Human pallial MGE-type GABAergic interneuron cell therapy for chronic focal epilepsy

Mesial temporal lobe epilepsy (MTLE) is the most common focal epilepsy. One-third of patients have drug-refractory seizures and are left with suboptimal therapeutic options such as brain tissue-destructive surgery. Here, we report the development and characterization of a cell therapy alternative for drug-resistant MTLE, which is derived from a human embryonic stem cell line and comprises cryopreserved, post-mitotic, medial ganglionic eminence (MGE) pallial-type GABAergic interneurons. Single-dose intrahippocampal delivery of the interneurons in a mouse model of chronic MTLE resulted in consistent mesiotemporal seizure suppression, with most animals becoming seizure-free and surviving longer. The grafted interneurons dispersed locally, functionally integrated, persisted long term, and significantly reduced dentate granule cell dispersion, a pathological hallmark of MTLE. These disease-modifying effects were dose-dependent, with a broad therapeutic range. No adverse effects were observed. These findings support an ongoing phase 1/2 clinical trial (NCT05135091) for drug-resistant MTLE.

Linking the Mechanics of Chewing to Biology of the Junctional Epithelium

The capacity of a tissue to continuously alter its phenotype lies at the heart of how an animal is able to quickly adapt to changes in environmental stimuli. Within tissues, differentiated cells are rigid and play a limited role in adapting to new environments; however, differentiated cells are replenished by stem cells that are defined by their phenotypic plasticity. Here we demonstrate that a Wnt-responsive stem cell niche in the junctional epithelium is responsible for the capability of this tissue to quickly adapt to changes in the physical consistency of a diet. Mechanical input from chewing is required to both establish and maintain this niche. Since the junctional epithelium directly attaches to the tooth surface via hemidesmosomes, a soft diet requires minimal mastication, and consequently, lower distortional strains are produced in the tissue. This reduced strain state is accompanied by reduced mitotic activity in both stem cells and their progeny, leading to tissue atrophy. The atrophied junctional epithelium exhibits suboptimal barrier functions, allowing the ingression of bacteria into the underlying connective tissues, which in turn trigger inflammation and mild alveolar bone loss. These data link the mechanics of chewing to the biology of tooth-supporting tissues, revealing how a stem cell niche is responsible for the remarkable adaptability of the junctional epithelium to different diets.

[Use of drugs in pregnant dogs and puppies - what should be considered from a pharmacological point of view?]

The administration of drugs to pregnant bitches may not only pose a potential danger to the bitch but also to the fetuses. In this context, the extent of the risk also depends on the stage of gestation. Although a number of agents are known to have a fetotoxic and/or teratogenic potential, insufficient knowledge concerning their potential risk to the embryo/fetus is available for the majority of drugs.Arranged in groups of active substances, a selection of drugs that are assumed to be safe according to the current state of knowledge is provided. Drugs for which there is insufficient knowledge or which are clearly contraindicated are also mentioned.The second part of this overview covers the use of medicines in puppies. The physiology of a puppy does not simply correspond to that of a small dog. Only very few drugs posess valid pharmacological data for use in puppies. However, a subset of restrictions on the use of pharmacologic agents may be derived from the specifics of puppy physiology.The recommendations made do not claim to be complete and no guarantee for accuracy is provided, as these have been compiled from a literature review. In certain cases, the recommendations are even contradictory.Veterinarians are therefore encouraged to report all adverse events associated with treatments of pregnant bitches and puppies within the framework of pharmacovigilance. This is the only way to close gaps in knowledge about the treatment of these special patients.

Outcome of Communication Training in Veterinary Studies: Influence on the Perception of the Relevance of Veterinary Competencies and Self-Assessment of Communication Skills

Since communication skills contribute significantly to professional success among veterinarians, there is a particular focus on developing communication classes in veterinary curricula. At Freie Universität Berlin, an e-learning course covering the basics of communication and a practical communication course based on role plays with and without simulation persons have been established. The outcome of these communication courses on the assessment of the relevance of several veterinary competencies and on the self-assessment of communication skills using the SE-12 questionnaire was investigated. For this purpose, students were surveyed before and after the e-learning course as well as before and after the practical course. Veterinarians were also surveyed on the relevance of veterinary competencies. The relevance of communicative competencies for professional success was rated significantly higher by the students after completing the practical course than by the other students and the veterinarians. Self-assessment of communication skills showed little increase after the e-learning course, but a significant increase after the practical course. Thus, an effective outcome of the communication classes was observed mainly after the practical course. However, the effect of the e-learning course cannot be ruled out since the students participating in the practical course have also completed the e-learning course beforehand.

A Model for Epilepsy of Infectious Etiology using Theiler's Murine Encephalomyelitis Virus

One of the main causes of epilepsy is an infection of the central nervous system (CNS); approximately 8% of patients who survive such an infection develop epilepsy as a consequence, with rates being significantly higher in less economically developed countries. This work provides an overview of modeling epilepsy of infectious etiology and using it as a platform for novel antiseizure compound testing. A protocol of epilepsy induction by non-stereotactic intracerebral injection of Theiler's murine encephalomyelitis virus (TMEV) in C57BL/6 mice is presented, which replicates many of the early and chronic clinical symptoms of viral encephalitis and subsequent epilepsy in human patients. The clinical evaluation of mice during encephalitis to monitor seizure activity and detect the potential antiseizure effects of novel compounds is described. Furthermore, histopathological consequences of viral encephalitis and seizures such as hippocampal damage and neuroinflammation are shown, as well as long-term consequences such as spontaneous epileptic seizures. The TMEV model is one of the first translational, infection-driven, experimental platforms to allow for the investigation of the mechanisms of epilepsy development as a consequence of CNS infection. Thus, it also serves to identify potential therapeutic targets and compounds for patients at risk of developing epilepsy following a CNS infection.

Not Part of the Temporal Lobe, but Still of Importance? Substantia Nigra and Subthalamic Nucleus in Epilepsy

The most researched brain region in epilepsy research is the temporal lobe, and more specifically, the hippocampus. However, numerous other brain regions play a pivotal role in seizure circuitry and secondary generalization of epileptic activity: The substantia nigra pars reticulata (SNr) and its direct input structure, the subthalamic nucleus (STN), are considered seizure gating nuclei. There is ample evidence that direct inhibition of the SNr is capable of suppressing various seizure types in experimental models. Similarly, inhibition via its monosynaptic glutamatergic input, the STN, can decrease seizure susceptibility as well. This review will focus on therapeutic interventions such as electrical stimulation and targeted drug delivery to SNr and STN in human patients and experimental animal models of epilepsy, highlighting the opportunities for overcoming pharmacoresistance in epilepsy by investigating these promising target structures.

The circadian dynamics of the hippocampal transcriptome and proteome is altered in experimental temporal lobe epilepsy

Gene and protein expressions display circadian oscillations, which can be disrupted in diseases in most body organs. Whether these oscillations occur in the healthy hippocampus and whether they are altered in epilepsy are not known. We identified more than 1200 daily oscillating transcripts in the hippocampus of control mice and 1600 in experimental epilepsy, with only one-fourth oscillating in both conditions. Comparison of gene oscillations in control and epilepsy predicted time-dependent alterations in energy metabolism, which were verified experimentally. Although aerobic glycolysis remained constant from morning to afternoon in controls, it increased in epilepsy. In contrast, oxidative phosphorylation increased in control and decreased in epilepsy. Thus, the control hippocampus shows circadian molecular remapping, which is altered in epilepsy. We suggest that the hippocampus operates in a different functioning mode in epilepsy. These alterations need to be considered when studying epilepsy mechanisms, designing drug treatments, and timing their delivery.

