Protein kinase C gamma associates directly with the GluR4 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit. Effect on receptor phosphorylation

The role of post-translational modifications in synaptic AMPA receptor activity

AMPA-type receptors for the neurotransmitter glutamate are very dynamic entities, and changes in their synaptic abundance underlie different forms of synaptic plasticity, including long-term synaptic potentiation (LTP), long-term depression (LTD) and homeostatic scaling. The different AMPA receptor subunits (GluA1-GluA4) share a common modular structure and membrane topology, and their intracellular C-terminus tail is responsible for the interaction with intracellular proteins important in receptor trafficking. The latter sequence differs between subunits and contains most sites for post-translational modifications of the receptors, including phosphorylation, O-GlcNAcylation, ubiquitination, acetylation, palmitoylation and nitrosylation, which affect differentially the various subunits. Considering that each single subunit may undergo modifications in multiple sites, and that AMPA receptors may be formed by the assembly of different subunits, this creates multiple layers of regulation of the receptors with impact in synaptic function and plasticity. This review discusses the diversity of mechanisms involved in the post-translational modification of AMPA receptor subunits, and their impact on the subcellular distribution and synaptic activity of the receptors.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Declining Levels of Specialized Synaptic Surface Proteins in nNOS-Expressing Interneurons in Mice Treated Prenatally with Valproic Acid

Autism spectrum disorder (ASD) is a heterogeneous group of neurodevelopmental disorder characterized by impaired social interaction, and repetitive or restricted interests and behaviors. Membrane proteins are a significant part of the proteins in cell and play key functions in synaptic transmission. We have recently shown that neuronal nitric oxide synthase (nNOS) expression was reduced in the basolateral amygdala (BLA) of mice following postnatal valproic acid (VPA) exposure. In the current study, we utilized a label-free proteomics approach to identify and quantify surface protein expression in nNOS-positive interneurons between VPA-treated and control mice. Western blot was used to confirm the expression of selected membrane proteins. Our proteomics data revealed differentially expressed surface proteins in nNOS interneurons, e.g. Narp, AMPA-type glutamate (AMPA) receptor subunit GluA4 and Protein kinase C gamma (PKCγ), which were validated by Western blotting in mice treated with VPA. This work will pave the way for further elucidation of the mechanisms of these differentially membrane proteins in nNOS interneurons-medicated ASD.

Alterations of transcriptome signatures in head trauma-related neurodegenerative disorders

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease that is associated with repetitive traumatic brain injury (TBI). CTE is known to share similar neuropathological features with Alzheimer’s disease (AD), but little is known about the molecular properties in CTE. To better understand the neuropathological mechanism of TBI-related disorders, we conducted transcriptome sequencing analysis of CTE including AD and CTE with AD (CTE/AD) post-mortem human brain samples. Through weighted gene co-expression network analysis (WGCNA) and principal component analysis (PCA), we characterized common and unique transcriptome signatures among CTE, CTE/AD, and AD. Interestingly, synapse signaling-associated gene signatures (such as synaptotagmins) were commonly down-regulated in CTE, CTE/AD, and AD. Quantitative real-time PCR (qPCR) and Western blot analyses confirmed that the levels of synaptotagmin 1 (SYT1) were markedly decreased in CTE and AD compared to normal. In addition, calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase A (PKA), protein kinase C (PKC), and AMPA receptor genes that play a pivotal role in memory function, were down-regulated in head trauma-related disorders. On the other hand, up-regulation of cell adhesion molecules (CAMs) associated genes was only found in CTE. Our results indicate that dysregulation of synaptic transmission- and memory function-related genes are closely linked to the pathology of head injury-related disorder and AD. Alteration of CAMs-related genes may be specific pathological markers for the CTE pathology.

Catalpol in Diabetes and its Complications: A Review of Pharmacology, Pharmacokinetics, and Safety

This review aimed to provide a general view of catalpol in protection against diabetes and diabetic complications, as well as its pharmacokinetics and safety concerns. The following databases were consulted with the retrieval of more than 100 publications through June 2019: PubMed, Chinese National Knowledge Infrastructure, WanFang Data, and web of science. Catalpol exerts an anti-diabetic effect in different animal models with an oral dosage ranging from 2.5 to 200 mg/kg in rats and 10 to 200 mg/kg in mice. Besides, catalpol may prevent the development of diabetic complications in kidney, heart, central nervous system, and bone. The underlying mechanism may be associated with an inhibition of inflammation, oxidative stress, and apoptosis through modulation of various cellular signaling, such as AMPK/PI3K/Akt, PPAR/ACC, JNK/NF-κB, and AGE/RAGE/NOX4 signaling pathways, as well as PKCγ and Cav-1 expression. The pharmacokinetic profile reveals that catalpol could pass the blood-brain barrier and has a potential to be orally administrated. Taken together, catalpol is a well-tolerated natural compound with promising pharmacological actions in protection against diabetes and diabetic complications via multi-targets, offering a novel scaffold for the development of anti-diabetic drug candidate. Further prospective and well-designed clinical trials will shed light on the potential of clinical usage of catalpol.

Knockdown of Myo-Inositol Transporter SMIT1 Normalizes Cholinergic and Glutamatergic Function in an Immortalized Cell Line Established from the Cerebral Cortex of a Trisomy 16 Fetal Mouse, an Animal Model of Human Trisomy 21 (Down Syndrome)

The Na⁺/myo-inositol cotransporter (SMIT1) is overexpressed in human Down syndrome (DS) and in trisomy 16 fetal mice (Ts16), an animal model of the human condition. SMIT1 overexpression determines increased levels of intracellular myo-inositol, a precursor of phophoinositide synthesis. SMIT1 is overexpressed in CTb cells, an immortalized cell line established from the cerebral cortex of a Ts16 mouse fetus. CTb cells exhibit impaired cytosolic Ca²⁺ signals in response to glutamatergic and cholinergic stimuli (increased amplitude and delayed time-dependent kinetics in the decay post-stimulation), compared to our CNh cell line, derived from the cerebral cortex of a euploid animal. Considering the role of myo-inositol in intracellular signaling, we normalized SMIT1 expression in CTb cells using specific mRNA antisenses. Forty-eight hours post-transfection, SMIT1 levels in CTb cells reached values comparable to those of CNh cells. At this time, decay kinetics of Ca²⁺ signals induced by either glutamate, nicotine, or muscarine were accelerated in transfected CTb cells, to values similar to those of CNh cells. The amplitude of glutamate-induced cytosolic Ca²⁺ signals in CTb cells was also normalized. The results suggest that SMIT1 overexpression contributes to abnormal cholinergic and glutamatergic Ca²⁺ signals in the trisomic condition, and knockdown of DS-related genes in our Ts16-derived cell line could constitute a relevant tool to study DS-related neuronal dysfunction.

