Hydrocortisone cardioprotection in ischaemia/reperfusion injury involves antioxidant mechanisms

BACKGROUND

Glucocorticoid (GR) and mineralocorticoid (MR) receptors are highly expressed in cardiac tissue, and both can be activated by corticosteroids. MR activation, in acute myocardial infarction (AMI), worsens cardiac function, and increase NHE activity contributing to the deleterious process. In contrast, effects of GR activation are not fully understood, probably because of the controversial scenario generated by using different doses or potencies of corticosteroids.

AIMS

We tested the hypothesis that an acute dose of hydrocortisone (HC), a low-potency glucocorticoid, in a murine model of AMI could be cardioprotective by regulating NHE1 activity, leading to a decrease in oxidative stress.

MATERIALS AND METHODS

Isolated hearts from Wistar rats were subjected to regional ischemic protocol. HC (10 nmol/L) was added to the perfusate during early reperfusion. Infarct size and oxidative stress were determined. Isolated papillary muscles from non-infarcted hearts were used to evaluate HC effect on sodium-proton exchanger 1 (NHE1) by analysing intracellular pH recovery from acute transient acidosis.

RESULTS

HC treatment decreased infarct size, improved cardiac mechanics, reduced oxidative stress after AMI, while restoring the decreased level of the pro-fusion mitochondrial protein MFN-2. Co-treatment with the GR-blocker Mifepristone avoided these effects. HC reduced NHE1 activity by increasing the NHE1 pro-inhibiting Ser648 phosphorylation site and its upstream kinase AKT. HC restored the decreased AKT phosphorylation and anti-apoptotic BCL-2 protein expression detected after AMI.

CONCLUSIONS

Our results provide the first evidence that acute HC treatment during early reperfusion induces cardioprotection against AMI, associated with a non-genomic HC-triggered NHE1 inhibition by AKT and antioxidant action that might involves mitochondrial dynamics improvement.

NhaA facilitates the maintenance of bacterial envelope integrity and the evasion of complement attack contributing to extraintestinal pathogenic Escherichia coli virulence

Extraintestinal pathogenic Escherichia coli (ExPEC) is responsible for severe bloodstream infections in humans and animals. However, the mechanisms underlying ExPEC's serum resistance remain incompletely understood. Through the transposon-directed insertion-site sequencing approach, our previous study identified nhaA, the gene encoding a Na+/H+ antiporter, as a crucial factor for infection in vivo. In this study, we investigated the role of NhaA in ExPEC virulence utilizing both in vitro models and systemic infection models involving avian and mammalian animals. Genetic mutagenesis analysis revealed that nhaA deletion resulted in filamentous bacterial morphology and rendered the bacteria more susceptible to sodium dodecyl sulfate, suggesting the role of nhaA in maintaining cell envelope integrity. The nhaA mutant also displayed heightened sensitivity to complement-mediated killing compared to the wild-type strain, attributed to augmented deposition of complement components (C3b and C9). Remarkably, NhaA played a more crucial role in virulence compared to several well-known factors, including Iss, Prc, NlpI, and OmpA. Our findings revealed that NhaA significantly enhanced virulence across diverse human ExPEC prototype strains within B2 phylogroups, suggesting widespread involvement in virulence. Given its pivotal role, NhaA could serve as a potential drug target for tackling ExPEC infections.

Physiological and Molecular Function of the Sodium/Hydrogen Exchanger NHA2 (SLC9B2)

NHA2, also known as SLC9B2, is an orphan intracellular Na+/H+ exchanger (NHE) that has been associated with arterial hypertension and diabetes mellitus in humans. The objective of this NCCR TransCure project was to define the physiological and molecular function of NHA2, to develop a high resolution kinetic transport assay for NHA2 and to identify specific and potent compounds targeting NHA2. In this review, we summarize the results of this highly interdisciplinary and interfaculty effort, led by the groups of Proffs. Jean-Louis Reymond, Christoph von Ballmoos and Daniel Fuster.

Loss of SALT OVERLY SENSITIVE 1 prevents virescence in chloroplast K+/H+ EFFLUX ANTIPORTER-deficient mutants

Defects in two plastid K⁺/H⁺ EFFLUX ANTIPORTERs in Arabidopsis can be relieved by loss of a plasma membrane Na⁺/H⁺ exchanger, presumably by altering plant K⁺ transport.

Prolonged NHE Activation may be both Cause and Outcome of Cytokine Release Syndrome in COVID-19

The release of cytokines and chemokines such as IL-1β, IL-2, IL-6, IL-7, IL-10, TNF-α, IFN-γ, CCL2, CCL3, and CXCL10 is increased in critically ill patients with COVID-19. Excessive cytokine release during COVID-19 is related to increased morbidity and mortality. Several mechanisms are put forward for cytokine release syndrome during COVID-19. Here we have mentioned novel pathways. SARS-CoV-2 increases angiotensin II levels by rendering ACE2 nonfunctional. Angiotensin II causes cytokine release via AT₁ and AT₂ receptors. Moreover, angiotensin II potently stimulates the Na+/H+ exchanger (NHE). It is a pump found in the membranes of many cells that pumps Na+ inward and H+ outward. NHE has nine isoforms. NHE1 is the most common isoform found in endothelial cells and many cells. NHE is involved in keeping the intracellular pH within physiological limits. When the intracellular pH is acidic, NHE is activated, bringing the intracellular pH to physiological levels, ending its activity. Sustained NHE activity is highly pathological and causes many problems. Prolonged NHE activation in COVID-19 may cause a decrease in intracellular pH through H+ ion accumulation in the extracellular area and subsequent redox reactions. The activation reduces the intracellular K+ concentration and leads to Na+ and Ca²⁺ overload. Increased ROS can cause intense cytokine release by stimulating NF-κB and NLRP3 inflammasomes. Cytokines also cause overstimulation of NHE. As the intracellular pH decreases, SARS-CoV-2 rapidly infects new cells, increasing the viral load. This vicious circle increases morbidity and mortality in patients with COVID-19. On the other hand, SARS-CoV-2 interaction with NHE3 in intestinal tissue is different from other tissues. SARS-CoV-2 can trigger CRS via NHE3 inhibition by disrupting the intestinal microbiota. This review aimed to help develop new treatment models against SARS-CoV-2- induced CRS by revealing the possible effects of SARS-CoV-2 on the NHE.

Post-translational regulation of the membrane transporters contributing to salt tolerance in plants

This review article summarises the role of membrane transporters and their regulatory kinases in minimising the toxicity of Na+ in the plant under salt stress. The salt-tolerant plants keep their cytosolic level of Na+ up to 10-50mM. The first line of action in this context is the generation of proton motive force by the plasma membrane H+-ATPase. The generated proton motive force repolarises the membrane that gets depolarised due to passive uptake of Na+ under salt stress. The proton motive force generated also drives the plasma membrane Na+/H+ antiporter, SOS1 that effluxes the cytosolic Na+ back into the environment. At the intracellular level, Na+ is sequestered by the vacuole. Vacuolar Na+ uptake is mediated by Na+/H+ antiporter, NHX, driven by the electrochemical gradient for H+, generated by tonoplast H+ pumps, both H+ATPase and PPase. However, it is the expression of the regulatory kinases that make these transporters active through post-translational modification enabling them to effectively manage the cytosolic level of Na+, which is essential for tolerance to salinity in plants. Yet our knowledge of the expression and functioning of the regulatory kinases in plant species differing in tolerance to salinity is scant. Bioinformatics-based identification of the kinases like OsCIPK24 in crop plants, which are mostly salt-sensitive, may enable biotechnological intervention in making the crop cultivar more salt-tolerant, and effectively increasing its annual yield.

