Increased neuronal excitability and seizures in the Na(+)/H(+) exchanger null mutant mouse

Leukocyte differential gene expression prognostic value for high versus low seizure frequency in temporal lobe epilepsy

BACKGROUND

This study was performed to test the hypothesis that systemic leukocyte gene expression has prognostic value differentiating low from high seizure frequency refractory temporal lobe epilepsy (TLE).

METHODS

A consecutive series of patients with refractory temporal lobe epilepsy was studied. Based on a median baseline seizure frequency of 2.0 seizures per month, low versus high seizure frequency was defined as ≤ 2 seizures/month and > 2 seizures/month, respectively. Systemic leukocyte gene expression was analyzed for prognostic value for TLE seizure frequency. All differentially expressed genes were analyzed, with Ingenuity® Pathway Analysis (IPA®) and Reactome, to identify leukocyte gene expression and biological pathways with prognostic value for seizure frequency.

RESULTS

There were ten males and six females with a mean age of 39.4 years (range: 16 to 62 years, standard error of mean: 3.6 years). There were five patients in the high and eleven patients in the low seizure frequency cohorts, respectively. Based on a threshold of twofold change (p < 0.001, FC > 2.0, FDR < 0.05) and expression within at least two pathways from both Reactome and Ingenuity® Pathway Analysis (IPA®), 13 differentially expressed leukocyte genes were identified which were all over-expressed in the low when compared to the high seizure frequency groups, including NCF2, HMOX1, RHOB, FCGR2A, PRKCD, RAC2, TLR1, CHP1, TNFRSF1A, IFNGR1, LYN, MYD88, and CASP1. Similar analysis identified four differentially expressed genes which were all over-expressed in the high when compared to the low seizure frequency groups, including AK1, F2R, GNB5, and TYMS.

CONCLUSIONS

Low and high seizure frequency TLE are predicted by the respective upregulation and downregulation of specific leukocyte genes involved in canonical pathways of neuroinflammation, oxidative stress and lipid peroxidation, GABA (γ-aminobutyric acid) inhibition, and AMPA and NMDA receptor signaling. Furthermore, high seizure frequency-TLE is distinguished prognostically from low seizure frequency-TLE by differentially increased specific leukocyte gene expression involved in GABA inhibition and NMDA receptor signaling. High and low seizure frequency patients appear to represent two mechanistically different forms of temporal lobe epilepsy based on leukocyte gene expression.

Employing the sustained-release properties of poly(lactic-co-glycolic acid) nanoparticles to reveal a novel mechanism of sodium-hydrogen exchanger 1 in neuropathic pain

Neuropathic pain is caused by injury or disease of the somatosensory system, and its course is usually chronic. Several studies have been dedicated to investigating neuropathic pain-related targets; however, little attention has been paid to the persistent alterations that these targets, some of which may be crucial to the pathophysiology of neuropathic pain. The present study aimed to identify potential targets that may play a crucial role in neuropathic pain and validate their long-term impact. Through bioinformatics analysis of RNA sequencing results, we identified Slc9a1 and validated the reduced expression of sodium-hydrogen exchanger 1 (NHE1), the protein that Slc9a1 encodes, in the spinal nerve ligation (SNL) model. Colocalization analysis revealed that NHE1 is primarily co-localized with vesicular glutamate transporter 2-positive neurons. In vitro experiments confirmed that poly(lactic-co-glycolic acid) nanoparticles loaded with siRNA successfully inhibited NHE1 in SH-SY5Y cells, lowered intracellular pH, and increased intracellular calcium concentrations. In vivo experiments showed that sustained suppression of spinal NHE1 expression by siRNA-loaded nanoparticles resulted in delayed hyperalgesia in naïve and SNL model rats, whereas amiloride-induced transient suppression of NHE1 expression yielded no significant changes in pain sensitivity. We identified Slc9a1, which encodes NHE1, as a key gene in neuropathic pain. Utilizing the sustained release properties of nanoparticles enabled us to elucidate the chronic role of decreased NHE1 expression, establishing its significance in the mechanisms of neuropathic pain.

Stress-induced reduction of Na+/H+ exchanger isoform 1 promotes maladaptation of neuroplasticity and exacerbates depressive behaviors

Major depression disorder (MDD) is a neuropsychiatric disorder characterized by abnormal neuronal activity in specific brain regions. A factor that is crucial in maintaining normal neuronal functioning is intracellular pH (pHi) homeostasis. In this study, we show that chronic stress, which induces depression-like behaviors in animal models, down-regulates the expression of the hippocampal Na⁺/H⁺ exchanger isoform 1, NHE1, a major determinant of pHi in neurons. Knockdown of NHE1 in CA1 hippocampal pyramidal neurons leads to intracellular acidification, promotes dendritic spine loss, lowers excitatory synaptic transmission, and enhances the susceptibility to stress exposure in rats. Moreover, E3 ubiquitin ligase cullin4A may promote ubiquitination and degradation of NHE1 to induce these effects of an unbalanced pHi on synaptic processes. Electrophysiological data further suggest that the abnormal excitability of hippocampal neurons caused by maladaptation of neuroplasticity may be involved in the pathogenesis of this disease. These findings elucidate a mechanism for pHi homeostasis alteration as related to MDD.

Real time imaging of single extracellular vesicle pH regulation in a microfluidic cross-flow filtration platform

Extracellular vesicles (EVs) are cell-derived membranous structures carrying transmembrane proteins and luminal cargo. Their complex cargo requires pH stability in EVs while traversing diverse body fluids. We used a filtration-based platform to capture and stabilize EVs based on their size and studied their pH regulation at the single EV level. Dead-end filtration facilitated EV capture in the pores of an ultrathin (100 nm thick) and nanoporous silicon nitride (NPN) membrane within a custom microfluidic device. Immobilized EVs were rapidly exposed to test solution changes driven across the backside of the membrane using tangential flow without exposing the EVs to fluid shear forces. The epithelial sodium-hydrogen exchanger, NHE1, is a ubiquitous plasma membrane protein tasked with the maintenance of cytoplasmic pH at neutrality. We show that NHE1 identified on the membrane of EVs is functional in the maintenance of pH neutrality within single vesicles. This is the first mechanistic description of EV function on the single vesicle level.

Riazanski et al describe a platform to capture extracellular vesicles (EVs) using a nanoporous silicon nitride membrane, investigate the expression of NHE1 protein on the surface of EVs and monitor the transport of Na+ and H+ at the single EV level. The authors report a mechanistic function of the proteins found in EVs and specifically identify NHE1 on a single EV, where it maintains pH neutrality within single vesicles.

