PSD-95 regulates synaptic transmission and plasticity in rat cerebral cortex

Glutamate receptor expression and chronic glutamate toxicity in rat motor cortex

In addition to the loss of spinal motor neurons, amyotrophic lateral sclerosis (ALS) is also associated with degeneration of corticospinal layer V pyramidal neurons and decreased glutamate transport in the cortex. We characterized the glutamate receptors on corticospinal neurons in acutely isolated rat motor cortex slices and found that the synaptic inputs to the corticospinal layer V neurons had a lesser proportional contribution from NMDA receptors relative to AMPA receptors than did layer II/III pyramidal neurons. The synaptic I(AMPA) was also more inwardly rectified, indicating a greater Ca(2+)-permeable component, in layer V. In a cortical organotypic slice culture model, blockade of glutamate transporters elevated glutamate in the media and led to pyramidal neuron loss in both layers. The loss of layer V pyramidal neurons was attenuated by antagonists of AMPA/kainate or Ca(2+)-permeable AMPA receptors, suggesting their therapeutic potential in the protection of the motor cortex in ALS.

Synapse-specific and developmentally regulated targeting of AMPA receptors by a family of MAGUK scaffolding proteins

Trafficking of AMPA receptors (AMPA-Rs) to and from synapses controls the strength of excitatory synaptic transmission. However, proteins that cluster AMPA-Rs at synapses remain poorly understood. Here we show that PSD-95-like membrane-associated guanylate kinases (PSD-MAGUKs) mediate this synaptic targeting, and we uncover a remarkable functional redundancy within this protein family. By manipulating endogenous neuronal PSD-MAGUK levels, we find that both PSD-95 and PSD-93 independently mediate AMPA-R targeting at mature synapses. We also reveal unanticipated synapse heterogeneity as loss of either PSD-95 or PSD-93 silences largely nonoverlapping populations of excitatory synapses. In adult PSD-95 and PSD-93 double knockout animals, SAP-102 is upregulated and compensates for the loss of synaptic AMPA-Rs. At immature synapses, PSD-95 and PSD-93 play little role in synaptic AMPA-R clustering; instead, SAP-102 dominates. These studies establish a PSD-MAGUK-specific regulation of AMPA-R synaptic expression that establishes and maintains glutamatergic synaptic transmission in the mammalian central nervous system.

Long-term potentiation in the juvenile superior colliculus requires simultaneous activation of NMDA receptors and L-type Ca2+ channels and reflects addition of newly functional synapses

The visual layers of the rodent superficial superior colliculus (sSC) have been the focus of many development studies of the molecular bases of retinotopic map formation, the role of early retinal waves in this process, and the development of glutamate synapses. Previous studies have documented long-term potentiation (LTP), believed to be critical to developmental synapse refinement, in the rodent sSC. However, the means of induction and the preparations used have varied widely, and thus cellular changes underlying this LTP remain ambiguous. Whole-cell and perforated patch clamping were used in this study to elucidate the cellular mechanism of electrically evoked LTP in the juvenile rat sSC. This LTP required relatively low-frequency stimulation (20 Hz) and simultaneous activation of NMDA receptors and L-type Ca2+ channels. Experiments focused on narrow-field vertical neurons, a documented excitatory cell type in the stratum griseum superficiale using bipolar stimulation in the stratum opticum. Strontium (Sr2+) replacement of calcium (Ca2+) was applied to study evoked quantal events before and after LTP induction at the same synapses. Paired-pulse ratio and coefficient of variance analyses examined presynaptic release. Increases in quantal frequency were invariably found in the absence of increases in quantal amplitude and probability of release. These data suggest that electrically stimulated LTP, in the juvenile sSC after eye opening, selectively involves the addition or stabilization of AMPA receptors at the large number of silent synapses known to appear in the sSC after eye opening.

Synapse-specific regulation of AMPA receptor function by PSD-95

PSD-95 is a major protein found in virtually all mature excitatory glutamatergic synapses in the brain. Here, we have addressed the role of PSD-95 in controlling glutamatergic synapse function by generating and characterizing a PSD-95 KO mouse. We found that the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)subtype of glutamate receptor (AMPAR)-mediated synaptic transmission was reduced in these mice. Two-photon (2P) uncaging of MNI-glutamate onto individual spines suggested that the decrease in AMPAR function in the PSD-95 KO mouse stems from an increase in the proportion of "silent" synapses i.e., synapses containing N-methyl-d-aspartate (NMDA) receptors (NMDARs) but no AMPARs. Unexpectedly, the silent synapses in the KO mouse were located onto morphologically mature spines. We also observed that a significant population of synapses appeared unaffected by PSD-95 gene deletion, suggesting that the functional role of PSD-95 displays synapse-specificity. In addition, we report that the decay of NMDAR-mediated current was slower in KO mice: The contribution of NR2B subunit containing receptors to the NMDAR-mediated synaptic current was greater in KO mice. The greater occurrence of silent synapses might be related to the greater magnitude of potentiation after long-term potentiation induction observed in these mice. Together, these results suggest a synapse-specific role for PSD-95 in controlling synaptic function that is independent of spine morphology.