Disruption of the sodium-dependent citrate transporter SLC13A5 in mice causes alterations in brain citrate levels and neuronal network excitability in the hippocampus

In addition to tissues such as liver, the plasma membrane sodium-dependent citrate transporter, NaCT (SLC13A5), is highly expressed in brain neurons, but its function is not understood. Loss-of-function mutations in the human SLC13A5 gene have been associated with severe neonatal encephalopathy and pharmacoresistant seizures. The molecular mechanisms of these neurological alterations are not clear. We performed a detailed examination of a Slc13a5 deletion mouse model including video-EEG monitoring, behavioral tests, and electrophysiologic, proteomic, and metabolomic analyses of brain and cerebrospinal fluid. The experiments revealed an increased propensity for epileptic seizures, proepileptogenic neuronal excitability changes in the hippocampus, and significant citrate alterations in the CSF and brain tissue of Slc13a5 deficient mice, which may underlie the neurological abnormalities. These data demonstrate that SLC13A5 is involved in brain citrate regulation and suggest that abnormalities in this regulation can induce seizures. The present study is the first to (i) establish the Slc13a5-knockout mouse model as a helpful tool to study the neuronal functions of NaCT and characterize the molecular mechanisms by which functional deficiency of this citrate transporter causes epilepsy and impairs neuronal function; (ii) evaluate all hypotheses that have previously been suggested on theoretical grounds to explain the neurological phenotype of SLC13A5 mutations; and (iii) indicate that alterations in brain citrate levels result in neuronal network excitability and increased seizure propensity.

Automated quantification of EEG spikes and spike clusters as a new read out in Theiler's virus mouse model of encephalitis-induced epilepsy

Intracerebral infection of C57BL/6 mice with Theiler's murine encephalomyelitis virus (TMEV) replicates many features of viral encephalitis-induced epilepsy in humans, including neuroinflammation, early (insult-associated) and late (spontaneous) seizures, neurodegeneration in the hippocampus, and cognitive and behavioral alterations. Thus, this model may be ideally suited to study mechanisms involved in encephalitis-induced epilepsy as potential targets for epilepsy prevention. However, spontaneous recurrent seizures (SRS) occur too infrequently to be useful as a biomarker of epilepsy, e.g., for drug studies. This prompted us to evaluate whether epileptiform spikes or spike clusters in the cortical electroencephalogram (EEG) may be a useful surrogate of epilepsy in this model. For this purpose, we developed an algorithm that allows efficient and large-scale EEG analysis of early and late seizures, spikes, and spike clusters in the EEG. While 77% of the infected mice exhibited early seizures, late seizures were only observed in 33% of the animals. The clinical characteristics of early and late seizures did not differ except that late generalized convulsive (stage 5) seizures were significantly longer than early stage 5 seizures. Furthermore, the frequency of SRS was much lower than the frequency of early seizures. Continuous (24/7) video-EEG monitoring over several months following infection indicated that the latent period to onset of SRS was 61 (range 16-91) days. Spike and spike clusters were significantly more frequent in infected mice with late seizures than in infected mice without seizures or in mock-infected sham controls. Based on the results of this study, increases in EEG spikes and spike clusters in groups of infected mice may be used as a new readout for studies on antiepileptogenic or disease-modifying drug effects in this model, because the significant increase in average spike counts in mice with late seizures obviously indicates a proepileptogenic alteration.

Anticonvulsant effects after grafting of rat, porcine, and human mesencephalic neural progenitor cells into the rat subthalamic nucleus

Cell transplantation based therapy is a promising strategy for treating intractable epilepsies. Inhibition of the subthalamic nucleus (STN) or substantia nigra pars reticulata (SNr) is a powerful experimental approach for remote control of different partial seizure types, when targeting the seizure focus is not amenable. Here, we tested the hypothesis that grafting of embryonic/fetal neural precursor cells (NPCs) from various species (rat, human, pig) into STN or SNr of adult rats induces anticonvulsant effects. To rationally refine this approach, we included NPCs derived from the medial ganglionic eminence (MGE) and ventral mesencephalon (VM), both of which are able to develop a GABAergic phenotype. All VM- and MGE-derived cells showed intense migration behavior after grafting into adult rats, developed characteristics of inhibitory interneurons, and survived at least up to 4 months after transplantation. By using the intravenous pentylenetetrazole (PTZ) seizure threshold test in adult rats, transient anticonvulsant effects were observed after bilateral grafting of NPCs derived from human and porcine VM into STN, but not after SNr injection (site-specificity). In contrast, MGE-derived NPCs did not cause anticonvulsant effects after grafting into STN or SNr (cell-specificity). Neither induction of status epilepticus by lithium-pilocarpine to induce neuronal damage prior to the PTZ test nor pretreatment of MGE cells with retinoic acid and potassium chloride to increase differentiation into GABAergic neurons could enhance anticonvulsant effectiveness of MGE cells. This is the first proof-of-principle study showing anticonvulsant effects by bilateral xenotransplantation of NPCs into the STN. Our study highlights the value of VM-derived NPCs for interneuron-based cell grafting targeting the STN.

A combination of NMDA and AMPA receptor antagonists retards granule cell dispersion and epileptogenesis in a model of acquired epilepsy

Epilepsy may arise following acute brain insults, but no treatments exist that prevent epilepsy in patients at risk. Here we examined whether a combination of two glutamate receptor antagonists, NBQX and ifenprodil, acting at different receptor subtypes, exerts antiepileptogenic effects in the intrahippocampal kainate mouse model of epilepsy. These drugs were administered over 5 days following kainate. Spontaneous seizures were recorded by video/EEG at different intervals up to 3 months. Initial trials showed that drug treatment during the latent period led to higher mortality than treatment after onset of epilepsy, and further, that combined therapy with both drugs caused higher mortality at doses that appear safe when used singly. We therefore refined the combined-drug protocol, using lower doses. Two weeks after kainate, significantly less mice of the NBQX/ifenprodil group exhibited electroclinical seizures compared to vehicle controls, but this effect was lost at subsequent weeks. The disease modifying effect of the treatment was associated with a transient prevention of granule cell dispersion and less neuronal degeneration in the dentate hilus. These data substantiate the involvement of altered glutamatergic transmission in the early phase of epileptogenesis. Longer treatment with NBQX and ifenprodil may shed further light on the apparent temporal relationship between dentate gyrus reorganization and development of spontaneous seizures.

Refinement of a model of acquired epilepsy for identification and validation of biomarkers of epileptogenesis in rats

In rodent models in which status epilepticus (SE) is used to induce epilepsy, typically most animals develop spontaneous recurrent seizures (SRS). The SE duration for induction of epileptogenesis depends on the type of SE induction. In models with electrical SE induction, the minimum duration of SE to induce epileptogenesis in >90% of animals ranges from 3-4h. A high incidence of epilepsy is an advantage in the search of antiepileptogenic treatments, whereas it is a disadvantage in the search for biomarkers of epileptogenesis, because it does not allow a comparison of potential biomarkers in animals that either develop or do not develop epilepsy. The aim of this project was the refinement of an established SE rat model so that only ~50% of the animals develop epilepsy. For this purpose, we used an electrical model of SE induction, in which a self-sustained SE develops after prolonged stimulation of the basolateral amygdala. Previous experiments had shown that the majority of rats develop SRS after 4-h SE in this model so that the SE reduced duration to 2.5h by administering diazepam. This resulted in epilepsy development in only 50% of rats, thus reaching the goal of the project. The latent period to onset of SRS wa s >2weeks in most rats. Development of epilepsy could be predicted in most rats by behavioral hyperexcitability, whereas seizure threshold did not differentiate rats that did and did not develop SRS. The refined SE model may offer a platform to identify and validate biomarkers of epileptogenesis.

