Late-phase Long-term Potentiation Expression Research Articles

Elevated progranulin contributes to synaptic and learning deficit due to loss of fragile X mental retardation protein.

Fragile X syndrome is an inheritable form of intellectual disability caused by loss of fragile X mental retardation protein (FMRP, encoded by the FMR1 gene). Absence of FMRP caused overexpression of progranulin (PGRN, encoded by GRN), a putative tumour necrosis factor receptor ligand. In the present study, we found that progranulin mRNA and protein were upregulated in the medial prefrontal cortex of Fmr1 knock-out mice. In Fmr1 knock-out mice, elevated progranulin caused insufficient dendritic spine pruning and late-phase long-term potentiation in the medial prefrontal cortex of Fmr1 knock-out mice. Partial progranulin knock-down restored spine morphology and reversed behavioural deficits, including impaired fear memory, hyperactivity, and motor inflexibility in Fmr1 knock-out mice. Progranulin increased levels of phosphorylated glutamate ionotropic receptor GluA1 and nuclear factor kappa B in cultured wild-type neurons. Tumour necrosis factor receptor 2 antibody perfusion blocked the effects of progranulin on GluA1 phosphorylation; this result indicates that tumour necrosis factor receptor 2 is required for progranulin-mediated GluA1 phosphorylation and late-phase long-term potentiation expression. However, high basal level of progranulin in Fmr1 knock-out mice prevented further facilitation of synaptic plasticity by exogenous progranulin. Partial downregulation of progranulin or tumour necrosis factor receptor 2/nuclear factor kappa B signalling restored synaptic plasticity and memory deficits in Fmr1 knock-out mice. These findings suggest that elevated PGRN is linked to cognitive deficits of fragile X syndrome, and the progranulin/tumour necrosis factor receptor 2 signalling pathway may be a putative therapeutic target for improving cognitive deficits in fragile X syndrome.

Prevention by Regular Exercise of Acute Sleep Deprivation-Induced Impairment of Late Phase LTP and Related Signaling Molecules in the Dentate Gyrus.

The dentate gyrus (DG) and CA1 regions of the hippocampus are intimately related physically and functionally, yet they react differently to insults. The purpose of this study was to determine the protective effects of regular treadmill exercise on late phase long-term potentiation (L-LTP) and its signaling cascade in the DG region of the hippocampus of rapid eye movement (REM) sleep-deprived rats. Adult Wistar rats ran on treadmills for 4weeks then wereacutely sleep deprived for 24h using the modified multiple platform method. After sleep deprivation, the rats were anesthetized and L-LTP was induced in the DG region. Extracellular field potentials from the DG were recorded in vivo, and levels of L-LTP-related signaling proteins were assessed both before and after L-LTP expression using immunoblot analysis. Sleep deprivation reduced the basal levels of phosphorylated cAMP response element-binding protein (P-CREB) as well as other upstream modulators including calcium/calmodulin kinase IV (CaMKIV) and brain-derived neurotrophic factor (BDNF) in the DG of the hippocampus. Regular exercise prevented impairment of the basal levels of P-CREB and total CREB as well as those of CaMKIV in sleep-deprived animals. Furthermore, regular exercise prevented sleep deprivation-induced inhibition of L-LTP and post-L-LTP downregulation of P-CREB and BDNF levels in the DG. The current findings show that our exercise regimen prevents sleep deprivation-induced deficits in L-LTP as well as the basal and poststimulation levels of key signaling molecules.

Nucleolar integrity is required for the maintenance of long-term synaptic plasticity.

Long-term memory (LTM) formation requires new protein synthesis and new gene expression. Based on our work in Aplysia, we hypothesized that the rRNA genes, stimulation-dependent targets of the enzyme Poly(ADP-ribose) polymerase-1 (PARP-1), are primary effectors of the activity-dependent changes in synaptic function that maintain synaptic plasticity and memory. Using electrophysiology, immunohistochemistry, pharmacology and molecular biology techniques, we show here, for the first time, that the maintenance of forskolin-induced late-phase long-term potentiation (L-LTP) in mouse hippocampal slices requires nucleolar integrity and the expression of new rRNAs. The activity-dependent upregulation of rRNA, as well as L-LTP expression, are poly(ADP-ribosyl)ation (PAR) dependent and accompanied by an increase in nuclear PARP-1 and Poly(ADP) ribose molecules (pADPr) after forskolin stimulation. The upregulation of PARP-1 and pADPr is regulated by Protein kinase A (PKA) and extracellular signal-regulated kinase (ERK)—two kinases strongly associated with long-term plasticity and learning and memory. Selective inhibition of RNA Polymerase I (Pol I), responsible for the synthesis of precursor rRNA, results in the segmentation of nucleoli, the exclusion of PARP-1 from functional nucleolar compartments and disrupted L-LTP maintenance. Taken as a whole, these results suggest that new rRNAs (28S, 18S, and 5.8S ribosomal components)—hence, new ribosomes and nucleoli integrity—are required for the maintenance of long-term synaptic plasticity. This provides a mechanistic link between stimulation-dependent gene expression and the new protein synthesis known to be required for memory consolidation.

