An essential role for the RNA helicase DDX6 in NMDA receptor-dependent gene silencing and dendritic spine shrinkage

MicroRNAs (miRNAs) repress translation of target mRNAs by associating with Argonaute (Ago) proteins in the RNA-induced silencing complex (RISC) to modulate protein expression. Specific miRNAs are required for NMDA receptor (NMDAR)-dependent synaptic plasticity by repressing the translation of proteins involved in dendritic spine morphogenesis. Rapid NMDAR-dependent silencing of Limk1 is essential for spine shrinkage and requires Ago2 phosphorylation at S387. Not all gene silencing events are modulated by S387 phosphorylation, and the mechanisms that govern the selection of specific mRNAs for silencing downstream of S387 phosphorylation are unknown. Here, we show that NMDAR-dependent S387 phosphorylation causes a rapid and transient increase in the association of Ago2 with Limk1, but not Apt1 mRNA. The specific increase in Limk1 mRNA binding to Ago2 requires recruitment of the helicase DDX6 to RISC. Furthermore, we show that DDX6 is required for NMDAR-dependent silencing of Limk1 via miR-134, but not Apt1 via miR-138, and is essential for NMDAR-dependent spine shrinkage. This work defines a novel mechanism for the rapid transduction of NMDAR stimulation into miRNA-mediated translational repression of specific genes to control dendritic spine morphology.

Ca2+ Regulates Dimerization of the BAR Domain Protein PICK1 and Consequent Membrane Curvature

Bin-Amphiphysin-Rvs (BAR) domain proteins are critical regulators of membrane geometry. They induce and stabilize membrane curvature for processes, such as clathrin-coated pit formation and endosomal membrane tubulation. BAR domains form their characteristic crescent-shaped structure in the dimeric form, indicating that the formation of the dimer is critical to their function of inducing membrane curvature and suggesting that a dynamic monomer–dimer equilibrium regulated by cellular signaling would be a powerful mechanism for controlling BAR domain protein function. However, to the best of our knowledge, cellular mechanisms for regulating BAR domain dimerization remain unexplored. PICK1 is a Ca²⁺-binding BAR domain protein involved in the endocytosis and endosomal recycling of neuronal AMPA receptors and other transmembrane proteins. In this study, we demonstrated that PICK1 dimerization is regulated by a direct effect of Ca²⁺ ions via acidic regions in the BAR domain and at the N-terminus. While the cellular membrane tubulating activity of PICK1 is absent under basal conditions, Ca²⁺ influx causes the generation of membrane tubules that originate from the cell surface. Furthermore, in neurons, PICK1 dimerization increases transiently following NMDA receptor stimulation. We believe that this novel mechanism for regulating BAR domain dimerization and function represents a significant conceptual advance in our knowledge about the regulation of cellular membrane curvature.

c-Abl regulates a synaptic plasticity-related transcriptional program involved in memory and learning

Memory consolidation requires activation of a gene expression program that allows de novo protein synthesis. But the molecular mechanisms that favour or restrict that program are poorly understood. The kinase c-Abl can modulate gene expression through transcription factors and chromatin modifiers. Here, we show that c-Abl ablation in the brain improves learning acquisition and memory consolidation in mice. Its absence also affects gene expression profiles in the mouse hippocampus. We found that genes involved in synaptic plasticity and actin cytoskeleton dynamics, such as Arp2 and Thorase, are up-regulated at the mRNA and protein levels in trained c-Abl KO mice and by a chemical-LTP stimulus. Trained c-Abl KO mice also show that dendritic spines are larger than in wild-type mice and present at a higher density. These results indicate that c-Abl kinase is an important part of the mechanism that limits or restricts signalling of relevant gene programs involved in morphological and functional spine changes upon neuronal stimulation.

