Characterizing the binding and function of TARP γ8-selective AMPA receptor modulators

Various pharmacological agents in the pipeline against intractable epilepsy

Epilepsy is a noncommunicable chronic neurological disorder affecting people of all ages, with the highest prevalence in low and middle-income countries. Despite the pharmacological armamentarium, the plethora of drugs in the market, and other treatment options, 30%-35% of individuals still show resistance to the current medication, termed intractable epilepsy/drug resistance epilepsy, which contributes to 50% of the mortalities due to epilepsy. Therefore, the development of new drugs and agents is needed to manage this devastating epilepsy. We reviewed the pipeline of drugs in "ClinicalTrials. gov," which is the federal registry of clinical trials to identify drugs and other treatment options in various phases against intractable epilepsy. A total of 31 clinical trials were found regarding intractable epilepsy. Among them, 48.4% (15) are about pharmacological agents, of which 26.6% are in Phase 1, 60% are in Phase 2, and 13.3% are in Phase 3. The mechanism of action or targets of the majority of these agents are different and are more diversified than those of the approved drugs. In this article, we summarized various pharmacological agents in clinical trials, their backgrounds, targets, and mechanisms of action for the treatment of intractable epilepsy. Treatment options other than pharmacological ones, such as devices for brain stimulation, ketogenic diets, gene therapy, and others, are also summarized.

Preclinical Evaluation of Azabenzimidazole-Based PET Radioligands for γ-8 Dependent Transmembrane AMPA Receptor Regulatory Protein Imaging

AMPA glutamate receptors (AMPARs) play a pivotal role in excitatory neurotransmission, particularly in the hippocampus where the TARP γ-8 subunit is enriched and serves as a target for emerging anti-epileptic drugs. To enable in vivo visualization of TARP γ-8 distribution and expression by positron emission tomography (PET), this study focuses on the development of novel 18 F-labeled TARP γ-8 inhibitors and their corresponding precursors, stemming from the azabenzimidazole scaffold. The resulting radioligands [18 F]TARP-2204 and [18 F]TARP-2205 were successfully synthesized with acceptable radiochemical yield, high molar activity, and excellent radiochemical purity. In vitro autoradiography demonstrates high level of specific binding of [18 F]TARP-2205 to TARP γ-8 in both rat and nonhuman primate brain tissues. However, unexpected radiodefluorination in PET imaging studies of rodents emphasizes the need for further structural refinement. This work serves as an excellent starting point for the development of future 18 F-labeled TARP γ-8 PET tracers, offering valuable insights into medicinal chemistry design, radiosynthesis and subsequent PET evaluation.

Modulatory mechanisms of TARP γ8-selective AMPA receptor therapeutics

AMPA glutamate receptors (AMPARs) mediate excitatory neurotransmission throughout the brain. Their signalling is uniquely diversified by brain region-specific auxiliary subunits, providing an opportunity for the development of selective therapeutics. AMPARs associated with TARP γ8 are enriched in the hippocampus, and are targets of emerging anti-epileptic drugs. To understand their therapeutic activity, we determined cryo-EM structures of the GluA1/2-γ8 receptor associated with three potent, chemically diverse ligands. We find that despite sharing a lipid-exposed and water-accessible binding pocket, drug action is differentially affected by binding-site mutants. Together with patch-clamp recordings and MD simulations we also demonstrate that ligand-triggered reorganisation of the AMPAR-TARP interface contributes to modulation. Unexpectedly, one ligand (JNJ-61432059) acts bifunctionally, negatively affecting GluA1 but exerting positive modulatory action on GluA2-containing AMPARs, in a TARP stoichiometry-dependent manner. These results further illuminate the action of TARPs, demonstrate the sensitive balance between positive and negative modulatory action, and provide a mechanistic platform for development of both positive and negative selective AMPAR modulators.

