Dendrite development: a surprising origin

Neurons extend elaborate dendrites studded with spines. Unexpectedly, this cellular sculpting is regulated by the origin recognition complex—the core machinery for initiating DNA replication.

Diffusional trapping of GluR1 AMPA receptors by input-specific synaptic activity

Synaptic activity regulates the postsynaptic accumulation of AMPA receptors over timescales ranging from minutes to days. Indeed, the regulated trafficking and mobility of GluR1 AMPA receptors underlies many forms of synaptic potentiation at glutamatergic synapses throughout the brain. However, the basis for synapse-specific accumulation of GluR1 is unknown. Here we report that synaptic activity locally immobilizes GluR1 AMPA receptors at individual synapses. Using single-molecule tracking together with the silencing of individual presynaptic boutons, we demonstrate that local synaptic activity reduces diffusional exchange of GluR1 between synaptic and extraynaptic domains, resulting in postsynaptic accumulation of GluR1. At neighboring inactive synapses, GluR1 is highly mobile with individual receptors frequently escaping the synapse. Within the synapse, spontaneous activity confines the diffusional movement of GluR1 to restricted subregions of the postsynaptic membrane. Thus, local activity restricts GluR1 mobility on a submicron scale, defining an input-specific mechanism for regulating AMPA receptor composition and abundance.

Emerging roles for ubiquitin and protein degradation in neuronal function

Alterations in cellular structure and synapse composition are central to proper nervous system function. Recent work has identified the ubiquitin-proteasome system (UPS) as a key regulator of neuronal biology. The UPS is essential for the growth and development of immature neurons and is a critical mediator of synaptic adaptability in mature neurons. Furthermore, proteinaceous deposits that accumulate in diverse neurodegenerative disorders are enriched in components of the UPS, suggesting that UPS dysfunction may be pivotal for pathogenesis. Here, we summarize existing knowledge about the role of the UPS in brain function, highlighting recent work delineating its importance in neuronal development, plasticity, and degeneration.

In vivo light-induced activation of neural circuitry in transgenic mice expressing channelrhodopsin-2

Channelrhodopsin-2 (ChR2) is a light-gated, cation-selective ion channel isolated from the green algae Chlamydomonas reinhardtii. Here, we report the generation of transgenic mice that express a ChR2-YFP fusion protein in the CNS for in vivo activation and mapping of neural circuits. Using focal illumination of the cerebral cortex and olfactory bulb, we demonstrate a highly reproducible, light-dependent activation of neurons and precise control of firing frequency in vivo. To test the feasibility of mapping neural circuits, we exploited the circuitry formed between the olfactory bulb and the piriform cortex in anesthetized mice. In the olfactory bulb, individual mitral cells fired action potentials in response to light, and their firing rate was not influenced by costimulated glomeruli. However, in piriform cortex, the activity of target neurons increased as larger areas of the bulb were illuminated to recruit additional glomeruli. These results support a model of olfactory processing that is dependent upon mitral cell convergence and integration onto cortical cells. More broadly, these findings demonstrate a system for precise manipulation of neural activity in the intact mammalian brain with light and illustrate the use of ChR2 mice in exploring functional connectivity of complex neural circuits in vivo.

Phosphatidylinositol-4,5-bisphosphate regulates NMDA receptor activity through alpha-actinin

Phosphatidylinositol-4,5-bisphosphate (PIP2) has been shown to regulate many ion channels, transporters, and other signaling proteins, but it is not known whether it also regulates neurotransmitter-gated channels. The NMDA receptors (NMDARs) are gated by glutamate and serve as a critical control point in synaptic function. Here we demonstrate that PIP2 supports NMDAR activity. In Xenopus oocytes, overexpression of phospholipase Cgamma (PLCgamma) or preincubation with 10 microm wortmannin markedly reduced NMDA currents. Stimulation of the epidermal growth factor receptor (EGFR) promoted the formation of an immunocomplex between PLCgamma and NMDAR subunits. Stimulation of EGFR or the PLCbeta-coupled M1 acetylcholine receptor produced a robust transient inhibition of NMDA currents. Wortmannin application blocked the recovery of NMDA currents from the inhibition. Using mutagenesis, we identified the structural elements on NMDAR intracellular tails that transduce the receptor-mediated inhibition, which pinpoint to the binding site for the cytoskeletal protein alpha-actinin. Mutation of the PIP2-binding residues of alpha-actinin dramatically reduced NMDA currents and occluded the effect of EGF. Interestingly, EGF or wortmannin affected the interaction between NMDAR subunits and alpha-actinin, suggesting that this protein mediates the effect of PIP2 on NMDARs. In mature hippocampal neurons, expression of the mutant alpha-actinin reduced NMDA currents and accelerated inactivation. We propose a model in which alpha-actinin supports NMDAR activity via tethering their intracellular tails to plasma membrane PIP2. Thus, our results extend the influence of PIP2 to the NMDA ionotropic glutamate receptors and introduce a novel mechanism of "indirect" regulation of transmembrane protein activity by PIP2.

