Chloride transport in cultured fetal human retinal pigment epithelium

Apical CLC-2 in retinal pigment epithelium is crucial for survival of the outer retina

Knockout of the chloride channel protein 2 (CLC‐2; CLCN2) results in fast progressing blindness in mice. Retinal Pigment Epithelium (RPE) and photoreceptors undergo, in parallel, rapid, and profound morphological changes and degeneration. Immunohistochemistry and electron microscopy of the outer retina and electroretinography of the CLC‐2 KO mouse demonstrated normal morphology at postnatal day 2, followed by drastic changes in RPE and photoreceptor morphology and loss of vision during the first postnatal month. To investigate whether the RPE or the photoreceptors are the primary cause of the degeneration, we injected lentiviruses carrying HA‐tagged CLC‐2 with an RPE‐specific promotor in the subretinal space of CLC‐2‐KO mice at the time of eye opening. As expected, CLC‐2‐HA was expressed exclusively in RPE; strikingly, this procedure rescued the degeneration of both RPE and photoreceptors. Light response in transduced eyes was also recovered. Only a fraction of RPE was transduced with the lentivirus; however, the entire RPE monolayer appears healthy, even the RPE cells not expressing the CLC‐2‐HA. Surprisingly, in contrast with previous physiological observations that postulate that CLC‐2 has a basolateral localization in RPE, our immunofluorescence experiments demonstrated CLC‐2 has an apical distribution, facing the subretinal space and the photoreceptor outer segments. Our findings suggest that CLC‐2 does not play the postulated role in fluid transport at the basolateral membrane. Rather, they suggest that CLC‐2 performs a critical homeostatic role in the subretinal compartment involving a chloride regulatory mechanism that is critical for the survival of both RPE and photoreceptors.

Mechanisms of Ion Transport Across the Mouse Retinal Pigment Epithelium Measured In Vitro

Purpose

To examine ion transport across the mouse retinal pigment epithelium (RPE), measured by the short-circuit current (I_SC) and transepithelial resistance (TER).

Methods

Sheets of RPE from mice (C57BL6/J) with retina, choroid, and sclera attached were mounted in Ussing chambers (0.031-cm² aperture) and Krebs solution. The I_SC and TER were recorded with voltage clamps. Receptors implicated in ion transport were blocked or stimulated by ligands applied to both sides.

Results

The mean initial I_SC was −12.0 ± 3.9 µA/cm² (basolateral negative), and mean TER was 67.1 ± 8.0 ohm·cm². RPE preparations remained stable for 3 hours, with I_SC decreasing by 0.078 ± 0,033 µA/cm²/hr. Adenosine triphosphate (100 µM) increased I_SC by 2.22 ± 0.41 µA/cm² (P = 0.003). Epinephrine (100 µM) increased I_SC by 1.14 ± 0.19 µA/cm² (P = 0.011). Bumetanide (100 µM) reduced I_SC by 1.72 ± 0.73 µA/cm² (P = 0.027). Ouabain (1 mM) induced a biphasic response: an I_SC increase from −7.9 ± 2.4 to −15.49 ± 2.12 µA/cm² and then a decrease to −3.7 ± 2.2 µA/cm². Ouabain increased TER by 15.3 ± 4.8 ohm·cm². These compounds were added sequentially. Apical [K⁺]_o at zero mM transiently increased I_SC by 3.36 ± 1.06 µA/cm². Ba⁺⁺ decreased I_SC from −10.4 ± 3.1 to −6.6 ± 1.8 µA/cm² (P = 0.01). Ba⁺⁺ reversed the K⁺-free response, with I_sc decreasing further from −5.65 ± 1.24 to −3.37 ± 0.79 µA/cm² (P = 0.029).

Conclusions

The I_SC and TER can be recorded from the mouse RPE for 3 hours. Adrenergic and purinergic receptors affect murine RPE ion transport. Sodium–potassium adenosine triphosphatase plays a role in net ion transport across mouse RPE, and Na-K-2Cl cotransporter activity partly accounts for transepithelial ion transport. Mimicking light-induced changes, low subretinal [K⁺]_o increases ion transport transiently, dependent on K⁺ channels.

High-yield, automated intracellular electrophysiology in retinal pigment epithelia

BACKGROUND

Recent advancements with induced pluripotent stem cell-derived (iPSC) retinal pigment epithelium (RPE) have made disease modeling and cell therapy for macular degeneration feasible. However, current techniques for intracellular electrophysiology - used to validate epithelial function - are painstaking and require manual skill; limiting experimental throughput.

