The use of Xenopus laevis oocytes for the functional characterization of heterologously expressed membrane proteins

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Cannabidiol Strengthening of Gastric Tight Junction Complexes Analyzed in an Improved Xenopus Oocyte Assay

Cannabidiol (CBD), the non-psychoactive compound derived from the cannabis plant, has gained attention in recent years as a remedy against gastrointestinal disorders ranging from nausea and inflammation to abdominal pain. Recent advances demonstrated an effect on inflammatory pathways and barrier proteins. However, information on possible direct effects is scarce and needs to be addressed, as applications are currently increasing in popularity. To accomplish this, we have employed Xenopus laevis oocytes as a heterologous expression system for analysis of the direct effects on stomach-specific claudins and further developed tight junction (TJ) protein interaction assays. Human claudin-4, claudin-5, and claudin-18.2 were expressed in Xenopus oocytes, clustered in pairs to form contact areas, and analyzed in a two-cell model approach, including measurement of the contact area and contact strength. CLDN4/5/18 + CLDN4/5/18 oocyte pairs were incubated with 20 µM CBD or with 40 µM CBD and were compared to cells without CBD treatment (ctrl). For interaction analysis, the contact area was measured after 24 h and 48 h. Whereas CBD did not affect the size of the protein interaction area, Double Orbital Challenge experiments revealed an increased contact strength after 24 h incubation with CBD. In addition, the Xenopus oocyte experiments were accompanied by an analysis of claudin-4, -5, and -18 expression in gastric epithelium by immunoblotting and immunohistochemistry. Claudin-4, -5, and -18 were strongly expressed, indicating a major role for gastric epithelial barrier function. In summary, our study shows direct effects of 40 µM CBD on Xenopus oocytes heterologously expressing a stomach-specific claudin combination, indicating a supportive and beneficial effect of CBD on gastric TJ proteins.

Screening of Saccharomyces cerevisiae metabolite transporters by 13C isotope substrate labeling

The transportome of Saccharomyces cerevisiae comprises approximately 340 membrane-bound proteins, of which very few are well-characterized. Elucidating transporter proteins' function is essential not only for understanding central cellular processes in metabolite exchange with the external milieu but also for optimizing the production of value-added compounds in microbial cell factories. Here, we describe the application of 13C-labeled stable isotopes and detection by targeted LC-MS/MS as a screening tool for identifying Saccharomyces cerevisiae metabolite transporters. We compare the transport assay's sensitivity, reproducibility, and accuracy in yeast transporter mutant cell lines and Xenopus oocytes. As proof of principle, we analyzed the transport profiles of five yeast amino acid transporters. We first cultured yeast transporter deletion or overexpression mutants on uniformly labeled 13C-glucose and then screened their ability to facilitate the uptake or export of an unlabeled pool of amino acids. Individual transporters were further studied by heterologous expression in Xenopus oocytes, followed by an uptake assay with 13C labeled yeast extract. Uptake assays in Xenopus oocytes showed higher reproducibility and accuracy. Although having lower accuracy, the results from S. cerevisiae indicated the system's potential for initial high-throughput screening for native metabolite transporters. We partially confirmed previously reported substrates for all five amino acid transporters. In addition, we propose broader substrate specificity for two of the transporter proteins. The method presented here demonstrates the application of a comprehensive screening platform for the knowledge expansion of the transporter-substrate relationship for native metabolites in S. cerevisiae.

Do Amino Acid Antiporters Have Asymmetric Substrate Specificity?

Amino acid antiporters mediate the 1:1 exchange of groups of amino acids. Whether substrate specificity can be different for the inward and outward facing conformation has not been investigated systematically, although examples of asymmetric transport have been reported. Here we used LC-MS to detect the movement of ¹²C- and ¹³C-labelled amino acid mixtures across the plasma membrane of Xenopus laevis oocytes expressing a variety of amino acid antiporters. Differences of substrate specificity between transporter paralogs were readily observed using this method. Our results suggest that antiporters are largely symmetric, equalizing the pools of their substrate amino acids. Exceptions are the antiporters y⁺LAT1 and y⁺LAT2 where neutral amino acids are co-transported with Na⁺ ions, favouring their import. For the antiporters ASCT1 and ASCT2 glycine acted as a selective influx substrate, while proline was a selective influx substrate of ASCT1. These data show that antiporters can display non-canonical modes of transport.

Targeting Insect Olfaction in vivo and in vitro Using Functional Imaging

Insects decode volatile chemical signals from its surrounding environment with the help of its olfactory system, in a fast and reliable manner for its survival. In order to accomplish this task, odorant receptors (ORs) expressed in olfactory sensory neurons (OSNs) in the fly's antenna process such odor information. In order to study such a sophisticated process, we require access to the sensory neurons to perform functional imaging. In this article, we present different preparations to monitor odor information processing in Drosophila melanogaster OSNs using functional imaging of their Ca2+ dynamics. First, we established an in vivo preparation to image specific OSN population expressing the fluorescent Ca2+ reporter GCaMP3 during OR activation with airborne odors. Next, we developed a method to extract and to embed OSNs in a silica hydrogel with OR activation by dissolved odors. The odor response dynamics under these different conditions was qualitatively similar which indicates that the reduction of complexity did not affect the concentration dependence of odor responses at OSN level.

Patch Clamp: The First Four Decades of a Technique That Revolutionized Electrophysiology and Beyond

Forty years ago, the introduction of a new electrophysiological technique, the patch clamp, revolutionized the fields of Cellular Physiology and Biophysics, providing for the first time the possibility of describing the behavior of a single protein, an ion-permeable channel of the cell plasma membrane, in its physiological environment. The new approach was actually much more potent and versatile than initially envisaged, and it has evolved into several different modalities that have radically changed our knowledge of how cells (not only the classical "electrically excitable "ones, such as nerves and muscles) use electrical signaling to modulate and organize their activity. This review aims at telling the history of the background from which the new technique evolved and at analyzing some of its more recent developments.

In Vivo and In Vitro Characterization of Cyclase and Phosphodiesterase Rhodopsins

Rhodopsins with enzymatic activity were found in microbes, in 2004 hypothetically from sequence data and since 2014 by experimental proof. So far three different types are known: light-activated guanylyl cyclase opsins (Cyclop) in fungi, light-inhibited two-component guanylyl cyclase opsins (2c-Cyclop) in green algae, and rhodopsin phosphodiesterases (RhoPDE) in choanoflagellates. They are integral membrane proteins with eight transmembrane helices (TM), different to the other microbial (type I) rhodopsins with 7 TM. Therefore, we propose a classification as type Ib rhodopsins for opsins with 8 TM and type Ia for the ones with 7 TM. To characterize those rhodopsins or their mutants, the expression in Xenopus laevis oocytes proved to be an efficient strategy. Functional analysis was initially performed "in oocyte" (in vivo), but more detailed characterization can be obtained with an in vitro assay. In this chapter, we describe procedures how to extract membranes from oocytes after cRNA microinjection and heterologous protein expression. Enzymatic activity of these membranes is then analyzed under different illumination conditions. In addition, fluorescent labeling of the rhodopsins is employed to quantify the expression level and the absolute activity of designed mutants. We discuss strengths and pitfalls, associated with this expression system, and strategies for selecting potentially useful optogenetic tools.

