CREB and the stability of memory

There is a great deal of evidence that CREB

CREB

CREB is a cellular transcription factor. It binds to certain DNA sequences called cAMP response elements , thereby increasing or decreasing the transcription of the downstream genes....

 has a role in the molecular steps that stabilize memory in the brain. This evidence includes CREB manipulations in several learning paradigms across multiple brain regions and multiple species. For example, there is compelling evidence that CREB has a role in emotional memory. This form of memory can be studied in rodents with experimental paradigms that depend on fear conditioning, an amygdala

Amygdala

The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...

 dependent behavior. The evidence that supports a role for CREB in emotional memory falls into three experimental categories: negative manipulations (where the levels of CREB were lowered), positive manipulations (where the levels of CREB were increased), and non-interventions (where the endogenous levels of CREB were tracked before and after learning).

Knockout

Electrophysiological studies in Aplysia

Aplysia

Aplysia is a genus of medium-sized to extremely large sea slugs, specifically sea hares, which are one clade of large sea slugs, marine gastropod mollusks. The general description of sea hares can be found in the article on the superfamily Aplysioidea....

 indicated that decreasing CREB function blocked long, but not short-term changes in synaptic function . Changes in synaptic function (i.e. synaptic plasticity

Synaptic plasticity

In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...

) are required for learning and memory Accordingly, a line of mice with a targeted disruption of the α and δ isoforms of CREB showed intact short-term memory, but disrupted long-term memory in several behavioral tasks, including contextual conditioning and spatial learning in the water maze, two hippocampal-dependent learning tasks. Accordingly, hippocampal electrophysiological studies revealed that the CREB mutation disrupted the stability of synaptic plasticity Genetic studies in Drosophila

Drosophila

Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

 also uncovered a role for CREB in memory, suggesting that this transcription factor has an evolutionary conserved role in memory

Antisense

Antisense oligonucleotides against hippocampal CREB mRNA can lower the levels of CREB within 6 hours of infusion and impair spatial memory. Interestingly, tests given immediately after training showed that the oligonucleotides against CREB do not disrupt short-term memory

Dominant negative

Another strategy for interfering with CREB function is the use of a dominant negative transgenic strategy. In this strategy, a fragment of the CREB gene was expressed from a transgene in mice The resulting transgenic protein was engineered to interfere with the normal function of CREB so that it competes with wild type CREB for binding sites in the DNA and lacks the domains required for making functional complexes with the transcriptional machinery. To carefully regulate when the dominant negative CREB fragment interfered with CREB function, this DNA was used to generate a fusion protein that also included a mutated ligand binding domain (LBD) of the estrogen receptor

Estrogen receptor

Estrogen receptor refers to a group of receptors that are activated by the hormone 17β-estradiol . Two types of estrogen receptor exist: ER, which is a member of the nuclear hormone family of intracellular receptors, and the estrogen G protein-coupled receptor GPR30 , which is a G protein-coupled...

 that bound tamoxifen

Tamoxifen

Tamoxifen is an antagonist of the estrogen receptor in breast tissue via its active metabolite, hydroxytamoxifen. In other tissues such as the endometrium, it behaves as an agonist, hence tamoxifen may be characterized as a mixed agonist/antagonist...

 rather than estrogen

Estrogen

Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

. When exposed to tamoxifen, the dominant negative fragment changed the conformation of the fusion protein, became active and could therefore interfere with CREB binding sites. One advantage of this inducible transgenic system is that the altered protein is constitutively present and can therefore be rapidly activated following the administration of tamoxifen.

Use of the LBD system to knock down CREB protein function during training produced a deficit in long-term but not short-term memory using both the contextual freezing and tone fear paradigms. Impairing CREB function on test did not impair retrieval of the consolidated memory.

RNAi

siRNAs

Small interfering RNA

Small interfering RNA , sometimes known as short interfering RNA or silencing RNA, is a class of double-stranded RNA molecules, 20-25 nucleotides in length, that play a variety of roles in biology. The most notable role of siRNA is its involvement in the RNA interference pathway, where it...

 can induce a selective degradation of the mRNA of the protein of interest. Infusion of siRNAs against CREB have produced deficits in both contextual conditioning and forward trace conditioning

Activation

CRE dependent Activation Following Learning

Impey et al. produced a transgenic line of CRE-lacZ reporter mice. These mice had a CRE fused to the reporter gene lacZ that produces a protein that is easily visualized. Mice that were trained with a context protocol showed higher levels of CRE mediated transcription in areas CA1 and CA3 of the hippocampus relative to mice that were either untrained or that did not associate the context with the shock due to latent inhibition

Latent inhibition

Latent inhibition is a technical term used in Classical conditioning. A stimulus that has not had any significance in the past takes longer to acquire meaning than a new stimulus...

.

Similarly CRE-lacZ mice that were trained with a tone protocol showed higher levels of CRE dependent gene transcription in the amygdala than either mice with no training or mice in the unpaired group. There was no difference in CRE dependent gene expression in the hippocampus of animals trained with a tone protocol.

Replacement/Rescue

Given that CREB knockout animals show deficits in fear conditioning, an important question is whether that deficit can be rescued by the replacement of CREB. Han et al. addressed this question by infusing a herpes simplex virus expressing CREB into the amygdala of CREB knockout mice.

Surprisingly, the expression of CREB in the amygdala rescued the deficit, indicating that CREB in this structure is critical for memory for tone conditioning.

Overexpression

The role of overexpression of CREB has not been examined systematically in fear conditioning. Overexpression has been used in other conditioning paradigms and has produced a mixed set of results. Josselyn et al. (2001) used viral transfection

Transfection

Transfection is the process of deliberately introducing nucleic acids into cells. The term is used notably for non-viral methods in eukaryotic cells...

 to overexpress CREB in the basolateral amygdala

Amygdala

The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...

 of wild-type rats. CREB over expression increased the fear potentiated startle

Fear potentiated startle

Fear potentiated startle is a reflexive physiological reaction to a neutral stimulus or environment as a result of fear conditioning, a process that occurs when a benign stimulus comes to evoke fear and anxiety upon being paired with a traumatic or fear-provoking stimulus/event...

 response compared to animals receiving a control virus suggesting that CREB levels are limiting during the acquisition of fear potentiated startle and that these levels are related to the strength of this form of memory.

A more recent paper from Restivo et al. showed used a similar viral approach in the hippocampus and demonstrated that additional CREB expression could also enhance contextual fear conditioning, a result consistent with a role of the hippocampus in this form of conditioning.

While viral CREB rescued conditioning deficits in CREB knockout animals, additional CREB did not seem to enhance memory of the wild-type controls. This may be due to a ceiling effect

Ceiling effect

The term ceiling effect has two distinct meanings, referring to the level at which an independent variable no longer has an effect on a dependent variable, or to the level above which variance in an independent variable is no longer measured or estimated...

on the fear conditioning parameters used in the study.

Chronic enhancement of CREB, using genetic manipulations in mice, did not seem to enhance memory in a water maze task (Viosca et al.. In this experiment CREB overexpression was made inducible with the tetracycline transgenic dox system. While additional CREB did not enhance acquisition, additional CREB did interfere with retrieval, suggesting that there may be an optimal level of CREB activation that is compatible with normal memory function. Several recent papers

suggest that CREB is one factor that controls intrinsic excitability which provides an additional mechanism by which CREB can contribute to a gating function on memory acquisition and expression.
Enhanced CREB-dependent gene expression increases the excitability of neurons in the basal amygdala and primes the consolidation of contextual and cued fear memory.