Structure and function of the NHE1 isoform of the Na+/H+ exchanger

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.

Proanthocyanidins Maintain Cardiac Ionic Homeostasis in Aldosterone-Induced Hypertension and Heart Failure

Excess aldosterone promotes pathological remodeling of the heart and imbalance in cardiac ion homeostasis of sodium, potassium and calcium. Novel treatment with proanthocyanidins in aldosterone-treated rats has resulted in downregulation of cardiac SGK1, the main genomic aldosterone-induced intracellular mediator of ion handling. It therefore follows that proanthocyanidins could be modulating cardiac ion homeostasis in aldosterone-treated rats. Male Wistar rats received aldosterone (1 mg kg⁻¹ day⁻¹) +1% NaCl for three weeks. Half of the animals in each group were simultaneously treated with the proanthocyanidins-rich extract (80% w/w) (PRO80, 5 mg kg⁻¹ day⁻¹). PRO80 prevented cardiac hypertrophy and decreased calcium content. Expression of ion channels (ROMK, NHE1, NKA and NCX1) and calcium transient mediators (CAV1.2, pCaMKII and oxCaMKII) were reduced by PRO80 treatment in aldosterone-treated rats. To conclude, our data indicate that PRO80 may offer an alternative treatment to conventional MR-blockade in the prevention of aldosterone-induced cardiac pathology.

The Ph paradigm in cancer

Malignant tissues show a peculiar feature regarding pH: while normal tissues have a higher extracellular pH than intracellular pH, in cancer is exactly the opposite. This phenomenon is called the inversion of the pH gradient and is now considered a hallmark of malignancy. For some time, this inverted pH gradient was believed to be a secondary effect of cancer. Now, it is becoming clear that pH inversion is not an innocent consequence, but a key player in the etiopathogenesis of cancer. Therefore, addressing this issue as part of an integral treatment of neoplasia should be a necessary step for improving cancer patients' outcomes. However, the knowledge acquired in this regard through basic research has not reached bedside treatments. The most striking fact is that there are repurposed drugs and nutraceuticals with low or no toxicity that can modify the pH gradient inversion. However, these drugs have not even been tested in cancer treatment.

Myocardial Impact of NHE1 Regulation by Sildenafil

The cardiac Na⁺/H⁺ exchanger (NHE1) is a membrane glycoprotein fundamental for proper cell functioning due its multiple housekeeping tasks, including regulation of intracellular pH, Na⁺ concentration, and cell volume. In the heart, hyperactivation of NHE1 has been linked to the development of different pathologies. Several studies in animal models that reproduce the deleterious effects of ischemia/reperfusion injury or cardiac hypertrophy have conclusively demonstrated that NHE1 inhibition provides cardioprotection. Unfortunately, NHE1 inhibitors failed to reproduce these effects in the clinical arena. The reasons for those discrepancies are not apparent yet. However, a reasonable clue to consider would be that drugs that completely abolish the exchanger activity, including that its essential housekeeping function may not be the best therapeutic approach. Therefore, interventions tending to specifically reduce its hyperactive state without affecting its basal activity emerge as a novel potential gold standard. In this regard, a promising goal seems to be the modulation of the phosphorylation state of the cytosolic tail of the exchanger. Recent own experiments demonstrated that Sildenafil, a phosphodiesterase 5A inhibitor drug that has been widely used for the treatment of erectile dysfunction is able to decrease NHE1 phosphorylation, and hence reduce its hyperactivity. In connection, growing evidence demonstrates cardioprotective properties of Sildenafil against different cardiac pathologies, with the distinctive characteristic of directly affecting cardiac tissue without altering blood pressure. This mini-review was aimed to focus on the regulation of NHE1 activity by Sildenafil. For this purpose, experimental data reporting Sildenafil effects in different animal models of heart disease will be discussed.

Targets of cGMP/cGKI in Cardiac Myocytes

The 3',5'-cyclic guanosine monophosphate (cGMP)-dependent protein kinase type I (cGKI aka PKGI) is a major cardiac effector acting downstream of nitric oxide (NO)-sensitive soluble guanylyl cyclase and natriuretic peptides (NPs), which signal through transmembrane guanylyl cyclases. Consistent with the wide distribution of the cGMP-generating guanylyl cyclases, cGKI, which usually elicits its cellular effects by direct phosphorylation of its targets, is present in multiple cardiac cell types including cardiomyocytes (CMs). Although numerous targets of cGMP/cGKI in heart were identified in the past, neither their exact patho-/physiological functions nor cell-type specific roles are clear. Herein, we inform about the current knowledge on the signal transduction downstream of CM cGKI. We believe that better insights into the specific actions of cGMP and cGKI in these cells will help to guide future studies in the search for predictive biomarkers for the response to pharmacological cGMP pathway modulation. In addition, targets downstream of cGMP/cGKI may be exploited for refined and optimized diagnostic and therapeutic strategies in different types of heart disease and their causes. Importantly, key functions of these proteins and particularly sites of regulatory phosphorylation by cGKI should, at least in principle, remain intact, although upstream signaling through the second messenger cGMP is impaired or dysregulated in a stressed or diseased heart state.

Acid enhancement of ROS generation by complex-I reverse electron transport is balanced by acid inhibition of complex-II: Relevance for tissue reperfusion injury

Generation of mitochondrial reactive oxygen species (ROS) is an important process in triggering cellular necrosis and tissue infarction during ischemia-reperfusion (IR) injury. Ischemia results in accumulation of the metabolite succinate. Rapid oxidation of this succinate by mitochondrial complex II (Cx-II) during reperfusion reduces the co-enzyme Q (Co-Q) pool, thereby driving electrons backward into complex-I (Cx-I), a process known as reverse electron transport (RET), which is thought to be a major source of ROS. During ischemia, enhanced glycolysis results in an acidic cellular pH at the onset of reperfusion. While the process of RsET within Cx-I is known to be enhanced by a high mitochondrial trans-membrane ΔpH, the impact of pH itself on the integrated process of Cx-II to Cx-I RET has not been fully studied. Using isolated mouse heart and liver mitochondria under conditions which mimic the onset of reperfusion (i.e., high [ADP]), we show that mitochondrial respiration (state 2 and state 3) as well as isolated Cx-II activity are impaired at acidic pH, whereas the overall generation of ROS by Cx-II to Cx-I RET was insensitive to pH. Together these data indicate that the acceleration of Cx-I RET ROS by ΔpH appears to be cancelled out by the impact of pH on the source of electrons, i.e. Cx-II. Implications for the role of Cx-II to Cx-I RET derived ROS in IR injury are discussed.

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ROS from complex I (Cx-I) reverse electron transport (RET) is enhanced at acidic pH.

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Mitochondrial complex II (Cx-II) activity is inhibited at acidic pH.

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These effects cancel out, yielding no net pH response of Cx-II to Cx-I RET ROS.

Acquiring Resistance Against a Retroviral Infection via CRISPR/Cas9 Targeted Genome Editing in a Commercial Chicken Line

Genome editing technology provides new possibilities for animal breeding and aid in understanding host-pathogen interactions. In poultry, retroviruses display one of the most difficult pathogens to control by conventional strategies such as vaccinations. Avian leukosis virus subgroup J (ALV-J) is an oncogenic, immunosuppressive retrovirus that causes myeloid leukosis and other tumors in chickens. Severe economic losses caused by ALV-J remain an unsolved problem in many parts of the world due to inefficient eradication strategies and lack of effective vaccines. ALV-J attachment and entry are mediated through the specific receptor, chicken Na⁺/H⁺ exchanger type 1 (chNHE1). The non-conserved amino acid tryptophan 38 (W38) in chNHE1 is crucial for virus entry, making it a favorable target for the introduction of disease resistance. In this study, we obtained ALV-J-resistance in a commercial chicken line by precise deletion of chNHE1 W38, utilizing the CRISPR/Cas9-system in combination with homology directed repair. The genetic modification completely protected cells from infection with a subgroup J retrovirus. W38 deletion did neither have a negative effect on the development nor on the general health condition of the gene edited chickens. Overall, the generation of ALV-J-resistant birds by precise gene editing demonstrates the immense potential of this approach as an alternative disease control strategy in poultry.

