Dual MAP kinase pathways mediate opposing forms of long-term plasticity at CA3-CA1 synapses

Effects of Cannabis on Glutamatergic Neurotransmission: The Interplay between Cannabinoids and Glutamate

There has been a significant increase in the consumption of cannabis for both recreational and medicinal purposes in recent years, and its use can have long-term consequences on cognitive functions, including memory. Here, we review the immediate and long-term effects of cannabis and its derivatives on glutamatergic neurotransmission, with a focus on both the presynaptic and postsynaptic alterations. Several factors can influence cannabinoid-mediated changes in glutamatergic neurotransmission, including dosage, sex, age, and frequency of use. Acute exposure to cannabis typically inhibits glutamate release, whereas chronic use tends to increase glutamate release. Conversely, the postsynaptic alterations are more complicated than the presynaptic effects, as cannabis can affect the glutamate receptor expression and the downstream signaling of glutamate. All these effects ultimately influence cognitive functions, particularly memory. This review will cover the current research on glutamate-cannabis interactions, as well as the future directions of research needed to understand cannabis-related health effects and neurological and psychological aspects of cannabis use.

Novel crosstalk mechanisms between GluA3 and Epac2 in synaptic plasticity and memory in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease which accounts for the most cases of dementia worldwide. Impaired memory, including acquisition, consolidation, and retrieval, is one of the hallmarks in AD. At the cellular level, dysregulated synaptic plasticity partly due to reduced long-term potentiation (LTP) and enhanced long-term depression (LTD) underlies the memory deficits in AD. GluA3 containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are one of key receptors involved in rapid neurotransmission and synaptic plasticity. Recent studies revealed a novel form of GluA3 involved in neuronal plasticity that is dependent on cyclic adenosine monophosphate (cAMP), rather than N-methyl-d-aspartate (NMDA). However, this cAMP-dependent GluA3 pathway is specifically and significantly impaired by amyloid beta (Aβ), a pathological marker of AD. cAMP is a key second messenger that plays an important role in modulating memory and synaptic plasticity. We previously reported that exchange protein directly activated by cAMP 2 (Epac2), acting as a main cAMP effector, plays a specific and time-limited role in memory retrieval. From electrophysiological perspective, Epac2 facilities the maintenance of LTP, a cellular event closely associated with memory retrieval. Additionally, Epac2 was found to be involved in the GluA3-mediated plasticity. In this review, we comprehensively summarize current knowledge regarding the specific roles of GluA3 and Epac2 in synaptic plasticity and memory, and their potential association with AD.

Voltage-Gated Na+ Channels in Alzheimer's Disease: Physiological Roles and Therapeutic Potential

Alzheimer's disease (AD) is the most common cause of dementia and is classically characterized by two major histopathological abnormalities: extracellular plaques composed of amyloid beta (Aβ) and intracellular hyperphosphorylated tau. Due to the progressive nature of the disease, it is of the utmost importance to develop disease-modifying therapeutics that tackle AD pathology in its early stages. Attenuation of hippocampal hyperactivity, one of the earliest neuronal abnormalities observed in AD brains, has emerged as a promising strategy to ameliorate cognitive deficits and abate the spread of neurotoxic species. This aberrant hyperactivity has been attributed in part to the dysfunction of voltage-gated Na+ (Nav) channels, which are central mediators of neuronal excitability. Therefore, targeting Nav channels is a promising strategy for developing disease-modifying therapeutics that can correct aberrant neuronal phenotypes in early-stage AD. This review will explore the role of Nav channels in neuronal function, their connections to AD pathology, and their potential as therapeutic targets.

Early chronic suppression of microglial p38α in a model of Alzheimer's disease does not significantly alter amyloid-associated neuropathology

The p38 alpha mitogen-activated protein kinase (p38α) is linked to both innate and adaptive immune responses and is under investigation as a target for drug development in the context of Alzheimer's disease (AD) and other conditions with neuroinflammatory dysfunction. While preclinical data has shown that p38α inhibition can protect against AD-associated neuropathology, the underlying mechanisms are not fully elucidated. Inhibitors of p38α may provide benefit via modulation of microglial-associated neuroinflammatory responses that contribute to AD pathology. The present study tests this hypothesis by knocking out microglial p38α and assessing early-stage pathological changes. Conditional knockout of microglial p38α was accomplished in 5-month-old C57BL/6J wild-type and amyloidogenic AD model (APPswe/PS1dE9) mice using a tamoxifen-inducible Cre/loxP system under control of the Cx3cr1 promoter. Beginning at 7.5 months of age, animals underwent behavioral assessment on the open field, followed by a later radial arm water maze test and collection of cortical and hippocampal tissues at 11 months. Additional endpoint measures included quantification of proinflammatory cytokines, assessment of amyloid burden and plaque deposition, and characterization of microglia-plaque dynamics. Loss of microglial p38α did not alter behavioral outcomes, proinflammatory cytokine levels, or overall amyloid plaque burden. However, this manipulation did significantly increase hippocampal levels of soluble Aβ42 and reduce colocalization of Iba1 and 6E10 in a subset of microglia in close proximity to plaques. The data presented here suggest that rather than reducing inflammation per se, the net effect of microglial p38α inhibition in the context of early AD-type amyloid pathology is a subtle alteration of microglia-plaque interactions. Encouragingly from a therapeutic standpoint, these data suggest no detrimental effect of even substantial decreases in microglial p38α in this context. Additionally, these results support future investigations of microglial p38α signaling at different stages of disease, as well as its relationship to phagocytic processes in this particular cell-type.

Inhibition of p38α MAPK restores neuronal p38γ MAPK and ameliorates synaptic degeneration in a mouse model of DLB/PD

Alterations in the p38 mitogen-activated protein kinases (MAPKs) play an important role in the pathogenesis of dementia with Lewy bodies (DLB) and Parkinson's disease (PD). Activation of the p38α MAPK isoform and mislocalization of the p38γ MAPK isoform are associated with neuroinflammation and synaptic degeneration in DLB and PD. Therefore, we hypothesized that p38α might be associated with neuronal p38γ distribution and synaptic dysfunction in these diseases. To test this hypothesis, we treated in vitro cellular and in vivo mouse models of DLB/PD with SKF-86002, a compound that attenuates inflammation by inhibiting p38α/β, and then investigated the effects of this compound on p38γ and neurodegenerative pathology. We found that inhibition of p38α reduced neuroinflammation and ameliorated synaptic, neurodegenerative, and motor behavioral deficits in transgenic mice overexpressing human α-synuclein. Moreover, treatment with SKF-86002 promoted the redistribution of p38γ to synapses and reduced the accumulation of α-synuclein in mice overexpressing human α-synuclein. Supporting the potential value of targeting p38 in DLB/PD, we found that SKF-86002 promoted the redistribution of p38γ in neurons differentiated from iPS cells derived from patients with familial PD (carrying the A53T α-synuclein mutation) and healthy controls. Treatment with SKF-86002 ameliorated α-synuclein-induced neurodegeneration in these neurons only when microglia were pretreated with this compound. However, direct treatment of neurons with SKF-86002 did not affect α-synuclein-induced neurotoxicity, suggesting that SKF-86002 treatment inhibits α-synuclein-induced neurotoxicity mediated by microglia. These findings provide a mechanistic connection between p38α and p38γ as well as a rationale for targeting this pathway in DLB/PD.

Contributions of extracellular-signal regulated kinase 1/2 activity to the memory trace

The ability to learn from experience and consequently adapt our behavior is one of the most fundamental capacities enabled by complex and plastic nervous systems. Next to cellular and systems-level changes, learning and memory formation crucially depends on molecular signaling mechanisms. In particular, the extracellular-signal regulated kinase 1/2 (ERK), historically studied in the context of tumor growth and proliferation, has been shown to affect synaptic transmission, regulation of neuronal gene expression and protein synthesis leading to structural synaptic changes. However, to what extent the effects of ERK are specifically related to memory formation and stabilization, or merely the result of general neuronal activation, remains unknown. Here, we review the signals leading to ERK activation in the nervous system, the subcellular ERK targets associated with learning-related plasticity, and how neurons with activated ERK signaling may contribute to the formation of the memory trace.

