NMDA receptors mediate calcium-dependent, bidirectional changes in dendritic PICK1 clustering

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Memantine as a neuroprotective agent in ischemic stroke: Preclinical and clinical analysis

The primary mechanism for neuron death after an ischemic stroke is excitotoxic injury. Excessive depolarization leads to NMDA-mediated calcium entry to the neuron and, subsequently, cellular death. Therefore, the inhibition of the NMDA channel has been proposed as a neuroprotective measure in ischemic stroke. The high morbimortality associated with stroke warrants new therapies that can improve the functional prognosis of patients. Memantine is a non-competitive NMDA receptor antagonist which has gained attention as a potential drug for ischemic stroke. Here we analyze the available preclinical and clinical evidence concerning the use of memantine following an ischemic stroke. Preclinical evidence shows inhibition of the excitotoxic cascade, as well as improved outcomes in terms of motor and sensory function with the use of memantine. The available clinical trials of high-dose memantine in patients poststroke have found that it can improve patients' NIHSS and Barthel index and help patients with poststroke aphasia and intracranial hemorrhage. These results suggest that memantine has a clinically relevant neuroprotective effect; however, small sample sizes and other study shortcomings limit the impact of these findings. Even so, current studies show promising results that should serve as a basis to promote future research to conclusively determine if memantine does improve the outcomes of patients' post-ischemic stroke. We anticipate that future trials will fill current gaps in knowledge, and these latter results will broaden the therapeutic arsenal for clinicians looking to improve the prognosis of patients poststroke.

Kainate and AMPA receptors in epilepsy: Cell biology, signalling pathways and possible crosstalk

Epilepsy is caused when rhythmic neuronal network activity escapes normal control mechanisms, resulting in seizures. There is an extensive and growing body of evidence that the onset and maintenance of epilepsy involves alterations in the trafficking, synaptic surface expression and signalling of kainate and AMPA receptors (KARs and AMPARs). The KAR subunit GluK2 and AMPAR subunit GluA2 are key determinants of the properties of their respective assembled receptors. Both subunits are subject to extensive protein interactions, RNA editing and post-translational modifications. In this review we focus on the cell biology of GluK2-containing KARs and GluA2-containing AMPARs and outline how their regulation and dysregulation is implicated in, and affected by, seizure activity. Further, we discuss role of KARs in regulating AMPAR surface expression and plasticity, and the relevance of this to epilepsy. This article is part of the special issue on 'Glutamate Receptors - Kainate receptors'.

AMPA receptors and their minions: auxiliary proteins in AMPA receptor trafficking

To correctly transfer information, neuronal networks need to continuously adjust their synaptic strength to extrinsic stimuli. This ability, termed synaptic plasticity, is at the heart of their function and is, thus, tightly regulated. In glutamatergic neurons, synaptic strength is controlled by the number and function of AMPA receptors at the postsynapse, which mediate most of the fast excitatory transmission in the central nervous system. Their trafficking to, at, and from the synapse, is, therefore, a key mechanism underlying synaptic plasticity. Intensive research over the last 20 years has revealed the increasing importance of interacting proteins, which accompany AMPA receptors throughout their lifetime and help to refine the temporal and spatial modulation of their trafficking and function. In this review, we discuss the current knowledge about the roles of key partners in regulating AMPA receptor trafficking and focus especially on the movement between the intracellular, extrasynaptic, and synaptic pools. We examine their involvement not only in basal synaptic function, but also in Hebbian and homeostatic plasticity. Included in our review are well-established AMPA receptor interactants such as GRIP1 and PICK1, the classical auxiliary subunits TARP and CNIH, and the newest additions to AMPA receptor native complexes.

Endocytosis and lysosomal degradation of GluA2/3 AMPARs in response to oxygen/glucose deprivation in hippocampal but not cortical neurons

Global cerebral ischemia results in oxygen and glucose deprivation (OGD) and consequent delayed cell death of vulnerable neurons, with hippocampal CA1 neurons more vulnerable than cortical neurons. Most AMPA receptors (AMPARs) are heteromeric complexes of subunits GluA1/GluA2 or GluA2/GluA3, and the presence of GluA2 renders AMPARs Ca²⁺-impermeable. In hippocampal CA1 neurons, OGD causes the synaptic expression of GluA2-lacking Ca²⁺-permeable AMPARs, contributing to toxic Ca²⁺ influx. The loss of synaptic GluA2 is caused by rapid trafficking of GluA2-containing AMPARs from the cell surface, followed by a delayed reduction in GluA2 mRNA expression. We show here that OGD causes endocytosis, lysosomal targeting and consequent degradation of GluA2- and GluA3-containing AMPARs, and that PICK1 is required for both OGD-induced GluA2 endocytosis and lysosomal sorting. Our results further suggest that GluA1-containing AMPARs resist OGD-induced endocytosis. OGD does not cause GluA2 endocytosis in cortical neurons, and we show that PICK1 binding to the endocytic adaptor AP2 is enhanced by OGD in hippocampal, but not cortical neurons. We propose that endocytosis of GluA2/3, caused by a hippocampal-specific increase in PICK1-AP2 interactions, followed by PICK1-dependent lysosomal targeting, are critical events in determining changes in AMPAR subunit composition in the response to ischaemia.