Molecular organization and assembly of the postsynaptic density of excitatory brain synapses

The postsynaptic density (PSD) is a postsynaptic membrane specialization at excitatory synapses. The PSD is made of macromolecular multiprotein complexes, which contain a variety of synaptic proteins including membrane, scaffolding, and signaling proteins. By coaggregating with postsynaptic cell adhesion molecules, PSD proteins promote the formation and maturation of excitatory synapses. PSD proteins organize signaling pathways to coordinate structural and functional changes in synapses, and they regulate trafficking and recycling of glutamate receptors, which determines synaptic strength and plasticity. Synaptic activity dynamically regulates the assembly of the PSD through mechanisms including protein phosphorylation, palmitoylation, and protein degradation. PSD proteins associate with diverse motor proteins, suggesting that they function as adaptors linking motors to their specific cargoes.

SPIN90/WISH interacts with PSD-95 and regulates dendritic spinogenesis via an N-WASP-independent mechanism

SPIN90/WISH (SH3 protein interacting with Nck, 90 kDa/Wiskott-Aldrich syndrome protein (WASP) interacting SH3 protein) regulates actin polymerization through its interaction with various actin-regulating proteins. It is highly expressed in the brain, but its role in the nervous system is largely unknown. We report that it is expressed in dendritic spines where it associates with PSD-95. Its overexpression increased the number and length of dendritic filopodia/spines via an N-WASP-independent mechanism, and knock down of its expression with small interfering RNA reduced dendritic spine density. The increase in spinogenesis is accompanied by an increase in synaptogenesis in contacting presynaptic neurons. Interestingly, PSD-95-induced dendritic spinogenesis was completely abolished by knock down of SPIN90/WISH. Finally, in response to chemically induced long-term potentiation, SPIN90/WISH associated with PSD-95 and was redistributed to dendritic spines. Our results suggest that SPIN90/WISH associates with PSD-95, and so becomes localized to dendritic spines where it modulates actin dynamics to control dendritic spinogenesis. They also raise the possibility that SPIN90/WISH is a downstream effector of PSD-95-dependent synaptic remodeling.

Alternative N-terminal domains of PSD-95 and SAP97 govern activity-dependent regulation of synaptic AMPA receptor function

PSD-95 and SAP97 are scaffolding proteins that have been implicated in regulating AMPA receptor incorporation and function at synapses. Gain- and loss-of-function approaches, however, have generated conflicting results. To minimize adaptations during development and potential dominant-negative effects of overexpression, we have combined silencing of endogenous PSD-95 in mature neurons with heterologous expression of specific SAP97 or PSD-95 isoforms. We find that both PSD-95 and SAP97 contain alternative N termini expressing either double cysteines that normally are palmitoylated (alpha-isoforms) or an L27 domain (beta-isoforms). Whereas alpha-isoforms of PSD-95 and SAP97 influence AMPA receptor-mediated synaptic strength independent of activity, the effects of beta-isoforms are regulated by activity in a CaMKII-dependent manner. Importantly, the synaptic effects of the beta-isoforms are masked by the endogenous alpha-isoform of PSD-95. These results demonstrate that the different N termini of the predominant endogenous forms of PSD-95 (alpha-isoform) and SAP97 (beta-isoform) govern their role in regulating synaptic function.

SGK protein kinase facilitates the expression of long-term potentiation in hippocampal neurons

Previous studies showed that the serum- and glucocorticoid-inducible kinase (sgk) gene plays an important role in long-term memory formation. The present study further examined the role of SGK in long-term potentiation (LTP). The dominant-negative mutant of sgk, SGKS422A, was used to inactivate SGK. Results revealed a time-dependent increase in SGK phosphorylation after tetanization with a significant effect observed 3 h and 5 h later. Transfection of SGKS422A impaired the expression, but not the induction, of LTP. Furthermore, the constitutively active sgk, SGKS422D, up-regulated postsynaptic density-95 expression in the hippocampus. These results together support the role of SGK in neuronal plasticity.

Dual role of the exocyst in AMPA receptor targeting and insertion into the postsynaptic membrane

Intracellular membrane trafficking of glutamate receptors at excitatory synapses is critical for synaptic function. However, little is known about the specialized trafficking events occurring at the postsynaptic membrane. We have found that two components of the exocyst complex, Sec8 and Exo70, separately control synaptic targeting and insertion of AMPA-type glutamate receptors. Sec8 controls the directional movement of receptors towards synapses through PDZ-dependent interactions. In contrast, Exo70 mediates receptor insertion at the postsynaptic membrane, but it does not participate in receptor targeting. Thus, interference with Exo70 function accumulates AMPA receptors inside the spine, forming a complex physically associated, but not yet fused with the postsynaptic membrane. Electron microscopic analysis of these complexes indicates that Exo70 mediates AMPA receptor insertion directly within the postsynaptic density, rather than at extrasynaptic membranes. Therefore, we propose a molecular and anatomical model that dissects AMPA receptor sorting and synaptic delivery within the spine, and uncovers new functions of the exocyst at the postsynaptic membrane.