The MK2 cascade mediates transient alteration in mGluR-LTD and spatial learning in a murine model of Alzheimer's disease

A key aim of Alzheimer disease research is to develop efficient therapies to prevent and/or delay the irreversible progression of cognitive impairments. Early deficits in long-term potentiation (LTP) are associated with the accumulation of amyloid beta in rodent models of the disease; however, less is known about how mGluR-mediated long-term depression (mGluR-LTD) is affected. In this study, we have found that mGluR-LTD is enhanced in the APP_swe /PS1dE9 mouse at 7 but returns to wild-type levels at 13 months of age. This transient over-activation of mGluR signalling is coupled with impaired LTP and shifts the dynamic range of synapses towards depression. These alterations in synaptic plasticity are associated with an inability to utilize cues in a spatial learning task. The transient dysregulation of plasticity can be prevented by genetic deletion of the MAP kinase-activated protein kinase 2 (MK2), a substrate of p38 MAPK, demonstrating that manipulating the mGluR-p38 MAPK-MK2 cascade at 7 months can prevent the shift in synapse dynamic range. Our work reveals the MK2 cascade as a potential pharmacological target to correct the over-activation of mGluR signalling.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Behind the scenes: Are latent memories supported by calcium independent plasticity?

N-methyl-D-aspartate receptors (NMDARs) can be considered to be the de facto "plasticity" receptors in the brain due to their central role in the activity-dependent modification of neuronal morphology and synaptic transmission. Since the 1980s, research on NMDARs has focused on the second messenger properties of calcium and the downstream signaling pathways that mediate alterations in neural form and function. Recently, NMDARs were shown to drive activity-dependent synaptic plasticity without calcium influx. How this "nonionotropic" plasticity occurs in vitro is becoming clearer, but research on its involvement in behavior and cognition is in its infancy. There is a partial overlap in the downstream signaling molecules that are involved in ionotropic and nonionotropic NMDAR-dependent plasticity. Given this, and prior studies of the cognitive impacts of ionotropic NMDAR plasticity, a preliminary model explaining how NMDAR nonionotropic plasticity affects learning and memory can be established. We hypothesize that nonionotropic NMDAR plasticity takes part in latent memory encoding in immature rodents through nonassociative depression of synaptic efficacy, and possibly shrinking of dendritic spines. Further, the late postnatal alteration in NMDAR composition in the hippocampus appears to reduce nonionotropic signaling and remove a restriction on memory retrieval. This framework substantially alters the canonical model of NMDAR involvement in spatial cognition and hippocampal maturation and provides novel and exciting inroads for future studies.

The role of pathological tau in synaptic dysfunction in Alzheimer's diseases

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by progressive cognitive decline, accompanied by amyloid-β (Aβ) overload and hyperphosphorylated tau accumulation in the brain. Synaptic dysfunction, an important pathological hallmark in AD, is recognized as the main cause of the cognitive impairments. Accumulating evidence suggests that synaptic dysfunction could be an early pathological event in AD. Pathological tau, which is detached from axonal microtubules and mislocalized into pre- and postsynaptic neuronal compartments, is suggested to induce synaptic dysfunction in several ways, including reducing mobility and release of presynaptic vesicles, decreasing glutamatergic receptors, impairing the maturation of dendritic spines at postsynaptic terminals, disrupting mitochondrial transport and function in synapses, and promoting the phagocytosis of synapses by microglia. Here, we review the current understanding of how pathological tau mediates synaptic dysfunction and contributes to cognitive decline in AD. We propose that elucidating the mechanism by which pathological tau impairs synaptic function is essential for exploring novel therapeutic strategies for AD.

When Good Kinases Go Rogue: GSK3, p38 MAPK and CDKs as Therapeutic Targets for Alzheimer's and Huntington's Disease

Alzheimer's disease (AD) is a mostly sporadic brain disorder characterized by cognitive decline resulting from selective neurodegeneration in the hippocampus and cerebral cortex whereas Huntington's disease (HD) is a monogenic inherited disorder characterized by motor abnormalities and psychiatric disturbances resulting from selective neurodegeneration in the striatum. Although there have been numerous clinical trials for these diseases, they have been unsuccessful. Research conducted over the past three decades by a large number of laboratories has demonstrated that abnormal actions of common kinases play a key role in the pathogenesis of both AD and HD as well as several other neurodegenerative diseases. Prominent among these kinases are glycogen synthase kinase (GSK3), p38 mitogen-activated protein kinase (MAPK) and some of the cyclin-dependent kinases (CDKs). After a brief summary of the molecular and cell biology of AD and HD this review covers what is known about the role of these three groups of kinases in the brain and in the pathogenesis of the two neurodegenerative disorders. The potential of targeting GSK3, p38 MAPK and CDKS as effective therapeutics is also discussed as is a brief discussion on the utilization of recently developed drugs that simultaneously target two or all three of these groups of kinases. Multi-kinase inhibitors either by themselves or in combination with strategies currently being used such as immunotherapy or secretase inhibitors for AD and knockdown for HD could represent a more effective therapeutic approach for these fatal neurodegenerative diseases.

Age-related changes in hippocampal-dependent synaptic plasticity and memory mediated by p75 neurotrophin receptor

The plasticity mechanisms in the nervous system that are important for learning and memory are greatly impacted during aging. Notably, hippocampal-dependent long-term plasticity and its associative plasticity, such as synaptic tagging and capture (STC), show considerable age-related decline. The p75 neurotrophin receptor (p75NTR ) is a negative regulator of structural and functional plasticity in the brain and thus represents a potential candidate to mediate age-related alterations. However, the mechanisms by which p75NTR affects synaptic plasticity of aged neuronal networks and ultimately contribute to deficits in cognitive function have not been well characterized. Here, we report that mutant mice lacking the p75NTR were resistant to age-associated changes in long-term plasticity, associative plasticity, and associative memory. Our study shows that p75NTR is responsible for age-dependent disruption of hippocampal homeostatic plasticity by modulating several signaling pathways, including BDNF, MAPK, Arc, and RhoA-ROCK2-LIMK1-cofilin. p75NTR may thus represent an important therapeutic target for limiting the age-related memory and cognitive function deficits.

Diversity and versatility of p38 kinase signalling in health and disease

The ability of cells to deal with different types of stressful situations in a precise and coordinated manner is key for survival and involves various signalling networks. Over the past 25 years, p38 kinases — in particular, p38α — have been implicated in the cellular response to stress at many levels. These span from environmental and intracellular stresses, such as hyperosmolarity, oxidative stress or DNA damage, to physiological situations that involve important cellular changes such as differentiation. Given that p38α controls a plethora of functions, dysregulation of this pathway has been linked to diseases such as inflammation, immune disorders or cancer, suggesting the possibility that targeting p38α could be of therapeutic interest. In this Review, we discuss the organization of this signalling pathway focusing on the diversity of p38α substrates, their mechanisms and their links to particular cellular functions. We then address how the different cellular responses can be generated depending on the signal received and the cell type, and highlight the roles of this kinase in human physiology and in pathological contexts.

p38α — the best-characterized member of the p38 kinase family — is a key mediator of cellular stress responses. p38α is activated by a plethora of signals and functions through a multitude of substrates to regulate different cellular behaviours. Understanding context-dependent p38α signalling provides important insights into p38α roles in physiology and pathology.

