Phosphorylation and modulation of a kainate receptor (GluR6) by cAMP-dependent protein kinase

Temporal characterisation and electrophysiological implications of TBI-induced serine/threonine kinase activity in mouse cortex

Traumatic brain injury (TBI) remains the leading cause of death and disability worldwide with no existing effective treatment. The early phase after TBI induction triggers numerous molecular cascades to regulate adaptive processes and cortical network activity. Kinases play a particularly prominent role in modifying peptide substrates, which include ion channels, receptors, transcription factors and inflammatory mediators. This study aimed to better understand the post-injury serine/threonine kinome; (1) Which kinases conduct phosphorylation-induced alterations of target peptides following unilateral TBI in mouse cortex? (2) How do these kinases effectuate pathological network hyperexcitability, which has detrimental long-term outcomes? We used a serine/threonine kinase assay at 4 h, 24 h and 72 h post-TBI to identify hyper-/hypo-active/phosphorylated kinases and peptides in the ipsilateral and contralateral cortical hemispheres relative to sham-operated controls. We pharmacologically mimicked the changes seen in ERK1/2 and PKC kinase activity, and using microelectrode array recordings we explored their significant electrophysiological implications on spontaneous and evoked cortical activity. We then used these findings to manipulate key kinase activity changes at 24 h post-TBI to rescue the hyperexcitability that is seen in the contralateral cortical network at this timepoint back to sham level. The contribution of specific downstream peptide target channel/receptor subunits was also shown. We conclude that volatile kinase activity has potent implications on cortical network activity after the injury and that these kinases and/or their peptide substrates should be more seriously considered as therapeutic targets for the clinical treatment of TBI.

Structural and compositional diversity in the kainate receptor family

The kainate receptors (KARs) are members of the ionotropic glutamate receptor family and assemble into tetramers from a pool of five subunit types (GluK1–5). Each subunit confers distinct functional properties to a receptor, but the compositional and stoichiometric diversity of KAR tetramers is not well understood. To address this, we first solve the structure of the GluK1 homomer, which enables a systematic assessment of structural compatibility among KAR subunits. Next, we analyze single-cell RNA sequencing data, which reveal extreme diversity in the combinations of two or more KAR subunits co-expressed within the same cell. We then investigate the composition of individual receptor complexes using single-molecule fluorescence techniques and find that di-heteromers assembled from GluK1, GluK2, or GluK3 can form with all possible stoichiometries, while GluK1/K5, GluK2/K5, and GluK3/K5 can form 3:1 or 2:2 complexes. Finally, using three-color single-molecule imaging, we discover that KARs can form tri- and tetra-heteromers.

Selvakumar et al. use cryo-electron microscopy, single-cell RNA sequencing analysis, and single-molecule fluorescence techniques to investigate the stoichiometric and assembly diversity of kainate receptors (KARs). The work gives insight into KAR molecular diversity and expands the potential KAR subunit combinations to include a variety of di-, tri-, and tetra-heteromers.

Metabotropic actions of kainate receptors modulating glutamate release

Presynaptic kainate (KA) receptors (KARs) modulate GABA and glutamate release in the central nervous system of mammals. While some of the actions of KARs are ionotropic, metabotropic actions for these receptors have also been seen to modulate both GABA and glutamate release. In general, presynaptic KARs modulate glutamate release through their metabotropic actions in a biphasic manner, with low KA concentrations producing an increase in glutamate release and higher concentrations of KA driving weaker release of this neurotransmitter. Different molecular mechanisms are involved in this modulation of glutamate release, with a G-protein independent, Ca²⁺-calmodulin adenylate cyclase (AC) and protein kinase A (PKA) dependent mechanism facilitating glutamate release, and a G-protein, AC and PKA dependent mechanism mediating the decrease in neurotransmitter release. Here, we describe the events underlying the KAR modulation of glutamatergic transmission in different brain regions, addressing the possible functions of this modulation and proposing future research lines in this field.

Non-canonical Mechanisms of Presynaptic Kainate Receptors Controlling Glutamate Release

A metabotropic modus operandi for kainate receptors (KARs) was first discovered in 1998 modulating GABA release. These receptors have been also found to modulate glutamate release at different synapses in several brain regions. Mechanistically, a general biphasic mechanism for modulating glutamate release by presynaptic KARs with metabotropic actions has emerged, with low KA concentrations invoking an increase in glutamate release, whereas higher concentrations of KA mediate a decrease in the release of this neurotransmitter. The molecular mechanisms underpinning the opposite modulation of glutamate release are distinct, with a G-protein-independent, adenylate cyclase (AC)- and protein kinase A (PKA)-dependent mechanism mediating the facilitation of glutamate release, while a G-protein dependent mechanism (with or without protein kinase recruitment) is involved in the decrease of neurotransmitter release. In the present review, we revisit the mechanisms underlying the non-canonical modus operandi of KARs effecting the bimodal control of glutamatergic transmission in different brain regions, and address the possible functions that this modulation may support.

Commemorating John F. MacDonald and the Art of Being a Mentor

John F. MacDonald was a close friend and mentor whose life was ended far too soon on April 22, 2014. To those who knew him, John was an endearing blend of fiery Scotsman, compassionate socialist, dedicated family man, and tireless investigator. Those close to him valued his loyalty and friendship, relished his biting wit, and puzzled at his self-deprecating manner. His career spanned a remarkable period of discovery from the early identification of excitatory amino acid, to the molecular cloning and characterization of glutamate receptors and the elucidation of mechanisms responsible for regulating their function. A true pioneer in each of these areas, John's research has had a lasting impact on our understanding of excitatory synaptic transmission and its plasticity. Our intent in commemorating John's work is to focus on some notable discoveries that highlight the impact and innovative aspects of John's work. In doing so, we also wish to highlight just how greatly our understanding of the glutamate transmitter systems has advanced since the late 1970s, when John first launched his independent neuroscience career.

Identification of new phosphorylation sites of AMPA receptors in the rat hippocampus--A resource for neuroscience research

PURPOSE

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors (AMPARs) are glutamate-gated ion channels that mediate the majority of fast excitatory synaptic transmissions in the mammalian brain. A series of phosphorylation sites have been predicted or identified and knowledge on phosphorylations is mandatory for understanding receptor biology and functions.

EXPERIMENTAL DESIGN

Immunoprecipitation from extracted hippocampal rat proteins was carried out using an antibody against the AMPAR GluA1 subunit, followed by identification of GluA1 and binding partners by MS. Bands from SDS-PAGE were picked, peptides were generated by trypsin and chymotrypsin digestion and identified by MS/MS (LTQ Orbitrap Velos).

RESULTS

Using Mascot as a search engine, phosphorylation sites S506, S645, S720, S849, S863, S895, T858, Y228, Y419, and T734 were found on GluA1; S357, S513, S656, S727, T243, T420, T741, Y 143, Y301,Y426 on GluA2; S301, S516, S657, S732, T222, and T746 were observed on GluA3; and S514, S653 was phosphorylated on GluA4.

CONCLUSIONS AND CLINICAL RELEVANCE

A series of additional protein modifications were observed and in particular, tyrosine and tryptophan nitrations on GluA1 were detected that may raise questions on additional regulation mechanisms for AMPARs in addition to phosphorylations. The findings are relevant for interpretation of previous work and design of future studies using AMPAR serving as a resource for neuroscience research and indeed, phosphorylations and PTMs per se would have to be respected when neuropathological and neurological disorders are being studied.