Plasticity-induced growth of dendritic spines by exocytic trafficking from recycling endosomes

Dendritic spines are micron-sized membrane protrusions receiving most excitatory synaptic inputs in the mammalian brain. Spines form and grow during long-term potentiation (LTP) of synaptic strength. However, the source of membrane for spine formation and enlargement is unknown. Here we report that membrane trafficking from recycling endosomes is required for the growth and maintenance of spines. Using live-cell imaging and serial section electron microscopy, we demonstrate that LTP-inducing stimuli promote the mobilization of recycling endosomes and vesicles into spines. Preventing recycling endosomal transport abolishes LTP-induced spine formation. Using a pH-sensitive recycling cargo, we show that exocytosis from recycling endosomes occurs locally in spines, is triggered by activation of synaptic NMDA receptors, and occurs concurrently with spine enlargement. Thus, recycling endosomes provide membrane for activity-dependent spine growth and remodeling, defining a novel membrane trafficking mechanism for spine morphological plasticity and providing a mechanistic link between structural and functional plasticity during LTP.

Organelles and trafficking machinery for postsynaptic plasticity

Neurons are among the largest and most complex cells in the body. Their immense size and intricate geometry pose many unique cell-biological problems. How is dendritic architecture established and maintained? How do neurons traffic newly synthesized integral membrane proteins over such long distances to synapses? Functionally, protein trafficking to and from the postsynaptic membrane has emerged as a key mechanism underlying various forms of synaptic plasticity. Which organelles are involved in postsynaptic trafficking, and how do they integrate and respond to activity at individual synapses? Here we review what is currently known about long-range trafficking of newly synthesized postsynaptic proteins as well as the local rules that govern postsynaptic trafficking at individual synapses.

Endocytosis and synaptic removal of NR3A-containing NMDA receptors by PACSIN1/syndapin1

A key step in glutamatergic synapse maturation is the replacement of developmentally expressed N-methyl-D-aspartate receptors (NMDARs) with mature forms that differ in subunit composition, electrophysiological properties and propensity to elicit synaptic plasticity. However, the mechanisms underlying the removal and replacement of synaptic NMDARs are poorly understood. Here we demonstrate that NMDARs containing the developmentally regulated NR3A subunit undergo rapid endocytosis from the dendritic plasma membrane in cultured rat hippocampal neurons. This endocytic removal is regulated by PACSIN1/syndapin1, which directly and selectively binds the carboxy-terminal domain of NR3A through its NPF motifs and assembles a complex of proteins including dynamin and clathrin. Endocytosis of NR3A by PACSIN1 is activity dependent, and disruption of PACSIN1 function causes NR3A accumulation at synaptic sites. Our results reveal a new activity-dependent mechanism involved in the regulation of NMDAR expression at synapses during development, and identify a brain-specific endocytic adaptor that confers spatiotemporal and subunit specificity to NMDAR endocytosis.

Apolipoprotein E Receptor 2 Interactions with the N-Methyl-D-aspartate Receptor

In our previous studies we showed that apoE treatment of neurons activated ERK 1/2 signaling, and activation was blocked by treatment with inhibitors of the low density lipoprotein receptor family, the N-methyl-d-aspartate (NMDA) receptor antagonist MK 801, and calcium channel blockers. We hypothesized an interaction between the low density lipoprotein receptor family members and the NMDA receptor. In the present study, we confirmed through co-immunoprecipitation experiments an interaction between the apoE receptor, ApoEr2, and NMDAR1 through their extracellular domains. We also found that the PDZ1 domain of PSD95, a postsynaptic scaffolding protein, interacted with the C terminus of ApoEr2 via an alternatively spliced, intracellular exon. This interaction between ApoEr2 and PSD95 in neurons was modulated by NMDA receptor activation and an ApoEr2 ligand. We also found that the PDZ2 domain of PSD95 interacted with the NR2A and NR2B subunits of NMDA receptors. Full-length PSD95 increased cell surface levels of ApoEr2 and its cleavage, resulting in increases in secreted ApoEr2 and C-terminal fragments of ApoEr2. These studies suggest that ApoEr2 can form a multiprotein complex with NMDA receptor subunits and PSD95.

Polarized secretory trafficking directs cargo for asymmetric dendrite growth and morphogenesis

Proper growth of dendrites is critical to the formation of neuronal circuits, but the cellular machinery that directs the addition of membrane components to generate dendritic architecture remains obscure. Here, we demonstrate that post-Golgi membrane trafficking is polarized toward longer dendrites of hippocampal pyramidal neurons in vitro and toward apical dendrites in vivo. Small Golgi outposts partition selectively into longer dendrites and are excluded from axons. In dendrites, Golgi outposts concentrate at branchpoints where they engage in post-Golgi trafficking. Within the cell body, the Golgi apparatus orients toward the longest dendrite, and this Golgi polarity precedes asymmetric dendrite growth. Manipulations that selectively block post-Golgi trafficking halt dendrite growth in developing neurons and cause a shrinkage of dendrites in mature pyramidal neurons. Further, disruption of Golgi polarity produces neurons with symmetric dendritic arbors lacking a single longest principal dendrite. These results define a novel polarized organization of neuronal secretory trafficking and demonstrate a mechanistic link between directed membrane trafficking and asymmetric dendrite growth.