Diversity of AMPA Receptor Ligands: Chemotypes, Binding Modes, Mechanisms of Action, and Therapeutic Effects

L-Glutamic acid is the main excitatory neurotransmitter in the central nervous system (CNS). Its associated receptors localized on neuronal and non-neuronal cells mediate rapid excitatory synaptic transmission in the CNS and regulate a wide range of processes in the brain, spinal cord, retina, and peripheral nervous system. In particular, the glutamate receptors selective to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) also play an important role in numerous neurological disorders and attract close attention as targets for the creation of new classes of drugs for the treatment or substantial correction of a number of serious neurodegenerative and neuropsychiatric diseases. For this reason, the search for various types of AMPA receptor ligands and studies of their properties are attracting considerable attention both in academic institutions and in pharmaceutical companies around the world. This review focuses mainly on the advances in this area published since 2017. Particular attention is paid to the structural diversity of new chemotypes of agonists, competitive AMPA receptor antagonists, positive and negative allosteric modulators, transmembrane AMPA regulatory protein (TARP) dependent allosteric modulators, ion channel blockers as well as their binding sites. This review also presents the studies of the mechanisms of action of AMPA receptor ligands that mediate their therapeutic effects.

Biphenyl scaffold for the design of NMDA-receptor negative modulators: molecular modeling, synthesis, and biological activity

NMDA (N-methyl-d-aspartate) receptor antagonists are promising tools for the treatment of a wide variety of central nervous system impairments including major depressive disorder. We present here the activity optimization process of a biphenyl-based NMDA negative allosteric modulator (NAM) guided by free energy calculations, which led to a 100 times activity improvement (IC50 = 50 nM) compared to a hit compound identified in virtual screening. Preliminary calculation results suggest a low affinity for the human ether-a-go-go-related gene ion channel (hERG), a high affinity for which was earlier one of the main obstacles for the development of first-generation NMDA-receptor negative allosteric modulators. The docking study and the molecular dynamics calculations suggest a completely different binding mode (ifenprodil-like) compared to another biaryl-based NMDA NAM EVT-101.

Imaging of Transmembrane AMPA Receptor Regulatory Proteins by Positron Emission Tomography

The transmembrane α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptor regulatory protein γ-8 (TARP γ-8) constitutes an auxiliary subunit of AMPA receptors, which mediates various brain functions including learning and memory. TARP γ-8 has emerged as a promising therapeutic target for central nervous system disorders. Despite considerable efforts, previously reported TARP γ-8 PET radioligands, such as [11C]TARP-1903 and [11C]TARP-1811 series, were plagued by limited brain uptake and/or high nonspecific binding in vivo. Herein, we developed two novel 11C-labeled probes, [11C]8 and [11C]15 (also named as [11C]TARP-2105), of which the latter exhibited a reasonable brain uptake as well as specific binding toward TARP γ-8 both in vitro and in vivo, as confirmed by blocking experiments with the commercially available TARP γ-8 inhibitor, JNJ-55511118 in the TARP γ-8-rich hippocampus. Overall, [11C]15 exhibited promising tracer characteristics and proved to be a lead positron-emission tomography ligand for the non-invasive quantification of TARP γ-8 in the mammalian brain.

Glioma‑neuronal interactions in tumor progression: Mechanism, therapeutic strategies and perspectives (Review)

An increasing body of evidence has become available to reveal the synaptic and functional integration of glioma into the brain network, facilitating tumor progression. The novel discovery of glioma-neuronal interactions has fundamentally challenged our understanding of this refractory disease. The present review aimed to provide an overview of how the neuronal activities function through synapses, neurotransmitters, ion channels, gap junctions, tumor microtubes and neuronal molecules to establish communications with glioma, as well as a simplified explanation of the reciprocal effects of crosstalk on neuronal pathophysiology. In addition, the current state of therapeutic avenues targeting critical factors involved in glioma-euronal interactions is discussed and an overview of clinical trial data for further investigation is provided. Finally, newly emerging technologies, including immunomodulation, a neural stem cell-based delivery system, optogenetics techniques and co-culture of neuron organoids and glioma, are proposed, which may pave a way towards gaining deeper insight into both the mechanisms associated with neuron- and glioma-communicating networks and the development of therapeutic strategies to target this currently lethal brain tumor.