Regulation of AMPAR expression by microRNAs

AMPA receptors (AMPARs) are the major excitatory neurotransmitter receptor in the brain, and their expression at synapses is a critical determinant of synaptic transmission and therefore brain function. Synaptic plasticity involves increases or decreases in synaptic strength, caused by changes in the number or subunit-specific subtype of AMPARs expressed at synapses, and resulting in modifications of functional connectivity of neuronal circuits, a process which is thought to underpin learning and the formation or loss of memories. Furthermore, numerous neurological disorders involve dysregulation of excitatory synaptic transmission or aberrant recruitment of plasticity processes. MicroRNAs (miRNAs) repress the translation of target genes by partial complementary base pairing with mRNAs, and are the core component of a mechanism widely used in a range of cell processes for regulating protein translation. MiRNA-dependent translational repression can occur locally in neuronal dendrites, close to synapses, and can also result in relatively rapid changes in protein expression. MiRNAs are therefore well-placed to regulate synaptic plasticity via the local control of AMPAR subunit synthesis, and can also result in synaptic dysfunction in the event of dysregulation in disease. Here, I will review the miRNAs that have been identified as playing a role in physiological or pathological changes in AMPAR subunit expression at synapses, focussing on miRNAs that target mRNAs encoding AMPAR subunits, and on miRNAs that target AMPAR accessory proteins involved in AMPAR trafficking and hence the regulation of AMPAR synaptic localisation.

Mechanisms of AMPA Receptor Endosomal Sorting

The regulation of synaptic AMPA receptors (AMPARs) is critical for excitatory synaptic transmission, synaptic plasticity and the consequent formation of neural circuits during brain development and their modification during learning and memory processes. The number of synaptic AMPARs is regulated through endocytosis, exocytosis and endosomal sorting that results in recycling back to the plasma membrane or degradation in the lysosome. Hence, endo-lysosomal sorting is vitally important in maintaining AMPAR expression at the synapse, and the dynamic regulation of these trafficking events is a key component of synaptic plasticity. A reduction in synaptic strength such as in long-term depression (LTD) involves AMPAR sorting to lysosomes to reduce synaptic AMPAR number, whereas long-term potentiation (LTP) involves an increase in AMPAR recycling to increase the number of AMPARs at synapses. Here, we review our current understanding of the endosomal trafficking routes taken by AMPARs, and the mechanisms involved in AMPAR endosomal sorting, focussing on the numerous AMPAR associated proteins that have been implicated in this complex process. We also discuss how these events are dysregulated in brain disorders.

The Regulation of AMPA Receptor Endocytosis by Dynamic Protein-Protein Interactions

The precise regulation of AMPA receptor (AMPAR) trafficking in neurons is crucial for excitatory neurotransmission, synaptic plasticity and the consequent formation and modification of neural circuits during brain development and learning. Clathrin-mediated endocytosis (CME) is an essential trafficking event for the activity-dependent removal of AMPARs from the neuronal plasma membrane, resulting in a reduction in synaptic strength known as long-term depression (LTD). The regulated AMPAR endocytosis that underlies LTD is caused by specific modes of synaptic activity, most notably stimulation of NMDA receptors (NMDARs) and metabotropic glutamate receptors (mGluRs). Numerous proteins associate with AMPAR subunits, directly or indirectly, to control their trafficking, and therefore the regulation of these protein-protein interactions in response to NMDAR or mGluR signaling is a critical feature of synaptic plasticity. This article reviews the protein-protein interactions that are dynamically regulated during synaptic plasticity to modulate AMPAR endocytosis, focussing on AMPAR binding proteins and proteins that bind the core endocytic machinery. In addition, the mechanisms for the regulation of protein-protein interactions are considered, as well as the functional consequences of these dynamic interactions on AMPAR endocytosis.