Ubiquitin and protein turnover in synapse function

Enduring modification of synapses is central to long-lasting neural circuit plasticity. Such adaptations include rapid posttranslational modification of existing synaptic proteins over periods of minutes and persisting changes in the abundance of synaptic proteins over hours to days. Recently, ubiquitination and protein degradation have emerged as additional mechanisms for modifying the function and molecular composition of synapses. These recent findings raise intriguing questions as to how enduring changes at synapses are accomplished in the face of robust, ongoing molecular turnover.

Subcellular localization of spastin: implications for the pathogenesis of hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is a group of clinically and genetically heterogeneous diseases characterized by neuronal degeneration that is maximal at the distal ends of the longest axons of the central nervous system. The most common cause of autosomal dominant HSP is mutation of a novel gene encoding spastin, a protein whose function is still being elucidated. One clue concerning spastin function is its intracellular localization. Here, we describe a novel anti-spastin antiserum designed to a unique epitope contained within all splicing isoforms. The antiserum exhibits specific immunostaining of recombinant spastin in intact, fixed cells. Using this reagent, we find that endogenous spastin is located at the centrosome in a variety of cell types at all points in the cell cycle. This localization is resistant to microtubule disruption, suggesting that spastin may be an integral centrosomal protein. In addition to the centrosome, spastin also localizes at discrete focal regions along the axons of primary cultured neurons. These data lend additional support to the emerging hypothesis that spastin plays a role in microtubule dynamics, with a crucial role in microtubule organization.

Homeostatic plasticity and NMDA receptor trafficking

Learning, memory and brain development are associated with long-lasting modifications of synapses that are guided by specific patterns of neuronal activity. Such modifications include classical Hebbian plasticities (such as long-term potentiation and long-term depression), which are rapid and synapse-specific, and others, such as synaptic scaling and metaplasticity, that work over longer timescales and are crucial for maintaining and orchestrating neuronal network function. The cellular mechanisms underlying Hebbian plasticity have been well studied and involve rapid changes in the trafficking of highly mobile AMPA receptors. An emerging concept is that activity-dependent alterations in NMDA receptor trafficking contribute to homeostatic plasticity at central glutamatergic synapses.

Synapse formation: astrocytes spout off

Trillions of synapses form as the brain develops. This intimate contact between two neurons has now been shown to be facilitated by thrombospondins - large extracellular matrix proteins secreted by astrocytes.

Neurabin/protein phosphatase-1 complex regulates dendritic spine morphogenesis and maturation

The majority of excitatory synapses in the mammalian brain form on filopodia and spines, actin-rich membrane protrusions present on neuronal dendrites. The biochemical events that induce filopodia and remodel these structures into dendritic spines remain poorly understood. Here, we show that the neuronal actin- and protein phosphatase-1-binding protein, neurabin-I, promotes filopodia in neurons and nonneuronal cells. Neurabin-I actin-binding domain bundled F-actin, promoted filopodia, and delayed the maturation of dendritic spines in cultured hippocampal neurons. In contrast, dimerization of neurabin-I via C-terminal coiled-coil domains and association of protein phosphatase-1 (PP1) with neurabin-I through a canonical KIXF motif inhibited filopodia. Furthermore, the expression of a neurabin-I polypeptide unable to bind PP1 delayed the maturation of neuronal filopodia into spines, reduced the synaptic targeting of AMPA-type glutamate (GluR1) receptors, and decreased AMPA receptor-mediated synaptic transmission. Reduction of endogenous neurabin levels by interference RNA (RNAi)-mediated knockdown also inhibited the surface expression of GluR1 receptors. Together, our studies suggested that disrupting the functions of a cytoskeletal neurabin/PP1 complex enhanced filopodia and impaired surface GluR1 expression in hippocampal neurons, thereby hindering the morphological and functional maturation of dendritic spines.

Endocytosis and degradative sorting of NMDA receptors by conserved membrane-proximal signals

Regulation of the abundance of NMDA receptors (NMDARs) at excitatory synapses is critical during changes in synaptic efficacy underlying learning and memory as well as during synapse formation throughout neural development. However, the molecular signals that govern NMDAR delivery, maintenance, and internalization remain unclear. In this study, we identify a conserved family of membrane-proximal endocytic signals, two within the NMDAR type 1 (NR1) subunit and one within the NR2A and NR2B subunits, necessary and sufficient to drive the internalization of NMDARs. These endocytic motifs reside in the region of NMDAR subunits immediately after the fourth membrane segment, a region implicated in use-dependent rundown and NMDA channel inactivation. Although endocytosis driven by the distal C-terminal domain of NR2B is followed by rapid recycling, internalization mediated by membrane-proximal motifs selectively targets receptors to late endosomes and accelerates degradation. These results define a novel conserved signature of NMDARs regulating internalization and postendocytic trafficking.

Recycling endosomes supply AMPA receptors for LTP

Long-term potentiation (LTP) of synaptic strength, the most established cellular model of information storage in the brain, is expressed by an increase in the number of postsynaptic AMPA receptors. However, the source of AMPA receptors mobilized during LTP is unknown. We report that AMPA receptors are transported from recycling endosomes to the plasma membrane for LTP. Stimuli that triggered LTP promoted not only AMPA receptor insertion but also generalized recycling of cargo and membrane from endocytic compartments. Thus, recycling endosomes supply AMPA receptors for LTP and provide a mechanistic link between synaptic potentiation and membrane remodeling during synapse modification.