Molecular mechanisms controlling synaptic recruitment of GluA4 subunit-containing AMPA-receptors critical for functional maturation of CA1 glutamatergic synapses

Synaptic recruitment of AMPA receptors (AMPARs) represents a key postsynaptic mechanism driving functional development and maturation of glutamatergic synapses. At immature hippocampal synapses, PKA-driven synaptic insertion of GluA4 is the predominant mechanism for synaptic reinforcement. However, the physiological significance and molecular determinants of this developmentally restricted form of plasticity are not known. Here we show that PKA activation leads to insertion of GluA4 to synaptic sites with initially weak or silent AMPAR-mediated transmission. This effect depends on a novel mechanism involving the extreme C-terminal end of GluA4, which interacts with the membrane proximal region of the C-terminal domain to control GluA4 trafficking. In the absence of GluA4, strengthening of AMPAR-mediated transmission during postnatal development was significantly delayed. These data suggest that the GluA4-mediated activation of silent synapses is a critical mechanism facilitating the functional maturation of glutamatergic circuitry during the critical period of experience-dependent fine-tuning. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

DNA methylation is critical for tooth agenesis: implications for sporadic non-syndromic anodontia and hypodontia

Hypodontia is caused by interactions among genetic, epigenetic, and environmental factors during tooth development, but the actual mechanism is unknown. DNA methylation now appears to play a significant role in abnormal developments, flawed phenotypes, and acquired diseases. Methylated DNA immunoprecipitation (MeDIP) has been developed as a new method of scanning large-scale DNA-methylation profiles within particular regions or in the entire genome. Here, we performed a genome-wide scan of paired DNA samples obtained from 4 patients lacking two mandibular incisors and 4 healthy controls with normal dentition. We scanned another female with non-syndromic anodontia and her younger brother with the same gene mutations of the PAX9,MSX1,AXIN2 and EDA, but without developmental abnormalities in the dentition. Results showed significant differences in the methylation level of the whole genome between the hypodontia and the normal groups. Nine genes were spotted, some of which have not been associated with dental development; these genes were related mainly to the development of cartilage, bone, teeth, and neural transduction, which implied a potential gene cascade network in hypodontia at the methylation level. This pilot study reveals the critical role of DNA methylation in hypodontia and might provide insights into developmental biology and the pathobiology of acquired diseases.

The Membrane Proximal Region of AMPA Receptors in Lateral Amygdala is Essential for Fear Memory Formation

The membrane proximal region (MPR) of AMPA receptor (AMPAR) is needed for receptor trafficking and synaptic plasticity. However, its roles in long-term memory formation are not known. To assess the possible roles of AMPAR-MPR in rat lateral amygdala (LA) in short- and long-term fear memory formation, we used glutamate receptors (GluAs)-MPR competitive peptides MPR(DD) and MPR(AA). The MPR(DD) peptide is derived from GluA1 MPR and was previously shown to impair synaptic plasticity and to inhibit GluA1 containing AMPAR insertion into the synapse in an activity-dependent manner. The MPR(AA) peptide is derived from GluA2/4 MPR, and this receptor fragment was shown to be essential for GluA4 protein interaction needed for its insertion into the neuronal membrane and synapse. The peptides were linked to a TAT peptide (TAT-MPR(DD) and TAT-MPR(AA)) to facilitate internalization into LA cells. Infusion of the TAT-MPR(DD) peptide into LA 30 min before fear conditioning led to a significant impairment of long-term fear memory formation. Injection of TAT-MPR(DD) peptide into LA 30 min before fear conditioning impaired short-term fear memory formation. The TAT-MPR(DD) peptide had no effect on memory retrieval when injected into LA 30 min before fear memory test. Infusion of the TAT-MPR(AA) peptide into LA 30 min before fear conditioning led to a significant impairment of long-term fear memory formation. In contrast, the TAT-MPR(AA) had no effect on short-term fear memory formation. A TAT-control peptide had no effect on short- or long-term fear memory. These results show that the AMPAR-MPR in LA is needed for fear memory formation and that the MPR region of GluA1 is essential for acquisition of memory, whereas the MPR region of GluA4 is essential for long-term fear memory consolidation.

'Medusa head ataxia': the expanding spectrum of Purkinje cell antibodies in autoimmune cerebellar ataxia. Part 2: Anti-PKC-gamma, anti-GluR-delta2, anti-Ca/ARHGAP26 and anti-VGCC

Serological testing for anti-neural autoantibodies is important in patients presenting with idiopathic cerebellar ataxia, since these autoantibodies may indicate cancer, determine treatment and predict prognosis. While some of them target nuclear antigens present in all or most CNS neurons (e.g. anti-Hu, anti-Ri), others more specifically target antigens present in the cytoplasm or plasma membrane of Purkinje cells (PC). In this series of articles, we provide a detailed review of the clinical and paraclinical features, oncological, therapeutic and prognostic implications, pathogenetic relevance, and differential laboratory diagnosis of the 12 most common PC autoantibodies (often referred to as 'Medusa head antibodies' due their characteristic somatodendritic binding pattern when tested by immunohistochemistry). To assist immunologists and neurologists in diagnosing these disorders, typical high-resolution immunohistochemical images of all 12 reactivities are presented, diagnostic pitfalls discussed and all currently available assays reviewed. Of note, most of these antibodies target antigens involved in the mGluR1/calcium pathway essential for PC function and survival. Many of the antigens also play a role in spinocerebellar ataxia. Part 1 focuses on anti-metabotropic glutamate receptor 1-, anti-Homer protein homolog 3-, anti-Sj/inositol 1,4,5-trisphosphate receptor- and anti-carbonic anhydrase-related protein VIII-associated autoimmune cerebellar ataxia (ACA); part 2 covers anti-protein kinase C gamma-, anti-glutamate receptor delta-2-, anti-Ca/RhoGTPase-activating protein 26- and anti-voltage-gated calcium channel-associated ACA; and part 3 reviews the current knowledge on anti-Tr/delta notch-like epidermal growth factor-related receptor-, anti-Nb/AP3B2-, anti-Yo/cerebellar degeneration-related protein 2- and Purkinje cell antibody 2-associated ACA, discusses differential diagnostic aspects, and provides a summary and outlook.

