Impaired KCC2 Function Triggers Interictal-Like Activity Driven by Parvalbumin-Expressing Interneurons in the Isolated Subiculum In Vitro

The decreased expression of the KCC2 membrane transporter in subicular neurons has been proposed to be a key epileptogenic event in temporal lobe epilepsy (TLE). Here, we have addressed this question in a reduced model in vitro and have studied the properties and mechanistic involvement of a major class of interneurons, that is, parvalbumin-expressing cells (PVs). When exposed to the KCC2 blocker VU0463271, mouse subicular slices generated hypersynchronous discharges that could be recorded electrophysiologically and visualized as clusters of co-active neurons with calcium imaging. The pharmacological profile of these events resembled interictal-like discharges in human epileptic tissue because of their dependence on GABAA and AMPA receptors. On average, PVs fired before pyramidal cells (PCs) and the area of co-active clusters was comparable to the individual axonal spread of PVs, suggesting their mechanistic involvement. Optogenetic experiments confirmed this hypothesis, as the flash-stimulation of PVs in the presence of VU0463271 initiated interictal-like discharges, whereas their optogenetic silencing suppressed network hyper-excitability. We conclude that reduced KCC2 activity in subicular networks in vitro is sufficient to induce interictal-like activity via altered GABAergic signaling from PVs without other epilepsy-related changes. This conclusion supports an epileptogenic role for impaired subicular KCC2 function during the progression of TLE.

[Different Membrane Environments Generate Multiple Functions of P-type Ion Pumps]

P-type ion pumps (P-type ATPases) are involved in various fundamental biological processes. For example, the gastric proton pump (H+,K+-ATPase) and sodium pump (Na+,K+-ATPase) are responsible for secretion of gastric acid and maintenance of cell membrane potential, respectively. In this review, we summarize three topics of our studies. The first topic is gastric H+,K+-ATPase associated with Cl--transporting proteins (Cl-/H+ exchanger ClC-5 and K+-Cl- cotransporter KCC4). In gastric parietal cells, we found that ClC-5 is predominantly expressed in intracellular tubulovesicles and that KCC4 is predominantly expressed in the apical membrane. Gastric acid (HCl) secretion may be accomplished by the two different complexes of H+,K+-ATPase and Cl--transporting protein. The second topic focuses on the Na+,K+-ATPase α1-isoform (α1NaK) associated with the volume-regulated anion channel (VRAC). In the cholesterol-enriched membrane microdomains of human cancer cells, we found that α1NaK has a receptor-like (non-pumping) function and that binding of low concentrations (nM level) of cardiac glycosides to α1NaK activates VRAC and exerts anti-cancer effects without affecting the pumping function of α1NaK. The third topic is the Na+,K+-ATPase α3-isoform (α3NaK) in human cancer cells. We found that α3NaK is abnormally expressed in the intracellular vesicles of attached cancer cells and that the plasma membrane translocation of α3NaK upon cell detachment contributes to the survival of metastatic cancer cells. Our results indicate that multiple functions of P-type ion pumps are generated by different membrane environments and their associated proteins.

KCC2 is required for the survival of mature neurons but not for their development

The K+/Cl- cotransporter KCC2 (SLC12A5) allows mature neurons in the CNS to maintain low intracellular Cl- levels that are critical in mediating fast hyperpolarizing synaptic inhibition via type A γ-aminobutyric acid receptors (GABAARs). In accordance with this, compromised KCC2 activity results in seizures, but whether such deficits directly contribute to the subsequent changes in neuronal structure and viability that lead to epileptogenesis remains to be assessed. Canonical hyperpolarizing GABAAR currents develop postnatally, which reflect a progressive increase in KCC2 expression levels and activity. To investigate the role that KCC2 plays in regulating neuronal viability and architecture, we have conditionally ablated KCC2 expression in developing and mature neurons. Decreasing KCC2 expression in mature neurons resulted in the rapid activation of the extrinsic apoptotic pathway. Intriguingly, direct pharmacological inhibition of KCC2 in mature neurons was sufficient to rapidly induce apoptosis, an effect that was not abrogated via blockade of neuronal depolarization using tetrodotoxin (TTX). In contrast, ablating KCC2 expression in immature neurons had no discernable effects on their subsequent development, arborization, or dendritic structure. However, removing KCC2 in immature neurons was sufficient to ablate the subsequent postnatal development of hyperpolarizing GABAAR currents. Collectively, our results demonstrate that KCC2 plays a critical role in neuronal survival by limiting apoptosis, and mature neurons are highly sensitive to the loss of KCC2 function. In contrast, KCC2 appears to play a minimal role in mediating neuronal development or architecture.

SLC16A1-AS1 enhances radiosensitivity and represses cell proliferation and invasion by regulating the miR-301b-3p/CHD5 axis in hepatocellular carcinoma

Hepatocellular carcinoma (HCC), a common type of human malignancies, leads to increasing incidence and fairly high mortality. An increasing number of studies have verified that long noncoding RNAs (lncRNAs) played key roles in the development of multiple human cancers. As a biomarker, SLC16A1-AS1 has been reported in non-small cell lung cancer (NSCLC) and oral squamous cell carcinoma (OSCC). Thus, we decided to investigate whether SLC16A1-AS1 exerts its biological function in HCC. In this study, we discovered that SLC16A1-AS1 was obviously downregulated in HCC tissues and cells. Overexpression of SLC16A1-AS1 inhibited HCC cell proliferation, invasion, and epithelial-mesenchymal transition (EMT) process as well as promoted cell apoptosis. Moreover, SLC16A1-AS1 was confirmed to enhance the radiosensitivity of HCC cells. Molecular mechanism exploration suggested that SLC16A1-AS1 served as a sponge for miR-301b-3p and CHD5 was the downstream target gene of miR-301b-3p in HCC cells. Rescue assays implied that CHD5 knockdown could recover the effects of SLC16A1-AS1 overexpression on HCC cellular processes. In brief, our study clarified that SLC16A1-AS1 acted as a tumor suppressor in HCC by targeting the miR-301b-3p/CHD5 axis, which may be a promising diagnostic biomarker and provide promising treatment for HCC patients.

Hepatitis B Virus Entry into Cells

Hepatitis B virus (HBV), an enveloped partially double-stranded DNA virus, is a widespread human pathogen responsible for more than 250 million chronic infections worldwide. Current therapeutic strategies cannot eradicate HBV due to the persistence of the viral genome in a special DNA structure (covalently closed circular DNA, cccDNA). The identification of sodium taurocholate co-transporting polypeptide (NTCP) as an entry receptor for both HBV and its satellite virus hepatitis delta virus (HDV) has led to great advances in our understanding of the life cycle of HBV, including the early steps of infection in particular. However, the mechanisms of HBV internalization and the host factors involved in this uptake remain unclear. Improvements in our understanding of HBV entry would facilitate the design of new therapeutic approaches targeting this stage and preventing the de novo infection of naïve hepatocytes. In this review, we provide an overview of current knowledge about the process of HBV internalization into cells.

KCC2 antagonism and gabaergic synchronization in the entorhinal cortex in the absence of ionotropic glutamatergic receptor signalling

γ-Aminobutyric acid (GABA), which is released by interneurons, plays an active role in generating interictal epileptiform spikes during blockade of ionotropic glutamatergic signalling, but it remains unclear whether and how the K+-Cl- cotransporter 2 (KCC2) influences these paroxysmal events. Therefore, we employed tetrode recordings in the in vitro rat entorhinal cortex (EC) to analyze the effects of the KCC2 antagonist VU0463271 on 4-aminopyridine (4AP)-induced interictal spikes that were pharmacologically isolated by applying ionotropic glutamatergic receptor antagonists. After the addition of VU0463271, these interictal spikes continued to occur at similar rates as in control (i.e., during application of 4AP with ionotropic glutamatergic receptor antagonists) but were smaller and shorter. Despite the absence of ionotropic glutamatergic receptor signalling, both interneurons and principal cells increased their firing during interictal spikes. Moreover, we found that KCC2 antagonism increased interneuron firing but decreased principal cell firing during the interictal spike rising phase; in contrast, during the falling phase, interneuron firing decreased in the presence of VU0463271 while no change was observed in principal cell firing. Overall, our results show that KCC2 antagonism enhances interneuron excitability at the onset of interictal spikes generated by the EC neuronal networks during blockade of ionotropic glutamatergic transmission but disrupts later neuronal recruitment.