Lateral organization of endocytic machinery in dendritic spines

Postsynaptic membrane trafficking plays an important role in synaptic plasticity, but the organization of trafficking machinery within dendritic spines is poorly understood. We use immunocytochemical analysis of rat hippocampal neurons to show that proteins mediating endocytosis are systematically arrayed within dendritic spines, tangential to the synapse. Thus, previously unrecognized lateral domains of the spine organize endocytic protein machinery at sites removed from the postsynaptic density.

Secretory trafficking in neuronal dendrites

The neuronal secretory pathway represents the intracellular route for proteins involved in synaptic transmission and plasticity, as well as lipids required for outgrowth and remodelling of dendrites and axons. Although neurons use the same secretory compartments as other eukaryotic cells, the enormous distances involved, as well as the unique morphology of the neuron and its signalling requirements, challenge canonical models of secretory pathway organization. Here, we review evidence for a distributed secretory pathway in neurons, suggest mechanisms that may regulate secretory compartment distribution, and discuss the implications of a distributed secretory pathway for neuronal morphogenesis and neural-circuit plasticity.

Learning from NMDA receptor trafficking: clues to the development and maturation of glutamatergic synapses

Activity-dependent changes in excitatory transmission allow the brain to develop, mature, learn and retain memories, and underlie many pathological states of the central nervous system. A principal mechanism by which neurons regulate excitatory transmission is by altering the number and composition of glutamate receptors at the postsynaptic plasma membrane. The dynamic trafficking of glutamate receptors to and from synaptic sites involves a complex series of events including receptor assembly, trafficking through secretory compartments, membrane insertion and endocytic cycling. While these events have become widely appreciated as critical processes regulating AMPA-type glutamate receptors during synaptic plasticity, the mechanisms that control the trafficking of NMDA-type glutamate receptors (NMDARs) are only now beginning to be understood. Until recently, NMDARs were considered immobile receptors, tightly anchored to the postsynaptic membrane. Here, we review recent evidence that challenges this view, focusing on the role that activity plays in altering NMDAR trafficking and how such dynamic regulation of NMDARs may impact on the plasticity of neural circuits.

Microanatomy of dendritic spines: emerging principles of synaptic pathology in psychiatric and neurological disease

Psychiatric and neurologic disorders ranging from mental retardation to addiction are accompanied by structural and functional alterations of synaptic connections in the brain. Such alterations include abnormal density and morphology of dendritic spines, synapse loss, and aberrant synaptic signaling and plasticity. Recent work is revealing an unexpectedly complex biochemical and subcellular organization of dendritic spines. In this review, we highlight the molecular interplay between functional domains of the spine, including the postsynaptic density, the actin cytoskeleton, and membrane trafficking domains. This research points to an emerging level of analysis--a microanatomical understanding of synaptic physiology--that will be critical for discerning how synapses operate in normal physiologic states and for identifying and reversing microscopic changes in psychiatric and neurologic disease.

Coordinated PKA and PKC phosphorylation suppresses RXR-mediated ER retention and regulates the surface delivery of NMDA receptors

Endoplasmic reticulum (ER) retention mediated by the RXR (Arg-X-Arg) motif is an important quality control mechanism used by G-protein coupled receptors and ion channels, including N-methyl-D-aspartate (NMDA) receptors, to ensure the proper assembly and trafficking of multimeric complexes. During assembly, RXR motifs are masked by intersubunit interactions thereby allowing ER release. Here, we find that PKA and PKC phosphorylation sites flanking the RXR motif of the NMDA receptor NR1 subunit suppress ER retention and regulate receptor forward trafficking. These sites are differentially phosphorylated during the trafficking of NR1 subunits in vivo, and phosphorylation at these sites occurs in early secretory compartments. In addition, residues near the RXR motif not involved in phosphorylation are also required for ER retention. These results indicate that ER retention of NMDA receptors is tightly regulated, and suggest that coordinated phosphorylation by PKA and PKC mediates release of receptors from the ER for subsequent traffic to synapses. Phosphorylation-induced ER export of RXR-containing channels and receptors may serve as a novel quality control mechanism for creating a readily releasable pool of receptors sensitive to the activation of intracellular signaling pathways.

Age-related regulation of dendritic endocytosis associated with altered clathrin dynamics

The protein composition of the neuronal plasma membrane is regulated by clathrin-mediated endocytosis and changes drastically over the neuronal lifespan. Here, we utilize the transition out of the period of early postnatal growth as a model system to study age-related changes in endocytosis. Previously, we have found that the dynamic behavior of endocytic clathrin coats in dendrites changes during this period, and that clathrin coat lifetime increases in older neurons. In this study, we examine endocytosis in neuronal dendrites by measuring transferrin (Tf) uptake, and find that it is markedly reduced in older neurons in culture. This decrease was not due to a reduction in transferrin receptor protein levels, nor to a decrease in the expression of endocytic proteins. However, imaging of endocytosis in living dendrites demonstrated that cargo transport through clathrin-coated pits was slower during internalization. Thus, endocytic function in dendrites is altered in older neurons, suggesting that as neurons age, protein trafficking mechanisms are controlled to complement maturational requirements.

