CREB and memory

Reactive oxygen species and nitric oxide mediate plasticity of neuronal calcium signaling

Reactive oxygen species (ROS) and nitric oxide (NO) are important participants in signal transduction that could provide the cellular basis for activity-dependent regulation of neuronal excitability. In young rat cortical brain slices and undifferentiated PC12 cells, paired application of depolarization/agonist stimulation and oxidation induces long-lasting potentiation of subsequent Ca(2+) signaling that is reversed by hypoxia. This potentiation critically depends on NO production and involves cellular ROS utilization. The ability to develop the Ca(2+) signal potentiation is regulated by the developmental stage of nerve tissue, decreasing markedly in adult rat cortical neurons and differentiated PC12 cells.

Reversible downregulation of protein kinase A during olfactory learning using antisense technique impairs long-term memory formation in the honeybee, Apis mellifera

In this study, we examined the role of cAMP-dependent protein kinase (PKA) in associative olfactory learning of the honeybee, Apis mellifera. In the bee, specific interference with molecules to clarify their role in a certain behavior is difficult, because genetic approaches, such as mutants or transgenic animals, are not feasible at the moment. As a new approach in insects in vivo, we report the use of short antisense oligonucleotides. We show that phosphorothioate-modified oligodeoxynucleotides complementary to the mRNA of a catalytic subunit of PKA directly injected into the bee brain cause a reversible and specific downregulation of both the amount of the catalytic subunit and of PKA activity by 10-15%. The amounts of the regulatory subunit of PKA, as well as PKC, are not affected. The slight "knockdown" of PKA activity during the training procedure, a classical olfactory conditioning of the proboscis extension reflex, neither affects acquisition nor memory retention 3 or 6 hr after training. However, it causes an impairment of long-term memory retention 24 hr after training. Downregulation of PKA 3 hr after training has no detectable effect on memory formation. We conclude that PKA contributes to the induction of a long-term memory 24 hr after training when activated during learning. Second, we demonstrate that the antisense technique is feasible in honeybees in vivo and provides a new and powerful tool for the study of the molecular basis of learning and memory formation in insects.

Activation and degradation of the transcription factor C/EBP during long-term facilitation in Aplysia

Long-term facilitation (LTF) of the sensory-to-motor synapses that mediate defensive reflexes in Aplysia requires induction of the transcription factor Aplysia CCAAT/enhancer binding protein (ApC/EBP) as an early response gene. We examined the time course of ApC/ EBP DNA binding during the induction of LTF: Binding activity was detected within 1 h of the sensitization treatment with serotonin, reached a maximum at 2 h, and decreased after 6 h. How are DNA binding and the turnover of ApC/EBP regulated? We find that phosphorylation of ApC/EBP by mitogen-activated protein (MAP) kinase is essential for binding. MAP kinase appears to be activated through protein kinase C. We also showed that ApC/EBP is degraded through the ubiquitin-proteasome pathway but that phosphorylation by MAP kinase renders it resistant to proteolysis. Thus, phosphorylation by MAP kinase is required for ApC/EBP to act as a transcription activator as well as to assure its stability early in the consolidation phase, when genes essential for the development of LTF begin to be expressed.

Alternative exon splicing of cyclic AMP response element-binding protein in peripheral sensory and sympathetic ganglia of the rat

Alternative splicing patterns of cyclic AMP response element-binding protein (CREB) in dorsal root ganglia, lumbar sympathetic ganglia and several peripheral tissues of the rat have been investigated by an exon-flanking polymerase chain reaction strategy. A series of RT-PCR with primer pairs flanking all possible alternative splicing sites (corresponding to a genomic region with at least one full exon and two flanking introns) has revealed multiple tissue specific splice variants. These include some novel transcripts that lack the phosphorylation site and part of the leucine zipper region which is crucial for dimerization and DNA binding. Some isoforms previously reported as testis-specific were also detected in rat peripheral ganglia and other tissues. Notably, splicing patterns are specific for some regions. Some of the splice variants indicate inhibitory functions due to lacking phosphorylation sites or partially missing DNA-binding or leucine zipper domains. These findings suggest a complex expression and functional regulation of CREB in peripheral tissues including dorsal root and sympathetic ganglia.

