Synaptic Incorporation of AMPA Receptors during LTP Is Controlled by a PKC Phosphorylation Site on GluR1

Expression Changes of NMDA and AMPA Receptor Subunits in the Hippocampus in rats with Diabetes Induced by Streptozotocin Coupled with Memory Impairment

Cognitive impairment in diabetes (CID) is a severe chronic complication of diabetes mellitus (DM). It has been hypothesized that diabetes can lead to cognitive dysfunction due to expression changes of excitatory neurotransmission mediated by N-methyl-D-aspartate receptors (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR); however, the pathogenesis involved in this has not been fully understood, especially at early phase of DM. Here, we sought to determine the cognitive changes and aim to correlate this with the expression changes of NMDAR and AMPAR of glutamate signaling pathways in the rat hippocampus from early phase of DM and in the course of the disease progression. By Western blot analysis and immunofluorescence labeling, the hippocampus in diabetic rats showed a significant increase in protein expression NMDAR subunits NR1, NR2A and NR2B and AMPAR subunit GluR1. Along with this, behavioral test by Morris water maze showed a significant decline in their performance when compared with the control rats. It is suggested that NR1, NR2A, NR2B and GluR1are involved in learning and memory and that their expression alterations maybe correlated with the occurrence and development of CID in diabetic rats induced by streptozotocin.

Preferential generation of Ca2+-permeable AMPA receptors by AKAP79-anchored protein kinase C proceeds via GluA1 subunit phosphorylation at Ser-831

AMPA-type glutamate receptors (AMPARs) mediate fast excitatory neurotransmission in the mammalian central nervous system. Preferential AMPAR subunit assembly favors heteromeric GluA1/GluA2 complexes. The presence of the GluA2 subunit generates Ca²⁺-impermeable (CI) AMPARs that have linear current-voltage (I-V) relationships. However, diverse forms of synaptic plasticity and pathophysiological conditions are associated with shifts from CI to inwardly rectifying, GluA2-lacking, Ca²⁺-permeable (CP) AMPARs on time scales ranging from minutes to days. These shifts have been linked to GluA1 phosphorylation at Ser-845, a protein kinase A (PKA)-targeted site within its intracellular C-terminal tail, often in conjunction with protein kinase A anchoring protein 79 (AKAP79; AKAP150 in rodents), which targets PKA to GluA1. However, AKAP79 may impact GluA1 phosphorylation at other sites by interacting with other signaling enzymes. Here, we evaluated the ability of AKAP79, its signaling components, and GluA1 phosphorylation sites to induce CP-AMPARs under conditions in which CI-AMPARs normally predominate. We found that GluA1 phosphorylation at Ser-831 is sufficient for the appearance of CP-AMPARs and that AKAP79-anchored protein kinase C (PKC) primarily drives the appearance of these receptors via this site. In contrast, other AKAP79-signaling components and C-terminal tail GluA1 phosphorylation sites exhibited a permissive role, limiting the extent to which AKAP79 promotes CP-AMPARs. This may reflect the need for these sites to undergo active phosphorylation/dephosphorylation cycles that control their residency within distinct subcellular compartments. These findings suggest that AKAP79, by orchestrating phosphorylation, represents a key to a GluA1 phosphorylation passcode, which allows the GluA1 subunit to escape GluA2 dominance and promote the appearance of CP-AMPARs.

Molecular Mechanisms of Early and Late LTP

LTP is the most intensively studied cellular model of the memory and generally divided at least two distinct phases as early and late. E-LTP requires activation of CaMKII that initiates biochemical events and trafficking of proteins, which eventually potentiate synaptic transmission, and is independent of de novo protein synthesis. In contrast, L-LTP requires gene expression and local protein synthesis regulated via TrkB receptor- and functional prions CPEB2-3-mediated translation. Maintenance of LTP for longer periods depends on constitutively active PKMζ. Throughout this review, current knowledge about early and late phases of LTP will be reviewed.

Phosphorylation Induces Conformational Rigidity at the C-Terminal Domain of AMPA Receptors

The intracellular C-terminal domain (CTD) of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor undergoes phosphorylation at specific locations during long-term potentiation. This modification enhances conductance through the AMPA receptor ion channel and thus potentially plays a crucial role in modulating receptor trafficking and signaling. However, because the CTD structure is largely unresolved, it is difficult to establish if phosphorylation induces conformational changes that might play a role in enhancing channel conductance. Herein, we utilize single-molecule Förster resonance energy transfer (smFRET) spectroscopy to probe the conformational changes of a section of the AMPA receptor CTD, under the conditions of point-mutated phosphomimicry. Multiple analysis algorithms fail to identify stable conformational states within the smFRET distributions, consistent with a lack of well-defined secondary structure. Instead, our results show that phosphomimicry induces conformational rigidity to the CTD, and such rigidity is electrostatically tunable.

PKCε Inhibits Neuronal Dendritic Spine Development through Dual Phosphorylation of Ephexin5

Protein kinase C (PKC)-dependent mechanisms promote synaptic function in the mature brain. However, the roles of PKC signaling during synapse development remain largely unknown. Investigating each brain-enriched PKC isoform in early neuronal development, we show that PKCε acutely and specifically reduces the number of dendritic spines, sites of eventual synapse formation on developing dendrites. This PKCε-mediated spine suppression is temporally restricted to immature neurons and mediated through the phosphorylation and activation of Ephexin5, a RhoA guanine nucleotide exchange factor (GEF) and inhibitor of hippocampal synapse formation. Our data suggest that PKCε acts as an early developmental inhibitor of dendritic spine formation, in contrast to its emerging pro-synaptic roles in mature brain function. Moreover, we identify a substrate of PKCε, Ephexin5, whose early-elevated expression in developing neurons may in part explain the mechanism by which PKCε plays seemingly opposing roles that depend on neuronal maturity.

AKAP150 Palmitoylation Regulates Synaptic Incorporation of Ca2+-Permeable AMPA Receptors to Control LTP

Ca2+-permeable AMPA-type glutamate receptors (CP-AMPARs) containing GluA1 but lacking GluA2 subunits contribute to multiple forms of synaptic plasticity, including long-term potentiation (LTP), but mechanisms regulating CP-AMPARs are poorly understood. A-kinase anchoring protein (AKAP) 150 scaffolds kinases and phosphatases to regulate GluA1 phosphorylation and trafficking, and trafficking of AKAP150 itself is modulated by palmitoylation on two Cys residues. Here, we developed a palmitoylation-deficient knockin mouse to show that AKAP150 palmitoylation regulates CP-AMPAR incorporation at hippocampal synapses. Using biochemical, super-resolution imaging, and electrophysiological approaches, we found that palmitoylation promotes AKAP150 localization to recycling endosomes and the postsynaptic density (PSD) to limit CP-AMPAR basal synaptic incorporation. In addition, we found that AKAP150 palmitoylation is required for LTP induced by weaker stimulation that recruits CP-AMPARs to synapses but not stronger stimulation that recruits GluA2-containing AMPARs. Thus, AKAP150 palmitoylation controls its subcellular localization to maintain proper basal and activity-dependent regulation of synaptic AMPAR subunit composition.

