Organization and regulation of proteins at synapses

Neuronal pentraxin-2 (NPTX2) serum levels during an acute psychotic episode in patients with schizophrenia

BACKGROUND

Neuronal pentraxin-2 (NPTX2, an immediate-early gene), which regulates synapse activity and neuroplasticity, plays an essential role in the neurodevelopmental process. NPTX2 possibly enhances the accumulation of amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptors (AMPAR) on the postsynaptic membranes and stimulates excitatory synaptogenesis. We aimed to evaluate the plasma concentrations of NPTX2 of patients with schizophrenia in acute psychotic episodes compared with matched community-based controls.

METHODS

Ninety-three (93) patients diagnosed with schizophrenia according to DSM-5 and 83 healthy controls were included. The patients, all of which were in acute psychotic episodes, were recruited from the inpatient clinic. The patients were assessed by the Positive and Negative Syndrome Scale (PANSS) and Clinical Global Impression- Severity (CGIS) scale, whereas the healthy subjects were evaluated with Structured Clinical Interview for DSM-5 (SCID-5) to exclude any major psychiatric diagnoses.

RESULTS

NPTX2 serum concentrations were significantly higher in the schizophrenia group (p < 0.001). NPTX2 levels negatively correlated with age (p = 0.004) and PANSS-positive symptom scores (p < 0.001). The most determinant factors in predicting the change in NPTX2 levels were PANSS-positive symptom and general psychopathology scores.

CONCLUSIONS

We conclude that NPTX2 could be involved in schizophrenia pathophysiology and valuable as a synapse-derived and glutamate-related biomarker. Further studies in larger samples assessing NPTX2 levels in remitted schizophrenia patients and combining neuroimaging techniques and cognitive evaluations with blood samples are needed.

Nicotine Facilitates Facial Stimulation-Evoked Mossy Fiber-Granule Cell Long-Term Potentiation in vivo in Mice

Nicotine is a psychoactive component of tobacco that plays critical roles in the regulation of neuronal circuit function and neuroplasticity and contributes to the improvement of working memory performance and motor learning function via nicotinic acetylcholine receptors (nAChRs). Under in vivo conditions, nicotine enhances facial stimulation-evoked mossy fiber-granule cell (MF-GrC) synaptic transmission, which suggests that nicotine regulates MF-GrC synaptic plasticity in the mouse cerebellar cortex. In this study, we investigated the effects of nicotine on facial stimulation-induced long-term potentiation (LTP) of MF-GrC synaptic transmission in urethane-anesthetized mice. Our results showed that facial stimulation at 20 Hz induced an MF-GrC LTP in the mouse cerebellar granular layer that was significantly enhanced by the application of nicotine (1 μM). Blockade of α4β2 nAChRs, but not α7 nAChRs, during delivery of 20 Hz facial stimulation prevented the nicotine-induced facilitation of MF-GrC LTP. Notably, the facial stimulation-induced MF-GrC LTP was abolished by an N-methyl-D-aspartate (NMDA) receptor antagonist, but it was restored by additional application of nicotine during delivery of 20 Hz facial stimulation. Furthermore, antagonism of α4β2 nAChRs, but not α7 nAChRs, during delivery of 20 Hz facial stimulation prevented nicotine-induced MF-GrC LTP. Moreover, inhibition of nitric oxide synthase (NOS) abolished the facial stimulation-induced MF-GrC LTP, as well as the effect of nicotine on it. Our results indicated that 20 Hz facial stimulation induced MF-GrC LTP via an NMDA receptor/nitric oxide (NO) cascade, but MF-GrC LTP was enhanced by nicotine through the α4β2 AChR/NO signaling pathway. These results suggest that nicotine-induced facilitation of MF-GrC LTP may play a critical role in the improvement of working memory performance and motor learning function.

Facial Stimulation Induces Long-Term Potentiation of Mossy Fiber-Granule Cell Synaptic Transmission via GluN2A-Containing N-Methyl-D-Aspartate Receptor/Nitric Oxide Cascade in the Mouse Cerebellum

Long-term synaptic plasticity in the cerebellar cortex is a possible mechanism for motor learning. Previous studies have demonstrated the induction of mossy fiber-granule cell (MF-GrC) synaptic plasticity under in vitro and in vivo conditions, but the mechanisms underlying sensory stimulation-evoked long-term synaptic plasticity of MF-GrC in living animals are unclear. In this study, we investigated the mechanism of long-term potentiation (LTP) of MF-GrC synaptic transmission in the cerebellum induced by train of facial stimulation at 20 Hz in urethane-anesthetized mice using electrophysiological recording, immunohistochemistry techniques, and pharmacological methods. Blockade of GABA_A receptor activity and repetitive facial stimulation at 20 Hz (240 pulses) induced an LTP of MF-GrC synapses in the mouse cerebellar cortical folium Crus II, accompanied with a decrease in paired-pulse ratio (N2/N1). The facial stimulation-induced MF-GrC LTP was abolished by either an N-methyl-D-aspartate (NMDA) receptor blocker, i.e., D-APV, or a specific GluNR2A subunit-containing NMDA receptor antagonist, PEAQX, but was not prevented by selective GluNR2B or GluNR2C/D subunit-containing NMDA receptor blockers. Application of GNE-0723, a selective and brain-penetrant-positive allosteric modulator of GluN2A subunit-containing NMDA receptors, produced an LTP of N1, accompanied with a decrease in N2/N1 ratio, and occluded the 20-Hz facial stimulation-induced MF-GrC LTP. Inhibition of nitric oxide synthesis (NOS) prevented the facial stimulation-induced MF-GrC LTP, while activation of NOS produced an LTP of N1, with a decrease in N2/N1 ratio, and occluded the 20-Hz facial stimulation-induced MF-GrC LTP. In addition, GluN2A-containing NMDA receptor immunoreactivity was observed in the mouse cerebellar granular layer. These results indicate that facial stimulation at 20 Hz induced LTP of MF-GrC synaptic transmission via the GluN2A-containing NMDA receptor/nitric oxide cascade in mice. The results suggest that the sensory stimulation-evoked LTP of MF-GrC synaptic transmission in the granular layer may play a critical role in cerebellar adaptation to native mossy fiber excitatory inputs and motor learning behavior in living animals.

Long noncoding RNA GM12371 acts as a transcriptional regulator of synapse function

Neuronal functions of long noncoding RNAs (lncRNAs) are poorly understood. Here we describe identification and function of lncRNA GM12371 in regulating synaptic transmission, synapse density, and dendritic arborization in primary hippocampal neurons. GM12371 expression is regulated by cAMP signaling and is critical for the activity regulated synaptic transmission. Importantly, GM12371 is associated with transcriptionally active chromatin and regulates expression of several genes involved in neuronal growth and development. Taken together, these results suggest that GM12371 acts as a transcriptional regulator of synapse function.

Despite the growing evidence suggesting that long noncoding RNAs (lncRNAs) are critical regulators of several biological processes, their functions in the nervous system remain elusive. We have identified an lncRNA, GM12371, in hippocampal neurons that is enriched in the nucleus and necessary for synaptic communication, synapse density, synapse morphology, and dendritic tree complexity. Mechanistically, GM12371 regulates the expression of several genes involved in neuronal development and differentiation, as well as expression of specific lncRNAs and their cognate mRNA targets. Furthermore, we find that cAMP-PKA signaling up-regulates the expression of GM12371 and that its expression is essential for the activity-dependent changes in synaptic transmission in hippocampal neurons. Taken together, our data establish a key role for GM12371 in regulating synapse function.

