Oxidant and antioxidant modulation of chloride channels expressed in human retinal pigment epithelium

Identification of novel key molecular signatures in the pathogenesis of experimental diabetic retinopathy

Deep mining of the molecular mechanisms underlying diabetic retinopathy (DR) is critical for the development of novel therapeutic targets. This study aimed to identify key molecular signatures involved in experimental DR on the basis of integrated bioinformatics analysis. Four datasets consisting of 37 retinal samples were downloaded from the National Center of Biotechnology Information Gene Expression Omnibus. After batch-effect adjustment, bioinformatics tools such as Networkanalyst, Enrichr, STRING, and Metascape were used to evaluate the differentially expressed genes (DEGs), perform enrichment analysis, and construct protein-protein interaction networks. The hub genes were identified using Cytoscape software. The DEGs of interest from the meta-analysis were confirmed by quantitative reverse transcription-polymerase chain reaction in diabetic rats and a high-glucose-treated retinal cell model, respectively. A total of 743 DEGs related to lens differentiation, insulin resistance, and high-density lipoprotein (HDL) cholesterol metabolism were obtained using the meta-analysis. Alterations of dynamic gene expression in the chloride ion channel, retinol metabolism, and fatty acid metabolism were involved in the course of DR in rats. Importantly, H3K27m3 modifications regulated the expression of most DEGs at the early stage of DR. Using an integrated bioinformatics approach, novel molecular signatures were obtained for different stages of DR progression, and the findings may represent distinct therapeutic strategies for DR patients.

Novel roles for voltage-gated T-type Ca2+ and ClC-2 channels in phagocytosis and angiogenic factor balance identified in human iPSC-derived RPE

Human-induced pluripotent stem cell (hiPSC)-derived retinal pigment epithelium (RPE) is a powerful tool for pathophysiological studies and preclinical therapeutic screening, as well as a source for clinical cell transplantation. Thus, it must be validated for maturity and functionality to ensure correct data readouts and clinical safety. Previous studies have validated hiPSC-derived RPE as morphologically characteristic of the tissue in the human eye. However, information concerning the expression and functionality of ion channels is still limited. We screened hiPSC-derived RPE for the polarized expression of a panel of L-type (Ca_V 1.1, Ca_V 1.3) and T-type (Ca_V 3.1, Ca_V 3.3) Ca²⁺ channels, K⁺ channels (Maxi-K, Kir4.1, Kir7.1), and the Cl^- channel ClC-2 known to be expressed in native RPE. We also tested the roles of these channels in key RPE functions using specific inhibitors. In addition to confirming the native expression profiles and function of certain channels, such as L-type Ca²⁺ channels, we show for the first time that T-type Ca²⁺ channels play a role in both phagocytosis and vascular endothelial growth factor (VEGF) secretion. Moreover, we demonstrate that Maxi-K and Kir7.1 channels are involved in the polarized secretion of VEGF and pigment epithelium-derived factor (PEDF). Furthermore, we show a novel localization for ClC-2 channel on the apical side of hiPSC-derived RPE, with an overexpression at the level of fluid-filled domes, and demonstrate that it plays an important role in phagocytosis, as well as VEGF and PEDF secretion. Taken together, hiPSC-derived RPE is a powerful model for advancing fundamental knowledge of RPE functions.

SLC26A7 constitutes the thiocyanate-selective anion conductance of the basolateral membrane of the retinal pigment epithelium

Anion channels in the retinal pigment epithelium (RPE) play an essential role in the transport of Cl^- between the outer retina and the choroidal blood to regulate the ionic composition and volume of the subretinal fluid that surrounds the photoreceptor outer segments. Recently, we reported that the anion conductance of the mouse RPE basolateral membrane is highly selective for the biologically active anion thiocyanate (SCN^-), a property that does not correspond with any of the Cl^- channels that have been found to be expressed in the RPE to date. The purpose of this study was to determine the extent to which SLC26A7, a SCN^- permeable-anion exchanger/channel that was reported to be expressed in human RPE, contributes to the RPE basolateral anion conductance. We show by quantitative RT-PCR that Slc26a7 is highly expressed in mouse RPE compared with other members of the Slc26 gene family and Cl^- channel genes known to be expressed in the RPE. By applying immunofluorescence microscopy to mouse retinal sections and isolated cells, we localized SLC26A7 to the RPE basolateral membrane. Finally, we performed whole cell and excised patch recordings from RPE cells acutely isolated from Slc26a7 knockout mice to show that the SCN^- conductance and permeability of its basolateral membrane are dramatically smaller relative to wild-type mouse RPE cells. These findings establish SLC26A7 as the SCN^--selective conductance of the RPE basolateral membrane and provide new insight into the physiology of an anion channel that may participate in anion transport and pH regulation by the RPE.

Mouse retinal pigment epithelial cells exhibit a thiocyanate-selective conductance

The basolateral membrane anion conductance of the retinal pigment epithelium (RPE) is a key component of the transepithelial Cl^- transport pathway. Although multiple Cl^- channels have been found to be expressed in the RPE, the components of the resting Cl^- conductance have not been identified. In this study, we used the patch-clamp method to characterize the ion selectivity of the anion conductance in isolated mouse RPE cells and in excised patches of RPE basolateral and apical membranes. Relative permeabilities ( P_A/ P_Cl) calculated from reversal potentials measured in intact cells under bi-ionic conditions were as follows: SCN^- >> ClO₄^- > [Formula: see text] > I^- > Br^- > Cl^- >> gluconate. Relative conductances ( G_A/ G_Cl) followed a similar trend of SCN^- >> ClO₄^- > [Formula: see text] > I^- > Br^- ≈Cl^- >> gluconate. Whole cell currents were highly time-dependent in 10 mM external SCN^-, reflecting collapse of the electrochemical potential gradient due to SCN^- accumulation or depletion intracellularly. When the membrane potential was held at -120 mV to minimize SCN^- accumulation in cells exposed to 10 mM SCN^-, the instantaneous current reversed at -90 mV, revealing that P_SCN/ P_Cl is approximately 500. Macroscopic current recordings from outside-out patches demonstrated that both the basolateral and apical membranes exhibit SCN^- conductances, with the basolateral membrane having a larger SCN^- current density and higher relative permeability for SCN^-. Our results suggest that the RPE basolateral and apical membranes contain previously unappreciated anion channels or electrogenic transporters that may mediate the transmembrane fluxes of SCN^- and Cl^-.

The slow light and dark oscillation of the clinical electro-oculogram

BACKGROUND

The standing potential of the eye exhibits a slow damped oscillation under light and dark conditions that continues for at least 80 minutes. However, our understanding of the relationship between the slow dark and light oscillation has not been previously studied. The aim of this study was to explore through regression analysis a model of these oscillations in order to establish if they may have the same underlying cellular generators.

