Two-electrode Voltage-clamp Recordings in Xenopus laevis Oocytes: Reconstitution of Abscisic Acid Activation of SLAC1 Anion Channel via PYL9 ABA Receptor

Cellular mechanisms underlying carry-over effects after magnetic stimulation

Magnetic fields are widely used for neuromodulation in clinical settings. The intended effect of magnetic stimulation is that neural activity resumes its pre-stimulation state right after stimulation. Many theoretical and experimental works have focused on the cellular and molecular basis of the acute neural response to magnetic field. However, effects of magnetic stimulation can still last after the termination of the magnetic stimulation (named "carry-over effects"), which could generate profound effects to the outcome of the stimulation. However, the cellular and molecular mechanisms of carry-over effects are largely unknown, which renders the neural modulation practice using magnetic stimulation unpredictable. Here, we investigated carry-over effects at the cellular level, using the combination of micro-magnetic stimulation (µMS), electrophysiology, and computation modeling. We found that high frequency magnetic stimulation could lead to immediate neural inhibition in ganglion neurons from Aplysia californica, as well as persistent, carry-over inhibition after withdrawing the magnetic stimulus. Carry-over effects were found in the neurons that fired action potentials under a variety of conditions. The carry-over effects were also observed in the neurons when the magnetic field was applied across the ganglion sheath. The state of the neuron, specifically synaptic input and membrane potential fluctuation, plays a significant role in generating the carry-over effects after magnetic stimulation. To elucidate the cellular mechanisms of such carry-over effects under magnetic stimulation, we simulated a single neuron under magnetic stimulation with multi-compartment modeling. The model successfully replicated the carry-over effects in the neuron, and revealed that the carry-over effect was due to the dysfunction of the ion channel dynamics that were responsible for the initiation and sustaining of membrane excitability. A virtual voltage-clamp experiment revealed a compromised Na conductance and enhanced K conductance post magnetic stimulation, rendering the neurons incapable of generating action potentials and, therefore, leading to the carry over effects. Finally, both simulation and experimental results demonstrated that the carry-over effects could be controlled by disturbing the membrane potential during the post-stimulus inhibition period. Delineating the cellular and ion channel mechanisms underlying carry-over effects could provide insights to the clinical outcomes in brain stimulation using TMS and other modalities. This research incentivizes the development of novel neural engineering or pharmacological approaches to better control the carry-over effects for optimized clinical outcomes.

An affordable Three-Dimensional (3D) Printed Recording Chamber for Two-Electrode Voltage Clamp Electrophysiology

Two electrode voltage-clamp (TEVC) electrophysiology in Xenopus oocytes is a common approach to studying the physiology and pharmacology of membrane transport proteins. Undergraduates may learn to use TEVC methodology in neuroscience or physiology courses and/or in faculty-mentored research experiences. Challenges with the methodology include the cost of commercially available recording chambers, especially when a lab needs multiple copies, and the additional time and expertise needed to use agar bridges and to stabilize solution flow and minimize noise from solution aspiration. Offering a low-cost and accessible recording chamber that overcomes these challenges would lower the barriers to success for undergraduates while also supporting publication-quality recordings. To address these issues, we developed a recording chamber using stereolithography, a 3D printing process. The physiology (PhISio) recording chamber features two options for solution aspiration that allow for individual preferences, optimizes placement of pre-made agar bridges to achieve laminar flow and reduce the time delays in initiating daily experiments, and minimizes the challenges of changing solution height and aspiration noise during perfusion. We compared the functionality of the PhISio chamber with a commercially available Warner Instruments RC-1Z chamber in electrophysiological recordings of inwardly rectifying potassium channels expressed in Xenopus oocytes. The PhISio chamber produced equivalent results to the RC-1Z chamber with respect to time-dependent solution changes and has several operational advantages for both new and experienced electrophysiologists, providing an affordable and convenient alternative to commercially available TEVC recording chambers.

Extracellular malate induces stomatal closure via direct activation of guard-cell anion channel SLAC1 and stimulation of Ca2+ signalling

Plants secrete malate from guard cells to apoplast under stress conditions and exogenous malate induces stomatal closure. Malate is considered an extracellular chemical signal of stomatal closure. However, the molecular mechanism of malate-induced stomatal closure is not fully elucidated. We investigated responses of stomatal aperture, ion channels, and cytosolic Ca²⁺ to malate. A treatment with malate induced stomatal closure in Arabidopsis thaliana wild-type plants, but not in the mutants deficient in the slow (S-type) anion channel gene SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1). The treatment with malate increased S-type anion currents in guard-cell protoplasts of wild-type plants but not in the slac1 mutant. In addition, extracellular rather than intracellular application of malate increased the S-type currents of constitutively active mutants of SLAC1, which have kinase-independent activities, in a heterologous expression system using Xenopus oocytes. The treatment with malate transiently increased cytosolic Ca²⁺ concentration in the wild-type Arabidopsis guard cells and the malate-induced stomatal closure was inhibited by the Ca²⁺ channel blocker and the Ca²⁺ chelator. These results indicate that extracellular malate directly activates SLAC1 and simultaneously stimulates Ca²⁺ signalling in guard cells, resulting in steady and solid activation of SLAC1 for stomatal closure.

