Single-cell genetic expression of mutant GABAA receptors causing Human genetic epilepsy alters dendritic spine and GABAergic bouton formation in a mutation-specific manner

Ion channels in neurodevelopment: lessons from the Integrin-KCNB1 channel complex

Ion channels modulate cellular excitability by regulating ionic fluxes across biological membranes. Pathogenic mutations in ion channel genes give rise to epileptic disorders that are among the most frequent neurological diseases affecting millions of individuals worldwide. Epilepsies are triggered by an imbalance between excitatory and inhibitory conductances. However, pathogenic mutations in the same allele can give rise to loss-of-function and/or gain-of-function variants, all able to trigger epilepsy. Furthermore, certain alleles are associated with brain malformations even in the absence of a clear electrical phenotype. This body of evidence argues that the underlying epileptogenic mechanisms of ion channels are more diverse than originally thought. Studies focusing on ion channels in prenatal cortical development have shed light on this apparent paradox. The picture that emerges is that ion channels play crucial roles in landmark neurodevelopmental processes, including neuronal migration, neurite outgrowth, and synapse formation. Thus, pathogenic channel mutants can not only cause epileptic disorders by altering excitability, but further, by inducing morphological and synaptic abnormalities that are initiated during neocortex formation and may persist into the adult brain.

Distinct Functional Alterations and Therapeutic Options of Two Pathological De Novo Variants of the T292 Residue of GABRA1 Identified in Children with Epileptic Encephalopathy and Neurodevelopmental Disorders

Mutations of GABA_AR have reportedly led to epileptic encephalopathy and neurodevelopmental disorders. We have identified a novel de novo T292S missense variant of GABRA1 from a pediatric patient with grievous global developmental delay but without obvious epileptic activity. This mutation coincidentally occurs at the same residue as that of a previously reported GABRA1 variant T292I identified from a pediatric patient with severe epilepsy. The distinct phenotypes of these two patients prompted us to compare the impacts of the two mutants on the receptor function and to search for suitable therapeutics. In this study, we used biochemical techniques and patch-clamp recordings in HEK293 cells overexpressing either wild-type or mutated rat recombinant GABA_ARs. We found that the α1T292S variant significantly increased GABA-evoked whole-cell currents, shifting the dose-response curve to the left without altering the maximal response. In contrast, the α1T292I variant significantly reduced GABA-evoked currents, shifting the dose-response curve to the right with a severely diminished maximum response. Single-channel recordings further revealed that the α1T292S variant increased, while the α1T292I variant decreased the GABA_AR single-channel open time and open probability. Importantly, we found that the T292S mutation-induced increase in GABA_AR function could be fully normalized by the negative GABA_AR modulator thiocolchicoside, whereas the T292I mutation-induced impairment of GABA_AR function was largely rescued with a combination of the GABA_AR positive modulators diazepam and verapamil. Our study demonstrated that α1T292 is a critical residue for controlling GABA_AR channel gating, and mutations at this residue may produce opposite impacts on the function of the receptors. Thus, the present work highlights the importance of functionally characterizing each individual GABA_AR mutation for ensuring precision medicine.

Impact of Pharmacological and Non-Pharmacological Modulators on Dendritic Spines Structure and Functions in Brain

Dendritic spines are small, thin, hair-like protrusions found on the dendritic processes of neurons. They serve as independent compartments providing large amplitudes of Ca2+ signals to achieve synaptic plasticity, provide sites for newer synapses, facilitate learning and memory. One of the common and severe complication of neurodegenerative disease is cognitive impairment, which is said to be closely associated with spine pathologies viz., decreased in spine density, spine length, spine volume, spine size etc. Many treatments targeting neurological diseases have shown to improve the spine structure and distribution. However, concise data on the various modulators of dendritic spines are imperative and a need of the hour. Hence, in this review we made an attempt to consolidate the effects of various pharmacological (cholinergic, glutamatergic, GABAergic, serotonergic, adrenergic, and dopaminergic agents) and non-pharmacological modulators (dietary interventions, enriched environment, yoga and meditation) on dendritic spines structure and functions. These data suggest that both the pharmacological and non-pharmacological modulators produced significant improvement in dendritic spine structure and functions and in turn reversing the pathologies underlying neurodegeneration. Intriguingly, the non-pharmacological approaches have shown to improve intellectual performances both in preclinical and clinical platforms, but still more technology-based evidence needs to be studied. Thus, we conclude that a combination of pharmacological and non-pharmacological intervention may restore cognitive performance synergistically via improving dendritic spine number and functions in various neurological disorders.