Overexpression of KcNHX1 gene confers tolerance to multiple abiotic stresses in Arabidopsis thaliana

Abiotic stresses such as drought, salinity, and heat affect plant growth and development. Karelinia caspica is a unique perennial herb that grows in desert area for a long time and has strong tolerance to environmental stresses. In order to explore the functions of the Na⁺/H⁺ antiporter gene from eremophyte K. caspica (KcNHX1) in the abiotic stress response of K. caspica and the underlying regulatory mechanisms, we constructed a vector overexpressing KcNHX1 and transformed it into Arabidopsis thaliana. The physiological results showed that the overexpression of KcNHX1 in A. thaliana not only enhanced the plant's tolerance to salt stress, but also enhanced its tolerance to drought and heat stress at the seedling stage. In addition, KcNHX1-overexpressing plants exhibited enhanced reproductive growth under high temperature, which was mediated by increased auxin accumulation. Taken together, our results indicate that KcNHX1 from an eremophyte can be used as a candidate gene to improve multiple stress tolerance in other plants.

Pharmacological inhibition of sodium-calcium exchange activates NADPH oxidase and induces infection-independent NETotic cell death

In addition to its function of innate immunity against invading pathogens, neutrophil extracellular traps (NETs) promote thrombosis, autoimmune disease, and cancer metastasis; therefore, unnecessary exposure to the triggers of infection-independent NET generation should be avoided. We herein show that inhibition of forward-mode Na⁺/Ca²⁺ exchange by amiloride analogs, 5-(N-ethyl-N-isopropyl)amiloride (EIPA) and 5-(N-Methyl-N-isobutyl)amiloride (MIA), triggers NETotic cell death independently of infectious stimuli. Isolated human neutrophils treated with EIPA and MIA undergo NETotic cell death by an increase of intracellular Ca²⁺ following activation of NADPH oxidase and the resultant upregulation of intracellular ROS. EIPA- and MIA-mediated intracellular Ca²⁺ increase is attributed to the competitive binding of EIPA and MIA against Na⁺ to Na⁺/Ca²⁺ exchanger 1 (NCX1). These results demonstrate a new mechanism of infection-independent NET generation and implicate NCX1 as a physiologic regulator of intracellular calcium balance and NETotic cell death.

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Two of the amiloride analogs, EIPA and MIA, induce NETotic cell death without infectious stimuli.

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EIPA and MIA inhibit the forward-mode Na⁺/Ca²⁺ exchange and promote the intracellular Ca²⁺ overload.

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Intracellular Ca²⁺ overload by EIPA and MIA activates NADPH oxidase, elevates intracellular ROS level, and induces resultant NETotic cell death.

ROLE OF Β-ADRENOCEPTORS IN REGULATION OF ERYTHROCYTES' RHEOLOGICAL FUNCTIONS (REVIEW)

In this review, we discuss the role of adrenoceptors in the regulation of the rheological functions of erythrocytes. β-adrenoceptors play an important role in the regulation of erythrocytes functions and metabolism. They participate in the modification of transport membrane proteins (Na+/K+-ATPase, Ca+2-ATPase, Na+/K+/2Cl-cotransporter, Na+/H+-antiporter, CAT-1, Ca2+-dependent K+ channels (Gard channels), the activity of adenylate cyclase and cAMP, AMP-dependent activation of the L-arginine/NOS system and erythrocyte NOS) and by this way modulate the cells volume, rheological properties (deformability, aggregability), intensity of NO synthesis and ATP reliase. These properties of erythrocytes determine, that, in addition to the transport of gases, they play the oxygen sensors role and can participate in the mechanisms of vasorelaxation and maintenance of a normal level of microcirculation.

Carbonic anhydrases enhance activity of endogenous Na-H exchangers and not the electrogenic Na/HCO3 cotransporter NBCe1-A, expressed in Xenopus oocytes

KEY POINTS

According to the HCO 3 - metabolon hypothesis, direct association of cytosolic carbonic anhydrases (CAs) with the electrogenic Na/HCO3 cotransporter NBCe1-A speeds transport by regenerating/consuming HCO 3 - . The present work addresses published discrepancies as to whether cytosolic CAs stimulate NBCe1-A, heterologously expressed in Xenopus oocytes. We confirm the essential elements of the previous experimental observations, taken as support for the HCO 3 - metabolon hypothesis. However, using our own experimental protocols or those of others, we find that NBCe1-A function is unaffected by cytosolic CAs. Previous conclusions that cytosolic CAs do stimulate NBCe1-A can be explained by an unanticipated stimulatory effect of the CAs on an endogenous Na-H exchanger. Theoretical analyses show that, although CAs could stimulate non- HCO 3 - transporters (e.g. Na-H exchangers) by accelerating CO2 / HCO 3 - -mediated buffering of acid-base equivalents, they could not appreciably affect transport rates of NBCe1 or other transporters carrying HCO 3 - , CO 3 = , or NaCO 3 - ion pairs.

ABSTRACT

The HCO 3 - metabolon hypothesis predicts that cytosolic carbonic anhydrase (CA) binds to NBCe1-A, promotes HCO 3 - replenishment/consumption, and enhances transport. Using a short step-duration current-voltage (I-V) protocol with Xenopus oocytes expressing eGFP-tagged NBCe1-A, our group reported that neither injecting human CA II (hCA II) nor fusing hCA II to the NBCe1-A carboxy terminus affects background-subtracted NBCe1 slope conductance (GNBC ), which is a direct measure of NBCe1-A activity. Others - using bovine CA (bCA), untagged NBCe1-A, and protocols keeping holding potential (Vh ) far from NBCe1-A's reversal potential (Erev ) for prolonged periods - found that bCA increases total membrane current (ΔIm ), which apparently supports the metabolon hypothesis. We systematically investigated differences in the two protocols. In oocytes expressing untagged NBCe1-A, injected with bCA and clamped to -40 mV, CO2 / HCO 3 - exposures markedly decrease Erev , producing large transient outward currents persisting for >10 min and rapid increases in [Na+ ]i . Although the CA inhibitor ethoxzolamide (EZA) reduces both ΔIm and d[Na+ ]i /dt, it does not reduce GNBC . In oocytes not expressing NBCe1-A, CO2 / HCO 3 - triggers rapid increases in [Na+ ]i that both hCA II and bCA enhance in concentration-dependent manners. These d[Na+ ]i /dt increases are inhibited by EZA and blocked by EIPA, a Na-H exchanger (NHE) inhibitor. In oocytes expressing untagged NBCe1-A and injected with bCA, EIPA abolishes the EZA-dependent decreases in ΔIm and d[Na+ ]i /dt. Thus, CAs/EZA produce their ΔIm and d[Na+ ]i /dt effects not through NBCe1-A, but endogenous NHEs. Theoretical considerations argue against a CA stimulation of HCO 3 - transport, supporting the conclusion that an NBCe1-A- HCO 3 - metabolon does not exist in oocytes.