Potassium channels act as chemosensors for solute transporters

Potassium channels form physical complexes with solute transporters in vivo, yet little is known about their range of possible signaling modalities and the underlying mechanisms. The KCNQ2/3 potassium channel, which generates neuronal M-current, is voltage-gated and its activity is also stimulated by binding of various small molecules. KCNQ2/3 forms reciprocally regulating complexes with sodium-coupled myo-inositol transporters (SMITs) in mammalian neurons. Here, we report that the neurotransmitter γ-aminobutyric acid (GABA) and other small molecules directly regulate myo-inositol transport in rat dorsal root ganglia, and by human SMIT1-KCNQ2/3 complexes in vitro, by inducing a distinct KCNQ2/3 pore conformation. Reciprocally, SMIT1 tunes KCNQ2/3 sensing of GABA and related metabolites. Ion permeation and mutagenesis studies suggest that SMIT1 and GABA similarly alter KCNQ2/3 pore conformation but via different KCNQ subunits and molecular mechanisms. KCNQ channels therefore act as chemosensors to enable co-assembled myo-inositol transporters to respond to diverse stimuli including neurotransmitters, metabolites and drugs.

The role of LuloPAT amino acid/proton symporters in midgut alkalinization in the sandfly Lutzomyia longipalpis (Diptera - Psychodidae)

In Lutzomyia longipalpis females, which are the main vectors of Leishmania infantum in the Americas, hematophagy is crucial for ovary development. The control of pH in the midgut during blood digestion is important to the functioning of the digestive enzymes, which release amino acids in the luminal compartment that are then transported through the enterocytes to the hemolymph for delivery to the ovary and other organs. In the present work, we investigated transport systems known as LuloPATs that are present in the midgut of L. longipalpis but not in other organs. These transporters achieve symport of amino acids with H⁺ ions, and one of them (LuloPAT1) is orthologous to a transporter described in Aedes aegypti. According to our results, the transcription levels of LuloPAT1 increased significantly immediately after a blood meal. Based on the variation of the fluorescence of fluorescein with the pH of the medium, we developed a technique that shows the acidification of the cytoplasm of gut cells when amino acids are cotransported with H⁺ from the lumen into the enterocytes. In our experiments, the midguts of the sandflies were dissected and opened longitudinally so that added amino acids could enter the enterocytes via the lumen (PAT carriers are apical). LuloPAT1 transporters are part of a complex of mechanisms that act synergistically to promote gut alkalinization as soon as blood intake by the vector occurs. In dissected but not longitudinally opened midguts, added amino acids could only enter through the basolateral region of enterocytes. However, alkalinization of the lumen was observed because the entry of some amino acids into the cytoplasm of enterocytes triggers a luminal alkalinization mechanism independent of LuloPATs. These findings provide new perspectives that will enable the characterization of the set of signaling pathways involved in pH regulation within the L. longipalpis midgut.

The Year in Basic Thyroidology

Basic research in 2019 yielded exciting discoveries and advancements in thyroidology. Specifically, there have been breakthroughs in our understanding of the molecular actions of thyroid hormone and thyroid hormone receptors, thyroid hormone metabolism and transport, autoimmunity, and thyroid cancer. Next, I summarize important studies published over the past year and whose major data I presented during the 89th American Thyroid Association annual meeting at the opening plenary session The Year in Thyroidology.

Lactate Buildup at the Site of Chronic Inflammation Promotes Disease by Inducing CD4+ T Cell Metabolic Rewiring

Accumulation of lactate in the tissue microenvironment is a feature of both inflammatory disease and cancer. Here, we assess the response of immune cells to lactate in the context of chronic inflammation. We report that lactate accumulation in the inflamed tissue contributes to the upregulation of the lactate transporter SLC5A12 by human CD4+ T cells. SLC5A12-mediated lactate uptake into CD4+ T cells induces a reshaping of their effector phenotype, resulting in increased IL17 production via nuclear PKM2/STAT3 and enhanced fatty acid synthesis. It also leads to CD4+ T cell retention in the inflamed tissue as a consequence of reduced glycolysis and enhanced fatty acid synthesis. Furthermore, antibody-mediated blockade of SLC5A12 ameliorates the disease severity in a murine model of arthritis. Finally, we propose that lactate/SLC5A12-induced metabolic reprogramming is a distinctive feature of lymphoid synovitis in rheumatoid arthritis patients and a potential therapeutic target in chronic inflammatory disorders.

Phloem-specific overexpression of AtOPT6 in Arabidopsis enhances Zn transport into shoots

The Arabidopsis oligopeptide transporter AtOPT6 is membrane transport protein that mediated transport of glutathione in both the reduced (GSH) and oxidized (GSSG) forms. In this study, the role of AtOPT6 in glutathione distribution throughout the plant was investigated. We found that transgenic Arabidopsis overexpressing AtOPT6 under the control of a phloem-specific promoter of sucrose-proton symporter 2 (pSUC2), remarkably increased AtOPT6 transcript levels, ranging from 30- to 40-fold in shoots and 6- to 10-fold in roots, relative to the wild type. AtOPT6-overexpressing lines could elevate the foliar glutathione content; however, glutathione content in the phloem did not change. We observed that the ratio of shoot glutathione content to total glutathione content increased in AtOPT6-overexpressing lines, but not in transgenic Arabidopsis with elevated foliar GSH synthesis. These results indicate the possibility that loading and unloading of glutathione in phloem tissues are enhanced in AtOPT6-overexpressing lines under the control of pSUC2. The results of heavy metal analysis revealed that transgenic Arabidopsis overexpressing AtOPT6 under the control of pSUC2 could promote the transport of Zn into shoots as effectively as transgenic Arabidopsis with elevated foliar GSH synthesis, or wild-type plants with exogenous foliar application of GSH.

KCC2 Regulates Dendritic Spine Formation in a Brain-Region Specific and BDNF Dependent Manner

KCC2 is the major chloride extruder in neurons. The spatiotemporal regulation of KCC2 expression orchestrates the developmental shift towards inhibitory GABAergic drive and the formation of glutamatergic synapses. Whether KCC2's role in synapse formation is similar in different brain regions is unknown. First, we found that KCC2 subcellular localization, but not overall KCC2 expression levels, differed between cortex and hippocampus during the first postnatal week. We performed site-specific in utero electroporation of KCC2 cDNA to target either hippocampal CA1 or somatosensory cortical pyramidal neurons. We found that a premature expression of KCC2 significantly decreased spine density in CA1 neurons, while it had the opposite effect in cortical neurons. These effects were cell autonomous, because single-cell biolistic overexpression of KCC2 in hippocampal and cortical organotypic cultures also induced a reduction and an increase of dendritic spine density, respectively. In addition, we found that the effects of its premature expression on spine density were dependent on BDNF levels. Finally, we showed that the effects of KCC2 on dendritic spine were dependent on its chloride transporter function in the hippocampus, contrary to what was observed in cortex. Altogether, these results demonstrate that KCC2 regulation of dendritic spine development, and its underlying mechanisms, are brain-region specific.