PICK1 regulates AMPA receptor endocytosis via direct interactions with AP2 α-appendage and dynamin

Clathrin-mediated endocytosis (CME) is used to internalize a diverse range of cargo proteins from the cell surface, often in response to specific signals. In neurons, the rapid endocytosis of GluA2-containing AMPA receptors (AMPARs) in response to NMDA receptor (NMDAR) stimulation causes a reduction in synaptic strength and is the central mechanism for long-term depression, which underlies certain forms of learning. The mechanisms that link NMDAR activation to CME of AMPARs remain elusive. PICK1 is a BAR domain protein required for NMDAR-dependent reductions in surface GluA2; however, the molecular mechanisms involved are unclear. In this study, we show that PICK1 makes direct, NMDAR-dependent interactions with the core endocytic proteins AP2 and dynamin. PICK1-AP2 interactions are required for clustering AMPARs at endocytic zones in dendrites in response to NMDAR stimulation and for consequent AMPAR internalization. We further show that PICK1 stimulates dynamin polymerization. We propose that PICK1 is a cargo-specific endocytic accessory protein required for efficient, activity-dependent AMPAR endocytosis.

Protein kinase C regulates AMPA receptor auxiliary protein Shisa9/CKAMP44 through interactions with neuronal scaffold PICK1

Synaptic α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole‐propionate (AMPA) receptors are essential mediators of neurotransmission in the central nervous system. Shisa9/cysteine‐knot AMPAR modulating protein 44 (CKAMP44) is a transmembrane protein recently found to be present in AMPA receptor‐associated protein complexes. Here, we show that the cytosolic tail of Shisa9/CKAMP44 interacts with multiple scaffold proteins that are important for regulating synaptic plasticity in central neurons. We focussed on the interaction with the scaffold protein PICK1, which facilitates the formation of a tripartite complex with the protein kinase C (PKC) and thereby regulates phosphorylation of Shisa9/CKAMP44 C‐terminal residues. This work has implications for our understanding of how PICK1 modulates AMPAR‐mediated transmission and plasticity and also highlights a novel function of PKC.

The PICK1 Ca2+ sensor modulates N-methyl-d-aspartate (NMDA) receptor-dependent microRNA-mediated translational repression in neurons

MicroRNAs (miRNAs) are important regulators of localized mRNA translation in neuronal dendrites. The presence of RNA-induced silencing complex proteins in these compartments and the dynamic miRNA expression changes that occur in response to neuronal stimulation highlight their importance in synaptic plasticity. Previously, we demonstrated a novel interaction between the major RNA-induced silencing complex component Argounaute-2 (Ago2) and the BAR (bin/amphiphysin/rvs) domain protein PICK1. PICK1 recruits Ago2 to recycling endosomes in dendrites, where it inhibits miRNA-mediated translational repression. Chemical induction of long-term depression via NMDA receptor activation causes the dissociation of Ago2 from PICK1 and a consequent increase in dendritic miRNA-mediated gene silencing. The mechanism that underlies the regulation of PICK1-Ago2 binding is unknown. In this study, we demonstrate that the PICK1-Ago2 interaction is directly sensitive to Ca²⁺ ions so that high [Ca²⁺]_free reduces PICK1 binding to Ago2. Mutating a stretch of C-terminal Ca²⁺-binding residues in PICK1 results in a complete block of NMDA-induced PICK1-Ago2 disassociation in cortical neurons. Furthermore, the same mutant also blocks NMDA-stimulated miRNA-mediated gene silencing. This study defines a novel mechanism whereby elevated [Ca²⁺] induced by NMDA receptor activation modulates Ago2 and miRNA activity via PICK1. Our work suggests a Ca²⁺-dependent process to regulate miRNA activity in neurons in response to the induction of long-term depression.

PICK1 mediates synaptic recruitment of AMPA receptors at neurexin-induced postsynaptic sites

In the CNS, synapse formation and maturation play crucial roles in the construction and consolidation of neuronal circuits. Neurexin and neuroligin localize on the opposite sides of synaptic membrane and interact with each other to promote the assembly and specialization of synapses. However, the excitatory synapses induced by the neurexin-neuroligin complex are initially immature synapses that lack AMPA receptors. Previously, PICK1 (protein interacting with C kinase 1) was shown to cluster and regulate the synaptic localization of AMPA receptors. Here, we report that during synaptogenesis induced by neurexin in cultured neurons from rat hippocampus, PICK1 recruited AMPA receptors to immature postsynaptic sites. This synaptic recruitment of AMPA receptors depended on the interaction between GluA2 and PICK1, and on the lipid-binding ability of PICK1, but not the interaction between PICK1 and neuroligin. Last, our results demonstrated that the recruitment of GluA2 to synapses could be prevented by ICA69 (islet cell autoantigen 69 kDa), a key binding partner of PICK1. Our study showed that PICK1, being negatively regulated by ICA69, could facilitate synapse maturation.

PICK1 links Argonaute 2 to endosomes in neuronal dendrites and regulates miRNA activity

MicroRNAs fine-tune gene expression by inhibiting the translation of mRNA targets. Argonaute (Ago) proteins are critical mediators of microRNA-induced post-transcriptional silencing and have been shown to associate with endosomal compartments, but the molecular mechanisms that underlie this process are unclear, especially in neurons. Here, we report a novel interaction between Ago2 and the BAR-domain protein, PICK1. We show that PICK1 promotes Ago2 localization at endosomal compartments in neuronal dendrites and inhibits Ago2 function in translational repression following neuronal stimulation. We propose that PICK1 provides a link between activity-dependent endosomal trafficking and local regulation of translation in neurons.

