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Keywords:

  • conditioned taste aversion;
  • erk;
  • fear conditioning;
  • learning;
  • LTP

Abstract

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

The mitogen-activated protein kinase (MAP kinase, MAPK) cascade, as the name implies, was originally discovered as a critical regulator of cell division and differentiation. As further details of this signaling cascade were worked out, it became clear that the MAPK cascade is in fact a prototype for a family of signaling cascades that share the motif of three serially linked kinases regulating each other by sequential phosphorylation. Thus, a revised nomenclature arose that uses the term MAPK to refer to the entire superfamily of signaling cascades (comprising the erks, the JNKs and the p38 stress activated protein kinases), and specifies the prototype MAPK as the extracellular signal-regulated kinase (erk). The two erk MAPK isoforms, p44 MAPK and p42 MAPK, are referred to as erk1 and erk2, respectively.The erks are abundantly expressed in neurons in the mature central nervous system, raising the question of why the prototype molecular regulators of cell division and differentiation are present in these non-dividing, terminally differentiated neurons. This review will describe the beginnings of an answer to this question. Interestingly, the general model has begun to emerge that the erk signaling system has been co-opted in mature neurons to function in synaptic plasticity and memory. Moreover, recent insights have led to the intriguing prospect that these molecules serve as biochemical signal integrators and molecular coincidence detectors for coordinating responses to extracellular signals in neurons. In this review I will first outline the essential components of this signal transduction cascade, and briefly describe recent results implicating the erks in mammalian synaptic plasticity and learning. I will then proceed to outline recent results implicating the erks as molecular signal integrators and, potentially, coincidence detectors. Finally, I will speculate on what the critical downstream effectors of the erks are in neurons, and how they might provide a readout of the integrated signal.


The erk MAPK Cascade

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

Regulation of the extracellular signal-regulated kinase (erk) cascade is complex (reviewed in Grewal et al. 1999; see Fig. 1). The erk cascade, like the other MAPK cascades, is distinguished by a characteristic core cascade of three kinases. The first kinase is a so-called MAP kinase kinase kinase (MAPKKK, Raf-1 and B-Raf in the erk cascade) which activates the second, a MAP kinase kinase (MAPKK, MEK in the erk cascade), by serine/threonine phosphorylation. MAPKKs (MEKs) are dual specificity kinases which in turn activate a MAP kinase (p44 MAPK = erk1, p42MAPK = erk2) by phosphorylating both a threonine and a tyrosine residue.

image

Figure 1. MAP kinase signaling through Raf-1 and B-Raf. p42 MAPK, erk2; AC, adenylyl cyclase; GFR, growth factor receptor tyrosine kinase. See text for details and description.

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As mentioned above, regulation of cell proliferation is the best-studied function of erk1 and erk2; the enzymes are activated in response to growth factors and tumor promoters (Hoshi et al. 1988; Ahn and Krebs 1990; Miyasaka et al. 1990) and control cell cycle progression (Gotoh et al. 1991; Posada and Cooper 1992). The ubiquitous Raf-1 pathway is activated by Ras, which is stimulated by growth factor tyrosine kinase receptors acting through Grb2 and Sos; PKC also activates this pathway by interacting with either Ras or Raf-1 (see Fig. 1). Activation of Raf-1 leads to activation of MEK and consequently the erks. Interestingly, a family of phorbol ester-binding Ras/Rap guanine nucleotide exchange factors (GEFs) was recently discovered that allows the second messenger diacylglycerol to achieve erk activation independent of PKC activation (Ebinu et al. 1998; Kawasaki et al. 1998b).

The Ras/Raf-1 pathway is inhibited by PKA, which prevents Raf-1 activation and attenuates its activity. In an important breakthrough, Stork and coworkers discovered that cAMP can be positively coupled to erk activation in neurons via Rap-1 and B-Raf (Vossler et al. 1997). The B-Raf pathway is stimulated by cyclic AMP-dependent protein kinase (PKA) and signals through the Ras homolog, Rap1 (Fig. 1). In addition, more recently a cAMP-responsive GEF was discovered that can also lead to erk activation independent of PKA (de Rooij et al. 1998; Kawasaki et al. 1998a).

Despite this complexity, one simplifying feature of the cascade is that erk (both erk1 and erk2) activity is exclusively regulated by MEK, the upstream dual-specificity kinase that phosphorylates the erks. This dual phosphorylation is both necessary and sufficient for erk activation. This attribute has been capitalized upon to create three pharmacologic tools used to investigate the erk MAPK cascade experimentally: the MEK inhibitors PD098059 (Alessi et al. 1996); U0126 (DeSilva et al. 1998); and SL327 (Atkins et al. 1998). These agents are very effective at blocking erk activation and lend themselves well to studies in vitro, and SL327 can be injected peripherally and still cross the blood–brain barrier (Atkins et al. 1998; Selcher et al. 1999). In addition, the dual phosphorylation and activation of erks can be monitored using commercially available phospho-site antibodies, allowing for a relatively convenient method of directly assaying erk activation in cell extracts (see, for example, Roberson et al. 1999).

MAPK in LTP

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

Long-term potentiation (LTP) is a robust and long-lasting form of synaptic plasticity that is the leading candidate for a cellular mechanism contributing to mammalian learning and memory. Recent advances have given us a much more detailed understanding of the signal transduction mechanisms operating to elicit LTP, and one fact that has become clear is that erk activation plays a critical role in LTP induction. Initial studies in this area focused on NMDA receptor-dependent LTP in area CA1, using hippocampal slices in vitro (English and Sweatt 1996, 1997; Atkins et al. 1998; Impey et al. 1998; Winder et al. 1999; Wu et al. 1999). In addition, recent data have shown a necessity for erk activation in the induction of NMDA receptor-independent LTP (Coogan et al. 1999), LTP in the dentate gyrus in vitro (Coogan et al. 1999), LTP in vivo (McGahon et al. 1999; Davis et al. 2000; Rosenblum et al. 2000) and LTP at the amygdalar inputs into the insular cortex (Jones et al. 1999).