A novel role for cyclic guanosine 3',5'monophosphate signaling in synaptic plasticity: a selective suppressor of protein kinase A-dependent forms of long-term potentiation

At excitatory synapses onto hippocampal CA1 pyramidal cells, activation of cyclic AMP-dependent protein kinase and subsequent down-regulation of protein phosphatases has a crucial role in the induction of long-term potentiation by low-frequency patterns of synaptic stimulation. Because the second messenger cyclic guanosine 3',5'monophosphate can regulate the activity of different forms of the cyclic AMP degrading enzyme phosphodiesterase, we examined whether increases in cyclic guanosine 3',5'monophosphate can modulate long-term potentiation induction in the mouse hippocampal CA1 region through effects on cyclic AMP signaling. Using the cyclic guanosine 3',5'monophosphate-specific phosphodiesterase inhibitor zaprinast or the nitric oxide donor S-nitroso-D,L-penicillamine to elevate cyclic guanosine 3',5'monophosphate levels we found that increases in cyclic guanosine 3',5'monophosphate strongly inhibit the induction of long-term potentiation by low-frequency patterns of synaptic stimulation where protein kinase A activation is required for long-term potentiation induction. In contrast, zaprinast and S-nitroso-D,L-penicillamine had no effect on the induction of long-term potentiation by high-frequency patterns of synaptic stimulation that induce long-term potentiation in a protein kinase A-independent manner. Directly activating protein kinase A with the phosphodiesterase-resistant cyclic AMP analog 8-Br-cAMP, blocking all phosphodiesterases with 3-isobutyl-1-methylxanthine, or inhibiting protein phosphatases rescued long-term potentiation induction in zaprinast-treated slices. Together, these results suggest that increases in cyclic guanosine 3',5'monophosphate inhibit long-term potentiation by activating phosphodiesterases that interfere with the protein kinase A-mediated suppression of protein phosphatases needed for long-term potentiation induction. Consistent with the notion that this cyclic guanosine 3',5'monophosphate-mediated inhibitory pathway is recruited by some patterns of synaptic activity, blocking cyclic guanosine 3',5'monophosphate production strongly facilitated the induction of long-term potentiation by long trains of theta-frequency synaptic stimulation. Together, our results indicate that increases in cyclic guanosine 3',5'monophosphate can act as a long-term potentiation suppressor mechanism that selectively constrains the induction of protein kinase A-dependent forms of long-term potentiation induced by low-frequency patterns of synaptic stimulation.

Auxiliary Subunits Assist AMPA-Type Glutamate Receptors

Glutamate, the major excitatory neurotransmitter in the brain, acts primarily on two types of ionotropic receptors: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and N-methyl-d-aspartate (NMDA) receptors. Work over the past decade indicates that regulated changes in the number of synaptic AMPA receptors may serve as a mechanism for information storage. Recent studies demonstrate that a family of small transmembrane AMPA receptor regulatory proteins (TARPs) controls both AMPA receptor trafficking and channel gating. TARPs provide the first example of auxiliary subunits of ionotropic receptors. Here we review the pivotal role that TARPs play in the life cycle of AMPA receptors.

Synapse-specific regulation of AMPA receptor subunit composition by activity

We examined the changes that arise when neurotransmitter release is inhibited in a subpopulation of hippocampal neurons in coculture with normally active neighbors. Subsets of neurons were presynaptically silenced by chronic expression of tetanus toxin light chain tagged with cyan fluorescent protein (TNTCFP). Surprisingly, silenced neurons formed as many presynaptic terminals as their active neighbors when grown together on glial microislands. However, silenced neurons could not recruit the AMPA-type glutamate receptor subunit GluR1 as efficiently when competing with active neighbors. The immunofluorescence intensity ratio of GluR1 at synaptic puncta versus shaft was reduced by 22% opposite TNTCFP-expressing terminals compared with active neighbors. In contrast, this effect is abolished when vesicular release is blocked in all neurons. Local presynaptic inhibition by TNTCFP did not change the synaptic level of the AMPA receptor subunits GluR2 or GluR2/3 or of the PSD95 (postsynaptic density 95) family scaffolding proteins. Thus, neurotransmitter release selectively regulates the AMPA receptor population on a synapse-by-synapse basis but is not essential for an axon to efficiently compete for synaptic territory in a simple model system. These results demonstrate precise input specificity of postsynaptic receptor composition via differential activity among neighbor synapses.

Change in the shape and density of dendritic spines caused by overexpression of acidic calponin in cultured hippocampal neurons

Dendritic spines are morphing structures believed to provide a cellular substrate for synaptic plasticity. It has been suggested that the actin cytoskeleton is the target of molecular mechanisms regulating spine morphology. Here we hypothesized that acidic calponin, an actin-binding protein, is one of the key regulators of actin filaments during spine plasticity. Our data showed that the overexpression of acidic calponin-GFP (green fluorescent protein) in primary cultures of rat hippocampal neurons causes an elongation of spines and an increase of their density as compared with those of GFP-expressing neurons. These effects required the actin-binding domains of acidic calponin. The close apposition of the presynatic marker synaptophysin to these long spines and the presence of specific postsynaptic markers actin, PSD-95, NR1, and GluR1 suggested the existence of functional excitatory synaptic contacts. Indeed, electrophysiological data showed that the postsynaptic overexpression of acidic calponin enhanced the frequency of miniature excitatory postsynaptic currents as compared with that of GFP-expressing neurons, but did not affect their properties such as amplitude, rise time, and half width. Studies in heterologous cells revealed that acidic calponin reorganized the actin filaments and stabilized them. Taken together, these findings show that acidic calponin regulates dendritic spine morphology and density, likely via regulation of the actin cytoskeleton reorganization and dynamic. Furthermore, the acidic calponin-induced spines are able to establish functional glutamatergic synapses. Such data suggest that acidic calponin is a key factor in the regulation of spine plasticity and synaptic activity.