Tenuous Inhibitory GABAergic Signaling in the Reticular Thalamus

Maintenance of a low intracellular Cl- concentration ([Cl-]i) is critical for enabling inhibitory neuronal responses to GABAA receptor-mediated signaling. Cl- transporters, including KCC2, and extracellular impermeant anions ([A]o) of the extracellular matrix are both proposed to be important regulators of [Cl-]i Neurons of the reticular thalamic (RT) nucleus express reduced levels of KCC2, indicating that GABAergic signaling may produce excitation in RT neurons. However, by performing perforated patch recordings and calcium imaging experiments in rats (male and female), we find that [Cl-]i remains relatively low in RT neurons. Although we identify a small contribution of [A]o to a low [Cl-]i in RT neurons, our results also demonstrate that reduced levels of KCC2 remain sufficient to maintain low levels of Cl- Reduced KCC2 levels, however, restrict the capacity of RT neurons to rapidly extrude Cl- following periods of elevated GABAergic signaling. In a computational model of a local RT network featuring slow Cl- extrusion kinetics, similar to those we found experimentally, model RT neurons are predisposed to an activity-dependent switch from GABA-mediated inhibition to excitation. By decreasing the activity threshold required to produce excitatory GABAergic signaling, weaker stimuli are able to propagate activity within the model RT nucleus. Our results indicate the importance of even diminished levels of KCC2 in maintaining inhibitory signaling within the RT nucleus and suggest how this important activity choke point may be easily overcome in disorders such as epilepsy.SIGNIFICANCE STATEMENT Precise regulation of intracellular Cl- levels ([Cl-]i) preserves appropriate, often inhibitory, GABAergic signaling within the brain. However, there is disagreement over the relative contribution of various mechanisms that maintain low [Cl-]i We found that the Cl- transporter KCC2 is an important Cl- extruder in the reticular thalamic (RT) nucleus, despite this nucleus having remarkably low KCC2 immunoreactivity relative to other regions of the adult brain. We also identified a smaller contribution of fixed, impermeant anions ([A]o) to lowering [Cl-]i in RT neurons. Inhibitory signaling among RT neurons is important for preventing excessive activation of RT neurons, which can be responsible for generating seizures. Our work suggests that KCC2 critically restricts the spread of activity within the RT nucleus.

Role of KCC2-dependent potassium efflux in 4-Aminopyridine-induced Epileptiform synchronization

A balance between excitation and inhibition is necessary to maintain stable brain network dynamics. Traditionally, seizure activity is believed to arise from the breakdown of this delicate balance in favor of excitation with loss of inhibition. Surprisingly, recent experimental evidence suggests that this conventional view may be limited, and that inhibition plays a prominent role in the development of epileptiform synchronization. Here, we explored the role of the KCC2 co-transporter in the onset of inhibitory network-induced seizures. Our experiments in acute mouse brain slices, of either sex, revealed that optogenetic stimulation of either parvalbumin- or somatostatin-expressing interneurons induced ictal discharges in rodent entorhinal cortex during 4-aminopyridine application. These data point to a proconvulsive role of GABAA receptor signaling that is independent of the inhibitory input location (i.e., dendritic vs. somatic). We developed a biophysically realistic network model implementing dynamics of ion concentrations to explore the mechanisms leading to inhibitory network-induced seizures. In agreement with experimental results, we found that stimulation of the inhibitory interneurons induced seizure-like activity in a network with reduced potassium A-current. Our model predicts that interneuron stimulation triggered an increase of interneuron firing, which was accompanied by an increase in the intracellular chloride concentration and a subsequent KCC2-dependent gradual accumulation of the extracellular potassium promoting epileptiform ictal activity. When the KCC2 activity was reduced, stimulation of the interneurons was no longer able to induce ictal events. Overall, our study provides evidence for a proconvulsive role of GABAA receptor signaling that depends on the involvement of the KCC2 co-transporter.

Humic Acid Confers HIGH-AFFINITY K+ TRANSPORTER 1-Mediated Salinity Stress Tolerance in Arabidopsis

Excessive salt disrupts intracellular ion homeostasis and inhibits plant growth, which poses a serious threat to global food security. Plants have adapted various strategies to survive in unfavorable saline soil conditions. Here, we show that humic acid (HA) is a good soil amendment that can be used to help overcome salinity stress because it markedly reduces the adverse effects of salinity on Arabidopsis thaliana seedlings. To identify the molecular mechanisms of HA-induced salt stress tolerance in Arabidopsis, we examined possible roles of a sodium influx transporter HIGH-AFFINITY K+ TRANSPORTER 1 (HKT1). Salt-induced root growth inhibition in HKT1 overexpressor transgenic plants (HKT1-OX) was rescued by application of HA, but not in wild-type and other plants. Moreover, salt-induced degradation of HKT1 protein was blocked by HA treatment. In addition, the application of HA to HKT1-OX seedlings led to increased distribution of Na+ in roots up to the elongation zone and caused the reabsorption of Na+ by xylem and parenchyma cells. Both the influx of the secondary messenger calcium and its cytosolic release appear to function in the destabilization of HKT1 protein under salt stress. Taken together, these results suggest that HA could be applied to the field to enhance plant growth and salt stress tolerance via post-transcriptional control of the HKT1 transporter gene under saline conditions.

N-Ethylmaleimide increases KCC2 cotransporter activity by modulating transporter phosphorylation

K+/Cl- cotransporter 2 (KCC2) is selectively expressed in the adult nervous system and allows neurons to maintain low intracellular Cl- levels. Thus, KCC2 activity is an essential prerequisite for fast hyperpolarizing synaptic inhibition mediated by type A γ-aminobutyric acid (GABAA) receptors, which are Cl--permeable, ligand-gated ion channels. Consistent with this, deficits in the activity of KCC2 lead to epilepsy and are also implicated in neurodevelopmental disorders, neuropathic pain, and schizophrenia. Accordingly, there is significant interest in developing activators of KCC2 as therapeutic agents. To provide insights into the cellular processes that determine KCC2 activity, we have investigated the mechanism by which N-ethylmaleimide (NEM) enhances transporter activity using a combination of biochemical and electrophysiological approaches. Our results revealed that, within 15 min, NEM increased cell surface levels of KCC2 and modulated the phosphorylation of key regulatory residues within the large cytoplasmic domain of KCC2 in neurons. More specifically, NEM increased the phosphorylation of serine 940 (Ser-940), whereas it decreased phosphorylation of threonine 1007 (Thr-1007). NEM also reduced with no lysine (WNK) kinase phosphorylation of Ste20-related proline/alanine-rich kinase (SPAK), a kinase that directly phosphorylates KCC2 at residue Thr-1007. Mutational analysis revealed that Thr-1007 dephosphorylation mediated the effects of NEM on KCC2 activity. Collectively, our results suggest that compounds that either increase the surface stability of KCC2 or reduce Thr-1007 phosphorylation may be of use as enhancers of KCC2 activity.

Short-Chain Fatty Acid Transporters: Role in Colonic Homeostasis

Short-chain fatty acids (SCFA; acetate, propionate, and butyrate) are generated in colon by bacterial fermentation of dietary fiber. Though diffusion in protonated form is a significant route, carrier-mediated mechanisms constitute the major route for the entry of SCFA in their anionic form into colonic epithelium. Several transport systems operate in cellular uptake of SCFA. MCT1 (SLC16A1) and MCT4 (SLC16A3) are H+-coupled and mediate electroneutral transport of SCFA (H+: SCFA stoichiometry; 1:1). MCT1 is expressed both in the apical membrane and basolateral membrane of colonic epithelium whereas MCT4 specifically in the basolateral membrane. SMCT1 (SLC5A8) and SMCT2 (SLC5A12) are Na+-coupled; SMCT1-mediated transport is electrogenic (Na+: SCFA stoichiometry; 2:1) whereas SMCT2-mediated transport is electroneutral (Na+: SCFA stoichiometry; 1:1). SMCT1 and SMCT2 are expressed exclusively in the apical membrane. An anion-exchange mechanism also operates in the apical membrane in which SCFA entry in anionic form is coupled to bicarbonate efflux; the molecular identity of this exchanger however remains unknown. All these transporters are subject to regulation, notably by their substrates themselves; this process involves cell-surface receptors with SCFA as signaling molecules. There are significant alterations in the expression of these transporters in ulcerative colitis and colon cancer. The tumor-associated changes occur via transcriptional regulation by p53 and HIF1α and by promoter methylation. As SCFA are obligatory for optimal colonic health, the transporters responsible for the entry and transcellular transfer of these bacterial products in colonic epithelium are critical determinants of colonic function under physiological conditions and in disease states. © 2018 American Physiological Society. Compr Physiol 8:299-314, 2018.