Codon Optimization of Insect Odorant Receptor Genes May Increase Their Stable Expression for Functional Characterization in HEK293 Cells

Insect odorant receptor (OR) genes are routinely expressed in Human Embryonic Kidney (HEK) 293 cells for functional characterization ("de-orphanization") using transient or stable expression. However, progress in this research field has been hampered because some insect ORs are not functional in this system, which may be due to insufficient protein levels. We investigated whether codon optimization of insect OR sequences for expression in human cells could facilitate their functional characterization in HEK293 cells with stable and inducible expression. We tested the olfactory receptor co-receptor (Orco) proteins from the bark beetles Ips typographus ("Ityp") and Dendroctonus ponderosae ("Dpon"), and six ItypORs previously characterized in Xenopus laevis oocytes and/or HEK cells. Western blot analysis indicated that codon optimization yielded increased cellular protein levels for seven of the eight receptors. Our experimental assays demonstrated that codon optimization enabled functional characterization of two ORs (ItypOR25 and ItypOR29) which are unresponsive when expressed from wildtype (non-codon optimized) genes. Similar to previous Xenopus oocyte recordings, ItypOR25 responded primarily to the host/conifer monoterpene (+)-3-carene. ItypOR29 responded primarily to (+)-isopinochamphone and similar ketones produced by fungal symbionts and trees. Codon optimization also resulted in significantly increased responses in ItypOR49 to its pheromone ligand (R)-(-)-ipsdienol, and improved responses to the Orco agonist VUAA1 in ItypOrco. However, codon optimization did not result in functional expression of DponOrco, ItypOR23, ItypOR27, and ItypOR28 despite higher protein levels as indicated by Western blots. We conclude that codon optimization may enable or improve the functional characterization of insect ORs in HEK cells, although this method is not sufficient for all ORs that are not functionally expressed from wildtype genes.

Coordinated action of multiple transporters in the acquisition of essential cationic amino acids by the intracellular parasite Toxoplasma gondii

Intracellular parasites of the phylum Apicomplexa are dependent on the scavenging of essential amino acids from their hosts. We previously identified a large family of apicomplexan-specific plasma membrane-localized amino acid transporters, the ApiATs, and showed that the Toxoplasma gondii transporter TgApiAT1 functions in the selective uptake of arginine. TgApiAT1 is essential for parasite virulence, but dispensable for parasite growth in medium containing high concentrations of arginine, indicating the presence of at least one other arginine transporter. Here we identify TgApiAT6-1 as the second arginine transporter. Using a combination of parasite assays and heterologous characterisation of TgApiAT6-1 in Xenopus laevis oocytes, we demonstrate that TgApiAT6-1 is a general cationic amino acid transporter that mediates both the high-affinity uptake of lysine and the low-affinity uptake of arginine. TgApiAT6-1 is the primary lysine transporter in the disease-causing tachyzoite stage of T. gondii and is essential for parasite proliferation. We demonstrate that the uptake of cationic amino acids by TgApiAT6-1 is ‘trans-stimulated’ by cationic and neutral amino acids and is likely promoted by an inwardly negative membrane potential. These findings demonstrate that T. gondii has evolved overlapping transport mechanisms for the uptake of essential cationic amino acids, and we draw together our findings into a comprehensive model that highlights the finely-tuned, regulated processes that mediate cationic amino acid scavenging by these intracellular parasites.

The causative agent of toxoplasmosis, Toxoplasma gondii, is a versatile intracellular parasite that can proliferate within nucleated cells of warm-blooded organisms. In order to survive, T. gondii parasites must scavenge the cationic amino acids lysine and arginine from their hosts. In a previous study, we demonstrated that a plasma membrane-localized protein called TgApiAT1 facilitates the uptake of arginine into the parasite. We found that parasites lacking TgApiAT1 could proliferate when cultured in medium containing high concentrations of arginine, suggesting the existence of an additional uptake pathway for arginine. In the present study, we demonstrate that this second uptake pathway is mediated by TgApiAT6-1, a protein belonging to the same solute transporter family as TgApiAT1. We show that TgApiAT6-1 is the major lysine transporter of the parasite, and that it is critical for parasite proliferation. Furthermore, we demonstrate that TgApiAT6-1 can transport arginine into parasites under conditions in which arginine concentrations are high and lysine concentrations are comparatively lower. These data support a model for the finely-tuned acquisition of essential cationic amino acids that involves multiple transporters, and which likely contributes to these parasites being able to survive and proliferate within a wide variety of host cell types.

Experimental Approaches to Identify Selective Picomolar Inhibitors for Carbonic Anhydrase IX

BACKGROUND

Carbonic anhydrases (CAs) regulate pH homeostasis via the reversible hydration of CO₂, thereby emerging as essential enzymes for many vital functions. Among 12 catalytically active CA isoforms in humans, CA IX has become a relevant therapeutic target because of its role in cancer progression. Only two CA IX inhibitors have entered clinical trials, mostly due to low affinity and selectivity properties.

OBJECTIVE

The current review presents the design, development, and identification of the selective nano- to picomolar CA IX inhibitors VD11-4-2, VR16-09, and VD12-09.

METHODS AND RESULTS

Compounds were selected from our database, composed of over 400 benzensulfonamides, synthesized at our laboratory, and tested for their binding to 12 human CAs. Here we discuss the CA CO₂ hydratase activity/inhibition assay and several biophysical techniques, such as fluorescent thermal shift assay and isothermal titration calorimetry, highlighting their contribution to the analysis of compound affinity and structure- activity relationships. To obtain sufficient amounts of recombinant CAs for inhibitor screening, several gene cloning and protein purification strategies are presented, including site-directed CA mutants, heterologous CAs from Xenopus oocytes, and native endogenous CAs. The cancer cell-based methods, such as clonogenicity, extracellular acidification, and mass spectrometric gas-analysis are reviewed, confirming nanomolar activities of lead inhibitors in intact cancer cells.

CONCLUSIONS

Novel CA IX inhibitors are promising derivatives for in vivo explorations. Furthermore, the simultaneous targeting of several proteins involved in proton flux upon tumor acidosis and the disruption of transport metabolons might improve cancer management.

