Induction Of Late-phase Long-term Potentiation Research Articles

The role of 19S proteasome associated deubiquitinases in activity-dependent hippocampal synaptic plasticity

Posttranslational modification and degradation of proteins by the ubiquitin-proteasome system (UPS) is crucial to synaptic transmission. It is well established that 19S proteasome associated deubiquitinases (DUBs) reverse the process of ubiquitination by removing ubiquitin from their substrates. However, their potential contribution to hippocampal synaptic plasticity has not been addressed in detail. Here, we report that inhibition of the 19S proteasome associated DUBs, ubiquitin C-terminal hydrolase 5 (UCHL5) and ubiquitin-specific peptidase 14 (USP14) by b-AP15 results in an accumulation of polyubiquitinated proteins and a reduction of monomeric ubiquitin without overt effects on 26S proteasome activity. b-AP15 led to a suppression of mTOR-p70S6K signaling and an increase in levels of p-p38 MAPK, two pathways essentially involved in establishing various forms of activity-dependent plasticity. Additionally, b-AP15 impaired the induction of late-phase long-term potentiation (L-LTP), induced the transformation of mGluR-mediated protein synthesis-independent long-term depression (early-LTD) to L-LTD and promoted heterosynaptic stabilization through synaptic tagging/capture (STC) in the hippocampal CA1 region of mice. The activity of 19S proteasome associated DUBs was also required for the enhancement of short-term potentiation (STP) induced by brain-derived neurotrophic factor (BDNF). Altogether, these results indicate an essential role of 19S proteasome associated DUBs in regulating activity-dependent hippocampal synaptic plasticity.

Age-dependent changes in brain hydration and synaptic plasticity

Aging in humans and animals is associated with gradual and variable changes in some cognitive functions, but what causes them and explains individual variations remains unclear. Hydration decreases with aging but whether dehydration contributes to cognitive dysfunction is not known. The brain hydration of aging mice was determined by colloidosmotic-pressure titration. Dehydration increased with age from ∼76 mmHg at 6 weeks to ∼105 mmHg at 40 weeks, or a progressive ∼10 percent loss of brain water but seemed to level off afterward. When we adjusted dehydration in hippocampal slices of <8-week-old mice to the levels seen in mice 40 weeks and older, their basal synaptic responses were amplified at all stimulus voltages tested, but induction of late-phase long-term potentiation was impaired. Our results document progressive brain dehydration with age in inbred mice to levels at which in vitro synaptic plasticity appears dysregulated. They also suggest that dehydration contributes to some of the changes in synaptic plasticity observed with aging, possibly due to adjustments in neuronal excitation mechanisms.

Mapping Gene Expression in Excitatory Neurons during Hippocampal Late-Phase Long-Term Potentiation.

The persistence of long-lasting changes in synaptic connectivity that underlie long-term memory require new RNA and protein synthesis. To elucidate the temporal pattern of gene expression that gives rise to long-lasting neuronal plasticity, we analyzed differentially-expressed (DE) RNAs in mouse hippocampal slices following induction of late phase long-term potentiation (L-LTP) specifically within pyramidal excitatory neurons using Translating Ribosome Affinity Purification RNA sequencing (TRAP-seq). We detected time-dependent changes in up- and down-regulated ribosome-associated mRNAs over 2 h following L-LTP induction, with minimal overlap of DE transcripts between time points. TRAP-seq revealed greater numbers of DE transcripts and magnitudes of LTP-induced changes than RNA-seq of all cell types in the hippocampus. Neuron-enriched transcripts had greater changes at the ribosome-loading level than the total RNA level, while RNA-seq identified many non-neuronal DE mRNAs. Our results highlight the importance of considering both time course and cell-type specificity in activity-dependent gene expression during memory formation.

Social Isolation During Adolescence Strengthens Retention of Fear Memories and Facilitates Induction of Late-Phase Long-Term Potentiation.