Implications of the N-terminal heterogeneity for the neuronal K-Cl cotransporter KCC2 function

The neuron-specific K-Cl cotransporter KCC2 maintains the low intracellular chloride concentration required for the fast hyperpolarizing responses of the inhibitory neurotransmitters γ-aminobutyric acid (GABA) and glycine. The two KCC2 isoforms, KCC2a and KCC2b differ by their N-termini as a result of alternative promoter usage. Whereas the role of KCC2b in mediating the chloride transport is unequivocal, the physiological role of KCC2a in neurons has remained obscure. We show that KCC2a isoform can decrease the intracellular chloride concentration in cultured neurons and attenuate calcium responses evoked by application of the GABAA receptor agonist muscimol. While the biotinylation assay detected both KCC2 isoforms at the cell surface of cultured neurons, KCC2a was not detected at the plasma membrane in immunostainings, suggesting that the N-terminal KCC2a epitope is masked. Confirming this hypothesis, KCC2a surface expression was detected by the C-terminal KCC2 pan antibody but not by the N-terminal KCC2a antibody in KCC2b-deficient neurons. One possible cause for the epitope masking is the binding site of Ste20-related proline-alanine-rich kinase (SPAK) in the KCC2a N-terminus. SPAK, a known regulator of K-Cl cotransporters, was co-immunoprecipitated in a complex with KCC2a but not KCC2b isoform. Moreover, SPAK overexpression decreased the transport activity of KCC2a but not that of KCC2b, as revealed by rubidium flux assay in HEK293 cells. Thus, our data indicate that both KCC2 isoforms perform as chloride cotransporters in neuronal cells, while their N-terminal heterogeneity could play an important role in fine-tuning of the K-Cl transport activity.

Molecular insights into the normal operation, regulation, and multisystemic roles of K+-Cl- cotransporter 3 (KCC3)

Long before the molecular identity of the Na+-dependent K+-Cl- cotransporters was uncovered in the mid-nineties, a Na+-independent K+-Cl- cotransport system was also known to exist. It was initially observed in sheep and goat red blood cells where it was shown to be ouabain-insensitive and to increase in the presence of N-ethylmaleimide (NEM). After it was established between the early and mid-nineties, the expressed sequence tag (EST) databank was found to include a sequence that was highly homologous to those of the Na+-dependent K+-Cl- cotransporters. This sequence was eventually found to code for the Na+-independent K+-Cl- cotransport function that was described in red blood cells several years before. It was termed KCC1 and led to the discovery of three isoforms called KCC2, KCC3, and KCC4. Since then, it has become obvious that each one of these isoforms exhibits unique patterns of distribution and fulfills distinct physiological roles. Among them, KCC3 has been the subject of great attention in view of its important role in the nervous system and its association with a rare hereditary sensorimotor neuropathy (called Andermann syndrome) that affects many individuals in Quebec province (Canada). It was also found to play important roles in the cardiovascular system, the organ of Corti, and circulating blood cells. As will be seen in this review, however, there are still a number of uncertainties regarding the transport properties, structural organization, and regulation of KCC3. The same is true regarding the mechanisms by which KCC3 accomplishes its numerous functions in animal cells.

Role of the Bile Acid Transporter SLC10A1 in Liver Targeting of the Lipid-Lowering Thyroid Hormone Analog Eprotirome

The thyroid hormone (TH) analog eprotirome (KB2115) was developed to lower cholesterol through selective activation of the TH receptor (TR) β1 in the liver. Interestingly, eprotirome shows low uptake in nonhepatic tissues, explaining its lipid-lowering action without adverse extrahepatic thyromimetic effects. Clinical trials have shown marked decreases in serum cholesterol levels. We explored the transport of eprotirome across the plasma membrane by members of three TH transporter families: monocarboxylate transporters MCT8 and MCT10; Na-independent organic anion transporters 1A2, 1B1, 1B3, 1C1, 2A1, and 2B1; and Na-dependent organic anion transporters SLC10A1 to SLC10A7. Cellular transport was studied in transfected COS1 cells using [14C]eprotirome and [125I]TH analogs. Of the 15 transporters tested initially, the liver-specific bile acid transporter SLC10A1 showed the highest eprotirome uptake (greater than a sevenfold induction after 60 minutes) as well as TRβ1-mediated transcriptional activity. Uptake of eprotirome by SLC10A1 was Na+ dependent and saturable with a Michaelis constant of 8 μM. Eprotirome transport was inhibited by known substrates for SLC10A1 (e.g., cholate and taurocholate), and by TH analogs such as triiodothyropropionic acid and triiodothyroacetic acid. However, no significant SLC10A1-mediated transport was observed of these [125I]TH analogs. We also studied the plasma disappearance and biliary excretion of [14C]eprotirome injected in control and Slc10a1 knockout mice. Although eprotirome is also transported by mouse Slc10a1, the pharmacokinetics of eprotirome were not affected by Slc10a1 deficiency. In conclusion, we have demonstrated that the liver-specific bile acid transporter SLC10A1 effectively transports eprotirome. However, Slc10a1 does not appear to be critical for the liver targeting of this TH analog in mice. Therefore, the importance of SLC10A1 for liver uptake of eprotirome in humans remains to be elucidated.

[Advances in studies of ileal apical sodium-dependent bile acid transporter]

Bile acids play critical roles in the regulation of metabolism and absorption of lipids. The ileal apical sodium-dependent bile acid transporter (ASBT) located at the enterocyte brush border is responsible for the reuptake of bile acids and the maintenance of bile acid homeostasis. Recently, a number of investigations have been made concerning the regulation and control of ASBT and the relationship between ASBT and intestinal inflammation, tumorigenesis, diabetes mellitus and hyperlipemia, which suggests ASBT as a potential therapeutic target of these diseases. In this review, advances in the study of above-mentioned issues were summarized.

A new appraisal of iodine refractory thyroid cancer

Thyroid cancer incidence is increasing all over the world - mostly due to an increase in the detection of small tumors that were previously undetected. A small percentage of these tumors lose the ability to uptake and/or to respond to radioiodine (RAI) therapy, especially in metastatic patients. There are several new therapeutic options that have emerged in the last 5 years to treat RAI refractory thyroid cancer patients, however, it is very important to properly identify RAI refractory patients and to clarify those appropriate for these treatments. In this review, we discuss the RAI refractory definitions and the criteria that have been suggested based on RAI uptake in the post therapy scan, as well as the response after RAI therapy and the possible molecular mechanisms involved in this process. We offer a review of the therapeutic options available at the moment and the therapeutic considerations based on a patient's individualized personal characteristics, primary tumor histology, tumor burden and location and velocity of lesion growth.

Arylhydrocarbon receptor-dependent mIndy (Slc13a5) induction as possible contributor to benzo[a]pyrene-induced lipid accumulation in hepatocytes

Non-alcoholic fatty liver disease is a growing problem in industrialized and developing countries. Hepatic lipid accumulation is the result of an imbalance between fatty acid uptake, fatty acid de novo synthesis, β-oxidation and secretion of triglyceride-rich lipoproteins from the hepatocyte. A central regulator of hepatic lipid metabolism is cytosolic citrate that can either be derived from the mitochondrium or be taken up from the blood via the plasma membrane sodium citrate transporter NaCT, the product of the mammalian INDY gene (SLC13A5). mINDY ablation protects against diet-induced steatosis whereas mINDY expression is increased in patients with hepatic steatosis. Diet-induced hepatic steatosis is also enhanced by activation of the arylhyrocarbon receptor (AhR) both in humans and animal models. Therefore, the hypothesis was tested whether the mINDY gene might be a target of the AhR. In accordance with such a hypothesis, the AhR activator benzo[a]pyrene induced the mINDY expression in primary cultures of rat hepatocytes in an AhR-dependent manner. This induction resulted in an increased citrate uptake and citrate incorporation into lipids which probably was further enhanced by the benzo[a]pyrene-dependent induction of key enzymes of fatty acid synthesis. A potential AhR binding site was identified in the mINDY promoter that appears to be conserved in the human promoter. Elimination or mutation of this site largely abolished the activation of the mINDY promoter by benzo[a]pyrene. This study thus identified the mINDY as an AhR target gene. AhR-dependent induction of the mINDY gene might contribute to the development of hepatic steatosis.