NMDA receptor-dependent LTP in area CA1 is generally divided into at least two phases: early LTP (E-LTP), which lasts about 60–90 min; and late LTP (L-LTP), which is longer lasting and is blocked by inhibitors of protein and RNA synthesis. This latter observation has given rise to the model that L-LTP is dependent on changes in gene expression for its induction. Given the prominent role of erk in regulating gene expression, it is intriguing that strong evidence exists that erk activation is necessary for L-LTP; three structurally distinct MEK inhibitors all block late LTP (English and Sweatt 1996, 1997; Impey et al. 1998; Atkins et al. 1999; Wu et al. 1999; and unpublished observations; see also Liu et al. 1999). However, the effects of inhibitors of MAPK activation are not limited to L-LTP: E-LTP is attenuated also (see, for example, Fig. 2).

image

Figure 2. Proposed involvement of Kv4.2 and CREB in the induction of early and late-phase LTP. Results shown are from extracellular recordings from area CA1 (Atkins et al. 1998). Two 1-s, 100-Hz stimuli were delivered at the arrow, in vehicle or MEK inhibitor-treated (SL327) rat hippocampal slices. One current model is that MAPK plays a dual role in LTP: modulating E-LTP induction through regulating voltage-dependent potassium channels; and triggering L-LTP through regulating CREB phosphorylation. Other possible sites of action are regulating local protein synthesis and regulating other ion channels.

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MAPK in learning and memory

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

The discovery that erk activation is necessary for synaptic plasticity in vitro rapidly led to the evaluation of the role of erk activation in learning in vivo. In this section I will highlight exciting recent discoveries of a role for erk activation in three forms of mammalian learning: fear conditioning, Morris water maze learning and conditioned taste aversion.

Fear conditioning

Fear conditioning is a robust form of classical conditioning exhibited by rodents. Cued (tone) conditioning is used as an index of general associative learning that is amygdala-dependent but hippocampus-independent. Context-dependent conditioning is used as a variant to assess a hippocampus-dependent form of associative learning.

Contextual fear conditioning results in the activation of MAPK in the hippocampus. Atkins et al. (1998) trained animals with the contextual fear conditioning protocol, pairing five foot shocks with the context, then assayed the hippocampus for changes in erk phosphorylation 1 h post-training. A significant increase in erk2 activation in trained animals was observed 1 h following contextual fear conditioning. Identical sham-training of the animals (by placing them in the fear conditioning apparatus in the same manner as the trained animals, but without being shocked), resulted in no change in erk2 activation. To control for potential changes in erk2 activation due to the shock itself, animals were placed in the fear conditioning box and immediately shocked. This protocol elicits no learning in response to the context, nor did it result in increased erk activation. These control experiments indicate that the increase in erk activation during training is not a non-specific response to the context, handling or foot shock.

To test whether another associative conditioning paradigm also led to erk activation, Atkins et al. (1998) paired a tone and shock three times in a unique context, then assayed for MAPK activation. This protocol resulted in robust associative conditioning to both the cue and the context. This behavioral training protocol resulted in a significant increase in erk1 and erk2 activation at 1 h post-training, indicating that the erk MAPK cascade is activated during cued plus contextual fear conditioning. Delivery of the tone alone in the unique context had no effect on erk activation.

These data demonstrate that the erk cascade is activated with fear conditioning. Is this effect NMDA receptor-dependent? Administration of the NMDA receptor antagonist MK801 to animals prior to training resulted in an attenuation of learning with both protocols, and erk activation was attenuated when the hippocampi were assayed 1 h following training with the contextual fear conditioning protocol or the cued and contextual fear conditioning protocol. These data indicate a necessity for NMDA receptor activation for the learning-associated erk activation.

Having observed activation of erk in response to behavioral associative conditioning using the fear conditioning paradigm, it was of interest to determine if blockade of erk activation could cause a blockade of learning in this situation. Fortunately, as described above, the MEK inhibitor SL327 can be administered intraperitoneally and achieve effective concentrations in the CNS. Administration of SL327 either before or immediately after behavioral training led to a blockade of learning, i.e. the animals exhibited essentially no contextual or cued fear conditioning (Atkins et al. 1998). These data strongly support the hypothesis that erk activation is a necessary component of the biochemical cascades utilized to establish behavioral plasticity.

One limitation to the experiments described above using SL327 is that the drug is administered intraperitoneally and thus inhibits MEK throughout the animal. In a significant refinement, Schafe et al. (1999) provided additional strong evidence that MEK activation is required for fear conditioning. These investigators utilized PD98059 infused intraventricularly and observed selective blockade of long-term, but not short-term, fear conditioning. Also, Walz et al. (1999, 2000), using a similar approach of cortical and limbic infusion of PD98059, observed significant effects of MEK inhibition on step-down inhibitory avoidance, a learning paradigm with similarities to associative fear conditioning. Overall these studies, combined with those by Atkins et al. (1998) and a more recent study by Selcher et al. (1999) using MEK inhibition in mice, provide convincing evidence of a necessity for erk activation in mammalian fear conditioning, a robust form of associative learning.

Morris water maze

The Morris water maze is a now-classic test of spatial learning in rodents. This task assesses an animal's capacity to remember spatial cues that are used to locate a hidden underwater platform. The ‘visible platform’ variant also is used as a control to assess the animal's ability to navigate using non-spatial cues. In pioneering studies by Blum et al. (1999) using PD98059 infusion into the hippocampus, the necessity for hippocampal erk activation for spatial memory formation was clearly demonstrated. These observations were later confirmed by Selcher et al. (1999) using SL327 in mice. The effects of MEK inhibition were selective for the hidden platform test while no effect was seen in the visible platform; these data indicate that the MEK inhibition did not non-specifically interfere with the animal's ability to physically execute the task. Dash and coworkers also nicely demonstrated erk activation in the hippocampus when animals underwent Morris maze training, directly demonstrating that erk activation occurs in the hippocampus during learning. Thus, similar to fear conditioning, spatial learning in rodents involves erk activation in the CNS, an effect that is necessary for the formation of lasting memories.