14-3-3 Proteins and Other Candidates form Protein-Protein Interactions with the Cytosolic C-terminal End of SOS1 Affecting Its Transport Activity

The plasma membrane transporter SOS1 (SALT-OVERLY SENSITIVE1) is vital for plant survival under salt stress. SOS1 activity is tightly regulated, but little is known about the underlying mechanism. SOS1 contains a cytosolic, autoinhibitory C-terminal tail (abbreviated as SOS1 C-term), which is targeted by the protein kinase SOS2 to trigger its transport activity. Here, to identify additional binding proteins that regulate SOS1 activity, we synthesized the SOS1 C-term domain and used it as bait to probe Arabidopsis thaliana cell extracts. Several 14-3-3 proteins, which function in plant salt tolerance, specifically bound to and interacted with the SOS1 C-term. Compared to wild-type plants, when exposed to salt stress, Arabidopsis plants overexpressing SOS1 C-term showed improved salt tolerance, significantly reduced Na⁺ accumulation in leaves, reduced induction of the salt-responsive gene WRKY25, decreased soluble sugar, starch, and proline levels, less impaired inflorescence formation and increased biomass. It appears that overexpressing SOS1 C-term leads to the sequestration of inhibitory 14-3-3 proteins, allowing SOS1 to be more readily activated and leading to increased salt tolerance. We propose that the SOS1 C-term binds to previously unknown proteins such as 14-3-3 isoforms, thereby regulating salt tolerance. This finding uncovers another regulatory layer of the plant salt tolerance program.

The Interplay of Dysregulated pH and Electrolyte Imbalance in Cancer

Cancer cells and tissues have an aberrant regulation of hydrogen ion dynamics driven by a combination of poor vascular perfusion, regional hypoxia, and increased the flux of carbons through fermentative glycolysis. This leads to extracellular acidosis and intracellular alkalinization. Dysregulated pH dynamics influence cancer cell biology, from cell transformation and tumorigenesis to proliferation, local growth, invasion, and metastasis. Moreover, this dysregulated intracellular pH (pHi) drives a metabolic shift to increased aerobic glycolysis and reduced mitochondrial oxidative phosphorylation, referred to as the Warburg effect, or Warburg metabolism, which is a selective feature of cancer. This metabolic reprogramming confers a thermodynamic advantage on cancer cells and tissues by protecting them against oxidative stress, enhancing their resistance to hypoxia, and allowing a rapid conversion of nutrients into biomass to enable cell proliferation. Indeed, most cancers have increased glucose uptake and lactic acid production. Furthermore, cancer cells have very dysregulated electrolyte balances, and in the interaction of the pH dynamics with electrolyte, dynamics is less well known. In this review, we highlight the interconnected roles of dysregulated pH dynamics and electrolytes imbalance in cancer initiation, progression, adaptation, and in determining the programming and reprogramming of tumor cell metabolism.

Physiological Significance of Ion Transporters and Channels in the Stomach and Pathophysiological Relevance in Gastric Cancer

Gastric cancer (GC) is a highly invasive and fatal malignant disease that accounts for 5.7% of new global cancer cases and is the third leading cause of cancer-related death. Acid/base homeostasis is critical for organisms because protein and enzyme function, cellular structure, and plasma membrane permeability change with pH. Various ion transporters are expressed in normal gastric mucosal epithelial cells and regulate gastric acid secretion, ion transport, and fluid absorption, thereby stabilizing the differentiation and homeostasis of gastric mucosal epithelial cells. Ion transporter dysfunction results in disordered ion transport, mucosa barrier dysfunction, and acid/base disturbances, causing gastric acid-related diseases such as chronic atrophic gastritis (CAG) and GC. This review summarizes the physiological functions of multiple ion transporters and channels in the stomach, including Cl⁻ channels, Cl⁻/HCO₃⁻ exchangers, sodium/hydrogen exchangers (NHEs), and potassium (K⁺) channels, and their pathophysiological relevance in GC.

Protocol for Investigating the Interactions Between Intrinsically Disordered Proteins and Membranes by Neutron Reflectometry

Several intrinsically disordered proteins (IDPs) exhibit high affinity for lipid membranes. Among the different biophysical methods to probe protein-lipid interaction, neutron reflectometry (NR) can provide direct and structural detailed information on the location of the IDP with respect to the membrane. Supported lipid bilayers are commonly used as cell membrane models in such experiments. NR measurements can be collected on the supported lipid bilayer before and after the interaction with the IDP to characterize whether the protein molecules are mainly located on the membrane surface (interaction with the lipid headgroups), are penetrating into the hydrophobic region of the membrane (interaction with the lipid acyl chains), or are not interacting at all with the membrane. The lipid composition of the supported lipid bilayer can easily be tuned; hence the NR experiments can be designed to investigate selective IDP-lipid interactions.This chapter will describe the fundamental steps for performing an NR experiment and the subsequent data analysis aimed at characterizing IDP-lipid bilayer interactions. The specific case of an intrinsically disordered region (IDR) from the membrane protein Na⁺/H⁺ exchanger isoform 1 (NHE1) will be used as an example, but the same protocol can be easily adapted to other IDPs.

Rapid isolation and expansion of skin-derived precursor cells from human primary fibroblast cultures

Skin-derived precursor (SKP) cells have self-renewal and multipotent abilities and are found in the dermis. SKP cells have been isolated previously from pre-established dermal fibroblast cultures. In these procedures, long-term culture and low yield remain the crucial aspects requiring improvement. In this study, we exposed pre-established dermal fibroblasts to 30-min acid stress prior to isolating SKP cells (termed pH-SKP) and compared the yield to the previously published trypsin- and no-stress methods. Spheroid formation was confirmed and analyzed at days 3, 5 and 7. Stemness was investigated by immunohistochemistry for the stem cell markers Nestin, CD9, vimentin and NG2. Multipotency was investigated by differentiation into adipocytes, smooth muscle cells and fibroblasts. The pH-SKP spheroid yield at day 5 was four- and threefold higher than those obtained using trypsin- and no-stress methods, respectively. The expression of stem cell markers Nestin, CD9, vimentin and NG2 were significantly expressed in pH-SKPs compared to the fibroblast origin. Successful pH-SKP spheroid formation and differentiation were achieved and validated in 11 distinct human primary fibroblast lines. These results demonstrate that acute acidic stress treatment of dermal fibroblast cultures greatly improves SKP isolation, growth, yield and multipotency compared to previous methods.

Summary: Exposure of human dermal fibroblast cultures to a short period of acidic stress results in rapid isolation of skin-derived precursor (SKP) cells under the three-dimensional SKP culture conditions.