METHODS

Healthy participants undertook recordings of the standing potential using the electro-oculogram for 100 minutes. To explore the light oscillation, participants (n = 8) were dilated and performed an extended electro-oculogram protocol consisting of 15 minutes dark adaptation and 85 minutes of white light adaptation at 100 cd/m² . For the dark oscillation, participants (n = 11) undertook the electro-oculogram for 100 minutes in complete darkness. Both sessions began with pre-adaptation to 30 cd/m² of white light for five minutes. Non-parametric statistics were used to evaluate all data.

RESULTS

Ratios of the dark and light oscillations showed a significantly greater dampening of the dark oscillation compared to the light oscillation (p < 0.000). Regression analysis using a five-factor damped sine function revealed significant differences in the parameters governing the dampening (p = 0.005) and period (p = 0.009) of the functions (R² > 0.874). There were no significant differences in the dark trough amplitude.

CONCLUSION

The results support a different underlying physiological mechanism for the light and dark oscillation of the clinical electro-oculogram. Future work will need to establish how the dark oscillation and dark trough of the clinical electro-oculogram arise.

Retinal pigment epithelium in the pathogenesis of age-related macular degeneration and photobiomodulation as a potential therapy?

The retinal pigment epithelium (RPE) comprises a monolayer of cells located between the neuroretina and the choriocapillaries. The RPE serves several important functions in the eye: formation of the blood-retinal barrier, protection of the retina from oxidative stress, nutrient delivery and waste disposal, ionic homeostasis, phagocytosis of photoreceptor outer segments, synthesis and release of growth factors, reisomerization of all-trans-retinal during the visual cycle, and establishment of ocular immune privilege. Age-related macular degeneration (AMD) is the leading cause of blindness in developed countries. Dysfunction of the RPE has been associated with the pathogenesis of AMD in relation to increased oxidative stress, mitochondrial destabilization and complement dysregulation. Photobiomodulation or near infrared light therapy which refers to non-invasive irradiation of tissue with light in the far-red to near-infrared light spectrum (630-1000 nm), is an intervention that specifically targets key mechanisms of RPE dysfunction that are implicated in AMD pathogenesis. The current evidence for the efficacy of photobiomodulation in AMD is poor but its safety profile and proposed mechanisms of action motivate further research as a novel therapy for AMD.

Expression of anoctamins in retinal pigment epithelium (RPE)

The anoctamin (ANO, TMEM16) family of Ca²⁺-activated Cl^- channels consists of ten members with different cellular functions (ANO1-10). ANO1 is a Ca²⁺-activated Cl^- channel in secretory epithelial cells of exocrine pancreas, salivary glands, or enterocytes. Expression of ANO1 also promotes cell proliferation and migration of tumor cells. ANO6 is essential for Ca²⁺-dependent scrambling of membrane phospholipids in platelets, red blood cells, and lymphocytes. ANO10 modulates Ca²⁺ signals in macrophages and has a role in cerebellar ataxia and other neurological disorders. All three anoctamins have been proposed to control intracellular Ca²⁺ signals. Anoctamins may also form the basolateral Ca²⁺-activated Cl^- channel in the retinal pigment epithelium (RPE). We show that native human, bovine, porcine, and mouse RPEs express ANO1, ANO6, and ANO10. Growth arrested and confluent RPR cells expressed ANO1 in the plasma membrane, whereas ANO6 and ANO10 were found in the primary cilium. Ussing chamber experiments showed that the application of ATP to the apical (retinal) side of porcine RPE induced a Ca²⁺-activated Cl^- secretion. Activation was inhibited by basolateral (choroidal) administration of the ANO inhibitors AO1, niflumic acid (NFA), and 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS). The results suggest that ANO1 is responsible for basolateral Ca²⁺-dependent Cl^- secretion in RPE, whereas ANO6 and ANO10 may have different functions, such as modulating Ca²⁺ signals.

Pharmacology of the retinal pigment epithelium, the interface between retina and body system

The retinal pigment epithelium (RPE) is a close, interactive partner to the photoreceptors as well as an interface with the endothelium of the choroid and thus with the body's circulatory system. To fulfill these roles, the RPE communicates with neighboring tissue by secretion of a large variety of factors and is able to react to secreted factors via a plethora of transmembrane receptors. Clinically relevant local pharmacological effects are caused by anti-VEGF-A treatment in choroidal neovascularization or by carboanhydrase inhibitors reducing fluid accumulation in the macula. Being exposed to the bloodstream, the RPE reacts to systemic disease, such as diabetes or hypertension, but also to systemic pharmacological intervention, for example to hypotensive drugs acting on the renin-angiotensin-system. Sustained pharmacological treatments, in particular, cause side effects at the RPE with consequences for both RPE function and photoreceptor survival. Among these are systemic inhibition of angiotensin-converting enzyme, insulin treatment in diabetes and anti-VEGF-A therapy. Given the special anatomical and functional relationships of the RPE, pharmacological intervention targeting either the eye or the body systemically should take potential alteration of RPE and subsequently photoreceptor function into account.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

Stem cells as source for retinal pigment epithelium transplantation

Inherited maculopathies, age related macular degeneration and some forms of retinitis pigmentosa are associated with impaired function or loss of the retinal pigment epithelium (RPE). Among potential treatments, transplantation approaches are particularly promising. The arrangement of RPE cells in a well-defined tissue layer makes the RPE amenable to cell or tissue sheet transplantation. Different cell sources have been suggested for RPE transplantation but the development of a clinical protocol faces several obstacles. The source should provide a sufficient number of cells to at least recover the macula area. Secondly, cells should be plastic enough to be able to integrate in the host tissue. Tissue sheets should be considered as well, but the substrate on which RPE cells are cultured needs to be carefully evaluated. Immunogenicity can also be an obstacle for effective transplantation as well as tumorigenicity of not fully differentiated cells. Finally, ethical concerns may represent drawbacks when embryo-derived cells are proposed for RPE transplantation. Here we discuss different cell sources that became available in recent years and their different properties. We also present data on a new source of human RPE. We provide a protocol for RPE differentiation of retinal stem cells derived from adult ciliary bodies of post-mortem donors. We show molecular characterization of the in vitro differentiated RPE tissue and demonstrate its functionality based on a phagocytosis assay. This new source may provide tissue for allogenic transplantation based on best matches through histocompatibility testing.

Branch retinal vein occlusion in an asymptomatic adult with cystic fibrosis

PURPOSE

This article aims to describe a case of asymptomatic branch retinal vein occlusion (BRVO) in a patient with cystic fibrosis (CF) and discuss the possible link between the two.