Conditioned taste aversion

Some types of learning are so critical to an animal's survival that they are subserved by extremely robust learning mechanisms. One striking example of this is conditioned taste aversion (CTA). Generally, if an animal consumes a novel foodstuff that subsequently causes sickness, even after a single such experience the animal will exhibit a life-long aversion to that particular food. The survival value of this type of learning is obvious: for an animal having survived a single exposure to a potentially deadly substance their subsequent life-long avoidance of the compound greatly increases their chances of survival.

An exciting series of recent experiments have begun to examine in some detail the molecular and cellular events associated with the formation of this type of gustatory memory. CTA is largely limited to novel tastes, and it is well established that changes in the insular cortex are necessary for conditioned taste aversion (Gallo et al. 1992). In the earliest studies of erk activation in CTA, Berman et al. (1998) discovered that novel saccharin activates the erk MAP kinase pathway in insular cortex of rats. Equally importantly, these investigators demonstrated that erk activation in the insular cortex is necessary for the formation of stable CTA, as infusion of the MEK inhibitor PD098059 into insular cortex blocks this learning.

Until recently little was known about upstream regulators of MAP kinase in the insular cortex. However, two key findings set the stage for experiments to investigate this issue: cholinergic activation arising from nucleus basalis is necessary for taste memory formation; and infusion of cholinergic agonists produces effects that are indistinguishable from those seen in novel taste exposure (Rosenblum et al. 1996, 1997). With these findings in mind Tim Bliss and collaborators (Jones et al. 1999; Rosenblum et al. 2000) investigated the role of the muscarinic cholinergic system in regulating MAPK in the insular cortex. They found that cholinergic receptors can regulate erk activation, and that erk activation is involved in LTP in the insular cortex.

Thus in CTA erk is activated in the insular cortex, and erk activation is necessary for the formation of CTA memory. Erk activation is also necessary for synaptic plasticity (LTP) in the insular cortex. Furthermore, in CTA muscarinic receptors appear to play a key role in learning and erk activation. These observations are especially intriguing given the rapid formation and permanence of CTA, an issue we will return to later.

Erks are responsive to a variety of cell-surface signals in the hippocampus

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

As described above, hippocampal erk activation is critical for LTP and various forms of hippocampus-dependent memory formation. In the next section we will explore in more detail the plethora of signaling mechanisms operating to control erk activation in the hippocampus. We will focus largely on hippocampal area CA1, which is the CNS subregion for which the most information is currently available.

Initial insights into the possible regulation of the erk cascade in neurons came from studies using cell cultures of embryonic neurons or pheochromocytoma cells (PC12 cells). For example, several studies have demonstrated that pharmacological stimulation of the n-methyl-d-aspartate subtype of glutamate receptor leads to the activation of erk2 in both cortical and hippocampal neurons in culture (Bading and Greenberg 1991; Fiore et al. 1993; Kurino et al. 1995; Xia et al. 1996). In addition, stimulation of muscarinic receptors elicits p42 MAPK activation in cortical neurons (Stratton et al. 1989; Wang and Durkin 1995). Finally, stimulation of protein kinase C produces a robust activation of erk2 in both hippocampal and cortical neurons (Stratton et al. 1989). In the first studies of CA1 pyramidal neurons in acute hippocampal slices several of these findings were reiterated. Thus, both pharmacologic NMDA receptor activation and PKC stimulation lead to erk2 activation in this preparation (English and Sweatt 1996). Also, activation of NMDA receptors using LTP-inducing physiologic stimulation similarly results in erk2 activation (English and Sweatt 1996).

As described above more recent work demonstrated the capacity of the cAMP cascade to activate erk (Vossler et al. 1997). Activation of the cAMP cascade also leads to secondary activation of MAPK in hippocampal area CA1 (Martin et al. 1997; Roberson et al. 1999). In addition, activation of β-adrenergic receptors (βARs) using isoproterenol application leads to MAPK activation in area CA1, an effect attenuated by PKA inhibition (Roberson et al. 1999). An important implication of these data is that the β-adrenergic system may utilize MAPK as part of its neuromodulatory cascade in area CA1. This finding is especially interesting because βARs modulate LTP induction in area CA1 (Thomas et al 1996) and, in fact, Winder et al. (1999) have recently observed that the βAR modulation of LTP induction is blocked by MEK inhibitors.

In additional recent studies it was also observed that hippocampal erk activation occurs in response to activation of metabotropic glutamate receptors, muscarinic acetylcholine receptors and DA receptors (Roberson et al. 1999). Moreover, erk activation by DA was blocked by the PKA inhibitor H89, and MAPK activation by metabotropic receptor and muscarinic receptor agonists was blocked by the PKC inhibitor chelerythrine (Roberson et al. 1999).

As described above, initial studies of erk activation in the hippocampus focused on NMDA receptors coupling to erk. Given the more recently documented importance of erk activation in NMDA receptor-dependent LTP and learning, it is important to understand how NMDA receptors couple to erk activation in the hippocampus. This question is unanswered at present. However, the neuromodulatory receptor agonist studies described above, coupled with studies indicating that both the PKA and PKC pathways can elicit hippocampal erk activation, suggest that either (or both) PKA and PKC might be utilized to couple NMDA receptors to erk activation. Also consistent with this possibility is the known property of NMDA receptors to flux calcium, as both PKC and hippocampal adenylyl cyclase are calcium-responsive enzymes. In direct biochemical studies both the PKA and PKC pathways have been found to be activated in an NMDA receptor-dependent fashion in response to LTP-inducing stimulation; thus it remains to be determined which of these pathways operate to cause erk activation in LTP and learning. It is an intriguing possibility that given a sufficient stimulus, either pathway alone may be capable of erk activation in synaptic plasticity, allowing for a fail-safe functional redundancy in the system.