NMDAR-dependent Argonaute 2 phosphorylation regulates miRNA activity and dendritic spine plasticity

MicroRNAs (miRNAs) repress translation of target mRNAs by associating with Argonaute (Ago) proteins to form the RNA-induced silencing complex (RISC), underpinning a powerful mechanism for fine-tuning protein expression. Specific miRNAs are required for NMDA receptor (NMDAR)-dependent synaptic plasticity by modulating the translation of proteins involved in dendritic spine morphogenesis or synaptic transmission. However, it is unknown how NMDAR stimulation stimulates RISC activity to rapidly repress translation of synaptic proteins. We show that NMDAR stimulation transiently increases Akt-dependent phosphorylation of Ago2 at S387, which causes an increase in binding to GW182 and a rapid increase in translational repression of LIMK1 via miR-134. Furthermore, NMDAR-dependent down-regulation of endogenous LIMK1 translation in dendrites and dendritic spine shrinkage requires phospho-regulation of Ago2 at S387. AMPAR trafficking and hippocampal LTD do not involve S387 phosphorylation, defining this mechanism as a specific pathway for structural plasticity. This work defines a novel mechanism for the rapid transduction of NMDAR stimulation into miRNA-mediated translational repression to control dendritic spine morphology.

Cortactin regulates endo-lysosomal sorting of AMPARs via direct interaction with GluA2 subunit

AMPA receptor (AMPAR) trafficking is a key determinant of synaptic strength and synaptic plasticity. Under basal conditions, constitutive trafficking maintains surface AMPARs by internalization into the endosomal system, where the majority are sorted and targeted for recycling back to the plasma membrane. NMDA receptor (NMDAR)-dependent Long-Term Depression (LTD) is characterised by a reduction in synaptic strength, and involves endosomal sorting of AMPARs away from recycling pathways to lysosomes. The mechanisms that determine whether AMPARs are trafficked to lysosomes or to recycling endosomes, especially in response to NMDAR stimulation, are unclear. Here, we define a role for the actin-regulatory protein cortactin as a mediator of AMPAR endosomal sorting by direct interaction with the GluA2 subunit. Disrupting GluA2-cortactin binding in neurons causes the targeting of GluA2/A3-containing receptors to lysosomes and their consequent degradation, resulting in a loss of surface and synaptic GluA2 under basal conditions and an occlusion of subsequent LTD expression. Furthermore, we show that NMDAR stimulation causes a dissociation of endogenous cortactin from GluA2 via tyrosine phosphorylation of cortactin. These results demonstrate that cortactin maintains GluA2/A3 levels by directing receptors away from lysosomes, and that disrupting GluA2-cortactin interactions to target GluA2/A3 to lysosomes is an essential component of LTD expression.

Differential regulation of the Rac1 GTPase-activating protein (GAP) BCR during oxygen/glucose deprivation in hippocampal and cortical neurons

Brain ischemia causes oxygen and glucose deprivation (OGD) in neurons, triggering a cascade of events leading to synaptic accumulation of glutamate. Excessive activation of glutamate receptors causes excitotoxicity and delayed cell death in vulnerable neurons. Following global cerebral ischemia, hippocampal CA1 pyramidal neurons are more vulnerable to injury than their cortical counterparts, but the mechanisms that underlie this difference are unclear. Signaling via Rho-family small GTPases, their upstream guanine nucleotide exchange factors, and GTPase-activating proteins (GAPs) is differentially dysregulated in response to OGD/ischemia in hippocampal and cortical neurons. Increased Rac1 activity caused by OGD/ischemia contributes to neuronal death in hippocampal neurons via diverse effects on NADPH oxidase activity and dendritic spine morphology. The Rac1 guanine nucleotide exchange factor Tiam1 mediates an OGD-induced increase in Rac1 activity in hippocampal neurons; however, the identity of an antagonistic GAP remains elusive. Here we show that the Rac1 GAP breakpoint cluster region (BCR) associates with NMDA receptors (NMDARs) along with Tiam1 and that this protein complex is more abundant in hippocampal compared with cortical neurons. Although total BCR is similar in the two neuronal types, BCR is more active in hippocampal compared with cortical neurons. OGD causes an NMDAR- and Ca²⁺-permeable AMPAR-dependent deactivation of BCR in hippocampal but not cortical neurons. BCR knockdown occludes OGD-induced Rac1 activation in hippocampal neurons. Furthermore, disrupting the Tiam1–NMDAR interaction with a fragment of Tiam1 blocks OGD-induced Tiam1 activation but has no effect on the deactivation of BCR. This work identifies BCR as a critical player in Rac1 regulation during OGD in hippocampal neurons.