High Sodium-Induced Oxidative Stress and Poor Anticrystallization Defense Aggravate Calcium Oxalate Crystal Formation in Rat Hyperoxaluric Kidneys

Enhanced sodium excretion is associated with intrarenal oxidative stress. The present study evaluated whether oxidative stress caused by high sodium (HS) may be involved in calcium oxalate crystal formation. Male rats were fed a sodium-depleted diet. Normal-sodium and HS diets were achieved by providing drinking water containing 0.3% and 3% NaCl, respectively. Rats were fed a sodium-depleted diet with 5% hydroxyl-L-proline (HP) for 7 and 42 days to induce hyperoxaluria and/or calcium oxalate deposition. Compared to normal sodium, HS slightly increased calcium excretion despite diuresis; however, the result did not reach statistical significance. HS did not affect the hyperoxaluria, hypocalciuria or supersaturation caused by HP; however, it increased calcium oxalate crystal deposition soon after 7 days of co-treatment. Massive calcium oxalate formation and calcium crystal excretion in HS+HP rats were seen after 42 days of treatment. HP-mediated hypocitraturia was further exacerbated by HS. Moreover, HS aggravated HP-induced renal injury and tubular damage via increased apoptosis and oxidative stress. Increased urinary malondialdehyde excretion, in situ superoxide production, NAD(P)H oxidase and xanthine oxidase expression and activity, and decreased antioxidant enzyme expression or activity in the HS+HP kidney indicated exaggerated oxidative stress. Interestingly, this redox imbalance was associated with reduced renal osteopontin and Tamm-Horsfall protein expression (via increased excretion) and sodium-dependent dicarboxylate cotransporter NaDC-1 upregulation. Collectively, our results demonstrate that a HS diet induces massive crystal formation in the hyperoxaluric kidney; this is not due to increased urinary calcium excretion but is related to oxidative injury and loss of anticrystallization defense.

The Arabidopsis R-SNARE VAMP721 Interacts with KAT1 and KC1 K+ Channels to Moderate K+ Current at the Plasma Membrane

SNARE (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor) proteins drive vesicle traffic, delivering membrane and cargo to target sites within the cell and at its surface. They contribute to cell homeostasis, morphogenesis, and pathogen defense. A subset of SNAREs, including the Arabidopsis thaliana SNARE SYP121, are known also to coordinate solute uptake via physical interactions with K(+) channels and to moderate their gating at the plasma membrane. Here, we identify a second subset of SNAREs that interact to control these K(+) channels, but with opposing actions on gating. We show that VAMPs (vesicle-associated membrane proteins), which target vesicles to the plasma membrane, also interact with and suppress the activities of the inward-rectifying K(+) channels KAT1 and KC1. Interactions were evident in yeast split-ubiquitin assays, they were recovered in vivo by ratiometric bimolecular fluorescence complementation, and they were sensitive to mutation of a single residue, Tyr-57, within the longin domain of VAMP721. Interaction was also recovered on exchange of the residue at this site in the homolog VAMP723, which normally localizes to the endoplasmic reticulum and otherwise did not interact. Functional analysis showed reduced channel activity and alterations in voltage sensitivity that are best explained by a physical interaction with the channel gates. These actions complement those of SYP121, a cognate SNARE partner of VAMP721, and lead us to propose that the channel interactions reflect a "hand-off" in channel control between the two SNARE proteins that is woven together with vesicle fusion.

Selective inhibition of KCC2 leads to hyperexcitability and epileptiform discharges in hippocampal slices and in vivo

GABA(A) receptors form Cl(-) permeable channels that mediate the majority of fast synaptic inhibition in the brain. The K(+)/Cl(-) cotransporter KCC2 is the main mechanism by which neurons establish low intracellular Cl(-) levels, which is thought to enable GABAergic inhibitory control of neuronal activity. However, the widely used KCC2 inhibitor furosemide is nonselective with antiseizure efficacy in slices and in vivo, leading to a conflicting scheme of how KCC2 influences GABAergic control of neuronal synchronization. Here we used the selective KCC2 inhibitor VU0463271 [N-cyclopropyl-N-(4-methyl-2-thiazolyl)-2-[(6-phenyl-3-pyridazinyl)thio]acetamide] to investigate the influence of KCC2 function. Application of VU0463271 caused a reversible depolarizing shift in E(GABA) values and increased spiking of cultured hippocampal neurons. Application of VU0463271 to mouse hippocampal slices under low-Mg(2+) conditions induced unremitting recurrent epileptiform discharges. Finally, microinfusion of VU0463271 alone directly into the mouse dorsal hippocampus rapidly caused epileptiform discharges. Our findings indicated that KCC2 function was a critical inhibitory factor ex vivo and in vivo.

Contribution of the potassium-chloride cotransporter KCC2 to the strength of inhibition in the neonatal rodent spinal cord in vitro

In healthy mature motoneurons (MNs), KCC2 cotransporters maintain the intracellular chloride concentration at low levels, a prerequisite for postsynaptic inhibition mediated by GABA and glycine. KCC2 expression in lumbar MNs is reduced after spinal cord injury (SCI) resulting in a depolarizing shift of the chloride equilibrium potential. Despite modeling studies indicating that such a downregulation of KCC2 function would reduce the strength of postsynaptic inhibition, physiological evidence is still lacking. The present study aimed at investigating the functional impact of a modification of KCC2 function. We focused on a well characterized disynaptic inhibitory pathway responsible for reciprocal inhibition between antagonistic muscles. We performed in vitro extracellular recordings on spinal cords isolated from rodents at the end of the first postnatal week. Genetic reduction of KCC2 expression, pharmacological blockade of KCC2, as well as SCI-induced downregulation of KCC2 all resulted in a reduction of the strength of reciprocal inhibition. We then tried to restore endogenous inhibition after SCI by means of zinc ions that have been shown to boost KCC2 function in other models. Zinc chloride indeed hyperpolarized the chloride equilibrium potential in MNs and increased reciprocal inhibition after neonatal SCI. This study demonstrates that the level of KCC2 function sets the strength of postsynaptic inhibition and suggests that the downregulation of KCC2 after SCI likely contributes to the high occurrence of flexor-extensor cocontractions in SCI patients.

KCC2-mediated regulation of respiration-related rhythmic activity during postnatal development in mouse medulla oblongata

GABA acts as inhibitory neurotransmitter in the adult central nervous system but as excitatory neurotransmitter during early postnatal development. This shift in GABA's action from excitation to inhibition is caused by a decrease in intracellular chloride concentration ([Cl(-)]i), which in turn is caused by changes in the relative expression levels of the K(+)-Cl(-) co-transporter (KCC2) and the Na(+), K(+)-2Cl(-) co-transporter (NKCC1) proteins. Previous studies have used slices containing the medullary pre-Bötzinger complex (pre-BötC) to record respiration-related rhythmic activity (RRA) from the hypoglossal nucleus (12 N). The role of GABAergic transmission in the regulation of medullary RRA neonatally, however, is yet to be determined. Here, we examined how GABA and chloride co-transporters contribute to RRA during development in the 12 N where inspiratory neurons reside. We recorded extracellular RRA in medullary slices obtained from postnatal day (P) 0-7 mice. RRA was induced by soaking slices in artificial cerebrospinal fluid (aCSF) containing 8mM-K(+). Application of GABA significantly increased the frequency of RRA after P3, whereas application of a KCC2 blocker (R (+)-[(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-indenyl-5-yl)oxy]acetic acid (DIOA)) significantly decreased the frequency of RRA after P1. In addition, dense KCC2 immunolabeling was seen in the superior longitudinalis (SL) of the 12 N, which is responsible for retraction of the tongue, from P0 and P7. These results indicate that GABA administration can increase RRA frequency during the first week following birth. This in turn suggests that decreasing [Cl(-)]i levels caused by increasing KCC2 levels in the 12 N could play important roles in regulating the frequency of RRA during development.