Pathophysiology of and therapeutic options for a GABRA1 variant linked to epileptic encephalopathy

We report the identification of a de novo GABRA1 (R214C) variant in a child with epileptic encephalopathy (EE), describe its functional characterization and pathophysiology, and evaluate its potential therapeutic options. The GABRA1 (R214C) variant was identified using whole exome sequencing, and the pathogenic effect of this mutation was investigated by comparing wild-type (WT) α1 and R214C α1 GABA_A receptor-expressing HEK cells. GABA-evoked currents in these cells were recorded using whole-cell, outside-out macro-patch and cell-attached single-channel patch-clamp recordings. Changes to surface and total protein expression levels of WT α1 and R214C α1 were quantified using surface biotinylation assay and western blotting, respectively. Finally, potential therapeutic options were explored by determining the effects of modulators, including diazepam, insulin, and verapamil, on channel gating and receptor trafficking of WT and R214C GABA_A receptors. We found that the GABRA1 (R214C) variant decreased whole-cell GABA-evoked currents by reducing single channel open time and both surface and total GABA_A receptor expression levels. The GABA-evoked currents in R214C GABA_A receptors could only be partially restored with benzodiazepine (diazepam) and insulin. However, verapamil treatment for 24 h fully restored the function of R214C mutant receptors, primarily by increasing channel open time. We conclude that the GABRA1 (R214C) variant reduces channel activity and surface expression of mutant receptors, thereby contributing to the pathogenesis of genetic EE. The functional restoration by verapamil suggests that it is a potentially new therapeutic option for patients with the R214C variant and highlights the value of precision medicine in the treatment of genetic EEs.

PKC/CREB pathway mediates the expressions of GABAA receptor subunits in cultured hippocampal neurons after low-Mg2+ solution treatment

OBJECTIVE

To investigate the potential effects of the PKC/CREB pathway on the expressions of GABAA receptor subunits α1, γ2, and δ in cultured hippocampal neurons using a model of epilepsy that employed conditions of low magnesium (Mg2+).

METHODS

A total of 108 embryonic rats at the age of 18 embryonic days (E18)prepared from adult female SD rats were used as experimental subjects. Primary rat hippocampal cultures were prepared from the embryonic 18 days rats. The cultured hippocampal neurons were then treated with artificial cerebrospinal fluid containing low Mg2+ solutions to generate a low Mg2+ model of epilepsy. The low Mg2+ stimulation lasted for 3 h and then returned to in maintenance medium for 20 h. The changes of the GABAA receptor subunit α1, γ2, δ were observed by blocking or activating the function of the CREB. The quantification of the GABAA receptor subunit α1, γ2, δ and the CREB were determined by a qRT-PCR and a Western blot method.

RESULTS

After the neurons were exposed to a low-Mg2+ solution for 3 h, GABAA receptor mRNA expression markedly increased compared to the control, and then gradually decreased. In contrast, CREB mRNA levels exhibited a dramatic down-regulation 3 h after terminating low-Mg2+ treatment, and then peaked at 9 h. Western blot analyses verified that staurosporine suppressed CREB phosphorylation (p-CREB). The mRNA expression of GABAA receptor subunit α1 increased only in the presence of staurosporine, whereas the expressions of subunits γ2 and δ significantly increased in the presence of either KG-501 or staurosporine. Furthermore, phorbol 12-myristate 13-acetate (PMA) decreased the expressions of GABAA subunits α1, γ2, and δ when administered alone. However, the administration of either KG-501 or staurosporine reversed the inhibitory effects of PMA.

CONCLUSIONS

The PKC/CREB pathway may negatively regulate the expressions of GABAA receptor subunits α1, γ2, and δ in cultured hippocampal neurons in low Mg2+ model of epilepsy.

Inhibitory synapse deficits caused by familial α1 GABAA receptor mutations in epilepsy