CASE REPORT

A young adult (aged 35 years) with CF who presented for routine ocular examination was found to have a superior temporal BRVO in the left eye. Visual acuity was unaffected, measuring -0.06 LogMAR, and intraocular pressure was 10 mm Hg. Optical coherence tomography showed no macular involvement. Regarding the patient's general health, blood pressure was within the normal range, and there was no diabetes. Exploratory blood tests revealed elevated fibrinogen levels.

CONCLUSIONS

It is hypothesized that BRVO occurred secondary to raised fibrinogen levels, a common feature in CF resulting from chronic pulmonary infection and inflammation. Practitioners should be aware of the possible link between BRVO and CF.

Cystic fibrosis transmembrane conductance regulator contributes to reacidification of alkalinized lysosomes in RPE cells

The role of the cystic fibrosis transmembrane conductance regulator (CFTR) in lysosomal acidification has been difficult to determine. We demonstrate here that CFTR contributes more to the reacidification of lysosomes from an elevated pH than to baseline pH maintenance. Lysosomal alkalinization is increasingly recognized as a factor in diseases of accumulation, and we previously showed that cAMP reacidified alkalinized lysosomes in retinal pigmented epithelial (RPE) cells. As the influx of anions to electrically balance proton accumulation may enhance lysosomal acidification, the contribution of the cAMP-activated anion channel CFTR to lysosomal reacidification was probed. The antagonist CFTR(inh)-172 had little effect on baseline levels of lysosomal pH in cultured human RPE cells but substantially reduced the reacidification of compromised lysosomes by cAMP. Likewise, CFTR activators had a bigger impact on cells whose lysosomes had been alkalinized. Knockdown of CFTR with small interfering RNA had a larger effect on alkalinized lysosomes than on baseline levels. Inhibition of CFTR in isolated lysosomes altered pH. While CFTR and Lamp1 were colocalized, treatment with cAMP did not increase targeting of CFTR to the lysosome. The inhibition of CFTR slowed lysosomal degradation of photoreceptor outer segments while activation of CFTR enhanced their clearance from compromised lysosomes. Activation of CFTR acidified RPE lysosomes from the ABCA4(-/-) mouse model of recessive Stargardt's disease, whose lysosomes are considerably alkalinized. In summary, CFTR contributes more to reducing lysosomal pH from alkalinized levels than to maintaining baseline pH. Treatment to activate CFTR may thus be of benefit in disorders of accumulation associated with lysosomal alkalinization.

ZIP2 and ZIP4 mediate age-related zinc fluxes across the retinal pigment epithelium

Decreases in systemic and cellular levels of zinc (Zn(2+)) during normal aging correlate with several age-related pathologies including age-related macular degeneration. Zn(2+) homeostasis in tissues is not only dependent on dietary intake but also on optimal expression and function of its influx (ZIP) and efflux (ZnT) transporters. We recently showed that many of the Zn(2+) transporters are expressed by the retinal pigment epithelial (RPE) cells. In this study, we present evidence that RPE cells contain less endogenous Zn(2+) with increased aging and transport this ion vectorially with greater transport from the basal to apical direction. Expression of two Zn(2+) influx transporters, ZIP2 and ZIP4, is reduced as a function of RPE age. Gene silencing of ZIP2 and ZIP4 in RPE cells from young donors or their overexpression in cells from older donors confirms that these two transporters are essential in controlling Zn(2+) influx and sequestration in RPE cells. Both transporters are distributed on the basal surface of the RPE where they are likely to control Zn(2+) homeostasis in the outer retina.

Bestrophins and retinopathies

Best vitelliform macular dystrophy (BVMD, also called Best's disease) is a dominantly inherited, juvenile-onset form of macular degeneration, which is characterized by abnormal accumulation of yellow pigment in the outer retina and a depressed electro-oculogram light peak (LP). Over 100 disease-causing mutations in human bestrophin-1 (hBest1) are closely linked to BVMD and several other retinopathies. However, the physiological role of hBest1 and the mechanisms of retinal pathology remain obscure partly because hBest1 has been described as a protein with multiple functions including a Ca2+-activated Cl- channel, a Ca2+ channel regulator, a volume-regulated Cl- channel, and a HCO3- channel. This review focuses on how dysfunction of hBest1 is related to the accumulation of yellow pigment and a decreased LP. The dysfunction of hBest1 as a HCO3- channel or a volume-regulated Cl- channel may be associated with defective regulation of the subretinal fluid or phagocytosis of photoreceptor outer segments by retinal pigment epithelium cells, which may lead to fluid and pigment accumulation.

Inhibition of aldose reductase attenuates endotoxin signals in human non-pigmented ciliary epithelial cells

Chronic inflammatory diseases such as autoimmune and bacterial infections are associated with an elevated risk of ocular inflammation. Ciliary epithelial cells that play an important role in maintaining aqueous humor dynamics and homeostasis of anterior segment of eye are continuously exposed to inflammatory markers during infections and injury. Lipopolysacchharide (LPS), a Gram-negative bacterial endotoxin, dysregulates aqueous humor (AqH) homeostasis by inducing inflammatory changes. We have investigated how inhibition of a polyol pathway enzyme, aldose reductase (AR), alters LPS-induced inflammatory changes in human non-pigmented ciliary epithelial cells (hNPECs). The stimulation of hNPECs with LPS (1 microg/ml) caused increased secretion of inflammatory markers such as PGE(2) and NO in the culture medium as well as increased expression of COX-2 and iNOS proteins in cell extracts. LPS also increased phosphorylation of MAPKs (ERK1/2) and SAPK/JNK and activation of redox-sensitive transcription factors NF-kappaB and AP-1 in hNPECs and inhibition of AR by zopolrestat and sorbinil ameliorated these changes. Further, LPS-induced decrease in the expression of Na/K-ATPase in hNPECs was restored by AR inhibitors. Similar results were observed in ciliary bodies of LPS-injected rats. Taken together, our results suggest that AR plays an important role in the LPS-induced inflammatory changes in hNPECs and that inhibition of AR could be a novel therapeutic approach for ocular inflammation.