Urea retranslocation from senescing Arabidopsis leaves is promoted by DUR3-mediated urea retrieval from leaf apoplast

In plants, urea derives either from root uptake or protein degradation. Although large quantities of urea are released during senescence, urea is mainly seen as a short-lived nitrogen (N) catabolite serving urease-mediated hydrolysis to ammonium. Here, we investigated the roles of DUR3 and of urea in N remobilization. During natural leaf senescence urea concentrations and DUR3 transcript levels showed a parallel increase with senescence markers like ORE1 in a plant age- and leaf age-dependent manner. Deletion of DUR3 decreased urea accumulation in leaves, whereas the fraction of urea lost to the leaf apoplast was enhanced. Under natural and N deficiency-induced senescence DUR3 promoter activity was highest in the vasculature, but was also found in surrounding bundle sheath and mesophyll cells. An analysis of petiole exudates from wild-type leaves revealed that N from urea accounted for >13% of amino acid N. Urea export from senescent leaves further increased in ureG-2 deletion mutants lacking urease activity. In the dur3 ureG double insertion line the absence of DUR3 reduced urea export from leaf petioles. These results indicate that urea can serve as an early metabolic marker for leaf senescence, and that DUR3-mediated urea retrieval contributes to the retranslocation of N from urea during leaf senescence.

The sodium/multivitamin transporter: a multipotent system with therapeutic implications

The Na(+)/multivitamin transporter (SMVT) is a member of the solute:sodium symporter family that catalyzes the Na(+)-dependent uptake of the structurally diverse water-soluble vitamins pantothenic acid (vitamin B5) and biotin (vitamin H), α-lipoic acid-a vitamin-like substance with strong antioxidant properties-and iodide. The organic substrates of SMVT play central roles in the cellular metabolism and are, therefore, essential for normal human health and development. For example, biotin deficiency leads to growth retardation, dermatological disorders, and neurological disorders. Animal studies have shown that biotin deficiency during pregnancy is directly correlated to embryonic growth retardation, congenital malformation, and death of the embryo. This chapter focuses on the structural and functional features of the human isoform of SMVT (hSMVT); the discovery of which was greatly facilitated by the cloning and expression of hSMVT in tractable expression systems. Special emphasis will be given to mechanistic implications of the transport process of hSMVT that will inform our understanding of the molecular determinants of hSMVT-mediated transport in dynamic context to alleviate the development and optimization of hSMVT as a multipotent platform for drug delivery.

Monocarboxylate transporter 1 deficiency and ketone utilization

Ketoacidosis is a potentially lethal condition caused by the imbalance between hepatic production and extrahepatic utilization of ketone bodies. We performed exome sequencing in a patient with recurrent, severe ketoacidosis and identified a homozygous frameshift mutation in the gene encoding monocarboxylate transporter 1 (SLC16A1, also called MCT1). Genetic analysis in 96 patients suspected of having ketolytic defects yielded seven additional inactivating mutations in MCT1, both homozygous and heterozygous. Mutational status was found to be correlated with ketoacidosis severity, MCT1 protein levels, and transport capacity. Thus, MCT1 deficiency is a novel cause of profound ketoacidosis; the present work suggests that MCT1-mediated ketone-body transport is needed to maintain acid-base balance.

NIS mediates iodide uptake in the female reproductive tract and is a poor prognostic factor in ovarian cancer

CONTEXT

The sodium iodide symporter (NIS) mediates active transport of iodide into the thyroid and the lactating mammary glands and is highly expressed in thyroid and breast carcinomas. NIS is clinically very relevant because it allows the treatment with radioiodine of thyroid cancer patients.

OBJECTIVE

In this study we wanted to explore whether NIS is expressed in the ovary and in ovarian cancer.

METHODS/PATIENTS

Methods included NIS and paired box 8 expression and function in ovarian cancer patients and rats by immunochemistry, immunoblot, RT-PCR, and iodide uptake.

RESULTS

Here we demonstrate for the first time that NIS is expressed in the ovary and fallopian tube and actively accumulates significant levels of radioiodide in vivo. In a large survey of menstruating women receiving radioiodide for medical purposes, 15% showed significant uptake in the normal reproductive tract. Ovarian NIS activity is influenced by the estrous cycle stage in rats, being up-regulated during peak levels of estrogens occurring immediately before the ovulation. We unveil that the regulatory mechanism underlying this phenomenon is based on the functional cooperation of estrogen receptor-α and paired box 8. We also show that NIS is highly expressed in ovarian cancer, predicting a poor prognosis in these patients.

CONCLUSIONS

These results provide the basis that will help minimize the impact of therapeutic doses of radioiodide on gonadal function. We also suggest that NIS is a new ovarian cancer marker, opening a door for the use of radioiodide in the diagnosis and treatment of ovarian cancer patients.

Therapeutic restoration of spinal inhibition via druggable enhancement of potassium-chloride cotransporter KCC2-mediated chloride extrusion in peripheral neuropathic pain

Peripheral neuropathic pain, typified by the development of spontaneous pain or pain hypersensitivity following injury to the peripheral nervous system, is common, greatly impairs quality of life, and is inadequately treated with available drugs. Maladaptive changes in chloride homeostasis due to a decrease in the functional expression of the potassium-chloride cotransporter KCC2 in spinal cord dorsal horn neurons are a major contributor to the central disinhibition of γ-aminobutyric acid type A receptor- and glycine receptor-mediated signaling that characterizes neuropathic pain. A compelling novel analgesic strategy is to restore spinal ionotropic inhibition by enhancing KCC2-mediated chloride extrusion. We review the data on which this theory of alternative analgesia is based, discuss recent high-throughput screens that have searched for small-molecule activators of KCC2, and propose other strategies of KCC2 activation based on recent developments in the basic understanding of KCC2's functional regulation. Exploiting the chloride-dependent functional plasticity of the γ-aminobutyric acid and glycinergic system by targeting KCC2 may be a tenable method of restoring ionotropic inhibition not only in neuropathic pain but also in other "hyperexcitable" diseases of the nervous system such as seizures and spasticity.

Baculovirus vector-mediated transfer of sodium iodide symporter and plasminogen kringle 5 genes for tumor radioiodide therapy

Background

Both tumor cells and their supporting endothelial cells should be considered for targeted cell killing when designing cancer treatments. Here we investigated the feasibility of combining radioiodide and antiangiogenic therapies after baculovirus-mediated transfer of genes encoding the sodium iodide symporter (NIS) and plasminogen kringle 5 (K5).

Methods

A recombinant baculovirus containing the NIS gene under control of the human telomerase reverse transcriptase (hTERT) promoter and the K5 gene driven by the early growth response 1 (Egr1) promoter was developed. Dual-luciferase reporter assay was performed to confirm the activation of hTERT transcription. NIS and K5 gene expression were identified by Western blot and Real-Time PCR. Functional NIS activity in baculovirus-infected Hela cells was confirmed by the uptake of ¹²⁵I and cytotoxicity of ¹³¹I. The apoptotic effect of ¹³¹I-induced K5 on baculovirus-infected human umbilical vein endothelial cells (HUVECs) was analyzed by a flow cytometry-based assay. In vivo, NIS reporter gene imaging and therapeutic experiments with ¹³¹I were performed. Finally, the microvessel density (MVD) in tumors after treatment was determined by CD31 immunostaining.

Results

The activation of hTERT transcription was specifically up-regulated in tumor cells. NIS gene expression markedly increased in baculovirus-infected HeLa cells, but not in MRC5 cells. The Hela cells showed a significant increase of ¹²⁵I uptake, which was inhibited by NaClO₄, and a notably decreased cell survival rate by ¹³¹I treatment. Expression of the K5 gene induced by ¹³¹I was elevated in a dose- and time-dependent manner and resulted in the apoptosis of HUVECs. Furthermore, ¹³¹I SPECT imaging clearly showed cervical tumor xenografts infected with recombinant baculovirus. Following therapy, tumor growth was significantly retarded. CD31 immunostaining confirmed a significant decrease of MVD.

Conclusion

The recombinant baculovirus supports a promising strategy of NIS-based raidoiodide therapy combined with K5-based antiangiogenic therapy by targeting both the tumor and its supporting vessels.

Maturation of AMPAR composition and the GABAAR reversal potential in hPSC-derived cortical neurons

Rodent-based studies have shown that neurons undergo major developmental changes to ion channel expression and ionic gradients that determine their excitation-inhibition balance. Neurons derived from human pluripotent stem cells theoretically offer the potential to study classical developmental processes in a human-relevant system, although this is currently not well explored. Here, we show that excitatory cortical-patterned neurons derived from multiple human pluripotent stem cell lines exhibit native-like maturation changes in AMPAR composition such that there is an increase in the expression of GluA2(R) subunits. Moreover, we observe a dynamic shift in intracellular Cl- levels, which determines the reversal potential of GABAAR-mediated currents and is influenced by neurotrophic factors. The shift is concomitant with changes in KCC2 and NKCC1 expression. Because some human diseases are thought to involve perturbations to AMPAR GluA2 content and others in the chloride reversal potential, human stem-cell-derived neurons represent a valuable tool for studying these fundamental properties.

