TrkB receptor interacts with mGlu2 receptor and mediates antipsychotic-like effects of mGlu2 receptor activation in the mouse.

Metabotropic glutamate receptor 2 (mGlu2) attracts particular attention as a possible target for a new class of antipsychotics. However, the signaling pathways transducing the effects of mGlu2 in the brain remain poorly characterized. Here, we addressed this issue by identifying native mGlu2 interactome in mouse prefrontal cortex. Nanobody-based affinity purification and mass spectrometry identified 149 candidate mGlu2 partners, including the neurotrophin receptor TrkB. The later interaction was confirmed both in cultured cells and prefrontal cortex. mGlu2 activation triggers phosphorylation of TrkB on Tyr816 in primary cortical neurons and prefrontal cortex. Reciprocally, TrkB stimulation enhances mGlu2-operated Gi/o protein activation. Furthermore, TrkB inhibition prevents the rescue of behavioral deficits by glutamatergic antipsychotics in phencyclidine-treated mice. Collectively, these results reveal a cross-talk between TrkB and mGlu2, which is key to the behavioral response to glutamatergic antipsychotics.

Modulation of Hippocampal Network Oscillation by PICK1-Dependent Cell Surface Expression of mGlu3 Receptors.

Metabotropic glutamate receptor Type 3 (mGlu3) controls the sleep/wake architecture, which plays a role in the glutamatergic pathophysiology of schizophrenia. Interestingly, mGlu3 receptor expression is decreased in the brain of schizophrenic patients. However, little is known about the molecular mechanisms regulating mGlu3 receptors at the cell membrane. Subcellular receptor localization is strongly dependent on protein-protein interactions. Here we show that mGlu3 interacts with PICK1 and that this scaffolding protein is important for mGlu3 surface expression and function in hippocampal primary cultures. Disruption of their interaction via an mGlu3 C-terminal mimicking peptide or an inhibitor of the PDZ domain of PICK1 altered the functional expression of mGlu3 receptors in neurons. We next investigated the impact of disrupting the mGlu3-PICK1 interaction on hippocampal theta oscillations in vitro and in vivo in WT male mice. We found a decreased frequency of theta oscillations in organotypic hippocampal slices, similar to what was previously observed in mGlu3 KO mice. In addition, hippocampal theta power was reduced during rapid eye movement sleep, non-rapid eye movement (NREM) sleep, and wake states after intraventricular administration of the mGlu3 C-terminal mimicking peptide. Targeting the mGlu3-PICK1 complex could thus be relevant to the pathophysiology of schizophrenia.SIGNIFICANCE STATEMENT Dysregulation of the glutamatergic system might play a role in the pathophysiology of schizophrenia. Metabotropic glutamate receptors Type 3 (mGlu3) have been proposed as potential targets for schizophrenia. Understanding the molecular mechanisms regulating mGlu3 receptor at the cell membrane is critical toward comprehending how their dysfunction contributes to the pathogenesis of schizophrenia. Here we describe that the binding of the signaling and scaffolding protein PICK1 to mGlu3 receptors is important for their localization and physiological functions. The identification of new proteins that associate specifically to mGlu3 receptors will advance our understanding of the regulatory mechanisms associated with their targeting and function and ultimately might provide new therapeutic strategies to counter these psychiatric conditions.

Loss of Nogo-A, encoded by the schizophrenia risk gene Rtn4, reduces mGlu3 expression and causes hyperexcitability in hippocampal CA3 circuits.

Recent investigations of Nogo-A, a well characterized protein inhibitor of neurite outgrowth in the brain, have revealed additional functions including a role in neuropsychiatric disorders such as schizophrenia. Here we examined Nogo-A functions in mouse CA3 hippocampal circuitry. Patch clamp recordings showed that the absence of Nogo-A results in a hyperactive network. In addition, mGlu3 metabotropic glutamate receptors, which exhibit mutations in certain forms of schizophrenia, were downregulated specifically in the CA3 area. Furthermore, Nogo-A-/- mice showed disordered theta oscillations with decreased incidence and frequency, similar to those observed in mGlu3-/- mice. As disruptions in theta rhythmicity are associated with impaired spatial navigation, we tested mice using modified Morris water maze tasks. Mice lacking Nogo-A exhibited altered search strategies, displaying greater dependence on global as opposed to local reference frames. This link between Nogo-A and mGlu3 receptors may provide new insights into mechanisms underlying schizophrenia.