The plasma-membrane polyamine transporter PUT3 is regulated by the Na+ /H+ antiporter SOS1 and protein kinase SOS2

In Arabidopsis, the plasma membrane transporter PUT3 is important to maintain the cellular homeostasis of polyamines and plays a role in stabilizing mRNAs of some heat-inducible genes. The plasma membrane Na⁺ /H⁺ transporter SOS1 and the protein kinase SOS2 are two salt-tolerance determinants crucial for maintaining intracellular Na⁺ and K⁺ homeostasis. Here, we report that PUT3 genetically and physically interacts with SOS1 and SOS2, and these interactions modulate PUT3 transport activity. Overexpression of PUT3 (PUT3OE) results in hypersensitivity of the transgenic plants to polyamine and paraquat. The hypersensitivity of PUT3OE is inhibited by the sos1 and sos2 mutations, which indicates that SOS1 and SOS2 are required for PUT3 transport activity. A protein interaction assay revealed that PUT3 physically interacts with SOS1 and SOS2 in yeast and plant cells. SOS2 phosphorylates PUT3 both in vitro and in vivo. SOS1 and SOS2 synergistically activate the polyamine transport activity of PUT3, and PUT3 also modulates SOS1 activity by activating SOS2 in yeast cells. Overall, our findings suggest that both plasma-membrane proteins PUT3 and SOS1 could form a complex with the protein kinase SOS2 in response to stress conditions and modulate the transport activity of each other through protein interactions and phosphorylation.

Declined cadmium accumulation in Na+/H+ antiporter (NHX1) transgenic duckweed under cadmium stress

Cadmium (Cd) is a serious threat to plants health. Though some genes have been reported to get involved in the regulation of tolerance to Cd, the mechanisms underlying this process are not fully understood. Na⁺/H⁺ antiporter (NHX1) plays an important role in Na⁺/H⁺ trafficking. The salt and cadmium stress tolerance were found to be enhanced by NHX1 in duckweed according to our previous study, however, its function in Cd²⁺ flux under Cd stress has not been studied. Here we explored the Cd²⁺ flux in wild type (WT) and NHX1 transgenic duckweed (NHX1) under Cd stress. We found that the Cd²⁺ influx in NHX1 duckweed was significantly declined, followed by an increased Cd²⁺ efflux after 20 min treatment of Cd, which resulted a less accumulation of Cd in NHX1. Reversely, inhibition of NHX1 by amiloride treatment, enhanced Cd²⁺ influx in NHX1 duckweed, subsequently delayed Cd²⁺ efflux in both genotypes of duckweed under Cd²⁺ shock. H⁺ efflux in NHX1 duckweed was lower compare with that in WT with 20 min Cd²⁺ shock. NHX1 also increased the pH value with Cd²⁺ stress in the transgenic rhizoid. These finding suggested a new function of NHX1 in regulation of Cd²⁺ and H⁺ flow during short-term Cd²⁺ shock.

Clostridium difficile toxins A and B decrease intestinal SLC26A3 protein expression

Clostridium difficile infection (CDI) is the primary cause of nosocomial diarrhea in the United States. Although C. difficile toxins A and B are the primary mediators of CDI, the overall pathophysiology underlying C. difficile-associated diarrhea remains poorly understood. Studies have shown that a decrease in both NHE3 (Na+/H+ exchanger) and DRA (downregulated in adenoma, Cl-/[Formula: see text] exchanger), resulting in decreased electrolyte absorption, is implicated in infectious and inflammatory diarrhea. Furthermore, studies have shown that NHE3 is depleted at the apical surface of intestinal epithelial cells and downregulated in patients with CDI, but the role of DRA in CDI remains unknown. In the current studies, we examined the effects of C. difficile toxins TcdA and TcdB on DRA protein and mRNA levels in intestinal epithelial cells (IECs). Our data demonstrated that DRA protein levels were significantly reduced in response to TcdA and TcdB in IECs in culture. This effect was also specific to DRA, as NHE3 and PAT-1 (putative anion transporter 1) protein levels were unaffected by TcdA and TcdB. Additionally, purified TcdA and TcdA + TcdB, but not TcdB, resulted in a decrease in colonic DRA protein levels in a toxigenic mouse model of CDI. Finally, patients with recurrent CDI also exhibited significantly reduced expression of colonic DRA protein. Together, these findings indicate that C. difficile toxins markedly downregulate intestinal expression of DRA which may contribute to the diarrheal phenotype of CDI. NEW & NOTEWORTHY Our studies demonstrate, for the first time, that C. difficile toxins reduce DRA protein, but not mRNA, levels in intestinal epithelial cells. These findings suggest that a downregulation of DRA may be a critical factor in C. difficile infection-associated diarrhea.

Investigations of the signaling cascade involved in diuretic hormone stimulation of Malpighian tubule fluid secretion in Rhodnius prolixus

In insects, the excretory system is comprised of the Malpighian tubules (MTs) and the hindgut, which collectively function to maintain ionic and osmotic balance of the haemolymph and rid the organism of toxic compounds or elements in excess. Secretion by the Malpighian tubules of insects is regulated by a variety of hormones including peptidergic factors as well as biogenic amines. In Rhodnius prolixus, two endogenous diuretic hormones have been identified; the biogenic amine serotonin (5-hydroxytryptamine, 5-HT) and the corticotropin releasing factor-related peptide, RhoprCRF. Both factors significantly increase secretion by MTs and are known to elevate intracellular levels of cAMP. Interestingly, applying sub-maximal doses of these two diuretic factors in combination on isolated MTs in vitro reveals synergistic effects as rates of fluid secretion are significantly higher than would be expected if rates of secretion from MTs treated with each factor alone were summed. This observed synergism suggests that different downstream targets may be activated by the two diuretic factors, but that some cellular elicitors may be shared since cAMP is elevated in response to either diuretic hormone. This study investigated the signaling cascade involved in the diuretic hormone regulation of Malpighian tubule fluid secretion. Bioassays were performed in physiological as well as modified salines (e.g. calcium-free) alone or in the presence of a variety of pharmacological compounds that interfere with prospective intracellular targets, such as the apical cation/H(+) exchanger. Intriguingly, only amiloride yielded differential effects on the two diuretics with 5HT-stimulated secretion being blocked, whereas in contrast, RhoprCRF-stimulated secretion was unaffected. In addition, experiments examining the role of extracellular and intracellular calcium on fluid secretion rate showed that both diuretics are dependent on intracellular calcium availability. Finally, fluid secretion stimulated by either diuretic hormone was also sensitive to inhibition of cAMP-dependent protein kinase A. Taken together, these results suggest that each diuretic hormone activates pathways dependent upon intracellular calcium and cAMP.

Skin fibroblasts as a tool for identifying the risk of nephropathy in the type 1 diabetic population

Human fibroblasts in culture have been employed as an in vitro system to investigate some pathophysiological mechanisms of diabetes mellitus also associated with the development of diabetic nephropathy. In fact, there is increasing evidence that genetic factors either convey the risk of, or protect from, diabetic nephropathy and that the expression profiles and/or the behaviour of the cultured skin fibroblasts from type 1 diabetic patients could reflect these genetic influences. On the other hand, alterations could be attributable not only to changes in DNA sequence, but also to epigenetic factors. Our aim is to make a critical overview of the studies involving primary cultures of skin fibroblasts as tools to investigate the pathophysiology of diabetic nephropathy performed until now in this area. Cultured skin fibroblasts could be useful not only for the identification of patients at risk of developing diabetic renal disease, but also for a better understanding of the complex multifactorial mechanisms leading to the long-term complications in diabetes.