Overall, then, hippocampal erk activation is regulated by a wide variety of neurotransmitter receptors coupled to either PKA or PKC: NMDA receptors, adrenergic receptors, DA receptors, muscarinic acetylcholine receptors and metabotropic glutamate receptors. These findings indicate an unexpected richness of diversity in erk regulation in the hippocampus, and suggest the possibility of a broad role for the erk cascade in both short-term and long-term forms of hippocampal synaptic plasticity.

Moreover, regulation of erk activation in the hippocampus is not limited to neurotransmitter receptors. One of the most widely studied activators of hippocampal erks is brain-derived neurotrophic factor (BDNF). In an important series of studies, Bai Lu and coworkers documented that BDNF receptors couple to erk activation in hippocampal neurons, and demonstrated that the erk activation contributes to BDNF-induced synaptic plasticity in area CA1 (Gottschalk et al. 1998, 1999; Pozzo-Miller et al. 1999). In addition, there are several other intriguing possible regulators of erk in the hippocampus that have not so far been directly demonstrated to play this role. For example, Mary Kennedy's laboratory identified a novel GTPase activating protein, SynGAP, that potentially links Ca/calmodulin activation to erk stimulation via CaMKII inhibition of SynGAP's GTPase-regulating activity (Chen et al. 1998). Reactive oxygen species can lead to erk activation in the hippocampus by mechanisms that are still being worked out (Kanterewicz et al. 1998). Ras is potentially a key regulator of hippocampal erk, acting through RasGRF, src or pyk2 (see Grewal et al. 1999 for a review). These latter possibilities are especially intriguing given recent findings of roles for src and RasGRF in synaptic plasticity and learning (Orban et al. 1999; Manabe et al. 2000).

Finally, in an interesting recent study Perkinton et al. (1999) demonstrated AMPA receptor coupling to erk activation in striatal neurons. These authors observed that calcium-fluxing AMPA receptors led to calcium-dependent erk activation in these cells, via a novel PI 3-kinase-mediated mechanism. While this pathway has not yet been investigated in the hippocampus, it is worthwhile to consider that a similar mechanism might play a role in NMDA receptor-independent LTP in area CA1. Such a mechanism in area CA1 would presumably involve voltage-gated calcium channels as the source of calcium for erk activation, as calcium channel blockers inhibit NMDA receptor-independent LTP in area CA1.

As can be readily seen from the above discussion, the MEK/erk cascade is positioned to play a key role in integrating a variety of cell-surface signals (See Fig. 3). This signal transduction cascade likely serves to ‘funnel’ signals into common downstream targets, in order to achieve a coordinated and cohesive output at the cellular level. Especially intriguing is the possibility that this signal integration may not simply serve to sum up signals, but rather in some cases serves to allow synergistic effects or coincidence detection. Recent results suggest that these sophisticated types of biochemical signal processing are indeed occuring. For example, Watabe et al. (2000) recently reported synergistic activation of hippocampal erks by convergent activation of beta-adrenergic receptors and muscarinic acetylcholine receptors. As mentioned above, Winder et al. (1999) have demonstrated that modulation of LTP induction by beta receptor activation requires erk activation, an effect likely to be achieved through signal integration. Finally, Rosenblum et al. (2000) have recently reported that erk is a key integrator of ionotropic and metabotropic receptor signaling in the dentate gyrus.

image

Figure 3. erk integration of diverse cell-surface signaling mechanisms. See text for description. VGCC, voltage-gated calcium channels; A/K, AMPA/Kainate subtype of glutamate receptor; PLC, phospholipase C; AC, adenylyl cyclase.

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Targets of the integrated signal

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

Given that erks are serving an important role for integrating signals in the hippocampus, what is the read-out of the integrated signal? In other words, what are the downstream effectors of erk that contribute to the induction of synaptic plasticity and learning? While this is very much an open question at the moment, in the remaining space I would like to speculate on two potential key targets of erk: the transcription factor CREB(cAMP response element binding protein) and the potassium channel Kv4.2. Our current working model is that CREB plays a key role in regulating the induction of gene expression-dependent L-LTP, and that Kv4.2 plays a key role in regulating the calcium influx necessary for generating the autonomously active forms of PKC and CaMKII that underlie E-LTP (see Fig. 2). The data suggesting the first part of this working model are substantial and this is a vigorous area of contemporary research; the data suggesting the latter part of the model are much more speculative at present.

Erk as a regulator of neuronal gene expression

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

The activation of erks can lead to the activation of several transcription factors, including CREB, Elk-1 and c-Myc. Historically prominent among the transcription factors regulated by the erks is Elk-1, which when phosphorylated at multiple sites by erk cooperates with serum response factor to drive transcription of serum response element (SRE)-controlled genes. In the interest of saving space, I direct readers to an excellent review by Treisman (1996) for additional details on the role of this cascade in transcriptional regulation by Elk-1. However, at this point I would be remiss if I did not point out the exciting results of Berman et al. (1998) that Elk-1 is phosphorylated in insular cortex in response to novel taste. These investigators also have shown that erk mediates this effect. Thus, while I will focus in the remaining sections on LTP in the hippocampus, the most convincing evidence to date for erk regulation of gene expression in a behavioral learning model comes from investigations of the effects of CTA in the insular cortex.

Another substrate extensively studied in relation to erk's role in regulating gene expression is the transcription factor CREB. CREB activates transcription of genes when phosphorylated at Ser133; the erk cascade is most likely coupled to CREB phosphorylation via activation of a member of the pp90rsk family of S6 kinases, RSK2. Ser133 of CREB is not a substrate for erk; erk's effect is indirect through activating RSK2. Interestingly, the RSK2 gene when mutated in humans causes a form of mental retardation, Coffin–Lowry Syndrome (Trivier et al. 1996). Phosphorylation of Ser133 by PKA, RSK2 or other kinases recruits the CREB binding protein, CBP, to the initiator complex and thereby promotes transcription. Many genes are activated by CREB, including other transcription factors such as c-fos through which CREB signaling can indirectly activate an expanded range of genes.

