Peter V. Nguyen

Genetic Demonstration of a Role for PKA in the Late Phase of LTP and in Hippocampus-Based Long-Term Memory

To explore the role of protein kinase A (PKA) in the late phase of long-term potentiation (L-LTP) and memory, we generated transgenic mice that express R(AB), an inhibitory form of the regulatory subunit of PKA, only in the hippocampus and other forebrain regions by using the promoter from the gene encoding Ca2+/ calmodulin protein kinase IIalpha. In these R(AB) transgenic mice, hippocampal PKA activity was reduced, and L-LTP was significantly decreased in area CA1, without affecting basal synaptic transmission or the early phase of LTP. Moreover, the L-LTP deficit was paralleled by behavioral deficits in spatial memory and in long-term but not short-term memory for contextual fear conditioning. These deficits in long-term memory were similar to those produced by protein synthesis inhibition. Thus, PKA plays a critical role in the consolidation of long-term memory.

A Macromolecular Synthesis-Dependent Late Phase of Long-Term Potentiation Requiring cAMP in the Medial Perforant Pathway of Rat Hippocampal Slices

Memory storage consists of a short-term phase that is independent of new protein synthesis and a long-term phase that requires the synthesis of new proteins and RNA. A cellular representation of these two phases has been demonstrated recently for long-term potentiation (LTP) in both the Schaffer collateral and the mossy fibers of the hippocampus, a structure widely thought to contribute to memory consolidation. By contrast, much less information is available about the medial perforant pathway (MPP), one of the major inputs to the hippocampus. We found that both a short-lasting and a long-lasting potentiation (L-LTP) can be induced in the MPP of rat hippocampal slices by applying repeated tetanization in reduced levels of magnesium. This potentiation was dependent on the activation of NMDA receptors. The early, transient phase of LTP in the MPP did not require either protein or RNA synthesis, and it was independent of protein kinase A activation. By contrast, L-LTP required the synthesis of proteins and RNA, and was selectively blocked by inhibitors of cAMP-dependent protein kinase (PKA). Forskolin, an adenylate cyclase activator, also induced a L-LTP that was attenuated by inhibition of transcription. Our results demonstrate that, like LTP in the Schaffer collateral and mossy fiber pathways, MPP LTP also consists of a late phase that is dependent on protein and RNA synthesis and PKA activity. Thus, cAMP-mediated transcription appears to be a common mechanism for the late form of LTP in all three pathways within the hippocampus.

β-adrenergic receptor activation facilitates induction of a protein synthesis-dependent late phase of long-term potentiation

Long-term potentiation (LTP) is activity-dependent enhancement of synaptic strength that can critically regulate long-term memory storage. Like memory, LTP exhibits at least two mechanistically distinct temporal phases. Early LTP (E-LTP) does not require protein synthesis, whereas the late phase of LTP (L-LTP), like long-term memory, requires protein synthesis. Hippocampal β-adrenergic receptors can regulate expression of both E-LTP and long-term memory. Although β-adrenergic receptor activation enhances the ability of subthreshold stimuli to induce E-LTP, it is unclear whether such activation can facilitate induction of L-LTP. Here, we use electrophysiological recording methods on mouse hippocampal slices to show that when synaptic stimulation that is subthreshold for inducing L-LTP is paired with β-adrenergic receptor activation, the resulting LTP persists for over 6 h in area CA1. Like L-LTP induced by multiple trains of high-frequency electrical stimulation, this LTP requires protein synthesis. Unlike tetanus-induced L-LTP, however, L-LTP induced by β-adrenergic receptor activation during subthreshold stimulation appears to involve dendritic protein synthesis but not somatic transcription. Maintenance of this LTP also requires activation of extracellular signal-regulated kinases (ERKs). Thus, β-adrenergic receptor activation elicits a type of L-LTP that requires translation and ERK activation but not transcription. This form of L-LTP may be a cellular mechanism for facilitation of behavioral long-term memory during periods of heightened emotional arousal that engage the noradrenergic modulatory system.

Regulation of hippocampus-dependent memory by cyclic AMP-dependent protein kinase.

The hippocampus is crucial for the consolidation of new declarative long-term memories. Genetic and behavioral experimentation have revealed that several protein kinases are critical for the formation of hippocampus-dependent long-term memories. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of hippocampus-dependent memory. We review evidence that PKA is required for hippocampus-dependent memory in mammals, and we highlight some of the proteins that have been implicated as targets of PKA. Future directions and open questions regarding the role of PKA in memory storage are also described.

