al. Targeting of the CREB gene leads to up-regulation of

a novel CREB mRNA isoform. Embo J 1996; 155:1098-1106.

12. Blenis J, Chung J, Erikson E et al. Distinct mechanisms for the activation of the RSK kinases/

MAP2 kinase/pp90rsk and pp70-S6 kinase signaling systems are indicated by inhibition of protein

synthesis. Cell Growth Differ 1991; 26:279-285.

13. Bourtchouladze R, Abel T, Berman N et al. Different training procedures recruit either one or two

critical periods for contextual memory consolidation, each of which requires protein synthesis and

PKA. Learning Memory 1998; 5:365-374.

14. Bourtchouladze R, Frenguelli B, Blendy J et al. Deficient long-term memory in mice with a targeted mutation of the cAMP-responsive element-binding protein. Cell 1994; 791:59-68.

15. Bunsey M, Eichenbaum H. Selective damage to the hippocampal region blocks long-term retention of a natural and nonspatial stimulus-stimulus association. Hippocampus 1995; 56:546-556.

16. Byers D, Davis RL, Kiger Jr JA. Defect in cyclic AMP phosphodiesterase due to the dunce mutation of learning in Drosophila melanogaster. Nature 1981; 289:79-81.

17. Cammarota M, Bevilaqua LR, Ardenghi P et al. Learning-associated activation of nuclear MAPK,

CREB and Elk-1, along with Fos production, in the rat hippocampus after a one-trial avoidance

learning: Abolition by NMDA receptor blockade. Mol Brain Res 2000; 761:36-46.

18. Carew TJ, Castellucci VF, Kandel ER. An analysis of dishabituation and sensitization of the

gill-withdrawal reflex in Aplysia. Int J Neurosci 1971; 2:79-98.

19. Carlezon Jr WA, Boundy VA, Haile CN et al. Sensitization to morphine induced by viral-mediated gene transfer. Science 1997; 277:812-814.

20. Carlezon Jr WA, Nestler EJ, Neve RL. Herpes simplex virus-mediated gene transfer as a tool for

neuropsychiatric research. Crit Rev Neurobiol 2000; 141:47-67.

21. Carlezon Jr WA, Thome J, Olson VG et al. Regulation of cocaine reward by CREB. Science 1998;

282:2272-2275.

22. Casadio A, Martin KC, Giustetto M et al. A transient, neuron-wide form of CREB-mediated

long-term facilitation can be stabilized at specific synapses by local protein synthesis. Cell 1999;

992:221-237.

23. Chen RH, Sarnecki C, Blenis J. Nuclear localization and regulation of erk- and rsk-encoded protein kinases. Mol Cell Biol 1992; 123:915-927.

24. Chrivia JC, Kwok RP, Lamb N et al. Phosphorylated CREB binds specifically to the nuclear protein CBP. Nature 1993; 365:855-859.

25. Cleary LJ, Lee WL, Byrne JH. Cellular correlates of long-term sensitization in Aplysia. J Neurosci

1998; 18:5988-5998.

26. Cole TJ, Copeland NG, Gilbert DJ et al. The mouse CREB (cAMP responsive element binding

protein) gene: Structure, promoter analysis, and chromosomal localization. Genomics 1992;

134:974-982.

27. Danielian PS, White R, Hoare SA et al. Identification of residues in the estrogen receptor that

confer differential sensitivity to estrogen and hydroxytamoxifen. Mol Endocrinol 1993; 72:232-240.

28. Dash PK, Hochner B, Kandel ER. Injection of the cAMP-responsive element into the nucleus of

Aplysia sensory neurons blocks long-term facilitation. Nature 1990; 345:718-721.

29. Dash PK, Karl KA, Colicos MA et al. cAMP response element-binding protein is activated by

Ca2+/calmodulin- as well as cAMP-dependent protein kinase. Proc Natl Acad Sci USA 1991;

811:5061-5065.

30. Davis HP, Squire LR. Protein synthesis and memory. Psychol Bull 1984; 6:518-559.

31. Deisseroth K, Tsien RW. Dynamic multiphosphorylation passwords for activity-dependent gene

expression. Neuron 2002; 34:179-182.

32. Dudai Y. Consolidation: Fragility on the road to the engram. Neuron 1996; 173:367-370.

33. Enslen H, Sun P, Brickey D et al. Characterization of Ca2+/calmodulin-dependent protein kinase

IV. Role in transcriptional regulation. J Biol Chem 1994; 269:15520-15537.

34. Falls WA, Kogan JH, Silva AJ et al. Fear-potentiated startle, but not prepulse inhibition of startle,

is impaired in CREBalphadelta-/- mutant mice. Behav Neurosci 2000; 1145:998-1004.

35. Feil R, Brocard J, Mascrez B et al. Ligand-activated site specific recombination in mice. Proc Natl

Acad Sci USA 1996; 93:10887-10890.

36. Finkbeiner S, Tavazoie SF, Maloratsky A et al. CREB: A major mediator of neuronal neurotrophin

responses. Neuron 1997; 195:1031-1047.

37. Foulkes NS, Borrelli E, Sassone-Corsi P. CREM gene: Use of alternative DNA-binding domains

generates multiple antagonists of cAMP-induced transcription. Cell 1991; 644:739-749.

38. Foulkes NS, Mellstrom B, Benusiglio E et al. Developmental switch of CREM function during

spermatogenesis: From antagonist to activator. Nature 1992; 355:80-84.