Given the observations described above concerning a possible role for altered gene expression in late stages of LTP, it is of great interest to evaluate the capacity of the MAPK cascade to regulate phosphorylation of the transcription factor CREB in area CA1. In addition, in earlier studies Impey et al. (1996) constructed a transgenic mouse that allows the monitoring of CREB activation through the insertion into the animal's genome of a CRE-driven β-galactosidase gene. Using this mouse model, Impey et al. directly demonstrated CREB activation in area CA1 after LTP-inducing stimulation. Interestingly, but unexpectedly, tetanus-induced LTP in these transgenic animals is NMDA receptor-independent. Thus, the results of Impey et al. (1996) serendipitously provide a strong implication that CREB activation is also involved in NMDA receptor-independent LTP. Overall these studies indicate the importance of evaluating the mechanisms whereby erk regulates CREB phosphorylation in area CA1.

As already described, activation of PKA by application of forskolin to hippocampal slices results in erk activation in area CA1, and Roberson et al. (1999) observed that this manipulation also elicits increased CREB phosphorylation. Because PKA can directly phosphorylate Ser133 in CREB, it was important to determine if the increased CREB phosphorylation was due to a direct PKA effect versus an effect using MAPK as an intermediary. In order to assess this, Roberson et al. (1999) determined the effects of MEK inhibition on forskolin stimulation of CREB phosphorylation in area CA1. Surprisingly, the MEK inhibitor U0126 blocked CREB phosphorylation in response to forskolin application. While this was unexpected, this effect has recently been confirmed by Lu et al. (1999). Thus, these data demonstrate that activation of MAPK results in increased CREB phosphorylation in area CA1; interestingly, these data also indicate that the cAMP pathway utilizes the MAPK cascade as an obligatory intermediate in regulating CREB phosphorylation in area CA1.

Not only PKA but also PKC can regulate erk in area CA1, and Roberson et al. (1999) determined if erk activation via this route also led to increased CREB phosphorylation. Application of phorbol diacetate (PDA) to hippocampal slices resulted in a robust increase in CREB phosphorylation, indicating that activation of PKC in area CA1 results in increased CREB phosphorylation. Thus while the PKA and CaM-kinase IV pathways have generally been thought of as the chief regulators of CREB phosphorylation in this hippocampal subregion, these data indicate that the PKC pathway plays this role as well. To determine whether regulation of CREB phosphorylation by PKC involves erk as an intermediate, Roberson et al. determined the effects of MEK inhibition on PDA-stimulated CREB phosphorylation. MEK inhibition significantly attenuated phorbol ester-elicited increases in CREB phosphorylation, demonstrating that erk is a necessary component for maximal coupling of PKC activation to CREB phosphorylation in area CA1.

The studies described above indicate that erk is a critical regulator of CREB phosphorylation in area CA1. In a more direct test of this idea, Impey et al. (1998) demonstrated CREB phosphorylation in area CA1 in response to LTPinducing stimulation, an effect that was blocked by erk inhibition. Moreover, recent elegant studies by Caboche and coworkers have demonstrated the existence of a similar role for erk in regulating CREB activation in the dentate gyrus (Davis et al. 2000). In addition, Caboche and coworkers (Vanhoutte et al. 1999) and Perkinton et al. (1999) have documented a role for erk in regulating CREB phosphorylation in striatal neurons. Overall, these studies strongly support the idea that in LTP and other forms of synaptic plasticity the erk cascade may contribute to CREB regulation of gene expression by both the PKA and PKC signal transduction systems.

Kv4.2

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References

The Shal-type K+ channel Kv4.2 is a voltage-dependent K+ channel abundantly expressed in hippocampal pyramidal neurons in area CA1, where its expression is limited to the somatic and dendritic regions (Sheng et al. 1992). In addition, ultrastructural studies suggest that Kv4.2 is predominantly localized to the subsynaptic compartment in neurons (Alonso and Widmer (1997). In the hippocampus, voltage-dependent, transient K+ channels of the Shaker superfamily (Kv channels) can regulate both the excitability of pyramidal neurons and the magnitude of the excitatory post-synaptic potential (EPSP) produced in response to synaptic activity (Hoffman et al. 1997). These channels control back-propagating action potentials that play a critical role in regulating LTP induction, most likely by regulating calcium influx due to NMDA receptor activation or voltage-dependent calcium channel activation. Thus, Kv4.2 appears to be ideally suited, in terms of both its biophysical properties and its subcellular localization, for regulating pyramidal neuron excitability and controlling the induction of synaptic plasticity in the hippocampus.

Our current working hypothesis is that erk regulation of Kv4.2 activation plays a critical role in LTP induction by controlling the voltage-dependent activation of the channel. Any effect of erk to decrease voltage-dependent activation of Kv4.2 would lead to increased excitability, increased action potential back-propagation and augmented LTP induction through enhancing the calcium influx necessary to trigger E-LTP. While we are in the early stages of investigating this hypothesis, several pieces of evidence support the idea. For example, Hoffman and Johnston (1998) recently showed that voltage-dependent activation of these currents (in the dendrites of CA1 pyramidal neurons) is decreased by either PKC or PKA, known activators of MAPK in CA1 pyramidal neurons (English and Sweatt 1996; Martin et al. 1997; Impey et al. 1998; Lu et al. 1999; Roberson et al. 1999). Furthermore, we found evidence that Kv4.2 is a substrate for MAPK (Adams et al. 2000). Finally, Liu et al. (1999) recently observed that MEK inhibition blocks LTP-associated CaMKII autophosphorylation, an effect possibly due to a necessity of erk activity for LTP-associated membrane depolarization and NMDA receptor activation. Thus, while the mechanisms underlying the regulation of neuronal excitability in the induction of LTP are unknown, recent studies indicate that phosphorylation of the K+ channel Kv4.2 is a very appealing explanation.