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na⁺–K⁺–Cl⁻ cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na⁺, K⁺, and Cl⁻ ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na⁺, Cl⁻, Ca²⁺, Mg²⁺, and HCO₃⁻ loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na⁺–K⁺–Cl⁻ cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Characterization of Schistosoma mansoni Glutamate-Gated Chloride Channels

Electrophysiology is the standard method for characterizing ion channel function. Two-electrode voltage clamp is a robust and relatively simple version which can be applied to the characterization of glutamate-gated chloride channels from Schistosoma mansoni, a potential schistomicidal target. Here, the method is described in detail, with an emphasis on the investigation of S. mansoni. GluCls.

The transportome of the malaria parasite

Membrane transport proteins, also known as transporters, control the movement of ions, nutrients, metabolites, and waste products across the membranes of a cell and are central to its biology. Proteins of this type also serve as drug targets and are key players in the phenomenon of drug resistance. The malaria parasite has a relatively reduced transportome, with only approximately 2.5% of its genes encoding transporters. Even so, assigning functions and physiological roles to these proteins, and ascertaining their contributions to drug action and drug resistance, has been very challenging. This review presents a detailed critique and synthesis of the disruption phenotypes, protein subcellular localisations, protein functions (observed or predicted), and links to antimalarial drug resistance for each of the parasite's transporter genes. The breadth and depth of the gene disruption data are particularly impressive, with at least one phenotype determined in the parasite's asexual blood stage for each transporter gene, and multiple phenotypes available for 76% of the genes. Analysis of the curated data set revealed there to be relatively little redundancy in the Plasmodium transportome; almost two-thirds of the parasite's transporter genes are essential or required for normal growth in the asexual blood stage of the parasite, and this proportion increased to 78% when the disruption phenotypes available for the other parasite life stages were included in the analysis. These observations, together with the finding that 22% of the transportome is implicated in the parasite's resistance to existing antimalarials and/or drugs within the development pipeline, indicate that transporters are likely to serve, or are already serving, as drug targets. Integration of the different biological and bioinformatic data sets also enabled the selection of candidates for transport processes known to be essential for parasite survival, but for which the underlying proteins have thus far remained undiscovered. These include potential transporters of pantothenate, isoleucine, or isopentenyl diphosphate, as well as putative anion-selective channels that may serve as the pore component of the parasite's 'new permeation pathways'. Other novel insights into the parasite's biology included the identification of transporters for the potential development of antimalarial treatments, transmission-blocking drugs, prophylactics, and genetically attenuated vaccines. The syntheses presented herein set a foundation for elucidating the functions and physiological roles of key members of the Plasmodium transportome and, ultimately, to explore and realise their potential as therapeutic targets.

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.

The tyrosine transporter of Toxoplasma gondii is a member of the newly defined apicomplexan amino acid transporter (ApiAT) family

Apicomplexan parasites are auxotrophic for a range of amino acids which must be salvaged from their host cells, either through direct uptake or degradation of host proteins. Here, we describe a family of plasma membrane-localized amino acid transporters, termed the Apicomplexan Amino acid Transporters (ApiATs), that are ubiquitous in apicomplexan parasites. Functional characterization of the ApiATs of Toxoplasma gondii indicate that several of these transporters are important for intracellular growth of the tachyzoite stage of the parasite, which is responsible for acute infections. We demonstrate that the ApiAT protein TgApiAT5-3 is an exchanger for aromatic and large neutral amino acids, with particular importance for L-tyrosine scavenging and amino acid homeostasis, and that TgApiAT5-3 is critical for parasite virulence. Our data indicate that T. gondii expresses additional proteins involved in the uptake of aromatic amino acids, and we present a model for the uptake and homeostasis of these amino acids. Our findings identify a family of amino acid transporters in apicomplexans, and highlight the importance of amino acid scavenging for the biology of this important phylum of intracellular parasites.

The Apicomplexa comprise a large number of parasitic protozoa that have obligate intracellular lifestyles and cause significant human and animal diseases, including malaria, cryptosporidiosis, toxoplasmosis, coccidiosis in poultry, and various cattle fevers. Apicomplexans must scavenge essential nutrients from their hosts in order to proliferate and cause disease, including a range of amino acids. The direct uptake of these nutrients is presumed to be mediated by transporter proteins located in the plasma membrane of intracellular stages, although the identities of these proteins are poorly defined. Using a combination of bioinformatic, genetic, cell biological, and physiological approaches, we have characterized an apicomplexan-specific family of plasma membrane-localized transporter proteins that we have called the Apicomplexan Amino acid Transporters (ApiATs). We show that TgApiAT5-3, a member of the family in the apicomplexan Toxoplasma gondii, is an exchanger for aromatic and large neutral amino acids. In particular, it is critical for uptake of tyrosine, and for parasite virulence in a mouse infection model. We conclude that ApiATs are a family of plasma membrane transporters that play crucial roles in amino acid scavenging by apicomplexan parasites.

Rhinella marina oocytes: a suitable alternative expression system for functional characterization of aquaglyceroporins

Amphibian oocytes have been extensively used for heterologous expression of membrane proteins for studying their biochemical and biophysical properties. So far, Xenopus laevis is the main amphibian used as oocytes source to express aquaglyceroporins in order to assess water and solutes permeability. However, this well-established amphibian model represents a threat to the biodiversity in many countries, especially in those from tropical regions. For that reason, the import of Xenopus laevis is subjected to strict control, which essentially has restricted its use in these regions. Therefore, a wider variety of expression systems for aquaglyceroporins is needed. Rhinella marina is extensively distributed in the Americas and its native range spreads from South America to Texas, US. Here we report the use of Rhinella marina oocytes as an alternative expression system for aquaglyceroporins and demonstrated its suitability to determine the permeability to water and non-ionic solutes. Rhinella marina oocytes were able to functionally express channels from human and the protozoan pathogen Trypanosoma brucei, two very distant organisms on the evolutionary scale. Permeability values obtained from Rhinella marina oocytes expressing members of aquaporin family were similar and comparable to those values reported in the literature for the same channels expressed in Xenopus laevis oocytes.

Inhibitory effects of Kampo medicines, Keishibukuryogan and Shakuyakukanzoto, on the substrate uptake activities of solute carrier organic anion transporters

The aim of this study was to assess the effects of Keishibukuryogan (K-06) and Shakuyakukanzoto (TJ-68), commercial herbal medicines, on the substrate uptake activities of renal organic anion transporters. We performed transporter uptake and cell viability assays in Xenopus oocytes and HEK293 human kidney embryonic cells treated with K-06 or TJ-68. K-06 and TJ-68 markedly inhibited the substrate uptake activities of URAT1, OAT1, and OAT3, while they did not exhibit non-cytotoxic effects. Our findings demonstrated that K-06 and TJ-68 inhibited the substrate uptake activities of renal transporters, suggesting their mechanism of action as nephroprotective agents.

