Nitric oxide potentiation of the homomeric ρ1 GABA(C) receptor function

Nitrergic modulation of ion channel function in regulating neuronal excitability

Nitric oxide (NO) signaling in the brain provides a wide range of functional properties in response to neuronal activity. NO exerts its effects through different signaling pathways, namely, through the canonical soluble guanylyl cyclase-mediated cGMP production route and via post-translational protein modifications. The latter pathways comprise cysteine S-nitrosylation and 3-nitrotyrosination of distinct tyrosine residues. Many ion channels are targeted by one or more of these signaling routes, which leads to their functional regulation under physiological conditions or facilities their dysfunction leading to channelopathies in many pathologies. The resulting alterations in ion channel function changes neuronal excitability, synaptic transmission, and action potential propagation. Transient and activity-dependent NO production mediates reversible ion channel modifications via cGMP and S-nitrosylation signaling, whereas more pronounced and longer-term NO production during conditions of elevated oxidative stress leads to increasingly cumulative and irreversible protein 3-nitrotyrosination. The complexity of this regulation and vast variety of target ion channels and their associated functional alterations presents a challenging task in assessing and understanding the role of NO signaling in physiology and disease.

Redox Post-translational Modifications of Protein Thiols in Brain Aging and Neurodegenerative Conditions-Focus on S-Nitrosation

Reactive oxygen species and reactive nitrogen species (RONS) are by-products of aerobic metabolism. RONS trigger a signaling cascade that can be transduced through oxidation-reduction (redox)-based post-translational modifications (redox PTMs) of protein thiols. This redox signaling is essential for normal cellular physiology and coordinately regulates the function of redox-sensitive proteins. It plays a particularly important role in the brain, which is a major producer of RONS. Aberrant redox PTMs of protein thiols can impair protein function and are associated with several diseases. This mini review article aims to evaluate the role of redox PTMs of protein thiols, in particular S-nitrosation, in brain aging, and in neurodegenerative diseases. It also discusses the potential of using redox-based therapeutic approaches for neurodegenerative conditions.

Transient receptor potential (TRP) channels: Biosensors for redox environmental stimuli and cellular status

Transient receptor potential (TRP) channels are a family of cation channels that depolarizes the membrane potential and regulates intracellular concentrations of cations such as Ca²⁺. TRP channels are also known to function as "biosensors" to detect changes of the surrounding environment and cellular status. Lines of evidence have unveiled that numerous proteins are subject to redox modification and subsequent signaling. For example, TRPM2, TRPC5, TRPV1, and TRPA1 are known as redox sensors activated by hydrogen peroxide (H₂O₂), nitric oxide (NO), and electrophiles. Thus, these channels facilitate the influx of cations which in turn triggers the appropriate cellular responses against environmental redox stimuli and cellular redox status. In this review, we focus on the recent findings regarding the functions of TRP channels in relation to other ion channels, and other proteins which also go through redox modification of cysteine (Cys) residues. We aim to understand the structural and molecular basis of the redox-sensing mechanisms of TRP channels in exerting various functions under physiological conditions as well as pathological conditions such as cancer malignancy. Their future potential as drug targets will also be discussed.

Negative modulation of the GABAA ρ1 receptor function by l-cysteine

l-Cysteine is an endogenous sulfur-containing amino acid with multiple and varied roles in the central nervous system, including neuroprotection and the maintenance of the redox balance. However, it was also suggested as an excitotoxic agent implicated in the pathogenesis of neurological disorders such as Parkinson's and Alzheimer's disease. l-Cysteine can modulate the activity of ionic channels, including voltage-gated calcium channels and glutamatergic NMDA receptors, whereas its effects on GABAergic neurotransmission had not been studied before. In the present work, we analyzed the effects of l-cysteine on responses mediated by homomeric GABA_A ρ1 receptors, which are known for mediating tonic γ-aminobutyric acid (GABA) responses in retinal neurons. GABA_A ρ1 receptors were expressed in Xenopus laevis oocytes and GABA-evoked chloride currents recorded by two-electrode voltage-clamp in the presence or absence of l-cysteine. l-Cysteine antagonized GABA_A ρ1 receptor-mediated responses; inhibition was dose-dependent, reversible, voltage independent, and susceptible to GABA concentration. Concentration-response curves for GABA were shifted to the right in the presence of l-cysteine without a substantial change in the maximal response. l-Cysteine inhibition was insensitive to chemical protection of the sulfhydryl groups of the ρ1 subunits by the irreversible alkylating agent N-ethyl maleimide. Our results suggest that redox modulation is not involved during l-cysteine actions and that l-cysteine might be acting as a competitive antagonist of the GABA_A ρ1 receptors.

