Mutations M287L and Q266I in the glycine receptor α1 subunit change sensitivity to volatile anesthetics in oocytes and neurons, but not the minimal alveolar concentration in knockin mice

Metabolomics and Whole-Exome Sequencing in Patients with Differential Sensitivity to Sevoflurane: A Protocol for a Prospective Observational Trial

Introduction: Different sensitivity to volatile anesthetics in Drosophila, nematodes and mice is related to mutation of energy metabolism genes. In clinical practice, we find that the end-tidal sevoflurane concentration (ET_sevo) differs among patients at the same depth of anesthesia, indicating that the sensitivity to sevoflurane varies among patients. However, the underlying mechanism remains unclear. The sensitivity of an anesthetic is associated with the postoperative outcomes of patients and the mechanism of action of volatile anesthetics. We therefore propose this protocol to determine whether differences in metabolite profile and genetic variations contribute to patients' sensitivity to volatile anesthetics. Methods and Analysis: This is a single-centre, prospective observational study. 720 patients undergoing abdominal surgery were included. General anesthesia was induced with inhaled sevoflurane, a bolus of sufentanil (0.2-0.4 μg/kg) and cis-atracurium (0.2-0.3 mg/kg). The end-tidal sevoflurane concentration (ET_sevo) was adjusted to maintain a BIS (bispectral index) value between 40-60. The mean ET_sevo from 20 min after endotracheal intubation to 2 h after the beginning of surgery (steady state) was calculated for each patient. Patients were further divided into a high-sensitivity group (mean ET_sevo - SD) and a low-sensitivity group (mean ET_sevo + SD) to investigate the sensitivity to sevoflurane. Cases were paired from the high-sensitivity group (group H) and low-sensitivity group (group L) according to age, sex, body mass index (BMI), ASA physical status classification, vital signs, BIS, ephedrine use, sufentanildose, and cis-atracurium dose at anesthesia induction and steady state. Differences in metabolite levels, single nucleotide polymorphisms (SNPs) and protein-coding gene sequence variations between group H and group L will be determined through plasma metabolomics, whole-exome sequencing (WES), genome-wide association study (GWAS), and bioinformatics analyses. These results will be analysed to determine the reasons for the differential sensitivity to sevoflurane in humans. Ethics and Dissemination: This prospective observational study protocol has received ethical approval from the Ethical Committee of West China Hospital of Sichuan University on May 19, 2017 (Approval No. 78). Informed consent will be obtained before patient enrolment. The results will be submitted to international peer-review journals. Trial Registration Number: ChiCTR1800014327.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Anesthetic-sensitive ion channel modulation is associated with a molar water solubility cut-off

Background

NMDA receptor modulation by hydrocarbons is associated with a molar water solubility cut-off. Low-affinity phenolic modulation of GABA_A receptors is also associated with a cut-off, but at much lower molar solubility values. We hypothesized that other anesthetic-sensitive ion channels exhibit distinct cut-off effects associated with hydrocarbon molar water solubility, and that cut-off values are comparatively similar between related receptors than phylogenetically distant ones.

Methods

Glycine or GABA_A receptors or TREK-1, TRESK, Na_v1.2, or Na_v1.4 channels were expressed separately in frog oocytes. Two electrode voltage clamp techniques were used to study current responses in the presence and absence of hydrocarbon series from eight functional groups with progressively increasing size at saturated aqueous concentrations. Null response (cut-off) was defined by current measurements that were statistically indistinguishable between baseline and hydrocarbon exposure.

Results

Ion channels exhibited cut-off effects associated with hydrocarbon molar water solubility in the following order of decreasing solubility: Na_v1.2 ≈ Na_v1.4 ≳ TRESK ≈ TREK-1 > GABA_A >> glycine. Previously measured solubility cut-off values for NMDA receptors were intermediate between those for Na_v1.4 and TRESK.

Conclusions

Water solubility cut-off responses were present for all anesthetic-sensitive ion channels; distinct cut-off effects may exist for all cell surface receptors that are sensitive to volatile anesthetics. Suggested is the presence of amphipathic receptor sites normally occupied by water molecules that have dissociation constants inversely related to the cut-off solubility value. Poorly soluble hydrocarbons unable to reach concentrations sufficient to out-compete water for binding site access fail to modulate the receptor.

Mutations at beta N265 in γ-aminobutyric acid type A receptors alter both binding affinity and efficacy of potent anesthetics

Etomidate and propofol are potent general anesthetics that act via GABAA receptor allosteric co-agonist sites located at transmembrane β+/α- inter-subunit interfaces. Early experiments in heteromeric receptors identified βN265 (M2-15') on β2 and β3 subunits as an important determinant of sensitivity to these drugs. Mechanistic analyses suggest that substitution with serine, the β1 residue at this position, primarily reduces etomidate efficacy, while mutation to methionine eliminates etomidate sensitivity and might prevent drug binding. However, the βN265 residue has not been photolabeled with analogs of either etomidate or propofol. Furthermore, substituted cysteine modification studies find no propofol protection at this locus, while etomidate protection has not been tested. Thus, evidence of contact between βN265 and potent anesthetics is lacking and it remains uncertain how mutations alter drug sensitivity. In the current study, we first applied heterologous α1β2N265Cγ2L receptor expression in Xenopus oocytes, thiol-specific aqueous probe modification, and voltage-clamp electrophysiology to test whether etomidate inhibits probe reactions at the β-265 sidechain. Using up to 300 µM etomidate, we found both an absence of etomidate effects on α1β2N265Cγ2L receptor activity and no inhibition of thiol modification. To gain further insight into anesthetic insensitive βN265M mutants, we applied indirect structure-function strategies, exploiting second mutations in α1β2/3γ2L GABAA receptors. Using α1M236C as a modifiable and anesthetic-protectable site occupancy reporter in β+/α- interfaces, we found that βN265M reduced apparent anesthetic affinity for receptors in both resting and GABA-activated states. βN265M also impaired the transduction of gating effects associated with α1M236W, a mutation that mimics β+/α- anesthetic site occupancy. Our results show that βN265M mutations dramatically reduce the efficacy/transduction of anesthetics bound in β+/α- sites, and also significantly reduce anesthetic affinity for resting state receptors. These findings are consistent with a role for βN265 in anesthetic binding within the β+/α- transmembrane sites.

Open-channel structures of the human glycine receptor α1 full-length transmembrane domain

Glycine receptors play a major role in mediating fast inhibitory neurotransmission in the spinal cord and brain stem, yet their high-resolution structures remain unsolved. We determined open-channel structures of the full-length transmembrane domain (TMD) of the human glycine receptor α1-subunit (hGlyR-α1) using nuclear magnetic resonance (NMR) spectroscopy and electron micrographs. hGlyR-α1 TMD spontaneously forms pentameric Cl(-)-conducting channels, with structures sharing overall topology observed in crystal structures of homologous bacterial and nematode pentameric ligand-gated ion channels (pLGICs). However, the mammalian hGlyR-α1 structures present several distinctive features, including a shorter, pore-lining TM2 helix with helical unwinding near the C-terminal end, a TM3 helical kink at A288 that partially overlaps with the homologous ivermectin-binding site in GluCl, and a highly dynamic segment between S267(15') of TM2 and A288 that likely affects allosteric modulations of channel function. Our structures provide additional templates for identifying potential drug targets in GlyRs and other mammalian pLGICs.