Influence of the TARP γ8-Selective Negative Allosteric Modulator JNJ-55511118 on AMPA Receptor Gating and Channel Conductance

AMPA-type gultamate receptors (AMPARs) mediate excitatory signaling in the brain and are therapeutic targets for the treatment of diverse neurological disorders. The receptors interact with a variety of auxiliary subunits, including the transmembrane AMPAR regulatory proteins (TARPs). The TARPs influence AMPAR biosynthesis and trafficking and enhance receptor responses by slowing desensitization and deactivation and increasing single-channel conductance. TARP γ8 has an expression pattern that is distinct from that of other TARPs, being enriched in the hippocampus. Recently, several compounds have been identified that selectivity inhibit γ8-containing AMPARs. One such inhibitor, JNJ-55511118, has shown considerable promise for the treatment of epilepsy. However, key details of its mechanism of action are still lacking. Here, using patch-clamp electrophysiological recording from heterologously expressed AMPARs, we show that JNJ-55511118 inhibits peak currents of γ8-containing AMPARs by decreasing their single-channel conductance. The drug also modifies hallmark features of AMPAR pharmacology, including the TARP-dependent actions of intracellular polyamines and the partial agonist kainate. Moreover, we find that JNJ-55511118 reduces the influence of γ8 on all biophysical measures, aside from its effect on the recovery from desensitization. The drug is also effective when applied intracellularly, suggesting it may access its binding site from within the membrane. Additionally, we find that AMPARs incorporating TARP γ2 mutated to contain the JNJ-55511118 binding site, exhibit greater block than seen with AMPARs containing γ8, potentially reflecting differences in TARP stoichiometry. Taken together, our data provide new insight into the mechanism by which γ8-selective drugs inhibit AMPARs. SIGNIFICANCE STATEMENT: Although modulation of AMPA-type glutamate receptors shows promise for the treatment various neurological conditions, the absence of subtype-selective drugs has hindered adoption of this therapeutic strategy. We made patch-clamp recordings to characterize the actions of the γ8-selective AMPAR inhibitor JNJ-55511118 on GluA2(Q) receptors expressed in HEK cells. We report that JNJ-55511118 inhibits AMPAR-mediated currents by reducing single-channel conductance, providing clear insight into the mechanism of action of this important class of AMPAR modulators.

Dysfunction of Glutamatergic Synaptic Transmission in Depression: Focus on AMPA Receptor Trafficking

Pharmacological and anatomical evidence suggests that abnormal glutamatergic neurotransmission may be associated with the pathophysiology of depression. Compounds that act as NMDA receptor antagonists may be a potential treatment for depression, notably the rapid-acting agent ketamine. The rapid-acting and sustained antidepressant effects of ketamine rely on the activation of AMPA receptors (AMPARs). As the key elements of fast excitatory neurotransmission in the brain, AMPARs are crucially involved in synaptic plasticity and memory. Recent efforts have been directed toward investigating the bidirectional dysregulation of AMPAR-mediated synaptic transmission in depression. Here, we summarize the published evidence relevant to the dysfunction of AMPAR in stress conditions and review the recent progress toward the understanding of the involvement of AMPAR trafficking in the pathophysiology of depression, focusing on the roles of AMPAR auxiliary subunits, key AMPAR-interacting proteins, and posttranslational regulation of AMPARs. We also discuss new prospects for the development of improved therapeutics for depression.