NEW METHOD

A five-stage algorithm, leveraging advances in automated patch clamping, systematically derived and optimized, improves yield and reduces skill when compared to conventional, manual techniques.

RESULTS

The automated algorithm improves yield per attempt from 17% (manually, n = 23) to 22% (automated, n = 120) (chi-squared, p = 0.004). Specifically for RPE, depressing the local cell membrane by 6 μm and electroporating (buzzing) just prior to this depth (5 μm) maximized yield.

COMPARISON WITH EXISTING METHOD

Conventionally, intracellular epithelial electrophysiology is performed by manually lowering a pipette with a micromanipulator, blindly, towards a monolayer of cells and spontaneously stopping when the magnitude of the instantaneous measured membrane potential decreased below a predetermined threshold. The new method automatically measures the pipette tip resistance during the descent, detects the cell surface, indents the cell membrane, and briefly buzzes to electroporate the membrane while descending, overall achieving a higher yield than conventional methods.

CONCLUSIONS

This paper presents an algorithm for high-yield, automated intracellular electrophysiology in epithelia; optimized for human RPE. Automation reduces required user skill and training while, simultaneously, improving yield. This algorithm could enable large-scale exploration of drug toxicity and physiological function verification for numerous kinds of epithelia.

Directional Fluid Transport across Organ-Blood Barriers: Physiology and Cell Biology

Directional fluid flow is an essential process for embryo development as well as for organ and organism homeostasis. Here, we review the diverse structure of various organ-blood barriers, the driving forces, transporters, and polarity mechanisms that regulate fluid transport across them, focusing on kidney-, eye-, and brain-blood barriers. We end by discussing how cross talk between barrier epithelial and endothelial cells, perivascular cells, and basement membrane signaling contribute to generate and maintain organ-blood barriers.

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.

Stimulation of aquaporin-mediated fluid transport by cyclic GMP in human retinal pigment epithelium in vitro

PURPOSE

The retinal pigment epithelium (RPE) expresses aquaporin-1 (AQP1) and components of the natriuretic peptide signaling pathway. We hypothesized that stimulation of the natriuretic signaling pathway in RPE with atrial natriuretic peptide (ANP) and with membrane-permeable analogs of cGMP would induce a net apical-to-basal transport of fluid.

METHODS

The hypothesis was tested using human RPE cultures that retain properties seen in vivo. Confluent monolayers were treated with ANP or membrane-permeable cGMP analogs in the presence of anantin, H-8, and an AQP1 inhibitor, AqB013. Fluid movement from the apical to basal chambers was measured by weight and used to calculate net fluid transport.

RESULTS

Our results demonstrated a 40% increase in net apical-to-basal fluid transport by ANP (5 μM) that was inhibited completely by the ANP receptor antagonist anantin and a 60% increase in net apical-to-basal fluid transport in response to the extracellularly applied membrane-permeable cGMP analog pCPT-cGMP (50 μM), which was not affected by the protein kinase G inhibitor H-8. The aquaporin antagonist AqB013 (20 μM) inhibited the cGMP-stimulated RPE fluid flux.

CONCLUSIONS

The effect of cGMP is consistent with an enhancement of the net fluid flux in RPE mediated by AQP1 channels. Pharmacologic activation of cGMP signaling and concomitant stimulation of fluid uptake from the subretinal space could offer insights into a new approach to treating or reducing the risk of retinal detachment.

The retinal pigment epithelium: something more than a constituent of the blood-retinal barrier--implications for the pathogenesis of diabetic retinopathy

The retinal pigment epithelium (RPE) is an specialized epithelium lying in the interface between the neural retina and the choriocapillaris where it forms the outer blood-retinal barrier (BRB). The main functions of the RPE are the following: (1) transport of nutrients, ions, and water, (2) absorption of light and protection against photooxidation, (3) reisomerization of all-trans-retinal into 11-cis-retinal, which is crucial for the visual cycle, (4) phagocytosis of shed photoreceptor membranes, and (5) secretion of essential factors for the structural integrity of the retina. An overview of these functions will be given. Most of the research on the physiopathology of diabetic retinopathy has been focused on the impairment of the neuroretina and the breakdown of the inner BRB. By contrast, the effects of diabetes on the RPE and in particular on its secretory activity have received less attention. In this regard, new therapeutic strategies addressed to modulating RPE impairment are warranted.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.