Temporal Quantitative Proteomics of mGluR-induced Protein Translation and Phosphorylation in Neurons

At neuronal synapses, activation of group I metabotropic glutamate receptors (mGluR1/5) triggers a form of long-term depression (mGluR-LTD) that relies on new protein synthesis and the internalization of AMPA-type glutamate receptors. Dysregulation of these processes has been implicated in the development of mental disorders such as autism spectrum disorders and therefore merit a better understanding on a molecular level. Here, to study mGluR-induced signaling pathways, we integrated quantitative phosphoproteomics with the analyses of newly synthesized proteins via bio-orthogonal amino acids (azidohomoalanine) in a pulsed labeling strategy in cultured hippocampal neurons stimulated with DHPG, a specific agonist for group I mGluRs. We identified several kinases with important roles in DHPG-induced mGluR activation, which we confirmed using small molecule kinase inhibitors. Furthermore, changes in the AMPA receptor endocytosis pathway in both protein synthesis and protein phosphorylation were identified, whereby Intersectin-1 was validated as a novel player in this pathway. This study revealed several new insights into the molecular pathways downstream of group I mGluR activation in hippocampal neurons, and provides a rich resource for further analyses.

Involvement of p38 MAPK in Synaptic Function and Dysfunction

Many studies have revealed a central role of p38 MAPK in neuronal plasticity and the regulation of long-term changes in synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD). However, p38 MAPK is classically known as a responsive element to stress stimuli, including neuroinflammation. Specific to the pathophysiology of Alzheimer’s disease (AD), several studies have shown that the p38 MAPK cascade is activated either in response to the Aβ peptide or in the presence of tauopathies. Here, we describe the role of p38 MAPK in the regulation of synaptic plasticity and its implication in an animal model of neurodegeneration. In particular, recent evidence suggests the p38 MAPK α isoform as a potential neurotherapeutic target, and specific inhibitors have been developed and have proven to be effective in ameliorating synaptic and memory deficits in AD mouse models.

Adenosine A1 Receptor-Mediated Synaptic Depression in the Developing Hippocampal Area CA2

Immunolabeling for adenosine A1 receptors (A1Rs) is high in hippocampal area CA2 in adult rats, and the potentiating effects of caffeine or other A1R-selective antagonists on synaptic responses are particularly robust at Schaffer collateral synapses in CA2. Interestingly, the pronounced staining for A1Rs in CA2 is not apparent until rats are 4 weeks old, suggesting that developmental changes other than receptor distribution underlie the sensitivity of CA2 synapses to A1R antagonists in young animals. To evaluate the role of A1R-mediated postsynaptic signals at these synapses, we tested whether A1R agonists regulate synaptic transmission at Schaffer collateral inputs to CA2 and CA1. We found that the selective A1R agonist CCPA caused a lasting depression of synaptic responses in both CA2 and CA1 neurons in slices obtained from juvenile rats (P14), but that the effect was observed only in CA2 in slices prepared from adult animals (~P70). Interestingly, blocking phosphodiesterase activity with rolipram inhibited the CCPA-induced depression in CA1, but not in CA2, indicative of robust phosphodiesterase activity in CA1 neurons. Likewise, synaptic responses in CA2 and CA1 differed in their sensitivity to the adenylyl cyclase activator, forskolin, in that it increased synaptic transmission in CA2, but had little effect in CA1. These findings suggest that the A1R-mediated synaptic depression tracks the postnatal development of immunolabeling for A1Rs and that the enhanced sensitivity to antagonists in CA2 at young ages is likely due to robust adenylyl cyclase activity and weak phosphodiesterase activity rather than to enrichment of A1Rs.

Changes in Nup62 content affect contact-induced differentiation of cultured myoblasts

Differentiation of cultured skeletal myoblasts is induced by extrinsic signals that include reduction in ambient mitogen concentration and increased cell density. Using an established murine myoblast cell line (C2C12), we have found that experimental reduction of the nucleoporin p62 (Nup62) content of myoblasts enhances differentiation in high-mitogen medium, while forced expression of Nup62 inhibits density-induced differentiation. In contrast, differentiation of myoblasts induced by low-mitogen medium was unaffected by ectopic Nup62 expression. Further analyses suggested that Nup62 content affects density-induced myoblast differentiation through a mechanism involving activation of p38 MAP kinase. Nuclear pore complex (NPC) composition, in particular changes in NUP62 content, may be altered during viral infection, differentiation, and in neoplastic growth. The results support a functional role for changes in Nup62 composition in NPCs and density-induced myogenic differentiation, and suggest a link between loss of Nup62 content and induction of an intracellular stress signaling pathways.

BRAG2a Mediates mGluR-Dependent AMPA Receptor Internalization at Excitatory Postsynapses through the Interaction with PSD-95 and Endophilin 3

Brefeldin A-resistant ArfGEF 2 (BRAG2) [or Iqsec1 (IQ motif and Sec7 domain-containing protein 1)] is a guanine nucleotide exchange factor for ADP ribosylation factor 6 (Arf6), a small GTPase implicated in the membrane trafficking between the plasma membrane and endosomes. BRAG2 regulates Arf6-dependent endocytosis of AMPA receptors (AMPARs) through the direct interaction during the hippocampal long-term depression. However, the molecular mechanism by which the BRAG2-Arf6 pathway links AMPARs to the endocytic machinery remains elusive. Herein, using mouse brains of both sexes, we demonstrated that BRAG2a, an alternative isoform with a long C-terminal insert containing a proline-rich domain and type I PDZ-binding motif, was selectively localized to the excitatory postsynaptic density (PSD). Using yeast two-hybrid screening, we identified PSD-95 and endophilin 1/3 as BRAG2a-binding partners in the brain. The interaction with PSD-95 was required for synaptic targeting of BRAG2a. In cultured hippocampal neurons, stimulation of group I metabotropic glutamate receptors (mGluRs) increased the interaction of BRAG2a with endophilin 3 and concomitant Arf6 activation in a time-dependent manner. Knockdown of BRAG2 in cultured hippocampal neurons blocked the mGluR-dependent decrease in surface AMPAR levels, which was rescued by introducing wild-type BRAG2a, but not wild-type BRAG2b or BRAG2a mutants lacking the ability to activate Arf6 or to interact with endophilin 3 or PSD-95. Further postembedding immunoelectron microscopic analysis revealed the preorganized lateral distribution of BRAG2a, Arf6, and endophilin 3 for efficient endocytosis at the postsynaptic membrane. Together, the present findings unveiled a novel molecular mechanism by which BRAG2a links AMPARs to the clathrin-dependent endocytic pathway through its interaction with PSD-95 and endophilin 3.SIGNIFICANCE STATEMENT BRAG2/Iqsec1 is a GDP/GTP exchange factor for ADP ribosylation factor 6 (Arf6), a small GTPase implicated in the membrane trafficking between the plasma membrane and endosomes, and regulates Arf6-dependent endocytosis of AMPARs through direct interaction during hippocampal long-term depression, one of the mechanisms of synaptic plasticity related to learning and memory. However, the molecular mechanism by which the BRAG2-Arf6 pathway links AMPARs to the endocytic machinery remains elusive. Here, we identified isoform-specific mechanisms of BRAG2-mediated AMPAR internalization. We demonstrated that the interaction of BRAG2a isoform with PSD-95 and endophilin 3 was required for the mGluR-dependent decrease in surface AMPARs in hippocampal neurons. These results unveiled a novel molecular mechanism by which BRAG2 links AMPARs to the clathrin-mediated endocytic machinery at postsynaptic sites.

