Slc38a1 Conveys Astroglia-Derived Glutamine into GABAergic Interneurons for Neurotransmitter GABA Synthesis.

Tayyaba Qureshi,Tor Paaske Utheim,Vidar Gundersen,Kaja Nordengen,Jon Storm-Mathisen,Bjørnar Hassel,Leiv Otto Watne,Mona Bjørkmo,Farrukh Abbas Chaudhry

doi:10.3390/cells9071686

Abstract

GABA signaling is involved in a wide range of neuronal functions, such as synchronization of action potential firing, synaptic plasticity and neuronal development. Sustained GABA signaling requires efficient mechanisms for the replenishment of the neurotransmitter pool of GABA. The prevailing theory is that exocytotically released GABA may be transported into perisynaptic astroglia and converted to glutamine, which is then shuttled back to the neurons for resynthesis of GABA—i.e., the glutamate/GABA-glutamine (GGG) cycle. However, an unequivocal demonstration of astroglia-to-nerve terminal transport of glutamine and the contribution of astroglia-derived glutamine to neurotransmitter GABA synthesis is lacking. By genetic inactivation of the amino acid transporter Solute carrier 38 member a1 (Slc38a1)—which is enriched on parvalbumin+ GABAergic neurons—and by intraperitoneal injection of radiolabeled acetate (which is metabolized to glutamine in astroglial cells), we show that Slc38a1 mediates import of astroglia-derived glutamine into GABAergic neurons for synthesis of GABA. In brain slices, we demonstrate the role of Slc38a1 for the uptake of glutamine specifically into GABAergic nerve terminals for the synthesis of GABA depending on demand and glutamine supply. Thus, while leaving room for other pathways, our study demonstrates a key role of Slc38a1 for newly formed GABA, in harmony with the existence of a GGG cycle.

Highlights

GABA, the principal fast inhibitory neurotransmitter in the central nervous system (CNS), is pivotal to regulate neuronal excitability and function [1]
We find that the specific activity of glutamine was approximately 3× higher than that of glutamate in both Solute carrier 38 member a1 (Slc38a1)+/+ and Slc38a1−/− mice: the glutamine/glutamate specific activity ratio was 2.9 ± 0.7 and 2.9 ± 0.6 in Slc38a1+/+
From [14 C]acetate is significantly reduced in Slc38a1−/− mice (Figure 1—data given as radioactivity/mg protein to focus on newly formed amino acids), implicating a role for Slc38a1 in accumulating glutamine in GABAergic neurons for GABA synthesis

Summary

Introduction

GABA, the principal fast inhibitory neurotransmitter in the central nervous system (CNS), is pivotal to regulate neuronal excitability and function [1]. Classical GABA signaling depends on the quantal release of this neurotransmitter from nerve terminals, activation of postsynaptic GABAA receptors and termination of synaptic neurotransmission by removal of GABA from the synaptic cleft. Such short-lived and phasic signaling regulates action potential firing, synchronizes ensembles of neuronal networks and controls the efficacy and plasticity of excitatory inputs onto principal neurons. Investigation of Slc38a1−/− mice showed significantly reduced levels of glutamine, glutamate, GABA and aspartate in total brain lysates by high-performance liquid chromatography (HPLC), compared to Slc38a1+/+ mice, suggesting that Slc38a1 is pivotal for the synthesis of GABA. We investigated whether GABA synthesis in GABAergic interneurons depends on astroglia-derived glutamine, using radiolabeled acetate, and evaluated contribution of Slc38a1 for de novo synthesis of GABA in nerve endings of GABAergic interneurons

Animal Handling

Labeling Experiments with 14 C- and 13 C-Labeled Acetate

Preparation and Incubation of In Vitro Hippocampal Slices

Immunoperoxidase Processing and Analyses of Rat Brain Slices

Results

Discussion