Tyrosine phosphorylation of GluK2 up-regulates kainate receptor-mediated responses and downstream signaling after brain ischemia

Although kainate receptors play important roles in ischemic stroke, the molecular mechanisms underlying postischemic regulation of kainate receptors remain unclear. In this study we demonstrate that Src family kinases contribute to the potentiation of kainate receptor function. Brain ischemia and reperfusion induce rapid and sustained phosphorylation of the kainate receptor subunit GluK2 by Src in the rat hippocampus, implicating a critical role for Src-mediated GluK2 phosphorylation in ischemic brain injury. The NMDA and kainate receptors are involved in the tyrosine phosphorylation of GluK2. GluK2 binds to Src, and the tyrosine residue at position 590 (Y590) on GluK2 is a major site of phosphorylation by Src kinases. GluK2 phosphorylation at Y590 is responsible for increases in whole-cell currents and calcium influx in response to transient kainate stimulation. In addition, GluK2 phosphorylation at Y590 facilitates the endocytosis of GluK2 subunits, and the activation of JNK3 and its substrate c-Jun after long-term kainate treatment. Thus, Src phosphorylation of GluK2 plays an important role in the opening of kainate receptor channels and downstream proapoptosis signaling after brain ischemia. The present study reveals an additional mechanism for the regulation of GluK2-containing kainate receptors by Src family kinases, which may be of pathological significance in ischemic stroke.

Kainate receptor activation induces glycine receptor endocytosis through PKC deSUMOylation

Surface expression and regulated endocytosis of glycine receptors (GlyRs) play a critical function in balancing neuronal excitability. SUMOylation (SUMO modification) is of critical importance for maintaining neuronal function in the central nervous system. Here we show that activation of kainate receptors (KARs) causes GlyR endocytosis in a calcium- and protein kinase C (PKC)-dependent manner, leading to reduced GlyR-mediated synaptic activity in cultured spinal cord neurons and the superficial dorsal horn of rat spinal cord slices. This effect requires SUMO1/sentrin-specific peptidase 1 (SENP1)-mediated deSUMOylation of PKC, indicating that the crosstalk between KARs and GlyRs relies on the SUMOylation status of PKC. SENP1-mediated deSUMOylation of PKC is involved in the kainate-induced GlyR endocytosis and thus plays an important role in the anti-homeostatic regulation between excitatory and inhibitory ligand-gated ion channels. Altogether, we have identified a SUMOylation-dependent regulatory pathway for GlyR endocytosis, which may have important physiological implications for proper neuronal excitability.

Maintenance of proper membrane excitability is vital to neuronal function and in several neuronal types this relies on a balance between receptor-mediated excitation and inhibition. Here the authors report a crosstalk between excitatory kainate receptors and inhibitory glycine receptors that relies on the SUMOylation status of PKC.

Trafficking of kainate receptors

Ionotropic glutamate receptors (iGluRs) mediate the vast majority of excitatory neurotransmission in the central nervous system of vertebrates. In the protein family of iGluRs, kainate receptors (KARs) comprise the probably least well understood receptor class. Although KARs act as key players in the regulation of synaptic network activity, many properties and functions of these proteins remain elusive until now. Especially the precise pre-, extra-, and postsynaptic localization of KARs plays a critical role for neuronal function, as an unbalanced localization of KARs would ultimately lead to dysregulated neuronal excitability. Recently, important advances in the understanding of the regulation of surface expression, function, and agonist-dependent endocytosis of KARs have been achieved. Post-translational modifications like PKC-mediated phosphorylation and SUMOylation have been reported to critically influence surface expression and endocytosis, while newly discovered auxiliary proteins were shown to shape the functional properties of KARs.

The prostaglandin EP1 receptor potentiates kainate receptor activation via a protein kinase C pathway and exacerbates status epilepticus

Prostaglandin E2 (PGE2) regulates membrane excitability, synaptic transmission, plasticity, and neuronal survival. The consequences of PGE2 release following seizures has been the subject of much study. Here we demonstrate that the prostaglandin E2 receptor 1 (EP1, or Ptger1) modulates native kainate receptors, a family of ionotropic glutamate receptors widely expressed throughout the central nervous system. Global ablation of the EP1 gene in mice (EP1-KO) had no effect on seizure threshold after kainate injection but reduced the likelihood to enter status epilepticus. EP1-KO mice that did experience typical status epilepticus had reduced hippocampal neurodegeneration and a blunted inflammatory response. Further studies with native prostanoid and kainate receptors in cultured cortical neurons, as well as with recombinant prostanoid and kainate receptors expressed in Xenopus oocytes, demonstrated that EP1 receptor activation potentiates heteromeric but not homomeric kainate receptors via a second messenger cascade involving phospholipase C, calcium and protein kinase C. Three critical GluK5 C-terminal serines underlie the potentiation of the GluK2/GluK5 receptor by EP1 activation. Taken together, these results indicate that EP1 receptor activation during seizures, through a protein kinase C pathway, increases the probability of kainic acid induced status epilepticus, and independently promotes hippocampal neurodegeneration and a broad inflammatory response.

Activation of group I metabotropic glutamate receptors potentiates heteromeric kainate receptors

Kainate receptors (KARs), a family of ionotropic glutamate receptors, are widely expressed in the central nervous system and are critically involved in synaptic transmission. KAR activation is influenced by metabotropic glutamate receptor (mGlu) signaling, but the underlying mechanisms are not understood. We undertook studies to examine how mGlu modulation affects activation of KARs. Confocal immunohistochemistry of rat hippocampus and cultured rat cortex revealed colocalization of the high-affinity KAR subunits with group I mGlu receptors. In hippocampal and cortical cultures, the calcium signal caused by activation of native KARs was potentiated by activation of group I mGlu receptors. In Xenopus laevis oocytes, activation of group I mGlu receptors potentiated heteromeric but not homomeric KAR-mediated currents, with no change in agonist potency. The potentiation of heteromeric KARs by mGlu1 activation was attenuated by GDPβS, blocked by an inhibitor of phospholipase C or the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), prolonged by the phosphatase inhibitor okadaic acid, but unaffected by the tyrosine kinase inhibitor lavendustin A. Protein kinase C (PKC) inhibition reduced the potentiation by mGlu1 of GluK2/GluK5, and conversely, direct activation of PKC by phorbol 12-myristate,13-acetate potentiated GluK2/GluK5. Using site-directed mutagenesis, we identified three serines (Ser833, Ser836, and Ser840) within the membrane proximal region of the GluK5 C-terminal domain that, in combination, are required for mGlu1-mediated potentiation of KARs. Together, these data suggest that phosphorylation of key residues in the C-terminal domain changes the overall charge of this domain, resulting in potentiated agonist responses.