Enriched environment promotes behavioral and morphological recovery in a mouse model for the fragile X syndrome

Fragile X syndrome, the most frequent form of hereditary mental retardation, is due to a mutation of the fragile X mental retardation 1 (FMR1) gene on the X chromosome. Like fragile X patients, FMR1-knockout (FMR1-KO) mice lack the normal fragile X mental retardation protein (FMRP) and show both cognitive alterations and an immature neuronal morphology. We reared FMR1-KO mice in a C57BL/6 background in enriched environmental conditions to examine the possibility that experience-dependent stimulation alleviates their behavioral and neuronal abnormalities. FMR1-KO mice kept in standard cages were hyperactive, displayed an altered pattern of open field exploration, and did not show habituation. Quantitative morphological analyses revealed a reduction in basal dendrite length and branching together with more immature-appearing spines along apical dendrites of layer five pyramidal neurons in the visual cortex. Enrichment largely rescued these behavioral and neuronal abnormalities while increasing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit 1 (GluR1) levels in both genotypes. Enrichment did not, however, affect FMRP levels in the WT mice. These data suggest that FMRP-independent pathways activating glutamatergic signaling are preserved in FMR1-KO mice and that they can be elicited by environmental stimulation.

MEMBRANE-ASSOCIATED GUANYLATE KINASES REGULATE ADHESION AND PLASTICITY AT CELL JUNCTIONS

Tissue development, differentiation, and physiology require specialized cellular adhesion and signal transduction at sites of cell-cell contact. Scaffolding proteins that tether adhesion molecules, receptors, and intracellular signaling enzymes organize macromolecular protein complexes at cellular junctions to integrate these functions. One family of such scaffolding proteins is the large group of membrane-associated guanylate kinases (MAGUKs). Genetic studies have highlighted critical roles for MAGUK proteins in the development and physiology of numerous tissues from a variety of metazoan organisms. Mutation of Drosophila discs large (dlg) disrupts epithelial septate junctions and causes overgrowth of imaginal discs. Similarly, mutation of lin-2, a related MAGUK in Caenorhabditis elegans, blocks vulval development, and mutation of the postsynaptic density protein PSD-95 impairs synaptic plasticity in mammalian brain. These diverse roles are explained by recent biochemical and structural analyses of MAGUKs, which demonstrate their capacity to assemble well--efined--yet adaptable--protein complexes at cellular junctions.

The expanding role of PSD-95: a new link to addiction

One feature of addiction is the enhanced locomotor response known as behavioral sensitization that occurs with prolonged exposure to psychostimulants. In a recent study, Yao et al. have analyzed four distinct animal models that share this common phenotype. Evaluation of gene expression in the striatum reveals that postsynaptic density (PSD)-95, a synaptic scaffolding protein, is downregulated in each model, suggesting an essential role for this protein in addiction.

Bidirectional synaptic plasticity regulated by phosphorylation of stargazin-like TARPs

Synaptic plasticity involves protein phosphorylation cascades that alter the density of AMPA-type glutamate receptors at excitatory synapses; however, the crucial phosphorylated substrates remain uncertain. Here, we show that the AMPA receptor-associated protein stargazin is quantitatively phosphorylated and that stargazin phosphorylation promotes synaptic trafficking of AMPA receptors. Synaptic NMDA receptor activity can induce both stargazin phosphorylation, via activation of CaMKII and PKC, and stargazin dephosphorylation, by activation of PP1 downstream of PP2B. At hippocampal synapses, long-term potentiation and long-term depression require stargazin phosphorylation and dephosphorylation, respectively. These results establish stargazin as a critical substrate in the bidirectional control of synaptic strength, which is thought to underlie aspects of learning and memory.

A proteasome-sensitive connection between PSD-95 and GluR1 endocytosis

Synaptic transmission at excitatory synapses can be regulated by changing the number of synaptic glutamate receptors (GluRs) through endocytosis and exocytosis. The endocytosis of GluRs has recently been shown to require the activity of the ubiquitin-proteasome system (UPS): proteasome inhibitors or dominant negative forms of ubiquitin block the ligand-stimulated internalization of GluRs. We have examined whether PSD-95 is a potential target of the UPS. Following neurotransmitter stimulation, PSD-95 levels are negatively correlated with the magnitude of internalized GluR1 in individual neurons. Neurotransmitter stimulation also results in a proteasome-dependent decrease in dendritic PSD-95. Consistent with the idea that PSD-95 degradation is important for GluR internalization, overexpression of PSD-95 can inhibit neurotransmitter-stimulated GluR1 endocytosis. If PSD-95 is a direct target for proteasomal degradation, then the polyubiquitination of PSD-95 is expected. Using experimental conditions that favor the detection of polyubiquitination, however, no ubiquitination of PSD-95 was detected. It is possible that the polyubiquitination of PSD-95 is short-lived and thus difficult to detect. Alternatively, the regulation of PSD-95 levels by the proteasome important for ligand-stimulated GluR endocytosis may be accomplished via an intermediate protein.