The small GTPase Arf1 modulates Arp2/3-mediated actin polymerization via PICK1 to regulate synaptic plasticity

Inhibition of Arp2/3-mediated actin polymerization by PICK1 is a central mechanism to AMPA receptor (AMPAR) internalization and long-term depression (LTD), although the signaling pathways that modulate this process in response to NMDA receptor (NMDAR) activation are unknown. Here, we define a function for the GTPase Arf1 in this process. We show that Arf1-GTP binds PICK1 to limit PICK1-mediated inhibition of Arp2/3 activity. Expression of mutant Arf1 that does not bind PICK1 leads to reduced surface levels of GluA2-containing AMPARs and smaller spines in hippocampal neurons, which occludes subsequent NMDA-induced AMPAR internalization and spine shrinkage. In organotypic slices, NMDAR-dependent LTD of AMPAR excitatory postsynaptic currents is abolished in neurons expressing mutant Arf1. Furthermore, NMDAR stimulation downregulates Arf1 activation and binding to PICK1 via the Arf-GAP GIT1. This study defines Arf1 as a critical regulator of actin dynamics and synaptic function via modulation of PICK1.

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The Arf1-PICK1-Arp2/3 pathway regulates actin polymerization

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NMDAR activation activates the Arf-GAP GIT1 to deactivate Arf1

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Arf1 controls NMDAR-dependent, PICK1-mediated AMPAR trafficking and LTD

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A noncanonical role is described for Arf1 in vesicle traffic, distinct from COPI regulation

AMPA receptor trafficking and the mechanisms underlying synaptic plasticity and cognitive aging

Even in healthy individuals there is an inexorable agerelated decline in cognitive function. This is due, in large part, to reduced synaptic plasticity caused by changes in the molecular composition of the postsynaptic membrane. AMPA receptors (AMPARs) are glutamate-gated cation channels that mediate the overwhelming majority of fast excitatory transmission in the brain. Changes in AMPAR number and/or function are a core feature of synaptic plasticity and age-related cognitive decline, AMPARs are highly dynamic proteins that are subject to highly controlled trafficking, recycling, and/or degradation and replacement. This active regulation of AMPAR synthesis, targeting, synaptic dwell time, and degradation is fundamentally important for memory formation and storage. Further, aberrant AMPAR trafficking and consequent detrimental changes in synapses are strongly implicated in many brain diseases, which represent a vast social and economic burden. The purpose of this article is to provide an overview of the molecular and cellular AMPA receptor trafficking events that control synaptic responsiveness and plasticity, and highlight what is known currently known about how these processes change with age and disease.

Molecular mechanisms underlying activity-dependent AMPA receptor cycling in retinal ganglion cells

On retinal ganglion cells (RGCs) transmit light encoded information to the brain and receive excitatory input from On cone bipolar cells (CBPs). The synaptic CBP input onto On RGCs is mediated by AMPA-type glutamate receptors (AMPARs) that include both those lacking a GluA2 subunit, and are therefore permeable to Ca(2+), and those that possess at least one GluA2 subunit and are Ca(2+)-impermeable. We have previously demonstrated in electrophysiological studies that periods of low synaptic activity, brought about by housing animals in darkness, enhance the proportion of GluA2-lacking AMPARs at the On CBP-On RGC synapse by mobilizing surface GluA2 containing receptors into a receptor pool that rapidly cycles in and out of the membrane. AMPAR cycling induction by reduced synaptic activity takes several hours. This delay suggests that changes in expression of proteins which regulate AMPAR trafficking may mediate the altered mobility of GluA2 AMPARs in RGCs. In this study, we test the hypothesis that AMPAR trafficking proteins couple synaptic activity to AMPAR cycling in RGCs. Immunocytochemical and biochemical analyses confirmed that darkness decreases surface GluA2 in RGCs and changed the expression levels of three proteins associated with GluA2 trafficking. GRIP was decreased, while PICK1 and Arc were increased. Knockdown of GRIP with siRNA elevated constitutive AMPAR cycling, mimicking effects of reduced synaptic activity, while knockdown of PICK1 and Arc blocked increases in constitutive GluA2 trafficking. Our results support a role for correlated, activity-driven changes in multiple AMPAR trafficking proteins that modulate GluA2 cycling which can in turn affect synaptic AMPAR composition in RGCs.

PICK1 mediates transient synaptic expression of GluA2-lacking AMPA receptors during glycine-induced AMPA receptor trafficking

The number and subunit composition of postsynaptic AMPA receptors (AMPARs) is a key determinant of synaptic transmission. The vast majority of AMPARs contain GluA2 subunit, which renders the channel impermeable to calcium. However, a small proportion are GluA2 lacking and therefore calcium permeable (CP-AMPARs). It has been proposed recently that long-term potentiation (LTP) involves not only an increase in the total number of AMPARs at the synapse but also a transient switch to CP-AMPARs in the first few minutes after LTP induction. The molecular mechanisms that underlie this switch to CP-AMPARs and the subsequent switch back to calcium-impermeable AMPARs are unknown. Here, we show that endogenous GluA1 is rapidly inserted at the synaptic plasma membrane of rat hippocampal neurons immediately after stimulation with elevated glycine, a treatment known to induce LTP. In contrast, GluA2 is restricted from trafficking to the cell surface by a glycine-induced increase in PICK1-GluA2 binding on endosomal compartments. Between 5 and 20 min after stimulus, activation of CP-AMPARs triggers a release of GluA2 from PICK1, allowing GluA2-containing AMPARs to traffic to the synaptic plasma membrane. These results define a PICK1-dependent mechanism that underlies transient alterations in the subunit composition and calcium permeability of synaptic AMPARs that is important during the early phase after stimulation with glycine and therefore is likely to be important during the expression of LTP.

