Non-canonical Signaling, the Hidden Life of Ligand-Gated Ion Channels

Unveiling the intracellular dynamics of α4β2 nAChR-mediated ERK activation through the interplay of arrestin, Gβγ, and PKCβII

AIMS

In contrast to G protein-coupled receptors or receptor tyrosine kinases, the mechanism underlying ERK activation through nicotine acetylcholine receptors (nAChRs), members of the ligand-gated ion channel family, remains poorly elucidated. This study aimed to delineate the signaling pathway responsible for ERK activation by the α4β2 nAChR subtype, which is implicated in nicotine addiction and various mental disorders.

MATERIALS AND METHODS

Loss-of-function strategies and mutants of arrestin2/PKCβII with distinct functional characteristics were employed to identify the cellular components and processes involved in ERK activation.

KEY FINDINGS

ERK activation via α4β2 nAChR was observed within the nucleus and necessitated the nuclear translocation of arrestin2 and PKCβII, which exhibited mutual augmentation. Activation of PKCβII by α4β2 nAChR stimulation facilitated the nuclear translocation of arrestin2 by enhancing its interaction with importin β1. Apart from scaffolding ERK activation in the nucleus, arrestin2, in cooperation with GRK2, facilitated the activation of the Src/Syk/PKCβII signaling cascade, leading to the nuclear entry of PKCβII in a Gβγ-dependent manner. Upon nuclear localization, PKCβII underwent ubiquitination by Mdm2 and interacted with MEK1, resulting in ERK activation. In summary, α4β2 nAChR-mediated ERK activation in the nucleus involves the nuclear translocation of arrestin2 and PKCβII, which is reciprocally facilitated via positive feedback augmentation.

SIGNIFICANCE

As α4β2 nAChRs play a pivotal role in various cellular processes including drug addiction and mental disorders, our findings will offer insights into understanding the pathogenesis of α4β2 nAChR-related disorders and may facilitate the development of targeted therapeutic interventions.

d-Serine Inhibits Non-ionotropic NMDA Receptor Signaling

NMDA-type glutamate receptors (NMDARs) are widely recognized as master regulators of synaptic plasticity, most notably for driving long-term changes in synapse size and strength that support learning. NMDARs are unique among neurotransmitter receptors in that they require binding of both neurotransmitter (glutamate) and co-agonist (e.g., d-serine) to open the receptor channel, which leads to the influx of calcium ions that drive synaptic plasticity. Over the past decade, evidence has accumulated that NMDARs also support synaptic plasticity via ion flux-independent (non-ionotropic) signaling upon the binding of glutamate in the absence of co-agonist, although conflicting results have led to significant controversy. Here, we hypothesized that a major source of contradictory results might be attributed to variable occupancy of the co-agonist binding site under different experimental conditions. To test this hypothesis, we manipulated co-agonist availability in acute hippocampal slices from mice of both sexes. We found that enzymatic scavenging of endogenous co-agonists enhanced the magnitude of long-term depression (LTD) induced by non-ionotropic NMDAR signaling in the presence of the NMDAR pore blocker MK801. Conversely, a saturating concentration of d-serine completely inhibited LTD and spine shrinkage induced by glutamate binding in the presence of MK801 or Mg2+ Using a Förster resonance energy transfer (FRET)-based assay in cultured neurons, we further found that d-serine completely blocked NMDA-induced conformational movements of the GluN1 cytoplasmic domains in the presence of MK801. Our results support a model in which d-serine availability serves to modulate NMDAR signaling and synaptic plasticity even when the NMDAR is blocked by magnesium.

Kainate receptors regulate synaptic integrity and plasticity by forming a complex with synaptic organizers in the cerebellum

Kainate (KA)-type glutamate receptors (KARs) are implicated in various neuropsychiatric and neurological disorders through their ionotropic and metabotropic actions. However, compared to AMPA- and NMDA-type receptor functions, many aspects of KAR biology remain incompletely understood. Our study demonstrates an important role of KARs in organizing climbing fiber (CF)-Purkinje cell (PC) synapses and synaptic plasticity in the cerebellum, independently of their ion channel or metabotropic functions. The amino-terminal domain (ATD) of the GluK4 KAR subunit binds to C1ql1, provided by CFs, and associates with Bai3, an adhesion-type G protein-coupled receptor expressed in PC dendrites. Mice lacking GluK4 exhibit no KAR-mediated responses, reduced C1ql1 and Bai3 levels, and fewer CF-PC synapses, along with impaired long-term depression and oculomotor learning. Remarkably, introduction of the ATD of GluK4 significantly improves all these phenotypes. These findings demonstrate that KARs act as synaptic scaffolds, orchestrating synapses by forming a KAR-C1ql1-Bai3 complex in the cerebellum.

3-Hydroxykynurenine targets kainate receptors to promote defense against infection

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxykynurenine (3-HK) protects from lethal bacterial infection. 3-HK, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages through a systemic mechanism that targets kainate-sensitive glutamate receptors. These findings reveal a new pathway by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with important implications for the coordination between the immune and nervous systems in pathological conditions.

Kainate receptor channel opening and gating mechanism

Kainate receptors, a subclass of ionotropic glutamate receptors, are tetrameric ligand-gated ion channels that mediate excitatory neurotransmission1-4. Kainate receptors modulate neuronal circuits and synaptic plasticity during the development and function of the central nervous system and are implicated in various neurological and psychiatric diseases, including epilepsy, depression, schizophrenia, anxiety and autism5-11. Although structures of kainate receptor domains and subunit assemblies are available12-18, the mechanism of kainate receptor gating remains poorly understood. Here we present cryo-electron microscopy structures of the kainate receptor GluK2 in the presence of the agonist glutamate and the positive allosteric modulators lectin concanavalin A and BPAM344. Concanavalin A and BPAM344 inhibit kainate receptor desensitization and prolong activation by acting as a spacer between the amino-terminal and ligand-binding domains and a stabilizer of the ligand-binding domain dimer interface, respectively. Channel opening involves the kinking of all four pore-forming M3 helices. Our structures reveal the molecular basis of kainate receptor gating, which could guide the development of drugs for treatment of neurological disorders.

The fallacy of functional nomenclature in the kingdom of biological multifunctionality: physiological and evolutionary considerations on ion channels

Living organisms are multiscale complex systems that have evolved high degrees of multifunctionality and redundancy in the structure-function relationship. A number of factors, only in part determined genetically, affect the jobs of proteins. The overall structural organization confers unique molecular properties that provide the potential to perform a pattern of activities, some of which are co-opted by specific environments. The variety of multifunctional proteins is expanding, but most cases are handled individually and according to the still dominant 'one structure-one function' approach, which relies on the attribution of canonical names typically referring to the first task identified for a given protein. The present topical review focuses on the multifunctionality of ion channels as a paradigmatic example. Mounting evidence reports the ability of many ion channels (including members of voltage-dependent, ligand-gated and transient receptor potential families) to exert biological effects independently of their ion conductivity. 'Functionally based' nomenclature (the practice of naming a protein or family of proteins based on a single purpose) is a conceptual bias for three main reasons: (i) it increases the amount of ambiguity, deceiving our understanding of the multiple contributions of biomolecules that is the heart of the complexity; (ii) it is in stark contrast to protein evolution dynamics, largely based on multidomain arrangement; and (iii) it overlooks the crucial role played by the microenvironment in adjusting the actions of cell structures and in tuning protein isoform diversity to accomplish adaptational requirements. Biological information in protein physiology is distributed among different entwined layers working as the primary 'locus' of natural selection and of evolutionary constraints.