Brain inflammation, neurodegeneration and seizure development following picornavirus infection markedly differ among virus and mouse strains and substrains

Infections, particularly those caused by viruses, are among the main causes of acquired epilepsy, but the mechanisms causing epileptogenesis are only poorly understood. As a consequence, no treatment exists for preventing epilepsy in patients at risk. Animal models are useful to study epileptogenesis after virus-induced encephalitis and how to interfere with this process, but most viruses that cause encephalitis in rodents are associated with high mortality, so that the processes leading to epilepsy cannot be investigated. Recently, intracerebral infection with Theiler's murine encephalomyelitis virus (TMEV) in C57BL/6 (B6) mice was reported to induce early seizures and epilepsy and it was proposed that the TMEV mouse model represents the first virus infection-driven animal model of epilepsy. In the present study, we characterized this model in two B6 substrains and seizure-resistant SJL/J mice by using three TMEV (sub)strains (BeAn-1, BeAn-2, DA). The idea behind this approach was to study what is and what is not necessary for development of acute and late seizures after brain infection in mice. Receiver operating characteristic (ROC) curve analysis was used to determine which virus-induced brain alterations are associated with seizure development. In B6 mice infected with different TMEV virus (sub)strains, the severity of hippocampal neurodegeneration, amount of MAC3-positive microglia/macrophages, and expression of the interferon-inducible antiviral effector ISG15 were almost perfect at discriminating seizing from non-seizing B6 mice, whereas T-lymphocyte brain infiltration was not found to be a crucial factor. However, intense microglia/macrophage activation and some hippocampal damage were also observed in SJL/J mice. Overall, the TMEV model provides a unique platform to study virus and host factors in ictogenesis and epileptogenesis.

Novel combinations of phenotypic biomarkers predict development of epilepsy in the lithium-pilocarpine model of temporal lobe epilepsy in rats

The discovery and validation of biomarkers in neurological and neurodegenerative diseases is an important challenge for early diagnosis of disease and for the development of therapeutics. Epilepsy is often a consequence of brain insults such as traumatic brain injury or stroke, but as yet no biomarker exists to predict the development of epilepsy in patients at risk. Given the complexity of epilepsy, it is unlikely that a single biomarker is sufficient for this purpose, but a combinatorial approach may be needed to overcome the challenge of individual variability and disease heterogeneity. The goal of the present prospective study in the lithium-pilocarpine model of epilepsy in rats was to determine the discriminative utility of combinations of phenotypic biomarkers by examining their ability to predict epilepsy. For this purpose, we used a recent model refinement that allows comparing rats that will or will not develop spontaneous recurrent seizures (SRS) after pilocarpine-induced status epilepticus (SE). Potential biomarkers included in our study were seizure threshold and seizure severity in response to timed i.v. infusion of pentylenetetrazole (PTZ) and behavioral alterations determined by a battery of tests during the three weeks following SE. Three months after SE, video/EEG monitoring was used to determine which rats had developed SRS. To determine whether a biomarker or combination of biomarkers performed better than chance at predicting epilepsy after SE, derived data underwent receiver operating characteristic (ROC) curve analyses. When comparing rats with and without SRS and sham controls, the best intergroup discrimination was obtained by combining all measurements, resulting in a ROC area under curve (AUC) of 0.9592 (P<0.01), indicating an almost perfect discrimination or accuracy to predict development of SRS. These data indicate that a combinatorial biomarker approach may overcome the challenge of individual variability in the prediction of epilepsy.

Effective termination of status epilepticus by rational polypharmacy in the lithium-pilocarpine model in rats: Window of opportunity to prevent epilepsy and prediction of epilepsy by biomarkers

The pilocarpine rat model, in which status epilepticus (SE) leads to epilepsy with spontaneous recurrent seizures (SRS), is widely used to study the mechanisms of epileptogenesis and develop strategies for epilepsy prevention. SE is commonly interrupted after 30-90min by high-dose diazepam or other anticonvulsants to reduce mortality. It is widely believed that SE duration of 30-60min is sufficient to induce hippocampal damage and epilepsy. However, resistance to diazepam develops during SE, so that an SE that is longer than 30min is difficult to terminate, and SE typically recurs several hours after diazepam, thus forming a bias for studies on epileptogenesis or antiepileptogenesis. We developed a drug cocktail, consisting of diazepam, phenobarbital, and scopolamine that allows complete and persistent SE termination in the lithium-pilocarpine model. A number of novel findings were obtained with this cocktail. (a) In contrast to previous reports with incomplete SE suppression, a SE of 60min duration did not induce epilepsy, whereas epilepsy with SRS developed after 90 or 120min SE; (b) by comparing groups of rats with 60 and 90min of SE, development of epilepsy could be predicted by behavioral hyperexcitability and decrease in seizure threshold, indicating that these read-outs are suited as biomarkers of epileptogenesis; (c) CA1 damage was prevented by the cocktail, but rats exhibited cell loss in the dentate hilus, which was related to development of epilepsy. These data demonstrate that the duration of SE needed for induction of epileptogenesis in this model is longer than previously thought.

The Concise Guide to PHARMACOLOGY 2013/14: overview

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties from the IUPHAR database. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. This compilation of the major pharmacological targets is divided into seven areas of focus: G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors & Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and GRAC and provides a permanent, citable, point-in-time record that will survive database updates.

Anticonvulsant Effects by Bilateral and Unilateral Transplantation of GABA-Producing Cells into the Subthalamic Nucleus in an Acute Seizure Model

Neural transplantation of GABA-producing cells into key structures within seizure-suppressing circuits holds promise for medication-resistant epilepsy patients not eligible for resection of the epileptic focus. The substantia nigra pars reticulata (SNr), a basal ganglia output structure, is well known to modulate different seizure types. A recent microinjection study by our group indicated that the subthalamic nucleus (STN), which critically regulates nigral activity, might be a more promising target for focal therapy in epilepsies than the SNr. As a proof of principle, we therefore assessed the anticonvulsant efficacy of bilateral and unilateral allografting of GABA-producing cell lines into the STN using the timed intravenous pentylenetetrazole seizure threshold test, which allows repeated seizure threshold determinations in individual rats. We observed (a) that grafted cells survived up to the end of the experiments, (b) that anticonvulsant effects can be induced by bilateral transplantation into the STN using immortalized GABAergic cells derived from the rat embryonic striatum and cells additionally transfected to obtain higher GABA synthesis than the parent cell line, and (c) that anticonvulsant effects were observed even after unilateral transplantation into the STN. Neither grafting of control cells nor transplantation outside the STN induced anticonvulsant effects, emphasizing the site and cell specificity of the observed anticonvulsant effects. To our knowledge, the present study is the first showing anticonvulsant effects by grafting of GABA-producing cells into the STN. The STN can be considered a highly promising target region for modulation of seizure circuits and, moreover, has the advantage of being clinically established for functional neurosurgery.

