Identification of the Acid-Sensitive Site Critical for Chloral Hydrate (CH) Activation of the Proton-Activated Chloride Channel.

The transmembrane protein TMEM206 was recently identified as the molecular basis of the extracellular proton-activated Cl- channel (PAC), which plays an essential role in neuronal death in ischemia-reperfusion. The PAC channel is activated by extracellular acid, but the proton-sensitive mechanism remains unclear, although different acid-sensitive pockets have been suggested based on the cryo-EM structure of the human PAC (hPAC) channel. In the present study, we firstly identified two acidic amino acid residues that removed the pH-dependent activation of the hPAC channel by neutralization all the conservative negative charged residues located in the extracellular domain of the hPAC channel and some positively charged residues at the hotspot combined with two-electrode voltage-clamp (TEVC) recording in the Xenopus oocytes system. Double-mutant cycle analysis and double cysteine mutant of these two residues proved that these two residues cooperatively form a proton-sensitive site. In addition, we found that chloral hydrate activates the hPAC channel depending on the normal pH sensitivity of the hPAC channel. Furthermore, the PAC channel knock-out (KO) male mice (C57BL/6J) resist chloral hydrate-induced sedation and hypnosis. Our study provides a molecular basis for understanding the proton-dependent activation mechanism of the hPAC channel and a novel drug target of chloral hydrate.SIGNIFICANCE STATEMENT Proton-activated Cl- channel (PAC) channels are widely distributed in the nervous system and play a vital pathophysiological role in ischemia and endosomal acidification. The main discovery of this paper is that we identified the proton activation mechanism of the human proton-activated chloride channel (hPAC). Intriguingly, we also found that anesthetic chloral hydrate can activate the hPAC channel in a pH-dependent manner. We found that the chloral hydrate activates the hPAC channel and needs the integrity of the pH-sensitive site. In addition, the PAC channel knock-out (KO) mice are resistant to chloral hydrate-induced anesthesia. The study on PAC channels' pH activation mechanism enables us to better understand PAC's biophysical mechanism and provides a novel target of chloral hydrate.

Identification of Sodium- and Chloride-Sensitive Sites in the Slack Channel.

The Slack channel (KCNT1, Slo2.2) is a sodium-activated and chloride-activated potassium channel that regulates heart rate and maintains the normal excitability of the nervous system. Despite intense interest in the sodium gating mechanism, a comprehensive investigation to identify the sodium-sensitive and chloride-sensitive sites has been missing. In the present study, we identified two potential sodium-binding sites in the C-terminal domain of the rat Slack channel by conducting electrophysical recordings and systematic mutagenesis of cytosolic acidic residues in the rat Slack channel C terminus. In particular, by taking advantage of the M335A mutant, which results in the opening of the Slack channel in the absence of cytosolic sodium, we found that among the 92 screened negatively charged amino acids, E373 mutants could completely remove sodium sensitivity of the Slack channel. In contrast, several other mutants showed dramatic decreases in sodium sensitivity but did not abolish it altogether. Furthermore, molecular dynamics (MD) simulations performed at the hundreds of nanoseconds timescale revealed one or two sodium ions at the E373 position or an acidic pocket composed of several negatively charged residues. Moreover, the MD simulations predicted possible chloride interaction sites. By screening predicted positively charged residues, we identified R379 as a chloride interaction site. Thus, we conclude that the E373 site and the D863/E865 pocket are two potential sodium-sensitive sites, while R379 is a chloride interaction site in the Slack channel.SIGNIFICANCE STATEMENT The research presented here identified two distinct sodium and one chloride interaction sites located in the intracellular C-terminal domain of the Slack (Slo2.2, KCNT1) channel. Identification of the sites responsible for the sodium and chloride activation of the Slack channel sets its gating property apart from other potassium channels in the BK channel family. This finding sets the stage for future functional and pharmacological studies of this channel.

The Slack Channel Regulates Anxiety-Like Behaviors via Basolateral Amygdala Glutamatergic Projections to Ventral Hippocampus.