D-Serine inhibits non-ionotropic NMDA receptor signaling

NMDA-type glutamate receptors (NMDARs) are widely recognized as master regulators of synaptic plasticity, most notably for driving long-term changes in synapse size and strength that support learning. NMDARs are unique among neurotransmitter receptors in that they require binding of both neurotransmitter (glutamate) and co-agonist (e.g. d -serine) to open the receptor channel, which leads to the influx of calcium ions that drive synaptic plasticity. Over the past decade, evidence has accumulated that NMDARs also support synaptic plasticity via ion flux-independent (non-ionotropic) signaling upon the binding of glutamate in the absence of co-agonist, although conflicting results have led to significant controversy. Here, we hypothesized that a major source of contradictory results can be attributed to variable occupancy of the co-agonist binding site under different experimental conditions. To test this hypothesis, we manipulated co-agonist availability in acute hippocampal slices from mice of both sexes. We found that enzymatic scavenging of endogenous co-agonists enhanced the magnitude of LTD induced by non-ionotropic NMDAR signaling in the presence of the NMDAR pore blocker, MK801. Conversely, a saturating concentration of d -serine completely inhibited both LTD and spine shrinkage induced by glutamate binding in the presence of MK801. Using a FRET-based assay in cultured neurons, we further found that d -serine completely blocked NMDA-induced conformational movements of the GluN1 cytoplasmic domains in the presence of MK801. Our results support a model in which d -serine inhibits ion flux-independent NMDAR signaling and plasticity, and thus d -serine availability could serve to modulate NMDAR signaling even when the NMDAR is blocked by magnesium.

Significance Statement

NMDARs are glutamate-gated cation channels that are key regulators of neurodevelopment and synaptic plasticity and unique in their requirement for binding of a co-agonist (e.g. d -serine) in order for the channel to open. NMDARs have been found to drive synaptic plasticity via non-ionotropic (ion flux-independent) signaling upon the binding of glutamate in the absence of co-agonist, though conflicting results have led to controversy. Here, we found that d -serine inhibits non-ionotropic NMDAR-mediated LTD and LTD-associated spine shrinkage. Thus, a major source of the contradictory findings might be attributed to experimental variability in d -serine availability. In addition, the developmental regulation of d -serine levels suggests a role for non-ionotropic NMDAR plasticity during critical periods of plasticity.

Two Signaling Modes Are Better than One: Flux-Independent Signaling by Ionotropic Glutamate Receptors Is Coming of Age

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamatergic transmission can be mediated by ionotropic glutamate receptors (iGluRs), which mediate rapid synaptic depolarization that can be associated with Ca2+ entry and activity-dependent change in the strength of synaptic transmission, as well as by metabotropic glutamate receptors (mGluRs), which mediate slower postsynaptic responses through the recruitment of second messenger systems. A wealth of evidence reported over the last three decades has shown that this dogmatic subdivision between iGluRs and mGluRs may not reflect the actual physiological signaling mode of the iGluRs, i.e., α-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA) receptors (AMPAR), kainate receptors (KARs), and N-methyl-D-aspartate (NMDA) receptors (NMDARs). Herein, we review the evidence available supporting the notion that the canonical iGluRs can recruit flux-independent signaling pathways not only in neurons, but also in brain astrocytes and cerebrovascular endothelial cells. Understanding the signaling versatility of iGluRs can exert a profound impact on our understanding of glutamatergic synapses. Furthermore, it may shed light on novel neuroprotective strategies against brain disorders.

Postsynaptic receptors regulate presynaptic transmitter stability through transsynaptic bridges

Stable matching of neurotransmitters with their receptors is fundamental to synapse function and reliable communication in neural circuits. Presynaptic neurotransmitters regulate the stabilization of postsynaptic transmitter receptors. Whether postsynaptic receptors regulate stabilization of presynaptic transmitters has received less attention. Here, we show that blockade of endogenous postsynaptic acetylcholine receptors (AChR) at the neuromuscular junction destabilizes the cholinergic phenotype in motor neurons and stabilizes an earlier, developmentally transient glutamatergic phenotype. Further, expression of exogenous postsynaptic gamma-aminobutyric acid type A receptors (GABAA receptors) in muscle cells stabilizes an earlier, developmentally transient GABAergic motor neuron phenotype. Both AChR and GABAA receptors are linked to presynaptic neurons through transsynaptic bridges. Knockdown of specific components of these transsynaptic bridges prevents stabilization of the cholinergic or GABAergic phenotypes. Bidirectional communication can enforce a match between transmitter and receptor and ensure the fidelity of synaptic transmission. Our findings suggest a potential role of dysfunctional transmitter receptors in neurological disorders that involve the loss of the presynaptic transmitter.

The kainate receptor GluK2 mediates cold sensing in mice

Thermosensors expressed in peripheral somatosensory neurons sense a wide range of environmental temperatures. While thermosensors detecting cool, warm and hot temperatures have all been extensively characterized, little is known about those sensing cold temperatures. Though several candidate cold sensors have been proposed, none has been demonstrated to mediate cold sensing in somatosensory neurons in vivo, leaving a knowledge gap in thermosensation. Here we characterized mice lacking the kainate-type glutamate receptor GluK2, a mammalian homolog of the Caenorhabditis elegans cold sensor GLR-3. While GluK2 knockout mice respond normally to heat and mechanical stimuli, they exhibit a specific deficit in sensing cold but not cool temperatures. Further analysis supports a key role for GluK2 in sensing cold temperatures in somatosensory DRG neurons in the periphery. Our results reveal that GluK2-a glutamate-sensing chemoreceptor mediating synaptic transmission in the central nervous system-is co-opted as a cold-sensing thermoreceptor in the periphery.

Mechanisms of NMDA receptor regulation

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels widely expressed in the central nervous system that play key role in brain development and plasticity. On the downside, NMDAR dysfunction, be it hyperactivity or hypofunction, is harmful to neuronal function and has emerged as a common theme in various neuropsychiatric disorders including autism spectrum disorders, epilepsy, intellectual disability, and schizophrenia. Not surprisingly, NMDAR signaling is under a complex set of regulatory mechanisms that maintain NMDAR-mediated transmission in check. These include an unusual large number of endogenous agents that directly bind NMDARs and tune their activity in a subunit-dependent manner. Here, we review current knowledge on the regulation of NMDAR signaling. We focus on the regulation of the receptor by its microenvironment as well as by external (i.e. pharmacological) factors and their underlying molecular and cellular mechanisms. Recent developments showing how NMDAR dysregulation participate to disease mechanisms are also highlighted.