REM Sleep Regulating Mechanisms in the Cholinergic Cell Compartment of the Brainstem

Rapid eye movement (REM) sleep is a highly evolved yet paradoxical behavioral state (highly activated brain in a paralyzed body) in mammalian species. Since the discovery of REM sleep and its physiological distinction from other sleep states¹, a vast number of studies in neurosciences have been dedicated toward understanding the mechanisms and functions of this behavioral state. Collectively, studies have shown that each of the physiological events that characterize the behavioral state of REM sleep is executed by distinct cell groups located in the brainstem. These cell groups are discrete components of a widely distributed network, rather than a single REM sleep center. The final activity within each of these executive cell groups is controlled by the ratio of cholinergic neurotransmission emanating from the pedunculopontine tegmentum (PPT) to aminergic neurotransmission emanating from the locus coeruleus (LC) and raphe nucleus (RN). In this review, we summarize the most recent findings on the cellular and molecular mechanisms in the PPT cholinergic cell compartment that underlie the regulation of REM sleep. This up-to-date review should allow clinicians and researchers to better understand the effects of drugs and neurologic disease on REM sleep.

Cellular and chemical neuroscience of mammalian sleep

Extraordinary strides have been made toward understanding the complexities and regulatory mechanisms of sleep over the past two decades thanks to the help of rapidly evolving technologies. At its most basic level, mammalian sleep is a restorative process of the brain and body. Beyond its primary restorative purpose, sleep is essential for a number of vital functions. Our primary research interest is to understand the cellular and molecular mechanisms underlying the regulation of sleep and its cognitive functions. Here I will reflect on our own research contributions to 50 years of extraordinary advances in the neurobiology of slow-wave sleep (SWS) and rapid eye movement (REM) sleep regulation. I conclude this review by suggesting some potential future directions to further our understanding of the neurobiology of sleep.

Platelet-derived growth factor selectively inhibits NR2B-containing N-methyl-D-aspartate receptors in CA1 hippocampal neurons

Platelet-derived growth factor (PDGF) beta receptor activation inhibits N-methyl-d-aspartate (NMDA)-evoked currents in hippocampal and cortical neurons via the activation of phospholipase Cgamma, PKC, the release of intracellular calcium, and a rearrangement of the actin cytoskeleton. In the hippocampus, the majority of NMDA receptors are heteromeric; most are composed of 2 NR1 subunits and 2 NR2A or 2 NR2B subunits. Using NR2B- and NR2A-specific antagonists, we demonstrate that PDGF-BB treatment preferentially inhibits NR2B-containing NMDA receptor currents in CA1 hippocampal neurons and enhances long-term depression in an NR2B subunit-dependent manner. Furthermore, treatment of hippocampal slices or cultures with PDGF-BB decreases the surface localization of NR2B but not of NR2A subunits. PDGFbeta receptors colocalize to a higher degree with NR2B subunits than with NR2A subunits. After neuronal injury, PDGFbeta receptors and PDGF-BB are up-regulated and PDGFbeta receptor activation is neuroprotective against glutamate-induced neuronal damage in cultured neurons. We demonstrate that the neuroprotective effects of PDGF-BB are occluded by the NR2B antagonist, Ro25-6981, and that PDGF-BB promotes NMDA signaling to CREB and ERK1/2. We conclude that PDGFbetaR signaling, by preferentially targeting NR2B receptors, provides an important mechanism for neuroprotection by growth factors in the central nervous system.

Molecular determinants of kainate receptor trafficking

Glutamate receptors of the kainate subtype are ionotropic receptors that play a key role in the modulation of neuronal network activity. The role of kainate receptors depends on their precise membrane and subcellular localization in presynaptic, extrasynaptic and postsynaptic domains. These receptors are composed of the combination of five subunits, three of them having several splice variants. The subunits and splice variants show great divergence in their C-terminal cytoplasmic tail domains, which have been implicated in intracellular trafficking of homomeric and heteromeric receptors. Differential trafficking of kainate receptors to specific neuronal compartments likely relies on interactions between the different kainate receptor subunits with distinct subsets of protein partners that interact with C-terminal domains. These C-terminal domains have also been implicated in the degradation of kainate receptors. Finally, the phosphorylation of the C-terminal domain regulates receptor trafficking and function. This review summarizes our knowledge on the regulation of membrane delivery and trafficking of kainate receptors implicating C-terminal domains of the different isoforms and focuses on the identification and characterization of the function of interacting partners.

Identification of C-terminal domain residues involved in protein kinase A-mediated potentiation of kainate receptor subtype 6

Glutamate receptors are the major excitatory receptors in the vertebrate CNS and have been implicated in a number of physiological and pathological processes. Previous work has shown that glutamate receptor function may be modulated by protein kinase A (PKA)-mediated phosphorylation, although the molecular mechanism of this potentiation has remained unclear. We have investigated the phosphorylation of specific amino acid residues in the C-terminal cytoplasmic domain of the rat kainate receptor subtype 6 (GluR6) as a possible mechanism for regulation of receptor function. The C-terminal tail of rat GluR6 can be phosphorylated by PKA on serine residues as demonstrated using [gamma-32P]ATP kinase assays. Whole cell recordings of transiently transfected human embryonic kidney (HEK) 293 cells showed that phosphorylation by PKA potentiates whole cell currents in wildtype GluR6 and that removal of the cytoplasmic C-terminal domain abolishes this potentiation. This suggested that the C-terminal domain may contain residue(s) involved in the PKA-mediated potentiation. Single mutations of each serine residue in the C-terminal domain (S815A, S825A, S828A, and S837A) and a truncation after position 855, which removes all threonines (T856, T864, and T875) from the domain, do not abolish PKA potentiation. However, the S825A/S837A mutation, but no other double mutation, abolishes potentiation. These results demonstrate that phosphorylation of the C-terminal tail of GluR6 by PKA leads to potentiation of whole cell response, and the combination of S825 and S837 in the C-terminal domain is a vital component of the mechanism of GluR6 potentiation by PKA.

Activation of pedunculopontine tegmental PKA prevents GABAB receptor activation-mediated rapid eye movement sleep suppression in the freely moving rat

The pedunculopontine tegmental (PPT) GABAergic system plays a crucial role in the regulation of rapid eye movement (REM) sleep. I recently reported that the activation of PPT GABA(B) receptors suppressed REM sleep by inhibiting REM-on cells. One of the important mechanisms for GABA(B) receptor activation-mediated physiological action is the inhibition of the intracellular cAMP-dependent protein kinase A (cAMP-PKA) signaling pathway. Accordingly, I hypothesized that the PPT GABA(B) receptor activation-mediated REM sleep suppression effect could be mediated through inhibition of cAMP-PKA activation. To test this hypothesis, a GABA(B) receptor selective agonist, baclofen hydrochloride (baclofen), cAMP-PKA activator, Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine (SpCAMPS), and vehicle control were microinjected into the PPT in selected combinations to determine effects on sleep-waking states of chronically instrumented, freely moving rats. Microinjection of SpCAMPS (1.5 nmol) induced REM sleep within a short latency (12.1 +/- 3.6 min) compared with vehicle control microinjection (60.0 +/- 6.5 min). On the contrary, microinjection of baclofen (1.5 nmol) suppressed REM sleep by delaying its appearance for approximately 183 min; however, the suppression of REM sleep by baclofen was prevented by a subsequent microinjection of SpCAMPS. These results provide evidence that the activation of cAMP-PKA within the PPT can successfully block the GABA(B) receptor activation-mediated REM sleep suppression effect. These findings suggest that the PPT GABA(B) receptor activation-mediated REM sleep regulating mechanism involves inactivation of cAMP-PKA signaling in the freely moving rat.