Review and Hypothesis: A Potential Common Link Between Glial Cells, Calcium Changes, Modulation of Synaptic Transmission, Spreading Depression, Migraine, and Epilepsy-H. Front Cell Neurosci 2021;15:693095. [PMID: 34539347 PMCID: PMC8446203 DOI: 10.3389/fncel.2021.693095]

There is significant evidence to support the notion that glial cells can modulate the strength of synaptic connections between nerve cells, and it has further been suggested that alterations in intracellular calcium are likely to play a key role in this process. However, the molecular mechanism(s) by which glial cells modulate neuronal signaling remains contentiously debated. Recent experiments have suggested that alterations in extracellular H+ efflux initiated by extracellular ATP may play a key role in the modulation of synaptic strength by radial glial cells in the retina and astrocytes throughout the brain. ATP-elicited alterations in H+ flux from radial glial cells were first detected from Müller cells enzymatically dissociated from the retina of tiger salamander using self-referencing H+-selective microelectrodes. The ATP-elicited alteration in H+ efflux was further found to be highly evolutionarily conserved, extending to Müller cells isolated from species as diverse as lamprey, skate, rat, mouse, monkey and human. More recently, self-referencing H+-selective electrodes have been used to detect ATP-elicited alterations in H+ efflux around individual mammalian astrocytes from the cortex and hippocampus. Tied to increases in intracellular calcium, these ATP-induced extracellular acidifications are well-positioned to be key mediators of synaptic modulation. In this article, we examine the evidence supporting H+ as a key modulator of neurotransmission, review data showing that extracellular ATP elicits an increase in H+ efflux from glial cells, and describe the potential signal transduction pathways involved in glial cell-mediated H+ efflux. We then examine the potential role that extracellular H+ released by glia might play in regulating synaptic transmission within the vertebrate retina, and then expand the focus to discuss potential roles in spreading depression, migraine, epilepsy, and alterations in brain rhythms, and suggest that alterations in extracellular H+ may be a unifying feature linking these disparate phenomena.

Decreased Brain pH and Pathophysiology in Schizophrenia

Postmortem studies reveal that the brain pH in schizophrenia patients is lower than normal. The exact cause of this low pH is unclear, but increased lactate levels due to abnormal energy metabolism appear to be involved. Schizophrenia patients display distinct changes in mitochondria number, morphology, and function, and such changes promote anaerobic glycolysis, elevating lactate levels. pH can affect neuronal activity as H⁺ binds to numerous proteins in the nervous system and alters the structure and function of the bound proteins. There is growing evidence of pH change associated with cognition, emotion, and psychotic behaviors. Brain has delicate pH regulatory mechanisms to maintain normal pH in neurons/glia and extracellular fluid, and a change in these mechanisms can affect, or be affected by, neuronal activities associated with schizophrenia. In this review, we discuss the current understanding of the cause and effect of decreased brain pH in schizophrenia based on postmortem human brains, animal models, and cellular studies. The topic includes the factors causing decreased brain pH in schizophrenia, mitochondria dysfunction leading to altered energy metabolism, and pH effects on the pathophysiology of schizophrenia. We also review the acid/base transporters regulating pH in the nervous system and discuss the potential contribution of the major transporters, sodium hydrogen exchangers (NHEs), and sodium-coupled bicarbonate transporters (NCBTs), to schizophrenia.

Role of Genetic Mutations of the Na+/H+ Exchanger Isoform 1, in Human Disease and Protein Targeting and Activity

The mammalian Na⁺/H⁺ exchanger isoform one (NHE1) is a plasma membrane protein that is ubiquitously present in human cells. It functions to regulate intracellular pH removing an intracellular proton in exchange for one extracellular sodium and is involved in heart disease and in promoting metastasis in cancer. It is made of a 500 amino acid membrane domain plus a 315 amino acid, regulatory cytosolic tail. The membrane domain is thought to have 12 transmembrane segments and a large membrane-associated extracellular loop. Early studies demonstrated that in mice, disruption of the NHE1 gene results in locomotor ataxia and a phenotype of slow-wave epilepsy. Defects included a progressive neuronal degeneration. Growth and reproductive ability were also reduced. Recent studies have identified human autosomal homozygous recessive mutations in the NHE1 gene (SLC9A1) that result in impaired development, ataxia and other severe defects, and explain the cause of the human disease Lichtenstein-Knorr syndrome. Other human mutations have been identified that are stop codon polymorphisms. These cause short non-functional NHE1 proteins, while other genetic polymorphisms in the NHE1 gene cause impaired expression of the NHE1 protein, reduced activity, enhanced protein degradation or altered kinetic activation of the protein. Since NHE1 plays a key role in many human physiological functions and in human disease, genetic polymorphisms of the protein that significantly alter its function and are likely play significant roles in varying human phenotypes and be involved in disease.

The Na+/H+ Exchanger Nhe1 Modulates Network Excitability via GABA Release

Brain functions are extremely sensitive to pH changes because of the pH-dependence of proteins involved in neuronal excitability and synaptic transmission. Here, we show that the Na+/H+ exchanger Nhe1, which uses the Na+ gradient to extrude H+, is expressed at both inhibitory and excitatory presynapses. We disrupted Nhe1 specifically in mice either in Emx1-positive glutamatergic neurons or in parvalbumin-positive cells, mainly GABAergic interneurons. While Nhe1 disruption in excitatory neurons had no effect on overall network excitability, mice with disruption of Nhe1 in parvalbumin-positive neurons displayed epileptic activity. From our electrophysiological analyses in the CA1 of the hippocampus, we conclude that the disruption in parvalbumin-positive neurons impairs the release of GABA-loaded vesicles, but increases the size of GABA quanta. The latter is most likely an indirect pH-dependent effect, as Nhe1 was not expressed in purified synaptic vesicles itself. Conclusively, our data provide first evidence that Nhe1 affects network excitability via modulation of inhibitory interneurons.

Intracellular pH Regulation in iPSCs-derived Astrocytes from Subjects with Chronic Mountain Sickness

Chronic Mountain Sickness (CMS) occurs in high-altitude residents with major neurological symptoms such as migraine headaches, dizziness and cognitive deficits. Recent work demonstrated that highlanders have increased intracellular pH (pH_i) in their brain cells, perhaps for the sake of adaptation to hypoxemia and help to facilitate glycolysis, DNA synthesis, and cell cycle progression. Since there are well adapted (non-CMS) and maladapted (CMS) high-altitude dwellers, it is not clear whether pH_i is differently regulated in these two high-altitude populations. In this work, we obtained induced pluripotent stem cell (iPSC)-derived astrocytes from both CMS and non-CMS highlanders who live in the Peruvian Andes (>14,000 ft) and studied pH_i regulation in these astrocytes using pH-sensitive dye BCECF. Our results show that the steady-state pH_i (ss pH_i) is lower in CMS astrocytes compared with non-CMS astrocytes. In addition, the acid extrusion following an acid loading is faster and the pH_i dependence of H⁺ flux rate becomes steeper in CMS astrocytes. Furthermore, the Na⁺ dependency of ss pH_i is stronger in CMS astrocytes and the Na⁺/H⁺ exchanger (NHE) inhibitors blunted the acid extrusion in both CMS and non-CMS astrocytes. We conclude that (a) NHE contributes to the ss pH_i stabilization and mediates active acid extrusion during the cytosolic acidosis in highlanders; (b) acid extrusion becomes less pH_i sensitive in non-CMS (versus CMS) astrocytes which may prevent NHE from over-activated in the hypoxia-induced intracellular acidosis and render the non-CMS astrocytes more resistant to hypoxemia challenges.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.