Pharmacological evidence for a metabotropic glutamate receptor heterodimer in neuronal cells.

Metabotropic glutamate receptors (mGluRs) are mandatory dimers playing important roles in regulating CNS function. Although assumed to form exclusive homodimers, 16 possible heterodimeric mGluRs have been proposed but their existence in native cells remains elusive. Here, we set up two assays to specifically identify the pharmacological properties of rat mGlu heterodimers composed of mGlu2 and 4 subunits. We used either a heterodimer-specific conformational LRET-based biosensor or a system that guarantees the cell surface targeting of the heterodimer only. We identified mGlu2-4 specific pharmacological fingerprints that were also observed in a neuronal cell line and in lateral perforant path terminals naturally expressing mGlu2 and mGlu4. These results bring strong evidence for the existence of mGlu2-4 heterodimers in native cells. In addition to reporting a general approach to characterize heterodimeric mGluRs, our study opens new avenues to understanding the pathophysiological roles of mGlu heterodimers.

Activation of Group II Metabotropic Glutamate Receptors Promotes LTP Induction at Schaffer Collateral-CA1 Pyramidal Cell Synapses by Priming NMDA Receptors.

It is well established that selective activation of group I metabotropic glutamate (mGlu) receptors induces LTD of synaptic transmission at Schaffer collateral-CA1 synapses. In contrast, application of 1S,3R-ACPD, a mixed agonist at group I and group II mGlu receptors, induces LTP. Using whole-cell recordings from CA1 pyramidal cells and field recordings in the hippocampal CA1 region, we investigated the specific contribution of group II mGlu receptors to synaptic plasticity at Schaffer collateral-CA1 synapses in acute slices of adult mice. Pharmacological activation of group II mGlu receptors (mGlu2 and mGlu3 receptors) with the specific agonist LY354740 in conjunction with electrical stimulation induced postsynaptic LTP. This form of plasticity requires coactivation of NMDA receptors (NMDARs). Group II mGlu receptor activation led to PKC-dependent phosphorylation of the GluN1 subunit. We found that both synaptic and extrasynaptic NMDARs, which are differentially modulated by mGlu2 and mGlu3 receptors, contribute to LTP induction. Furthermore, LTP initiated by activation of group II mGlu receptors was not occluded by LTP induced with high-frequency trains of stimuli. However, the phosphorylation of NMDARs mediated by group II mGlu receptor activation led to a priming effect that enhanced subsequent high-frequency stimulation-induced LTP. These findings reveal a novel metaplastic mechanism through which group II mGlu receptors modulate synaptic function at the Schaffer collateral input to CA1 pyramidal cells, thereby lowering the threshold to induce plasticity. SIGNIFICANCE STATEMENT: The group II metabotropic glutamate (mGlu II) receptors exert a well characterized action on presynaptic neuron terminals to modulate neurotransmitter release. Here, we show that these receptors also have postsynaptic effects in promoting the induction of synaptic plasticity. Using an electrophysiological approach including field and whole-cell patch recording in hippocampi from wild-type and transgenic mice, we show that activation of group II mGlu receptors enhances NMDA receptor (NMDAR)-mediated currents through PKC-dependent phosphorylation. This priming of NMDARs lowers the threshold for the induction of LTP of synaptic transmission. These findings may also provide new insights into the mechanisms through which drugs targeting mGlu II receptors alleviate hypoglutamatergic conditions such as those occurring in certain brain disorders such as schizophrenia.

Calsyntenin-1 regulates targeting of dendritic NMDA receptors and dendritic spine maturation in CA1 hippocampal pyramidal cells during postnatal development.

