Evaluation of microtransplantation of rat brain neurolemma into Xenopus laevis oocytes as a technique to study the effect of neurotoxicants on endogenous voltage-sensitive ion channels

Determination of Multiple Neurotransmitters through LC-MS/MS to Confirm the Therapeutic Effects of Althaea rosea Flower on TTX-Intoxicated Rats

Tetrodotoxin (TTX) inhibits neurotransmission in animals, and there is no specific antidote. In clinical practice in China, Althaea rosea (A. rosea flower) extract has been used to treat TTX poisoning. In this work, the efficacy of the ethyl acetate fraction extract of A. rosea flower in treating TTX poisoning in rats was investigated. A high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed to determine nine neurotransmitters in rat brain tissue, including γ-aminobutyric acid (GABA), dopamine (DA), 5-hydroxytryptamine (5-HT), noradrenaline (NE), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxyindole-3-acetic acid (5-HIAA), epinephrine (E), and tyramine (Tyn). The detoxifying effect of A. rosea flower was verified by comparing the changes in neurotransmitters' content in brain tissue before and after poisoning in rats. The assay was performed in multiple reaction monitoring mode. The quantification method was performed by plotting an internal-standard working curve with good linearity (R² > 0.9941) and sensitivity. Analyte recoveries were 94.04-107.53% (RSD < 4.21%). Results indicated that the levels of 5-HT, DA, E, and NE in the brains of TTX-intoxicated rats decreased, whereas the levels of GABA, Tyn, and 5-HIAA showed an opposite trend, and HVA and DOPAC were not detected. The levels of all seven neurotransmitters returned to normal after the gavage administration of ethyl acetate extract of A. rosea flower to prove that the ethyl acetate extract of A. rosea flower had a therapeutic effect on TTX poisoning. The work provided new ideas for studies on TTX detoxification.

Determination of potential toxicodynamic differences of pyrethroid insecticides on native voltage-sensitive sodium channels in juvenile versus adult rat brain

Microtransplantation of neurolemma tissue fragments from mammalian brain into the plasma membrane of Xenopus laevis oocytes is a tool to examine the endogenous structure and function of various ion channels and receptors associated with the central nervous system. Microtransplanted neurolemma can originate from a variety of sources, contain ion channels and receptors in their native configuration, and are applicable to examine diseases associated with different channelopathies. Here, we examined potential age-related differences in voltage-sensitive sodium channel (VSSC) expression and concentration-dependent responses to pyrethroids following the microtransplantation of juvenile or adult rat brain tissue (neurolemma) into X. laevis oocytes. Using automated western blotting, adult neurolemma exhibited a 2.5-fold higher level of expression of VSSCs compared with juvenile neurolemma. The predominant isoform expressed in both tissues was Na_v1.2. However, adult neurolemma expressed 2.8-fold more Na_v1.2 than juvenile and expressed Na_v1.6 at a significantly higher level (2.2-fold). Microtransplanted neurolemma elicited ion currents across the plasma membrane of oocytes following membrane depolarization using two electrode voltage clamp electrophysiology. A portion of this current was sensitive to tetrodotoxin (TTX) and this TTX-sensitive current was abolished when external sodium ion was replaced by choline ion, functionally demonstrating the presence of native VSSC. Increasing concentrations of permethrin or deltamethrin exhibited concentration-dependent increases in inward TTX-sensitive current in the presence of niflumic acid from both adult and juvenile tissues following a pulsed depolarization of the oocyte plasma membrane. Concentration-dependent response curves illustrate that VSSCs associated with juvenile neurolemma were up to 2.5-fold more sensitive to deltamethrin than VSSCs in adult neurolemma. In contrast, VSSCs from juvenile neurolemma were less sensitive to permethrin than adult VSSCs at lower concentrations (0.6-0.8-fold) but were more sensitive at higher concentrations (up to 2.4-fold). Nonetheless, because the expected concentrations in human brains following realistic exposure levels are approximately 21- (deltamethrin) to 333- (permethrin) times below the threshold concentration for response in rat neurolemma-injected oocytes, age-related differences, if any, are not likely to be toxicologically relevant.

Xenopus Oocytes as a Powerful Cellular Model to Study Foreign Fully-Processed Membrane Proteins

The use of Xenopus oocytes in electrophysiological and biophysical research constitutes a long and successful story, providing major advances to the knowledge of the function and modulation of membrane proteins, mostly receptors, ion channels, and transporters. Earlier reports showed that these cells are capable of correctly expressing heterologous proteins after injecting the corresponding mRNA or cDNA. More recently, the Xenopus oocyte has become an outstanding host-cell model to carry out detailed studies on the function of fully-processed foreign membrane proteins after their microtransplantation to the oocyte. This review focused on the latter overall process of transplanting foreign membrane proteins to the oocyte after injecting plasma membranes or purified and reconstituted proteins. This experimental approach allows for the study of both the function of mature proteins, with their native stoichiometry and post-translational modifications, and their putative modulation by surrounding lipids, mostly when the protein is purified and reconstituted in lipid matrices of defined composition. Remarkably, this methodology enables functional microtransplantation to the oocyte of membrane receptors, ion channels, and transporters from different sources including human post-mortem tissue banks. Despite the large progress achieved over the last decades on the structure, function, and modulation of neuroreceptors and ion channels in healthy and pathological tissues, many unanswered questions remain and, most likely, Xenopus oocytes will continue to help provide valuable responses.