AMPA Receptor Trafficking for Postsynaptic Potentiation

Long-term potentiation (LTP) of excitatory synaptic strength, which has long been considered a synaptic correlate for learning and memory, requires a fast recruitment of additional α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors (AMPARs) to the postsynaptic sites. As cell biological concepts have been applied to the field and genetic manipulation and microscopic imaging technologies have been advanced, visualization of the trafficking of AMPARs to synapses for LTP has been investigated intensively over the last decade. Recycling endosomes have been reported as intracellular storage organelles to supply AMPARs for LTP through the endocytic recycling pathway. In addition, exocytic domains in the spine plasma membrane, where AMPARs are inserted from the intracellular compartment, and nanodomains, where diffusing AMPARs are trapped and immobilized inside synapses for LTP, have been described. Furthermore, cell surface lateral diffusion of AMPARs from extrasynaptic to synaptic sites has been reported as a key step for AMPAR location to the synaptic sites for LTP. This review article will discuss recent findings and views on the reservoir(s) of AMPARs and their trafficking for LTP expression by focusing on the exocytosis and lateral diffusion of AMPARs, and provide some future directions that need to be addressed in the field of LTP.

Stress-induced changes in social dominance are scaled by AMPA-type glutamate receptor phosphorylation in the medial prefrontal cortex

The establishment and maintenance of social dominance are critical for social stability and the survival and health of individual animals. Stress lead to depression and a decrease in the social status of depressed persons is a risk factor for suicide. Therefore, we explored the mechanistic and behavioral links among stress, depression, and social dominance and found that mice subjected to chronic restraint stress (CRS), an animal model of stress-induced depression, showed decreased social dominance as measured by a dominance tube test. Importantly, this submissive behavior was occluded by the antidepressant, fluoxetine, a selective serotonin reuptake inhibitor. It is known that social dominance is controlled by synaptic efficacy in the medial prefrontal cortex (mPFC) and that AMPA-type glutamate receptor (AMPA-R) is a key molecule for synaptic efficacy. We found that the phosphorylation on AMPA-R was bidirectionally changed by CRS and fluoxetine in the mPFC of mice with CRS. Moreover, we found a strong correlation between social dominance and AMPA-R phosphorylation that regulates synaptic efficacy by modulating the synaptic targeting of AMPA-R. Our correlational analysis of the behavior and biochemistry of the CRS model suggests that AMPA-R phosphorylation in the mPFC may serve as a biomarker of social dominance related to stress.

Phosphorylation of the AMPAR-TARP Complex in Synaptic Plasticity

Synaptic plasticity has been considered a key mechanism underlying many brain functions including learning, memory, and drug addiction. An increase or decrease in synaptic activity of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) complex mediates the phenomena as shown in the cellular models of synaptic plasticity, long-term potentiation (LTP), and depression (LTD). In particular, protein phosphorylation shares the spotlight in expressing the synaptic plasticity. This review summarizes the studies on phosphorylation of the AMPAR pore-forming subunits and auxiliary proteins including transmembrane AMPA receptor regulatory proteins (TARPs) and discusses its role in synaptic plasticity.

Of local translation control and lipid signaling in neurons

Fine-tuned regulation of new proteins synthesis is key to the fast adaptation of cells to their changing environment and their response to external cues. Protein synthesis regulation is particularly refined and important in the case of highly polarized cells like neurons where translation occurs in the subcellular dendritic compartment to produce long-lasting changes that enable the formation, strengthening and weakening of inter-neuronal connection, constituting synaptic plasticity. The changes in local synaptic proteome of neurons underlie several aspects of synaptic plasticity and new protein synthesis is necessary for long-term memory formation. Details of how neuronal translation is locally controlled only start to be unraveled. A generally accepted view is that mRNAs are transported in a repressed state and are translated locally upon externally cued triggering signaling cascades that derepress or activate translation machinery at specific sites. Some important yet poorly considered intermediates in these cascades of events are signaling lipids such as diacylglycerol and its balancing partner phosphatidic acid. A link between these signaling lipids and the most common inherited cause of intellectual disability, Fragile X syndrome, is emphasizing the important role of these secondary messages in synaptically controlled translation.

Spinal PKCα inhibition and gene-silencing for pain relief: AMPAR trafficking at the synapses between primary afferents and sensory interneurons

Upregulation of Ca²⁺-permeable AMPA receptors (CP-AMPARs) in dorsal horn (DH) neurons has been causally linked to persistent inflammatory pain. This upregulation, demonstrated for both synaptic and extrasynaptic AMPARs, depends on the protein kinase C alpha (PKCα) activation; hence, spinal PKC inhibition has alleviated peripheral nociceptive hypersensitivity. However, whether targeting the spinal PKCα would alleviate both pain development and maintenance has not been explored yet (essential to pharmacological translation). Similarly, if it could balance the upregulated postsynaptic CP-AMPARs also remains unknown. Here, we utilized pharmacological and genetic inhibition of spinal PKCα in various schemes of pain treatment in an animal model of long-lasting peripheral inflammation. Pharmacological inhibition (pre- or post-treatment) reduced the peripheral nociceptive hypersensitivity and accompanying locomotive deficit and anxiety in rats with induced inflammation. These effects were dose-dependent and observed for both pain development and maintenance. Gene-therapy (knockdown of PKCα) was also found to relieve inflammatory pain when applied as pre- or post-treatment. Moreover, the revealed therapeutic effects were accompanied with the declined upregulation of CP-AMPARs at the DH synapses between primary afferents and sensory interneurons. Our results provide a new focus on the mechanism-based pain treatment through interference with molecular mechanisms of AMPAR trafficking in central pain pathways.

Purinergic modulation of glutamate transmission: An expanding role in stress-linked neuropathology

Chronic stress has been extensively linked to disturbances in glutamatergic signalling. Emerging from this field of research is a considerable number of studies identifying the ability of purines at the pre-, post-, and peri-synaptic levels to tune glutamatergic neurotransmission. While the evidence describing purinergic control of glutamate has continued to grow, there has been relatively little attention given to how chronic stress modulates purinergic functions. The available research on this topic has demonstrated that chronic stress can not only disturb purinergic receptors involved in the regulation of glutamate neurotransmission, but also perturb glial-dependent purinergic signalling. This review will provide a detailed examining of the complex literature relating to glutamatergic-purinergic interactions with a focus on both neuronal and glial contributions. Once these detailed interactions have been described and contextualised, we will integrate recent findings from the field of stress research.