Cooperation of histone deacetylase inhibitors SAHA and valproic acid in promoting sodium/iodide symporter expression and function in rat Leydig testicular carcinoma cells

The presence of the sodium/iodide symporter (NIS) is the prerequisite for the use of the radioiodine in the treatment of thyroid cancer. Thus, stimulators of NIS expression and function are currently investigated in cellular models of various human malignancies, also including extrathyroid cancers. In this study, we analyzed the effects of the histone deacetylase inhibitors (HDACi), suberoylanilide hydroxamic acid (SAHA) and valproic acid (VPA), on NIS expression and function in rat Leydig testicular carcinoma cells (LC540). LC540 cells were exposed to SAHA 3 μM and VPA 3 mM (alone and in combination), and cell viability evaluated by MTT assay and cell counting, NIS mRNA and protein levels by using, respectively, real-time RT-PCR and western blotting. NIS function was evaluated by iodide uptake assay. We found that both HDACi were able to stimulate the transcription of NIS gene, but not its protein expression, while the association of SAHA and VPA increased both NIS transcript and protein levels, resulting in significant sixfold enhancement of radioiodine uptake capacity of LC540 cells. These data demonstrate the presence of an epigenetic control of NIS expression in Leydig tumor cells, suggesting the possibility to use the combination of these two HDACi for a radioiodine-based treatment of these malignancies.

Regulation of monocarboxylic acid transporter-1 by cAMP dependent vesicular trafficking in brain microvascular endothelial cells

In this study, a detailed characterization of Monocarboxylic Acid Transporter-1 (Mct1) in cytoplasmic vesicles of cultured rat brain microvascular endothelial cells shows them to be a diverse population of endosomes intrinsic to the regulation of the transporter by a brief 25 to 30 minute exposure to the membrane permeant cAMP analog, 8Br-cAMP. The vesicles are heterogeneous in size, mobility, internal pH, and co-localize with discreet markers of particular types of endosomes including early endosomes, clathrin coated vesicles, caveolar vesicles, trans-golgi, and lysosomes. The vesicular localization of Mct1 was not dependent on its N or C termini, however, the size and pH of Mct1 vesicles was increased by deletion of either terminus demonstrating a role for the termini in vesicular trafficking of Mct1. Using a novel BCECF-AM based assay developed in this study, 8Br-cAMP was shown to decrease the pH of Mct1 vesicles after 25 minutes. This result and method were confirmed in experiments with a ratiometric pH-sensitive EGFP-mCherry dual tagged Mct1 construct. Overall, the results indicate that cAMP signaling reduces the functionality of Mct1 in cerebrovascular endothelial cells by facilitating its entry into a highly dynamic vesicular trafficking pathway that appears to lead to the transporter's trafficking to autophagosomes and lysosomes.

PAT1 (SLC36A1) shows nuclear localization and affects growth of smooth muscle cells from rats

The proton-coupled amino acid transporter 1 (PAT1) is a transporter of amino acids in small intestinal enterocytes. PAT1 is, however, also capable of regulating cell growth and sensing the availability of amino acids in other cell types. The aim of the present study was to investigate the localization and function of PAT1 in smooth muscle cells (SMCs). The PAT1 protein was found in smooth muscles from rat intestine and in the embryonic rat aorta cell line A7r5. Immunolocalization and cellular fractionation studies revealed that the majority of the PAT1 protein located within the cell nucleus of A7r5 cells. These results were confirmed in primary SMCs derived from rat aorta and colon. A 3'-untranslated region of the PAT1 transcript directed the nuclear localization. Neither cellular starvation nor cell division altered the nuclear localization. In agreement, uptake studies of l-proline, a PAT1 substrate, in A7r5 cells suggested an alternative role for PAT1 in SMCs than in transport. To shed light on the function of PAT1 in A7r5 cells, experiments with downregulation of the PAT1 level by use of a siRNA approach were conducted. The growth rates of the cells were evaluated, and knockdown of PAT1 led to induced cellular growth, suggesting a role for PAT1 in regulating cellular proliferation of SMCs.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Involvement of the K+-Cl- co-transporter KCC2 in the sensitization to morphine-induced hyperlocomotion under chronic treatment with zolpidem in the mesolimbic system

Benzodiazepines are commonly used as sedatives, sleeping aids, and anti-anxiety drugs. However, chronic treatment with benzodiazepines is known to induce dependence, which is considered related to neuroplastic changes in the mesolimbic system. This study investigated the involvement of K(+) -Cl(-) co-transporter 2 (KCC2) in the sensitization to morphine-induced hyperlocomotion after chronic treatment with zolpidem [a selective agonist of γ-aminobutyric acid A-type receptor (GABAA R) α1 subunit]. In this study, chronic treatment with zolpidem enhanced morphine-induced hyperlocomotion, which is accompanied by the up-regulation of KCC2 in the limbic forebrain. We also found that chronic treatment with zolpidem induced the down-regulation of protein phosphatase-1 (PP-1) as well as the up-regulation of phosphorylated protein kinase C γ (pPKCγ). Furthermore, PP-1 directly associated with KCC2 and pPKCγ, whereas pPKCγ did not associate with KCC2. On the other hand, pre-treatment with furosemide (a KCC2 inhibitor) suppressed the enhancing effects of zolpidem on morphine-induced hyperlocomotion. These results suggest that the mesolimbic dopaminergic system could be amenable to neuroplastic change through a pPKCγ-PP-1-KCC2 pathway by chronic treatment with zolpidem.

The SLC12 family of electroneutral cation-coupled chloride cotransporters

The SLC12 family encodes electroneutral cation-coupled chloride cotransporters that are critical for several physiological processes including cell volume regulation, modulation of intraneuronal chloride concentration, transepithelial ion movement, and blood pressure regulation. Members of this family are the targets of the most commonly used diuretic drugs, have been shown to be the causative genes for inherited disease such as Gitelman, Bartter and Andermann syndromes, and potentially play a role in polygenic complex diseases like arterial hypertension, epilepsy, osteoporosis, and cancer.

Enhanced GABAergic activity in the mouse primary somatosensory cortex is insufficient to alleviate chronic pain behavior with reduced expression of neuronal potassium-chloride cotransporter

The correct balance between excitation and inhibition is crucial for brain function and disrupted in several pathological conditions. Excitatory neuronal circuits in the primary somatosensory cortex (S1) are modulated by local inhibitory neurons with the balance of this excitatory and inhibitory activity important for function. The activity of excitatory layer 2/3 neurons (L2/3) in the S1 cortex is increased in chronic pain, but it is not known how the local interneurons, nor the balance between excitation and inhibition, may change in chronic pain. Using in vivo two-photon calcium imaging and electrophysiology, we report here that the response of L2/3 local inhibitory neurons to both sensory stimulation and to layer 4 electrical stimulation increases in inflammatory chronic pain. Local application into L2/3 of a GABA(A) receptor blocker further enhanced the activity of S1 excitatory neurons and reduced pain thresholds, whereas local application of the GABA(A) receptor modulators (muscimol and diazepam) transiently alleviated the allodynia. This illustrates the importance of the local inhibitory pathways in chronic pain sensation. A reduction in the expression and function of the potassium-chloride cotransporter 2 occurred during chronic pain, which reduces the efficacy of the inhibitory inputs to L2/3 excitatory neurons. In summary, both excitatory and inhibitory neuronal activities in the S1 are enhanced in the chronic pain model, but the increased inhibition is insufficient to completely counterbalance the increased excitation and alleviate the symptoms of chronic pain.

