A Kinetic Model for Brain Plasma Gamma-Aminobutyric Acid Transporters

Abstract

Gamma-aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. Regulation of synaptic and extra-synaptic GABA concentrations in the brain is carried out by four high-affinity GABA transporter (GAT) isoforms found on the plasma membrane of glia and neurons. GABA transporters are responsible for maintaining low resting levels of GABA in the central nervous system, as well as for modulating synaptic and extra-synaptic GABAergic neurotransmission. Potentiation of GABAergic neurotransmission via inhibition or reversal of GATs is believed to have therapeutic value in treating epileptic seizures. GABA transporters are Na+-dependent and Cl–-facilitated transporters that couple the co-translocation of Na+, Cl–, and GABA across the plasma membrane. The ion/GABA transport coupling ratio (i.e., stoichiometry) is 3 Na+ : 1 Cl– : 1 GABA and is believed to be fixed. Transport is driven by the Na+ and Cl– electrochemical gradients as well as by the GABA concentration gradient. Under most physiological conditions, GABA is removed from the extracellular space and transported across the plasma membrane into neurons and glia (i.e., GABA uptake). Under pathophysiological conditions and extreme physiological conditions, GABA transporters work in reverse, leading to the release of GABA from neurons and glia into the extracellular space. In this study, we have developed a 14-state kinetic model for the GABA transporters that qualitatively and quantitatively describes all experimentally known functional features of the transporter. The model describes Na+/Cl–/GABA cotransport across the plasma membrane as a series of partial reactions, including binding/dissociation events when the transporter binding sites face the intracellular and extracellular compartments, and conformational changes of the empty or substrate-bound carrier to expose the binding sites to the intracellular or extracellular compartment. Each partial reaction is modeled as a first-order binding/dissociation chemical reaction with associated forward and reverse rate constants. The effect of the membrane potential on rate constants was treated according to the Eyring theory of transition states, assuming a single and symmetrical energy barrier for each reaction step. Steady-state electrogenic behavior of the transporter was modeled by the net transport of charge across the plasma membrane. Presteady-state electrogenic behavior was modeled as movement of charge into, out of, and within the membrane electric field. Model simulations based on a single set of rate constants quantitatively predict the transporter ion/GABA stoichiometry, electrogenic properties, forward and reverse transport modes and rates, absolute dependence of GABA transport on Na+, partial dependence of GABA transport on Cl–, steady-state functional features and kinetic parameters, and presteady-state functional features and kinetic parameters. To our knowledge, this is the first kinetic model that adequately predicts all known functional features of the GABA transporters.