Cellular Receptors of Amyloid β Oligomers (AβOs) in Alzheimer's Disease

It is estimated that Alzheimer’s disease (AD) affects tens of millions of people, comprising not only suffering patients, but also their relatives and caregivers. AD is one of age-related neurodegenerative diseases (NDs) characterized by progressive synaptic damage and neuronal loss, which result in gradual cognitive impairment leading to dementia. The cause of AD remains still unresolved, despite being studied for more than a century. The hallmark pathological features of this disease are senile plaques within patients’ brain composed of amyloid beta (Aβ) and neurofibrillary tangles (NFTs) of Tau protein. However, the roles of Aβ and Tau in AD pathology are being questioned and other causes of AD are postulated. One of the most interesting theories proposed is the causative role of amyloid β oligomers (AβOs) aggregation in the pathogenesis of AD. Moreover, binding of AβOs to cell membranes is probably mediated by certain proteins on the neuronal cell surface acting as AβO receptors. The aim of our paper is to describe alternative hypotheses of AD etiology, including genetic alterations and the role of misfolded proteins, especially Aβ oligomers, in Alzheimer’s disease. Furthermore, in this review we present various putative cellular AβO receptors related to toxic activity of oligomers.

Regulator of G-Protein Signaling 4 (RGS4) Controls Morphine Reward by Glutamate Receptor Activation in the Nucleus Accumbens of Mouse Brain

Crosstalk between G-protein signaling and glutamatergic transmission within the brain reward circuits is critical for long-term emotional effects (depression and anxiety), cravings, and negative withdrawal symptoms associated with opioid addiction. A previous study showed that Regulator of G-protein signaling 4 (RGS4) may be implicated in opiate action in the nucleus accumbens (NAc). However, the mechanism of the NAc-specific RGS4 actions that induce the behavioral responses to opiates remains largely unknown. The present study used a short hairpin RNA (shRNA)-mediated knock-down of RGS4 in the NAc of the mouse brain to investigate the relationship between the activation of ionotropic glutamate receptors and RGS4 in the NAc during morphine reward. Additionally, the shRNA-mediated RGS4 knock-down was implemented in NAc/striatal primary-cultured neurons to investigate the role that striatal neurons have in the morphine-induced activation of ionotropic glutamate receptors. The results of this study show that the NAc-specific knockdown of RGS4 significantly increased the behaviors associated with morphine and did so by phosphorylation of the GluR1 (Ser831) and NR2A (Tyr1325) glutamate receptors in the NAc. Furthermore, the knock-down of RGS4 enhanced the phosphorylation of the GluR1 and NR2A glutamate receptors in the primary NAc/striatal neurons during spontaneous morphine withdrawal. These findings show a novel molecular mechanism of RGS4 in glutamatergic transmission that underlies the negative symptoms associated with morphine administration.

Regulation of AMPA receptor trafficking and exit from the endoplasmic reticulum

A fundamental property of the brain is its ability to modify its function in response to its own activity. This ability for self-modification depends to a large extent on synaptic plasticity. It is now appreciated that for excitatory synapses, a significant part of synaptic plasticity depends upon changes in the post synaptic response to glutamate released from nerve terminals. Modification of the post synaptic response depends, in turn, on changes in the abundances of AMPA receptors in the post synaptic membrane. In this review, we consider mechanisms of trafficking of AMPA receptors to and from synapses that take place in the early trafficking stages, starting in the endoplasmic reticulum (ER) and continuing into the secretory pathway. We consider mechanisms of AMPA receptor assembly in the ER, highlighting the role of protein synthesis and the selective properties of specific AMPA receptor subunits, as well as regulation of ER exit, including the roles of chaperones and accessory proteins and the incorporation of AMPA receptors into COPII vesicles. We consider these processes in the context of the mechanism of mGluR LTD and discuss a compelling role for the dendritic ER membrane that is found proximal to synapses. The review illustrates the important, yet little studied, contribution of the early stages of AMPA receptor trafficking to synaptic plasticity.

Protein kinase A mediates scopolamine-induced mTOR activation and an antidepressant response

BACKGROUND

Clinical reports have shown that scopolamine produces a rapid (3-4 d) and potent anti-depressive response without severe adverse effects. Animal experiments have proven that scopolamine induces mTOR pathway activation in an AMPAR dependent manner. The present study aimed to determine the role of PKA in scopolamine-induced potentiation of AMPAR, as well as in mTOR pathway activation and rapid antidepressant effects.

METHODS

We utilized electrophysiological recording, Western blotting, and behavior tests to examine the effects of scopolamine, the selective M2 cholinergic receptor antagonist methoctramine, and H89, a PKA specific inhibitor on AMPAR potentiation, mTOR pathway activation, and behavioral responses in a rat depression model of learned helplessness.

RESULTS

Scopolamine (1μM) rapidly increased AMPAR-fEPSP amplitudes and membrane GluA1 expression in CA1 region of hippocampal slices, both of which were abolished by H89. Moreover, scopolamine promoted AMPAR phosphorylation on GluA1 ser845, a PKA site involved in GluA1 membrane insertion. H89 disrupted both GluA1 ser845 phosphorylation and mTOR activation, as well as the antidepressant effects of scopolamine as determined via forced swim test. Additionally, methoctramine mimicked the effects of scopolamine on phosphorylation and counter-depressive action in a PKA-dependent manner.

LIMITATIONS

Only one test was used to evaluate depressive behavior, and gene knock-out rats were not yet utilized to refine our hypotheses.

CONCLUSION

Our findings revealed that PKA pathway is necessary for scopolamine-induced synaptic plasticity and mTOR pathway activation, and indicated that a potential M2-PKA mechanism underlies scopolamine's antidepressant effects. Such findings suggest that GluA1 ser845 phosphorylation may be a trigger event for scopolamine's actions, and that PKA may represent a novel target for the treatment of depressive symptoms.

Potential for therapeutic targeting of AKAP signaling complexes in nervous system disorders

A common feature of neurological and neuropsychiatric disorders is a breakdown in the integrity of intracellular signal transduction pathways. Dysregulation of ion channels and receptors in the cell membrane and the enzymatic mediators that link them to intracellular effectors can lead to synaptic dysfunction and neuronal death. However, therapeutic targeting of these ubiquitous signaling elements can lead to off-target side effects due to their widespread expression in multiple systems of the body. A-kinase anchoring proteins (AKAPs) are multivalent scaffolding proteins that compartmentalize a diverse range of receptor and effector proteins to streamline signaling within nanodomain signalosomes. A number of essential neurological processes are known to critically depend on AKAP-directed signaling and an understanding of the role AKAPs play in nervous system disorders has emerged in recent years. Selective targeting of AKAP protein-protein interactions may be a means to uncouple pathologically active signaling pathways in neurological disorders with a greater degree of specificity. In this review we will discuss the role of AKAPs in both regulating normal nervous system function and dysfunction associated with disease, and the potential for therapeutic targeting of AKAP signaling complexes.