The ClC-3 Cl-/H+ antiporter becomes uncoupled at low extracellular pH

Adenovirus expressing ClC-3 (Ad-ClC-3) induces Cl(-)/H(+) antiport current (I(ClC-3)) in HEK293 cells. The outward rectification and time dependence of I(ClC-3) closely resemble an endogenous HEK293 cell acid-activated Cl(-) current (ICl(acid)) seen at extracellular pH <or= 5.5. ICl(acid) was present in smooth muscle cells from wild-type but not ClC-3 null mice. We therefore sought to determine whether these currents were related. ICl(acid) was larger in cells expressing Ad-ClC-3. Protons shifted the reversal potential (E(rev)) of I(ClC-3) between pH 8.2 and 6.2, but not pH 6.2 and 5.2, suggesting that Cl(-) and H(+) transport become uncoupled at low pH. At pH 4.0 E(rev) was completely Cl(-) dependent (55.8 +/- 2.3 mV/decade). Several findings linked ClC-3 with native ICl(acid); 1) RNA interference directed at ClC-3 message reduced native ICl(acid); 2) removal of the extracellular "fast gate" (E224A) produced large currents that were pH-insensitive; and 3) wild-type I(ClC-3) and ICl(acid) were both inhibited by (2-sulfonatoethyl)methanethiosulfonate (MTSES; 10-500 microm)-induced alkanethiolation at exposed cysteine residues. However, a ClC-3 mutant lacking four extracellular cysteine residues (C103_P130del) was completely resistant to MTSES. C103_P130del currents were still acid-activated, but could be distinguished from wild-type I(ClC-3) and from native ICl(acid) by a much slower response to low pH. Thus, ClC-3 currents are activated by protons and ClC-3 protein may account for native ICl(acid). Low pH uncouples Cl(-)/H(+) transport so that at pH 4.0 ClC-3 behaves as an anion-selective channel. These findings have important implications for the biology of Cl(-)/H(+) antiporters and perhaps for pH regulation in highly acidic intracellular compartments.

Glutathione and its transporters in ocular surface defense

Glutathione (GSH) is an abundant antioxidant ubiquitous in nearly all cell types. Deficiency of GSH has been linked to ocular disease and viral infection. Other established vital roles of GSH include detoxification and immunoprotection. Endogenous GSH plays a protagonist's role in safeguarding active transport processes compartmentalized at the interface between conjunctival mucosa and the tear film. Optimal electrokinetic transport across the conjunctival epithelium requires the mucosal presence of GSH. Glutathione is the most abundant known endogenous antioxidant molecule in tear fluid, mainly derived from conjunctival secretion. Conjunctival GSH transport, a major kinetic component of GSH turnover, occurs through multiple functionally distinct mechanisms. Cell membrane potential regulates conjunctival GSH efflux, while conjunctival GSH uptake requires extracellular Na(+). Significant modulation of GSH, its constituent amino acids, and functions of associated transporters occurs in the conjunctival epithelium with viral inflammatory disease. Topical conjunctival delivery of GSH, its metabolic precursors, or pharmacologic stimulation of endogenous conjunctival GSH secretion carry potential in alleviating viral-inflammatory conjunctivitis.

Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies

This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl(-) channels and as regulators of voltage-gated Ca(2+) channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl(-) channels, but bestrophins remain enigmatic because it is not clear that the Cl(-) channel function can explain Best disease. In addition to functioning as a Cl(-) channel, hBest1 also is able to regulate voltage-gated Ca(2+) channels. Some bestrophins are activated by increases in intracellular Ca(2+) concentration, but whether bestrophins are the molecular counterpart of Ca(2+)-activated Cl(-) channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

Molecular mechanisms of epithelial cell-specific expression and regulation of the human anion exchanger (pendrin) gene

Pendrin, a Cl(-)/anion exchanger encoded by the gene PDS, is highly expressed in the kidney, thyroid, and inner ear epithelia and is essential for bicarbonate secretion, iodide accumulation, and endolymph ion balance, respectively. This study aimed to define promoter regulatory elements essential for renal, thyroid, and inner ear epithelial cell-specific expression of human PDS (hPDS) and to explore the effect of ambient pH and aldosterone on hPDS promoter activity. Endogenous pendrin mRNA and protein were detected in renal HEK293, thyroid LA2, and inner ear VOT36 epithelial cell lines, but not in the fibroblast cell line, NIH3T3. A 4.2-kb hPDS 5'-flanking DNA sequence and consecutive 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the above cell lines. Distinct differences in expression/activity of deduced positive/negative regulatory elements within the hPDS promoter between HEK293, LA2, and VOT36 cells were demonstrated, with only basal activity in NIH3T3 cells. Acidic pH (7.0-7.1) decreased and alkaline pH (7.6-7.7) increased hPDS promoter activity in transfected HEK293 and VOT36, but not in LA2 cells. Aldosterone (10(-8) M) reduced hPDS promoter activity in HEK293 but had no effect in LA2 and VOT36 cells. These pH and aldosterone-induced effects on the hPDS promoter occurred within 96-bp and 89-bp regions, respectively, which likely contain distinct response elements to these modulators. Acidic pH and aldosterone decreased, and alkaline pH increased, endogenous pendrin mRNA level in HEK293 cells. In conclusion, pendrin-mediated HCO3(-) secretion in the renal tubule and anion transport in the endolymph may be regulated transcriptionally by systemic pH and aldosterone.

Transcriptional control of terminal nephron differentiation

Terminal differentiation of epithelial cells into more specialized cell types is a critical step in organogenesis. Throughout the process of terminal differentiation, epithelial progenitors acquire or upregulate expression of renal function genes and cease to proliferate, while expression of embryonic genes is repressed. This exquisite coordination of gene expression is accomplished by signaling networks and transcription factors which couple the external environment with the new functional demands of the cell. While there has been much progress in understanding the early steps involved in renal epithelial cell differentiation, a major gap remains in our knowledge of the factors that control the steps of terminal differentiation. A number of signaling molecules and transcription factors have been recently implicated in determining segmental nephron identity and functional differentiation. While some of these factors (the p53 gene family, hepatocyte nuclear factor-1beta) promote the terminal epithelial differentiation fate, others (Notch, Brn-1, IRX, KLF4, and Foxi1) tend to regulate differentiation of specific nephron segments and individual cell types. This review summarizes current knowledge related to these transcription factors and discusses how diverse cellular signals are integrated to generate a transcriptional output during the process of terminal differentiation. Since these transcriptional processes are accompanied by profound changes in nuclear chromatin structure involving the genes responsible for creating and maintaining the differentiated cell phenotype, future studies should focus on identifying the nature of these epigenetic events and factors, how they are regulated temporally and spatially, and the chromatin environment they eventually reside in.

Overexpression of CLC-3 in HEK293T cells yields novel currents that are pH dependent

ClC-3 is a member of the ClC family of anion channels/transporters. Recently, the closely related proteins ClC-4 and ClC-5 were shown to be Cl−/H+antiporters ( 39 , 44 ). The function of ClC-3 has been controversial. We studied anion currents in HEK293T cells expressing wild-type or mutant ClC-3. The basic biophysical properties of ClC-3 currents were very similar to those of ClC-4 and ClC-5, and distinct from those of the swelling-activated anion channel. ClC-3 expression induced currents with time-dependent activation that rectified sharply in the outward direction. The reversal potential of the current shifted by −48.3 ± 2.5 mV per 10-fold (decade) change in extracellular Cl−concentration, which did not conform to the behavior of an anion-selective channel based upon the Nernst equation, which predicts a −58.4 mV/decade shift at 22°C. Manipulation of extracellular pH (6.35–8.2) altered reversal potential by 10.2 ± 3.0 mV/decade, suggesting that ClC-3 currents were coupled to proton movement. Mutation of a specific glutamate residue (E224A) changed voltage dependence in a manner similar to that observed in other ClC Cl−/H+antiporters. Mutant currents exhibited Nernstian changes in reversal potential in response to altered extracellular Cl−concentration that averaged −60 ± 3.4 mV/decade and were pH independent. Thus ClC-3 overexpression induced a pH-sensitive conductance in HEK293T cells that is biophysically similar to ClC-4 and ClC-5.