Editor's Highlight: Congener-Specific Disposition of Chiral Polychlorinated Biphenyls in Lactating Mice and Their Offspring: Implications for PCB Developmental Neurotoxicity

Chiral polychlorinated biphenyl (PCB) congeners have been implicated by laboratory and epidemiological studies in PCB developmental neurotoxicity. These congeners are metabolized by cytochrome P450 (P450) enzymes to potentially neurotoxic hydroxylated metabolites (OH-PCBs). The present study explores the enantioselective disposition and toxicity of 2 environmentally relevant, neurotoxic PCB congeners and their OH-PCB metabolites in lactating mice and their offspring following dietary exposure of the dam. Female C57BL/6N mice (8-weeks old) were fed daily, beginning 2 weeks prior to conception and continuing throughout gestation and lactation, with 3.1 µmol/kg bw/d of racemic 2,2',3,5',6-pentachlorobiphenyl (PCB 95) or 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) in peanut butter; controls received vehicle (peanut oil) in peanut butter. PCB 95 levels were higher than PCB 136 levels in both dams and pups, consistent with the more rapid metabolism of PCB 136 compared with PCB 95. In pups and dams, both congeners were enriched for the enantiomer eluting second on enantioselective gas chromatography columns. OH-PCB profiles in lactating mice and their offspring were complex and varied according to congener, tissue and age. Developmental exposure to PCB 95 versus PCB 136 differentially affected the expression of P450 enzymes as well as neural plasticity (arc and ppp1r9b) and thyroid hormone-responsive genes (nrgn and mbp). The results suggest that the enantioselective metabolism of PCBs to OH-PCBs may influence neurotoxic outcomes following developmental exposures, a hypothesis that warrants further investigation.

C-terminal splice variants of P/Q-type Ca2+ channel CaV2.1 α1 subunits are differentially regulated by Rab3-interacting molecule proteins

Voltage-dependent Ca²⁺ channels (VDCCs) mediate neurotransmitter release controlled by presynaptic proteins such as the scaffolding proteins Rab3-interacting molecules (RIMs). RIMs confer sustained activity and anchoring of synaptic vesicles to the VDCCs. Multiple sites on the VDCC α₁ and β subunits have been reported to mediate the RIMs-VDCC interaction, but their significance is unclear. Because alternative splicing of exons 44 and 47 in the P/Q-type VDCC α₁ subunit Ca_V2.1 gene generates major variants of the Ca_V2.1 C-terminal region, known for associating with presynaptic proteins, we focused here on the protein regions encoded by these two exons. Co-immunoprecipitation experiments indicated that the C-terminal domain (CTD) encoded by Ca_V2.1 exons 40-47 interacts with the α-RIMs, RIM1α and RIM2α, and this interaction was abolished by alternative splicing that deletes the protein regions encoded by exons 44 and 47. Electrophysiological characterization of VDCC currents revealed that the suppressive effect of RIM2α on voltage-dependent inactivation (VDI) was stronger than that of RIM1α for the Ca_V2.1 variant containing the region encoded by exons 44 and 47. Importantly, in the Ca_V2.1 variant in which exons 44 and 47 were deleted, strong RIM2α-mediated VDI suppression was attenuated to a level comparable with that of RIM1α-mediated VDI suppression, which was unaffected by the exclusion of exons 44 and 47. Studies of deletion mutants of the exon 47 region identified 17 amino acid residues on the C-terminal side of a polyglutamine stretch as being essential for the potentiated VDI suppression characteristic of RIM2α. These results suggest that the interactions of the Ca_V2.1 CTD with RIMs enable Ca_V2.1 proteins to distinguish α-RIM isoforms in VDI suppression of P/Q-type VDCC currents.

CaMKII phosphorylation of neuroligin-1 regulates excitatory synapses

Neuroligins are postsynaptic cell adhesion molecules that are important for synaptic function through their trans-synaptic interaction with neurexins (NRXNs). The localization and synaptic effects of neuroligin-1 (NL-1, also called NLGN1) are specific to excitatory synapses with the capacity to enhance excitatory synapses dependent on synaptic activity or Ca(2+)/calmodulin kinase II (CaMKII). Here we report that CaMKII robustly phosphorylates the intracellular domain of NL-1. We show that T739 is the dominant CaMKII site on NL-1 and is phosphorylated in response to synaptic activity in cultured rodent neurons and sensory experience in vivo. Furthermore, a phosphodeficient mutant (NL-1 T739A) reduces the basal and activity-driven surface expression of NL-1, leading to a reduction in neuroligin-mediated excitatory synaptic potentiation. To the best of our knowledge, our results are the first to demonstrate a direct functional interaction between CaMKII and NL-1, two primary components of excitatory synapses.

Regulators of synaptic transmission: roles in the pathogenesis and treatment of epilepsy

Synaptic transmission is the communication between a presynaptic and a postsynaptic neuron, and the subsequent processing of the signal. These processes are complex and highly regulated, reflecting their importance in normal brain functioning and homeostasis. Sustaining synaptic transmission depends on the continuing cycle of synaptic vesicle formation, release, and endocytosis, which requires proteins such as dynamin, syndapin, synapsin, and synaptic vesicle protein 2A. Synaptic transmission is regulated by diverse mechanisms, including presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors and signaling, and modulators of neurotransmission. Neurotransmitters released presynaptically can bind to their postsynaptic receptors, the inhibitory γ-aminobutyric acid (GABA)ergic receptors or the excitatory glutamate receptors. Once released, glutamate activates a variety of postsynaptic receptors including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA), kainate, and metabotropic receptors. The activation of the receptors triggers downstream signaling cascades generating a vast array of effects, which can be modulated by a numerous auxiliary regulatory subunits. Moreover, different neuropeptides such as neuropeptide Y, brain-derived neurotrophic factor (BDNF), somatostatin, ghrelin, and galanin, act as regulators of diverse synaptic functions and along with the classic neurotransmitters. Abnormalities in the regulation of synaptic transmission play a critical role in the pathogenesis of numerous brain diseases, including epilepsy. This review focuses on the different mechanisms involved in the regulation of synaptic transmission, which may play a role in the pathogenesis of epilepsy: the presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors, and modulators of neurotransmission, including the mechanism by which drugs can modulate the frequency and severity of epileptic seizures.

Casein kinase 2 regulates the NR2 subunit composition of synaptic NMDA receptors

N-methyl-D-aspartate (NMDA) receptors (NMDARs) play a central role in development, synaptic plasticity, and neurological disease. NMDAR subunit composition defines their biophysical properties and downstream signaling. Casein kinase 2 (CK2) phosphorylates the NR2B subunit within its PDZ-binding domain; however, the consequences for NMDAR localization and function are unclear. Here we show that CK2 phosphorylation of NR2B regulates synaptic NR2B and NR2A in response to activity. We find that CK2 phosphorylates NR2B, but not NR2A, to drive NR2B-endocytosis and remove NR2B from synapses resulting in an increase in synaptic NR2A expression. During development there is an activity-dependent switch from NR2B to NR2A at cortical synapses. We observe an increase in CK2 expression and NR2B phosphorylation over this same critical period and show that the acute activity-dependent switch in NR2 subunit composition at developing hippocampal synapses requires CK2 activity. Thus, CK2 plays a central role in determining the NR2 subunit content of synaptic NMDARs.

