Properties of GABAergic Neurons Containing Calcium-Permeable Kainate and AMPA-Receptors

Channelopathies in epilepsy: an overview of clinical presentations, pathogenic mechanisms, and therapeutic insights

Pathogenic variants in genes encoding ion channels are causal for various pediatric and adult neurological conditions. In particular, several epilepsy syndromes have been identified to be caused by specific channelopathies. These encompass a spectrum from self-limited epilepsies to developmental and epileptic encephalopathies spanning genetic and acquired causes. Several of these channelopathies have exquisite responses to specific antiseizure medications (ASMs), while others ASMs may prove ineffective or even worsen seizures. Some channelopathies demonstrate phenotypic pleiotropy and can cause other neurological conditions outside of epilepsy. This review aims to provide a comprehensive exploration of the pathophysiology of seizure generation, ion channels implicated in epilepsy, and several genetic epilepsies due to ion channel dysfunction. We outline the clinical presentation, pathogenesis, and the current state of basic science and clinical research for these channelopathies. In addition, we briefly look at potential precision therapy approaches emerging for these disorders.

Peculiarities of ion homeostasis in neurons containing calcium-permeable AMPA receptors

Glutamate excitotoxicity accompanies numerous brain pathologies, including traumatic brain injury, ischemic stroke, and epilepsy. Disturbances of the ion homeostasis, mitochondria dysfunction, and further cell death are considered the main detrimental consequences of excitotoxicity. It is well known that neurons demonstrate different vulnerability to pathological exposures. In this regard, neurons containing calcium-permeable AMPA receptors (CP-AMPARs) may show higher susceptibility to excitotoxicity due to an additional pathway of Ca2+ influx. Here, we demonstrate that neurons containing CP-AMPARs are characterized by the higher amplitude of the glutamate-induced elevation of intracellular Ca2+ concentration ([Ca2+]i) and slower restoration of [Ca2+]i level compared to non-CP-AMPA neurons. Moreover, we have found that NASPM, an antagonist of CP-AMPARs, significantly decreases the amplitude of the [Ca2+]i elevation induced by glutamate or selective AMPARs agonist, 5-fluorowillardiine. In contrast, the antagonists of NMDARs or KARs affect insignificantly. We have also described some peculiarities of Na+, K+, and H+ intracellular dynamics in neurons containing CP-AMPARs. In particular, the amplitude of [Na+]i elevation was lower compared to non-CP-AMPA neurons, whereas the amplitude of [K+]i decrease was higher. We have shown the significant inverse correlation between [K+]i and [Ca2+]i and between intracellular pH and [Na+]i in CP-AMPARs-containing and non-CP-AMPA neurons upon glutamate excitotoxicity. Our data indicate that CP-AMPARs-mediated Ca2+ influx and slow removal of Ca2+ from the cytosol may underlie the vulnerability of the CP-AMPARs-containing neurons to glutamate excitotoxicity. Further studies of the mechanisms mediating the disturbances in ion homeostasis are crucial for developing new approaches for protecting these neurons at brain pathologies.

Calcium-permeable AMPA and kainate receptors of GABAergic neurons

This Commentary presents a brief discussion of the action of glutamate calcium permeable receptors present with neurons on the release of the neurotransmitter gamma-aminobutyric acid (GABA). In particular, Glutamate sensitive Kainic Acid Receptors (KARs) and α-Amino-3-hydroxy-5-Methyl-4-isoxazole Propionic Acid Receptor (AMPARs) are Na+ channels that typically cause neuronal cells to depolarize and release GABA. Some of these receptors are also permeable to Ca2+ and are hence involved in the calcium-dependent release of GABA neurotransmitters. Calcium-permeable kainate and AMPA receptors (CP-KARs and CP-AMPARs) are predominantly located in GABAergic neurons in the mature brain and their primary role is to regulate GABA release. AMPARs which do not contain the GluA2 subunit are mainly localized in the postsynaptic membrane. CP-KAR receptors are located mainly in the presynapse. GABAergic neurons expressing CP-KARs and CP-AMPARs respond to excitation earlier and faster, suppressing hyperexcitation of other neurons by the advanced GABA release due to an early rapid [Ca2+]i increase. CP-AMPARs have demonstrated a more pronounced impact on plasticity compared to NMDARs because of their capacity to elevate intracellular Ca2+ levels independently of voltage. GABAergic neurons that express CP-AMPARs contribute to the disinhibition of glutamatergic neurons by suppressing GABAergic neurons that express CP-KARs. Hence, the presence of glutamate CP-KARs and CP-AMPARs is crucial in governing hyperexcitation and synaptic plasticity in GABAergic neurons.