Hippocampal protein kinase C family members in spatial memory retrieval in the mouse

Although a few individual members of the protein kinase C (PKC) family were studied in spatial memory no systematic approach was carried out to concomitantly determine all described PKC family members in spatial memory of the mouse. It was therefore the aim of the current study to link hippocampal PKCs to memory retrieval in the Morris water maze (MWM). CD1 mice were trained (n=9) or untrained (n=9) in the MWM, hippocampi were taken 6h following the test for memory retrieval and PKCs were determined in mouse hippocampi by immunoblotting. The trained animals learned the spatial memory task and kept memory at the probe trial. PKCs alpha and epsilon were comparable between groups while PKCs beta, delta, gamma (two forms, i.e. two bands on Western blotting), zeta (2 forms) were higher in trained mice and theta (2 forms) were lower in trained mice. PKC gamma (1 form) was significantly correlating with the time spent in the target quadrant (r=0.7933; P=0.0188). Changes of hippocampal levels of PKCs beta, delta, gamma, zeta and theta were paralleling memory retrieval of the MWM task but correlations revealed that spatial memory retrieval was only linked to one form of PKC gamma. Results are also in agreement with a recent publication showing that PKM zeta is not required for memory formation. These findings may be relevant for the interpretation of previous work and the design of future work on the protein kinase C family in spatial memory of the mouse.

Signaling pathways mediating alcohol effects

Ethanol's effects on intracellular signaling pathways contribute to acute effects of ethanol as well as to neuroadaptive responses to repeated ethanol exposure. In this chapter we review recent discoveries that demonstrate how ethanol alters signaling pathways involving several receptor tyrosine kinases and intracellular tyrosine and serine-threonine kinases, with consequences for regulation of cell surface receptor function, gene expression, protein translation, neuronal excitability and animal behavior. We also describe recent work that demonstrates a key role for ethanol in regulating the function of scaffolding proteins that organize signaling complexes into functional units. Finally, we review recent exciting studies demonstrating ethanol modulation of DNA and histone modification and the expression of microRNAs, indicating epigenetic mechanisms by which ethanol regulates neuronal gene expression and addictive behaviors.

Contactin-associated protein 1 (Caspr1) regulates the traffic and synaptic content of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors

Glutamate receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type mediate fast excitatory synaptic transmission in the CNS. Synaptic strength is modulated by AMPA receptor binding partners, which regulate receptor synaptic targeting and functional properties. We identify Contactin-associated protein 1 (Caspr1) as an AMPA receptor interactor. Caspr1 is present in synapses and interacts with AMPA receptors in brain synaptic fractions. Coexpression of Caspr1 with GluA1 increases the amplitude of glutamate-evoked currents. Caspr1 overexpression in hippocampal neurons increases the number and size of synaptic GluA1 clusters, whereas knockdown of Caspr1 decreases the intensity of synaptic GluA1 clusters. Hence, Caspr1 is a regulator of the trafficking of AMPA receptors to synapses.

CNL, a ricin B-like lectin from mushroom Clitocybe nebularis, induces maturation and activation of dendritic cells via the toll-like receptor 4 pathway

A novel lectin, isolated from the basidiomycete mushroom Clitocybe nebularis and termed C. nebularis lectin (CNL), exhibits an immunostimulatory effect on the most potent antigen-presenting cells, the dendritic cells (DCs). Treatment of human monocyte-derived DCs with CNL in doses from 1 to 10 μg/ml resulted in a dose-dependent induction of overall DC maturation characteristics. Exposure of DCs to CNL for 48 hr resulted in extensive up-regulation of co-stimulatory molecules CD80 and CD86, as well as of the maturation marker CD83 and HLA-DR molecules. Such CNL-matured DCs (CNL-DCs) were capable of inducing a T helper type 1-polarized response in naive CD4+  CD45RA+ T cells in 5-day allogeneic co-cultures. The allostimulatory potential of CNL-DCs was significantly increased relative to untreated controls, as was their capacity to produce several pro-inflammatory cytokines such as interleukin-6, interleukin-8 and tumour necrosis factor-α. By using a specific Toll-like receptor 4 (TLR4) signalling inhibitor, CLI-095, as well as Myd88 inhibitory peptide, we have shown that DC activation by CNL is completely dependent on the TLR4 activation pathway. Furthermore, activation of TLR4 by CNL was confirmed via TLR4 reporter assay. Measurement of p65 nuclear factor-κB and p38 mitogen-activated protein kinase (MAPK) phosphorylation levels following CNL stimulation of DCs revealed primarily an increase in nuclear factor-κB activity, with less effect on the induction of p38 MAPK signalling than of lipopolysaccharide-matured DCs. The CNL had the ability to activate human DCs in such a way as to subsequently direct T helper type 1 T-cell responses. Our results encourage the use of mushroom-derived lectins for use in therapeutic strategies with aims such as to strengthen anti-tumour immune responses.

Tracheal occlusion conditioning in conscious rats modulates gene expression profile of medial thalamus

The thalamus may be the critical brain area involved in sensory gating and the relay of respiratory mechanical information to the cerebral cortex for the conscious awareness of breathing. We hypothesized that respiratory mechanical stimuli in the form of tracheal occlusions would modulate the gene expression profile of the thalamus. Specifically, it was reasoned that conditioning to the respiratory loading would induce a state change in the medial thalamus consistent with a change in sensory gating and the activation of molecular pathways associated with learning and memory. In addition, respiratory loading is stressful and thus should elicit changes in gene expressions related to stress, anxiety, and depression. Rats were instrumented with inflatable tracheal cuffs. Following surgical recovery, they underwent 10 days (5 days/week) of transient tracheal occlusion conditioning. On day 10, the animals were sacrificed and the brains removed. The medial thalamus was dissected and microarray analysis of gene expression performed. Tracheal obstruction conditioning modulated a total of 661 genes (p < 0.05, log₂ fold change ≥0.58), 250 genes were down-regulated and 411 up-regulated. There was a significant down-regulation of GAD1, GAD2 and HTR1A, HTR2A genes. CCK, PRKCG, mGluR4, and KCJN9 genes were significantly up-regulated. Some of these genes have been associated with anxiety and depression, while others have been shown to play a role in switching between tonic and burst firing modes in the thalamus and thus may be involved in gating of the respiratory stimuli. Furthermore, gene ontology and pathway analysis showed a significant modulation of learning and memory pathways. These results support the hypothesis that the medial thalamus is involved in the respiratory sensory neural pathway due to the state change of its gene expression profile following repeated tracheal occlusions.