Ciliary neurotrophic factor signaling in the rat orbitofrontal cortex ameliorates stress-induced deficits in reversal learning

Deficits in cognitive flexibility, i.e. the ability to modify behavior in response to changes in the environment, are present in several psychiatric disorders and are often refractory to treatment. However, improving treatment response has been hindered by a lack of understanding of the neurobiology of cognitive flexibility. Using a rat model of chronic stress (chronic intermittent cold stress, CIC) that produces selective deficits in reversal learning, a form of cognitive flexibility dependent on orbitofrontal cortex (OFC) function, we have previously shown that JAK2 signaling is required for optimal reversal learning. In this study we explore the molecular basis of those effects. We show that, within the OFC, CIC stress reduces the levels of phosphorylated JAK2 and of ciliary neurotrophic factor (CNTF), a promoter of neuronal survival and an activator of JAK2 signaling, and that neutralizing endogenous CNTF with an intra-OFC microinjection of a specific antibody is sufficient to produce reversal-learning deficits similar to stress. Intra-OFC delivery of recombinant CNTF to CIC-stressed rats, at a dose that induces JAK2 and Akt but not STAT3 or ERK, ameliorates reversal-learning deficits, and Akt blockade prevents the positive effects of CNTF. Further analysis revealed that CNTF may exert its beneficial effects by inhibiting GSK3β, a substrate of Akt and a regulator of protein degradation. We also revealed a novel mechanism of CNTF action through modulation of p38/Mnk1/eIF4E signaling. This cascade controls translation of select mRNAs, including those encoding several plasticity-related proteins. Thus, we suggest that CNTF-driven JAK2 signaling corrects stress-induced reversal learning deficits by modulating the steady-state levels of plasticity-related proteins in the OFC.

The MK2 cascade regulates mGluR-dependent synaptic plasticity and reversal learning

The ability to either erase or update the memories of a previously learned spatial task is an essential process that is required to modify behaviour in a changing environment. Current evidence suggests that the neural representation of such cognitive flexibility involves the balancing of synaptic potentiation (acquisition of memories) with synaptic depression (modulation and updating previously acquired memories). Here we demonstrate that the p38 MAPK/MAPK-activated protein kinase 2 (MK2) cascade is required to maintain the precise tuning of long-term potentiation and long-term depression at CA1 synapses of the hippocampus which is correlated with efficient reversal learning. Using the MK2 knockout (KO) mouse, we show that mGluR-LTD, but not NMDAR-LTD, is markedly impaired in mice aged between 4 and 5 weeks (juvenile) to 7 months (mature adult). Although the amplitude of LTP was the same as in wildtype mice, priming of LTP by the activation of group I metabotropic receptors was impaired in MK2 KO mice. Consistent with unaltered LTP amplitude and compromised mGluR-LTD, MK2 KO mice had intact spatial learning when performing the Barnes maze task, but showed specific deficits in selecting the most efficient combination of search strategies to perform the task reversal. Findings from this study suggest that the mGluR-p38-MK2 cascade is important for cognitive flexibility by regulating LTD amplitude and the priming of LTP.

Astrocytic p38α MAPK drives NMDA receptor-dependent long-term depression and modulates long-term memory

NMDA receptor-dependent long-term depression (LTD) in the hippocampus is a well-known form of synaptic plasticity that has been linked to different cognitive functions. The core mechanism for this form of plasticity is thought to be entirely neuronal. However, we now demonstrate that astrocytic activity drives LTD at CA3-CA1 synapses. We have found that LTD induction enhances astrocyte-to-neuron communication mediated by glutamate, and that Ca²⁺ signaling and SNARE-dependent vesicular release from the astrocyte are required for LTD expression. In addition, using optogenetic techniques, we show that low-frequency astrocytic activation, in the absence of presynaptic activity, is sufficient to induce postsynaptic AMPA receptor removal and LTD expression. Using cell-type-specific gene deletion, we show that astrocytic p38α MAPK is required for the increased astrocytic glutamate release and astrocyte-to-neuron communication during low-frequency stimulation. Accordingly, removal of astrocytic (but not neuronal) p38α abolishes LTD expression. Finally, this mechanism modulates long-term memory in vivo.

How astrocytes influence neuronal plasticity remains unclear, as they are typically considered as modulators of core mechanisms driven by neuronal components. Here, authors show that Long-term depression (LTD) induction in the hippocampus triggers calcium signaling in the astrocyte and enhances SNARE-dependent astrocytic glutamate release, which is then responsible for the activation of postsynaptic NMDA receptors and synaptic depression.

Neurochemical and molecular mechanisms underlying the retrieval-extinction effect

Extinction within the reconsolidation window, or 'retrieval-extinction', has received much research interest as a possible technique for targeting the reconsolidation of maladaptive memories with a behavioural intervention. However, it remains to be determined whether the retrieval-extinction effect-a long-term reduction in fear behaviour, which appears resistant to spontaneous recovery, renewal and reinstatement-depends specifically on destabilisation of the original memory (the 'reconsolidation-update' account) or represents facilitation of an extinction memory (the 'extinction-facilitation' account). We propose that comparing the neurotransmitter systems, receptors and intracellular signalling pathways recruited by reconsolidation, extinction and retrieval-extinction will provide a way of distinguishing between these accounts.

Depotentiation of Long-Term Potentiation Is Associated with Epitope-Specific Tau Hyper-/Hypophosphorylation in the Hippocampus of Adult Rats

It is well-known that some kinases which are involved in the induction of synaptic plasticity probably modulate tau phosphorylation. However, how depression of potentiated synaptic strength contributes to tau phosphorylation is unclear because of the lack of experiments in which depotentiation of LTP was induced. Field excitatory postsynaptic potential (fEPSP) and population spike (PS) were recorded from the dentate gyrus in response to the perforant pathway stimulation. To induce LTP, high-frequency stimulation (HFS) was used, while, for depotentiation of LTP, low-frequency stimulation (LFS) consisting of 900 pulses at 1 Hz was applied 5 min after tetanization. In some experiments, a neutral protocol at 0.033 Hz was applied throughout the experiment without any induction of synaptic plasticity. One-hertz depotentiation protocol was able to decrease fEPSP slope which was previously increased by HFS, whereas no significant change in fEPSP slope and PS amplitude was observed in neutral protocol experiments. Relative to saline infusion, LTP was lower in magnitude and was more reversed by subsequent LFS in the presence of ERK1/2 inhibitor. Western blot experiments indicated that tau protein was hyperphosphorylated at ser416 epitope but rather hypophosphorylated at thr231 epitope in the whole hippocampus upon depotentiation of LTP. These changes concomitantly occurred with a notable increase in the levels of total tau and in the levels of phosphorylated form of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). ERK1/2 inhibition resulted in a decrease in phosphorylation of tau at p416Tau when ERK1/2 was inhibited. These findings indicate that some forms of long-term plastic changes might be related with epitope-specific tau phosphorylation and ERK1/2 activation in the hippocampus. Therefore, we emphasize that tau may be crucial for physiological learning as well as Alzheimer's disease pathology.

Socio-demographic, Clinical, and Genetic Determinants of Quality of Life in Lung Cancer Patients

Patient reported health-related quality of life (QOL) is a major component of the overall well-being of cancer patients, with links to prognosis. In 6,420 lung cancer patients, we identified patient characteristics and genetic determinants of QOL. Patient responses from the SF-12 questionnaire was used to calculate normalized Physical Component Summary (PCS) and Mental Component Summary (MCS) scores. Further, we analyzed 218 single nucleotide polymorphisms (SNPs) in the p38 MAPK signaling pathway, a key mediator of response to cellular and environmental stress, as genetic determinants of QOL in a subset of the study population (N = 641). Trends among demographic factors for mean PCS and MCS included smoking status (PCS P_trend < 0.001, MCS P_trend < 0.001) and education (PCS P_trend < 0.001, MCS P_trend < 0.001). Similar relationships were seen for MCS. The homozygous rare genotype of MEF2B: rs2040562 showed an increased risk of a poor MCS (OR: 3.06, 95% CI: 1.05–8.92, P = 0.041). Finally, survival analysis showed that a low PCS or a MCS was associated with increased risks of five-year mortality (HR = 1.63, 95% CI: 1.51–1.77, HR = 1.23, 95% CI: 1.16–1.32, respectively) and there was a significant reduction in median survival time (P_log-rank < 0.001). These findings suggest that multiple factors contribute to QOL in lung cancer patients, and baseline QOL can impact survival.