ERK and mTOR Signaling Couple β-Adrenergic Receptors to Translation Initiation Machinery to Gate Induction of Protein Synthesis-dependent Long-term Potentiation

β-Adrenergic receptors critically modulate long-lasting synaptic plasticity and long-term memory in the mammalian hippocampus. Persistent long-term potentiation of synaptic strength requires protein synthesis and has been correlated with some forms of hippocampal long-term memory. However, the intracellular processes that initiate protein synthesis downstream of the β-adrenergic receptor are unidentified. Here we report that activation of β-adrenergic receptors recruits ERK and mammalian target of rapamycin signaling to facilitate long-term potentiation maintenance at the level of translation initiation. Treatment of mouse hippocampal slices with a β-adrenergic receptor agonist results in activation of eukaryotic initiation factor 4E and the eukaryotic initiation factor 4E kinase Mnk1, along with inhibition of the translation repressor 4E-BP. This coordinated activation of translation machinery requires concomitant ERK and mammalian target of rapamycin signaling. Taken together, our data identify distinct signaling pathways that converge to regulate β-adrenergic receptor-dependent protein synthesis during long-term synaptic potentiation in the hippocampus. We suggest that β-adrenergic receptors play a crucial role in gating the induction of long-lasting synaptic plasticity at the level of translation initiation, a mechanism that may underlie the ability of these receptors to influence the formation of long-lasting memories.

Activation of exchange protein activated by cyclic-AMP enhances long-lasting synaptic potentiation in the hippocampus

cAMP is a critical second messenger implicated in synaptic plasticity and memory in the mammalian brain. Substantial evidence links increases in intracellular cAMP to activation of cAMP-dependent protein kinase (PKA) and subsequent phosphorylation of downstream effectors (transcription factors, receptors, protein kinases) necessary for long-term potentiation (LTP) of synaptic strength. However, cAMP may also initiate signaling via a guanine nucleotide exchange protein directly activated by cAMP (Epac). The role of Epac in hippocampal synaptic plasticity is unknown. We found that in area CA1 of mouse hippocampal slices, activation of Epac enhances maintenance of LTP without affecting basal synaptic transmission. The persistence of this form of LTP requires extracellular signal-regulated protein kinase (ERK) and new protein synthesis, but not transcription. Because ERK is involved in translational control of long-lasting plasticity and memory, our data suggest that Epac is a crucial link between cAMP and ERK during some forms of protein synthesis-dependent LTP. Activation of Epac represents a novel signaling pathway for rapid regulation of the stability of enduring forms of LTP and, perhaps, of hippocampus- dependent long-term memories.

Homosynaptic and Heterosynaptic Inhibition of Synaptic Tagging and Capture of Long-Term Potentiation by Previous Synaptic Activity

Long-term potentiation (LTP) is an enhancement of synaptic strength that may contribute to information storage in the mammalian brain. LTP expression can be regulated by previous synaptic activity, a process known as “metaplasticity.” Cell-wide occurrence of metaplasticity may regulate synaptic strength. However, few reports have demonstrated metaplasticity at synapses that are silent during activity at converging synaptic inputs. We describe a novel form of cell-wide metaplasticity in hippocampal area CA1. Low-frequency stimulation (LFS) decreased the stability of long-lasting LTP [“late” LTP (L-LTP)] induced later at the same inputs (homosynaptic inhibition) and at other inputs converging on the same postsynaptic cells (heterosynaptic inhibition). Significantly, heterosynaptic inhibition of L-LTP also occurred across basal and apical dendrites (“heterodendritic” inhibition). Because transient early LTP (E-LTP) was not affected by previous LFS, we examined the effects of LFS on the consolidation of E-LTP to L-LTP. The duration of E-LTP induced at one set of inputs can be extended by capturing L-LTP-associated gene products generated by previous activity at other inputs to the same postsynaptic neurons. LFS applied homosynaptically or heterosynaptically before L-LTP induction did not impair synaptic capture by subsequent E-LTP stimulation, suggesting that LFS does not impair L-LTP-associated transcription. In contrast, LFS applied just before E-LTP (homosynaptically or heterosynaptically) prevented synaptic tagging, and capture of L-LTP expression. Thus, LFS inhibits synaptic tagging to impair expression of subsequent L-LTP. Such anterograde inhibition represents a novel way in which synaptic activity can regulate the expression of future long-lasting synaptic plasticity in a cell-wide manner.