Dopamine reduces cell surface Na+/H+ exchanger-3 protein by decreasing NHE3 exocytosis and cell membrane recycling

The intrarenal autocrine-paracrine dopamine (DA) system mediates a significant fraction of the natriuresis in response to a salt load. DA inhibits a number of Na⁺ transporters to effect sodium excretion, including the proximal tubule Na⁺/H⁺ exchanger-3 (NHE3). DA represent a single hormone that regulates NHE3 at multiple levels, including translation, degradation, endocytosis, and protein phosphorylation. Because cell surface NHE3 protein is determined by the balance between exocytotic insertion and endocytotic retrieval, we examined whether DA acutely affects the rate of NHE3 exocytosis in a cell culture model. DA inhibited NHE3 exocytosis at a dose-dependent manner with a half maximal around 10^-6 M. The DA effect on NHE3 exocytosis was blocked by inhibition of protein kinase A and by brefeldin A, which inhibits endoplasmic reticulum-to-Golgi transport. NHE3 directly interacts with the ε-subunit of coatomer protein based on yeast-two-hybrid and coimmunoprecipitation. Because NHE3 has been shown to be recycled back to the cell membrane after endocytosis, we measured NHE3 recycling using a biochemical reinsertion assay and showed that reinsertion of NHE3 back to the membrane is also inhibited by DA. In conclusion, among the many mechanisms by which DA reduces apical membrane NHE3 and induces proximal tubule natriuresis, one additional mechanism is inhibition of exocytotic insertion and reinsertion of NHE3 in the apical cell surface.

Application of Hanging Drop Technique for Kidney Tissue Culture

Background/Aims

The hanging drop technique is a well-established method used in culture of animal tissues. However, this method has not been used in adult kidney tissue culture yet. This study was to explore the feasibility of using this technique for culturing adult kidney cortex to study the time course of RNA viability in the tubules and vasculature, as well as the tissue structural integrity.

Methods

In each Petri dish with the plate covered with sterile buffer, a section of mouse renal cortex was cultured within a drop of DMEM culture medium on the inner surface of the lip facing downward. The tissue were then harvested at each specific time points for Real-time PCR analysis and histological studies.

Results

The results showed that the mRNA level of most Na⁺ related transporters and cotransporters were stably maintained within 6 hours in culture, and that the mRNA level of most receptors found in the vasculature and glomeruli were stably maintained for up to 9 days in culture. Paraffin sections of the cultured renal cortex indicated that the tubules began to lose tubular integrity after 6 hours, but the glomeruli and vasculatures were still recognizable up to 9 days in culture.

Conclusions

We concluded that adult kidney tissue culture by hanging drop method can be used to study gene expressions in vasculature and glomeruli.

NHE1 is upregulated in gastric cancer and regulates gastric cancer cell proliferation, migration and invasion

Na+/H+ exchanger isoform 1 (NHE1) is known to play a key role in regulating intracellular pH and osmotic homeostasis and is involved in the development and progression of several types of cancer. However, the function and specific mechanism of NHE1 in gastric cancer (GC) are not clearly understood. In the present study, we report that NHE1 is overexpressed in tissues and cell lines from GC patients, and knockdown or inhibition of NHE1 suppressed GC cell proliferation via regulation of G1/S and G2/M cell cycle phase transitions, concomitant with a marked decrease in positive cell cycle regulators, including cyclin D1 and cyclin B1. Likewise, NHE1 was required for GC cell migration and invasion through the regulation of epithelial-mesenchymal transition (EMT) proteins, and NHE1 inhibition resulted in an acidic intracellular environment, providing possible mechanisms underlying NHE1-mediated GC progression both in vitro and in vivo. These data highlight the important role of NHE1 in GC progression and suggest that NHE1 may be a useful target for GC therapy.

Membrane transport piece by piece: production of transmembrane peptides for structural and functional studies

Membrane proteins are involved in all cellular processes from signaling cascades to nutrient uptake and waste disposal. Because of these essential functions, many membrane proteins are recognized as important, yet elusive, clinical targets. Recent advances in structural biology have answered many questions about how membrane proteins function, yet one of the major bottlenecks remains the ability to obtain sufficient quantities of pure and homogeneous protein. This is particularly true for human membrane proteins, where novel expression strategies and structural techniques are needed to better characterize their function and therapeutic potential. One way to approach this challenge is to determine the structure of smaller pieces of membrane proteins that can be assembled into models of the complete protein. This unit describes the rationale for working with single or multiple transmembrane segments and provides a description of strategies and methods to express and purify them for structural and functional studies using a maltose binding protein (MBP) fusion. The bulk of the unit outlines a detailed methodology and justification for producing these peptides under native-like conditions.

Amiloride and guggulsterone suppression of esophageal cancer cell growth in vitro and in nude mouse xenografts

Esophageal adenocarcinoma is increasing in the US and Western countries and frequent gastresophageal reflux or gastresophageal reflux disease carrying gastric acid and bile acid could contribute to esophageal adenocarcinogenesis. This study was designed to detect the expression of gastric acid-inducing gene Na + /H + exchanger-1 (NHE-1) ex vivo and then to explore targeting of NHE-1 expression or activity to control esophageal cancer cell viability in vitro and in nude mouse xenografts. The data showed that NHE-1 was highly expressed in esophageal adenocarcinoma tissues (66 of 101 cases [65.3%], but not in normal esophageal squamous cell epithelium (1 of 26 cases [3.8%]). Knockdown of NHE-1 expression using NHE-1 shRNA or inhibition of NHE-1 activity using the NHE-1 inhibitor amiloride suppressed viability and induced apoptosis in esophageal cancer cells. Molecularly, amiloride inhibited expression of cyclooxygenase-2 and matrix metallopeptidase-9 but not NHE-1 mRNA in esophageal cancer cells. A combination of amiloride and guggulsterone (a natural bile acid receptor inhibitor) showed more than additive effects in suppressing esophageal cancer cell growth in vitro and in nude mouse xenografts. This study suggests that inhibition of NHE-1 expression or activity or combination of amiloride and guggulsterone could be useful in control of esophageal adenocarcinoma.

Regulation of intracellular pH in cnidarians: response to acidosis in Anemonia viridis

The regulation of intracellular pH (pHi) is a fundamental aspect of cell physiology that has received little attention in studies of the phylum Cnidaria, which includes ecologically important sea anemones and reef-building corals. Like all organisms, cnidarians must maintain pH homeostasis to counterbalance reductions in pHi, which can arise because of changes in either intrinsic or extrinsic parameters. Corals and sea anemones face natural daily changes in internal fluids, where the extracellular pH can range from 8.9 during the day to 7.4 at night. Furthermore, cnidarians are likely to experience future CO₂-driven declines in seawater pH, a process known as ocean acidification. Here, we carried out the first mechanistic investigation to determine how cnidarian pHi regulation responds to decreases in extracellular and intracellular pH. Using the anemone Anemonia viridis, we employed confocal live cell imaging and a pH-sensitive dye to track the dynamics of pHi after intracellular acidosis induced by acute exposure to decreases in seawater pH and NH₄Cl prepulses. The investigation was conducted on cells that contained intracellular symbiotic algae (Symbiodinium sp.) and on symbiont-free endoderm cells. Experiments using inhibitors and Na⁺-free seawater indicate a potential role of Na⁺/H⁺ plasma membrane exchangers (NHEs) in mediating pHi recovery following intracellular acidosis in both cell types. We also measured the buffering capacity of cells, and obtained values between 20.8 and 43.8 mM per pH unit, which are comparable to those in other invertebrates. Our findings provide the first steps towards a better understanding of acid-base regulation in these basal metazoans, for which information on cell physiology is extremely limited.