The CBL-CIPK Pathway in Plant Response to Stress Signals

Plants need to cope with multitudes of stimuli throughout their lifecycles in their complex environments. Calcium acts as a ubiquitous secondary messenger in response to numerous stresses and developmental processes in plants. The major Ca2+ sensors, calcineurin B-like proteins (CBLs), interact with CBL-interacting protein kinases (CIPKs) to form a CBL-CIPK signaling network, which functions as a key component in the regulation of multiple stimuli or signals in plants. In this review, we describe the conserved structure of CBLs and CIPKs, characterize the features of classification and localization, draw conclusions about the currently known mechanisms, with a focus on novel findings in response to multiple stresses, and summarize the physiological functions of the CBL-CIPK network. Moreover, based on the gradually clarified mechanisms of the CBL-CIPK complex, we discuss the present limitations and potential prospects for future research. These aspects may provide a deeper understanding and functional characterization of the CBL-CIPK pathway and other signaling pathways under different stresses, which could promote crop yield improvement via biotechnological intervention.

Effect of OBPs on the response of olfactory receptors

The technique of two-electrode voltage-clamp (TEVC) recording from the heterologous expression system of olfactory receptors (ORs) in Xenopus laevis oocytes has been widely used to deorphanize insect ORs, that is to identify specific ligands for each of them. However, there is a controversial issue on whether ORs are activated by the odorant/OBP complex or the odorant alone. The mechanism of interaction among odorants, odorant-binding proteins (OBPs) and ORs remains largely unknown, due to the limitations in the use of scientific and innovative methods. In this chapter, the modified Xenopus oocytes expression system combined with TEVC technique is used to approach this issue. We describe the experimental strategies and provide detailed protocols for recording the signals generated by ORs in response to odorant/OBP complex at different concentrations. Results obtained by this approach have revealed that the presence of OBPs in the system affects the selectivity and sensitivity responses of ORs. Such studies help understanding the molecular mechanism of odorant detection in peripheral nervous system.

MAP3Kinase-dependent SnRK2-kinase activation is required for abscisic acid signal transduction and rapid osmotic stress response

Abiotic stresses, including drought and salinity, trigger a complex osmotic-stress and abscisic acid (ABA) signal transduction network. The core ABA signalling components are snf1-related protein kinase2s (SnRK2s), which are activated by ABA-triggered inhibition of type-2C protein-phosphatases (PP2Cs). SnRK2 kinases are also activated by a rapid, largely unknown, ABA-independent osmotic-stress signalling pathway. Here, through a combination of a redundancy-circumventing genetic screen and biochemical analyses, we have identified functionally-redundant MAPKK-kinases (M3Ks) that are necessary for activation of SnRK2 kinases. These M3Ks phosphorylate a specific SnRK2/OST1 site, which is indispensable for ABA-induced reactivation of PP2C-dephosphorylated SnRK2 kinases. ABA-triggered SnRK2 activation, transcription factor phosphorylation and SLAC1 activation require these M3Ks in vitro and in plants. M3K triple knock-out plants show reduced ABA sensitivity and strongly impaired rapid osmotic-stress-induced SnRK2 activation. These findings demonstrate that this M3K clade is required for ABA- and osmotic-stress-activation of SnRK2 kinases, enabling robust ABA and osmotic stress signal transduction.

Cryo-EM structure of OSCA1.2 from Oryza sativa elucidates the mechanical basis of potential membrane hyperosmolality gating