Social isolation during the vulnerable period of adolescence produces emotional dysregulation that often manifests as abnormal behavior in adulthood. The enduring consequence of isolation might be caused by a weakened ability to forget unpleasant memories. However, it remains unclear whether isolation affects unpleasant memories. To address this, we used a model of associative learning to induce the fear memories and evaluated the influence of isolation mice during adolescence on the subsequent retention of fear memories and its underlying cellular mechanisms. Following adolescent social isolation, we found that mice decreased their social interaction time and had an increase in anxiety-related behavior. Interestingly, when we assessed memory retention, we found that isolated mice were unable to forget aversive memories when tested 4 weeks after the original event. Consistent with this, we observed that a single train of high-frequency stimulation (HFS) enabled a late-phase long-term potentiation (L-LTP) in the hippocampal CA1 region of isolated mice, whereas only an early-phase LTP was observed with the same stimulation in the control mice. Social isolation during adolescence also increased brain-derived neurotrophic factor (BDNF) expression in the hippocampus, and application of a tropomyosin-related kinase B (TrkB) receptor inhibitor ameliorated the facilitated L-LTP seen after isolation. Together, our results suggest that adolescent isolation may result in mental disorders during adulthood and that this may stem from an inability to forget the unpleasant memories via BDNF-mediated synaptic plasticity. These findings may give us a new strategy to prevent mental disorders caused by persistent unpleasant memories.Electronic supplementary materialThe online version of this article (doi:10.1007/s12035-014-8917-0) contains supplementary material, which is available to authorized users.

Apolipoprotein E4 impairs in vivo hippocampal long-term synaptic plasticity by reducing the phosphorylation of CaMKIIα and CREB.

Inheritance of the apolipoprotein E genotype ε4 (APOE4) is a powerful risk factor for most cases of late-onset Alzheimer's disease (AD). However, the effects of ApoE4 on the long-term synaptic plasticity and its underlying mechanism have not clearly investigated. In the present study, we examined the effects of ApoE4 on the hippocampal late-phase long-term potentiation (L-LTP) and investigated its probable molecular mechanisms by using in vivo field potential recording, immunohistochemistry, and western blotting. The results showed that: (1) intra-hippocampal injection of 0.2 μg ApoE4, but not ApoE2, before high frequency stimulations (HFSs) attenuated the induction of hippocampal L-LTP in the CA1 region, while injection of the same concentration of ApoE4 after HFSs, even at a higher concentration (2 μg), did not affect the long term synaptic plasticity; (2) ApoE4 injection did not affect the paired pulse facilitation in the hippocampal CA1 region; (3) ApoE4 injection before, not after, HFSs significantly decreased the levels of phosphorylated Ca2+/calmodulin-dependent protein kinase IIα (p-CaMKIIα) and phosphorylated cAMP response element-binding protein (p-CREB) in the hippocampus. These results demonstrated for the first time that ApoE4 could impair hippocampal L-LTP by reducing p-CaMKIIα and p-CREB, suggesting that the ApoE4-induced suppression of hippocampal long-term synaptic plasticity may contribute to the cognitive impairments in genetic AD; and both CaMKIIα and CREB are important intracellular targets of the neurotoxic ApoE4.

Simulations suggest pharmacological methods for rescuing long-term potentiation

Congenital cognitive dysfunctions are frequently due to deficits in molecular pathways that underlie the induction or maintenance of synaptic plasticity. For example, Rubinstein–Taybi syndrome (RTS) is due to a mutation in cbp, encoding the histone acetyltransferase CREB-binding protein (CBP). CBP is a transcriptional co-activator for CREB, and induction of CREB-dependent transcription plays a key role in long-term memory (LTM). In animal models of RTS, mutations of cbp impair LTM and late-phase long-term potentiation (LTP). As a step toward exploring plausible intervention strategies to rescue the deficits in LTP, we extended our previous model of LTP induction to describe histone acetylation and simulated LTP impairment due to cbp mutation. Plausible drug effects were simulated by model parameter changes, and many increased LTP. However no parameter variation consistent with a effect of a known drug class fully restored LTP. Thus we examined paired parameter variations consistent with effects of known drugs. A pair that simulated the effects of a phosphodiesterase inhibitor (slowing cAMP degradation) concurrent with a deacetylase inhibitor (prolonging histone acetylation) restored normal LTP. Importantly these paired parameter changes did not alter basal synaptic weight. A pair that simulated the effects of a phosphodiesterase inhibitor and an acetyltransferase activator was similarly effective. For both pairs strong additive synergism was present. The effect of the combination was greater than the summed effect of the separate parameter changes. These results suggest that promoting histone acetylation while simultaneously slowing the degradation of cAMP may constitute a promising strategy for restoring deficits in LTP that may be associated with learning deficits in RTS. More generally these results illustrate how the strategy of combining modeling and empirical studies may provide insights into the design of effective therapies for improving long-term synaptic plasticity and learning associated with cognitive disorders.