The ammonia transporter RhCG modulates urinary acidification by interacting with the vacuolar proton-ATPases in renal intercalated cells

Ammonium, stemming from renal ammoniagenesis, is a major urinary proton buffer and is excreted along the collecting duct. This process depends on the concomitant secretion of ammonia by the ammonia channel RhCG and of protons by the vacuolar-type proton-ATPase pump. Thus, urinary ammonium content and urinary acidification are tightly linked. However, mice lacking Rhcg excrete more alkaline urine despite lower urinary ammonium, suggesting an unexpected role of Rhcg in urinary acidification. RhCG and the B1 and B2 proton-ATPase subunits could be co-immunoprecipitated from kidney. In ex vivo microperfused cortical collecting ducts (CCD) proton-ATPase activity was drastically reduced in the absence of Rhcg. Conversely, overexpression of RhCG in HEK293 cells resulted in higher proton secretion rates and increased B1 proton-ATPase mRNA expression. However, in kidneys from Rhcg-/- mice the expression of only B1 and B2 subunits was altered. Immunolocalization of proton-ATPase subunits together with immuno-gold detection of the A proton-ATPase subunit showed similar localization and density of staining in kidneys from Rhcg+/+ and Rhcg-/-mice. In order to test for a reciprocal effect of intercalated cell proton-ATPases on Rhcg activity, we assessed Rhcg and proton-ATPase activities in microperfused CCD from Atp6v1b1-/- mice and showed reduced proton-ATPase activity without altering Rhcg activity. Thus, RhCG and proton-ATPase are located within the same cellular protein complex. RhCG may modulate proton-ATPase function and urinary acidification, whereas proton-ATPase activity does not affect RhCG function. This mechanism may help to coordinate ammonia and proton secretion beyond physicochemical driving forces.

Aromatic dipeptide Trp-Ala can be transported by Arabidopsis peptide transporters AtPTR1 and AtPTR5

Dipeptides with an aromatic residue at the N-terminal position induced lower inward currents or blocked leak currents in Xenopus oocytes expressing the proton-coupled peptide transporter AtPTR1 or AtPTR5 of Arabidopsis thaliana compared with dipeptides with an aromatic residue at the C-terminal position. Here, AtPTR1 and AtPTR5 were expressed in a yeast mutant of peptide transporter (ptr2) with tryptophan auxotrophy. Growth assays showed that Trp-Ala could be transported by both AtPTR1 and AtPTR5 as efficiently as Ala-Trp. Our data suggested that the previous finding in Xenopus oocytes might be an artifact of heterologous expression, and that AtPTR1 and AtPTR5 expressed in yeast could transport dipeptides with an aromatic residue at the N-terminal position.

Advances in methods for identification and characterization of plant transporter function

Transport proteins are crucial for cellular function at all levels. Numerous importers and exporters facilitate transport of a diverse array of metabolites and ions intra- and intercellularly. Identification of transporter function is essential for understanding biological processes at both the cellular and organismal level. Assignment of a functional role to individual transporter proteins or to identify a transporter with a given substrate specificity has notoriously been challenging. Recently, major advances have been achieved in function-driven screens, phenotype-driven screens, and in silico-based approaches. In this review, we highlight examples that illustrate how new technology and tools have advanced identification and characterization of plant transporter functions.

Oxidative stress induces stem cell proliferation via TRPA1/RyR-mediated Ca2+ signaling in the Drosophila midgut

Precise regulation of stem cell activity is crucial for tissue homeostasis and necessary to prevent overproliferation. In the Drosophila adult gut, high levels of reactive oxygen species (ROS) has been detected with different types of tissue damage, and oxidative stress has been shown to be both necessary and sufficient to trigger intestinal stem cell (ISC) proliferation. However, the connection between oxidative stress and mitogenic signals remains obscure. In a screen for genes required for ISC proliferation in response to oxidative stress, we identified two regulators of cytosolic Ca²⁺ levels, transient receptor potential A1 (TRPA1) and ryanodine receptor (RyR). Characterization of TRPA1 and RyR demonstrates that Ca²⁺ signaling is required for oxidative stress-induced activation of the Ras/MAPK pathway, which in turns drives ISC proliferation. Our findings provide a link between redox regulation and Ca²⁺ signaling and reveal a novel mechanism by which ISCs detect stress signals.

Aquaglyceroporins Are the Entry Pathway of Boric Acid in Trypanosoma brucei

The boron element possesses a range of different effects on living beings. It is essential to beneficial at low concentrations, but toxic at excessive concentrations. Recently, some boron-based compounds have been identified as promising molecules against Trypanosoma brucei, the causative agent of sleeping sickness. However, until now, the boron metabolism and its access route into the parasite remained elusive. The present study addressed the permeability of T. brucei aquaglyceroporins (TbAQPs) for boric acid, the main natural boron species. To this end, the three TbAQPs were expressed in Saccharomyces cerevisiae and Xenopus laevis oocytes. Our findings in both expression systems showed that all three TbAQPs are permeable for boric acid. Especially TbAQP2 is highly permeable for this compound, displaying one of the highest conductances reported for a solute in these channels. Additionally, T. brucei aquaglyceroporin activities were sensitive to pH. Taken together, these results establish that TbAQPs are channels for boric acid and are highly efficient entry pathways for boron into the parasite. Our findings stress the importance of studying the physiological functions of boron and their derivatives in T. brucei, as well as the pharmacological implications of their uptake by trypanosome aquaglyceroporins.

Primary Brain Calcification Causal PiT2 Transport-Knockout Variants can Exert Dominant Negative Effects on Wild-Type PiT2 Transport Function in Mammalian Cells

Primary brain calcification (PBC) is a neurodegenerative disorder characterized by calcium-phosphate deposits in the basal ganglia and often also other areas of the brain. The prevalent clinical manifestations are cognitive impairment, neuropsychiatric symptoms, and movement disorders. In recent years, monoallelic variants in SLC20A2, which encodes the type III sodium-dependent inorganic phosphate (P_i) transporter 2 (PiT2), have been linked to the familial form of PBC in 40–50% of the families reported worldwide as well as to sporadic cases of PBC. Further insight into the disease mechanism is, however, needed. Based on co-expression studies of wild-type and variant PiT2 in Xenopus laevis oocytes, the molecular disease mechanism associated with SLC20A2 missense variants has formerly been suggested to be haploinsufficiency. We have here used mammalian cells isolated from a Slc20a2^−/− mouse and co-expression of human wild-type and variant PiT2. Two of the variants studied have both been reported twice in unrelated PBC cases: PiT2D28N in two sporadic cases and PiT2E575K in a familial and a sporadic case. We find that in mammalian cells, the analyzed SLC20A2 missense variants can exert their effect in a dominant negative manner resulting in decreased wild-type PiT2 P_i transport. Thus, compared to monoallelic lack of functional PiT2 protein expression, which reasonably points towards haploinsufficiency, certain SLC20A2 missense variants may be more detrimental for cellular P_i uptake and potentially contribute to an earlier disease onset and/or a more severe phenotype as observed for Slc20a2^−/− mice compared to Slc20a2^+/− mice.