Sensing and responding to environmental water deficiency and osmotic stresses are essential for the growth, development, and survival of plants. Recently, an osmolality-sensing ion channel called OSCA1 was discovered that functions in sensing hyperosmolality in Arabidopsis Here, we report the cryo-electron microscopy (cryo-EM) structure and function of an OSCA1 homolog from rice (Oryza sativa; OsOSCA1.2), leading to a model of how it could mediate hyperosmolality sensing and transport pathway gating. The structure reveals a dimer; the molecular architecture of each subunit consists of 11 transmembrane (TM) helices and a cytosolic soluble domain that has homology to RNA recognition proteins. The TM domain is structurally related to the TMEM16 family of calcium-dependent ion channels and lipid scramblases. The cytosolic soluble domain possesses a distinct structural feature in the form of extended intracellular helical arms that are parallel to the plasma membrane. These helical arms are well positioned to potentially sense lateral tension on the inner leaflet of the lipid bilayer caused by changes in turgor pressure. Computational dynamic analysis suggests how this domain couples to the TM portion of the molecule to open a transport pathway. Hydrogen/deuterium exchange mass spectrometry (HDXMS) experimentally confirms the conformational dynamics of these coupled domains. These studies provide a framework to understand the structural basis of proposed hyperosmolality sensing in a staple crop plant, extend our knowledge of the anoctamin superfamily important for plants and fungi, and provide a structural mechanism for potentially translating membrane stress to transport regulation.

Antinociceptive effects of new pyrazoles compounds mediated by the ASIC-1α channel, TRPV-1 and μMOR receptors

Pyrazoles are potent medicinal scaffolds and exhibit a wide spectrum of biological activities, such as analgesic, anti-inflammatory and antipyretic. In this paper we report on research we have performed with the aim of continuing the biological evaluation of the regio-isomeric pyrazole compounds, LQFM-020 (fluorine, para position), LQFM-021 (fluorine, meta position), and LQFM-039 (fluorine, ortho position) in models of pain induced by acidified saline, capsaicin, and formalin. We also investigated the mechanisms of action of these compounds via electrophysiological analyses using the two-electrode voltage-clamp technique and heterologous expression in Xenopus laevis oocytes. This enabled us to study different potassium channel subtypes: the ASIC-1α channel, TRPV-1, and μMOR receptors. Our results indicate that LQFM-020, LQFM-021, and LQFM-039 (15, 30 or 60 mg.kg^-1) compounds inhibited the nociceptive response induced by acidified saline in a dose-dependent manner. The dose of 30 mg.kg^-1 inhibited the nociceptive response induced by capsaicin by 53.3%, 51.4%, and 52.1%, respectively. In addition, we found that naloxone reverses the antinociceptive effect produced by the compounds in both phases of the formalin test. In electrophysiological analyses, we observed that the LQFM-020, LQFM-021, and LQFM-039 compounds did not modulate voltage-gated K + channel subtypes. In contrast, all the compounds tested inhibited the ASIC-1α channel at pH 4.5, with IC50-values of 96.1, 91.6, and 235.2 μM, respectively. All compounds also inhibited the TRPV-1 channel with IC50-values of 139.1, 212.5, and 159.1 μM, respectively. In contrast to the ASIC-1α and TRPV-1 targets, all compounds showed agonist activity on the μMOR receptor with an EC50-value of 117.4, 98.9, and 86.3 μM, respectively. We thus conclude that the ASIC-1α, TRPV-1, and μMOR channels are targets that are directly involved in the antinociceptive effect of LQFM-020, LQFM-021, and LQFM-039. Furthermore, the modifications of the fluorine positions in the phenyl analogs do not change the analgesic effect. However, LQFM-039 showed lower interaction with ASIC-1α channel.

Cytosolic malate and oxaloacetate activate S-type anion channels in Arabidopsis guard cells

Intracellular malate-starch interconversion plays an important role in stomatal movements. We investigated whether malate or oxaloacetate from the cytosolic membrane side regulate anion channels in the plasma membrane of Arabidopsis thaliana guard cells. Physiological concentrations of cytosolic malate have been reported in the range of 0.4-3 mM in leaf cells. Guard cell patch clamp and two-electrode oocyte voltage-clamp experiments were pursued. We show that a concentration of 1 mM cytosolic malate greatly activates S-type anion channels in Arabidopsis thaliana guard cells. Interestingly, 1 mM cytosolic oxaloacetate also activates S-type anion channels. Malate activation was abrogated at 10 mM malate and in SLAC1 anion channel mutant alleles. Interestingly, malate activation of S-type anion currents was disrupted at below resting cytosolic-free calcium concentrations ([Ca2+ ]cyt ), suggesting a key role for basal [Ca2+ ]cyt signaling. Cytosolic malate was not able to directly activate or enhance SLAC1-mediated anion currents in Xenopus oocytes, whereas in positive controls, cytosolic NaHCO3 enhanced SLAC1 activity, suggesting that malate may not directly modulate SLAC1. Cytosolic malate activation of S-type anion currents was impaired in ost1 and in cpk5/6/11/23 quadruple mutant guard cells. Together these findings show that these cytosolic organic anions function in guard cell 'plasma membrane' ion channel regulation.