Biallelic CHP1 mutation causes human autosomal recessive ataxia by impairing NHE1 function

Objective:

To ascertain the genetic and functional basis of complex autosomal recessive cerebellar ataxia (ARCA) presented by 2 siblings of a consanguineous family characterized by motor neuropathy, cerebellar atrophy, spastic paraparesis, intellectual disability, and slow ocular saccades.

Methods:

Combined whole-genome linkage analysis, whole-exome sequencing, and focused screening for identification of potential causative genes were performed. Assessment of the functional consequences of the mutation on protein function via subcellular fractionation, size-exclusion chromatography, and fluorescence microscopy were done. A zebrafish model, using Morpholinos, was generated to study the pathogenic effect of the mutation in vivo.

Results:

We identified a biallelic 3-bp deletion (p.K19del) in CHP1 that cosegregates with the disease. Neither focused screening for CHP1 variants in 2 cohorts (ARCA: N = 319 and NeurOmics: N = 657) nor interrogating GeneMatcher yielded additional variants, thus revealing the scarcity of CHP1 mutations. We show that mutant CHP1 fails to integrate into functional protein complexes and is prone to aggregation, thereby leading to diminished levels of soluble CHP1 and reduced membrane targeting of NHE1, a major Na⁺/H⁺ exchanger implicated in syndromic ataxia-deafness. Chp1 deficiency in zebrafish, resembling the affected individuals, led to movement defects, cerebellar hypoplasia, and motor axon abnormalities, which were ameliorated by coinjection with wild-type, but not mutant, human CHP1 messenger RNA.

Conclusions:

Collectively, our results identified CHP1 as a novel ataxia-causative gene in humans, further expanding the spectrum of ARCA-associated loci, and corroborated the crucial role of NHE1 within the pathogenesis of these disorders.

Emerging roles of Na⁺/H⁺ exchangers in epilepsy and developmental brain disorders

Epilepsy is a common central nervous system (CNS) disease characterized by recurrent transient neurological events occurring due to abnormally excessive or synchronous neuronal activity in the brain. The CNS is affected by systemic acid-base disorders, and epileptic seizures are sensitive indicators of underlying imbalances in cellular pH regulation. Na(+)/H(+) exchangers (NHEs) are a family of membrane transporter proteins actively involved in regulating intracellular and organellar pH by extruding H(+) in exchange for Na(+) influx. Altering NHE function significantly influences neuronal excitability and plays a role in epilepsy. This review gives an overview of pH regulatory mechanisms in the brain with a special focus on the NHE family and the relationship between epilepsy and dysfunction of NHE isoforms. We first discuss how cells translocate acids and bases across the membrane and establish pH homeostasis as a result of the concerted effort of enzymes and ion transporters. We focus on the specific roles of the NHE family by detailing how the loss of NHE1 in two NHE mutant mice results in enhanced neuronal excitability in these animals. Furthermore, we highlight new findings on the link between mutations of NHE6 and NHE9 and developmental brain disorders including epilepsy, autism, and attention deficit hyperactivity disorder (ADHD). These studies demonstrate the importance of NHE proteins in maintaining H(+) homeostasis and their intricate roles in the regulation of neuronal function. A better understanding of the mechanisms underlying NHE1, 6, and 9 dysfunctions in epilepsy formation may advance the development of new epilepsy treatment strategies.

Regulation of neuronal pH by the metabotropic Zn(2+)-sensing Gq-coupled receptor, mZnR/GPR39

Synaptically released Zn(2+) acts as a neurotransmitter, in part, by activating the postsynaptic metabotropic Zn(2+)-sensing Gq protein-coupled receptor (mZnR/GPR39). In previous work using epithelial cells, we described crosstalk between Zn(2+) signaling and changes in intracellular pH and/or extracellular pH (pHe). As pH changes accompany neuronal activity under physiological and pathological conditions, we tested whether Zn(2+) signaling is involved in regulation of neuronal pH. Here, we report that up-regulation of a major H(+) extrusion pathway, the Na(+)/H(+) exchanger (NHE), is induced by mZnR/GPR39 activation in an extracellular-regulated kinase 1/2-dependent manner in hippocampal neurons in vitro. We also observed that changes in pHe can modulate neuronal mZnR/GPR39-dependent signaling, resulting in reduced activity at pHe 8 or 6.5. Similarly, Zn(2+)-dependent extracellular-regulated kinase 1/2 phosphorylation and up-regulation of NHE activity were absent at acidic pHe. Thus, our results suggest that when pHe is maintained within the physiological range, mZnR/GPR39 activation can up-regulate NHE-dependent recovery from intracellular acidification. During acidosis, as pHe drops, mZnR/GPR39-dependent NHE activation is inhibited, thereby attenuating further H(+) extrusion. This mechanism may serve to protect neurons from excessive decreases in pHe. Thus, mZnR/GPR39 signaling provides a homeostatic adaptive process for regulation of intracellular and extracellular pH changes in the brain. We show that the postsynaptic metabotropic Zn(2+)-sensing Gq protein-coupled receptor (mZnR/GPR39) activation induces up-regulation of a major neuronal H(+) extrusion pathway, the Na(+)/H(+) exchanger (NHE), thereby enhancing neuronal recovery from intracellular acidification. Changes in extracellular pH (pHe), however, modulate neuronal mZnR/GPR39-dependent signaling, resulting in reduced activity at pHe 8 or 6.5. This mechanism may serve to protect neurons from excessive decreases in pHe during acidosis. Hence, mZnR/GPR39 signaling provides a homeostatic adaptive process for regulation of intracellular and extracellular pH changes in the brain.

Intracellular pH regulation by acid-base transporters in mammalian neurons

Intracellular pH (pH_i) regulation in the brain is important in both physiological and physiopathological conditions because changes in pH_i generally result in altered neuronal excitability. In this review, we will cover 4 major areas: (1) The effect of pH_i on cellular processes in the brain, including channel activity and neuronal excitability. (2) pH_i homeostasis and how it is determined by the balance between rates of acid loading (J_L) and extrusion (J_E). The balance between J_E and J_L determine steady-state pH_i, as well as the ability of the cell to defend pH_i in the face of extracellular acid-base disturbances (e.g., metabolic acidosis). (3) The properties and importance of members of the SLC4 and SLC9 families of acid-base transporters expressed in the brain that contribute to J_L (namely the Cl-HCO₃ exchanger AE3) and J_E (the Na-H exchangers NHE1, NHE3, and NHE5 as well as the Na⁺- coupled HCO₃⁻ transporters NBCe1, NBCn1, NDCBE, and NBCn2). (4) The effect of acid-base disturbances on neuronal function and the roles of acid-base transporters in defending neuronal pH_i under physiopathologic conditions.

