Loop G in the GABAA receptor α1 subunit influences gating efficacy

Mutations of α1F45 residue of GABAA receptor loop G reveal its involvement in agonist binding and channel opening/closing transitions

GABA_A receptors (GABA_ARs) mediate inhibitory neurotransmission in the mammalian brain. Recently, numerous GABA_AR static structures have been published, but the molecular mechanisms of receptor activation remain elusive. Loop G is a rigid β-strand belonging to an extensive β-sheet that spans the regions involved in GABA binding and the interdomain interface which is important in receptor gating. It has been reported that loop G participates in ligand binding and gating of GABA_ARs, however, it remains unclear which specific gating transitions are controlled by this loop. Analysis of macroscopic responses revealed that mutation at the α₁F45 residue (loop G midpoint) resulted in slower macroscopic desensitization and accelerated deactivation. Single-channel analysis revealed that these mutations also affected open and closed times distributions and reduced open probability. Kinetic modeling demonstrated that mutations affected primarily channel opening/closing and ligand binding with a minor effect on preactivation. Thus, α₁F45 residue, in spite of its localization close to binding site, affects late gating transitions. In silico structural analysis suggested an important role of α₁F45 residue in loop G stability and rigidity as well as in general structure of the binding site. We propose that the rigid β-sheet comprising loop G is well suited for long range communication within GABA_AR but this mechanism becomes impaired when α₁F45 is mutated. In conclusion, we demonstrate that loop G is crucial in controlling both binding and gating of GABA_ARs. These data shed new light on GABA_AR activation mechanism and may also be helpful in designing clinically relevant modulators.

The Functional Role of Spontaneously Opening GABAA Receptors in Neural Transmission

Ionotropic type of γ-aminobutyric acid receptors (GABA_ARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABA_ARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABA_ARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABA_ARs (s-GABA_ARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABA_ARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABA_ARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABA_ARs functional properties and experimental approaches that allow isolation of s-GABA_ARs effects from those of conventional (GABA-dependent) GABA_ARs.

Amino acid substitutions in the human homomeric β3 GABAA receptor that enable activation by GABA

GABA_A receptors (GABA_ARs) are pentameric ligand-gated ion channels that mediate synaptic inhibition throughout the central nervous system. The α₁β₂γ₂ receptor is the major subtype in the brain; GABA binds at the β₂(+)α₁(-) interface. The structure of the homomeric β₃ GABA_AR, which is not activated by GABA, has been solved. Recently, four additional heteromeric structures were reported, highlighting key residues required for agonist binding. Here, we used a protein engineering method, taking advantage of knowledge of the key binding residues, to create a β₃(+)α₁(-) heteromeric interface in the homomeric human β₃ GABA_AR that enables GABA-mediated activation. Substitutions were made in the complementary side of the orthosteric binding site in loop D (Y87F and Q89R), loop E (G152T), and loop G (N66D and A70T). The Q89R and G152T combination enabled low-potency activation by GABA and potentiation by propofol but impaired direct activation by higher propofol concentrations. At higher concentrations, GABA inhibited gating of β₃ GABA_AR variants containing Y87F, Q89R, and G152T. Reversion of Phe⁸⁷ to tyrosine abolished GABA's inhibitory effect and partially recovered direct activation by propofol. This tyrosine is conserved in homomeric GABA_ARs and in the Erwinia chrysanthemi ligand-gated ion channel and may be essential for the absence of an inhibitory effect of GABA on homomeric channels. This work demonstrated that only two substitutions, Q89R and G152T, in β₃ GABA_AR are sufficient to reconstitute GABA-mediated activation and suggests that Tyr⁸⁷ prevents inhibitory effects of GABA.

Feature Article: Selective modulation of tonically active GABAA receptor functional subgroups by G-proteins and protein kinase C

IMPACT STATEMENT

Here we study intracellular mechanisms which regulate inhibitory signaling delivered through continuously (tonically) open ionotropic receptors of γ-aminobutyric acid (GABA) of dentate gyrus granule cells (DGCs). We found that, apart of classical GABA-A receptors (GABA_ARs) which can be activated by GABA binding, a significant part of tonic inhibitory current is delivered by newly discovered spontaneously opening GABA_ARs (s-GABA_ARs), which enter active state without binding of GABA. We have also found that conventional GABA_ARs and s-GABA_ARs are regulated by different intracellular mechanisms, which may overlap and thus induce various signaling repercussions. Our results demonstrate that s-GABA_ARs play a key role in the mechanism that implements DGCs functional role in the brain. On top of that, since regulatory mechanisms under study are affected in a number of pathological states, our results may have broad implications for treatment of neurological disorders.

Spontaneously opening GABAA receptors play a significant role in neuronal signal filtering and integration

Continuous (tonic) charge transfer through ionotropic receptors of γ-aminobutyric acid (GABAARs) is an important mechanism of inhibitory signalling in the brain. The conventional view has been that tonic GABA-ergic inhibitory currents are mediated by low concentrations of ambient GABA. Recently, however, it was shown that the GABA-independent, spontaneously opening GABAARs (s-GABAARs), may contribute significantly to the tonic GABAAR current. One of the common approaches to temporal lobe epilepsy (TLE) therapy is an increase of GABA concentration in the cerebrospinal fluid to augment tonic current through GABAARs. Such an increase, however, generates multiple side effects, which impose significant limitations on the use of correspondent drugs. In contrast, activation/deactivation of s-GABAARs in a GABA-independent manner may provide a mechanism of regulation of tonic conductance without modification of extracellular GABA concentration, thus avoiding connected side effects. Although s-GABAARs have been detected in our earlier work, it is unclear whether they modulate neural signalling, or, due to their independence from the neurotransmitter, they provide just a stable background effect without much impact on neural crosstalk dynamics. Here, we focused on the causal relationship between s-GABAAR activity and signal integration in the rat's dentate gyrus granule cells to find that s-GABAARs play an important role in neural signal transduction. s-GABAARs shape the dynamics of phasic inhibitory responses, regulate the action potential generation machinery and control the coincidence detection window pertinent to excitatory input summation. Our results demonstrate that tonic inhibition delivered by s-GABAARs contributes to the key mechanisms that ensure implementation of neural signal filtering and integration, in a GABA-independent manner. This makes s-GABAAR a new and important actor in the regulation of long-term neural plasticity and a perspective target for TLE therapy.