Characterization of cytoskeletal protein 4.1R interaction with NHE1 (Na(+)/H(+) exchanger isoform 1)

NHE1 (Na(+)/H(+) exchanger isoform 1) has been reported to be hyperactive in 4.1R-null erythrocytes [Rivera, De Franceschi, Peters, Gascard, Mohandas and Brugnara (2006) Am. J. Physiol. Cell Physiol. 291, C880-C886], supporting a functional interaction between NHE1 and 4.1R. In the present paper we demonstrate that 4.1R binds directly to the NHE1cd (cytoplasmic domain of NHE1) through the interaction of an EED motif in the 4.1R FERM (4.1/ezrin/radixin/moesin) domain with two clusters of basic amino acids in the NHE1cd, K(519)R and R(556)FNKKYVKK, previously shown to mediate PIP(2) (phosphatidylinositol 4,5-bisphosphate) binding [Aharonovitz, Zaun, Balla, York, Orlowski and Grinstein (2000) J. Cell. Biol. 150, 213-224]. The affinity of this interaction (K(d) = 100-200 nM) is reduced in hypertonic and acidic conditions, demonstrating that this interaction is of an electrostatic nature. The binding affinity is also reduced upon binding of Ca(2+)/CaM (Ca(2+)-saturated calmodulin) to the 4.1R FERM domain. We propose that 4.1R regulates NHE1 activity through a direct protein-protein interaction that can be modulated by intracellular pH and Na(+) and Ca(2+) concentrations.

Na+/H+-exchanger-1 inhibition counteracts diabetic cataract formation and retinal oxidative-nitrative stress and apoptosis

The Na⁺-H⁺-exchanger-1 (NHE-1) controls intracellular pH and glycolytic enzyme activities, and its expression and activity are increased by diabetes and high glucose. NHE-1-dependent upregulation of the upper part of glycolysis, under conditions of inhibition (lens) or insufficient activation (retina) of glyceraldehyde 3-phosphate dehydrogenase, underlies diversion of the excessive glycolytic flux towards several pathways contributing to oxidative stress, a causative factor in diabetic cataractogenesis and retinopathy. This study evaluated the role for NHE-1 in diabetic cataract formation and retinal oxidative stress and apoptosis. Control and streptozotocin-diabetic rats were maintained with or without treatment with the NHE-1 inhibitor cariporide (Sanofi-Aventis, 10 mgkg⁻¹d⁻¹) for 3.5 months. In in vitro studies, bovine retinal pericytes and endothelial cells were cultured in 5 or 30 mM glucose, with or without 10 μM cariporide, for 7 days. A several-fold increase of the by-product of glycolysis, α-glycerophosphate, indicative of activation of the upper part of glycolysis, was present in both rat lens and retina at an early (1-month) stage of streptozotocin-diabetes. Cariporide did not affect diabetic hyperglycemia and counteracted lens oxidative-nitrative stress and p38 MAPK activation, without affecting glucose or sorbitol pathway intermediate accumulation. Cataract formation (indirect ophthalmoscopy and slit-lamp examination) was delayed, but not prevented. The number of TUNEL-positive cells per flat-mounted retina was increased 4.4-fold in diabetic rats (101±17 vs. 23±8 in controls, P<0.01), and this increase was attenuated by cariporide (45±12, P<0.01). Nitrotyrosine and poly(ADP-ribose) fluorescence and percentage of TUNEL-positive cells were increased in pericytes and endothelial cells cultured in 30 mM glucose, and these changes were at least partially prevented by cariporide. In conclusion, NHE-1 contributes to diabetic cataract formation, and retinal oxidative-nitrative stress and apoptosis. The findings identify a new therapeutic target for diabetic ocular complications.

G Protein-regulated inducer of neurite outgrowth (GRIN) modulates Sprouty protein repression of mitogen-activated protein kinase (MAPK) activation by growth factor stimulation

Gα(o/i) interacts directly with GRIN (G protein-regulated inducer of neurite outgrowth). Using the yeast two-hybrid system, we identified Sprouty2 as an interacting partner of GRIN. Gα(o) and Sprouty2 bind to overlapping regions of GRIN, thus competing for GRIN binding. Imaging experiments demonstrated that Gα(o) expression promoted GRIN translocation to the plasma membrane, whereas Sprouty2 expression failed to do so. Given the role of Sprouty2 in the regulation of growth factor-mediated MAPK activation, we examined the contribution of the GRIN-Sprouty2 interaction to CB1 cannabinoid receptor regulation of FGF receptor signaling. In Neuro-2A cells, a system that expresses all of the components endogenously, modulation of GRIN levels led to regulation of MAPK activation. Overexpression of GRIN potentiated FGF activation of MAPK and decreased tyrosine phosphorylation of Sprouty2. Pretreatment with G(o/i)-coupled CB1 receptor agonist attenuated subsequent FGF activation of MAPK. Decreased expression of GRIN both diminished FGF activation of MAPK and blocked CB1R attenuation of MAPK activation. These observations indicate that Gα(o) interacts with GRIN and outcompetes GRIN from bound Sprouty. Free Sprouty then in turn inhibits growth factor signaling. Thus, here we present a novel mechanism of how G(o/i)-coupled receptors can inhibit growth factor signaling to MAPK.

SiRNA-mediated silencing of the NHE1 gene inhibits cell invasion and migration in human hepatocellular carcinoma cell line MHCC97-H

AIM: To investigate the effect of small interfering RNA (siRNA)-mediated silencing of the Na⁺/H⁺ exchanger 1 (NHE1) gene on cell invasion and migration in human hepatocellular carcinoma cell line MHCC97-H.

METHODS: After MHCC97-H cells were transfected with NHE1-specific siRNA, the levels of NHE1 mRNA and protein were detected by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, respectively. Cell proliferation was detected by MTT assay. The impact of NHE1 knockdown on cell invasion and migration was determined by Transwell chamber assay. The changes in morphology cytoskeleton and pseudopodia were observed by immunofluorescence.

RESULTS: Compared to negative control and mock control cells, the levels of NHE1 mRNA and protein in MHCC97-H cells transfected with NHE1-specific siRNA decreased significantly (P < 0.05). NHE1 knockdown significantly suppressed the invasion and migration of MHCC97-H cells compared to negative control and mock control cells (34.1 ± 5.2 vs 56.9 ± 6.1, 57.2 ± 6.1; 120.2 ± 12.8 vs 235.2 ± 16.8, 231.9 ± 14.7; all P < 0.05). However, there were no significant differences in cell migration and invasion between the two control groups. Cell proliferation showed no significant differences among the three groups 48 or 72 h after transfection. Compared to the two control groups, deletion of NHE1 decreased the number of membrane pseudopodia and disrupted the cross-linked actin network in MHCC97-H cells．

CONCLUSION: Deletion of NHE1 inhibits cell invasion and migration by influencing cytoskeleton rearrangement and pseudopodia formation in human hepatocellular carcinoma cell line MHCC97-H.

Role of Na+/H+ exchanger 1 in tumor invasion and metastasis

Mammalian Na⁺/H⁺ exchanger 1 (NHE1) is a membrane protein that regulates intracellular pH homeostasis by extruding one intracellular proton in exchange for one extracellular sodium. It has been demonstrated that NHE1 expression and activity are increased in tumor cells and contribute to the acidification of extracellular microenvironment. NHE1 and extracellular microenvironment play an important role in tumor invasion. In addition, NHE1 activation is involved in the formation of pseudopodia and invadopodia in tumor cells and therefore increases tumor invasion. Therefore, NHE1 may be a potential target for tumor therapy.