Co-expression of Pennisetum glaucum vacuolar Na⁺/H⁺ antiporter and Arabidopsis H⁺-pyrophosphatase enhances salt tolerance in transgenic tomato

Salinity is one of the major abiotic stresses affecting plant productivity. Tomato (Solanum lycopersicum L.), an important and widespread crop in the world, is sensitive to moderate levels of salt in the soil. To generate tomato plants that can adapt to saline soil, AVP1, a vacuolar H(+)-pyrophosphatase gene from Arabidopsis thaliana, and PgNHX1, a vacuolar Na(+)/H(+) antiporter gene from Pennisetum glaucum, were co-expressed by Agrobacterium tumefaciens-mediated transformation. A sample of transformants was self-pollinated, and progeny were evaluated for salt tolerance in vitro and in vivo. It is reported here that co-expression of AVP1 and PgNHX1 confers enhanced salt tolerance to the transformed tomato compared with the AVP1 and PgNHX1 single gene transgenic plants and the wild-type. These transgenic plants grew well in the presence of 200 mM NaCl while wild-type plants exhibited chlorosis and died within 3 weeks. The transgenic line co-expressing AVP1 and PgNHX1 retained more chlorophyll and accumulated 1.4 times more proline as a response to stress than single gene transformants. Moreover, these transgenic plants accumulated a 1.5 times higher Na(+) content in their leaf tissue than the single gene transformants. The toxic effect of Na(+) accumulation in the cytosol is reduced by its sequestration into the vacuole. The physiological analysis of the transgenic lines clearly demonstrates that co-expression of AVP1 and PgNHX1 improved the osmoregulatory capacity of double transgenic lines by enhanced sequestration of ions into the vacuole by increasing the availability of protons and thus alleviating the toxic effect of Na(+).

Ezrin and EBP50 redistribute apically in rat uterine epithelial cells at the time of implantation and in response to cell contact

Uterine epithelial cells (UECs) undergo extensive morphological remodelling in preparation for an implanting blastocyst. This remodelling involves changes in the actin cytoskeleton and surface structures including microvilli. Ezrin and ezrin-radixin-moesin-binding protein-50-kDa (EBP50) link actin filaments to intra-membranous adhesion molecules and are important molecules in polarised epithelia. The current study is the first to describe the colocalisation and molecular association of ezrin and EBP50 in rat UECs by using immunofluorescence microscopy and immunoprecipitation techniques. These proteins have also been localised in relation to uterine epithelial cytoskeletal rearrangement during early pregnancy in the rat and to the effect of apical surface contact between opposing epithelial cells, blastocyst contact and contact with a silicon filament. Immunofluorescence microscopy has revealed that ezrin and EBP50 respond to contact between opposing epithelial cells and increase apically on day 6 of pregnancy. This apical distribution is also observed in UECs in contact with a silicon filament. Ezrin and EBP50 are however absent within the implantation chamber itself, seemingly mimicking the events that take place in leucocyte-endothelium binding. Thus, ezrin and EBP50 occur apically in UECs at the time of implantation in the rat and in response to a substitute blastocyst (filament) suggesting a role for these proteins in the cytoskeletal rearrangements that facilitate uterine receptivity and blastocyst-epithelial adhesion. Their loss within the implantation chamber possibly allows the subsequent invasion of the embryo.

Transport characteristics of zolmitriptan in a human intestinal epithelial cell line Caco-2

The intestinal absorption characteristics and the efflux mechanisms of zolmitriptan, a new generation and highly selective 5-HT1B/1D receptor agonist used in the acute oral treatment of migraine, were investigated. A human intestinal cell line, Caco-2, was used as an in-vitro model of the intestinal mucosa to assess transepithelial transport of zolmitriptan. In the Caco-2 cells, the absorptive transport of zolmitriptan was pH dependent and the transport was enhanced at weakly alkali pH on the apical side. No concentration dependence and saturation were observed for the apical-to-basolateral and basolateral-to-apical transport of zolmitriptan at a concentration of 0.1–10 mM. The permeability ratio value was about 1.5-2.6 at a concentration of 0.1–2.0 mM. Inhibition experiments using verapamil, nifedipine and nimodipine as inhibitors were studied and indicated that P-glycoprotein participated in the transport of zolmitriptan. Inhibition of the Na+-H+ exchanger with amiloride resulted in a significant increase in absorption and a slight inhibition in secretion. This suggests that the Na+-H+ exchanger may be involved in the transport of zolmitriptan. The results indicated that the transport of zolmitriptan was mediated by both passive diffusion and active transport. A series of drug-drug interaction experiments were carried out between zolmitriptan and some drugs that may be co-administrated with zolmitriptan in the clinic. The results indicated that flunarizine, cetirizine, propranolol and atenolol potently decreased both the apical-to-basolateral and basolateral-to-apical transport rate of zolmitriptan. Cimetidine and aspirin slightly inhibited the apical-to-basolateral transport of zolmitriptan, but significantly decreased the basolateral-to-apical transport of zolmitriptan. Thus, the absorption drug-drug interactions should be considered when these drugs are co-administrated with zolmitriptan in the clinic.

DA1Receptor Mediated Regulation of Na+-H+Antiport Activity in Rat Renal Cortical Brush Border Membrane Vesicles

Our previous studies indicate that dopamine (DA) plays an important role in regulating renal sodium (Na+) metabolism during high Na+ intake, and that DA1 receptors are involved in natriuretic response to acute volume expansion. It has also been shown that in addition to the changes in renal hemodynamics, the natriuretic response produced by exogenously administered DA and DA1 receptor agonists appears to be due to alterations in renal tubular sodium transport mechanisms. This study was designed to investigate the DA1 receptor-mediated changes in Na(+)-H+ antiport activity in tubular brush border membranes of rat kidney. The Na(+)-H+ antiport activity, measured as the amiloride-sensitive Na+ influx in BBMV, was inhibited by 37%, 46%, 33%, and 42% by 1 microM DA, SKF 82958, SKF 38393, and fenoldopam respectively. The DA1 antagonist SCH 23390 increased the antiport activity when given alone, while when administered with an agonist it attenuated the effects of the agonist on the antiporter. DA2 agonists and antagonists failed to affect the antiport activity. These results indicate that the inhibitory effects of DA and DA receptor agonists on Na(+)-H+ antiport activity in renal cortical BBMV were mediated by the DA1 receptors.

Refined genomic localization of the genetic lesion in the osteopetrosis (op) rat and exclusion of three positional and functional candidate genes, Clcn7, Atp6v0c, and Slc9a3r2

Osteopetrosis is a disease characterised by a generalized skeletal sclerosis resulting from a reduced osteoclast-mediated bone resorption. Several spontaneous mutations lead to osteopetrotic phenotypes in animals. Moutier et al. (1974) discovered the osteopetrosis (op) rat as a spontaneous, lethal, autosomal recessive mutant. op rats have large nonfunctioning osteoclasts and severe osteopetrosis. Dobbins et al. (2002) localized the disease-causing gene to a 1.5-cM genetic interval on rat chromosome 10, which we confirm in the present report. We also refined the genomic localization of the disease gene and provide statistical evidence for a disease-causing gene in a small region of rat chromosome 10. Three strong functional candidate genes are within the delineated region. Clcn7 was previously shown to underlie different forms of osteopetrosis, in both human and mice. ATP6v0c encodes a subunit of the vacuolar H(+)-ATPase or proton pump. Mutations in TCIRG1, another subunit of the proton pump, are known to cause a severe form of osteopetrosis. Given the critical role of proton pumping in bone resorption, the Slc9a3r2 gene, a sodium/hydrogen exchanger, was also considered as a candidate for the op mutation. RT-PCR showed that all 3 genes are expressed in osteoclasts, but sequencing found no mutations either in the coding regions or in intron splice junctions. Our ongoing mutation analysis of other genes in the candidate region will lead to the discovery of a novel osteopetrosis gene and further insights into osteoclast functioning.