Activity-Dependent mRNA Splicing Controls ER Export and Synaptic Delivery of NMDA Receptors

Activity-dependent targeting of NMDA receptors (NMDARs) is a key feature of synapse formation and plasticity. Although mechanisms for rapid trafficking of glutamate receptors have been identified, the molecular events underlying chronic accumulation or loss of synaptic NMDARs have remained unclear. Here we demonstrate that activity controls NMDAR synaptic accumulation by regulating forward trafficking at the endoplasmic reticulum (ER). ER export is accelerated by the alternatively spliced C2' domain of the NR1 subunit and slowed by the C2 splice cassette. This mRNA splicing event at the C2/C2' site is activity dependent, with C2' variants predominating upon activity blockade and C2 variants abundant with increased activity. The switch to C2' accelerates NMDAR forward trafficking by enhancing recruitment of nascent NMDARs to ER exit sites via binding of a divaline motif within C2' to COPII coats. These results define a novel pathway underlying activity-dependent targeting of glutamate receptors, providing an unexpected mechanistic link between activity, mRNA splicing, and membrane trafficking during excitatory synapse modification.

Neuronal Polarity and Trafficking

Among the most morphologically complex cells, neurons are masters of membrane specialization. Nowhere is this more striking than in the division of cellular labor between the axon and the dendrites. In morphology, signaling properties, cytoskeletal organization, and physiological function, axons and dendrites (or more properly, the somatodendritic compartment) are radically different. Such polarization of neurons into domains specialized for either receiving (dendrites) or transmitting (axons) cellular signals provides the underpinning for all neural circuitry. The initial specification of axonal and dendritic identity occurs early in neuronal life, persists for decades, and is manifested by the presence of very different sets of cell surface proteins. Yet, how neuronal polarity is established, how distinct axonal and somatodendritic domains are maintained, and how integral membrane proteins are directed to dendrites or accumulate in axons remain enduring and formidable questions in neuronal cell biology.

Dual modes of endoplasmic reticulum-to-Golgi transport in dendrites revealed by live-cell imaging

Organelles of the neuronal secretory pathway are critical for the addition of membrane that accompanies neuronal development, as well as for the proper localization of plasma membrane proteins necessary for polarity, synaptic transmission, and plasticity. Here, we demonstrate that two organizations of the secretory pathway exist in neurons: one requiring processing of membrane and lipids in the Golgi complex of the cell body and one in which endoplasmic reticulum (ER)-to-Golgi trafficking is localized to dendrites. Using time-lapse imaging of green fluorescent protein-tagged cargo proteins and compartment markers, we show that organelles of the secretory pathway, including ER, ER exit sites, and Golgi, are present and engage in trafficking in neuronal dendrites. We find that ER-to-Golgi trafficking involves highly mobile vesicular carriers that traffic in both the anterograde and retrograde directions throughout the dendritic arbor. Dendritic Golgi outposts, which appear developmentally during the phase of process outgrowth, are involved in the trafficking of both integral membrane proteins and the secreted neuronal growth factor BDNF. This distributed dendritic Golgi represents an organization of the secretory pathway unique among mammalian cells.

Activity level controls postsynaptic composition and signaling via the ubiquitin-proteasome system

Experience-dependent remodeling of the postsynaptic density (PSD) is critical for synapse formation and plasticity in the mammalian brain. Here, in cultured rat hippocampal neurons, I found long-lasting, global changes in the molecular composition of the PSD dictated by synaptic activity. These changes were bidirectional, reversible, modular, and involved multiple classes of PSD proteins. Moreover, activity-dependent remodeling was accompanied by altered protein turnover, occurred with corresponding increases or decreases in ubiquitin conjugation of synaptic proteins and required proteasome-mediated degradation. These modifications, in turn, reciprocally altered synaptic signaling to the downstream effectors CREB (cyclic AMP response element binding protein) and ERK-MAPK (extracellular signal regulated kinase-MAP kinase). These results indicate that activity regulates postsynaptic composition and signaling through the ubiquitin-proteasome system, providing a mechanistic link between synaptic activity, protein turnover and the functional reorganization of synapses.

Dynamics and regulation of clathrin coats at specialized endocytic zones of dendrites and spines

Endocytosis is a fundamental mechanism by which neurons control intercellular signaling, nutrient uptake, and synaptic transmission. This process is carried out by the assembly of clathrin coats and the budding of clathrin-coated vesicles from the neuronal plasma membrane. Here, we demonstrate that in young neurons, clathrin assembly and disassembly occur rapidly, locally, and repeatedly at "hot spots" throughout dendrites and at the tips of dendritic filopodia. In contrast, clathrin coats in mature dendrites reside in stable, long-lasting zones at sites of endocytosis, where clathrin undergoes continuous exchange with local cytosolic pools. In dendritic spines, endocytic zones lie lateral to the postsynaptic density (PSD) where they develop and persist independent of synaptic activity, akin to the PSD itself. These results reveal the presence of a novel specialization dedicated to endocytosis near the postsynaptic membrane.

Molecular morphogens for dendritic spines

Three protein components of the postsynaptic density--Shank, Homer and SPAR--have been found to regulate the structural and molecular organization of dendritic spines. These new studies reveal linkages between receptor complexes, the actin cytoskeleton and signaling molecules that help shape spines.