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. In vivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

Understanding the neuronal synapse and challenges associated with the mitochondrial dysfunction in mild cognitive impairment and Alzheimer's disease

Synaptic mitochondria are crucial for maintaining synaptic activity due to their high energy requirements, substantial calcium (Ca2+) fluctuation, and neurotransmitter release at the synapse. To provide a continuous energy supply, neurons use special mechanisms to transport and distribute healthy mitochondria to the synapse while eliminating the damaged mitochondria from the synapse. Along the neuron, mitochondrial membrane potential (ψ) gradient exists and is highest in the somal region. Lower ψ in the synaptic region renders mitochondria more vulnerable to oxidative stress-mediated damage. Secondly, mitochondria become susceptible to the release of cytochrome c, and mitochondrial DNA (mtDNA) is not shielded from the reactive oxygen species (ROS) by the histone proteins (unlike nuclear DNA), leading to activation of caspases and pronounced oxidative DNA base damage, which ultimately causes synaptic loss. Both synaptic mitochondrial dysfunction and synaptic failure are crucial factors responsible for Alzheimer's disease (AD). Furthermore, amyloid beta (Aβ) and hyper-phosphorylated Tau, the two leading players of AD, exaggerate the disease-like pathological conditions by reducing the mitochondrial trafficking, blocking the bi-directional transport at the synapse, enhancing the mitochondrial fission via activating the mitochondrial fission proteins, enhancing the swelling of mitochondria by increasing the influx of water through mitochondrial permeability transition pore (mPTP) opening, as well as reduced ATP production by blocking the activity of complex I and complex IV. Mild cognitive impairment (MCI) is also associated with decline in cognitive ability caused by synaptic degradation. This review summarizes the challenges associated with the synaptic mitochondrial dysfunction linked to AD and MCI and the role of phytochemicals in restoring the synaptic activity and rendering neuroprotection in AD.

Structure-Activity Relationship and Solubility Studies of N1-Substituted Quinoxaline-2,3-diones as Kainate Receptor Antagonists

Kainate receptors are a class of ionotropic glutamate receptors that respond to the excitatory neurotransmitter glutamate in the central nervous system and play an important role in the development of neurodegenerative disorders and the regulation of synaptic function. In the current study, we investigated the structure- activity relationship of the series of quinoxaline-2,3-diones substituted at N1, 6, and 7 positions, as ligands of kainate homomeric receptors GluK1-3 and GluK5. Pharmacological characterization showed that all derivatives obtained exhibited micromolar affinity at GluK3 receptors with Ki values in the range 0.1-4.4 μM range. The antagonistic properties of the selected analogues: N-(7-fluoro-6-iodo-2,3-dioxo-3,4-dihydroquinoxalin-1(2H)-yl)-3-sulfamoylbenzamide, N-(7-(1H-imidazol-1-yl)-6-iodo-2,3-dioxo-3,4-dihydroquinoxalin-1(2H)-yl)-3-sulfamoylbenzamide and N-(7-(1H-imidazol-1-yl)-2,3-dioxo-6-(phenylethynyl)-3,4-dihydroquinoxalin-1(2H)-yl)-3-sulfamoylbenzamide at GluK3 receptors, were confirmed by an intracellular calcium imaging assay. To correlate in vitro affinity data with structural features of the synthesized compounds and to understand the impact of the substituent in N1 position on ability to form additional protein-ligand interactions, molecular modeling and docking studies were carried out. Experimental solubility studies using UV spectroscopy detection have shown that 7-imidazolyl-6-iodo analogues with a sulfamoylbenzamide moiety at the N1 position are the best soluble compounds in the series, with molar solubility in TRISS buffer at pH 9 more than 3-fold higher compared to NBQX, a known AMPA/kainate antagonist.

A tryptophan metabolite modulates the host response to bacterial infection via kainate receptors

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxy-kynurenine protects from lethal gram-negative bacterial infection. 3-hydroxy-kynurenine, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages by targeting kainate-sensitive glutamate receptors. These findings reveal new mechanisms by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with significant implications for the coordination between the immune and nervous systems in pathological conditions.

α7 Nicotinic Acetylcholine Receptors May Improve Schwann Cell Regenerating Potential via Metabotropic Signaling Pathways

BACKGROUND

Schwann cells (SCs) are glial cells involved in peripheral axon myelination. SCs also play a strategic role after peripheral nerve injury, regulating local inflammation and axon regeneration. Our previous studies demonstrated the presence of cholinergic receptors in SCs. In particular, the α7 nicotinic acetylcholine receptors (nAChRs) are expressed in SCs after peripheral axotomy, suggesting their involvement in the regulation of SC-regenerating properties. To clarify the role that α7 nAChRs may play after peripheral axon damage, in this study we investigated the signal transduction pathways triggered by receptor activation and the effects produced by their activation.

METHODS

Both ionotropic and metabotropic cholinergic signaling were analyzed by calcium imaging and Western blot analysis, respectively, following α7 nAChR activation. In addition, the expression of c-Jun and α7 nAChRs was evaluated by immunocytochemistry and Western blot analysis. Finally, the cell migration was studied by a wound healing assay.

RESULTS

Activation of α7 nAChRs, activated by the selective partial agonist ICH3, did not induce calcium mobilization but positively modulated the PI3K/AKT/mTORC1 axis. Activation of the mTORC1 complex was also supported by the up-regulated expression of its specific p-p70 S6KThr389 target. Moreover, up-regulation of p-AMPKThr172, a negative regulator of myelination, was also observed concomitantly to an increased nuclear accumulation of the transcription factor c-Jun. Cell migration and morphology analyses proved that α7 nAChR activation also promotes SC migration.

CONCLUSIONS

Our data demonstrate that α7 nAChRs, expressed by SCs only after peripheral axon damage and/or in an inflammatory microenvironment, contribute to improve the SCs regenerating properties. Indeed, α7 nAChR stimulation leads to an upregulation of c-Jun expression and promotes Schwann cell migration by non-canonical pathways involving the mTORC1 activity.

NMDA receptor functions in health and disease: Old actor, new dimensions

N-Methyl-D-aspartate ionotropic glutamate receptors (NMDARs) play key roles in synaptogenesis, synaptic maturation, long-term plasticity, neuronal network activity, and cognition. Mirroring this wide range of instrumental functions, abnormalities in NMDAR-mediated signaling have been associated with numerous neurological and psychiatric disorders. Thus, identifying the molecular mechanisms underpinning the physiological and pathological contributions of NMDAR has been a major area of investigation. Over the past decades, a large body of literature has flourished, revealing that the physiology of ionotropic glutamate receptors cannot be restricted to fluxing ions, and involves additional facets controlling synaptic transmissions in health and disease. Here, we review newly discovered dimensions of postsynaptic NMDAR signaling supporting neural plasticity and cognition, such as the nanoscale organization of NMDAR complexes, their activity-dependent redistributions, and non-ionotropic signaling capacities. We also discuss how dysregulations of these processes may directly contribute to NMDAR-dysfunction-related brain diseases.