PKCgamma in Vc and C1/C2 is involved in trigeminal neuropathic pain

The aim of the present study was to clarify the involvement of protein kinase Cγ (PKCγ) in the facial neuropathic pain following infraorbital nerve injury. We analyzed the change in PKCγ expression in the trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1/C2) following chronic constriction injury of the infraorbital nerve (ION-CCI). We also studied ION-CCI-mediated mechanical nocifensive behavior in rats. The mechanical head-withdrawal threshold significantly decreased 1 to 14 days after ION-CCI compared with that before ION-CCI and in sham rats. The expression of PKCγ was significantly larger in the ipsilateral Vc compared with the contralateral side in ION-CCI rats 3, 7, and 14 days after ION-CCI. Intrathecal (i.t.) administration of the PKCγ inhibitor chelerythrine prevented an increase in the PKCγ expression in the ipsilateral Vc. Moreover, i.t. administration of chelerythrine annulled ION-CCI-mediated reduction in the head-withdrawal threshold. Taken together, these findings suggest that PKCγ expression in the Vc played an important role in the mechanism of orofacial static mechanical allodynia following trigeminal nerve injury.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Analysis of the potential role of GluA4 carboxyl-terminus in PDZ interactions

Background

Specific delivery to synapses of α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors with long-tailed subunits is believed to be a key event in many forms of activity-dependent changes in synaptic strength. GluA1, the best characterized long-tailed AMPA receptor subunit, contains a C-terminal class I PDZ binding motif, which mediates its interaction with scaffold and trafficking proteins, including synapse-associated protein 97 (SAP97). In GluA4, another long-tailed subunit implicated in synaptic plasticity, the PDZ motif is blocked by a single proline residue. This feature is highly conserved in vertebrates, whereas the closest invertebrate homologs of GluA4 have a canonical class I PDZ binding motif. In this work, we have examined the role of GluA4 in PDZ interactions.

Methodology/Principal Findings

Deletion of the carboxy-terminal proline residue of recombinant GluA4 conferred avid binding to SAP97 in cultured cells as shown by coimmunoprecipitation, whereas wild-type GluA4 did not associate with SAP97. Native GluA4 and SAP97 coimmunoprecipitated from mouse brain independently of the GluA1 subunit, supporting the possibility of in vivo PDZ interaction. To obtain evidence for or against the exposure of the PDZ motif by carboxyterminal processing of native GluA4 receptors, we generated an antibody reagent specific for proline-deleted GluA4 C-terminus. Immunoprecipitation and mass spectrometric analyses indicated that the carboxyl-terminus of native GluA4 AMPA receptors is intact and that the postulated single-residue cleavage does not occur to any significant extent.

Conclusion/Significance

We conclude that native GluA4 receptors are not capable of canonical PDZ interactions and that their association with SAP97 is likely to be indirect.

Protein Kinase C-γ Activation in the Early Streptozotocin Diabetic Rat Lens

PURPOSE

The purpose of this study is to demonstrate the early activation of the protein kinase C-gamma (PKC-gamma) pathway in the streptozotocin (STZ)-induced diabetic rat lens.

METHODS

Twelve-week-old male and female Sprague-Dawley rats were injected with 80 mg/kg (body weight) of STZ (N-[methylnitrosocarbamoyl]-D-glucosamine) intraperitoneally. Very high glucose (VHG) diabetes was defined as a nonfasting blood glucose level of at least 450 mg/dl, confirmed by daily monitoring with Accu-Check Advantage test strips, and occurred about 2 weeks after STZ administration. All assayed lenses were from VHG or age-matched control rats, harvested within 24 hr of VHG detection. PKC-gamma activation was measured by enzyme activity assay and by Western blotting to show autophosphorylation on Thr514. Cellular insulin-like growth factor-1 (IGF-1), PKC-gamma phosphorylation of Cx43 on Ser368, and activation of phospholipase C-gamma 1 (PLC-gamma 1), extracellular signal-regulated kinase (ERK1/2), and caspase-3 were determined by Western blotting. Endogenous diacylglycerol (DAG) levels were measured with a DAG assay kit. Lens gap junction activity was determined by the microinjection/Lucifer yellow dye transfer assay. Electron microscopy was applied to affirm fiber cell damage in the VHG diabetic lenses.

RESULTS

In the lenses of VHG diabetic rats, PKC-gamma enzyme was activated. PKC-gamma could be further activated by 400 nM phorbol-12-myristate-13-acetate (PMA), but the PKC-gamma protein levels remained constant. No elevation of IGF-1 level was observed. Western blots showed that activation of PKC-gamma may be due to activation of PLC-gamma 1, which synthesized endogenous DAG, a native PKC activator. The level of PKC-gamma -catalyzed phosphorylation of Cx43 on Ser368 and resulting inhibition of lens gap junction dye transfer activity was increased in the VHG diabetic lenses. At this early time period, the diabetic lens showed no activation of either caspase-3 or ERK1/2. Only a single fiber cell layer deep within the cortex (approximately 90 cell layers from capsule surface) showed vacuoles and damaged cell connections.

CONCLUSIONS

Early activation of PLC-gamma 1 and elevated DAG were observed within VHG diabetic lenses. These were correlated with activation of PKC-gamma, phosphorylation of Cx43 on Ser368, and inhibition of dye transfer. Abnormal signaling from PKC-gamma to Cx43 in the epithelial cells/early fiber cells, observed within VHG diabetic lenses, may be responsible for fiber cell damage deeper in the lens cortex.

AMPA receptor trafficking and synaptic plasticity require SQSTM1/p62

SQSTM1/p62 is a multidomain/scaffold for the atypical protein kinase Cs (aPKC). Phosphorylation of AMPA receptors by PKC has been shown to regulate their insertion in the postsynaptic membrane. Here, we directly tested whether p62 could interact with AMPA receptor subunits and influence their trafficking and phosphorylation. GluR1 receptor intracellular loop L2-3 and the ZZ-type zinc finger domain of p62 are essential for the interaction between these two proteins. In this context, both p62 and aPKC-mediated phosphorylation were necessary for surface delivery of the receptor. Our findings reveal that p62 is the first protein identified that interacts with a region of the GluR receptor other than the C-terminal tail. Furthermore, mice deficient in p62 displayed impaired hippocampal CA1 long-term potentiation (LTP), along with diminished surface expression of GluR1 and phosphorylation of S818. Lastly, we identify a conserved sequence (ISExSL) shared by all p62 interacting-aPKC substrates. These findings support a model where p62 interaction and aPKC phosphorylation act together to mediate AMPA receptor trafficking and long-term synaptic plasticity in the hippocampus.