Protein kinase C epsilon delays latency until anoxic depolarization through arc expression and GluR2 internalization

Global cerebral ischemia is a debilitating injury that damages the CA1 region of the hippocampus, an area important for learning and memory. Protein kinase C epsilon (PKCɛ) activation is a critical component of many neuroprotective treatments. The ability of PKCɛ activation to regulate AMPA receptors (AMPARs) remains unexplored despite the role of AMPARs in excitotoxicity after brain ischemia. We determined that PKCɛ activation increased expression of a protein linked to learning and memory, activity-regulated cytoskeleton-associated protein (arc). Also, arc is necessary for neuroprotection and confers protection through decreasing AMPAR currents via GluR2 internalization. In vivo, activation of PKCɛ increased arc expression through a BDNF/TrkB pathway, and decreased GluR2 mRNA levels. In hippocampal cultured slices, PKCɛ activation decreased AMPAR current amplitudes in an arc- and GluR2-dependent manner. Additionally, PKCɛ activation triggered an arc- and GluR2 internalization-dependent delay in latency until anoxic depolarization. Inhibiting arc also blocked PKCɛ-mediated neuroprotection against lethal oxygen and glucose deprivation. These data characterize a novel PKCɛ-dependent mechanism that for the first time defines a role for arc and AMPAR internalization in conferring neuroprotection.

Amyloid β oligomers (AβOs) in Alzheimer's disease

The causative role of amyloid β 1-42 (Aβ42) aggregation in the pathogenesis of Alzheimer's disease (AD) has been under debate for over 25 years. Primarily, scientific efforts have focused on the dyshomeostasis between production and clearance of Aβ42. This imbalance may result from mutations either in genes for the substrate, i.e., amyloid precursor protein or in genes encoding presenilin, the enzyme of the reaction that generates Aβ42. Currently, it is supposed that soluble oligomers of amyloid beta (AβOs) and not fibrillar Aβ42 within neuritic plaques may be the toxic factors acting on a very early stage of AD, perhaps even initiating pathological cascade. For example, soluble AβOs isolated from AD patients' brains reduced number of synapses, inhibited long-term potentiation, and enhanced long-term synaptic depression in brain regions responsible for memory in animal models of AD. Concentrations of AβOs in the cerebrospinal fluid (CSF) of AD patients are often reported higher than in non-demented controls, and show a negative correlation with mini-mental state examination scores. Furthermore, increased Aβ42/oligomer ratio in the CSF of AD/MCI patients indicated that the presence of soluble AβOs in CSF may be linked to lowering of natively measured monomeric Aβ42 by epitopes masking, and hence, concentrations of AβOs in the CSF are postulated to as useful AD biomarkers.

The genetics of PKMζ and memory maintenance

Elucidating the molecular mechanisms that maintain long-term memory is a fundamental goal of neuroscience. Accumulating evidence suggests that persistent signaling by the atypical protein kinase C (PKC) isoform protein kinase Mζ (PKMζ) might maintain synaptic long-term potentiation (LTP) and long-term memory. However, the role of PKMζ has been challenged by genetic data from PKMζ-knockout mice showing intact LTP and long-term memory. Moreover, the PKMζ inhibitor peptide ζ inhibitory peptide (ZIP) reverses LTP and erases memory in both wild-type and knockout mice. Data from four papers using additional isoform-specific genetic approaches have helped to reconcile these conflicting findings. First, a PKMζ-antisense approach showed that LTP and long-term memory in PKMζ-knockout mice are mediated through a compensatory mechanism that depends on another ZIP-sensitive atypical isoform, PKCι/λ. Second, short hairpin RNAs decreasing the amounts of individual atypical isoforms without inducing compensation disrupted memory in different temporal phases. PKCι/λ knockdown disrupted short-term memory, whereas PKMζ knockdown specifically erased long-term memory. Third, conditional PKCι/λ knockout induced compensation by rapidly activating PKMζ to preserve short-term memory. Fourth, a dominant-negative approach in the model system Aplysia revealed that multiple PKCs form PKMs to sustain different types of long-term synaptic facilitation, with atypical PKM maintaining synaptic plasticity similar to LTP. Thus, under physiological conditions, PKMζ is the principal PKC isoform that maintains LTP and long-term memory. PKCι/λ can compensate for PKMζ, and because other isoforms could also maintain synaptic facilitation, there may be a hierarchy of compensatory mechanisms maintaining memory if PKMζ malfunctions.

The AMPA Receptor Subunit GluA1 is Required for CA1 Hippocampal Long-Term Potentiation but is not Essential for Synaptic Transmission

AMPA receptors mediate the majority of excitatory glutamatergic transmission in the mammalian brain and are heterotetramers composed of GluA1-4 subunits. Despite genetic studies, the roles of the subunits in synaptic transmission and plasticity remain controversial. To address this issue, we investigated the effects of cell-specific removal of GluA1 in hippocampal CA1 pyramidal neurons using virally-expressed GluA1 shRNA in organotypic slice culture. We show that this shRNA approach produces a rapid, efficient and selective loss of GluA1, and removed > 80% of surface GluA1 from synapses. This loss of GluA1 caused a modest reduction (up to 57%) in synaptic transmission and when applied in neurons from GluA3 knock-out mice, a similar small reduction in transmission occurred. Further, we found that loss of GluA1 caused a redistribution of GluA2 to synapses that may compensate functionally for the absence of GluA1. We found that LTP was absent in neurons lacking GluA1, induced either by pairing or by a theta-burst pairing protocol previously shown to induce LTP in GluA1 knock-out mice. Our findings demonstrate a critical role of GluA1 in CA1 LTP, but no absolute requirement for GluA1 in maintaining synaptic transmission. Further, our results indicate that GluA2 homomers can mediate synaptic transmission and can compensate for loss of GluA1.

Peptides Acting as Cognitive Enhancers

The aim of this paper is to present an overview of three peptides that, by improving synaptic function, enhance learning and memory in laboratory rodents. We summarize their structure, their mechanisms of action, and their effects on synaptic and cognitive function. First we describe FGL, a peptide derived from the neural cell adhesion molecule which improves cognition by the activation of the PKC pathway that triggers an activity-dependent delivery of AMPA receptors to the synapses. Then we describe PTD4-PI3KAc peptide that by activating PI3K signaling pathway it promotes synapse and spine formation and enhances hippocampal dependent memory. Lastly, we describe a new peptide derived from the well-known tumor suppressor PTEN that prevents pathological interactions between PTEN and PDZ proteins at synapses during exposure to Amyloid beta. This action prevents memory deterioration in mouse model of Alzheimer's disease. Together, this review indicates how learning and memory can be improved by manipulating synaptic function and number through pharmacological treatment with peptides, and it establishes synaptic function as a valid target for cognitive enhancement.

Molecular and synaptic mechanisms regulating drug-associated memories: Towards a bidirectional treatment strategy

The successful treatment of substance use disorders is dependent on the establishment of a long-term abstinent state. Relapse can be suppressed by interfering with memories of drug use that are evoked by re-exposure to drug-associated contexts and cues. Two strategies for accomplishing this goal are either to prevent drug-memory reconsolidation or to induce the formation of a competing, extinction memory. However, clinical attempts to prolong abstinence by behavioral modification of drug-related memories have had limited success. One approach to improve behavioral treatment strategies is to identify the molecular mechanisms that regulate these memory processes and then use pharmacological tools as supplements to improve efficacy. Still, due to the involvement of several overlapping signaling cascades in both reconsolidation and extinction, it is difficult to specifically modify one of the two processes. For example, attempting to elicit extinction may instead initiate reconsolidation, resulting in the unintentional strengthening of drug-related memories. A better approach is to identify diverging components of the two processes, whereby a single medication would simultaneously weaken reconsolidation and enhance extinction. This review will provide an overview of the neural substrates that are involved in the regulation of drug-associated memories, and will discuss emerging approaches to pharmacologically weaken these memories, including recent efforts to precisely and bidirectionally target reconsolidation and extinction. Ultimately, pharmacologically-enhanced memory-based approaches have the potential to produce more informed relapse-prevention therapies.