Calsyntenin-1 is a transmembrane cargo-docking protein important for kinesin-1-mediated fast transport of membrane-bound organelles that exhibits peak expression levels at postnatal day 7. However, its neuronal function during postnatal development remains unknown. We generated a knock-out mouse to characterize calsyntenin-1 function in juvenile mice. In the absence of calsyntenin-1, synaptic transmission was depressed. To address the mechanism, evoked EPSPs were analyzed revealing a greater proportion of synaptic GluN2B subunit-containing receptors typical for less mature synapses. This imbalance was due to a disruption in calsyntenin-1-mediated dendritic transport of NMDA receptor subunits. As a consequence of increased expression of GluN2B subunits, NMDA receptor-dependent LTP was enhanced at Schaffer collateral-CA1 pyramidal cell synapses. Interestingly, these defects were accompanied by a decrease in dendritic arborization and increased proportions of immature filopodia-like dendritic protrusions at the expense of thin-type dendritic spines in CA1 pyramidal cells. Thus, these results highlight a key role for calsyntenin-1 in the transport of NMDA receptors to synaptic targets, which is necessary for the maturation of neuronal circuits during early development.

Selective silencing of individual dendritic branches by an mGlu2-activated potassium conductance in dentate gyrus granule cells.

Group II metabotropic glutamate receptors (mGlu-IIs) modulate hippocampal information processing through several presynaptic actions. We describe a novel postsynaptic inhibitory mechanism mediated by the mGlu2 subtype that activates an inwardly rectifying potassium conductance in the dendrites of DG granule cells of rats and mice. Data from glutamate-uncaging experiments and simulations indicate that mGlu2-activated potassium conductance uniformly reduces the peak amplitude of synaptic inputs arriving in the distal two-thirds of dendrites, with only minor effects on proximal inputs. This unique shunting profile is consistent with a peak expression of the mGlu2-activated conductance at the transition between the proximal and middle third of the dendrites. Further simulations under various physiologically relevant conditions showed that when a shunting conductance was activated in the proximal third of a single dendrite, it effectively modulated input to this specific branch while leaving inputs in neighboring dendrites relatively unaffected. Therefore, the restricted expression of the mGlu2-activated potassium conductance in the proximal third of DG granule cell dendrites represents an optimal localization for achieving the opposing biophysical requirements for uniform yet selective modulation of individual dendritic branches.

Perisynaptic chondroitin sulfate proteoglycans restrict structural plasticity in an integrin-dependent manner.

During early postnatal development of the CNS, neuronal networks are configured through the formation, elimination, and remodeling of dendritic spines, the sites of most excitatory synaptic connections. The closure of this critical period for plasticity correlates with the maturation of the extracellular matrix (ECM) and results in reduced dendritic spine dynamics. Chondroitin sulfate proteoglycans (CSPGs) are thought to be the active components of the mature ECM that inhibit functional plasticity in the adult CNS. These molecules are diffusely expressed in the extracellular space or aggregated as perineuronal nets around specific classes of neurons. We used organotypic hippocampal slices prepared from 6-d-old Thy1-YFP mice and maintained in culture for 4 weeks to allow ECM maturation. We performed live imaging of CA1 pyramidal cells to assess the effect of chondroitinase ABC (ChABC)-mediated digestion of CSPGs on dendritic spine dynamics. We found that CSPG digestion enhanced the motility of dendritic spines and induced the appearance of spine head protrusions in a glutamate receptor-independent manner. These changes were paralleled by the activation of ß1-integrins and phosphorylation of focal adhesion kinase at synaptic sites, and were prevented by preincubation with a ß1-integrin blocking antibody. Interestingly, microinjection of ChABC close to dendritic segments was sufficient to induce spine remodeling, demonstrating that CSPGs located around dendritic spines modulate their dynamics independently of perineuronal nets. This restrictive action of perisynaptic CSPGs in mature neural tissue may account for the therapeutic effects of ChABC in promoting functional recovery in impaired neural circuits.

Neuron to astrocyte communication via cannabinoid receptors is necessary for sustained epileptiform activity in rat hippocampus.