Cell survival promoted by the Ras-MAPK signaling pathway by transcription-dependent and -independent mechanisms

A mechanism by which the Ras-mitogen-activated protein kinase (MAPK) signaling pathway mediates growth factor-dependent cell survival was characterized. The MAPK-activated kinases, the Rsks, catalyzed the phosphorylation of the pro-apoptotic protein BAD at serine 112 both in vitro and in vivo. The Rsk-induced phosphorylation of BAD at serine 112 suppressed BAD-mediated apoptosis in neurons. Rsks also are known to phosphorylate the transcription factor CREB (cAMP response element-binding protein) at serine 133. Activated CREB promoted cell survival, and inhibition of CREB phosphorylation at serine 133 triggered apoptosis. These findings suggest that the MAPK signaling pathway promotes cell survival by a dual mechanism comprising the posttranslational modification and inactivation of a component of the cell death machinery and the increased transcription of pro-survival genes.

pCREB in the neonate rat olfactory bulb is selectively and transiently increased by odor preference-conditioned training

Early olfactory preference learning in rat pups occurs when novel odors are paired with tactile stimulation, for example stroking. cAMP-triggered phosphorylation of cAMP response element binding protein (pCREB) has been implicated as a mediator of learning and memory changes in various animals (Frank and Greenberg 1994). In the present study we investigate whether CREB is phosphorylated in response to conditioned olfactory training as might be predicted given the proposed role of the phosphorylated protein in learning. On postnatal day 6, pups were trained for 10 min using a standard conditioned olfactory learning paradigm in which a conditioned stimulus, Odor, was either used alone or paired with an unconditioned stimulus, Stroking (using a fine brush to stroke the pup). In some instances stroking only was used. The pups were sacrificed at 0, 10, 30, or 60 min after the training. Using Western blot analysis, we observed that the majority of olfactory bulbs in conditioned pups (Odor + Stroking) had a greater increase in pCREB activation at 10 min after training than pups given nonlearning training (Odor only or Stroking only). The phosphorylated protein levels were low at 0 min and at 60 min after training. This is in keeping with the slightly delayed and short-lived activation period for this protein. The localization of pCREB increases within the olfactory bulb as seen by immunocytochemistry. Naive pups were not exposed to odor or training. There was a significantly higher level of label in mitral cell nuclei within the dorsolateral quadrant of the bulb of pups undergoing odor-stroke pairing. No significant differences were observed among nonlearning groups (Naive, Odor only, or Stroking only) or among any training groups in the granule or periglomerular cells of the dorsolateral region. The localized changes in the nuclear protein are consistent with studies showing localized changes in the bulb in response to a learned familiar odor. The present study demonstrates that selective increases in pCREB occur as an early step following pairing procedures that normally lead to the development of long-term olfactory memories in rat pups. These results support the hypothesized link between pCREB and memory formation.

Electrical activity and gene expression in the development of vertebrate neural circuits

Over the past several decades, anatomical and electrophysiological analyses have demonstrated that the electrical activity of neurons is required for development of the precise patterns of synaptic connectivity found in the adult central nervous system. However, knowledge of the molecular cascades that underlie activity-dependent synaptic development remains rudimentary. As a result, many fundamental issues remain unresolved. Recent advances in differential cloning have begun to provide the tools and insight necessary to bring a molecular level of understanding to principles of activity-dependent synaptic development established via classic systems approaches.

Experience-dependent increase in cAMP-responsive element binding protein in synaptic and nonsynaptic mitochondria of the rat hippocampus

Cyclic AMP-responsive element binding protein (CREB) plays a pivotal role in the formation of long-term memory in Drosophila, Aplysia, mice and rats. Recently, we were able to demonstrate that CREB and its serine 133 phosphorylated form p-CREB are localized in synaptic and nonsynaptic mitochondria of the rat brain. Here we report on the effect of a one-trial inhibitory avoidance training procedure on mitochondrial CREB from the rat hippocampus. This aversively motivated training task is associated with a time-dependent increase (34-35%) in both p-CREB and CREB immunoreactivities detected in synaptic mitochondria of the hippocampus. In nonsynaptic mitochondria, p-CREB levels increased in both trained and shocked animals. In addition to CREB, two CRE-element binding repressors, CREB-2 and CREM-1, were also detected in purified brain mitochondria. No changes were observed in CREB-2 and CREM-1 immunoreactivities in hippocampal synaptic mitochondria after an inhibitory avoidance training. Taken together the present findings represent the first evidence showing that brain mitochondrial CREB may participate in plasticity-dependent changes associated with a behavioural training procedure.