The α9α10 acetylcholine receptor: a non-neuronal nicotinic receptor

Within the superfamily of pentameric ligand-gated ion channels, cholinergic nicotinic receptors (nAChRs) were classically identified to mediate synaptic transmission in the nervous system and the neuromuscular junction. The α9 and α10 nAChR subunits were the last ones to be identified. Surprisingly, they do not fall into the dichotomic neuronal/muscle classification of nAChRs. They assemble into heteropentamers with a well-established function as canonical ion channels in inner ear hair cells, where they mediate central nervous system control of auditory and vestibular sensory processing. The present review includes expression, pharmacological, structure-function, molecular evolution and pathophysiological studies, that define receptors composed from α9 and α10 subunits as distant and distinct members within the nAChR family. Thus, although α9 and α10 were initially included within the neuronal subdivision of nAChR subunits, they form a distinct clade within the phylogeny of nAChRs. Following the classification of nAChR subunits based on their main synaptic site of action, α9 and α10 should receive a name in their own right.

The Role of Glutamate Receptors in Epilepsy

Glutamate is an essential excitatory neurotransmitter in the central nervous system, playing an indispensable role in neuronal development and memory formation. The dysregulation of glutamate receptors and the glutamatergic system is involved in numerous neurological and psychiatric disorders, especially epilepsy. There are two main classes of glutamate receptor, namely ionotropic and metabotropic (mGluRs) receptors. The former stimulate fast excitatory neurotransmission, are N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), and kainate; while the latter are G-protein-coupled receptors that mediate glutamatergic activity via intracellular messenger systems. Glutamate, glutamate receptors, and regulation of astrocytes are significantly involved in the pathogenesis of acute seizure and chronic epilepsy. Some glutamate receptor antagonists have been shown to be effective for the treatment of epilepsy, and research and clinical trials are ongoing.

Signaling of nicotinic acetylcholine receptors in mononuclear phagocytes

Nicotinic acetylcholine receptors are not only expressed by the nervous system and at the neuro-muscular junction but also by mononuclear phagocytes, which belong to the innate immune system. Mononuclear phagocyte is an umbrella term for monocytes, macrophages, and dendritic cells. These cells play pivotal roles in host defense against infection but also in numerous often debilitating diseases that are characterized by exuberant inflammation. Nicotinic acetylcholine receptors of the neuronal type dominate in these cells, and their stimulation is mainly associated with anti-inflammatory effects. Although the cholinergic modulation of mononuclear phagocytes is of eminent clinical relevance for the prevention and treatment of inflammatory diseases and neuropathic pain, we are only beginning to understand the underlying mechanisms on the molecular level. The purpose of this review is to report and critically discuss the current knowledge on signal transduction mechanisms elicited by nicotinic acetylcholine receptors in mononuclear phagocytes.

The role of glutamate receptors in the regulation of the tumor microenvironment

Glutamate, as one of the most important carbon sources in the TCA cycle, is central in metabolic processes that will subsequently influence tumor progression. Several factors can affect the expression of glutamate receptors, playing either a tumor-promoting or tumor-suppressor role in cancer. Thus, the activation of glutamate receptors by the ligand could play a role in tumor development as ample studies have demonstrated the expression of glutamate receptors in a broad range of tumor cells. Glutamate and its receptors are involved in the regulation of different immune cells' development and function, as suggested by the receptor expression in immune cells. The activation of glutamate receptors can enhance the effectiveness of the effector's T cells, or decrease the cytokine production in immunosuppressive myeloid-derived suppressor cells, increasing the antitumor immune response. These receptors are essential for the interaction between tumor and immune cells within the tumor microenvironment (TME) and the regulation of antitumor immune responses. Although the role of glutamate in the TCA cycle has been well studied, few studies have deeply investigated the role of glutamate receptors in the regulation of cancer and immune cells within the TME. Here, by a systematic review of the available data, we will critically assess the physiopathological relevance of glutamate receptors in the regulation of cancer and immune cells in the TME and provide some unifying hypotheses for futures research on the role of glutamate receptors in the immune modulation of the tumor.

Synaptic Plasticity Abnormalities in Fetal Alcohol Spectrum Disorders

The brain's ability to strengthen or weaken synaptic connections is often termed synaptic plasticity. It has been shown to function in brain remodeling following different types of brain damage (e.g., drugs of abuse, alcohol use disorders, neurodegenerative diseases, and inflammatory conditions). Although synaptic plasticity mechanisms have been extensively studied, how neural plasticity can influence neurobehavioral abnormalities in alcohol use disorders (AUDs) is far from being completely understood. Alcohol use during pregnancy and its harmful effects on the developing offspring are major public health, social, and economic challenges. The significant attribute of prenatal alcohol exposure on offspring is damage to the central nervous system (CNS), causing a range of synaptic structural, functional, and behavioral impairments, collectively called fetal alcohol spectrum disorder (FASD). Although the synaptic mechanisms in FASD are limited, emerging evidence suggests that FASD pathogenesis involves altering a set of molecules involved in neurotransmission, myelination, and neuroinflammation. These studies identify several immediate and long-lasting changes using many molecular approaches that are essential for synaptic plasticity and cognitive function. Therefore, they can offer potential synaptic targets for the many neurobehavioral abnormalities observed in FASD. In this review, we discuss the substantial research progress in different aspects of synaptic and molecular changes that can shed light on the mechanism of synaptic dysfunction in FASD. Increasing our understanding of the synaptic changes in FASD will significantly advance our knowledge and could provide a basis for finding novel therapeutic targets and innovative treatment strategies.

Kainate Receptor Antagonists: Recent Advances and Therapeutic Perspective

Since the 1990s, ionotropic glutamate receptors have served as an outstanding target for drug discovery research aimed at the discovery of new neurotherapeutic agents. With the recent approval of perampanel, the first marketed non-competitive antagonist of AMPA receptors, particular interest has been directed toward 'non-NMDA' (AMPA and kainate) receptor inhibitors. Although the role of AMPA receptors in the development of neurological or psychiatric disorders has been well recognized and characterized, progress in understanding the function of kainate receptors (KARs) has been hampered, mainly due to the lack of specific and selective pharmacological tools. The latest findings in the biology of KA receptors indicate that they are involved in neurophysiological activity and play an important role in both health and disease, including conditions such as anxiety, schizophrenia, epilepsy, neuropathic pain, and migraine. Therefore, we reviewed recent advances in the field of competitive and non-competitive kainate receptor antagonists and their potential therapeutic applications. Due to the high level of structural divergence among the compounds described here, we decided to divide them into seven groups according to their overall structure, presenting a total of 72 active compounds.