GluA4 Dependent Plasticity Mechanisms Contribute to Developmental Synchronization of the CA3-CA1 Circuitry in the Hippocampus

During the course of development, molecular mechanisms underlying activity-dependent synaptic plasticity change considerably. At immature CA3-CA1 synapses in the hippocampus, PKA-driven synaptic insertion of GluA4 AMPA receptors is the predominant mechanism for synaptic strengthening. However, the physiological significance of the developmentally restricted GluA4-dependent plasticity mechanisms is poorly understood. Here we have used microelectrode array (MEA) recordings in GluA4 deficient slice cultures to study the role of GluA4 in early development of the hippocampal circuit function. We find that during the first week in culture (DIV2-6) when GluA4 expression is restricted to pyramidal neurons, loss of GluA4 has no effect on the overall excitability of the immature network, but significantly impairs synchronization of the CA3 and CA1 neuronal populations. In the absence of GluA4, the temporal correlation of the population spiking activity between CA3-CA1 neurons was significantly lower as compared to wild-types at DIV6. Our data show that synapse-level defects in transmission and plasticity mechanisms are efficiently compensated for to normalize population firing rate at the immature hippocampal network. However, lack of the plasticity mechanisms typical for the immature synapses may perturb functional coupling between neuronal sub-populations, a defect frequently implicated in the context of developmentally originating neuropsychiatric disorders.

Protein Kinase C γ Contributes to Central Sensitization in a Rat Model of Chronic Migraine

Protein kinase C γ (PKCγ) is a critical regulator of central sensitization and is widely recognized to be involved in the pathogenesis of chronic migraine (CM). However, the function of PKCγ in CM remains unknown. This study investigated the role of PKCγ on pathogenesis of CM. We repeated infusions of inflammatory soup (IS) on the intact dura of conscious rats to model recurrent trigeminovascular or dural nociceptor activation assumed to occur in patients with CM. The von Frey test was then used to detect changes in pain threshold. QT-PCR, western blotting, and double immunofluorescence staining were performed to detect the expression and location of PKCγ in the trigeminal nucleus caudalis (TNC) and the expressions of calcitonin gene-related peptide (CGRP), c-Fos, and phosphorylation level of GluR1 subunit at serine 831. Chelerythrine chloride (CHE) and phorbol 12-myristate 13-acetate (PMA) were administrated to investigate the role of PKCγ in central sensitization. We found that repeated infusions of IS induced mechanical allodynia. PKCγ was significantly increased in TNC after CM. Furthermore, inhibition of PKCγ by CHE relieved allodynia and reduced the expression of CGRP and c-Fos. Activation of PKCγ by PMA aggravated allodynia and increased the expression of CGRP and c-Fos. In addition, inhibition of PKCγ reduced the phosphorylation level of GluR1; in contrast, activation of PKCγ increased the phosphorylation level of GluR1. These results suggest PKCγ-induced GluR1 phosphorylation might participate in central sensitization in a rat model of CM. We suggest that PKCγ is a potential therapeutic target for CM.

Assembly, Secretory Pathway Trafficking, and Surface Delivery of Kainate Receptors Is Regulated by Neuronal Activity

Ionotropic glutamate receptor (iGluR) trafficking and function underpin excitatory synaptic transmission and plasticity and shape neuronal networks. It is well established that the transcription, translation, and endocytosis/recycling of iGluRs are all regulated by neuronal activity, but much less is known about the activity dependence of iGluR transport through the secretory pathway. Here, we use the kainate receptor subunit GluK2 as a model iGluR cargo to show that the assembly, early secretory pathway trafficking, and surface delivery of iGluRs are all controlled by neuronal activity. We show that the delivery of de novo kainate receptors is differentially regulated by modulation of GluK2 Q/R editing, PKC phosphorylation, and PDZ ligand interactions. These findings reveal that, in addition to short-term regulation of iGluRs by recycling/endocytosis and long-term modulation by altered transcription/translation, the trafficking of iGluRs through the secretory pathway is under tight activity-dependent control to determine the numbers and properties of surface-expressed iGluRs.

Memory and synaptic plasticity are impaired by dysregulated hippocampal O-GlcNAcylation

O-GlcNAcylated proteins are abundant in the brain and are associated with neuronal functions and neurodegenerative diseases. Although several studies have reported the effects of aberrant regulation of O-GlcNAcylation on brain function, the roles of O-GlcNAcylation in synaptic function remain unclear. To understand the effect of aberrant O-GlcNAcylation on the brain, we used Oga^+/− mice which have an increased level of O-GlcNAcylation, and found that Oga^+/− mice exhibited impaired spatial learning and memory. Consistent with this result, Oga^+/− mice showed a defect in hippocampal synaptic plasticity. Oga heterozygosity causes impairment of both long-term potentiation and long-term depression due to dysregulation of AMPA receptor phosphorylation. These results demonstrate a role for hyper-O-GlcNAcylation in learning and memory.

Conventional protein kinase C in the brain: 40 years later

Protein kinase C (PKC) is a family of enzymes whose members transduce a large variety of cellular signals instigated by the receptor-mediated hydrolysis of membrane phospholipids. While PKC has been widely implicated in the pathology of diseases affecting all areas of physiology including cancer, diabetes, and heart disease-it was discovered, and initially characterized, in the brain. PKC plays a key role in controlling the balance between cell survival and cell death. Its loss of function is generally associated with cancer, whereas its enhanced activity is associated with neurodegeneration. This review presents an overview of signaling by diacylglycerol (DG)-dependent PKC isozymes in the brain, and focuses on the role of the Ca2+-sensitive conventional PKC isozymes in neurodegeneration.