Cadmium accumulation in the human retina: effects of age, gender, and cellular toxicity

Tobacco smoking and aging are among the few factors linked to age-related macular degeneration (AMD), a major cause of blindness in the elderly. Recent studies indicate that cadmium (Cd), an environmental toxic trace metal, is approximately four-fold higher in the retinas of smokers compared to non-smokers. In this study, we determined the effects of age and gender on Cd accumulation in human retinal tissues, specifically the neural retina, retinal pigment epithelium (RPE), and choroid. Cadmium levels in cultured RPE cells or retinal tissues isolated from frozen donor eyes were measured using inductively coupled plasma mass spectrometry (ICP-MS) and graphite furnace atomic absorption spectrophotometry (GF-AAS). Cadmium uptake in cultured human RPE cells (ARPE-19) was also assessed using GF-AAS. Toxic effects of cadmium were determined from cell loss (measured as a decrease in cell density) and lactate dehydrogenase release (an indicator of membrane disruption). In "young" eyes (< 55 years) Cd was highest in the retinal pigment epithelium and lowest in the neural retina. Cd was higher in all tissues in aged eyes (>or=55 years) and was significantly higher in the neural retina and RPE in older females. Cultured RPE cells exposed to Cd showed altered cell morphology, decreased cell survival, elevated ROS levels and concentration-dependent disruption of membrane integrity. We conclude that cadmium is accumulated differently in the neural retinal and RPE of older men and women. The deleterious effects of Cd on RPE cells indicate that this environmental toxin is a potentially important factor in age-related retinal disease.

CLC chloride channels and transporters: a biophysical and physiological perspective

Chloride-transporting proteins play fundamental roles in many tissues in the plasma membrane as well as in intracellular membranes. They have received increasing attention in the last years because crucial, and often unexpected and novel, physiological functions have been disclosed with gene-targeting approaches, X-ray crystallography, and biophysical analysis. CLC proteins form a gene family that comprises nine members in mammals, at least four of which are involved in human genetic diseases. The X-ray structure of the bacterial CLC homolog, ClC-ec1, revealed a complex fold and confirmed the anticipated homodimeric double-barreled architecture of CLC-proteins with two separate Cl-ion transport pathways, one in each subunit. Four of the mammalian CLC proteins, ClC-1, ClC-2, ClC-Ka, and ClC-Kb, are chloride ion channels that fulfill their functional roles-stabilization of the membrane potential, transepithelial salt transport, and ion homeostasisin the plasma membrane. The other five CLC proteins are predominantly expressed in intracellular organelles like endosomes and lysosomes, where they are probably important for a proper luminal acidification, in concert with the V-type H+-ATPase. Surprisingly, ClC-4, ClC-5, and probably also ClC-3, are not Cl- ion channels but exhibit significant Cl-/H+ antiporter activity, as does the bacterial homolog ClC-ec1 and the plant homolog AtCLCa. The physiological significance of the Cl-/H+ antiport activity remains to be established.

Light and alcohol evoked electro-oculograms in cystic fibrosis

Cystic fibrosis (CF) is caused by a defect in the cystic fibrosis transmembrane conductance regulator (CFTR) which is a chloride channel. CFTR is expressed in the retinal pigment epithelium (RPE) where it is believed to be important in generating the fast oscillations (FOs) and potentially contributing to the light-electro-oculogram (EOG). The role of CFTR in the alcohol-EOG is unknown. We recruited six individuals with CF (three homozygotes for Delta508 and three heterozygous for Delta508) and recorded the light- and alcohol-EOGs as well as the FOs and compared them to a control group. The results showed that in the CF group the amplitude of the alcohol- and light-EOGs were normal. However, the time to peak of the light- and alcohol-rises were significantly faster than in the control group. We conclude that CFTR is not primarily responsible for the alcohol- or light-rises but is involved in altering the timing of these responses. The FOs showed differences between the homozygotes, heterozygotes and the controls. The amplitudes were significantly higher and the time to the dark troughs were significantly slower in the heterozygote group compared to both controls and the homozygotes. In contrast, the homozygotes did not differ in either amplitude or the timing of the FOs compared to the controls.

Tubulitis and epithelial cell alterations in mouse kidney transplant rejection are independent of CD103, perforin or granzymes A/B

One of the defining lesions of kidney allograft rejection is epithelial deterioration and invasion by inflammatory cells (tubulitis). We examined epithelial changes and their relationship to effector T cells and to CD103/E-cadherin interactions in mouse kidney allografts. Rejecting allografts showed interstitial mononuclear infiltration from day 5. Loss of epithelial mass, estimated by tubular surface area, and tubulitis were minimal through day 7 and severe by day 21. Tubules in day 21 allografts manifested severe reduction of E-cadherin and Ksp-cadherin by immunostaining with redistribution to the apical membrane, indicating loss of polarity. By flow cytometry T cells isolated from allografts were 25% CD103+. Laser capture microdissection and RT-PCR showed increased CD103 mRNA in the interstitium and tubules. However, allografts in hosts lacking CD103 developed tubulitis, cadherin loss, and epithelial deterioration similar to wild-type hosts. The loss of cadherins and epithelial mass was also independent of perforin and granzymes A and B. Thus rejection is characterized by severe tubular deterioration associated with CD103+ T cells but not mediated by CD103/cadherin interactions or granzyme-perforin cytotoxic mechanisms. We suggest that alloimmune effector T cells mediate epithelial injury by contact-independent mechanisms related to delayed type hypersensitivity, followed by invasion of the altered epithelium to produce tubulitis.