The Kv10.1 Channel: A Promising Target in Cancer

Carcinogenesis is a multistage process involving the dysregulation of multiple genes, proteins, and pathways that make any normal cell acquire a cancer cell phenotype. Therefore, it is no surprise that numerous ion channels could be involved in this process. Since their discovery and subsequent cloning, ion channels have been established as therapeutic targets in excitable cell pathologies (e.g., cardiac arrhythmias or epilepsy); however, their involvement in non-excitable cell pathologies is relatively recent. Among all ion channels, the voltage-gated potassium channels Kv10.1 have been established as a promising target in cancer treatment due to their high expression in tumoral tissues compared to low levels in healthy tissues.

Chemogenetic Silencing of Differentiating Cortical Neurons Impairs Dendritic and Axonal Growth

Electrical activity is considered a key driver for the neurochemical and morphological maturation of neurons and the formation of neuronal networks. Designer receptors exclusively activated by designer drugs (DREADDs) are tools for controlling neuronal activity at the single cell level by triggering specific G protein signaling. Our objective was to investigate if prolonged silencing of differentiating cortical neurons can influence dendritic and axonal maturation. The DREADD hM4Di couples to G_i/o signaling and evokes hyperpolarization via GIRK channels. HM4Di was biolistically transfected into neurons in organotypic slice cultures of rat visual cortex, and activated by clozapine-N-oxide (CNO) dissolved in H₂O; controls expressed hM4Di, but were mock-stimulated with H₂O. Neurons were analyzed after treatment for two postnatal time periods, DIV 5-10 and 10-20. We found that CNO treatment delays the maturation of apical dendrites of L2/3 pyramidal cells. Further, the number of collaterals arising from the main axon was significantly lower, as was the number of bouton terminaux along pyramidal cell and basket cell axons. The dendritic maturation of L5/6 pyramidal cells and of multipolar interneurons (basket cells and bitufted cells) was not altered by CNO treatment. Returning CNO-treated cultures to CNO-free medium for 7 days was sufficient to recover dendritic and axonal complexity. Our findings add to the view that activity is a key driver in particular of postnatal L2/3 pyramidal cell maturation. Our results further suggest that inhibitory G protein signaling may represent a factor balancing the strong driving force of neurotrophic factors, electrical activity and calcium signaling.

Reduced d-serine levels drive enhanced non-ionotropic NMDA receptor signaling and destabilization of dendritic spines in a mouse model for studying schizophrenia

Schizophrenia is a psychiatric disorder that affects over 20 million people globally. Notably, schizophrenia is associated with decreased density of dendritic spines and decreased levels of d-serine, a co-agonist required for opening of the N-methyl-d-aspartate receptor (NMDAR). We hypothesized that lowered d-serine levels associated with schizophrenia would enhance ion flux-independent signaling by the NMDAR, driving destabilization and loss of dendritic spines. We tested our hypothesis using the serine racemase knockout (SRKO) mouse model, which lacks the enzyme for d-serine production. We show that activity-dependent spine growth is impaired in SRKO mice, but can be acutely rescued by exogenous d-serine. Moreover, we find a significant bias of synaptic plasticity toward spine shrinkage in the SRKO mice as compared to wild-type littermates. Notably, we demonstrate that enhanced ion flux-independent signaling through the NMDAR contributes to this bias toward spine destabilization, which is exacerbated by an increase in synaptic NMDARs in hippocampal synapses of SRKO mice. Our results support a model in which lowered d-serine levels associated with schizophrenia enhance ion flux-independent NMDAR signaling and bias toward spine shrinkage and destabilization.

Comparison of the Anti-inflammatory Properties of Two Nicotinic Acetylcholine Receptor Ligands, Phosphocholine and pCF3-diEPP

Activation of nicotinic acetylcholine receptors (nAChRs) expressed by innate immune cells can attenuate pro-inflammatory responses. Silent nAChR agonists, which down-modulate inflammation but have little or no ionotropic activity, are of outstanding clinical interest for the prevention and therapy of numerous inflammatory diseases. Here, we compare two silent nAChR agonists, phosphocholine, which is known to interact with nAChR subunits α7, α9, and α10, and pCF3-N,N-diethyl-N′-phenyl-piperazine (pCF3-diEPP), a previously identified α7 nAChR silent agonist, regarding their anti-inflammatory properties and their effects on ionotropic nAChR functions. The lipopolysaccharide (LPS)-induced release of interleukin (IL)-6 by primary murine macrophages was inhibited by pCF3-diEPP, while phosphocholine was ineffective presumably because of instability. In human whole blood cultures pCF3-diEPP inhibited the LPS-induced secretion of IL-6, TNF-α and IL-1β. The ATP-mediated release of IL-1β by LPS-primed human peripheral blood mononuclear leukocytes, monocytic THP-1 cells and THP-1-derived M1-like macrophages was reduced by both phosphocholine and femtomolar concentrations of pCF3-diEPP. These effects were sensitive to mecamylamine and to conopeptides RgIA4 and [V11L; V16D]ArIB, suggesting the involvement of nAChR subunits α7, α9 and/or α10. In two-electrode voltage-clamp measurements pCF3-diEPP functioned as a partial agonist and a strong desensitizer of classical human α9 and α9α10 nAChRs. Interestingly, pCF3-diEPP was more effective as an ionotropic agonist at these nAChRs than at α7 nAChR. In conclusion, phosphocholine and pCF3-diEPP are potent agonists at unconventional nAChRs expressed by monocytic and macrophage-like cells. pCF3-diEPP inhibits the LPS-induced release of pro-inflammatory cytokines, while phosphocholine is ineffective. However, both agonists signal via nAChR subunits α7, α9 and/or α10 to efficiently down-modulate the ATP-induced release of IL-1β. Compared to phosphocholine, pCF3-diEPP is expected to have better pharmacological properties. Thus, low concentrations of pCF3-diEPP may be a therapeutic option for the treatment of inflammatory diseases including trauma-induced sterile inflammation.

Ion flux-independent NMDA receptor signaling

NMDA receptors play vital roles in a broad array of essential brain functions, from synaptic transmission and plasticity to learning and memory. Historically, the fundamental roles of NMDARs were attributed to their specialized properties of ion flux. More recently, it has become clear that NMDARs also signal in an ion flux-independent manner. Here, we review these non-ionotropic NMDAR signaling mechanisms that have been reported to contribute to a broad array of neuronal functions and dysfunctions including synaptic transmission and plasticity, cell death and survival, and neurological disorders.

Distinct molecular pathways govern presynaptic homeostatic plasticity

Presynaptic homeostatic plasticity (PHP) stabilizes synaptic transmission by counteracting impaired neurotransmitter receptor function through neurotransmitter release potentiation. PHP is thought to be triggered by impaired receptor function and to involve a stereotypic signaling pathway. However, here we demonstrate that different receptor perturbations that similarly reduce synaptic transmission result in different responses at the Drosophila neuromuscular junction. While receptor inhibition by the glutamate receptor (GluR) antagonist γ-D-glutamylglycine (γDGG) is not compensated by PHP, the GluR inhibitors Philanthotoxin-433 (PhTx) and Gyki-53655 (Gyki) induce compensatory PHP. Intriguingly, PHP triggered by PhTx and Gyki involve separable signaling pathways, including inhibition of distinct GluR subtypes, differential modulation of the active-zone scaffold Bruchpilot, and short-term plasticity. Moreover, while PHP upon Gyki treatment does not require genes promoting PhTx-induced PHP, it involves presynaptic protein kinase D. Thus, synapses not only respond differentially to similar activity impairments, but achieve homeostatic compensation via distinct mechanisms, highlighting the diversity of homeostatic signaling.