Mechanism of inducible nitric oxide synthase exclusion from mycobacterial phagosomes

Mycobacterium tuberculosis is sensitive to nitric oxide generated by inducible nitric oxide synthase (iNOS). Consequently, to ensure its survival in macrophages, M. tuberculosis inhibits iNOS recruitment to its phagosome by an unknown mechanism. Here we report the mechanism underlying this process, whereby mycobacteria affect the scaffolding protein EBP50, which normally binds to iNOS and links it to the actin cytoskeleton. Phagosomes harboring live mycobacteria showed reduced capacity to retain EBP50, consistent with lower iNOS recruitment. EBP50 was found on purified phagosomes, and its expression increased upon macrophage activation, paralleling expression changes seen with iNOS. Overexpression of EBP50 increased while EBP50 knockdown decreased iNOS recruitment to phagosomes. Knockdown of EBP50 enhanced mycobacterial survival in activated macrophages. We tested another actin organizer, coronin-1, implicated in mycobacterium-macrophage interaction for contribution to iNOS exclusion. A knockdown of coronin-1 resulted in increased iNOS recruitment to model latex bead phagosomes but did not increase iNOS recruitment to phagosomes with live mycobacteria and did not affect mycobacterial survival. Our findings are consistent with a model for the block in iNOS association with mycobacterial phagosomes as a mechanism dependent primarly on reduced EBP50 recruitment.

Mycobacterium tuberculosis infects one third of the world's population, with the majority of infected individuals being asymptomatic while running a lifetime risk of developing active disease. The key to the success of M. tuberculosis as a recalcitrant human pathogen is its ability to parasitize macrophages and persist in these cells or their derivatives for long periods of time. We still do not have complete knowledge of the full repertoire of M. tuberculosis determinants that allow it to evade bactericidal mechanisms of the macrophage. Here we report the mechanism by which M. tuberculosis eludes effective elimination by nitric oxide, a radical with antimycobacterial properties that is generated by the inducible form of nitric oxide synthase. It was generally assumed that nitric oxide synthase, upon induction by the major anti-tuberculosis cytokine interferon gamma, simply homogeneously fills up the macrophage like a sack and generates nitric oxide throughout the cell. The present study shows that nitric oxide synthase is not randomly distributed in macrophages, and that its positioning in the cell is dictated by interactions with the scaffolding protein EBP50, shown here to be induced during macrophage activation. Thus, not only do the phagocytic cells increase the amount of nitric oxide synthase, but they also have a system to deliver and keep this enzyme in the vicinity of phagosomes. This is of significance, as nitric oxide is a highly reactive radical, and its generation somewhere else in the cell would lead to it being spent by the time it diffuses to the site of intended action, such as mycobacterium-laden phagosomes. It turns out, as this study shows, that M. tuberculosis interferes with the process of EBP50-guided positioning of the inducible nitric oxide synthase, thus avoiding delivery and accumulation of this enzyme and its noxious products near the phagosome where nitric oxide would have the best chance of inhibiting intracellular mycobacteria.

Ionic mechanisms of intracellular pH regulation in the nervous system

Two separate mechanisms responsible for intracellular pH (pHi) regulation in neuronal membranes of the nervous system have been studied so far: they are Na+/H+ and Na+-H+-HCO3-/Cl- exchange. The involvement of these mechanisms in pHi regulation of neurons and glial cells has been investigated in the leech central nervous system using ion-selective microelectrodes. The amiloride-sensitive Na+/H+ exchange is the predominant mechanism of pHi regulation in nominally HCO3- free, Hepes-buffered saline of both neurons and glial cells of this nervous system. In the presence of CO2-HCO3- buffer, however, the SITS-sensitive Na+-H+-HCO3-/Cl- exchanger contributes to acid extrusion in neurons and probably also in glial cells. Unlike neuronal pHi, glial pHi increases when Hepes is replaced by CO2-HCO3- as the extracellular buffer, and decreases again on return to Hepes buffer. The glial alkalinization occurs in the opposite direction, as would be expected from the CO2 movement across the cell membrane and its hydration to form carbonic acid which dissociates into H+ and HCO3- ions. The expected acidification, however, is observed in neurons, and is reduced by acetazolamide and ethoxzolamide, inhibitors of carbonic anhydrase, which catalyses the formation of carbonic acid. On the other hand, these drugs are shown to produce no change of the CO2-HCO3- -induced alkalinization in glial cells. The observations suggest that Na+-HCO3- co-transport across the glial cell membrane, mediating the influx of HCO3- ions into the cell interior, could be responsible for the unusual alkalinization. Further evidence for the activation of Na+-HCO3- co-transport, as a third mechanism involved in pHi homeostasis of the nervous system, is presented.

Movement of acid equivalents across the mammalian smooth muscle cell membrane

Factors affecting intracellular pH (pHi) in the smooth muscle of guinea pig ureter have been investigated using pH-sensitive microelectrodes. Associated acids and bases appear to have free passage across the cell membrane but results suggest very low permeability to charged acid equivalents, thus implicating carrier-mediated movements in many of the observed pHi transients. Recovery from acidosis in the nominal absence of CO2 was inhibited by removal of Na+ and by the presence of amiloride, indicating that Na+/H+ exchange was responsible. The presence of CO2 resulted in a faster recovery from acidosis but, since intracellular buffering power was not increased, not a substantially faster effective extrusion of protons. Surprisingly, amiloride no longer caused discernable inhibition. Recovery from moderate acidosis remained Na+ dependent but was not inhibited by DIDS or acetazolamide or by the absence of Cl-, suggesting a dominant Na+-, and HCO3(-)-dependent mechanism unlike any hitherto described. Recovery from alkalosis was inhibited by DIDS and Cl(-)-free conditions, indicating that Cl-/HCO3- exchange was involved. Results suggest reversal of this mechanism on extreme acidosis. Experiments in vascular smooth muscle with fluorescent indicators confirm the presence of Na+/H+ exchange but provide conflicting evidence about the presence and properties of the HCO3(-)-dependent mechanism.

Regulation of intracellular pH in cardiac muscle

Intracellular pH (pHi) in sheep cardiac Purkinje fibres is controlled by sarcolemmal Na+/H+ and Cl-/HCO3- exchange. At normal pHo (7.4), Na+/H+ exchange mediates an acid efflux whenever pHi falls and Cl-/HCO3- exchange mediates an equivalent acid influx in response to a rise in pHi. Intracellular pH is also influenced by Ca2+i, which can activate force development leading to the anaerobic production of lactic acid. This is evident after an increase in stimulation rate which reversibly reduces both pHi and extracellular surface pH (pHs). Rate-dependent pHi changes are inhibited following inhibition of glycolysis, indicating that they are caused by accumulation of lactic acid. In some cases, the efflux of lactic acid may provide a faster method for recovery of pHi from a metabolic acidosis than that provided by Na+/H+ exchange. Finally, direct pHi measurement in isolated mammalian ventricular myocytes suggests that the intrinsic intracellular buffering power (beta) of ventricular tissue may be considerably lower than previously believed. An accurate knowledge of beta is essential for calculating net membrane fluxes of acid equivalents from changes in pHi.

