Epilepsy, E/I Balance and GABAA Receptor Plasticity

GABAA receptors mediate most of the fast inhibitory transmission in the CNS. They form heteromeric complexes assembled from a large family of subunit genes. The existence of multiple GABAA receptor subtypes differing in subunit composition, localization and functional properties underlies their role for fine-tuning of neuronal circuits and genesis of network oscillations. The differential regulation of GABAA receptor subtypes represents a major facet of homeostatic synaptic plasticity and contributes to the excitation/inhibition (E/I) balance under physiological conditions and upon pathological challenges. The purpose of this review is to discuss recent findings highlighting the significance of GABAA receptor heterogeneity for the concept of E/I balance and its relevance for epilepsy. Specifically, we address the following issues: (1) role for tonic inhibition, mediated by extrasynaptic GABAA receptors, for controlling neuronal excitability; (2) significance of chloride ion transport for maintenance of the E/I balance in adult brain; and (3) molecular mechanisms underlying GABAA receptor regulation (trafficking, posttranslational modification, gene transcription) that are important for homoeostatic plasticity. Finally, the relevance of these findings is discussed in light of the involvement of GABAA receptors in epileptic disorders, based on recent experimental studies of temporal lobe epilepsy (TLE) and absence seizures and on the identification of mutations in GABAA receptor subunit genes underlying familial forms of epilepsy

Molecular and synaptic organization of GABAA receptors in the cerebellum: Effects of targeted subunit gene deletions

GABAA receptors form heteromeric GABA-gated chloride channels assembled from a large family of subunit genes. In cerebellum, distinct GABAA receptor subtypes, differing in subunit composition, are segregated between cell types and synaptic circuits. The cerebellum therefore represents a useful system to investigate the significance of GABAA receptor heterogeneity. For instance, studies of mice carrying targeted deletion of major GABAA receptor subunit genes revealed the role of α subunit variants for receptor assembly, synaptic targeting, and functional properties. In addition, these studies unraveled mandatory association between certain subunits and demonstrated distinct pharmacology of receptors mediating phasic and tonic inhibition. Although some of these mutants have a profound loss of GABAA receptors, they exhibit only minor impairment of motor function, suggesting activation of compensatory mechanisms to preserve inhibitory networks in the cerebellum. These adaptations include an altered balance between phasic and tonic inhibition, activation of voltage-independent K+ conductances, and upregulation of GABAA receptors in interneurons that are not affected directly by the mutation. Deletion of the α1 subunit gene leads to complete loss of GABAA receptors in Purkinje cells. A striking alteration occurs in these mice, whereby presynaptic GABAergic terminals are preserved in the molecular layer but make heterologous synapses with spines, characterized by a glutamatergic-like postsynaptic density. During development of α1% mice, GABAergic synapses are initially formed but are replaced upon spine maturation. These findings suggest that functional GABAA receptors are required for long-term maintenance of GABAergic synapses in Purkinje cell

Reporting statistical methods and statistical results in EJN

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73132/1/j.1460-9568.2008.06581.x.pd

Molecular and functional heterogeneity of GABAergic synapses

Knowledge of the functional organization of the GABAergic system, the main inhibitory neurotransmitter system, in the CNS has increased remarkably in recent years. In particular, substantial progress has been made in elucidating the molecular mechanisms underlying the formation and plasticity of GABAergic synapses. Evidence available ascribes a key role to the cytoplasmic protein gephyrin to form a postsynaptic scaffold anchoring GABAA receptors along with other transmembrane proteins and signaling molecules in the postsynaptic density. However, the mechanisms of gephyrin scaffolding remain elusive, notably because gephyrin can auto-aggregate spontaneously and lacks PDZ protein interaction domains found in a majority of scaffolding proteins. In addition, the structural diversity of GABAA receptors, which are pentameric channels encoded by a large family of subunits, has been largely overlooked in these studies. Finally, the role of the dystrophin-glycoprotein complex, present in a subset of GABAergic synapses in cortical structures, remains ill-defined. In this review, we discuss recent results derived mainly from the analysis of mutant mice lacking a specific GABAA receptor subtype or a core protein of the GABAergic postsynaptic density (neuroligin-2, collybistin), highlighting the molecular diversity of GABAergic synapses and its relevance for brain plasticity and function. In addition, we discuss the contribution of the dystrophin-glycoprotein complex to the molecular and functional heterogeneity of GABAergic synapse

Molecular heterogeneity of the dystrophin-associated protein complex in the mouse kidney nephron: differential alterations in the absence of utrophin and dystrophin