Astrocytes are integral functional components of synapses, regulating transmission and plasticity. They have also been implicated in the pathogenesis of epilepsy, although their precise roles have not been comprehensively characterized. Astrocytes integrate activity from neighboring synapses by responding to neuronally released neurotransmitters such as glutamate and ATP. Strong activation of astrocytes mediated by these neurotransmitters can promote seizure-like activity by initiating a positive feedback loop that induces excessive neuronal discharge. Recent work has demonstrated that astrocytes express cannabinoid 1 (CB1) receptors, which are sensitive to endocannabinoids released by nearby pyramidal cells. In this study, we tested whether this mechanism also contributes to epileptiform activity. In a model of 4-aminopyridine induced epileptic-like activity in hippocampal slice cultures, we show that pharmacological blockade of astrocyte CB1 receptors did not modify the initiation, but significantly reduced the maintenance of epileptiform discharge. When communication in astrocytic networks was disrupted by chelating astrocytic calcium, this CB1 receptor-mediated modulation of epileptiform activity was no longer observed. Thus, endocannabinoid signaling from neurons to astrocytes represents an additional significant factor in the maintenance of epileptiform activity in the hippocampus.

Rapid and reversible formation of spine head filopodia in response to muscarinic receptor activation in CA1 pyramidal cells.

A key feature at excitatory synapses is the remodelling of dendritic spines, which in conjunction with receptor trafficking modifies the efficacy of neurotransmission. Here we investigated whether activation of cholinergic receptors, which can modulate synaptic plasticity, also mediates changes in dendritic spine structure. Using confocal time-lapse microscopy in mouse slice cultures we found that brief activation of muscarinic receptors induced the emergence of fine filopodia from spine heads in all CA1 pyramidal cells examined. This response was widespread occurring in 48% of imaged spines, appeared within minutes, was reversible, and was blocked by atropine. Electron microscopic analyses showed that the spine head filopodia (SHFs) extend along the presynaptic bouton. In addition, the decay time of miniature EPSCs was longer after application of the muscarinic acetylcholine receptor agonist methacholine (MCh). Both morphological and electrophysiological changes were reduced by preventing microtubule polymerization with nocodazole. This extension of SHFs during cholinergic receptor activation represents a novel structural form of subspine plasticity that may regulate synaptic properties by fine-tuning interactions between presynaptic boutons and dendritic spines.

Enhancement of CA3 hippocampal network activity by activation of group II metabotropic glutamate receptors.

Impaired function or expression of group II metabotropic glutamate receptors (mGluRIIs) is observed in brain disorders such as schizophrenia. This class of receptor is thought to modulate activity of neuronal circuits primarily by inhibiting neurotransmitter release. Here, we characterize a postsynaptic excitatory response mediated by somato-dendritic mGluRIIs in hippocampal CA3 pyramidal cells and in stratum oriens interneurons. The specific mGluRII agonists DCG-IV or LCCG-1 induced an inward current blocked by the mGluRII antagonist LY341495. Experiments with transgenic mice revealed a significant reduction of the inward current in mGluR3(-/-) but not in mGluR2(-/-) mice. The excitatory response was associated with periods of synchronized activity at theta frequency. Furthermore, cholinergically induced network oscillations exhibited decreased frequency when mGluRIIs were blocked. Thus, our data indicate that hippocampal responses are modulated not only by presynaptic mGluRIIs that reduce glutamate release but also by postsynaptic mGluRIIs that depolarize neurons and enhance CA3 network activity.

Activation conditions for the induction of metabotropic glutamate receptor-dependent long-term depression in hippocampal CA1 pyramidal cells.

Two forms of homosynaptic long-term depression (LTD) are distinguished in hippocampal CA1 pyramidal cells, one which is NMDA receptor dependent and the other metabotropic glutamate receptor (mGluR) dependent. Although the molecular processes involved in mGluR-LTD are well characterized, the conditions of circuit activation required for its induction remain unclear. We show that mGluR-LTD cannot be induced in synaptically coupled CA3-CA1 pyramidal cell pairs. Experiments to address the underlying mechanisms indicate that, even when glutamate transporters are blocked, one presynaptic cell releases insufficient glutamate to evoke an mGluR-mediated current in a connected CA1 cell. These findings imply that extrasynaptic diffusion is not a limiting factor and are consistent with a sparse distribution of functional mGluRs in the dendritic tree of pyramidal cells. Thus, the discharge of multiple Schaffer collaterals to a targeted cell is necessary for mGluR-LTD. Our experiments indicate that approximately eight CA3 inputs to a CA1 pyramidal cell must be activated to induce mGluR-LTD.