Purification of Human and Mammalian Membrane Proteins Expressed in Xenopus laevis Frog Oocytes for Structural Studies

This protocol describes the isolation of recombinant human and mammalian membrane proteins expressed in Xenopus laevis frog oocytes for structural studies. The cDNA-derived cRNA of the desired genes is injected into several hundreds of oocytes, which are incubated for several days to allow protein expression. Recombinant proteins are then purified via affinity chromatography. The novelty of this method comes from the design of a plasmid that produces multi-tagged proteins and, most importantly, the development of a protocol for efficiently discarding lipids, phospholipids, and lipoproteins from the oocyte egg yolk, which represent the major contaminants in protein purifications. Thus, the high protein purity and good yield obtained from this method allows protein structure determination by transmission electron microscopy of single detergent-solubilized protein particles and of 2D crystals of membrane protein embedded in lipid bilayers. Additionally, a radiotracer assay for functional analysis of the expressed target proteins in oocytes is described. Overall, this method is a valuable option for structural studies of mammalian and particularly human proteins, for which other expression systems often fail.

Voltage Clamp Fluorometry of P-Type ATPases

Voltage clamp fluorometry has become a powerful tool to compare partial reactions of P-type ATPases such as the Na(+),K(+)-ATPase and H(+),K(+)-ATPase with conformational dynamics of these ion pumps. Here, we describe the methodology to heterologously express membrane proteins in X. laevis oocytes and site-specifically label these proteins with one or more fluorophores. Fluorescence changes are measured simultaneously with current measurements under two-electrode voltage clamp conditions.

Ondansetron attenuates the activity of excitatory amino acid transporter type 3 expressed in Xenopus oocytes

The purpose of this study was to evaluate the effect of ondansetron on excitatory amino acid transporter type 3 (EAAT3) and to elucidate the roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in the effect. EAAT3 was expressed in Xenopus oocytes following the injection of rat EAAT3 mRNAs. Using the two-electrode voltage clamping method, the inward currents induced by L-glutamate were measured for 1 min in the presence and absence of ondansetron (1-1000 μM). Different concentrations of L-glutamate (3-300 μM) were used to determine the kinetic characteristics of EAAT3. To identify the involvement of PKC and PI3K in the effect, oocytes were exposed to a PKC activator and to PKC inhibitors and PI3K inhibitors, and L-glutamate-induced currents were recorded. Ondansetron decreased EAAT3 activity in a dose-dependent manner. In a kinetic study, ondansetron (10 μM for 3 min) reduced Vmax, but not Km compared with the control group. The PKC activator abolished the ondansetron-induced decrease in EAAT3 activity. The PKC inhibitors (staurosporine and chelerythrine) and ondansetron had not additive or synergistic effects on EAAT3 activity. The PI3K inhibitors (wortmannin and LY294002) decreased the EAAT3 response, although there were no differences among the groups comprising ondansetron, PI3K inhibitors, and PI3K inhibitors plus ondansetron. Our results demonstrate that ondansetron attenuates EAAT3 activity and this effect seems to be mediated by PKC and PI3K.

The Drosophila neuropeptides PDF and sNPF have opposing electrophysiological and molecular effects on central neurons

Neuropeptides have widespread effects on behavior, but how these molecules alter the activity of their target cells is poorly understood. We employed a new model system in Drosophila melanogaster to assess the electrophysiological and molecular effects of neuropeptides, recording in situ from larval motor neurons, which transgenically express a receptor of choice. We focused on two neuropeptides, pigment-dispersing factor (PDF) and small neuropeptide F (sNPF), which play important roles in sleep/rhythms and feeding/metabolism. PDF treatment depolarized motor neurons expressing the PDF receptor (PDFR), increasing excitability. sNPF treatment had the opposite effect, hyperpolarizing neurons expressing the sNPF receptor (sNPFR). Live optical imaging using a genetically encoded fluorescence resonance energy transfer (FRET)-based sensor for cyclic AMP (cAMP) showed that PDF induced a large increase in cAMP, whereas sNPF caused a small but significant decrease in cAMP. Coexpression of pertussis toxin or RNAi interference to disrupt the G-protein Gαo blocked the electrophysiological responses to sNPF, showing that sNPFR acts via Gαo signaling. Using a fluorescent sensor for intracellular calcium, we observed that sNPF-induced hyperpolarization blocked spontaneous waves of activity propagating along the ventral nerve cord, demonstrating that the electrical effects of sNPF can cause profound changes in natural network activity in the brain. This new model system provides a platform for mechanistic analysis of how neuropeptides can affect target cells at the electrical and molecular level, allowing for predictions of how they regulate brain circuits that control behaviors such as sleep and feeding.

Novel TASK channels inhibitors derived from dihydropyrrolo[2,1-a]isoquinoline

TASK channels belong to the family of K(+) channels with 4 transmembrane segments and 2 pore domains (4TM/2P) per subunit. These channels have been related to apoptosis in cerebellar granule neurons (CGN), as well as cancer in other tissues. TASK current is regulated by hormones, neurotransmitters, anesthetics and divalent cations, which are not selective. Recently, there has been found some organic compounds that inhibit TASK current selectively. In order to find other modulators, we report here a group of five dihydropyrrolo[2,1-a]isoquinolines (DPIs), four of them with putative anticancer activity, that were evaluated on TASK-1 and TASK-3 channels. The compounds 1, 2 and 3 showed IC50 < 320 μM on TASK-1 and TASK-3, intermediate activity on TASK-1/TASK-3 heterodimer, moderate effect over hslo and TREK-1 (500 μM), and practically not inhibition on Shaker-IR, herg and IRK2.1 potassium channels, when they were expressed heterologously in Xenopus laevis oocytes. In rat CGN, 500 μM of these three compounds induced a decrement by >39% of the TASK-carried leak current. Finally, only compound 1 showed significant protection (∼36%) against apoptotic death of CGN induced by K(+) deprivation. These results suggest that DPI compounds could be potential candidates for designing new selective inhibitors of TASK channels.