LIM kinase mediates estrogen action on the actin depolymerization factor Cofilin

The ovarian hormone estrogen increases the axospinous synapse density in the hippocampal CA1 region of young female rats but fails to do so in aged rats. This estrogen-mediated alteration of spine synapse structures suggests the coincident requirement for the structural reorganization of the underlying actin cytoskeleton network. Actin reorganization is known to require the deactivation of Cofilin, an actin depolymerization factor. Cofilin is deactivated by LIM kinase (LIMK), and LIMK activity is modulated by the phosphorylation of specific residues. We have previously demonstrated that estrogen is able to increase phosphorylated LIMK (pLIMK) immunoreactivity (IR) in the hippocampus in vivo and that this estrogen-stimulated pLIMK-IR is decreased in the aged brain. Because Cofilin phosphorylation allows for actin filament elongation and spine synapse growth, we sought to determine if estrogen acts through Cofilin and if such estrogen action requires the observed LIMK activity. Using both hippocampal neurons and the NG108-15 neuroblastoma cell line, we demonstrate here that estrogen stimulates the phosphorylation of Cofilin in vitro. Furthermore, this estrogen action on Cofilin requires LIMK. Lastly, while initiating the phosphorylation of LIMK and Cofilin, estrogen can also stimulate the formation of filopodial extensions, an early step in the formation of nascent spines, demonstrating that estrogen can alter the actin-dependent neuronal morphology. This linkage of estrogen communication to Cofilin via LIMK provides the functionality to the age-sensitive pLIMK-IR that we have observed in vivo.

Glutamate receptors and signal transduction in learning and memory

The plasticity of the central nervous system helps form the basis for the neurobiology of learning and memory. Long-term potentiation (LTP) is the main form of synaptic plasticity, reflecting the activity level of the synaptic information storage process, and provides a good model to study the underlying mechanisms of learning and memory. The glutamate receptor-mediated signal pathway plays a key role in the induction and maintenance of LTP, and hence the regulation of learning and memory. The progress in the understanding of the glutamate receptors and related signal transduction systems in learning and memory research are reviewed in this article.

Epac mediates PACAP-dependent long-term depression in the hippocampus

Extensive work has shown that activation of the cAMP-dependent protein kinase A (PKA) is crucial for long-term depression (LTD) of synaptic transmission in the hippocampus, a phenomenon that is thought to be involved in memory formation. Here we studied the role of an alternative target of cAMP, the exchange protein factor directly activated by cyclic AMP (Epac). We show that pharmacological activation of Epac by the selective agonist 8-(4-chlorophenylthio)-2'-O-methyl-cAMP (8-pCPT) induces LTD in the CA1 region. Paired-pulse facilitation of synaptic responses remained unchanged after induction of this LTD, suggesting that it depended on postsynaptic mechanisms. The 8-pCPT-induced LTD was blocked by the Epac signalling inhibitor brefeldin-A (BFA), Rap-1 antagonist geranylgeranyltransferase inhibitor (GGTI) and p38 mitogen activated protein kinase (P38-MAPK) inhibitor SB203580. This indicated a direct involvement of Epac in this form of LTD. As for other forms of LTD, a mimetic peptide of the PSD-95/Disc-large/ZO-1 homology (PDZ) ligand motif of the AMPA receptor subunit GluR2 blocked the Epac-LTD, suggesting involvement of PDZ protein interaction. The Epac-LTD also depended on mobilization of intracellular Ca(2+), proteasome activity and mRNA translation, but not transcription, as it was inhibited by thapsigargin, lactacystin and anisomycin, but not actinomycin-D, respectively. Finally, we found that the pituitary adenylate cyclase activating polypeptide (PACAP) can induce an LTD that was mutually occluded by the Epac-LTD and blocked by BFA or SB203580, suggesting that the Epac-LTD could be mobilized by stimulation of PACAP receptors. Altogether these results provided evidence for a new form of hippocampal LTD.

Size dependence of protein diffusion very close to membrane surfaces: measurement by total internal reflection with fluorescence correlation spectroscopy

The diffusion coefficients of nine fluorescently labeled antibodies, antibody fragments, and antibody complexes have been measured in solution very close to supported planar membranes by using total internal reflection with fluorescence correlation spectroscopy (TIR-FCS). The hydrodynamic radii (3-24 nm) of the nine antibody types were determined by comparing literature values with bulk diffusion coefficients measured by spot FCS. The diffusion coefficients very near membranes decreased significantly with molecular size, and the size dependence was greater than that predicted to occur in bulk solution. The observation that membrane surfaces slow the local diffusion coefficient of proteins in a size-dependent manner suggests that the primary effect is hydrodynamic as predicted for simple spheres diffusing close to planar walls. The TIR-FCS data are consistent with predictions derived from hydrodynamic theory. This work illustrates one factor that could contribute to previously observed nonideal ligand-receptor kinetics at model and natural cell membranes.

Altered calcium homeostasis in motor neurons following AMPA receptor but not voltage-dependent calcium channels' activation in a genetic model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a late-onset progressive neurodegenerative disease characterized by a substantial loss of motor neurons in the spinal cord, brain stem and motor cortex. By combining electrophysiological recordings with imaging techniques, clearance/buffering capacity of cultured spinal cord motor neurons after a calcium accumulation has been analyzed in response to AMPA receptors' (AMPARs') activation and to depolarizing stimuli in a genetic mouse model of ALS (G93A). Our studies demonstrate that the amplitude of the calcium signal in response to AMPARs' or voltage-dependent calcium channels' activation is not significantly different in controls and G93A motor neurons. On the contrary, in G93A motor neurons, the [Ca(2+)](i) recovery to basal level is significantly slower compared to control neurons following AMPARs but not voltage-dependent calcium channels' activation. This difference was not observed in G93A cultured cortical neurons. This observation is the first to indicate a specific alteration of the calcium clearance linked to AMPA receptors' activation in G93A motor neurons and the involvement of AMPA receptor regulatory proteins controlling both AMPA receptor functionality and the sequence of events connected to them.

Dynamic aspects of CNS synapse formation

The mammalian central nervous system (CNS) requires the proper formation of exquisitely precise circuits to function correctly. These neuronal circuits are assembled during development by the formation of synaptic connections between thousands of differentiating neurons. Proper synapse formation during childhood provides the substrate for cognition, whereas improper formation or function of these synapses leads to neurodevelopmental disorders, including mental retardation and autism. Recent work has begun to identify some of the early cellular events in synapse formation as well as the molecular signals that initiate this process. However, despite the wealth of information published on this topic in the past few years, some of the most fundamental questions about how, whether, and where glutamatergic synapses form in the mammalian CNS remain unanswered. This review focuses on the dynamic aspects of the early cellular and molecular events in the initial assembly of glutamatergic synapses in the mammalian CNS.

The CNS synapse revisited: gaps, adhesive welds, and borders

Although processes leading up to the point of synapse formation are fairly well understood, the precise sequence of events in which the membranes of two separate cells "lock in" to form a mature synaptic junctional complex is poorly understood. A careful study of the molecules operating at the synapse indicates that their roles are more multifarious than once imagined. In this review we posit that the synapse is a functional organelle with poorly defined boundaries and a complex biochemistry. The role of adhesion molecules, including the integration of their signaling and adhesive properties in the context of synaptic activity is discussed.