Mesoscale Architecture Shapes Initiation and Richness of Spontaneous Network Activity

Spontaneous activity in the absence of external input, including propagating waves of activity, is a robust feature of neuronal networks in vivo and in vitro The neurophysiological and anatomical requirements for initiation and persistence of such activity, however, are poorly understood, as is their role in the function of neuronal networks. Computational network studies indicate that clustered connectivity may foster the generation, maintenance, and richness of spontaneous activity. Since this mesoscale architecture cannot be systematically modified in intact tissue, testing these predictions is impracticable in vivo Here, we investigate how the mesoscale structure shapes spontaneous activity in generic networks of rat cortical neurons in vitro In these networks, neurons spontaneously arrange into local clusters with high neurite density and form fasciculating long-range axons. We modified this structure by modulation of protein kinase C, an enzyme regulating neurite growth and cell migration. Inhibition of protein kinase C reduced neuronal aggregation and fasciculation of axons, i.e., promoted uniform architecture. Conversely, activation of protein kinase C promoted aggregation of neurons into clusters, local connectivity, and bundling of long-range axons. Supporting predictions from theory, clustered networks were more spontaneously active and generated diverse activity patterns. Neurons within clusters received stronger synaptic inputs and displayed increased membrane potential fluctuations. Intensified clustering promoted the initiation of synchronous bursting events but entailed incomplete network recruitment. Moderately clustered networks appear optimal for initiation and propagation of diverse patterns of activity. Our findings support a crucial role of the mesoscale architectures in the regulation of spontaneous activity dynamics.SIGNIFICANCE STATEMENT Computational studies predict richer and persisting spatiotemporal patterns of spontaneous activity in neuronal networks with neuron clustering. To test this, we created networks of varying architecture in vitro Supporting these predictions, the generation and spatiotemporal patterns of propagation were most variable in networks with intermediate clustering and lowest in uniform networks. Grid-like clustering, on the other hand, facilitated spontaneous activity but led to degenerating patterns of propagation. Neurons outside clusters had weaker synaptic input than neurons within clusters, in which increased membrane potential fluctuations facilitated the initiation of synchronized spike activity. Our results thus show that the intermediate level organization of neuronal networks strongly influences the dynamics of their activity.

Cocaine self-administration induces changes in synaptic transmission and plasticity in ventral hippocampus

Allowing rats extended access to cocaine self-administration is thought to recapitulate several key aspects of cocaine addiction in humans. Understanding the mechanisms that underlie drug-induced neuroadaptations that persist in the brain after protracted periods of abstinence is crucial towards the goal of developing therapeutic interventions for this disease state. We have employed both whole-cell voltage clamp and extracellular recording technique to assess changes in neurotransmission and long-term potentiation (LTP) in stratum radiatum of the CA1 region using the rat ventral hippocampal slice preparation. Rats allowed to self-administer cocaine daily, including 'long access' (6 hours) sessions, exhibited an increase in the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/N-methyl-d-aspartate current ratio and enhanced excitatory transmission following 3-5 weeks of abstinence. Inhibitory transmission was also significantly decreased in long-access animals, and the AMPA/N-methyl-d-aspartate ratio measured in the absence of GABAergic blockers was greatly enhanced. We also observed a significant reduction of LTP magnitude evoked in the long-access cocaine rats. These findings suggest the presence of synergistic effects of enhanced AMPA and diminished gamma-aminobutyric acid neurotransmission under physiological conditions in the CA1 region of cocaine-taking animals, supporting the conclusion that persisting enhancement of AMPA-mediated transmission and/or inhibition of gamma-aminobutyric acid-mediated transmission promoted a chronic state of potentiation that partially occluded further LTP. This increased output from the ventral hippocampus to other limbic areas would be among the drug-induced neuroadaptations that persist following abstinence from cocaine self-administration and therefore may contribute to the disease state of addiction.

Melatonin Alleviates the Epilepsy-Associated Impairments in Hippocampal LTP and Spatial Learning Through Rescue of Surface GluR2 Expression at Hippocampal CA1 Synapses

Epilepsy-associated cognitive impairment is common, and negatively impacts patients' quality of life. However, most antiepileptic drugs focus on the suppression of seizures, and fewer emphasize treatment of cognitive dysfunction. Melatonin, an indolamine synthesized primarily in the pineal grand, is reported to be neuroprotective against several central nervous system disorders. In this study, we investigated whether melatonin could reverse cognitive dysfunction in lithium-pilocarpine treated rats. Chronic treatment with melatonin (8 mg/kg daily for 15 days) after induction of status epilepticus significantly alleviated seizure severity, reduced neuronal death in the CA1 region of the hippocampus, improved spatial learning (as measured by the Morris water maze test), and reversed LTP impairments, compared to vehicle treatment. Furthermore, we found that melatonin rescued the decreased surface levels of GluR2 in the CA1 region observed in epilepsy, which might be the underlying mechanism of the neuroprotective and synapse-modulating function of melatonin. Our study provides experimental evidence for the possible clinical utility of melatonin as an adjunctive therapy to prevent epilepsy-associated cognitive impairments.

Long-Term Potentiation: From CaMKII to AMPA Receptor Trafficking

For more than 20 years, we have known that Ca(2+)/calmodulin-dependent protein kinase (CaMKII) activation is both necessary and sufficient for the induction of long-term potentiation (LTP). During this time, tremendous effort has been spent in attempting to understand how CaMKII activation gives rise to this phenomenon. Despite such efforts, there is much to be learned about the molecular mechanisms involved in LTP induction downstream of CaMKII activation. In this review, we highlight recent developments that have shaped our current thinking about the molecular mechanisms underlying LTP and discuss important questions that remain in the field.

A critical role for VEGF and VEGFR2 in NMDA receptor synaptic function and fear-related behavior

Vascular endothelial growth factor (VEGF) is known to be required for the action of antidepressant therapies but its impact on brain synaptic function is poorly characterized. Using a combination of electrophysiological, single-molecule imaging and conditional transgenic approaches, we identified the molecular basis of the VEGF effect on synaptic transmission and plasticity. VEGF increases the postsynaptic responses mediated by the N-methyl-D-aspartate type of glutamate receptors (GluNRs) in hippocampal neurons. This is concurrent with the formation of new synapses and with the synaptic recruitment of GluNR expressing the GluN2B subunit (GluNR-2B). VEGF induces a rapid redistribution of GluNR-2B at synaptic sites by increasing the surface dynamics of these receptors within the membrane. Consistently, silencing the expression of the VEGF receptor 2 (VEGFR2) in neural cells impairs hippocampal-dependent synaptic plasticity and consolidation of emotional memory. These findings demonstrated the direct implication of VEGF signaling in neurons via VEGFR2 in proper synaptic function. They highlight the potential of VEGF as a key regulator of GluNR synaptic function and suggest a role for VEGF in new therapeutic approaches targeting GluNR in depression.

CAMK2γ antagonizes mTORC1 activation during hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is one of the most deadly cancers that still lacks effective treatments. Dysregulation of kinase signaling has frequently been reported to contribute to HCC. In this study, we used bioinformatic approaches to identify kinases that regulate gene expression changes in human HCCs and two murine HCC models. We identified a role for calcium/calmodulin-dependent protein kinases II gamma isoform (CAMK2γ) in hepatocarcinogenesis. CAMK2γ^-/- mice displayed severely enhanced chemical-induced hepatocarcinogenesis compared with wild-type controls. Mechanistically, CAMK2γ deletion potentiates hepatic activation of mechanistic target of rapamycin complex 1 (mTORC1), which results in hyperproliferation of hepatocytes. Inhibition of mTORC1 by rapamycin effectively attenuates the compensatory proliferation of hepatocytes in CAMK2γ^-/- livers. We further demonstrated that CAMK2γ suppressed growth factor- or insulin-induced mTORC1 activation by inhibiting IRS1/AKT signaling. Taken together, our results reveal a novel mechanism by which CAMK2γ antagonizes mTORC1 activation during hepatocarcinogenesis.