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.

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.

Identification of novel nesprin-1 binding partners and cytoplasmic matrin-3 in processing bodies

Several new nesprin-1 binding partners were identified, of which many are well-characterized RNA-binding proteins involved in various forms of nuclear RNA-processing events. Matrin-3 was one such protein identified, and a new cytoplasmic localization for matrin-3 is shown.

Nesprins are highly conserved spectrin repeat–containing scaffold proteins predominantly known to function at the nuclear envelope (NE). However, nesprin isoforms are emerging with localizations and scaffolding functions at sites away from the NE, suggesting their functions are more diverse than originally thought. In this study, we combined nesprin-1 coimmunoprecipitations with mass spectrometry to identify novel nesprin-1 binding partners for isoforms that localize to subcellular compartments beyond the NE. We show that one of these interactors, matrin-3 (matr3), localizes to mRNA processing bodies (PBs), where we have previously shown a nesprin-1 isoform to localize. Furthermore, we show that Matr3 is part of PB mRNP complexes, is a regulator of miRNA-mediated gene silencing, and possibly shuttles to stress granules in stressed cells. More importantly, we identify a new C-terminally truncated Matr3 isoform that is likely to be involved in these functions and PB localization. This study highlights several novel nesprin-1 binding partners and a new function and localization for Matr3 in cytoplasmic RNA granules.

Chitin and carbon nanotube composites as biocompatible scaffolds for neuron growth

The design of biocompatible implants for neuron repair/regeneration ideally requires high cell adhesion as well as good electrical conductivity. Here, we have shown that plasma-treated chitin carbon nanotube composite scaffolds show very good neuron adhesion as well as support of synaptic function of neurons. The addition of carbon nanotubes to a chitin biopolymer improved the electrical conductivity and the assisted oxygen plasma treatment introduced more oxygen species onto the chitin nanotube scaffold surface. Neuron viability experiments showed excellent neuron attachment onto plasma-treated chitin nanotube composite scaffolds. The support of synaptic function was evident on chitin/nanotube composites, as confirmed by PSD-95 staining. The biocompatible and electrically-conducting chitin nanotube composite scaffold prepared in this study can be used for in vitro tissue engineering of neurons and, potentially, as an implantable electrode for stimulation and repair of neurons.

Protein interacting with C kinase 1 suppresses invasion and anchorage-independent growth of astrocytic tumor cells

Astrocytic tumors are the most common form of primary brain tumor. Astrocytic tumor cells infiltrate the surrounding CNS tissue, allowing them to evade removal upon surgical resection of the primary tumor. Dynamic changes to the actin cytoskeleton are crucial to cancer cell invasion, but the specific mechanisms that underlie the particularly invasive phenotype of astrocytic tumor cells are unclear. Protein interacting with C kinase 1 (PICK1) is a PDZ and BAR domain-containing protein that inhibits actin-related protein 2/3 (Arp2/3)-dependent actin polymerization and is involved in regulating the trafficking of a number of cell-surface receptors. Here we report that, in contrast to other cancers, PICK1 expression is down-regulated in grade IV astrocytic tumor cell lines and also in clinical cases of the disease in which grade IV tumors have progressed from lower-grade tumors. Exogenous expression of PICK1 in the grade IV astrocytic cell line U251 reduces their capacity for anchorage-independent growth, two-dimensional migration, and invasion through a three-dimensional matrix, strongly suggesting that low PICK1 expression plays an important role in astrocytic tumorigenesis. We propose that PICK1 negatively regulates neoplastic infiltration of astrocytic tumors and that manipulation of PICK1 is an attractive possibility for therapeutic intervention.