Protein interacting with C kinase 1 (PICK1) reduces reinsertion rates of interaction partners sorted to Rab11-dependent slow recycling pathway

The scaffolding protein PICK1 (protein interacting with C kinase 1) contains an N-terminal PSD-95/Discs large/ZO-1 (PDZ) domain and a central lipid-binding Bin/amphiphysin/Rvs (BAR) domain. PICK1 is thought to regulate trafficking of its PDZ binding partners but different and even opposing functions have been suggested. Here, we apply ELISA-based assays and confocal microscopy in HEK293 cells with inducible PICK1 expression to assess in an isolated system the ability of PICK1 to regulate trafficking of natural and engineered PDZ binding partners. The dopamine transporter (DAT), which primarily sorts to degradation upon internalization, did not form perinuclear clusters with PICK1, and PICK1 did not affect DAT internalization/recycling. However, transfer of the PICK1-binding DAT C terminus to the β(2)-adrenergic receptor, which sorts to recycling upon internalization, led to formation of PICK1 co-clusters in Rab11-positive compartments. Furthermore, PICK1 inhibited Rab11-mediated recycling of the receptor in a BAR and PDZ domain-dependent manner. In contrast, transfer of the DAT C terminus to the δ-opioid receptor, which sorts to degradation, did not result in PICK1 co-clusters or any change in internalization/recycling. Further support for a role of PICK1 determined by its PDZ cargo was obtained for the PICK1 interaction partner prolactin-releasing peptide receptor (GPR10). GPR10 co-localized with Rab11 and clustered with PICK1 upon constitutive internalization but co-localized with the late endosomal marker Rab7 and did not cluster with PICK1 upon agonist-induced internalization. Our data suggest a selective role of PICK1 in clustering and reducing the recycling rates of PDZ domain binding partners sorted to the Rab11-dependent recycling pathway.

Regulation of AMPA receptor trafficking and synaptic plasticity

AMPA receptors (AMPARs) mediate the majority of fast excitatory synaptic transmission in the brain. Dynamic changes in neuronal synaptic efficacy, termed synaptic plasticity, are thought to underlie information coding and storage in learning and memory. One major mechanism that regulates synaptic strength involves the tightly regulated trafficking of AMPARs into and out of synapses. The life cycle of AMPARs from their biosynthesis, membrane trafficking, and synaptic targeting to their degradation are controlled by a series of orchestrated interactions with numerous intracellular regulatory proteins. Here we review recent progress made toward the understanding the regulation of AMPAR trafficking, focusing on the roles of several key intracellular AMPAR interacting proteins.

New insights in endosomal dynamics and AMPA receptor trafficking

The trafficking mechanisms that control the density of synaptic AMPA-type glutamate receptors have received significant attention because of their importance for regulating excitatory synaptic transmission and synaptic plasticity in the hippocampus. AMPA receptors are synthesized in the neuronal cell body and reach their postsynaptic targets after a complex journey involving multiple transport steps along different cytoskeleton structures and through various stages of the endocytic pathway. Dendritic spines are important sites for AMPA receptor trafficking and contain the basic components of endosomal recycling. On induction of synaptic plasticity, internalized AMPA receptors undergo endosomal sorting and cycle through early endosomes and recycling endosomes back to the plasma membrane (model for long-term potentiation) or target for degradation to the lysosomes (model for long-term depression). Exciting new studies now provide insight in actin-mediated processes that controls endosomal tubule formation and receptor sorting. This review describes the path of AMPA receptor internalization up to sites of recycling and summarizes recent studies on actin-mediated endosomal receptor sorting.

mGluR and NMDAR activation internalize distinct populations of AMPARs

Activation of metabotropic- (mGluRs) or NMDA-type glutamate receptors (NMDARs) each can induce long-term depression (LTD) of synaptic transmission in CA1 hippocampal neurons. These two forms of LTD are triggered by diverse signaling pathways yet both are expressed by the internalization of AMPA-type glutamate receptors (AMPARs). An unanswered question remains as to whether the convergence of the mGluR and NMDAR signaling pathways on AMPAR endocytosis renders these two forms of plasticity functionally equivalent, with both pathways inducing endocytosis of the same population of synaptic AMPARs. We now report evidence that these pathways couple to the endocytosis of distinct populations of AMPARs defined by their mobility in the membrane surface. NMDAR activation enhances removal of surface AMPARs that rapidly cycle into and out of the membrane surface, while activation of mGluRs with DHPG results in the internalization of a non-mobile population of AMPARs. Glutamate Receptor Interacting Proteins 1 and 2 (GRIP1/2) play a key role in defining the non-cycling receptor population. GRIP1/2 knockdown with siRNA increases the proportion of rapidly cycling surface AMPARs and inhibits mGluR- but not NMDAR-mediated AMPAR internalization. Additionally, we find that mGluR activation dissociates surface AMPARs from GRIP1/2 while stimulation of NMDARs elicits the loss of membrane receptors not bound to GRIP1/2. We propose that these two receptor pathways can drive the endocytosis of distinct populations of AMPARs: NMDARs activation induces the endocytosis of rapidly cycling surface AMPARs not directly associated with GRIP1/2 while mGluR activation induces the endocytosis of non-cycling GRIP-bound surface AMPARs.