Protein kinases paralleling late-phase LTP formation in dorsal hippocampus in the rat

Hippocampal long term potentiation (LTP), representing a cellular model for learning and memory formation, can be dissociated into at least two phases: a protein-synthesis-independent early phase, lasting about 4h and a protein-synthesis-dependent late phase LTP lasting 6h or longer, or even days. A large series of protein kinases have been shown to be involved and herein, a distinct set of protein kinases proposed to be involved in memory retrieval in previous work was tested in dorsal hippocampus of the rat following induction of late-phase LTP. A bipolar stimulation electrode was chronically implanted into the perforant path, while two monopolar recording electrodes were implanted into the dentate gyrus of the dorsal hippocampus. The recording electrode was measuring extracellular excitatory postsynaptic potentials, while the other one measured population spikes. Protein kinases were determined by immunoblotting and immunoflourescence on hippocampal areas showed the distribution pattern of protein kinases PKN1 and NEK7.Induction of LTP was proven, elevated levels for protein kinases PKN1, RPS6KB1, STK4, CDC42BPB, PRKG, TLK, BMX and decreased levels for NEK7, MAK14 and PLK1 were observed. A remarkable overlap of protein kinases observed in spatial memory processes with those proposed in LTP formation was demonstrated. The findings may be relevant for design of future studies on protein kinases and for the interpretation of previous work.

Proteasome Modulates Positive and Negative Translational Regulators in Long-Term Synaptic Plasticity

Proteolysis by the ubiquitin-proteasome pathway appears to have a complex role in synaptic plasticity, but its various functions remain to be elucidated. Using late phase long-term potentiation (L-LTP) in the hippocampus of the mouse as a model for long-term synaptic plasticity, we previously showed that inhibition of the proteasome enhances induction but blocks maintenance of L-LTP. In this study, we investigated the possible mechanisms by which proteasome inhibition has opposite effects on L-LTP induction and maintenance. Our results show that inhibiting phosphatidyl inositol-3 kinase or blocking the interaction between eukaryotic initiation factors 4E (eIF4E) and 4G (eIF4G) reduces the enhancement of L-LTP induction brought about by proteasome inhibition suggesting interplay between proteolysis and the signaling pathway mediated by mammalian target of rapamycin (mTOR). Also, proteasome inhibition leads to accumulation of translational activators in the mTOR pathway such as eIF4E and eukaryotic elongation factor 1A (eEF1A) early during L-LTP causing increased induction. Furthermore, inhibition of the proteasome causes a buildup of translational repressors, such as polyadenylate-binding protein interacting protein 2 (Paip2) and eukaryotic initiation factor 4E-binding protein 2 (4E-BP2), during late stages of L-LTP contributing to the blockade of L-LTP maintenance. Thus, the proteasome plays a critical role in regulating protein synthesis during L-LTP by tightly controlling translation. Our results provide novel mechanistic insights into the interplay between protein degradation and protein synthesis in long-term synaptic plasticity.

Adenylyl cyclase subtype 1 is essential for late-phase long term potentiation and spatial propagation of synaptic responses in the anterior cingulate cortex of adult mice.