Coincident pre- and postsynaptic activation induces dendritic filopodia via neurotrypsin-dependent agrin cleavage.

The synaptic serine protease neurotrypsin is essential for cognitive function, as its deficiency in humans results in severe mental retardation. Recently, we demonstrated the activity-dependent release of neurotrypsin from presynaptic terminals and proteolytical cleavage of agrin at the synapse. Here we show that the activity-dependent formation of dendritic filopodia is abolished in hippocampal neurons from neurotrypsin-deficient mice. Administration of the neurotrypsin-dependent 22 kDa fragment of agrin rescues the filopodial response. Detailed analyses indicated that presynaptic action potential firing is necessary for the release of neurotrypsin, whereas postsynaptic NMDA receptor activation is necessary for the neurotrypsin-dependent cleavage of agrin. This contingency characterizes the neurotrypsin-agrin system as a coincidence detector of pre- and postsynaptic activation. As the resulting dendritic filopodia are thought to represent precursors of synapses, the neurotrypsin-dependent cleavage of agrin at the synapse may be instrumental for a Hebbian organization and remodeling of synaptic circuits in the CNS.

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

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

Exchange protein activated by cAMP (Epac) mediates cAMP activation of p38 MAPK and modulation of Ca2+-dependent K+ channels in cerebellar neurons.

The exchange factor directly activated by cAMP (Epac) is a newly discovered direct target for cAMP and a guanine-nucleotide exchange factor for the small GTPase Rap. Little is known about the neuronal functions of Epac. Here we show that activation of Epac by specific cAMP analogs or by the pituitary adenylate cyclase-activating polypeptide induces a potent activation of the Ca2+-sensitive big K+ channel, slight membrane hyperpolarization, and increased after-hyperpolarization in cultured cerebellar granule cells. These effects involve activation of Rap and p38 MAPK, which mobilizes intracellular Ca2+ stores. These findings reveal a cAMP Epac-dependent and protein kinase A-independent signaling cascade that controls neuronal excitability.

Insulin-like growth factor-1 inhibits adult supraoptic neurons via complementary modulation of mechanoreceptors and glycine receptors.

In the CNS, insulin-like growth factor-1 (IGF-1) is mainly known for its trophic effect both during development and in adulthood. Here, we show than in adult rat supraoptic nucleus (SON), IGF-1 receptor immunoreactivity is present in neurons, whereas IGF-1 immunoreactivity is found principally in astrocytes and more moderately in neurons. In vivo application of IGF-1 within the SON acutely inhibits the activity of both vasopressin and oxytocin neurons, the two populations of SON neuroendocrine cells. Recordings of acutely isolated SON neurons showed that this inhibition occurs through two rapid and reversible mechanisms, both involving the neuronal IGF-1 receptor but different intracellular messengers. IGF-1 inhibits Gd3+-sensitive and osmosensitive mechanoreceptor cation current via phosphatidylinositol-3 (PI3) kinase activation. IGF-1 also potentiates taurine-activated glycine receptor (GlyR) Cl- currents by increasing the agonist sensitivity through a extremely rapid (within a second) PI3 kinase-independent mechanism. Both mechanoreceptor channels and GlyR, which form the excitatory and inhibitory components of SON neuron osmosensitivity, are active at rest, and their respective inhibition and potentiation will both be inhibitory, leading to strong decrease in neuronal activity. It will be of interest to determine whether IGF-1 is released by neurons, thus participating in an inhibitory autocontrol, or astrocytes, then joining the growing family of glia-to-neuron transmitters that modulate neuronal and synaptic activity. Through the opposite and complementary acute regulation of mechanoreceptors and GlyR, IGF-1 appears as a new important neuromodulator in the adult CNS, participating in the complex integration of neural messages that regulates the level of neuronal excitability.