Calcium binding to PICK1 is essential for the intracellular retention of AMPA receptors underlying long-term depression

NMDA receptor (NMDAR)-dependent long-term depression (LTD) in the hippocampus is mediated primarily by the calcium-dependent removal of AMPA receptors (AMPARs) from the postsynaptic density. The AMPAR-binding, PDZ (PSD-95/Dlg/ZO1) and BAR (Bin/amphiphysin/Rvs) domain-containing protein PICK1 has been implicated in the regulation of AMPAR trafficking underlying several forms of synaptic plasticity. Using a strategy involving small hairpin RNA-mediated knockdown of PICK1 and its replacement with recombinant PICK1, we performed a detailed structure-function analysis of the role of PICK1 in hippocampal synaptic plasticity and the underlying NMDAR-induced AMPAR trafficking. We found that PICK1 is not necessary for maintenance of the basal synaptic complement of AMPARs or expression of either metabotropic glutamate receptor-dependent LTD or NMDAR-dependent LTP. Rather, PICK1 function is specific to NMDAR-dependent LTD and the underlying AMPAR trafficking. Furthermore, although PICK1 does not regulate the initial phase of NMDAR-induced AMPAR endocytosis, it is required for intracellular retention of internalized AMPARs. Detailed biophysical analysis of an N-terminal acidic motif indicated that it is involved in intramolecular electrostatic interactions that are disrupted by calcium. Mutations that interfered with the calcium-induced structural changes in PICK1 precluded LTD and the underlying NMDAR-induced intracellular retention of AMPARs. These findings support a model whereby calcium-induced modification of PICK1 structure is critical for its function in the retention of internalized AMPARs that underlies the expression of hippocampal NMDAR-dependent LTD.

Developmental regulation of protein interacting with C kinase 1 (PICK1) function in hippocampal synaptic plasticity and learning

AMPA-type glutamate receptors (AMPARs) mediate the majority of fast excitatory neurotransmission in the mammalian central nervous system. Modulation of AMPAR trafficking supports several forms of synaptic plasticity thought to underlie learning and memory. Protein interacting with C kinase 1 (PICK1) is an AMPAR-binding protein shown to regulate both AMPAR trafficking and synaptic plasticity at many distinct synapses. However, studies examining the requirement for PICK1 in maintaining basal synaptic transmission and regulating synaptic plasticity at hippocampal Schaffer collateral-cornu ammonis 1 (SC-CA1) synapses have produced conflicting results. In addition, the effect of PICK1 manipulation on learning and memory has not been investigated. In the present study we analyzed the effect of genetic deletion of PICK1 on basal synaptic transmission and synaptic plasticity at hippocampal Schaffer collateral-CA1 synapses in adult and juvenile mice. Surprisingly, we find that loss of PICK1 has no significant effect on synaptic plasticity in juvenile mice but impairs some forms of long-term potentiation and multiple distinct forms of long-term depression in adult mice. Moreover, inhibitory avoidance learning is impaired only in adult KO mice. These results suggest that PICK1 is selectively required for hippocampal synaptic plasticity and learning in adult rodents.

Endosomal sorting of AMPA receptors in hippocampal neurons

An important mechanism for the regulation of excitatory synaptic transmission in the hippocampus involves tight control of AMPAR [AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor] trafficking to alter the number or subtype of synaptic receptors. This is achieved via the multiple stages of the endosomal system. AMPARs constitutively cycle through early endosomes and recycling endosomes to maintain synaptic receptor numbers. However, on induction of synaptic plasticity, subtle alterations are made to this cycle by the action of specific AMPAR-interacting proteins and also via a number of additional proteins that regulate endosomal sorting more generally. During long-term depression, receptors are diverted to late endosomes and lysosomes rather than recycling back to the plasma membrane, hence reducing the number of receptors at the synapse. The increased number of synaptic AMPARs after induction of LTP (long-term potentiation) originates from the recycling compartment. In addition, transient changes in subunit composition may arise as a result of retention of AMPAR subtypes within the endosome during LTP. Aberrant trafficking after pathological insults such as oxygen/glucose deprivation or mechanical trauma also involves alterations in synaptic AMPAR subunit composition, leading to calcium influx that ultimately results in cell death.

Burst firing induces postsynaptic LTD at developing mossy fibre-CA3 pyramid synapses

Synaptic development is an activity-dependent process utilizing coordinated network activity to drive synaptogenesis and subsequent refinement of immature connections. Hippocampal CA3 pyramidal neurons (PYRs) exhibit intense burst firing (BF) early in development, concomitant with the period of mossy fibre (MF) development. However, whether developing MF-PYR synapses utilize PYR BF to promote MF synapse maturation remains unknown. Recently, we demonstrated that transient tonic depolarization of postsynaptic PYRs induces a persistent postsynaptic form of long-term depression (depolarization-induced long-term depression, DiLTD) at immature MF-PYR synapses. DiLTD induction is NMDAR independent but does require postsynaptic Ca(2+) influx through L-type voltage gated Ca(2+) channels (L-VGCCs), and is expressed as a reduction in AMPAR function through the loss of GluR2-lacking AMPARs present at immature MF-PYR synapses. Here we examined whether more physiologically relevant phasic L-VGCC activation by PYR action potential (AP) BF activity patterns can trigger DiLTD. Using combined electrophysiological and Ca(2+) imaging approaches we demonstrate that PYR BF effectively drives L-VGCC activation and that brief periods of repetitive PYR BF, produced by direct current injection or intrinsic network activity induces NMDAR-independent LTD by promoting Ca(2+) influx through the activated L-VGCCs. This BF induced LTD, just like DiLTD, is specific for developing MF-PYR synapses, is PICK1 dependent, and is expressed postsynaptically. Our results demonstrate that DiLTD can be induced by phasic L-VGCC activation driven by PYR BF, suggesting the engagement of natural PYR network activity patterns for MF synapse maturation.