Long-term potentiation (LTP) is a key cellular mechanism for pathological pain in the central nervous system. LTP contains at least two different phases: early-phase LTP (E-LTP) and late-phase LTP (L-LTP). Among several major cortical areas, the anterior cingulate cortex (ACC) is a critical brain region for pain perception and its related emotional changes. Periphery tissue or nerve injuries cause LTP of excitatory synaptic transmission in the ACC. Our previous studies have demonstrated that genetic deletion of calcium-stimulated adenylyl cyclase 1 (AC1) or pharmacological application of a selective AC1 inhibitor NB001 blocked E-LTP in the ACC. However, the effect of AC1 on L-LTP, which requires new protein synthesis and is important for the process of chronic pain, has not been investigated. Here we tested the effects of NB001 on the ACC L-LTP and found that bath application of NB001 (0.1 μM) totally blocked the induction of L-LTP and recruitment of cortical circuitry without affecting basal excitatory transmission. In contrast, gabapentin, a widely used analgesic drug for neuropathic pain, did not block the induction of L-LTP and circuitry recruitment even at a high concentration (100 μM). Gabapentin non-selectively decreased basal synaptic transmission. Our results provide strong evidence that the selective AC1 inhibitor NB001 can be used to inhibit pain-related cortical L-LTP without affecting basal synaptic transmission. It also provides basic mechanisms for possible side effects of gabapentin in the central nervous system and its ineffectiveness in some patients with neuropathic pain.

Chronic Nicotine Treatment Reverses Hypothyroidism-Induced Impairment of L-LTP Induction Phase: Critical Role of CREB

We have previously shown that adult onset hypothyroidism impairs late-phase long-term potentiation (L-LTP) and reduces basal protein levels of cyclic-AMP response element binding protein (CREB), mutagen-activated protein kinase (MAPKp42/44), and calcium calmodulin kinase IV (CaMKIV) in area Cornu Ammonis 1 (CA1) of the hippocampus. These changes were reversed by chronic nicotine treatment. In the present study, levels of signaling molecules important for L-LTP were determined in CA1 area of the hippocampus during the induction phase. Standard multiple high-frequency stimulation (MHFS) was used to evoke L-LTP in the CA1 area of the hippocampus of hypothyroid, nicotine-treated hypothyroid, nicotine, and sham control anaesthetized adult rats. Chronic nicotine treatment reversed hypothyroidism-induced impairment of L-LTP at the induction phase. Five minutes after MHFS, Western blotting showed an increase in the levels of P-CREB, and P-MAPKp42/44 in sham-operated control, nicotine, and nicotine-treated hypothyroid animals, but not in hypothyroid animals. The protein levels of total CREB, total MAPK p42/44, BDNF, and CaMKIV were not altered in all groups 5min after MHFS. Therefore, normalized phosphorylation of essential kinases such as P-CREB and P-MAPK p42/44 in the CA1 area of nicotine-treated hypothyroid animals plays a crucial role in nicotine-induced rescue of L-LTP induction during hypothyroidism.

Levothyroxin replacement therapy restores hypothyroidism induced impairment of L-LTP induction: Critical role of CREB

Cyclic-AMP response element binding protein (CREB) is a transcription factor crucial for late phase long-term potentiation (L-LTP) induction and maintenance. Upon multiple high frequency stimulation (MHFS), large Ca(2+) influx activates adenylyl cyclase. This, in turn, activates PKA, which by itself or through MAPK p42/p44 can activate (phosphorylate) CREB. Upon phosphorylation, P-CREB activates multiple genes essential for L-LTP generation. Calcium calmodulin kinase IV (CaMKIV) is also activated by calcium and can directly activate CREB. We have shown previously that hypothyroidism impairs L-LTP and reduces the basal protein levels of CREB, MAPK p42/p44, and CaMKIV in area CA1 of the hippocampus. In the present study, levels of these signaling molecules were determined in area CA1 during the induction and maintenance phases of L-LTP. Standard MHFS was used to evoke L-LTP in the CA1 area of hypothyroid, levothyroxin treated hypothyroid and sham control anesthetized adult rats. Chronic levothyroxin treatment reversed hypothyroidism-induced L-LTP impairment. Five minutes after MHFS, western blotting showed an increase in the levels of P-CREB, and P-MAPK p42/p44 in sham-operated control, and levothyroxin treated hypothyroid animals, but not in hypothyroid animals. The protein levels of total CREB, total MAPK p42/p44, BDNF and CaMKIV were not altered in all groups five minutes after MHFS. Four hours after MHFS, the levels of P-CREB, and P-MAPK p42/p44 remained unchanged in hypothyroid animals, while they were elevated in sham-operated control, and levothyroxin treated hypothyroid animals. We conclude that respective normalized phosphorylation of essential kinases such as P-CREB and P-MAPK p42/p44 is correlated with restoration of normal L-LTP induction and maintenance in the CA1 area of levothyroxin-treated hypothyroid animals.