Molecular determinants of NMDA receptor internalization

Although synaptic AMPA receptors have been shown to rapidly internalize, synaptic NMDA receptors are reported to be static. It is not certain whether NMDA receptor stability at synaptic sites is an inherent property of the receptor, or is due to stabilization by scaffolding proteins. In this study, we demonstrate that NMDA receptors are internalized in both heterologous cells and neurons, and we define an internalization motif, YEKL, on the distal C-terminus of NR2B. In addition, we show that the synaptic protein PSD-95 inhibits NR2B-mediated internalization, and that deletion of the PDZ-binding domain of NR2B increases internalization in neurons. This suggests an involvement for PSD-95 in NMDA receptor regulation and an explanation for NMDA receptor stability at synaptic sites.

An NMDA receptor ER retention signal regulated by phosphorylation and alternative splicing

Formation of mature excitatory synapses requires the assembly and delivery of NMDA receptors to the neuronal plasma membrane. A key step in the trafficking of NMDA receptors to synapses is the exit of newly assembled receptors from the endoplasmic reticulum (ER). Here we report the identification of an RXR-type ER retention/retrieval motif in the C-terminal tail of the NMDA receptor subunit NR1 that regulates receptor surface expression in heterologous cells and in neurons. In addition, we show that PKC phosphorylation and an alternatively spliced consensus type I PDZ-binding domain suppress ER retention. These results demonstrate a novel quality control function for alternatively spliced C-terminal domains of NR1 and implicate both phosphorylation and potential PDZ-mediated interactions in the trafficking of NMDA receptors through early stages of the secretory pathway.

Reinsertion or degradation of AMPA receptors determined by activity-dependent endocytic sorting

Both acute and chronic changes in AMPA receptor (AMPAR) localization are critical for synaptic formation, maturation, and plasticity. Here I report that AMPARs are differentially sorted between recycling and degradative pathways following endocytosis. AMPAR sorting occurs in early endosomes and is regulated by synaptic activity and activation of AMPA and NMDA receptors. AMPAR intemalization triggered by NMDAR activation is Ca2+-dependent, requires protein phosphatases, and is followed by rapid membrane reinsertion. Furthermore, NMDAR-mediated AMPAR trafficking is regulated by PKA and accompanied by dephosphorylation and rephosphorylation of GluR1 subunits at a PKA site. In contrast, activation of AMPARs without NMDAR activation targets AMPARs to late endosomes and lysosomes, independent of Ca2+, protein phosphatases, or PKA. These results demonstrate that activity regulates AMPAR endocytic sorting, providing a potential mechanistic link between rapid and chronic changes in synaptic strength.

Localization and stabilization of ionotropic glutamate receptors at synapses

Appropriate targeting and clustering of ionotropic glutamate receptors (iGluRs) is critical for the formation and maintenance of excitatory synapses. Recent studies have demonstrated that the synaptic localization of iGluR subtypes is remarkably heterogeneous and subject to regulation over time scales ranging from minutes to months. These findings, together with the identification of key protein binding partners of iGluRs, have opened a window onto the complex cell biology of iGluR membrane trafficking. In this article, we review recent findings on the cellular and molecular mechanisms involved in localizing iGluRs at synapses and discuss their implications for synaptogenesis and synaptic plasticity.

Synapse structure: glutamate receptors connected by the shanks

A family of proteins has been identified whose members, the Shanks, physically link two major receptor complexes at excitatory synapses - NMDA receptors and metabotropic glutamate receptors.

Regulation of morphological postsynaptic silent synapses in developing hippocampal neurons

Many excitatory synapses are thought to be postsynaptically 'silent', possessing functional NMDA but lacking functional AMPA glutamate receptors. The acquisition of AMPA receptors at silent synapses may be important in synaptic plasticity and neuronal development. Here we characterize a possible morphological correlate of silent synapses in cultured hippocampal neurons. Initially, most excitatory synapses contained NMDA receptors, but only a few contained detectable AMPA receptors. Synapses progressively acquired AMPA receptors as the cultures matured. AMPA receptor blockade increased the number, size and fluorescent intensity of AMPA receptor clusters and rapidly induced the appearance of AMPA receptors at 'silent' synapses. In contrast, NMDA receptor blockade increased the size, intensity and number of NMDA receptor clusters and decreased the number of AMPA receptor clusters, resulting in an increase in the proportion of 'silent' synapses. These results suggest that the number of silent synapses is regulated during development and by changes in synaptic activity.

Activity-dependent modulation of synaptic AMPA receptor accumulation

Both theoretical and experimental work have suggested that central neurons compensate for changes in excitatory synaptic input in order to maintain a relatively constant output. We report here that inhibition of excitatory synaptic transmission in cultured spinal neurons leads to an increase in mEPSC amplitudes, accompanied by an equivalent increase in the accumulation of AMPA receptors at synapses. Conversely, increasing excitatory synaptic activity leads to a decrease in synaptic AMPA receptors and a decline in mEPSC amplitude. The time course of this synaptic remodeling is slow, similar to the metabolic half-life of neuronal AMPA receptors. Moreover, inhibiting excitatory synaptic transmission significantly prolongs the half-life of the AMPA receptor subunit GluR1, suggesting that synaptic activity modulates the size of the mEPSC by regulating the turnover of postsynaptic AMPA receptors.