Participation of calcium-permeable AMPA receptors in the regulation of epileptiform activity of hippocampal neurons

Introduction

Epileptiform activity is the most striking result of hyperexcitation of a group of neurons that can occur in different brain regions and then spread to other sites. Later it was shown that these rhythms have a cellular correlate in vitro called paroxysmal depolarization shift (PDS). In 13-15 DIV neuron-glial cell culture, inhibition of the GABA(A) receptors induces bursts of action potential in the form of clasters PDS and oscillations of intracellular Ca2+ concentration ([Ca2+]i). We demonstrate that GABAergic neurons expressing calcium-permeable AMPA receptors (CP-AMPARs) as well as Kv7-type potassium channels regulate hippocampal glutamatergic neurons' excitability during epileptiform activity in culture.

Methods

A combination of whole-cell patch-clamp in current clamp mode and calcium imaging microscopy was used to simultaneously register membrane potential and [Ca2+]i level. To identify GABAergic cell cultures were fixed and stained with antibodies against glutamate decarboxylase GAD 65/67 and neuron-specific enolase (NSE) after vital [Ca2+]i imaging.

Results and discussion

It was shown that CP-AMPARs are involved in the regulation of the PDS clusters and [Ca2+]i pulses accompanied them. Activation of CP-AMPARs of GABAergic neurons is thought to cause the release of GABA, which activates the GABA(B) receptors of other GABAergic interneurons. It is assumed that activation of these GABA(B) receptors leads to the release of beta-gamma subunits of Gi protein, which activate potassium channels, resulting in hyperpolarization and inhibition of these interneurons. The latter causes disinhibition of glutamatergic neurons, the targets of these interneurons. In turn, the CP-AMPAR antagonist, NASPM, has the opposite effect. Measurement of membrane potential in GABAergic neurons by the patch-clamp method in whole-cell configuration demonstrated that NASPM suppresses hyperpolarization in clusters and individual PDSs. It is believed that Kv7-type potassium channels are involved in the control of hyperpolarization during epileptiform activity. The blocker of Kv7 channels, XE 991, mimicked the effect of the CP-AMPARs antagonist on PDS clusters. Both drugs increased the duration of the PDS cluster. In turn, the Kv7 activator, retigabine, decreased the duration of the PDS cluster and Ca2+ pulse. In addition, retigabine led to deep posthyperpolarization at the end of the PDS cluster. The Kv7 channel is believed to be involved in the formation of PDS, as the channel blocker reduced the rate of hyperpolarization in the PDS almost three times. Thus, GABAergic neurons expressing CP-AMPARs, regulate the membrane potential of innervated glutamatergic neurons by modulating the activity of postsynaptic potassium channels of other GABAergic neurons.

mRNA editing of kainate receptor subunits: what do we know so far?

Kainate receptors (KARs) are considered one of the key modulators of synaptic activity in the mammalian central nervous system. These receptors were discovered more than 30 years ago, but their role in brain functioning remains unclear due to some peculiarities. One such feature of these receptors is the editing of pre-mRNAs encoding GluK1 and GluK2 subunits. Despite the long history of studying this phenomenon, numerous questions remain unanswered. This review summarizes the current data about the mechanism and role of pre-mRNA editing of KAR subunits in the mammalian brain and proposes a perspective of future investigations.