KRIP6: a novel BTB/kelch protein regulating function of kainate receptors

Whereas many interacting proteins have been identified for AMPA and NMDA glutamate receptors, fewer are known to directly bind and regulate function of kainate receptors. Using a yeast two-hybrid screen for interacting partners of the C-terminal domain of GluR6a, we identified a novel neuronal protein of the BTB/kelch family, KRIP6. KRIP6 binds to the GluR6a C-terminal domain at a site distinct from the PDZ-binding motif and it co-immunoprecipitates with recombinant and endogenous GluR6. Co-expression of KRIP6 alters GluR6 mediated currents in a heterologous expression system reducing peak current amplitude and steady-state desensitization, without affecting surface levels of GluR6. Endogenous KRIP6 is widely expressed in brain and overexpression of KRIP6 reduces endogenous kainate receptor-mediated responses evoked in hippocampal neurons. Taken together, these results suggest that KRIP6 can directly regulate native kainate receptors and provide the first evidence for a BTB/kelch protein in direct functional regulation of a mammalian glutamate receptor.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

pkaPS: prediction of protein kinase A phosphorylation sites with the simplified kinase-substrate binding model

Background

Protein kinase A (cAMP-dependent kinase, PKA) is a serine/threonine kinase, for which ca. 150 substrate proteins are known. Based on a refinement of the recognition motif using the available experimental data, we wished to apply the simplified substrate protein binding model for accurate prediction of PKA phosphorylation sites, an approach that was previously successful for the prediction of lipid posttranslational modifications and of the PTS1 peroxisomal translocation signal.

Results

Approximately 20 sequence positions flanking the phosphorylated residue on both sides have been found to be restricted in their sequence variability (region -18...+23 with the site at position 0). The conserved physical pattern can be rationalized in terms of a qualitative binding model with the catalytic cleft of the protein kinase A. Positions -6...+4 surrounding the phosphorylation site are influenced by direct interaction with the kinase in a varying degree. This sequence stretch is embedded in an intrinsically disordered region composed preferentially of hydrophilic residues with flexible backbone and small side chain. This knowledge has been incorporated into a simplified analytical model of productive binding of substrate proteins with PKA.

Conclusion

The scoring function of the pkaPS predictor can confidently discriminate PKA phosphorylation sites from serines/threonines with non-permissive sequence environments (sensitivity of ~96% at a specificity of ~94%). The tool "pkaPS" has been applied on the whole human proteome. Among new predicted PKA targets, there are entirely uncharacterized protein groups as well as apparently well-known families such as those of the ribosomal proteins L21e, L22 and L6.

Availability

The supplementary data as well as the prediction tool as WWW server are available at .

Reviewers

Erik van Nimwegen (Biozentrum, University of Basel, Switzerland), Sandor Pongor (International Centre for Genetic Engineering and Biotechnology, Trieste, Italy), Igor Zhulin (University of Tennessee, Oak Ridge National Laboratory, USA).

Modulation of D2R-NR2B Interactions in Response to Cocaine

Dopamine-glutamate interactions in the neostriatum determine psychostimulant action, but the underlying molecular mechanisms remain elusive. Here we found that dopamine stimulation by cocaine enhances a heteroreceptor complex formation between dopamine D2 receptors (D2R) and NMDA receptor NR2B subunits in the neostriatum in vivo. The D2R-NR2B interaction is direct and occurs in the confined postsynaptic density microdomain of excitatory synapses. The enhanced D2R-NR2B interaction disrupts the association of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) with NR2B, reduces NR2B phosphorylation at a CaMKII-sensitive site (Ser1303), and inhibits NMDA receptor-mediated currents in medium-sized striatal neurons. Furthermore, the regulated D2R-NR2B interaction is critical for constructing behavioral responsiveness to cocaine. Our findings here uncover a direct and dynamic D2R-NR2B interaction in striatal neurons in vivo. This type of dopamine-glutamate integration at the receptor level may be responsible for synergistically inhibiting the D2R-mediated circuits in the basal ganglia and fulfilling the stimulative effect of psychostimulants.

Kainate receptor-interacting proteins and membrane trafficking

Kainate receptors are composed of several subunits and splice variants, but the relevance of this diversity is still not well understood. The subunits and splice variants show great divergence in their C-terminal cytoplasmic tail region, which has been identified as a region of interaction with a number of protein partners. Differential trafficking of kainate receptors to neuronal compartments is likely to rely on interactions with distinct subsets of protein partners. This review summarizes our knowledge of the regulation of trafficking of kainate receptors and focuses on the identification and characterization of functions of interacting partners.

Activation of pedunculopontine tegmental protein kinase A: a mechanism for rapid eye movement sleep generation in the freely moving rat

Cells in the pedunculopontine tegmentum (PPT) play a key role in the generation of rapid eye movement (REM) sleep, but its intracellular signaling mechanisms remain unknown. In the current studies, the role of PPT intracellular protein kinase A (PKA) in the regulation of REM sleep was evaluated by comparing PKA subunit [catalytic (PKA(C alpha)) and regulatory (PKA(RI), PKA(RII alpha), and PKA(RII beta)) types] expression and activity in the PPT at normal, high, and low REM sleep conditions. To compare anatomical specificity, REM sleep-dependent expressions of these PKA subunits were also measured in the medial pontine reticular formation (mPRF), medial prefrontal cortex (mPFC), and anterior hypothalamus (AHTh). The results of these PKA subunit expression and activity studies demonstrated that the expression of PKA(C alpha) and PKA activity in the PPT increased and decreased during high and low REM sleep, respectively. Conversely, PKA(C alpha) expression and PKA activity decreased with high REM sleep in the mPRF. Expression of PKA(C alpha) also decreased in the mPFC and remained unchanged in the AHTh with high REM sleep. These subunit expression and PKA activity data reveal a positive relationship between REM sleep and increased PKA activity in the PPT. To test this molecular evidence, localized activation of cAMP-dependent PKA activity was blocked using a pharmacological technique. The results of this pharmacological study demonstrated that the localized inhibition of cAMP-dependent PKA activation in the PPT dose-dependently suppressed REM sleep. Together, these results provide the first evidence that the activation of the PPT intracellular PKA system is involved in the generation of REM sleep.

Kainate receptors

Kainate receptors form a family of ionotropic glutamate receptors that appear to play a special role in the regulation of the activity of synaptic networks. This review first describes briefly the molecular and pharmacological properties of native and recombinant kainate receptors. It then attempts to outline the general principles that appear to govern the function of kainate receptors in the activity of synaptic networks under physiological conditions. It subsequently describes the way that kainate receptors are involved in synaptic integration, synaptic plasticity, the regulation of neurotransmitter release and the control of neuronal excitability, and the manner in which they might play an important role in synaptogenesis and synaptic maturation. These functions require the proper subcellular localization of kainate receptors in specific functional domains of the neuron, necessitating complex cellular and molecular trafficking events. We show that our comprehension of these mechanisms is just starting to emerge. Finally, this review presents evidence that implicates kainate receptors in pathophysiological conditions such as epilepsy, excitotoxicity and pain, and that shows that these receptors represent promising therapeutic targets.