The Hair Cell α9α10 Nicotinic Acetylcholine Receptor: Odd Cousin in an Old Family

Nicotinic acetylcholine receptors (nAChRs) are a subfamily of pentameric ligand-gated ion channels with members identified in most eumetazoan clades. In vertebrates, they are divided into three subgroups, according to their main tissue of expression: neuronal, muscle and hair cell nAChRs. Each receptor subtype is composed of different subunits, encoded by paralogous genes. The latest to be identified are the α9 and α10 subunits, expressed in the mechanosensory hair cells of the inner ear and the lateral line, where they mediate efferent modulation. α9α10 nAChRs are the most divergent amongst all nicotinic receptors, showing marked differences in their degree of sequence conservation, their expression pattern, their subunit co-assembly rules and, most importantly, their functional properties. Here, we review recent advances in the understanding of the structure and evolution of nAChRs. We discuss the functional consequences of sequence divergence and conservation, with special emphasis on the hair cell α9α10 receptor, a seemingly distant cousin of neuronal and muscle nicotinic receptors. Finally, we highlight potential links between the evolution of the octavolateral system and the extreme divergence of vertebrate α9α10 receptors.

Downregulation of kainate receptors regulating GABAergic transmission in amygdala after early life stress is associated with anxiety-like behavior in rodents

Early life stress (ELS) is a well-characterized risk factor for mood and anxiety disorders. GABAergic microcircuits in the amygdala are critically implicated in anxiety; however, whether their function is altered after ELS is not known. Here we identify a novel mechanism by which kainate receptors (KARs) modulate feedforward inhibition in the lateral amygdala (LA) and show that this mechanism is downregulated after ELS induced by maternal separation (MS). Specifically, we show that in control rats but not after MS, endogenous activity of GluK1 subunit containing KARs disinhibit LA principal neurons during activation of cortical afferents. GluK1 antagonism attenuated excitability of parvalbumin (PV)-expressing interneurons, resulting in loss of PV-dependent inhibitory control and an increase in firing of somatostatin-expressing interneurons. Inactivation of Grik1 expression locally in the adult amygdala reduced ongoing GABAergic transmission and was sufficient to produce a mild anxiety-like behavioral phenotype. Interestingly, MS and GluK1-dependent phenotypes showed similar gender specificity, being detectable in male but not female rodents. Our data identify a novel KAR-dependent mechanism for cell-type and projection-specific functional modulation of the LA GABAergic microcircuit and suggest that the loss of GluK1 KAR function contributes to anxiogenesis after ELS.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

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

Kainate receptors and synaptic plasticity

Synaptic plasticity has classically been characterized to involve the NMDA and AMPA subtypes of glutamate receptors, with NMDA receptors providing the key trigger for the induction of long-term plasticity leading to changes in AMPA receptor expression. Here we review the more subtle roles played by kainate receptors, which contribute critical postsynaptic signalling as well as playing major presynaptic auto-receptor roles. We focus on two research areas: plasticity of kainate receptors themselves and the contribution they make to the plasticity of synaptic transmission. This article is part of the special issue on Glutamate Receptors - Kainate receptors.

Kainate and AMPA receptors in epilepsy: Cell biology, signalling pathways and possible crosstalk

Epilepsy is caused when rhythmic neuronal network activity escapes normal control mechanisms, resulting in seizures. There is an extensive and growing body of evidence that the onset and maintenance of epilepsy involves alterations in the trafficking, synaptic surface expression and signalling of kainate and AMPA receptors (KARs and AMPARs). The KAR subunit GluK2 and AMPAR subunit GluA2 are key determinants of the properties of their respective assembled receptors. Both subunits are subject to extensive protein interactions, RNA editing and post-translational modifications. In this review we focus on the cell biology of GluK2-containing KARs and GluA2-containing AMPARs and outline how their regulation and dysregulation is implicated in, and affected by, seizure activity. Further, we discuss role of KARs in regulating AMPAR surface expression and plasticity, and the relevance of this to epilepsy. This article is part of the special issue on 'Glutamate Receptors - Kainate receptors'.

The diverse and complex modes of action of anti-NMDA receptor autoantibodies

NMDA receptors are ligand-gated ion channels that are found throughout the brain and are required for both brain development and many higher order functions. A variety of human patients with diverse clinical phenotypes have been identified that carry autoantibodies directed against NMDA receptor subunits. Here we focus on two general classes of autoantibodies, anti-GluN1 antibodies associated with anti-NMDA receptor encephalitis and anti-GluN2 antibodies associated with systemic lupus erythematosus (SLE). These two general classes of anti-NMDA receptor autoantibodies display a wide range of pathophysiological mechanisms from altering synaptic composition to gating of NMDARs. While we have made progress in understanding how these autoantibodies work at the molecular and cellular level, many unanswered questions remain including their long-term actions on brain function, the significance of clonal variations, and their effects on different NMDA receptor-expressing cell types in local circuits. This information will be needed to define fully the transition from anti-NMDA receptor autoantibodies to a clinical phenotype.

Kainate receptor modulation of glutamatergic synaptic transmission in the CA2 region of the hippocampus

Kainate (KA) receptors (KARs) are important modulators of synaptic transmission. We studied here the role of KARs on glutamatergic synaptic transmission in the CA2 region of the hippocampus where the actions of these receptors are unknown. We observed that KA depresses glutamatergic synaptic transmission at Schaffer collateral-CA2 synapses; an effect that was antagonized by NBQX (a KA/AMPA receptors antagonist) under condition where AMPA receptors were previously blocked. The study of paired-pulse facilitation ratio, miniature responses, and fluctuation analysis indicated a presynaptic locus of action for KAR. Additionally, we determined the action mechanism for this depression of glutamate release mediated by the activation of KARs. We found that inhibition of protein kinase A suppressed the effect of KAR activation on evoked excitatory post-synaptic current, an effect that was not suppressed by protein kinase C inhibitors. Furthermore, in the presence of Pertussis toxin, the depression of glutamate release mediated by KAR activation was not present, invoking the participation of a G_i/o protein in this modulation. Finally, the KAR-mediated depression of glutamate release was not suppressed by treatments that affect calcium entry trough voltage-dependent calcium channels or calcium release from intracellular stores. We conclude that KARs present at these synapses mediate a depression of glutamate release through a mechanism that involves the activation of G protein and protein kinase A.