CREB



503



39. Frost WN, Castelluci VG, Hawkins RD et al. The monosynaptic connections made by the sensory

neurons of the gill– and siphon–withdrawal reflex participate in the storage of long–term memory

for sensitization. Proc Nat Acad Sci USA 1985; 82:8266.

40. Galef Jr BG, Wigmore SW. Transfer of information concerning distant foods: A laboratory investigation of the ‘information-centre’ hypothesis. Anim Behav 1983; 31:748-758.

41. Galef Jr BG, Mason JR, Preti G et al. Carbon disulfide: A semiochemical mediating socially-induced

diet choice in rats. Physiol Behav 1988; 42:119-124.

42. Gass P, Wolfer DP, Balschun D et al. Deficits in memory tasks of mice with CREB mutations

depend on gene dosage. Learning Memory 1998; 5:274-288.

43. Gerlai R. Gene targeting: Technical confounds and potential solutions in behavioral brain research.

Behav Brain Res 2001; 125:13-21.

44. Gilman AG. G proteins: Transducers of receptor-generated signals. Annu Rev Biochem 1987;

56:615-49.

45. Goelet P, Castellucci VF, Schacher S et al. The long and short of long-term memory - a molecular

framework. Nature 1986; 322:419-422.

46. Gonzalez GA, Montminy MR. Cyclic AMP stimulates somatostatin gene transcription by phosphorylation of CREB at serine 133. Cell 1989; 594:675-680.

47. Gonzalez GA, Yamamoto KK, Fischer WH et al. A cluster of phosphorylation sites on the cyclic

AMP-regulated nuclear factor CREB predicted by its sequence. Nature 1989; 337:749-752.

48. Guzowski JF, McGaugh JL. Antisense oligodeoxynucleotide-mediated disruption of hippocampal

cAMP response element binding protein levels impairs consolidation of memory for water maze

training. Proc Natl Acad Sci USA 1997; 946:2693-2698.

49. Hagiwara M, Alberts A, Brindle P et al. Transcriptional attenuation following cAMP induction

requires PP-1-mediated dephosphorylation of CREB. Cell 1992; 701:105-113.

50. Hagiwara M, Brindle P, Harootunian A et al. Coupling of hormonal stimulation and transcription

via the cyclic AMP-responsive factor CREB is rate limited by nuclear entry of protein kinase A.

Mol Cell Biol 1993; 138:4852-4859.

51. Hai T, Liu F, Coukos W et al. Transcription factor ATF cDNA clones: An extensive family of

leucine zipper proteins able to selectively form DNA-binding heterodimers. Genes and Dev 1989;

3:2083-2090.

52. Hall J, Thomas KL, Everitt BJ. Fear memory retrieval induces CREB phosphorylation and Fos

expression within the amygdala. Eur J Neurosci 2001; 137:1453-1458.

53. Harum KH, Alemi L, Johnston MV. Cognitive impairment in Coffin-Lowry syndrome correlates

with reduced RSK2 activation. Neurology 2001; 562:207-214.

54. Hoeffler JP, Meyer TE, Waeber G et al. Multiple adenosine 3',5'-cyclic monophosphate response

element DNA-binding proteins generated by gene diversification and alternative exon splicing. Mol

Endocrinol 1990; 46:920-930.

55. Hoeffler JP, Meyer TE, Yun Y et al. Cyclic AMP-responsive DNA-binding protein: Structure based

on a cloned placental cDNA. Science 1988; 242:1430-1433.

56. Hummler E, Cole TJ, Blendy JA et al. Targeted mutation of the cAMP response element binding

protein (CREB) gene: Compensation within the CREB/ATF family of transcription factors. Proc

Natl Acad Sci USA 1994; 91:5647-5651.

57. Impey S, Goodman RH. CREB signaling—timing is everything. Sci STKE 2001; 82:E1.

58. Impey S, Mark M, Villacres EC et al. Induction of CRE-mediated gene expression by stimuli that

generate long-lasting LTP in area CA1 of the hippocampus. Neuron 1996; 165:973-982.

59. Impey S, Smith DM, Obrietan K et al. Stimulation of cAMP response element (CRE)-mediated

transcription during contextual learning. Nature Neurosci 1998; 1:595–601.

60. Josselyn SA, Shi C, Carlezon Jr WA et al. Long-term memory is facilitated by cAMP response

element-binding protein overexpression in the amygdala. J Neurosci 2001; 21:2404-2412.

61. Kaang BK, Kandel ER, Grant SG. Activation of cAMP-responsive genes by stimuli that produce

long-term facilitation in Aplysia sensory neurons. Neuron 1993; 103:427-435.

62. Kandel ER, Klein M, Bailey CH et al. Serotonin, cyclic AMP, and the modulation of the calcium

current during behavioral arousal. In: Jacobs BL, Gelperin A, eds. Serotonin Neurotransmission

and Behavior. Cambridge, MA: MIT Press, 1981:211-254.

63. Kida S, Josselyn SA, de Ortiz SP et al. CREB required for the stability of new and reactivated fear

memories. Nature Neurosci 2002; 5:348-355.

64. Kim JJ, Baxter MG. Multiple brain-memory systems: The whole does not equal the sum of its

parts. Trends Neurosci 2001; 246:324-330.

65. Kogan JH, Frankland PW, Silva AJ. Long-term memory underlying hippocampus-dependent social

recognition in mice. Hippocampus 2000; 10:47-56.



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