Dynamic Regulation of the GABAA Receptor Function by Redox Mechanisms

Oxidizing and reducing agents, which are currently involved in cell metabolism and signaling pathways, can regulate fast inhibitory neurotransmission mediated by GABA receptors in the nervous system. A number of in vitro studies have shown that diverse redox compounds, including redox metabolites and reactive oxygen and nitrogen species, modulate phasic and tonic responses mediated by neuronal GABAA receptors through both presynaptic and postsynaptic mechanisms. We review experimental data showing that many redox agents, which are normally present in neurons and glia or are endogenously generated in these cells under physiologic states or during oxidative stress (e.g., hydrogen peroxide, superoxide and hydroxyl radicals, nitric oxide, ascorbic acid, and glutathione), induce potentiating or inhibiting actions on different native and recombinant GABAA receptor subtypes. Based on these results, it is thought that redox signaling might represent a homeostatic mechanism that regulates the function of synaptic and extrasynaptic GABAA receptors in physiologic and pathologic conditions.

Enhancement of tonic and phasic GABAergic currents following nitric oxide synthase inhibition in hippocampal CA1 pyramidal neurons

Nitric oxide (NO) is involved in synaptic plasticity in the hippocampus through different presynaptic and postsynaptic mechanisms that include the modulation of the GABAergic neurotransmission. Inhibitory synapses on hippocampal pyramidal neurons are known to possess the molecular machinery for retrograde NO-signaling, but the modulation of GABAARs function by NO in these neurons and the mechanisms of action involved have not been fully characterized. Here we show that suppression of the endogenous NO generation by the nitric oxide synthase (NOS) inhibitor L-NAME produces significant and reversible increases in the magnitude of both tonic and phasic GABAergic currents in CA1 hippocampal pyramidal neurons. GABA-evoked chloride currents were measured in the presence or absence of L-NAME using whole-cell patch-clamp recordings in acute hippocampal slices from young adult mice. Enhancement of the tonic GABA responses induced by L-NAME was insensitive to TTX and decreased by co-incubation with the NO donor DEA/NO. Applications of DEA/NO alone did not produce significant effects on tonic GABA responses. L-NAME treatment also increased the amplitude of phasic GABAergic currents evoked by GABA-puffs. Our results indicate that the extent of tonic and phasic inhibition mediated by GABAA receptors in CA1 hippocampal pyramidal neurons is affected by endogenous NO production.

An intracellular redox sensor for reactive oxygen species at the M3-M4 linker of GABAA ρ1 receptors

BACKGROUND AND PURPOSE

Reactive oxygen species (ROS) are normally involved in cell oxidative stress but also play a role as cellular messengers in redox signalling; for example, modulating the activity of neurotransmitter receptors and ion channels. However, the direct actions of ROS on GABAA receptors were not previously demonstrated. In the present work, we studied the effects of ROS on GABAA ρ1 receptor function.

EXPERIMENTAL APPROACH

GABAA ρ1 receptors were expressed in oocytes and GABA-evoked responses electrophysiologically recorded in the presence or absence of ROS. Chemical protection of cysteines by selective sulfhydryl reagents and site-directed mutagenesis studies were used to identify protein residues involved in ROS actions.

KEY RESULTS

GABAA ρ1 receptor-mediated responses were significantly enhanced in a concentration-dependent and reversible manner by H₂O₂. Potentiating effects were attenuated by a free radical scavenger, lipoic acid or an inhibitor of the Fenton reaction, deferoxamine. Each ρ1 subunit contains only three cysteine residues, two extracellular at the Cys-loop (C¹⁷⁷ and C¹⁹¹) and one intracellular (C³⁶⁴) at the M3-M4 linker. Mutant GABAA ρ1 receptors in which C³⁶⁴ was exchanged by alanine were completely insensitive to modulation, implying that this site, rather than a cysteine in the Cys-loop, is essential for ROS modulation.

CONCLUSION AND IMPLICATIONS

Our results show that the function of GABAA ρ1 receptors is enhanced by ROS and that the intracellular C³⁶⁴ is the sensor for ROS actions.

Advantages of an antagonist: bicuculline and other GABA antagonists

The convulsant alkaloid bicuculline continues to be investigated more than 40 years after the first publication of its action as an antagonist of receptors for the inhibitory neurotransmitter GABA. This historical perspective highlights key aspects of the discovery of bicuculline as a GABA antagonist and the sustained interest in this and other GABA antagonists. The exciting advances in the molecular biology, pharmacology and physiology of GABA receptors provide a continuing stimulus for the discovery of new antagonists with increasing selectivity for the myriad of GABA receptor subclasses. Interesting GABA antagonists not structurally related to bicuculline include gabazine, salicylidene salicylhydrazide, RU5135 and 4-(3-biphenyl-5-(4-piperidyl)-3-isoxazole. Bicuculline became the benchmark antagonist for what became known as GABAA receptors, but not all ionotropic GABA receptors are susceptible to bicuculline. In addition, not all GABAA receptor antagonists are convulsants. Thus there are still surprises in store as the study of GABA receptors evolves.