Dendritic Spine Elimination: Molecular Mechanisms and Implications

Dynamic modification of synaptic connectivity in response to sensory experience is a vital step in the refinement of brain circuits as they are established during development and modified during learning. In addition to the well-established role for new spine growth and stabilization in the experience-dependent plasticity of neural circuits, dendritic spine elimination has been linked to improvements in learning, and dysregulation of spine elimination has been associated with intellectual disability and behavioral impairment. Proper brain function requires a tightly regulated balance between spine formation and spine elimination. Although most studies have focused on the mechanisms of spine formation, considerable progress has been made recently in delineating the neural activity patterns and downstream molecular mechanisms that drive dendritic spine elimination. Here, we review the current state of knowledge concerning the signaling pathways that drive dendritic spine shrinkage and elimination in the cerebral cortex and we discuss their implication in neuropsychiatric and neurodegenerative disease.

Why do so many genetic insults lead to Purkinje Cell degeneration and spinocerebellar ataxia?

The genetically heterozygous spinocerebellar ataxias are all characterized by cerebellar atrophy and pervasive Purkinje Cell degeneration. Up to date, more than 35 functionally diverse spinocerebellar ataxia genes have been identified. The main question that remains yet unsolved is why do some many genetic insults lead to Purkinje Cell degeneration and spinocerebellar ataxia? To address this question it is important to identify intrinsic pathways important for Purkinje Cell function and survival. In this review, we discuss the current consensus on shared mechanisms underlying the pervasive Purkinje Cell loss in spinocerebellar ataxia. Additionally, using recently published cell type specific expression data, we identified several Purkinje Cell-specific genes and discuss how the corresponding pathways might underlie the vulnerability of Purkinje Cells in response to the diverse genetic insults causing spinocerebellar ataxia.

Identification of 22q13 genes most likely to contribute to Phelan McDermid syndrome

Chromosome 22q13.3 deletion (Phelan McDermid) syndrome (PMS) is a rare genetic neurodevelopmental disorder resulting from deletions or other genetic variants on distal 22q. Pathological variants of the SHANK3 gene have been identified, but terminal chromosomal deletions including SHANK3 are most common. Terminal deletions disrupt up to 108 protein-coding genes. The impact of these losses is highly variable and includes both significantly impairing neurodevelopmental and somatic manifestations. The current review combines two metrics, prevalence of gene loss and predicted loss pathogenicity, to identify likely contributors to phenotypic expression. These genes are grouped according to function as follows: molecular signaling at glutamate synapses, phenotypes involving neuropsychiatric disorders, involvement in multicellular organization, cerebellar development and functioning, and mitochondrial. The likely most impactful genes are reviewed to provide information for future clinical and translational investigations.

Differences in the expression of long noncoding RNAs at different time points in the PTSD-like syndrome rat hippocampus

The aim of this study was to characterize the expression profiles at different time points in the PTSD-like syndrome rat hippocampus and perform analyses. PTSD rat models were made as reported by Rau, and we collected the hippocampus at different time points. The lncRNAs at different time points were compared by microarray and listed. We used quantitative real-time PCR to confirm the lncRNA profiling expression data. Bioinformatics analysis was performed on EU056364_P1. Compare with control, a total of 948 lncRNAs and 2514 mRNAs were found (fold-change > 2.0) among the four time points. Additionally, bioinformatics analysis of EU056364_P1 suggested it might be involved in memory development through the target gene Camk2a.This study revealed different lncRNAs expressed at different time points in PTSD and explored the targets of PTSD.

Unconventional NMDA Receptor Signaling

In the classical view, NMDA receptors (NMDARs) are stably expressed at the postsynaptic membrane, where they act via Ca²⁺ to signal coincidence detection in Hebbian plasticity. More recently, it has been established that NMDAR-mediated transmission can be dynamically regulated by neural activity. In addition, NMDARs have been found presynaptically, where they cannot act as conventional coincidence detectors. Unexpectedly, NMDARs have also been shown to signal metabotropically, without the need for Ca²⁺ This review highlights novel findings concerning these unconventional modes of NMDAR action.

The SEK-1 p38 MAP Kinase Pathway Modulates Gq Signaling in Caenorhabditis elegans

Gq is a heterotrimeric G protein that is widely expressed in neurons and regulates neuronal activity. To identify pathways regulating neuronal Gq signaling, we performed a forward genetic screen in Caenorhabditis elegans for suppressors of activated Gq. One of the suppressors is an allele of sek-1, which encodes a mitogen-activated protein kinase kinase (MAPKK) in the p38 MAPK pathway. Here, we show that sek-1 mutants have a slow locomotion rate and that sek-1 acts in acetylcholine neurons to modulate both locomotion rate and Gq signaling. Furthermore, we find that sek-1 acts in mature neurons to modulate locomotion. Using genetic and behavioral approaches, we demonstrate that other components of the p38 MAPK pathway also play a positive role in modulating locomotion and Gq signaling. Finally, we find that mutants in the SEK-1 p38 MAPK pathway partially suppress an activated mutant of the sodium leak channel, NCA-1/NALCN, a downstream target of Gq signaling. Our results suggest that the SEK-1 p38 pathway may modulate the output of Gq signaling through NCA-1(unc-77).

Recent Advances in the Inhibition of p38 MAPK as a Potential Strategy for the Treatment of Alzheimer's Disease

P38 mitogen-activated protein kinase (MAPK) is a crucial target for chronic inflammatory diseases. Alzheimer’s disease (AD) is characterized by the presence of amyloid plaques and neurofibrillary tangles in the brain, as well as neurodegeneration, and there is no known cure. Recent studies on the underlying biology of AD in cellular and animal models have indicated that p38 MAPK is capable of orchestrating diverse events related to AD, such as tau phosphorylation, neurotoxicity, neuroinflammation and synaptic dysfunction. Thus, the inhibition of p38 MAPK is considered a promising strategy for the treatment of AD. In this review, we summarize recent advances in the targeting of p38 MAPK as a potential strategy for the treatment of AD and envision possibilities of p38 MAPK inhibitors as a fundamental therapeutics for AD.

Characterization and reversal of Doxorubicin-mediated biphasic activation of ERK and persistent excitability in sensory neurons of Aplysia californica

Doxorubicin (DOX), a common chemotherapeutic agent, impairs synaptic plasticity. DOX also causes a persistent increase in basal neuronal excitability, which occludes serotonin-induced enhanced excitability. Therefore, we sought to characterize and reverse DOX-induced physiological changes and modulation of molecules implicated in memory induction using sensory neurons from the marine mollusk Aplysia californica. DOX produced two mechanistically distinct phases of extracellular signal-regulated kinase (ERK) activation, an early and a late phase. Inhibition of MEK (mitogen-activated protein kinase (MAPK)/ERK kinase) after DOX treatment reversed the late ERK activation. MEK inhibition during treatment enhanced the late ERK activation possibly through prolonged downregulation of MAPK phosphatase-1 (MKP-1). Unexpectedly, the late ERK activation negatively correlated with excitability. MEK inhibition during DOX treatment simultaneously enhanced the late activation of ERK and blocked the increase in basal excitability. In summary, we report DOX-mediated biphasic activation of ERK and the reversal of the associated changes in neurons, a potential strategy for reversing the deleterious effects of DOX treatment.

Antidyskinetic Effects of MEK Inhibitor Are Associated with Multiple Neurochemical Alterations in the Striatum of Hemiparkinsonian Rats

L-DOPA-induced dyskinesia (LID) represents one of the major problems of the long-term therapy of patients with Parkinson's disease (PD). Although, the pathophysiologic mechanisms underlying LID are not completely understood, activation of the extracellular signal regulated kinase (ERK) is recognized to play a key role. ERK is phosphorylated by mitogen-activated protein kinase kinase (MEK), and thus MEK inhibitor can prevent ERK activation. Here the effect of the MEK inhibitor PD98059 on LID and the associated molecular changes were examined. Rats with unilateral 6-OHDA lesions of the nigrostriatal pathway received daily L-DOPA treatment for 3 weeks, and abnormal involuntary movements (AIMs) were assessed every other day. PD98059 was injected in the lateral ventricle daily for 12 days starting from day 10 of L-DOPA treatment. Striatal molecular markers of LID were analyzed together with gene regulation using microarray. The administration of PD98059 significantly reduced AIMs. In addition, ERK activation and other associated molecular changes including ΔFosB were reversed in rats treated with the MEK inhibitor. PD98059 induced significant up-regulation of 418 transcripts and down-regulation of 378 transcripts in the striatum. Tyrosine hydroxylase (Th) and aryl hydrocarbon receptor nuclear translocator (Arnt) genes were down-regulated in lesioned animals and up-regulated in L-DOPA-treated animals. Analysis of protein levels showed that PD98059 reduced the striatal TH. These results support the association of p-ERK1/2, ΔFosB, p-H3 to the regulation of TH and ARNT in the mechanisms of LID, and pinpoint other gene regulatory changes, thus providing clues for identifying new targets for LID therapy.