Topography and motion of acid-sensing ion channel intracellular domains

Acid-sensing ion channels (ASICs) are trimeric cation-selective channels activated by decreases in extracellular pH. The intracellular N and C terminal tails of ASIC1 influence channel gating, trafficking, and signaling in ischemic cell death. Despite several X-ray and cryo-EM structures of the extracellular and transmembrane segments of ASIC1, these important intracellular tails remain unresolved. Here, we describe the coarse topography of the chicken ASIC1 intracellular domains determined by fluorescence resonance energy transfer (FRET), measured using either fluorescent lifetime imaging or patch clamp fluorometry. We find the C terminal tail projects into the cytosol by approximately 35 Å and that the N and C tails from the same subunits are closer than adjacent subunits. Using pH-insensitive fluorescent proteins, we fail to detect any relative movement between the N and C tails upon extracellular acidification but do observe axial motions of the membrane proximal segments toward the plasma membrane. Taken together, our study furnishes a coarse topographic map of the ASIC intracellular domains while providing directionality and context to intracellular conformational changes induced by extracellular acidification.

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.

Regulation and dysregulation of neuronal circuits by KARs

Kainate receptors (KARs) constitute a family of ionotropic glutamate receptors (iGluRs) with distinct physiological roles in synapses and neuronal circuits. Despite structural and biophysical commonalities with the other iGluRs, AMPA receptors and NMDA receptors, their role as post-synaptic receptors involved in shaping EPSCs to transmit signals across synapses is limited to a small number of synapses. On the other hand KARs regulate presynaptic release mechanisms and control ion channels and signaling pathways through non-canonical metabotropic actions. We review how these different KAR-dependent mechanisms concur to regulate the activity and plasticity of neuronal circuits in physiological conditions of activation of KARs by endogenous glutamate (as opposed to pharmacological activation by exogenous agonists). KARs have been implicated in neurological disorders, based on genetic association and on physiopathological studies. A well described example relates to temporal lobe epilepsy for which the aberrant recruitment of KARs at recurrent mossy fiber synapses takes part in epileptogenic neuronal activity. In conclusion, KARs certainly represent an underestimated actor in the regulation of neuronal circuits, and a potential therapeutic target awaiting more selective and efficient genetic tools and/or ligands.

Glycine agonism in ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the majority of excitatory neurotransmission in the vertebrate CNS. Classified as AMPA, kainate, delta and NMDA receptors, iGluRs are central drivers of synaptic plasticity widely considered as a major cellular substrate of learning and memory. Surprisingly however, five out of the eighteen vertebrate iGluR subunits do not bind glutamate but glycine, a neurotransmitter known to mediate inhibitory neurotransmission through its action on pentameric glycine receptors (GlyRs). This is the case of GluN1, GluN3A, GluN3B, GluD1 and GluD2 subunits, all also binding the D amino acid d-serine endogenously present in many brain regions. Glycine and d-serine action and affinities broadly differ between glycinergic iGluR subtypes. On 'conventional' GluN1/GluN2 NMDA receptors, glycine (or d-serine) acts in concert with glutamate as a mandatory co-agonist to set the level of receptor activity. It also regulates the receptor's trafficking and expression independently of glutamate. On 'unconventional' GluN1/GluN3 NMDARs, glycine acts as the sole agonist directly triggering opening of excitatory glycinergic channels recently shown to be physiologically relevant. On GluD receptors, d-serine on its own mediates non-ionotropic signaling involved in excitatory and inhibitory synaptogenesis, further reinforcing the concept of glutamate-insensitive iGluRs. Here we present an overview of our current knowledge on glycine and d-serine agonism in iGluRs emphasizing aspects related to molecular mechanisms, cellular function and pharmacological profile. The growing appreciation of the critical influence of glycine and d-serine on iGluR biology reshapes our understanding of iGluR signaling diversity and complexity, with important implications in neuropharmacology.

β-Arrestin1 and GPCR kinase2 play permissive roles in Src-mediated endocytosis of α4β2 nicotinic ACh receptors

BACKGROUND AND PURPOSE

The α4β2 nicotinic ACh receptor (nAChR), a subtype of the ligand-gated ion channel, is abundantly expressed in the brain and is implicated in several neurological disorders. The endocytosis of nAChRs plays important roles in the pathogenesis of neurological diseases, but the underlying molecular mechanisms remain poorly understood.

EXPERIMENTAL APPROACH

Loss-of-function approaches and mutants of α4β2 nAChRs that display different endocytic properties were used to identify the cellular components and processes responsible for endocytosis. The signalling cascade that leads to endocytosis was deduced via protein interactions in predicted cellular components. The endocytosis of α4β2 nAChRs was determined and crosschecked using an ELISA and radioligand assay.

KEY RESULTS

Endocytosis of α4β2 nAChRs occurred through clathrin-mediated endocytosis in a dynamin-dependent manner. 14-3-3η-dependent Src-mediated phosphorylation of the nAChR α4 subunit at Y575 was required for nAChR endocytosis, and this occurred with the assistance of β-arrestin1 and GPCR kinase 2 (GRK2) without the need for kinase activity. Endocytosis triggered the mouse double minute 2 homologue-mediated ubiquitination and subsequent down-regulation of α4β2 nAChRs.

CONCLUSIONS AND IMPLICATIONS

α4β2 nAChR, an ionophore receptor, employs the metabotropic signalling pathway required for endocytosis, which leads to ubiquitination and down-regulation. Further, GRK2 and β-arrestin1, usually associated with GPCR signalling, are involved in the endocytosis of α4β2 nAChRs via different mechanisms. Considering the functional and pathological implications of nAChR endocytosis, results obtained in this study are crucial for the progression of basic research and clinical investigations.

Kainate receptors in the developing neuronal networks

Kainate receptors (KARs) are highly expressed in the immature brain and have unique developmentally regulated functions that may be important in linking neuronal activity to morphogenesis during activity-dependent fine-tuning of the synaptic connectivity. Altered expression of KARs in the developing neural network leads to changes in glutamatergic connectivity and network excitability, which may lead to long-lasting changes in behaviorally relevant circuitries in the brain. Here, we summarize the current knowledge on physiological and morphogenic functions described for different types of KARs at immature neural circuitries, focusing on their roles in modulating synaptic transmission and plasticity as well as circuit maturation in the rodent hippocampus and amygdala. Finally, we discuss the emerging evidence suggesting that malfunction of KARs in the immature brain may contribute to the pathophysiology underlying developmentally originating neurological disorders.

Losing balance: Kainate receptors and psychiatric disorders comorbidities

Cognition and behavior are tightly linked to synaptic function. A growing body of evidence suggests that aberrant neurotransmission, caused by changes in synaptic protein expression levels, may be a major cause underlying different brain disorders. These changes in expression result in abnormal synaptic organization or function, leading to impaired neurotransmission and unbalanced circuit operations. Here, we review the data supporting the involvement of mutations in genes coding for kainate receptor (KAR) subunits in the pathogenesis of psychiatric disorders and Down syndrome (DS). We show that most of these mutations do not affect the biophysical properties or the receptors, but rather alter subunit expression levels. On the basis of reports studying KAR genes mutations in mouse models of autism spectrum disorders and DS, we illustrate how deviations from the physiological regulatory role that these receptors play in neurotransmitter release and plasticity give rise to synaptic alterations that lead to behavioral and cognitive deficits underlying these disorders.