Consequences of inhibition of bumetanide metabolism in rodents on brain penetration and effects of bumetanide in chronic models of epilepsy

The diuretic bumetanide, which acts by blocking the Na-K-Cl cotransporter (NKCC), is widely used to inhibit neuronal NKCC1, particularly when NKCC1 expression is abnormally increased in brain diseases such as epilepsy. However, bumetanide poorly penetrates into the brain and, in rodents, is rapidly eliminated because of extensive oxidation of its N-butyl sidechain, reducing the translational value of rodent experiments. Inhibition of oxidation by piperonyl butoxide (PBO) has previously been reported to increase the half-life and diuretic activity of bumetanide in rats. Here we studied whether inhibition of bumetanide metabolism by PBO also increases brain levels of bumetanide in rats, and whether this alters pharmacodynamic effects in the kindling model of epilepsy. Furthermore, we studied the effects of PBO in mice. Mice eliminated bumetanide less rapidly than rats (elimination half-life 47 min vs. 13 min). Pretreatment with PBO increased the half-life in mice to average values (70 min) previously determined in humans, and markedly elevated brain levels of bumetanide. In rats, the increase in plasma and brain levels of bumetanide by PBO was less marked than in mice. PBO significantly increased the diuretic activity of bumetanide in rats and, less effectively, in mice. In epileptic mice, bumetanide (with PBO) did not suppress spontaneous seizures. In the rat kindling model, bumetanide (with or without PBO) did not exert anticonvulsant effects on fully kindled seizures, but dose-dependently altered kindling development. These data indicate that PBO offers a simple means to enhance the translational properties of rodent experiments with bumetanide, particularly when using mice.

Proconvulsant actions of intrahippocampal botulinum neurotoxin B in the rat

Botulinum neurotoxins (BoNTs) may affect the excitability of brain circuits by inhibiting neurotransmitter release at central synapses. There is evidence that local delivery of BoNT serotypes A and E, which target SNAP-25, a component of the release machinery specific to excitatory synapses, can inhibit seizure generation. BoNT serotype B (BoNT/B) targets VAMP2, which is expressed in both excitatory and inhibitory terminals. Here we assessed the effects of unilateral intrahippocampal infusion of BoNT/B in the rat on intravenous pentylenetetrazol (PTZ) seizure thresholds, and on the expression of spontaneous behavioral and electrographic seizures. Infusion of BoNT/B (500 and 1,000 unit) by convection-enhanced delivery caused a reduction in myoclonic twitch and clonic seizure thresholds in response to intravenous PTZ beginning about 6 days after the infusion. Handling-evoked and spontaneous convulsive seizures were observed in many BoNT/B-treated animals but not in vehicle-treated controls. Spontaneous electrographic seizure discharges were recorded in the dentate gyrus of animals that received local BoNT/B infusion. In addition, there was an increased frequency of interictal epileptiform spikes and sharp waves at the same recording site. BoNT/B-treated animals also exhibited tactile hyperresponsivity in comparison with vehicle-treated controls. This is the first demonstration that BoNT/B causes a delayed proconvulsant action when infused into the hippocampus. Local infusion of BoNT/B could be useful as a focal epilepsy model.

Mechanism and putative structure of B(0)-like neutral amino acid transporters

The Na(+)-dependent transport of neutral amino acids in epithelial cells and neurons is mediated by B(0)-type neutral amino acid transporters. Two B(0)-type amino acid transporters have been identified in the neurotransmitter transporter family SLC6, namely B(0)AT1 (SLC6A19) and B(0)AT2 (SLC6A15). In contrast to other members of this family, B(0)-like transporters are chloride-independent. B(0)AT1 and B(0)AT2 preferentially bind the substrate prior to the Na(+)-ion. The Na(+)-concentration affects the K ( m ) of the substrate and vice versa. A kinetic scheme is proposed that is consistent with the experimental data. An overlapping binding site of substrate and cosubstrate has been demonstrated in the bacterial orthologue LeuT( Aa ) from Aquifex aeolicus, which elegantly explains the mutual effect of substrate and cosubstrate on each other's K ( m )-value. LeuT( Aa ) is sequence-related to transporters of the SLC6 family, allowing homology modeling of B(0)-like transporters along its structure.

Plasmodium permeomics: membrane transport proteins in the malaria parasite

Membrane transport proteins are integral membrane proteins that mediate the passage across the membrane bilayer of specific molecules and/or ions. Such proteins serve a diverse range of physiological roles, mediating the uptake of nutrients into cells, the removal of metabolic wastes and xenobiotics (including drugs), and the generation and maintenance of transmembrane electrochemical gradients. In this chapter we review the present state of knowledge of the membrane transport mechanisms underlying the cell physiology of the intraerythrocytic malaria parasite and its host cell, considering in particular physiological measurements on the parasite and parasitized erythrocyte, the annotation of transport proteins in the Plasmodium genome, and molecular methods used to analyze transport protein function.

Neutral amino acid transport in epithelial cells and its malfunction in Hartnup disorder

Hartnup disorder is an autosomal recessive abnormality of renal and gastrointestinal neutral amino acid transport. A corresponding transport activity has been characterized in kidney and intestinal cells and named system B(0). The failure to resorb amino acids in this disorder is thought to be compensated by a protein-rich diet. However, in combination with a poor diet and other factors, more severe symptoms can develop in Hartnup patients, including a photosensitive pellagra-like skin rash, cerebellar ataxia and other neurological symptoms. Homozygosity mapping in a Japanese family and linkage analysis on six Australian pedigrees placed the Hartnup disorder gene at a locus on chromosome 5p15. This fine mapping facilitated a candidate gene approach within the interval, which resulted in the cloning and characterization of a novel member of the sodium-dependent neurotransmitter transporter family (B(0)AT1, SLC6A19) from mouse and human kidney, which shows all properties of system B(0). Flux experiments and electrophysiological recording showed that the transporter is Na(+) dependent and Cl(-) independent, electrogenic and actively transports most neutral amino acids. In situ hybridization showed strong expression in intestinal villi and in the proximal tubule of the kidney. Expression of B(0)AT1 was restricted to kidney, intestine and skin. A total of ten mutations have been identified in SLC6A19 that co-segregate with disease in the predicted recessive manner, with the majority of affected individuals being compound heterozygotes. These mutations lead to altered neutral amino acid transport function compared to the wild-type allele in vitro. One of the mutations occurs in members of the original Hartnup family described in 1956, thereby defining SLC6A19 as the 'Hartnup'-gene.

The serine/threonine kinases SGK2 and SGK3 are potent stimulators of the epithelial Na+ channel alpha,beta,gamma-ENaC

The serum- and glucocorticoid-inducible kinase 1 (SGK1) has been identified as a signalling molecule up-regulated by aldosterone, which stimulates the renal epithelial Na(+) channel ENaC. It is therefore thought to participate in the antinatriuretic action of this hormone. More recently, two isoforms, SGK2 and SGK3, have been cloned. The present study was performed to establish whether SGK2 and SGK3 influence ENaC activity similarly to SGK1. Dual-electrode voltage-clamp experiments in Xenopus laevis oocytes expressing alpha,ss,gamma-ENaC with or without SGK1, SGK2 or SGK3 revealed a stimulatory effect of all three kinases on the amiloride-sensitive current (I(Na)). To establish whether the SGK isoforms exert their effects through direct phosphorylation, we replaced the serine at the SGK consensus site of alphaENaC (alpha(S622A)ENaC) by site-directed mutagenesis. alpha(S622A),beta,gamma-ENaC was up-regulated similar to wild-type ENaC, suggesting that SGK isoforms do not act via direct phosphorylation of the transport proteins. In conclusion, SGK2 and SGK3 mimic the function of SGK1 and are likely to participate in the regulation of ENaC activity.