Elevated hydrostatic pressure activates sodium/hydrogen exchanger-1 in rat optic nerve head astrocytes

Optic nerve head astrocytes become abnormal in eyes that have elevated intraocular pressure, and cultured astrocytes display altered protein expression after being subjected for > or = 1 days to elevated hydrostatic pressure. Here we show that 2-h elevated hydrostatic pressure (15 or 30 mmHg) causes phosphorylation of ERK1/2, ribosomal S6 protein kinase (p90(RSK)), and Na/H exchanger (NHE)1 in cultured rat optic nerve head astrocytes as judged by Western blot analysis. The MEK/ERK inhibitor U0126 abolished phosphorylation of NHE1 and p90(RSK) as well as ERK1/2. To examine NHE1 activity, cytoplasmic pH (pH(i)) was measured with BCECF and, in some experiments, cells were acidified by 5-min exposure to 20 mM ammonium chloride. Although baseline pH(i) was unaltered, the rate of pH(i) recovery from acidification was fourfold higher in pressure-treated astrocytes. In the presence of either U0126 or dimethylamiloride (DMA), an NHE inhibitor, hydrostatic pressure did not change the rate of pH(i) recovery. The findings are consistent with NHE1 activation due to phosphorylation of ERK1/2, p90(RSK), and NHE1 that occurs in response to hydrostatic pressure. These responses may precede long-term changes of protein expression known to occur in pressure-stressed astrocytes.

Effect of NHE1 antisense gene transfection on the biological behavior of SGC-7901 human gastric carcinoma cells

AIM: To study the effect of type 1 Na⁺/H⁺ exchanger (NHE1) antisense human gene transfection on the biological behavior of gastric carcinoma cell line SGC-7901.

METHODS: Antisense NHE1 eukaryotic expression on vector pcDNA3.1 was constructed by recombinant DNA technique and transfected into gastric carcinoma cell line SGC-7901 with DOTAP liposome transfection method. Morphological changes of cells were observed with optic and electron microscopes. Changes in cell proliferative capacity, apoptosis, intracellular pH (pH_i), cell cycle, clone formation in two-layer soft agar, and tumorigenicity in nude mice were examined.

RESULTS: Antisense eukaryotic expressing vectors were successfully constructed and transfected into SGC-7901. The transfectant obtained named 7901-antisense (7901-AS) stablely produced antisense NHE1. There was a significant difference between the pH_i of 7901-AS cells (6.77 ± 0.05) and that of 7901-zeo cells and SGC-7901 cells (7.24 ± 0.03 and 7.26 ± 0.03, P < 0.01). Compared with SGC-7901 and 7901-zeo cells, 7901-AS cells mostly showed cell proliferation inhibition, G₁/G₀ phase arrest, increased cell apoptotic rate, recovery of contact inhibition, and density contact. The tumorigenicity in nude mice and cloning efficiency in the two-layer soft agar were clearly inhibited.

CONCLUSION: NHE1 antisense gene significantly restrains the malignant behavior of human gastric carcinoma cells, suppresses cell growth and induces cell apoptosis, and partially reverses the malignant phenotypes of SGC-7901. These results suggest a potential role for human tumor gene therapy.

Shear stress and 17β-estradiol modulate cerebral microvascular endothelial Na-K-Cl cotransporter and Na/H exchanger protein levels

Ion transporters of blood-brain barrier (BBB) endothelial cells play an important role in regulating the movement of ions between the blood and brain. During ischemic stroke, reduction in cerebral blood flow is accompanied by transport of Na and Cl from the blood into the brain, with consequent brain edema formation. We have shown previously that a BBB Na-K-Cl cotransporter (NKCC) participates in ischemia-induced brain Na and water uptake and that a BBB Na/H exchanger (NHE) may also participate. While the abrupt reduction of blood flow is a prominent component of ischemia, the effects of flow on BBB NKCC and NHE are not known. In the present study, we examined the effects of changes in shear stress on NKCC and NHE protein levels in cerebral microvascular endothelial cells (CMECs). We have shown previously that estradiol attenuates both ischemia-induced cerebral edema and CMEC NKCC activity. Thus, in the present study, we also examined the effects of estradiol on NKCC and NHE protein levels in CMECs. Exposing CMECs to steady shear stress (19 dyn/cm2) increased the abundance of both NKCC and NHE. Estradiol abolished the shear stress-induced increase in NHE but not NKCC. Abrupt reduction of shear stress did not alter NKCC or NHE abundance in the absence of estradiol, but it decreased NKCC abundance in estradiol-treated cells. Our results indicate that changes in shear stress modulate BBB NKCC and NHE protein levels. They also support the hypothesis that estradiol attenuates edema formation in ischemic stroke in part by reducing the abundance of BBB NKCC protein.

DNA damage-induced Bcl-xL deamidation is mediated by NHE-1 antiport regulated intracellular pH

The pro-survival protein Bcl-x_L is critical for the resistance of tumour cells to DNA damage. We have previously demonstrated, using a mouse cancer model, that oncogenic tyrosine kinase inhibition of DNA damage–induced Bcl-x_L deamidation tightly correlates with T cell transformation in vivo, although the pathway to Bcl-x_L deamidation remains unknown and its functional consequences unclear. We show here that rBcl-x_L deamidation generates an iso-Asp⁵²/iso-Asp⁶⁶ species that is unable to sequester pro-apoptotic BH3-only proteins such as Bim and Puma. DNA damage in thymocytes results in increased expression of the NHE-1 Na/H antiport, an event both necessary and sufficient for subsequent intracellular alkalinisation, Bcl-x_L deamidation, and apoptosis. In murine thymocytes and tumour cells expressing an oncogenic tyrosine kinase, this DNA damage–induced cascade is blocked. Enforced intracellular alkalinisation mimics the effects of DNA damage in murine tumour cells and human B-lineage chronic lymphocytic leukaemia cells, thereby causing Bcl-x_L deamidation and increased apoptosis. Our results define a signalling pathway leading from DNA damage to up-regulation of the NHE-1 antiport, to intracellular alkalanisation to Bcl-x_L deamidation, to apoptosis, representing the first example, to our knowledge, of how deamidation of internal asparagine residues can be regulated in a protein in vivo. Our findings also suggest novel approaches to cancer therapy.

Cell survival and cell death (apoptosis) are controlled by a finely tuned ensemble of pro-survival and pro-apoptotic proteins. When the two types of protein are balanced, cells survive. But if the pro-survival proteins dominate, there is a danger that cells with damaged DNA will stay alive, leading to malignancy. One of the key pro-survival proteins, Bcl-x_L, acts by blocking the actions of pro-apoptotic proteins. We show here that DNA damage results in an important modification of Bcl-x_L. Specifically, when the amide groups are removed from two critical asparagine (amino acid) residues, Bcl-x_L can no longer block pro-apoptotic proteins, leading to cell death. Surprisingly, Bcl-x_L deamidation is catalysed not by an enzyme, but by increased pH inside the cell due to the up-regulation of an NHE-1 transporter that moves positive ions across the cell membrane. Indeed, artificially increasing pH causes Bcl-x_L deamidation and apoptosis in the absence of initial DNA damage. Exploring this novel pathway may ultimately suggest approaches to cancer therapy, especially when malignant cells are resistant to chemotherapy or radiotherapy.

Until now, the mechanisms and functional implications for DNA damage-induced Bcl-x_L deamidation were unknown. Here the authors provide important new insights into this phenomenon and its impact on cell survival.