Imaging of Xenopus laevis oocyte plasma membrane in physiological-like conditions by atomic force microscopy

Xenopus laevis oocytes are an interesting model for the study of many developmental mechanisms because of their dimensions and the ease with which they can be manipulated. In addition, they are widely employed systems for the expression and functional study of heterologous proteins, which can be expressed with high efficiency on their plasma membrane. Here we applied atomic force microscopy (AFM) to the study of the plasma membrane of X. laevis oocytes. In particular, we developed and optimized a new sample preparation protocol, based on the purification of plasma membranes by ultracentrifugation on a sucrose gradient, to perform a high-resolution AFM imaging of X. laevis oocyte plasma membrane in physiological-like conditions. Reproducible AFM topographs allowed visualization and dimensional characterization of membrane patches, whose height corresponds to a single lipid bilayer, as well as the presence of nanometer structures embedded in the plasma membrane and identified as native membrane proteins. The described method appears to be an applicable tool for performing high-resolution AFM imaging of X. laevis oocyte plasma membrane in a physiological-like environment, thus opening promising perspectives for studying in situ cloned membrane proteins of relevant biomedical/pharmacological interest expressed in this biological system.

Progesterone increases the activity of glutamate transporter type 3 expressed in Xenopus oocytes

Progesterone is an important sex hormone for pregnancy and also has neuroprotective and anticonvulsant effects. It is well-known that full-term parturients become more susceptible to volatile anesthetics. Glutamate transporters are important for preventing neurotoxicity and anesthetic action in the central nervous system. We investigated the effects of progesterone on the activity of glutamate transporter type 3 (EAAT3), the major neuronal EAAT. EAAT3 was expressed in Xenopus laevis oocytes by injecting its mRNA. Oocytes were incubated with diluted progesterone for 72 h. Two-electrode voltage clamping was used to measure membrane currents before, during, and after applying 30 μML-glutamate. Progesterone (1-100 nM) significantly increased EAAT3 activity in a dose-dependent manner. Our kinetic study showed that the Vmax was increased in the progesterone group compared with that in the control group (2.7 ± 0.2 vs. 3.6 ± 0.2μC for control group vs. progesterone group; n=18-23; P<0.05), however, Km was unaltered (46.7 ± 10.2μM vs. 55.9 ± 10.5μM for control group vs. progesterone group; n=18-23; P>0.05). Phorbol-12-myristate-13-acetate, a protein kinase C (PKC) activator, did not change progesterone-enhanced EAAT3 activity. Inhibitors of PKC or phosphatidylinositol 3-kinase (PI3K) abolished the progesterone-induced increases in EAAT3 activity. Our results suggest that progesterone enhances EAAT3 activity and that PKC and PI3K are involved in mediating these effects. These effects of progesterone may contribute to its anticonvulsant and anesthesia-related properties.

Functional validation of virtual screening for novel agents with general anesthetic action at ligand-gated ion channels

GABA(A) receptors play a crucial role in the actions of general anesthetics. The recently published crystal structure of the general anesthetic propofol bound to Gloeobacter violaceus ligand-gated ion channel (GLIC), a bacterial homolog of GABA(A) receptors, provided an opportunity to explore structure-based ligand discovery for pentameric ligand-gated ion channels (pLGICs). We used molecular docking of 153,000 commercially available compounds to identify molecules that interact with the propofol binding site in GLIC. In total, 29 compounds were selected for functional testing on recombinant GLIC, and 16 of these compounds modulated GLIC function. Active compounds were also tested on recombinant GABA(A) receptors, and point mutations around the presumed binding pocket were introduced into GLIC and GABA(A) receptors to test for binding specificity. The potency of active compounds was only weakly correlated with properties such as lipophilicity or molecular weight. One compound was found to mimic the actions of propofol on GLIC and GABA(A), and to be sensitive to mutations that reduce the action of propofol in both receptors. Mutant receptors also provided insight about the position of the binding sites and the relevance of the receptor's conformation for anesthetic actions. Overall, the findings support the feasibility of the use of virtual screening to discover allosteric modulators of pLGICs, and suggest that GLIC is a valid model system to identify novel GABA(A) receptor ligands.

Electrophysiological characterization of membrane transport proteins

Active transport in biological membranes has been traditionally studied using a variety of biochemical and biophysical techniques, including electrophysiology. This review focuses on aspects of electrophysiological methods that make them particularly suited for the investigation of transporter function. Two major approaches to electrical recording of transporter activity are discussed: (a) artificial planar lipid membranes, such as the black lipid membrane and solid supported membrane, which are useful for studies on bacterial transporters and transporters of intracellular compartments, and (b) patch clamp and voltage clamp techniques, which investigate transporters in native cellular membranes. The analytical power of these methods is highlighted by several examples of mechanistic studies of specific membrane proteins, including cytochrome c oxidase, NhaA Na(+)/H(+) exchanger, ClC-7 H(+)/Cl(-) exchanger, glutamate transporters, and neutral amino acid transporters. These examples reveal the wealth of mechanistic information that can be obtained when electrophysiological methods are used in combination with rapid perturbation approaches.

Electrophysiological characterization of recombinant and native P2X receptors

ATP acts as a fast neurotransmitter by activating a family of ligand-gated ion channels, the P2X receptors. Functional homomeric P2X(3) and heteromeric P2X(2/3) receptors are highly localized on primary sensory afferent neurons that transmit nociceptive sensory information. Activation of these P2X(3)-containing channels may provide a specific mechanism whereby ATP, released via synaptic transmission or by cellular injury, elicits pain. The experimental procedures described in this unit are useful for the electorphysiological characterization of P2X receptors. In addition, these protocols provide methods for the evaluation of ligands that interact with P2X receptors that are either natively expressed on excitable cells or cloned and expressed in heterologous cell systems. These methods are derived from standard electrophysiological principles and procedures that are applicable to a wide variety of ligand-gated ion channels. Specific attention is given here to the reliable electrophysiological measurement of both quickly (P2X(3)) and more slowly (P2X(2) and P2X(2/3)) desensitizing receptors.

Introduction to section V: assessment of CFTR function

This chapter introduces the various techniques to asses the function of CFTR. The numerous functional interactions of CFTR and cellular properties affected by CFTR will be described initially. This will be followed by sections explaining the importance of patch clamping and double electrode voltage clamp experiments in Xenopus oocytes for expression analysis of CFTR, and the Ussing chamber technique to analyze CFTR in polarized epithelia. It is concluded that examining CFTR function should occur at different levels, starting with the intact epithelium and ending with isolated CFTR proteins.