Alterations in long-term seizure susceptibility and the complex of PSD-95 with NMDA receptor from animals previously exposed to perinatal hypoxia

PURPOSE

Perinatal hypoxia is an important cause of brain injury in the newborn and has consequences that are potentially devastating and life-long, such as an increased risk of epilepsy in later life. The postsynaptic density (PSD) is a cytoskeletal specialization involved in the anchoring of neurotransmitter receptors and in regulating the response of postsynaptic neurons to synaptic stimulation. The postsynaptic protein PSD-95 binds to the N-methyl-D-aspartate receptor (NMDAR) subunit, and hence activates cascades of NMDAR-mediated events, such as cyclic adenosine monophosphate (cAMP)-responsive element binding protein phosphorylation at serine-133 (pCREB(Serine-133)). Here we studied the effect of perinatal hypoxia on protein interactions involving PSD-95 and the NMDAR, as well as pCREB(Ser-133) expression at an age when the animals show increased seizure susceptibility.

METHODS

Rats were assigned randomly to the control rats or the rats exposed to transient global hypoxia at postnatal day 10 (P10). At P45, some rats from both groups were treated with pentylenetetrazol (PTZ) intraperitoneally to test the seizure threshold, and others were studied for neuronal loss, pCREB(Serine-133), PSD-95, and NMDAR expressions in the midbrain, temporal cortex, and hippocampal CA1 subfield by using immunohistochemistry, co-immunoprecipitation, and immunoblotting techniques, respectively.

RESULTS

The rats with prior exposure to perinatal hypoxia exhibited increased seizure susceptibility to PTZ, compared with the control rats. Associated with this long-term change in seizure susceptibility, selective neuronal loss was observed in the midbrain region while pCREB(Ser-133) expression was reduced in the midbrain, temporal cortex, and hippocampal CA1 subfield. Perinatal hypoxia led to a decrease in PSD-95 expression in the both midbrain and hippocampal CA1 subfield, with the exception of temporal cortex. Furthermore, the association between PSD-95 and NMDAR subunits (NR1, NR2A, and NR2B) in the hippocampal CA1 was also markedly altered by perinatal hypoxia.

CONCLUSIONS

This study demonstrates that the decrease in several protein complexes that are essential components of the postsynaptic apparatus is associated with the observed increase in seizure susceptibility in adult rats with prior exposure to perinatal hypoxia. The results indicate that reductions in PSD-95 expression, PSD-95 binding of NMDAR subunits, and subsequent NMDAR-mediated CREB phosphorylation, particularly in hippocampal CA1, are long-term consequences of perinatal hypoxia and may, at least in part, contribute to perinatal hypoxia-induced reduction in seizure threshold.

Synthetic multivalent ligands as probes of signal transduction

Cell-surface receptors acquire information from the extracellular environment and coordinate intracellular responses. Many receptors do not operate as individual entities, but rather as part of dimeric or oligomeric complexes. Coupling the functions of multiple receptors may endow signaling pathways with the sensitivity and malleability required to govern cellular responses. Moreover, multireceptor signaling complexes may provide a means of spatially segregating otherwise degenerate signaling cascades. Understanding the mechanisms, extent, and consequences of receptor co-localization and interreceptor communication is critical; chemical synthesis can provide compounds to address the role of receptor assembly in signal transduction. Multivalent ligands can be generated that possess a variety of sizes, shapes, valencies, orientations, and densities of binding elements. This Review focuses on the use of synthetic multivalent ligands to characterize receptor function.

Regulation and dysregulation of glutamate transporters

Glutamate is the primary excitatory neurotransmitter in the central nervous system. During synaptic activity, glutamate is released into the synaptic cleft and binds to glutamate receptors on the pre- and postsynaptic membrane as well as on neighboring astrocytes in order to start a number of intracellular signaling cascades. To allow for an efficient signaling to occur, glutamate levels in the synaptic cleft have to be maintained at very low levels. This process is regulated by glutamate transporters, which remove excess extracellular glutamate via a sodium-potassium coupled uptake mechanism. When extracellular glutamate levels rise to about normal, glutamate overactivates glutamate receptors, triggering a multitude of intracellular events in the postsynaptic neuron, which ultimately results in neuronal cell death. This phenomenon is known as excitotoxicity and is the underlying mechanisms of a number of neurodegenerative diseases. A dysfunction of the glutamate transporter is thought to contribute to cell death during excitotoxicity. Therefore, efforts have been made to understand the regulation of glutamate transporter function. Transporter activity can be regulated in different ways, including through gene expression, transporter protein targeting and trafficking and through posttranslational modifications of the transporter protein. The identification of these mechanisms has helped to understand the role of glutamate transporters during pathology and will aid in the development of therapeutic strategies with the transporter as a desirable target.

AMPA receptor synaptic targeting regulated by stargazin interactions with the Golgi-resident PDZ protein nPIST

Regulation of AMPA receptors (AMPARs) at synapses plays a critical role in alterations of synaptic strength in the brain. Stargazin, an AMPAR-interacting protein, is critical for clustering and regulation of synaptic AMPARs. Stargazin interacts with AMPARs via its extracellular domain and with PDZ [postsynaptic density-95 (PSD-95)/Discs large (Dlg)/zona occludens-1 (ZO-1)] proteins via its C-terminal PDZ-binding motif, and these interactions are necessary for stargazin and AMPAR synaptic targeting. By studying the expression of stargazin mutant constructs in cultured hippocampal neurons, we identified a novel domain corresponding to residues 243-283 within the cytoplasmic C terminus of stargazin that is also required for stargazin and AMPAR synaptic clustering. To identify proteins that interact with this stargazin synaptic clustering domain, we performed a yeast two-hybrid assay and found that this stargazin domain binds to nPIST (neuronal isoform of protein-interacting specifically with TC10), a Golgi-enriched protein implicated in trafficking of transmembrane proteins. Using in situ hybridization, immunohistochemistry, coimmunoprecipitation studies, and biochemical fractionation, we found that stargazin and nPIST colocalize and interact in the brain. Finally, by studying AMPAR clustering in transfected hippocampal neurons, we found that overexpression of nPIST enhances AMPAR synaptic clustering, whereas transfection of a dominant-negative nPIST construct attenuates AMPAR synaptic clustering. These studies identify a novel stargazin domain necessary for synaptic clustering of AMPARs and suggest that nPIST and stargazin interactions play a critical role in AMPAR trafficking to the synapse.

Total internal reflection with fluorescence correlation spectroscopy: nonfluorescent competitors

Total internal reflection with fluorescence correlation spectroscopy is a method for measuring the surface association/dissociation rate constants and absolute densities of fluorescent molecules at the interface of a planar substrate and solution. This method can also report the apparent diffusion coefficient and absolute concentration of fluorescent molecules very close to the surface. Theoretical expressions for the fluorescence fluctuation autocorrelation function when both surface association/dissociation kinetics and diffusion through the evanescent wave, in solution, contribute to the fluorescence fluctuations have been published previously. In the work described here, the nature of the autocorrelation function when both surface association/dissociation kinetics and diffusion through the evanescent wave contribute to the fluorescence fluctuations, and when fluorescent and nonfluorescent molecules compete for surface binding sites, is described. The autocorrelation function depends in general on the kinetic association and dissociation rate constants of the fluorescent and nonfluorescent molecules, the surface site density, the concentrations of fluorescent and nonfluorescent molecules in solution, the solution diffusion coefficients of the two chemical species, the depth of the evanescent field, and the size of the observed area on the surface. Both general and approximate expressions are presented.