Functional importance of charged residues within the putative intracellular loops in pH regulation by Na+/ H+ exchanger NHE1

The plasma membrane Na+/H+ exchanger 1 is activated in response to various extrinsic factors, and this process is regulated by an intracellular pH-sensing mechanism. To identify the candidate residues responsible for intracellular pH regulation, we analyzed the functional properties of engineered Na+/H+ exchanger 1 mutants with charge-reversal mutations of charged residues located in the intracellular loops. Na+/H+ exchanger 1 mutants with mutations at 11 positions were well expressed in the plasma membrane, but that with E247R was not, suggesting that Glu247 is important for the functional expression of Na+/H+ exchanger 1. Charge-reversal mutations of Glu131 (E131R, E131K) and Arg327 (R327E) resulted in a shift in the intracellular pH dependence of the exchange activity measured by 22Na+ uptake to the acidic side, and it abolished the response to growth factors and a hyperosmotic medium; however, mutations of Asp448 (D448R) and Arg500 (R500E) slightly shifted it to the alkaline side. In E131R, in addition to the change in intracellular pH dependence, the affinities for extracellular Na+, Li+ and the inhibitor 5-(N-ethyl-N-isopropyl)amiloride significantly increased. Furthermore, charge-conserved mutation of E131 (E131D) was found to have no effect, whereas charge neutralization (E131Q) resulted in a slight acidic shift of exchange. These results support the view that the multiple charged residues identified in this study, along with several basic residues reported previously, participate in the regulation of the intracellular pH sensing of Na+/H+ exchanger 1. In addition, Glu131 may also be important for cation transport.

p21-Activated kinase-1 and its role in integrated regulation of cardiac contractility

We review here a novel concept in the regulation of cardiac contractility involving variations in the activity of the multifunctional enzyme, p21-activated kinase 1 (Pak1), a member of a family of proteins in the small G protein-signaling pathway that is activated by Cdc42 and Rac1. There is a large body of evidence from studies in noncardiac tissue that Pak1 activity is key in regulation of a number of cellular functions, including cytoskeletal dynamics, cell motility, growth, and proliferation. Although of significant potential impact, the role of Pak1 in regulation of the heart has been investigated in only a few laboratories. In this review, we discuss the structure of Pak1 and its sites of posttranslational modification and molecular interactions. We assemble an overview of the current data on Pak1 signaling in noncardiac tissues relative to similar signaling pathways in the heart, and we identify potential roles of Pak1 in cardiac regulation. Finally, we discuss the current state of Pak1 research in the heart in regard to regulation of contractility through functional myofilament and Ca(2+)-flux modification. An important aspect of this regulation is the modulation of kinase and phosphatase activity. We have focused on Pak1 regulation of protein phosphatase 2A (PP2A), which is abundant in cardiac muscle, thereby mediating dephosphorylation of sarcomeric proteins and sensitizing the myofilaments to Ca(2+). We present a model for Pak1 signaling that provides a mechanism for specifically affecting cardiac cellular processes in which regulation of protein phosphorylation states by PP2A dephosphorylation predominates.

Carbonic anhydrase inhibition prevents and reverts cardiomyocyte hypertrophy

Hypertrophic cardiomyocyte growth contributes substantially to the progression of heart failure. Activation of the plasma membrane Na+-H+ exchanger (NHE1) and Cl- -HCO3- exchanger (AE3) has emerged as a central point in the hypertrophic cascade. Both NHE1 and AE3 bind carbonic anhydrase (CA), which activates their transport flux, by providing H+ and HCO3-, their respective transport substrates. We examined the contribution of CA activity to the hypertrophic response of cultured neonatal and adult rodent cardiomyocytes. Phenylephrine (PE) increased cell size by 37 +/- 2% and increased expression of the hypertrophic marker, atrial natriuretic factor mRNA, twofold in cultured neonatal rat cardiomyocytes. Cell size was also increased in adult cardiomyocytes subjected to angiotensin II or PE treatment. These effects were associated with increased expression of cytosolic CAII protein and the membrane-anchored isoform, CAIV. The membrane-permeant CA inhibitor, 6-ethoxyzolamide (ETZ), both prevented and reversed PE-induced hypertrophy in a concentration-dependent manner in neonate cardiomyocytes (IC50=18 microm). ETZ and the related CA inhibitor methazolamide prevented hypertrophy in adult cardiomyocytes. In addition, ETZ inhibited transport activity of NHE1 and the AE isoform, AE3, with respective EC50 values of 1.2 +/- 0.3 microm and 2.7 +/- 0.3 microm. PE significantly increased neonatal cardiomyocyte Ca2+ transient frequency from 0.33 +/- 0.4 Hz to 0.77 +/- 0.04 Hz following 24 h treatment; these Ca2+ -handling abnormalities were completely prevented by ETZ (0.28 +/- 0.07 Hz). Our study demonstrates a novel role for CA in mediating the hypertrophic response of cardiac myocytes to PE and suggests that CA inhibition represents an effective therapeutic approach towards mitigation of the hypertrophic phenotype.

AICAR inhibits the Na+/H+ exchanger in rat hearts--possible contribution to cardioprotection

AICAR (5-amino-1-beta-D: -ribofuranosyl-imidazole-4-carboxamide) is an adenosine analog which improves the recovery of the heart after ischemia. In some tissues AICAR enters cells and stimulates AMP-activated protein kinase (AMPK). We explored the mechanism of cardioprotection in isolated rat hearts. We confirmed that AICAR (0.5 mM) applied 10 min prior to a 30-min period of ischemia and present throughout ischemia and reperfusion caused a substantial improvement in the recovery of developed pressure on reperfusion. However, adenosine (100 microM) produced no improvement, suggesting that the mechanism of action of AICAR was not increased endogenous adenosine production. Measurements of intracellular sodium concentration ([Na(+)](i)) showed that AICAR prevented the rapid rise of [Na(+)](i), which normally occurs on reperfusion. Inhibitors of the cardiac sodium-hydrogen exchanger (NHE1) also protect the heart from ischemic damage and also prevent the rapid rise of [Na(+)](i) on reperfusion, suggesting that AICAR might cause the inhibition of NHE1. We tested this possibility on isolated rat ventricular myocytes in which the recovery of pH(i) after NH(4)Cl exposure provides a measure of NHE1 activity. AICAR (0.5 micromM) inhibited NHE1 activity in response to an acid load by about 80%. To test whether the AICAR-induced inhibition of NHE1 arose through adenosine, we used the adenosine receptor blocker 8-sulfophenyltheophylline (8-SPT) and found that it had no measureable effect. To test whether the AICAR-induced inhibition of NHE1 might occur through the activation of AMPK, we measured the activity of two isoforms of AMPK. Surprisingly, activity was reduced, whereas in many other tissues AICAR increases AMPK activity. Furthermore, this effect of AMPK was blocked by 8-SPT, suggesting that the inhibition of AMPK arose through an adenosine-receptor-related pathway. We conclude that AICAR inhibits NHE1 through an unidentified pathway. This inhibition may make a contribution to the cardioprotective effects of AICAR.