Computational modeling reveals dendritic origins of GABA(A)-mediated excitation in CA1 pyramidal neurons

GABA is the key inhibitory neurotransmitter in the adult central nervous system, but in some circumstances can lead to a paradoxical excitation that has been causally implicated in diverse pathologies from endocrine stress responses to diseases of excitability including neuropathic pain and temporal lobe epilepsy. We undertook a computational modeling approach to determine plausible ionic mechanisms of GABA(A)-dependent excitation in isolated post-synaptic CA1 hippocampal neurons because it may constitute a trigger for pathological synchronous epileptiform discharge. In particular, the interplay intracellular chloride accumulation via the GABA(A) receptor and extracellular potassium accumulation via the K/Cl co-transporter KCC2 in promoting GABA(A)-mediated excitation is complex. Experimentally it is difficult to determine the ionic mechanisms of depolarizing current since potassium transients are challenging to isolate pharmacologically and much GABA signaling occurs in small, difficult to measure, dendritic compartments. To address this problem and determine plausible ionic mechanisms of GABA(A)-mediated excitation, we built a detailed biophysically realistic model of the CA1 pyramidal neuron that includes processes critical for ion homeostasis. Our results suggest that in dendritic compartments, but not in the somatic compartments, chloride buildup is sufficient to cause dramatic depolarization of the GABA(A) reversal potential and dominating bicarbonate currents that provide a substantial current source to drive whole-cell depolarization. The model simulations predict that extracellular K(+) transients can augment GABA(A)-mediated excitation, but not cause it. Our model also suggests the potential for GABA(A)-mediated excitation to promote network synchrony depending on interneuron synapse location - excitatory positive-feedback can occur when interneurons synapse onto distal dendritic compartments, while interneurons projecting to the perisomatic region will cause inhibition.

Vhc1, a novel transporter belonging to the family of electroneutral cation-Cl(-) cotransporters, participates in the regulation of cation content and morphology of Saccharomyces cerevisiae vacuoles

Cation-Cl(-) cotransporters (CCCs) are integral membrane proteins which catalyze the coordinated symport of Cl(-) with Na(+) and/or K(+) ions in plant and mammalian cells. Here we describe the first Saccharomyces cerevisiae CCC protein, encoded by the YBR235w open reading frame. Subcellular localization studies showed that this yeast CCC is targeted to the vacuolar membrane. Deletion of the YBR235w gene in a salt-sensitive strain (lacking the plasma-membrane cation exporters) resulted in an increased sensitivity to high KCl, altered vacuolar morphology control and decreased survival upon hyperosmotic shock. In addition, deletion of the YBR235w gene in a mutant strain deficient in K(+) uptake produced a significant growth advantage over the parental strain under K(+)-limiting conditions, and a hypersensitivity to the exogenous K(+)/H(+) exchanger nigericin. These results strongly suggest that we have identified a novel yeast vacuolar ion transporter mediating a K(+)-Cl(-) cotransport and playing a role in vacuolar osmoregulation. Considering its identified function, we propose to refer to the yeast YBR235w gene as VHC1 (vacuolar protein homologous to CCC family 1).

Effect of heterozygous deletion of WNK1 on the WNK-OSR1/ SPAK-NCC/NKCC1/NKCC2 signal cascade in the kidney and blood vessels

BACKGROUND

We found that a mechanism of hypertension in pseudohypoaldosteronism type II (PHAII) caused by a WNK4 missense mutation (D561A) was activation of the WNK-OSR1/SPAK-NCC signal cascade. However, the pathogenic effect of intronic deletions in WNK1 genes also observed in PHAII patients remains unclear. To understand the pathophysiological roles of WNK1 in vivo, WNK1(+/-)mice have been analyzed, because homozygous WNK1 knockout is embryonic lethal. Although WNK1(+/-) mice have been reported to have hypotension, detailed analyses of the WNK signal cascade in the kidney and other organs of WNK1(+/-) mice have not been performed.

METHOD

We assess the effect of heterozygous deletion of WNK1 on the WNK-OSR1/SPAK-NCC/NKCC1/NKCC2 signal cascade in the kidney and blood vessels.

RESULTS

Contrary to the previous report, the blood pressure of WNK1(+/-) mice was not decreased, even under a low-salt diet. Under a WNK4(D561A/+) background, the heterozygous deletion of the WNK1 gene did not reduce the high blood pressure either. We then evaluated the phosphorylation status of OSR1, SPAK, NCC, NKCC1, and NKCC2 in the kidney, but no significant decrease in the phosphorylation was observed in WNK1(+/-) mice or WNK1(+/-)WNK4(D561A/+) mice. In contrast, a significant decrease in NKCC1 phosphorylation in the aorta and a decreased pressure-induced myogenic response in the mesenteric arteries were observed in WNK1(+/-) mice.

CONCLUSION

The contribution of WNK1 to total WNK kinase activity in the kidney may be small, but that WNK1 may play a substantial role in the regulation of blood pressure in the arteries.

Molecular and functional expression of cation-chloride cotransporters in dorsal root ganglion neurons during postnatal maturation

GABA depolarizes and excites central neurons during early development, becoming inhibitory and hyperpolarizing with maturation. This "developmental shift" occurs abruptly, reflecting a decrease in intracellular Cl(-) concentration ([Cl(-)](i)) and a hyperpolarizing shift in Cl(-) equilibrium potential due to upregulation of the K(+)-Cl(-) cotransporter KCC2b, a neuron-specific Cl(-) extruder. In contrast, primary afferent neurons (PANs) are depolarized by GABA throughout adulthood because of expression of NKCC1, a Na(+)-K(+)-2Cl(-) cotransporter that accumulates Cl(-) above equilibrium. The GABA(A)-mediated depolarization of PANs determines presynaptic inhibition in the spinal cord, a key mechanism gating somatosensory information. Little is known about developmental changes in Cl(-) transporter expression and Cl(-) homeostasis in PANs. Whether NKCC1 is expressed in PANs of all phenotypes or is restricted to subpopulations (e.g., nociceptors) is debatable. Likewise, whether PANs express KCC2s is controversial. We investigated NKCC1 and K(+)-Cl(-) cotransporter expression in rat and mouse dorsal root ganglion (DRG) neurons with molecular methods. Using fluorescence imaging microscopy, we measured [Cl(-)](i) in acutely dissociated rat DRG neurons (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and classified with phenotypic markers. DRG neurons of all sizes express two NKCC1 mRNAs, one full-length and a shorter splice variant lacking exon 21. Immunolabeling with validated antibodies revealed ubiquitous expression of NKCC1 in DRG neurons irrespective of postnatal age and phenotype. As maturation progresses [Cl(-)](i) decreases gradually, persisting above equilibrium in >95% mature neurons. DRG neurons express mRNAs for KCC1, KCC3s, and KCC4, but not for KCC2s. Mechanisms underlying PANs' developmental changes in Cl(-) homeostasis are discussed and compared with those of central neurons.

Overexpression of monocarboxylate transporter-1 (SLC16A1) in mouse pancreatic β-cells leads to relative hyperinsulinism during exercise

Exercise-induced hyperinsulinism (EIHI) is an autosomal dominant disorder characterized by inappropriate insulin secretion in response to vigorous physical exercise or pyruvate injection. Activating mutations in the monocarboxylate transporter-1 (MCT1, SLC16A1) promoter have been linked to EIHI. Expression of this pyruvate transporter is specifically repressed (disallowed) in pancreatic β-cells, despite nearly universal expression across other tissues. It has been impossible to determine, however, whether EIHI mutations cause MCT1 expression in patient β-cells. The hypothesis that MCT1 expression in β-cells is sufficient to cause EIHI by allowing entry of pyruvate and triggering insulin secretion thus remains unproven. Therefore, we generated a transgenic mouse capable of doxycycline-induced, β-cell-specific overexpression of MCT1 to test this model directly. MCT1 expression caused isolated islets to secrete insulin in response to pyruvate, without affecting glucose-stimulated insulin secretion. In vivo, transgene induction lowered fasting blood glucose, mimicking EIHI. Pyruvate challenge stimulated increased plasma insulin and smaller excursions in blood glucose in transgenic mice. Finally, in response to exercise, transgene induction prevented the normal inhibition of insulin secretion. Forced overexpression of MCT1 in β-cells thus replicates the key features of EIHI and highlights the importance of this transporter's absence from these cells for the normal control of insulin secretion.