GABA, but Not Bestrophin-1, Is Localized in Astroglial Processes in the Mouse Hippocampus and the Cerebellum

The pentameric chloride channel BEST1 is activated by extracellular GABA

Bestrophin-1 (BEST1) is a chloride channel expressed in the eye and other tissues of the body. A link between BEST1 and the principal inhibitory neurotransmitter γ-aminobutyric acid (GABA) has been proposed. The most appreciated receptors for extracellular GABA are the GABAB G-protein-coupled receptors and the pentameric GABAA chloride channels, both of which have fundamental roles in the central nervous system. Here, we demonstrate that BEST1 is directly activated by GABA. Through functional studies and atomic-resolution structures of human and chicken BEST1, we identify a GABA binding site on the channel's extracellular side and determine the mechanism by which GABA binding stabilizes opening of the channel's central gate. This same gate, "the neck," is activated by intracellular [Ca2+], indicating that BEST1 is controlled by ligands from both sides of the membrane. The studies demonstrate that BEST1, which shares no structural homology with GABAA receptors, is a GABA-activated chloride channel. The physiological implications of this finding remain to be studied.

The pentameric chloride channel BEST1 is activated by extracellular GABA

Bestrophin 1 (BEST1) is chloride channel expressed in the eye, central nervous system (CNS), and other tissues in the body. A link between BEST1 and the principal inhibitory neurotransmitter γ-aminobutyric acid (GABA) has been proposed. The most appreciated receptors for extracellular GABA are the GABAB G-protein coupled receptors and the pentameric GABAA chloride channels, both of which have fundamental roles in the CNS. Here, we demonstrate that BEST1 is directly activated by GABA. Through functional studies and atomic-resolution structures of human and chicken BEST1, we identify a GABA binding site on the channel's extracellular side and determine the mechanism by which GABA binding induces opening of the channel's central gate. This same gate is activated by intracellular [Ca2+], indicating that BEST1 is controlled by ligands from both sides of the membrane. The studies demonstrate that BEST1, which shares no structural homology with GABAA, is a GABA-activated chloride channel. The physiological implications of this finding remain to be studied.

Assessment of astrocytes as a mediator of memory and learning in rodents

The classical view of astrocytes is that they provide supportive functions for neurons, transporting metabolites and maintaining the homeostasis of the extracellular milieu. This view is gradually changing with the advent of molecular genetics and optical methods allowing interrogation of selected cell types in live experimental animals. An emerging view that astrocytes additionally act as a mediator of synaptic plasticity and contribute to learning processes has gained in vitro and in vivo experimental support. Here we focus on the literature published in the past two decades to review the roles of astrocytes in brain plasticity in rodents, whereby the roles of neurotransmitters and neuromodulators are considered to be comparable to those in humans. We outline established inputs and outputs of astrocytes and discuss how manipulations of astrocytes have impacted the behavior in various learning paradigms. Multiple studies suggest that the contribution of astrocytes has a considerably longer time course than neuronal activation, indicating metabolic roles of astrocytes. We advocate that exploring upstream and downstream mechanisms of astrocytic activation will further provide insight into brain plasticity and memory/learning impairment.

Dual Role for Astroglial Copper-Assisted Polyamine Metabolism during Intense Network Activity

Astrocytes serve essential roles in human brain function and diseases. Growing evidence indicates that astrocytes are central players of the feedback modulation of excitatory Glu signalling during epileptiform activity via Glu-GABA exchange. The underlying mechanism results in the increase of tonic inhibition by reverse operation of the astroglial GABA transporter, induced by Glu-Na+ symport. GABA, released from astrocytes, is synthesized from the polyamine (PA) putrescine and this process involves copper amino oxidase. Through this pathway, putrescine can be considered as an important source of inhibitory signaling that counterbalances epileptic discharges. Putrescine, however, is also a precursor for spermine that is known to enhance gap junction channel communication and, consequently, supports long-range Ca2+ signaling and contributes to spreading of excitatory activity through the astrocytic syncytium. Recently, we presented the possibility of neuron-glia redox coupling through copper (Cu+/Cu2+) signaling and oxidative putrescine catabolism. In the current work, we explore whether the Cu+/Cu2+ homeostasis is involved in astrocytic control on neuronal excitability by regulating PA catabolism. We provide supporting experimental data underlying this hypothesis. We show that the blockade of copper transporter (CTR1) by AgNO3 (3.6 µM) prevents GABA transporter-mediated tonic inhibitory currents, indicating causal relationship between copper (Cu+/Cu2+) uptake and the catabolism of putrescine to GABA in astrocytes. In addition, we show that MnCl2 (20 μM), an inhibitor of the divalent metal transporter DMT1, also prevents the astrocytic Glu-GABA exchange. Furthermore, we observed that facilitation of copper uptake by added CuCl2 (2 µM) boosts tonic inhibitory currents. These findings corroborate the hypothesis that modulation of neuron-glia coupling by copper uptake drives putrescine → GABA transformation, which leads to subsequent Glu-GABA exchange and tonic inhibition. Findings may in turn highlight the potential role of copper signaling in fine-tuning the activity of the tripartite synapse.

Astroglial asthenia and loss of function, rather than reactivity, contribute to the ageing of the brain

Astroglia represent a class of heterogeneous, in form and function, cells known as astrocytes, which provide for homoeostasis and defence of the central nervous system (CNS). Ageing is associated with morphological and functional remodelling of astrocytes with a prevalence of morphological atrophy and loss of function. In particular, ageing is associated with (i) decrease in astroglial synaptic coverage, (ii) deficits in glutamate and potassium clearance, (iii) reduced astroglial synthesis of synaptogenic factors such as cholesterol, (iv) decrease in aquaporin 4 channels in astroglial endfeet with subsequent decline in the glymphatic clearance, (v) decrease in astroglial metabolic support through the lactate shuttle, (vi) dwindling adult neurogenesis resulting from diminished proliferative capacity of radial stem astrocytes, (vii) decline in the astroglial-vascular coupling and deficient blood-brain barrier and (viii) decrease in astroglial ability to mount reactive astrogliosis. Decrease in reactive capabilities of astroglia are associated with rise of age-dependent neurodegenerative diseases. Astroglial morphology and function can be influenced and improved by lifestyle interventions such as intellectual engagement, social interactions, physical exercise, caloric restriction and healthy diet. These modifications of lifestyle are paramount for cognitive longevity.