PDZ domain proteins of synapses

PDZ domains are protein-interaction domains that are often found in multi-domain scaffolding proteins. PDZ-containing scaffolds assemble specific proteins into large molecular complexes at defined locations in the cell. In the postsynaptic density of neuronal excitatory synapses, PDZ proteins such as PSD-95 organize glutamate receptors and their associated signalling proteins and determine the size and strength of synapses. PDZ scaffolds also function in the dynamic trafficking of synaptic proteins by assembling cargo complexes for transport by molecular motors. As key organizers that control synaptic protein composition and structure, PDZ scaffolds are themselves highly regulated by synthesis and degradation, subcellular distribution and post-translational modification.

A balance between excitatory and inhibitory synapses is controlled by PSD-95 and neuroligin

Factors that control differentiation of presynaptic and postsynaptic elements into excitatory or inhibitory synapses are poorly defined. Here we show that the postsynaptic density (PSD) proteins PSD-95 and neuroligin-1 (NLG) are critical for dictating the ratio of excitatory-to-inhibitory synaptic contacts. Exogenous NLG increased both excitatory and inhibitory presynaptic contacts and the frequency of miniature excitatory and inhibitory synaptic currents. In contrast, PSD-95 overexpression enhanced excitatory synapse size and miniature frequency, but reduced the number of inhibitory synaptic contacts. Introduction of PSD-95 with NLG augmented synaptic clustering of NLG and abolished NLG effects on inhibitory synapses. Interfering with endogenous PSD-95 expression alone was sufficient to reduce the ratio of excitatory-to-inhibitory synapses. These findings elucidate a mechanism by which the amounts of specific elements critical for synapse formation control the ratio of excitatory-to-inhibitory synaptic input.

SKCa channels mediate the medium but not the slow calcium-activated afterhyperpolarization in cortical neurons

Many neurons, including pyramidal cells of the cortex, express a slow afterhyperpolarization (sAHP) that regulates their firing. Although initial findings suggested that the current underlying the sAHP could be carried through SK(Ca) channels, recent work has uncovered anomalies that are not congruent with this idea. Here, we used overexpression and dominant-negative strategies to assess the involvement of SK(Ca) channels in mediating the current underlying the sAHP in pyramidal cells of the cerebral cortex. Pyramidal cells of layer V exhibit robust AHP currents composed of two kinetically and pharmacologically distinguishable currents known as the medium AHP current (I(mAHP)) and the slow AHP current (I(sAHP)). I(mAHP) is blocked by the SK(Ca) channel blockers apamin and bicuculline, whereas I(sAHP) is resistant to these agents but is inhibited by activation of muscarinic receptors. To test for a role for SK(Ca) channels, we overexpressed K(Ca)2.1 (SK1) and K(Ca)2.2 (SK2), the predominant SK(Ca) subunits expressed in the cortex, in pyramidal cells of cultured brain slices. Overexpression of K(Ca)2.1 and K(Ca)2.2 resulted in a fourfold to fivefold increase in the amplitude of I(mAHP) but had no detectable effect on I(sAHP). As an additional test, we examined I(sAHP) in a transgenic mouse expressing a truncated SK(Ca) subunit (SK3-1B) capable of acting as a dominant negative for the entire family of SK(Ca)-IK(Ca) channels. Expression of SK3-1B profoundly inhibited I(mAHP) but again had no discernable effect on I(sAHP). These results are inconsistent with the proposal that SK(Ca) channels mediate I(sAHP) in pyramidal cells and indicate that a different potassium channel mediates this current.

Receptor compartmentalization and trafficking at glutamate synapses: a developmental proposal

This article focuses on NMDA receptor subunit changes that occur in the forebrain and midbrain during development, namely the switch from predominance of NMDA receptors rich in NR2B subunits to that of NMDA receptors rich in NR2A subunits. We review the potential roles in brain plasticity of two membrane-associated guanylate kinases (MAGUKs), SAP102 and PSD95, which form a scaffold for the ion-passing glutamate receptors at the postsynaptic density, and we consider the known functional significance of these molecules in subunit switching. In addition, based on recent analyses of the synaptic location of glutamate receptors, activity-dependent changes in developing visual neurons, and extensive data on MAGUKs, we propose a model of glutamatergic synaptic differentiation. In this model, different NMDA receptor scaffolding and signaling complexes effect the trafficking and synaptic localization of NR2A-rich and NR2B-rich receptors, leading to tangential compartmentalization of these receptors and their movement between synaptic and extrasynaptic compartments.