Phosphorylation and Recruitment of Syk by Immunoreceptor Tyrosine-based Activation Motif-based Phosphorylation of Tamalin

Tamalin is a scaffold protein that forms a multiple protein assembly including metabotropic glutamate receptors (mGluRs) and several postsynaptic and protein-trafficking scaffold proteins in distinct mode of protein-protein association. In the present investigation, we report that tamalin possesses a typical immunoreceptor tyrosine-based activation motif (ITAM), which enables Syk kinase to be recruited and phosphorylated by the Src family kinases. Coimmunoprecipitation analysis of rat brain membrane fractions showed that tamalin is present in a multimolecular protein assembly comprising not only mGluR1 but also c-Src, Fyn, and a protein phosphatase, SHP-2. The protein association of both tamalin and c-Src, as determined by truncation analysis of mGluR1 in COS-7 cells, occurred at the carboxyl-terminal tail of mGluR1. Mutation analysis of tyrosine with phenylalanine in COS-7 cells revealed that paired tyrosines at the ITAM sequence of tamalin are phosphorylated preferentially by c-Src and Fyn, and this phosphorylation can recruit Syk kinase and enables it to be phosphorylated by the Src family kinases. The phosphorylated tyrosines at the ITAM sequence of tamalin were highly susceptible to dephosphorylation by protein-tyrosine phosphatases in COS-7 cells. Importantly, tamalin was endogenously phosphorylated and associated with Syk in retinoic acid-treated P19 embryonal carcinoma cells that undergo neuron-like differentiation. The present investigation demonstrates that tamalin is a novel signaling molecule that possesses a PDZ domain and a PDZ binding motif and mediates Syk signaling in an ITAM-based fashion.

Regulation of single NMDA receptor channel activity by alpha-actinin and calmodulin in rat hippocampal granule cells

The NMDA receptor is modulated by changes in the intracellular calcium concentration, through activation of various intracellular calcium-dependent proteins. We have investigated regulation of single NMDA receptor channel activity by the calcium-sensing proteins alpha-actinin and calmodulin. Both of these proteins bind to the NMDA receptor NR1 subunit C-terminus at the C0 region where they compete for occupation of the C0 site and contribute to calcium-dependent inactivation of NMDA receptor-mediated whole-cell currents. Calmodulin has also been shown to bind to the neighbouring C1 region where it has been shown to reduce single channel open time. To investigate regulation of single NMDA channel activity by alpha-actinin and calmodulin, we selected concentrations of these two proteins that would result in maximal binding to the C0 region and/or the C1 region in the case of calmodulin. Alpha-actinin binding was found to predominantly decrease single channel shut time, resulting in an increased open probability (P(open)), whereas calmodulin binding reduced single channel mean open time, resulting in an overall reduction in P(open). The physiological implications of these findings are discussed.

Temporal dynamics of NMDA receptor-induced changes in spine morphology and AMPA receptor recruitment to spines

Recent studies have shown that the activation of NMDA receptors can induce rapid changes in dendritic morphology and synaptic recruitment of AMPA receptors in dendritic spines. Here, we analyze the time course of NMDA receptor-induced changes in dendrite morphology and recruitment of AMPA receptors to synapses in cultured neurons. Activation of NMDA receptors causes a rapid transient increase in the size of preexisting spines and then the gradual formation of new dendritic protrusions and spines. NMDA receptor activation also induced GFP-tagged AMPA receptors to cluster in dendrites and to be inserted into the surface of dendritic spines. These results indicate that NMDA receptor activation induces several phases of dendritic plasticity, initial expansion of dendritic spines, followed by the de novo formation of spines and AMPA receptor dendritic clustering and surface expression on spines. Each of these forms of plasticity may have significant effects on the efficacy of synaptic transmission.

Association of gephyrin and glycine receptors in the human brainstem and spinal cord: an immunohistochemical analysis

Gephyrin is a postsynaptic clustering molecule that forms a protein scaffold to anchor inhibitory neurotransmitter receptors at the postsynaptic membrane of neurons. Gephyrin was first identified as a protein component of the glycine receptor complex and is also colocalized with several GABAA receptor subunits in rodent brain. We have studied the distribution of gephyrin and glycine receptor subunits in the human brainstem and spinal cord using immunohistochemistry at light and confocal laser scanning microscopy levels. This study demonstrates the novel localization of gephyrin with glycine receptors in the human brainstem and spinal cord. Colocalization of immunoreactivities for gephyrin and glycine receptor subunits was detected in the dorsal and ventral horns of the spinal cord, the hypoglossal nucleus and the medial vestibular nucleus of the medulla. The results clearly establish that gephyrin is ubiquitously distributed and is colocalized, with a large proportion of glycine receptor subunits in the human brainstem and spinal cord. We therefore suggest that gephyrin functions as a clustering molecule for major subtypes of glycine receptors in the human CNS.

Amino acid neurotransmitter release and learning: a study of visual imprinting

The intermediate and medial part of the hyperstriatum ventrale (IMHV) is an area of the domestic chick forebrain that stores information acquired through the learning process of imprinting. The effects of visual imprinting on the release of the amino acids aspartate, arginine, citrulline, gamma-aminobutyric acid (GABA), glutamate, glycine and taurine from the left and right IMHVs in vitro were measured at 3.5, 10 and 24 h after training. Chicks were exposed to an imprinting stimulus for 1 h, their preferences measured 10 min afterward and a preference score calculated as a measure of the strength of learning. Potassium stimulation was used to evoke amino acid release from the IMHVs of trained and untrained chicks in the presence and absence of extracellular Ca2+. Ca2+-dependent, K+-evoked release of glutamate was significantly (34.4%) higher in trained than in untrained chicks. This effect was not influenced by time after training or by side (left or right IMHV). Training influenced the evoked release of GABA and taurine from the left IMHV at both 3.5 and 10 h. The training effects at the two times were statistically homogeneous so data (< or = 10 h group) were combined for each amino acid respectively. For this < or = 10 h group, evoked release increased significantly with preference score. In contrast, for the 24 h group, evoked release of GABA and taurine was not significantly correlated with preference score. There were no significant correlations between preference score and GABA or taurine release in the right IMHV at any time, nor in the absence of extracellular calcium. No significant effects of training condition, time or side were observed for any other amino acid in the study. The present findings suggest that soon after chicks have been exposed to an imprinting stimulus glutamatergic excitatory transmission in IMHV is enhanced, and remains enhanced for at least 24 h. In contrast, the learning-related elevations in taurine and GABA release are not sustained over this period. The change in GABA release may reflect a transient increase in inhibitory transmission in the left IMHV.