Biphasic Regulation of p38 MAPK by Serotonin Contributes to the Efficacy of Stimulus Protocols That Induce Long-Term Synaptic Facilitation

The MAPK isoforms ERK and p38 MAPK are believed to play opposing roles in long-term synaptic facilitation (LTF) induced by serotonin (5-HT) in Aplysia. To fully understand their roles, however, it is necessary to consider the dynamics of ERK and p38 MAPK activation. Previous studies determined that activation of ERK occurred ∼45 min after a 5-min pulse of 5-HT treatment. The dynamics of p38 MAPK activation following 5-HT are yet to be elucidated. Here, the activity of p38 MAPK was examined at different times after 5-HT, and the interaction between the ERK and p38 MAPK pathways was investigated. A 5-min pulse of 5-HT induced a transient inhibition of p38 MAPK, followed by a delayed activation between 25 and 45 min. This activation was blocked by a MAPK kinase inhibitor, suggesting that similar pathways are involved in activation of ERK and p38 MAPK. ERK activity decreased shortly after the activation of p38 MAPK. A p38 MAPK inhibitor blocked this decrease in ERK activity, suggesting a causal relationship. The p38 MAPK activity ∼45 min after different stimulus protocols was also characterized. These data were incorporated into a computational model for the induction of LTF. Simulations and empirical data suggest that p38 MAPK, together with ERK, contributes to the efficacy of spaced stimulus protocols to induce LTF, a correlate of long-term memory (LTM). For example, decreased p38 MAPK activity ∼45 min after the first of two sensitizing stimuli might be an important determinant of an optimal interstimulus interval (ISI) for LTF induction.

Metabotropic glutamate receptor dependent long-term depression in the cortex

Metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), a type of synaptic plasticity, is characterized by a reduction in the synaptic response, mainly at the excitatory synapses of the neurons. The hippocampus and the cerebellum have been the most extensively studied regions in mGluR-dependent LTD, and Group 1 mGluR has been reported to be mainly involved in this synaptic LTD at excitatory synapses. However, mGluR-dependent LTD in other brain regions may be involved in the specific behaviors or diseases. In this paper, we focus on five cortical regions and review the literature that implicates their contribution to the pathogenesis of several behaviors and specific conditions associated with mGluR-dependent LTD.

Hippocampal metabotropic glutamate receptor long-term depression in health and disease: focus on mitogen-activated protein kinase pathways

Group I metabotropic glutamate receptor (mGluR) dependent long-term depression (LTD) is a major form of synaptic plasticity underlying learning and memory. The molecular mechanisms involved in mGluR-LTD have been investigated intensively for the last two decades. In this 60th anniversary special issue article, we review the recent advances in determining the mechanisms that regulate the induction, transduction and expression of mGluR-LTD in the hippocampus, with a focus on the mitogen-activated protein kinase (MAPK) pathways. In particular we discuss the requirement of p38 MAPK and extracellular signal-regulated kinase 1/2 (ERK 1/2) activation. The recent advances in understanding the signaling cascades regulating mGluR-LTD are then related to the cognitive impairments observed in neurological disorders, such as fragile X syndrome and Alzheimer's disease. mGluR-LTD is a form of synaptic plasticity that impacts on memory formation. In the hippocampus mitogen-activated protein kinases (MAPKs) have been found to be important in mGluR-LTD. In this 60th anniversary special issue article, we review the independent and complementary roles of two classes of MAPK, p38 and ERK1/2 and link this to the aberrant mGluR-LTD that has an important role in diseases. This article is part of the 60th Anniversary special issue.

Non-Ionotropic NMDA Receptor Signaling Drives Activity-Induced Dendritic Spine Shrinkage

The elimination of dendritic spine synapses is a critical step in the refinement of neuronal circuits during development of the cerebral cortex. Several studies have shown that activity-induced shrinkage and retraction of dendritic spines depend on activation of the NMDA-type glutamate receptor (NMDAR), which leads to influx of extracellular calcium ions and activation of calcium-dependent phosphatases that modify regulators of the spine cytoskeleton, suggesting that influx of extracellular calcium ions drives spine shrinkage. Intriguingly, a recent report revealed a novel non-ionotropic function of the NMDAR in the regulation of synaptic strength, which relies on glutamate binding but is independent of ion flux through the receptor (Nabavi et al., 2013). Here, we tested whether non-ionotropic NMDAR signaling could also play a role in driving structural plasticity of dendritic spines. Using two-photon glutamate uncaging and time-lapse imaging of rat hippocampal CA1 neurons, we show that low-frequency glutamatergic stimulation results in shrinkage of dendritic spines even in the presence of the NMDAR d-serine/glycine binding site antagonist 7-chlorokynurenic acid (7CK), which fully blocks NMDAR-mediated currents and Ca(2+) transients. Notably, application of 7CK or MK-801 also converts spine enlargement resulting from a high-frequency uncaging stimulus into spine shrinkage, demonstrating that strong Ca(2+) influx through the NMDAR normally overcomes a non-ionotropic shrinkage signal to drive spine growth. Our results support a model in which NMDAR signaling, independent of ion flux, drives structural shrinkage at spiny synapses.

SIGNIFICANCE STATEMENT

Dendritic spine elimination is vital for the refinement of neural circuits during development and has been linked to improvements in behavioral performance in the adult. Spine shrinkage and elimination have been widely accepted to depend on Ca(2+) influx through NMDA-type glutamate receptors (NMDARs) in conjunction with long-term depression (LTD) of synaptic strength. Here, we use two-photon glutamate uncaging and time-lapse imaging to show that non-ionotropic NMDAR signaling can drive shrinkage of dendritic spines, independent of NMDAR-mediated Ca(2+) influx. Signaling through p38 MAPK was required for this activity-dependent spine shrinkage. Our results provide fundamental new insights into the signaling mechanisms that support experience-dependent changes in brain structure.

Doxorubicin attenuates serotonin-induced long-term synaptic facilitation by phosphorylation of p38 mitogen-activated protein kinase

Doxorubicin (DOX) is an anthracycline used widely for cancer chemotherapy. Its primary mode of action appears to be topoisomerase II inhibition, DNA cleavage, and free radical generation. However, in non-neuronal cells, DOX also inhibits the expression of dual-specificity phosphatases (also referred to as MAPK phosphatases) and thereby inhibits the dephosphorylation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38 MAPK), two MAPK isoforms important for long-term memory (LTM) formation. Activation of these kinases by DOX in neurons, if present, could have secondary effects on cognitive functions, such as learning and memory. The present study used cultures of rat cortical neurons and sensory neurons (SNs) of Aplysia to examine the effects of DOX on levels of phosphorylated ERK (pERK) and phosphorylated p38 (p-p38) MAPK. In addition, Aplysia neurons were used to examine the effects of DOX on long-term enhanced excitability, long-term synaptic facilitation (LTF), and long-term synaptic depression (LTD). DOX treatment led to elevated levels of pERK and p-p38 MAPK in SNs and cortical neurons. In addition, it increased phosphorylation of the downstream transcriptional repressor cAMP response element-binding protein 2 in SNs. DOX treatment blocked serotonin-induced LTF and enhanced LTD induced by the neuropeptide Phe-Met-Arg-Phe-NH2. The block of LTF appeared to be attributable to overriding inhibitory effects of p-p38 MAPK, because LTF was rescued in the presence of an inhibitor (SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole]) of p38 MAPK. These results suggest that acute application of DOX might impair the formation of LTM via the p38 MAPK pathway.