Cerebral localization and regulation of the cell volume-sensitive serum- and glucocorticoid-dependent kinase SGK1

The serum- and glucocorticoid-dependent kinase SGK1 is regulated by alterations of cell volume, whereby cell shrinkage increases and cell swelling decreases the transcription, expression and activity of SGK1. The kinase is expressed in all human tissues studied including the brain. The present study was performed to localize the sites of SGK1 transcription in the brain, to elucidate the influence of the hydration status on SGK1 transcription and to explore the functional significance of altered SGK1 expression. Northern blot analysis of human brain showed SGK1 to be expressed in all cerebral structures examined: amygdala, caudate nucleus, corpus callosum, hippocampus, substantia nigra, subthalamic nucleus and thalamus. In situ hybridization and immunohistochemistry in the rat revealed increased expression of SGK1 in neurons of the hippocampal area CA3 after dehydration, compared with similar slices from brains of euvolaemic rats. Additionally, several oligodendrocytes, a few microglial cells, but no astrocytes, were positive for SGK1. The abundance of SGK1 mRNA in the temporal lobe, including hippocampus, was increased by dehydration and SGK1 transcription in neuroblastoma cells was stimulated by an increase of extracellular osmolarity. Co-expression studies in Xenopus laevis oocytes revealed that SGK1 markedly increased the activity of the neuronal K+ channel Kv1.3. As activation of K+ channels modifies excitation of neuronal cells, SGK1 may participate in the regulation of neuronal excitability.

Serum- and glucocorticoid-dependent kinase, cell volume, and the regulation of epithelial transport

Ample pharmacological evidence points to a role of kinases in the regulation of cell volume. Given the limited selectivity of most inhibitors, however, the specific molecules involved have remained largely elusive. The search for cell volume regulated genes in liver HepG2 cells led to the discovery of the human serum- and glucocorticoid-dependent serine/threonine kinase hsgk1. Transcription and expression of hsgk1 is markedly and rapidly upregulated by osmotic and isotonic cell shrinkage. The effect of osmotic cell shrinkage on hsgk1 is mediated by p38 kinase. Further stimuli of hsgk1 transcription include glucocorticoids, aldosterone, TGF-beta1, serum, increase of intracellular Ca2+ and phorbolesters, whereas cAMP downregulates hsgk1 transcription. The hsgk1 protein is expressed in several epithelial tissues including human pancreas, intestine, kidney, and shark rectal gland. Co-expression of hsgk1 with the renal epithelial Na+-channel ENaC or the Na+/K+/2Cl(-)-cotransporter NKCC2 (BSC1) in Xenopus oocytes, accelerates insertion of the transport proteins into the cell membrane and thus, stimulates channel or transport activity. Thus, hsgk1 participates in the regulation of transport by steroids and secretagogues increasing intracellular Ca2+-activity. The stimulation of hsgk1 transcription by TGF-beta1 may further bear pathophysiological relevance.

Function and structure of heterodimeric amino acid transporters

Heterodimeric amino acid transporters are comprised of two subunits, a polytopic membrane protein (light chain) and an associated type II membrane protein (heavy chain). The heavy chain rbAT (related to b(0,+) amino acid transporter) associates with the light chain b(0,+)AT (b(0,+) amino acid transporter) to form the amino acid transport system b(0,+), whereas the homologous heavy chain 4F2hc interacts with several light chains to form system L (with LAT1 and LAT2), system y(+)L (with y(+)LAT1 and y(+)LAT2), system x (with xAT), or system asc (with asc1). The association of light chains with the two heavy chains is not unambiguous. rbAT may interact with LAT2 and y(+)LAT1 and vice versa; 4F2hc may interact with b(0,+)AT when overexpressed. 4F2hc is necessary for trafficking of the light chain to the plasma membrane, whereas the light chains are thought to determine the transport characteristics of the respective heterodimer. In contrast to 4F2hc, mutations in rbAT suggest that rbAT itself takes part in the transport besides serving for the trafficking of the light chain to the cell surface. Heavy and light subunits are linked together by a disulfide bridge. The disulfide bridge, however, is not necessary for the trafficking of rbAT or 4F2 heterodimers to the membrane or for the functioning of the transporter. However, there is experimental evidence that the disulfide bridge in the 4F2hc/LAT1 heterodimer plays a role in the regulation of a cation channel. These results highlight complex interactions between the different subunits of heterodimeric amino acid transporters and suggest that despite high grades of homology, the interactions between rbAT and 4F2hc and their respective partners may be different.

Na+ transport by the neural glutamine transporter ATA1

Transfer of glutamine between astrocytes and neurons is an essential part of the glutamate-glutamine cycle in the brain. Here we have investigated how the neural glutamine transporter (rATA1/GlnT) works. Rat ATA1 was expressed in Xenopus laevis oocytes and examined using two-electrode voltage-clamp recordings, ion-sensitive microelectrodes and tracer flux experiments. Glutamine transport via rATA1 was electrogenic and caused inward currents that did not reverse at positive holding potentials. Currents were induced by a variety of neutral amino acids in the following relative order Ala>Ser/Gln/Asn/His/Cys/Met >MeAIB/Gly>Thr/Pro/Tyr/Val, where MeAIB is the amino acid analogue N-methylaminoisobutyric acid. The uptake of glutamine and the corresponding currents depended on Na+ and pH. Hill-coefficient and flux studies with 22NaCl indicated a cotransport stoichiometry 1 Na+ per transport cycle. The transporter also showed uncoupled Na+ transport, particularly when alanine was used as the substrate. Although substrate uptake increased strongly with increasing pH, no change of intracellular pH was observed during transport. A decrease of the intracellular pH similarly inhibited glutamine transport via ATA1, suggesting that the pH dependence was an allosteric effect on the transporter.

Transfer of glutamine between astrocytes and neurons

The export of glutamine from astrocytes, and the uptake of glutamine by neurons, are integral steps in the glutamate-glutamine cycle, a major pathway for the replenishment of neuronal glutamate. We review here the functional and molecular identification of the transporters that mediate this transfer. The emerging picture of glutamine transfer in adult brain is of a dominant pathway mediated by system N transport (SN1) in astrocytes and system A transport (SAT/ATA) in neurons. The participating glutamine transporters are functionally and structurally related, sharing the following properties: (a) unlike many neutral amino acid transporters which have proven to be obligate exchangers, these glutamine transporters mediate net substrate transfer energized by coupling to ionic gradients; (b) they are sensitive to small pH changes in the physiological range; (c) they are susceptible to adaptive and humoral regulation; (d) they are related structurally to the AAAP (amino acid and auxin permeases) family of transporters. A key difference between SN1 and the SAT/ATA transporters is the ready reversibility of glutamine fluxes via SN1 under physiological conditions, which allows SN1 both to sustain a glutamine concentration gradient in astrocytes and to mediate the net outward flux of glutamine. It is likely that the ASCT2 transporter, an obligate exchanger of neutral amino acids, displaces the SN1 transporter as the main carrier of glutamine export in proliferating astrocytes.