Loss of protein kinase Cgamma in knockout mice and increased retinal sensitivity to hyperbaric oxygen

OBJECTIVE

To determine if loss of protein kinase Cgamma (PKCgamma) results in increased structural damage to the retina by hyperbaric oxygen (HBO), a treatment used for several ocular disorders.

METHODS

Six-week-old mice were exposed in vivo to 100% HBO 3 times a week for 8 weeks. Eyes were dissected, fixed, embedded in Epon, sectioned, stained with toluidine blue O, and examined by light microscopy.

RESULTS

The thicknesses of the inner nuclear and ganglion cell layers were increased. Destruction of the outer plexiform layer was observed in the retinas of the PKCgamma-knockout mice relative to control mice. Exposure to HBO caused significant degradation of the retina in knockout mice compared with control mice. Damage to the outer segments of the photoreceptor layer and ganglion cell layer was apparent in central retinas of HBO-treated knockout mice.

CONCLUSIONS

Protein kinase Cgamma-knockout mice had increased retinal sensitivity to HBO. Results demonstrate that PKCgamma protects retinas from HBO damage.

CLINICAL RELEVANCE

Care should be taken in treating patients with HBO, particularly if they have a genetic disease, such as spinocerebellar ataxia type 14, a condition in which the PKCgamma is mutated and nonfunctional.

Temporal phases of activity-dependent plasticity and memory are mediated by compartmentalized routing of MAPK signaling in aplysia sensory neurons

An activity-dependent form of intermediate memory (AD-ITM) for sensitization is induced in Aplysia by a single tail shock that gives rise to plastic changes (AD-ITF) in tail sensory neurons (SNs) via the interaction of action potential firing in the SN coupled with the release of serotonin in the CNS. Activity-dependent long-term facilitation (AD-LTF, lasting >24hr) requires protein synthesis dependent persistent mitogen-activated protein kinase (MAPK) activation and translocation to the SN nucleus. We now show that the induction of the earlier temporal phase (AD-ITM and AD-ITF), which is translation and transcription independent, requires the activation of a compartmentally distinct novel signaling cascade that links second messengers, MAPK and PKC into a unified pathway within tail SNs. Since both AD-ITM and AD-LTM require MAPK activity, these collective findings suggest that presynaptic SNs route the flow of molecular information to distinct subcellular compartments during the induction of activity-dependent long-lasting memories.

Determination of conventional protein kinase C isoforms involved in high intraocular pressure-induced retinal ischemic preconditioning of rats

Evidence indicates that conventional protein kinase C (cPKC) plays a pivotal role in the development of retinal ischemic preconditioning (IPC). In this study, the effect of high intraocular pressure (IOP)-induced retinal IPC on cPKC isoform-specific membrane translocation and protein expression were observed. We found that cPKCgamma membrane translocation increased significantly at the early stage (20min-1h), while the protein expression levels of cPKCalpha and gamma were markedly elevated in the delayed retinal IPC (12-168h) of rats. The increased protein expressions of cPKCalpha at 72h and cPKCgamma at 24h after IPC were further confirmed by immunofluorescence staining. In addition, we found that cPKCgamma co-localized with retinal ganglion cell (RGC)-specific marker, neurofilaments heavy chain (NF-H) by using double immunofluorescence labeling. These results suggest that increased cPKCgamma membrane translocation and up-regulated protein expressions of cPKCalpha and gamma are involved in the development of high IOP-induced rat retinal IPC.

Protein kinase C-dependent and independent signaling pathways regulate synaptic GluR1 and GluR4 AMPAR subunits during in vitro classical conditioning

Protein kinase C (PKC) signal transduction pathways have been implicated in mechanisms of synaptic plasticity and learning, however, the roles of the different PKC family isoforms remain to be clarified. Previous studies showed that NMDAR-mediated trafficking of GluR4-containing AMPARs supports conditioning and that the mitogen-activated protein kinases (MAPKs) have a central role in the synaptic delivery of GluR4 subunits. Here, an in vitro model of classical conditioning in pond turtles, Pseudemys scripta elegans, was used to assess the role of PKC isoforms in mechanisms underlying this form of learning. We show that the PKC antagonists chelerythrine and bisindolylmaleimide I attenuated conditioned response (CR) acquisition and expression, as did the PKCzeta pseudosubstrate peptide inhibitor ZIP. Analysis of protein expression revealed that PKCzeta is activated in early stages of conditioning followed shortly afterward by increased levels of PKCalpha/beta and activation of ERK MAPK. Data also suggest that PKCzeta is upstream from and activates ERK. Finally, protein localization studies using confocal imaging indicate that inhibitors of ERK, but not PKC, suppress colocalization of GluR1 with synaptophysin while inhibitors of PKC and ERK attenuate colocalization of GluR4 with synaptophysin. Together, these data suggest that acquisition of conditioning proceeds by two stages of AMPAR trafficking. The first is PKC-independent and ERK-dependent synaptic delivery of GluR1 subunits to activate silent synapses. This is followed by PKC-dependent and ERK-dependent synthesis and delivery of GluR4 subunits that supports the acquisition of CRs. Therefore, there is a selective role for PKC and MAPK signaling pathways in multistep AMPAR trafficking that mediates acquisition of classical conditioning.

Role of the carboxyl terminal di-leucine in phosphorylation and internalization of C5a receptor

The carboxyl tail of G protein-coupled receptors contains motifs that regulate receptor interactions with intracellular partners. Activation of the human neutrophil complement fragment C5a receptor (C5aR) is terminated by phosphorylation of the carboxyl tail followed by receptor internalization. In this study, we demonstrated that bulky hydrophobic residues in the membrane-proximal region of the C5aR carboxyl tail play an important role in proper structure and function of the receptor: Substitution of leucine 319 with alanine (L319A) resulted in receptor retention in the endoplasmic reticulum, whereas a L318A substitution allowed receptor transport to the cell surface, but showed slow internalization upon activation, presumably due to a defect in phosphorylation by both PKC and GRK. Normal agonist-induced activation of ERK1/2 and intracellular calcium release suggested that the L318A mutation did not affect receptor signaling. Binding of GRK2 and PKCbetaII to intracellular loop 3 of C5aR in vitro indicated that mutagenesis of L318 did not affect kinase binding. Limited proteolysis with trypsin revealed a conformational difference between wild type and mutant receptor. Our studies support a model in which the L318/L319 stabilizes an amphipathic helix (Q305-R320) in the membrane-proximal region of C5aR.

