Plasma membrane transporters GAT-1 and GAT-3 contribute to heterogeneity of GABAergic synapses in neocortex

γ-Aminobutyric acid transporter and GABAA receptor mechanisms in Slc6a1+/A288V and Slc6a1+/S295L mice associated with developmental and epileptic encephalopathies

We have previously characterized the molecular mechanisms for variants in γ-aminobutyric acid transporter 1-encoding solute carrier family 6-member 1 (SLC6A1) in vitro and concluded that a partial or complete loss of γ-aminobutyric acid uptake due to impaired protein trafficking is the primary aetiology. Impairment of γ-aminobutyric acid transporter 1 function could cause compensatory changes in the expression of γ-aminobutyric acid receptors, which, in turn, modify disease pathophysiology and phenotype. Here we used different approaches including radioactive 3H γ-aminobutyric acid uptake in cells and synaptosomes, immunohistochemistry and confocal microscopy as well as brain slice surface protein biotinylation to characterize Slc6a1+/A288V and Slc6a1+/S295L mice, representative of a partial or a complete loss of function of SLC6A1 mutations, respectively. We employed the γ-aminobutyric acid transporter 1-specific inhibitor [3H]tiagabine binding and GABAA receptor subunit-specific radioligand binding to profile the γ-aminobutyric acid transporter 1 and GABAA receptor expression in major brain regions such as cortex, cerebellum, hippocampus and thalamus. We also determined the total and surface expression of γ-aminobutyric acid transporter 1, γ-aminobutyric acid transporter 3 and expression of GABAA receptor in the major brain regions in the knockin mice. We found that γ-aminobutyric acid transporter 1 protein was markedly reduced in cortex, hippocampus, thalamus and cerebellum in both mutant mouse lines. Consistent with the findings of reduced γ-aminobutyric acid uptake for both γ-aminobutyric acid transporter 1(A288V) and γ-aminobutyric acid transporter 1(S295L), both the total and the γ-aminobutyric acid transporter 1-mediated 3H γ-aminobutyric acid reuptake was reduced. We found that γ-aminobutyric acid transporter 3 is only abundantly expressed in the thalamus and there was no compensatory increase of γ-aminobutyric acid transporter 3 in either of the mutant mouse lines. γ-Aminobutyric acid transporter 1 was reduced in both somatic regions and nonsomatic regions in both mouse models, in which a ring-like structure was identified only in the Slc6a1+/A288V mouse, suggesting more γ-aminobutyric acid transporter 1 retention inside endoplasmic reticulum in the Slc6a1+/A288V mouse. The [3H]tiagabine binding was similar in both mouse models despite the difference in γ-aminobutyric acid uptake function and γ-aminobutyric acid transporter 1 protein expression for both mutations. There were no differences in GABAA receptor subtype expression, except for a small increase in the expression of α5 subunits of GABAA receptor in the hippocampus of Slc6a1S295L homozygous mice, suggesting a potential interaction between the expression of this GABAA receptor subtype and the mutant γ-aminobutyric acid transporter 1. The study provides the first comprehensive characterization of the SLC6A1 mutations in vivo in two representative mouse models. Because both γ-aminobutyric acid transporter 1 and GABAA receptors are targets for anti-seizure medications, the findings from this study can help guide tailored treatment options based on the expression and function of γ-aminobutyric acid transporter 1 and GABAA receptor in SLC6A1 mutation-mediated neurodevelopmental and epileptic encephalopathies.