Postsynaptic mechanisms are essential for forskolin-induced potentiation of synaptic transmission

It has been demonstrated that stimulation of protein kinase A (PKA) results in enhanced synaptic transmission in the hippocampus and other brain areas. To investigate mechanisms of the PKA-mediated potentiation of synaptic transmission, we used rat hippocampal embryonic cultures. In low-density cultures, paired recordings under the perforated patch demonstrated that 15-min forskolin treatment produced long-lasting potentiation of evoked excitatory postsynaptic currents (eEPSCs) mediated by the cAMP/PKA pathway. eEPSC amplitudes increased to 240 +/- 10% of baseline after 15 min of forskolin treatment (early). After forskolin washout, eEPSCs declined to a potentiated level. Potentiation was sustained for > or = 85 min after forskolin washout and, 60 min after forskolin washout, constituted 152 +/- 7% of baseline (late potentiation). Disruption of presynaptic processes with the whole cell configuration and internal solution containing PKA inhibitor peptide did not affect forskolin-induced potentiation. Disruption of postsynaptic processes, in contrast, impaired early potentiation and abolished late potentiation. Study of mEPSCs confirmed the contribution of postsynaptic mechanisms. Forskolin-induced enhancement of mEPSC frequency observed under the perforated patch was attenuated by the whole cell configuration. Forskolin also induced an increase of mEPSC amplitudes in the perforated patch, but not in the whole cell, experiments. Potentiation of eEPSCs was not activity dependent, persisting in the absence of stimulation. NMDA receptor blockade did not abolish forskolin-induced potentiation. In summary, we demonstrate that forskolin-induced potentiation of eEPSCs was mediated by postsynaptic mechanisms, presumably by upregulation of AMPA receptors by phosphorylation.

Functional significance of the kainate receptor GluR6(M836I) mutation that is linked to autism

Previous studies revealed a linkage of the kainate receptor GluR6 with autism, a pervasive developmental disorder. Mutational screening in autistic patients disclosed the amino acid exchange M836I in a highly conserved domain of the cytoplasmic C-terminal region of GluR6. Here, we show that this mutation leads to GluR6 gain-of-function. By using the two-electrode voltage clamp technique we observed a significant increase of current amplitudes of mutant GluR6 compared to wild type GluR6. Western blotting of oocytes injected with mutant or wild type GluR6 cRNA and transfection of EGFP-tagged GluR6 receptors into COS-7 cells revealed an enhanced plasma membrane expression of GluR6(M836I) compared to wild type GluR6. Membrane expression of GluR6(M836I) but not of wild type GluR6 seems to be regulated by Rab11 as indicated by our finding that GluR6(M836I) but not wild type GluR6 showed increased current amplitudes and protein expression when coexpressed with Rab11. Furthermore, injection of GTP plus Rab11A protein into oocytes increased current amplitudes in GluR6(M836I) but not in wild type GluR6. By contrast, Rab5 downregulated the currents in oocytes expressing wild type GluR6 but had only little, statistically not significant effects on currents in oocytes expressing GluR6(M836I). Our data on altered functional properties of GluR6(M836I) provide a functional basis for the postulated linkage of GluR6 to autism. Furthermore, we identified new mechanisms determining the plasma membrane abundance of wild type GluR6 and GluR6(M836I).

Calcium/calmodulin-dependent protein kinase II (CaMKII), through NMDA receptors and L-Voltage-gated channels, modulates the serine phosphorylation of GluR6 during cerebral ischemia and early reperfusion period in rat hippocampus

Recent studies have shown that GluR6 is involved in the modulation of neuronal cell death. It has been shown that PKA can phosphorylate recombinant GluR6 homomeric receptors and that this phosphorylation of GluR6 was suggested to underlie an enhancement of whole-cell current responses. Here, we try to find out whether brain ischemia and reperfusion could induce any change in the serine phosphorylation of GluR6. Our results showed that the serine phosphorylation of GluR6 increased in hippocampus during brain ischemia and early reperfusion period. Then, we used several drugs to investigate the mechanism of modulating the serine phosphorylation of GluR6. KT5720, a specific cell-permeable inhibitor of protein kinase A (PKA), had no effect on the increase in serine phosphorylation of GluR6 induced by brain ischemia or reperfusion. On the other hand, KN-62, a selective inhibitor of rat brain Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), diminished the increase in serine phosphorylation of GluR6. Moreover, our results showed that either MK801 (a NMDA receptor antagonist) or Nifedipine (a L-type Ca2+ channel (L-VGCC) blocker) decreased the increase in serine phosphorylation. In conclusion, our results suggest that CaMKII, activated through NMDA receptors and L-VGCCs, mediated the serine phosphorylation of GluR6 during brain ischemia and early reperfusion period.

Co-assembly of two GluR6 kainate receptor splice variants within a functional protein complex

Kainate receptors (KAR) are composed of several distinct subunits and splice variants, but the functional relevance of this diversity remains largely unclear. Here we show that two splice variants of the GluR6 subunit, GluR6a and GluR6b, which differ in their C-terminal domains, do not show distinct functional properties, but coassemble as heteromers in vitro and in vivo. Using a proteomic approach combining affinity purification and MALDI-TOF mass spectrometry, we found that GluR6a and GluR6b interact with two distinct subsets of cytosolic proteins mainly involved in Ca(2+) regulation of channel function and intracellular trafficking. Guided by these results, we provide evidence that the regulation of native KAR function by NMDA receptors depends on the heteromerization of GluR6a and GluR6b and interaction of calcineurin with GluR6b. Thus, GluR6a and GluR6b bring in close proximity two separate subsets of interacting proteins that contribute to the fine regulation of KAR trafficking and function.

Glucocorticoid adrenal steroids and glucocorticoid-inducible kinase isoforms in the regulation of GluR6 expression

Generation of memory is enhanced during stress, an effect attributed to stimulation of neuronal learning by adrenal glucocorticoids. The glucocorticoid-dependent genes include the serum- and glucocorticoid-inducible kinase SGK1. SGK1 is activated through the phosphatidylinositol 3 kinase (PI3-kinase) pathway by growth factors such as insulin-like growth factor-1 (IGF1) or tumour growth factor beta (TGF-beta). Previously, a fourfold higher expression of SGK1 has been observed in fast-learning rats as compared with slow-learning rats. The mechanisms linking glucocorticoids or SGK1 with neuronal function have, however, remained elusive. We show here that treatment of mice with the glucocorticoid dexamethasone (238 microg day-1 for 8-20 days) enhances hippocampal expression of GluR6. Immunohistochemistry reveals significantly enhanced GluR6 protein abundance at neurones but not at astrocytes in mice. Immunohistochemistry and patch clamp on hippocampal neurones in primary culture reveal upregulation of GluR6 protein abundance and kainate-induced currents following treatment with dexamethasone (1 microm) and TGF-beta (1 microm). In Xenopus oocytes expressing rat GluR6, coexpression of SGK1 strongly increases glutamate-induced current at least partially by increasing the abundance of GluR6 protein in the plasma membrane. The related kinases SGK2 and SGK3 similarly stimulate GluR6, but are less effective than SGK1. The observations point to a novel mechanism regulating GluR6 which contributes to the regulation of neuronal function by glucocorticoids.