Epilepsy is a spectrum of neurological disorders with many causal factors. The GABA type-A receptor (GABA_AR) is a major genetic target for heritable human epilepsies. Here we examine the functional effects of three epilepsy-causing mutations to the α1 subunit (α1^T10'I, α1^D192N and α1^A295D) on inhibitory postsynaptic currents (IPSCs) mediated by the major synaptic GABA_AR isoform, α1β2γ2L. We employed a neuron - HEK293 cell heterosynapse preparation to record IPSCs mediated by mutant-containing GABA_ARs in isolation from other GABA_AR isoforms. IPSCs were recorded in the presence of the anticonvulsant drugs, carbamazepine and midazolam, and at elevated temperatures (22, 37 and 40°C) to gain insight into mechanisms of febrile seizures. The mutant subunits were also transfected into cultured cortical neurons to investigate changes in synapse formation and neuronal morphology using fluorescence microscopy. We found that IPSCs mediated by α1^T10'Iβ2γ2L, α1^D192Nβ2γ2L GABA_ARs decayed faster than those mediated by α1β2γ2L receptors. IPSCs mediated by α1^D192Nβ2γ2L and α1^A295Dβ2γ2L receptors also exhibited a heightened temperature sensitivity. In addition, the α1^T10'Iβ2γ2L GABA_ARs were refractory to modulation by carbamazepine or midazolam. In agreement with previous studies, we found that α1^A295Dβ2γ2L GABA_ARs were retained intracellularly in HEK293 cells and neurons. However, pre-incubation with 100nM suberanilohydroxamic acid (SAHA) induced α1^A295Dβ2γ2L GABA_ARs to mediate IPSCs that were indistinguishable in magnitude and waveform from those mediated by α1β2γ2L receptors. Finally, mutation-specific changes to synaptic bouton size, synapse number and neurite branching were also observed. These results provide new insights into the mechanisms of epileptogenesis of α1 epilepsy mutations and suggest possible leads for improving treatments for patients harbouring these mutations.

Chronic benzodiazepine treatment decreases spine density in cortical pyramidal neurons

The adult brain retains a substantial capacity for synaptic reorganization, which includes a wide range of modifications from molecular to structural plasticity. Previous reports have demonstrated that the structural remodeling of excitatory neurons seems to occur in parallel to changes in GABAergic neurotransmission. The function of neuronal inhibitory networks can be modified through GABAA receptors, which have a binding site for benzodiazepines (BZ). Although BZs are among the most prescribed drugs, is not known whether they modify the structure and connectivity of pyramidal neurons. In the present study we wish to elucidate the impact of a chronic treatment of 21 days with diazepam (2mg/kg, ip), a BZ that acts as an agonist of GABAA receptors, on the structural plasticity of pyramidal neurons in the prefrontal cortex of adult mice. We have examined the density of dendritic spines and the density of axonal en passant boutons in the cingulate cortex. Although no significant changes were observed in their anxiety levels, animals treated with diazepam showed a decrease in the density of spines in the apical dendrites of pyramidal neurons. Most GFP-expressing en passant boutons in the upper layers of the cingulate cortex had an extracortical origin and no changes in their density were detected after diazepam treatment. These results indicate that the chronic potentiation of GABAergic synapses can induce the structural remodeling of postsynaptic elements in pyramidal neurons.

Models of drug-induced epileptiform synchronization in vitro

Models of epileptiform activity in vitro have many advantages for recording and experimental manipulation. Neural tissues can be maintained in vitro for hours, and in neuronal or organotypic slice cultures for several weeks. A variety of drugs and other agents increase activity in these in vitro conditions, in many cases resulting in epileptiform activity, thus providing a direct model of symptomatic seizures. We review these preparations and the experimental manipulations used to induce epileptiform activity. The most common of drugs used are GABAA receptor antagonists and potassium channel blockers (notably 4-aminopyridine). Muscarinic agents also can induce epileptiform synchronization in vitro, and include potassium channel inhibition amongst their cellular actions. Manipulations of extracellular ions are reviewed in another paper in this special issue, as are ex vivo slices prepared from chronically epileptic animals and from people with epilepsy. More complex slices including extensive networks and/or several connected brain structures can provide insights into the dynamics of long range connections during epileptic activity. Visualization of slices also provides opportunities for identification of living neurons and for optical recording/stimulation and manipulation. Overall, the analysis of the epileptiform activity induced in brain tissue in vitro has played a major role in advancing our understanding of the cellular and network mechanisms of epileptiform synchronization, and it is expected to continue to do so in future.

Organotypic Spinal Cord Culture: a Proper Platform for the Functional Screening

Recent improvements in organotypic slice culturing and its accompanying technological innovations have made this biological preparation increasingly useful ex vivo experimental model. Among organotypic slice cultures obtained from various central nervous regions, spinal cord slice culture is an absorbing model that represents several unique advantages over other current in vitro and in vivo models. The culture of developing spinal cord slices, as allows real-time observation of embryonic cells behaviors, is an instrumental platform for developmental investigation. Importantly, due to the ability of ex vivo models to recapitulate different aspects of corresponding in vivo conditions, these models have been subject of various manipulations to derive disease-relevant slice models. Moreover spinal cord slice cultures represent a potential platform for screening of different pharmacological agents and evaluation of cell transplantation and neuroregenerative materials. In this review, we will focus on studies carried out using the ex vivo model of spinal cord slice cultures and main advantages linked to practicality of these slices in both normal and neuropathological diseases and summarize them in different categories based on application.