Regular Exercise Prevents Sleep Deprivation Associated Impairment of Long-Term Memory and Synaptic Plasticity in The CA1 Area of the Hippocampus

The present study aimed to investigate the effects of treadmill exercise on sleep deprivation (S-D)-induced impairment of hippocampal dependent long-term memory, late phase long-term potentiation (L-LTP) and its signaling cascade in the cornu ammonis 1 (CA1) area. Animals were conditioned to run on treadmills for 4 weeks then deprived of sleep for 24 h using the columns-in-water method. We tested the effect of exercise and/or S-D on behavioral performance using a post-learning paradigm in the radial arm water maze (RAWM) and in vivo extracellular recording in the CA1 area. The levels of L-LTP-related molecules in the CA1 area were then assessed both before and after L-LTP induction. After 24 h of S-D, spatial long-term memory impairment in the RAWM and L-LTP suppression was prevented by 4 weeks of regular exercise. Regular exercise also restored the S-D-associated decreases in the basal levels of key signaling molecules such as: calcium/calmodulin kinase IV (CaMKIV), mitogen-activated protein kinase (MAPK/ERK), phosphorylated cAMP response element-binding protein (P-CREB) and brain derived neurotrophic factor (BDNF), in the CA1 area. After L-LTP induction, regular exercise also prevented the S-D-induced down regulation of BDNF and P-CREB protein levels. The results suggest that our exercise protocol may prevent 24-h S-D-induced impairments in long-term memory and LTP by preventing deleterious changes in the basal and post-stimulation levels of P-CREB and BDNF associated with S-D.

Long-term potentiation of synaptic transmission in the adult mouse insular cortex: multielectrode array recordings

The insular cortex (IC) is widely believed to be an important forebrain structure involved in cognitive and sensory processes such as memory and pain. However, little work has been performed at the cellular level to investigate the synaptic basis of IC-related brain functions. To bridge the gap, the present study was designed to characterize the basic synaptic mechanisms for insular long-term potentiation (LTP). Using a 64-channel recording system, we found that an enduring form of late-phase LTP (L-LTP) could be reliably recorded for at least 3 h in different layers of IC slices after theta burst stimulation. The induction of insular LTP is protein synthesis dependent and requires activation of both GluN2A and GluN2B subunits of the NMDA receptor, L-type voltage-gated calcium channels, and metabotropic glutamate receptor 1. The paired-pulse facilitation ratio was unaffected by insular L-LTP induction, and expression of insular L-LTP required the recruitment of postsynaptic calcium-permeable AMPA receptors. Our results provide the first in vitro report of long-term multichannel recordings of L-LTP in the IC in adult mice and suggest its potential important roles in insula-related memory and chronic pain.

Effects of subchronic aluminum exposure on spatial memory, ultrastructure and L-LTP of hippocampus in rats

Epidemiological investigations have indicated that aluminum (Al), as an important environmental neurotoxicant, could cause damage to the cognitive function which was closely related with neurodegenerative diseases. Long-term potentiation (LTP) is one form of synaptic plasticity in association with cognitive function. Previous studies have demonstrated that Al impaired early phase long-term potentiation (E-LTP) in vivo and in vitro. However, Al-induced damage to late phase long-term potentiation (L-LTP) has poorly been studied. The present study was designed to observe the effects of subchronic Al exposure on the spatial memory, hippocampus ultrastructure and L-LTP in rats. Pregnant Wistar rats were assigned to four groups. Neonatal rats were exposed to Al by parental lactation from parturition to weaning for 3 weeks and then fed with the distilled water containing 0, 0.2%, 0.4% and 0.6% aluminum chloride (AlCl3) respectively from weaning to postnatal 3 months. The levels of Al in blood and hippocampus were quantitated by atomic absorption spectrophotometer. Morris water maze test was performed to study spatial memory. The induction and maintenance of L-LTP in area of Schaffer collateral- CA1 synapse was recorded by extracellular microelectrode recording technology in hippocampus of experimental rats. Hippocampus was collected for transmission electron microscopy observation. The results showed that the Al concentrations in blood and hippocampus of Al-exposed rats were higher than those of the control rats. Al could impair spatial memory ability of rats. Neuronal and synaptic ultrastructure from Al-exposed rats presented pathological changes; the incidence of L-LTP has a decrease trend while population spike (PS) amplitude was much smaller significantly stimulated by high-frequency stimulation (HFS) in Al-exposed rats. Our findings showed that Al exposure caused spatial memory damage, under which the neuronal and synaptic ultrastructure changes maybe were their morphological basis and the impaired L-LTP of hippocampus could be their electrophysiological basis.