Novel role of brain stem pedunculopontine tegmental adenylyl cyclase in the regulation of spontaneous REM sleep in the freely moving rat

Physiological activation of kainate receptors and GABA(B) receptors within the pedunculopontine tegmentum (PPT) is involved in regulation of rapid-eye-movement (REM) sleep. Because these two types of receptors may also directly and/or indirectly activate the intracellular cyclic adenosine monophosphate (cAMP) signaling pathway, we hypothesized that this signaling pathway may be involved in the PPT to regulate spontaneous REM sleep. To test this hypothesis, four different doses (0.25, 0.50, 0.75, and 1.0 nmol) of a specific adenylyl cyclase (AC) inhibitor, 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ22536), were microinjected bilaterally (100 nl/site) into the PPT, and the effects on REM sleep in freely moving chronically instrumented rats were quantified. By comparing alterations in the patterns of REM sleep after control injections of vehicle or one of the four different doses of SQ22536, the contributions made by each dose of SQ22536 to REM sleep were evaluated. The results demonstrated that the local microinjection of AC inhibitor SQ22536 into the PPT decreased the total amount of REM sleep for 3 h and increased slow-wave sleep (SWS) for 2 h in a dose-dependent manner. This reduction in REM sleep was due to increased latency and decreased frequency of REM sleep episodes. These results provide evidence that inhibition of AC within the PPT can successfully reduce REM sleep. These findings suggest that activation of the cAMP-signaling pathway within the cholinergic cell compartment of the PPT is an intracellular biochemical/molecular step for generating REM sleep in the freely moving rat.

Activity-dependent endocytic sorting of kainate receptors to recycling or degradation pathways

Kainate receptors (KARs) play important roles in the modulation of neurotransmission and plasticity, but the mechanisms that regulate their surface expression and endocytic sorting remain largely unknown. Here, we show that in cultured hippocampal neurons the surface expression of GluR6-containing KARs is dynamically regulated. Furthermore, internalized KARs are sorted into recycling or degradative pathways depending on the endocytotic stimulus. Kainate activation causes a Ca2+- and PKA-independent but PKC-dependent internalization of KARs that are targeted to lysosomes for degradation. In contrast, NMDAR activation evokes a Ca2+-, PKA- and PKC-dependent endocytosis of KARs to early endosomes with subsequent reinsertion back into the plasma membrane. These results demonstrate that GluR6-containing KARs are subject to activity-dependent endocytic sorting, a process that provides a mechanism for both rapid and chronic changes in the number of functional receptors.

The effects of corticotropin-releasing factor on anoxia-induced changes in evoked potentials in living slices of rat olfactory cortex

Experiments were performed to address the dynamics of evoked focal potentials in slices of rat olfactory cortex during 10-min anoxia and subsequent reoxygenation. These experiments showed that perfusion with corticotropin-releasing factor (CRF)--corticoliberin--at concentrations of 1, 10, and 100 nM had no effect on changes in EPSP parameters before or during anoxia. However, CRF (10 and 100 nM) significantly aided recovery of the amplitude and slope of EPSP during reoxygenation. Application of the competitive NMDA receptor blocker APV (50 microM) during reoxygenation did not eliminate the protective effects of CRF on neuronal activity.

Expression of functional kainate and AMPA receptors in developing lateral superior olive neurons of the rat

A functional analysis of AMPA and kainate receptors (AMPARs and KARs) in the lateral superior olive (LSO), a major nucleus in the auditory brainstem, has not been performed so far, to our knowledge. Here we investigated the presence and characteristics of such receptors in the rat LSO by means of whole-cell patch-clamp recordings in combination with pharmacology. Current responses evoked by 200 microM AMPA were completely blocked by the specific AMPAR antagonist GYKI 52466 (100 microM). Properties of the AMPAR-mediated currents (latency, activation time constant, and peak amplitude) remained constant between postnatal day 3 (P3) and P10. Current responses evoked by 100 microM KA were not completely blocked by 100 microM GYKI 52466, indicating that the residual component was mediated by KARs. Throughout development, two groups of KAR-mediated currents (fast I(KA) and slow I(KA)) were distinguished because they had significantly different mean activation time constants. Moreover, the mean peak amplitude of fast I(KA) was significantly higher than that of slow I(KA). The differentiation into fast I(KA) and slow I(KA) can be explained by the existence of two groups of LSO neurons displaying different KAR densities, distributions, and/or diverse types with differences in conductance. Application of the specific KAR subunit agonists SYM 2081 (10 microM), ATPA (10 microM), or iodowillardiine (1 microM) evoked currents in almost all cells tested, showing that GluR5 subunits are a component of functional KARs in LSO neurons. Electrical stimulation of ipsilateral input fibers in the presence of KAR antagonists (NS-102 and GAMS), modulators (WGA), or GYKI 52466 revealed the presence of synaptic KARs in LSO neurons.

Presynaptic kainate receptor facilitation of glutamate release involves protein kinase A in the rat hippocampus

We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors in the rat hippocampus using isolated nerve terminal (synaptosome) and slice preparations. In hippocampal nerve terminals, kainate (KA) produced an increase of glutamate release at concentrations of agonist ranging from 10 to 1000 microm. In hippocampal slices, KA at low nanomolar concentrations (20-50 nm) also produced an increase of evoked excitatory postsynaptic currents (eEPSCs) at mossy fibre-CA3 synapses. In both, synaptosomes and slices, the effect of KA was antagonized by CNQX, and persisted after pretreatment with a cocktail of antagonists for other receptors whose activation could potentially have produced facilitation of release. These data indicate that the facilitation of glutamate release observed is mediated by the activation of presynaptic glutamate receptors of the kainate type. Mechanistically, the observed effects of KA appear to be the same in synaptosomal and slice preparations. Thus, the effect of KA on glutamate release and mossy fibre-CA3 synaptic transmission was occluded by the stimulation of adenylyl cyclase by forskolin and suppressed by the inhibition of protein kinase A by H-89 or Rp-Br-cAMP. We conclude that kainate receptors present at presynaptic terminals in the rat hippocampus mediate the facilitation of glutamate release through a mechanism involving the activation of an adenylyl cyclase-second messenger cAMP-protein kinase A signalling cascade.

Ionotropic Glutamate Receptor Recognition and Activation

Ionotropic glutamate receptors are the major excitatory neurotransmitters in mammalian brain but are found throughout the animal kingdom as well as in plants and bacteria. A great deal of progress in understanding the structure of these essential neurotransmitter receptors has been made since the first examples were cloned and sequenced in 1989. The atomic structure of the ligand-binding domain of several ionotropic glutamate receptors has been determined, and a great deal of progress has been made in relating the structural properties of the binding site to the function of the intact receptor. In addition, the identification of glutamate receptors from a wide variety of organisms ranging from several types of bacteria to Arabidopsis to a range of animal species has made glutamate receptors a molecular laboratory for studying the evolution of proteins. The fact that glutamate receptors are a particularly ancient intercellular signaling molecule suggests a potential role in the transition from single celled to multicellular organisms. This review focuses on the structure and dynamics of ionotropic glutamate receptors and their relation to the function and evolution of these proteins.

Kainate receptors and synaptic transmission

Excitatory glutamatergic transmission involves a variety of different receptor types, each with distinct properties and functions. Physiological studies have identified both post- and presynaptic roles for kainate receptors, which are a subtype of the ionotropic glutamate receptors. Kainate receptors contribute to excitatory postsynaptic currents in many regions of the central nervous system including hippocampus, cortex, spinal cord and retina. In some cases, postsynaptic kainate receptors are co-distributed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, but there are also synapses where transmission is mediated exclusively by postsynaptic kainate receptors: for example, in the retina at connections made by cones onto off bipolar cells. Modulation of transmitter release by presynaptic kainate receptors can occur at both excitatory and inhibitory synapses. The depolarization of nerve terminals by current flow through ionotropic kainate receptors appears sufficient to account for most examples of presynaptic regulation; however, a number of studies have provided evidence for metabotropic effects on transmitter release that can be initiated by activation of kainate receptors. Recent analysis of knockout mice lacking one or more of the subunits that contribute to kainate receptors, as well as studies with subunit-selective agonists and antagonists, have revealed the important roles that kainate receptors play in short- and long-term synaptic plasticity. This review briefly addresses the properties of kainate receptors and considers in greater detail the physiological analysis of their contributions to synaptic transmission.