Vacuolar Na+/H+ antiporter cation selectivity is regulated by calmodulin from within the vacuole in a Ca2+- and pH-dependent manner

The selective movement of ions between intracellular compartments is fundamental for eukaryotes. Arabidopsis thaliana Na(+)/H(+) exchanger 1 (AtNHX1), the most abundant vacuolar Na(+)/H(+) antiporter in A. thaliana, has important roles affecting the maintenance of cellular pH, ion homeostasis, and the regulation of protein trafficking. Previously, we have shown that the AtNHX1 C-terminal hydrophilic region localized in the vacuolar lumen plays an important role in regulating the antiporter's activity. Here, we have identified A. thaliana calmodulin-like protein 15 (AtCaM15), which interacts with the AtNHX1 C terminus. When expressed in yeast, AtCaM15 is localized in the vacuolar lumen. The transient expression of AtCaM15 in Arabidopsis leaf protoplasts showed that AtCaM15 is present in the central vacuole. The binding of AtCaM15 to AtNHX1 was Ca(2+)- and pH-dependent and decreased with increasing pH values. Our results also show that the binding of AtCaM15 to AtNHX1 modified the Na(+)/K(+) selectivity of the antiporter, decreasing its Na(+)/H(+) exchange activity. Taken together, the presence of a vacuolar calmodulin-like protein acting on the vacuolar-localized AtNHX1 C terminus in a Ca(2+)- pH-dependent manner suggests the presence of signaling entities acting within the vacuole.

Differential contribution of troponin I phosphorylation sites to the endothelin-modulated contractile response

Cardiac troponin I is a phosphorylation target for endothelin-activated protein kinase C. Earlier work in cardiac myocytes expressing nonphosphorylatable slow skeletal troponin I provided evidence that protein kinase C-mediated cardiac troponin I phosphorylation accelerates relaxation. However, replacement with the slow skeletal isoform also alters the myofilament pH response and the Ca2+ transient, which could influence endothelin-mediated relaxation. Here, differences in the Ca2+ transient could not explain the divergent relaxation response to endothelin in myocytes expressing cardiac versus slow skeletal troponin I nor could activation of Na+/H+ exchange. Three separate clusters within cardiac troponin I are phosphorylated by protein kinase C, and we set out to determine the contribution of the Thr144 and Ser23/Ser24 clusters to the endothelin-mediated contractile response. Myocyte replacement with a cardiac troponin I containing a Thr144 substituted with the Pro residue found in slow skeletal troponin I resulted in prolonged relaxation in response to acute endothelin compared with control myocytes. Ser23/Ser24 also is a target for protein kinase C phosphorylation of purified cardiac troponin I, and although this cluster was not acutely phosphorylated in intact myocytes, significant phosphorylation developed within 1 h after adding endothelin. Replacement of Ser23/Ser24 with Ala indicated that this cluster contributes significantly to relaxation during more prolonged endothelin stimulation. Overall, results with these mutants provide evidence that Thr144 plays an important role in the acute acceleration of relaxation, whereas Ser23/Ser24 contributes to relaxation during more prolonged activation of protein kinase C by endothelin.

Rho GTPases dictate the mobility of the Na/H exchanger NHE3 in epithelia: role in apical retention and targeting

Proximal tubular reabsorption of filtered sodium by the sodium/hydrogen exchanger isoform 3 (NHE3), located on the apical membrane, is fundamental to the maintenance of systemic volume and pH homeostasis. NHE3 is finely regulated by a variety of hormones and by changes in ionic composition and volume, likely requiring redistribution of the exchangers. We analyzed the subcellular distribution and dynamics of the exchangers by generating an epithelial line expressing NHE3 tagged with an exofacial epitope, which enabled us to monitor exchanger mobility and traffic in intact cells. Using determinations of fluorescence recovery after photobleaching in combination with dynamic measurements of subcellular distribution, we found that, in renal epithelial cells, NHE3 exists in four distinct subcompartments: a virtually immobile subpopulation that is retained on the apical membrane by interaction with the actin cytoskeleton in a manner that depends on the sustained activity of Rho GTPases; a mobile subpopulation on the apical membrane, which can be readily internalized; and two intracellular compartments that can be differentiated by their rate of exchange with the apical pool of NHE3. We provide evidence that detachment of the immobile fraction from its cytoskeletal anchorage leads to rapid internalization. These observations suggest that modulation of the mobile fraction of NHE3 on the apical membrane can alter the number of functional exchangers on the cell surface and, consequently, the rate of transepithelial ion transport. Regulation of the interaction of NHE3 with the actin cytoskeleton can therefore provide a new mode of regulation of sodium and hydrogen transport.

Na+/H+ Exchanger Inhibitor Prevented Endothelial Dysfunction Induced by High Glucose

The aims of this study were to examine whether cariporide, a selective Na+/H+ exchanger inhibitor, has protective effects against endothelial dysfunction induced by high glucose in vitro and to investigate the potential mechanisms. Exposure of rat aorta rings to high glucose (44 mmol/L) for 6 hours caused an inhibition of acetylcholine-induced endothelium-dependent relaxation but had no effect on sodium nitroprusside-induced endothelium-independent relaxation. Treatment with cariporide (0.01, 0.1, 1 micromol/L) of aortic rings incubated with high-glucose medium attenuated the impaired endothelium-dependent relaxation in a dose-dependent manner. Furthermore, high glucose resulted in an increase of malondialdehyde and a decrease of superoxide dismutase activity in rat aorta rings, and these effects were reversed by cariporide. In addition, cariporide was able to inhibit the activation of Na+/H+ exchanger induced by high glucose in cultured endothelial cells. These findings suggest that the endothelial dysfunction induced by high glucose in vitro is caused by the activation of Na+/H+ exchanger.

Expression and membrane localization of MCT isoforms along the length of the human intestine

Recent studies from our laboratory and others have demonstrated the involvement of monocarboxylate transporter (MCT)1 in the luminal uptake of short-chain fatty acids (SCFAs) in the human intestine. Functional studies from our laboratory previously demonstrated kinetically distinct SCFA transporters on the apical and basolateral membranes of human colonocytes. Although apical SCFA uptake is mediated by the MCT1 isoform, the molecular identity of the basolateral membrane SCFA transporter(s) and whether this transporter is encoded by another MCT isoform is not known. The present studies were designed to assess the expression and membrane localization of different MCT isoforms in human small intestine and colon. Immunoblotting was performed with the purified apical and basolateral membranes from human intestinal mucosa obtained from organ donor intestine. Immunohistochemistry studies were done on paraffin-embedded sections of human colonic biopsy samples. Immunoblotting studies detected a protein band of approximately 39 kDa for MCT1, predominantly in the apical membranes. The relative abundance of MCT1 mRNA and protein increased along the length of the human intestine. MCT4 (54 kDa) and MCT5 (54 kDa) isoforms showed basolateral localization and were highly expressed in the distal colon. Immunohistochemical studies confirmed that human MCT1 antibody labeling was confined to the apical membranes, whereas MCT5 antibody staining was restricted to the basolateral membranes of the colonocytes. We speculate that distinct MCT isoforms may be involved in SCFA transport across the apical or basolateral membranes in polarized colonic epithelial cells.

Calcitonin receptor-stimulating peptide-1 regulates ion transport and growth of renal epithelial cell line LLC-PK1

Calcitonin receptor-stimulating peptide-1 (CRSP-1) is a peptide recently identified from porcine brain by monitoring the cAMP production through an endogenous calcitonin (CT) receptor in the renal epithelial cell line LLC-PK(1). Here we investigated the effects of CRSP-1 on the ion transport and growth of LLC-PK(1) cells. CRSP-1 inhibited the growth of LLC-PK(1) cells with a higher potency than porcine CT. CRSP-1 enhanced the uptake of (22)Na(+) into LLC-PK(1) cells more strongly than did CT and slightly reduced the (45)Ca(2+) uptake. The enhancement of the (22)Na(+) uptake was abolished by 5-(N-ethyl-N-isopropyl) amiloride, a strong Na(+)/H(+) exchanger (NHE) inhibitor for NHE1, even at a concentration of 1x10(-8)M, although other ion transporter inhibitors did not affect the (22)Na(+) uptake. These results indicate that CRSP-1 enhances the (22)Na(+) uptake by the specific activation of NHE1. Taken together, CRSP-1 is considered to be a new regulator for the urinary ion excretion and renal epithelial cell growth.