Differential role of cation and anion exchange in lymphocyte pH regulation

In lymphocytes, the Na+/H+ antiport is well suited to function in cytoplasmic pH (pHi) regulation. It is activated by departures from the physiological pHi and is thermodynamically poised to compensate for the tendency of the cells to become acidic. The driving force for H+ (equivalent) efflux is indirectly provided by the Na+ pump. Lymphocytes also possess a cation-independent anion (Cl-/HCO3-) exchange system, which, under the appropriate conditions, tends to restore pHi after an alkali load. Unlike the cation antiport, the source of energy driving the anion exchanger, i.e. the factors that determine the transmembrane Cl- distribution, is not well understood. The contribution of conductive pathways appears to be minimal, resulting in a marked difference between the membrane potential and ECl-. Instead, ECl- is very similar to EH+. Moreover, changes in the distribution of Cl- lead to alterations in the transmembrane delta pH and vice versa, suggesting a relationship between these parameters. Evidence is presented which suggests that the transmembrane distribution of HCO3-, dictated by delta pH, is a major determinant of the intracellular Cl- concentration, a process mediated by the anion exchanger. Thus, if Cl- is driven by the gradient of HCO3-, the cation-independent anion exchanger cannot play an active role in determining pHi. Instead, Cl-/HCO3- exchange may simply stabilize pHi by increasing the dynamic buffering power of the cells. Cation-independent Cl-/HCO3- exchange could be involved in pHi regulation only if coupled to a separate mechanism of intracellular Cl- accumulation, such as Na+-K+-2Cl- co-transport or an inward Cl- pump, which have not been detected in lymphoid cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Epithelial pH and ion transport regulation by proton pumps and exchangers

This study reports on the interaction between transepithelial Na+ transport and H+ secretory and intracellular pH (pHi) regulating mechanisms in the model 'tight' epithelium of frog skin. We have used 22Na isotope fluxes and fixed end-point titration to measure undirectional Na+ fluxes, net Na absorption (J(net)Na) and proton secretion (J(net)H), and electrophysiological techniques (double-barrelled ion-sensitive microelectrodes and cell membrane current--voltage relations) to determine intracellular activities of Na+, Cl- and H+ and the conductance of apical membranes to Na+ (gNa) and of basolateral membranes to K+ (gK). In dilute mucosal solutions or in the absence of a permeant anion (Cl-) or counter-current (open-circuit conditions) to accompany Na+ uptake, the J(net)Na is electrically coupled to J(net)H via an electrogenic apical H+-ATPase (located in mitochondria-rich cells). Both fluxes proceed via mitochondria-rich cells and are inhibited by blockers of carbonic anhydrase and H+-ATPase and stimulated by aldosterone and acid load. In high NaCl-containing mucosal solutions or in short-circuit conditions, the J(net)Na becomes uncoupled from J(net)H and proceeds mainly via the principal cells in the epithelium, in which pHi is regulated by basolateral Na+/H+ and Cl-/HCO3- exchangers. Under these conditions, J(net)Na, gNa and gK vary directly and in parallel with pHi, when pHi is changed by permeable weak acids or bases. There is also co-variance between gNa and pHi accompanying spontaneous variations in J(net)Na and when Na+ transport is stimulated by aldosterone or inhibited with ouabain. We conclude that the level of intracellular H+, modulated by H+ pump and Na+/H+ and Cl-/HCO3- exchangers provides an intrinsic regulation of epithelial Na+ transport.

Aldosterone-controlled linkage between Na+/H+ exchange and K+ channels in fused renal epithelial cells

Aldosterone maintains acid-base balance and K+ homeostasis by controlling H+ and K+ secretion in renal epithelial cells. We have shown recently in the amphibian distal nephron that aldosterone activates a Na+/H+ exchange system in the luminal cell membrane, leading to transepithelial H+ secretion and cytoplasmic alkalinization. Since H+ secretory fluxes are paralleled by K+ secretion, it was postulated that the hormone-induced increase of intracellular pH activates the luminally located K+ channels. In 'giant' cells fused from individual cells of the distal nephron, we measured simultaneously cytoplasmic pH and cell membrane K+ conductance during acidification of the cell cytoplasm. The experiments demonstrate that cell membrane K+ conductance is half-maximal at an intracellular pH of 7.42, and that a positive cooperative interaction exists between K+ channel proteins and H+ ions (Hill coefficient = 6.5). Moreover, the cellular K+ conductance is most sensitive to cytoplasmic pH in the range modified by aldosterone. This supports the hypothesis that intracellular H+ activity, regulated by the Na+/H+ exchanger, serves as the signal to couple aldosterone-induced K+ secretory flux to H+ secretion in renal tubules.

Ciclosporin-induced hypertension is associated with increased sodium transporter of the loop of Henle (NKCC2)

BACKGROUND

Hypertension induced by cyclosporine is associated with renal sodium and water retention. Using immunoblotting of kidney homogenates, we investigated the regulation of sodium and water transport proteins in a rat model of cyclosporine-induced hypertension.

METHODS

Rats were treated with cyclosporine (25 mg/kg/day intraperitoneally) during 7 days. Control rats received vehicle.

RESULTS

Cyclosporine-treated rats had an increase in blood pressure with a decrease in renal sodium excretion compared with control rats. There were no differences either in sodium intake or in plasma creatinine levels between the two groups of rats. These data suggest that the decrease in sodium excretion in the cyclosporine-treated rats was due to an increase in renal sodium absorption. The densitometric analysis of the renal immunoblot showed an increase in the Na-K-2Cl cotransporter of the loop of Henle (NKCC2) in cyclosporine-treated rats (178% +/- 36) compared with control rats (100% +/- 18; P < 0.05*). This protein rise was associated with an increase in the NKCC2 mRNA pointing to a transcriptional regulation of this sodium transporter. There were no statistically significant changes in the sodium proton exchange (NHE-3) of the proximal tubule although in this renal segment, aquaporin-1 was increased in cyclosporine-treated rats compared with control rats (control 100% +/- 6 vs cyclosporine 119% +/- 6; P < 0.05*).

CONCLUSIONS

Our results pointed to the thick ascending limb of the loop of Henle as an important site of sodium retention in cyclosporine-induced hypertension. This data may have potential clinical implications for the treatment of hypertension induced by cyclosporine.

The developmental regulation of CD81 in the rat retina

PURPOSE

The tetraspanin CD81 is expressed in Müller glial cells and retinal pigment epithelium (RPE). CD81 and other members of the tetraspanin family link extracellular interactions of cells into intracellular cascades. This study examined the developmental expression of CD81 and protein-protein interactions linking CD81 to intracellular proteins.

METHODS

We used synthetic peptides of the C-terminal intracellular domains of CD81 to probe fusion proteins of PDZ domains blotted to nitrocellulose membranes, then confirmed the relationships between the PDZ proteins using immunoprecipitation methods. Colocalization of the associated proteins was analyzed across development, using double-label immunohistochemical methods in the retina of Sprague-Dawley rats.

RESULTS

The C-terminal intracellular sequences of CD81 bound to three putative PDZ domains that potentially represented domains on Sap97 and EBP50. In immunoprecipitation experiments using RPE cells, CD81 coprecipitated with both proteins, EBP50 and Sap97. Like CD81, EBP50 and Sap97 are expressed at low levels immediately after birth and upregulated during the first two postnatal weeks, reaching almost adult levels at postnatal day 20. In the RPE layer, synapse-associated protein 97 (Sap97) and CD81 were associated with the basolateral surface of the cells; ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) localizing with CD81 was found on microvilli at the inner surface of RPE cells.