The dystrophin-associated protein complex (DPC) consisting of syntrophin, dystrobrevin, and dystroglycan isoforms is associated either with dystrophin or its homolog utrophin. It is present not only in muscle cells, but also in numerous tissues, including kidney, liver, and brain. Using high-resolution immunofluorescence imaging and Western blotting, we have investigated the effects of utrophin and dystrophin gene deletion on the formation and membrane anchoring of the DPC in kidney epithelial cells, which co-express utrophin and low levels of the C-terminal dystrophin isoform Dp71. We show that multiple, molecularly distinct DPCs co-exist in the nephron; these DPCs have a segment-specific distribution and are only partially associated with utrophin in the basal membrane of tubular epithelial cells. In utrophin-deficient mice, a selective reduction of β2-syntrophin has been observed in medullary tubular segments, whereas α1-syntrophin and β1-syntrophin are retained, concomintant with an upregulation of β-dystroglycan, β-dystrobrevin, and Dp71. These findings suggest that β2-syntrophin is dependent on utrophin for association with the DPC, and that loss of utrophin is partially compensated by Dp71, allowing the preservation of the DPC in kidney epithelial cells. This hypothesis is confirmed by the almost complete loss of all DPC proteins examined in mice lacking full-length utrophin and all C-terminal dystrophin isoforms (utrophin0/0/mdx3Cv). The DPC thus critically depends on these proteins for assembly and/or membrane localization in kidney epithelial cell

EJN in the digital age: introducing the ‘EJN blog’

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/88053/1/j.1460-9568.2011.07945.x.pd

GABAergic Synaptogenesis: A Case for Cooperation

Multiple cell-adhesion molecules contribute to synapse formation by mediating trans-synaptic interactions with presynaptic signaling molecules. In this issue of Neuron, Li et al. (2017) report cooperativity between Neuroligin2 and Slitrk3, exerting distinct effects on GABAergic synapse formation in immature and mature neurons

Selective changes in GABAA receptor subtypes in white matter neurons of patients with focal epilepsy

Mapping the distribution of GABAA receptor subtypes represents a promising approach to characterize alterations in cortical circuitry associated with neurological disorders. We previously reported subtype-selective changes in GABAA receptor expression in the grey matter of patients with focal epilepsy. In the present follow-up study, we focused on the subcortical white matter in the same tissue specimens obtained at surgery from 9 patients with temporal lobe epilepsy (TLE) and hippocampal sclerosis, 12 patients with TLE associated with neocortical lesions and 5 patients with frontal lobe epilepsy; post-mortem tissue from 4 subjects served as controls. The subunit composition and distribution of three major GABAA receptor subtypes were determined immunohistochemically with subunit-specific antibodies. In all cases, a majority of neurons in the white matter was distinctly labelled, allowing detailed visualization of their dendritic arborization and revealing a differential, cell type-specific expression pattern of α-subunit variants. In controls, α1-subunit staining was most prominent, displaying a gradient that decreased with depth, in parallel with the density of NeuN-positive cells. Subsets of pyramidal cells were α3-subunit-positive, and α2-subunit-labelled neurons were rare. In 19 of the 26 patients with focal epilepsy, no changes were detected as compared with controls. In five patients with TLE, striking changes in the dendritic arborization of a subset of white matter neurons were seen with the α1-subunit antibody. In two further patients with TLE, we observed a disorganized dendritic network immuno-positive for the α1-subunit, cell clusters selectively expressing the α2-subunit and small neuronal aggregates that expressed all subunits and appeared to connect to neighbouring white matter neurons. All seven patients with anomalies in the white matter had a selective reduction in α3-containing GABAA receptors in the superficial layers of the grey matter. These results demonstrate a distinct organization of GABAA receptors in human white matter neurons, consistent with an inhibitory network that is likely to be integrated functionally with the overlying grey matter. The altered dendritic morphology and changes in GABAA receptor expression in the white matter of a subset of patients with focal epilepsy are suggestive for a rewiring of neuronal circuit

Characterization and Turnover of CD73/IP3R3-positive Microvillar Cells in the Adult Mouse Olfactory Epithelium

The main olfactory epithelium consists of 4 major cell types: sensory neurons, supporting cells, microvillar cells, and basal progenitor cells. Several populations of microvillar olfactory cells have been described, whose properties are not yet fully understood. In this study, we aimed to clarify the classification of microvillar cells by introducing a specific marker, CD73. Furthermore, we investigated the turnover of CD73-microvillar cells during adult life. Using direct and indirect immunofluorescence in adult main olfactory epithelium, we first demonstrate that ecto-5′-nucleotidase (CD73) is a reliable marker for microvillar cells reported previously to express phospholipase C β2 (PLC β2) along with type 3 IP3 receptors (IP3R3) and transient receptor potential channels 6 (TRPC6), as well as for cells labeled by transgenic expression of tauGFP driven by the IP3R3 promoter. The ubiquitous CD73 immunoreactivity in the microvilli of these 2 cell populations indicates that they correspond to the same cell type (CD73-microvillar cell), endowed with a signal transduction cascade mobilizing Ca++ from intracellular stores. These microvillar cells respond to odors, possess a basal process, and do not degenerate after bulbectomy, suggesting that they contribute to cellular homeostasis in the olfactory epithelium. Next, we examined whether CD73-microvillar cells undergo turnover in the adult olfactory epithelium. By combining CD73 immunofluorescence and BrdU pulse labeling, we show delayed BrdU incorporation in a small fraction of CD73-positive microvillar cells, which persists for several weeks after BrdU administration. These findings indicate that CD73-microvillar cells likely differentiate from proliferating progenitor cells and have a slow turnover despite their apical position in the olfactory epithelium. These combined properties are unique among olfactory cells, in line with the possibility that they might regulate cellular homeostasis driven by extracellular ATP and adenosin