Multiple components of eIF4F are required for protein synthesis-dependent hippocampal long-term potentiation

Persistent forms of synaptic plasticity are widely thought to require the synthesis of new proteins. This feature of long-lasting forms of plasticity largely has been demonstrated using inhibitors of general protein synthesis, such as either anisomycin or emetine. However, these drugs, which inhibit elongation, cannot address detailed questions about the regulation of translation initiation, where the majority of translational control occurs. Moreover, general protein synthesis inhibitors cannot distinguish between cap-dependent and cap-independent modes of translation initiation. In the present study, we took advantage of two novel compounds, 4EGI-1 and hippuristanol, each of which targets a different component of the eukaryotic initiation factor (eIF)4F initiation complex, and investigated their effects on long-term potentiation (LTP) at CA3-CA1 synapses in the hippocampus. We found that 4EGI-1 and hippuristanol both attenuated long-lasting late-phase LTP induced by two different stimulation paradigms. We also found that 4EGI-1 and hippuristanol each were capable of blocking the expression of newly synthesized proteins immediately after the induction of late-phase LTP. These new pharmacological tools allow for a more precise dissection of the role played by translational control pathways in synaptic plasticity and demonstrate the importance of multiple aspects of eIF4F in processes underlying hippocampal LTP, laying the foundation for future studies investigating the role of eIF4F in hippocampus-dependent memory processes.

Fear conditioning occludes late-phase long-term potentiation at thalamic input synapses onto the lateral amygdala in rat brain slices

Late-phase long-term potentiation (L-LTP) of excitatory synaptic transmission at thalamic input synapses onto the lateral amygdala (T-LA synapses) has been proposed as a cellular substrate for long-term fear memory. This notion is evidenced primarily by previous reports in which the same pharmacological treatments block both T-LA L-LTP and the consolidation of fear memory. In this study, we report that fear conditioning occludes L-LTP at T-LA synapses in brain slices prepared after fear memory consolidation. L-LTP was restored either when synaptic depotentiation was induced prior to L-LTP induction in brain slices prepared from conditioned rats or when brain slices were prepared from conditioned rats that had been exposed to subsequent fear extinction, which is a behavior paradigm known to induce in vivo synaptic depotentiation at T-LA synapses. These results suggest that fear conditioning recruits L-LTP-like mechanisms that are reversible and saturable at T-LA synapses.

TrkB as a Potential Synaptic and Behavioral Tag

Late-phase long-term potentiation (L-LTP), a cellular model for long-term memory (LTM), requires de novo protein synthesis. An attractive hypothesis for synapse specificity of long-term memory is "synaptic tagging": synaptic activity generates a tag, which "captures" the PRPs (plasticity-related proteins) derived outside of synapses. Here we provide evidence that TrkB, the receptor of BDNF (brain-derived neurotrophic factor), may serve as a "synaptic tag." TrkB is transiently activated by weak theta-burst stimulation (TBS) that induces only early-phase LTP (E-LTP). This TrkB activation is independent of protein synthesis, and confined to stimulated synapses. Induction of L-LTP by strong stimulation in one synaptic pathway converts weak TBS-induced E-LTP to L-LTP in a second, independent pathway. Transient inhibition of TrkB around the time of weak TBS to the second pathway diminished L-LTP in that pathway without affecting the first one. Behaviorally, weak training, which induces short-term memory (STM) but not LTM, can be consolidated into LTM by exposing animals to novel but not familiar environment 1 h before training. Inhibition of TrkB during STM training blocked such consolidation. These results suggest TrkB as a potential tag for synapse-specific expression of L-LTP and LTM.