Induction and expression of GluA1 (GluR-A)-independent LTP in the hippocampus

Long-term potentiation (LTP) at hippocampal CA3-CA1 synapses is thought to be mediated, at least in part, by an increase in the postsynaptic surface expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors induced by N-methyl-d-aspartate (NMDA) receptor activation. While this process was originally attributed to the regulated synaptic insertion of GluA1 (GluR-A) subunit-containing AMPA receptors, recent evidence suggests that regulated synaptic trafficking of GluA2 subunits might also contribute to one or several phases of potentiation. However, it has so far been difficult to separate these two mechanisms experimentally. Here we used genetically modified mice lacking the GluA1 subunit (Gria1(-/-) mice) to investigate GluA1-independent mechanisms of LTP at CA3-CA1 synapses in transverse hippocampal slices. An extracellular, paired theta-burst stimulation paradigm induced a robust GluA1-independent form of LTP lacking the early, rapidly decaying component characteristic of LTP in wild-type mice. This GluA1-independent form of LTP was attenuated by inhibitors of neuronal nitric oxide synthase and protein kinase C (PKC), two enzymes known to regulate GluA2 surface expression. Furthermore, the induction of GluA1-independent potentiation required the activation of GluN2B (NR2B) subunit-containing NMDA receptors. Our findings support and extend the evidence that LTP at hippocampal CA3-CA1 synapses comprises a rapidly decaying, GluA1-dependent component and a more sustained, GluA1-independent component, induced and expressed via a separate mechanism involving GluN2B-containing NMDA receptors, neuronal nitric oxide synthase and PKC.

Membrane localization is critical for activation of the PICK1 BAR domain

The PSD-95/Discs-large/ZO-1 homology (PDZ) domain protein, protein interacting with C kinase 1 (PICK1) contains a C-terminal Bin/amphiphysin/Rvs (BAR) domain mediating recognition of curved membranes; however, the molecular mechanisms controlling the activity of this domain are poorly understood. In agreement with negative regulation of the BAR domain by the N-terminal PDZ domain, PICK1 distributed evenly in the cytoplasm, whereas truncation of the PDZ domain caused BAR domain-dependent redistribution to clusters colocalizing with markers of recycling endosomal compartments. A similar clustering was observed both upon truncation of a short putative alpha-helical segment in the linker between the PDZ and the BAR domains and upon coexpression of PICK1 with a transmembrane PDZ ligand, including the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluR2 subunit, the GluR2 C-terminus transferred to the single transmembrane protein Tac or the dopamine transporter C-terminus transferred to Tac. In contrast, transfer of the GluR2 C-terminus to cyan fluorescent protein, a cytosolic protein, did not elicit BAR domain-dependent clustering. Instead, localizing PICK1 to the membrane by introducing an N-terminal myristoylation site produced BAR domain-dependent, but ligand-independent, PICK1 clustering. The data support that in the absence of PDZ ligand, the PICK1 BAR domain is inhibited through a PDZ domain-dependent and linker-dependent mechanism. Moreover, they suggest that unmasking of the BAR domain's membrane-binding capacity is not a consequence of ligand binding to the PDZ domain per se but results from, and coincides with, recruitment of PICK1 to a membrane compartment.

Corequirement of PICK1 binding and PKC phosphorylation for stable surface expression of the metabotropic glutamate receptor mGluR7

The presynaptic metabotropic glutamate receptor (mGluR) mGluR7 modulates excitatory neurotransmission by regulating neurotransmitter release and plays a critical role in certain forms of synaptic plasticity. Although the dynamic regulation of mGluR7 surface expression governs a form of metaplasticity in the hippocampus, little is known about the molecular mechanisms regulating mGluR7 trafficking. We now show that mGluR7 surface expression is stabilized by both PKC phosphorylation and by receptor binding to the PDZ domain-containing protein PICK1. Phosphorylation of mGluR7 on serine 862 (S862) inhibits CaM binding, thereby increasing mGluR7 surface expression and receptor binding to PICK1. Furthermore, in mice lacking PICK1, PKC-dependent increases in mGluR7 phosphorylation and surface expression are diminished, and mGluR7-dependent plasticity at mossy fiber-interneuron hippocampal synapses is impaired. These data support a model in which PICK1 binding and PKC phosphorylation act together to stabilize mGluR7 on the cell surface in vivo.

An essential role for PICK1 in NMDA receptor-dependent bidirectional synaptic plasticity

PICK1 is a calcium-sensing, PDZ domain-containing protein that interacts with GluR2 and GluR3 AMPA receptor (AMPAR) subunits and regulates their trafficking. Although PICK1 has been principally implicated in long-term depression (LTD), PICK1 overexpression in CA1 pyramidal neurons causes a CaMK- and PKC-dependent potentiation of AMPAR-mediated transmission and an increase in synaptic GluR2-lacking AMPARs, mechanisms associated with NMDA receptor (NMDAR)-dependent long-term potentiation (LTP). Here, we directly tested whether PICK1 participates in both hippocampal NMDAR-dependent LTP and LTD. We show that the PICK1 potentiation of AMPAR-mediated transmission is NMDAR dependent and fully occludes LTP. Conversely, blockade of PICK1 PDZ interactions or lack of PICK1 prevents LTP. These observations demonstrate an important role for PICK1 in LTP. In addition, deletion of PICK1 or blockade of PICK1 PDZ binding prevented NMDAR-dependent LTD. Thus, PICK1 plays a critical role in bidirectional NMDAR-dependent long-term synaptic plasticity in the hippocampus.