PICK1 links AMPA receptor stimulation to Cdc42

Rho-family GTPases control numerous cell biological processes via effects on actin dynamics, such as cell migration, cell adhesion, morphogenesis and vesicle traffic. In neurons, they are involved in dendritic spine morphogenesis and other aspects of neuronal morphology via regulation of the actin cytoskeleton. The Rho-family member Cdc42 regulates dendritic spine morphology via its effector N-WASP, which activates the actin-nucleating Arp2/3 complex. Excitatory synaptic transmission is known to regulate actin dynamics in dendritic spines to bring about changes in spine morphology or motility, however, the details of the signalling pathways that transduce glutamate receptor activation to Rho GTPase function are unclear. PICK1 is a PDZ and BAR domain protein that interacts with the Arp2/3 complex and the GTPase Arf1 to regulate actin polymerisation in dendritic spines. PICK1 also binds AMPA receptor subunits GluA2/3 and is involved in GluA2-dependent AMPAR trafficking. Here, we show that PICK1 binds Rac1 and Cdc42, via distinct but overlapping binding sites. Furthermore, AMPAR stimulation deactivates Cdc42 and alters its detergent solubility in neurons via a PICK1-dependent process. This work suggests a novel role for PICK1 in transducing AMPAR stimulation to Cdc42 function in neurons.

Actin-dependent mechanisms in AMPA receptor trafficking

The precise regulation of AMPA receptor (AMPAR) number and subtype at the synapse is crucial for the regulation of excitatory neurotransmission, synaptic plasticity and the consequent formation of appropriate neural circuits for learning and memory. AMPAR trafficking involves the dynamic processes of exocytosis, endocytosis and endosomal recycling, all of which involve the actin cytoskeleton. The actin cytoskeleton is highly dynamic and highly regulated by an abundance of actin-binding proteins and upstream signaling pathways that modulate actin polymerization and depolymerization. Actin dynamics generate forces that manipulate membranes in the process of vesicle biogenesis, and also for propelling vesicles through the cytoplasm to reach their destination. In addition, trafficking mechanisms exploit more stable aspects of the actin cytoskeleton by using actin-based motor proteins to traffic vesicular cargo along actin filaments. Numerous studies have shown that actin dynamics are critical for AMPAR localization and function. The identification of actin-binding proteins that physically interact with AMPAR subunits, and research into their mode of action is starting to shed light on the mechanisms involved. Such proteins either regulate actin dynamics to modulate mechanical forces exerted on AMPAR-containing membranes, or associate with actin filaments to target or transport AMPAR-containing vesicles to specific subcellular regions. In addition, actin-regulatory proteins that do not physically interact with AMPARs may influence AMPAR trafficking by regulating the local actin environment in the dendritic spine.

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.

Distinct subunit-specific α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor trafficking mechanisms in cultured cortical and hippocampal neurons in response to oxygen and glucose deprivation

Brain ischemia occurs when the blood supply to the brain is interrupted, leading to oxygen and glucose deprivation (OGD). This triggers a cascade of events causing a synaptic accumulation of glutamate. Excessive activation of glutamate receptors results in excitotoxicity and delayed cell death in vulnerable neurons. Following global cerebral ischemia, hippocampal CA1 pyramidal neurons are more vulnerable to injury than their cortical counterparts. The mechanisms that underlie this difference are unclear. Cultured hippocampal neurons respond to OGD with a rapid internalization of AMPA receptor (AMPAR) subunit GluA2, resulting in a switch from GluA2-containing Ca(2+)-impermeable receptors to GluA2-lacking Ca(2+)-permeable subtypes (CP-AMPARs). GluA2 internalization is a critical component of OGD-induced cell death in hippocampal neurons. It is unknown how AMPAR trafficking is affected in cortical neurons following OGD. Here, we show that cultured cortical neurons are resistant to an OGD insult that causes cell death in hippocampal neurons. GluA1 is inserted at the plasma membrane in both cortical and hippocampal neurons in response to OGD. In contrast, OGD causes a rapid endocytosis of GluA2 in hippocampal neurons, which is absent in cortical neurons. These data demonstrate that populations of neurons with different vulnerabilities to OGD recruit distinct cell biological mechanisms in response to insult, and that a crucial aspect of the mechanism leading to OGD-induced cell death is absent in cortical neurons. This strongly suggests that the absence of OGD-induced GluA2 trafficking contributes to the relatively low vulnerability of cortical neurons to ischemia.