Assembly and cell surface expression of KA-2 subunit-containing kainate receptors

Kainate receptors (KARs) modulate synaptic transmission at both pre-synaptic and post-synaptic sites. The overlap in the distribution of KA-2 and GluR6/7 subunits in several brain regions suggests the co-assembly of these subunits in native KARs. The molecular mechanisms that control the assembly and surface expression of KARs are unknown. Unlike GluR5-7, the KA-2 subunit is unable to form functional homomeric KAR channels. We expressed the KA-2 subunit alone or in combination with other KAR subunits in HEK-293 cells. The cell surface expression of the KAR subunit homo- and heteromers were analysed using biotinylation and agonist-stimulated cobalt uptake. While GluR6 or GluR7 homomers were expressed on the cell surface, KA-2 alone was retained within the endoplasmic reticulum. We found that the cell surface expression of KA-2 was dramatically increased by co-expression with either of the low-affinity KAR subunits GluR5-7. However, co-expression with other related ionotropic glutamate receptor subunits (GluR1 and NR1) does not facilitate the cell surface expression of KA-2. The analysis of subcellular fractions of neocortex revealed that synaptic KARs have a relatively high KA-2 content compared to microsomal ones. Thus, KA-2 is likely to contain an endoplasmic reticulum retention signal that is shielded on assembly with other KAR subunits.

Regulation of kainate receptors by protein kinase C and metabotropic glutamate receptors

Kainate receptors have recently been shown to be involved in synaptic transmission, to regulate transmitter release and to mediate synaptic plasticity in different regions of the CNS. However, very little is known about endogenous mechanisms that can control native kainate receptor signalling. In this study we have found that GluR5-containing kainate receptor-mediated actions can be modulated by activation of protein kinase C (PKC) but not protein kinase A (PKA). However, both PKA and PKC directly phosphorylate the GluR5 subunit of kainate receptors. Metabotropic glutamate (mGlu) receptors are well known to be involved in synaptic transmission, regulation of transmitter release and synaptic plasticity in a variety of brain regions. We now demonstrate that kainate receptor signalling is enhanced by activation of group I mGlu receptors, in a PKC-dependent manner. These data demonstrate for the first time that kainate receptor function can be modulated by activation of metabotropic glutamate receptors and have implications for understanding mechanisms of synaptic transmission, plasticity and disorders such as epilepsy.

Modulation of AMPA receptors by cAMP-dependent protein kinase in preBötzinger complex inspiratory neurons regulates respiratory rhythm in the rat

We hypothesize that phosphorylation of AMPA receptors or associated synaptic proteins modulates the excitability of respiratory neurons in the preBötzinger Complex (preBötC), affecting respiratory rhythm. Using neonatal rat medullary slices that spontaneously generate respiratory rhythm, we examined the role of the cAMP-PKA pathway (PKA: cAMP-dependent protein kinase) in modulating glutamatergic synaptic transmission, the excitability of inspiratory neurons in the preBötC and respiratory rhythm. Microinjection of forskolin, an activator of adenylate cyclase, into the preBötC with or without the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), decreased the period (increased the frequency) of respiratory-related rhythmic motor output in the hypoglossal nerve (XIIn) to 84 % (without IBMX) and to 72 % (with IBMX) of the pre-injection baseline. In the presence of MK-801, a non-competitive NMDA receptor antagonist, microinjection of forskolin plus IBMX decreased the period to 66 % of baseline levels. Microinjection of Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS), a PKA inhibitor, increased the period to 145 % of baseline levels. Concurrent microinjection of Rp-cAMPS and forskolin had no effect on the period. Bath application of 7beta-deacetyl-7beta-[gamma-(morpholino)butyryl]-forskolin hydrochloride (7Db-forskolin, a water-soluble derivative of forskolin): (1) decreased the period to 67 % of baseline levels without affecting the amplitude of integrated XIIn inspiratory discharge, (2) induced a tonic inward current of 29 pA and enhanced inspiratory drive current (the amplitude increased to 183 % and the integral increased to 184 % of baseline) in voltage-clamped (holding potential = -60 mV) preBötC inspiratory neurons and (3) increased the frequency to 195 % and amplitude to 118 % of spontaneous excitatory postsynaptic currents (sEPSCs) during expiratory periods. Dideoxy-forskolin did not have these effects. Intracellular perfusion with the catalytic subunit of PKA (cPKA) into preBötC inspiratory neurons progressively enhanced inspiratory drive currents and, in the presence of TTX, increased the inward currents induced by local ejection of AMPA; the latter currents were blocked by 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulphonamide (NBQX, an AMPA/kainate receptor antagonist). The effects of cPKA were blocked by co-application of PKA inhibitor (6-22) amide (PKI). These results suggest that phosphorylation of postsynaptic AMPA receptors through the cAMP-PKA pathway modulates both tonic and phasic excitatory amino acid synaptic transmission and excitability of inspiratory neurons in the preBötC and, therefore, regulates respiratory rhythm. Moreover, the basal level of endogenous PKA activity appears to be a determinant of resting respiratory frequency.

Rapid and differential regulation of AMPA and kainate receptors at hippocampal mossy fibre synapses by PICK1 and GRIP

We identified four PDZ domain-containing proteins, syntenin, PICK1, GRIP, and PSD95, as interactors with the kainate receptor (KAR) subunits GluR5(2b,) GluR5(2c), and GluR6. Of these, we show that both GRIP and PICK1 interactions are required to maintain KAR-mediated synaptic function at mossy fiber-CA3 synapses. In addition, PKC alpha can phosphorylate ct-GluR5(2b) at residues S880 and S886, and PKC activity is required to maintain KAR-mediated synaptic responses. We propose that PICK1 targets PKC alpha to phosphorylate KARs, causing their stabilization at the synapse by an interaction with GRIP. Importantly, this mechanism is not involved in the constitutive recycling of AMPA receptors since blockade of PDZ interactions can simultaneously increase AMPAR- and decrease KAR-mediated synaptic transmission at the same population of synapses.