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Structural Heterogeneity of the GABAergic Tripartite Synapse

The concept of the tripartite synapse describes the close interaction of pre- and postsynaptic elements and the surrounding astrocyte processes. For glutamatergic synapses, it is established that the presence of astrocytic processes and their structural arrangements varies considerably between and within brain regions and between synapses of the same neuron. In contrast, less is known about the organization of astrocytic processes at GABAergic synapses although bi-directional signaling is known to exist at these synapses too. Therefore, we established super-resolution expansion microscopy of GABAergic synapses and nearby astrocytic processes in the stratum radiatum of the mouse hippocampal CA1 region. By visualizing the presynaptic vesicular GABA transporter and the postsynaptic clustering protein gephyrin, we documented the subsynaptic heterogeneity of GABAergic synaptic contacts. We then compared the volume distribution of astrocytic processes near GABAergic synapses between individual synapses and with glutamatergic synapses. We made two novel observations. First, astrocytic processes were more abundant at the GABAergic synapses with large postsynaptic gephyrin clusters. Second, astrocytic processes were less abundant in the vicinity of GABAergic synapses compared to glutamatergic, suggesting that the latter may be selectively approached by astrocytes. Because of the GABA transporter distribution, we also speculate that this specific arrangement enables more efficient re-uptake of GABA into presynaptic terminals.

Interactions of Glutamate and Gamma Amino Butyric Acid with the Insulin-like growth factor system in Traumatic Brain Injury (TBI) and/or Cardiovascular Accidents (CVA or stroke): A systematic review

The brain maintains homeostasis of neural excitation in part through the receptor-mediated signaling of Glutamate (Glu) and Gamma Amino Butyric Acid (GABA), but localized injuries cause cellular release of excess Glu leading to neurotoxicity. The literature strongly supports the role of Insulin-like growth factor-1 (IGF-1) in adult brain neuroprotection and repair, and research supporting the existence of molecular interactions between Glu, GABA, and IGF-1 in vitro and in normal animals raises the question of whether and/or how the Glu/GABA system interacts with IGF-1 post-injury. This systematic review was undertaken to explore works addressing this question among adults with a history of traumatic brain injury (TBI) and/or cerebrovascular accident (CVA; stroke). The literature was searched for human and animal studies and only four animal papers met inclusion criteria. The SYRCLE criteria was used to evaluate risk of bias; results varied between categories and papers. All the included studies, one on TBI and three on stroke, supported the molecular relationship between the excitatory and IGF-1 systems; two studies provided direct, detailed molecular evidence. The results point to the importance of research on the role of this protective system in pathological brain injury; a hypothetical proposal for future studies is presented.

Astroglial cells as neuroendocrine targets in forebrain development: Implications for sex differences in psychiatric disease

Astroglial cells are the most abundant cell type in the mammalian brain. They are implicated in almost every aspect of brain physiology, including maintaining homeostasis, building and maintaining the blood brain barrier, and the development and maturation of neuronal networks. Critically, astroglia also express receptors for gonadal sex hormones, respond rapidly to gonadal hormones, and are able to synthesize hormones. Thus, they are positioned to guide and mediate sexual differentiation of the brain, particularly neuronal networks in typical and pathological conditions. In this review, we describe astroglial involvement in the organization and development of the brain, and consider known sex differences in astroglial responses to understand how astroglial cell-mediated organization may play a role in forebrain sexual dimorphisms in human populations. Finally, we consider how sexually dimorphic astroglial responses and functions in development may lead to sex differences in vulnerability for neuropsychiatric disorders.

A Reappraisal of GAT-1 Localization in Neocortex

γ-Aminobutyric acid (GABA) transporter (GAT)-1, the major GABA transporter in the brain, plays a key role in modulating GABA signaling and is involved in the pathophysiology of several neuropsychiatric diseases, including epilepsy. The original description of GAT-1 as a neuronal transporter has guided the interpretation of the findings of all physiological, pharmacological, genetic, or clinical studies. However, evidence published in the past few years, some of which is briefly reviewed herein, does not seem to be consistent with a neurocentric view of GAT-1 function and calls for more detailed analysis of its localization. We therefore performed a thorough systematic assessment of GAT-1 localization in neocortex and subcortical white matter. In line with earlier work, we found that GAT-1 was robustly expressed in axon terminals forming symmetric synapses and in astrocytic processes, whereas its astrocytic expression was more diffuse than expected and, even more surprisingly, immature and mature oligodendrocytes and microglial cells also expressed the transporter. These data indicate that the era of “neuronal” and “glial” GABA transporters has finally come to a close and provide a wider perspective from which to view GABA-mediated physiological phenomena. In addition, given the well-known involvement of astrocytes, oligodendrocytes, and microglial cells in physiological as well as pathological conditions, the demonstration of functional GAT-1 in these cells is expected to provide greater insight into the phenomena occurring in the diseased brain as well as to prompt a reassessment of earlier findings.