Anxiety disorders are a series of mental disorders characterized by anxiety and fear, but the molecular basis of these disorders remains unclear. In the present study, we find that the global Slack KO male mice exhibit anxious behaviors, whereas the Slack Y777H male mice manifest anxiolytic behaviors. The expression of Slack channels is rich in basolateral amygdala (BLA) glutamatergic neurons and downregulated in chronic corticosterone-treated mice. In addition, electrophysiological data show enhanced excitability of BLA glutamatergic neurons in the Slack KO mice and decreased excitability of these neurons in the Slack Y777H mice. Furthermore, the Slack channel deletion in BLA glutamatergic neurons is sufficient to result in enhanced avoidance behaviors, whereas Kcnt1 gene expression in the BLA or BLA-ventral hippocampus (vHPC) glutamatergic projections reverses anxious behaviors of the Slack KO mice. Our study identifies the role of the Slack channel in controlling anxious behaviors by decreasing the excitability of BLA-vHPC glutamatergic projections, providing a potential target for anxiolytic therapies.SIGNIFICANCE STATEMENT Anxiety disorders are a series of mental disorders characterized by anxiety and fear, but the molecular basis of these disorders remains unclear. Here, we examined the behaviors of loss- and gain-of-function of Slack channel mice in elevated plus maze and open field tests and found the anxiolytic role of the Slack channel. By altering the Slack channel expression in the specific neuronal circuit, we demonstrated that the Slack channel played its anxiolytic role by decreasing the excitability of BLA-vHPC glutamatergic projections. Our data reveal the role of the Slack channel in the regulation of anxiety, which may provide a potential molecular target for anxiolytic therapies.

Two types of peptides derived from the neurotoxin GsMTx4 inhibit a mechanosensitive potassium channel by modifying the mechanogate.

Atrial fibrillation is the most common sustained cardiac arrhythmia in humans. Current atrial fibrillation antiarrhythmic drugs have limited efficacy and carry the risk of ventricular proarrhythmia. GsMTx4, a mechanosensitive channel-selective inhibitor, has been shown to suppress arrhythmias through the inhibition of stretch-activated channels (SACs) in the heart. The cost of synthesizing this peptide is a major obstacle to clinical use. Here, we studied two types of short peptides derived from GsMTx4 for their effects on a stretch-activated big potassium channel (SAKcaC) from the heart. Type I, a 17-residue peptide (referred to as Pept 01), showed comparable efficacy, whereas type II (i.e., Pept 02), a 10-residue peptide, exerted even more potent inhibitory efficacy on SAKcaC compared with GsMTx4. We identified through mutagenesis important sequences required for peptide functions. In addition, molecular dynamics simulations revealed common structural features with a hydrophobic head followed by a positively charged protrusion that may be involved in peptide channel-lipid interactions. Furthermore, we suggest that these short peptides may inhibit SAKcaC through a specific modification to the mechanogate, as the inhibitory effects for both types of peptides were mostly abolished when tested with a mechano-insensitive channel variant (STREX-del) and a nonmechanosensitive big potassium (mouse Slo1) channel. These findings may offer an opportunity for the development of a new class of drugs in the treatment of cardiac arrhythmia generated by excitatory SACs in the heart.

The neuropeptide GsMTx4 inhibits a mechanosensitive BK channel through the voltage-dependent modification specific to mechano-gating.

The cardiac mechanosensitive BK (Slo1) channels are gated by Ca2+, voltage, and membrane stretch. The neuropeptide GsMTx4 is a selective inhibitor of mechanosensitive (MS) channels. It has been reported to suppress stretch-induced cardiac fibrillation in the heart, but the mechanism underlying the specificity and even the targeting channel(s) in the heart remain elusive. Here, we report that GsMTx4 inhibits a stretch-activated BK channel (SAKcaC) in the heart through a modulation specific to mechano-gating. We show that membrane stretching increases while GsMTx4 decreases the open probability (Po) of SAKcaC. These effects were mostly abolished by the deletion of the STREX axis-regulated (STREX) exon located between RCK1 and RCK2 domains in BK channels. Single-channel kinetics analysis revealed that membrane stretch activates SAKcaC by prolonging the open-time duration (τO) and shortening the closed-time constant (τC). In contrast, GsMTx4 reversed the effects of membrane stretch, suggesting that GsMTx4 inhibits SAKcaC activity by interfering with mechano-gating of the channel. Moreover, GsMTx4 exerted stronger efficacy on SAKcaC under membrane-hyperpolarized/resting conditions. Molecular dynamics simulation study revealed that GsMTx4 appeared to have the ability to penetrate deeply within the bilayer, thus generating strong membrane deformation under the hyperpolarizing/resting conditions. Immunostaining results indicate that BK variants containing STREX are also expressed in mouse ventricular cardiomyocytes. Our results provide common mechanisms of peptide actions on MS channels and may give clues to therapeutic suppression of cardiac arrhythmias caused by excitatory currents through MS channels under hyper-mechanical stress in the heart.