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

The antagonistic modulation of Arp2/3 activity by N-WASP, WAVE2 and PICK1 defines dynamic changes in astrocyte morphology

Astrocytes exhibit a complex, branched morphology, allowing them to functionally interact with numerous blood vessels, neighboring glial processes and neuronal elements, including synapses. They also respond to central nervous system (CNS) injury by a process known as astrogliosis, which involves morphological changes, including cell body hypertrophy and thickening of major processes. Following severe injury, astrocytes exhibit drastically reduced morphological complexity and collectively form a glial scar. The mechanistic details behind these morphological changes are unknown. Here, we investigate the regulation of the actin-nucleating Arp2/3 complex in controlling dynamic changes in astrocyte morphology. In contrast to other cell types, Arp2/3 inhibition drives the rapid expansion of astrocyte cell bodies and major processes. This intervention results in a reduced morphological complexity of astrocytes in both dissociated culture and in brain slices. We show that this expansion requires functional myosin II downstream of ROCK and RhoA. Knockdown of the Arp2/3 subunit Arp3 or the Arp2/3 activator N-WASP by siRNA also results in cell body expansion and reduced morphological complexity, whereas depleting WAVE2 specifically reduces the branching complexity of astrocyte processes. By contrast, knockdown of the Arp2/3 inhibitor PICK1 increases astrocyte branching complexity. Furthermore, astrocyte expansion induced by ischemic conditions is delayed by PICK1 knockdown or N-WASP overexpression. Our findings identify a new morphological outcome for Arp2/3 activation in restricting rather than promoting outwards movement of the plasma membrane in astrocytes. The Arp2/3 regulators PICK1, and N-WASP and WAVE2 function antagonistically to control the complexity of astrocyte branched morphology, and this mechanism underlies the morphological changes seen in astrocytes during their response to pathological insult.

PICK1 inhibition of the Arp2/3 complex controls dendritic spine size and synaptic plasticity

Activity-dependent remodelling of dendritic spines is essential for neural circuit development and synaptic plasticity, but the precise molecular mechanisms that regulate this process are unclear. Activators of Arp2/3-mediated actin polymerisation are required for spine enlargement; however, during long-term depression (LTD), spines shrink via actin depolymerisation and Arp2/3 inhibitors in this process have not yet been identified. Here, we show that PICK1 regulates spine size in hippocampal neurons via inhibition of the Arp2/3 complex. PICK1 knockdown increases spine size, whereas PICK1 overexpression reduces spine size. NMDA receptor activation results in spine shrinkage, which is blocked by PICK1 knockdown or overexpression of a PICK1 mutant that cannot bind Arp2/3. Furthermore, we show that PICK1-Arp2/3 interactions are required for functional hippocampal LTD. This work demonstrates that PICK1 is a novel regulator of spine dynamics. Via Arp2/3 inhibition, PICK1 has complementary yet distinct roles during LTD to regulate AMPA receptor trafficking and spine size, and therefore functions as a crucial factor in both structural and functional plasticity.

PICK1-mediated glutamate receptor subunit 2 (GluR2) trafficking contributes to cell death in oxygen/glucose-deprived hippocampal neurons