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.

Chronic psychosocial stress accelerates impairment of long‐term memory and late‐phase long‐term potentiation in an at‐risk model of Alzheimer's disease

Although it is generally agreed that Aβ contributes to the pathogenesis of AD, its precise role in AD and the reason for the varying intensity and time of onset of the disease have not been elucidated. In addition to genetic factors, environmental issues such as stress may also play a critical role in the etiology of AD. This study examined the effect of chronic psychosocial stress in an at-risk (treatment with a subpathogenic dose of Aβ; "subAβ") rat model of AD on long-term memory by three techniques: memory tests in the radial arm water maze, electrophysiological recordings of synaptic plasticity in anesthetized rats, and immunoblot analysis of learning- and long-term memory-related signaling molecules. Chronic psychosocial stress was induced using a rat intruder model. The subAβ rat model of AD was induced by continuous infusion of 160 pmol/day Aβ(1-42) via a 14-day i.c.v. osmotic pump. All tests showed that subAβ rats were not different from control rats. Result from behavioral tests and electrophysiological recordings showed that infusion of subAβ in chronically stressed rats (stress/subAβ group) caused significant impairment of cognitive functions and late-phase long-term potentiation (L-LTP). Molecular analysis of various signaling molecules after expression of L-LTP, revealed an increase in the levels of p-CREB in control, stress, and subAβ rats, but not in the stress/subAβ rats. These findings suggest that the chronic stress-induced molecular alteration may accelerate the impairment of cognition and synaptic plasticity in individuals "at-risk" for AD.

Nicotine prevents disruption of the late phase LTP‐related molecular cascade in adult‐onset hypothyroidism

We have shown previously that chronic nicotine treatment reverses adult-onset hypothyroidism-induced impairment of late-phase long-term potentiation (L-LTP) in area CA1 of the hippocampus. In the present study, basal and stimulated levels of signaling molecules essential for the expression of L-LTP were determined in area CA1. Immunoblots analysis showed that chronic nicotine treatment of hypothyroid rats prevented the reduction in the basal protein levels of adenylyl cyclase I (ACI), mitogen-activated protein kinases [MAPKp44/42 (ERK1/2)], calcium-calmodulin-dependent protein kinase IV (CaMKIV), and cyclic-AMP response element binding protein [CREB; phosphorylated (P-) and total]. A significant increase in the levels of P-CREB, P-MAPKp44, P-MAPKp42 and brain derived neurotrophic factor (BDNF) was seen 4 h after multiple train high frequency stimulation (MHFS) in nicotine-treated hypothyroid and control animals, but not in hypothyroid animals. The levels of total CREB, total MAPKp44, total MAPKp42, and CaMKIV were elevated in all groups 4 h after MHFS. These findings suggest that prevention of the reduced basal level of CaMKIV, MAPKp44/42, and CREB by nicotine along with the regained ability of MHFS to induce MAPKp44/42 and CREB phosphorylation in nicotine treated hypothyroid animals may be responsible for the reversal of L-LTP impairment by chronic nicotine treatment in this disease model.

A critical role of CREB in the impairment of late-phase LTP by adult onset hypothyroidism