Microtransplantation of Postmortem Native Synaptic mGluRs Receptors into Xenopus Oocytes for Their Functional Analysis

Metabotropic glutamate receptors (mGluRs) are membrane receptors that play a central role in the modulation of synaptic transmission and neuronal excitability and whose dysregulation is implicated in diverse neurological disorders. Most current understanding about the electrophysiological properties of such receptors has been determined using recombinant proteins. However, recombinant receptors do not necessarily recapitulate the properties of native receptors due to the lack of obligated accessory proteins, some of which are differentially expressed as function of developmental stage and brain region. To overcome this limitation, we sought to microtransplant entire synaptosome membranes from frozen rat cortex into Xenopus oocytes, and directly analyze the responses elicited by native mGluRs. We recorded ion currents elicited by 1 mM glutamate using two electrodes voltage clamp. Glutamate produced a fast ionotropic response (6 ± 0.3 nA) in all microtransplanted oocytes (n = 218 oocytes) and a delayed oscillatory response (52 ± 7 nA) in 73% of them. The participation of Group 1 mGluRs was confirmed by the presence of metabotropic oscillations during the administration of (±)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD; Group 1 mGluR agonist), and the absence of oscillations during co-administration of N-(1-adamantyl)quinoxaline-2-carboxamide (NPS 2390; Group 1 mGluR antagonist). Since both mGluR1 and mGluR5 belong to Group 1 mGluRs, further investigation revealed that mGluR1 antagonism with LY 456236 has little effect on metabotropic oscillations, while mGluR5 antagonism with 100 µM AZD 9272 has significant reduction of metabotropic currents elicited by ACPD and glutamate. We confirmed the expression of mGluR1 and mGluR5 in native synaptosomes by immunoblots, both of which are enhanced when compared to their counterpart proteins in rat cortex tissue lysates. Finally, these results demonstrate the merit of using microtransplantation of native synaptosomes for the study of mGluRs and the contribution of mGluR5 to the metabotropic glutamate signaling, providing a better tool for the understanding of the role of these receptors in neurological disorders.

Nicotinic acetylcholine receptors in the synganglion of the tick Ixodes ricinus: Functional characterization using membrane microtransplantation

Nicotinic acetylcholine receptors are an important class of excitatory receptors in the central nervous system of arthropods. In the ticks Ixodes ricinus, the functional and pharmacological properties of nicotinic receptors located in their neurons are still unknown. The objective of this study was to characterize the pharmacological properties of tick nicotinic receptors using membrane microtransplantation in Xenopus laevis oocytes and two-electrodes voltage clamp method. The membranes microtransplanted were extracted from the tick synganglion. We found that oocytes microtransplanted with tick synganglion membranes expressed nicotinic acetylcholine receptor subtypes which were activated by acetylcholine (1 mM) and nicotine (1 mM). Currents induced by pressure application of acetylcholine and nicotine were diminished by 10 nM α-bungarotoxin and methyllycaconitine, suggesting that they expressed two subtypes of nicotinic receptors, α-bungarotoxin-sensitive and -insensitive, respectively. In addition, we found that nicotine receptors expressed in the synganglion membranes were poorly sensitive to the neonicotinoid insecticides clothianidin (CLT), imidacloprid (IMI), acetamiprid (ACE) and thiamethoxam (TMX), in agreement with their lack of activity as acaricides. Interestingly, current amplitudes were strongly potentialized in the presence of 1 μM PNU-120596. CLT was more active as an agonist than IMI, TMX and ACE. Finally, we demonstrated that microtransplantation of purified membrane from the tick synganglion can be a valuable tool for the development and screening of compounds targeting tick nicotinic acetylcholine receptor subtypes.

Preservation of global synaptic excitatory to inhibitory ratio during long postmortem intervals

The study of postsynaptic excitation to inhibition (E/I ratio) imbalances in human brain diseases, is a highly relevant functional measurement poorly investigated due to postmortem degradation of synaptic receptors. We show that near-simultaneous recording of microtransplanted synaptic receptors after simulated morgue conditions allows the determination of the postsynaptic E/I ratio for at least 120 h after death, expanding the availability and use of human diseased tissue stored in brain banks.

Using Xenopus oocytes in neurological disease drug discovery

Introduction: Neurological diseases present a difficult challenge in drug discovery. Many of the current treatments have limited efficiency or result in a variety of debilitating side effects. The search of new therapies is of a paramount importance, since the number of patients that require a better treatment is growing rapidly. As an in vitro model, Xenopus oocytes provide the drug developer with many distinct advantages, including size, durability, and efficiency in exogenous protein expression. However, there is an increasing need to refine the recent breakthroughs.Areas covered: This review covers the usage and recent advancements of Xenopus oocytes for drug discovery in neurological diseases from expression and functional measurement techniques to current applications in Alzheimer's disease, painful neuropathies, and amyotrophic lateral sclerosis (ALS). The existing limitations of Xenopus oocytes in drug discovery are also discussed.Expert opinion: With the rise of aging population and neurological disorders, Xenopus oocytes, will continue to play an important role in understanding the mechanism of the disease, identification and validation of novel molecular targets, and drug screening, providing high-quality data despite the technical limitations. With further advances in oocytes-related techniques toward an accurate modeling of the disease, the diagnostics and treatment of neuropathologies will be becoming increasing personalized.