Calmodulin mediates calcium-dependent inactivation of N-methyl-D-aspartate receptors

Ca2+ influx through N-methyl-D-aspartate (NMDA) receptors activates signal transduction pathways critical for many forms of synaptic plasticity in the brain. NMDA receptor-mediated Ca2+ influx also downregulates the gating of NMDA channels through a process called Ca2+-dependent inactivation (CDI). Recent studies have demonstrated that the calcium binding protein calmodulin directly interacts with NMDA receptors, suggesting that calmodulin may play a role in CDI. We report here that the mutation of a specific calmodulin binding site in the CO region of the NR1 subunit of the NMDA receptor blocks CDI. Moreover, intracellular infusion of a calmodulin inhibitory peptide markedly reduces CDI of both recombinant and neuronal NMDA receptors. Furthermore, this inactivating effect of calmodulin can be prevented by coexpressing a region of the cytoskeletal protein alpha-actinin2 known to interact with the CO region of NR1. Taken together, these results demonstrate that the binding of Ca2+/calmodulin to NR1 mediates CDI of the NMDA receptor and suggest that inactivation occurs via Ca2+/calmodulin-dependent release of the receptor complex from the neuronal cytoskeleton.

Splice variant-specific interaction of the NMDA receptor subunit NR1 with neuronal intermediate filaments

NMDA receptors are excitatory neurotransmitter receptors critical for synaptic plasticity and neuronal development in the mammalian brain. These receptors are found highly concentrated in the postsynaptic membrane of glutamatergic synapses. To investigate the molecular mechanisms underlying NMDA receptor localization, we used the yeast two-hybrid system to identify proteins expressed in the brain that interact with the NMDA receptor subunit NR1. Here we report that the 68 kDa neurofilament subunit NF-L directly interacts with the NR1 subunit. This interaction occurs between the cytoplasmic C-terminal domain of NR1 and the rod domain of NF-L. However, NR1 splice variants lacking the first C-terminal exon cassette (C1) failed to associate with NF-L. Immunogold electron microscopy revealed a preferential localization of NR1 at the ends of in vitro-assembled neurofilaments. Overexpression of C1 cassette-containing NR1 constructs in fibroblast cells disrupted the assembly of recombinant neurofilaments. In addition, NR1 and NF-L cofractionated in detergent-treated rat brain synaptic plasma membranes. Furthermore, NR1 and NF-L colocalize in the dendrites and growth cones of cultured hippocampal neurons. These results demonstrate the splice variant-specific association of NR1 with neurofilaments and suggest a possible mechanism for anchoring or localizing NMDA receptors in the neuronal plasma membrane.

The development of excitatory synapses in cultured spinal neurons

Immunohistochemical studies of synapses in the CNS have demonstrated that glutamate receptors (GluRs) are concentrated at postsynaptic sites in vivo and in vitro (Baude et al., 1995). The mechanisms leading to receptor clustering at excitatory synapses are far less understood than those governing acetylcholine receptor accumulation at the neuromuscular junction () or glycine receptor aggregation at central inhibitory synapses (). Using cultured rat spinal cord neurons, we demonstrate that clustering of the AMPA receptor subunit GluR1 is among the earliest events in excitatory synapse formation in vitro, coincident with the onset of miniature EPSCs and in many cases preceding presynaptic vesicle accumulation. Postsynaptic receptor clustering is induced in a highly specific and reiterative pattern, independent of receptor activation, by contact with a subset of axons capable of inducing receptor clusters. The subunit composition of AMPA receptor clusters varied significantly between neurons but was invariant within a given neuron. The presence of either GluR2 or GluR3 was common to all receptor clusters. Neither high-affinity glutamate transporters nor NMDA receptors appeared to be concentrated with AMPA receptor subunits at these excitatory synapses.

Characterization of protein kinase A and protein kinase C phosphorylation of the N-methyl-D-aspartate receptor NR1 subunit using phosphorylation site-specific antibodies

Modulation of N-methyl-D-aspartate receptors in the brain by protein phosphorylation may play a central role in the regulation of synaptic plasticity. To examine the phosphorylation of the NR1 subunit of N-methyl-D-aspartate receptors in situ, we have generated several polyclonal antibodies that recognize the NR1 subunit only when specific serine residues are phosphorylated. Using these antibodies, we demonstrate that protein kinase C (PKC) phosphorylates serine residues 890 and 896 and cAMP-dependent protein kinase (PKA) phosphorylates serine residue 897 of the NR1 subunit. Activation of PKC and PKA together lead to the simultaneous phosphorylation of neighboring serine residues 896 and 897. Phosphorylation of serine 890 by PKC results in the dispersion of surface-associated clusters of the NR1 subunit expressed in fibroblasts, while phosphorylation of serine 896 and 897 has no effect on the subcellular distribution of NR1. The PKC-induced redistribution of the NR1 subunit in cells occurs within minutes of serine 890 phosphorylation and reverses upon dephosphorylation. These results demonstrate that PKA and PKC phosphorylate distinct residues within a small region of the NR1 subunit and differentially affect the subcellular distribution of the NR1 subunit.