Frog oocytes to unveil the structure and supramolecular organization of human transport proteins

Structural analyses of heterologously expressed mammalian membrane proteins remain a great challenge given that microgram to milligram amounts of correctly folded and highly purified proteins are required. Here, we present a novel method for the expression and affinity purification of recombinant mammalian and in particular human transport proteins in Xenopus laevis frog oocytes. The method was validated for four human and one murine transporter. Negative stain transmission electron microscopy (TEM) and single particle analysis (SPA) of two of these transporters, i.e., the potassium-chloride cotransporter 4 (KCC4) and the aquaporin-1 (AQP1) water channel, revealed the expected quaternary structures within homogeneous preparations, and thus correct protein folding and assembly. This is the first time a cation-chloride cotransporter (SLC12) family member is isolated, and its shape, dimensions, low-resolution structure and oligomeric state determined by TEM, i.e., by a direct method. Finally, we were able to grow 2D crystals of human AQP1. The ability of AQP1 to crystallize was a strong indicator for the structural integrity of the purified recombinant protein. This approach will open the way for the structure determination of many human membrane transporters taking full advantage of the Xenopus laevis oocyte expression system that generally yields robust functional expression.

Alcohol-binding sites in distinct brain proteins: the quest for atomic level resolution

Defining the sites of action of ethanol on brain proteins is a major prerequisite to understanding the molecular pharmacology of this drug. The main barrier to reaching an atomic-level understanding of alcohol action is the low potency of alcohols, ethanol in particular, which is a reflection of transient, low-affinity interactions with their targets. These mechanisms are difficult or impossible to study with traditional techniques such as radioligand binding or spectroscopy. However, there has been considerable recent progress in combining X-ray crystallography, structural modeling, and site-directed mutagenesis to define the sites and mechanisms of action of ethanol and related alcohols on key brain proteins. We review such insights for several diverse classes of proteins including inwardly rectifying potassium, transient receptor potential, and neurotransmitter-gated ion channels, as well as protein kinase C epsilon. Some common themes are beginning to emerge from these proteins, including hydrogen bonding of the hydroxyl group and van der Waals interactions of the methylene groups of ethanol with specific amino acid residues. The resulting binding energy is proposed to facilitate or stabilize low-energy state transitions in the bound proteins, allowing ethanol to act as a "molecular lubricant" for protein function. We discuss evidence for characteristic, discrete alcohol-binding sites on protein targets, as well as evidence that binding to some proteins is better characterized by an interaction region that can accommodate multiple molecules of ethanol.

Synthesis, maturation, and trafficking of human Na+-dicarboxylate cotransporter NaDC1 requires the chaperone activity of cyclophilin B

Renal excretion of citrate, an inhibitor of calcium stone formation, is controlled mainly by reabsorption via the apical Na(+)-dicarboxylate cotransporter NaDC1 (SLC13A2) in the proximal tubule. Recently, it has been shown that the protein phosphatase calcineurin inhibitors cyclosporin A (CsA) and FK-506 induce hypocitraturia, a risk factor for nephrolithiasis in kidney transplant patients, but apparently through urine acidification. This suggests that these agents up-regulate NaDC1 activity. Using the Xenopus lævis oocyte and HEK293 cell expression systems, we examined first the effect of both anti-calcineurins on NaDC1 activity and expression. While FK-506 had no effect, CsA reduced NaDC1-mediated citrate transport by lowering heterologous carrier expression (as well as endogenous carrier expression in HEK293 cells), indicating that calcineurin is not involved. Given that CsA also binds specifically to cyclophilins, we determined next whether such proteins could account for the observed changes by examining the effect of selected cyclophilin wild types and mutants on NaDC1 activity and cyclophilin-specific siRNA. Interestingly, our data show that the cyclophilin isoform B is likely responsible for down-regulation of carrier expression by CsA and that it does so via its chaperone activity on NaDC1 (by direct interaction) rather than its rotamase activity. We have thus identified for the first time a regulatory partner for NaDC1, and have gained novel mechanistic insight into the effect of CsA on renal citrate transport and kidney stone disease, as well as into the regulation of membrane transporters in general.

Inhibition of voltage-gated K+ channels in dendritic cells by rapamycin

Rapamycin, an inhibitor of the serine/threonine kinase mammalian target of rapamycin (mTOR), is a widely used immunosuppressive drug. Rapamycin affects the function of dendritic cells (DCs), antigen-presenting cells participating in the initiation of primary immune responses and the establishment of immunological memory. Voltage-gated K(+) (Kv) channels are expressed in and impact on the function of DCs. The present study explored whether rapamycin influences Kv channels in DCs. To this end, DCs were isolated from murine bone marrow and ion channel activity was determined by whole cell patch clamp. To more directly analyze an effect of mTOR on Kv channel activity, Kv1.3 and Kv1.5 were expressed in Xenopus oocytes with or without the additional expression of mTOR and voltage-gated currents were determined by dual-electrode voltage clamp. As a result, preincubation with rapamycin (0-50 nM) led to a gradual decline of Kv currents in DCs, reaching statistical significance within 6 h and 50 nM of rapamycin. Rapamycin accelerated Kv channel inactivation. Coexpression of mTOR upregulated Kv1.3 and Kv1.5 currents in Xenopus oocytes. Furthermore, mTOR accelerated Kv1.3 channel activation and slowed down Kv1.3 channel inactivation. In conclusion, mTOR stimulates Kv channels, an effect contributing to the immunomodulating properties of rapamycin in DCs.

PfCHA is a mitochondrial divalent cation/H+ antiporter in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is capable of adapting to vastly different extracellular Ca(2+) environments while maintaining tight control of its intracellular Ca(2+) concentration. The mechanisms underpinning Ca(2+) homeostasis in this important pathogen are only partly understood. Here we have functionally expressed the putative Ca(2+)/H(+) antiporter PfCHA in Xenopus laevis oocytes. Our data suggest that PfCHA mediates H(+)-coupled Ca(2+) and Mn(2+) exchange. The apparent dissociation constant K(M) for Ca(2+) of 2.2 +/- 0.7 mM and the maximal velocity V(max) of 0.6 +/- 0.1 nmol per oocyte per hour are consistent with PfCHA being a low-affinity high-capacity Ca(2+) carrier. In the parasite, PfCHA was found to localize to the mitochondrion. Physiological studies conducted with live parasitized erythrocytes, and using Fluo-4 and Rhod-2 to monitor cytoplasmic and mitochondrial Ca(2+) dynamics, suggest that the mitochondrion serves as a dynamic Ca(2+) store and that PfCHA functions as a Ca(2+) efflux system expelling excess Ca(2+) from the mitochondrion. PfCHA lacks appreciable homologies to the human mitochondrial Ca(2+)/H(+) exchanger and might represent an evolutionary divergent class of mitochondrial cation antiporter, which, in turn, might provide novel opportunities for intervention.