Calcium-stimulated adenylyl cyclase activity is critical for hippocampus-dependent long-term memory and late phase LTP

It is hypothesized that Ca2+ stimulation of calmodulin (CaM)-activated adenylyl cyclases (AC1 or AC8) generates cAMP signals critical for late phase LTP (L-LTP) and long-term memory (LTM). However, mice lacking either AC1 or AC8 exhibit normal L-LTP and LTM. Here, we report that mice lacking both enzymes (DKO) do not exhibit L-LTP or LTM. To determine if these defects are due to a loss of cAMP increases in the hippocampus, DKO mice were unilaterally cannulated to deliver forskolin. Administration of forskolin to area CA1 before training restored normal LTM. We conclude that Ca2+-stimulated adenylyl cyclase activity is essential for L-LTP and LTM and that AC1 or AC8 can produce the necessary cAMP signal.

A late phase of cerebellar long-term depression requires activation of CaMKIV and CREB

Recently, it has been shown that cerebellar LTD has a late phase that may be blocked by protein synthesis inhibitors. To understand the mechanisms underlying the late phase, we interfered with the activation of transcription factors that might couple synaptic activation to protein synthesis. Particle-mediated transfection of cultured Purkinje neurons with an expression vector encoding a dominant inhibitory form of CREB resulted in a nearly complete blockade of the late phase. Kinases that activate CREB were inhibited, and LTD was assessed. Inhibition of PKA or the MAPK/RSK cascades were without effect on the late phase, while constructs designed to interfere with CaMKIV function attenuated the late phase. These results indicate that the activation of CaMKIV and CREB are necessary to establish a late phase of cerebellar LTD.

Drosophila larval neuromuscular junction's responses to reduction of cAMP in the nervous system

We investigated the effects of chronically lowered cyclic adenosine monophosphate (cAMP) on the morphology and physiology of the Drosophila larval neuromuscular junction, using two fly lines in which cAMP was significantly lower than normal in the nervous system: (a) transgenic flies in which the dunce (dnc) gene product was overexpressed in the nervous system, and (b) flies mutant for the rutabaga gene (rut1) which have reduced adenylyl cyclase activity. In comparison with controls, larvae with reduced cAMP exhibited a smaller number of synaptic varicosities. This effect was more pronounced in transgenic larvae, in which the reduction of neural cAMP was more pronounced. Synaptic transmission was also reduced in both cases, as evidenced by smaller excitatory junctional potentials (EJPs). Synaptic currents recorded from individual synaptic varicosities of the neuromuscular junction indicated almost normal transmitter release properties in transgenic larvae and a modest impairment in rut1 larvae. Thus, reduction in EJP amplitude in transgenic larvae is primarily due to reduced innervation, while in rut1 larvae it is attributable to the combined effects of reduced innervation and a mild impairment of transmitter release. We conclude that the major effect of chronically lowered cAMP is reduction of innervation rather than impairment of transmitter release properties.

Signaling routes to CREM and CREB: plasticity in transcriptional activation

The CREB and CREM transcription factors are activated by phosphorylation of a key serine residue by kinases stimulated by cyclic AMP, Ca2+, growth factors and stress signals. Phosphorylation allows recruitment of CREB-binding protein (CBP), a large co-activator that contacts the general transcriptional machinery. Studies of the physiological roles played by CREB and CREM have uncovered novel routes of transcriptional activation. For example, in male germ cells CREM is not phosphorylated but associates with ACT, a member of the LIM-only class of proteins that has intrinsic transcriptional activity. Thus, in some circumstances, CREM can bypass the classical requirement for phosphorylation and association with CBP.

Multiple splice patterns of cyclic AMP response element-binding protein mRNA in the central nervous system of the rat

The alternative splicing pattern of cyclic AMP response element-binding protein (CREB) in the central nervous system (CNS) of the rat has been investigated by an exon-flanking polymerase chain reaction (PCR) strategy. A series of RT-PCR studies with primer pairs flanking all possible alternative splicing sites (corresponding to a genomic region with at least one full exon and two flanking introns) has revealed multiple splice patterns in nine regions of the rat CNS. These include some novel transcripts that lack the phosphorylation site and a segment of the leucine zipper region which is crucial for dimerization and DNA binding. Some isoforms previously reported as testis-specific were also detected in the rat CNS. The findings from this study, which include differential splicing patterns among CNS regions, suggest a complex expression and functional regulation of CREB in the CNS.