Modification of Hypoxic Respiratory Response by Protein Tyrosine Kinase in Brainstem Ventral Respiratory Neuron Group

Protein tyrosine kinase (PTK) mediated the tyrosine phosphorylation modification of neuronal receptors and ion channels. Whether such modification resulted in changes of physiological functions was not sufficiently studied. In this study we examined whether the hypoxic respiratory response—which is the enhancement of breathing in hypoxic environment could be affected by the inhibition of PTK at brainstem ventral respiratory neuron column (VRC). Experiments were performed on urethane anesthetized adult rabbits. Phrenic nerve discharge was recorded as the central respiratory motor output. Hypoxic respiratory response was produced by ventilating the rabbit with 10% O2-balance 90% N2 for 5 minutes. The responses of phrenic nerve discharge to hypoxia were observed before and after microinjecting PTK inhibitor genistein, AMPA receptor antagonist CNQX, or inactive PTK inhibitor analogue daidzein at the region of ambiguus nucleus (NA) at levels 0–2 mm rostral to obex where the inspiratory subgroup of VRC were recorded. Results were as follows: 1. the hypoxic respiratory response was significantly attenuated after microinjection of genistein and/or CNQX, and no additive effect (i.e., further attenuation of hypoxic respiratory response) was observed when genistein and CNQX were microinjected one after another at the same injection site. Microinjection of daidzein had no effect on hypoxic respiratory response. 2. Fluorescent immunostaining showed that hypoxia significantly increased the number of phosphotyrosine immunopositive neurons in areas surrounding NA and most of these neurons were also immunopositive to glutamate AMPA receptor subunit GluR1. These results suggested that PTK played an important role in regulating the hypoxic respiratory response, possibly through the tyrosine phosphorylation modification of glutamate AMPA receptors on the respiratory neurons of ventral respiratory neuron column.

Disrupting GluA2 phosphorylation potentiates reinstatement of cocaine seeking

Addiction is associated with changes in synaptic plasticity mediated, in part, by alterations in the trafficking and stabilization of AMPA receptors at synapses within the nucleus accumbens. Exposure to cocaine can lead to protein kinase C-mediated phosphorylation of GluA2 AMPA subunits and this phosphorylation event leads to the internalization of GluA2-containing AMPARs, which are calcium-impermeable. However, it is not clear whether this internalization is necessary for the expression of addictive phenotypes. Utilizing a mouse with a point mutation within the GluA2 subunit c-terminus, the current study demonstrates that disrupting PKC-mediated GluA2 phosphorylation potentiates reinstatement of both cue-induced cocaine seeking and cocaine conditioned reward without affecting operant learning, food self-administration or cocaine sensitization. Electrophysiological recordings revealed increased GluA2-mediated AMPA transmission as evidenced by increased sEPSC amplitude without any changes in sEPSC frequency or rectification. In support of this increase in GluA2 activity mediating the augmented cocaine reinstatement, we found that accumbal overexpression of GluA2 recapitulated this behavioral effect in wildtype mice while not altering reinstatement behavior in the GluA2 K882A knock-in mice. In addition, disrupting GluA2 phosphorylation was associated with blunted long-term depression in the nucleus accumbens, mimicking the anaplasticity seen following cocaine self-administration. Taken together these results indicate that disrupting GluA2 phosphorylation and increasing GluA2-mediated transmission in the nucleus accumbens leads to increased vulnerability to cocaine relapse. Further, these results indicate that modulating GluA2-containing AMPAR trafficking can contribute to addictive phenotypes in the absence of alterations in GluA2-lacking receptors. These results highlight the GluA2 phosphorylation site as a novel target for the development of cocaine addiction therapeutics.

Gq Protein-Coupled Membrane-Initiated Estrogen Signaling Rapidly Excites Corticotropin-Releasing Hormone Neurons in the Hypothalamic Paraventricular Nucleus in Female Mice

CRH neurons in the hypothalamic paraventricular nucleus (PVN) play a central role in regulating the hypothalamus-pituitary-adrenal (HPA) axis and are directly influenced by 17β-estradiol (E2). Although compelling evidence has suggested the existence of membrane-associated estrogen receptors (mERs) in hypothalamic and other central nervous system neurons, it remains unknown whether E2 impacts CRH neuronal excitability through this mechanism. The purpose of the current study is to examine the existence and function of mER signaling in PVN CRH neurons. Whole-cell recordings were made from CRH neurons identified by Alexa Fluor 594 labeling and post hoc immunostaining in ovariectomized female mice. E2 (100nM) rapidly suppressed the M-current (a voltage-dependent K(+) current) and potentiated glutamatergic excitatory postsynaptic currents. The putative Gq-coupled mER (Gq-mER) characterized in hypothalamic proopiomelanocortin neurons initiates a phospholipase C-protein kinase C-protein kinase A pathway; therefore, we examined the involvement of this pathway using selective inhibitors. Indeed, the ER antagonist ICI 182780 and inhibitors of Gq-phospholipase C-protein kinase C-protein kinase A blocked E2's actions, suggesting dependence on the Gq-mER. Furthermore, STX, a selective ligand for the Gq-mER, mimicked E2's actions. Finally, to examine the in vivo effect of Gq-mER activation, E2 or STX injection increased c-fos expression in CRH neurons in the PVN, suggesting CRH neuronal activation. This corresponded to an increase in plasma corticosterone. We conclude that the Gq-mER plays a critical role in the rapid regulation of CRH neuronal activity and the HPA axis. Our findings provide a potential underlying mechanism for E2's involvement in the pathophysiology of HPA-associated mood disorders.

Synaptic GluN2B/CaMKII-α Signaling Induces Synapto-Nuclear Transport of ERK and Jacob

A central pathway in synaptic plasticity couples N-Methyl-D-Aspartate-receptor (NMDAR)-signaling to the activation of extracellular signal-regulated kinases (ERKs) cascade. ERK-dependency has been demonstrated for several forms of synaptic plasticity as well as learning and memory and includes local synaptic processes but also long-distance signaling to the nucleus. It is, however, controversial how NMDAR signals are connected to ERK activation in dendritic spines and nuclear import of ERK. The synapto-nuclear messenger Jacob couples NMDAR-dependent Ca²⁺-signaling to CREB-mediated gene expression. Protein transport of Jacob from synapse to nucleus essentially requires activation of GluN2B-containing NMDARs. Subsequent phosphorylation and binding of ERK1/2 to and ERK-dependent phosphorylation of serine 180 in Jacob encodes synaptic but not extrasynaptic NMDAR activation. In this study we show that stimulation of synaptic NMDAR in hippocampal primary neurons and induction of long-term potentiation (LTP) in acute slices results in GluN2B-dependent activation of CaMKII-α and subsequent nuclear import of active ERK and serine 180 phosphorylated Jacob. On the contrary, no evidence was found that either GluN2A-containing NMDAR or RasGRF2 are upstream of ERK activation and nuclear import of Jacob and ERK.