Regulation of the expression of human organic anion transporter 3 by hepatocyte nuclear factor 1alpha/beta and DNA methylation

Human organic anion transporter 3 (hOAT3/SLC22A8) is predominantly expressed in the proximal tubules of the kidney and plays a major role in the urinary excretion of a variety of organic anions. The promoter region of hOAT3 was characterized to elucidate the mechanism underlying the tissue-specific expression of hOAT3. The minimal promoter of hOAT3 was identified to be located approximately 300 base pairs upstream of the transcriptional start site, where there are canonical TATA and hepatocyte nuclear factor (HNF1) binding motifs, which are conserved in the rodent Oat3 genes. Transactivation assays revealed that HNF1alpha and HNF1beta markedly increased hOAT3 promoter activity, where the transactivation potency of HNF1beta was lower than that of HNF1alpha. Mutations in the HNF1 binding motif prevented the transactivation. Electrophoretic mobility shift assays demonstrated binding of the HNF1alpha/HNF1alpha homodimer or HNF1alpha/HNF1beta heterodimer to the hOAT3 promoter. It was also demonstrated that the promoter activity of hOAT3 is repressed by DNA methylation. Moreover, the expression of hOAT3 was activated de novo by forced expression of HNF1alpha alone or both HNF1alpha and HNF1beta together with the concomitant DNA demethylation in human embryonic kidney 293 cells that lack expression of endogenous HNF1alpha and HNF1beta, whereas forced expression of HNF1beta alone could not activate the expression of hOAT3. This suggests a synergistic action of the HNF1alpha/HNF1alpha homodimer or HNF1alpha/HNF1beta heterodimer and DNA demethylation for the constitutive expression of hOAT3. These results indicate that the tissue-specific expression of hOAT3 might be regulated by the concerted effect of genetic (HNF1alpha and HNF1beta) and epigenetic (DNA methylation) factors.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Functional abnormalities in the retinal pigment epithelium of CFTR mutant mice

In response to light, the mouse retinal pigment epithelium (RPE) generates a series of slow changes in potential that are referred to as the c-wave, fast oscillation (FO) and light peak (LP) of the electroretinogram (ERG). While the FO is known to reflect a Cl(-) conductance generated at the basal membrane of the RPE, the specific channel (s) underlying this potential has not been identified. In the present study we examined two strains of mice with cftr mutations to define the contribution that cystic fibrosis transmembrane regulator (CFTR)-mediated Cl(-) conductance makes to the mouse ERG. Responses obtained from cftr(Delta508/Delta508) mice exhibited an overall reduction in all components generated by the RPE in response to light without alteration of the luminance response function. Responses obtained from cftr(-/-) mice were also reduced in amplitude. These results illustrate the usefulness of ERG analysis of mice deficient in ion channels that are expressed in the RPE, and indicate that CFTR contributes to the generation of RPE-driven ERG components, but that it is not the sole generator of any one of these components.

Oxidative stress in ARPE-19 cultures: do melanosomes confer cytoprotection?

The pigment melanin has antioxidant properties that could theoretically reduce oxidative damage to the retinal pigment epithelium (RPE), perhaps protecting against retinal diseases with an oxidative stress component like age-related macular degeneration. To determine whether melanin confers cytoprotection on RPE cells, melanosomes or control particles were introduced by phagocytosis into the human cell line ARPE-19 and oxidative stress was induced chemically (H2O2 or tert-butyl hydroperoxide) or with visible light. Since the iron-binding capacity of melanin is important for its antioxidant function, experiments were performed to confirm that the melanosomes were not iron saturated. Cytotoxicity was assessed by measures of plasma or lysosomal membrane integrity, mitochondrial function, and cell-substrate reattachment. Oxidative stress protocols were critically evaluated to produce modest cytotoxicity, which might allow detection of a small cytoprotective effect as expected for melanosomes. Particle internalization alone had no effect on baseline metabolic activity or on major RPE antioxidants. Particles were tested in multiple oxidative stress experiments in which culture conditions known to affect stress-induced cytotoxicity, notably culture density, were varied. No testing condition or outcome measure revealed a consistent protective (or cytotoxic) effect of melanosomes, indicating that measures of lysosome stability or whole cell viability do not demonstrate an antioxidant role for RPE melanosomes. If the melanosome, an insoluble particle, performs a cytoprotective function within cells, its effects may be limited to the local environment of the organelle and undetectable by conventional methods.

Looking chloride channels straight in the eye: bestrophins, lipofuscinosis, and retinal degeneration

Recent evidence suggests that Cl(-) ion channels are important for retinal integrity. Bestrophin Cl(-) channel mutations in humans are genetically linked to a juvenile form of macular degeneration, and disruption of some ClC Cl(-) channels in mice leads to retinal degeneration. In both cases, accumulation of lipofuscin pigment is a key feature of the cellular degeneration. Because Cl(-) channels regulate the ionic environment inside organelles in the endosomal-lysosomal pathway, retinal degeneration may result from defects in lysosomal trafficking or function.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

ClC-5 chloride channel alters expression of the epithelial sodium channel (ENaC)

ClC-5 chloride channels and epithelial sodium channels (ENaC) are present in many cell types including airway and retinal epithelia. Since ENaC activity is known to be affected by chloride transport, we co-injected Xenopus oocytes with cRNAs encoding ENaC and ClC-5 to investigate whether channel currents are impacted by heterologous co-expression of these proteins. ClC-5 currents were not detectably affected by co-expression with ENaC, whereas amiloride-sensitive ENaC currents were significantly lower compared to control oocytes expressing ENaC alone. Co-expression of ENaC with cRNA sequences encoding non-conducting fragments of ClC-5 revealed that the amino acid sequence region between positions 347 and 647 was sufficient for inhibition of ENaC currents. Co-expression of ENaC and another transport protein, the sodium dicarboxylate co-transporter (NaDC-1), did not affect ENaC currents. To test whether the inhibitory effects of ClC-5 were specific for ENaC, ClC-5 was also co-expressed with CFTR. CFTR currents were also inhibited by co-expression with ClC-5, whereas ClC-5 currents were unaffected. Western blot analysis of biotinylated oocyte surface membranes revealed that the co-expression of ClC-5 with ENaC, CFTR, or NaDC-1 decreased the abundance of these proteins at the surface membrane. We conclude that overexpression of ClC-5, specifically amino acids 347-647, can alter the normal translation or trafficking of ENaC and other ion transport proteins by a mechanism that is independent of the chloride conductance of ClC-5.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Angiotensin II (AT1) receptors and NADPH oxidase regulate Cl- current elicited by beta1 integrin stretch in rabbit ventricular myocytes