The Drosophila dCREB2 gene affects the circadian clock

We report the role of dCREB2, the Drosophila homolog of CREB/CREM, in circadian rhythms. dCREB2 activity cycles with a 24 hr rhythm in flies, both in a light:dark cycle and in constant darkness. A mutation in dCREB2 shortens circadian locomotor rhythm in flies and dampens the oscillation of period, a known clock gene. Cycling dCREB2 activity is abolished in a period mutant, indicating that dCREB2 and Period affect each other and suggesting that the two genes participate in the same regulatory feedback loop. We propose that dCREB2 supports cycling of the Period/Timeless oscillator. These findings support CREB's role in mediating adaptive behavioral responses to a variey of environmental stimuli (stress, growth factors, drug addiction, circadian rhythms, and memory formation) in mammals and long-term memory formation and circadian rhythms in Drosophila.

Molecular mechanisms of synaptic plasticity and memory

To unravel the molecular and cellular bases of learning and memory is one of the most ambitious goals of modern science. The progress of recent years has not only brought us closer to understanding the molecular mechanisms underlying stable, long-lasting changes in synaptic strength, but it has also provided further evidence that these mechanisms are required for memory formation.

Memory Consolidation for Contextual and Auditory Fear Conditioning Is Dependent on Protein Synthesis, PKA, and MAP Kinase

Fear conditioning has received extensive experimental attention. However, little is known about the molecular mechanisms that underlie fear memory consolidation. Previous studies have shown that long-term potentiation (LTP) exists in pathways known to be relevant to fear conditioning and that fear conditioning modifies neural processing in these pathways in a manner similar to LTP induction. The present experiments examined whether inhibition of protein synthesis, PKA, and MAP kinase activity, treatments that block LTP, also interfere with the consolidation of fear conditioning. Rats were injected intraventricularly with Anisomycin (100 or 300 μg), Rp-cAMPS (90 or 180 μg), or PD098059 (1 or 3 μg) prior to conditioning and assessed for retention of contextual and auditory fear memory both within an hour and 24 hr later. Results indicated that injection of these compounds selectively interfered with long-term memory for contextual and auditory fear, while leaving short-term memory intact. Additional control groups indicated that this effect was likely due to impaired memory consolidation rather than to nonspecific effects of the drugs on fear expression. Results suggest that fear conditioning and LTP may share common molecular mechanisms.

Activation of protein kinase A contributes to the expression but not the induction of long-term hyperexcitability caused by axotomy of Aplysia sensory neurons

Nociceptive sensory neurons (SNs) in Aplysia provide useful models to study both memory and adaptive responses to nerve injury. Induction of long-term memory in many species, including Aplysia, is thought to depend on activation of cAMP-dependent protein kinase (PKA). Because Aplysia SNs display similar alterations in models of memory and after nerve injury, a plausible hypothesis is that axotomy triggers memory-like modifications by activating PKA in damaged axons. The present study disproves this hypothesis. SN axotomy was produced by (1) dissociation of somata from the ganglion [which is shown to induce long-term hyperexcitability (LTH)], (2) transection of neurites of dissociated SNs growing in vitro, or (3) peripheral nerve crush. Application of the competitive PKA inhibitor Rp-8-CPT-cAMPS at the time of axotomy failed to alter the induction of LTH by each form of axotomy, although the inhibitor antagonized hyperexcitability produced by 5-HT application. Strong activation of PKA in the nerve by coapplication of a membrane-permeant analog of cAMP and a phosphodiesterase inhibitor was not sufficient to induce LTH of either the SN somata or axons. Furthermore, nerve crush failed to activate axonal PKA or stimulate its retrograde transport. Therefore, PKA activation plays little if any role in the induction of LTH by axotomy. However, the expression of LTH was reduced by intracellular injection of the highly specific PKA inhibitor PKI several days after nerve crush. This suggests that long-lasting activation of PKA in or near the soma contributes to the maintenance of long-term modifications produced by nerve injury.

Is the Ras-MAPK signalling pathway necessary for long-term memory formation?