The effect of intracellular acidification on the relationship between cell volume and membrane potential in amphibian skeletal muscle

The relationship between cell volume (V(c)) and membrane potential (E(m)) in Rana temporaria striated muscle fibres was investigated under different conditions of intracellular acidification. Confocal microscope xz-scanning monitored the changes in V(c), whilst conventional KCl and pH-sensitive microelectrodes measured E(m) and intracellular pH (pH(i)), respectively. Applications of Ringer solutions with added NH(4)Cl induced rapid reductions in V(c) that rapidly reversed upon their withdrawal. These could be directly attributed to the related alterations in extracellular tonicity. However: (1) a slower and persistent decrease in V(c) followed the NH(4)Cl withdrawal, leaving V(c) up to 10% below its resting value; (2) similar sustained decreases in resting V(c) were produced by the addition and subsequent withdrawal of extracellular solutions in which NaCl was isosmotically replaced with NH(4)Cl; (3) the same manoeuvres also produced a marked intracellular acidification, that depended upon the duration of the preceding exposure to NH(4)Cl, of up to 0.53 +/- 0.10 pH units; and (4) the corresponding reductions in V(c) similarly increased with this exposure time. These reductions in V(c) persisted and became more rapid with Cl(-) deprivation, thus excluding mechanisms involving either direct or indirect actions of pH(i) upon Cl(-)-dependent membrane transport. However they were abolished by the Na(+),K(+)-ATPase inhibitor ouabain. The E(m) changes that accompanied the addition and withdrawal of NH(4)(+) conformed to a Nernst equation modified to include realistic NH(4)(+) permeability terms, and thus the withdrawal of NH(4)(+) restored E(m) to close to control values despite a persistent change in V(c). Finally these E(m) changes persisted and assumed faster kinetics with Cl(-) deprivation. The relative changes in V(c), E(m) and pH(i) were compared to predictions from the recent model of Fraser and Huang published in 2004 that related steady-state values of V(c) and E(m) to the mean charge valency (z(x)) of intracellular membrane-impermeant anions, X(-)(i). By assuming accepted values of intracellular buffering capacity (beta(i)), intracellular acidification was shown to produce quantitatively predictable decreases in V(c). These findings thus provide experimental evidence that titration of the anionic z(x) by increased intracellular [H(+)] causes cellular volume decrease in the presence of normal Na(+),K(+)- ATPase activity, with Cl(-)-dependent membrane fluxes only influencing the kinetics of such changes.

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Amiloride peptide conjugates: prodrugs for sodium-proton exchange inhibition

Inhibition of the sodium-proton exchanger (NHE) plays an important role in reducing tissue damage during ischemic reperfusion injury; however, pharmacological inhibitors of NHE have restricted access to acutely ischemic tissues because of severely compromised tissue perfusion. We describe the syntheses, characterization, and NHE inhibitory activities of a novel class of amiloride derivatives where peptides are conjugated to the amiloride C(5) amino group. These new peptide-C(5)-amiloride conjugates are inactive; however, peptide residues were chosen such that selective cleavage by neutral endopeptidase 24.11 (enkephalinase) liberates an amino acid-C(5)-amiloride conjugate that inhibits NHE in a glial cell line. These results confirm the feasibility of using peptide-amiloride conjugates as NHE inhibitor prodrugs. We envision the design of analogous peptide-amiloride prodrugs that can be administered prior to ischemic events and subsequently activated by endopeptidases selectively expressed by ischemic tissues.

Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion

The renal proximal tubule of vertebrates performs an essential role in controlling plasma SO42−concentration ([SO42−]). Although net tubular SO42−reabsorption is the predominate control process in terrestrial vertebrates, a facilitated secretory flux is also present. In contrast, marine teleosts obtain excess SO42−from drinking, and increased plasma [SO42−] is prevented predominately through net tubular secretion. Tubular SO42−secretion is accomplished by at least two electroneutral anion exchange processes in series. Movement of SO42−into the cell across the basolateral membrane is pH dependent, suggesting SO42−/OH−exchange. Luminal HCO3−and Cl−can facilitate SO42−movement out of the cell across the brush-border membrane. The molecular identities of the anion exchangers are unknown but are probably homologues of SO42−transporters in the mammalian SLC26 gene family. In all species tested, glucocorticoids increase renal SO42−excretion. Whereas glucocorticoids downregulate SO42−reabsorptive mechanisms in terrestrial vertebrates, they may also stimulate a mediated secretory flux. In the marine teleost, cortisol increases the level of SO42−/HCO3−exchange at the brush-border membrane, tubular carbonic anhydrase (CA) activity, CAII protein, and a proportion of tubular SO42−secretion that is CA dependent. CA activity is required for about one-half of this net SO42−secretion but is also required for about one-half of the net reabsorption in bird proximal epithelium. A CA-SO42−/anion exchanger metabolon arrangement is proposed that may speed both the secretory and reabsorptive processes.

Functional analysis of amino acids of the Na+/H+ exchanger that are important for proton translocation

The Na+/H+ exchanger is an integral membrane protein found in the plasma membrane of eukaryotic and prokaryotic cells. In eukaryotes it functions to exchange one proton for a sodium ion. In mammals it removes intracellular protons while in plants and fungal cells the plasma membrane form removes intracellular sodium in exchange for extracellular protons. In this study we used the Na+/H+ exchanger of Schizosaccharomyces pombe (Sod2) as a model system to study amino acids critical for activity of the protein. Twelve mutant forms of the Na+/H+ exchanger were examined for their ability to translocate protons as assessed by a Cytosensor microphysiometer. Mutation of the amino acid Histidine 367 resulted in defective proton translocation. The acidic residues Asp145, Asp178, Asp266 and Asp267 were important in the proton translocation activity of the Na+/H+ exchanger. Mutation of amino acids His98, His233 and Asp241 did not significantly impair proton translocation by the Na+/H+ exchanger. These results confirm that polar amino acids are important in proton flux activity of Na+/H+ exchangers.

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.

Topological analysis of a plant vacuolar Na+/H+ antiporter reveals a luminal C terminus that regulates antiporter cation selectivity

We conducted an analysis of the topology of AtNHX1, an Arabidopsis thaliana vacuolar Na+/H+ antiporter. Several hydrophilic regions of the antiporter were tagged with a hemagglutinin epitope, and protease protection assays were conducted to determine the membrane topology of the antiporter by using yeast as a heterologous expression system. The overall structure of AtNHX1 is distinct from the human Na+/H+ antiporter NHE1 or any known Na+/H+ antiporter. It is comprised of nine transmembrane domains and a hydrophilic C-terminal domain. Three hydrophobic regions do not appear to span the tonoplast membrane, yet appear to be membrane associated. Our results also indicate that, whereas the N terminus of AtNHX1 is facing the cytosol, almost the entire C-terminal hydrophilic region resides in the vacuolar lumen. Deletion of the hydrophilic C terminus resulted in a dramatic increase in the relative rate of Na+/H+ transport. The ratio of Na+/K+ transport was twice that of the unmodified AtNHX1. This altered ratio resulted from a relatively small decrease in K+/H+ transport with a large increase in Na+/H+ transport. The vacuolar localization of the C terminus of the AtNHX1, taken together with the regulation of the antiporter selectivity by its C terminus, demonstrates the existence of luminal vacuolar regulatory mechanisms of the antiporter activity.