Rapamycin-induced phosphaturia

BACKGROUND

The mammalian target of rapamycin (mTOR) is known to stimulate a variety of transport mechanisms including the intestinal phosphate transporter NaPi-IIb. The present study was performed to elucidate whether mTOR similarly regulates the major renal tubular phosphate transporter NaPi-IIa.

METHODS

To this end, NaPi-IIa was expressed in Xenopus oocytes with or without mTOR and phosphate transport estimated from phosphate-induced (1 mM) current (I(pi)).

RESULTS

As a result, I(pi) was observed in NaPi-IIa-expressing but not in H(2)O-injected Xenopus oocytes. Co-expression of mTOR significantly enhanced I(pi) in NaPi-IIa-expressing Xenopus oocytes, an effect abrogated by treatment with rapamycin (50 nM for the last 24 h of incubation). In a second series of experiments, the effect of rapamycin was analysed in mice. The in vivo administration of rapamycin (3 microg/g body weight/day) for 3 days resulted in phosphaturia in mice despite a tendency of plasma phosphate concentration to decrease.

CONCLUSIONS

mTOR contributes to the regulation of renal phosphate transport, and rapamycin thus influences phosphate balance.

Regulation of renal tubular glucose reabsorption by Akt2/PKBβ

Akt/PKB is known to regulate the facilitative glucose carrier GLUT4. Nothing is known, however, of the role of Akt/PKB in the regulation of renal epithelial transport. To explore whether Akt2/PKBβ influences the Na(+)-coupled glucose cotransporter SGLT1, human SGLT1 was expressed in Xenopus laevis oocytes with or without Akt/PKB, and electrogenic glucose transport was determined by dual-electrode voltage clamp. The coexpression of Akt/PKB in SGLT1-expressing oocytes was followed by an increase in glucose-induced currents. To study the functional significance of Akt/PKB-sensitive renal glucose transport, further experiments were performed in gene-targeted mice lacking functional Akt2/PKBβ (akt2(-/-)) and in their wild-type littermates (akt2(+/+)). Plasma glucose concentration was significantly higher in akt2(-/-) mice than in akt2(+/+) mice but was virtually identical to the plasma glucose concentration in fructose-treated akt2(+/+) mice. Urinary glucose excretion was significantly higher in akt2(-/-) mice compared with akt2(+/+) mice with or without fructose treatment. Moreover, the glucose-induced depolarization of proximal tubular cells was significantly smaller in isolated, perfused renal tubules from akt2(-/-) mice than in those from akt2(+/+) mice. In conclusion, Akt2/PKBβ plays a role in the regulation of renal glucose transport.

Chloroquine transport via the malaria parasite's chloroquine resistance transporter

The emergence and spread of chloroquine-resistant Plasmodium falciparum malaria parasites has been a disaster for world health. Resistance is conferred by mutations in the Chloroquine Resistance Transporter (PfCRT), an integral membrane protein localized to the parasite's internal digestive vacuole. These mutations result in a marked reduction in the accumulation of chloroquine (CQ) by the parasite. However, the mechanism by which this occurs is unclear. We expressed both wild-type and resistant forms of PfCRT at the surface of Xenopus laevis oocytes. The resistant form of PfCRT transported CQ, whereas the wild-type protein did not. CQ transport via the mutant PfCRT was inhibited by CQ analogs and by the resistance-reverser verapamil. Thus, CQ resistance is due to direct transport of the drug via mutant PfCRT.

PIP5K2A-dependent regulation of excitatory amino acid transporter EAAT3

INTRODUCTION

According to previous observations, the gene encoding the phosphatidylinositol-4-phosphate 5-kinase II alpha (PIP5K2A) is associated with schizophrenia. Specifically, the mutation (N251S)PIP5K2A has been discovered in schizophrenic patients but not in healthy individuals. A defect of the excitatory amino acid transporter EAAT3 has similarly been implicated in the development of schizophrenia. The present study thus explored whether PIP5K2A is involved in the regulation of EAAT3 activity.

MATERIALS AND METHODS

EAAT3 was expressed in Xenopus oocytes either without or with PIP5K2A, and EAAT3 transporter activity was estimated from the glutamate (2-mM)-induced current (I(glu)) in dual electrode voltage clamp experiments. EAAT3 protein abundance in the cell membrane was estimated by Western blotting and confocal microscopy.

RESULTS

In EAAT3-expressing oocytes, I(glu) was enhanced by coexpression of wild type PIP5K2A. Coexpression of the schizophrenia-associated mutant (N251S)PIP5K2A significantly decreased I(glu) in oocytes expressing EAAT3 with or without additional expression of wild type PIP5K2A. Thus, (N251S)PIP5K2A exerts a dominant inhibitory effect.

DISCUSSION

Membrane abundance of EAAT3 was increased by wild type PIP5K2A and decreased by (N251S)PIP5K2A in both EAAT3-expressing oocytes and human embryonic kidney cells. The present observations disclose a novel mechanism of EAAT3 regulation, which may contribute to the deranged regulation of excitability in schizophrenic patients.

Characterization of D150E and G196D aquaporin-2 mutations responsible for nephrogenic diabetes insipidus: importance of a mild phenotype

Aquaporin-2 (AQP2) is a water channel responsible for the final water reabsorption in renal collecting ducts. Alterations in AQP2 function induce nephrogenic diabetes insipidus (NDI), a condition characterized by severe polyuria and polydipsia. Three patients affected with severe NDI, who were compound heterozygous for the AQP2 mutations D150E and G196D, are presented here along with a mildly affected D150E homozygous patient from another family. Using Xenopus oocytes as an expression system, these two mutations (G196D and D150E) were compared with the wild-type protein (AQP2-wt) for functional activity (water flux analysis), protein maturation, and plasma membrane targeting. AQP2-wt induces a major increase in water permeability (P(f) = 47.4 +/- 12.2 x 10(-4) cm/s) whereas D150E displays intermediate P(f) values (P(f) = 12.5 +/- 3.0 x 10(-4) cm/s) and G196D presents no specific water flux, similar to controls (P(f) = 2.1 +/- 0.8 x 10(-4) cm/s and 2.2 +/- 0.7 x 10(-4) cm/s, respectively). Western blot and immunocytochemical evaluations show protein targeting that parallels activity levels with AQP2-wt adequately targeted to the plasma membrane, partial targeting for D150E, and complete sequestration of G196D within intracellular compartments. When coinjecting AQP2-wt with mutants, no (AQP2-wt + D150E) or partial (AQP2-wt + G196D) reduction of water flux were observed compared with AQP2-wt alone, whereas complete loss of function was found when both mutants were coinjected. These results essentially recapitulate the clinical profiles of the family members, showing a typical dominant negative effect when G196D is coinjected with either AQP2-wt or D150E but not between AQP2-wt and D150E mutant.