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

The SLC9 gene family encodes Na(+)/H(+) exchangers (NHEs). These transmembrane proteins transport ions across lipid bilayers in a diverse array of species from prokaryotes to eukaryotes, including plants, fungi, and animals. They utilize the electrochemical gradient of one ion to transport another ion against its electrochemical gradient. Currently, 13 evolutionarily conserved NHE isoforms are known in mammals [22, 46, 128]. The SLC9 gene family (solute carrier classification of transporters: www.bioparadigms.org) is divided into three subgroups [46]. The SLC9A subgroup encompasses plasmalemmal isoforms NHE1-5 (SLC9A1-5) and the predominantly intracellular isoforms NHE6-9 (SLC9A6-9). The SLC9B subgroup consists of two recently cloned isoforms, NHA1 and NHA2 (SLC9B1 and SLC9B2, respectively). The SLC9C subgroup consist of a sperm specific plasmalemmal NHE (SLC9C1) and a putative NHE, SLC9C2, for which there is currently no functional data [46]. NHEs participate in the regulation of cytosolic and organellar pH as well as cell volume. In the intestine and kidney, NHEs are critical for transepithelial movement of Na(+) and HCO3(-) and thus for whole body volume and acid-base homeostasis [46]. Mutations in the NHE6 or NHE9 genes cause neurological disease in humans and are currently the only NHEs directly linked to human disease. However, it is becoming increasingly apparent that members of this gene family contribute to the pathophysiology of multiple human diseases.

SLC9/NHE gene family, a plasma membrane and organellar family of Na⁺/H⁺ exchangers

This brief review of the human Na/H exchanger gene family introduces a new classification with three subgroups to the SLC9 gene family. Progress in the structure and function of this gene family is reviewed with structure based on homology to the bacterial Na/H exchanger NhaA. Human diseases which result from genetic abnormalities of the SLC9 family are discussed although the exact role of these transporters in causing any disease is not established, other than poorly functioning NHE3 in congenital Na diarrhea.

Minireview: pH and synaptic transmission

As a general rule a rise in pH increases neuronal activity, whereas it is dampened by a fall of pH. Neuronal activity per se also challenges pH homeostasis by the increase of metabolic acid equivalents. Moreover, the negative membrane potential of neurons promotes the intracellular accumulation of protons. Synaptic key players such as glutamate receptors or voltage-gated calcium channels show strong pH dependence and effects of pH gradients on synaptic processes are well known. However, the processes and mechanisms that allow controlling the pH in synaptic structures and how these mechanisms contribute to normal synaptic function are only beginning to be resolved.

From Acupuncture to Interaction between δ-Opioid Receptors and Na (+) Channels: A Potential Pathway to Inhibit Epileptic Hyperexcitability

Epilepsy is one of the most common neurological disorders affecting about 1% of population. Although the precise mechanism of its pathophysiological changes in the brain is unknown, epilepsy has been recognized as a disorder of brain excitability characterized by recurrent unprovoked seizures that result from the abnormal, excessive, and synchronous activity of clusters of nerve cells in the brain. Currently available therapies, including medical, surgical, and other strategies, such as ketogenic diet and vagus nerve stimulation, are symptomatic with their own limitations and complications. Seeking new strategies to cure this serious disorder still poses a big challenge to the field of medicine. Our recent studies suggest that acupuncture may exert its antiepileptic effects by normalizing the disrupted neuronal and network excitability through several mechanisms, including lowering the overexcited neuronal activity, enhancing the inhibitory system, and attenuating the excitatory system in the brain via regulation of the interaction between δ -opioid receptors (DOR) and Na(+) channels. This paper reviews the progress in this field and summarizes new knowledge based on our work and those of others.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

The Na(+)/H (+) exchanger NHE5 is sorted to discrete intracellular vesicles in the central and peripheral nervous systems

The pH milieu of the central and peripheral nervous systems is an important determinant of neuronal excitability, function, and survival. In mammals, neural acid-base homeostasis is coordinately regulated by ion transporters belonging to the Na(+)/H(+) exchanger (NHE) and bicarbonate transporter gene families. However, the relative contributions of individual isoforms within the respective families are not fully understood. This report focuses on the NHE family, specifically the plasma membrane-type NHE5 which is preferentially transcribed in brain, but the distribution of the native protein has not been extensively characterized. To this end, we generated a rabbit polyclonal antibody that specifically recognizes NHE5. In both central (cortex, hippocampus) and peripheral (superior cervical ganglia, SCG) nervous tissue of mice, NHE5 immunostaining was punctate and highly concentrated in the somas and to lesser amounts in the dendrites of neurons. Very little signal was detected in axons. Similarly, in primary cultures of differentiated SCG neurons, NHE5 localized predominantly to vesicles in the somatodendritic compartment, though some immunostaining was also evident in punctate vesicles along the axons. NHE5 was also detected predominantly in intracellular vesicles of cultured SCG glial cells. Dual immunolabeling of SCG neurons showed that NHE5 did not colocalize with markers for early endosomes (EEA1) or synaptic vesicles (synaptophysin), but did partially colocalize with the transferrin receptor, a marker of recycling endosomes. Collectively, these data suggest that NHE5 partitions into a unique vesicular pool in neurons that shares some characteristics of recycling endosomes where it may serve as an important regulated store of functional transporters required to maintain cytoplasmic pH homeostasis.

Intracellular pH in the resistance vasculature: regulation and functional implications

Net acid extrusion from vascular smooth muscle (VSMCs) and endothelial cells (ECs) in the wall of resistance arteries is mediated by the Na(+),HCO(3)(-) cotransporter NBCn1 (SLC4A7) and the Na(+)/H(+) exchanger NHE1 (SLC9A1) and is essential for intracellular pH (pH(i)) control. Experimental evidence suggests that the pH(i) of VSMCs and ECs modulates both vasocontractile and vasodilatory functions in resistance arteries with implications for blood pressure regulation. The connection between disturbed pH(i) and altered cardiovascular function has been substantiated by a genome-wide association study showing a link between NBCn1 and human hypertension. On this basis, we here review the current evidence regarding (a) molecular mechanisms involved in pH(i) control in VSMCs and ECs of resistance arteries at rest and during contractions, (b) implications of disturbed pH(i) for resistance artery function, and (c) involvement of disturbed pH(i) in the pathogenesis of vascular disease. The current evidence clearly implies that pH(i) of VSMCs and ECs modulates vascular function and suggests that disturbed pH(i) either consequent to disturbed regulation or due to metabolic challenges needs to be taken into consideration as a mechanistic component of artery dysfunction and disturbed blood pressure regulation.

Genetic variants in SLC9A9 are associated with measures of attention-deficit/hyperactivity disorder symptoms in families

OBJECTIVE

A family was previously identified that cosegregates a pericentric inversion, inv(3)(p14 : q21), with an early-onset developmental condition, characterized by impulsive behavior and intellectual deficit. The inversion breakpoints lie within DOCK3 and SLC9A9 at the p-arm and q-arm, respectively. Based on this report, these genes were selected to be evaluated in a family-based attention-deficit/hyperactivity disorder (AD/HD) association study.

METHODS

Conners' Parent (CPRS) and Teacher (CTRS) Rating Scales of AD/HD symptoms and Conners' Continuous Performance Test (CPT) measures were collected and a minimal number of tagging single-nucleotide polymorphisms (SNPs) in each gene were selected for analysis. Analyses were performed on families who met research criteria for AD/HD. Using the program, QTDT, each tagging SNP was tested for association with T-scores from the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV) subscales according to the CTRS and CPRS, and five CPT measures.