Direct stretch of β1 integrin activates an outwardly rectifying, tamoxifen-sensitive Cl⁻ current (Cl⁻ SAC) via focal adhesion kinase (FAK) and/or Src. The characteristics of Cl⁻ SAC resemble those of the volume-sensitive Cl⁻ current, I_Cl,swell. Because myocyte stretch releases angiotensin II (AngII), which binds AT1 receptors (AT1R) and stimulates FAK and Src in an autocrine-paracrine loop, we tested whether AT1R and their downstream signaling cascade participate in mechanotransduction. Paramagnetic beads coated with mAb for β1-integrin were applied to myocytes and pulled upward with an electromagnet while recording whole-cell anion current. Losartan (5 μM), an AT1R competitive antagonist, blocked Cl⁻ SAC but did not significantly alter the background Cl⁻ current in the absence of integrin stretch. AT1R signaling is mediated largely by H₂O₂ produced from superoxide generated by sarcolemmal NADPH oxidase. Diphenyleneiodonium (DPI, 60 μM), a potent NADPH oxidase inhibitor, rapidly and completely blocked both Cl⁻ SAC elicited by stretch and the background Cl⁻ current. A structurally unrelated NADPH oxidase inhibitor, 4-(2-aminoethyl) benzenesulfonyl fluoride (AEBSF, 0.5 and 2 mM), also rapidly and completely blocked Cl⁻ SAC as well as a large fraction of the background Cl⁻ current. With continuing integrin stretch, Cl⁻ SAC recovered upon washout of AEBSF (2 mM). In the absence of stretch, exogenous AngII (5 nM) activated an outwardly rectifying Cl⁻ current that was rapidly and completely blocked by DPI (60 μM). Moreover, exogenous H₂O₂ (10, 100, and 500 μM), the eventual product of NADPH oxidase activity, also activated Cl⁻ SAC in the absence of stretch, whereas catalase (1,000 U/ml), an H₂O₂ scavenger, attenuated the response to stretch. Application of H₂O₂ during NADPH oxidase inhibition by either DPI (60 μM) or AEBSF (0.5 mM) did not fully reactivate Cl⁻ SAC, however. These results suggest that stretch of β1-integrin in cardiac myocytes elicits Cl⁻ SAC by activating AT1R and NADPH oxidase and, thereby, producing reactive oxygen species. In addition, NADPH oxidase may be intimately coupled to the channel responsible for Cl⁻ SAC, providing a second regulatory pathway.

Role of the Hepatocyte Nuclear Factor-1β (HNF-1β) C-terminal Domain in Pkhd1 (ARPKD) Gene Transcription and Renal Cystogenesis

Hepatocyte nuclear factor-1beta (HNF-1beta) is a homeodomain-containing transcription factor that regulates tissue-specific gene expression in the kidney and other epithelial organs. Mutations of HNF-1beta produce congenital cystic abnormalities of the kidney, and previous studies showed that HNF-1beta regulates the expression of the autosomal recessive polycystic kidney disease (ARPKD) gene, Pkhd1. Here we show that the C-terminal region of HNF-1beta contains an activation domain that is functional when fused to a heterologous DNA-binding domain. An HNF-1beta deletion mutant lacking the C-terminal domain interacts with wild-type HNF-1beta, binds DNA, and functions as a dominant-negative inhibitor of a chromosomally integrated Pkhd1 promoter. The activation of the Pkhd1 promoter by wild-type HNF-1beta is stimulated by sodium butyrate or coactivators CREB (cAMP-response element)-binding protein (CBP) and P/CAF. The interaction with CBP and P/CAF requires the C-terminal domain. Expression of an HNF-1beta C-terminal deletion mutant in transgenic mice produces renal cysts, increased cell proliferation, and dilatation of the ureter similar to mice with kidney-specific inactivation of HNF-1beta. Pkhd1 expression is inhibited in cystic collecting ducts but not in non-cystic proximal tubules, despite transgene expression in this nephron segment. We conclude that the C-terminal domain of HNF-1beta is required for the activation of the Pkhd1 promoter. Deletion mutants lacking the C-terminal domain function as dominant-negative mutants, possibly by preventing the recruitment of histone acetylases to the promoter. Cyst formation correlates with inhibition of Pkhd1 expression, which argues that mutations of HNF-1beta produce kidney cysts by down-regulating the ARPKD gene, Pkhd1. Expression of HNF-1alpha in proximal tubules may protect against cystogenesis.

Mitochondrial fatty acid beta-oxidation in the human eye and brain: implications for the retinopathy of long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency

The retinal pigment epithelium (RPE) and the choriocapillaris are affected early in the retinopathy associated with long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency. RPE in culture possesses the machinery needed for mitochondrial fatty acid beta-oxidation in vitro. To further elucidate pathogenesis of LCHAD retinopathy, we performed immunohistochemistry of the human eye and brain with antibodies to beta-oxidation enzymes. Human eye and brain sections were stained with antibodies to medium-chain (MCAD) and very long-chain acyl-CoA dehydrogenase (VLCAD), short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), and mitochondrial trifunctional protein (MTP) harboring LCHAD. Antibodies to 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) and cytochrome c oxidase subunit I (COX I) were used as a reference. VLCAD, MTP, MCAD, SCHAD, MHBD, and COX I antibodies labeled most retinal layers and tissues of the human eye actively involved in oxidative metabolism (extraocular and intraocular muscle, the RPE, the corneal endothelium, and the ciliary epithelium). MTP and COX I antibodies labeled the inner segments of photoreceptors. The choriocapillaris was labeled only with SCHAD and MCAD antibodies. In the brain, the choroid plexus and nuclei of the brain stem were most intensely labeled with beta-oxidation antibodies, whereas COX I antibodies strongly labeled neurons in several regions of the brain. Mitochondrial fatty acid beta-oxidation likely plays a role in ocular energy production in vivo. The RPE rather than the choriocapillaris could be the critical affected cell layer in LCHAD retinopathy. Reduced energy generation in the choroid plexus may contribute to the cerebral edema observed in patients with beta-oxidation defects.

Reduced renal ClC-5 Cl− channel expression in spontaneously hypertensive rats with microalbuminuria

Mutations in a renal-specific Cl(-) channel, ClC-5, result in low-molecular-weight proteinuria. Herein we studied ClC-5 expression in the kidneys of spontaneously hypertensive rats (SHR) to identify possible causes of their increased urinary excretion of albumin. The amount of ClC-5 protein was significantly reduced in 3-month-old SHR as compared with normotensive Wistar/Kyoto (WKY) rats. The ClC-5 protein level was partially restored by short term administration of perindopril, an inhibitor of angiotensin-converting enzyme. Corresponding to the increase in ClC-5 expression, the albuminuria in SHR improved to the control level. These results implicate the ClC-5 Cl(-) channel reduction in the development of albuminuria in the early stage of essential hypertension.