Abstract

Our understanding of the molecular mechanisms for synaptic plasticity and learning have increased dramatically over the last decade. These new insights have been not only exciting but also somewhat bewildering, as the complexity of the molecular underpinnings of synaptic plasticity have become clear. This review has been an attempt to refine from a disparate literature one example of how the neuron may deal with coordinating its responses to the wide variety of cell surface signals involved in plasticity and learning. Thus, we have examined the role of the erk cascade in coordinating responses to neuronal second messengers and cell surface receptors. In the broader context it is interesting to note that this role in neurons reiterates the role of the extracellular signal-regulated kinase cascade in coordinating other complex cellular responses such as cell division and differentiation.

References

  1. Top of page
  2. Abstract
  3. The erk MAPK Cascade
  4. MAPK in LTP
  5. MAPK in learning and memory
  6. Erks are responsive to a variety of cell-surface signals in the hippocampus
  7. Targets of the integrated signal
  8. Erk as a regulator of neuronal gene expression
  9. Kv4.2
  10. References
  • Adams P., Anderson A., Varga A., Dinley K., Cook R., Pfaffinger P. J. & Sweatt J. D. (2000 ) The A-type potassium channel Kv4.2 is a substrate for the nitrogen-activated protein kinase ERK. J. Neurochem. 75, 22772287.
  • Ahn N. G. & Krebs E. G. (1990) Evidence for an epidermal growth factor-stimulated protein kinase cascade in Swiss 3T3 cells. Activation of serine peptide kinase activity by myelin basic protein kinases in vitro. J. Biol. Chem. 265, 1149511501.
  • Alessandrini A., Crews C. M. & Erikson R. L. (1992) Phorbol ester stimulates a protein-tyrosine/threonine kinase that phosphorylates and activates the Erk-1 gene product. Proc. Natl Acad. Sci. USA 89, 82008204.
  • Alessi D. R., Cuenda A., Cohen P., Dudley D. T. & Saltiel A. R. ( 1996 ) PD 098059 is a specific inhibitor of the activation of nitrogen-activated protein kinase in vitro and in vivo. J. Biol Chem. 270, 2784927494.
  • Alonso G. & Widmer H. (1997) Clustering of Kv4.2 potassium channels in postsynaptic membrane of rat supraoptic neurons: an ultrastructural study. Neuroscience 77, 617621.DOI: 10.1016/s0306-4522(96)00561-1
  • Atkins C. M., Selcher J. C., Petraitis J. J., Trzaskos J. M. & Sweatt J. D. (1998) The MAP kinase cascade is required for mammalian associative learning. Nature Neurosci. 1, 602609.
  • Bading H. & Greenberg M. E. (1991) Stimulation of protein tyrosine phosphorylation by NMDA receptor activation. Science 253, 912914.
  • Berman D. E., Hazvi S., Rosenblum K., Seger R. & Dudai Y. (1998) Specific and differential activation of mitogen-activated protein kinase cascades by unfamiliar taste in the insular cortex of the behaving rat. J. Neurosci. 18, 1003710044.
  • Blum S., Moore A. N., Adams F. & Dash P. K. 1999 A mitogen-activated protein kinase cascade in the CA1/CA2 subfield of the dorsal hippocampus is essential for long-term spatial memory. J. Neurosci. 19, 35353544.
  • Chen H. J., Rojas-Soto M., Oguni A. & Kennedy M. B. (1998) A synaptic Ras-GTPase activating protein (p135 SynGAP) inhibited by CaM kinase II. Neuron 20, 895904.
  • Coogan A. N., O'leary D. M. & O'connor J. J. (1999) P42/44 MAP kinase inhibitor PD98059 attenuates multiple forms of synaptic plasticity in rat dentate gyrus in vitro. J. Neurophysiol. 81, 103110.
  • Davis S., Vanhoutte P., Pages C., Caboche J. & Laroche S. (2000) The MAPK/ERK cascade targets both Elk-1 and cAMP response element-binding protein to control long-term potentiation-dependent gene expression in the dentate gyrus in vivo. J. Neurosci. 20, 45634572.
  • DeSilva D. R., Jones E. A., Favata M. F., Jaffee B. D., Magolda R. L., Trzaskos J. M. & Scherle P. A. (1998) Inhibition of mitogen-activated protein kinase kinase blocks T cell proliferation but does not induce or prevent anergy. J. Immunol. 160, 41754181.
  • Ebinu J. O., Bottorff D. A., Chan E. Y. W., Stang S. L., Dunn R. J. & Stone J. C. (1998) RasGRP, a ras guanyl nucleotide-releasing protein with calcium- and diacylglycerol-binding motifs. Science 280, 10821086.DOI: 10.1126/science.280.5366.1082
  • English J. D. & Sweatt J. D. (1996) Activation of p42 mitogen-activated protein kinase in hippocampal long term potentiation. J. Biol. Chem. 271, 2432924332.
  • English J. D. & Sweatt J. D. (1997) A requirement for the mitogen-activated protein kinase cascade in hippocampal long-term potentiation. J. Biol. Chem. 272, 1910319106.
  • Fiore R. S., Murphy T. H., Sanghera J. S., Pelech S. L. & Baraban J. M. (1993) Activation of p42 mitogen-activated protein kinase by glutamate receptor stimulation in rat primary cortical cultures. J. Neurochem. 61, 16261633.
  • Gallo M., Roldan G. & Bures J. (1992) Differential involvement of gustatory insular cortex and amygdala in the acquisition and retrieval of conditioned taste aversion in rats. Behav. Brain Res. 52, 9197.
  • Gotoh Y., Nishida E., Matsuda S., Shiina N., Kosako H., Shiokawa K., Akiyama T., Ohta K. & Sakai H. (1991) In vitro effects on microtubule dynamics of purified Xenopus M phase-activated MAP kinase. Nature 349, 251254.