Molecular mechanisms of calcium-dependent neurodegeneration in excitotoxicity

Excitotoxicity contributes to neuronal degeneration in many acute CNS diseases, including ischemia, trauma, and epilepsy, and may also play a role in chronic diseases, such as amyotrophic lateral sclerosis (ALS). Key mediators of excitotoxic damage are Ca ions (Ca(2+)), which under physiological conditions govern a multitude of cellular processes, including cell growth, differentiation, and synaptic activity. Consequently, homeostatic mechanisms exist to maintain a low intracellular Ca(2+) ion concentration so that Ca(2+) signals remain spatially and temporally localized. This permits multiple independent Ca-mediated signaling pathways to occur in the same cell. In excitotoxicity, excessive synaptic release of glutamate can lead to the disregulation of Ca(2+) homeostasis. Glutamate activates postsynaptic receptors, including the ionotropic N-methyl-D-aspartate (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) proprionate (AMPA), and kainate receptors. Upon their activation, these open their associated ion channel to allow the influx of Ca(2+) and Na(+) ions. Although physiological elevations in intracellular Ca(2+) are salient to normal cell functioning, the excessive influx of Ca(2+) together with any Ca(2+) release from intracellular compartments can overwhelm Ca(2+)-regulatory mechanisms and lead to cell death. Although Ca(2+) disregulation is paramount to neurodegeneration, the exact mechanism by which Ca(2+) ions actually mediate excitotoxicity is less clear. One hypothesis outlined in this review suggests that Ca(2+)-dependent neurotoxicity occurs following the activation of distinct signaling cascades downstream from key points of Ca(2+) entry at synapses, and that triggers of these cascades are physically co-localized with specific glutamate receptors. Thus, we summarize the importance of Ca(2+) regulation in mammalian neurons and the excitotoxicity hypothesis, and focus on the molecular determinants of glutamate receptor-mediated excitotoxic mechanisms.

Synapse number and synaptic efficacy are regulated by presynaptic cAMP and protein kinase A

The mechanisms by which neurons regulate the number and strength of synapses during development and synaptic plasticity have not yet been defined fully. This lack of fundamental knowledge in the fields of neurodevelopment and synaptic plasticity can be attributed, in part, to compensatory mechanisms by which neurons accommodate for the loss of function in their synaptic partners. This is generally achieved either by scaling up neuronal transmitter release capabilities or by enhancing the postsynaptic responsiveness. Here, we demonstrate that regulation of synaptic strength and number between identified Lymnaea neurons visceral dorsal 4 (VD4, the presynaptic cell) and left pedal dorsal 1 (LPeD1, the postsynaptic cell) requires presynaptic activation of a cAMP-PKA-dependent signal. Experimental activation of the cAMP-PKA pathway resulted in reduced synaptic efficacy, whereas inhibition of the cAMP-PKA cascade permitted hyperinnervation and an overall enhancement of synaptic strength. Because synaptic transmission between VD4 and LPeD1 does not require a cAMP-PKA pathway, our data show that these messengers may play a novel role in regulating the synaptic efficacy during early synaptogenesis and plasticity.

Activation of PI3-kinase is required for AMPA receptor insertion during LTP of mEPSCs in cultured hippocampal neurons

Hippocampal CA1 homosynaptic long-term potentiation (LTP) is expressed specifically at activated synapses. Increased insertion of postsynaptic alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid receptors (AMPARs) appears to be crucial for CA1 LTP. However, the mechanism underlying AMPAR insertion during LTP remains largely unknown. We now report that phosphatidylinositol 3-kinase (PI3K) is complexed with AMPARs at synapses and activated by selective stimulation of synaptic N-methyl-D-aspartate (NMDA) receptors. Activation of the AMPAR-associated PI3K is required for the increased cell surface expression of AMPARs and LTP. Thus, our results strongly suggest that the AMPAR-PI3K complex may constitute a critical molecular signal responsible for AMPAR insertion at activated CA1 synapses during LTP, and consequently, this lipid kinase may serve to determine the polarity of NMDA receptor-dependent synaptic plasticity.

Synchronized formation and remodeling of postsynaptic densities: long-term visualization of hippocampal neurons expressing postsynaptic density proteins tagged with green fluorescent protein

To explore mechanisms governing the formation and remodeling of postsynaptic density (PSD), we used dissociated cultures of hippocampal neurons isolated from transgenic embryos expressing green fluorescent protein (GFP)-tagged PSD proteins PSD-Zip45 (Homer 1c) and PSD-95. Expression of GFP-tagged PSD molecules was stable, and the remodeling process of PSDs could be followed for >1 week. A higher expression level of GFP-PSD-Zip45 enabled us to quantitatively analyze the amount of PSD-Zip45 clusters during development. Repetitive imaging of the same cell populations between 11 and 17 d in culture revealed an increase of the average PSD-Zip45 cluster density from 0.32 to 0.73/microm. Newly generated dendrites rapidly acquired GFP-PSD-Zip45 clusters, and their density reached the level of parental dendrites within a few days. Temporal profiles of GFP-PSD-Zip45 cluster density showed a variety of patterns. Some dendrites showed a monotonous increase of clusters, whereas others showed complex patterns, including short decremental stages. Analysis of long-term remodeling of PSD-95-GFP clusters confirmed that the decremental stages were not specific to the PSD-Zip45 clusters. Comparison of the temporal profiles of the cluster density among neurons indicated synchronization of both GFP-PSD-Zip45 and PSD-95 clustering within individual cells. Furthermore, activation of cAMP-dependent protein kinase suppressed the decremental stages of cluster remodeling. These observations suggest the presence of signaling mechanisms that can induce synchronized addition or elimination of PSD proteins throughout dendritic arborization of a single neuron.

Synaptic targeting of PSD-Zip45 (Homer 1c) and its involvement in the synaptic accumulation of F-actin

PSD-Zip45/Homer1c, which contains an enabled/VASP homology 1 (EVH1) domain and leucine zipper motifs, is a postsynaptic density (PSD) scaffold protein that interacts with metabotropic glutamate receptors and the shank family. We studied the molecular mechanism underlying the synaptic targeting of PSD-Zip45 in cultured hippocampal neurons. The EVH1 domain and the extreme C-terminal leucine zipper motif were molecular determinants for its synaptic targeting. The overexpression of the mutant of the EVH1 domain or deletion of the extreme C-terminal leucine zipper motif markedly suppressed the synaptic localization of endogenous shank but not PSD-95 or GKAP. In contrast, an overexpressed GKAP mutant lacking shank binding activity had no effect on the synaptic localization of shank. Actin depolymerization by latrunculin A reduced the synaptic localization of PSD-Zip45, shank, and F-actin but not of PSD-95 or GKAP. Overexpression of PSD-Zip45 enhanced the accumulation of synaptic F-actin. Additionally, overexpression of PSD-Zip45 and an isoform of shank induced synaptic enlargement in association with the further accumulation of synaptic F-actin. The EVH1 domain and extreme C-terminal leucine zipper motif of PSD-Zip45 were also critical for these events. Thus, these data suggest that the PSD-Zip45-shank and PSD-95-GKAP complexes form different synaptic compartments, and PSD-Zip45 alone or PSD-Zip45-shank is involved in the synaptic accumulation of F-actin.

Phosphorylation of spinophilin modulates its interaction with actin filaments

Spinophilin is a protein phosphatase 1 (PP1)- and actin-binding protein that modulates excitatory synaptic transmission and dendritic spine morphology. We report that spinophilin is phosphorylated in vitro by protein kinase A (PKA). Phosphorylation of spinophilin was stimulated by treatment of neostriatal neurons with a dopamine D1 receptor agonist or with forskolin, consistent with spinophilin being a substrate for PKA in intact cells. Using tryptic phosphopeptide mapping, site-directed mutagenesis, and microsequencing analysis, we identified two major sites of phosphorylation, Ser-94 and Ser-177, that are located within the actin-binding domain of spinophilin. Phosphorylation of spinophilin by PKA modulated the association between spinophilin and the actin cytoskeleton. Following subcellular fractionation, unphosphorylated spinophilin was enriched in the postsynaptic density, whereas a pool of phosphorylated spinophilin was found in the cytosol. F-actin co-sedimentation and overlay analysis revealed that phosphorylation of spinophilin reduced the stoichiometry of the spinophilin-actin interaction. In contrast, the ability of spinophilin to bind to PP1 remained unchanged. Taken together, our studies suggest that phosphorylation of spinophilin by PKA modulates the anchoring of the spinophilin-PP1 complex within dendritic spines, thereby likely contributing to the efficacy and plasticity of synaptic transmission.