Oxygen and glucose deprivation (OGD) induces delayed cell death in hippocampal CA1 neurons via Ca²⁺/Zn²⁺-permeable, GluR2-lacking AMPA receptors (AMPARs). Following OGD, synaptic AMPAR currents in hippocampal neurons show marked inward rectification and increased sensitivity to channel blockers selective for GluR2-lacking AMPARs. This occurs via two mechanisms: a delayed down-regulation of GluR2 mRNA expression and a rapid internalization of GluR2-containing AMPARs during the OGD insult, which are replaced by GluR2-lacking receptors. The mechanisms that underlie this rapid change in subunit composition are unknown. Here, we demonstrate that this trafficking event shares features in common with events that mediate long term depression and long term potentiation and is initiated by the activation of N-methyl-d-aspartic acid receptors. Using biochemical and electrophysiological approaches, we show that peptides that interfere with PICK1 PDZ domain interactions block the OGD-induced switch in subunit composition, implicating PICK1 in restricting GluR2 from synapses during OGD. Furthermore, we show that GluR2-lacking AMPARs that arise at synapses during OGD as a result of PICK1 PDZ interactions are involved in OGD-induced delayed cell death. This work demonstrates that PICK1 plays a crucial role in the response to OGD that results in altered synaptic transmission and neuronal death and has implications for our understanding of the molecular mechanisms that underlie cell death during stroke.

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.

NSF binds calcium to regulate its interaction with AMPA receptor subunit GluR2

N-ethylmaleimide-sensitive fusion protein (NSF) is essential for numerous Ca(2+)-triggered vesicle trafficking events. It functions as a molecular chaperone to regulate trafficking protein complexes such as the soluble NSF attachment protein (SNAP) receptor complex and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-protein interacting with C-kinase (PICK1) complex. AMPAR trafficking is fundamental to processes of synaptic plasticity, which may underlie learning and memory. Changes in synaptic strength brought about by AMPAR trafficking are triggered by a post-synaptic influx of Ca(2+), which may have numerous molecular targets including PICK1. NSF binds AMPAR subunit glutamate receptor subunit 2 (GluR2) and functions to maintain receptors at the synapse. In this study, it was showed that NSF is a Ca(2+)-binding protein and that GluR2-NSF interactions are inhibited by the presence of 15 mumol/L Ca(2+). NSF Ca(2+)-binding is reciprocally inhibited by the presence of GluR2 C-terminus. Mutant of NSF that binds Ca(2+) with reduced affinity and binds GluR2 with reduced sensitivity to Ca(2+) was identied. In addition, the interaction of betaSNAP with PICK1 is sensitive to Ca(2+). This study demonstrates that the GluR2-NSF-betaSNAP-PICK1 complex is regulated directly by Ca(2+), allowing for the transduction of Ca(2+) signals into concerted alterations in protein-protein interactions to bring about changes in AMPAR trafficking during synaptic plasticity.

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.

Picking out the details of cerebellar LTD

Cerebellar long-term depression is thought to underlie motor learning and is mediated by internalization of AMPA receptors from the neuronal plasma membrane. In this issue of Neuron, Steinberg et al. provide firm evidence that PICK 1 and the C terminus of GluR2 are central to this process by analyzing three different transgenic mice.

PICK1 is a calcium-sensor for NMDA-induced AMPA receptor trafficking

Regulation of AMPA receptor (AMPAR) trafficking results in changes in receptor number at the postsynaptic membrane, and hence modifications in synaptic strength, which are proposed to underlie learning and memory. NMDA receptor-mediated postsynaptic Ca2+ influx enhances AMPAR internalisation, but the molecular mechanisms that trigger such trafficking are not well understood. We investigated whether AMPAR-associated protein-protein interactions known to regulate receptor surface expression may be directly regulated by Ca2+. PICK1 binds the AMPAR GluR2 subunit and is involved in AMPAR internalisation and LTD. We show that PICK1 is a Ca2+-binding protein, and that PICK1-GluR2 interactions are enhanced by the presence of 15 muM Ca2+. Deletion of an N-terminal acidic domain in PICK1 reduces its ability to bind Ca2+, and renders the GluR2-PICK1 interaction insensitive to Ca2+. Overexpression of this Ca2+-insensitive mutant occludes NMDA-induced AMPAR internalisation in hippocampal neurons. This work reveals a novel postsynaptic Ca2+-binding protein that provides a direct mechanistic link between NMDAR-mediated Ca2+ influx and AMPAR endocytosis.