CONCLUSIONS

These results support the hypothesis that CD81 is associated with the final stages of RPE cell maturation, establishing key molecular interactions linking the cell membrane proteins into macromolecular complexes containing PDZ protein scaffolds.

Asymmetric ERM activation at the Schwann cell process tip is required in axon-associated motility

Axon-associated Schwann cell (SC) motility and process dynamics are crucial in the development and regeneration of the peripheral nervous system (PNS). The bipolar morphology of SCs represents an unexplored conundrum in terms of directed motility. Using fluorescence time-lapse microscopy of transfected SCs within myelinating dorsal root ganglion (DRG) explants, we demonstrate cycling of SCs between bipolar and highly motile, unipolar morphologies as a result of asymmetric process retraction and extension. Unipolar SC motility appears nucleotaxic in nature, similar to the movement of neurons on radial glia during cortical development. We also show that asymmetric process retraction is associated with the activation of ERM (ezrin/radixin/moesin) proteins and subsequent recruitment of ezrin-binding phospho-protein 50 kDa (EBP50) at the retracting process tip. This activation occurs in response to localized synthesis of phosphatidylinositol (4,5)-bisphosphate (PIP2) at this site. Finally, we demonstrate that the activation of ERM proteins at the SC process tip is essential for motility and the maintenance of SC polarity, as ERM disruption yields a dysfunctional, multi-polar cell. These results demonstrate that specializations at the tips of SC processes regulate their dynamics, which in turn is associated with directed motility in these cells.

Kidney Vacuolar H+-ATPase: Physiology and Regulation

The vacuolar H(+)-ATPase is a multisubunit protein consisting of a peripheral catalytic domain (V(1)) that binds and hydrolyzes adenosine triphosphate (ATP) and provides energy to pump H(+) through the transmembrane domain (V(0)) against a large gradient. This proton-translocating vacuolar H(+)-ATPase is present in both intracellular compartments and the plasma membrane of eukaryotic cells. Mutations in genes encoding kidney intercalated cell-specific V(0) a4 and V(1) B1 subunits of the vacuolar H(+)-ATPase cause the syndrome of distal tubular renal acidosis. This review focuses on the function, regulation, and the role of vacuolar H(+)-ATPases in renal physiology. The localization of vacuolar H(+)-ATPases in the kidney, and their role in intracellular pH (pHi) regulation, transepithelial proton transport, and acid-base homeostasis are discussed.

Regulation of the rat brain Na+-driven Cl-/HCO3- exchanger involves protein kinase A and a multiprotein signaling complex

The Na(+)-driven Cl(-)/HCO(3)(-) exchanger (NCBE) plays an important role in the regulation of intracellular pH (pH(i)). We previously identified two variants of NCBE from rat brain of which the variant with a carboxyterminal PSD-95/Dlg/ZO-1 (PDZ) motif (rb2NCBE) colocalized with the actin cytoskeleton. Increased rb2NCBE activity by PKA inhibition and reduction by forskolin and cAMP agonist suggest PKA regulation of NCBE. Disruption of actin filaments also decreased rb2NCBE activity. EBP50 and FLAG-rb2NCBE were reciprocally co-immunoprecipitated from rb2NCBE transfected cells. It is concluded that NCBE activity is inhibited by PKA and depends on the integrity of the actin cytoskeleton within a multiprotein complex at the plasma membrane.

Beyond mere markers: functions for CD34 family of sialomucins in hematopoiesis

CD34, podocalyxin, and endoglycan are members of a family of single-pass transmembrane proteins that show distinct expression on early hematopoietic precursors and vascular-associated tissue. In spite of the fact that the expression of CD34 on these early progenitors has been known for over 20 yr and used clinically in hematopoietic stem cell transplantation for more than 15 yr, little is known about its exact role or function. More recently, CD34 expression has been shown to distinguish activated early progenitors from quiescent cells. With the subsequent identification of podocalyxin and endoglycan as related family members also expressed on early progenitor cells, attention is slowly shifting toward understanding how these molecules might contribute to progenitor function and behavior. In this review we examine the existing evidence and propose testable models to reveal the importance of these molecules for stem and progenitor cell function.

Structural basis for NHERF recognition by ERM proteins

The Na+/H+ exchanger regulatory factor (NHERF) is a key adaptor protein involved in the anchoring of ion channels and receptors to the actin cytoskeleton through binding to ERM (ezrin/radixin/moesin) proteins. NHERF binds the FERM domain of ERM proteins, although NHERF has no signature Motif-1 sequence for FERM binding found in adhesion molecules. The crystal structures of the radixin FERM domain complexed with the NHERF-1 and NHERF-2 C-terminal peptides revealed a peptide binding site of the FERM domain specific for the 13 residue motif MDWxxxxx(L/I)Fxx(L/F) (Motif-2), which is distinct from Motif-1. This Motif-2 forms an amphipathic alpha helix for hydrophobic docking to subdomain C of the FERM domain. This docking causes induced-fit conformational changes in subdomain C and affects binding to adhesion molecule peptides, while the two binding sites are not overlapped. Our studies provide structural paradigms for versatile ERM linkages between membrane proteins and the cytoskeleton.

Regulation of TRPV5 and TRPV6 by associated proteins

The epithelial Ca2+ channels TRPV5 and TRPV6 are the most Ca2+-selective members of the TRP channel superfamily. These channels are the prime target for hormonal control of the active Ca2+ flux from the urine space or intestinal lumen to the blood compartment. Insight into their regulation is, therefore, pivotal in our understanding of the (patho)physiology of Ca2+ homeostasis. The recent elucidation of TRPV5/6-associated proteins has provided new insight into the molecular mechanisms underlying the regulation of these channels. In this review, we describe the various means of TRPV5/6 regulation, the role of channel-associated proteins herein, and the relationship between both processes.

The association of NHERF adaptor proteins with g protein-coupled receptors and receptor tyrosine kinases

The sodium-hydrogen exchanger regulatory factors (NHERF-1 and NHERF-2) are a family of adaptor proteins characterized by the presence of two tandem PDZ protein interaction domains and a C-terminal domain that binds the cytoskeleton proteins ezrin, radixin, moesin, and merlin. The NHERF proteins are highly expressed in the kidney, small intestine, and other organs, where they associate with a number of transporters and ion channels, signaling proteins, and transcription factors. Recent evidence has revealed important associations between the NHERF proteins and several G protein-coupled receptors such as the beta2-adrenergic receptor, the kappa-opioid receptor, and the parathyroid hormone receptor, as well as growth factor tyrosine kinase receptors such as the platelet-derived growth factor receptor and the epidermal growth factor receptor. This review summarizes the emerging data on the biochemical mechanisms, physiologic outcomes, and potential clinical implications of the assembly and disassembly of receptor/NHERF complexes.

Lysophosphatidic acid inhibits cholera toxin-induced secretory diarrhea through CFTR-dependent protein interactions

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells that line the airway, gut, and exocrine glands; it is well established that CFTR plays a pivotal role in cholera toxin (CTX)-induced secretory diarrhea. Lysophosphatidic acid (LPA), a naturally occurring phospholipid present in blood and foods, has been reported to play a vital role in a variety of conditions involving gastrointestinal wound repair, apoptosis, inflammatory bowel disease, and diarrhea. Here we show, for the first time, that type 2 LPA receptors (LPA₂) are expressed at the apical surface of intestinal epithelial cells, where they form a macromolecular complex with Na⁺/H⁺ exchanger regulatory factor–2 and CFTR through a PSD95/Dlg/ZO-1–based interaction. LPA inhibited CFTR-dependent iodide efflux through LPA₂-mediated G_i pathway, and LPA inhibited CFTR-mediated short-circuit currents in a compartmentalized fashion. CFTR-dependent intestinal fluid secretion induced by CTX in mice was reduced substantially by LPA administration; disruption of this complex using a cell-permeant LPA₂-specific peptide reversed LPA₂-mediated inhibition. Thus, LPA-rich foods may represent an alternative method of treating certain forms of diarrhea.