Membrane and cytoskeleton dynamics during axonal elongation and stabilization

Proper nervous activities are gradually developing events. Reflecting this, embryonic neurons start differentiation by sprouting multiple extensions, neurites, which do not bear clear axonal or dendritic structural and molecular characteristics. Later in development one of these multiple neurites elongates further, generating a morphologically polarized neuron with a single long axon and many short dendrites. Still, despite such morphological differences these processes can switch destiny, further reflecting their immaturity. Final and irreversible axonal and dendritic commitment occurs after both axons and dendrites have elongated considerably. Recent evidence suggests that the transition from axonal immaturity to maturity reflects changes in the mechanisms used by neurons to control the precise membrane and cytoskeleton polarization. This chapter provides an overview of how these mechanisms contribute to the formation of an axon.

Direct effects of calmodulin on NMDA receptor single-channel gating in rat hippocampal granule cells

NMDA receptors are glutamate-sensitive ion channel receptors that mediate excitatory synaptic transmission and are widely implicated in synaptic plasticity and integration of synaptic activity in the CNS. This is in part attributable to the high calcium permeability of the ion channel, which allows receptor activation to influence the intracellular calcium concentration and also the slow time course of NMDA receptor-mediated synaptic currents. NMDA receptor activity is also regulated by the intracellular calcium concentration through activation of various calcium-dependent proteins, including calmodulin, calcineurin, protein kinase C, and alpha-actinin-2. Here, we have shown that calmodulin reduces the duration of native NMDA receptor single-channel openings from 3.5 +/- 0.6 msec to 1.71 +/- 0.2 msec in agreement with previous studies on recombinant NMDA receptors (Ehlers et al., 1996). NMDA receptor single-channel amplitudes and shut times were not affected. However, calmodulin reduced the duration of groups of channel openings called superclusters, which determine the slow time course of synaptic currents, from 121 +/- 25.4 msec to 60.4 +/- 11.6 msec. In addition, total open time, number of channel openings, and charge transfer per supercluster were all reduced by calmodulin. A 68% decrease in charge transfer per supercluster suggests that calmodulin activation will significantly reduce calcium influx during synaptic transmission. These results suggest that calmodulin-dependent inhibition of NMDA receptors will reduce the amplitude and time course of excitatory synaptic currents and thus affect synaptic plasticity and integration of synaptic activity in the CNS.

NMDA receptor 2 (NR2) C-terminal control of NR open probability regulates synaptic transmission and plasticity at a cerebellar synapse

The C-terminal domain of NMDA receptor 2 (NR2) subunits has been proposed to play a critical role in regulating NMDA receptor localization and function in postsynaptic densities. However, the mechanism of this regulation is not completely understood. In this paper we show that C-terminal truncation of NR2A and NR2C subunits in mice (NR2A/C(DeltaC/DeltaC)) impairs synaptic transmission and plasticity at the cerebellar mossy fiber-granule cell relay. Activation of synaptic NMDA receptors could be distinguished from that of extrasynaptic receptors by using the glutamate scavenger glutamate pyruvate transaminase and the open channel blocker MK801. NR2A/C(DeltaC/DeltaC) mice exhibited a specific reduction in synaptic NMDA receptor activation attributable to a severalfold decrease in channel open probability but not channel conductance. Immunodetection revealed normal developmental expression of NR subunit proteins. Quantitative immunogold analyses with an antibody to NR1 indicated that the reduction in receptor activation is not attributed to a reduced number of NR1-containing receptors in postsynaptic densities. Thus, NR2A/NR2C subunits and particularly their C termini regulate synaptic NMDA receptor activation and function by enhancing channel open probability, which is critical for long-term potentiation induction.

Multiple spatiotemporal modes of actin reorganization by NMDA receptors and voltage-gated Ca2+ channels

Cytoskeleton is believed to contribute to activity-dependent processes underlying neuronal plasticity, such as regulations of cellular morphology and localization of signaling proteins. However, how neuronal activity controls actin cytoskeleton remains obscure. Taking advantage of confocal imaging of enhanced GFP-actin in the primary culture of hippocampal neurons, we show that synaptic activity induces multiple types of actin reorganization, both at the spines and at the somatic periphery. Activation of N-methyl-d-aspartate receptors, accompanied with a local rise in [Ca(2+)]i, was sufficient to trigger a slow and sustained recruitment of actin into dendritic spines. In contrast, opening of voltage-gated Ca(2+) channels rapidly and reversibly enhanced cortical actin at the somatic periphery but not in the spines, in keeping with a high transient rise in somatic [Ca(2+)]i. These data suggest that spatiotemporal dynamics of [Ca(2+)]i, triggered by activation of N-methyl-d-aspartate receptors and voltage-gated Ca(2+) channels, provides the molecular basis for activity-dependent actin remodeling.

Phosphorylation of the postsynaptic density-95 (PSD-95)/discs large/zona occludens-1 binding site of stargazin regulates binding to PSD-95 and synaptic targeting of AMPA receptors

Dynamic regulation of AMPA-type receptors at the synapse is proposed to play a critical role in alterations of the synaptic strength seen in cellular models of learning and memory such as long-term potentiation in the hippocampus. Stargazin, previously identified as an AMPA receptor (AMPAR)-interacting protein, is critical for surface expression and synaptic targeting of AMPARs. Stargazin interacts with postsynaptic density-95/discs large/zona occludens-1 (PDZ) proteins via a C-terminal PDZ binding motif. Interestingly, the C terminal of stargazin also predicts phosphorylation at a threonine residue critical for PDZ protein binding. Because protein phosphorylation regulates synaptic plasticity, we characterized this site and the effects of stargazin phosphorylation on AMPAR function. In vitro peptide phosphorylation assays and Western blot analysis with phospho-stargazin-specific antibodies indicate that the critical threonine within the stargazin PDZ binding site is phosphorylated by protein kinase A. This phosphorylation disrupts stargazin interaction and clustering with postsynaptic density-95 (PSD-95) in transfected COS-7 cells. Furthermore, a stargazin construct with a Thr-to-Glu mutation that mimics phosphorylation fails to cluster at synaptic spines and downregulates synaptic AMPAR function in cultured hippocampal neurons. These data suggest that phosphorylation of the stargazin PDZ ligand can disrupt stargazin interaction with PSD-95 and thereby regulate synaptic AMPAR function.