Domain contributions to signaling specificity differences between Ras-guanine nucleotide releasing factor (Ras-GRF) 1 and Ras-GRF2

Ras-GRF1 (GRF1) and Ras-GRF2 (GRF2) constitute a family of similar calcium sensors that regulate synaptic plasticity. They are both guanine exchange factors that contain a very similar set of functional domains, including N-terminal pleckstrin homology, coiled-coil, and calmodulin-binding IQ domains and C-terminal Dbl homology Rac-activating domains, Ras-exchange motifs, and CDC25 Ras-activating domains. Nevertheless, they regulate different forms of synaptic plasticity. Although both GRF proteins transduce calcium signals emanating from NMDA-type glutamate receptors in the CA1 region of the hippocampus, GRF1 promotes LTD, whereas GRF2 promotes θ-burst stimulation-induced LTP (TBS-LTP). GRF1 can also mediate high frequency stimulation-induced LTP (HFS-LTP) in mice over 2-months of age, which involves calcium-permeable AMPA-type glutamate receptors. To add to our understanding of how proteins with similar domains can have different functions, WT and various chimeras between GRF1 and GRF2 proteins were tested for their abilities to reconstitute defective LTP and/or LTD in the CA1 hippocampus of Grf1/Grf2 double knock-out mice. These studies revealed a critical role for the GRF2 CDC25 domain in the induction of TBS-LTP by GRF proteins. In contrast, the N-terminal pleckstrin homology and/or coiled-coil domains of GRF1 are key to the induction of HFS-LTP by GRF proteins. Finally, the IQ motif of GRF1 determines whether a GRF protein can induce LTD. Overall, these findings show that for the three forms of synaptic plasticity that are regulated by GRF proteins in the CA1 hippocampus, specificity is encoded in only one or two domains, and a different set of domains for each form of synaptic plasticity.

Cocaine facilitates protein synthesis-dependent LTP: the role of metabotropic glutamate receptors

Cocaine addiction alters synaptic plasticity in many brain areas involved in learning and memory processes, including the hippocampus. Long-term potentiation (LTP) is one of the best studied examples of hippocampal synaptic plasticity and it is considered as one of the molecular basis of learning and memory. We previously demonstrated that in the presence of cocaine, a long lasting form of hippocampal LTP is induced by a single pulse of high frequency stimulation, which in normal conditions evokes only an early form of LTP. In this study, we further explore the molecular basis of this modulation of synaptic plasticity by cocaine. By performing pharmacological experiments on hippocampal slices, we were able to show that cocaine converts early LTP to a form of LTP dependent on protein synthesis, probably through the cAMP-dependent protein kinase and extracellular signal-regulated kinase signaling cascades. We also found that metabotropic glutamate receptors are involved in this phenomenon. These studies further clarify the molecular machinery used by cocaine to alter synaptic plasticity and modulate learning and memory processes.

mGlu5 receptors and cellular prion protein mediate amyloid-β-facilitated synaptic long-term depression in vivo

NMDA-type glutamate receptors (NMDARs) are currently regarded as paramount in the potent and selective disruption of synaptic plasticity by Alzheimer’s disease amyloid β-protein (Aβ). Non-NMDAR mechanisms remain relatively unexplored. Here we describe how Aβ facilitates NMDAR-independent long-term depression of synaptic transmission in the hippocampus in vivo. Synthetic Aβ and Aβ in soluble extracts of Alzheimer’s disease brain usurp endogenous acetylcholine muscarinic receptor-dependent long-term depression, to enable long-term depression that required metabotropic glutamate-5 receptors (mGlu5Rs). We also find that mGlu5Rs are essential for Aβ-mediated inhibition of NMDAR-dependent long-term potentiation in vivo. Blocking Aβ binding to cellular prion protein with antibodies prevents the facilitation of long-term depression. Our findings uncover an overarching role for Aβ-PrP^C-mGlu5R interplay in mediating both LTD facilitation and LTP inhibition, encompassing NMDAR-mediated processes that were previously considered primary.

In Alzheimer's disease, the soluble amyloid beta peptide is known to modulate synaptic function by forming a complex with prion proteins and metabotropic glutamate receptors. Here, Hu et al. show that amyloid beta signalling via this complex facilitates the induction of long-term depression at synapses.

Blockade of peripheral P2Y1 receptors prevents the induction of thermal hyperalgesia via modulation of TRPV1 expression in carrageenan-induced inflammatory pain rats: involvement of p38 MAPK phosphorylation in DRGs

Although previous reports have suggested that P2Y1 receptors (P2Y1Rs) are involved in cutaneous nociceptive signaling, it remains unclear how P2Y1Rs contribute to peripheral sensitization. The current study was designed to delineate the role of peripheral P2Y1Rs in pain and to investigate potential linkages to mitogen-activated protein kinase (MAPK) in DRGs and Transient Receptor Potential Vanilloid 1 (TRPV1) expression in a rodent inflammatory pain model. Following injection of 2% carrageenan into the hind paw, expressions of P2Y1 and TRPV1 and the phosphorylation rates of both p38 MAPK and ERK but not JNK were increased and peaked at day 2 post-injection. Blockade of peripheral P2Y1Rs by the P2Y1R antagonist, MRS2500 injection (i.pl, D0 to D2) significantly reduced the induction of thermal hyperalgesia, but not mechanical allodynia. Simultaneously, MRS2500 injections suppressed upregulated TRPV1 expression and DRG p38 phosphorylation, while pERK signaling was not affected. Furthermore, inhibition of p38 activation in the DRGs by SB203580 (a p38 inhibitor, i.t, D0 to D2) prevented the upregulation of TRPV1 and a single i.t injection of SB203580 reversed the established thermal hyperalgesia, but not mechanical allodynia. Lastly, to identify the mechanism of action of P2Y1Rs, we repeatedly injected the P2Y1 agonist, MRS2365 into the naïve rat's hind paw and observed a dose-dependent increase in TRPV1 expression and p38 MAPK phosphorylation. These data demonstrate a sequential role for P2Y1R, p38 MAPK and TRPV1 in inflammation-induced thermal hyperalgesia; thus, peripheral P2Y1Rs activation modulates p38 MAPK signaling and TRPV1 expression, which ultimately leads to the induction of thermal hyperalgesia.

Discovering dominant pathways and signal-response relationships in signaling networks through nonparametric approaches

A signaling pathway is a sequence of proteins and passenger molecules that transmits information from the cell surface to target molecules. Understanding signal transduction process requires detailed description of the involved pathways. Several methods and tools resolved this problem by incorporating genomic and proteomic data. However, the difficulty of obtaining prior knowledge of complex signaling networks limited the applicability of these tools. In this study, based on the simulation of signal flow in signaling network, we introduce a method for determining dominant pathways and signal response to stimulations. The model uses topology-weighted transit compartment approach and comprises four main steps which include weighting the edges, simulating signal transduction in the network (weighting the nodes), finding paths between initial and target nodes, and assigning a significance score to each path. We applied the proposed model to eighty-three signaling networks by using biologically derived source and sink molecules. The recovered dominant paths matched many known signaling pathways and suggesting a promising index to analyze the phenotype essentiality of molecule encoding paths. We also modeled the stimulus-response relations in long and short-term synaptic plasticity based on the dominant signaling pathway concept. We showed that the proposed method not only accurately determines dominant signaling pathways, but also identifies effective points of intervention in signal transduction.