  • Gottschalk W., Pozzo-Miller L. D., Figurov A. & Lu B. (1998) Presynaptic modulation of synaptic transmission and plasticity by brain-derived neurotrophic factor in the developing hippocampus. J. Neurosci. 18, 68306839.
  • Gottschalk W. A., Jiang H., Tartaglia N., Feng L., Figurov A. & Lu B. (1999) Signaling mechanisms mediating BDNF modulation of synaptic plasticity in the hippocampus. Learn Mem. 6, 243256.
  • Grewal S. S., York R. D. & Stork P. J. (1999) Extracellular-signal-regulated kinase signalling in neurons. Curr. Opin. Neurobiol. 9, 544553.DOI: 10.1016/s0959-4388(99)00010-0
  • Hoffman D. & Johnston D. (1998) Downregulation of transient K+ channels in dendrites of hippocampal CA1 pyramidal neurons by activation of PKA and PKC. J. Neurosci. 18, 35213528.
  • Hoffman D., Magee J. C., Colbert C. M. & Johnston D. (1997) Potassium channel regulation of signal propagation in dendrites of hippocampal pyramidal neurons. Nature 387, 1997.
  • Hoshi M., Nishida E. & Sakai H. (1988) Activation of a Ca2+-inhibitable protein kinase that phosphorylates microtubule-associated protein 2 in vitro by growth factors, phorbol esters, and serum in quiescent cultured human fibroblasts. J. Biol. Chem. 263, 53965401.
  • Impey S., Mark M., Villacres E. C., Poser S., Chavkin C. & Storm D. R. (1996) Induction of CRE-mediated gene expression by stimuli that generate long-lasting LTP in area CA1 of the hippocampus. Neuron 16, 973982.
  • Impey S., Obrietan K., Wong S. T., Poser S., Yano S., Wayman G., Deloulme J. C., Chan G. & Storm D. R. (1998) Cross talk between ERK and PKA is required for Ca2+ stimulation of CREB-dependent transcription and ERK nuclear translocation. Neuron 21, 869883.
  • Johnston D., Hoffman D. A., Magee J. C., Poolos N. P., Watanabe S., Colbert C. M. & Migliore M. (2000) dendritic potassium channels in hippocampal pyramidal neurons. J. Physiol. (Lon.) in press.
  • Jones M. W., French P. J., Bliss T. V. & Rosenblum K., (1999) Molecular mechanisms of long-term potentiation in the insular cortex in vivo. J. Neurosci. 19, RC36.
  • Kanterewicz B. I., Knapp L. T. & Klann E. (1998) Stimulation of p42 and p44 mitogen-activated protein kinases by reactive oxygen species and nitric oxide in hippocampus. J. Neurochem. 70, 10091016.
  • Kawasaki H., Springett G. M., Mochizuki N., Toki S., Nakaya M., Matsuda M., Housman D. E. & Graybiel A. M. (1998a) A family of cAMP-binding proteins that directly activate Rap1. Science 282, 22752279.DOI: 10.1126/science.282.5397.2275
  • Kawasaki H., Springett G. M., Toki S., Canales J. J., Harlan P., Blumenstiel J. P., Chen E. J., Bany I. A., Mochizuki N., Ashbacher A., Matsuda M., Housman D. E. & Graybiel A. M. (1998b) A Rap guanine nucleotide exchange factor enriched highly in the basal ganglia. Proc. Natl Acad. Sci. USA 95, 1327813283.DOI: 10.1073/pnas.95.22.13278
  • Kurino M., Fukunaga K., Ushio Y. & Miyamoto E. (1995) Activation of mitogen-activated protein kinase in cultured rat hippocampal neurons by stimulation of glutamate receptors. J. Neurochem. 65, 12821289.
  • Liu J., Fukunaga K., Yamamoto H., Nishi K. & Miyamoto E. (1999) Differential roles of Ca2+/calmodulin-dependent protein kinase II and mitogen-activated protein kinase activation in hippocampal long-term potentiation. J. Neurosci. 19, 82928299.
  • Lu Y., Kandel E. R. & Hawkins R. D. (1999) Nitric oxide signaling contributes to late-phase LTP and CREB phosphorylation in the hippocampus. J. Neurosci. 19, 1025010261.
  • Manabe T., Aiba A., Yamada A., Ichise T., Sakagami H., Kondo H. & Katsuki M. (2000) Regulation of long-term potentiation by H-Ras through NMDA receptor phosphorylation. J. Neurosci. 20, 25042511.
  • Martin K. C., Michael D., Rose J. C., Barad M., Casadio A., Zhu H. & Kandel E. R. (1997) MAP kinase translocates into the nucleus of the presynaptic cell and is required for long-term facilitation in Aplysia. Neuron 18, 899912.
  • McGahon B., Maguire C., Kelly A. & Lynch M. A. (1999) Activation of p42 mitogen-activated protein kinase by arachidonic acid and trans-1-amino-cyclopentyl-1,3-dicarboxylate impacts on long-term potentiation in the dentate gyrus in the rat: analysis of age-related changes. Neuroscience 90, 11671175.DOI: 10.1016/s0306-4522(98)00528-4
  • Miyasaka T., Miyasaka J. & Saltiel A. R. (1990) Okadaic acid stimulates the activity of microtubule associated protein kinase in PC-12 pheochromocytoma cells. Biochem. Biophys. Res. Comm. 168, 12371243.
  • Orban P. C., Chapman P. F. & Brambilla R. (1999) Is the Ras-MAPK signalling pathway necessary for long-term memory formation? Trends Neurosci. 22, 3844.DOI: 10.1016/s0166-2236(98)01306-x
  • Perkinton M. S., Sihra T. S. & Williams R. J. (1999) Ca(2+)-permeable AMPA receptors induce phosphorylation of cAMP response element-binding protein through a phosphatidylinositol 3-kinase-dependent stimulation of the mitogen-activated protein kinase signaling cascade in neurons. J. Neurosci. 19, 58615874.
  • Posada J. & Cooper J. A. (1992) Requirements for phosphorylation of MAP kinase during meiosis in Xenopus oocytes. Science 255, 212215.