RESULTS

After adjusting for multiple testing, a SNP in the 3' UTR of SLC9A9, rs1046706, remained significantly associated (false discovery rate, q value <0.05) with scores on the DSM-IV hyperactive-impulsive and total symptom subscales according to the CTRS and errors of commission on the CPT. In addition, an intronic SLC9A9 SNP, rs2360867, remained significantly associated with errors of commission.

CONCLUSION

Our results suggest that SLC9A9 may be related to hyperactive-impulsive symptoms in AD/HD and the disruption of SLC9A9 may be responsible for the behavioral phenotype observed in the inversion family. The association with SLC9A9 is particularly interesting as it was recently implicated in a genome-wide association study for AD/HD. Further investigation of the role of SLC9A9 in AD/HD and other behavioral disorders is warranted.

Modulatory effects of neuropsychopharmaca on intracellular pH of hippocampal neurones in vitro

BACKGROUND AND PURPOSE

The intracellular pH (pHi) of neurones is tightly regulated by, for example, membrane-bound acid-exchangers and loaders. Nevertheless, excessive bioelectric activity lowers steady-state pHi. In turn, even a moderate acidification can inhibit neuronal activity, a process believed to be part of a negative feedback loop controlling neuronal excitation. As moclobemide, an antidepressant, and also some antiepileptic drugs can reduce neuronal pHi in hippocampus slices in vitro, we screened a panel of currently used neuropsychopharmaca for comparable effects.

EXPERIMENTAL APPROACH

BCECF-AM loaded hippocampal slices were superfused with 16 different neuroleptics, antidepressants and antiepileptics under bicarbonate-buffered conditions. Changes in steady-state pHi of CA3 neurones were measured fluorometrically.

KEY RESULTS

The antipsychotics haloperidol, clozapine, ziprasidone, and the antidepressants amitriptyline, doxepin, trimipramine, citalopram, mirtazapine, as well as the anticonvulsive drug tiagabine reversibly reduced the steady-state pHi by up to 0.35 pH-units in concentrations of 5-50 microM. In contrast, venlafaxine, the anticonvulsants carbamazepine, clonazepam, gabapentin, lamotrigine, zonisamide, and the mood stabilizer lithium had no effect on neuronal pHi.

CONCLUSION AND IMPLICATIONS

These data substantiate the view that clinically relevant concentrations of neuroleptics and antidepressants can mediate changes in neuronal pHi, which may contribute to their pharmacological mode of action. Effects on pHi should be taken into account when therapeutic or even harmful effects of these drugs are evaluated.

Gain-of-function mutation of Nav1.5 in atrial fibrillation enhances cellular excitability and lowers the threshold for action potential firing

Genetic mutations of the cardiac sodium channel (SCN5A) specific only to the phenotype of atrial fibrillation have recently been described. However, data on the biophysical properties of SCN5A variants associated with atrial fibrillation are scarce. In a mother and son with lone atrial fibrillation, we identified a novel SCN5A coding variant, K1493R, which altered a highly conserved residue in the DIII-IV linker and was located six amino acids downstream from the fast inactivation motif of sodium channels. Biophysical studies of K1493R in tsA201 cells demonstrated a significant positive shift in voltage-dependence of inactivation and a large ramp current near resting membrane potential, indicating a gain-of-function. Enhanced cellular excitability was observed in transfected HL-1 atrial cardiomyocytes, including spontaneous action potential depolarizations and a lower threshold for action potential firing. These novel biophysical observations provide molecular evidence linking cellular "hyperexcitability" as a mechanism inducing vulnerability to this common arrhythmia.

Switch in the expression of mGlu1 and mGlu5 metabotropic glutamate receptors in the cerebellum of mice developing experimental autoimmune encephalomyelitis and in autoptic cerebellar samples from patients with multiple sclerosis

Recent evidence suggests that changes in the expression of membrane receptors/ion channels in cerebellar Purkinje cells contribute to the onset of cerebellar motor symptoms in patients with multiple sclerosis (MS). We examined the expression of group-I metabotropic glutamate receptors (mGlu1 and mGlu5 receptors) in the cerebellum of mice developing experimental autoimmune encephalomyelitis (EAE) and in autoptic cerebellar samples of MS patients. EAE was induced in mice by immunization with the 35-55 fragment of MOG (myelin oligodendrocyte glycoprotein). EAE mice showed a progressive loss of mGlu1a receptors in the cerebellum, associated with an increased expression of mGlu5 receptors. These changes were restricted to Purkinje cells and their dendritic arborization, as shown by immunohistochemistry. A reduced expression of mGlu1a receptors in cerebellar Purkinje cells was also found in 7 of 9 MS patients. In addition, a light/moderate to very strong mGlu5 receptor immunoreactivity was detected in Purkinje cells of 8 MS patients, but was always absent in non-MS control patients. In EAE mice, an acute treatment with the mGlu1 receptor enhancer, 9H-xanthene-9-carboxylic acid (4-trifluoromethyl-oxazol-2-yl)-amide (RO0711401), significantly improved motor coordination, whereas treatment with the mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and 6-methyl-2-(phenylazo)-3-pyridinol (SIB-1757), had no effect. We conclude that mGlu1 receptor enhancers improve motor symptoms associated with EAE and might be helpful as symptomatic drugs in patients with MS.

Neurotoxic Effects of Zoniporide: A Selective Inhibitor of the Na+/H+ Exchanger Isoform 1

Zoniporide, an inhibitor of the Na+-H+ exchanger-1, was administered by continuous intravenous infusion to rats and dogs for up to 1 month. In 1-month studies, histological and functional changes were observed in select portions of the peripheral nervous system; however, these findings were not detected in 2-week studies at similar or higher doses. In the 1-month rat study, there was dose-dependent, minimal, focal, or multifocal nerve fiber (axonal) degeneration in the spinal cord and/or sciatic nerve. In a follow-up rat study, findings included slowing of caudal nerve conduction velocity and axonal degeneration in the spinal cord (dorsal funiculus), dorsal roots, dorsal root ganglia (DRG), radial, sciatic, and tibial nerves. In the 1-month dog study, there was impairment of the patellar reflex and associated postural reaction changes, minimal to marked proximal nerve fiber degeneration in the DRG, and minimal nerve fiber degeneration in the dorsal roots and funiculi of the spinal cord. Minimal nerve fiber degeneration of equivocal significance was noted in various peripheral nerves. Taken together, these findings were consistent with a specific effect on peripheral sensory nerve fibers. These studies demonstrated that zoniporide produces clinical, electrophysiologic, and microscopic evidence of peripheral sensory axonopathy and establishes the importance of careful preclinical evaluation of neurological function.