Cholecystokinin-A receptors regulate photic input pathways to the circadian clock

Daily behaviors are strongly dominated by internally generated circadian rhythms, but the underlying mechanisms remain unclear. In mammals, photoentrainment of behaviors to light-dark cycles involves signaling from both intrinsically photosensitive retinal ganglion cells and classic photoreceptor pathways to the suprachiasmatic nucleus (SCN). How classic photoreceptor pathways work with the photosensitive ganglion cells, however, is not fully understood. Although cholecystokinin (CCK) peptide has been shown to be present in a variety of vertebrate retinas, its function at a systems level is also unknown. In the present study we examined a possible role of CCK-A receptors in photoentrainment using CCK-A receptor knockout mice. The lacZ reporter gene within a gene-knockout cassette revealed precise localization of CCK-A receptors in the circadian clock system. We demonstrated that CCK-A receptors were located predominately on glycinergic amacrine cells but were rarely found on SCN neurons. Moreover, Ca(2+) imaging analysis demonstrated that the CCK-A agonist, CCK-8 sulfate (CCK-8s), mobilized intracellular Ca(2+) in amacrine cells but not glutamate-receptive SCN neurons. Furthermore, light pulse-induced mPer1/mPer2 gene expression in SCN, behavioral phase shifts, and the pupillary reflex were significantly reduced in CCK-A receptor knockout mice. These data indicate a novel function of CCK-A receptors in the nonimage-forming photoreception presumably via amacrine cell-mediated signal transduction pathways.

Posttranslational modifications and receptor-associated proteins in AMPA receptor trafficking and synaptic plasticity

AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission in the mammalian brain. It is widely believed that the long-lasting, activity-dependent changes in synaptic strength, including long-term potentiation and long-term depression, could be the molecular and cellular basis of experience-dependent plasticities, such as learning and memory. Those changes of synaptic strength are directly related to AMPAR trafficking to and away from the synapse. There are many forms of synaptic plasticity in the mammalian brain, while the prototypic form, hippocampal CA1 long-term potentiation, has received the most intense investigation. After synthesis, AMPAR subunits undergo posttranslational modifications such as glycosylation, palmitoylation, phosphorylation and potential ubiquitination. In addition, AMPAR subunits spatiotemporally associate with specific neuronal proteins in the cell. Those posttranslational modifications and receptor-associated proteins play critical roles in AMPAR trafficking and regulation of AMPAR-dependent synaptic plasticity. Here, we summarize recent studies on posttranslational modifications and associated proteins of AMPAR subunits, and their roles in receptor trafficking and synaptic plasticity.

Protein kinase C gamma mutations in the C1B domain cause caspase-3-linked apoptosis in lens epithelial cells through gap junctions

Failure to control oxidative stress is closely related to aging and to a diverse range of human diseases. We have reported that protein kinase C gamma (PKCgamma) acts as a primary oxidative stress sensor in the lens. PKCgamma has a Zn-finger C1B stress switch domain, residues 101-150. Mutation, H101Y, in the C1B domain of PKCgamma proteins causes a failure of the PKCgamma oxidative stress response [Lin, D., Takemoto, D.J., 2005. Oxidative activation of protein kinase Cgamma through the C1 domain. Effects on gap junctions. J. Biol. Chem. 280, 13682-13693]. Some human neurodegenerative spinocerebellar ataxia type 14 are caused by mutations in the PKCgamma C1B domain. In the current study we have investigated the effects of these mutations on lens epithelial cell responses to oxidative stress. The results demonstrate that PKCgamma C1B mutants had lower basal enzyme activities and were not activated by H(2)O(2). Furthermore, the PKCgamma mutations caused a failure of endogenous wild type PKCgamma to be activated by H(2)O(2). These PKCgamma mutations abolished the effect of H(2)O(2) on phosphorylation of Cx43 and Cx50 by H(2)O(2) activation of PKCgamma. The cells with PKCgamma C1B mutations had more Cx43 and/or Cx50 gap junction plaques which were not decreased by H(2)O(2). Since open gap junctions could have a bystander effect this could cause apoptosis to occur. H(2)O(2) (100 microM, 3 h) activated a caspase-3 apoptotic pathway in the lens epithelial cells but was more severe in cells expressing PKCgamma mutations. The presence of 18alpha-glycyrrhetinic acid (AGA), an inhibitor of gap junctions, decreased Cx43 and Cx50 protein levels and gap junction plaque number. This reduction in gap junctions by AGA resulted in inhibition of H(2)O(2)-induced apoptosis. Our results demonstrate that there is a dominant negative effect of PKCgamma C1B mutations on endogenous PKCgamma which results in loss of control of gap junctions. Modeled structures suggest that the severity of C1B mutation effects may be related to the extent of loss of C1B structure. Mutations in the C1B domain of PKCgamma result in increased apoptosis in lens epithelial cells. This can be prevented by a gap junction inhibitor. Thus, propagation of apoptosis from cell-to-cell in lens epithelial cells may be through open gap junctions. The control of gap junctions requires PKCgamma.

Phosphorylation of glutamate receptors: a potential mechanism for the regulation of receptor function and psychostimulant action

Ionotropic glutamate receptors, N-methyl-d-aspartate receptors (NMDARs) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs), are densely distributed in the mammalian brain and actively regulate a variety of cellular activities. Expression and function of these receptors are also under a tight regulation by many molecular mechanisms. Protein phosphorylation represents one of the important mechanisms for the posttranslational modulation of these receptors. Constitutive and regulatory phosphorylation occurs at distinct sites (serine, threonine, or tyrosine) on the intracellular C-terminal domain of almost all subunits capable of assembling a functional channel. Several key protein kinases, such as protein kinase A, protein kinase C, Ca(2+)/calmodulin-dependent protein kinases, and tyrosine kinases are involved in the site-specific catalyzation and regulation of NMDAR and AMPAR phosphorylation. Through the phosphorylation mechanism, these protein kinases as well as protein phosphatases control biochemical properties (biosynthesis, delivery, and subunit assembling), subcellular distribution, and interactions of these receptors with various synaptic proteins, which ultimately modify the efficacy and strength of excitatory synapses containing NMDARs and AMPARs and many forms of synaptic plasticity. Emerging evidence shows that psychostimulants (cocaine and amphetamine) are among effective agents that profoundly alter the phosphorylation status of both receptors in striatal neurons in vivo. Thus, psychostimulants may modulate NMDAR and AMPAR function through the phosphorylation mechanism to shape the excitatory synaptic plasticity related to additive properties of drugs of abuse.