NMDAR in cultured astrocytes: Flux-independent pH sensor and flux-dependent regulator of mitochondria and plasma membrane-mitochondria bridging

Glutamate N-methyl-D-aspartate (NMDA) receptor (NMDAR) is critical for neurotransmission as a Ca²⁺ channel. Nonetheless, flux-independent signaling has also been demonstrated. Astrocytes express NMDAR distinct from its neuronal counterpart, but cultured astrocytes have no electrophysiological response to NMDA. We recently demonstrated that in cultured astrocytes, NMDA at pH6 (NMDA/pH6) acting through the NMDAR elicits flux-independent Ca²⁺ release from the Endoplasmic Reticulum (ER) and depletes mitochondrial membrane potential (mΔΨ). Here we show that Ca²⁺ release is due to pH6 sensing by NMDAR, whereas mΔΨ depletion requires both: pH6 and flux-dependent NMDAR signaling. Plasma membrane (PM) NMDAR guard a non-random distribution relative to the ER and mitochondria. Also, NMDA/pH6 induces ER stress, endocytosis, PM electrical capacitance reduction, mitochondria-ER, and -nuclear contacts. Strikingly, it also produces the formation of PM invaginations near mitochondria along with structures referred to here as PM-mitochondrial bridges (PM-m-br). These and earlier data strongly suggest PM-mitochondria communication. As proof of the concept of mass transfer, we found that NMDA/pH6 provoked mitochondria labeling by the PM dye FM-4-64FX. NMDA/pH6 caused PM depolarization, cell acidification, and Ca²⁺ release from most mitochondria. Finally, the MCU and microtubules were not involved in mΔΨ depletion, while actin cytoskeleton was partially involved. These findings demonstrate that NMDAR has concomitant flux-independent and flux-dependent actions in cultured astrocytes.

Senescence of Normal Human Fibroblasts Relates to the Expression of Ionotropic Glutamate Receptor GluR6/Grik2

BACKGROUND/AIM

Glutamate receptor GRIK2, previously designated as GluR6, is best described in neuronal cells. However, its biological relevance in non-neuronal cells is not well understood. We have investigated the expression of this important protein in normal human fibroblasts as a function of cell proliferation.

MATERIALS AND METHODS

We introduced expression constructs of all five isoforms (A-E) of GRIK2 in normal human fibroblasts and investigated the cells for the presence and localization of GRIK2, as well as for cell proliferation and senescence over a period of 24 days.

RESULTS

The expression of GRIK2-A isoform led to immediate cessation of cell proliferation. However, the cell numbers increased by 1.5- to 9.0-fold in 24 days upon transfection with B, C, D and E isoforms, after which they entered a state of senescence. The decreased proliferation was reflected by incorporation of BrdU in only 2-8% of transfected cells even after culturing them for 16 days.

CONCLUSION

Our results are indicative of an association between GRIK2 and aging of fibroblasts.

Amyloid Beta Peptide (Aβ1-42) Reverses the Cholinergic Control of Monocytic IL-1β Release

Amyloid-β peptide (Aβ_1-42), the cleavage product of the evolutionary highly conserved amyloid precursor protein, presumably plays a pathogenic role in Alzheimer's disease. Aβ_1-42 can induce the secretion of the pro-inflammatory cytokine intereukin-1β (IL-1β) in immune cells within and out of the nervous system. Known interaction partners of Aβ_1-42 are α7 nicotinic acetylcholine receptors (nAChRs). The physiological functions of Aβ_1-42 are, however, not fully understood. Recently, we identified a cholinergic mechanism that controls monocytic release of IL-1β by canonical and non-canonical agonists of nAChRs containing subunits α7, α9, and/or α10. Here, we tested the hypothesis that Aβ_1-42 modulates this inhibitory cholinergic mechanism. Lipopolysaccharide-primed monocytic U937 cells and human mononuclear leukocytes were stimulated with the P2X7 receptor agonist 2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate triethylammonium salt (BzATP) in the presence or absence of nAChR agonists and Aβ_1-42. IL-1β concentrations were measured in the supernatant. Aβ_1-42 dose-dependently (IC₅₀ = 2.54 µM) reversed the inhibitory effect of canonical and non-canonical nicotinic agonists on BzATP-mediated IL-1β-release by monocytic cells, whereas reverse Aβ_42-1 was ineffective. In conclusion, we discovered a novel pro-inflammatory Aβ_1-42 function that enables monocytic IL-1β release in the presence of nAChR agonists. These findings provide evidence for a novel physiological function of Aβ_1-42 in the context of sterile systemic inflammation.

Nicotinic acetylcholine receptors: Conventional and unconventional ligands and signaling

Postsynaptic nAChRs in the peripheral nervous system are critical for neuromuscular and autonomic neurotransmission. Pre- and peri-synaptic nAChRs in the brain modulate neurotransmission and are responsible for the addictive effects of nicotine. Subtypes of nAChRs in lymphocytes and non-synaptic locations may modulate inflammation and other cellular functions. All AChRs that function as ligand-gated ion channels are formed from five homologous subunits organized to form a central cation channel whose opening is regulated by ACh bound at extracellular subunit interfaces. nAChR subtype subunit composition can range from α7 homomers to α4β2α6β2β3 heteromers. Subtypes differ in affinities for ACh and other agonists like nicotine and in efficiencies with which their channels are opened and desensitized. Subtypes also differ in affinities for antagonists and for positive and negative allosteric modulators. Some agonists are "silent" with respect to channel opening, and AChRs may be able to signal metabotropic pathways by releasing G-proteins independent of channel opening. Electrophysiological studies that can resolve single-channel openings and molecular genetic approaches have allowed characterization of the structures of ligand binding sites, the cation channel, and the linkages between them, as well as the organization of AChR subunits and their contributions to function. Crystallography and cryo-electron-microscopy are providing increasing insights into the structures and functions of AChRs. However, much remains to be learned about both AChR structure and function, the in vivo functional roles of some AChR subtypes, and the development of better pharmacological tools directed at AChRs to treat addiction, pain, inflammation, and other medically important issues. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Elevated PSD-95 Blocks Ion-flux Independent LTD: A Potential New Role for PSD-95 in Synaptic Plasticity

We recently demonstrated that NMDA receptors (NMDARs) are capable of ion-flux independent signaling through conformational change in the NMDAR intracellular domain resulting in long-term depression of synaptic transmission (LTD). Here we show that PSD-95 overexpression blocks agonist induced conformational movement in the NMDAR intracellular domain as well as LTD that is NMDAR-dependent and ion-flux independent. Interestingly, previous studies indicate that overexpressed PSD-95 does not block NMDAR-dependent LTD. These data support a model where ion-flux independent LTD is predominant in young animals, which have synapses with low amounts of PSD-95, whereas only ion flux dependent LTD occurs at more mature synapses, which have more PSD-95 that would block ion-flux independent LTD. These results may reconcile different findings regarding ion-flux independent LTD.