Association of 4F2hc with light chains LAT1, LAT2 or y+LAT2 requires different domains

Heterodimeric amino acid transporters are comprised of a type-II membrane protein named the heavy chain (4F2hc or rBAT) that may associate with a number of different polytopic membrane proteins, called light chains. It is thought that the heavy chain is mainly involved in the trafficking of the complex to the plasma membrane, whereas the transport process itself is catalysed by the light chain. The 4F2 heavy chain (4F2hc) associates with at least six different light chains to induce distinct amino acid-transport activites. To test if the light chains are specifically recognized and to identify domains involved in the recognition of light chains, C-terminally truncated mutants of 4F2hc were constructed and co-expressed with the light chains LAT1, LAT2 and y(+)LAT2. The truncated isoform T1, comprised of only 133 amino acids that form the cytosolic N-terminus and the transmembrane helix, displayed only a slight reduction in its ability to promote LAT1 expression at the membrane surface compared with the 529 amino acid wild-type 4F2hc protein. Co-expression of increasingly larger 4F2hc mutants caused a delayed translocation of LAT1. In contrast to the weak effects of 4F2hc truncations on LAT1 expression, surface expression of LAT2 and y(+)LAT2 was almost completely lost with all truncated heavy chains. Co-expression of LAT1 together with the other light chains did not result in displacement of LAT2 and y(+)LAT2. The results suggest that extracellular domains of the heavy chain are responsible mainly for recognition of light chains other than LAT1 and that the extracellular domain ensures proper translocation to the plasma membrane.

Effect of NaPi-mediated phosphate transport on intracellular pH

Extracellular pH has been shown previously to influence transport via type-II Na+/phosphate (NaPi) transporters by modifying the affinity of the carrier for Na+ and by altering the availability of divalent and monovalent phosphate. As the transport of monovalent phosphate would be expected to acidify, and that of divalent phosphate to alkalinize the cell interior, the effect of phosphate transport on cytosolic pH was studied using ion selective microelectrodes in Xenopus oocytes expressing NaPi-3 or NaPi-5. At an alkaline extracellular pH (pHe) of 8.0, addition of phosphate elicited a strong inward current, depolarization of the cell membrane and cytosolic alkalinization. At pHe 6.0 the phosphate-induced inward current and depolarization were reduced and the alkalinization completely abolished. In conclusion, at alkaline pHe phosphate transport is enhanced and the transport of divalent phosphate prevails. At pHe 6.0, phosphate transport is attenuated and is accomplished by transport of both divalent and monovalent phosphate.

Acute regulation of the betaine/GABA transporter BGT-1 expressed in Xenopus oocytes by extracellular pH

Besides uptake of Na(+) and Cl(-), mammalian cells counteract osmotic cell shrinkage also by Na(+)-coupled uptake of osmolytes, e. g., myo-inositol, taurine or betaine. The expression of the corresponding transporters is transcriptionally regulated by the ambient pH and osmolarity and is increased upon cell shrinkage, a process requiring hours. The present study has been performed to disclose rapid regulation by pH of osmolyte transport via BGT-1. Transport of GABA was investigated by using the two-electrode voltage-clamp technique with BGT-1 expressing Xenopus oocytes. GABA was used as a substrate, because of the low oocyte endogenous transport activity. Extracellular acidification to pH 5.5 reversibly decreased and extracellular alkalinization to pH 8.5 increased GABA-induced currents. Kinetic analysis revealed that extracellular alkalinization increases the affinity for Cl(-) as reflected by a decrease of the apparent K(m)-value for Cl(-) from >500 mM to 55.8 +/- 4.7 mM upon an increase of the pH from 7.0 to 8.5. The apparent K(m)- values for Na(+) and GABA remained unaltered in the pH range from 6.0 to 8.5. Instead, alkalinization increased the maximal current induced by saturating Na(+) and GABA concentrations. The results are compatible with a model of interference of H(+) ions with Cl(-) binding and a pH-dependent reduction of V(max) for Na(+) and GABA.

3-[123I]iodo-alpha-methyl-L-tyrosine transport and 4F2 antigen expression in human glioma cells

3[(123)I]iodo-alpha-methyl-L-tyrosine is a tracer of amino acid transport in brain tumors using single-photon emission-computed tomography and predominantly transported by amino acid transport system L. The 4F2 antigen has been identified to be linked to system L-like transport and is assumed to be a part of the transporter protein. We demonstrated that system L-mediated transport of IMT and 4F2 antigen expression are dependent on proliferation rate of human glioma cells and significantly correlated with each other.

Isozyme pattern of glycogen phosphorylase in the rat nervous system and rat astroglia-rich primary cultures: electrophoretic and polymerase chain reaction studies

Of the three isozymes of glycogen phosphorylase (GP) known, the brain (B) and muscle (M) isoforms have been reported to occur in brain. We investigated the regional and cellular occurrence of the three isozymes in various parts of the rat nervous system, fetal brain and astroglia-rich primary cultures by means of electrophoresis of native proteins with subsequent activity stain and by reverse transcriptase polymerase chain reaction. In the cortex, cerebellum, olfactory bulb, brainstem, spinal cord and dorsal root ganglia, both mRNA and enzyme protein were found for the B and M isozymes. In addition, the liver (L) isoform mRNA was detected in fetal brain and cultured astrocytes. Our studies indicate that there is no regional difference in distribution pattern between brain regions, spinal cord and dorsal root ganglia. In immature brain and cultured glial cells, the additional presence of the L isozyme is possible. These results support the idea that astrocytes express two or even three GP isozymes simultaneously.

Functional and pharmacological characterization of human Na(+)-carnitine cotransporter hOCTN2

L-Carnitine is essential for the translocation of acyl-carnitine into the mitochondria for beta-oxidation of long-chain fatty acids. It is taken up into the cells by the recently cloned Na(+)-driven carnitine organic cation transporter OCTN2. Here we expressed hOCTN2 in Xenopus laevis oocytes and investigated with two-electrode voltage- clamp and flux measurements its functional and pharmacological properties as a Na(+)-carnitine cotransporter. L-carnitine transport was electrogenic. The L-carnitine-induced currents were voltage and Na(+) dependent, with half-maximal currents at 0.3 +/- 0.1 mM Na(+) at -60 mV. Furthermore, L-carnitine-induced currents were pH dependent, decreasing with acidification. In contrast to other members of the organic cation transporter family, hOCTN2 functions as a Na(+)-coupled carnitine transporter. Carnitine transport was stereoselective, with an apparent Michaelis-Menten constant (K(m)) of 4.8 +/- 0.3 microM for L-carnitine and 98.3 +/- 38.0 microM for D-carnitine. The substrate specificity of hOCTN2 differs from rOCT-1 and hOCT-2 as hOCTN2 showed only small currents with classic OCT substrates such as choline or tetraethylammonium; by contrast hOCTN2 mediated transport of betaine. hOCTN2 was inhibited by several drugs known to induce secondary carnitine deficiency. Most potent blockers were the antibiotic emetine and the ion channel blockers quinidine and verapamil. The apparent IC(50) for emetine was 4.2 +/- 1.2 microM. The anticonvulsant valproic acid did not induce a significant inhibition of carnitine transport, pointing to a different mode of action. In summary, hOCTN2 mediates electrogenic Na(+)-dependent stereoselective high-affinity transport of L-carnitine and Na(+). hOCTN2 displays transport properties distinct from other members of the OCT family and is directly inhibited by several substances known to induce systemic carnitine deficiency.