We have shown previously that adult onset hypothyroidism impairs late-phase long-term potentiation (L-LTP) and reduces the protein levels of mitogen-activated protein kinases (MAPKp44/42 (ERK1/2)) in area CA1 of the hippocampus. In the present study, basal and stimulated levels of signaling molecules essential for the expression of L-LTP were determined in area CA1 of the hippocampus. L-LTP was evoked by multiple train high-frequency stimulation (MHFS) in area CA1 of the hippocampus of thyroidectomized and sham control anesthetized adult rats. Immunoblot analysis showed reduction in the basal protein levels of adenylyl cyclase I (ACI), calcium calmodulin-dependent protein kinase IV (CaMKIV), and cyclic-AMP response element-binding protein (CREB; phosphorylated (P-) and total) in hypothyroid rats. A significant increase in the levels of P-CREB, P-MAPKp44 and P-MAPKp42 was seen 4 h after MHFS in sham-operated control animals, but not in hypothyroid animals. The levels of total CREB, total MAPKp44, total MAPKp42 and CaMKIV were elevated in both groups 4 h after MHFS. Our results suggest that in adult hypothyroid rats, the reduced basal level of CaMKIV, MAPKp44/42 and CREB along with the failure of MHFS to induce MAPKp44/42 and CREB phosphorylation may be responsible for L-LTP impairment in the CA1 area during hypothyroidism.

Cleavage of proBDNF by tPA/plasmin is essential for long-term hippocampal plasticity.

Long-term memory is thought to be mediated by protein synthesis-dependent, late-phase long-term potentiation (L-LTP). Two secretory proteins, tissue plasminogen activator (tPA) and brain-derived neurotrophic factor (BDNF), have been implicated in this process, but their relationship is unclear. Here we report that tPA, by activating the extracellular protease plasmin, converts the precursor proBDNF to the mature BDNF (mBDNF), and that such conversion is critical for L-LTP expression in mouse hippocampus. Moreover, application of mBDNF is sufficient to rescue L-LTP when protein synthesis is inhibited, which suggests that mBDNF is a key protein synthesis product for L-LTP expression.

Dopamine D1/D5 receptor activation fails to initiate an activity-independent late-phase LTP in rat hippocampus

The role of dopamine in the hippocampus remains poorly defined. Numerous studies have suggested that it acts as a neuromodulator of late-phase long-term potentiation (L-LTP) in CA1, while other reports controversially indicate that D1/D5 receptor (D1/D5R) activation may directly initiate activity-independent LTP. We have further investigated this putative role of dopamine in area CA1 in rat hippocampal slices using field potential recording techniques. Application of the dopamine D1/D5 receptor agonists SKF 38393 and 6-bromo-APB at 100 μM for 20 min did not induce an activity-independent L-LTP. Varying the incubation conditions still did not permit either SKF 38393 or an alternative D1/D5R agonist, 6-chloro-PB, to induce L-LTP. To further determine if intracellular mechanisms, which may act to limit the expression of LTP, were preventing D1/D5R-induced L-LTP expression, we inhibited protein phosphatase 1 activity by reducing cyclin-dependent kinase 5 (cdk5) inhibition of inhibitor 1. Inhibition of cdk5 by roscovitine (10 μM, 40 min) did not facilitate the ability of SKF 38393 to induce L-LTP in CA1. Biochemical experiments confirmed that the concentration of agonist used significantly elevated intracellular cAMP levels, suggesting that effective D1/D5R activation was achieved. Furthermore, coactivation with NMDA receptors (NMDAR) resulted in a synergistic increase in cAMP. These findings demonstrate that D1/D5R activation in CA1 initiates intracellular second messenger accumulation, but that this is insufficient to induce an activity-independent L-LTP.

MAPK, CREB and zif268 are all required for the consolidation of recognition memory.

There has been nearly a century of interest in the idea that encoding and storage of information in the brain requires changes in the efficacy of synaptic connections between neurons that are activated during learning. Recent research into the molecular mechanisms of long-term potentiation (LTP) has brought about new knowledge that has provided valuable insights into the neural mechanisms of memory storage. The evidence indicates that rapid activation of the genetic machinery can be a key mechanism underlying the enduring modification of neural networks required for the stability of memories. In recent years, a wealth of experimental data has highlighted the importance of mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signalling in the regulation of gene transcription in neurons. Here, we briefly review experiments that have shown MAPK/ERK, cAMP response element-binding protein (CREB) and the immediate early gene (IEG) zif268 are essential components of a signalling cascade required for the expression of late phase LTP and of certain forms of long-term memory. We also present experiments in which we have assessed the role of these three molecules in recognition memory. We show that pharmacological blockade of MAPK/ERK phosphorylation, functional inactivation of CREB in an inducible transgenic mouse and inactivation of zif268 in a mutant mouse result in a similar deficit in long-term recognition memory. In the continuing debate about the role of LTP mechanisms in memory, these findings provide an important complement to the suggestion that synaptic changes brought about by LTP and memory consolidation and storage share, at least in part, common underlying molecular mechanisms.