Regulation of pH in the mammalian central nervous system under normal and pathological conditions: facts and hypotheses

The maintenance of pH homeostasis in the CNS is of key importance for proper execution and regulation of neurotransmission, and deviations from this homeostasis are a crucial factor in the mechanism underlying a spectrum of pathological conditions. The first few sections of the review are devoted to the brain operating under normal conditions. The article commences with an overview of how extrinsic factors modelling the brain at work: neurotransmitters, depolarising stimuli (potassium and voltage changes) and cyclic nucleotides as major signal transducing vehicles affect pH in the CNS. Further, consequences of pH alterations on the major aspects of CNS function and metabolism are outlined. Next, the major cellular events involved in the transport, sequestration, metabolic production and buffering of protons that are common to all the mammalian cells, including the CNS cells. Since CNS function reflects tight interaction between astrocytes and neurons, the pH regulatory events pertinent to either cell type are discussed: overwhelming evidence implicates astrocytes as a key player in pH homeostasis in the brain. The different classes of membrane proteins involved in proton shuttling are listed and their mechanisms of action are given. These include: the Na+/H+ exchanger, different classes of bicarbonate transporters acting in a sodium-dependent- or -independent mode, monocarboxylic acid transporters and the vacuolar-type proton ATPase. A separate section is devoted to carbonic anhydrase, which is represented by multiple isoenzymes capable of pH buffering both in the cell interior and in the extracellular space. Next, impairment of pH regulation and compensatory responses occurring in brain affected by different pathologies: hypoxia/ischemia, epilepsy, hyperammonemic encephalopathies, cerebral tumours and HIV will be described. The review is limited to facts and plausible hypotheses pertaining to phenomena directly involved in pH regulation: changes in pH that accompany metabolic stress but have no distinct implications for the pH regulatory mechanisms are not dealt with. In most cases, the vast body of knowledge derived from in vitro studies remains to be verified in in vivo settings.

Mice with a targeted disruption of the Cl-/HCO3- exchanger AE3 display a reduced seizure threshold

Neuronal activity results in significant pH shifts in neurons, glia, and interstitial space. Several transport mechanisms are involved in the fine-tuning and regulation of extra- and intracellular pH. The sodium-independent electroneutral anion exchangers (AEs) exchange intracellular bicarbonate for extracellular chloride and thereby lower the intracellular pH. Recently, a significant association was found with the variant Ala867Asp of the anion exchanger AE3, which is predominantly expressed in brain and heart, in a large cohort of patients with idiopathic generalized epilepsy. To analyze a possible involvement of AE3 dysfunction in the pathogenesis of seizures, we generated an AE3-knockout mouse model by targeted disruption of Slc4a3. AE3-knockout mice were apparently healthy, and neither displayed gross histological and behavioral abnormalities nor spontaneous seizures or spike wave complexes in electrocorticograms. However, the seizure threshold of AE3-knockout mice exposed to bicuculline, pentylenetetrazole, or pilocarpine was reduced, and seizure-induced mortality was significantly increased compared to wild-type littermates. In the pyramidal cell layer of the hippocampal CA3 region, where AE3 is strongly expressed, disruption of AE3 abolished sodium-independent chloride-bicarbonate exchange. These findings strongly support the hypothesis that AE3 modulates seizure susceptibility and, therefore, are of significance for understanding the role of intracellular pH in epilepsy.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Down-regulation of delta-opioid receptors in Na+/H+ exchanger 1 null mutant mouse brain with epilepsy

Mice lacking Na+/H+ exchanger 1 (NHE1) show a unique epilepsy phenotype although the underlying mechanisms remain unclear. Since expression of delta-opioid receptor (DOR) may be involved in control of epileptic activity, we conducted immunohistochemistry and autoradiography to investigate whether DOR expression is dys-regulated in the brain of NHE1 null mouse. Immunohistochemistry showed a decline in DOR expression in hippocampus and cortex. Autoradiographic results confirmed that the density of DOR was decreased in most cortical and hippocampal regions such as striate and temporal cortex, hippocampal CA1 and CA3 regions (reduced by 27.7 +/- 6.4%, 29.4 +/- 5.1%, 40.7 +/- 4.4% and 20.6 +/- 5.7%, respectively, P < 0.05). These data demonstrate that NHE1 null mutation leads to a reduction of DOR expression in the cortical and hippocampal regions, which provides a new clue for the genesis of epilepsy.

Characterization of the ion transport activity of the budding yeast Na+/H+ antiporter, Nha1p, using isolated secretory vesicles

The Saccharomyces cerevisiae Nha1p, a plasma membrane protein belonging to the monovalent cation/proton antiporter family, plays a key role in the salt tolerance and pH regulation of cells. We examined the molecular function of Nha1p by using secretory vesicles isolated from a temperature sensitive secretory mutant, sec4-2, in vitro. The isolated secretory vesicles contained newly synthesized Nha1p en route to the plasma membrane and showed antiporter activity exchanging H+ for monovalent alkali metal cations. An amino acid substitution in Nha1p (D266N, Asp-266 to Asn) almost completely abolished the Na+/H+ but not K+/H+ antiport activity, confirming the validity of this assay system as well as the functional importance of Asp-266, especially for selectivity of substrate cations. Nha1p catalyzes transport of Na+ and K+ with similar affinity (12.7 mM and 12.4 mM), and with lower affinity for Rb+ and Li+. Nha1p activity is associated with a net charge movement across the membrane, transporting more protons per single sodium ion (i.e., electrogenic). This feature is similar to the bacterial Na+/H+ antiporters, whereas other known eukaryotic Na+/H+ antiporters are electroneutral. The ion selectivity and the stoichiometry suggest a unique physiological role of Nha1p which is distinct from that of other known Na+/H+ antiporters.

A novel epilepsy mutation in the sodium channel SCN1A identifies a cytoplasmic domain for beta subunit interaction

A mutation in the sodium channel SCN1A was identified in a small Italian family with dominantly inherited generalized epilepsy with febrile seizures plus (GEFS+). The mutation, D1866Y, alters an evolutionarily conserved aspartate residue in the C-terminal cytoplasmic domain of the sodium channel alpha subunit. The mutation decreased modulation of the alpha subunit by beta1, which normally causes a negative shift in the voltage dependence of inactivation in oocytes. There was less of a shift with the mutant channel, resulting in a 10 mV difference between the wild-type and mutant channels in the presence of beta1. This shift increased the magnitude of the window current, which resulted in more persistent current during a voltage ramp. Computational analysis suggests that neurons expressing the mutant channels will fire an action potential with a shorter onset delay in response to a threshold current injection, and that they will fire multiple action potentials with a shorter interspike interval at a higher input stimulus. These results suggest a causal relationship between a positive shift in the voltage dependence of sodium channel inactivation and spontaneous seizure activity. Direct interaction between the cytoplasmic C-terminal domain of the wild-type alpha subunit with the beta1 or beta3 subunit was first demonstrated by yeast two-hybrid analysis. The SCN1A peptide K1846-R1886 is sufficient for beta subunit interaction. Coimmunoprecipitation from transfected mammalian cells confirmed the interaction between the C-terminal domains of the alpha and beta1 subunits. The D1866Y mutation weakens this interaction, demonstrating a novel molecular mechanism leading to seizure susceptibility.