Eye Opening Rapidly Induces Synaptic Potentiation and Refinement

NMDA receptor (NMDAR)-mediated increases in AMPA receptor (AMPAR) currents are associated with long-term synaptic potentiation (LTP). Here, we provide evidence that similar changes occur in response to normal increases in sensory stimulation during development. Experiments discriminated between eye opening-induced and age-dependent changes in synaptic currents. At 6 hr after eye opening (AEO), a transient population of currents mediated by NR2B-rich NMDARs increase significantly, and silent synapses peak. Sustained increases in evoked and miniature AMPAR currents occur at 12 hr AEO. Significant changes in AMPAR:NMDAR evoked current ratios, contacts per axon, and inputs per cell are present at 24 hr AEO. The AMPAR current changes are those seen in vitro during NMDAR-dependent LTP. Here, they are a consequence of eye opening and are associated with a new wave of synaptic refinement. These data also suggest that new NR2B-rich NMDAR currents precede and may initiate this developmental synaptic potentiation and functional tuning.

Hallucinogens

Hallucinogens (psychedelics) are psychoactive substances that powerfully alter perception, mood, and a host of cognitive processes. They are considered physiologically safe and do not produce dependence or addiction. Their origin predates written history, and they were employed by early cultures in a variety of sociocultural and ritual contexts. In the 1950s, after the virtually contemporaneous discovery of both serotonin (5-HT) and lysergic acid diethylamide (LSD-25), early brain research focused intensely on the possibility that LSD or other hallucinogens had a serotonergic basis of action and reinforced the idea that 5-HT was an important neurotransmitter in brain. These ideas were eventually proven, and today it is believed that hallucinogens stimulate 5-HT(2A) receptors, especially those expressed on neocortical pyramidal cells. Activation of 5-HT(2A) receptors also leads to increased cortical glutamate levels presumably by a presynaptic receptor-mediated release from thalamic afferents. These findings have led to comparisons of the effects of classical hallucinogens with certain aspects of acute psychosis and to a focus on thalamocortical interactions as key to understanding both the action of these substances and the neuroanatomical sites involved in altered states of consciousness (ASC). In vivo brain imaging in humans using [(18)F]fluorodeoxyglucose has shown that hallucinogens increase prefrontal cortical metabolism, and correlations have been developed between activity in specific brain areas and psychological elements of the ASC produced by hallucinogens. The 5-HT(2A) receptor clearly plays an essential role in cognitive processing, including working memory, and ligands for this receptor may be extremely useful tools for future cognitive neuroscience research. In addition, it appears entirely possible that utility may still emerge for the use of hallucinogens in treating alcoholism, substance abuse, and certain psychiatric disorders.

Postsynaptic density 95 controls AMPA receptor incorporation during long-term potentiation and experience-driven synaptic plasticity

The regulated delivery of AMPA-type glutamate receptors (AMPARs) to synapses is an important mechanism underlying synaptic plasticity. Here, we ask whether the synaptic scaffolding protein PSD-95 (postsynaptic density 95) participates in AMPAR incorporation during two forms of synaptic plasticity. In hippocampal slice cultures, the expression of PSD-95-green fluorescent protein (PSD-95-GFP) increases AMPAR currents by selectively delivering glutamate receptor 1 (GluR1)-containing receptors to synapses, thus mimicking long-term potentiation (LTP). Mutational analysis shows that the N terminal of PSD-95 including the first two PDZ [PSD-95/Discs large (Dlg)/zona occludens-1 (ZO-1)] domains is necessary and sufficient to mediate this effect. Further supporting a role in synaptic plasticity, wild-type PSD-95 occludes LTP and dominant negative forms block LTP. Moreover, we demonstrate that PSD-95 also participates in AMPAR delivery during experience-driven plasticity in vivo. In the barrel cortex from experience-deprived animals, the expression of PSD-95-GFP selectively increases AMPAR currents, mimicking experience-driven plasticity. In nondeprived animals, PSD-95-GFP produces no additional potentiation, indicating common mechanisms between PSD-95-mediated potentiation and experience-driven synaptic strengthening. A dominant negative form of PSD-95 blocks experience-driven potentiation of synapses. Pharmacological analysis in slice cultures reveals that PSD-95 acts downstream of other signaling pathways involved in LTP. We conclude that PSD-95 controls activity-dependent AMPAR incorporation at synapses via PDZ interactions not only during LTP in vitro but also during experience-driven synaptic strengthening by natural stimuli in vivo.

Postsynaptic density protein 95 mediates Ca2+/calmodulin-dependent protein kinase II-activated serine phosphorylation of neuronal nitric oxide synthase during brain ischemia in rat hippocampus

Recent study has indicated that postsynaptic density protein 95 (PSD95) promotes Ca2+/calmodulin-dependent protein kinase II (CaMKII)-mediated serine phosphorylation of neuronal nitric oxide synthase (nNOS). To investigate whether PSD95 is involved in the brain ischemia-induced enhancement of serine phosphorylation of nNOS by CaMKII in rat hippocampus, we examined the interactions among CaMKIIalpha, PSD95 and nNOS, and the effects of suppression of PSD95 expression on both the increased serine phosphorylation of nNOS and the interactions mentioned above by immunoprecipitation and immunoblotting. The following results were observed: (1) brain ischemia increased markedly the interactions of CaMKIIalpha and nNOS with PSD95. (2) Intracerebroventricular infusion of PSD95 antisense oligodeoxynucleotides, but not missense oligodeoxynucleotides or vehicle, not only significantly decreased the protein level of PSD95 but also attenuated the elevated serine phosphorylation of nNOS and the interactions among CaMKIIalpha, PSD95 and nNOS induced by 15 min ischemia. These data suggested that PSD95 is important for facilitating nNOS serine phosphorylation by CaMKII.