Neurofilament-M interacts with the D1 dopamine receptor to regulate cell surface expression and desensitization

We used the yeast two-hybrid assay to identify novel proteins that interact with the D(1) dopamine receptor. The third cytoplasmic loop (residues 217-273) of the rat D(1) receptor was used as bait to identify clones encoding interacting proteins from a rat brain cDNA library. This identified two clones encoding the C terminus of rat neurofilament-M (NF-M) (residues 782-846). The NF-M clone did not interact with the third cytoplasmic loops of the rat D(2), D(3), or D(4) receptors, but showed weak interaction with that of the D(5) receptor. Coexpression of full-length NF-M with the D(1) receptor in HEK-293 cells resulted in >50% reduction of receptor binding accompanied by a reduction in D(1) receptor-mediated cAMP accumulation. NF-M had no effect on the expression of other dopamine receptor subtypes. Using a D(1) receptor-green fluorescent protein chimera and confocal fluorescence microscopy, we found that NF-M reduced D(1) receptor expression at the cell surface and promoted accumulation of the receptor in the cytosol. Interestingly, the D(1) receptors that were expressed at the cell surface in the presence of NF-M were resistant to agonist-induced desensitization. Cellular colocalization of NF-M and the D(1) receptor in the rat brain was examined by epifluorescence microscopy. These experiments showed that approximately 50% of medium-sized striatal neurons expressed both proteins. Colocalization was also observed in pyramidal cells and interneurons within the frontal cortex. Similar immunohistochemical analyses using NF-M-deficient mice showed decrements in D(1) receptor expression compared with control mice. These results suggest that NF-M interacts with the D(1) receptor in vivo and may modify its expression and regulation.

Synaptically targeted narp plays an essential role in the aggregation of AMPA receptors at excitatory synapses in cultured spinal neurons

Neuronal activity regulated pentraxin (Narp) has been implicated in the aggregation of AMPA-type glutamate receptors (GluR) at excitatory synapses. In the present paper, we examine the role of endogenous Narp in excitatory synapse formation by using novel, dominant-negative Narp mutants (dnNarp) that selectively bind endogenous Narp and prevent its accumulation at synapses. Axons from neurons transfected with wild-type Narp showed an increase in their ability to cluster AMPA receptors on spinal neurons, whereas axons from neurons transfected with dnNarp showed a marked decrease in their ability to induce GluR1 clusters on contacted dendrites. Despite their marked effect at excitatory synapses, dnNarp and wild-type Narp had no effect on the postsynaptic clustering of the inhibitory protein gephyrin or the percentage of contacts associated with staining for the presynaptic vesicle proteins GAD or synaptophysin. Use of the dnNarp mutants to suppress endogenous Narp expression by postsynaptic dendrites showed a complementary role for dendritic Narp in the clustering of synaptic AMPA receptors, as well as a reduction in the total number of excitatory synapses on transfected neurons. Together these experiments suggest an important role for Narp in the formation of excitatory synapses in cultured spinal neurons.

Deletion of the C-terminal domain of the NR2B subunit alters channel properties and synaptic targeting of N-methyl-D-aspartate receptors in nascent neocortical synapses

Channel properties and synaptic targeting of N-methyl-D-aspartate (NMDA) receptors determine their importance in synaptic transmission, long-term synaptic plasticity, and developmental reorganization of synaptic circuits. To investigate the involvement of the C-terminal domain of the NR2B subunit in regulating channel properties and synaptic localization, we analyzed gene-targeted mice expressing C-terminally truncated NR2B subunits (NR2B(DeltaC/DeltaC) mice; Sprengel et al. [1998] Cell 92:279-89). Because homozygous NR2B(DeltaC/DeltaC) mice die perinatally, we studied embryonic neocortical neurons differentiating in culture. At early stages in vitro, neurons predominantly expressed NR1/NR2B receptors, as shown by the NR2B subunit-specific antagonist ifenprodil. At these nascent synapses, NMDA excitatory postsynaptic currents (EPSCs) in neurons from NR2B(DeltaC/DeltaC) mice showed a strong-amplitude reduction to 20% of control, but AMPA EPSCs were unaltered. Analysis of the MK-801 block of NMDA receptor-mediated whole-cell currents revealed a decreased peak open probability of NMDA receptor channels (to about 60%) in neurons from NR2B(DeltaC/DeltaC) mice, although their single channel conductance was unchanged. To study effects on synaptic targeting, we determined the fraction of synaptically localized NMDA receptors relative to the whole-cell NMDA receptor population. In neurons from NR2B(DeltaC/DeltaC) mice, the synaptic NMDA receptor fraction was drastically reduced, suggesting that the C-terminal domain of the NR2B subunit plays a major role in synaptic targeting of NMDA receptors at nascent synapses. With increasing time in culture, the reduction in NMDA EPSCs in neurons from NR2B(DeltaC/DeltaC) mice diminished. This is explained by the expression of additional NMDA receptor subtypes containing NR2A subunits at more mature synapses.

Expression of a variant form of the glutamate transporter GLT1 in neuronal cultures and in neurons and astrocytes in the rat brain

To identify glutamate transporters expressed in forebrain neurons, we prepared a cDNA library from rat forebrain neuronal cultures, previously shown to transport glutamate with high affinity and capacity. Using this library, we cloned two forms, varying in the C terminus, of the glutamate transporter GLT1. This transporter was previously found to be localized exclusively in astrocytes in the normal mature brain. Specific antibodies against the C-terminal peptides were used to show that forebrain neurons in culture express both GLT1a and GLT1b proteins. The pharmacological properties of glutamate transport mediated by GLT1a and GLT1b expressed in COS-7 cells and in neuronal cultures were indistinguishable. Both GLT1a and GLT1b were upregulated in astrocyte cultures by exposure to dibutyryl cAMP. We next investigated the expression of GLT1b in vivo. Northern blot analysis of forebrain RNA revealed two transcripts of approximately 3 and 11 kb that became more plentiful with developmental age. Immunoblot analysis showed high levels of expression in the cortex, hippocampus, striatum, thalamus, and midbrain. Pre-embedding electron microscopic immunocytochemistry with silver-enhanced immunogold detection was used to localize GLT1b in vivo. In the rat somatosensory cortex, GLT1b was clearly expressed in neurons in presynaptic terminals and dendritic shafts, as well as in astrocytes. The presence of GLT1b in neurons may offer a partial explanation for the observed uptake of glutamate by presynaptic terminals, for the preservation of input specificity at excitatory synapses, and may play a role in the pathophysiology of excitotoxicity.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Interaction of the synaptic protein PICK1 (protein interacting with C kinase 1) with the non-voltage gated sodium channels BNC1 (brain Na+ channel 1) and ASIC (acid-sensing ion channel)

Neuronal members of the degenerin/epithelial Na(+) channel (DEG/ENaC) family of cation channels include the mammalian brain Na(+) channel 1 (BNC1), acid-sensing ion channel (ASIC) and dorsal-root acid-sensing ion channel (DRASIC). Their response to acidic pH, their sequence similarity to nematode proteins involved in mechanotransduction and their modulation by neuropeptides suggest that they may function as receptors for a number of different stimuli. Using the yeast two-hybrid assay, we found that the PDZ domain-containing protein PICK1 (protein interacting with C kinase) interacts specifically with the C-termini of BNC1 and ASIC, but not DRASIC or the related alphaENaC or betaENaC. In both the yeast two-hybrid system and mammalian cells, deletion of the BNC1 and ASIC C-termini abolished the interaction with PICK1. Likewise, mutating the PDZ domain in PICK1 abolished its interaction with BNC1 and ASIC. In addition, in a heterologous expression system PICK1 altered the distribution of BNC1 channels; this effect was dependent on the PDZ domain of PICK1 and the C-terminus of BNC1. We found crude synaptosomal fractions of brain to be enriched in ASIC, suggesting a possible synaptic localization. Moreover, in transfected hippocampal neurons ASIC co-localized with PICK1 in a punctate pattern at synapses. These data suggest that PICK1 binds ASIC and BNC1 via its PDZ domain. This interaction may be important for the localization and/or function of these channels in both the central and peripheral nervous systems.