PICK1: a multi-talented modulator of AMPA receptor trafficking

AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor trafficking is a fundamental mechanism for regulating synaptic strength, and hence may underlie cellular processes involved in learning and memory. PICK1 (protein interacting with C-kinase) is a PDZ and BAR domain-containing protein that has recently emerged as a key regulator of AMPA receptor traffic. Via the PDZ domain, PICK1 interacts directly with AMPA receptor subunits and is involved in the regulated removal of AMPA receptors from the synaptic plasma membrane. PICK1 has the ability to functionally interact with a number of cellular processes, including calcium signaling, actin polymerisation and phospholipid membrane architecture. In this review, I summarize recent findings that describe the importance of PICK1 in neurons and its specific molecular characteristics that enable it to regulate AMPA receptor trafficking.

Developmental expression of Ca2+-permeable AMPA receptors underlies depolarization-induced long-term depression at mossy fiber CA3 pyramid synapses

Many central excitatory synapses undergo developmental alterations in the molecular and biophysical characteristics of postsynaptic ionotropic glutamate receptors via changes in subunit composition. Concerning AMPA receptors (AMPARs), glutamate receptor 2 subunit (GluR2)-containing, Ca2+-impermeable AMPARs (CI-AMPARs) prevail at synapses between mature principal neurons; however, accumulating evidence indicates that GluR2-lacking, Ca2+-permeable AMPARs (CP-AMPARs) contribute at these synapses early in development. Here, we used a combination of imaging and electrophysiological recording techniques to investigate potential roles for CP-AMPARs at developing hippocampal mossy fiber-CA3 pyramidal cell (MF-PYR) synapses. We found that transmission at nascent MF-PYR synapses is mediated by a mixed population of CP- and CI-AMPARs as evidenced by polyamine-dependent inwardly rectifying current-voltage (I-V) relationships, and partial philanthotoxin sensitivity of synaptic events. CP-AMPAR expression at MF-PYR synapses is transient, being limited to the first 3 postnatal weeks. Moreover, the expression of CP-AMPARs is regulated by the PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain-containing protein interacting with C kinase 1 (PICK1), because MF-PYR synapses in young PICK1 knock-out mice are philanthotoxin insensitive with linear I-V relationships. Strikingly, MF-PYR transmission via CP-AMPARs is selectively depressed during depolarization-induced long-term depression (DiLTD), a postsynaptic form of MF-PYR plasticity observed only at young MF-PYR synapses. The selective depression of CP-AMPARs during DiLTD was evident as a loss of postsynaptic CP-AMPAR-mediated Ca2+ transients in PYR spines and reduced rectification of MF-PYR synaptic currents. Preferential targeting of CP-AMPARs during DiLTD is further supported by a lack of DiLTD in young PICK1 knock-out mice. Together, these findings indicate that the transient participation of CP-AMPARs at young MF-PYR synapses dictates the developmental window to observe DiLTD.

AMPAR exocytosis through NO modulation of PICK1

The activation of NMDA receptors (NMDARs) triggers long-term changes in AMPA receptor-mediated synaptic transmission in the CNS. These long-lasting changes occur via the addition or removal of AMPA receptors (AMPARs) at the synaptic membrane and are mediated by a number of regulatory proteins including the GluR2 AMPAR-interacting proteins n-ethylmaleimide sensitive factor (NSF) and Protein Interacting with C Kinase (PICK1). We have shown that the potent activation of NMDARs drives unclustering of PICK1 and PICK1-GluR2 dissociation in dendrites resulting in increased surface delivery of AMPARs. Here we show that the dispersal of PICK1 is mediated by the actions of NSF. We find that elevated NMDAR signaling leads to the S-nitrosylation of NSF and increased NSF-GluR2 association. Both NMDAR-dependent unclustering of PICK1 and the delivery of surface AMPARs are dependent on release of nitric oxide (NO). Our data suggest that NMDAR activation can drive the surface delivery of AMPARs from a pool of intracellular AMPARs retained by PICK1 through the NO-dependent modification of NSF.

Differential regulation of AMPA receptor trafficking by neurabin-targeted synaptic protein phosphatase-1 in synaptic transmission and long-term depression in hippocampus

Filamentous actin binding protein neurabin I (NrbI) targets protein phosphatase-1 (PP1) to specific postsynaptic microdomains, exerting critical control over AMPA receptor (AMPAR)-mediated synaptic transmission. NrbI-targeted synaptic PP1, which promotes synaptic depression upon long-term depression (LTD) stimuli, serves to prevent synaptic depression under basal conditions. The present studies investigate this opposite regulation of AMPAR trafficking during basal synaptic transmission and LTD by expressing NrbI or NrbI mutant, which is defective in PP1 binding, in hippocampal slice or neuron cultures. We find that expression of the NrbI mutant to interfere with PP1 targeting dramatically reduces basal synaptic transmission, which is correlated with the reduction in surface expression of AMPA subtype glutamate receptor (GluR) 1 and GluR2 subunits. Biochemical analysis demonstrates that the NrbI mutant selectively increases the phosphorylation of GluR2 at C-terminal consensus PKC site, serine 880, which is known to favor GluR2 interaction with PDZ (postsynaptic density 95/Discs large/zona occludens 1) protein PICK1 (protein interacting with C kinase-1). Inhibition of PKC activity or GluR2-PICK1 interaction completely reverses the synaptic depression in neurons expressing the NrbI mutant, suggesting that NrbI-targeted synaptic PP1 stabilizes the basal transmission by negatively controlling PKC phosphorylation of GluR2 and the subsequent PICK1-mediated decrease in GluR2-containing AMPAR surface expression. Distinct from basal transmission, blocking GluR2-PICK1 interaction or PKC activity produces minimal effects on LTD in NrbI-expressing neurons. Instead, NrbI-targeted PP1 facilitates LTD by dephosphorylating GluR1 at both serine 845 and serine 831, with GluR2 serine 880 phosphorylation unaltered. Our studies thus elucidate that NrbI-targeted PP1, in response to distinct synaptic activities, regulates the synaptic trafficking of specific AMPAR subunits.