Identification of PSD-95 as a Regulator of Dopamine-Mediated Synaptic and Behavioral Plasticity

To identify the molecular mechanisms underlying psychostimulant-elicited plasticity in the brain reward system, we undertook a phenotype-driven approach using genome-wide microarray profiling of striatal transcripts from three genetic and one pharmacological mouse models of psychostimulant or dopamine supersensitivity. A small set of co-affected genes was identified. One of these genes encoding the synaptic scaffolding protein PSD-95 is downregulated in the striatum of all three mutants and in chronically, but not acutely, cocaine-treated mice. At the synaptic level, enhanced long-term potentiation (LTP) of the frontocortico-accumbal glutamatergic synapses correlates with PSD-95 reduction in every case. Finally, targeted deletion of PSD-95 in an independent line of mice enhances LTP, augments the acute locomotor-stimulating effects of cocaine, but leads to no further behavioral plasticity in response to chronic cocaine. Our findings uncover a previously unappreciated role of PSD-95 in psychostimulant action and identify a molecular and cellular mechanism shared between drug-related plasticity and learning.

Functional excitatory synapses in HEK293 cells expressing neuroligin and glutamate receptors

The discovery that neuroligin is a key protein involved in synapse formation offers the unprecedented opportunity to induce functional synapses between neurons and heterologous cells. We took this opportunity recording for the first-time synaptic currents in human embryonic kidney 293 (HEK293) cells transfected with neuroligin and the N-methyl-d-aspartate or AMPA receptor subunits in a co-culture with rat cerebellar granule cells. These currents were similar to synaptic currents recorded in neurons, and their decay kinetics was determined by the postsynaptic subunit combination. Although neuroligin expression was sufficient to detect functional synapses, cotransfection of HEK293 cells with Postsynaptic density-95/synapse-associated protein-90 (PSD-95) significantly increased current frequency. Our results support the central role of neuroligin in the formation of CNS synapses, validate the proposal that PSD-95 allows synaptic maturation, and provide a unique experimental model to study how molecular components determine functional properties of excitatory synapses.

Neuronal-NOS adaptor protein expression after spreading depression: implications for NO production and ischemic tolerance

Cortical spreading depression (CSD) is characterized by slowly propagating waves of neuronal/astrocytic depolarization and metabolic changes, followed by a period of quiescent neuronal and electroencephalographic activity. CSD acts as a preconditioning stimulus in brain, reducing cell death when elicited up to several days prior to an ischemic insult. Precise mechanisms associated with this neuroprotection are not known, although CSD increases the expression of a number of potentially neuroprotective genes/proteins. The nitric oxide (NO) system may be of particular importance, as it is acutely activated and chronically up-regulated in cerebral cortex by CSD, and NO can ameliorate and exacerbate cell death under different conditions. Several molecules have recently been identified that modulate the production and/or cellular actions of NO, but it is not known whether their expression is altered by CSD. Therefore, the present study examined the effect of CSD on the spatiotemporal expression of PIN, CAPON, PSD-95, Mn-SOD and Cu/Zn-SOD mRNA in the rat brain. In situ hybridization using specific [35S]-labelled oligonucleotides revealed that levels of PIN mRNA were significantly increased in the cortex and claustrum ( approximately 30-180%; p </= 0.01) after 6 h and 1 and 2 days, but were again equivalent to contralateral (control) cortical values at 7, 14 and 28 days. CAPON mRNA levels were increased ( approximately 30-180%; p </= 0.05) in the ipsilateral cortical hemisphere at 6 h and 2 days post treatment, but not at the other times examined. In contrast, levels of PSD-95, Mn- and Cu/Zn-SOD mRNA were not altered at any time after CSD. These results suggest that following CSD, nNOS activity and NO levels may be tightly regulated by both transcriptional and translational alterations in a range of nNOS adaptor proteins, which may contribute to CSD-induced neuroprotection against subsequent ischemia.

Ubiquitination Regulates PSD-95 Degradation and AMPA Receptor Surface Expression

PSD-95 is a major scaffolding protein of the postsynaptic density, tethering NMDA- and AMPA-type glutamate receptors to signaling proteins and the neuronal cytoskeleton. Here we show that PSD-95 is regulated by the ubiquitin-proteasome pathway. PSD-95 interacts with and is ubiquitinated by the E3 ligase Mdm2. In response to NMDA receptor activation, PSD-95 is ubiquitinated and rapidly removed from synaptic sites by proteasome-dependent degradation. Mutations that block PSD-95 ubiquitination prevent NMDA-induced AMPA receptor endocytosis. Likewise, proteasome inhibitors prevent NMDA-induced AMPA receptor internalization and synaptically induced long-term depression. This is consistent with the notion that PSD-95 levels are an important determinant of AMPA receptor number at the synapse. These data suggest that ubiquitination of PSD-95 through an Mdm2-mediated pathway is critical in regulating AMPA receptor surface expression during synaptic plasticity.