Release of ATP from retinal pigment epithelial cells involves both CFTR and vesicular transport

The retinal pigment epithelium (RPE) faces the photoreceptor outer segments and regulates the composition of the interstitial subretinal space. ATP enhances fluid movement from the subretinal space across the RPE. RPE cells can themselves release ATP, but the mechanisms and polarity of this release are unknown. The RPE expresses the cystic fibrosis transmembrane conductance regulator (CFTR), and CFTR is associated with ATP release in other epithelial cells. However, an increasing number of reports have suggested that the exocytotic pathway contributes to release. In the present study, we examined the involvement of CFTR and the vesicular pathway in ATP release from RPE cells. Release from cultured human ARPE-19 cells and across the apical membrane of fresh bovine RPE cells in an eyecup was studied. A cAMP cocktail to activate CFTR triggered ATP release from fresh and cultured RPE cells. Release from both RPE preparations was largely prevented by the broad-acting blocker glibenclamide and the specific thiazolidinone CFTR inhibitor CFTR-172. The block by CFTR-172 was enhanced by preincubation and prevented ATP release with 3.5 microM IC(50). The rise in intracellular Ca(2+) accompanying hypotonic challenge was prevented by CFTR-172. The vesicular transport inhibitor brefeldin A prevented ATP release after stimulation with both hypotonic and cAMP conditions, suggesting vesicular insertion was also involved. These results show an intimate involvement of CFTR in ATP release from RPE cells which can autostimulate receptors on the apical membrane to modify Ca(2+) signaling. The requirement for both CFTR and vesicular transport pathways suggests vesicular insertion of CFTR may underlie the release of ATP.

The human paracellin-1 gene (hPCLN-1): renal epithelial cell-specific expression and regulation

Tubular reabsorption of Mg2+ is mediated by the tight junction protein paracellin-1, which is encoded by the gene PCLN-1 (CLDN16) and exclusively expressed in the kidney. Tubular Mg2+ reclamation is modulated by many hormones and factors. The aim of this study was to define regulatory elements essential for renal tubular cell-specific expression of human PCLN-1 (hPCLN-1) and to explore the effect of Mg2+ transport modulators on the paracellin-1 gene promoter. Endogenous paracellin-1 mRNA and protein were detected in renal cell lines opossom kidney (OK), HEK293, and MDCT, but not in the fibroblast cell line NIH3T3. A 7.5-kb hPCLN-1 5'-flanking DNA sequence along with seven 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the renal and nonrenal cells. The highest levels of luciferase activity resulted from transfection of a 5'-flanking 2.5-kb fragment (pJ2M). This activity was maximal in OK cells, was orientation dependent, and was absent in NIH3T3 cells. Mg2+ deprivation significantly increased pJ2M-driven activity in transfected OK cells, whereas Mg2+ load decreased it compared with conditions of normal Mg2+. Deletion analysis along with electrophoretic mobility-shift assay demonstrated that OK cells contain nuclear proteins, which bind a 70-bp region between -1633 and -1703 of major functional significance. Deleting this 70-bp segment, which contains a single peroxisome proliferator-response element (PPRE), or mutating the PPRE, caused a 60% reduction in luciferase activity. Stimulating the 70-bp sequence with 1,25(OH)2 vitamin D decreased luciferase activity by 52%. This effect of 1,25(OH)2 vitamin D was abolished in the absence of PPRE or in the presence of mutated PPRE. We conclude that the PPRE within this 70-bp DNA region may play a key role in the cell-specific and regulatory activity of the hPCLN-1 promoter. Ambient Mg2+ concentration and 1,25(OH)2 vitamin D may modulate paracellular, paracellin-1-mediated, Mg2+ transport at the transcriptional level. 1,25(OH)2 vitamin D exerts its activity on the hPCLN-1 promoter likely via the PPRE site.

Oxidative-stress-related proteome changes in Helicobacter pylori-infected human gastric mucosa

Helicobacter pylori infection leads to gastroduodenal inflammation, peptic ulceration and gastric carcinoma. Proteomic analysis of the human gastric mucosa from the patients with erosive gastritis, peptic ulcer or gastric cancer, which were either infected or not with H. pylori, was used to determine the differentially expressed proteins by H. pylori in the human gastric mucosa in order to investigate the pathogenic mechanism of H. pylori -induced gastric diseases. Prior to the experiment, the expression of the main 18 proteins were identified in the gastric mucosa and used for a proteome map of the human gastric mucosa. Using two-dimensional electrophoresis of the protein isolated from the H. pylori -infected tissues, Coomassie Brilliant Blue staining and computerized analysis of the stained gel, the expression of eight proteins were altered in the H. pylori -infected tissues compared with the non-infected tissues. MS analysis (matrix-assisted laser desorption/ionization-time of flight MS) of the tryptic fragment and a data search allowed the the identification of the four increased proteins (78 kDa glucose-regulated protein precursor, endoplasmin precursor, aldehyde dehydrogenase 2 and L-lactate dehydrogenase B chain) and the four decreased proteins (intracellular chloride channel protein 1, glutathione S-transferase, heat-shock protein 60 and cytokeratin 8) caused by H. pylori infection in the gastric mucosa. These proteins are related to cell proliferation, carcinogenesis, cytoskeletal function and cellular defence mechanism. The common feature is that these proteins are related to oxidative-stress-mediated cell damage. In conclusion, the established gastric mucosal proteome map might be useful for detecting the disease-related protein changes. The H. pylori -induced alterations in protein expression demonstrate the involvement of oxidative stress in the pathogenesis of H. pylori -induced gastric diseases, including inflammation, ulceration and carcinogenesis.

Chloride currents in acutely isolated Xenopus retinal pigment epithelial cells

The retinal pigment epithelium (RPE) regulates the ionic composition of the fluid surrounding the photoreceptors by transport mechanisms that utilize Cl- channels. Cl- currents in RPE cells, however, remain incompletely characterized. The purpose of this study was to identify the Cl- currents in acutely isolated Xenopus RPE cells using whole-cell patch clamp. We describe three different Cl- currents. (1) An inwardly rectifying Cl- current, ICl,ir, activates slowly with hyperpolarization (tauact = ~1 s at -80 mV, V1/2= -94 +/- 3 mV), is blocked by Zn2+ (IC50 =185 microM), is stimulated by acid (ICl,ir is 5 times larger at pH 6 than pH 8), and is blocked by DIDS in a voltage-dependent manner. ICl,ir closely resembles cloned ClC-2currents. (2) An outwardly rectifying Cl- current, ICl,Ca, is stimulated by elevated cytosolic free [Ca2+]. With 1 microM free [Ca2+]i in the patch pipette, ICl,Ca activates slowly with depolarization (tauact =325 ms at 100 mV) and deactivates upon hyperpolarization. ICl,Ca is not blocked by 1 mM Zn2+ or 10 microM Gd3+ but is blocked by DIDS. High extracellular [Ca2+] (10 mM) also activates ICl,Ca. (3) A non-rectifying current is activated by elevation of cytoplasmic cAMP with forskolin and IBMX. In addition to these three Cl- currents, Xenopus RPE cells exhibit a non-selective background current (Ibkg) which has a linear I-V relationship and is voltage insensitive. This current is blocked by Zn2+ (IC50 of 5.3 microM) or 10 microM Gd3+. This description provides new insights into the physiology of Cl- channels involved in salt and fluid transport by the retinal pigment epithelium.