Caffeine regulates GABA transport via A1R blockade and cAMP signaling

Caffeine is the most consumed psychostimulant drug in the world, acting as a non-selective antagonist of adenosine receptors A₁R and A_2AR, which are widely expressed in retinal layers. We have previously shown that caffeine, when administered acutely, acts on A₁R to potentiate the NMDA receptor-induced GABA release. Now we asked if long-term caffeine exposure also modifies GABA uptake in the avian retina and which mechanisms are involved in this process. Chicken embryos aged E11 were injected with a single dose of caffeine (30 mg/kg) in the air chamber. Retinas were dissected on E15 for ex vivo neurochemical assays. Our results showed that [³H]-GABA uptake was dependent on Na⁺ and blocked at 4 °C or by NO-711 and caffeine. This decrease was observed after 60 min of [³H]-GABA uptake assay at E15, which is accompanied by an increase in [³H]-GABA release. Caffeine increased the protein levels of A₁R without altering ADORA1 mRNA and was devoid of effects on A_2AR density or ADORA2A mRNA levels. The decrease of GABA uptake promoted by caffeine was reverted by A₁R activation with N6-cyclohexyl adenosine (CHA) but not by A_2AR activation with CGS 21680. Caffeine exposure increased cAMP levels and GAT-1 protein levels, which was evenly expressed between E11-E15. As expected, we observed an increase of GABA containing amacrine cells and processes in the IPL, also, cAMP pathway blockage by H-89 decreased caffeine mediated [³H]-GABA uptake. Our data support the idea that chronic injection of caffeine alters GABA transport via A₁R during retinal development and that the cAMP/PKA pathway plays an important role in the regulation of GAT-1 function.

Diverse Actions of Astrocytes in GABAergic Signaling

An imbalance of excitatory and inhibitory neurotransmission leading to over excitation plays a crucial role in generating seizures, while enhancing GABAergic mechanisms are critical in terminating seizures. In recent years, it has been reported in many studies that astrocytes are deeply involved in synaptic transmission. Astrocytes form a critical component of the “tripartite” synapses by wrapping around the pre- and post-synaptic elements. From this location, astrocytes are known to greatly influence the dynamics of ions and transmitters in the synaptic cleft. Despite recent extensive research on excitatory tripartite synapses, inhibitory tripartite synapses have received less attention, even though they influence inhibitory synaptic transmission by affecting chloride and GABA concentration dynamics. In this review, we will discuss the diverse actions of astrocytic chloride and GABA homeostasis at GABAergic tripartite synapses. We will then consider the pathophysiological impacts of disturbed GABA homeostasis at the tripartite synapse.

Feedback adaptation of synaptic excitability via Glu:Na+ symport driven astrocytic GABA and Gln release

Glutamatergic transmission composed of the arriving of action potential at the axon terminal, fast vesicular Glu release, postsynaptic Glu receptor activation, astrocytic Glu clearance and Glu→Gln shuttle is an abundantly investigated phenomenon. Despite its essential role, however, much less is known about the consequences of the mechanistic connotations of Glu:Na⁺ symport. Due to the coupled Na⁺ transport, Glu uptake results in significantly elevated intracellular astrocytic [Na⁺] that markedly alters the driving force of other Na⁺-coupled astrocytic transporters. The resulting GABA and Gln release by reverse transport through the respective GAT-3 and SNAT3 transporters help to re-establish the physiological Na⁺ homeostasis without ATP dissipation and consequently leads to enhanced tonic inhibition and replenishment of axonal glutamate pool. Here, we place this emerging astrocytic adjustment of synaptic excitability into the centre of future perspectives. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Expression of functional inhibitory neurotransmitter transporters GlyT1, GAT-1, and GAT-3 by astrocytes of inferior colliculus and hippocampus