Organization of postsynaptic density proteins and glutamate receptors in axodendritic and dendrodendritic synapses of the rat olfactory bulb

Glutamate neurotransmission in the olfactory bulb involves both axodendritic synapses and dendrodendritic reciprocal synapses and possibly also extrasynaptic receptors. By using a sensitive immunogold procedure, we have investigated the organization of two synaptic scaffolding molecules, PSD-95 and PSD-93, as well as N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptors, at these heterogeneous glutamate signaling sites. Immunolabeling for PSD-95 and PSD-93 was present in all major types of putative glutamatergic synapse, suggesting that these proteins are essential components of the synaptic signaling apparatus. The linear density and the subsynaptic distribution of PSD-95/PSD-93 gold particles did not differ significantly between axodendritic and dendrodendritic synapses. Antibodies recognizing NMDA and AMPA receptor subunits also labeled asymmetric synapses throughout the olfactory bulb. Immunolabeling for the AMPA receptor subunits GluR2/3 was similar in all types of synapse. In contrast, immunogold signals for the NR1 subunit of NMDA receptors varied significantly among different synapse populations, with olfactory nerve synapses in the glomerular layer showing the lowest labeling intensity. Although the lateral dendrites of mitral and tufted cells have been reported to respond to glutamate, they did not display significant plasma membrane labeling for the NR1 subunit or for PSD-95, suggesting that the physiological effects of glutamate at these sites are mediated by NMDA autoreceptors that are not clustered and occur only at a low density on the dendritic surface. Our quantitative analysis of olfactory bulb synapses indicates that the density of NMDA receptors is not determined by the complement of PSD-95/PSD-93. The latter molecules appear to be expressed in an all-or-none fashion and may form a standard lattice common to different types of glutamatergic synapse.

Postsynaptic density-95 mimics and occludes hippocampal long-term potentiation and enhances long-term depression

Previous studies have shown that overexpression of the protein PSD-95 (postsynaptic density-95) selectively enhances AMPA receptor-mediated synaptic responses in hippocampal pyramidal cells. To determine whether this effect is related to synaptic plasticity at these synapses, we examined whether PSD-95 expression mimics long-term potentiation (LTP), and also whether it influences LTP and long-term depression (LTD) in hippocampal slice cultures. Using simultaneous recording from transfected or infected cells and control pyramidal cells, we found that PSD-95, similar to LTP, increases the amplitude and frequency of miniature EPSCs. It also converts silent synapses to functional synapses, as does LTP. In addition, LTP is completely occluded in cells expressing PSD-95, whereas LTD is greatly enhanced. These results suggest that common mechanisms are involved in controlling synaptic AMPA receptors by PSD-95 and synaptic plasticity.

Postsynaptic density-95 mimics and occludes hippocampal long-term potentiation and enhances long-term depression

Previous studies have shown that overexpression of the protein PSD-95 (postsynaptic density-95) selectively enhances AMPA receptor-mediated synaptic responses in hippocampal pyramidal cells. To determine whether this effect is related to synaptic plasticity at these synapses, we examined whether PSD-95 expression mimics long-term potentiation (LTP), and also whether it influences LTP and long-term depression (LTD) in hippocampal slice cultures. Using simultaneous recording from transfected or infected cells and control pyramidal cells, we found that PSD-95, similar to LTP, increases the amplitude and frequency of miniature EPSCs. It also converts silent synapses to functional synapses, as does LTP. In addition, LTP is completely occluded in cells expressing PSD-95, whereas LTD is greatly enhanced. These results suggest that common mechanisms are involved in controlling synaptic AMPA receptors by PSD-95 and synaptic plasticity.

Suppression of postsynaptic density protein 95 expression attenuates increased tyrosine phosphorylation of NR2A subunits of N-methyl-D-aspartate receptors and interactions of Src and Fyn with NR2A after transient brain ischemia in rat hippocampus

The effects of suppression of postsynaptic density protein 95 (PSD-95) expression on the increased tyrosine phosphorylation of N-methyl-D-aspartate receptor subunit NR2A and interactions of Src and Fyn with NR2A after brain ischemia were investigated by immunoprecipitation and immunoblotting. Transient (15 min) brain ischemia was induced by the four-vessel occlusion method in Sprague-Dawley rats. Intracerebroventricular infusion of PSD-95 antisense oligonucleotides (every 24 h for 3 days before ischemia), but not missense oligonucleotides or vehicle, not only markedly decreased the protein level of PSD-95 but also attenuated the elevated tyrosine phosphorylation of NR2A and interactions of Src and Fyn with NR2A induced by 6 h of reperfusion following ischemia in the hippocampus. The protein levels of NR2A, Src and Fyn had no differences under the above conditions. These data suggested that PSD-95 is critical for facilitating NR2A tyrosine phosphorylation by Src family kinases in postischemic brain.