Postnatal Deamidation of 4E-BP2 in Brain Enhances Its Association with Raptor and Alters Kinetics of Excitatory Synaptic Transmission

The eIF4E-binding proteins (4E-BPs) repress translation initiation by preventing eIF4F complex formation. Of the three mammalian 4E-BPs, only 4E-BP2 is enriched in the mammalian brain and plays an important role in synaptic plasticity and learning and memory formation. Here we describe asparagine deamidation as a brain-specific posttranslational modification of 4E-BP2. Deamidation is the spontaneous conversion of asparagines to aspartates. Two deamidation sites were mapped to an asparagine-rich sequence unique to 4E-BP2. Deamidated 4E-BP2 exhibits increased binding to the mammalian target of rapamycin (mTOR)-binding protein raptor, which effects its reduced association with eIF4E. 4E-BP2 deamidation occurs during postnatal development, concomitant with the attenuation of the activity of the PI3K-Akt-mTOR signaling pathway. Expression of deamidated 4E-BP2 in 4E-BP2(-/-) neurons yielded mEPSCs exhibiting increased charge transfer with slower rise and decay kinetics relative to the wild-type form. 4E-BP2 deamidation may represent a compensatory mechanism for the developmental reduction of PI3K-Akt-mTOR signaling.

Temporal Sensitivity of Protein Kinase A Activation in Late-Phase Long Term Potentiation

Protein kinases play critical roles in learning and memory and in long term potentiation (LTP), a form of synaptic plasticity. The induction of late-phase LTP (L-LTP) in the CA1 region of the hippocampus requires several kinases, including CaMKII and PKA, which are activated by calcium-dependent signaling processes and other intracellular signaling pathways. The requirement for PKA is limited to L-LTP induced using spaced stimuli, but not massed stimuli. To investigate this temporal sensitivity of PKA, a computational biochemical model of L-LTP induction in CA1 pyramidal neurons was developed. The model describes the interactions of calcium and cAMP signaling pathways and is based on published biochemical measurements of two key synaptic signaling molecules, PKA and CaMKII. The model is stimulated using four 100 Hz tetani separated by 3 sec (massed) or 300 sec (spaced), identical to experimental L-LTP induction protocols. Simulations show that spaced stimulation activates more PKA than massed stimulation, and makes a key experimental prediction, that L-LTP is PKA-dependent for intervals larger than 60 sec. Experimental measurements of L-LTP demonstrate that intervals of 80 sec, but not 40 sec, produce PKA-dependent L-LTP, thereby confirming the model prediction. Examination of CaMKII reveals that its temporal sensitivity is opposite that of PKA, suggesting that PKA is required after spaced stimulation to compensate for a decrease in CaMKII. In addition to explaining the temporal sensitivity of PKA, these simulations suggest that the use of several kinases for memory storage allows each to respond optimally to different temporal patterns.

Protective effect of S14G-humanin against beta-amyloid induced LTP inhibition in mouse hippocampal slices

Synaptic dysfunction induced by amyloid-beta protein (Aβ) has been shown to play a critical role in cognitive deficits of Alzheimer's disease (AD). Currently, however there is no clinical causative therapy for the disease. S14G-humanin (HNG) is best known for its strong neuroprotective ability against AD-related insults in vitro, and several in vivo studies have shown its effectiveness in ameliorating the cognitive impairment, but the precise mechanism of HNG on neuroprotection still remains to be elucidated. The present study examined the effects of HNG on Aβ-induced inhibition of hippocampal long-term potentiation (LTP) in mouse hippocampal slices. The results disclosed that soluble Aβ 25–35 significantly inhibited the induction of early-phase LTP (E-LTP) and late-phase LTP (L-LTP) in the hippocampal CA1 region without affecting the basal synaptic transmission, while HNG significantly ameliorated such inhibition of E-LTP and L-LTP in a dose-dependent manner. In addition, the reduction of phosphorylated CREB trigged by Aβ 25–35 was restored by HNG during L-LTP induction, possibly attributing to the improvement of the L-LTP inhibition. Collectively, our findings add to the evidence that soluble Aβ-induced LTP inhibition may represent an early pathological event of AD, and demonstrate for the first time that HNG may improve LTP inhibition by subneurotoxic concentration of soluble Aβ, suggesting that HNG may have therapeutic potential for Aβ-induced synaptic dysfunction closely associated with cognitive deficits in the early stage of AD.