Acquisition of contextual discrimination involves the appearance of a RAS-GRF1/p38 mitogen-activated protein (MAP) kinase-mediated signaling pathway that promotes long term potentiation (LTP)

RAS-GRF1 is a guanine nucleotide exchange factor with the ability to activate RAS and RAC GTPases in response to elevated calcium levels. We previously showed that beginning at 1 month of age, RAS-GRF1 mediates NMDA-type glutamate receptor (NMDAR)-induction of long term depression in the CA1 region of the hippocampus of mice. Here we show that beginning at 2 months of age, when mice first acquire the ability to discriminate between closely related contexts, RAS-GRF1 begins to contribute to the induction of long term potentiation (LTP) in the CA1 hippocampus by mediating the action of calcium-permeable, AMPA-type glutamate receptors (CP-AMPARs). Surprisingly, LTP induction by CP-AMPARs through RAS-GRF1 occurs via activation of p38 MAP kinase rather than ERK MAP kinase, which has more frequently been linked to LTP. Moreover, contextual discrimination is blocked by knockdown of Ras-Grf1 expression specifically in the CA1 hippocampus, infusion of a p38 MAP kinase inhibitor into the CA1 hippocampus, or the injection of an inhibitor of CP-AMPARs. These findings implicate the CA1 hippocampus in the developmentally dependent capacity to distinguish closely related contexts through the appearance of a novel LTP-supporting signaling pathway.

Calbindin-D28k and calretinin in chicken inner retina during postnatal development and neuroplasticity by dim red light

Members of the family of calcium binding proteins (CBPs) are involved in the buffering of calcium (Ca2+) by regulating how Ca2+ can operate within synapses or more globally in the entire cytoplasm and they are present in a particular arrangement in all types of retinal neurons. Calbindin D28k and calretinin belong to the family of CBPs and they are mainly co-expressed with other CBPs. Calbindin D28k is expressed in doubles cones, bipolar cells and in a subpopulation of amacrine and ganglion neurons. Calretinin is present in horizontal cells as well as in a subpopulation of amacrine and ganglion neurons. Both proteins fill the soma at the inner nuclear layer and the neuronal projections at the inner plexiform layer. Moreover, calbindin D28k and calretinin have been associated with neuronal plasticity in the central nervous system. During pre and early postnatal visual development, the visual system shows high responsiveness to environmental influences. In this work we observed modifications in the pattern of stratification of calbindin immunoreactive neurons, as well as in the total amount of calbindin through the early postnatal development. In order to test whether or not calbindin is involved in retinal plasticity we analyzed phosphorylated p38 MAPK expression, which showed a decrease in p-p38 MAPK, concomitant to the observed decrease of calbindin D28k. Results showed in this study suggest that calbindin is a molecule related with neuroplasticity, and we suggest that calbindin D28k has significant roles in neuroplastic changes in the retina, when retinas are stimulated with different light conditions.

The intersections of NMDAR-dependent synaptic plasticity and cell survival

The discovery of a requirement for N-methyl d-aspartate receptor (NMDAR) activation in long-term potentiation (LTP) set off an explosion of interest in the mechanisms of NMDAR-dependent synaptic plasticity. Meanwhile other research has advanced our understanding of how NMDAR activation regulates neuronal death and survival. Surprisingly, there have been few attempts to correlate these important areas of research. Here we review current knowledge of the various mechanisms of NMDAR-dependent synaptic plasticity that are shared with neuronal survival and death, while drawing comparisons with the proneurotrophin/neurotrophin receptor and intracellular signaling systems. Our conclusion is that NMDAR-dependent LTP and long-term depression (LTD) share many common mechanisms with cell survival and cell death, respectively. The intersections of plasticity and cell survival may represent novel avenues for neuroprotection. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Unique gene alterations are induced in FACS-purified Fos-positive neurons activated during cue-induced relapse to heroin seeking

Cue-induced heroin seeking after prolonged withdrawal is associated with neuronal activation and altered gene expression in prefrontal cortex (PFC). However, these previous studies assessed gene expression in all neurons regardless of their activity state during heroin seeking. Using Fos as a marker of neural activity, we describe distinct molecular alterations induced in activated versus non-activated neurons during cue-induced heroin seeking after prolonged withdrawal. We trained rats to self-administer heroin for 10 days (6 h/day) and assessed cue-induced heroin seeking in extinction tests after 14 or 30 days. We used fluorescent-activated cell sorting (FACS) to purify Fos-positive and Fos-negative neurons from PFC 90 min after extinction testing. Flow cytometry showed that Fos-immunoreactivity was increased in less than 10% of sparsely distributed PFC neurons. mRNA levels of the immediate early genes fosB, arc, egr1, and egr2, as well as npy and map2k6, were increased in Fos-positive, but not Fos-negative, neurons. In support of these findings, double-label immunohistochemistry indicated substantial coexpression of neuropeptide Y (NPY)- and Arc-immunoreactivity in Fos-positive neurons. Our data indicate that cue-induced relapse to heroin seeking after prolonged withdrawal induces unique molecular alterations within activated PFC neurons that are distinct from those observed in the surrounding majority of non-activated neurons.

Arf6-GEF BRAG1 regulates JNK-mediated synaptic removal of GluA1-containing AMPA receptors: a new mechanism for nonsyndromic X-linked mental disorder

Activity-dependent modifications of excitatory synapses contribute to synaptic maturation and plasticity, and are critical for learning and memory. Consequently, impairments in synapse formation or synaptic transmission are thought to be responsible for several types of mental disabilities. BRAG1 is a guanine nucleotide exchange factor for the small GTP-binding protein Arf6 that localizes to the postsynaptic density of excitatory synapses. Mutations in BRAG1 have been identified in families with X-linked intellectual disability (XLID). These mutations mapped to either the catalytic domain or an IQ-like motif; however, the pathophysiological basis of these mutations remains unknown. Here, we show that the BRAG1 IQ motif binds apo-calmodulin (CaM), and that calcium-induced CaM release triggers a reversible conformational change in human BRAG1. We demonstrate that BRAG1 activity, stimulated by activation of NMDA-sensitive glutamate receptors, depresses AMPA receptor (AMPA-R)-mediated transmission via JNK-mediated synaptic removal of GluA1-containing AMPA-Rs in rat hippocampal neurons. Importantly, a BRAG1 mutant that fails to activate Arf6 also fails to depress AMPA-R signaling, indicating that Arf6 activity is necessary for this process. Conversely, a mutation in the BRAG1 IQ-like motif that impairs CaM binding results in hyperactivation of Arf6 signaling and constitutive depression of AMPA transmission. Our findings reveal a role for BRAG1 in response to neuronal activity with possible clinical relevance to nonsyndromic XLID.

MSK1 regulates environmental enrichment-induced hippocampal plasticity and cognitive enhancement

Environmental enrichment (EE) has marked beneficial effects on cognitive capacity. Given the possibility that this form of neuronal plasticity could function via the actuation of the same cellular signaling pathways that underlie learning/memory formation, we examined whether the MAPK cascade effector, mitogen/stress-activated kinase 1 (MSK1), could play a role in this process. MSK1 functions as a key signaling intermediate that couples changes in neuronal activity into inducible gene expression, neuronal plasticity, and learning/memory. Here, we show that MSK1 is expressed in excitatory cell layers of the hippocampus, progenitor cells of the subgranular zone (SGZ), and adult-born immature neurons. MSK1(-/-) mice exhibit reduced spinogenesis and decreased dendritic branching complexity in hippocampal neurons, compared with WT mice. Further, in MSK1(-/-) mice, progenitor cell proliferation within the SGZ was significantly reduced and, correspondingly, the number of immature neurons within the dentate gyrus was significantly reduced. Consistent with prior work, MSK1(-/-) mice displayed deficits in both spatial and recognition memory tasks. Strikingly, cognitive enhancement resulting from a 40-d period of EE was markedly reduced in MSK1(-/-) animals. MSK1(-/-) mice exhibited reduced levels of EE-induced spinogenesis and SGZ progenitor proliferation. Taken together, these data reveal that MSK1 serves as a critical regulator of hippocampal physiology and function and that MSK1 serves as a key conduit by which enriching stimuli augment cellular plasticity and cognition.