Evolutionary origins of eukaryotic sodium/proton exchangers

More than 200 genes annotated as Na+/H+ hydrogen exchangers (NHEs) currently reside in bioinformation databases such as GenBank and Pfam. We performed detailed phylogenetic analyses of these NHEs in an effort to better understand their specific functions and physiological roles. This analysis initially required examining the entire monovalent cation proton antiporter (CPA) superfamily that includes the CPA1, CPA2, and NaT-DC families of transporters, each of which has a unique set of bacterial ancestors. We have concluded that there are nine human NHE (or SLC9A) paralogs as well as two previously unknown human CPA2 genes, which we have named HsNHA1 and HsNHA2. The eukaryotic NHE family is composed of five phylogenetically distinct clades that differ in subcellular location, drug sensitivity, cation selectivity, and sequence length. The major subgroups are plasma membrane (recycling and resident) and intracellular (endosomal/TGN, NHE8-like, and plant vacuolar). HsNHE1, the first cloned eukaryotic NHE gene, belongs to the resident plasma membrane clade. The latter is the most recent to emerge, being found exclusively in vertebrates. In contrast, the intracellular clades are ubiquitously distributed and are likely precursors to the plasma membrane NHE. Yeast endosomal ScNHX1 was the first intracellular NHE to be described and is closely related to HsNHE6, HsNHE7, and HsNHE9 in humans. Our results link the appearance of NHE on the plasma membrane of animal cells to the use of the Na+/K(+)-ATPase to generate the membrane potential. These novel observations have allowed us to use comparative biology to predict physiological roles for the nine human NHE paralogs and to propose appropriate model organisms in which to study the unique properties of each NHE subclass.

Na+/H+ exchanger 1 deficiency alters gene expression in mouse brain

Na(+)/H(+) exchanger 1 (NHE1) is well known to function as a major regulator of intracellular pH (pH(i)). It is activated by low pH(i) and exchanges extracellular Na(+) for intracellular H(+) to maintain cellular homeostasis. Despite the fact that we now have evidence suggesting other roles for NHE1, there has been no comprehensive study investigating its role as a signaling molecule. Toward this aim, we used in this study NHE1 null mutant mice and cDNA microarrays to investigate the effects of NHE1 on global gene expression in various regions of the brain, e.g., cortex, hippocampus, brain stem-diencephalon, and cerebellum. We found that a total of 35 to 79 genes were up- or downregulated in each brain region, with the majority being downregulated. The effect of NHE1 null mutation on gene expression is region specific, and only 11 genes were changed in all brain regions studied. Further analysis of the cis-regulatory regions of downregulated genes revealed that transcription suppressors, BCL6 and E4BP4, were probable candidates that mediated the inhibitory effect of NHE1 null mutation. One of the genes, MCT-13, was not only downregulated in the NHE1 null mutant brain but also in tissue cultures treated with an NHE1 inhibitor. We conclude that 1) a relatively small number of genes were altered in the NHE1 null mouse brain; 2) the effects of NHE1 null mutation on gene expression are region specific; and 3) several genes implicated in neurodegeneration have altered expression, potentially offering a molecular explanation for the phenotype of the NHE1 null mouse.

Lobster hepatopancreatic epithelial single cell suspensions as models for electrogenic sodium–proton exchange

Sodium-proton antiporters, also called Na+/H+ exchangers (NHE), are vital transmembrane proteins involved in multiple cellular functions including transepithelial ion transport and Na+ homeostasis of cells throughout the biological kingdom. Na+/H+ exchange is accelerated by cytosolic acidification and also by osmotically induced cell shrinking, thereby promoting recovery of the physiological pHi and volume. Eight isoforms of Na+/H+ exchangers have been cloned and characterized to date and share the same overall structure, but exhibit differences with respect to cellular localization, kinetic variables and plasma membrane targeting, in polarized epithelial cells. The electrogenic Na+ absorption across tight epithelia from invertebrates follow significantly different principles from the electroneutral Na+/H+ antiporter found in vertebrates. In all invertebrate cells examined, the antiporter displayed a 2Na+/1H+ transport stoichiometry and this transport was markedly inhibited by exogenous calcium and zinc. Na+/H+ exchangers (NHE) are present in crustacean hepatopancreatic cell type suspensions and are believed to function in acid-base regulation by driving the extrusion of protons across the hepatopancreatic epithelium in exchange for Na+ in the sea water. A brief review of current knowledge about Na+/H+ exchangers has been presented. In addition, understanding of hepatopancreatic Na+/H+ exchange is described as obtained after isolation of purified E-, R-, F- and B-cell suspensions from the whole organ by centrifugal elutriation.

The antiepileptic drug levetiracetam selectively modifies kindling-induced alterations in gene expression in the temporal lobe of rats

Gene expression profiling by microarrays is a powerful tool for identification of genes that may encode key proteins involved in molecular mechanisms underlying epileptogenesis. Using the Affymetrix oligonucleotide microarray, we have surveyed the expression levels of more than 26,000 genes and expressed sequence tags (ESTs) in the amygdala-kindling model of temporal lobe epilepsy. Furthermore, the effect of the antiepileptic drug levetiracetam (LEV) on kindling-induced alterations of gene expression was studied. Treatment of rats with LEV during kindling acquisition significantly suppressed kindling development. For gene expression profiling, six groups of rats were included in the present study: (i) and (ii) sham-operated rats treated with saline or LEV; (iii) and (iv) electrode-implanted but non-kindled rats treated with saline or LEV; (v) and (vi) kindled rats treated with saline or LEV. Treatment was terminated after 11 or 12 daily amygdala stimulations, when all vehicle-treated rats had reached kindling criterion, i.e. a stage 5 seizure. Twenty-four hours later, the ipsilateral temporal lobe was dissected for mRNA preparation. Six temporal lobe preparations from each group were analysed for differential gene expression. In control (non-kindled) rats, LEV treatment was devoid of any significant effect on gene expression. In saline-treated kindled rats, a large number of genes were observed to display mRNA expression alterations compared with non-kindled rats. LEV treatment induced marked effects on gene expression from kindled rats. Previously described epilepsy-related genes, such as neuropeptide Y (NPY), thyrotropin-releasing hormone (TRH) and glial fibrillary acidic protein (GFAP) were confirmed to be up-regulated by kindling and partially normalized by LEV treatment. Real-time quantitative polymerase chain reaction confirmed NPY, TRH and GFAP expression data from chip experiments. Furthermore, a number of novel genes were identified from the gene chip experiments. A subgroup of these genes demonstrated correlation between expression changes and kindled phenotype measurements. In summary, this study identified many genes with potentially important roles in epileptogenesis and highlighted several important issues in using the gene chip technology for the study of animal models of CNS disorders.

Disruption of a novel member of a sodium/hydrogen exchanger family and DOCK3 is associated with an attention deficit hyperactivity disorder-like phenotype

BACKGROUND

Attention deficit hyperactivity disorder (ADHD) is a complex condition with high heritability. However, both biochemical investigations and association and linkage studies have failed to define fully the underlying genetic factors associated with ADHD. We have identified a family co-segregating an early onset behavioural/developmental condition, with features of ADHD and intellectual disability, with a pericentric inversion of chromosome 3, 46N inv(3)(p14:q21).