Mechanisms underlying TARP modulation of the GluA1/2-γ8 AMPA receptor

AMPA-type glutamate receptors (AMPARs) mediate rapid signal transmission at excitatory synapses in the brain. Glutamate binding to the receptor’s ligand-binding domains (LBDs) leads to ion channel activation and desensitization. Gating kinetics shape synaptic transmission and are strongly modulated by transmembrane AMPAR regulatory proteins (TARPs) through currently incompletely resolved mechanisms. Here, electron cryo-microscopy structures of the GluA1/2 TARP-γ8 complex, in both open and desensitized states (at 3.5 Å), reveal state-selective engagement of the LBDs by the large TARP-γ8 loop (‘β1’), elucidating how this TARP stabilizes specific gating states. We further show how TARPs alter channel rectification, by interacting with the pore helix of the selectivity filter. Lastly, we reveal that the Q/R-editing site couples the channel constriction at the filter entrance to the gate, and forms the major cation binding site in the conduction path. Our results provide a mechanistic framework of how TARPs modulate AMPAR gating and conductance.

AMPA glutamate receptors, mediate the majority of excitatory signaling in the brain. Here the authors show how the auxiliary subunit TARP-γ8 shapes gating kinetics, ion conductance and rectification properties of the heteromeric GluA1/2 AMPA receptor.

The role of the TNFα-mediated astrocyte signaling pathway in epilepsy

Epilepsy is a common disease  in the central nervous system. There is growing evidence that epilepsy is associated with glial cells, including astrocytes. Tumor necrosis factor α (TNFα) is a “master regulator” of proinflammatory cytokine production and is secreted by microglia and astrocytes. TNFα secreted by microglia can activate astrocytes. Additionally, TNFα can regulate neuron activity and induce epilepsy by increasing the glutamate release, reducing the expression of γ-aminobutyric acid, inducing neuroinflammation and affecting the synaptic function in astrocytes. This review summarizes the signaling pathways and receptors of TNFα acting on astrocytes that are related to epilepsy and provides insights into the potential therapeutic strategies of epilepsy for clinical practice.

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.

Slow excitatory synaptic currents generated by AMPA receptors

Decades of literature indicate that the AMPA-type glutamate receptor is among the fastest acting of all neurotransmitter receptors. These receptors are located at excitatory synapses, and conventional wisdom says that they activate in hundreds of microseconds, deactivate in milliseconds due to their low affinity for glutamate and also desensitize profoundly. These properties circumscribe AMPA receptor activation in both space and time. However, accumulating evidence shows that AMPA receptors can also activate with slow, indefatigable responses. They do so through interactions with auxiliary subunits that are able promote a switch to a high open probability, high-conductance 'superactive' mode. In this review, we show that any assumption that this phenomenon is limited to heterologous expression is false and rather that slow AMPA currents have been widely and repeatedly observed throughout the nervous system. Hallmarks of the superactive mode are a lack of desensitization, resistance to competitive antagonists and a current decay that outlives free glutamate by hundreds of milliseconds. Because the switch to the superactive mode is triggered by activation, AMPA receptors can generate accumulating 'pedestal' currents in response to repetitive stimulation, constituting a postsynaptic mechanism for short-term potentiation in the range 5-100 Hz. Further, slow AMPA currents span 'cognitive' time intervals in the 100 ms range (theta rhythms), of particular interest for hippocampal function, where slow AMPA currents are widely expressed in a synapse-specific manner. Here, we outline the implications that slow AMPA receptors have for excitatory synaptic transmission and computation in the nervous system.

Drug treatment of epilepsy: From serendipitous discovery to evolutionary mechanisms

Epilepsy is a chronic brain disorder caused by abnormal firing of neurons. Up to now, using antiepileptic drugs is the main method of epilepsy treatment. The development of antiepileptic drugs lasted for centuries. In general, most agents entering clinical practice act on the balance mechanisms of brain "excitability-inhibition". More specifically, they target voltage-gated ion channels, GABAergic transmission and glutamatergic transmission. In recent years, some novel drugs representing new mechanisms of action have been discovered. Although there are about 30 available drugs in the market, it is still in urgent need of discovering more effective and safer drugs. The development of new antiepileptic drugs is into a new era: from serendipitous discovery to evolutionary mechanism-based design. This article presents an overview of drug treatment of epilepsy, including a series of traditional and novel drugs.