Regulation of glutamatergic neurotransmission in the striatum by presynaptic adenylyl cyclase-dependent processes

The aim here was to examine the possible roles of adenylyl cyclase- and protein kinase A (PKA)-dependent processes in ionotropic glutamate receptor (iGluR)-mediated neurotransmission using superfused mouse striatal slices and a non-metabolized L-glutamate analogue, D-[3H]aspartate. The direct and indirect presynaptic modulation of glutamate release and its susceptibility to changes in the intracellular levels of cyclic AMP (cAMP), Ca(2+) and calmodulin (CaM) and in protein phosphorylation was characterized by pharmacological manipulations. The agonists of iGluRs, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate, stimulated the basal release of D-[3H]aspartate, while N-methyl-D-aspartate (NMDA) was without effect. Both the AMPA- and kainate-mediated responses were accentuated by the beta-adrenoceptor agonist isoproterenol. These facilitatory effects were mimicked by the permeable cAMP analogue dibutyryl-cAMP. The beta-adrenoceptor antagonist propranolol, the adenylyl cyclase inhibitor MDL12,330A, the inhibitor of PKA and PKC, H-7, and the PKA inhibitor H-89 abolished the isoproterenol effect on the kainate-evoked release. The dibutyryl-cAMP-induced potentiation was also attenuated by H-7. Isoproterenol, propranolol and MDL12,330A failed to affect the basal release of D-[3H]aspartate, but dibutyryl-cAMP was inhibitory and MDL12,330A activatory. In Ca(2+)-free medium, the kainate-evoked release was enhanced, being further accentuated by the CaM antagonists calmidazolium and trifluoperazine, though these inhibited the basal release. The potentiating effect of calmidazolium on the kainate-stimulated release was counteracted by both MDL12,330A and H-7. We conclude that AMPA- and kainate-evoked glutamate release from striatal glutamatergic terminals is potentiated by beta-adrenergic receptor-mediated adenylyl cyclase activation and cAMP accumulation. Glutamate release is enhanced if the Ca(2+)- and CaM-dependent, kainate-evoked processes do not prevent the excessive accumulation of intracellular cAMP.

AMPA receptors and kainate receptors encode different features of afferent activity

Postsynaptic kainate receptors (KARs) have been found in the CNS along with AMPA receptors (AMPARs), but because KAR-mediated EPSCs are much smaller and slower than AMPAR-mediated EPSCs, it remains unclear whether these postsynaptic KARs are functionally significant. In this study we measured KAR- and AMPAR-mediated EPSPs in hippocampal interneurons, and then we used these EPSPs in a model to examine the effects of afferent firing on each receptor. In this model the KARs generated a large tonic depolarization when activated by a small population of afferent fibers firing asynchronously at physiologically relevant firing rates (1-5 Hz). At 3-5 Hz this tonic depolarization exceeded the peak depolarization mediated by AMPARs in response to the same afferent activity. We also found that, unlike AMPARs, KARs did not generate large oscillations in membrane potential during theta rhythms. When simulated EPSCs were injected into interneurons to mimic afferents firing at 5 Hz, we found that currents simulating KARs elicited more spiking than currents simulating AMPARs. We also found that simulated AMPARs, but not KARs, could transmit presynaptic theta rhythms into postsynaptic spiking at the theta rhythm. Our results suggest that synaptically activated KARs have a strong influence on membrane potential and that AMPARs and KARs differ in their ability to encode temporal information.

Molecular constituents and phosphorylation-dependent regulation of the post-synaptic density

The post-synaptic density (PSD) contains receptors with associated signaling- and scaffolding-proteins that organize signal-transduction pathways near the post-synaptic membrane. The PSD plays an important role in synaptic plasticity, and protein phosphorylation is critical to the regulation of PSD function, including learning and memory. Recently, studies have investigated the protein constituents of the PSD and substrate proteins for various protein kinases by proteomic analysis. The present review focuses on the molecular properties of PSD proteins, and substrates of protein kinases and their regulation by phosphorylation in order to understand the role of PSD in synaptic plasticity.

Increased sensitivity to N-methyl-D-aspartate receptor-mediated excitotoxicity in a mouse model of Huntington's disease

Previous work suggests N-methyl-D-aspartate receptor (NMDAR) activation may be involved in degeneration of medium-sized spiny striatal neurons in Huntington's disease (HD). Here we show that these neurons are more vulnerable to NMDAR-mediated death in a YAC transgenic FVB/N mouse model of HD expressing full-length mutant huntingtin, compared with wild-type FVB/N mice. Excitotoxic death of these neurons was increased after intrastriatal injection of quinolinate in vivo, and after NMDA but not AMPA exposure in culture. NMDA-induced cell death was abolished by an NR2B subtype-specific antagonist. In contrast, NMDAR-mediated death of cerebellar granule neurons was not enhanced, consistent with cell-type and NMDAR subtype specificity. Moreover, increased NMDA-evoked current amplitude and caspase-3 activity were observed in transgenic striatal neurons. Our data support a role for NR2B-subtype NMDAR activation as a trigger for selective neuronal degeneration in HD.

cGMP-induced presynaptic depression and postsynaptic facilitation at glutamatergic synapses in visual cortex

The mechanisms by which the intracellular messenger cGMP can modulate synaptic efficacy remain poorly understood. Here we report that cGMP, acting through cGMP-dependent protein kinase (PKG), has multiple rapid and reversible effects on synaptic transmission in slices and cultures of rodent visual cortex. Extracellular application of the membrane permeable cGMP analog 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) and the PKG specific activator beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate sp-isomer (Sp-8-Br-PET-cGMPS) reduced stimulus-evoked EPSPs in slices. In cortical cultures, both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. In both slices and cultures, intracellular perfusion of the postsynaptic neurons with a pseudosubstrate inhibitory peptide specific for PKG had no effect on the reduction in EPSPs and EPSCs, indicating that the inhibition occurred at presynaptic sites. Whole-cell calcium currents in cultured cortical neurons were also reduced by both analogs, which may account for the effect on synaptic release. To determine whether cGMP was also acting at postsynaptic sites, we applied exogenous kainate/AMPA and NMDA to the recorded cells directly. cGMP and its analogs showed little effect on the postsynaptic kainate/AMPA responses but produced a dramatic enhancement of NMDA responses. cGMP-induced NMDA potentiation was prevented by the specific PKG inhibitory peptide infused into the postsynaptic cell. In summary, cGMP, acting through PKG, had depressive presynaptic and facilitatory postsynaptic actions at excitatory synapses in the visual cortex. We suggest that these opposing actions may be useful for altering the balance of synaptic inputs to cortical neurons in ways that enhance signals important for synaptic facilitation and neuronal plasticity.

Linkage and association of the glutamate receptor 6 gene with autism

A genome scan was previously performed and pointed to chromosome 6q21 as a candidate region for autism. This region contains the glutamate receptor 6 (GluR6 or GRIK2) gene, a functional candidate for the syndrome. Glutamate is the principal excitatory neurotransmitter in the brain and is directly involved in cognitive functions such as memory and learning. We used two different approaches, the affected sib-pair (ASP) method and the transmission disequilibrium test (TDT), to investigate the linkage and association between GluR6 and autism. The ASP method, conducted with additional markers on the 51 original families and in eight new sibling pairs, showed a significant excess of allele sharing, generating an elevated multipoint maximum LOD score (ASPEX MLS = 3.28). TDT analysis, performed in the ASP families and in an independent data set of 107 parent-offspring trios, indicated a significant maternal transmission disequilibrium (TDTall P = 0.0004). Furthermore, TDT analysis (with only one affected proband per family) showed significant association between GluR6 and autism (TDT association P = 0.008). In contrast to maternal transmission, paternal transmission of GluR6 alleles was as expected in the absence of linkage, suggesting a maternal effect such as imprinting. Mutation screening was performed in 33 affected individuals, revealing several nucleotide polymorphisms (SNPs), including one amino acid change (M867I) in a highly conserved domain of the intracytoplasmic C-terminal region of the protein. This change is found in 8% of the autistic subjects and in 4% of the control population and seems to be more maternally transmitted than expected to autistic males (P = 0.007). Taken together, these data suggest that GluR6 is in linkage disequilibrium with autism.