METHODS

We hypothesised that the inversion breakpoints affect a gene or genes that cause the observed phenotype. Large genomic clones (P1 derived/yeast/bacterial artificial chromosomes) were assembled into contigs across the two inversion breakpoints using molecular and bioinformatic technologies. Restriction fragments crossing the junctions were identified by Southern analysis and these fragments were amplified using inverse PCR.

RESULTS

The amplification products were subsequently sequenced to reveal that the breakpoints lay within an intron of the dedicator of cytokinesis 3 (DOCK3) gene at the p arm breakpoint, and an intron of a novel member of the solute carrier family 9 (sodium/hydrogen exchanger) isoform 9 (SLC9A9) at the q arm. Both genes are expressed in the brain, but neither of the genes has previously been implicated in developmental or behavioural disorders.

CONCLUSION

These two disrupted genes are candidates for involvement in the pathway leading to the neuropsychological condition in this family.

Expression of Na+/H+ and HCO3−-dependent transporters in Na+/H+ exchanger isoform 1 null mutant mouse brain

Acid-base transporters, such as the sodium-hydrogen exchangers (NHEs) and bicarbonate-dependent transporters, play an important role in the regulation of intracellular pH (pH(i)) in the CNS. Previous studies from our laboratory have shown that the absence of the major NHE isoform 1 (NHE1) reduced the steady-state pH(i) and recovery rate from an acid load in the hippocampal neurons not only in HEPES but also in HCO(3)(-) solutions (Yao et al., 1999). The purpose of the current study was to determine whether the NHE1 null mutation affects the expression of pH-regulatory transporters in the mouse CNS. Immunoblotting and semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) were performed to examine the protein and mRNA levels of NHE1-4, electrogenic sodium-bicarbonate cotransporter 1 variants (NBCe1), and brain-specific anion exchanger 3 (AE3) in four brain regions (cerebral cortex, hippocampus, cerebellum and brainstem-diencephalon). NHE1 null mutant mice were compared with their wild type controls at the average age of approximately 4 weeks. Our results revealed that the NHE1 null mutation caused a significant increase in NHE3 in the cerebellum (84% for protein, 105% for mRNA), an increase in NBCe1 expression in the brainstem-diencephalon (approximately 40-50% for protein, 9-15% for mRNA), as well as a decrease in AE3 in the hippocampus (approximately 60% for protein, 24% for mRNA). We conclude that the NHE1 null mutation does alter the expression of other membrane transporters at both protein and mRNA levels. The alteration is region-specific. An increase in acid extruders (e.g. NHE3) and a decrease in acid loaders (e.g. AE3) suggest that there are some compensatory mechanisms that occur in NHE1 null mutant mice.

Na+ channel expression and neuronal function in the Na+/H+ exchanger 1 null mutant mouse

Mice lacking Na(+)/H(+) exchanger 1 (NHE1) suffer from recurrent seizures and die early postnatally. Although the mechanisms for seizures are not well established, our previous electrophysiological work has shown that neuronal excitability and Na(+) current density are increased in hippocampal CA1 neurons of these mutant mice. However, it is unknown whether this increased density is related to altered expression or functional regulation of Na(+) channels. In this work, we asked three questions: is the increased excitability limited to CA1 neurons, is the increased Na(+) current density related to an increased Na(+) channel expression, and, if so, which Na(+) channel subtype(s) is upregulated? Using neurophysiological, autoradiographic, and immunoblotting techniques, we showed that both CA1 and cortical neurons have an increase in membrane excitability and Na(+) current density; Na(+) channel density is selectively upregulated in the hippocampus and cortex (P < 0.05); and Na(+) channel subtype I is significantly increased in the hippocampus and Na(+) channel subtype II is increased in the cortex. Our results demonstrate that mice lacking NHE1 upregulate their Na(+) channel expression in the hippocampal and cortical regions selectively; this leads to an increase in Na(+) current density and membrane excitability. We speculate that neuronal overexcitability due to Na(+) channel upregulation in the hippocampus and cortex forms the basis of epileptic seizures in NHE1 mutant mice.

Alterations in Na+/H+ exchanger and Na+/HCO3- cotransporter immunoreactivities within the gerbil hippocampus following seizure

In this study, a chronological and comparative analysis of the immunoreactivities of Na(+)/H(+) exchanger 1 (NHE1), Na(+)/HCO(3)(-) cotransporter (NBC) and Na(+)/Ca(2+) exchanger (NCE) was conducted in order to identify the effects of spontaneous seizure on their protein expression levels using the gerbil model. The distribution of NHE1 and NBC immunoreactivity in the hippocampus of seizure-resistant (SR) gerbils was similar to that observed in the pre-seizure group of seizure-sensitive (SS) gerbils. From 30 min to 3 h after the onset of the seizure, both NHE1 and NBC immunoreactivities were elevated in the hippocampus, as compared to the pre-seizure group of SS gerbils. At 6 h postictal, these immunoreactivities in the hippocampus had reduced to the pre-seizure level. However, NCE immunoreactivity within the hippocampus was unaltered. These findings suggest that the changes in both NHE1 and NBC immunoreactivity within the hippocampus following seizure may affect tissue excitability and play a role in the reduction of the seizure activity in the gerbil.

Clathrin-mediated endocytosis and recycling of the neuron-specific Na+/H+ exchanger NHE5 isoform. Regulation by phosphatidylinositol 3'-kinase and the actin cytoskeleton

Mammalian Na+/H+ exchangers (NHEs) are a family of integral membrane proteins that play central roles in sodium, acid-base, and cell volume homeostasis. The recently cloned NHE5 isoform is expressed predominantly in brain, but its functional and cellular properties are poorly understood. To facilitate its characterization, an epitope-tagged construct of NHE5 was ectopically expressed in nonneuronal and neuronal cells. In NHE-deficient Chinese hamster ovary AP-1 cells, NHE5 localized at the plasmalemma, but a significant fraction accumulated intracellularly in vesicles that concentrated in a juxtanuclear region. Similarly, in nerve growth factor-differentiated neuroendocrine PC12 cells and primary hippocampal neurons, immunolabeling of NHE5 was detected in endomembrane vesicles in the perinuclear region of the cell body but also along the processes. More detailed characterization in AP-1 cells using organelle-specific markers showed that NHE5 co-localized with internalized transferrin, a marker of recycling endosomes. Transient transfection of a dominant negative mutant of dynamin-1, which inhibits clathrin-mediated endocytosis, blocked uptake of transferrin as well as internalization of NHE5. Likewise, wortmannin inhibition of phosphatidylinositol 3'-kinase, a lipid kinase implicated in endosomal traffic, induced coalescence of vesicles containing NHE5 and caused a pronounced inhibition of plasmalemmal Na+/H+ exchange. By contrast, disruption of the F-actin cytoskeleton with cytochalasin D increased cell surface NHE5 activity and abundance. These observations demonstrate that NHE5 is localized to the recycling endosomal pathway and is dynamically regulated by phosphatidylinositol 3'-kinase and by the state of F-actin assembly.