Ca++-switch induction of RPE differentiation

Cultured retinal pigment epithelial (RPE) cells are commonly used as a model of the tissue to study their involvement in visual diseases. Unfortunately, cultured RPE often lose their differentiated phenotype reducing their usefulness as a model of the RPE in vivo. In this study, we used a Ca++-switch protocol to initiate the patterned expression of several phenotypic and functional markers of RPE differentiation. Cultured RPE cells from adult donors were maintained through at least six serial passages prior to assay to minimize their differentiated properties. The cells were then subjected to the Ca++-switch protocol and maintained at confluence for up to 4 months. Paired control and Ca++-switch cells were examined for phenotype, pigmentation, and the expression of tyrosinase, CRABP, myocilin, and bestrophin by western blot analysis. The Ca++-switch protocol led to a rapid restriction of N-cadherin to lateral cell borders, and to expression of tyrosinase by day 4. After 8 weeks, the experimental RPE monolayers began to accumulate visible pigment, and after 12 weeks CRABP expression was observed. Myocilin was observed at 4 months after the Ca++-switch but bestrophin was not detected at any time point. Our results suggest this protocol may drive epithelial morphogenesis in RPE cells. We note two specific differences in cells plated in low Ca++, reduced spreading on the substrate and coordinated development of cadherin adhesion when the Ca++-concentration is returned to normal. Thus, we suggest that this method produces phenotypic changes through multiple cell signalling pathways.

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.

HPAF-II, a cell culture model to study pancreatic epithelial cell structure and function

OBJECTIVES

Epithelial cells have distinct apical and basolateral plasma membrane domains separated by tight junctions. This phenotype is essential for the directional transport functions of epithelial cells. Here we characterized a well-differentiated pancreatic epithelial cell line to establish a useful model for understanding the mechanisms involved in the regulation of junctional complexes, polarity, and disease processes in the pancreas.

METHODS

Immunofluorescence of cell junction marker proteins and electron microscopy were used to determine the presence of tight junctions, adherens junctions, and desmosomes. The functionality of tight junctions was tested by transepithelial resistance measurements and transepithelial permeability studies of nonionic molecules. Tight junction function in polarity was determined by laser scanning confocal microscopy.

RESULTS

Immunofluorescence analysis in HPAF-II cells revealed tight junction localization of ZO-1, occludin, and claudin-4; adherens junction localization of E-cadherin and beta-catenin; and desmosomal localization of desmocollin. Transmission electron microscopy showed the presence of tight junctions, adherens junctions, and des-mosomes, and freeze-fracture electron microscopy revealed the presence of distinct anastomosing tight junction strands. Transepithelial electrical resistance and permeability measurements revealed functional tight junctions. In addition, 3-dimensional images of the monolayer generated by laser scanning confocal microscopy revealed that HPAF-II cells show polarity. Immunoblotting and RT-PCR analyses revealed high expression levels of E-cadherin and Na,K-ATPase beta-subunit but low levels of the transcription factor Snail in HPAF-II cells compared with MiaPaCa-2 cells.

CONCLUSION

The HPAF-II cell line is a well-differentiated human pancreatic carcinoma cell line that should be useful as a model for studies aimed at understanding epithelial polarity, regulation of junctional complexes, and disease processes in pancreas.

Na,K-ATPase inhibition alters tight junction structure and permeability in human retinal pigment epithelial cells

Na,K-ATPase regulates a variety of transport functions in epithelial cells. In cultures of human retinal pigment epithelial (RPE) cells, inhibition of Na,K-ATPase by ouabain and K(+) depletion decreased transepithelial electrical resistance (TER) and increased permeability of tight junctions to mannitol and inulin. Electrophysiological studies demonstrated that the decrease in TER was due to an increase in paracellular shunt conductance. At the light microscopy level, this increased permeability was not accompanied by changes in the localization of the tight junction proteins ZO-1, occludin, and claudin-3. At the ultrastructural level, increased tight junction permeability correlated with a decrease in tight junction membrane contact points. Decreased tight junction membrane contact points and increased tight junction permeability were reversible in K(+)-repletion experiments. Confocal microscopy revealed that in control cells, Na,K-ATPase was localized at both apical and basolateral plasma membranes. K(+) depletion resulted in a large reduction of apical Na,K-ATPase, and after K(+) repletion the apical Na,K-ATPase recovered to control levels. These results suggest a functional link exists between Na,K-ATPase and tight junction function in human RPE cells.