Metaplasticity of the late-phase of long-term potentiation: a critical role for protein kinase A in synaptic tagging

The late‐phase of long‐term potentiation (L‐LTP) in hippocampal area CA1 requires gene expression and de novo protein synthesis but it is expressed in an input‐specific manner. The ‘synaptic tag’ theory proposes that gene products can only be captured and utilized at synapses that have been ‘tagged’ by previous activity. The mechanisms underlying synaptic tagging, and its activity dependence, are largely undefined. Previously, we reported that low‐frequency stimulation (LFS) decreases the stability of L‐LTP in a cell‐wide manner by impairing synaptic tagging. We show here that a phosphatase inhibitor, okadaic acid, blocked homosynaptic and heterosynaptic inhibition of L‐LTP by prior LFS. In addition, prior LFS homosynaptically and heterosynaptically impaired chemically induced synaptic facilitation elicited by forskolin/3‐isobutyl‐1‐methylxanthine, suggesting that there is a cell‐wide dampening of cAMP/protein kinase A (PKA) signaling concurrent with phosphatase activation. We propose that prior LFS impairs expression of L‐LTP by inhibiting synaptic tagging through its actions on the cAMP/PKA pathway. In support of this notion, we show that hippocampal slices from transgenic mice that have genetically reduced hippocampal PKA activity display impaired synaptic capture of L‐LTP. An inhibitor of PKA, KT‐5720, also blocked synaptic capture of L‐LTP. Moreover, pharmacological activation of the cAMP/PKA pathway can produce a synaptic tag to capture L‐LTP expression, resulting in persistent synaptic facilitation. Collectively, our results show that PKA is critical for synaptic tagging and for input‐specific L‐LTP. PKA‐mediated signaling can be constrained by prior episodes of synaptic activity to regulate subsequent L‐LTP expression and perhaps control the integration of multiple synaptic events over time.

Genetic and pharmacological demonstration of a role for cyclic AMP‐dependent protein kinase‐mediated suppression of protein phosphatases in gating the expression of late LTP

Protein kinases and phosphatases play antagonistic roles in regulating hippocampal long‐term potentiation (LTP), with kinase inhibition and phosphatase activation both impairing LTP. The late phase of LTP (L‐LTP) requires activation of cAMP‐dependent protein kinase (PKA) for its full expression. One way in which PKA may critically modulate L‐LTP is by relieving an inhibitory constraint imposed by protein phosphatases. Using mutant PKA mice [R(AB) transgenic mice] that have genetically reduced hippocampal PKA activity, we show that deficient L‐LTP in area CA1 of mutant hippocampal slices is rescued by acute application of two inhibitors of protein phosphatase‐1 and protein phosphatase‐2A (PP1/2A) (okadaic acid and calyculin A). Furthermore, synaptic facilitation induced by forskolin, an adenylyl cyclase activator, was impaired in R(AB) transgenics and was also rescued by a PP1/2A inhibitor in mutant slices. Inhibition of PP1/2A did not affect early LTP (E‐LTP) or basal synaptic transmission in mutant and wildtype slices. Our data show that genetic inhibition of PKA impairs L‐LTP by reducing PKA‐mediated suppression of PP1/2A.

Viagra for your synapses: Enhancement of hippocampal long-term potentiation by activation of beta-adrenergic receptors

Beta-adrenergic receptors (beta-ARs) critically modulate long-lasting synaptic plasticity and long-term memory storage in the mammalian brain. Synaptic plasticity is widely believed to mediate memory storage at the cellular level. Long-term potentiation (LTP) is one type of synaptic plasticity that has been linked to memory storage. Activation of beta-ARs can enhance LTP and facilitate long-term memory storage. Interestingly, many of the molecular signaling pathways that are critical for beta-adrenergic modulation of LTP mirror those required for the persistence of memory. In this article, we review the roles of signaling cascades and translation regulation in enabling beta-ARs to control expression of long-lasting LTP in the rodent hippocampus. These include the cyclic-AMP/protein kinase-A (cAMP-PKA) and extracellular signal-regulated protein kinase cascades, two key pathways known to link transmitter receptors with translation regulation. Future research directions are discussed, with emphasis on defining the roles of signaling complexes (e.g. PSD-95) and glutamatergic receptors in controlling the efficacy of beta-AR modulation of LTP.