Genetic and pharmacological experiments have recently implicated several protein kinase cascades in LTP and memory formation. The small GTPases of the Ras subfamily are activated by multiple extracellular stimuli and, via a complex array of downstream effectors, they control a variety of cellular events that culminate in gene transcription. In the well-characterized Aplysia gill-withdrawal reflex, activation of the Ras-dependent mitogen-activated protein kinase (MAPK) cascade is essential for the long-term, but not the short-term, facilitation process. In addition, in the rodent hippocampus, specific inhibition of the MAPK pathway significantly impairs the induction of LTP, which implicates this signalling cascade in hippocampal-dependent behaviour. Mice that lack the neuronal-specific Ras regulator, Ras-GRF (guanine-releasing factor), have severely impaired LTP in the amygdala and a corresponding deficit in long-term memory for aversive events. The results obtained from these different systems demonstrate the involvement of Ras-dependent signalling in neuronal plasticity and behaviour and raise a number of intriguing questions.

CRE-mediated gene transcription in neocortical neuronal plasticity during the developmental critical period

Neuronal activity-dependent processes are believed to mediate the formation of synaptic connections during neocortical development, but the underlying intracellular mechanisms are not known. In the visual system, altering the pattern of visually driven neuronal activity by monocular deprivation induces cortical synaptic rearrangement during a postnatal developmental window, the critical period. Here, using transgenic mice carrying a CRE-lacZ reporter, we demonstrate that a calcium- and cAMP-regulated signaling pathway is activated following monocular deprivation. We find that monocular deprivation leads to an induction of CRE-mediated lacZ expression in the visual cortex preceding the onset of physiologic plasticity, and this induction is dramatically downregulated following the end of the critical period. These results suggest that CRE-dependent coordinate regulation of a network of genes may control physiologic plasticity during postnatal neocortical development.

Ca2+-calmodulin and protein kinase Cs: a hypothetical synthesis of their conflicting convergences on shared substrate domains

Evidence is accumulating that suggests that Ca2+-calmodulin (Ca2+-CaM) and the protein kinase Cs (PKCs) obstruct each other's actions because of the embedding of PKC phosphorylation sites in CaM or Ca2+-CaM-binding domains of a growing number of crucial substrates in neurons (and other cells). These substrates include the CaM storage proteins (neurogranin, neuromodulin), the membrane-associated MARCKS (myristoylated alanine-rich C-kinase substrate) protein, the NMDA receptor RI subunit and the autoinhibitory domain of the plasma membrane Ca2+ pump. In this review, the emerging data are woven into a hypothetical picture of the conflicting, timing-dependent convergence of two major signalers on neuronal functions.

Cyclic GMP pathway is critical for inducing long-term sensitization of nociceptive sensory neurons

Noxious stimulation can trigger persistent sensitization of somatosensory systems that involves memory-like mechanisms. Here we report that noxious stimulation of the mollusc Aplysia produces transcription-dependent, long-term hyperexcitability (LTH) of nociceptive sensory neurons that requires a nitric oxide (NO)-cyclic GMP-protein kinase G (PKG) pathway. Injection of cGMP induced LTH, whereas antagonists of the NO-cGMP-PKG pathway prevented pinch-induced LTH. Co-injection of calcium/cAMP-responsive-element (CRE) blocked both pinch-induced LTH and cAMP-induced LTH, but antagonists of protein kinase A (PKA) failed to block pinch-induced LTH. Thus the NO-cGMP-PKG pathway and at least one other pathway, but not the cAMP-PKA pathway, are critical for inducing LTH after brief, noxious stimulation.

Neurobiology of the Caenorhabditis elegans genome

Neurotransmitter receptors, neurotransmitter synthesis and release pathways, and heterotrimeric GTP-binding protein (G protein)-coupled second messenger pathways are highly conserved between Caenorhabditis elegans and mammals, but gap junctions and chemosensory receptors have independent origins in vertebrates and nematodes. Most ion channels are similar to vertebrate channels but there are no predicted voltage-activated sodium channels. The C. elegans genome encodes at least 80 potassium channels, 90 neurotransmitter-gated ion channels, 50 peptide receptors, and up to 1000 orphan receptors that may be chemoreceptors. For many gene families, C. elegans has both conventional members and divergent outliers with weak homology to known genes; these outliers may provide insights into previously unknown functions of conserved protein families.