Molecular mechanisms of water transport in the eye

The four major sites for ocular water transport, the corneal epithelium and endothelium, the ciliary epithelium, and the retinal pigment epithelium, are reviewed. The cornea has an inherent tendency to swell, which is counteracted by its two surface cell layers, the corneal epithelium and endothelium. The bilayered ciliary epithelium secretes the aqueous humor into the posterior chamber, and the retinal pigment epithelium transports water from the retinal to the choroidal site. For each epithelium, ion transport mechanisms are associated with fluid transport, but the exact molecular coupling sites between ion and water transport remain undefined. In the retinal pigment epithelium, a H+-lactate cotransporter transports water. This protein could be the site of coupling between salt and water in this epithelium. The distribution of aquaporins does not suggest a role for these proteins in a general model for water transport in ocular epithelia. Some water-transporting membranes contain aquaporins, others do not. The ultrastructure is also variable among the cell layers and cannot be fitted into a general model. On the other hand, the direction of cotransport in symporters complies with the direction of fluid transport in both the corneal epi- and endothelium, as well as the ciliary epithelium and retinal pigment epithelium.

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

Retinal pigment epithelium (RPE) possesses regulated chloride channels that are crucial for transepithelial fluid and ion transport. At present, little is known about the molecular nature of chloride channels in human adult RPE (haRPE) or the effects of oxidative stress on membrane conductance properties. In the present study, we assessed ClC channel and cystic fibrosis transmembrane conductance regulator (CFTR) expression and membrane chloride conductance properties in haRPE cells. ClC-5, ClC-3, ClC-2, and CFTR mRNA expression was confirmed with RT-PCR analysis, and protein expression was detected with Western blot analysis and immunofluorescence microscopy. Whole cell recordings of primary cultures of haRPE showed an outwardly rectifying chloride current that was inhibited by the oxidant H(2)O(2). The inhibitory effects of H(2)O(2) were reduced in cultured human RPE cells that were incubated with precursors of glutathione synthesis or that were stably transfected to overexpress glutathione S-transferase. These findings indicate a possible role for ClC channels in haRPE cells and suggest possible redox modulation of human RPE chloride conductances.

Effects of a non-steroidal (ketorolac tromethamine) and a steroidal (dexamethasone) anti-inflammatory drug on refractive state and ocular growth

Topical steroidal anti-inflammatory drugs (SAID) and non-steroidal anti-inflammatory drugs (NSAID) are known to affect fluid balance. The effects of twice daily topical applications of Maxidex (dexamethasone, a SAID), Acular (ketorolac, a NSAID), and saline were examined biometrically on the development of refractive errors and eye growth in chicks raised from days 3-12 wearing either a monocular +10 D, 0 D, or -10 D lens. Biometric analysis showed that neither SAID nor NSAID nor saline affected refractive error compensation but that the anti-inflammatory drugs affected eye growth. In chicks reared with a +10 D lens, dexamethasone induced a decrease in axial length (AL), vitreous chamber (VC) and anterior chamber (AC) depth, while ketorolac only induced a decrease in AC. In -10 D lens chicks dexamethasone again induced a decrease in AL and VC, but did not affect AC depth, whereas ketorolac only induced an increase in AC depth. Taken together, these results suggest that anti-inflammatory drugs can induce changes in ocular size without affecting refractive state and, as such, have implications for the management of progressive myopia.

The Royal College of Surgeons rat: an animal model for inherited retinal degeneration with a still unknown genetic defect

The Royal College of Surgeons (RCS) rat is the first known animal with inherited retinal degeneration. Despite the fact that the genetic defect is not known, the RCS rat is widely used for research in hereditary retinal dystrophies. This review tries to summarize observations which have been made in the RCS rat and to make an attempt to formulate candidate genes which may the cause for the retinal degeneration in this rat strain. The genetic defect in RCS rats causes the inability of the retinal pigment epithelium (RPE) to phagocytose shed photoreceptor outer segments. In normal rats or humans, this circadian process is regulated by both the cyclic adenosine monophosphate (cAMP) and the calcium/ inositol phosphate systems. The calcium/inositol phosphate system seems to be linked to the phagocytosis receptors which recognize photoreceptor outer membranes to initialize phagocytosis. The cAMP system appeared as modulator of the regulation of phagocytosis. An increase in the intracellular cAMP concentration is an 'off' signal for phagocytosis. In RPE cells from RCS rats many observations have been made which indicate a changed second messenger metabolism concerning both the cAMP and the calcium/inositol phosphate systems. The genetic defect seems to concern a protein which is involved in the initialization of a second messenger pathway. We conclude that the genes coding for the phagocytosis receptor or for proteins which are linked to receptors (for example G proteins) are good candidates for defective genes in RCS rats.