  • Pozzo-Miller L. D., Gottschalk W., Zhang L., McDermott K., Du J., Gopalakrishnan R., Oho C., Sheng Z. H. & Lu B. (1999) Impairments in high-frequency transmission, synaptic vesicle docking, and synaptic protein distribution in the hippocampus of BDNF knockout mice. J. Neurosci. 19, 49724983.
  • Roberson* E. D., English* J. D., Adams J. P., Selcher J. C. & Sweatt J. D. (1999) The mitogen-activated protein kinase cascade couples PKA and PKC to CREB phosphorylation in area CA1 of hippocampus. J. Neurosci. 19, 43374348 (*denotes equal contributions).
  • De Rooij J., Zwartkruis F. J., Verheijen M. H., Cool R. H., Nijman S. M., Wittinghofer A. & Bos J. L. (1998) Epac is a Rap1 guanine-nucleotide-exchange factor directly activated by cyclic AMP. Nature 396, 474477.
  • Rosenblum K., Meiri N. & Dudai Y. (1993) Taste memory: the role of protein synthesis in gustatory cortex. Behav. Neural Biol. 59, 4956.
  • Rosenblum K., Berman D. E., Hazvi S. & Dudai Y. (1996) Carbachol mimics effects of sensory input on tyrosine phosphorylation in cortex. Neuroreport 7, 14011404.
  • Rosenblum K., Berman D. E., Hazvi S., Lamprecht R. & Dudai Y. (1997) NMDA receptor and the tyrosine phosphorylation of its 2B subunit in taste learning in the rat insular cortex. J. Neurosci. 17, 51295135.
  • Rosenblum K., Futter M., Jones M., Hulme E. C. & Bliss T. V. P. (2000) ERKI/II Regulation by the Muscarinic Acetylcholine Receptors in Neurons. J. Neurosci. 20, 977985.
  • Schafe G. E., Nadel N. V., Sullivan G. M., Harris A. & LeDoux J. E. 1999 Memory consolidation for contextual and auditory fear conditioning is dependent on protein synthesis, PKA and MAP kinase. Learn Mem. 6, 97110.
  • Selcher J., Atkins C. M., Trzaskos J. M., Paylor R. & Sweatt J. D. 1999 A necessity for MAP kinase activation in mammalian spatial learning. Learn. Mem. 6, 478 - 490.
  • Sheng M., Tsaur M. L. & January L. Y. (1992) Subcellular segregation of two A-type K+ channel proteins in rat central neurons. Neuron 9, 271284.
  • Stratton K. R., Worley P. F., Huganir R. L. & Baraban J. M. (1989) Muscarinic agonists and phorbol esters increase tyrosine phosphorylation of a 40-kilodalton protein in hippocampal slices. Proc. Natl Acad. Sci. USA 86, 24892501.
  • Thomas M. J., Moody T. D., Makhinson M. & O'dell T. J. (1996) Activity-dependent beta-adrenergic modulation of low frequency stimulation induced LTP in the hippocampal CA1 region. Neuron 17, 475482.
  • Treisman R. (1996) Regulation of transcription by MAP kinase cascades. Curr. Opin. Cell Biol. 8, 205215.
  • Trivier E., De Cesare D., Jacquot S., Pannetier S., Zackai E., Young I., Mandel J. L., Sassone-Corsi P. & Hanauer A. (1996) Mutations in the kinase Rsk-2 associated with Coffin–Lowry syndrome. Nature 384, 567570.
  • Vanhoutte P., Barnier J. V., Guibert B., Pages C., Besson M. J., Hipskind R. A. & Caboche J. (1999) Glutamate induces phosphorylation of Elk-1 and CREB, along with c-fos activation, via an extracellular signal-regulated kinase-dependent pathway brain slices. Mol. Cell Biol. 19, 136146.
  • Vossler M. R., Yao H., York R. D., Pan M.-G., Rim C. S. & Stork P. J. S. (1997) cAMP activates MAP kinase and Elk-1 through a B-Raf- and Rap1-dependent pathway. Cell 89, 7382.
  • Walz R., Roesler R., Quevedo J., Rockenbach I. C., Amaral O. B., Vianna M. R., Lenz G., Medina J. H. & Izquierdo I. (1999) Dose-dependent impairment of inhibitory avoidance retention in rats by immediate post-training infusion of a mitogen-activated protein kinase kinase inhibitor into cortical structures. Behav. Brain Res. 105, 219223.DOI: 10.1016/s0166-4328(99)00077-7
  • Walz R., Roesler R., Quevedo J., Sant'anna M. K., Madruga M., Rodrigues C., Gottfried C., Medina J. H. & Izquierdo I. (2000) Time-dependent impairment of inhibitory avoidance retention in rats by posttraining infusion of a mitogen-activated protein kinase kinase inhibitor into cortical and limbic structures. Neurobiol. Learn. Mem. 73, 1120.DOI: 10.1006/nlme.1999.3913
  • Wang Y. & Durkin J. P. (1995) Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, but not n-methyl-d-aspartate, activates mitogen-activated protein kinase through G-protein beta gamma subunits in rat cortical neurons. J. Biol. Chem. 270, 2278322787.
  • Watabe A. M., Zaki P. A. & O’Dell T. J. 2000 Coactivation of beta-adrenergic and cholinergic receptors enhances the introduction of long-term potentiation and synergistically activates mitogen-activated protein kinase in the hippocampal CA1 region. J. Neurosci. 20, 59245931.
  • Winder D. G., Martin K., Muzzo I., Rohrer D., Chruscinski A., Kobilka B. & Kandel E. (1999) ERK plays a novel regulatory role in the induction of LTP by theta frequency stimulation and its regulation by beta-adrenergic receptors in CA1 pyramidal cells. Neuron 24, 715726.
  • Wu S. P., Lu K. T., Chang W. C. & Gean P. W. (1999) Involvement of mitogen-activated protein kinase in hippocampal long-term potentiation. J. Biomed Sci. 6, 409417.
  • Xia Z., Dudek H., Miranti C. K. & Greenberg M. E. (1996) Calcium influx via the NMDA receptor induces immediate early gene transcription by a MAP kinase/ERK-dependent mechanism. J. Neurosci. 16, 54255436.