NSF ATPase and alpha-/beta-SNAPs disassemble the AMPA receptor-PICK1 complex

AMPA receptor (AMPAR) trafficking is crucial for synaptic plasticity that may be important for learning and memory. NSF and PICK1 bind the AMPAR GluR2 subunit and are involved in trafficking of AMPARs. Here, we show that GluR2, PICK1, NSF, and alpha-/beta-SNAPs form a complex in the presence of ATPgammaS. Similar to SNARE complex disassembly, NSF ATPase activity disrupts PICK1-GluR2 interactions in this complex. Alpha- and beta-SNAP have differential effects on this reaction. SNAP overexpression in hippocampal neurons leads to corresponding changes in AMPAR trafficking by acting on GluR2-PICK1 complexes. This demonstrates that the previously reported synaptic stabilization of AMPARs by NSF involves disruption of GluR2-PICK1 interactions. Furthermore, we are reporting a non-SNARE substrate for NSF disassembly activity.

GABAC receptor sensitivity is modulated by interaction with MAP1B

GABA(C) receptors contain rho subunits and mediate feedback inhibition from retinal amacrine cells to bipolar cells. We previously identified the cytoskeletal protein MAP1B as a rho1 subunit anchoring protein. Here, we analyze the structural basis and functional significance of the MAP1B-rho1 interaction. Twelve amino acids at the C terminus of the large intracellular loop of rho1 (and also rho2) are sufficient for interaction with MAP1B. Disruption of the MAP1B-rho interaction in bipolar cells in retinal slices decreased the EC(50) of their GABA(C) receptors, doubling the receptors' current at low GABA concentrations without affecting their maximum current at high concentrations. Thus, anchoring to the cytoskeleton lowers the sensitivity of GABA(C) receptors and provides a likely site for functional modulation of GABA(C) receptor-mediated inhibition.

GABA receptor rho1 subunit interacts with a novel splice variant of the glycine transporter, GLYT-1

Ionotropic gamma-aminobutyric acid (GABA(A) and GABA(C)) receptors mediate fast synaptic inhibition in the central nervous system. GABA(C) receptors are expressed predominantly in the retina on bipolar cell axon terminals, and are thought to mediate feedback inhibition from GABAergic amacrine cells. Utilizing the yeast two-hybrid system, we previously identified MAP1B as a binding partner of the GABA(C) receptor rho1 subunit. Here we describe the isolation of an additional rho1 interacting protein: a novel C-terminal variant of the glycine transporter GLYT-1. We show that GLYT-1 exists as four alternatively spliced mRNAs which encode proteins expressing one of two possible intracellullar N- and C-terminal domains. Variants containing the novel C terminus efficiently transport glycine when expressed in COS cells, but with unusual kinetics. We have confirmed the interaction between the novel C terminus and rho1 subunit and demonstrated binding in heterologous cells. This interaction may be crucial for the integration of GABAergic and glycinergic neurotransmission in the retina.

The protein MAP-1B links GABA(C) receptors to the cytoskeleton at retinal synapses

The ionotropic type-A and type-C receptors for the neurotransmitter gamma-aminobutyric acid (GABA(A) and GABA(C) receptors) are the principal sites of fast synaptic inhibition in the central nervous system, but it is not known how these receptors are localized at GABA-dependent synapses. GABA(C) receptors, which are composed of rho-subunits, are expressed almost exclusively in the retina of adult vertebrates, where they are enriched on bipolar cell axon terminals. Here we show that the microtubule-associated protein 1B (MAP-1B) specifically interacts with the GABA(C) rho1 subunit but not with GABA(A) receptor subunits. Furthermore, GABA(C) receptors and MAP-1B co-localize at postsynaptic sites on bipolar cell axon terminals. Co-expression of MAP-1B and the rho1 subunit in COS cells results in a dramatic redistribution of the rho1 subunit. Our observations suggest a novel mechanism for localizing ionotropic GABA receptors to synaptic sites. This mechanism, which is specific for GABA(C) but not GABA(A) receptors, may allow these receptor subtypes, which have distinct physiological and pharmacological properties, to be differentially localized at inhibitory synapses.