The low-affinity monocarboxylate transporter MCT4 is adapted to the export of lactate in highly glycolytic cells

Transport of lactate and other monocarboxylates in mammalian cells is mediated by a family of transporters, designated monocarboxylate transporters (MCTs). The MCT4 member of this family has recently been identified as the major isoform of white muscle cells, mediating lactate efflux out of glycolytically active myocytes [Wilson, Jackson, Heddle, Price, Pilegaard, Juel, Bonen, Montgomery, Hutter and Halestrap (1998) J. Biol. Chem. 273, 15920-15926]. To analyse the functional properties of this transporter, rat MCT4 was expressed in Xenopus laevis oocytes and transport activity was monitored by flux measurements with radioactive tracers and by changes of the cytosolic pH using pH-sensitive microelectrodes. Similar to other members of this family, monocarboxylate transport via MCT4 is accompanied by the transport of H(+) across the plasma membrane. Uptake of lactate strongly increased with decreasing extracellular pH, which resulted from a concomitant drop in the K(m) value. MCT4 could be distinguished from the other isoforms mainly in two respects. First, MCT4 is a low-affinity MCT: for L-lactate K(m) values of 17+/-3 mM (pH-electrode) and 34+/-5 mM (flux measurements with L-[U-(14)C]lactate) were determined. Secondly, lactate is the preferred substrate of MCT4. K(m) values of other monocarboxylates were either similar to the K(m) value for lactate (pyruvate, 2-oxoisohexanoate, 2-oxoisopentanoate, acetoacetate) or displayed much lower affinity for the transporter (beta-hydroxybutyrate and short-chain fatty acids). Under physiological conditions, rat MCT will therefore preferentially transport lactate. Monocarboxylate transport via MCT4 could be competitively inhibited by alpha-cyano-4-hydroxycinnamate, phloretin and partly by 4, 4'-di-isothiocyanostilbene-2,2'-disulphonic acid. Similar to MCT1, monocarboxylate transport via MCT4 was sensitive to inhibition by the thiol reagent p-chloromercuribenzoesulphonic acid.

The heterodimeric amino acid transporter 4F2hc/y+LAT2 mediates arginine efflux in exchange with glutamine

The cationic amino acid arginine, due to its positive charge, is usually accumulated in the cytosol. Nevertheless, arginine has to be released by a number of cell types, e.g. kidney cells, which supply other organs with this amino acid, or the endothelial cells of the blood-brain barrier which release arginine into the brain. Arginine release in mammalian cells can be mediated by two different transporters, y(+)LAT1 and y(+)LAT2. For insertion into the plasma membrane, these transporters have to be associated with the type-II membrane glycoprotein 4F2hc [Torrents, Estevez, Pineda, Fernandez, Lloberas, Shi, Zorzano and Palacin (1998) J. Biol. Chem. 273, 32437-32445]. The present study elucidates the function and distribution of y(+)LAT2. In contrast to y(+)LAT1, which is expressed mainly in kidney epithelial cells, lung and leucocytes, y(+)LAT2 has a wider tissue distribution, including brain, heart, testis, kidney, small intestine and parotis. When co-expressed with 4F2hc in Xenopus laevis oocytes, y(+)LAT2 mediated uptake of arginine, leucine and glutamine. Arginine uptake was inhibited strongly by lysine, glutamate, leucine, glutamine, methionine and histidine. Mutual inhibition was observed when leucine or glutamine was used as substrate. Inhibition of arginine uptake by neutral amino acids depended on the presence of Na(+), which is a hallmark of y(+)LAT-type transporters. Although arginine transport was inhibited strongly by glutamate, this anionic amino acid was only weakly transported by 4F2hc/y(+)LAT2. Amino acid transport via 4F2hc/y(+)LAT2 followed an antiport mechanism similar to the other members of this new family. Only preloaded arginine could be released in exchange for extracellular amino acids, whereas marginal release of glutamine or leucine was observed under identical conditions. These results indicated that arginine has the highest affinity for the intracellular binding site and that arginine release may be the main physiological function of this transporter.

Swelling-induced taurine release without chloride channel activity in Xenopus laevis oocytes expressing anion channels and transporters

Taurine is an important osmolyte involved in cell volume regulation. During regulatory volume decrease it is released via a volume-sensitive organic osmolyte/anion channel. Several molecules have been suggested as candidates for osmolyte release. In this study, we chose three of these, namely ClC-2, ClC-3 and ICln, because of their expression in rat astrocytes, a cell type which is known to release taurine under hypotonic stress, and their activation by hypotonic shock. As all three candidates were also suggested to be chloride channels, we investigated their permeability for both chloride and taurine under isotonic and hypotonic conditions using the Xenopus laevis oocyte expression system. We found a volume-sensitive increase of chloride permeability in ClC-2-expressing oocytes only. Yet, the taurine permeability was significantly increased under hypotonic conditions in oocytes expressing any of the tested candidates. Further experiments confirmed that the detected taurine efflux does not represent unspecific leakage. These results suggest that ClC-2, ClC-3 and ICln either participate in taurine transport themselves or upregulate an endogenous oocyte osmolyte channel. In either case, the taurine efflux of oocytes not being accompanied by an increased chloride flux suggests that taurine and chloride can be released via two separate pathways.

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

The oocytes of the South African clawed frog X. laevis are widely used for the expression of heterologous proteins. The functional characterization of membrane proteins in particular has significantly profited from the use of this expression system. Heterologous cRNA can easily be injected and protein expression and function be studied with several techniques. This review will give a short overview into the variety of methods applicable. They span from different electrophysiological methods such as two electrode voltage clamp, patch clamp and ion-selective electrodes over cytochemistry to protein biochemistry. In spite of the wide usage of Xenopus oocytes, caution should be taken interpreting the results of protein expression. Heterologous proteins may either interact with endogenous proteins, the background of endogenous protein function may be relatively high, or altered protein behaviour may occur due to differences of the ambient temperature or altered cellular environment.

The heterodimeric amino acid transporter 4F2hc/LAT1 is associated in Xenopus oocytes with a non-selective cation channel that is regulated by the serine/threonine kinase sgk-1

System L is the major Na(+)-independent amino acid transporter of mammalian cells. It is constituted of the type II membrane protein 4F2hc (CD98) which is covalently linked to the polytopic membrane protein LAT1 via a disulfide bridge. The transporter is known to be regulated by the mineral corticoid aldosterone in Xenopus A6 cells. To understand the regulation of the transporter, the 4F2hc/LAT1 heterodimer was functionally expressed in Xenopus laevis oocytes and its transport properties were analysed using flux measurements and the two-electrode voltage-clamp technique. Expression of 4F2hc/LAT1 resulted in a rapid increase in a Na(+)-independent neutral amino acid antiport activity and simultaneously gave rise to a cation conductance. The cation channel was non-rectifying and non-selective, conducting Li(+) > Cs(+) = Na(+) > K(+). After replacement of Na(+) by NMDG, however, the currents were suppressed almost completely. The cation channel was not inhibited by amiloride, Ba2(+), TEA, Hoe293B, flufenamic acid or substrates of the system L amino acid transporter. Significant inhibition, however, was observed in the presence of La3(+), Gd3(+) and quinidine. Channel activity was upregulated by coexpression of 4F2hc/LAT1 with the aldosterone-regulated protein kinase sgk-1. The cation conductance was sensitive to changes in the redox potential, being inhibited following incubation of the oocytes with DTE for 30 min. Mutation of either of the disulfide bridge-constituting cysteines to serine resulted in a loss of ion channel activity whereas amino acid transport was unaffected. It is concluded that the 4F2hc/LAT1 heterodimer regulates a closely associated cation channel or even constitutes a cation channel itself.