Biology of AMPA receptor interacting proteins - From biogenesis to synaptic plasticity

AMPA-type glutamate receptors mediate the majority of excitatory synaptic transmission in the central nervous system. Their signaling properties and abundance at synapses are both crucial determinants of synapse efficacy and plasticity, and are therefore under sophisticated control. Unique to this ionotropic glutamate receptor (iGluR) is the abundance of interacting proteins that contribute to its complex regulation. These include transient interactions with the receptor cytoplasmic tail as well as the N-terminal domain locating to the synaptic cleft, both of which are involved in AMPAR trafficking and receptor stabilization at the synapse. Moreover, an array of transmembrane proteins operate as auxiliary subunits that in addition to receptor trafficking and stabilization also substantially impact AMPAR gating and pharmacology. Here, we provide an overview of the catalogue of AMPAR interacting proteins, and how they contribute to the complex biology of this central glutamate receptor.

[Ligands of the AMPA-subtype glutamate receptors: mechanisms of action and novel chemotypes]

Ionotropic glutamate receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype play a key role in synaptic plasticity representing one of the mechanisms for learning and memory formation. They can also serve as targets for the development of novel classes of pharmaceuticals for the treatment or substantive correction of many serious neurodegenerative and psychoneurological disorders. The search and studies of various types of AMPA receptor ligands attract considerable attention from academic organizations and pharmaceutical companies all over the world. This review mainly focuses on recent advances in this field. The architecture and operational mechanism of the receptor as well as its major binding sites and ligand types are considered. Special attention is paid to the studies of mechanisms of action and novel chemotypes of AMPA receptor agonists and competitive antagonists, positive and negative allosteric modulators, auxiliary protein-dependent allosteric modulators, and ion channel blockers.

Ca2+ -permeable AMPA receptors and their auxiliary subunits in synaptic plasticity and disease

AMPA receptors are tetrameric glutamate-gated ion channels that mediate a majority of fast excitatory neurotransmission in the brain. They exist as calcium-impermeable (CI-) and calcium-permeable (CP-) subtypes, the latter of which lacks the GluA2 subunit. CP-AMPARs display an array of distinctive biophysical and pharmacological properties that allow them to be functionally identified. This has revealed that they play crucial roles in diverse forms of central synaptic plasticity. Here we summarise the functional hallmarks of CP-AMPARs and describe how these are modified by the presence of auxiliary subunits that have emerged as pivotal regulators of AMPARs. A lasting change in the prevalence of GluA2-containing AMPARs, and hence in the fraction of CP-AMPARs, is a feature in many maladaptive forms of synaptic plasticity and neurological disorders. These include modifications of glutamatergic transmission induced by inflammatory pain, fear conditioning, cocaine exposure, and anoxia-induced damage in neurons and glia. Furthermore, defective RNA editing of GluA2 can cause altered expression of CP-AMPARs and is implicated in motor neuron damage (amyotrophic lateral sclerosis) and the proliferation of cells in malignant gliomas. A number of the players involved in CP-AMPAR regulation have been identified, providing useful insight into interventions that may prevent the aberrant CP-AMPAR expression. Furthermore, recent molecular and pharmacological developments, particularly the discovery of TARP subtype-selective drugs, offer the exciting potential to modify some of the harmful effects of increased CP-AMPAR prevalence in a brain region-specific manner.

Auxiliary subunits of the AMPA receptor: The Shisa family of proteins

AMPA receptors mediate fast synaptic transmission in the CNS and can assemble with several types of auxiliary proteins in a spatio-temporal manner, from newly synthesized AMPA receptor tetramers to mature AMPA receptors in the cell membrane. As such, the interaction of auxiliary subunits with the AMPA receptor plays a major role in the regulation of AMPA receptor biogenesis, trafficking, and biophysical properties. Throughout the years, various 'families' of proteins have been identified and today the approximate full complement of AMPAR auxiliary proteins is known. This review presents the current knowledge on the most prominent AMPA-receptor-interacting auxiliary proteins, highlights recent results regarding the Shisa protein family, and provides a discussion on future research that might contribute to the discovery of novel pharmacological targets of auxiliary subunits.