Sound tuning of amygdala plasticity in auditory fear conditioning

Various auditory tones have been used as conditioned stimuli (CS) for fear conditioning, but researchers have largely neglected the effect that different types of auditory tones may have on fear memory processing. Here, we report that at lateral amygdala (LA) synapses (a storage site for fear memory), conditioning with different types of auditory CSs (2.8 kHz tone, white noise, FM tone) recruits distinct forms of long-term potentiation (LTP) and inserts calcium permeable AMPA receptor (CP-AMPAR) for variable periods. White noise or FM tone conditioning produced brief insertion (<6 hr after conditioning) of CP-AMPARs, whereas 2.8 kHz tone conditioning induced more persistent insertion (≥6 hr). Consistently, conditioned fear to 2.8 kHz tone but not to white noise or FM tones was erased by reconsolidation-update (which depends on the insertion of CP-AMPARs at LA synapses) when it was performed 6 hr after conditioning. Our data suggest that conditioning with different auditory CSs recruits distinct forms of LA synaptic plasticity, resulting in more malleable fear memory to some tones than to others.

Extensive phosphorylation of AMPA receptors in neurons

Regulation of AMPA receptor (AMPAR) function is a fundamental mechanism controlling synaptic strength during long-term potentiation/depression and homeostatic scaling. AMPAR function and membrane trafficking is controlled by protein-protein interactions, as well as by posttranslational modifications. Phosphorylation of the GluA1 AMPAR subunit at S845 and S831 play especially important roles during synaptic plasticity. Recent controversy has emerged regarding the extent to which GluA1 phosphorylation may contribute to synaptic plasticity. Here we used a variety of methods to measure the population of phosphorylated GluA1-containing AMPARs in cultured primary neurons and mouse forebrain. Phosphorylated GluA1 represents large fractions from 12% to 50% of the total population under basal and stimulated conditions in vitro and in vivo. Furthermore, a large fraction of synapses are positive for phospho-GluA1-containing AMPARs. Our results support the large body of research indicating a prominent role of GluA1 phosphorylation in synaptic plasticity.

ApoE4 expression accelerates hippocampus-dependent cognitive deficits by enhancing Aβ impairment of insulin signaling in an Alzheimer's disease mouse model

The apolipoprotein E4 (ApoE4) is the strongest genetic risk factor for Alzheimer's disease (AD). The AD brain was shown to be insulin resistant at end stage, but the interplay between insulin signaling, ApoE4 and Aβ across time, and their involvement in memory decline is unclear. To investigate insulin response in the ageing mouse hippocampus, we crossed the human ApoE-targeted replacement mice with the mutant human amyloid precursor protein (APP) mice (ApoExAPP). While hippocampal Aβ levels were comparable between ApoE3xAPP and ApoE4xAPP mice at 26 weeks, insulin response was impaired in the ApoE4xAPP hippocampus. Insulin treatment was only able to stimulate insulin signaling and increased AMPA-GluR1 phosphorylation in forskolin pre-treated hippocampal slices from ApoE3xAPP mice. In ApoE4xAPP mice, insulin dysfunction was also associated with poorer spatial memory performance. Using dissociated hippocampal neuron in vitro, we showed that insulin response in ApoE3 and ApoE4 neurons increased AMPA receptor-mediated miniature excitatory postsynaptic current (mEPSC) amplitudes and GluR1-subunit insertion. Pre-treatment of ApoE3 neurons with Aβ42 did not affect insulin-mediated GluR1 subunit insertion. However, impaired insulin sensitivity observed only in the presence of ApoE4 and Aβ42, attenuated GluR1-subunit insertion. Taken together, our results suggest that ApoE4 enhances Aβ inhibition of insulin-stimulated AMPA receptor function, which accelerates memory impairment in ApoE4xAPP mice.

Molecular mechanisms controlling synaptic recruitment of GluA4 subunit-containing AMPA-receptors critical for functional maturation of CA1 glutamatergic synapses

Synaptic recruitment of AMPA receptors (AMPARs) represents a key postsynaptic mechanism driving functional development and maturation of glutamatergic synapses. At immature hippocampal synapses, PKA-driven synaptic insertion of GluA4 is the predominant mechanism for synaptic reinforcement. However, the physiological significance and molecular determinants of this developmentally restricted form of plasticity are not known. Here we show that PKA activation leads to insertion of GluA4 to synaptic sites with initially weak or silent AMPAR-mediated transmission. This effect depends on a novel mechanism involving the extreme C-terminal end of GluA4, which interacts with the membrane proximal region of the C-terminal domain to control GluA4 trafficking. In the absence of GluA4, strengthening of AMPAR-mediated transmission during postnatal development was significantly delayed. These data suggest that the GluA4-mediated activation of silent synapses is a critical mechanism facilitating the functional maturation of glutamatergic circuitry during the critical period of experience-dependent fine-tuning. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

ERK-GluR1 phosphorylation in trigeminal spinal subnucleus caudalis neurons is involved in pain associated with dry tongue

BACKGROUND

Dry mouth is known to cause severe pain in the intraoral structures, and many dry mouth patients have been suffering from intraoral pain. In development of an appropriate treatment, it is crucial to study the mechanisms underlying intraoral pain associated with dry mouth, yet the detailed mechanisms are not fully understood. To evaluate the mechanisms underlying pain related to dry mouth, the dry-tongue rat model was developed. Hence, the mechanical or heat nocifensive reflex, the phosphorylated extracellular signal-regulated kinase and phosphorylated GluR1-IR immunohistochemistries, and the single neuronal activity were examined in the trigeminal spinal subnucleus caudalis of dry-tongue rats.

RESULTS

The head-withdrawal reflex threshold to mechanical, but not heat, stimulation of the tongue was significantly decreased on day 7 after tongue drying. The mechanical, but not heat, responses of trigeminal spinal subnucleus caudalis nociceptive neurons were significantly enhanced in dry-tongue rats compared to sham rats on day 7. The number of phosphorylated extracellular signal-regulated kinase-immunoreactive cells was also significantly increased in the trigeminal spinal subnucleus caudalis following noxious stimulation of the tongue in dry-tongue rats compared to sham rats on day 7. The decrement of the mechanical head-withdrawal reflex threshold (HWT) was reversed during intracisternal administration of the mitogen-activated protein kinase kinase 1 inhibitor, PD98059. The trigeminal spinal subnucleus caudalis neuronal activities and the number of phosphorylated extracellular signal-regulated kinase-immunoreactive cells following noxious mechanical stimulation of dried tongue were also significantly decreased following intracisternal administration of PD98059 compared to vehicle-administrated rats. Increased number of the phosphorylated GluR1-IR cells was observed in the trigeminal spinal subnucleus caudalis of dry-tongue rats, and the number of phosphorylated GluR1-IR cells was significantly reduced in PD98059-administrated rats compared to the vehicle-administrated tongue-dry rats.

CONCLUSIONS

These findings suggest that the pERK-pGluR1 cascade is involved in central sensitization of trigeminal spinal subnucleus caudalis nociceptive neurons, thus resulting in tongue mechanical hyperalgesia associated with tongue drying.