Interaction of the N-methyl-D-aspartate receptor complex with a novel synapse-associated protein, SAP102

Ionotropic glutamate receptors are known to cluster at high concentration on the postsynaptic membrane of excitatory synapses, but the mechanism by which this occurs is poorly understood. Studies on the neuromuscular junction and central inhibitory synapses suggest that clustering of neurotransmitter receptors requires its interaction with a cytoplasmic protein. Recently, in vitro studies have shown that members of the N-methyl--aspartate (NMDA) class of glutamate receptors interact with a synapse-associated protein, SAP90 (PSD-95). However, evidence for the in vivo interaction of NMDA receptors with SAPs is still lacking. In the present study, we demonstrate the specific interaction between SAP102, a novel synapse-associated protein, and the NMDA receptor complex from the rat cortical synaptic plasma membranes using co-immunoprecipitation techniques. No association was observed between SAP102 and GluR1, a member of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate class of glutamate receptors. To identify the domain on the NMDA receptor responsible for this interaction, we constructed hexahistidine fusion proteins from different regions of the NR1a and NR2 subunits of the NMDA receptor. Immunoblot overlay experiments showed that while the C-terminal domain of the NR2 subunit displayed strong binding, the NR1a intracellular C-terminal tail did not interact with SAP102. The site of interaction was more precisely located to the last 20 amino acids of the NR2 subunit as indicated by the interaction of the synthetic peptide with SAP102. In summary, we demonstrate here for the first time an in vivo interaction between the native NMDA receptor complex and a synapse-associated protein. These results suggest that SAP102 may play an important role in NMDA receptor clustering and immobilization at excitatory synapses.

Synaptic targeting of glutamate receptors

The proper targeting and clustering of neurotransmitter receptors at appropriate postsynaptic sites are principal requirements for the formation of functional synapses. Recently, new studies have begun to elucidate the mechanisms underlying the targeting and clustering of glutamate receptors at excitatory synapses in the brain. Members of the SAP90/PSD-95 family of proteins have emerged as potential regulators of glutamate-receptor membrane distribution. Further, targeting motifs within glutamate receptor subunits have been identified. These findings provide important clues in the effort to understand the molecular features of synaptic organization.

Effect of dopaminergic drugs on the in vivo binding of [3H]WIN 35,428 to central dopamine transporters

[11C]WIN 35,428 (also designated [11C]CFT) is now being used in several positron emission tomography (PET) centers to image dopamine (DA) transporter sites in the mammalian brain. Whether and to what extent in vivo WIN 35,428 binding is influenced by intra- and extrasynaptic dopamine levels are largely unknown. The purpose of the present study was to evaluate the effects of various drugs, known to affect DA levels and release, on the binding of [3H]WIN 35,428 to central DA transporters in the mouse brain. D-Amphetamine, which releases DA from neurons and blocks the DA transporter directly, inhibited striatal [3H]WIN 35,428 binding in dose-dependent manner. Similarly, alpha-methyl-DL-p-tyrosine, an inhibitor of tyrosine hydroxylase, blocked [3H]WIN 35,428 binding, possibly via competitive inhibition by the metabolite p-hydroxyamphetamine. Specific binding of [3H]WIN 35,428 was insensitive to changes in synaptic DA levels caused by pretreatment of the animals with high doses of D2 receptor agonists (apomorphine, bromocriptine), antagonists (haloperidol) or the inhibitor of dopaminergic neuron firing gamma-butyrolactone (GBL). High doses (> 50 mg/kg) of L-DOPA (in combination with benserazide), however, reduced [3H]WIN 35,428 binding significantly, yet for a relatively short time (approximately 2.5 h). Chronic treatment with L-deprenyl elicited no changes in in vivo [3H]WIN 35,428 accumulation in the striatum. Neurotoxic damage of DA neurons caused by administration of high doses of amphetamine was detected in the striatum by a significant reduction in [3H]WIN 35,428 binding 7 days after cessation of amphetamine treatment. Thus, [3H]WIN 35,428 binding was only affected by neurotoxic loss of neurons, by administration of uptake inhibitors, or by some treatments which significantly elevate DA levels. Compounds which inhibit DA release or deplete DA acutely do not increase [3H]WIN 35,428 binding, suggesting that normal or "resting" levels of DA are not sufficient to alter [3H]WIN 35,428 binding in vivo. These findings are important for our understanding of the function and regulation of the DA transporter, as well as the in vivo binding of the radioligand [3H/11 C]WIN 35,428. Moreover, they will be important for the interpretation of PET studies in which [11C]WIN 35,428 is used to assess the integrity of dopaminergic neurons.

Inactivation of NMDA receptors by direct interaction of calmodulin with the NR1 subunit

NMDA (N-methyl-D-aspartate) receptors are excitatory neurotransmitter receptors in the brain critical for synaptic plasticity and neuronal development. These receptors are Ca2+-permeable glutamate-gated ion channels whose physiological properties are regulated by intracellular Ca2+. We report here the purification of a 20 kDa protein identified as calmodulin that interacts with the NR1 subunit of the NMDA receptor. Calmodulin binding to the NR1 subunit is Ca2+ dependent and occurs with homomeric NR1 complexes, heteromeric NR1/NR2 subunit complexes, and NMDA receptors from brain. Furthermore, calmodulin binding to NR1 causes a 4-fold reduction in NMDA channel open probability. These results demonstrate that NMDA receptor function can be regulated by direct binding of calmodulin to the NR1 subunit, and suggest a possible mechanism for activity-dependent feedback inhibition and Ca2+-dependent inactivation of NMDA receptors.