PKC Anchoring to GluR4 AMPA Receptor Subunit Modulates PKC-Driven Receptor Phosphorylation and Surface Expression

Changes in the synaptic content of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors lead to synaptic efficacy modifications, involved in synaptic plasticity mechanisms believed to underlie learning and memory formation. Early in development, GluR4 is highly expressed in the hippocampus, and GluR4-containing AMPA receptors are inserted into synapses. During synapse maturation, the number of AMPA receptors at the synapse is dynamically regulated, and both addition and removal of receptors from postsynaptic sites occur through regulated mechanisms. GluR4 delivery to synapses in rat hippocampal slices was shown to require protein kinase A (PKA)-mediated phosphorylation of GluR4 at serine 842 (Ser842). Protein kinase C (PKC) can also phosphorylate Ser842, and we have shown that PKCgamma can associate with GluR4. Here we show that activation of PKC in retina neurons, or in human embryonic kidney 293 cells cotransfected with GluR4 and PKCgamma, increases GluR4 surface expression and Ser842 phosphorylation. Moreover, mutation of amino acids R821A, K825A and R826A at the GluR4 C-terminal, within the interacting region of GluR4 with PKCgamma, abolishes the interaction between PKCgamma and GluR4 and prevents the stimulatory effect of PKCgamma on GluR4 Ser842 phosphorylation and surface expression. These data argue for a role of anchored PKCgamma in Ser842 phosphorylation and targeting to the plasma membrane. The triple GluR4 mutant is, however, phosphorylated by PKA, and it is targeted to the synapse in CA1 hippocampal neurons in organotypic rat hippocampal slices. The present findings show that the interaction between PKCgamma and GluR4 is specifically required to assure PKC-driven phosphorylation and surface membrane expression of GluR4.

Differential activation of protein kinase C isoforms following chemical ischemia in rat cerebral cortex slices

The aim of the current study was to characterize the effects of chemical ischemia and reperfusion at the transductional level in the brain. Protein kinase C isoforms (alpha, beta(1), beta(2), gamma, delta and epsilon) total levels and their distribution in the particulate and cytosolic compartments were investigated in superfused rat cerebral cortex slices: (i) under control conditions; (ii) immediately after a 5-min treatment with 10mM NaN(3), combined with 2mM 2-deoxyglucose (chemical ischemia); (iii) 1h after chemical ischemia (reperfusion). In control samples, all the PKC isoforms were detected; immediately after chemical ischemia, PKC beta(1), delta and epsilon isoforms total levels (cytosol+particulate) were increased by 2.9, 2.7 and 9.9 times, respectively, while alpha isoform was slightly reduced and gamma isoform was no longer detectable. After reperfusion, the changes displayed by alpha, beta(1), gamma, delta and epsilon were maintained and even potentiated, moreover, an increase in beta(2) (by 41+/-12%) total levels became significant. Chemical ischemia-induced a significant translocation to the particulate compartment of PKC alpha isoform, which following reperfusion was found only in the cytosol. PKC beta(1) and delta isoforms particulate levels were significantly higher both in ischemic and in reperfused samples than in the controls. Conversely, following reperfusion, PKC beta(2) and epsilon isoforms displayed a reduction in their particulate to total level ratios. The intracellular calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, 1mM, but not the N-methyl-d-asparate receptor antagonist, MK-801, 1muM, prevented the translocation of beta(1) isoform observed during ischemia. Both drugs were effective in counteracting reperfusion-induced changes in beta(2) and epsilon isoforms, suggesting the involvement of glutamate-induced calcium overload. These findings demonstrate that: (i) PKC isoforms participate differently in neurotoxicity/neuroprotection events; (ii) the changes observed following chemical ischemia are pharmacologically modulable; (iii) the protocol of in vitro chemical ischemia is suitable for drug screening.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Phosphorylation-dependent interactions of α-Actinin-1/IQGAP1 with the AMPA receptor subunit GluR4

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors play key roles in excitatory synaptic transmission and synaptic plasticity in the CNS. Although a variety of proteins has been characterized to interact with AMPA receptors and regulate their function, little is known about the regulation of the AMPA receptor subunit GluR4. To understand the molecular mechanisms of GluR4 functional regulation, the yeast two-hybrid system was used to identify GluR4-interacting molecules. alpha-Actinin-1 and IQGAP1 were identified to be GluR4-specific binding partners. Both proteins interact specifically with GluR4 and co-cluster with GluR4 individually in neurons. Mapping experiments revealed that alpha-Actinin-1 and IQGAP1 bind to the same region within the C-terminus of GluR4 that contains a previously identified PKA phosphorylation site, Ser842, phosphorylation of which is regulated by synaptic activity. Interestingly, the phosphorylation of Ser842 differentially regulates interactions of GluR4 with alpha-Actinin-1 and IQGAP1; phosphorylation strongly inhibits interaction of GluR4 with alpha-Actinin-1 but has little effect on its interaction with IQGAP1. These results suggest that alpha-Actinin-1 and IQGAP1 regulate GluR4 functions via their specific associations with GluR4. In addition, our data indicate that activity-dependent phosphorylation of GluR4 may regulate its synaptic targeting through phosphorylation-dependent interactions with alpha-Actinin-1 and IQGAP1.

Activated PKA and PKC, but not CaMKIIα, are required for AMPA/Kainate-mediated pain behavior in the thermal stimulus model

Secondary mechanical allodynia resulting from a thermal stimulus (52.5 degrees C for 45s) is blocked by intrathecal (i.t.) pretreatment with calcium-permeable AMPA/KA receptor antagonists, but not NMDA receptor antagonists. Spinal sensitization is presumed to underlie thermal stimulus-evoked secondary mechanical allodynia. We investigated whether this spinal sensitization involves activation and phosphorylation of calcium-dependent protein kinases (PKA, PKC and CaMKIIalpha), and examined if the noxious stimulus increases phosphorylated AMPA GLUR1 (pGLUR1 Ser-845 and pGLUR1 Ser-831). Secondary mechanical allodynia after thermal stimulation was not altered by i.t. pretreatment with control vehicles (saline or 5% DMSO). Comparable allodynia was observed after pretreatment with a selective CaMKIIalpha inhibitor (17 and 34nmol KN-93). In marked contrast, pretreatment with either a PKA (10nmol H89) or PKC (30nmol chelerythrine) inhibitor blocked allodynia. Western immunoblot analyses supported behavioral findings and revealed a thermal stimulus-evoked increase in spinal phosphorylated PKA and PKC, but not CaMKIIalpha. There was no increase in any of the total protein kinases. Although thermal stimulation did not change either pGLUR1 Ser-845 or pGLUR1 Ser-831, it was associated with an increase in cytosolic total GLUR1. Pretreatment with a selective calcium-permeable AMPA/KA receptor antagonist (5nmol joro spider toxin), but not an NMDA receptor antagonist (25nmol d-2-amino-5-phosphonovalerate, AP-5), blocked thermal stimulus-evoked increases in phosphorylated PKA and PKC, in addition to increased cytosolic GLUR1. These findings indicate that spinal sensitization in the thermal stimulus model does not involve CaMKIIalpha activation or AMPA GLUR1 receptor phosphorylation, and differs from that occurring in NMDAr-dependent pain states.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Electrophysiology and plasticity in isolated postsynaptic densities