The activation of GABAergic neurons in the hypothalamic tuberomammillary nucleus attenuates sevoflurane and propofol-induced anesthesia in mice.

Background: The histaminergic neurons in the hypothalamic tuberomammillary nucleus (TMN) have been suggested to play a vital role in maintaining a rising state. But the neuronal types of the TMN are in debate and the role of GABAergic neurons remains unclear. Methods: In the present study, we examined the role of TMN GABAergic neurons in general anesthesia using chemogenetics and optogenetics strategies to regulate the activity of TMN GABAergic neurons. Results: The results indicated that either chemogenetic or optogenetic activation of TMN GABAergic neurons in mice decreased the effect of sevoflurane and propofol anesthesia. In contrast, inhibition of the TMN GABAergic neurons facilitates the sevoflurane anesthesia effect. Conclusion: Our results suggest that the activity of TMN GABAergic neurons produces an anti-anesthesia effect in loss of consciousness and analgesia.

The Slack Channel Deletion Causes Mechanical Pain Hypersensitivity in Mice.

The role of the Slack (also known as Slo2.2, KNa1.1, or KCNT1) channel in pain-sensing is still in debate on which kind of pain it regulates. In the present study, we found that the Slack-/- mice exhibited decreased mechanical pain threshold but normal heat and cold pain sensitivity. Subsequently, X-gal staining, in situ hybridization, and immunofluorescence staining revealed high expression of the Slack channel in Isolectin B4 positive (IB4+) neurons in the dorsal root ganglion (DRG) and somatostatin-positive (SOM+) neurons in the spinal cord. Patch-clamp recordings indicated the firing frequency was increased in both small neurons in DRG and spinal SOM+ neurons in the Slack-/- mice whereas no obvious slow afterhyperpolarization was observed in both WT mice and Slack-/- mice. Furthermore, we found Kcnt1 gene expression in spinal SOM+ neurons in Slack-/- mice partially relieved the mechanical pain hypersensitivity of Slack-/- mice and decreased AP firing rates of the spinal SOM+ neurons. Finally, deletion of the Slack channel in spinal SOM+ neurons is sufficient to result in mechanical pain hypersensitivity in mice. In summary, our results suggest the important role of the Slack channel in the regulation of mechanical pain-sensing both in small neurons in DRG and SOM+ neurons in the spinal dorsal horn.

Up-regulation of antioxidative proteins TRX1, TXNL1 and TXNRD1 in the cortex of PTZ kindling seizure model mice.

Oxidative stress has been considered as one of pathogenesis of brain damage led by epilepsy. Reducing oxidative stress can ameliorate brain damage during seizures. However, expression levels of important antioxidative enzymes such as thioredoxin-1 (TRX1), thioredoxin-like 1 protein (TXNL1) and thioredoxin reductase 1 (TXNRD1) during seizures have not been investigated. In this study, we examined protein and mRNA expression levels of TRX1, TXNL1 and TXNRD1 in different brain regions in PTZ induced seizure model mice. We found that protein expression levels of TRX1, TXNL1 and TXNRD1 are simultaneously up-regulated by 2- or 3-fold in the cortex of both acute and chronic seizure model mice. But there is no unified expression pattern change of these enzymes in the hippocampus, cerebellum and diencephalon in the seizure model mice. Less extent up-regulation of mRNA expression of these enzymes were also observed in the cortex of seizure mice. These data suggest that antioxidative enzymes may provide a protective effect against oxidative stress in the cortex during seizures.