Neutral amino acid transporter ASCT2 displays substrate-induced Na+ exchange and a substrate-gated anion conductance

The neutral amino acid transporter ASCT2 mediates electroneutral obligatory antiport but at the same time requires Na(+) for its function. To elucidate the mechanism, ASCT2 was expressed in Xenopus laevis oocytes and transport was analysed by flux studies and two-electrode voltage clamp recordings. Flux studies with (22)NaCl indicated that the uptake of one molecule of glutamine or alanine is accompanied by the uptake of four to seven Na(+) ions. Similarly to the transport of amino acids, the Na(+) uptake was mediated by an obligatory Na(+) exchange mechanism that depended on the presence of amino acids but was not stoichiometrically coupled to the amino acid transport. Other cations could not replace Na(+) in this transport mechanism. When NaCl was replaced by NaSCN in the transport buffer, the superfusion of oocytes with amino acid substrates resulted in large inward currents, indicating the presence of a substrate-gated anion channel in the ASCT2 transporter. The K(m) for glutamine derived from these experiments is in good agreement with the K(m) derived from flux studies; it varied between 40 and 90 microM at holding potentials of -60 and -20 mV respectively. The permeability of the substrate-gated anion conductance decreased in the order SCN(-)>>NO(3)(-)>I(-)>Cl(-) and also required the presence of Na(+).

The astroglial ASCT2 amino acid transporter as a mediator of glutamine efflux

Glutamine release from astrocytes is an essential part of the glutamate-glutamine cycle in the brain. Uptake of glutamine into cultured rat astrocytes occurs by at least four different routes. In agreement with earlier studies, a significant contribution of amino acid transport systems ASC, A, L, and N was detected. It has not been determined whether these systems are also involved in glutamine efflux or whether specific efflux transporters exist. We show here that ASCT2, a variant of transport system ASC, is strongly expressed in rat astroglia-rich primary cultures but not in neuron-rich primary cultures. The amino acid sequence of rat astroglial ASCT2 is 83% identical to that of mouse ASCT2. In Xenopus laevis oocytes expressing rat ASCT2, we observed high-affinity uptake of [U-14C]glutamine (Km = 70 microM) that was Na(+)-dependent, concentrative, and unaffected by membrane depolarization. When oocytes were preloaded with [U-14C]glutamine, no glutamine efflux was detected in the absence of extracellular amino acids. Neither lowering intracellular pH nor raising the temperature elicited efflux. However, addition of 0.1 mM unlabeled alanine, serine, cysteine, threonine, glutamine, or leucine to the extracellular solution resulted in a rapid release of glutamine from the ASCT2-expressing oocytes. Amino acids that are not recognized as substrates by ASCT2 were ineffective in this role. Extracellular glutamate stimulated glutamine release weakly at pH 7.5 but was more effective on lowering pH to 5.5, consistent with the pH dependence of ASCT2 affinity for glutamate. Our findings suggest a significant role of ASCT2 in glutamine efflux from astrocytes by obligatory exchange with extracellular amino acids. However, the relative contribution of this pathway to glutamine release from cells in vivo or in vitro remains to be determined.

Helix 8 and helix 10 are involved in substrate recognition in the rat monocarboxylate transporter MCT1

Transport of lactate, pyruvate, and the ketone bodies, acetoacetate and beta-hydroxybutyrate, is mediated in many mammalian cells by the monocarboxylate transporter MCT1. To be accepted as a substrate, a carboxyl group and an unpolar side chain are necessary. Site-directed mutagenesis of the rat MCT1 was used to identify residues which are involved in substrate recognition. Helices 8 and 10 but not helix 9 were found to contain critical residues for substrate recognition. Mutation of arginine 306 to threonine in helix 8 resulted in strongly reduced transport activity. Concomitantly, saturable transport was transformed into a nonsaturable dependence of transport activity on lactate concentration, suggesting that binding of the substrate was strongly impaired. Furthermore, proton translocation in the mutant was partially uncoupled from monocarboxylate transport. Mutation of phenylalanine 360 to cysteine in helix 10 resulted in an altered substrate side chain recognition. In contrast to the wild-type transporter, monocarboxylates with more bulky and polar side chains were recognized by the mutated MCT1. Mutation of selected residues in helix 9 and helix 11 (C336A, H337Q, and E391Q) did not cause alterations of the transport properties of MCT1. It is suggested that substrate binding occurs in the carboxy-terminal half of MCT1 and that helices 8 and 10 are involved in the recognition of different parts of the substrate.

Characterization of the high-affinity monocarboxylate transporter MCT2 in Xenopus laevis oocytes

Observations on lactate transport in brain cells and cardiac myocytes indicate the presence of a high-affinity monocarboxylate transporter. The rat monocarboxylate transporter isoform MCT2 was analysed by expression in Xenopus laevis oocytes and the results were compared with the known characteristics of lactate transport in heart and brain. Monocarboxylate transport via MCT2 was driven by the H(+) gradient over the plasma membrane. Uptake of lactate strongly increased with decreasing pH, showing half-maximal stimulation at pH 7.2. A wide variety of monocarboxylates and ketone bodies, including lactate, pyruvate, beta-hydroxybutyrate, acetoacetate, 2-oxoisovalerate and 2-oxoisohexanoate, were substrates of MCT2. All substrates had a high affinity for MCT2. For lactate a K(m) value of 0.74+/-0.07 mM was determined at pH 7.0. For the other substrates, K(i) values between 100 microM and 1 mM were measured for inhibition of lactate transport, which is about one-tenth of the corresponding values for the ubiquitously expressed monocarboxylate transporter isoform MCT1. Monocarboxylate transport via MCT2 could be inhibited by alpha-cyano-4-hydroxycinnamate, anion-channel inhibitors and flavonoids. It is suggested that cells which express MCT2 preferentially use lactate and ketone bodies as energy sources.

Functional characterization of the Betaine/gamma-aminobutyric acid transporter BGT-1 expressed in Xenopus oocytes

Betaine is an osmolyte accumulated in cells during osmotic cell shrinkage. The canine transporter mediating cellular accumulation of the osmolyte betaine and the neurotransmitter gamma-aminobutyric acid (BGT-1) was expressed in Xenopus oocytes and analyzed by two-electrode voltage clamp and tracer flux studies. Exposure of oocytes expressing BGT-1 to betaine or gamma-aminobutyric acid (GABA) depolarized the cell membrane in the current clamp mode and induced an inward current under voltage clamp conditions. At 1 mM substrate the induced currents decreased in the following order: betaine = GABA > diaminobutyric acid = beta-alanine > proline = quinidine > dimethylglycine > glycine > sarcosine. Both the Vmax and Km of GABA- and betaine-induced currents were voltage-dependent, and GABA- and betaine-induced currents and radioactive tracer uptake were strictly Na+-dependent but only partially dependent on the presence of Cl-. The apparent affinity of GABA decreased with decreasing Na+ concentrations. The Km of Na+ also depended on the GABA and Cl- concentration. A decrease of the Cl- concentration reduced the apparent affinity for Na+ and GABA, and a decrease of the Na+ concentration reduced the apparent affinity for Cl- and GABA. A comparison of 22Na+-, 36Cl--, and 14C-labeled GABA and 14C-labeled betaine fluxes and GABA- and betaine-induced currents yielded a coupling ratio of Na+/Cl-/organic substrate of 3:1:1 or 3:2:1. Based on the data, a transport model of ordered binding is proposed in which GABA binds first, Na+ second, and Cl- third. In conclusion, BGT-1 displays significant functional differences from the other members of the GABA transporter family.