Phospholipase C is required for changes in postsynaptic structure and function associated with NMDA receptor-dependent long-term depression

NMDA receptor (NMDAR)-dependent hippocampal synaptic plasticity underlying learning and memory coordinately regulates dendritic spine structure and AMPA receptor (AMPAR) postsynaptic strength through poorly understood mechanisms. Induction of long-term depression (LTD) activates protein phosphatase 2B/calcineurin (CaN), leading to dendritic spine shrinkage through actin depolymerization and AMPAR depression through receptor dephosphorylation and internalization. The scaffold proteins A-kinase-anchoring protein 79/150 (AKAP79/150) and postsynaptic density 95 (PSD95) form a complex that controls the opposing actions of the cAMP-dependent protein kinase (PKA) and CaN in regulation of AMPAR phosphorylation. The AKAP79/150-PSD95 complex is disrupted in hippocampal neurons during LTD coincident with internalization of AMPARs, decreases in PSD95 levels, and loss of AKAP79/150 and PKA from spines. AKAP79/150 is targeted to spines through binding F-actin and the phospholipid phosphatidylinositol-(4,5)-bisphosphate (PIP2). Previous electrophysiological studies have demonstrated that inhibition of phospholipase C (PLC)-catalyzed hydrolysis of PIP2 inhibits NMDAR-dependent LTD; however, the signaling mechanisms that link PLC activation to alterations in dendritic spine structure and AMPAR function in LTD are unknown. We show here that NMDAR stimulation of PLC in cultured hippocampal neurons is necessary for AKAP79/150 loss from spines and depolymerization of spine actin. Importantly, we demonstrate that NMDAR activation of PLC is also necessary for decreases in spine PSD95 levels and AMPAR internalization. Thus, PLC signaling is required for structural and functional changes in spine actin, PSD scaffolding, and AMPAR trafficking underlying postsynaptic expression of LTD.

PICK1 is not a susceptibility gene for schizophrenia in a Japanese population: association study in a large case-control population

The protein interacting with C-kinase 1 (PICK1) has been implicated in the susceptibility to schizophrenia. PICK1 interacts with enzymes and receptors that play roles in the pathogenesis of schizophrenia via glutamatergic dysfunction. Recently, two studies reported associations between schizophrenia and two PICK1 gene polymorphisms, rs3952 in Chinese and Japanese populations and rs2076369 in a Japanese population. We attempted to confirm these associations in a case-control study of 1765 Japanese patients with schizophrenia and 1851 Japanese control subjects. Neither polymorphism was associated with schizophrenia (rs3952, p=0.755; rs2076369, p=0.997). A haplotype block with these polymorphisms spanning the 5' region of the PICK1 gene showed high linkage disequilibrium in the Japanese population (D'=0.98, r(2)=0.34); however, neither haplotype was significantly associated with schizophrenia. We conclude that the common haplotypes and polymorphisms of the PICK1 gene identified thus far are unlikely to contribute to genetic susceptibility to schizophrenia in the Japanese population.

Molecular mechanisms for regulation of AMPAR trafficking by PICK1

AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor trafficking is a fundamental mechanism for regulating synaptic strength, and hence may underlie cellular processes involved in learning and memory. PICK1 (protein that interacts with protein C-kinase) has recently emerged as a key regulator of AMPAR (AMPA receptor) traffic, and the precise molecular mechanisms of PICK1's action are just beginning to be unravelled. In this review, I summarize recent findings that describe some important molecular characteristics of PICK1 with respect to AMPAR cell biology.

Determination of the role of conventional, novel and atypical PKC isoforms in the expression of morphine tolerance in mice

This study comprehensively determines the role of all the major PKC isoforms in the expression morphine tolerance. Pseudosubstrate and receptors for activated C-kinase (RACK) peptides inhibit only a single PKC isoform, while previously tested chemical PKC inhibitors simultaneously inhibit multiple isoforms making it impossible to determine which PKC isoform mediates morphine tolerance. Tolerance can result in a diminished effect during continued exposure to the same amount of substance. In rodents, morphine pellets provide sustained exposures to morphine leading to the development of tolerance by 72 h. We hypothesized that administration of the PKC isoform inhibitors i.c.v. would reverse tolerance and reinstate antinociception in the tail immersion and hot plate tests from the morphine released solely from the pellet. Inhibitors to PKC alpha, gamma and epsilon (100-625 pmol) dose-dependently reinstated antinociception in both tests. The PKC beta(I), beta(II), delta, theta, epsilon, eta and xi inhibitors were inactive (up to 2500 pmol). In other mice, the degree of morphine tolerance was determined by calculating ED50 and potency-ratio values following s.c. morphine administration. Morphine s.c. was 5.6-fold less potent in morphine-pelleted vs. placebo-pelleted mice. Co-administration of s.c. morphine with the inhibitors i.c.v. to either PKC alpha (625 pmol), gamma (100 pmol) or epsilon (400 pmol) completely reversed the tolerance so that s.c. morphine was equally potent in both placebo- and morphine-pelleted mice. The PKC beta(I), beta(II), delta, theta, epsilon, eta and xi inhibitors were inactive. Thus, PKC alpha, gamma and epsilon appear to contribute to the expression of morphine tolerance in mice.