The organization and regulation of excitatory synapses in the mammalian CNS entails complex molecular and cellular processes. In the postsynaptic membrane, scaffolding proteins bring together glutamate receptors with multiple regulatory proteins involved in signal transduction. This gives rise to an elaborate postsynaptic structure known as the postsynaptic density (PSD). This protein network plays a critical role in the regulation of glutamate receptor function and thus in synaptic plasticity. To study this regulation, we have developed a system in which ionotropic glutamate receptors (iGluRs) can be recorded, in the steady state, by the patch clamp technique in isolated PSDs incorporated into giant liposomes. In this preparation, ionotropic glutamate receptors maintain their characteristic physiological and pharmacological properties. The recordings reflect the presence of channel clusters, as multiple conductance and subconductance states are observed. Each of the receptor subtypes is activated by a specific set of kinases that are activated differentially by Ca(2+): the "kainate receptor kinases" are active even in the presence of EGTA, i.e. they are not calcium-dependent; the "N-methyl-D-aspartate receptor (NMDAR) channel kinases" are active in the presence of submicromolar calcium concentrations, whereas the "alpha-amino-3- hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor kinases" need microM calcium for activation. The NMDA receptor showed its characteristic voltage-dependent Mg(2+) blockade, and activation by phosphorylation was in part a consequence of a relief of Mg(2+) blockade. These results allow us to propose a model in which phosphorylation of NMDA receptors can contribute to a long-lasting and self-maintained change in synaptic function. The experimental approach we present will allow us to test the functional consequence of activation of the multiple signal transduction pathways thought to regulate excitatory neurotransmission in the adult CNS.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors signaling complexes in Bergmann glia

Glutamate, the major excitatory neurotransmitter, induces a wide array of signals from the membrane to the nucleus regulating gene expression. In Bergmann glia, Ca2+ -permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole- propionic acid (AMPA) receptors are involved in the short- and long-term interactions between these cells and the neurons that they surround. After activation, AMPA receptors become tyrosine phosphorylated and by these means form multiprotein signaling complexes. To characterize these events, cultured chick Bergmann glia cells as well as chick cerebellar slices were exposed to glutamate, and, by using a combination of immunoprecipitation assays coupled to Western blot analysis, we identified several signaling proteins that become associated with these receptors. A dose- and time-dependent association among AMPA receptors, the focal adhesion kinase pp125FAK, the phosphatidylinositol-3 kinase and paxillin was found. These results extend the concept of the transducisome to AMPA receptors and provide a framework in which a plausible control of the cytoskeletal network by glutamate is taking place, most possibly through AMPA receptors.

Ser1928 Is a Common Site for Cav1.2 Phosphorylation by Protein Kinase C Isoforms

Voltage-dependent Ca(2+) channel (Ca(v)1.2, L-type Ca(2+) channel) function is highly regulated by hormones and neurotransmitters in large part through the activation of kinases and phosphatases. Regulation of Ca(v)1.2 by protein kinase C (PKC) is of significant physiologic importance, mediating, in part, the cardiac response to hormonal regulation. Although PKC has been reported to mediate activation and/or inhibition of Ca(v)1.2 function, the molecular mechanisms mediating the response have not been definitively elucidated. We show that PKC forms a macromolecular complex with the alpha(1c) subunit of Ca(v)1.2 through direct interaction with the C terminus. This interaction leads to phosphorylation of the channel in response to activators of PKC. We identify Ser(1928) as the residue that is phosphorylated by PKC in vitro and in vivo. Ser(1928) has been identified previously as the site mediating, in part, the protein kinase A up-regulation of channel activity. Thus, the protein kinase A and PKC signaling pathways converge on the Ca(v)1.2 complex at Ser(1928) to increase channel activity. Our results identify two mechanisms leading to regulation of Ca(v)1.2 activity by PKC: pre-association of the channel with PKC isoforms and phosphorylation of specific sites within the alpha(1c) subunit.

Sulfate- and size-dependent polysaccharide modulation of AMPA receptor properties

Previous work found evidence that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors interact with and are functionally regulated by the glycosaminoglycan heparin. The present study tested whether dextran species affect ligand binding, channel kinetics, and calcium permeability of AMPA receptors. Dextran sulfate of 500 kDa markedly reduced high affinity [3H]AMPA binding in solubilized hippocampal membranes. In isolated receptors reconstituted in a lipid bilayer, the same dextran sulfate prolonged the lifetime of open states exhibited by AMPA-induced channel fluctuations. The large polysaccharide further changed the single channel kinetics by increasing the open channel probability five- to sixfold. Such modulation of channel activity corresponded with enhanced levels of calcium influx as shown in hippocampal neurons loaded with Fluo3AM dye. With an exposure time of <1 min, AMPA produced a dose-dependent increase in intracellular calcium that was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX). Dextran sulfate, at the same concentration range that modified ligand binding (EC50 of 5-10 nM), enhanced the AMPA-induced calcium influx by as much as 60%. The enhanced influx was blocked by CNQX, although unchanged by the N-methyl-D-aspartate (NMDA) receptor antagonist AP5. Confocal microscopy showed that the increase in calcium occurred in neuronal cell bodies and their processes. Interestingly, smaller 5-8-kDa dextran sulfate and a non-sulfated dextran of 500 kDa had little or no effect on the binding, channel, and calcium permeability properties. Together, these findings suggest that synaptic polysaccharide species modulate hippocampal AMPA receptors in a sulfate- and size-dependent manner.

Regulation of AMPA receptor activity, synaptic targeting and recycling: role in synaptic plasticity

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors for the neurotransmitter glutamate are oligomeric structures responsible for most fast excitatory responses in the central nervous system. The activity of AMPA receptors can be directly regulated by protein phosphorylation, which may also affect the interaction with intracellular proteins and, consequently, their recycling and localization to defined postsynaptic sites. This review focuses on recent advances in understanding the dynamic regulation of AMPA receptors, on a short- and long-term basis, and its implications in synaptic plasticity.