Regulation of albumin endocytosis by PSD95/Dlg/ZO-1 (PDZ) scaffolds. Interaction of Na+-H+ exchange regulatory factor-2 with ClC-5

The constitutive reuptake of albumin from the glomerular filtrate by receptor-mediated endocytosis is a key function of the renal proximal tubules. Both the Cl- channel ClC-5 and the Na+-H+ exchanger isoform 3 are critical components of the macromolecular endocytic complex that is required for albumin uptake, and therefore the cell-surface levels of these proteins may limit albumin endocytosis. This study was undertaken to investigate the potential roles of the epithelial PDZ scaffolds, Na+-H+ exchange regulatory factors, NHERF1 and NHERF2, in albumin uptake by opossum kidney (OK) cells. We found that ClC-5 co-immunoprecipitates with NHERF2 but not NHERF1 from OK cell lysate. Experiments using fusion proteins demonstrated that this was a direct interaction between an internal binding site in the C terminus of ClC-5 and the PDZ2 module of NHERF2. In OK cells, NHERF2 is restricted to the intravillar region while NHERF1 is located in the microvilli. Silencing NHERF2 reduced both cell-surface levels of ClC-5 and albumin uptake. Conversely, silencing NHERF1 increased cell-surface levels of ClC-5 and albumin uptake, presumably by increasing the mobility of NHE3 in the membrane and its availability to the albumin uptake complex. Surface biotinylation experiments revealed that both NHERF1 and NHERF2 were associated with the plasma membrane and that NHERF2 was recruited to the membrane in the presence of albumin. The importance of the interaction between NHERF2 and the cytoskeleton was demonstrated by a significant reduction in albumin uptake in cells overexpressing an ezrin binding-deficient mutant of NHERF2. Thus NHERF1 and NHERF2 differentially regulate albumin uptake by mechanisms that ultimately alter the cell-surface levels of ClC-5.

Astrocytic and neuronal localization of the scaffold protein Na+/H+ exchanger regulatory factor 2 (NHERF-2) in mouse brain

The Na+/H+ exchanger regulatory factor 2 (NHERF-2) is a scaffold protein that regulates cellular signaling by forming protein complexes. Several proteins known to interact with NHERF-2 are abundantly expressed in the central nervous system, but little is known about NHERF-2 localization in the brain. By using immunohistochemistry combined with light and electron microscopy, we found that many populations of astrocytes, as well as some populations of neurons, were immunopositive for NHERF-2 throughout the mouse brain. Quantitative analysis of the subcellular distribution of NHERF-2 immunostaining in four brain structures, cerebral cortex, hippocampus, striatum, and cerebellar cortex, showed that NHERF-2 was expressed mainly in astrocytic processes but was also sometimes observed in both pre- and postsynaptic neuronal elements. NHERF-2 immunostaining was associated mainly with the plasma membrane of neurons and astrocytes. However, NHERF-2 immunoreactivity was also observed in association with synaptic vesicles in putative glutamatergic axon terminals. The subcellular localization of NHERF-2 in brain is consistent with a role for NHERF-2 in forming complexes between cell surface and cytosolic proteins, and the preferential expression of NHERF-2 in astrocytes suggests that this scaffold protein may play an important role in astrocytic physiology.

Hypophosphatemia and calcium nephrolithiasis

Our knowledge of phosphate balance under physiological and pathological situations has increased substantially during the last decade thanks to the molecular identification of three dissimilar families of sodium-phosphate cotransport systems, two of them almost exclusively expressed in epithelia whereas the third one has a ubiquitous expression. Intracellular proteins such as NHERF1 (sodium-proton exchanger regulatory factor 1) can interact with phosphate transporters through PDZ domains thus regulating the expression of the transporters at the membrane. Moreover, newly acknowledged paracrine/endocrine peptides, such as fibroblast growth factor 23 (FGF23), also affect the activity of phosphate transporters. Renal phosphate leak, related to invalidation (in the mouse) or to mutations (in humans) of the renal phosphate transporter NPT2a, leads to hypophosphatemia on the one hand, and to nephrolithiasis or bone demineralization on the other hand. Similar features are observed during invalidation of NHERF or in case of overproduction of FGF23. These observations highlight the importance of phosphate homeostasis in common diseases such as renal stones or bone loss.

Interaction of the epithelial Ca2+ channels TRPV5 and TRPV6 with the intestine- and kidney-enriched PDZ protein NHERF4

The epithelial Ca(2+) channels TRPV5 and TRPV6 constitute the apical Ca(2+) influx pathway in epithelial Ca(2+) transport. PDZ proteins have been demonstrated to play a crucial role in the targeting or anchoring of ion channels and transporters in the apical domain of the cell. In this study, we describe the identification of NHERF4 (Na-P(i) Cap2/IKEPP/PDZK2) as a novel TRPV5- and TRPV6-associated PDZ protein. NHERF4 was identified using two separate yeast two-hybrid screens with the carboxyl termini of TRPV5 and TRPV6 as bait. Binding of the carboxyl termini of TRPV5 and TRPV6 with NHERF4 was confirmed by GST pull-down assays using in-vitro-translated NHERF4 or lysates of Xenopus laevis oocytes expressing NHERF4. Furthermore, the interaction was confirmed by GST pull-down and co-immunoprecipitation assays using in-vitro-translated full-length TRPV5 and Xenopus oocytes or HEK293 cells co-expressing NHERF4 and TRPV5/TRPV6, respectively. The fourth PDZ domain of NHERF4 was sufficient for the interaction, although PDZ domain 1 also contributed to the binding. The binding site for NHERF4 localized in a conserved region in the carboxyl terminus of TRPV5 and was distinct from the binding site of the PDZ protein NHERF2. NHERF4 predominantly localized at the plasma membrane of X. laevis oocytes and HeLa cells. This localization was independent of the presence of TRPV5. Therefore, we hypothesize a role for this novel PDZ protein as a putative plasma membrane scaffold for the epithelial Ca(2+) channels.

A microdeletion in Xp11.3 accounts for co-segregation of retinitis pigmentosa and mental retardation in a large kindred

In a previous report, Aldred et al. [1994] described a 5-generation family in which severe retinitis pigmentosa (RP) co-segregates with mild-moderate mental retardation as an X-linked recessive phenotype mapping to the broad interval between Xp21-q21. We re-examined this family, initially analyzing RP2, a gene in the disease interval that was identified as a cause of RP after the initial report of this family. We found that the male propositus lacked the 5' three exons of RP2 and that RP2 marks the centromeric boundary of a 1.27 Mb deletion that includes two other annotated genes (SLC9A7, CHST7), one predicted transcript encoding a zinc finger protein (FLJ20344) and two highly conserved miRNAs (mir221, mir222). We conclude that this family is segregating a contiguous gene deletion and that the absence of a functional RP2 accounts, at least in part, for the retinal degeneration while deletion of one or more of the other genes is likely responsible for the mental retardation phenotype.