Neuronal inhibition is mediated by glycine and/or GABA. Inferior colliculus (IC) neurons receive glycinergic and GABAergic inputs, whereas inhibition in hippocampus (HC) predominantly relies on GABA. Astrocytes heterogeneously express neurotransmitter transporters and are expected to adapt to the local requirements regarding neurotransmitter homeostasis. Here we analyzed the expression of inhibitory neurotransmitter transporters in IC and HC astrocytes using whole-cell patch-clamp and single-cell reverse transcription-PCR. We show that most astrocytes in both regions expressed functional glycine transporters (GlyTs). Activation of these transporters resulted in an inward current (I_Gly) that was sensitive to the competitive GlyT1 agonist sarcosine. Astrocytes exhibited transcripts for GlyT1 but not for GlyT2. Glycine did not alter the membrane resistance (R_M) arguing for the absence of functional glycine receptors (GlyRs). Thus, I_Gly was mainly mediated by GlyT1. Similarly, we found expression of functional GABA transporters (GATs) in all IC astrocytes and about half of the HC astrocytes. These transporters mediated an inward current (I_GABA) that was sensitive to the competitive GAT-1 and GAT-3 antagonists NO711 and SNAP5114, respectively. Accordingly, transcripts for GAT-1 and GAT-3 were found but not for GAT-2 and BGT-1. Only in hippocampal astrocytes, GABA transiently reduced R_M demonstrating the presence of GABA_A receptors (GABA_ARs). However, I_GABA was mainly not contaminated by GABA_AR-mediated currents as R_M changes vanished shortly after GABA application. In both regions, I_GABA was stronger than I_Gly. Furthermore, in HC the I_GABA/I_Gly ratio was larger compared to IC. Taken together, our results demonstrate that astrocytes are heterogeneous across and within distinct brain areas. Furthermore, we could show that the capacity for glycine and GABA uptake varies between both brain regions.

Voluntary Exercise Induces Astrocytic Structural Plasticity in the Globus Pallidus

Changes in astrocyte morphology are primarily attributed to the fine processes where intimate connections with neurons form the tripartite synapse and participate in neurotransmission. Recent evidence has shown that neurotransmission induces dynamic synaptic remodeling, suggesting that astrocytic fine processes may adapt their morphologies to the activity in their environment. To illustrate such a neuron-glia relationship in morphological detail, we employed a double transgenic Olig2^CreER/WT; ROSA26-GAP43-EGFP mice, in which Olig2-lineage cells can be visualized and traced with membrane-targeted GFP. Although Olig2-lineage cells in the adult brain usually become mature oligodendrocytes or oligodendrocyte precursor cells with NG2-proteoglycan expression, we found a population of Olig2-lineage astrocytes with bushy morphology in several brain regions. The globus pallidus (GP) preferentially contains Olig2-lineage astrocytes. Since the GP exerts pivotal motor functions in the indirect pathway of the basal ganglionic circuit, we subjected the double transgenic mice to voluntary wheel running to activate the GP and examined morphological changes of Olig2-lineage astrocytes at both the light and electron microscopic levels. The double transgenic mice were divided into three groups: control group mice were kept in a cage with a locked running wheel for 3 weeks, Runner group were allowed free access to a running wheel for 3 weeks, and the Runner-Rest group took a sedentary 3-week rest after a 3-week running period. GFP immunofluorescence analysis and immunoelectron microscopy revealed that astrocytic fine processes elaborated complex arborization in the Runner mice, and reverted to simple morphology comparable to that of the Control group in the Runner-Rest group. Our results indicated that the fine processes of the Olig2-lineage astrocytes underwent plastic changes that correlated with overall running activities, suggesting that they actively participate in motor functions.